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C synthesized by a simple co⿿precipitation method
Accepted Manuscript Title: Electrochemical properties of nanostructured Li2 FeSiO4 /C synthesized by a simple co-precipitation method Author: Xuefei Du Hailei Zhao Yao Lu Chunhui Gao Qing Xia Zijia Zhang PII: DOI: Reference:
S0013-4686(15)30960-9 http://dx.doi.org/doi:10.1016/j.electacta.2015.12.039 EA 26188
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
5-10-2015 29-11-2015 4-12-2015
Please cite this article as: Xuefei Du, Hailei Zhao, Yao Lu, Chunhui Gao, Qing Xia, Zijia Zhang, Electrochemical properties of nanostructured Li2FeSiO4/C synthesized by a simple co-precipitation method, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2015.12.039 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Electrochemical properties of nanostructured Li2FeSiO4/C synthesized by a simple co-precipitation method a
Xuefei Dua, Hailei Zhaoa,b* [email protected], Yao Lu , Chunhui Gaoa, Qing Xiaa, Zijia Zhanga a
School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing
100083, China b
Beijing Key Lab of New Energy Materials and Technologies, Beijing 100083, China
*
Corresponding author at: School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China. Tel.: +86 10 82376837; Fax.: +86 10 82376837.
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Graphical abstract
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Highlights
Nanostructured Li2FeSiO4/C is synthesized by a simple co-precipitation method.
Thermodynamic calculation provides guidance for the synthesis of pure Li2FeSiO4.
Li2FeSiO4/C shows high reversible capacity and excellent cycling performance.
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Abstract Nanostructured Li2FeSiO4/C cathode material is successfully synthesized through a simple co-precipitation method by using Fe3+ salt as iron source and polyethylene glycol as surfactant. Thermodynamic calculation is carried out to get phase predominance diagram as functions of oxygen partial pressure and temperature for Fe-O-C system, which provides effective guidance for synthesis parameter selection of pure phase Li2FeSiO4. The synthesized Li2FeSiO4/C nanoparticles show an average size of 150 nm, which are composed of ultra-small Li2FeSiO4 nanocrystals in 10-25 nm dispersing in amorphous carbon matrix. The in situ formed carbon network and the Li2FeSiO4 nanocrystals provide a fast transport of electron and lithium ion and thus ensure a quick electrode reaction, leading to an excellent electrochemical performance. The synthesized Li2FeSiO4/C exhibits a specific capacity of 190 mAh g-1 at 0.1 C, realizing reversible extraction/insertion of 1.37 Li+, taking into account of 16.1 wt% carbon content in the composite. This work offers a simple, scalable, and low cost approach for the synthesis of high performance Li2FeSiO4/C cathode material for lithium ion batteries.
Keywords: co-precipitation; lithium iron orthosilicate; cathode material; electrochemical properties; lithium ion batteries.
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1. Introduction Lithium ion batteries (LIBs) are one of the most promising technologies for the storage of electrical energy [1-3]. Cathode materials are a key part of the LIBs. Since lithium iron orthosilicate (Li2FeSiO4) was first reported in 2005 [4], it has received great attention due to its advantageous material properties, such as low cost, high thermal stability through strong Si-O bonding, good cycle stability and environmental benignity [5,6]. In addition, Li2FeSiO4 can theoretically offer a high capacity of 333 mAh g-1 by allowing reversible extraction of two lithium ions per formula unit [7]. Thus, Li2FeSiO4 is regarded as an ideal cathode material for next generation LIBs. Unfortunately, the inherent low electronic conductivity and sluggish lithium ion diffusivity limit the electrochemical activity of Li2FeSiO4 electrode [8,9]. Several strategies, including cation-doping [10-15], carbon-coating [16-18] and nano-architecturing [19-21], have been adopted to overcome these obstacles. It has been demonstrated that carbon coating is an effective method to improve the electronic conductivity of Li2FeSiO4 while nano-architecture is an efficient way to enhance the diffusion kinetics of lithium ions because of the shortened diffusion path. Usually these two methods are combined together. Thus far, numerous synthesis or processing routes have been proposed and developed to engineer Li2FeSiO4 nanoparticles along with conductive carbon coating, such as sol-gel [22-27],
hydrothermal
[28-30], spray
pyrolysis
[31],
combustion
[32,33]
and
microwave-solvothermal [34] methods. Hasegawa et al. [27] prepared monolithic Li2FeSiO4-carbon composites by sol-gel method and investigated the effect of pore size on their electrochemical performance. The composite with the smaller macropores exhibited the 5
higher capacity of about 120 mAh g-1 at C/16. Dahbi et al. [32] synthesized Li2FeSiO4/C nanocomposite by combustion method. Due to the ~100 nm particle size and high specific surface area, the obtained material delivered a discharge capacity of above 88 mAh g-1 at the rate of 0.5 C for 100 cycles. Additionally, Li2FeSiO4/C hollow sphere was successfully synthesized via hydrothermal method [30]. The thin wall of Li2FeSiO4 hollow spheres reduced the lithium ion diffusion path and thus was favorable for the electrode reaction kinetic, leading to a high specific capacity of 152 mAh g-1 at the rate of 0.05 C. Many methods have been reported to synthesize the Li2FeSiO4 material. Sol-gel process usually requires a carefully controlled and time consuming drying procedure. Combustion method causes violent reaction easily, so it leads to safety concern. Spray pyrolysis is highly successful in synthesizing nanoparticles, but the equipment and energy required to vaporize the reagents tend to be costly. Moreover, high pressure equipment and long reaction time are indispensable in hydrothermal and solvothermal methods, which would lead to high cost during synthesis. Therefore, it is significant to explore a simple, low cost and feasible synthetic method for the Li2FeSiO4/C cathode material. In the regard of synthesis, one important problem with Li2FeSiO4 is the appearance of Fe3+-based impurities, such as Fe2O3 or Fe3O4, in the final product [34-40]. To avert this problem, some works employ Fe2+ salt as starting material to prepare Li2FeSiO4 cathode material with the aim of guarantying the oxide state of Fe2+ ion in Li2FeSiO4 [18,25,40-43]. Generally, Fe2+ salt is much more expensive than Fe3+ salt, therefore Fe3+ salt would be of great benefit for the application of Li2FeSiO4. From the viewpoint of thermodynamics, the oxidation state of iron ions in a compound should mainly depend on the calcining 6
temperature and oxygen partial pressure. By controlling the temperature and atmosphere of sample synthesis, the final oxidation state of Fe ions is possible to be fixed. In this study, we developed a simple co-precipitation method for the first time to synthesize nanosized Li2FeSiO4/C composite. The co-precipitation method, which allows for the mixing of starting ingredients at the atomic level, could get precursor of Li2FeSiO4/C at ambient atmosphere by employing low-cost Fe3+ salt as iron source and polyethylene glycol (PEG200) as dispersant and carbon source. Based on the thermodynamic calculation, the predominant phase diagram of Fe-O system as a function of temperature and oxygen partial pressure (CO/CO2 ratio) was obtained so that the suitable synthesis parameters for pure phase Li2FeSiO4 was proposed. The addition of PEG200 was beneficial to preventing the aggregation of the precipitates during co-precipitation process as well as forming continuous carbon network in post-calcination. The phase crystal structure, particle morphology and electrochemical properties of the synthesized Li2FeSiO4/C cathode material were investigated. In view of the advantages of low energy consumption, mild reaction condition, simple operation, high yield and short production period, the developed co-precipitation route in this work is full of great potentiality for large scale production of Li2FeSiO4-based cathode materials. 2. Experimental 2.1 Preparation of materials The precursor of Li2FeSiO4/C was synthesized according to a co-precipitation method. First, a certain amount of PEG200 was dissolved in distilled water, and the precipitating reagent ammonia water (NH3·H2O) was added dropwise until the pH of the mixed solution 7
reached about 10. Then an ethanol and deionized water mixed solution of stoichiometric Fe(NO3)3·9H2O and tetraethyl orthosilicate (TEOS) was dropped into the above PEG200 solution under vigorous stirring, immediately, umber precipitate was formed. Subsequently, stoichiometric CH3COOLi·2H2O was dispersed in the above solution. After stirring for about 30 min, the resulting mixture was evaporated by a rotary evaporator under vacuum at 80 °C, the excess solvent was removed and the final wet precursor was obtained. The wet precursor was then calcined at 700 °C for 10 h in flowing CO/CO2 gas (50:50 in vol.) to obtain Li2FeSiO4/C powders. 2.2 Characterizations The crystal structure of the as-prepared Li2FeSiO4/C sample was characterized by powder X-ray diffraction (XRD, Rigaku, D/max-A) with Cu Κα radiation. The morphology of the samples was observed by field emission scanning electron microscope (FE-SEM, CARL ZEISS, SUPRA55, 10 kV) and transmission electron microscope (TEM, Tecnai F20, 200 kV) equipped with high resolution transmission electron microscope (HRTEM). The carbon content in the Li2FeSiO4/C composite was determined by thermogravimetric analysis (TG, Netzsch, STA409C). The valence state of iron ions was confirmed by X-ray photoelectron spectroscopy (XPS, PHI 5000C ESCA system). The surface species were identified by Fourier transformed infrared spectroscopy (FTIR, WQF-510A, Beijing Rayleigh Analytical Instrument Co., Ltd.). The magnetization was measured using a superconducting quantum interference device magnetometer (Quantum Design SQUID-VSM). The electrical conductivity of Li2FeSiO4/C composite was measured by a traditional ohmic resistance method with keithley 2010 multimeter. 8
For electrochemical measurements, the cathode electrode was fabricated by mixing Li2FeSiO4/C composite, acetylene black and PVDF in a weight ratio of 80:10:10 in N-methyl-pyrrolidone (NMP). After being mixed uniformly, the slurry was coated onto aluminum foil and dried at 120°C for 10 h in vacuum. The electrode was then pressed and disks were punched out. The testing cells were assembled in an argon-filled glove box. Lithium metal foil and Celgard 2400 membrane were used as the counter electrode and separator, respectively. A solution of 1 mol L-1 LiPF6 in EC:DMC (1:1, v/v) was employed as the electrolyte. The charge-discharge tests at a constant current mode were performed in a voltage range of 1.5-4.8 V on a LAND CT2001A battery test system (Wuhan, China). The cyclic voltammograms (CV) were performed at different scan rates with Arbin instruments (Model BT 2000). All the electrochemical measurements were carried out at room temperature. 3. Results and discussion As mentioned above, the calcination temperature and oxygen partial pressure are key factors that affect the oxidation state of Fe ions in final products. However, seldom works about Li2FeSiO4 synthesis concerns the optimization of processing parameters from the viewpoint of thermodynamics. Actually, the reduction of Fe2O3 follows a series of stepwise reactions and different reduction products are generated at different temperatures and oxygen partial pressures [44-46]. When the temperature is below 843 K, the reduction consequence should be Fe2O3→Fe3O4→Fe. While when the temperature is above 843 K, there is a more complex four-step pathway Fe2O3→Fe3O4→FeO→Fe. According to the Ellingham diagrams [47], the standard Gibbs free energy changes (ΔGΘ) of reactions (1)-(4) as a function of 9
temperature can be derived and used to calculate the predominance diagram of iron oxides under different temperatures and oxygen partial pressures (Fig. 1(a)). 4Fe3O 4 (s) + O 2 (g) = 6Fe 2 O3 (s) (1) 6FeO(s) + O 2 (g) = 2Fe3O 4 (s) (2) 2Fe(s) + O 2 (g) = 2FeO(s)
(3)
3 1 Fe(s) + O2 (g) = Fe3O4 (s) 2 2
(4)
When the temperature is above 843 K, FeO will be stabilized as the intermediate of reduction reactions if the oxygen partial pressure is limited between the lines of reactions (2) and (3). The oxygen partial pressure at high temperature is usually controlled by CO/CO2 gas mixture in practical application, based on the thermodynamic equilibrium of reaction (5).
CO(g) +
1 O 2 (g) = CO 2 (g) (5) 2
Accordingly, Fig. 1(a) can be converted to Fig. 1(b), a phase stability diagram of Fe-O-C system as a function of temperature and CO/(CO2+CO) ratio. The stable regions of Fe with different oxidation states are clearly illustrated. The synthesis temperature of Li2FeSiO4 in reported literatures [4,20,48-51] usually falls into the range of 873 to 1173 K, which overlaps with FeO predominant area, forming the shadowed pink area in Fig. 1(b), where FeO is the stable phase and Li2FeSiO4 is supposed to be synthesized in a stoichiometric batch. Based on the calculated result and taking into account the fact that temperature has strong influence on particle size and morphologies, which will in turn affect the electrochemical properties, the synthesis parameters for Li2FeSiO4 powder can be determined. The formation mechanism of Li2FeSiO4/C nanocomposite was illustrated in Fig. 2. Once Fe3+ and TEOS were added dropwise to NH3·H2O solution containing a certain amount of 10
PEG200, hydroxides of iron and silicon were formed immediately. As a nonionic surfactant, the PEG200 with hydrophilic oxygen atoms can link to the free hydroxyls on the surface of precipitates by hydrogen bonds [52], inhibiting the aggregation of the precipitate particles. The active oxygen atoms on PEG200 chains will also keep Li ions homogeneously distributing on the particle surface of hydroxide precipitates after CH3COOLi·2H2O was introduced [53]. In the subsequent calcination process, the Li ions will diffuse to the inside of particles and react with the iron and silicon hydroxides to form Li2FeSiO4, while the PEG200 is pyrolyzed into carbon completely and coated onto the surface of Li2FeSiO4 particles to further limit the grain growth and particle aggregation effectively. The carbon coating layer on Li2FeSiO4 particle surface can facilitate the electron conduction and improve the electrode reaction in a charge/discharge process. Fig. 3 shows XRD pattern of the as-prepared Li2FeSiO4/C nanoparticles. All peaks can be indexed to the monoclinic structure of Li2FeSiO4 with space group P21/n, as reported by Sirisopanaporn et al. [48]. No impurity of iron oxides can be observed in the XRD pattern of as-prepared sample, indicating that the designed synthetic condition for Li2FeSiO4 is feasible. It is worth noting that no diffraction peaks ascribable to carbon are detected, probably due to amorphous feature of the carbon. The phase purity of Li2FeSiO4/C nanocomposite was further confirmed by Fourier transform infrared spectroscopy (FTIR) and magnetic moment measurements. FTIR was performed to investigate whether the Li2SiO3 impurity existed, depending on the different characteristic peaks of Si-O in Li2FeSiO4 and Li2SiO3. As illustrated in Fig. 4(a), the bands around 897 and 524 cm-1 can be attributed to the Si-O stretching and bending vibrations in 11
(SiO4)4- of Li2FeSiO4, respectively [30,54]. While the characteristic absorption peaks of Si-O-Si from Li2SiO3 at 1100 and 780 cm-1 are not observed, indicating the absence of Li2SiO3 impurity [6,55]. Moreover, magnetic moment measurement is an effective method for determining Fe3O4 existence. Fig. 4(b) shows the plot of magnetic moment vs applied magnetic field for as-prepared Li2FeSiO4/C. The linear curve suggests that there is no magnetic phase impurity such as Fe3O4 in the product [55]. The results of XRD, FTIR and magnetic moment demonstrate that the as-prepared Li2FeSiO4/C is in a pure phase. To confirm the valence state of Fe ions in the Li2FeSiO4 phase, XPS examination was carried out for the synthesized Li2FeSiO4/C composite. As shown in Fig. 5(a), in the full XPS spectrum of Li2FeSiO4/C, the peaks can be clearly assigned to Fe 2p, C 1s, O 1s, Si 2p, Si 2s and Li 1s, respectively. The binding energies are calibrated by taking carbon C1s peak (284.6 eV) as reference. With respect to the Fe spectrum, as illustrated in Fig. 5(b), the binding energies located at 711.8 and 725.5 eV are ascribable to Fe 2p3/2 and Fe 2p1/2, respectively [56], confirming that the valence of Fe is +2. The particle morphology and microstructure of the as-prepared products were characterized by FE-SEM, TEM and HR-TEM. As shown in Fig. 6(a), the Li2FeSiO4/C nanocomposite exhibits a spherical particle shape with an average diameter of approximately 150 nm. TEM observation indicates that the single particle is consisted of Li2FeSiO4 nanocrystals embedded in amorphous carbon matrix (Fig. 6(b)). HRTEM image in Fig. 6(c) reveals that the thickness of carbon on the particle surface of Li2FeSiO4 is about several nanometers. The carbon layer is in situ formed by the pyrolysis of PEG200, therefore, it disperses homogeneously and keeps well contact with Li2FeSiO4 nanocrystals, as illustrated 12
in Fig. 6(d). It can be noted that there is amorphous carbon not only at the surface, but also inside, of the Li2FeSiO4/C nanoparticles. The inner amorphous carbon connects with outer carbon layer, forming a continuous carbon network which provides an effective conductive channel for the electron transfer. The lattice fringe with d-spacing of 0.28 nm and 0.34 nm can be observed in Fig. 6(e), which is consistent with the (112) and (102) lattice spacing of monoclinic Li2FeSiO4, respectively. The well-resolved lattice fringes also demonstrate the good crystallinity of Li2FeSiO4 gains. The primary grain size distribution of Li2FeSiO4 was analyzed by Nano Measurer 1.2 Software (Fudan University), which exhibits a normal distribution with grain size range of 10-25 nm (Fig. 6(f)). The carbon contents in Li2FeSiO4/C cathode material were revealed by TG-DTA examination from room temperature to 800 °C with a heating rate of 10 °C min-1 in air. The TG-DTA curves of Li2FeSiO4/C are shown in Fig. 7. There are two weight-lossing processes in the TG curve. The first one at about 100 °C is likely due to the loss of water adsorbed on the surface of material, the second one at 200~400 °C is assigned to the oxidation of Li2FeSiO4/C. In accord with the TG data, an endothermic peak centered at 100 °C and a strong exothermic peak at about 380 °C can be observed in the DTA curve correspondingly. The oxidation of Li2FeSiO4 can be written as reaction (6). According to reaction (6) and taking the weight after its first loss around 100 °C as the calculation base, the carbon contents in Li2FeSiO4/C composite can be calculated based on the analysis of the result of TG, which is 16.1 %. Li 2 FeSiO 4 (s) + nC(s) + (0.25+n)O 2 (g) = Li 2SiO 3 (s) + 0.5Fe 2 O 3 (s) + nCO 2 (g) (6)
When the as-prepared Li2FeSiO4/C was assembled into batteries as cathode material, it 13
showed excellent electrochemical performance. The galvanostatic charge/discharge measurements were performed at 0.1 C in the potential range of 1.5-4.8 V (vs.Li+/Li). The initial three charge-discharge curves of Li2FeSiO4/C are presented in Fig. 8(a). The first charge profile is far different from those of following cycles, this phenomenon is usually caused by the structure rearrangement in the first charge process [55]. After the structure rearrangement, Li2FeSiO4/C transforms to a more stable phase and its charge/discharge voltage plateaus are focused on 2.8 and 2.6 V in the subsequent two cycles. Moreover, the discharge capacities of the intial three cycles all exceed 185 mAh g-1, indicating that more than one Li+ can be extracted from the Li2FeSiO4/C composite. In order to get more detailed analysis of charge-discharge characteristics, the differential capacities (dQ/dV) were calculated and the dQ/dV vs. voltage curves are shown in Fig. 8(b). In the first charge process, a wide oxidation peak at about 4.3 V corresponds to the oxidation of Fe3+ to Fe4+, which is coinsident with other groups’ reports [9,57] . This peak becomes extemely weak in the second and third cycles. After the first cycle, the Li2FeSiO4/C sample has a pair of redox peaks (at about 2.6-2.8 V) in the dQ/dV curves, which can be attributed to the redox reaction of Fe2+/Fe3+ couple. The separation between the oxidation and reduction peaks is narrow, indicating that the as-parpared Li2FeSiO4/C cathode material has good electrochemical reversibility. Besides, the reduction peak located at 1.8 V has not be explained clearly until now and some researchers thought this peak could be attributed to some extra side reactions [54]. Fig. 9(a) shows the cycle performance of Li2FeSiO4/C at 0.1 C. The discharge capacity reaches 227 mAh g-1 in the first cycle, but it drops to 191 mAh g-1 for the 2nd and 172.1 mAh 14
g-1 for the 5th cycle. The capacity fading and poor coulombic efficiency in the initial 4 cycles should result from the decomposition of electrolyte at high voltage and the formation of a solid electrolyte interphase (SEI) [55,58,59]. Fortunately, after 5 cycles, the specific capacity stops fading and tends to increase gradually up to 20 cycles, and then it keeps at about 190 mAh g-1 for next 60 cycles. Besides the coulombic efficiency reaches almost 100% after 11th cycle. This phenomenon indicates that a stable SEI film is formed, which can prevent further side reaction occurring at the electrode-electrolyte interface [60,61]. Considering that there is 16.1 wt% carbon in Li2FeSiO4/C composite, the actual specific capacity delivered by Li2FeSiO4 should be 227 mAh g-1, corresponding to 1.37 Li+ being extracted from Li2FeSiO4. The excellent electrochemical performance of Li2FeSiO4/C is ascribed to the smaller particle size and narrow particle size distribution as well as continuous carbon network pyrolyzed from PEG200. The carbon network that contacts firmly with Li2FeSiO4 particles due to its in situ formation feature provides a good electronic connection and ensures the majority of
active Li2FeSiO4 particles to participate in the electrode reaction. The measured electrical conductivity of the synthesized Li2FeSiO4/C composite is about 8×10-8 S cm-1, which is higher than the reported pure Li2FeSiO4 material [8]. Fig. 9(b) gives the rate capability of Li2FeSiO4/C under varied current density for every 10 cycles. At different rates of 0.1 C, 0.2 C, 0.5 C, 1 C and 2 C, the prepared Li2FeSiO4/C nanocomposite delivers specific capacities of 190, 175.3, 136.6, 98.0 and 76.9 mAh g-1, respectively. When the current rate returns to 0.1 C, the specific capacity can be recovered to its original value, indicating excellent reversibility in electrode reaction for Li2FeSiO4/C cathode material over cycling at high current density. The long-term cycling stability of 15
Li2FeSiO4/C was further studied at a current rate of 0.5 C with initial 5 cycles at 0.1 C. As plotted in Fig. 9(c), the specific capacities of 189.5 and 151.2 mAh g-1 are first measured at rate of 0.1 C and 0.5 C, respectively. Thereafter, the electrode shows a stable cycling performance for 50 cycles and then declines slightly. After 100 cycles, the capacity tends to stable and delivers about 90% capacity retention at 0.5 C after 400 cycles, revealing a good long-term performance. In order to well understand the kinetic characteristic of as-prepared Li2FeSiO4/C nanocomposite, cyclic voltammetry (CV) analysis was carried out at an increasing scan rate from 0.2 to 0.6 mV s-1. With the increasing scan rate, the peak current increases, meanwhile the cathodic and anodic peaks shift to lower and higher potentials, respectively (Fig. 10(a)). The diffusion coefficient of lithium ions (DLi) can be calculated from a linear relationship between peak current Ip (A) and the square root of the scan rate ν1/2 (V1/2 s-1/2) from the CVs, according to the following equation [62]: 1/2 Ip =2.69 105 n 3/2 AD1/2 Li C Li υ
(7)
where n is the number of electrons per reaction species, A is the electrode area (0.5 cm2 in this study) and CLi is the bulk concentration of the lithium ion in the electrode (0.040 mol cm-3 for Li2FeSiO4). From the slope of the fitting line in Fig. 10(b), the DLi of the as-prepared Li2FeSiO4/C nanocomposite is 1.58×10-12 cm2 s-1, which is larger than the previous carbon coated nanocomposites [17,63,64] and metallic ions (Ni2+, Cu2+, Zn2+, Mg2+, Al3+) doped Li2FeSiO4 [14,15,65]. This is mainly attributed to the smaller particle size and homogeneous particle distribution.
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4. Conclusions
In summary, Li2FeSiO4/C nanocomposite was successfully synthesized for the first time via co-precipitation method by employing low cost Fe3+ salt as iron source. The predominance phase diagram based on the thermodynamic calculation could provide suitable parameters for the synthesis of Fe2+-based oxides, with which pure phase Li2FeSiO4 was synthesized without any other iron oxides impurities. Nanosized Li2FeSiO4/C particles (~150 nm) with nanocrystals Li2FeSiO4 (10-25 nm) dispering in amorphous carbon matrix were obtained. The PEG200 plays crucial roles in the precipitation process, which can not only disperse the precipitates and prevent the primary particles from growth, but also introduce carbon source to form conductive network in Li2FeSiO4 particles. The good electronic conduction network and the small Li2FeSiO4 particle size endow the synthesized Li2FeSiO4/C composite with excellent electrochemical performance. It delivers a reversible capacity of 190 mAh g-1 at 0.1 C, corresponding to the extraction of 1.37 Li+ from Li2FeSiO4 lattice. The Li2FeSiO4/C electrode also demonstrates a good long-term cycling stability, 90% capacity retention being achieved at 0.5 C over 400 cycles. This work proposes a low cost, simple and fast fabrication method for Li2FeSiO4/C cathode material, which is suitable for large-scale production. Acknowledgements
This work was financially supported by National Basic Research Program of China (2013CB934003), “863”program (2013AA050902), National Nature Science Foundation of China (21273019)， Guangdong Industry-Academy-Research Alliance (2013C2FC0015) and Program of Introducing Talents of Discipline to Universities (B14003) .
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26
Figure Captions
Fig. 1. (a) Phase stability diagram of Fe-O-C system under different oxygen partial pressures and (b) different CO volume fractions in CO/CO2 mixture.
27
Fig. 2. Schematic illustration of preparation process of Li2FeSiO4/C via co-precipitation method.
28
Fig. 3. XRD patterns of as-prepared Li2FeSiO4/C, standard ICSD# 261339 of Li2FeSiO4, standard PDF# 89-2648 of FeO, standard PDF# 89-0599 of Fe2O3 and standard PDF# 89-4319 of Fe3O4. Inset is the crystal structure model of monoclinic Li2FeSiO4 with space group P21/n.
29
Fig. 4. (a) FTIR spectrum and (b) magnetic moment as a function of applied field of the prepared Li2FeSiO4/C composite.
30
Fig. 5. (a) Wide scan XPS spectrum for all elements of the synthesized Li2FeSiO4/C composite and (b) XPS core-level spectra and fitting results of Fe 2p for Li2FeSiO4/C composite.
31
Fig. 6. (a) FESEM, (b, c) TEM, (d, e) HRTEM images and (f) particle size distribution of the synthesized Li2FeSiO4/C powders.
32
Fig. 7. TG-DTA curves of Li2FeSiO4/C composite.
33
Fig. 8. (a) Initial three charge-discharge curves and (b) corresponding differential capacity curves of the Li2FeSiO4/C electrode at rate of 0.1 C in voltage range of 1.5-4.8 V.
34
Fig. 9. (a) Cycle performance at current rate of 0.1 C in voltage window of 1.5-4.8 V, (b) rate performance of Li2FeSiO4/C at various current rates from 0.1 C to 2 C and (c) long-term cycling performance of Li2FeSiO4/C at a current rate of 0.5 C with initial 5 cycles at 0.1 C.
35
Fig. 10 (a) Cyclic voltammograms of Li2FeSiO4/C at different scan rates and (b) plot of anodic peak current as a funcion of the square root of the scan rate.
36
S0013-4686(15)30960-9 http://dx.doi.org/doi:10.1016/j.electacta.2015.12.039 EA 26188
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
5-10-2015 29-11-2015 4-12-2015
Please cite this article as: Xuefei Du, Hailei Zhao, Yao Lu, Chunhui Gao, Qing Xia, Zijia Zhang, Electrochemical properties of nanostructured Li2FeSiO4/C synthesized by a simple co-precipitation method, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2015.12.039 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Electrochemical properties of nanostructured Li2FeSiO4/C synthesized by a simple co-precipitation method a
Xuefei Dua, Hailei Zhaoa,b* [email protected], Yao Lu , Chunhui Gaoa, Qing Xiaa, Zijia Zhanga a
School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing
100083, China b
Beijing Key Lab of New Energy Materials and Technologies, Beijing 100083, China
*
Corresponding author at: School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China. Tel.: +86 10 82376837; Fax.: +86 10 82376837.
1
Graphical abstract
2
Highlights
Nanostructured Li2FeSiO4/C is synthesized by a simple co-precipitation method.
Thermodynamic calculation provides guidance for the synthesis of pure Li2FeSiO4.
Li2FeSiO4/C shows high reversible capacity and excellent cycling performance.
3
Abstract Nanostructured Li2FeSiO4/C cathode material is successfully synthesized through a simple co-precipitation method by using Fe3+ salt as iron source and polyethylene glycol as surfactant. Thermodynamic calculation is carried out to get phase predominance diagram as functions of oxygen partial pressure and temperature for Fe-O-C system, which provides effective guidance for synthesis parameter selection of pure phase Li2FeSiO4. The synthesized Li2FeSiO4/C nanoparticles show an average size of 150 nm, which are composed of ultra-small Li2FeSiO4 nanocrystals in 10-25 nm dispersing in amorphous carbon matrix. The in situ formed carbon network and the Li2FeSiO4 nanocrystals provide a fast transport of electron and lithium ion and thus ensure a quick electrode reaction, leading to an excellent electrochemical performance. The synthesized Li2FeSiO4/C exhibits a specific capacity of 190 mAh g-1 at 0.1 C, realizing reversible extraction/insertion of 1.37 Li+, taking into account of 16.1 wt% carbon content in the composite. This work offers a simple, scalable, and low cost approach for the synthesis of high performance Li2FeSiO4/C cathode material for lithium ion batteries.
Keywords: co-precipitation; lithium iron orthosilicate; cathode material; electrochemical properties; lithium ion batteries.
4
1. Introduction Lithium ion batteries (LIBs) are one of the most promising technologies for the storage of electrical energy [1-3]. Cathode materials are a key part of the LIBs. Since lithium iron orthosilicate (Li2FeSiO4) was first reported in 2005 [4], it has received great attention due to its advantageous material properties, such as low cost, high thermal stability through strong Si-O bonding, good cycle stability and environmental benignity [5,6]. In addition, Li2FeSiO4 can theoretically offer a high capacity of 333 mAh g-1 by allowing reversible extraction of two lithium ions per formula unit [7]. Thus, Li2FeSiO4 is regarded as an ideal cathode material for next generation LIBs. Unfortunately, the inherent low electronic conductivity and sluggish lithium ion diffusivity limit the electrochemical activity of Li2FeSiO4 electrode [8,9]. Several strategies, including cation-doping [10-15], carbon-coating [16-18] and nano-architecturing [19-21], have been adopted to overcome these obstacles. It has been demonstrated that carbon coating is an effective method to improve the electronic conductivity of Li2FeSiO4 while nano-architecture is an efficient way to enhance the diffusion kinetics of lithium ions because of the shortened diffusion path. Usually these two methods are combined together. Thus far, numerous synthesis or processing routes have been proposed and developed to engineer Li2FeSiO4 nanoparticles along with conductive carbon coating, such as sol-gel [22-27],
hydrothermal
[28-30], spray
pyrolysis
[31],
combustion
[32,33]
and
microwave-solvothermal [34] methods. Hasegawa et al. [27] prepared monolithic Li2FeSiO4-carbon composites by sol-gel method and investigated the effect of pore size on their electrochemical performance. The composite with the smaller macropores exhibited the 5
higher capacity of about 120 mAh g-1 at C/16. Dahbi et al. [32] synthesized Li2FeSiO4/C nanocomposite by combustion method. Due to the ~100 nm particle size and high specific surface area, the obtained material delivered a discharge capacity of above 88 mAh g-1 at the rate of 0.5 C for 100 cycles. Additionally, Li2FeSiO4/C hollow sphere was successfully synthesized via hydrothermal method [30]. The thin wall of Li2FeSiO4 hollow spheres reduced the lithium ion diffusion path and thus was favorable for the electrode reaction kinetic, leading to a high specific capacity of 152 mAh g-1 at the rate of 0.05 C. Many methods have been reported to synthesize the Li2FeSiO4 material. Sol-gel process usually requires a carefully controlled and time consuming drying procedure. Combustion method causes violent reaction easily, so it leads to safety concern. Spray pyrolysis is highly successful in synthesizing nanoparticles, but the equipment and energy required to vaporize the reagents tend to be costly. Moreover, high pressure equipment and long reaction time are indispensable in hydrothermal and solvothermal methods, which would lead to high cost during synthesis. Therefore, it is significant to explore a simple, low cost and feasible synthetic method for the Li2FeSiO4/C cathode material. In the regard of synthesis, one important problem with Li2FeSiO4 is the appearance of Fe3+-based impurities, such as Fe2O3 or Fe3O4, in the final product [34-40]. To avert this problem, some works employ Fe2+ salt as starting material to prepare Li2FeSiO4 cathode material with the aim of guarantying the oxide state of Fe2+ ion in Li2FeSiO4 [18,25,40-43]. Generally, Fe2+ salt is much more expensive than Fe3+ salt, therefore Fe3+ salt would be of great benefit for the application of Li2FeSiO4. From the viewpoint of thermodynamics, the oxidation state of iron ions in a compound should mainly depend on the calcining 6
temperature and oxygen partial pressure. By controlling the temperature and atmosphere of sample synthesis, the final oxidation state of Fe ions is possible to be fixed. In this study, we developed a simple co-precipitation method for the first time to synthesize nanosized Li2FeSiO4/C composite. The co-precipitation method, which allows for the mixing of starting ingredients at the atomic level, could get precursor of Li2FeSiO4/C at ambient atmosphere by employing low-cost Fe3+ salt as iron source and polyethylene glycol (PEG200) as dispersant and carbon source. Based on the thermodynamic calculation, the predominant phase diagram of Fe-O system as a function of temperature and oxygen partial pressure (CO/CO2 ratio) was obtained so that the suitable synthesis parameters for pure phase Li2FeSiO4 was proposed. The addition of PEG200 was beneficial to preventing the aggregation of the precipitates during co-precipitation process as well as forming continuous carbon network in post-calcination. The phase crystal structure, particle morphology and electrochemical properties of the synthesized Li2FeSiO4/C cathode material were investigated. In view of the advantages of low energy consumption, mild reaction condition, simple operation, high yield and short production period, the developed co-precipitation route in this work is full of great potentiality for large scale production of Li2FeSiO4-based cathode materials. 2. Experimental 2.1 Preparation of materials The precursor of Li2FeSiO4/C was synthesized according to a co-precipitation method. First, a certain amount of PEG200 was dissolved in distilled water, and the precipitating reagent ammonia water (NH3·H2O) was added dropwise until the pH of the mixed solution 7
reached about 10. Then an ethanol and deionized water mixed solution of stoichiometric Fe(NO3)3·9H2O and tetraethyl orthosilicate (TEOS) was dropped into the above PEG200 solution under vigorous stirring, immediately, umber precipitate was formed. Subsequently, stoichiometric CH3COOLi·2H2O was dispersed in the above solution. After stirring for about 30 min, the resulting mixture was evaporated by a rotary evaporator under vacuum at 80 °C, the excess solvent was removed and the final wet precursor was obtained. The wet precursor was then calcined at 700 °C for 10 h in flowing CO/CO2 gas (50:50 in vol.) to obtain Li2FeSiO4/C powders. 2.2 Characterizations The crystal structure of the as-prepared Li2FeSiO4/C sample was characterized by powder X-ray diffraction (XRD, Rigaku, D/max-A) with Cu Κα radiation. The morphology of the samples was observed by field emission scanning electron microscope (FE-SEM, CARL ZEISS, SUPRA55, 10 kV) and transmission electron microscope (TEM, Tecnai F20, 200 kV) equipped with high resolution transmission electron microscope (HRTEM). The carbon content in the Li2FeSiO4/C composite was determined by thermogravimetric analysis (TG, Netzsch, STA409C). The valence state of iron ions was confirmed by X-ray photoelectron spectroscopy (XPS, PHI 5000C ESCA system). The surface species were identified by Fourier transformed infrared spectroscopy (FTIR, WQF-510A, Beijing Rayleigh Analytical Instrument Co., Ltd.). The magnetization was measured using a superconducting quantum interference device magnetometer (Quantum Design SQUID-VSM). The electrical conductivity of Li2FeSiO4/C composite was measured by a traditional ohmic resistance method with keithley 2010 multimeter. 8
For electrochemical measurements, the cathode electrode was fabricated by mixing Li2FeSiO4/C composite, acetylene black and PVDF in a weight ratio of 80:10:10 in N-methyl-pyrrolidone (NMP). After being mixed uniformly, the slurry was coated onto aluminum foil and dried at 120°C for 10 h in vacuum. The electrode was then pressed and disks were punched out. The testing cells were assembled in an argon-filled glove box. Lithium metal foil and Celgard 2400 membrane were used as the counter electrode and separator, respectively. A solution of 1 mol L-1 LiPF6 in EC:DMC (1:1, v/v) was employed as the electrolyte. The charge-discharge tests at a constant current mode were performed in a voltage range of 1.5-4.8 V on a LAND CT2001A battery test system (Wuhan, China). The cyclic voltammograms (CV) were performed at different scan rates with Arbin instruments (Model BT 2000). All the electrochemical measurements were carried out at room temperature. 3. Results and discussion As mentioned above, the calcination temperature and oxygen partial pressure are key factors that affect the oxidation state of Fe ions in final products. However, seldom works about Li2FeSiO4 synthesis concerns the optimization of processing parameters from the viewpoint of thermodynamics. Actually, the reduction of Fe2O3 follows a series of stepwise reactions and different reduction products are generated at different temperatures and oxygen partial pressures [44-46]. When the temperature is below 843 K, the reduction consequence should be Fe2O3→Fe3O4→Fe. While when the temperature is above 843 K, there is a more complex four-step pathway Fe2O3→Fe3O4→FeO→Fe. According to the Ellingham diagrams [47], the standard Gibbs free energy changes (ΔGΘ) of reactions (1)-(4) as a function of 9
temperature can be derived and used to calculate the predominance diagram of iron oxides under different temperatures and oxygen partial pressures (Fig. 1(a)). 4Fe3O 4 (s) + O 2 (g) = 6Fe 2 O3 (s) (1) 6FeO(s) + O 2 (g) = 2Fe3O 4 (s) (2) 2Fe(s) + O 2 (g) = 2FeO(s)
(3)
3 1 Fe(s) + O2 (g) = Fe3O4 (s) 2 2
(4)
When the temperature is above 843 K, FeO will be stabilized as the intermediate of reduction reactions if the oxygen partial pressure is limited between the lines of reactions (2) and (3). The oxygen partial pressure at high temperature is usually controlled by CO/CO2 gas mixture in practical application, based on the thermodynamic equilibrium of reaction (5).
CO(g) +
1 O 2 (g) = CO 2 (g) (5) 2
Accordingly, Fig. 1(a) can be converted to Fig. 1(b), a phase stability diagram of Fe-O-C system as a function of temperature and CO/(CO2+CO) ratio. The stable regions of Fe with different oxidation states are clearly illustrated. The synthesis temperature of Li2FeSiO4 in reported literatures [4,20,48-51] usually falls into the range of 873 to 1173 K, which overlaps with FeO predominant area, forming the shadowed pink area in Fig. 1(b), where FeO is the stable phase and Li2FeSiO4 is supposed to be synthesized in a stoichiometric batch. Based on the calculated result and taking into account the fact that temperature has strong influence on particle size and morphologies, which will in turn affect the electrochemical properties, the synthesis parameters for Li2FeSiO4 powder can be determined. The formation mechanism of Li2FeSiO4/C nanocomposite was illustrated in Fig. 2. Once Fe3+ and TEOS were added dropwise to NH3·H2O solution containing a certain amount of 10
PEG200, hydroxides of iron and silicon were formed immediately. As a nonionic surfactant, the PEG200 with hydrophilic oxygen atoms can link to the free hydroxyls on the surface of precipitates by hydrogen bonds [52], inhibiting the aggregation of the precipitate particles. The active oxygen atoms on PEG200 chains will also keep Li ions homogeneously distributing on the particle surface of hydroxide precipitates after CH3COOLi·2H2O was introduced [53]. In the subsequent calcination process, the Li ions will diffuse to the inside of particles and react with the iron and silicon hydroxides to form Li2FeSiO4, while the PEG200 is pyrolyzed into carbon completely and coated onto the surface of Li2FeSiO4 particles to further limit the grain growth and particle aggregation effectively. The carbon coating layer on Li2FeSiO4 particle surface can facilitate the electron conduction and improve the electrode reaction in a charge/discharge process. Fig. 3 shows XRD pattern of the as-prepared Li2FeSiO4/C nanoparticles. All peaks can be indexed to the monoclinic structure of Li2FeSiO4 with space group P21/n, as reported by Sirisopanaporn et al. [48]. No impurity of iron oxides can be observed in the XRD pattern of as-prepared sample, indicating that the designed synthetic condition for Li2FeSiO4 is feasible. It is worth noting that no diffraction peaks ascribable to carbon are detected, probably due to amorphous feature of the carbon. The phase purity of Li2FeSiO4/C nanocomposite was further confirmed by Fourier transform infrared spectroscopy (FTIR) and magnetic moment measurements. FTIR was performed to investigate whether the Li2SiO3 impurity existed, depending on the different characteristic peaks of Si-O in Li2FeSiO4 and Li2SiO3. As illustrated in Fig. 4(a), the bands around 897 and 524 cm-1 can be attributed to the Si-O stretching and bending vibrations in 11
(SiO4)4- of Li2FeSiO4, respectively [30,54]. While the characteristic absorption peaks of Si-O-Si from Li2SiO3 at 1100 and 780 cm-1 are not observed, indicating the absence of Li2SiO3 impurity [6,55]. Moreover, magnetic moment measurement is an effective method for determining Fe3O4 existence. Fig. 4(b) shows the plot of magnetic moment vs applied magnetic field for as-prepared Li2FeSiO4/C. The linear curve suggests that there is no magnetic phase impurity such as Fe3O4 in the product [55]. The results of XRD, FTIR and magnetic moment demonstrate that the as-prepared Li2FeSiO4/C is in a pure phase. To confirm the valence state of Fe ions in the Li2FeSiO4 phase, XPS examination was carried out for the synthesized Li2FeSiO4/C composite. As shown in Fig. 5(a), in the full XPS spectrum of Li2FeSiO4/C, the peaks can be clearly assigned to Fe 2p, C 1s, O 1s, Si 2p, Si 2s and Li 1s, respectively. The binding energies are calibrated by taking carbon C1s peak (284.6 eV) as reference. With respect to the Fe spectrum, as illustrated in Fig. 5(b), the binding energies located at 711.8 and 725.5 eV are ascribable to Fe 2p3/2 and Fe 2p1/2, respectively [56], confirming that the valence of Fe is +2. The particle morphology and microstructure of the as-prepared products were characterized by FE-SEM, TEM and HR-TEM. As shown in Fig. 6(a), the Li2FeSiO4/C nanocomposite exhibits a spherical particle shape with an average diameter of approximately 150 nm. TEM observation indicates that the single particle is consisted of Li2FeSiO4 nanocrystals embedded in amorphous carbon matrix (Fig. 6(b)). HRTEM image in Fig. 6(c) reveals that the thickness of carbon on the particle surface of Li2FeSiO4 is about several nanometers. The carbon layer is in situ formed by the pyrolysis of PEG200, therefore, it disperses homogeneously and keeps well contact with Li2FeSiO4 nanocrystals, as illustrated 12
in Fig. 6(d). It can be noted that there is amorphous carbon not only at the surface, but also inside, of the Li2FeSiO4/C nanoparticles. The inner amorphous carbon connects with outer carbon layer, forming a continuous carbon network which provides an effective conductive channel for the electron transfer. The lattice fringe with d-spacing of 0.28 nm and 0.34 nm can be observed in Fig. 6(e), which is consistent with the (112) and (102) lattice spacing of monoclinic Li2FeSiO4, respectively. The well-resolved lattice fringes also demonstrate the good crystallinity of Li2FeSiO4 gains. The primary grain size distribution of Li2FeSiO4 was analyzed by Nano Measurer 1.2 Software (Fudan University), which exhibits a normal distribution with grain size range of 10-25 nm (Fig. 6(f)). The carbon contents in Li2FeSiO4/C cathode material were revealed by TG-DTA examination from room temperature to 800 °C with a heating rate of 10 °C min-1 in air. The TG-DTA curves of Li2FeSiO4/C are shown in Fig. 7. There are two weight-lossing processes in the TG curve. The first one at about 100 °C is likely due to the loss of water adsorbed on the surface of material, the second one at 200~400 °C is assigned to the oxidation of Li2FeSiO4/C. In accord with the TG data, an endothermic peak centered at 100 °C and a strong exothermic peak at about 380 °C can be observed in the DTA curve correspondingly. The oxidation of Li2FeSiO4 can be written as reaction (6). According to reaction (6) and taking the weight after its first loss around 100 °C as the calculation base, the carbon contents in Li2FeSiO4/C composite can be calculated based on the analysis of the result of TG, which is 16.1 %. Li 2 FeSiO 4 (s) + nC(s) + (0.25+n)O 2 (g) = Li 2SiO 3 (s) + 0.5Fe 2 O 3 (s) + nCO 2 (g) (6)
When the as-prepared Li2FeSiO4/C was assembled into batteries as cathode material, it 13
showed excellent electrochemical performance. The galvanostatic charge/discharge measurements were performed at 0.1 C in the potential range of 1.5-4.8 V (vs.Li+/Li). The initial three charge-discharge curves of Li2FeSiO4/C are presented in Fig. 8(a). The first charge profile is far different from those of following cycles, this phenomenon is usually caused by the structure rearrangement in the first charge process [55]. After the structure rearrangement, Li2FeSiO4/C transforms to a more stable phase and its charge/discharge voltage plateaus are focused on 2.8 and 2.6 V in the subsequent two cycles. Moreover, the discharge capacities of the intial three cycles all exceed 185 mAh g-1, indicating that more than one Li+ can be extracted from the Li2FeSiO4/C composite. In order to get more detailed analysis of charge-discharge characteristics, the differential capacities (dQ/dV) were calculated and the dQ/dV vs. voltage curves are shown in Fig. 8(b). In the first charge process, a wide oxidation peak at about 4.3 V corresponds to the oxidation of Fe3+ to Fe4+, which is coinsident with other groups’ reports [9,57] . This peak becomes extemely weak in the second and third cycles. After the first cycle, the Li2FeSiO4/C sample has a pair of redox peaks (at about 2.6-2.8 V) in the dQ/dV curves, which can be attributed to the redox reaction of Fe2+/Fe3+ couple. The separation between the oxidation and reduction peaks is narrow, indicating that the as-parpared Li2FeSiO4/C cathode material has good electrochemical reversibility. Besides, the reduction peak located at 1.8 V has not be explained clearly until now and some researchers thought this peak could be attributed to some extra side reactions [54]. Fig. 9(a) shows the cycle performance of Li2FeSiO4/C at 0.1 C. The discharge capacity reaches 227 mAh g-1 in the first cycle, but it drops to 191 mAh g-1 for the 2nd and 172.1 mAh 14
g-1 for the 5th cycle. The capacity fading and poor coulombic efficiency in the initial 4 cycles should result from the decomposition of electrolyte at high voltage and the formation of a solid electrolyte interphase (SEI) [55,58,59]. Fortunately, after 5 cycles, the specific capacity stops fading and tends to increase gradually up to 20 cycles, and then it keeps at about 190 mAh g-1 for next 60 cycles. Besides the coulombic efficiency reaches almost 100% after 11th cycle. This phenomenon indicates that a stable SEI film is formed, which can prevent further side reaction occurring at the electrode-electrolyte interface [60,61]. Considering that there is 16.1 wt% carbon in Li2FeSiO4/C composite, the actual specific capacity delivered by Li2FeSiO4 should be 227 mAh g-1, corresponding to 1.37 Li+ being extracted from Li2FeSiO4. The excellent electrochemical performance of Li2FeSiO4/C is ascribed to the smaller particle size and narrow particle size distribution as well as continuous carbon network pyrolyzed from PEG200. The carbon network that contacts firmly with Li2FeSiO4 particles due to its in situ formation feature provides a good electronic connection and ensures the majority of
active Li2FeSiO4 particles to participate in the electrode reaction. The measured electrical conductivity of the synthesized Li2FeSiO4/C composite is about 8×10-8 S cm-1, which is higher than the reported pure Li2FeSiO4 material [8]. Fig. 9(b) gives the rate capability of Li2FeSiO4/C under varied current density for every 10 cycles. At different rates of 0.1 C, 0.2 C, 0.5 C, 1 C and 2 C, the prepared Li2FeSiO4/C nanocomposite delivers specific capacities of 190, 175.3, 136.6, 98.0 and 76.9 mAh g-1, respectively. When the current rate returns to 0.1 C, the specific capacity can be recovered to its original value, indicating excellent reversibility in electrode reaction for Li2FeSiO4/C cathode material over cycling at high current density. The long-term cycling stability of 15
Li2FeSiO4/C was further studied at a current rate of 0.5 C with initial 5 cycles at 0.1 C. As plotted in Fig. 9(c), the specific capacities of 189.5 and 151.2 mAh g-1 are first measured at rate of 0.1 C and 0.5 C, respectively. Thereafter, the electrode shows a stable cycling performance for 50 cycles and then declines slightly. After 100 cycles, the capacity tends to stable and delivers about 90% capacity retention at 0.5 C after 400 cycles, revealing a good long-term performance. In order to well understand the kinetic characteristic of as-prepared Li2FeSiO4/C nanocomposite, cyclic voltammetry (CV) analysis was carried out at an increasing scan rate from 0.2 to 0.6 mV s-1. With the increasing scan rate, the peak current increases, meanwhile the cathodic and anodic peaks shift to lower and higher potentials, respectively (Fig. 10(a)). The diffusion coefficient of lithium ions (DLi) can be calculated from a linear relationship between peak current Ip (A) and the square root of the scan rate ν1/2 (V1/2 s-1/2) from the CVs, according to the following equation [62]: 1/2 Ip =2.69 105 n 3/2 AD1/2 Li C Li υ
(7)
where n is the number of electrons per reaction species, A is the electrode area (0.5 cm2 in this study) and CLi is the bulk concentration of the lithium ion in the electrode (0.040 mol cm-3 for Li2FeSiO4). From the slope of the fitting line in Fig. 10(b), the DLi of the as-prepared Li2FeSiO4/C nanocomposite is 1.58×10-12 cm2 s-1, which is larger than the previous carbon coated nanocomposites [17,63,64] and metallic ions (Ni2+, Cu2+, Zn2+, Mg2+, Al3+) doped Li2FeSiO4 [14,15,65]. This is mainly attributed to the smaller particle size and homogeneous particle distribution.
16
4. Conclusions
In summary, Li2FeSiO4/C nanocomposite was successfully synthesized for the first time via co-precipitation method by employing low cost Fe3+ salt as iron source. The predominance phase diagram based on the thermodynamic calculation could provide suitable parameters for the synthesis of Fe2+-based oxides, with which pure phase Li2FeSiO4 was synthesized without any other iron oxides impurities. Nanosized Li2FeSiO4/C particles (~150 nm) with nanocrystals Li2FeSiO4 (10-25 nm) dispering in amorphous carbon matrix were obtained. The PEG200 plays crucial roles in the precipitation process, which can not only disperse the precipitates and prevent the primary particles from growth, but also introduce carbon source to form conductive network in Li2FeSiO4 particles. The good electronic conduction network and the small Li2FeSiO4 particle size endow the synthesized Li2FeSiO4/C composite with excellent electrochemical performance. It delivers a reversible capacity of 190 mAh g-1 at 0.1 C, corresponding to the extraction of 1.37 Li+ from Li2FeSiO4 lattice. The Li2FeSiO4/C electrode also demonstrates a good long-term cycling stability, 90% capacity retention being achieved at 0.5 C over 400 cycles. This work proposes a low cost, simple and fast fabrication method for Li2FeSiO4/C cathode material, which is suitable for large-scale production. Acknowledgements
This work was financially supported by National Basic Research Program of China (2013CB934003), “863”program (2013AA050902), National Nature Science Foundation of China (21273019)， Guangdong Industry-Academy-Research Alliance (2013C2FC0015) and Program of Introducing Talents of Discipline to Universities (B14003) .
17
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26
Figure Captions
Fig. 1. (a) Phase stability diagram of Fe-O-C system under different oxygen partial pressures and (b) different CO volume fractions in CO/CO2 mixture.
27
Fig. 2. Schematic illustration of preparation process of Li2FeSiO4/C via co-precipitation method.
28
Fig. 3. XRD patterns of as-prepared Li2FeSiO4/C, standard ICSD# 261339 of Li2FeSiO4, standard PDF# 89-2648 of FeO, standard PDF# 89-0599 of Fe2O3 and standard PDF# 89-4319 of Fe3O4. Inset is the crystal structure model of monoclinic Li2FeSiO4 with space group P21/n.
29
Fig. 4. (a) FTIR spectrum and (b) magnetic moment as a function of applied field of the prepared Li2FeSiO4/C composite.
30
Fig. 5. (a) Wide scan XPS spectrum for all elements of the synthesized Li2FeSiO4/C composite and (b) XPS core-level spectra and fitting results of Fe 2p for Li2FeSiO4/C composite.
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Fig. 6. (a) FESEM, (b, c) TEM, (d, e) HRTEM images and (f) particle size distribution of the synthesized Li2FeSiO4/C powders.
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Fig. 7. TG-DTA curves of Li2FeSiO4/C composite.
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Fig. 8. (a) Initial three charge-discharge curves and (b) corresponding differential capacity curves of the Li2FeSiO4/C electrode at rate of 0.1 C in voltage range of 1.5-4.8 V.
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Fig. 9. (a) Cycle performance at current rate of 0.1 C in voltage window of 1.5-4.8 V, (b) rate performance of Li2FeSiO4/C at various current rates from 0.1 C to 2 C and (c) long-term cycling performance of Li2FeSiO4/C at a current rate of 0.5 C with initial 5 cycles at 0.1 C.
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Fig. 10 (a) Cyclic voltammograms of Li2FeSiO4/C at different scan rates and (b) plot of anodic peak current as a funcion of the square root of the scan rate.
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