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. Diverse material’s properties can be monitored in operando regime: the electrochemical mechanism (single-phase intercalation vs two-phase intercalation, conversion reaction, pseudocapacity), phase transformations and stability, crystal structure transformations, volume change between the charged and discharged states, oxidation states of the involved atomic species (i.e. d-metals and oxygen), local crystal and electronic structure etc. A vital need in the operando methods stems from their inherent advantage in studying the electrochemical response of the materials at the non-equilibrium dynamic conditions mimicking a real battery. As an example, for olivine-type cathode materials application of X-ray diffraction in operando regime revealed a number of intriguing properties, e.g. dependence of the phase transformation behavior during Li+ (de)intercalation on particle’s size, charge/discharge rate and composition in d-cation sublattice [1-3]. To successfully meet the challenge of comprehensive operando studies of different types of materials at different regimes, we created a novel type of synchrotron electrochemical cell [4]. Single crystal sapphire X-ray windows provide good signal-to-noise ratio, excellent electrochemical contact because of constant pressure between the electrodes and perfect electrochemical stability at high potentials due to the inert and non-conductive nature of the sapphire windows. A number of diffraction and spectroscopic experiments was performed using the cell. We studied phase transitions, crystal structure evolution, changes of valence state and local coordination of Fe and other cations in phosphate, flouruphosphate and oxide materials for Li-ion and Na-ion batteries. The obtained results will be discussed in details in the present report. Acknowledgements: the work was supported by Russian Science Foundation (grant No. 17-73-30006) and NGP (grant No. 2016-1/NGP). This work was carried out within the framework of the MSU–Skoltech Center for Electrochemical Energy Storage and Moscow State University Program of Development.