Abstract: Talk mainly is devoted to the recent publication by speaker and colleagues on intercalation crystal chemistry in the case of (Na,Li)₂FePO₄F. The type of the intercalating cation plays crucial role in the phase transformation regime of the cathode materials. Layered Na₂FePO₄F demonstrates two two-phase transitions in the Na-ion cell, which has confirmations in the literature. In contrast, desodiation of Na₂FePO₄F in the Li-ion cell demonstrates sloping charge profile suggesting a solid-solution mechanism with no charge ordered intermediate phase formation. This difference in desodiation behaviors is ascribed to a different activity of the A1 and A2 alkali positions which have in their coordination spheres different number of the “dangling” oxygen atoms (bonded only to the alkali and P cations, but not included into the FeO₄F₂ octahedra). Operando x-ray powder diffraction with a novel electrochemical cell, ex-situ synchrotron x-ray powder diffraction and ⁵⁷Fe Mössbauer spectroscopy studies corroborated with density functional theory calculations allowed to conclude that the bonding to the “dangling” oxygen atoms is an important factor which must be taken into account while discussing electrochemical activity of alkali cations in the polyanion structures. Another topic of the talk will be synthesis, evaluation and investigation of other oxoanionic compounds obtained during the speaker’s latest research, including the iso-structural Na₂MPO₄F (M=Co, Mn, Mg), fluoride K₂FeF₄, borate KMBO₃ (M=Fe, Mn, Mg), different phosphates of iron, manganese and vanadium and finally layered oxide (K,Li)Ni_0.33Mn_0.33Co_0.33O₂.

Abstract: Sodium-ion batteries are considered as the most perspective candidates to replace lithium-ion batteries in high-scale applications in the future. Most of scientific reports are focused on development of cathode materials, such as layered sodium-metal oxides, phosphates and polyanion compounds. In this work we present mixed-valent vanadate Na9V14O35 as possible cathode material. The features of the crystal structure, synthetic approaches and electrochemical behavior of this compound will be discussed.