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Lithium‐excess cation‐disordered rocksalt materials are a promising class of transition metal‐based cathodes that exhibit high specific capacity and energy density. The exceptional performance is achieved through participation of anionic redox in addition to cationic redox reactions in the electrochemistry. However, anionic redox reactions accompanied by oxygen evolution, accelerated electrolyte breakdown, and structural evolution lead to voltage hysteresis and low initial Coulombic efficiency. Herein, an Al2O3 layer with varying thickness has been coated onto a high‐entropy disordered rocksalt oxyfluoride cathode through atomic layer deposition to enhance battery performance. The results indicate that the utilization of a uniform Al2O3 coating improves the capacity retention and rate capability of the cathode, with the performance being strongly dependent on the layer thickness. Further investigation into cathode–electrolyte interfacial reactions reveals that the thin protecting Al2O3 coating can reduce the decomposition of electrolyte on the cathode surface but cannot prevent bulk phase degradation during prolonged cycling. These findings highlight the need for optimized coating design on the disordered rocksalt cathode to improve battery performance. [ABSTRACT FROM AUTHOR]

Li‐excess cation‐disordered rocksalt oxides can boost the energy density of rechargeable batteries by using anionic redox, but the inferior reversibility of anionic redox hinders its use for practical applications. Herein, a binary system of Li3NbO4–LiMnO2 is targeted and the systematic study on factors affecting electrode reversibility, i.e., percolation probability, electronic conductivity, defect concentrations, and electrolyte solutions, is conducted. A Mn‐rich sample, Li1.1Nb0.1Mn0.8O2, delivers a smaller reversible capacity compared with a Li‐rich sample, Li1.3Nb0.3Mn0.4O2, because of the limitation of ionic migration associated with insufficient percolation probability for disordered oxides. Nevertheless, a larger reversible capacity of Li1.3Nb0.3Mn0.4O2 originates from excessive activation of anionic redox, leading to the degradation of electrode reversibility. The superior performance of Li1.1Nb0.1Mn0.8O2, including Li ion migration kinetics and electronic transport properties, is unlocked by the enrichment of structural defects for nanosized oxides. Moreover, electrode reversibility is further improved by using a highly concentrated electrolyte solution with LiN(SO2F)2 through the surface stabilization on high‐voltage exposure. Superior capacity retention, >100 cycles, is achieved for nanosized Li1.1Nb0.1Mn0.8O2. The electrolyte decomposition and surface stabilization mechanisms are also carefully examined, and it is revealed that the use of highly concentrated electrolyte solution can effectively prevent lattice oxygen being further oxidized and transition metal ion dissolution. [ABSTRACT FROM AUTHOR]

Lithium‐excess cation‐disordered rocksalt materials are a promising class of transition metal‐based cathodes that exhibit high specific capacity and energy density. The exceptional performance is achieved through participation of anionic redox in addition to cationic redox reactions in the electrochemistry. However, anionic redox reactions accompanied by oxygen evolution, accelerated electrolyte breakdown, and structural evolution lead to voltage hysteresis and low initial Coulombic efficiency. Herein, an Al2O3 layer with varying thickness has been coated onto a high‐entropy disordered rocksalt oxyfluoride cathode through atomic layer deposition to enhance battery performance. The results indicate that the utilization of a uniform Al2O3 coating improves the capacity retention and rate capability of the cathode, with the performance being strongly dependent on the layer thickness. Further investigation into cathode–electrolyte interfacial reactions reveals that the thin protecting Al2O3 coating can reduce the decomposition of electrolyte on the cathode surface but cannot prevent bulk phase degradation during prolonged cycling. These findings highlight the need for optimized coating design on the disordered rocksalt cathode to improve battery performance.

Cation‐disordered metal oxides as cathode materials for Li ion batteries have been overlooked from early studies due to to the restriction of Li ion diffusion, leading to poor electrochemical performance. However, the discovery of a new disordered rocksalt (DRX) structured material Li1.211Mo0.467Cr0.3O2 with a high capacity of >260 mAh g−1 at 0.05 C opened new research prospects in this emerging field and established DRX materials as a promising alternative with wider choices of transition metal elements compared with currently widely used layered cathode materials. Some of the major obstacles of the DRX materials include γ‐LiFeO2 type cation short‐range‐order that impedes Li ion diffusion, irreversible oxygen loss, and transition metal dissolution, which also present challenges for appropriate characterization techniques. Several performance optimization strategies have been employed, including fluorine incorporation, high entropy modification, and surface coating. This review article focuses on advancements in characterization techniques to uncover underlying mechanisms of Li ion diffusion and degradation of the DRX cathode materials to address the abovementioned challenges and provide inspiration for future studies of this class of materials. [ABSTRACT FROM AUTHOR]

Cation-disordered rocksalt (DRX) oxides are a promising new class of high-energy-density cathode materials for next-generation Li-ion batteries. However, their capacity fade presents a major challenge. Partial fluorine (F) substitution into the oxygen (O) lattice appears to be an effective strategy for improving the cycling stability, but the underlying atomistic mechanism remains elusive. Here, using a combination of advanced transmission electron microscopy based imaging and spectroscopy techniques, the structural and chemical evolution upon cycling of Mn-based DRX cathodes with an increasing F content (Li-Mn-Nb-O-Fx , x = 0, 0.05, 0.2) are probed. The atomic origin behind the beneficial effect of high-level fluorination for enhancing the surface stability of the DRX is revealed. It is discovered that, due to the reduced O redox activity while with increasing F concentration, F in the DRX lattice mitigates the formation of an O-deficient surface layer upon cycling. For low F-substituted DRX, the O loss near the surface results in the formation of an amorphous cathode-electrolyte interphase layer and nanoscale voids after extended cycling. Increased F concentration in the DRX lattice minimizes both O loss and the interfacial reactions between DRX and the liquid electrolyte, enhancing the surface stability of DRX. These results provide guidance on the development of next-generation cathode materials through anion substitution.

Cation-disordered rocksalt (DRX) materials have emerged as a class of novel high-capacity cathodes for Li-ion batteries. However, the commercialization of DRX cathodes will require reducing their capacity decay, which has been associated with oxygen loss during cycling. Recent studies show that fluorination of DRX cathodes can effectively reduce oxygen loss and improve cycling stability; however, the underlying atomic-scale mechanisms remain elusive. Herein, using a combination of electrochemical measurements, scanning transmission electron microscopy, and electron energy loss spectroscopy, the correlation between the electrochemical properties and structural evolution in Mn-redox-based DRX cathodes, Li1.2Ti0.4–xMn0.4+xO2.0-xFx (x = 0 and 0.2) is examined. It is found that fluorination strongly suppresses structural amorphization and void formation initiated from the particle surface, therefore greatly enhancing the cyclability of the cathode. A novel rocksalt-to-spinel-like structural transformation in the DRX bulk is further revealed, which surprisingly contributes to a gradual capacity increase during cycling. The results provide important insight for the design of novel DRX cathodes with high capacity and long cycle life.

We present a methodology for accessing the cation short-range ordering (SRO) in disordered rocksalt (DRX) materials by decomposing their pair distribution function (PDF) pattern in terms of a set of basis PDFs, pre-determined via Metropolis non-negative matrix factorisation analysis. These basis PDFs, underpinned by structure modelling, point to specific SRO types that subsequently enable identification and quantification of their presence in the DRX structure. Rapid identification of the evolution of SRO addresses a key bottleneck in the analysis and understanding of high energy density characteristics of DRX materials.

A disordered rocksalt Li-excess cathode material, Li 1.25 Nb 0.25 Mn 0.5 O 2 , was synthesized and investigated. It shows a large initial discharge capacity of 287 mAh g − 1 in the first cycle, which is much higher than the theoretical capacity of 146 mAh g − 1 based on the Mn 3+ /Mn 4+ redox reaction. In situ X-ray diffraction (XRD) demonstrates that the compound remains cation-disordered during the first cycle. Electron energy loss spectroscopy (EELS) suggests that Mn and O are likely to both be redox active, resulting in the large reversible capacity. Our results show that Li 1.25 Nb 0.25 Mn 0.5 O 2 is a promising cathode material for high capacity Li-ion batteries and that reversible oxygen redox in the bulk may be a viable way forward to increase the energy density of lithium-ion batteries. [ABSTRACT FROM AUTHOR]

[Display omitted] Cation-disordered Li-excess oxides/oxyfluorides have opened the door for designing alternative, high energy cathodes. However, achieving long cycle life and high rate capability represents a major challenge for disordered rocksalt cathodes (DRX). Herein, we develop Li 2 Mn 3/4 Cr 1/4 O 2 F (LMCOF) DRX materials through a distinct redox mechanochemical method. The LMCOF contains trivalent Cr3+ and Mn3+, which allows for simultaneously accessing stable Mn/Cr dual redox at a narrow voltage window acceptable for conventional carbonate electrolytes. Coupled with the mitigated and reversible oxygen chemical changes, the LMCOF delivers high capacity, rate capability, as well as long cycle life upon extensive 1,000 cycles at various current densities. The multiscale synchrotron and neutron scattering, spectroscopic, and imaging analyses demonstrate that the capacity originates from the Mn/Cr dual redox with minimal contribution from oxygen redox, and that the good cycle life is attributed to the stable global crystal structure and local oxygen chemical environment. [ABSTRACT FROM AUTHOR]