The increased energy density in Li-ion batteries is particularly dependent on the cathode materials that so far have been limiting the overall battery performance. A new class of materials, Li-rich disordered rock salts, has recently been brought forward as promising candidates for next-generation cathodes because of their ability to reversibly cycle more than one Li-ion per transition metal. Several variants of these Li-rich cathode materials have been developed recently and show promising initial capacities, but challenges concerning capacity fade and voltage decay during cycling are yet to be overcome. Mechanisms behind the significant capacity fade of some materials must be understood to allow for the design of new materials in which detrimental reactions can be mitigated. In this study, the origin of the capacity fade in the Li-rich material Li2VO2F is investigated, and it is shown to begin with degradation of the particle surface that spreads inward with continued cycling.

Lithium-rich transition metal disordered rock salt (DRS) oxyfluorides have the potential to lessen one large bottleneck for lithium ion batteries by improving the cathode capacity. However, irreversible reactions at the electrode/electrolyte interface have so far led to fast capacity fading during electrochemical cycling. Here, we report the synthesis of two new Li-rich transition metal oxyfluorides Li 2 V 0.5 Ti 0.5 O 2 F and Li 2 V 0.5 Fe 0.5 O 2 F using the mechanochemical ball milling procedure. Both materials show substantially improved cycling stability compared to Li 2 VO 2 F. Rietveld refinements of synchrotron X-ray diffraction patterns reveal the DRS structure of the materials. Based on density functional theory (DFT) calculations, we demonstrate that substitution of V 3+ with Ti 3+ and Fe 3+ favors disordering of the mixed metastable DRS oxyfluoride phase. Hard X-ray photoelectron spectroscopy shows that the substitution stabilizes the active material electrode particle surface and increases the reversibility of the V 3+ /V 5+ redox couple. This work presents a strategy for stabilization of the DRS structure leading to improved electrochemical cyclability of the materials. † Electronic supplementary information (ESI) available: PXRD pattern of ceramic synthesis attempts; structural parameters of the Rietveld renements; PXRD pattern of Li 2 VO 2 F with Rietveld renement; Williamson-Hall-plots; TEM and EDX analysis; SQS of Li 2 TMO 2 F and Li 2 TM1 0.5 TM2 0.5 O 2 F; ordered structures of Li 2 TM1 0.5 TM2 0.5 O 2 F; table of energy difference between the ordered/decomposed state and disordered state; table of oxidation states of TMs; voltage proles of Li 2 VO 2 F, Li 2 V 0.5 Ti 0.5 O 2 F and Li 2 V 0.5 Fe 0.5 O 2 F half-cells cycled up to 4.1 V; PXRD pattern of cycled electrodes; HAXPES Fe 2p peak tting; HAXPES survey of Li 2 V 0.5 Fe 0.5 O 2 F and Li 2 VO 2 F and uorine plasmon overlaps with the Fe 2p 3/2 peak; core level photoelectron spectra of Fe 2p and Ti 2p; cycling performance of Li 2 VO 2 F, Li 2 V 0.5 Ti 0.5 O 2 F and Li 2 V 0.5 Fe 0.5 O 2 F half-cells cycled up to 4.5 V. See

Pure tysonite La1-xBaxF3-x solid solutions for x < 0.15 were prepared by solid state synthesis in a platinum tube under an azote atmosphere with subsequent quenching for 0.07 ≤x < 0.15. The solid solutions were studied by X-ray, electron and neutron diffractions and by (19)F NMR and impedance spectroscopy. The evolution of the cell parameters obeying Vegard's rule was determined for 0 < x≤ 0.15 and atomic position parameters were accurately refined for x = 0.03, 0.07 and 0.10. The chemical pressure induced by large Ba(2+) cations leads to an increase of the unit cell parameters. Fluorine environment and mobilities are discussed on the basis of the results of neutron diffraction and (19)F solid state NMR. The F1 subnetwork is lacunar; fluorine exchange occurs according to the order: F1-F1 and F1-F2,3. 2D EXSY NMR spectra of La0.97Ba0.03F2.97 reveal, for the first time, a chemical exchange between F2 and F3 sites that requires two successive jumps. The ionic conductivity was evaluated from sintered pellets and different shaping methods were compared. The only structural features which could explain the conductivity maximum are a crossover together with a smaller dispersion of F1-F1,2,3 distances at x = 0.05-0.07.