Prussian blue analogues with Na2NixCoyMnzFe(CN)6-multimetallic structures as positive and hydrogen vanadate as negative electrodes in aqueous Na-ion batteries for solar energy storage applications

Abstract

Low-cost and environmentally benign aqueous Na-ion batteries are the suitable choice for large-scale solar energy storage applications. Na-ion batteries with Prussian blue analogues (PBAs) as positive and hydrogen vanadate (H₂V₃O₈/HVO) as negative electrodes in hydrogel electrolytes exhibit excellent durability, good power capability and moderate energy density. In this work, three variants of PBAs, namely Na₂Ni_0.33Co_0.33Mn_0.33Fe(CN)₆ (PBA-1), Na₂Ni_0.5Co_0.2Mn_0.3Fe(CN)₆ (PBA-2) and Na₂Ni_0.2Co_0.5Mn_0.3Fe(CN)₆ (PBA-3), were synthesized by tuning the compositions of nitrogen-coordinated transition metals such as Ni, Co and Mn. The trend of capacity in PBAs is the function of the Ni content as Ni²⁺ is electrochemically inert. PBA-1, PBA-2 and PBA-3 exhibited specific capacities of 100, 75 and 120 mA h g⁻¹, respectively, at 100 mA g⁻¹. Electrochemical impedance spectroscopy analysis illustrated solid-state electronic conductivity, and PBA-1 exhibited the highest electronic conductivity of 23 × 10⁻⁴ S cm⁻¹ compared to PBA-2 (15 × 10⁻⁴ S cm⁻¹) and PBA-3 (7.4 × 10⁻⁴ S cm⁻¹). Although Ni does not contribute to the capacity, it improves the cycling life of the active material. PBA-3 demonstrated 86% capacity retention after 500 cycles at 300 mA g⁻¹, whereas PBA-1 and PBA-2 respectively showed 91% and 92% retention. The lower Ni content in PBA-3 is accountable for the comparatively poor cycling performance. However, based on the optimum performances of PBAs, a 1.2 V rated PBA-1//HVO full cell was assembled, which delivered a specific capacity of 65 mA h g⁻¹ (at 100 mA g⁻¹), an energy density of 78 W h kg⁻¹ (at 121 W kg⁻¹), a power density of 520 W kg⁻¹ (at 31 W h kg⁻¹) and a good cycling life (81% capacity retention after 500 cycles at 300 mA g⁻¹). A 5 V/4 mA h prototype device was also developed for the practical demonstration of solar energy storage applications. Based on the energy storage performances, cost, safety and environmental issues, this Na-ion battery could be scaled up for large-scale production.