Exploring a non-aqueous hybrid zinc/potassium battery with monovalent ion (de)-insertion at high voltage in potassium/sodium manganese(II) hexacyanoferrates(II)

Abstract

Non-aqueous zinc batteries as a complement to lithium ion batteries continue attracting the attention for energy storage in stationary systems because of the high volumetric energy density and competitive production costs. Unfortunately, the rapid development of such technology is hindered by technical issues such as sluggish diffusion of Zn²⁺ within the host material. In this work we demonstrate that is possible to use a non-aqueous hybrid Zn/K battery circumventing the Zn²⁺ insertion effects into two host structures by using monovalent ions. For this approach two different structures with different alkali earth metals (K and Na) are tested as the cubic (N-MF: Na_1.9Mn[Fe(CN)₆]_0.95·3.9H₂O) and monoclinic (K-MF: K_1.9Mn[Fe(CN)₆]_0.94·0.61H₂O) PBAs. Intriguing phase changes and voltage of the reactions are disclosed. A single voltage plateau occurred at 2 V in charge and 1 V on discharge for K-MF. Voltage plateaux between 2.05 and 2.65 and 3.05 V on charge, and 1.2–0.95 V on discharge for N-MF are observed. Two active redox centres Fe^2+/3+ and Mn^2+/3+ trigger a multi-electron reaction with 141.2 (K-MF) and 117.7 mA h g⁻¹ (N-MF). Eventually, a wide electrochemical voltage can be used for N-MF cathode to 3.25 V (vs. Zn²⁺/Zn) with a characteristic phase transformation from cubic to monoclinic on cycling, reporting data to enhance the voltage and energy density. Therefore, the results reported here try to promote fundamental progress within the field of non-aqueous Zn/K batteries.