Effect of Mn Content in Co_1-xMn_xFe[CN]₆ as Cathode Material for Rechargeable Aqueous Zinc-Ion Batteries

Aqueous rechargeable zinc-ion batteries are often regarded as an attractive alternative to lithium-ion batteries, mainly because of their low cost, material abundancy and environmental friendliness. While metallic zinc has been demonstrated to be a suitable anode material with high theoretical energy density, the research for a cathodic material which has a high capacity and long-term stability in aqueous electrolytes is still ongoing.<br/>Prussian blue analogues (PBA) are a noteworthy class of cathodic materials with an open framework structure and generic formula A_xM’[M’’[CN]₆]_y●nH₂O, where A are guest cations M’ and M’’ are transition metals. In the crystalline lattice, the M’ atoms are coordinated to the nitrogen atoms of the CN group, while M’’ atoms are coordinated to the carbon atoms.[1]<br/>Zinc-ion batteries featuring PBAs as cathode materials exhibit good specific capacity and outstanding operating voltage. For example, in 2015 <i>R. Trócoli</i> et al.[2] reported an operating voltage of 1.73V for a zinc-ion battery based on CuFe[CN]₆. After 100 cycles in 20mM ZnSO₄, the device was able to retain 96.3% of its maximum capacity of 55 mAh g^-1. More recently, <i>L. Ma et al. </i>[3] developed a zinc-ion battery based on CoFe[CN]₆ which operated above 1.7V. The device was able to sustain a current as high as 6 A g^-1 with a high capacity of 109 mAh g^-1.<br/>One of the main advantages of PBAs is their easy synthetic route, which involves a simple coprecipitation reaction. By partially substituting some of the M' transition metal atoms in the precursor solution, it's possible to tune of the electrochemical properties and cyclability. For instance, <i>G.Kasiri et al.</i> [4] increased the capacity retention at 1000 cycles of CuFe[CN]₆ from 74.35% to 85.54% by introducing zinc, with the best results being achieved by Cu_0.93Zn_0.07Fe[CN]₆.<br/>The aim of this work is to highlight the properties of Co_1-xMn_xFe[CN]₆ obtained by of the introduction of Mn atoms during the synthesis of CoFe[CN]₆. As shown by <i>M. Li et al.</i> [5] in their recent work on MnFe[CN]₆, Mn can dissolve in the ZnSO₄ electrolyte. In the long term, this phenomenon leads to material degradation, but in the short term the Mn dissolution reduces the diffusion resistance and increases the capacity of the battery. By reducing Mn dissolution to involve only a small fraction of the material, we can reduce the diffusion resistance of CoFe[CN]₆, without capacity loss in the long run.<br/>The material is synthesized with a co-precipitation reaction, where the Mn and Co precursors are mixed in the appropriate stoichiometric ratio. For the material characterization XRD, SEM, EDS, ICP-OES and TGA analysis on the dried powders are carried out. Electrochemical characterization is performed with CV in a three electrode cell, by drop casting an ink with the active material on a glassy carbon electrode. Coin cells are assembled with the PBA ink as the cathodic material, zinc as the anodic material and their performance is assessed in charge-discharge cycles at different C-rates.<br/><br/><b>Bibliography</b>:<br/>[1] G. Zampardi and F. La Mantia, “Prussian blue analogues as aqueous Zn-ion batteries electrodes: Current challenges and future perspectives,” <i>Curr. Opin. Electrochem.</i>, vol. 21, pp. 84–92, Jun. 2020.<br/>[2] R. Trócoli and F. La Mantia, “An Aqueous Zinc-Ion Battery Based on Copper Hexacyanoferrate,” <i>ChemSusChem</i>, vol. 8, no. 3, pp. 481–485, Feb. 2015.<br/>[3] L. Ma <i>et al.</i>, “Achieving High-Voltage and High-Capacity Aqueous Rechargeable Zinc Ion Battery by Incorporating Two-Species Redox Reaction,” <i>Adv. Energy Mater.</i>, vol. 9, no. 45, p. 1902446, Dec. 2019.<br/>[4] G. Kasiri, J. Glenneberg, A. Bani Hashemi, R. Kun, and F. La Mantia, “Mixed copper-zinc hexacyanoferrates as cathode materials for aqueous zinc-ion batteries,” <i>Energy Storage Mater.</i>, vol. 19, pp. 360–369, May 2019.<br/>[5] M. Li <i>et al.</i>, “Electrochemical performance of Manganese Hexacyanoferrate cathode material in aqueous Zn-ion battery,” <i>Electrochim. Acta</i>, p. 139414, Oct. 2021.