Introduction

Nowadays, facing the challenges of ecological environment and natural resources’ shortage, it is urgent for all industry to pursue “green” or sustainable development [1,2,3,4,5,6,7,8]. In the field of energy conversion and storage, lithium ion batteries (LIBs) is considered to be one of most efficient energy storage devices which possess high energy density, high power density, and long cycling life [9,10,11,12,13]. The anode material plays an important role in LIB. Graphite, as the dominating anode material for LIBs nowadays, possesses the theory capacity of 372 mAh g−1 which could not satisfy the requirement of LIB with high capacity [14, 15]. Silicon will become a new generation of anode material due to the advantages of high theoretical capacity (4200 mAh g−1) and low lithiation/delithiation potential [16]. However, silicon has poor cyclic performance because of poor electrical conductivity and large volume expansion (300%) during charge-discharge process [17]. In order to solve these problems, extensive researches have been carried out [18,19,20,21,22,23,24]. Designing silicon nanostructures and synthesizing Si-based composite are the primary means. Special silicon nanostructures including nanotube, nanowire, nanosphere, and nanofilm have been reported.

Silicon/carbon composite is one of the most promising composites due to its excellent electrical conductivity and structure stability of carbon [25, 26]. However, most current preparation technologies for Si-based materials are not suited for large-scale production because of the complicated process and the expensive costing. Graphite and coal tar pitch are important carbon sources to synthesize Si/C composite due to high electronic conductivity, excellent cohesiveness, and structure stability. However, simple operation by mixing three raw materials (silicon, graphite, and coal tar pitch) is unable to provide a stable structure because the raw materials could not have a perfect cladding layer.

Herein, compact-structured silicon/carbon composites are prepared via a scalable heating procedure after mixing raw materials. At the temperature (180 °C) over the softening temperature (110 °C), the coal tar pitch has a good fluidity and it is easy to form a uniform coating on the surface of materials. At the same time, the fluidic coal tar pitch could also fill the voids between particles to form compact-structured silicon/carbon composites, which is in favor of constructing conductive networks. The structure and electrochemical performance are characterized, and the relationship between them is also discussed.

Experimental

Material synthesis

All raw materials were commercial products. Through the theoretical calculation, the specific capacity design of composite should not be more than 867 mAh g−1 to avoid overlarge electrode volume variation [27]. In this study, the content of the silicon was less than 16%. The precursor was prepared by liquid solidification [28]. The weight ratio of Si, graphite, and coal tar pitch pyrolysis carbon was designed to 15:75:10. Commercial nano-silicon powders (> 99.9%,~ 80 nm, Zhongning Materials Technology Co., Ltd., Zhejiang, China) was dispersed in ethanol by sonication for 1 h to get a homogeneous dispersed suspension. Then, coal tar pitch (China Power Investment Corporation Ningxia Qingtongxia Energy Aluminum Co., Ltd.) and flake graphite powders (> 99%, ~ 1 μm, Qingdao Tianhe Graphite Co., Ltd., Shandong, China) were dispersed into the above-mentioned suspension by sonication for 2 h. After dispersing uniformly, the mixture was evaporated by stirring at 90 °C as the precursor of silicon/carbon composite. At last, the precursor was sintered at two heat-treatment stages in tube furnace and the heating rate is 5 °C min−1. The first heat-treatment stage sustained for 4 h at 180 °C (S180) in vacuum and the second heat-treatment stage sustained for 3 h at 850 °C in Ar. The materials treated at 120 °C (S120) and 200 °C (S200) during the first heat-treatment stage, and the material without the first heat-treatment stage (S0) were conducted by the same method. The preparation of compact-structured silicon/carbon composites is shown in Scheme 1.

Scheme 1
scheme 1

Preparation of compact structured silicon/carbon composites

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Material characterization

Thermogravimetric (TG) analysis of the precursor was performed on a SDT Q600 TG (TE, USA) apparatus from 0 to 900 °C at a heating rate of 5 °C min−1 in Ar and air atmosphere, respectively. The crystal structure of the composite was characterized by the powder X-ray diffraction (XRD, Rint-2000, Rigaku) in the range of 2θ = 10°–80°. The morphology of the composite was observed by scanning electron microscopy (SEM, JEOL, JSM-5600LV). The microstructure of the composite was examined by a Tecnai G12 transmission electron microscope (TEM). Nitrogen adsorption/desorption isotherms at 77.3 K was obtained using a Micromeritics ASAP 2460 surface area and porosity analyzer. The electronic conductivities of all samples were tested using ST-2722 semiconductor resistivity of the powder tester by four-point probe method.

Electrochemical measurement

The working electrode was made up of active material, Super P, and LA132 binder at a weight ratio of 87:5:8. The slurry dispersed in deionized water was spread on a copper foil and dried at 80 °C for 4 h in a vacuum-drying chamber. Electrodes were punched in the form of 12-mm-diameter disks. The thickness of electrode was controlled by the coating machine, and the mass of the active materials was kept around 1.2 mg. The button cells were carried out in an argon-filled glove box using 1 M LiPF6 in a mixture of EC/EMC/DMC (1:1:1 by volume) as the electrolyte. The electrochemical performances of button cells were tested by a Neware Battery Test System with a cut-off voltage range from 0.01 to 2.00 V (vs. Li/Li+) at room temperature. Cyclic voltammetry (CV) was recorded using a CHI660A electrochemical analyzer at a scanning rate of 0.1 mV s−1. Electrochemical impedance spectroscopy (EIS) was performed using a CHI660A electrochemical analyzer by applying 5 mV AC voltage in the frequency range of 10−2~105 Hz.

Results and discussion

Material characterizations

Thermogravimetric (TG) analysis of the precursors was performed in Ar, and the result is shown in Fig. 1a. The weight loss of precursor corresponds to the pyrolysis of the coal tar pitch, because graphite and Si are steady in argon atmosphere below 900 °C [29]. There is a little weight loss between 0 and 200 °C, which is resulted from the removal of the absorbed water in the coal tar pitch. The large weight loss from 200 to 900 °C is due to the decomposition of coal tar pitch including the release of low-molecular-weight hydrocarbons, polycondensation of aromatic molecules, and so on. The mass loss rate is changeless above 550 °C. The pyrolysis temperature is designed at 850 °C to obtain silicon/carbon composites, and the yield of the precursor retains 89.4%. TG analysis was also carried out in air atmosphere to determine the content of silicon in composite S180. Figure 1b shows the TG curves of S180 and bare silicon in air atmosphere. The weight ratio of bare silicon sample increases to 111.7 wt% at 850 °C due to the oxidation of silicon under air atmosphere, while that of S180 to 19.7 wt% due to the oxidation reaction of carbon to produce carbon oxide gas. Based on these mass loss percentage data, the silicon content is calculated to be approximately 16.7% which is slightly higher than the designed result. And the carbon content is approximately 83.3%.

Fig. 1
figure 1

a TG curve of the precursor in Ar. b TG curves of the composite S180 and silicon in air

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The XRD patterns of silicon/carbon composites are shown in Fig. 2. It is clear that four samples have the same features corresponding to silicon and graphite. The peaks at around 26.4°, 42.5°, 44.5°, 54.5°, and 78.5° are related to the (002), (100), (101), (004), and (110) diffractions of graphite (PDF no. 08-0415). The peaks at 28.4°, 47.3°, and 56.1° are correlated with the (111), (220), and (311) diffractions of silicon (PDF no. 27-1402) [30]. There is no other phase in the XRD patterns which indicates that the silicon/carbon composites are the blend of silicon, graphite, and amorphous coal tar pitch pyrolysis carbon.

Fig. 2
figure 2

X-ray diffraction patterns of the samples with different treatments

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The surface morphologies of silicon/carbon composites are illustrated by SEM (as shown in Fig. S1). The result shows that morphologies of the whole silicon/carbon composites are irregular and micron-sized particles. There are much more silicon particles distributing in the composite S180 (Fig. S1C) than other composites illustrating well integration between silicon, graphite, and coal tar pitch pyrolysis carbon. Figure 3 presents the morphology, element distribution, and microstructure of the silicon/carbon composite S180. As shown in Fig. 3a–c, S180 shows the irregular morphology, and the silicon and carbon elements are distributed uniformly on the particle surface, which demonstrates that all the raw materials are dispersed well in the composite. TEM and HRTEM are combined to further discern the microstructure of S180. It could be seen that nano-silicon particles with the size of ~ 80 nm are compactly embedded in the matrix (Fig. 3d). As shown in Fig. 3e, the crystalline plane spacing of 0.336 and 0.313 nm corresponding to graphite (002) and silicon (111), respectively, is observed in HRTEM image (Fig. 3e). Additionally, the amorphous structure marked in yellow cycles is observed not only in the carbon layer but also in the interspace between graphite and silicon which is ascribed to the filling the voids between particles by fluidic coal tar pitch over the softening temperature.

Fig. 3
figure 3

a SEM image. b, c EDAX spectra dots. d TEM image. e HRTEM image of sample S180

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In order to further determine the compactness and connection between all components in all samples, the specific surface area (SSA) was obtained by the Brunauer-Emmett-Teller equation according to nitrogen adsorption/desorption isotherms as shown in Fig. 4. Meanwhile, the textural properties of all samples are shown in Table 1. In Fig. 4, type II isotherms could be observed for S0, S120, S180, and S200, which indicates that four samples mainly contain large pores and have small SSA. The SSA and pore volume are decreased with the increase of temperature at the first heat-treatment stage. S180 has a smaller SSA of 4.0 m2 g−1 and a pore volume of 0.056 cm3 g−1 than S0 with an SSA of 7.9 m2 g−1 and a pore volume of 0.059 cm3 g−1. It demonstrates that the fluidic coal tar pitch at the first heat-treatment period fills the voids between particles reducing the SSA area and the pore volume.

Fig. 4
figure 4

Nitrogen adsorption/desorption isotherm linear plot of all samples

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Table 1 Textural properties and electronic conductivities of all samples
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Electrochemical performance

Electrochemical measurements were carried out at room temperature. Figure 5a displays the first charge-discharge curves of all the composites at 0.1 A g−1. The initial reversible capacities of S0, S120, S180, and S200 are 805.7, 705.6, 602.4, and 591.8 mAh g−1, respectively. And the corresponding initial charge-discharge efficiencies are 82.0, 83.1, 82.3, and 84.7%, respectively. Compared with S0, S180 displays lower capacity and slightly higher initial charge-discharge efficiency due to lower SSA and more compact structure probably.

Fig. 5
figure 5

a The first charge-discharge curves of all samples at 0.1 A g−1. b Cycling profiles of all samples at 0.1 A g−1. c Rate capability of all samples. d CV curves of the sample S180 for the first 3 cycles at the scanning rate of 0.1 mV s−1. Inset: CV curves at high magnification

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Figure 5b presents the cyclic stability of all samples at 0.1 A g−1 for 50 cycles. S180 displays excellent cycle performance, of which the capacity retention could reach 93.4% (562 mAh g−1) after 50 cycles. S200, S120, and S0 could reach 91.1, 57.6, and 36.8%, respectively. Even at high current density of 1 A g−1 as shown in Fig. S2, the capacity retention of S180 could keep 71.5% (343 mAh g−1) after 100 cycles, which is higher than others. The rate capabilities of S0, S120, S180, and S200 at 0.1, 0.2, 0.5, 1, and 2 A g−1 are presented in Fig. 5c. At 0.1 A g−1, the charge capacities (3rd cycle) of S0, S120, S180, and S200 are 730, 652, 607, and 596 mAh g−1, respectively. With the rise of current density, the capacity decreases. At 2 A g−1, S180 exhibits the capacity retention of 46% with respect to initial capacity at 0.1 A g−1, which is higher than that of S0 (23%), S120 (33%), and S200 (38%). S180 possesses favorable cyclic stability and rate capability due to the compact structure. The graphite served as good dispersive matrix for silicon contributes to high electronic conductivity for the silicon/carbon composite. The amorphous structure derived from coal tar pitch in the carbon layer and the interspace between graphite and silicon not only buffers the volume change of silicon but also connects graphite with silicon to construct conductive networks. Compared with the silicon/carbon composites consisting of silicon, graphite, and pitch pyrolysis carbon reported in literature (Table S1) [31,32,33,34], our silicon/carbon composites have competitive electrochemical performances. Additionally, the preparation method is simple and practicable, which is suitable for mass production.

Figure 5d shows the CV curves of S180 for the first 3 cycles at 0.1 mV s−1. As shown in inset, the broad cathodic peak at around ~ 0.8 V appears in the first cycle but disappears in the following two cycles, which is related to the formation of SEI layer on the surface of electrode due to the decomposition of electrolyte [35]. The reduction peaks between 0.01 and 0.25 V correspond to the lithium intercalation into graphite and alloying action with silicon. The oxidation peaks at 0.15 and 0.18 V correspond to the deintercalation of lithium ions into graphite while peaks at 0.25 and 0.49 V correspond to the dealloying process of silicon [36].

Additionally, we also characterize the morphologies of electrodes before and after cycling for 50 cycles at 0.1 A g−1. As shown in Fig. 6, there are obvious cracks on the electrodes of S120 and S0 after cycling, while the electrodes of samples S180 and S200 are still integrated. It is proved that the first heating procedure plays a key role in the formation of compact structured silicon/carbon composites, which is conductive to the cyclic stability of materials.

Fig. 6
figure 6

SEM images of electrodes before and after 50 cycles for sample S0 (a1) and (a2); sample S120 (b1) and (b2); sample S180 (c1) and (c2); sample S200 (d1) and (d2)

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The EIS measurements are conducted after initial cycling. The Nyquist plots of all samples are shown in Fig. 7, which consist of a semicircle at high frequency and a linear slope at low frequency corresponding to the charge transfer resistance (R2) and the Warburg resistance (Wo), respectively [37]. In addition, the intercept along X-axis indicates the electrolyte resistance (R1). The plots are fitted with the aid of an appropriate electric equivalent circuit as shown in the inset of Fig. 7, and the fitting dates are shown in Table 2. It could obviously be seen that S200 and S180 possessed smaller R1 and R2 than S120 and S0, which indicates higher exchange current density and higher electronic conductivity. The same results could also be confirmed by actual electronic conductivities of all samples. As shown in Table 1, the electronic conductivities of S0, S120, S180, and S200 are 2638, 2874, 3246, and 3322 S m−1. S200 and S180 possess higher electronic conductivity than S120 and S0, which is ascribed to that the coal tar pitch with a good fluidity at the temperature over the softening temperature is able to connect graphite with silicon to construct conductive networks.

Fig. 7
figure 7

Nyquist plots of all samples after the initial cycle

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Table 2 Electrochemical impedance parameters of samples
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Conclusions

In summary, the compact-structured silicon/carbon composites with high electrochemical performance were synthesized by sintering at two heat-treatment procedures (at 180 °C for 4 h and at 850 °C for 3 h). The specific surface area and pore volume were reduced due to the filling the voids between particles by fluidic coal tar pitch at the first heat-treatment stage. As-prepared silicon/carbon composites exhibited good electrochemical performance due to the compact structure. The results indicate that the preparation method sintering at two heat-treatment periods could be a promising method for silicon/carbon composites.