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

One-Step Surface-to-Bulk Modification of High-Voltage and Long-Life LiCoO2 Cathode with Concentration Gradient Architecture

Raising the charging cut-off voltage of layered oxide cathodes can improve their energy density. However, it inevitably introduces instabilities regarding both bulk structure and surface/interface. Herein, exploiting the unique characteristics of high-valence Nb5+ element, a synchronous surface-to-bulk-modified LiCoO2 featuring Li3 NbO4 surface coating layer, Nb-doped bulk, and the desired concentration gradient architecture through one-step calcination is achieved. Such a multifunctional structure facilitates the construction of high-quality cathode/electrolyte interface, enhances Li+ diffusion, and restrains lattice-O loss, Co migration, and associated layer-to-spinel phase distortion. Therefore, a stable operation of Nb-modified LiCoO2 half-cell is achieved at 4.6 V (90.9% capacity retention after 200 cycles). Long-life 250 Wh kg-1 and 4.7 V-class 550 Wh kg-1 pouch cells assembled with graphite and thin Li anodes are harvested (both beyond 87% after 1600 and 200 cycles). This multifunctional one-step modification strategy establishes a technological paradigm to pave the way for high-energy density and long-life lithium-ion cathode materials.

Metal-organic frameworks (MOFs) for energy production and gaseous fuel and electrochemical energy storage applications

The increasing energy demands in society and industrial sectors have inspired the search for alternative energy sources that are renewable and sustainable, also driving the development of clean energy storage and delivery systems. Various solid-state materials (e.g., oxides, sulphides, polymer and conductive nanomaterials, activated carbon and their composites) have been developed for energy production (water splitting-H2 production), gaseous fuel (H2 and CH4) storage and electrochemical energy storage (batteries and supercapacitors) applications. Nevertheless, the low surface area, pore volume and conductivity, and poor physical and chemical stability of the reported materials have resulted in higher requirements and challenges in the development of energy production and energy storage technologies. Thus, to overcome these issues, the development of metal-organic frameworks (MOFs) has attracted significant attention. MOFs are a class of porous materials with extremely high porosity and surface area, structural diversity, multifunctionality, and chemical and structural stability, and thus they can be used in a wide range of applications. In the present review, we precisely discuss the interesting properties of MOFs and the various methodologies for their synthesis, and also the future dependence on the valorization of solid waste for the recovery of metals and organic ligands for the synthesis of new classes of MOFs. Subsequently, the utilization of these interesting characteristics for energy production (water splitting), storage of gaseous fuels (H2 and CH4), and electrochemical storage (batteries and supercapacitors) applications are described. However, although MOFs are efficient materials with versatile uses, they still have many challenges, limiting their practical applications. Therefore, finally, we highlight the challenges associated with MOFs and show the way forward in overcoming them for the development of these highly porous materials with large-scale practical utility.

Zinc-Doping Strategy on P2-Type Mn-Based Layered Oxide Cathode for High-Performance Potassium-ion Batteries

Mn-based layered oxide is extensively investigated as a promising cathode material for potassium-ion batteries due to its high theoretical capacity and natural abundance of manganese. However, the Jahn-Teller distortion caused by high-spin Mn3+ (t2g 3 eg 1 ) destabilizes the host structure and reduces the cycling stability. Here, K0.02 Na0.55 Mn0.70 Ni0.25 Zn0.05 O2 (denoted as KNMNO-Z) is reported to inhibit the Jahn-Teller effect and reduce the irreversible phase transition. Through the implementation of a Zn-doping strategy, higher Mn valence is achieved in the KNMNO-Z electrode, resulting in a reduction of Mn3+ amount and subsequently leading to an improvement in cyclic stability. Specifically, after 1000 cycles, a high retention rate of 97% is observed. Density functional theory calculations reveals that low-valence Zn2+ ions substituting the transition metal position of Mn regulated the electronic structure around the MnO bonding, thereby alleviating the anisotropic coupling between oxidized O2- and Mn4+ and improving the structural stability. K0.02 Na0.55 Mn0.70 Ni0.25 Zn0.05 O2 provided an initial discharge capacity of 57 mAh g-1 at 100 mA g-1 and a decay rate of only 0.003% per cycle, indicating that the Zn-doped strategy is effective for developing high-performance Mn-based layered oxide cathode materials in PIBs.

Research Progress in Metal-Organic Framework Based Nanomaterials Applied in Battery Cathodes

Metal-Organic Frameworks have attracted profound attention the latest years for use in environmental applications. They can offer a broad variety of functions due to their tunable porosity, high surface area and metal activity centers. Not more than ten years ago, they have been applied experimentally for the first time in energy storage devices, such as batteries. Specifically, MOFs have been investigated thoroughly as potential materials hosting the oxidizing agent in the cathode electrode of several battery systems such as Lithium Batteries, Metal-Ion Batteries and Metal-Air Batteries. The aim of this review is to provide researchers with a summary of the electrochemical properties and performance of MOFs recently implemented in battery cathodes in order to provide fertile ground for further exploration of performance-oriented materials. In the following sections, the basic working principles of each battery system are briefly defined, and special emphasis is dedicated to MOF-based or MOF-derived nanomaterials, especially nanocomposites, which have been tested as potential battery cathodes.

Exploring the Effect of a MnO2 Coating on the Electrochemical Performance of a Li1.2Mn0.54Ni0.13Co0.13O2 Cathode Material

The effect of electrochemically active MnO2 as a coating material on the electrochemical properties of a Li1.2Mn0.54Ni0.13Co0.13O2 (LTMO) cathode material is explored in this article. The structural analysis indicated that the layered structure of the LTMO was unchanged after the modification with MnO2. The morphology inspection demonstrated that the rod-like LTMO particles were encapsulated by a compact coating layer. The MnO2 layer was able to hinder the electrolyte solution from corroding the LTMO particles and optimized the formation of a solid electrolyte interface (SEI). Meanwhile, lithium ions were reversibly inserted into and extracted from MnO2, which afforded an additional capacity. Compared with the bare LTMO, the MnO2-coated sample exhibited enhanced electrochemical performance. After the MnO2 coating, the first discharge capacity rose from 224.2 to 239.1 mAh/g, and the initial irreversible capacity loss declined from 78.2 to 46.0 mAh/g. Meanwhile, the cyclic retention climbed up to 88.2% after 100 cycles at 0.5 C, which was more competitive than that of the bare LTMO with a value of 71.1%. When discharging at a high current density of 2 C, the capacity increased from 100.5 to 136.9 mAh/g after the modification. These investigations may be conducive to the practical application of LTMO in prospective automotive Li-ion batteries.

Metal-Organic Frameworks and Their Derivatives: Designing Principles and Advances toward Advanced Cathode Materials for Alkali Metal Ion Batteries

Metal-organic frameworks (MOFs) and their derivatives have attracted enormous attention in the field of energy storage, due to their high specific surface area, tunable structure, highly ordered pores, and uniform metal sites. Compared with the wide research of MOFs and their related materials on anode materials for alkali metal ion batteries, few works are on cathode materials. In this review, design principles for promoting the electrochemical performance of MOF-related materials in terms of component/structure design, composite fabrication, and morphology engineering are presented. By summarizing the advancement of MOFs and their derivatives, Prussian blue and its analogs, and MOF surface coating, challenges and opportunities for future outlooks of MOF-related cathode materials are discussed.

Multiscale Deficiency Integration by Na-Rich Engineering for High-Stability Li-Rich Layered Oxide Cathodes

Lithium-rich manganese-based (LRM) layered oxides are considered as one of the most promising cathode materials for next-generation high-energy-density lithium-ion batteries (LIBs) because of their high specific capacity (>250 mAh g^-1). However, they also go through severe capacity decay, serious voltage fading, and poor rate capability during cycling. Herein, a multiscale deficiency integration, including surface coating, subsurface defect construction, and bulk doping, is realized in a Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ cathode material by facile Na-rich engineering through a sol-gel method. This multiscale design can significantly improve the bulk and surface structural stability and diffusion rate of Li⁺ ions of electrode materials. Specifically, an outstanding specific capacity of 201 mAh g^-1 is delivered at 1C of the designed cathode material after 400 cycles, relating to a large capacity retention of 89.0%. Meanwhile, the average voltage is retained up to 3.13 V with a large voltage retention of 89.6% and the energy density is maintained at 627.4 Wh kg^-1. In situ X-ray diffraction (XRD), ex situ transmission electron microscopy (TEM) investigations, and density functional theory (DFT) calculations are conducted to explain the greatly enhanced electrochemical properties of a LRM cathode. We believe that this strategy would be a meaningful reference of LRM cathode materials for the research in the future.

Facile formation of tetragonal-Nb2O5 microspheres for high-rate and stable lithium storage with high areal capacity

Niobium pentoxide (Nb₂O₅) has attracted great attention as an anode for lithium-ion battery, which is attributed to the high-rate and good stability performances. In this work, TT-, T-, M-, and H-Nb₂O₅ microspheres were synthesized by a facile one-step thermal oxidation method. Ion and electron transport properties of Nb₂O₅ with different phases were investigated by both electrochemical analyses and density functional theoretical calculations. Without nanostructuring and carbon modification, the tetragonal Nb₂O₅ (M-Nb₂O₅) displays preferable rate capability (121 mAh g^-1 at 5 A g^-1), enhanced reversible capacity (163 mAh g^-1 at 0.2 A g^-1) and better cycling stability (82.3% capacity retention after 1000 cycles) when compared with TT-, T-, and H-Nb₂O₅. Electrochemical analyses further reveal the diffusion-controlled Li⁺ intercalation kinetics and in-situ X-ray diffraction analysis indicates superior structural stability upon Li⁺ intercalation/deintercalation. Benefiting from the intrinsic fast ion/electron transport, a high areal capacity of 2.24 mAh cm^-2 is obtained even at an ultrahigh mass loading of 22.51 mg cm^-2. This work can promote the development of Nb₂O₅ materials for high areal capacity and stable lithium storage towards practical applications.

Advances in metal-organic framework coatings: versatile synthesis and broad applications

Metal-organic frameworks (MOFs) as a new kind of porous crystalline materials have attracted much interest in many applications due to their high porosity, diverse structures, and controllable chemical structures. However, the specific geometrical morphologies, limited functions and unsatisfactory performances of pure MOFs hinder their further applications. In recent years, an efficient approach to synthesize new composites to overcome the above issues has been achieved, by integrating MOF coatings with other functional materials, which have synergistic advantages in many potential applications, including batteries, supercapacitors, catalysis, gas storage and separation, sensors, drug delivery/cytoprotection and so on. Nevertheless, the systemic synthesis strategies and the relationships between their structures and application performances have not been reviewed comprehensively yet. This review emphasizes the recent advances in versatile synthesis strategies and broad applications of MOF coatings. A comprehensive discussion of the fundamental chemistry, classifications and functions of MOF coatings is provided first. Next, by modulating the different states (e.g. solid, liquid, and gas) of metal ion sources and organic ligands, the synthesis methods for MOF coatings on functional materials are systematically summarized. Then, many potential applications of MOF coatings are highlighted and their structure-property correlations are discussed. Finally, the opportunities and challenges for the future research of MOF coatings are proposed. This review on the deep understanding of MOF coatings will bring better directions into the rational design of high-performance MOF-based materials and open up new opportunities for MOF applications.

Hollow Fe3O4/carbon with surface mesopores derived from MOFs for enhanced lithium storage performance

Hollow metal-organic frameworks (MOFs) and their derivatives have attracted more and more attention due to their high specific surface area and perfect morphological structure, which determine their large potential application in energy storage and catalysis fields. However, few researchers have carried out further modification on the outer shell of hollow MOFs, such as the perforation modification, which will endow hollow nanomaterials derived from MOFs with multifunctionality. In this paper, hollow MOFs of MIL-53(Fe) with perforated outer surface are successfully synthesized by using SiO₂ nanospheres as the template via a self-assembly process induced by the coordination polymerization. The tightly packed mesopore structure makes the carbon outer shell of MOFs thinner, thus realizing the in-situ transformation from MOFs to hollow Fe₃O₄/carbon, which exhibits perfect capacity approaching 1270 mA h g^-1 even after 200 cycles at 0.1 A g^-1, as an anode material in lithium ion batteries (LIBs) application. This research provides a new strategy for the design and preparation of MOFs and their derivatives with multifunctionality for the energy applications.

Rational Design of Unique ZnO/ZnS@N-C Heterostructures for High-Performance Lithium-Ion Batteries

Conversion-type anodes with high theoretical capacity have attracted enormous interest for lithium storage, although their extremely poor conductivity and volume variations during lithiation-delithiation processes seriously limit their practical applications. Herein, a facile strategy to fabricate ZnO/ZnS@N-C heterostructures decorated on carbon nanotubes (ZnO/ZnS@N-C/CNTs) with metal-organic framework assistance is developed. The as-prepared anodes display higher reversible capacity of 1020.6 mAh g^-1 at 100 mA g^-1 after 200 cycles and excellent high-cyclability with 386.6 mAh g^-1 at 1000 mA g^-1 over 400 cycles. The conductive CNT network and N-doped carbon shell could successfully improve the electrical conductivity and avoid the aggregation of ultrasmall ZnO/ZnS nanoparticles. The results calculated from density functional theory also suggest that the ZnO/ZnS heterostructures could promote electron-transfer capability.

Local Electric-Field-Driven Fast Li Diffusion Kinetics at the Piezoelectric LiTaO3 Modified Li-Rich Cathode-Electrolyte Interphase

As one of the most promising cathodes for next-generation lithium ion batteries (LIBs), Li-rich materials have been extensively investigated for their high energy densities. However, the practical application of Li-rich cathodes is extremely retarded by the sluggish electrode-electrolyte interface kinetics and structure instability. In this context, piezoelectric LiTaO3 is employed to functionalize the surface of Li1.2Ni0.17Mn0.56Co0.07O2 (LNMCO), aiming to boost the interfacial Li+ transport process in LIBs. The results demonstrate that the 2 wt% LiTaO3-LNMCO electrode exhibits a stable capacity of 209.2 mAh g-1 at 0.1 C after 200 cycles and 172.4 mAh g-1 at 3 C. Further investigation reveals that such superior electrochemical performances of the LiTaO3 modified electrode results from the additional driving force from the piezoelectric LiTaO3 layer in promoting Li+ diffusion at the interface, as well as the stabilized bulk structure of LNMCO. The supplemented LiTaO3 layer on the LNMCO surface herein, sheds new light on the development of better Li-rich cathodes toward high energy density applications.

A New Type of Li-Rich Rock-Salt Oxide Li2 Ni1/3 Ru2/3 O3 with Reversible Anionic Redox Chemistry

Li-rich oxide cathodes are of prime importance for the development of high-energy lithium-ion batteries (LIBs). Li-rich layered oxides, however, always undergo irreversible structural evolution, leading to inevitable capacity and voltage decay during cycling. Meanwhile, Li-rich cation-disordered rock-salt oxides usually exhibit sluggish kinetics and inferior cycling stability, despite their firm structure and stable voltage output. Herein, a new Li-rich rock-salt oxide Li₂ Ni_1/3 Ru_2/3 O₃ with Fd-3m space group, where partial cation-ordering arrangement exists in cationic sites, is reported. Results demonstrate that a cathode fabricated from Li₂ Ni_1/3 Ru_2/3 O₃ delivers a large capacity, outstanding rate capability as well as good cycling performance with negligible voltage decay, in contrast to the common cations disordered oxides with space group Fm-3m. First principle calculations also indicate that rock-salt oxide with space group Fd-3m possesses oxygen activity potential at the state of delithiation, and good kinetics with more 0-TM (TM = transition metals) percolation networks. In situ Raman results confirm the reversible anionic redox chemistry, confirming O^2- /O^- evolution during cycles in Li-rich rock-salt cathode for the first time. These findings open up the opportunity to design high-performance oxide cathodes and promote the development of high-energy LIBs.

Understanding the Air-Exposure Degradation Chemistry at a Nanoscale of Layered Oxide Cathodes for Sodium-Ion Batteries

Undesired reactions between layered sodium transition-metal oxide cathodes and air impede their utilization in practical sodium-ion batteries. Consequently, a fundamental understanding of how layered oxide cathodes degrade in air is of paramount importance, but it has not been fully understood yet. Here a comprehensive study on a model material NaNi_0.7Mn_0.15Co_0.15O₂ reveals its reaction chemistry with air and the dynamic evolution of the degradation species upon air exposure. We find that besides the extraction of Na⁺ ions from the crystal lattice to form NaOH, Na₂CO₃, and Na₂CO₃·H₂O in contact with air, nickel ions gradually dissolve from the bulk to form NiO and accumulate on the particle surface as revealed by subnanometer surface-sensitive time-of-flight secondary ion mass spectroscopy. The degradation species on the surface are insulating, leading to an increase in interfacial resistance and declined electrochemical performance. We also demonstrate a feasible surface coating strategy for suppressing the unfavorable degradation process. Understanding the degradation mechanism at a nanoscale can facilitate the future development of high-energy cathodes for sodium-ion batteries.

Enhancing the Structural Stability of Ni-Rich Layered Oxide Cathodes with a Preformed Zr-Concentrated Defective Nanolayer

Nickel-rich layered oxides (NLOs) exhibit great potential to meet the ever-growing demand for further increases in the energy density of Li-ion batteries because of their high specific capacities. However, NLOs usually suffer from severe structural degradation and undesired side reactions when cycled above 4.3 V. These effects are strongly correlated with the surface structure and chemistry of the active NLO materials. Herein, we demonstrate a preformed cation-mixed ( Fm3̅ m) surface nanolayer (∼5 nm) that shares a consistent oxygen framework with the layered lattice through Zr modification, in which Ni cations reside in Li slabs and play the role of a "pillar". This preformed nanolayer alleviates the detrimental phase transformations upon electrochemical cycling, effectively enhancing the structural stability. As a result, the Zr-modified Li(Ni_0.8Co_0.1Mn_0.1)_0.985Zr_0.015O₂ material exhibits a high reversible discharge capacity of ∼210 mA h/g at 0.1 C (1 C = 200 mA/g) and outstanding cycling stability with a capacity retention of 93.2% after 100 cycles between 2.8 and 4.5 V. This strategy may be further extended to design and prepare other high-performance layered oxide cathode materials.

Porous CaFe2O4 as a promising lithium ion battery anode: a trade-off between high capacity and long-term stability

Metal oxides are considered as attractive candidates as anode materials for lithium ion batteries (LIBs) due to their high capacities compared to commercialized graphite. However, fast capacity fading, which is caused by inherent large volume expansions and agglomeration of active particles upon cycling, is a great challenge. Herein, we propose the design of porous CaFe2O4 electrode material to address the above issue. Compared to pristine iron oxides, CaFe2O4 exhibits a distinct trade-off in terms of high capacity and long-term stability, which is beneficial to the potential practical applications. Such a trade-off effect is attributed to the synergistic effect between the porous structure and the in situ formed CaO nanograins during charging/discharging processes. This work provides an effective strategy in achieving anode materials with high capacity and long-term stability for next-generation LIBs.