Engineering Silicon Anodes for Sulfide Solid-State Batteries: Addressing Contact Instability through Anode–Electrolyte Interaction

The demand for high-energy-density and safe energy storage has accelerated the development of all-solid-state batteries (ASSBs), especially those employing sulfide solid electrolytes (SSEs). Silicon (Si) is a promising anode material for ASSBs due to its exceptional theoretical capacity (3579 mAh·g⁻¹). However, its application is limited by severe chemo-mechanical challenges, such as >300% volume expansion during lithiation and subsequent interfacial instability with SSEs. To address these limitations, we propose an aluminum-silicon (Al-Si) alloy anode architecture. The Al phase not only enhances electrical conductivity and mechanical compliance but also forms the β-LiAl phase upon lithiation, offering moderate capacity (990 mAh·g⁻¹) and minimal volume change (~96%). The Al-Si wet anode, composed of 99 wt% active material, exhibits early expansion saturation ( ~SOC 30%) followed by stable volumetric behavior. Using operando stress monitoring, synchrotron X-ray nanoimaging, and digital twin simulations, we elucidate the unique electro-chemo-mechanical evolution of the Al-Si anode during cycling. To take advantage of this behavior, an electrochemical prelithiation process is introduced. The process pre-expands the anode and forms lithiated Al-Si phases before full-cell assembly, significantly reducing stress accumulation. The resulting prelithiated Al-Si (Li-Al-Si) anode shows only ~37.5% volume change in the first cycle—substantially lower than untreated Si or Al-Si. Paired with a high-loading NCM811 cathode (8.2 mg·cm⁻²), full cells deliver >100 mAh·g⁻¹ over 250 cycles. However, notable capacity fade is observed over extended cycling, revealing the limitations of planar (2D) interfacial contact between SSEs and wet-processed anodes. In response, we design a composite anode incorporating a hydride-based SE, 3LiBH₄–LiI (LBHI), as a compliant anolyte. This enables the formation of a 3D percolated interface with the active material. Compared to the conventional argyrodite-type SE, Li₆PS₅Cl (LPSCl), LBHI exhibits superior mechanical resilience and electrochemical compatibility with Al-Si, tolerating volume fluctuations and suppressing interfacial degradation. In contrast, LPSCl systems undergo plastic deformation and undesirable reactions with Si, leading to capacity loss. The optimized LBHI-integrated Al-Si composite anode is validated in ASSBs using an NCM811 cathode with a high areal capacity of 6 mAh·cm⁻². This configuration delivers a remarkable capacity retention of 81.6% after 300 cycles and approximately 70% after 500 cycles at a high rate of 1C/-1C. This represents an outstanding performance compared to previous studies using Si-based anodes paired with NCM cathodes in ASSBs. This work identifies elastic recovery of LBHI as the dominant factor enabling long-term cycling stability, by maintaining robust interfacial contact and suppressing mechanical disconnection. These findings demonstrate the potential of Si-based ductile active material-the elastic resilient anolyte strategies to resolve interfacial and mechanical degradation modes common to high-capacity Si-based anodes in solid-state systems. This work represents the first demonstration of a practical Al-Si alloy anode for high-loading sulfide-based ASSBs, addressing key failure mechanisms. Beyond offering an effective route toward stable long-term cycling of Si-based anodes, the framework developed here provides a broader insight into how tailored anode–electrolyte compatibility and mechanical compliance can unlock the performance limits of next-generation ASSBs.