Improved electrolyte and its application in LiNi1/3Mn1/3Co1/3O2–Graphite full cells

Abstract Lithium oxalatodifluoroborate (LiODFB) has been synthesized and used as a novel electrolyte additive. Standard and modified electrolytes were flame-sealed in NMR tubes and stored at 60 °C for 3 months. Multiple nuclear NMR (1H, 11B, 13C, 19F, 31P) studies confirmed that the modified electrolyte (2% LiODFB added) showed no signs of decomposition as that of regular electrolyte, which is possibly due to the –F of LiPF6 and oxalate of LiODFB ligand exchange effect. The high temperature stabilization mechanism of the added LiODFB was studied using quantum mechanical calculations. Electrochemical tests of LiNi1/3Mn1/3Co1/3O2 (NMC)–Graphite full-cells with and without LiODFB as the electrolyte additive were conducted. When cycling with the NMC–Graphite full-cell at elevated temperature (60 °C), the 100th cycle capacity retention rate of the modified electrolyte was 60%, compared to 27% with the standard electrolyte. The EIS study indicates the full-cells with LiODFB have much lower interfacial impedance than the standard cells. Theoretical calculations reveal that LiODFB generates a layer of thin and resilient SEI on the graphite surface at a higher reduction potential than ethylene carbonate (EC) due to its higher ring strain and protects graphite from the toxic Mn2+ resulting in improved electrochemical performance of NMC–Graphite based cells.