The impact of templating and macropores in hard carbons on their properties as negative electrode materials in sodium-ion batteries

Abstract

Due to the abundance of sodium and the comparable working principle to lithium-ion technology, sodium-ion batteries (SIBs) are of high interest as sustainable electochemical energy storage devices. Non-graphitizing (“hard”) carbons are widely investigated as negative electrode materials due to their high sodium storage capacity close to the potential of Na/Na⁺, excellent safety, and simple synthesis pathways from abundant resources. The accumulation of sodium in “closed pores” that are inaccessible to the electrolyte solvent molecules is typically considered as a main source of capacity. While several features such as the microstructure of hard carbons (controllable by e.g. precursors or carbonization conditions) on their electrochemical properties have been widely investigated, the influence of macropores and domain sizes of hard carbons received less research attention. We investigate the use of polystyrene as a sacrificial template to introduce additional internal porosity into glucose-derived hard carbons, in the form of micrometer-sized channels, and the effect on the sodium storage properties and irreversible processes. Despite no significant changes in the microstructure of the hard carbons, pronounced differences in their electrochemical signature can be resolved at working potentials above, but also below 0 V vs. Na/Na⁺ for different polystyrene amounts and thus macropore contents. Carbons with higher polystyrene amount exhibit higher reversible sodium storage capacity, but also suffer from a more pronounced capacity loss due to irreversible reactions in the first cycle(s). Furthermore, their sodium nucleation overpotential is shifted to higher values in capacity limited measurements, which is likely due to the higher electric resistance. The necessary time to achieve full sodiation capacity increases with a lower content of macropores.