Design of Conductive Binders for LiFePO<sub>4</sub> Cathodes with Long-Term Cycle Life
Shu Huang, Heng Chen, Ming Chen, Youyuan Huang, Xueqin He, Haitao Zhuo, Shaojun Chen
Abstract
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.