Gel polymer electrolytes in lithium ion batteries

The concept of the GPE was first proposed by Feuillard and Perche (1975). It contains a small amount of organic liquid known as a plasticizer, such as cyclic carbonic acid ester and chain-like ester (propylene carbonate, dimethyl carbonate, diethyl carbonate, etc.). Until now it has been the most common approach for the ion conductivity to reach the magnitude of 10 S/ cm at room temperature. Plasticizer-containing polyacrylonitrile (PAN) or poly(methyl methacrylate) (PMMA) polymer host was reported to provide 

To date, several polymer hosts have been used in GPEs that include PEG, PPO, PAN, PMMA, poly(vinyl chloride) (PVC), PVdF, poly(vinylidene fluoride-hexafluoro propylene) (PVdF-HFP), etc. Furthermore, the Bellcore method and phase inversion method have been developed for the preparation of GPEs. The Bellcore process is critical and developed from the fabrication of PVdF-HFP copolymers. It involves the plasticization of PVdF-HFP copolymers, subsequent plasticizer removal, and the final reswelling in an electrolyte solution. Low-boiling solvents, such as diethyl ether or methanol, are successfully employed to remove dibutyl phthalate (DBF) from the polymer matrix, leaving a pore structure in the polymer layers which is then refilled with the liquid electrolyte during the cell activation process (Song et af, 1999). 

Phase inversion operation is easier with regard to the Bellcore process. The polymer is dissolved in a mixture of a volatile solvent and a non-solvent such that the amount of the non-solvent is low enough to allow solubilization and high enough to allow phase separation upon evaporation. The resulting solution is spread as a film on a glass substrate and the solvent is allowed to evaporate at ambient temperature to form a polymer electrolyte membrane. Finally, the membrane should be allowed to swell in an electrolyte solution to produce the GPE (Stephan, 2006). 

Ito et al. (1987) investigated the ion conductivity of PEO-LiCFsSOs complexes using PEG as plasticizer. The increase in the ion conductivity with the PEG content was mainly attributed to the reduction of crystallinity as well as the increase of free volume of the system. 

Among the polymer hosts used for GPEs so far, the PAN-based electrolyte offers a homogeneous, hybrid electrolyte film in which the salt and the plasticizer are molecularly dispersed. As reported by Abraham and Alamgir (1990,1993), for instance, a typical electrolyte comprising 38 mol% EC, 33 mol% PC and 8 mol% LiC104 immobilized in 21 mol% PAN showed an ion conductivity of 1.1 x lO S/cm at -10°C and 1.71 x 10 S/ cm at 20 °C. 

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