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Application of Carbon Nanotubes in Lithium-ion Anode Materials

2019-08-14 08:53

Description

Lithium-ion battery anode materials widely used at present are usually graphite or some other forms of carbon. The most common lithium-ion battery anode materials mainly include artificial graphite and natural graphite, as well as mesocarbon microspheres.


Among them, artificial graphite and natural graphite are the most widely used, but in recent years, mesophase carbon microspheres are gradually being widely used due to their advantages in capacity.


In bulk graphite, lithium is discontinuously embedded between the graphite layers (segmentation) to form the largest compound LiC6 (theoretical capacity is 372 mAh/g).


When the lithium content is low, the voltage curve drops with a large slope. When the voltage is lower than 100mV, two voltage platforms appear during the continuous embedding of lithium. Under the action of impedance and chemical polarization, the voltage There will be a small lag.

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The advantage of low voltage is that it can increase the voltage of the battery and increase the energy density. The disadvantage is that at a lower voltage, lithium may be precipitated, causing safety problems.


The use of carbon nanotubes can improve the cycle performance of the battery, but it will affect the voltage curve of the battery (the voltage range becomes wider and the voltage hysteresis becomes larger).


Carbon nanotubes are typical nanomaterials and have a wide range of applications due to their excellent mechanical and electrical properties. The use of carbon nanotubes in lithium ion batteries was first reported in Landi et al. Single-walled carbon nanotubes (SWNTs) are single-layer graphene tubes with a diameter of a few nanometers and a length in the submicron to micrometer range. Gao et al. first studied the electrochemical embedding of lithium in SWNTs. They observed a capacity of 600 mAh/g in the initial cycle, but their capacity increased to 1000 mAh/g after ball milling of SWNTs.

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It has been suggested that this may be caused by ball milling causing a certain degree of damage to the wall of the carbon nanotubes, thereby causing the disorder of the SWNT, thereby increasing the embedding point of lithium and increasing the passage of lithium ions into the interior of the carbon nanotube.


Carbon nanotubes are characterized by a wide range of cyclic voltages (0~3V vs Li/Li+), significant voltage hysteresis, and large irreversible capacity during initial lithium insertion (this may be due to the higher specific surface area of the carbon nanotubes causing the electrolyte to The surface is decomposed more, and some of the lithium embedded in the inside of the carbon nanotubes becomes inactive and becomes dead lithium, causing capacity loss.


Multi-walled carbon nanotubes (MWNTs) are made of graphene rolled into concentric cylinders, and researchers have also studied its electrochemical insertion characteristics of lithium.


The lithium-ion voltage and cycle voltage of MWNT are very similar to SWNT: high irreversible capacity (mainly around 1V, mainly due to decomposition of electrolyte on the high specific surface area of carbon nanotubes), wide voltage range (0~3V vs Li/) Li+), as well as significant voltage hysteresis (the difference between delithiation and lithium intercalation is 1V or more).


The reversible capacity is only 100~400mAh/g, which is lower than SWNT, and there is no obvious improvement compared with the traditional graphite negative electrode.


However, MWNT is very sensitive to high temperature treatment. After high temperature treatment, the degree of graphitization is increased, the irreversible capacity is remarkably reduced, and the reversible capacity is improved. This is mainly due to the low surface area of the sample and the decrease in the disorder of the sample after high temperature treatment.


At the same time, due to cost issues, carbon nanotubes still have a long way to go, but as a conductive agent to increase the conductivity of the electrode and reduce the polarization of the battery, it has been used in practical production.