Sodium-ion batteries (SIB) represent a competitive alternative to lithium-ion batteries, especially for large-scale electric energy storage to stabilize the power grid. Their suitability is underlined by lower costs, environmental benignity and higher natural abundance of sodium resources. Common cathode materials for SIBs are layered transition metal oxides like Na2/3Fe1/2MnO2 and oxoanion compounds like NaFePO4. Prussian Blue (PB) NaxFe[Fe(CN)6]1−y and Prussian blue analogues (PBA) NaxMe[Fe(CN)6]1−y (Me = Mn, Ni, Cu, V, etc.) are promising alternative cathode materials for Na-ion batteries due to their high-theoretical capacity of about 170 mAh/g [1].

The presence of interstitial water, vacancies and various doping elements in PB and PBA, which depend on different synthesis methods and drying processes, influences the crystal structure and the electroche-mical properties [2]. Herein, we investigate the change of PB’s structure by different synthesis conditions using X-ray diffraction, Infrared/Raman spectroscopy, inductively coupled plasma atomic emission spec¬troscopy (composition), thermal gravimetric analysis (water content) and scanning electron microscope (morphology). The presence of interstitial water in the PB cathode leads to the water accumulation in the electrolyte, where it induces side reactions with carbonate-based solvents [3]. To systematically investi¬gate the impact of the electrolyte composition and the water uptake from the PB cathode on the electrochemical performance of the PB cathode material, its specific capacity, rate capacity and cycle sta¬bility are evaluated in various electrolyte systems. These include NaPF6 dissolved in alkyl carbonate- and alkyl ether-based solvents, as well as the solid-state electrolyte NASICON (Na3Zr2Si2PO12). For the electrolyte compatibility, in terms of both C-rate capability and cycling stability, follows the hierarchy: NASICON > NaPF6 + ether-based solvent > NaPF6 + carbonate-based solvent. In the case of NaClO4 the difference are not as significant. Notably, with the NASICON electrolyte, the specific capacity retains 98% of its initial value even after 1800 charge-discharge cycles, demonstrating excep¬tional cycling stability.

The cycling behavior of the PB and PBA cathode material is studied by in-Situ Raman spectroscopy and XRD to characterize the (reversible) transformations of the crystal structure of these materials correspon-ding to the cathode potential and the sodium uptake (sodiation/de-sodiation). The frequency of the cya-nide stretching vibration mode v(CN) is sensitive to the oxidation state of coordinating metal ions of the PB/PBA Keggin structure [4]. In addition, we have performed coulometric titration experiments to investigate the variation of the sodium activity as a function of composition in the equilibrium state. The results by combing titration, XRD and Raman data helps to quantify the ranges of stoichiometry of the different phases/structures that are involved in the charge/discharge process (phase equilibria between trigonal and cubic phases).

Literature:
[1] D. O. Ojwang, ACS applied materials & interfaces 2021, 13, 10054-10063.
[2] F. M. Maddar, J. Mater. Chem. A, 2023, 11, 15778-15791.
[3] L. Ge, ACS nano, 2024, 18, 3542-3552.
[4] W. Ren, Adv. Funct. Materials, 2019, 29.