Academic literature on the topic 'Disordered Rocksalt'

The development of next-generation nickel and cobalt-free Li-ion batteries with significantly greater energy density for electric vehicles is largely contingent on the discovery of new cathode materials. A number of novel candidates with a disordered rocksalt crystal structure have recently been reported to exhibit high capacities, offering a highly promising new avenue for cathode research.1–5 However, disordered rocksalt cathode materials generally suffer from voltage and capacity fade over cycling. For example, the Mn-based archetypal oxyfluoride Li2MnO2F shows 254 mAh g-1 discharge capacity in the first cycle but fades to 104 mAh g-1 after 100 cycles. For improving these materials leading to practical devices, it is vital to develop an understanding of the fading mechanisms. Various explanations have been proposed for the gradual deterioration in performance of disordered rocksalt oxide and oxyfluoride cathodes. These include O-loss and surface densification4,6, growth of a CEI layer7, phase segregation6,8. In this study, we examine the main degradation mechanisms in Li2MnO2F over cycling and demonstrate that the capacity and voltage retention can be improved through compositional control, figure below. This understanding points the way to manganese oxyfluorides with better cycling performance. House, R. A. et al. Lithium manganese oxyfluoride as a new cathode material exhibiting oxygen redox. Energy Environ. Sci. 11, 926–932 (2018). Lee, J. et al. Reversible Mn2+/Mn4+ double redox in lithium-excess cathode materials. Nature 556, 185–190 (2018). Yabuuchi, N. et al. Origin of stabilization and destabilization in solid-state redox reaction of oxide ions for lithium-ion batteries. Nat. Commun. 7, 1–10 (2016). Li, L. et al. Fluorination‐Enhanced Surface Stability of Cation‐Disordered Rocksalt Cathodes for Li‐Ion Batteries. Adv. Funct. Mater. 2101888, 2101888 (2021). Chen, R. et al. Disordered lithium-rich oxyfluoride as a stable host for enhanced Li+ intercalation storage. Adv. Energy Mater. 5, (2015). Chen, D., Kan, W. H. & Chen, G. Understanding Performance Degradation in Cation-Disordered Rock-Salt Oxide Cathodes. Adv. Energy Mater. 9, 1–15 (2019). Källquist, I. et al. Degradation Mechanisms in Li2VO2F Li-Rich Disordered Rock-Salt Cathodes. Chem. Mater. 31, 6084–6096 (2019). Chen, D., Ahn, J., Self, E., Nanda, J. & Chen, G. Understanding cation-disordered rocksalt oxyfluoride cathodes. J. Mater. Chem. A 2, 7826–7837 (2021). Figure 1

A “concerted-densification” based failure mechanism, involving atomic-level changes in both transition-metal cationic sublattice and oxygen/fluorine anionic sublattice, is proposed for the degradation of F-DRX cathode materials.

In recent years, cation-disordered Li-excess rocksalts (DRX) have emerged as a promising new class of high-energy cathode materials for lithium-ion batteries. [1] Aside from the desirable Co-free chemistry, these compounds offer exceptionally large charge storage capacities by utilizing the redox reactions of both cationic transition-metals and anionic oxygen in the lattice. While early research focused on DRX oxides, which met with significant challenges in voltage stability and capacity retention upon cycling [2-3], recent studies shifted towards oxyfluorides with a substantial level of F substitution. It was found that incorporating F into the anionic sublattice can reduce oxygen gas release, impedance rise and capacity fade, consequently improving cathode cycling stability. [4-5] To this end, developing synthesis methods to incorporate large F content in the lattice as well as designing and optimizing oxyfluoride chemistry for both high energy density and cycling stability are imperative. While high F substitution levels (up to 30-40 at.%) in DRX have been achieved through mechanochemical synthesis, the method has limitations in industrial application due to poor scalability. Solid-state synthesis, on the other hand, are readily scalable and often offers drop-in replacement in materials processing. In this presentation, we show our recent effort in developing calcination-based fluorination approach to achieve high-level fluorination of Mn-redox-active DRX materials. [6] The unique behavior of capacity rise upon cycling of a new class of Mn-rich DRX oxyfluoride cathodes will be reported. Our understanding in how chemistry can impact local and long-range structures and their evolution during electrochemical cycling will also be presented, as well as perspectives on future directions in DRX development. References Lee, J.; Urban, A.; Li, X.; Su, D.; Hautier, G.; Ceder, G. Unlocking the Potential of Cation-Disordered Oxides for Rechargeable Lithium Batteries. Science 2014, 343, 519. Yabuuchi, N.; Takeuchi, M.; Nakayama, M.; Shiiba, H.; Ogawa, M.; Nakayama, K.; Ohta, T.; Endo, D.; Ozaki, T.; Inamasu, T.; Sato, K.; Komaba, S., High-Capacity Electrode Materials for Rechargeable Lithium Batteries: Li3NbO4-based System with Cation-Disordered Rocksalt Structure. Natl. Acad. Sci. 2015, 112, 7650. Chen, D.; Kan, W. H.; Chen, G. Understanding Performance Degradation in Cation-Disordered Rock-Salt Oxide Cathodes. Energy Mater. 2019, 9, 1901255. Lee, J.; Papp, J. K.; Clément, R. J.; Sallis, S.; Kwon, D.-H.; Shi, T.; Yang, W.; McCloskey, B. D.; Ceder, G. Mitigating oxygen loss to improve the cycling performance of high capacity cation-disordered cathode materials. Commun. 2017, 8, 981. Lun, Z.; Ouyang, B.; Kitchaev, D. A.; Clément, R. J.; Papp, J. K.; Balasubramanian, M.; Tian, Y.; Lei, T.; Shi, T.; McCloskey, B. D.; Lee, J.; Ceder, G. Improved Cycling Performance of Li-Excess Cation-Disordered Cathode Materials upon Fluorine Substitution. Energy Mater. 2018, 9,1802959. Ahn, J.; Chen, D.; Chen, G.. A Fluorination Method for Improving Cation-Disordered Rocksalt Cathode Performance. Energy Mater. 2020, 10, 2001671.

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.

Dans les batteries Li-ion commerciales, les électrodes positives sont majoritairement composées de matériaux à base d’oxydes lamellaires de nickel ou de cobalt. Remplacer ces deux cations est devenu une nécessité pour des raisons éthiques, écologiques et de raréfaction ou bien économique. Avec une structure tridimensionnelle stable, les matériaux de structure type NaCl sur-lithiés ont montré des capacités spécifiques réversibles dépassant 200 mAh.g-1. Cette haute capacité est atteinte pour un désordre local proche d’une répartition statistique des sites cationiques afin de permettre une bonne diffusion du lithium tout en maintenant une conductivité électronique importante. La mécanosynthèse à haute énergie permet d’obtenir des matériaux fortement désordonnés. Bien que leurs capacités initiales soient élevées, ces matériaux, souffrent d'une réaction irréversible d'oxydation de l'oxygène à haute tension. Il a été découvert que la fluoration était une manière intéressante de supprimer cette oxydation tout en préservant les capacités élevées. Dans cette étude, il a d'abord été proposé de comprendre comment choisir entre différents ensembles de précurseurs pour obtenir Li2MnO2F, matériau désordonné de structure type NaCl, puis d'étudier l'impact de la fluoration sur les propriétés structurales, morphologiques et électrochimiques de Li1.25Mn0.5+y/2Nb0.25-y/2O2-yFy (0 ≤ y ≤ 0.5) à différentes échelles. Une étude par Résonance Magnétique Nucléaire des noyaux 7Li et 19F en rotation à l’angle magique, couplée à la Diffraction des Rayons X et à la fonction de distribution de paires, a permis d’étudier finement la structure des matériaux à différentes échelles. Des expériences ex-situ et in-situ menées par absorption des rayons X au synchrotron ont permis d’étudier les mécanismes rédox impliqués lors du cyclage. Dans la première partie, il a été démontré qu'il était possible de suivre l'avancement de la mécanosynthèse par DRX et par RMN. Ces analyses ont mis en évidence la nécessité d'une réaction prolongée. Dans un second temps, il a été constaté que la nature des précurseurs, lorsque peu de variations étaient apportées (Mn2O3 et Li2O par LiMnO2), avait un faible impact sur les propriétés structurales et électrochimiques de Li2MnO2F. La seconde partie de l'étude a démontré par TEM-EDX qu’il était possible d’introduire de façon homogène des fractions de fluor supérieures à 0.2 dans Li1.25Mn0.5+y/2Nb0.25-y/2O2-yFy grâce à la mécanosynthèse. Si à l’échelle de la particule la fluoration semble homogène, l’introduction de fractions importantes de fluor semble induire un écart à la distribution statistique de l’environnement du lithium face aux cations qui l’entoure (6% de lithium en plus). Cette fluoration plus importante mène aussi à une diminution du paramètre de maille observée par DRX et PDF et tend à créer des environnements riches en lithium et niobium autour du fluor comme cela a été constaté par RMN en étudiant les rapports d’environnements diamagnétiques sur paramagnétiques. La morphologie n’évolue pas avec la fluoration et consiste en un ensemble de particules primaires nanométriques agglomérées en agrégats de plusieurs centaines de nanomètres. D’un point de vue électrochimique il a été montré que deux mécanismes se succédaient en charge : d’abord l’oxydation du manganèse +III en +IV puis l’oxydation de l’oxygène à partir de 4.5 V vs Li+/Li. La contribution irréversible de cette seconde oxydation a pu être atténuée pour les matériaux Li1.25Mn0.55Nb0.2O1.9F0.1, Li1.25Mn0.6Nb0.15O1.8F0.2, Li1.25Mn0.65Nb0.1O1.7F0.3 et supprimée pour des taux de fluoration supérieurs. Ainsi, il a été montré qu'une composition optimale, pouvait être atteinte pour des taux de fluoration compris entre 0.2 et 0.4 dans Li1.25Mn0.5+y/2Nb0.25-y/2O2-yFy, où cette gamme de matériaux présentait la polarisation la plus basse et une bonne rétention de capacité tout en préservant une capacité élevée supérieure à 200 mAh.g-1 en décharge après 20 cycles
In commercial Li-ion batteries, positive electrodes are primarily composed of layered nickel or cobalt oxide-based materials. Replacing these two cations has become a necessity due to ethical, ecological, and economic reasons, as well as concerns about their scarcity. To increase capacity, structural stability, and use transition metals that are less critical, such as manganese, disordered materials with a NaCl-type structure have emerged as one of the solutions among many explored possibilities. With a stable three-dimensional structure, over-lithiated disordered rocksalt materials have demonstrated reversible specific capacities exceeding 200 mAh.g-1. One commonly employed synthesis method is high-energy mechanosynthesis, which yields highly disordered materials. However, these materials, like their layered counterparts, suffer from irreversible oxygen oxidation at high voltage. It has been discovered that fluorination is an interesting approach to suppress this oxidation while preserving high capacities. In this study, the first goal was to understand how to choose between different precursor sets to obtain Li2MnO2F, a disordered material with a disordered rocksalt type structure. Subsequently, the impact of fluorination on the structural, morphological, and electrochemical properties of Li1.25Mn0.5+y/2Nb0.25-y/2O2-yFy (0 ≤ y ≤ 0.5) was studied at different scales. Nuclear Magnetic Resonance (NMR) studies of 7Li and 19F nuclei under magic angle spinning, coupled with X-ray Diffraction (XRD) and Pair Distribution Function (PDF) analysis, allowed detailed examination of the materials' structures at various scales. Ex-situ and in-situ experiments conducted via X-ray Absorption Spectroscopy (XAS) at the synchrotron enabled the study of redox mechanisms involved during cycling. In the first part, it was demonstrated that the progress of mechanosynthesis could be tracked by XRD and at a local scale by NMR. These analyses highlighted the necessity for an extended reaction period. Subsequently, it was observed that the nature of the precursors, with minimal variations (replacing Mn2O3 and Li2O with LiMnO2), had a negligible impact on the structural and electrochemical properties of Li2MnO2F. The second part of the study showed through TEM-EDX that it was possible to uniformly introduce fluorine fractions greater than 0.2 into Li1.25Mn0.5+y/2Nb0.25-y/2O2-yFy via mechanosynthesis. While at the particle scale fluorination seemed uniform, introducing significant fractions of fluorine appeared to deviate the surrounding cations repartition from the statistical distribution around lithium (6% more lithium). This increased fluorination also led to a decrease in the lattice parameter observed by XRD and PDF, tending to create lithium and niobium-rich environments around fluorine, as noted with NMR by studying diamagnetic-to-paramagnetic environment ratios. Morphology did not change with fluorination and consisted of agglomerated nanometric primary particles in clusters of several hundred nanometres. Electrochemically, it was shown that two redox mechanisms occurred: first, the oxidation of manganese from +III to +IV, followed by oxygen oxidation starting from 4.5 V vs Li+/Li. The irreversible contribution of this second irreversible contribution to the capacity could be mitigated for materials Li1.25Mn0.55Nb0.2O1.9F0.1, Li1.25Mn0.6Nb0.15O1.8F0.2, Li1.25Mn0.65Nb0.1O1.7F0.3 and eliminated for higher fluorination rates. Thus, it was demonstrated that an optimal composition could be achieved for fluorination amount between 0.2 and 0.4 in Li1.25Mn0.5+y/2Nb0.25-y/2O2-yFy. Within this range, these materials exhibited the lowest polarisation and good capacity retention while preserving a high capacity exceeding 200 mAh.g-1 after 20 discharge cycles