Sodium Alginate Binders for Bivalency Aqueous Batteries

Environmental friendly sodium alginate (SA) cannot be used as a binder in aqueous batteries due to its high solubility in water. A water-insoluble polyvinylidene difluoride (PVDF) binder has been widely applied for an aqueous battery, in which the toxic and expensive organic solvent of N-methy-2-pyrrolidone (NMP) is required during the coating process. Herein, we report that the water-soluble SA can be utilized as a binder in aqueous Zn batteries because SA could cross-link with the Zn²⁺ ion to form a water-insoluble and mechanically super strong binder for electrodes. Aqueous Zn||LiFePO₄ cells are assembled to demonstrate the performance of the SA binder for LiFePO₄ cathodes. Due to the high adhesion strength of cross-linked Zn-SA, LiFePO₄ with the SA binder displays a high capacity retention of 93.7% with a high Coulombic efficiency of nearly 100% after 100 cycles at a 0.2 C rate, while the capacity of LiFePO₄ with the PVDF binder quickly decays to 84.7% after 100 cycles at 0.2 C. In addition, the LiFePO₄ cathode with the SA binder also has smaller redox polarization, faster ion diffusion rate, and more favorable electrochemical kinetics than that with the PVDF binder.