Direct visualization and mechanistic insights into initial lithium plating in anode-free lithium metal batteries

Abstract

Anode-free lithium metal batteries (AFBs), which use bare Cu current collectors, represent a promising energy storage technology that offers higher energy density than conventional lithium-ion batteries. Without a lithium metal anode, AFBs are easier to assemble and more cost-effective. However, they suffer from rapid capacity loss and short cycle life limiting their practical applications. A major challenge in their development lies in achieving an understanding of the cycling protocols and mechanisms needed to control the morphology and microstructure of the initial lithium anode growth on the Cu current collector (Cu-CC). In this study, we observed a significant pressure difference between the annular edge and center regions of the Cu-CC in coin cell type AFBs, which dramatically influenced the microstructural morphology of the initial lithium metal growth process. We demonstrated that lithium metal plated in the high-pressure annular edge region forms large, dense grains with a void-free internal structure and a smooth, flat surface, whereas in the low-pressure center region, lithium metal plating consists of small, loose, and tortuous grains with abundant voids and a rough surface. The pressure difference does not affect the solid electrolyte interphase (SEI) composition in these regions. This study provides a unified view on initial lithium metal plating on the bare Cu current collector in AFBs for achieving a uniform and dense lithium metal microstructure.