The Effect of Hard Carbon Negative Electrode Materials on Sodium Batteries

Hard carbon is carbon that will not graphitise after high-temperature treatment, and its internal crystal arrangement is disordered and the layer spacing is large, which enables the hard carbon anode to store more charge in the same volume, and improves the energy density and endurance of sodium-ion batteries. The expansion and contraction of the hard carbon anode is 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 power generation from renewable energy sources such as solar and wind, research on new materials for energy storage batteries is also deepening. At the 15th China International Battery Technology Exhibition in Shenzhen, a company released a new generation of hard carbon anode materials for sodium-ion batteries, whose first-time charging and discharging efficiency can reach 90 per cent.

China is rich in sodium resources, sodium-ion battery is considered to be the most suitable for large-scale energy storage of new batteries, and is expected to alleviate the shortage of lithium resources and uneven distribution of the development of energy storage and other constraints. Compared with other anode materials for sodium-ion batteries, what are the advantages of hard carbon materials? China's hard carbon materials industry development status? How far away from the large-scale application, there is still a long way to go? With these questions, 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 electrode, negative electrode, electrolyte, battery separator, etc. Its working principle is similar to that of lithium-ion battery. As the main body of sodium storage in the battery, the anode material of sodium-ion battery realises the embedding or dislodging of sodium ions in the process of charging and discharging, so the capacity of the battery is positively correlated with the anode's ability of storing sodium ions, and the selection of anode material has a decisive role in the development of sodium-ion battery.

Zhou Xiangyang, professor of Central South University, said that from the classification of anode materials for sodium-ion batteries, they 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 industrialisation; second, alloy-based anode materials, with high theoretical capacity, but serious volume expansion after the electron embedded sodium, and poor cycling performance; third, metal oxides and sulphides-based anode materials, with high theoretical capacity, but poor electrical conductivity; fourth, embedded titanium-based anode materials, with small volume change but low capacity; fifth, organic-based anode materials, with low cost, but poor electrical conductivity and easy to dissolve in the electrolyte. Fifth, organic anode materials, low cost, but poor conductivity and easy to dissolve in the electrolyte.

Carbon-based anode materials have excellent electrical conductivity, as well as flexible preparation methods, low cost, and environmental friendliness, which make them the primary choice of anode materials for sodium-ion batteries. Among them, hard carbon and soft carbon materials in amorphous carbon are considered as potential anode materials for sodium-ion batteries. Soft carbon refers to the carbon that can be graphitised after high-temperature treatment, which is usually obtained by processing and manufacturing low-cost anthracite coal as precursor, but it has low specific capacity for sodium storage, slower charging speed, and poor low-temperature performance.

Hard carbon is carbon that will not be graphitised after high temperature treatment, and its internal crystal arrangement is disordered and the layer spacing is large, which allows hard carbon anode to store more charge in the same volume, increasing the energy density and range of the battery. Since the pore structure of hard carbon is larger and can hold more sodium ions, the expansion and contraction of the electrode is more uniform during the discharge process, which increases the cycling stability, charging and discharging performance of the hard carbon anode, and prolongs the cycling service life of the sodium ion battery.