Research On Functional Electrolytes For Lithium Ion Battery

Ionic liquids (ILs) have received considerable attentions for application as potential electrolytes or electrolyte additives in lithium ion batteries due to their unique properties, such as negligible volatility, non-flammability and high thermal stability. In this thesis, N-methyl-N-propylcyanide morpholinium bis(trifluoromethylsulfonyl) imide (CNMorTFSI) was used as conventional electrolyte 1M LiPF6/EC:DMC(1:1, Vol) additive for LiMn2O₄/Li battery. Cycling test at 1C rate between 2.5-4.0V revealed that for the LiMn2O₄/Li cell, no deterioration of cell performance was observed when 10wt% CNMorTFSI was added to the conventional electrolyte, moreover, the performance was superior to that of the cell with 10wt% 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide (EtMeImTFSI) as additive.Lithium iron phosphate (LiFePO₄), which has a high theoretical capacity, low cost, and low environmental impact, is considered to be the most promising cathode material for lithium ion batteries. In this thesis, LiFePO₄/MCMB cells with bi-solvent, tri-solvent or tetra-solvent electrolyte systems incorporating various additives were tested, wherein, cycling performances was tested under 25°C, 55°C and -20°C, respectively.The cell performances of LiFePO₄/MCMB with 1mol L^-1 LiPF6 /EC:EMC (1:2, Vol) were tested. The results revealed that the cells with SEI forming improver additives exhibited better cycling performances than that without additives at 25°C, but showed unfavorable cycling performances at 55°C.The cell performances of LiFePO₄/MCMB with 1mol L^-1 LiPF6 /EC:EMC:DMC(1:1:1, Vol) were tested. The results revealed that the cells with one SEI forming improver additive exhibited better cycling performances than that without additive at 25°C; addition of binary additives improved the cycling performances at 25°C but the cells showed unfavorable battery performances at 55°C or -20°C. LiFePO₄/MCMB cell with 1mol L^-1 LiPF6 /EC:EMC:DMC(1:1:1, Vol)+2.0wt% PA1+2.0wt% DA1+0.7wt% LA1 exhibited better performances at 25°C, 55°C and -20°C than that with commercialized electrolyte 8022; this electrolyte could be used as an alternate conventional electrolyte.The cell performances of LiFePO₄/MCMB with 1mol L^-1 LiPF6 /EC:DMC:DEC:EMC (1:1:1:3, Vol) were also tested. The cells exhibited unfavorable performances without and with additives at 25°C and 55°C.