Abstract: Li- and Mn-rich cathode materials have been considered as promising candidates for application of lithium-ion batteries in vehicles due to their high capacity and high energy density (capacity around 250 mAh/g). However, these materials suffer from a serious problem of voltage fade. The average discharge voltage keeps decreasing as the cycling goes on and results in decreased energy output after limited cycles. In order to solve this problem, many efforts have been put in to understand the origin of such phenomenon, mostly from structural and chemical points of view. In this work, the important roles of crystallinity, or the microstructure of the cathode material  in voltage fade are studied. The study is carried out on Li₂Ru_0.5Mn_0.5O₃ as an example. The material is electrochemically prelithiated to around Li_3.2Ru_0.5Mn_0.5O₃ first and then delithiated to Li₂Ru_0.5Mn_0.5O₃ . It is shown that even though the crystal structure mostly remains the same, the crystallinity is severely damaged by breaking the sub-micro domains into very fine nano-sized domains. Such loss of crystallinity causes much more significant voltage fade in the following cycling compared with those normally cycled samples. The more severe voltage fade caused by low crystallinity can be attributed to the accelerated oxygen loss due to the increased defects and surface area of the prelithiated sample. These results indicate the great importance of crystallinity of the cathode materials.

Acknowledgement

The work at Brookhaven National Laboratory was supported by the U.S. Department of Energy, the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies under Contract No. DE-SC0012704. Use of the National Synchrotron Light Source II (NSLS-II) and Center for Functional Nanomaterials at Brookhaven National Laboratory were supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contracts No. DE-SC0012704.