Migration Of Transition Metal Ions Research Articles

High-performance lithium batteries achieved by electrospun MXene-Enhanced cation-selective membranes

Conventional separators applied in lithium batteries face limitations like low porosity, poor electrolyte wettability, lower cation selectivity, and thermal instability, which impact battery performance and safety characteristics. Besides the challenge in separator design, transition metal ion migration from positive electrodes during cycling greatly accelerates capacity fading because of unrestricted diffusion of transition metal cation across the separator. This study explores an innovative strategy for the separator design by incorporating MXene (spraying MXene solution on both sides of the separator), a two-dimensional material, into electrospun Polyacrylonitrile/Polyetherimide membranes with the function of cation selection. Li+ can be accelerated, and its transference number increases along with the anion limitation in the MXene layer. Ni, Co, and Mn ion dissolution from the positive electrode is inhibited due to the cation selectivity of the MXene layer. Leveraging electrospinning advantages, the resultant membranes exhibit high porosity, excellent liquid absorption, mechanical strength, and superior thermal properties. Benefiting from MXene's layered structure and excellent adsorption ability, the membrane significantly inhibits the dissolution of transition metal ions while enabling smooth lithium deposition due to its cation selectivity. Eventually, the Li||NMC811 batteries deliver outstanding cyclic performance (91.3 % capacity retention after 200 cycles) and robust lithium dendrite suppression (800 h of stable cycling). Moreover, the MXene-modified membrane demonstrates exceptional electrolyte wettability, significantly improving ionic transference number (0.67) and conductivity (1.6 mS/cm). Modification of membrane surfaces with MXene offers insights into addressing transition metal ion migration from the positive electrode material perspective, providing a promising avenue for high-performance LIBs.

PO43--doped layer @ spinel @ rGO sandwich-structured lithium-rich manganese-based cathode material with enhancing rate capability and cycle stability for Li-ion battery

The Li-rich Mn-based cathode (LRM) material faces challenges like low initial coulombic efficiency (ICE) and reduced capacity retention, which limit their commercial viability in high energy density lithium-ion batteries (LIBs). These challenges primarily arise from the irreversible migration of transition metal ions (TMn+) and the escape of lattice oxygen. Herein, this study proposes a novel approach to enhance Li-rich cathodes by implementing a PO43--doped layer @ spinel @ rGO sandwich structure, which effectively addresses the aforementioned challenges. Through the synergistic effects of PO43- doping and rGO surface modification, the modified samples demonstrate remarkable electrochemical properties. These include an impressive initial coulombic efficiency (ICE) of 87.54%, and a superior capacity of 187.12 mAhg−1 and 149.44 mAhg−1 at 25°C and 45°C, respectively, following 200 cycles at 1 C. Additionally, they exhibit an outstanding rate capability of 82.3 mAhg−1 at 10 C and a minimal voltage decrease of 0.504 V after 200 cycles. The synergetic strategy presented in this work serves as an inspiring source for the exploration of high energy density and long-cycle cathode materials.

Modification Strategies and Challenges of High‐Performance Lithium‐Rich Manganese‐Based Cathode Materials

Lithium‐rich manganese‐based (LRM) cathode materials have recently gained increasing attention for their remarkable reversible capacity of up to 378 mAh g−1. However, the development of these materials is hindered by several challenges, including oxygen deficiency, migration of transition metal ions, and structural changes from lamellar to spinel phases. As a result, these limitations result in a low initial Coulomb efficiency, capacity/voltage decay, and inadequate cycle life. To address these issues, the modification of layered lithium‐rich materials proves to be an effective approach. In this review, the structure and electrochemical properties of layered LRM materials are introduced and various systematic modification strategies are discussed. Herein, the current state and future prospects of several strategies such as doping, surface coating, and structure control are delved into. Furthermore, development ideas for achieving high‐capacity and long‐cycle‐layered LRM cathode materials are presented.

Insight into Nb-sbustitution effects on Li1.17Mn0.57Ni0.24Nb0.02O2 cathode material for lithium-ion batteries application

Lithium-rich layered oxides (LLOs) have been intensively studied due to their high energy density, but still face the drawbacks such as capacity decay and voltage decay, and poor cycling stability. In this work, the Nb substituted Li1.17Mn0.57Ni0.24Nb0.02O2 (LMN-Nb) is successfully prepared, and the effects of Nb substitution on the microstructure, voltage attenuation, cycling stability and electrochemical kinetics are characterized and investigated compared with the host material Li1.17Mn0.58Ni0.25O2 (LMN). A proper amount of Nb substitution not only extends the interlayer spacing and increases conductivity of LMN-Nb, but also inhibits the release of lattice oxygen, the migration of transition metal ions into lithium layer and the dissolution of transition metal ions in the electrolyte. As a result, LMN-Nb exhibits significantly improved initial coulombic efficiency, specific capacity and excellent cycling stability (99.1 % remaining after 150 cycles at 2C rate). In addition, the LMN-Nb also shows good electrochemical performance in pouch-type cell (full cell). This results demonstrate that the strategy of using 4d/5d transition metals substitution to design high-performance LLOs is feasible in the following work.

Enhancement of performance and kinetics of layered Li2MnO3 by Fe doping

Layered Li2MnO3 exhibits great potential as a high-capacity cathode used in lithium-ion batteries, but materials synthesized by traditional solid-phase methods exhibit poor activity and poor cycling stability. In this work, Li2MnO3 nanoparticles were synthesized by solid state method combined with high-energy ball milling, in which Fe was doped at transition metal sites to obtain a Li-rich cathode material (Li1.32Fe0.034Mn0.64O2). We systematically studied the effects of Fe doping and calcination temperature on the crystal structure and electrochemical properties and conducted kinetic studies using the galvanostatic intermittent titration technique (GITT), electrical impedance spectroscopy (EIS) and density functional theory (DFT) calculations. Our results demonstrate that the performance was significantly improved, its discharge capacity is about 235 mAh g−1 after 10 cycles compared 138 mAh g−1 of the pristine sample. The GITT and EIS results show that the Fe-doped sample had smaller charge transfer impedance, larger lithium-ion diffusion coefficient and lower internal polarization than the pristine. The activation energy results show the Fe-doped sample has larger activation energy at the late discharging period due to the anion redox reaction activation, indicting its O activation is more active than Li2MnO3. Moreover, the DOS results showed that the Fe-doped sample had a narrow band-gap, indicating good electrical conductivity. The CI-NEB results showed that the Fe-doped sample had a high transition metal ion migration energy barrier, indicating that its structure was stable. In conclusion, Fe doping is helpful to active the anion redox reaction in Li2MnO3 and improve its electrochemical performance as a low cost and high-capacity cathode materials.

Synergetic effects of Na+-Mo6+ co-doping on layered Li1.2Mn0.54Ni0.13Co0.13O2 cathode for high performance lithium-ion batteries

The Li-rich Mn-based material, with high energy density, less toxicity and low cost, is regarded as one of the potential cathode candidates for the next generation lithium-ion batteries (LIBs). However, some issues need to be addressed before large-scale commercialized application, such as capacity attenuation, low initial coulomb efficiency and poor rate performance, caused by the structure defects and oxygen escape from the crystal lattice at high voltage. Here, the Na+ and Mo6+ dopants are introduced to Li1.2Mn0.54Ni0.13Co0.13O2 by sol-gel method, with slight Li+ displaced by Na+ and tiny Mo6+ substituting transition metal ions, respectively. The doping effects on the Li layer spacing, Li/Ni mixing and oxygen vacancy defects have been examined. Regarding the doped samples, the valence states of transition metal elements and electrochemical performance are systematically studied. The Na+ and Mo6+ co-doping not only expands the lithium layer spacing and reduces cationic disorder to promote the Li+ diffusion and enhance the rate capability, but also stabilizes the oxygen skeleton, inhibits oxygen loss, and suppresses the migration of transition metal ions to improve the cyclic performance. As a result, Li1.18Na0.02(Mn0.54Ni0.13Co0.13)0.98Mo0.02O2 delivers 122.8 mAh.g−1 at 5 C and 262.9 mAh.g−1 at 0.2 C, retaining 90% capacity after 150 cycles.

Fundamentals of Ion‐Exchange Synthesis and Its Implications in Layered Oxide Cathodes: Recent Advances and Perspective

AbstractLayered oxide cathodes such as Ni‐rich ternary and Li‐rich layered cathode materials have been widely used for lithium‐ion batteries owing to their excellent Li+ transport properties, high energy density, and relatively low cost. However, such layered cathode materials synthesized by high‐temperature sintering face inherent issues such as low structural stability, irreversible migration of transition metal ions, and irreversible redox reactions of oxygen anions. To make a breakthrough from the perspective of material synthesis, a new ion‐exchange synthesis has emerged in recent years, which is a promising strategy for synthesis of Li‐ion cathodes. Herein, the fundamentals of ion‐exchange synthesis and their implications in layered oxide cathodes for lithium‐ion batteries is presented. Specifically, ion‐exchange synthesis and mechanisms of ion exchange are introduced in detail, followed by a discussion of the reduction of synthetic temperature, the synthesis of novel crystal structures, the inhibited migration of transition metal ions, the increased reversibility of anionic redox, and the optimized surface reconstruction. Finally, a summary and outlook is provided for ion‐exchange synthesis of layered oxide cathodes for lithium‐ion batteries. It is anticipated that this ion‐exchange synthesis will facilitate the commercialization of high‐performance cathode materials for next generation Li‐ion batteries.

Structural insights into lithium-deficient type Li-rich layered oxide for high-performance cathode

As one of the promising candidate cathode materials for the high-performance lithium-ion batteries, Li-rich layered oxides still suffer from a series of critical drawbacks, such as voltage decay, oxygen release, irreversible migration of transition metal ions, etc. In this work, Li-deficient method has been confirmed as an effective approach to improve the overall electrochemical performances of Li-rich cathode. The optimized lithium-deficient Li-rich layered cathode exhibits splendid discharge capacity of ∼297 mAh/g at 0.1 C and prominent rate performance of ∼143 mAh/g at 5 C. Subsequently, neutron diffraction in combination with Raman spectroscopy is applied to explore and clarify the underlying mechanism for improved performances. It was found that the lithium-deficient induced nickel migration and oxygen vacancy play an significant role in improving electrochemical performances, because migration of nickel into Li layer is able to expand the Li layer spacing and reduce the Li/Ni antisite, leading to facilitated diffusion of lithium ions. Moreover, the formation of oxygen vacancy is able to promote anionic redox processes and suppress the gas release, thus leading to higher capacity. The results present valuable structural insights into the influence of lithium deficiency and provide guidance for the development of Li-rich cathode materials.

Phosphorylation of Li-Rich Mn-Based Layered Oxides for Anion Redox and Structural Stability.

Li-rich Mn-based layered oxides are proposed to be candidates for high-energy Li-ion batteries. However, their large-scale production is still hampered by poor rate capability and severe voltage decay. It was mainly attributed to the irreversible oxygen loss, which induces transition metal ion migration, electrolyte consumption, and structural evolution. Herein, we propose an effective strategy of phosphorylation, in which the phosphate ion is induced to remove the surface labile oxygen. It urges the Li2MnO3 component to transform to the spinel-like structure and promotes the anionic redox process, thus facilitating lithium-ion diffusion and improving structural stability. As a result, the Li2MnO3 component is more prone to be activated, with the capacity increased by 18% in comparison with the pristine one. It also exhibits a superior capacity retention of 86.1% after 150 extended cycles and better rate performance delivering a capacity of 148.1 mA h g-1 even at 10 C. The effective phosphorylation opens a new way to tune anion redox chemistry and obtain structurally stable materials.

In-Situ Gas Analysis of Sodium Nickel Manganese Oxide Cathodes Synthesized By Eutectic Method

Sodium-ion batteries 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 sodium-ion batteries compared with lithium-ion batteries diminishes their practical value proposition. Among the many sodium-based cathodes, layered transition metal oxides with high sodium content can achieve energy densities comparable with the lithium-ion battery technology. For some layered transition metal oxide cathodes, when they are charged to above 4 V, a long plateau is often observed before they reach to fully charged state, which involves structural transformations that lead to irreversible reactions. Apart from structural transformations, lattice oxygen anion redox has also been proposed as a mechanism from both experimental results such as X-ray absorption spectroscopy, resonant inelastic X-ray scattering studies and simulation efforts. Reversible lattice oxygen anion redox is often accompanied by the generation of peroxo (O2 2-) or superoxo (O2-) related species. Irreversible lattice oxygen anion redox is often accompanied by the generation of molecular O2, along with transition metal ion migration, lattice distortion, and rapid capacity decay. In this work, in-situ gas analysis was performed to evaluate the gas generation in Na x NiyMn1-yO2 cathodes synthesized by eutectic method in sodium half-cell configuration. Operando x-ray diffraction and neutron diffraction were performed to assess the structural changes related to the high voltage plateau and transition metal ion migration in Na x NiyMn1-yO2 cathodes. The results unveil that in-plane honeycomb cationic ordering can help reduce the activity of irreversible oxygen anion redox which is critical for future design of layered transition metal oxide cathode that are prone to achieve high-energy for durable sodium-ion batteries.

Enhanced cyclic performance of O2-type Mn-based layered oxide via Al doping for lithium-ion battery

The layered lithium nickel manganese oxide Li y Ni x Mn 1−x O 2 cathode materials with O2 structure exhibit good cyclic stability owing to their special arrangement of oxygen layers. However, the O2-type oxides generally present limited discharge capacity because of Li + ions defects in the lithium layer (as in Li y Ni x Mn 1−x O 2 , y < 1). Herein, an O2-type layered lithium nickel manganese oxide LiNi 0.33 Mn 0.65 Al 0.02 O 2 with high capacity and good capacity retention is obtained by introducing lithium and aluminum into the transition metal layer. The results of structural characterization and performance tests show that the optimal amount of Li + and Al 3+ in the transition metal layer effectively inhibits the disordered migration of transition metal ions and increases the electrochemical stability. The LiNi 0.33 Mn 0.65 Al 0.02 O 2 cathode delivers an initial discharge capacity of 153 mAh g −1 and the capacity only attenuates 13% after 100 cycles at the current density of 200 mA g −1 , which are higher than that of undoped oxide LiNi 0.33 Mn 0.67 O 2 . • O2-type Mn-based oxides with Li and Al in transition metal layer were synthesized. • Al doping amount has important effect on the performance of LiNi 0.33 Mn 0.67−x Al x O 2 . • LiNi 0.33 Mn 0.65 Al 0.02 O 2 has high stability and high initial capacity at 200 mA g −1 .

Kinetic Origin of Planar Gliding in Single-Crystalline Ni-Rich Cathodes.

Single-crystalline Ni-rich cathodes with high capacity have drawn much attention for mitigating cycling and safety crisis of their polycrystalline analogues. However, planar gliding and intragranular cracking tend to occur in single crystals with cycling, which undermine cathode integrity and therefore cause capacity degradation. Herein, we intensively investigate the origin and evolution of the gliding phenomenon in single-crystalline Ni-rich cathodes. Discrete or continuous gliding forms are revealed with new surface exposure including the gliding plane (003) and reconstructed (-108) under surface energy drive. It is also demonstrated that the gliding process is the in-plane migration of transition metal ions, and reducing oxygen vacancies will increase the migration energy barrier by which gliding and microcracking can be restrained. The designed cathode with less oxygen deficiency exhibits outstanding cycling performance with an 80.8% capacity retention after 1000 cycles in pouch cells. Our findings provide an insight into the relationship between defect control and chemomechanical properties of single-crystalline Ni-rich 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.

A facile and high-effective oxygen defect engineering for improving electrochemical performance of lithium-rich manganese-based cathode materials

The irreversible evolution of oxygen for lithium-rich manganese-based cathode (LRMC) is the main challenge which will induce accelerated migration of transition metal ions and structural transformation, thus resulting in serious voltage attenuation and capacity decay. Herein, a facile and high-effective oxygen vacancy engineering of hydrogenation treatment is proposed for introducing oxygen vacancy on the surface of LRMC to suppress the irreversible evolution of oxygen. X-ray diffraction (XRD) Rietveld refinement, X-ray photoelectron spectroscopy and electron paramagnetic spectroscopy demonstrate the successful introduction of oxygen vacancy on the surface of LRMC. The changes of physicochemical properties of LRMC after hydrogenation treatment are investigated through systematical electrochemical performance tests and scanning electron microscopy and transmission electron microscopy. The investigative results reveal that the presence of oxygen vacancy can significantly improve the structural stability, electrical conductivity and Li+ diffusion kinetics of the LRMC materials. Especially, the optimized H500-LRMC shows excellent electrochemical performances including the enhanced cyclic stability with a capacity retention of 221.8 mAh g−1 and a capacity retention rate of 94.11% after 100 cycles. Therefore, the well-designed oxygen vacancy engineering through hydrogenation treatment is a promising strategy for the development and industrialization of the high-performance LRMC.

Suppressing Voltage Fading of Li‐Rich Oxide Cathode via Building a Well‐Protected and Partially‐Protonated Surface by Polyacrylic Acid Binder for Cycle‐Stable Li‐Ion Batteries

AbstractLi‐rich manganese based oxides (LRMOs) are considered an attractive high‐capacity cathode for advanced Li‐ion batteries; however, their poor cyclability and gradual voltage fading have hindered their practical applications. Herein, an efficient and facile strategy is proposed to stabilize the lattice structure of LRMOs by surface modification of polyacrylic acid (PAA). The PAA‐coated LRMO electrode exhibits only 104 mV of the voltage fading after 100 cycles and 88% capacity retention over 500 cycles. The structural stability is attributed to the carboxyl groups in PAA chains reacting with oxygen species on the surface of LRMO to form a uniform and tightly coated film, which significantly suppresses the dissolution of transition metal elements from the cathode materials into the electrolyte. Importantly, a H+/Li+ exchange reaction takes place between the LRMO and PAA, generating a proton‐doped surface layer. Density functional theory calculations and experimental evidence demonstrates that the H+ ions in the surface lattice efficiently inhibit the migration of transition metal ions, leading to a stabilized lattice structure. This surface modification approach may provide a new route to building a stable Li‐rich oxide cathode with high capacity retention and low voltage fading for practical Li‐ion battery applications.

Atomic-scale structural evolution of electrode materials in Li-ion batteries: a review

Owing to the high spatial resolution at the atomic scale, the transmission electron microscopy (TEM) or scanning transmission electron microscopy is demonstrated as a promising characterization method to unveil the charge storage mechanism of electrode materials in Li-ion batteries. The structural evolution of electrode materials during charge/discharge process can be directly observed by using TEM. The detailed analysis establishes a relationship between the structure of electrode material and battery performance. Herein, we present a brief review of the atomic-scale characterization in Li-ion batteries, including Li (de)insertion mechanism (both cations and anions charge-compensation mechanism), migration of transition metal ions, and surface phase transition. The in-depth microscopic analysis reveals the detailed structural characteristics, which influence the properties of LIBs, establish the structure–function relationship, and facilitate the development of Li-ion batteries.

Practical Realization of O3-Type NaNi0.5Mn0.3Co0.2O2 Cathodes for Sodium-Ion Batteries

In this work, we report the practical realization of 3.6 V Sodium-ion coin and multi-electrode pouch-type cells using O3-type layered NaNi0.5Mn0.3Co0.2O2 cathodes and dextrose derived hard carbon anodes in one-unit cell. The manuscript provides a thorough study of NaNi0.5Mn0.3Co0.2O2 cathodes along with hard carbon anodes and its practical difficulty in the development of Sodium-ion cells. NaNi0.5Mn0.3Co0.2O2 cathodes deliver an initial discharge capacity of ∼136 mAhg−1 with a moderate capacity retention of 87 mAhg−1 at 0.1C rate after 200 cycles. Similarly, the hard carbon anodes show a reversible capacity of about 280 mAhg−1 at C/10 rate. The diffusion coefficient of Sodium-ion calculated is in the order of 10−11–10−12 cm2s−1 suggesting good reversibility, capacity retention and C-rate performance of the cathode. Further, no change in peak voltage in dQ/dV plots, XRD patterns, and CVs dictates the crystalline phase stability of NaNi0.5Mn0.3Co0.2O2 during cycling. NaNi0.5Mn0.3Co0.2O2 retains good structural stability during long cycling and decreased interlayer transition metal ion migration. Engineering aspects for assembling and testing O3-Type NaNi0.5Mn0.3Co0.2O2-hard carbon in the full cell is also investigated thoroughly. Herein, we have taken the research output and challenges from the coin-type cell and applied further to develop single and multi-electrode pouch type-cell of various configurations.

An in-depth study of Sn substitution in Li-rich/Mn-rich NMC as a cathode material for Li-ion batteries.

Layered Li-rich/Mn-rich NMC (LMR-NMC) is characterized by high initial specific capacities of more than 250 mA h g-1, lower cost due to a lower Co content and higher thermal stability than LiCoO2. However, its commercialisation is currently still hampered by significant voltage fade, which is caused by irreversible transition metal ion migration to emptied Li positions via tetrahedral interstices upon electrochemical cycling. This structural change is strongly correlated with anionic redox chemistry of the oxygen sublattice and has a detrimental effect on electrochemical performance. In a fully charged state, up to 4.8 V vs. Li/Li+, Mn4+ is prone to migrate to the Li layer. The replacement of Mn4+ for an isovalent cation such as Sn4+ which does not tend to adopt tetrahedral coordination and shows a higher metal-oxygen bond strength is considered to be a viable strategy to stabilize the layered structure upon extended electrochemical cycling, hereby decreasing voltage fade. The influence of Sn4+ on the voltage fade in partially charged LMR-NMC is not yet reported in the literature, and therefore, we have investigated the structure and the corresponding electrochemical properties of LMR-NMC with different Sn concentrations. We determined the substitution limit of Sn4+ in Li1.2Ni0.13Co0.13Mn0.54-xSnxO2 by powder X-ray diffraction and transmission electron microscopy to be x≈ 0.045. The limited solubility of Sn is subsequently confirmed by density functional theory calculations. Voltage fade for x = 0 and x = 0.027 has been comparatively assessed within the 3.00 V-4.55 V (vs. Li/Li+) potential window, from which it is concluded that replacing Mn4+ by Sn4+ cannot be considered as a viable strategy to inhibit voltage fade within this window, at least with the given restricted doping level.

Evolution mechanism of phase transformation of Li-rich cathode materials in cycling

Conventional Li-rich layered oxides are composed of the intergrowth of LiMO2R3‾m and Li2MnO3 C2/m phases, and provide high capacity as cathode materials for Li-ion rechargeable battery. However, the mechanism behind capacity and voltage fading still needs further understanding. In this work, based on intensive analysis on crystalline phase and interface of the classical Li-rich cathode material, Li1.2Ni0.13Co0.13Mn0.54O2 during charge-discharge processes, especially in the initial charge process, evolution mechanism of oxygen loss-induced phase transformation is illustrated in details. The irreversible anionic redox contributes to the large voltage plateau at 4.5 V during initial charging. As a result, oxygen vacancies forming on the surface reduce the barrier of the migration of transition metal ions into Li layer, causing severe cation mixing and initiating the phase transformation from a layered structure to a spinel structure. Meanwhile, the removal of Li+ and O2− from Li2MnO3 component brings about changes in lattice parameters. The evolution mechanism can be described in stages: starting from the first charge as the initiation stage; Li/transition metal cations mixing will continue to increase due to the existence of oxygen vacancies in the next few cycles, accompanying with accelerated phase transformation process; then it becomes relatively moderate under the protection of CEI film while still with inevitable structural changes over long cycling, reaching the accumulation stage.

Unveiling the benefits of potassium doping on the structural integrity of Li-Mn-rich layered oxides during prolonged cycling by dual-mode EPR spectroscopy.

The oxygen redox process in Li- and Mn-rich layered oxides will inevitably lead to the generation of oxygen vacancies on the surface and their subsequent injection into the bulk lattice, which incurs poor kinetics, capacity decrease, and voltage fading. Herein, this predicament is effectively alleviated by bulk doping of K+, which is intrinsically stable in the lattice to inhibit the generation of oxygen vacancies in the deep delithiated state. More importantly, the benefits of K+ doping on the structural reversibility during prolonged cycling were studied by electron paramagnetic resonance (EPR) spectroscopy in both perpendicular and parallel polarization modes and high-resolution transmission electron microscopy. The results elucidate that the migration of transition-metal ions and oxygen vacancies and the reduction of Mn-ions are mitigated after K+ doping. Consequently, the growth of Li-poor nanovoids in the bulk lattice is greatly diminished and the structural transition from layered to spinel phases is effectively delayed.