Graphene nanohybrid with superior lithium storage capability

Electrochimica Acta 132 (2014) 483–489

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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

Facile synthesis of CoSnO3 /Graphene nanohybrid with superior lithium storage capability Yiqi Cao ∗ , Lei Zhang, Duolei Tao, Dexuan Huo, Kunpeng Su Institute of Materials Physics, Hangzhou Dianzi University, Hangzhou 310018, People’s Republic of China

a r t i c l e

i n f o

Article history: Received 19 January 2014 Received in revised form 1 March 2014 Accepted 10 March 2014 Available online 2 April 2014 Keywords: Graphene In situ route Nanohybrid Enhanced electrochemical performance

a b s t r a c t CoSnO3 /G nanohybrid has been synthesized by thermal annealing of CoSn(OH)6 /G, prepared by a facile in situ solvothermal route using CoSO4 , K2 SnO3 and GO as precursors and has been investigated as a promising high-performance anode material for Li-ion batteries. The CoSnO3 nanocrystals are uniformly dispersed and immobilized by graphene nanosheets reduced from GO. The CoSnO3 /G composite exhibits superior cycling stability and rate capability compared to bare CoSnO3 . The improvement in electrochemical performance can be attributed to the combined conducting, confining and dispersing effects of graphene for the CoSnO3 nanocrystals. © 2014 Elsevier Ltd. All rights reserved.

1. Introduction Modern society depends on the widespread use of reliable devices for electrochemical energy storage and conversion [1,2]. Rechargeable Li-ion batteries (LIBs) have been the most popular power source for high consumer electronics for many years, which are considered as the best systems for hybrid electric vehicles (HEVs) and electric vehicles (EVs) due to their superior advantages such as high energy density, long cycle life, no memory effect and environmental benignity [3–5]. In order to meet the increasing requirements for HEVs and EVs applications, a great deal of research interest has been shifted to seek high-performance materials that can store and deliver more energy efficiency. So far, metal oxides such as Mn3 O4 [6], Co3 O4 [7], NiO [3], SnO2 [8] and FeOx [9] have been investigated as promising anode materials due to their much higher capacity than that of commercial graphite anodes (372 mAh/g). Among them, a special interest has been paid to Sn-based oxides since first reported by Idota et al. that tin-based amorphous oxide could yield a stable capacity over 600 mAh/g [10]. The lithium storage of Sn-based oxides mainly replies on the reversible alloying-dealloying reaction between lithium and metal nanocrystals. A major drawback of these materials, however, is the severe electrode pulverization caused by large volume changes (358%) during Li-insertion/extraction, which greatly hampers long-term cycle stability and rate performance.

∗ Corresponding author. Tel.: +8613600517449. E-mail address: [email protected] (Y. Cao). http://dx.doi.org/10.1016/j.electacta.2014.03.048 0013-4686/© 2014 Elsevier Ltd. All rights reserved.

Great efforts have been made to improve the electrochemical performance of these Sn-based oxides. One commonly used approach to enhance the electrode stability is to form a composite with a matrix, for example the oxides or carbonaceous materials [11–16]. The confining matrix, may buffer the volume changes of the active materials during repeated discharge and charge process, while preventing them from aggregation, thus possessing long time cycle stability, which has been demonstrated in a series of Snbased oxides reported by previous work such as Li2 O-CuO-SnO2 [10] multideck-cages, coaxial SnO2 @C hollow nanospheres [17] and graphene-SnO2 composites [18–26]. Nevertheless, achieving satisfactory cycling ability and high capacity is still high desirable for next-generation LIBs and still remains a great challenge for these materials. Graphene nanosheet, a new two-dimensional one-atom-thick carbon material [27], is considered as an ideal matrix to support nanoparticles due to its appealing characteristics such as superior electronic conductivity [28], large specific surface area [29] and excellent mechanical strength [30]. Herein, we chose CoSnO3 as the active material owing to its high lithium storage capacity and report a facile in situ solvothermal method to synthesize a CoSnO3 /G nanohybrid, where CoSnO3 nanoparticles are uniformly anchoring on graphene nanosheets. The nanohybrid exhibited remarkably improved electrochemical properties compared with bare CoSnO3 , indicating promising applications as an advanced anode material for high-performance LIBs. The CoSnO3 /G nanocomposite showed high reversible capacities and cycling stability at high current density over a wide voltage range of 0.005 − 3 V. The CoSnO3 /G electrode can deliver a charge capacity

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Fig. 1. (a) XRD patterns of CoSn(OH)6 , CoSn(OH)6 /G, CoSnO3 and CoSnO3 /G, (b) XPS survey spectrum of CoSnO3 /G, (c) C1s XPS of CoSnO3 /G and GO, and (d) Raman spectra of CoSnO3 /G, CoSn(OH)6 /G, graphene and GO.

of 380 mAh/g at 1600 mA/g and maintain a charge capacity of 650 mAh/g after 50 cycles at 400 mA/g. The excellent electrochemical properties of CoSnO3 /G can be attributed to the combined buffering, conducting, and immobilizing effects of graphene and the synergistic between graphene nanosheets and CoSnO3 nanoparticles. 2. Experimental 2.1. Preparation of CoSnO3 /G hybrid Graphite oxide (GO, 50 mg), prepared by a modified Hummer’s method, was added into 50 mL of deionized water (DIW) with sonication to form a uniform dispersion. Then, 10 mL DIW solution of CoSO4 (0.5 mmol) was added slowly to the above dispersion under stirring, followed by adding 10 mL DIW solution of K2 SnO3 (0.5 mmol) dropwise. After vigorous stirring, the mixed dispersion was transferred into a 100 mL Teflon-lined stainless steel autoclave and heated in an electric oven at 180 ◦ C for 20 h. The resulting product of CoSn(OH)6 /G was collected by centrifugation, washed with deionized water and absolute ethanol for several times and dried at 60 ◦ C under vacuum for 8 h. After annealing in N2 at 300 ◦ C for 4 h, CoSnO3 /G composite were obtained by thermal-induced dehydration of CoSn(OH)6 /G. A control experiment was carried out to prepare bare CoSn(OH)6 and CoSnO3 using the same route without adding GO. 2.2. Materials Characterization The X-ray diffraction (XRD) patterns of the products were collected on a Rigaku D/Max-2550pc powder diffractometer

˚ X-ray photoelecequipped with Cu K␣ radiation (␭ = 1.541 A). tron spectroscopy (XPS) analysis was performed on a KRATOS AXIS ULTRA-DLD spectrometer with a monochromatic Al K␣ radiation (hv = 1486.6 eV). The morphologies and microstructures of the products were characterized by field emission scanning electron microscopy (FE-SEM) on a FEI-sirion microscope, transmission electron microscopy (TEM) on a JEM 2100F microscope. The Raman spectra were measured on a Jobin-Yvon Labor Raman HR-800 using Ar-ion laser of 514.5 nm. Thermogravimetric (TG) analysis was conducted on a DSCQ1000 instrument from 30 to 500 ◦ C at a heating rate of 10 ◦ C min−1 in air.

2.3. Electrochemical measurements The electrochemical properties of the products were evaluated using CR2025-type coin cells. The working electrodes were made by spreading the slurry composed of 75 wt% active material (CoSnO3 /G or CoSnO3 ), 15 wt% poly(vinylidene fluoride) (PVDF) and 10 wt% acetylene black on Ni foam current collector. The working electrodes were then dried at 100 ◦ C under vacuum overnight. The electrodes were then assembled into half cells in an Ar-filled glove box using Li foil as the counter electrode and polypropylene microporous film (Celgard 2300) as the separator. The electrolyte used was 1 M LiPF6 dissolved in ethylene carbonate (EC)/dimethyl carbonate (DMC) (1:1 in volume). The cells were charged at various current densities and discharged at 50 mA g−1 between 0.005 and 3 V vs. Li/Li+ on a Neware battery cycler (Shenzhen, China). The capacity of CoSnO3 /G was calculated based on the total mass of CoSnO3 and graphene. Cyclic voltammetry (CV) experiments were conducted on an Arbin BT2000 system over the voltage range

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0.005 − 3.0 V vs. Li/Li+ at 0.1 mV s−1 . All of the electrochemical measurements were performed at 25 ◦ C. 3. Results and discussion Fig. 1a shows the XRD patterns of precursor of CoSn(OH)6 and CoSn(OH)6 /G, and the final product CoSnO3 and CoSnO3 /G. For the precursor, all the diffraction peaks can be assigned to perovskitetype CoSn(OH)6 (JCPDS card no. 13-356, space Pn3 m). For the final products, being consistent with the literature report, the XRD analysis proves the formation of amorphous CoSnO3 , which has been further proved by the selected area electron diffraction (SAED) pattern of the bare CoSnO3 (Figure S3), as characterized by a plain pattern with a broad band between 33o –35o [31], No signals from possible impurities such as Co2 SnO4 or SnO2 are detected since they can only form at much higher annealing temperatures. The formation of CoSnO3 and conversion of GO into graphene are further checked by XPS and Raman analyses. Fig. 2b presents the XPS survey spectrum of CoSnO3 /G. The detected elements are Sn, Co, C and O as expected for the CoSnO3 /G sample. The peaks at 486.8 and 495.2 eV correspond to the Sn3d5/2 and Sn3d3/2 , respectively, of Sn4+ (See the Supporting Information, Figure S1) [32]. The peaks at 26.5, 716.9, 758.8 are related to the Sn4d, Sn3p3/2 and Sn3p1/2 bands, respectively [33]. The C1s XPS of GO and CoSnO3 /G is given in Fig. 1c. The spectra can be fitted into four peaks for carbon atoms in different functional groups: non-oxygenated carbon (C–C 285.6 eV or C = C, 284.8 eV), carbon in C-O bonds (286.3 eV), carbonyl carbon (C = O, 287.6 eV) and carboxylate carbon (O-C = O, 289. 0 eV) [34,35]. Note that the peak intensity of the oxygenated carbon shows a significant decrease after the solvothermal reactions, indicating a sufficient reduction of GO to graphene. The presence of O peaks and the C1s spectrum analysis also indicates that a small

Fig. 2. TG curves of CoSn(OH)6 /G and CoSnO3 /G.

amount of oxygen-containing groups still exist, which possibly plays an important role in fixing the dispersing CoSnO3 nanoparticles through chemical and/or physical absorption, even though they have a negative effect on the electronic conductivity of the hybrid. The Raman spectra of GO, graphene, CoSn(OH)6 /G and CoSnO3 /G are presented in Fig. 1d. In the Raman spectra of the four samples, two bands at 1350 and 1580 cm−1 appear, corresponding to the disordered (D) band and graphitic (G) band, respectively, of carbonbased materials [36]. Compared to GO (D/G intensity ratio, 1.17), graphene (D/G intensity ratio, 1.62), CoSn(OH)6 /G (D/G intensity ratio, 1.75) and CoSnO3 /G (D/G intensity ratio, 1.61) all exhibit an increased D/G intensity ratio, caused by a reduction of the average size of the sp2 domains, which can signify the reduction of GO to graphene [34]. Note that the G peak shows an asymmetric

Fig. 3. SEM images of (a) CoSn(OH)6 /G and (b) CoSnO3 /G, (c) TEM image of CoSnO3 /G, (d) SEM image of bare CoSnO3 .

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feature. It could be concluded based on the above analyses that a CoSnO3 /G nanohybrid has been formed by this solvothermal route. Fig. 2 shows the TG curves of CoSn(OH)6 /G and CoSnO3 /G in air between 30 ◦ C and 800 ◦ C. The weight loss before 100 ◦ C is due to the loss of the absorbed water trapped within the graphene nanosheets [37]. The weight of the absorbed water is excluded in the weight of graphene since CoSn(OH)6 /G and CoSnO3 /G will be dried at 100 ◦ C under vacuum before the electrochemical tests. The continuous weight loss between 100 ◦ C and 800 ◦ C signifies the removal of the residual oxygen-containing groups and the combustion of the carbon skeleton into carbon oxides. The graphene contents of CoSn(OH)6 /G and CoSnO3 /G are estimated to 29 wt.% and 19 wt.% based on the TG analysis. The morphology was investigated by SEM and TEM. A typical SEM image of CoSn(OH)6 /G hybrid is shown in Fig. 3a. It is clear that the CoSn(OH)6 nanoparticles (NPs) are confined in between the graphene sheets. The NPs are also attached firmly on the surface of graphene even though undergone vigorous sonication, indicating a strong interaction between graphene and CoSn(OH)6 . The reduced graphene oxide prepared by hummer’s method contains residual oxygen group, which has been identified by C1s XPS in Fig. 1c. Such oxygen present on graphene has been proved in playing important role to anchor the NPs on graphene. The bond formed between graphene and NPs limits the agglommmation of NPs generated during lithiation, favoring the electrochemical performance of the composite [38,39]. Fig. 3b gives the SEM images CoSnO3 /G prepared by heating CoSn(OH)6 /G in Ar. In contrast to the loosely stacked CoSn(OH)6 NPs and graphene nanosheets, CoSnO3 /G exhibits dense structure, forming a unique sandwich structure. The CoSnO3 NPs in CoSnO3 /G sandwich are homogeneously dispersed in between graphene due to the confining effect. In contrast, the bare CoSnO3 NPs are connecting with each other and tend to aggregate without the immobilizing effect of graphene (see Fig. 3d). The aggregation of CoSnO3 NPs may decrease the surface area and further decrease the contact area between the electrode and electrolyte, making it difficult for Lithium ion to transport into the electrode. On the other hand, the direct restacking of graphene is prevented by the attached CoSnO3 NPs as the spacers. Consequently, a high active surface area of graphene could be maintained. Otherwise, aggregation of the graphene nanosheets occurs owing to the hydrophobic nature. Fig. 3c shows a typical TEM image of CoSnO3 /G. The uniform distribution of CoSnO3 NPs on graphene nanosheets is confirmed by the TEM observation. It is worth noting that no free CoSnO3 NPs appear even though after a long time of intensive ultrasonication, indicating a strong interaction between the CoSnO3 NPs and graphene nanosheets. To investigate the graphene on the electrode kinetics of CoSnO3 , CV measurements were performed. Fig. 4a and 4b compare the CV plots of CoSnO3 /G and bare CoSnO3 scanned at 0.1 mV s−1 between 0.005 and 3 V vs Li/Li+ . CV curves are almost identical to that of tin oxides. In this sense, Sn may contribute most of the capacity for lithium storage of the two electrodes. Although electrochemical inactive, Co also determines the electrochemical properties of the two electrodes, serving as buffer component against the volume change of the active material. For CoSnO3 /G, apart from the first scan, the plots are almost overlapped during the subsequent scan, indicating the reversible of the CoSnO3 /G electrode. In contrast, the peak intensity of bare CoSnO3 is on the decrease with cycling (Fig. 4a), indicating degraded electrode kinetics with cycling. It suggests that the improved reversibility of CoSnO3 /G is ascribed to the introduction of graphene, which has been intensively investigated as the promising building block to form composites due to its large specific surface area, high mechanical strength, high electronic conductivity, and the many-electron effect since first reported by Novoselov et al. [40].

Fig. 4. CV plots of (a) bare CoSnO3 and (b) CoSnO3 /G at 0.1 mV/s.

To highlight the critical role that graphene plays in improving electrochemical properties of CoSnO3 , we compared the electrochemical properties of CoSnO3 /G and bare CoSnO3 . Fig. 5a and 5b compare the voltage profiles between CoSnO3 and CoSnO3 /G charged-discharged at a current density of 50 mA g−1 within a cut-off voltage 0.005–3.0 V vs Li/Li+ . The charge is defined as the de-lithiation process of CoSnO3 and graphene, while discharge is defined as the lithiation process of CoSnO3 and graphene. The specific capacity of CoSnO3 /G is calculated based on the total weight of CoSnO3 and graphene. As shown in the figure, CoSnO3 /G yields the first charge and discharge capacities of 1048 and 1721 mAh g−1 , respectively, with the first irreversible loss of 39%, while the first charge and discharge capacities of bare CoSnO3 are 982 and 1408 mAh g−1 , respectively, with the first irreversible loss of 30%. The large capacity loss in the first cycle for bare CoSnO3 and CoSnO3 /G is mainly attributed to the initial irreversible formation of Li2 O, and other irreversible processes such as trapping of some lithium in the lattice, inevitable formation of a solid electrolyte interface (SEI) layer and electrolyte decomposition, which are all quite common for most anode materials. The reason for the large difference in the first and second discharge values of the composite might be because of the presence of graphene with some oxygenated groups that causes the formation of thick SEI film on the electrode surface in the first discharge, which we think improves the electrode/electrolyte contact [41,42]. The reactions of CoSnO3 with Li can be written as: CoSnO3 + 6Li+ + 6e− → Co + Sn + 3Li2 O

(1)

Sn + 4.4Li ↔ Li4.4 Sn

(2)

Co + Li2 O ↔ 2Li+ + 2e− + CoO

(3)

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Fig. 5. Charge and discharge curves of (a) bare CoSnO3 and (b) CoSnO3 /G, (c) cycle performance of CoSnO3 /G at current densities of 200 mA/g and 400 mA/g, (d) rate capability of bare CoSnO3 and CoSnO3 /G.

The theoretical reversible capacity of CoSnO3 should be 697 mAh g−1 . For bare CoSnO3 , the higher obtainable capacity is possibly due to the reversible reactions (at least partly) of Eq. (1), which corresponds to the quasiplateau at around 2.0 V (see Fig. 4 and Fig. 5). For CoSnO3 /G, except for the above reason of bare CoSnO3 , the extremely high capacity of CoSnO3 /G can be mainly explained by the synergistic effect between CoSnO3 and graphene. On one hand, the introduced graphene can uniformly disperse the CoSnO3 NPs, maximizing the exposure of the active material to the electrolyte and facilitating the rapid Li-ion transport across the electrode/electrolyte interface. On the other hand, the CoSnO3 NPs can act as spacers to restrain the direct stacking of the graphene sheets, increasing the contact area of graphene with the electrolyte. In this case, the Li-ion storage in graphene also obeys an absorption mechanism in addition to the intercalation mechanism, similar to the case for nongraphitizable hard carbon. In addition, the double-layer capacitance of graphene also contributes to the capacity above 0.5 V, as evidenced by the sloping feature of the voltage profiles. In order to further explore the synergistic effect between graphene and CoSnO3 NPs, the electrochemical performances of bare graphene are provided in Figure S4 and Figure S5. From the discharge-charge and CV curves, the first discharge profile exhibited a plateau at about 0.6 V, indicative of the formation of SEI film due to the presence of some functional groups on the graphene surface, and the plateau disappeared in the subsequent cycles, this result is in accordance with the CV results, which explained the large irreversible loss of the composite in the first cycle. Moreover, it is obvious from Fig. 4Sb, it retained a reversible discharge and charge capacity of 100-200 mAh g−1 during the following 50 cycles. In addition, CoSnO3 /G exhibits a better reversibility than

bare CoSnO3 , evidenced from its almost overlapped charge or discharge curves after the first cycle. To further demonstrate the advantages of CoSnO3 /G for lithium storage, the cycling performance of bare CoSnO3 and CoSnO3 /G is also investigated under identical test conditions. Fig. 5c shows the cycling stability of CoSnO3 /G charged at 200 and 400 mA/g and discharged at 50 mA/g for 50 cycles. Obviously, the CoSnO3 /G electrode exhibits high reversible capacity and excellent cycling stability. At a relatively high current density of 200 mA/g, CoSnO3 /G can deliver an initial charge capacity as high as 830 mAh/g, while maintaining a charge capacity of 724 mAh/g in the 50th cycle, with high capacity retention of 87%. Even at 400 mA/g, the CoSnO3 /G composite can still yield an initial charge capacity as high as 820 mAh/g, the charge capacity can be maintained at 649 mA/g in the 50th cycle, revealing its excellent high-rate cycling stability. We have also calculated the coulomb efficiency of the CoSnO3 /G composite charged at 400 mA/g for 50 cycles (see Fig. 5c). The initial coulomb efficiency is 55%. Low initial coulomb efficiency has been proved to be solvable through supplying additional Li resources such as combining the anode with Li-rich Li2.6 Co0.4 N or prelithiation. After the first cycle, the coulomb efficiency keeps higher than 98% for the remaining cycles. For bare CoSnO3 , its charge capacity decreases rapidly from 947 to 266 mAh g−1 after 50 cycles at a low charge-discharge current of 50 mA/g (See the Supporting Information, Figure S2). The enhanced cycling stability can be attributed to the in situ incorporated graphene that not only buffers the volume changes during the lithiation/delithiation reactions but also restrains the agglomeration of CoSnO3 NPs upon long-term cycling. The rate capability of CoSnO3 /G and CoSnO3 is evaluated to further investigate the effect of graphene incorporation on the

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Combining the charge performance and rate capability stated in Fig. 6a, the specific energy and power of the CoSnO3 /G anode can be assessed by integration (Fig. 6b). The calculations were normalized by the total weight of CoSnO3 /G. At a current density of 50 mA/g, the CoSnO3 /G electrodes deliver a maximum energy density of 605 Wh/Kg. As the charge rate increases, the specific power rises significantly. At 3200 mA/g, the specific power reached 627 W/Kg. Based on the above results, suggesting that CoSnO3 /G shows a promising application as a high-capacity anode in LIBs. 4. Conclusion CoSnO3 /G nanohybrid has been synthesized by thermal annealing of CoSn(OH)6 /G, prepared by a facile in situ solvothermal route and has been investigated as a promising high-performance anode material for Li-ion batteries. The CoSnO3 /G nanohybrid exhibits an improved cycling stability compared to bare CoSnO3 due to the effective buffering and immobilizing effects of graphene. Even at 400 mA/g, the CoSnO3 /G composite can still yield an initial charge capacity as high as 820 mAh/g, the charge capacity can be maintained at 649 mA/g in the 50th cycle, revealing its excellent high-rate cycling stability. The unique nanobox-on-sheet structure not only offers 2D conducting networks but also facilitates the rapid Li-ion transport, leading to enhanced rate capability. Even at a current density as high as 1600 mA/g, CoSnO3 /G still yields a charge capacity of 372 mAh/g. Due to the synergistic effect between CoSnO3 and graphene, the hybrid also exhibits a higher reversible capacity compared with the theoretical values. The excellent electrochemical performance of CoSnO3 /G makes it a promising anode for LIBs. Fig. 6. (a) Charge curves of the CoSnO3 /G electrode at different current density, (b) the corresponding specific energy and power density.

electrochemical performance of CoSnO3 (Fig. 5d). It is obvious that the CoSnO3 /G hybrid displays a much better rate capability than bare CoSnO3 . The charge capacities of CoSnO3 /G at 50, 100, 200, 400, 800, 1600, 3200 mA/g are 1088, 957, 829, 706, 539, 372 and 248 mAh/g, respectively, greatly higher than those of bare CoSnO3 . When the current density reaches 800 mA/g, the CoSnO3 /G electrode can still retain a charge capacity of 539 mAh/g. Even at a current density as high as 1600 mA/g, CoSnO3 /G still yields a charge capacity of 372 mAh/g. By contrast, the charge capacities of bare CoSnO3 drop dramatically with increasing the current density. The results clearly demonstrate that the graphene plays an important role in improving the rate performance of CoSnO3 . It is believed that the enhanced rate capability originates from the two factors: first, the highly conductive graphene supplies 2D electronically conducting networks for the CoSnO3 NPs; second, the CoSnO3 NPs can act as spacers to prevent the direct restacking of the graphene sheets. Such a hybrid structure is favorable for maximizing the contact with the electrolyte for both CoSnO3 and graphene, leading to rapid Li ion diffusion both at the electrode–electrolyte interface and within the bulk electrode. As a result, electrode kinetics can be remarkably enhanced by graphene incorporation. To further confirm the important role of graphene on the enhancement of conductivity, electrochemical impedance spectroscopy (EIS) measurements were carried out (SI, Figure S6) and the impedance plots were fitted by the equivalent circuit (SI, the inset in Figure S6). As shown in Figure S6, the graphene electrode exhibits a small charge transfer resistance (Rct) of 31.2 , while the CoSnO3 electrode exhibits a large Rct of 120.4 . After graphene incorporation, Rct of the CoSnO3 /G electrode is reduced to 37.5 , which is comparable with that of the graphene electrode. As a result, the introduction of graphene improves the Li-ion and electronic conductivity [43–45].

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