POLYACRYLONITRILE-BASED ELECTROLYTES WITH TERNARY SOLVENT MIXTURES AS PLASTICIZERS

Polyacrylonitrile (PAN)-based electrolytes with improved low temperature conductivity can be prepared using carefully selected plasticizer compositions from ternary solvent mixtures consisting of propylene carbonate (PC), ethylene carbonate (EC), and butylene carbonate (BC) or PC, EC, and 3-methyl-2-oxazolidinone (MEOX). All the electrolytes were prepared as freestanding films. A number of solid polymer electrolyte compositions potentially useful for ambient temperature applications were identified. The solid polymer electrolyte composition with 21.0 mole percent (m/o) PAN:37.8 m/o EC:22.9 m/o PC:12.3 m/o BC:6.0 m/o LiAsF6 exhibited conductivities of 1.12 X 10(-4) S cm(-1) at -40 degrees C and 2.88 X 10(-3) S cm(-1) at 25 degrees C. Two other electrolytes contained MEOX; one with 21.0 m/o PAN:33:8 m/o EC:27.7 m/o PC:11.5 m/o MEOX:6.0 m/o LiAsF6 showed conductivities of 1.14 X 10(-4) S cm(-1) at -40 degrees C and 2.98 X 10(-3) S cm(-1) at 25 degrees C and the other with 21.0 m/o PAN:10.8 m/o EC:8.7 m/o PC:53.4 m/o MEOX:6.0 m/o LiAsF6 had conductivities of 1.56 X 10(-1) S cm(-1) at -40 degrees C and 3.10 X 10(-3) S cm(-1) at 25 degrees C. Cyclic voltammetry of the electrolytes on Al indicated small oxidative currents of the order of 0.5 mu A/cm(2) at 4.2 V vs. Li+/Li. Pt, Ni, and carbon showed oxidative currents of the order of 1, 30, and 60 mu A/cm(2), respectively, at the same potential. Alloy formation and plating were evident on Al at 0.15 and -0.20 V, respectively Platinum showed similar behavior with alloy formation at 0.45 V and Li plating at 0.05 V. Carbon showed an onset of Li intercalation around 1.5 V followed by Li plating at -0.1 V. Nickel showed a simple Li plating-stripping process at -0.05 and 0.15 V vs. Li+/Li, respectively. The rechargeability of the Li/solid polymer electrolyte/Li0.8Mn2O4 cell showed short cycle life in electrolytes containing BC with cell failure caused by internal soft shorts on charge. In contrast, cells with MEOX-containing polymer, electrolytes showed vastly improved cyclability. A typical cell retained more than 80% of the second cycle capacity through 140 cycles at 0.1 mA/cm(2).