Abstract
Sulfonated poly(ether ether ketone) (SPEEK) was synthesized and used as an ion-conductive binder for LiFePO4 cathodes. In order to further increase its ionic conductivity, SPEEK was lithiated by adding various amounts of LiClO4 (Li-SPEEK). For comparison, SPEEK, Li-SPEEK, and conventional polyvinylidene fluoride (PVDF) binders were used to fabricate LiFePO4 cathodes. At a high discharge rate (10 C), the LiFePO4 cathodes using Li-SPEEK with optimal amounts of LiClO4 exhibited higher discharge capacity (108 mAh/g) than the LiFePO4 cathode with a PVDF binder (80 mAh/g). This can be attributed to the lower charge-transfer resistance, resulting from the higher ionic conductivity because of the presence of Li+ ions in the Li-SPEEK binder. The electrochemical impedance spectra showed more than 50% reduction in charge-transfer resistance for Li-SPEEK. In addition, the LiFePO4 cathodes using Li-SPEEK and conventional binders exhibited comparable in 100 charge-discharge cycles at 1 C, showing capacity retention higher than 88%. These findings indicate that Li-SPEEK is a promising candidate as an alternative binder for high-rate applications.