Sodium-ion batteries (SIBs) have been identified as one of the most promising alternative technologies for future stationary large-scale storage and light electromobility applications. Indeed, the potential cost decrease with respect to the lithium-ion technology, and the sustainability provided by using Earth abundant elements lead to lower economic and geopolitical impact.^[1,2]

Great achievements in terms of materials development have been reported in the recent years. However, further work is still needed to fully understand the structure-function correlation in several SIB materials. Easy-synthesis Prussian blue analogues (PBAs) have attracted much attention as potential cathode material in SIBs. Their open framework structure with large interstitial spaces allows a range of different compositions and enables fast Na⁺ insertion/extraction. However, a key factor affecting the electrochemical performance of PBAs is the crystal water in the structure arising from aqueous synthesis environments.^[3] Thus, many have investigated improved synthesis methods aiming at reducing crystal water and associated structural defects.^[4] Goodenough et al^[5] firstly reported the improved electrochemical stability of PBAs through dehydration converting Na-rich monoclinic Na₂MnFe(CN)₆ to a rhombohedral structure. More recent studies, including Zhou et al^[6] and Chou et al^[7], further highlighted the negative impact of structural water on electrochemical performance. So far, the water sensitivity of PBAs has hindered aqueous electrode processing as demonstrated by the several studies conducted on N-methyl pyrrolidone-based electrodes. A more comprehensive understanding of the effect of crystal water and the induced degradation processes is necessary for obtaining high-performance electrode materials produced through environmentally friendly aqueous processing.

Herein, we investigate the electrochemical behaviour of an aqueous-processed Fe-based Prussian white (PW) cathode material (Na_2-x Fe[Fe(CN)₆]_1-y·zH₂O).^[8] The use of such material and processing matches the requirements for low toxicity, cost, and resource abundance in large-scale applications. We uniquely show that, by carefully tuning the water content in the PW structure, the electrochemical performance can be significantly improved. As structural water is removed by tuning the temperature and vacuum conditions during the electrode preparation, the crystallinity of the structure can be controlled, and optimal electrochemical behaviour can be achieved. Moreover, with the combination of cyclic voltammetry, impedance spectroscopy, X-Ray diffraction, X-ray absorption spectroscopy and scanning electron microscopy, we elucidate the activity of the low spin/ high spin Fe^2+/3+redox reactions, the Na⁺ transport properties and the electrode’s structural and morphological changes. This allows to understand holistically the effect of crystal water on the electrochemical performance of PW cathodes. We then demonstrate that our approach is scalable and can be applied to different cell formats representing a step forward in the upscale of sustainable and low-cost sodium-ion cells.

References

[1] I. Hasa et al, Journal of Power Sources 2021, 482, 228872.

[2] C. Vaalma et al, Nature reviews materials 2018, 3, 1.

[3] W.-J. Li et al, Small 2019, 15, 1900470.

[4] J. Qian et al, Advanced Energy Materials 2018, 8, 1702619.

[5] J. Song et al, J Am Chem Soc 2015, 137, 2658.

[6] J. Hu et al, ACS Applied Materials & Interfaces 2022, 14, 12234.

[7] W. Wang et al, Advanced Functional Materials 2022, 2111727.

[8] D. O. Ojwang et al, ACS Applied Materials & Interfaces 2021, 13, 10054.