Li-rich Layered Cathode Materials Research Articles

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Machine learning-based evaluation of functional characteristics of Li-rich layered oxide cathode materials using the data of XPS and XRD spectra

Li-ion batteries are the most wide-spread electrochemical energy storage systems. Cathode materials are a major focus because they are considered the limiting element of overall battery performance. Li-rich layered oxides are among the actively investigated cathode materials designated for using in Li-ion batteries of new generation and approaching to the imposed high requirements of high energy and power densities. However, the inherent complexity of their structure resulting in the well-known problems with the structural stability has hindered the widespread use of Li-rich layered oxides as the cathode materials. In this study, we have used machine learning (ML) methods to obtain the models able to predict the electrochemical and structural characteristics of these materials. The data from XPS and XRD spectra were used as the additional descriptors to evaluate the target functional characteristics: initial discharge capacity, coulombic efficiency and capacity fade. One of the capacities of ML to impute the missing data were exploited to solve the problem of incomplete experimental data. The obtained models have shown the reasonable predictive performance. The analysis of the experimental data and the results of modeling have feed to further considerations concerning the complexity of the multifold processes that determine the target functional characteristics of these materials.

Facilitating structural stability of the Li-rich layered cathode materials by cation rearrangements and Li vacancies

The anomalously high capacities of Li-rich materials are a result of contributions from both cationic and anionic redox processes, however, anionic redox also leads to capacity loss and structural degradation. In this study, the content of Li-O-Li configuration and the types of Li-O-TM configuration are regulated by introducing the Li deficiencies and changing the ratio of Ni/Co/Mn. The results of XRD, Raman spectra, XPS, HR-TEM and in-situ XRD characterizations indicate that the generation of the spinel structure and crystal defects strengthen the structural stability in the modified Li-rich materials. Besides, the density functional theory (DFT) calculation result prove that increasing the ratio of Ni/(Co+Mn) further enhance the electron contribution of the Ni ions and the electronic conductivity of as-prepared samples. The LR1.8–811 sample presents outstanding cycling performance and energy density, that is, the capacity retention is 98.64% and energy density is 764.7 Wh kg−1 after 100 cycles at a rate of 0.2C, respectively. These improvements are ascribed to the less content of Li-O-Li configuration and the more content of Li-O-Ni configuration in the LR1.8–811 material, which mitigated the oxygen redox and enhanced the structural stability.

SeO2-infused grain boundaries effectively improve rate and stability performance of Li-rich manganese-based layered cathode materials

For polycrystalline lithium-rich manganese-based layered oxides (LMLO), the presence of porous microstructures always leads to the greater oxygen release and lower conductivity, impairing their rate and cycling stability performance for real applications. Here, by temperature-controlled annealing, we propose a facile and scalable method to infuse molten SeO2 into the intracrystalline grain boundary of the polycrystalline Li1.2Mn0.54Ni0.13Co0.13O2 (LMNCO) particles. Then the uniform surface and double nanolayer structure composed of interconnected Li2SeO4 and robust Li2NixCoyO4 are revealed on primary particles. Accordingly, SeO2-infused LMNCO(Se-LMNCO) gains an upraised rate performance with 97 % capacity retenting rate after charge and discharge under 0.1–5 C then returned to 0.1 C and an improved cycling stability with a capacity retention of 80.0 % over 200 cycles at 1 C (1 C=250 mAh g−1). Our work highlights the significance of surface engineering inside the secondary LMLO particles and the improvement of electrochemical rate and stability performance toward lithium-ion batteries with LMLO electrode.

Mitigated lattice distortion and oxygen loss of Li-rich layered cathode materials through anion/cation regulation by Ti4+-substitution

Lithium-rich layered cathode material (LLM) can meet the requirement of power lithium-ion energy storage devices due to the great energy density. However, the de/intercalation of Li+ will cause the irreversible loss of lattice oxygen and trigger transition metal (TM) ions migrate to Li+ vacancies, resulting in capacity decay. Here we brought Ti4+ in substitution of TM ions in Li1.2Mn0.54Ni0.13Co0.13O2, which could stabilize structure and expand the layer spacing of LLM. Moreover, optimized Ti-substitution can regulate the anions and cations of LLM, enhance the interaction with lattice oxygen, increase Ni3+ and Co3+, and improve Mn4+ coordination, improving reversibility of oxygen redox activation, maintaining the stable framework and facilitating the Li+ diffusion. Furthermore, we found 5% Ti-substitution sample delivered a high discharge capacity of 244.2 mAh/g at 50 mA/g, an improved cycling stability to 87.3% after 100 cycles and enhanced rate performance. Thereby Ti-substitution gives a new pathway to achieve high reversible cycle retention for LLMs.

Constructing an Inhomogeneous Surface to Suppress the Capacity Decay of Li-Rich Layered Cathode Materials

Lithium-rich layered oxides with superior capacity over 250 mA h g–1 have been regarded as one of the most promising cathode materials to address the problem of low endurance of electric vehicles. Unfortunately, their practical application has been blocked for decades by severe voltage decay and capacity fading, which mainly originate from structure evolution of layered to spinel-like phase and undesirable cathode–electrolyte interfacial reactions. Herein, the inhomogeneous distribution of LiMO2 and Li2MnO3 components on the surface of Li1.2Mn0.6Ni0.2O2 (LMNO) is constructed by a facile NH3·H2O-assisted Mn- and Ni-treatment method, demonstrated by Raman results. As supported by dQ/dV curves and electrochemical impedance spectroscopy data, the inhomogeneous surface improves the corrosion resistance against the electrolyte and enhances the surface–interface stability. Compared with the pristine one (P-LMNO), the Mn- and Ni-treated samples (M-LMNO and N-LMNO) deliver excellent cycling stability. After 200 cycles, the discharge capacity of the M-LMNO and N-LMNO samples is still as large as 154.5 and 174.0 mA h g–1 at 200 mA g–1, respectively, which is about twice as large as that of the P-LMNO sample. This work proposes a facile and effective surface-treated strategy for Li-rich layered cathodes to alleviate structure evolution and suppress the erosion of electrolytes.

Effects of Lithium Source and Content on the Properties of Li-Rich Layered Oxide Cathode Materials

Lithium-rich layered oxide (LLO) are considered high-capacity cathode materials for next-generation lithium-ion batteries. In this study, LLO cathode materials were synthesized via the hydroxide coprecipitation method followed by a two-step lithiation process using different lithium contents and lithium sources. The effects of lithium content and lithium source on structure and electrochemical performance were investigated. This study demonstrated the clear impact of Li/TM ratio on electrochemical performance. Lower Li/TM ratio reduced the irreversible capacity loss in the first cycle and provided better cycling stability among all samples. The best results exhibited an initial discharge capacity of 279.65 mAh g−1 and reached a discharge capacity of 231.9 mAh g−1 (82.9% capacity retention) after 30 cycles. The sample using Li2CO3 as lithium source exhibits better electrochemical performance than the sample using LiOH as lithium source. Therefore, it is important to choose the appropriate lithium source and optimal lithium content for improving structural properties and electrochemical performance of LLO.

On the disparity in reporting Li-rich layered oxide cathode materials.

Lithium-rich layered oxides are considered one of the most promising cathode materials for next generation lithium-ion batteries due to their extraordinary specific capacity of over 280 mA h g-1 and superior energy density of over 1000 W h kg-1. Despite the excellent performance, LRLOs still suffer from low Coulombic efficiency, serious capacity/voltage decay upon cycling, voltage hysteresis, short lifespan, and poor rate capability. Driven by the thirst for high-energy-density battery technologies, various strategies have been developed to address these issues with great progress being achieved in the past several years. However, the emerging disparity among the published results severely precludes meaningful comparisons between different LRLOs and material modification strategies, which has become an impediment to the development and commercialization of LRLOs. Although the significance of standardization has been recognized in the battery community, the standardization of LRLOs is worth particular attention due to their complicated compositions and unique electrochemical properties. This perspective analyzes the underlying parameters that can cause varied and even controversial results observed in LRLOs, from the synthesis procedure to the electrochemical evaluation procedure, followed by preliminary suggestions for the standard protocols of chemical compositions, synthesis pathways, calcination conditions, electrode preparation, battery fabrication, and battery testing. Hopefully, this perspective can help build a reliable baseline for LRLO research, thus aligning the huge research effort toward the practical applications of LRLOs.

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.

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 Li1.2Ni0.2Mn0.6O2 (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.

Improved electrochemical kinetics and interfacial stability of cobalt-free lithium-rich layered oxides via thiourea treatment

Co-free Li-rich layered oxides have emerged as practically feasible cathode materials for high-energy–density Li-ion batteries due to their extra anionic redox behaviour and the market competitiveness of inexpensive manganese. However, the low initial coulombic efficiency, poor rate performance, and structural degradation have limited their practical applications. Herein, a facile strategy is reported to improve electrochemical kinetics and structural stability by a one-step treatment with thiourea at a moderate temperature. The decomposition reaction integrated nitrogen doped carbon coating, oxygen vacancies, and a built-in S-doping spinel-like/rock-salt structure on surfaces. The built-in surface reconstruction layer with lower TM-O hybridization and covalency suppresses the irreversible oxygen evolution while promoting cation redox. Surface reconstruction layer satisfies the essential requirements of efficient diffusion kinetics, and inhibits corrosion of the electrolyte and the dissolution of transition metal ions. As a result, the initial coulombic efficiency, rate performance, and structural degradation are greatly improved. After 100 cycles at 0.1C, the treated materials exhibit a capacity retention of 96.7% (vs. 86.0% for the pristine sample) with excellent structural stability. This study sheds some new light on designing high-performance Li-rich layered oxide cathode materials by the incorporation of surface reconstitution.

In2O3 surface modification of a Li-rich layered cathode material for boosting electrochemical performance

Lithium-rich layered cathode materials have high specific capacities. However, some drawbacks such as low initial Coulombic efficiency (ICE) and severe capacity/voltage decay during repeated cycling hinder their practical applications. Herein, a pristine Li1.2Mn0.54Ni0.13Co0.13O2 (LMNCO) material is synthesized by the carbonate coprecipitation method and then the In2O3 surface modified LMNCO materials with different amounts of In2O3 are prepared by the facile solvent evaporation and calcination method. The In2O3-modified LMNCO material with 2 wt% In2O3 demonstrates the highest ICE and the best rate capability and cycle stability, which are much superior to those of the pristine LMNCO. This surface modified material exhibits the specific capacities of 268.2 and 109.1 mAh g−1 at the current rates of 0.1C and 5C, respectively. After cycling at 1C rate for 200 cycles, the discharge specific capacity is declined from 176.2 to 140.9 mAh g−1, giving capacity retention of 80.0%. The improved electrochemical performance is ascribed to the surface modification of a moderate amount of In2O3. This surface modification can enhance the electrical conduction and hence the electrochemical reaction activity of the active material without hindering the lithium ion transport through the electrode surface. Moreover, it can suppress the undesirable increasing growing of the solid electrolyte interphase film during the repeated charge/discharge cycling due to the surface side reactions between the active material and the electrolyte.

The Stabilizing Effect of Li4Ti5O12 Coating on Li1.1Ni0.35Mn0.55O2 Cathode for Liquid and Solid–State Lithium-Metal Batteries

Li–rich layered cathode materials with high energy density suffer from severe capacity decay during cycling, which is associated with volume change and electrolyte corrosion during (de)lithiation. A Li+ ionic conducting Li4Ti5O12 coating with high structural integrity is developed on Li1.1Ni0.35Mn0.55O2 cathodes via a dry powder coating method. The electrochemical performances of Li4Ti5O12–coated Li1.1Ni0.35Mn0.55O2 cathodes in liquid and solid–state lithium batteries were investigated. The initial discharge capacity of Li4Ti5O12–coated Li1.1Ni0.35Mn0.55O2 in the liquid electrolyte has been improved from 116.5 mA h g−1 to 123.7 mA h g−1 at 0.1°C. An impressive cyclability with a high capacity retention of 89.3% was achieved in solid–state lithium batteries. These results demonstrate that the Li4Ti5O12 coating plays an essential role in enhancing the specific capacity and better performance for Li1.1Ni0.35Mn0.55O2 cathode.

In Situ Reconstruction of the Spinel Interface on a Li-Rich Layered Cathode Material with Enhanced Electrochemical Performances through HEPES and Heat Treatment Strategy

In Situ Reconstruction of the Spinel Interface on a Li-Rich Layered Cathode Material with Enhanced Electrochemical Performances through HEPES and Heat Treatment Strategy