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Nov 29, 2011 - Choi D, Kumt PN (2007) J Power Sources 163:1064. 25. Xu ZH, Xu L, Lai QY, Ji XY (2007) Mater Res Bull 42:883. 26. Li XL, Wang WD, Shi CW ...
J Mater Sci (2012) 47:3076–3081 DOI 10.1007/s10853-011-6139-7

Synthesis of LiFePO4/C composite as a cathode material for lithium-ion battery by a novel two-step method Le Zhang • Hongfa Xiang • Xuefeng Zhu Weishen Yang • Haihui Wang



Received: 23 September 2011 / Accepted: 16 November 2011 / Published online: 29 November 2011 Ó Springer Science+Business Media, LLC 2011

Abstract In this study, LiFePO4/C is synthesized via a novel two-step method. The first step is the synthesis of nano-sized intermediate FePO4 by a modified sol–gel method. A fast and full combustion procedure is involved to remove carbon and control the size of the intermediate particles. The second step is to prepare LiFePO4/C by combining solid-state reaction with controllable carbon coating. This two-step method is facile to prepare nanosized LiFePO4 and easy to optimize the carbon content for surface coating. X-ray diffraction shows that the LiFePO4/ C composite possesses good crystallinity. Spherical morphology with a diameter of 30–150 nm is observed by scanning electron microscope and transmission electron microscope. Electrochemical measurements indicate that the LiFePO4/C composite exhibits discharge capacities of 162, 144, 126, and 106 mAh g-1 at 0.1, 1, 2, and 5C, respectively. No capacity fading is observed in 50 cycles.

Introduction In recent years, lithium-ion batteries (LIBs) have played an important role in power tools, electronic and telecommunication devices [1, 2]. LiCoO2 is the widely applied cathode material in the commercial LIBs owing to its good electrochemical properties and the convenience of preparation, but L. Zhang  H. Xiang  H. Wang (&) School of Chemistry & Chemical Engineering, South China University of Technology, Wushan Road, Guangzhou 510640, China e-mail: [email protected] X. Zhu  W. Yang State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China

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it suffers from high cost, toxicity, and unsafety [3–7]. LiFePO4 with the olivine structure has attracted a large interest after it was reported by Goodenough and his co-workers [8]. It is believed that LiFePO4 will be the best candidate of the LIB’s cathode material because of its high energy density, low cost, environmental compatibility, and intrinsic thermal safety [9–12]. However, the large-scale application of LiFePO4 in high-power batteries is limited by its poor inherent ionic and electronic conductivities [13, 14]. A few methods could be attempted to solve this problem. One method is to produce nano-sized LiFePO4 particles, which is helpful for Li ion transport by shortening diffusion path and increasing the electrode/electrolyte contact area [15, 16]. Another method is to enhance the conductivity of LiFePO4 by conductive carbon coating [17–19]. During the carbon coating, the carbon content could make a great influence on the performance of LiFePO4 [20]. The low carbon content is not enough to form a continuous conductive layer to improve the electronic conductivity, while the high carbon content will reduce the tap density of this material. Therefore, it is important to control the content of carbon. In previous studies, sol–gel method has commonly been employed for preparing nano-sized LiFePO4 [21–29]. In the sol–gel method, the precursors can be mixed at a molecular level [30], but it is difficult to control the carbon content for the coating. In a typical solid-state reaction, it is convenient to control the carbon content [31–34], but the size of the product is determined by the raw materials, which usually is large [35]. To prepare nano-sized LiFePO4/C composite with a controllable carbon content, the combination of the sol–gel method and the solid-state method is proposed. In this article, the nano-sized LiFePO4/C composite was synthesized using a two-step method: (1) FePO4 was synthesized by a sol–gel method with a combustion process in air; therefore, the nano-sized

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intermediate FePO4 particles without carbon was prepared; (2) LiFePO4/C was synthesized via the solid-state method using the as-prepared nano-sized FePO4 intermediate, Li2CO3 and glucose as raw materials. During the process, the carbon content is determined by glucose, and the ferric can be reduced to the ferrous by carbon at high temperature. Therefore, the combination of the sol–gel method and the solid-state reaction could not only obtain nano-sized LiFePO4, but also control the carbon content for the carbon coating.

carbonate (1:1 in volume). CR2032-type coin cells were assembled in an argon-filled glove box (Mikrouna, Super1220). Galvanostatic charge–discharge tests were conducted between 2.0 and 4.3 V versus Li/Li? on a battery test system (Neware Electronic Co., China). Cyclic voltammograms (CV) were performed using an electrochemical work station (Zahner IM6ex). In this article, 1C means 170 mA g-1. Constant current charging was used, and the discharge rate was the same as the charge rate during every cycle.

Experimental

Results and discussion

The FePO4 was synthesized by a sol–gel combustion method. Stoichiometric amounts of Fe(NO3)3 and NH4H2PO4 were dissolved in citric acid solution under continuous stirring. Subsequently, ethylene glycol and NH4NO3 were added to the mixed solution. Molar ratios of targeted FePO4 to citric acid to ethylene glycol to NH4NO3 were 1:1.5:1.5:1.5. This solution was stirred and evaporated at 120 °C to obtain a light yellow gel. Here, citric acid was employed as a chelating reagent. Ethylene glycol and NH4NO3 were used as a combustion improver, and as an oxidant, respectively. Both of them could help a complete combustion. Finally, the gel was ignited and burnt out in air to get the intermediate FePO4. LiFePO4/C was synthesized by the solid-state method. The as-prepared nanosize intermediate FePO4, Li2CO3, and glucose all were homogeneously mixed in acetone. After ball-milling, the resulting slurry was calcined at 700 °C for 8 h in a N2/H2 (90/10 in volume) atmosphere. The structure analysis of the samples was performed by X-ray diffraction (XRD, Bruker, D8 ADVANCE, Cu, Ka radiation). The diffraction patterns were recorded in the 2h range from 10 to 60°. The carbon content was determined by an element analyzer (Vario EL III). The morphology of the powders was observed by a scanning electron microscope (SEM, Quanta 200F) and a transmission electron microscope (TEM, FEI, Tecnai G2 F30 S-Twin). The specific surface area was obtained by Micromeritics, ASAP2020. The electrochemical measurements were carried out by fabricating CR2032 coin cells. A metallic lithium foil was served as the anode. The cathode was prepared by mixing LiFePO4/C with Super P carbon and poly(vinylidene fluoride) (PVDF) at a weight ratio of 80:12:8 in N-methyl2-pyrrolidinone (NMP) to form a slurry. Then, the resultant slurry was uniformly pasted on Al foil with a blade, dried at 80 °C in vacuum. The mass loading was about 1.5 mg cm-2. The Celgard 2400 microporous membrane was used as a separator, and the electrolyte used was a solution of 1 M LiPF6/ethylene carbonate ? dimethyl

Figure 1 shows the XRD patterns of the intermediate FePO4 (a) and the as-prepared LiFePO4/C composite (b). It is found that the intermediate mainly is FePO4, while small amounts of impurities are detected. It has been reported that some ferric impurities such as Fe2O3 in the intermediate do not affect the purity of the final product LiFePO4 [4]. As shown in Fig. 1b, the XRD pattern of LiFePO4/C is in full accordance with the LiFePO4 olivine structure indexed by orthorhombic Pnmb without any impurities detected. Moreover, no diffraction peaks of carbon were found in the final product indicating that carbon generated from glucose is amorphous in the LiFePO4/C composite. Figure 2 shows the SEM images of the intermediate FePO4 (a, b) and the LiFePO4/C composite (c, d). In Fig. 2a, b, the FePO4 exhibits a uniform nano-grade microstructure, which could be helpful to prepare nanosized LiFePO4 fine powders. The size of FePO4 is 100–200 nm. As shown in Fig. 2c, d, the LiFePO4 inherits the morphology feature of the intermediate FePO4. The obtained LiFePO4 powder is composed of agglomerated particles with size distributions in the range 200–400 nm.

Fig. 1 XRD patterns of the intermediate (a) and the LiFePO4/C (b)

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Fig. 2 SEM images of the intermediate (a, b) and synthesized LiFePO4/C composite (c, d)

The size of the primary particle is 30–80 nm. The size of LiFePO4 is a little larger than the FePO4 due to the growth of the particle in the process of solid-state reaction. Figure 3 shows the TEM images of the intermediate FePO4 (a, b) and the LiFePO4/C composite (c–f). As shown in Fig. 3a, b, the FePO4 is composed of many nano-sized particles with the diameter of 20–150 nm. The morphology of intermediate plays an important role for the formation of sphere LiFePO4 particles. In Fig. 3c, the LiFePO4 particles are coated and connected by carbon derived from the glucose. From Fig. 3d, it is clear that the surface of the LiFePO4 particle is completely coated by carbon. The diameter of the spherical primary particle is in the range of 30–150 nm. The morphology of carbon is well observed in Fig. 3e, f. The coating is well distributed, and the thickness of the coating is in the range of 2.5–5.5 nm, indicating that carbon could inhibit the particle growth, and help in obtaining smaller particle. Hence, the electronic conductivity of LiFePO4 can be increased, and the electrochemical performance can be improved because of fine carbon coating. The N2 adsorption–desorption isotherms of the final product are shown in Fig. 4 and the BrunauerEmmett-Teller (BET) specific surface area is 36.4 m2 g-1. Figure 5 shows the cycling performance of LiFePO4/C composite at 5C (=850 mA g-1) with various glucose contents. The glucose contents are 0, 15, 25, and 35 wt%, respectively. It was found that the 50th discharge capacities are 20, 85, 105, and 108 mAh g-1, respectively. The cycling performance becomes better with increasing carbon contents from 0 to 25 wt%. It was also found that the

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cycling performances of the samples with 25 and 35 wt% glucose are almost the same. However, the tap density of the sample with 35 wt% glucose is smaller than that of 25 wt% because of the higher carbon content. Hence, the carbon content of the product with 25 wt% glucose is optimal. According to the element analysis results, the carbon content in the product with 25 wt% glucose is 6.8%. Figure 6 shows CV profiles of the LiFePO4/C composite with 25 wt% glucose in the first three cycles at a scanning rate of 0.2 mV s-1 from 2.0 to 4.3 V. The profiles of all the three cycles are almost reduplicate. There is only one peak pair for each cycle. The sharp oxidation and reduction peaks are attributed to the two-phase reaction of the Fe2?/ Fe3? redox couple, corresponding to lithium insertion and extraction in LiFePO4 crystal structure [4, 36]. The polarization in the first cycle is obvious and the difference between the oxidation and reduction peaks is 0.50 V in the first cycle. After the first cycle reaction, the difference between the oxidation and reduction peaks reduces to 0.30 V, implying that lithium diffusion and electrochemical kinetics reach an improved condition [37]. The initial charge/discharge curves of the LiFePO4/C composite with 25 wt% glucose are shown in Fig. 7a. The long and flat voltage plateaus at 3.5 and 3.4 V (vs. Li?/Li) indicate the two-phase transition between FePO4 and LiFePO4. The narrow gap (0.1 V) between the charge and discharge voltage plateaus demonstrates indirectly a low polarization [38, 39]. In the first cycle, the LiFePO4/C exhibits a charge capacity of 165 mAh g-1 and a discharge capacity of 161 mAh g-1 at 0.1C (=17 mA g-1) with a

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Fig. 3 TEM images of the intermediate (a, b) and synthesized LiFePO4/C composite (c, d, e, f)

high coulombic efficiency of 97.6%. Figure 7b shows the typical electrochemical performance of the LiFePO4/C at different rates. At 0.1C, it delivers a capacity of 162 mAh g-1 which is close to the theoretical capacity of 170 mAh g-1. The specific capacity gradually decreases with increasing current rate. The capacities are 154, 145, and 124 mAh g-1 at 0.2, 1, and 2C, respectively. Even at a high rate of 5C, a capacity of 107 mAh g-1 is still obtained. The voltage plateaus are 3.4 V indicating that the polarization is relatively small. The rate performance of the

LiFePO4/C is excellent. The possible reasons for this excellent rate performance would be the uniform intermediate, the nano-sized particle, and the well-distributed carbon coating. The cycling performances of the LiFePO4/C cathode materials with 25 wt% glucose at different rates ranging from 0.1 to 5C are shown in Fig. 8. The initial discharge capacities are 160, 145, 127, and 109 mAh g-1 at 0.1, 1, 2, and 5C, respectively. After 50 cycles, the capacities become 162, 144, 126, and 106 mAh g-1, respectively.

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Fig. 4 Nitrogen adsorption–desorption isotherms of the final product

Fig. 5 Cycling performance of LiFePO4/C composite at 5C (=850 mA g-1) with various glucose contents. The glucose contents are 0, 15, 25, and 35 wt%, respectively

Fig. 7 Initial charge/discharge curves of LiFePO4/C composite with 25 wt% glucose at 0.1C (=17 mA g-1) between 4.3 and 2.0 V (a) and typical discharge curves at different rates ranging from 0.1 to 5C (b). The discharge rates are the same as the charge rates in cycles

The capacity retention is almost 100%. It can also be found that the capacity is very stable except for the initial two cycles. The reason for the unstable initial two cycles could be that the contact time between the cathode material and the electrolyte is too short, resulting in an insufficient ‘‘activation’’ [35]. The good cycling behavior of the LiFePO4/C confirms the excellent reversibility of lithium insertion and extraction reaction.

Conclusions

Fig. 6 CV profiles of the LiFePO4/C in the first three cycles at 0.2 mV s-1 between 2.0 and 4.3 V

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In this article, the olivine-type LiFePO4/C cathode material is successfully synthesized by a novel two-step method. The XRD patterns show that the LiFePO4 has good crystallinity and high phase purity. The diameter of the primary particle is in the range of 30–150 nm. The particle is well coated by amorphous carbon, and the thickness of the

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Fig. 8 Plot of the discharge capacities versus the cycle number at 0.1, 1, 2, and 5C between 2.0 and 4.3 V (the same charge and discharge rates are used)

coating is 2.5–5.5 nm. The as-prepared LiFePO4/C exhibits a good electrochemical performance. At the rates of 0.1, 0.2, 1, 2, and 5C, the as-prepared LiFePO4/C has specific capacities of 162, 154, 145, 124, and 107 mAh g-1, respectively. It also shows an excellent capacity retention, and no capacity loss is observed during 50 cycles. Acknowledgements This study was supported by the National Science Foundation of China (grant No. 21006033), Program for New Century Excellent Talents in Chinese Ministry of Education (No. NECT-07-0307), and the Fundamental Research Funds for the Central Universities, SCUT (2009220038).

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