Focusing on the core contradiction between the high capacity characteristics of Li-rich manganese-based cathode materials（LRMOs）and their practical application， this review systematically elaborates the multiscale chemical processes involved in their entire chain from synthesis to implementation. Leveraging its unique anionic redox mechanism， LRMOs can deliver ultra-high discharge capacity， making them an ideal cathode system for next-generation high-energy-density lithium-ion batteries. This article highlights the complexation-displacement reactions and nucleation-growth mechanisms during precursor co-precipitation， the lithiation kinetics and phase-transformation behavior during sintering， the functional mechanisms of atomic layer deposition surface engineering， as well as the nature and regulation strategies of lattice-oxygen redox. Furthermore， it outlines the interfacial chemical challenges and corresponding strategies faced by LRMOs in solid-state battery systems. Finally， prospects are provided for future research directions in precise synthesis， oxygen-behavior regulation， and solid-state interface engineering， aiming to promote the practical application and industrialization of LRMOs. By clarifying the intrinsic correlations among material synthesis， structural evolution， and electrochemical performance， this review aims to offer theoretical foundations and technical insights for the rational design and performance optimization of LRMOs.
Contents
1 Introduction
2 Multiscale chemical control in precursor synthesis
2.1 Fundamental principles of complexation-displacement reactions
2.2 Thermodynamic and kinetic mechanisms of precursor nucleation and growth
3 Multiscale structural evolution driven by sintering kinetics
3.1 Kinetic controlling factors in precursor lithiation processes
3.2 Phase transformation processes and microstructural evolution
4 Surface modification strategies via atomic layer deposition
4.1 Principles and technical advantages of ALD technology
4.2 ALD deposition morphology and structure-property relationship
4.3 Frontier challenges and development pathways in ALD interface engineering
5 Electrochemical principles of LRMOs
5.1 Models of oxygen redox
5.2 Strategies for regulating oxygen activity
5.3 Generation， construction， and utilization of oxygen vacancies
The rapid development of the electric vehicle industry has created an increasingly urgent demand for high-energy-density power batteries[1-3]. Compared to new energy battery systems such as solar and hydrogen energy, lithium-ion batteries offer wider application scenarios and higher energy conversion efficiency, making them the optimal choice for power batteries[4-7]. The energy density of lithium-ion batteries primarily depends on the cathode and anode materials[8]. Currently, the specific capacity of commercialized cathode materials is far lower than that of graphite anodes (373 mAh/g), which is the key bottleneck limiting battery energy density[9-11]. Lithium-rich manganese-based cathode materials (LRMOs) possess ultra-high discharge specific capacity (>300 mAh/g) and can theoretically achieve an energy density of 700 Wh/kg. With advantages such as low cost and good thermal stability, they are considered the ideal choice for next-generation power battery cathodes[12-15].
The general molecular formula for lithium-rich manganese-based cathode materials is usually written asxLi2MnO3·(1-x)LiTMO2(TM = Ni, Co, Mn, etc.). Its crystal structure is composed of a rhombohedral LiTMO2(space group R-3m) and a monoclinic Li2MnO3(space group C2/m) composite at the nanoscale. The LiTMO2phase is similar to conventional layered cathode materials, consisting of alternating transition metal layers and lithium layers. It is primarily responsible for providing charge compensation through the redox reactions of transition metal ions in the low voltage range (<4.45 V) during lithium ion deintercalation, thereby contributing capacity. In contrast, the Li2MnO3phase exhibits a unique LiMn6superlattice ordered structure, where additional lithium ions are embedded in the transition metal layers, forming a 180° Li—O—Li linear configuration. This linear configuration contains non-bonding active oxygen electrons, which can provide charge compensation for lithium ion deintercalation above 4.45 V, thereby contributing extra capacity[12,16-17]. The two phases coexist and are uniformly distributed at the nanoscale, forming coherent or semi-coherent interfaces. This structural feature not only determines the initial electrochemical behavior of the material but also profoundly affects lithium ion diffusion pathways, lattice oxygen activity, and structural stability. Therefore, deeply understanding the two-phase structural characteristics of LRMOs is of great significance for revealing the source of their high capacity, elucidating performance degradation mechanisms, and guiding structural optimization of the materials.
In recent years, domestic and international scholars have published multiple high-quality reviews on LRMOs. Chen et al.[18]systematically summarized the synthesis methods of LRMOs; Chen et al.[19]comprehensively reviewed the electrochemical performance and modification strategies of LRMOs; Jin et al.[20]focused on interfacial degradation mechanisms and optimization methods; Zhuo et al.[21]systematically elaborated on the high-capacity mechanisms of LRMOs from the perspective of anionic redox; Zheng et al.[22]explored the challenges and solutions facing LRMOs in commercial applications from a practical standpoint. The aforementioned reviews have provided in-depth elaborations from dimensions such as synthesis, modification, interfacial reactions, oxygen ion redox mechanisms, and commercialization challenges, laying an important foundation for the development of this field (Figure 1).
Unlike the single-dimensional discussions in the aforementioned works, this paper takes "multi-scale chemical processes" as the main thread, connecting the entire chain from precursor synthesis to solid-state application. It systematically integrates the complexation regulation and nucleation mechanisms in precursor co-precipitation, the lithiation kinetics and phase transition behaviors during sintering, the interfacial regulation principles of atomic layer deposition surface modification, the electrochemical essence of lattice oxygen redox, and the interfacial chemical challenges in solid-state battery systems. The aim is to reveal the intrinsic correlations among synthesis conditions, microstructure, and electrochemical performance, emphasizing the coupling and transmission relationships between key chemical processes, thereby providing a more systematic and comprehensive theoretical framework for the rational design and full-chain optimization of LRMOs.
2 Multi-scale chemical regulation in precursor synthesis
The electrochemical performance of LRMOs mainly depends on the physicochemical properties of the precursors[23-24]. Among various preparation methods, the co-precipitation method is widely adopted due to its advantages in component uniformity, particle size controllability, and scalable production[25-26]. The co-precipitation method is typically conducted in a reactor equipped with a precision control system, where mixed salt solutions, complexing agents, and precipitants are delivered to the reactor via independent feeding systems. By precisely controlling parameters such as reaction temperature, pH value, reactant concentration, and feeding rate, the morphology, size, and density of precursor particles can be effectively regulated[27-28]. Combined with the multi-stage stirring system and flow-guiding structure inside the reactor, elemental segregation within the precursors is effectively suppressed, the overall sphericity of the particles is improved, and the uniformity of the lithiation reaction during the subsequent sintering process is also ensured.
2.1 Basic principles of complexation displacement reactions
Figure 2aThe core principle of the co-precipitation method is using a complexing agent to transform an uncontrollable precipitation process into a "controlled release" controllable process[29]. Therefore, it is necessary to deeply understand the basic principles of thermodynamics and kinetics governing precipitation regulated by complexation displacement reactions[30-32]. The solubility product (Ksp) is a key thermodynamic parameter determining precipitation behavior, and its magnitude depends solely on the nature of the substance and temperature. Taking the carbonate system as an example, at 298 K, the precipitation equilibria of three ions and their correspondingKspare shown in Equations (1) to (3).
Ni2+ + CO32- ⇌ NiCO3（s）（Ksp1=6.60×10-9）
Co2+ + CO32- ⇌ CoCO3（s）（Ksp2=1.40×10-13）
Mn2+ + CO32- ⇌ MnCO3（s）（Ksp3=2.24×10-11）
it can be seen that the threeKspvalues differ significantly. Without intervention, this will inevitably lead to uneven distribution of precursor elements, inconsistent particle sizes, and irregular morphological growth, subsequently triggering a series of performance defects. Therefore, a complexing agent needs to be introduced to achieve controlled precipitation[33]. Taking ammonia water as an example (Figure 2b), NH3can form soluble complex ions TM(NH3)n2+with transition metal ions (Equation (4)). Subsequently, the complex ions undergo a displacement precipitation reaction with CO32-, slowly releasing NH3while achieving uniform precipitation of the three metal ions[34](Equation (5)). When the pH is controlled around 8, the equivalent solubility products of Ni2+, Co2+, and Mn2+Kʺspare regulated to the same order of magnitude (1.3×10-7.94, 1.1×10-7.81, 1.0×10-8), enabling controlled implementation of uniform co-precipitation.
furthermore, complexing agent molecules can specifically adsorb onto specific crystal planes of newly formed precipitates, altering the relative growth rates of these planes, thereby guiding particle growth toward the expected morphology[35]. Moreover, this adsorption effect generates steric hindrance between particles, inhibiting hard agglomeration of primary particles and facilitating the formation of loose, porous secondary spherical structures. This not only promotes uniform mixing of the precursor with the lithium source but also provides favorable mesoporous channels for lithium-ion transport[36].
In summary, through ingenious chemical equilibrium design, the complexation displacement reaction reshapes the co-precipitation process from both thermodynamic and kinetic perspectives, which is the cornerstone for achieving the controlled preparation of LRMO precursors.
2.2 Thermodynamic and kinetic mechanisms of precursor nucleation and growth
During the co-precipitation process, nano-sized (10~100 nm) primaryparticlesare first precipitated, subsequently forming micron-sized (1~10 μm) secondary particles through agglomeration[37-38]. Secondary particles constitute the main morphology of mature precursors; therefore, deeply understanding the nucleation and growth mechanisms from the perspectives of chemical reaction thermodynamics and kinetics is key to achieving precise structural control of the precursors.
2.2.1 Nucleation mechanism of primary particles
The nucleation modes of primary particles can be classified into homogeneous nucleation and heterogeneous nucleation. In the early stage of the reaction, high concentrations of metal ions create a highly supersaturated environment, thereby generating a large number of initial nuclei. Whether these nuclei can stably exist and continue to grow depends on the competition between the reduction in volumetric free energy and the increase in surface free energy during the nucleation process. The total change in system free energy (ΔGho) can be expressed by Equation (6).
where:σis the specific surface energy of the solid-liquid interface;ΔGVis the change in free energy per unit volume. Whenr<r*, the nucleus is unstable and tends to dissolve; whenr>r*, the nucleus can grow stably[39](Figure 2c). The critical radiusr*is not a constant value; its magnitude is influenced by reaction conditions such as supersaturation and temperature[35,40].
As the reaction proceeds, the surface of nuclei in the solution becomes active sites for ion adsorption and arrangement; therefore, subsequent nucleation behavior preferentially occurs on the surface of existing nuclei, i.e., heterogeneous nucleation. The total free energy (ΔGho) change can be expressed as Equation (7).
thus, the energy barrier required for heterogeneous nucleation is lower than that for homogeneous nucleation, causing nuclei to preferentially grow on existing particle surfaces. By selecting different substrates or adding morphology regulators, nucleus growth can be guided along specific directions, thereby achieving precise control over the material's microstructure.[37,41-42].
2.2.2 Growth and densification of secondary particles
The growth of secondary particles typically follows the Diffusion-Limited Aggregation (DLA) or Reaction-Limited Aggregation (RLA) models. In the DLA model, the Brownian motion rate of particles is much higher than their surface reaction rate; upon collision, they rapidly and irreversibly adhere, forming open, branched, loose fractal structures.[43]. In contrast, in the RLA model, an energy barrier exists between particles, meaning not every collision results in successful adhesion; particles have a probability of rearranging, ultimately forming denser aggregates with higher sphericity.[44]. By regulating solution ionic strength, pH value, and surfactants, a transition from the DLA to the RLA mechanism can be achieved, thereby controlling the final morphology of the secondary particles.
Structural densification is a key step accompanying the agglomeration process, with its core mechanism being Ostwald ripening. Small-sized particles or sharp edges with large radii of curvature continuously dissolve, and the dissolved material re-crystallizes and deposits on particle surfaces or contact points with small radii of curvature and low energy. Within secondary particles, the radius of curvature at contact points is negative, resulting in an equilibrium solubility lower than that of convex surfaces; thus, deposition occurs preferentially at these locations. Connections between particles are strengthened, porosity is reduced, and ultimately, spherical secondary particles with smooth surfaces and dense structures are formed.[45-46].
In summary, the synergistic growth of primary and secondary particles jointly determines the physicochemical properties of the precursor. By optimizing the synthesis process, the size and structure of the particles can be regulated, thereby laying the foundation for the subsequent preparation of high-performance LRMOs. However, the precipitation thermodynamics and kinetics of different elements, as well as the influence of novel complexing agents and surfactants, remain directions requiring further exploration in the future.
3 Multi-scale structural evolution driven by sintering kinetics
LRMOs are nanocomposites composed of two phases; therefore, their sintering process involves multiple steps including lithium source decomposition, transition metal oxidation, lithium ion diffusion, and phase transformation. Essentially, it is a non-equilibrium kinetic process where the system evolves toward a thermodynamically stable state.[47]. Although in-situ characterization techniques have gradually revealed the dynamic mechanisms of the sintering process, further in-depth research is still required regarding aspects such as competitive and synergistic mechanisms of multiple reactions.[48-49]. Therefore, deeply analyzing the chemical mechanisms during the sintering process is key to achieving precise control over the phase composition and microstructure of LRMOs.
3.1 Kinetic control factors in the precursor lithiation process
The core of the lithiation reaction is Li+intercalating into the transition metal oxide lattice, ultimately forming a solid-state reaction process with a stable layered crystal structure[50]. In the initial stage, the lithium source and precursor undergo decomposition, and their decomposition pathways are closely related to the types of lithium source and precursor, influencing subsequent thermodynamics and kinetics (Figure 3). Then Li+diffuses according to the Arrhenius law[51]. Finally, through complex phase transitions and structural reorganization, Li+reacts chemically with the oxide under high-temperature conditions (>700 °C) to form a new phase[52-54].
3.2 Phase transformation process and microstructure evolution
3.2.1 Precursor decomposition chemistry and the formation of intermediate phases
The decomposition pathways of precursors with different chemical properties determine the thermodynamic and kinetic starting points for subsequent phase transitions. Among them, hydroxide precursors decompose at lower temperatures (300 °C), generating transition metal oxides rich in oxygen ion vacancies. These vacancies provide channels for Li+rapid intercalation, and their phase transition pathway generally follows an evolution sequence from a rock-salt structure to a lithium-deficient layered structure, and finally to a two-phase composite structure[55]. In contrast, carbonate precursors require higher temperatures (400–600 °C) to decompose, forming a locally reducing microenvironment; their phase transition often proceeds via a spinel intermediate phase, followed by transformation into the target phase through Li+intercalation and structural rearrangement[56]. Studies have shown that high-Mn precursors are more prone to forming spinel intermediate phases, whereas high-Ni precursors tend to generate rock-salt intermediate phases, which directly leads to different distribution patterns and crystallite sizes of the two phases in the final product[57].
3.2.2 Effect of sintering process on phase transition thermodynamics/kinetics
Sintering temperature is the core parameter for regulating phase composition and microstructure. From a thermodynamic perspective, LiTMO2and Li2MnO3exhibit competing Gibbs free energies of formation across different temperature ranges. In the low-temperature range (<600 °C), amorphous or low-crystallinity intermediate phases tend to form; the medium-temperature range (700~800 °C) favors the formation of the LiTMO2phase; whereas in the high-temperature range (>800 °C), the nucleation and growth driving forces for the Li2MnO3phase are enhanced[52]. From a kinetic perspective, according to the Arrhenius equation, temperature exponentially affects the mutual diffusion rate of Li+and TM ions. At a specific temperature, the mutual diffusion rates of Li+and TM ions reach an optimal match, promoting the formation of a nano-composite structure with uniformly distributed dual phases[58]. Therefore, precise temperature control is key to synthesizing high-performance LRMOs. Li et al.[52]revealed, through synchrotron X-ray diffraction (SXRD) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), the existence of an oriented phase transition from LiTMO2to Li2MnO3in the medium-temperature range. The driving force for this transition originates from vacancies in the transition metal layer. The LiTMO2phase formed in the early stage of the reaction contains numerous vacancies due to insufficient Li/O intercalation, which promotes the migration of TM ions into the lithium layer, thereby generating vacancies in the TM layer. These vacancies are subsequently occupied by continuously intercalated Li+, triggering local Li/TM disordering, and ultimately forming the Li2MnO3phase with an ordered TM-TM-Li superlattice arrangement through structural rearrangement.
3.2.3 Partial pressure of oxygen (P</sub>) effects
In the sintering atmosphere, P</sub> acts as a "chemical scissor" that directly acts on the material's oxygen sublattice, enabling precise tailoring of material defects and surface structures. According to defect chemistry principles, the oxygen exchange between the material and the atmosphere at high temperatures can be represented by equilibrium equation (8).
reducingP</sub> shifts the equilibrium to the right, generating more oxygen vacancies and accompanying electrons, thereby stabilizing lattice oxygen[59]. This is because whenP</sub> is low, part of Mn4+ is reduced to Mn3+. On one hand, Mn3+ can introduce local structural distortions via the Jahn-Teller effect, preventing excessive aggregation of the Li2MnO3 phase, which facilitates lithium-ion extraction during electrochemical activation; on the other hand, the presence of Mn3+ can regulate the local chemical environment of the transition metal layer. However, the content of Mn3+ must be precisely controlled to avoid excessive lattice distortion[54,60]. Furthermore, pre-introducing controllable oxygen vacancies increases the energy barrier for irreversible oxygen release[61-62]. Finally, sintering under lowP</sub> may induce the preferential exposure of LiMn6 superstructure facets with low symmetry; their unique local coordination environments serve not only as channels for rapid Li+ diffusion but also stabilize coordinated lattice oxygen through strong Mn—O covalent bonds[63].
In summary, the sintering process of LRMOs is a complex solid-state chemical reaction involving multiple steps and coupled variables. Through synergistic optimization of the process and atmosphere, the phase composition, nano-domain structure, defect types, and concentration of the material can be precisely regulated, thereby targetedly addressing core issues such as difficult initial activation, irreversible oxygen release, and voltage decay during cycling.
4 Surface modification strategies based on atomic layer deposition
Surface coating is an important modification strategy to overcome the inherent problems of LRMOs. Compared with traditional coating methods, atomic layer deposition (ALD), by virtue of its self-limiting surface reaction characteristics, can deposit angstrom (Å)-level uniform ultra-thin coatings layer by layer on complex three-dimensional structures[64]. For LRMOs with complex pore structures, the applicability and advantages of the ALD method are even more prominent[65].
4.1 Process Principles and Technical Advantages of ALD Technology
Through saturated adsorption and reaction at surface active sites, ALD technology enables uniform film growth with atomic-level precision. Taking typical Al2O3deposition as an example (Figure 4a~d), a complete reaction includes the following steps: (1) Use inert gas purging to remove physically adsorbed water from the material surface; (2) Introduce trimethylaluminum (Al(CH3)3) vapor into the reactor to interact with hydroxyl groups on the material surface; (3) Increase the vacuum level inside the reactor and use inert gas purging to remove excess Al(CH3)3vapor; (4) Introduce water vapor as an oxygen source to undergo hydrolysis with the adsorbed Al(CH3)3molecules, generating Al2O3films; (5) Use inert gas purging to remove water vapor. The process advantages of ALD stem from its layer-by-layer, self-limiting gas-solid surface reaction mechanism. This characteristic ensures excellent three-dimensional conformality of the coating, enabling uniform coverage on the outer surface, inner pore walls, and even primary particle interfaces of cathode materials with high specific surface area and porous structures, thereby constructing a complete and continuous protective layer. Furthermore, by precisely controlling the number of cycles, one can avoid increased ion transport impedance due to excessively thick coatings or failure to effectively suppress interfacial side reactions due to excessively thin coatings. The process flexibility of ALD is also reflected in its wide range of precursor choices, which can adapt to various functional coating systems, providing multiple means for the rational design of interface engineering.
4.2 Morphology of ALD coatings and their structure-activity relationship
In practical applications, influenced by the combined effects of surface chemical state, microstructure, and process parameters, ALD coatings can exhibit two typical morphological modes: complete uniform coverage and incomplete island-like coverage.
Among them, the core advantage of uniform coating lies in constructing a continuous, dense, and thickness-uniform interfacial layer, thereby effectively isolating direct contact between the cathode material and the electrolyte (Figure 4e,f). Zhang et al.[66]prepared a uniform Al2Ni0.13Co0.13Mn0.54O2 surface with a uniform Al2O3 coating, enabling the material, after 50 cycles at a 0.33 C rate at room temperature, to increase its capacity retention from 68% to 82%; furthermore, at a high temperature of 55 °C, the Al2O3-coated material also exhibited superior cycling stability compared to the uncoated material.
As for island-like coating, its unique discontinuous structure enables selective modification and local regulation of the material surface (Fig. 4g,h). Hoskins et al.[67]prepared a non-uniform island-like Al2O3coating layer on the surface of NMC111 cathode materials by controlling the number of ALD cycles (<10). Compared with uniform thick films, the non-uniform coating under low cycle numbers effectively suppressed the dissolution of transition metals while retaining unobstructed channels for lithium-ion insertion and extraction, thereby significantly improving the cycling stability of the battery without sacrificing capacity. This study demonstrates that island-like coating can achieve electrochemical performance superior to the traditional concept of complete coverage, providing a new design paradigm for surface modification engineering. The aforementioned structure-activity principles are equally applicable to the surface modification design of LRMOs.
4.3 Frontier Challenges and Development Pathways of ALD Interface Engineering
Although ALD technology has demonstrated unique advantages in surface modification, it still faces numerous challenges in practical applications. For instance, the uneven distribution of active sites on LRMOs can easily lead to non-uniform initial coating and may be accompanied by irreversible damage to the subsurface lattice, ultimately affecting the uniformity and density of ultra-thin coatings. Furthermore, LRMOs undergo certain lattice volume changes, continuous evolution of oxygen species, and migration of transition metal ions during working cycles. These dynamic processes impose mechanical stress and chemical impact on the nanoscale coating-substrate interface, potentially causing coating cracking or delamination, or inducing inter-diffusion of elements between the coating and substrate to form high-impedance interfacial phases. More importantly, the high process costs and slow deposition rates place significant economic and efficiency pressures on the industrial application of ALD technology, severely hindering its transfer from the laboratory to industry. Therefore, how to precisely construct a multifunctional protective layer that integrates physical barriers, rapid ion conduction, and electronic structure modulation, while effectively reducing process costs, is a crucial research direction for future breakthroughs in the industrial bottlenecks of ALD technology and achieving a performance leap in LRMOs.
In summary, ALD technology provides an effective pathway for atomic-scale precise modification of LRMO surfaces, demonstrating significant potential especially in suppressing interfacial side reactions and stabilizing crystal structures. However, this technology still faces multiple challenges in practical applications, including surface chemical heterogeneity, diffusion limitations within pores, high industrialization costs, and interfacial stability under dynamic electrochemical environments. Future research should focus on developing novel deposition processes, optimizing coating uniformity within complex pores, and exploring gradient and multi-layer composite ALD interface designs to synergistically enhance the mechanical adaptability, ionic conductivity, and electrochemical stability of the coatings.
5 Electrochemical principles of lithium-rich manganese-based cathode materials
The lattice oxygen activity of LRMOs originates from their unique atomic configuration. From the perspective of crystal chemistry, when Li+enters the TM layer, it forms a Li—O—Li structure with Li in the Li layer. This configuration causes hybridization between O 2p and Mn 3d orbitals; the resulting antibonding orbitals preferentially lose electrons during charging, thereby activating lattice oxygen. However, due to the redox activity of lattice oxygen, LRMOs also face a series of issues such as voltage hysteresis, poor rate capability, and voltage decay. Therefore, deeply understanding the electrochemical principles of lattice oxygen redox is the foundation for identifying reasonable solution strategies and promoting the commercialization of LRMOs.
5.1 Lattice oxygen oxidation model
Figure 5aandFigure 5bshow the electrochemical curves and major reaction stages during the first charge-discharge cycle of typical LRMOs. When the voltage is below 4.45 V, the electrochemical reactions of LRMOs are identical to those of conventional layered cathode materials, involving only the oxidation of transition metals. As the voltage further increases, a distinct voltage plateau appears in the electrochemical curve. This stage corresponds to the activation of the Li2MnO3phase, where lattice oxygen undergoes oxidation to participate in charge compensation. However, there remains considerable debate regarding whether the oxidation of lattice oxygen is fully reversible and how intermediate products exist in the reversible reaction. To address this, researchers have proposed a series of hypotheses concerning lattice oxygen oxidation models. Marie et al.[68]employed high-resolution resonant inelastic X-ray scattering (RIXS) technology to intuitively and quantitatively characterize molecular oxygen trapped by transition metal vacancy clusters. In the first cycle, the generated molecular oxygen can be completely reduced back to O2-. As cycling extends, the amount of formed molecular oxygen gradually decreases, and the difficulty of reducing it during the discharge process also increases. Meanwhile, accompanied by the enlargement of internal voids and changes in the material's microstructure, resulting cracks allow molecular oxygen to penetrate to the surface, leading to oxygen release. This reduction in reversibility is the root cause of structural transformation and voltage decay in LRMOs. Li et al.[69]used various in-situ testing techniques to visually characterize peroxide dimers and suggested that the redox of lattice oxygen can maintain a certain degree of reversibility over long cycles. Additionally, some researchers believe that after oxidation, lattice oxygen undergoes displacement, causing distortion and deformation of the lattice oxygen framework. However, this sub-lattice distortion process during redox reactions was previously impossible to observe directly and could only be understood holistically by observing coordinated cations. With the development of high-resolution transmission electron microscopy (HRTEM) capabilities, direct observation of lattice oxygen has become possible. Song et al.[70]utilized electron ptychography to image the displacement of lattice oxygen in LRMOs during the initial charge-discharge process. The displacement of bulk lattice oxygen caused by oxidation did not fully recover during the subsequent discharge process, whereas near-surface lattice oxygen returned to its initial position accompanying the phase transformation. This inconsistency between internal and external lattice oxygen displacement is considered the answer to explaining the voltage hysteresis and voltage decay observed in LRMOs.
Although diverse representation methods provide researchers with different interpretative perspectives, the core mechanism of various lattice oxygen oxidation models remains the same. Asshown in Figure 5c, when lattice oxygen is activated, electrons are removed from the non-bonding O 2p orbitals, causing the valence state of the lattice oxygen to change from -2 to -n (0 <n< 2). Due to changes in the local charge environment, oxygen ions in the bulk lattice shorten their distance to neighboring oxygen ions to maintain the minimum energy of the lattice system. This shortening of distance can be understood as oxidized lattice oxygen forming O—O dimers through a reduction coupling mechanism, thereby inducing lattice distortion within the bulk. When observed from a long-range perspective, this distortion can be regarded as a displacement of lattice oxygen, meaning the lattice oxygen framework undergoes a disordered transition. When significant structural changes occur, the distance between oxygen atoms in the O—O dimer further shortens, forming stable covalent bonds and separating from the lattice; the resulting molecular oxygen becomes trapped in vacancy clusters within the bulk. The Bruce team[68,71]utilized high-resolution resonant inelastic X-ray scattering (HR-RIXS) technology and17O magic-angle spinning nuclear magnetic resonance spectroscopy (17O MAS NMR) to intuitively characterize molecular oxygen trapped by metal vacancy clusters within the bulk. As cycling prolongs, the formed molecular oxygen gradually decreases, and the difficulty of reducing it during the discharge process increases. Simultaneously, accompanied by the enlargement of internal voids and changes in the material's microstructure, generated cracks allow molecular oxygen to penetrate to the surface, leading to oxygen release. When this phenomenon occurs on the surface of cathode particles, it results in oxygen release. Therefore, the structural evolution brought about by different lattice oxygen oxidation models is very similar. At high states of charge, the internal structure of LRMOs forms a vast network of mobile dislocations. These line defects disturb the stacking of lattice oxygen layers, creating new local environments for Li ions, which is subsequently reflected in the discharge curves[72-74]. Consequently, the local lattice deviates from its original equilibrium state, providing rapid diffusion channels for oxidized lattice oxygen species and inducing irreversible oxygen loss on the particle surface[75]. Meanwhile, local stress changes in the bulk lattice force TM migration, causing the structure to transition from ordered to disordered. This transformation in microstructure manifests macroscopically as a series of electrochemical issues such as voltage hysteresis, voltage decay, and capacity decay.
5.2 Lattice Oxygen Activity Regulation Strategy
From the discussion in the previous section, it is evident that the root cause of electrochemical issues in LRMOs lies in the oxidized lattice oxygen. Therefore, effectively regulating the activity of lattice oxygen is a necessary means to improve the current predicament of LRMOs.
The key to regulating lattice oxygen activity lies in modulating the local environment of lattice oxygen. The most common method is doping with heterogeneous elements to adjust the covalency of TM—O bonds, thereby stabilizing lattice oxygen activity[76]. Although this method can effectively stabilize lattice oxygen, it sacrifices part of the lattice oxygen activity. Furthermore, as indicated by the above discussion results, the co-precipitation process requires the solubility products and ionic radii of various ions to be relatively close, which also limits the widespread use of doping strategies. Therefore, other approaches need to be considered. Yin et al.[77]prepared LRMOs with different lattice oxygen activities by regulating the ratio of TM ions. The two materials exhibited significant differences in terms of local structural stability and electrochemical performance. In subsequent work, the structural characteristics of LRMOs were systematically studied, unraveling the structural factors affecting lattice oxygen activity[78]. The activity of lattice oxygen is highly susceptible to the local coordination environment of Co; the incorporation of Co ions into the Li2MnO3phase enhances the activation of lattice oxygen[79]. Wei et al.[80]regulated the size and distribution of the Li2MnO3phase by controlling the amount of lithium source, successfully achieving oxygen release suppression without affecting lattice oxygen activity. Similarly, Gong et al.[81]also regulated the Li content, utilizing the local ferromagnetic characteristics of the Ni—O—Ni configuration to improve the reversibility of LRMOs redox reactions, which not only suppressed irreversible oxygen release but also enhanced the initial Coulombic efficiency of the electrode.
In addition to regulating the lattice oxygen activity of raw materials, how to fully utilize this activity represents another research direction. Zhou et al.[82]deeply activated LRMOs, utilizing the resulting disordered structure to buffer local stresses generated during subsequent charge-discharge cycles, thereby suppressing voltage decay. Qiu et al.[73]demonstrated through multi-scale characterization that lattice oxygen activation induces multi-dimensional structural defects, causing LRMOs to form metastable structures after long-term cycling. The essence of this metastable structure is the structural disorder arising from lattice oxygen displacement caused by lattice oxygen activity, which may subsequently lead to phase transitions in the material structure, resulting in capacity loss and voltage decay. Qiu et al.[73]and Zhang et al.[83]employed heat treatment and high-pressure treatment, respectively, on LRMOs after long-term cycling, using external energy injection to restore order to the electrochemically induced disordered structures. Among them, the heat-treated samples were able to re-exhibit charging plateaus, proving that their structures could basically return to the initial state and that lattice oxygen activity was restored. Subsequent results indicate that materials possessing lattice oxygen activity exhibit significant negative thermal expansion phenomena[84]. By regulating lattice oxygen activity, it is possible not only to precisely control the thermal expansion coefficient of materials but also to establish a practical framework for designing functional materials with zero thermal expansion characteristics. Meanwhile, electrochemical methods can also induce and drive the disorder-order transition of materials, prompting lithium in the transition metal layers to re-occupy sites and lattice oxygen displacement to recover, thereby achieving a significant rebound in average discharge voltage and providing a new strategy for mitigating voltage decay.
5.3 Generation, Construction, and Utilization of Oxygen Vacancies
From the previous discussion, it is known that the oxidation of lattice oxygen at high voltage inevitably generates oxygen vacancies[85]. The root cause of this phenomenon lies in the unique Li—O—Li structure in LRMOs, which endows the lattice oxygen with non-hybridized O 2p orbitals. Since the electrons within these orbitals possess higher energy, they spontaneously migrate during charging, leaving localized electron holes on the oxygen atoms and forming highly active, unstable lattice oxygen at high voltages. When Li2MnO3 components are activated, the extraction of Li ions disrupts the originally stable TMO6 octahedral structure; the resulting low-coordination environment makes it easier for lattice oxygen to escape from the lattice[86-88]. This series of changes in structure and local environment provides the thermodynamic and kinetic basis for the generation of oxygen vacancies.
After their formation, oxygen vacancies induce the migration of transition metal ions and the generation of a spinel structure, ultimately resulting in issues such as reduced initial Coulombic efficiency, diminished cycling stability, and voltage decay.[89]. To address this dilemma, researchers have explored various approaches. One option is to suppress the formation of oxygen vacancies; for instance, Peng et al.[90]enhanced the strength of TM—O bonds by employing surface coating and elemental doping, thereby stabilizing lattice oxygen on the surface and within the bulk phase respectively, and maintaining the overall crystal structure during electrochemical cycling. Another option involves designing, constructing, and utilizing lattice oxygen vacancies. Without compromising the overall structural integrity, actively removing "unstable" lattice oxygen from the particle surface can improve the initial efficiency of LRMOs, increase the formation energy of subsequent oxygen vacancies, regulate the rigidity of the crystal structure, and alleviate stress accumulation during charge-discharge processes. Furthermore, oxygen vacancies can effectively modulate the electronic structure of materials, lower the diffusion energy barrier for lithium ions, and thus enhance the rate performance of the materials.[91-93].
Currently, methods for constructing lattice oxygen vacancies mainly include the use of reducing agents and ion doping. For example, Qiu et al.[62]achieved precise fabrication of oxygen vacancies on the surface of LRMOs via gas-solid surface modification, increasing the initial discharge capacity of the material to 301 mAh/g, with significant improvements in initial Coulombic efficiency, voltage stability, and capacity stability. The ion doping method for constructing oxygen vacancies generally involves substituting original transition metals (TM) and lattice oxygen in the material with high-valence cations or anions, utilizing the property of local charge neutrality to introduce oxygen vacancies.[94]. For instance, Wang et al.[95]adopted a complex concentrated anion doping (CCA) strategy to simultaneously introduce F-, Br-, and S2-—three types of anions—into the oxygen lattice of LRMOs to construct oxygen vacancies. The modified material exhibited "zero-strain" characteristics during electrochemical processes, and the fabricated pouch cells achieved ultra-low voltage decay and excellent energy retention. It is important to note that although actively constructing oxygen vacancies can enhance electrochemical performance, exceeding the controllable concentration range can induce detrimental structural collapse, leading to counterproductive results.
In summary, the activity and reversible transformation of lattice oxygen are central to overcoming the application bottlenecks of LRMOs. Future research should combine in-situ spectroscopic techniques with multi-scale simulations to further reveal the regulation mechanisms of different components and structures on oxygen activity, with particular attention to the retention and re-reduction behavior of oxygen species, as well as the impact of interface engineering on their reaction pathways, thereby providing a theoretical basis for designing LRMOs with high capacity, low hysteresis, and long cycle life.
6 Prospects for Solid-State Applications of Lithium-Rich Manganese-Based Cathode Materials
Due to the high operating voltage of LRMOs, current liquid electrolytes inevitably undergo decomposition, causing harmful chemical issues on the surface of cathode particles, which severely limits the industrial application of LRMOs[96-98]. With characteristics such as a wide electrochemical window, high mechanical strength, and excellent thermal stability, the development of solid-state electrolytes (SEs) offers an opportunity to address this issue[99]. Integrating LRMO materials with solid-state electrolytes to construct all-solid-state lithium batteries (ASSLBs) is a key future development direction for safer, high-energy-density batteries.
However, shifting from the traditional "liquid-solid" interface to a "solid-solid" interface reaction system poses many challenges for ASSLBs, thereby imposing many new intrinsic requirements on LRMOs. First, unlike liquid electrolytes that can wet and buffer the volume changes of cathode particles, the rigid solid-solid contact within the ASSLB system requires LRMOs particles to have good mechanochemical stability and exhibit smaller absolute volume strain during charge-discharge cycles[100-101]. Second, due to the lack of liquid electrolyte penetration in ASSLBs, lithium-ion transport relies entirely on solid-state diffusion, requiring higher ion transport efficiency in LRMOs[102-103]. Third, solid-state electrolytes are extremely sensitive to oxidizing species; highly reactive oxygen species generated by LRMOs at high voltages will irreversibly oxidize the solid-state electrolyte, forming high-impedance interfacial phases, thus necessitating enhanced surface electrochemical stability[104].
To address the aforementioned issues, researchers have proposed a series of interface regulation strategies aimed at constructing solid-solid interfaces that possess both high ionic conductivity and chemical/electrochemical stability (Figure 6). At the interface level, through precise coating design at the atomic scale, synergistic regulation of physical isolation, chemical stability, and ion transport is achieved[105-106]. At the composite electrode level, LRMOs, solid-state electrolytes, and conductive materials are three-dimensionally compounded at the micro-nano scale to construct a continuous synergistic ion/electron conduction network, optimizing interfacial charge transfer and bulk ion transport kinetics[107-108]. At the battery level, small amounts of polymer electrolytes or ionic liquids are introduced to construct "semi-solid" batteries[109]. This strategy can optimize physical contact at the interface; the resulting continuous Li+transport channels reduce interfacial impedance. Meanwhile, wetting agent molecules form a stable interfacial film on the surface of cathode particles through chemical adsorption and coordination, suppressing irreversible lattice oxygen release and subsequent side reactions with the solid-state electrolyte. Although this semi-solid strategy may appear to be a compromise, it is currently a practical transitional solution that balances performance and safety[110].
In addition, the development of LRMOs for ASSLBs is shifting from post-synthesis modification to intrinsic design, focusing on addressing the three core issues mentioned above. Wu et al.[111]successfully synthesized single-crystal Li with preferential exposure of the [010] facet by regulating lithium salt dosage and sintering protocols.1.2Ni0.13Co0.13Mn0.54O2, eliminating grain boundaries between primary particles. This not only enhanced the material's intrinsic mechanical strength to resist volume changes but also prevented oxygen release and structural degradation along grain boundaries, resulting in significantly superior initial Coulombic efficiency and cycling stability in sulfide-based ASSLBs compared to their polycrystalline counterparts. Ji et al.[112]employed a gradient doping and surface reconstruction strategy to design a concentration gradient of high-valence ions (such as Ta5+, Nb5+) from the particle surface to the bulk. This constructed a thin layer on the surface that combines structural rigidity and oxygen stability, suppressing oxygen release and blocking direct side reactions with the solid-state electrolyte. Consequently, the capacity retention of all-solid-state batteries after 300 cycles increased from 63% to 78%, while the growth of interfacial impedance was significantly reduced. Song et al.[113]constructed a three-dimensional composite cathode of LMROs and sulfide solid-state electrolytes at the nanoscale, significantly shortening the solid-state diffusion distance of Li+within the LMROs and improving charge transfer kinetics. This nano-composite structure achieved a capacity retention rate as high as 97.1% after 1300 cycles at 30 °C, with voltage decay significantly suppressed.
In summary, applying LRMOs to all-solid-state battery systems is a key pathway to fundamentally resolve their compatibility issues with liquid electrolytes. Although solidification effectively suppresses chain side reactions triggered by TM dissolution and oxygen release, it also introduces numerous new challenges. Future research should focus on developing multi-scale, multifunctional interface engineering technologies, precisely elucidating the dynamic processes and regulation mechanisms of interfacial reactions, and promoting the practical application of LRMOs in high-energy-density all-solid-state batteries.
7 Conclusion and Outlook
Since their discovery, researchers have conducted a comprehensive chemical process analysis of LRMOs, ranging from synthesis to application. This paper systematically elaborates on key stages including their synthesis, modification, and application processes, and provides a detailed analysis of a series of chemical issues arising at each stage, such as coordination chemistry, physical chemistry, and electrochemistry. Although relevant research has made progress, significant challenges remain. Therefore, to achieve the practical application of LRMOs, future research should focus on the following aspects.
(1) Precise chemical control of the synthesis process: In-depth study of reaction thermodynamics and kinetics in co-precipitation and sintering to establish quantitative "method-structure-performance" relationships, achieving intrinsic regulation of lattice oxygen activity and phase composition.
(2) Develop synergistic modification strategies: Combine multi-element doping with multifunctional coating to develop a "co-modification" pathway that can simultaneously stabilize the bulk structure and the electrode interface.
(3) Revealing the anionic redox mechanism: Leveraging multi-scale in-situ characterization and theoretical simulations to clarify the evolution pathways of oxygen species (O2-/O2n-/O2) and their synergistic effects with TM, elucidating the scientific essence of voltage hysteresis and voltage decay.
(4) Addressing solid-state battery interface challenges: Design artificial interfacial layers that combine high ionic conductivity, potential stability, and good contact; develop three-dimensional composite electrode structures to promote the application of LRMOs in high-energy-density solid-state batteries.
With continuous breakthroughs in analytical characterization technologies, collaborative research across scales and mechanisms facilitates the precise prediction and resolution of a series of chemical challenges faced by LRMOs, thereby effectively promoting their commercial application.
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