Controlling and Understanding the Impact of Closed Pore Size on Sodium Storage in Hard Carbons via Controlled Pyrolysis of Molecular Precursors

Abstract

Hard carbon is the most widely applied material for sodium-ion battery negative electrodes. Although capacities comparable to those of lithium/graphite can be achieved, the underlying sodium storage mechanisms remain poorly understood. From a simplified perspective, a two-step process is commonly observed: first, sodium adsorbs to the polar sites of the carbon (“sloping region”) and then fills the small voids in the material (“plateau region”). In order to study the impact of the molecular size of precursors on the microstructure of carbon materials and their pore geometry, a systematic series of cyclodextrin-based hard carbons has been synthesized. It is found that the type of precursors used influences the resulting materials’ pore structure, which at higher temperatures can be converted to a closed pore system. This pore conversion enables a large, low-potential sodiation plateau. Indeed, up to 75% of the total capacity is measured at potentials below 0.1 V versus Na⁺/Na. Additionally, the plateau region can be extended by up to 16% by additionally considering reversible capacity below 0 V versus Na⁺/Na, which means quasimetallic sodium can be stabilized within such structural motifs. Finally, gas physisorption measurements are related to charge–discharge data to identify the architecture of pores relevant to energy storage.