Design of Conductive Binders for LiFePO4 Cathodes with Long-Term Cycle Life

The design of new binder systems is an effective way to improve electrochemical performances of electrodes. In this work, a novel ionic copolymer, poly(acrylic acid/2-hydroxyethyl acrylate/1-vinyl-3-ethylimidazolium hexafluorophosphate) (AA-HA-VEH), was synthesized by photopolymerization for use as a cross-linked and conductive binder of LiFePO4 cathodes. The chemical structure of the as-obtained copolymer was verified by FT-IR, 1H NMR, and XPS. The results from peeling and morphological analysis indicated that AA-HA-VEH binder could offer strong adhesion/cohesion to maintain an integrated LiFePO4 cathode structure during hundreds of cycles without cracks and delamination. Compared to PVDF-cathodes with a specific capacity of 17.51 mAh g–1, the bonded LiFePO4 cathodes presented an excellent rate performance with a high specific capacity of 98.35 mAh g–1 at 5C rate. Moreover, the AA-HA-VEH cathodes maintained remarkable cycle stability with retention of 97.01% at 0.5C after 400 cycles. The introduction of the ionic structure of the AA-HA-VEH decreased the impedance and increased the ionic conductivity by nearly 2- to 4-fold. Therefore, the ionic units effectively contributed to the transport of Li+ ions between the cathode interfaces with low polarization. In summary, these findings provided novel insights into the rational design of binders that might be useful for the practical long-term cycling of Lithium-ion batteries.