Unveiling the irreversible structural evolution upon rehydration of Manganese-based Prussian White: an in-situ X-ray diffraction study

Abstract

Manganese-based Prussian White Na2-xMn[Fe(CN)6]y, □1-y, zH2O (0≤x≤2, 0≤y≤1) (PW) is a promising cathode material for sodium-ion batteries, due to variety of its composition, intercalation properties, and good electrochemical performance. However, water-induced structural transformations limit its practical application and remain poorly understood. To unravel how water content governs structure transformations in relation to electrochemical performance, a rehydration of a heat-treated Na1.67Mn[Fe(CN)6]0.88, □0.12 compound was monitored by in situ synchrotron x-ray diffraction performed under a controlled atmosphere. At a dew point of -8°C and a flow rate of 30 mL.min-1, the original rhombohedral (dehydrated) phase transforms in 20 minutes into a newly formed disordered monoclinic structure. Water uptake induces a significant expansion of the cell volume, and the enhanced structural disorder. Regarding the electrochemical performance, promising first discharge capacity of 145 mAh.g-1 is obtained for the dehydrated PW, corresponding to 85% of its theoretical capacity (170 mAh.g-1). Surprisingly, the rehydrated compound demonstrates rather high capacity retention of 64%, while the hydrated compound retains only 14% of its initial capacity over 100 cycles at C/10 rate in a voltage range of 2.5 - 4 V vs Na+/Na. This study provides new quantitative insights into the impact of exposure to humidity of PW and on its structural integrity after a heat treatment. The present work will help to implement cost-effective PW cathode materials in practice.