Electrochemical performance enhancement of porous Si lithium-ion battery anode by integrating with optimized carbonaceous materials

Journal Pre-proof Electrochemical performance enhancement of porous Si lithium-ion battery anode by integrating with optimized carbonaceous materials Wentao Yang, Hangjun Ying, Shunlong Zhang, Rongnan Guo, Jianli Wang, WeiQiang Han PII:

S0013-4686(20)30078-5

DOI:

https://doi.org/10.1016/j.electacta.2020.135687

Reference:

EA 135687

To appear in:

Electrochimica Acta

Received Date: 18 November 2019 Revised Date:

4 January 2020

Accepted Date: 9 January 2020

Please cite this article as: W. Yang, H. Ying, S. Zhang, R. Guo, J. Wang, W.-Q. Han, Electrochemical performance enhancement of porous Si lithium-ion battery anode by integrating with optimized carbonaceous materials, Electrochimica Acta (2020), doi: https://doi.org/10.1016/ j.electacta.2020.135687. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.

Author contributions Wentao Yang: Conceptualization, Methodology, Validation, Investigation, Writing Original Draft. Jianli Wang: Resources, Investigation Shunlong Zhang: Resources, Writing - Review & Editing, Rongnan Guo: Resources, Writing - Review & Editing, Hangjun Ying: Writing: Review & Editing. Wei-Qiang Han: Writing: Conceptualization, Review & Editing, Funding acquisition

Electrochemical performance enhancement of porous Si lithium-ion battery anode by integrating with optimized carbonaceous materials Wentao Yang, Hangjun Ying, Shunlong Zhang, Rongnan Guo, Jianli Wang, and Wei-Qiang Han* School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, P. R. China E-mail: [email protected]

Abstract Silicon (Si) is regarded as a promising negative material for Li-ion batteries (LIBs), but the poor cyclic performance limits its practical applications. Herein, we used several different carbonaceous materials as carbon sources to form Si/C composites, which was formed by etched Fe-Si alloys followed by mechanical ball mill mixing with carbon. It is found that the artificial graphite as the carbonaceous material can form a well-distributed mixture during the ball milling, and then uniformly coated with amorphous carbon pyrolyzed by the phenolic resin. The pore structure of Si particles can provide rapid diffusion for lithium ions, resulting in improving the electrochemical properties. The well-coated carbon layer promotes the formation of stable SEI layer on the composites surface, which is advantageous for the long cycle performance. The carbonaceous materials (artificial graphite, flake graphite and soft carbon) have remarkable influence on the electrochemical performance of Si/C composites. The silicon/graphite-artificial graphite (Si/C-AG) exhibits the best performance among these three Si/C composites. It delivers a specific capacity of 445 mA h g-1 at 0.5 A g-1 with a retention of 94% after 200 cycles. This work would be helpful with choosing suitable carbonaceous materials for the Si/C composites. Keywords: Li-ion battery, anode material, porous silicon, carbon

1. Introduction Lithium ion batteries (LIBs) have been applied to portable electrical devices, electric vehicles (EVs), hybrid electric vehicles (HEVs), and smart electric grid systems owing to their advantages of high energy density, good rate capability, long cycle life and low self-discharge rate.[1-3] However, as the most important component in commercial LIBs, graphite cannot satisfy the demanding requirements due to its low capacity of 372 mA h g-1 and 850 mA h cm-3, and safety problems caused by the low operation potential.[4-6] Therefore, developing new anode materials is the key to improve the performance of lithium ion batteries. Substantial efforts have been dedicated to find alternative high-performance anode materials.[7-9] For example, Li4Ti5O12 has been used in energy storage batteries because of high operating potential (1.55 V vs. Li/Li+) and no volume variation during Li insertion/extraction.[10-12] Li3VO4 could provide a theoretical capacity of 592 mA h g-1 with a lower work potential (0.5 V vs. Li/Li+) and shows fantastic performance under high current density.[13, 14] Silicon (Si) was once considered to be the most likely choice owing to the advantages of high specific capacity (Li15Si4, 3580 mA h g-1), low charge potential (~0.4 V vs. Li/Li+) and abundant reserves.[15, 16] However, the severe volume change (~300%) of Si during lithiation/delithiation processes leads to severe material pulverization, and subsequent capacity degradation.[17, 18] And its poor electronic conductivity limits the transmission of Li ions, resulting in

inferior rate performance.[17, 18] And its poor

electronic conductivity limits the transmission of Li ions, Si shows inferior rate performance as anode in LIBs.[16] Those defects make Si cannot show absolute superiority compared to other anode materials. Solving these problems of Si is still a hot issue in Si-based anodes studies. In order to solve the problems of silicon electrode capacity loss caused by large volume change during the lithiation/delithiation processes, lots of researches have been expanded. There are two main modification approaches, one is to design and construct different structures of silicon, such as nanoparticles,[19, 20] porous structure,[21-23] nanowires, [24-26] nanotubes,[27] yolk-shell constructions,[28, 29]

etc. These structures exhibit fast electron movement, good structural stability, and short ion diffusion path. However, the drawbacks of uneconomical synthesis process, low tap density, and accelerated side reaction still limit the practical application of these materials.[30, 31] The micro-sized porous Si materials have exhibits promising prospect of practical application, and many studies about preparing high-performance micro-sized porous Si with a large-scale and low-cost method have made great breakthroughs.[16, 32-34] Design of Si-based composites is the other modification approach of Si anode materials. The silicon particles are dispersed in or coated with inactive or less active materials to release the mechanical stress resulting from the volume expansion. Carbonaceous materials, including amorphous carbon,[34-37] graphene,[38] CNTs[39, 40] or reduced graphene oxide,[41, 42] are the representative buffering materials in Si-based composites. Si-based composites combined with carbonaceous materials have obtained satisfactory cycling stability and rate performance. Though, due to complex synthesis of materials or expensive raw material and equipment, it can’t completely replace graphite. Thus, designing a silicon-carbon composite with low-cost raw materials, and simple preparation method has high practical value. In this study, we designed and synthesized Si/C@C structure composites by a simple ball milling and carbonization method, to explore the effects of different commercial carbon materials on the electrochemical properties of micro-sized porous Si. Artificial graphite, flake graphite and Soft carbon are mixed with porous Si and coated with amorphous carbon pyrolyzed by the phenolic resin to obtain Si/C@C composites. It is found that the artificial graphite has the best modification effect on the electrochemical properties of porous Si. Comparing to the flake graphite and soft carbon, the composites combined with artificial graphite have more regular morphology and are uniformly coated with amorphous carbon to form a three-dimensional carbon skeleton, which is advantageous to structure stability and electronic transmission. As a result, the Si/C-AG composites show excellent cycle stability and rate capacity.

2. Experimental 2.1 Material preparation Artificial graphite (AG), micro-sized porous silicon prepared from Fe-Si alloy by acid etching method,[16] amorphous carbon source viz. phenolic resin was used as raw materials for preparing Si/C-AG composite anodes. Micro-sized porous silicon and artificial graphite were added into the phenolic resin ethanol solution with a mass fraction of 25% and milled at the speed of 400 rpm for 10 h in a high energy ball mill machine with 40 g iron ball. The atomic ratio of Si to artificial graphite was 1:4. The mixture above was collected into corundum boat and pyrolyzed in tube furnace at 800 for 3h under Ar/H2 (95%/5%, vol) atmosphere at a heating rate of 5 composite was cooled to room temperature (25

min-1. Then the

). And then Si/C-AG composites

was obtained. The composite synthesized by the same progress without phenolic resin was named Si/AG. In order to compare the effect of different carbon materials, artificial graphite was replaced by flake graphite (FG) and soft carbon (SC) to prepare Si/C-FG and Si/C-SC composites. 2.2 Material characterization The structure of the samples was characterized by X-ray diffraction (Shimadzu XRD-6000) with filtered Cu Kα radiation from 2θ = 20° to 60°. Raman spectra were operated on a Renish in Via Raman system. The Surface chemistry and bonding characterizations were conducted with X-ray photoelectron spectroscopy (XPS) in Thermo Fisher Scientific ESCALAB 250Xi. The specific surface areas and pore volume measurements were taken using Micromeritics ASAP 2020 Plus HD88 on Brunauer-Emmett-Teller (BET) method. The scanning electron microscopy (SEM) image was obtained by Zeiss Ultra 55 field-emission microscopy. 2.3 Electrochemical measurements The electrochemical performance was estimated by assembly of 2032-type coin cell with the Si/C composites as working electrodes. Active material, conductive additives (Super P), and polyvinylidene fluoride with a mass ration of 8:1:1 were mixed and stirred for over 10 h to form a homogeneous slurry. Then the slurry was

uniformly scraped onto the Cu foil. after being dried in the vacuum at 70

for 12 h, it

was can into 14 mm diameter circular disk. The loading of each electrode was ca.1.0 mg cm-2. Finally, the half-cells were assembled in an Ar-filled glovebox with the lithium foil as counter/reference electrode, 1.0 M LiPF6 solution in a mixture of fluoroethylene

carbonate/dimethyl

carbonate/ethyl

methyl

carbonate

(FEC/DMC/EMC, 1 : 1 : 1 in volume) as the electrolyte, and a polypropylene film (Celgard-2400) was used as a separator. The galvanostatic charge/discharge tests were conducted on a LAND instrument at different current density between 0.01 and 1.2 V. the cyclic voltammetry (CV) was performed at the scan rate of 0.1 mV s-1 with a potential window of 0.01-1.5V vs. Li/Li+. EIS was recorded with the frequency ranging from 1 MHz to 0.001 Hz in alternating current voltage of 5 mV.

3. Result and discussion The XRD patterns of as-prepared composites of the Si/C-AG, Si/C-FG, Si/C-SC and the raw materials are shown in Figure 1. The distinct peaks appearing at 28.4°,47.3°,56.1° belong to (111), (220) and (311) lattice face of silicon (PDF#27-1402), respectively. The peaks around 26° and 54° can be assigned to the diffraction planes of graphite. The flake graphite, whose peaks centered at 26.6° and 54.8°, belongs to graphite-3R (PDF#26-1079). And the artificial graphite, whose peaks appear at 26.4° and 54.5°, belongs to graphite-2H (PDF#41-1487) (Figure S1). Soft carbon is amorphous carbon, only a hump at around 26° can be observed. The peaks of silicon, graphite and soft carbon can be observed at the XRD patterns of Si/C-FG, Si/C-AG and Si/C-SC composites, suggesting the successful combination of Si and carbon. The intensity of the silicon peaks is weaker, which can be explained by the low content of silicon in composites. As observed in Figure S2, the content of Si in Si/C composites is calculated to be about 21.5 ωt% by TGA.

Figure 1. (a) XRD patterns of Si, AG and Si/C-AG. (b) XRD patterns of Si, FG and Si/C-FG. (c) XRD patterns of Si, SC and Si/C-SC. (d) Raman spectrum of Si, AG and Si/C-AG.

Raman spectroscopy was applied to further study the structure of the samples. As shown in Figure 1d and Figure S3, the pure silicon shows a strong adsorption bands at a Raman shift of 515 cm-1 and a weak adsorption band at 950 cm-1. While the graphite shows two characteristic bands at 1350 and 1580 cm-1, which are identified to the D-band and G-band. The D-band arises from disordered structure of graphite and usually found in the disorder or amorphous carbon. Whereas the G-band is related to bond stretching of the in-plane sp2 carbon atoms.[40] The ratio (R) of ID:IG can reflect the graphitization degree of carbon material. A lower R value indicates a higher

graphitization degree of carbon.[21, 38] The R value of artificial graphite is 0.27, indicating a high degree of graphitization. Comparing to the artificial graphite, the R value increased to 0.60 in Si/C-AG (Figure 1d), and the R value also arises from 0.27 of flake graphite to 0.58 of Si/C-FG (Figure S3a), suggesting the increase of disordered carbon in the composites after ball milling and pyrolyzation of resin.

Figure 2. The SEM images of Si(a), AG(b), Si/AG(c), and Si/C-AG(d).

The microscopic morphology of the raw materials and composites was observed by the SEM. Figure 2a exhibits the SEM image of pure silicon material. As we can see, the particles size of silicon is micron-sized and the agglomeration is not obvious due to the large size and low surface energy.[43, 44] Figure 2b is the SEM image of artificial graphite with bulk shapes and the particle size is approximate 10 µm. The flake graphite has a layered structure with smooth surface and an irregular shape with sharp boundaries (Figure S4a). The particle size of the flake graphite relatively ranges from 1 to 10 µm and the thickness is about hundreds of nanometers. Compared to the flake graphite, artificial graphite is thicker and the particle size is much more homogeneous. The soft carbon has spherical shapes and the size is about 3-10 nm

(Figure S4b). Figure 2c is the typical SEM image of Si/AG sample, the artificial graphite has undergone obvious breakage due to ball milling process, and as the EDX analysis results shown in Figure S5, the graphite and silicon are well mixed. The Si/C-AG is well coated by the carbon pyrolyzed from resin. (Figure 2d) The Si/C-FG is somehow similar to Si/C-AG, but the carbon coating is not complete, a part of flake graphite structure without coating can be observed (Figure S4c). The regular block shape and even particle size of artificial graphite are more beneficial to the uniform coating of amorphous carbon. The amorphous carbon obtained by pyrolysis of phenolic resin can provide effective constraint force to prevent the separation of silicon and graphite particles. Trustingly, this complete carbon layer can enhance the structural strength of the composite and provide convenience for the transmission of electron, resulting in the improvement of the electrochemical performance.[45, 46]

Figure 3. (a) TEM image of Si/C-AG, (b) HR-TEM images of Si/C-AG (b), (c) SEAD pattern of the Si/C-AG (c), (d-f) EDX results of Si/C-AG.

The structure of the porous Si and Si/C composites was investigated by the TEM. The porous structure of micro-Si prepared by acid etching method can be clearly observed by high-magnification TEM images (Figure S6ab). Figure 3a and Figure S7

show the good combination of Si particles and carbon materials and uniform coating of amorphous carbon on the surface. The elliptical-shaped Si particles are of 50-500 nm in diameter after ball milling, which are smaller than that of pure Si particles. This indicates that the ball milling process shows a significant effect on grain refinement of Si. After compositing with graphite and coated by amorphous carbon, there exists more void space between the Si particles and AG or amorphous carbon, and the pore sizes are around 5-30 nm, which is advantageous to the transport of Li ions and alleviation of volume expansion of Si (Figure S6cd). Figure 3b shows the HR-TEM images of the Si/C-AG composites, a lattice distance of 0.191 nm is corresponding to the d-spacing value of the Si (220) crystal plane. While the lattice distance of 0.355 nm is corresponding to the graphite (002) crystal plane. As the selected area electron diffraction (SEAD) patterns show in Figure 3c, crystalline Si are clearly visible, and bright rings belong to the (111) and (220) planes of crystal Si, respectively. Presence of amorphous carbon is verified from the diffuse haloes in the SEAD. The result of EDX further indicates that the uniform distribution of Si in the carbon matrix.

Figure 4. the XPS spectrum of Si/C-FG, Si/C-AG and Si/C-SC composites.

Figure 4 shows the XPS spectra of Si/C-FG, Si/C-AG and Si/C-SC. The peaks of C 1s, and O 1s could be clearly found. The O 1s peaks can be mainly ascribed to C-O, C=O and Si-O types of O atoms (Figure S8 a-c).[47] The peaks located at 155 eV and 105 eV can be assigned to Si 2s and Si 2p (Figure 4b). However, because of coating

of amorphous carbon, these peaks can hardly be observed and the peaks of Si/C-AG are further lower than the others. This phenomenon echoes the above SEM images, suggesting the more uniform coating of AG as the carbon sources36-38. In contrast, there are some silicon particles exposed on the surface of the material when used the other carbon sources.[48-50] The Si 2p can be ascribed to Si-O, which indicates the surface of Si is partially oxidized during the ball milling process (Figure S8 d-f). The Brunauer-Emmett-Teller (BET) result is shown in Figure 5 and Figure S9. The specific area of artificial graphite, Si, and Si/C-AG composite are 4, 33 and 165 m2 g-1, respectively. As exhibited in Figure 5a and S9a, the Si and Si/C-AG, Si/C-FG, Si/C-SC show the type-

adsorption, indicating the presence of mesopores and

macropores.[51] While the N2 absorption and desorption of artificial graphite are almost coincident. Figure 5b indicates the pore size distribution of the samples. The average pore diameters are mainly from 5 to 20 nm, which is consistent with the TEM images results. It is obvious that there are more pores in Si/C-AG than raw material because of the ball-milling and carbon coating processes. The Si/C-FG and Si/C-SC samples have more macropores (Figure S9b). As previous discussed, more pores can provide favorable transport routes for electrolyte and facilitate the diffusion of Li+ and improve the rate performance.

Figure 5. (a) The N2 absorption and desorption curves of Si, AG and Si/C-AG and (b) the corresponding pore size distribution.

Figure 6a shows the first charge/discharge profiles of Si/C-FG, Si/C-AG,

Si/C-SC and Figure S10bc are that of Si, graphite and soft carbon with voltage between 0.01-1.2 V (vs. Li/Li+) and at a current density of 0.05 A g-1. It can be seen that the first discharge specific capacities of Si/C-FG, Si/C-AG and Si/C-SC are 800, 916 and 1041 mA h g-1, and the initial coulombic efficiency (CE) are 59, 64 and 44 %, respectively. And for the flake graphite, artificial graphite, soft carbon and pure Si, the discharge specific capacities are 458, 414, 326, and 3453 mA h g-1. The value of CE can be used to estimate the effective utilization efficiency of the active material in the discharge/charge cycles. The decrease of initial CE of Si after ball milling and carbon coating can be explained by the increase of amorphous carbon generating from ball milling and pyrolysis of resin, which will provide more irreversible capacity. There is more amorphous carbon in Si/C-SC composites than Si/C-AG and Si/C-FG. The amorphous carbon will bring lithium storage on the surface and irreversible lithium insertion in defects, which delivers a larger discharging capacity above 0.1 V in Si/C-SC.[52, 53] Figure 6b shows the specific capacity and columbic efficiency vs. the cycle number of pure Si and Si/AG at a current density of 0.5 A g-1 after activation at 0.05 A g-1 in the initial two cycles. The capacity of porous Si electrode drops dramatically in the first 10 cycles and after 100 cycles it declines to 1461 mA h g-1, only maintaining 58 % of the third cycle value. The Si/AG also only remains a specific capacity of 651 mA h g-1 and maintaining 64 % of the third cycle value. The cycling performance of Si/C-AG Si/C-FG and Si/C-SC at the current of 0.5 A g-1 is exhibited in Figure 6e. After coating of amorphous carbon, the cycling performance of the composites is obviously improved, which can be ascribed to the anchoring effect of amorphous carbon layer on the Si particles. The Si/C-FG, Si/C-AG and Si/C-SC deliver a specific discharge capacity of 415, 445 and 321 mA h g-1 after 200 cycles respectively, which is 104, 94 and 81 % of the third cycle value. The reversible specific capacity of Si/C-FG and Si/C-AG anode continues to increase in the initial tens of cycles, which can be attributed to the activation of active materials.[54] The cycling performance of Si/C-FG, Si/C-AG and Si/C-SC anodes is

further assessed at a large current density of 1 A g-1 (Figure 6f). The initial discharge specific capacities of Si/C-FG, Si/C-AG and Si/C-SC are 329, 369 and 275 mA h g-1, respectively. After 500 cycles, the reversible specific capacities of Si/C-FG and Si/C-AG slowly decline to 303 and 334 mA h g-1, with a capacity retention of 92.1 and 90.5%. However, the specific capacity of Si/C-SC is only 183 mA h g-1 after 500 cycles, even worse than graphite (Figure S11b). As shown in Figure 6d and S13a, the Si/C-AG also exhibits the best rate performance among these three samples. The discharge capacities of Si/C-AG are 563, 549, 532, 493, 424, and 350 mA h g-1 at 0.05, 0.1, 0.2, 0.5, 1, 2 A g-1, respectively. When the current density returns to 0.05 A g-1, the average specific capacity can nearly recover to original level (560 mA h g-1, with almost 99.5% recovery).

Figure 6. Electrochemical performance: (a) the charge/discharge voltage profiles for the first cycle of Si/C-FG, Si/C-AG and Si/C-SC composites, (b) cycling performance

of pure Si and Si/AG, (c) CV curves of the initial five cycles of Si/C-AG, (d) rate performance of Si/C-FG, Si/C-AG and Si/C-SC composites, (e) cycling performance of Si/C-FG, Si/C-AG and Si/C-SC composites at 0.5 A g-1, (f) cycling performance of Si/C-FG, Si/C-AG and Si/C-SC composites at 1A g-1.

Figure 6c, Figure S12 and S13b show the CV profiles of Si/C-FG, Si/C-AG, Si/C-SC composite and pure Si anode. In the cathodic curves of the first cycle, a weak peak at around 0.8 to 1.0 V can be ascribed to the formation of SEI layer.[55] Obvious peaks are observed at 0.26, 0.18 and 0.08 V, which are ascribed to lithiation of crystalline Si and graphite, respectively.[56] In terms of the anodic curves, distinct peaks located at 0.24, 0.26 and 0.46 V are ascribed to delithiation of Li ion from graphite, LixSi, respectively. Cathodic curve in the first cycle is slightly different with the latter cycles, which may be caused by the formation of SEI layer in the initial lithiation process. The intensity of the peak gradually increases in the subsequent cycle, indicating that the reactivity of the material is continuously enhanced and the capacity increases.[57] The CV curves of Si/C-SC are quite different from those of Si/C-FG and Si/C-AG (Figure S12). The decrease of the peak intensity corresponding to the lithiation/delithiation of carbon suggests the weak phase transition in amorphous carbon during the electrochemical process.

Figure 7. the Nyquist plots of the Si/C-FG, Si/C-AG and Si/C-SC before cycling (a), after 100 cycle(b).

Figure 7 shows the electrochemical impedance spectroscopy (EIS) of Si/C-FG, Si/C-AG and Si/C-SC. The semicircle in the high-frequency region is attributed to the charge transfer (Rct) and a linear tail in the low frequency region is ascribed to the solid-state diffusion of lithium into the bulk of electrode material. [58]The fitted Rct values of the Si/C-FG, Si/C-AG and Si/C-SC before cycles are 75, 50 and 145 Ω, respectively, indicating the best electrical conductivity of Si/C-AG. After 100 cycles, the fitted values of the Si/C-FG, Si/C-AG and Si/C-SC are 19, 12 and 22 Ω. The Si/C-AG electrode still exhibits the best kinetics, suggesting the best structural stability of Si/C-AG (Figure 7b). The EIS curve of Si/C-SC presents two semicircles, which can be explained by the continual increase of the thickness of SEI layer during cycling.[59]

Figure 8. SEM images of prepared electrode. (a) Si/C-FG, (b) Si/C-AG, (c) Si/C-SC. SEM images of electrode after 100 cycles. (dg) Si/C-FG, (eh) Si/C-AG, (fi)Si/C-SC.

Figure 8a-c present the SEM images of fresh electrode prepared by Si/C-FG, Si/C-AG and Si/C-SC, respectively. The electrode slurry was manually coated onto

the Cu foil and without additional roller-compression process, so all the fresh electrodes show a porous morphology. After 100 charge/discharge cycles, the porous structures of Si/C-FG and Si/C-AG electrodes still exist (Figure 8de). Furthermore, spherical nano-particles appear on the surface of composites, which strengthen the contact between the particles (Figure 8gh). This morphology variation of electrode in cycling was defined as electrochemical activation and electrochemical reconstruction, and these electrochemical activation and reconstruction will give the self-adaptive high reaction activity, electronic conductivity and stability. This is consistent with the electrochemical performance, including the increase of capacity in the initial cycles, decline of Rct and excellent cyclic performance of Si/C-FG and Si/C-AG anodes.[60, 61] However, the Si/C-SC shows a different surface morphology (Figure 7fi), the anode material is covered by dense SEI film, consistent with the EIS results, which is also responsible for the loss of specific capacity.

4. Conclusion In conclusion, we have synthesized three Si/C composites by combining micro-sized porous Si with three commercial carbon materials, including artificial graphite, flake graphite and soft carbon. The structure and morphology properties, as well as electrochemical performance are studied systematically. All of the Si/C composites show a porous structure, which can not only release the suppression of volume expansion related to silicon but also provide transport routes for electrolyte and facilitate the diffusion of Li+. Compared to flake graphite and soft carbon, artificial graphite is the optimal matrix for porous Si, it can form a well-distributed mixture during the ball milling. The amorphous carbon coating on the surface of the composites can remarkably improve the cycling stability of Si/AG composite. The well coating amorphous carbon layer of Si/C-AG can promote the formation of stable SEI layer on the composites surface, which is advantageous for the long cycle life and better rate performance. The Si/C-AG composite exhibits a reversible specific capacity of 473 mA h g-1 and maintain 445 mA h g-1 after 200 cycles at 0.5 A g-1. It also shows excellent rate capability with 350 mA h g-1 retention at a high current

density of 2 A g-1. This work will provide the reference for the selection of carbonaceous materials on facile and large-scale synthesis of Si/C composites.

Conflicts of interest There are no conflicts to declare.

Acknowledgements This work has received financial support from the National Natural Science Foundation of China (Grant No. 51901206)

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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: