Mn4+-Substituted Li-Rich Li1.2Mn0.43+Mnx4+Ti0.4-xO2 Materials with High Energy Density

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.

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.

Regulating the Anion Redox and Suppressing the Structural Distortion of Cation-Disordered Rock-Salt Cathode Materials to Improve Cycling Durability through Chlorine Substitution

Owing to the capacity boost from anion redox activities, cation-disordered rock-salt oxides are considered as potential candidates for the next-generation of high energy density Li-ion cathode materials. Unfortunately, the anion redox process that affords ultra-high specific capacity often triggers irreversible O₂ release, which brings about structural degradation and rapid capacity decay. In this study, we present a partial chlorine (Cl) substitution strategy to synthesize a new cation-disordered rock-salt compound of Li_1.225Ti_0.45Mn_0.325O_1.9Cl_0.1 and investigate the impact of Cl substitution on the oxygen redox process and the structural stability of cation-disordered rock-salt cathodes. We find that partial replacement of O^2- by Cl^- expands the cell volume and promotes anion redox reaction reversibility, thus increasing the Li⁺ ion diffusion rate and suppressing irreversible lattice oxygen loss. As a result, the Li_1.225Ti_0.45Mn_0.325O_1.9Cl_0.1 cathode exhibits significantly improved cycling durability at high current densities, compared with the pristine Li_1.225Ti_0.45Mn_0.325O₂ cathode. This work demonstrates the promising feasibility of the Cl substitution process for advanced cation-disordered rock-salt cathode materials.

Structural Stabilization of Cation-Disordered Rock-Salt Cathode Materials: Coupling between a High-Ratio Inactive Ti4+ Cation and a Mn2+/Mn4+ Two-Electron Redox Pair

Cation-disordered rock-salt cathode materials are featured by their extraordinarily high specific capacities in lithium-ion batteries primarily contributed by anion redox reactions. Unfortunately, anion redox reactions can trigger oxygen release in this class of materials, leading to fast capacity fading and major safety concern. Despite the capability of absorbing structural distortions, high-ratio d⁰ transition-metal cations are considered to be unfavorable in design of a new cation-disordered rock-salt structure because of their electrochemically inactive nature. Herein, we report a new cation-disordered rock-salt compound of Li_1.2Ti_0.6Mn_0.2O₂ with the stoichiometry of Ti⁴⁺ as high as 0.6. The capacity reducing effect by the low-ratio active transition-metal center can be balanced by using a Mn²⁺/Mn⁴⁺ two-electron redox couple. The strengthened networks of strong Ti-O bonds greatly retard the oxygen release and improve the structural stability of cation-disordered rock-salt cathode materials. As expected, Li_1.2Ti_0.6Mn_0.2O₂ delivers significantly improved electrochemical performances and thermal stability compared to the low-ratio Ti⁴⁺ counterpart of Li_1.2Ti_0.4Mn_0.4O₂. Theoretical simulations further reveal that the improved electrochemical performances of Li_1.2Ti_0.6Mn_0.2O₂ are attributed to its lower Li⁺ diffusion energy barrier and enhanced unhybridized O 2p states compared to Li_1.2Ti_0.4Mn_0.4O₂. This concept might be helpful for the improvement of structural stability and electrochemical performances of other cation-disordered rock-salt metal oxide cathode materials.

Understanding anion-redox reactions in cathode materials of lithium-ion batteries throughin situcharacterization techniques: a review

As the demand for rechargeable lithium-ion batteries (LIBs) with higher energy density increases, the interest in lithium-rich oxide (LRO) with extraordinarily high capacities is surging. The capacity of LRO cathodes exceeds that of conventional layered oxides. This has been attributed to the redox contribution from both cations and anions, either sequentially or simultaneously. However, LROs with notable anion redox suffer from capacity loss and voltage decay during cycling. Therefore, a fundamental understanding of their electrochemical behaviors and related structural evolution is a prerequisite for the successful development of high-capacity LRO cathodes with anion redox activity. However, there is still controversy over their electrochemical behavior and principles of operation. In addition, complicated redox mechanisms and the lack of sufficient analytical tools render the basic study difficult. In this review, we aim to introduce theoretical insights into the anion redox mechanism andin situanalytical instruments that can be used to prove the mechanism and behavior of cathodes with anion redox activity. We summarized the anion redox phenomenon, suggested mechanisms, and discussed the history of development for anion redox in cathode materials of LIBs. Finally, we review the recent progress in identification of reaction mechanisms in LROs and validation of engineering strategies to improve cathode performance based on anion redox through various analytical tools, particularly,in situcharacterization techniques. Because unexpected phenomena may occur during cycling, it is crucial to study the kinetic properties of materialsin situunder operating conditions, especially for this newly investigated anion redox phenomenon. This review provides a comprehensive perspective on the future direction of studies on materials with anion redox activity.

Enhanced stability of vanadium-doped Li1.2Ni0.16Co0.08Mn0.56O2 cathode materials for superior Li-ion batteries

The high-valence V5+ can improve the discharge capacity and coulomb efficiency and inhibit the voltage attenuation of cathode materials.

Regulating Anion Redox and Cation Migration to Enhance the Structural Stability of Li-Rich Layered Oxides

Lithium-rich manganese-based layered oxide cathodes (LLOs) with oxygen redox reactions are considered to be potential candidates for the next generation of high-energy-density Li-ion batteries. However, the oxygen redox process that enables ultrahigh specific capacity usually leads to irreversible O₂ release and cation migration, which induce structure degradation and severe capacity/voltage losses and thus limit the commercial application of LLOs. Herein, we successfully synthesized chlorine (Cl)-doped Co-free LLOs (Li_1.2Mn_0.53Ni_0.27O_1.976Cl_0.024) and analyzed the effect of anion doping on oxygen redox and structure stability of LLOs. Cl doping has been proven to decrease the irreversible lattice oxygen loss to enhance the redox reversibility of oxygen and inhibit the transition-metal migration during cycles, which substantially enhances the capacity and voltage retention and improves the rate capability during cycling. This work provides new insights for the development of high-performance TM oxide cathode materials with reversible oxygen redox.