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Application of graphene in lithium-sulfur battery

Release date:2018-08-23  source:粉体网  Browse times:820
      With the rapid development of industries such as portable electronic devices and electric vehicles, the demand for high-energy density batteries is becoming more and more urgent. However, in traditional lithium-ion batteries, the positive electrode materials are generally more dense due to the "intercalation" lithium storage mechanism. Low, unable to meet the rapidly growing market demand. Therefore, the exploration and development of new high-energy density secondary batteries has become a research hotspot in the field of energy storage, and lithium-sulfur batteries are one of them.

1. Introduction to lithium-sulfur batteries

The working principle of lithium-sulfur battery is based on the reversible redox reaction of sulfur and Li+. The electrochemical reaction between the two is as follows:
The theoretical specific capacity of sulfur-based positive electrode based on this reaction is as high as 1675 mAh/g, which is 10 times that of traditional lithium ion battery cathode materials. At the same time, sulfur storage is abundant and the cost is low. Therefore, lithium-sulfur batteries have received extensive attention, but sulfur and polysulfide itself. The defects of nature make the lithium-sulfur battery still have many problems.

First, sulfur is an insulator, and its conductivity is poor, which brings difficulties to the charge transfer process. Secondly, lithium polysulfide can be dissolved in the electrolyte, and it is easy to migrate to the metal lithium side and is reduced to insoluble Li2S deposition on the surface of the metal lithium electrode. "shuttle effet "Phenomenon; again, when soluble lithium polysulfide is completely reduced to insoluble sulfide, it will hinder the efficient transmission of electrons and ions; finally, after the conversion of elemental sulfur to insoluble sulfide, the volume effect will be caused by the difference in density of the two substances. , to reduce electrode stability. Therefore, lithium-sulfur batteries have disadvantages such as low actual capacity, poor cycle performance, and poor signal performance.

Second, the application of graphene in lithium-sulfur batteries

In order to obtain the high-performance lithium-sulfur battery, the researchers conducted a variety of methods for the composite and modification of sulfur cathodes, and designed and prepared a series of composite sulfur cathode materials with novel structure and excellent performance. Among them, carbon materials have been widely used due to their high conductivity, rich structure and large specific surface area, and graphene, a new carbon material, has excellent performance in improving the performance of lithium-sulfur batteries.

Graphene is an excellent electronic conductor, and has the advantages of high mechanical strength and large specific surface area. At the same time, chemically modified graphene and graphene derivatives have a series of surface functional groups capable of providing many active sites for the load, so graphene It has been widely used in composite sulfur cathode materials.

On the one hand, graphene is used as a conductive carrier for the sulfur positive electrode to compensate for the poor conductivity of sulfur; on the other hand, graphene can also inhibit the dissolution of polysulfide by reasonable structural design and surface modification. In addition, in recent research, scientists have also found that the design of graphene functional coatings can slow the shuttle of polysulfides between the positive and negative electrodes and inhibit the "shuttle effet" phenomenon.

1. Research progress of graphene/sulfur composite cathode materials

The extremely high conductivity of graphene can make up for the poor conductivity of sulfur particles. Therefore, graphene materials are mostly designed as conductive substrates or conductive networks that support sulfur. For example, graphene foam structure can achieve uniformity of graphene and sulfur at the nanometer scale. Composite, capable of providing fast and efficient electron transport channels for sulfur, while nanopores are also effective in binding polysulfides.

Although the three-dimensional graphene obtained under the conventional conditions is rich in structure, it is extremely fluffy and the apparent density is very low, which leads to a serious shortage of the volumetric energy density of the composite electrode material after sulfur loading. For this reason, the academician of the Shenyang Institute of metals, Chinese Academy of Sciences, used the CVD method. A three-dimensional porous graphene foam was obtained on the foamed nickel.
Figure 1 (a) Preparation process of flexible graphene/sulfur composite; (b, c, d, e) graphene/sulfur composite electrode material photo and flexible display

The method can not only support a high proportion of sulfur, but also the sulfur content can be regulated in the range of 3.3 to 10.1 mg/cm2, especially the electrode with a loading of 10.1 mg/cm2, which can obtain a very high specific area capacity (13.4 mAh). /cm2).

In addition, considering the unique two-dimensional sheet-like nanostructure of graphene, the use of graphene nanosheets as a wrapping material to construct a composite electrode material with a "core shell" structure is also an important way to fix polysulfides and alleviate their dissolution. It is a very effective method to uniformly load sulfur on the surface of the carbon nanofibers and then coat the sulfur surface with graphene.
Figure 2 Preparation of a graphene/S/carbon nanofiber composite electrode with coaxial structure

2. Application of graphene functional coating in lithium-sulfur battery

In order to improve the cycle stability of lithium-sulfur batteries, in addition to regulating the composition and structure of sulfur cathode materials to inhibit the dissolution of polysulfides, it is also an important way to weaken the "shuttle effect" through the design of the pole piece structure. For example, adding a buffer layer between the sulfur positive electrode and the separator can greatly increase the life of the lithium-sulfur battery.
Figure 3 Schematic diagram of graphene membrane coating effectively blocking polysulfide migration

Innovative pole piece structure design of graphene/sulfur/graphene-separator, on the one hand, the current collector is changed from traditional Al foil to graphene; on the other hand, the diaphragm is modified to change the original diaphragm and direct contact with sulfur positive electrode. The way, a layer of graphene material is coated on the surface of the diaphragm.

With a conventional pole piece structure, after the polysulfide dissolves in the electrolyte during the cycle, it passes through the separator and enters the metal Li side. In this novel structure, the graphene layer exists between the separator and the positive electrode material. It can effectively prevent the migration of polysulfides. In addition, due to the excellent mechanical properties of the graphene material, the graphene modified membrane can effectively alleviate the volume change of the sulfur positive electrode during charging and discharging, and maintain the integrity of the pole piece structure.


Electrochemical energy storage plays an important role in today's production and life, whether it is mass storage of renewable energy or high-density storage of portable equipment, cost, energy storage density, stability and other indicators of electrochemical energy storage devices and materials. Both put forward higher requirements.

Due to its theoretical specific capacity and specific energy, lithium-sulfur batteries are cheap and easy to obtain, and will be highly competitive in the future electrochemical energy storage field. If the application of graphene can improve the actual capacity and cycle performance of lithium-sulfur batteries. The shortcomings of poor performance and poor signal performance, in the near future, the performance of lithium-sulfur batteries may bring us more surprises.

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