In contrast, referring to FIG. 17B, the battery with PVA electrolyte exposed in air degrades more rapidly, only retaining 54% of initial capacitance (37 mAh g^?1) after merely 875 cycles.

In addition, the battery performances were studied by leaving at the open circuit state for 3-7 days. With reference to FIGS. 18A and 18B, discharge capacity of the battery with the PANa hydrogel electrolyte is well maintained after 3-day standstill at the open circuit, and 81% of the initial capacity (135 mAh g^?1) is still delivered after 7-day standstill. The open circuit voltage of the battery is almost stable during the whole week. On the other hand, with reference to FIGS. 18C and 18D, battery performances with the PVA electrolyte are totally different. Only 63% of the initial capacity (23.5 mAh g^?1) is reserved after 3-day standstill at the open circuit, even worse the battery totally fails after 7-day standstill. Its open circuit voltage gradually decreases in the first 3 days and drops terribly to 0.5 V during a week.

With reference to FIG. 19, the battery with the PANa hydrogel electrolyte can continue charge/discharge up to 15000 cycles after the 7-day standstill at the open circuit state, indicating its stable feature.

Advantageously, the rechargeable Zn//NiCo battery based on the PANa hydrogel electrolyte has the features of a high-capacity, ultralong-life, stable, low-cost and safe energy storage device.