Prussian whites, such as the here tested Na_xMn_0.8Fe_1.2(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.

References:

1. N. Tapia-Ruiz et al., JPhys Energy, 3, 031503 (2021).
2. Q. Liu et al., Adv. Funct. Mater., 30, 1–15 (2020).
3. D. O. Ojwang et al., ACS Appl. Mater. Interfaces, 13, 10054–10063 (2021).
4. J. Song et al., J. Am. Chem. Soc., 137, 2658–2664 (2015).
5. L. Yang et al., J. Power Sources, 448, 227421 (2020).
6. D. Pritzl et al., J. Electrochem. Soc., 166, A4056–A4066 (2019).

Acknowledgements

The authors acknowledge the financial support of NSERC and Tesla Canada under the auspices of the Industrial Research Chair program.

Figure 1: panel a: First cycle capacity of Na/PW half-cells with cathodes either made from an as-received CAM in a NMP-based slurry (black lines), or 1 week wet-stored material made with the same slurry process (blue lines) dried at low temperatures (120 °C). First cycle capacity of Na/PW half-cells with cathodes made from either 1 week stored material heated to 150 °C (orange lines) or of an as-received material, but the slurry was done in a water-based slurry and the electrodes were dried at 150 °C (green lines)). The half-cells were cycled at 30 °C (CC mode, C/20), using an FEC/DMC (2:8) electrolyte with 1.5 m NaPF₆.