Exploring the Role of Crystal Water in Potassium Manganese Hexacyanoferrate as a Cathode Material for Potassium-Ion Batteries

Abstract

The Prussian Blue analogue K2−δMn[Fe(CN)6]1−ɣ∙nH2O is regarded as a key candidate for potassium-ion battery positive electrode materials due to its high specific capacity and redox potential, easy scalability, and low cost. However, various intrinsic defects, such as water in the crystal lattice, can drastically affect electrochemical performance. In this work, we varied the water content in K2−δMn[Fe(CN)6]1−ɣ∙nH2O by using a vacuum/air drying procedure and investigated its effect on the crystal structure, chemical composition and electrochemical properties. The crystal structure of K2−δMn[Fe(CN)6]1−ɣ∙nH2O was, for the first time, Rietveld-refined, based on neutron powder diffraction data at 10 and 300 K, suggesting a new structural model with the Pc space group in accordance with Mössbauer spectroscopy. The chemical composition was characterized by thermogravimetric analysis combined with mass spectroscopy, scanning transmission electron microscopy microanalysis and infrared spectroscopy. Nanosized cathode materials delivered electrochemical specific capacities of 130–134 mAh g−1 at 30 mA g−1 (C/5) in the 2.5–4.5 V (vs. K+/K) potential range. Diffusion coefficients determined by potentiostatic intermittent titration in a three-electrode cell reached 10−13 cm2 s−1 after full potassium extraction. It was shown that drying triggers no significant changes in crystal structure, iron oxidation state or electrochemical performance, though the water level clearly decreased from the pristine to air- and vacuum-dried samples.

Ice-Assisted Synthesis of Highly Crystallized Prussian Blue Analogues for All-Climate and Long-Calendar-Life Sodium Ion Batteries

“…The peaks at ∼493, ∼1610, and ∼3447 cm −1 are assigned to the vibrations of O−H and H−O−H bending modes arising from the free surface water and the interstitial water, respectively, while the peaks at ∼600 and ∼2081 cm −1 are associated with the vibrations of the cyanide groups in [Fe(CN) 6 ] 3−/4− . 38 The FTIR results indicate the existence of the water molecules inside and on the surface of both asprepared samples. The vibration state of CN − is highly sensitive to the valence state of Fe, so Raman spectroscopy was employed to show the valence state of the Fe (Figure S5a).…”

“…Figure S4 shows the Fourier transform infrared spectroscopy (FT-IR) spectra of the samples. The peaks at ∼493, ∼1610, and ∼3447 cm –1 are assigned to the vibrations of O–H and H–O–H bending modes arising from the free surface water and the interstitial water, respectively, while the peaks at ∼600 and ∼2081 cm –1 are associated with the vibrations of the cyanide groups in [Fe­(CN) 6 ] 3–/4– . The FTIR results indicate the existence of the water molecules inside and on the surface of both as-prepared samples.…”

Poly(ethylene oxide)-Based Electrolytes for Solid-State Potassium Metal Batteries with a Prussian Blue Positive Electrode

“…These shortcomings are often compensated by higher electrode potentials of commonly used cathode materials based on iron, manganese, or vanadium [e.g., Prussian blue analogues (PBAs) K 2 M­[Fe­(CN) 6 ] (M = Fe, Mn, etc.) or polyanionic compounds, such as KVOPO 4 and KVPO 4 F], − with the potential to boost the PIB cell voltage over 4 V. , However, using high-voltage materials puts high demands on the anodic stability of the electrolyte, as previously shown by Kim et al…”

Active material and interphase structures governing performance in sodium and potassium ion batteries

“…Future studies would clarify the effects of volume change and crystal water release on the capacity degradation to develop long-life PBA materials for SIBs. In contrast to the significant effect of interstitial water of Na–PBAs, the interstitial water has no significant influence on the electrochemical performance of KMnFe–PBA in K half-cells, 180 which could be due to the fact that the large K + and water molecules hardly occupy the same interstitial sites of the PBA simultaneously.…”

“…However, K-rich PBAs have monoclinic structures, regardless of the presence of interstitial H 2 O. 174,175,179,180 This phenomenon is explained by the large ionic radius of K + , which increases the Pauli repulsion in the lattice and prevents lattice shrinkage, 178 and the formation of a monoclinic phase with a larger lattice volume than that of the rhombohedral phase. Furthermore, most K-rich PBAs contain signicantly less interstitial water 174,175,179 than Na-rich PBAs, which typically contain about 2 mol interstitial water per formula unit.…”