Lithium Morphology Evolution through Crosslinked Poly(ethylene oxide) Solid Polymer Electrolyte

Abstract

Solid-state, lithium metal batteries are promising candidates for developing the safe, energy-dense devices needed to transition to an electrified economy. However, lithium is highly reactive, making it thermodynamically unstable when in contact with many electrolyte materials. Achieving uniform Li plating and stripping during cycling is the key for enabling high energy Li metal batteries. The lithium stripping and plating mechanism is complicated as it can be affected by the cathode, electrolyte and lithium anode, and the resulting solid electrolyte interphase (SEI). In particular, the mechanism is not well understood in solid polymer electrolytes.In this work, we investigate lithium morphology evolution through a solid polymer electrolyte at different stages of battery cycling. Crosslinked poly(ethylene oxide) (xPEO) solid polymer electrolyte is used as a model electrolyte and full cells using single crystal LiNi0.6Mn0.2Co0.2O2 (NMC622) cathode, dry xPEO electrolyte and lithium from two commercial sources are assembled. Our results show that different lithium sources lead to different Coulombic efficiencies and capacity fade rate of the full cells assembled. The lithium morphology evolution at different stages of cycling is examined using scanning electron microscopy and the lithium plating/stripping mechanism are compared between these two commercial lithium anodes. Furthermore, the lithium morphology is compared to a gel polymer composite electrolyte with the same host polymer (xPEO). The dry solid polymer produces a smoother morphology than the gel polymer composite electrolyte without pit formation. A better understanding of the roles of each of these components (pristine Li surface chemistry, microstructure, dry vs gel polymer electrolyte) is essential to control for uniform lithium stripping and plating.