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EnglishViews: 20 Author: Site Editor Publish Time: 2022-07-28 Origin: Site
Compared with the traditional lithium ion battery, the theoretical energy density of lithium sulfur battery can reach 2680Wh/kg at the average voltage of 2.15V, which is about 6 times of the energy density of the current lithium ion battery. Therefore, it is considered as one of the most potential systems to replace the traditional lithium ion battery. In addition, compared with the expensive cathode materials of traditional lithium-ion batteries, sulfur is abundant, inexpensive, and environmentally friendly, giving it unparalleled advantages in large-scale energy storage system applications.
The structure diagram and typical charging and discharging curves of lithium sulfur batteries are shown in the following figure. The negative lithium metal is oxidized to release lithium ions and electrons, and a protective SEI film is formed on the contact surface with the electrolyte. Lithium ions and electrons move to the positive electrode through electrolyte and external load, respectively; Elemental sulfur is reduced to discharge product lithium sulfide at the positive electrode.
Elemental sulfur is commonly found in nature in the form of annular S8. In the discharge process, from solid ring S8 to liquid long chain polysulfide Li2Sn (4<n≤8) and short chain polysulfide Li2Sn (2<n≤4), and finally to solid discharge products Li2S2 and Li2S. There are two discharge platforms 2.3V (high plateau) and 2.1V (low plateau) on the discharge curve, in which the long-chain polysulfide has high solubility in the electrolyte, so the electrochemical reaction rate of the process is fast, while Li2S2 and Li2S are almost insoluble in the electrolyte, and the reaction rate is slow. In the charging process, Li2S is directly oxidized to S8 through intermediate polysulfide, thus forming a complete reversible REDOX reaction.
The invention of lithium-sulfur batteries dates back to the 1960s, when Herbert and Ulam filed a battery patent that first proposed sulfur as an electrode material for energy storage devices. Today, the research work of lithium sulfur battery has been carried out for more than half a century, but due to its inherent defects, its commercial application still faces many difficult challenges:
(1) Dissolution of LiPSs and "shuttle effect";
(2) Insulation of elemental sulfur and its discharge products;
(3) Large volume change;
(4) Unstable SEI and safety problems caused by lithium anode;
(5) Low sulfur load (area) or low sulfur content (%);
(6) The ratio of electrolyte to active sulfur (E/S) is greater than 10μL/mg in most cases, hindering the lithium sulfur battery to achieve high energy density;
(7) Self-discharge phenomenon.
In order to solve the above problems, lithium-sulfur battery research is usually carried out in the following aspects:
(1) Sulfur compound positive structure design, effectively control the reaction path of sulfide. One or more conductive/ionic materials are used to compound with sulfur to construct an electronic/ionic conductive network, and the active sulfur is evenly dispersed into the host material to improve electrode conductivity and reduce polarization loss. Select reasonable physical/chemical adsorption materials to effectively anchor LiPSs and prevent their "shuttle effect"; The optimization and development of binders to stabilize the structure of the positive electrode, reduce the content of inactive substances and the amount of electrolyte, improve the dispersion of active substances in the positive electrode, and anchor LiPSs, especially the development of functional binders such as ion/electron conduction; Design of new fluid collector and construction of binder free structure.
(2) Electrolyte optimization. By selecting salt components, solvents and new electrolyte additives matching with sulfur positive electrode structure, the electrolyte system is optimized, and the reaction kinetics and thermodynamic process of sulfide are effectively regulated at micro and macro levels.
(3) Development and use of functional diaphragm/interlayer. In lithium-sulfur batteries, the functional diaphragm/interlayer can reduce the diffusion of LiPSs to the lithium negative electrode by physical blocking or chemisorption, regulate the transport behavior of lithium ions, guide the deposition of lithium, and realize the effective protection of the lithium negative electrode. In addition, functional diaphragm/interlayer can increase the positive conductive interface, enhance the kinetics of LiPSs transformation, and improve the utilization of active substances.
(4) lithium anode protection. LiPSs is oxidized, and the strongly reducing lithium metal can react with the LiPSs shuttling to the negative electrode, leading to the severe corrosion of lithium metal. Especially in the case of low E/S ratio and high sulfur load, the high concentration gradient of LiPSs in the cell will aggravate the erosion reaction. In addition, during repeated cycles, SEI will be destroyed by huge volume changes, and the decomposition products of LiPSs will also become SEI components, making the control of SEI more complicated.
At present, lithium-ion batteries are still the majority of battery researches, and lithium-ion batteries are the main ones in new energy vehicles. However, the actual capacity value of lithium-ion batteries is close to the theoretical value, on the contrary, lithium-sulfur batteries can make up for this drawback. However, the poor conductivity of sulfur cathode materials, the dissolution of polysulfide and the volume change of sulfur in the charging and discharging process limit the commercialization and large-scale use of lithium sulfur batteries. For the research of lithium sulfur battery, there is a great application space in the future, its high theoretical specific capacity, high energy density and other significant advantages will make it replace lithium ion battery and become the mainstream.
