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.
Hydrogel
electrolytes have attracted great attention because the
properties of integrated conductivity and mechanical flexibility endow
them with potential applications in flexible solid-state batteries.
However, hydrogels are easy to freeze when the temperature drops below
zero, resulting in the reduction of ionic conductivity and mechanical
strength of hydrogels. Herein, an antifreezing polymer hydrogel electrolyte
was developed by introducing a zwitterionic group and salt into the
double-network polymer. The polymer hydrogel electrolyte possessed
a perfect ionic conductivity of 10.38 mS cm–1 at
−20 °C and could remain unfrozen until −50 °C.
A flexible solid-state Zn//LiFePO4 battery assembled with
the prepared hydrogel electrolyte exhibited a discharge capacity of
126.1 and 75.1 mAh g–1 at 20 and −20 °C
at 1 C, respectively. In addition, it showed quite perfect cycling
performance and maintained a capacity retention of 72.6% after 1000
cycles with a favorable Coulombic efficiency (∼100%) at −20
°C at 1 C. The hydrogel electrolyte with perfect antifreezing
performance is supposed to broaden the range of working temperature
and practical applications of the zinc ion battery.
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