WNEVC 2021 | Yu Hailong, Senior Engineer, Institute of Physics, Chinese Academy of Sciences: Technical Challenges of All-Solid-State Lithium Secondary Batteries

From September 15 to 17, 2021, the "3rd World New Energy Vehicle Conference" (WNEVC 2021) was grandly held at the Hainan International Convention and Exhibition Center, co-organized by the China Association for Science and Technology, the People's Government of Hainan Province, the Ministry of Science and Technology, the Ministry of Industry and Information Technology, the Ministry of Ecology and Environment, the Ministry of Housing and Urban-Rural Development, the Ministry of Transport, the State Administration for Market Regulation, and the National Energy Administration. The conference was themed "Promoting marketization in an all-round way, accelerating cross-industry integration, and working together to achieve carbon neutrality", and invited representatives from all walks of life from all over the world to discuss.

At the theme summit "Key Technologies of Power Batteries and Construction of Green and Efficient Industrial Ecosystem" held on the morning of September 17, Yu Hailong, a senior engineer at the Institute of Physics, Chinese Academy of Sciences, gave a special speech. He introduced:

He introduced the advantages and disadvantages of sulfide solid electrolytes, oxide solid electrolytes, and polymer solid electrolytes, and introduced the power battery research of Bollore in France and Toyota in Japan. He pointed out that solid-state batteries must be originally innovated in material structure, have innovative structures, make application pilots, and then match new technological innovations, integrate preparation methods and batteries, and finally form a new industrial ecology.

The following is a transcript of the live speech:

Dear leaders, experts, and business colleagues, I am Yu Hailong from the Institute of Physics, Chinese Academy of Sciences. Today I will be giving a report on "Technical Challenges of All-Solid-State Lithium Secondary Batteries" on behalf of our researcher Huang Xuejie.

My report will mainly focus on the problems and progress faced by all-solid-state batteries, which may be quite popular nowadays.

The first part is that from 2010 to 2030, China's lithium-ion batteries have gone through about three generations of development. At the beginning, the energy density was only about 100 to 200 watt-hours per kilogram. At this stage, we think it is a relatively traditional system, based on a power battery method of iron-lithium to graphite. In the second generation, we can realize ternary materials from lithium batteries, and the introduction of this alloy material can raise its theoretical energy density to within 400 watt-hours per kilogram. This will be slightly delayed in the industry, but this prototype can be realized with the support of the Ministry of Industry and Information Technology and the Ministry of Science and Technology. When it comes to the third generation, when the possibility of more than 400 watt-hours per kilogram is further proposed, the introduction of metal aluminum has become indispensable. No matter what kind of material system, whether it is a positive electrode material system or a higher 87 ternary system, it is actually irreplaceable. At this time, 500 watt-hours is its biggest technology, and it will also bring great safety issues.

So how do we get to more than 500? There is only one idea - the introduction of all-solid-state batteries. The next question is when did all-solid-state batteries become popular? The concept of this battery has been around for more than 60 years. All-solid-state batteries are actually a concept that was proposed earlier than lithium-ion batteries. In the past two years, we can see that since 2008 and 2009, the number of global literature based on all-solid-state batteries has increased significantly. During the same period, the number of global patents for all-solid-state batteries also grew rapidly from 2008 and 2009, reaching a level of about 500 per year in 2018 and 2019.

Then everyone will ask, what is an all-solid-state battery? Just hearing the concept, as the name suggests, compared with traditional liquid batteries, its structure does not contain any liquid components. Why do we now think that this liquid battery often has some safety problems? Mainly when it has thermal runaway or some extreme conditions, the electronic liquid used in it is a low-flash point, flammable organic component, which may cause an irreversible thermal runaway in the battery in the future.

Based on this problem, we use an all-solid-state electrolyte, which may be an oxide, sulfide or polymer system. Regardless of which system it is, its thermal stability is much higher than the traditional carbonate properties. At this time, its fire and explosion problems are also controlled, and even in the case of a short circuit, it will bring a relatively safe heat release effect.

The difficulty in solid-state batteries is that after we replace the liquid electrolyte that originally achieved ion conduction at the interface between the positive and negative electrodes of the battery with a solid electrolyte, the first problem it faces is the contact problem. It will not occupy too much in the electrode to achieve a complete conduction. Under the action of the rear electrode, it may face more challenging problems such as insufficient electrode liquid retention and poor fluidity. Both the positive and negative electrodes need to be transmitted by solid batteries, and they need to be transmitted through contact at the solid-solid interface. In this process, how to mix them to a uniform degree and suppress their interface separation becomes a very critical issue.

Solid-state batteries also have a relatively big advantage. They can broaden the scope of application of their materials. They may be the only solution for the use of metallic lithium. The application of metallic lithium in long cycles will definitely be a key technology for future power batteries with energy density exceeding 400 watt-hours.

Next, we will emphasize its advantages. The first is the inhibitory functionality between metal lithium. It is definitely far better than the liquid mode. It does not catch fire or burn. Its safety is definitely higher than the current liquid lithium battery. It also does not have a continuous interface reaction. It comes from the fact that there is no solvent inside the solid-state battery, so the by-products will not dissolve in the interface, so it will have better stability and cycle characteristics. At the same time, the problems of drying up and leakage will no longer exist. The high-temperature life has a significant improvement, and it may even be better than before, because its thermal stability is higher, the ionic conductivity is higher, and the reduction of inactive substances will be improved to a certain extent. More importantly, it is possible to achieve series connection inside the battery cell, which can increase the voltage of the single cell inside the battery from the original 3 to 4 volts to three or four hundred volts or even 1 kilovolt. Its rate characteristics are maintained, and the voltage characteristics are improved. It will be better for high-voltage modularization and system design. It is a customizable approach in the future.

What is the most critical part of solid-state batteries? It is the solid electrolyte. The solid electrolyte is the basic core of the battery. Thanks to the progress in the electrolyte field in the past 20 years, the room temperature ion conductivity of some solid electrolytes has exceeded the level of previous organic electrolytes. In this case, it is possible to achieve commercialization or productization.

The classification of electrolytes, the first is the solid electrolyte of sulfide, which is composed of lithium silver sulfur, including LPS, LGPS based on gauge, tin and other materials. This concept is that it is the highest community among the existing solid electrolytes, and it is also higher than the current liquid electrolyte. However, this type of sulfide solid electrolyte has its own defects, that is, the anti-oxidation and reduction stability is poor, that is, its electrochemical window is very narrow, about only about 1.5 volts, about a very narrow window between 1.5 volts and 3 volts. In this case, it will oxidize when charged with a higher voltage, and it will be reduced when placed at a lower potential, so this is a big problem. Another problem is that it is highly sensitive to moisture. We all know that sulfide and hydrogen sulfide are very smelly. When the solid electrolyte of sulfide is taken out of the air or when doing experiments, the laboratory next door will scold it. This problem is also very difficult to solve. However, Toyota of Japan has been working in this area for more than ten years. Their main technical routes are also based on the thinking of sulfide solid electrolytes. The remaining structural problems that are inherently brought can also be solved by optimizing some subsequent means.

The second category is the oxide solid electrolyte, which is also quite distinctive, with LLTO and LLZO as its characteristics, and it also includes LIPON. It is higher than other solid electrolytes, but it cannot reach the high ionic conductivity of sulfides, but its electrochemical window stability is good, significantly higher than that of sulfides, but there is a more troublesome thing here is that it is not easy to prepare into a chemical electrolyte, it often uses micron or submicron electrode materials, what problem will be encountered here? It is difficult to press into a sheet, and it cannot be made thin after being pressed into a sheet. After being made thin, there are lattice hard particles between it and the positive electrode particles, and there is no way to form an integrated shape. So we have one way to do it by sintering, another is to mix it with a polymer, and the third is to directly use it as a target material and perform a physical product on the positive or negative electrode. However, among these three methods, only the polymer mixture method is better, but the residual polymer may have a certain impact on the performance, and may even lose some of the solid-state properties.