Study on the Cycling Performance of Lithium Polymer Cells assembled with Gel Polymer Electrolyte Containing Ionic Liquid

Abstract

In part Ⅰ, Gel polymer electrolytes containing a mixture of organic solvent and ionic liquid were prepared and their electrochemical properties were investigated. The gel polymer electrolytes exhibited good electrochemical stability and stable interfacial properties toward lithium metal. Using these gel polymer electrolytes, lithium metal polymer cells composed of a lithium anode and a LiFePO4 cathode were assembled, and their cycling performances were evaluated. The cells showed good cycling performance comparable to that of a cell assembled with gel polymer electrolyte containing only organic liquid electrolyte. The Li/LiFePO4 cells assembled with a gel polymer electrolyte containing ionic liquid are expected to be one of the promising candidates for high energy density lithium batteries with improved safety. In part Ⅱ, Gel polymer electrolytes containing 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide and a small amount of additive (vinylene carbonate or fluoroethylene carbonate) were prepared, and their electrochemical properties were investigated. The cathodic limit of the gel polymer electrolytes could be extended to 0 V vs. Li by the formation of a protective solid electrolyte interphase on the electrode surface. Using these gel polymer electrolytes, lithium metal polymer cells composed of a lithium anode and a LiNi1/3Co1/3Mn1/3O2 cathode were assembled, and their cycling performances were evaluated at room temperature. The cells showed good cycling performance, comparable to that of a cell assembled with gel polymer electrolyte containing standard liquid electrolyte (1.0 M LiPF6 in ethylene carbonate/diethylene carbonate). Flammability tests and differential scanning calorimetry studies showed that the presence of the ionic liquid in the gel polymer electrolyte considerably improved the safety and thermal stability of the cells.