Ni-rich Layered Oxide Cathode Materials Research Articles

Gradient doping Mg and Al to stabilize Ni-rich cathode materials for rechargeable lithium-ion batteries

Nickel-rich layered oxides are attracting extensive interest as cathode materials to build high-energy lithium-ion batteries, but their intrinsic structural instability and interfacial parasitic reactions deteriorate the cycling performance. Herein, we report concentration-gradient Mg and Al doped LiNi0.95Co0.03Al0.01Mg0.01O2 (NCAMg) as a stable and high-capacity cathode material. The obtained NCAMg microparticles exhibit outstanding cyclability (capacity retention of 95.6% after 100 cycles) and remarkable rate capability (172.9 mAh g−1 at 10 C). The gradient Mg and Al doping boosts the electrochemical performance by restraining the undesirable H2–H3 phase conversion and the anisotropic volume change. Moreover, the enrichment of Mg and Al at particle shell enhances the Li-ion diffusivity, inhibits the growth of microcracks, and significantly improves the thermal stability over LiNiO2. The results indicate that Mg and Al gradient co-doping is an efficient and practical route to upgrade Ni-rich layered oxide cathode materials for LIBs.

Spinel LiNi0.5Mn1.5O4 shell enables Ni-rich layered oxide cathode with improved cycling stability and rate capability for high-energy lithium-ion batteries

Increasing the charging cut-off voltage is an effective strategy to achieve high energy density Ni-rich layered oxide cathode materials, however the dramatic deterioration of cycling lifespan and thermal stability still remain great challenges in large-scale practical applications. Herein, a rationally designed core-shell structure LiNi0.9Co0.1O2@LiNi0.5Mn1.5O4 composite composed of spinel structure Mn-rich LiNi0.5Mn1.5O4 shell and layered structure Ni-rich LiNi0.9Co0.1O2 core is proposed to enhance the structure stability and thermal stability under high-voltage application scenario. Compared to the pristine LiNi0.9Co0.1O2 cathode, the core-shell structure LiNi0.9Co0.1O2@LiNi0.5Mn1.5O4 composite with optimized LiNi0.5Mn1.5O4 shell exhibits high reversible specific capacity (195.5 mA h g − 1 at 0.5 C), remarkable cycling stability (86% capacity retention after 100 cycles) and superior rate capability (162.5 mA h g − 1 at 5.0 C) within 2.8 V-4.5 V. The comprehensively improved electrochemical performance is mainly attributed to the Mn-rich LiNi0.5Mn1.5O4 shell, which not only effectively prevents the phase transition and microstructural collapse, but also ameliorates the Li+ transport ability, high-voltage charging capability and thermal stability. Such novel core-shell structure LiNi0.9Co0.1O2@LiNi0.5Mn1.5O4 composite as promising cathode is highly expected to greatly promote the practical application of high energy density lithium-ion batteries.

Probing the Air Storage Failure Mechanism of Ni-Rich Layered Cathode Materials

Ni-rich layered oxide cathode materials, such as LiNi0.83Co0.12Mn0.05O2 (NCM811), exhibit high specific capacity and low cost, and become cathode material preference of high-energy-density Li-ion batteries. However, these cathode materials are not stable and will form Li-poor reconstructed layers and alkaline compounds (Li2CO3, LiOH) on the surface during the storage and processing in humid air, resulting in serious deterioration of electrochemical properties. During the past two decades, the consensus on the surface instability mechanism during humid air storage has not been reached. The main controversy focuses on the unstable octahedron mechanism and the Li/H exchange mechanism. Herein, we investigate the instability mechanism in the humid air by conducting scanning electronic microscopy, scanning transmission electron microscopy, and x-ray photoelectron spectroscopy analysis on NCM811 samples stored in designed atmospheres, etc., and realize that the surface instability of the NCM811 during storage should be mainly originated from Li/H exchange when it contacts with moisture.

Hydrophobic surface coating against chemical environmental instability for Ni-rich layered oxide cathode materials

Ni-rich layered oxide LiNi0.8Co0.1Mn0.1O2 (NCM) as a most promising cathode material for Li-ion batteries suffers easily from the deterioration of electrochemical performance during storage because of the chemical environmental instability. Herein, a simple and scalable strategy is reported to overcome these issues by a hydrophobic coating with dihexadecyl phosphate (DHP) on NCM particle surface, which is composed of an outer-layer hydrophobic alkyl surface and an inner-layer phosphate-based coating with O=POLi covalent bonds. The former layer can alleviate the absorption of H2O/CO2 on the particle surface in the humid environment, and the later can improve Li+ transportation coefficient during charge–discharge process. As a result, profiting from these two synergistic effects, the DHP-coated NCM shows excellent rate performance and cycling stability, particularly, after 14 days of exposure to air, the DHP-coated NCM delivers the initial capacity of 168.7 mAh g−1 at 2 C rate, but only 134.7 mAh g−1 for the pristine NCM. This work provides an effective approach to improve the environmental stability in Ni-rich layered oxide cathodes.

Magnesium Substitution in Ni‐Rich NMC Layered Cathodes for High‐Energy Lithium Ion Batteries (Adv. Energy Mater. 8/2022)

Li-Ion Batteries In article number 2103045, Aurora Gomez-Martin, Richard Schmuch, and co-workers report the effect of Mg substitution on mitigating the structural challenges of Ni-rich layered oxide cathode materials upon cycling in lithium-ion batteries. While partial substitution of Mg in Li sites linearly reduces the initial capacities at a given upper cut-off cell voltage, the attainable cycle life is significantly extended.

Crucial role of Ni-doping to interfacial Li2MnO3 layer of High-performance Ni-rich layered cathode in Lithium-Ion batteries

The surface instability of Ni3+ in Ni-rich layered oxide cathode materials is recognized as an obstacle in high-energy–density lithium-ion batteries. Researchers have previously attempted to solve this issue using a protective layer with a stable substance. Despite the popularity Ni-rich layered oxides, their exceptionally unstable surface has not been investigated comprehensively. Ni-rich layered oxides include lithium impurities and have a fragile surface, forming a NiO bi-phase. In this study, we perform Li2MnO3 coating to enable Ni doping via simple stirring and heat treatment combined, while considering the surface states of Ni-rich layered oxide, where lithium impurities are inevitable and a NiO bi-phase may exist. It is discovered that the tailoring interface consuming surface NiO is critical for mitigating the surface resistance. Among the samples with Li2MnO3 coating, only the sample prepared via 800 °C heating indicates the presence of Ni-doped Li2MnO3 based on electrochemical de-lithiation at 4.65 V vs. Li/Li+. It is effective in reducing NiO and stabilizing the surface for a high cycle life of 88.3% at the 100th cycle and a high rate capability of 76.9% at 5C, whereas a Li2MnO3-coated sample exhibits a cycle life of 70.4% at the 100th cycle and a rate capability of 29.1% at 5C. The surface is investigated via X-ray photoelectron spectroscopy, scanning transmission electron microscopy, and time-of-flight secondary ion mass spectroscopy analyses.

Microcracking of Ni-rich layered oxide does not occur at single crystal primary particles even abused at 4.7 V

There is a controversial issue based on the particle cracking of the Ni-rich layered oxide cathode materials whether it occurs at the primary particles or the grain boundary. Herein, we found that the microcracking of NMC811 does not occur at single crystalline primary particles even abused at a severe upper cell voltage of 4.7 V having a lot of gas evolution since the single-crystal NMC811 has superior mechanical stability. The capacity retentions determined at 1C rate and a 100% state of charge (SOC) are 80% and 50% after 1000 cycles for single crystal and polycrystal NMC811, respectively.

Comprehensive study of tantalum doping on morphology, structure, and electrochemical performance of Ni-rich cathode materials

Due to their high specific capacity, nickel (Ni)-rich layered oxide cathode materials are considered to be promising for use as cathode materials in lithium (Li)-ion batteries. However, such materials exhibit poor cycling performance, which hinders their commercial applications. Herein, Ta bulk doping in Ni-rich layered oxide cathode materials is comprehensively analysed using experimental results and density functional theory calculations. It is observed that Ta5+ uniformly distributes in the bulk of the cathode materials. Further studies reveal that Ta prefers to enter both Li and TM sites, and when the doping amount increases Ta doped at TM site dominates. After Ta doping, the energy of the (001) facets is lowered, which thus affects the primary particle morphology, resulting in a preferential growth mode that produce elongated and radially oriented primary particles. A Ni-rich cathode material doped with 1 wt% of Ta shows the best cycling and rate performance, which indicates that a certain level of Ta doping is beneficial towards improving the structural stability and electrochemical performance of Ni-rich cathode materials.

Elucidating the Effect of the Dopant Ionic Radius on the Structure and Electrochemical Performance of Ni-Rich Layered Oxides for Lithium-Ion Batteries.

The merits of Ni-rich layered oxide cathodes in specific capacity and material cost accelerate their practical applications in electric vehicles and grid energy storage. However, detrimental structural deterioration occurs inevitably during long-term cycling, leading to potential instability and capacity decay of the cathodes. In this work, we investigate the effect of the doped cation radius on the electrochemical performance and structural stability of Ni-rich cathode materials by doping with Mg and Ca ions in LiNi0.8Co0.1Mn0.1O2. The results reveal that an increase in the doping ion radius can enlarge the interlayer spacing but lead to the collapse of the layered structure if the ion radius is too large, which undermines the cycling stability of the cathode material. Compared with the Ca-doped sample and the pristine material, Mg-doped LiNi0.8Co0.1Mn0.1O2 presents improved structural stability and superior thermal stability due to the pillar and glue roles of medium-sized Mg ions in the lithium layer. The results of this study suggest that a suitable ionic radius of the dopant is critical for stabilizing the structure and improving the electrochemical properties of Ni-rich layered oxide cathode materials.

Residual Li Compounds-Selective Washing Process for Ni-Rich Layered Oxide Cathode Materials for Li-Ion Batteries

The industrial manufacturing process for Ni-rich layered oxides for Li-ion batteries includes a washing step to remove residual Li compounds, such as LiOH and Li2CO3, from the oxide surface. However, Ni-rich layered oxides are deformed during a conventional water-based washing process, leading to the degradation of the oxide surface. In this regard, a significant challenge for Ni-rich layered oxides is to remove residual Li compounds without surface damage during a washing process. Herein, the residual Li compounds-selective washing process is introduced as a promising strategy for LiNi0.945Co0.04Al0.015O2 to suppress the degradation of the oxide surface during washing. The selectively hydrophobic LiNi0.945Co0.04Al0.015O2 powders are obtained through selective coating only on the LiNi0.945Co0.04Al0.015O2 surface with hydrophobic polydimethylsiloxane, leaving residual Li compounds uncoated. For this reason, the oxide surface is hydrophobic, whereas the residual Li compounds surface is hydrophilic. As a result, only residual Li compounds are selectively exposed to water and removed during washing, mitigating contact between the oxide surface and water. Eventually, this method suppresses the degradation of the LiNi0.945Co0.04Al0.015O2 surface during washing, resulting in the improved electrochemical performance compared to LiNi0.945Co0.04Al0.015O2 obtained through a conventional washing technique.

Online Monitoring of Transition-Metal Dissolution from a High-Ni-Content Cathode Material.

The dissolution of transition metals (TMs) from cathode materials and their deposition on the anode represents a serious degradation process and, with that, a shortcoming of lithium-ion batteries. It occurs particularly at high charge voltages (>4.3 V), contributing to severe capacity loss and thus impeding the increase of cell voltage as a simple measure to increase energy density. We present here for the first time the online detection of dissolved TMs from a Ni-rich layered oxide cathode material with unprecedented potential and time resolution in potentiodynamic scans. To this aid, we used the coupling of an electroanalytical flow cell (EFC) with inductively coupled plasma mass spectrometry (ICP-MS), which is demonstrated to be an ideal tool for a fast performance assessment of new cathode materials from initial cycles. The simultaneous analysis of electrochemical and dissolution data allows hitherto hidden insights into the processes' characteristics and underlying mechanisms.

Surface-dependent stress-corrosion cracking in Ni-rich layered oxide cathodes

Structural degradation is the principal driving force for rapid voltage decay and capacity fading of Ni-rich layered oxide (NLO) cathode materials upon cycling, but its working mechanism is not yet fully elucidated. In this work, multi-scale electron microscopy/spectroscopy techniques and theoretical calculations are applied on both polycrystalline and single-crystal NLOs. We discover that both the intergranular and intragranular cracks initiate along polar (001) basal plane, while surface structure evolution and transition metal dissolution occur on nonpolar (104) fresh surface. A new chain stress corrosion mechanism from anisotropic elastic (001) tensile deformation, microcrack generation, nonpolar surface reconstruction, HF attack to metal dissolution is proposed to paint the full picture of the structural degradation of NLOs. This surface-dependent stress-corrosion coupling effect indicates that severe intergranular cracking that accumulates within the polycrystalline NLO aggregates accounts mostly for the fast voltage decay and capacity fading, whereas minor intragranular cracking and less surface damage lead to substantial improvements on cyclability and reversible capacity of single-crystal NLOs. The surface-dependent stress-corrosion cracking in both polycrystalline and single-crystal NLOs provides grain-boundary engineering clues on designing new cathode materials with high energy density and long cycle life.

Nanowelding to Improve the Chemomechanical Stability of the Ni-Rich Layered Cathode Materials

To satisfy the increasing energy density requirements for electric vehicles and grid-scale energy storage systems, Ni-rich layered oxide cathode materials are often fabricated as micron-sized secondary spherical particles consisting of nanosized single crystals. Unfortunately, the hierarchical structure inevitably induces intergranular cracks and parasitic reactions at the cathode-electrolyte interphase, aggravating chemomechanical instability and seriously hindering their practical application. Here, we propose a nanowelding strategy to build consolidation points at the grain boundary of the primary particles, which dramatically enhances the capacity retention and chemomechanical stability. Meanwhile, the oxygen vacancies in the ceria-based solid electrolyte possessing oxygen adsorbing and storage capability can restrain the active oxygenates in the surficial lattice to avoid oxygen evolution. Experimental characterization further confirms that this unique architecture can effectively prevent the liquid electrolyte from penetrating into the active material along the grain boundary and consequently eliminate the adverse effects, including intergranular cracks, cathode electrolyte interface formation and growth, and the layered structure-rock salt phase irreversible transition. This finding provides a promising approach to realize the rapid commercialization of highly stabilized nickel-rich cathode materials for high-performance lithium-ion batteries.

Fundamental and solutions of microcrack in Ni-rich layered oxide cathode materials of lithium-ion batteries

Ni-rich layered transition metal oxide is one of the most promising cathode materials for the next generation lithium-based automotive batteries due to its excellent electrochemical performances. Nevertheless, its further applications are capped by the structural/interfacial instability during the prolonged charging/discharging, leading to severe performance fading and serious safety concerns. Here, we provide a comprehensive review about challenges and solutions to modify Ni-rich layered cathodes specifically for microcrack failure. Firstly, the mechanism of microcrack formation and evolution are concluded thoroughly. Secondly, recent advances in stabilizing the structure/interface of Ni-rich cathodes are summarized such as surface coating, cation/anion doping, composition tailoring, morphology engineering and electrolytes optimization. Furthermore, strategies to mitigate the microcrack and then boost the electrochemical performance of Ni-rich cathodes at the chemical & mechanical engineering level are presented. More importantly, outlook and perspectives to facilitate the practical application of Ni-rich layered cathodes toward electrical vehicle application are provided as well. The information compiled herein can promote the understanding of the microcracks of Ni-rich cathode materials, including its fundamental science, characterization method, and strategies for designing crack-free Ni-rich cathode materials. Furthermore, we provide a new idea for material modification, which is perspective strategy from crystallographic, structural mechanics and chemistry under the guidance of “chemical-mechanical” effect. • The mechanism of microcrack formation and evolution are concluded. • Recent advances in stabilizing the structure/interface of Ni-rich cathodes are summarized. • Strategies to mitigate the microcrack and then boost the electrochemical performance of Ni-rich cathodes are presented. • Main challenges and the future horizons are outlooked.

Surface-Dependent Stress-Corrosion Cracking in Ni-Rich Layered Oxide Cathodes

Structural degradation is the principal driving force for rapid voltage decay and capacity fading of Ni-rich layered oxide (NLO) cathode materials upon cycling, but its working mechanism is not yet fully elucidated. Here we apply multi-scale electron microscopy/spectroscopy techniques and theoretical calculations on both polycrystalline and single-crystal NLOs, and describe their structural evolution upon cycling. We discover that both the intergranular and intragranular cracks initiate along polar (001) basal plane due to its large elastic anisotropy upon cycling and surface structure evolution and transition metal dissolution occur on nonpolar (104) fresh surface. With this surface-dependent stress-corrosion coupling effect, severe intergranular cracking that accumulates within the polycrystalline NLO aggregates accounts mostly for the fast voltage decay and capacity fading, whereas minor intragranular cracking and less surface damage lead to substantial improvements on cyclability and reversible capacity of single-crystal NLOs. A universal understanding of the surface-dependent degradation in both polycrystalline and single-crystal NLOs provides clues on designing new cathode materials with high energy density and long cycle life through grain-boundary engineering.

Hydrothermal preparing agglomerate LiNi0.8Co0.1Mn0.1O2 cathode material with submicron primary particle for alleviating microcracks

Ni-rich layered oxides LiNixTM1-xO2 (TM = Co, Mn, Al, x > 0.6) cathode materials for lithium-ion batteries have an anisotropic volume change during lithium deintercalation process, which easily causes microcracks and rapid capacity decay. In the paper, carbonate precursor prepared by a urea-based hydrothermal method is lithiated at different temperatures of 750–850 °C to obtain agglomerate LiNi0.8Co0.1Mn0.1O2 (NCM811) cathode material with different primary sizes. The sample synthesized at 750 °C shows broadening diffraction peaks due to its near-nano grain size, which results in the merging of (006)/(012) and (018)/(110) peaks. The effect of anisotropic volume shrinkage is effectively weakened by fine grain range, which alleviates formation of intergranular crack that is commonly considered as an important reason for capacity decay of Ni-rich layered oxide cathode materials. In addition, two new types of intragranular microcracks are first identified in cyclic NCM811. One extends along the spinel phase and layered phase interface, and the other extends along the vertical (003) R crystal plane. The difference in atomic plane spacing during the electrochemical cycles plays a key role in the growth of intragranular microcracks.

Improved high-voltage performance of LiNi0.87Co0.1Al0.03O2 by Li+-conductor coating

Ni-rich layered oxide cathode material is extensively studied as promising cathode material in high-energy lithium ion batteries. However, capacity decay, poor rate performance and thermal instability are critical challenges for commercialization. Herein, Ni-rich layered oxide cathode material (LiNi0.87Co0.1Al0.03O2) is synthesized via oxide precursor and further modified by amorphous Li+-ion conductor Li2O-BPO4 (LBP) coating. The LBP coating layer can facilitate the transfer of Li+ across the solid-electrolyte interface, reduce the side reaction between the cathode-and electrolyte as well as decrease cation mixing. Therefore, the NCA coated by LBP displays the improved cycling performance due to the increased interfacial stability, the decreased polarization and the intensified Li+ transport kinetics. Especially, 0.5 wt% LBP coated NCA (NCA@LBP-2) possesses the good capacity retention of 92.3% at 1C in 2.7–4.3 V and 76.2% at high-voltage in 2.7–4.7 V. Moreover, the sample shows an outstanding rate capability even after cycled at 10C. Hence, this effective coating strategy on Ni-rich cathode material improves the cycling stability and rate capability under high-voltage condition and shows a good application prospect in the practical production of NCA.

Enhanced cycling stability of nickel-rich layered oxide by tantalum doping

Ni-rich layered oxide cathode material is considered as a promising cathode material in Li-ion batteries owing to their high specific capacity. However, it suffers from poor cycling performance which hinder their commercial applications. The unstable oxygen retention is one of the main causes of structural deformation as well as lower cycling stability. Herein, a simple high-temperature solid state method is utilized to synthesize surface tantalum doped Ni-rich layered oxide cathode material (LiNi0.88Co0.09Al0.03O2) due to the stronger bond between Ta–O which intensifies the TM-O layer due to the positioning of Ta5+ in the TM layer. In the meantime, based on density functional theory (DFT) calculations it has been confirmed that the Ta-doped structure exhibits metallic behavior and lower Fermi level therefore it acquires better phase stability. The 1 wt% Ta-doped NCA (T2-NCA) reveals the best cycling performance of about 93% (4.3V at 1C). Even, it retains good cycling stability of 75% at high voltage (4.5 V) and 97.7% at high C-rate (5C). Therefore, a small amount of Ta surface doping on NCA plays an unneglectable role for enhancing the initial discharge capacity and cycling stability.

Uniform Coating of Se on Selenophilic Surfaces of Nickel-Rich Layered Oxide Cathode Materials for High Performance Li-Ion Batteries

A novel and uniform Se coating layer on the selenophilic surface of the Ni-rich layered metal oxide (LNCM) cathode material was constructed via a facile melt-diffusion approach. The high selenophilicity of the LNCM surfaces should attribute to the strong interactions of the exposed Ni, Co, and Mn atoms with Se coating through the formation of metal–Se bonds, resulting in the conformal and homogenous deposition of Se layers. The uniform Se coating layers could not only protect the Ni-rich LNCM cathode materials from the side reaction with the electrolytes but also reduce the degree of Li+/Ni2+ cation disorder. Therefore, owing to the multifunctional effects of Se coating, the Se-coated LNCM811 cathode materials exhibit excellent cyclic stability and rate performances with a high initial capacity of 205 mA h g–1 at 0.2 C and the high reversible capacities of 173 mA h g–1 at the 100th cycle at 0.2 C, 132 mA h g–1 at 200th cycle at 2 C, and 103 mA h g–1 at 300th cycle at 5 C. This present work indicates that the Se coating on the advanced Ni-rich layered metal oxide cathode material through the facile melt-diffusion approach should be promising for the high performance Li-ion battery.

Sulfonate-Based Artificial Cathode–Electrolyte Interface to Enhance Electrochemical Performance of Ni-Rich Layered Oxide Cathode Materials

Despite significant progress in the field of Li-ion battery development over the past 30 years, challenges associated with the gradual shift from small-scale to large-scale battery applications nec...