Discharge Rate‐Driven Li2O2 Growth Exhibits Unconventional Morphology Trends in Solid‐State Li‐O2 Batteries

Abstract Solid‐state lithium oxygen batteries (LOBs) are known for their enhanced safety, higher electrochemical stability, and improved energy density compared to liquid‐state LOBs. However, the investigation of solid‐state LOBs is limited with little understanding of their discharge and charge processes. In this work, a polymer‐based solid‐state LOB is used to investigate the effect of discharge rate on lithium peroxide (Li 2 O 2 ) formation, the oxygen evolution reaction (OER), and cycle performance. Notably, we observe a counterintuitive trend: Li 2 O 2 particle size increases with increasing discharge current density, in contrast to liquid systems. This behavior arises from inherent space charge layers that restrict Li⁺ transport under high current, and spatially heterogeneous active sites at the solid electrolyte–cathode interface, directly evidenced by small angle X‐ray scattering (SAXS), which govern nucleation accessibility and promote site‐selective Li 2 O 2 growth. Furthermore, higher current densities improve ORR and OER efficiency but accelerate anode degradation, while lower currents promote side reactions. These opposing effects result in a trade‐off that defines an optimal discharge rate (0.1 mA cm⁻ 2 ) for maximizing cycle life. This study provides a new mechanistic perspective on discharge‐driven processes in solid‐state LOBs and offers practical guidelines for performance optimization in future high‐energy battery systems.