Synchrotron-based operando X-ray diffraction and X-ray absorption spectroscopy study of LiCo0.5Fe0.5PO4 mixed d-metal olivine cathode.

Lithium-ion batteries based on high-voltage cathode materials, such as LiCoPO4, despite being promising in terms of specific power, still suffer from poor cycle life due to the lower stability of common non-aqueous electrolytes at higher voltages. One way to overcome this issue might be decreasing the working potential of the battery by doping LiCoPO4 by Fe, thus reducing electrolyte degradation upon cycling. However, such modification requires a deep understanding of the structural behavior of cathode material upon lithiation/delithiation. Here we used a combination of operando synchrotron-based XRD and XAS to investigate the dynamics of d-metal local atomic structure and charge state upon cycling of LiCo0.5Fe0.5PO4 mixed d-metal olivine cathode material. Principal components analysis (PCA) of XAS data allowed the extraction of spectra of individual phases in the material and their concentrations. For both Co and Fe two components were extracted, they correspond to fully lithiated and delithiated phases of LixMPO4 (where M = Fe, Co). Thus, we were able to track the phase transitions in the material upon charge and discharge and quantitatively analyze the M2+/M3+ electrochemical conversion rate for both Fe and Co. Rietveld's refinement of XRD data allowed us to analyze the changes in the lattice of cathode material and their reversibility upon (de)lithiation during cycling. The calculation of DFT and Bader charge analysis expects the oxygen redox procedure combined with d-metals redox, which supplements iron charge variations and dominates at high voltages when x < 0.75 in LixCoFePO4.

Effect of transition metal cations on the local structure and lithium transport in disordered rock-salt oxides.

Lithium-excess oxides Li1.2Ti0.4Mn0.4O2 and Li1.3Nb0.3Mn0.4O2 with a disordered rock-salt structure and Mn3+/Mn4+ as a redox couple were compared to analyze the effect of different d0 metal ions on the local structure and Li+ ion migration. These cathode materials were obtained by mechanochemically assisted solid-state synthesis. Using XRD, 7Li NMR and EPR spectroscopy and transmission electron microscopy it was shown that the Mn ions are prone to form clusters, while d0 metal ions are evenly distributed in the crystal lattice. The presence of Nb5+ ions contributes to the formation of noticeably larger Mn clusters and larger gaps in the Li+ migration maps as compared to Ti4+. These results were confirmed by the geometrical-topological method, BVSE simulation and DFT calculations, and are in good agreement with the Li diffusion coefficient determined by GITT, which is 1.5 orders of magnitude higher in Li1.2Ti0.4Mn0.4O2 than that in Li1.3Nb0.3Mn0.4O2.

Crystal structure and migration paths of alkaline ions in NaVPO4F.

A sample with a nominal composition 'NaVPO4F' is prepared by mechanochemically assisted solid-state synthesis using quenching. A detailed study of its crystal and local structure is conducted by means of XRD and FTIR and solid-state 31P NMR spectroscopies in comparison with Na3V2(PO4)2F3. It is shown that the as-prepared 'NaVPO4F' has a multiphase composition, including NaVPO4F as the main phase and Na3V2(PO4)2F3 and Na2.57V4P4O17F as the side products. The crystal structure of NaVPO4F is described in the monoclinic C2/c space group. It is characterized by negligible V3+/V4+ oxidation with the corresponding F-/O2- substitution and the presence of structural disordering. Using the Voronoi-Dirichlet partition (VDP) method, the Na+ and Li+ migration pathways in Tavorite-like NaVPO4F and closely related LiVPO4F (with the triclinic structure, P1[combining macron] S.G.) are analyzed. While the Na+ migration is suppressed in both cases, the Na+/Li+ ion exchange in NaVPO4F with the formation of monoclinic LiVPO4F could occur, but is difficult due to the sodium immobility rather than the instability of the lithium derivatives as was concluded from the DFT calculations.

A new active Li-Mn-O compound for high energy density Li-ion batteries.

The search for new materials that could improve the energy density of Li-ion batteries is one of today's most challenging issues. Many families of transition metal oxides as well as transition metal polyanionic frameworks have been proposed during the past twenty years. Among them, manganese oxides, such as the LiMn2O4 spinel or the overlithiated oxide Li[Li1/3Mn2/3]O2, have been intensively studied owing to the low toxicity of manganese-based materials and the high redox potential of the Mn(3+)/Mn(4+) couple. In this work, we report on a new electrochemically active compound with the 'Li4Mn2O5' composition, prepared by direct mechanochemical synthesis at room temperature. This rock-salt-type nanostructured material shows a discharge capacity of 355 mAh g(-1), which is the highest yet reported among the known lithium manganese oxide electrode materials. According to the magnetic measurements, this exceptional capacity results from the electrochemical activity of the Mn(3+)/Mn(4+) and O(2-)/O(-) redox couples, and, importantly, of the Mn(4+)/Mn(5+) couple also.