Improved electrolyte and its application in LiNi sub(1/3)Mn sub(1/3)Co sub(1/3)O sub(2)-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 [degrees]C for 3 months. Multiple nuclear NMR ( super(1)H, super(11)B, super(13)C, super(19)F, super(31)P) 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 LiPF sub(6) 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 LiNi sub(1/3)Mn sub(1/3)Co sub(1/3)O sub(2) (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 [degrees]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 Mn super(2+) resulting in improved electrochemical performance of NMC-Graphite based cells.