What are Lithium-Sulfur Batteries?

Lithium-sulfur batteries are a type of lithium batteries, and as of 2013, they are still in the scientific research stage. Lithium-sulfur battery is a lithium battery with sulfur as the positive electrode and lithium metal as the negative. Elemental sulfur is abundant in the earth and has the characteristics of low price and environmental friendliness. Lithium-sulfur batteries using sulfur as the positive electrode material have higher theoretical specific capacity and theoretical specific energy of the battery, reaching 1675m Ah / g and 2600Wh / kg respectively ^[1] , which are much higher than those of lithium cobaltate batteries widely used commercially Capacity (<150mAh / g). And sulfur is an environmentally friendly element, which basically has no pollution to the environment, and is a very promising lithium battery.

Lithium-sulfur battery

Lithium-sulfur battery is

A typical lithium-sulfur battery generally uses elemental sulfur as the positive electrode and a metallic lithium sheet as the negative electrode. Its reaction mechanism is different from the ion deintercalation mechanism of the lithium ion battery, but an electrochemical mechanism.

Lithium-sulfur batteries use sulfur as the positive electrode reaction material and lithium as the negative electrode. During discharge, the negative electrode reacts as lithium loses electrons and changes to lithium ions. The positive electrode reacts as sulfur reacts with lithium ions and electrons to form sulfides. The potential difference between the positive electrode and negative electrode reactions is the discharge voltage provided by the lithium-sulfur battery. Under the action of an applied voltage, the positive and negative reactions of the lithium-sulfur battery proceed in the reverse direction, which is the charging process. According to the amount of elemental sulfur that can be completely converted into S ² per unit mass, the theoretical specific discharge mass specific capacity of sulfur is 1675 mAh / g, and the theoretical specific discharge mass specific capacity of single element lithium is 3860 mAh / g. g. The theoretical discharge voltage of a lithium-sulfur battery is 2.287V. When sulfur and lithium completely react to form lithium sulfide (Li ₂ S). The theoretical discharge mass specific energy of the corresponding lithium-sulfur battery is 2600 Wh / kg.

The charge and discharge reactions of sulfur electrodes are relatively complicated. As of 2013, there is no clear understanding of the intermediate products produced by the sulfur electrodes during the charge and discharge reactions. The charge and discharge reactions of the lithium negative electrode and the sulfur positive electrode are shown in formulas (1-1) to (1-4). The discharge process of the sulfur electrode mainly includes two steps, which respectively correspond to two discharge platforms. The ring structure of formula (1-2) corresponding to S8 becomes a chain structure of Sn ^2- (3n7) ions, and combines with Li + to form Li2Sn. This reaction corresponds to the vicinity of 2.4-2.1V on the discharge curve. Discharge platform. The formula (1-3) corresponds to the chain structure of Sn ^2- ions changed to S2- and S2 ^2- and combines with Li ^{+ to} form Li ₂ S ₂ and Li ₂ S. This reaction corresponds to a voltage near 2.1-1.8V in the discharge curve. Long discharge platform, this platform is the main discharge area of lithium-sulfur batteries. Yuan Lixia et al. Studied the electrochemical reaction process of sulfur cathode in lithium-sulfur batteries. They believe that the discharge at the potential range of 2.5-2.05V corresponds to the reduction of elemental sulfur to generate soluble polysulfides and the further reduction of polysulfides, and the reduction of soluble polysulfides at the potential range of 2.05-1.5V corresponds to the reduction of soluble polysulfides to form lithium sulfide solid films. It covers the surface of the conductive carbon substrate. During charging, Li ₂ S and Li ₂ S _{2 in the} sulfur electrode are oxidized by S ₈ and Sm ^2- (6m7), and cannot be completely oxidized to S _8. This charging reaction corresponds to 2.5 ~ 2.4V in the charging curve. Nearby charging platform.

There are three main problems with lithium-sulfur batteries: 1. Lithium polysulfide compounds dissolve in the electrolyte; 2. Sulfur, as a non-conductive substance, has very poor electrical conductivity, which is not conducive to the high rate performance of the battery; 3. Sulfur during the charge and discharge process The expansion and contraction of the volume is very large, which may cause battery damage.

The main problems with lithium-sulfur batteries are:

First, the elemental sulfur has poor electronic and ionic conductivity. The sulfur material has extremely low electrical conductivity (5.0 × 10-30S · cm-1) at room temperature. The final products of the reaction, Li2S2 and Li2S, are also electronic insulators, which is not good for batteries. High rate performance

Second, the intermediate discharge products of lithium-sulfur batteries will be dissolved in the organic electrolyte, which increases the viscosity of the electrolyte and reduces ionic conductivity. Polysulfide ions can migrate between the positive and negative electrodes, resulting in loss of active materials and waste of electrical energy. (Shuttle effect). The dissolved polysulfide diffuses across the separator to the negative electrode, reacts with the negative electrode, and destroys the solid electrolyte interface film (SEI film) of the negative electrode.

Third, the final discharge product of lithium-sulfur batteries, Li2Sn (n = 1 ~ 2), is electrically insulated and insoluble in the electrolyte, and is deposited on the surface of the conductive skeleton; part of the lithium sulfide detaches from the conductive skeleton and cannot be converted into sulfur through a reversible charging process Or higher-order polysulfide, which caused a great attenuation of capacity.

Fourth, the density of sulfur and lithium sulfide are 2.07 and 1.66 gcm-3, respectively. During the charge and discharge process, the volume expansion / contraction is as high as 79%. This expansion will lead to changes in the shape and structure of the positive electrode, leading to sulfur Disengagement from the conductive skeleton, resulting in capacity degradation; this volume effect is not significant under button batteries, but in large batteries, the volume effect will be amplified, which will cause significant capacity attenuation, which may cause battery damage and huge volume Changes can destroy electrode structure

Fifth, lithium-sulfur batteries use metallic lithium as the negative electrode. In addition to the high activity of metallic lithium itself, the metallic lithium negative electrode undergoes a volume change during charge and discharge, and is easy to form dendrites.

Sixth, there are many researches on the scale of lithium-sulfur batteries in the laboratory, and the sulfur load per unit area is generally below 3.0mg · cm-2. Research on high-load pole pieces is of great value for obtaining high-performance lithium-sulfur batteries. . ^[2]

The main solution is to start with the electrolyte and cathode materials. The first is the electrolyte, which mainly uses an ether electrolyte as the battery electrolyte. Adding some additives to the electrolyte can effectively alleviate the dissolution of lithium polysulfide compounds. The second is the cathode material, which is mainly composed of sulfur and carbon materials, or sulfur and organic compounds, can solve the problem of sulfur non-conductivity and volume expansion.

In recent decades, in order to improve the utilization of active material sulfur, limit the dissolution of lithium polysulfide, and poor battery cycle performance, researchers have conducted a lot of exploration and research in the modification of electrolytes and composite positive electrode materials. For the modification of electrolytes, solid electrolytes, gel electrolytes, or LiNO3 ionic liquids are added to the electrolyte to limit the dissolution of lithium polysulfide generated during the electrode reaction and reduce the "shuttle effect", which improves the The utilization rate of the active material sulfur, thereby achieving the purpose of improving the cycle performance of the lithium sulfur battery. For the modification of the sulfur-based composite positive electrode material, a high-performance sulfur-based composite positive electrode material is mainly prepared by combining a matrix material with good conductivity and a specific structure with elemental sulfur. Among them, the introduced matrix material should have the following functions:

(1) Good electrical conductivity;

(2) The active material sulfur can be uniformly dispersed on the matrix material to ensure high utilization of the active material;

(3) To inhibit the dissolution of sulfur and polysulfides. The study found that by combining the active substance sulfur with activated carbon, mesoporous carbon, nano carbon fibers (CNF), multi-walled carbon nanotubes (MWCNTs), graphene, polyacrylonitrile (PAN), polyaniline (PAn), and polypyrrole (PPy ), Polythiophene (PTh) and other matrix materials with specific structures to prepare sulfur-based composite cathode materials can significantly improve the cycle performance and rate performance of lithium-sulfur batteries. ^[1]

On August 22, 2014, researcher Chen Jian from the Dalian Institute of Chemical Technology of the Chinese Academy of Sciences led an advanced secondary battery research team and made important progress in high specific energy lithium secondary batteries. He successfully developed a lithium-sulfur battery with a rated capacity of 15 Ah and formed a small battery. Batch preparation capabilities.