Fluorinated High-Voltage Electrolytes To Stabilize Nickel-Rich Lithium Batteries

As state-of-the-art (SOA) lithium-ion (Li-ion) batteries approach their specific energy limit (similar to 250 Wh kg(-1)), layer-structured, nickel-rich (Ni-rich) lithium transition metal oxide-based cathode materials, e. g., LiNi0.8Mn0.1Co0.1O2 (NMC811), have attracted great interest owing to their practical high specific capacities (>200 mAhg(-1)). Coupled with their high average discharge voltages (similar to 4 V vs Li/Li+), Ni-rich cathode-based lithium batteries possess a great potential to achieve much higher specific energies (>350 Wh kg(-1) at the cell level) than the SOA Li-ion counterparts. In addition, Ni-rich oxides are low-cost battery cathode materials due to their low cobalt contents. However, Ni-rich cathode-based lithium batteries suffer quick capacity degradations upon cycling, particularly at high upper cutoff voltages (e.g., >= 4.5 V vs Li/Li+), due to crystal structure changes of the active cathode materials and parasitic side reactions at the electrolyte/electrode interfaces. In this study, a fluorinated-solventbased, high-voltage stable electrolyte (HVE), i.e., 1 M Li bis(trifluoromethanesulfonyl)imide (LiTFSI) in fluoroethylene carbonate (FEC), bis(2,2,2-trifluoroethyl) carbonate (FDEC), and methyl (2,2,2-trifluoroethyl) carbonate (FEMC) with Li difluoro(oxalate)borate (LiDFOB) additive, was formulated and evaluated in Li/NMC811 battery cells. To the best of our knowledge, this class of electrolyte has not been investigated for Ni-rich cathode-based lithium batteries. Li/NMC811 cells with HVE exhibited a superior long-term cycle performance stability, maintaining similar to 80% capacity after similar to 500 cycles at a high cutoff voltage of 4.5 V (vs Li/Li+) than a baseline carbonate-solvent-based electrolyte (BE). The superior cycle stability of the Li/NMC811 cells is attributed to the inherently high-voltage stability of HVE, supported by the physical and electrochemical analyses. This conclusion is supported by our density functional theory (DFT) modeling where HVE shows a less tendency of deprotonation/oxidation than BE, leading to the observed cycle stability. The findings in this study are important to help tackle the technical challenges facing Ni-rich cathodebased lithium batteries to realize their high energy density potentials with a long cycle life.

Automated summary

In this study, a high-voltage stable electrolyte was formulated to improve the cycle stability of nickel-rich cathode-based lithium batteries. The electrolyte showed high voltage stability at high cutoff voltages and reduced structure changes and parasitic reactions in the battery, leading to improved cycle stability.