Hybrid additives of lithium difluorophosphate (LIDFP) and adiponitrile (ADN): improving the high-voltage behaviors of NCM811 lithium-ion batteries

Expanding the charge/discharge voltage range of the high‑nickel cathode material NCM811 is one of the most cost-effective ways to increase the specific energy density of lithium-ion batteries (LIBs). However, carbonate-based electrolytes fail to adapted to the high-voltage environment is susceptible to objectionable oxidative decomposition. Therefore, two widely used electrolyte additives, lithium difluorophosphate (LiDFP) and adiponitrile (ADN), were combined to enhance the performance of LIBs at 4.5 V through their synergistic effect. The results show that the specific discharge capacity and capacity retention rate (CRR) of the Li||NCM811 half-cell with 5ADN electrolyte at 150 cycles under 4.5 V are 161.9 mAh/g and 76.5 %, significantly higher than the 118 mAh/g and 60.1 % of the standard electrolyte cell (STD). Similarly, graphite||NCM811 full-cell also has high specific discharge capacity and excellent CRR compared with STD cell. Further confirmed by physical analysis methods, the LiF-rich interfacial film between the cathode and the electrolyte called cathode electrolyte interphase (CEI) film, is more robust and denser. Thereby, the integrity of the cathode material is well protected, effectively inhibiting the dissolution of transition metal (TM) from the cathode material, and, ultimately, resulting in excellent high-voltage electrochemical performance of the battery. In addition, theoretical calculations show that LiDFP and ADN have larger adsorption energies for Li + and Co 4+ , which can effectively reduce the deposition of Co 4+ to the anode and increase the dissociation of lithium salts. This work provides a novel insight into the effective application of high-voltage electrolytes.