稀有金属(英文版) 2015,34(10),731-737

收稿日期：10 March 2013

基金：financially supported by the National Natural Science Foundation of China(No.51203041);the Higher School in Hebei Province Science and Technology Research Project(No.ZH201206);the Scientific Research Foundation of Ministry Education for Returned Overseas Students;

Spindle LiFePO₄ particles as cathode of lithium-ion batteries synthesized by solvothermal method with glucose as auxiliary reductant

Li Ren Xing-En Li Fang-Fang Wang Yang Han

Polymer Research Institute,College of Chemical Engineering,Hebei University of Technology

Abstract：

The well-distribution spindle Li Fe PO4(LFP)nanoparticles as cathode of lithium secondary batteries were synthesized by a solvothermal reaction route at low temperature(180 °C) in which the ascorbic acid was used as reducing agent. In order to guarantee that the p H values of thermal systems were not affected too much and the reducibility of the system was enhanced at the same time,glucose was chosen as an auxiliary reductant in this reaction. The obtained powders were characterized by X-ray diffraction(XRD), scanning electron microscopy(SEM),and laser particle analyzer. The results show that the carbon-coated uniform spindle olivine Li Fe PO4/C-glucose particles(glucose as auxiliary reductant, LFP/C-G) are prepared with the size 500–600 nm and without any impurity phases. Their electrochemical properties were evaluated by electrochemical impedance spectroscopy,cyclic voltammetry, and galvanostatic charge/discharge tests. LFP/C-G has a higher conductivity and better reversible capability than carbon-coated LFP(LFP/C). The highest discharge capacity of LFP/C-G is 161.3 mAh·g-1at0.1C and 108.6 mAh·g-1at 5.0C, respectively. The results imply that the neat crystal nanostructure of LFP/C-G has excellent capacity retention and cycling stability.The adding of glucose is the key factor for the welldistribution and neat crystal structure of nanoparticles,thus the electrochemical performances of materials are improved.

Keyword：

Lithium-ion battery; LiFePO4; Spindle; Solvothermal; Glucose;

Author： Li Ren,e-mail:liren@hebut.edu.cn;

Received： 10 March 2013

1 Introduction

With the development of battery technology, the rechargeable lithium-ion batteries get more and more attention. The battery is mainly composed of positive, negative, and electrolyte. In these three component parts, the positive electrode material is a dominating factor for devoting to the performance of lithium secondary battery [1]. The traditional anode materials, such as Li Co O₂, Li Ni O₂and so on, are high cost, toxicity, and low safety at high temperature, etc. Therefore, people want to replace them by a kind of material with advantages of environment-friendly, high performance, and low price. Since lithium iron phosphate (Li Fe PO₄, LFP) was proposed by Goodenough coworkers [2] in 1997, it attracted much attention due to the low toxicity, low cost, good thermal stability, and high theoretical capacity (170 m Ah?g^-1), etc. However, Li Fe PO₄suffers two disadvantages, such as low lithium-ion diffusion and low electronic conductivity through the Li Fe PO₄/Fe PO₄interface [3], so the application of Li Fe PO₄in Li-ion battery technologies is hindered. To overcome the two disadvantages of Li Fe PO₄, several methods were attempted, including coating conductive materials (carbon, metal, polymer or metal oxide) on the particles [4–10], doping with supervalent cation into the olivine structure [11, 12], minimizing the particle size by different synthesis routes, etc. [13–15].

Among these resolved methods, minimizing particle size and forming uniform nanocrystal structure would be the most basic and important way to shorten the path of lithium-ion diffusion in materials. Faster diffusion of lithium ions across the Li Fe PO₄/Fe PO₄interface is the sole factor for improving the electrochemical kinetics of this material [13, 16]. To date, the hydrothermal synthesis within numerous synthesis methods [17–19] has mild synthesis environment, low energy consumption, and simplicity, etc. [20]. Many researchers reported hydrothermal methods to synthesize nanoparticles with good electrochemical behavior [21–23]. But a series of reaction conditions of hydrothermal method, for example, p H value, temperature, and mole ratio of materials have certain influence on the performance of the product. Our previous work showed that the p H value indeed had impact on the product performance, and Chen et al. [24] also showed that p H value of the system had an important influence on the morphology and electrochemical performance of Li Fe PO₄.

In this paper, a simple and effective solvothermal method was reported by using both ascorbic acid and glucose as reductive agents. Well-distribution spindle particles were prepared with the particle size around 500–600 nm without agglomeration. In the preparation process, ascorbic acid was used as main reducing agent to avoid the oxidation of ferric ions. In order to control the p H value of the system and guarantee enough reducibility at the same time, glucose was also used for its weak reduction. The glucose itself acted as not only a reducing agent but also a stabilizer to limit particle growth and prohibit agglomeration. The residual glucose on the surface of the final product can be used as carbon source to coat on particle surface. Meanwhile, it has little influence on p H value of the system.

2 Experimental

2.1 Synthesis of Li Fe PO4

The pure LFP powders were synthesized through solvothermal method using Li OH?H₂O, Fe SO₄?7H₂O, H₃PO₄, and ascorbic acid as raw materials. All reagents were analytical grade. In a typical synthesis process, 13.48 g ferrous sulfate (Fe SO₄?7H₂O), 5.54 g phosphoric acid (H₃PO₄), and 0.42 g ascorbic acid were mixed together and dissolved in 40 ml distilled water, 80 ml lithium hydroxide (Li OH?H₂O, 1.8 mol?L) aqueous solution was added to the mixture slowly under vigorous stirring. Then, 190 ml ethanol was poured into the mixed solution. The molar ratio of Li?, Fe^2?, and PO₄^3-was controlled as 3:1:1. After stirring, the resulted suspension was rapidly transferred into a teflon-linked autoclave and placed into an electric furnace, treated at 180 °C for 5 h. After reaction, the gray resultant was washed sequentially with distilled water and ethanol for several times, collected by centrifugal separation, and finally dried at 80 °C for 12 h under vacuum.

The pure Li Fe PO₄-glucose (LFP-G) powders were synthesized through the same ways showed above but only adding 2.53 g glucose as auxiliary reductant.

Then, the carbon-coated LFP (LFP/C) or carbon-coated LFP-G composite marterials (LFP/C-G) were obtained by sintering the LFP or LFP-G powder and glucose at 350 °C for 1 h, and further at 650 °C for 6 h under nitrogen atmosphere, respectively.

2.2 Characterizations

The obtained powders were subjected to the following characterizations. X-ray diffraction (XRD, Smart Lab(3), Smart) was employed to determine the crystal structure and phase purity of prepared samples, using Cu Ka radiation. XRD patterns were collected by a step-scanning mode in the range of 10°–80° with a step time of 5(°)?min^-1at room temperature. Powder morphologies were observed by a scanning electron microscopy (SEM, LEO1530VP, LEO). The size distribution of prepared samples was detected by laser particle analyzer (ZS90/MAL1053626, Malvern).

2.3 Electrochemical test

The electrochemical behavior of cathode material LFP/C or LFP/C-G was evaluated with coin-type cell (2430 type). Cathode slurry was prepared by mixing LFP/C or LFP/C-G powder, acetylene black, and polyvinylidene fluoride (PVDF) in the weight ratio of 85:5:10 with N-methyl pyrrolidone (NMP). The obtained slurry was stirred by the slurry machine (PT-1600E, KINEMATICA AG) for 0.5 h and coated onto an aluminum current collector with a thickness of 90 lm. The resulting electrode films were pressed with 20 MPa after dried at 120 °C for 24 h under vacuum, then cut into round plates (d = 14 mm). These film-type cathodes were assembled in argon-filled glove box using lithium metal as anodes. The galvanostatic charge–discharge experiment was conducted by using a battery testing system (CT-2001A, LAND) from 2.3 to 3.8 V (vs. Li?/Li) at room temperature. Cyclic voltammetry was performed at a scan rate of 0.1 m V?s^-1from 2.4 to 4.3 V (vs. Li?/Li) on an electrochemical workstation (CHI-650B, Shanghai Chenhua Instrument Limited Corporation). In electrochemical impedance spectroscopic (EIS) measurements, the excitation potential applied to the cells was 5 m V and the frequency ranged from 100 k Hz to 10 m Hz.

3 Results and discussion

3.1 XRD analysis

The synthesis of pure LFP is very important to its utilization as a cathode of lithium battery. The role of the glucose additive could be revealed from XRD patterns of LFP, LFP/C, LFP-G, and LFP/C-G (Fig. 1). The XRD patterns of LFP and LFP-G were prepared only by hydrothermalreaction. Compared with the diffraction peaks of the two materials, the adding of glucose is of great importance to form pure LFP. The two patterns of LFP/C and LFP/C-G with a series of sharp and symmetric diffraction peaks indicate that the two samples are highly crystallized and have a guiding function to form ordered orthorhombic olivine crystal-structured lithium iron phosphate (space group Pmnb, PDF#40-1499). In the patterns of LFP and LFP/C, the small diffraction peaks of Fe PO₄, Li₃PO₄, and Li₃Fe₂(PO₄)₃could be observed. But there are almost no such impurity peaks in XRD patterns of LFP-G and LFP/CG. The lattice parameters and unit cell volume were obtained by Rietveld refinement. The unit cell parameters for the orthorhombic cell of LFP/C-G is a = 1.0316 nm, b = 0.6000 nm, and c = 0.4691 nm. The unit cell volume is 0.29041 nm³, which is quite close to the result of a previous report [25]. The results in Fig. 1 indicate that the glucose additives effectively restrain Fe^2?oxidation and prevent the generation of other impurities, thus promote the formation of pure LFP. The role of Glucose is to make up for the deficiency of reducibility of the ascorbic acid in the system and to keep the p H of the system.

Fig.1 XRD patterns of LFP, LFP/C, LFP-G, and LFP/C-G. Labeled peaks indicating some impurity phases (Fe PO4, Li3PO4, and Li3Fe2(PO4)3)

3.2 Morphologies

Figure 2 shows the SEM images of LFP/C and LFP/C-G particles. The two materials display completely different surface morphologies. It could be seen from Fig. 2a that LFP/C is made of spherical-like particles with the particle size around 500–800 nm and the size distribution is quite broad, while the shape of LFP/C-G particles in Fig. 2b is more uniform like spindle. The average size of the spindle particle is around 500–600 nm with less agglomeration. The result shows that under solvothermal conditions, the glucose plays an important role in controlling not only the size but also the growth direction of the LFP crystal. Glucose is a polyhydroxy aldehyde, the adding of glucose to the starting solution can provide a number of hydrogen bonds. The cations in the solution may be trapped in the network of the hydrogen bonds. Such situation should promote nucleation and formation of well-organized structure, meanwhile the crystal growth of LFP is controlled. The small size, well-distribution, and ordered structure would contribute to better electrochemical properties of the cathode materials.

3.3 Particle analysis

Laser particle analyzer was used to determine the particle size and distribution. Figure 3 shows the particle size distribution curves of LFP/C and LFP/C-G. It can be seen clearly that there are two distribution peaks in the image of LFP/C in Fig. 3a, while the particle size distribution of LFP/C-G has a lognormal shape with a high degree of uniformity in Fig. 3b. The result of the particle size analysis supports the SEM observations. Generally, when using glucose as auxiliary reducing agent in solvothermalprocess, it is helpful to form smaller size and uniform distribution of LFP particles.

Fig.2 SEM images of a LFP/C and b LFP/C-G

Fig.3 Particle size distribution curves of a LFP/C and b LFP/C-G composites

Fig.4 Initial charge and discharge curves of LFP/C-G composites synthesized at different dosages of glucose at 0.2C rate

3.4 Electrochemical performance analysis

Figure 4 shows the initial charge and discharge curves of LFP/C-G samples synthesized at different dosages of glucose at 0.2C rate. As shown in Fig. 4, LFP/C-G samples synthesized at different molar ratios of Fe to glucose at 2:1, 3.75:1.00, 4:1, and 6:1, and the corresponding initial discharge capacities are 144.4, 150.4, 144.6, and 134.2 m Ah?g^-1, respectively. The sample with a molar ratio of Fe to glucose at 3.75:1.00 has the highest charge–discharge specific capacity of 150.4 m Ah?g^-1at 0.2C. Either too much or too little glucose would lead to the decline of electrochemical performance of the final products. When adding too much glucose, the cations would be surrounded by many hydroxyls, the crystal formation process would be hindered. Also, thick carbon layer would be left on particle surface which prevents the diffusion of lithium ions. While small amount of glucose is not enough to make up for the lackof reducing agent of system, which leads to incomplete and impure particle crystal structure, and hence results in poor electrochemical performance.

Fig.5 EIS result of LFP/C and LFP/C-G composite in frequency range between 100 k Hz and 10 m Hz

Figure 5 shows the electrochemical impedance spectroscopy results of LFP/C and LFP/C-G composites. All the spectra have a semicircle which referred to the chargetransfer resistance of electrochemical reaction in high-frequency region, and a sloping line refers to the thin-layer ionic diffusion-controlled Warburg impedance in low-frequency region. The intercept impedance on the real axis corresponds to the solution resistance, and the solution resistance is almost the same. This phenomenon may be due to the same electrolyte used in this study [26–29]. From the spectrum diagram in Fig. 5, it shows clearly that the transfer resistance of LFP/C-G (86.5 X) is much smaller than that of LFP/C (129.4 X). This implies that the conductivity of LFP/C-G is higher than that of LFP/C. The increased conductivity owes to its complete crystal type and uniform particle size distribution of LFP/C-G. The complete and uniform carbon coating could be beneficial to form orderly charge-transfer channels, hence decreases thecharge-transfer resistance. The uneven LFP/C particle size leads to higher resistance. Furthermore, as shown in Fig. 5, the curve of LFP/C-G has a steep slope of the inclined line in low frequency, indicating the low Warburg impedance, accounting for a good diffusion process of lithium ions in active substances solid.

Fig.6 Electrochemical properties of as-prepared cathode materials: a initial charge–discharge curves of LFP/C and LFP/C-G composites at a rate of 0.1C (17 m A?g-1), and b cycle performance of LFP/C and LFP/C-G composites at 0.1C (17 m A?g-1)

In order to study the properties of cathode materials, a series of electrochemical tests were made. The initial charge–discharge curves of LFP/C and LFP/C-G composites are shown in Fig. 6a. When the cell is cycled between 2.3 and 3.8 V at 0.1C rate, the LFP/C shows an initial discharge capacity of 123.4 m Ah?g^-1, while the LFP/C-G shows a much higher initial discharge capacity of 157.3 m Ah?g^-1. Both curves of LFP/C and LFP/C-G have a flat voltage plateau. The small electric potential difference between charge and discharge process indicates that both the materials had excellent cycling stability. This may be due to the relatively complete conducting layer and pure phase, which makes a better transfer and diffusion channel for lithium ions. Figure 6b shows the cycle performance of LFP/C and LFP/C-G composites at 0.1C current rate at room temperature. The discharge capacities of LFP/C and LFP/C-G are 157.3 and 123.4 m Ah?g^-1, respectively. The discharge capacities do not decline after 20 cycles. It indicates that both the materials have excellent cycling stability, which is consistent with the results of the initial charge– discharge curves of LFP/C and LFP/C-G composites shown in Fig. 6a. The highest discharge capacity of LFP/C is 130.2 m Ah?g^-1, while LFP/C-G is 161.3 m Ah?g^-1, corresponding to 94.9 % of the theoretical capacity. The shape of spindle particles, the uniform size distribution, and less agglomeration are helpful for improving the electrochemical performance of LFP/C-G. It should also be noted that the faster diffusion of lithium ions through the interface of Li Fe PO₄/Fe PO₄is the sole factor to improve the electrochemical kinetics of this material [13, 16]. The orderednanocrystal structure may shorten the migration and transportation path of lithium ions and electrons, the diffusion velocity of lithium ions increases and the electrochemical properties of cathode materials enhance.

Fig.7 Cyclic voltammetry tests of LFP/C-G composites at scan rate of 0.1 m V?s-1in potential window 2.4–4.2 V

Figure 7 shows the CV profiles of the LFP/C-G sample. In each cycle, it is just found one anodic peak and one cathodic peak, which corresponds to the two-phase charge/ discharge reaction of the Fe^2?/Fe^3?redox couple. The good overlap of the peaks, the small voltage interval between anodic and cathodic peaks, and the high peak currents indicate that LFP/C-G has excellent coulombic efficiency and reversible capability.

Figure 8 shows the discharge capacity and cycling performance of LFP/C-G at 0.2C, 1.0C, and 5.0C between 2.3 and 4.2 V at room temperature. The LFP/C-G cell shows a capacity of near 141.2 m Ah?g^-1at 0.2C, 140.2 m Ah?g^-1at 1.0C, and 108.6 m Ah?g^-1at 5.0C. The good ratio performance should be attributed to its neat spindle nanocrystal structure, which makes the intercalation and deintercalation of lithium ions easier.

Fig.8 Cycle capability of LFP/C-G composites at different discharge rates

4 Conclusion

The well-distributed spindle LFP/C-G nanoparticles as cathode of lithium secondary batteries were successfully synthesized through a solvothermal process with the adding of glucose. The XRD results indicate that LFP/C-G has a good crystal structure and without any impurity phase. SEM and particle analysis reveal that LFP/C-G has a highly ordered orthorhombic olivine structure and uniform dispersion. The small particle size and neat crystal structure of LFP/C-G would be helpful to form even and unhindered charge–discharge channel which shortens the diffusion path of lithium ions in cathode internal. The electrochemical performance test shows the highest discharge capacity of LFP/C-G is 161.3 m Ah?g^-1at 0.1C corresponding to 94.9 % of its theoretical capacity and exhibits excellent capacity retention and cycling stability at higher current rate.

The above results show that the glucose has a very important role in forming well-distributed spindle LFP nanoparticles. Glucose itself acts as not only a reducing agent but also as a stabilizer to limit the particle growth and prohibit agglomeration. The results also illustrate that small particle size, neat crystal structure, and uniform dispersion of LFP could effectively increase the transfer and diffusion of lithium ion, so as to improve the comprehensive electrochemical performance of cathode materials.