Hard Carbon Anode Material Achieves “New Talent” in Energy Storage

Hard carbon/graphite composite anode materials

Hard carbon will not graphitize after high-temperature treatment. Its internal crystal arrangement is disordered, and the layer spacing is large. This makes the hard carbon anode store more charge in the same volume. It improves the energy density and endurance of sodium-ion batteries. The expansion and contraction of the hard carbon anode are more uniform during the discharge process, which increases its cycling stability, charging and discharging performance and prolongs the cycling service life of the sodium ion battery.

With the rapid expansion of solar, wind and other renewable energy generation, the research on new materials for energy storage batteries is also deepening. At the 15th Shenzhen China International Battery Technology Exhibition, a company released a new generation of sodium-ion battery hard carbon anode materials; its first charge and discharge efficiency can reach 90%.

With China’s abundant sodium resources, sodium-ion batteries are considered to be the most suitable new type of batteries for large-scale energy storage. They are expected to alleviate the problems caused by the shortage of lithium resources as well as the uneven distribution of the limited development of energy storage. Compared with other anode materials for sodium-ion batteries, what are the advantages of hard carbon materials? What is the status of China’s hard carbon materials industry development? How far away from the large-scale application? There is still a long way to go. With these questions, the Science and Technology Daily reporter interviewed the relevant experts.

Hard carbon is the anode material of choice for sodium-ion batteries

Sodium-ion battery is mainly composed of positive electrodes, a negative electrode, electrolytes, diaphragm, etc. Its working principle is similar to that of lithium-ion batteries. As the main body of sodium storage in the battery, the anode material of the sodium-ion battery realizes the embedding or disengagement of sodium ions during the charging and discharging process, so the capacity of the battery is positively correlated with the anode’s ability to store sodium ions. The selection of anode materials has a decisive role in the development of sodium-ion batteries.

Zhou Xiangyang, a professor at Central South University, said that the classification of anode materials for sodium-ion batteries can be roughly divided into five categories. First, carbon-based anode materials, mainly including graphite, amorphous carbon, nanocarbon, etc., of which amorphous carbon is the most likely to take the lead in industrialization; second, alloy anode materials, theoretical capacity is high, but the volume of electrons embedded in the sodium expansion is serious, and the cycle performance is poor; third, metal oxides and sulfide-based anode materials, theoretical capacity is high, but the conductivity is poor; fourth, embedded type of titanium-based anode materials, the volume of change in small but low capacity; fifth, organic-based anode materials, the volume of which is low; fifth, organic-based anode materials, the volume of which is low. Fifth, organic anode materials have low cost but poor conductivity and are easy to dissolve in the electrolyte.

Carbon-based anode materials have excellent electrical conductivity, and at the same time, the preparation method is flexible, low-cost and environmentally friendly, so they have become the primary choice of anode materials for sodium-ion batteries. Among them, hard carbon and soft carbon materials in amorphous carbon are considered potential anode materials for sodium-ion batteries. Soft carbon refers to the carbon that can be graphitized after high-temperature treatment, which is usually obtained by processing and manufacturing low-cost anthracite as a precursor. Still, it has a low specific capacity for sodium storage, a slow charging speed, and poor low-temperature performance.

Hard carbon is carbon that will not be graphitized after high-temperature treatment; its internal crystal arrangement is disordered, and the layer spacing is large, which makes the hard carbon anode store more charge in the same volume and improves the energy density and endurance of the battery. Because the pore structure of hard carbon is larger, it can hold more sodium ions. Hence, the electrode expands and contracts more evenly during the discharge process, which increases the cycling stability and charge/discharge performance of the hard carbon anode and extends the cycling service life of the sodium-ion battery.

Zhou Xiangyang said that by comparing the performance of different kinds of carbon anode materials, it can be found that hard carbon is the preferred solution of anode materials for the commercialization of sodium-ion batteries and is expected to take the lead in industrialization.

Biomass Goes Mainstream for Preparing Hard Carbon Materials

“Hard carbon precursor, raw material sources, are abundant, and precursor selection and process technology accumulation are key factors in the development of hard carbon anode materials.” Zhou Xiangyang said.

Hard carbon/graphite composite anode materials
Hard carbon/graphite composite anode materials

The precursors for preparing hard carbon materials are commonly biomass, synthetic polymers, fossil fuels, etc. The hard carbon materials prepared by different precursors have significant performance differences, and the cost composition of hard carbon materials is also significantly different due to the different sources of raw materials for precursors. Among them, biomass has a wide range of raw material sources, such as coconut shells, fruit shells, grapefruit peels, plant and animal tissues, etc. The cost is relatively low, which makes it the first choice for the preparation of hard carbon materials at present. Synthetic polymers mainly include phenolic resins, polyacrylonitrile and other chemically synthesized materials, which have good electrochemical properties, controllable raw materials and good product consistency, but at a higher cost. Fossil fuels mainly include asphalt, coal tar and related mixtures, with low cost from a wide range of raw material sources but lower product capacity. Due to the highly volatile substances contained in asphalt, etc., additional exhaust gas and wastewater treatment are required during the production process, thus increasing the production cost.

At present, the hard carbon preparation process is multiplexed, and there are constantly hard carbon negative electrode materials being developed. For example, a team led by Chen Chengmeng, a researcher at the Shanxi Institute of Coal Chemistry, Chinese Academy of Sciences, prepared starch into hard carbon-negative electrode materials through a chemical reaction, and its results were published in the academic journal Energy Storage Materials.

How do you prepare starch into hard carbon? The process can be roughly divided into three steps: first of all, the use of corn starch and maleic anhydride to prepare an oxygen-rich esterified starch; and then in the reactor input hydrogen and argon gas mixture, and esterified starch for hydrogen reduction reaction, the reaction product starch used as a precursor of the final product; finally argon as a protective gas, the starch precursor at 1100 ℃ for the high-temperature carbonization reaction, to complete the hard carbon material preparation.

Chen Chengmeng’s team also realized the regulation of the microstructure of hard carbon by changing the reaction temperature in the tube furnace and adjusting the oxygen content in the precursor of the reaction product, confirming the effect of oxygen content on the electrochemical properties of hard carbon anode materials.

Chen Chengmeng emphasized that although the team’s research has laid the foundation for the subsequent development of high-performance hard carbon materials, the microstructure and electrochemical properties of the materials still need to be explored in depth.

In addition, Prof. Yongyao Xia and others from Fudan University sequentially immersed the shell biomass material into a hydroalcoholic solution and sulfuric acid solution and stirred it to get the suspension; then, the suspension was dispersed in water, filtered and dried to get the precursor. They warmed the precursor under inert gas protection for pre-carbonization, cooled it and ball-milled it to get pre-carbon powder. They warmed the pre-carbon powder under inert gas protection for high-temperature carbonization, cooled it, and got highly efficient biomass hard carbon anode materials for sodium-ion batteries.

Hard Carbon Negative Electrode Material Industry Market Size Will Continue to Grow

Sodium-ion batteries have become a hot spot for research and industrialization at home and abroad. National Development and Reform Commission, the National Energy Board and nine other departments issued the “14th Five-Year” Renewable Energy Development Plan, which put forward research and development reserves for sodium-ion batteries, liquid metal batteries, solid-state lithium-ion batteries, metal-air batteries, lithium-sulfur batteries and other high-energy-density energy storage technologies.

Zhou Xiangyang said that at present, researchers of hard carbon sodium storage mechanisms proposed a variety of models, but its sodium storage mechanism has not yet reached a unified understanding. Therefore, further research is needed to reveal the constitutive relationship between hard carbon materials and the electrochemical reaction mechanism to provide theoretical guidance and scientific basis for improving the performance of hard carbon. In addition, the effects of physical parameters of hard carbon materials, such as particle size, vibration density, and mass loading, on the electrochemical performance also need to be further explored in order to synergistically improve the performance of the materials when they are used in sodium-ion battery systems.

The “China Hard Carbon Anode Industry Market Special Survey and Investment Prospect Analysis Report 2023-2029” released by Beijing Wisdom Research Consulting Co., Ltd. points out that with the national support for the development of new energy vehicles and energy storage equipment, the market scale of China’s hard carbon anode material industry will grow further. According to the market forecast, in 2025, the market size of China’s hard carbon negative electrode materials industry will reach 8.65 billion yuan, and the average annual growth rate of the hard carbon negative electrode materials industry in the next five years will reach 15.3%.

At present, due to the relatively short development time of the domestic hard carbon negative electrode material industry, most enterprises and research institutions are still in the stage of technology research and development and optimization. However, major domestic manufacturers are actively laying out the production of hard carbon-negative electrode materials. In April this year, Guangdong Rong Sodium New Energy Technology Co., Ltd. announced that its annual output of 10,000 tons of hard carbon anode material precursor production project was officially put into production in the graphite and graphene industrial park of Yong’an City, Fujian Province, which mainly uses plant-based biomass as raw material. For its part, Ningbo Sugo Co., Ltd. said that the hard carbon anode materials applied to sodium-ion batteries have achieved tonnage sales in China, and it is expected that the scale of mass production will reach 1,000 tons this year.

Ye Yindan, a researcher at the Bank of China Research Institute, believes that sodium-ion batteries are better than lithium e-batteries in terms of low temperature, safety, fast charging and other performance indicators. However, there is still room for improvement in their energy density, cycle life and so on. However, considering the rich source of materials, there is still a large potential for development. With the breakthrough of key technologies of sodium-ion batteries, such as hard carbon anode materials, and the rapid growth of energy storage demand, the application scenario and scale of sodium-ion batteries will also be developed rapidly.

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