GUO Hong-yi, LI Yao-yu, WANG Chun-lei, HE Lei, LI Chen, GUO Yong-qiang, ZHOU Ying*

（Liaoning Key Laboratory of Energy Materials and Chemical Engineering, Carbon Research Laboratory, College of Chemical Engineering,Dalian University of Technology, Dalian 116024, China）

Abstract： Hard carbon anode materials for sodium ion batteries were prepared from petroleum pitch by air oxidation stabilization followed by carbonization. The effects of the oxidation stabilization temperature on the compositions and microstructures of the oxidized samples, as well as on the morphology, microstructure and sodium storage property of the carbonized samples were investigated. Results show that air oxidation introduces a large number of oxygen-containing functional groups, induces dehydrogenation condensation and oxidative crosslinking reactions, and transforms the petroleum asphalt from thermoplastic to thermosetting. The air oxidation stabilization treatment effectively hinders the inherent tendency of asphalt to graphitize during high temperature carbonization, resulting in carbons with randomly oriented carbon layers with more defects. Electrochemical tests show that o-PDC-350-1400(oxidation stabilization at 350 °C, carbonization at 1 400 °C) has a high charging specific capacity of 276.8 mAh g−1 at 100 mA g−1 and a high initial coulombic efficiency of 73.38%. Compared with sample PDC-1400 that was directly carbonized at 1 400 °C, the charging specific capacity was increased by about 1.8 times and the initial coulombic efficiency was increased by 22%. The charging specific capacity of o-PDC-350-1400 after 200 cycles reached 170.2 mAh g−1, indicating good cycling stability.

Key words: Petroleum asphalt；Oxidation stabilization；Hard carbon；Sodium ion battery

1 Introduction

With the rapid development of new energy technology, high requirements have been put forward for energy conversion and storage devices. Sodium-ion batteries (SIBs) show much potential in the largescale energy storage systems because of high abundance, easy availability and low price of sodium resource. The radius of sodium ion is relatively large, so the suitable electrode material is one of the key issues to realize the practical application of SIBs. Among the anode materials, hard carbons have been regarded as the most promising materials due to the advantage of wide source of precursors, high reversible capacity and low operating voltage[1,2]. The common precursors of hard carbons include biomass[3–5], organic polymers, commercial carbon molecular sieves and activated carbons[6]. However, the quality, supply and price hinder their applications in the field of large-scale energy storage.

The petroleum pitch, a byproduct of crude oil processing, has been regarded as an excellent carbon precursor owing to its low price, abundance of polycyclic aromatic hydrocarbons and high carbon residue. It is widely used to prepare mesophase carbon microspheres, carbon fibers, activated carbons,needle coke and artificial graphite anode[7]. Generally,the carbonization of asphalt goes through the process of liquid phase, and the resulting product shows a tendency of graphitization at high temperature. Chenet al.mixed lignin and asphalt through ball mill, and inhibited the graphitization tendency in the asphalt carbonization process by means of emulsification. The prepared amorphous carbon exhibited a reversible capacity of 254 mAh g−1and an initial coulombic efficiency (ICE) of 82%[8]. They also successfully prepared the soft/hard carbon anode material by carbonization of phenolic resin and asphalt, which delivered a reversible capacity of 284 mAh g−1and an ICE of 88%. After assembled with O3-Na0.9[Cu0.22Fe0.30Mn0.48]O2to the full cell, it showed an ICE of 80%and an energy density of 195 Wh kg−1[9]. Liuet al.widened the interlayer spacing of graphite crystallite and increased the defect degree of soft carbon to improve the electrochemical performance of the electrode material by introducing P element. The obtained anode material delivered a reversible capacity of 251 mAh g−1. Moreover, a high capacity retention ratio of 80.1% over 200 cycles at 100 mA g−1can be obtained[10]. Oxidative stabilization is a commonly used technology in the process of preparing carbon materials from asphalt. Under the atmosphere of hot air, the unstable components in asphalt are decomposed and small molecules are released, and oxidative dehydrogenation occurs at the same time. Oxygen molecules in air and asphalt molecules are crosslinked to form stable macromolecules by forming C＝O (aldehyde or ketone) and ―C―O―C― (ester or ether) bonds, which lead to an increase of the oxygen content greatly[11]. Air oxidation introduces oxygen-containing groups easily, resulting in the increase of softening point, the oxygen content and the more quinoline insoluble (alpha resin) in oxidized asphalt,which is difficult to be graphitized.

In this paper, hard carbons were successfully prepared by pretreating the petroleum pitch under air followed by carbonization at a high temperature. The effects of the oxidation stabilization temperature on the composition and microstructure of the oxidized samples, as well as its effects on the morphology, microstructure and sodium storage properties of the carbonized samples were investigated.

2 Experimental

2.1 Preparation of hard carbons

The blocky petroleum pitch was smashed into powder and dried at 80 °C for 12 h. 20 g of raw material was heated with a heating rate of 2 °C min−1under air atmosphere. Then, sample was kept at the specified oxidation stabilization temperature for 2 h and cooled to room temperature to obtain oxidized samples, which were denoted as o-P-T, where T represents the oxidation stabilization temperature. After pulverized, the o-P-T samples were carbonized at 1 400 °C for 3 h to obtain carbonized samples, which were denoted as o-PDC-T-1400. For comparison, the petroleum pitch was carbonized under the same conditions without pre-oxidation treatment, which were denoted as PDC-1400.

2.2 Characterization of microstructure and performance

The microstructures of the samples were characterized by powder X-ray diffraction (XRD, SmartLab 9 kW, wavelength 0.154 06 nm, tube pressure 45 kV,tube current 200 mA), DXR Raman microscopy (the excitation wavelength of the laser was 532 nm), Fourier transform infrared (FT-IR, ThermoFisher 6700)and the element analysis (Vario EL). The morphologies of the samples were examined by scanning electron microscopy (SEM, Hitachi SU8200) and transmission electron microscopy (TEM, TF30). Nitrogen adsorption was carried out at −196 °C with an ASAP2020 physical adsorption apparatus produced by Micromeritics. The specific surface areas of the samples were calculated by the Brunauer-Emmet-Teller (BET）method, and the pore size distribution curves were plotted according to the non local density functional theory （NLDFT） model.

To prepare the working electrode, 80 wt.% of active material, 10 wt.% of acetylene black and 10 wt.% of polyvinylidene fluoride (PVDF) within proper amount of N-methyl-2-pyrrolidone (NMP)were mixed to form a slurry that was coated on a copper foil with a thickness of 50 μm and eventually dried at 120 °C for 12 h under vacuum. The half cell was assembled by filling the CR2016 coin cell up with a 12 mm diameter anode slice, a glass fiber separator (Whatman GF/D), a sodium foil and the electrolyte (1 mol L−1NaClO4dissolved in ethylene carbonate (EC)/diethyl carbonate (DEC) with 1∶1 volumetric ratio) in an argon-filled glove box with moisture and oxygen levels less than 0.1×10−6. All the cells were held for 12 h for electrochemical performance testing. The rate performance and cyclic performance tests were performed on the Land battery test system(CT2001A). The cyclic voltammetry test was performed at 0.1 mV s−1on the VSP multi-channel electrochemical workstation of Bio-logic, France.

3 Results and discussion

Table 1 lists the element analysis results of the samples treated at different oxidation stabilization temperatures. With increasing the temperature, the oxygen content increased. The oxygen content of the o-P-350 reached 27.42%. The C/H ratio, as an important parameter to characterize the aromatic structure of asphalt, was about 1.5 at 200 °C and 2.09 at 250 °C,implying that the aromatic ring number increased with the oxidation temperature and that not only oxidation but also dehydrogenation condensation occurred during the oxidation stabilization processing[12]. Due to dehydrogenation condensation and/or oxidation crosslinking of the polycyclic aromatic hydrocarbon molecules in asphalt, larger molecules were formed,which reduced the volatile components of small molecules in pyrolysis. With the increase of the oxidation temperature (T≤300 °C), the yield of oxidized asphalt increased first and then decreased. When the temperature was relatively low (150 °C), the oxidation weight gain of the sample was not obvious, and the carbonization yield was similar to that of the sample without oxidation stabilization, which was about 50%. When the temperature increased to 250 °C,the oxidation weight gain reached 9.4%, and the carbonization yield increased to 72.3% while the temperature reached 350 °C, the oxidation weight loss of the sample was 7.1%, and the carbon yield was 61.5%. If the temperature continued to rise to 400 °C, the loss rate of the sample increased rapidly and the oxidation weight loss reached 54.9%, indicating that the asphalt had been over-oxidized. Therefore, the higher oxida-tion temperature is not considered in this paper.

Table 1 Element analysis, weight gain ratio and carbon yield of samples after oxidation stabilization.

XRD is an important method to analyze the asphalt structure. Through the analysis and calculation of the peak ofγand the peak of (002), the information of the aromatic carbon ratio, the interlayer spacing of the aromatic, the distance of saturated component and the crystal size could be obtained. The(002) peak in the XRD pattern was caused by the diffraction of the interlayer spacing in the graphite-like structure. For the attribution of theγpeak, Ergun and Wenderet al. believed that it may come from the condensation of the naphthenic rings[13], while Ebertet al[14]. believed that it may be mainly caused by the alkyl chain part. In short, it was caused by the diffraction of the saturated part of the asphalt molecule. According to the method in relative reference[15], the XRD patterns of asphalt and o-P-T were separated by applying the Gauss function from 10° to 30° and the results are shown in Fig. 1(a). The 2θvalue of peak(002) was 25°, and the 2θvalue of peakγwas 20°.According to the formula as followed, the microstructural parameters of asphalt were calculated, and the microcrystalline structure parameters are listed in Table 2.

Farepresents the aromatic carbon ratio in asphalt structure, which can reflect the degree of aromaticity to some extent. It could be found from the data in Table 2 that with the increase of the oxidation treatment temperature, the aromatic carbon content of the treated-asphalt increased, indicating that dehydrogenation condensation occurred in the saturated component. When the oxidation temperature reached 350 °C,the oxidation cross-linking extent increased and theFavalue of o-P-350 slightly reduced while the layer spacing of the aromatic (Dm) and the distance of saturated component (Dr) did not change significantly.With the increase of the oxidation temperature,La, the width of the aromatic layer, decreased from 1.113 to 0.827 nm, andLc, the packing thickness of the aromatic layers, decreased from 0.544 to 0.405 nm, indicating that during oxidation stabilization, the spacing between the aromatic layers generated by dehydrogenation aromatization was small, and the growth of aromatic layer size was effectively inhibited due to the cross-linking effect of oxygen.

Fig. 1 (a)X-ray diffraction patterns and (b) FT-IR analysis of petroleum asphalt and oxidized petroleum asphalt treated at different oxidation stabilization temperatures.

Table 2 Crystalline parameters of pristine petroleum asphalt and samples after oxidation stabilization.

In order to study the effect of oxidation on the microstructural transformation of asphalt, FT-IR characterization of o-P-T was conducted and the group changes in the process of oxidation stabilization were recorded. Fig. 1(b) shows that the stretching vibration absorption peak corresponding to the O―H bond appeared in all samples at 3 430 cm−1, indicating that the samples contain hydroxyl groups or adsorbed water.The absorption peaks at 2 970 and 2 920 cm−1corresponded to the stretching vibration of ―CH3and the anti-symmetric stretching vibration of ―CH2respectively, indicating the presences of methylene and methyl groups in the samples. For the raw asphalt and o-P-150, a stretching vibration peak corresponding to aromatic hydrogen appeared at 3 040 cm−1. When the oxidation stabilization temperature exceeded 200 °C,the above-mentioned peak disappeared, indicating that dehydrogenation condensation reactions occurred.With the further increase of the temperature, the absorption peaks of the bending vibration corresponding to methyl and methylene groups at 1 380 and 1 460 cm−1gradually decreased, implying that dehydrogenation aromatization, oxidation and/or adipose chain breaking reactions occurred. Moreover, when the oxidation stabilization temperature increased (T≥200 °C), the C―O expansion vibration peak became stronger at 1 100 cm−1, and a new carbonyl (C＝O)absorption peak generated at 1 700 cm−1, indicating that the introduced oxygen mainly exists in the form of aromatic ether and aromatic aldehyde/ketone or ester. Due to the poor thermal stability of oxygen-containing groups, a large number of free radicals were generated after dissociation[16,17], which greatly enhanced the reactivity of samples during carbonization and hindered the formation and aggregation of intermediate phases. The dissociation of C＝O and C―O bonds could generate biphenyl structures, and the planar structure of the crosslinked macromolecules was restricted because the rotatable single bonds were limited by the steric hindrance effect of the polycyclic aromatic hydrocarbons. When the oxidative stabilization temperature was relatively low, the amount of the introduced oxygen-containing groups was lacking,which hardly transformed the petroleum asphalt from thermoplastic to thermosetting one. With increasing the oxidation temperature, the free radical content grew with increasing the oxygen-containing groups,and the viscosity of the system increased greatly until it lost fluidity, indicating that the transition process achieved.

Fig. 2 (a) XRD patterns and (b) Raman spectra of o-PDC-T-1400 series.

To evaluate the effect of oxidation stabilization on the crystal structure of hard carbons, the samples with different oxidation temperatures and the same carbonization temperature (o-PDC-T-1400) were characterized by XRD and Raman spectroscopy. As shown in Fig. 2(a), the unoxidized sample (PDC-1400) and the samples treated at lower oxidation stabilization temperatures (o-PDC-150-1400 and o-PDC-200-1400) showed sharp diffraction peak of graphite(002) crystal plane at 2θ=25° and the peaks corresponding to (100)/(110) and (004) planes at 2θ=42°and 52°, respectively. o-PDC-150-1400 and o-PDC-200-1400 showed largeLaandLcvalues close to that of the untreated PDC-1400 (Table 3), indicating a high graphitization tendency. When the oxidation stabilization temperatureT≥ 250 °C, the (002) peak of the sample significantly widened, forming the"steamed bun peak". The (004) peak of the graphite located near 53° disappeared, and theLaandLcvalues significantly decreased, demonstrating that the oxidation stabilization at higher temperatures than 250 °C did hinder the graphitization trend of the raw asphalt.From Fig. 2(b), all samples exhibitDbands at1 360 cm−1andGbands at 1 590 cm−1. Table 3 lists the ratios of the absolute strength ofDbands andGbands of a series of o-PDC-T-1400 samples. For the PDC-1400 and the samples pre-oxidized at low temperatures, theID/IGvalues were small (～0.95) while with the further increase of the oxidation stabilization temperature, theID/IGvalues gradually increased, indicating that the number of defects increased and the part of graphitic structure of the material decreased.

Table 3 Crystalline parameters and electrochemical performance of o-PDC-T-1400 series.

The SEM and HRTEM images of PDC-1400 and o-PDC-350-1400 are shown in Fig. 3. As shown in Fig. 3(a), PDC-1400 was a highly oriented layerstacked structure with a thickness of about 3 μm and a lateral size of about 15 μm. The o-PDC-350-1400, exhibited in Fig. 3(c), maintained the irregular particle morphology of the raw asphalt, indicating that the sample transformed from thermoplastic to thermosetting one after oxidation stabilization, and the carbonization changed to solid-phase carbonization process.In HRTEM photographs, a graphite-like structure existed in PDC-1400, as shown in the white box region in Fig. 3(b), which reflected the graphitization degree.As shown in Fig. 3(d), o-PDC-350-1400 exhibited an obvious disordered microstructure, belonging to typical amorphous characteristics. From the selected area electron diffraction (SAED), the diffraction rings of o-PDC-350-1400 became fuzzy compared with PDC-1400, confirming that the oxidation stabilization treatment could hinder the graphitization tendency of asphalt and lead to the formation of hard carbon materials with a disordered structure, which was consistent with the results of XRD and Raman analysis.

Fig. 3 SEM images of (a) PDC-1400 and (c) o-PDC-350-1400, HRTEM images of (b) PDC-1400 and (d) o-PDC-350-1400 (inserts are the selected area electron diffraction patterns).

To study the effect of oxidation stabilization on the pore structure of hard carbon materials, physical adsorption tests were carried out for PDC-1400 and o-PDC-350-1400. As shown in Fig. 4, the specific surface area of the oxidized sample o-PDC-350-1400 increased compared with that of the unoxidized sample.Meanwhile, the average pore size changed from less than 5 nm for the unoxidized sample to 30 nm for the oxidized sample. Such large-sized mesopores could reduce the phenomenon of "queued sodium storage"and facilitate the rapid transmission of Na+.

Fig. 4 (a) Nitrogen adsorption curves and (b) pore size distributions of PDC-1400 and o-PDC-350-1400.

The electrochemical performance of the prepared samples is shown in Fig. 5. Fig. 5(a) exhibits the initial charge/discharge curves of o-PDC-T-1400 at the current density of 100 mA g−1. The PDC-1400 and the samples oxidized at low temperatures, such as o-PDC-150-1400 and o-PDC-200-1400, had low specific capacities (<100 mAh g−1, Table 3) while o-PDC-350-1400 was up to 276.8 mAh g−1with the increased oxidation temperature. The capacity below 0.1 V was attributed to the platform capacity, and that above 0.1 V belonged to the slope capacity[18]. As shown in Fig. 5(b), the platform capacities of the samples, as PDC-1400, o-PDC-150-1400 and o-PDC-200-1400, were very small, and the specific capacity mainly came from the slope capacity. Based on the results of XRD, Bommier Cet al. discovered that the(002) peak of hard carbon reversibly changed when the sodium storage occurred in the platform area and considered that Na+intercalation between graphitelike layers occurred during the process, which meant that the platform capacity was related to the microstructure parameters of graphite-like layers[19]. Table 3 clearly shows that whenTwas 150 and 200 °C, the low-temperature oxidation treatment had little impact on the carbon microstructural parameters (Lc,La,d002)of asphalt, and the specific capacity as well as the initial coulombic efficiency almost unchanged. When the oxidation temperature exceeded 250 °C,Lcreduced from 3.484 to 1.313 nm andLareduced from 5.292 to 3.440 nm whiled002increased from 0.347 to 0.363 nm. The structure transformation directly confirmed that the original carbon layers were stripped and the microcrystalline size decreased, which contributed to the intercalation of Na+in multiple directions, the sharply increased capacity of the platform area, the incremental reversible sodium storage capacity and the improved initial efficiency. The specific capacity increased from 99.8 mAh/g of o-PDC-150-1400 to 276.8 mAh g−1of o-PDC-350-1400, and the corresponding coulombic efficiency increased from 51.92% to 73.38%. Fig. 5(c) shows the rate capability of o-PDC-T-1400 at different current densities and it can be seen that o-PDC-350-1400 exhibited the most excellent rate performance.

Fig. 5 (a) Galvanostatic charge/discharge profiles, (b) column charts of slope capacity and platform capacity distribution and (c) rate performance under different current densities of the o-PDC-T-1400 series.

To further study the sodium storage mechanism and stability of hard carbons with different microstructures, cyclic voltammetry test and cyclic performance test were carried out for PDC-1400 and o-PDC-350-1400. The first three CV curves of PDC-1400 and o-PDC-350-1400 are shown in Fig. 6 (a)and (b), respectively. Two obvious reduction peaks appeared at 0.88 and 0.26 V in the first cycle of PDC-1400 and two weak reduction peaks appeared at 0.80 and 0.37 V in the same process of o-PDC-350-1400,corresponding to the irreversible reaction during the formation of SEI and the side reaction of the surface functional groups[20]. In the second and third scanning,the CV curve was in a high degree of coincidence,which indicated that the electrode had good reactivity reversibility and cycling stability. A strong oxidation peak appeared at around 2.7 V in the first cycle of the two materials, attributed to the reaction of sodium ion in defect points in the materials and oxygen-containing functional groups[21]. From Fig. 6(c), although the charge capacity fluctuated slightly, o-PDC-350-1400 showed the excellent cyclic performance in that the capacity of 170.2 mAh g−1retained after 200 cycles and the coulombic efficiency was up to 99.71%. For comparison, the abovementioned two values of PDC-1400 were 86.4 mAh g−1and 99.57%, respectively.

Fig. 6 CV curves of (a) PDC-1400 and (b) o-PDC-350-1400 at a scan rate of 0.1 mV s−1 , (c) cyclic performance of the PDC-1400 and o-PDC-350-1400 under a current density of 30 mA g−1 .

4 Conclusion

Hard carbon anode materials for SIBs were prepared successfully by air oxidation stabilization and carbonization from petroleum asphalt, and the electrochemical sodium storage performance was studied.With the increase of the oxidation stabilization temperature (T≥ 250 °C), the oxygen content in asphalt increased significantly and the free radical density during carbonization in the carbon precursor increased. Moreover, the cross-linking degree was enhanced so that the sample transformed from the thermoplastic to the thermosetting. The three-dimensional crosslinked structure generated by oxidation stabilization hindered the graphitization tendency of asphalt in high temperature pyrolysis, and the hard carbon was obtained based on that. Compared with the carbon materials directly carbonized at high temperature (PDC-1400), the hard carbon (o-pdc-350-1400)showed the excellent electrochemical sodium storage performance. At the current density of 100 mA g−1,the specific capacity increased from 99.7 to 276.8 mAh g−1and the initial coulombic efficiency increased from 51.45% to 73.38%.

Acknowledgements

The authors acknowledge the financial support by the National Natural Science Foundation of China,NSFC (21576047, U1510204, 21776040).