In the pursuit of next-generation energy storage solutions, all-solid-state lithium batteries (ASSLBs) have emerged as a promising technology due to their enhanced safety, wide operating temperature range, and potential for high energy density. Unlike conventional liquid electrolytes, solid-state electrolytes (SSEs) eliminate flammability risks, enabling theoretical energy densities exceeding 500 Wh·kg−1. Among various SSEs, sulfide-based electrolytes stand out for their ultra-high ionic conductivity, often approaching 10−2 S·cm−1, and low elastic modulus, which facilitates cold pressing during cell assembly. However, the compatibility of sulfide SSEs with electrode materials remains a critical challenge. Specifically, silicon anodes, which offer a high theoretical capacity of 3,759 mAh·g−1 and a low working potential of ~0.3 V versus Li+/Li, face significant issues in all-solid-state battery configurations. These include severe side reactions at the interface with sulfide SSEs and electrical/ionic contact loss due to the substantial volume changes (~400%) during lithiation and delithiation. This work addresses these challenges by developing a novel in-situ coating strategy for silicon nanoparticles, resulting in improved interfacial stability and electrochemical performance in sulfide-based all-solid-state batteries.
The instability between silicon and sulfide SSEs, such as Li3PS4, arises from thermodynamic driving forces that promote side reactions, leading to the formation of unstable interfacial phases like Li-Si-P compounds. These reactions increase interfacial impedance and deplete active lithium, resulting in poor initial Coulombic efficiency (ICE) and rapid capacity decay. Additionally, the mechanical stress induced by silicon’s volume expansion can cause pulverization and contact loss within the electrode structure. To overcome these limitations, we propose an in-situ liquid-phase method to coat silicon nanoparticles with a conformal LixSiSy layer. This coating serves multiple purposes: it acts as a barrier against side reactions, provides partial pre-lithiation to enhance initial reversibility, and buffers mechanical stress during cycling. The resulting composite electrodes demonstrate high ICE and stable cycling performance without external pressure, marking a significant advancement for silicon anodes in all-solid-state batteries.
To evaluate the thermodynamic stability of the silicon-sulfide interface, we calculated the mutual reaction energy between Si and Li3PS4 using first-principles methods. The reaction energy, ΔEr, is defined as:
$$ \Delta E_r = E_{\text{products}} – E_{\text{reactants}} $$
where Eproducts and Ereactants represent the total energies of the reaction products and initial materials, respectively. For the Si/Li3PS4 system, the highest reaction energy was found to be 0.124 eV·atom−1, indicating thermodynamic instability. When considering lithiated silicon (e.g., Li22Si5), the reaction energy with Li3PS4 increases to 0.532 eV·atom−1, highlighting greater instability during battery operation. In contrast, the LixSiSy coating layer, composed of phases like Li4SiS4 and SiS2, exhibits significantly lower reaction energies with Li3PS4, as summarized in Table 1. This reduction in reactivity underscores the protective role of the coating in all-solid-state battery systems.
| Material Pair | Highest Reaction Energy (eV·atom−1) | Stability Assessment |
|---|---|---|
| Si / Li3PS4 | 0.124 | Unstable |
| Li22Si5 / Li3PS4 | 0.532 | Highly Unstable |
| Li4SiS4 / Li3PS4 | 0.045 | Stable |
| SiS2 / Li3PS4 | 0.032 | Stable |
The synthesis of LixSiSy-coated silicon (Si@LixSiSy) involves a two-step liquid-phase reaction conducted in an argon-filled glovebox. First, commercial silicon nanoparticles (average size ~226 nm) are dispersed in a tetrahydrofuran (THF) solution containing SiCl4 (5 mmol·L−1) and stirred for 4 hours to form a thin SiClx layer on the surface. This step is represented by the reaction:
$$ \text{Si} + \text{SiCl}_4 \rightarrow \text{Si}@\text{SiCl}_x $$
Subsequently, a Li2S8/THF solution (2.5 mmol·L−1) is added, and the mixture is stirred for 12 hours to facilitate the formation of the LixSiSy layer. The reaction can be generalized as:
$$ \text{Si}@\text{SiCl}_x + \text{Li}_2\text{S}_8 \rightarrow \text{Si}@\text{LixSiSy} + \text{LiCl} $$
The precipitate is washed with THF to remove soluble by-products like LiCl, yielding the final Si@LixSiSy product. For electrode preparation, composite materials are made by ball-milling Si@LixSiSy or bare Si with Li3PS4 and carbon nanotubes (CNTs) in a weight ratio of 60:30:10 at 400 rpm for 2 hours. The resulting Si@LixSiSy-Li3PS4-C and Si-Li3PS4-C electrodes are used in all-solid-state battery cells with a multilayer SSE configuration (Li3PS4-Li10GeP2S12-Li3PS4) and lithium metal as the counter electrode.
Material characterization confirms the successful formation of the LixSiSy coating. X-ray diffraction (XRD) patterns show the presence of crystalline silicon with a cubic structure (ICSD No. 00-027-1402), while Raman spectroscopy reveals a shift in the silicon phonon band from 517 cm−1 to 512 cm−1, indicating partial amorphization. Additional peaks at approximately 300 and 400 cm−1 are attributed to the LixSiSy layer. High-energy X-ray photoelectron spectroscopy (HEXPS) at different photon energies (3, 4, and 8 keV) provides depth-dependent chemical analysis, with peaks at 1,843.5 eV (SiS2), 1,842.3 eV (Li4SiS4 or Li2SiS3), and 1,839.0 eV (Si). The intensity of silicon-related peaks increases with probing depth, confirming the gradient composition of the coating layer. Scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) mapping demonstrate uniform distribution of silicon and sulfur in the Si@LixSiSy particles, with no significant changes in morphology compared to pristine silicon.
Electrochemical performance evaluations are conducted in all-solid-state battery cells under a current density of 0.13 mA·cm−2 and a voltage range of 0.005–2.0 V versus Li+/Li at 25°C. The Si@LixSiSy-Li3PS4-C electrode delivers an initial discharge capacity of 1,616 mAh·g−1 and a charge capacity of 1,252 mAh·g−1, corresponding to an ICE of 77.5%. In contrast, the bare Si-Li3PS4-C electrode shows a higher initial discharge capacity of 2,015 mAh·g−1 but a lower charge capacity of 1,126 mAh·g−1, yielding an ICE of only 55.9%. This improvement in ICE for the coated electrode is attributed to the LixSiSy layer’s dual role in suppressing side reactions and providing partial pre-lithiation. Cycling stability is also enhanced; after 30 cycles, the Si@LixSiSy-Li3PS4-C electrode retains a reversible capacity of 990 mAh·g−1, while the bare Si electrode decays to 140 mAh·g−1. The capacity decay rate for the coated electrode is 11.2% per cycle, compared to 72% for the uncoated one, underscoring the benefits of the coating in all-solid-state batteries.
| Electrode Type | Initial Discharge Capacity (mAh·g−1) | Initial Charge Capacity (mAh·g−1) | Initial Coulombic Efficiency (%) | Capacity After 30 Cycles (mAh·g−1) | Decay Rate (% per cycle) |
|---|---|---|---|---|---|
| Si-Li3PS4-C | 2,015 | 1,126 | 55.9 | 140 | 72.0 |
| Si@LixSiSy-Li3PS4-C | 1,616 | 1,252 | 77.5 | 990 | 11.2 |
Differential capacity (dQ/dV) analysis provides insights into the reaction mechanisms. For the Si-Li3PS4-C electrode, reduction peaks shift to lower voltages, and oxidation peaks shift to higher voltages over cycles, indicating increasing overpotentials and capacity degradation. In contrast, the Si@LixSiSy-Li3PS4-C electrode maintains stable peak positions with minimal intensity reduction, reflecting improved interfacial stability. The reduction peaks at 0.18 V and 0.03 V correspond to the alloying of amorphous silicon, while oxidation peaks at 0.31 V and 0.50 V represent dealloying processes. The LixSiSy coating mitigates mechanical stress by acting as a buffer, which can be modeled using the stress-strain relationship in composite materials:
$$ \sigma = E \cdot \epsilon $$
where σ is stress, E is the elastic modulus, and ε is strain. The low elastic modulus of LixSiSy (compared to silicon) reduces stress concentrations during volume changes, preserving electrode integrity in all-solid-state batteries.
Ionic conductivity plays a crucial role in the performance of all-solid-state batteries. The effective ionic conductivity, σeff, of the composite electrode can be estimated using the Bruggeman model for porous media:
$$ \sigma_{\text{eff}} = \sigma_0 \cdot \phi^{3/2} $$
where σ0 is the intrinsic ionic conductivity of the SSE, and φ is the volume fraction of the electrolyte. In our electrodes, the inclusion of LixSiSy and Li3PS4 ensures continuous ion transport pathways, with σeff values typically ranging from 10−4 to 10−3 S·cm−1. This facilitates efficient lithium-ion diffusion, contributing to the high rate capability observed in cycling tests.
In conclusion, the in-situ formation of a LixSiSy coating on silicon nanoparticles effectively addresses key challenges in all-solid-state batteries, including interfacial instability and mechanical stress from volume changes. The coating layer, composed of Li4SiS4 and SiS2, provides thermodynamic stability against sulfide SSEs like Li3PS4, while partial pre-lithiation enhances initial reversibility. Electrochemical tests demonstrate a significant improvement in ICE and cycling stability, with the Si@LixSiSy-Li3PS4-C electrode achieving 77.5% ICE and retaining 990 mAh·g−1 after 30 cycles without external pressure. These results highlight the potential of tailored interface engineering to enable high-performance silicon anodes in all-solid-state batteries, paving the way for commercial applications. Future work will focus on optimizing coating thickness and exploring the effects of external pressure on long-term cycling behavior.
The development of reliable all-solid-state batteries hinges on overcoming material incompatibilities, and this study underscores the importance of protective coatings for silicon anodes. By leveraging simple liquid-phase methods, we can achieve conformal layers that enhance both ionic and mechanical properties. As research progresses, such strategies will be critical for realizing the full potential of all-solid-state batteries in energy storage systems, offering safer and more efficient alternatives to conventional lithium-ion technologies.