Tracking Electrochemical-Cycle-Induced Surface Structure Evolutions of Cathode Material LiMn₂O₄ with Improved...

Linking surface structure evolution to the capacity fading of cathode materials has been a problem in lithium ion batteries. Most of the strategies used to solve this problem are focused on the differences between the unaged and aged materials, leading to the loss of intermediate dynamic change information during cycling. Raman spectroscopy is a convenient, nondestructive, and highly sensitive tool for characterizing the surface/near-surface region structure. In this work, we improved an operando Raman system, which is able to record in situ and in real time a series of Raman spectra during charging/discharging cycles and is even able to record very weak Raman peaks without the use of SRES enhancement, which facilitates sample preparation. These series of Raman spectra revealed an inherent correlation between the electrode potential/Li content and the surface structure changes of the as-prepared pure LiMn₂O₄ film, including the biphase reaction, the evolution of the peroxo O-O bond, and the formation of the Mn₃O₄ surface phase. They were the first to show that the number of peroxo O-O bonds was decreased with an increasing number of cycles and that this decrease was accompanied by an increase in the Mn₃O₄ phase. With the help of the data measured by XPS, c-AFM, electrochemical testing equipment, and the calculation based on density functional theory, the causes of the capacity fading of the material are discussed. This work not only showed a direct correlation between the surface structure evolution and the capacity fading of the LiMn₂O₄ but also could provide an alternative operando Raman system that could be widely used for the in situ characterization of battery electrode materials.