新型炭材料

2020, 35(2): .

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Abstract:

A review of silicon/carbon composite anode materials with an encapsulated structure for lithium-ion rechargeable batteries

WEI Jian, QIN Cong-min, SU Huan, WANG Jia-min, LI Xue-ting

2020, 35(2): 97-111.

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Abstract:
Si/C composites as anode materials have received increasing attention owing to their high energy density, low self-discharge efficiency and long cycle life in lithium-ion rechargeable batteries (LIBs). However, severe volume expansion and an unstable solid electrolyte interface in the lithiation-delithiation of silicon are major obstacles to their commercial applications. The production methods, electrochemical performance, specific capacity and cycling performance of Si/C composite anode materials are summarized, and indicate that by forming an encapsulated structure, the Si volume expansion and cracking of the carbon layer during lithiation can be greatly avoided and the cycling stability of LIBs can be effectively improved. Si encapsulated by carbon is a promising strategy for preparing Si-based anode materials to replace graphite for high-capacity LIBs.

An asymmetric supercapacitor based on a NiO/Co₃O₄@NiCo cathode and an activated carbon anode

LI Jing, ZOU Pei-chao, YAO Wen-tao, LIU Peng, KANG Fei-yu, YANG Cheng

2020, 35(2): 112-120. doi: 10.1016/S1872-5805(20)60478-4

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Abstract:
Pseudocapacitive metal oxide active materials are promising for use in high performance supercapacitors. However, their poor electrical conductivity greatly hinders their practical application. We formed NiO and Co₃O₄ active materials on the surface of a highly conductive NiCo nanowire membrane current collector by a simple air oxidation method to fabricate a self-supported flexible NiO/Co₃O₄@NiCo electrode. This significantly improves electron transport at the interface between the current collector and the active NiO/Co₃O₄. Furthermore, the reticular nanowire network facilitates ion transportation and releases strain caused during charge and discharge. Due to this unique structural characteristic, the NiO/Co₃O₄@NiCo electrode delivers a high specific capacitance of 1.36 F cm^-2 at a current density of 5 mA cm^-2, and excellent cycling stability with a capacitance retention of 96.95% after 10 000 cycles. An asymmetric supercapacitor was assembled using the NiO/Co₃O₄@NiCo as the cathode and an activated carbon electrode as the anode, which delivered an energy density of 0.32 mWh cm^-2 at a power density of 8 mW cm^-2. Even at a high power density of 40 mW cm^-2, an energy density of 0.17 mWh cm^-2 was achieved, suggesting its promising use as an efficient electrode for high performance supercapacitors.

Highly efficient formation of Mn₃O₄-graphene oxide hybrid aerogels for use as the cathode material of high performance lithium ion batteries

GAO Feng, QIN Shi-hui, ZANG Yun-hao, GU Jian-feng, QU Jiang-ying

2020, 35(2): 121-130. doi: 10.1016/S1872-5805(20)60479-6

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Abstract:
We report a combined hydrothermal treatment and freeze-drying method to fabricate Mn₃O₄-graphene oxide (GO) hybrid aerogels for use as the cathode material of lithium ion batteries. Results indicate that the Mn₃O₄-GO hybrids show much better lithium storage capacity and rate capability than Mn₃O₄/reduced GO powder obtained by calcination of the hydrothermally treated sample dried at 300 ℃ for 30 min under an argon atmosphere. The stronger interaction between GO and Mn₃O₄ compared with that between reduced GO and Mn₃O₄ is beneficial for the improvement of utilization rate of Mn₃O₄ and therefore the capacity. Also the higher porosity of the Mn₃O₄-GO hybrids than that of the Mn₃O₄/reduced GO allows faster ion diffusion and therefore a higher rate capability. A typical Mn₃O₄-GO hybrid with a Mn₃O₄ content of 70 wt.% exhibits the highest specific capacity of 1 073 mA h g^-1 at 100 mA g^-1 and excellent cycling stability with a capacity retention rate of 85% of after 200 cycles at 800 mA g^-1. The method is promising for the large-scale, environmentally friendly production of MnO_x-GO hybrids for lithium ion batteries.

Optimization of the preparation conditions of KOH-activated, PAN-based carbon ellipsoids by orthogonal experimental analysis

LIU Yuan-ming, QIN Xian-ying, ZHANG Shao-qiong, ZHANG Zhe-xu, KANG Fei-yu, LI Bao-hua

2020, 35(2): 131-139. doi: 10.1016/S1872-5805(20)60480-2

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The optimal preparation conditions for activated carbons from polyacrolynitrile ellipsoid powder by KOH activation were investigated by orthogonal experimental analysis. Results indicate that the activation temperature, holding time, KOH/carbon mass ratio all have a significant impact on the specific surface area, but a small impact on the total pore volume and mean pore size. The importance of factors determining (a) specific surface area are activation temperature > activation time > KOH/carbon mass ratio, (b) total pore volume are activation temperature > KOH/carbon mass ratio > activation time, and (c) the mean pore size are activation time > KOH/carbon mass ratio > activation temperature. The optimal combination of conditions (activation temperature of 800 ℃, holding time of 4 h and KOH/carbon mass ratio of 4:1) from orthogonal experimental analysis was validated experimentally. Under these conditions the specific surface area, total pore volume and mean pore size of the PAN-based porous carbon reached 2 517 m² g^-1, 1.07 cm³ g^-1 and 1.70 nm, respectively.

Preparation and electrochemical properties of heteroatom-doped graphene hydrogels

XU Hui, YANG Shu-hua, ZHU Yuan-qiang, FAN Xin, CHEN Min-zhang

2020, 35(2): 140-147.

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Graphene hydrogel electrode materials doped with nitrogen, phosphorus and sulfur were prepared by a hydrothermal method. SEM, TEM, Raman spectroscopy, XRD and XPS were used to characterize their microstructures and electrochemical properties for use as supercapacitor electrodes. The results indicate that nitrogen, phosphorus and sulfur are doped in the graphene lattice and that they improve the electrochemical performance of graphene with phosphorus being the best of them. Because of their having the largest atomic radius, P atoms produce the largest graphene lattice distortion and significantly increase the specific surface area, which is favorable for fast ion transfer. The capacitance of the P-doped graphene is 388 F/g at a current density of 1 mA/cm² in a 1 mol/L H₂SO₄ electrolyte. A symmetrical cell using the P-doped graphene as the electrodes delivers an energy density of 25.2 Wh/kg at a current density of 1 A/g. The high capacitance of the doped graphene is mainly due to the Faraday pseudocapacitance provided by the heteroatoms.

Surface modification of multiwall carbon nanotubes by electrochemical anodic oxidation

ZHANG Wei-song, LIU Yu-ting, WU Gang-ping

2020, 35(2): 155-164. doi: 10.1016/S1872-5805(20)60481-4

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The surface modification of multiwall carbon nanotubes (MWCTNs) was achieved by electrochemical anodic oxidation in NaOH and H₂SO₄ electrolytes. Their defect structures, functional groups, morphology and dispersibility in aqueous solutions were characterized by SEM, TEM, XPS, FTIR, Raman spectroscopy, zeta potential analysis and a stability test of their suspensions. Results indicate that anodic oxidation with the NaOH electrolyte removes more amorphous carbon, introduces fewer defects and more oxygen-containing functional groups (mainly -OH), and produces shorter nanotubes, as a result of which the resulting nanotubes are more stable when dispersed in aqueous solutions compared with those oxidized by the H₂SO₄ electrolyte. MWCTNs are more easily oxidized in the NaOH electrolyte, and the numbers of oxygen-containing functional groups and hydroxyl groups increase continuously with the oxidation degree. In the H₂SO₄ electrolyte, however, the numbers of oxygen-containing functional groups and hydroxyl groups increase and level off with the oxidation degree.

Reactive molecular dynamics simulation of the laser-induced carbonization of pre-oxidized polyacrylonitrile fibers

YAO Liang-bo, YANG Wei-min, TAN Jing, CHENG Li-sheng

2020, 35(2): 176-183.

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The transition of polyacrylonitrile (PAN) molecules to graphite-like structures is at the heart of the carbonization of PAN-based carbon fibers. Studies of the laser-induced carbonization of pre-oxidized PAN fibers can greatly help in revealing the carbonization mechanism. The differences between the laser-induced and heat-induced carbonization of fibers at 2500 K were investigated by reactive molecular dynamics simulation. The energy evolution, chemical reaction rates, the fluctuation of atom position and forces on atoms were analyzed. Results indicate that the simulation using a ReaxFF reactive force field can provide detailed information for the reactions, which is helpful for insight into the formation of graphitic structures in the carbonization. The thermal shock stress caused by rapid laser heating in the carbonization results in an amplification of the fluctuation of atom positions, which increases the number of active atoms and the atomistic activity. As a result, the probability of collision and chemical reaction between atoms is increased, which promotes the formation of graphite structures. A more graphitic structure is achieved by the laser irradiation heating of PAN-based carbon fibers compared with traditional heating.

Effect of the average grain size of green pitch coke on the microstructure and properties of self-sintered graphite blocks

ZHAO Hong-chao, HE Zhao, GUO Xiao-hui, LIAN Peng-fei, LIU Zhan-jun

2020, 35(2): 184-192. doi: 10.1016/S1872-5805(20)60483-8

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Three self-sintered graphite blocks (SGs) were prepared from green pitch cokes with different average grain sizes (5.1, 3.1 and 1.9 μm) using cold isostatic compaction. The microstructures and properties of the SGs were characterized by XRD, SEM, mercury porosimetry, mechanical and thermal tests. Results indicated that the microstructures and properties of the SGs were closely related to the green coke granularity. When decreasing the average grain size of the green coke from 5.1 to 1.9 μm, the density, mechanical properties, Shore hardness and coefficient of thermal expansion increased while the degree of graphitization, crystallite diameter, thermal conductivity, median pore diameter and porosity decreased. The median pore diameters of the SGs were all in the nanometer range, which implied that they could effectively inhibit the infiltration of molten salt and might be used in molten salt reactors.

2020 Vol. 35, No. 2

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