Fabrication of cubic spinel MnCo2O4 nanoparticles embedded in graphene sheets with their improved lithium-ion and sodium-ion storage properties

Journal of Power Sources 326 (2016) 252e263

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Fabrication of cubic spinel MnCo2O4 nanoparticles embedded in graphene sheets with their improved lithium-ion and sodium-ion storage properties Chang Chen a, b, c, Borui Liu d, Qiang Ru a, b, c, *, Shaomeng Ma a, b, c, Bonan An a, b, c, Xianhua Hou a, b, c, Shejun Hu a, b, c a

Guangdong Provincial Key Laboratory of Quantum Engineering and Quantum Materials, School of Physics and Telecommunication Engineering, South China Normal University, Guangzhou, 510006, PR China Engineering Research Center of Materials and Technology for Electrochemical Energy Storage (Ministry of Education), Guangzhou, 510006, PR China c Guang dong Engineering Technology Research Center of Efficient Green Energy and Environmental Protection Materials, Guangzhou 510006, China d Department of Mechanical Engineering, The University of Texas at Austin, Austin, TX, 78712, United States b

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


MCO/GS nanocomposites are synthesized by facile hydrothermal methods.
MCO/GS nanocomposites display special nanoparticles-on-sheets hybrid structures.
MCO/GS electrodes exhibit excellent electrochemical performance in LIBs and NIBs.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 19 April 2016 Received in revised form 6 June 2016 Accepted 29 June 2016

Cubic Spinel MnCo2O4/graphene sheets (MCO/GS) nanocomposites are synthesized by a facile hydrothermal method with a subsequent annealing process. Nano-sized MnCo2O4 particles are evenly embedded in paper-like graphene sheets, possessing a unique nanoparticles-on-sheets hybrid nanostructure, with particle size around 20e50 nm. Owing to the special nanoparticles-on-sheets structures, MCO/GS nanocomposites have an outstanding electrochemical performance for rechargeable energy storage devices. As an anode material for lithium-ion batteries, MCO/GS electrodes exhibit high reversible discharge capacities (1350.4 mAh g1 at the initial rate of 100 mA g1), excellent rate capability (462.1 mAh g1 at a current rate of 4000 mA g1) and outstanding cycling performance (584.3 mAh g1 at 2000 mA g1 after 250 cycles). Meanwhile, as an anode material for sodium-ion batteries, MCO/GS electrodes also exhibit comparably promising electrochemical characteristics. Greatly improved electrochemical properties can be assigned to the special advantageous nanostructures. Besides, the existence of graphene sheets is beneficial to the transportation of ions/electrons during battery operation. The outstanding electrochemical performance demonstrates that the lithium/sodium storage capability of MCO/GS nanocomposites is highly promising for high-capacity batteries. © 2016 Elsevier B.V. All rights reserved.

Keywords: MnCo2O4 Graphene sheets Electrochemical performance Lithium-ion batteries Sodium-ion batteries

* Corresponding author. Guangdong Provincial Key Laboratory of Quantum Engineering and Quantum Materials, School of Physics and Telecommunication Engineering, South China Normal University, Guangzhou, 510006, PR China. E-mail address: [email protected] (Q. Ru). http://dx.doi.org/10.1016/j.jpowsour.2016.06.131 0378-7753/© 2016 Elsevier B.V. All rights reserved.

C. Chen et al. / Journal of Power Sources 326 (2016) 252e263

1. Introduction In recent years, the ever-increasing global demands for energy source and the negative impacts of environmental pollution have urged people to seek for new clean energy solutions. As a result, the development of rechargeable batteries is receiving increasing attention [1,2]. Applications of lithium-ion batteries (LIBs) with high-energy density and high-energy storage efficiency are successively expanding from portable electronics to high-power electric vehicles (EVs). Currently, graphite is commonly used as an anode material in most commercial LIBs, while graphitic anodes can only have one Li atom per six carbon atoms (LiC6) intercalated, thus possessing a limited theoretical specific capacity of 372 mAh g1 [3], which cannot meet future demands for more advanced batteries and hence limits its large-scale applications in LIBs. It is therefore, necessary to find better alternative anode materials which possess faster electron transportation and lithium-ion diffusion, larger capacity and higher Coulombic efficiency. Tremendous efforts have been devoted to further research on exploring new anode materials [4,5]. Among various anode materials for the next-generation LIBs, transition metal oxides (TMOs) have attracted broad attention, owing to their relatively high specific capacity, high metallurgical achievability, large resource stock, low cost, and environmental benignity [6e10]. A large number of TMOs materials such as Fe2O3 [11], SnO2 [12], ZnO [13], Co3O4 [14] and TiO2 [15], have attracted tremendous attention because of their high theoretical specific capacities, ranging from 500 mAh g1 up to 1500 mAh g1. However, the intrinsic drawbacks of them are a large obstacle to their practical applications, such as electrochemical reaction induced electrode cracking and severe partial aggregation, which may lead to rapid capacity loss and poor cycling stability [16e18]. Huge efforts have been paid to solve these intractable problems, such that by using nanostructure constructions [19,20], element doping [21], carbon-based composites [22e24] and compound metal oxides [25], and so on. For example, Wang et al. have synthesized nitrogen-doped porous carbon-Co3O4 nanocomposites, demonstrating an initial discharge capacity of 1223 mAh g1 at a current density of 100 mA g1, and a high reversible capacity of 1060 mAh g1 after 100 cycles [26]. Unitary metal oxides, such as SnO2 (782 mAh g1), have also been intensively investigated, but the vast enormous volume expansion and structural change of lithiation/delithiation process, still inevitably result in significant capacity fading during cycling. Lin et al. have synthesized graphene nanoribbons and SnO2 nanoparticles and used them as the anode material for LIBs. The anode exhibits good reversible charge capacity (825 mAh g1 at a current density of 100 mA g1) and excellent power performance (580 mAh g1 at a current density of 2 A g1) [27]. The high electrochemical performance was ascribed to SnO2 distributed among the graphene nanoribbons. Carbon matrices in these composites acted as conductive additives which can not only buffer the volume changes but also provide a conductive network for SnO2 to improve the electronic conductivity during cycling [28,29]. Specifically, many binary transition metal oxides with cubic spinel structures, such as FeMn2O4 [30], CuCo2O4 [31,32], NiCo2O4 [33] and CoMn2O4 [34], etc, have been heavily investigated because of their favoruable synergetic effects [35,36]. Li et al. have adopted a hydrothermal method of the growth of MnCo2O4 (MCO) nanowire array. As anode materials, the MCO nanowire arrays displayed a high specific discharge capacity of 1288.6 mAh g1 at 100 mA g1, with capacity retention of 92.7% after 50 cycles. Therefore, MCO electrodes with excellent performance are now highly desirable for LIBs application. However, there are still several issues to resolve:

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low conductivity of MCO-based electrodes materials, enormous volume change associated with Liþ charge/discharge processes, low material utilization and inferior structure stability [37]. Graphene, as a novel two-dimensional carbon material with sp2-bond carbon atoms arranged in a honey combed network of the hexagonal members of graphene layers, possesses high electrons conductivity and large specific surface area (up to 2600 cm2 g1), which has been the most promising research hotspot since it was discovered in 2004 [38,39]. In this regard, it is favor to prepare the cubic spinel MnCo2O4 nanoparticles anchored on graphene sheets by turning the reaction solvent. The synthesis process of MCO/GS nanocomposites can be illustrated in Fig. 1. The resulting materials are assembled as anodes for LIBs and sodium-ion batteries (NIBs), which are expected to exhibit outstanding electrochemical performance because of the high integration of several advantageous structural features. 2. Experimental 2.1. Preparation of graphene oxide (GO) All the reagents are analytically bare, commercially available, and used without further purification. GO employed here was synthesized from natural flake graphite by a modified Hummers method. Firstly, 1 g natural graphite powders were added into 23 mL of 98 wt% H2SO4 aqueous solution (Aladdin, 99%) and 10 mL of 98 wt% HNO3 aqueous solution (Aladdin, 99%), vigorous stirring for 1 h in ice bath, keeping temperature at 0  C. Subsequently, 4 g of KMnO4 (Aladdin, 98%) was added gradually under continuously stirring. After being reacted for 40 min at 35  C, 70 mL of deionized water and 8 mL of 10 wt% H2O2 aqueous solutions were added dropwise into the solution. While the solution color changed to golden yellow, continuous reaction was conducted for another 15 min under magnetic stirring. Afterwards, the mixture was centrifuged (8000 rpm) and washed with 80 mL of 5 wt% HCl aqueous solution (Aladdin, 99%) for two times, and then washed with deionized water and absolute ethanol for several times. Finally, the resultant solution was collected and dried at 60  C in the vacuum, GO is subsequently obtained. 2.2. Preparation of MCO/GS nanocomposites The MCO/GS nanocomposites were synthesized by using a facile hydrothermal process. Firstly, dried GO of 60 mg was added into deionized (DI) water under vigorous ultrasonic treatment for 40 min to form a homogeneous solution. Then 5 mL of NH3$H2O (Aladdin, 25%) was added into the above solution, dropwise, subsequently stirring for another 1 h, centrifuging and washing several times in order to obtain graphene sheets materials (GS). Secondly, 60 mg GS materials were dispersed in 60 mL ethylene glycol (EG) (C2H6O2, Aladdin, 99.9%) and 1 mL DI water with an ultrasonic 30 min to obtain GS solution. Simultaneously, 0.190 g MnCl2 $4H2O (Aladdin, 99.9%) and 0.714 g CoCl2$6H2O (Aladdin, 99.9%) were dissolved in 40 mL ethylene glycol with a trace amount of 1 mL DI water, 0.288 g of NaOH (Aladdin, 99%) was dissolved in 5 mL of DI water and added dropwise into the reaction mixture under vigorous stirring, and then mixed with GS solution (the theoretical weight ratio of MCO to GS was 6:1). The mixture precursor solution was then transferred to a 150 mL Teflon-lined stainless steel autoclave and kept at 200  C for 20 h. After cooling to the room temperature, the final solution was collected by centrifugation, washed with DI water and absolute ethanol for several times. Afterwards, the resulting materials were dried at 60  C under vacuum situation overnight. Furthermore, the as-prepared samples were

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Fig. 1. Schematic preparation process of MCO/GS nanocomposites and MCO/GS electrodes for LIBs and NIBs.

annealed to 400  C with a temperature growth of 2  C min1 and kept for 2 h under the argon atmosphere. For the purpose of comparison, bare MCO nanoparticles were also synthesized with a similar route without adding GS materials. Besides, GS materials were also prepared. 2.3. Material characterization The crystal structures and morphology of MCO/GS nanocomposites were checked by X-ray diffraction employing a scanning rate of 0.033 s1 in a 2q range from 10 to 85 (XRD; PANalytical X’Pert PRO, CuKa radiation, l ¼ 0.15406 nm), scanning electron microscopy (SEM; ZEISS ULTRA 55) and transmission electron microscopy (TEM; JEM-2100 HR). Thermogravimetric analysis (TG, PerkinElemer, Inc; USA) are conducted from the room temperature to 680  C in air at the heating rate of 10  C min1. Fourier transformed infrared spectroscopy (FT-IR) was recorded with the wave from 500 cm1 to 4000 cm1 (WQF-410, Beijing Secondary Optical Instruments; China). Raman spectroscopy measurement was carried out on a Labor Raman HR-800 Raman spectrometer system with the wave of 1000 cm1 to 1800 cm1, with a 632.8 nm wavelength laser light. Surface analyses of samples are carried out with X-ray photoelectron spectrum (XPS, ESCALAB 250 with 150 W Al Ka probe beam); all binding energies are referenced to the C 1 s peak (284.6 eV). 2.4. Electrochemical measurements The electrochemical performance was tested by using half-cells (CR2430) with Celgard 2400 film as a separator and Li/Na foil as a counter electrode as well as a reference electrode. Working electrode was made by mixing 80 wt% active materials (MCO/GS in synthesized sample and bare MCO in the control sample), 10 wt% acetylene black as conducting agent and 10 wt% LA132 as a binder. The working electrode slurry was well dispersed via coating machine and fabricated on the copper foils of 13 mm thickness and dried at 80  C for 12 h under vacuum. The electrode was pressed and punched. The electrolyte for the lithium-ion batteries was 1.0 M LiPF6 in a mixture of ethylene carbonate (EC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC) (1:1:1 by volume, provided by Chei Industries Inc, South Korea), and the electrolyte for the sodium-ion batteries was 1 M NaClO4 in a mixture of DEC and EC (1:1 by volume) with 5% FEC as an additive. The half-cells were assembled in an argon filled glove box, wherein oxygen and

water concentrations were limited to below 0.5 ppm. The charge/discharge test was conducted at 100 mA g1 for galvanostatic measurements in 0.01e3.0 V (vs. Li/Liþ) by NEWARE Battery Test System. The cyclic voltammograms at a scan rate of 0.2 mV s1 between 0.01 V and 3.00 V, and the electrochemical impedance spectroscopy (EIS) were carried out by CHI604D Electrochemistry System with a frequency range of 100 kHz e 0.01 Hz. All the electrochemical measurements were carried out at 25  C. 3. Results and discussion 3.1. Structural and morphological characterization The XRD patterns are used to identify the phase formation of the individual constituents as well as the nanocomposites. Fig. 2(a) shows the XRD patterns of bare MCO nanoparticles and MCO/GS nanocomposites, the standard XRD patterns of cubic spinel MnCo2O4 is also included in Fig. 2(a). According to the diffraction pattern above, both diffraction peaks of bare MCO nanoparticles and MCO/GS nanocomposites can be well indexed as a spinel nanostructure phase of MCO (JCPDS card no. 23e1237, space groups Fd-3m (227)). XRD patterns exhibit the characteristic peaks at 18.5e18.9 , 30.5e30.7 and 36.0e36.2 , 43.8e44.1 and 57.9e58.2 as well as 63.6e63.8 , those can be indexed to the panel of (111), (220), (311), (400), (511) and (440), which is well matched with the standard JCPDS file no. 23e1237. The sharp intensity peaks denote high crystallinity of bare MCO nanoparticles and MCO/GS nanocomposites [40]. Besides, there are no other peaks from another crystallized phase which can be probed, indicating the high purity of bare MCO nanoparticles and MCO/GS nanocomposites [41,42]. Raman spectrum is widely used to evaluate the disordered and defected structures of graphene-based composites. The structural changes from graphite oxide to graphene are shown in the Raman spectrum during the chemical reduction process. Fig. 2(b) shows the Raman spectrum of GO materials and MCO/GS nanocomposites. Both samples display two prominent peaks, the D band (1342.6 cm1) and the G band (1582.6 cm1), corresponding to the D line and the G line of carbon materials, respectively. The D line is a breathing mode of k-point phonons of A1g symmetry, corresponding to the vibration of six-membered sp2 carbon rings [43]. Meanwhile, the G line is assigned to the doubly degenerate zone center E2g mode, representing the in-plane vibration of sp2 carbon atoms [44,45]. As can be seen from Fig. 2(b), both GO materials and MCO/GS nanocomposites have an obvious 2D band and a slight

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Fig. 2. (a) XRD patterns of bare MCO nanoparticles and MCO/GS nanocomposites; (b) Raman spectrum of MCO/GS nanocomposites and GO materials. The inset of (b): the partial enlarged patterns of 2D band and (D þ G) band which are special structure of oxidized carbon; (c) FT-IR spectrum of MCO/GS nanocomposites and GO materials; (d) TGA curves of MCO/GS nanocomposites, bare MCO nanoparticles and GS materials.

(D þ G) band. Wherein, the 2D band (sometimes referred to as G’ band) at 2597.4 cm1 is ascribed to the second-order double resonant process between nonequivalent k-points in the Brillouin zone of graphene sheets, involving the vibration caused by. scattering of phonons at the two zone boundary. Besides, the frequencies of 2D band is approximately twice of that of the D peak (u2D~uD) [43e45]. The (D þ G) band at 2871.2 cm1 is ascribed to the disorder of graphene. It can be proved that disorder carbon structure in MCO/GS nanocomposites which is associated with the partial coating of MnCo2O4 nano-sized particles inserted into graphene layers [44,46,47]. Subsequently, FT-IR analysis is employed to identify the presence of functional groups and the phase formation of GO materials and MCO/GS nanocomposites. As it can be seen from Fig. 2(c), the absorption peaks related to oxygen containing are displayed in FTIR spectrum. In detail, the strong and wide absorption band at 3396.4 cm1 is attributed to the surfaceeOH vibration of surface carbocyclic in graphene layers and the OeH symmetric stretching vibration of absorbed water molecules [48]. For GO materials, the bands at 1720.4 cm1 and 1235.1 cm1 are assigned to the vibration of C]O and CeO ineCOOH functional group, which is situated in GO layer surface [49]. The symmetrical and acute band at 1625.4 cm1 is attributed to C]C skeleton vibration, OeH bending vibration and epoxide group stretching vibration. All the functional groups vibration prove that GO materials contain abundant hydrophilic hydroxyl, carboxyl and epoxide groups, which are consistent with the theory values [44,48e50]. However, in the FT-IR spectrum of MCO/GS nanocomposites, the above absorption bands disappear or diminish greatly, indicating the successful reduction of GO materials. The FT-IR spectrum of MCO/GS nanocomposites display the especial functional groups at 671.1 cm1 and 588.2 cm1, which can be assigned to the stretching vibration mode of MneO bond and CoeO bond in the tetragonal MnCo2O4 spinel, these results indicate the crystallization process of the MnCo2O4 spinel structures [41,51]. TGA curve is performed to determine the amount of carbon

present in the samples. Fig. 2(d) shows the TGA curves of MCO/GS nanocomposites, bare MCO nanoparticles and GS materials under the air condition at the range of 30e680  C. GS materials are thermally unstable and start to lose mass upon 200  C, and there is a significant drop in mass around 420e580  C. The former mass loss is assigned to the removal of the residual water groups and the decomposition of labile oxygen-containing functional groups, and the latter mass loss is assigned to pyrolysis of the carbon skeleton of graphene sheets in GS materials. Relatively, there is a slight mass loss of bare MCO materials range from 30 to 680  C, attributing to the volatilization of tiny amounts of residual water groups [52e55]. Besides, the TGA curve of MCO/GS nanocomposites is displayed in Fig. 2(d), a weight loss of 0.9 wt% before 200  C is assigned to the evaporation of the residual water, adhering to the surface of graphene sheets and some other volatile substances, which is existed in MCO/GS nanocomposites. The weight loss 6.8 wt% between 200 and 650  C can be ascribed to the removal of the combustion of the carbon containing functional groups and the residual oxygen containing functional groups into graphene sheets [56,57]. Original content of graphene sheets in MCO/GS nanocomposites is evaluated to be about 7.5 wt% based on TGA curve. The morphology and structures of the resulting samples are observed by SEM in Fig. 3. Fig. 3(a) shows the natural graphite, disorder and chunk. As can be seen from Fig. 3(b), graphene sheets display non-uniformly and a thin film. A majority of bare MCO nanoparticles are shown in Fig. 3(c) and (d). The particle size varies in the range of 20e50 nm, which display an irregular shape and disorder shape with serious agglomeration. Fig. 3(e) and (f) show the SEM of synthesized MCO/GS nanocomposites. The obtained graphene sheets have a non-uniform morphology, and the bare MCO nanoparticles are distributed on graphene layers during the facile solvothermal reactions. MCO nanoparticles are wrapped by graphene sheets, and therefore, on the other hand, graphene sheets are also separated by MCO nanoparticles. It can be seen that the incorporating of MCO nanoparticles on graphene sheets expedite fast electron transportation through graphene sheets to MCO

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Fig. 3. The morphology and structure analysis of MnCo2O4-based products by SEM. SEM images of (a) nature graphite. Scale bar, 50 mm; (b) graphene sheets (GS). Scale bar, 500 nm; (c and d) bare MnCo2O4 nanoparticles. Scale bar, c (2 mm) and d (500 nm); (e and f) MCO/GS nanocomposites. Scale bar, e (2 mm) and f (500 nm). (g) EDS microanalysis of selected MCO/GS nanocomposites.

nanoparticles to improve the electrochemical performance, these special structures of graphene sheets provide an elastic buffer for releasing the volume expansion of MCO nanoparticles during Liþ/ Naþ insertion/extraction process to enhance cyclic stability [20,56,58]. Fig. 3(g) displays the energy dispersive spectroscopy (EDS) of MCO/GS nanocomposites. It can be seen that the strong peaks for few elements such as Mn, Co, C and O are present in MCO/ GS nanocomposites, indicating that MCO/GS nanocomposites are relatively bare MCO and graphene sheets, and the amount of impurities is negligible. Thus the atomic ratio of Mn and Co is about 1:2.27. The microstructures of MCO/GS nanocomposites are further studied by TEM and high-resolution TEM (HRTEM). As shown in Fig. 4(a), graphene sheets exhibit a non-uniformly structure of film with well transparency and disordered wrinkles, those results are the typical characteristics of graphene sheets. Fig. 4(b) and (c) display the TEM images of MCO/GS nanocomposites. Clearly bare MCO nanoparticles display a spinel sphere shape of a size of 20e50 nm, which is in agreement with the SEM observations above. The size of graphene sheets is as large as several microns,

and there is no observation of MCO nanoparticles outside the graphene sheets undergo vigorous ultra-sonication in ethanol used for TEM measurement, those results indicate that there is a strong interaction between graphene sheets and MCO nanoparticles, the binding effect of graphene sheets can immobilize MCO nanoparticles and prevent their movement and agglomeration [29,58,59]. Fig. 4(d) displays the high-resolution TEM (HRTEM) lattice resolved image of individual MCO nanoparticles on graphene sheets. The marked inter-planer distances (d-value) of the planes are 0.119 nm, 0.125 nm, 0.126 nm and 0.251 nm, which are corresponding to the inter-planar spacing of (444), (622), (533) and (311) planes of MCO nanoparticles (JCPDS NO. 23e1237). What’s more, it can be concluded from the XRD and SEM. The measured lattice interlayer distance from graphene in MCO/GS nanocomposites (0.362 nm) is larger than that of the natural graphite (0.335 nm), which is attributed to an expansion effect induced by the remaining oxygen containing species on graphene sheets. In order to determine the elemental composition and valence states of elements, the as-prepared MCO/GS nanocomposites are investigated by XPS spectra. All types of the binding energies are

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Fig. 4. (a) TEM images of graphene sheets. Scale bar, 0.5 mm. (b, c and d) TEM images of MCO/GS nanocomposites. Scale bar, b (200 nm), c (20 nm) and d (10 nm).

referenced to the C 1s peak (284.6 eV). The survey spectrum in Fig. 5(a) displays the presence of C 1s, O 1s and Co 2p as well as Mn 2p from the testing sample that are absence of other impurities. Fig. 5(b) shows the O 1s region and the O 1s emission spectrum can be divided into three photoelectron peaks (530.1 eV, 531.9 eV and 539.3 eV). The binding energy of 530.1 eV is ascribed to M  O bonds (M ¼ Co, Mn); 531.9 eV and 539.3 eV are assigned to the oxygen in residual oxygen containing groups (CeO and OeH) which reside in the surface of graphene, respectively [60]. Fig. 5(c) displays the high resolution C 1s region; the spectrum is further separated into two peaks (284.7 eV and 287.3 eV), which is consistent with the existence of carbon-containing groups (CeC, C]C, CeO and OeC]O). The OeC]O bond is attributed to the residual carboxyl groups situated on the surface of graphene sheets which are not reduced. The OeH bond is assigned to the residual water and free hydroxyl groups. Overall, the analysis of O 1s region and C 1s region indicates that there are a few hydroxyl groups, and carboxyl groups existing on the surface of graphene [60,61]. The following Table 1 displays the related functional groups and relevant parameters in C 1s region and O 1s region, respectively. Fig. 5(d) and (e) show the high resolution XPS spectra of Co 2p region and Mn 2p region, respectively. By using a Gaussian fitting theory, Co 2p can be best fitted by considering two spin-orbit doublets characteristic of Co3þ and Co2þ and two shakes up satellites (defined as sat.), which clearly demonstrate the existence of Co with different valence states (Co2þ and Co3þ). The two peaks at 781.4 eV (Co 2p3/2) and 795.4 eV (Co 2p1/2) are ascribed to the Co3þ, meanwhile, the observation of the satellite peaks which are situated on 790.3 eV, and 805.4 eV indicate the presence of Co2þ. Therefore, the Co cation valence should be a mixed valence of þ2 and þ 3. Similarly, the XPS spectrum of the Mn 2p region can be divided into four peaks after refined fitting. 643.2 eV and 652.7 eV can be attributed to the existence of Mn2þ, and two peaks situated at 644.2 eV and 65.4 eV can be assigned to Mn3þ. In conclusion, it can be concluded that both Co2þ/Co3þ and Mn2þ/Mn3þ are existed in MCO/GS nanocomposites [62]. The multiple valence cations maybe provide more stability electrochemical activity, which is beneficial to the improvement to lithium-ion batteries and sodiumion batteries. The atom ratio of Co and Mn is calculated by the area

of deconvoluted peaks, and the proportion is about 2:1, which is well consistent with the above analysis of EDS. 3.2. Electrochemical performance of lithium-ion batteries Cyclic voltammetry measurements are conducted in order to investigate the redox reactions taking place at the anode electrodes. MCO/GS electrodes are used as potential anode materials for reversible Liþ storage. Fig. 6(a) shows the cyclic voltammogram (CV) profiles of MCO/GS electrodes for the 1st, 2nd and 5th cycles at a scan rate of 0.2 mV s1. CV curve is accordance with the previous reported MCO electrodes [34,37,63]. The first cycle is substantially different from those of the subsequent one, presenting an excellent reversible electrochemical performance except for the irreversible reactivity in the first cycle. The entire electrochemical process can be classified as follows [34,37e41,63]. MnCo2O4 þ 8Liþ þ 8e / Mn þ 2Co þ 4Li2O

(1)

Mn þ Li2O 4 MnO þ 2Liþ þ 2e

(2)

Co þ Li2O 4 CoO þ 2Liþ þ 2e

(3)

CoO þ 1/3Li2O 4 1/3Co3O4 þ 2/3 Liþ þ 2/3e

(4)

According to the previous studies, there is a broad and weak peak which is observed at 1.42 V (vs. Li/Liþ) in the first cycle of the cathodic process. The peak is attributed to the Liþ intercalation into the lattice followed by MCO crystal structure destruction and the reduction process of Co3þ to Co2þ. Subsequently, an obvious peak situated at 0.56 V (vs. Li/Liþ) is assigned to the formation of the respective nano-sized metal particles (reduction of Co2þ and Mn2þ to metallic Co0 and Mn0, respectively), accompanied by the partly irreversible decomposition of the organic electrolyte to form a solid electrolyte interphase film (SEI) and the formation of the amorphous Li2O matrix. Thus, the initial electrochemical reaction is based on the Eq. (1) [36,37,61e63]. In the anodic scan, three broad oxidation peaks are observed at 1.38 V, 1.72 V and 2.21 V (vs. Li/Liþ). The two peaks at 1.38 V and 2.21 V are due to the oxidation of Mn

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Fig. 5. XPS spectra of MCO/GS nanocomposites: (a) survey spectrum; (b) O 1s region; (c) C 1s region; (d) Co 2p region; (e) Mn 2p region. Table 1 The related functional groups and relevant parameters in MCO/GS nanocomposites through XPS fitting process. Element

Peak

Position

FWHM

Area

C 1s

CeC/C]C OeC]O/CeO CoeO/MneO OeH CeO

284.7 287.3 530.1 531.9 539.3

1.642 3.123 0.978 3.046 12.146

10,669 1528 39,729 27,282 1165

O 1s

and Co to Mn2þ and Co2þ, and the peak at 1.72 V corresponds to the oxidation of Co2þ to Co3þ, which is based on Eqs. (2)e(4), which is in accordance with electrochemical reaction mechanisms of MnO, CoO and Co3O4 that have been previously reported, respectively [62e64]. The reduction peak in the 2nd and the 5th cycles can be observed to gradually move to 0.68 V (vs. Li/Liþ) and become much broader, which is different from the irreversible electrochemical reaction against the first discharge cycle [37,63].

Fig. 6. (a) Cyclic voltammograms of MCO/GS electrodes in several cycles, and the inset of Fig. 6(a) shows the partial pattern of a weak reducing peak. The electrochemical performance of bare MCO electrodes and MCO/GS electrodes at 100 mA g1: (b) The initial charge/discharge curves; (c) The 20th and 100th charge/discharge curves. (d) Rate performance of bare MCO electrodes and MCO/GS electrodes at various current densities between 100 and 4000 mA g1.

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In the following 2nd and 5th cycles, there are two redox couples of peaks at the potential for 0.68/1.72 V (vs. Li/Liþ) and 1.32/2.21 V (vs. Li/Liþ), which appears to the reversible reactions to some extent according to Eqs. (2) and (3), confirming to the redox reaction to Mn/Mn2þ and Co/Co2þ. In the subsequent 2nd and the 5th cycles, the reduction peak is gradually moved to about 0.68 V with a decrease intensity and become much broader, which is obviously different from the first cycle. The phenomenon reflect that different electrochemical mechanisms controlled the two process, such as the formation and growth of SEI film and some irreversible reactions during the first cathodic process [34,37,42,64]. The slight shift of the reduction peak at the higher potential value of the subsequent cycles may be assigned to some activation process caused by Liþ insertion in the first cycle, showing the slightly easier reduction in the following cycles [40,65]. Furthermore, charge/discharge curves of bare MCO electrodes and MCO/GS electrodes are also examined. Fig. 6(b) and (c) display the 1st, 20th and 100th charge/discharge curves of two electrodes based on anode material for LIBs at a current density of 100 mA g1 at the room temperature. It can be seen in Fig. 6(b) that the bare MCO electrodes show an extended potential plateau at about 0.74 V. Meanwhile, for MCO/GS electrodes, the plateau at 0.78 V is corresponding to Liþ, intercalation into the MCO crystal’s lattice followed by crystal structure destruction and the formation of metallic particles. Both curves are followed by two sloping potentials at about 0.35 V and below 0.21 V. Three flat plateaus are observed in 1.34 V, 1.68 V and 2.23 V during the charging process, respectively, which are assigned to the formation of MnO, Co3O4 and CoO. In conclusion, all of the results are consistent with the above CV curve analysis [37,40,63e66]. The first charge/discharge capacity is 962.5/1350.4 mAh g1 for

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MCO/GS electrodes with a Coulombic efficiency of 71.3%, 854.8/ 1182.5 mAh g1 for bare MCO electrodes with a Coulombic efficiency of 72.1%. The irreversibility capacity loss in the first delithiation/lithiation process is due to the irreversible electrode reaction such as the formation of the Li2O, the electrochemically driven electrolyte degradation and the inorganic solid electrolyte interface (SEI) film [63,64,66]. Meanwhile, the initial discharge capacity of bare MCO electrodes is much higher than the theoretical values (906 mAh g1). The formation of solid electrolyte interphase (SEI) films at the electrolyte interface may contribute to the extra capacity of the initial process of discharge [37,60,67]. Fig. 6(c) exhibits the 20th and 100th charge/discharge process of Liþ voltage profiles for bare MCO electrodes and MCO/GS electrodes at the current density of 100 Ma g. In the 20th cycle, the charge/ discharge capacity is 893.5/896.6 mAh g1 for MCO/GS electrodes and 633.1/644.6 mAh g1 for bare MCO electrodes, 834.8/ 841.1 mAh g1 for MCO/GS electrodes and 358.4/362.6 mAh g1 for bare MCO electrodes after the 100th cycles. At the 20th and the 100th cycles, the Coulombic efficiency for two electrodes increases up to more than 95%. It can be noted that bare MCO electrodes display a rapid capacity fading between the 20th cycles and the 100th cycles, showing its poor capacity retention and electrode polarization on the process of cycling. However, MCO/GS electrodes show mild capacity increasing and good reversibility. Improved electrochemical performance is ascribed to the synergistic effect between the spinel sphere MCO particles and graphene sheets in MCO/GS electrodes. More importantly, the improved cycling performance can be assigned to the buffering effect of graphene sheets that effectively alleviate the large-volume changes of the oxidation/ reduction reaction against Liþ and MCO nanoparticles [37,60,64,68,69].

Fig. 7. (a) Cyclic performance and the corresponding Coulombic efficiency of bare MCO electrodes and MCO/GS electrodes at 200 mA g1; (b) Cycling performance and the corresponding Coulombic efficiencies of MCO/GS electrodes at 2000 mA g1. (c) Nyquist plots measured in the frequency for bare MCO electrodes and MCO/GS electrodes.

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The rate performance of electrodes in the process of the Liþ delithiation is investigated with different current density of every 5 cycles. As it can be seen in Fig. 6(d), MCO/GS electrodes exhibit larger rate capacities than bare MCO electrodes, whose average reversible charge capacities are 1128.5, 982.4 and 852.7 mA g1, 726.5 and 592.1 mAh g1 for 100, 500 and 1000 mA g1, 1500 and 2000 mA g1, whereas 1102.4, 722.7 and 508.8 mA g1, 371.1 and 262.8 mAh g1 for bare MCO electrodes, respectively. Even at a current density of 4000 mA g1, the capacities of MCO/GS electrodes can maintain at 462.1 mAh g1, while only 175.3 mAh g1 for bare MCO electrodes. When the current density decreased from 4000 to 100 mA g1 after the 55th rate cycles, the specific capacity of MCO/GS electrodes rebound upon 692.4 mAh g1, but only 329.4 mAh g1 for bare MCO electrodes. As a result, the superior rate performance can be attributed to the paper-like structure of graphene sheets in MCO/GS electrodes that effectively inhibit the volume change of the transportation of Liþ. The rate performance of MCO/GS electrodes is better than bare MCO electrodes. Fig. 7(a) shows the cyclic performance of Liþ delithiation process and the Coulombic efficiency versus cycle number (#) of bare MCO electrodes and MCO/GS electrodes at a current density of 200 mA g1. Bare MCO electrodes display an irreversible capacity loss from 786.2 mAh g1 to 364.5 mAh g1 after 50 cycles with the capacity retention of 46.4%, exhibiting poor capacity retention of respective cycling, which is assigned to volume expansion and worse electrical conduction during alloying/de-alloying process. The initial charge capacity of MCO/GS electrodes is 812.6 mAh g1. After 50 cycles, the cyclic capacity of MCO/GS electrodes reaches at 865.1 mAh g1, which is much higher than bare MCO electrodes. Compared with bare MCO electrodes, MCO/GS electrodes exhibit low initial Coulombic efficiency in the first charge/discharge cycle (MCO/GS electrodes: 68.4%, bare MCO electrodes: 72.6%). The explanation is interpreted on the basis of the reaction of Liþ with oxygenated functional groups which is situated on the surface of graphene. Besides, owing to their larger specific surface areas, graphene sheets generate more SEI films, which hinder the movement towards Liþ and electrons, and further affect the reaction rate of bare MCO nanoparticles and Liþ [65,69]. After 50 cycles, MCO/GS electrodes show a superior capacity retention and high Coulombic efficiency comparing with bare MCO electrodes, indicating that the graphene sheets provided a higher electrical conductivity in MCO/GS electrodes. Graphene sheets can serve as an ideal volume buffering matrix for not only strengthening electrical conductivity and enhancing rapid electron transfer to electrochemical reaction, but also effectively stabilizing the electrode structure during the lithium intercalation/extraction processes [37,63,68]. Therefore, MCO/GS electrodes exhibit higher reversible capacity and more improved cyclic stability than bare MCO electrodes. The long term cycling performance measurement of MCO/GS electrodes is shown in Fig. 7(b) at a current density of 2000 mA g1 for more than 250 cycles cyclic performance. It can be observed that it had a high initial charge capacity (632.7 mAh g1) at the first cycle accompanied with a slow decay process, however, maintained a stable value of 584.3 mAh g1 after the 250 cycles with capacity retention of 92.3%. The phenomenon is consistent with previous reports and can be assigned to the electrochemical activation of anode materials and heterogeneous storage of Liþ lithiation/delithiation with materials. The Coulombic efficiency of the first cycle is 65.7%, which then are added to 99.5% after 250 cycles. These structures of MCO/GS electrodes exhibit an excellent cycling performance, indicating that MCO/GS electrodes can be a promising candidate anode material for LIBs. Electrochemical impedance spectrum (EIS) is used to evaluate the resistance of electrons transfer as well as impedance of the cell

during cycling, which shows the contribution to electrolyte resistance, surface film resistance and solid-state diffusion of Liþ through the bulk of the active material. EIS curve is employed to explain that the existence of graphene can improve the transmission efficiency of electrons and ions. The characteristic of EIS spectrum of two electrodes are tested after the initial five charge/ discharge cycles. Fig. 7(c) shows the Nyquist plot of bare MCO electrodes and MCO/GS electrodes. It contains a semicircle at a higher frequency region and a long low slope line of a lower frequency region. The intercept on the Z real axis in the high frequency region is assigned to the solid electrolyte interphase (SEI) layer resistance, ohmic resistance and layer contact resistance (Re), while the semicircle in the middle frequency region is attributed to the charge transfers impedance on electrode/electrolyte interface (Rct) and the slope line of the low frequency region corresponds to the Warburg impedance (W), which is ascribed to the solid-state diffusion of Liþ in the electrode materials [70,71]. Besides, CPE1 and CPE2 are the constant phase elements, which are defined to account for the nonhomogeneous nature of the composite electrodes. The equivalent circuits of related electrodes are analyzed by fitting, obviously, MCO/GS electrodes exhibit a lower Rct (64.2 U) than that of bare MCO electrodes (146.2 U), suggesting much faster charge transfer channels on the surface of MCO/GS electrodes. Meanwhile, the existing of graphene sheets can provide a rapid route for Liþ transport and shorten the total diffusion distance. On the basis of the above analysis, the unique structures of MCO/GS nanocomposites not only shorten the diffusion paths of Liþ, but also facilitate easy circulation of the electrolyte, further improve the electrochemical performance of reversible lithium storage [72], thus lead to an outstanding improvement on the rate performance (Table 2). It is supportive that the method of graphene sheets coating is one of the most important strategies to provide fast ion/ electron transfer and excellent electrochemical performance for energy storage.

3.3. Electrochemical performance of sodium-ion batteries Furthermore, bare MCO electrodes and MCO/GS electrodes are also investigated as an anode for sodium-ion batteries to explore their electrochemical performance of Naþ insertion/extraction, respectively. Fig. 8(a) displays the first five CV curves of MCO/GS electrodes. It can be seen that there are two evident reduction peaks, which are situated at 0.23 V and 1.02 V in the cathodic process, respectively. Those peaks are ascribed to the formation of Na2O and the reduction of Mn2þ to metallic Mn, Co3þ to Co2þ as well as Co2þ to metallic Co (the reaction can be described as: MnCo2O4 þ 8Naþ þ 8e / Mn þ 2Co þ 4Na2O), continuously coupled with the partial decomposition of the electrolyte to form the SEI layers [73,74]. Two broad peaks at 1.43 V and 0.84 V can be assigned to the oxidation of metallic Mn to Mn2þ, metallic Co to Co2þ and Co2þ to Co3þ in the anodic process, respectively. In the following cyclic CV curves, it can be found that the cathodic peak moves to 0.45 V, broader and gentler, suggesting that different electrochemical mechanisms grasp the following process (the irreversible reaction and the formation/decomposition of SEI

Table 2 Equivalent circuit parameters obtained from fitting the experimental impedance spectra. Electrodes

Rct (U cm2)

Re (U cm2)

W (U s1/2)

CPE1 (F)

CPE2 (F)

bare MCO MCO/GS

146.2 64.2

25.4 6.8

0.543 0.486

1.42E-5 1.38E-5

1.44E-5 1.41E-5

C. Chen et al. / Journal of Power Sources 326 (2016) 252e263

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Fig. 8. Electrochemical characterization of Sodium-ion batteries: (a) Typical CV curves of MCO/GS electrodes; (b) The 1st, 5th and 10th charge/discharge voltage profiles of MCO/GS electrodes at 50 mA g1; (c) Rate performance of bare MCO electrodes and MCO/GS electrodes at various current density between 50 mA g1 and 200 mA g1; (d) Cyclic performances and the related corresponding Coulombic efficiency of bare MCO electrodes and MCO/GS electrodes at 50 mA g1.

layers) [74,75]. In conclusion, the mechanism of Naþ insertion/ extraction in MCO/GS electrodes is similar to that of Liþ insertion/ extraction in TMOs, which has been verified by some early researchers. Fig. 8(b) shows the galvanostatic charge/discharge curve of MCO/GS electrodes in sodium-ion batteries for the 1st, 5th and 10th

cycles at the current density of 50 mA g1. The initial curve exhibits a large distinct plateau at 0.67 V, followed by a gentle decrease to the second plateau at 0.25 V, corresponding to the reduction of MnCo2O4, which is well consistent with CV analysis. The initial charge/discharge specific capacity of MCO/GS electrodes arrives at 613.8/912.6 mAh g1, while its initial Coulombic efficiency is 67.3%.

Table 3 Summarization of the reported electrochemical performance with special morphologies of MCO and their applications. Electrodes

Application

Current density (A g1)

Initial discharge capacity (mAh g1)

Cyclic number (#)

Reversible capacity (mAh g1)

Reference

Nanowires MCO/rGO nanosheets MCO/P-doped hierarchical porous carbon MCO-G hybrid materials Nanowires MCO grow on nickel foam Flower-like MCO Mesoporous MnO/MCO composite Mesoporous nanosheets MCO @ polypyrrole composites Flower like MCO microsphere Flake-like MCO mesoporous

LieO2 LieO2 LieO2 LieO2 NIBs Super-capacitors LIBs

0.2 0.2 0.8 0.2 0.2 0.5 0.2

11092.1 13150 3784 1708 675 497 F g1 1398

35 200 40 45 40 5000 100

1000 1000 2743 1038 126 298.2 F g1 910

[79] [80] [81] [82] [83] [84] [85]

Super-capacitors LIBs Super-capacitors LIBs LIBs LIBs Super-capacitors LIBs LIBs Super-capacitors LIBs LieO2 Super-capacitors LIBs NIBs

1 0.1 1 0.4 0.4 0.2 12 0.4 1.0 1.0 0.1 0.25 400 W kg1 2 0.05

235.7 F g1 1460 1487 F g1 755 1033.3 1445 310 F g1 1845 1034 151.2 1433.3 4861 42.1 Wh kg1 632.7 912.6

2000 100 2000 100 50 40 3000 200 1000 300 60 50 1000 250 50

220.6 F g1 952 1387.4 F g1 610 766.7 860 330 F g1 851 740 118.8 F g1 900 1000 37.2 Wh kg1 584.3 301.9

[86] [87]

Multi-porous MCO Multi-porous core-shell structured MCO Yolk-shell structured MCO Porous MCO nanorods Porous MCO microsphere Porous urchin-like MnCo2O4.5 Core-shell ellipsoidal MCO Multi-porous MCO microspheres Stacked CuCO2O4/MCO on graphite paper Nanoparticles MCO/Graphene sheets

[88] [89] [90] [91] [92] [93] [94] [95] [96] Our work

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The initial capacity loss is ascribed to the irreversible formation of Na2O and SEI layers. In the following cycles, the charge/discharge capacity is 424.9/430.2 mAh g1 in the 5th cycle and 388.9/ 397.8 mAh g1 in the 10th cycle. Herein, capacity maintenance is 47.1% in the initial 10 cycles. As it can be seen in Fig. 8(c), MCO/GS electrodes also display a better rate performance than bare MCO electrodes. The rate capability is tested by a multiple steps galvanostatic strategies at different current density between 50 mA g1 and 200 mA g1. When the current density is changed range from 50 mA g1 to 100 mA g1 as well as 200 mA g1, MCO/GS electrodes display a high charge/discharge capacity of 756.3/895.2, 309.5/342.7, 203.3/ 218.1 mAh g1, while only 682.5/762.4, 300.2/346.3, 115.6/ 141.8 mAh g1 to bare MCO electrodes. Then the current density is reversed back to 50 mA g1, the specific Naþ insertion/extraction capacity of MCO/GS electrodes can recover a value of 290.9/ 301.6 mAh g1, however, 139.2/145.5 mAh g1 for bare MCO electrodes. In conclusion, the superior rate stability can be ascribed to the existence structure of graphene in MCO/GS electrodes, which possesses a smooth channel for ion/electron transportation. The rate stability of MCO/GS electrodes is better than bare MCO electrodes. Fig. 8(d) shows the cyclic performance and its corresponding Coulombic efficiency of bare MCO electrodes and MCO/GS electrodes at 50 mA g1. It can be observed that the initial charge/ discharge capacity of MCO/GS electrodes is 613.75/912.6 mAh g1, while 588.8/839.6 mAh g1 for bare MCO electrodes. In the following cycles, the special capacity of bare MCO electrodes suffers from a rapidly decreasing trend, whereas, relatively slow decreasing to MCO/GS electrodes. The decreasing cyclic performance is attributed to the partial formation/decomposition of SEI layers and initial reversible reactions, which is well consistent with the above describe, besides, the Coulombic efficiencies of MCO/GS electrodes and bare MCO electrodes are surpassing over 90% [73,75,76]. After 50 cycles, MCO/GS electrodes exhibit a reversible capacity (300.2/301.9 mAh g1) with capacity retention of 33.1%, which is better than bare MCO electrodes (151.0/152.6 mAh g1 with the capacity retention of 18.2%). Herein, we can define that the existence of graphene sheets alleviates the volume change and hinders the aggregation of MnCo2O4 nanoparticles, as well as provides an extra ion/electron during the electrochemical performance process. In conclusion, on the basis of the above analysis, all the results are supportive that graphene coating is one of the greatest strategies to improve the electrochemical performance of transition metal oxides [77,78]. In addition, the summarization of the reported electrochemical stabilities of MnCo2O4 electrodes with special morphologies are displayed in Table 3. It can be seen that our synthesized nano-sized particles-sheets structure of MCO/ GS electrodes show the most excellent Li insertion/extraction cyclic performance in LIBs and a favourable electrochemical properties in NIBs, whatever methods and morphologies MCO are synthesized. Besides, all of the prepared special morphologies of MCO materials, element doping and compositing with other materials can significantly improve the electrochemical properties of bare MCO electrodes. 4. Conclusions In conclusion, we have presented a facile way to synthesize MCO/GS nanocomposites through a simple hydrothermal and subsequent process of carbonization. The synthesized MCO/GS nanocomposites as anode electrodes exhibit large reversible capacity, excellent rate performance and perfect cycling stability for LIBs and NIBs, which can be ascribed to the non-uniformly and paper-like graphene buffering matrix. Graphene sheets show

enhanced electrochemical properties in nanocomposites. Furthermore, MCO/GS nanocomposites are demonstrated to be one of the greatest potential applications as an anode electrode for lithiumion batteries and sodium-ion batteries. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 51101062 and 51171065), The Natural Science Foundation of Guangdong Province (Grant No. S2012020010937), The Project Supported by Guangdong Natural Science Foundation (No. 2014A030313436) and the Foundation for Distinguished Young Talents in Higher Education of Guangdong, China (Grant No. LYM09052). The Scientific and Technological Plan of Guangzhou City (Grant No. 201607010274 and 201505050909107), The Scientific and Technological Plan of Guangdong Province (Grant No. 2016A050503040 and 2016B010114002). References [1] J.W. Fergus, J. Power Source 4 (2010) 939e954. [2] G. Jeong, Y.U. Kim, H. Kim, Y.J. Kim, H.J. Sohn, Energy Environ. Sci. 4 (2011) 1986e2002. [3] P. Poizot, S. Laruelle, S. Grugeon, L. Dupont, J.M. Tarascon, Nature 407 (2000) 496e499. [4] X. Zang, Q. Chen, P. Li, Y. Yan, X.W. Lou, X. Wang, J. Am. Chem. Soc. 135 (2013) 9480e9485. [5] S. Goriparti, E. Miele, F. De Angelis, E. Di Fabrizio, R. Proietti Zaccaria, C. Capiglia, J. Power Sources 257 (2014) 421e442. [6] H. Liu, Z. Bi, X.G. Sun, R.R. Unocic, M.P. Paranthaman, S. Dai, G.M. Brown, Adv. Mater 23 (2011) 3450e3454. [7] X. Feng, Y. Liang, L. Zhi, A. Thomas, D. Wu, I. Lieberwirth, U. Kolb, K. Mullen, Adv. Funct. Mater. 19 (2009) 2125e2129. [8] C.W. Sun, F. Li, C. Ma, Y. Wang, Y.L. Ren, W. Yang, Z.H. Ma, J.Q. Li, Y.J. Chen, Y. Kim, L.Q. Chen, J. Mater. Chem. A 2 (2014) 7188e7196. [9] X. Zang, Q. Chen, P. Li, Y. He, X. Li, M. Zhu, X. Li, K. Wang, M. Zhong, D. Du, H. Zhu, Small 10 (2014) 2538e2588. [10] B. Rajagopalan, E.S. Oh, J.S. Chung, J. Power Sources 275 (2015) 702e711. [11] S.M. Xu, C.M. Hessel, H. Ren, R.B. Yu, Q. Jin, M. Yang, H.J. Zhao, D. Wang, Energy Environ. Sci. 6 (2013) 1352e1361. [12] X.W. Lou, Y. Wang, C.L. Yuan, J.Y. Lee, L.A. Archer, Adv. Mater. 17 (2006) 2325e2329. [13] J.P. Liu, Y.Y. Li, R.M. Ding, J. Jiang, Y.Y. Hu, X.X. Ji, Q.B. Chi, Z.H. Zhu, X.T. Huang, J. Phys. Chem. C 13 (2009) 5336e5339. [14] J.M. Xu, J.S. Wu, L.L. Luo, X.Q. Chen, H.B. Qin, V. Dravid, S.B. Mi, C.L. Jia, J. Power Sources 274 (2015) 816e822. [15] D.H. Wang, D.W. Choi, J. Li, Z.G. Yang, Z.M. Nie, R. Kou, D.H. Hu, C.M. Wang, L.V. Saraf, J.G. Zhang, ACS Nano 4 (2009) 907e914. [16] C. Yu, J. Yang, C.T. Zhao, X.M. Fan, G. Wang, J.S. Qiu, Nanoscale 6 (2014) 3097e3104. [17] J. Zhu, S. Chen, H. Zhou, X. Wang, Nano Res. 5 (2012) 11e19. [18] P. Meduri, C. Pendyala, V. Kumar, G.U. Sumanasekera, M.K. Sunkara, Nano Lett. 6 (2009) 612e616. [19] K.Y. Wang, G.H. Liu, N. Hoivik, E. Johannessen, H. Jakobsen, Chem. Soc. Rev. 43 (2014) 1476e1550. [20] J.P. Cheng, X. Chen, J.S. Wu, F. Liu, X.B. Zhang, CrystEngComm 14 (2012) 6702e6709. [21] Z. Yang, Z. Yao, G.F. Li, G.Y. Fang, H.G. Nie, Z. Liu, X.M. Zhou, X. Chen, S.M. Huang, ACS Nano 1 (2012) 205e211. [22] L. Wang, Y. Yu, P.C. Chen, J. Power Source 2 (2008) 717e723. [23] N. Brun, K. Sakaushi, L.H. Yu, L. Giebeler, J. Eckert, M.M. Titirici, Phys. Chem. Chem. Phys. 16 (2013) 6080e6087. [24] M.K. Datta, J. Maranchi, S.J. Chung, R. Epur, K. Kadakia, P. Jampani, P.N. Kumta, Electrochim Acta 13 (2011) 4717e4723. [25] Z.Y. Wang, L. Zhou, X.W. Lou, Adv. Mater. 14 (2012) 1903e1911. [26] L. Wang, Y.L. Zheng, X.H. Wang, S.H. Chen, F.G. Xu, L. Zuo, J.F. Wu, L.L. Sun, Z. Li, H.Q. Hou, Y.H. Song, ACS Appl. Mater. Interfaces 6 (2014) 7117e7125. [27] J. Lin, Z.W. Peng, C.S. Xiang, G.D. Ruan, Z. Yan, D. Natelson, J.M. Tour, ACS Nano 7 (2013) 6001e6006. [28] C.M. Ban, Z.C. Wu, D.T. Gillaspie, L. Chen, Y.F. Yan, J.L. Blackburn, A.C. Dillon, Adv. Mater. 22 (2010) 145e149. [29] C. Chen, Q. Ru, S.J. Hu, B.N. An, X. Song, Electrochim Acta 151 (2015) 203e213. [30] R. Patrice, L. Dupont, L. Aldon, J.C. Jumas, E. Wang, J.M. Tarascon, Chem. Mater. 14 (2004) 2772e2782. [31] Y. Sharma, N. Sharma, G.V.S. Rao, B.V.R. Chowdari, J. Power Sources 1 (2007) 495e501. [32] J.L. Gautier, E. Trollund, E. Rios, P. Nkeng, G. Poillerat, J. Electronal. Chem. 1

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