Figure 1. Unique in-situ package strategy for a lithium-sulfur cathode structure: (a) Unwrapped carbon/sulfur material. (b) Perfectly wrapped carbon/sulfur material (before battery assembly). (c) Defective carbon/sulfur materials on the wrap (before battery assembly). (d) In-situ wrapped carbon/sulfur material.
Figure 2. (a) In-situ package flow chart. (b) Transmission electron micrographs of unwrapped carbon/sulfur materials, (c) carbon/sulfur materials with deficient cladding, and (d) carbon/sulfur materials in-situ. (e) Long cycle performance chart of in-situ wrapped in-situ wrapped carbon/sulfur material.
With the development of society and science and technology, the demand for electrochemical energy storage technology is increasing day by day. Researchers are all looking for next-generation secondary batteries with higher specific energy. Lithium-sulfur batteries use sulfur as the positive electrode active material, and realize energy storage and release based on the reversible electrochemical reaction between sulfur and lithium. The theoretical mass specific energy can reach 2,600 Wh/kg, which is 3 to 5 times that of current lithium ion batteries. It is expected to be used in power batteries, portable electronic products and other fields, but the internal polysulfide shuttle effect causes short cycle life problems that will limit its practical application in the future.
Recently, the research group of the Suzhou Institute of Nanotechnology and Nanobionics of the Chinese Academy of Sciences Chen Lijiu has made new progress in the research of lithium-sulfur battery cathode materials. The researchers demonstrated a new strategy that is different from the conventional sulfur cathode material package. The conventional coating strategy is to prepare a coating layer outside the particles of the sulfur positive electrode material, and then prepare this material into a positive electrode and assemble it into a battery with an electrolyte and the like. The conventional coating strategy has an insurmountable contradiction: if the material particles have been coated with a perfect coating before assembling the battery, the electrolyte will hardly diffuse into the material, resulting in the internal sulfur not being able to participate in the charge-discharge process; and if If the material is not perfectly covered, the polysulfide in the middle of charge and discharge will still diffuse out of the positive electrode material, causing a shuttle effect. In this new work, the researchers preliminarily grown an imperfect pore-containing precoating layer on the carbon/sulfur composite particles (completed in the material preparation process), and then prepared the positive electrode and the special material containing this material. The electrolyte of the additive is assembled together into a battery. As the electrolyte infiltrates the carbon/sulfur particles, the additive reacts with the pre-coating layer to form a dense coating in-situ outside the particles.
This in-situ coating strategy avoids the disadvantages of conventional methods, which not only achieves the infiltration of electrolyte and materials, but also limits the diffusion of polysulfides. The results of the study show that the Coulomb efficiency and cycle life of lithium-sulfur batteries using this new coating strategy are significantly improved. The assembled battery exhibits excellent cycle stability under conditions of high discharge rate, circulating 1000 times at a current density of 1C, and the capacity decay rate of only one cycle is only 0.03%. Related results have been published on Nature Communications (8,479,2017).
This work has been supported by the special pilot project of the Chinese Academy of Sciences, the key R&D program of the Ministry of Science and Technology, and the National Natural Science Foundation of China.
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