Staged Lithiation/Delithiation of Silicon Anode in All-Solid-State Batteries Revealed by High-Stack-Pressure Operando NMR Spectroscopy

All-solid-state lithium-ion batteries (ASSLBs) with Si anodes are promising candidates for achieving high energy and improved safety. However, the chemical evolution of Si anodes during electrochemical cycling in ASSLBs remains poorly understood at the microscopic level due to the amorphous nature of intermediate phases and, more importantly, the lack of operando characterization techniques compatible with the high stack pressures required in solid cells. Here, we develop quantitative operando ⁷Li NMR spectroscopy to study micro-Si electrodes in LiCoO₂|Li₆PS₅Cl|Si full cells under a high stack pressure of 95 MPa. The designed operando NMR setup enables real-time monitoring and tracking of the lithium chemical states of the Si anode, and the NMR spectra are carefully interpreted and discussed despite the effect of bulk magnetic susceptibility. The assignment of three resolved lithium silicides (Li_3.75Si, Li_3.25Si, and Li_2.33Si) permits us to capture a multistep concurrent reaction and the dynamic phase transition, which points to an asymmetric lithiation/delithiation mechanism involving four stages, including interface nucleation to bulk diffusion in lithiation and interface extraction to bulk dissolution in delithiation. Meanwhile, the operando NMR technique quantified the temperature-dependent formation rates of the silicide phases, revealing a mechanism by which temperature regulates the phase transition pathways.