Lithium anode alloying of lithium-sulfur batteries
The alloying method is mainly to use lithium-containing alloys to replace metal lithium as the negative electrode, and it is expected that the characteristics of the lithium alloy structure can be used to slow down or inhibit dendrites and improve the low coulombic efficiency of the lithium negative electrode. From the earlier lithium aluminum alloys to lithium boron alloys, they have certain effects in ensuring the stability of the negative electrode structure, inhibiting lithium dendrites, and improving battery stability. The addition of a small amount of aluminum in lithium metal will not change the dissolution/deposition behavior of lithium, but will enhance the interface stability of the negative electrode, reduce the interface impedance, reduce the growth of dendrites on the negative electrode surface, and improve the poor cycle stability of the negative electrode and low Coulomb efficiency. It plays a positive role in the improvement of battery performance, especially the high temperature cycle performance of the battery. In the lithium-sulfur battery system, the introduction of the lithium aluminum alloy layer can inhibit the “shuttle effect” of the soluble polysulfide ions in the battery, and improve the stability of the anode interface and the electrochemical performance of the battery.
The lithium-boron alloy with a fibrous structure can maintain the original structure after lithium removal, and maintain a good surface morphology, achieve efficient and stable lithium dissolution/deposition, while reducing the reaction impedance of the system,and effectively improve battery cycle stability. When the lithium-boron alloy is applied to a lithium-sulfur battery, defects such as dendrites on the surface of the negative electrode are suppressed, and the battery exhibits a higher capacity and better cycle stability. In lithium-boron alloys, with the increase of boron content, the alloy is more likely to form a fiber network skeleton structure, so that the negative electrode has a larger specific surface area, which can effectively reduce the actual current density of the negative electrode, create a uniform electric field distribution, and inhibit the formation of lithium dendrites. In addition, this fibrous structure is also conducive to the rapid formation of a stable SEI film on the surface of the negative electrode, reducing the occurrence of pulverization and destruction of the negative electrode surface, reducing the occurrence of side reactions, and effectively improving the performance of the lithium-sulfur battery. Theoretical calculations prove that the negative electrode material with large specific surface area and nanostructure can effectively reduce the actual current density on the negative electrode and reduce the possibility of lithium dendrites. The lithium-boron alloy with 3D nanostructure can effectively control the size of deposited lithium, avoiding the excessive growth of lithium;
The abundant voids in the alloy allow the electrolyte to be infiltrated, and it is easy to quickly form a relatively stable SEI film on the electrode surface, and alleviate the difference in the concentration of Li+ at the electrode interface.
In addition, some of the more common alloy anodes for lithium-ion batteries (such as lithium tin alloys, lithium silicon alloys, etc.) can also replace metal lithium as lithium-sulfur battery anodes. Among them, the use of lithium tin alloy can reduce the side reaction between polysulfide ions and the negative electrode to a certain extent, so that the shuttle effect in the battery system is partially suppressed, so as to improve the coulombic efficiency and capacity of the battery. However, in actual use, the effect of lithium-silicon alloy is not particularly ideal, and it is easy to cause rapid decay of battery capacity. This is mainly due to the serious volume effect of both the sulfur cathode and the silicon anode during the cycle. However, the trial of lithium-silicon alloys still proves the possibility of replacing metal lithium with lithium-containing alloys.
At the same time, alloys composed of other metal elements and lithium may also be used as a negative electrode material for secondary batteries instead of metal lithium. Taking lithium-magnesium alloy as an example, when the magnesium content is high, the intermediate phase will be produced in the alloy and form the basic framework of the alloy. In the process of repeated discharge/charge, the framework of the alloy remains stable. In addition, the diffusion coefficient of lithium in lithium-magnesium alloys is relatively high, which can avoid the generation of lithium dendrites at high current densities. Similarly, the lithium-zinc alloy has a higher exchange current density between lithium ion deposition and dissolution, and enables the negative electrode to form a stable SEI film in the electrolyte, and improves the efficiency of negative electrode dissolution/deposition. In the same way, other metal elements such as Pb, Cd, Bi, Sb and As can form alloys with lithium, and have the potential to replace metal lithium as negative electrode materials, especially in lithium-sulfur battery systems.