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Effect of Graphene Modified Cu Current Collector on the Performance of Li4Ti5O12 Anode for Lithium-ion Batteries Jiangmin Jiang, Ping Nie, Bing Ding, Wenxin Wu, Zhi Chang, Yuting Wu, Hui Dou, and Xiaogang Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b10038 • Publication Date (Web): 13 Oct 2016 Downloaded from http://pubs.acs.org on October 17, 2016

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ACS Applied Materials & Interfaces

Effect of Graphene Modified Cu Current Collector on the Performance of Li4Ti5O12 Anode for Lithium-ion Batteries

Jiangmin Jiang, Ping Nie, Bing Ding, Wenxin Wu, Zhi Chang, Yuting Wu, Hui Dou and Xiaogang Zhang∗ Jiangsu Key Laboratory of Materials and Technology for Energy Conversion, College of Material Science and Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing, 210016, P. R. China

ABSTRACT: Interface design between current collector and electroactive materials plays a key role in the electrochemical process for lithium-ion batteries. Here, a thin graphene film has been successfully synthesized on the surface of Cu current collector by a large-scale low-pressure chemical vapor deposition (LPCVD) process. The modified Cu foil was used as a current collector to support spinel Li4Ti5O12 anode directly. Electrochemical test results demonstrated that graphene coating Cu foil could effectively improve overall Li storage performance of Li4Ti5O12 anode. Especially under high current rate (e.g. 10 C), the Li4Ti5O12 electrode using modified current collector maintained a favorable capacity, which is 32% higher than that electrode using bare current collector. In addition, cycling performance has been improved by using the new type current collector. The enhanced performance can be attributed to the reduced internal resistance and improved charge transfer kinetics of graphene film by increasing electron collection and decreasing lithium ion interfacial diffusion. Furthermore, the graphene film

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adhered on Cu foil surface could act as an effective protective film to avoid oxidization, which can effectively improve chemical stability of Cu current collector.

KEYWORDS: Cu current collector, graphene, Li4Ti5O12, low-pressure chemical vapor deposition, lithium-ion battery.


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1. INTRODUCTION Future success in the global effort to change energy usage away from fossil fuels to renewable sources depends on efficient and flexible electrical energy storage devices, such as rechargeable lithium-ion batteries (LIBs), which have attracted considerable attention owing to their great potential for electric, hybrid electric vehicles and smart grids.1-8 However, with the increasing demand of excellent performance in terms of high power density, better safety and longer cycles life, new electrode materials and novel electrode manufacturing still need to be explored.9-16 Commercialized graphite is widely used as an anode material in LIBs since it has a high theoretical capacity and long cycle life. However, relatively poor rate capability and the solid electrolyte interface (SEI) generated during cycling usually limit its development. More seriously, the dendritic lithium formation may cause a security problem under the lower lithium insertion voltage.17 Thus, developing advanced anode alternatives is a necessary approach in LIBs. Spinel Li4Ti5O12 has been considered as one of the most promising anode materials, which exhibits a flat and relatively high voltage plateau at about 1.55 V (vs. Li/Li+), thus avoiding the formation of SEI and suppressing dendritic lithium deposition. Additionally, the spinel Li4Ti5O12 shows excellent cycling performance with almost zero volume change during the charge-discharge process.18 Unfortunately, the low electronic conductivity (ca. 10–13 S cm–1) and lithium diffusion coefficient (ca. 10–9 to 10–16 cm2 s–1) of Li4Ti5O12 anode dramatically limit the overall capacity under high charge-discharge current rates.19 Various research efforts have been attempted to address this issue, the spinel Li4Ti5O12 has been composited with carbon-based material to enhance the electrical conductivity and reduce the Li ion transport path length.20 Moreover, various nanostructured Li4Ti5O12 with different morphology has been synthesized,



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such as nanoparticles,21 nanosheets,22 noaowires23 and porous networks.24 However, the electrode is a completed system, the electrochemical performance not only depends on Li4Ti5O12 active material, but on other component, such as the current collector. The current collector interface design as well as interface interacting between active material and conductive substrate plays a key role in electrochemical process.25 Therefore, it could be an effective method to improve the electrochemical performance of Li4Ti5O12 through modifying Cu current collector. Generally, aluminum foil was used for cathode and copper foil for anode current collectors, respectively. The current collector plays a vital bridge function in supporting active materials, binders, conductive additives and electronically connecting overall structure with the external circuit. Since minimizing the overall resistance of electrochemical system is the basic principle for achieving better electrochemical performance.26 Therefore, the interface characteristic of current collector has a significant effect on improving the specific capacity, rate performance and long cycle life. However, few researchers pay attention to the interface effect of current collector on electrochemical performance, although it is an indispensable constituent structure in LIBs. The previous reported methods about modified current collector mainly include chemical and electrochemical etching,27,28 electro-deposition,29 carbon-coating30-32 and three-dimensional nano-structure design.33-36 Taberna et al modified nano-architecture Cu foil surface by electro-deposition. Although good rate capacity and long cyclic life has been achieved, but the large-scale production has been restricted by expensive anodized aluminium (AAO).37 Kang et al reported the interface of current collector coated by protective oxide thin layer, which improved the anti-corrosion ability of current collector for LIBs. However, the preparation process needed many chemical reagents which made it a very complicated process. In addition, electrochemical performance was not obviously improved with this thin layer.38


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In this communication, the interface of Cu current collector was modified by robust graphene film, which is produced by a simple and large scale LPCVD technology. We compared and discussed the effects of two types of Cu current collector as well as the difference in interfacial property. The use of modified current collector in Li4Ti5O12 half cells both significantly reduced charge transfer resistance, and largely enhanced reversible specific capacity, especially under high current rates. Moreover, the cycling performance has been improved by the new type Cu current collector. Also, the robust graphene film can act as an effective protective film adhered on the Cu foil surface to prevent from being oxidization during the charge-discharge cycling. 2. EXPERIMENTAL 2.1. In situ growth of graphene film on Cu current collector The large-area graphene growth on the surface of Cu current collector was performed by using a LPCVD method. The Cu foil (46 µm, Chinalco Shanghai Copper Co., Ltd) was dipped into acetone solution for several minutes to eliminate the surface oil and other organic impurities, then washed with deionized water and ethanol several times to remove the residual acetone solution and dried under nitrogen gas flow. During the growth, the pre-processed Cu foil was loaded in the quartz tube, it was annealed at 1050 °C under 10 sccm H2 and 50 sccm Ar with a pressure of 450 mTorr for 60 min, and then, 5 sccm of CH4 was introduced in the furnace with a pressure of 95 mTorr for 5 min at the same temperature. After that, the temperature was quickly cooled down to room temperature under the flow of H2 and Ar mixed gases to get the graphene film modified Cu current collector.39 2.2. Characterizations



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The microstructural properties of the prepared graphene film and spinel Li4Ti5O12 anode (Commercially available from Shenzhen BTR New Energy Material Co Ltd) were carried out with a field emission scanning electron microscopy (FE-SEM, JEOL JSM-6308LV). The crystal structure was characterized by X-ray diffractometer (Bruker D8 Advance) with Cu Kα radiation. The optical image was obtained with Optical Microscope (OLYMPUS BX51) at room temperature. Atomic force microscopy (AFM) images were taken in tapping mode with the SPM Dimension 3100 from Veeco under ambient conditions. Raman spectra were conducted on the HORIBA Scientific Lab RAM HR with a 532.4 nm laser. The X-ray photoelectron spectroscopy (XPS) analysis was performed on a Perkin-Elmer PHI 550 spectrometer with Al Kα (1486.6 eV). 2.3. Electrochemical measurements The modified and pristine Cu foils were used as current collector directly. The slurry of commercialized Li4Ti5O12 (LTO) powder (80 wt.%), polyvinylidene fluoride (PVDF) binder (10 wt.%) and acetylene black (10 wt.%) dissolved in N-methylpyrrolidone (NMP) and then pasted on Cu current collector using a medical blade technique. Then, the electrode was dried under vacuum at 110 °C for 12 h and punched into 12 mm diameter disc. The total mass loading on the current collector was nearly 2 mg. CR-2016 coin cells were assembled in an argon-filled glove box with the LTO as working electrode, lithium metal as counter electrode and polypropylene film as separator. The electrolytes were 1mol L–1 LiPF6 in a volumetric ratio with 1:1 mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC). The electrodes were denoted as P-Cu-LTO and G-Cu-LTO using pristine Cu foil (P-Cu) as well as graphene coating Cu foil (G-Cu) as current collectors. Galvanostatically charge-discharge experiments were performed at different current densities between 1.0~2.5 V (vs. Li/Li+) using a CT2001A cell test instrument (LAND Electronic Co.). The electrochemical impedance spectrum (EIS) was measured by using


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a CHI 660E electrochemical workstation in the frequency range 10–2~106 Hz. After charge-discharge cycling, the electrode was extracted from coin-type cells inside the glove box then washed by methanol and vacuum drying for the morphologies characterization. 3. RESULTS AND DISCUSSION Figure 1a shows the schematic illustration of graphene film grown on the surface of Cu current collector by LPCVD process. Generally, a piece of pre-processed Cu foil was loaded in a quartz-tube chamber, then the introduction of CH4 gas at the appropriate growth temperature. It is noted that the growth mechanism of graphene on Cu foil is a surface catalysis, which is different from that of other metal substrate such as Ni with a precipitation process. By precisely controlling the time and gas flow, a complete G-Cu was obtained and could be directly used as anode current collector without complex transfer process. Therefore, this method effectively reduced the defects of graphene film. Because of high transparent and high outstanding conductivity of the graphene film, the G-Cu has a smoother and brighter surface compared to the P-Cu (Figure S1). As shown in optical microcopy image (Figure 1b), after in situ growth of graphene film on the surface of Cu foil, the Cu grain boundaries can be clearly observed after annealing. As determined by atomic force microscopy (AFM), the graphene film with a uniform thickness covered on the Cu foil surface (Figure S2). Figure 1c presents the typical Raman spectrum of the current collector after LPCVD process. There are two peaks centered at 1580 cm–1 and 2690 cm–1, corresponding to the G and 2D bands of graphene.40 The D band was inconspicuous and near the background level, indicating the low defect density of the graphene due to the appropriate synthesis condition and without complex transfer process. On the other hand, Raman spectrum has the following characteristics: (i) a peak intensity ratio of D band to G band (ID/IG) is less than ~0.1; (ii) a sharp and symmetric single peak for the 2D bond; (iii) the



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value of IG/I2D is less than ~1. Since the second-order Raman 2D band was sensitive to the number of layers of graphene, the value of IG/I2D is < 0.36 for 1 layer and >1 for 2 layers.41 Consequently the main monolayer graphene we synthesized is determined by the value of IG/I2D of 0.53. Thanks to the low solubility of carbon on Cu surface in the high temperature, an atomically thin graphene film had been obtained by the self-limiting growth mechanism.42 Figure 2 shows the surface morphologies of Cu current collector. The P-Cu surface has parallel streaks, which made by mechanical rolling operation (Figure 2a). The SEM images of G-Cu show the presence of graphene wrinkles and graphene domains (Figure 2b). The wrinkles were concerned with the thermal expansion coefficient difference between Cu foil and graphene, indicating that the graphene film coating on the Cu foil surface is continuous. Figure 2c shows the image of a single graphene domain layer, which consists of two different size graphene domains. All of graphene domain has hexagonal shape, a black spot in the center of graphene domain, which is the nucleation site for graphene in situ growth. The graphene film robustly adhered on the surface of Cu current collector. After 10 min ultrasonic treatment at room temperature in ethanol solution, only few white cracks were appeared (Figure 2d), which demonstrates that the external force is difficult to strip the graphene film from Cu foil surface. To investigate the effect of the graphene film between the current collector and LTO anode (Figure S3), electrochemical properties were systematically studied by using P-Cu and G-Cu as the current collectors. The EIS spectra of both cells were measured after one cycle charge-discharge at the stable potential of 1.55 V (vs. Li/Li+). Figure 3 shows the Nyquist plots and the equivalent circuit model used for fitting the results. Both of EIS spectra consist of an intercept at high frequency corresponding to the electrolyte resistance, a depressed semicircle at the high-middle frequency range corresponding to charge transfer resistance and an inclined line


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in the low frequency range corresponding to Warburg impedance.43 As shown in the model for fitting the EIS spectra (inset of Figure 3), Rs and Rct represent solution resistance and charge-transfer resistance, respectively. CPE represents the passivation film capacitance and the double layer capacitance. By using G-Cu, the Rs and Rct are 2.19 and 91.17 Ω, which are smaller than that using of P-Cu (2.41 Ω for Rs and 190.3 Ω for Rct). This suggests that the P-Cu shows lower electric conductivity compared with the G-Cu. In other words, the G-Cu is capable of decreasing the internal resistance of electrodes and enhancing the ability of collecting electron, thus improving the electrochemical kinetics. To further verify the effect of the graphene film coating Cu current collector, galvanostatically charge-discharge test was carried out. As shown in Figure 4a and b, both cells were tested under different current density at 0.1, 2, 5 and 10 C (1 C = 175 mAh g–1). Although similar and typical charge-discharge profiles were exhibited, the G-Cu-LTO delivered higher discharge specific capacities especially at high current rates. At current rates of 0.1, 2, 5 and 10 C, the discharge capacities were 154.7, 131.7, 120.3 and 107.1 mAh g–1, respectively. In comparison, the P-Cu-LTO delivered only 151.1, 125.9, 102.7, 83.7 mAh g–1 respectively by using P-Cu. The discharge rate capabilities as shown in Figure 4c, there was no obviously difference for discharge capacity when tested at low current rates. However, the G-Cu-LTO exhibit higher reversible specific capacity than that using of P-Cu-LTO at high current rates. The LTO anode material has a better electronic contact with the G-Cu, which is well consistent with the EIS results (Figure 3). Therefore, the interface of current collector modified with graphene film is able to make LTO anode deliver higher specific capacity retention especially under high current densities. The cycling performance is another significant factor for LIBs. Figure 4d shows the cycling performance of LTO anode at a current rate of 2 C. It can be clearly seen that the cycling



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stability was improved by using the new type current collector. The initial discharge capacities were 137.3 and 137.2 mAh g–1 with the G-Cu-LTO and P-Cu-LTO, and the initial Coulombic efficiency was 98.7% and 89.4% respectively. After cycling for 200 cycles, the specific capacity retention of G-Cu-LTO was 96.9%, which is much higher than that of P-Cu-LTO (only 85.1%). The polarization of LTO electrode can be characterized by the voltage or IR drop. The IR drop of G-Cu-LTO was 0.69 V, which is lower than that of P-Cu-LTO with 0.83 V (Figure 4e). Both of the electrodes display smaller electrochemical impedance after long cycles. This could be attributed to the robust architecture of LTO electrode and destruction of the passivation layer on the lithium metal surface.44 Figure 4f shows that the charge transfer resistance of G-Cu-LTO was smaller than P-Cu-LTO after 200 cycles, indicating the graphene film can effectively reduce polarization. Therefore, the graphene film played a continuing role in charge transport and reduced polarization during the cycling process. In order to further clarify the different electrochemical performance of LTO electrodes supported by P-Cu and G-Cu, the morphologies of the electrodes were carefully characterized after cycling for 200 cycles. Figure 5 shows the SEM images of the electrodes by using different Cu current collectors. After cycling for 200 cycles, the surface of the P-Cu-LTO was severely damaged and depth cracks appeared after repeated Li+ intercalation-deintercalation (Figure 5a). These cracks prevented the transportation of electrons and diffusion of Li+, inevitably leading to capacity fading. However, the surface of G-Cu-LTO is still completed with porous structure (Figure 5b), which benefit for electrolyte infiltrating into the LTO anode and providing short channel for Li+ transportation. As observed from the cross-sectional SEM images, there was a huge gap between the active material layer and current collector for the P-Cu-LTO (Figure 5c), which mainly caused by the electrode polarization after long cycling.45 In contrast, the slurry


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layer adhered to the current collector firmly by using G-Cu (Figure 5d), indicating better adhesion between active material and current collector. This strong adhesion helps maintain a good electrical contact, which may be the reason for the better electrochemical performance, especially, the outstanding rate performance when using the G-Cu as current collector. The capacity contribution of the graphene film is shown in Figure. S4. For comparison, we tested both blank Cu current collectors by galvanostatically charge-discharge at current density of 10 µA cm–2 between 0.01~3 V (vs. Li/Li+). The initial discharge capacities of G-Cu and P-Cu were 13.4 and 9.9 µAh cm–2, respectively. However, the reversible capacity was only 3.7 and 4.2 µAh cm–2 in the second discharge process (Figure S4a and c). The result demonstrated the atomic thin graphene go against lithium storage unless that it includes abundant defects.46 Therefore, the electrochemical contribution can be negligible compared to the high quantity of LTO anode material. There are distinct differences for the different blank Cu foils in the capacity and cycling performance owing to the atomic thin graphene with disparate lithium storage mechanism, which adsorbed/desorbed reversibly per surface area by amount of Li+ ion. Thus, the specific capacity generated due to the electrochemical response from the Cu substrate.47 Although negligible capacities for the two current collectors, both of the blank Cu foils showed better cycling performance (Figure S4b and d). Therefore, the growth of thin graphene film on Cu current collector is an effectively method for surface modification. Graphene as an effective corrosion inhibitor in the environmental and ocean for metals has been reported.48,49 However, less study has been focused on the chemical stability of current collector in electrolyte. During long time cycles, the surface of Cu current collector was partially oxidized to copper oxide with the reduction reaction of electrolyte.50 The G-Cu and P-Cu after long cycling (100 cycles at 1C) were studied by XPS analysis in this paper. XPS is a sensitive



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technology limited to few nanometers, the surface micro-changes of Cu current collector can be clearly detected. It is interesting that the G-Cu ensured protection against oxidation in LiPF6-base electrolyte. Figure 6 shows the XPS spectrum of different Cu foils surface after charge-discharge cycling. The Cu 2p3/2 and Cu 2p1/2 lined position at 932.8 and 952.8 eV, respectively (Figure 6a), which indicated that the G-Cu was only copper phase after long cycling. However, the peaks position at 943.6 and 962.4 eV assigned to satellite peaks in P-Cu (Figure 6b), revealing the presence of trace amounts of CuO on the surface.31 It should be noted that, the P-Cu was cleaned by acetone and anhydrous ethanol before using as anode current collector. This indicated that the graphene film coating has able to avoid interface oxidization of current collector in the electrolyte, thus leading to better cycling performance during the charge-discharge process. Moreover, the graphene film still remained after the cycling due to strong adhesion. Figure S5 shows the C1s peak of the G-Cu after cycling, which can be ascribe to the thin graphene film still coated on surface of Cu foil. Therefore, the robust graphene can behave as a protection film and maintain integrity with the long cycles. In order to future understand the kinetic effect of LTO anode using the G-Cu and P-Cu. The lithiation-delithiation reactions of the LTO is following:51

Li 4 Ti 5O12 + 3Li + + 3e − ←  → Li 7 Ti 5O12

The electrochemical reaction is a two-phase reaction among the spinel structured LTO and rock-salt structured Li7Ti5O12. The spinel LTO has a low electrical conductivity. Its electronic conductivity was significantly improved after the lithiation process due to the mixed valence state of titanium. During lithiation process, the better electronic conductivity Li7Ti5O12 was


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formed on the surface of LTO particles, then, it would continue to develop to the core. During delithiation, the electrical insulating LTO was formed on the particle surface in turn. Thus the delithation process was not electronic favorable by the low electronic conductivity of the particle. Consequently, the electron transfer through the interface interaction between the LTO and current collectors is the limited step in the electrochemical reactions.52 The electrochemical properties could be significantly enhanced by using graphene modified current collector for the LIBs. The functions of electrons transport could be achieved, additionally, the electronic conductivity and chemical stability of the current collector has been improved by this unique design.8,15 As mentioned above, compared with the P-Cu, the kinetics was improved by using G-Cu as current collector. Therefore, G-Cu can effectively improve overall Li storage performance of spinel LTO anode. Figure 7 schematically illustrates the collecting electron ability and Li+ interfacial diffusion on different Cu current collectors. Due to the high electronic conductivity and chemical stability of graphene film, the G-Cu enhanced electron collecting ability compared to P-Cu. During charge-discharge process, there was a thin oxide film formed on the surface of P-Cu, which hampered the electron transfer between the current collectors and LTO anode. Although this oxide film is semiconducting contrast to insulating aluminum oxide, but also has some serious kinetic problems especially under high current rates. Consequently, the G-Cu-LTO exhibits better rate performance. In addition, the graphene film is a superior Li+ interfacial diffusion barrier, which can effectively prevent intermixing at the interface between the Cu current collector and LTO anode materials. It should be noted that the stable interface when coating with graphene film can effectively relieve the fatigue related with the repeated interfacial



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mismatching during cycling.53 Thus, the stable interface of G-Cu was also the reason for the better cycling performance in batteries. 4. CONCLUSIONS In summary, the current collector interfacial properties have been successfully improved with graphene film produced by large-scale preparation strategy using LPCVD technique. The electrochemical test results indicate the G-Cu can effectively reduce the internal resistance and improve the utilization efficiency of LTO anode material, which enhances the specific capacity, rate capability, cycle stability especially at high current density. In addition, the G-Cu could effectively improve kinetic by increasing electronic collecting and decreasing lithium ion interfacial diffusion. Furthermore, the robust graphene film on the interface could act as a protection film to avoid oxidizing after long cycling in the electrolyte. Owing to the advantages of effective and industrial production, the present method could be expanded to other metal foils for rechargeable battery application.


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ASSOCIATED CONTENT Supporting Information. The photos of P-Cu and G-Cu. AFM image of G-Cu surface. FE-SEM and XRD pattern of spinel LTO anode material. Charge-discharge curves and cycling performance of both blank current collectors. XPS spectrum of C 1s for the G-Cu after cycling. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected], Tel: +86 025 52112902; fax: +86 025 52112626. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Basic Research Program of China (973 Program) (No. 2014CB239701), National Natural Science Foundation of China (No. 51372116, 51672128), Natural Science Foundation of Jiangsu Province (BK20151468), Prospective Joint Research Project of Cooperative Innovation Fund of Jiangsu Province (BY-2015003-7), and a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). Ping Nie acknowledages financial support from Funding for Outstanding Doctoral Dissertation in NUAA (no. BCXJ14-12), Funding of Jiangsu Innovation Program for Graduate Education (no. KYLX_0254).



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FIGURES Figure 1. (a) Schematic diagram for grew of graphene film on Cu foil surface by LPCVD, (b) Optical image of as-synthesized graphene film on Cu foil, (c) Raman spectrum of graphene film. Figure 2. FE-SEM images of (a) the P-Cu, (b-c) the G-Cu, (d) the G-Cu after ultrasonic treatment. Figure 3.EIS spectra of LTO electrodes used P-Cu and G-Cu after one charge-discharge cycle at the stable potential of 1.55 V (vs Li/Li+); the inserted image is corresponding equivalent circuit model. Figure 4. Comparison of electrochemical performance of LTO electrodes using P-Cu and G-Cu: galvanostatic charge-discharge curves of electrodes using (a) P-Cu and (b) G-Cu, (c) discharge rate capabilities, (d) cycling performance at 2 C. (e) IR drop and (f) EIS spectra after cycling for 200 cycles at 2 C. Figure 5. SEM images of surface morphology by the LTO electrode used with (a) P-Cu, (b) G-Cu. Cross-sectional SEM images of the electrode surface morphology used with (c) P-Cu, (d) G-Cu after 200 cycles at current rate of 2 C. Figure 6.XPS spectrum of different Cu foils after cycles. (a) The Cu2p of G-Cu. (b) The Cu2p of P-Cu. Figure 7. Schematic illustration of electrons and lithium ions transfer process on the interface of (a) the G-Cu and (b) the P-Cu.


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Figure 1.



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Figure 2.


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Figure 3.



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Figure 4.


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Figure 5.



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Figure 6.


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Figure 7.



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TOC Figure


Effect of Graphene Modified Cu Current Collector ... - ACS Publications

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