Characterising the Structure and Electrochemical Behaviour of Manganese Hexacyanochromate: Highlighting the Role and Importance of Structural Water at Low Potentials.
Authors: Wheeler Samuel , Pasta Mauro 
Published: February 2020

In recent years Prussian blue analogue materials have received much attention as battery materials. Two distinct transition metal sites connected through cyanide ligands create an open-framework structure containing interstitial sites capable of containing a range of alkali metal cations.1 Compositional control, as well as defect and water content, allow tunability of many properties. They show excellent electrochemical properties and are some of the most promising candidate cathode materials for sodium- and potassium-ion batteries.2 Operating in both aqueous and organic electrolytes they have shown high rate capability and extremely long cycle lives attributed to negligible change in lattice constant on cycling and fast ionic mobility.3

A majority of current literature focuses on hexacyanoferrate's as cathode materials, operating at around 0.8 V vs. SHE. However, to utilise the excellent properties in a full cell you need an anode with comparable cycle life and kinetics. Through exchanging Fe with Mn, manganese hexacyanomanganate had a redox couple at 0 V vs. SHE. This, in combination with copper hexacyanoferrate produced a full cell with an average discharge voltage of 1 V and long cycle life.4 Efforts to produce lower potential PBA materials have often resulted in loss of the cubic structure and reversible capacity ascribable to a conversion-type mechanism.5

In this work we investigate manganese hexacyanochromate as a low potential PBA material. Through substitution of Cr into the structure we have lowered the reduction potential to -0.86 V vs. SHE, whilst operating in a high voltage aqueous electrolyte. This is the lowest reduction potential reported with the confirmation that the open-framework structure is maintained, and reversible capacity arises from sodium insertion into the material. Two types of water in the structure are characterised through high resolution XRD, TGA and FTIR, and the crucial role water plays in electrochemical activity and structural integrity is highlighted. It was found that when operating in a non-aqueous electrolyte the material is dehydrated and during electrochemical reduction the crystalline structure is lost. This study isolates the important role water plays within the structure and that it is vital to consider when investigating low potential PBA materials for alkali-ion batteries.

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Figure 1