Since the discovery of carbon nanotubes, research on the application of carbon nanotubes in secondary batteries has not stopped. Due to its special tubular graphite structure and unique ballistic electron conduction effect, the electrical conductivity of carbon nanotubes at room temperature can be as high as 103s/cm/ (MK), the thermal conductivity can reach 5800 W/ (MK), MWCNTs (multi-wall carbon The thermal conductivity of nanotubes can reach 3000 W/(MK). At the same time, the directional growth of the three-dimensional carbon nanotube array has excellent mechanical properties.
Carbon nanotubes are also widely used, including electronics (transistors, sensors, etc.), biomedical fields, aerospace (research spacecraft lenses, composite material reinforcements, functional materials), military fields (biochemical protective clothing and mines, Explosives detectors), energy fields (supercapacitors, lithium-ion batteries and solar thermal photovoltaic equipment) and lasers. Today we will take a look at the application of carbon nanotubes in lithium-ion batteries.
Application of carbon nanotubes in cathode materials of lithium ion batteries
Carbon nanotubes are used as a conductive agent in positive electrode materials, mainly used to improve the capacity, rate and cycle performance of batteries.
◆ Lithium cobaltate/carbon nanotube composite cathode material
The performance differences of batteries when multi-walled carbon nanotubes (MWCNTs), carbon black (CB) and carbon fiber (CF) are used as conductive agents in LiCoO2 are compared. At a rate of 2 C, the capacity of LiCoO2/MWCNT battery decays during cycling The battery is extremely small, and the battery containing carbon black and carbon fiber is attenuated by 10% and 30% after 20 cycles. Tests show that the battery with MWCNT has the lowest impedance and the highest conductivity.
◆ Lithium manganate/carbon nanotube composite cathode material
The multi-walled carbon nanotube/lithium manganate nanocomposite was synthesized by the solution gel method. The capacity retention rate of the composite material reached 99% after 20 cycles, while the capacity retention rate of pure LiMn2O4 nanoparticles after 20 cycles 90%, indicating that the composite material has better cycle stability. This is because the MWCNT forms a conductive network inside the electrode, which makes charge transfer easier, and the AC impedance test results show that the composite material has a lower impedance.
◆ Lithium iron phosphate/carbon nanotube composite cathode material
The electrochemical performance of the composite electrode prepared by mixing LiFePO4 particles with MWCNT was studied. Scanning electron microscopy showed that the one-dimensional MWCNT and LiFePO4 particles formed a three-dimensional network, which effectively improved the conductivity of electrons between the active material and the current collector.
Application of carbon nanotubes in anode materials for lithium ion batteries
◆ Pure carbon nanotubes as anode materials
The unique shape and high specific capacity of carbon nanotubes are very beneficial to their application in lithium ion batteries. When lithium ions can be inserted between the tubes, the electrode capacity will be greatly improved. So some people try to directly use carbon nanotubes as negative electrode materials.
The carbon nanotubes synthesized by the template method are prepared into a nanoporous carbon film, so that the carbon nanotubes can be embedded with lithium ions both inside and outside the tube. The carbon nanotube film electrode exhibited 490m Ah·g-1.
The structure of carbon nanotubes and their degree of graphitization determine the specific capacity and cycle life of carbon nanotube films. Low-graphitized carbon nanotubes show a higher specific capacity, but the stability is worse than that of highly graphitized carbon nanotubes. This is because lithium is often preferentially inserted into regions with low graphitization, such as the graphite layer boundary or the surface of single-layer graphite. However, graphitized carbon nanotubes are generally superior to amorphous carbon nanotubes in their ability to dissolve lithium. Different preparation conditions lead to different microstructures and chemical compositions of carbon nanotubes, thus determining their electrochemical performance.
However, due to the large specific surface area of carbon nanotubes, when it is used directly as a negative electrode material, the formation of SEI film results in a huge loss of specific capacity in the first ring, making the final capacity of carbon nanotubes very limited. So people have to find ways to compound carbon nanotubes with other materials to improve the performance of the materials.
◆ Carbon nanotube composite anode material
The chemical inertness of carbon nanotubes allows it to maintain its structural stability in many chemical reactions, so many researchers grow negative materials on carbon nanotubes in situ to synthesize composite electrodes. In order to successfully prepare carbon nanotube composite electrode, it is necessary to overcome the van der Waals attraction between carbon nanotubes, so surface dispersants or weak oxidants are essential, they can weaken the interaction between carbon nanotubes. The negative electrode material grown in situ is coated on the surface of the carbon nanotubes to obtain further surface modification, which not only promotes the dispersion of the carbon nanotubes but also strengthens the chemical force between the carbon nanotubes and the electrode material.
In addition, the researchers have also tried to prepare tin dioxide/carbon nanotube composite anode materials and transition metal oxide/carbon nanotube composite anode materials, which have successfully improved the electrochemical performance of the electrode materials, which are large-capacity and highly stable lithium The development of ion batteries provides a possible solution.