Next Generation Energy Storage: An Examination of Lignin-based Carbon Composite Anodes for Sodium Ion Batteries through Modeling and Simulation

The current energy market relies heavily on fossil fuel sources; however, we are amidst a momentous shift towards wind, solar, and water based renewable energies. Large-scale energy storage allows renewable energy to be stored and supply the grid with consistent energy despite changing weather conditions. Improvements to large-scale energy storage in terms of cost, safety, and sustainability are crucial to wide-scale adoption. A promising candidate for large-scale energy storage are sodium-ion batteries using hard carbon anodes. Sodium is globally available, cheaper, and more sustainable than lithium, but requires a different anode structure. A sustainable hard carbon anode with excellent Li-ion performance has been manufactured from lignin, a byproduct of the paper and bio-ethanol industries. The carbon composite generated from lignin is composed of nanoscale crystallites dispersed in an amorphous graphene matrix whose structure is highly dependent on manufacturing process; however, the sodium-ion storage mechanisms for these lignin-based hard carbons are not well known.

The purpose of the following work is to elucidate the Na-ion storage mechanisms for these lignin-based hard carbons and develop process-structure-property-performance (PSPP) relationships for them so an optimal Na-ion anode can be manufactured. To this end, reactive molecular dynamics simulations of lignin-based carbon composites were conducted with both lithium and sodium to compare the binding energies and mechanisms as well as their respective diffusive properties. It was found that lithium-ions prefer to localize in the hydrogen dense interfacial regions of the carbon composites while sodium prefer to adsorb to the surfaces of graphene fragments as well as the outer faces and edge-intercalation positions of the crystallites. At higher porosity, sodium shows a tendency to aggregate in the porous regions along curved planes of graphene, which gives the Na-ions the highest diffusion rate of all systems studied.

To aid in determining the PSPP relationships of LBCCs, synchrotron x-ray scattering was performed, and models were created and refined using the Hierarchical Decomposition of the Radial Distribution Function (HDRDF) technique and software (now highly generalized). PSPP relationships with respect to processing temperature were quantitatively and qualitatively determined for the lignin-based carbon composites.