1. What is a lithium sulfur battery?
Lithium sulfur battery is a kind of lithium battery, which uses sulfur as the cathode of the battery and metal lithium as the anode. Elemental sulfur is abundant in the earth and has the characteristics of low price and environmental friendliness. Lithium sulfur battery using sulfur as the cathode material have higher material theoretical specific capacity and battery theoretical specific energy, much higher than the specific capacity of widely used commercial lithium cobalt oxide batteries and lithium battery companies in the world have also begun to deploy related products. Lithium sulfur battery is a very promising lithium battery.
2. Advantages of lithium sulfur battery
Lithium sulfur battery is light in weight
Its lightweight characteristics are beneficial to the improvement of the overall energy density of the battery. Based on the common reaction of the three types of graphene, the all-graphene sulfur cathode can establish up to 90% active material utilization and excellent cycle stability.
Lithium sulfur battery has good conductivity
The lithium sulfur battery uses graphene with high pore volume as the sulfur carrier, a part of graphene oxide as the spacer layer, high-conductivity graphene as the current collector, and an all-graphene-based cathode structure design. The use of high pore volume graphene allows the electrode material to achieve a sulfur content of 80 wt% and a sulfur loading of 5 mg/cm-2, which greatly improves the electrical conductivity.
Low cost and wide range of material sources
Lithium sulfur battery have many advantages, such as simple and clear structural features, a wide range of material sources, low product cost, low environmental damage, and extremely strong battery life, and , many company managers believe that it is the most suitable similar product for lithium-ion batteries used as kayak battery, golf cart batteries, etc.
Special structural features, strong endurance and high stability
The lithium sulfur battery uses the unique bridge structure characteristics, and innovatively equips the sulfur cathode, so that it has stronger stress load and stability performance, and the endurance and stability have been greatly improved.
3. Lithium sulfur battery charge and discharge principle
A typical lithium sulfur battery generally uses elemental sulfur as the cathode and metal lithium sheet as the anode. Its reaction mechanism is different from the ion deintercalation mechanism used in general lithium-ion batteries, but an electrochemical mechanism. Lithium sulfur batteries use sulfur as the cathode reaction material and lithium as the anode.
During discharge, the anode reaction is that lithium loses electrons to become lithium ions, the cathode reaction is that sulfur reacts with lithium ions and electrons to form sulfide, and the potential difference between the cathode and anode reaction is the discharge voltage provided by the lithium sulfur battery. Under the action of an applied voltage, the cathode and anode reactions of the lithium sulfur battery are reversed, which is the charging process.
According to the amount of electricity that the unit mass of elemental sulfur can completely change into S2-, it can be concluded that the theoretical discharge mass specific capacity of sulfur is 1675 mAh/g. Similarly, it can be concluded that the theoretical discharge mass specific capacity of elemental lithium is 3860 mAh/g. The theoretical discharge voltage of a lithium-sulfur battery is 2.287V, when sulfur reacts completely with lithium to form lithium sulfide (Li2S). The theoretical discharge mass specific energy of the corresponding lithium-sulfur battery is 2600 Wh/kg.
4. Problems with lithium sulfur battery
The electronic conductivity and ionic conductivity of elemental sulfur are poor, and the conductivity of the sulfur material at room temperature is extremely low (5.0×10-30S·cm-1). The final products of the reaction, Li2S2 and Li2S, are also electronic insulators, which is not conducive to the high rate performance of the battery.
The intermediate discharge products of the lithium sulfur battery will dissolve into the organic electrolyte, increasing the viscosity of the electrolyte and reducing the ionic conductivity. Polysulfide ions can migrate between positive and negative electrodes, resulting in loss of active material and waste of electrical energy (Shuttle effect). The dissolved polysulfides diffuse across the separator to the anode, react with the anode, and destroy the solid electrolyte interfacial film (SEI film) of the anode.
The final discharge product of lithium sulfur battery, Li2Sn (n=1~2), is electronically insulating and insoluble in electrolyte, and deposited on the surface of the conductive framework. Part of the lithium sulfide is separated from the conductive framework and cannot be reacted into sulfur or high-order polysulfides through the reversible charging process, resulting in a great decrease in capacity.
The densities of sulfur and lithium sulfide are 2.07 and 1.66 g cm-3, respectively, with up to 79% volume expansion/contraction during charge and discharge. This expansion can lead to changes in the morphology and structure of the cathode, resulting in the detachment of sulfur from the conductive framework, resulting in capacity fading. This volume effect is not significant under button batteries, but it will be amplified in large batteries, resulting in significant capacity fading, which may lead to battery damage, and huge volume changes will destroy the electrode structure.
In order to solve these problems, scientists are trying a variety of methods, the following introduces the method of optimizing the battery performance by changing the cathode and anode materials of lithium sulfur battery.
5. Application of porous support materials in lithium sulfur battery
The use of porous materials as the sulfur carrier of lithium sulfur battery can effectively accommodate the volume change of sulfur during charging and discharging, which plays an important role in protecting the electrode structure and improving the cycle performance. Among them, carbon-based materials, as porous carriers, can play the role of porous materials in buffering the volume change of active substances. Moreover, the high specific surface area and high electrical conductivity of the carbon material also enable the conductive substrate to fully contact the active material and effectively improve its utilization efficiency.
At the same time, through the regulation of the pore structure of the material, the respective advantages of different pores can be brought into play, which can have an important impact on further improving the performance of lithium sulfur battery. As typical representatives of carbon materials, although carbon nanotubes and graphene materials lack pore structure. However, the construction of its pore structure can also be achieved by improving the material preparation method. By using anodized aluminum as a template, chemists can grow carbon tubes up to 200 nm in diameter. Meanwhile, the CuO nanoparticles located inside the larger carbon tubes can be reduced to Cu and act as catalysts to promote the growth of small-diameter carbon nanotubes.
The structure of the large carbon tube and the small carbon tube not only helps to increase the loading of sulfur, but also provides sufficient space for accommodating the volume change of sulfur during the cycle. As a cathode material for lithium sulfur battery, Li2S has a high theoretical specific capacity of 1166 mAh g-1, which can avoid the volume expansion effect during lithium intercalation. However, it still faces similar problems with the use of sulfur cathodes in terms of cycling stability and rate capability. In order to improve the performance of the Li2S cathode material, a method similar to that used to improve the performance of the sulfur cathode can also be adopted.