Inhibiting Voltage Decay in Li-Rich Layered Oxide Cathode: From O3-Type to O2-Type Structural Design.

Li-rich layered oxide (LRLO) cathodes have been regarded as promising candidates for next-generation Li-ion batteries due to their exceptionally high energy density, which combines cationic and anionic redox activities. However, continuous voltage decay during cycling remains the primary obstacle for practical applications, which has yet to be fundamentally addressed. It is widely acknowledged that voltage decay originates from the irreversible migration of transition metal ions, which usually further exacerbates structural evolution and aggravates the irreversible oxygen redox reactions. Recently, constructing O2-type structure has been considered one of the most promising approaches for inhibiting voltage decay. In this review, the relationship between voltage decay and structural evolution is systematically elucidated. Strategies to suppress voltage decay are systematically summarized. Additionally, the design of O2-type structure and the corresponding mechanism of suppressing voltage decay are comprehensively discussed. Unfortunately, the reported O2-type LRLO cathodes still exhibit partially disordered structure with extended cycles. Herein, the factors that may cause the irreversible transition metal migrations in O2-type LRLO materials are also explored, while the perspectives and challenges for designing high-performance O2-type LRLO cathodes without voltage decay are proposed.

On the Elusive Crystallography of Lithium-Rich Layered Oxides: Novel Structural Models.

Lithium-rich layered oxides (LRLOs) are one of the most attractive families among future positive electrode materials for the so-called fourth generation of lithium-ion batteries (LIBs). Their electrochemical performance is enabled by the unique ambiguous crystal structure that is still not well understood despite decades of research. In the literature, a clear structural model able to describe their crystallographic features is missing thereby hindering a clear rationalization of the interplay between synthesis, structure, and functional properties. Here, the structure of a specific LRLO, Li1.28 Mn0.54 Ni0.13 Co0.02 Al0.03 O2 , using synchrotron X-ray diffraction (XRD), neutron diffraction (ND), and High-Resolution Transmission Electron Microscopy (HR-TEM), is analyzed. A systematic approach is applied to model diffraction patterns of Li1.28 Mn0.54 Ni0.13 Co0.02 Al0.03 O2 by using the Rietveld refinement method considering the R 3 ¯ $\bar{3}$ m and C2/m unit cells as the prototype structures. Here, the relative ability of a variety of structural models is compared to match the experimental diffraction pattern evaluating the impact of defects and supercells derived from the R 3 ¯ $\bar{3}$ m structure. To summarize, two possible models able to reconcile the description of experimental data are proposed here for the structure of Li1.28 Mn0.54 Ni0.13 Co0.02 Al0.03 O2 : namely a monoclinic C2/m defective lattice (prototype Li2 MnO3 ) and a monoclinic defective supercell derived from the rhombohedral R 3 ¯ $\bar{3}$ m unit cell (prototype LiCoO2 ).

Improvement of stability and capacity of Co-free, Li-rich layered oxide Li1.2Ni0.2Mn0.6O2 cathode material through defect control.

Layered oxides based on manganese (Mn), rich in lithium (Li), and free of cobalt (Co) are the most promising cathode candidates used for lithium-ion batteries due to their high capacity, high voltage and low cost. These types of material can be written as xLi2MnO3·(1 - x) LiTMO2 (TM = Ni,Mn,etc.). Though, Li2MnO3 is known to have poor cycling stability and low capacity, which hinder its industrial application commercially. In this work, Li1.2Ni0.2Mn0.6O2 materials with different amounts of structural defects was successfully synthesized using powder metallurgy followed by different cooling processes in order to improve its electrochemical properties. Microstructural analyses and electrochemical measurements were carried out on the study samples synthesized by a combination of X-ray diffraction, transmission electron microscopy, and cyclic voltammetry. It is found that the disorder of the transition metal layer in Li2MnO3 promotes its electrochemical activity, whereas the Li/Ni antisites of the Li layer maintain the stability of its local structure. The material with optimal amount of structural defects had an initial capacity of 188.2 mAh g-1, while maintaining an excellent specific capacity of 144.2 mAh g-1 after 500 cycles at 1C. In comparison, Li1.2Ni0.2Mn0.6O2 without structural defect only gives a capacity of 40.8 mAh g-1 after cycling. This microstructural control strategy provides a simple and effective route to develop high-performance Co-free, Li-rich Mn-based cathode materials and scale-up manufacturing.

Feasibility to Improve the Stability of Lithium-Rich Layered Oxides by Surface Doping.

Li-rich layer-structured oxides are considered promising cathode materials for their specific capacities above 250 mAh·g-1. However, the drawbacks such as poor rate performance, fast capacity fading, and the continuous transition metal (TM) migration into the Li layer hinder their commercial applications. To address these issues, surface doping of Ti and Zr was conducted to the Li- and Mn-rich layered oxide (LMR), Li1.2Mn0.54Ni0.13Co0.13O2. The drop of the average discharge potentials of the Ti- and Zr-doped LMR was reduced by 593 and 346 mV in 100 cycles, respectively. With aberration-corrected scanning transmission electron microscopy and electron energy loss spectroscopy, we clarified that Ti4+ and Zr4+ ions are located near the surface of the material, anchor the surface oxygen, and stabilize the LMR structure. The difference in the strengths of the Ti-O and Zr-O bonds and the doping-resultant electronic structures were determined with density functional theory (DFT) calculations and soft X-ray absorption spectroscopy (SXAS), responsible for the electrochemical performance of surface-doped materials. These findings verify our modification strategies to enhance the cycling performances of the promising LMR cathode materials.

Phase Compatible NiFe2O4 Coating Tunes Oxygen Redox in Li-Rich Layered Oxide.

Li-rich layered oxides have attracted intense attention for lithium-ion batteries, as provide substantial capacity from transition metal cation redox simultaneous with reversible oxygen-anion redox. However, unregulated irreversible oxygen-anion redox leads to critical issues such as voltage fade and oxygen release. Here, we report a feasible NiFe2O4 (NFO) surface-coating strategy to turn the nonbonding coordination of surface oxygen into metal-oxygen decoordination. In particular, the surface simplex M-O (M = Ni, Co, Mn from MO6 octahedra) and N-O (N = Ni, Fe from NO6 octahedra) bonds are reconstructed in the form of M-O-N bonds. By applying both in operando and ex situ technologies, we found this heterostructural interface traps surface lattice oxygen, as well as restrains cation migration in Li-rich layered oxide during electrochemical cycling. Therefore, surface lattice oxygen behavior is significantly sustained. More interestingly, we directly observe the surface oxygen redox decouple with cation migration. In addition, the NFO-coating blocks HF produced from electrolyte decomposition, resulting in reducing the dissolution of Mn. With this strategy, higher cycle stability (91.8% at 1 C after 200 cycles) and higher rate capability (109.4 mA g-1 at 1 C) were achieved in this work, compared with pristine Li-rich layered oxide. Our work offers potential for designing electrode materials utilizing oxygen redox chemistry.

Synergetic effect of high Ni ratio and low oxygen defect interface zone of single crystals on the capacity retention of lithium rich layered oxides.

Li-rich layered oxides (LLOs) are promising cathode materials for Li-ion batteries owing to their high capacities (>250 mAh g-1), however, they suffered from severe capacity and voltage fading caused by irreversible oxygen loss and phase changes. Herein, the structural stability of single crystalline and polycrystalline Li1.14Ni0.32Mn0.44Co0.04O2 was compared in detail. It was found that the stability of oxidized oxygen ions on the near surface was improved in single crystals, which retarded oxygen loss from surface and surficial phase changes, possibly owing to the facet regulating and low surface curvature. In addition, the formation-migration of Mn3+, one of the crucial factors that caused capacity fading of LLOs, can be mitigated by increasing Ni3+ ratio. Under the synergistic effect of low oxygen defects on the near surface and high Ni3+ ratio, stable cycling performances and higher thermal stability were obtained.

Effect of Different Composition on Voltage Attenuation of Li-Rich Cathode Material for Lithium-Ion Batteries.

Li-rich layered oxide cathode materials have become one of the most promising cathode materials for high specific energy lithium-ion batteries owning to its high theoretical specific capacity, low cost, high operating voltage and environmental friendliness. Yet they suffer from severe capacity and voltage attenuation during prolong cycling, which blocks their commercial application. To clarify these causes, we synthesize Li1.5Mn0.55Ni0.4Co0.05O2.5 (Li1.2Mn0.44Ni0.32Co0.04O2) with high-nickel-content cathode material by a solid-sate complexation method, and it manifests a lot slower capacity and voltage attenuation during prolong cycling compared to Li1.5Mn0.66Ni0.17Co0.17O2.5 (Li1.2Mn0.54Ni0.13Co0.13O2) and Li1.5Mn0.65Ni0.25Co0.1O2.5 (Li1.2Mn0.52Ni0.2Co0.08O2) cathode materials. The capacity retention at 1 C after 100 cycles reaches to 87.5% and the voltage attenuation after 100 cycles is only 0.460 V. Combining X-ray diffraction (XRD), scanning electron microscope (SEM), and transmission electron microscopy (TEM), it indicates that increasing the nickel content not only stabilizes the structure but also alleviates the attenuation of capacity and voltage. Therefore, it provides a new idea for designing of Li-rich layered oxide cathode materials that suppress voltage and capacity attenuation.

Novel MOF shell-derived surface modification of Li-rich layered oxide cathode for enhanced lithium storage.

Li-rich layered oxide materials have attracted increasing attention because of their high specific capacity (>250 mAh g-1). However, these materials typically suffer from poor cycling stability and low rate performance. Herein, we propose a facile and novel metal-organic-framework (MOF) shell-derived surface modification strategy to construct NiCo nanodots decorated (∼5 nm in diameter) carbon-confined Li1.2Mn0.54Ni0.13Co0.13O2 nanoparticles (LLO@C&NiCo). The MOF shell is firstly formed on the surface of as-prepared Li1.2Mn0.54Ni0.13Co0.13O2 nanoparticles via low-pressure vapor superassembly and then is in situ converted to the NiCo nanodots decorated carbon shell after subsequent controlled pyrolysis. The obtained LLO@C&NiCo cathode exhibits enhanced cycling and rate capability with a capacity retention of 95% after 100 cycles at 0.4 C and a high capacity of 159 mAh g-1 at 5 C, respectively, compared with those of LLO (75% and 105 mAh g-1). The electrochemical impedance spectroscopy and selected area electron diffraction analyses after cycling demonstrate that the thin C&NiCo shell can endow LLO with high electronic conductivity and structural stability, indicating the undesired formation of the spinel phase initiated from the particle surface is efficiently suppressed. Therefore, this presented strategy may open a new avenue on the design of high-performance electrode materials for energy storage.

Tailoring Anisotropic Li-Ion Transport Tunnels on Orthogonally Arranged Li-Rich Layered Oxide Nanoplates Toward High-Performance Li-Ion Batteries.

High-performance Li-rich layered oxide (LRLO) cathode material is appealing for next-generation Li-ion batteries owing to its high specific capacity (>300 mAh g-1). Despite intense studies in the past decade, the low initial Coulombic efficiency and unsatisfactory cycling stability of LRLO still remain as great challenges for its practical applications. Here, we report a rational design of the orthogonally arranged {010}-oriented LRLO nanoplates with built-in anisotropic Li+ ion transport tunnels. Such a novel structure enables fast Li+ ion intercalation and deintercalation kinetics and enhances structural stability of LRLO. Theoretical calculations and experimental characterizations demonstrate the successful synthesis of target cathode material that delivers an initial discharge capacity as high as 303 mAh g-1 with an initial Coulombic efficiency of 93%. After 200 cycles at 1.0 C rate, an excellent capacity retention of 92% can be attained. Our method reported here opens a door to the development of high-performance Ni-Co-Mn-based cathode materials for high-energy density Li-ion batteries.