Thermal analysis of a solid polymer electrolyte and a subsequent electrochemical investigation of a lithium polymer battery

A notable slow recrystallization process following heating above the liquidus line for P(EO)nLiTFSI electrolytes with n values ranging from 5 to 12 is shown to be correlated to a slow nucleation process as concluded from extensive differential scanning calorimetry (DSC) measurements. This concentration anomaly motivated the assembly of a room temperature Li/P(EO)8LiTFSI/LixMnO2 cell, containing a preheated amorphous electrolyte, which initially delivered a cathode capacity of 27 mA h/g at a current density of 0.1 mA/cm2 and 60 mA h/g at 0.05 mA/cm2 as compared to typical discharge capacities of 95–120 mA h/g for similar polymer systems at 85 °C and liquid systems at room temperature. The delivered capacity decreased during cell cycling and reached a limiting value of 10 mA h/g after cycle 15. Li/P(EO)nLiTFSI/LixMnO2 cells (n=6, 8, 20) were also cycled extensively at 85 °C, and an analysis of the results indicates that the solid electrolyte interfacial (SEI) resistance, formed between the lithium negative electrode and the electrolyte, remains constant during cycling (RSEI≈3–7×10−3 Ω m2) and is of the same magnitude as the bulk electrolyte resistance. Furthermore, it is shown that severe concentration gradients develop over the electrolytes during cell operation, and that the marked diminishing cathode capacity for the n=8 cell at 20 °C, and also for the n=20 cell at 85 °C, with increasing current density, appears to be related to a limiting current situation. Losses in energy and power densities at steady state conditions for n=6 and n=8 cells at 85 °C appear mainly related to the development of a concentration overpotential, which lowers the working potential curve and causes the cutoff potential to be reached sooner.