First-principles calculation on the lithium storage properties of high-entropy MXene Ti3C2(N0.25O0.25F0.25S0.25)2.

Experimental studies had shown that a variety of surface functional groups exist simultaneously on the surface of Ti3C2Tx MXenes. However, current theoretical calculations on MXenes, used as anode materials for lithium-ion batteries, consider only one surface functional group, which fails to take into account the actual situation. In this study, combining the characteristics of high-entropy materials and two-dimensional MXene material, a model of MXene with multiple surface functional groups was constructed, and its electrochemical performance as an anode material for lithium-ion batteries was further explored. The Ti3C2(N0.25O0.25F0.25S0.25)2 monolayer exhibited metallic properties. Meanwhile, Li atoms could be stably adsorbed on the surface and the diffusion energy barrier of Li on the surface was only 0.17 eV. First-principles calculation showed that Ti3C2(N0.25O0.25F0.25S0.25)2 monolayer had good rate performance and low open-circuit voltage (1 V), corresponding to a lithium storage capacity of 385.38 mA h g-1. The results of our work might inspire further studies on the Li storage performance of high-entropy MXenes experimentally and theoretically.

Phenediamine bridging phthalocyanine-based covalent organic framework polymers used as anode materials for lithium-ion batteries.

In this study, phenediamine bridging phthalocyanine-based covalent organic framework materials (CoTAPc-PDA, CoTAPc-BDA and CoTAPc-TDA) with increasingly-widening pore sizes are prepared by reacting cobalt octacarboxylate phthalocyanine with p-phenylenediamine (PDA), benzidine (BDA) and 4,4''-diamino-p-terphenyl (TDA), respectively. The effects of frame size on the morphology structure and its electrochemical properties were explored. X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET) and transmission electron microscopy (TEM) images show that the pore sizes of the CoTAPc-PDA, CoTAPc-BDA and CoTAPc-TDA are about 1.7 nm, 2.0 nm and 2.3 nm, respectively, which are close to the simulated results after geometric conformation optimization using Material Studio software. In addition, the specific surface areas of CoTAPc-PDA, CoTAPc-BDA and CoTAPc-TDA are 62, 81 and 137 m2 g-1, respectively. With increase in the frame size, the specific surface area of the corresponding material increases, which is bound to produce different electrochemical behaviors. Consequently, the initial capacities of the CoTAPc-PDA, CoTAPc-BDA and CoTAPc-TDA electrodes in lithium-ion batteries (LIBs) are 204, 251 and 382 mA h g-1, respectively. As the charge and discharge processes continue, the active points in the electrode material are continuously activated, leading to a continuous increase in charge and discharge capacities. After 300 cycles, the CoTAPc-PDA, CoTAPc-BDA and CoTAPc-TDA electrodes exhibit capacities of 519, 680 and 826 mA h g-1, respectively, and after 600 cycles, the capacities are maintained at 602, 701 and 865 mA h g-1, respectively, with a stable capacity retention rate at a current density of 100 mA g-1. The results show that the large-size frame structure materials have a larger specific surface area and more favorable lithium ion transmission channels, which produce greater active point utilization and smaller charge transmission impedance, thus showing larger charge and discharge capacity and superior rate capability. This study fully confirms that frame size is a key factor affecting the properties of organic frame electrodes, providing design ideas for the development of high-performance organic frame electrode materials.

Induction and Maintenance of Local Structural Durability for High-Energy Nickel-Rich Layered Oxides.

Nickel-rich layered oxides are one of the most promising cathode candidates for next-generation high-energy-density lithium-ion batteries. However, due to similar ion radius between Li+ and Ni2+ (0.76 and 0.69 Å), the Li+ /Ni2+ mixing phenomenon seriously hinders the migration of Li+ and increases kinetic barrier of Li+ diffusion, resulting in limited rate capability. In this work, the introduction of Ce4+ to effectively improve electrochemical properties of Ni-rich cathode materials is proposed. The LiNi0.8 Co0.15 Al0.05 O2 (LNCA) is modified with an additional precursor oxidization process using an appropriate amount of (NH4 )2 Ce(NO3 )6 . The Ce(NO3 )6 2- easily obtains electrons and generates reduction reactions, while Ni(OH)2 is prone to electron loss and oxidation reaction. The participation of (NH4 )2 Ce(NO3 )6 can promote the oxidation of Ni2+ to Ni3+ , thereby reducing the Li+ /Ni2+ mixing and increasing the structural stability of LNCA samples. Ce4+ cation doping can impede Li+ /Ni2+ mixing of LNCA cathode materials upon the long-term cycles. Both rate performance and long-term cyclability of Li[Ni0.8 Co0.15 Al0.05 ]0.97 Ce0.03 O2 (LNCA-Ce0.03) sample are significantly improved. Besides, a practical pouch cell based on the cathode presents sufficient gravimetric energy density (≈300 Wh kg-1 ) and cycling stability (capacity retention of 81.3% after 500 cycles at 1 C).