Accelerated Short Circuiting in Anode-Free Solid-State Batteries Driven by Local Lithium Depletion

“Anode-free” solid-state batteries (SSBs), which have no active material at the anode and undergo in situ lithium plating during the first charge, can exhibit extremely high energy density (~1500 Wh L-1). However, there is a lack of understanding of lithium plating/stripping mechanisms at bare solid-state electrolyte (SSE) interfaces since excess lithium is often used. Here, we demonstrate that commercially relevant quantities of lithium (> 5 mAh cm-2) can be reliably plated at relatively high current densities (1 mA cm-2) using the sulfide SSE Li6PS5Cl. Investigations of lithium plating/stripping mechanisms, in conjunction with cryo-focused ion beam (FIB) and ex situ synchrotron tomography, reveal that the cycling stability of these cells is intrinsically limited by spatially uneven plating/stripping. Local lithium depletion toward the end of stripping decreases electrochemically active area, which results in high local current densities and void formation, accelerating subsequent filament growth and short circuiting compared to lithium-excess cells. Despite this governing degradation mode, we show that anode-free cells exhibit comparable Coulombic efficiencies to lithium-excess cells before short circuiting, and improved resistance to short-circuiting is achieved by avoiding local lithium depletion through retention of lithium at the interface. These new insights provide a foundation for engineering future high-energy anode-free SSBs.