Synthesis of N-Ethyl Carboxylic Acid Functionalized Polyethylenimine as Self-Crosslinking Aqueous Binder for LiFePO4 Cathode of Lithium-Ion Batteries

N-Ethyl carboxylic acid functionalized polyethyleneimine (N-CEPEI) has been explored as a novel water-solution binder for LiFePO₄ (LFP) cathodes, which is synthesized via Michael addition reaction of acrylic acid with the primary and secondary amines from PEI, followed by subsequent in situ condensation. The N-CEPEI binder facilitates the formation of a 3D polymer networks, which exhibits a higher diffusion efficacy of lithium ions and better mechanical strength compared to the commercial poly(vinylidene difluoride) (PVDF) binder, and thus maintains the structural integrity of LFP electrode. The electrochemical performance of the LFP electrode utilizing N-CEPEI binder is evaluated through cyclic voltammetry, electrochemical impedance spectroscopy, and long-cycle-life testing, and the results are compared with those of electrodes using PVDF and PEI binder. The optimal LFP electrode with N-CEPEI binder exhibits superior cycling stability and rate capability, delivering a capacity of 139.60 mAh g⁻¹ with a capacity retention of 94.8% after 400 cycles at 1 C, as compared with 86.6% for PVDF-LFP electrode. Even at a high rate of 5 C, the N-CEPEI-LFP electrode maintains a capacity of 80 mAh g⁻¹ after 500 cycles. This work highlights the potential of N-CEPEI as an effective water-solution binder for LFP-based lithium-ion batteries.