Processability of Prussian White Cathode Active Materials for Sodium Ion Batteries-Towards a Green Electrode Preparation

Abstract

Prussian whites, such as the here tested NaxMn0.8Fe1.2(CN)6 (PW), are promising candidates as Na-Ion cathode active materials for cost-effective and safe battery technologies. The use of abundant elements such as sodium, iron or manganese obtained in a sustainable way makes this technology extremely attractive, especially in grid storage applications where high energy densities are not of major concern.1,2 However, PWs still suffer from low electronic conductivity, limited capacity retention, and a high sensitivity towards moisture which results in structural changes and loss of cyclable Na.3,4 Different approaches can be utilized to mitigate these issues, such as surface coatings or strict handling of the materials under inert conditions, starting from the storage of PWs powders to slurry preparations and to the handling of electrodes.3,5 In this study, we investigated the changes in surface chemistry during ambient storage (here called wet storage) and during subsequent heating of stored material. ATR-IR and TGA-MS data of materials stored for different times ranging from 1 h to up to a week showed a sharp increase in moisture-content in the material. The increase plateaued after 24 h of storage. XRD analysis of stored material showed a clear trend of structural changes upon even small storage times (<1h). Furthermore, surface hydroxides and carbonates were found via ATR-IR and a reannealing at low temperatures (<200 °C) showed the reversible release of adsorbed and interstitial water, but surface hydroxides and carbonates were not easily removed and stayed on the surface.By conducting electrochemical testing in coin-cells we analyzed the difference in cycling performance of as-received and ambient stored materials, and materials washed in water. As seen in earlier studies we also saw a drastic change in the charge profile after storage, due to changes in crystal structure.4 Interestingly, we found that this detrimental effect is reversible if the electrodes are dried at elevated temperatures. The pH of water after washing PW materials was neutral indicating the absence of major ion exchange.6 This finding, combined with the effective drying of hydrated electrodes resulted in the formulation of a water-based slurry process.As seen in Figure 1, a cell built with electrodes made from a 1 week wet-stored CAM which were dried at 120 °C (blue lines) showed a drastic different charge profile, compared to a cell built with an as-received electrode (black lines, panel a). Interestingly a cell built with CAM stored for 1 week and heated to 150 °C (orange lines, panel b) and a cell built with a cathode from a H2O-based coating which was dried at 150 °C (green lines, panel b), looks almost identical to the NMP-cells. Therefore, we assume that most water can be removed easily, and a water-based slurry process is possible.Cycling tests of water-based and environmentally more sustainable hard-carbon/PW full-cells reveal a similar cycling stability in comparison to cells with electrodes made from a traditional NMP process.