Operando Synchrotron X-Ray Diffraction and Mössbauer Spectroscopy of the Cathode Materials for Li-Ion and Na-Ion Batteries
Authors: Drozhzhin Oleg A. , Zakharkin Maxim V. , Sumanov Vasiliy D. , Sobolev Alexey V. , Presniakov Igor A. , Chernyshov Dmitry , Stevenson Keith J. , Abakumov Artem M. , Antipov Evgeny V. 
Published: February 2020

Application of different diffraction and spectroscopic techniques to operando studies on the electrode materials became an integral part of the metal-ion battery research. Combination of novel operando electrochemical cell with sapphire windows [1], synchrotron X-ray diffraction (SXRD) and Mössbauer spectroscopy (MS) has enabled us to study crystal structure evolution and phase transformation behavior during electrochemical cycling of several cathode materials for Li-ion and Na-ion batteries. Two of them are given below.
Phase transformations and evolution of the Fe cations environment in LiFePO4, LiFe1/2Mn1/2PO4 and LiFe1/3Mn1/3Co1/3PO4 during Li+ (de)intercalation were studied in operando regime by means of synchrotron SXPD and MS. Presence of a certain amount of Li+ in M2 position in the crystal structure of the initial phosphates leads to appearing of additional component in MS spectra of all studied compounds, corresponding to ferric ions in the same position with distorted second coordination sphere. Evolution of MS spectra during charge/discharge revealed clear connection between relative contribution of this component and the mechanism of Li+ (de)intercalation. Extended single-phase regions with large Li+ non-stoichiometry in triphilite and heterosite phases of Li1-xFePO4 observed by means of SXPD appear due to Li-Fe defects existing in Li-rich olivines and acting as a “diluting” agent preventing two-phase spinodal decomposition. These features can be regarded as an additional merit of Li-rich olivines [2] as a promising cathodes for high-power Li-ion batteries.
Unusual phase transformation behavior was also observed for Na4MnV(PO4)3 cathode material with NASICON-type structure by means of operando SXPD. In contrast with other NASICON-type phosphates, Na4MnV(PO4)3↔ Na3MnV(PO4)3 transition proceeds via single phase mechanism. The next step Na3MnV(PO4)3↔ Na2MnV(PO4)3 is biphasic. Further desodiation results in appearance of additional voltage plateau at ~3.9 V vs. Na/Na+, associated with re-distribution of Na atoms over available positions and activation of Na1 site, which previously was considered as inactive. During reverse process, all extracted Na cations are introduced back via continuous solid solution region. The demonstrated behavior is in part similar with Li-ion monoclinic Li3-xV2(PO4)3, where fully deintercalated Li+ cations introduce disorder in the structure which leads to single-phase intercalation mechanism [3].
This work was supported by the Russian Science Foundation (Grant No. 17-73-30006).

Reference
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[1] Drozhzhin, O.A., Tereshchenko, I.V., Emerich, H., Antipov, E.V., Abakumov, A.M., Chernyshov, D., J. Synchrotron Rad., (2018) 25 (2), pp. 468-472.
[2] K.-Y. Park, I. Park, H. Kim, G. Yoon, H. Gwon, Y. Cho, Y.-S. Yun, J.-J. Kim, S. Lee, D. Ahn, Y. Kim, H. Kim, I. Hwang, W.-S. Yoong, K. Kang, Energy Environ. Sci., 2016,9, 2902-2915
[3] Zakharkin M. V., Drozhzhin O. A., Tereshchenko I. V., Chernyshov D., Abakumov A. M., Antipov E. V., Stevenson K. J., ACS Applied Energy Materials, 1 (2018) pp. 5842-5846.

Figure 1