In the pursuit of next-generation energy storage technologies, all-solid-state lithium batteries (ASSLBs) have emerged as a promising candidate due to their enhanced safety, wide operating temperature range, and potential for high energy density exceeding 500 Wh·kg−1. As a researcher focused on advancing battery materials, I have investigated the challenges and solutions for silicon anodes in sulfide-based solid-state batteries. Silicon offers a high theoretical capacity of 3,759 mAh·g−1 and a low working potential, making it attractive for high-energy solid-state batteries. However, practical applications face significant hurdles, including severe side reactions with sulfide solid-state electrolytes and electrical/ionic contact loss due to volume changes during lithiation and delithiation. These issues lead to unstable interfacial phases, increased impedance, active lithium depletion, and poor cycling stability. In this study, I developed a simple liquid-phase method to in-situ form a LixSiSy coating on silicon nanoparticles, resulting in composite electrodes that achieve high initial Coulombic efficiency and stable cyclability in sulfide-based all-solid-state batteries. The improved performance stems from the protective layer blocking direct contact between silicon and the electrolyte, reducing side reactions, and mitigating mechanical stress from volume changes.
The thermodynamic stability between silicon and sulfide solid-state electrolytes, such as Li3PS4, is a critical factor in all-solid-state batteries. My calculations using reaction energy analysis reveal that silicon is not thermodynamically stable with Li3PS4, with a maximum reaction energy of 0.124 eV·atom−1. This instability worsens during lithiation; for example, the lithiated product Li22Si5 exhibits a higher reaction energy of 0.532 eV·atom−1 with Li3PS4. To address this, I designed an artificial LixSiSy coating layer, which shows significantly reduced reaction energies, as summarized in Table 1. The mutual reaction energy (ΔEr) can be expressed by the formula: $$ \Delta E_r = \frac{E_{\text{total}} – (E_{\text{Si}} + E_{\text{Li3PS4}})}{N_{\text{atoms}}} $$ where Etotal is the energy of the interface system, ESi and ELi3PS4 are the energies of the isolated components, and Natoms is the number of atoms in the system. This approach ensures a stable interface, crucial for the longevity of solid-state batteries.
| Material Pair | Maximum Reaction Energy (eV·atom−1) | Stability Assessment |
|---|---|---|
| Si and Li3PS4 | 0.124 | Unstable |
| Li22Si5 and Li3PS4 | 0.532 | Highly Unstable |
| Li4SiS4 and Li3PS4 | 0.045 | Stable |
| SiS2 and Li3PS4 | 0.038 | 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 powder is dispersed in a SiCl4/THF solution and stirred to form a thin SiClx layer on the surface. Then, Li2S8/THF solution is added, leading to the in-situ formation of the LixSiSy coating. The reactions can be represented as: $$ \text{Si} + \text{SiCl}_4 \rightarrow \text{Si@SiCl}_x $$ $$ \text{Si@SiCl}_x + \text{Li}_2\text{S}_8 \rightarrow \text{Si@LixSiSy} + \text{LiCl} $$ The product is washed with THF to remove byproducts like LiCl, resulting in a conformal coating that enhances interfacial stability in all-solid-state batteries. This method is scalable and effective for producing high-quality anode materials for solid-state batteries.
Morphological and structural characterizations confirm the success of the coating. Scanning electron microscopy (SEM) images show that the Si@LixSiSy particles retain a similar size and shape to pristine silicon, with uniform distribution of silicon and sulfur elements. X-ray diffraction (XRD) patterns indicate the presence of crystalline silicon, while Raman spectroscopy reveals a shift in the phonon band from 517 cm−1 to 512 cm−1, suggesting partial amorphization due to the coating. Additional peaks at 300 and 400 cm−1 correspond to the glassy LixSiSy layer. High-energy X-ray photoelectron spectroscopy (HEXPS) at different depths (using photon energies of 3, 4, and 8 keV) confirms the surface composition, with peaks assigned to SiS2, Li4SiS4 or Li2SiS3, and silicon. The intensity of silicon peaks increases with depth, validating the gradient nature of the coating. This layered structure is essential for maintaining stable interfaces in solid-state batteries.
For electrochemical evaluation, composite electrodes were prepared by ball-milling Si@LixSiSy or bare silicon with Li3PS4 and carbon nanotubes (CNTs) in a 60:30:10 weight ratio. The electrodes were assembled into all-solid-state batteries with a multilayer solid-state electrolyte configuration (Li3PS4-Li10GeP2S12-Li3PS4) and lithium metal as the counter electrode. Cycling tests were performed at 0.13 mA·cm−2 and 25°C without external pressure. The Si@LixSiSy-Li3PS4-C electrode delivered an initial discharge capacity of 1,616 mAh·g−1 and a charge capacity of 1,252 mAh·g−1, corresponding to an initial Coulombic efficiency (ICE) of 77.5%. In contrast, the bare Si-Li3PS4-C electrode showed a lower ICE of 55.9% with discharge and charge capacities of 2,015 and 1,126 mAh·g−1, respectively. The improved ICE is attributed to the LixSiSy coating, which reduces side reactions and provides partial pre-lithiation. Over 30 cycles, the Si@LixSiSy-based electrode maintained a reversible capacity of 990 mAh·g−1, while the bare silicon electrode decayed to 140 mAh·g−1, highlighting the enhanced cycling stability in all-solid-state batteries.
The dQ/dV analysis further elucidates the reaction mechanisms. For the Si@LixSiSy-Li3PS4-C electrode, 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 the dealloying process. These peaks remain stable over cycles, indicating minimal overpotential growth. In contrast, the bare silicon electrode shows shifting peaks and decreasing intensities, signifying capacity degradation. The LixSiSy coating acts as a buffer, alleviating mechanical stress from volume changes, which is described by the stress-strain relationship: $$ \sigma = E \cdot \epsilon $$ where σ is stress, E is the elastic modulus, and ε is strain. The low elastic modulus of LixSiSy ensures effective stress relief, contributing to the durability of 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) |
|---|---|---|---|---|
| Si-Li3PS4-C | 2,015 | 1,126 | 55.9 | 140 |
| Si@LixSiSy-Li3PS4-C | 1,616 | 1,252 | 77.5 | 990 |
The interfacial stability and ionic conductivity play vital roles in the performance of all-solid-state batteries. The LixSiSy coating not only prevents side reactions but also enhances ionic transport. The ionic conductivity (σi) can be modeled using the Arrhenius equation: $$ \sigma_i = A \exp\left(-\frac{E_a}{kT}\right) $$ where A is the pre-exponential factor, Ea is the activation energy, k is Boltzmann’s constant, and T is temperature. For Si@LixSiSy, the coating layer facilitates lithium-ion diffusion, reducing interfacial resistance. This is crucial for achieving high-rate capability in solid-state batteries. Moreover, the partial pre-lithiation effect can be quantified by the pre-lithiation degree (α), defined as: $$ \alpha = \frac{Q_{\text{pre}}}{Q_{\text{total}}} $$ where Qpre is the capacity contributed by pre-lithiation and Qtotal is the theoretical capacity. In this case, α is estimated to be around 0.1–0.2, which significantly improves initial reversibility.
Long-term cycling performance underscores the importance of the LixSiSy coating in mitigating capacity fade. The capacity retention rate (R) after n cycles is given by: $$ R = \frac{C_n}{C_1} \times 100\% $$ where Cn is the capacity at cycle n and C1 is the initial capacity. For the Si@LixSiSy electrode, R after 30 cycles is approximately 79%, compared to only 12.4% for bare silicon. This improvement is due to the stable interface and mechanical buffering, which minimize contact loss and side reactions. The reduction in capacity decay rate from 72% for bare silicon to 11.2% for coated silicon demonstrates the efficacy of this approach for all-solid-state batteries.
In conclusion, the in-situ formation of a LixSiSy coating on silicon nanoparticles via a simple liquid-phase method effectively addresses the key challenges of silicon anodes in sulfide-based all-solid-state batteries. The coating enhances interfacial stability, reduces side reactions, provides partial pre-lithiation, and buffers volume changes, leading to high initial Coulombic efficiency and stable cycling performance. This strategy highlights the potential of engineered interfaces for advancing solid-state battery technology. Future work should explore the effects of external pressure and scale-up processes to further optimize these systems for commercial applications in all-solid-state batteries.