Dielectric Polarization in Inverse Spinel-Structured Mg2 TiO4 Coating to Suppress Oxygen Evolution of Li-Rich Cathode Materials

Dual Modification for Low-Strain Ni-Rich Cathodes Toward Superior Cyclability in Pouch Full Cells

Rapid capacity fading, interfacial instability, and thermal runaway due to oxygen loss are critical obstacles hindering the practical application and commercialization of Ni-rich cathodes (LiNi0.8Co0.1Mn0.1O2, NCM811). Herein, a Sn4+/F- codoping and LiF-coated Ni-rich cathode, denoted as NCM811-SF, is structurally fabricated that demonstrates very high cyclic and thermal stabilities. The introduction of Sn4+ regulates the local electronic structure and facilitates the conversion of the layered structure into a spinel phase; F- captures lithium impurities to form LiF coatings and forms TM-F bonds to reduce Ni/Li disordering. The compositionally complex codoping strategy reduces the internal structure strain, inhibits the Li+/Ni2+ intermixing during cycling and degradation of the nanoscale structure, and further improves the thermal stability and the crystal structure. The cathodic electrode showed a little volume shift at 2.8-4.5 V, which significantly decreased lattice flaws and fractures generated by local strain, based on detailed analyses performed using COMSOL simulations, X-ray diffraction, and scanning transmission electron microscopy. Benefiting from this, after 300 cycles, our as-prepared NCM811-SF cathode maintains 85.4% of its initial capacity at 4.5 V and has an excellent reversible capacity equal to 169 mAh·g-1 at 1 C. In addition, the NCM811-SF/graphite cell in a pouch-type complete cell retained 94.8% of its starting capacity following 500 cycles. These findings underscore the effectiveness of introducing the Sn-O and TM-F bonds in improving the durability and electrochemical efficiency of the cathode material, which makes it a good choice for high-efficiency Li-ion batteries.

Invoking Interfacial Engineering Boosts Structural Stability Empowering Exceptional Cyclability of Ni-Rich Cathode

The cycling stability of LiNi0.8Co0.1Mn0.1O2 under high voltages is hindered by the occurrence of hybrid anion- and cation-redox processes, leading to oxygen escape and uncontrolled phase collapse. In this study, an interfacial engineering strategy involving a straightforward mechanical ball milling and low-temperature calcination, employing a Se-doped and FeSe2&Fe2O3-modified approach is proposed to design a stable Ni-rich cathode. Se2- are selectively adsorbed within oxygen vacancies to form O─TM─Se bond, effectively stabilizing lattice oxygen, and preventing structural distortion. Simultaneously, the Se-NCM811//FeSe2//Fe2O3 self-assembled electric field is activated, improving interfacial charge transfer and coupling. Furthermore, FeSe2 accelerates Li+ diffusion and reacts with oxygen to form Fe2O3 and SeO2. The Fe2O3 coating mitigates hydrofluoric acid erosion and acts as an electrostatic shield layer, limiting the outward migration of oxygen anions. Impressively, the modified materials exhibit significantly improved electrochemical performance, with a capacity retention of 79.7% after 500 cycles at 1C under 4.5 V. Furthermore, it provides an extraordinary capacity retention of 94.6% in 3-4.25 V after 550 cycles in pouch-type full battery. This dual-modification approach demonstrates its feasibility and opens new perspective for the development of stable lithium-ion batteries operating at high voltages.

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.

Engineering Reversible Lattice Structure for High-Capacity Co-Free Li-Rich Cathodes with Negligible Capacity Degradation

Co-free Li-rich Mn-based cathode materials are garnering great interest because of high capacity and low cost. However, their practical application is seriously hampered by the irreversible oxygen escape and the poor cycling stability. Herein, a reversible lattice adjustment strategy is proposed by integrating O vacancies and B doping. B incorporation increases TM─O (TM: transition metal) bonding orbitals whereas decreases the antibonding orbitals. Moreover, B doping and O vacancies synergistically increase the crystal orbital bond index values enhancing the overall covalent bonding strength, which makes TM─O octahedron more resistant to damage and enables the lattice to better accommodate the deformation and reaction without irreversible fracture. Furthermore, Mott-Hubbard splitting energy is decreased due to O vacancies, facilitating electron leaps, and enhancing the lattice reactivity and capacity. Such a reversible lattice, more amenable to deformation and forestalling fracturing, markedly improves the reversibility of lattice reactions and mitigates TM migration and the irreversible oxygen redox which enables the high cycling stability and high rate capability. The modified cathode demonstrates a specific capacity of 200 mAh g-1 at 1C, amazingly sustaining the capacity for 200 cycles without capacity degradation. This finding presents a promising avenue for solving the long-term cycling issue of Li-rich cathode.

Constructing Nano-Interlayer Inhibiting Interfacial Degradation toward High-Voltage PEO-Based All-Solid-State Lithium Batteries

The interfacial instability between PEO-based solid electrolyte (SPE) and high-voltage cathode materials inhibits the longevity of high-energy-density all-solid-state polymer lithium metal batteries (ASSPLBs). Herein, for the first time it is demonstrated, that contact loss caused by gas generation from interfacial side reactions between the high-voltage cathode and solid polymer electrolyte (SPE) can also arise in ASSPLBs. To alleviate the interfacial side reactions, a LiNb0.6Ti0.5O3 (LNTO) layer is well coated on LiNi0.83Co0.07Mn0.1O2 (NCM83), denoted as (CNCM83). The LNTO layer with low electronic conductivity reduces the decomposition drive force of SPE. Furthermore, Ti and Nb in the LNTO layer spontaneously migrate inside the NCM83 surface to form a strong Ti/Nb─O bond, stalling oxygen evolution in high-voltage cathodes. The interfacial degradation phenomena, including SPE decomposition, detrimental phase transition and intragranular cracks of NCM83, and void formation between cathode and SPE, are effectively mitigated by the LNTO layer. Therefore, the growth rate of interfacial resistance (RCEI) decreases from 37.6 Ω h-0.5 for bare NCM83 to 2.4 Ω h-0.5 for CNCM83 at 4.2 V. Moreover, 4.2 V PEO-based ASSPLBs achieve impressive cyclability with high capacity retention of 135 mAh g-1 (75%) even after 300 cycles at 0.5 C.

Routes to high-performance layered oxide cathodes for sodium-ion batteries

Sodium-ion batteries (SIBs) are experiencing a large-scale renaissance to supplement or replace expensive lithium-ion batteries (LIBs) and low energy density lead-acid batteries in electrical energy storage systems and other applications. In this case, layered oxide materials have become one of the most popular cathode candidates for SIBs because of their low cost and comparatively facile synthesis method. However, the intrinsic shortcomings of layered oxide cathodes, which severely limit their commercialization process, urgently need to be addressed. In this review, inherent challenges associated with layered oxide cathodes for SIBs, such as their irreversible multiphase transition, poor air stability, and low energy density, are systematically summarized and discussed, together with strategies to overcome these dilemmas through bulk phase modulation, surface/interface modification, functional structure manipulation, and cationic and anionic redox optimization. Emphasis is placed on investigating variations in the chemical composition and structural configuration of layered oxide cathodes and how they affect the electrochemical behavior of the cathodes to illustrate how these issues can be addressed. The summary of failure mechanisms and corresponding modification strategies of layered oxide cathodes presented herein provides a valuable reference for scientific and practical issues related to the development of SIBs.

Oxygen vacancy chemistry in oxide cathodes

Secondary batteries are a core technology for clean energy storage and conversion systems, to reduce environmental pollution and alleviate the energy crisis. Oxide cathodes play a vital role in revolutionizing battery technology due to their high capacity and voltage for oxide-based batteries. However, oxygen vacancies (OVs) are an essential type of defect that exist predominantly in both the bulk and surface regions of transition metal (TM) oxide batteries, and have a crucial impact on battery performance. This paper reviews previous studies from the past few decades that have investigated the intrinsic and anionic redox-mediated OVs in the field of secondary batteries. We focus on discussing the formation and evolution of these OVs from both thermodynamic and kinetic perspectives, as well as their impact on the thermodynamic and kinetic properties of oxide cathodes. Finally, we offer insights into the utilization of OVs to enhance the energy density and lifespan of batteries. We expect that this review will advance our understanding of the role of OVs and subsequently boost the development of high-performance electrode materials for next-generation energy storage devices.

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).

Enhancing Structure Stability by Mg/Cr Co-Doped for High-Voltage Sodium-Ion Batteries

P2-Na2/3Ni1/3Mn2/3O2 cathode materials have garnered significant attention due to their high cationic and anionic redox capacity under high voltage. However, the challenge of structural instability caused by lattice oxygen evolution and P2-O2 phase transition during deep charging persists. A breakthrough is achieved through a simple one-step synthesis of Cr, Mg co-doped P2-NaNMCM, resulting in a bi-functional improvement effect. P2-NaNMCM-0.01 exhibits an impressive capacity retention rate of 82% after 100 cycles at 1 C. In situ X-ray diffraction analysis shows that the "pillar effect" of Mg mitigates the weakening of the electrostatic shielding and effectively suppresses the phase transition of P2-O2 during the charging and discharging process. This successfully averts serious volume expansion linked to the phase transition, as well as enhances the Na+ migration. Simultaneously, in situ Raman spectroscopy and ex situ X-ray photoelectron spectroscopy tests demonstrate that the strong oxygen affinity of Cr forms a robust TM─O bond, effectively restraining lattice oxygen evolution during deep charging. This study pioneers a novel approach to designing and optimizing layered oxide cathode materials for sodium-ion batteries, promising high operating voltage and energy density.

Ni/Mg Dual Concentration-Gradient Surface Modification to Enhance Structural Stability and Electrochemical Performance of Li-Rich Layered Oxides

Li-rich layered oxides (LRLOs), with the advantages of high specific capacity and low cost, are considered as candidates for the next-generation cathode of lithium-ion batteries (LIBs). Unfortunately, sluggish kinetics and interfacial degradation lead to capacity loss and voltage decay of the material during cycling. To address these issues, we propose a Ni/Mg dual concentration-gradient modification strategy for LRLOs. From the center to the surface of the modified materials, the contents of Ni and Mg are gradually increased while the content of Mn is decreased. The high Ni content on the surface increases the proportion of cationic redox, elevating the operating voltage and accelerating reaction kinetics. Moreover, the doped Mg on the surface of the material acting as a stabilizing pillar suppresses the migration of transition metals, stabilizing the layered structure. Therefore, the material with the Ni/Mg dual concentration-gradients delivers a superior electrochemical performance, exhibiting a suppressed voltage decay of 2.8 mV per cycle during 200 cycles (1 C, 2-4.8 V) and an excellent rate capability of 94.84 mAh/g at 7C. This study demonstrates a synergic design to construct high-performance LRLO cathode materials for LIBs.

Suppressing Bulk Strain and Surface O2 Release in Li-Rich Cathodes by Just Tuning the Li Content

Layered oxides represent a prominent class of cathodes employed in lithium-ion batteries. The structural degradation of layered cathodes causes capacity decay during cycling, which is generally induced by anisotropic lattice strain in the bulk of cathode particle and oxygen release at the surface. However, particularly in lithium-rich layered oxides (LLOs) that undergo intense oxygen redox reactions, the challenge of simultaneously addressing bulk and surface issues through a singular modification technique remains arduous. Here a thin (1-nm) and coherent spinel-like phase is constructed on the surface of LLOs particle to suppress bulk strain and surface O2 release by just adjusting the amount of lithium source during synthesis. The spinel-like phase hinders the surface O2 release by accommodating O2 inside the surface layer, while the trapped O2 in the bulk impedes strain evolution by ≈70% at high voltages compared with unmodified LLOs. Consequently, the enhanced structural stability leads to an improved capacity retention of 97.6% and a high Coulombic efficiency of ≈99.5% after 100 cycles at 0.1°C. These findings provide profound mechanistic insights into the functioning of surface structure and offer guidance for synthesizing high-capacity cathodes with superior cyclability.

Rational construction of graphitic carbon nitride composited Li-rich Mn-based oxide cathode materials toward high-performance Li-ion battery

Li-rich Mn-based oxides (LRMOs) are considered as one of the most-promising cathode materials for next generation Li-ion batteries (LIBs) because of their high energy density. Nevertheless, the intrinsic shortcomings, such as the low first coulomb efficiency, severe capacity/voltage fade, and poor rate performance seriously limit its commercial application in the future. In this work, we construct successfully g-C3N4 coating layer to modify Li1.2Mn0.54Ni0.13Co0.13O2 (LMNC) via a facile solution. The g-C3N4 layer can alleviate the side-reaction between electrolyte and LMNC materials, and improve electronic conduction of LMNC. In addition, the g-C3N4 layer can suppress the collapse of structure and improve cyclic stability of LMNC materials. Consequently, g-C3N4 (4 wt%)-coated LMNC sample shows the highest initial coulomb efficiency (78.5%), the highest capacity retention ratio (78.8%) and the slightest voltage decay (0.48 V) after 300 loops. Besides, it also can provide high reversible capacity of about 300 and 93 mAh g-1 at 0.1 and 10C, respectively. This work proposes a novel approach to achieve next-generation high-energy density cathode materials, and g-C3N4 (4 wt%)-coated LMNC shows an enormous potential as the cathode materials for next generation LIBs with excellent performance.

Artificial Post-Cycled Structure Modulation Towards Highly Stable Li-Rich Layered Cathode

High-capacity Li-rich layered oxides (LLOs) suffer from severe structure degradation due to the utilization of hybrid anion- and cation-redox activity. The native post-cycled structure, composed of progressively densified defective spinel layer (DSL) and intrinsic cations mixing, is deemed as the hindrance of the rapid and reversible de/intercalation of Li+ . Herein, the artificial post-cycled structure consisting of artificial DSL and inner cations mixing is in situ constructed, which would act as a shield against the irreversible oxygen emission and undesirable transition metal migration by suppressing anion redox activity and modulating cation mixing. Eventually, the modified DSL-2% Li-rich cathode demonstrates remarkable electrochemical properties with a high discharge capacity of 187 mAh g-1 after 500 cycles at 2 C, and improved voltage stability. Even under harsh operating conditions of 50 °C, DSL-2% can provide a high discharge capacity of 168 mAh g-1 after 250 cycles at 2 C, which is much higher than that of pristine LLO (92 mAh g-1 ). Furthermore, the artificial post-cycled structure provides a novel perspective on the role of native post-cycled structure in sustaining the lattice structure of the lithium-depleted region and also provides an insightful universal design principle for highly stable intercalated materials with anionic redox activity.

A Mechanistic Insight into the Oxygen Redox of Li-Rich Layered Cathodes and their Related Electronic/Atomic Behaviors Upon Cycling

Li-rich cathodes are extensively investigated as their energy density is superior to Li stoichiometric cathode materials. In addition to the transition metal redox, this intriguing electrochemical performance originates from the redox reaction of the anionic sublattice. This new redox process, the so-called anionic redox or, more directly, oxygen redox in the case of oxides, almost doubles the energy density of Li-rich cathodes compared to conventional cathodes. Numerous theoretical and experimental investigations have thoroughly established the current understanding of the oxygen redox of Li-rich cathodes. However, different reports are occasionally contradictory, indicating that current knowledge remains incomplete. Moreover, several practical issues still hinder the real-world application of Li-rich cathodes. As these issues are related to phenomena resulting from the electronic to atomic evolution induced by unstable oxygen redox, a fundamental multiscale understanding is essential for solving the problem. In this review, the current mechanistic understanding of oxygen redox, the origin of the practical problems, and how current studies tackle the issues are summarized.

Enhancing Ionic Transport and Structural Stability of Lithium-Rich Layered Oxide Cathodes via Local Structure Regulation

Lithium-rich manganese-based layered oxides (LRM) have garnered considerable attention as cathode materials due to their superior performance. However, the inherent structural degradation and obstruction of ion transport during cycling lead to capacity and voltage decay, impeding their practical applications. Herein, an Sb-doped LRM material with local spinel phase is reported, which has good compatibility with the layered structure and provides 3D Li+ diffusion channels to accelerate Li+ transport. Additionally, the strong Sb-O bond enhances the stability of the layered structure. Differential electrochemical mass spectrometry indicates that highly electronegative Sb doping effectively suppresses the release of oxygen in the crystal structure and mitigates successive electrolyte decomposition, thereby reducing structural degradation of the material. As a result of this dual-functional design, the 0.5 Sb-doped material with local spinel phases exhibits favorable cycling stability, retaining 81.7% capacity after 300 cycles at 1C, and an average discharge voltage of 1.87 mV per cycle, which is far superior to untreated material with retention values of 28.8% and 3.43 mV, respectively. This study systematically introduces Sb doping and regulates local spinel phases to facilitate ion transport and alleviate structural degradation of LRM, thereby suppressing capacity and voltage fading, and improving the electrochemical performance of batteries.

Anion-Cation Dual-Ion Multisite Doping Stabilizes the Crystal Structure of Li-Rich Layered Oxides

Li-rich layered oxide (LLOs) cathode materials are gaining increasing attention as lithium-ion batteries (LIBs) pursue greater energy density. However, LLOs still suffer from severe capacity fading and voltage decay due to their unstable crystal structure. Hence, the anion-cation dual-ion multisite doping strategy based on Mg and S atoms is used to stabilize the crystal structures of LLOs. Mg substitutes Li atoms in the Li and transition-metal (TM) layers, while S atoms occupy tetrahedral interstitial sites and lattice O sites, all of which contribute to the crystal structure stability of LLOs. Theoretical calculations show that Mg/S dual-ion multisite doping successfully reduces the energy levels of the TM 3d-O 2p and isolated O 2p orbitals, which effectively stabilizes the lattice oxygen. Therefore, multisite-doped samples exhibit promising electrochemical performance. This strategy provides a new approach to enhance the crystal structure stability of LLOs for the design of high-energy-density Li-ion batteries.

Chip-based in situ TEM investigation of structural thermal instability in aged layered cathode

Thermally induced oxygen release is an intrinsic structural instability in layered cathodes, which causes thermal runaway issues and becomes increasingly critical with the continuous improvement in energy density. Furthermore, thermal runaway events always occur in electrochemically aged cathodes, where the coupling of the thermal and electrochemical effect remains elusive. Herein, we report the anomalous segregation of cobalt metal in an aged LiCoO2 cathode, which is attributed to the local exposure of the high-energy (100) surface of LiCoO2 and weak interface Co-O dangling bonds significantly promoting the diffusion of Co. The presence of the LCO-Co interface severely aggregated the oxygen release in the form of dramatic Co growth. A unique particle-to-particle oxygen release pathway was also found, starting from the isolated high reduction areas induced by the cycling heterogeneity. This study provides atomistic insight into the robust coupling between the intrinsic structural instability and electrochemical cycling.

Lithium-Ion Conductor Li2ZrO3-Coated Primary Particles To Optimize the Performance of Li-Rich Mn-Based Cathode Materials

A lithium-rich manganese-based cathode material (LRMC) is currently considered as one of the most promising next-generation materials for lithium-ion batteries, which has received much attention, but the LRMC still faces some key scientific issues to break through, such as poor rate capacity, rapid voltage, capacity decay, and low first coulomb efficiency. In this work, homogeneous Li₂ZrO₃ (LZO) was successfully coated on the surface of Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ (LRO) by molten salt-assisted sintering technology. Li₂ZrO₃ has good chemical and electrochemical stability, which can effectively inhibit the side reaction between electrode materials and electrolytes and reduce the dissolution of transition metal ions. Thus, the as-prepared LRO@LZO composites are expected to improve the cycling performance. It can be found that the discharge specific capacity of LRO is 271 mAh g^-1 at 0.1 C, and the capacity retention rate is still 93.7% after 100 cycles at 1 C. In addition, Li₂ZrO₃ is an excellent lithium-ion conductor, which is prone to increasing the lithium-ion transfer rate and improving the rate capacity of LRO. Therefore, this study provides a new solution to improve the structure stability and electrochemical performance of LRMCs.

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.

Oxygen-deficient TiO2-x interlayer enabling Li-rich Mn-based layered oxide cathodes with enhanced reversible capacity and cyclability

The unique anion redox mechanism of Li-rich Mn-based layered oxide (LMLO) cathodes endows them with a higher specific capacity compared with conventional cathodes. However, the irreversible anion redox reactions can cause structural degradation and sluggish electrochemical kinetics in the cathode, resulting in a poor electrochemical performance in the batteries. Thus, to address these issues, a single-sided conductive oxygen-deficient TiO_2-x interlayer was applied on a commercial Celgard separator as a coating layer towards the LMLO cathode. After coating TiO_2-x, the initial coulombic efficiency (ICE) of the cathode increased from 92.1% to 95.8%, the capacity retention improved from 84.2% to 91.7% after 100 cycles, and the rate performance of the cathode was significantly enhanced from 91.3 mA h g^-1 to 203.9 mA h g^-1 at 5C. Operando differential electrochemical mass spectroscopy (DEMS) showed that the coating layer could restrain the release of oxygen in the battery, especially from the initial formation process. The X-ray photoelectron spectroscopy (XPS) results demonstrated that the favorable oxygen absorption by the TiO_2-x interlayer benefitted the suppression of side reactions and cathode structural evolution and favored the formation of a uniform cathode-electrolyte interphase on the LMLO cathode. This work provides an alternative path to address the issue of oxygen release in LMLO cathodes.

Construction of Inorganic-Rich Cathode Electrolyte Interphase on Co-Free Cathodes

Lithium-rich layered oxides (LRLOs), with the chemical formula of xLi₂MnO₃·(1 - x)LiMO₂, delivering higher specific discharge capacity, are potential cathode materials for lithium-ion batteries. However, the dissolution of transition metal ions and the instability of the cathode-electrolyte interphase (CEI) hinder the commercial application of LRLOs. Herein, a simple and affordable method is developed for the construction of a robust CEI layer by quenching a kind of cobalt-free LRLO, Li_1.2Ni_0.15Fe_0.1Mn_0.55O₂ (denoted as NFM), in 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether solvent. This robust CEI, with well-distributed LiF, TMF_x, and partial organic component CF_x, performs as a physical barrier to prevent NFM from direct contact with the electrolyte, suppresses the oxygen release, and ensures the CEI layer stability. The customized CEI with LiF and TMF_x-rich phase considerably enhances the NFM cycle stability and the initial coulomb efficiency and inhibits voltage fading. This work provides a valuable strategy for designing stable interface chemistry on the cathode of lithium-ion batteries.

Nb2O5 Coating to Improve the Cyclic Stability and Voltage Decay of Li-Rich Cathode Material for Lithium-Ion Battery

The commercialization of lithium manganese oxide (LMO) is seriously hindered by several drawbacks, such as low initial Coulombic efficiency, the degradation of the voltage and capacity during cycling, and the poor rating performance. Developing a simple and scalable synthesis for engineering with surface coating layers is significant and challenging for the commercial prospects of LMO oxides. Herein, we have proposed an efficient engineering strategy with a Nb₂O₅ coating layer. We dissolved niobate (V) ammonium oxalate hydrate and stoichiometric rich LMO (RLM) in deionized water and stirred constantly. Then, the target product was calcined at high temperature. The discharge capacity of the Nb₂O₅ coating RLM is increased from 195 mAh·g^-1 (the RLM without Nb₂O₅) to 215 mAh·g^-1 at a coating volume ratio of 0.010. The average voltage decay was 4.38 mV/cycle, which was far lower than the 7.50 mV/cycle for the pure LMO. The electrochemical kinetics results indicated that the performance was superior with the buffer engineering by the Nb₂O₅ coating of RLM, which provided an excellent lithium-ion conduction channel, and improved diffusion kinetics, capacity fading, and voltage decay. This reveals the strong potential of the Nb₂O₅ coating in the field of cathode materials for lithium-ion batteries.

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).

Regulation of Interfacial Lattice Oxygen Activity by Full-Surface Modification Engineering towards Long Cycling Stability for Co-Free Li-Rich Mn-Based Cathode

The construction of a protective layer for stabilizing anion redox reaction is the key to obtaining long cycling stability for Li-rich Mn-based cathode materials. However, the protection of the exposed surface/interface of the primary particles inside the secondary particles is usually ignored and difficult, let alone the investigation of the impact of the surface engineering of the internal primary particles on the cycling stability. In this work, an efficient method to regulate cycling stability is proposed by simply adjusting the distribution state of the boron nickel complexes coating layer. Theoretical calculation and experimental results display that the full-surface boron nickel complexes coating layer can not only passivate the activity of interface oxygen and improve its stability but also play the role of sharing voltage and protective layer to gradually activate the oxygen redox reaction during cycling. As a result, the elaborately designed cobalt-free Li-rich Mn-based cathode displays the highest discharge-specific capacity retentions of 91.1% after 400 cycles at 1 C and 94.3% even after 800 cycles at 5 C. In particular, the regulation strategy has well universality and is suitable for other high-capacity Li-rich cathode materials.

Tuning the Li+ Solvation Structure by a "Bulky Coordinating" Strategy Enables Nonflammable Electrolyte for Ultrahigh Voltage Lithium Metal Batteries

In battery electrolyte design principles, tuning Li⁺ solvation structure is an effective way to connect electrolyte chemistry with interfacial chemistry. Although recent proposed solvation tuning strategies are able to improve battery cyclability, a comprehensive strategy for electrolyte design remains imperative. Here, we report a solvation tuning strategy by utilizing molecular steric effect to create a "bulky coordinating" structure. Based on this strategy, the designed electrolyte generates an inorganic-rich solid electrolyte interphase (SEI) and cathode-electrolyte interphase (CEI), leading to excellent compatibility with both Li metal anodes and high-voltage cathodes. Under an ultrahigh voltage of 4.6 V, Li/NMC811 full-cells (N/P = 2.0) hold an 84.1% capacity retention over 150 cycles and industrial Li/NMC811 pouch cells realize an energy density of 495 Wh kg^-1. This study provides innovative insights into Li⁺ solvation tuning for electrolyte engineering and offers a promising path toward developing high-energy Li metal batteries.

In Situ Formed Heterostructure Interface and Well-Tuned Electronic Structure Ensuring Long Cycle Stability for 4.9 V High-Voltage Li-Rich Layered Oxide Cathodes

High-voltage lithium-rich manganese-based layered oxides (LMLOs) are considered as the most competitive cathode materials for next-generation high-energy-density lithium-ion batteries (LIBs). However, LMLOs still suffer from irreversible lattice oxygen release, uncontrollable interface side reactions, and surface structural degradation. Herein, we propose an integration strategy combining La/Al codoping and Li_xCoPO₄ nanocoating to improve the electrochemical performance of LMLOs comprehensively. La/Al codoping regulates the electronic structure to enhance the redox activity of anions and cations and inhibit structural degradation. The Li_xCoPO₄ nanocoating formed by in situ reaction with the surface residual lithium can not only promote Li-ion migration but also reduce interfacial side reactions. The induced Layered@Rocksalt@Li_xCoPO₄ heterostructure suppresses lattice volume variation and structural degradation during cycling. Under the synergistic effect of the heterostructure interface and well-tuned electronic structure, the capacity retention rate of comodified LMLO materials reaches 80.06% after 500 cycles (2.0-4.65 V) and 75.1% after 340 cycles at 1C under a high cut-off voltage of 4.9 V.

Role of Oxygen in the Ti3AlC2 MAX Phase in the Oxide Formation and Conductivity of Ti3C2-Based MXene Nanosheets

Ti₃C₂T_x MXene, a two-dimensional transition metal carbide, has attracted substantial interest due to its unique physical properties and a wide range of potential applications. Although the properties of devices using MXene have been substantially enhanced in recent years, it is not fully understood how the oxygen concentration in Ti₃AlC₂ MAX affects oxide formation in Ti₃C₂-based MXene nanosheets and their fundamental properties. To this end, we compared two types of MAX phases: MAX with low oxygen content (LO-MAX) and MAX synthesized by a conventional process. Since the conventional MAX synthesis employs metal (Ti) as a primary material, it is referred to as metal-based MAX (MB-MAX) from here. The oxygen content of the LO-MAX was only 0.56 wt %, which was about 20% compared to that of MAX synthesized using conventional methods. We compared the properties of MXene nanosheets prepared from the LO-MAX with MXene nanosheets obtained from the MB-MAX. Microscopic and chemical analyses revealed smooth and wrinkle-free morphology and small amounts of oxygen in MXene nanosheets prepared from LO-MAX (LO-MXene). The LO-MXene nanosheet film exhibited an exceptionally high conductivity of 10,540 S/cm and an ultralow surface roughness of 1.7 nm, which originated from inhibited surface oxide formation. Moreover, the inhibition of oxide formation strengthened the function of -O or -OH groups on the surface of MXene, thereby facilitating strong hydrogen bonding to the polymer with hydroxyl groups. To clearly reveal these properties, we prepared a pressure sensor by coating these MXene nanosheets on nylon/polyester fibers. The fabricated sensor exhibited a high sensitivity of up to 85.6/kPa and excellent stretch stability and reliability. These results clearly revealed that lowering the oxygen content in MAX can make a decisive contribution to improving the fundamental properties of MXene nanosheets prepared therefrom.

Suppressing the Dynamic Oxygen Evolution of Sodium Layered Cathodes through Synergistic Surface Dielectric Polarization and Bulk Site-Selective Co-Doping

Utilizing anionic redox activity within layered oxide cathode materials represents a transformational avenue for enabling high-energy-density rechargeable batteries. However, the anionic oxygen redox reaction is often accompanied with irreversible dynamic oxygen evolution, leading to unfavorable structural distortion and thus severe voltage decay and rapid capacity fading. Herein, it is proposed and validated that the dynamic oxygen evolution can be effectively suppressed through the synergistic surface CaTiO₃ dielectric coating and bulk site-selective Ca/Ti co-doping for layered Na_2/3 Ni_1/3 Mn_2/3 O₂ . The surface dielectric coating layer not only suppresses the surface oxygen release but more importantly inhibits the bulk oxygen migration by creating a reverse electric field through dielectric polarization. Meanwhile, the site-selective doping of oxygen-affinity Ca into Na layers and Ti into transition metal layers effectively stabilizes the bulk oxygen through modulating the O 2p band center and the oxygen migration barrier. Such a strategy also leads to a reversible structural evolution with a low volume change because of the enhanced structural integrality and improved oxygen rigidity. Because of these synergistic advantages, the designed electrode exhibits greatly suppressed voltage decay and capacity fading upon long-term cycling. This study affords a promising strategy for regulating the dynamic oxygen evolution to achieve high-capacity layered cathode materials.

Integrating a Ferroelectric Interface with a Well-Tuned Electronic Structure in Lithium-Rich Layered Oxide Cathodes for Enhanced Lithium Storage

Li-rich layered oxides (LLOs) are considered promising candidates for new high-energy-density cathode materials for next-generation power batteries. However, their large-scale applications are largely hindered by irreversible Li/O loss, structural degradation, and interfacial side reactions during cycling. Herein, we demonstrate an integration strategy that tunes the electronic structure by La/Al codoping and constructs a ferroelectric interface on the LLOs surface through Bi_0.5Na_0.5TiO₃ (BNT) coating. Experimental characterization reveals that the synergistic effect of the ferroelectric interface and the well-tuned electronic structure can not only promote the diffusion of Li⁺ and hinder the migration of O^n- but also suppress the lattice volume changes and reduce interfacial side reactions at high voltages up to 4.9 V vs Li⁺/Li. As a result, the modified material shows enhanced initial capacities and retention rates of 224.4 mAh g^-1 and 78.57% after 500 cycles at 2.0-4.65 V and 231.7 mAh g^-1 and 85.76% after 200 cycles at 2.0-4.9 V at 1C, respectively.

Mechano-Graded Electrodes Mitigate the Mismatch between Mechanical Reliability and Energy Density for Foldable Lithium-Ion Batteries

Flexible lithium-ion batteries (LIBs) with high energy density are highly desirable for wearable electronics. However, difficult to achieve excellent flexibility and high energy density simultaneously via the current approaches for designing flexible LIBs. To mitigate the mismatch, mechano-graded electrodes with gradient-distributed maximum allowable strain are proposed to endow high-loading-mass slurry-coating electrodes with brilliant intrinsic flexibility without sacrificing energy density. As a proof-of-concept, the up-graded LiNi_1/3 Mn_1/3 Co_1/3 O₂ cathodes (≈15 mg cm^-2 , ≈70 µm) and graphite anodes (≈8 mg cm^-2 , ≈105 µm) can tolerate an extremely low bending radius of 400 and 600 µm, respectively. Finite element analysis (FEA) reveals that, compared with conventionally homogeneous electrodes, the flexibility of the up-graded electrodes is enhanced by specifically strengthening the upper layer and avoiding crack initiation. Benefiting from this, the foldable pouch cell (required bending radius of ≈600 µm) successfully realizes a remarkable figure of merit (FOM, energy density vs bending radius) of 121.3 mWh cm^-3 . Moreover, the up-graded-electrodes-based pouch cells can deliver a stable power supply, even under various deformation modes, such as twisting, folding, and knotting. This work proposes new insights for harmonizing the mechanics and electrochemistry of energy storage devices to achieve high energy density under flexible extremes.

Mitigation of Oxygen Evolution and Phase Transition of Li-Rich Mn-Based Layered Oxide Cathodes by Coating with Oxygen-Deficient Perovskite Compounds

Li-rich Mn-based layered oxide cathodes with a high discharge capacity hold great promise for high energy density lithium-ion batteries. However, application is hampered by voltage and capacity decay and gas evolution during cycling due to interfacial side reactions. Here, we report coating by oxygen-deficient perovskite La0.9Sr0.1CoO3 using the Pechini process. X-ray photoelectron spectroscopy and scanning transmission electron microscopy both exhibit a uniform coating layer with a high oxygen vacancy concentration. The coating effectively mitigates the first cycle irreversible capacity loss and voltage decay while increasing cyclability. Optimized coating improves capacity retention from 55.6% to 84.8% after 400 cycles at 2 C. Operando differential electrochemical mass spectroscopy shows that such a coating can significantly mitigate the release of oxygen and carbon dioxide. Electrochemical impedance spectroscopy and post-mortem analysis indicate that the coating layer forms a stable interface and restricts structure evolution and cation mixing during cycling, conferring these cathode materials with better cycling and voltage stability. The perovskite can be applied to other cathodes with high voltage and capacity to suppress interfacial side reactions toward developing stable high energy density batteries.

Strain-regulated Gibbs free energy enables reversible redox chemistry of chalcogenides for sodium ion batteries

Manipulating the reversible redox chemistry of transition metal dichalcogenides for energy storage often faces great challenges as it is difficult to regulate the discharged products directly. Herein we report that tensile-strained MoSe₂ (TS-MoSe₂) can act as a host to transfer its strain to corresponding discharged product Mo, thus contributing to the regulation of Gibbs free energy change (ΔG) and enabling a reversible sodium storage mechanism. The inherited strain results in lattice distortion of Mo, which adjusts the d-band center upshifted closer to the Fermi level to enhance the adsorbability of Na₂Se, thereby leading to a decreased ΔG of the redox chemistry between Mo/Na₂Se and MoSe₂. Ex situ and in situ experiments revealed that, unlike the unstrained MoSe₂, TS-MoSe₂ shows a highly reversible sodium storage, along with an evidently improved reaction kinetics. This work sheds light on the study on electrochemical energy storage mechanism of other electrode materials.

Manipulating the redox chemistry of transition metal dichalcogenides still faces challenges. Here the authors report that tensile-strained MoSe2 can pass on the strain to its sodiated product Mo, and thus regulate the Gibbs free energy in the charging process to enable the reversible sodium storage.

Tuning Electrochemical Performance of Li-Rich Layered Cathode Materials with a Solid Phase Fusion Strategy

Li-rich layered cathode materials (LRMs) have attracted extensive attention because of their high theoretical specific capacity. However, their practical application is limited by the severe depreciation of capacity and voltage during cycling. Herein, high electrical conductivity MoS₂ is constructed on Li_1.2Ni_0.2Mn_0.6O₂ (LLNM) surface through solid phase fusion technology (SFT). Extraordinarily, the MoS₂ modified layer lessens the interface side reaction and stabilizes the surface structure of LLNM. Meanwhile, the strong electron conductivity of MoS₂ speeds up electron transit at the surface. The results demonstrate that LLNM-M10 exhibits a remarkable electrochemical performance as it retains 183.3 mA h g^-1 at 1 C after 250 cycles. More crucially, the modified electrode exhibits an exceptional low-temperature performance of 120.3 mA h g^-1 at 0.1 C and -10 °C. Therefore, this presented strategy may provide a new method for further application of Li-rich layered cathode materials.

Restraining the escape of lattice oxygen enables superior cyclic performance towards high-voltage Ni-rich cathodes

Layered Ni-rich cathodes, operating at high voltage with superior cyclic performance, are required to develop future high-energy Li-ion batteries. However, the worst lattice oxygen escape at the high-voltage region easily causes structural instability, rapid capacity fading and safety issues upon cycling. Here, we report a dual-track strategy to fully restrain the escape of lattice oxygen from Ni-rich cathodes within 2.7-4.5 V by one-step Ta doping and CeO₂ coating according to their different diffusion energy barriers. The doped Ta can alleviate the charge compensation of oxygen anions as a positive charge centre to reduce the lattice oxygen escape and induce the formation of elongated primary particles, significantly inhibiting microcrack generation and propagation. Additionally, the layer of CeO₂ coating effectively captures the remaining escaped oxygen and then the captured oxygen feeds back into the lattice during subsequent discharge. The resultant Ni-rich cathode enables a capacity of 231.3 mAh g^-1 with a high initial coulombic efficiency of 93.5%. A pouch-type full cell comprising this cathode and a graphite anode exhibits >1000 times life cycles at 1C in the 2.7-4.5 V range, with 90.9% capacity retention.

Tuning the Electronic, Ion Transport, and Stability Properties of Li-rich Manganese-based Oxide Materials with Oxide Perovskite Coatings: A First-Principles Computational Study

Lithium-rich manganese-based oxides (LRMO) are regarded as promising cathode materials for powering electric applications due to their high capacity (250 mAh g^-1) and energy density (∼900 Wh kg^-1). However, poor cycle stability and capacity fading have impeded the commercialization of this family of materials as battery components. Surface modification based on coating has proven successful in mitigating some of these problems, but a microscopic understanding of how such improvements are attained is still lacking, thus impeding systematic and rational design of LRMO-based cathodes. In this work, first-principles density functional theory (DFT) calculations are carried out to fill out such a knowledge gap and to propose a promising LRMO-coating material. It is found that SrTiO₃ (STO), an archetypal and highly stable oxide perovskite, represents an excellent coating material for Li_1.2Ni_0.2Mn_0.6O₂ (LNMO), a prototypical member of the LRMO family. An accomplished atomistic model is constructed to theoretically estimate the structural, electronic, oxygen vacancy formation energy, and lithium-transport properties of the LNMO/STO interface system, thus providing insightful comparisons with the two integrating bulk materials. It is found that (i) electronic transport in the LNMO cathode is enhanced due to partial closure of the LNMO band gap (∼0.4 eV) and (ii) the lithium ions can easily diffuse near the LNMO/STO interface and within STO due to the small size of the involved ion-hopping energy barriers. Furthermore, the formation energy of oxygen vacancies notably increases close to the LNMO/STO interface, thus indicating a reduction in oxygen loss at the cathode surface and a potential inhibition of undesirable structural phase transitions. This theoretical work therefore opens up new routes for the practical improvement of cost-affordable lithium-rich cathode materials based on highly stable oxide perovskite coatings.

Formulating High-Rate and Long-Cycle Heterostructured Layered Oxide Cathodes by Local Chemistry and Orbital Hybridization Modulation for Sodium-Ion Batteries

It is still very urgent and challenging to simultaneously develop high-rate and long-cycle oxide cathodes for sodium-ion batteries (SIBs) because of the sluggish kinetics and complex multiphase evolution during cycling. Here, the concept of accurately manipulating structural evolution and formulating high-performance heterostructured biphasic layered oxide cathodes by local chemistry and orbital hybridization modulation is reported. The P2-structure stoichiometric composition of the cathode material shows a layered P2- and O3-type heterostructure that is explicitly evidenced by various macroscale and atomic-scale techniques. Surprisingly, the heterostructured cathode displays excellent rate performance, remarkable cycling stability (capacity retention of 82.16% after 600 cycles at 2 C), and outstanding compatibility with hard carbon anode because of the integrated advantages of intergrowth structure and local environment regulation. Meanwhile, the formation process from precursors during calcination and the highly reversible dynamic structural evolution during the Na⁺ intercalation/deintercalation process are clearly articulated by a series of in situ characterization techniques. Also, the intrinsic structural properties and corresponding electrochemical behavior are further elucidated by the density of states and electron localization function of density functional theory calculations. Overall, this strategy, which finely tunes the local chemistry and orbitals hybridization for high-performance SIBs, will open up a new field for other materials.

Li-Deficient Materials-Decoration Restrains Oxygen Evolution Achieving Excellent Cycling Stability of Li-Rich Mn-Based Cathode

With the increasing demand for high energy density and rapid charging performance, Li-rich materials have been the up and coming cathodes for next-generation lithium-ion batteries. However, because of oxygen evolution and structural instability, the commercialization of Li-rich materials is extremely retarded by their poor electrochemical performances. In this work, Li-deficient materials Li_0.3NbO₂ and (Nb_0.62Li_0.15)TiO₃ are applied to functionalize the surface of Li_1.2Mn_0.54Ni_0.13Co_0.13O₂, aiming to suppress oxygen evolution and increase structural stability in LIBs. In addition, a fast Li-ion transport channel is beneficial to enhance Li⁺ diffusion kinetics. The results demonstrate that the electrodes decorated with Li_0.3NbO₂ and (Nb_0.62Li_0.15)TiO₃ materials exhibit more stable cycling stability after long-term cycling and outstanding rate capability.

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.

Effects of Ru doping on the structural stability and electrochemical properties of Li2MoO3 cathode materials for Li-ion batteries

The Li₂MoO₃ (LMO) material is one promising cathode material for lithium-ion batteries due to its high specific capacity and absence of oxygen release. However, its surface instability in air and poor conductivity have limited its application. To solve these problems, the Ru element has been successfully introduced into the LMO lattice with the aid of the molten salt method. XRD and TEM characterization showed that the introduction of Ru does not change the crystal structure but expands the crystal plane spacing of the {001} facets, which is further evidenced by density functional theory (DFT) calculations. XPS and EDS tests indicated that the introduction of Ru inhibits the oxidation of Mo species and leads to a more uniform distribution of the material. In addition, DFT calculations revealed that covalent interactions are formed between Mo_4d/Ru_4d and O_2p orbitals, leading to a significant reduction of the band gap. Therefore, Ru-doped samples exhibit good electrochemical performances. The initial discharge capacity of an LMRO-2 sample reaches 299.1 mA h g^-1 at a 1C rate, and the capacity remains at 125.2 mA h g^-1 after 100 cycles. In comparison, the initial discharge capacity of pure phase sample LMO is only 250.5 mA h g^-1 under the same conditions, and the capacity remains only at 76.5 mA h g^-1 after 100 cycles. The present results confirmed that Ru doping is an effective strategy to improve the performance of the LMO cathode material.

Facilitating Reversible Cation Migration and Suppressing O2 Escape for High Performance Li-Rich Oxide Cathodes

High-capacity Li-rich Mn-based oxide cathodes show a great potential in next generation Li-ion batteries but suffer from some critical issues, such as, lattice oxygen escape, irreversible transition metal (TM) cation migration, and voltage decay. Herein, a comprehensive structural modulation in the bulk and surface of Li-rich cathodes is proposed through simultaneously introducing oxygen vacancies and P doping to mitigate these issues, and the improvement mechanism is revealed. First, oxygen vacancies and P doping elongates OO distance, which lowers the energy barrier and enhances the reversible cation migration. Second, reversible cation migration elevates the discharge voltage, inhibits voltage decay and lattice oxygen escape by increasing the Li vacancy-TM antisite at charge, and decreasing the trapped cations at discharge. Third, oxygen vacancies vary the lattice arrangement on the surface from a layered lattice to a spinel phase, which deactivates oxygen redox and restrains oxygen gas (O₂ ) escape. Fourth, P doping enhances the covalency between cations and anions and elevates lattice stability in bulk. The modulated Li-rich cathode exhibits a high-rate capability, a good cycling stability, a restrained voltage decay, and an elevated working voltage. This study presents insights into regulating oxygen redox by facilitating reversible cation migration and suppressing O₂ escape.

Enabling the High-Voltage Operation of Layered Ternary Oxide Cathodes via Thermally Tailored Interphase

Layered ternary oxides LiNi_x Mn_y Co_z O₂ are promising cathode candidates for high-energy lithium-ion batteries (LIBs), but they usually suffer from the severe interfacial parasitic reactions at voltages above 4.3 V versus Li⁺ /Li, which greatly limit their practical capacities. Herein, using LiNi_1/3 Mn_1/3 Co_1/3 O₂ (NMC111) as the model system, a novel high-temperature pre-cycling strategy is proposed to realize its stable cycling in 3.0-4.5 V by constructing a robust cathode/electrolyte interphase (CEI). Specifically, performing the first five cycles of NMC111 at 55 °C helps to yield a uniform CEI layer enriched with fluorine-containing species, Li₂ CO₃ and poly(CO₃ ), which greatly suppresses the detrimental side reactions during extended cycling at 25 °C, endowing the cell with a capacity retention of 92.3% at 1C after 300 cycles, far surpassing 62.0% for the control sample without the thermally tailored CEI. This work highlights the critical role of temperature on manipulating the interfacial properties of cathode materials, opening a new avenue for developing high-voltage cathodes for Li-ion batteries.

Interfacial Reviving of the Degraded LiNi0.8 Co0.1 Mn0.1 O2 Cathode by LiPO3 Repair Strategy

Nickel-rich cathode materials, owing to their high energy density and low cost, are considered to be one of the cathodes with the most potential in next-generation lithium-ion batteries. Unfortunately, this kind of cathode with highly active surface is easy to react with H₂ O and CO₂ when exposed to ambient air, resulting in the formation of lithium impurities and interfacial phase transition as well as deterioration of the electrochemical properties. In this work, the evolution mechanism of the structure and interface of LiNi_0.8 Co_0.1 Mn_0.1 O₂ during air-exposure is systematically investigated. Furthermore, a facile reviving strategy is proposed to restore the degraded LiNi_0.8 Co_0.1 Mn_0.1 O₂ by using LiPO₃ as the repair agent. The lithium impurities on the surface of the degraded sample can transform into the repair/coating layer, and part of the rock salt phase on the subsurface can revive to layered phase after repair heat treatment. As a result, the optimized cathode delivers an initial discharge capacity of 198.3 mAh g^-1 at 0.1C and a capacity retention of 85.5% after 50 cycles. Although slightly lower than the bare sample (201 mAh g^-1 and 88%), they are obviously higher than the exposed samples (166.5 mAh g^-1 and 40.4%). The regenerated electrochemical properties should be attributed to the multifunctional repair layer that can efficiently reduce the surface lithium impurities, prevent the corrosion of electrolyte, and improve the interfacial Li⁺ diffusion kinetics. This work can effectively reduce the waste of the degraded Ni-rich ternary materials and realize the transformation of "waste" into wealth.

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.

Enabling Anionic Redox Stability of P2-Na5/6 Li1/4 Mn3/4 O2 by Mg Substitution

Oxygen-based anionic redox reactions have recently emerged as a lever to increase the capacity of Mn-rich layered oxide cathodes in addition to the charge compensation based on cationic redox reactions for sodium-ion batteries. Unfortunately, the irreversibility of anionic redox often aggravates irreversible structure change and poor cycling performance. Here, a stable anionic redox is achieved through substituting Na ions by Mg ions in P2-type Na_0.83 Li_0.25 Mn_0.75 O₂ . Density functional theory (DFT) calculations reveal that Mg substitution effectively decreases the oxygen chemical potential, causing an improvement in lattice oxygen stability. Moreover, at a highly desodiated state, Mg ions that remain in the lattice and interact with O 2p orbitals can decrease the undercoordinated oxygen and the nonbonded, electron-deficient O 2p states, facilitating the reversibility of oxygen redox. When cycled in the voltage range of 2.6-4.5 V where only anionic redox occurs for charge compensation, Na_0.773 Mg_0.03 Li_0.25 Mn_0.75 O₂ presents a much better reversibility, giving a 4 times better cycle stability than that of Na_0.83 Li_0.25 Mn_0.75 O₂ . Experimentally, Na_0.773 Mg_0.03 Li_0.25 Mn_0.75 O₂ exhibits a ≈1.1% volume expansion during sodium insertion/extraction, suggestive of a "zero-strain" cathode. Overall, the work opens a new avenue for enhancing anionic reversibility of oxygen-related Mn-rich cathodes.

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.

Precise surface control of cathode materials for stable lithium-ion batteries

The increasing demand for high-energy Li-ion batteries (LIBs) continues to push the development of electrode materials, particularly cathode materials, towards their capacity limits. Despite the enormous success, the stability and reliability of LIBs are becoming a serious concern due to the much-aggravated side reactions between electrode materials and organic electrolytes. How to stabilize the cathode/electrolyte interface is therefore an imperative and urgent task drawing considerable attention from both academia and industry. An active treatment on the surface of cathode materials, usually by introducing an inert protection layer, to diminish their side reaction with electrolytes turns out to be a reasonable and effective strategy. This Feature Article firstly outlines our synthesis efforts for the construction of a uniform surface nanocoating on various cathode materials. Different wet chemical routes have been designed to facilitate the control of growth kinetics of targeted coating species so that a precise surface coating could be achieved with nanometer accuracy. Furthermore, we showed the possibility to transform the outer coating layer into a surface doping effect through surface solid reaction at high temperature. A detailed discussion on the structure-performance relationship of these surface-controlled cathode materials is introduced to probe the stabilization mechanism. Finally, perspectives on the development tendency of high-energy cathodes for stable LIBs are provided.

An Active-Oxygen-Scavenging Oriented Cathode-Electrolyte-Interphase for Long-Life Lithium-Rich Cathode Materials

Lithium-rich layered oxides with high energy density are promising cathode materials, thus having attracted a large number of researchers. However, the materials cannot be commercialized for application so far. The crucial problem is the releasing of lattice oxygen at high voltage and resulting consequence, such as decomposition of electrolyte, irreversible phase transition of crystal structure, capacity degradation, and voltage decay. Therefore, capturing active-oxygen and further constructing a cathode-electrolyte-interface (CEI) protective layer via the scavenging effects should be a fundamental step to solve these issues. Herein, β-carotene with antioxidant properties is used as a scavenging molecule to achieve this goal. The control of active oxygen species effectively alleviates the decomposition of carbonate electrolyte under high voltage. The introduction of β-carotene additives can also be adjusted in situ to generate a customized CEI film, which is a double-layer structure with external organic components and internal inorganic components. Moreover, the β-carotene-containing electrolyte system exhibits better thermal stability. Benefited from these, Lithium-rich cathode of β-carotene-containing electrolyte shows outstanding long-life cycle stability, with 93.4% capacity retention rate after 200 cycles at 1 C; this electrochemical stability is superior to other electrolyte additive systems reported at present.

Harmonious Dual-Riveting Interface Induced from Niobium Oxides Coating Toward Superior Stability of Li-Rich Mn-Based Cathode

Ni²⁺/Ni⁴⁺ and O^2-/O_n^2- redoxs endow the Li-rich layered oxide of Li_1.2Mn_0.6Ni_0.2O₂ (LMNO) with a considerable specific capacity and higher voltage. However, during the repeated de-/lithiation, the constant structure degradation initiated from transition metal ion dissolvement and oxygen escape leads to rapid capacity decay, which severely hinders the commercial application of LMNO. Herein, Nb₂O₅ and LiNbO₃ are fabricated on the outside of the LMNO substrate. With the appropriate ion radius, a small amount of Nb⁵⁺ enters the substrate, which could enlarge the crystal spacing and facilitate the fast Li⁺ transfer and, more importantly, change the valence state of Mn and induce the formation a Fd3̅m transition phase on the interface between the coating layer and the interior LMNO. Density functional theory (DFT) calculation has proven that the transition phase could build double-way chemical bonds both inside and outside, and the LiNbO₃ coated LMNO composite (LMNO@LNO) possesses a more stable and harmonious interface due to the higher bonding strength between LiNbO₃ and the transition phase. Therefore, LMNO@LNO demonstrates the most outstanding rate capability and long-tern cycling stability (decay rate of 0.041% per cycle during 1000 cycling at 5 C). This work provides a new inspiration for the coating materials selection and the interface stability research for the LMNO cathodes.