Types and Properties of Solid Battery Electrolytes

A solid-state battery (SSB) is an electrical battery that uses a solid electrolyte (solectro) to conduct ions between the electrodes, instead of the liquid or gel polymer electrolytes found in conventional batteries.^[3] Solid-state batteries theoretically offer much higher energy density than the typical lithium-ion or lithium polymer batteries.^[4]
The development of electric vehicles in the 21st century has reignited people's interest in solid-state batteries. Major automobile companies have plans to implement the application of solid-state batteries before 2030 to improve the overall performance of electric vehicles.

There are three main types of solid electrolytes:

1. Polymer Electrolyte

Polymers are easy to synthesize and process, and were the first to be commercialized, but their electrical conductivity at room temperature is low and their overall performance improvement is limited, which restricts their large-scale application and development.

The polymer solid electrolyte is formed by the complexation of polymer and lithium salt, with a small amount of inert filler added. Lithium ions move through the polymer in sections and are transferred by continuous complexation and decomplexation.

The main polymer used is polyethylene oxide ( PEO ), polysiloxane (PS), polyacrylonitrile (PAN), polymethyl methacrylate (PMMA) and other materials can also be used, but they also have problems such as low room temperature ion conductivity and brittle texture, and are still in the research and development modification stage. Lithium salts are mainly used LiTFSI , good dispersibility and stability in polymers. Inert fillers are mainly oxides, such as TiO2, Al2O3, ZrO2, SiO2, etc., which play a role in reducing the crystallinity of polymers and improving mechanical properties.

Currently, the large-scale application of polymers is restricted. It is expected that they will be compounded with inorganic solid electrolytes in the future to achieve performance breakthroughs in the application end by combining the advantages of both.

2. Oxide Electrolyte

Oxides have both electrical conductivity and stability, are moderately difficult to mass produce, and are currently developing rapidly.

Oxide electrolytes are compounds containing lithium, oxygen and other components (phosphorus/titanium/aluminum/lanthanum/germanium/zinc/zirconium), which can be divided into two types: crystalline and amorphous. The amorphous type is mainly LiPON type, and the crystalline type can be divided into perovskite type (LLTO), antiperovskite type, GARNET type (LLZO), NASICON type (LATP), and LISICON type.

Overall, oxides have good thermal stability, a wide electrochemical window, and high mechanical strength, but their disadvantages are average conductivity, high brittleness and difficulty in processing, and poor interface contact. In terms of mass production, the preparation of oxide systems is moderately difficult, and many new players and domestic companies have chosen this route. It is expected that they will be the first to be installed on a large scale in semi-solid batteries by combining with polymers.

3. Sulfide electrolyte

Sulfide has the highest electrical conductivity and good processing performance, and has the greatest potential, but it is still in the research and development stage. Sulfide has the highest ion conductivity, is soft and easy to process, and can be squeezed to increase the interface contact, thereby improving battery performance.

According to the crystal structure, sulfides can also be divided into two types: crystalline and amorphous. The amorphous state is mainly LPS type (thiophosphoric acid), and the crystalline state can be divided into Argyrodite type (Argyrodite), LGPS type (lithium germanium phosphorus sulfur), and Thio-LISICON type (Thio-lithium fast ion conductor).

However, sulfide solid electrolytes have disadvantages such as high cost, poor electrochemical stability, poor air stability (produces H2S when in contact with water), and difficult production process, which limit their application in high energy density (high voltage, lithium metal) batteries. They are still in the research and development stage, but have the greatest potential for subsequent development. After technological breakthroughs, they may become the mainstream route in the future.