Hard carbon

Hard carbon is a solid form of carbon that cannot be converted to graphite by heat-treatment, even at temperatures as high as 3000 °C. It is also known as char, or non-graphitizing carbon. More colloquially it can be described as charcoal.

Hard carbon is produced by heating carbonaceous precursors to approximately 1000 °C in the absence of oxygen. Among the precursors for hard carbon are polyvinylidene chloride (PVDC), lignin and sucrose. Other precursors, such as polyvinyl chloride (PVC) and petroleum coke, produce soft carbon, or graphitizing carbon. Soft carbon can be readily converted to graphite by heating to 3000 °C.

The physical properties of the two classes of carbons are quite different. Hard carbon is a low density material, with extremely high microporosity, while soft carbon has little microporosity. Hard carbon is extensively used as anode materials in lithium-ion batteries and sodium-ion batteries.

Manufacturers of hard carbon include Xiamen Tob New Energy (China), Kuraray (Japan) and Stora Enso (Finland).

Plasma-derived Hard Carbon is an emerging area for SIB batteries. Plasma methods are scalable and sustainable manufacturing sources of hard carbon to meet its increasing industrial demands for energy storage applications. Plasma-derived hard carbon uplift Coulombic efficiency and specific capacity by 33% and 44%. Spark sintering plasma has shown initial Coulombic efficiency of ~90% reversible capacity of ~300 mAh/g and rate capacity of 136.6 mAh/g at 5 A/g. The future aspects of plasma methods to perform multi-material doping, in-situ nanoarchitecture fabrications, and challenges around SIB functioning in extreme environments, and the development of real-time robust monitoring and diagnostic tools to make safe, stable, and high-performance SIB with long life. Further, a data-driven manufacturing framework suggests integrating material informatics with experimental protocols for virtual synthesis of hard carbon; estimating material formulations, manufacturing methods, process-property-performance relationship, and limitations before physical manufacturing of high-performance sodium batteries.