High-energy and high-power Zn–Ni flow batteries with semi-solid electrodes

Abstract

Flow battery technology offers a promising low-cost option for stationary energy storage applications. Aqueous zinc–nickel battery chemistry is intrinsically safer than non-aqueous battery chemistry (e.g. lithium-based batteries) and offers comparable energy density. In this work, we show how combining high power density and low-yield stress electrodes can minimize energy loss due to pumping, and have demonstrate methods to achieve high energy and power density for ZnO/Ni(OH)₂ electrodes by changing composition and optimizing testing protocols. Firstly, mechanically stable and homogeneous Ni(OH)₂/carbon and ZnO/Zn flowable electrodes in 7 M KOH electrolyte were designed using a microgel dispersion as the suspending matrix. By determining the critical volume fractions for conductivity percolation, colloidal suspensions with 6.2 vol% of carbon and 23.1 vol% of Zn were selected for preparing catholytes and anolytes to ensure that these semi-solid electrodes possess high voltage and high coulombic efficiencies. The resulting flowable electrodes exhibited non-Newtonian rheology with a yield stress of approximately ∼200 Pa, which assists in maintaining mechanical stability of the suspensions. An energy density of up to 134 W h L_catholyte⁻¹ and power density up to ∼159 mW cm_geo.⁻² was demonstrated for semi-solid ZnO/Ni(OH)₂ electrodes, and coulombic efficiency of 94% was achieved during cycling by optimizing the charging protocol to 60% SOC of Ni(OH)₂. Lastly, semi-solid ZnO and Ni(OH)₂ flow cells were built and tested using an intermittent mode of operation. The high energy and power densities, high coulombic efficiency, and negligible pumping loss of the Zn–Ni semi-solid electrodes developed in the present work present a promising system for further development.