Xinxiang Zhang

High sulfur loading composite wrapped by 3D nitrogen-doped graphene as a cathode material for lithium–sulfur batteries

A porous three-dimensional nitrogen-doped graphene (3D-NG) was introduced as an interconnected framework for sulfur in lithium–sulfur batteries. The 3D-NG-sulfur composite (3D-NGS) with a high sulfur content of 87.6 wt% was synthesized via a facile one-pot solution method and sulfur was well dispersed within it. The as-designed 3D-NGS composite exhibits excellent rate capability and cyclability. The discharge specific capacity is 792 mA h g−1 after 145 cycles at a current density of 600 mA g−1 and the capacity fading rate is 0.05% per cycle. Even at a high rate of 1500 mA g−1, the composite still shows a good cycle performance with a capacity of 671 mA h g−1 after 200 cycles. The outstanding electrochemical performance can be attributed to the flexible porous 3D structure and N-doping in graphene. The flexible 3D-NG can provide a conductive framework for electron transport and alleviate the volume effect during cycling. N-doping can facilitate the penetration of Li ions across the graphene and restrain sulfur due to the strong chemical bonding between S and the nearby N atoms.

Dual core–shell structured sulfur cathode composite synthesized by a one-pot route for lithium sulfur batteries

Lithium–sulfur batteries are promising electrochemical devices for future energy conversion and storage. Its theoretical capacity is 1675 mA h g−1, much higher than that of conventional lithium-ion batteries. However, it suffers from rapid capacity decay and low energy efficiency. In this work, we introduce a novel dual core–shell structured sulfur composite with multi-walled carbon nanotubes (MWCNTs) and polypyrrole (PPy), MWCNTs@S@PPy, as a cathode material for Li–S batteries. The composite is synthesized via a facile one-pot method. In the structure, MWCNTs and PPy work as a combined conductive framework to provide access to Li+ ingress and egress for reaction with sulfur, and to inhibit the diffusion of polysulfide out of the cathode, and hence reduce the capacity decay. Meanwhile, LiNO3 additive is added into the electrolyte to improve the coulombic efficiency. The as-designed MWCNTs@S@PPy composite shows excellent rate capability and cyclability. The initial discharge specific capacity is as high as 1517 mA h g−1, and remains at 917 mA h g−1 after 60 cycles at a current density of 200 mA g−1. Even at a high current density of 1500 mA g−1, the composite still shows a good cycle performance with a capacity of 560 mA h g−1 after 200 cycles.

Fabrication of high tap density LiFe0.6Mn0.4PO4/C microspheres by a double carbon coating–spray drying method for high rate lithium ion batteries

Spherical LiFe0.6Mn0.4PO4/C particles with high tap density were successfully synthesized by sintering spherical precursor powders prepared by a modified spray drying method with a double carbon coating process. The obtained secondary spheres were made of carbon-coated nanocrystallines (∼100 nm), exhibiting a high tap density of 1.4 g cm−3. The LiFe0.6Mn0.4PO4/C microspheres had a reversible capacity of 160.2 mAh g−1 at 0.1C, and a volume energy density of 801.5 Wh L−1 which is nearly 1.4 times that of their nano-sized counterparts. This spherical material showed remarkable rate capability by maintaining 106.3 mAh g−1 at 20C, as well as excellent cycleablity with 98.9% capacity retention after 100 cycles at 2C and 200 cycles at 5C. The excellent electrochemical performance and processability of the LiFe0.6Mn0.4PO4/C microspheres make them very attractive as cathode materials for use in high rate battery application.

Monodisperse Li1.2Mn0.6Ni0.2O2 microspheres with enhanced lithium storage capability

Monodisperse spherical Mn0.75Ni0.25(OH)2 precursors built up from plate-like primary particles have been successfully synthesized by the control of pH values during a co-precipitation reaction. The size of spherical particles, namely the secondary particles, is observed to decrease with increasing pH value from 9.0 to 11.0, and is accompanied by a series of shape changes of the primary particles from close-packed plates to well-exposed nanoplates, and then to nanoparticles. Further lithiation of these hydroxide precursors produces the final lithium-rich layered Li1.2Mn0.6Ni0.2O2 cathode materials without destroying the morphology of the precursors. Electrochemical measurements show that the spherical cathode material assembled from well-exposed nanoplates exhibits superior rate capability and good cyclability compared to other electrode materials, which can be attributed to its uniform particle size and the favorable shape which facilitates the diffusion of lithium ions. Through the control of the sample morphologies, we provide a simple and effective way to enhance the lithium storage capability of lithium-rich layered oxide cathode materials for high-performance lithium-ion batteries.

Superior performance of nanoscaled Fe3O4 as anode material promoted by mosaicking into porous carbon framework

A nanoscale Fe3O4/porous carbon-multiwalled carbon nanotubes (MWCNTs) composite is synthesized through a simple hard-template method by using Fe2O3 nanoparticles as the precursor and SiO2 nanoparticles as the template. The composite shows good cycle performance (941 mAh g-1 for the first cycle at 0.1 C, with 106% capacity retention at the 80th cycle) and high rate capability (71% capacity retained at 5 C rate). Its excellent electrical properties can be attributed to the porous carbon framework structure, which is composed of carbon and MWCNTs. In this composite, the porous structure provides space for the change in Fe3O4 volume during cycling and shortens the lithium ion diffusion distance, the MWCNTs increase the electron conductivity, and the carbon coating reduces the risk of side reactions. The results provide clear evidences for the utility of porous carbon framework to improve the electrochemical performances of nanosized transition-metal oxides as anode materials for lithium-ion batteries.