Structural evolution of NM (Ni and Mn) lithium-rich layered material revealed by in-situ electrochemical Raman spectroscopic study

Li-rich layered materials are one of promising candidates of cathode materials for energy storage in electric vehicles (EVs) due to their high energy density. The practical application of these materials relies on the in-depth understanding of the crystal structures and reaction mechanisms during the electrochemical processes to overcome the potential decay issue. In this work, in-situ electrochemical Raman spectroscopy has been developed and used to investigate the structural evolution of the Li-rich layered material (0.5LiNi0.5Mn0.5O2·0.5Li2MnO3). An electrochemical Raman spectroscopic cell with an excellent air-tightness and optical signal collection efficiency has been designed and used for in-situ investigation of the NM Li-rich material during the very first two electrochemical cycles. We found that the reactions of Ni2+ to Ni3+ and Ni3+ to Ni4+ appearing in the potential range of from 3.70 V to 4.45 V show a good reversibility. The in-situ Raman spectra after the first two electrochemical cycles also indicate the activation of Li2MnO3 changes the ionic local coordination structure and increases the ionic disorder of the pristine NM Li-rich layered material. This structural change has a great impact on the subsequent electrochemical cycles. The in-situ Raman spectroscopy results can help to improve the performance of NM Li-rich layered materials.