Graphene, as a super-material, has attracted wide attention due to its excellent electrochemical and mechanical properties. In 2015, state leaders paid a special visit to the National Institute of Graphene at the University of Manchester in England during their visit to the UK Spoke highly of this super-material with special properties, then Huawei announced that it would invest millions of pounds and the Institute on the application of graphene in information and communication technology research prospects in the domestic A shares The market also set off a concept of graphene material speculation storm, and graphene-related stocks have all rose, there are many battery companies have announced that they developed the world's first graphene battery, "although these are Battery manufacturers released smoke bombs, but also won the eye of everyone, has aroused widespread concern.
Xiaobian as a senior practitioner of lithium-ion battery industry, these so-called graphene batteries can not hide Xiaobian eyes.In fact, most of the graphene battery is only a small amount of graphene as a lithium-ion battery conductive agent , Add less than 1%, in essence, or lithium-ion battery, but as a gimmick publicity, and some time ago speculation Huawei graphene batteries, graphene is only as a lithium-ion battery cooling aids, Enhance lithium-ion battery work ability under high temperature, graphene does not participate in the electrochemical reaction within the lithium-ion battery, in the strict sense can only be called graphene enhanced lithium-ion battery.
In fact, under the existing technical capabilities, taking into account factors such as cost constraints, graphene is currently the main application as a lithium-ion battery conductive agent and auxiliary cooling means.Traditional lithium-ion battery conductive agent, such as carbon black SP, carbon fiber VGCF, etc., contact with the active material is point contact, limiting the conductive effect of play, increasing the amount of conductive agent addition, but graphene is a sheet-like structure, the contact with the active material is point-to-surface contact can be maximized Play the role of conductive agent and reduce the amount of conductive agent to enhance the energy density of lithium-ion batteries, but even the best materials also have shortcomings, the sheet-like structure of graphene, lithium ion diffusion will form an obstacle in the larger operating current Density, will cause the Li + diffusion impedance increases, resulting in the battery capacity performance decline Xiaobian today we take graphene as a lithium-ion battery conductive agent advantages and disadvantages analysis.
Yang Quanhong from Tianjin University, a senior scholar in the graphene industry, published an article in Nano Energy in 2012 on the advantages and disadvantages of graphene as a conducting agent for lithium-ion batteries. In his research, Yang adopted a commercial 10Ah LiFePO4 / graphite 'square lithium ion battery.Research shows that substituting a small amount of (1%) graphene for the traditional conductive agent in lithium ion battery not only can enhance the proportion of the chemical substance, but also reduce the lithium ion battery However, due to the flaky structure of graphene, it will greatly impede the rapid diffusion of Li +. Therefore, lithium ion battery will greatly polarize during high current charge and discharge (> 3C), affecting the lithium ion Battery discharge capacity.This study shows that graphene as a conductive agent suitable for use in some lithium-ion battery charge and discharge rate of less demanding situations, the addition of graphene can significantly increase the proportion of active material, reduce the electrode impedance, Lithium-ion battery to enhance the energy density, but in some graphene is not suitable for application in power-type batteries (charge-discharge ratio> 3C) as a conductive agent.
In the experiment, Yang Quanhong's team produced two kinds of batteries, one is a common control cell, using 7% carbon black and 3% conductive graphite, and the experimental group using 1% graphene and 1% carbon black as the conductive The experimental results show that under the same coating amount, the experimental group using graphene battery capacity (0.5C charge and discharge) should be significantly higher than that of the control cell, and the cycling performance of both close, indicating that graphene can Set up a more efficient conductive network, thereby reducing the amount of conductive agent to improve the capacity of lithium-ion battery (10%), reduce the battery polarization and enhance the battery energy density.
In the subsequent experiments of magnification, it was found that the graphene conductive agent cells in the experimental group showed higher capacity and smaller polarization at 0.5C, 1C and 2C charge-discharge magnification than the control cell, However, when the charge-discharge rate increased to 3C, the battery capacity of the experimental group dropped rapidly to below 4Ah, while the capacity of the control battery remained at about 9Ah, and the discharge rate was continuously increased to 4C. In the experimental group, Too large, has been unable to discharge, while the control group battery is relatively stable.
EIS analysis showed that the ohmic impedance of the graphene-filled experimental cells was significantly lower than that of the control cells, mainly due to the graphene's flaky structure can make good contact with the active material particles and reduce the contact resistance, but in the The charge exchange impedance of the high frequency band in the experimental group was significantly higher than that in the control group, indicating that the addition of graphene affected the diffusion of Li + in the electrode.The simulation results show that the diffusion of Li + is mainly caused by the sheet structure of graphene Resulting in obstruction, resulting in Li + diffusion path extension, resulting in the addition of graphene lithium-ion battery under high current polarization increases, resulting in decreased discharge capacity of the battery while the traditional carbon black, conductive carbon fiber and conductive graphite and other materials Due to the smaller cross section, it hinders the diffusion of Li + and has little effect on the high-current discharge performance of Li-ion batteries.
The study shows that although graphene as a conductive agent can significantly improve the conductivity of the electrode, reducing the amount of conductive agent to improve the energy density of lithium-ion batteries, but not all of the lithium-ion batteries suitable for food graphene as a conductive agent, in Some of the charge-discharge current less demanding areas, such as energy storage, electronic equipment, working current is smaller, suitable for the use of graphene to replace the traditional conductive agent.But in some charge and discharge current demanding areas, such as high-power Batteries, power batteries, etc., due to the graphene will cause lithium ion batteries in the case of high current, the polarization increases, it is not suitable for the use of graphene as a conductive agent.
