Li-rich Layered Cathode Research Articles

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 mAhg-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.

Bifunctional modulation of Li-rich layered oxide cathode material via W-modification for improved electrochemical performance

Lithium-rich layered oxides (LRLOs) have raised the greatest attention to the high-energy-density Li-ion batteries. Nevertheless, the undesirable interfacial side reactions and the structure transformation cause fast voltage fading and poor capacity retention of LRLO cathodes, which extremely prevent their practical application. Surface coating or lattice doping has been an effective strategy to improve its electrochemical properties. However, the general performance of the material can only be limitedly promoted. Herein, we proposed a bifunctional modulation of LRLO cathodes coupled by bulk-phase W-doping and surficial Li2WO4 nanocoating of LRLOs using a facile synchronous lithiation strategy under a high-temperature solid-state treatment. The modified material (2 mol% W6+ modified) showed a distinguished discharge capability of 260.5 mAh/g at 0.1 C after 50 cycles. Meanwhile, a competitive capacity retention of 95.38% at 1 C after 100 cycles was achieved. The splendid electrochemical performance can be the result of the increased interlayer spacing for enhancing Li+ diffusion kinetics and the stronger W–O bond for alleviating oxygen release after W-doping and Li2WO4 nanocoating, which not only accelerates the Li+ transfer rate as its fast Li+ transfer channels but also act as a transitional interface layer to restrain the interfacial adverse reactions.

Boosting cationic and anionic redox activity of Li-rich layered oxide cathodes via Li/Ni disordered regulation

Lithium-rich layered oxides (LLOs) are increasingly recognized as promising cathode materials for next-generation high-energy-density lithium-ion batteries (LIBs). However, they suffer from voltage decay and low initial Coulombic efficiency (ICE) due to severe structural degradation caused by irreversible O release. Herein, we introduce a three-in-one strategy of increasing Ni and Mn content, along with Li/Ni disordering and TM–O covalency regulation to boost cationic and anionic redox activity simultaneously and thus enhance the electrochemical activity of LLOs. The target material, Li1.2Ni0.168Mn0.558Co0.074O2 (L1), exhibits an improved ICE of 87.2% and specific capacity of 293.2 mA h g−1 and minimal voltage decay of less than 0.53 mV cycle−1 over 300 cycles at 1C, compared to Li1.2Ni0.13Mn0.54Co0.13O2 (Ls) (274.4 mA h g−1 for initial capacity, 73.8% for ICE and voltage decay of 0.84 mV/cycle over 300 cycles at 1C). Theoretical calculations reveal that the density of states (DOS) area near the Fermi energy level for L1 is larger than that of Ls, indicating higher anionic and cationic redox reactivity than Ls. Moreover, L1 exhibits increased O-vacancy formation energy due to higher Li/Ni disordering of 4.76% (quantified by X-ray diffraction Rietveld refinement) and enhanced TM–O covalency, making lattice O release more difficult and thus improving electrochemical stability. The increased Li/Ni disordering also leads to more Ni2+ presence in the Li layer, which acts as a pillar during Li+ de-embedding, improving structural stability. This research not only presents a viable approach to designing low-Co LLOs with enhanced capacity and ICE but also contributes significantly to the fundamental understanding of structural regulation in high-performance LIB cathodes.

Comprehensive Review of Li-Rich Mn-Based Layered Oxide Cathode Materials for Lithium-Ion Batteries: Theories, Challenges, Strategies and Perspectives.

Lithium-rich manganese-based layered oxide cathode materials (LLOs) have always been considered as the most promising cathode materials for achieving high energy density lithium-ion batteries (LIBs). However, in practical applications, LLOs often face some key problems, such as low initial coulombic efficiency, capacity/voltage decay, poor rate performance and poor cycle stability. It seriously shortens the lifespan of lithium-ion batteries and hinder the large-scale commercial application of LLOs. Herein, firstly, the basic theories of LLOs were systematically reviewed, including the structural characteristics, the working mechanism of LLOs, the preparation methods of LLOs (liquid phase co-precipitate method, sol-gel method, hydrothermal synthesis method, solid phase method, low heat solid-phase method, high temperature solid-state method etc.), and electrochemical characteristics of LLOs (first charge discharge characteristics and reversible efficiency, cycling performance, high and low temperature performance and thermal stability etc.). Then, key challenges faced by LLOs were systematically discussed. Finally, the LLOs modification strategies used to address these challenges (element doping, surface modification, defect engineering, structural and morphological control etc.) were elaborated in detail. This important review provides potential insights and directions for further improving the electrochemical performance of LLOs, and provides a necessary theoretical basis for accelerating the large-scale commercial application of LLOs. It possesses important scientific research value and far-reaching social significance.

Enhancing the electrochemical properties of Li-rich layered oxide cathodes by a facile Fe/Ti integrated modification strategy

Li-rich layered oxide cathodes have gained much attention due to their high specific capacity, high operating voltage, and environmental friendliness. However, poor rate performance, rapid capacity and voltage decay during cycling have hindered its commercial application. In this work, a Fe/Ti integrated modification strategy is proposed to construct a LiFeTiO4 spinel phase coating on the surface of Li1.2Mn0.54Ni0.13Co0.13O2, effectively reducing interfacial side reactions and suppressing the irreversible oxygen release, while doping with Fe3+ and Ti4+ ions can effectively broaden the lithium diffusion channel and enhance the structural stability. As a result, the modified sample with 2 wt% of Fe/Ti demonstrates excellent cycling stability with a superior capacity retention of 91% after 400 cycles at 1C and good rate performance with a high specific discharge capacity of 216 mAh g−1 at 2C. Moreover, the initial Coulombic efficiency of the modified sample is also improved. This integrated modification strategy offers a new design to promote the large-scale application of Li-rich Mn-based cathode materials.

Unveiling the role of fluorinated interface on anionic redox chemistry in Li-rich layered oxide cathode materials towards high-energy Li metal batteries

The utilization of Lithium-rich materials (LR) with oxygen anionic redox (OAR) activity as cathode materials in Li metal batteries has gradually emerged as a prevailing trend in designing high-specific energy batteries. However, the interface design of LR and its influence mechanism on anionic redox chemistry are still ambiguous. Herein, in-situ interface engineering with an LiF-rich cathode-electrolyte interface (CEI) using all-fluorinated electrolyte was proposed and the role of the CEI on OAR was clarified. The shielding effect on high-valence oxygen ions is revealed that the dense, uniform, thin, and robust LiF-rich CEI protects high-valence oxygen ions from the electrolyte, inhibits electrolyte decomposition and interface thickening, and ameliorates structural degradation and oxygen loss of LR. And a more critical effect of CEI on the OAR process is demonstrated that the LiF-rich CEI layer prevents molecular oxygen in the bulk from escaping into the electrolyte as O2 gas and makes the oxidized oxygen exists more in the form of peroxo-like oxygen dimer O2n−. Besides the stabilization of OAR in the cathode side, the all-fluorinated electrolyte along with its enhancement in the Li anode enable high safety, excellent cycling stability, and fast charging performance for high energy lithium metal batteries.

Structure engineering with sodium doping for cobalt-free Li-rich layered oxide toward improving electrochemical stability

Structure engineering of the Li-rich layered cathodes to overcome insufficient structural stability and the rapid decay of capacity and voltage is crucial for commercializing of the materials for the lithium-ion batteries. Alkali metal element doping at the lithium sites has proven to be a feasible approach to boost the performance of the Li-rich layered oxides. Herein, the Na+-doping strategy in the lithium slabs is introduced to modify the structure of the cobalt-free layered Li-rich oxide, Li1.2Ni0.2Mn0.6O2. It is revealed that the doped Na+ ions can promote the activation of the Li2MnO3 phase, endowing the materials with high initial discharge capacity of 284.2 mAh g−1 at 0.1C. Due to the pillaring effect of the doped Na+ ions in the lithium slabs and the induced formation of oxygen vacancies, the electrochemical stability of the material is significantly improved, providing a capacity retention of 94.0 % after 100 cycles at 0.5C. The voltage decay per cycle is only 2.0 mV, less than 3.2 mV of the Li1.2Ni0.2Mn0.6O2. The results suggest that the facile strategy of introducing Na+ ions into the lithium slabs is an efficient approach for optimizing structure design of the Li-rich layered oxides for the lithium-ion batteries.

One-Step Realization of Layered/Spinel Heterostructures and Na Doping by Sodium Dodecyl Sulfate-Assisted Sol-Gel Method for Li-Ion Batteries.

Li-rich layered oxide cathodes have attracted extensive attention due to their high energy density. However, due to the low initial Coulombic efficiency and the capacity fading and voltage fading during cycling, its practical application is still a great challenge. Here, we report the one-step realization of layered/spinel heterostructures and Na doping by the sodium dodecyl sulfate (SDS)-assisted sol-gel method. The spinel phase provides 3D diffusion channels for Li-ions, and sodium doping changes the layered lattice constant and expands the layer spacing. Therefore, the designed Li1.15Mn0.54Ni0.13Co0.13Na0.05O2 (SDS-2) cathode possesses excellent electrochemical performance such as higher initial Coulombic efficiency and rate capacity and also alleviates voltage decay. The initial discharge-specific capacity of SDS-2 is 298.8 mAh g-1 at 0.1 C, and the discharge-specific capacity can reach 111.7 mAh g-1 at 10 C. This strategy can provide new insights into the design and synthesis of high-performance Li-rich layered oxide cathode materials.

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.

Phase Stability of Li-rich Layered Cathodes: Insight into the Debate over Solid Solutions vs Phase Separation

Phase Stability of Li-rich Layered Cathodes: Insight into the Debate over Solid Solutions vs Phase Separation

Perspectives on the Practicability of Li-Rich NMC Layered Oxide Cathodes.

Li-rich NMCs layered oxides, with the general formula of Li[LixNiyMnzCo1- x - y - z]O2, are known for their exceptionally high capacities but remain yet to be practicalized in the real world. They have attracted enormous research attention due to their complex structure and intriguing redox mechanisms, with a particular focus on anionic redox over the past decade. While fundamental understandings are fruitful, the practical considerations are emphasized here by providing perspectives on how Li-rich NMCs are limited by practical roadblocks and guidelines on how to cope with these limitations. It is also demonstrated that, via a techno-economic analysis, Li-rich NMCs have material cost ($/kg) highly dependent on the lithium price, but still preserve the dominance of lower pack cost ($/kWh) than other cathode candidates principally owing to their larger material energy densities. In addition to their pure application in electric-vehicle batteries, using them as "electrode additive" or "sacrificial agent" can further multiply their practicalities in assortment of scenarios, which is further discussed.

Activation of Oxygen Redox by Inhibited Dynamic Phase Transition for High-Energy Li-Rich Layered Oxide Cathode

Li-rich layered oxides have garnered tremendous research interest as a promising high-energy cathode by harnessing combined cationic and anionic redox. Despite the great promise of oxygen redox, however, the distinct activity of oxygen redox in Li-rich materials has inhibited the precise understanding of the oxygen redox activation mechanism. Herein, we unravel the interplay between de/activation of oxygen redox and the phase transition behaviors of Li-rich cathodes through a comprehensive investigation of two Ru-based model Li-rich layered oxides, Li1.2Ni0.2Mn0.3Ru0.3O2 and Li1.2Ni0.2Ru0.6O2. It is corroborated by synchrotron-based X-ray absorption spectroscopy analyses that the reversible oxygen redox occurs inLi1.2Ni0.2Mn0.3Ru0.3O2 but the reaction is not triggered in Li1.2Ni0.2Ru0.6O2 despite high relevancein structure and stoichiometry. Operando X-ray diffraction elucidates that the O3-type Li1.2Ni0.2Ru0.6O2 undergoes a phase transition into the T3-type phase during charging while there is negligible structural change in O3-type Li1.2Ni0.2Mn0.3Ru0.3O2. First-principles studies reveal that the theoretical redox potential to extract Li+ from T3-type phase is significantly higher than that of O3-type Li1.2Ni0.2Ru0.6O2, inhibiting the activation of the oxygen redox.Our findings indicate the significance of the dynamic structural evolution on governing the activation of oxygen redox, thereby offering guidance for further design of Li-rich layered oxides toward the full utilization of the oxygen redox.

Highly reversible transition metal migration in superstructure-free Li-rich oxide boosting voltage stability and redox symmetry

The further practical applications of Li-rich layered oxides are impeded by voltage decay and redox asymmetry, which are closely related to the structural degradation involving irreversible transition metal migration. It has been demonstrated that the superstructure ordering in O2-type materials can effectively suppress voltage decay and redox asymmetry. Herein, we elucidate that the absence of this superstructure ordering arrangement in a Ru-based O2-type oxide can still facilitate the highly reversible transition metal migration. We certify that Ru in superstructure-free O2-type structure can unlock a quite different migration path from Mn in mostly studied cases. The highly reversible migration of Ru helps the cathode maintain the structural robustness, thus realizing terrific capacity retention with neglectable voltage decay and inhibited oxygen redox asymmetry. We untie the knot that the absence of superstructure ordering fails to enable a high-performance Li-rich layered oxide cathode material with suppressed voltage decay and redox asymmetry.

Regulating anionic redox reversibility in Li-rich layered cathodes via diffusion-induced entropy-assisted surface engineering

Cobalt-free lithium- and manganese-rich layered oxides (LMROs) are regarded as effective cathode materials for lithium storage due to their high capacity and cost-effectiveness. However, challenges with structural degradation, resulting in poor cyclability and rate performance, have hindered their widespread use. Essentially, rapid structural degradation arises from irreversible and complex redox reactions, triggering oxygen release, transition metal migration, and electrolyte decomposition. To tackle this issue, we propose an epitaxial entropy-assisted construction approach to develop a sturdy surface with adaptable composition, stabilising the surface crystal structure and preventing undesirable interface reactions. This distinct reconstructed surface comprises diverse heterogeneous elements and composite microstructures. The heteroatom-doped layer, with multi-doping sites like Li, O site, and tetrahedral positions, effectively manages the chemical environment and electronic structure of surface lattice oxygen. This epitaxial entropy stabilisation approach, stemming from multi-element synergy, effectively controls redox progress to limit oxygen release and curb transition metal migration, reducing structural decay. Additionally, the composite coated layer, containing oxygen defects and heterogeneous spinel phases, can hinder electrolyte corrosion and promote Li+ transport. Using these epitaxial entropy surface modifications, the LMRO cathode demonstrates regulated anionic redox reversibility and enhanced cycling stability across diverse operational conditions. This epitaxial entropy-assisted surface engineering offers a promising avenue for stabilising high-energy cathode materials.

Regulating the Electron Distribution of Metal-Oxygen for Enhanced Oxygen Stability in Li-rich Layered Cathodes.

Li-rich Mn-based layered oxides (LLO) hold great promise as cathode materials for lithium-ion batteries (LIBs) due to their unique oxygen redox (OR)chemistry, which enables additional capacity. However, the LLOs face challenges related to the instability of their ORprocess due to the weak transition metal (TM)-oxygen bond, leading to oxygen loss and irreversible phase transition that results in severe capacity and voltage decay. Herein, a synergistic electronic regulation strategy of surface and interior structures to enhance oxygen stability is proposed. In the interior of the materials, the local electrons around TM and O atoms may be delocalized by surrounding Mo atoms, facilitating the formation of stronger TM─O bonds at high voltages. Besides, on the surface, the highly reactive O atoms with lone pairs of electrons are passivated by additional TM atoms, which provides a more stable TM─O framework. Hence, this strategy stabilizes the oxygen and hinders TM migration, which enhances the reversibility in structural evolution, leading to increased capacity and voltage retention. This work presents an efficient approach to enhance the performance of LLOs through surface-to-interior electronic structure modulation, while also contributing to a deeper understanding of their redox reaction.

A Li-rich layered oxide cathode with remarkable capacity and prolonged cycle life

Introducing a facile ion-exchange method coupled with low-temperature thermal treatment, we have developed a strategy to enhance the cycling performance of Lithium-rich manganese-based layered oxides (LLOs). This approach results in the formation of a thin spinel phase layer, which is covered by a fast-ion-conducting Li3PO4 phase on the surface of pristine 0.5Li2MnO3·0.5LiMn1/3Co1/3Ni1/3O2 secondary particles of aggregated original nanoparticles, and an increase in oxygen vacancies. After undergoing 200 cycles at a rate of 1 C, the resulting cathode exhibits a high capacity of 204.7 mAh/g with a retention rate of up to 94.4 %, which is significantly superior to that of the original materials. The long-term cycling stability of the modified cathode is also evident at higher rates such as 2, 5, and 8 C. The electrochemical analysis suggests that the surface modification and oxygen vacancies can enhance the Li+ diffusion coefficient, improve anion redox reversibility and increase the capacity contribution from spinel oxidation, thereby improving the electrochemical performance of the cathode during cycling.

Highly structural stability from small-sized Li2MnO3-like domains in Co-free Li-rich layered oxide cathodes

Highly structural stability from small-sized Li2MnO3-like domains in Co-free Li-rich layered oxide cathodes

Triggering cationic/anionic hybrid redox stabilizes high-temperature Li-rich cathodes materials via three-in-one strategy

Efficiently advancing lithium-ion batteries toward high energy density entails overcoming challenges in Li-rich layered oxides, including severe voltage and capacity fading, irreversible oxygen escape and compromised thermal robustness due to the uncontrollable oxygen anionic redox (OAR). Herein, a three-in-one modification strategy, aim at strengthening the reversibility of OAR and structural integrity of MnO6 octahedron, is rationally introduced on Li1.2Mn0.53Ni0.20Co0.07O2 cathode through a universal H3BO3-treatment. Through the introduction of oxygen vacancies and gradient B-doping, the energy level of unhybridized O 2p states is decreased to narrow the band energy gap with transition metal (TM) band, which triggers cationic/anionic hybrid redox at high voltage, thereby stabilizing the peroxo-like (O2)n- species and hindering irreversible oxygen escape, while the lithium borate nano-coating serves to mitigate side reactions, particularly during charging at elevated temperatures. As a result, the modulated cathode exhibits notable capacity retention of 96.3 % after 500 cycles at 1 C with limited voltage fading rate (1.73 mV per cycle) and even stable cyclic performance at high temperature (60 °C). This approach provides an effective and straightforward method to tackle the voltage decay and capacity fading of high-energy Li-rich layered cathode materials.

Band Structure Engineering Promotes Anionic Redox Reversibility of Cobalt-Free Li-Rich Layered Oxides Cathodes.

Li-rich layered oxides cathodes (LLOs) have prevailed as the promising high-energy-density cathode materials due to their distinctive anionic redox chemistry. However, uncontrollable anionic redox process usually leads to structural deterioration and electrochemical degradation. Herein, a Mo/Cl co-doping strategy is proposed to regulate the relative position of energy band for modulating the anionic redox chemistry and strengthening the structural stability of Co-free Li1.16Mn0.56Ni0.28O2 cathodes. The incorporation of Mo with high d state orbit and Cl with low electronegativity can narrow the band energy gap between bonding and antibonding bands via increasing the filled lower-Hubbard band (LHB) and decreasing the non-bonding O 2p energy bands, promoting the anionic redox reversibility. In addition, strong covalent Mo─O and Mn─Cl bonding further increases the covalency of Mn─O band to further stabilize the O2 n- species and enhance the reversible distortion of MnO6 octahedron. The strengthening electronic conductivity, together with the epitaxial structure Li2MoO4 facilitates the fast Li+ kinetics. As a result, the dual doping material exhibits enhanced anionic redox reversibility and suppressed oxygen release with increased cyclic stability and excellent rate performance. This strategy provides some guidance to design high-energy-density LLOs with desirable anionic redox reversibility and stable crystal structure via band structure engineering.

Controllable oxygen vacancies (in surface and bulk) to suppress the voltage decay of Li-rich layered cathode

Li-rich Mn-Based layered oxide is defined as a potential cathode material for its high specific capacity while several significant drawbacks such as inferior cycling stability and voltage decay limit its wide application. In this work, a suite of characterization techniques is employed to comprehensively investigate the crystal structure, surface morphology and chemical state of materials. The results suggest the successful preparation of layered oxide with controllable oxygen vacancies. Specifically, 1) Surface oxygen vacancies can act as a deterrent to irreversible oxygen release, consequently suppressing the voltage decay. 2) Stable lattice oxygen can enhance structural stability during cycling. 3) Swing-like sintering methodology and inducing surface oxygen vacancies technique are connected to synthesize the layered oxide with little lattice oxygen vacancies and abundant surface oxygen vacancies. The as-prepared cathode exhibits a commendable capacity retention of 85 %, and managed to alleviate the voltage attenuation significantly to 3.81 mV/cycle over 150 cycles at 0.5C. This concept of pre-constructing controllable oxygen vacancies provides important groundwork for developing Li-rich cathode with mitigated voltage decay and enhanced cycle life.