Behind the high heat of the development of the solid-state batteries

Cell energy needs to be improved

Lithium ion batteries with high voltage layered transition metal oxides as the cathode and graphite as the anode. If a silicon-based alloy is introduced instead of pure graphite as the anode, the upper limit of the energy density theory can be increased.

In order to further improve the energy density, lithium metal must be used as the anode, and the gram capacity of the metal lithium anode is about 10 times that of graphite. Lithium metal cathode is difficult to achieve in the current traditional liquid battery system. In the field of rechargeable batteries, there are a series of technical problems with liquid batteries, and there is still no effective solution to them.

For example, there are many interfacial side reactions between lithium metal and liquid electrolyte, and the instability of SEI membrane leads to poor cycle life. The uneven deposition and dissolution of lithium metal leads to the uneven formation of lithium dendrites and pores.

To use lithium metal as the cathode, it is necessary to replace the liquid electrolyte with poor thermal stability, flammable and leaky, and easy to decompose on the surface of lithium metal to shorten battery life, with a solid electrolyte. After removing the electrolyte, the anode and anode and the electrolyte of the lithium battery are solid.

Thermal runaway of lithium-ion batteries

Since lithium metal will have problems such as pulverization and dendrite growth after multiple charges and discharges, current lithium batteries need to use graphite as lithium carrier in the anode. And a separator is added between the anode and cathode to prevent battery safety accidents such as short circuits between the anode and cathode .

● Development environment

The development path at this stage is clear. The top 10 solid state battery manufacturers have clarified the development goals and industrial technology plans of solid-state batteries, focusing on improving battery energy density and transforming into solid-state batteries from 2020 to 2025, and developing all-solid-state battery that can be used commercially in 2030.

Basic concept of solid-state battery

Such batteries essentially replace the electrolyte and separator of liquid batteries with solid electrolytes, while pursuing the application of lithium metal cathode. Build high battery capacity in solid electrolytes, consistent with the core requirements of liquid electrolytes.

● High conductivity

The function of the electrolyte is that during battery charging and discharging, the index that determines the smooth transport of lithium ions is called ion conductivity. Low ion conductivity means poor electrolyte conductivity, so lithium ions cannot move smoothly between the anode and cathode of the battery.

● Good chemical stability

Does not react with the internal materials of the battery. Non-flammable, high temperature resistant, non-corrosive, and non-volatile, eliminating electrolyte leakage, short circuit and other safety hazards in traditional lithium-ion batteries.

● High migration number of lithium ions

Theoretically, lithium-metal solid-state batteries are more efficient in grouping, use fewer materials, and have a simpler structure, and the production process is expected to be simplified.

Correspondingly, the battery pack protection system, cooling system, BMS, etc. can be simplified. Therefore, after mass production, it is expected to achieve lower costs than conventional lithium-ion batteries in terms of materials and production processes.

Solid state battery classification

● Electrolyte material

According to the choice of electrolyte materials, solid-state batteries can be divided into three system electrolytes: polymers, oxides and sulfides. By doping the liquid electrolyte, the problem of low conductivity can be improved to some extent.

However, due to the extremely active characteristics of lithium metal, new interface and stability problems will occur between the liquid electrolyte and the lithium metal cathode. Therefore, the choice of anode material is likely not to cross directly with lithium metal.

Instead, it is to use the gradual method of graphite doped with silicon and silicon instead of graphite, looking for material systems that improve energy density while maintaining stability and safety.

● Anode and cathode materials

According to the different anode and cathode materials, solid-state batteries can be divided into solid-state lithium-ion batteries (following the current lithium-ion battery material system, such as graphite+silicon-carbon anode, ternary cathode, etc.) and solid-state lithium metal batteries.

● Liquid electrolyte content

According to the classification of electrolytes containing liquid electrolyte content, lithium batteries can be divided into four categories: liquid, semi-solid, quasi-solid, and all-solid. It is expected that by 2025, semi-solid-state batteries can achieve small-scale mass production, and by 2030, all-solid-state batteries can be commercialized.

Solid-state electrolyte type

Polymer electrolytes have low conductivity at room temperature. After heating, ionic conductivity is greatly increased, but it consumes energy and increases the cost, which increases the difficulty of commercialization.

The oxide electrolyte has good overall performance, and top 10 battery electrolyte companies have mass produced LiPON thin film solid state batteries in small quantities. LLZO type lithium-rich electrolyte lithium anode has good compatibility and is considered one of the most attractive solid electrolyte materials.

The important factor restricting its development is the large interface impedance between the electrolyte and the electrode, and the interface reaction causes the battery capacity to decay. Sulfide solid electrolyte has the highest conductivity, the highest research difficulty, and the greatest development potential.

Although the interface stability of sulfide electrolyte and lithium electrode is poor, it is favored by many enterprises due to its extremely high ionic conductivity. In particular, Japanese and Korean companies have invested a lot of money in research.

The disadvantages are poor adaptability to the cathode, high sensitivity to water or air, and the possibility of producing toxic substances in the manufacturing process, which increases the overall manufacturing cost.