Research On The New Solid Polymer Electrolytes For Lithium Ion Battery

As lithium batteries have high energy densities and their environmental pollution isnot that serious as traditional lead-acid batteries and nickel-cadmium batteries, they arewidely used in the field of mobile communication and digital electronic goods. However,the existence of organic solvents in the commercial electrolytes gives rise to manysafety problems such as liquid leakage and the flammability of liquid electrolytes. Solidpolymer electrolytes overcome the safety issues of organic liquid electrolytes. Amongthe different kinds of solid polymer electrolytes, polyether （e.g., poly（ethylene oxide）,PEO） based ones have attracted most research attentions. However, the low chainmobility and the existence of crystallization limit the ionic conductivity of polymerelectrolytes, which is far below the practical requirement （ionic conductivity higher than10-3S/cm at ambient temperature）.A kind of commercial ethylene oxide-epichlorohydrin copolymer [P（EO/EH）s] andits related polymer derivatives obtained via chemical modification are studied in thispaper to improve the ionic conductivities and thermal properties of polymer electrolytes,on account of these polymer matrixes’ advantages of low glass transition temperature（Tg） and being amorphous at ambient temperature.Firstly, a series of P（EO/EH）s-LiCF3SO3polymer electrolytes with different saltconcentrations were prepared by solution casting method. It is found that the Tgin thissystem increases slower with the increment of salt concentration than in theP（EO/EH）s-LiClO4polymer electrolytes. In addition, although the dissociation abilityof LiCF3SO3in the matrix of P（EO/EH）s is weaker than that of LiCF3SO3in PEO, theionic conductivities of P（EO/EH）s-LiCF3SO3polymer electrolytes are nearly two ordersof magnitude higher than that of PEO-LiCF3SO3ones and the system does not exhibit aconductivity hysteresis in heating and cooling scans. Both the increase of ionicconductivities and the lack of conductivity hysteresis could be attributed to theamorphism of P（EO/EH）s.A kind of comb-shaped poly（oxyethylene） with oxyethylene units in side chains hasbeen synthesized via chemical modification on P（EO/EH）s with a kind of commerciallyused alcohol ether carboxylate called AE9C-Na. A series of polymer electrolytes wereprepared by dissolving LiN（SO₂CF₃）₂at different ratios in the polymer matrix. It is found that the ionic conductivities of this system could reach4.3×10-5S/cm at30℃and6.2×10^-4S/cm at80℃, which increase by an order of magnitude compared with thatof PEO based ones at corresponding temperatures. The melting region of these polymerelectrolytes ranges from-10℃to10℃because the synthesized comb-shaped polymeris crystalline due to the introduction of ether side chain, which is part of the crystallineAE9C-Na. However, these polymer electrolytes are amorphous above the ambienttemperature.To prepare a kind of branched polyether with ether side chains, the methoxyethanolbased alcohol ether carboxylates, which are synthesized through the basification andetherification of methoxyethanol, conduct nucleophile substitution reaction withP（EO/EH）s to introduce ether side chains to the polymer backbone. It is found that theionic conductivities of the branched polyether based polymer electrolytes complexedwith LiCF3SO3could reach4.1×10-5S/cm at30℃and6.7×10^-4S/cm at80℃, whichincrease by three orders of magnitude compared with that of PEO-LiCF3SO3ones atcorresponding temperatures. Both the non-crystallinity of these polymer electrolytesand the highly mobile ether side chains on the branched polyether contribute to theincrease of ionic conductivity.