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

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

Structural insights into composition design of Li-rich layered cathode materials for high-energy rechargeable battery

The Li-rich layered oxide is considered as one of the most promising cathode materials for high energy density batteries, due to its ultrahigh capacity derived from oxygen redox. Although incorporating over-stoichiometric Li into layered structure can generate Li₂MnO₃-like domain and enhance the oxygen redox activity thermodynamically, the fast and complete activation of the Li₂MnO₃-like domain remains challenging. Herein, we performed a systematic study on structural characteristics of Li-rich cathode materials to decipher the factors accounting for activation of oxygen redox. We reveal that the activation of Li-rich cathode materials is susceptible to local Co coordination environments. The Co ions can intrude into Li₂MnO₃-like domain and modulate the electronic structure, thereby facilitating the activation of Li-rich layered cathode materials upon first charging, leading to higher reversible capacity. In contrast, Li₂MnO₃-like domain hardly contains any Ni ions which contribute little to the activation process. The optimum composition design of this class of materials is discussed and we demonstrate a small amount of Co/Mn exchange in Li₂MnO₃-like domain can significantly promote the oxygen redox activation. Our findings highlight the vital role of Co ions in the activation of oxygen redox Li-rich layered cathode materials and provide new insights into the pathway toward achieving high-capacity Li-rich layered cathode materials.     

Excess-Li Localization Triggers Chemical Irreversibility in Li- and Mn-Rich Layered Oxides

Li- and Mn-rich layered oxides (LMRs) have emerged as practically feasible cathode materials for high-energy-density Li-ion batteries due to their extra anionic redox behavior and market competitiveness. However, sluggish kinetics regions (<3.5 V vs Li/Li⁺) associated with anionic redox chemistry engender LMRs with chemical irreversibility (first-cycle irreversibility, poor rate properties, voltage fading), which limits their practical use. Herein, the structural origin of this chemical irreversibility is revealed through a comparative study involving Li_1.15Mn_0.51Co_0.17Ni_0.17O₂ with relatively localized and delocalized excess-Li in its lattice system. Operando fine-interval X-ray absorption spectroscopy is used to simultaneously observe the interplay between transition-metal–oxygen (TM-O) redox chemistry and TM migration behavior in real time. Density functional theory calculations show that excess-Li localization in the LMR structure attenuates TM-O covalency and stability, leading to overall chemical irreversibility. Hence, the delocalized excess-Li system is proposed as an alternative design for practically feasible LMR cathodes with restrained TM migration and sustainable O-redox chemistry.     

Stacking Order Induced Anion Redox Regulation for Layer-Structured Na0.75Li0.2Mn0.7Cu0.1O2 Cathode Materials

Stacking order plays a key role in defining the electrochemical behavior and structural stability of layer-structured cathode materials. However, the detailed effects of stacking order on anionic redox in layer-structured cathode materials have not been investigated specifically and are still unrevealed. Herein, two layered cathodes with the same chemical formula but different stacking orders: P2-Na_0.75Li_0.2Mn_0.7Cu_0.1O₂ (P2-LMC) and P3-Na_0.75Li_0.2Mn_0.7Cu_0.1O₂ (P3-LMC) are compared. It is found that P3 stacking order is beneficial to improve the oxygen redox reversibility compared with P2 stacking order. By using synchrotron hard and soft X-ray absorption spectroscopies, three redox couples of Cu²⁺/Cu³⁺, Mn^3.5+/Mn⁴⁺, and O²⁻/O⁻ are revealed to contribute charge compensation in P3 structure simultaneously, and two redox couples of Cu²⁺/Cu³⁺ and O²⁻/O⁻ are more reversible than those in P2-LMC due to the higher electronic densities in Cu 3d and O 2p orbitals in P3-LMC. In situ X-ray diffraction reveals that P3-LMC exhibits higher structural reversibility during charge and discharge than P2-LMC, even at 5C rate. As a result, P3-LMC delivers a high reversible capacity of 190.3 mAh g⁻¹ and capacity retention of 125.7 mAh g⁻¹ over 100 cycles. These findings provide new insight into oxygen-redox-involved layered cathode materials for SIBs.     

Rational design of thermally stable polymorphic layered cathode materials for next generation lithium rechargeable batteries

Classical layered transition metal oxides have remained the preferred cathode materials for commercial lithium-ion batteries. Variation in the transition metal composition and local ordering can greatly affect the structure stability. In classical layered cathodes, high concentrations of electrochemically inert Mn elements usually act as a pillar to stabilize the structure. When excess amount of Li and Mn are present in the layered structure, the capacity of the Li-rich layered oxide (molar ratio of lithium over transition metal is larger than one by design) can exceed that expected from transition metal redox. However, the over lithiation in the classical layered structure results in safety issues, which remains challenging for the commercialization of Li-rich layered oxides. To characterize the safety performance of a series of Li-rich layered cathodes, we utilize differential scanning calorimeter and thermal gravimetric analysis; this is coupled with local structural changes using in situ temperature dependent synchrotron X-ray diffraction and X-ray adsorption spectroscopy. These methods demonstrate that the gradual decrease of the Mn–M (M = Ni, Co, Mn and Li) coordination number directly reduces structural stability and accelerates oxygen release. For safety characterization tests in practice, we evaluate the thermal runaway process through accelerating rate calorimeter in 1.0 Ah pouch cells to confirm this trend. Using the insights obtained in this work, we design a polymorphic composition to improve the thermal stability of Li-rich layered cathode material, which outperforms Ni-rich layered oxides in terms of both electrochemical and safety performances.     

Countering the Segregation of Transition‐Metal Ions in LiMn1/3Co1/3Ni1/3O2 Cathode for Ultralong Life and High‐Energy Li‐Ion Batteries

High‐voltage layered lithium transition‐metal oxides are very promising cathodes for high‐energy Li‐ion batteries. However, these materials often suffer from a fast degradation of cycling stability due to structural evolutions. It seriously impedes the large‐scale application of layered lithium transition‐metal oxides. In this work, an ultralong life LiMn_1/3Co_1/3Ni_1/3O₂ microspherical cathode is prepared by constructing an Mn‐rich surface. Its capacity retention ratio at 700 mA g^?1 is as large as 92.9% after 600 cycles. The energy dispersive X‐ray maps of electrodes after numerous cycles demonstrate that the ultralong life of the as‐prepared cathode is attributed to the mitigation of TM‐ions segregation. Additionally, it is discovered that layered lithium transition‐metal oxide cathodes with an Mn‐rich surface can mitigate the segregation of TM ions and the corrosion of active materials. This study provides a new strategy to counter the segregation of TM ions in layered lithium transition‐metal oxides and will help to the design and development of high‐energy cathodes with ultralong life.     

钠离子电池正极材料P2-Nax[Mg0.33Mn0.67]O2的电化学活性研究

钠离子电池具有成本低廉、原料分布广泛等优点, 是锂离子电池正极材料的最佳替代材料。在具有层状结构的P2相NaMnO₂正极材料中, 对过渡金属层进行二元固溶可有效提升电极材料的电化学性能。本研究利用库仑模型构建了Mg离子固溶的Na_x[Mg_0.33Mn_0.67]O₂结构模型。通过第一性原理计算发现, 在钠离子含量小于0.67时, Na_x[Mg_0.33Mn_0.67]O₂的放电电压达到3.0 V。电子态密度和电荷布居分析共同表明, Mg的固溶激活了P2相Na_x[Mg_0.33Mn_0.67]O₂中晶格氧的电化学活性, 使体系的电化学反应机制从阴阳离子协同电化学反应转变为可逆阴离子电化学反应。这一机理为钠离子电池电极材料的设计提供了一种全新方法, 也为其它离子电池的优化和探索提供了全新的思路。     

An Ordered Ni6‐Ring Superstructure Enables a Highly Stable Sodium Oxide Cathode

Sodium‐based layered oxides are among the leading cathode candidates for sodium‐ion batteries, toward potential grid energy storage, having large specific capacity, good ionic conductivity, and feasible synthesis. Despite their excellent prospects, the performance of layered intercalation materials is affected by both a phase transition induced by the gliding of the transition metal slabs and air‐exposure degradation within the Na layers. Here, this problem is significantly mitigated by selecting two ions with very different M? O bond energies to construct a highly ordered Ni₆‐ring superstructure within the transition metal layers in a model compound (NaNi_2/3Sb_1/3O₂). By virtue of substitution of 1/3 nickel with antimony in NaNiO₂, the existence of these ordered Ni₆‐rings with super‐exchange interaction to form a symmetric atomic configuration and degenerate electronic orbital in layered oxides can not only largely enhance their air stability and thermal stability, but also increase the redox potential and simplify the phase‐transition process during battery cycling. The findings reveal that the ordered Ni₆‐ring superstructure is beneficial for constructing highly stable layered cathodes and calls for new paradigms for better design of layered materials.     

Role of Redox‐Inactive Transition‐Metals in the Behavior of Cation‐Disordered Rocksalt Cathodes

Owing to the capacity boost from oxygen redox activities, Li‐rich cation‐disordered rocksalts (LRCDRS) represent a new class of promising high‐energy Li‐ion battery cathode materials. Redox‐inactive transition‐metal (TM) cations, typically d⁰ TM, are essential in the formation of rocksalt phases, however, their role in electrochemical performance and cathode stability is largely unknown. In the present study, the effect of two d⁰ TM (Nb⁵⁺ and Ti⁴⁺) is systematically compared on the redox chemistry of Mn‐based model LRCDRS cathodes, namely Li_1.3Nb_0.3Mn_0.4O₂ (LNMO), Li_1.25Nb_0.15Ti_0.2Mn_0.4O₂ (LNTMO), and Li_1.2Ti_0.4Mn_0.4O₂ (LTMO). Although electrochemically inactive, d⁰ TM serves as a modulator for oxygen redox, with Nb⁵⁺ significantly enhancing initial charge storage contribution from oxygen redox. Further studies using differential electrochemical mass spectroscopy and resonant inelastic X‐ray scattering reveal that Ti⁴⁺ is better in stabilizing the oxidized oxygen anions (O^n?, 0 < n < 2), leading to a more reversible O redox process with less oxygen gas release. As a result, much improved chemical, structural and cycling stabilities are achieved on LTMO. Detailed evaluation on the effect of d⁰ TM on degradation mechanism further suggests that proper design of redox‐inactive TM cations provides an important avenue to balanced capacity and stability in this newer class of cathode materials.     

Critical Role of Cations in Lithium Sites on Extended Electrochemical Reversibility of Co‐Rich Layered Oxide

Only a very limited amount of the high theoretical energy density of LiCoO₂ as a cathode material has been realized, due to its irreversible deterioration when more than 0.6 mol of lithium ions are extracted. In this study, new insights into the origin of such low electrochemical reversibility, namely the structural collapse caused by electrostatic repulsion between oxygen ions during the charge process are suggested. By incorporating the partial cation migration of LiNiO₂, which produces a screen effect of cations in the 3b‐Li site, the phase distortion of LiCoO₂ is successfully delayed which in turn expands its electrochemical reversibility. This study elucidates the relationship between the structural reversibility and electrochemical behavior of layered cathode materials and enables new design of Co‐rich layered materials for cathodes with high energy density.     

A High‐Capacity O2‐Type Li‐Rich Cathode Material with a Single‐Layer Li2MnO3 Superstructure

A high capacity cathode is the key to the realization of high‐energy‐density lithium‐ion batteries. The anionic oxygen redox induced by activation of the Li₂MnO₃ domain has previously afforded an O3‐type layered Li‐rich material used as the cathode for lithium‐ion batteries with a notably high capacity of 250–300 mAh g^?1. However, its practical application in lithium‐ion batteries has been limited due to electrodes made from this material suffering severe voltage fading and capacity decay during cycling. Here, it is shown that an O2‐type Li‐rich material with a single‐layer Li₂MnO₃ superstructure can deliver an extraordinary reversible capacity of 400 mAh g^?1 (energy density: ≈1360 Wh kg^?1). The activation of a single‐layer Li₂MnO₃ enables stable anionic oxygen redox reactions and leads to a highly reversible charge–discharge cycle. Understanding the high performance will further the development of high‐capacity cathode materials that utilize anionic oxygen redox processes.     

A New Type of Li‐Rich Rock‐Salt Oxide Li2Ni1/3Ru2/3O3 with Reversible Anionic Redox Chemistry

Li‐rich oxide cathodes are of prime importance for the development of high‐energy lithium‐ion batteries (LIBs). Li‐rich layered oxides, however, always undergo irreversible structural evolution, leading to inevitable capacity and voltage decay during cycling. Meanwhile, Li‐rich cation‐disordered rock‐salt oxides usually exhibit sluggish kinetics and inferior cycling stability, despite their firm structure and stable voltage output. Herein, a new Li‐rich rock‐salt oxide Li₂Ni_1/3Ru_2/3O₃ with Fd‐3m space group, where partial cation‐ordering arrangement exists in cationic sites, is reported. Results demonstrate that a cathode fabricated from Li₂Ni_1/3Ru_2/3O₃ delivers a large capacity, outstanding rate capability as well as good cycling performance with negligible voltage decay, in contrast to the common cations disordered oxides with space group Fm‐3m. First principle calculations also indicate that rock‐salt oxide with space group Fd‐3m possesses oxygen activity potential at the state of delithiation, and good kinetics with more 0‐TM (TM = transition metals) percolation networks. In situ Raman results confirm the reversible anionic redox chemistry, confirming O^2?/O^? evolution during cycles in Li‐rich rock‐salt cathode for the first time. These findings open up the opportunity to design high‐performance oxide cathodes and promote the development of high‐energy LIBs.     

Comprehensive understanding of Li/Ni intermixing in layered transition metal oxides

With the development of high energy density battery technology, layered transition metal oxide cathode materials, particularly Ni-rich layered cathodes of Li-ion batteries are urgently required due to its high energy density. However, Li/Ni intermixing inevitably occurs in Ni-rich cathode materials and affects the materials in terms of structure and performance. This review comprehensively summarizes the causes of Li/Ni intermixing and analyzes its inevitability due to ionic radius, Ni migration, magnetic interactions, and thermal stability. In addition, the effect of Li/Ni intermixing on materials is summarized, particularly its benefits, which have not yet been comprehensively examined. Finally, the methods for regulating Li/Ni intermixing that corresponds to its causes are presented in detail. This review can help researchers fully understand Li/Ni intermixing and propose solutions for the current shortcomings of Li/Ni intermixing research and directions for future studies.     

Cathodes Coating Layer with Li-Ion Diffusion Selectivity Employing Interactive Network of Metal-Organic Polyhedras for Li-Ion Batteries (Small 5/2023)

Surface modification of cathodes using Ni-rich coating layers prevents bulk and surface degradation for the stable operation of Li–ion batteries at high voltages. However, insulating and dense inorganic coating layers often impede charge transfer and ion diffusion kinetics. In this study, the fabrication of dual functional coating materials using metal–organic polyhedra (MOP) with 3D networks within microporous units of Li–ion batteries for surface stabilization and facile ion diffusion is proposed. Zr-based MOP is modified by introducing acyl groups as a chemical linkage (MOPAC), and MOPAC layers are homogenously coated by simple spray coating on the cathode. The coating allow the smooth transport of electrons and ions. MOPAC effectively suppress side reactions between the cathode and electrolyte and protect active materials against aggressive fluoride ions by forming a Li–ion selective passivation film. The MOPAC-coated Ni-rich layered cathode exhibited better cycle retention and enhanced kinetic properties than pristine and MOP-coated cathodes. Reduction of undesirable gas evolution on the cathode by MOPAC is also verified. Microporous MOPAC coating can simultaneously stabilize both the bulk and surface of the Ni-rich layered cathode and maintain good electrochemical reaction kinetics for high-performance Li–ion batteries.     

Thermal stress-induced charge and structure heterogeneity in emerging cathode materials

Nickel-rich layered oxide cathode materials are attractive near-term candidates for boosting the energy density of next generation lithium-ion batteries. The practical implementation of these materials is, however, hindered by unsatisfactory capacity retention, poor thermal stability, and oxygen release as a consequence of structural decomposition, which may have serious safety consequences. The undesired side reactions are often exothermic, causing complicated electro-chemo-mechanical interplay at elevated temperatures. In this work, we explore the effects of thermal exposure on chemically delithiated LiNi_0.8Mn_0.1Co_0.1O₂ (NMC-811) at a practical state-of-charge (50% Li content) and an over-charged state (25% Li content). A systematic study using a suite of advanced synchrotron radiation characterization tools reveals the dynamics of thermal behavior of the charged NMC-811, which involves sophisticated structural and chemical evolution; e.g. lattice phase transformation, transition metal (TM) cation migration and valence change, and lithium redistribution. These intertwined processes exhibit a complex 3D spatial heterogeneity and, collectively, form a valence state gradient throughout the particles. Our study sheds light on the response of NMC-811 to elevated temperature and highlights the importance of the cathode’s thermal robustness for battery performance and safety.     

Direct Visualization of the Reversible O2−/O− Redox Process in Li‐Rich Cathode Materials

Conventional cathodes of Li‐ion batteries mainly operate through an insertion–extraction process involving transition metal redox. These cathodes will not be able to meet the increasing requirements until lithium‐rich layered oxides emerge with beyond‐capacity performance. Nevertheless, in‐depth understanding of the evolution of crystal and excess capacity delivered by Li‐rich layered oxides is insufficient. Herein, various in situ technologies such as X‐ray diffraction and Raman spectroscopy are employed for a typical material Li_1.2Ni_0.2Mn_0.6O₂, directly visualizing O^?? O^? (peroxo oxygen dimers) bonding mostly along the c‐axis and demonstrating the reversible O^2?/O^? redox process. Additionally, the formation of the peroxo O? O bond is calculated via density functional theory, and the corresponding O? O bond length of ≈1.3 Å matches well with the in situ Raman results. These findings enrich the oxygen chemistry in layered oxides and open opportunities to design high‐performance positive electrodes for lithium‐ion batteries.     

Recent applications of electron magnetic resonance (EMR) techniques in heterogeneous catalysis

EMR techniques have been extensively used in the past year to explore problems relevant to heterogeneous catalysis, including surface defects and radicals, redox processes with supported transition metal ions and in situ studies at elevated temperatures. The combination of these techniques with computational methods, can now be used to provide detailed structural information of active sites even in polycrystalline materials. Developments in high field EMR and the utilisation of pulsed EMR methods are providing marked improvements in sensitivity and spatial resolution of paramagnetic surface states.     

Soft-combustion (wet-chemical) synthesis of a new 4-V class cathode-active material,LiVMoO6, for Li-ion batteries

A new, 4-V class, lithiated transition metal oxide cathode, LiVMoO₆, has been synthesized by a novel soft-combustion (wet chemical) low temperature (LT) method that presents advantages compared to the classical ceramic method, namely, in terms of phase purity, surface texture and size, preparation time, costs and electrochemical performances of the resulting products. The structural properties of the newly synthesized product have been examined by means of X-ray diffraction studies (XRD). The thermal reactions which occur during the soft-combustion of the precursor mixture have been examined by DTA/TG techniques. It has been found that the layered LiVMoO₆ can only be obtained upon calcining the precursor at 540°C, beyond which the compound will thermally be reduced to LiVMoO₅ which exhibits inferior structural characteristics for the intercalation/deintercalation reactions. The product (LiVMoO₆) thus prepared exhibits submicrometre spherical grains (<1 μm) whose specific surface area is 5.01 m²/g. The intercalation/deintercalation (redox) kinetics of the above product has been studied and its suitability as cathode material in actual electrochemical cells is discussed in the light of electrochemical properties.     

A Ternary Hybrid-Cation Room-Temperature Liquid Metal Battery and Interfacial Selection Mechanism Study

The dendrite-free sodium–potassium (Na–K) liquid alloy composed of two alkali metals is one of the ideal alternatives for Li metal as an anode material while maintaining large capacity, low potential, and high abundance. However, Na- or K-ion batteries have limited cathode materials that can deliver stably large capacity. Combining advantages of both, a hybrid-cation liquid metal battery is designed for a Li-ion-insertion-based cathode to deliver stable high capacity using a Na–K liquid anode to avoid dendrites. The mechanical property of the Na–K alloy is confirmed by simulation and experimental characterization, which leads to stable cycling performance. The charge carrier selection principle in this ternary hybrid-cation system is investigated, showing consistency with the proposed interfacial layer formation and ion distribution mechanism for the electrochemical process as well as the good stability. With Li ions contributing stable cycling as the cathode charge carrier, the K ion working as charge carrier on the anode, and Na as the medium to liquefy K metal, such a ternary hybrid battery system not only inherits the rich battery chemistry of Li-insertion cathodes but also broadens the understanding of alkali metal alloys and hybrid-ion battery chemistry.     

高镍系三元层状氧化物正极材料容量衰减机理的研究进展

高镍系三元层状氧化物正极材料因其高比容量、低廉的价格以及较好的环境友好性而受到广泛关注, 但是其固有的一些缺点, 如循环过程中结构稳定性差、高温稳定性差以及储存性能差等极大地限制了其在各领域的广泛应用。本文着重总结并讨论近年来对高镍系三元层状氧化物正极材料循环过程容量衰减机理的研究进展, 并对高镍系三元层状氧化物正极材料的进一步改性作了简要的展望。