Speech Title: Progress and Prospects of High Specific Energy Supercharging Solid State Battery Technology
Speaker: Professor Yang Xiaoguang from Beijing Institute of Technology
Speech time: May 15, 2025 14:10-14:40
At this seminar, Professor Yang Xiaoguang discussed the development history, current pain points, and future directions of battery technology, with a focus on the technological progress of supercharging batteries and solid-state batteries, as well as their breakthroughs in energy density, safety, fast charging performance, and other aspects.

The report is divided into three parts, with the following content:
1、 History and Pain Points of Battery Development
Under the dual promotion of policies and technology, the fields of electric vehicles, energy storage, and consumer electronics, including the currently popular low altitude economy and humanoid robots, are undergoing an irreversible transformation towards electrification. The importance of batteries as the core component of this transformation is self-evident. Looking back at the development history of batteries, we can find that the first prototype of human batteries was born in 1800, when the energy density of batteries was less than 10Wh/kg. The invention of lead-acid batteries can be traced back to 1859, with an energy density of 30-50Wh/kg. Subsequently, the emergence of nickel chromium batteries promoted the development of applications such as military telegraph machines. In 1989, the launch of the Toyota Prius marked the beginning of the era of hybrid electric vehicles. In 1991, Sony commercialized lithium-ion batteries, initially applied to 3C digital products, and later expanded to the field of electric vehicles, foreshadowing the direction of future electrification trends. It is worth noting that the widely used lithium-ion batteries are still based on graphite negative electrode materials, with an upper limit of 300Wh/kg for cell energy density. For the next generation application scenarios such as low altitude economy and robots, this energy density is still insufficient. Therefore, developing battery technology with higher energy density has become an urgent task. From a historical perspective, the development of battery technology mainly pursues the improvement of energy density, and every breakthrough in energy density brings opportunities for the development of new industries. Further reviewing the development trend of lithium-ion batteries, the 2019 Nobel Prize in Chemistry was awarded to three pioneers who invented lithium-ion batteries. Professor Stanley Whittingham used lithium metal and titanium sulfide to create a battery with a voltage of 2V. Although the energy density was insufficient, reversible charging and discharging were achieved. Professor Goodenough has increased the positive electrode material to lithium cobalt oxide, raised the voltage platform to 4V, and improved the energy density. However, he still uses metallic lithium as the negative electrode, which has issues with dendrite formation, lifespan, and safety. Until Professor Yoshino proposed carbon based negative electrode materials, especially graphite based materials, the commercialization of lithium-ion batteries was truly achieved. Energy density is the foundation, while meeting safety and lifespan requirements is the key to successful commercialization.

With the help of Professor Li Hong's famous energy density development trend chart, we can observe that the energy density of lithium-ion batteries has continued to increase since 1991. At present, we are committed to further improving battery performance by increasing the nickel content in ternary materials. However, the energy density of graphite based lithium-ion batteries has reached a level of 300Wh/kg. To further improve, it is necessary to switch to using silicon-based negative electrode materials or metal lithium negative electrodes. In this process, how to solve the limitations of battery life and safety has become a key issue that we urgently need to address.

We must emphasize that in addition to energy density, batteries also need to meet many other requirements in practical applications. In terms of energy density, the pursuit is the improvement of volumetric energy density and mass energy density. For example, mobile phones pursue volumetric energy density, while electric vehicles and eVTOLs pursue power density, which is particularly important. In addition, we also need to pay attention to the fast charging performance, service life, safety, and adaptability under extreme temperature conditions of the battery. Of course, cost factors are also an important driving force. Although energy density is our goal, we need to conduct research and development tailored to specific situations to balance energy density with the requirements in different application scenarios.

Here, we use a metaphor to illustrate the concept of energy density. Imagine a fixed parking lot whose capacity represents energy density. To increase energy density, it is necessary to reduce the size of parking spaces in order to accommodate more vehicles. In other words, an increase in energy density means that more energy can be accommodated in a limited space. Regarding the process of charging and discharging, it can be analogized as a vehicle entering and leaving a parking space. When the vehicle comes to a parking space, it is charging, and when it leaves the parking space, it is discharging. If parking spaces are designed to be more compact or the number of vehicles increases, the time required to find a parking space will be extended, and the risk of collisions during parking will also increase. This is similar to the challenge faced by current lithium-ion battery technology: while pursuing higher energy density, it is difficult to balance safety and high charge discharge rate requirements, forming an almost "impossible triangle" dilemma.

Before 2019, we continued to pursue ternary materials, especially high nickel ternary materials. The national subsidy policy for battery energy density led to a significant decline in the market share of lithium iron phosphate batteries. The main reason for this phenomenon is that the subsidy policy at that time tended towards high-energy density batteries. However, with the gradual withdrawal of subsidy policies and the real rise of electric vehicles, we have observed that the share of lithium iron phosphate batteries in the electric vehicle market has gradually increased since 2019. This trend indicates that although we pursue high energy density, what is more crucial is that lithium iron phosphate batteries have lower costs and higher safety. In the market, we must achieve low cost and high security in order to truly promote commercialization. In addition, we have learned that early lithium-ion batteries attempted to use metal lithium negative electrodes, but there have always been serious technical bottlenecks such as dendrite growth and short cycle life. The US Department of Energy's BATTERY500 project has invested $75 million to develop a new generation of batteries based on metal lithium anodes, with a target energy density of 500Wh/kg and a lifespan of 1000 cycles. As of 2024, the latest data shows that the technology has only made progress at a low charging rate of 0.1C, and the metal lithium battery system still faces significant challenges in terms of charging and discharging efficiency, cycle life, and safety.

In recent years, fast charging technology has rapidly emerged and become a focus of attention in the industry. However, we should be aware that fast charging technology is closely related to the energy density of batteries. High energy density batteries face significant lithium deposition challenges during the charging process, which directly leads to a sharp reduction in battery life. When the energy density of the battery is low and the electrode manufacturing is thin, the implementation of fast charging technology is relatively easy. However, in order to achieve fast charging while increasing energy density, the lifespan of the battery will be significantly reduced, indicating a contradiction between energy density and fast charging technology. In addition, low temperature environments pose additional challenges to the operation of electric vehicles. According to the data from the Big Data Platform of Beijing Institute of Technology, the distribution of electric vehicles in China shows that the penetration rate of electrification in the Northeast and Northwest regions is still a problem, mainly due to the inability of batteries to discharge effectively under low temperature conditions, resulting in a significant decrease in driving range. At the same time, the issue of charging safety in low-temperature environments cannot be ignored, as it can easily lead to lithium deposition in batteries. As a key component of batteries, electrolytes with high activity and low viscosity, such as carboxylic acid electrolytes, need to be used to improve their performance in low-temperature environments. These electrolytes have high activity at low temperatures but often have low boiling points, accompanied by insufficient stability at high or room temperatures, resulting in a contradiction between stability and activity.

In summary, electric vehicles have diverse performance requirements in various application scenarios. If we pursue high energy density, power density, and fast charging characteristics, we must use materials with excellent performance and high activity. However, at the same time, the lifespan, safety, and cost-effectiveness of batteries must also be considered, which requires battery materials to have excellent stability. In other words, the batteries we need must have both the ability to sprint short distances and the ability to complete long-distance endurance races, which is a big contradiction. To solve this contradiction, we believe that it is difficult to achieve solely from the material level. Collaborative efforts must be made from multiple aspects such as material, battery design, system integration, and operation and maintenance management.

2、 Introduction to Supercharging Battery Technology

Next, let's report on some of the technological advancements we have made in recent years. Firstly, let me emphasize why fast charging is necessary. According to the statistics of private car charging on the North China University of Technology's big data platform, it can be seen that when using only slow charging technology, most users start charging when the remaining battery is 40%, due to concerns about the possibility of long-distance travel the next day. Therefore, in reality, 40% of the electricity is not used, resulting in waste. However, with fast charging technology, we are confident in using the vehicle's power even lower before charging. Just like gasoline cars do not have range anxiety, but if refueling takes two to three hours, we will also have range anxiety. Therefore, we believe that the combination of high energy density and fast charging technology is the only way to truly solve the problem of range anxiety. For other scenarios, such as ride hailing and heavy-duty truck operations, the demand for fast charging is equally urgent.

In fact, since 2018, the United States has been implementing the so-called rapid charging plan, deploying 2000 supercharging stations on American highways. The European Union has also invested heavily in supercharging infrastructure construction in the past few years. Our country has increased its promotion efforts since last year, such as Shenzhen, which has made the supercharging city its business card. Beijing announced in January this year that it will build 1000 supercharging stations, and we are actively building a high-power charging network. The definition of supercharging is that for a battery pack with a power greater than 350kW and a 60 degree charge, it can replenish a range of 300 kilometers in about 10 minutes. But even after the completion of the charging station construction, our current electric vehicle batteries still face some challenges. Taking the United States as an example, they annually bring cars from the European and American markets to California supercharging stations for testing, to evaluate their actual performance at a charging power of 350kW. We observed that almost all vehicles can only achieve high-power charging in a low battery state, and as the battery level increases, its charging power gradually decreases. It can be understood that when the battery is low, the parking lot is relatively spacious and easy to park, but when the number of vehicles increases, we need more time to park carefully to ensure the actual charging effect. At present, actual test results show that a range of 150-200 kilometers can be replenished after 10 minutes of charging, corresponding to 30-40% battery life. However, achieving the goal of charging to 80% still faces challenges.

The main problem we face in this process is that under normal charging conditions, lithium ions need to be embedded inside the graphite negative electrode, which means the car needs to be parked in the parking space. In high-power charging or low-temperature environments, lithium ions are difficult to fully embed, resulting in their precipitation on the surface of the battery. The precipitated metallic lithium has high activity, which significantly reduces battery capacity and may cause short circuits between the positive and negative electrodes of the battery, thereby increasing the risk of safety accidents such as fire or explosion. How to avoid lithium deposition is a key safety factor, and the higher the energy density, the greater the risk of lithium deposition, posing a challenge to high energy density fast charging technology.

From a basic principle analysis, the occurrence of lithium precipitation in lithium-ion batteries mainly depends on the diffusion of lithium ions in the electrolyte, the reaction process at the interface between the electrode and the electrolyte, and the diffusion process inside the electrode material. These three processes are interconnected, and any obstruction in any link may lead to the occurrence of lithium deposition phenomenon. How to avoid lithium deposition at the material level? We can use negative electrode materials with smaller particle size or higher specific surface area to accelerate reaction and diffusion rates, improve activity, but at the same time increase side reactions, leading to a decrease in battery calendar life and cycle life. In addition, we can change the electrolyte and use a carboxylic acid ester electrolyte for better transmission. However, based on our experience, stability issues may arise at high or room temperatures, resulting in a contradiction between activity and stability.

On the other hand, our expertise is more focused on the fields of battery cells and mass transfer. The rates at the three levels mentioned earlier are all related to temperature. If we raise the temperature from 20 ° C to 60 ° C, the solid-phase diffusion increases by 6 times, the electrolyte conductivity increases by 2 times, and the reaction kinetics increases by 12 times, we can increase the activity through thermal stimulation. At low temperatures, the activity will definitely decrease by the same factor. Therefore, low-temperature charging is difficult. In addition, high temperatures are beneficial for fast charging, but there is a contradiction between lithium deposition and loss of active materials due to the problems of faster lithium deposition and rapid material decay. For many years, we have believed that high temperatures are the forbidden zone for battery operation, and the optimal working temperature is around 25 ℃. However, it seems that this may not be the case now.

Firstly, we started developing a battery simulation life prediction model. This model is based on the electrochemical, thermal, and mechanical coupling mechanisms inside the battery to predict the attenuation mechanism under different operating modes. We have noticed that as the charging rate increases and the battery energy density increases, the optimal operating temperature of the battery is not always around 20-30 ℃. In fact, under high energy density or high-power charging conditions, the optimal operating temperature tends to shift towards higher temperature regions. This discovery suggests that by adjusting the operating temperature of the battery, it provides us with a new dimension to solve such problems on another dimension at the material level.

Based on this idea, we have developed the so-called thermal regulation speed thermal overcharging technology. At the beginning, we developed a new battery structure with Professor Wang Chaoyang in the United States, which added micro heating devices inside the battery to achieve very low energy consumption and fast heating. Later on, we proposed that the charging and discharging temperatures could be separated, and charging in high-temperature areas could avoid lithium deposition. However, high temperatures are harmful to material degradation, so we need to control the high-temperature working time. We proposed a 10 minute high-temperature charging and discharging at ambient temperature for the rest of the time. This not only avoids lithium deposition but also slows down material degradation. From the perspective of operation and maintenance control, we can eliminate or slow down the contradiction between lithium deposition and active material degradation mentioned earlier. At that time, the data we measured was based on a 10Ah, 175Wh/kg battery, which rose from 20 ° C to 60 ° C in just over 20 seconds. After charging at 6C, the normal discharge temperature quickly dropped, and we separated the charging and discharging temperatures.

In further cycling experiments, we observed a significant improvement in the battery's cycling life with increasing charging temperature in the overcharged state. Through disassembly and analysis, we noticed that under low-temperature overcharging conditions, the battery exhibited significant lithium deposition, while high-temperature charging effectively avoided this problem. We compared this asymmetric temperature regulated battery with a battery operating at 60 ℃. The latter operates at a temperature of 60 ℃ for 2 hours per cycle, while the thermal control battery operates for 10 minutes. By comparing the attenuation curve with the working time at 60 ℃, we found that the two are very consistent, indicating that the attenuation of the battery is mainly affected by the performance of the material at high temperatures without lithium evolution. Based on this, we further developed high-temperature resistant materials and combined them with thermal control methods to achieve fast charging. The energy density of the second-generation battery has been increased to 215Wh/kg. Lithium iron phosphate is used on the positive electrode side for ternary coating, and high particle size and low specific surface area materials are used on the negative electrode, significantly increasing high-temperature stability. Even though the energy density increases and overcharging becomes more difficult, the battery can still achieve a lifespan of 2500 cycles under 6C charging conditions. Based on this material system design, the energy density of the third-generation battery has been increased to 265Wh/kg, and a dual salt electrolyte has been selected on the basis of positive and negative electrode coating, achieving a cycle life of 2000 cycles of 4C charging.

The following is the work we have not yet published. We have recently developed two high-temperature resistant electrolytes that can achieve 1C/1C cycles and 2000 cycles at a constant temperature of 60 ℃