What are solid state batteries

Solid-state batteries are lithium-ion batteries that use a solid electrolyte instead of a traditional liquid electrolyte. Based on the amount of solid electrolyte used, they can be divided into semi-solid-state batteries and all-solid-state batteries. Generally, a 10% liquid content in the battery is used as the dividing line between semi-solid-state and liquid batteries, while all-solid-state batteries use a completely solid electrolyte, reducing the liquid content to 0%.

Solid-state lithium-ion batteries mainly consist of a positive electrode, a negative electrode, and a solid electrolyte. The most fundamental difference is that the electrolyte and separator of a liquid battery are replaced with a solid electrolyte, achieving the elimination or reduction of the need for a separator and electrolyte.

1. The positive electrode typically uses lithium metal or similar materials. When lithium ions move from the solid electrolyte to the positive electrode, the positive electrode material undergoes an oxidation reaction, releasing electrons

2. The negative electrode generally uses a lithium alloy or similar materials. When lithium ions move from the solid electrolyte to the negative electrode, the negative electrode material undergoes a reduction reaction, accepting electrons.

3. The solid electrolyte is composed of conductive solid materials, such as lithium-containing inorganic salts, polymers, or ceramic materials. This electrolyte exhibits high ion mobility, low electrical resistance, and high chemical stability.

Based on electrolyte classification, batteries can be subdivided into four main categories: liquid (25wt%), semi-solid (5-10wt%), quasi-solid (0-5wt%), and all-solid (0wt%). Semi-solid, quasi-solid, and all-solid are collectively referred to as solid-state batteries. For automakers adopting solid-state batteries, safety is a short-term driving factor, while energy density is a long-term driving factor.

Semi-Solid-State Batteries: Compared to liquid batteries, semi-solid-state batteries reduce the amount of liquid electrolyte and increase the use of composite electrolytes of oxides and polymers. Oxides are mainly added through separator coating and positive and negative electrode coating, while polymers are filled in a framework network. Furthermore, the negative electrode has been upgraded from a graphite system to a pre-lithiated silicon-based negative electrode or a lithium metal negative electrode, and the positive electrode has been upgraded from high-nickel to high-nickel + high-voltage, lithium-rich manganese-based, etc. The separator is still retained and coated with a solid electrolyte coating. The lithium salt has been upgraded from LiPF6 to LiTFSI, achieving an energy density of over 350 Wh/kg. While semi-solid-state batteries reduce the amount of liquid electrolyte used, they still pose a flammability risk.

All-solid-state batteries, compared to liquid batteries, eliminate the traditional liquid electrolyte, using oxides, sulfides, polymers, etc., as solid electrolytes. The positive and negative electrodes are separated in the form of a thin film, thus replacing the function of a separator. Oxides are currently making the fastest progress, sulfides have the greatest future potential, and polymers have a lower performance ceiling. The negative electrode has upgraded from graphite systems to pre-lithiated silicon-based negative electrodes and lithium metal negative electrodes, while the positive electrode has upgraded from high-nickel to ultra-high-nickel, lithium nickel manganese oxide, and lithium-rich manganese-based positive electrodes. Energy densities can reach 500 Wh/kg.

Based on the materials and properties of the solid-state electrolyte, solid-state batteries can be divided into several main categories, including sulfide, oxide, and polymer solid-state batteries.

Sulfide Solid-State Batteries: Sulfide solid-state batteries use inorganic sulfide materials as electrolytes. These materials typically exhibit high lithium-ion conductivity, approaching or exceeding that of traditional liquid electrolytes.

Sulfide solid-state electrolytes have attracted considerable attention due to their high ionic conductivity; for example, the conductivity of Li10GeP2S12 (LGPS) electrolyte can reach 1.2 × 10^-2 S/cm. However, sulfide electrolytes are sensitive to moisture, readily reacting with water to generate toxic H2S gas, and undergoing irreversible chemical reactions with oxygen and water vapor in the air, leading to a decrease in ionic conductivity and structural damage.

Therefore, the development of sulfide solid-state electrolytes is challenging and requires stringent production environments.

Oxide Solid-State Batteries: Oxide solid-state batteries use oxide materials as electrolytes. These materials generally have lower ionic conductivity but possess good mechanical properties and chemical stability.

A representative oxide electrolyte is Li7La3Zr2O12 (LLZO) with a garnet-type structure, exhibiting high ionic conductivity, reaching 10^-4 S/cm at room temperature. The dense morphology of oxide electrolytes gives them higher mechanical strength, good stability in air, and the ability to withstand high voltages. However, due to their high mechanical strength, oxide electrolytes have poor deformation and flexibility, making the electrolyte sheet prone to brittleness and resulting in high solid-solid interface contact losses, thus limiting their applications.

Polymer Solid-State Batteries:Polymer solid-state batteries consist of a polymer matrix and lithium salts. They exhibit low ionic conductivity at room temperature, but this significantly improves when heated above 60°C.

Polymer electrolytes are lightweight, elastic, and have excellent machinability. Their manufacturing process is similar to existing lithium-ion batteries, making mass production easy. However, polymer electrolytes have low ionic conductivity at room temperature and are susceptible to short circuits caused by lithium dendrite penetration, resulting in limited thermal stability

Combined Solid-State Batteries:Besides the three main types of solid-state batteries mentioned above, there are also combined solid-state batteries, such as composite solid-state electrolytes, which are electrolytes obtained by combining sulfide/oxide and polymer electrolytes. This composite electrolyte combines the advantages of inorganic and organic solid-state electrolytes, possessing both high lithium-ion conductivity and electrochemical stability.

In addition, there are chloride solid-state electrolytes, which combine the high ionic conductivity and deformability of sulfides with the stability of oxides for high-voltage cathode materials, but large-scale commercialization is not yet feasible

Solid-state electrolytes, unlike liquid electrolytes, lack fluidity, resulting in poor contact between solid particles. Combined with electrochemical instability, this leads to numerous interface problems. However, compared to liquid batteries, solid-state batteries offer potential advantages:

High safety: Non-volatile and non-flammable solid-state electrolytes offer higher safety compared to organic electrolytes.

Good temperature adaptability: All-solid-state batteries can operate over a wider temperature range, especially at higher temperatures.

High energy density: All-solid-state batteries hold promise for solving the safety issues of lithium metal anodes (lithium dendrites). This could further improve the energy density of lithium-ion batteries compared to the graphite and silicon-carbon anodes currently used in commercial lithium batteries.

Simplified cell, module, and system design: Due to the non-fluid nature of solid-state electrolytes, internal series connection is possible.

Solid-state battery research has a long history.

Between 1831 and 1834, Michael Faraday discovered the solid electrolytes silver sulfide and lead fluoride, laying the foundation for solid-state ionics. In the late 1950s, scientists discovered a silver-conducting electrochemical system using solid electrolytes.

In 1967, scientists discovered the rapid ion conduction of β-alumina, which could be used in alumina, initiating the development of novel solid-state electrochemical devices with higher energy densities, such as the molten sodium/β-alumina/sulfur battery developed by Ford Motor Company in the United States and NGK in Japan. During system development, organic solid electrolytes (polyethylene oxide (PEO)) and inorganic solid electrolytes (NASICON) were discovered.

In the 1990s, Oak Ridge National Laboratory in the United States developed a novel solid electrolyte: lithium phosphorus oxynitride (LiPON), which can be used to manufacture thin-film lithium-ion batteries. In 2011, Kamaya et al. demonstrated the first solid electrolyte (LAGP), capable of achieving volumetric ionic conductivity exceeding that of its liquid electrolyte counterparts at room temperature. In 2017, John Goodenough, co-inventor of the lithium-ion battery, introduced a solid-state battery that uses a glass electrolyte and an alkali metal anode composed of lithium, sodium, or potassium.

Solid-state batteries, as a crucial direction for future energy technology, have a very broad development prospect. With technological advancements, policy support, and market expansion, solid-state batteries are expected to achieve large-scale commercial applications.

Technological Advancements: With the continuous development of materials science, electrochemistry, and other fields, the technical challenges of solid-state batteries will be gradually resolved. For example, through material composites and interface optimization, the ionic conductivity and fast-charging performance of solid-state batteries can be improved.

Policy Support: The Chinese government attaches great importance to the development of the solid-state battery industry and has introduced a series of supportive policies. These policies provide a clear and broad market prospect and a favorable production and operating environment for the solid-state battery industry.

Market Demand: The rapid growth of the new energy vehicle market and the increasing demand for energy storage provide a vast market space for solid-state batteries. In the future, with the popularization of new energy vehicles and the widespread application of energy storage systems, the demand for solid-state batteries will increase significantly.

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