Selecting the molecular components of a pitch to produce a hard carbon anode with a high sodium storage capacity

Abstract

Pitch is an excellent precursor for the production of hard carbon, with pre-oxidation a crucial process in the fabrication. The structural changes in the different molecular components of pitch during thermochemical treatment are a key factor in determining the sodium-ion storage of pitch-based hard carbon anodes. We investigated the effects of the different molecular structures in the asphaltene precursor, including aromatic rings and aliphatic chains, on the sodium-ion storage behavior of the resulting carbon. We found that polar oxygen functional groups limit the steric hindrance caused by the aromatic rings in pitch, and thus facilitate the introduction of cross-linked structures. During high-temperature carbonization, aromatic rings form a rigid carbon framework that prevents the rearrangement of ordered carbon layers, leading to a short-range disordered carbon structure and promotes the production of closed pores. For example, a material prepared from asphaltene, which contains a large number of oxygen-containing functional groups and macromolecular aromatic rings, using pre-oxidation at 300 °C and carbonization at 1200 °C had a reversible capacity of 316.7 mAh g−1 when used as the anode for sodium ion batteries. Our research provides a theoretical basis for the selection of raw materials for the development of high-quality pitch-based hard carbons.

1. Introduction

Concerns regarding environmental pollution and energy depletion are propelling the expansion of renewable energy, leading to an increased demand for advanced energy storage solutions. Sodium-ion batteries (SIBs) present a viable alternative to widely used lithium-ion batteries (LIBs) due to their lower cost and enhanced safety profile. However, the development of high-performance, cost-effective, and high-stability anode materials still remains a significant challenge for the commercialization of SIBs[1–3]. Hard carbon is distinguished among anode materials owing to its abundance, low cost, and wide operating voltage range[4–5].

Hard carbon materials are generally synthesized through the pyrolysis of various precursors, including biomass[6], fossil fuels[7], organic small molecules[8], and polymer[9]. Petroleum pitch, a byproduct of the petroleum industry, is cost-effective and has a higher carbon content compared with other precursors, contributing to reducing electrode expenses[10]. However, not only the provenance or batch of pitch gives rise to pronounced compositional variations, but the carbon derived from the direct carbonization of petroleum pitch tends to be highly graphitized with small interlayer spacings, leading to limited sodium-ion storage performance. Converting pitch into hard carbon materials requires introduction of additional cross-linking structures to prevent its melting during high-temperature carbonization. This approach prevents the formation of viscous liquid intermediates and effectively suppresses the rearrangement of carbon layers. Extensive studies have investigated a variety of modification methods for petroleum pitch, such as potassium permanganate treatment[11], acid treatment[12], template interference[13] and co-carbonization[14]. Nonetheless, these methods often involve complex processes that are not suitable for large-scale industrial production. The pre-oxidation process entails the thermal treatment of pitch in an atmosphere rich in air or oxygen, facilitating the incorporation of oxygen-containing functional groups without the necessity for dopants or intricate procedures. This method lends itself to automation, making it suitable for large-scale production, thereby significantly lowering material production costs and improving manufacturing efficiency.