Solid-state Batteries and Their Main Materials

What is a solid-state battery? The traditional lithium-ion battery includes four major components: positive electrode, negative electrode, electrolyte, and separator. A solid-state battery replaces the electrolyte with a solid electrolyte. Compared with traditional lithium-ion batteries, the key difference of solid-state batteries lies in that the electrolyte has changed from liquid to solid, with both safety and high energy density. Solid-state electrolyte batteries are the ultimate form of lithium and sodium batteries, which can completely solve safety issues and are undoubtedly the protagonist in the second half of the new energy market. The solid-state battery industry chain is roughly similar to that of liquid lithium batteries. The upstream includes raw materials, mining, machinery and equipment, and basic materials. The main difference between the two lies in the types of negative electrode materials and electrolytes. The positive electrode materials are almost the same. If it is fully developed into a full solid-state battery, the separator will also be completely replaced. The midstream of the industry chain is the processing and preparation process of battery packs, and the downstream application areas of the industry chain include new energy vehicles, energy storage systems, consumer electronics, etc.

The advantages of solid-state batteries are:

(1) Solid-state electrolytes are used to replace liquid electrolytes and separators. Solid-state electrolytes have a very high ignition point, which improves the thermal stability of the battery;

(2) The voltage platform of solid-state batteries is 5V, higher than the 4.3V of liquid batteries, which can match high-voltage electrode materials, and the battery energy density and specific capacity are better than liquid batteries;

(3) Solid-state electrolytes are not fluid, so there is no leakage, simplifying the battery pack design, reducing the weight and volume of the battery, and the energy density is expected to exceed 300Wh/kg.

Solid-state electrolyte

Solid-state electrolyte is the core component of solid-state lithium-ion batteries, which can serve as the separator and electrolyte of the battery at the same time. The core role of the electrolyte is to transmit Li+ between the positive and negative electrodes. Ideal solid-state electrolytes should have high ionic conductivity, low interface impedance, stable structure, high safety, high mechanical strength, and low price. Currently, based on different electrolytes, it can be mainly divided into polymer solid-state electrolytes and inorganic solid-state electrolytes. The representative system of the former is PEO polyethylene oxide; the latter is the oxide, sulfide, and halide systems.

Cathode materials

The main cathode materials for solid-state batteries are: Lithium Cobalt Oxide, Lithium Iron Phosphate, Lithium Nickel Cobalt Oxide, and Lithium Cobalt Aluminum Oxide.

（1）Lithium Cobalt Oxide: A commonly used cathode material in lithium-ion batteries, which can provide high energy density and long cycle life, but there are safety issues.

（2）Lithium Iron Phosphate: Compared with Lithium Cobalt Oxide, Lithium Iron Phosphate has better safety and longer life, but its energy density is lower.

（3）Lithium Nickel Cobalt Oxide: High energy density and long cycle life, but the material cost is high, and there are safety issues.

（4）Lithium Cobalt Aluminum Oxide: High energy density, but the cycle life is slightly lower than Lithium Nickel Cobalt Oxide.

（5）Combinations of multiple materials in solid-state electrolytes: For example, Lithium Manganese Oxide (LiMn2O4) and Lithium Titanate (Li4Ti5O12) can provide higher safety and longer life, but the energy density is relatively low.

Anode materials

The negative electrode materials for solid-state batteries mainly include three types: lithium metal, carbon materials, and silicon materials.

（1）Lithium metal is mainly used in solid-state lithium-ion batteries and solid-state lithium-sulfur batteries. Among them, solid-state lithium-ion batteries are a type of high-energy-density battery that can be applied in fields such as electric vehicles and drones. On the other hand, solid-state lithium-sulfur batteries are batteries with high energy density and high safety, suitable for applications in aerospace, military, and other fields.

（2）Carbon materials are primarily used in solid-state lithium-ion batteries. Carbon nanotubes, a common carbon material, have a high specific surface area and excellent electrochemical performance, making them suitable for use in high-performance solid-state lithium-ion batteries.

（3）Silicon is a novel negative electrode material that offers high specific capacity and low cost. In solid-state batteries, silicon can react with solid-state electrolytes to form lithium ions, enabling the charging and discharging of the battery. Compared to lithium metal and carbon materials, silicon has a higher specific capacity, but its cycle stability is relatively poor, prone to volume expansion and structural damage. Silicon materials are mainly used in solid-state lithium-ion batteries. Among them, silicon nanowires, a common silicon material, possess a high specific surface area and excellent electrochemical performance, suitable for use in high-performance solid-state lithium-ion batteries.

Separator

Separator material is an essential component of solid-state batteries, primarily used to isolate the positive and negative electrodes to prevent electronic conduction. The composition of separator materials mainly includes polymers, nanoscale powders, and so on. Research suggests that a double-layer coating can be used as an alternative to a separator, with an inorganic solid-state electrolyte layer coated on both sides of the negative electrode sheet, and an organic polymer layer coated on the surface of the inorganic solid-state electrolyte layer. Currently, there are views that sulfide and oxide all-solid-state batteries do not require a separator. Additionally, various published patents for solid-state batteries have proposed the concept of composite separators, such as inorganic-organic composite separators.

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