To address the severe safety concern raised by flammable organic electrolytes, aqueous batteries utilizing water-based electrolytes have attracted intensive attention recently. Depending on the electrochemical stability window of the aqueous electrolytes, various cathode candidates are employed and tested under different electrochemical protocols, of which the LiMn₂O₄ (LMO) material is regarded as one of the most promising alternatives because of its high energy density and low cost. However, despite its relatively good stability against water, LMO still suffers from significant chemical degradation and structural distortion in dilute aqueous electrolytes, which can be ascribed to the Mn dissolution behavior during electrochemical cycling. The investigation of dynamic dissolution/redeposition (D/R) behavior of Mn species in LMO can not only reveal the Mn dissolution mechanism in aqueous media and quantitatively correlates the Mn loss with performance decay but also guides the direction of future LMO development. More importantly, understanding the D/R response to the electrochemical potential and electrode architecture (e.g., electrode thickness) can create new insights into composite electrode design and modifications of the testing protocol.

Herein, using in-situ X-ray fluorescence microscopy combined with X-ray absorption spectroscopy and transmission electron microscopy, we demonstrate that simultaneous Mn D/R behaviors and gradually expanded inactive LMO domains lead to the continuous LMO degradation in the aqueous electrolytes during cycling. The in-depth analysis of the chemical environment evolution and distinct D/R patterns reveal a voltage-dependent LMO degradation mechanism. The Jahn-Teller (J-T) distortion is the root cause of Mn dissolution at a low voltage range (0.3 – 1.0 V vs. Ag/AgCl). When the voltage is higher than 1.0 V vs. Ag/AgCl, surface reconstruction and J-T distortion coexist and compete. The surface reconstruction-induced Mn dissolution dominates the medium voltage range while the J-T distortion-induced Mn dissolution governs the high voltage range (1.2 – 1.5 V vs. Ag/AgCl). We also discover that the concentration-dependent redox reactions at electrode scale and voltage-dependent degradation driving force collectively result in spatially heterogeneous Mn dissolution behaviors, which highlights the correlation between local electrode architecture and Mn D/R patterns. Our study, for the first time, reveals the LMO degradation mechanisms from a dynamic perspective in aqueous electrolytes. We hope this study can help the community to understand the Mn dissolution behavior in the aqueous system and provides strategies to improve the performance of aqueous batteries from materials and electrodes levels.