ИСТИНА

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 examples are given below. Li-rich iron-containing olivines Li1+δM1-δPO4 (δ~0.03-0.06) prepared by solvothermal method via Li3PO4 precursor demonstrate excellent electrochemical characteristics such as C-rate capability (140 mAh/g at 10C charge for the Li1.04Fe0.96PO4/C material) and low voltage hysteresis (14 mV at C/300 rate for the same sample) between lithiation and delithiation. Phase transformations and evolution of the Fe cations coordination environment during Li+ (de)intercalation are studied in operando regime by means of synchrotron X-ray powder diffraction (SXPD) and 57FeMössbauer spectroscopy (MS). Presence of a certain amount of Li+ in M2 position in the crystal structure of the initial phosphates leads to additional component in MS spectra of all studied compounds, corresponding to ferric ions in the M2position with distorted second coordination sphere. Evolution of the MS spectra during charge/discharge reveals clear relationship between relative fraction of this component and the mechanism of Li+ (de)intercalation. Extended single-phase regions with large Li+ non-stoichiometry in triphylite 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. Increased thermodynamic stability of the intermediate Li1-xFePO4 phases was also shown for Li-rich olivines by DFT calculations. These features can be regarded as an additional merit of Li-rich olivines rendering them 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]. These and other examples will be discussed in more detail in the presentation. This work was supported by the Russian Science Foundation (Grant No. 17-73-30006).