High‐entropy oxide: A future anode contender for lithium‐ion battery

Abstract

Revolutionary changes in energy storage technology have put forward higher requirements on next‐generation anode materials for lithium‐ion battery. Recently, a new class of materials with complex stoichiometric ratios, high‐entropy oxide (HEO), has gradually emerging into sight and embracing the prosperity. The ideal elemental adjustability and attractive synergistic effect make HEO promising to break through the integrated performance bottleneck of conventional anodes and provide new impetus for the design and development of electrochemical energy storage materials. Here, the research progress of HEO anodes is comprehensively reviewed. The driving force behind phase stability, the role of individual cations, potential mechanisms for controlling properties, as well as state‐of‐the‐art synthetic strategies and modification approaches are critically evaluated. Finally, we envision the future prospects and related challenges in this field, which will bring some enlightening guidance and criteria for researchers to further unlock the mysteries of HEO anodes.

INTRODUCTION

High-entropy oxide (HEO) refers to the single-phase oxide obtained by solid solution of five or more elements on the same sub-lattice in a molar ratio of 5%–35%. The increment of mixed configuration entropy caused by the increase of constituent elements is sufficient to overcome the single-phase compound formation enthalpy, thereby preventing the generation of detrimental intermetallic compounds.¹ The earliest HEO species was the rock salt (R-HEO) type (MgCoNiCuZn)O reported by Rost et al.² This seminal work demonstrates that entropy can facilitate the reversible transition between multiphase mixtures and quinary single-phase oxide solid solutions. This is particularly interesting as Cu ions in octahedral coordination are expected to exhibit Jahn-Teller distortion due to the asymmetry of electron occupation, resulting in four short and two long Cu²⁺-O²⁻ separations. However, the as-prepared (MgCoNiCuZn)O maintains a robust single phase even with 20% CuO. That is, entropy factor is excellent in supporting the structural stability of multicomponent oxides due to its high tolerance to lattice distortion. More precisely, the new phase formed due to configurational entropy should be called “entropy stabilized”. In contrast, multicomponent conventional solid solutions have a high entropy but are generally not entropy stabilized. The key difference is the presence of an entropy-driven phase transition at the critical temperature. Since then, a series of other HEOs types, such as fluorite, spinel, perovskite, O3 and P2 layered structures, have been discovered and prepared.^3–8 HEO generally needs to satisfy the mixing entropy greater than 1.5R. Of note, mixing entropy is often represented by configuration entropy in the literature because it is dominant and easy to quantify. The molar configurational entropy for HEO family can be calculated by Equation (1). In particular, Equation (2) is applicable to the case where each component is in an equimolar ratio.⁹ [Image Omitted. See PDF] [Image Omitted. See PDF]where R is the molar gas constant (8.314 J mol⁻¹ K⁻¹), N refers to the number of components, xi and xj represents the mole fraction of cations and anions, respectively. HEOs break the inherent notion of doped oxides. For one thing, high configurational entropy favors the formation of simple solid solution structures. For another, the principal elements tend to be chaotically arranged, with their chemical composition in a disordered state, leading to some interesting performance gains.¹⁰

In recent years, HEOs have attracted increasing attention in the field of electrochemical energy storage, especially as anodes for lithium-ion battery (LIB).^11–14 In addition to the inherent multi-electron redox mechanism and high safety profile of transition metal oxides,^15,16 the advantages of HEOs are also reflected in: (i) Abundant and flexible component design reduces any single element dependence and opens new opportunities for tailoring/fine-tuning electrochemical behavior; (ii) The thermodynamically high-entropy effect and kinetically hysteretic diffusion effect enable HEOs to maintain a stable structure even under harsh operating conditions, thus suppressing electrode volume expansion and improving cycling capability; (iii) The multi-element synergy and inherently complex surface of HEOs can provide nearly continuous adsorption energy, which is ideal for multistep tandem reactions; (iv) The highly disordered and distorted lattice creates numerous internal defects in the structure, which facilitates electron and ion migration; and (v) The electronic structure of HEO, especially the Fermi level related to the electrode potential, can be tuned by changing the stoichiometry. Given the fascinating prospects exhibited by HEO anodes, it is imperative to present a timely survey of this emerging field to extract potential advantages and challenges. This review shares our unique viewpoints through an overview of the research status of HEO anodes and specific descriptions regarding material design and electrochemical behavior. Guidelines for successfully searching and preparing such HEOs, as well as future outlooks for improving screening efficiency and optimizing certain properties, will be drawn in the following chapters. A timeline of HEOs evolution, lithium storage applications, and important advances is depicted in Figure 1. Since their birth in 2015, the compositions and properties of HEOs have been extensively investigated as their structures are considered to be stabilized by maximizing configurational entropy through highly disordered coupling elements. With the deployment of HEOs in battery anode, more targeted electrode material design, optimization and electrochemical research have come in a continuous stream. Together with the high-throughput computation/experiment and advanced synthesis/characterization enabled by modern technology, it opens an unprecedented window of opportunity to explore novel HEOs in an efficient manner.

FIGURE 1. Key progress timeline of HEO for LIB anode.2,17–22

LIB ANODE APPLICATION Outline

In 2018, a high-entropy (MgCoNiCuZn)O was first-ever reported with tempting reversible lithium storage function.¹⁷ Although only micron-sized particles were produced by nebulized spray pyrolysis, R-HEO still exhibited favorable long-cycle stability (over 300 cycles) with a specific capacity >650 mAh g⁻¹ (0.2 A g⁻¹). The voltage distribution of the GCD curve indicated a conversion-type reaction with an almost linear voltage decay over the entire potential range. Since then, researchers have carried out a multitude of fruitful studies in this blooming field. The types of HEO electrodes have been extended from rock salt to spinel (S-HEO) and even perovskite (P-HEO) structure. The spinel structure has three-dimensional Li⁺ diffusion channels, and the random distribution of cations at the two Wyckoff sites (8b and 16c) will further enhance the configurational entropy and form mixed valence states, which are conductive to accelerate the reversible redox reactions.²³ Therefore, S-HEO is considered as a more promising candidate for LIB anode. Conversely, although the perovskite structure also contains mixed lattice sites, more inert oxygen and electrochemically inert metals result in the dilution of electroactive elements, thus releasing less electron transformation. For example, the P-HEO [(Bi,Na)_1/5(La,Li)_1/5(Ce,K)_1/5Ca_1/5Sr_1/5]TiO₃ prepared by Yan et al.²⁴ delivered a discharge capacity of only 84 mAh g⁻¹ at 0.1 A g⁻¹. Apparently, high-entropy effect and multi-cation synergistic effect are not always positive. The specific role of each participating element in the Li⁺ intercalation/deintercalation process needs to be carefully elucidated.

Individual element functionality and component design

According to the mechanism first proposed by Sarkar et al., the rock-salt lattice of (MgCoNiCuZn)O is preserved during electrochemical cycling and serves as a permanent substrate for the conversion reaction (Figure 2A).²⁵ This implies that a portion of active elements are utilized to provide capacity, while others act as structure stabilizers. Qiu et al. speculated that Mg induces structural stabilization because it is inactive within the given potential window. The “spectator effect” derived by MgO alleviates the volume expansion of HEO anode while suppressing the agglomeration of active nanocrystals.²⁶ Wang and co-workers demonstrated the contribution of Mg in maintaining the crystal structure and mitigating electrode powdering/cracking via in-situ transmission X-ray microscope.²⁷ Accordingly, the Mg-free sample exhibits rapid capacity fading at high rates. Subsequently, Chen's group found that the role of Ti in (FeCoNiTiMn)O₄ was similar to that of Mg (Figure 2B). Along with the uptake of Li⁺, Ti preserves the spinel structure by forming LiTi₂O₄, while the remaining active ions are reduced.²⁸ Similar conceptions could even be generalized to sodium-ion batteries.^29–31 Also, Al has been deployed as a structural pillar,³² as Al substitution is found to pronouncedly alleviate the structural deformation of the cathode material under deep charged state.³³ It is worth mentioning that the retention of spinel framework was also observed in (FeCoNiCrMn)₃O₄ without inert elements. Based on the STEM-EELS results, Huang et al. proposed that the pristine lattice was separated into two new spinel phases (Cr_xFe_3-xO₄ and LiNi_xCo_1-xO₂) and Mn nanoparticles when discharged to 0.5 V (vs. Li⁺/Li). Further lithiation to 0.01 V resulted in the partial precipitation of metallic Fe, Cr, Co, and Ni (Figure 2C). The separation of Mn may be related to the electrolyte-induced corrosion phenomenon. During the subsequent delithiation process, Mn was integrated with Cr_xFe_3-xO₄ to form Mn_xCr_yFe_3-x-yO_4.³⁴

FIGURE 2. (A) Schematics of the lithiation/delithiation mechanism of R-HEO. Reproduced with permission.17 Copyright 2018, Springer Nature. (B) The stabilizing effect of Ti on the structure. Reproduced with permission.28 Copyright 2020, The Royal Society of Chemistry. (C) Schematics of the lithiation/delithiation mechanism of stabilizer-free S-HEO. Reproduced with permission.34 Copyright 2021, Elsevier. (D) The electronic band structures of S-HEO. Reproduced with permission.37 Copyright 2022, Elsevier

By comparison, the rational collocation of active components is the core to tailor the lithium storage performance of HEOs. Mn, V, Mo, and Cr are believed to provide high electron capacity, especially Cr offers three-electron redox (Cr⁶⁺/Cr³⁺) with high voltage.³⁵ Chen et al. claimed that Mn enjoys a fully reversible charge storage capability because it can recover to the initial valence after battery charging.²⁸ Duan et al. found that the introduction of Zn could promote the reversible reaction of Li₂O with transition metals and reduce electrode polarization.³⁶ Xiao et al. confirmed that Ni in S-HEO exerts an important effect in shortening the band gap and adjusting the electronic structure, because the site occupied by Ni is pretty close to the Fermi level (Figure 2D).³⁷ Sarkar et al. investigated the resulting changes in electrochemical behavior by removing specific elements from R-HEO.¹⁷ R-HEO-Co (extracting Co from R-HEO) completely fails after 10 cycles, implying that Co should be responsible for the high capacity of HEO. R-HEO-Cu exhibits the lowest discharge potential, which renders it as an interesting choice for primary anode applications. The lack of Zn in R-HEO leads to a stepwise oxidation process, suggesting that the parent rock-salt structure separates out the secondary phase after delithiation. In addition, the potential improvement of electrical conductivity by Cu dendrites and the capacity gain caused by Li/Zn alloying may also be involved upon cycling. It should be pointed out that the combination of elements in practical applications needs to consider tradeoffs between electrochemical performance and processability, safety, environmental friendliness, cost, and so forth.³⁸

With regard to element selection and material discovery, rational electroactive HEOs are generally based on experience drawn from known conventional oxides. However, for the vast component space of high entropy materials, the trial-and-error method lacking any pre-screening requires extensive time and cost for experimental evaluation. More efficient scientific strategies, such as computational chemistry and high-throughput experiments, need to be further aided in material prediction and optimization.^39,40 Lun et al. evaluated the chemical compatibility of 23 cations using ab initio DFT calculation (Figure 3A).³⁵ All redox-active species were found to be well chemically compatible except Cr³⁺. Species such as Zn²⁺, Al³⁺, Mg²⁺, and Ga³⁺ have poor compatibility with other transition metals and should be kept at a lower concentration. As a proof of concept, the authors synthesized 12 cation-containing R-HEO according to this guideline. Leong et al. predicted the phase stability of HEOs using an empirical biplots of d-Valence versus Ghosh electronegativity (Figure 3B),⁴¹ which offers the possibility of finding compositions in the stable region to suppress electrochemically induced phase transitions (e.g., spinel to rock salt). Furthermore, the authors distinguish the overlapping regions in Figure 3B by Voronoi tessellation (Figure 3C). Based on these models, it may be possible to gain further insight into what drives phase transition in oxide materials through correlation with mechanisms as diverse as ligand formation and electron transfer. In terms of developing autonomous and automated screening protocols, Velasco et al. mapped visual landscapes related to the phase-property (crystallographic phase, oxygen vacancy concentration and band gap, Figure 3D–F) of 91 samples through high-throughput experiments combined with Machine Learning.¹⁸ These samples are derived from stoichiometric tunable fluorite HEO (CeLaSmPrY)O₂. The systematic elaboration of phase-property diagrams of the integrated multicomponent systems will deepen the understanding of material properties evolution and controlling factors, and allow the search for the most promising target composition. For example, if single-phase Fm$\overline{3}$m, high defect concentration and narrow bandgap are desired, phase-property diagrams can determine the optimal chemical composition required, or at least narrow the compositional region of interest. The most common limitation in machine learning is acquiring enough data for reliable parameterization. Fortunately, high-throughput experiments provide the benefit to create substantial raw data.

FIGURE 3. (A) Chemical compatibility of cations in R-HEO. Reproduced with permission.35 Copyright 2021, Springer Nature. (B) Phase stability regions in HEO compositions (d-valence vs. Ghosh electronegativity). (C) The Voronoi tessellation diagram from (B) to delineate overlapping regions. Reproduced with permission.41 Copyright 2021, MDPI. (D) Isothermal crystallographic phase map, (E) oxygen vacancy concentration landscape and (F) direct band gap landscape obtained from multicomponent phase-property of (CeLaSmPrY)O2 system. Reproduced with permission.18 Copyright 2021, Wiley-VCH

In brief, due to complicated atomic arrangement and unpredictable element interaction, it is indispensable to carefully decouple the implication of a single element on the intrinsic properties and electrochemical behavior of materials. The combination of electroactive and structural supporting elements seems to be a more reasonable choice, as they are responsible for the lithium storage capacity and cyclability, respectively. Additionally, most reports only involve 5-cation equimolar HEO. The number and stoichiometry of elements that may alter the crystal structure and voltage window have not been fully appreciated.^42,43 In terms of routine experiments, a common strategy is to choose a baseline model and then add, subtract or replace a specific element to verify its functionality. (MgCoNiCuZn)O and (FeCoNiCrMn)₃O₄ are the two most dominant models. For broader HEO oriented design in the future, including synthesizability, thermodynamic/dynamic stability and optimization of specific desired properties, high-throughput computation and experimentation will accelerate the generation of multitudinous results and stimulate the identification of target products. For example, screening HEOs with narrow band gaps is beneficial for electronic conductivity; The high defect concentration helps to improve the ion diffusion and rate capacity of the electrode; Tuning the defect configuration, electronic structure and interface state of multi-principal structure are expected to regulate the selective catalysis of electrolyte, thereby ameliorating the composition and stability of SEI films.

Electrochemical mechanism

The exploration of electrochemical derivation mechanism is of major significance for material design, performance optimization and establishment of structure–activity relationship. Breitung et al. studied the gassing behavior of HEO half cells,⁴⁴ and they found that H₂ and C₂H₄ were released at low cutoff potentials. In detail, H₂ is mainly from the decomposition of trace water in the battery components while C₂H₄ can be attributed to the reduction of ethylene carbonate in the electrolyte. The amount of evolved gas was appreciably reduced after the second cycle, while the onset potential of C₂H₄ evolution shifted to a lower position, indicating the formation of a rather stable SEI. For the first time, Schweidler et al.⁴⁵ monitored the capacity attenuation mechanism of HEO anode by acoustic emission technology. The acoustic responses in the first lithiation cycle (0.3–0.01 V vs. Li⁺/Li) and each subsequent delithiation cycle (>1 V) can be respectively assigned to chemical mechanical decay (including particle fracture and pulverization) and progressive crack formation/propagation (Figure 4A), which offers the possibility to limit mechanical decay by adjusting the potential window. Regarding the Li⁺ insertion/extraction mechanism, although most literatures supposed that HEO anodes follow the combination of entropy-mediated phase stabilization effect and partially reversible redox reactions of metal ions, many additional scientific issues remain to be clarified. Luo et al.⁴⁶ observed that Cu and Ni were hardly re-oxidized upon first charging ([FeCuNiCrMn]₃O₄-based half cell), which is anomalous in known HEOs since they are usually regarded as active elements. Sun et al.⁴⁷ and Patra et al.⁴⁸ found that S-HEO undergoes a spinel to rock salt transformation in the first half-cycle, and the two phases coexist in subsequent reactions. This finding seems to run contrary to the single-phase retention capability of HEO. Supplementary testing is required to further verify whether the postmortem active material is a complex solid solution or an entropy-stabilized compound. Collectively, the existing forms of cations after electrochemical cycling, the contribution of extra rock-salt structure to lithium storage and the underlying trigger of phase transition are important directions in future research. Moreover, there are some opposing voices against the entropy stabilization effect. Quartarone's group teased out the lithiation mechanism of R-HEO by operando X-ray absorption spectroscopy. They proposed that the reduction reaction is a multistep procedure. The HEO structure remained intact at 60% lithiation capacity and subsequently collapsed irreversibly. From the second cycle, the capacity contribution comes only from the alloying/dealloying of Li/Zn and Li/Mg.^49,50 The divergence on the lithium storage mechanism may stem from different synthetic methods. Various synthetic strategies are often accompanied by enormous differences in product phase purity, element uniformity, crystallinity, morphology and particle size. In addition, single analysis tools, especially ex-situ analysis, can only obtain limited information. Because lithiated compounds are very sensitive to air and moisture exposure. On the other hand, the electrodes may self-discharge once the applied potential is unloaded.

FIGURE 4. (A) Acoustic activity (hit density) as a function of time and voltage. Reproduced with permission.45 Copyright 2021, Springer Nature. (B) Cycling performance of S-HEO synthesized by alloy oxidation. Reproduced with permission.37 Copyright 2022, Elsevier. (C) Schematic of HEO preparation through a solution-based route. Reproduced with permission.53 Copyright 2022, Elsevier. (D) Schematic of HEO preparation through laser scanning ablation technique. Reproduced with permission.20 Copyright 2022, Springer Nature. (E) Schematic diagram of the full cell using HEO anode and the GCD curve of a corresponding pouch cell. Reproduced with permission.58 Copyright 2019, Elsevier

Advanced synthesis and modification

The performance comparison of available HEO-based anodes is summarized in Table 1. As can be seen, the performance of electrode materials strongly depends on the synthesis method and experimental conditions. A typical example is the (FeCoNiCrMn)₃O₄ obtained by direct oxidation of high-entropy alloy FeCoNiCrMn delivers outstanding rate capability and long-cycle stability (Figure 4B),³⁷ spectacularly outperforming the counterparts prepared by other methods. The authors speculated that the superior performance is related to the reasonable secondary particle structure. Because the high reactivity and large specific surface area of primary nanoparticles often trigger undesired side reactions and excessive SEI formation. Nguyen et al. found that S-HEO with balanced crystallinity and particle size showed optimized capacity retention by controlling the annealing temperature.⁵¹ Chen et al. reported the size-dependent pseudocapacitive effect of R-HEO nanocrystals. As expected, the small size grains (46.3 nm) have more promising rate capability (358 mAh g⁻¹ at 2 A g⁻¹).⁵² Moreover, developing green, fast, and scalable synthetic routes is a prerequisite for intended functional applications. In this context, some inspiring progress has been documented very recently. Kim et al. proposed an energy-efficient, mass-producible solution-based scheme using high-entropy layered double hydroxides as precursors (Figure 4C).⁵³ This approach highlights the customizability of the metal cation ratios as they are fully inherited from the reaction solution. Similarly, Ritter et al. provided an instructive example of aqueous electrochemical synthesis in which fast kinetics brought by strong current densities endow high-entropy particles with robust phase stability.⁵⁴ Phakatkar et al. reported a flame spray pyrolysis strategy, which is achieved by ultrafast evaporation and condensation (on the millisecond scale) of aerosol precursors in a high temperature flame.⁵⁵ Furthermore, Wang and collaborators developed a laser scanning ablation technique—the intense light field in laser focus enables precise synthesis of various HEO nanoparticles within 5 ns (Figure 4D),²⁰ which opens up tangible opportunities for exploring the broad component space of HEO.

TABLE 1 A comparison of Li storage performance of HEO anode.

[Table omitted. See PDF]

In general, solid-state reaction represented by ball milling is still the main method to produce HEO powders. Although it is featured by a simple process and high yield, the powders obtained by the solid-phase method are not fine enough as electrode materials, and more importantly, the impurities are always inevitable. A variety of liquid-phase synthesis methods have been widely developed in the past few years, including sol–gel, hydrothermal/solvothermal treatment, solution combustion, molten salt method, microwave, sonochemistry, electrochemistry, dealloying, spray pyrolysis, and so forth.^4,5 Obviously, the HEOs obtained by liquid-phase methods show high phase purity, favorable dispersibility, ideal stoichiometric ratio and controllable particle morphology. Notwithstanding, liquid-phase methods generally follow a dissolution-precipitation procedure. The ineluctable local concentration gradients may lead to elemental segregation and broad particle size distribution. Beyond that, the lately proposed rapid synthesis methods such as Joule heating,⁵⁶ laser ablation²⁰ and droplet-to-particle pyrolysis⁵⁷ have greatly improved the work efficiency, but still face challenges in terms of cost and yield. Of particular note, the synthesis of most HEOs requires the assistance of extreme conditions (e.g., instantaneous heating, quenching, high voltage, electric field and laser field) to suppress phase separation compared to low- and medium-entropy oxides.

Apart from exploring new HEO anodes, the necessary modification of electrode materials is an effective means to make up for their own flaws and thus improve the electrochemical performance. In this regard, the incorporation of additional low-valent cations into the already complex components is a high-profile approach. The substitution possibility of such heterovalent elements can broaden the phase space and greatly stimulate the development potential of new materials. A pioneering work reported by Bérardan et al. validated that (MgCoNiCuZn)_1-xLi_xO enjoys a huge dielectric constant (2 × 10⁵ for 5% Li)⁵⁹ and a lithium content-dependent ultrahigh Li⁺ conductivity (10⁻³ S cm⁻¹ for 30% Li).²¹ After that, Moździerz et al.⁶⁰ and Ma et al.⁶¹ also found the superior electronic conductivity of (MgCoNiCuZn)_1-xLi_xO series, based on which a hypothetical model of mixed ionic-electronic transport was put forward. It should be noted that dopants tend to alter charge balance. Two latent repair mechanisms have been documented, namely (i) defects, possible oxygen vacancies, and (ii) self-charge compensation, achieved by oxidation of certain variable valence elements, such as Co²⁺ to Co^3+.62–64 Although the contribution ratios of these two mechanisms are variable across the whole Li concentration range, they can still partially account for the changes in electronic properties of R-HEO. Based on the remarkable functional properties, (MgCoNiCuZn)_0.7Li_0.3O performs satisfactorily as an active electrode material, which may have an intercalation-like region at higher potentials and conversion/alloying reaction at lower potentials.⁶⁰ Our group found that the abundant oxygen vacancies in (LiMgCoNiCuZn)O not only act as charge carriers to accelerate ion/electron transport and electrochemical reaction kinetics, but also provide additional Coulomb attraction that facilitates the preferential aggregation of Li around the vacancies.⁶⁵ It should be pointed out that the disordered atomic distribution in the HEO lattice leading to the introduction of vacancies at particular locations with accurate concentrations remains challenging. In addition, due to the synergy, it is difficult to distinguish the respective contributions of doping and oxygen vacancies. Continued exploration of controllable and sustainable modification strategies is warranted. Recently, the Li doping pathway has also been extended to S-HEO group,^36,66 but the specific role of Li and its direct relationship to the electrochemical behaviors needs to be further clarified.

Considering the promotion of electron transport by oxygen vacancies, the researchers also designed defect-rich HEOs for ad hoc electrochemical applications by means of plasma treatment⁶⁷ and acid etching.⁶⁸ Furthermore, carbon-coated/modified HEO is also an interesting design inspiration.^69–71 For instance, the R-HEO@Graphene anode constructed by Guo et al. can deliver a high reversible capacity of 1001 mAh g⁻¹ at 0.1 A g⁻¹ and maintain 460 mAh g⁻¹ after 1000 cycles at 1 A g^−1.72 Graphene further buffers the volume expansion of the electrode and ensures superior electrical conductivity. Beyond that, the morphology modulation of electrode materials is also an aspect that cannot be ignored, such as but not limited to HEO nanofibers,^73,74 nanosheets,^67,75 porous network,⁷⁶ hollow microspheres.⁷⁷ The geometry often involves specific effects on material specific surface area, charge accommodation space, electron transfer paths, volume changes, and so forth. Wei et al. synthesized 2D R-HEO nanonets with large specific surface area (25 m² g⁻¹) based on in-situ carbon template. Due to the high reactivity of the material and its adequate contact with the electrolyte, the R-HEO electrode manifests a rate capacity of 240 mAh g⁻¹ (2 C) and a capacity retention of 92% after 200 cycles (0.2 C).⁷⁸ Unfortunately, reports on the application of modified HEOs to LIBs are still extremely limited, and more efforts are invited to achieve breakthroughs in electrochemical performance.

The full cell is a pivotal demonstration of the practical application potential of electrode materials. The (FeCoNiCrMn)₃O₄||LiFePO₄ full cell assembled by Zhao et al. exhibits a specific discharge capacity of 136 mAh g⁻¹ (1–4 V vs. Li⁺/Li, 0.1 A g⁻¹) and a stable cyclic performance over 100 cycles (capacity retention 91%) in the temperature range of 0–50°C.⁷⁹ Wang et al. systematically studied the electrochemical performance of (MgCoNiCuZn)O||LiNi_1/3Co_1/3Mn_1/3O₂ battery.⁵⁸ The coin full cell maintained 256 mAh g⁻¹ at a current density of 0.12 A g⁻¹ for 100 cycles (0.5–4.5 V vs. Li⁺/Li). Meanwhile, a high energy density of 240 Wh kg⁻¹ was achieved at a power density of 320 W kg⁻¹. Particularly, the established pouch cells with an electrode area of 2 × 4 cm² can be utilized to continuously supply power to the LED array (32 LEDs operating at 2 V and 64 mA) (Figure 4E), proving the feasibility as energy storage device.

OUTLOOK AND PERSPECTIVE

The development of LIBs technology highly relies on the innovative anode materials with both high energy density and power density. Although just in its infancy, the introduction of the high-entropy concept has proven effective in designing and exploring high-performance oxide anodes. Of special note, the cycling stability of HEOs is appreciably better than that of their medium-entropy counterparts, which is benefited from the entropy-driven structure retention ability. Rock salt and spinel compounds are the major candidates, while other HEO structures need to be further verified for their utility as LIB anodes. Regarding the component design, some basic principles should be considered: (i) Introducing appropriate electrochemically inert elements (e.g., Mg, Ti, Al) into the structure may help maintain a stable framework and improve cycling performance; (ii) Metals with low redox voltages are preferred to improve overall energy density; (iii) Mixed valence states, especially high-valence elements, are conductive to increase oxygen vacancy concentration and the number of electron transport; and (iv) Selecting elements with weaker bond polarity should alleviate the low energy efficiency resulted from large voltage hysteresis of the conversion anode. Despite the potential advantages, the following statements about the existing problems and possible improvements of HEO still need to be highlighted.

1. Combining advanced characterization techniques

At present, the identification and characterization of HEOs are mainly performed by XRD, TEM and EDS, which are inaccurate and insufficient. It is necessary to further confirm the cation distribution and phase purity through more reliable and precise techniques. Moreover, the overall performance of HEO is not a simple superposition of single oxides. More advanced measurements are required to deconstruct the association between element combination, structure and property/performance. This mainly covers synchrotron radiation related technology, neutron scattering/diffraction method, atomic-resolution electron microscopy analysis and operando characterization instruments. For example, synchrotron X-ray technology can detect the electronic and structural features of materials at different depths almost non-destructively through spectroscopy, scattering and imaging functions, providing a powerful reference for analyzing the structural evolution mechanism, phase transition process and charge compensation behavior of electrode materials. A feasible future attempt lies in identifying the functional units of HEOs and reveal the endogenous structure–activity correlation with the aid of various characterization techniques.

1. Theoretical calculation is an efficient auxiliary tool

While the high-entropy concept allows entirely new material compositions and uniquely tailored material properties, the complex interactions of bound ions make it difficult to predict material behavior through simple chemical understanding. The lattice stabilization and distortion are governed by a combination of components (electronegativity and valence electron concentration, chemical compatibility), structural characteristics (ionic radii, structural displacement of ion pairs) and formation energy. Scientists have developed descriptors such as entropy forming ability (EFA value), Goldschmid tolerance factor and size disorder factor based on computational materials science and materials informatics to evaluate the synthesizability of various HEOs.^5,80 For instance, DFT simulation on account of Special Quantum Structure algorithm has been employed to theoretically validate the formation possibility of new HEOs. Nevertheless, DFT is not applicable for big data virtual screening due to the disorder of solid-solution structures and the complexity of multi-component systems. In contrast, Machine Learning can rapidly build models by mapping the relationship between target attributes and material property parameters, which may be a desirable option for predicting the synthesizability of HEOs. Moreover, thermodynamic studies and molecular dynamics simulations help to examine phase stability by analyzing the competing relationship between entropy and enthalpy, particularly stable composition and temperature range, as well as transition trends and critical conditions. Additionally, the functional properties of HEOs are also bound up with the microstructure, surface/interface states and internal defects. The theoretical calculations are instrumental in analyzing the electrochemical reaction kinetics of electrode materials from the atomic scale, and offer a theoretical basis for revealing the complex redox process of HEOs.

1. Focusing on the deep unscrambling of HEO, especially concerning working mechanism

There are many unsolved mysteries about HEO. For example, some HEO types (e.g., most S-HEOs) fail to exhibit temperature-dependent reversible phase transition characteristics, which calls into question the universality of the high-entropy effect. In addition, the solid solution mechanism has not been validated. It may be based on a certain unary oxide as a matrix for sequential incorporation, or it may be that the multiphase mixture is highly miscible under extreme conditions like high temperature. These findings are critical for understanding the structural stability of HEO. Furthermore, in most reports, the electrochemical performance is simply attributed to the “cocktail effect” or multi-element synergy of high-entropy materials, but the specific synergistic mechanism remains elusive. The incompletely reversible redox reactions and the spinel–rock salt–amorphous transition of the active substances arguably imply a change in the lithium storage mechanism. Besides, lattice distortion is also an interesting topic. Because the already heavily distorted HEO structure remains marvelously intact after undergoing volume expansion and redox of transition metals upon electrochemical cycling. What is the tolerance limit for this lattice stabilization? Unraveling the interactions of atoms, lattices, metastable structures, stacking faults and strain in HEOs may help uncover these mysteries.

1. The overall performance of HEOs remains to be improved to meet the ever-increasing demand

As a whole, the exploration of HEO anodes remains in the primary stage. Many electrochemical properties, including electrical conductivity, initial coulombic efficiency and rate performance, deserve further optimization. Therefore, new classes of HEOs, especially with multiple Wyckoff sites, should be continuously developed. Besides, mixed anionic, polymeric anionic or polymeric cationic systems are also an attractive direction. For instance, HEO materials have been rapidly expanded to oxychloride,²² oxyfluoride,⁸¹ NASICON,⁸² fluorophosphate⁸³ and dittmarite analogue.⁸⁴ These atypical HEO categories not only enrich the known high-entropy library, but also demonstrate the positive contribution of anionic sites to structural entropy, which may provide new and insightful solutions for the engineering design of materials. Beyond that, the escalation of HEO synthesis methods, modification of material morphology and structure, and even coupling with additional functional materials are highly potential boosters that will lead the next wave of innovation in energy storage technologies.

ACKNOWLEDGMENTS

This work supported by National Natural Science Foundation of China (Grant number 52072274, 52104309) and Natural Science Foundation of Hubei Province (2021CFB011). The authors would also like to thank the Analytical & Testing Center of Wuhan University of Science and Technology.

CONFLICT OF INTEREST

The authors declare no conflicts of interest.