Introduction of lithium sulfur battery – is it better

1. What is a lithium sulfur battery
2. Charging and discharging principle of lithium sulfur battery
3. Advantages of lithium sulfur battery
4. Are lithium sulfur batteries better than other lithium ion batteries
5. Problems with lithium sulfur battery
6. Application of porous carrier materials in lithium sulfur battery
7. Anode current collector design

Previously, we have written lithium titanate cells, solid state battery, vanadium battery and so on. In this post, we will learn about lithium sulfur battery, compared with the traditional lithium ion battery, the theoretical energy density of lithium sulfur battery can reach 2680Wh/kg at the average voltage of 2.15V, which is about 6 times of the energy density of the current lithium-ion battery. Therefore, it is considered as one of the most potential systems to replace the traditional lithium-ion battery.

In addition, compared with the expensive cathode materials of traditional lithium-ion batteries, sulfur is abundant, inexpensive, and environmentally friendly, giving it unparalleled advantages in large-scale energy storage system applications. Let’s see what lithium sulfur battery made of, how it works, is it better than other batteries and what the problems are.

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.

The lithium sulfur battery using sulfur as the cathode material has high material theoretical specific capacity and battery theoretical specific energy, reaching 1675mAh/g and 2600Wh/kg, respectively, it is much higher than the specific capacity (<150mAh/g) of the widely used lithium cobalt oxide battery. Lithium sulfur battery is a very promising lithium battery.

Charging and discharging principle of lithium sulfur battery

A typical lithium sulfur battery generally uses elemental sulfur as the positive electrode and metal lithium sheet as the negative electrode. Its reaction mechanism is different from the ion deintercalation mechanism used in general lithium ion batteries, but an electrochemical mechanism. The lithium sulfur battery uses sulfur as the positive electrode reaction material and lithium as the negative electrode.

During discharge, the negative electrode reaction is that lithium loses electrons and becomes lithium ions, and the positive electrode reaction is that sulfur reacts with lithium ions and electrons to form sulfides. The potential difference between the positive electrode and the negative electrode reaction is the discharge voltage provided by the lithium sulfur battery. Under the action of the applied voltage, the positive and negative reactions of the lithium sulfur battery are reversed, that 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 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. Let’s take a look at the charging and discharging performance of lithium sulfur battery:

Advantages of lithium sulfur battery

1.Lithium sulfur battery light weight

Its lightweight characteristics are beneficial to the improvement of the overall energy density of the battery. According to 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.

2.Lithium sulfur battery has good electrical conductivity

The lithium sulfur battery uses graphene with high pore volume as the sulfur carrier, a part of graphene oxide as the spacer layer, highly conductive graphene as the current collector, and is designed with an all-graphene-based cathode structure. 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.

3. Low cost and wide range of material sources

Lithium sulfur battery has many advantages, such as simple and clear structural features, wide range of material sources, low product cost, low environmental damage and extremely strong battery life, etc. Many company managers believe that it is the most suitable similar product for lithium-ion batteries.

4. Special structural features, strong endurance and high stability

Using the unique bridge structure features, the sulfur cathode is innovatively equipped, so that it has stronger stress load and stability performance, and the endurance and stability have been greatly improved.

Are lithium sulfur batteries better than other lithium ion batteries

Although lithium-sulfur batteries have great advantages in energy density than other lithium-ion batteries, and mainly use sulfur and lithium as production raw materials, the production cost is low. However, the stability of the vulcanized polymer of lithium-sulfur batteries is relatively poor, and the number of cycles of current lithium-sulfur batteries is much lower than that of ordinary lithium iron phosphate batteries, which greatly increases the cost of lithium-sulfur batteries.

Especially for the energy storage industry, the first priority is safety, and the second is the number of cycles. At present, lithium-ion batteries, especially lithium iron phosphate batteries, are the best choice in the energy storage industry.

Problems with lithium sulfur battery

● The electronic conductivity and ionic conductivity of elemental sulfur are poor, the conductivity of sulfur materials 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, it is not conducive to the high rate performance of the battery.

● The intermediate discharge products of 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.

● Li2Sn (n=1~2), the final discharge product of lithium sulfur battery, 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 battery capacity.

● The densities of sulfur and lithium sulfide are 2.07 and 1.66 g·cm-3, respectively, and there is a volume expansion/contraction of up to 79% during the charging and discharging process. 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 to optimize the battery performance by changing the anode and cathode materials of lithium sulfur battery.

Application of porous carrier materials in lithium sulfur battery

Using 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 exerted, which can have an important impact on further improving the performance of lithium sulfur battery.

Using MgO as a hard template, chemists designed and fabricated porous graphene nanosheets (HPCR) with a hierarchical pore structure, the HPCR material has macroporous and mesoporous structures, which can not only facilitate the transport of substances through the macroporous structure, but also realize the accommodation of sulfur by using the mesoporous structure. Thus, under the condition of higher sulfur loading, the cycle stability of lithium sulfur battery is improved.

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. Chemists can grow carbon tubes up to 200 nm in diameter by using anodized aluminum as a template; 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.

This structure of large carbon tubes and small carbon tubes not only helps to increase the loading of sulfur (up to 85.2%), but also provides sufficient space for accommodating the volume change of sulfur during the cycle. Chemists used double metal hydroxides of Mg and Al as templates, the mesoporous graphene (DTG) with a specific surface area of 1628 m2·g-1 and a pore volume of 2.0 cm3·g-1 was prepared for use as a lithium sulfur battery cathode.

The stacking phenomenon of graphene during the preparation process is avoided, and sufficient space is provided for the accommodation of sulfur. 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 the lithium intercalation process, but 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. For example, a porous carbon-coated Li2S composite was constructed to be used as a lithium sulfur battery cathode, utilizing the pore structure and good electrical conductivity of the carbon-based material coating.

This not only helps to alleviate the rapid capacity decay of the battery caused by the “shuttle effect” of polysulfides, but also forms a good path for electron conduction and provides a channel for material transport, thus improving the overall performance of the lithium sulfur battery. If you want to know lithium battery cathode material related industry information, please refer to top 10 lithium battery cathode material manufacturers.

Anode current collector design

In terms of current collector design, the main idea is to optimize the lithium extraction and deposition process during battery cycling, so that the deposition on the current collector is more uniform, thereby suppressing the formation of lithium dendrites.

In specific implementation, on the one hand, the use of three-dimensional current collectors, such as three-dimensional Cu, Ni and graphene, can reduce the local current density, so that lithium can be uniformly deposited on the current collector, and lithium dendrites can be avoided;

At the same time, the three-dimensional framework structure of the current collector also helps to accommodate the volume change of lithium during the extraction and deposition process, and ensures the stability of the electrode structure. Chemists modified copper foil by hydrogen bubble dynamic template method to prepare three-dimensional porous copper as an improved Li metal anode current collector.

Compared with conventional copper foils, the porous structure of this three-dimensional copper current collector can reduce the local current density, suppress the growth of lithium dendrites, and accommodate the volume change of metallic lithium during cycling.

The current collector has both low overpotential and high Coulombic efficiency when tested at different current densities, showing more stable cycling performance. On the other hand, by modifying the substrate material and introducing lithiophilic substances, the nucleation energy barrier of lithium can be reduced and the affinity for lithium can be improved, so that the uniform deposition of lithium on the electrode surface can be achieved. For example, the negative electrode of a battery can use a carbon material as a substrate.

By depositing materials such as silicon or silver on it, it can provide a nucleation site for the growth of lithium, and significantly improve the interaction between the substrate material and lithium, it reduces the nucleation energy barrier of lithium deposition, promotes its uniform deposition, inhibits the formation of dendrites, and exhibits low overpotential in the process of lithium deposition and extraction, and has better cycle performance.

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