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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.

Chemical short-range-order has been widely noticed to dictate the electrochemical properties of Li-excess cation-disordered rocksalt oxides, a class of cathode based on earth abundant elements for next-generation high-energy-density batteries. Existence of short-range-order is normally evidenced by a diffused intensity pattern in reciprocal space, however, derivation of local atomic arrangements of short-range-order in real space is hardly possible. Here, by a combination of aberration-corrected scanning transmission electron microscopy, electron diffraction, and cluster-expansion Monte Carlo simulations, we reveal the short-range-order is a convolution of three basic types: tetrahedron, octahedron, and cube. We discover that short-range-order directly correlates with Li percolation channels, which correspondingly affects Li transport behavior. We further demonstrate that short-range-order can be effectively manipulated by anion doping or post-synthesis thermal treatment, creating new avenues for tailoring the electrochemical properties. Our results provide fundamental insights for decoding the complex relationship between local chemical ordering and properties of crystalline compounds. Derivation of atomic arrangements of short-range-order from diffused intensity pattern in reciprocal space for Li-excess cation-disordered rocksalt cathode remains as a challenge. Here, the authors reveal the short-range-order is a convolution of three basic types: tetrahedron, octahedron, and cube. [ABSTRACT FROM AUTHOR]

The recently developed Li‐excess cation‐disordered rock salts (DRXs) exhibit an excellent chemical diversity for the development of alternative Co/Ni‐free high‐energy cathodes. Herein, the synthesis of a highly fluorinated DRX cathode, Li1.2Mn0.6Ti0.2O1.8F0.2, based on cost‐effective and earth‐abundant transition metals, via a solid‐state reaction, is reported. The fluorinated DRX cathode using ammonium fluoride precursor exhibits more uniform particle size and delivers a specific discharge capacity of 233 mAh g−1 and specific energy of 754 Wh kg−1, with 206 mAh g−1 retained after 200 cycles. The combined synchrotron X‐ray absorption spectroscopy and resonant inelastic X‐ray scattering spectroscopy analysis reveals that the remarkable cycling performance is attributed to the high fluorination and thus enhanced Mn content, enabling the utilization of more Mn redox than the oxide analog. This study demonstrates a great promise to develop next‐generation cost‐effective DRX cathodes with enhanced capacity retention for high‐energy Li‐ion batteries. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Cation‐disordered rocksalts (DRXs) have emerged as a new class of high‐capacity Li‐ion cathode materials. One unique advantage of the DRX chemistry is the structural flexibility that substantially lessens the elemental constraints in the crystal lattice, such as Li content, choice of transition metal redox center paired with appropriate d0 metal, and incorporation of F anion, which allows optimization of the key redox reactions. Herein, a series of the DRX oxyfluorides based on the Mn redox have been designed and synthesized. By tailoring the stoichiometry of the DRX compositions, high‐capacity cycling by promoting the cationic Mn2+/Mn4+ redox reactions while suppressing those from anionic O is successfully demonstrated. A highly fluorinated DRX compound, Li1.2Mn0.625Nb0.175O1.325F0.675 (M0.625F0.675), delivers a capacity of ≈170 mAh g−1 at C/3 for 100 cycles. This work showcases the concept of balancing the cationic and anionic redox reactions in the DRX cathodes for improved electrochemical performance through the rational composition design. [ABSTRACT FROM AUTHOR]

The demand for high energy‐density, mass‐producible cathode materials has spurred the exploration of new material structures and compositions. Lithium‐excess, cation‐disordered rocksalt (DRX) materials are a new class of transition metal oxides that display high capacity and environmental friendly composition. These materials achieve their high capacities partially through oxygen redox, which leads to oxygen loss and detrimental reactivity with the electrolyte. It has previously been shown that oxygen loss can be suppressed by partial substitution of the lattice oxygen for fluorine, but the explicit mechanism behind this effect remains unknown. In this work, differential electrochemical mass spectrometry (DEMS) and titration mass spectrometry are used to quantify the primary electrochemical reactions occurring during the first cycle in DRX materials. Comparing a DRX oxide and a DRX oxyfluoride, it is shown that fluorination limits oxygen redox and suppresses oxygen loss. Additionally, DEMS is coupled with fluoride‐scavenging to demonstrate that small amounts of fluorine dissolve from DRX oxyfluorides during the first cycle. Finally, these techniques are extended over the first several cycles, demonstrating that CO2 evolution persists and fluoride dissolution continues to a diminishing extent during the first few cycles. These findings motivate surface modifications to control interfacial reactivity and improve long‐term cycling. [ABSTRACT FROM AUTHOR]

An electrochemically active nanostructured silver copper fluoride (SCF) perovskite, AgCuF 3 , was synthesized via a mechanochemical reaction between AgF and CuF 2 precursors. Phase composition and electrochemical properties of the SCF perovskites produced under various synthetic parameters were studied. The optimum SCF perovskite sample exhibited an appreciable electrochemical performance through the use of conductive carbon matrix in a primary lithium half cell. A high specific capacity of 270 mAh g −1 was achieved at a cutoff voltage of 2 V with 190 mAh g −1 above 3 V, leading to a total volumetric energy density of 3666 Wh L −1 at >3 V and 4848 Wh L −1 at >2 V. [ABSTRACT FROM AUTHOR]

Extensive efforts have been devoted to unraveling the true cause of voltage decay in Li, Mn‐rich layered oxides. An initial consensus was reached on structural rearrangement, then leaned toward the newly discovered lattice oxygen activity. It is challenging to differentiate their explicit roles because these events typically coexist during the electrochemical reaction of most Li‐rich layered oxides. Here, the voltage decay behavior is probed in Li1.2Ni0.2Ru0.6O2, a structurally and electrochemically relevant compound to Li, Mn‐rich layered oxide, but of no oxygen activity. Such intriguing characteristics allow the explicit decoupling of the contribution of transition metal migration and lattice oxygen activity to voltage decay in Li‐rich layered oxides. The results demonstrate that the microstructural evolution, mainly originating from transition metal migration, is a direct cause of voltage decay, and lattice oxygen activity likely accelerates the decay. [ABSTRACT FROM AUTHOR]

Despite being one of the most promising cathode materials for lithium-ion batteries, LiNi 1/3 Co 1/3 Mn 1/3 O 2 has been plagued by its low electronic conductivity and Li + /Ni 2+ cation mixing, which lead to poor rate capacity and cycle stability. In this paper, hollow peanut-like hierarchical mesoporous LiNi 1/3 Co 1/3 Mn 1/3 O 2 has been synthesized by a simple self-template solid-state method, and the influence of calcination times on the electrochemical performances was also investigated. The results suggest that L2 sample calcined at 850 °C for 10 h shows the highest rate capability and superior cycle stability as compared to L1 sample calcined for 8 h and L3 sample calcined for 12 h. The initial discharge capacity of L2 is 155.1 mAh g −1 , and maintains 107.8 mAh g −1 after 500 cycles at 1 C. Surprisingly, its initial discharge capacity is 144.7 mAh g −1 even when cycled at 10 C, and still retains 97 mAh g −1 after 300 cycles. The significant performance improvement of L2 is ascribed to the hollow peanut-like hierarchical mesoporous structure, which can not only buffer the volume changes during the repeated charge/discharge processes, but also shorten the ion transport distance. [ABSTRACT FROM AUTHOR]

LiNi 1/3 Co 1/3 Mn 1/3 O 2 cathode coated with a thin layer of graphene (~8 nm) is successfully synthesized by self-assembly and pyrolysis of polyelectrolyte layers on the surface of NMC particles. The graphene coated NMCs still possess a layered structure with good crystallinity and demonstrate a superior electrochemical performance (e.g., rate capability and cycling stability). The best graphene coated NMC cathode is prepared at a calcination temperature of 800 °C, exhibiting a capacity retention of ~90% vs . 78% for pristine NMC @ cycle 100 and 1 C rate. The improvement in cycling performance is further enlarged after 500 cycles (74% vs . 51%). This can be attributed to the dual functions of graphene coating in enhancing electronic conductivity and protecting NMC surface from the contact with electrolyte during the electrochemical reaction. [ABSTRACT FROM AUTHOR]

One method of increasing the energy density of Li-ion batteries is to access reversible Li intercalation in conventional layered transition metal oxide cathode materials at high potentials (4.3–5 V vs. Li/Li+), and thus allow more electrochemical capacity per volume of active material. This comes at the cost of increased interfacial reactivity and often results in capacity fade over many cycles. Tracing gas evolution during electrochemical lithium extraction and insertion provides a useful strategy to understand this high voltage reactivity. In this study, we examine outgassing during Li extraction in well-known layered oxides (LiCoO 2 , LCO; LiNiO 2 , LNO; and Li 2 MnO 3 , LMO). We highlight key differences in the outgassing of each material. Whereas negligible O 2 release is found in LCO and LNO, even with voltage holds at 5 V vs. Li/Li+, O 2 release is found to account for a large portion of the electrochemical capacity and persists over many cycles in LMO. CO 2 outgassing is observed to varying extents, following the trend of LMO ≈ LNO > LCO, and is due to a combination of residual solid carbonate oxidation and electrolyte degradation. Taken together, these results show the importance of quantitative analysis in understanding the role of transition metal composition on the chemistry of conventional Li-ion battery materials. [ABSTRACT FROM AUTHOR]

• La 0.7 Ca 0.3 MnO 3 films sputtered from He-treated target surface. • He+ ion penetration in the cathode target is the crucial step for the He treatment. • He treatment increased the films' transition temperature and c-axis lattice parameter. • Direct film exposition to He plasma was more effective than He target treatment. We previously reported that out-of-plane lattice of in-plane tensile strained (001) La 0.7 Ca 0.3 MnO 3 thin films can be stretched by He treatment of the films via adding helium gas in an Ar/O 2 mixed sputtering atmosphere. To gain more insight into the He treatment process, in this study, we carried out He plasma treatment on a target and He atom adsorption on a substrate surface respectively before/between sputtering deposition of La 0.7 Ca 0.3 MnO 3 films. The films deposited after the target pre-treatment show remarkable changes in the out-of-plane lattice and magnetotransport properties, while the films deposited through the latter method changes only slightly, suggesting that the He+ ion penetration in the cathode target plays a key role in the He treatment of the manganite films. Furthermore, we placed the as-grown films at the cathode position and exposed them directly to He discharge plasma. After the He plasma post-treatment, the out-of-plane lattice parameter increases from 3.823 to 3.851Å, accompanied by an increase in the metal-insulator transition temperature from 260 to 290 K. The results demonstrate that the He plasma treatment is a simple strategy for the single-axis control of epitaxial thin films. [ABSTRACT FROM AUTHOR]