LiNi _0.5 Mn _1.5 O _4−δ surface is doped with Ti ion maintaining the spinel structure at 500 °C, higher annealing temperatures cause Ti diffusion from surface towards the core.

The revival of the Na-ion battery concept has prompted an intense search for new high capacity Na-based positive electrodes. Recently, emphasis has been placed on manipulating Na-based layered compounds to trigger the participation of the anionic network. We further explored this direction and show the feasibility of achieving anionic-redox activity in 3-dimensional Na-based compounds. A new 3D β-Na_1.7IrO₃ phase was synthesized in a two-step process, which involves first the electrochemical removal of Li from β-Li₂IrO₃ to produce β-IrO₃, which is subsequently reduced by electrochemical Na insertion. We show that β-Na_1.7IrO₃ can reversibly uptake nearly 1.3 Na⁺ per formula unit through an uneven voltage profile characterized by the presence of four plateaus related to structural transitions. Surprisingly, the β-Na_1.7IrO₃ phase was found to be stable up to 600°C, while it could not be directly synthesized via conventional synthetic methods. Although these Na-based iridate phases are of limited practical interest, they help to understand how introducing highly polarizable guest ions (Na⁺) into host rocksalt-derived oxide structures affects the anionic redox mechanism.

The Sn⁴⁺ substitution limit in Li_1.2Ni_0.13Co_0.13Mn_0.54−xSn_xO₂ is around x = 0.045. For x = 0.027 the honeycomb ordering and O₃ structure is preserved. For x = 0 and x = 0.027 similar voltage fade has been obtained in the 3 V–4.55 V vs. Li/Li⁺ window.

The success and continued expansion of research on metal-oxo clusters owe largely to their structural richness and wide range of functions. However, while most of them known to date are negatively charged polyoxometalates, there is only a handful of cationic ones, much less functional ones. Here, we show an all-inorganic hydroxyiodide [H _10.7 Sb _32.1 O ₄₄ ][H _2.1 Sb _2.1 I ₈ O ₆ ][Sb _0.76 I ₆ ] ₂ ·25H ₂ O ( HSbOI ), forming a face-centered cubic structure with cationic Sb ₃₂ O ₄₄ clusters and two types of anionic clusters in its interstitial spaces. Although it is submicrometer in size, electron diffraction tomography of HSbOI allowed the construction of the initial structural model, followed by powder Rietveld refinement to reach the final structure. The cationic cluster is characterized by the presence of acidic protons on its surface due to substantial Sb ³⁺ deficiencies, which enables HSbOI to serve as an excellent solid acid catalyst. These results open up a frontier for the exploration and functionalization of cationic metal-oxo clusters containing heavy main group elements.

Today researches in the field of cathode materials for Li-ion batteries are directed towards the development of new materials with high power and energy densities, especially to answer the requirements of large scale applications. In this respect, fluoride-phosphate materials are very attractive as they demonstrate wide structural diversity. Furthermore, they are expected to exploit the advantages of both anion species: the increase in the working potential due to inductive effect of the polyanion group and higher ionicity of the M−F bond, faster kinetics arising from lower affinity of lithium to the F-anion.¹ In the search for new cathode materials for Li-ion batteries we have explored the A₂MPO₄F and AMPO₄F (A=Li, Na, K, and M=Co, Mn, Fe, V) fluorophosphates families. The fluoride-phosphate phases were prepared through different synthetic routes depending on the chemistry of transition metal. Combination of X-ray/neutron diffraction and electron microscopy were applied for a thorough structural characterization. It was shown that in these systems the substitution on the alkali metal site leads to the wide range of adopted structures. Thus, in the Li₂FePO₄F system synthesis of sodium-containing counterpart, NaLiFePO₄F, followed by subsequent electrochemical exchange of Na by Li leads to the cathode material with a 3D “stacked” structure.² In the (Li,Na)₂CoPO₄F system it provides a new low-temperature polymorph adopting a complex layered structure, in contrast to the high-temperature phase with 3D “stacked” arrangement. The introduction of large potassium into the AMPO₄F system gives a raise to a new cathode material with a KTP structure, which exhibits excellent rate capability.³ Reversible electrochemical activity demonstrated by these fluoride-phosphate materials in Li-cell will be discussed. A special attention will be given to the structure peculiarities and their impact on electrochemical properties, which is important for possible practical applications of these materials. The work was partly supported by Russian Science Foundation (grant 16-19-00190), Skoltech Center for Electrochemical Energy Storage and Moscow State University Development Program up to 2020. References Antipov, E. V.; Khasanova, N. R.; Fedotov, S. S. International Union of Crystallography Journal 2015, 2, 85-94 Khasanova N. R., Drozhzhin O. A., Storozhilova D. A., Delmas C., Antipov E. V. Chem. Mater., 2012, 24, 4271–4273 Fedotov, S. S.; Khasanova, N. R.; Samarin, A. Sh.; Drozhzhin, O. A.; Batuk, D.; Karakulina, O. M.; Hadermann, J.; Abakumov, A. M; Antipov, E. V. Chem. Mater. 2016, DOI: 10.1021/acs.chemmater.5b04065