Enhanced rate performance and high potential as well as decreased strain of LiNi0.6Co0.2Mn0.2O2 by facile fluorine modification

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.

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.

A review on nickel-rich nickel-cobalt-manganese ternary cathode materials LiNi0.6Co0.2Mn0.2O2 for lithium-ion batteries: performance enhancement by modification

The new energy era has put forward higher requirements for lithium-ion batteries, and the cathode material plays a major role in the determination of electrochemical performance. Due to the advantages of low cost, environmental friendliness, and reversible capacity, high-nickel ternary materials are considered to be one of ideal candidates for power batteries now and in the future. At present, the main design idea of ternary materials is to fully consider the structural stability and safety performance of batteries while maintaining high energy density. Ternary materials currently face problems such as low lithium-ion diffusion rate and irreversible collapse of the structure, although the battery performance can be improved utilizing coating, ion doping, etc., the actual demand requires a more effective modification method based on the intrinsic properties of the material. Based on the summary of the current research status of the ternary material LiNi0.6Co0.2Mn0.2O2 (NCM622), a comparative study of the modification paths of the material was conducted from the level of molecular action mechanism. Finally, the major problems of ternary cathode materials and the future development direction are pointed out to stimulate more innovative insights and facilitate their practical applications.

Formation of LiF Surface Layer During Direct Fluorination of High-Capacity Co-Free Disordered Rocksalt Cathodes

Disordered rocksalt (DRX) cathodes have attracted interest due to their high capacity and compositional flexibility (e.g., Co-free chemistries). However, the sloping voltage profile and gradual capacity fade during cycling have hindered the widespread adoption of these materials. Simulations predict that fluorine substitution in DRX cathodes will improve their capacity, rate performance, and cyclability. In this study, we use a fluidized bed reactor to fluorinate a model Li-rich DRX composition (Li_1.15Ni_0.375Ti_0.375Mo_0.1O₂, NTMO) to investigate how fluorine content impacts the cathode's structure and electrochemical performance. Instead of substituting O with F to form oxyfluoride phases, direct fluorination of DRX cathodes leads to the formation of LiF surface films, which improves the specific energy and capacity retention. This study demonstrates the feasibility of direct fluorination to improve the electrochemical performance of high-voltage cathodes by tuning the material's surface chemistry.

Enhanced Electrochemical Performance and Safety of Silicon by a Negative Thermal Expansion Material of ZrW2O8

Silicon (Si) faces big challenges in serious volume changes for applications in spite of its high theoretical capacity. Herein, a novel and facile method was proposed to decrease the volume change by simultaneously in situ absorbing the generated heat of only Si using a negative thermal expansion (NTE) material of ZrW₂O₈. The Si modified with 2 wt % of ZrW₂O₈ exhibits excellent structural integrity, electrochemical performance, and safety under various conditions, especially at elevated temperatures. Its reversible capacities can remain 1187.2 mA h g^-1 after 50 cycles and 643.8 mA h g^-1 after 100 cycles at 2 A g^-1 (∼199 and ∼190% higher than that of Si, respectively) at 25 °C. In addition, 930.6 mA h g^-1 is maintained after 50 cycles at 60 °C (∼219% higher than that of Si). As current densities increase to 2 and 4 A g^-1, the values still remain 1389.4 and 757.5 mA h g^-1, respectively, much higher than that of Si. Furthermore, the strain of Si is reduced by 37.2% using ZrW₂O₈ at 60 °C. Various products were analyzed, and the possible enhanced mechanism was discussed using multiple techniques. These findings exhibit significant potential for the improvement of energy materials using NTE materials by combining thermal effects and volume changes as well as the improved interface behavior.

Synergistic Effect of F- Doping and LiF Coating on Improving the High-Voltage Cycling Stability and Rate Capacity of LiNi0.5Co0.2Mn0.3O2 Cathode Materials for Lithium-Ion Batteries

A commercial LiNi_0.5Co_0.2Mn_0.3O₂ (LNCM) cathode material is purposefully modified using a small account of LiPF₆ as one precursor via a simple means at low calcination temperature in air. X-ray diffraction, scanning electron microscopy, and transmission electron microscopy images reveal that this modification process keeps the layered bulk structure of LNCM even though the surface components have obviously been modified. Electron energy loss spectroscopy and X-ray photoelectron spectroscopy with different etching depths further prove the formation of LiF and F^- doping on the LNCM surface, which simultaneously triggers partial Ni³⁺ reduction to Ni²⁺; and the metal-oxygen bond is partially replaced by a higher energy metal-fluorine bond. The modified material (LNCM-2) retains 93.7% of its initial capacity and delivers 179.4 mAh g^-1 at a current density of 0.5 C after 100 stable cycles at 3.0-4.5 V. Meanwhile, LNCM-2 is able to maintain capacity retention up to 81.1% after 300 cycles at 5 C, much better than the original LNCM (35.1%) in the commercial electrolyte. Remarkably, 90% of initial capacity is retained for LNCM-2 with considerably improved Coulombic efficiency (>99.5%) at 5 C after 300 cycles within a voltage range of 3-4.5 V compared with the primary LNCM using succinonitrile-based electrolyte. Consequently, these results fully demonstrate the advantages of synergistic effect between F^- doping and LiF coating.