Sodium ion batteries (NIBs) are one of the versatile technologies for low cost rechargeable batteries. O3-type layered sodium transition metal oxides (NaMO2, M = transition metal ions) are one of the most promising positive electrode materials, capacity-wise. However, the use of O3 phases is limited due to their low redox voltage and associated multiple phase transitions which are detrimental for long cycling. Herein, we proposed a simple strategy to successfully combat these issues. It consists in the introduction of a larger, non-transition metal ion Sn 4+ in NaMO2 to prepare a series of NaNi0.5Mn0.5-ySnyO2 (y=0-0.5) compositions with attractive electrochemical performances, namely for y=0.5, which shows a single phase transition from O3  P3 at the very end of the oxidation process. Na-ion NaNi0.5Sn0.5O2/C coin cells are shown to deliver an average cell voltage of 3.1 V with an excellent capacity retention as compared to an average step-wise voltage of ~2.8 V and limited capacity retention for the pure NaNi0.5Mn0.5O2 phase. This study potentially shows the way to manipulate the O3 NaMO2 for facilitating their practical use in NIBs.

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This study reports on the synthesis and characterization of MAX phases in the (Zr,Ti)AlC system. The MAX phases were synthesized by reactive hot pressing and pressureless sintering in the 1350-1700 °C temperature range. The produced ceramics contained large fractions of 211 and 312 (n = 1, 2) MAX phases, while strong evidence of a 413 (n = 3) stacking was found. Moreover, (Zr,Ti)C, ZrAl, ZrAl, and ZrAl were present as secondary phases. In general, the lattice parameters of the hexagonal 211 and 312 phases followed Vegard's law over the complete Zr-Ti solid solution range, but the 312 phase showed a non-negligible deviation from Vegard's law around the (Zr,Ti)AlC stoichiometry. High-resolution scanning transmission electron microscopy combined with X-ray diffraction demonstrated ordering of the Zr and Ti atoms in the 312 phase, whereby Zr atoms occupied preferentially the central position in the close-packed MX octahedral layers. The same ordering was also observed in 413 stackings present within the 312 phase. The decomposition of the secondary (Zr,Ti)C phase was attributed to the miscibility gap in the ZrC-TiC system.

We demonstrate that changes in the unit cell structure of lithium battery cathode materials during electrochemical cycling in liquid electrolyte can be determined for particles of just a few hundred nanometers in size using in situ transmission electron microscopy (TEM). The atomic coordinates, site occupancies (including lithium occupancy), and cell parameters of the materials can all be reliably quantified. This was achieved using electron diffraction tomography (EDT) in a sealed electrochemical cell with conventional liquid electrolyte (LP30) and LiFePO crystals, which have a well-documented charged structure to use as reference. In situ EDT in a liquid environment cell provides a viable alternative to in situ X-ray and neutron diffraction experiments due to the more local character of TEM, allowing for single crystal diffraction data to be obtained from multiphased powder samples and from submicrometer- to nanometer-sized particles. EDT is the first in situ TEM technique to provide information at the unit cell level in the liquid environment of a commercial TEM electrochemical cell. Its application to a wide range of electrochemical experiments in liquid environment cells and diverse types of crystalline materials can be envisaged.

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Switching between solid solution and two-phase regimes in the cathode materials during lithium (de)insertion : combined PITT, in situ XRPD and electron diffraction tomography study Electrochimica acta -

Layered Li(M,Li)O2 (where M is a transition metal) ordered rock-salt-type structures are used in advanced metal-ion batteries as one of the best hosts for the reversible intercalation of Li ions. Besides the conventional redox reaction involving oxidation/reduction of the M cation upon Li extraction/insertion, creating oxygen-located holes because of the partial oxygen oxidation increases capacity while maintaining the oxidized oxygen species in the lattice through high covalency of the M-O bonding. Typical degradation mechanism of the Li(M,Li)O2 electrodes involves partially irreversible M cation migration toward the Li positions, resulting in gradual capacity/voltage fade. Here, using LiRhO2 as a model system (isostructural and isoelectronic to LiCoO2), for the first time, we demonstrate an intimate coupling between the oxygen redox and M cation migration. A formation of the oxidized oxygen species upon electrochemical Li extraction coincides with transformation of the layered Li1-xRhO2 structure into the γ-MnO2-type rutile-ramsdellite intergrowth LiyRh3O6 structure with rutile-like [1 × 1] channels along with bigger ramsdellite-like [2 × 1] tunnels through massive and concerted Rh migration toward the empty positions in the Li layers. The oxidized oxygen dimers with the O-O distances as short as 2.26 Å are stabilized in this structure via the local Rh-O configuration reminiscent to that in the μ-peroxo-μ-hydroxo Rh complexes. The LiyRh3O6 structure is remarkably stable upon electrochemical cycling illustrating that proper structural implementation of the oxidized oxygen species can open a pathway toward deliberate employment of the anion redox chemistry in high-capacity/high-voltage positive electrodes for metal-ion batteries.

T ransition-metal (TM) cation migration has always been considered detrimental for the performance of cathode (positive electrode) materials for metal-ion batteries. In layered oxides based on the rock-salt structure, the TM cations can migrate from their original octahedral sites toward the empty octahedra in the Li layers upon charge and return upon discharge, as observed in the Li-rich Li 2 Ru 1−y Ti y O 3 phases. 1 This process is only partially reversible and leaves a fraction of the TM cations trapped at the tetrahedral interstices of the close-packed oxygen array, resulting in a gradual voltage fade. 1,2 Migration of the TM cations to the Li positions can also induce irreversible phase transformations (e.g., layered Li 0.5 MO 2 to spinel LiM 2 O 4 ) and negatively affect the electrode kinetics due to blocking the Li diffusion pathway. 3−7 Although some cationdisordered materials can demonstrate facile Li diffusion due to the presence of percolation diffusion channels, 8 the perfect ordering of the TM and alkali cations is generally regarded beneficial for the electrochemical performance. 9,10 Compared to the layered oxides much less is known on the TM-alkali metal disorder in the polyanion cathodes. Olivinestructured LiFePO 4 as the commercially deployed material is, perhaps, most scrutinized in this aspect. Owing to the similar ionic radii of the Li + and Fe 2+ cations in the octahedral oxygen environment (0.76 and 0.78 Å, respectively), 11 antisite Li/Fe disorder is possible corresponding to the Kroger−Vink equation: