ZnAl_xCo_2-xO₄ Spinels as Cathode Materials for Non-Aqueous Zn Batteries with an Open Circuit Voltage of ≤2 V

Rechargeable Zn batteries are promising energy storage alternatives for Li-ion batteries in part because of the high specific and volumetric capacities of Zn anodes, as well as their low cost, improved prospects for safety, and the fact that they are environmentally friendly. Development efforts, however, have focused mostly on aqueous electrolyte systems, which are intrinsically limited by the narrow electrochemical potential window of water. As a consequence, the use of alternative non-aqueous electrolytes has attracted a growing level of interest with the hope that they may provide higher operational voltages, which potentially could provide viable pathways to high-energy and high-power density Zn batteries. With regard to the latter, the considerable progress made in developing useful non-aqueous electrolyte chemistries for Zn anodes has not been matched by correlated progress regarding the development of useful cathode materials. In this work, a new series of spinels, ZnAl_xCo_2-xO₄, are reported and their utility as cathode materials for non-aqueous Zn-ion batteries is demonstrated. Full cells constructed using this new spinel as a cathode paired with a metal anode showed capacities over 100 cycles of 114 mAh/g and an onset potential of 1.95 V, which is the highest OCV yet reported for a non-aqueous Zn-ion battery system. The data show that the Zn²⁺ ions reversibly intercalate into the spinel structure during the charge-discharge processes, a compositional transformation directly correlated with a reversible conversion between Co⁴⁺ and Co³⁺ oxidation states within the lattice. The data illustrate that the Al³⁺-doped spinel structure is a robust candidate material for use in non-aqueous Zn batteries, suggesting guidelines for the design of more efficient multivalent cathode materials.