Mesoporous Li2FeSiO4@ordered mesoporous carbon composites cathode material for lithium-ion batteries

Accepted Manuscript Mesoporous Li2FeSiO4@ordered mesoporous carbon Composites Cathode Material for Lithium-Ion Batteries Hailong Qiu, Kai Zhu, Haoming Li, Tingting Li, Tong Zhang, Huijuan Yue, Yingjin Wei, Fei Du, Wang Chunzhong, Gang Chen, Dong Zhang PII: DOI: Reference:

S0008-6223(15)00154-2 http://dx.doi.org/10.1016/j.carbon.2015.02.056 CARBON 9724

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Carbon

Received Date: Accepted Date:

1 November 2014 16 February 2015

Please cite this article as: Qiu, H., Zhu, K., Li, H., Li, T., Zhang, T., Yue, H., Wei, Y., Du, F., Chunzhong, W., Chen, G., Zhang, D., Mesoporous Li2FeSiO4@ordered mesoporous carbon Composites Cathode Material for Lithium-Ion Batteries, Carbon (2015), doi: http://dx.doi.org/10.1016/j.carbon.2015.02.056

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Mesoporous Li2FeSiO4@ordered mesoporous carbon Composites Cathode Material for Lithium-Ion Batteries Hailong Qiu[a] ,Kai Zhu[a], Haoming Li[a], Tingting Li[a], Tong Zhang[a], Huijuan Yue[b], Yingjin Wei[a], Fei Du[a], Wang Chunzhong[a], Gang Chen[a][c], and Dong Zhang[a]* [a] Key Laboratory of Physics and Technology for Advance Batteries (Ministry of Education), College of Physics, Jilin University, Changchun 130012 (P. R. China) [b] State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012 (P. R. China) [c] State Key Laboratory of Surperhard Materials, Jilin University, Changchun 130012 (P. R. China) Abstract The mesoporous Li2FeSiO4@ordered mesoporous carbon (CMK-3) has been firstly synthesized by a sol-gel method. The structural properties of the samples are characterized by X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy and nitrogen adsorption-desorption. The composite is then evaluated as a cathode material for lithium ion batteries. It exhibits greatly improved electrochemical performance compared with bulk Li2FeSiO4 and shows an excellent rate capability (160, 148, 129, 110, 90, 66 and 50 mAh g−1 at 0.1, 0.2, 0.5, 1, 2, 5 and 10 C, respectively) with significantly enhanced cycling performance. The greatly enhanced lithium storage properties of the Li2FeSiO4@CMK-3 composites may be attributed to the interpenetrating conductive carbon network, ordered mesoporous structure, and small uniform Li2FeSiO4 nanocrystallites that increase the ionic and electronic conduction throughout the electrode. 1. Introduction Lithium-ion batteries (LIBs) have been widely used in portable electronics, electric vehicles and aerospace energy storage. As an important component of lithium-ion batteries, cathode materials play a decisive role in improving electrochemical performance and reducing the cost of the whole cell. For practical applications, new *Corresponding author. Tel: +86 431 85155126. E-mail address: [email protected] (Dong Zhang)

kinds of cathode materials with higher capacity and power density are required. Recently, Li2FeSiO4 (LFS) has been drawn considerable attention due to its lower cost, high safety, environment-friendly and its high theoretical capacity with reversibly inserting/de-inserting two lithium-ions (332mAh g−1)

[1-9]

. However, as a polyanion

cathode material, LFS suffers from low electronic conductivity, which limits its electrochemical performance and commercial utilization. In order to overcome this weakness, many approaches have been reported, including coating with conductive materials [10], metal or nonmetal ion doping [11-15] and particle downsizing [16, 17]. As the most well-known member of the ordered mesoporous carbon family, CMK-3 has attracted much attention for used as conductive and porous matrix for LIBs due to its uniform pore diameter, large pore volume, interconnected porous structure, and high conductivity. Some mesoporous electrode materials, such as Li4Ti5O12 [18], MoS2 [19]

, SnO2 [20], V2O3 [21], LiFePO4 [22] and Li3V2(PO4)2 [23], have been synthesized via

using CMK-3 as conductive and mesoporous matrix to improve their electrochemical performance. In this work, we report a facile strategy to prepare mesoporous LFS@CMK-3 composites. In this LFS@CMK-3 electrode, the nano-sized LFS as the active material and the ordered mesoporous CMK-3 serves as the nanostructured electrode matrix. The CMK-3 not only acts as the hard template for LFS nanoparticles that prevents agglomeration and size growth during the heat treatment process, but also creates a conductive network in the LFS@CMK-3 electrode to improve its conductivity. By comparison to the bulk LFS, the as-prepared LFS@CMK-3 composites exhibit improved reversible capacity and excellent rate capability. 2. Experimental section 2.1. Synthesis of LFS@CMK-3 composites and LFS All the agents used in the experiment are analytical grade and used without further purification. The overall fabrication procedure of LFS@CMK-3 composites are schematically illustrated in Fig.1. CMK-3 was prepared by nanocasting templated 2

SBA-15 and the procedure followed was essentially according to Ref 22. The LFS@CMK-3 composites were synthesized by a sol-gel method. In a typical synthesis procedure, stoichiometric amounts of tetraethyl orthosilicate (TEOS), Fe(NO3)3·9H2O and LiNO3 were dispersed in ethanol and deionized water solution. Then 1.5 g citric acid and 0.1 g CMK-3 were added to the mixed solution with continuous stirring at room temperature for 12 h. The solution was then dried at 70 °C to evaporate the solvent and the obtained dry gel powders were treated at 650 °C for 10 h in a flowing nitrogen atmosphere to yield LFS@CMK-3 composites. Similar procedures were employed to synthesize bulk LFS with CMK-3 omitted.

Fig. 1– Schematic illustration of the synthesis of mesoporous LFS@CMK-3 composites. 2.2. Material characterizations X-ray diffraction (XRD) of the synthesized material was measured on a Bruker AXS D8 X-ray diffractometer with a Cu-Kα X-ray source operating at 40 kV and 100 mA. The morphologies of the materials were examined using a field emission scanning electron microscope (FE-SEM, JEOL JSM-6700F) and a transmission electron microscope (TEM, FEI Tecnai G2). Nitrogen adsorption and desorption isotherms were carried out at 77 K on a Micromeritics ASAP 2010 instrument. Specific surface area calculations were made using the Brunauer–Emmett–Teller (BET) method. The 3

pore size distribution (PSD) curves were calculated from the isotherm using the BJH (Barrett–Joyner–Halenda) algorithm. The carbon amounts were determined by Elementar Vario EL cube elemental analyzer. The electrical conductivity of the materials was measured by Solartron 1260 frequency response analyzer on sintered ceramic pellets. Silver paste was used as electrodes on opposite sides of each tablet, with an electrode area of 0.25 cm2. 2.3. Electrochemical measurements The electrochemical experiments were performed using 2032 type coin cells, with metallic lithium foil served as the counter electrode. The working electrode was made from a mixture of 70 wt% of active material, 20 wt% of super P conductive additive and 10 wt% of polyvinylidene difluoride (PVDF) binder which was pasted onto an aluminum current collector. The working electrode and counter electrode were separated by a Celgard 2320 membrane. 1 M LiPF6 solution dissolved in EC/DEC (1/1 vol %) was used as the electrolyte. The battery cells were assembled in an argon-filled glovebox. Galvonostatic charge–discharge was measured on a LAND-2100 (Wuhan, China) battery tester in the potential range of 1.5~4.8V versus Li/Li+. The charge/discharge specific capacities mentioned in this paper were calculated on the mass of LFS by excluding the carbon content. Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) were performed on a Bio-Logic VSP multichannel potentiostatic–galvanostatic system. The impedance spectra were recorded by applying an alternating-current voltage of 5 mV in the frequency range from 1MHz to 5 mHz. 3．Results and discussion 3.1 Structure and morphology analysis The SAXRD of SBA-15 and CMK-3 is shown in the Fig. S1. The result shows that SBA-15 and CMK-3 are ordered mesoporous structure. Fig. 2 shows the XRD patterns of the LFS and LFS@CMK-3 samples (the XRD of the different quantity of CMK-3 is in the Fig. S2). All the identified peaks can be perfectly indexed to the 4

monoclinic structure reported by Nishimura et al. (S.G. P21/n)

[17, 24]

without any

reflection peak from impurity phases, suggesting that the samples are pure. No diffraction peaks corresponding to carbon were found in the XRD pattern, most likely due to its low content or low graphitization degree. The lattice parameters of the samples are determined by the Celref program and described in Table 1. The average crystallite size (D) of the materials is calculated using Scherrer’s formula [25], D = Kλ /B cos θ, where λ is the wavelength of the X-ray radiation, B is the angular width at half of the maximum intensity of the (-103) peak, θ is the Bragg angle of the (-103) peak, and K is a constant, which is 0.9. Calculation shows that the average crystallite size of LFS (25.4 nm) is a little bit larger than that of LFS@CMK-3 (16.2 nm). Elemental analysis confirms that the carbon content in LFS and LFS@CMK-3 composite is 5.95% and 17.65%, respectively. Table 1– The lattice parameters of the LFS and LFS@CMK-3. Samples

a (Å)

B (Å)

c (Å)

β (°)

LFS

8.3094

5.0252

8.2748

98.59

LFS@CMK-3

8.3410

5.0237

8.2609

98.73

Fig. 2– X-ray diffraction pattens of LFS and LFS@CMK-3. 5

Fig. 3– (a) FE-SEM image and (b) TEM image of CMK template, (c) FE-SEM image and (d) TEM image of LFS@CMK-3, (e) FE-SEM image and (f) TEM image of LFS. The morphology and microstructure of the as-prepared CMK-3 mesoporous carbon templates, the mesoporous LFS@CMK-3 composites and bulk LFS are studied by FE-SEM and TEM. It is seen from the FE-SEM image (Fig. 3a) that the CMK-3 has a 6

rod-like morphology, deriving from the mesoporous SBA-15 template. As shown in Fig. 3b, CMK-3 has open and parallel one dimensional channel structures with a uniform pore size of several nanometers, indicating the successful synthesis of CMK-3. Such structural features make CMK-3 an ideal template for the synthesis of mesoporous carbon-hybrid composites. After LFS loading, the apparent morphology of the CMK-3 was mostly retained as shown in Fig. 3c and Fig. 3d. One can see that LFS nanoparticles with particles size of less than 50 nm are not only embedded in the conductive matrix partly blocking the channels, but also dispersed on the CMK-3 surface after impregnation and calcinations process. The LFS@CMK-3 composites have a nearly same mesoporous structure as that of CMK-3, which is beneficial to penetration of electrolyte into the mesopores and contact between electrolyte and LFS active nanoparticles. In contrast, the bulk LFS obtained without the presence of CMK-3 exhibits aggregated particles with a large particle size of more than 100 nm and the irregular structure (Fig. 3e, Fig. 3f), suggesting that the CMK-3 matrix limits the growth and prevents the agglomeration of LFS during the process of crystallization. The specific surface area and pore size distribution of the samples were depicted by nitrogen adsorption–desorption isotherms as shown in Fig. 4. The samples exhibited a typical IV-type isotherm curve with a H4 hysteresis loop in the range of 0.5–1.0 P/P0, which was typical characteristic of mesoporous materials. The pore size distribution plots are calculated from the desorption isotherm using the Barrett–Joyner–Halenda (BJH) model (inset of Fig. 4). The BET surface area and pore volume are found to decrease significantly from 1243 m2 g-1 and 1.53 cm3 g-1 for CMK-3 to 87 m2 g-1 and 0.12 cm3 g-1 for LFS@CMK-3 composites. The pore size distribution for the CMK-3 is shown in the inset of Fig. 4a and indicates the formation of randomly distributed pores with size of from 4 to 10 nm and dominant at 5.5 nm. From analysis of the isotherms (inset of Fig. 4b), the LFS@CMK-3 have a well-defined 5 nm mesopore distribution and the intensity was lower. Pore-filling would significantly 7

reduce the surface and pore size

[26]

. The N2 adsorption–desorption isotherms of the

LFS is shown in the Fig. S3. The sample exhibited a typical IV-type isotherm curve with a H3 hysteresis loop in the range of 0.5–1.0 P/P0. The BET surface area for LFS is 10.7832 m2 g-1. These results strongly prove that LFS nanoparticles were embedded inside the pore channels of CMK-3, which is in good agreement with the TEM analysis. The composites with an inner mesoporous structure allow the electrolyte to easily penetrate into the mesopores and make contact with the embedded LFS nanocrystals, leading to significant improvement in electrochemical performance.

Fig. 4– N2 adsorption–desorption isotherms of (a) CMK-3 and (b) LFS@CMK-3 composites. Inset: pore size distributions from the adsorption branch through the BJH method.

8

The electronic conductivity of LFS is measured to be 5.6×10-4 S cm-1, which is very close to the result of the Ref 8. In comparison, electronic conductivity of the as-prepared LFS@CMK-3 rises to 3.2×10-3 S cm-1, which is 10 times higher than that of LFS. It depicts that the CMK-3 used as electrode matrix is favorable for the improvement of the electrical conductivity. The electrochemical performance of the LFS and LFS@CMK-3 materials were studied in the potential window of 1.5~4.8 V under various C rates (1C = 166 mAh g-1). Fig. 5a shows the initial two and fifth charge-discharge cycle profiles of the materials at the 0.1 C. Despite their different specific capacities, both materials show similar charge/discharge curves indicating the same electrochemical reaction occurring in the Li+ insertion/de-insertions processes. As shown, the charge profile of the first cycle is totally different from the charge profiles of the others, two potential plateaus (∼3.25 and ∼4.3 V) can be observed, which agrees well with the previous reports

[3, 6, 27]

. The first voltage plateau (3.25 V) corresponds to the Fe2+/Fe3+ redox

couple. Subsequently, the 3.25V potential plateau shifts to 3.0 V in the second charge process, suggesting a structural rearrangement during the first charge process. On the further charge-discharge, the 3.0V potential plateaus are almost the same that suggests little or no subsequent change in structure. The second voltage plateau (∼4.3V) should correspond to the Fe3+/Fe4+ redox couple and disappeared in the following charge processes, which was proved by Lv et al

[28]

. In addition, the first discharge

and charge capacities of LFS@CMK-3 are 182.2 and 156 mAh g-1, while those of bulk LFS are 95.4 and 86.2 mAh g-1. Obviously, the participation of CMK-3 considerably increases the charge/discharge capacities. The rate dependent cycling performance of the LFS and LFS@CMK-3 samples was investigated by galvanostatic cycling at various charge–discharge rates from 0.1 to 10 C and then back to 0.1 C, each for 10 cycles, which is shown in Fig. 5b. As seen clearly, the discharge capacity decreases with increasing the rate for both samples because of the increased polarization and the decreased utilization of active materials 9

at high current density. Compared to the LFS material, the LFS@CMK-3 composites deliver a relatively higher capacities and better rate capacities (Fig. S4 further shows rate dependent cycling performances of the different quantity of CMK-3.). For example, The maximum discharge capacity measured for the LFS@CMK-3 composite at a rate of 0.1, 0.2, 0.5, 1, 2, 5 and 10 C is 160, 148, 129, 110, 90, 66 and 50 mAh g−1, respectively, while that of LFS is 90, 75, 56, 43, 32, 17 and 10 mAh g−1, respectively. More interesting, even after cycling at an extremely high C-rate of 10 C, the capacity of LFS@CMK-3 composites can recover to the initial value (about 160 mAh g−1). The excellent rate performance for LFS@CMK-3 composites may be attributed to the following reasons: 1) the charge transfer is efficient as CMK-3 maintains a continuous and conductive network. 2) Lithium ion diffusion rate is improved by decreasing the diffusion distance due to the reducing particle size.

Fig. 5– (a) Charge–discharge profiles of LFS and LFS@CMK-3 at 0.1C rate, and (b) rate dependent cycling performances of LFS and LFS@CMK-3. To further investigate the cycling stability of the LFS@CMK-3 composites, it was tested at a rate of 1 C for 100 cycles and 2 C for 500 cycles, and the results are given in Fig. 6. (The cycle performance of different proportions of active material, conductive additive and binder at 2 C was shown in Fig. S5.) It is seen that the material shows good rate capability and excellent cyclability in 100 cycles. The specific discharge capacity of this composite still stabilized around 100 and 79 mAh 10

g−1 at a rate of 1 and 2C, retaining over 99% and 96% of initial capacity, respectively. Obviously, the electrochemical performance demonstrated that CMK-3 conductive matrix forms a stable network for LFS@CMK-3 composites that makes the Li+ insertion/de-insertion process in the relative high rates. In addition, LFS nanoparticles are connect directly to the carbon matrix, constructing a superior conductive framework that result for enhancement of the electron conductivity and efficient charge transport of the composite. After 100 cycles at 2 C, the specific discharge capacity of this composite starts to decrease and is 55.2 mAh g−1, retaining 73% of initial capacity after 500 cycles. The attenuation of capacity may be due to part of the electrode composite falling off from the current collector, according to the cross-section images of the LFS@CMK-3 electrode and after 120 cycles (Fig. 7).

Fig. 6– Cycling performance of the LFS@CMK-3 sample at the rate of 1 C and 2 C.

Fig .7– FE-SEM images of the LFS@CMK-3 electrode (a) and after 120 cycles (b) at 2 C. 11

In order to understand the effect of CMK-3 incorporation on the electrochemical performance of LFS, we carried out AC impedance measurements after the second cycle at a rate of 0.1 C, the Nyquist curves of LFS and LFS@CMK-3 electrodes are shown in Fig. 8a. As shown, the impedance plot consists of a depressed semicircle and a straight line. It is well known, the intercept at the highest frequency is due to the internal resistance (Rs) of the cell arising from the electrolyte, separator, current collector, etc. The semicircle in the middle frequency range is attributed to the charge-transfer resistance (Rct). The straight line in the low frequency is associated with lithium ion diffusion in the electrode bulk. Based on this, the Nyquist plots are simulated using the equivalent circuit, as shown in the insets of Fig. 8a. The lithium ion diffusion coefficient (DLi) could be calculated from the low frequency plots according to the following equation [29-31]: DLi = R2T2/2A2n4F4C2σ2 (1) where R is the gas constant, T is the absolute temperature, A is the surface area of the cathode, n is the number of electrons per molecule during oxidation, F is the Faraday constant, C is the concentration of lithium ion, σ is the Warburg factor which is relative with Zre. Zre = RD + RL +σω-1/2 (2) where ω is frequency. The relationship between Zre and the reciprocal square root of frequency in the low frequency are shown in Fig. 8b. All the parameters obtained and calculated from EIS are shown in Table 2. It can be observed that the exchange current density ( i  RT / nFRct ) and the lithium ion diffusion coefficient of the LFS@CMK-3 composites are higher than those of the LFS. Furthermore, the LFS@CMK-3 composites exhibit a smaller charge-transfer resistance than that of the LFS. Therefore, the charge-transfer reaction is stronger in the LFS@CMK-3 electrode than in the LFS electrode. The above results then well explains the better rate performance of the LFS@CMK-3 composites than that of the LFS. The larger chemical diffusion coefficients of LFS@CMK-3 should be attributed to CMK-3 that 12

provides an ordered passageway for Li+ rapid transfer and large sufficient electrolyte/electrode contact area.

Fig. 8– (a) Nyquist plots of the LFS and LFS@CMK-3. Inset: equivalent circuit models, (b) Linear fitting of the Z′ vs ω−1/2 relationship of the LFS and LFS@CMK-3 samples. Table 2– Impedance parameters of the samples. Samples

Rct (Ω)

σ (Ω S-1/2)

DLi (cm2 s-1)

i (mA cm-2)

LFS

138.7

106.26

1.94×10-14

0.185

LFS@CMK-3

85.14

42.71

1.20×10-13

0.302

As shown in Fig. 9, the CV of the LFS and LFS@CMK-3 were measured at 0.1 mV s-1 in the voltage window of 1.5~4.8 V. Obviously, the LFS@CMK-3 shows the 13

similar shape of CV curve with that of LFS electrode, demonstrating that the electrochemical reaction of both materials is same. The peaks of Fe2+/Fe3+ redox couple of both samples can be observed around 3.25 V/2.6 V (for the first cycle) and 3.0 V/2.6 V (for the second cycle). And the shift of oxidation peaks between the first and the second cycles proves the structural rearrangement. The oxidation peak around 4.3 V can be ascribe to the Fe3+/Fe4+ redox couples. The results agree well with the plateau in the charge/discharge curves (Fig. 5a) and the previous reports [8].

Fig. 9– CV curves of the LFS (a) and LFS@CMK-3 (b) at 0.1 mV s-1 in the voltage window of 1.5~4.8 V. The different scan Cyclic Voltammetry (DSCV) of the LFS and LFS@CMK-3 were also performed from 0.15 to 0.35 mV s-1 after two cycles at 0.1 C. As shown Fig.10 (a) and (b), all the CV curves show a couple of cathodic/anodic peaks, which are attributed to the Li+ insertion/de-insertion from the samples. The polarization of the LFS@CMK-3 is lower than that of LFS at various scan rates due to the better electrical conductivity of LFS@CMK-3, as shown in Table 3. Table 3–The polarization of the LFS and LFS@CMK-3 at various scan rates. Scan rate (mV S-1)

0.15

0.2

0.25

0.3

0.35

LFS (V)

0.602

0.598

0.656

0.689

0.737

LFS@CMK-3 (V)

0.444

0.502

0.570

0.622

0.691

14

Fig.10–CV curves of the LFS (a) and LFS@CMK-3 (b) at different scan rates from 0.15 to 0.35 mV s-1 after two cycles at 0.1 C. Linear fits of the peak current Ip for the oxidation (c) and reduction (d) peaks as functions of the square root of the scan rate v1/2. As we known, the CV technique has been used to evaluate the diffusion kinetics of Li+ insertion materials via equations 3 and 4, expressed as Ip = 2.69×105 n 3/2ADLi1/2v1/2C0* (3) DLi=[k/(2.69×105 n 3/2A C0*)]2 (4) Where Ip is the peak current, k is the slope of the linear fit of the peak current (Ip) as a function of the square root of the scan rate (v1/2), A is the surface area of the electrode (cm2), C0* is the concentration of Li+ ions (mol cm-3), DLi is the chemical Li+ diffusion coefficient, n is the number of electrons involved in the redox process (n = 1 for Fe2+/Fe3+ redox pair), v is the potential scan rate (V s-1). 15

From the Fig.10 (c) and (d), the peak current (Ip) shows a linear relationship versus square root of the scan rate (v1/2). Therefore, we can easily calculate the Li+ diffusion coefficient DLi according to Equation 4 (Table 4). The results show that, with CMK-3 incorporation, the diffusion coefficient of lithium ions (DLi) was increased from 10-12 cm2 s-1 to 10-11 cm2 s-1. Here it should be noted that the calculated value is two orders of magnitude higher than that of the EIS measurement, the difference may be due to different testing mechanism [25]. Table 4–The calculated lithium diffusion coefficients for LFS and LFS@CMK-3 at scan rates from 0.15 to 0.35 mV s-1. DLi (cm2 s-1)

Oxidation (charge)

Reduction (discharge)

LFS

3.78×10-12

4.27×10-12

LFS@CMK-3

3.49×10-11

4.10×10-11

4. Conclusions We have successfully fabricated the mesoporous LFS@CMK-3 composites by a sol-gel method. LFS has particle size ranging from several nanometers to a dozen nanometers and are embedded in the mesoporous channels as well dispersed on the CMK-3 matrix surface. Compared to the bulk LFS, the LFS@CMK-3 composites exhibit much improved electrochemical performance in terms of specific capacity and rate capability. The results should be attributed to the nanosized particles, special mesoporous structure, and the existence of conductive carbon matrix for LFS@CMK-3 composites, which can enhance the electronic conductivity and lithium ion diffusion. Acknowledgements This study was supported by the 973 Program (No. 2015CB251103), the National Natural Science Foundation of China (No. 51272088，No. 21201073), the Science and Technology Development Planning of Jilin Province (No. 201215028, No. 20150204030GX).

16

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