(117r) The Impact of Electrolyte Degradation on Lithium Ion Battery Performance

This study explores the effects of heat and water contamination in the electrolyte solutions containing LiPF₆ salts commonly used in lithium ion batteries. Evidence is presented that verifies the occurrence of unwanted side reactions which degrade the performance of the battery. The behavior of LiTFSI based electrolyte was also examined as a control. The equilibration of LiPF₆ to form PF₅ (g) in-situ results in ring opening of the electrolyte component, ethylene carbonate (EC), and its reaction with H₂O to form HF and PF₃=O, strong Lewis acids. Several solutions with different water contents were prepared and placed under heat (50^oC) with intermittent analysis to determine variations in water content, viscosity, composition, anion presence, and oligomer/polymer formation. Electrolyte solutions prepared with LiPF₆ exhibited a 10-15% increase in viscosity over a one week period under heat in part due to the high rate of formation of transesterification products as well as the production of ethylene oxide based oligomers observed in the GC analyses. Also detected was the formation of large PEO-like polymers and their further reaction products with the Lewis acids, PF₅, PF₃=O and HF. Analysis with HPCE exhibited peaks of the phosphate anion that increased with time under heat revealing further evidence of the breakdown of LiPF₆ and its reaction with the ethers in solution. Cells (anode/Li) were assembled with the electrolyte solutions at different stages of the analysis and cycled. Impedance spectroscopy followed each lithium insertion and extraction. This analysis displayed increased interfacial impedance with pre-heated electrolyte solutions as compared to those stored at room temperature possibly due to the presence of high molecular weight polymers on the electrodes. These results coupled with previous studies indicate that LiPF₆ is unstable in the carbonate solvents commonly used to prepare electrolytes for lithium ion batteries and needs to be replaced.