Investigation on solvent-free solid polymer electrolytes for advanced lithium batteries and their performance

Seven solvent-free solid polymer electrolytes based on cross-linked polyethers and doped with LiN(CF3SO2)2, were investigated for usage in large-scale rechargeable lithium batteries. They are classified into two categories: one contains five poly(ethyleneoxide-co-propyleneoxide) (poly(EO-co-PO)) network polymers with different degrees of cross-linking and precursor molecular weight and the other includes two polyethers with borate structure having a Lewis acid functionality as an anion trap. The ionic conductivity, direct current characteristics, thermal properties and transference numbers were measured. To evaluate the battery performance for the polymer electrolytes, test cells of the typical Li/solvent-free solid polymer electrolyte/LiCoO2, were fabricated. High discharge performance was obtained for the solvent-free electrolytes made of the less cross-linked poly(EO-co-PO) and for the PEO including borate structure. A 10 Wh class lithium battery was entirely fabricated from solid components, and 500 charge–discharge cycles with 100% capacity retention was demonstrated. The present limitations and problems for further improvement of polymer batteries are also discussed.