Chinese Journal of Chemical Physics  2016, Vol. 29 Issue (6): 699-702

#### The article information

Bin Cheng, Lian-sheng Jiao, Zhong-feng Tang, Sheng-jie Zhang, Hong-li Chen, Chun-hua Chen

Nano-Li3V2(PO4)3/C Synthesized by Thermal Polymerization Method as Cathode Material for Lithium Ion Batteries

Chinese Journal of Chemical Physics, 2016, 29(6): 699-702

http://dx.doi.org/10.1063/1674-0068/29/cjcp1604091

### Article history

Accepted on: August 6, 2016
Nano-Li3V2(PO4)3/C Synthesized by Thermal Polymerization Method as Cathode Material for Lithium Ion Batteries
Bin Chenga, Lian-sheng Jiaob, Zhong-feng Tanga, Sheng-jie Zhangb, Hong-li Chenb, Chun-hua Chena
Dated: Received on April 28, 2016; Accepted on August 6, 2016
a. CAS Key Laboratory of Materials for Energy Conversions, Department of Materials Science and Engineering & Collaborative Innovation Center of Suzhou Nano Science and Technology, University of Science and Technology of China, Hefei 230026, China;
b. Department of Chemistry, Hebei Normal University for Nationalities, Chengde 067000, China
*Author to whom correspondence should be addressed. Chun-hua Chen, E-mail:cchchen@ustc.edu.cn
Abstract: A nano-Li3V2(PO4)3/C powder was successfully prepared by a thermal polymerization method. The particle sizes of the intermediate product powder and the final product Li3V2(PO4)3 are all less than 200 nm. The carbon is partially coated on the surface of Li3V2(PO4)3 particles and the rest exists between particles with a total carbon content of 4.6wt%. This nano-Li3V2(PO4)3/C sample shows a discharge capacity of 124 mAh/g without capacity fading after 100 cycles at 0.1 C in the voltage rang of 3.0-4.3 V. Excellent rate performance is also achieved with a capacity of 80 mAh/g at 20 C in 3.0-4.3 V and 100 mAh/g at 10 C in 3.0-4.8 V. This study suggests that the thermal polymerization method is suitable to synthesize nano-Li3V2(PO4)3/C materials.
Key words: Lithium vanadium phosphate    Thermal polymerization    Nanoparticles    Acrylic acid
Ⅰ. INTRODUCTION

Recently lithium ion batteries (LIBs) have attracted much attention due to their high energy density and high power density [1]. However, with the rapid development of the hybrid electric vehicles and electric vehicles, the electrochemical performances, especially the rate capability, of current LIBs cannot meet the constantly increasing demands [2]. The cathode material is one of the crucial factors that determine the performance of a LIB. Compared with other V-based cathode materials, just like LiVOPO4 [3], the monoclinic Li3V2(PO4)3 (LVP) is very promising because of its good ionic mobility, high operating voltage, high theoretical capacity, excellent cycling capability, and thermal stability [4]. But its low intrinsic electronic conductivity (10-8 S/cm) strongly limits its applications. The use of nano-sized powders is an effective way to improve the rate performance [5]. Special morphologies, such as nanoparticles [6, 7], nanowires [8], nanoplates [9], and porous structure [10], were applied to achieve high rate capability because at least one direction of the charge transport was shorted. The introduction of carbon in the forms of surface coating [7], matrix [6], graphene sheets [11], and their mixtures [12] is also widely used to enhance the electric conductivity and improve the rate performance.

On the other hand, it is well established that the syntheses of metal oxide nanostructures using polymer precursors are advantageous for controlled morphology, pore size, etc. [13]. Herein we use a thermal polymerization method (TP) [14] to synthesize a nanoscale precursor of LVP, and then introduce carbon to obtain the final powder. Benefited from the formation of polymers and their removal, the oxides are separated effectively and finally sintered to be uniform nanoparticles. Actually, this method is usually used to synthesize metal oxide materials, such as LiCoO2 [15], LiNi1/3Co1/3Mn1/3O2 [16], and lithium-rich cathode material [14]. After some modifications, TP is applied to synthesize a phosphate material.

Ⅱ. EXPERIMENTS

The nano-Li3V2(PO4)3/C (LVPC$_{\textrm{TP}}$) was synthesized by a thermal polymerization method. NH4VO3 (2 mmol) and oxalic acid (4 mmol) were dissolved in 40 mL distilled water, then NH4H2PO4 (3 mmol), LiAc·2H2O (3.15 mmol) and nitric acid (2 g) were added in the solution. Then acrylic acid (AA) was added to form AA-H2O (1:1, V/V) solution. This solution was heated at 180 ℃ for 10 h to form a fluffy xerogel. After grinding, the polymer product was treated at 450 ℃ in air to remove the polyacrylic acid to obtain a LVP precursor, which was then mixed with glucose dispersed in ethanol (precursor:glucose=5:1, W/W). Such a suspension was grinded under an infrared lamp until it was dried completely. The obtained powder was caicined in argon at 350 ℃ for 5 h, then at 750 ℃ for 8 h to yield a LVPC$_{\textrm{TP}}$ powder. For comparison, another Li3V2(PO4)3/C (LVPC) powder was similarly prepared without using acrylic acid and nitric acid. Also, glucose was added into the initial solution which was continuously stirred and heated at 50 ℃ until it was dried completely. The obtained powder was calcined in argon at 350 ℃ for 5 h and then at 750 ℃ for 8 h.

The LVPC$_{{\textrm{TP}}}$ and LVPC powders were analyzed by X-ray diffraction (XRD) using a diffractometer (Philips X'Pert Pro Super, Cu Kα radiation). Thermogravimetric analysis (TGA) of the composites was conducted in air at a heating rate of 10 ℃/min from 30 ℃ to 750 ℃ using a thermal analyzer (TGA50H). The powders were observed under a scanning electron microscope (SEM, JSM-6700F) and a high-resolution transmission electron microscope (HRTEM, JEOL-2010).

The electrochemical characterization was evaluated using CR2032 coin cells. An electrode laminate was prepared by mixing an active material, acetylene black, and poly (vinylidenedifluoride) with a weight ratio of 80:10:10 in NMP to form a homogeneous slurry, which was coated on an aluminum foil. The laminate was dried at 120 ℃ for several hours. Discs about 14 mm in diameter of the laminates were punched, dried at 70 ℃ for 2 h in a vacuum oven. The weight of active material is about 4 mg and all capacity values are calculated by the whole mass of active material including carbon residue in the composites. Afterwards, CR2032 coin-type half-cells with Li as the counter electrode were assembled in an argon-filled glove box. The electrolyte was 1 mol/L LiPF6 in ethylene carbonate/dimethyl carbonate (1:1, V/V), while a polypropylene micro-porous film (Celgard 2300) was used as the separator. The cells were tested on a NEWWARE BTS-610 multichannel battery test system at different C-rates in voltage ranges of 3.0-4.3 and 3.0-4.8 V. Electrochemical impedance spectroscopy (EIS) measurements were performed on a CHI660d electrochemical work station over a frequency range from 0.01 Hz to 100 kHz with the AC amplitude of 5.0 mV. The half cells with different cathode materials were discharged to 4.0 V after one cycle of activation and once more charge step.

Ⅲ. RESULTS AND DISCUSSION

During the thermal polymerization process, oxalic acid is used to help dissolution of NH4VO3 while nitric acid is added to generate gases during the heating and result in a fluffy xerogel. Acrylic acid is polymerized at 180 ℃.

In the XRD patterns of LVPC$_{{\textrm{TP}}}$ and LVPC powders (Fig. 1(a)), all the diffraction peaks can be indexed well to the monoclinic Li3V2(PO4)3 with a space group P21/n, that is very close to the pattern of Li3Fe2(PO4)3 (JCPDS: 80-1515). The cell parameters are a=8.61 Å, b=12.05 Å, c=8.62 Å, $\beta$=89.94°, which are consistent with a previous report [17]. Both samples have good crystallinity without impurity peaks being detected. According to the TGA curves (Fig. 1(b)), the carbon contents are 4.6wt% for LVPC$_{{\textrm{TP}}}$ and 5.4wt% for LVPC, respectively.

 FIG. 1 (a) XRD patterns and (b) TGA curves of LVPC and LVPCTP composites.

The particle morphology of LVPC is irregular with particles larger than 1 µm (Fig. 2(a)). After removing the polyacrylic acid, the LVP precursor displays a uniform particle size distribution with a primary particle size of about 200 nm (Fig. 2(b)). After the carbon coating, LVPC$_{{\textrm{TP}}}$ keeps the particle morphology of LVP precursor (Fig. 2(c)). Obviously, the presence of carbon blocks the further growth of the particles. From its TEM image (Fig. 2(d)), some carbon is coated on the surface of the LVP particles and the rest exists between particles.

 FIG. 2 SEM images of (a) LVPC, (b) LVP precursor, and (c) LVPCTP. (d) TEM image of LVPCTP.

The electrochemical performances of LVPC$_{{\textrm{TP}}}$ and LVPC in the voltage range of 3.0-4.3 and 3.0-4.8 V vs. Li+/Li are shown in Fig. 3. In 3.0-4.3 V, the first charge-discharge curve (Fig. 3(a)) shows that the lithium extraction-insertion process includes three two-phase electrochemical plateaus. The initial discharge capacities are about 124 mAh/g for LVPC$_{{\textrm{TP}}}$ and 122 mAh/g for LVPC, respectively. In Fig. 3(b), LVPC$_{{\textrm{TP}}}$ exhibits excellent cyclability without capacity fading after 100 cycles at 0.1 C, while the LVPC only maintains 91% of the first discharge capacity. As to rate performance (Fig. 3(c)), the LVPC$_{{\textrm{TP}}}$ electrode delivers discharge capacities of 122, 121, 120, 116, 110, 80 mAh/g at 0.1, 0.5, 1, 5, 10, 20 C, respectively. This rate capability of LVPC$_{{\textrm{TP}}}$ is much better than that of LVPC, whose specific capacities are 124, 118, 115, 96, 57, 2 mAh/g at the same current rates. This excellent rate performance of LVPC$_{{\textrm{TP}}}$ can be ascribed to the short lithium ion diffusion path in the nanoparticles and better electron conductivity resulted from the carbon coating layer and carbon matrix.

 FIG. 3 Electrochemical performance of LVPC and LVPCTP in the voltage range of (a-c) 3.0-4.3 V and (d-f) 3.0-4.8 V. (a, d) the voltage profiles of the 1st cycle, (b, e) cycling performance at 0.1 C, (c, f) rate performance.

Figure 3(d) shows the voltage profiles of the first cycle at the range of 3.0-4.8 V. During the charge process, we can see a profile with three plateaus before 4.3 V. Then at a plateau at about 4.53 V, the third lithium is extracted from LVP lattice. However, a solid solution behavior with a long oblique line, unlike the discharge curve between 3.0-4.3 V, is observed for the insertion of the initial two lithium ions. The insertion of the last lithium exhibits a two-phase behavior at about 3.5 V. The initial specific discharge capacities of LVPC$_{{\textrm{TP}}}$ and LVPC are about 166 and 165 mAh/g at 0.1 C, with capacity retention of 82.8% and 77.5% after 100 cycles, respectively. For the rate performance in the voltage of 3.0-4.8 V (Fig. 3(f)), LVPC$_{{\textrm{TP}}}$ delivers specific capacities of 167, 155, 147, 126, 100 mAh/g at 0.1, 0.5, 1, 5, 10 C, respectively, which is much better than LVPC.

The excellent rate performance of LVPC$_{\textrm{TP}}$ can be verified by EIS analysis (Fig. 4). The spectrum of LVPC$_{\textrm{TP}}$ shows a semi-circle with smaller radius compared to that of the LVPC, indicating that LVPC$_{\textrm{TP}}$ has lower charge-transfer resistance. This property can be attributed to its nanoscale particle size and mixed carbon modification with both carbon coating layer and carbon matrix. The nano-size particles and carbon can facilitate the lithium ions diffusion and electron conduction.

 FIG. 4 Electrochemical impedance spectra of LVPCTP/Li and LVPC/Li cells.
Ⅳ. CONCLUSION

A nano-Li3V2(PO4)3/C is successfully prepared by a thermal polymerization method. The LVP nanoparticles can decrease the lithium ion diffusion length and the carbon can effectively improve the electronic conduction. As a cathode material for lithium-ion batteries, this cathode powder shows great cycle and rate performances.

Ⅴ. ACKNOWLEDGMENTS

This work was supported by the National Natural Science Foundation of China (No.51577175), Hefei Center of Materials Science and Technology (No.2014FXZY006) and Education Ministry of Anhui Province (No.KJ2014ZD36). We are also grateful to Elementec Ltd. in Suzhou.

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a. 中国科学技术大学材料科学与工程系, 中国科学院能量转换材料重点实验室, 合肥 230026;
b. 河北民族师范学院化学系, 承德 067000