The oxidation reaction mechanism and its kinetics for a carbonaceous precursor prepared from ethylene tar for use as an anode material for lithium-ion batteries

Abstract

The oxidation reaction mechanism and its kinetics for ethylene tar were investigated in order to obtain a suitable anode material for Li-ion batteries. The oxidation of ethylene tar was divided into 3 stages (350–550, 550–700 and 700–900 K) according to the thermogravimetric curve. To reveal the oxidation reaction mechanism, the components of the gases evolved at different stages were analyzed by mass spectrometry and infrared technology. Based on these results the reaction was divided into 4 stages (323–400, 400–605, 605–750 and 750–860 K) to perform simulation calculations of the kinetics. Using the iso-conversion method (Coats-Redfern) to analyze the linear regression rates (R2) between 17 common reaction kinetics models and experimental data, an optimum reaction kinetics model for expressing the oxidation of ethylene tar was determined and the results were as follows. (1) During oxidation, the side chains of aromatic compounds first react with oxygen to form alcohols and aldehydes, leaving peroxy-radicals on aromatic rings. Subsequently, the aromatic compounds with peroxy-radicals undergo polymerization/condensation reactions to form larger molecules. (2) A fourth-order reaction model was used to describe the first 3 stages in the oxidation process, and the activation energies are 47.33, 18.69 and 9.00 kJ·mol−1 at 323–400, 400–605, 605–750 K, respectively. A three-dimensional diffusion model was applied to the fourth stage of the oxidation process, and the activation energy is 88.37 kJ·mol−1 at 750–860 K. A high softening point pitch was also produced for use as a coating of the graphite anode, and after it had been applied the capacity retention after 300 cycles increased from 51.54% to 79.07%.

1. Introduction

Functional carbon materials have been playing an important role in electrical applications, thermal management (e.g., heat conduction and insulation), energy storage (e.g., electrode materials), environmental protection (e.g., gas capture), and various other fields[1–4]. It is well known that high-softening-point petroleum pitch is an important carbonaceous precursor[5]. For example, petroleum pitch with a high softening point can be applied as the coating material to the anode materials of lithium-ion batteries, raw materials for pitch-based carbon fiber and pitch-based spherical activated carbon[6–8].

Among these, ethylene tar is an important resource for preparing high softening point petroleum pitches[9–10]. The raw material for ethylene production is mainly naphtha (atmospheric crude oil fraction with an initial distillation point of ～200 °C). The C―H bond and C―C bond of naphtha are broken at high temperatures (600–800 °C) to produce ethylene and propylene. Meanwhile, olefins can also polymerize and cyclize to produce aromatic hydrocarbons. Therefore, the main components of ethylene tar are monocyclic, polycyclic aromatic hydrocarbons and heavy aromatic distillates, which are valuable resources rich in aromatic hydrocarbons. As the byproduct of hydrocarbons cracking in ethylene production (ca. 15% in yield), ethylene tar is produced in large amounts every year with the increasing demand for ethylene. Thus, it is regarded as an ideal material for producing high-quality pitch.

To increase the softening point and carbon yield of synthetic pitch[11–14], the air oxidation method has been widely applied to prepare high-quality pitch[15–18]. Several researchers[11,13,19] have reported that the air oxidation method can cause polymerization/condensation reactions between aromatic compounds. However, the mechanism and kinetics related to the growth of molecular mass are still sparsely reported. SugioŌtani et al.[20–21] assumed that oxygen was taken up by the aliphatic carbon atoms in an α-position to aromatic rings, giving rise to peroxy-radical. Peroxy-radical is an intermediate of polymerization/condensation reaction, which is accompanied by the production and emission of water[22–24]. Nevertheless, the mechanism of the air oxidation reaction seems to be related to the experimental conditions[25] and the chemical properties of the raw materials[11,13].