SP cathode material for lithium-ion batteries

SP cathode material for lithium-ion batteries

Journal of Alloys and Compounds 633 (2015) 456–462 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: www.e...

2MB Sizes 1 Downloads 87 Views

Journal of Alloys and Compounds 633 (2015) 456–462

Contents lists available at ScienceDirect

Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom

Synthesis and electrochemical performance of spherical-like Li2FeSiO4/C/SP cathode material for lithium-ion batteries Bing Ren a, Yunhua Xu a,⇑, Yinglin Yan a, Rong Yang a, Juan Wang b a b

School of Materials Science and Engineering, Xi’an University of Technology, Xi’an 710048, PR China Shaanxi Key Laboratory of Nano-materials and Technology, Xi’an University of Architecture and Technology, Xi’an 710055, PR China

a r t i c l e

i n f o

Article history: Received 9 October 2014 Received in revised form 12 January 2015 Accepted 18 January 2015 Available online 24 January 2015 Keywords: Cathode Spherical lithium iron silicate Two-step precipitation method Growth mechanism

a b s t r a c t The novel spherical Li2FeSiO4/C/SP is successfully synthesized by two-step precipitation method. The structure and morphology of the as-prepared samples are characterized by means of XRD, SEM, TEM and HRTEM. The results show that the product is consisted of spherical Li2FeSiO4 microparticles coated with carbon layer and dispersive nanoparticle which is conductive carbon black. The growth mechanism of the spherical Li2FeSiO4/C/SP composites is discussed in particularly. The electrochemical performance is demonstrated by the electrochemical impedance spectra and charge–discharge curves. The product presents excellent electrochemical performance. This is the result of the uniform spherical morphology improved lithium-ion diffusion rate (1.271  1012 cm2 s1) by shortening diffusion pathways of lithium ion, and the carbon coating enhance the electronic conductivity. It is found that the spherical Li2FeSiO4/C/ SP composites show a high capacity as 171.8 mA h g1 at 0.1 C and high coulombic efficiency as 93.6%. The outstanding high-rate capability is also presented as 113.7 mA h g1 at 5 C. Moreover, it exhibits very stable discharge specific capacity at different current densities from 0.2 to 5 C. These indicate that the spherical Li2FeSiO4/C/SP is a very promising candidate for cathode material in lithium-ion batteries. Ó 2015 Elsevier B.V. All rights reserved.

1. Introduction In recent years, the lithium-ion batteries have become the most promising candidate for electric vehicles (EVs) and hybrid electric vehicles (HEVs) [1]. In the research area of lithium-ion batteries (LIBs), great efforts have been devoted to increasing the capacity and improving the security of the cathode materials [2]. Many works have been devoted to the study of lithium iron silicate (Li2FeSiO4) as a kind of cathode material, first reported by Nytén et al. [3], due to its high theoretical capacity (potential capacity of 332 mA h g1) [4], high chemical and electrochemical stability and good cycle performance [5,6]. Furthermore, Li2FeSiO4 materials are also featured with virtues of low cost, high safety and nontoxicity [7,8]. However, the slow lithium-ion diffusion rate [9,10] and low electronic conductivity [2,11] of Li2FeSiO4 lead to the poor rate capability which inhibits its further use in large-scale battery systems. To overcome the demerits, tremendous efforts have been made to enhance the electronic and ionic conductivity of Li2FeSiO4 particles through various material processing approaches, such as particle size reduction [12,13], typical carbon

⇑ Corresponding author. Tel.: +86 29 82202531; fax: +86 29 82202531. E-mail address: [email protected] (Y. Xu). http://dx.doi.org/10.1016/j.jallcom.2015.01.108 0925-8388/Ó 2015 Elsevier B.V. All rights reserved.

coating fabrication of carbonaceous matrices [14–16] and metalion doping [17–19]. Recently, many researches are attempting to enhance the electrochemical properties of cathode materials by investigating various architectures, such as shuttle-like [20], dumbbell-like [21,22], flower-like [23], rod-like [24], bowl-like [25], spindle-like [26,27], and porous materials [28]; which present high capacity, high-rate and good cyclic-performance. As we know, spherical particles and particles in nanometer scale have been reported to provide high discharge capacity and good capacity retention [29–31], because the spherical particles with a reasonable size distribution generally possess higher energy density and reduce the contact interface of electrode with the electrolyte [32,33]. It also can shorten the lithium ion transport distances and enhance ionic diffusion rate [29,34]. Thus, it can be expected that preparing spherical powders is an effective way to improve the electrochemical performance. In addition, coating the particles with carbonaceous conductors is an effective way to improve the electronic conductivity. Up to now, various methods have been developed to prepare Li2FeSiO4, including solid-state reaction [35], sol–gel [13], hydrothermal [12], microwave synthesis [36], combustion synthesis [37], and Pechini synthesis [38]. However, almost no research related to the Li2FeSiO4 cathode materials synthesized by precipitation method. The precipitation method offers mild synthesis

B. Ren et al. / Journal of Alloys and Compounds 633 (2015) 456–462


conditions, high degree of crystalline and narrow particle size distribution of products. Extraordinarily, architecture electrode materials with specific morphologies from nano-scale building blocks could be easily obtained via precipitation method. In this work, we report a kind of spherical Li2FeSiO4/C/SP material synthesized via the two-step precipitation method, which is expected to have high lithium-ion diffusion rate as cathode materials for lithium-ion batteries. Moreover, the mechanism of its formation and the electrochemical performance are investigated. 2. Experimental 2.1. Preparation of materials Li2FeSiO4/C/SP composites were synthesized by two-step precipitation of SiO2 and Fe(II) precursor material in the presence of de-ionized water at room temperature. Mono-dispersed SiO2 spheres were synthesized by base-catalyzed hydrolysis of TEOS, generally, 0.025 mol of TEOS was added into a solution containing 25 ml of ethanol and 20 ml of deionized water. 12 ml of aqueous ammonia was dropped into the as-prepared mixture at room temperature. Under vigorous stirring for about 2 h to fabricate the mono-dispersed SiO2 spheres. The particle size of SiO2 was around 300–400 nm. Oxalic acid solution as carbon source and reducer was added to the solution. Meanwhile oxalic acid could decrease the pH of solution. The pH should be reach to 5 by adding oxalic acid. Then an aqueous solution of FeSO47H2O was dropwise added into the white turbid solution. The SiO2 and Fe(II) precursor particles was obtained after separating by the centrifugal separator and washing by acetone for several times. Then the SiO2 and Fe(II) precursor particles, Li2CO3 and Super P (a kind of nanoscale conductive carbon black, provided by TIMCAL Graphite & Carbon) as another carbon source (10 wt.% compared to the Li2FeSiO4) were mixed under vacuum ball milling and dried at 80 °C in vacuum for 1 h. The powders were treated at 410 °C for 2 h and then 800 °C for 1 h, 4 h, 8 h, 12 h in Ar gas atmosphere. 2.2. Characterization The crystal structure of the as-synthesized powders was characterized by X-ray powder diffraction (XRD) using D/MAX2550V X-ray diffractometer (Rigaku, Japan) with Cu Ka radiation (k = 1.5406 Å) at 40 kV and 40 mA. A simultaneous thermogravimetric–differential thermal analysis (TG–DSC) apparatus STA 449F3 (TA instrument) was used for the thermal characterization. The morphology of the assynthesized powders was examined using a Quanta 200 scanning electron microscope (SEM) (FEI, The Netherlands). The detailed effect of ultrasonic irradiation on the morphology and particle size distribution was observed with transmission electron microscopy (TEM and HRTEM) (FEI, Tecnai G2 F20). The amount of C and Super P in the composite of Li2FeSiO4/C/SP was measured by elemental analysis instrument (Vario micro). Electrochemical measurement was performed using CR2032 coin-type cell. The positive electrode was prepared by mixing 85 wt.% active materials (Li2FeSiO4/C/ SP), 10 wt.% conductive carbon black and 5 wt.% poly-vinylidenefluoride (PVDF) dissolved in N-methylpyrrolidone (NMP). The total carbon content in the electrode is calculated as 14.55 wt.%. The slurry was coated on an Al foil and dried at 120 °C for 8 h. A lithium foil was used as the anode. For the evaluation of the electrochemical property of Li2FeSiO4 in a liquid electrolyte, the electrolyte used was a solvent mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) in a volume ratio of 1:1 containing 1 M of LiPF6. The cell was assembled in an argon-filled glove box. A Celgard 2400 microporous polypropylene was used as the separator membrane. Galvanostatic charge–discharge measurements were performed at room temperature. The electrochemical properties (charge–discharge, cycling capacity, rate performance) of the cells prepared were measured using a BT2000 Arbin Testing System at various currents (0.1 C, 0.2 C, 0.5 C, 1 C, 2 C, and 5 C) with a charge-discharge voltage limit of 1.5–4.3 V. Electrochemical impedance measurement was performed using a Par 2273 Potentiostats-Electrochemistry Workstation in the frequency range 10 mHz to 100 MHz with an amplitude of 10 mV at room temperature.

3. Results and discussions 3.1. Microstructure and morphology A typical XRD pattern of the as-synthesized Li2FeSiO4/C/SP powders heat treated at 800 °C for 8 h is shown in Fig. 1. The distinguished sharp intense peaks correspond to the highly crystalline nature. As reported previously, there are at least three typical structures for Li2FeSiO4: (i) Li3PO4-based orthorhombic structure (Space Group/SG: Pmn21) [3,39], (ii) monoclinic structure (SG:

Fig. 1. XRD patterns of spherical Li2FeSiO4/C/SP obtained at 8 h.

P21/n) [40], and (iii) orthorhombic structure (SG: Pmnb) [41]. The diffraction peaks of the Li2FeSiO4/C/SP are consistent with the previous literatures [20]. The Li2FeSiO4/C/SP are identified as an orthorhombic structure with SG of Pmn21. No characteristic peaks related to any impurity (e.g., Fe3O4, Fe2O3) are observed in the XRD pattern. The reduction nature of oxalic acid prevents the Fe2+ ions from oxidizing to Fe3+. It greatly improved the purity of the product. No crystalline carbon can be obviously detected in the XRD pattern, revealing that the carbon is probably existed as amorphous state or the diffraction peak intensity of crystalline carbon is too low to be detected. It is can be seen that the XRD pattern of Li2FeSiO4/C/SP was not influenced by the introduction of Super P. The morphological images of as-synthesized Li2FeSiO4/C/SP are shown in Fig. 2. Fig. 2a is the SEM image of the sample. The Li2FeSiO4 particles exists as homogeneous sphere-like particles with smooth surfaces. The diameters of the spheres distribute within the range of 500–600 nm. As can be seen in Fig. 2a, a large number of fine particles are dispersed in the void space among spherical Li2FeSiO4 particles. The fine particles should be Super P. Although some Super P particles aggregated together, they distribute in the sample uniformly. To illustrate the detail information on the morphology and particle size distribution of the sample, TEM and HRTEM images are shown in Fig. 2b and c. In Fig. 2b, the nanoparticles, which are distributed around the spherical Li2FeSiO4 particle, are Super P. The particle size of Super P is around 20–40 nm. It can be seen that the as-obtained spherical Li2FeSiO4 adopted a spherical morphology and the diameter of the particles ranged from 500 to 600 nm, which agrees with the SEM results. There is some lighter shadow around the Li2FeSiO4 particle. So the HRTEM image of the edge of spherical particle is shown in Fig. 2c. It can be seen that there is amorphous carbon coated on the surface of the particle and the thickness is 10 nm. The carbon layer was generated by the decomposition of oxalic acid during the heat treatment. The carbon coating thickness and its homogeneity played important roles in determining the electrochemical performance of Li2FeSiO4. The quantity of C and Super P in the composite of Li2FeSiO4/C/SP was measured to be 5.35%. It is widely known that the spherical particles and carbon coating can improve the electrochemical performance. Spherical particles have been reported to provide high discharge capacity and good capacity retention [30,42], because they have higher energy density, reduce the contact interface of electrode with the electrolyte [43,44] and shorten the lithium-ion diffusion distances. Also the surrounding Super P and a uniform amorphous carbon layer can lead to better electrochemical performance. Super P can


B. Ren et al. / Journal of Alloys and Compounds 633 (2015) 456–462

indicate that the synthesis method obtained the sample with spherical morphology and uniform carbon coating. It is believed that the production is very suitable as an electrode material to improve the electrochemical properties of lithium-ion batteries, which have been confirmed by the following electrochemical characterization. 3.2. Growth mechanism According to the two-step precipitation experimental process, a possible reaction mechanism of the Li2FeSiO4 precursor can be proposed. The potential mechanism includes the following reaction (reactions (1)–(4)). Firstly, the mixture of water, ethanol, TEOS and ammonia rapidly produced SiO2 spherical particles (steps (1) and (3)).

SiðOC2 H5 Þ4 þ 4H2 O ! SiðOHÞ4 þ 4C2 H5 OH


SiðOHÞ4 ! SiO2 þ 2H2 O


SiðOHÞ4 þ SiðOC2 H5 Þ4 ! 2SiO2 þ 4C2 H5 OH


Next, with oxalic acid and FeSO47H2O solution added to the above solution, the color turned to yellow. This means FeC2O4 was synthesized by the reaction as step (4).

FeSO4 þ H2 C2 O4 ! FeC2 O4 þ H2 SO4

Fig. 2. Images of SEM (a), TEM (b) and HRTEM (c) of spherical Li2FeSiO4/C/SP obtained at 8 h.

acts as bridges, interconnecting the isolated Li2FeSiO4 particle, and providing valid conductive network, the uniform amorphous carbon layer could effectively restrict the growth of the particles and increase the powder conductivity, therefore the electronic transport properties can be improved [15,45,48]. The results


After added Li2CO3 and Super P to the solution, the precursor was obtained through filtration and ball milling. In order to determine the reaction condition of as-synthesized Li2FeSiO4, the SEM and TG/DSC measurements were applied to characterize the precursors. The SEM image is presented in the insert of Fig. 3. It can be seen that the precursor is consisted of spherical SiO2, blocky FeC2O4 and irregular Li2CO3. The TG/DSC curves are shown in Fig. 3. There are three weight losses corresponding to the three endothermic peaks. At 202 °C, the weight loss can be associated with the desorption of water and carbonization of precursor [35,49]. The ferrous oxalate materials were decomposed and carbonized before 405 °C. At 716 °C, the weight loss can be associated with the decomposing of lithium carbonate. According to the TG/DSC results, the heat treatment process we adopted was keeping at 410 °C at first and then rise to 800 °C for synthesizing.

Fig. 3. The TG/DTA curves for the precursor recorded over the temperature range from ambient to 1000 °C at a heating rate of 5 °C min1 in Ar atmosphere at 100 mL min1 flow rate, the insert is the SEM image of Li2FeSiO4/C/SP precursor.

B. Ren et al. / Journal of Alloys and Compounds 633 (2015) 456–462


To discover the morphology evolution with heat treatment time, the samples treated at 410 °C for 2 h and then 800 °C for 1 h, 4 h, 8 h, 12 h were carried out. The XRD patterns and morphologies of these samples are shown in Fig. 4. As can be seen, when the heat treatment time is 1 h, the morphology of sample is irregular and the phases are mainly composed of iron oxide (Fe3O4) and Li2SiO3 crystals which were identified by XRD analyses. In Fig. 5b, the bulk FeC2O4 particle was disappeared. The morphology of the sample was not uniform. The size distribution was around 100–500 nm. When the duration time extended to 4 h, the morphology turns to spherical particles and tiny irregular particles those are Li2FeSiO4, Li2SiO3 and Fe3O4 according to the corresponding XRD results. As shown in Fig. 5c, spherical particles reappeared and the size was about 300–400 nm. In addition, some irregular particles remained and its size was 100 nm. Further extended the reaction time to 8 h, the impurities are disappeared and the sample is pure Li2FeSiO4 with orthorhombic structure. Furthermore, the morphology of sample was perfect sphere as can be seen in Fig. 5d. The diameter of the sphere was 500–600 nm. There were a bit of small particles around the sphere. If the reaction time reached 12 h, the particles of the sample grew larger and aggregated to 1–2 lm; the morphology is shown as coralloid–like structure in Fig. 5e. Based on the above results, a possible formation of the Li2FeSiO4 was attributed to steps (5)–(8), which is schematically illustrated in Fig. 5. First of all, the FeC2O4 decomposited at 410 °C as the reaction (5).

FeC2 O4 ! FeO þ CO þ CO2

Fig. 4. Phase and morphology of the samples with different heat-treat time.


When the temperature raised from 410 °C to 800 °C, the Li2SiO3 phase occurred at first was a result from the reaction between Li2CO3 and SiO2 used as starting materials (steps (6 and 7)) [46]. After 4 h at 800 °C, the sphere-like Li2FeSiO4 (Fig. 5c) had already obtained through reaction (8). As the reaction progress, the SiO2

Fig. 5. Schematic of formation for the spherical Li2FeSiO4.


B. Ren et al. / Journal of Alloys and Compounds 633 (2015) 456–462

particle collapsed to Li2SiO3 and then reconstructed to Li2FeSiO4 presented as a sphere shape as well. Meanwhile the particle size was increasing from 300–400 nm to 500–600 nm.

Li2 CO3 ! Li2 O þ CO2


Li2 O þ SiO2 ! Li2 SiO3


Li2 SiO3 þ FeO ! Li2 FeSiO4


When the heat-treatment time increased to 8 h, the Li2FeSiO4 particles grew to perfect sphere as shown in Fig. 5d. However, with the reaction time increased to 12 h, the spherical particles grew larger and agglomerated to coralloid–like structure (Fig. 5e). The corresponding SEM images of samples prepared with different heat-treatment time are shown at the bottom of Fig. 5. 3.3. Electrochemical performance To understand the effect of the different Li2FeSiO4/C/SP architectures on the electrochemical performance of electrodes, electrochemical impedance spectra measurements are carried out mainly to measure the lithium ion diffusion. Fig. 6a shows the EIS data of samples obtained under different heat treatment conditions. As shown, each Nyquist plot can be divided into three regions of an intercept at high frequency, followed by a depressed semicircle in the middle-high frequency and a sloping line in the low frequency. Based on the literature [19], an intercept at Zre axis in the high frequency corresponds to the ohmic resistance (Re), which represents the resistance of the electrolyte. The semicircle in the middle-high frequency range indicates the charge transfer resistance (Rct) and the double layer capacitance (Cdl) between the electrolyte and electrode. The straight line in the low frequency represented the Warburg impedance (Zw), which was associated with lithium-ion diffusion in the electrode material. A simplified equivalent circuit model (the inset of Fig. 6a) was constructed to analyze the impedance spectra. The lithium ion diffusion coefficient can be calculated from the low frequency line according to the following Eq. (9) [19,47].

R2 T 2 2A2 n4 F 4 C 4



where R is the gas constant, T is the absolute temperature, A is the surface area of the cathode, n is the charge-transfer number, F is the Faraday constant, C is the concentration of lithium ions in the

Table 1 Impedance parameters for the different samples. Sample (heat time) (h)

Re (X)

Rct (X)

r (X cm2 s1/2)

D (cm2 s1)

i (mA cm2)

4 8 12

9.46 2.69 13.23

64.0 17.1 205.7

61.7 22.7 218.6

1.526  1013 1.271  1012 1.437  1014

1.08  103 4.06  103 3.39  104

cathode, and r is the Warburg factor which is relative with Zre by the following:

Z re ¼ RX þ Rct þ rx1=2


where x is frequency. The relationship between Zre and x1/2 in the low frequency are shown in Fig. 6b. The lithium ion diffusion coefficients for the different samples are obtained and the results are listed in Table 1. It can be observed that the exchange current density (i = RT/nFRct) [47] of the spherical Li2FeSiO4/C/SP composite architectures obtained at 800 °C for 8 h is the highest among that of other samples. Furthermore, the spherical Li2FeSiO4/C/SP exhibited the highest lithium ion diffusion coefficient. The sample obtained at 800 °C for 12 h exhibited the lowest lithium ion diffusion coefficient. The lithium ion diffusion coefficient of sample obtained at 800 °C for 4 h is lower than that obtained at 800 °C for 8 h, but it is larger than that of previous diatomic metallic ions (Ni2+, Cu2+, Zn2+) doped Li2FeSiO4 [17]. Therefore, the EIS results implied three points: one is the excellent electrochemical performance of the Li2FeSiO4/C/SP could be mainly contributed to the enhanced lithium-ion diffusion rate ascribed to the unique morphology, architecture and suitable crystal size; the second point is the smaller particles size, higher specific surface area, improves the diffusion of lithium ions by means of shortening the lithium ion diffusion pathway and enhancing the infiltration of the liquid electrolyte; and the last point is the thin and uniform carbon coating of samples offer smaller hindrance for lithium ions diffusion, because the carbon coating network is an intrinsically inert material for lithium ion diffusion. The charge–discharge curves in different cycles and cycle performances at 0.1 C (1 C = 166 mA h g1) of samples obtained under different heat treatment conditions are shown in Fig. 7. In the 1st cycle, the samples heated for 4 h, 8 h, and 12 h deliver discharge specific capacity of 159.1, 171.8, and 149.2 mA h g1 respectively. The corresponding coulombic efficiency are 93.4% (4 h), 93.6% (8 h) and 97.7% (12 h). The sample heated for 8 h shows the highest

Fig. 6. Electrochemical impedance spectra of the different samples at open circuit potentials (a), the insert is the equivalent circuit used for fitting the experimental EIS data; the relationship curve between Zre and x1/2 in the low frequency (b).

B. Ren et al. / Journal of Alloys and Compounds 633 (2015) 456–462


Fig. 7. Charge–discharge curves ((a) 4 h, (b) 8 h, and (c) 12 h) and cycle performances (d) of different samples at 0.1 C (1 C = 166 mA h g1).

Fig. 8. Electrochemical performances of the spherical Li2FeSiO4/C/SP obtained at 8 h. (a) Charge–discharge curves of the spherical Li2FeSiO4/C/SP at various rates and (b) cyclic performance of the spherical Li2FeSiO4/C/SP at different C rates.

initial charge capacity, 183.4 mA h g1 and discharge capacity, 171.8 mA h g1. And the sample obtained after 4 h heat treatment presents initial charge capacity of 170.3 mA h g1. Such high capacities are resulted by more than one lithium ion extract/insert to the cathode material in the first charge–discharge (166 mA h g1 þ based on single ion reaction: Li2 FeSiO4 ! LiFeSiO4 þ Li þ e and 1 based on double ions reaction: Li2 FeSiO4 ! 332 mA h g þ FeSiO4 þ 2Li þ 2e ). In the 2nd cycle, the discharge specific capacities of the samples decrease to 147.5 mA h g1 (4 h), 162.3 mA h g1 (8 h), 137.3 mA h g1 (12 h). The charge curves are totally different from those of the 1st cycle. The charge plateaus drop off the curve might be ascribed to the extraction of O (Li2O) as

a result of oxidation of oxide ion and the structural rearrangement during the first electrochemical cycling [3,20]. Meanwhile, they caused irreversible capacity loss. After three cycles, the charge–discharge curves exhibit better stability which attribute to the ending of the structure rearrangement. Obviously, the spherical Li2FeSiO4/ C/SP architectures obtained at 8 h had a higher capacity and much better cycle property. The charge–discharge results (Fig. 7b) show that the capacity of as-synthesized samples maintain at above 150 mA h g1 in 10th cycle. It can be see the Li2FeSiO4/C/SP obtained at 800 °C for 8 h than 4 h and 12 h had a higher and wider plateau at above 3.0 V. These results indicate that the spherical Li2FeSiO4/C/SP has great electrochemical properties as cathode


B. Ren et al. / Journal of Alloys and Compounds 633 (2015) 456–462

materials for lithium-ion batteries, which is verified by the EIS results as discussed above. The samples after the initial 3 cycles was performed at different C-rates given to eliminating the effect of structure rearrangement. The typical charge–discharge curves and cyclic performance under various current densities are presented in Fig. 8. As shown in Fig. 8a, at a low rate of 0.2 C, the spherical Li2FeSiO4/C/SP electrode delivered wider plateau at about 3.0 V. With the increasing of Crate from 0.2 to 5 C, the plateau was shorten, and the charge specific capacities under different current density were 163.9 mA h g1 (0.2 C), 158.0 mA h g1 (0.5 C), 150.2 mA h g1 (1 C), 1 1 139.1 mA h g (2 C), and 124.5 mA h g (5 C), which correspond to the discharge specific capacities of 160.3 mA h g1 (0.2 C), 152.4 mA h g1 (0.5 C), 144.6 mA h g1 (1 C), 132.3 mA h g1 (2 C) and 113.7 mA h g1 (5 C). The results indicate that spherical Li2FeSiO4/C/SP possesses excellent rate performance. As shown in Fig. 8b, it is found that the spherical Li2FeSiO4/C/SP electrodes exhibit a very stable discharge capacity with almost no discernible capacity decay at different current densities from 0.2 to 5 C rates. These rate performance and cycle performance indicate great electrochemical properties of the spherical Li2FeSiO4/C/SP, which is attributed to the improvement of lithium ionic conductivity by the uniform spherical morphology and carbon-coating. 4. Conclusions The spherical Li2FeSiO4/C/SP was synthesized via two-step precipitation method. The synthesized spherical architecture consisting of a large amount of nano-spherical primary particles connected with carbon layer and Super P verified by SEM, TEM and XRD techniques. The novel Li2FeSiO4/C/SP, as cathode materials for lithium ion batteries, performed high reversible capacity and coulombic efficency in the first cycle, enhanced cycling performance and C-rate capability, attributed to the unique morphology and uniform carbon coating. The intrinsic reasons for that are spherical morphology improved lithium-ion diffusion rate by shortening diffusion pathways of lithium ion, the carbon coating enhanced the electronic conductivity. The prepared uniform spherical Li2FeSiO4/C/SP material is very promising material proposed as a cathode for a commercial lithium-ion batteries. Acknowledgments This work was supported by the Science Foundation of Shaanxi Provincial Department of Education (Nos. 12JS059, 12JS060, 13JS055 and 13JS063), China. References [1] M. Armand, J.M. Tarascon, Nature 451 (2008) 652–657. [2] R. Dominko, J. Power Sources 184 (2008) 462–468. [3] A. Nytén, A. Abouimrane, M. Armand, T. Gustafsson, J.O. Thomas, Electrochem. Commun. 7 (2005) 156–160. [4] A. Nytén, S. Kamali, L. Häggström, T. Gustafsson, J.O. Thomas, J. Mater. Chem. 16 (2006) 2266–2272. [5] Z. Yan, S. Cai, L. Miao, X. Zhou, Y. Zhao, J. Alloys Comp. 511 (2012) 101–106. [6] R. Dominko, C. Sirisopanaporn, C. Masquelier, D. Hanzel, I. Arcon, M. Gaberscek, J. Electrochem. Soc. 157 (2010) A1309–A1316.

[7] M. Nadherna, R. Dominko, D. Hanzel, J. Reiter, M. Gaberscek, J. Electrochem. Soc. 156 (2009) A619–A626. [8] T. Kojima, A. Kojima, T. Miyuki, Y. Okuyama, T. Sakai, J. Electrochem. Soc. 158 (2011) A1340–A1346. [9] J. Bai, Z. Gong, D. Lv, Y. Li, H. Zou, Y. Yang, J. Mater. Chem. 22 (2012) 12128– 12132. [10] Z.L. Gong, Y.X. Li, G.N. He, J. Li, Y. Yang, Electrochem. Solid-State Lett. 11 (2008) A60–A63. [11] R. Dominko, D.E. Conte, D. Hanzel, M. Gaberscek, J. Jamnik, J. Power Sources 178 (2007) 842–847. [12] Y. Xu, Y. Li, S. Liu, H. Li, Y. Liu, J. Power Sources 220 (2012) 103–107. [13] M. Gaberscek, R. Dominko, J. Jamnik, Electrochem. Commun. 9 (2007) 2778– 2783. [14] Z. Hai, W. Xiaozhen, Z. Ling, Z. Youxiang, Electrochim. Acta 117 (2014) 34–40. [15] X.B. Huang, X. Li, H.Y. Wang, Z.L. Pan, M.Z. Qu, Z.L. Yu, Electrochim. Acta 55 (2010) 7362–7366. [16] J. Yang, X. Kang, L. Hu, X. Gong, D. He, T. Peng, S. Mu, J. Alloys Comp. 572 (2013) 158–162. [17] C. Denga, S. Zhang, S.Y. Yang, B.L. Fu, L. Ma, J. Power Sources 196 (2011) 386– 392. [18] H. Guo, X. Cao, X. Li, L. Li, X. Li, Z. Wang, W. Peng, Q. Li, Electrochim. Acta 55 (2010) 8036–8042. [19] H. Hao, J. Wang, J. Liu, T. Huang, A. Yu, J. Power Sources 210 (2012) 397–401. [20] J. Yang, X. Kang, D. He, T. Peng, L. Hu, S. Mu, J. Power Sources 242 (2013) 171– 178. [21] H. Yang, X.-L. Wu, M.-H. Cao, Y.-G. Guo, J. Phys. Chem. C 113 (2009) 3345– 3351. [22] Won-Hee Ryu, Sung-Jin Lim, Won-Keun Kim, HyukSang Kwona, J. Power Sources 257 (2014) 186–191. [23] J.W. Min, C.J. Yim, W.B. Im, Ceram. Int. 40 (2014) 2029–2034. [24] L. Zhang, B. Wu, N. Li, D. Mu, C. Zhang, F. Wu, J. Power Sources 240 (2013) 644– 652. [25] L. Yi-Ju, L. Yun-Fei, S. Jing, L. Xiao-Yan, W. Yan-Xuan, Electrochim. Acta 119 (2014) 155–163. [26] C. Su, L. Xu, B. Wu, C. Zhang, Electrochim. Acta 56 (2011) 10204–10209. [27] Y. Xia, W. Zhang, H. Huang, Y. Gan, J. Tian, X. Tao, J. Power Sources 196 (2011) 5651–5658. [28] Z. Zheng, Y. Wang, A. Zhang, T. Zhang, F. Cheng, Z. Tao, J. Chen, J. Power Sources 198 (2012) 229–235. [29] Z. Yang, D. Choi, S. Kerisit, K.M. Rosso, D. Wang, J. Zhang, G. Graff, J. Liu, J. Power Sources 192 (2009) 588–598. [30] S.H. Ju, Y.C. Kang, J. Power Sources 178 (2008) 387–392. [31] X. Yang, X. Wang, G. Zou, L. Hu, H. Shu, S. Yang, J. Power Sources 232 (2013) 338–347. [32] J.G. Li, X.M. He, M.S. Fan, R.S. Zhao, C.Y. Jiang, C.R. Wan, Ionics 12 (2006) 77–80. [33] D. Arumugam, K.G. Paruthimal, J. Electroanal. Chem. 24 (2008) 197–204. [34] D.W. Wang, F. Li, M. Liu, G.Q. Lu, H.M. Cheng, Angew. Chem. Int. Ed. 120 (2007) 379–382. [35] L.M. Li, H.J. Guo, X.H. Li, Z.M. Wang, W.J. Peng, K.X. Xiang, X. Cao, J. Power Sources 189 (2009) 45–50. [36] Z. Dong Peng, Y. Bing Cao, G. Rong Hu, K. Du, X. Guang Gao, Z. Wei Xiao, Chin. Chem. Lett. 20 (2009) 1000–1004. [37] M. Dahbi, S. Urbonaite, T. Gustafsson, J. Power Sources 205 (2012) 456–462. [38] R. Dominko, I. Arcˇon, A. Kodre, D. Hanzˇel, M. Gaberšcˇek, J. Power Sources 189 (2009) 51–58. [39] K. Zaghib, A. Ait Salah, N. Ravet, A. Mauger, F. Gendron, C.M. Julien, J. Power Sources 160 (2006) 1381–1386. [40] S.I. Nishimura, S. Hayase, R. Kanno, M. Yashima, N. Nakayama, A. Yamada, J. Am. Chem. Soc. 130 (2008) 13212–13213. [41] A. Boulineau, C. Sirisopanaporn, R. Dominko, A.R. Armstrong, P.G. Bruce, C. Masquelier, Dalton Trans. 39 (2010) 6310–6316. [42] H.L. Zhu, Z.Y. Chen, S. Ji, V. Linkov, Solid State Ionics 179 (2008) 1788–1793. [43] H.E. Wang, D. Qian, Z.G. Lu, Y.K. Li, J. Alloys Comp. 517 (2012) 186–191. [44] M.Y. Son, Y.J. Hong, S.H. Choi, Y.C. Kang, Electrochim. Acta 103 (2013) 110–118. [45] Y. Liu, C. Cao, Electrochim. Acta 55 (2010) 4694–4699. [46] O. Kamon-in, W. Klysubun, W. Limphirat, S. Srilomsak, N. Meethong, Physica B 416 (2013) 69–75. [47] Y. Atef, K.R. Shenouda, Murali, J. Power Sources 176 (2008) 332–339. [48] Xiaobing Huang, Xing Li, Haiyan Wang, Zhonglai Pan, Qu Meizhen, Yu ZuoLong, Solid State Ionics 181 (2010) 1451–1455. [49] Mu-Rong Yang, Wei-Hsin Ke, She-Huang Wu, J. Power Sources 146 (2005) 539–543.