J Solid State Electrochem (2015) 19:415–421 DOI 10.1007/s10008-014-2603-z

ORIGINAL PAPER

Synthesis and electrochemical performance of Li2FeSiO4/C cathode material using ascorbic acid as an additive Ming Li & Lu-Lu Zhang & Xue-Lin Yang & Yun-Hui Huang & Hua-Bin Sun & Shi-Bing Ni & Hua-Chao Tao

Received: 28 May 2014 / Revised: 28 July 2014 / Accepted: 1 August 2014 / Published online: 27 August 2014 # Springer-Verlag Berlin Heidelberg 2014

Abstract Carbon-coated Li2FeSiO4 composite (LFS/C-AA) was synthesized via a refluxing-assisted solid-state reaction by using ascorbic acid as additive and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy, galvanostatic charge/discharge measurements, and electrochemical impedance spectra (EIS) tests. The results show that ascorbic acid can to some extent prohibit the oxidation of Fe2+ during the synthesis process, and the pyrolytic carbon from ascorbic acid shows higher electronic conductivity and improves the degree of graphitization of residual carbon in the LFS/C-AA composite. Compared with LFS/C prepared without ascorbic acid, LFS/C-AA displays better electrochemical performance. The desirable property is attributed to the reduced particle size, the enhanced electronic conductivity, and the improved diffusion coefficient of lithium ions. M. Li : L.
Keywords Lithium-ion battery . Cathode . Lithium iron silicate . Ascorbic acid

Introduction Because of serious environmental pollution and rapid depletion of fossil fuels, Li-ion batteries (LIBs) have drawn increasing attention in recent years for use as the power supply in electric vehicles (EVs) or hybrid electric vehicles (HEVs). It is generally known that the performance of LIBs highly depend on the structure and properties of electrode materials and electrolytes, especially cathode materials [1]. Li2FeSiO4 (LFS), as a new kind of polyanion cathode materials for LIBs [2], has attracted wide interest due to its low cost (the abundance of iron and silicon in crust), environmental benignity (the nontoxicity of elements), high thermal stability (the strong Si-O bonding), high safety (the polyanionic structure), and high theoretical capacity (166 mAh g−1 for one Li+ ion exchange, and 332 mAh g−1 for two Li+ ions exchange) [3–6]. Unfortunately, LFS suffers from poor electronic conductivity and slow lithium ion diffusion as other polyanion cathode materials (i.e., LiFePO4 [7–9] and Li3V2(PO4)3 [10–12]), which limit its practical application in LIBs. Therefore, much effort has been made to improve the electrochemical performance of LFS, such as carbon coating [2–6, 13–17], nanostructure designing [4, 18–22], and metal cation doping [13, 16, 23–26]. Carbon coating is one of the most effective methods to improve the electronic conductivity, but the modification effect depends not only on the amount of residual carbon but also on the type of carbon source. So far, various carbon sources, such as glucose [14], sucrose [4, 5, 15, 16], citric acid [13, 17], carbon nanotubes [27–31], graphene, and reduced graphene oxide [3, 6], have been successfully employed. Nowadays, Li2FeSiO4/C has generally been synthesized by sol-gel process [3, 13, 24, 26, 32, 33],

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hydrothermal method [34–36], and solid-state reaction [27, 37–39]. Compared with traditional solid-state reaction, solution method (i.e., sol-gel process, hydrothermal method, refluxing process, and so on) can mix the starting materials in a molecular level, which is conducive to homogeneous reaction. However, compared with hydrothermal method, refluxing process is easier to operate in closed system just because the raw materials can react mildly in open system. In addition, compared with sol-gel process, large amount of H2O, CO2, and other small molecule gases can be avoided during the subsequent sintering process by using a refluxing process because of no chelating agent. To our knowledge, refluxing process has been successfully used to synthesize Li 2 FeSiO 4 [5, 40, 41]. In this work, we also used a refluxing-assisted solid-state reaction to prepare LFS/C, but we chose ascorbic acid only as an additive not as a carbon source and a reducing agent. Ascorbic acid, as a carbon source and a reducing agent, has also been successfully used in the modification of LFS [32, 33, 42] and other cathode materials (i.e., LiFePO4 [43–46], LiMnPO4 [43, 47] and LiCoPO4 [43, 48], and LiFeBO3 [49]). For instance, Yan Z et al. [32] synthesized LFS/C composites via sol-gel method with Lascorbic acid as carbon additive. It was found that the structure of residual carbon in the LFS/C composite is graphene-rich with obviously lower disordered/graphene (D/G) ratio, and the LFS/C exhibited better electrochemical performance than the LFS/C composite synthesized with sucrose as a carbon additive. Devaraju MK et al. [42] used ascorbic acid as a reducing agent to prepare LFS/C with enhanced electrochemical performance, but this LFS/C composite was prepared by hydrothermal method in closed system at high pressure. However, using ascorbic acid as a carbon source or as a reducing agent is unfavorable for large-scale production because of the higher price of ascorbic acid than that of conventional carbon source (i.e., glucose, sucrose, etc.). In this work, we used ascorbic acid only as an additive to prepare LFS/C (denoted as LFS/C-AA) via a refluxingassisted solid-state reaction. On the one hand, ascorbic acid can inhibit the oxidation of Fe2+ during the synthesis process; on the other hand, the pyrolytic carbon from ascorbic acid shows higher electronic conductivity and improves the degree of graphitization of residual carbon in LFS/C-AA. More importantly, ascorbic acid is only used as additive rather than carbon source, so the dose can be largely decreased, which is advantageous in reducing the packing cost for large-scale production. For comparison, another LFS/C composite was also prepared via the same process without ascorbic acid (denoted as LFS/C). The as-obtained LFS/C composites were characterized with X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Raman spectra. The effect of ascorbic acid on the electrochemical performance of LFS was also investigated.

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Experimental Li2FeSiO4/C composites were synthesized via a refluxingassisted solid-state reaction. All chemicals were of analytical grade and used without further purification. A stoichiometric amount of tetraethyl orthosilicate (TEOS), CH3COOLi·2H2O, and FeC2O4·2H2O were dispersed in ethanol with a small amount of ascorbic acid. Here, ascorbic acid served as an additive. The above mixture was refluxed at 80 °C for 24 h under stirring till a brown gel was formed. The resulting wet gel was dried at 50 °C overnight to obtain the dry gel. Then, sucrose as a carbon source was finely ground with the obtained dry gel in acetone for 7 h. After drying, the above mixture was calcined at 350 °C for 5 h and then sintered at 650 °C for 10 h under flowing nitrogen gas. After natural cooling down to room temperature, the powders were ground and sieved to obtain the final Li2FeSiO4/C (LFS/C-AA). For comparison, another Li2FeSiO4/C composite (LFS/C) was synthesized via the same process without ascorbic acid. The phase identification of the obtained samples was performed by powder X-ray diffraction (XRD, Rigaku Ultima IV) employing Cu-kα radiation (λ=1.5406 Å). Diffraction patterns were scanned over the range of 2θ between 10° and 90° in a step of 0.02°. The morphology was observed with a field-scanning electron microscope (FSEM, JSM-7500F, JEOL) and a transmission electron microscope (TEM, JEM-2100, JEOL). Carbon coating on both samples was characterized by Raman spectrometry (VERTEX 70, Bruker). Electrical conductivity was measured with a standard four-probe method by RTS resistivity measurement system (RTS-8, China) on disk-shaped pellets with diameter of 8 mm and thickness of about 1.0 mm. The electrochemical properties of the obtained samples were measured in CR2025 coin cells using lithium foil as counter and reference electrodes. The coin cells were prepared as described in Ref. [6]. The working electrodes were prepared by mixing active materials (75 wt%), acetylene black (15 wt%), and polyvinylidene fluoride (PVDF, 10 wt%) in N-methyl pyrrolidinone (0.02 g mL−1) on an aluminum foil (20 μm in thickness) which was used as the current collectors. The loading of the active materials on the electrode was 1.8 mg cm−2. Galvanostatic charge-discharge measurements were performed in a voltage range of 1.5–4.6 V on a battery test system (LAND CT2001A, China). All reported capacities are quoted with respect to the mass of the obtained samples including the coating carbon. Electrochemical impedance spectroscopy (EIS) measurement was performed on an electrochemical working station (CHI614C, China) over a frequency range between 0.01 Hz and 100 kHz.

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Results and discussion Figure 1 shows the XRD patterns of LFS/C and LFS/C-AA samples. It can be clearly seen that LFS/C-AA shows a same XRD pattern as LFS/C and the main diffraction peaks of both samples are well indexed as the monoclinic structure of LFS (JCPDS, No.77-4374) with space group P21. It is indicated that the addition of ascorbic acid during the synthesis process has no inherent effect on the LFS phase formation. Noting that a weak diffraction peaks around 30° for Fe2SiO4 and two weak diffraction peaks around 43.4° and 53.0° for Fe7SiO10 were observed. Among the two impurities, a small amount of Fe7SiO10 can enhance the electronic conductivity of LFS [50]. By carefully observing, another weak peak at 21.2° corresponding to Fe3O4 was detected only in the XRD pattern of LFS/C. But despite the absence of Fe3O4 ([Fe3+]=50 %) in the XRD pattern of LFS/C-AA, the presence of Fe7SiO10 (i.e., a mixed valence compound as Fe2+5Fe3+2SiO10) with relatively low content of Fe3+ (28.57 %) demonstrates ascorbic acid can to some extent inhibit the oxidation of Fe2+. It must be pointed out that no peaks for crystalline carbon are observed, suggesting that the carbon in the composite is amorphous. The amount of residual carbon in both LFS/C and LFS/C-AA samples is approximately 6 wt% as determined by a carbonsulfur analyzer (CS600, LECO, USA). Figure 2 shows the SEM images of LFS/C and LFS/C-AA samples. As shown in Fig. 2, there is no significant difference between the morphology of both samples. The particles for both samples present irregular granular shapes with some agglomeration and with a receivable size distribution ranging from ∼100 to 500 nm. In order to further investigate the influence of AA on microstructure for the LFS/C samples, TEM images are shown in Fig. 3. It can be clearly seen that the LFS/C-AA sample (30–100 nm) (Fig. 3c) exhibits smaller particle size than LFS/C (80–200 nm) (Fig. 3a), which demonstrates that the addition of ascorbic acid can reduce the particle size of LFS. Decrease of particle size is favorable to diffusion of

Fig. 1 XRD patterns of the LFS/C and LFS/C-AA samples

Fig. 2 SEM images of a, c LFS/C and b, d LFS/C-AA

lithium ions due to the shortened lithium ion diffusion pathway. From Fig. 3b, d, we can also see that the LFS particles in both samples are intimately connected with a continuous carbon network and coated with a thin carbon layer about 3– 5 nm. Moreover, the lattice fringes with interplannar distance of 0.2651 and 0.3649 nm are in accordance with the d-spacing of the (−212) planes of LFS (Fig. 3b) and the (111) planes of LFS (Fig. 3d), respectively. Noting that the lattice fringes with interplannar distance of 0.3240 nm related to the d-spacing of the (002) planes of graphite is also observed (Fig. 3d), which indicates that the graphite structure of carbon is easily detected in the LFS/C-AA sample. As we all know, each molecule of ascorbic acid consists of an almost planar five-membered ring plus a side chain [32]. This kind of structure was readily transformed to graphite in view of the principle of lowest energy [32, 51, 52], which means that ascorbic acid may act as a template for the formation of graphite precursor upon

Fig. 3 TEM images of a, b LFS/C and c, d LFS/C-AA samples

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Fig. 4 a, b Charge/discharge profiles and c cycle performance curves of LFS/C and LFS/C-AA samples. d Discharge capacity of LFS/C and LFS/C-AA at different C-rate

heating [32]. Therefore, the degree of graphitization of the carbon layer on the surface of the LFS/C-AA particles was promoted. To further study the structure of residual carbon in both samples, Raman spectra are discussed later. To investigate the effect of AA on the electrochemical performance of LFS, galvanostatic charge-discharge tests on the LFS/C and LFS/C-AA electrodes were performed in the voltage range of 1.5–4.6 V (vs. Li+/Li). The electrodes in turn were charged/discharged at 0.1 C (1 C=166 mA g−1) for 5 cycles, 1 C for 50 cycles, and then back to 0.1 C for 5 cycles (Fig. 4a–c). Figure 4a shows the first two charge/discharge profiles of LFS/C and LFS/C-AA samples. Two voltage plateaus, located around 3.3 and 4.3 V, are observed in the initial charge process of both LFS/C and LFS/C-AA. For LFS/CAA, the charge capacities of these two voltage regions are 44 and 176 mAh g−1, respectively, which is higher than that for LFS/C (34 and 106 mAh g−1, respectively). Obviously, the further Li+ can extract at higher voltage [18]. The lower voltage plateau at ~3.3 V relates to a two-phase transformation process from α to β, i.e., Li2FeSiO4(α)→LiFeSiO4(β)+Li+ + e−, which corresponds to the oxidation of Fe2+ to Fe3+ [53]. The higher voltage plateau at ~4.3 V indicates another twoFig. 5 a EIS curves and b relationship between Z′ and ω−1/2 in the low-frequency region of LFS/C and LFS/C-AA samples

phase transformation process, as LiFeSiO4(β)→LiyFeSiO4 (γ)+(1−y)Li+ +(1–y)e− (0≤y<1) [53], which corresponds to the Fe3+/Fe4+ redox couple [18]. Subsequently, the lower potential plateau at ~3.3 V shifts to ~2.8 V in the second charge process, indicative of a structural rearrangement [54] and the decreased polarization of the cathode [50]. On the further charge processes (Fig. 4b), the potential plateaus are stabilized at ~2.8 V, suggesting little or no subsequent change in structure and less polarization. Compared with LFS/C (117.3 mAh g−1), LFS/C-AA exhibits a significantly enhanced initial discharge capacity of 163.3 mAh g−1 at 0.1 C, which is much higher than the LFS/C composite obtained by sol-gel method with L-ascorbic acid as carbon additive (135.3 mAh g−1) [32]. Even with carbon nanotube modification, the capacity reported in Ref. [29] can only reach 153 mAh g−1. As shown in Fig. 4b, the two electrodes exhibit much larger potential difference at 1 C (6th) than that at 0.1 C (60th), indicative of the increased polarization at high C-rate. In addition, LFS/C-AA also delivers a higher capacity of 91.1 mAh g−1 than LFS/C (68.0 mAh g−1) at 1 C. As shown in Fig. 4c, when charged/discharged back to 0.1 C for another 5 cycles, both electrodes show a discharge capacity close to

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Table 1 EIS parameters of LFS/C and LFS/C-AA samples Sample

Rct (Ω)

δ (Ω s1/2)

i (mA cm−2)

DLi (cm2 s−1)

LFS/C LFS/C-AA

58.21 23.25

211.18 54.00

0.441 1.105

3.1×10−13 5.1×10−12

the first five cycles at 0.1 C, which demonstrates the two LFS/ C composites are tolerant to varied charge/discharge currents. LFS/C-AA still exhibits a discharge capacity of 157.4 mAh g −1 , which is much higher than LFS/C. Figure 4d shows the rate capability for LFS/C and LFS/CAA. The data were obtained by testing in a mode that the cell was charged at a low current density of 0.1 C to 4.6 V and discharged at different rates (0.5, 1, 2, 5, and 0.5 C) to 1.5 V, and the cell ran for 10 cycles at each current density. In all cases, LFS/C-AA delivers higher capacity than other LFS/C samples. Compared with the LFS/C composite by using AA as carbon source [33], LFS/C-AA shows a comparable or better electrochemical performance. In this work, AA is only used as an additive, and the dose is largely decreased, which is advantageous in reducing the packing cost. It is worth mentioning that a large irreversible capacity in the first two cycles can be seen in LFS/C-AA sample (Fig. 4a). To our knowledge, the existence of a solid electrolyte interface (SEI) layer on the surface of cathode has been proven [55–58]. The higher the specific surface area, the more the SEI layers could be formed during the initial cycling process, which is indicative of irreversibly consuming more lithium ion for the formation of SEI layers. Thus, due to the reduced particle size (shown in Fig. 3), the LFS/C-AA sample exhibits a large irreversible capacity. Nevertheless, according to “radial model” and “mosaic model” reported by Andersson [59], the smaller the particle, the faster the lithium ions and electrons insert/extract. Thus, the LFS/C-AA sample delivers much higher discharge capacity than the LFS/C sample in the subsequent cycles at all rates. The electrochemical impedance spectroscopies (EIS) for the LFS/C and LFS/C-AA composites are shown in Fig. 5. Both curves consist of a small intercept, a depressed semicircle, and an inclined line. The small intercept at the Z′ axis in the high-frequency region corresponds to the ohmic resistance, representing the resistance Fig. 6 Raman spectra of a carbon yielded from pyrolysis of AA and sucrose and b LFS/C and LFS/CAA samples

Table 2 ID/IG obtained from Raman spectra of LFS/C and LFS/C-AA samples Sample

C-Sucrose

C-AA

LFS/C

LFS/C-AA

ID/IG

1.53

0.77

1.97

1.73

of the electrolyte. The depressed semicircle in the medium frequency region is related to the charge-transfer resistance and the double-layer capacitance between the electrolyte and cathode. The inclined line in the low frequency region is the Warburg impedance (Rct), which is associated with Li+ ion diffusion in the cathode-active particles [6]. The smaller the diameter, the lower the chargetransfer resistance is. Both EIS curves were fitted by an equivalent circuit composed of “R(C(RW))” (the insert in Fig. 5a) using the ZSimpWin program [12], and the fitting results were shown Table 1. From Fig. 5a and Table 1, it can be clearly seen that LFS/C-AA (Rct =23.25 Ω) shows much lower charge-transfer resistance than LFS/C (Rct = 58.21 Ω). The exchange current density (i) and the diffusion coefficient of lithium ions (DLi) can be obtained according to the following equations [6, 32, 33]: . i ¼ RT nFRct ð1Þ . DLi ¼ R2 T 2 2A2 n4 F 4 C 2Li δ2

ð2Þ

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, and CLi is the concentration of lithium ion. δ is the Warburg coefficient which is related to Z′ [6, 32, 33]: . Z 0 ¼ RC þ Rct þ δω

−1

2

ð3Þ

where ω is the angular frequency in the low frequency region, both RC and Rct are kinetics parameters independent

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of frequency, so δ is also the slope for the plot of Z′ vs. the reciprocal square root of the lower angular frequencies (ω−1/2). To obtain the Warburg coefficient (δ), the linear fitting of Z′ vs. ω−1/2 in the low frequency region of LFS/C and LFS/C-AA samples was shown in Fig. 5b. As listed in Table 1, LFS/C-AA shows higher exchange current density (i=1.105 mA cm−2) and increased diffusion coefficient of lithium ions (DLi =5.1×10−12 cm2 s−1) than LFS/ C (i = 0.441 mA cm−2, and DLi = 3.1 × 10−13 cm2 s−1). Higher exchange current density and increased diffusion coefficient of lithium ions indicate faster kinetics of the cell reactions in the LFS/C-AA electrode, which agrees well with the results in Fig. 4. Raman spectroscopy is an essential tool for studying the structural properties of carbonaceous materials [60, 61]. To investigate the structure of the carbon yielded from pyrolysis of sucrose and AA (labeled as C-sucrose and C-AA, respectively), and the residual carbon in LFS/ C and LFS/C-AA particles, Raman spectra were shown in Fig. 6a, b, respectively. All the four samples exhibit two most prominent peaks (~1,300 and 1,590 cm−1), which correspond to the E2g and A1g vibration modes or the D (disordered) and G (graphene) bands, respectively. The origin of the D-band is associated with the breakage of symmetry that occurs at the edges of graphite sheets and point defects, so that the D/G peak intensity ratio can be used as rough measure of the graphene domain size [62]. The lower the D/G band intensity ratio, the higher the degree of graphitization of carbon is. The D/G band intensity ratio is listed in Table 2. Obviously, the D/G band intensity ratio for C-AA (0.77) is much lower than that for C-sucrose (1.53), that is to say, the degree of graphitization in C-AA is higher than that in C-sucrose. Higher degree of graphitization in C-AA means higher electronic conductivity for C-AA, which is verified by the electronic conductivity measurements (i.e., 3.23 × 10−4 S cm−1 for C-AA, but 1.90×10−4 S cm−1 for Csucrose). As we know, the structure of the residual carbon has an influence on the electrochemical behavior of LFS samples. The lower the D/G band intensity ratio, the higher the electronic conductivity of the residual carbon is. Furthermore, electrode utilization rises as D/G ratios and the amorphous carbon content decreases [52]. As shown in Table 2, the ID/IG value (i.e., 1.73 for LFS/CAA, but 1.97 for LFS/C) indicates the residual carbon with more graphite structure and an enhanced electronic conductivity for LFS/C-AA. This result is in good agreement with the measured electronic conductivity (i.e., 1.68 × 10−4 S cm−1 for LFS/C-AA, but 1.35 × 10−5 S cm−1 for LFS/C). Obviously, AA addition is beneficial to improve the degree of graphitization of residual carbon in LFS, thus an enhanced electronic conductivity and better electrochemical performance.

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Conclusions Li2FeSiO4/C composite was successfully synthesized via a refluxing-assisted solid-state reaction by using ascorbic acid as an additive and sucrose as a carbon source, and its electrochemical performance was also investigated. The XRD results show that ascorbic acid can to some extent inhibit the oxidation of Fe2+. TEM images show that the addition of ascorbic acid can reduce the particle size of LFS. Raman spectra further reveal that the pyrolytic carbon from ascorbic acid can improve the degree of graphitization of residual carbon in LFS/ C-AA, thus an enhanced electronic conductivity. EIS results demonstrate that LFS/C-AA shows higher exchange current density and diffusion coefficient of lithium ions than the LFS/ C sample prepared without ascorbic acid. As a result, LFS/CAA delivers higher discharge capacity of 163.3 mAh g−1 than LFS/C. This work demonstrates that using a small amount of ascorbic acid as an additive is an efficient way to improve the electrochemical performance of LFS and other cathode materials (i.e., LiFePO4, Li3V2(PO4)3, etc.) in lithium-ion batteries. Acknowledgments This work was supported by the National Science Foundation of China (51302153, 51302152, 51272128); the Key Project of Hubei Provincial Department of Education (D20131303); the Opening Project of CAS Key Laboratory of Materials for Energy Conversion (CKEM131404); the Scientific Fund of China Three Gorges University (KJ2012B043); and the Research Innovation Foundation of Master Dissertation of China Three Gorges University (2013CX028). Moreover, the authors are grateful to Dr. Jianlin Li at Three Gorges University for his kind support to our research.

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