A Universal Method for Enhancing the Structural Stability of Ni-Rich Cathodes Via the Synergistic Effect of Dual-Element Cosubstitution

A review of Ni-based layered oxide cathode materials for alkali-ion batteries

Compared with the costly and toxic LiCoO2 cathode in lithium-ion batteries (LIBs), nickel-based layered oxide (NLO) cathode materials exhibit the advantages of high capacity, natural abundance, environment-friendliness, and low cost, displaying tremendous application potentials in power batteries for automobiles and aircrafts. This review comprehensively introduces the challenges faced by NLO cathode materials in all alkali-ion batteries (AIBs) in their material synthesis, cation mixing, particle cracking, phase changes, cation dissolution of Mn, and oxygen loss Various strategies, including heteroatom doping, surface coating, and concentration gradient, are applied to tackle these problems by developing layered LiNi1-xMxO2 (M: metal; 0 < x < 1) and LiNixCoyMnzO2 (x + y + z = 1) materials. The successful commercial application of NLO cathode materials in LIBs has further driven their developments in sodium/potassium-ion batteries via the synthesis of (Na/K)Ni1-xMxO2. Moreover, many sophisticated techniques, including in situ X-ray diffraction, scanning/transmission electron microscopy, operando neutron diffraction, and elemental analysis, are used to simultaneously monitor real-time phase changes, lattice variations, structural distortions, and elemental dissolutions of NLO-based materials. Furthermore, density functional theory (DFT) calculations are discussed as a powerful tool for predicting structural evolution, energy band structures, optimal doping concentrations, and ion diffusion pathways, thereby guiding the reasonable design of these materials. Finally, this review provides perspectives on future research directions and modification strategies for NLO cathode materials in AIBs, aiming to accelerate their deployment in electric vehicles and other energy storage devices. These efforts are expected to contribute significantly to the advancement of sustainable energy technologies and the global pursuit for carbon neutrality.

High-Entropy Rock-Salt Surface Layer Stabilizes the Ultrahigh-Ni Single-Crystal Cathode

Single-crystalline Ni-rich layered oxides are one of the most promising cathode materials for lithium-ion batteries due to their superior structural stability. However, sluggish lithium-ion diffusion kinetics and interfacial issues hinder their practical applications. These issues intensify with increasing Ni content in the ultrahigh-Ni regime (≥90%), significantly threatening the practical viability of the single-crystalline strategy for ultrahigh-Ni layered oxide cathodes. Herein, by developing a high-entropy coating strategy, we successfully constructed an epitaxial lattice-coherent high-entropy rock-salt layer (∼3 nm) via Zr and Al doping on the surface of the single-crystalline cathode LiNi0.92Co0.05Mn0.03O2 through an in situ modification process. The surface high-entropy rock-salt layer with tailored Ni valence and lattice coherence not only greatly improves lithium-ion diffusion kinetics but also suppresses interface parasitic reactions and surface structural degradations. The high-entropy surface layer-stabilized ultrahigh-Ni single-crystalline cathode (SC-Ni92-ZA) demonstrates significantly improved rate and cycling performances (127.5 mAh g-1 at 20C, capacity retention of 74.9% after 500 cycles at 1C) in a half-cell. The SC-Ni92-ZA exhibits a capacity retention of 87.1% after 600 cycles at 1C in a full-cell. This epitaxial lattice-coherent high-entropy coating strategy develops a promising avenue for developing high-capacity, long-life cathode materials.

A multifunctional dual cation doping strategy to stabilize high-voltage medium-nickel low-cobalt lithium layered oxide cathode

High-voltage medium-nickel low-cobalt lithium layered oxide cathode materials are intriguing for lithium-ion batteries (LIBs) applications because of their relatively low cost and high capacity. Unfortunately, high charging voltage induces bulk layered structure decline and interface environment deterioration, low cobalt content reduces lithium diffusion kinetics, severely limiting the performance liberation of this kind of cathode. Here, a multifunctional Al/Zr dual cation doping strategy is employed to enhance the electrochemical performance of LiNi0.6Co0.05Mn0.35O2 (NCM) cathode at a high charging cut-off voltage of 4.5 V. On the one hand, Al/Zr co-doping weakens the Li+/Ni2+ mixing through magnetic interactions due to the inexistence of unpaired electrons for Al3+ and Zr4+, thereby increasing the lithium diffusion rate and suppressing the harmful coexistence of H1 and H2 phases. On the other hand, they enhance the lattice oxygen framework stability due to strong Al-O and Zr-O bonds, inhibiting the undesired H2 to H3 phase transition and interface lattice oxygen loss, thereby enhancing the stability of the bulk structure and cathode-electrolyte interface. As a result, Al/Zr co-doped NCM (NCMAZ) shows a 94.2 % capacity retention rate after 100 cycles, while that of NCM is only 79.4 %. NCMAZ also exhibits better rate performance than NCM, with output capacities of 92 mAh/g and 59 mAh/g at a high current density of 5C, respectively. The modification strategy will make the high-voltage medium-nickel low-cobalt cathode closer to practical applications.

LiNbO3 Coating and F- Doping Stabilize the Crystal Structure and Ameliorate the Interface of LiNi0.88Co0.06Mn0.03Al0.03O2 to Improve the Electrochemical Properties and Safety Capability

Ni-rich layered materials Li[NixCoyMnzAl1-x-y-z]O2 (x > 0.8) are regarded as the competitive cathode for practical applications in lithium-ion batteries owing to the large discharging capacity. Nevertheless, the strong oxidation activity, the poor structure, and the thermal stability at the electrode-electrolyte interface would lead to much trouble, for example, inferior electrochemical properties and acute safety issues. To ameliorate the above problems, this work reports a strategy for the double modification of F- doping and LiNbO3 covering in LiNi0.88Co0.06Mn0.03Al0.03O2 cathode via using high-temperature calcining and ball-milling technology. As a result, the cathodes after F- doping and LiNbO3 covering not only demonstrate a more stabilized crystal structure and particle interface but also reduce the release of high-activity oxygen species to ameliorate the thermal runaway. The electrochemical tests show that the LiNbO3-F--modified cathode displays a superior rate capability of 159.3 mAh g-1 at 10.0 C and has the predominant capability retention of 92.1% in the 200th cycle at 25 °C, much superior than those (125.4 mAh g-1 and 84.0%) of bare cathode. Thus, the F- doped and LiNbO3-coated Ni-rich oxides could be a promising cathode to realize the high capacity and a stabilized interface.

Why the Synthesis Affects Performance of Layered Transition Metal Oxide Cathode Materials for Li-Ion Batteries

The limited cyclability of high-specific-energy layered transition metal oxide (LiTMO2) cathode materials poses a significant challenge to the industrialization of batteries incorporating these materials. This limitation can be attributed to various factors, with the intrinsic behavior of the crystal structure during the cycle process being a key contributor. These factors include phase transition induced cracks, reduced Li active sites due to Li/Ni mixing, and slower Li+ migration. In addition, the presence of synthesis-induced heterogeneous phases and lattice defects cannot be disregarded as they also contribute to the degradation in performance. Therefore, gaining a profound understanding of the intricate relationship among material synthesis, structure, and performance is imperative for the development of LiTMO2. This paper highlights the pivotal role of structural play in LiTMO2 materials and provides a comprehensive overview of how various control factors influence the specific pathways of structural evolution during the synthesis process. In addition, it summarizes the scientific challenges associated with diverse modification approaches currently employed to address the cyclic failure of materials. The overarching goal is to provide readers with profound insights into the study of LiTMO2.

Modification of the Ni-Rich Layered Cathode Material by Hf Addition: Synergistic Microstructural Engineering and Surface Stabilization

The rapid decline of the reversible capacity originating from microcracks and surface structural degradation during cycling is still a serious obstacle to the practical utilization of Ni-rich LiNixCoyAl1-x-yO2 (x ≥ 0.8) cathode materials. In this research, a feasible Hf-doping method is proposed to improve the electrochemical performance of LiNi0.9Co0.08Al0.02O2 (NCA90) through microstructural optimization and structural enhancement. The addition of Hf refines the primary particles of NCA90 and develops them into a short rod shape, making them densely arranged along the radial direction, which increases the secondary particle toughness and reduces their internal porosity. Moreover, Hf-doping stabilizes the layered structure and suppresses the side reactions through the introduction of robust Hf-O bonding. Multiple advantages of Hf-doping allowed significant improvement of the cycling stability of LiNi0.895Co0.08Al0.02Hf0.005O2 (NCA90-Hf0.5), with a reversible capacity retention rate of 95.3% after 100 cycles at 1 C, as compared with only 82.0% for the pristine NCA90. The proposed synergetic strategy combining microstructural engineering and crystal structure enhancement can effectively resolve the inherent capacity fading of Ni-rich layered cathodes, promoting their practical application for next-generation lithium-ion batteries.

Enhanced mechanical strength of a highly de-lithiated single-crystal Ni-rich cathode to suppress irreversible planar gliding

The mechanical properties of de-lithiated single-crystal Ni-rich cathodes are causing extensive concern. Here, we first show that the compression hardness of single crystal Ni-rich cathode particles decreases significantly at highly de-lithiated states by micro-compression testing. Thus, phase-boundary hardening was introduced to inhibit the planar gliding, resulting in excellent electrochemical performance.

Integrated Modification Strategy Enables Remarkable Cyclability and Thermal Stability of Ni-Rich Cathode Materials for Lithium-Ion Batteries

Structural degradation and surface chemical instability are dominant issues of Ni-rich layered cathodes, which trigger capacity fading and safety concerns, hindering the extensive application of Ni-rich cathodes toward high-energy, long-life lithium-ion batteries. Here, by combining trace Ta doping and an ultrathin Zr-Y mixed oxide coating, an integrated modification strategy significantly improves the cycling and thermal stability of Ni-rich LiNi_0.88Co_0.10Al_0.02O₂ (NCA) cathodes. The integrated modified Ni-rich cathode provides an unprecedented comprehensive performance with a high discharge capacity of 212.2 mA h g^-1 at 0.1 C, an 88.6% cycling retention after 500 cycles at 1 C, and a high exothermic peak temperature of 261 °C compared with the pristine NCA cathode (67.4% capacity retention for 500 cycles and 221 °C for the exothermic peak). Further mechanism studies illustrate that a dual-structural surface constructed of a rock salt surface induced by Ta doping and ultrathin Zr-Y mixed oxide coating jointly suppresses surface side reactions between cathodes and electrolytes. Moreover, trace Ta doping in the bulk stabilizes the bulk structure and prevents mechanical cracks. This study highlights the importance of comprehensive modification of the bulk and surface for improving the electrochemical performance and provides a potential optimizing strategy for the commercialization of high-capacity Ni-rich cathode materials.

Precise surface control of cathode materials for stable lithium-ion batteries

The increasing demand for high-energy Li-ion batteries (LIBs) continues to push the development of electrode materials, particularly cathode materials, towards their capacity limits. Despite the enormous success, the stability and reliability of LIBs are becoming a serious concern due to the much-aggravated side reactions between electrode materials and organic electrolytes. How to stabilize the cathode/electrolyte interface is therefore an imperative and urgent task drawing considerable attention from both academia and industry. An active treatment on the surface of cathode materials, usually by introducing an inert protection layer, to diminish their side reaction with electrolytes turns out to be a reasonable and effective strategy. This Feature Article firstly outlines our synthesis efforts for the construction of a uniform surface nanocoating on various cathode materials. Different wet chemical routes have been designed to facilitate the control of growth kinetics of targeted coating species so that a precise surface coating could be achieved with nanometer accuracy. Furthermore, we showed the possibility to transform the outer coating layer into a surface doping effect through surface solid reaction at high temperature. A detailed discussion on the structure-performance relationship of these surface-controlled cathode materials is introduced to probe the stabilization mechanism. Finally, perspectives on the development tendency of high-energy cathodes for stable LIBs are provided.

Gospel for Improving the Lithium Storage Performance of High-Voltage High-Nickel Low-Cobalt Layered Oxide Cathode Materials

High-voltage high-nickel low-cobalt lithium layered oxide cathodes show great application prospects for lithium-ion batteries due to their low cost and high capacity. However, deterioration of the bulk structure and the electrode-electrolyte interface will significantly endanger the cycle life and thermal stability of the battery as the nickel content and voltage increase. We present here a lattice doping strategy to greatly improve the cell performance by doping a small dose of Ti (2 mol %) in LiNi_0.6Co_0.05Mn_0.35O₂. Through density functional theory calculations, we know that the diffusion energy barrier of Li⁺ decreases and the activation energy of surface lattice oxygen atom loss increases after Ti doping, thereby improving the rate performance and inhibiting the undesired phase transition. The battery in situ X-ray diffraction (XRD) pattern demonstrates that Ti doping tunes the H1-H2 phase-transition process from a two-phase reaction to a single-phase reaction and inhibits the undesired H2-H3 phase transition, minimizing the mechanical degradation. The variable temperature in situ XRD reveals delayed phase-transition temperature to improve thermal stability. These improvements can be attributed to Ti doping to passivate the reactivity of the layered oxide cathode, which is fundamentally related to the strong Ti-O bond and no unpaired electrons for Ti⁴⁺. This work provides valuable strategic guidelines for the use of high-voltage high-nickel low-cobalt cathodes in lithium-ion batteries.

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 LiNi_0.8Co_0.1Mn_0.1O₂. 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 LiNi_0.8Co_0.1Mn_0.1O₂ 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.

Synergistic Effect of Microstructure Engineering and Local Crystal Structure Tuning to Improve the Cycling Stability of Ni-Rich Cathodes

Ultrahigh Ni-rich layered oxides have been regarded as one of the most promising cathode candidates. However, cycling instability induced by interfacial reactions and irreversible H2-H3 lattice distortion is yet to be demonstrated by an effective strategy that could construct a stable grain interface and microstructure. Here, Ni-rich cathode LiNi_0.92Co_0.05Mn_0.03O₂ is modified by B and Ti to realize the synchronous regulation of a microstructure and the oxygen framework robustness. Compared with the large equiaxed crystalline grains for the pristine cathode, highly elongated grains with a strong radially oriented crystallographic texture in which the (003) facet is maximized are produced for Ti and B-modified LiNi_0.92Co_0.05Mn_0.03O₂. With the suppressed H2-H3 phase transition and cation mixing provided by radially oriented grains and turned local crystal oxygen framework robustness during cycling, the co-modified cathode exhibits enhanced Li⁺ diffusion kinetics and a capacity retention of 78.3% after 100 cycles, which outperformed the 38.5% for the pristine cathode. The improved cycling performance suggests the significance of the turned microstructure and local crystal structure in suppressing internal strain and crystal structure degradation. The synchronous realization of microstructure engineering and local crystal structure turning by optimal element combination would provide a heuristic solution for the construction of high perform Ni-rich cathodes.