Jointly Improving Anionic-Cationic Redox Reversibility of Lithium-Rich Manganese-Based Cathode Materials by N Surface Doping

The lithium-rich manganese-based layer oxide (LRMO) with high specific capacity (∼300 mAh g-1) and economic feasibility is accepted as the cathode material for high energy density rechargeable batteries. Accompanied by the additional anionic redox reactions during the initial charging process, LRMO presents oxygen release, sluggish Li+ diffusion, and irreversible transition metal ion (TM) migration, which is responsible for its severe structural deterioration and rapid capacity/voltage decay. Here, the N doping strategy is proposed via feasible treatment of oxygen-vacancy-containing Li1.16Ni0.21Mn0.63O2-δ (LNMO) particles. The as obtained LNMO-N samples demonstrate doping N, partially reduced Mn/Ni cations, and oxygen vacancies on the surface. The DFT calculations and experimental results demonstrate that N replacing the crystal oxygen sites on the surface reduces the energy barrier for diffusion, thereby enhancing the kinetics of Li+ diffusion and improving the reversibility of transition metal migration. Furthermore, N doping induces stacking faults and a more flexible structure. Therefore, LNMO-N exhibits a significantly improved anionic-cationic redox reaction reversibility with a high discharge specific capacity of 296.6 mAh g-1 at 20 mA g-1 within the range of 2.0 to 4.8 V and an impressive initial Coulombic efficiency of 85.9%. Moreover, the rate capability is obviously improved with a remarkable capacity of 215.1 mAh g-1 at 200 mA g-1 in 200 cycles with a capacity retention of 72.52% and exceptional performance of 141.4 mAh g-1 even at a higher current density of 1000 mA g-1.

Recent advances in lithium-rich manganese-based cathodes for high energy density lithium-ion batteries

The development of society challenges the limit of lithium-ion batteries (LIBs) in terms of energy density and safety. Lithium-rich manganese oxide (LRMO) is regarded as one of the most promising cathode materials owing to its advantages of high voltage and specific capacity (more than 250 mA h g^-1) as well as low cost. However, the problems of fast voltage/capacity fading, poor rate performance and the low initial Coulombic efficiency severely hinder its practical application. In this paper, we review the latest research advances of LRMO cathode materials, including crystal structure, electrochemical reaction mechanism, existing problems and modification strategies. In this review, we pay more attention to recent progress in modification methods, including surface modification, doping, morphology and structure design, binder and electrolyte additives, and integration strategies. It not only includes widely studied strategies such as composition and process optimization, coating, defect engineering, and surface treatment, but also introduces many relatively novel modification methods, such as novel coatings, grain boundary coating, gradient design, single crystal, ion exchange method, solid-state batteries and entropy stabilization strategy. Finally, we summarize the existing problems in the development of LRMO and put forward some perspectives on the further research.

The Importance of Structural Uniformity and Chemical Homogeneity in Cobalt-Free Lithium Excess Nickel Manganese Oxide Cathodes

This study explores the relationships between material quench rate during processing and the resulting structural and electrochemical properties of Li[Ni_0.25 Li_0.167 Mn_0.583 ]O₂ . Samples of this lithium-rich material are prepared with highly contrasting postfiring cooling methods: a rapid water emersion quench or closed-door oven cooling. The contrasting approaches result in samples with different structural, chemical, and electrochemical behaviors; after cycling the rapidly quenched material yields greater capacity, greater stability, and initially lower, but more stable voltages than the slower cooled samples. Through the use of scanning tunneling electron microscopy, X-Ray Diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) it is demonstrated that rapidly quenched powders are more structurally uniform and chemically homogenous before cycling. By comparing these precycling sample to postcycling samples, it is then examined how this increased structural uniformity and chemical homogeneity leads to the superior electrochemical properties of the rapidly quenched samples.

Enhanced cycling stability and storage performance of Na0.67Ni0.33Mn0.67-xTixO1.9F0.1 cathode materials by Mn-rich shells and Ti doping

We propose a synergistic strategy of titanium doping and surface coating with a Mn-rich shell to modify the Na-rich manganese-oxide-based cathode material Na_0.67Ni_0.33Mn_0.67-xTi_xO_1.9F_0.1 in sodium-ion batteries and elucidate the underlying mechanism for enhanced material performance. First, it is found that the electrochemical performance of the proposed cathode material can be effectively improved when the Ti doping amount is x = 0.3. In addition to doping, the cathode material coated with a manganese-rich shell was prepared by a liquid coating method. The as-prepared Mn@Ti-doped-Na_0.67Ni_0.33Mn_0.37Ti_0.3O_1.9F_0.1 exhibited excellent electrochemical performance, delivering 169 mAh/g discharge capacity. The charge-discharge cycle test was carried out at a current density of 2C, and the sample not only provides a reversible capacity of 119 mAh/g but also has a capacity retention rate of 71 % after 500 charge-discharge cycles. The Ti doping and surface coating with a Mn-rich shell are shown to improve the specific discharge capacity, cycling stability and rate capability of the cathode material and mitigate voltage decay. These results validate our design principle and provide a novel approach to enhance the performance of cathode materials in sodium-ion batteries.

Understanding and Control of Activation Process of Lithium-Rich Cathode Materials

Lithium-rich materials (LRMs) are among the most promising cathode materials toward next-generation Li-ion batteries due to their extraordinary specific capacity of over 250 mAh g−1 and high energy density of over 1 000 Wh kg−1. The superior capacity of LRMs originates from the activation process of the key active component Li2MnO3. This process can trigger reversible oxygen redox, providing extra charge for more Li-ion extraction. However, such an activation process is kinetically slow with complex phase transformations. To address these issues, tremendous effort has been made to explore the mechanism and origin of activation, yet there are still many controversies. Despite considerable strategies that have been proposed to improve the performance of LRMs, in-depth understanding of the relationship between the LRMs’ preparation and their activation process is limited. To inspire further research on LRMs, this article firstly systematically reviews the progress in mechanism studies and performance improving attempts. Then, guidelines for activation controlling strategies, including composition adjustment, elemental substitution and chemical treatment, are provided for the future design of Li-rich cathode materials. Based on these investigations, recommendations on Li-rich materials with precisely controlled Mn/Ni/Co composition, multi-elemental substitution and oxygen vacancy engineering are proposed for designing high-performance Li-rich cathode materials with fast and stable activation processes.

Graphical abstract

The “Troika” of composition adjustment, elemental substitution, and chemical treatment can drive the Li-rich cathode towards stabilized and accelerated activation.

Oxygen Loss in Layered Oxide Cathodes for Li-Ion Batteries: Mechanisms, Effects, and Mitigation

Layered lithium transition metal oxides derived from LiMO₂ (M = Co, Ni, Mn, etc.) have been widely adopted as the cathodes of Li-ion batteries for portable electronics, electric vehicles, and energy storage. Oxygen loss in the layered oxides is one of the major factors leading to cycling-induced structural degradation and its associated fade in electrochemical performance. Herein, we review recent progress in understanding the phenomena of oxygen loss and the resulting structural degradation in layered oxide cathodes. We first present the major driving forces leading to the oxygen loss and then describe the associated structural degradation resulting from the oxygen loss. We follow this analysis with a discussion of the kinetic pathways that enable oxygen loss, and then we address the resulting electrochemical fade. Finally, we review the possible approaches toward mitigating oxygen loss and the associated electrochemical fade as well as detail novel analytical methods for probing the oxygen loss.

Recent Development of Nickel-Rich and Cobalt-Free Cathode Materials for Lithium-Ion Batteries

The exponential growth in the production of electric vehicles requires an increasing supply of low-cost, high-performance lithium-ion batteries. The increased production of lithium-ion batteries raises concerns over the availability of raw materials, especially cobalt for batteries with nickel-rich cathodes, in which these constraints can impact the high price of cobalt. The reliance on cobalt in these cathodes is worrisome because it is a high-cost, rare material, with an unstable supply chain. This review describes the need and feasibility of developing cobalt-free high-nickel cathode materials for lithium-ion batteries. The new type of cathode material, LiNi1−x−yMnxAlyO2 promises a completely cobalt-free composition with almost the same electrochemical performance as that of the conventional high-nickel cathode. Therefore, this new type of cathode needs further research for its commercial applications.

Li1.2Mn0.54Ni0.13Co0.13O2 nanosheets with porous structure as a high-performance cathode material for lithium-ion batteries

The morphological and structural optimizations of electrode materials are efficient ways to enhance their electrochemical performance. Herein, we report a facile co-precipitation and subsequent calcination method to fabricate Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ nanosheets consisting of interconnected primary nanoparticles and open holes through the full thickness. By comparing the nanosheets and the agglomerated nanoparticles, the effects of the morphology and structure on the electrochemical performance are investigated. Specifically, the nanosheets exhibit a discharge capacity of 210 mA h g⁻¹ at 0.5C with a capacity retention of 85% after 100 cycles. The improved electrochemical performance could be attributed to their morphological and structural improvements, which may facilitate sufficient electrolyte contacts, short diffusion paths and good structural integrity during the charge/discharge process. This work provides a feasible approach to fabricate lithium-rich layered oxide cathode materials with 2D morphology and porous structure, and reveals the relationships between their morphology, structure and electrochemical performance.

Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ nanosheets with interconnected nanoparticles and open holes are prepared by co-precipitation and calcination processes, exhibiting improved electrochemical performance compared to agglomerated nanoparticles.

Controllable preparation of one-dimensional Li1.2Mn0.54Ni0.13Co0.13O2 cathode materials for high-performance lithium-ion batteries

Lithium-rich layered oxides are attractive candidates of high-energy-density cathode materials for high-performance lithium ion batteries because of their high specific capacity and low cost. Nevertheless, their unsatisfactory rate capability and poor cycling stability have strongly hindered commercial applications in lithium ion batteries, mainly due to the ineffectiveness of the complicated synthesis techniques to control their morphologies and sizes. In this work, the Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ cathode materials with a one-dimensional rod-like morphology were synthesized via a facile co-precipitation route followed by a post-calcination treatment. By reasonably adding NH₃·H₂O in the co-precipitation reaction, the sizes of the metal oxalate precursors could be rationally varied. The electrochemical measurements displayed that the Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ short rods delivered a high capacity of 286 mA h g^-1 at 0.1C and excellent capacity retention of 85% after 100 cycles, which could be contributed to the improvement of the electrolyte contact, Li⁺ diffusion, and structural stability of the one-dimension porous structure.

Controllable fabrication of Li-rich layered oxide Li1.2Mn0.54Ni0.13Co0.13O2 microspheres for enhanced electrochemical performance

Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ microspheres assembled by nanoplates are prepared by a co-precipitation and calcination method using metal oxalate microspheres as a template, exhibiting improved electrochemical properties compared to the nanoplates.

Breaking Free from Cobalt Reliance in Lithium-Ion Batteries

The exponential growth in demand for electric vehicles (EVs) necessitates increasing supplies of low-cost and high-performance lithium-ion batteries (LIBs). Naturally, the ramp-up in LIB production raises concerns over raw material availability, where constraints can generate severe price spikes and bring the momentum and optimism of the EV market to a halt. Particularly, the reliance of cobalt in the cathode is concerning owing to its high cost, scarcity, and centralized and volatile supply chain structure. However, compositions suitable for EV applications that demonstrate high energy density and lifetime are all reliant on cobalt to some degree. In this work, we assess the necessity and feasibility of developing and commercializing cobalt-free cathode materials for LIBs. Promising cobalt-free compositions and critical areas of research are highlighted, which provide new insight into the role and contribution of cobalt.

Stabilizing Li-rich NMC Materials by Using Precursor Salts with Acetate and Nitrate Anions for Li-ion Batteries

Lithium-rich layered oxide cathode materials of Li1.2Mn0.5100Ni0.2175Co0.0725O2 have been synthesized using metal salts with acetate and nitrate anions as precursors in glycerol solvent. The effects of the precursor metal salts on particle size, morphology, cationic ordering, and ultimately, the electrode performance of the cathode powders have been studied. It was demonstrated that the use of cornstarch as a gelling agent with nitrate-based metal salts results in a reduction of particle size, leading to higher surface area and initial discharge capacity. However, the cornstarch gelling effect was minimized when acetate salts were used. As observed in the Fourier-transform infrared spectroscopy analysis, cornstarch can react with acetates to form acetyl groups during the synthesis, effectively preventing the cornstarch gel from capping the particles, thus leading to larger particles. A tradeoff was found when nitrate and acetate salts were mixed in the synthesis. It was shown that the new cathode powder has the best cationic ordering and capacity retention, promising a much stable Li-rich cathode material for lithium-ion batteries.

Understanding the Discrepancy of Defect Kinetics on Anionic Redox in Lithium-Rich Cathode Oxides

Reversible anionic (oxygen) redox in lithium-rich cathode oxides has been becoming a blooming research topic to further boost the energy density in lithium-ion batteries. There are numerous experimental observations and theoretical calculations to illustrate the importance of defects on anionic redox activity, but how the defects on the surface and bulk control the kinetics of anionic redox is not well understood. Here, we uncover this intriguing ambiguity on the correlation among defects states, Li-ion diffusion, and oxygen redox reaction. It is found that the surface-defective microstructure has fast Li-ion diffusion to achieve superior cationic redox activities/kinetics, whereas the bulk-defective microstructure corresponds to a slow Li-ion diffusion to result in poor cationic redox activities/kinetics. By contrast, both surface and bulk defects can be of benefit to the enhancement of oxygen redox activities/kinetics. Moreover, a positive correlation is also established among charge-transfer resistance, interface reaction charge-transfer activation energy, and oxygen redox activity in these electrode materials. This study on defect-anionic activity provides a new insight for controlling anionic redox reaction in lithium-rich cathode materials for real-world application.

Investigation of Fluorine and Nitrogen as Anionic Dopants in Nickel-Rich Cathode Materials for Lithium-Ion Batteries

Advanced lithium-ion batteries are of great interest for consumer electronics and electric vehicle applications; however, they still suffer from drawbacks stemming from cathode active material limitations (e.g., insufficient capacities and capacity fading). One approach for alleviating such limitations and stabilizing the active material structure may be anion doping. In this work, fluorine and nitrogen are investigated as potential dopants in Li_1.02(Ni_0.8Co_0.1Mn_0.1)_0.98O₂ (NCM) as a prototypical nickel-rich cathode active material. Nitrogen doping is achieved by ammonia treatment of NCM in the presence of oxygen, which serves as an unconventional and new approach. The crystal structure was investigated by means of Rietveld and pair distribution function analysis of X-ray diffraction data, which provide very precise information regarding both the average and local structure, respectively. Meanwhile, time-of-flight secondary-ion mass spectroscopy was used to assess the efficacy of dopant incorporation within the NCM structure. Moreover, scanning electron microscopy and scanning transmission electron microscopy were conducted to thoroughly investigate the dopant influences on the NCM morphology. Finally, the electrochemical performance was tested via galvanostatic cycling of half- and full-cells between 0.1 and 2 C. Ultimately, a dopant-dependent modulation of the NCM structure was found to enable the enhancement of the electrochemical performance, thereby opening a route to cathode active material optimization.

Influence of Ni/Mn distributions on the structure and electrochemical properties of Ni-rich cathode materials

To reveal the influence of element distribution on the structure and electrochemical performances of Ni-rich layered cathode materials, LiNi0.68Co0.13Mn0.19O2 (NCM) with four types of Ni/Mn distributions (homogeneous, core-shell, multi-shell and concentration-gradient structures) is designed and synthesized with a combination of co-precipitation and high-temperature solid-state method. Ni/Mn distributions of the as-prepared NCM cathode materials are investigated with focused ion beam (FIB) and energy disperse X-ray spectrum (EDS) line scanning on the cross-section of single particles, which illustrate that NCM materials with the desired Ni/Mn distributions are successfully prepared. For the three spherical heterogeneous NCM materials, the center is the Ni-rich component while the surface is the Mn-rich component. Ni/Mn distributions between the center and surface components are in different forms. Studies imply that the heterogeneous samples exhibit smaller cation disordering, lower charge transfer resistance, higher Li+ diffusion coefficient and higher structural stability than the homogeneous one. Therefore, the heterogeneous samples, especially the multi-shell and concentration-gradient ones, display improved cycling and thermal stability compared to the homogeneous one. These results manifest that multi-shell and concentration-gradient structures are effective strategies to modify the layered NCM cathode materials for Li-ion batteries.

Horizons for Li-Ion Batteries Relevant to Electro-Mobility: High-Specific-Energy Cathodes and Chemically Active Separators

Li-ion batteries (LIBs) today face the challenge of application in electrified vehicles (xEVs) which require increased energy density, improved abuse tolerance, prolonged life, and low cost. LIB technology can significantly advance through more realistic approaches such as: i) stable high-specific-energy cathodes based on Li_{1+ x} Ni_y Co_z Mn_w O₂ (NCM) compounds with either Ni-rich (x = 0, y → 1), or Li- and Mn-rich (0.1 < x < 0.2, w > 0.5) compositions, and ii) chemically active separators and binders that mitigate battery performance degradation. While the stability of such cathode materials during cell operation tends to decrease with increasing specific capacity, active material doping and coatings, together with carefully designed cell-formation protocols, can enable both high specific capacities and good long-term stability. It has also been shown that major LIB capacity fading mechanisms can be reduced by multifunctional separators and binders that trap transition metal ions and/or scavenge acid species. Here, recent progress on improving Ni-rich and Mn-rich NCM cathode materials is reviewed, as well as in the search for inexpensive, multifunctional, chemically active separators. A realistic overview regarding some of the most promising approaches to improving the performance of rechargeable batteries for xEV applications is also presented.

Unravelling the Structure and Electrochemical Performance of Li-Cr-Mn-O Cathodes: From Spinel to Layered

To explore a new series of cathode materials with high electrochemical performance, the spinel-layered (1 - x)[LiCrMnO₄]· x[Li₂MnO₃·LiCrO₂] ( x = 0, 0.25, 0.5, 0.75, and 1) composites are synthesized with the sol-gel method. X-ray diffraction, high-resolution transmission electron microscopy, selected area electron diffraction, and Raman spectra reveal that the structure of the (1 - x)[LiCrMnO₄]· x[Li₂MnO₃·LiCrO₂] cathode materials evolves from spinel to hybrid spinel-layered and layered structures with the increase of the Li concentration. Test results reveal that the structure and electrochemical performance of (1 - x)[LiCrMnO₄]· x[Li₂MnO₃·LiCrO₂] ( x = 0.25, 0.5 and 0.75) composites have the characteristics of both spinel ( x = 0) and Li-rich layered phases ( x = 1). In particular, x = 0.5 and 0.75 electrodes exhibit relatively high capacity retention and rate capability, which is mainly ascribed to the synergistic effect of the spinel and Li-rich layered phases, the 3D Li-ion diffusion channels of the spinel phase, and the low charge-transfer resistance ( R_ct) and Warburg diffusion impedance ( W_o).

On the Ageing of High Energy Lithium-Ion Batteries-Comprehensive Electrochemical Diffusivity Studies of Harvested Nickel Manganese Cobalt Electrodes

This paper examines the impact of the characterisation technique considered for the determination of the Li+ solid state diffusion coefficient in uncycled as in cycled Nickel Manganese Cobalt oxide (NMC) electrodes. As major characterisation techniques, Cyclic Voltammetry (CV), Galvanostatic Intermittent Titration Technique (GITT) and Electrochemical Impedance Spectroscopy (EIS) were systematically investigated. Li+ diffusion coefficients during the lithiation process of the uncycled and cycled electrodes determined by CV at 3.71 V are shown to be equal to 3.48×10−10 cm2·s−1 and 1.56×10−10 cm2·s−1 , respectively. The dependency of the Li+ diffusion with the lithium content in the electrodes is further studied in this paper with GITT and EIS. Diffusion coefficients calculated by GITT and EIS characterisations are shown to be in the range between 1.76×10−15 cm2·s−1 and 4.06×10−12 cm2·s−1, while demonstrating the same decreasing trend with the lithiation process of the electrodes. For both electrode types, diffusion coefficients calculated by CV show greater values compared to those determined by GITT and EIS. With ageing, CV and EIS techniques lead to diffusion coefficients in the electrodes at 3.71 V that are decreasing, in contrast to GITT for which results indicate increasing diffusion coefficient. After long-term cycling, ratios of the diffusion coefficients determined by GITT compared to CV become more significant with an increase about 1 order of magnitude, while no significant variation is seen between the diffusion coefficients calculated from EIS in comparison to CV.

Tetrathiafulvalene as a Conductive Film-Making Additive on High-Voltage Cathode

Tetrathiafulvalene (TTF) is investigated as a conductive film-making additive on an overlithiated layered oxide (OLO) cathode. When the OLO/graphite cell is cycled at high voltage, carbonate-based electrolyte without the additive decomposes continuously to form a thick and highly resistant surface film on the cathode. In contrast, TTF added into the electrolyte becomes oxidized before the electrolyte solvents, creating a thinner film on the cathode surface. This film inhibits further electrolyte decomposition through cycling and stabilizes the interface between the cathode and the electrolyte. The cells containing the OLO cathode with TTF-added electrolyte afforded enhanced capacity retention and rate capability, making TTF a prospective electrolyte additive for higher energy density lithium-ion cells.

Advances in electrode materials for Li-based rechargeable batteries

We summarize strategies to enhance the performance of electrode materials for Li-based batteries through nanoengineering and surface coating, and introduce new trends in developing alternative materials, battery concepts and cell configurations.