Enhancement of Electrochemical Stability about Silicon/Carbon Composite Anode Materials for Lithium Ion Batteries

Silicon/carbon (Si/C) composite anode materials are successfully synthesized by mechanical ball milling followed by pyrolysis method.The structure and morphology of the composite are characterized by X-ray diffraction and scanning electron microscopy and transmission electron microscope, respectively. The results show that the composite is composed of Si, flake graphite, and phenolic resin-pyrolyzed carbon, and Si and flake graphite are enwrapped by phenolic resin-pyrolyzed carbon, which can provide not only a good buffering matrix but also a conductive network. The Si/C composite also shows good electrochemical stability, in which the composite anode material exhibits a high initial charge capacity of 805.3mAh g at 100mAg and it can still deliver a high charge capacity of 791.7mAh g when the current density increases to 500mAg. The results indicate that it could be used as a promising anode material for lithium ion batteries.


Introduction
Rechargeable lithium ion batteries are key components of portable electronic devices and electrical vehicles due to their high energy density, long cycle life, and high power [1][2][3].In this area, the anode materials play an important role in lithium ion batteries.Though graphite is now widely used as the commercial anode material, the low theoretical capacity about 372 mAh g −1 is still an obstacle for the lithium ion batteries to further develop [4,5].In this regard, Si has attracted much attention as anode material for its high theoretical capacity about 4200 mAh g −1 [6,7].However, some shortcomings, such as the poor capacity retention resulting from the low intrinsic electronic conductivity, the over three times volume change during the Li insertion and extraction processes and the unstable solid electrolyte interface (SEI), are still the thresholds that prevent the materials from practical application [8][9][10][11].Therefore, various forms of Si-based materials have been explored to overcome these disadvantages, such as limiting the volume change by coating various materials [12,13], buffering the volume expansion by constructing new frame work for Si grains [14,15], and synthesizing nanodispersed Si particles to create enough interspace in anode materials by chemical vapor deposition (CVD) and thermal vapor deposition (TVD) [16,17].And carbon coating has been considered as the most effective and feasible method to improve the performance of Si-based materials, which promotes extensive efforts in the development of synthesis methods, such as ball milling, mechanical milling, combination of pyrolysis with mechanical milling, sol-gel, pyrolysis, and CVD/TVD [18,19].Among these techniques, the mechanical milling combining with pyrolysis is a flexible and scalable process to prepare Si/C composite material at present.Therefore, the Si/graphite composite materials coated with carbon are prepared via mechanical milling followed by pyrolysis in this work.The obtained Si/C composite used as anode material for lithium ion batteries is studied in respect of structural, morphological, and electrochemical properties.The effect of carbon coating and mechanical milling on the electrochemical performance is also investigated in the paper.

Properties Characterization.
The powder X-ray diffraction (XRD, Rint-2000, Rigaku, Japan) using Cu K radiation was employed to identify the crystalline phase of the synthesized materials.Thermal analysis of the as-prepared samples was performed on a SDT Q600 TG-DTA (TE, USA) apparatus between room temperature and 1 000 ∘ C at a heating rate of 10 ∘ C min −1 in argon atmosphere.The morphology of the samples was observed by scanning electron microscope (SEM, JEOL, JSM-5612LV) with an accelerating voltage of 20 kV, and the microstructure of the composite was investigated by transmission electron microscope (TEM, Tecnai G12, 200 kV).
The electrochemical performance was evaluated via a standard Li//LiPF 6 (EC : EMC : DMC = 1 : 1 : 1; v/v/v)//Si/C CR2025 coin cell.To fabricate the working electrode, 80 wt.% active materials, 10 wt.% acetylene black, and 10 wt.% poly(vinylidene fluoride) were firstly blended in N-methyl pyrrolidinone to obtain homogeneous slurry, and then the slurry was spread uniformly on a copper foil and dried at 120 ∘ C for 12 h in the air; the desirable working electrode can be obtained after being cut into round pieces with an area of 1.54 cm 2 .The whole assembling processes of the cell were carried out in a dry argon-filled glove box.The chargedischarge tests of the cell were performed on a Neware battery tester (Neware, Shenzhen) with cut-off voltage of 0.01-2.00V at room temperature.The electrochemical impedance measurement was performed on a CHI660A electrochemical workstation (Chenhua, Shanghai), in which the impedance spectra were recorded with AC amplitude of 5 mV from 0.01 Hz to 100 kHz.

Thermal Analysis of the Precursor.
The TG-DTA curves of the precursor are shown in Figure 1.It can be obviously observed that the TG curve presents three stages of weight loss at 25∼260, 260∼560, and 560∼1000 ∘ C, which represent the loss of absorbed water, the decomposition of phenolic resin to form excess phenolic resin-pyrolyzed carbon, and the further evaporation of phenolic resin, respectively.DTA curve displays two distinctly corresponding endothermic peaks, which correspond to the aforementioned first two stages.As for the third stage, the weight loss is not remarkable in Figure 1.It is worthy to note that the change of weight loss becomes insignificant when the pyrolysis temperature is above 750 ∘ C. Therefore, in order to obtain desirable Si/C composites, the blended precursor was pyrolyzed at 750 ∘ C in this work.220), (311), (400), and (331) plane of Si, respectively [20].These results indicate that the pyrolysis process would not destroy the basic structure of the composite material.Furthermore, the broadened diffraction peak around 2 = 23 ∘ comes from the amorphous phenolic resin-pyrolyzed carbon as shown on the upper right corner in Figure 2. The results suggest that no other impurity phase is detected in the composite and the composite is composed of flake graphite, nano-Si, and phenolic resin-pyrolyzed carbon, which can be explained that the desirable composite materials can be successfully prepared by the mechanical milling followed by pyrolysis method.It should be noted that Si/C material is looser than the precursor, which can be mainly attributed to the decomposition of phenolic resin during pyrolysis [21,22].Figures 3(c) and 3(d) show the TEM and HRTEM images of the Si/C composite, respectively.It can be obviously observed that the composite particles are composed of flake graphite, nano-Si, and phenolic resin-pyrolyzed carbon, which are in concordance with the results of XRD analysis.Furthermore, the phenolic resin-pyrolyzed carbon is coated on the surface of the composite sphere perfectly, which not only provides a good buffering matrix but also constructs the connection network of flake graphite and Si particles [23].

Electrochemical Performance.
The cut-off voltages range of the cells was chosen as 0.01-2.00V, and the discharge current density is limited at 100 mA g −1 .The voltage profiles of the Si/C composite for the 1st, 2nd, and 3rd cycle are presented in Figure 4(a), respectively.It can be seen that the first charge capacity (reversible capacity corresponding to lithium extraction) of the composite is about 805.3 mAh g −1 , and the initial columbic efficiency is 74.26%, which is a little higher than the value of the previous report [23,24].As known to us, the irreversible capacity of the first cycle is mainly attributed to the formation of a solid electrolyte interphase (SEI) film on the surface of electrode at 0.6∼0.8V.
It can be also seen from Figure 4(a) that there is a distinct potential platform below 0.2 V during the first insertion process, which can be assigned to the alloying process of the composite with lithium and the insertion of lithium ions into the carbon host.And the shift of the subsequent discharge curves presented in the following cycles may be ascribed to the typical phase transformation of silicon from crystal to amorphous [23,25,26].Another significant plateau at 0.45 V can also be found in Figure 4 80.69% after 20 cycles, respectively.Therefore, there are reasons to believe that the Si/C composite possesses better electrochemical performance, and this is associated with the effective attachment between Si, graphite, and phenolic resin-pyrolyzed carbon, which can provide good electronic conductivity and avoid direct contact between Si particles and the electrolyte to improve the electrochemical stability of the assembled cell.
The electrochemical impedance spectra of the composite anodes after different cycles are shown in Figure 5(a).It can be obviously seen that all plots are composed of a compressed semicircle in high frequency and an inclined line in low frequency, which are attributed to the charge transfer process and lithium diffusion process, respectively.To investigate the charge-discharge behavior of the electrodes, the pattern in the impedance spectra can be fitted using the equivalent circuit described as   in series with parallel (CPE 1  ct ) and  1 elements  diffusion-controlled resistance, respectively [27].It is obviously observed that the EIS curves shown in Figure 5(a) can be well fitted by the equivalent circuit demonstrated in Figure 5(b).Furthermore, the resistance of   ,  ct , and  1 increases obviously in the 20 cycles, which is due to the destruction of the electrode.However, the resistance can remain stable value after 20 cycles, which can be attributed to the buffering of the Si/C composite material to improve the electrochemical stability of the electrodes.

Conclusions
Si/C composite anode material was successfully synthesized using the simple mechanical milling followed by pyrolysis method.The particles exhibit scaly shape and are micron in dimension; the phenolic resin-pyrolyzed carbon is coated on the surface of the composite sphere perfectly and constructs the connection network of flake graphite and nano-Si particles in the composite.Therefore, this composite shows good electrochemical performance, in which the composite exhibits not only high specific capacity with high coulombic efficiency in first cycle but also good cycle and rate performance.The primary results indicate that the as-prepared Si/C composite material can be a promising anode material for high energy density and power demanding lithium-ion batteries.

Figure 2 .
Figure 2. As demonstrated in Figure 2, both XRD patterns of the precursor and Si/C composite show the diffraction peaks of flake graphite (2 = 26.6,42.5, 43.5, 54.7, and 77.6 ∘ , PDF#41-1487) and silicon (2 = 28.4,47.3, 56.1, 69.1, and 76.4 ∘ , PDF#27-1407), which corresponds to the (002), (100), (101), (004), and (110) plane of flake graphite and the (111), (220), (311), (400), and (331) plane of Si, respectively[20].These results indicate that the pyrolysis process would not destroy the basic structure of the composite material.Furthermore, the broadened diffraction peak around 2 = 23 ∘ comes from the amorphous phenolic resin-pyrolyzed carbon as shown on the upper right corner in Figure2.The results suggest that no other impurity phase is detected in the composite and the composite is composed of flake graphite, nano-Si, and phenolic resin-pyrolyzed carbon, which can be explained that the desirable composite materials can be successfully prepared by the mechanical milling followed by pyrolysis method.

Figure 3 :
Figure 3: SEM images of the precursor (a) and the Si/C composite (b), TEM (c), and HRTEM (d) images of the Si/C composite.

1 )(mAh g − 1 )Figure 4 :
Figure 4: (a) The voltage profiles of Si/C composites at 100 mA g −1 ; (b) the charge-discharge cycling curves of the Si/C composite with and without ball milling at 100 mA g −1 ; (c) initial charge-discharge curves of the composites at different rates; (d) cycling curves of the composites at different rates.
, in which   is attributed to the ohmic resistance of the electrolyte and electrodes,  ct represents the charge transfer resistance of electrochemical reactions, and CPE 1 and  1 are the capacitance of the interface and Warburg

Figure 5 :
Figure 5: EIS curves of Si/C during different cycles (a) and the equivalent circuit for the impedance spectra (b).