Vacancy influence on manganese hexacyanoferrate: a structural perspective

Manganese Hexacyanoferrates (AxMn[Fe(CN)6]1-y,□y∙H2O, where A: alkali metal ions, 0≤ x≤2; □: Fe(CN)6 vacancies, y<1)) are promising positive electrode materials for non-aqueous batteries, including Na-ion batteries, due to their large specific capacity (>130 mAhg-1), high discharge potential and sustainability. Typically, the electrochemical reaction of MnHCF associates with phase and structural changes, due to the Jahn-Teller (JT) distortion of Mn sites upon the charge process.

To understand the effect of the MnHCF structure on its electrochemical performance, two MnHCF materials with the same phase structure (monoclinic), similar particle size, but different [Fe(CN)6]4- vacancy content (4% and 11%) were synthesized and tested as cathode material in organic Na-ion battery. The electrochemical results show that the sample with lower vacancy content (4%) exhibits relatively higher capacity retention of 99.1% and 92.6% at 2nd and 10th cycles, respectively, with respect to 97.4% and 79.3% in sample with higher vacancy content (11%).

In order to interpret the different electrochemical performance of the two MnHCF materials, ex-situ X-ray absorption spectroscopy (XAS) data and ex-situ X-ray diffraction (XRD) data were collected to study their local structural and crystal structure changes during the early stages of cycling. The XAS experiments were performed at the XAFS beamline of Elettra, probing both the Fe and Mn K-edges. X-ray diffraction (XRD) data were recorded at the MCX beamline at Elettra synchrotron, Trieste (Italy).

The Mn environment in the pristine electrode consists of a symmetric MnN6 octahedral, with Mn-N distance being 2.18 Å and 2.19 Å for MnHCF-P and MnHCF-A, respectively. After the first charge, the four equatorial Mn-N distances contract to 1.95 Å and 1.96 Å for MnHCF-P and MnHCF-A electrodes, respectively, i.e., shrinking by 10.55% and 10.50%, respectively (see Figure 1). Meanwhile, the axial Mn-N distances remain roughly constant. However, a more insightful point is the comparison of the Mn-N distance at the discharged state. The MnHCF-A electrodes experience an irreversible configuration change after the first charge and discharge cycle, which is also kept for the 2nd cycle, with the 4 equatorial Mn-N bonds being always shorter than the axial ones. This irreversible change, however, enables MnHCF-A electrodes to undergo less local structure variation during the charge and discharge process. In fact, based on the data above, the low-vacancy-content MnHCF-A experiences less effect from the JT-distortion and less structural variations during charge/discharge process.

Figure 1 of the top story by Li et al. Chem. Sus. Chem. 2023

Figure 1: Evolution of Mn first shell: Mn-N distances in both axial and equatorial direction for MnHCF-P/A samples at different charge/discharge states.

As seen in Figure 2, a reversible change of XRD patterns between charged (C1, C2, C10) and discharged (D1, D2, D10) electrodes is observed. For the charged electrodes, the monoclinic phase exhibits better fitting results than the tetragonal phase. The cell parameters, beta value and volume exhibit the same change trend: decreasing upon charge and increasing during discharge. While for MnHCF-P electrodes, the variation of volume value and β values is larger than MnHCF-A electrodes, indicating that a rather larger structure rearrangement is required by the Na+ extraction/insertion in MnHCF-P samples.

Figure 2 of the top story by Li et al. Chem. Sus. Chem. 2023

Figure 2: (a) Ex-situ XRD patterns of MnHCF-P at different charge-discharge states (C1, D1, C2, D2 and C10, D10); (b)Pawley refinement of MnHCF-P C1 electrodes based on tetragonal phase; (c) crystal structure of tetragonal phase; (d) Pawley refinement of MnHCF-P C1 electrodes based on monoclinic phase; (e) crystal structure of monoclinic phase.

The results show that a weaker cooperative JT-distortion effect and relatively smaller crystal structure modification occurring for the material with lower vacancies, which explains the better electrochemical performance in cycled electrodes.

This research was conducted by the following research team:

Min Li1, Mattia Gaboardi2, Angelo Mullaliu3,4, Mariam Maisuradze1, Xilai Xue3,4, Giuliana Aquilanti2, Jasper Rikkert Plaisier2, Stefano Passerini3,4,5, Marco Giorgetti1

1 Department of Industrial Chemistry “Toso Montanari”, University of Bologna, Bologna, Italy
2 Elettra - Sincrotrone Trieste S.C.p.A., Trieste, Italy
3 Helmholtz Institute Ulm (HIU), Ulm, Germany
4 Karlsruhe Institute of Technology (KIT), Karlsruhe, Germany
5 Sapienza University of Rome, Department of Chemistry, Rome, Italy

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M. Li, M. Gaboardi, A. Mullaliu, M. Maisuradze, X. Xue, G. Aquilanti, J.R. Plaisier, S. Passerini, M. Giorgetti, "Influence of Vacancies in Manganese Hexacyanoferrate Cathode for Organic Na‐ion Batteries: A Structural Perspective", ChemSusChem., e202300201 (2023); DOI: 10.1002/cssc.202300201

Last Updated on Wednesday, 10 May 2023 17:54