There is an urgent need for low-cost, resource-friendly, high-energy-density cathode materials for lithium-ion batteries to satisfy the rapidly increasing need for electrical energy storage. To replace the nickel and cobalt, which are limited resources and are associated with safety problems, in current lithium-ion batteries, high-capacity cathodes based on manganese would be particularly desirable owing to the low cost and high abundance of the metal, and the intrinsic stability of the Mn oxidation state. Here we present a strategy of combining high-valent cations and the partial substitution of fluorine for oxygen in a disordered-rocksalt structure to incorporate the reversible Mn/Mn double redox couple into lithium-excess cathode materials. The lithium-rich cathodes thus produced have high capacity and energy density. The use of the Mn/Mn redox reduces oxygen redox activity, thereby stabilizing the materials, and opens up new opportunities for the design of high-performance manganese-rich cathodes for advanced lithium-ion batteries.

Cation-disordered rocksalt oxides and oxyfluorides are promising high energy density lithium-ion cathodes, yet require a detailed understanding of the impact of disorder and short-range order on the structural and electrochemical properties.

Sodium transition metal oxides (NaMO 2 ) with a P2 structure exhibit good Na + ion conductivity and are promising sodium-ion battery cathode materials. Manganese-based compounds have a high working potential vs. Na + /Na, and high capacity. Yet, the layered nature of these materials means that they are prone to structural rearrangements at high voltage/low Na contents, the phase transformations and Na + ion/vacancy ordering transitions resulting in capacity fade and poor reversibility. This review discusses the latest advances in the field and focuses mainly on recent work on Na y Mn 1-x M x O 2 (x, y ≤ 1, M = Ni, Mg, Li) compounds. We compare the different lithium and sodium transition metal layered oxides (P2, O3, etc.) and describe the structures and mechanisms observed on alkali (de)intercalation. The strategies used to enhance the electrochemical properties and stabilize the structural framework of sodium transition metal oxides are reviewed. We show how X-ray diffraction and 7 Li/ 23 Na solid-state Nuclear Magnetic Resonance can be combined to provide a detailed insight into the structural and electronic processes occurring upon electrochemical cycling of these materials. Why Are We Interested in Na-Ion Battery Cathodes?Together, the increasing demand for energy and the threat from Global Warming make electrical energy storage (EES) a world wide strategic priority. EES is expected to play a key role in the decarbonization of electric power sources. The two billion people worldwide not currently served by a reliable electricity supply are likely to be connected via local grids, for which EES is essential. While Li-ion batteries (LIBs) will play a role, a key challenge is to develop lower cost batteries that deliver safe, reliable storage with high cycle and calendar life. In this regard, Na-ion batteries (NIBs) are potentially important. NIBs operate in a similar fashion to LIBs, offering both advantages and disadvantages. Concerning the former, the ability to use Al instead of Cu as a current collector at the anode could substantially reduce cost. Na is also far more abundant in the Earth's crust than Li, and more widely distributed geographically. Regarding the disadvantages, the standard operating potential of Na metal is 300 mV more positive than Li metal. This generally translates into lower operating potentials for Na-ion systems, although the potential of a full cell depends on the difference in the chemical potentials of Na in the anode and cathode materials. There are considerable opportunities for scientists to develop new combinations of anode, electrolyte and cathode that are optimized for a variety of applications with different criteria such as high energy density, high power, and high operating potential. This has encouraged a rapid growth of interest in research into NIB components. Hard carbons show some promise as anodes, [1][2][3] but more must be done to improve performance. The cathode remains a major challenge and it is to this that we direct the current review. A comparative plot sho...

Large-scale electric energy storage is fundamental to the use of renewable energy. Recently, research and development efforts on room-temperature sodium-ion batteries (NIBs) have been revitalized, as NIBs are considered promising, low-cost alternatives to the current Li-ion battery technology for large-scale applications. Herein, we introduce a novel layered oxide cathode material, NaNiMnO. This new compound provides a high reversible capacity of 138 mAh g and an average potential of 3.25 V vs Na/Na with a single smooth voltage profile. Its remarkable rate and cycling performances are attributed to the elimination of the P2-O2 phase transition upon cycling to 4.5 V. The first charge process yields an abnormally excess capacity, which has yet to be observed in other P2 layered oxides. Metal K-edge XANES results show that the major charge compensation at the metal site during Na-ion deintercalation is achieved via the oxidation of nickel (Ni) ions, whereas, to a large extent, manganese (Mn) ions remain in their Mn state. Interestingly, electron energy loss spectroscopy (EELS) and soft X-ray absorption spectroscopy (sXAS) results reveal differences in electronic structures in the bulk and at the surface of electrochemically cycled particles. At the surface, transition metal ions (TM ions) are in a lower valence state than in the bulk, and the O K-edge prepeak disappears. On the basis of previous reports on related Li-excess LIB cathodes, it is proposed that part of the charge compensation mechanism during the first cycle takes place at the lattice oxygen site, resulting in a surface to bulk transition metal gradient. We believe that by optimizing and controlling oxygen activity, Na layered oxide materials with higher capacities can be designed.

Cation-disordered rocksalt-type high-entropy cathodes for Li-ion batteries.

Li-substituted layered P2–Na0.80[Li0.12Ni0.22Mn0.66]O2 is investigated as an advanced cathode material for Na-ion batteries. Both neutron diffraction and nuclear magnetic resonance (NMR) spectroscopy are used to elucidate the local structure, and they reveal that most of the Li ions are located in transition metal (TM) sites, preferably surrounded by Mn ions. To characterize structural changes occurring upon electrochemical cycling, in situ synchrotron X-ray diffraction is conducted. It is clearly demonstrated that no significant phase transformation is observed up to 4.4 V charge for this material, unlike Li-free P2-type Na cathodes. The presence of monovalent Li ions in the TM layers allows more Na ions to reside in the prismatic sites, stabilizing the overall charge balance of the compound. Consequently, more Na ions remain in the compound upon charge, the P2 structure is retained in the high voltage region, and the phase transformation is delayed. Ex situ NMR is conducted on samples at different states of charge/discharge to track Li-ion site occupation changes. Surprisingly, Li is found to be mobile, some Li ions migrate from the TM layer to the Na layer at high voltage, and yet this process is highly reversible. Novel design principles for Na cathode materials are proposed on the basis of an atomistic level understanding of the underlying electrochemical processes. These principles enable us to devise an optimized, high capacity, and structurally stable compound as a potential cathode material for high-energy Na-ion batteries.

There is much interest in Na-ion batteries for grid storage because of the lower projected cost compared with Li-ion. Identifying Earth-abundant, low-cost, and safe materials that can function as intercalation cathodes in Na-ion batteries is an important challenge facing the field. Here we investigate such a material, β-NaMnO2, with a different structure from that of NaMnO2 polymorphs and other compounds studied extensively in the past. It exhibits a high capacity (of ca. 190 mA h g(-1) at a rate of C/20), along with a good rate capability (142 mA h g(-1) at a rate of 2C) and a good capacity retention (100 mA h g(-1)after 100 Na extraction/insertion cycles at a rate of 2C). Powder XRD, HRTEM, and (23)Na NMR studies revealed that this compound exhibits a complex structure consisting of intergrown regions of α-NaMnO2 and β-NaMnO2 domains. The collapse of the long-range structure at low Na content is expected to compromise the reversibility of the Na extraction and insertion processes occurring upon charge and discharge of the cathode material, respectively. Yet stable, reproducible, and reversible Na intercalation is observed.

Recent progress in the understanding of percolation theory points to cation-disordered lithium-excess transition metal oxides as high-capacity lithium-ion cathode materials. Nevertheless, the oxygen redox processes required for these materials to deliver high capacity can trigger oxygen loss, which leads to the formation of resistive surface layers on the cathode particles. We demonstrate here that, somewhat surprisingly, fluorine can be incorporated into the bulk of disordered lithium nickel titanium molybdenum oxides using a standard solid-state method to increase the nickel content, and that this compositional modification is very effective in reducing oxygen loss, improving energy density, average voltage, and rate performance. We argue that the valence reduction on the anion site, offered by fluorine incorporation, opens up significant opportunities for the design of high-capacity cation-disordered cathode materials.