Are graphene conductors suitable for lithium-ion batteries for energy storage?

As a super material, with excellent electrochemical and mechanical properties, it has attracted widespread attention. In 2015, General Secretary Xi Jinping visited the National Graphite Institute of the University of Manchester in the United Kingdom during his visit to the United Kingdom. The company later announced that it would invest millions of pounds in a study with the Institute on the future of graphene applications in information and communications technology. In the domestic A-share market, there has also been a storm of conceptual speculation on graphene materials. All the stocks related to the graphene concept have soared, and many battery companies have announced that they have developed "the world's first graphene battery.", Although these are smoke bombs released by battery manufacturers, they have also attracted the attention of everyone and attracted widespread attention.

In fact, most graphene batteries use only a small amount of graphene as a conductive agent for lithium-ion batteries, adding less than 1 %, and it is essentially a lithium-ion battery. It is only a publicity stunt with graphene. Huawei graphene batteries, Simply using graphene as a heat dissipation aid for lithium-ion batteries to enhance the ability of lithium-ion batteries to work at high temperatures, graphene does not participate in the electrochemical reactions inside lithium-ion batteries. Strictly speaking, it can only be called graphene enhanced lithium-ion batteries.

In fact, under the existing technical capabilities, taking into account the limitations of costs and other factors, graphene is currently mainly used as a conductive agent and an auxiliary heat dissipation method for lithium ion batteries. Traditional lithium-ion battery conductors, such as carbon black SP, carbon fiber VGCF, etc., are in contact with active substances, limiting the use of conductive properties and increasing the amount of conductive agents added. Instead, graphene is a flaky structure. The contact with the active material is point-surface contact. It can maximize the role of conductive agents, reduce the amount of conductive agents, and increase the energy density of lithium-ion batteries, but the best material There are also shortcomings, graphene's flaky structure, It will hinder the diffusion of lithium ions. At a large working current density, Li +'s diffusion impedance will increase, resulting in a decrease in the multiplier performance of the battery. Today, the small editor took everyone to analyze the advantages and disadvantages of graphene as a conductive agent for lithium ion batteries.

Teacher Yangquanhong of Tianjin University is a senior scholar in the graphene industry. In an article published in Nano Energy in 2012, he studied the advantages and disadvantages of graphene as a conductive agent for lithium-ion batteries. In Ms. Yang's study, a commercial 10Ah "LiFePO4/graphite" square lithium ion battery was used. Studies have shown that replacing traditional conductive agents in lithium ion batteries with a small amount(1 %) of graphene can not only increase the proportion of chemical substances, but also significantly reduce the impedance of lithium ion batteries, but due to graphene The sheet structure, It will create a great obstacle to the rapid diffusion of Li +, so it is charged and discharged at high currents(& GT; When 3C), it will cause a great polarization of lithium ion batteries and affect the discharge capacity of lithium ion batteries. This study shows that graphene is suitable for application as a conductive agent in some cases where the charge and discharge ratio of lithium ion batteries is not high. The addition of graphene can significantly increase the proportion of active substances, reduce electrode impedance, and increase the energy density of lithium ion batteries., However, some graphene are not suitable for use in power batteries(charge and discharge doubling rate & GT; 3C) as a conductive agent.

In the experiment, Yangquanhong's team produced two types of batteries, one of which was an ordinary control group battery, using 7 % carbon black and 3 % conductive graphite. The experimental group used 1 % graphene and 1 % carbon black. As a conductive agent. The test results show that under the same amount of coating, the battery capacity of the experimental group using graphene(0.5 C charge and discharge) is significantly higher than that of the control group battery, and the cyclic properties of the two are similar, indicating that graphene can be built. More efficient conductive network, In order to reduce the amount of conductive agents, increase the capacity of lithium-ion batteries(10 %), reduce the polarization of batteries, and increase the energy density of batteries.

In the subsequent doubling experiment, it was found that the graphene conductive battery in the experimental group had higher capacity and smaller polarization than the control group battery at the charge and discharge rates of 0.5 C, 1C, and 2C. However, when the charge and discharge doubling rate was increased to 3C, the battery capacity in the experimental group quickly dropped below 4Ah, while the battery capacity in the control group remained at about 9Ah, and the discharge doubling rate continued to be increased to 4C. Graphene conductive battery due to too much polarization, Discharge was no longer possible, but the control group was relatively stable.

The EIS analysis showed that the ohmic impedance of the added graphene experimental group batteries was significantly lower than that of the control group batteries. This was mainly due to the fact that the graphene flaky structure was able to form good contact with the active material particles and reduce the contact resistance. However, at the high-frequency charge exchange impedance, the experimental group batteries were significantly higher than the control group batteries, indicating that the addition of graphene affected the diffusion of Li + within the electrode. The simulation results show that mainly because the graphene flaky structure hinders the diffusion of Li +, resulting in the extension of Li +'s spreading path, resulting in an increase in polarization of the lithium-ion battery added with graphene in the large current, resulting in the battery discharge capacity. Decline. Traditional materials such as carbon black, conductive carbon fiber, and conductive graphite have small cross-sections, so they have less obstacles to the diffusion of Li + and have little effect on the performance of large current discharge in lithium-ion batteries.

The study shows that although graphene as a conductive agent can significantly increase the conductivity of the electrode, reduce the amount of conductive agent, and increase the energy density of lithium ion batteries, not all lithium ion batteries are suitable for using graphene as a conductive agent., In some areas where the demand for charge and discharge currents is not high, such as energy storage, electronic equipment, etc., the working current is small and it is suitable for replacing traditional conductive agents with graphene. However, in some areas where charging and discharging currents are required to be high, such as high-power batteries, power batteries, etc., graphene will cause lithium ion batteries to increase polarization under high current conditions and is not suitable for graphene as a conductive agent.

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