Li-rich Layered Oxide Cathode Research Articles

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

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

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

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

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

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

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

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

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.

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

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

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

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

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

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

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

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

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

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

Weak σ–π–σ interaction stabilizes oxygen redox towards high-performance Li–rich layered oxide cathodes

The aggregation of Li2MnO3–like domains in Li-rich layered oxides (LLOs) causes severe capacity/voltage fading, which seriously impedes their commercial applications. Here, we design Co–free LLO models with well–dispersed Li2MnO3–like domains (D–LNMO) and aggregated Li2MnO3–like domains (A–LNMO) to investigate the oxygen redox process and structural stability. It is found that low oxygen partial pressure can disperse Li2MnO3–like domains by forming stable ONiMn4+Mn3+Li3 coordination configurations so that D-LNMO is predominant. Moreover, a novel oxygen oxidation mechanism involving a weak σ–π–σ interaction where oxygen redox in OTM2MnLi3 (TM = Ni, Mn) configurations is triggered by O in Li-O-Li configurations is revealed. Specifically, the lattice oxygen at the interface of Li2MnO3–like domains and LiTMO2 domains can be activated, which is beyond conventional Li-O-Li configuration. Due to the abundance of interfacial lattice oxygen in D–LNMO, more lattice oxygen participates in charge compensation, thereby relieving the oxidation load of oxygen ions, suppressing lattice oxygen release, and delaying irreversible structural transformation. Consequently, D-LNMO possesses highly reversible oxygen redox and exceptional structural stability, exhibiting superior cycling stability of high capacity. The findings provide new perspectives and concepts for designing high-energy Li-rich cathodes.

Stabilizing Li-rich layered oxide cathode interface by using silicon-based electrolyte additive

In this study, we investigate the efficacy of 2,4,6,8-tetramethyl-2,4,6,8-tetravinylcyclotetrasiloxane (ViD4) as an electrolyte additive to enhance the electrochemical stability of Li-rich (LRO)/Li cells. The LRO/Li cell in the 1 vol% ViD4 electrolyte displays a mere 27.9 % capacity loss after 100 cycles at 0.5C (1C = 300 mAh−1), in comparison with the 66 % observed in the baseline electrolyte. Theoretical calculations reveal that ViD4 possesses a lower calculated oxidation potential than the electrolyte solution, signifying its preferential oxidation propensity. Physical characterization results demonstrate the formation of a uniform ViD4-derived film spanning 2–3 nm on the LRO cathode surface. This film enhances the stability of the cathode/electrolyte interface and safeguards the structural integrity of LRO. Moreover, ViD4 acts as a scavenger for hydrogen fluoride (HF), which is a decomposition product of LiPF6. Theoretical calculations verify the feasibility of ViD4 in effectively eliminating HF.

Manipulated Fluoro‐Ether Derived Nucleophilic Decomposition Products for Mitigating Polarization‐Induced Capacity Loss in Li‐Rich Layered Cathode

AbstractElectrolyte engineering is a fascinating choice to improve the performance of Li‐rich layered oxide cathodes (LRLO) for high‐energy lithium‐ion batteries. However, many existing electrolyte designs and adjustment principles tend to overlook the unique challenges posed by LRLO, particularly the nucleophilic attack. Here, we introduce an electrolyte modification by locally replacing carbonate solvents in traditional electrolytes with a fluoro‐ether. By benefit of the decomposition of fluoro‐ether under nucleophilic O‐related attacks, which delivers an excellent passivation layer with LiF and polymers, possessing rigidity and flexibility on the LRLO surface. More importantly, the fluoro‐ether acts as “sutures”, ensuring the integrity and stability of both interfacial and bulk structures, which contributed to suppressing severe polarization and enhancing the cycling capacity retention from 39 % to 78 % after 300 cycles for the 4.8 V‐class LRLO. This key electrolyte strategy with comprehensive analysis, provides new insights into addressing nucleophilic challenge for high‐energy anionic redox related cathode systems.

Manipulated Fluoro-Ether Derived Nucleophilic Decomposition Products for Mitigating Polarization-Induced Capacity Loss in Li-Rich Layered Cathode.

Electrolyte engineering is a fascinating choice to improve the performance of Li-rich layered oxide cathodes (LRLO) for high-energy lithium-ion batteries. However, many existing electrolyte designs and adjustment principles tend to overlook the unique challenges posed by LRLO, particularly the nucleophilic attack. Here, we introduce an electrolyte modification by locally replacing carbonate solvents in traditional electrolytes with a fluoro-ether. By benefit of the decomposition of fluoro-ether under nucleophilic O-related attacks, which delivers an excellent passivation layer with LiF and polymers, possessing rigidity and flexibility on the LRLO surface. More importantly, the fluoro-ether acts as "sutures", ensuring the integrity and stability of both interfacial and bulk structures, which contributed to suppressing severe polarization and enhancing the cycling capacity retention from 39 % to 78 % after 300 cycles for the 4.8 V-class LRLO. This key electrolyte strategy with comprehensive analysis, provides new insights into addressing nucleophilic challenge for high-energy anionic redox related cathode systems.

New information about the cyclable capacity fading process of a pouch cell with Li-rich layered oxide cathodes.

Most studies investigate the cyclable capacity fading mechanism of Li-rich layered oxides (LLOs) from the microscopic structure level, lacking discussions about how the structure degradation influences the performance of the pouch cell precisely and quantitatively. Based on the analysis of the evolution of key parameters during the whole cycling period, a new transition-type fading mechanism is proposed. From the early-to-middle stage of the cycling period, polarization increases, most of which is interface-related, causing about 67% of the whole capacity loss. From the middle-to-late stage of the cycling period, active material losses turn out to be the dominating factor, inducing about 61% of the total capacity loss. Diffusion-related polarization, replacing the interface type, is responsible for most of the increased overpotential. Relative analysis confirms that during the early stage, the increase of the charge transfer resistance, induced by CEI (cathode electrolyte interface) growth and initial surface layered-structure degradation, is the main source of interface polarization. As the cycling evolves to the late stage, severe bulky structure degradation, including lattice-oxygen release, Li/Ni mixture and generation of a new spinel phase, turns out to be the major factor, causing further capacity fading.

Ultrafast synthesis of cation/anion co-doped Li-rich layered oxide cathodes with Li+/Ni2+ mixing structural defects

An ultrafast nickel foil-based thermal shock method is reported to synthesize cation/anion co-doped Li-rich layered oxide with Li+/Ni2+ mixing in seconds.

Review on the synthesis of Li-rich layered oxide cathodes

Based on the solid-phase reaction mechanism, this paper analyzes the synthesis process of LLOs, and summarizes the factors affecting the sintering process of LLOs.

Building oxygen-rich vacancy coating for fast Li-ion diffusion kinetics and stable cycling of lithium-rich layered oxide cathode at wide-temperature

As a cathode with a high energy density for Li-ion batteries, Li-rich layered oxide (LLO) cathodes suffer from short lifespans and poor rate performance, mainly due to their irreversible lattice oxygen evolution leading to severe structural degradation and oxygen release. Herein, we propose the use of Zn-doped SnO2 coated with LLO cathode to tackle these issues. Compared to the traditional method of coating oxides on the surface of lithium-rich cathodes, Zn doping of SnO2 coating can artificially create abundant oxygen vacancies for LLO cathode, which can provide a buffer boundary for active oxygen generated by lithium-rich cathode during circulation, effectively promoting the reversible reaction of oxygen redox couple and stabilizing lattice oxygen. The experimental results show that the surface of the LLO cathode's Zn-doped SnO2 is rich in oxygen vacancies could significantly boost the kinetics of lithium ion transfer, and effectively inhibit layer-to-spinel phase transition, and lattice oxygen release. By taking advantage of the Zn-doped SnO2 coating, the capacity stability of LLO cathode is raised from 58.1 % to 86.7 % at 1C after 500 cycles and also obtains outstanding rate performance and cycling durability at wide-temperature (−10 to 55 °C).

Multifunctional surface structure with enhanced electron and ion conductivities for superior electrochemical performance of Co-free Li-rich Mn-based layered oxide cathodes

Co-free Li-rich Mn-based layered oxides (LRMLOs) are attractive cathode materials for lithium-ion batteries due to their low cost, high energy density and environmental friendliness. However, they also suffer from the drawbacks of low initial Coulombic efficiency, rapid voltage fading and capacity decay, and poor rate performance as the general LRMLO cathode, which prevent their practical applications. In the present work, a multilayer surface coating composed of Li2B4O7/B2O3 and a spinel heterostructure rich-in oxygen vacancies are in-situ formed at the surface of the Co-free LRMLO particles by a facile thermal reduction treatment with NH4B5O8. The multilayer surface improves greatly the electron and ion conductivities, and plays significantly synergistic roles on suppressing oxygen release, reducing the dissolution of Mn and Ni elements, retarding the structure transformation from layered to spinel, and preventing the side reaction of the active material with electrolyte during cycling for the cathodes. As a result, the overall electrochemical properties of the cathodes are significantly improved. With an optimized addition of 10 wt% NH4B5O8, the initial Coulombic efficiency of the cathode is as high as 90.0%, and a capacity of 275 mAh g-1 is obtained at 0.1 C (1 C = 200 mA g-1) with the capacity retention of 98.5% after 100 cycles. A capacity of 194 mAh g-1 is maintained after 300 cycles at 1 C, with the capacity retention of 92.8%. A capacity of 117 mAh g-1 is obtained at a high rate of 20 C. In addition, the voltage fading during cycling is also greatly inhibited. The structure evolution of the cathode material during cycling is systematically investigated and the mechanism for the improved electrochemical properties is proposed. The present work provides a new surface structure design strategy and the method for enhancing the overall electrochemical performance of Co-free Li-rich Mn-based layered oxide cathodes.