Lithium Compensation to Mitigate Capacity Fading in High Voltage LiNi0.5Mn1.5O4/graphite Li-ion Cells

High voltage cathodes such as the high voltage spinel LiNi_0.5Mn_1.5O₄ have become the spotlight of the Li-ion battery research recently due to their ability to deliver high energy. However, conventional electrolytes based on ethylene carbonate and linear carbonates decompose severely on such high voltage cathodes due to the highly oxidative nature of the fully charged electrode. While efforts have been made to improve the cell performance by developing better electrolytes and additives, none can achieve the high capacity retention seen in the traditional Li-ion batteries with lower voltage limits. In this study, through Li-metal half cells the mechanism of capacity fading in the high voltage Li-ion battery was examined, and the direct cause for the rapid capacity fading on the cathode has been determined to be the active lithium loss due to electrolyte decomposition. The mechanism is similar to what is observed on the graphite anode during the SEI formation process, so prelithiation has been proposed to mitigate the capacity fading in the high voltage LNMO/graphite full cell. Prelithiation of the graphite anodes electrochemically, by physically contact with lithium metal or simply using lithium metal as an anode component have all shown significant improvement in capacity retention for the high voltage LNMO/graphite full cell with conventional carbonate electrolytes at both ambient and elevated cycling temperature. This method has shown even better cycle performance in combination with the  previously reported fluorinated high voltage electrolyte.