The potassium storage performance of carbon nanosheets derived from heavy oils

Abstract

As by-products of petroleum refining, heavy oils are characterized by a high carbon content, low cost and great variability, making them competitive precursors for the anodes of potassium ion batteries (PIBs). However, the relationship between heavy oil composition and potassium storage performance remains unclear. Using heavy oils containing distinct chemical groups as the carbon source, namely fluid catalytic cracking slurry (FCCS), petroleum asphalt (PA) and deoiled asphalt (DOA), three carbon nanosheets (CNS) were prepared through a molten salt method, and used as the anodes for PIBs. The composition of the heavy oil determines the lamellar thicknesses, sp3-C/sp2-C ratio and defect concentration, thereby affecting the potassium storage performance. The high content of aromatic hydrocarbons and moderate amount of heavy component moieties in FCCS produce carbon nanosheets (CNS-FCCS) that have a smaller layer thickness, larger interlayer spacing (0.372 nm), and increased number of folds than in CNS derived from the other three precursors. These features give it faster charge/ion transfer, more potassium storage sites and better reaction kinetics. CNS-FCCS has a remarkable K+ storage capacity (248.7 mAh g−1 after 100 cycles at 0.1 A g−1), long cycle lifespan (190.8 mAh g−1 after 800 cycles at 1.0 A g−1) and excellent rate capability, ranking it among the best materials for this application. This work sheds light on the influence of heavy oil composition on carbon structure and electrochemical performance, and provides guidance for the design and development of advanced heavy oil-derived carbon electrodes for PIBs.

1. Introduction

The excessive reliance on fossil fuels has resulted in severe energy crises and environmental pollution issues, necessitating the urgent development of green energy storage technologies[1]. Over the past few decades, lithium-ion batteries (LIBs) have been extensively used in energy storage systems owing to their elevated voltage, high energy density, and extended lifespan[2,3]. Nevertheless, the continual use of LIBs has raised various concerns, such as the scarcity of lithium resources and elevated costs, shedding doubt on the sustainable use of lithium to meet future energy needs[4]. Given the abundant reserves and low potassium cost in the Earth’s crust, potassium-ion batteries (PIBs), which follow a rocking-chair mechanism akin to LIBs, have emerged as a promising alternative[5,6]. However, the larger radius of K+ (1.38 Å) compared with Li+ (0.76 Å) makes it more susceptible to electrode material expansion and collapse[7–9]. Therefore, the key to driving the development of PIBs lies in the search for high-performance and cost-effective anode materials to meet the requirements of PIBs.

In recent years, there has been extensive exploration of various materials as anodes for PIBs[10,11], including carbon-based materials[12,13], organic materials[14,15], transition metal compounds[16,17], etc. Especially, carbon-based materials have garnered significant attention among these options due to their abundance, environmental friendliness, and excellent conductivity[18–20]. As by-products in petroleum refining, heavy oils such as asphalt, fluid catalytic cracking slurry, and petroleum coke, etc. are characterized by their high content of aromatic hydrocarbons, low cost, and abundant tunability, which make them competitive precursors for constructing carbonaceous electrodes for PIBs[21–23]. For instance, Quan et al.[24] synthesized micro-sized and robust pitch-derived carbon (PC)/expanded graphite (EG) composites (EGCs) with abundant edge defect sites on a large scale by melt impregnation and confined pyrolysis techniques. The EGC delivered a reversible capacity of 204.8 mAh g−1 after 70 cycles.