In addition to protecting lithium from the alloying point of view, surface modification of lithium is also a common and effective means. By constructing various modified layers on the lithium surface in situ or ex situ to solve the problems faced by lithium in practical electrochemical reactions, it provides a certain solution for the commercialization of lithium secondary batteries, especially lithium-sulfur secondary batteries.
A simple mechanical treatment method was used to roll out some regular-shaped pits on the lithium surface, and then apply them to the battery for testing. The pretreated lithium surface acquires defects, increases the specific surface area, and avoids the increase of the specific surface area caused by the structural change of lithium itself during the cycling process. After repeated dissolution/deposition of lithium, the surface can still maintain the intact morphology, which is mainly attributed to the effective surface pretreatment that enables the dissolution and deposition of lithium to preferentially occur in the pits. This is inconsistent with the generally accepted idea of keeping the surface of lithium flat. However, this method of “doing the opposite” to protect lithium is still worth considering and learning from. However, the actual effect in lithium-sulfur batteries still needs to be verified and studied.
The reaction between the substance and the metal lithium can also form a protective layer on the surface of the metal lithium in situ, so that the lithium remains stable in the electrolyte, reducing the direct contact between the electrolyte, polysulfide ions and lithium, avoiding the irreversible loss of active materials, and improving the stability of the battery during the electrochemical reaction process. For example, a Li3N layer is formed on the lithium surface by means of the reaction of nitrogen gas and metal lithium. The Li3N layer is a good lithium ion conductor, which can shorten the diffusion distance from Li+ to lithium, improve the properties of the lithium/electrolyte interface, and promote efficient and stable electrochemical reactions. The use of Li3N-modified lithium-containing lithium-sulfur batteries exhibits higher discharge capacity and better cycling performance. Based on the reaction of polyphosphoric acid with lithium, a stable Li3PO4 layer can be formed in situ on the lithium surface. This “artificial” SEI film effectively reduces the surface structure fluctuation of lithium during the dissolution/deposition process and suppresses dendrite generation. At the same time, in the sulfur-containing battery system, the side reaction between polysulfide ions and lithium is alleviated, and the battery structure and electrochemical performance are steadily improved. Modification of lithium with trimethylchlorosilane can in situ form a uniform and dense protective layer on the lithium surface. The role of the protective layer is not only reflected in reducing the interfacial impedance and polarization potential of lithium, but also enables the lithium-sulfur battery to achieve high charge-discharge efficiency (98%) and high capacity (760 mA·h/g) in a lithium nitrate-free electrolyte. After 100 cycles of the protected lithium, the surface is flat, and there is no obvious dendrite and structural fracture damage. This proves the feasibility and reliability of trimethylchlorosilane to protect lithium, and also provides a reference for other halogen-containing silanes for lithium protection.
The protective layer was prepared in situ or ex situ on the surface of metallic lithium by other means, and applied in lithium-sulfur batteries. The conductive polymer (polyethylenedioxythiophene, PEDOT) layer is obtained by in-situ formation of a polymer protective layer or chemical polymerization on the lithium surface by means of the cross-linking reaction initiated by an organic substance under the action of ultraviolet light. The polymer layer obtained by surface polymerization can reduce the interfacial impedance of lithium, reduce the morphology change of lithium before and after cycling, and prevent the generation of lithium dendrites, which can have a positive impact on the performance of lithium electrodes and batteries; the conductive polymer layer constructed on the lithium surface is conducive to the uniformization of the electric field on the lithium surface and reduces the possibility of dendritic growth during lithium deposition, and it can also block polysulfide ions to prevent them from damaging lithium, maintain a relatively stable physical and chemical state of lithium, and achieve the dual results of lithium protection and battery performance improvement. In addition, a protective layer can be constructed on the lithium surface by spin coating, blade coating, atomic layer deposition, and magnetron sputtering. The protective layer obtained by spin coating can reduce the erosion and destruction of lithium by electrolyte and polysulfide ions, and at the same time form a relatively uniform and stable electric field distribution on the lithium surface to ensure uniform deposition/dissolution of lithium; atomic Layer Deposition (ALD) technology can precisely control the thickness of the lithium surface protective layer and enhance the stability of lithium in air, sulfur and organic solvent environments. When it is applied to sulfur-containing batteries, the irreversible capacity decreases in the first week, and the capacity retention rate is significantly improved, and the capacity retention rate for 100 cycles is still close to 90%. The method of magnetron sputtering can also build a more uniform protective layer on the surface of metal lithium, slow down the tendency of the SEI film on the lithium surface to grow thicker with the cycle, reduce the contact and side reactions between lithium and polysulfide ions, and steadily improve the cycle stability of the lithium-sulfur battery. Using a scraper to coat the lithium surface SEI film component on the lithium surface to form a protective layer can avoid the decomposition of the electrolyte on the lithium side and reduce the interfacial impedance of lithium, which is of guiding significance for the use of other SEI film components to construct a protective layer. With the help of electrochemical methods, a stable SEI film can also be formed on the surface of metal lithium in advance, and then used in batteries. This cleverly designed ex-situ method can form a dense SEI film, and can reduce the loss of electrolyte, active materials and lithium caused by the formation of the SEI film in the lithium-sulfur battery, and improve the sulfur utilization rate and system stability. It provides a new idea for lithium protection.