Disordered Rocksalt Structure Research Articles

Structure, Composition and Electrochemical Properties of Disordered Rock-Salt LiCoO2 Thin Films Prepared by Ion Beam Assisted Deposition

Ion beam assisted deposition is a method used to deposit thin films while irradiating the growing film with an ion beam. This technique is known to be effective in crystallization and orientation control at low temperatures. In this study, LiCoO2 (LCO) thin films were prepared at temperatures below 100 °C using ion beam assisted pulsed laser deposition. The structure, composition, and electrochemical properties of these films were characterized. The results indicated that LCO thin films with a disordered rock salt structure and triaxial orientation were successfully grown on polycrystalline aluminum and quartz glass substrates using ion beam assisted deposition. Furthermore, the diffusion coefficient of Li ions and the charge/discharge capacity were significantly improved compared to LCO thin films with a disordered rock salt structure prepared without the ion beam assisted process.

Is There Hope for Ni-Based Disordered Rock Salt Cathode Materials?

Li-ion batteries achieved in the last 3 decades extremely high energy densities, making them the energy storage devices of choice for the electrification of our society. Despite this improvement, their energy density is still low as compared to gasoline, motivating further research towards materials with better performances. State of the art positive electrodes are nowadays layered materials with a ratio Li:M of 1:1, with M being one or multiple transition metals. LiNiO2 is a noteworthy example. Disordered rock salt (DRX) materials were instead thought to be electrochemically inactive until the development of a percolation theory,1 which demonstrated that lithium-rich materials (Li:M>1.1) with a cation disordered rock-salt structure (DRX) can achieve high reversible specific capacities. The general formula of these lithium-rich compounds can be written as Li1+y(MM′)1−yO2 where the Li content ranges usually from 1.05 to 1.33. With M is indicated a redox active specie (Mn, Ni, Fe, Cr, V) and M’ is typically a d0 element, which is redox inactive during charge and discharge and well accommodates the disordered structure.2 Materials with such structures have been shown to deliver extremely high specific capacities, resulting in equally high specific energies.3 In our recent work we investigated two DRX compounds based on Ni-redox. First, Li2NiO3, which is layered (with monoclinic distortion), but that decomposes to a disordered rock salt structure with cycling. The material displays interesting defect chemistry but fast capacity fading.4 More recently, we developed a class of Li-rich compounds with the chemical composition Li2yNiyTi2-3yO2 (y = 0.50, 0.55, 0.60, 0.67). These samples were prepared via solid-state reaction at three different temperatures (700 °C, 800 °C, 900 °C). By X-ray and neutron diffraction, we demonstrated a pure phase solid solution at high temperature, yet, by decreasing the synthesis temperature and increasing the Li content, phase separation occurs into a rock salt-type and a Li2TiO3 phase. The synthesis process has been also verified by DFT and by in situ X-ray diffraction. In all DRX materials synthesized, the electrochemical performances revealed a strong voltage hysteresis between charge and discharge and fast capacity fade.5,6 This raises then the question of whether any Ni-based DRX material can provide high capacity, high rate capability and stability, while avoiding the commonly encountered large hysteresis, or whether this can only be achieved in Mn-based compounds. In this presentation, by discussing our own work and recent literature reports, we aim at answering this question. Bibliography Urban, Alexander, Jinhyuk Lee, and Gerbrand Ceder. “The Configurational Space of Rocksalt-Type Oxides for High-Capacity Lithium Battery Electrodes.” Advanced Energy Materials4, 13: 1400478.Chen, Dongchang, Juhyeon Ahn, and Guoying Chen. “An Overview of Cation-Disordered Lithium-Excess Rocksalt Cathodes.” ACS Energy Letters, 2021, 1358–76.Clément et al., Cation-disordered rocksalt transition metal oxides and oxyfluorides for high energy lithium-ion cathodes, Energy & Environmental Science, 2020, 13 (2), 345-373Bianchini et al., From LiNiO2 to Li2NiO3: Synthesis, Structures and Electrochemical Mechanisms in Li-Rich Nickel Oxides, Chem. Mater. 2020, 32, 9211−9227Reitano, et al. Manuscript submitted, 2024.Li, Biao, Khagesh Kumar, Indrani Roy, Anatolii V. Morozov, Olga V. Emelyanova, Leiting Zhang, Tuncay Koç, et al. “Capturing Dynamic Ligand-to-Metal Charge Transfer with a Long-Lived Cationic Intermediate for Anionic Redox.” Nature Materials21, 10: 1165–74.

Enhanced Electrochemical Performance of Disordered Rocksalt Cathodes in a Localized High‐Concentration Electrolyte

Enhanced Electrochemical Performance of Disordered Rocksalt Cathodes in a Localized High‐Concentration Electrolyte

Enhancing the Electrochemical Performances of Disordered Li1.23Ni0.3Nb0.3Fe0.16O0.85F0.15 Cathode Material for Lithium-Ion Batteries by LiNbOx Coating.

For the next generation of lithium-ion batteries (LIBs), it is primary to seek high capacity and long-lifetime electrode materials. Li-excess disordered rock-salt structure (DRS) cathodes have gained much attention due to their high specific capacity. However, Li-excess can lead to a decrease in the structural stability of an electrode material. A new Li-rich DRS oxyfluorides, Li1.23Ni0.3Nb0.3Fe0.16O0.85F0.15 (F0.15) with a series amounts of LiNbOx (LN) coating (0, 5, 10, and 15 wt % denoted as F0.15-LN0, F0.15-LN5, F0.15-LN10, and F0.15-LN15, respectively), are successfully synthesized and evaluated as cathode materials in LIBs. Among them, F0.15-LN10 exhibits the highest initial discharge specific capacity of 296.1 mAh g-1 (at a current density of 20 mA g-1) with the capacity retention rate of 14% higher than that of the uncoated F0.15 after 80 cycles. Even at 300 mA g-1, F0.15-LN10 still delivers the highest discharge specific capacity of 130 mAh g-1. After 20 cycles, the charge-transfer impedance of F0.15-LN10 remained the smallest. The characterizations indicate that LN coating reduces the surface polarization of the cathode materials, slows the interfacial side reactions between the electrolyte and the electrode, and speeds up the Li+ diffusion. These results demonstrate that LN coating is an effective strategy to improve the electrochemical performance.

Influence of Synthesis Parameters on the Short-Range Structure and Electrochemical Performances of Li2MnO2F.

In the last years, disordered rocksalt structure (DRS) materials were proposed as a positive electrode for lithium-ion batteries. In particular, the fluorinated DRS materials were proposed to be more stable upon cycling than pure oxide counterparts. These materials are mainly obtained by mechanosynthesis in order to incorporate a significant number of F ions and maintain a disordered structure. Since the local structural arrangement is crucial for battery application, we aim to monitor its evolution upon the synthesis of Li2MnO2F from two sets of precursors: Mn2O3, Li2O, and LiF or LiMnO2 and LiF. The synthesis progress was thus followed, by 7Li and 19F MAS NMR coupled to XRD to probe the structure at different scales. This allowed us to identify an optimal milling time to reach the final compounds. We show that they exhibit similar morphology (by SEM), medium- and short-range orders (by XRD, 7Li and 19F NMR, EXAFS), and average Mn oxidation degree (by XANES). The electrochemical performances of the two compounds are almost similar, with high specific capacities of 319 mAh·g-1 ("from LiMnO2") and 304 mAh·g-1 ("from Mn2O3") for the first charge to 4.8 V vs Li+/Li, proving their interest as post-NMC candidates as positive electrode materials.

Nb-doping of cation-disordered rocksalt oxides with B2O3 surface modification for superior performance cathode

As the growing price and inadequate reserves of cobalt, the demand for cobalt-free materials with superior energy density to apply in Li-ion batteries has increased distinctly. Li-rich cation disordered rocksalt (DRX) material has aroused research hotspot because of high specific capacity and wide compositional space. However, irreversible oxygen loss as well as sluggish kinetic raise the problem in large polarization and inevitable capacity decay in cycling process, which have impeded the commercialization of these electrode materials. In this work, we have synthesized cobalt-free boron trioxide (B2O3)-modified Li1.2Ni0.3Ti0.3Nb0.2O2 compounds with cation disordered rocksalt structure through mechanochemical method. It is a fact that B2O3-modified cathodes possess outstanding cyclic stability as well as rate properties. Particularly, the Li1.2Ni0.3Ti0.3Nb0.2O2 electrode with 2.0 wt% B2O3 displayed a competitive capacity of 192.4 mAh/g at 10 mA g−1 and retained 134.7 mAh/g after 100 charge/discharge times. The redox mechanism is explored through ex-situ XPS measurement, verifying the reversible cationic redox of Ni2+/Ni4+ accompanying small Ti4+/Ti3+ reduction and anionic O2−/O− evolution, which results in unusual high capacity. Such an optimized surface modification combines with the Nb doping, can promote Li ion transport, reduce polarization, protect cathode from reacting with electrolyte. These findings provide important implications to improve performance of DRX cathodes.

Tolerance Factor and Phase Stability of the KCoO2-Type AMN2 Nitrides.

In order to help understand the structural stability of KCoO2-type ternary nitrides AMN2, referring to perovskite structure, a tolerance factor t is proposed to describe the size effect on the phase/symmetry options of the experimentally accessible AMN2 nitrides. This leads to a range of t values above 0.946 for structurally stable KCoO2-type AMN2 nitrides with t values around 0.970 for the orthorhombic and tetragonal phase boundary. In contrast, most of AMN2 nitrides exhibit α-NaFeO2-type structure with t ∼ 0.898-0.946 and cations ordered or disordered rocksalt structure while t below 0.898. Employing the proposed criterion, the structure formation for other ternary AMN2 compositions with lanthanum and alkaline earth cations for the A sites were predicted, which was testified through the synthesis attempts and complemented by formation energy evaluations. The efforts to synthesize the ternary Lanthanide and alkaline earth-based AMN2 nitrides were unsuccessful, which could associate the structural instability with the large formation energies of lanthanide nitrides LaMN2 and the greater tolerance factor of 1.048 for BaTiN2. The experimentally already synthesized AMN2 nitrides could be categorized into three types with different tolerance factors, and scarce AMN2 nitrides with lower formation energies would be accessible using different synthetic routes beyond the traditional solid-state synthesis method.

The Nature of d0 Ion Effect on the Electrochemical Activity of the O2–/O–-Redox-Couple in Oxyfluorides with the Disordered Rock-Salt Structure

The effect of the nature of the transition metal ion (electron configuration 3d0 and 4d0) on the local structure and electrochemical properties of lithium-rich oxyfluorides with disordered rock-salt structure Li1 + x(MеMn3+)1 – xO2 – yFy, where Mе = Ti4+, Nb5+, 0.2 ≤ x ≤ 0.288 and 0.05 ≤ y ≤ 0.15 is studied. The compounds are thoroughly investigated by the methods of X-ray diffraction analysis, scanning electron microscopy, granulometry, electron spin resonance spectroscopy, and galvanostatic cycling. The galvanostatic-cycling curves of the compounds have two plateaus in the voltage regions of 3.3–3.4 and 4.1–4.3 V. They can be attributed to redox-processes involving two couples: Mn3+/Mn4+ and O2–/O–. In the case of Ti-containing oxyfluorides with disordered rock-salt structure, with the increasing of fluorine content the contribution from O2–/O–-couple during the electrochemical process decreases. In both systems of the oxyfluorides with disordered rock-salt structure we observed formation of paramagnetic clusters Mn3+–O–Mn4+ whose number increased with the increasing of Mn content. The largest clusterization is observed for the sample Li1.266Nb0.217Mn0.55O1.85F0.15. At the same time, the diffusion coefficient for Nb-containing oxyfluorides with disordered rock-salt structure is lower by order of magnitude than for the Ti-containing ones. This may be connected with the strongest clustering of Mn3+ ions, which hinders the Li+ ion macrodiffusion and, as a consequence, deteriorates the kinetics of the process.

The Nature of d0 Ion Effect on the Electrochemical Activity of the O2–/O–-Redox-Couple in Oxyfluorides with the Disordered Rock-Salt Structure

The Nature of d0 Ion Effect on the Electrochemical Activity of the O2–/O–-Redox-Couple in Oxyfluorides with the Disordered Rock-Salt Structure

Activity of Metal-Fluorine States upon Delithiation of Disordered Rocksalt Oxyfluorides

The capacity of transition-metal oxides as Li-ion battery cathodes is limited by instabilities that arise when high states of charge are achieved. Oxyfluorides with a disordered rocksalt structure have emerged as attractive alternatives, but the role of F in their electrochemical function, particularly when cationic redox produces high formal oxidation states, remains to be ascertained. Using X-ray absorption spectroscopy, we confirm the existence of Mn–F covalent interactions in Li2MnO2F and Li2Mn2/3Nb1/3O2F. New unoccupied states evolve from hybrid 3d-2p states of both Mn–F and Mn–O bonds when the phases are delithiated, particularly in the presence of Mn(IV). The results challenge the assumption of F as anion whose covalent states with the metal are tapped at very high potentials, providing instead a nuanced picture of redox compensation in oxyfluorides. They suggest the existence of unique knobs of design of battery cathodes by manipulating the covalent interactions between transition metals and two different anions.

A near dimensionally invariable high-capacity positive electrode material.

Delivering inherently stable lithium-ion batteries is a key challenge. Electrochemical lithium insertion and extraction often severely alters the electrode crystal chemistry, and this contributes to degradation with electrochemical cycling. Moreover, electrodes do not act in isolation, and this can be difficult to manage, especially in all-solid-state batteries. Therefore, discovering materials that can reversibly insert and extract large quantities of the charge carrier (Li+), that is, high capacity, with inherent stability during electrochemical cycles is necessary. Here lithium-excess vanadium oxides with a disordered rocksalt structure are examined as high-capacity and long-life positive electrode materials. Nanosized Li8/7Ti2/7V4/7O2 in optimized liquid electrolytes deliver a large reversible capacity of over 300 mAh g-1 with two-electron V3+/V5+ cationic redox, reaching 750 Wh kg-1 versus metallic lithium. Critically, highly reversible Li storage and no capacity fading for 400 cycles were observed in all-solid-state batteries with a sulfide-based solid electrolyte. Operando synchrotron X-ray diffraction combined with high-precision dilatometry reveals excellent reversibility and a near dimensionally invariable character during electrochemical cycling, which is associated with reversible vanadium migration on lithiation and delithiation. This work demonstrates an example of an electrode/electrolyte couple that produces high-capacity and long-life batteries enabled by multi-electron transition metal redox with a structure that is near invariant during cycling.

Studies of Synthesis Temperature Effects on Cubic Lithated-Spinel to Layered Phase Transformation and Electrochemical Performance of LiMn0.5Ni0.5O2 Cathode Materials

Recently, our group has reported a novel Li-ion battery cathode, ‘LT-LiMn0.5Ni0.5O2’, where LT refers to its relatively low synthesis temperature (400 °C). By collecting high energy X-ray diffraction and high-angle, annular-dark-field (HAADF) scanning-transmission electron microscopy (STEM) data, we confirm that the LT-LiMn0.5Ni0.5O2 features a unique partially disordered rock salt structure with predominant lithiated-spinel-like character. The electrochemical data indicates that the Li/LT-LiMn0.5Ni0.5O2 cells shows good cycle stability when operate between 2.5-5.0V. The 1st cycle specific discharge capacity was determined to be 225mAh/g. Two distinct voltage plateaus can be identified at approximately at ~4.6 V and ~3.5 V during discharge. The ~1 voltage separation of the plateaus is caused by the octahedral and tetrahedral site energy difference during lithium extraction and insertion.1 The discovery of LT-LiMn0.5Ni0.5O2 not only expand the compositional space for the known lithated-spinel family but also provide a practical way to increase the operational capacity of traditional spinels such like LiMn2O4 and LiMn1.5Ni0.5O2.2-5 It is well known that when synthesized at 900°C, LiMn0.5Ni0.5O2 features a layered structure.6 The existence of lithated-spinel like LiMn0.5Ni0.5O2 at low temperature (400°C) immediately raise another question: How does the synthesis temperature affect the structure and electrochemical performance of LiMn0.5Ni0.5O2? We synthesized a series of different temperature of LiMn0.5Ni0.5O2 powders. By collecting high energy X-ray diffraction and high-resolution transmission electron microscopy (HR-TEM) data, we confirm that, the LiMn0.5Ni0.5O2 exhibits a lithated spinel → disordered layered → ordered layered structure transformation as temperature increase from 400-900 °C. The Rietveld refinements results also reveals a gradual decrease of the Li/Ni exchange ratio accompanied by the synthesis temperature changes. The evolution of specific discharge capacity for different temperature LiMn0.5Ni0.5O2 shows a parabola-like curve behavior with the lowest discharge capacity at 700 °C. This unique behavior is likely to be related to the diffusion channels reconstruction along with the structure changes. 3D diffusion channels are incorporated with 2D diffusion channels at low temperature (400°C) for lithated-spinel LiMn0.5Ni0.5O2. Within the middle temperature range (500°C-700°C), the 3D channels destroy rapidly while 2D diffusion channels are not well established due to the highly disorder ratio resulting in a decrease of capacity. At higher temperature (800-900°C), the 2D diffusion channels are more well established enabling an increase of the capacity. Shi, B.; Gim, J.; Li, L.; Wang, C.; Vu, A.; Croy, J. R.; Thackeray, M. M.; Lee, E., LT-LiMn 0.5 Ni 0.5 O 2: a unique co-free cathode for high energy Li-ion cells. Chemical Communications 2021, 57 (84), 11009-11012.Thackeray, M. M.; Lee, E.; Shi, B.; Croy, J. R., Review–From LiMn2O4 to Partially-Disordered Li2MnNiO4: The Evolution of Lithiated-Spinel Cathodes for Li-Ion Batteries. Journal of The Electrochemical Society 2022, 169 (2), 020535.Gummow, R.; Thackeray, M.; David, W.; Hull, S., Structure and electrochemistry of lithium cobalt oxide synthesised at 400 C. Materials research bulletin 1992, 27 (3), 327-337.Lee, E.; Blauwkamp, J.; Castro, F. C.; Wu, J.; Dravid, V. P.; Yan, P.; Wang, C.; Kim, S.; Wolverton, C.; Benedek, R., Exploring lithium-cobalt-nickel oxide spinel electrodes for≥ 3.5 V Li-ion cells. ACS applied materials & interfaces 2016, 8 (41), 27720-27729.Lee, E.; Kwon, B. J.; Dogan, F.; Ren, Y.; Croy, J. R.; Thackeray, M. M., Lithiated Spinel LiCo1–x Al x O2 as a Stable Zero-Strain Cathode. ACS Applied Energy Materials 2019, 2 (9), 6170-6175.Yang, X.-Q.; McBreen, J.; Yoon, W.-S.; Grey, C. P., Crystal structure changes of LiMn0. 5Ni0. 5O2 cathode materials during charge and discharge studied by synchrotron based in situ XRD. Electrochemistry communications 2002, 4 (8), 649-654

Solution-Grown Ternary Semiconductors: Nanostructuring and Stereoelectronic Lone Pair Distortions in I–V–VI2 Materials

Alkali pnictogen dichalcogenides─I–V–VI2 or APnCh2─have been identified as promising semiconducting materials for energy conversion devices. However, the controlled nanoscale synthesis and our understanding of the effects of cation ordering and stereochemically active lone pairs on the structures of these ternary compounds remain underdeveloped. Here, we use solution-phase chemistry to synthesize a family of APnCh2 materials, including LiSbSe2, NaSbS2, NaSbSe2, NaBiS2, and NaBiSe2. Our approach utilizes alkali metal hydrides (AH) or carboxylates, A(O2CR), PnPh3, and elemental chalcogens as synthetic precursors and oleylamine or 1-octadecene as solvents. Synthetic manipulation via fine-tuning of reaction temperature enables control over the degree of ordering caused by the Sb 5s2 lone pair-induced distortions in NaSbS2. Pair distribution function analysis demonstrates that the structure of the Sb-containing phases deviates much more from a disordered rock salt structure than that of the Bi-containing phases. This local distortion, induced by the Sb lone pair, leads to a previously unreported noncentrosymmetric NaSbS2 crystal structure, which is additionally supported by second-harmonic generation measurements. Infrared and multinuclear solid-state NMR spectroscopies show that oleylamine or chelating carboxylates and, in some cases, unreacted precursors (LiH and PnPh3) remain bound to the nanocrystalline surfaces. A deeper understanding of the local atomic environment, long-range ordering, surface chemistry, and optoelectronic properties of these materials may speed up their fundamental study and application.

Rational material design of Li-excess metal oxides with disordered rock salt structure

This review article summarizes recent research development on a new class of electrode materials with a cation-disordered rock salt structure for energy storage applications. Historically, oxide-based electrode materials with the disordered rock salt structure are regarded as “electrochemically inactive.” However, recent experimental and theoretical research reveals that many oxides with the disordered rock salt structure can be utilized as high-capacity electrode materials, which deliver a much larger reversible capacity compared with traditional and cation ordered layered materials used for practical battery applications. For these emerging electrode materials, higher energy density is achieved relying on anionic and/or cationic redox as multi-electron reactions. Moreover, this anionic/cationic redox for Li-excess materials with the rock salt structure is effectively activated for nano-sized materials. These new trends for the material design on high-capacity electrode materials are highlighted and the future direction to design Li/Na insertion materials for energy storage applications is outlooked.

High Energy Density Electrode Materials with the Disordered Rocksalt Structure

High Energy Density Electrode Materials with the Disordered Rocksalt Structure

Exploring the Origin of Anionic Redox Activity in Super Li-Rich Iron Oxide-Based High-Energy-Density Cathode Materials

Super alkali-rich materials (alkali:transition metal ratio of ≥2:1), such as Li5FeO4, exhibit the potential to realize anionic redox upon deep delithiation. Li5FeO4 was shown to undergo reversible cycling between Li4FeO3.5 and Li3FeO3.5 with combined cation/anion redox, a remarkable capacity of 189 mAh/g, and no O2 release. However, the impact of phase transformations on the reaction thermodynamics and the correlation between the structural changes and reaction reversibility remain unclear. In this study, we use first-principles calculations to examine the delithiation and (re)lithiation reactions of Li5FeO4. We show that the experimentally observed charge and discharge processes go through non-equilibrium pathways. Upon delithiation, the compound undergoes a phase transformation from Li5FeO4, with tetrahedrally coordinated (Td) Fe ions, to a delithiated disordered rocksalt structure, with octahedral (Oh) Fe ions. Fe-ion migration has an asymmetric kinetic barrier that makes Td → Oh migration facile, whereas the reverse process has a much larger barrier, explaining the difficulties in reaction reversibility. We further elucidate the transition metal and O redox sequences during the charge cycle and identify the complex electrochemistry associated with the dual participation of cationic redox (Fe3+/Fe4+) and anionic redox (O2–/O–/O0). Armed with this knowledge, we conduct high-throughput screening of known alkali-rich transition metal oxides by evaluating their potential to enable reversible anionic redox, with multiple candidates proposed for further experimental trials. Our work provides a useful guide for the further development of super alkali-rich anionic-redox-active electrodes for high-energy-density batteries.

Research Trend in Rock Salt Structured High Entropy Cathode

Development of lithium-ion rechargeable batteries with high energy storage capability are required in timely manner. Recently, it has been experimentally and computationally proven that oxides with the disordered rock salt structure can be charged and discharged in the Li-ion battery system. In particular, the high entropy disordered rock salt cathode has unique structure property, where both Li-ion and transition metal are randomly located on the cation sites. Such disordering in metal sites can migrate the Li-ion in a percolating way albeit with sluggish kinetics. Therefore, the high entropy disordered rock salt structure has attracted great attention due to its high energy density and stable structure. In this paper, we introduce a simple and effective strategy in the selection of transition metals for high entropy cathodes to achieve desired electrochemical properties.

(Invited) Electron Microscopy Probing of Atomic Level Process on the Fading of Intercalation Cathode for Lithium Ion Batteries

Electron microscopy and spectroscopy diagnosis, both in-situ and ex-situ, appear to be one of the essential methods for gaining insights as how an electrode material evolves and eventually fails, therefore feeding back for designing and creating new materials with enhanced battery performances. In this presentation, we will focus on recent progress on ex-situ, in-situ and operando S/TEM studies for probing into the structural and chemical evolution of cathode materials for lithium ion batteries, such as layer structured transition metal oxide, lithium and manganese rich layered structure, and disordered rock salt structure (DRX). A broad spectrum of observed phenomenon, such the surface reconstruction, intragranular cracking, intergranular cracking, invariant layer sliding, anionic redox, oxygen loss, vacancy injection, and void formation, appears to be contributing to the fading of the electrode in one way or another, while the intrinsic cause of each of these phenomena remains rather obscure. We will glean through several recent key observations that yield insight on tracking the root cause of the fading of these class of cathode materials, paving path for tailoring cathode with good lattice stability toward stable cycling of battery.

Li2NiO2F a New Oxyfluoride Disordered Rocksalt Cathode Material

Lithium-rich disordered rocksalts such as Li1.3Nb0.3Mn0.4O2 and Li2MnO2F are being investigated as high energy density cathodes for next generation Li-ion batteries. They can support the (de) lithiation of lithium ions over large compositional ranges while preserving the same overall structure. Here, we present a new Ni-rich oxyfluoride cathode, Li2NiO2F, with a disordered rocksalt structure. Li2NiO2F and can deliver a discharge capacity of 200 mAh g−1 at an average voltage of 3.2 V.

Kinetic Rejuvenation of Li-Rich Layered and Disordered-Rocksalt Li-Ion Battery Cathodes upon Oxygen Redox

Li-rich cathode materials (x>1 in LixTM2-xO2, TM=transition metal) with a layered structure or a disordered-rocksalt structure have received tremendous attention as advanced battery cathodes [1,2,3]. These materials can deliver substantially higher gravimetric capacity (>250 mAh/g) and energy density (>800 Wh/kg) compared to traditional materials (e.g., LiCoO2; ~170 mAh/g, 660 Wh/kg). Moreover, they improve the Li-ion batteries' sustainability by removing or greatly reducing scarce and environmentally unfriendly elements in their structure. In these materials, the theoretical TM-redox capacity is typically limited to ~140 mAh/g because of the limited amount of redox-active TMs in their crystal structure [1,2,3]. As a result, to achieve a very high capacity (> 250 mAh/g), O-redox must additionally take place in the materials, which occurs in local Li-rich environments (e.g., OLi4TM2, OLi5TM1) around oxygen where there are non-bonding O 2p orbitals [4]. In Li-rich materials containing multiple TMs, the local electroneutrality demands the TM-sites in the OLi4TM2 or OLi5TM1-unit to be occupied by high-valent TM-cations (e.g., Ti4+, Mn4+) instead of low-valent ones (e.g., Ni2+), which often results in medium-range-order (MRO) of cations in the Li-rich layered cathodes or short-range-order (SRO) in the Li-rich disordered-rocksalt cathodes [1,5]. For instance, this tendency leads to the rise of the Li2MnO3-like MRO in the layered Li- and Mn-rich cathodes, and because the non-bonding O 2p orbitals are primarily in the Li2MnO3-like domains, O-redox also takes place within the domains [1,4]. Unfortunately, the radical oxygen ion generated upon oxygen oxidation is highly mobile, and as the oxygen is covalently bonded with TMs, this increased O-mobility often accompanies TM-migration, triggering reversible or irreversible structural changes in the Li-rich materials [1]. This structural changes result in oxygen loss or unwanted phase transformation, which leads to voltage- and capacity-decay. Therefore, minimizing the structural rearrangement while using combined TM- and O-redox has been widely considered a holy grail to developing Li-rich cathodes with high capacity and stability. For instance, Bruce et al. showed that by changing the superstructure from the honeycomb to ribbon structure, one could suppress the Mn-migration upon O-redox in the layered Na-(Li, Mn)-O cathode to reduce voltage hysteresis [6]. Moreover, Li-rich disordered-rocksalt cathodes also experience structural damages upon O-oxidation; hence, its mitigation by various methods (e.g., fluorination) has been at the center of its research [3,7].In this presentation, we demonstrate that, in contrast to the prevailing opinion, the O-redox-assisted structural change can be highly beneficial to Li-rich cathodes' performance by substantially reducing the internal Li-transport resistance [8]. We synthesized highly-Li-rich Co-free layered cathodes, which possess the Li2MnO3-type medium-range-order (MRO). These materials' initial capacities are low; yet, they undergo a 'rejuvenation' (activation) upon extended cycling with substantially reduced hysteresis and increased capacity, which correlates with the MRO disruption and associated volume expansion initiated by O-redox-facilitating TM-migration. Moreover, we show that virtually the same process occurs for a disordered-rocksalt-type Li-rich cathode (Li1.2Ni1/3Ti1/3Mo2/15O2), suggesting the universality of this process.Furthermore, we use this knowledge to inform a molten-salt treatment to pre-disturb MRO, expand crystal volume before cycling, and endow a surface gradient composition for our Li-rich Co-free layered cathodes, such that the treated materials can achieve high capacity (>230 mAh/g) from the very first cycle with excellent rate capability (154 mAh/g at 2 A/g) and outstanding capacity/voltage-retention (~4 % capacity-loss; ~140 mV voltage-loss after 200 cycles at 100 mA/g; >210 mAh/g). From these results, we explain the mechanism and universality of the rejuvenation process in various charge-ordered oxides and propose guidelines for designing advanced Li-rich cathode materials with combined transition metal- and oxygen-redox activities. References M. Thackeray et al., J. Mater. Chem. 17, 3112–3125 (2007).N. Yabuuchi et al., PNAS 112, 7650–7655 (2014).J. Lee et al., Nature 556, 185–190 (2018).Seo et al., Nature Chem. 8, 692–697 (2016).H. Ji et al., Nat. Commun. 10, 592 (2019).R. House et al., Nature 577, 502–508 (2020).Z. Lun et al., Adv. Energy. Mater. 1802959 (2019).J. Lee et al., ACS Appl. Energy Mater. 3, 7931–7943 (2020).