Enhanced Surface Chemical and Structural Stability of Ni-Rich Cathode Materials by Synchronous Lithium-Ion Conductor Coating for Lithium-Ion Batteries

Lithium-Ion Conductive Coatings for Nickel-Rich Cathodes for Lithium-Ion Batteries

Nickel (Ni)-rich cathodes are among the most promising cathode materials of lithium batteries, ascribed to their high-power density, cost-effectiveness, and eco-friendliness, having extensive applications from portable electronics to electric vehicles and national grids. They can boost the wide implementation of renewable energies and thereby contribute to carbon neutrality and achieving sustainable prosperity in the modern society. Nevertheless, these cathodes suffer from significant technical challenges, leading to poor cycling performance and safety risks. The underlying mechanisms are residual lithium compounds, uncontrolled lithium/nickel cation mixing, severe interface reactions, irreversible phase transition, anisotropic internal stress, and microcracking. Notably, they have become more serious with increasing Ni content and have been impeding the widespread commercial applications of Ni-rich cathodes. Various strategies have been developed to tackle these issues, such as elemental doping, adding electrolyte additives, and surface coating. Surface coating has been a facile and effective route and has been investigated widely among them. Of numerous surface coating materials, have recently emerged as highly attractive options due to their high lithium-ion conductivity. In this review, a thorough and comprehensive review of lithium-ion conductive coatings (LCCs) are made, aimed at probing their underlying mechanisms for improved cell performance and stimulating new research efforts.

Two birds with one stone: One-pot concurrent Ta-doping and -coating on Ni-rich LiNi0.92Co0.04Mn0.04O2 cathode materials with fiber-type microstructure and Li+-conducting layer formation

A novel scalable Taylor-Couette reactor (TCR) synthesis method was employed to prepare Ta-modified LiNi0.92Co0.04Mn0.04O2 (T-NCM92) with different Ta contents. Through experiments and density functional theory (DFT) calculations, the phase and microstructure of Ta-modified NCM92 were analyzed, showing that Ta provides a bifunctional (doping and coating at one time) effect on LiNi0.92Co0.04Mn0.04O2 cathode material through a one-step synthesis process via a controlling suitable amount of Ta and Li-salt. Ta doping allows the tailoring of the microstructure, orientation, and morphology of the primary NCM92 particles, resulting in a needle-like shape with fine structures that considerably enhance Li+ ion diffusion and electrochemical charge/discharge stability. The Ta-based surface-coating layer effectively prevented microcrack formation and inhibited electrolyte decomposition and surface-side reactions during cycling, thereby significantly improving the electrochemical performance and long-term cycling stability of NCM92 cathodes. Our as-prepared NCM92 modified with 0.2 mol% Ta (i.e., T2-NCM92) exhibits outstanding cyclability, retaining 84.5 % capacity at 4.3 V, 78.3 % at 4.5 V, and 67.6 % at 45 ℃ after 200 cycles at 1C. Even under high-rate conditions (10C), T2-NCM92 demonstrated a remarkable capacity retention of 66.9 % after 100 cycles, with an initial discharge capacity of 157.6 mAh g-1. Thus, the Ta modification of Ni-rich NCM92 materials is a promising option for optimizing NCM cathode materials and enabling their use in real-world electric vehicle (EV) applications.

Atomic-Scale Characterization of Microscale Battery Particles Enabled by a High-Throughput Focused Ion Beam Milling Technique

The cathode materials in lithium-ion batteries (LIBs) require improvements to address issues such as surface degradation, short-circuiting, and the formation of dendrites. One such method for addressing these issues is using surface coatings. Coatings can be sought to improve the durability of cathode materials, but the characterization of the uniformity and stability of the coating is important to assess the performance and lifetime of these materials. For microscale particles, there are, however, challenges associated with characterizing their surface modifications by transmission electron microscopy (TEM) techniques due to the size of these particles. Often, techniques such as focused ion beam (FIB)-assisted lift-out can be used to prepare thin cross sections to enable TEM analysis, but these techniques are very time-consuming and have a relatively low throughput. The work outlined herein demonstrates a FIB technique with direct support of microscale cathode materials on a TEM grid that increases sample throughput and reduces the processing time by 60-80% (i.e., from >5 to ∼1.5 h). The demonstrated workflow incorporates an air-liquid particle assembly followed by direct particle transfer to a TEM grid, FIB milling, and subsequent TEM analysis, which was illustrated with lithium nickel cobalt aluminum oxide particles and lithium manganese nickel oxide particles. These TEM analyses included mapping the elemental composition of cross sections of the microscale particles using energy-dispersive X-ray spectroscopy. The methods developed in this study can be extended to high-throughput characterization of additional LIB cathode materials (e.g., new compositions, coating, end-of-life studies), as well as to other microparticles and their coatings as prepared for a variety of applications.

An effective tellurium surface modification strategy to enhance the capacity and rate capability of Ni-rich LiNi0.8Co0.1Mn0.1O2 cathode material

LiNi0.8Co0.1Mn0.1O2 (NCM) layered oxide is contemplated as an auspicious cathode candidate for commercialized lithium-ion batteries. Regardless, the successful commercial utilization of these materials is impeded by technical issues like structural degradation and poor cyclability. Elemental doping is among the most viable strategies for enhancing electrochemical performance. Herein, the preparation of surface tellurium-doped NCM is done by utilizing the methodology solid-state route at high temperatures. Surface doping of the Te ions leads to structural stability owing to the inactivation of oxygen at the surface via the binding of slabs of transition metal-oxygen. Remarkably, 1 wt% of Te doping in NCM exhibits enhanced electrochemical characteristics with an excellent discharge capacity, i.e., 225.8 mAh/g (0.1C), improved rate-capability of 156 mAh/g (5C) with 82.2% retention in capacity (0.5C) over 100 cycles within 2.7-4.3V as compared to all other prepared electrodes. Hence, the optimal doping of Te is favorable for enhancing capacity, cyclability along with rate capability of NCM.

Concerted Effect of Ion- and Electron-Conductive Additives on the Electrochemical and Thermal Performances of the LiNi0.8Co0.1Mn0.1O2 Cathode Material Synthesized by a Taylor-Flow Reactor for Lithium-Ion Batteries

To address the issue that a single coating agent cannot simultaneously enhance Li+-ion transport and electronic conductivity of Ni-rich cathode materials with surface modification, in the present study, we first successfully synthesized a LiNi0.8Co0.1Mn0.1O2 (NCM811) cathode material by a Taylor-flow reactor followed by surface coating with Li-BTJ and dispersion of vapor-grown carbon fibers treated with polydopamine (PDA-VGCF) filler in the composite slurry. The Li-BTJ hybrid oligomer coating can suppress side reactions and enhance ionic conductivity, and the PDA-VGCFs filler can increase electronic conductivity. As a result of the synergistic effect of the dual conducting agents, the cells based on the modified NCM811 electrodes deliver superior cycling stability and rate capability, as compared to the bare NCM811 electrode. The CR2032 coin-type cells with the NCM811@Li-BTJ + PDA-VGCF electrode retain a discharge specific capacity of ∼92.2% at 1C after 200 cycles between 2.8 and 4.3 V (vs Li/Li+), while bare NCM811 retains only 84.0%. Moreover, the NCM811@Li-BTJ + PDA-VGCF electrode-based cells reduced the total heat (Qt) by ca. 7.0% at 35 °C over the bare electrode. Remarkably, the Li-BTJ hybrid oligomer coating on the surface of the NCM811 active particles acts as an artificial cathode electrolyte interphase (ACEI) layer, mitigating irreversible surface phase transformation of the layered NCM811 cathode and facilitating Li+ ion transport. Meanwhile, the fiber-shaped PDA-VGCF filler significantly reduced microcrack propagation during cycling and promoted the electronic conductance of the NCM811-based electrode. Generally, enlightened with the current experimental findings, the concerted ion and electron conductive agents significantly enhanced the Ni-rich cathode-based cell performance, which is a promising strategy to apply to other Ni-rich cathode materials for lithium-ion batteries.

Exploring the Role of an Electrolyte Additive in Suppressing Surface Reconstruction of a Ni-Rich NMC Cathode at Ultrahigh Voltage via Enhanced In Situ and Operando Characterization Methods

Vinylene carbonate (VC) is a widely used electrolyte additive in lithium-ion batteries for enhanced solid electrolyte interphase formation on the anode side. However, the cathode electrolyte interphase (CEI) formation with VC has received a lot less attention. This study presents a comprehensive investigation employing advanced in situ/operando-based Raman and X-ray absorption spectroscopy (XAS) to explore the effect of electrolyte composition on the CEI formation and suppression of surface reconstruction of LixNiyMnzCo1-y-zO2 (NMC) cathodes. A novel chemical pathway via VC polymerization is proposed based on experimental results. In situ Raman spectra revealed a new peak at 995 cm-1, indicating the presence of C-O semi-carbonates resulting from the radical polymerization of VC. Operando Raman analysis unveiled the formation of NiO at 490 cm-1 in the baseline system under ultrahigh voltage (up to 5.2 V). However, this peak was conspicuously absent in the VC electrolyte, signifying the effectiveness of VC in suppressing surface reconstruction. Further investigation was carried out utilizing in situ XAS compared X-ray absorption near edge structure spectra from cells of 3 and 20 cycles in both electrolytes at different operating voltages. The observed shift at the Ni K-edge confirmed a more substantial reduction of Ni in the baseline electrolyte compared to that in the VC electrolyte, thus indicating less CEI protection in the former. A sophisticated extended X-ray absorption fine structure analysis quantitatively confirmed the effective suppression of rock-salt formation with the VC electrolyte during the charging process, consistent with the operando Raman results. The in situ XAS results thus provided additional support for the key findings of this study, establishing the crucial role of VC polymerization in enhancing CEI stability and mitigating surface reconstruction on NMC cathodes. This work clarifies the relationship between the enhanced CEI layer and NMC degradation and inspires rational electrolyte design for long-cycling NMC cathodes.

Advancements and Challenges in High-Capacity Ni-Rich Cathode Materials for Lithium-Ion Batteries

Nowadays, lithium-ion batteries are undoubtedly known as the most promising rechargeable batteries. However, these batteries face some big challenges, like not having enough energy and not lasting long enough, that should be addressed. Ternary Ni-rich Li[NixCoyMnz]O2 and Li[NixCoyAlz]O2 cathode materials stand as the ideal candidate for a cathode active material to achieve high capacity and energy density, low manufacturing cost, and high operating voltage. However, capacity gain from Ni enrichment is nullified by the concurrent fast capacity fading because of issues such as gas evolution, microcracks propagation and pulverization, phase transition, electrolyte decomposition, cation mixing, and dissolution of transition metals at high operating voltage, which hinders their commercialization. In order to tackle these problems, researchers conducted many strategies, including elemental doping, surface coating, and particle engineering. This review paper mainly talks about origins of problems and their mechanisms leading to electrochemical performance deterioration for Ni-rich cathode materials and modification approaches to address the problems.

An effective co-modification strategy to enhance the cycle stability of LiNi0.8Co0.1Mn0.1O2 for lithium-ion batteries

Ni-rich cathode materials suffer from rapid capacity fading caused by interface side reactions and bulk structure degradation. Previous studies show that Co is conducive to bulk structure stability and sulfate can react with the residual lithium (LiOH and Li2CO3) on the surface of Ni-rich cathode materials and form a uniform coating to suppress the side reactions between the cathode and electrolyte. Here, CoSO4 is utilized as a modifier for LiNi0.8Co0.1Mn0.1O2 (NCM811) cathode materials. It reacts with the residual lithium on the surface of the NCM811 cathode to form Li-ion conductive Li2SO4 protective layers and Co doping simultaneously during the high-temperature sintering process, which can suppress the side reactions between the Ni-rich cathode and electrolyte and effectively prevent the structural transformation. As a result, the co-modified NCM811 cathode with 3 wt% CoSO4 exhibits an improved cycling performance of 81.1% capacity retention after 200 cycles at 1C and delivers an excellent rate performance at 5C of 187.4 mA h g-1, which is 10.2% higher than that of the pristine NCM811 cathode.

Osmotically Delaminated Silicate Nanosheet-Coated NCM for Ultra-Stable Li+ Storage and Chemical Stability Toward Long-Term Air Exposure

To ensure the safety and performance of lithium-ion batteries (LIBs), a rational design and optimization of suitable cathode materials are crucial. Lithium nickel cobalt manganese oxides (NCM) represent one of the most popular cathode materials for commercial LIBs. However, they are limited by several critical issues, such as transition metal dissolution, formation of an unstable cathode-electrolyte interphase (CEI) layer, chemical instability upon air exposure, and mechanical instability. In this work, coating fabricated by self-assembly of osmotically delaminated sodium fluorohectorite (Hec) nanosheets onto NCM (Hec-NCM) in a simple and technically benign aqueous wet-coating process is reported first. Complete wrapping of NCM by high aspect ratio (>10 000) nanosheets is enabled through an electrostatic attraction between Hec nanosheets and NCM as well as by the superior mechanical flexibility of Hec nanosheets. The coating significantly suppresses mechanical degradation while forming a multi-functional CEI layer. Consequently, Hec-NCM delivers outstanding capacity retention for 300 cycles. Furthermore, due to the exceptional gas barrier properties of the few-layer Hec-coating, the electrochemical performance of Hec-NCM is maintained even after 6 months of exposure to the ambient atmosphere. These findings suggest a new direction of significantly improving the long-term stability and activity of cathode materials by creating an artificial CEI layer.

Aqueous Cold Sintering of Li-Based Compounds

Aqueous cold sintering of two lithium-based compounds, the electrolyte Li_6.25La₃Zr₂Al_0.25O₁₂ (LLZAO) and cathode material LiCoO₂ (LCO), is reported. For LLZAO, a relative density of ∼87% was achieved, whereas LCO was sintered to ∼95% with 20 wt % LLZAO as a flux/binder. As-cold sintered LLZAO exhibited a low total conductivity (10^-8 S/cm) attributed to an insulating grain boundary blocking layer of Li₂CO₃. The blocking layer was reduced with a post-annealing process or, more effectively, by replacing deionized water with 5 M LiCl during cold sintering to achieve a total conductivity of ∼3 × 10^-5 S/cm (similar to the bulk conductivity). For LCO-LLZAO composites, scanning electron microscopy and X-ray computer tomography indicated a continuous LCO matrix with the LLZAO phase evenly distributed but isolated throughout the ceramics. [001] texturing during cold sintering resulted in an order of magnitude difference in electronic conductivity between directions perpendicular and parallel to the c-axis at room temperature. The electronic conductivity (∼10^-2 S/cm) of cold sintered LCO-LLZAO ceramics at room temperature was comparable to that of single crystals and higher than those synthesized via either conventional sintering or hot pressing.

Outstanding Electrochemical Performance of Ni-Rich Concentration-Gradient Cathode Material LiNi0.9Co0.083Mn0.017O2 for Lithium-Ion Batteries

The full-concentrationgradient LiNi_0.9Co_0.083Mn_0.017O₂ (CG-LNCM), consisting of core Ni-rich LiNi_0.93Co_0.07O₂, transition zone LiNi_1-x-yCo_xMn_yO_2, and outmost shell LiNi_1/3Co_1/3Mn_1/3O₂ was prepared by a facile co-precipitation method and high-temperature calcination. CG-LNCM was then investigated with an X-ray diffractometer, ascanning electron microscope, a transmission electron microscope, and electrochemical measurements. The results demonstrate that CG-LNCM has a lower cation mixing of Li⁺ and Ni²⁺ and larger Li⁺ diffusion coefficients than concentration-constant LiNi_0.9Co_0.083Mn_0.017O₂ (CC-LNCM). CG-LNCM presents a higher capacity and a better rate of capability and cyclability than CC-LNCM. CG-LNCM and CC-LNCM show initial discharge capacities of 221.2 and 212.5 mAh g^-1 at 0.2C (40 mA g^-1) with corresponding residual discharge capacities of 177.3 and 156.1 mAh g^-1 after 80 cycles, respectively. Even at high current rates of 2C and 5C, CG-LNCM exhibits high discharge capacities of 165.1 and 149.1 mAh g^-1 after 100 cycles, respectively, while the residual discharge capacities of CC-LNCM are as low as 148.8 and 117.9 mAh g^-1 at 2C and 5C after 100 cycles, respectively. The significantly improved electrochemical performance of CG-LNCM is attributed to its concentration-gradient microstructure and the composition distribution of concentration-gradient LiNi_0.9Co_0.083Mn_0.017O₂. The special concentration-gradient design and the facile synthesis are favorable for massive manufacturing of high-performance Ni-rich ternary cathode materials for lithium-ion batteries.

Electrochemically Engineering a Single-Crystal Nickel-Rich Layered Cathode

Nickel-rich layered electrode material has been attracting significant attention owing to its high specific capacity as a cathode for lithium-ion batteries. Generally, the high-nickel ternary precursors obtained by traditional coprecipitation methods are micron-scale. In this work, the submicrometer single-crystal LiNi_0.8Co_0.1Mn_0.1O₂ (NCM) cathode is efficiently prepared by electrochemically anodic oxidation followed by a molten-salt-assisted reaction without the need of extreme alkaline environments and complex processes. More importantly, when prepared under optimal voltage (10 V), single-crystal NCM exhibits a moderate particle size (∼250 nm) and strong metal-oxygen bonds due to reasonable and balanced crystal nucleation/growth rate, which are conducive to greatly enhancing the Li⁺ diffusion kinetics and structure stability. Given that a good discharge capacity of 205.7 mAh g^-1 at 0.1 C (1 C = 200 mAh g^-1) and a superior capacity retention of 87.7% after 180 cycles at 1 C are obtained based on the NCM electrode, this strategy is effective and flexible for developing a submicrometer single-crystal nickel-rich layered cathode. Besides, it can be adopted to elevate the performance and utilization of nickel-rich cathode materials.

Insight into the Surface Reconstruction-Induced Structure and Electrochemical Performance Evolution for Ni-Rich Cathodes with Postannealing after Washing

Ni-rich layered LiNixCoyAlzO2 (NCA, x ≥ 0.8) oxides have attracted wide attention as cathode materials for lithium-ion batteries due to their higher energy density and lower cost. However, the increase in the capacity for Ni-rich cathodes can cause faster capacity decay and increase sensitivity to ambient air exposure during the storage process. Especially, the residual lithium on the surface of Ni-rich cathodes will cause severe flatulence during cycling which greatly reduces the safety performance of the battery. Washing is an effective method to reduce residual lithium, but it will seriously damage the surface phase structure of Ni-rich materials. Here, we introduce a designed method involving two steps, washing and high-temperature annealing, which can ingeniously modify the surface phase structure of Ni-rich cathodes. The results show that the residual lithium content can be significantly reduced. The thin NiO-like rock-salt phase formed on the surface of Ni-rich cathode annealed at 600 °C improves the diffusion kinetics of Li+, reduces the polarization, and improves the electrochemical performance of Ni-rich materials, while the thick spinel-like phase formed at 400 °C hinders the diffusion kinetics of Li+, significantly increases the polarization, and eventually leads to the structural degradation of Ni-rich materials. As a result, the discharge capacity of the cathode annealed at 600 °C still retains 174.48 mA h g-1 after 100 cycles, with a capacity retention of 92.04%, much larger than the cathode annealed at 400 °C, for which the discharge capacity drops to 107.77 mA h g-1, with a capacity retention of 65.78%.

Enhanced Electrochemical Performance of the LiNi0.5Mn1.5O4 Cathode Material by the Construction of Uniform Lithium Silicate Nanoshells

In order to alleviate the rapid capacity decay caused by the instability of the crystal structure and electrode/electrolyte interface, a series of Li₂SiO₃-coated LiNi_0.5Mn_1.5O₄ materials have been prepared via the lithium acetate-assisted sol-gel method followed by a short-term calcination process. During the sol-gel process, TEOS is hydrolyzed, condensed, and polymerized with the assistance of lithium acetate to form a Li⁺-embedded [Si-O-Si]_n network structure to ensure the uniformity of the coating. By changing the amount of TEOS and lithium acetate, the coating thickness can be precisely controlled, whose effects on the structural and electrochemical properties of LiNi_0.5Mn_1.5O₄ materials are intensively investigated. The results show that the material with an appropriate thickness of Li₂SiO₃ coating exhibits a larger primary particle size and reduced secondary particle agglomeration. The uniform Li₂SiO₃ coating with appropriate thickness can not only improve Li⁺ ion diffusion kinetics but also suppress side reactions and CEI growth at the electrode/electrolyte interface. Besides, the interaction of Li₂SiO₃ with HF can alleviate electrode corrosion and the dissolution of transition metal ions. All the abovementioned factors together promote the significant improvement of the electrochemical performance of Li₂SiO₃-coated LiNi_0.5Mn_1.5O₄ materials.

F-Doped Ni-Rich Layered Cathode Material with Improved Rate Performance for Lithium-Ion Batteries

Ni-rich layered cathode materials for lithium-ion batteries have received widespread attention due to their large capacity and low cost; however, the structural stability of the material needs to be improved. Herein, F-doped and undoped cathode materials prepared with an advanced co-precipitation method were used to measure the effect of F doping on the material. Compared to the undoped sample, the F-doped cathode materials exhibited an improved rate performance, because the porous structure of F-doped cathode materials is favorable for the infiltration of the electrolyte and the material, and the F-doped cathode material has a larger (003) crystal plane and a smaller Li+ migration barrier energy. This simple F-doping treatment strategy provides a promising way to improve the performance of Ni-rich layered cathode materials for lithium-ion batteries.

Atomic layer deposition of lithium zirconium oxides for the improved performance of lithium-ion batteries

Recently there has been increasing interest to develop lithium-containing films as solid-state electrolytes or surface coatings for lithium-ion batteries (LIBs) and related systems. In this study, we for the first time investigated the thin film growth of lithium zirconium oxides (Li_xZr_yO or LZOs) through combining two individual atomic layer deposition (ALD) processes of ZrO₂ and LiOH, i.e., sub-ALD of ZrO₂ and LiOH. We revealed that the hygroscopic nature of the LiOH component has a big impact on the growth of LZOs. We found that an increased temperature to 225 °C was more effective than an elongated purge to mitigate the adverse effects of physisorbed H₂O. We further discovered that, during the resultant LZO super-ALD processes, the growth of sub-ALD LiOH has been promoted while the growth of sub-ALD ZrO₂ has been inhibited. In this study, a suite of instruments has been applied to characterize the LZO super-ALD processes and the resultant LZO films, including in situ quartz crystal microbalance (QCM), scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM), atomic force microscopy (AFM), synchrotron-based X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). Furthermore, we applied the resulting LZO films over LiNi_0.6Mn_0.2Co_0.2O₂ (NMC622) cathodes in LIBs and demonstrated that the LZO coating films could evidently improve the lithium-ion insertion and extraction rates of the NMC622 electrodes up to 3.4 and 2.6 times, respectively. The LZO-coated NMC622 cathodes exhibited much better performance than the uncoated NMC622 ones.

Demonstrating Apparently Inconspicuous but Sensitive Impacts on the Rollover Failure of Lithium-Ion Batteries at a High Voltage

Layered oxides, such as Li[Ni_0.5Co_0.2Mn_0.3]O₂ (NCM523), are promising cathode materials for operation at a high voltage, i.e., high-energy lithium-ion batteries. The instability-reasoned transition metal dissolution remains a major challenge, which initiates electrode cross-talk, alteration of the solid electrolyte interphase, and enhanced Li-metal dendrite formation at the graphite anode, consequently leading to rollover failure. In this work, relevant impacts on this failure mechanism are highlighted. For example, a conventional coating of NCM523 with aluminum oxide as a typical high-voltage modification improves kinetic aspects but can only postpone the rollover failure to later charge/discharge cycles. Interestingly, a similar effect on the rollover failure is observed merely after modification of the cell formation protocol, i.e., the first cycles. Further influences of specific test protocols are highlighted and show that the rollover failure even disappears at C-rates above 2C, which can be attributed to a more homogeneous distribution of Li-metal dendrite formation. It is worth noting that a variation of anode porosity can reveal similar effects, as, e.g., variations in anode processing also impact Li dendrite distribution and the appearance of rollover failure. Overall, the rollover failure is a valid but complex phenomenon, which sensitively depends on apparently inconspicuous parameters and should not be disregarded.

Dual-Modified Compact Layer and Superficial Ti Doping for Reinforced Structural Integrity and Thermal Stability of Ni-Rich Cathodes

Nickel-rich layered oxides have been regarded as a potential cathode material for high-energy-density lithium-ion batteries because of the high specific capacity and low cost. However, the rapid capacity fading due to interfacial side reactions and bulk structural degradation seriously encumbers its commercialization. Herein, a highly stable hybrid surface architecture, which integrates an outer coating layer of TiO₂&Li₂TiO₃ and a surficial titanium doping by incorporated Ti₂O₃, is carefully designed to enhance the structural stability and eliminate lithium impurity. Meanwhile, the surficial titanium doping induces a nanoscale cation-mixing layer, which suppresses transition-metal-ion migration and ameliorates the reversibility of the H2 → H3 phase transition. Also, the Li₂TiO₃ coating layer with three-dimensional channels promotes ion transportation. Moreover, the electrochemically stable TiO₂ coating layer restrains side reactions and reinforces interfacial stability. With the collaboration of titanium doping and TiO₂&Li₂TiO₃ hybrid coating, the sample with 1 mol % modified achieves a capacity retention of 93.02% after 100 cycles with a voltage decay of only 0.03 V and up to 84.62% at a high voltage of 3.0-4.5 V. Furthermore, the ordered occupation of Ni ions in the Li layer boosts the thermal stability by procrastinating the layered-to-rock salt phase transition. This work provides a straightforward and economical modification strategy for boosting the structural and thermal stability of nickel-rich cathode materials.

Understanding the Stabilizing Effects of Nanoscale Metal Oxide and Li-Metal Oxide Coatings on Lithium-Ion Battery Positive Electrode Materials

Nickel-rich layered oxides, such as LiNi_0.6Co_0.2Mn_0.2O₂ (NMC622), are high-capacity electrode materials for lithium-ion batteries. However, this material faces issues, such as poor durability at high cut-off voltages (>4.4 V vs Li/Li⁺), which mainly originate from an unstable electrode-electrolyte interface. To reduce the side reactions at the interfacial zone and increase the structural stability of the NMC622 materials, nanoscale (<5 nm) coatings of TiO_x (TO) and Li_xTi_yO_z (LTO) were deposited over NMC622 composite electrodes using atomic layer deposition. It was found that these coatings provided a protective surface and also reinforced the electrode structure. Under high-voltage range (3.0-4.6 V) cycling, the coatings enhance the NMC electrochemical behavior, enabling longer cycle life and higher capacity. Cyclic voltammetry, X-ray photoelectron spectroscopy, and X-ray diffraction analyses of the coated NMC electrodes suggest that the enhanced electrochemical performance originates from reduced side reactions. In situ dilatometry analysis shows reversible volume change for NMC-LTO during the cycling. It revealed that the dilation behavior of the electrode, resulting in crack formation and consequent particle degradation, is significantly suppressed for the coated sample. The ability of the coatings to mitigate the electrode degradation mechanisms, illustrated in this report, provides insight into a method to enhance the performance of Ni-rich positive electrode materials under high-voltage ranges.

Addressing Unfavorable Influence of Particle Cracking with a Strengthened Shell Layer in Ni-Rich Cathodes

Ni-rich layered materials are widely accepted as pivotal cathode materials to realize low-cost high-energy-density batteries. However, they still suffer from the intrinsic mechanically induced degradation due to the large lattice deformation. Here, we fabricate a strengthened shell layer on polycrystalline secondary particles to address the unfavorable influence of particle cracking instead of suppressing their bulky pulverization. This tough layer, constructed via welding LiNi_0.8Co_0.1Mn_0.1O₂ primary particles with a Nb-based ceramic, increases Young's modulus of the particles 2.6 times. This layer allows the particles work properly with the intact spherical morphology even after bulk cracking. It seems that this tough skin stops the bulky flaws growing into perforated fissures and keeps the electrodes from quick polarization. This approach demonstrates that, besides addressing the intrinsic challenges directly, appropriate particle engineering is another efficient way to exploit the potentials of Ni-rich cathodes and power batteries made out of them.

Ultrathin Li-Si-O Coating Layer to Stabilize the Surface Structure and Prolong the Cycling Life of Single-Crystal LiNi0.6Co0.2Mn0.2O2 Cathode Materials at 4.5 V

Single-crystal LiNi_1-x-yCo_xMn_yO₂ cathode materials can effectively suppress intergranular cracks that usually is seen in commercial polycrystal LiNi_1-x-yCo_xMn_yO₂ cathode materials. However, the surface structure degradation for single-crystal LiNi_1-x-yCo_xMn_yO₂ cathode materials is still aggravated at a higher cutoff voltage (over 4.5 V). In this work, we prepare single-crystal LiNi_0.6Co_0.2Mn_0.2O₂ cathode materials via a solid-state method and then coat an ultrathin Li-Si-O layer on their surface by a wet coating method. The results show that the single-crystal LiNi_0.6Co_0.2Mn_0.2O₂ cathode materials with a Li-Si-O coating layer deliver excellent cycling performance even at a higher cutoff voltage of 4.5 V. The optimized Li-Si-O-modified sample displays a capacity retention of 90.6% after 100 cycles, whereas only 68.0% for unmodified single-crystal LiNi_0.6Co_0.2Mn_0.2O₂. Further analysis of the cycled electrodes reveals that the surface structure degradation is the main reason for the decrease of electrochemical performance of single-crystal LiNi_0.6Co_0.2Mn_0.2O₂ at a high voltage (4.5 V). In contrast, with Li-Si-O coating, this phenomenon can be suppressed effectively to maintain interfacial stability and prolong the cycling life.