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 inexpensive and environmentally friendly. 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 (<150 mAh/g).
I. Charging and discharging principle of lithium-sulfur batteries
A typical lithium-sulfur battery uses elemental sulfur as the positive electrode and lithium metal as the negative electrode. Its reaction mechanism differs from the ion insertion/extraction mechanism of lithium-ion batteries; it operates on an electrochemical mechanism. In a lithium-sulfur battery, sulfur is the positive electrode reactant, and lithium is 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 an applied voltage, the positive and negative electrodes of a lithium-sulfur battery react 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 the lithium-sulfur battery is 2600 Wh/kg.
II. Problems with Lithium-Sulfur Batteries
The main problems with lithium-sulfur batteries are:
First, elemental sulfur has poor electronic and ionic conductivity. Sulfur materials have extremely low conductivity at room temperature (5.0 × 10⁻³⁰ S·cm⁻¹). The final products of the reaction, Li₂S₂ and Li₂S, are also electronic insulators, which is detrimental to the high-rate performance of the battery.
Secondly, intermediate discharge products in lithium-sulfur batteries dissolve into the organic electrolyte, increasing its viscosity and reducing ionic conductivity. Polysulfide ions can migrate between the positive and negative electrodes, leading to loss of active material and wasted energy (Shuttle effect). Dissolved polysulfides can diffuse across the separator to the negative electrode, reacting with it and damaging the solid electrolyte interphase (SEI) film.
Third, the final discharge product of lithium-sulfur batteries, Li2Sn (n=1~2), is electronically insulated and insoluble in the electrolyte, and is deposited on the surface of the conductive framework. Some lithium sulfides detach from the conductive framework and cannot be converted into sulfur or higher-order polysulfides through the reversible charging process, resulting in a significant capacity decay.
Fourth, sulfur and lithium sulfide have densities of 2.07 and 1.66 g·cm⁻³, respectively, and exhibit volume expansion/contraction of up to 79% during charge and discharge. This expansion leads to changes in the morphology and structure of the positive electrode, causing sulfur to detach from the conductive framework, thus resulting in capacity decay. This volume effect is not significant in coin cells, but it is amplified in large batteries, producing significant capacity decay and potentially damaging the battery. The huge volume change can also destroy the 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 volume changes during charging and discharging and is prone to dendrite formation.
Sixth, there is a lot of laboratory-scale research on lithium-sulfur batteries. The sulfur loading per unit area is generally below 3.0 mg·cm-2. Research on high-loading electrodes is of great value for obtaining high-performance lithium-sulfur batteries.
