Five configurations of solid-state batteries and their current R&D levels and key technologies to be tackled

In recent years, with the rapid increase in the penetration rate of electric vehicles, solid-state batteries have received extensive attention and attention from domestic and foreign companies and researchers, and have developed rapidly. At present, there are many market participants in solid-state batteries, including car companies, battery companies, investment institutions, and scientific research institutions. The

so-called solid-state battery is a battery that uses solid electrodes and solid electrolytes. Since the electrolyte and diaphragm are replaced by solid electrolytes, first of all, the thermal stability of solid-state batteries is higher and there will be no internal short circuit; secondly, solid electrolytes have higher intrinsic safety; and theoretically, solid-state batteries have a more stable interface and a longer service life; in addition, the energy density of solid-state batteries can be further improved.

Difficulties faced by solid-state batteries

From the current advancement speed, we are still some distance away from all-solid-state batteries, and it is expected that commercial use will not be achieved until after 2030. Because there are still several major difficulties that have not been resolved:
First, from the material point of view, solid-state electrolytes cannot take into account both high ionic conductivity and stability;
second, from the interface point of view, rigid solid-solid contact cannot ensure efficient transmission of ions at the atomic level;
third, in terms of dendrite growth, lithium dendrite growth can easily pierce the solid electrolyte;
fourth, in terms of battery manufacturing, the cost of existing solid electrolytes is relatively high.

In other words, from the perspective of materials, interfaces, dendrite growth and battery manufacturing, the current all-solid-state batteries cannot meet current needs. High-safety, high-energy-density all-solid-state batteries are still some distance away from us, but the existing commercial liquid batteries cannot meet our growing demand for battery energy density, so semi-solid-state batteries came into being. Semi-solid-state batteries

are more suitable for the current battery development path, because semi-solid-state batteries are safer than liquid batteries; they are more compatible with existing battery production lines and easy to industrialize; and they can solve interface problems.

Recently, Dr. Zhu Gaolong, director of solid-state battery research and development at Sichuan New Energy Vehicle Innovation Center Co., Ltd., the workstation of Academician Ouyang Minggao, made an online sharing of key technical issues of solid-state batteries. He divided solid-state batteries into 5 configurations and explained in detail the current research and development level of these 5 configurations and the key technologies that need to be overcome.

Solid-state battery configuration 1 and key technologies that need to be overcome

Whether it is a liquid battery, a semi-solid battery, or a solid-state battery, the structure is the same. Generally, there are positive and negative electrodes on both sides (as shown in Figure 1), and the electrolyte in the middle is used to complete the transport of negative ions and realize energy storage and conversion. Battery cells are divided into cylindrical, soft-pack and square shell types; battery cells are combined in series and parallel into a module or PACK to meet the needs of automobiles or other applications.

Figure 1: Schematic diagram of a single cell battery structure

Solid-state battery configuration 1 is actually equivalent to the configuration of a liquid battery, with the introduction of a solid electrolyte coating. The coating can be on the diaphragm or on the positive and negative electrodes.

Figure 2: Solid-state battery configuration 1 structure

Dr. Zhu Gaolong pointed out that although this looks no different from a liquid battery, it actually contains a lot of know-how, which is mainly reflected in material selection, spacing design, and material formulation.

Figure 3: Characteristics of solid-state battery configuration 1

But the advantage is that the equipment used in Configuration 1 is highly compatible with existing equipment. In addition, in terms of technical maturity, many manufacturers have accumulated a long period of experience, and many key points have been resolved. For example, the thickness of the coating can now be distributed very evenly, from tens of nanometers to several nanometers, or even hundreds of microns, and a very consistent distribution can be achieved; in addition, the coating formula, including the particle size, structure, ratio, adhesive, and auxiliary ingredients of the electrolyte, can be well regulated.

The characteristic of Configuration 1 is that it can improve battery safety while ensuring electrical performance, but the problem is that the safety improvement is not enough. Zhu Gaolong lamented that if the technology can improve the safety of nickel 90 to the safety index of nickel 50, the energy density of the battery will be greatly improved.

The technical difficulties faced by solid-state battery configuration 2

are in solid-state battery configuration 1. We applied a layer of ceramic coating to the diaphragm, which greatly improved the safety, but there is still a large amount of organic electrolyte in the liquid battery. The organic electrolyte will react with the oxygen released from the positive electrode to generate a lot of heat, resulting in safety failure of the battery.

Therefore, in two types of solid-state battery configurations, in-situ solidification technology was introduced, that is, a colloidal/solid electrolyte was introduced to solidify the electrolyte, preventing the electrolyte from flowing and preventing continuous reaction during the oxygen release process, thereby further improving safety performance.

Figure 4: Key technical points encountered in solid-state battery configuration 2

However, the technology is not yet fully mature. The main difficulty is that after curing, the overall battery safety or pass rate will be greatly improved, but the hot box test may not meet the requirements, and the battery performance, especially the rate performance, will decay rapidly.

Therefore, in the curing formula, such as monomer selection, after curing, whether it is colloidal or completely solid; ion conductivity, the selection of diaphragm as the substrate, and the performance of ion transport in the diaphragm need special attention; in addition, attention should also be paid to the initiator of the solid electrolyte. For example, although some initiators can make the monomer cure well, the initiator itself is not very stable after curing, which makes the safety of semi-solid batteries worse than that of liquid batteries.

Zhu Gaolong emphasized that the imagination here is very beautiful, but the choice of materials here is very critical.

The biggest feature of Configuration 2 at present is that the needle puncture pass rate has been greatly improved. Therefore, the in-situ curing technology has received widespread attention from enterprises. For example, Weilai has made a lot of achievements in the equipment, methods, and formulas of in-situ curing.

He pointed out in particular that each company has a layout in this regard. How to efficiently make the solid electrolyte after curing, and still play the ability of ion transport and battery safety performance, has become a difficulty we are going to overcome.

The current research and development level of solid-state battery configuration 3, that is, the key technology to be tackled,
has been upgraded to solid-state battery configuration 2. The sealant is used on the single-chip battery cell to seal both sides so that the electrolyte cannot flow around. In this way, the battery cell can be connected in series, thereby reducing the use of unnecessary structural parts, greatly improving the storage efficiency of the solid-state battery, and thus increasing the energy density of the battery cell. This is another configuration, namely solid-state battery configuration 3.

Figure 5: Current research and development level of semi-solid-state batteries and key technologies to be tackled

The key point of configuration 3 is the control of the amount of electrolyte and the sealing technology. Such technology is similar to structural innovation. For example, BYD's blade battery, CATL's CTP/CTC battery, etc.

Its characteristics are that the needle puncture pass rate and hot box performance are greatly improved. The problem encountered is that the solid electrolyte has a large density, energy density is lost, and the energy density is balanced with safety performance.

Technical difficulties faced by solid-state battery configuration 4

Solid-state battery configuration 4 mainly replaces the diaphragm and electrolyte in the liquid battery with a solid electrolyte. After years of experiments, everyone found that the diaphragm and electrolyte in the liquid battery are the main factors affecting the safety performance of the battery. Therefore, the most effective way to improve the safety performance of the battery is to reduce the amount of electrolyte, so it is easy to think of a new type of lithium-ion conductive polymer electrolyte, an inorganic ceramic electrolyte, or an electrolyte mixed with a polymer electrolyte membrane and an inorganic solid electrolyte.

The requirement is that this electrolyte needs to be stable, highly consistent, and have good compatibility with the positive and negative electrodes. Also here, the diaphragm and the electrolyte in the diaphragm are replaced with a polymer electrolyte or an inorganic solid electrolyte. In-situ curing technology can also be used on the positive electrode, or colloidal or solid electrolytes can be directly added, and sealing technology can also be used, which is equivalent to making an overall upgrade in the original solid-state battery.

In 2015, French battery manufacturer Bollore produced a polymer electrolyte solid-state battery. Based on previous development experience, Bollore actually uses polymer electrolytes, which soften at 60°C, similar to the properties of jelly or colloid. In this case, the interface contact with the positive and negative electrodes is very good, and at 60°C, it has a relatively high ion conductivity, which will form a situation similar to a liquid battery. That is, there is good contact, especially between the electrode and the active material, and between the electrode and the electrolyte. This ensures the rapid and efficient operation of ion directional transport, thereby realizing its application in the whole vehicle.