Widespread commercialization of sodium-ion batteries (SIB) is limited by the shortcomings of existing electrode materials, so the search and testing of various sodium compounds suitable for SIB are relevant. This paper presents the results of a study of the sodium diffusion mechanisms in quasi-layered oxides Na1-xV1-xMo1+xO6, which are potentially promising for applications for SIB. A simple synthesis procedure has been developed, which makes it possible to obtain compounds in a wide range of compositions up to x = 0.2. To elucidate the mechanisms of sodium diffusion, we applied a comprehensive approach that combines material characterization at the “macro” (XRD, impedance spectroscopy) and “atomic-scale” levels (NMR, ab-initio calculations). Our results reveal rather fast sodium dynamics: Ionic conductivity reaches the values of 10–3 S/cm at T > 730 K. It has been found moreover that the diffusion mechanism changes with increasing temperature. At T < 625 K, sodium motion occurs mainly along the crystallographic b axis due to atomic jumps with the shortest jump length ≈ 3.6 Å and activation energy Ea 1 eV. With increasing temperature, another type of jumps along a axis (in the ab plane) with a jump length of ≈ 5 Å and a barrier value of 2 eV is also activated.

An entry from the Inorganic Crystal Structure Database, the world’s repository for inorganic crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the joint CCDC and FIZ Karlsruhe Access Structures service and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

Министерство науки и высшего образования Российской Федерации Российское химическое общество им.Д.И.Менделеева Секция по химической термодинамике и термохимии Научного совета РАН по физической химии Сибирское Отделение Российской Академии Наук Институт неорганической химии им.А.В.Николаева СО РАН

Discussion about the mechanism of pseudocapacitance led to the hypothesis that on the surface of pseudocapacitive materials there is an intermediate layer responsible for charge accumulation. Cyclic voltammetry revealed the presence of a hydroxylated layer on the surface of NiCo2O4 film. It was proven that the hydroxylated layer is formed as a result of the interaction of NiCo2O4 with alkali and consists of anionic metal complexes at the phase boundary. The redox switching process is considered as the loss of protons from water molecules or the removal of OH- ions in the environment of a metal ion. The presence of structurally bound hydroxyl groups on the surface was confirmed by TG-DSC, NMR H-1 MAS and XPS. According to the results of ARXPS measurements, the thickness of the surface layer is similar to 0.4 nm.

Development of the technologies for energy storage and conversion requires a search for compounds with high diffusion of alkali and alkaline-earth ions. Here, we present the results of comprehensive studies, including synthesis, powder X-ray diffraction, experiments on impedance and Na-23 NMR spectroscopy, as well as ab initio calculations, which were carried out to explore the sodium diffusion in scheelite-like Na5M(MoO4)(4) with M = Y, La, Bi, and in related solid solutions Na5-xM1-xZrx(MoO4)(4) (0.05 <= x <= 0.1), which were synthesized for the first time. Our investigations reveal that the Na-ion mobility increases in the sequence Y -> La -> Bi and with growing x. For Na4.9Bi0.9Zr0.1(MoO4)(4) the highest ion conductivity was found: similar to 10(-4) S/cm at T = 450 degrees C, which is comparable to that of the NASICON-type molybdates. From the temperature variations of the Na-23 NMR spectra and DFT calculations, the mechanism of sodium-ion diffusion was established at the atomic-scale level.

The work reviews the structure, non-stoichiometry, and ionic mobility of molybdates, tungstates, and other compounds crystallizing in the structure type of alluaudite (Na, Ca)(Fe, Mn, Mg)3(PO4)3 with the general Moore′s crystal chemical formula X(2)X(1)M(1)M(2)2(TO4)3, where X are large cations Na+, Ca2+, K+, Pb2+, etc., with the coordination number 8; M are octahedral cations, T = P, As, V, S, Mo, W. Using this formula and the corresponding site occupancies, possible limits of double molybdate and tungstate compositions of the alluaudite family are determined. Various types of distortions (superstructures) of alluaudite are considered; several groups of phases with different symmetries, numbers of anions in the unit cell, and vector relations with the unit cell of the original alluaudite structure are distinguished. It is shown that chains of partially defective positions X(2) and X(1) aligned along axis c play a key role in the transport of sodium cations in the alluaudite type phases. Phosphates and sulfates with alluaudite structure exhibit mainly 1D transport of sodium ions; however, calculations of the bond-valence sum maps, NMR data, and ab initio calculations show that 2D transport in the (100) plane is possible in complex molybdates and tungstates due to the transport of Na+ ions between X(2)–X(2) and X(1)–X(1) channels through the bridging site M(1). It is shown that the family of alluaudite-related (pseudo)orthorhombic triple molybdates Na10Cs4M5(MoO4)12 (M = Mn, Co) and Na25Cs8R5(MoO4)24 (R = Fe, Sc, In) also exhibits 2D diffusion of sodium ions via successive zigzag ion hoppings and that 3D transport may appear at elevated temperatures.

Complex sodium-containing oxides are of interest for the search and development of new materials for many practical applications. This paper presents the results of multiscale experimental and theoretical studies aimed to explore the mechanism of sodium-ion diffusion in the scheelite-related Na2Zr(MoO4)(3) and Na4Zr(MoO4)(4). These molybdates were synthesized by the precursor (formate) method. Ab initio modeling of sodium migration predicts a barrier of 1.0-1.2 eV for long-range sodium diffusion, which is confirmed by the impedance spectroscopy and the Na-23 NMR experiments. Our results allow us to assume the existence in Na4Zr(MoO4)(4) of an additional, much faster motional process (with energy barrier of 0.5-0.6 eV) associated with localized ion jumps.

Cationic transport in triple molybdate Na25Cs8Sc5(MoO4)24 with an alluaudite structure has been studied. According to 23Na NMR, the ion jump frequency is as high as $$\tau _{d}^{{-1}}$$ ~ 104‒105 s–1 at Т = 550‒600 K, and the activation energy for sodium diffusion is Ea ~ 0.9 eV. The mobility of Na+ ions is slower than that found in double molybdate Na5Sc(MoO4)4, which is caused by structural features. In Na25Cs8Sc5(MoO4)24, some of sodium positions are replaced by Cs+ ions preventing the sodium diffusion in a channel along the c axis. As a result, the 2D mechanism of Na+ transport typical of the Na5R(MoO4)4 alluaudite family does not occur. The most probable mechanism of sodium diffusion in Na25Cs8Sc5(MoO4)24 involves successive atomic jumps along a zigzag path in the ab plane.

Research data for the diffusion mechanisms of Na+ ions in Na1 – xMg1 – xAl1 + x(XO4)3 (X = Mo, W) compounds with the NASICON-type structure (space group R $$\bar {3}$$ c, Z = 6) are reported. Solid solutions in the homogeneity range 0.1 ≤ x ≤ 0.5 for X = Mo and 0.4 ≤ x ≤ 0.6 for X = W have been prepared by solid-state synthesis. Conductivity measurements and NMR spectroscopy data indicate fast sodium diffusion in the studied samples: the ionic conductivity reaches the values of about 10–3 S/cm at T > 800 K. The frequency of elementary ionic jumps is on the order of 104 s–1 at T ≈ 500 K, and the activation energy is equal to 0.8–0.9 eV. The results have shown that the ionic conductivity in molybdates is higher than in tungstates. The growth of magnesium concentration increases the concentration of local coordinations Mg2+–Na+–Mg2+, acting as traps for moving sodium ions. The above conclusions are supported by ab initio calculations according to which the barrier for sodium diffusion from the Mg2+–Na+–Mg2+ position is expected to be higher than those for the Mg2+–Na+–Al3+ and Al3+–Na+–Al3+ ones.

Рассмотрены особенности строения, нестехиометрия и ионная подвижность молибдатов, вольфраматов и других соединений, кристаллизующихся в структурном типе аллюодита (Na, Ca)(Fe, Mn, Mg)3(PO4)3 с общей кристаллохимической формулой по П. Муру X(2)X(1)M(1)M(2)2(TO4)3, где X — крупные катионы Na+, Ca2+, K+, Pb2+ и др. с КЧ = 8; M — октаэдрические катионы, T = P, As, V, S, Mo, W. На основе этой формулы и заселенностей соответствующих позиций выведены возможные пределы составов двойных молибдатов и вольфраматов типа аллюодита. Рассмотрены виды искажения (сверхструктуры) аллюодита, выделено несколько групп фаз, различающихся симметрией, числом анионов в ячейке и векторной связью с ячейкой исходной структуры аллюодита. Показано, что ключевую роль в переносе катионов натрия в фазах типа аллюодита играют цепочки частично дефектных позиций X(2) и X(1), идущие вдоль оси c. Для фосфатов и сульфатов со структурой аллюодита характерен преимущественно одномерный транспорт ионов натрия, однако, согласно данным расчетов карт сумм валентных усилий, ЯМР спектроскопии и ab initio расчетов, в сложных молибдатах и вольфраматах есть возможность двухмерной проводимости в плоскости (100) за счет перетока ионов Na+ между каналами X(2)—X(2) и X(1)—X(1) через мостиковую позицию M(1). Показано, что в семействе родственных аллюодиту (псевдо)ромбических тройных молибдатов Na10Cs4M5(MoO4)12 (M = Mn, Co) и Na25Cs8R5(MoO4)24 (R = Fe, Sc, In) также возможна двухмерная диффузия ионов натрия через последовательные зигзагообразные ионные перескоки с вероятной реализацией трехмерного транспорта при повышенных температурах.

Alluaudite-type compounds are currently considered as a promising class of materials for sodium-ion batteries, and understanding of the diffusion processes in them is very important. Using the Na-23 MAS NMR and ab initio calculations, we established the mechanism of sodium diffusion in Na4-2xM1-x(MoO4)(3) (M = Mg, Zn, Cd) depending on the type of M-element and x. A comparison of the results obtained for various alluaudite-type compounds shows the crucial effect of the M-cation on the Na-ion dynamics in this class of materials. Higher concentration and charge of M-element increase the concentration of vacancies in the Na-sublattice and enhance the sodium mobility. Moreover, the Na dynamics increases with the M-ion size. The occupancy of the M site and the type of T atom in the TO4 group also determine the mechanism of sodium diffusion in alluaudites, whether it is one-dimensional or two-dimensional. These findings may help a deeper understanding of sodium diffusion processes in alluaudite-type compounds and their development as materials for sodium-ion batteries.

Glasses in 30Li(2)O-(70-x)B2O3-xV(2)O(5) system (x = 30, 40, 47.5 mol%) are obtained by melt quenching method. Their structure has been studied by a set of experimental and simulation methods such as nuclear magnetic resonances, X-ray diffraction, and molecular dynamics. The coordination numbers of lithium and boron ions are determined from NMR data and confirmed by XRD analysis; the obtained data are compared with the simulation results. Based on the experimental and simulation results, the model of the glass network is constructed. According to the experimental data and simulation results, boron coordination in the glasses is 3 and 4, and vanadium coordination is equal to 5 and 6.

Alluaudite phases are very attractive as both cathode and electrolyte materials for rechargeable sodiumion batteries. In this work, the combined experimental and DFT studies have been performed to establish the diffusion mechanism in alluaudite-like compound Na5In(MoO4)(4). The ionic conductivity was found to reach 3.3 x 10(-4) S/cm at 687 K, with an activation energy of 0.66 eV. The sodium diffusion mechanisms have been revealed from the analysis of the Na-23 NMR spectra along with the DFT estimations of Naion migration barriers. Our results predict that one-dimensional diffusion of sodium in the separate channels along the c-axis is accompanied by the cross-linking jumps providing two-dimensional diffusion in the bc-plane. It is clearly demonstrated that the indium deficiency favors 2D diffusion, but sharply increases the energy barrier for 1D diffusion. Comparison of our results for Na5In(MoO4)(4) and related Na5Sc(MoO4)(4) shows that the type and deficiency of M-metal in the NaxMy(MoO4)(x) alluaudites can control the sodium diffusion. The present work highlights the key aspects of cation influence on the diffusion properties in alluaudite materials.

The structure and conductivity of the new Li- and Mg-codoped bismuth niobates Bi1.5Mg1-xLixNb1.5O7-delta (0 <= x <= 0.50) with the pyrochlore structure have been investigated. The samples were synthesized by the method of organic-inorganic precursors combustion. A structural characterization was performed using Li-7 nuclear magnetic resonance (NMR) spectroscopy in combination with the fitting of X-ray diffraction patterns. The Li+ cations dynamics were studied by temperature-dependent Li-7 NMR lineshape analysis. The measurements have shown that the Li+ cations are distributed in one of two possible sublattices in the structure and are not mobile up to 120 degrees C. According to the results of structural analysis of the Bi1.5Mg1-xLixNb1.5O7 (x = 0.25; 0.50) pyrochlores the lithium atoms are distributed in the bismuth sites. The electrical properties were investigated by impedance spectroscopy method in the air, oxygen and "wet" atmospheres in the 25-750 degrees C temperature range. The activation energy value of dc conductivity is about 1.2-1.3 eV for the all samples and corresponds to the ionic (oxygen) conductivity at T > 400 degrees C. Electronic (p-type) conductivity was determined at T < 360 degrees C. H-1 MAS NMR data and the results of the comparison of the conductivity of Li- and Mg-codoped bismuth niobates in dry and "wet" atmospheres point to the proton conductivity up to 500 degrees C. The dielectric permittivity epsilon' values increase with lithium content from 86 (x = 0) to 143 (x = 0.5) at the same dielectric loss tan delta = 0.002 (1 MHz, 25 degrees C), TCC values vary from -590 to -530 ppm/degrees C in the 25-280 degrees C temperature range.

Двойные молибдаты индия и скандия со структурой аллюодита получены методом твердофазного синтеза. Уточнена кристаллическая структура индийсодержащего соединения и определены оптические характеристики Na5R(MoO4)4, (R = Sc, In). В рамках неэмпирического метода изучена электронная структура молибдатов Na5R(MoO4)4, (R = Sc, In) с учетом позиционного разупорядочения Na/Sc(In). Расчеты мнимой части диэлектрической функции предсказывают величину оптической щели ~3.8 эВ, в соответствии с результатами, полученными из спектров поглощения. Установлена сильная зависимость энергии образования вакансий натрия от типа позиции и концентрации Sc(In), что может привести к различным механизмам диффузии в соединениях со структурой аллюодита с большим и малым содержанием металла R.

Double molybdates of indium and scandium with alluaudite structure are prepared by the solid-phase synthesis method. The crystal structure of the indium containing compound is refined and optical characteristics of Na5R(Mo04)4(R = Sc, In) are determined. Electronic structures of Na5R(Mo04)4(R = Sc, In) molybdates are studied within the ab initio method taking account of Na/Sc(In) positional disordering. Calculations of the imaginary part of dielectric function predict the optical gap of ~3.8 eV, in accordance with absorption spectroscopy data. It is established that formation energy of sodium vacancies strongly depends on sodium position and Sc(In) concentration. As a result, various diffusion mechanisms may be activated in alluaudite-type compounds with high and low contents of metal R.

The research uses the method of high-temperature thermogravimetric analysis to study the processes of interaction of the gas phase in the temperature range 300–950 °C in the partial pressure ranges of oxygen 8.1–50.7 kPa, water 6.1–24.3 kPa and hydrogen 4.1 kPa with La1–xSrxScO3–α oxides (x = 0; 0.04; 0.09). In the case of an increase in the partial pressure of water vapor at a constant partial pressure of oxygen (or hydrogen) in the gas phase, the apparent level of saturation of protons is shown to increase. An increase in the apparent level of saturation of protons of the sample also occurs with an increase in the partial pressure of oxygen at a constant partial pressure of water vapor in the gas phase. The paper discusses the causes of the observed processes. The research uses the hydrogen isotope exchange method with the equilibration of the isotope composition of the gas phase to study the incorporation of hydrogen into the structure of proton-conducting oxides based on strontium-doped lanthanum scandates. The concentrations of protons and deuterons were determined in the temperature range of 300–800 °C and a hydrogen pressure of 0.2 kPa for La0.91Sr0.09ScO3–α oxide. The paper discusses the role of oxygen vacancies in the process of incorporation of protons and deuterons from the atmosphere of molecular hydrogen into the structure of the proton conducting oxides La1–xSrxScO3–α (x = 0; 0.04; 0.09). The proton magnetic resonance method was used to study the local structure in the temperature range 23–110 °C at a rotation speed of 10 kHz (MAS) for La0.96Sr0.04ScO3–α oxide after thermogravimetric measurements in an atmosphere containing water vapor, and after exposures in molecular hydrogen atmosphere. The existence of proton defects incorporated into the volume of the investigated proton oxide from both the atmosphere containing water and the atmosphere containing molecular hydrogen is unambiguously shown. The paper considers the effect of the contributions of the volume and surface of La0.96Sr0.04ScO3–α oxide on the shape of the proton magnetic resonance spectra.

Effects on polycaproamide structure of a polyfluorinated alcohol immobilized on montmorillonite clay nanoparticles were studied using x-ray structure analysis and broad-line PMR spectroscopy. An exfoliated composite formed and caused the crystal and molecular structures of this heterochain polymer to reorganize. A composite based on the polyfluorinated alcohol changed the mobility of macromolecular chain segments and increased the fraction of planar trans-conformations.

Prepared by the precursor method, lithium-doped cadmium oxide samples were synthesized using mixed formates of Cdi(1-x)Li(x)(HCOO)(2)center dot 2H(2)O (0 <= x <= 0.075) as a precursor. NMR spectra were obtained on the lithium nuclei incorporated in cadmium oxide; and it is demonstrated that there is only one type of impurity centers to be found. The first-principle method using a projector augmented wave (PAW) approach is applied to carry out electronic band structure calculations and to analyze optical absorption spectra for the cases of lithium atoms occupying interstitial sites and of lithium atoms substituting for cadmium or oxygen atoms. Calculations are carried out to determine energy-efficient preferences for sites of impurity centers, which indicate as the most probable the presence of lithium atoms at the interstitial sites, with vacancies observed in the cadmium sub lattice. The presence of lithium atoms at the interstitial sites appears to be explaining the conductivity increase and the red-shift of the absorption edge observed in experiments.