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.