High Voltage Electrolytes to Stabilize Ni-Rich Lithium Battery Performance
Authors: Poches Christopher , Razzaq Amir Abdul , Studer Haiden , Li Xuguang , Pupek Krzysztof , Xing Weibing 
Published: November 2022

State-of-the-art (SOA) lithium-ion (Li-ion) batteries are approaching their specific energy density limit (~250 Wh kg−1).1 Layered structured, nickel-rich (Ni-rich or high Ni content) lithium transition metal oxides, e.g., LiNi0.8Mn0.1Co0.1O2 (NMC811), have attracted great interests2 owning to their practically deliverable high specific capacity >200 mAh/g. Coupled with high average discharge voltages (~4V vs. Li/Li+), Ni-rich cathode-based lithium batteries possess a great potential to achieve much higher specific energies, e.g., >350 Wh/kg at cell level targeted for electric drive vehicles,3 than SOA Li-ion batteries. In addition, Ni-rich oxides are economically viable as low-cost battery cathode materials due to their low cobalt content. However, Ni-rich cathode-based Li-ion batteries exhibit a quick capacity degradation upon cycling particularly at high charge cutoff voltages (e.g., 4.5V vs. Li/Li+) and at elevated temperatures. Possible degradation mechanisms of Ni-rich based Li cells include structural changes of the material (large c-axis shrinkage at high potentials)4 and parasitic reactions that arise from the interactions between the electrolytes and highly reactive delithiated cathode surface (due to high oxidation state Ni4+ ions).5,6 Therefore, R&D efforts are needed to tackle technical challenges facing the Ni-rick based Li batteries before they become commercially viable.
We will present our efforts of developing high voltage electrolytes to afford stable electrochemical performance of Ni-rich cathode-based Li cells. Figure 1 shows the electrochemical performance of NMC-811 cathode, paired Li metal anode, in conventional Li-ion battery electrolyte (Baseline electrolyte) and the high voltage electrolyte developed in this study, evaluated at C/4 rate during the formation and 1 C rate during cycling, between 2.5V and 4.5V, at room temperature. The cell with the high voltage electrolyte maintained ~80% capacity retention after 400 cycles. In contract, the cell with the baseline electrolyte experieced a large capacity fade with only ~25% capacity retention after 400 cycles. The superior cycle stability of the high votage electrolyte, Ni-rich based cell is attributed to the inharently high-voltage stable, multi-functional. Electrolyte chemical structures and their correlation with the electrochemical stability will be discussed.

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Acknowledgement

This material is based upon work supported by the Naval Air Warfare Center Weapons Division, China Lake, CA under Contract No N6893622C0017. Any opinions, findings and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the Naval Air Warfare Center Weapons Division, China Lake, CA.

Figure 1