Henghui Zhou

Self-supported Li4Ti5O12 nanosheet arrays for lithium ion batteries with excellent rate capability and ultralong cycle life

Facile fabrication of well-aligned Li4Ti5O12 (LTO) nanosheet arrays grown directly on conductive Ti foil was achieved by hydrothermal growth in LiOH solution. The reaction between Ti foil and LiOH led to the growth of vertically aligned, rectangular lithium titanate oxide hydrate (H-LTO) nanosheet arrays, which could be converted into LTO nanosheet arrays through topotactic transformation via thermal decomposition. An appropriate LiOH concentration was essential for the formation of densely aligned H-LTO nanosheet arrays on the substrate. It was proposed that the formation of the H-LTO nanosheet arrays was through kinetics-controlled growth during the hydrothermal metal corrosion process. When used as a binder-free anode for LIBs, the self-supported LTO nanosheet arrays standing on Ti foil exhibited an excellent rate capability (a reversible capacity of 163 mA h g−1 and 78 mA h g−1 at 20 C and 200 C, respectively) and an outstanding cycling performance (a capacity retention of 124 mA h g−1 after 3000 cycles at 50 C). Furthermore, a flexible lithium ion battery, which could be fully recharged within 30 s and was able to light an LED, was assembled by using the LTO nanosheet arrays as the anode.

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.

Monodispersed mesoporous Li4Ti5O12 submicrospheres as anode materials for lithium-ion batteries: morphology and electrochemical performances

Although nanosizing Li4Ti5O12 (LTO) materials is an effective way to improve their rate performances, their low tap density and first cycle coulombic efficiency limit their practical applications. To tackle these problems while preserving the advanced rate performances, monodispersed mesoporous LTO submicrospheres are developed here. These submicrospheres are synthesized via a solvothermal method using TiO2 submicrospheres and LiOH as precursors followed by a mild calcinations. The roles of the solvent used in the solvothermal process and calcination temperature are systematically investigated and optimized. The LTO submicrospheres fabricated by the solvothermal process using a water-ethanol (60 vol%) solvent followed by a calcination process at 600 °C reveal a large sphere size of 660 ± 30 nm with a small primary particle size of 20-100 nm, a large specific surface area of 15.5 m(2) g(-1), an appropriate pore size of 4.5 nm and an ultra-high tap density of 1.62 g cm(-3). Furthermore, they show high crystallinity and no blockage of Li(+) ion transportation pathways. Due to the novel morphology and ideal crystal structure, these submicrospheres exhibit outstanding electrochemical performances. They display a high first cycle coulombic efficiency of 93.5% and a high charge capacity of 179 mA h g(-1) at 0.5 C between 1.0 and 2.5 V (vs. Li/Li(+)), surpassing the theoretical capacity of LTO. Their charge capacity at 10 C is as high as 109 mA h g(-1) with a capacity retention of 97.8% over 100 cycles. Therefore, this LTO material can be a superior and practical candidate for the anodes of high-power lithium-ion batteries.

Li4Ti5O12-based anode materials with low working potentials, high rate capabilities and high cyclability for high-power lithium-ion batteries: a synergistic effect of doping, incorporating a conductive phase and reducing the particle size

Doping, incorporating a conductive phase and reducing the particle size are three strategies for improving the rate capability of Li4Ti5O12 (LTO). Thus, the synergistic employment of these three strategies is expected to more efficiently improve the rate capability. To achieve this goal, Fe2+ doped LTO/multiwall carbon nanotube (MWCNT) composites were prepared by post-mixing MWCNTs with Fe2+ doped LTO particles from a solid-state reaction, while Cr3+ doped LTO/MWCNT composites were fabricated by a facile one-step solid-reaction using MWCNT premixing. Fe2+/Cr3+ doping not only remarkably improves the electronic conductivity and Li+ ion diffusion coefficient in LTO but also lowers its working potential. The carbon existed in the material fabrication processes leads to the reduction of the particle size. The introduction of MWCNTs in the Fe2+/Cr3+ doped LTO/MWCNT composite significantly enhances the electrical conduction between Fe2+/Cr3+ doped LTO particles. As a result of this novel synergistic strategy, performances of Li3.8Fe0.3Ti4.9O12/MWCNT and LiCrTiO4/MWCNT composites are comprehensively improved. The Li3.8Fe0.3Ti4.9O12/MWCNT composite shows a working potential of 8.9 mV lower than that of pristine LTO. At 10 C, its capacity is up to 106 mA h g−1 with an unexpected capacity retention of 117% after 200 cycles in a potential window of 1.0–2.5 V (vs. Li/Li+). The corresponding values for LiCrTiO4/MWCNT composites are 46.2 mV, 120 mA h g−1 and 95.9%. In sharp contrast, the pristine counterpart shows a very disappointing capacity of only 11 mA h g−1 at 10 C. Therefore, the novel Li3.8Fe0.3Ti4.9O12/MWCNT and LiCrTiO4/MWCNT composites possess great potential for applications in high-power lithium-ion batteries.

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.

Branched CNT@SnO2 nanorods@carbon hierarchical heterostructures for lithium ion batteries with high reversibility and rate capability

A novel hierarchical heterostructure consisting of carbon-coated SnO2 mesocrystalline nanorods radially aligned on carbon nanotubes (CNTs) was designed and fabricated by a two-step growth process. SnO2 nanorods were first grown directly on CNTs through a facile solvothermal reaction, which were subsequently coated with a thin layer of carbon to form a branched CNT@SnO2@carbon sandwich-type heterostructure. When used as an anode material in lithium ion batteries, the branched CNT@SnO2@C heterostructures exhibited highly reversible lithium storage behavior and excellent rate capability. The reversible capacity of the CNT@SnO2@C heterostructure reached 984 mA h g−1 at a current density of 720 mA g−1, and retained 590 mA h g−1 at 3.6 A g−1 and 420 mA h g−1 at 7.2 A g−1. This superior performance might be ascribed to the improved mechanical capability and high loading content of SnO2 of the branched architecture, the good electrical conductivity of the CNT backbones and the carbon layer, and the high electrochemical reactivity of the 1D mesocrystalline SnO2 nanorods.

Robust α-Fe2O3 nanorod arrays with optimized interstices as high-performance 3D anodes for high-rate lithium ion batteries

Self-supported α-Fe2O3 nanorod arrays consisting of mesocrystalline nanorod bundles with tunable interstices were prepared by solution-phase growth coupled with chemical etching. The existence of acetic acid and sulfate ions in the hydrothermal system promoted the direct growth of α-Fe2O3 nanorod bundles with a mesocrystalline structure on a Ti substrate. The robust α-Fe2O3 nanorod arrays with optimized interstices are able to offer reduced lengths for electron transport and ion diffusion, and enough spaces to accommodate lithiation-induced volume expansion, leading to novel three-dimensional (3D) anodes with significantly improved rate capability and cyclability. When used as binder-free anodes for lithium ion batteries (LIBs), the α-Fe2O3 nanorod arrays retained a reversible capacity of 801 mA h g−1 after 500 cycles at 5 C (namely, 5 A g−1), and achieved practically valuable capacities of 499 mA h g−1 and 350 mA h g−1 at high rates of 20 C and 30 C, respectively. Furthermore, a flexible full battery with high capacity and fast charging capability was assembled using the α-Fe2O3 nanorod arrays as the anode, demonstrating their potential applications in flexible electronic devices.

In situ quantization of ferroferric oxide embedded in 3D microcarbon for ultrahigh performance sodium-ion batteries

Unlike conventional carbon coating strategies which only focus on the macrodimension to enhance electrical conductivity and alleviate volume variation for high-capacity metal oxide anode materials, a hierarchically raspberry-like microstructure embedded with three-dimensional carbon-coated Fe3O4 quantum dots is built for ultrafast rechargeable sodium ion batteries. Taking advantage of using metal organic frameworks (MOFs) as templates, it realizes an in situ quantization process in which Fe3O4 quantum dots are formed and uniformly embedded in microcarbon coating protection. Due to the short diffusion length and integrated hierarchical conductive network, the electrode combines supercapacitor-like rate performance (e.g., less than 6 minutes to full charge/discharge) and battery-like capacity (e.g., maintaining >90% of theoretical capacity). An interesting surface-induced process which imitates pseudocapacitive behaviors in supercapacitors is analyzed in detail. This proof-of-concept study and insightful understanding on sodium storage in this investigation may inherently solve the widely encountered problems existing in high-capacity metal oxide anode materials and point out new directions for the future development of ultrafast rechargeable sodium ion batteries.