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The P2- and O3-Na0.67Co0.33Ti0.67O2 cathodes undergo a very slow transformation during charge and discharge in sodium batteries due to a valence and spin-state transition of cobalt cations. As a result, a big potential hysteresis of 2.5–3 V arises.

Iron-doped potassium titanate with a hollandite structure is synthesized by the solid-phase method. To reduce the contact resistance and to increase the diffusion coefficient of potassium ions, a conductive carbon coating is applied to the surface of the titanate synthesized in an argon atmosphere. The formation of the composite material is confirmed by the results of X-ray phase analysis, scanning electron microscopy, and IR spectroscopy. The half-cells with potassium metal in a CR2025 case, which are assembled on the basis of the synthesized composite, are tested electrochemically with subsequent calculation of the specific capacitance, power density, and energy density. It is found that the specific capacitance of an electrode with a carbon coating is 10–15% higher with low sweep rates (1 and 3 mV s–1). It is shown that the use of carbon coatings increases the diffusion coefficient of potassium to 1.77 × 10–14 cm2 s–1/2.

The hollandite-like copper doped potassium titanate was synthesized by two-step method, including the potassium polytitanate precursor modification with copper compounds in aqueous solution and the following calcination at 900 °C. The structure of material obtained was studied using the XRD and SEM analysis. The Rietveld method was used to estimate the crystalline unit cell parameters and calculate the exact stoichiometry of K1.53(Cu0.76Ti7.24)O16 by refining the atomic occupancies for chemical elements. High-k ceramic materials based on this shollandite-like solid solution were sintered at different temperatures varied in the range from 1000 to 1100 °C. The influence of sintering conditions on electric properties of the obtained ceramics was investigated in a wide frequency range. The contribution of different processes on permittivity was discussed using the electron-pinned defect-dipole (EPDD) model, and the internal barrier layer capacitance (IBLC) model. The optimal conditions to produce modified potassium polytitanate, used as intermediate, as well as optimal sintering conditions which allowed obtaining mono-phase hollandite-like ceramic materials were recognized. It was found that such ceramic materials had high permittivity (e’ ≈ 0.9 × 104 at 10 Hz).

A series of solid solutions (Li2Fe1-yMny)SO with a cubic antiperovskite structure was successfully synthesized. The composition (Li2Fe0.5Mn0.5)SO was intensively studied as a cathode in Li-ion batteries showing a reversible specific capacity of 120 mA h g-1 and almost a 100% Coulombic efficiency after 50 cycles at 0.1C meaning extraction/insertion of 1 Li per formula unit during 10 h. Operando X-ray absorption spectroscopy confirmed the redox activity of both Fe2+ and Mn2+ cations during battery charge and discharge, while operando synchrotron X-ray diffraction studies revealed a reversible formation of a second isostructural phase upon Li-removal and insertion at least for the first several cycles. In comparison to (Li2Fe)SO, the presence of Mn stabilizes the crystal structure of (Li2Fe0.5Mn0.5)SO during battery operation, although post mortem TEM studies confirmed a gradual amorphization after 50 cycles. A lower specific capacity of (Li2Fe0.5Mn0.5)SO in comparison to (Li2Fe)SO is probably caused by slower kinetics, especially in the two-phase region, as confirmed by Li-diffusion coefficient measurements.

Materials futures 1(3), 035104 (2022). doi:10.1088/2752-5724/ac8ab9, P2-type layered oxides with the general Na-deficient composition NaxTMO2 (x < 1, TM: transition metal) are a promising class of cathode materials for sodium-ion batteries. The open Na+ transport pathways present in the structure lead to low diffusion barriers and enable high charge/discharge rates. However, a phase transition from P2 to O2 structure occurring above 4.2 V, combined with metal dissolution at low potentials upon discharge, results in rapid capacity degradation. In this work, we demonstrate the positive effect of configurational entropy on the stability of the crystal structure during battery operation. Three different compositions of layered P2-type oxides were synthesized by solid-state chemistry, Na0.67(Mn0.55Ni0.21Co0.24)O2, Na0.67(Mn0.45Ni0.18Co0.24Ti0.1Mg0.03)O2, and Na0.67(Mn0.45Ni0.18Co0.18Ti0.1Mg0.03Al0.04Fe0.02)O2 with low, medium and high configurational entropy, respectively. The high-entropy cathode material shows lower structural transformation and Mn dissolution upon cycling in a wide voltage range from 1.5 to 4.6 V. Advanced operando techniques and post-mortem analysis were used to probe the underlying reaction mechanism thoroughly. Overall, the high-entropy strategy is a promising route for improving the electrochemical performance of P2 layered oxide cathodes for advanced sodium-ion battery applications., Published by IOP Science, Bristol

Journal of materials chemistry / A 9(40), 23095 - 23105 (2021). doi:10.1039/D1TA05130J, The development of electrode materials with multielectron redox functionality is imperative for next-generation Li-ion batteries with a high gravimetric capacity. Within this context, a Li-rich (Li$_2$Fe)SO antiperovskite cathode is a promising candidate exhibiting such multielectron cationic and anionic redox features, resulting in a reversible extraction/insertion of about 1.2 Li$^+$ per formula unit. However, it suffers from poor structural and cycling stabilities which hinder its practical application. Herein, we systematically investigate the effect of anionic substitution of S with Se on the structural, thermal and electrochemical properties of the (Li$_2$Fe)SO cathode. With increasing the Se content, higher thermal stability and lower sensitivity to moist air were obtained. Multi-stage cationic and anionic redox processes characterized the electrochemical activity of all the prepared (Li$_2$Fe)S$_{1-x}$Se$_x$O solid solutions. The cationic redox process was shifted to higher potentials while the anionic redox process was shifted to lower potentials upon the increase of the Se content. Among the various synthesized compositions, (Li$_2$Fe)S$_{0.7}$Se$_{0.3}$O exhibited the best electrochemical performance with a high discharge capacity of ∼245 mA h g$^{−1}$ and an outstanding cycling stability at 0.1C current rate as well as nearly 100% capacity recovery after rate capability tests of 50 cycles. To deeply characterize (Li$_2$Fe)S$_{0.7}$Se$_{0.3}$O, various ex situ and in situ techniques were applied. In contrast to (Li$_2$Fe)SO, the substituted (Li$_2$Fe)S$_{0.7}$Se$_{0.3}$O material remains crystalline without the evolution of secondary phases or superstructures after the first charge/discharge cycle highlighting its enhanced structural stability. Similar to (Li$_2$Fe)SO, both the cation (Fe) and anions (S/Se) from (Li$_2$Fe)S$_{0.7}$Se$_{0.3}$O participate in the redox process., Published by RSC, London ˜[u.a.]œ