Polymer gel electrolytes for rechargeable lithium polymer batteries

Two types of novel polymer gel electrolytes have been formulated and their morphological, mechanical, thermal and electrochemical properties were systematically characterized and correlated. The first type gel electrolyte is the homogenous rubber like stereocomplexed poly(methyl methacrylate) (PMMA) gel electrolyte; the second type gel electrolyte is a microporous poly(vinylidene fluoride)-co-(hexafluoropropylene) copolymer (PVDF-HFP) film filled with electrolyte solution.; Due to association between isotactic PMMA (i-PMMA) and syndiotactic sequences of atactic PMMA (a-PMMA), strong rubber-like gels are formed in solvents which ordinarily do not form gels with high molecular weight a-PMMA, namely, dimethyl carbonate (DMC) and diethyl carbonate (DEC). Thus, a PMMA gel electrolyte with a typical polymer content of 15–20 wt%, 1M lithium salt, and a ratio of i-PMMA to a-PMMA of about 1:3 has an ionic conductivity in the range of 1 × 10−3–4 × 10−3 S/cm and possesses a dynamic elastic modulus (104 ∼ 10 5MPa) one order of magnitude higher than the electrolytes containing only atactic PMMA. The differential scanning calorimetry (DSC) of these gels show that the physical crosslinks formed by stereocomplexed syndiotactic and isotactic triads melt in the range of 65°C–85°C. The stereocomplexed PMMA gel electrolytes show good electrochemical stability and cell performance in both rechargeable lithium metal and lithium ion batteries.; The PVDF-HFP electrolyte films were obtained as the result of phase separation between PVDF-HFP and PEO oligomer additives, which were cast from a common solvent. Morphology studies shows that as the result of spinodal decomposition, an interpenetrating bi-continuous phase evolves with micro-phase separation between PVDF-HFP and PEO oligomer phases. The ionic conductivities of the methanol extracted microporous films formed from PVDF-HFP/PEO oligomer blends are more than 70% higher than those of the nano-porous films prepared from PVDF-HFP/di-butyl plthalate (DBP) blends. The cells prepared using extracted microporous PVDF-HFP/PEO oligomer blend films as separators show more than 40% higher specific discharge capacity at C/2 rate, 70% higher rate capability than those using extracted nano-porous PVDF separators.