Enhanced electrochemical performance of Ni-rich cathode material by N-doped LiAlO2 surface modification 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.

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.

Effects of Oxygen Pressurization on Li+/Ni2+ Cation Mixing and the Oxygen Vacancies of LiNi0.8Co0.15Al0.05O2 Cathode Materials

Ni-rich cathode materials are a low-cost and high-energy density solution for high-power lithium-ion batteries. However, Li⁺/Ni²⁺ cation mixing and oxygen vacancies are inevitably formed during the high-temperature calcination process, resulting in a poor crystal structure that adversely affects the electrochemical performance. In this work, the LiNi_0.8Co_0.15Al_0.05O₂ cathode material with a regular crystal structure was prepared through oxygen pressurization during lithiation-calcination, which effectively solved the problems caused by the high calcination temperature, such as oxygen loss and a reduction of Ni³⁺. The co-effect of oxygen pressure and calcination temperature on the properties of Ni-rich materials was systematically explored. Oxygen pressurization increased the redox conversion temperature, thus promoting the oxidation of Ni²⁺ and reducing Li⁺/Ni²⁺ cation mixing. Moreover, due to the strong oxidizing environment provided by the elevated calcination temperature and oxygen pressurization, the LiNi_0.8Co_0.15Al_0.05O₂ material synthesized under 0.4 MPa oxygen pressure and a calcination temperature of 775 °C exhibited few oxygen vacancies, which in turn suppressed the formation of microcracks during the electrochemical cycling. An additional feature of the LiNi_0.8Co_0.15Al_0.05O₂ material was the small specific surface area of the particles, which reduced both the contact area with the electrolyte and side reactions. As a result, the LiNi_0.8Co_0.15Al_0.05O₂ material exhibited remarkable electrochemical performance, with an initial discharge capacity of 191.6 mA h·g^-1 at 0.1 C and a capacity retention of 94.5% at 0.2 C after 100 cycles.

Thermochemical Cyclization Constructs Bridged Dual-Coating of Ni-Rich Layered Oxide Cathodes for High-Energy Li-Ion Batteries

Enhancing microstructural and electrochemical stabilities of Ni-rich layered oxides is critical for improving the safety and cycle-life of high-energy Li-ion batteries. Here we propose a thermochemical cyclization strategy where heating polyacrylonitrile with LiNi_0.8Co_0.1Mn_0.1O₂ can simultaneously construct a cyclized polyacrylonitrile outer layer and a rock-salt bridge-like inner layer, forming a compact dual-coating of LiNi_0.8Co_0.1Mn_0.1O₂. Systematic studies demonstrate that the mild cyclization reaction between polyacrylonitrile and LiNi_0.8Co_0.1Mn_0.1O₂ induces a desirable "layered to rock-salt" structural transformation to create a nano-intermedium that acts as the bridge for binding cyclized polyacrylonitrile to layered LiNi_0.8Co_0.1Mn_0.1O₂. Because of the improvement of the structural and electrochemical stability and electrical properties, this cathode design remarkably enhances the cycling performance and rate capability of LiNi_0.8Co_0.1Mn_0.1O₂, showing a high reversible capacity of 183 mAh g^-1 and a high capacity retention of 83% after 300 cycles at 1 C rate. Notably, this facile and scalable surface engineering makes Ni-rich cathodes potentially viable for commercialization in high-energy Li-ion batteries.

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.