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.