Solid electrolyte for lithium-sulfur batteries
As an important branch of the electrolyte system, solid electrolyte occupies a place in actual battery production and application. The solid electrolyte has better safety and mechanical properties than liquid electrolytes, and its shape and size can achieve controllable design. However, the ionic conductivity of solid electrolytes is generally lower than that of liquid electrolytes, as shown in Figure 1. Solid electrolytes are often divided into polymer electrolytes and inorganic solid electrolytes.
As a typical representative of polymer electrolytes, gel polymer electrolytes have been sought after by people once they came out, and they have quickly been popularized and applied in lithium-ion batteries. As we all know, the ion transport mechanism of gel electrolytes is similar to that of liquid electrolytes. The application of common gel polymer electrolytes to lithium-sulfur battery systems is not ideal. On the contrary, the capacity of the battery decays faster than that of the liquid electrolyte. This is mainly due to the lack of restriction on polysulfide ions by the gel polymer electrolyte, and it is easier to cause the polysulfide ions to migrate out of the positive electrode side to cause shuttle and electrochemical inactivation. Therefore, people are actively looking for new gel polyelectrolyte systems in order to stabilize lithium-sulfur batteries, and adding some inorganic particles to the gel electrolyte has become an effective means to improve battery performance. When the gel electrolyte containing inorganic nanoparticles is used as the electrolyte of the lithium-sulfur battery, the problems of the traditional gel electrolyte can be obviously avoided; at the same time, the polysulfide ion shuttle phenomenon is alleviated, and the battery performance does not appear to be greatly and rapidly degraded. However, this system uses carbonate and sulfided polyacrylonitrile cathodes, and its applicability to conventional high-sulfur cathodes is worth studying.
Another important representative of polymer electrolytes—solid polymer electrolytes has become one of the alternatives to liquid electrolytes due to its more excellent safety performance and better interface compatibility. The solid polymer electrolyte omits the solvent and only contains the polymer matrix and the lithium salt. When in contact with lithium, it has better interface compatibility. However, its room temperature conductivity (ion) is generally relatively low. In actual work, especially when working with high current density, the deposition and dissolution of lithium on the surface will be greatly restricted, and it will easily lead to uneven lithium deposition and induce the generation of dendrites. In view of the low conductivity of solid polymer electrolytes, people often use methods such as adding inorganic lithium ion conductors or non-lithium ion conductor particles to artificially control the crystallinity of the polymer, and strive to increase the amorphous area in the polymer. Although the solid polymer electrolyte can limit the production and diffusion of polysulfides in the lithium-sulfur battery system, the utilization of sulfur will be greatly restricted when the solid state does not contain solvents. At the same time, if the surface properties are very different, it may cause some interface problems when matching with lithium, which will affect the stability of lithium.
Inorganic solid electrolytes have better thermal stability and mechanical properties than polymer electrolytes, and can truly realize solvent-free and improve battery safety. Due to the high strength of the material, the occurrence of lithium dendritic puncture can be avoided. After the scientists studied the electrode reaction mechanism of lithium-sulfur batteries using inorganic solid electrolyte systems, some inorganic solid electrolytes (such as LiBH4 and glassy P2S5, -Li2S, etc.) were applied to lithium-sulfur batteries. As a result, it was found that the use of inorganic solid electrolytes can not only achieve high capacity, but also high coulombic efficiency and excellent cycle stability. All-solid-state lithium-sulfur batteries based on inorganic solid electrolytes can overcome many problems in traditional liquid lithium-sulfur batteries, such as dendrites and shuttles, but there are also a series of difficult problems. For example, the low room temperature conductivity limits the possibility of using lithium-sulfur batteries under different current conditions; the interface matching between the electrolyte and the positive and negative electrodes is poor and has a large interface impedance; the preparation process has strict requirements on water and oxygen, and the battery assembly process is cumbersome.