Lithium Deficiencies Engineering in Li-Rich Layered Oxide Li1.098Mn0.533Ni0.113Co0.138O2 for High-Stability Cathode

Toward High-Performance Li-Rich Mn-Based Layered Cathodes: A Review on Surface Modifications

Lithium-rich manganese-based layered oxides (LRMOs) have received attention from both the academic and the industrial communities in recent years due to their high specific capacity (theoretical capacity ≥250 mAh g-1), low cost, and excellent processability. However, the large-scale applications of these materials still face unstable surface/interface structures, unsatisfactory cycling/rate performance, severe voltage decay, etc. Recently, solid evidence has shown that lattice oxygen in LRMOs easily moves and escapes from the particle surface, which inspires significant efforts on stabilizing the surface/interfacial structures of LRMOs. In this review, the main issues associated with the surface of LRMOs together with the recent advances in surface modifications are outlined. The critical role of outside-in surface decoration at both atomic and mesoscopic scales with an emphasis on surface coating, surface doping, surface structural reconstructions, and multiple-strategy co-modifications is discussed. Finally, the future development and commercialization of LRMOs are prospected. Looking forward, the optimal surface modifications of LRMOs may lead to a low-cost and sustainable next-generation high-performance battery technology.

Enhancing the Reaction Kinetics and Stability of Co-Free Li-Rich Cathode Materials via a Multifunctional Strategy

Co-free Li-rich layered oxides (CFLLOs) with anionic redox activity are among the most promising cathode materials for high-energy-density and low-cost lithium-ion batteries (LIBs). However, irreversible oxygen release often causes severe structural deterioration, electrolyte decomposition, and the formation of unstable cathode-electrolyte interface (CEI) film with high impedance. Additionally, the elimination of cobalt elements further deteriorates the reaction kinetics, leading to reduced capacity and poor rate performance. Here, a multifunctional strategy is proposed, incorporating Li2MnO3 phase content regulation, micro-nano structure design, and heteroatom substitution. The increased content of Li2MnO3 phase enhances the capacity through oxygen redox. The smaller nanoscale primary particles induce greater tensile strain and introduce more grain boundaries, thereby improving the reaction kinetics and reactivity, while the larger micron-sized secondary particles help to reduce interfacial side reactions. Furthermore, Na⁺ doping modulates the local coordination environment of oxygen, stabilizing both the anion framework and the crystal structure. As a result, the designed cathode exhibits enhanced rate performance, delivering a capacity of 158 mAh g⁻¹ at 5.0 C and improved cyclic stability, with a high capacity retention of 99% after 400 cycles at 1.0 C. This multifunctional strategy holds great promise for advancing the practical application of CFLLOs in next-generation LIBs.

Cut-off voltage influencing the voltage decay of single crystal lithium-rich manganese-based cathode materials in lithium-ion batteries

The voltage decay of Li-rich layered oxide cathode materials results in the deterioration of cycling performance and continuous energy loss, which seriously hinders their application in the high-energy-density lithium-ion battery (LIB) market. However, the origin of the voltage decay mechanism remains controversial due to the complex influences of transition metal (TM) migration, oxygen release, indistinguishable surface/bulk reactions and the easy intra/inter-crystalline cracking during cycling. We investigated the direct cause of voltage decay in micrometer-scale single-crystal Li1.2Mn0.54Ni0.13Co0.13O2 (SC-LNCM) cathode materials by regulating the cut-off voltage. The redox of TM and O2- ions can be precisely controlled by setting different voltage windows, while the cracking can be restrained, and surface/bulk structural evaluation can be monitored because of the large single crystal size. The results show that the voltage decay of SC-LNCM is related to the combined effect of cation rearrangement and oxygen release. Maintaining the discharge cutoff voltage at 3 V or the charging cutoff voltage at 4.5 V effectively mitigates the voltage decay, which provides a solution for suppressing the voltage decay of Li-rich and Mn-based layered oxide cathode materials. Our work provides significant insights into the origin of the voltage decay mechanism and an easily achievable strategy to restrain the voltage decay for Li-rich and Mn-based cathode materials.

Functional Composite Dual-Phase In Situ Self-Reconstruction Design for High-Energy-Density Li-Rich Cathodes

The unique anionic redox mechanism provides, high-capacity, irreversible oxygen release and voltage/capacity degradation to Li-rich cathode materials (LRO, Li1.2Mn0.54Co0.13Ni0.13O2). In this study, an integrated stabilized carbon-rock salt/spinel composite heterostructured layers (C@spinel/MO) is constructed by in situ self-reconstruction, and the generation mechanism of the in situ reconstructed surface is elucidated. The formation of atomic-level connections between the surface-protected phase and bulk-layered phase contributes to electrochemical performance. The best-performing sample shows a high increase (63%) of capacity retention compared to that of the pristine sample after 100 cycles at 1C, with an 86.7% reduction in surface oxygen release shown by differential electrochemical mass spectrometry. Soft X-ray results show that Co3+ and Mn4+ are mainly reduce in the carbothermal reduction reaction and participate in the formation of the spinel/MO rock-salt phase. The results of oxygen release characterized by Differential electrochemical mass spectrometry (DEMS) strongly prove the effectiveness of surface reconstruction.

High-Performance Single-Crystal Lithium-Rich Layered Oxides Cathode Materials via Na2WO4-Assisted Sintering

Single-crystal lithium-rich layered oxides (LLOs) with excellent mechanical properties can enhance their crystal structure stability. However, the conventional methods for preparing single-crystal LLOs, require large amounts of molten salt additives, involve complicated washing steps, and increase the difficulty of large-scale production. In this study, a sodium tungstate (Na2WO4)-assisted sintering method is proposed to fabricate high-performance single-crystal LLOs cathode materials without large amounts of additives and additional washing steps. During the sintering process, Na2WO4 promotes particle growth and forms a protective coating on the surface of LLOs particles, effectively suppressing the side reactions at the cathode/electrolyte interface. Additionally, trace amounts of Na and W atoms are doped into the LLOs lattice via gradient doping. Experimental results and theoretical calculations indicate that Na and W doping stabilizes the crystal structure and enhances the Li+ ions diffusion rate. The prepared single-crystal LLOs exhibit outstanding capacity retention of 82.7% (compared to 65.0%, after 200 cycles at 1 C) and a low voltage decay rate of 0.76 mV per cycle (compared to 1.80 mV per cycle). This strategy provides a novel pathway for designing the next-generation high-performance cathode materials for Lithium-ion batteries (LIBs).

From Oxygen Redox to Sulfur Redox: A Paradigm for Li-Rich Layered Cathodes

The utilization of anionic redox chemistry provides an opportunity to further improve the energy density of Li-ion batteries, particularly for Li-rich layered oxides. However, oxygen-based hosts still suffer from unfavorable structural rearrangement, including the oxygen release and transition metal (TM)-ion migration, in association with the tenuous framework rooted in the ionicity of the TM-O bonding. An intrinsic solution, by using a sulfur-based host with strong TM-S covalency, is proposed here to buffer the lattice distortion upon the highly activating sulfur redox process, and it achieves howling success in stabilizing the host frameworks. Experimental results demonstrate the prolonged preservation of the layered sulfur lattice, especially the honeycomb superlattice, during the Li+ extraction/insertion process in contrast to the large structural degeneration in Li-rich oxides. Moreover, the Li-rich sulfide cathodes exhibited a negligible overpotential of 0.08 V and a voltage drop of 0.13 mV/cycle, while maintaining a substantial reversible capacity upon cycling. These superior electrochemical performances can be unambiguously ascribed to the much shorter trajectories of sulfur in comparison to those of oxygen revealed by molecular dynamics simulations at a large scale (∼30 nm) and a long time scale (∼300 ps) via high-dimensional neural network potentials during the delithiation process. Our findings highlight the importance of stabilizing host frameworks and establish general guidance for designing Li-rich cathodes with durable anionic redox chemistry.

Stabilized Anionic Redox by Rational Structural Design from Surface to Bulk for Long-Life Fast-Charging Li-Rich Oxide Cathodes

On account of high capacity and high voltage resulting from anionic redox, Li-rich layered oxides (LLOs) have become the most promising cathode candidate for the next-generation high-energy-density lithium-ion batteries (LIBs). Unfortunately, the participation of oxygen anion in charge compensation causes lattice oxygen evolution and accompanying structural degradation, voltage decay, capacity attenuation, low initial columbic efficiency, poor kinetics, and other problems. To resolve these challenges, a rational structural design strategy from surface to bulk by a facile pretreatment method for LLOs is provided to stabilize oxygen redox. On the surface, an integrated structure is constructed to suppress oxygen release, electrolyte attack, and consequent transition metals dissolution, accelerate lithium ions transport on the cathode-electrolyte interface, and alleviate the undesired phase transformation. While in the bulk, B doping into Li and Mn layer tetrahedron is introduced to increase the formation energy of O vacancy and decrease the lithium ions immigration barrier energy, bringing about the high stability of surrounding lattice oxygen and outstanding ions transport ability. Benefiting from the specific structure, the designed material with the enhanced structural integrity and stabilized anionic redox performs an excellent electrochemical performance and fast-charging property..

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.

A Gradient Doping Strategy toward Superior Electrochemical Performance for Li-Rich Mn-Based Cathode Materials

Lithium-rich layered oxides (LLOs) are concerned as promising cathode materials for next-generation lithium-ion batteries due to their high reversible capacities (larger than 250 mA h g^-1 ). However, LLOs suffer from critical drawbacks, such as irreversible oxygen release, structural degradation, and poor reaction kinetics, which hinder their commercialization. Herein, the local electronic structure is tuned to improve the capacity energy density retention and rate performance of LLOs via gradient Ta⁵⁺ doping. As a result, the capacity retention elevates from 73% to above 93%, and the energy density rises from 65% to above 87% for LLO with modification at 1 C after 200 cycles. Besides, the discharge capacity for the Ta⁵⁺ doped LLO at 5 C is 155 mA h g^-1 , while it is only 122 mA h g^-1 for bare LLO. Theoretical calculations reveal that Ta⁵⁺ doping can effectively increase oxygen vacancy formation energy, thus guaranteeing the structure stability during the electrochemical process, and the density of states results indicate that the electronic conductivity of the LLOs can be boosted significantly at the same time. This strategy of gradient doping provides a new avenue to improve the electrochemical performance of the LLOs by modulating the local structure at the surface.

Thermodynamically Stable Dual-Modified LiF&FeF3 layer Empowering Ni-Rich Cathodes with Superior Cyclabilities

Pushing the limit of cutoff potentials allows nickel-rich layered oxides to provide greater energy density and specific capacity whereas reducing thermodynamic and kinetic stability. Herein, a one-step dual-modified method is proposed for in situ synthesizing thermodynamically stable LiF&FeF₃ coating on LiNi_0.8 Co_0.1 Mn_0.1 O₂ surfaces by capturing lithium impurity on the surface to overcome the challenges suffered. The thermodynamically stabilized LiF&FeF₃ coating can effectively suppress the nanoscale structural degradation and the intergranular cracks. Meanwhile, the LiF&FeF₃ coating alleviates the outward migration of O^α- (α<2), increases oxygen vacancy formation energies, and accelerates interfacial Li⁺ diffusion. Benefited from these, the electrochemical performance of LiF&FeF₃ modified materials is improved (83.1% capacity retention after 1000 cycles at 1C), even under exertive operational conditions of elevated temperature (91.3% capacity retention after 150 cycles at 1C). This work demonstrates that the dual-modified strategy can simultaneously address the problems of interfacial instability and bulk structural degradation and represents significant progress in developing high-performance lithium-ion batteries (LIBs).

Improving the long-term electrochemical performances of Li-rich cathode material by encapsulating a three-in-one nanolayer

With large specific capacity, wide voltage window, and high energy density, Li-rich layered oxides have been considered as a promising cathode candidate for advanced lithium-ion batteries (LIBs). However, their commercial application is challenging due to severe capacity degradation and voltage fading caused by irreversible oxygen evolution and phase transition upon repeated cycling. This work proposes an effective strategy to improve the long-term electrochemical performances of Li_1.2Mn_0.56Ni_0.17Co_0.07O₂ (LMNCO) by constructing multifunctional nanolayers composed of element-doping, layered-spinel heterostructural connection, and fast ion conductor shell via a facile method. The Li_0.09B_0.97PO₄ (LBPO) coating shell acts as a fast ion carrier and physical screen to promote Li⁺ diffusion and isolate side reactions at the cathode-electrolyte interface; moreover, two-phase transitional region provides three-dimensional channel to facilitate Li⁺ transport and inhibit phase transition. Besides, B³⁺ and PO₄^3--doping collaborates with oxygen vacancies to stabilize lattice oxygen and restrain oxygen evolution from the bulk active cathode. The optimized LMNCO@LBPO material exhibits a superior capacity retention of 78.6%, higher than that of the pristine sample (49.3%), with the mitigated voltage fading of 0.73 mV per cycle after 500 cycles at 1 C. This study opens up an avenue for the surface modification to the electrochemical properties and perspective application of Li-rich cathodes in high-performance LIBs.

Adjustable Mixed Conductive Interphase for Dendrite-Free Lithium Metal Batteries

Lithium (Li) metal batteries with high energy density are of great promise for next-generation energy storage; however, they suffer from severe Li dendritic growth and an unstable solid electrolyte interphase. In this study, a mixed ionic and electronic conductive (MIEC) interphase layer with an adjustable ratio assembled by ZnO and Zn nanoparticles is developed. During the initial cycle, the in situ formed Li₂O with high ionic conductivity and a lithiophilic LiZn alloy with high electronic conductivity enable fast Li⁺ transportation in the interlayer and charge transfer at the ion/electron conductive junction, respectively. The optimized interface kinetics is achieved by balancing the ion migration and charge transfer in the MIEC Li₂O-LiZn interphase. As a result, the symmetric cell with MIEC interphase delivers superior cycling stability of over 1200 h. Also, Li||Zn-ZnO@PP||LFP (LFP = LiFePO₄) full cells exhibit long cyclic life for 2000 cycles with a very high capacity retention of 91.5% at a high rate of 5 C and stable cycling for 350 cycles at a high LFP loading mass of 13.27 mg cm^-2.

Scalable Nitrate Treatment for Constructing Integrated Surface Structures to Mitigate Capacity Fading and Voltage Decay of Li-Rich Layered Oxides

Capacity fading and voltage decay is one of the biggest obstacles for the practical application of Li-rich layered oxides due to the serious surface-related detrimental reactions. Herein, we develop a versatile and scalable method to construct a robust surface-integrated structure. All the designed samples deliver outstanding capacity and voltage stability, among which the Zn-treated sample possesses the best electrochemical performance. Its capacity retention is larger than 90 % after 400 cycles with a voltage decay ratio as small as 0.73 mV per cycle. What is more, the rules of surface-integrated structure with different cations in terms of capacity and voltage stability is further deciphered by combining with density function theory (DFT) calculations. It is found that to obtain advanced Li-rich layered oxide cathodes, cations in Li-sites should firstly ensure the binding energy of the surface-integrated structure in a lower level and then provide Bader charge for the nearest O atoms as small as possible.

In Situ Induced Lattice-Matched Interfacial Oxygen-Passivation-Layer Endowing Li-Rich and Mn-Based Cathodes with Ultralong Life

The high capacity of Li-rich and Mn-based (LRM) cathode materials is originally due to the unique hybrid anion- and cation redox, which also induces detrimental oxygen escape. Furthermore, the counter diffusion of released oxygen (into electrolyte) and induced oxygen vacancies (into the interior bulk phase) that occurs at the interface will cause uncontrolled phase collapse and other issues. Therefore, due to its higher working voltage (>4.7 V) than the activation voltage of lattice oxygen in LRM (≈4.5 V), the anion-redox-free and structurally consistent cobalt-free LiNi_0.5 Mn_1.5 O₄ (LNMO) is selected to in situ construct a robust, crystal-dense and lattice-matched oxygen-passivation-layer (OPL) on the surface of LRM particles by the electrochemical delithiation to protect the core layered components. As expected, the modified sample displays continuously decreasing interfacial impedance and high specific capacity of 135.5 mAh g^-1 with a very small voltage decay of 0.67 mV per cycle after 1000 cycles at 2 C rate. Moreover, the stress accumulation during cycling is mitigated effectively. This semicoherent OPL strengthens the surface stability and interrupts the counter diffusion of oxygen and oxygen vacancies in LRM cathode materials, which would provide guidance for designing high-energy-density layered cathode materials.

Structural Origin of Suppressed Voltage Decay in Single-Crystalline Li-Rich Layered Li[Li0.2 Ni0.2 Mn0.6 ]O2 Cathodes

Lithium- and manganese-rich layered oxides (LMLOs, ≥ 250 mAh g^-1 ) with polycrystalline morphology always suffer from severe voltage decay upon cycling because of the anisotropic lattice strain and oxygen release induced chemo-mechanical breakdown. Herein, a Co-free single-crystalline LMLO, that is, Li[Li_0.2 Ni_0.2 Mn_0.6 ]O₂ (LLNMO-SC), is prepared via a Li⁺ /Na⁺ ion-exchange reaction. In situ synchrotron-based X-ray diffraction (sXRD) results demonstrate that relatively small changes in lattice parameters and reduced average micro-strain are observed in LLNMO-SC compared to its polycrystalline counterpart (LLNMO-PC) during the charge-discharge process. Specifically, the as-synthesized LLNMO-SC exhibits a unit cell volume change as low as 1.1% during electrochemical cycling. Such low strain characteristics ensure a stable framework for Li-ion insertion/extraction, which considerably enhances the structural stability of LLNMO during long-term cycling. Due to these peculiar benefits, the average discharge voltage of LLNMO-SC decreases by only ≈0.2 V after 100 cycles at 28 mA g^-1 between 2.0 and 4.8 V, which is much lower than that of LLNMO-PC (≈0.5 V). Such a single-crystalline strategy offers a promising solution to constructing stable high-energy lithium-ion batteries (LIBs).

Building Homogenous Li2 TiO3 Coating Layer on Primary Particles to Stabilize Li-Rich Mn-Based Cathode Materials

Li-rich Mn-based oxides (LRMOs) are promising cathode materials for next-generation lithium-ion batteries (LIBs) with high specific energy (≈900 Wh kg^-1 ) because of anionic redox contribution. However, LRMOs suffer from issues such as irreversible release of lattice oxygen, transition metal (TM) dissolution, and parasitic cathode-electrolyte reactions. Herein, a facile, scalable route to build homogenous and ultrathin Li₂ TiO₃ (LTO) coating layer on the primary particles of LRMO through molten salt (LiCl) assisted solid-liquid reaction between TiO₂ and Li_1.08 Mn_0.54 Co_0.13 Ni_0.13 O₂ is reported. The prepared LTO-coated Li_1.08 Mn_0.54 Co_0.13 Ni_0.13 O₂ (LTO@LRMO) exhibits 99.7% capacity retention and 95.3% voltage retention over 125 cycles at 0.2 C, significantly outperforming uncoated LRMO. Combined characterizations of differential electrochemical mass spectrometry, in situ X-ray diffraction, and ex situ X-ray photoelectron spectroscopy evidence significantly suppressed oxygen release, phase transition, and interfacial reactions. Further analysis of cycled electrodes reveals that the LTO coating layer inhibits TM dissolution and prevents the lithium anode from TM crossover effect. This study expands the primary particle coating strategy to upgrade LRMO cathode materials for advanced LIBs.

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.

Preparation and characterization of the Li1.12K0.05Mn0.57Ni0.24Nb0.02O2 cathode material with highly improved rate cycling performance for lithium ion batteries

In this work, Li_1.12K_0.05Mn_0.57Ni_0.24Nb_0.02O₂ (LMN-K/Nb) as a novel and high energy density cathode material is successfully synthesized and applied in lithium ion batteries. By combining interlayer exchange and elemental analysis, it can be confirmed that K⁺ and Nb⁵⁺ substitution is respectively in the lithium layer and transition metal (TM) layer since H⁺ replaces the cations that remain in the lithium layer rather than those in the TM layer. The effect of K⁺ and Nb⁵⁺ co-substitution on the kinetic behavior of insertion/extraction of Li⁺ is evaluated by electrochemical impedance spectroscopy (EIS), the galvanostatic intermittent titration technique (GITT) and galvanostatic charge/discharge (GCD). LMN-K/Nb delivers an initial capacity of 145 mA h g^-1 at 5C rate and 112 mA h g^-1 at 10C rate, and maintains 83.1% after 400 cycles at 5C rate and 82.5% at 10C rate. By post-mortem analysis of long-term cycled LMN-K/Nb, K⁺ and Nb⁵⁺ are recognized to play a role in suppressing the irreversible side reactions in LLOs during cycling. This work demonstrates that dual elemental substitution into the lithium layer and TM layer is a feasible strategy to enhance the performance of LLO cathode materials.

Challenges and Recent Advances in High Capacity Li-Rich Cathode Materials for High Energy Density Lithium-Ion Batteries

Li-rich cathode materials have attracted increasing attention because of their high reversible discharge capacity (>250 mA h g^-1 ), which originates from transition metal (TM) ion redox reactions and unconventional oxygen anion redox reactions. However, many issues need to be addressed before their practical applications, such as their low kinetic properties and inefficient voltage fading. The development of cutting-edge technologies has led to cognitive advances in theory and offer potential solutions to these problems. Herein, a recent in-depth understanding of the mechanisms and the frontier electrochemical research progress of Li-rich cathodes are reviewed. In addition, recent advances associated with various strategies to promote the performance and the development of modification methods are discussed. In particular, excluding Li-rich Mn-based (LRM) cathodes, other branches of the Li-rich cathode materials are also summarized. The consistent pursuit is to obtain energy storage devices with high capacity, reliable practicability, and absolute safety. The recent literature and ongoing efforts in this area are also described, which will create more opportunities and new ideas for the future development of Li-rich cathode materials.

Insights into the Enhanced Structural and Thermal Stabilities of Nb-Substituted Lithium-Rich Layered Oxide Cathodes

Lithium-rich manganese-based layered oxides (LLOs) are considered to be the most promising cathode materials for next-generation lithium-ion batteries (LIBs) for their higher reversible capacity, higher operating voltage, and lower cost compared with those of other commercially available cathode materials. However, irreversible lattice oxygen release and associated severe structural degradation that exacerbate under high temperature and deep delithiation hinder the large-scale application of LLOs. Herein, we propose a strategy to stabilize the layered lattice framework and improve the thermal stability of cobalt-free Li_1.2Mn_0.53Ni_0.27O₂ by doping with 4d transition metal niobium (Nb). Detailed atomic-scale imaging, in situ characterization, and DFT simulations confirm that the induced strong Nb-O bonds stabilize the oxygen lattice framework and restrains the fracture of TM-O bonds, thereby inhibiting the release of lattice oxygen and the continuous migration of TM ions to the lithium layer during the cycle. Furthermore, Nb doping also promotes the surface rearrangement to form a Ni-enrichment layered/rocksalt heterogeneous interface to enhance surface structural stability. As a result, the Nb-doped material delivers a capacity of 181.7 mAh g^-1 with retention of 85.5% after 200 cycles at 1C, extraordinary thermal stability with a capacity retention of 80.7% after 200 cycles at 50 °C, and superior rate capability.

A Universal Strategy toward the Precise Regulation of Initial Coulombic Efficiency of Li-Rich Mn-Based Cathode Materials

Li-rich Mn-based cathode materials (LRMs) are potential cathode materials for high energy density lithium-ion batteries. However, low initial Coulombic efficiency (ICE) severely hinders the commercialization of LRM. Herein, a facile oleic acid-assisted interface engineering is put forward to precisely control the ICE, enhance reversible capacity and rate performance of LRM effectively. As a result, the ICE of LRM can be precisely adjusted from 84.1% to 100.7%, and a very high specific capacity of 330 mAh g^-1 at 0.1 C, as well as outstanding rate capability with a fascinating specific capacity of 250 mAh g^-1 at 5 C, are harvested. Theoretical calculations reveal that the introduced cation/anion double defects can reduce the diffusion barrier of Li⁺ ions, and in situ surface reconstruction layer can induce a self-built-in electric field to stabilize the surface lattice oxygen. Moreover, this facile interface engineering is universal and can enhance the ICEs of other kinds of LRM effectively. This work provides a valuable new idea for improving the comprehensive electrochemical performance of LRM through multistrategy collaborative interface engineering technology.

Enhanced Activity and Reversibility of Anionic Redox by Tuning Lithium Vacancies in Li-Rich Cathode Materials

Li-rich Mn-based layered oxide cathodes (LLOs) are considered to be the most promising cathode candidates for lithium-ion batteries owing to their high-voltage platform and ultrahigh specific capacity originating from anionic redox. However, anionic redox results in many problems including irreversible oxygen release, voltage hysteresis, and so on. Although many efforts have been made to regulate anionic redox, a fundamental issue, the effect of lithium vacancies on anionic redox, is still unclear. Herein, we synthesized a series of LLO materials with different lithium vacancy contents by controlling the amount of lithium salt. Specifically, lithium-vacancy-type LLOs Li_1.11Ni_0.18Co_0.18Mn_0.53O₂ with a pompon morphology exhibit an ultrahigh specific capacity (293.9 mA h g^-1 at 0.1 C), an outstanding long-term cycling stability (173.5 mA h g^-1 after 300 cycles at 1 C), and an excellent rate performance (106 mA h g^-1 at 10 C). It reveals that lithium vacancy is a key factor to enhance anionic redox activity and reversibility. Lithium vacancies exhibit different inductive effects on the structure of the surface and bulk. Abundant surface oxygen vacancies and a surface spinel phase layer induced by lithium vacancies suppress irreversible oxygen release, while the bulk phase transformation and cation disorder combined with sufficient lithium vacancies in the bulk stabilize structure and improve anionic redox kinetic. The findings offer a significant theoretical guidance for the practical application of LLO materials.

Synergetic effect of high Ni ratio and low oxygen defect interface zone of single crystals on the capacity retention of lithium rich layered oxides

Li-rich layered oxides (LLOs) are promising cathode materials for Li-ion batteries owing to their high capacities (>250 mAh g^-1), however, they suffered from severe capacity and voltage fading caused by irreversible oxygen loss and phase changes. Herein, the structural stability of single crystalline and polycrystalline Li_1.14Ni_0.32Mn_0.44Co_0.04O₂ was compared in detail. It was found that the stability of oxidized oxygen ions on the near surface was improved in single crystals, which retarded oxygen loss from surface and surficial phase changes, possibly owing to the facet regulating and low surface curvature. In addition, the formation-migration of Mn³⁺, one of the crucial factors that caused capacity fading of LLOs, can be mitigated by increasing Ni³⁺ ratio. Under the synergistic effect of low oxygen defects on the near surface and high Ni³⁺ ratio, stable cycling performances and higher thermal stability were obtained.

Encouraging Voltage Stability upon Long Cycling of Li-Rich Mn-Based Cathode Materials by Ta-Mo Dual Doping

The Li-rich and Mn-based material xLi₂MnO₃·(1-x)LiMO₂ (M = Ni, Co, and Mn) is regarded as one of the new generations of cathode materials for Li-ion batteries due to its high energy density, low cost, and less toxicity. However, there still exist some drawbacks such as its high initial irreversible capacity, capacity/voltage fading, poor rate capability, and so forth, which seriously limit its large-scale commercial applications. In this paper, the Ta-Mo codoped Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ with high energy density is prepared via a coprecipitation method, followed by a solid-state reaction. The synthetic mechanism and technology, the effect of charge-discharge methods, the bulk doping and the surface structure design on the structure, morphology, and electrochemical performances of the Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ cathode are systematically investigated. The results show that Ta⁵⁺ and Mo⁶⁺ mainly occupy the Li site and transition-metal site, respectively. Both the appropriate Ta and Ta-Mo doping are conductive to increase the Mn³⁺ concentration and suppress the generation of Li/Ni mixing and the oxygen defects. The Ta-Mo codoped cathode sample can deliver 243.2 mA h·g^-1 at 1 C under 2.0-4.8 V, retaining 80% capacity retention after 240 cycles, and decay 1.584 mV per cycle in 250 cycles. The capacity retention can be still maintained to 80% after 410 cycles over 2.0-4.4 V, and the average voltage fading rate is 0.714 mV per cycle in 500 cycles. Compared with the pristine, the capacity and voltage fading of Ta-Mo codoped materials are effectively suppressed, which are mainly ascribed to the fact that the highly valence Ta⁵⁺ and Mo⁶⁺ that entered into the crystal lattice are favorable for maintaining the charge balance, and the strong bond energies of Ta-O and Mo-O can help to maintain the crystal structure and relieve the corrosion from the electrolyte during the charging/discharging process.

Addressing voltage decay in Li-rich cathodes by broadening the gap between metallic and anionic bands

Oxygen release and irreversible cation migration are the main causes of voltage fade in Li-rich transition metal oxide cathode. But their correlation is not very clear and voltage decay is still a bottleneck. Herein, we modulate the oxygen anionic redox chemistry by constructing Li2ZrO3 slabs into Li2MnO3 domain in Li1.21Ni0.28Mn0.51O2, which induces the lattice strain, tunes the chemical environment for redox-active oxygen and enlarges the gap between metallic and anionic bands. This modulation expands the region in which lattice oxygen contributes capacity by oxidation to oxygen holes and relieves the charge transfer from anionic band to antibonding metal-oxygen band under a deep delithiation. This restrains cation reduction, metal-oxygen bond fracture, and the formation of localized O2 molecule, which fundamentally inhibits lattice oxygen escape and cation migration. The modulated cathode demonstrates a low voltage decay rate (0.45 millivolt per cycle) and a long cyclic stability.

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.

Deciphering the effects of hexagonal and monoclinic structure distribution on the properties of Li-rich layered oxides

Uniform distribution of Li₂MnO₃ and LiMO₂ components in a Co-free Li-rich layered oxide is achieved by treating precursors with NH₃·H₂O, which expands the lattice parameter and promotes the activation of Li₂MnO₃, resulting in excellent electrochemical performance. What's more, it also contributes to the storage stability of Li-rich layered oxides.

Accelerating the activation of Li2MnO3 in Li-rich high-Mn cathodes to improve its electrochemical performance

Li-rich high-Mn oxides, xLi2MnO3·(1 - x)LiMO2 (x ≥ 0.5, M = Co, Ni, Mn…), have attracted extensive research interest due to their high specific capacity and low cost. However, slow Li2MnO3 activation and poor cycling stability have affected their electrochemical performance. Herein, to solve these problems, morphology regulation and LiAlF4 coating strategies have been synergistically applied to a Li-rich high-Mn material Li1.7Mn0.8Co0.1Ni0.1O2.7 (HM-811). This dual-strategy successfully promotes the activation process of the Li2MnO3 phase and thus improves the electrochemical performance of HM-811. Theoretical computation indicates that the LiAlF4 layer has a lower Li+ migration barrier than the HM-811 matrix, so it could boost the diffusion of Li+ ions and promote the activation of the Li2MnO3 phase. Benefiting from the morphology regulation and LiAlF4 coating, the HM-811 cathode shows a high initial charge capacity of >300 mA h g-1. In addition, the modified HM-811 could deliver superior electrochemical performance even at a low temperature of -20 °C. This work provides a new approach for developing high performance cathode materials for next-generation Li-ion batteries.

Multiscale Deficiency Integration by Na-Rich Engineering for High-Stability Li-Rich Layered Oxide Cathodes

Lithium-rich manganese-based (LRM) layered oxides are considered as one of the most promising cathode materials for next-generation high-energy-density lithium-ion batteries (LIBs) because of their high specific capacity (>250 mAh g^-1). However, they also go through severe capacity decay, serious voltage fading, and poor rate capability during cycling. Herein, a multiscale deficiency integration, including surface coating, subsurface defect construction, and bulk doping, is realized in a Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ cathode material by facile Na-rich engineering through a sol-gel method. This multiscale design can significantly improve the bulk and surface structural stability and diffusion rate of Li⁺ ions of electrode materials. Specifically, an outstanding specific capacity of 201 mAh g^-1 is delivered at 1C of the designed cathode material after 400 cycles, relating to a large capacity retention of 89.0%. Meanwhile, the average voltage is retained up to 3.13 V with a large voltage retention of 89.6% and the energy density is maintained at 627.4 Wh kg^-1. In situ X-ray diffraction (XRD), ex situ transmission electron microscopy (TEM) investigations, and density functional theory (DFT) calculations are conducted to explain the greatly enhanced electrochemical properties of a LRM cathode. We believe that this strategy would be a meaningful reference of LRM cathode materials for the research in the future.

Stabilizing Anionic Redox Chemistry in a Mn-Based Layered Oxide Cathode Constructed by Li-Deficient Pristine State

Li-rich cathode materials are of significant interest for coupling anionic redox with cationic redox chemistry to achieve high-energy-density batteries. However, lattice oxygen loss and derived structure distortion would induce serious capacity loss and voltage decay, further hindering its practical application. Herein, a novel Li-rich cathode material, O3-type Li_0.6 [Li_0.2 Mn_0.8 ]O₂ , is developed with the pristine state displaying both a Li excess in the transition metal layer and a deficiency in the alkali metal layer. Benefiting from stable structure evolution and Li migration processes, not only can high reversible capacity (≈329 mAh g^-1 ) be harvested but also irreversible/reversible anionic/cationic redox reactions are comprehensively assigned via the combination of in/ex situ spectroscopies. Furthermore, irreversible lattice oxygen loss and structure distortion are effectively restrained, resulting in long-term cycle stability (capacity drop of 0.045% per cycle, 500 cycles). Altogether, tuning the Li state in the alkali metal layer presents a promising way for modification of high-capacity Li-rich cathode candidates.

A review on the electrochemical reaction of Li-rich layered oxide materials

Lithium-rich layered oxide materials xLi2MnO3·(1 − x)LiMO2 (M = Mn, Co, Ni, Fe, Cr, etc.) as promising cathode candidates for high energy Li-ion batteries have been summarized in this review.

Function and Application of Defect Chemistry in High-Capacity Electrode Materials for Li-Based Batteries

Current commercial Li-based batteries are approaching their energy density limitation, yet still cannot satisfy the energy density demand of the high-end devices. Hence, it is critical to developing advanced electrode materials with high specific capacity. However, these electrode materials are facing challenges of severe structural degradation and fast capacity fading. Among various strategies, constructing defects in electrode materials holds great promise in addressing these issues. Herein, we summarize a series of significant defect engineering in the high-capacity electrode materials for Li-based batteries. The detailed retrospective on defects specification, function mechanism, and corresponding application achievements on these electrodes are discussed from the view of point, line, planar, volume defects. Defect engineering can not only stabilize the structure and enhance electric/ionic conductivity, but also act as active sites to improve the ionic storage and bonding ability of electrode materials to Li metal. We hope this review can spark more perspectives on evaluating high-energy-density Li-based batteries.

Surface Architecture Design of LiNi0.8 Co0.15 Al0.05 O2 Cathode with Synergistic Organics Encapsulation to Enhance Electrochemical Stability

Ni-rich LiNi_0.8 Co_0.15 Al_0.05 O₂ (NCA) material attracts extensive attention due to its high discharge specific capacity, but its distinct drawbacks of rapid capacity decline and poor cycle performance at elevated temperatures and high voltage during charge/discharge cycling restricts its widespread application. To solve these problems, a multifunctional coating layer composed of a lithium-ion-conductive lithium polyacrylate (LiPAA) inner layer and a cross-linked polymer outer layer from certain organic substances of silane-coupling agent (KH550) and polyacrylic acid (PAA) is successfully designed on the surface of NCA materials, which is favorable for eliminating residual lithium and improving lithium-ion conductivity, surface stability, and hydrophobicity of NCA materials. In addition, the amount of the coating material is also investigated. A series of characterization methods such as XRD, FTIR, SEM, TEM, and X-ray photoelectron spectroscopy are used to analyze the morphologies and structures for materials of pristine and modified NCA. It is revealed that the co-coating layer plays a vital part in reducing the surface residual alkalis and improving the stability of NCA particles; as a result, the modified NCA exhibits a greatly improved rate capability, cycle performance, and low polarization impedance.

Surface regulation enables high stability of single-crystal lithium-ion cathodes at high voltage

Single-crystal cathode materials for lithium-ion batteries have attracted increasing interest in providing greater capacity retention than their polycrystalline counterparts. However, after being cycled at high voltages, these single-crystal materials exhibit severe structural instability and capacity fade. Understanding how the surface structural changes determine the performance degradation over cycling is crucial, but remains elusive. Here, we investigate the correlation of the surface structure, internal strain, and capacity deterioration by using operando X-ray spectroscopy imaging and nano-tomography. We directly observe a close correlation between surface chemistry and phase distribution from homogeneity to heterogeneity, which induces heterogeneous internal strain within the particle and the resulting structural/performance degradation during cycling. We also discover that surface chemistry can significantly enhance the cyclic performance. Our modified process effectively regulates the performance fade issue of single-crystal cathode and provides new insights for improved design of high-capacity battery materials.