Investigating Cathode–Electrolyte Interfacial Degradation Mechanism to Enhance the Performance of Rechargeable Aqueous Batteries

The invention of Li-ion batteries (LIBs) marks a new era of energy storage and allows for the large-scale industrialization of electric vehicles. However, the flammable organic electrolyte in LIBs raises significant safety concerns and has resulted in numerous fires and explosion accidents. In the pursuit of more reliable and stable battery solutions, interests in aqueous batteries composed of high-energy cathodes and water-based electrolytes are surging. Limited by the narrow electrochemical stability window (ESW) of water, conventional aqueous batteries only achieve inferior energy densities. Current development mainly focuses on manipulating the properties of aqueous electrolytes through introducing excessive salts or secondary solvents, which enables an unprecedentedly broad ESW and more selections of electrode materials while also resulting in some compromises. On the other hand, the interaction between electrodes and aqueous electrolytes and associated electrode failure mechanism, as the key factors that govern cell performance, are of vital importance yet not fully understood. Owing to the high-temperature calcination synthesis, most electrode materials are intrinsically moisture-free and sensitive to the water-rich environment. Therefore, compared to the degradation behaviors in conventional LIBs, such as cracking and structure collapse, the electrode may suffer more severe damage during cycling and lead to rapid capacity decay. Herein, we adopted multi-scale characterization techniques to identify the failure modes at cathode–electrolyte interface and provide strategies for improving the cell capacity and life during prolonged cycling. In Chapter 1, we first provide a background introduction of conventional non-aqueous and aqueous batteries. We then show the current development of modern aqueous batteries through electrolyte modification and their merits and drawbacks. Finally, we present typical electrode failure mechanism in non-aqueous electrolytes and discuss how water can further impact the degradation behaviors. In Chapter 2, we prepare three types of aqueous electrolytes and systematically evaluate the electrochemical performance of LiNixMnyCo1-x-yO2, LiMn2O4 and LiFePO4 in the aqueous electrolytes. Combing surface- and bulk-sensitive techniques, we identify the roles played by surface exfoliation, structure degradation, transition metal dissolution and interface formation in terms of the capacity decay in different cathode materials. We also provide fundamental insights into the materials selection and electrolyte design in the aqueous batteries. In Chapter 3, we select LiMn2O4 as the material platform to study the transition metal dissolution behavior. Relying on the spatially resolved X-ray fluorescence microscopy, we discover a voltage-dependent Mn dissolution/redeposition (D/R) process during electrochemical cycling, which is confirmed to be related to the Jahn–Teller distortion and surface reconstruction at different voltages. Inspired by the findings, we propose an approach to stabilize the material performance through coating sulfonated tetrafluoroethylene (i.e., Nafion) on the particle, which can regulate the proton diffusion and Mn dissolution behavior. Our study discovers the dynamic Mn D/R process and highlights the impact of coating strategy in the performance of aqueous batteries. In Chapter 4, we investigate the diffusion layer formed by transition metals at the electrode–electrolyte interface. With the help of customized cells and XFM technique, we successfully track the spatiotemporal evolution of the diffusion layer during soaking and electrochemical cycling. The thickness of diffusion layer is determined to be at micron level, which can be readily diminished when gas is generated on the electrode surface. Our approach can be further expanded to study the phase transformation and particle agglomeration at the interfacial region and provide insights into the reactive complexes. In Chapter 5, we reveal the correlation between the electrolytic water decomposition and ion intercalation behaviors in aqueous batteries. In the Na-deficient system, we discover that overcharging in the formation process can introduce more cyclable Na ions into the full cell and allows for a boosted performance from 58 mAh/g to 124 mAh/g. The mechanism can be attributed to the water oxidation on the cathode and Na-ion intercalation on the anode when the charging voltage exceeds the normal oxidation potential of cathode. We emphasize the importance of unique formation process in terms of the cell performance and cycle life of aqueous batteries. In Chapter 6, we summarize the results of our work and propose perspectives of future research directions.