POLYACRYLONITRILE-BASED ELECTROLYTES WITH TERNARY SOLVENT MIXTURES ASPLASTICIZERS

Polyacrylonitrile (PAN)-based electrolytes with improved low temperatu re conductivity can be prepared using carefully selected plasticizer c ompositions from ternary solvent mixtures consisting of propylene carb onate (PC), ethylene carbonate (EC), and butylene carbonate (BC) or PC , EC, and 3-methyl-2-oxazolidinone (MEOX). All the electrolytes were p repared as freestanding films. A number of solid polymer electrolyte c ompositions potentially useful for ambient temperature applications we re identified. The solid polymer electrolyte composition with 21.0 mol e 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 a nd 2.88 X 10(-3) S cm(-1) at 25 degrees C. Two other electrolytes cont ained MEOX; one with 21.0 m/o PAN:33:8 m/o EC:27.7 m/o PC:11.5 m/o MEO X:6.0 m/o LiAsF6 showed conductivities of 1.14 X 10(-4) S cm(-1) at -4 0 degrees C and 2.98 X 10(-3) S cm(-1) at 25 degrees C and the other w ith 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 h ad 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 electroly tes 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 current s 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 form ation 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 s howed a simple Li plating-stripping process at -0.05 and 0.15 V vs. Li +/Li, respectively. The rechargeability of the Li/solid polymer electr olyte/Li0.8Mn2O4 cell showed short cycle life in electrolytes containi ng BC with cell failure caused by internal soft shorts on charge. In c ontrast, cells with MEOX-containing polymer, electrolytes showed vastl y improved cyclability. A typical cell retained more than 80% of the s econd cycle capacity through 140 cycles at 0.1 mA/cm(2).