An Enduring Na-Ion Solar Battery Configured with Na2Co0.5Ni0.5Fe(CN)6 Positive and NaTi2(PO4)3 Negative Electrodes in Na2SO4-SiO2 Hydrogel Electrolyte

Abstract

A successful solar energy storage is accomplished in a 5 V/5 mAh rated Na-ion battery, which is customized by series connection of three 1.8 V unit cells. The unit cell is constructed with open framework structured Na2Co0.5Ni0.5Fe(CN)6 positive and Natrium Super Ionic CONductor (NaSICON) type NaTi2(PO4)3 negative electrodes in 1 M Na2SO4-SiO2 hydrogel electrolyte. Both positive and negative active materials demonstrate Faradaic charge storage mechanism with appreciable diffusion coefficients, such as 5.17 × 10−15 and 1.13 × 10−15, respectively. The Na2Co0.5Ni0.5Fe(CN)6//NaTi2(PO4)3 full cell depicts a specific capacity of 48 mAh g−1 (@ 100 mA g−1), an energy density of 95.68 Wh kg−1 (@ 259.96 W kg−1) and a power density of 779.75 W kg−1 (@ 54.36 Wh kg−1). The cell also exhibits an excellent durability, i.e., 93% capacity retention with 99% Columbic efficiency at 200 mA g−1 upon 1000 cycles. However, the 5 V battery is performance tested under solar-charging cum constant-load-discharging experiment for one-month, and the device describes 85% capacity retention over the period. The good power capability and cycle life with moderate energy density make this battery chemistry suitable for solar energy storage applications.