Lithium-sulfur batteries are generally a secondary battery system in which elemental sulfur and metallic lithium are active materials for the positive and negative electrodes, respectively. Among them, the density of sulfur is 2.36g/cm3, the molar mass is 32.065g/mol, and the standard electrode potential is 0.142V (vs. standard hydrogen electrode). Sulfur is abundant in the earth’s crust (about 0.048%, mass 7), widely distributed, and mainly exists in the form of elemental substances and compounds. There are many allotropes of elemental sulfur, among which orthorhombic sulfur, monoclinic sulfur and elastic sulfur are more common. At room temperature, elemental sulfur stably exists in an orthorhombic crystalline state and is composed of typical crown-shaped S8 molecules. As the temperature increases, sulfur will undergo a series of physical and chemical changes, such as melting, sublimation, and crystal transformation. Especially in the temperature range of 150~160℃, the viscosity of liquid sulfur is the smallest, and the sulfur cathode material can be prepared by virtue of its good fluidity. Elemental sulfur is difficult to dissolve in water, and has low solubility in ethanol and ether, but it has high solubility in certain non-polar organic solvents such as carbon disulfide, carbon tetrachloride, toluene and benzene.
Lithium metal is a metal with metallic luster and soft texture. It is also the metal with the smallest density of 0.53g/cm3 and the most negative standard electrode potential (-3.045V, vs. standard hydrogen electrode). The content of lithium in the shell is about 0.0065%, and it mainly exists in the form of lithium minerals such as spodumene, lepidolite, spodumene and bauxite. Lithium metal has a typical body-centered cubic structure (the temperature is higher than -117°C), but when the temperature drops to -201°C, the body-centered cubic structure will partly transform into a face-centered cubic structure, and the higher the temperature, the greater the degree of transformation . In addition, lithium is easy to fuse with other metals (except iron) to form alloys. Lithium metal is chemically active and easily reacts with various substances.
Unlike the intercalation/extraction mechanism in conventional “rocking chair” lithium-ion batteries, the reaction mechanism of lithium-sulfur batteries is based on the conversion reaction between sulfur and lithium. Among them, the reaction of the positive and negative electrodes and the overall reaction are as follows:
Positive electrode: S+2Li++2e–↔Li2S (1-1)
Negative electrode: 2Li↔2Li++2e– (1-2)
Total response： 2Li+S↔Li2S (1-3)
However, in actual batteries, due to the difference in electrolyte system, there are some differences in the charging and discharging process of lithium-sulfur batteries. Among them, a lithium-sulfur battery using an ether electrolyte system during the discharge phase corresponds to the loss of electrons from the negative electrode metal lithium, which converts into lithium ions and enters the electrolyte;
The sulfur molecules on the positive electrode react with the electrons transferred from the external circuit and the lithium ions in the electrolyte, and the elemental sulfur changes from solid (S8) to liquid high-order polysulfides (S2-x, 4≤x≤8). Subsequently, the soluble high-order polysulfides continue to transform into low-order polysulfides (S2-x, 2≤x<4), and the low-order polysulfides can still react to form solid Li2S2, and finally form electronically insulating Li2S. Theoretically, sulfur can release a specific capacity of 1675mA·h/g. During the charging process, the lithium ions on the lithium side of the negative electrode are converted into electrons supplied by the external circuit and deposited on the surface;
The Li2S at the positive electrode loses electrons and turns into solid sulfur through a multi-step reaction to complete the charging process, as shown in Figures 1(a) and 1(b). For lithium-sulfur battery systems using ester electrolytes, the discharge voltage and charge voltage are significantly reduced, and the discharge process corresponds to an inclined plateau. It is generally believed that the dissolution/deposition reaction occurs on the lithium metal negative electrode, and the solid-to-solid conversion reaction (S and Li2S) occurs on the sulfur positive electrode side. In an all-solid-state lithium-sulfur battery using solid electrolyte, the reaction process is similar to that of an ester electrolyte system. The positive electrode still corresponds to the mutual transformation between S and Li2S solid phase, the discharge process is a typical inclined platform, and the discharge voltage platform is lower, and the charging voltage platform is higher; the negative electrode corresponds to the repeated dissolution-deposition process of metallic lithium, and lithium ions move back and forth between the positive and negative electrodes through the solid electrolyte.
(a) Schematic diagram of the reaction mechanism of lithium-sulfur battery;
(b) Charge and discharge curve;
(c) Schematic diagram of reaction mechanism in ester electrolyte;
(d) Charge and discharge curve;
(e) CV curve of all solid-state lithium-sulfur battery;
(f) XRD pattern of sulfur cathode.
Since elemental sulfur (conductivity at room temperature is 5×10-30S/cm) and the reaction product Li2S are both electronic insulating materials, it is not conducive to the electrochemical performance of the system; At the same time, the density difference between sulfur and Li2S will cause the volume change of the electrode during charge and discharge (about 80%), which is easy to form stress in the material and electrode, resulting in adverse consequences such as material pulverization and electrode structure damage, and adversely affecting the cycle stability of the battery.
In addition, the soluble polysulfide ions (ether electrolyte system) formed in the reaction are easy to diffuse and migrate to the negative side, and can react with surface active lithium, resulting in partial loss of positive and negative active substances, as shown in Figure 2 (a); During the charging process, the polysulfide ions on the negative electrode side can be converted into low-order polysulfide ions through the electrons, and return to the positive electrode side, and then converted into high-order polysulfide ions through the oxidation process; part of the high-order polysulfide ions can be transferred to the negative electrode side and reduced to low-order polysulfide ions. Such reciprocation causes a “shuttle effect”, as shown in Figure 2(b), which causes the battery charging process to not be completed smoothly, and seriously reduces the battery’s charging and discharging efficiency.