From Dead Lithium to Functional Fillers: An in Situ Conversion Strategy for High-Performance All-Solid-State Lithium Metal Batteries

Poly(ethylene oxide) (PEO) solid electrolytes offer great promise to realize all-solid-state lithium metal batteries with both high energy density and safety. However, it remains challenging to fabricate ultrathin PEO-based solid electrolytes that can operate at practical current densities with a long lifespan. Here, we develop a 19 μm-thick PEO-based solid electrolyte with a porous polyethylene support, which provides mechanical strength and blocks lithium dendrites. By repeatedly plating and stripping lithium at a high current density and low areal capacity, we ingeniously transform otherwise detrimental "dead lithium" into functional fillers within the PEO solid electrolytes. Results show that LiOH, Li₂CO₃, Li₂O, and LiF form on the surface of the "dead lithium", blocking electronic transport and thus rendering them as effective fillers. These in situ formed fillers simultaneously enhance lithium-ion transport and act as a barrier to suppress dendrite growth, thus facilitating uniform lithium deposition. As a result, this approach enables Li||Li symmetric cells to achieve a critical current density of as high as 1 mA cm^-2 and operate stably for 400 h at 0.5 mA cm^-2 and 0.5 mAh cm^-2 without short-circuits. Importantly, a precycled Li||LiFePO₄ full cell can retain 90.9% capacity after 600 cycles at 1C charging and 3C discharging.