Mechanisms and characteristics of mesocarbon microbeads prepared by co-carbonization of coal tar pitch and direct coal liquefaction residue
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DCLR-P was prepared by direct coal liquefaction residue (DCLR) with ash removal. In the present experiments, mesocarbon microbeads (MCMBs) were prepared by co-carbonization of coal tar pitch (CTP) and DCLR-P. With the increase of DCLR-P content, the yield of MCMBs increased from 47.8% to 56.8%. At the same time, the particle sizes distribution of MCMBs was narrowed, resulting in the decrease of D90/D10 ratio from 154.88 to 6.53. The results showed that DCLR-P had a positive effect on the preparation of MCMBs. 1H-NMR, FTIR, SEM and XRD were used to analyze the mechanisms and characteristics of MCMBs prepared by co-carbonization of CTP and DCLR-P. The results showed that the Proton Donor Quality Index (PDQI) of DCLR-P was 13.32, significantly higher than that of CTP (0.83). This indicated that DCLR-P had more naphthenic structure than CTP, which leads to hydrogen transferring in polycondensation reaction. The aliphatic structure of DCLR-P can improve the solubility and fusibility of mesophase, thereby making the structure of MCMBs more structured. The microstructure of the graphitized MCMBs had a substantially parallel carbon layer useful for its electrical performance. The performance of graphitized MCMBs as a negative electrode material for Li-ion batteries was tested. The particle sizes, tap density, specific surface area and initial charge–discharge efficiency of graphitized MCMBs met the requirements of CMB-I in GB/T-24533-2009. However, the initial discharge capacity of graphitized MCMB was only 296.3 mA h g−1 due to the low degree of graphitization of MCMBs.
KeywordsMesocarbon microbeads Direct coal liquefaction residue Mechanisms Characteristics
Mesocarbon microbeads (MCMBs) have been recognized as an exceptional precursor of lithium ion battery anode materials due to its uniform size, excellent sphericity, homogeneous shrinkage and unique microstructure (Chang et al. 1999; Alcantara et al. 2000; Wang et al. 2000; Hossain et al. 2003; Imanishi et al. 2008). MCMBs were typically prepared from coal tar pitch (CTP) with some additives. It is acknowledged that the microstructure of MCMBs varies with the different additives (Li et al. 2005; Zhang et al. 2005; Concheso et al. 2006; Liu et al. 2007; Song et al. 2008), and the microstructure of MCMBs had a great influence on its electrochemical performance (Korai et al. 1996). The microstructure of MCMBs is considered to be global type or Brooks-Taylor type (Bernhauer et al. 1994; Wang et al. 1999) with the polyaromatic molecules approximately parallel to each other and perpendicular to the surface of the sphere, which is suitable for the lithium ion insertion and desertion (Chang et al. 1999; Alcantara et al. 2000; Wang et al. 2000). A lot of researches have been carried out on the effects of different additives, such as carbon black (Korai et al. 1996; Wang et al. 1999), ferrocene (Bernhauer et al. 1994), flake graphite (Li et al. 2002), carbon nanotubes (Wang et al. 2008) and boron (Eichner et al. 1996). The effects of different additives on the preparation of MCMBs can be divided into physical effects and chemical effects (Li et al. 2005). The physical effects showed that the additives did not react with polyaromatic hydrocarbons, only affecting the formation of MCMBs physically such as its shape and sizes. Chemical effects showed that the additives can react with polyaromatic hydrocarbons catalytically and accelerate the formation of MCMBs. However, due to different characteristics of chemical additives, the effect mechanisms of chemical additives on the formation of MCMB is also different.
Direct coal liquefaction residue (DCLR) is one of the products in direct coal liquefaction process. It has the properties of high ash, high sulfur, and high aromatics. In order to improve the economic benefits of direct coal liquefaction, it is necessary to provide a method for high value application of DCLR. Similar to CTP, DCLR is also converted from coal. Therefore, DCLR can be used to prepare high value-added carbon materials such as carbon foam (Xiao et al. 2010), carbon fibers (Liu et al. 2015), and MCMBs (Chang 2017). However, Chang (2017) found that since the mesophase spheres are easy to melt during the synthesis, it is difficult to prepare a large number of MCMBs with uniform size by DCLR alone. In this paper, the MCMBs were prepared by CTP in the presence of DCLR, and the effects of DCLR on the formation and characteristics of MCMBs were also studied.
Properties of CTP and DCLR-P
Elemental analysis (%)
Chemical shift and hydrogen belonging in 1H-NMR
Hydrogen attached to aromatic ring
Hydrogen attached to first side-chain carbon adjacent to an aromatic ring
Naphthenic hydrogen or methyl protons two positions from an aromatic ring or non-cyclic methylene or methylene protons two or more positions from an aromatic ring
Terminal methyl protons of paraffins or alkyl side-chains three or more positions from an aromatic ring
1H-NMR analysis of CTP and DCLR-P
2.2 Preparation of MCMBs
The CTP with DCLR-P were sealed off in a 1L stainless-steel reactor within nitrogen atmosphere. The initial pressure was atmospheric pressure. The contents of DCLR-P in raw pitches were 0%, 10%, 20% and 30% in weight. The polycondensation reaction was carried out at 440 °C for 8 h, and then the mesophase pitch (MP) was obtained. According to the contents of DCLR-P in raw pitches, the mesophase pitches were labelled MP0, MP10, MP20, and MP30. MCMBs were separated from MP with the solvents of pyridine and ethanol at 115 °C for 2 h. The ratio of solvents to mesopahse pitches was 3:1. MCMBs obtained from MP0, MP10, MP20, and MP30 were labelled MCMBs0, MCMBs10, MCMBs20, and MCMBs30 respectively. The graphitized MCMBs were obtained by heat treatment at 1000 °C for 1 h under the protection of nitrogen and then up to 2600 °C without holding under the protection of argon.
The elemental analysis of samples was carried out using the Vario Micro cube elemental analyzer. The hydrogen spectra of nuclear magnetic resonance (1H-NMR) were recorded on Avance-300 NMR spectrometer using CDCl3 as solvent. Fourier transform infrared spectroscopy (FTIR) spectra were recorded on Nicolet Nexus 470 FTIR spectrometer. The frequency of scattering of each spectrum was 15 times s−1. KBr discs were prepared in the usual way from very well dried mixtures of about 1 mg sample and 100 mg KBr. The particle sizes of MCMBs were measured with a Malvern-2000 mastersizer using ethanol as dispersion medium. X-ray diffraction (XRD) analysis of the MCMBs was carried out using PANalytical-Empyrean diffractometer with Cu radiation. The optical textures of samples were observed using ZEISS QlmagerA2 polarized-light microscope. The samples were embedded in an epoxy resin and polished at the beginning. The scanning electron microscopy (SEM) was carried out using the nanoSEM-450 environmental scanning electron microscope. The MCMBs were coated with aurum in vacuum. The graphitization degree of graphitized MCMBs was characterized using Raman spectrometer (JY-HR800 532 nm).
3 Results and discussion
3.1 Yields and particle sizes of MCMBs
Particle sizes distribution of MCMBs
3.2 The mechanism of influence on the preparation of MCMBs with DCLR-P added
Structure parameters of FTIR spectra of DCLR-P, CTP and MP
PDQI of samples
3.3 X-ray diffraction analysis
The microstructure parameters of MCMBs
3.4 Morphologies of MCMBs and graphitized MCMBs
3.5 The performance of graphitized MCMBs10 as a negative electrode material for Li-ion batteries
The performance of graphitized MCMBs10 as a negative electrode material
15 ± 2
22 ± 2
31 ± 3
2.24 ± 0.02
0.7 ± 0.3
Microstructure parameters of MCMBs
The mechanisms of influence on the preparation of MCMBs with DCLR-P added were studied. Because of DCLR-P owing more aliphatic and naphthenic structure than CTP, the addition of DCLR-P can inhibit excessive polycondensation of CTP, and facilitates the formation of MCMB and promotes the nucleation and growth of MCMBs. Due to the addition of DCLR-P, the yield of MCMBs increases and the particle sizes are narrowed. Through XRD analysis, the addition of DCLR-P makes d002 decrease, and Lc and N become larger, this means the addition of DCLR-P is useful to make MCMBs structure more ordered. As can be seen from the SEM micrographs of the graphitized MCMBs with DCLR-P added, the microstructure of the graphitized MCMB has a substantially parallel carbon layers, which is useful for its electrical performance. The performance of graphitized MCMBs as a negative electrode material for Li-ion batteries was also tested. The particle sizes, tap density, specific surface area and initial charge–discharge efficiency of graphitized MCMBs met the requirements of CMB-I in GB/T-24533-2009. However, the initial discharge capacity of graphitized MCMB was only 296.3 mA h g−1 due to the low degree of graphitization of MCMBs. If the initial discharge capacity of graphitized MCMB could be improved, the graphitized MCMB may be an excellent anode material for Li-ion batteries.
Supported by National Key Research and Development Program of China (2018YFB0604601) and the Technology Innovation Fund of China coal research institute (2016CX01).
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