The China Chemical and Physical Power Sources Industry Association (CCPSIA) held the "Second Symposium on Hybrid Vehicle Market and Advanced Battery Technology Development" in Hangzhou from October 13th to 14th, 2016. The conference attracted widespread attention from numerous domestic and international companies in the battery, materials, equipment, and automotive sectors, with over 400 domestic and international guests in attendance. Since the "First Symposium on Hybrid Vehicle Market and Advanced Battery Technology Development" organized by CCPSIA in Beijing in April 2014, while pure electric vehicles and plug-in hybrid electric vehicles have achieved rapid development both domestically and internationally, hybrid vehicles, especially mild hybrid vehicles, have also experienced the anticipated rapid development internationally. In particular, the recent demand for the development of 48V battery systems and technologies has created a new hotspot in the battery industry.
The "Second Hybrid Vehicle Market and Advanced Battery Technology Seminar" provided a platform for exchange, helping participants understand the development trends and latest progress in the hybrid vehicle industry both domestically and internationally, and to discuss various battery technologies applicable to hybrid vehicle development and their latest advancements. It is hoped that this technical exchange will provide some reference and impetus for the development of my country's hybrid vehicle industry, accelerating the rapid development of various battery technologies for hybrid vehicles in my country.
At this meeting, Dr. Wang Shuangcai of Huzhou Chuangya Power Battery Materials Co., Ltd.
I delivered a special report on "Research and Development Progress of Low-Temperature Fast-Charging Power Battery Anode Materials". The following is a transcript based on shorthand notes. It has not been reviewed by me and is for reference only.
Abstract: 1. Overview of the current market status of power battery anode materials; 2. Development strategy of Chuangya products; 3. Performance testing and characterization of low-temperature fast-charging anode materials; 4. Mechanism exploration of low-temperature fast-charging anode materials; 5. Outlook and conclusions.
First: Background. The growth rate of new energy vehicle development in 2016 was relatively slower than the previous year. At the end of last year, some overly optimistic predictions were made, with the total number of new energy vehicles expected to reach 700,000 this year. According to the latest statistics, this number is expected to fall short of 700,000, estimated at around 500,000 to 600,000. The demand for power batteries has been growing rapidly in recent years. The future development trend of anode materials shows an average annual compound growth rate exceeding 24.5%. Graphite will remain the dominant anode material for lithium batteries, while power batteries will lean towards artificial graphite. New anode materials such as silicon-based materials will see some growth and application.
Second: Development strategy for low-temperature fast-charging anode materials. High-rate (fast discharge) applications have been successful in model aircraft and drones, and our material (GHMG-M) has consistently performed very well in the high-rate market. Chuangya currently has a production capacity of 10,000 tons/year, forming a relatively complete anode material industrial chain from R&D to production and sales. Chuangya's products are mainly artificial graphite, with a small portion of composite graphite, and new anode materials are also under development. Coating artificial graphite with amorphous carbon to improve its low-temperature and rate performance is a new topic in the field of artificial graphite. Amorphous carbon-coated PV-6 has already been shipped in batches, and the next-generation, higher-capacity coated product PV-9 is under intensive development.
Chuangya's product development focuses on two main areas: high-rate and high-specific-energy anode materials. In 2008, we launched the high-rate anode material GHMG-M. Power batteries have high requirements for low temperature and fast charging, so we developed low-temperature graphite anodes. Since the specific energy of batteries cannot meet the requirements for long-range electric vehicles, and power batteries require fast charging, we have also conducted some development work on fast-charging anode materials, building on our previous work. Other materials include composite graphite, which exhibits less electrode rebound during baking compared to artificial graphite, while also having higher capacity, making it very suitable for the requirements of cylindrical power batteries.
Development strategy for low-temperature anode materials: Appropriately reduce particle size; control the OI value of powder and electrode; pore design improves the liquid absorption performance of the anode at low temperatures; soft carbon coating reduces the Rct of the material; reducing graphitization increases the diffusion rate of Li+ at low temperatures. Development strategy for fast-charging anode materials: Hard carbon coating is more effective than soft carbon coating; control particle size; control the OI value through raw materials and processes. Low-temperature and fast-charging are definitely different, but for the sake of overall report coherence, these two sections are combined.
Natural graphite coating technology is mature, while coating with artificial graphite is a new research topic. Mesophase carbon microspheres may offer indirect coating. Artificial graphite coating methods include solid-phase and liquid-phase methods. While coating may weaken the efficiency of the anode material, it may improve cycle performance.
Application of low-temperature fast-charging negative electrode materials: Electrode fabrication includes matching of main and auxiliary materials, slurry dispersion, uniform coating, controlled compaction, and N/P ratio. Low-temperature fast-charging negative electrode materials have small particles, making slurry dispersion difficult, and the electrode sheets shed powder during rolling. We collaborated with our clients to solve the problems of small-particle artificial graphite coating sticking to the rollers and powder shedding during rolling.
Third: Performance of low-temperature fast-charging power battery anode materials. The SEM image below shows a typical high-rate material with a particle size controlled at around 8μm. PV-6, a low-temperature anode material, is obtained by coating the surface of GHMG-M with artificial graphite. The primary particles are relatively small, while the secondary particles have a high degree of composite structure. The upper right TEM image shows the coating layer, i.e., the structure of amorphous carbon, in the center. The upper right corner of the upper right image shows the artificial graphite core, where the graphite crystal layer can be seen. The coating layer is 50-60 nanometers thick. The upper left image has a higher magnification; the upper left corner shows the artificial graphite core, and the right side shows the amorphous carbon coating layer. Laman analysis results show that the ID/IG ratio of artificial graphite is generally below 0.2, while the ID/IG ratio after amorphous carbon coating is greater than 0.2.
Physical property analysis revealed that GHMG-M and PV-6 have relatively small primary particles, with secondary particles also controlled below 10μm. RAC1671-32 is a hard carbon coating material, and the specific surface area of the coating material is relatively low. In a full-electric battery design using LCO as the positive electrode, comparative battery tests were conducted on both GHMG-M and PV-6.
Battery performance: The top left corner icon shows that PV-6 is one percentage point lower than GHMG-M, its compaction density and liquid absorption performance are similar, its compaction density is slightly lower, and its cycle performance is improved.
PV-6 exhibits better rate performance than GHMG-M, primarily due to reduced polarization and increased capacity during rate charging and discharging. Improved coating reduces interfacial impedance, potentially benefiting the Li+ diffusion coefficient. Low-temperature rate charging capacity is improved with significantly reduced polarization; however, at -20℃, the constant-current charging capacity is less than 20%, indicating a substantial capacity decrease largely related to the electrolyte. The difference between 1C and 2C cycling at room temperature is minimal, while at 0℃, the degradation of artificial graphite is very significant, with a capacity retention of only 40%. A 12Ah lithium manganese oxide pouch battery retains 80% of its capacity after 1280 cycles of 4C charge and 5C discharge. Amorphous carbon coating improves the rate performance and low-temperature cycling performance of graphite. Some exploratory experiments were also conducted with hard carbon coating. The initial efficiency of hard carbon coated materials is slightly lower than that of artificial graphite, but hard carbon coating is more effective than soft carbon coating in improving fast-charging performance.
Fourth: Exploration of the mechanism of low-temperature fast-charging power battery anode materials. Improving the orientation of graphite, i.e., reducing the OI value, is beneficial to obtaining graphite anode materials with better kinetic performance. The OI value of artificial graphite is greater than 2, the OI value of natural graphite is greater than 5, the OI value of composite materials is 3-4, and the OI value of PV-6 material electrode sheets is the lowest.
Low-graphitization artificial graphite can be obtained by selecting raw materials and controlling the graphitization process; amorphous carbon coating will reduce the degree of graphitization.
EIS indicates that amorphous carbon coating reduces the material's impedance. CV testing shows that during film formation, the redox peak (0.7V) is more pronounced in PV-6, and SEI formation consumes more Li, resulting in a decrease in initial efficiency. The AC impedance of the full cell at 100% SOC is significantly reduced by amorphous carbon coating.
Fifth: Conclusions and Outlook. Low-temperature fast-charging anode materials will find widespread application in markets such as fast-charging buses, hybrid electric vehicles, drones, and model aircraft; TEM and Laman spectroscopy characterized the coating of artificial graphite with amorphous carbon; amorphous carbon coating modified artificial graphite can improve its rate performance and low-temperature performance; low alignment and low graphitization can improve the rate performance of the material. Amorphous carbon coating can reduce battery impedance.
