4 Types of Solid Electrolytes for Solid State Battery

Why are all-solid-state batteries an industry trend?

High security:

The safety issues of liquid batteries have always been criticized. The electrolyte is easily flammable under high temperature or severe impact. Under high current, lithium dendrites will also appear to pierce the separator and cause short circuit. Sometimes the electrolyte may undergo side reactions or decompose at high temperatures. The thermal stability of liquid electrolytes can only be maintained up to 100°C, while oxide solid electrolytes can reach 800°C, and sulfides and halides can also reach 400°C. Solid oxides are more stable than liquids, and due to their solid form, their impact resistance is much higher than that of liquids. Therefore, solid-state batteries can meet people's needs for safety.

High energy density：

At present, solid-state batteries have not achieved an energy density exceeding that of liquid batteries, but theoretically solid-state batteries can achieve very high energy density. Solid-state batteries do not need to be wrapped in liquid to prevent leakage like liquid batteries. Therefore, redundant shells, wrapping films, heat dissipation materials, etc. can be eliminated, and the energy density can be greatly improved.

High power:

Lithium ions in liquid batteries are carried by conduction, while lithium ions in solid-state batteries are by jump conduction, which is faster and has a higher charge and discharge rate. Fast charging has always been a difficulty in liquid battery technology, because lithium will be precipitated if the charging speed is too fast, but this problem does not exist in all-solid-state batteries.

Low temperature performance:

Liquid batteries generally work stably at -10°C to 45°C, but their cruising range seriously drops in winter. The operating temperature of solid electrolytes is between -30°C and 100°C, so there will be no reduction in battery life except in extremely cold areas, and no complex thermal management system is required.

Long life span:

Among liquid batteries, the average life of ternary batteries is 500-1000 cycles, and the life of lithium iron phosphate can reach 2000 cycles. The thin film all-solid state can reach 45,000 cycles in the future, and the 5C life span in the laboratory can reach 10,000 times. When the production cost of the same energy density can be converged, the cost-effectiveness of solid-state batteries is unparalleled.

Comparison of 4 solid inorganic electrolytes

The material types of solid electrolytes can be divided into four categories: oxides, sulfides, polymers, and halides. Each of these four types of electrolytes has different physical and chemical properties, which determines the difficulty of R&D, production, and industrialization and its future market position.

Oxide Electrolytes:

Advantages: The ionic conductivity is in the middle, and it has the best electrochemical stability, mechanical stability and thermal stability. It can be adapted to high-voltage cathode materials and metal lithium anodes. Excellent electronic conductivity and ion selectivity. At the same time, the degree of equipment continuity and manufacturing cost also have great advantages. The comprehensive ability is the most comprehensive.

Disadvantages: Reduction stability is slightly low, brittle, and may cause cracks.

Sulfide electrolyte:

Advantages: highest ion conductivity, small grain boundary resistance, good ductility, and good ion selectivity.

Disadvantages: poor chemical stability, will react with lithium metal, and easily react with moist air. The cost is higher and the mechanical properties are poor. At present, production still needs to be carried out in a glove box, making it difficult to mass produce on a large scale.

Sulfide electrolytes have high room temperature conductivity and good ductility, and their stability can be improved through doping and coating. Sulfide electrolytes currently come in three main forms: glass, glass-ceramics and crystals. Sulfide electrolytes have high room temperature conductivity, which can be close to that of liquid electrolytes (10-4-10-2 S/cm), moderate hardness, good interface physical contact, and good mechanical properties. They are important candidate materials for solid-state batteries. However, sulfide electrolytes have a narrow electrochemical window, poor interface stability with positive and negative electrodes, and are very sensitive to moisture. It can react with trace amounts of water in the air and release toxic hydrogen sulfide gas. Production, transportation, and processing have very high environmental requirements. Modification methods such as doping and coating can stabilize the interface between sulfide and positive and negative electrodes, making them suitable for various types of positive and negative electrode materials, and even used in lithium-sulfur batteries.

The preparation of sulfide electrolyte batteries has high environmental requirements. Sulfide electrolytes have high conductivity and are relatively soft, and can be produced by coating methods. The production process is not very different from the existing liquid battery production process, but in order to improve the interface contact of the battery, it is usually necessary to perform multiple hot pressings after coating and add a buffer layer to improve the interface contact. Sulfide electrolytes are very sensitive to moisture and can react with trace amounts of water in the air to generate toxic gas hydrogen sulfide, so the environmental requirements for battery manufacturing are very high.

Polymer electrolyte:

Advantages: good safety, good flexibility and interface contact, easy to form film.

Halide electrolyte:

Advantages: low electronic resistance, high ion selectivity, high reduction stability, and not easy to crack.

Disadvantages: It is still in the laboratory stage, has poor chemical stability and oxidative stability, and has high ion resistance.

Due to the prominent advantages and disadvantages of halides and polymers, the future global competition for solid-state batteries will mainly focus on oxides and sulfides. In fact, due to its poor chemical stability, the types of materials that can be selected for sulfide electrolytes are very narrow, but as long as suitable materials and process breakthroughs are found, this shortcoming can be made up for. 

However, from an industrialization perspective, complex processes will lead to higher costs and a scale ceiling, so oxide solid electrolytes are currently the mainstream in the development of solid-state batteries. From liquid batteries to solid-state batteries, there will be a semi-solid battery stage, and the most suitable one at this stage is the oxide path. It is because of its comprehensive performance and cost advantages. Semi-solid-state batteries can replace current liquid batteries more quickly, gradually taking advantage of the advantages and cost-effectiveness of solid-state batteries.

However, with the advancement of technology, it is still unclear whether the world will be dominated by oxides or sulfides in the future. The core of solid-state battery technology is the research and development of solid-state electrolytes. Although current solid electrolyte materials have made great progress, they still have problems such as poor conductivity, large interface resistance, and high preparation costs. Continued basic research and technological breakthroughs are needed to improve the conductivity and stability of solid electrolytes.