Na2 Ti3 O7 /C Nanofibers for High-Rate and Ultralong-Life Anodes in Sodium-Ion Batteries

Rigid organic molecule pillared Ti3C2 towards high rate capability and fast sodium ion storage

MXenes are promising two-dimensional layered anode materials for rechargeable batteries due to their outstanding electrical conductivity, high specific surface area, and tunable surface functional groups. However, serious self-stacking of the layered structure and the sluggish sodium diffusion kinetics lead to inferior rate capability and cycling stability. Herein, an organic molecular pillaring strategy is reported to enlarge the interlayer spacing of Ti3C2 through a dehydration condensation reaction between the -COOH groups of 3,3',4,4'-benzene tetracarboxylic acid (BTCA) molecules and the -NH2 groups of Ti3C2-NH2, which enables rigid organic BTCA molecules to be chemically pillared into the interlayers of Ti3C2 (Ti3C2-BTCA). The rigid organic BTCA molecules not only play a dual role of pillar and strain effects in Ti3C2 layers, but also expand the interlayer spacing. Therefore, they can significantly enhance the rate capability and cycling stability of Ti3C2. Ti3C2-BTCA exhibits a reversible capacity of 182.3 mA h g-1 at a current density of 0.1 A g-1 after 2000 cycles and maintains a reversible capacity of 77.9%. Moreover, the sodium diffusion coefficient of Ti3C2-BTCA is 6.6 × 10-7 cm2 s-1. Ti3C2-BTCA shows a relatively low sodium diffusion barrier and a high sodium diffusion coefficient compared with Ti3C2. Interlayer engineering based on the organic molecular pillaring strategy is significant and meaningful for expanding the interlayer spacing of Ti3C2. This work provides theoretical guidance and new perspectives for the development of Na+ storage materials with high-rate capability.

Boosting the Electrochemical Performance of V2 O3 by Anchoring on Carbon Nanotube Microspheres with Macrovoids for Ultrafast and Long-Life Aqueous Zinc-Ion Batteries

Zinc-ion batteries (ZIBs) are next-generation energy storage systems with high safety and environmental friendliness because they can be operated in aqueous systems. However, the search for electrode materials with ideal nanostructures and compositions for aqueous ZIBs is in progress. Herein, the synthesis of porous microspheres, consisting of V₂ O₃ anchored on entangled carbon nanotubes (p-V₂ O₃ -CNT) and their application as cathode for ZIBs is reported. From various analyses, it is revealed that V₂ O₃ phase disappears after the initial charge process, and Zn_{3+ x} (OH)_{2+3 x} V_{2- x} O_{7-3 x} ∙2H₂ O and zinc vanadate (Zn_y VO_z ) phases undergo zinc-ion intercalation/deintercalation processes from the second cycle. Additionally, the electrochemical performances of p-V₂ O₃ -CNT, V₂ O₃ -CNT (without macrovoids), and porous V₂ O₃ (without CNTs) microspheres are compared to determine the effects of nanostructures and conductive carbonaceous matrix on the zinc-ion storage performance. p-V₂ O₃ -CNT exhibits a high reversible capacity of 237 mA h g^-1 after 5000 cycles at 10 A g^-1 . Furthermore, a reversible capacity of 211 mA h g^-1 is obtained at an extremely high current density of 50 A g^-1 . The macrovoids in V₂ O₃ nanostructure effectively alleviate the volume changes during cycling, and the entangled CNTs with high electrical conductivity assist in achieving fast electrochemical kinetics.

Three-Dimensional Fast Na-Ion Transport in Sodium Titanate Nanoarchitectures via Engineering of Oxygen Vacancies and Bismuth Substitution

Layered sodium titanates (NTO), one of the most promising anode materials for advanced sodium-ion batteries (SIBs), feature high theoretical capacity and no serious safety concerns. The pristine NTO electrode, however, has unfavorable Na⁺ transport kinetics, due to the dominant two-dimensional (2D) Na-ion transport channels within the crystal along the low energy barrier octahedron layers, which impedes the practical application of this class of potential materials. Herein, an interesting concept of opening three-dimensional (3D) fast ion transport channels within the intrinsic NTO frameworks is proposed to enhance the electrochemical performance through a combination of oxygen vacancy generation and cation substitution strategies, by which the interlayer spacing of the NTO frameworks is expanded for fast 3D Na-ion transport. It is evidenced that the oxygen-deficient and bismuth-substituted HBNTO (Bi_xNa_2-xTi₃O_y, 0 < x < 2, 0 < y < 7, HBNTO) exhibits obvious enhancements on the reversible capacity (∼145% enhancement at 20 mAh g^-1 compared with NTO), the rate capability (∼200% enhancement at 500 mAh g^-1 compared with NTO), and the cycling stability (∼210% enhancement of retention capacity after 150 cycles at 20 mAh g^-1 compared with NTO). The molecular dynamic simulations and theoretical calculations demonstrate that the enhanced performance of HBNTO is contributed by the multiplied sodium diffusion pathways and the increased ion migration rates with the successful opening of 3D internal ion transport channels. This work demonstrates the effectiveness of the strategies in opening the 3D intercrystal ion transport channels for boosting the electrochemical performance of SIBs.