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

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.