Dehydration Conditions and Ultrafast Rehydration of Prussian White: Phase Transition Dynamics and Implications for Sodium-Ion Batteries

Abstract

Prussian White (PW) is a strategic cathode material for sodium-ion batteries, offering a high theoretical capacity and voltage. However, the crystalline structure and the electrochemical performance of PW strongly depend on the hydration level, which is difficult to control, leading to discrepancies in the results and interpretations presented in the literature. This work aims to provide a deeper insight into the dehydration process of PW materials and a better understanding of the impact of their fast rehydration, upon exposure to moisture, on their characterization. For this purpose, a Na_1.87Mn[Fe(CN)₆]_0.99·1.99H₂O sample was synthesized by a coprecipitation method and subsequently dehydrated to remove water. After thorough characterization, our findings show that drying parameters, such as temperature and pressure, strongly influence the post-drying result. Moreover, the dehydrated samples rehydrate within minutes of exposure to air, which may explain some discrepancies observed in the literature and highlights the necessity to work under fully air-tight conditions.

Figure 1

(a) PXRD patterns of the sample heated up to 150 °C under a vacuum pressure of 20 mbar (left) and 10^–2 mbar (right) with highlighted characteristic Bragg reflections of monoclinic (blue) and rhombohedral (red) phases, and (b) rhombohedral phase fraction evolution under low (dashed line) and medium vacuum (solid line) as a function of temperature, obtained from refinement of scale factors parameters.

Figure 2

(a) FT-IR spectra of R-MnFePW and M-MnFePW, in agreement with peaks indexation from Song et al. (b and c) TGA-MS of M-MnFePW (panel (b), solid blue line) or R-MnFePW (panel (c), solid red line) with derivative weight (solid black lines) and m/z signal of water (dashed lines).

Figure 3

(a) PXRD patterns of M-MnFePW and R-MnFePW measured in synchrotron (13 keV, λ = 0.9537 Å). (b) First electrochemical cycle and corresponding dQ/dV plot for both samples, and XAS measurements at (c) Fe and (d) Mn K-edges for both samples as well as references.

Figure 4

(a) PXRD patterns (λ_Cu Kα1 = 1.540598 Å, λ_Cu Kα2 = 1.544426 Å) of the powder sample heated up to 150 °C protected by Kapton and then exposed to air and (b) evolution of the intensity ratio of (012)_R/((012)_R + (011)_M), as a function of time for both powder and electrode samples.