# A Review of Lithium-Sulfur Batteries Based on Metal-Organic Frameworks: Progress and Prospects
### Overview of Lithium-Sulphur Batteries
#### 1.1.1 Reaction Mechanism of Lithium-Sulfur Batteries  
In contrast to the typical lithiation/de-lithiation process observed in lithium-ion batteries (LIBs), the sulfur cathode in lithium-sulfur batteries (LSBs) operates through a conversion-type mechanism. This mechanism involves the electrochemical cleavage and reformation of the S-S bond in S\({}_{8}\), enabling a complex, multistep, multi-electron, and multiphase redox reaction within LSBs [25; 26; 27]. The electrochemical process of multi-electron conversion at the sulfur cathode during both charging and discharging phases is depicted in Figure 1a. During discharge, sulfur undergoes a sequential reduction, initially forming soluble polysulfides (Li\({}_{2}\)S\({}_{8}\), Li\({}_{2}\)S\({}_{6}\), etc.), which are subsequently converted to sulfur ions in a lower valence state (S\({}^{2-}\)) or persulfide ions (S\({}_{2}\)\({}^{2-}\)), ultimately resulting in the formation of solid lithium sulfide. A conventional lithium-sulfur battery (LSB) typically comprises a lithium metal anode, an organic electrolyte, a separator, and a sulfur cathode [28]. The reaction mechanism can be delineated as follows:  
1. S\({}_{8}\) to Li\({}_{2}\)S\({}_{8}\): Solid cyclic S\({}_{8}\) is first lithiated to form liquid Li\({}_{2}\)S\({}_{8}\) and dissolved in the electrolyte, which is a'solid-liquid' conversion process. This phenomenon aligns with the initial plateau observed in the discharge profile;
2. Li\({}_{2}\)S\({}_{8}\) to Li\({}_{2}\)S\({}_{6}\): This stage represents a 'liquid-liquid' transformation, taking place at approximately 2.3 V (relative to Li\({}^{+}\)/Li), which is associated with the initial plateau in the discharge curve;
3. Li\({}^{+}\)S\({}_{6}\) to Li\({}_{2}\)S\({}_{4}\): This procedure is depicted by the second gradient. The initial three sulfur transformation steps contribute to approximately 25% of the theoretical discharge capacity, which is around 419 mAh g\({}^{-1}\);
4. Conversion of Li\({}_{2}\)S\({}_{4}\) to Li\({}_{2}\)S\({}_{2}\): The dissolved ions undergo precipitation to form a solid phase at approximately 2.05 V (relative to Li/Li), a process that manifests itself as a second discharge plateau;
5. Conversion of Li\({}^{+}\)S\({}_{2}\) to Li\({}_{2}\)S: During the discharge process, the accumulation of Li\({}_{2}\)S\({}_{2}\) on the electrode surface persists in undergoing reduction to form Li\({}_{2}\)S until the process concludes.  
Figure 1: **(a)** LBS reaction mechanism and problems (reprinted/adapted with permission from [28]; copyright 2024, Cell Reports Physical Science). **(b)** Discharge; and **(c)** Investigation of charging voltage profiles and associated redox reaction byproducts in LSBs (reprinted/adapted with permission from [29]; copyright 2023, _Journal of Energy Chemistry_).  
Approximately 75% of the theoretical discharge capacity comes from the last two processes, as shown in Figure 1b. The charging process of LSBs is opposite to the discharging process. A typical charging profile is depicted in Figure 1c, illustrating the oxidation of solid Li\({}_{2}\)S to form LiPSs, concluding with the regeneration of sulfur. The peak that occurs before the first charging plateau corresponds to the overpotential observed during the oxidation of Li\({}_{2}\)S\({}_{2}\)/Li\({}_{2}\)S to LiPSs. This overpotential arises from the difficulty of dissolving solid Li\({}_{2}\)S, which has a rather low conductivity of about 3.6 \(\times\) 10\({}^{-7}\) S cm\({}^{-1}\) in LSBs [29].