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

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Lithium‐rich layered oxides (LRLOs) exhibit specific capacities above 250 mAh g−1, i.e., higher than any of the commercially employed lithium‐ion‐positive electrode materials. Such high capacities result in high specific energies, meeting the tough requirements for electric vehicle applications. However, LRLOs generally suffer from severe capacity and voltage fading, originating from undesired structural transformations during cycling. Herein, the eco‐friendly, cobalt‐free Li1.2Ni0.2Mn0.6O2 (LRNM), offering a specific energy above 800 Wh kg−1 at 0.1 C, is investigated in combination with a lithium metal anode and a room temperature ionic liquid‐based electrolyte, i.e., lithium bis(trifluoromethanesulfonyl)imide and N‐butyl‐N‐methylpyrrolidinium bis(fluorosulfonyl)imide. As evidenced by electrochemical performance and high‐resolution transmission electron microscopy, X‐ray photoelectron spectroscopy, and online differential electrochemical mass spectrometry characterization, this electrolyte is capable of suppressing the structural transformation of the positive electrode material, resulting in enhanced cycling stability compared to conventional carbonate‐based electrolytes. Practically, the capacity and voltage fading are significantly limited to only 19% and 3% (i.e., lower than 0.2 mV per cycle), respectively, after 500 cycles. Finally, the beneficial effect of the ionic liquid‐based electrolyte is validated in lithium‐ion cells employing LRNM and Li4Ti5O12. These cells achieve a promising capacity retention of 80% after 500 cycles at 1 C.

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