Building better solid‐state batteries with silicon‐based anodes

Abstract

Silicon (Si)‐based solid‐state batteries (Si‐SSBs) are attracting tremendous attention because of their high energy density and unprecedented safety, making them become promising candidates for next‐generation energy storage systems. Nevertheless, the commercialization of Si‐SSBs is significantly impeded by enormous challenges including large volume variation, severe interfacial problems, elusive fundamental mechanisms, and unsatisfied electrochemical performance. Besides, some unknown electrochemical processes in Si‐based anode, solid‐state electrolytes (SSEs), and Si‐based anode/SSE interfaces are still needed to be explored, while an in‐depth understanding of solid–solid interfacial chemistry is insufficient in Si‐SSBs. This review aims to summarize the current scientific and technological advances and insights into tackling challenges to promote the deployment of Si‐SSBs. First, the differences between various conventional liquid electrolyte‐dominated Si‐based lithium‐ion batteries (LIBs) with Si‐SSBs are discussed. Subsequently, the interfacial mechanical contact model, chemical reaction properties, and charge transfer kinetics (mechanical–chemical kinetics) between Si‐based anode and three different SSEs (inorganic (oxides) SSEs, organic–inorganic composite SSEs, and inorganic (sulfides) SSEs) are systemically reviewed, respectively. Moreover, the progress for promising inorganic (sulfides) SSE‐based Si‐SSBs on the aspects of electrode constitution, three‐dimensional structured electrodes, and external stack pressure is highlighted, respectively. Finally, future research directions and prospects in the development of Si‐SSBs are proposed.

INTRODUCTION

Over the past few decades, commercial lithium-ion batteries (LIBs) with organic liquid electrolytes have played crucial roles in both portable electronics and the electric vehicle industry, which greatly improve the quality of our life.^[1-4] Although LIBs have experienced significant progress in recent years, the achievement of rechargeable Li batteries with both high security and high energy density is still ongoing.^[5,6] Anode materials, as a key component, play a dominant role in the energy density of rechargeable Li batteries.^[7,8] Accordingly, numerous efforts have been made to develop high-capacity anodes to enhance the energy density of rechargeable Li batteries.^[9-11] It should be mentioned that due to the ultrahigh theoretical capacity (3860 mA h g⁻¹) and the lowest electrochemical potential (−3.04 V vs. standard hydrogen electrode), the Li metal anode is considered the ultimate anode candidate.^[10-14] However, commercial LIBs with flammable organic liquid electrolytes suffer from potential safety concerns including electrolyte volatilization, premature cell failure, and even serious explosion problems, and their energy densities become the bottlenecks of further application (Figure 1A).^[16-18] The combination of Li metal anode with solid-state electrolytes (SSEs), by assembling Li metal solid-state batteries (LMSSBs), can provide great potential for achieving both high safety and improved energy density (Figure 1B).^[19,20]

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Nevertheless, the practical application of LMSSBs still faces huge obstacles including uncontrolled growth of Li dendrites that may penetrate the separator and short the cell, continuous side reactions that cause irreversible lithium loss (dead lithium), and interfacial instability with SSEs that leads to contact loss and large interfacial impedance, and so on.^[21-25] Accordingly, various strategies have been designed to solve the Li dendrite growth and interfacial issues in LMSSBs, such as the surface modification of Li metal anodes, the imposed external stack pressure for ensuring interfacial contact, and the structural and composition optimization for SSEs and Li metal anodes.^[26-30] The development of LMSSBs can be facilitated by such beneficial explorations, but the commercialization of LMSSBs remains a significant challenge. In this regard, greater attention has to be paid to investigating solid-state batteries (SSBs) along with other high-capacity anodes, especially alloy anodes.^[15,31,32] Owing to the merit of the high theoretical capacity (3579 mA h g⁻¹) of Si-based anode, environmental friendliness, and low cost, the study of silicon-based SSBs (Si-SSBs) has received increased interest.^[33] Importantly, as compared to the Li metal anode, the Si-based anode exhibits other advantages. This can be explained that many of the degradation modes can be avoided, when the Li metal anodes are used in SSBs, including dendrite growth that causes short-circuit (Figure 1C).^[34,35]

Currently, the Si-based anodes in commercial liquid electrolyte-dominated LIBs have been extensively investigated. The prevailing challenges of Si-based anodes for being used in LIBs have been thoroughly concluded in previous studies^[36-42]: (1) continuous formation of undesired solid-electrolyte interphases (SEI)^[43,44]; (2) particle pulverization and breakage of electric conductivity caused by large volume change (>300%)^[45-48]; and (3) poor rate performance induced by low electronic conductivity (σ_e < 10⁻⁵ S cm⁻¹) and Li⁺ diffusion rate (D_Li+, 10⁻¹⁴–10⁻¹³ cm² s⁻¹),^[49] as shown in Figure 2. To address these issues, various types of Si anodes have been exploited, including nano/micro-Si anodes, three-dimensional (3D) structured Si anodes, Si-based composite anodes, and so on.^[36,50-53] These strategies have been proven indispensable for enhancing electrochemical performance (Figure 2), and thereby they have been already used to enable high-performance Si-SSBs. Apart from the structural design of the Si-based anode, the interfacial compatibility between the Si-based anode and SSEs is also required to be considered in Si-SSBs to realize further improvement in electrochemical performance.^[54] This is because the flowable liquid electrolyte used in LIBs can ensure close contact between the liquid electrolyte and Si-based anode.^[55] In contrast, the SSEs cannot permeate through the Si-based anode during cycling in Si-SSBs and the contact condition between SSEs and Si-based anode is easy to deteriorate.^[35] Compared with LMSSBs, such drawback in Si-SSBs is even more severe, which is derived from its larger volume expansion and larger rigidity than those of Li metal. Moreover, due to the unsatisfied contact condition, the ion/electron transfer inside Si-SSBs is challenging. Similar to LMSSBs, external pressure is probably helpful for facilitating the ion/electron transfer inside Si-SSBs;^[56,57] however, the preparation process of Si-SSBs under external pressure is complicated. Fortunately, the limited interfacial contact between SSEs and Si-based anode can provide an opportunity for suppressing the continuous growth of SEI in Si-SSBs, which is conducive to achieving stable long-term cycling performance without the complicated structuring of Si-based materials.^[58] Additionally, the anisotropic volume expansion and anisotropic Li⁺ diffusion in Si anodes, as confirmed by in situ transmission electron microscopy (in situ TEM),^[59] can also cause a great challenge in the development of Si-SSBs, which may accelerate the deterioration of the interfacial contact condition.

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Recently, given their predicted high energy density and safety, the development of Si-SSBs has begun to be accelerated.^[34] Moreover, some companies, including the UNIGRID battery, are making great efforts to commercialize Si-SSBs. The history of Si-SSBs is much shorter than that of commercial liquid electrolyte-dominated LIBs. The first Si-SSB was proposed by Notten et al. in 2007,^[60] which was composed of a layer of lithium phosphorus oxynitride (LiPON) SSE, a layer of LiCoO₂ cathode, and a layer of Si thin film anode.^[60] Since then, extensive work related to Si-SSBs has been published, and thereby various inorganic (oxides) SSEs, organic–inorganic composite SSEs, and inorganic (sulfides) SSEs were employed to couple with Si-based anode to construct Si-SSBs (Figure 3). Therefore, it is an opportune time to systematically summarize the state-of-the-art progress of Si-SSBs and provide our perspectives on the future development of Si-SSBs. Different from previous review, we mainly focus on the different interfacial configuration characteristics and mechanisms between various types of SSEs including inorganic (oxides) SSEs, organic–inorganic composite SSEs, and inorganic (sulfides) SSEs and Si-based anode, as shown in Figure 4. Besides, the correlations between these interfacial characteristics and electrochemical performance^[72-74] are also elucidated.

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The properties of SSEs are extremely important for the performance improvement of Si-SSBs, which require the SSEs to have high ionic conductivity (>10⁻⁴ S cm⁻¹), broad electrochemical properties (excellent reduction stability and oxidation stability), good thermal/air stability, reasonable processing cost, scale-up capability, and mechanical suitability. However, it is quite difficult to develop such SSE to meet all these favorable properties. To understand the properties of various SSEs more comprehensively, radar plots of the different properties of inorganic (sulfides), inorganic (oxides), and organic–inorganic SSEs are compared in Figure 5. It is worth noting that the interfaces between these SSEs and Si-based anode are largely different, in which the contact between inorganic (oxides) SSE and Si-based anode acts as a rigid-to-rigid contact. In contrast, for the organic–inorganic composite SSEs and inorganic (sulfides) SSEs, deformation of these SSEs is tolerable, as a result, more stable interfaces can be established. It has been demonstrated that the interfacial challenges for Si-SSBs using organic–inorganic composite SSEs and inorganic (sulfides) SSEs are not as severe as that using inorganic (oxides) SSEs. Besides, for practical application, the thickness of the electrolyte should be strictly controlled (<20 μm), and it is extremely difficult to prepare thin electrolytes with rigid inorganic (oxides) SSEs at a considerable cost. Consequently, the organic–inorganic composite SSEs and inorganic (sulfides) SSEs show advantages as compared with inorganic (oxides) SSEs. However, for some organic–inorganic composite SSEs, the intrinsic ionic conductivity is inferior to inorganic (oxides) and inorganic (sulfides) SSEs.^[75-78] From this aspect, the inorganic (sulfides) SSEs exhibit great prospects for Si-SSBs with enhanced performance. It should be noted that the strict synthesis condition (high processing cost) for sulfide/oxide SSEs and their severe side effects will also be taken into consideration.^[54,79-81] Finally, the scale-up capability is crucial for commercialization; the organic–inorganic composite SSEs and inorganic (sulfides) SSEs are thus superior to the inorganic (oxides) SSEs, which are more promising for the deployment of Si-SSBs in the future.

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To clearly reveal the correlations between these different interfacial characteristics and electrochemical performance, the present review is divided into five sections. First, the comparisons of fabrication processes and test equipment of Si-SSBs with commercial liquid electrolyte-dominated Si-based LIBs are discussed. Second, the Si-SSBs using Si-based anode and inorganic (oxides) SSEs, organic–inorganic composite SSEs, inorganic (sulfides) SSEs, and other types of SSEs, as well as different types of formed interfaces, are systemically summarized. Eventually, the underlying challenges and future perspectives in the development of high-performance Si-SSBs are also proposed.

Si-SSBs versus COMMERCIAL LIQUID ELECTROLYTE-DOMINATED Si-BASED LIBs

Compared with the commercial liquid electrolyte-dominated LIBs, Si-SSBs show significant differences in both the battery configuration and the electrochemical test equipment. As a result, design principle and experience for liquid electrolyte-dominated LIBs may not feasible for Si-SSBs.^[35,72]

Structure and configuration

In general, commercial liquid electrolyte-dominated LIBs are composed of four critical components, including cathode, electrolyte, separator, and anode. Meanwhile, the SSEs serve both as an electrolyte and a separator in the Si-SSBs, and thus the structure of Si-SSBs is simplified as compared to that of the commercial liquid electrolyte-dominated LIBs.^[82]

Nevertheless, severe contact problems can be induced by the lack of liquid electrolytes in Si-SSBs. In the liquid electrolyte-dominated LIBs, the electrolyte can easily permeate through the whole electrode by the formation of 3D electrolyte–electrode contact to establish satisfactory contact conditions. However, the electrode–electrolyte contact in Si-SSBs is transformed from 3D to two-dimensional (2D) contact, and thereby the rapid and continuous ion/electron transfer in the whole electrode is challenging. Accordingly, two effective strategies have been proposed to enable favorable charge transfer in the Si-based anode capable of achieving the Si-SSBs with improved electrochemical performance: (1) reducing the thickness of Si anodes.^[69,71] When the thickness of Si anodes remains less than 1 μm, Li⁺ and electrons can transfer over the whole anodes without high resistance. With this method, pure Si anode can be applied in the SSBs, but the mass loading of pure Si anode is strictly limited. Moreover, the fabrication process of thin film Si anodes with a nanoscale thickness is complex, and thus the traditional slurry coating method cannot be applied; (2) introducing electronic conductive and ionic conductive agents in Si-based anodes for establishing ionic/electronic conductive network.^[61,83] This strategy is more feasible for the fabrication of high-loading Si anodes with a simple slurry coating process. Simultaneously, the introduction of conductive agents may sacrifice the energy density of the batteries. Reducing the content of additive agents is crucial for the commercialization of Si-SSBs.

In addition, it should be mentioned that, in practice, the cathode side, and some extra conductive agents, that is, SSEs, are necessary to obtain a satisfactory contact condition.^[27]

Testing condition

The testing condition for Si-SSBs may differ from the liquid electrolyte-dominated LIBs due to their poor contact condition inside Si-SSBs. Moreover, this contact condition in Si-SSBs is much more severe as compared to LMSSBs, which can be ascribed to the large volume variation and high mechanical stiffness of Si. To maintain good contact in Si-SSBs, external stack pressure is usually applied to guarantee good contact between SSEs and Si anodes. In addition, the applied external pressure is also beneficial for the internal contact inside Si anodes. It is worth noting that, by comparisons as discussed above, the contact condition is not such severe for the thin film Si-SSBs with nanoscale Si anodes. As a result, external pressure is not necessary for thin film Si-SSBs.

To apply external pressure during the cycling of Si-SSBs, testing dies are designed for applying pressure on them. After the assembled cells or electrode/electrolyte powder are placed inside the dies, external stack pressure can be applied to the cells by tightening the bolts on the dies.^[61] Furthermore, other equipment, such as force sensors, can be added to the testing dies to monitor the status of Si-SSBs during the cycling process.^[57]

For the Si-SSBs with organic–inorganic composite SSEs, the ionic conductivity of organic–inorganic composite SSEs should be taken into consideration. For example, poly(ethylene oxide) (PEO)/LLZO, as a kind of organic–inorganic composite SSEs, shows insufficient ionic conductivity at room temperature, and thus higher operating temperature is required.^[84] Additionally, due to the flexibility of organic–inorganic composite SSEs, Si-SSBs using organic–inorganic composite SSEs can be operated upon without any external pressure.

In conclusion, because of the interfacial challenges, the testing conditions for Si-SSBs are largely different from the commercial liquid electrolyte-dominated LIBs. It should be noted that high temperature and high pressure are not favorable for their practical application, and the improved ionic conductivity at room temperature and interfacial contact of Si-SSBs are expected for simplifying its operating condition.

INORGANIC (OXIDES) SSE-BASED Si-SSBs

Inorganic (oxides) SSEs, as a crucial series of SSEs, have been intensively applied as the component of LMSSBs.^[30,85,86] Coupling inorganic (oxides) SSEs with Si-based anodes is promising for achieving the Si-SSBs. However, obtaining compact contact between SSEs and Si-based anodes still remains a significant challenge. Unlike the LMSSBs, a contact between Li metal and SSEs can be realized conveniently, such as the molten Li.^[87,88] Even when the LMSSBs are assembled by cold tapping, the interfacial resistance between SSEs and Li metal anode is acceptable because the Li metal has enough plastic deformation capacity under external stress. In contrast, these strategies are not compatible with Si-SSBs for the intrinsic properties of Si (rigid and high melting temperature). Thus, how to address the issue of electrode–electrolyte interface becomes the key point for the realization of Si-SSBs containing inorganic (oxides) SSEs with an enhanced electrochemical performance. Ferraresi et al.^[69] and Guo and colleagues^[67] successfully assembled the Si-SSBs using the garnet type Ta-doped Li₇La₃Zr₂O₁₂ (LLZTO, a typical oxide SSE) via the sputtering method. As reported, a thin Si film (~50 nm) was successfully deposited on the surface of LLZTO via the procedure as shown in Figure 6A. Before the deposition of Si film, the LLZTO SSEs were treated by argon plasma etching to minimize the resistive species. Benefiting from this strategy, the excellent contact condition between Si anodes and LLZTO SSEs was achieved. The cycling performance of such batteries was also evaluated. Although a high initial charge capacity of 2702 mA h g⁻¹ can be obtained, it decreases to about 1200 mA h g⁻¹ after 100 cycles. Additionally, Chen et al. revealed that even with an original good contact between LLZTO and Si anodes, the interfacial condition can deteriorates after cycling (Figure 6B). It is found that when the thickness of Si anodes was less than 180 nm, the electrode–electrolyte interface remained stable during the cycling process. But once the thickness of Si anodes was increased to over 180 nm, the electrode–electrolyte contact condition deteriorated rapidly during the cycling process. Moreover, the interfacial contact condition became worse as the increase of the thickness of Si layer. These experimental results were in accordance with the corresponding electrochemical performance, indicating that the Si-SSBs assembled by the Si anodes with a thickness greater than 180 nm experienced faster capacity fading as compared to the Si-SSBs assembled by the thin Si anodes with a thickness less than 180 nm. In addition, the observation results of in situ scanning electron microscopy (in situ SEM) demonstrated the lithiation/delithiation process of thin Si anode, which was consistent with the results of the electrochemical test. Besides, the thin Si film with a thickness of about 45 nm remained in close contact with LLZTO SSEs during the cycling process.

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However, since the mass loading of Si anodes is vital for industrial production, the increase in the thickness of Si anodes is an essential condition. Ping et al.^[72] first demonstrated the feasibility of combing microscale-thick Si anodes with inorganic (oxides) SSEs (Figure 6C). In this work, plasma-enhanced chemical vapor deposition (PECVD) was employed to construct the amorphous Si anode on the surface of LLZAO (Li₇La₃Zr₂O₁₂ with 3% Al₂O₃). After the deposition process was completed, a layer of single-wall carbon nanotubes (CNTs) was coated on the surface of Si film and it worked as a current collector. Moreover, the electrochemical performance of the as-mentioned microscale Si anodes was assembled into SSBs and liquid LIBs for comparison. Intriguingly, the initial Coulombic efficiency (ICE) of Si anode in liquid LIBs was inferior to that in SSBs. To deeply understand such a phenomenon, finite element modeling was exerted to simulate the lithiation/delithiation process of Si at the electrolyte–electrode interface. As a result, the volume expansion of Si during lithiation was not only effectively constrained by the garnet SSEs but also the shrinkage of Si film was limited during delithiation. Meanwhile, such strong nanomechanical constraint was lacked in the liquid electrolyte, leading to the appearance of cracks, which was probably responsible for the inadequate ICE.

Nevertheless, microscale-thick Si anodes can only satisfy the application of some microscale devices such as monolithic Si-wafer or System-in-Package with no demand for high capacity, and thereby it is still far from meeting the demands for portable electronics and electrical vehicles. In addition, we have summarized the electrochemical performance of Si-SSBs with inorganic (oxides) SSEs in Table 1.

Table 1 Electrochemical performance of Si-SSBs with inorganic (oxides) SSEs.

Anode	SSE	Ionic conductivity	Initial capacity	ICE	Current	Cycling number	Capacity retention	References
Si film (90 nm)	LLZTO	-	3387 mAh g−1	60%	375 mA g−1	100	85%	[71]
Si film (50 nm)	LLZTO	>0.3 mS cm−1	2702 mAh g−1	54%	C/18	100	44%	[69]
Si film (150 nm)	LLZTO	-	~2600 mAh g−1	-	0.1 C	200	49.1%	[67]
Si film (1 μm)–CNTs	LLZO	0.4 mS cm−1	2685 mAh g−1	83.2%	2.5 mA g−1	2	-	[72]

ORGANIC–INORGANIC COMPOSITE SSE-BASED Si-SSBs

As mentioned above, for the SSBs containing Si anodes and rigid inorganic (oxides) SSEs, the cycle performance of batteries may be a conflict with the mass loading of Si anodes, it is critically difficult to enhance the mass loading and cycling performance of Si-SSBs using inorganic (oxides) SSEs. This may be ascribed to the rigid–rigid contact between inorganic (oxides) SSEs and Si anodes, indicating that avoiding rigid–rigid electrode–electrolyte contact can be conducive to solving this problem.

To understand the influences of contact conditions on the electrochemical performance of SSBs, Huo et al.^[67] constructed a soft–rigid interface (using poly(propylene carbonates) matrix with LLZTO particles as active additives) and a rigid–rigid interface (using LLZTO) for comparison. As presented in Figure 7A, the cycle performance showed obvious variation. When the Si anodes were coupled with rigid LLZTO SSEs, the cycling performance was greatly inferior to the SSBs with organic–inorganic composite SSEs, which was derived from the mechanical property differences between organic–inorganic composite SSE and garnet-LLZTO. Benefiting from the relatively low moduli of organic–inorganic composite SSEs, the organic–inorganic composite SSEs with good deformation capacity can be accommodated with strain pressure during the lithiation/delithiation process of Si anodes. However, due to the ultrahigh Young's modulus (~100 GPa), the garnet-LLZTO electrolyte exhibited the brittleness property. Consequently, when organic–inorganic composite SSE was applied, the electrolyte–electrode interface remained sustainable, with a capacity retention of 86.1% even after 200 cycles, which was higher than the battery using LLZTO SSE (49.1% capacity retention after 200 cycles).

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A sustainable electrolyte–electrode interface is feasible in the organic–inorganic composite SSE-based Si-SSBs, which can provide the possibility of using thick Si anodes prepared by traditional slurry casting methods. Bintang et al.^[89] successfully integrated Si anodes (fabricated by a slurry casting method) with the quasi-solid-state succinonitrile-based electrolyte (QS-SCN) by exploiting the unique phase property of QS-SCN, as shown in Figure 7B. During preparation, the QS-SCN was heated to the liquid state at 60°C, and then the Si electrode was impregnated into QS-SCN. After cooling, the QS-state electrode was successfully prepared. In addition, the cycling performance of as-assembled batteries can be enhanced by precycling treatment, which induces a stable and rigid SEI. The induced SEI made contributions to compensate the volume expansion of Si particles and maintained intimate contact with QS-SCN. Apart from the preparation of a stable SEI layer on the surface of Si particles, the coating method has been applied to modify the properties of Si anodes in Si-SSBs through the design of Si-based anodes. Zhang et al.^[68] recently developed a high-capacity Si-SSBs using a metal-organic framework (MOF) hosted Si (Si@MOF) anode and fiber-supported PEO/garnet composite electrolyte (Figure 7C). This novel anode is promising because the volume expansion of Si particles was effectively buffered by the MOF host, and the volume expansion of the Si anode was one of the main reasons for the failure of Si-SSBs. Consequently, the full cells assembled with Si@MOF anodes and LiFePO₄ cathode exhibited superior cycling performance with a capacity retention of 73.1% after 500 cycles at 0.5 C. Furthermore, Gu et al.^[70] developed a novel multilayer coating method for the deposition of SiO₂@Li₃PO₄@C layer on the surface of microsized Si, which was denoted as Si@SiO₂@Li₃PO₄@C (Figure 7D). The as-obtained Si@SiO₂@Li₃PO₄@C anodes exhibited excellent cycling performance when coupled with both liquid electrolyte and SSEs. In this work, Li_1.3Al_0.3Ti_1.7(PO₄)₃ (LATP) was mixed with PEO electrolyte for improving the electronic conductivity. Notably, the as-prepared SiO₂@Li₃PO₄@C layer not only accommodated the volume expansion of Si but also facilitated the Li⁺ transfer between the electrolyte and the inner Si core. Based on the cryo-TEM technique, a model that can describe the morphology and SEI evolution of microsized Si in both liquid and SSE was proposed. For the microsized Si, voids are usually formed inside microsized Si, but the evolution of these formed voids is closely related to the state of the electrolyte. When the voids are maintained by the microsized Si contact with the liquid electrolyte, the percolation of the liquid electrolyte led to the formation of SEI on the void surface. Eventually, the sponge-like structure can be generated due to the continued growth of SEI. Fortunately, this phenomenon may not happen when the microsized Si is coupled with SSEs because SSEs cannot be percolated into the inner microsized Si. As a result, SEI can only exist on the outer surface of microsized Si, suggesting that the uncontrolled growth of SEI may be suppressed in SSBs, which is beneficial for the cycling performance of batteries.

Compared with the Si-SSBs assembled with inorganic (oxides) SSEs, the electrolyte–electrode interfacial condition can be remarkably improved by the organic–inorganic composite SSEs, but challenges are still needed to be overcome for Si-SSBs using organic–inorganic composite SSEs. First, the synthesis process is usually complex. Additionally, for some solid polymer electrolytes (SPEs), such as PEO, the ionic conductivity at room temperature is not sufficient, so these Si-SSBs only can be cycled at an elevated temperature. Finally, the intrinsic ionic conductivity of organic-inorganic composite SSEs is not comparable to inorganic electrolytes, and thus the inorganic electrolyte particles or other agents are essential for enhancing the ionic conductivity of organic-inorganic composite SSEs. To better understand the progress of organic–inorganic composite SSE-based Si-SSBs, the electrochemical performance of representative works was concluded in Table 2.

Table 2 Electrochemical performance of Si-SSBs with organic–inorganic composite SSEs.

Anode	SSE	Ionic conductivity	Initial capacity	ICE	Current	Cycle number	Capacity retention	References
Si particles	QS-SCN (QS)	0.1 mS cm−1 (RT)	944.3 m Ah g−1	80%	0.02 C	100	-	[89]
Si-coated carbon nanofiber	PVDF-HFP (QS)	3 mS cm−1 (60°C) 0.2 mS cm−1 (RT)	4743 mAh g−1	76%	2.6 A g−1	>100	88%	[90]
Si NPs	PVDF-HFP (QS)	>1 mS cm−1 (RT)	-	-	0.25 C	100	80%	[91]
SiO	Poly(tetramethylene ether) glycol (QS)	0.24 mS cm−1 (RT)	3.0 mAh cm−2	86%	1 mA cm−2	300	70%	[92]
Si film 150 nm	PPCL-SPE (ASS)	0.42 mS cm−1 (RT)	2675 mAh g−1	-	0.1 C	200	86.1%	[67]
Si@MOF	PVDF/PEO/LLZTO (ASS)	0.081 mS cm−1 (RT)	1967 mAh g−1	72%	0.2 A g−1	50	73.3%	[68)
Si@SiO2@LPO@C	PEO@LATP (ASS)	0.053 mS cm−1 (RT)	2482.1 mAh g−1	88.7%	0.5 A g−1	200	44%	[70]
Si@Li3PO4@C	PVDF/PVDF-HFP/LiTFSI/LLZO/PC (ASS)	0.33 mS cm−1 (RT)	~1900 mAh g−1	83.3%	0.6 A g−1	120	83.3%	[93]
Si-N-MXene	PEO@LATP (ASS)	0.47 mS cm−1 (60°C)	1362 mAh g−1	82%	0.4 A g−1	90	64.7%	[94]

INORGANIC (SULFIDES) SSE-BASED Si-SSBs

Inorganic (sulfides) SSEs, with high ionic conductivity at room temperature (up to 10⁻² S cm⁻¹), belong to an emerging candidate for LMSSBs. Meanwhile, the combination of sulfide SSEs and Li metal in LMSSBs brings about abundant problems. First, the electrochemical window of sulfide SSEs is narrow, and side reactions usually occur when the sulfide SSEs and the Li metal are intimately contacted.^[95] Therefore, passivation layer is required to construct a robust electrolyte–electrode interface in LMSSBs. In contrast, the coating layer is not indispensable for the Si anodes because the contact between Si and sulfide SSEs is more stable. Moreover, Li dendrites are prone to generate in LMSSBs, resulting in boosting the failure of batteries.^[24] Meanwhile, the side reaction in Si-SSBs is more moderate because the deposition potential of Li dendrites is lower than the working potential of Si.

Notably, a certain degree of mechanical deformation is tolerable for sulfide SSEs under pressure. Accordingly, the dense sulfide SSEs disks can be fabricated by simple cold pressing, and the voids between sulfide particles can be filled by the deformation of sulfide SSEs. Furthermore, the unique mechanical properties of sulfide SSEs can make the contribution to alleviating the troublesome interfacial problems when integrated with Si anodes. In conclusion, sulfide SSEs with high ionic conductivity at room temperature and outstanding mechanical properties are more promising in Si-SSBs.

Composition-regulated electrodes for Si-SSBs

As a semiconductor, both the intrinsic electronic conductivity and the ionic conductivity of Si are not sufficient.^[96] For the thin film Si anode, the side effect of such insufficient ionic/electronic conductivity is acceptable, while for the sheet-type Si anodes, this drawback must be conquered. Accordingly, the introduction of other components that can deliver high ionic conductivity or electronic conductivity in anodes is necessary for the design of Si-SSBs.

Trevey et al.^[61] applied multiwalled carbon nanotubes (MWCNTs) as conductive additives to fabricate Si anodes for SSBs, as shown in Figure 8A. Compared with the traditional acetylene black, the MWCNTs with the merits of high surface area and improved conductive properties were conducive to better performance. In addition, Li₂S-P₂S₅ SSE was added to the Si composite electrode to construct ionic transport routines, the mass ratio of Si powder, SSE, and MWCNTs was controlled as 1:5:1. Both the electronic and ionic conductive networks were established inside Si anodes after the addition of MWCNTs and Li₂S-P₂S₅. As a result, the Si composite with MWCNTs as a conductive additive showed approximately a 100% capacity increase compared with those using AB. Moreover, based on the comparisons of the performance of Si nanoparticles (Si NPs) and bulk Si, it is found that the Si NPs displayed better reversible cycling performance than the bulk Si anodes. Notably, this configuration of Si composites with a large number of SSEs and conductive agents has been widely reported in later works of literature to construct sheet-type Si anodes. However, the energy density of SSBs is largely killed after the introduction of the above additives. A reduction in the number of additives is urgently required for the application of Si-SSBs. With the aim to reduce the content of SSEs in sheet-type Si anodes, Kim et al.^[97] used the liquid-infiltration method to construct ionic pathways inside Si anodes, instead of directly mechanically mixing the SSEs and Si particles (Figure 8B). First, the sheet-type Si anodes were made by conventional slurry coating method (the molar ratio of Si:binder:carbon = 80:10:10). After then, the Si anode disks were infiltrated into LPSCl ethanol solution, and the solidified LPSCl electrolyte can provide favorable ionic pathways for the electrode composite after evaporating the solvent. Eventually, the weight fraction of LPSCl in the electrode was decrease to about 50%, and the reversible capacities of this electrode were increased to over 3000 mA h g⁻¹ at 30°C when coupled with Li₃PS₄ electrolyte. Furthermore, two kinds of composites with microscale Si and nanoscale Si were fabricated for comparison. Interestingly, as the high external pressure was imposed during the cycling process, the capacity of anodes with micro-Si cannot be significantly degraded as compared to anodes with nano-Si.

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Apart from the introduction of SSEs in electrodes to establish ionic conductive pathways, the materials with both high ionic conductivity and electronic conductivity were selected as additives. Dunlap et al.^[100] utilized the mixed-conducting thermal treated polyacrylonitrile (PAN) as both a binder and conductive additive to fabricate the Si anodes for SSBs. In the initial state, the conduction of Li ion in this polymer was produced by the high polarity of nitrile groups in the PAN, while the PAN remained electronically isolative. Fortunately, after heating at a low temperature (avoiding the occurrence of carbonization), the nitrile groups were cyclized into a partly aromatic conjugated ladder structure or crosslink chain. The electronic conductivity of heat-treated PAN was induced by the formation of delocalized sp² π bonding, while the PAN was transferred into a mixed conductor. Thus, the thermally treated PAN and Si particles were directly mixed to satisfy the requirements for the construction of sheet-type Si anodes for SSBs. Finally, the content of Si in the electrode reached 70%, the slurry-coated sheet-type Si-PAN anodes were assembled into Si-SSBs against the high-voltage LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811) cathode, and LPSCl was selected as SSE.

Whether introducing SSEs or mixing conductors, they cannot make any contributions to the specific capacity of composite anodes. The introduction of these materials also brings about the sacrifice of energy density. From this aspect of view, the combination of Si with other active materials may be promising for high-performance Si-SSBs. Recently, Kim et al.^[98] developed a simple electrode configuration, which was mainly composed of graphite and silicon (Figure 8C). In this configuration, nano-Si was uniformly distributed in the graphite matrix, the mechanically compliant graphite can accommodate the volume expansion of Si and provide the electron transfer pathways. The high contact area between Si particles and graphite can promote the interdiffusion of Li⁺ and minimize the agglomeration of Si. Combined with LPSCl SSE, this optimized composite anode delivered high areal and volumetric capacities of 2.94 and 997 mAh cm⁻³, respectively. Meanwhile, this configuration can be further improved. Although graphite can deliver reversible capacity for the electrode, the theoretical capacity of graphite was still not sufficient. Besides, the stability of sulfide SSEs was also affected by the introduction of carbon. Eliminating the negative effects of carbon to enable sufficient electronic conductivity for anodes remains a challenge. Accordingly, Tan et al.^[65] innovatively fabricated the carbon-free high-loading Si anodes which contained 99.9 wt% micro-Si (μSi) and 0.1 wt% PVDF as the binder. The intrinsic electronic conductivity of μSi was comparable to the majority of cathode materials which can reach 10⁻⁶–10⁻⁴ S cm⁻¹. Hence, eliminating the carbonaceous materials in μSi anodes can be realized. With the purpose to clarify the influence of carbonaceous agents on the performance of batteries, two Si-SSE composite anodes (to enlarge and accelerate the side reactions) with different contents of carbon additives (0 and 20 wt%) were fabricated and assembled into full cells with LPSCl electrolyte and NCM811 cathode. As a result, the voltage profile of full cells with 20 wt% carbon presented a lower initial voltage plateau of 2.5 V compared with the cell without carbon (3.5 V), indicating that severe decomposition of SSE occurred. Besides, the severe decomposition process of SSE with carbon additives was further verified by the characterizations of X-ray diffraction and X-ray photoelectron spectroscopy (XPS). Recently, the side effect of carbonaceous materials in Si-SSBs was also proved by Cao et al.^[74] with the assistance of operando X-ray absorption near-edge structure (XANES) spectroscopy. However, the presence of carbon may contribute to the structural stability of the anode, which was confirmed by ex situ X-ray nanotomography and ex situ SEM.

Furthermore, titration gas chromatography (TGC) was exerted to quantify the amount of SEI in Si-SSBs assembled with NCM cathode and μSi anode by Tan et al.,^[65] and the status of batteries can be directly reflected by the amount of SEI. Consequently, after an initial cycle, the amount of SEI increased slightly during the subsequent cycles, suggesting that the SEI formation was expected to stabilize after the first cycle. The limited growth of SEI can also prompt the 2D plane interfacial contact between the SSE and the electrode, leading to the reduction in the electrolyte–electrode contact area. Besides, the morphologies of pristine, lithiated and delithiated Si anodes were observed via SEM equipped with a focused ion beam. As shown in Figure 8D, for the pristine stage, the porous structure existed in the μSi anode. Meanwhile, the charged state μSi anode showed densified structure, suggesting that the interconnection of the Li-Si structure can be obtained. After delithiation, the morphology of the anode consisted of large particles with voids instead of the initially porous structure. The energy dispersive spectrum (EDS) mappings also confirmed that the SSE cannot be permeated into the inner Si anode, indicating that the 2D SEI between LPSCl and Si anode remained. This phenomenon was consistent with the result of TGC. Owing to the carbon elimination and stable 2D electrode–electrolyte interface, the as-obtained NCM811//LPSCl//μSi full cells exhibited superior electrochemical performance with high areal current density, wide operating temperature, and high areal loadings. After 500 cycles under the current density of 5 mA cm⁻², 80% capacity was retained. This impressive design presented prospects for the commercialization of Si-SSBs and offers a pathway to address the interfacial challenges of Si anodes.

Recently, Na et al.^[99] directly applied the surface-treated monolithic 100% silicon wafer as anode for Si-SSBs without the addition of a binder. As shown in Figure 8E, the additive, electrolyte, and void-free electrodes had a high areal capacity of 10 mA h cm⁻² at room temperature. When coupled with sulfide SSE (LPSCl), such anodes delivered higher cycling performance than the liquid electrolyte, which can be ascribed to the formation of dense SEI in SSBs. Interestingly, the lithiation of Si was prone to occur along the [110] direction of Si, and only slightly along the [100] and [111] direction. Therefore, compared with the <100> wafer and <111> wafer, the grooved <110> wafer exhibited better reversible capacity.

Very recently, Yan et al. demonstrated that merely increase the content of Si may not beneficial for enhancing the electrochemical performance of Si-SSBs.^[66] As shown in Figure 9A, for pure Si anodes, the electrochemical sintering process occurred during the lithiation process, leading to the continuous growth of LiSi film associated with the formation of cracks. As a result, Li dendrites prefer to grow on the surface of LiSi film and eventually cause the soft shortcoming of batteries. Innovatively, hard carbon (HC) was introduced to stabilize the Si anodes. It has been shown that the mass ratio of Si and HC is critically important for the electrochemical performance of Si-SSBs. When the mass ratio of Si and HC is larger than 6:4, Li dendrites still occurred. For the anodes with too much HC (Si:HC = 2:8), the reversible capacity is severely scarified; thus, the mass ratio of Si and HC was optimized as 4:6 (noted as LiSH46). In this case, although the sintering process of Si still occurred, a dense, uniform, and coherent anode is also generated with fast Li⁺ transport. The introduction of HC can provide the graphene layer, pores, and surface, which is beneficial for accommodating the excessive Li and suppresses the growth of Li dendrites and the shortcoming is thus avoided. As a result, this innovative anode delivered excellent cycling performance when coupled with Li₆PS₅Cl electrolyte and LiCoO₂ or NCM811 cathodes. As shown in Figure 9B, for NCM811|LiSH46 Si-SSBs, 61.5% reversible capacity was retained after 5000 cycles at a current density of 1 C. More remarkably, for the full cells assembled with LiCoO₂ cathodes, 72.1% capacity retention was achieved even after 30,000 cycles at a high current density of 20 C.

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Apart from regulating the compositions of Si anodes, the modifications at the cathode side also contribute to the electrochemical performance of Si-SSBs. Cao et al.^[66] prepared a layer of lithium silicate (Li₂SiO_x) on the surface of single-crystal NCM811 through the sol–gel method for assembling Si-SSBs with LPSCl electrolyte and composite Si anode. As a result, the assembled Si-SSBs can deliver a high energy density of 285 Wh kg⁻¹, and after 1000 cycles under the current density of C/3, the reversible capacity reached 145 mA h g⁻¹.

As discussed above, reducing the conductive additives in Si anodes, enhancing the energy density of batteries, and reducing the production cost of Si anodes are crucial for Si-SSBs. However, reducing these additives may be detrimental to the performance of Si-SSBs. Although great progress has been made in improving the electrochemical performance of Si-SSBs, the commercialization of Si-SSBs is still challenging, indicating that further extensive investigations are required to be conducted.

3D structured Si anodes for inorganic (sulfides) SSE-based Si-SSBs

Structural design for anodes is another important strategy that enables high-performance anodes in liquid LIBs, such as hollow structure, porous structure, and 3D structured anodes, and the efficiency of these strategies has been proved in previous works.^[101-104] But the practicability for applying these structures in Si-SSBs has not been deeply understood. The feasibility of these strategies is needed to be reconsidered in Si-SSBs.

Trevey et al.^[62] constructed the 3D structured Si microrod array using the deep reactive ion etching method, and it was applied as anodes for SSBs with Li₂S-P₂S₅, as shown in Figure 10A. In addition, the electrochemical performance of Si rod arrays with various diameters was carried out for comparison. As a result, similar to liquid LIBs, the diameter of Si rods showed a noticeable effect on the cycling stability of Si-SSBs. However, the electrochemical performance of Si rod arrays cannot be satisfied, in which only 20 cycles were achieved. The production cost of 3D Si rod arrays cannot meet the requirements for commercialization. Nevertheless, deep insights into the structural design of Si anodes in SSB were provided.

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Additionally, a scalable physical vapor deposition was exerted to fabricate the 3D structured anodes,^[64] which decrease the production cost and enhance the mass loading of Si anodes compared with the deep reactive ion etching method. Interestingly, the columnar silicon anode (col-Si) exhibited 1D breathing behavior in the vertical direction, which can be compensated by the initial presence of pores in this structure and external stacking pressure (Figure 10B). Therefore, a 2D SEI was stabilized by the innovative structure and applied external pressure. The contact area between Si and SSEs was reduced by such stable 2D SEI, which was conducive to reducing side reactions. Besides, full cells with the integration of col-Si, LPSCl, and NCM displayed better electrochemical performance (82% capacity retention after 100 cycles) than the cells using liquid electrolytes.^[64]

The above strategies for establishing 3D structured Si without the addition of extra inactive agents in anodes. The Si electrodes fabricated by these strategies usually possess a higher specific capacity. Meanwhile, the ionic and electronic conductivity of pure Si is not sufficient, leading to fast capacity fading and unsatisfied rate performance. Therefore, the introduction of ionic conductive or electronic conductive material in electrodes may improve the performance of Si-based anodes. Carbonaceous materials were first considered to enhance the stability of Si anodes. As reported, carbon acted as the buffer layer to accommodate the volume expansion of Si and it was expected to provide an abundant electron transfer pathway.^[106-108] Electrospinning is a facial and efficient strategy for the fabrication of Si/C composite with 3D structure, which has been systematically studied as anode materials for the liquid LIBs. For example, as presented in Figure 10C, Kim et al.^[109] successfully realized the introduction of Si NPs into the carbon nanofibers using electrospinning by the formation of nanostructured Si/C fibers. The carbon fiber matrix not only provided more pathways for electron transfer but also hindered the volume expansion of Si NPs. However, merely delivering high electron conductivity is not sufficient because the ionic transfer is blocked. Thus, Li₂S-P₂S₅ powder was mixed with Si/C nanofibers with a mass ratio of 7:3 to guarantee the ionic conductivity of the electrode. In addition, the correlation between fiber size and electrochemical performance was also investigated. It is also found that the Si/C fibers with smaller diameters (~100 nm) delivered better electrochemical performance than micrometer diameter Si/C fibers. As commented by Kim et al., such a configuration was much better than that obtained by directly applying Si NPs, but there were also two shortcomings in such a configuration. First, a large number of solid electrolytes were required for the fabrication of electrodes, resulting in an increase in cost and a decrease in the energy density of the whole battery. More importantly, the Si/C fiber only has limited contact with solid electrolytes, leading to poor rate capability. To overcome these challenges, a coating layer of LPSCl was introduced on the surface of Si/C fibers (denoted as Si/CNF@LPSCl) by a simple liquid method. In this way, the intimate contact between solid electrolyte and Si/C fibers can provide a continuous ionic conductive pathway for the electrode (Figure 10C). The electrochemical performance further demonstrated the rationality of this speculation. Compared with the Si/CNF anodes, the Si/CNF@LPSCl electrode exhibited higher specific capacity and better rate performance.

Moreover, porous Si has also been applied for the Si-SSBs. Sakabe et al.^[110] compared the electrochemical difference between porous Si and nonporous Si anodes when coupled with sulfide SSEs (80Li₂S-20P₂S₅). As a result, the porous Si anodes delivered better cycling stability compared with nonporous Si anodes. Additionally, based on the results of ex situ SEM observation as reported in liquid LIBs,^[110] it indicated that the structural integrity was strengthened by the porous structure to accommodate the volume expansion of Si. These studies showed the promising prospect for 3D structure Si as anodes in Si-SSBs.

Influence of stress on Si-SSBs

Other inevitable challenges for Si-SSBs are the volumetric and structural changes of Si anodes during the cycling process, which can cause fast material and electrolyte–electrode interface deterioration. Moreover, for a sealed cell, the inner pressure is affected by the volume expansion of electrodes, and thereby the performance of batteries can be influenced. Thus, the research on the stress evolution for Si-SSBs during cycling is momentous and meaningful. To explore the effect of stress on Si-SSBs, the traditional coin cells, which are most widely used in liquid batteries, are difficult to be achieved. As discussed in the previous section, a test die that can adjust external pressure was developed to satisfy the demand for electrochemical tests of batteries. With this device, the effect of compressive stress on the electrochemical performance of Si anodes in SSBs was carried out by Piper et al. in 2013^[56] (Figure 11A). Half cells that assembled with Si composite, Li₂S-P₂S₅ electrolyte, and Li metal were exerted on the test die after insulation with different pressures of 3, 150, and 230 MPa, respectively. Interestingly, both the specific capacity and cycling stability of Si anodes were affected by external stress. With higher external stress, the cycling stability was obviously increased with the loss of specific capacity. Besides, it was documented that the overpotential of Si anodes was also associated with the external load, which can be evidenced by the peak shift in the differential capacity and cyclic voltammetric curves.^[56] Based on the experimental results, a model that described the lithiation process under different conditions was established. Under low external pressure conditions, the volume expansion of Si was accommodated by the presence of void space in the electrode and the lithiation process was similar to free volume expansion. By contrast, under higher external pressure, the voids were mechanically closed, and thus the expansion of Si was suppressed. Without the voids to balance the volume expansion of Si, extra energy is required to counteract the volumetric strain, resulting in a lower potential for the initial alloying process of Si.

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Apart from the anode side, Yamamoto et al.^[111] paid attention to the evolution of the cathode side in inorganic (sulfides) SSE-based Si-SSBs (Li₂S-P₂S₅ as SSE). As illustrated in Figure 11B, to verify the effect of volume variations on the electrochemical performance and microstructure evolution under pressure in SSBs, the active materials with different volume–change ratios were selected for investigation (graphite, silicon, and NCM111: LiNi_1/3Co_1/3Mn_1/3O₂). The volume–change ratio of the active materials with the decreasing order can be expressed as follows: Si > graphite > NCM111. For the half cells assembled with these three active materials, as the stack pressure was increased from 50 to 75 MPa, both the specific capacity and capacity retention were enhanced. Moreover, under the same external pressure, the capacity retention of Si anodes was much lower than those of NCM111 and graphite, indicating that the failure of Si anodes was induced by the change of volume. Nevertheless, the mutual interaction between cathodes and anodes cannot be studied via half cells, and thus full cells with Si/NCM111 and graphite/NCM111 were further assembled. By the microstructure observation of electrodes in cycled Si/NCM111 cells, obvious cracks/gaps between NCM111 cathode particles can be detected, while most of the NCM111 particles in graphite/NCM111 cells remained in close contact without any defects. This further supported that the cathodes also suffered from the large volume change in Si anodes. As for the morphology transformation in the Si anodes side, severe deformed Si (ameba-like) was first found after cycling. Based on the first principle calculation, Young's modulus was decreased from 90 to 40 GPa by the lithiation of Si (Si → Li_3.75Si). It demonstrated that the plastic deformation of Li_3.75Si was more feasible, especially under higher stack pressure. After the repeated lithiation/delithiation, ameba-like Si was formed.

As discussed above, the electrochemical performance and electrode evolutions were significantly influenced by the external pressure on Si-SSBs. Apart from the external pressure, the inner pressure created by the electrochemical reaction cannot be ignored, especially for those alloy anodes with large volume change when alloying with Li. Han et al. investigated the stack pressure evolution in full cells with three different alloy materials (Sb, Sn, and Si).^[57] For the full cell assembling, NCM111, argyrodite-type LPSCl was chosen as cathode and SSE, respectively. As shown in Figure 11C, the change in stress generated during the charge/discharge process with these alloying-type electrodes can reach the megapascal level. Besides, the change of stress was closely related to the state of charge. As mentioned above, owing to the negligible volume change of NCM111 cathode during the lithiation/delithiation process, the stress evolution was mainly derived from the volume change and structural evolution of anodes. Furthermore, these SSBs exhibited good long-term cycling performance even with cyclic-stress changes. This work highlighted that the nonnegligible megapascal-level stack pressure was generated during the cycling of the alloying-type electrode, which can provide insights into the design of optimal Si anodes in Si-SSBs.

The effects of external and inner pressure in Si-SSBs cannot be ignored, which significantly influence the electrochemical performance of batteries. Proper external stack pressure is important for stable Si-SSBs with excellent performance. However, the decrease of external pressure without the deterioration of performance is more appealing.

Sulfide electrolytes were considered as the most promising SSEs for Si-SSBs, and have been thoroughly investigated for several years (as concluded in Table 3). However, it is intrinsically moisture-sensitive and has high requirements for equipment, which results in great challenges for its commercialization.

Table 3 Electrochemical performance of Si-SSBs with inorganic (sulfides) SSEs.

Anode	SSE	Ionic conductivity	Initial capacity	ICE	Current	Cycling number	Capacity retention	References
n-Si/MWCNT	Li2S5-P2S5	-	2013 mAh g−1	-	120 mA g−1	120	~60%	[61]
Si-FeS	Li2S-P2S5	-	3360 mAh g−1	95%	0.05 C	120	80%	[83]
Si/LPS/AB/PPC	Li2S-P2S5	0.5 mS cm−1	3058 mAh g−1	90%	0.3 mA cm−2	375	56.9%	[112]
LPSCl-infiltrated Si	Li3PS4	0.14 mS cm−1	~3250 mA hg−1	>80%	0.2 C	30	~60%	[97]
Si-PAN	Li2S-P2S5	-	1500 mAh g−1	84%	0.1 C	200	~80%	[100]
Micro-Si	LPSCl	>1 mS cm−1	~3400 mAh g−1	77.5%	1 C	500	80%	[65]
Gr/μ-Si	LPSCl	11.6 mS cm−1	-	~75%	0.5 C	200	72.7%	[98]
Nano-Si	LPSCl	1.65 mS cm−1	2773 mAh g−1	85.6%	0.5 mA cm−2	200	65.1%	[66]
Si/CNF@LPSCl	LPSCl	-	1218 mAh g−1	~75%	0.5 C	50	83.4%	[106]
Si-SE-C	Li5.4PS4.4Cl1.6	~8 mS cm−1	3288 mAh g−1	88.7%	0.5 mA cm−2	50	55.8%	[74]
Si wafer	LPSCl	-	~10 mAh cm−2	~55%	0.5 mA cm−2	100	-	[99]
Si nanopowder	Li2S-P2S5	1.27 mS cm−1	~1300 mAh g−1	~45%	210 mA g−1	40	76.9%	[113]

Si-SSBs BASED ON OTHER SSEs

The reported works on Si-SSBs are mainly focused on the inorganic (oxides), organic–inorganic composite, and inorganic (sulfides) SSEs. Apart from these, some other SSEs are also promising to construct the Si-SSBs with high performance. For example, the first reported all-solid-state rechargeable Si-SSB was fabricated by the integration of a thin-film LiPON electrolyte and a thin-film Si anode (Figure 12A).^[60] By comparing the difference between solid-state Si batteries and liquid Si batteries, it is found that the undesired growth of SEI in liquid batteries can lead to faster capacity fading than SSBs. Additionally, with the assistance of the reactive ion/electrochemical etching technique, the concept of 3-D integrated Si-SSBs was first proposed.

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Meanwhile, the application of LiPON electrolyte is mostly applied in thin-film batteries due to the limited ionic conductivity.^[115,116] Besides, thin-film batteries with limited mass loading of Si cannot satisfy the requirement for high-performance energy storage devices but may be hopeful for the application of small autonomous devices, medical implants, and system-in-package devices, which own the limited inner space for install energy storage devices.^[117] This is one of the important reasons that most researchers are interested in the development of inorganic (sulfides) SSE-based Si-SSBs. Recently, Huang et al.^[114] systematically investigated the difference in the electrochemical stability between Si and three types of SSEs (75Li₂S-25P₂S₅, Li₂S-P₂S₅-LiI, and 3LiBH₄-LiI). Intriguingly, as shown in Figure 12B, the 3LiBH₄-LiI electrolyte showed superior electrochemical and chemical stability with Si anodes. The Si-SSBs assembled with 3LiBH₄-LiI SSE displayed an attractive ICE performance of 96.2%, which was the highest value of the previously reported Si anodes with SSEs. In contrast, because of the reaction between Si and P contented in sulfide SSEs, Si//75Li₂S-25P₂S₅//Li, and Si//Li₂S-P₂S₅-LiI//Li only delivered limited ICE of 75.9% and 77.6%, respectively.^[114]

DIAGNOSTIC AND CHARACTERIZATION TECHNIQUES FOR Si-SSBs

Characterizing an in-depth fundamental understanding of the degradation mechanisms at the solid|solid interfaces and solid electrodes in Si-SSBs is crucial for designing resilient, durable, high energy density Si-SSBs. Currently, even with great efforts from researchers, the underlying mechanism in Si-SSBs remain largely unknown. Advanced characterizations may provide constructive information on the fundamental mechanism of Si-SSBs, as concluded in Figure 11. Revealing the composition of SEI is crucial for enhancing the performance of both liquid LIBs and SSBs. In previous work, Tan et al.^[65] used the TGC technique to quantify the composition of SEI (Figure 13A)^[65] by this way, the SEI evolution during the initial several cycles was uncovered. Besides, the composition of SEI can be characterized at nanoscale by cryo-TEM, as conducted by Gu et al.^[70] As shown in Figure 13B, it is found that the SEI formed in SSBs is much condensed and intact compared with that formed in liquid LIBs. In addition, the structural evolution can be characterized as well. For example, with the help of in situ SEM (Figure 13C), the structural change of anodes was directly visualized by Chen et al.,^[71] which demonstrated that a stable electrolyte–electrode can be established with a thin film Si anode. As a nondestructive characterization method, X-ray nanotomography attracts attention and is a powerful technique for unveiling the structural evolution inside electrolytes/electrodes (Figure 13D).^[74] Finally, spectral characterizations play an important role in understanding the chemical state of Si-SSBs, including XANES, XPS, and so on. Facilitated by operando XANES, Cao et al.^[74] demonstrated that the presence of carbon accelerates the decomposition of SSEs (Figure 13E). With ex situ XPS, the bond information of Si anodes in Si-SSBs was acquired by Huang et al. (Figure 13F).^[114]

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All in all, advanced diagnostic and characterization techniques have provided significant insights into chemical, mechanical, and electrochemical transformation in Si-SSBs. However, numerous fundamental challenges with respect to the underlying physics of Si-SSBs are still unsolved. More advanced in situ and operando characterizations are urgently required to enable exquisite insight into material degradation processes in real time in Si-SSBs.

SUMMARY

Compared with the commercial liquid electrolyte-dominated LIBs, owing to the different properties of the solid–solid electrochemical interface, Si-SSBs show great potential for achieving high energy density and much-improved safety as well as long-term stability. In recent years, given their improved resistance to short-circuit and potential for long-term stability, even with the likely development of high-energy LMSSBs, Si-SSBs have seen a resurgence of interest and attracted broad attention from researchers. Nevertheless, with regard to the assembled Si-SSBs, the challenges of poor electrode/electrolyte interface are considered a critical constraint factor. In this review, we have systematically summarized the recent development in Si-SSBs focused on different interfacial configuration characteristics between various types of SSEs with Si-based anodes. The contact model, interfacial electrochemical stability, and charge carriers transport and their influences on the performance of Si-SSBs are carefully reviewed and discussed. Although considerable progress has been made in the development of Si-SSBs, the challenge of simultaneously maintaining chemical and mechanical stability between the Si-based anodes and SSEs during the battery operation remains to be addressed. The current challenges and several insightful future research directions are also proposed:

• (1)

  Up to now, compared with the commercial liquid electrolyte-dominated LIBs, the studies on Si-SSBs are still limited, especially for the fundamental operating mechanisms of Si-SSBs. An in-depth fundamental understanding of the basic mechanisms of Si-SSBs, such as the avoidance of the short-circuiting observed with Li metal and the chemomechanical stabilization of the SEI, morphology evolution, and interfacial reactions during the repeated cycles, will provide theoretical guidance for the design of both electrode/electrolyte materials with stability interfaces for high-performance Si-SSBs. In addition, the studies on Si-SSBs are mainly concentrated on the design of electrode structure and components, and the reactions occurred at the interface are usually ignored. It should be emphasized that the interfacial issues in Si-SSBs are still challenging because the Si anodes may not stably contact with some kinds of SSEs, and thereby further exploration is still needed to be conducted. The advanced characterization methods, including nuclear magnetic resonance (NMR), atomic force microscopy (AFM), in situ/ex situ TEM, X-ray computed tomography (XCT), time-of-flight secondary ion mass spectrometry (TOF-SIMS), in situ/ex situ optical microscopy (OM), Raman spectroscopy, and Fourier transform infrared spectrometer (FTIR) are probably favorable to reveal the interfacial evolution of Si-SSBs; Specifically, NMR, TOF-SIMS, XPS, FTIR, and Raman spectroscopy with the ability for unveiling the chemical information in Si-SSBs. SEM, TEM, OM, and XCT are powerful technologies for understanding the structural evolution in Si-SSBs. In situ AFM-related characterizations should be noticed due to the ability to monitor the dynamic evolution of mechanical properties and interface stability of Si anodes during cycling.

• (2)

  Compared with the liquid LIBs, the operating condition for Si-SSBs is more demanding, and thus the use cost of Si-SSBs is increased. For example, high external stack pressure or high temperature is necessary for most of high-performance Si-SSBs. When high external stack pressure is applied to ensure a satisfactory contact between electrode and electrolyte, alleviating the stack pressure and maintaining good contact simultaneously is challenging. Besides, the chemomechanical evolution inside the cells with high external stack pressure still remains elusive. Due to the insufficient ionic conductivity at room temperature, high operating temperature is required for a part of organic–inorganic composite SSE-based Si-SSBs. Accordingly, the development of organic–inorganic composite SSEs with high performance that deliver high ionic conductivity is urgent.

• (3)

  For the commercialization of Si-SSBs, the electrochemical performance of batteries is crucially important. Besides reversible capacity, the cycling stability, mass loading (>4 mA h cm⁻²), and low-temperature performance are also crucial.

• (4)

  The construction of a robust interface in Si-SSBs is also significantly important. Currently, a flexible interface seems to be more promising for achieving the Si-SSBs with high performance. Meanwhile, a flexible and mechanically robust interface is beneficial for establishing reliable contact between electrolytes and reducing the interfacial resistance of Si-SSBs. Moreover, the (electro)chemical between electrolytes and electrodes should be noticed as well.

• (5)

  New types of SSEs may exhibit outstanding electrochemical performance when coupled with Si anodes. For example, the thriving developed halide-type SSEs exhibit potential for constructing high-performance Si-SSBs. Similar to the sulfide electrolyte, the halide electrolyte with high ionic conductivity and can be processed by cold pressing.^[118,119] More importantly, unlike the sulfide electrolyte, the halide electrolyte is intrinsically stable when in contact with high-voltage cathodes. However, the reduction properties of halide electrolytes need to be improved urgently.

• (6)

  Apart from the anode side, the contact between cathodes and electrolytes should also be noticed. The expansion of the Si anode can exert pressure on the cathode, which may cause the failure of the cathode. Besides, reactions may occur between the cathodes and SSEs, especially for the high-voltage cathode.

• (7)

  It is demanding work to develop Si-based anodes that have low volume expansion. The large volume expansion of Si-based anodes during the cycling process can directly contribute to the deterioration of electrolyte–electrode interface, leading to the failure of Si-SSBs. Building favorable structures of Si-based anodes, such as conductive porous structures^[103] and designing Si-containing encapsulated structures and Si-containing functional second phases, exhibit low volume expansion during lithiation, and thus the robustness of electrolyte–electrode interface in Si-SSBs can be effectively enhanced.

• (8)

  Reducing the massive/volumetric ratio of SSEs in Si-SSBs is imperative. For commercial applications, the energy density of Si-SSBs is significantly important. SSEs only deliver ions and separate the anode from the cathode, which cannot contribute to the capacity of SSBs. Therefore, the use of SSEs should be minimized for practical application. Meanwhile, the decrease in SSEs is still challenging. First, the thickness SSEs used for assembling SSBs are large. Second, it is difficult to reduce the usage of SSE electrodes (including cathode and anode) because the ionic conductive network is crucial for both the electrode and the electrolyte.

As discussed above, with the great efforts of researchers, considerable advances have been made for realizing the commercialization of Si-SSBs. However, numerous challenges still remain, and perpetual efforts are still needed as illustrated in Figure 14. We envision this review has pointed a navigation for the further development of Si-SSBs and provides a useful insight into their challenges and future development of them. It is anticipated that the Si-SSBs with marvelous energy density, cycling performance, and safety will be successfully achieved in the near future.

[IMAGE OMITTED. SEE PDF]

ACKNOWLEDGMENTS

This work was financially supported by the National Natural Science Foundation of China (Grants Nos. 52072323, 52122211 and 21875155), the State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources (Grant No. LAPS22005), the Frontier Exploration Projects of Longmen Laboratory (Grant No. LMQYTSKT008), the Shenzhen Technical Plan Project (No. JCYJ20220818101003008), the support of High-Tech Industrialization Project of Tan Kah Kee Innovation Laboratory (Grant No. RD2021010101) and the “Double-First Class” Foundation of Materials and Intelligent Manufacturing Discipline of Xiamen University. L. Zhang and Q. Zhang acknowledge the support of the Nanqiang Young Top-notch Talent Fellowship at Xiamen University.

CONFLICT OF INTEREST STATEMENT

The authors declare no conflicts of interest.

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