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.

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.

research on lithium-ion battery cathodes has been largely dominated by layered rock salt materials in the Li x (Ni-Mn-Co-Al) 2−x O 2 (NMCA) compositional space, [3,4] in which redox activity is limited to Co and Ni. Cobalt in particular is expensive and relatively scarce compared to other 3d transition metals, such as Fe or Mn. [1,3,5] The fact that the cathode structure has to be layered and remain layered upon cycling greatly restricts the changes which can be made to NMCA-type rock salt chemistries.Recent progress in the development of Li percolation theory for rock salt compounds, in which Li transport still takes place even when the cations are disordered, has greatly enlarged the design space for cathode materials. [6,7] Lifting the requirement that cations form an ordered (layered) structure enables the use of various transition metal (TM) redox centers, including Mn 3+ /Mn 4+ , [8,9] Mn 2+ /Mn 4+ , [5,10] Cr 3+ /Cr 5+ , [6,11] Mo 3+ /Mo 6+ , [12] and V 3+ /V 5+ . [11,13] Because these compounds need Li excess to achieve Li percolation, [6,7] they typically also contain high valent charge compensators, such as Nb 5+ , [8,9] Sb 5+ , [14] Mo 6+ , [15,16] and Ti 4+ . [16][17][18] In addition, fluorine substitution is facile inThe recent discovery of Li-excess cation-disordered rock salt cathodes has greatly enlarged the design space of Li-ion cathode materials. Evidence of facile lattice fluorine substitution for oxygen has further provided an important strategy to enhance the cycling performance of this class of materials. Here, a group of Mn 3+ -Nb 5+ -based cation-disordered oxyfluorides, Li 1.2 Mn 3+ 0.6+0.5x Nb 5+ 0.2−0.5x O 2−x F x (x = 0, 0.05, 0.1, 0.15, 0.2) is investigated and it is found that fluorination improves capacity retention in a very significant way. Combining spectroscopic methods and ab initio calculations, it is demonstrated that the increased transition-metal redox (Mn 3+ /Mn 4+ ) capacity that can be accommodated upon fluorination reduces reliance on oxygen redox and leads to less oxygen loss, as evidenced by differential electrochemical mass spectroscopy measurements. Furthermore, it is found that fluorine substitution also decreases the Mn 3+ -induced Jahn-Teller distortion, leading to an orbital rearrangement that further increases the contribution of Mn-redox capacity to the overall capacity.

The discovery of facile Li transport in disordered, Li-excess rocksalt materials has opened a vast new chemical space for the development of high energy density, low cost Li-ion cathodes. We develop a strategy for obtaining optimized compositions within this class of materials, exhibiting high capacity and energy density as well as good reversibility, by using a combination of low-valence transition metal redox and a high-valence redox active charge compensator, as well as fluorine substitution for oxygen.Furthermore, we identify a new constraint on high-performance compositions by demonstrating the necessity of excess Li capacity as a means of counteracting high-voltage tetrahedral Li formation, Li-binding by fluorine and the associated irreversibility. Specifically, we demonstrate that 10-12% of Li capacity is lost due to tetrahedral Li formation, and 0.4-0.8 Li per F dopant is made inaccessible at moderate voltages due to Li-F binding. We demonstrate the success of this strategy by realizing a series of high-performance disordered oxyfluoride cathode materials based on Mn 2+/4+ and V 4+/5+ redox. Broader contextElectrochemical energy storage is a key component of modern energy systems, providing portable power to devices ranging from personal electronics to electric vehicles, and enabling grid-scale mitigation of the fluctuating availability of renewable energy sources. The central role of energy storage systems motivates the search for, and optimization of, low-cost, environmentally-benign materials which can reversibly provide high energy density. Cathode materials, which are presently the performance-limiting components in state-of-the-art Li-ion batteries, have been traditionally limited to Ni and Co-based layered oxides. The recent discovery of Li-percolation in disordered rocksalts has expanded the structural space of materials which may serve as a Li-ion electrode, while the demonstration of Mn 2+/4+ cathode electrochemistry and disordered rocksalt fluorination has opened to door to the use of cheap, environmentally-friendly chemistries. Here, we build on these demonstrations to derive optimization rules for designing disordered rocksalt oxyfluoride cathodes and provide an example of an optimized series of cathode materials.

2 Mn-based Li-excess cation-disordered rocksalt (DRX) oxyfluorides are promising candidates for 3 next-generation rechargeable battery cathodes owing to their large energy densities, earth-4 abundance of Mn and potential for low cost. In this work, we synthesized and electrochemically 5 tested four representative compositions in the Li-Mn-OF DRX chemical space with various Li 6 and F content. 7 material with high Li-excess (1.3333 per formula unit, Li x Mn2x O2y F y) and moderate fluorination 8 9 Higher fluorination (0.6667 per formula unit) at moderate Li-excess (1.25 per formula unit) can Wh/kg) initial capacity (specific energy) with more than 85% retained after 30 cycles. We show that the Li-site distribution (i.e., Li percolation properties) plays a more important role than the metal-redox capacity in determining the initial capacity, whereas the metal-redox capacity is more closely related to the cyclability of the materials. We apply these insights and generate a capacity map of the Li-Mn-OF chemical space, Li x Mn2x O2y F y (1.167 ≤ x ≤ 1.333, 0 ≤ y ≤ 0.667), which predicts both the accessible Li capacity and Mn-redox capacity. This map allows to design compounds which balance high capacity with good cyclability. activate Mn 2+ /Mn 4+ redox and there by balance capacity with cycle life, achieving 256 mAh/g (822 (0.3333 per formula unit) achieves 349 mAh g-1 initial capacity and 1068 Wh kg-1 specific energy. While all compositions tested achieve higher than 200 mAh g-1 initial capacity, the

K-ion batteries are promising alternative energy storage systems for largescale applications because of the globally abundant K reserves. K-ion batteries benefit from the lower standard redox potential of K/K + than that of Na/Na + and even Li/Li + , which can translate into a higher working voltage. Stable KC 8 can also be formed via K intercalation into a graphite anode, which contrasts with the thermodynamically unfavorable Na intercalation into graphite, making graphite a readily available anode for K-ion battery technology. However, to construct practical rocking-chair K-ion batteries, an appropriate cathode material that can accommodate reversible K release and storage is still needed. We show that stoichiometric KCrO 2 with a layered O3-type structure can function as a cathode for K-ion batteries and demonstrate a practical rocking-chair K-ion battery. In situ X-ray diffraction and electrochemical titration demonstrate that K x CrO 2 is stable for a wide K content, allowing for topotactic K extraction and reinsertion. We further explain why stoichiometric KCrO 2 is unique in forming the layered structure unlike other stoichiometric K-transition metal oxide compounds, which form nonlayered structures; this fundamental understanding provides insight for the future design of other layered cathodes for K-ion batteries.

Recent reports on high capacities delivered by Li-excess transition-metal oxide cathodes have triggered intense interest in utilizing reversible oxygen redox for high-energy battery applications. To control oxygen electrochemical activities, fundamental understanding of redox chemistry is essential yet has so far proven challenging. In the present study, micrometer-sized Li 1.3 Nb 0.3 Mn 0.4 O 2 single crystals were synthesized for the first time and used as a platform to understand the charge compensation mechanism during Li extraction and insertion. We explicitly demonstrate that the oxidation of O 2− to O n− (0 < n < 2) and O 2 loss from the lattice dominates at 4.5 and 4.7 V, respectively. While both processes occur in the first cycle, only the redox of O 2− /O n− participates in the following cycles. The lattice anion redox process triggers irreversible changes in Mn redox, which likely causes the voltage and capacity fade observed on this oxide. Two drastically different redox activity regions, a single-phase behavior involving only Mn 3+/4+ and a two-phase behavior involving O 2− /O n− (0 ≤ n < 2), were found in Li x Nb 0.3 Mn 0.4 O 2 (0 < x < 1.3). Morphological damage with particle cracking and fracturing was broadly observed when O redox is active, revealing additional challenges in utilizing O redox for high-energy cathode development. Recently, approaches to enable high-energy cathodes by utilizing redox reactions of both TM cations and oxygen anions have triggered intense interest. 5−7 One of the most studied examples is the lithium and manganese-rich (LMR) layered 49 oxides with a general formula of Li 1+x Mn 1−x−y−z Ni y Co z O 2. 8−10 50 Our recent work showed that, contrary to the common notion 51 of a nanocomposite structure, the oxide has a single monoclinic 52 phase (C2/m) with a large number of domains corresponding 53 to different variants. 11 To involve the O 2p electrons in the 54 following electrochemical reactions, the material typically 55 undergoes an initial activation process signaled by a unique 56 charging voltage profile that is much different from those of the 57 subsequent cycles. Recent studies by Luo et al. suggested the 58 formation of O − holes in the intermediates, as evidenced by the 59 progressive growth of a new peak on the O K-edge X-ray 60 absorption spectroscopy (XAS) along with the use of a number 61 of other characterization techniques, including isotopically 62 labeled differential electrochemical mass spectroscopy 63 (DEMS), X-ray absorption near edge structure (XANES), 64 and resonant inelastic X-ray scattering (RIXS). 9 However, this 65 remains controversial as experimental evidence is difficult to

Though Li 2 MnO 3 was originally considered to be electrochemically inert, its observed activation has spawned a new class of Li-rich layered compounds that deliver capacities beyond the traditional transition-metal redox limit. Despite progress in our understanding of oxygen redox in Li-rich compounds, the underlying origin of the initial charge capacity of Li 2 MnO 3 remains hotly contested. To resolve this issue, we review all possible charge compensation mechanisms including bulk oxygen redox, oxidation of Mn 4+ , and surface degradation for Li 2 MnO 3 cathodes displaying capacities exceeding 350 mAh g −1 . Using elemental and orbital selective X-ray spectroscopy techniques, we rule out oxidation of Mn 4+ and bulk oxygen redox during activation of Li 2 MnO 3 . Quantitative gas-evolution and titration studies reveal that O 2 and CO 2 release accounted for a large fraction of the observed capacity during activation with minor contributions from reduced Mn species on the surface. These studies reveal that, although Li 2 MnO 3 is considered critical for promoting bulk anionic redox in Li-rich layered oxides, Li 2 MnO 3 by itself does not exhibit bulk oxygen redox or manganese oxidation beyond its initial Mn 4+ valence.