Hydrothermal synthesis and characterization of Co2 ...

Electrochimica Acta 158 (2015) 446–456

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Hydrothermal synthesis and characterization of Co2.85Si0.15O4 solid solutions and its carbon composite as negative electrodes for Li-ion batteries S. Yuvaraj a , K. Karthikeyan b , L. Vasylechko c, R. Kalai Selvan a, * a b c

Solid State Ionics and Energy Devices Laboratory, Department of Physics, Bharathiar University, Coimbatore – 641 046, Tamil Nadu, India Department of Mechanical and Materials Engineering, University of Western Ontario, London, Ontario N6A 5B9, Canada Semiconductor Electronics Department, Lviv Polytechnic National University, 12 Bandera Street, Lviv 79013, Ukraine

A R T I C L E I N F O

A B S T R A C T

Article history: Received 15 October 2014 Received in revised form 21 December 2014 Accepted 15 January 2015 Available online 16 January 2015

Co2.85Si0.15O4 solid solution was successfully synthesized using a facile hydrothermal method for the first time. The structural and morphological features of prepared powders were thoroughly investigated by different techniques. The Rietveld refinement assured the formation of spinel structured Co2.85Si0.15O4 without any impurity phases. The FT-IR and Raman spectrums revealed the presence of SiO2, and carbon with the Co2.85Si0.15O4 solid solution. The X-ray photoelectron spectroscopy inferred that the Co exists in +2 and +3 oxidation state and Si exists in multivalence state. Surface morphological analysis demonstrated that the formation of cube shape Co2.85Si0.15O4 microparticles are embedded in to amorphous SiO2 matrix, which was confirmed using SAED pattern. The inactive nature of amorphous SiO2 in Co2.85Si0.15O4 was confirmed by CV studies. Consequently, it was electrochemically active while making composite with carbon, since it reduces SiO2 into SiOx. The cycling stability of Co2.85Si0.15O4@C composite provided superior electrochemical performance than the pristine Co2.85Si0.15O4. The composite delivers the specific capacity of 444 mAh g1 at 75 mA g1 after 50 cycles with feeble capacity fading. The EIS spectra corroborates the composite exhibit the lower charge transfer resistance (Rct) and solid electrolyte interphase resistance (RSEI) compared to pristine Co2.85Si0.15O4. This further confirmed that carbon composite enhanced the inherent conductive nature and rate capability of pristine Co2.85Si0.15O4 electrode. ã 2015 Elsevier Ltd. All rights reserved.

Keywords: Hydrothermal method Carbon composite Co2.85Si0.15O4 solid solutions Lithium ion batteries

1. Introduction Past two decades, Li-ion batteries are considering as one of the most powerful energy storage devices for large scale consumer electronics application such as laptops, mobile phones and medical application because of its high cell voltage, good cycling stability and high energy density. So far, there are many critical challenges and barriers still exist in the usage of Li-ion batteries in electric vehicles and hybrid electric vehicles such as safety related problems, low rate capability, low energy and power densities. Commercially used graphite anode has low theoretical gravimetric capacity of 372 mAh g1 that reduced the energy density and also suffers from Li-dendrite formation at high current rates [1–3]. To overcome these limitations, intense research had been focused on finding out alternative anode material instead of graphite along

* Corresponding author. E-mail address: [email protected] (R. K. Selvan). http://dx.doi.org/10.1016/j.electacta.2015.01.065 0013-4686/ ã 2015 Elsevier Ltd. All rights reserved.

with high energy as well as high rate capability. In past few years, transition metal oxides are considered as promising anode material such as MnO2, Fe2O3, Co3O4 and NiO due to their high theoretical capacity [4–7]. Tarascon et al. reported that Co3O4 exhibited well electrochemical properties among the transition metal oxides because it can accommodate eight electrons per formula unit during the conversion reaction mechanism (Co3O4 + 8Li+ + 8e $ 3Co + 4Li2O) and it delivered high theoretical capacity of 890 mAh g1 [6]. Nevertheless, there is a lot of problems exists while implementing Co3O4 as anode for Li-ion battery because of its toxic nature, high cost, poor electronic conductivity. Presently, SiO2 is considered as a promising negative electrode due to its high specific capacity, low cost, high abundance and eco-friendly in nature [8,9]. Generally, SiO2 was electrochemically inactive for lithium ion storage. However, it has been reported that SiO2 can accommodate lithium ion within its structure when the valence state of silicon was tuned [8]. Zhang et al. reported that carbon coated TiO2-SiO2 nanocomposites (272 mAh g1) showed enhanced specific capacity than

S. Yuvaraj et al. / Electrochimica Acta 158 (2015) 446–456

that of TiO2-SiO2 composite (77 mAh g1) and cycling stability via storing extra Li ions between the interface of TiO2 and SiO2 by changing the valence state of SiO2 [10]. In addition, the nonstoichiometric SiOx (0 < x < 2) delivered a higher capacity of about 1600 mAh g1 due to its lower oxygen content [11–13]. Zhou et al. studied the possibility of lithium ion storage behavior of Fe2O3 and SiO composite, in which the composite electrode delivered superior electrochemical performance than Fe2O3 electrode along with continuous capacity fading, resulting from volume changes of SiO [14]. Of late, many research works have been devoted to alleviate the problem related to volume changes and conductivity by modifying the surface chemistry of the active material by doping metal oxide and/or forming composite with carbonaceous materials like mesoporous carbon, CNT and graphene [15–18]. Among the conducting matrix, making carbon composite is the simple and low cost strategy to improve the electrochemical performance of the electrode material. In this regard, Zhang et al. has synthesized Co3O4/carbon composite by electrospinning technique and obtained the stable capacity of 534 mAh g1 when compared with Co3O4 nanoparticles (276 mAh g1 after 20 cycles) [15]. Similarly, Fe2O3–C composite delivered the stable reversible capacity of 820 mAh g1 at a current rate of 0.2 C up to 100 cycles [19]. As well as, when compared with Mn3O4 nanoparticles, microspheres of Mn3O4/C composite delivered the superior specific capacity of 622 mAh g1 up to 700 cycles [20]. Overall, the carbon composite has lot of advantages like, it makes good electrical conduction between the active material which leads to faster Li+ ion transport to and within the active material, it acts as a buffering matrix which accommodated the swelling of electrode material during charging-discharging process and it also involves the charge storage mechanism through insertion/de-insertion process. Further, these factors also enhance the coulombic efficiency of the active material [19,20]. In this line, here we are reporting the synthesis of Co2.85Si0.15O4 solid solutions and its carbon composite by simple and single step hydrothermal method. It is well known that the hydrothermal method is one of the simple strategies to form high crystalline uniform sized metal oxide particles at low temperature without post calcination step [21]. The prepared materials were analyzed through structural, morphological and electrochemical studies. Electrochemical studies clearly elucidated that Co2.85Si0.15O4/carbon composite (Co2.85Si0.15O4@C) exhibits improved lithium ion storage behavior than that of pristine Co2.85Si0.15O4.

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was dispersed into 0.15 M of aqueous glucose solution and sonicated for 30 mins to attain a homogeneous dispersion. Finally, the solution was transferred to autoclave and it was kept at 180 C for 12 hrs. After the process, the sample was washed with water and ethanol to remove the organic residues and the sample was dried at 80 C for 12 hrs. Finally, the dried sample was calcined at 600 C for 3 hrs under Ar atmosphere to obtain composite. 2.1. Characterization and electrochemical measurements Powder X-Ray diffraction measurement was carried out with D2 PHASER desktop diffractometer using Cu Ka radiation (l = 1.54186 Å). Thermogravimetric analysis (TGA) was carried out using Perkin Elmer STA 6000 with temperature range from 30 C to 800 C at heating rate of 10 C/min under nitrogen gas flow at 20 ml/min rate. Fourier transform infra-red spectroscopy (FT-IR) technique was carried out using Bruker Tensor 27 model with frequency range of 400 cm1–4000 cm1. Raman analysis was performed on Horiba Jobin Yvon with l = 514 nm laser source in the frequency range of 100 cm1–3000 cm1. The elemental oxidation state was analyzed using AXIS ULTA-AXIS 165 X-ray Photoelectron Spectrometer (Kratos Analytical). The morphological features were analyzed using FEI Quanta-250 Field Emission Scanning Electron Microscopy (FE-SEM)) and JEOL JEM 2100 High resolution transmission electron microscope (HR-TEM). The electrochemical performance was carried out in CR2032 coin type cell. The electrodes for electrochemical measurements were prepared by slurry-casting method on copper foil current collector. The slurry contained 75 wt % of Co2.85Si0.15O4 powder, 15 wt % carbon black as conductive additives and 10 wt % of poly(vinylidene) fluoride binder dissolved in N-methylpyrrolidinone (NMP) solvent. The obtained coated electrode was dried in a vacuum at 90 C overnight. Electrodes of the desired size for the cell fabrication were punched out from the slurry electrode. The coin cells were assembled in an ultra-pure argon filled glove box by sandwiching a cathode (Co2.85Si0.15O4 or composite), a lithium foil anode separated by polypropylene separator (Celgard 2400). 1 M LiPF6 dissolved in ethylene carbonate (EC) dimethyl carbonate (DMC) and diethyl carbonate (DEC) (1:1:1; v/v) was used as the electrolyte. The C-DC studies were performed between 0 and 3 V at different current rates in an Arbin BT-2000 Battery Test System at ambient temperature. The cyclic voltammetry (CV) and electrochemical impedance spectroscopic (EIS) measurements were conducted using an electrochemical analyzer (SP-150, Bio-Logic, France).

2. Experimental Details 3. Results and Discussion The Co2.85Si0.15O4 samples were prepared by using hydrothermal method. The starting materials Co(NO3)26H2O, C8H20O4Si (Tetraethyl orthosilicates, TEOS), ethanol and ammonia were purchased from Merck Pvt. Ltd Mumbai. In typical synthesis, 1.38 g of cobalt nitrate hexahydrate was dissolved in 35 ml of water and 0.52 ml of TEOS was dissolved into 15 ml of ethanol separately. The TEOS solution was added dropwise to the cobalt nitrate solution and stirred for 30 minutes. The ammonia solution was used to change the pH to 12. Subsequently, the mixture was poured into 75 ml of Teflon-lined stainless steel autoclave and the set up was kept at 220 C for 16 hrs and cooled to room temperature. The hydrothermally prepared samples were washed several times with water and ethanol to remove the residual compounds. Finally, the sample was dried at 80 C for 24 hrs to get the final product. Co2.85Si0.15O4@C composite was obtained through ex-situ hydrothermal method. The 0.3 g of prepared Co2.85Si0.15O4

3.1. Synthesis of Co2.85Si0.15O4 Initially, the Co(NO3)26H2O and C8H20O4Si are undergone hydrolysis reaction in the presence of water and ethanol medium and formed aqueous complexes of Co2+(H2O)6 (Eqs. (1) and (2)) and Si(OH)4 (Eq. (4)), respectively. Subsequently, during hydrothermal conditions, the Co2+(H2O)6 reacted with Si(OH)4 to produce the Co2.85Si0.15O4 solid solutions (Eq. (5)) and amorphous SiO2 phase (Eq. (5)) as by products. Normally, the nitrate containing ions easily formed as oxides due to their strong oxidative properties in supercritical conditions. This prevents the entering of Si ions in to the crystal structure. Therefore, the excess Si(OH)4 formed as amorphous silica. The similar type of observations has already been reported by Wang et al. [24]. The reaction mechanism for the formation of Co2.85Si0.15O4 solid solutions is as follows [22–24].

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S. Yuvaraj et al. / Electrochimica Acta 158 (2015) 446–456 H2 O

CoðNO3 Þ2 6H2 O ! Co2þ þ 2NO3 þ 6OH þ 6Hþ

(1)

Co2+ + 6H2O ! Co2+(H2O)6

(2)

NH3H2O ! NH4+ + OH

(3)

SiðOC2 H5 Þ4 þ 4H2 O

NH3

!

C2 H5 OH

SiðOHÞ4 þ 4C2 H5 OH

(4)

In general, (3 x)Co2+(H2O)6 + 8OH + Si(OH)4 ! Co3xSixO4 + (1 x)SiO2 + nH2O (5) 3.2. Structural and Thermal analysis Fig. 1(a) shows the XRD pattern of as-prepared Co2.85Si0.15O4 sample. The sharp and well defined diffraction peaks observed at 2u = 19.04, 31.32, 36.90, 38.52, 44.85, 55.68, 59.38 and 65.26 , corresponding to the lattice planes of (111), (2 2 0), (3 11), (2 2 2), (4 0 0), (4 2 2), (5 11) and (4 4 0) respectively, which inferred the formation of the cubic spinel structure without any impurity phases. The refined value of the lattice parameter a = 8.0919(6) Å was considerably smaller than the literature data for Co2SiO4 (a = 8.14 Å) [JCPDS 42-1462] but higher as the cell parameter of Co3O4 (a = 8.084 Å) [JCPDS 15-497]. This observation suggested the possible formation of Co2.85Si0.15O4 solid solution similar to the related iron system as reported by Woodland and co-workers [25–28].

In order to get the complete information about the structure of synthesized sample, a full-profile Rietveld refinement was carried out. The refinement of Co3xSixO4 structure has been performed in space group Fd3m in the structural model, in which the Co2+ and Co3+ ions occupy tetrahedral 8a (1/8 1/8 1/8) and octahedral 16d (1/2 1/2 1/2) positions respectively, and the oxygen species are located on the general 32e (x x x) sites. Full-profile Rietveld refinement of the lattice parameter and oxygen coordinates, together with isotropic displacement parameters of the atoms shows good fit between experimental and calculated profiles of Co3xSixO4 sample. However, the values of isotropic displacement parameters (adp’s) of Co ions in the tetrahedral and octahedral sites (Biso = 1.4 (8) and 0.4(6), respectively) proved that the tetrahedral sites can be partially occupied with Si atoms. Indeed, further structure refinement assuming mixed occupancy of the tetrahedral 8(a) site led to more reasonable values of adp's for the Co atoms in tetrahedral and octahedral sites (see Table 1) and the detectable decrease of the residuals. According to the results obtained, the refined composition of the Co3xSixO4 sample corresponds to the formula Co2.85Si0.15O4. Practically, the same composition of the sample can be evaluated from the comparison of the lattice parameter obtained for the Co2.85Si0.15O4 sample with the corresponding values of Co3O4 and Co2SiO4 (Fig. 1(c)). Since the refined composition Co2.85Si0.15O4 differs from the expected nominal Co2SiO4 one, it is believed that the remaining silica is presented in the sample as an amorphous phase, which is not observed in the XRD pattern. Graphical results of Rietveld refinement of Co2.85Si0.15O4 structure are presented in Fig. 1(b). Final structural parameters of Co2.85Si0.15O4 and corresponding residuals are presented in Table 1, selected interatomic distances and angles are collected in Table 2. In the Co2.85Si0.15O4 structure the Si and Co species are

Fig. 1. (a) XRD pattern of as-prepared Co2.85Si0.15O4, (b) Graphical results of Rietveld refinement of Co2.85Si0.15O4 structure. Experimental XRD pattern is shown in comparison with the calculated pattern. The difference between measured and calculated profiles is shown as a curve below the diagrams. Short vertical bars indicated the positions of diffraction maxima, (c) Compositional dependence of the lattice parameter in the Co3O4-Co2SiO4 system, (d) Coordination polyhedra of Co and Co/Si species and (e) view of spinel Co2.85Si0.15O4 structure as a packing of CoO6 octahedra and Co/SiO4 tetrahedra.

S. Yuvaraj et al. / Electrochimica Acta 158 (2015) 446–456 Table 1 Crystallographic data for Co2.85Si0.15O4 (S.G. Fd3 m,Z = 8, a = 8.0919(6) Å; RI = 0.056; Rp = 0.185). Atoms, Sites a

Co/Si , 8a Co2, 16d O, 32e a

X

y

Z

Biso/eq, Å2

1/8 1/2 0.264(2)

1/8 1/2 x

1/8 1/2 x

0.8(5) 0.6(4) 1.2(8)

Occupation: Co/Si = 0.85(6)Co + 0.15(6) Si.

Table 2 Selected bond lengths and bond angles with estimated standard deviations in parenthesis in Co2.85Si0.15O4 structure. Atoms

Distances (Å)

(Co/Si)O4 tetrahedra Co/Si—O 1.94(2) 4 O—O 3.17(2) 4 CoO6 octahedra Co—O 1.92(2) 6

2.55(2) 6 2.87(2) 6

O—O O—O

Atoms

Angles (degrees)

O—(Co/Si)—O O—O—O

109.47(7) 4 60.00(6) 12

O—Co—O O—Co—O O—Co—O O—O—O

83.25(7) 6 96.75(7) 6 180.00(7) 3 60.00(6) 6

O—O—O O—O—O

63.62(6) 12 52.76(6) 6

statistically located in the centers of tetrahedral 8a sites. The values of O—(Co/Si)—O interatomic angles correspond to the ideal intratetrahedral angle of 109.47, and the shape of tetrahedral faces does not deviate from the regular triangle (Table 2, Fig. 1(d)). The 16d sites in Co2.85Si0.15O4 structure are solely occupied with the cobalt ions, which are surrounded by six equidistanced oxygen species, forming distorted octahedral environment (Table 2, Fig. 1(d)). Deformation if CoO6 octahedra is reflected in the deviation of O—Co—O intraoctahedral angles from 90 as well as in the distortion of octahedral faces (six from eight), which deviate considerably from the regular triangle form (Table 2, Fig. 1(d)). Projection of as-described Co2.85Si0.15O4 structure along [1 0 1] axis is shown on the right panel of Fig. 1(e). The carbon content in composite was calculated by thermo gravimetric analysis. Fig. 2(a) shows the TGA curve of composite showing two distinct regions. The weight loss occurred during room temperature to 180 C corresponds to the evaporation of physically adsorbed water molecules in the sample. The second sharp weight loss noted between 280 and 410 C was resulted from the combustion of residual carbon on the composite, which is calculated about 55%.

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The surface functional groups were analyzed through FTIR spectra. Fig. 2(b) shows the FTIR spectra of Co2.85Si0.15O4 and composite in the region between 400 and 4000 cm1. Fig. 2(b) represents the four characteristic peaks of Co2.85Si0.15O4 solid solution centered at 453, 581, 664, 747 and 1014 cm1. The first intense band at 581 cm1 inferred that the vibration of Co3+ ion in the octahedral site and sharp intense peak at 664 cm1 indicated the Co2+ in tetrahedral site which further confirms the spinel structure of Co3O4 [29]. The observed band at 3432 cm1 assigned to the presence of —OH group, which has come from the physically adsorbed water molecule. The small peaks observed at 454 cm1 and 747 cm1 inferred that the rocking mode and bending vibration of Si—O—Si in the amorphous silica. The intense band at 1014 cm1 assigned to the stretching vibration of Si—O—Si [30]. The small peak at 1381 cm1 and 1639 cm1 is attributed to the existence of nitrate functional group which comes from the precursor and stretching vibration of C¼C respectively. The FT-IR spectra of composite indicated that the peak intensity was reduced due to the presence of amorphous carbon. An intense peak appeared at 1373 cm1 is an indication of in-plane bending vibrations of hydroxyl groups and peak at 1464 cm1 was attributed to the in-plane bending vibration of —CH2 functional groups. The weak peak observed at 2853 cm1 and 2926 cm1, inferred the symmetric and asymmetric stretching vibration of —CH2. The intense sharp peak at 1611 cm1 was ascribed to the stretching vibration of C¼C and small peak at 1694 cm1 corresponds to the stretching vibration of C¼O, which revealed that the carbon was surrounded by the active material during carbonization process [31]. Raman spectroscopy was carried out to find the nature of the carbon and vibrational frequency of Co2.85Si0.15O4 and composite shown in Fig. 2(c). In Co2.85Si0.15O4 solid solution, a peak at 220 cm1 was assigned to Si—C—C deformation mode and two small intense peaks at 269 and 341 cm1 indicates the SiO3 rocking mode. The peaks at 383 cm1 was assigned to the vibration of SiO3 symmetric deformation mode and two vibration bands were observed at 480 and 517 cm1 which indicates the antisymmetric deformation mode of SiO3 respectively [32]. The Raman vibration bands between 570 and 650 cm1 were strongly represented the Si—C stretching mode [32,33]. The strong bands at 819 cm1 was associated to the Si—O—Si symmetric stretching vibration and small peaks were observed between 1050 and 1170 cm1 assigned to the antisymmetric stretching vibration of Si—O—Si bond [33]. The vibration bands observed at 1410 and 1594 cm1 inferred the CH2 scissoring and C¼C stretching mode, respectively. The above vibrational frequencies were well matched with the reported organosilica nanotubes [32,33]. Similarly, the Raman spectrum was obtained for composite. Two broad and high intense bands

110 100

80 70 60 50 40 30

Intensity (a.u)

% Transmittance

Weight loss (%)

90

20 10 0

0

100

200

300

400

500

600

o Temperature ( C)

700

800 4000

3500

3000

2500

2000

1500

-1 Wavenumber (cm )

1000

500

200 400 600 800 1000 1200 1400 1600 1800 2000

-1 Raman shift (cm )

Fig. 2. (a) TGA curve of Co2.85Si0.15O4@C composite, (b) FTIR spectra of Co2.85Si0.15O4 and Co2.85Si0.15O4@C composite, (c) Raman spectra of Co2.85Si0.15O4@C composite.

450


Fig. 3. (a) shows the survey spectrum, (b) Co 2p region, (c) Si 2p region, (d) Oxygen 1s region and (e) C 1s spectra of Co2.85Si0.15O4@C.

revealed at 1386 and 1589 cm1 that correlated to the disorder carbon (D-band) and graphitic carbon (G-band), respectively. The intensity of the G-band is higher than that of D-band, revealed that high contribution of sp2-hybridized carbon present in the Co2.85Si0.15O4@C composite [34]. Fig. 3 shows the representative XPS spectra of composite to investigate the presence of elements and their oxidation state.

Fig. 3(a) shows the survey spectrum of the composite, which clearly elucidated that, the occurrence of Co, Si, O and C. Fig. 3(b) indicates the Co 2p region, which was deconvoluted into multiple peaks, it shows three sharp peaks at 779.09 eV, 780.8 eV and 794.7 eV which represents the spin-orbit doublets of Co 2p3/2 and Co 2p1/2 that indicates the presence of Co3+ and Co2+ oxidation state [35]. The satellite peaks were observed in the

Fig. 4. (a, b and c) shows the FE-SEM and TEM image of Co2.85Si0.15O4, (d, e and f) shows the FE-SEM and TEM image of Co2.85Si0.15O4@C composite and Inset figure of 3(c and f) indicates the SAED pattern of Co2.85Si0.15O4 and Co2.85Si0.15O4@C composite.


deconvoluted spectrum, which reveals the presence of Co2+ and Co3+ ions. Three intense peaks of 100.7 eV, 101.3 eV and 102.7 eV corresponding to Si—C, Si—O bond and oxidized silicon (SiOx (x > 1)), respectively, were observed in the deconvoluted spectrum of Si 2p (Fig. 3(c)), which revealed that Si exists in different valence state of Si+, Si2+ and Si3+ [36,37]. Fig. 3(d) shows the deconvoluted O 1 s spectrum where the sharp and high intensity peak situated at 530.0 eV inferred the metal-oxygen bond. The observed other two binding energies at 531.3 eV and 532.7 eV corresponds to the defect in the structure or chemisorbed hydroxyl groups under coordinated with the oxygen and physicochemical adsorbed water molecules on the surface of the particles [38]. Fig. 3(e) indicates the deconvoluted C 1 s spectrum that shows the four predominant peaks at 283.08 eV, 284.21 eV, 285.34 eV and 286.82 eV. The sharp and high intensity peak at 283.08 eV attributed to the binding energy of C—Si [39] and following high intensity peak at 284.21 eV ascribed to sp2 hybridized carbon of C—C bond. The peak at 285.34 eV was the characteristic peak of sp3 hybridized carbon of C¼C bond. Comparatively, the intensity of C¼C was low compared with C—C bond, which inferred that sp2 hybridized carbon was more prominent in the prepared sample. A small peak at 286.82 eV was assigned to the functional group of C—O bond [40]. Overall, the XPS spectra concluded that cobalt exist in +2 and +3 oxidation state, the presence of oxidized silicon (SiOx) and carbon in the prepared composite.

3.4. Electrochemical studies Fig. 5(a) shows the cyclic voltammogram of Co2.85Si0.15O4 at a scan rate of 0.075 mV s1 within a potential range of 0–3 V. In Fig. 5(a), the two cathodic peaks could be observed at 0.99 V and 0.7 V in first cycle, which corroborated the reduction reaction of Co2.85Si0.15O4, electrolyte decomposition and solid electrolyte interphase film formation [43]. However, second cycle is completely different from first cycle, confirming the reversible reaction mechanism of the active material. A sharp and broad anodic peak located at 2.09 V indicated the oxidation reaction of metallic cobalt and their corresponding cathodic peak was observed at 1.10 V and there is no obvious effect of Si in the second cycle and it may be the formation of Li4SiO4 inactive matrix in the first cycle at the same time minimum amount of Si on Co2.85Si0.15O4 solid solution and surrounding SiO2 amorphous matrix does not show any implication on Li+ ion storage and their electrochemical reaction mechanism may be possible as follows [44,45], Co3xSixO4 + 8Li+ + 8e ! (3x)Co + xLi4SiO4 + (4 4x)Li2O

(6)

(3x)Co + (4 4x)Li2O $ Co3xO(4 4x) + (8 8x)Li+ + (8 8x)e(7)

3.3. Morphological Analysis The size and morphological features of the prepared samples were analyzed through FE-SEM analysis. Fig. 4(a) shows the micrographs of Co2.85Si0.15O4 solid solution, which represented the formation of uniformly distributed cube shape Co2.85Si0.15O4 particles without any agglomeration and the particle size was in the range of 0.5 to 1 mm. It was well documented that the growth mechanism of the cube shape Co2.85Si0.15O4 was considered as three step growth process during hydrothermal method such as nucleation, dissolution and re-crystallization [41,42]. The elemental mapping is given in Fig. S1(c–f) indicated that the dense distribution of cobalt element on the surface of the Co2.85Si0.15O4 cubes. On the other hand, Si element was presented in both Co2.85Si0.15O4 cubes as well as the surroundings of the Co2.85Si0.15O4 particles, which infers the existence of amorphous silica in the prepared samples. The FE-SEM images of composite are shown in Fig. 4(d) which indicates the non-uniform shape of particles. This may be due to the carbonothermal reduction reaction since the sample containing 55% of carbon and EDAX spectra shown in Fig. S2(d–h)) indicates the solid solution of Co2.85Si0.15O4 was surrounded by both carbon and amorphous silica. The TEM images of Co2.85Si0.15O4 were illustrated in Fig. 4(b, c). It can be seen that the shape of the Co2.85Si0.15O4 particles was cube with micrometer in size around 700 nm. The observed dot patterns from the SAED image inferred the crystalline nature of Co2.85Si0.15O4 cubes. The calculated d-spacing values from the SAED pattern were 0.2965, 0.1489 and 0.0991 nm, which corresponds to the (2 2 0), (4 4 0) and (7 3 1) inter planes of the cubic system of Co2.85Si0.15O4 particles and it is confirmed through the standard XRD pattern. The EDAX spectra revealed the elements present in the compounds including cobalt, silicon and oxygen (Fig. S3 (a)). Fig. 4(e, f) shows the TEM images of composite. It shows that the particles are in non-uniform spherical shape due to the carbonothermal reduction reaction during hydrothermal conditions since the carbon content is high. EDAX spectrum is shown in Fig. S3 (b), it confirmed the presence of cobalt, silicon and carbon elements in the sample.

451

Fig. 5. (a) CV profile of Co2.85Si0.15O4 and (b) Co2.85Si0.15O4@C composite.

452


The CV profile of composite was shown in Fig. 5(b), which revealed the different electrochemical properties compared to pristine electrode as presented in Fig. 5(a). In the first cycle, the reduction reaction takes place at 1.24 V, 0.81 V and 0.01 V which is inferred that reduction of Co2.85Si0.15O4 into metallic cobalt, side reaction of electrolyte, SEI film formation and Li+ ions insertion into the carbon material, respectively [38,39] and two broad anodic peaks at 1.24 V and 2.30 V were attributed to the extraction of Li+ ions from the carbon matrix and the oxidation reaction of cobalt into cobalt oxide and Li2O formation. In second cycle, reduction peak shift from 1.25 V to 1.64 V represents the reversible reaction of cobalt oxide into Co formation followed by Li2O matrix formation and observed peak shift due to electrode polarization and cyclic voltammogram result is assured that carbon composite also participate in the electrochemical reaction as follows, [46,47], 2C + 2Li+ + 2e $ LiC2 + 4Li2O

(8)

The current area of the subsequent cycles decreases that reveal the capacity fading of Co2.85Si0.15O4 electrodes. But in composite, CV curve overlap with each other that indicates the good cycling stability of the material. Since electrochemical activity of silicon on lithium ion storage cannot be detected in CV studies and explained through dQ/dV plot which is given in Fig. 6(a, b). Fig. 6(a) shows the dQ/dV plot of Co2.85Si0.15O4 and it does not showed any reaction between the SiO2 matrix and lithium. In Co2.85Si0.15O4 embedded on SiO2 matrix, SiO2 acts as an inactive matrix for lithium storage because SiO2 consists interconnected rings of three to eight SiO4 tetrahedral units and joined together through bridging oxygen atom with a certain range of Si—O—Si bond angle and SiO2 exhibited in different polymorphic structure such as a-quartz, coesite, cristobalite and keatite which is mainly depends on the bond angle (u) of Si—O—Si and ring size n. The SiO2 polymorphic

structure is not favourable for energy storage application due to their charge transfer phenomena [48,49]. By reducing the bridging-oxygen bond angle could enable the SiO2 to storage energy within its structure. Ajioka et al. reported bond angle has redistributed in the 4-member rings and 7, 8 member rings after implantation to He+ ion beams. Unfortunately, it is not possible in a-quartz because of its most stable glass network formation [49]. Similar type of observations has been reported by Chang et al. [8]. He has prepared the amorphous silica by altering the valence state of SiO2 using ball milling and obtained improved electrochemical performances [8]. On the other hand, dQ/dV curve of composite showed different reaction mechanism as compared with pristine Co2.85Si0.15O4. Fig. 6(b) shows the dQ/dV curve of composite which indicated the peaks observed at 0.76 V, 0.34 V, 0.27 V and 0.15 V. The small peak at 0.76 V was attributed to the formation SEI film on the SiOx/C and electrolyte decomposition. The additional peak at 0.34 V corresponds to lithium ion reaction with SiO2 to form Si, inactive lithium silicate and simultaneous formation of amorphous Li2O matrix [11,50]. A sharp peak at 0.15 V is an indication of lithium alloy formation with silicon (LixSi) and possible electrochemical reaction is given below according to literature [9,50], 2SiOx + yLi+ + ye ! LiySiOx + Si

(9)

SiOx + yLi+ + ye ! LiyOx + Si

(10)

Si + xLi+ + xe $ LixSi

(11)

During the preparation of carbon composite, glucose acted as carbon source as well as reducing agent. It reduced the SiO2 to SiOx (oxidized silicon), lowering the bridging angle between the silicon

Fig. 6. (a,b) shows the dQ/dV curve of first discharging process (c,d) First five cycles of Charge–discharging curve . (e) Cycling stability curve and (f) Rate capability curve of the Co2.85Si0.15O4 and Co2.85Si0.15O4@C composite.


and oxygen (Si—O—Si), which facilitated Si—O breakage during discharge process [8]. In composite silicon exists in different oxidation states like +1, +2 and +3 which substantiate from the XPS silicon 2p spectra. Fig. 6(c, d) shows the first five charge-discharge curves of Co2.85Si0.15O4 and Co2.85Si0.15O4@C composite, respectively at a current density of 75 mA g1. In Fig. 6(c), the first discharge curve exhibited the two plateau regions at 1.2 V and 0.9 V, which attributed to the reduction reaction of active material and electrolyte decomposition. Both the samples were exhibited the higher discharge capacity in the first cycle and this irreversible capacity was resulted from SEI film formation and electrolyte decomposition [26,51,52]. The first discharge and charge capacity values of Co2.85Si0.15O4 was about 1103 and 625 mAh g1 and their corresponding coulombic efficiency was 57%. In the second cycle, the charge and discharge capacity values are 527 and 600 mAh g1 according to the reversible reaction of Eq. (7) and their coulombic efficiency is 88%. In subsequent cycles, discharge capacity was linearly decreased due to the formation of unstable SEI film [53]. In Fig. 6(d), carbon composite Co2.85Si0.15O4 sample, discharge curve had only one plateau followed by slanting region. The first plateau at 1.31 V infers the reduction reaction of Co3O4 into metallic cobalt and the formation of Li2O amorphous matrix. The slanting region is attributed to the electrolyte decomposition and insertion of lithium ions into the carbon matrix. In the first cycle, Co2.85Si0.15O4@C composite delivered the discharge capacity of 585 mAh g1 at 75 mA g1 current density. During cycling process, the capacity fading was almost negligible due to the presence of carbon, which enhanced conductivity of pristine Co2.85Si0.15O4 as well as the structural stability. The cycling stability of pristine and carbon coated sample at current density of 75 mA g1 in the voltage window of 0–3 V were given in Fig. 6(e). The Co2.85Si0.15O4 and composite maintained the specific capacity values of 153 and 444 mAh g1 after 50 cycles at current density of 75 mA g1. The pristine material displayed serious capacity fading and poor capacity retention of 26% after

453

50 cycles. The monotonous capacity fading of pristine material may be due to inability of the active material to accommodate the volume expansion it causes the destruction of active material, which leads to poor electrical conduction between the active material and unstable SEI layer. The destruction of SEI layer leave the electrode material to interact with electrolyte, which resulted the dissolution of active species thus reduced the structural stability [54,55]. In the case of carbon composite material, cycling stability was increased after few cycles, indicating the stable SEI layer formation occurred on the active material. Also, composite showed excellent cycling stability and capacity retention of 79% after 50 cycles. Enrichment of electrochemical performance is mainly attributed to the carbon and SiOx matrix which is additionally facilitated to Li-ion storage, prevented the agglomeration and maintaining good contact between the active material. Previously, Jin et al., reported that cube shape Co3O4 particles, Galvanostatic cycling results showed that the drastic decrease of discharge capacity of Co3O4 cube due to their larger particle size which offer longer transmission path for Li+ ions and high internal resistance thus leads to severe capacity fading which is well agreed with our pristine Co2.85Si0.15O4 cube particles [56]. However, the Co2.85Si0.15O4@C composite shows the good cycling stability as compared to cube shape Co3O4 [56] and pristine Co2.85Si0.15O4 particles. But moderate specific capacity was achieved for composite due to high weight percentage of carbon (55%). Whereas, the carbon matrix within the Co2.85Si0.15O4 particles tend to access more electrolytes within its structure, which almost eliminated the volume expansion during charging-discharging process and hence stable cyclic performance as achieved. Beside, carbon layer also acted as protection layer to restrict the direct interaction between the active material and electrolyte, which protected SEI layer and minimizing the Co dissolution during charging-discharging process. Fig. 6(f) presented the rate performance of the Co2.85Si0.15O4 and composite material at different current densities between 0 and 3 V. The composite electrode delivered the discharge

Fig. 7. (a,b) shows the EIS spectra of Co2.85Si0.15O4 sample before and after 50 cycles, (c,d) EIS spectra of Co2.85Si0.15O4@C sample before and after 50 cycles (e) Equivalent circuit and (f) Randles plot of Co2.85Si0.15O4 and Co2.85Si0.15O4@C composite before-after 50 cycles.

454


Table 3 EIS parameters. Sample Co2.85Si0.15O4 (before cycle) Co2.85Si0.15O4 (after 50 cycles) Co2.85Si0.15O4@C (before cycle) Co2.85Si0.15O4@C (after 50 cycles)

Rs (V) 5.60 8.49

RSEI (V) 2.61 122.0

41.28

0.66

10.71

44.16

CPESEI 106 (F) 57.64 13.09

Rct (V) 16.07 352.0

74.14 35.7

4.98 123.8

capacity values are 480, 414, 360, 302 and 284 mAh g1 at the current density of 75, 100, 150, 225 and 300 mA g1, respectively. On the other hand, pristine Co2.85Si0.15O4 electrode delivered much lower capacity values at the same current density. In addition, the pristine electrode restored the capacity of only about 120 mAh g1 when the current density was return to 75 mA g1, which indicated the poor rate capacity. The composite displayed the discharge capacity of 360 mAh g1 at 150 mA g1 which was higher compared to pristine one which reveals that good rate capability of the material. The composite electrode was retained 446 mAh g1 capacity when current density return to 75 mA g1 thus indicated the good reversibility as well as potential of the electrode material. Electrochemical Impedance spectroscopy (EIS) is an important tool to analyze the electrode reaction kinetics between the electrode and electrolyte interface. Fig. 7(a, b) shows the EIS spectra of Co2.85Si0.15O4 and composite measured in the frequency range of 100 KHz to 10 mHz. Generally, EIS consist of three regions; semicircle at high and medium frequency region and an inclined line at low frequency region [57]. The semicircle at high frequency region related to the solution resistance, attributing to the electrolyte decomposition occurs on the surface of the electrode material [53]. Another semi circle at medium frequency region is related to the charge transfer resistance between the electrode and electrolyte interface. Finally, an inclined line at angle of 45 at low frequency region was corresponding to the diffusion of Li ions in to the active material. EIS parameters were calculated through fitting according to the equivalent circuit presented in Fig. 7(e), and the fitting values are shown in Table 3. Fig. 7(a, b) shows the EIS spectra of before and after cycles of carbon free Co2.85Si0.15O4. As seen from table, the pristine and carbon composite electrode exhibits obvious variation in solution resistance (Rs) due to the high surface area of the carbon composite material, which allows absorbing more electrolytes within its porous structure. This aids to form more stable SEI layer and hence the difference in Rs is noted for the fresh cells in Table 3. It was reported that the Rs was mainly influenced by the SEI layer on the surface of the electrode [58]. This stable SEI layer formation also prevents direct contact between the active species and electrolyte, minimizing the dissolution of active species thereby enhancing the Li-ion storage capability of the composite Co2.85Si0.15O4@C electrode [59]. This justification is correlating well with the results obtained from the charge-discharge and rate performance studies. The carbon free electrode material has Rct and RSEI values of before and after cycles are 2.61, 16.07 and 122, 352 V respectively. The Rct and RSEI values significantly increased after cycles, which implied that the surface film resistance increased upon cycling process, representing the formation of thick SEI film. This insulating thick SEI layer reduced the capacity value by blocking the active species to be participated in the electrochemical reaction thus leads to the monotonous capacity fading during cycling process. Similarly, charge transfer

CPEdl 106 (F)

CPEdif 103 (F)

s (V.s1/2)

D (cm2.s1)

74.21

2.182

34.14

2.00 1013

61.59

0.759

255.71

3.56 1015

13.23

1.376

161.33

8.96 1015

16.26

8.82 1013

252.0

10.16

resistance also increased considerably after cycles due to the agglomeration and cracks of the surface of the electrode during cycling process and the loss of contact between the active materials and current collector [60]. Similar behavior was also observed for the composite electrode as presented in Fig. 7(c, d) . The Rct and RSEI values of before and after cycles were 0.66 and 4.98, 44.16 and 123.8 V, respectively. The stable SEI film formation was identified after ten cycles through cycling curve and then feeble capacity fading takes place, which assured that the value of Rct and RSEI was approximately same after the stable SEI film formation. The charge transfer resistance of carbon composite Co2.85Si0.15O4 electrode was decreased as compared to pristine electrode, which confirmed the presence of carbon effectively reduced the volume expansion during chargingdischarging process and enhanced the electronic conductivity. Further, Lithium ion diffusion coefficient was calculated using the following equation [53,61,62], D = R2T2/2 A2n4F4C2s2

(12)

Where R is gas constant, T is absolute temperature, A is the area of the electrode surface, n is the number of electron transfer, F is the Faraday's constant, C is the concentration of Li ions and s is the Warburg factor. s can be calculated using following relation, Zw = Rs + Rct + s v1/2

(13)

in Fig. 7(f) showed the relationship between the Zre and v the low frequency region. The calculated diffusion coefficient values before and after cycles for Co2.85Si0.15O4 and composite samples were 2.00 1013, 3.56 1015, 8.96 1015 and 8.82 1013 cm2 s1 respectively. It can be seen from Fig. 7(f) that the diffusion of Li-ions is maximum for carbon free electrode material because electrolyte fully utilizes the active material without any restriction and many side reactions takes place at the surface of the electrode. After cycles, the diffusion of Li-ions is decreased due to unstable formation of SEI film and deterioration of active material. In the case of carbon coated sample, before cycling the diffusion coefficient is minimum because of the large amount of carbon content in the composite, which hindered Li-ions diffusion. However, diffusion of Li ions was noticeably increased after initial cycles due to stable SEI film formation and electrolyte penetration into the active material leads to enhance the lithium ion diffusion [63,64]. Among the electrodes, composite showed the excellent Li ion diffusion coefficient. Hence, it is concluded that making carbon composite is one of the simple strategy to enhance the diffusion coefficient and cycling stability of the electrode material. 1/2

4. Conclusion In summary, Co2.85Si0.15O4 with and without carbon was successfully prepared using hydrothermal method. The structural and chemical composition was completely analyzed through


Rietveld refinement which assured that formation of Co2.85Si0.15O4 solid solution. The Raman spectra assured that the formation of graphitic carbon in composite. TEM images and its corresponding EDAX pattern was confirm the presence of carbon and Si in the composite. The electrochemical reaction could explained through dQ/dV curve that indicates that amorphous SiO2 matrix was inactive in pristine and it will be active for composite. The composite electrode showed the specific capacitance of 444 mAh g1 while pristine sample delivered only 153 mAh g1 after 50 cycles at 75 mA g1 between 0 and 3 V. The EIS results assured that the good electronic conductivity of carbon composite sample and calculated diffusion coefficient of pristine material and composite is 3.56 1015 cm2 s1 and 8.82 1013 cm2 s1 which indicated that making carbon composite favors for lithium ion diffusion into the active material. The above result described that the carbon contained sample shows the better cycling stability, good rate capability and electronic conductivity. Acknowledgement One of the authors (RKS) is grateful to UGC (No. 41-838/2012 (SR)) for their financial support under UGC-MRP. S. Yuvaraj would like to thank UGC-BSR (G2/5357/2013) programme, for providing the fellowship to carry out this work successfully. References [1] K. Park, A. Benayad, D.J. Kang, S.G. Doo, Nitridation-Driven conductive Li4Ti5O12 for lithium ion batteries, J. Am. Chem. Soc. 130 (2008) 14930. [2] J.S. Chen, Y.L. Tan, C.M. Li, Y.L. Cheah, D.Y. Luan, S. Madhavi, F.Y.C. Boey, L.A. Archer, X.W. Lou, Constructing hierarchical spheres from large ultrathin anatase TiO2 nanosheets with nearly 100% exposed (0 0 1) facets for fast reversible lithium storage, J. Am. Chem. Soc. 132 (2010) 6124. [3] L.F. Shen, X.G. Zhang, E. Uchaker, C.Z. Yuan, G.Z. Cao, Li4Ti5O12 Nanoparticles embedded in a mesoporous carbon matrix as a superior anode material for high rate lithium ion batteries, Adv. Energy Mater. 2 (2012) 691. [4] H. Xia, M. Lai, L. Lu, Nanoflaky MnO2/carbon nanotube nanocomposites as anode materials for lithium-ion batteries, J. Mater. Chem. 20 (2010) 6896. [5] Z. Xiao, Y. Xia, Z. Ren, Z. Liu, G. Xu, C. Chao, X. Li, G. Shena, G. Han, Facile synthesis of single-crystalline mesoporous a-Fe2O3 and Fe3O4 nanorods as anode materials for lithium-ion batteries, J. Mater. Chem. 22 (2012) 20566. [6] P. Poizot, S. Laruelle, S. Grugeon, L. Dupont, J.M. Tarascon, Nano-sized transition- metal oxides as negative-electrode materials for lithium-ion batteries, Nature 407 (2000) 496. [7] Y. Mao, Q. Kong, B. Guo, L. Shen, Z. Wang, L. Chen, Polypyrrole–NiO composite as high-performance lithium storage material, Electrochim. Acta 105 (2013) 162. [8] W.S. Chang, C.M. Park, J.H. Kim, Y.U. Kim, G. Jeong, H.J. Sohn, Quartz (SiO2): a new energy storage anode material for Li-ion batteries, Energy Environ. Sci. 5 (2012) 6895. [9] Z. Favors, W. Wang, H.H. Bay, A. George, M. Ozkan, C.S. Ozkan, Scalable synthesis of nano-silicon from beach sand for long cycle life Li-ion batteries, Sci. Rep. 4 (2013) 1. [10] J.J. Zhang, Z. Wei, T. Huang, Z.L. Liu, A.S. Yu, Carbon coated TiO2–SiO2 nanocomposites with high grain boundary density as anode materials for lithium-ion batteries, J. Mater. Chem. A 1 (2013) 7360. [11] J. Tu, W. Wang, L. Hu, H. Zhu, S. Jiao, A novel ordered SiOxCy film anode fabricated via electrodeposition in air for Li-ion batteries, J. Mater. Chem. A 2 (2014) 2467. [12] Y. Yao, J. Zhang, L. Xue, T. Huang, A. Yu, Carbon-coated SiO2 nanoparticles as anode material for lithium ion batteries, J. Power Sources 196 (2011) 10240. [13] J. Wang, H. Zhao, J. He, C. Wang, J. Wang, Nano-sized SiOx/C composite anode for lithium ion batteries, J. Power Sources 196 (2011) 4811. [14] M. Zhou, M.L. Gordin, S. Chen, T. Xu, J. Song, D. Lv, D. Wang, Enhanced performance of SiO/Fe2O3 composite as an anode for rechargeable Li-ion batteries, Electrochem. Commun. 28 (2013) 79. [15] P. Zhang, Z.P. Guo, Y. Huang, D. Jia, H.K. Liu, Synthesis of Co3O4/Carbon composite nanowires and their electrochemical properties, J. Power Sources 196 (2011) 6987. [16] X. Shen, D. Mu, S. Chen, B. Xu, B. Wu, F. Wu, Si/mesoporous carbon composite as an anode material for lithium ion batteries, J. Alloys Compd. 552 (2013) 60–64. [17] B. Wang, B. Luo, X. Li, L. Zhi, The dimensionality of Sn anodes in Li-ion batteries, Mater. Today 15 (2012) 544–552. [18] J.G. Ren, C. Wang, Q.H. Wu, X. Liu, Y. Yang, L. He, W. Zhang, A silicon nanowire– reduced graphene oxide composite as a high-performance lithium ion battery anode material, Nanoscale 6 (2014) 3353. [19] X. Zhang, H. Liu, S. Petnikota, S. Ramakrishna, H.J. Fan, Electrospun Fe2O3– carbon composite nanofibers as durable anode materials for lithium ion batteries, J. Mater. Chem. A 2 (2014) 10835.

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