Author’s Accepted Manuscript Carbon-coated isotropic natural graphite spheres as anode material for lithium-ion batteries Xuan Wu, Xuelin Yang, Fei Zhang, Liangting Cai, Lulu Zhang, Zhaoyin Wen www.elsevier.com/locate/ceri
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S0272-8842(17)30729-0 http://dx.doi.org/10.1016/j.ceramint.2017.04.123 CERI15103
To appear in: Ceramics International Received date: 15 December 2016 Revised date: 29 March 2017 Accepted date: 21 April 2017 Cite this article as: Xuan Wu, Xuelin Yang, Fei Zhang, Liangting Cai, Lulu Zhang and Zhaoyin Wen, Carbon-coated isotropic natural graphite spheres as anode material for lithium-ion batteries, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2017.04.123 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. 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.
Carbon-coated isotropic natural graphite spheres as anode material for lithium-ion batteries Xuan Wua, Xuelin Yanga*, Fei Zhanga, Liangting Caia, Lulu Zhanga, Zhaoyin Wenb
College of Materials and Chemical Engineering, Hubei Provincial Collaborative Innovation Center
for New Energy Microgrid, China Three Gorges University, 8 Daxue Road, Yichang, Hubei 443002, China b
Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Ding xi Road, Shanghai 200050, PR China
Abstract: Carbon coated isotropic natural graphite spheres (INGS/C) has been prepared by spray granulation using the by-product of spherical natural graphite (i.e., superfine graphite powders) as starting materials. In INGS/C, small graphite flakes are packed in disordered orientation and form a isotropic structure, which may effectively reduce local stress concentration appearing in the process of Li+ insertion/extraction. The pyrolytic carbon decomposed from polyvinyl alcohol (PVA) and citric acid is connecting the small graphite flakes and coating on the surface of INGS, respectively, thus forms a continuous conductive network to improve the electron transmission and maintain the
micro-structural integrity of isotropic natural graphite spheres as well. With the help of synergetic effect between the isotropic structure of INGS and the continuous conductive network, the INGS/C electrode exhibits superior rate capability of 346 mAh g-1 at 3 C at low potential below 0.25 V.
Keywords: Lithium-ion battery; Anode material; Natural graphite; Carbon coating; Isotropic.
1. Introduction Since Sony introduced a carbon material instead of lithium metal as anode electrode in the first commercial lithium ion cell , carbon materials (i.e., natural graphite [2, 3], artificial graphite , carbon nanofibers , carbon nanotubes  and graphene , etc.) have been widely investigated as alternative anodes in rechargeable lithium ion batteries. With the increasing application of lithium-ion batteries in portable electronic devices [8, 9], the anode materials with high capacity, long cycling life, excellent high rate performance and low cost have been attracted much attention in recent years. Among them, natural graphite has been regarded as the most desirable candidate due to its low cost and desirable potential plateau . In practical application, however, the large initial irreversible capacity loss, poor cycling life and unsatisfied rate
performance of natural graphite limit its large-scale application. The initial irreversible capacity and cycling life of graphite anode materials depend essentially on the stability of the solid electrolyte interface (SEI) layer formed in the first discharge process at ~0.75 V vs. Li+/Li [11, 12], and the rate capability is associated with electrical conductivity and the Li+ diffusion [13, 14]. As for conventional graphite anodes, the poor rate capability stems from the relatively low intrinsic conductivity of graphite and the long Li+ diffusion distance in the spherical natural graphite (SNG) particles (Φ 17-20 μm). Furthermore, a large number of graphite micro-powders (Φ3-8 μm) as a by-product will be obtained during the production process of SNG, leading to a low productivity ratio (~50%). The by-product of SNG (graphite micro-powders) is donated as PSNG. Up to now it’s still difficult to recycle the PSNG directly as anode active materials. In this paper, we fabricated a novel anode material of carbon-coated isotropic natural graphite spheres (INGS/C) by spray granulation using PSNG as starting material. The isotropic structure is designed to reduce local stress concentration appearing in the process of Li+ insertion/extraction to improve the structural stability, and the carbon coating is intended to improve the electrical conductivity. The structure, morphology and electrochemical performance of INGS/C were systematically investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electrochemical measurements. For comparison, the INGS composite without carbon-coating was also prepared by the same process. As a result, the INGS/C electrode is expected to exhibit better rate capability and cycling stability
than INGS. Meanwhile, the excellent charge transfer property of carbon-based materials promises versatile applications in upcoming nanotechnology-based disease diagnoses [15, 16, 17].
2. Experimental 2.1 Preparations of INGS Firstly, the superfine graphite powders was prepared by high energy ball milling using PSNG as starting material, and the particle size was examined by a laser mastersizer. The results show that the average diameter of the superfine graphite powders is below 1 μm (D50=0.75 μm, D90=2.21 μm). Secondly, the above superfine graphite powders (100 g), PVA (5 g) and deionized water (400 ml) were mixed by planetary ball milling at 400 rpm for 12 h (ball/graphite mass ratio of 20:1). The resulting slurry was dried instantly through a spraying granulator, and then sintered at 2800 oC for 3 h under the flow of argon in a medium-frequency induction furnace to obtain the INGS composite. To prepare the carbon-coated INGS composite, the above INGS (10 g), citric acid (1 g) and deionized water (150 ml) were mixed in a beaker and stirred for 4 h. The resulting mixture was dried (50 oC) in air overnight, followed by sintering at 900 oC for 3 h under the flow of nitrogen. The final product was referred as INGS/C. The schematic presentation of the process for INGS/C is illustrated in Fig. 1. 2.2 Materials characterization The phase of the obtained samples was examined by powder X-ray diffraction (XRD, Rigaku Ultima Ⅳ) using Cu-kα radiation. Diffraction patterns were obtained
over the range of 2 between 10 º and 90 º in a step of 8º·min-1. The morphology was observed using scanning electron microscopy (SEM, JEOL, JSM-7500F) and transmission electron microscopy (TEM, Tecnai G2 F30). The Raman spectrum was performed on Raman spectrometer (VERTEX 70, Bruker). N2 adsorption and desorption isotherms at 77 K were recorded with an ASAP 2020. 2.3 Electrochemical measurements Firstly, the working electrodes were fabricated by coating a slurry on a copper foil, the slurry was obtained by mixing active material (90 wt %) and polyvinylidene fluoride (PVDF, 10 wt. %) dissolved in N-methyl-2-pyrrolidinone. Then, the coated copper foil was dried and cut into disks (Φ 14 mm) and dried in vacuum (120 ºC) for 12 h. The electrodes were assembled into CR2025 coin cells in an argon-filled glove box (MIKROUNA, Super 1220/750) using lithium foil as the counter and reference electrodes, Celgard 2400 as the separator, and 1 M LiPF6 in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) (v/v=1:1) as the electrolyte. The galvanostatic charge/discharge measurements were carried out using a battery measurement system (LAND CT2001A) in a voltage range of 0.001-1.5 V vs. Li+/Li at room temperature. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements were performed using an electrochemical working station (CHI614C) at room temperature. CV curves were monitored at a scanning rate of 0.1 mV s-1 within a voltage range of 0-1.5 V, and EIS spectra were obtained in a frequency range from 0.01 Hz to 100 kHz.
3. Results and discussion
3.1 Structure and morphology Fig. 2 shows the XRD patterns of PSNG, INGS and INGS/C composites. For PSNG powders, only one broad but weak diffraction peak (26.4 °) can be detected, which demonstrates that the crystal structure of natural graphite has been damaged by high energy ball milling. For INGS and INGS/C, diffraction peaks located at 26.4°, 42.2°, 44.4°, 54.5° and 77.2° can be observed, corresponding to the (002), (110), (101), (004), and (110) planes of graphite (JCPDS, No. 41-1487). The average layer spacing for (002) plane was calculated to be 0.3361 nm, which indicates that the high-temperature heat treatment (2800 oC) is effective for the structural reconstruction of the milled PSNG with amorphous feature. The morphology of INGS and INGS/C are shown in Fig. 3. Obviously, the INGS (Fig. 3a) before heat treatment displays a regular, uniform and spherical structure resulting from spray drying technology. When the slurry were sprayed from nozzle, the milled PNGS were bonded together to form spherical precursor with the help of PVA. As shown in the cross-sectional SEM image (Fig. 3b) obtained by cutting the INGS by liquid nitrogen, it can be clearly seen that the INGS is composed of small graphite flakes packed in disordered orientation, and therefore the INGS will be of isotropic feature overall. Furthermore, as shown in Fig. 3e and Fig. 3f, this isotropic structure can also be well maintained in the electrode fabricated by using the INGS/C spheres. Compared with the traditional SNG anode material, the isotropic INGS can effectively alleviate
insertion/extraction of Li+. Furthermore, there are a large number of micropores in
INGS, which is benefit to electrolyte penetration and volume-expansion release. In order to explore the microstructure of INGS/C, HRTEM was used to check the carbon structure coating on the INGS. As shown in Fig. 3c, the pyrolytic carbon from PVA and citric acid is connecting the small graphite flakes in INGS and coating on the surface of INGS to form a continuous conductive network, which is good for improving the electron transmission. Furthermore, the lattice spacing of 0.3362 and 0.2065 nm corresponds to the (002) and (101) planes of graphite, respectively. It can be also seen clearly that the small graphite flakes are wrapped by amorphous carbon with a thickness of ~2 nm. Similar to many disordered nanowires, meanwhile, the pyrolytic carbon forms a continuous conductive network between the graphite flakes to improve the electrical conductivity [18, 19]. The schematic presentation of the structure of INGS/C is illustrated in Fig. 4. It is well known that the laser Raman spectroscopy is very important in analyzing the structure of carbon based materials. In the Raman spectra of INGS and INGS/C (Fig. 5), two obvious peaks were detected, the D-band at 1360 cm-1 is characteristic of a disordered sp2 phase and the G-band at 1580 cm-1 is the in-plane stretching vibration mode E2g of graphitic carbon. Furthermore, INGS/C exhibits much higher intensity ratio of D-band and G-band (0.2268) than INGS (0.0906), which demonstrates that amorphous carbon layer is coated on the surface of INGS particles, consistent with the TEM result (Fig. 3d). The nitrogen absorption/desorption isotherms of the as-obtained INGS (Fig. 6a) and INGS/C (Fig. 6b) both exhibit a typical IV type curve. The Barret-Joyner-Halenda
(BJH) pore-size-distribution curves shown in the inset of Fig. 6 demonstrate that both samples have micropores centered at 2 nm, and the Brunauer-Emmett-Teller (BET) specific surface area of INGS and INGS/C is 14.218 m2/g and 13.615 m2/g, respectively. The decreased specific surface after carbon coating can explain the increased initial reversible capacity efficiency . 3.2 Electrochemical properties of INGS and INGS/C Electrochemical characteristics of INGS and INGS/C electrodes are presented in Fig. 7. The discharge/charge profiles for the 1st, 2nd and 100th cycles at 0.3 C agree well with the graphite anode materials . In the first discharge process of INGS and INGS/C electrodes, Li+-insertion voltage drops rapidly and a short plateau appears near 0.75 V, and then a long plateau occurs between 0.2 and 0 V. The short plateau is related to the formation of solid electrolyte interface (SEI) film on the surface of the active particles [23,24], and the long plateau is attributed to multiphase insertion reaction from C to LixC . The discharge/charge curves are nearly coincident in the following cycles, indicating that the SEI film formed in the first cycle is stable and well kept in the following cycles. For Li+-extraction profile of anode material, the long plateau at low voltage is more important than high extraction capacity because high Li+-extraction plateau is unfavorable for improving energy density and selecting high-voltage electrolyte. Furthermore, the ratio of plateau capacity to total capacity (RPT) is also an important factor in maintaining stable work voltage of cell. RPT (< 0.25 V) of INGS and INGS/C electrodes in the first cycle are 92.26% and 92.56%. Fig. 7 also displays the cycling performance of the INGS and INGS/C electrodes
under long-term cycling up to 105 cycles. The INGS electrode exhibits a charge capacity of 338 mAh g-1 with a coloumbic efficiency of 77% in the first cycle and keeps a reversible capacity of 367 mAh g-1 after 105 cycles. The INGS composite with an enormous amount of small graphite flakes behaves large specific surface. The larger the specific surface is, the more the irreversible Li+-consumption for the formation of SEI will be. As a result, the low coloumbic efficiency of INGS can be ascribed to the irreversible Li+-consumption. As shown in Fig. 7b, with the help of carbon-coating, the coloumbic efficiency of INGS/C in the first cycle increases to 82%. Furthermore, the reversible capacity is also enhanced to 381 mAh g-1 in the first cycle and remains stable even after long-term cycling. Compared with pristine natural graphite , INGS/C with high RPT (< 0.25 V) exhibits much higher reversible capacity. On the one hand, the amorphous carbon layer with expanded interlayer space can provide more active sites for Li+-storage; on the other hand, the carbon coating layer can improve the structural stability of INGS . To further investigate the cycle stability of INGS/C, the INGS/C electrode was discharged/charged at 0.3 C for 800 cycles. As shown in Fig. 8, INGS/C exhibits an initial capacity of 437.4 mAh g-1 and retains a reversible capacity of 364.2 mAh g-1 even after 800 cycles. The capacity retention ratio is up to 97.8 %, demonstrating the excellent cycling stability of INGS/C. Fig. 9 shows the cycle voltammograms (CV) of the INGS and INGS/C electrodes, both CV curves match well with the charge/discharge profiles. One small peak can be observed at 0.7 V, but disappear in the subsequence cycles, correspending to the irreversible reduction of electrolyte and the formation of SEI layer . The three
typical reduction peaks at 0.21, 0.12, 0.09 V are attributed to the formation of stage-4, stage-2 and stage-1 graphite intercalation compound, respectively . Fig. 10 shows the electrochemical performance of the INGS and INGS/C electrodes at various charge/discharge rates. In Fig. 10a, the cells were first cycled at 0.3 C for 10 cycles followed by twice cycling with gradually increased discharge/charge rates from 0.6 C to 3 C and then back to 0.3 C. Obviously, the INGS/C electrode delivers reversible capacity of 391, 381, 378, 371 and 346 mAh g-1 at 0.3, 0.6, 1, 1.5 and 3 C, respectively, which is higher than INGS (358, 354, 348, 334 and 246 mAh g-1 at 0.3, 0.6, 1, 1.5 and 3 C, respectively). When the rate was reset to 0.3 C, the INGS/C electrode still retains the same reversible capacity as the value of 391 mAh g-1 at the beginning, indicative of excellent capacity recovery for INGS/C. For comparison, the SNG electrode was also tested at various charge/discharge rates. As shown in Fig. 10b, SNG delivers much lower capacity at various C rates than INGS and INGS/C. Especially, when the rate was increased to 3 C, the reversible capacity of SNG dropped to 153 mAh g-1, demonstrating fast capacity fading. The remarkable improvement of rate capability for INGS/C can be attributed to the following two factors: the one is the isotropic and porous spherical structure, which is beneficial to the penetration of electrolyte, decreasing of Li+ diffusion path, and stress relaxation at the state of deep-lithiation [28, 29]; the other is the pyrolytic carbon from PVA, which connects the neighboring graphite flakes to form a continuous conductive network in the electrode and consequently increases the electrical conductivity and improves its electrochemical stability.
The electrochemical impedance spectroscopy (EIS) measurements were tested to further demonstrate the great rate capability of INGS/C. As shown in Fig. 11a, all EIS spectra are composed of a small intercept in the high-frequency region, a depressed semicircle in the medium-frequency region and an inclined line in the low-frequency region. The smaller diameter of the semicircle is, the lower the charge-transfer resistance (Rct) is. All EIS curves can be fitted by an equivalent circuit composed of “R(C(R))(C(RW))” using the Zview program (inset in Fig. 11a) and the fitting results are shown in Table 1. It is found that the Rct value (27.93 Ω) of INGS/C upon 10 cycles is smaller than that of the fresh cell (34.43 Ω). To further study the reaction kinetics of INGS/C upon cycling, the Li+ diffusion coefficient (DLi+) was calculated using the following equations :
DLi R2T 2 / 2 A2 n4 F 4C 2 Li 2
where R is the gas constant, T is the absolute temperature, A is the surface area of the electrode, n is the number of electrons per molecule, F is the Faraday constant, CLi+ is the concentration of Li+, and δ is the Warburg coefficient. The real part of the impedance (Z’) is given by:
Z Rc Rct 1/2
where ω is the angular frequency in the low-frequency region. Both Rc and Rct are kinetics parameters, and δ is the slope for the plot of Z’ vs. the reciprocal square root of the lower angular frequencies (ω-1/2). Basing on the linear fitting results of Z’ vs. ω-1/2
(Fig. 11b), the calculated DLi+ value of INGS/C upon 10 cycles (1.70×10-12 cm2 s-1) is higher than that for the fresh cycle (7.33×10-13 cm2 s-1). The increased DLi+ value is indicative of an obvious activation and better kinetics of the electrochemical reaction upon cycling. Therefore, INGS/C with isotropic and porous spherical feature will be a promising alternative anode for lithium ion batteries.
4. Conclusions Carbon coated isotropic natural graphite spheres (INGS/C) were successfully prepared by spray granulation using the superfine natural graphite powders as starting materials. The INGS/C composite is composed of small graphite flakes packed in disordered orientation. Compared with SNG and INGS, the INGS/C electrode exhibits higher better rate capability and better cycling stability. The improved electrochemical performance is attributed to the isotropic and porous feature of INGS/C, which is helpful to electrolyte permeation and alleviating capacity fading caused by stress concentration during the deep insertion/extraction of Li+; moreover, carbon coating effectively improves the electrical conductivity of INGS/C. Our results demonstrate that the INGS/C has great potential to be a promising candidate for anodes in high energy density lithium ion batteries.
Acknowledgements This work was supported by the National Science Foundation of China (51272128, 51402168, 51572151), and the Outstanding Youth Science and Technology Innovation Team Project of Hubei Educational Committee (T201603). Moreover, the authors are
grateful to Dr. Jianlin Li at China Three Gorges University for his kind support to our research.
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Figure captions Figure 1 Schematic presentation of the process for INGS/C. Figure 2 XRD patterns of PSNG, INGS and INGS/C. Figure 3 SEM images of INGS (a, b) and INGS/C powders (c), HRTEM image of INGS/C powders (d), and SEM images of INGS/C electrode (e, f). Figure 4 Schematic presentation of the structure of INGS/C. Figure 5 Raman spectra of INGS and INGS/C. Figure 6 N2 sorption isotherm and BJH proe size distribution of INGS (a) and INGS/C (b). Figure 7 Electrochemical performance of INGS (a) and INGS/C (b) at 0.3 C. Figure 8 The long-term cycling property of INGS/C at 0.3 C. Figure 9 CV curves of INGS (a) and INGS/C (b). Figure 10 Rate capability of INGS and INGS/C (a) and conventional spherical graphite (b). Figure 11 EIS curves of INGS/C before cycling and after 10 cycles (a), and the relationship curves between Z’ and ω-1/2 in the low frequency region (b).
Table 1 EIS parameters of INGS/C before cycling and after 10 cycles. Sample
δ (Ω s-1/2)
DLi+ (cm2 s-1)