Selective control of composition in Prussian white for enhanced material properties

Sodium ion batteries based on Prussian blue analogues (PBAs) are ideal for large scale energy storage applications due to the ability to meet the huge volumes and low costs required. For Na2-xFe[Fe(CN)6]1-y.zH2O realising its commercial potential means fine control of the concentration of sodium, Fe(CN)6 vacancies and water content. To date, there is a huge variation in the literature of composition leading to variable electrochemical performance. In this work, we break down the synthesis of Prussian blue analogous (PBAs) into three steps for controlling the sodium, vacancy and water content via an inexpen-sive, scalable synthesis method. We produce rhombohedral Prussian white Na1.88(5)Fe[Fe(CN)6].0.18(9)H2O with an initial capacity of 158 mAhg-1 retaining 90% capacity after 50 cycles. Subsequent characterisation revealed that the increased polarisation on the 3 V plateau is coincident with a phase transition and reduced utilisation of the high spin Fe(III)/Fe(II) redox couple. This reveals a clear target...