Chunman Zheng

A 3D nanostructure of graphene interconnected with hollow carbon spheres for high performance lithium–sulfur batteries

To better suppress the capacity decay over cycling and improve the electrical insulation of the sulfur cathode for lithium–sulfur (Li–S) batteries, we designed a novel three-dimensional nanostructure of graphene interconnected with hollow carbon spheres (3D rGO–HCS) as the sulfur host. The 3D rGO–HCS nanostructure was first prepared via a hydrothermal self-assembly method followed by carbonization and etching of the SiO2 core, then sulfur was impregnated into the nanostructure by an in situ solution deposition method to obtain the S@rGO–HCS cathode. The as-prepared cathode material delivers a high discharge capacity of ∼770 mA h g−1 at 4 C rate. More importantly, it has a high capacity retention of 93.9% after 100 cycles and demonstrates a low capacity-decay rate of 0.052% per cycle after 400 cycles at 0.5 C rate. The superior comprehensive electrochemical performance of the S@rGO–HCS cathode is ascribed to the synergic effects from the 3D graphene-network design, including fast electron and ion transportation, efficient confinement of polysulfide dissolution and shuttling and successful maintenance of structural integrity.

Enhanced Li Storage Performance of LiNi0.5Mn1.5O4–Coated 0.4Li2MnO3·0.6LiNi1/3Co1/3Mn1/3O2 Cathode Materials for Li-Ion Batteries

In this study, Li-rich cathode, 0.4Li2MnO3·0.6LiNi1/3Co1/3Mn1/3O2 was synthesized by a resorcinol formaldehyde assisted sol-gel method for the first time. Then, the surface of the as-prepared Li-rich cathode was modified with different amounts of LiNi0.5Mn1.5O4 (5, 10, and 20 wt %) through a simple dip-dry approach. The structural and electrochemical characterizations revealed that the spinel LiNi0.5Mn1.5O4 coating not only can prevent electrolytes from eroding the Li-rich core but also can facilitate fast lithium ion transportation. As a result, the 20 wt % coated sample delivered an initial discharge capacity of 298.6 mAh g(-1) with a Coulombic efficiency of 84.8%, compared to 281.1 mAh g(-1) and 70.2%, respectively, for the bare sample. Particularly, the coated sample demonstrates a Li storage capacity of 170.7 mAh g(-1) and capacity retention of 94.4% after 100 cycles at a high rate of 5 C (1250 mA g(-1)), showing a prospect for practical lithium battery applications. More significantly, the synthetic method proposed in this work is facile and low-cost and possibly could be adopted for large-scale production of surface-modified cathode materials.

Graphene oxide wrapped hierarchical porous carbon–sulfur composite cathode with enhanced cycling and rate performance for lithium sulfur batteries

A graphene oxide sheet wrapped hierarchical porous carbon–sulfur (HPC–S@GO) composite was designed by a two-step method to improve the lithium sulfur battery performance. With this nanostructure design, the hierarchical porous carbon supplies an electronic transport pathway and provides a large pore volume to load the sulfur while the encapsulated graphene oxide sheet is effective in trapping sulfur and polysulfides during cycling. The obtained HPC–S@GO composite delivers prolonged cycling stability and an enhanced rate performance: the capacity fading is 0.12% per cycle over 400 cycles at 1 C (1672 mA g−1) rate, and the capacities at 0.2, 0.5, 1, 2 and 5 C rate are 1333.3, 896.9, 763.0, 669.5 and 505.6 mA h g−1, respectively.

A facile one-step hydrothermal synthesis of α-Fe2O3 nanoplates imbedded in graphene networks with high-rate lithium storage and long cycle life

In this study, we demonstrate a facile one-step hydrothermal strategy to build a nanostructure of α-Fe2O3 nanoplates imbedded in graphene networks, using water and glycerol as hydrothermal solvents. The graphene oxide was chemically reduced with Fe2+ and glycerol, and the obtained α-Fe2O3 nanoplates with a thickness of 20–30 nm and a side length of 100–300 nm are well wrapped by and in tight contact with the flexible conductive graphene networks. When used as the anode material for lithium ion batteries, the rGO/α-Fe2O3 nanoplate composite demonstrates high discharge capacities of ∼896 mA h g−1 up to 200 cycles at 5 C and ∼429 mA h g−1 up to 1000 cycles even at a 10 C rate. The excellent lithium storage performance could be attributed to the synergistic effects of the unique structures, which can provide fast electron transport and shorten the diffusion path of the Li ions as well as accommodate the volume change of the composite in the cycling.

Si/Si–O–C composite anode materials exhibiting good C rate performances prepared by a sol–gel method

Si/Si–O–C composite materials with nano-silicons uniformly distributed in an amorphous Si–O–C matrix are prepared using a sol–gel method. The resulting materials exhibit a large initial coulombic efficiency of 78%, and deliver a capacity of 715.9 mA h g−1 after 30 cycles at 3900 mA g−1. The high capacity originates from lithium storage in nanosilicons and the Si–O–C matrix, and the excellent C-rate performances are related to the buffering of the Si–O–C matrix.

Encapsulating sulfur into highly graphitized hollow carbon spheres as high performance cathode for lithium–sulfur batteries

Encapsulating sulfur into a highly graphitized hollow carbon sphere (GHCS) is proposed as sulfur cathode for the first time. After annealing the amorphous hollow carbon sphere (HCS) at a high temperature of 2600 °C, the obtained GHCS shows polyhedral morphology and few layers graphene characteristic with extremely low oxygen content. When used as sulfur cathode, the S@GHCS composite delivers a high discharge capacity of ∼800 mA h g−1 at 4C rate and high capacity retention of 93.7% after 240 cycles at 1C rate, demonstrating much better rate capability and cycling performance compared to those of S@HCS composite.

The significance of the stable Rhombohedral structure in Li-rich cathodes for lithium-ion batteries

The lithium-rich layered oxides offer higher capacity, but suffer from severe capacity fade and voltage decay. Aluminum doping can improve the stability of the structure, but the fundamental mechanism has not been fully revealed. In this work, cathode materials with different Al-doping contents are investigated. To characterize the structural evolutions upon cycling, ex situ XRD and ex situ TEM are performed. It is demonstrated that the voltage decay is attributed to the decrease of Rhombohedral phase, while capacity fade is possibly associated with the increase of surface impedance. Therefore, continuous voltage decay and capacity fade are observed for undoped Li-rich cathode. In comparison, the Al-doped cathodes have stable LiNi1/3Co1/3Mn1/3O2 and surface impedance, leading to superior capacity retention and a stable output voltage.

Identification of solid electrolyte interphase formed on graphite electrode cycled in trifluoroethyl aliphatic carboxylate-based electrolytes for low-temperature lithium-ion batteries

Trifluoroethyl aliphatic carboxylates with different length of carbon-chain in acyl groups have been introduced into carbonate-based electrolyte as co-solvents to improve the low-temperature performance of lithium-ion batteries, both in capacity retention and lowering polarization of graphite electrode. To identify the further influence of trifluoroethyl aliphatic carboxylates on graphite electrode, the components and properties of the surface film on graphite electrode cycled in different electrolytes are investigated using Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and electrochemical measurements. The IR and XPS results show that the chemical species of the solid electrolyte interphase (SEI) on graphite electrode strongly depend on the selection of co-solvent. For instance, among those species, the content of RCOOLi increases with an increasing number of carbon atoms in RCOOCH2CF3 molecule, wherein R was an alkyl with 1, 3, or 5 carbon atoms. We suggest that the thickness and components of the SEI film play a crucial role on the enhanced low-temperature performance of the lithium-ion batteries.