Chemical extraction of Zn from ZnMn2O4-based spinels

Abstract

With an aim to increase the energy density compared to current Li-ion batteries, there is immense interest in rechargeable batteries with multivalent cations, such as Mg²⁺ and Zn²⁺, as they could provide higher charge storage capacity. α-MnO₂ has been investigated as a cathode for Zn-ion batteries, but other structures could be explored. Accordingly, this study investigates the chemical extraction of Zn from the spinel compositions ZnMn_2−xNi_xO₄ (x = 0, 0.5, and 1) using H₂SO₄ and NO₂BF₄. Acid treatment is able to extract Zn from the structure through a Mn³⁺ disproportionation reaction that is dependent on the Mn³⁺ content. Zn extraction decreases with increasing Ni content due to the decrease in Mn³⁺ content; in fact, ZnMnNiO₄ does not lose any Zn upon acid treatment because all of the Mn ions are in the 4+ state. Treatment with NO₂BF₄, however, is unable to extract any Zn from the samples, unlike in the analogous LiMn₂O₄ spinel. This is believed to be due to the high electrostatic repulsion that Zn²⁺ ions would feel from other Zn²⁺ ions in neighboring tetrahedral sites as they have to diffuse through the unoccupied octahedral sites in the spinel lattice, and Zn²⁺ does not prefer octahedral sites. Because NO₂BF₄ more closely approximates the extraction mechanism experienced in a Zn-ion cell, it appears that the ideal spinel structure is not suitable as a Zn-ion battery cathode unless synthesis conditions or additional treatments were used to create vacancies in the structure that allows for easier Zn-ion diffusion without much electrostatic repulsion.