Recently, media outlets have been reporting on a breakthrough in lithium-sulfur batteries, potentially enabling electric vehicles to achieve a range of 1000 kilometers. So, what exactly is a lithium-sulfur battery, and how does it differ from the currently mainstream lithium-ion batteries?
Lithium-sulfur batteries are a type of lithium battery that uses sulfur as the positive electrode and metallic lithium as the negative electrode. Elemental sulfur is abundant on Earth and is characterized by its low cost and environmental friendliness. Lithium-sulfur batteries using sulfur as the positive electrode material have high theoretical specific capacity and theoretical specific energy, reaching 1675 mAh/g and 2600 Wh/kg respectively, far exceeding the capacity of commercially widely used lithium cobalt oxide batteries. Furthermore, sulfur is an environmentally friendly element with minimal pollution, making it a very promising lithium battery option.
In comparison, lithium-sulfur batteries have an energy density exceeding 500Wh/kg, which is significantly better than the maximum energy density of 300Wh/kg for lithium-ion batteries.
Lithium-sulfur batteries typically use elemental sulfur as the positive electrode and metallic lithium as the negative electrode. Their reaction mechanism differs from the ion insertion/extraction mechanism of lithium-ion batteries; it is an electrochemical mechanism.
Lithium-sulfur batteries use sulfur as the positive electrode reactant and lithium as the negative electrode. During discharge, the negative electrode reaction involves lithium losing electrons to become lithium ions, while the positive electrode reaction involves sulfur reacting with lithium ions and electrons to form sulfides. The potential difference between the positive 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 electrode reactions of the lithium-sulfur battery proceed in reverse, which is the charging process.
Based on the amount of electricity provided by the complete conversion of elemental sulfur to S2-, the theoretical discharge specific capacity of sulfur is 1675 mAh/g. Similarly, the theoretical discharge specific capacity of elemental lithium is 3860 mAh/g. The theoretical discharge voltage of a lithium-sulfur battery is 2.287V when sulfur and lithium completely react to form lithium sulfide (Li2S). The corresponding theoretical discharge specific energy of a lithium-sulfur battery is 2600 Wh/kg.
There are three main problems with lithium-sulfur batteries: 1. Lithium polysulfide compounds dissolve in the electrolyte; 2. Sulfur is a non-conductive substance with very poor conductivity, which is not conducive to the high-rate performance of the battery; 3. Sulfur undergoes significant volume expansion and contraction during charging and discharging, which may damage the battery.
