Optimizing Rotary Kiln Pyrolysis Kinetics to Control Hard Carbon Turbostratic Structure
Controlling rotary kiln pyrolysis kinetics optimizes hard carbon interlayer spacing and closed porosity, raising sodium-ion anode initial coulombic efficiency.
Pitch Stabilization
Meaning
Controlled oxidation at low temperatures prevents petroleum or coal derived solids from melting during subsequent high temperature carbonization. This step in the manufacturing of carbon materials involves heating the pitch in an air environment to create oxygen bridges between molecules. Pitch stabilization turns a thermoplastic substance that would flow when heated into a thermoset material that maintains its shape.
It is a necessary precursor to the creation of hard carbon or carbon fibers and stops once the material is sufficiently crosslinked.
Oxidative Crosslinking
Oxygen atoms react with the hydrocarbon chains in the pitch to form a rigid three dimensional network. During pitch stabilization, the temperature is typically kept between two hundred and three hundred degrees Celsius. If the temperature rises too quickly the pitch will melt before it can react and the final product will be a solid block rather than a fine powder or fiber.
This phase is the most time consuming part of the carbon production process and requires precise air flow control.
Shape Retention
Maintaining the physical form of the material is critical for the performance of the final battery anode. Effective pitch stabilization ensures that the spherical or fibrous shape of the particles is preserved throughout the furnace transit. This physical structure determines the surface area and the density of the electrode coating.
Any melting during the process would destroy the pore network required for sodium ion transport and storage.
Precursor Chemistry
The effectiveness of the stabilization step depends heavily on the chemical composition of the starting pitch. Materials with a high softening point require less oxygen to become stable and can be processed more quickly. Manufacturers often blend different types of pitch or add chemical catalysts to speed up the pitch stabilization reaction.
This chemical engineering reduces the energy cost of production and increases the throughput of the manufacturing facility.