What Is A Lithium-sulfur Battery?

Lithium-ion batteries (LiCo02) are single-electron deintercalation, while lithium-sulfur batteries are 8-electron redox, so lithium-sulfur batteries have the theory that they are 7-8 times the capacity of lithium-ion batteries. Although polymer lithium batteries have been widely used in 3C products, due to the limited energy density, that is, the limited battery life, they need to be charged frequently, which is a troublesome thing. The most intuitive feeling is that after changing the smart phone, everyone is charging every day, and even the charging treasure is not leaving the state. Today’s society needs a new type of lithium-ion battery with low cost, no pollution, stable performance, large specific capacity, and high energy density to meet the needs of longer battery life and faster charging speed.

Lithium-sulfur battery development history: Lithium-ion batteries have a history of more than 30 years, and lithium-sulfur batteries are younger. In 1962, Herbet and Ulam first proposed the use of sulfur as a cathode material and alkaline perchlorate as the electrolyte.

The early lithium-sulfur system was studied as a primary battery and even commercialized for a time, but it was later replaced by rechargeable batteries and put on hold. In 2009, Linda F. Nazar proposed a lithium-sulfur secondary rechargeable battery on Nature Materials, and used CMK-3 to achieve a high specific capacity of 1320mAh/g. Since then, lithium-sulfur batteries have truly opened a chapter in development.

The principle of lithium-sulfur battery: the positive electrode of lithium-sulfur battery is sulfur or sulfur-containing material, and the negative electrode is lithium. The average voltage is 2.1V. In theory, the lithium-sulfur system (Li-S) has a specific capacity of 1672mAh/g and an energy density of 2600Wh/kg. It is a traditional commercial lithium ion battery with LiCo02 as the positive electrode (theoretical specific capacity 273.8mAh/g , Energy density 360Wh/kg) about 7 times. Compared with ordinary lithium-ion batteries, the nature of the discharge of lithium-sulfur batteries is not simple lithium ion deintercalation, but a redox process accompanied by a large number of intermediate products. During the discharge process of lithium-sulfur discharge battery, elemental sulfur reacts with Li from the ring-opening of cyclic S8, and the conversion from long-chain Li2S8 to short-chain Li2S is accompanied by two obvious discharge platforms, the high potential discharge platform is 2.45V—- 2.1V, the process can be considered a large amount of S8 to S42- conversion, and low-potential discharge is 2.1V-1.7V, this process is a large amount of S42- into S22- and S2-. On the other hand, different conversion degrees also correspond to different capacitances.

The discharge reaction equation is as follows:

Positive electrode: S8+16Li+e-→8Li2S

Negative electrode: Li→Li++e-

Total reaction: 2Li+nS→Li2Sn→Li2S

Ordinary lithium-ion batteries are single-electron deintercalation, and lithium-sulfur batteries are 8-electron redox, so they have 7-8 times the theoretical capacity and energy density. Similar to traditional lithium-ion batteries, lithium-sulfur batteries consist of a positive electrode, a negative electrode, a separator, an electrolyte and a separator. Therefore, lithium-sulfur batteries are considered to be the most promising alternative to traditional lithium-ion batteries and become a new energy source for a new generation of energy storage equipment.

Sulfur cathode materials are a key factor restricting the development and application of lithium-sulfur batteries, so we focus on sulfur cathodes. At present, the sulfur cathode of the lithium-sulfur system also has several problems to be solved: shuttle effect, poor conductivity, and volume expansion.

1. Polysulfides dissolve during the discharge process (Li2Sx, 3＜x＜8), resulting in a complex disproportionation reaction and a "shuttle effect", causing a large amount of self-discharge, reducing the Coulomb efficiency and cycle performance, and causing irreversible capacity degradation;

2. The conductivity of elemental sulfur and the discharge product lithium sulfide is low, the conductivity of S (5×10-30S/cm, 25℃), the conductivity of Li2S/Li2S2 (~10-30S/cm), resulting in the utilization of sulfur only About 50-70%.

3. The transformation from orthorhombic α-S (ρ1=2.03g/cm3) to Li2S with inverse fluorite structure (ρ2=1.66g/cm3) has large volume expansion, destroys the electrode structure, and affects cycle stability.