Theoretical research overview on solid state battery

1. What is a solid state battery?
2. Solid-state battery film forming process
   1. Wet process
   2. Dry process
   3. Vapor deposition process
3. Solid state battery assembly process
4. Some solid-state battery manufacturers

Solid state battery can be divided into semi-solid state battery, solid state battery, and all-solid state battery. Compared with liquid lithium batteries, solid-state lithium batteries have higher safety, lower production costs, simpler packaging systems, and higher energy density. In addition, the solid state battery has a wide operating temperature range and supports super fast charging.

At present, there are many problems in solid state battery technology, such as improving the ionic conductivity of solid electrolytes and improving material stability. So although solid state lithium ion batteries have many advantages, there is still a long way to go before their real mass production applications.

What is a solid state battery?

According to the choice of electrolyte materials, solid state battery can be divided into three types of electrolytes: polymers, oxides, and sulfides. Among them, polymer electrolytes belong to organic electrolytes, oxides and sulfides belong to inorganic ceramic electrolytes.

According to the difference of positive and negative electrode materials, solid state battery can also be divided into solid-state lithium-ion batteries (the current lithium-ion battery material system is used, such as graphite + silicon carbon negative electrode, ternary positive electrode, etc.) and solid-state lithium metal battery (with metal lithium as the negative electrode). The solid electrolyte also plays the role of separating the positive and negative electrodes, that is, it acts as a separator, so the solid-state battery no longer needs a separator.

Solid-state battery film forming process

There are four main routes for solid state battery, namely polymer solid-state, oxide solid-state, sulfide solid-state and composite electrolyte solid-state. The solid electrolyte membrane is the unique structure of the solid state battery, which replaces the separator and electrolyte of the liquid battery, and the main body is a solid electrolyte. The film-forming process of solid electrolytes is the core of solid-state battery fabrication.

Different processes will affect the thickness and ionic conductivity of the solid electrolyte membrane. Excessive thickness of the solid electrolyte membrane will reduce the mass energy density and volumetric energy density of the solid state battery, and will also increase the internal resistance of the battery. If the solid electrolyte membrane is too thin, the mechanical properties will be deteriorated, which may cause a short circuit.

Wet process

Advantages of wet process: simple operation, mature process, easy to scale production.
Disadvantages of wet process: high cost, and the solvent used may be toxic, and the residual solvent will reduce the ionic conductivity of the solid electrolyte membrane.

According to different carriers, the wet process can be divided into mold-supported film formation, positive electrode-supported film-formation, and skeleton-supported film formation. Mold-supported film formation is often used to prepare polymer electrolyte membranes and composite electrolyte membranes.

The solid electrolyte solution is poured on the mold, and then the solvent is evaporated to obtain a solid electrolyte membrane. The thickness of the membrane is controlled by adjusting the volume and concentration of the solution. Positive electrode support film formation is often used in the preparation of inorganic electrolyte membranes and composite electrolyte membranes.

The solid electrolyte solution is directly poured on the surface of the positive electrode, and after the solvent is evaporated, a solid electrolyte membrane is formed on the surface of the positive electrode. Compared with the mold support, the cathode support can achieve a thinner solid electrolyte membrane and better interfacial contact.

Skeleton-supported film formation is often used in the preparation of composite electrolyte membranes. The solid electrolyte solution is injected into the framework, and the solvent is evaporated to form a solid electrolyte membrane with framework support.

Dry process

The dry process is to disperse the solid electrolyte and polymer binder into a high-viscosity mixture and then apply sufficient pressure to form a film.

Advantages of dry process: no solvent is used, no solvent residue is used for film formation, and ionic conductivity is high. The solid electrolyte and binder are directly mixed to form a film, and no drying is required; the cost is low.

Disadvantages of the dry process: the formed solid electrolyte membrane is usually thicker, which will reduce the energy density of the solid-state battery.

Vapor deposition process

Solid electrolyte membranes are prepared by vapor-phase methods such as chemical vapor deposition, physical vapor deposition, electrochemical vapor deposition, and vacuum sputtering deposition. These methods form ultrathin electrolyte membranes on electrodes. The gas-phase method has a higher cost and is only suitable for thin-film solid state battery.

Solid state battery assembly process

Solid state battery is usually integrated in pouch. The lamination can realize the integration of each component of the battery through the simple stacking of the positive electrode, the solid electrolyte membrane and the negative electrode, which is the most suitable process for the preparation of solid state battery. The lamination process is divided into segment lamination and integrated lamination according to the sequence of cutting and lamination.

The segmented lamination follows the liquid battery lamination process. The positive electrode, the solid electrolyte layer and the negative electrode are cut into specified sizes, stacked in sequence and then packaged.

The integrated lamination is to calender the positive electrode, the solid electrolyte membrane and the negative electrode into a 3-layer structure before cutting, and cut the 3-layer structure into multiple “positive electrode-solid electrolyte membrane-negative electrode” units according to the size requirements. Packed together after stacking.

In solid state battery, various interface problems are bound to exist between the components stacked together. For polymer solid state battery, the interface resistance between the polymer electrolyte membrane and the positive and negative electrodes can be solved by heating. For oxide and sulfide electrolyte membranes, a pressing treatment is required to improve the mechanical contact between the solid electrolyte and the electrodes.

Some solid-state battery manufacturers

In China, there are CATL, SVOLT, Gotion Hi-Tech, Jiangsu Qingtao, Taiwan ProLogium, BYD, Welion, Ganfeng, HIT, Jiangxi STAR, Jinlongyu, Wanxiang A123.

In the United States, there are Quantum Scape, Cymbet, Solid Power, Sakti3, Polyplus, Factorial Energy, Sparkz, Ionic Materials, 24M, etc. invested by the public.

Singapore has Solid Energy System (SES) and so on. France has Bolloré et al. South Korea has Hyundai, LG New Energy, Samsung SDI, South Korea’s SK On, Posco and so on. Japan has Toyota, Honda, Panasonic and so on.

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