Ultrathin spinel membrane-encapsulated layered lithium-rich cathode material for advanced Li-ion batteries

Exceptional Rate Performances of Li-Rich Mn-Based Cathodes Enabled by Boron-Based Additives-Driven Self-Optimized Interface

Li-rich Mn-based cathode material (LRM), as a promising cathode for high energy density lithium batteries, suffers from severe side reactions in conventional lithium hexafluorophosphate (LiPF6)-based carbonate electrolytes, leading to unstable interfaces and poor rate performances. Herein, a boron-based additives-driven self-optimized interface strategy is presented to dissolve low ionic conductivity LiF nanoparticles at the outer cathode electrolyte interface, leading to the optimized interfacial components, as well as the enhanced Li ion migration rate in electrolytes. Being attributed to these superiorities, the LRM||Li battery delivers a high-capacity retention of 92.19% at 1C after 200 cycles and a low voltage decay of 1.08 mV/cycle. This work provides a new perspective on the rational selection of functional additives with an interfacial self-optimized characteristic to achieve a long lifespan LRM with exceptional rate performances.

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.

Effects of Synthesis Conditions of Na0.44MnO2 Precursor on the Electrochemical Performance of Reduced Li2MnO3 Cathode Materials for Lithium-Ion Batteries

Li2MnO3 nanobelts have been synthesized via the molten salt method that used the Na0.44MnO2 nanobelts as both the manganese source and precursor template in LiNO3-LiCl eutectic molten salt. The electrochemical properties of Li2MnO3 reduced via a low-temperature reduction process as cathode materials for lithium-ion batteries have been measured and compared. Particularly investigated in this work are the effects of the synthesis conditions, such as reaction temperature, molten salt contents, and reaction time on the morphology and particle size of the synthesized Na0.44MnO2 precursor. Through repeated synthesis characterizations of the Na0.44MnO2 precursor, and comparing the electrochemical properties of the reduced Li2MnO3 nanobelts, the optimum conditions for the best electrochemical performance of the reduced Li2MnO3 are determined to be a molten salt reaction temperature of 850 °C and a molten salt amount of 25 g. When charge-discharged at 0.1 C (1 C = 200 mAh g-1) with a voltage window between 2.0 and 4.8 V, the reduced Li2MnO3 synthesized with reaction temperature of Na0.44MnO2 precursor at 850 °C and molten salt amounts of 25 g exhibits the best rate performance and cycling performance. This work develops a new strategy to prepare manganese-based cathode materials with special morphology.

In situ epitaxial growth and electrochemical conversion of LiNi0.5Mn1.5O4 thin layer on Ni-rich cathode materials for high voltage lithium-ion batteries

Ni-rich LiNi_xCo_yMn_1-x-yO₂ (0.5 < x < 1) cathode materials have attracted considerable interest due to their high energy density and low cost. However, they are subject to capacity fading during cycling, such as structural degradation and irreversible oxygen release, especially under high voltage. Herein, we report an in situ epitaxial growth strategy to construct a thin layer of LiNi_0.25Mn_0.75O₂ on the surface of LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811). Both of them share the same crystal structure. Interestingly, the LiNi_0.25Mn_0.75O₂ layer can be electrochemically converted into a stable spinel LiNi_0.5Mn_1.5O₄ (LNM) due to the Jahn-Teller effect under high voltage cycling. The derived LNM protective layer can effectively alleviate the harmful side reactions between the electrode and electrolyte and suppress oxygen release as well. Furthermore, the coating LNM layer can enhance Li⁺ ion diffusion due to its three-dimensional channels for Li⁺ ion transport. When used as half-cells with lithium as the anode, NCM811@LNM-1% realizes a large reversible capacity of 202.4 mA h g^-1 at 0.5 C, with high capacity retention of 86.52% at 0.5 C and 82.78% at 1 C, respectively, after 200 cycles in the voltage range of 2.8-4.5 V. Moreover, the assembled pouch full-cell with NCM811@LNM-1% as cathode and commercial graphite as an anode can deliver 11.63 mA h capacity with a high capacity retention of 80.05% after 139 cycles in the same voltage range. This work demonstrates a facile approach to the fabrication of NCM811@LNM cathode materials for enhancing performance in lithium-ion batteries under high voltage, rendering its promising applications.

In Situ Formed Heterostructure Interface and Well-Tuned Electronic Structure Ensuring Long Cycle Stability for 4.9 V High-Voltage Li-Rich Layered Oxide Cathodes

High-voltage lithium-rich manganese-based layered oxides (LMLOs) are considered as the most competitive cathode materials for next-generation high-energy-density lithium-ion batteries (LIBs). However, LMLOs still suffer from irreversible lattice oxygen release, uncontrollable interface side reactions, and surface structural degradation. Herein, we propose an integration strategy combining La/Al codoping and Li_xCoPO₄ nanocoating to improve the electrochemical performance of LMLOs comprehensively. La/Al codoping regulates the electronic structure to enhance the redox activity of anions and cations and inhibit structural degradation. The Li_xCoPO₄ nanocoating formed by in situ reaction with the surface residual lithium can not only promote Li-ion migration but also reduce interfacial side reactions. The induced Layered@Rocksalt@Li_xCoPO₄ heterostructure suppresses lattice volume variation and structural degradation during cycling. Under the synergistic effect of the heterostructure interface and well-tuned electronic structure, the capacity retention rate of comodified LMLO materials reaches 80.06% after 500 cycles (2.0-4.65 V) and 75.1% after 340 cycles at 1C under a high cut-off voltage of 4.9 V.

Stabilizing Li-Rich Layered Cathode Materials Using a LiCoMnO4 Spinel Nanolayer for Li-Ion Batteries

Lithium-rich cathodes have excess lithium in the transition metal layer and exhibit an extremely high specific capacity and good energy density. However, they still have some disadvantages. Here, we propose LiCoMnO₄, a new nanolayer coating material with a spinel structure, to modify the surface of lithium cathode oxide (Li_7/6Mn_1/2Ni_1/6Co_1/6O₂) with a layered structure. The designed cathode with nanolayer spinel coating delivers an excellent reversible capacity, outstanding rate capability, and superior cycling ability whilst exhibiting discharge capacities of 300, 275, 220, and 166 mAh g^-1 at rates of 0.1 C at 2.0-4.8 V formation and 0.1, 1, and 5 C, respectively, between 2.0 and 4.6 V. The cycling ability and voltage fading at a high operational voltage of 4.9 V were also investigated, with results showing that the nanolayer spinel coating can depress the surface of the lithium cathode oxide layer, leading to phase transformation that enhances the electrochemical performance.

Surface LiMn1.4Ni0.5Mo0.1O4 Coating and Bulk Mo Doping of Li-Rich Mn-Based Li1.2Mn0.54Ni0.13Co0.13O2 Cathode with Enhanced Electrochemical Performance for Lithium-Ion Batteries

To improve the initial Coulombic efficiency, cycling stability, and rate performance of the Li-rich Mn-based Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ cathode, the combination of LiMn_1.4Ni_0.5Mo_0.1O₄ coating with Mo doping has been successfully carried out by the sol-gel method and subsequent dip-dry process. This strategy buffers the electrodes from the corrosion of electrolyte and enhances the lattice parameter, which could inhibit the oxygen release and maintain the structural stability, thus improving the cycle stability and rate capability. After LiMn_1.4Ni_0.5Mo_0.1O₄ modification, the initial discharge capacity reaches 272.4 mAh g^-1 with a corresponding initial Coulombic efficiency (ICE) of 84.2% at 0.1C (1C = 250 mAh g^-1), far higher than those (221.5 mAh g^-1 and 68.9%) of the pristine sample. Besides, the capacity retention of the coated sample is enhanced by up to 66.8% after 200 cycles at 0.1C. Especially, the rate capability of the coated sample is 95.2 mAh g^-1 at 5C. XRD, SEM, TEM, XPS, and Raman spectroscopy are adopted to characterize the morphologies and structures of the samples. This coating strategy has been demonstrated to be an effective approach to construct high-performance energy storage devices.

Layered-rocksalt intergrown cathode for high-capacity zero-strain battery operation

The dependence on lithium-ion batteries leads to a pressing demand for advanced cathode materials. We demonstrate a new concept of layered-rocksalt intergrown structure that harnesses the combined figures of merit from each phase, including high capacity of layered and rocksalt phases, good kinetics of layered oxide and structural advantage of rocksalt. Based on this concept, lithium nickel ruthenium oxide of a main layered structure (R\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\bar{3}$$\end{document}3¯m) with intergrown rocksalt (Fm\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\bar{3}$$\end{document}3¯m) is developed, which delivers a high capacity with good rate performance. The interwoven rocksalt structure successfully prevents the anisotropic structural change that is typical for layered oxide, enabling a nearly zero-strain operation upon high-capacity cycling. Furthermore, a design principle is successfully extrapolated and experimentally verified in a series of compositions. Here, we show the success of such layered-rocksalt intergrown structure exemplifies a new battery electrode design concept and opens up a vast space of compositions to develop high-performance intergrown cathode materials.

The dependence on lithium-ion batteries leads to a pressing demand for advanced cathode materials. Here the authors report a new concept of layered-rocksalt intergrown structure that enables nearly zero-strain operation upon high-capacity cycling, offering tremendous opportunities to design new cathodes.

Accelerating the activation of Li2MnO3 in Li-rich high-Mn cathodes to improve its electrochemical performance

Li-rich high-Mn oxides, xLi2MnO3·(1 - x)LiMO2 (x ≥ 0.5, M = Co, Ni, Mn…), have attracted extensive research interest due to their high specific capacity and low cost. However, slow Li2MnO3 activation and poor cycling stability have affected their electrochemical performance. Herein, to solve these problems, morphology regulation and LiAlF4 coating strategies have been synergistically applied to a Li-rich high-Mn material Li1.7Mn0.8Co0.1Ni0.1O2.7 (HM-811). This dual-strategy successfully promotes the activation process of the Li2MnO3 phase and thus improves the electrochemical performance of HM-811. Theoretical computation indicates that the LiAlF4 layer has a lower Li+ migration barrier than the HM-811 matrix, so it could boost the diffusion of Li+ ions and promote the activation of the Li2MnO3 phase. Benefiting from the morphology regulation and LiAlF4 coating, the HM-811 cathode shows a high initial charge capacity of >300 mA h g-1. In addition, the modified HM-811 could deliver superior electrochemical performance even at a low temperature of -20 °C. This work provides a new approach for developing high performance cathode materials for next-generation Li-ion batteries.

Nano Polymorphism-Enabled Redox Electrodes for Rechargeable Batteries

Nano polymorphism (NPM), as an emerging research area in the field of energy storage, and rechargeable batteries, have attracted much attention recently. In this review, the recent progress on the composition and formation of polymorphs, and the evolution processes of different redox electrodes in rechargeable metal-ion, metal-air, and metal-sulfur batteries are highlighted. First, NPM and its significance for rechargeable batteries are discussed. Subsequently, the current NPM modulation strategies of different types of representative electrodes for their corresponding rechargeable battery applications are summarized. The goal is to demonstrate how NPM could tune the intrinsic material properties, and hence, improve their electrochemical activities for each battery type. It is expected that the analysis of polymorphism and electrochemical properties of materials could help identify some "processing-structure-properties" relationships for material design and performance enhancement. Lastly, the current research challenges and potential research directions are discussed to offer guidance and perspectives for future research on NPM engineering.

A chemically stabilized sulfur cathode for lean electrolyte lithium sulfur batteries

Lithium sulfur batteries (LSBs) are promising next-generation rechargeable batteries due to the high gravimetric energy, low cost, abundance, nontoxicity, and high sustainability of sulfur. However, the dissolution of high-order polysulfide in electrolytes and low Coulombic efficiency of Li anode require excess electrolytes and Li metal, which significantly reduce the energy density of LSBs. Quasi-solid-state LSBs, where sulfur is encapsulated in the micropores of carbon matrix and sealed by solid electrolyte interphase, can operate under lean electrolyte conditions, but a low sulfur loading in carbon matrix (<40 wt %) and low sulfur unitization (<70%) still limit the energy density in a cell level. Here, we significantly increase the sulfur loading in carbon to 60 wt % and sulfur utilization to ∼87% by dispersing sulfur in an oxygen-rich dense carbon host at a molecular level through strong chemical interactions of C-S and O-S. In an all-fluorinated organic lean electrolyte, the C/S cathode experiences a solid-state lithiation/delithiation reaction after the formation of solid electrolyte interphase in the first deep lithiation, completely avoiding the shuttle reaction. The chemically stabilized C/S composite retains a high reversible capacity of 541 mAh⋅g^-1 (based on the total weight of the C/S composite) for 200 cycles under lean electrolyte conditions, corresponding to a high energy density of 974 Wh⋅kg^-1 The superior electrochemical performance of the chemical bonding-stabilized C/S composite renders it a promising cathode material for high-energy and long-cycle-life LSBs.

Impact of surface coating on electrochemical and thermal behaviors of a Li-rich Li1.2Ni0.16Mn0.56Co0.08O2 cathode

Lithium-rich layered oxide materials are considered as potential cathode materials for future high-performance lithium-ion batteries (LIBs) owing to their high operating voltage and relatively high specific capacity. However, perceptible issues such as poor rate performance, poor capacity retention, and voltage degradation during cycling need to be improved before the successful commercialization of the material. In this report, zirconia coated Li1.2Ni0.16Mn0.56Co0.08O2 (NMC) (where ZrO2 = 1.0, 1.5 and 2.0 wt%) materials are synthesized using a sol-gel assisted ball milling approach. A comparison of structural, morphological and electrochemical properties is examined to elucidate the promising role of ZrO2 coating on the performance of the NMC cathode. A uniform and homogeneous ZrO2 coating is observed on the surface of NMC particles as evident by TEM elemental mapping images. The ZrO2 coated NMCs exhibit significantly improved electrochemical performance at a higher C-rate as compared to pristine material. 1.5% ZrO2 coated NMC demonstrates better cycling stability (95% capacity retention) than pristine NMC (77% capacity retention) after 50 cycles. All ZrO2 coated NMC materials demonstrated improved thermal stability compared to pristine material. The difference in onset temperature of 2 wt% ZrO2 coated and pristine NMC is 20 °C. The improved electrochemical performance of ZrO2 coated NMC can be attributed to the stabilization of its surface structure due to the presence of ZrO2.

In Situ Construction of Spinel Coating on the Surface of a Lithium-Rich Manganese-Based Single Crystal for Inhibiting Voltage Fade

Layered lithium-rich transition-metal oxides (LRMs) have been considered as the most promising next-generation cathode materials for lithium-ion batteries. However, capacity fading, poor rate performance, and large voltage decays during cycles hinder their commercial application. Herein, a spinel membrane (SM) was first in situ constructed on the surface of the octahedral single crystal Li_1.22Mn_0.55Ni_0.115Co_0.115O₂ (O-LRM) to form the O-LRM@SM composite with superior structural stability. The synergetic effects between the single crystal and spinel membrane are the origins of the enhancement of performance. On the one hand, the single crystal avoids the generation of inactive Li₂MnO₃-like phase domains, which is the main reason for capacity fading. On the other hand, the spinel membrane not only prevents the side reactions between the electrolyte and cathode materials but also increases the diffusion kinetics of lithium ions and inhibits the phase transformation on the electrode surface. Based on the beneficial structure, the O-LRM@SM electrode delivers a high discharge specific capacity and energy density (245.6 mA h g^-1 and 852.1 W h kg^-1 at 0.5 C), low voltage decay (0.38 V for 200 cycle), excellent rate performance, and cycle stability.

Electrode Degradation in Lithium-Ion Batteries

Although Li-ion batteries have emerged as the battery of choice for electric vehicles and large-scale smart grids, significant research efforts are devoted to identifying materials that offer higher energy density, longer cycle life, lower cost, and/or improved safety compared to those of conventional Li-ion batteries based on intercalation electrodes. By moving beyond intercalation chemistry, gravimetric capacities that are 2-5 times higher than that of conventional intercalation materials (e.g., LiCoO₂ and graphite) can be achieved. The transition to higher-capacity electrode materials in commercial applications is complicated by several factors. This Review highlights the developments of electrode materials and characterization tools for rechargeable lithium-ion batteries, with a focus on the structural and electrochemical degradation mechanisms that plague these systems.

Li3PO4 modification on a primary particle surface for high performance Li-rich layered oxide Li1.18Mn0.52Co0.15Ni0.15O2via a synchronous route

A Li-rich layered oxide, Li_1.18Mn_0.52Co_0.15Ni_0.15O₂, with Li₃PO₄ modification on the surface of a primary particle, was synthesized by a facile synchronous method.

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 Li_1.5Mn_0.55Ni_0.4Co_0.05O_2.5 (Li_1.2Mn_0.44Ni_0.32Co_0.04O₂) 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 Li_1.5Mn_0.66Ni_0.17Co_0.17O_2.5 (Li_1.2Mn_0.54Ni_0.13Co_0.13O₂) and Li_1.5Mn_0.65Ni_0.25Co_0.1O_2.5 (Li_1.2Mn_0.52Ni_0.2Co_0.08O₂) 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.

Improving the Structure Stability of LiNi0.8Co0.1Mn0.1O2 by Surface Perovskite-like La2Ni0.5Li0.5O4 Self-Assembling and Subsurface La3+ Doping

The commercialization of high-capacity Ni-rich cathode LiNi_0.8Co_0.1Mn_0.1O₂ is still hindered by some defects, such as moderate rate property and inferior high-voltage cycling stability. The main reason is that the structural transformation starts at the surface from layered to spinel and then to the rock salt phase, which will be aggravated under a higher voltage and gradually spread to the bulk region during cycling. Here, we fabricate the LiNi_0.8Co_0.1Mn_0.1O₂ surface with the perovskite-like La₂Ni_0.5Li_0.5O₄, which possesses good thermostability and Li⁺-ion diffusion kinetics, to strengthen its surface and subsurface lattice stability. First-principles theory has confirmed the well compatibility of La₂Ni_0.5Li_0.5O₄ with LiNi_0.8Co_0.1Mn_0.1O₂, thus affording unimpeded channels for fast Li⁺-ion transport in the same dimensions through these two crystal lattices. On the other hand, during the high-temperature synthesis process, La³⁺ ions are also doped into the subsurface lattice of LiNi_0.8Co_0.1Mn_0.1O₂. After La modification with the two above-mentioned effects, the structure stability of LiNi_0.8Co_0.1Mn_0.1O₂ at high operating voltages and after long cycles has been significantly enhanced. Specifically, at 2.75-4.5 V, the first discharge capacity at 0.2C of the La-modified sample is 229.3 mAh g^-1 and the 200th capacity retention ratio at 1C has been improved from 63.7 to 90.1%.

Reconstructing the Surface Structure of Li-Rich Cathodes for High-Energy Lithium-Ion Batteries

Reconstructing a favorable surface layer could contribute to superior charge transfer and stabilize bulk structure and thus achieve the excellent electrochemical performance of lithium- and manganese-rich oxides, but it is still challenging. In this work, the surface structures of Li-rich oxides have been successfully reconstructed via a facile strategy utilizing hydrothermal glucose carbonization and the subsequent reduction procedure. Surface microstructure and chemical state analyses reveal that the reconstruction process involves roughening of the surface connects with the extraction of lithium ions and the reduction of Mn ions as well as the formation of a spinel phase due to the distortion of oxygen anions or the presence of oxygen deficiencies. The reconstructed Co-free Li-rich oxide using 0.025 g of glucose exhibits superior electrochemical performance. Its maximum discharge capacities are 237 and 193 mAh/g at 100 and 600 mA/g, respectively, and their corresponding capacity retention ratios are higher than 93% at the 100th cycle. Furthermore, reconstructing the surface structure also enhances the discharge capacity and cycling performance of Co-containing Li-rich cathodes. The findings in this work would offer hints for surface structure reconstruction of many oxides used in energy and other fields.

The Effects of Trace Yb Doping on the Electrochemical Performance of Li-Rich Layered Oxides

Layered lithium-rich cathode materials are one of the most promising cathode materials owing to their higher mass energy density than the commercial counterparts. A series of trace Yb-doped lithium-rich cathode materials Li_1.2 Mn_0.54 Ni_0.13 Co_0.13-x Yb_x O₂ (0≤x≤0.050) were synthesized and the effects were investigated by XRD, X-ray photoelectron spectroscopy, and high-resolution TEM. The participation of Yb ions in electrochemical reactions and the larger binding energy of Yb-O than M-O (M=Mn, Ni, Co), which expands the lithium layer spacing and stabilizes the oxygen stacking, resulted in excellent performance of materials doped with a limited Yb content (x≤0.005). However, higher doping amounts (x>0.005) significantly increased the charge-transfer impedance and led to a sharp deterioration in electrochemical performance. The reason lies in the large difference in ionic radius between the transition metals (Mn, Co, and Ni) and Yb. There is an upper limit to the amount of Yb ions in the lattice. If the amount of Yb is higher than the limit, excess Yb ions enter the Li layers instead of staying in the transition-metal layers or even segregate on the surface and form electrochemically inert oxides.

Preparation and Performance of the Heterostructured Material with a Ni-Rich Layered Oxide Core and a LiNi0.5Mn1.5O4-like Spinel Shell

The LiNi_{1- x- y}Co _xAl _yO₂ (NCA)-layered materials are regarded as a research focus of power lithium-ion batteries (LIBs) because of their high capacity. However, NCA materials are still up against the defects of cation mixing and surface erosion of electrolytes. Herein, a novel design strategy is proposed to obtain a heterostructured cathode material with a high-capacity LiNi_0.88Co_0.09Al_0.03O₂ layer ( R3̅ m) core and a stable LiNi_0.5Mn_1.5O₄-like spinel ( Fd3̅ m) shell, which is prepared through spontaneous redox reaction of the precursor with KMnO₄ in an alkaline solution and subsequent calcination procedure. The structure, morphology, element distribution, and electrochemical performances of the as-prepared NCA are studied by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and electrochemical techniques. The results show that the LiNi_0.5Mn_1.5O₄-like spinel ( Fd3̅ m) shell layer with a robust cubic close-packed crystal structure is uniformly adhered to the surface of the NCA and can availably suppress the side reactions with the electrolyte and surface-phase transformation, which will facilitate insertion/extraction of Li⁺ ions during cycling. Benefiting from the enhanced structural stability and improved kinetics, the heterostructured NCA delivers a better cycling performance. The discharge specific capacity is as high as 153.7 mA h g^-1 at 10 C, and even at high charge voltage of 4.5 V, the capacity retention can still increase 11% at 1 C (200 mA g^-1) after 100 cycles. Besides, the material exhibits a prominent thermal stability of 248 °C at 4.3 V. Therefore, this novel structure design strategy can contribute to the development and commercialization of high-performance cathode materials for power LIBs.

Well-Wrapped Li-Rich Layered Cathodes by Reduced Graphene Oxide towards High-Performance Li-Ion Batteries

Layered lithium-rich manganese oxide (LLO) cathode materials have attracted much attention for the development of high-performance lithium-ion batteries. However, they have suffered seriously from disadvantages, such as large irreversible capacity loss during the first cycle, discharge capacity decaying, and poor rate performance. Here, a novel method was developed to coat the surface of 0.4Li2MnO3∙0.6LiNi1/3Co1/3Mn1/3O2 cathode material with reduced graphene-oxide (rGO) in order to address these drawbacks, where a surfactant was used to facilitate the well-wrapping of rGO. As a result, the modified LLO (LLO@rGO) cathode exhibits superior electrochemical performance including cycling stability and rate capability compared to the pristine LLO cathode. In particular, the LLO@rGO with a 0.5% rGO content can deliver a high discharge capacity of 166.3 mAh g-1 at a 5C rate. The novel strategy developed here can provide a vital approach to inhibit the undesired side reactions and structural deterioration of Li-rich cathode materials, and should be greatly useful for other cathode materials to improve their electrochemical performance.

Understanding the Discrepancy of Defect Kinetics on Anionic Redox in Lithium-Rich Cathode Oxides

Reversible anionic (oxygen) redox in lithium-rich cathode oxides has been becoming a blooming research topic to further boost the energy density in lithium-ion batteries. There are numerous experimental observations and theoretical calculations to illustrate the importance of defects on anionic redox activity, but how the defects on the surface and bulk control the kinetics of anionic redox is not well understood. Here, we uncover this intriguing ambiguity on the correlation among defects states, Li-ion diffusion, and oxygen redox reaction. It is found that the surface-defective microstructure has fast Li-ion diffusion to achieve superior cationic redox activities/kinetics, whereas the bulk-defective microstructure corresponds to a slow Li-ion diffusion to result in poor cationic redox activities/kinetics. By contrast, both surface and bulk defects can be of benefit to the enhancement of oxygen redox activities/kinetics. Moreover, a positive correlation is also established among charge-transfer resistance, interface reaction charge-transfer activation energy, and oxygen redox activity in these electrode materials. This study on defect-anionic activity provides a new insight for controlling anionic redox reaction in lithium-rich cathode materials for real-world application.

Tuning surface conductivity and stability for high-performance Li- and Mn-rich cathode materials

Li- and Mn-rich (LMR) layered materials with large specific capacities are one of the most promising cathodes for high-energy Li-ion batteries.

Crystal structural design of exposed planes: express channels, high-rate capability cathodes for lithium-ion batteries

Developing high-performance lithium ion batteries (LIBs) requires optimization of every battery component. Currently, the main problems lie in the mismatch of electrode capacities, especially the excessively low capacity of cathodes compared with that of anodes. Due to the anisotropy of the crystal structure, different crystal planes play different roles in the transmission of lithium ions. Among these, the {010} facets of layered-structure materials, the (110) planes of spinel cathodes and the (010) planes of olivine cathodes can provide open surface structures, which furnish express channels for the rapid and efficient transmission of lithium ions, leading to enhanced rate performance. However, due to the high-energy surfaces of these crystal planes, they tend to disappear in the synthetic process, forming thermodynamic equilibrium products dominated by low-energy and electrochemically-inactive planes. From the structure design of the material itself, preparing functional materials with specific morphologies and crystal structures is considered to be the most effective way to improve the cyclability and rate performance of LIB cathodes. In this review, we highlight the latest developments in selectively exposing the crystal planes of LIB cathode materials. The synthetic method, the corresponding electrochemical performance, especially the rate capability, and the growth mechanism have been systematically summarized for layered-structure cathodes of LiCoO2, LiNixCoyMn1-x-yO2 and Li2MnO3·LiMO2, spinel cathodes of LiMn2O4 and LiNi0.5Mn1.5O4, and olivine cathodes of LiFePO4. This in-depth discussion and understanding is beneficial for the rational design of well-performing LIB cathodes and can provide direction and perspectives for future work.

Improved Electrochemical Performances of LiCoO2 at Elevated Voltage and Temperature with an In Situ Formed Spinel Coating Layer

Although various cathode materials have been explored to improve the energy density of lithium-ion batteries, LiCoO₂ is still the first choice for 3C consumer electronics due to the high tap density and high volumetric energy density. However, only 0.5 mol of lithium ions can be extracted from LiCoO₂ to avoid side reactions and irreversible structure change, which typically occur at high voltage (>4.2 V). To improve the electrochemical performances of the LiCoO₂ cathode material at high cut-off voltage and elevated temperature for higher energy density, an in situ formed spinel interfacial coating layer of LiCo _xMn_{2- x}O₄ is achieved by the reaction of the surface region of the LiCoO₂ host. The capacity retention of the modified LiCoO₂ cycled at a high voltage of 4.5 V is significantly increased from 15.5 to 82.0% after 300 cycles at room temperature, due to the stable spinel interfacial inhibiting interfacial reactions between LiCoO₂ and the electrolyte as confirmed by impedance spectroscopy. We further demonstrated that LiCoO₂ with the spinel interfacial layer also exhibits an excellent cycling stability at a high temperature of 45 °C.

Design and Synthesis of Layered Na2Ti3O7 and Tunnel Na2Ti6O13 Hybrid Structures with Enhanced Electrochemical Behavior for Sodium-Ion Batteries

A novel complementary approach for promising anode materials is proposed. Sodium titanates with layered Na2Ti3O7 and tunnel Na2Ti6O13 hybrid structure are presented, fabricated, and characterized. The hybrid sample exhibits excellent cycling stability and superior rate performance by the inhibition of layered phase transformation and synergetic effect. The structural evolution, reaction mechanism, and reaction dynamics of hybrid electrodes during the sodium insertion/desertion process are carefully investigated. In situ synchrotron X-ray powder diffraction (SXRD) characterization is performed and the result indicates that Na+ inserts into tunnel structure with occurring solid solution reaction and intercalates into Na2Ti3O7 structure with appearing a phase transition in a low voltage. The reaction dynamics reveals that sodium ion diffusion of tunnel Na2Ti6O13 is faster than that of layered Na2Ti3O7. The synergetic complementary properties are significantly conductive to enhance electrochemical behavior of hybrid structure. This study provides a promising candidate anode for advanced sodium ion batteries (SIBs).

Composite-Structure Material Design for High-Energy Lithium Storage

High-energy storage devices are in demand for the rapid development of modern society. Until now, many kinds of energy storage devices, such as lithium-ion batteries (LIBs), sodium-ion batteries (NIBs), and so on, have been developed in the past 30 years. However, most of the commercially exploited and studied active electrode materials of these energy storage devices possess a single phase with low reversible capacity or unsatisfied cycle stability. Continuous and extensive research efforts are made to develop alternative materials with a higher specific energy density and long cycle life by element doping or surface modification. A novel strategy of forming composite-structure electrode materials by introducing structure units has attracted great attention in recent years. Herein, based on previous publications on these composite-structure materials, some important scientific points focusing on the design of composite-structure materials for better electrochemical performances reveal the distinction of composite structures based on average and local structure analysis methods, and an understanding of the relationship between these interior composite structures and their electrochemical performances is discussed thoroughly. The lithiation/delithiation mechanism and the remaining challenges and perspectives for composite-structure electrode materials are also elaborated.

Unravelling the Structure and Electrochemical Performance of Li-Cr-Mn-O Cathodes: From Spinel to Layered

To explore a new series of cathode materials with high electrochemical performance, the spinel-layered (1 - x)[LiCrMnO₄]· x[Li₂MnO₃·LiCrO₂] ( x = 0, 0.25, 0.5, 0.75, and 1) composites are synthesized with the sol-gel method. X-ray diffraction, high-resolution transmission electron microscopy, selected area electron diffraction, and Raman spectra reveal that the structure of the (1 - x)[LiCrMnO₄]· x[Li₂MnO₃·LiCrO₂] cathode materials evolves from spinel to hybrid spinel-layered and layered structures with the increase of the Li concentration. Test results reveal that the structure and electrochemical performance of (1 - x)[LiCrMnO₄]· x[Li₂MnO₃·LiCrO₂] ( x = 0.25, 0.5 and 0.75) composites have the characteristics of both spinel ( x = 0) and Li-rich layered phases ( x = 1). In particular, x = 0.5 and 0.75 electrodes exhibit relatively high capacity retention and rate capability, which is mainly ascribed to the synergistic effect of the spinel and Li-rich layered phases, the 3D Li-ion diffusion channels of the spinel phase, and the low charge-transfer resistance ( R_ct) and Warburg diffusion impedance ( W_o).

Understanding the effect of an in situ generated and integrated spinel phase on a layered Li-rich cathode material using a non-stoichiometric strategy

Recently, spinel-layered integrated Li-rich cathode materials have attracted great interest due to the large enhancement of their electrochemical performances. However, the modification mechanism and the effect of the integrated spinel phase on Li-rich layered cathode materials are still not very clear. Herein, we have successfully synthesized the spinel-layered integrated Li-rich cathode material using a facile non-stoichiometric strategy (NS-LNCMO). The rate capability (84 mA h g^-1vs. 28 mA h g^-1, 10 C), cycling stability (92.4% vs. 80.5%, 0.2 C), low temperature electrochemical capability (96.5 mA h g^-1vs. 59 mA h g^-1, -20 °C), initial coulomb efficiency (92% vs. 79%) and voltage fading (2.77 V vs. 3.02 V, 200 cycles@1 C) of spinel-layered integrated Li-rich cathode materials have been significantly improved compared with a pure Li-rich phase cathode. Some new insights into the effect of the integrated spinel phase on a layered Li-rich cathode have been proposed through a comparison of the structure evolution of the integrated and Li-rich only materials before and after cycling. The Li-ion diffusion coefficient of NS-LNCMO has been enlarged by about 3 times and almost does not change even after 100 cycles indicating an enhanced structure stability. The integration of the spinel phase not only enhances the structure stability of the layered Li-rich phase during charging-discharging but also expands the interslab spacing of the Li-ion diffusion layer, and elongates TM-O covalent bond lengths, which lowers the activation barrier of Li⁺-transportation, and alleviates the structure strain during the cycling procedure.