Recent Advances in Sn‐Based Heterojunction‐Type Anode Materials for Alkali‐Ion Batteries

Abstract

ABSTRACT

The urgent demand for clean energy solutions has intensified the search for advanced storage materials, with rechargeable alkali‐ion batteries (AIBs) playing a pivotal role in electrochemical energy storage. Enhancing electrode performance is critical to addressing the increasing need for high‐energy and high‐power AIBs. Next‐generation anode materials face significant challenges, including limited energy storage capacities and complex reaction mechanisms that complicate structural modeling. Sn‐based materials have emerged as promising candidates for AIBs due to their inherent advantages. Recent research has increasingly focused on the development of heterojunctions as a strategy to enhance the performance of Sn‐based anode materials. Despite significant advances in this field, comprehensive reviews summarizing the latest developments are still sparse. This review provides a detailed overview of recent progress in Sn‐based heterojunction‐type anode materials. It begins with an explanation of the concept of heterojunctions, including their fabrication, characterization, and classification. Cutting‐edge research on Sn‐based heterojunction‐type anodes for AIBs is highlighted. Finally, the review summarizes the latest advancements in heterojunction technology and discusses future directions for research and development in this area.

Introduction

With the development of society and economy, the demand for alkali-ion batteries (AIBs) with higher energy density, longer cycle life, better rate performance, and higher safety is rapidly increasing [1]. Extensive research on AIB anode materials and Sn-based anodes has garnered considerable interest due to their remarkable potential demonstrated in conversion and alloying reactions [2–5]. These materials boast a high theoretical specific capacity, are straightforward to synthesize, exhibit high safety, and are relatively low-cost. Thus, they are considered highly promising as anode materials for batteries [6–8]. However, the practical application of Sn-based anode materials faces significant challenges. During charge and discharge processes, Sn experiences volume expansion of up to 300%, which tests the material's structural integrity. Continuous ion intercalation and deintercalation can induce internal stress, which can potentially lead to material fragmentation and crack formation, compromising the battery's lifespan and initial Coulombic efficiency (ICE). Moreover, the low ionic diffusion rate of Sn-based anode materials restricts the diffusion of Li⁺, Na⁺, and K⁺ in the electrode material, leading to decreased charge and discharge rates as well as limited power performance of the battery. Addressing these challenges is crucial for advancing Sn-based anode materials and broader battery technology as a whole, necessitating ongoing research and innovation to develop more efficient, stable, and safe energy storage solutions.

Researchers have implemented various modification strategies to address these challenges [9, 10]. The first strategy involves the application of nanotechnology [11–14]. Downsizing the active material to the nanoscale creates multiple ion diffusion pathways, facilitates electrolyte penetration and ion movement, and significantly mitigates volume expansion during charge and discharge cycles, thereby substantially enhancing electrochemical performance [15]. The second approach focused on the architectural design of Sn-based anode materials, ranging from zero-dimensional (0D) to three-dimensional (3D) structures [16–18]. Intelligent structural design incorporates void spaces within the material to accommodate volumetric changes, whereas innovative structural features enhance both structural stability and kinetic properties. The third strategy involves the use of composite materials, including carbon-based materials, metallic materials, and metal compounds [19–21]. By combining two or more distinct phases of electrode materials, this strategy reinforces electrical conductivity and structural integrity, leading to enhanced rate capabilities and cycling endurance [22].

In addition to the aforementioned methods, the construction of heterojunctions is also an important method for modifying Sn-based anode materials. In recent years, heterojunction anode materials have seen considerable development. Characterized by distinctive structural attributes and adjustable properties, these materials demonstrate exceptional electrochemical performance. They surpass conventional non-heterojunction anode materials in performance, particularly in enhancing electrical conductivity, improving cycle stability, and increasing capacity. Researchers have rigorously reviewed the design strategies for these materials, projecting their future development [23–25]. Nevertheless, comprehensive reviews specifically addressing the application of Sn-based heterojunction-type anode materials are notably scarce.

In this review, we provide a comprehensive overview of the research status of Sn-based heterojunction-type anode materials, focusing on the definition of heterojunctions, comparative advantages over traditional composite materials, advanced technology and methodologies for fabricating and characterizing such heterojunction materials, application in AIBs, and modification methods. Additionally, we meticulously examine the utilization of Sn-based heterojunction-type anode materials in AIBs in recent years (Figure 1). Through these in-depth analyses and discussions, we aim to offer valuable insights to inspire researchers in this field in their future inquiries and innovations.

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Introduction of Heterojunctions

Definition of Heterojunction

The concept of heterojunctions, which originated in the field of semiconductors, was first applied to wide-bandgap semiconductor emitters [26, 27]. These structures consist of two or more different semiconductor materials bound together by van der Waals forces and chemical bonds, creating complex composite systems with intricate geometries and interfacial junctions [28, 29]. The differences in bandgap, Fermi level, and carrier concentration between the constituent materials generate built-in electric fields and alter charge distribution at the interfaces, which affects device performance. This unique physical phenomenon has not only expanded the performance potential of semiconductor devices but has also significantly impacted materials science and electronic engineering. The characteristics of heterojunctions offer substantial potential for advancing electronic device design, particularly in enhancing device efficiency and functionality.

In AIB anode research, Sn-based heterojunction-type anode materials have been shown to significantly improve the kinetic properties of materials, leading to higher energy density, longer cycle life, better rate performance, and improved safety. The built-in electric fields accelerate ion diffusion, enhance ion adsorption, strengthen the structural integrity of the electrode materials, and reduce energy barriers for ion migration. The redistribution of charge creates numerous electrochemically active sites for redox reactions, further enhancing the reversible energy storage capacity, cyclic stability, and rate performance of the electrode materials [30–34] If the reaction's kinetic process is slow, it often leads to reduced Coulombic efficiency and cyclic stability. In such cases, heterojunctions can be employed to improve the ICE of Sn-based anode materials.

Classification of Heterojunctions

With advancements in the study of heterojunctions, this concept has increasingly garnered attention from researchers. Various types of heterojunctions have been constructed and thoroughly investigated. This review categorizes heterojunctions into three main types based on the characteristics of the semiconductor materials that constitute them (Figure 2): homotype heterojunctions (p–p/n–n), anisotype heterojunctions (p–n/n–p), and Schottky junctions [37]. This classification aids in understanding the features and advantages of different heterojunctions and provides a theoretical foundation for designing and optimizing new materials with specific functions. Through in-depth analysis of these heterojunctions, we can gain a deeper understanding of their potential applications in Sn-based anode materials.

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The characteristics of homotype heterojunctions can be summarized as follows. (1) Band offset: due to differences in the band structures of the two materials, band offsets occur in homotype heterojunctions. In n–n junctions, the conduction band of the n-type semiconductor shifts upward to a higher energy level, creating a barrier. In p–p junctions, the valence band of the p-type semiconductor shifts downward to a lower energy level, also forming a barrier. (2) Tunneling effect: the barrier in a homotype heterojunction induces a tunneling effect. According to quantum mechanics, even if carriers do not have sufficient energy to directly overcome the barrier, they can still penetrate it through the tunneling effect, facilitating transfer from one semiconductor region to another. (3) Current transport characteristics: in a homotype heterojunction, the behavior of electrons and holes is significantly influenced by the barrier. For n–n homotype heterojunctions, electrons primarily transport through the barrier via tunneling, whereas for p–p junctions, holes are transported through a similar tunneling mechanism. These characteristics endow homotype heterojunctions with unique application value in the design of electronic devices, especially in situations requiring precise control of carrier transport and specific electrical characteristics. By thoroughly understanding and leveraging these characteristics, semiconductor device performance can be optimized, propelling the development of electronic technology.

Anisotype heterojunctions exhibit different electrical characteristics under forward and reverse bias conditions. (1) Forward bias: when the p-side is connected to a positive voltage and the n-side is grounded, the p–n junction allows current to pass. In this configuration, electrons from the n-type semiconductor transition to fill the holes in the p-type semiconductor, forming electron–hole pairs that generate current in the external circuit. (2) Reverse bias: if a reverse voltage is applied to the p–n junction, that is, the p-side is negative and the n-side is positive, the p–n junction enters a high-resistance state. This is because reverse bias causes the space charge region to expand, increasing the difficulty for charges to cross the junction.

Schottky junctions, also known as Schottky barriers or Schottky diodes, have some notable characteristics. (1) Diversity of Schottky barrier heights: when different metals and non-metallic materials with metallic properties contact different types of semiconductors, Schottky junctions with varying barrier heights are formed. These differences significantly affect the rectification characteristics and performance of the devices [23, 38]. (2) Dynamic changes in barrier height: the height of the Schottky barrier changes with the variation of external voltage. When the metal side is connected to a positive voltage, the electric field in the space charge region weakens, lowering the barrier and making it easier for carriers to pass through. Conversely, when the external voltage is negative, the barrier height increases, making it difficult for carriers to pass. (3) Current transport characteristics: compared to p–n junctions, current transport in Schottky junctions primarily depends on the majority carriers, resulting in minimal charge storage effects and extremely short reverse recovery times, which give Schottky junctions an advantage in high-frequency applications. (4) Comparison of electrical characteristics: the current–voltage (I–V) and capacitance–voltage (C–V) characteristics of Schottky junctions are similar to those of p–n junctions but with key differences. Schottky junctions have a lower forward voltage and a steeper slope in the forward conduction curve, indicating stronger conductivity. However, their reverse breakdown voltage is relatively lower.

The different properties of homotype heterojunctions, anisotype heterojunctions, and Schottky junctions have unique applications in Sn-based anode materials, where the type of heterojunction is carefully selected to meet specific application requirements (Table 1).

Table 1 Various characteristics of heterojunctions.

Heterojunction type	Key characteristics
Homotype heterojunction	Band offset (n–n: conduction band shifts up. p–p: valence band shifts down); tunneling effect; current transport via tunneling
Anisotype heterojunction	Different electrical characteristics under forward (current passes) and reverse bias (high resistance state)
Schottky junction	Variable Schottky barrier height; changes with applied voltage; current transport by majority carriers; short reverse recovery times

Heterostructures Versus Conventional Composites

Compared to traditional composite materials, heterojunction materials exhibit notable synergistic effects arising from interactions or coupling at the interfaces between different phases (Table 2) [39]. Traditional composites are straightforward blends of two or more materials, designed to combine their respective strengths and outperform a single material in terms of electrochemical performance. For instance, carbon coatings have been applied to anode surfaces to form composites, addressing the inadequate conductivity and structural instability of exposed anode materials. However, the interfacial interactions or couplings between the carbon coatings and the underlying anode materials are often neglected [40, 41].

Table 2 Heterostructures versus conventional composites.

Characterization	Conventional composites	Heterojunction material
Material composition	A simple mixture of two or more materials with a basic superposition of physical properties.	Includes at least one semiconductor material that maintains lattice continuity with significant changes in the electronic band structure.
Interface properties	Interactions or couplings between interfaces are usually ignored.	The change in electronic band structure at the interface leads to significant synergistic effects such as potential differences and the generation of built-in electric fields.
Mode of performance enhancement	Partial enhancement of a single material's properties is achieved through physical mixing.	Fusion of the excellent properties of both materials to achieve comprehensive performance beyond that of a single component.
Charge transfer efficiency	Usually does not involve significant enhancement of charge transfer efficiency.	Improved charge transfer efficiency through Fermi energy level balancing and charge redistribution.
Ion diffusion	No particular emphasis on the acceleration of ion diffusion or reduction of energy barriers.	The built-in electric field accelerates ion diffusion, lowers the ion diffusion energy barrier, and improves structural stability.
Application potential	Suitable for application scenarios that require simple enhancement of material properties.	Demonstrates great potential to enhance energy density, power density, cycle stability, and lifetime in the field of energy storage
Research and development trends	Relatively mature research, but with limited room for innovation	Promising research and application prospects with the development of materials science and nanotechnology.

In contrast, heterojunction materials are characterized by several unique attributes: (i) typically, one of the components is a semiconductor; (ii) the junction where two or more materials meet maintains lattice continuity; (iii) there is a pronounced change in the electronic band structure at the interface. The benefits of heterojunction materials over traditional composites include the following: (1) the creation of heterojunctions, which combine the superior properties of both materials, surpassing the capabilities of individual components; (2) the formation of heterojunction interfaces, which facilitates the migration of electrons and equalizes Fermi energy levels by lowering a higher level to match a lower one. This results in the redistribution of charge near the interface, inducing interface polarization and generating a potential difference. This potential difference leads to the emergence of built-in electric fields and further charge redistribution. (3) In energy storage applications, the built-in electric field can expedite ion diffusion in electrode materials, lower the energy threshold for ion diffusion, and enhance the structural stability of the electrodes. The charge redistribution can increase the efficiency of charge transfer, provide conduits for metal ion transport, and supply additional electrochemically active sites for redox reactions, thereby fortifying the electrode's kinetic performance and ion storage capacity [42–44]. Therefore, Sn-based heterojunction-type anode materials give advantages that Sn-based composites do not have, and the heterogeneous structure is expected to promote the development of Sn-based anode materials.

Preparation Methods of Heterojunctions

The art of crafting heterojunctions is crucial in the study and application of heterojunction technology. Developing efficient and straightforward fabrication methods is essential for maximizing the performance of Sn-based heterojunction-type anode materials and advancing both theoretical research and commercial applications (Table 3).

Table 3 Preparation methods of heterojunctions.

Method	Key features
Solid-phase method	Ball milling	High efficiency and scalability, enhanced material mixing and refinement, and in situ reactions
In situ treatment	Ability to create strong interfacial bonds; high reactivity at the substrate surface
Liquid-phase method	Hydrothermal/solvothermal	Precise control over dimensions and morphology, and no high-temp sintering required
Co-precipitation	Simple, efficient, mild conditions; allowable customization of compositions
Other methods	Vapor–solid reaction	High efficiency and scalability, and controlled temperature and atmosphere for tailored properties
Electrospinning	High efficiency, multifunction, and controlled particle size and morphology

Solid-Phase Method

The solid-phase method, widely used in heterostructure fabrication, is noted for its cost-effectiveness, straightforward manufacturing process, and ability to produce high-quality materials. This method includes techniques such as ball milling and in situ chemical transformation. The appeal of the solid-phase method lies in its user-friendly operation, economical pricing, and scalability, making it ideally suited for industrial applications. Additionally, this method adeptly retains the intrinsic crystal structure of materials, thereby preserving their electrochemical integrity. Through meticulous design and optimization of the solid-phase fabrication parameters, the performance of Sn-based heterojunction-type anode materials can be significantly enhanced, tailoring them to meet the demands of applications. This strategic approach not only bolsters the intrinsic capabilities of heterojunctions but also broadens their utility across diverse technological landscapes.

Ball Milling

The ball milling technique is renowned for its efficiency and scalability, making it a preferred method for synthesizing materials through physical processes. This technique operates with remarkable simplicity: materials with the desired composition are placed in a ball mill jar, where mechanical forces exerted by the milling process produce the target materials. This method enhances material mixing and refinement and facilitates in situ reactions to a significant extent.

For instance, Li et al. used the ball milling method to fabricate a heterostructure comprising SnO₂ and BaTiO₃ [45]. By introducing SnO₂ and BaTiO₃ powders into the ball mill jar and subjecting them to mechanical forces, they successfully encapsulated BaTiO₃ particles with SnO₂. Subsequent post-milling treatments yielded an anode material with enhanced performance characteristics. This application of ball milling highlights its substantial potential in preparing heterostructured materials, particularly where enhancing electrochemical performance and structural stability is crucial. By optimizing the conditions and parameters of the ball milling process, it is possible to precisely control the morphology and properties of the heterostructures, thereby providing a robust method for developing unique high-performance materials.

In Situ Treatment Method

The in situ treatment method is an effective technique for fabricating heterostructures by exploiting the high reactivity of the substrate's surface atoms. This method facilitates the formation of a second phase through chemical reactions on the substrate surface, leading to the creation of heterojunction materials with strong interfacial bonds. It enables the synthesis of materials at the atomic or molecular scale, resulting in heterostructures with unique properties.

Yang et al. demonstrated the efficacy of this method by integrating bimetallic sulfide (SnCo)S₂ nanocubes with 2D sulfur-doped graphene (SG) nanosheets to create a new heterostructure [46]. In this structure, the (SnCo)S₂ nanocubes are intricately interwoven with SG nanosheets, forming a distinctive composite system. The effective internal conversion within the heterostructure of the nanocubes results in (SnCo)S₂/SG nanocomposites with exceptional rate capability.

This study underscores the advantages of the in situ treatment method for synthesizing Sn-based heterojunction-type anode materials and demonstrates that precise control over the composition and architecture of materials can significantly enhance their electrochemical performance.

Liquid-Phase Method

Unlike the solid-phase method, the liquid-phase method includes techniques such as hydrothermal and solvothermal processes, as well as co-precipitation methods, each offering unique advantages for material fabrication. This approach is characterized by several key attributes. (1) Exceptional purity: conducted in a liquid medium, the liquid-phase method ensures homogeneous blending of components, resulting in high-purity products. (2) Precise morphology control: by meticulously adjusting reaction parameters—including temperature, pressure, pH, reaction time, and precursor concentration—this method allows for precise control over the morphology and dimensions of materials, from nanoscale particles to microscale structures. (3) Versatile structure formation: the liquid-phase synthesis is adaptable for creating a wide variety of materials, including oxides, sulfides, and phosphates, facilitating the fabrication of materials with diverse crystallographic structures and compositions.

The liquid-phase method is particularly well-suited for developing high-performance heterostructures, where precise control of material properties is essential. Researchers can leverage this technique to innovate and fabricate Sn-based heterojunction-type anode materials with unique architectures and superior properties, enhancing their electrochemical performance.

Hydrothermal and Solvothermal Methods

Hydrothermal synthesis involves high temperature and pressure reactions within sealed vessels, allowing for precise control over material dimensions and morphology by adjusting reaction time and temperature. This method is widely adopted for synthesizing nano-heterojunction materials [47–49]. Solvothermal synthesis, a variant that uses organic solvents instead of water, provides additional flexibility in reaction media [50, 51].

For example, Wang et al. used hydrothermal synthesis to create a SnS₂–SnS/rGO heterostructure, which was then integrated into LIBs as an anode material [52]. This heterostructure significantly improved the material's electrochemical performance by enhancing Li⁺ mobility and optimizing interfacial redox reactions.

The advantage of hydrothermal and solvothermal methods lies in their ability to produce crystalline powders directly, avoiding the need for high-temperature sintering and reducing the risk of particle agglomeration. However, achieving optimal material properties requires strict control over thermal and temporal conditions, which are critical to the success of these synthesis methods.

Co-Precipitation Method

The co-precipitation method is valued for its simplicity, efficiency, and high-quality results. This technique involves precipitating multiple metal precursors simultaneously, facilitating the one-step synthesis of composites under mild conditions. It is particularly effective for creating heterostructures with customized compositions and architectures.

Zhao et al. demonstrated the efficacy of the co-precipitation method by producing an SFS@SC nanocube, which integrates carbon-coating and sulfidation techniques [53]. This unique nanocube, featuring a Janus-like SnS–Fe_1−xS heterostructure, significantly enhanced the electrochemical performance by improving Na⁺ migration.

The co-precipitation method streamlines material synthesis and enables the development of high-performance heterostructures, offering researchers the ability to fine-tune material composition and structure for optimized performance in energy storage and conversion applications.

Other Methods

Vapor–Solid Reaction Method

The vapor–solid reaction method is a highly efficient synthesis technique conducted in a tubular furnace, where materials are produced through direct interactions between gases and solids. This method includes various gas–solid reactions such as sulfidation, phosphatization [54], and selenization [55]. Its simplicity, high efficiency, and scalability make it a preferred choice for fabricating heterojunction anode materials.

Bian et al. utilized the sulfidation approach to form a unique C@SnO₂/SnS heterostructure [56]. This material established an internal electric field at the interface, significantly enhancing its electrochemical activity by accelerating ion and electron transport.

The vapor–solid reaction method excels in controlling reaction parameters—temperature, atmosphere, and duration—to achieve materials with tailored structures and properties. It is a crucial tool for developing advanced Sn-based heterojunction-type anode materials.

Electrospinning Method

Electrospinning is a versatile technique known for its efficiency in producing coaxial heterojunction materials. It allows for precise control over particle size and morphology by adjusting parameters such as spinning solution composition, electrical conductivity, voltage, and collection distance [57]. Electrospinning is often combined with other techniques to enhance material properties.

Zhang et al. successfully prepared SnO₂/NiO heterojunction materials using electrospinning, which was then applied to SIBs, demonstrating excellent cycle stability [58]. This example underscores the potential of electrospinning for creating high-performance Sn-based heterojunction-type anode materials and highlights how innovations in material design and fabrication can significantly improve battery performance.

Given that no single method can meet all requirements, the integration of multiple techniques has become a trend in heterojunction material preparation. Combining methods such as gas–solid reactions with hydrothermal or solvothermal processes enables more precise control and optimization of material properties. This strategy expands the possibilities for material preparation and supports the development of new heterojunction materials with specific functionalities.

Characterization Methods for Heterojunctions

Extensive experimental data have shown that the development of heterostructures can significantly enhance the kinetic properties of materials. Accurate characterization of these heterostructures is crucial for optimizing their performance and understanding their fundamental properties. The following are key characterization techniques for heterojunctions: (1) band diagram analysis: band diagrams are essential for illustrating the distribution of electron energy levels in semiconductors. Because semiconductor heterojunctions consist of different materials, their band structures can vary significantly. Constructing band diagrams helps visualize the band alignment between materials, as well as the height and width of bandgap barriers. This provides valuable insights into the electronic properties of the heterojunctions. (2) Measurement of barrier heights and field distribution: the potential barriers within heterojunctions, which reflect the electrical potential differences between materials, directly affect the material's electronic behavior. Computational and simulation methods can be used to map the distribution of barrier heights and widths, revealing their variations across the heterojunction. (3) Assessment of band edge symmetry: the morphology and symmetry of band edges in heterojunctions critically impact their performance. In applications such as laser diodes and quantum wells, precise control over band edge symmetry is essential for optimal functionality. (4) Chemical composition analysis: the chemical composition of heterojunctions determines their characteristics and performance. The choice and ratio of materials can significantly affect the heterojunction's properties. Accurate control over the thickness and composition of the constituent materials allows for the customization of specific electronic and optical properties. (5) Characterization of structure and interface morphology: the structure and morphology of heterojunction interfaces are closely related to charge carrier transport efficiency and key performance metrics such as band alignment. Advanced techniques, such as transmission electron microscopy (TEM), provide detailed information on the structural and morphological characteristics of interfaces and heterojunctions, including quality and interface state density.

By integrating these characterization methods, researchers can comprehensively evaluate the physicochemical properties of heterojunctions, providing robust empirical support for material design and optimization. Given that no single technique can fully capture all aspects of heterointerfaces, a multifaceted approach combining various methods is often necessary to achieving thorough characterization. This section outlines our comprehensive approach to characterizing heterostructure interfaces, employing X-ray techniques, microscopic analyses, and computational simulations based on density functional theory (DFT).

X-Ray Techniques

X-ray characterization techniques are crucial for understanding the crystalline structure, electron transfer mechanisms, surface conditions, and chemical composition of materials [59]. XRD is fundamental for determining phase composition, crystallinity, and lattice parameters of materials. However, XRD alone often falls short of revealing the detailed features of heterointerfaces. A comprehensive assessment frequently requires the integration of additional techniques, such as XPS. XPS can provide information on the elemental composition, chemical state, molecular structure, and bonding of various compounds for the study of electronic materials [60].

For instance, Hu et al. used XRD to analyze a composite material with a heterointerface, specifically SnO₂/TiO₂. XRD data, as shown in Figure 3A, indicated the crystalline phases and heterojunction characteristics of the material [61]. However, to fully understand the heterojunction formation, more detailed analyses are necessary. XPS analysis, depicted in Figure 3B, revealed a shift of approximately 0.5 eV to higher binding energy for the Ti 2p peak, suggesting an electron-deficient state of TiO₂. Additionally, Figure 3C shows that the XPS peak for Ti in Li_xTiO₂ shifted to lower binding energy, indicating an electron-rich environment. These findings from XPS support the electronic interaction between SnO₂ and TiO₂, confirming the formation of the heterojunction.

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This multi-technique approach provides a robust scientific foundation for the strategic design and optimization of advanced materials, ensuring that the development of unique heterojunctions is both empirically grounded and theoretically sound.

Microscopy Techniques

Microscopy techniques are pivotal in the detailed characterization of materials, encompassing both optical and electron microscopy. Although optical microscopy (OM) provides valuable insights into macroscopic features, its resolution limitations restrict its effectiveness at the nanoscale. Conversely, electron microscopy, an advanced extension of OM, offers superior resolution and deeper insights into material structures, making it indispensable for nanoscale analysis [62].

Scanning electron microscopy (SEM) utilizes signals from a high-energy electron beam interacting with the material's surface, providing high-resolution images of surface morphology. SEM excels in visualizing surface features and is widely used in materials science for examining surface topography and texture [63–66]. However, SEM's capability to detect subtle heterointerfaces and confirm heterojunction formations may be limited, necessitating supplementary techniques for comprehensive analysis. TEM addresses the limitations of SEM by providing atomic-scale resolution and detailed crystallographic information. TEM allows for the examination of internal structures and interfaces at the nanoscale. Modern TEM setups are often coupled with spectroscopic techniques, such as energy-dispersive X-ray spectroscopy (EDS), for a more complete characterization of heterostructures.

In a study by Huang et al., a composite of SnSe₂-CoSe₂ microspheres embedded in a reduced graphene oxide (rGO) matrix was synthesized using an annealing selenization process (Figure 3D) [67]. SEM and TEM were employed to assess the morphology and microstructure of the rGO/SnSe₂–CoSe₂ composite. SEM images revealed that the annealed SnCo precursor microspheres had a rugged surface with an average diameter of approximately 700 nm (Figure 3E). TEM image demonstrated the uniform dispersion and layered structure of these microspheres within the rGO matrix (Figure 3F). High-resolution TEM (HRTEM) provided detailed lattice fringes of 0.225 nm and 0.262 nm, corresponding to the (102) and (210) planes of SnSe₂ and CoSe₂, respectively (Figure 3G). Additionally, EDS mapping identified the distribution of C, Se, Sn, and Co elements, confirming the presence of the heterostructures (Figure 3H).

The integration of multiple microscopy techniques enables a comprehensive understanding of material characteristics, providing valuable insights for advancing material science and practical applications.

DFT

DFT is a key tool in studying material properties, offering significant insights into structural stability, electronic configurations, diffusion kinetics, and adsorption dynamics [68]. DFT is particularly valuable in the analysis of heterostructures, providing essential information on their electronic and atomic-scale behaviors. In the design and synthesis process of heterojunctions, the predictive power of DFT enables scientists to anticipate the behavior of heterojunctions under specific conditions, thereby guiding experimental work, reducing the number of trial-and-error attempts, and accelerating the transition of heterojunctions from theory to practical application.

For instance, Li et al. synthesized a NiS₂@SnS₂ heterostructure and employed DFT calculations to explore its intrinsic properties [69]. Figure 3I shows the density of state (DOS) of the three models, which are conducted to investigate the electron state change brought by the interface regulation. The DOS in pure NiS₂, SnS₂, and NiS₂@SnS₂ possesses a discontinuous electron state near the Fermi level (EF), indicating the inferior conductivity of the exclusive pristine NiS₂ and SnS₂. With the interface regulation architecture, the NiS₂@SnS₂ shows a continuous electron state near the EF. The electron state is optimized by the heterostructure construction, thus increasing the electron transfer and lithium migration in the hybrids. The charge density difference map of NiS₂@SnS₂ is simulated to further illustrate the enhanced electron transfer by constructing the heterostructure. Figure 3J shows the charge distribution across the heterointerface, revealing electron transfer from SnS₂ to NiS₂, which induces a negative potential on the NiS₂ side and increases its affinity for Na⁺. This is further supported by the adsorption energy data in Figure 3K, which demonstrates a higher capacity for Na⁺ adsorption. These results underscore the effectiveness of DFT in elucidating the properties of complex material systems and highlight its predictive power in material design and synthesis.

In summary, although various characterization techniques are employed to reveal the characteristics of heterointerfaces, no single method provides a comprehensive view of these interfaces. Therefore, a multi-faceted approach, integrating different characterization methods, is essential for a complete understanding of heterostructures.

Application of Heterojunctions in Sn-Based Anode Materials

Sn-based anode materials, including SnO₂, SnO, SnS₂, SnS, SnSe₂, and SnSe, each with distinct n-type or p-type semiconductor properties, offer a diverse range of options for constructing heterojunctions. This diversity enables the strategic manipulation of electronic properties to optimize performance in energy storage applications. In this section, we provide a detailed examination of how these heterojunctions are employed to enhance the electrochemical performance of Sn-based anode materials in various energy storage systems (Table 4).

Table 4 Previously reported Sn-based heterojunction-type anode materials of LIBs/SIBs/PIBs.

Materials	Synthesis method	LIB/SIB/PIB	Cyclinga	ICE (%, 0.1 A g–1)	Refs.
CoS2–SnO2@rGO	Hydrothermal	LIB	521.4/300th/1.0	60	[70]
Sb2S3@SnS@C	Hydrothermal	SIB	542/200th/1.0 200/1300th/5.0	80	[71]
SnS2/Co3S4-rGO	Coprecipitation and hydrothermal	SIB	1141.8/50th/0.1	59	[72]
SnS2/Sb2S3@rGO	Hydrothermal	SIB	642/100th/0.2/	80	[33]
SnS2@MoS2@rGO	Hydrothermal	SIB	167/140th/6.4	76	[73]
SnS2–SnS/rGO	Hydrothermal	LIB	1310/200th/0.1	70	[52]
SnS-SnS2@CNTs	Solvothermal	SIB	413/50th/0.1	66	[74]
SnS-SnS2@GO	Solvothermal	SIB	450/100th/0.1	65	[75]
Sb2S3/SnS2/C	Hydrothermal	SIB	642/600th/1.0	71	[76]
SnS2/NiS2@CC	Solvothermal	SIB	588.9/100th/0.5	65	[77]
SnS/SnS2/rGO	In situ desulfurization	SIB	320.0/500th/1.0	84	[78]
SnS/MoS2/NS-CNs	Freeze-drying	SIB	287.2/800th/1.0	66	[79]
SnS2–Sb2S3@graphene	Sonochemical	SIB	622.1/100th/0.1	60	[80]
Co3S4/SnS	Hydrothermal	SIB	76/200th/0.05	71	[81]
ZnS-SnS/C	Hydrothermal	LIB	418/100th/0.2	67	[39]
MoS2/SnS2/NC	Hydrothermal	SIB	660/500th/2.0	57	[82]
SnS/ZnS@C	Solvothermal	SIB	485/250th/0.2	66	[83]
SnS2/NiS2@S-rGO	Solvothermal	SIB	380.9/4000th/1.0	88	[84]
SnS/SnS2@SG-K	Hydrothermal	SIB	372/500th/10	61	[85]
SnS2/Co3S4@CC	Solvothermal	SIB	910.1/100th/0.5	60	[86]
MoS2@SnS	Sulfurization	SIB	430.7/300th/1.0	81	[87]
SnS/SnS2@CC	Solvothermal	SIB	400/1000th/1.0	90	[88]
ZnO@SnO2	Hydrothermal	LIB	686/150th/0.1	83	[89]
Co3Sn2/SnO2@C@GN	Hydrothermal	LIB	605/300th/0.1	75	[90]
NiS/SnO2/MOF	Solvothermal	LIB	350/1000th/2.0	75	[91]
SnO2/Fe2O3	Hydrothermal	LIB	1022/100th/0.1	70	[51]
SbxOy/SnO2/rGO	Solvothermal	LIB	518.6/1000th/4.0	65	[92]
ZnS-SnO2@rGO	Solvothermal	SIB	313/400th/1.0	66	[93]
SnO2/Bi2O3/NF	In situ dipping	LIB	493/500th/1.0	83	[94]
SnO2/SnS/Sn@NG	Hydrothermal	SIB	480/200th/0.1	66	[95]
Mn2SnO4/SnO2@SG	Liquid deposition	LIB	1180.4/100th/0.1	81	[96]
CC@SnS2/SnO2	Hydrothermal	SIB	694.7/100th/0.2	75	[97]
p-Sb2S3/SnO2@rGO	Solvothermal	LIB	474/2000th/3.0	62	[98]
SnSe/SnO2@Gr	Hydrothermal	LIB	810/200th/0.2	50	[99]
C/SnO2/SnSe2@C	In situ gas-phase selenization	SIB	422.3/150th/0.1	58	[100]
SnSe@TiO2/C	Arc-discharge	SIB	318/1100th/1.0	55	[101]
MoSe2/SnSe2@C	Hydrothermal	SIB	591.4/110th/0.1	63	[102]
SnS@C@MoS2@NC	Hydrothermal and hot injection	PIB	471/100th/0.05 253/100th/1.0	78	[103]

Homotype Heterojunctions

Homotype heterojunctions, such as n–n or p–p types, are formed from two semiconductor materials of the same type. Although these heterojunctions are composed of semiconductors of the same type, they can exhibit diverse electronic transport properties. Variations in parameters such as bandgap, carrier concentration, and carrier mobility between the two materials lead to unique behavior at the junction. For instance, in n–n-type homotype heterojunctions, two n-type semiconductors with different bandgaps or carrier concentrations can interact to modify the band alignment and improve charge carrier transport efficiency. Similarly, in p–p-type heterojunctions, variations in hole concentration and mobility between the two p-type semiconductors can result in enhanced hole conductivity and overall performance improvements. These heterojunctions leverage the intrinsic properties of each semiconductor to optimize electronic transport characteristics and enhance the performance of devices, including energy storage systems. The careful design and engineering of homotype heterojunctions allow for tailored electronic properties, making them a valuable approach in the development of advanced materials for various applications.

In n–n-type homotype heterojunctions, a potential barrier forms at the interface when the conduction bands of n-type semiconductors shift to higher energy levels upon contact. Gui et al. have successfully employed an electron-attracting self-assembly strategy to fabricate the 1T/2H MoS₂@SnO₂ heterostructure [104]. Figure 4A outlines the process of electron-tracting self-assembly, where electrostatic forces guide the assembly and structural transformation of the component materials. The phase transformation of MoS₂ from 2H to 1T is illustrated in Figure 4B. SEM and TEM images depict the morphology of the 1T/2H MoS₂@SnO₂ heterostructure (Figure 4C–E). Initially, MoS₂ displays a smooth surface with a well-ordered layered structure. After exfoliation and self-assembly, the MoS₂ nanosheets become wrinkled and reduced in size, yet they retain the layered characteristics. Figure 4F,G reveals the atomic arrangements corresponding to the hexagonal 2H and tetragonal 1T phases of MoS₂, respectively. The lattice fringe spacing of 0.26 nm corresponds to the (101) plane of rutile-phase SnO₂. EDS mapping confirms the presence and uniform distribution of Sn, Mo, S, and O, which supports the successful synthesis of the 1T/2H MoS₂@ SnO₂ heterostructure (Figure 4H).

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Wang et al. demonstrated the successful synthesis of a MoSe₂/SnSe₂@C heterostructure composite using hydrothermal synthesis followed by thermal treatment [102]. The synthesis process for the MoSe₂/SnSe₂ heterostructure is illustrated in Figure 5A. Subsequently, the materials were characterized and their electrochemical properties were tested.

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HRTEM images reveal lattice spacings of 0.66 and 0.29 nm, corresponding to the (002) and (101) planes of MoSe₂ and SnSe₂, respectively (Figure 5B,C). Selected area electron diffraction (SAED) patterns further confirm the disordered lattice arrangement at the MoSe₂/SnSe₂@C heterointerface, verifying the successful formation of the heterostructure with coexisting MoSe₂ and SnSe₂ phases (Figure 5D). XPS analysis provides detailed information about the chemical composition of the materials. In Figure 5E, peaks at 228.9 and 232.1 eV in the Mo 3d spectra are attributed to Mo⁴⁺, whereas the peak at 229.4 eV corresponds to Se 3s. The Sn 3d spectra display peaks at 487.5 and 495.9 eV, corresponding to Sn 3d_5/2 and Sn 3d_3/2, respectively, which are indicative of the Sn⁴⁺ oxidation state (Figure 5F). The Se 3d spectra exhibit peaks at 55.2 and 56.1 eV, respectively, corresponding to Se 3d_5/2 and Se 3d_3/2 (Figure 5G). These chemical bonds enhance interface stability and mitigate volumetric expansion during cycling. Electron paramagnetic resonance (EPR) spectra reveal a g value of 2.001 for MoSe₂/SnSe₂@C, which is characteristic of selenium vacancies, indicating the presence of selenium defects (Figure 5H). Comparative tests using equal masses of the three materials show that the 2D heterojunction MoSe₂/SnSe₂@C exhibits significantly stronger peak in the EPR spectra, suggesting a higher density of selenium defects. These defects not only increase the material's defect density but also create additional active sites for electrochemical reactions, thereby enhancing the material's performance. Electrochemical testing demonstrates that at a current density of 0.1 A g⁻¹, the ICE is 89.2%, the MoSe₂/SnSe₂@C maintains a discharge capacity of 591.4 mAh g⁻¹ after 110 cycles, and the rate performance is also superior to that of the other two materials, indicating excellent cyclic stability and rate performance (Figure 5I,J).

In the domain of Sn-based anode materials, the exploration of p–p homojunction has been relatively limited. Notably, Lin et al. made significant progress by successfully fabricating the Sb₂S₃@SnS nanocomposite heterostructure [71]. Figure 6A illustrates the synthesis process of the Sb₂S₃@SnS heterostructure, providing a clear depiction of the material's formation mechanism. HRTEM images show a lattice fringe spacing of 0.283 nm, corresponding to the (013) plane of SnS (Figure 6B,C). SAED pattern confirms the (011), (110), and (211) planes of SnS, indicating the complete transformation of SnS₂ to SnS (Figure 6D). Electrochemical performance tests further validate the capabilities of this heterostructure, showing a high discharge capacity of 542 mAh g⁻¹ at a current density of 1.0 A g⁻¹ after 200 cycles, and retention of 200 mAh g⁻¹ after 1300 cycles at 5.0 A g⁻¹ (Figure 6E,F). These results highlight the potential of the Sb₂S₃@SnS heterostructure for electrochemical energy storage and underscore the importance of material design and structural optimization.

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To verify that heterojunctions improve the mechanical properties of the material, Lin et al. performed a DFT theoretical calculation for the material. The partial density of states (PDOS) diagrams for SnS indicate that states near the Fermi level are predominantly contributed by the p orbitals of S and Sn atoms, resulting in a 0.96 eV bandgap (Figure 6G,H). Upon forming the heterostructure with amorphous Sb₂S₃, the p orbitals of Sb interact with those of S and Sn, leading to a disappearance of the bandgap and a significant increase in electrical conductivity. Charge density calculations show that charge transfer at the Sb₂S₃@SnS interface is minimally affected by the amorphous Sb₂S₃ (Figure 6J). Bader charge analysis reveals that S ions in SnS and amorphous Sb₂S₃ carry a negative charge, whereas Sn and Sb ions have a positive charge. The formation of the Sb₂S₃@SnS heterostructure results in a marked reduction in the average number of valence electrons for Sb ions in the amorphous Sb₂S₃, with a relative increase for S ions (Figure 6I). Electrostatic potential calculations confirm the establishment of a directional electric field at the interface (Figure 6L). The enhanced electric field and synergistic effects of the Sb₂S₃@SnS heterostructure significantly improve Na⁺ transport and charge transfer within SnS. As illustrated in Figure 6K,M, Na⁺ diffusion pathways in SnS remain consistent across structures, with diffusion barriers of 0.74 and 0.17 eV for movement between equivalent positions, and the Sb₂S₃@SnS heterostructure lowers these barriers to 0.44 and 0.06 eV, respectively, indicating superior electrochemical performance for Na⁺ storage.

This comprehensive DFT analysis provides valuable insights into the influence of heterostructures on electrochemical properties, offering crucial theoretical foundations for designing and optimizing high-performance Sn-based anode materials.

Anisotype Heterojunction

p–n heterojunctions are pivotal components in semiconductor devices, constructed by combining p-type and n-type semiconductor materials. At the interface between these materials, differences in their band structures result in the formation of a space charge region with a non-uniform charge distribution. This space charge region is essential for the formation and function of the p–n junction.

In the field of energy storage research, p–n heterojunctions have garnered significant attention due to their unique electronic properties. Tang et al. synthesized MoS₂/SnS@C hollow hierarchical nanotubes with a self-supporting structure through a straightforward solvothermal reaction [105]. In this heterostructure, MoS₂ acts as the n-type semiconductor, whereas SnS serves as the p-type semiconductor. Figure 7A depicts the synthesis process of the MoS₂/SnS heterostructure. SEM and TEM images reveal the microstructure of the material at different scales, illustrating the internal MoS₂, intermediate SnS, and external carbon layers (Figure 7B,C). HRTEM images display lattice spacings of 0.612 and 0.404 nm, corresponding to the (002) and (110) planes of MoS₂ and SnS, respectively (Figure 7D). The SAED pattern confirms the lattice overlap between MoS₂ and SnS, validating the successful formation of the MoS₂/SnS heterostructure (Figure 7E). This heterostructure not only reduces the Na⁺ diffusion barrier but also enhances the capacitive performance of the composite material. Electrochemical performance tests show that at a current density of 5.0 A g⁻¹, the material maintains a reversible capacity of 292 mAh g⁻¹ after 2000 cycles. At a higher current density of 15 A g⁻¹, it exhibits a rate capability of 325 mAh g⁻¹ (Figure 7F,G). These results indicate that MoS₂/SnS@C exhibits outstanding electrochemical performance as an anode material for SIBs.

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Given the structural similarity between SnO₂ and SnO, researchers have investigated the feasibility of integrating these materials to create heterostructures. Yin et al. successfully capitalized on this structural similarity to develop a SnO₂/SnO heterostructure, where SnO₂ acts as the n-type semiconductor and SnO as the p-type semiconductor [106]. Figure 8A illustrates the synthesis process of the SnO₂/SnO heterostructure. SEM images reveal that the SnO₂/SnO heterostructure adopts a unique “biscuit-like” morphology (Figure 8B). This distinctive structure effectively mitigates volume fluctuations during charge and discharge cycles, thus preserving the material's structural integrity. HRTEM images display lattice spacings of 1.71 and 1.53 Å, corresponding to the (003) plane of SnO and the (002) plane of SnO₂, respectively (Figure 8C,D). The clear visualization of the heterojunction interface further facilitates the migration of Li⁺. EDS results confirm a uniform distribution of Sn, O, and C elements throughout the material, validating the successful synthesis of the SnO₂/SnO heterostructure and ensuring a homogeneous chemical composition (Figure 8E). Yin et al. promoted the development of Sn-based oxide anode materials by constructing SnO₂–SnO heterojunctions based on structural similarity.

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In the domain of Sn-based selenide research, Feng et al. achieved a significant advancement by successfully synthesizing a SnSe₂/SnSe heterostructure using a high-throughput wet chemical approach, which allows for efficient and scalable production [107]. In this heterostructure, SnSe₂ serves as the n-type semiconductor, whereas SnSe acts as the p-type semiconductor. Figure 9A illustrates the synthesis process of the SnSe₂/SnSe heterostructure. SEM images reveal that the SnSe₂/SnSe heterostructure exhibits a unique hierarchical nanoflower morphology, which helps accommodate volume changes during charge and discharge cycles, thereby maintaining structural stability (Figure 9B). High-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) images reveal a clear interface between SnSe₂ and SnSe, with lattice spacings of 0.33 nm and 0.41 nm corresponding to the (100) and (101) planes of SnSe₂ and SnSe, respectively (Figure 9C). XPS data further confirm the formation of the SnSe₂/SnSe heterostructure, with characteristic binding energies of Sn²⁺ and Sn⁴⁺ at 492.7, 484.4, 494.7, and 485.1 eV (Figure 9D). Raman spectroscopy further corroborates the formation of the SnSe₂/SnSe heterostructure (Figure 9E). These findings validate the successful synthesis of the SnSe₂/SnSe heterostructure via the assisted wet chemical method. The SnSe₂/SnSe anode material demonstrates exceptional high-rate performance and cycling stability, exhibiting a reversible capacity of 374.7 mAh g⁻¹ after 1000 cycles at a current density of 2.5 A g⁻¹ (Figure 9F,G).

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To investigate the structural stability, electron configuration, diffusion kinetics, and adsorption kinetics of the heterostructure, Feng et al. performed DFT theoretical calculations. DFT calculations and in situ characterizations elucidate the Li⁺ storage mechanism within SnSe₂/SnSe, providing insights into the interaction and storage of Li⁺ in this heterostructure. Observations of metallic characteristics in SnSe₂/SnSe contrast sharply with the semiconductor properties of SnSe₂ (1.10 eV) and SnSe (0.56 eV), indicating a significant enhancement in electronic conductivity (Figure 9J). This ultrahigh electronic conductivity in the heterostructure markedly accelerates charge transfer, thus improving battery performance. Differential charge density maps reveal charge accumulation at the heterojunction interface, suggesting the formation of an internal electric field that facilitates lithium atom migration (Figure 9H). Additionally, Figure 9I shows that the migration barriers for Li⁺ in SnSe₂/SnSe are significantly lower compared to those in pure SnSe₂ and SnSe, reflecting improved reaction kinetics and superior electrochemical performance. The unique electronic structure of the SnSe₂/SnSe heterostructure modifies electron distribution, whereas interface effects enhance ion transport, collectively contributing to the improved performance of LIBs. This work promoted the development of Sn-based selenide anode materials.

Wang et al. conducted an extensive investigation into the Na⁺ diffusion behavior within three heterostructures: Na₂O/Na₂Se, Na₂O/Na₂S, and Na₂S/Na₂Se [108]. Their simulations revealed that the Na₂S/Na₂Se heterostructure demonstrates the most stable diffusion behavior among the studied structures. Additionally, the Na₂S/Na₂Se heterostructure exhibits a robust interface binding energy of 25 meV Å⁻², indicating that metal sulfide/metal selenide heterostructures may possess superior cycling stability due to their structural resilience (Figure 10A–D). Building upon these theoretical findings, Wang et al. further synthesized a SnS/SnSe₂ heterostructure, with SnS functioning as the p-type semiconductor and SnSe₂ as the n-type semiconductor.

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XRD patterns confirmed the successful formation of the SnS/SnSe₂ heterostructure by showing characteristic peaks corresponding to both phases (Figure 10E). TEM images revealed lattice spacings of 0.325 and 0.293 nm, corresponding to the (021) plane of SnS and the (101) plane of SnSe₂, respectively (Figure 10G,H). XPS data further corroborated the heterostructure formation, showing the coexistence of Sn⁴⁺ and Sn²⁺ species as key evidence of the heterojunction (Figure 10F). The SnS/SnSe₂@3DC electrode demonstrated outstanding electrochemical performance, attributed to excellent ion diffusion at the heterojunction. As shown in Figure 10I, at a current density of 0.2 A g⁻¹, the ICE is 88.2%, and the SnS/SnSe₂@3DC maintains a discharge capacity of 529 mAh g⁻¹ after 100 cycles.

This study highlights the potential of p–n heterostructures to enhance energy storage material performance and offers new insights and approaches to address current challenges in energy storage technologies. At the same time, the theoretical and experimental methods adopted by Wang et al. provide a promising avenue for the development of Sn-based anode materials.

Schottky Heterojunctions

Schottky heterojunctions, representing a pivotal metal–semiconductor contact structure, play a critical role in electronic devices. In the context of Sn-based anode materials, the strategic design of heterostructured nanomaterials with distinctive compositions can yield Schottky junctions endowed with intrinsic built-in electric fields. These fields are crucial for significantly enhancing the electrochemical performance of the materials.

Li et al. demonstrated a method for synthesizing heterostructured SnO_x ultrafine nanoparticles supported on a carbon matrix (SnO_x@C/rGO) [109]. This synthesis involved solvent mixing followed by thermal annealing, where Sn acts as the metal, whereas SnO and SnO₂ serve as the semiconductors (Figure 11A). Li et al. then performed microscopic characterization and electrochemical performance tests of the material.

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XRD analysis was utilized to characterize the phase structures of both pretreated SnO_x@C/rGO and SnO_x@C (Figure 11B). XPS provided insight into the Sn 3d spectra, which were deconvoluted into six distinct peaks (Figure 11C). The principal peaks at binding energies of 487.2/495.6 eV, 486.5/494.2 eV, and 485.0/493.4 eV correspond to Sn 3d_5/2 and Sn 3d_3/2 in SnO₂, SnO, and elemental Sn, respectively. These observations confirm the presence of SnO₂, SnO, and a heterogeneous distribution of Sn within the SnO_x@C/rGO composite. Notably, Figure 11D illustrates overlapping regions, such as areas I, II, and III, where prominent heterojunctions are formed. The heterojunction in region I is composed of the (101) plane of SnO₂, the (200) plane of Sn, and the (101) plane of SnO, robustly supporting the presence of heterogeneous SnO_x. The SnO_x@C/rGO maintains a capacity of 447.8 mAh g⁻¹ after 1200 cycles at a current density of 5.0 A g⁻¹ (Figure 11E). This study underscores the potential of engineered heterojunctions in advancing the performance of anode materials for high-performance energy storage applications and promoting the application of Schottky heterojunctions in Sn-based anode electrode materials and provided ideas for the construction of multiple heterojunctions.

Schottky junctions are not only composed of metal and semiconductor materials, and conductive non-metallic materials with metal properties can also form Schottky junctions with semiconductor materials, such as graphene, and carbon nanotubes (CNTs) [110, 111]. Graphene, which consists of a single layer of carbon atoms with a high surface area and robust physical stability, and CNTs, which are defect-free 1D cylinders formed by rolling up one or more layers of graphene, are notable examples. Due to their narrow bandgaps and high carrier mobilities, many carbon-based materials exhibit metallic properties and are utilized to construct Schottky junctions with Sn-based semiconductor materials [25].

For instance, Wang et al. engineered a SnS/N-doped graphene (NG) heterojunction as a negative electrode material for SIBs, comprising alternating layers of SnS and NG nanosheets (Figure 12A) [112]. The morphology and microstructure of the SnS/NG nanoribbons were meticulously characterized using SEM and TEM. Figure 12B,C illustrates that the synthesized SnS nanoribbons have an approximate thickness of 30 nm. HRTEM images reveal a lattice spacing of 0.93 nm for SnS (Figure 12D). The increased interlayer spacing in SnS effectively lowers the barriers for Na⁺ insertion and extraction, while also providing additional active sites for Na⁺ storage. After 1200 cycles, the SnS/NG hybrid nanobelt anode shows a reversible capacity of 600 mAh g⁻¹ at 1.6 A g⁻¹ with a capacity retention of 95.3% (Figure 12E).

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Ran et al. successfully developed a biomimetic heterostructure, Sn₄P₃@CNT/C, using a combination of hydrothermal synthesis and thermal treatment (Figure 13A) [113]. Morphological characterization using SEM and TEM, as shown in Figure 13B,C, reveals the uniform distribution of nanoscale Sn₄P₃ particles on the CNT surface. This arrangement increases the contact area with the electrolyte and shortens the ion diffusion path, enhancing electrochemical performance. Additionally, the amorphous carbon coating on the Sn₄P₃ surface provides “infiltration pores” that buffer volume changes and enhance electrolyte penetration. The Sn₄P₃@CNT/C demonstrates superior electrochemical performance, maintaining a capacity of 742 mAh g^–1 after 500 cycles at 2 A g^–1 and excellent ICE (78%) (Figure 13D).

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MXene, characterized by its distinctive combination of metallic and ceramic properties, is frequently classified as metallic material. Wang et al. developed SnO₂ quantum dots (QDs)@delaminated Ti₃C₂T_x (d-Ti₃C₂T_x) (Figure 14A) [114]. Figure 14B shows the XRD patterns, which confirm the successful synthesis of pure SnO₂ QDs, d-Ti₃C₂T_x, and the SnO₂ QDs@d-Ti₃C₂T_x. SEM images indicate that the SnO₂ QDs on the d-Ti₃C₂T_x sheets are not readily visible at lower magnifications (Figure 14C). However, EDS spectra confirm the presence of Sn (Figure 14F). Elemental mapping reveals that SnO₂ QDs are uniformly distributed across the d-Ti₃C₂T_x layer surface, confirming the successful fabrication of the SnO₂ QDs@d-Ti₃C₂T_x. TEM images demonstrate that the d-Ti₃C₂T_x maintains its layered morphology, with SnO₂ QDs evenly anchored on the conductive d-Ti₃C₂T_x surface (Figure 14D). The MXene d-Ti₃C₂T_x nanosheets enhance electron and ion transport while preventing the aggregation of SnO₂ QDs and mitigating volume expansion during lithium insertion and extraction. The SnO₂ QDs also act as a filler to prevent the re-stacking of MXene nanosheets during synthesis and cycling. Notably, composite achieves a high reversible capacity of approximately 402 mAh g⁻¹ at a current density of 1.0 A g⁻¹, even after 100 cycles (Figure 14E).

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Combination of Sn-Based Heterojunction-Type Anode Materials With Other Modification Methods

The construction of heterojunctions provides an important concept for enhancing the dynamics of Sn-based anode materials. However, Sn-based materials undergo serious volume changes during charge and discharge, especially in lithium-ion batteries. The volume expansion can be as high as 300% due to the formation of an alloy between Sn and lithium during charging. This repeated expansion and contraction lead to structural destruction of the material and pulverization of the electrodes, and cause the cracking and reconstruction of the solid electrolyte interface (SEI). These processes lead to the constant consumption of alkali metal ions. The modification of heterojunction anode materials has certain limitations in enhancing the structural stability of these materials. To fully leverage the advantages of Sn-based anode materials, it is necessary to further improve their structural stability during the storage of alkali metal ions. Researchers will incorporate other modification methods while introducing heterostructures, such as nanoization, composite with carbon materials, and construction of special structures. The integration of heterojunctions with other modification strategies not only optimizes electrode material performance but also paves the way for the development of Sn-based anode materials.

Integration With Nanostructuring Technology

Nanostructuring is a pivotal technique for enhancing the cycling stability of Sn-based anode materials. Reducing particle size to the nanometer scale effectively decreases the absolute volume change of individual particles, with the reduction proportional to the cube of the particle diameter. This approach significantly alleviates volumetric strain and greatly improves structural stability [115–117]. To a certain extent, it also strengthens the stability of the SEI film. Furthermore, nanostructures shorten the charge diffusion pathways for ions and electrons, thus providing a high density of electrochemical active sites [118, 119]. The characteristic diffusion time (τ) for ions in active electrode materials can be expressed as τ = L²/D, where L represents the diffusion distance and D represents the diffusion coefficient. Because diffusion time decreases with the square of the diffusion distance (L²), reducing particle size can effectively enhance rate performance [120]. Additionally, the voids between nanoparticles facilitate electrolyte infiltration and offer buffer space to accommodate volume expansion.

Li et al. synthesized SnO₂/Sn@carbon heterostructured nanoparticles using a salt-template method followed by calcination [121]. SEM images of SnO₂/Sn@TA reveal that numerous nanoparticles, approximately 600 nm in size, are uniformly distributed on a carbon scaffold (Figure 15A,B). HRTEM images show lattice spacings of 0.274 nm corresponding to the (101) planes of Sn (Figure 15C). The SnO₂/Sn@TA demonstrates an exceptional capacity of 1059.7/1064.4 mAh g⁻¹ at 2.0 A g⁻¹ after 1500 cycles and maintains a rate performance of 399.3/404.5 mAh g⁻¹ at a current density of 6 A g⁻¹, with excellent ICE (88%) (Figure 15D,E). The enhanced electrochemical performance of SnO₂/Sn@TA is attributed to the abundant SnO₂/Sn heterojunctions and nanoscale dimensions, which mitigate volume expansion, prevent structural collapse and regeneration of SEI membrane, and shorten Li⁺ diffusion paths, thereby reducing the transport and diffusion barriers during lithiation/delithiation processes.

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Composite With Carbon Matrix

In recent years, carbon materials have garnered significant interest due to their high electrical conductivity, stability, and tunable structural properties. The introduction of carbon materials plays a crucial role in inhibiting volume expansion and enhancing the stability of the SEI film. Carbon nanomaterials, such as CNTs and graphene, have been extensively utilized as conductive additives and supports in electrode materials.

CNTs

Since their discovery by Iijima in 1991, CNTs have been widely studied for their exceptional surface area, electrical conductivity, and mechanical strength [122]. Although CNTs are not used directly as anodes in LIBs, SIBs, and PIBs, they play a critical role as conductive additives, significantly enhancing battery performance. The incorporation of CNTs with active materials creates a three-dimensional network structure that results in anode materials with high areal capacity and superior stability. Additionally, nanotubes derived from metal-organic frameworks (MOFs) or polymers represent another category of CNTs.

Li et al. reported a high-performance SnO₂/SnSe₂@CNT heterostructured anode material, synthesized by uniformly encapsulating SnO₂/SnSe₂ in nitrogen-doped CNTs [123]. The synthesis process is depicted in Figure 16A. The 3D structure and heterogeneous structure can be clearly observed by SEM and TEM.

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SEM and TEM images reveal the nanotube structure of the SnO₂/SnSe₂@C, with ultrafine nanoparticles evenly distributed on the inner walls of the carbon tubes (Figure 16B,C). The hollow tubular nanostructure not only accommodates the volume expansion of SnO₂/SnSe₂ but also enhances electrolyte infiltration and shortens the Na⁺ diffusion path. HRTEM images show lattice spacings of 0.334 and 0.291 nm, corresponding to the (110) plane of SnO₂ and the (101) plane of SnSe₂, respectively, confirming the presence of the heterojunction (Figure 16D). XPS analysis reveals additional selenium signals in the XPS spectrum of SnO₂/SnSe₂@C, confirming successful selenization treatment. The Sn 3d spectrum of SnO₂/SnSe₂@C shows peaks at 495.5 and 487 eV, attributed to Sn⁴⁺–O bonds, and peaks at 495 and 486.5 eV, associated with Sn⁴⁺–Se bonds, further validating the formation of the SnO₂/SnSe₂ heterostructure (Figure 16E,F). The construction of the heterostructure enhances charge transfer kinetics and stabilizes the electrode structure by increasing the additional boundary effect of the crystals. As shown in Figure 16G,H, the SnO₂/SnSe₂@C anode demonstrates exceptional rate performance (322 mAh g⁻¹ at 4 A g⁻¹), outstanding long-term cycling stability (87.7% capacity retention after 1000 cycles at 2.0 A g⁻¹), and excellent ICE (92%). This study not only underscores the potential of heterojunctions in enhancing the performance of Sn-based anode materials but also illustrates how leveraging the advantages of CNTs can improve material stability and cycling life.

Graphene

Graphene, a revolutionary 2D carbon nanomaterial, is distinguished by its single-atom-layer thickness and sp² hybridized carbon bonds. Since its isolation from graphite in 2004, graphene has garnered substantial interest due to its unique structural properties, exceptional mechanical strength, extensive specific surface area, and outstanding electrical conductivity.

In Figure 17, Kamboj et al. developed a straightforward method to synthesize a SnO–SnO₂ graphene oxide (rGO) [124]. This material integrates two distinct tin oxide phases (SnO and SnO₂) onto dispersed rGO nanosheets. The synthesis process of SnO-SnO₂@rGO is depicted in Figure 17A. Figure 17E proves the existence of two phases. Notably, the formation of Sn–O–C bonds, as evidenced by the O 1s XPS spectrum shown in Figure 17B–D, underscores the synergistic interaction between the carbon matrix and tin oxide nanoparticles. This interaction contributes to enhanced electrochemical performance. The p–n SnO-SnO₂@rGO heterojunction exhibits a high reversible capacity of up to 500 mAh g⁻¹ at a current density of 2.0 A g⁻¹ (Figure 17F).

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These findings highlight the significant potential of graphene in improving the performance of Sn-based heterojunction-type anode materials. The combination of heterojunctions with graphene enhances the electrochemical performance of Sn-based anodes, offering a promising approach for advanced energy storage applications.

Heteroatom doping of graphene is a prominent technique used to enhance the properties of this material for advanced energy storage applications. By incorporating heteroatoms such as nitrogen, boron, sulfur, or phosphorus into the graphene lattice, researchers can substantially modify its electronic properties and chemical reactivity. This approach not only improves the performance of graphene-based materials but also expands their potential applications. Li et al. have adeptly designed and synthesized a unique SnO₂-SnS₂@C embedded in a NG matrix, which is denoted as SnO₂-SnS₂@C/NG (Figure 18A) [125]. The synthesis process commenced with the hydrothermal preparation of SnO₂@C/reduced graphene oxide (rGO) as the precursor, followed by a sulfurization treatment to obtain the SnO₂-SnS₂@C/NG. XRD pattern reveals characteristic peaks of SnO₂–SnS₂ that match standard reference data, confirming the material's phase composition (Figure 18F). XPS analysis further validates successful nitrogen doping, supporting the effective synthesis of SnO₂-SnS₂@C/NG (Figure 18B–E). SEM images demonstrate that the SnO₂-SnS₂@C/NG features an ordered layered structure with porous gaps between layers, which enhances electrolyte penetration (Figure 18G). The ultrafine SnO₂–SnS₂ nanoparticles are uniformly embedded in a wrinkled carbon matrix, creating ample voids within the material. HRTEM images confirm the presence of heterojunctions, composed of (101) planes of SnO₂, (100) planes of SnS₂, and (110) planes of SnO₂, effectively validating the formation of SnO₂–SnS₂ heterostructures (Figure 18H,I). Owing to the synergistic effects of heterojunctions, nitrogen doping, and graphene oxide, the SnO₂-SnS₂@C/NG exhibits exceptional electrochemical performance. As illustrated in Figure 18J–L, this material achieves a high reversible capacity of 1201.2 mAh g⁻¹ at a current density of 0.1 A g⁻¹. It also demonstrates excellent rate capability and long-term stability, retaining a capacity of 1039.4 mAh g⁻¹ after 400 cycles at 0.5 A g⁻¹ and 944.3 mAh g⁻¹ after 950 cycles at 1.0 A g⁻¹.

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The research by Li et al. underscores the significant potential of integrating NG with heterojunction technology to enhance the performance of Sn-based anode materials. This innovative modification strategy provides valuable insights and a substantial reference for developing high-performance LIB anodes.

Other Carbon Materials

Beyond the previously discussed 3D carbon materials, 2D carbon nanosheets are also a significant focus in carbon-based composites. In Figure 19, Cui et al. made a significant breakthrough in SIB anodes by successfully constructing a bimetallic sulfide heterostructure, SnS/MoS₂, which was then integrated onto nitrogen and sulfur co-doped carbon nanosheets (NS-CNs) [79].

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This innovative SnS/MoS₂/NS-CNs structure not only creates an internal electric field within the nanocrystals, thereby effectively reducing ion diffusion resistance, but also enhances interfacial electron transport, significantly improving the material's electrochemical performance. SEM and TEM images reveal that SnS nanoparticles are embedded within the carbon nanosheets (Figure 19A–D). TEM images further highlight the layered structure and the uniform distribution of SnS nanoparticles within the carbon matrix. HRTEM images display clear lattice fringes with interplanar spacings of 0.29 and 0.28 nm, corresponding to the (101) and (111) planes of SnS, respectively, thus confirming the successful formation of the heterostructure (Figure 19E,F). EDS mapping shows a uniform distribution of Sn, Mo, S, O, C, and N elements throughout the material, indicating the successful incorporation of nitrogen and sulfur into the carbon nanosheets (Figure 19G). This doping enhances the material's structural stability and conductivity during charge–discharge processes. As a result, the SnS/MoS₂/NS-CNs anode material exhibits outstanding electrochemical performance, demonstrating a high-rate capability of 372.9 mAh g⁻¹ at a current density of 5.0 A g⁻¹ and maintaining a specific capacity of 287.2 mAh g⁻¹ after 800 cycles at 1.0 A g⁻¹ (Figure 19H,I).

In addition to inorganic carbon materials, organic substances can also be transformed into excellent carbon sources through thermal treatment. Liu et al. successfully synthesized a heterostructured material coated with polydopamine (PDA), designated as SnSe₂/ZnSe@PDA (Figure 20A) [55]. TEM images reveal a uniform PDA coating layer approximately 10 nm thick on the SnSe₂/ZnSe heterostructure surface (Figure 20B,C). The elasticity of the PDA coating effectively mitigates volume changes during charge-discharge cycles, significantly enhancing the material's structural stability. HRTEM images confirm the presence of a clear heterojunction interface between SnSe₂ and ZnSe, which enhances Na⁺ adsorption and improves electrochemical kinetics (Figure 20D). Lattice spacing measurements show spacings of 0.327 and 0.294 nm corresponding to the (111) plane of ZnSe and the (011) plane of SnSe₂, respectively. This heterostructure formation results in lattice reorganization at the interface, thereby enhancing the material's thermodynamic stability. By XRD, SEM, and TEM, Liu et al. determined the material microstructure, PDA coating layer, and heterojunction structure.

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To further determine the presence of heterojunctions, Liu et al. performed an XPS analysis which further confirms the formation of the heterostructure (Figure 20E,F). Following formation, the binding energy of Sn shifts to higher values, whereas that of Zn shifts to lower values, indicating lattice distortion. The electron transfer from SnSe₂ to ZnSe induces a negative potential on the ZnSe side, enhancing Na⁺ adsorption and improving the material's electrochemical performance. As shown in Figure 20G, this material exhibits a discharge capacity of 616 mAh g⁻¹ after 1000 cycles at a current density of 1.0 A g⁻¹ when used as an anode in SIBs. These studies not only highlight the potential of using organic substances as carbon sources for high-performance Sn-based heterojunction-type anode materials but also emphasize the importance of material interface engineering in optimizing electrochemical performance. This innovative structural design and material synthesis strategy hold promise for achieving higher energy densities and longer cycle lifetimes in future energy storage technologies. The combination of heterojunction and carbon material further improves the dynamic properties of the material, and the introduction of carbon material further enhances the structural stability of the material. The aforementioned research has promoted the research on the combination of heterojunction and carbon materials.

Integration With Morphological Structures

The design of different structures will further enhance the structural stability of the material, and the combination with heterojunction can significantly improve the conductivity, ionic diffusivity, and structural stability of the tin-based anode material. The excellent structural stability enhances the stability of SEI film and reduces the loss of Li⁺/Na⁺/K⁺ ions.

0D Structures

Rational structural design introduces void spaces within materials to accommodate the volumetric changes associated with Sn-based compounds. This innovative approach not only enhances the structural stability but also improves the kinetics of electrode materials. 0D materials, in contrast to bulk materials, offer an exceptionally high specific surface area, which provides numerous reactive sites and significantly boosts the electrochemical activity of the electrode materials [126]. Additionally, the small size effect of 0D materials mitigates issues related to volumetric expansion and particle agglomeration [127].

As shown in Figure 21, Zhu et al. synthesized a 0D SnO₂/MoS₂ heterojunction composite [128]. TEM images, depicted in Figure 21A,B, illustrate the presence of SnO₂ nanospheres with diameters of approximately 5 nm alongside external MoS₂ nanosheets. Furthermore, HRTEM images shown in Figure 21C reveal distinct lattice fringes corresponding to both SnO₂ and MoS₂. Specifically, interplanar spacings of 0.270 and 0.651 nm are attributed to the (101) and (002) planes of MoS₂, respectively, whereas the 0.335 nm spacing corresponds to the (110) plane of SnO₂, and the 0.261 nm spacing is associated with the (101) plane of SnO₂. These observations confirm the successful formation of a heterojunction between 0D SnO₂ and MoS₂. In electrochemical performance evaluations, the composite material demonstrated a remarkable discharge capacity of 803.6 mAh g⁻¹ after 200 cycles at a current density of 0.2 A g⁻¹ (Figure 21D).

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1D Structures

In general, 1D materials can effectively inhibit the accumulation of radial stress due to their ample internal space, which facilitates stress release. Consequently, 1D materials exhibit excellent cycling stability. Nanotubes are a typical example of 1D materials. Zhang et al. [58] successfully synthesized a SnO₂/NiO heterostructure and encapsulated it within carbon material to form a SnO₂/NiO@C composite. Figure 22A illustrates the synthesis process of the SnO₂/NiO@C composite, where PDA is transformed into a uniform carbon layer through high-temperature treatment. To verify the properties of the materials prepared by this method, Zhang et al. conducted relevant characterization tests and electrochemical performance tests.

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Figure 22B depicts the SnO₂/NiO@C material in the form of a 1D nanostructure, whereas the local magnification presented in Figure 22C indicates that the thickness of the carbon coating is approximately 5 nm. The formation of the carbon layer significantly enhances the structural stability and further improves the electrical conductivity of the material. Moreover, the hollow design effectively shortens the diffusion path for Na⁺. Figure 22D,E presents HRTEM and SAED images of SnO₂/NiO@C, respectively. The HRTEM image in Figure 22D shows distinct lattice fringes of SnO₂ and NiO, corresponding to their respective crystal planes. The SAED image in Figure 22E displays the intersecting lattice features of SnO₂ and NiO, further confirming the formation of the heterostructure. In electrochemical performance tests, as depicted in Figure 22F,G, the SnO₂/NiO@C anode material exhibited a discharge capacity of 320 mAh g⁻¹ after 200 cycles at a current density of 0.1 A g⁻¹. This study shows that the combination of heterojunction and 0D structure improves the material stability and electrical conductivity and further improves the performance of Sn-based anode materials.

2D Structures

The 2D MXene materials, which include transition metal carbides, nitrides, and carbonitrides, exhibit superior electronic conductivity compared to solution-processed graphene films [129]. The surface functional groups of MXene endow them with excellent solubility in various solvents [130]. Consequently, MXene possesses a unique combination of properties found in both graphene and graphene oxide. Additionally, MXene exhibits outstanding mechanical flexibility and favorable lithiation characteristics.

The 2D layered structure of MXene effectively reduces volumetric expansion, provides abundant reactive sites, and ensures sufficient electrolyte infiltration into the interlayer spaces, thereby preventing particle fragmentation and agglomeration.

Hou et al. synthesized a heterojunction VS₄/SnS₂@MXene anode material, confined within MXene nanosheets (Figure 23A) [131]. Figure 23B illustrates the morphological features of the VS₄/SnS₂@MXene, where VS₄/SnS₂ nanosheets are uniformly attached to the MXene surface. Moreover, the excellent integration between the ultrafine VS₄/SnS₂ nanosheets and the MXene is achieved. This 2D structure is advantageous for ion intercalation and deintercalation. The HRTEM image in Figure 23C reveals two distinct crystal planes within the material. The interplanar spacing of 0.589 nm matches the (001) plane of SnS₂, whereas the smaller spacing of 0.247 nm corresponds to the (402) plane of VS₄. Additionally, the SAED pattern in Figure 23D displays the (222) and (042) planes of VS₄ and the (001) and (110) planes of SnS₂, confirming the heterostructure of the composite. The VS₄/SnS₂@MXene anode material demonstrated a capacity of 1565.3 mAh g⁻¹ after 200 cycles at 0.1 A g⁻¹ and a specific capacity of 898 mAh g⁻¹ after 500 cycles at 5.0 A g⁻¹ (Figure 23E,F).

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3D Structures

The 3D materials often combine the advantages of low-dimensional materials, such as ample internal space, efficient ion and electron transport pathways, and large specific surface areas. Due to their unique benefits, core–shell structures have shown great potential in enhancing the electrochemical performance of Sn-based anode materials. Lin et al. ingeniously constructed a walnut-shaped core–shell MoS₂@SnS heterostructure composite [132]. Figure 24A presents a schematic of the synthesis process of the MoS₂@SnS composite, revealing the preparation method of this innovative structure.

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The morphology and microstructure of the MoS₂@SnS heterostructure were characterized in detail using SEM, TEM, and HRTEM (Figure 24B–D). The SEM image reveals that the synthesized MoS₂@SnS exhibits a walnut-shaped core–shell heterostructure. The HRTEM image shows an interplanar spacing of 0.32 nm, which corresponds to the (210) plane of orthorhombic SnS. This finding confirms the uniform distribution of SnS nanocrystals on the MoS₂ nanosheet substrate, forming a multilayered core–shell heterostructure. Further DFT calculations were performed to investigate the heterojunction structure of MoS₂@SnS (Figure 24E) and the diffusion energy barriers of Na⁺ in MoS₂, SnS, and MoS₂@SnS (Figure 24F). The results indicate that the built-in electric field at the heterojunction interface significantly promotes electron and ion transport as well as charge transfer processes, enhancing the rate performance of the material. Electrochemical performance tests show that the cycling stability of the MoS₂@SnS heterostructure surpasses that of individual MoS₂ and SnS materials, highlighting the significant effect of the core–shell heterostructure in improving electrochemical performance (Figure 24G).

In addition to conventional 3D structures, unique morphological designs can significantly enhance the electrochemical performance of Sn-based anode materials. Liu et al. [133] have made a noteworthy advancement by successfully fabricating, for the first time, a morphologically distinctive meadowfoam-like SnO₂/TiO₂ heterostructure. Figure 25A presents XRD patterns that reveal the phase composition and crystalline structure of the SnO₂/TiO₂ heterostructure, confirming its successful synthesis. SEM images in Figure 25B depict the meadowfoam-like morphology of the material, which facilitates effective electrolyte infiltration and thereby enhances electrochemical reaction efficiency. TEM images further reveal the hollow nature of the SnO₂/TiO₂ (Figure 25C). HRTEM images show distinct lattice spacings, providing additional confirmation of the heterostructure's formation (Figure 25D). This unique design not only promotes ion migration but also offers additional electrochemical active sites. When employed as an anode material in LIBs, the SnO₂/TiO₂ heterostructure demonstrates significantly improved electrochemical performance due to the synergistic effects of the heterojunction and hollow structure. As illustrated in Figure 25E,F, the material exhibits a discharge capacity of 152 mAh g⁻¹ after 250 cycles at a current density of 0.1 A g⁻¹, underscoring the effectiveness of the heterostructure design in enhancing battery performance. The innovative synthesis of this meadowfoam-like SnO₂/TiO₂ heterostructure provides valuable insights and guidance for the design and development of high-performance anode materials for LIBs.

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Conclusion

Optimizing Sn-based anode materials for AIBs can be achieved through a combination of heterojunctions, nanostructures, carbon composites, and structural design. These strategies address key challenges, improving the electrochemical performance, stability, and cycle life of Sn-based materials.

• 1.

  Integration with nanostructuring technology: Nanoscale heterojunctions significantly increase the material's surface area, providing more active sites for ion transport and SEI formation. This structure enhances lithium and sodium ion diffusion and improves interfacial reaction kinetics, leading to better capacity, rate performance, and cycle stability by mitigating volume changes during charge/discharge cycles.

• 2.

  Composite with carbon matrix: Combining Sn-based materials with conductive carbon materials like graphene or CNTs improves electrical conductivity and buffers the volume changes of Sn. Carbon materials act as a skeleton to maintain structural integrity, enhance charge transfer, and promote the formation of stable SEI films, improving ICE and long-term cycling stability.

• 3.

  Integration with morphological structures: Structural designs such as hollow or porous structures create better ion diffusion paths and buffer stress from volume expansion. Heterojunctions further improve charge distribution, reduce side reactions, and enhance structural durability over long cycles.

Challenges and Prospects

Despite significant advancements in heterojunction anode materials for battery applications, several challenges remain, which must be addressed to facilitate their broader commercial deployment. Ongoing research and development are essential to overcoming these obstacles and realizing the full potential of these materials.

• 1.

  Optimization of heterostructure construction methods:

  • i.

    Enhancement of synthesis methodologies: While hydrothermal and solvothermal methods are commonly used to construct heterostructures, they require precise control over synthesis conditions. To improve commercial viability, there is a need to develop more controllable, reproducible, and cost-effective synthesis technologies. Moreover, careful selection of heteromaterials is crucial for optimizing electrochemical performance and cost-efficiency.

  • ii.

    Elucidation of formation mechanisms: A deeper understanding of the mechanisms underlying heterostructure formation is vital for refining synthesis processes, enhancing material properties, and reducing manufacturing costs. Increased research focus is needed to achieve better control over these processes.

  • iii.

    Investigation of performance enhancement mechanisms: Although heterostructures can enhance electrochemical performance, the mechanisms behind these improvements are not fully understood. Further experimental and theoretical studies are required to elucidate these mechanisms and optimize material design.

  • iv.

    Advancement of characterization techniques: Current techniques such as XRD, XPS, SEM, TEM, and DFT are valuable but need to be complemented by new methods to more precisely characterize heterostructures and unlock their full potential.

  • v.

    Evaluation in battery applications: There is a discrepancy between electrochemical behaviors observed in half-cells versus full cells. Although much research focuses on half-cell testing, full-cell testing better simulates real-world conditions. Future studies should emphasize performance evaluation and optimization in full-cell configurations.

  • vi.

    Theory-guided design and material selection: Computational materials science offers a powerful approach to designing and selecting battery materials. Theoretical calculations can help decipher enhancement mechanisms in heterostructures and predict the properties of new materials, guiding experimental research.

• 2.

  Additional challenges in commercialization:

• i.

  Scaling up production: Transitioning from laboratory-scale synthesis to industrial-scale manufacturing involves addressing challenges related to process scalability, cost management, and maintaining consistent quality.

• ii.

  Environmental considerations and sustainability: Balancing performance improvements with environmental impact is crucial. This includes considering raw material sourcing, energy consumption during production, waste management, and recycling at the end of battery life.

• iii.

  Safety assessments: Ensuring the safety of battery materials is critical for commercial applications. Heterojunction materials must undergo rigorous safety evaluations to ensure stability and reliability under diverse operational conditions.

• iv.

  Interdisciplinary collaboration: The advancement of battery technology relies on collaboration across various disciplines, including materials science, chemistry, physics, and engineering. Interdisciplinary research teams are essential for addressing the comprehensive challenges associated with heterojunction materials and advancing their commercialization.

  To deal with the aforementioned challenges, we can start from the following aspects:

  • i.

    Scaling up production

    Optimize production processes to enhance efficiency and scalability. Develop standardized procedures for consistency and quality control. Invest in automation technologies to reduce costs and improve consistency.

  • ii.

    Environmental considerations and sustainability

    Utilize renewable or recyclable raw materials to minimize environmental impact. Implement green manufacturing methods to reduce the environmental footprint. Enhance waste management and recycling to increase material recovery rates.

  • iii.

    Safety assessments

    Conduct comprehensive safety evaluations of materials and battery systems. Establish standard testing procedures for rigorous material testing. Strengthen material database development for quick safety reference.

  • iv.

    Interdisciplinary collaboration

    Establish collaborative platforms for communication across disciplines. Support industry-academia partnerships to leverage resources and knowledge. Organize multi-disciplinary workshops and conferences for idea exchange.

Through these focused research and development efforts, heterojunction anode materials have the potential to overcome current limitations and achieve widespread application in high-performance battery technologies.

Conflicts of Interest

The authors declare no conflicts of interest.

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