Role of Redox-Inactive Transition-Metals in the Behavior of Cation-Disordered Rocksalt Cathodes

A green aqueous binder to enhance the electrochemical performance of Li-rich disordered rock salt cathode material

Li-rich disordered rock-salt oxides (DRX) are considered an attractive cathode material in the future battery field due to their excellent energy density and specific capacity. Nevertheless, anionic redox provides high capacity while causing O2 over-oxidation to O2, resulting in voltage hysteresis and capacity decay. Herein, the crystal structure of Li1.3Mn0.4Ti0.3O1.7F0.3 (LMTOF) cathode is stabilized by using sodium carboxymethylcellulose (CMC) binders replacing traditional polyvinylidene difluoride (PVDF) binders. The electrochemical impedance spectroscopy (EIS) and galvanostatic intermittent titration technique (GITT) reveal that the CMC-based LMTOF electrode has higher electronic conductivity and lithium-ion diffusion kinetics. Moreover, CMC has been demonstrated to improve the O2- reversibility, reduce the amounts of byproducts from electrolyte decomposition and suppress transition metal dissolution by Na+/Li+ exchange reaction. Furthermore, the CMC-based LMTOF electrode also exhibits less volume change upon lithiation/delithiation processes compared to the PVDF-based electrode, resulting in enhanced structural stability during cycling. Benefiting from these features, the CMC binders can effectively improve the cycling life and rate performance of the LMTOF cathode, and the CMC-based LMTOF electrode shows good capacity retention of 94.5 % after 30 cycles at 20 mA/g and 66.7 % after 100 cycles at 200 mA/g. This finding indicates that CMC as a binder can efficiently stabilize the structure and improve the electrochemical performance of Li-rich disordered rock-salt oxides cathode, making it possible for practical Li-ion battery applications.

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.

Correlating concerted cations with oxygen redox in rechargeable batteries

Rechargeable batteries currently power much of our world, but with the increased demand for electric vehicles (EVs) capable of traveling hundreds of miles on a single charge, new paradigms are necessary for overcoming the limits of energy density, particularly in rechargeable batteries. The emergence of reversible anionic redox reactions presents a promising direction toward achieving this goal; however this process has both positive and negative effects on battery performance. While it often leads to higher capacity, anionic redox also causes several unfavorable effects such as voltage fade, voltage hysteresis, sluggish kinetics, and oxygen loss. However, the introduction of cations with topological chemistry tendencies has created an efficient pathway for achieving long-term oxygen redox with improved kinetics. The cations serve as pillars in the crystal structure and meanwhile can interact with oxygen in ways that affect the oxygen redox process through their impact on the electronic structure. This review delves into a detailed examination of the fundamental physical and chemical characteristics of oxygen redox and elucidates the crucial role that cations play in this process at the atomic and electronic scales. Furthermore, we present a systematic summary of polycationic systems, with an emphasis on their electrochemical performance, in order to provide perspectives on the development of next-generation cathode materials.

Over-Stoichiometric Metastabilization of Cation-Disordered Rock Salts

Cation-disordered rock salts (DRXs) are well known for their potential to realize the goal of achieving scalable Ni- and Co-free high-energy-density Li-ion batteries. Unlike in most cathode materials, the disordered cation distribution may lead to more factors that control the electrochemistry of DRXs. An important variable that is not emphasized by research community is regarding whether a DRX exists in a more thermodynamically stable form or a more metastable form. Moreover, within the scope of metastable DRXs, over-stoichiometric DRXs, which allow relaxation of the site balance constraint of a rock salt structure, are particularly underexplored. In this work, these findings are reported in locating a generally applicable approach to "metastabilize" thermodynamically stable Mn-based DRXs to metastable ones by introducing Li over-stoichiometry. The over-stoichiometric metastabilization greatly stimulates more redox activities, enables better reversibility of Li deintercalation/intercalation, and changes the energy storage mechanism. The metastabilized DRXs can be transformed back to the thermodynamically stable form, which also reverts the electrochemical properties, further contrasting the two categories of DRXs. This work enriches the structural and compositional space of DRX families and adds new pathways for rationally tuning the properties of DRX cathodes.

Mitigating the Formation of Tetrahedral Zn in Layered Oxides Enables Reversible Lattice Oxygen Redox Triggering by the Na-O-Zn Configuration

Na-ion layered oxides with Na-O-A' local configurations (A' represents nonredox active cations such as Li⁺, Na⁺, Mg²⁺, Zn²⁺) are attractive cathode choices for energy-dense Na-ion batteries owing to the accumulation of cationic and anionic redox activities. However, the migration of A' would degrade the stability of the Na-O-A' configuration, bringing about drastic capacity decay and local structural distortions upon cycling. Herein, we uncover the close interplay between irreversible Zn migration and the inactivation of lattice oxygen redox (LOR) for layered oxides based on Na-O-Zn configuration by ²³Na solid-state NMR and Zn K-edge EXAFS techniques. We further design a Na_2/3Zn_0.18Ti_0.10Mn_0.72O₂ cathode in which irreversible Zn migration is effectively prevented, and the LOR reversibility is significantly enhanced. Theoretical insights demonstrate that the migrated Zn²⁺ is more inclined to occupy the tetrahedral site rather than the prismatic site and can be effectively minimized by incorporation of Ti⁴⁺ into the transition-metal layer. Our findings substantiate that the Na-O-Zn configuration can be utilized as an appropriate structure to achieve stable LOR by the cautious manipulating of intralayer cation arrangements.

Expandable Li Percolation Network: The Effects of Site Distortion in Cation-Disordered Rock-Salt Cathode Material

Cation-disordered rock-salt (DRX) materials receive intensive attention as a new class of cathode candidates for high-capacity lithium-ion batteries (LIBs). Unlike traditional layered cathode materials, DRX materials have a three-dimensional (3D) percolation network for Li⁺ transportation. The disordered structure poses a grand challenge to a thorough understanding of the percolation network due to its multiscale complexity. In this work, we introduce the large supercell modeling for DRX material Li_1.16Ti_0.37Ni_0.37Nb_0.10O₂ (LTNNO) via the reverse Monte Carlo (RMC) method combined with neutron total scattering. Through a quantitative statistical analysis of the material's local atomic environment, we experimentally verified the existence of short-range ordering (SRO) and uncovered an element-dependent behavior of transition metal (TM) site distortion. A displacement from the original octahedral site for Ti⁴⁺ cations is pervasive throughout the DRX lattice. Density functional theory (DFT) calculations revealed that site distortions quantified by the centroid offsets could alter the migration barrier for Li⁺ diffusion through the tetrahedral channels, which can expand the previously proposed theoretical percolating network of Li. The estimated accessible Li content is highly consistent with the observed charging capacity. The newly developed characterization method here uncovers the expandable nature of the Li percolation network in DRX materials, which may provide valuable guidelines for the design of superior DRX materials.

Unraveling the Nature and Role of Layered Cation Ordering in Cation-Disordered Rock-Salt Cathodes

Cation-disordered rock salts (DRXs), a new class of cathode materials for Li-ion batteries, have attracted a great amount of attention in recent years due to their fascinatingly simple cubic structure, highly diverse composition, and great electrochemical performance. As cations in DRXs are randomly distributed in a long range, how the cations are spatially arranged is an intriguing question for the community of solid-state materials chemistry. In this work, we report the vibrational structure of a series of Mn- and Fe-based DRXs with well-controlled compositions and reveal significant layered-like cation ordering in the DRXs. A scheme is proposed to describe how the layered-like anisotropy could exist in rock salt structures with an overall cubic diffraction pattern. Furthermore, we raise a model of Li-ion transport based on the proposed scheme, which complements the theory of Li percolation in DRXs. The electrochemical behavior of the DRX cathodes used in the study supports the scheme and clearly demonstrates the role of layered anisotropy in the battery performance of DRXs.

Enhancing the Reversibility of Lattice Oxygen Redox Through Modulated Transition Metal-Oxygen Covalency for Layered Battery Electrodes

Utilizing reversible lattice oxygen redox (OR) in battery electrodes is an essential strategy to overcome the capacity limitation set by conventional transition metal redox. However, lattice OR reactions are often accompanied with irreversible oxygen oxidation, leading to local structural transformations and voltage/capacity fading. Herein, it is proposed that the reversibility of lattice OR can be remarkably improved through modulating transition metal-oxygen covalency for layered electrode of Na-ion batteries. By developing a novel layered P2-Na_0.6 Mg_0.15 Mn_0.7 Cu_0.15 O₂ electrode, it is demonstrated that the highly electronegative Cu dopants can improve the lattice OR reversibility to 95% compared to 73% for Cu-free counterpart, as directly quantified through high-efficiency mapping of resonant inelastic X-ray scattering. Crucially, the large energetic overlap between Cu 3d and O 2p states dictates the rigidity of oxygen framework, which effectively mitigates the structural distortion of local oxygen environment upon (de)sodiation and leads to the enhanced lattice OR reversibility. The electrode also exhibits a completely solid-solution reaction with an ultralow volume change of only 0.45% and a reversible metal migration upon cycling, which together ensure the improved electrochemical performance. These results emphasize the critical role of transition metal-oxygen covalency for enhancing the reversibility of lattice OR toward high-capacity electrodes employing OR chemistry.

Adjusting Oxygen Redox Reaction and Structural Stability of Li- and Mn-Rich Cathodes by Zr-Ti Dual-Doping

Li- and Mn-rich cathodes (LMRs) with cationic and anionic redox reactions are considered as promising cathode materials for high-energy-density Li-ion batteries. However, the oxygen redox process leads to lattice oxygen loss and structure degradation, which would induce serious voltage fade and capacity loss and thus limit the practical application. High-valent and electrochemical inactive d⁰ element doping is an effective method to tune the crystal and electronic structures, which are the main factors for the electrochemical stability. Herein, noticeably inhibited oxygen loss, reduced voltage fade, enhanced rate performance, and improved structure stability and thermal stability of LMRs have been realized by Ti⁴⁺ and Zr⁴⁺ dual-doping. The underlying modulation mechanisms are unraveled by combining differential electrochemical mass spectrometry, soft X-ray absorption spectroscopies, in situ XRD measurements, etc. The dual-doping reduces the covalency of the TM-O bond, mitigates the irreversible oxygen release during the oxygen redox, and stabilizes the layered framework. The expanded lithium layer facilitates the lithium diffusion kinetics and structure stability. This study may result in the fundamental understanding of crystal and electronic structure evolution in LMRs and contribute to the development of high capacity cathodes.

A Novel Perovskite Electron-Ion Conductive Coating to Simultaneously Enhance Cycling Stability and Rate Capability of Li1.2 Ni0.13 Co0.13 Mn0.54 O2 Cathode Material for Lithium-Ion Batteries

Poor cycling stability and rate capability are two key issues needing to be solved for Li- and Mn-rich oxide cathode material for lithium-ion batteries (LIBs). Herein, a novel perovskite electron-ion mixed conductor Nd_0.6 Sr_0.4 CoO₃ (NSCO) is used as the coating layer on Li_1.2 Ni_0.13 Co_0.13 Mn_0.54 O₂ (LNCMO) to simultaneously enhance its cycling stability and rate capability. By coating 3 wt% NSCO, LNCMO-3NSCO exhibits an optimal cycling performance with a capacity retention of 99% at 0.1C (1C = 200 mA g^-1 ) after 60 cycles, 91% at 1C after 300 cycles, and 54% at 20C after 1000 cycles, much better than 78%, 63%, and 3% of LNCMO, respectively. Even at a high charge and discharge rate of 50C, LNCMO-3NSCO exhibits a discharge capacity of 53 mAh g^-1 and a mid-point discharge voltage of 2.88 V, much higher than those of LNCMO (24 mA h g^-1 and 2.40 V, respectively). Benefiting from the high electronic conductivity (1.46 S cm^-1 ) and ionic conductivity (1.48 × 10^-7  S cm^-1 ), NSCO coating not only suppresses transition metals dissolution and structure transformation, but also significantly enhances electronic conductivity and Li⁺ diffusion coefficient of LNCMO by an order of magnitude.

Demystifying the Lattice Oxygen Redox in Layered Oxide Cathode Materials of Lithium-Ion Batteries

The practical application of lithium-ion batteries suffers from low energy density and the struggle to satisfy the ever-growing requirements of the energy-storage Internet. Therefore, developing next-generation electrode materials with high energy density is of the utmost significance. There are high expectations with respect to the development of lattice oxygen redox (LOR)-a promising strategy for developing cathode materials as it renders nearly a doubling of the specific capacity. However, challenges have been put forward toward the deep-seated origins of the LOR reaction and if its whole potential could be effectively realized in practical application. In the following Review, the intrinsic science that induces the LOR activity and crystal structure evolution are extensively discussed. Moreover, a variety of characterization techniques for investigating these behaviors are presented. Furthermore, we have highlighted the practical restrictions and outlined the probable approaches of Li-based layered oxide cathodes for improving such materials to meet the practical applications.

Could Irradiation Introduce Oxidized Oxygen Signals in Resonant Inelastic X-ray Scattering of Battery Electrodes?

The characterization of oxidized oxygen states through high-efficiency mapping of resonant inelastic X-ray scattering (mRIXS) has become a crucial approach for studying the oxygen redox activities in high-energy battery cathodes. However, this approach has been recently challenged due to the concern of irradiation damage. Here we revisited a typical Li-rich electrode, Li_1.144Ni_0.136Mn_0.544Co_0.136O₂, in both lithiated and delithiated states and evaluated the X-ray irradiation effect in the lengthy mRIXS experiments. Our results show that irradiation cannot introduce any oxidized oxygen feature, and the features of oxidized oxygen are weakened with a high X-ray dose. The results confirm again that mRIXS detects the intrinsic oxidized oxygen state in battery electrodes. However, the distinct irradiation effects in different systems imply that irradiation could selectively target the least stable elemental or chemical states, which should be analyzed with caution in the study of active chemical states.

Advances in soft X-ray RIXS for studying redox reaction states in batteries

High-efficiency mapping of resonant inelastic X-ray scattering (mRIXS) for detecting and quantifying both cationic and anionic redox states in batteries.