Transition Metal Oxide Cathode Research Articles

‘Aqueous Processed’ O3-Type Transition Metal Oxide Cathodes Enabling Long-Term Cyclic Stability for Na-Ion Batteries

Among the potential cathode material classes for Na-ion batteries, O3-type layered NaxTMO2s (TM => transition metal ion) are of importance due to their high starting Na-content (of ~1 per formula unit; x). However, the O3-type NaxTMO2s suffer from multiple structural phase transformations during electrochemical charge/discharge cycles, TM-dissolution into electrolyte [1-2] and, more importantly, inherent sensitivity to moisture [3]. The moisture sensitivity of these ‘layered’ NaxTMO2s necessitates the usage of toxic/hazardous non-aqueous solvents like N-Methyl-2-pyrrolidone (NMP) during electrode preparation. Against this backdrop, a carefully designed composition has been developed in this work, which addresses the aforementioned problems, in particular, the air/water-instability. Partial/complete substitution of Ti-ion for Mn-ion in Na(Li0.05Mn0.5-xTixNi0.30Cu0.10Mg0.05)O2 eliminated the presence of Mn3+ (which dissolves in electrolyte) at the particle surface, supressed increment in impedance and voltage hysteresis during electrochemical cycling and, thus, significantly improved cyclic stability of Ti-substituted O3-type layered NaxTMO2s. The Mn-containing Na-TM-oxides were found to be extremely unstable in terms of phase/structure retention upon exposure to air and water; progressively evolving O’3 and P3 phases due to spontaneous Na-loss and thereby forming undesired NaOH and Na2CO3 phases on the particle surface (see Fig. 1a), causing increase in electrochemical impedance. By contrast, no phase/structural change occurred upon partial/complete Ti-substitution (for Mn-ion), even after 40 days of air-exposure and 12 h of soaking, as well as stirring, in water (viz., very stringent hydration condition) (see Fig. 1b). Such excellent stability against hydration, which was partly due to reduced Na-ion ‘inter-slab spacing’ in the presence of Ti-ion, was not reported earlier for O3-type Na-TM-oxides. The excellent stability of the optimized O3-type NaTMO2 enables the usage of environment/health-friendly and economical ‘aqueous-binder’ (viz., Na-alginate) and water (as solvent) for electrode preparation. Overall, the ‘aqueous-processed’ cathode exhibits first cycle capacity of ~125 mAh/g (between 2-4 V; vs. Na/Na+), with smooth electrochemical cycling profiles (see Fig. 2a) and excellent long-term cyclic stability, with a capacity retention of ~56% after 750 cycles at C/5 (see Fig. 2b). Overall, the present work, as published in ref. [4], has established important correlations between the composition, structure (viz., reduction in ‘inter-slab spacing’), stability against hydration (viz., in air and water), feasibility for health/environmental-friendly ‘aqueous processing’ of electrodes, electrochemical impedance, stability of average voltages and cyclic stability of O3-type Na-TM-oxide based cathode materials for Na-ion batteries. Keywords: Na-ion battery; layered transition metal oxide cathode; air/water-stability; aqueous processing; electrochemical behaviour Reference s : [1] S. Komaba, N. Yabuuchi, T. Nakayama, A.Ogata and T. Ishikawa, Inorg.chem, 51, 6211–6220 (2012).[2] P. F. Wang, Y. You, Y. X. Yin and Y. G. Guo, J. Mater. Chem. A, 4, 17660–17664 (2016).[3] H. R. Yao, P. F. Wang, Y. Gong, J. Zhang, X. Yu, L. Gu, C. Ouyang, Y. X. Yin, E. Hu, X. Q. Yang, E. Stavitski, Y. G. Guo and L. J. Wan, J. Am. Chem. Soc., 139, 8440–8443 (2017).[4] B. S. Kumar, A. Pradeep, A. Dutta, A. Mukhopadhyay., J. Mater. Chem. A, 8, 18064-18078 (2020). Figure 1

Understanding voltage hysteresis and decay during anionic redox reaction in layered transition metal oxide cathodes: A critical review

Understanding voltage hysteresis and decay during anionic redox reaction in layered transition metal oxide cathodes: A critical review

Electrochemical performance of LiNi1/3Co1/3Mn1/3O2 cathode recovered from pyrolysis residue of waste Li-ion batteries

Cycling performances of lithium-ion batteries using the cathodes of LiNi 1/3 Co 1/3 Mn 1/3 O 2 recovered from pyrolysis residue of a waste automotive lithium-ion battery stack (Recovered NCM111 including impurities), and commercial LiNi 0.5 Co 0.2 Mn 0.3 O 2 (Commercial NCM523). • A bare waste Li-ion battery stack was pyrolyzed at 800 °C under air-free conditions. • LiNi 1/3 Co 1/3 Mn 1/3 O 2 cathode was hydrometallurgically recovered from pyrolyzed stack. • The cathode included Al, Cu, and Fe impurities at a total of 4 mass% and residual F. • Cathode exhibited a specific capacity of 120 mAh g −1 and very high cycling stability. • High-level coexistence of Al, Cu, and Fe was not harmful to the recovered cathodes. A methodology for time-effective, automatic, and safe extraction of cathode active materials from waste lithium-ion battery (LIB) stacks without complex mechanical disassembly and using electrically and chemically harmless processes can be beneficial for the sustainable fabrication of LIB. Herein, we present a feasibility study on the recovery of ternary Li transition metal oxide (LTMO) cathode active materials from the residue of a waste automotive LIB stack using pyrolysis at 800 °C without exposure to air. Sequential processes from pyrolysis, residue grinding, classification (sieving), wet magnetic separation, acid leaching, hydroxide precipitation, and desalination to drying were used to prepare the precursor of the cathode active material. The precursor was mixed with Li 2 CO 3 and sintered in air at 800 °C, which yielded LiNi 1/3 Co 1/3 Mn 1/3 O 2 (NCM) with metallic impurities of Al, Cu, and Fe at a total of 4 mass% as well as excess Li and residual F. The electrochemical performance of waste LIB-derived LiNi 1/3 Co 1/3 Mn 1/3 O 2 (W-NCM) cathode active materials was evaluated in half-cell and full-cell configurations and compared with that of a commercial LIB cathode active material of LiNi 0.5 Co 0.2 Mn 0.3 O 2 (NCM) used in automobile applications. The Li-ion extraction/insertion specific capacity of W-NCM in the half-cell was 120 mAh/g at 15 mA g −1 , which was 70.4 % that of NCM. The specific capacity of W-NCM in a full-cell using a graphite anode at 0.1 C was 85 mAh/g, which was 69.1 % that of NCM. However, W-NCM in the full-cell exhibited much higher capacity retention (91.2 %) after 1000 charge–discharge cycles at 2 C, while the capacity retention of NCM was 34.0 %. The excellent cycling performance of W-NCM was attributed to the co-existence of metallic impurities. The proposed cathode recovery method may be further explored for applications in large-scale and automatic recycling of waste LIBs.

Molybdenum-Doped Li/Mn-Rich Layered Transition Metal Oxide Cathode Material Li1.2Mn0.6Ni0.1Co0.1O2 with High Specific Capacity and Improved Cyclic Stability for Rechargeable Li-Batteries

Molybdenum-Doped Li/Mn-Rich Layered Transition Metal Oxide Cathode Material Li_1.2Mn_0.6Ni_0.1Co_0.1O₂ with High Specific Capacity and Improved Cyclic Stability for Rechargeable Li-Batteries

Surface Reduction Stabilizes the Single-Crystalline Ni-Rich Layered Cathode for Li-Ion Batteries.

The surface of the layered transition metal oxide cathode plays an important role in its function and degradation. Modification of the surface structure and chemistry is often necessary to overcome the debilitating effect of the native surface. Here, we employ a chemical reduction method using CaI2 to modify the native surface of single-crystalline layered transition metal oxide cathode particles. High-resolution transmission electron microscopy shows the formation of a conformal cubic phase at the particle surface, where the outmost layer is enriched with Ca. The modified surface significantly improves the long-term capacity retention at low rates of cycling, yet the rate capability is compromised by the impeded interfacial kinetics at high voltages. The lack of oxygen vacancy generation in the chemically induced surface phase transformation likely results in a dense surface layer that accounts for the improved electrochemical stability and impeded Li-ion diffusion. This work highlights the strong dependence of the electrode's (electro)chemical stability and intercalation kinetics on the surface structure and chemistry, which can be further tailored by the chemical reduction method.

Spray-drying Al onto hydroxide precursors to prepare LiNi0.855Co0.095Al0.05O2 as a highly stable cathode for lithium-ion batteries

The very high specific discharge capacities of Ni> 90% cathodes are often undone by their extremely poor cycle life and thermal stability. Herein, Ni-poor, and Al-rich particle with double concentration gradients is fabricated by synthesizing particles of the Ni-rich hydroxide Ni0.9Co0.1(OH)2 in a highly efficient Taylor flow (TF) reactor and then depositing Al onto their surfaces using three methods: spray-drying and conventional wet and dry chemical coating. The uniformity of the surface distribution of Al on the Ni-rich transition metal oxide cathode materials affected the overall structural and electrochemical stability. Among our three systems, the cathode material prepared with the spray-dried Al exhibited the best performance, with an initial discharge capacity of 196.9 mA h g–1 and capacity retention of 93% after 100 cycles at a rate of 1 C. It also demonstrated superior electrochemical performance at higher C-rates. For example, at 10 C, it delivered an initial discharge capacity of 134.4 mA h g–1, compared with 56.8 mA h g–1 for the Al-free LiNi0.9Co0.1O2. We attribute this enhanced electrochemical performance to the presence of Al and its uniform distribution (through spray-drying processing) on the outer layer of the active material, with the Ni and Co elements remaining mainly within the core.

Chemical and structural evolution during solid-state synthesis of cobalt-free nickel-rich layered oxide cathode

In contrast to the surging need for the cobalt-free nickel-rich layered transition metal oxide cathodes in lithium-ion batteries, the chemical and structural change in the solid-state synthesis of such materials is still not clear enough. Herein, the solid-state synthetic process of LiNi0.94Mn0.04Al0.02O2 from room temperature to excessive temperature is systematically studied to present a detailed and full view of the structure transformation, valence variation, and impurity phase generation alongside the synthetic process. The results indicate three stages of the synthetic process, that is the decomposition-chemical combination stage below 500 °C, the regularization of R3¯m phase stage at 500∼800 °C, and the excessive calcining stage above 800 °C. Surprisingly, during the first stage below 500 °C, the precursor and alkali lithium start to react at a much lower temperature (e.g., 300 °C) than expected, accompanied by the integration of a considerable amount of Li element into the bulk of precursor and the oxidization of Ni. Specifically, the stoichiometric ratio of Li/TM in LixTMO2 is 37%, 55%, and ∼100% at 300, 400, and 500 °C, together with more than half of Ni oxidized to +3. However, on the third stage above 800 °C, R3¯m phase in LixTMO2 will become unstable and lead to the emergence of NiO-like rock-salt, accompanied by the reduction of a portion of Ni3+ and significant Li loss. The stoichiometric ratio of Li/TM in LixTMO2 is 79% at 1000 °C. These findings provide an insight into the solid-state synthesis of cobalt-free nickel-rich layered transition metal oxide cathodes.

Investigation of the Redox Activity in Mn-Based Oxyfluorides

Li-ion batteries offer excellent electrochemical performance, are widely used in vehicle electrification and portable devices, and have huge potential for grid storage applications. Their electrochemical behavior relies primarily on transition metal oxide cathodes. In a recent development, Mn has been used in Li-rich cathode materials with a cationic disordered rock-salt (DRX) structure [1,2]. These Mn-based DRX materials are promising candidates for next-generation Li-ion battery cathodes because of their large energy densities and basic favorable features of Mn, such as sustainability. Along with the use of Mn in a DRX structure, substitution of some of the oxygen by fluorine imparts improved cyclability [1-6]. It has been proposed that Li-site distribution played an important role in the initial capacity of these materials and the metal-redox capacity and reversibility was improved by fluorination [5,6]. However, the contribution of the different ions to this improvement has not been fully elucidated. Fluorine and oxygen have different bonding interactions with transition metals, raising the question of how they may comparatively participate in redox compensation. To address this question and determine the role of a mixture of anions in improving the energy density in such cathode materials, we conducted a deep dive into Li2MnO2F as a model cathode material [6]. We interrogated the covalent interaction between the oxygen 2p states, fluorine 2p states, and the transition metal 3d orbitals, and their respective contribution to the charge compensation mechanism using X-ray absorption spectroscopy (XAS). XAS allowed us to resolve the role of both oxygen and fluorine in the electrochemical activity and how covalent interactions are affected by redox, both in extent and in their reversibility.

Probing Crossover Degradation Effects in Nickel-Rich LiNixMnyCozO2 Lithium-Ion Battery Cathodes with Ultrasensitive on-Chip Electrochemistry Mass Spectrometry

Performance improvements in electric vehicle batteries are needed in order to reduce their cost and encourage greater use.1 This improvement is dependent on the properties (e.g. specific capacity and stability) of the cathode active material in the electric vehicle’s lithium ion battery.1 Lithium Nickel Cobalt Manganese Oxide (NMC) is a layered transition metal oxide that shows great promise as an electrode in lithium ion batteries for electric vehicles, with a high theoretical specific capacity and good stability in the layered structure.2 As the nickel content of the cathode material increases, so does the discharge capacity, however this increase comes at the cost of significantly decreased capacity retention.1 The mechanisms that contribute to this degradation are complex and interlinking and many of them are accompanied by some form of gas evolution. For example, the solid electrolyte interphase (SEI) formation at the cathode evolves CO2, which is able to crossover and contribute to solid electrolyte interphase formation at the anode.1 Similarly, upon electrochemical cycling a reactive form of oxygen can be evolved from the NMC lattice, resulting in a cascade of parasitic reactions within a lithium-ion battery.2 Herein, we probe these gas evolving degradation mechanisms through the development and use of a novel type of electrochemistry mass spectrometry (EC-MS) with unprecedented time resolution and sensitivity.3,4 The new technique, known as on-chip EC-MS, employs a microfabricated membrane chip to precisely control the transfer of volatile species from an electrochemical cell to a mass spectrometer. Its design also allows instantaneous gas exchange, particularly useful to simulate cross talk phenomena. This work demonstrates the first application of this technique to the study of lithium-ion batteries; this study uses a new cell design to facilitate operando measurements of gas evolution in lithium-ion batteries to provide important insight into these complex mechanisms.4 More specifically, the effect of transition metal dissolution on the stability of the anodic SEI is investigated by monitoring ethylene evolution. Isotopic labelling studies are also performed to probe the evolution and consumption of CO2 that is evolved from the cathode. Complementary and correlative ex situ surface sensitive analysis (such as x-ray photoelectron spectroscopy and secondary ion mass spectrometry) are carried out in order to develop a holistic understanding of the complex reactivity and chemistry evolution of lithium-ion batteries during operation.References(1) Jung, R.; Metzger, M.; Maglia, F.; Stinner, C.; Gasteiger, H. A. Chemical versus Electrochemical Electrolyte Oxidation on NMC111, NMC622, NMC811, LNMO, and Conductive Carbon. J. Phys. Chem. Lett. 2017, 8 (19), 4820–4825.(2) Wandt, J.; Freiberg, A. T. S.; Ogrodnik, A.; Gasteiger, H. A. Singlet Oxygen Evolution from Layered Transition Metal Oxide Cathode Materials and Its Implications for Lithium-Ion Batteries. Mater. Today 2018, 21 (8), 825-833.(3) Trimarco, D. B.; Scott, S. B.; Thilsted, A. H.; Pan, J. Y.; Pedersen, T.; Hansen, O.; Chorkendorff, I.; Vesborg, P. C. K; Stephens, I. E. L. Enabling Real-Time Detection of Electrochemical Desorption Phenomena with Sub-Monolayer Sensitivity. Electrochim. Acta 2018, 268, 520–530.(4) Thornton D. B.; Cavalca F.; Aguadero A.; Ryan M.; Stephens I.E.; UK Patent filed 17 September 2021

Hybrid Halide Solid Electrolytes and Bottom-up Cell Assembly Enable High Voltage Solid-State Lithium Batteries

Interface between halide based solid electrolytes and layered transition metal oxide cathodes has been found to be electro-chemically stable due to stability of chloride compounds, in particular, at >4V range. The extent of interfacial stability is correlated with the type of cationic and anionic species in the solid electrolyte compound, a fact supported by theoretical prediction and yet, not accurately measured in composite cathode mixtures. By altering the architecture of cathode into a dense additive-free structure, we have identified differences in interfacial stability of chloride compounds which are hidden in composite cathode formats. In this work, we report the use of dense cathode to track the electrochemical evolution of interface between a hybrid halide solid electrolyte composed of chloride and fluoride species. Introducing fluoride compounds is known to be a promising method to expand the oxidation stability while the nature of such expansion is found to be related to kinetics rather than thermodynamics, we report. Furthermore, fluorination of solid electrolyte is generally accompanied with loss of ionic conductivity due to strong electronegative fluoride ions. We demonstrate a fundamental change of solid-state battery assembly from conventional electrolyte pelletizing followed by electrode placement, to a bottom-up assembly route starting with dense cathode, thin (<20µm) layer of SE and anode addition, which compensates for the suppressed conductivity of fluorinated halide solid electrolytes. Through extensive characterization, compositional optimization, and electrochemical interfacial analysis, we demonstrate stable cycling of LiCoO2/hybrid halide solid electrolyte up to 4.4V vs. Li. Our findings pave the way for expanding the voltage stability of solid electrolytes without compromising the cell performance due to ionic conductivity overpotential issues.

Understanding the Degradation Mechanisms of Lithium Ion Batteries Using in-Situ Multi-Scale Diffraction Techniques

Advanced Li-ion batteries adopting new cathode chemistries are required for the successful widespread transition to electric vehicles (EVs) and renewable energy sources, aiming for high energy density, long cycle life, and good rate capability. Commercial candidates for EV batteries include Ni-rich Li(NixMnyCo1−x-y)O2 (NMC) cathodes, with Ni:Mn:Co ratios of 8:1:1 (NMC811) and higher. These are favored because of their high specific capacity (~ 200 mAh g-1)and reduced cobalt content. Despite all of the advantages, these materials suffer from a range of degradation modes, many of which are associated with the redox and crystallographic behavior at high states of charge. In particular, Ni-rich cathodes suffer from several limitations, such as rapid capacity fade in comparison to NMC stoichiometries with lower Ni content. In addition, they also have a lower onset voltage for oxygen release and subsequent surface reconstruction leading to the formation of spinel and rock salt phases which impede (de)lithiation and therefore the achievable capacity of the cell.1 Crystallographic properties of electrode materials are intrinsically linked to the electrochemical performance of the cell. NMC materials suffer from anisotropic changes in the crystal structure during cycling which induces strain and leads to issues such as crack formation, expediting degradation. One method to tackle capacity fade is to switch to single-crystal morphologies (particle size 1-3 μm) which have better mechanical stability than conventional polycrystalline morphologies (secondary agglomerate particles ~ 10 μm made up of primary particles which are 100 nm – 1 μm in size) and have less propensity to form extensive rock-salt layers. It is thought that the single-crystal morphology helps to reduce stress in the material as the anisotropic stress in polycrystalline cathodes is concentrated at grain boundaries. However, there is still a limited understanding of the subtle mechanistic differences between the two materials during cycling.2 A multi-scale approach is required to gain a more comprehensive understanding of the degradation mechanisms at play and how they initiate and propagate. In this work, synchrotron diffraction methods were employed at the crystal, particle and cell scale using a variety of techniques including in-situ Bragg Coherent Diffraction Imaging (BCDI), 3D-XRD and operando high-resolution XRD. Intra-particle, inter-particle and electrode level heterogeneities were observed during cycling, both in pristine and aged samples. It is believed that these heterogeneities accelerate the loss of performance at the cell level by inducing crack formation which can then be observed in X-ray computed tomography data acquired in simultaneous lab studies. The overarching goal of these investigations is to add to the understanding of complex degradation mechanisms for Ni-rich layered transition metal oxide cathodes, ultimately aiding in the informed development of future battery electrode materials.

Triggering Zn2+ Unsaturated Hydration Structure via Hydrated Salt Electrolyte for High Voltage and Cycling Stable Rechargeable Aqueous Zn Battery

AbstractOcean‐like free water in conventional aqueous electrolytes limits operating voltage and induces parasitic reactions in rechargeable Zn metal batteries (AZMBs). Herein, a hydrated salt electrolyte (HSE), which suppresses the presence of free water and triggers an unsaturated hydration structure (Zn(H2O)n2+, n &lt; 6), is proposed to circumvent the parasitic reactions occurring on the zinc anode and cathode and elevate the decomposition voltage (to ≈2.55 V vs Zn2+/Zn). In a full cell with a transition metal oxide cathode, the parasitic reactions of dissolution and diffusion of discharge products are inhibited, and high discharge capacity retention above 80% at 500 mA g‐1 and super‐stable Coulombic efficiency (average value ≈99.97%) are delivered. Moreover, when applied in a high‐voltage AZMB with zinc hexacyanoferrate cathode, the full cell yields an average output voltage above 1.8 V together with a 132.3 Wh kg‐1 energy density at 10 mA g‐1 and 1.78 V output voltage combining high energy density and power density (88.5 Wh kg‐1/106.2 W kg‐1). The exceptional electrochemical performance of HSE indicates rosy prospects for the prevention of parasitic reactions in AZMBs, a key step on the road to further commercialization of AZMBs.

Li8MnO6: A Novel Cathode Material with Only Anionic Redox.

In Li-excess transition metal-oxide cathode materials, anionic oxygen redox can offer high capacity and high voltages, although peroxo and superoxo species may cause oxygen loss, poor cycling performance, and capacity fading. Previous work showed that undesirable formation of peroxide and superoxide bonds was controlled to some extent by Mn substitution, and the present work uses density functional calculations to examine the reasons for this by studying the anionic redox mechanism in Li8MnO6. This material is obtained by substituting Mn for Sn in Li8SnO6 or for Zr in Li8ZrO6, and we also compare this to previous work on those materials. The calculations predict that Li8MnO6 is stable at room temperature (with a band gap of 3.19 eV as calculated by HSE06 and 1.82 eV as calculated with the less reliable PBE+U), and they elucidate the chemical and structural effects involved in the inhibition of oxygen release in this cathode. Throughout the whole delithiation process, only O2- ions are oxidized. The directional Mn-O bonds formed from unfilled 3d orbitals effectively inhibit the formation of O-O bonds, and the layered structure is maintained even after removing 3 Li per Li8MnO6 formula unit. The calculated average voltage for removal of 3 Li is 3.69 V by HSE06, and the corresponding capacity is 389 mAh/g. The high voltage of oxygen anionic redox and the high capacity result in a high energy density of 1436 Wh/kg. The Li-ion diffusion barrier for the dominant interlayer diffusion path along the c axis is 0.57 eV by PBE+U. These results help us to understand the oxygen redox mechanism in a new lithium-rich Li8MnO6 cathode material and contribute to the design of high-energy density lithium-ion battery cathode materials with favorable electrochemical properties based on anionic oxygen redox.

A novelty strategy induced pinning effect and defect structure in Ni-rich layered cathodes towards boosting its electrochemical performance

Layered Ni-rich transition metal oxide is treated as the most promising alternative cathode due to their high-capacity and flexible composition. However, the severe lattice strain and slow Li-ion migration kinetics severely restrict their practical application. Herein, a novelty strategy induced pinning effect and defect structure in layered Ni-rich transition metal oxide cathodes is proposed via a facile cation(iron ion)/anion(polyanion) co-doping method. Subsequently, the effects of pinning effect and defect structure on element valence state, crystal structure, morphology, lattice strain, and electrochemical performance during lithiation/delithiation are systematically explored. The detailed characterizations (soft X-ray absorption spectroscopy (sXAS), in-situ X-ray diffraction (XRD), etc.) and density functional theory (DFT) calculation demonstrate that the pinning effects built-in LiNi0.9Co0.05Mn0.05O2 materials by the dual-site occupation of iron ions on lithium and transition metal sites effectively alleviate the abrupt lattice strain caused by an unfavorable phase transition and the subsequent induction of defect structures in the Li layer can greatly reduce the lithium-ion diffusion barrier. Therefore, the modified LiNi0.9Co0.05Mn0.05O2 exhibits a high-capacity of 206.5 mAh g−1 and remarkably enhanced capacity retention of 93.9% after 100 cycles, far superior to ∼14.1% of the pristine cathodes. Besides, an excellent discharge capacity of 180.1 mAh g−1 at 10 C rate is maintained, illustrating its remarkable rate capability. This work reports a pinning effect and defect engineering method to suppress the lattice strain and alleviate lithium-ion kinetic barriers in the Ni-rich layered cathodes, providing a roadmap for understanding the fundamental mechanism of an intrinsic activity modulation and structural design of layered cathode materials.

Understanding the nucleation and growth of the degenerated surface structure of the layered transition metal oxide cathodes for lithium-ion batteries by operando Raman spectroscopy

• The surface degradation process of the lithium-layered transition metal oxide cathode is revealed by an operando Raman spectroscopy study. • The degradation process is visualized by Raman mapping with the spectral window of ∼600 cm −1 . • The degradation process not only involves the structural changes, but also the growth and merging between the degenerated layers on charge–discharge. • The present study offers crucial information to counter the capacity decay of the NMC batteries. The well-defined layered structure for Li + (de)intercalation of transition metal oxide cathodes plays a crucial role in delivering high capacity for lithium-ion batteries. Therefore, structural deformations of the layered structure are often considered as the evidence of deterioration in the battery performance and stability. In this study, the surface degradation mechanism of the layered LiNi 1/3 Co 1/3 Mn 1/3 O 2 cathode is examined on charge–discharge using an operando Raman cell. We revealed the surface degradation process of the cathode with the A 1g Raman spectral window (∼600 cm −1 ) in the voltage range of 2.8–4.35 V, and proposed the nucleation-merging mechanism. The spectral shifts and mappings have been compared between the early-stage and later-stage cycles (≥15 cycles). It has been identified that the degradation process not only involves the structural changes (i.e., nucleation of the degenerated layers), but also the growth/merging between the degenerated layers during charge and discharge. This observation provides key information on how the reconstructed surface can expand during cycling.

Preferential Extraction of Lithium from Spent Cathodes and the Regeneration of Layered Oxides for Li/Na-Ion Batteries

The preferentially selective extraction of Li+ from spent layered transition metal oxide (LiMO2, M = Ni, Co, Mn, etc.) cathodes has attracted extensive interest based on economic and recycling efficiency requirements. Presently, the efficient recycling of spent LiMO2 is still challenging due to the element loss in multistep processes. Here, we developed a facile strategy to selectively extract Li+ from LiMO2 scraps with stoichiometric H2SO4. The proton exchange reaction could be driven using temperature, accompanied by the generation of soluble Li2SO4 and MOOH precipitates. The extraction mechanism includes a two-stage evolution, including dissolution and ion exchange. As a result, the extraction rate of Li+ is over 98.5% and that of M ions is less than 0.1% for S-NCM. For S-LCO, the selective extraction result is even better. Finally, Li2CO3 products with a purity of 99.68% can be prepared from the Li+-rich leachate, demonstrating lithium recovery efficiencies as high as 95 and 96.3% from NCM scraps and S-LCO scraps, respectively. In the available cases, this work also represents the highest recycling efficiency of lithium, which can be attributed to the high leaching rate and selectivity of Li+, and even demonstrates the lowest reagent cost. The regenerated LiNi0.5Co0.24Mn0.26O2 and Na1.01Li0.001Ni0.38Co0.18Mn0.44O2 cathodes also deliver a decent electrochemical performance for Li-ion batteries (LIBs) and Na-ion batteries (NIBs), respectively. Our current work offers a facile, closed-loop, and scalable strategy for recycling spent LIB cathodes based on the preferentially selective extraction of Li+, which is superior to the other leaching technology in terms of its cost and recycling yield.

Ultrafast Synthesis of Layered Transition-Metal Oxide Cathodes from Metal-Organic Frameworks for High-Capacity Sodium-Ion Batteries.

Layered transition-metal oxides are promising candidate cathode materials for sodium-ion batteries due to their abundant raw materials and high theoretical capacity. Nevertheless, a long-time high-temperature heat treatment is required in traditional preparation methods, leading to low synthesis efficiency and waste of energy. Herein, an ultrafast preparation method of layered transition-metal oxides was proposed through minute calcination of metal-organic frameworks (MOFs). The homogeneous distribution of different atoms in MOFs allows fast phase transition during the calcination process. P'2-phase layered sodium manganese oxide was successfully obtained and demonstrated excellent electrochemical performance, with a high reversible capacity of 212 mA h g-1 and a cycling performance of 84% capacity retention after 100 cycles. Furthermore, this method can be expanded to a wide variety of MOF precursors and oxide electrode materials for different types of batteries. Our findings provide an efficient and cost-effective synthesis method for high-performance layered transition-metal oxide cathodes.

Correlation of Oxygen Anion Redox Activity to In‐Plane Honeycomb Cation Ordering in NaxNiyMn1−yO2 Cathodes

Sodium‐ion batteries (SIBs) are one of the most promising next‐generation energy storage systems because of their abundant and low‐cost component materials. However, the lower energy density of SIBs compared with lithium‐ion batteries diminishes their practical value proposition. Among the many sodium‐based cathodes, layered transition metal oxides with high sodium content have energy densities comparable with the lithium‐ion battery technology. When charged above 4.1 V, the sodium‐based cathodes often undergo transformations because the activation of oxygen anion redox causes irreversible oxygen release, transition metal ion migration, lattice distortion, and rapid capacity decay. Here, in situ gas analysis is performed to evaluate the lattice oxygen anion redox activity in NaxNiyMn1−yO2 cathodes with P2 and O3 structural orderings. Operando X‐ray diffraction and neutron diffraction are performed to assess the structural changes related to lattice oxygen redox and transition metal ion migration in NaxNiyMn1−yO2 cathodes. The results unveil that in‐plane honeycomb cationic ordering can help suppress oxygen anion redox activity, which is critical for the future design of layered transition metal oxide cathodes that are prone to achieve high‐energy for durable SIBs.

Coincident formation of trapped molecular O2 in oxygen-redox-active archetypical Li 3d oxide cathodes unveiled by EPR spectroscopy

• Molecular O 2 trapped in the bulk is the main oxidized oxygen species in Li 3 d oxide cathodes • The suppression of out-of-plane cation migration does not refrain the formation of molecular O 2 • The local structural reversibility on oxygen redox depends on the inhibition of cation disorder Li-excess 3 d transition metal oxides with additional capacity contribution via oxygen redox are promising high-energy-density cathodes for next-generation Li-ion batteries. However, the chemical state of oxidized oxygen in the bulk of charged materials has been manifested very challenging to clarify and remains elusive. We herein apply the electron paramagnetic resonance (EPR) spectroscopy to uncover the oxidized oxygen states formed in the bulk of archetypical Li 3 d oxide cathodes on oxygen redox, including O3-Li 1.2 Ni 0.2 Mn 0.6 O 2 and Li 1.2 Ni 0.13 Co 0.13 Mn 0.54 O 2 , O2-Li 1.033 Ni 0.2 Mn 0.6 O 2 , and disordered rocksalt Li 1.2 Ti 0.4 Mn 0.4 O 2 . The results substantiate the coincident formation of molecular O 2 trapped in the bulk of charged cathodes, which can be reduced back to O 2− on discharging. The implication is that in contrary to the conventional wisdom, the suppression of out-of-plane cation migration does not refrain the formation of molecular O 2 . Moreover, the NMR study suggests that the local structural reversibility on oxygen redox depends on the inhibition of cation disorder rather than the formation of specific oxidized oxygen. This study advances our basic understanding of oxygen redox in Li-excess 3 d transition metal oxide cathodes. The coincident formation of trapped molecular O 2 on charging in four archetypical oxygen-redox-active Li 3 d oxide cathodes was certified by advanced EPR spectroscopy.

Formation of LiF-rich Cathode-Electrolyte Interphase by Electrolyte Reduction.

The capacity of transition metal oxide cathode for Li‐ion batteries can be further enhanced by increasing the charging potential. However, these high voltage cathodes suffer from fast capacity decay because the large volume change of cathode breaks the active materials and cathode‐electrolyte interphase (CEI), resulting in electrolyte penetration into broken active materials and continuous side reactions between cathode and electrolytes. Herein, a robust LiF‐rich CEI was formed by potentiostatic reduction of fluorinated electrolyte at a low potential of 1.7 V. By taking LiCoO2 as a model cathode, we demonstrate that the LiF‐rich CEI maintains the structural integrity and suppresses electrolyte penetration at a high cut‐off potential of 4.6 V. The LiCoO2 with LiF‐rich CEI exhibited a capacity of 198 mAh g−1 at 0.5C and an enhanced capacity retention of 63.5 % over 400 cycles as compared to the LiF‐free LiCoO2 with only 17.4 % of capacity retention.