Introduction

Owing to excellent properties of graphene reported so far, such as Young’s modulus (>1060 GPa), electron conductivity (6000 S/cm), thermal conductivity (~3000 W/m K), light and inexpensive1,2,3,4, it is widely used in many applications, like sensors5, biomedicines6, mechanic resonators7, ultra-capacitors7, etc. By down-sizing graphite from micro to nano-scale, namely graphene, the applications would future widen to polymer composites, anode materials for Li-ion batteries, supercapacitors, hydrogen storage materials, adsorbers and catalysts8.

The synthesis of graphene has been carried out by various methods such as exfoliation and cleavage, ultrasonic exfoliation9,10, growth on SiC11,12, chemical vapor deposition13,14, molecular beam epitaxy, chemical synthesis15,16,17,18, chemical routes and other methods19,20,21. Mass production of graphene or few layer graphene is being hindered by the expensive cost and environmental threat of its conventional synthesis. For example, disadvantages of chemical vapor deposition are high cost and limitation of area. Chemical routes, so called Hummers’ method, suffered from usage of strong acid and oxidants during oxidation processes. A vast amount of waste acids resulted in environmental pollution and problems. Based on these issues, many researchers focus on green process to synthesize graphene or FLG, such as electrochemical exfoliation22,23,24, ultrasonicaion process25,26 and so on. However, many problems, such as low yields and poor quality of as-synthesized FLG still need to overcome.

Recently, there were reported in many kinds of literature by using the high pressure homogenizer27 and sonication28 to delaminate the graphene or using jet cavitation method and obtained few layer graphene29. Yi et al. showed the feasibility of delamination by JC for unmodified graphite and other layered materials and investigated the influence of feed concentration and processing time on yield and morphology of the product30,31,32. Moreover, Yi et al. demonstrated 10 L batch graphene production by a jet cavitation (JC) method and obtained few-layer graphene with low defect concentration29. The processing time, however, was quite long (8 h). Nacken et al. reported an environmentally friendly method for graphene production in large quantities (5 L batches) in processing times <3 h by top-down processing of isostatic and unmodified graphite in an industrial high pressure homogenizer27. According to the above researches, these solvents are organic-based solvents, like NMP, DMF or acetone in the preparation process, even the surfactants including sodium carboxymethyl cellulose (CMC), non-ionic surfactant TWEEN®80 (TW80) or sodium dodecyl sulfate (SDS) are used. In addition, the production of few-layer graphene was got after using centrifuge. In this paper, we propose a green process by using jet cavitation. Here we use an industrial low temperature ultra-high pressure continuous flow cell disrupter (LTHPD) as a delamination device for delamination of graphite suspension continuously by JC method. In this device the graphite suspension is pumped through a nozzle with a defined flow rate to adjust the system pressure. The cavitation and pressure yields a lateral force that induces exfoliation. Through the combination of velocity gradient-induced shear stress, turbulence-induced Reynolds shear stress, and shear effects that emerged from turbulence and flow-induced collisions, a lateral force is generated causing exfoliation. This normal force caused the bulk material to self-exfoliate to single or few layers through their lateral self-lubricating ability32. This method is simple, scalable and does not require toxic chemicals, graphite oxidation or ultrasound post processing to achieve exfoliation. In this study, we propose a green process by using jet cavitation. Here we use an industrial low temperature ultra-high pressure continuous flow cell disrupter (LTHPD) as a delamination device for delamination of graphite suspension continuously. This method is simple, scalable and does not require toxic chemicals, graphite oxidation or ultrasound post processing to achieve exfoliation.

Lithium-ion batteries are among the most widely used energy storage device today. Two-dimensional graphene has drawn much attention because of its admirable properties including conductivity, mechanical strength and high charge carrier mobility which make graphene a suitable electrode material for LIBs33,34,35,36. As for applications of graphene anode for Li-ion batteries, Lian et al.37, Jusef et al.38 and Wang et al.39 reported graphene nanosheets synthesized by chemical synthesis demonstrate good electrochemical performances as anodes in lithium-ion cells. However, it suffers from problems of poor electronic conductivity, larger hysteresis and irreversible capacity loss in the first cycle due to structural defects and functional groups in graphene nanosheets resulted from oxidation processes. In addition, Haiyan et al.40 reported the fabrication of anodes for lithium-ion batteries (LIBs) based on graphene nanoflakes. Moreover, the device cycled at 0.5 A/g of current densities present only 200 mAh/g. In 2011, Zhou et al.41 prepared foam-like graphene as an anode for Li-ion batteries. Its capacity at a current density of 0.2 A/g was about only 200 mAh/g.

To the best of our knowledge, there are few researches concerning electrochemical behavior and measurements of FLG synthesized from jet cavitation process for Li ion battery. Thus, in this study, we will discuss detail characterizations of FLG in terms of SEM, TEM, BET, AFM as well as Raman and XPS to understand surface morphologies, lateral size, thickness distribution, structural defects, functional groups of as-synthesized FLG. The corresponding electrochemical coin tests, such as C/D tests, cycle life as well as differential capacity plots and AC impedances are carried out.

Results and Discussion

Figure 1a shows images of pristine artificial graphite. The lateral size of graphite was about 15 μm. Figure 1b depicts the surface morphology of sample processed in DI water under 200 MPa for 3 cycles. Clearly, the morphology of graphite is thicker and irregular. After liquid exfoliaiton processes, the laterial size of graphite was decreased and delaminated to be few layer graphene (FLG) with thickness <10 nm. This indicates that the initial graphite can be effectively exfoliated into thinner sheets by JCD. Figure 1c shows a typical TEM image of FLG is transparent and folded which coincides well with the typical feature of the reported FLG. Figure 1d shows a typical high resolution TEM (HRTEM) image of FLG which illustrates 8~9 layers of FLG. Figure 1d shows a typical high resolution TEM (HRTEM) image of FLG, in which illustrates 8–9 layers of FLG. The result of HRTEM is corresponded with the AFM. It exhibits HRTEM image of the cross section view of stacked graphene layers. The interplanar distance was measured to be 0.344~0.348 nm corresponding to the spacing of the (002) planes, which is similar to that of graphite (d002 = 0.34 nm)39. In this study, FLG is prepared by JCD without chemical intercalation. Thus, d spacing in 002 plane was closed to 0.34 nm.

Figure 1
figure 1

SEM images of (a) graphite and (b) FLG; TEM image of (c) FLG and HRTEM image of (d) FLG, in which the lattice planes correspond to (002) planes with an interlayer distance of 0.344–0.348 nm.

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Figure 2a shows distribution of the thickness of FLG calculated from the obtained AFM analysis under 200 MPa pressure for 3 cycles. For the samples, more than 30 flakes were captured (Figure S1, S2 and S3 and Table S1). They are believed to be monolayers according to the fact that FLG are often measured to be 0.4–1 nm by AFM due to some external factors such as the AFM equipment and substrates42,43. The average thickness of FLG was 4 nm, which approached 8 layers of graphene (0.5 nm for one layer of graphene) in Fig. 2a. Up to 85% of the prepared FLG were less than 5 nm thick and most of FLG are belong to 10 layers. The N2 adsorption–desorption isotherms of graphene sheets are shown in Fig. 2b. The BET specific surface area of graphite and FLG were measured to be 0.98 and 5.91 m2/g, respectively. FLG demonstrated much larger specific surface area, which indicates that the evidence of graphite exfoliation after our processes.

Figure 2
figure 2

(a) Distribution of the thickness of FLG calculated from the obtained AFM analysis; (b) Nitrogen adsorption/desorption isotherms of graphite and FLG, inset shows the porosity distribution by Original Density Functional Theory Model; (c) Raman spectrum of graphite and FLG; (d) XPS spectrum of graphite and FLG.

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Raman spectra were analysis so as to examine the defect content of graphite and FLG. Typical spectra for graphite and FLG are shown in Fig. 2c. The features in the Raman spectra of graphitic materials are the alleged G band, D band and 2D band appearing at ~1582 cm−1, ~1350 cm−1 and ~2700 cm−1 29,44. The D band is consider the demonstration of the presence of defect and the degree can be estimated by the intensity ratio of the D peak to G peak (I D /I G ). The graphite and FLG of intensity ratio of the D peak to G peak (I D /I G ) increased from 0.208 to 0.350 through the evolution of I D /I G clearly revealed by comparison of curves. This result demonstrated that defect increased in FLG after exfoliation processes. Furthermore, the oxygen content of graphite and FLG were analysis by XPS is shown in Fig. 2d. Distinct C1s and O1s peaks can be seen from the survey scan. It was demonstrated that the oxygen content of graphite and FLG were 2.26 and 2.59%, respectively. The results indicated that it wasn’t leading to oxidation through exfoliation processes.

The delaminated of FLG might be applied as potential anode materials in LIBs. In this section, we test the graphite-based (graphite) and graphene-based (FLG) anode materials for lithium ion battery. During lithiation and de-lithiation, the anode may change its shape45. However, graphene is known to have good flexibiltiy compared to graphite. It is believed that graphene will retain the integrity of its shape during rapid lithiation process thus, providing better electrochemical performance. Figure 3a shows the galvanostatic discharge profiles (charing/discharge rate = 0.1 C) of graphite and FLG, respectively. The first discharge cycle obtains a reversible capacity of 373 mAh∙g−1 and 369 mAh∙g−1 for graphite and FLG at a current density of 0.1 C (0.035 A/g), corresponding to a coulombic efficiency of 93% and 90, respectively. The reversibility of FLG is comparable to graphite. The solid electrolyte interface (SEI) layer plays an essential role in the reversibility of the capacity. SEI layer is formed by the reaction between graphite and ethylene carbonate (EC) in the electrolyte. It provides kinetic stability to the cell and permits the use of graphite as anode material at the expense of some inevitable irreversible capacity46. Vital properties of energy storage devices such as irreversible capacity loss, cycle life, electrode corrosion, self-discharge rate and safety are highly influenced by the thickness of the SEI layer47. Similar to other literature, as-prepared FLG displayed an irreversible capacity ~0.75 V in the initial cycle attributable to the formation of SEI layer and the reaction of Li+ ions with residual H or O groups in the carbonaceous materials37. Lithium atoms are known to bind quasi-reversibly on the hydrogen terminated edges of graphene. In this study, our few layer graphene is not synthesized by Hummer’s method, namely, reduced graphene oxides. Thus, there is littele functional groups and structural defects in our FLG (As shown in Fig. 2c and d). We believe SEI formation is not a dominate issue for our FLG as anode materials for Li ion batteries48,49,50.

Figure 3
figure 3

(a) The 1st charge/discharge profiles for graphite and FLG at rate 0.1 C; (b) Rate performance of graphite and FLG at various current rate from of 0.1 C to 10 C. (c) Comparison of graphite and FLG rate performance; (d) Representative charge and discharge curves of graphite at various current densities; (e) Representative charge and discharge curves of FLG at various current densities; (f) Representative charge and discharge curves of graphite at various current densities.

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The capacity of FLG was equivalent to the capacity of graphite. The high rate discharge/charge properties are likely to originate in the shortened diffusion distance of lithium ions into the host position of the graphene sheets with fewer layers and porous structure51. Rate capability operated at different current rates from 0.1 C to 10 C run for five cycles each (Fig. 3b). As the figure depicts, FLG had average discharge capacities, followed by of 368, 366, 361, 356, 349, 322 and 200 mAh/g at current densities of 0.1, 0.2, 0.5, 1, 2, 5 and 10 C, respectively. An improvement of rate capability was observed with FLG as anodes. The reversible capacity was 368 mAh/g at a 0.1 C rate and maintained at 200 mAh/g even at 10 C, and the Coulombic efficiency of FLG reached to 99%. Compared to the rate capability of graphite and FLG, the specific capacity of FLG was higher than that of graphite at high current densities (2 C and 5 C) shown in Fig. 3c. When the current density goes back to 0.1 C, the specific capacity of the FLG also returns to 371 mAh/g. The rate capability and cycling stability of the graphite and FLG were evaluated by gradually increasing the current rate step-wise from 0.1 C to 10 C, and then returning back to 0.1 C. Figure 3d and e are representative charge and discharge curves of graphite and FLG at various current densities, respectively. These results demonstrated that the prepared graphene sheets with fewer layers have an intensive potential as a candidate of anode materials with high reversible capacity and high rate discharge/charge capability. Increased polarization, as shown in Fig. S4, was observed with increasing current density, particularly with respect to the plateaus at 0.5 V during discharge and 1.0 V during charge that are related to the reversible reaction (Fig. 3f).

Differential capacity plots (dQ/dV) of both graphite and FLG are shown in Fig. 4a and b. These results indicated that the graphite shows comparatively high polarization and the Li+ intercalation of FLG was faster than that of graphite. AC impedance of the graphite and FLG electrodes before cycling and after 2.5 cycles (3.5 V vs. Li+/Li) are shown in Fig. 4c. As can be seen, two semicircles at the high to medium frequencies and a straight sloping line at low frequency can be observed before cycling in both cases. The first semicircle represents surface films resistance and the second semicircle represents charge-transfer resistance, whereas the straight sloping line is associated with diffusion resistance through the bulk of the active material52. An equivalent circuit (inset in Fig. 4c) was used to analyze the measured impedance data, where Rs represents the total resistance of electrolyte, electrode, and separator. RSEI and CPE1 are the resistance and capacitance, respectively, of the solid electrolyte interface (SEI) formed on the electrode. RCT and CPE2 represent the charge-transfer resistance and the double layer capacitance, respectively, and W1 is the Warburg impedance related to the diffusion of lithium ions into the bulk electrode45,52. The fitting values from this equivalent circuit are presented in Table 1. As can be seen, the RS, RSEI and RCT of the graphite electrode are 5.9, 269.9 and 299.9 Ω, respectively, much higher than those of the FLG electrode (RS, RSEI and RCT of the FLG electrode are 6.1, 69.1 and 63.2 Ω, respectively), which means that both SEI resistance and charge-transfer resistance are significantly reduced in the presence of exfoliation. It is most likely that a more favorable SEI was formed for the FLG electrode than the graphite, which facilitates the lithium ion transfer at the interface between the electrolyte and the electrode. Besides, the thin characteristic was good for the reaction kinetics and diffusion of Li ion53. The Li+ diffusion coefficient can be calculated by the following equation54:

$${\rm{D}}=\frac{{{\rm{R}}}^{2}\,{{\rm{T}}}^{2}}{2{{\rm{A}}}^{2}{{\rm{n}}}^{4}{{\rm{C}}}^{2}{{\rm{\sigma }}}_{{\rm{w}}}^{2}}$$
(1)

where R is the gas constant (8.314 J K−1 mol−1), T is the absolute temperature (293.15 K) at room temperature, A is the surface area of the electrode (~1.54 cm2), n is the number of electrons per molecule during oxidization (n = 1), F is Faraday’s constant (96500 C mol−1), and C is the concentration of lithium ions (0.001 mol cm−3). According to the equation (1), diffusion coefficients of lithium among graphite and FLG were calculated to be 4.39 × 10−11 cm2/s and 2.67 × 10−9 cm2/s, respectively. Obviously, diffusivity of Li+ in FLG was much higher than graphite. The morphology of electrode materials is addressed as a key factor controlling rapid lithium storage in anisotropic systems such as graphite. The thickness of FLG was found to be 4–5 nm; such morphology favors short diffusion lengths for Li+ ions, while the thinner FLG provides connectivity for facile electron diffusion, resulting in high rate performances (Fig. 3b)53. Thus, the advantages of thinner FLG for high rate storage performances of battery electrode materials. These results demonstrated that the prepared graphene sheets with fewer layers have an intensive potential as a candidate of anode materials with high reversible capacity, good cycle performance and high rate discharge/charge capability.

Figure 4
figure 4

(a) AC impedance (inserting the fitted equivalent circuit) of graphite and FLG at the third cycle; (b) The relationship lines between Z′ vs. ω−1/2 in the low frequency region of graphite and FLG; (c) Charge-discharge differential capacity profiles of graphite at 0–3.5 V. (○) 0.1 C and (Δ) 5 C; (d) Charge-discharge differential capacity profiles of FLG at 0–3.5 V. (○) 0.1 C and (Δ) 5 C.

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Table 1 Impedance parameters calculated from equivalent circuit model for graphite and FLG.
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Conclusions

In summary, we develope a method to prepare FLG by JCD. SEM and TEM results indicated that it is transparent and folded, which coincides well with the typical feature of the reported FLG. Beyond the HRTEM and AFM presented in this study, the FLG exhibits thickness of the as-synthesized FLG approaches 10 layers of graphene. Furthermore, the obtained graphene were applied as anode material for lithium ion batteries. During the fast charge and discharge rates (5C), graphite delivered ~239.6 mAh/g while FLG exfoliated in DI water achieved ~322.2 mAh/g, respectively. The FLG presented the lower polarization and the Li+ intercalation of FLG is faster than that of graphite. The diffusion coefficients of lithium of FLG were increased from 4.39 × 10−11 cm2/s to 2.67 × 10−9 cm2/s. It shows great advantages and is thus proved to be a suitable convenient approach for massive production of graphene. As FLG has been successfully produced by this device and these results indicate that it is potential applications of FLG as anode material for lithium ion battery system.

Methods

Materials synthesis

The artificial graphite (graphite, purity >99%) was used as feed material for the delamination experiment. All materials were used as supplied without further purification. Deionized water was used for the preparation of all graphite suspensions (10 wt.%). All delamination experiments were carried out in a low temperature ultra-high pressure continuous flow cell disrupter (LTHPD, JNBIO, JN 10C, China). In this device, the graphite suspension is achieved by high pressure forcing the sample through a small orifice at high speed. At the effect of the shearing, hole and impact, graphite of the sample are crushed; substances of the sample are dispersed and emulsified. And the few layer graphene would be kept the suspension at the crash which operated in circulation cooling device at 14–16 °C circulating water bath. The principle and schematic illustration of JC device are shown in Fig. 5. De-ionized water was as a solvent. The graphene production was followed over 3 batch runs and the pressure was in 200 MPa by adapting the piston force through the nozzle. The obtained product of few layer graphene was dried using a vacuum oven at room temperature for battery tests.

Figure 5
figure 5

The principle of graphite delamination by JCD. The suspension is pumped through a nozzle and released into an expansion chamber. A counter pressure can be applied to the suspension by moving the piston after the expansion chamber.

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Characterization methods

Raman spectra were obtained with a micro-Raman spectroscopy system; a 532 nm laser was used as the excitation source. Scanning electron microscope (SEM) images were collected by a Hitachi S-4100. Atomic force microscope (AFM) images were captured by a Bruker Dimension Icon. The samples for AFM were prepared by dropping the dispersion directly onto freshly cleaved mica wafer with an injector. Bright-field transmission electron microscope (TEM) and high-resolution TEM (HRTEM) images were taken with a JEM-2100 operating at 200 kV. TEM specimens were made by diluting the suspension in alcohol and pipetting several drops onto holey carbon mesh grid. Brunauer–Emmett–Teller (BET) specific surface area was determined from N2 adsorption by using a Micromeritics TriStar 3000 (USA) analyzer at liquid nitrogen temperature. Spectra were all captured at a resolution of 4 cm−1, and by averaging 10 scans. X-ray photoelectron spectroscopy analysis was performed with a Thermo Scientific K-Alpha using a monochromated Al Kα X-ray source (1486 eV).

Electrochemical performance

The electrochemical performance of the products were measured by using CR2032 coin cells. The working electrode was composed of 92 wt.% active materials, 3 wt.% KS-6 (KS-6 is a commercially available synthetic graphite from Timcal® with lateral size of 6 μm (d90)), 1 wt.% Super P (Carbon black, 40 nm) and 4 wt.% PVdF, coating on the 10 μm copper foil, then dried at 120 °C for 8 h in vacuum system to remove the residual water. The electrolyte consisted of 1M LiPF6 in ethylene carbonate (EC) and ethyl methyl carbonate (EMC) (1:1 in volume ratio). The discharge/charge test were analysed by AcuTech System in the voltage range of 0.01–3.5 V at room temperature. The Cyclic voltammograms (CV) were measured by CH Instruments Analyser CHI 6273E at a scan rate of 0.001 mVs−1 between 0.01 V and 3.5 V, then tested the AC impedance in the frequency range from 1 Hz ~ 100000 Hz in litigation state of 0.001 V.