8 Characterization and Application of Nanomaterials (2021) Volume 4 Issue 2 doi:10.24294/can.v4i2.1331 ORIGINAL RESEARCH ARTICLE Electrospun Li3V2(PO4)3/carbon nanofiber as cathode materials for the high-performance lithium-ion batteries Mengxi Zhao1, Zhongpei Lu1, Lin Chen2, Xuefan Jiang3, Fan Yin1,2, Gang Yang1,2* 1 School of Chemistry and Material Engineering, Changshu Institute of Technology, Changshu 215500, Jiangsu, China. E-mail: gyang@cslg.cn 2 School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, Jiangsu, China 3 School of Physics and Electronic Engineering, Changshu Institute of Technology, Changshu 215500, Jiangsu, China ABSTRACT In this paper, a series of Li3V2(PO4)3/C composite nanofibers is prepared by a facile and environmentally friendly electrospinning method and calcined under different temperatures. The LVP nanofiber calcined under 900 oC exhibits the best electrochemical performance. The bicontinuous morphologies of LVP/CNF are the fibers shrunk and the LVP crystals simultaneously grown. At the range of 3.0–4.3 V, LVP/CNF obtained under 900 oC delivers the initial capacity of 135 mAh/g, close to the theoretical capacity of LVP. Even at high current density, the sample of LVP/CNF still pres- ents good electrochemical performance. Keywords: Lithium-ion Batteries; Cathode Material; Graphene; Nanocomposite; Electrochemical Performance ARTICLE INFO Received: 1 April 2021 Accepted: 20 May 2021 Available online: 28 May 2021 COPYRIGHT Copyright © 2021 Mengxi Zhao, et al. EnPress Publisher LLC. This work is licensed under the Creative Commons Attribution- NonCommercial 4.0 International License (CC BY-NC 4.0). https://creativecommons.org/licenses/by- nc/4.0/ 1. Introduction Lithium battery has the advantages of high specific energy, high battery voltage, wide working temperature range and long storage life. It has been widely used in small electrical appliances, such as porta- ble computers, cameras, electric tools, etc. In recent years, with the increasing shortage of non-renewable resources, such as oil and coal and the urgent needs of environmental protection, energy conservation and emission reduction, countries all over the world such as the United States, Japan Germany and France actively carry out the research on electric vehicle (EV) and hybrid electric vehicle (HEV), and put the development and application of lithium power battery on the important agenda. Because lithium battery has no pollution to the environment, it is more suitable to be used as the power supply of electric vehicle and large power reserve, so it has a very broad application prospect[1]. In recent years, great progress has been made in the research of positive and negative active materials for lithium batteries. At pres- ent, the most widely studied positive materials include layered lithium cobalt oxide LiCoO2, spinel oxide LiMn2O4, phosphate LiMPO4 (M = Mn, Fe, Co, etc.) and Li3V2(PO4)3, but these materials have their own shortcomings, It is difficult to meet the increasing requirements for lith- ium batteries[2–4]. Therefore, the existing electrode materials and search methods are improved. Finding new high-energy electrode materials 9 is the top priority of research work, especially new cathode materials, which have become the key factor restricting the further improvement of the overall performance of Lithium batteries. Monoclinic Li3V2(PO4)3 (LVP), as the cathode material of lithium-ion battery, has gradually attract- ed extensive attention because of its high theoretical capacity and energy density. The stable frame struc- ture of LVP provides a three-dimensional channel for the insertion/removal of Li+, and its theoretical ca- pacity is as high as 197 mAh/g in the voltage range of 3.0–4.8 V[5–7]. However, the electronic conductiv- ity of LVP is very poor. It affects the electrochemical performance of LVP and limits its commercial appli- cation. Researchers have proposed many methods to overcome the problem of electronic conductivity of LVP. In addition to effective carbon coating and met- al ion doping, reducing the particle size or embed- ding LVP particles into carbon Nanonetworks can also effectively improve the electronic conductivity of LVP[8,9]. As we all know, the migration rate of ions and electrons in LVP plays a key role in improving its electrochemical properties. Therefore, it is a very effective method to improve both ionic conductivity and electronic conductivity, because nanostructured LVP is conducive to the movement and intercalation/ de-intercalation of Li+ in active electrode materials. It can improve the ionic conductivity of LVP. Con- ductive carbon materials can effectively improve the electronic conductivity of LVP. Nanostructured LVP/C materials can meet the requirements of high electronic and ionic conductivity at the same time. In recent years, LVP nanocomposites with different morphologies, such as spherical particles, nanorods, nanobelts, nanoplates and films, have attracted more and more attention. It is proved that it is beneficial to improve the electrochemical properties of LVP. The preparation of nanofiber materials by elec- trospinning technology is a hot spot in the field of material science and technology in the world in recent ten years. Electrospinning has become the main way to effectively prepare nanofiber materials because of its simple manufacturing device, low spinning cost, wide variety of spinnable materials and controllable process. Electrospinning technol- ogy is a simple and low-cost method for preparing nanofibers, which has been widely used to prepare materials with one-dimensional nanostructures. With long length and porous structure, nanofibers obtained by electrospinning generally have a large specific surface volume ratio. Many nanofibers are randomly dispersed to form a multi space dense 3D structure. In recent years, the research on the preparation of olivine structure LiMPO4 (M = Mn, Fe, Co, etc.) nanofibers by electrospinning is increasing, and the obtained LiMPO4 fibers show good electrochemical properties[11–13]. However, there are few reports on the preparation of NASICON structure Li3V2(PO4)3. Considering that nanofibers are conducive to im- provement. In this paper, Li3V2(PO4)3 with nanofiber structure was prepared by electrospinning, which improved the conductivity and electrochemical prop- erties of Li3V2(PO4)3 as electrode material. 2. Experimental part Firstly, electrospinning precursor was syn- thesized, an appropriate amount of citric acid was dissolved in deionized water, NH4VO3 and stoichio- metric NH4H2PO3 and CH3COOLi·2H2O were add- ed, and the reaction was stirred in oil bath for 4 h to obtain uniform mixed sol A. Polyvinylpyrrolidone (PVP) was added to an appropriate amount of H2O and stirred for 4 h to obtain transparent viscous liq- uid B. Drop sol solution A into solution B and stir for 4 h to obtain uniform solution C. The precursor film is prepared by electrospinning method. The obtained white film is raised to 350 oC at a heating rate of 5 oC/min in nitrogen atmosphere and kept at a constant temperature for 4 h to decompose the mixture and release gas. Then it is cooled to room temperature with the furnace. After full grinding, it is burned in a high-temperature tubular furnace. In this process, PVP is carbonized. At the same time, lithium va- nadium phosphate (LVP) is generated under high temperature. In order to study the optimal calcina- tion temperature, the effects of heat treatment at four temperatures of 600 oC, 700 oC, 800 oC and 900 oC on the morphology and electrochemical properties of LVP/carbon nanofibers were tried. The samples were analyzed by X-ray diffraction 10 (XRD) using D/max-2200/PC X-ray diffractometer of RIGAKU (Science) Company in Japan. The test conditions were Cu target, tube pressure 40 kV. The tube current is 100 Ma, the continuous scanning speed is 4°/min, and the step width is 0.02°. The sample surface morphology is analyzed by JSM- 6700F scanning electron microscope produced by JEOL (Japan Electronics) Co., Ltd., the tube voltage is 10 kV and 15 kV, and the tube current is 10 μA. The electrochemical performance test process of the sample is as follows: weigh the synthesized cath- ode material, conductive carbon black and polytetra- fluoroethylene (PTFE) binder according to the mass percentage of 80:15:5, fully mix, and roll the materi- al into 40 by manual membrane rolling machine μm thin film, cut into a square positive electrode sheet with a side length of 6 mm. The negative electrode adopts high-purity metal lithium sheet with a purity of 99.9%, a thickness of 0.4 mm and a diameter of 10 mm. In order to reduce the impact of humidity on the battery performance, the positive electrode sheet needs to be vacuum dried at 120 oC for 12 h before simulating the battery assembly. The battery assembly is carried out in a glove box filled with high-purity argon. The diaphragm used is Celgard 2500 microporous film, and the battery shell is 2016 button type. Place the positive shell, positive plate, diaphragm, negative plate and negative shell in order from top to bottom, and inject an appropriate amount of electrolyte. The solvent in the electrolyte is a 1:1:1 mixture of vinyl carbonate, diethyl carbonate and di- methyl carbonate, and the solute is LiPF6. The charge discharge performance test of the assembled button battery with a concentration of 1 mol/l was carried out on the land CT2001A battery performance test system produced by Wuhan Lantian Electronics Co., Ltd. 3. Results and discussion Figure 1 shows the surface morphology of the precursor of LVP/C composite nanofibers after dry- ing in a 60 oC oven. It can be seen from Figure 1(a) that the fibers obtained by electrospinning are dis- tributed continuously, and each fiber is interwoven vertically and horizontally to form a dense nanofiber film. However, it can be seen that the diameter dis- tribution of the fibers is uneven. The fiber with larger diameter may be caused by the superposition of sev- eral fibers. Figure 1(b) shows the specific charac- teristics of the fiber more intuitively: the diameter is about 500 nm and the fiber surface is smooth. Figure 1. SEM image of the nanofiber precursors obtained from electrospinning. The LVP nanofiber film obtained by electrospin- ning was treated at high temperature in a nitrogen at- mosphere tubular furnace for 4 h. as shown in Figure 2, the SEM images after heat treatment at different temperatures. It can be seen from the figure that after high temperature treatment, the fibrous morpholo- gy of the precursor was not damaged, but different changes occurred on the basis of fiber dimension. However, temperature has a great influence on the morphology of the material. As shown in Figure 2(a) and 2(b), after calcination at 600 oC for 4 h, the fiber breaks, the diameter is about 600 nm, and small holes are formed on the fiber surface. Combined with XRD data analysis, we know that after calcina- tion at 600 oC, no LVP phase is formed. Therefore, it can be judged that the fiber with rough surface shown in Figures 2(a) and 2(b) is the carbon fiber obtained by PVP carbonization. After increasing the calcination temperature to 700 oC, the fiber surface changes significantly. As shown in Figures 2(c) and 2(d), the fiber does not break and the diameter be- comes smaller at 600 oC, with improved continuity. In addition to similar pores, needle and block parti- cles also appear on the fiber surface. It can be seen from the above analysis that the fiber is the carbide of PVP. Combined with the XRD analysis results, it can be inferred that the needle and block formed on the fiber surface should be the particles of LVP phase. When the temperature continues to rise to 800 oC, as shown in Figure 2(e) and 2(f), the diameter of the nanofibers obtained at 700 oC is close to that of 11 the nanofibers, but the needles disappear, the number of massive particles gradually increases, distributed on the surface of the nanofibers, and the particle sur- face is smooth, which indicates that the LVP phase formed gradually increases with the increase of the calcination temperature. Figure 2(g) and 2(h) are the SEM images after calcination for 4 h when the tem- perature rises to 900 oC, which can be clearly seen from the figure. The diameter of nanofibers is about 100 nm, and the LVP particles grow gradually. At this time, the PVP transformed carbon fibers still ex- ist continuously and play a supporting role, and the LVP particles are distributed on its surface. It can be seen that the sintering temperature has an effect on the formation of LVP phase, and the heat treatment temperature affects the change of morphology in the treatment of electrospinning precursors. Figure 2. SEM profile of LVP at different calcination tempera- tures. After high temperature treatment at different temperatures, the XRD diffraction patterns of each sample are shown in Figure 3. It can be seen from the figure that the XRD of the obtained LVP is ob- viously different. After high temperature treatment at 600 oC at the same heat treatment time, as shown in Figure 3(a), there is no diffraction peak of LVP, showing amorphous characteristics. It shows that it is not enough to react at 600 oC to form LVP phase. When the heat treatment temperature rises to 700 oC, it can be clearly seen from Figure 3(b), the XRD spectrum of the sample shows the characteristic diffraction peak of LVP, but the diffraction peaks of (002) and (111) crystal planes around 15° do not ap- pear, indicating that the temperature still needs to be increased. When the temperature rises to 800 oC, as shown in Figure 3(c). As shown in Figure 3(d), in addition to the enhancement of the diffraction peak intensity, the diffraction peaks of (002) and (111) crystal planes also began to appear. Figure 3(d) shows the XRD diffraction peaks of LVP samples obtained after calcination for 4 h after the tempera- ture increased to 900 oC, with monoclinic structure, belonging to P21/n space group, which corresponds to the standard spectral peaks one by one. From the analysis of XRD results, at the same time, Heat treatment temperature plays an important role in the formation of LVP phase. Figure 3. The XRD spectrogram of the LVP at the different cal- cination temperatures. After high temperature heat treatment at 700 oC, 800 oC, 900 oC, the obtained LVP nanofibers were ground and prepared into electrode sheets, and then assembled into batteries. The electrochemical properties were tested respectively in the voltage range of 3.0–4.3 V and 3.0–4.8 V. Figure 4 shows the constant magnification (0.1 C, 1 C = 133 MAH/ g) of LVP samples in the voltage range of 3.0–4.3 V. It can be seen from the charging curve that there are three platforms at 3.59, 3.68 and 4.09 V, but these three platforms are not obvious for the LVP samples calcined at 700 oC and 800 oC, which is consistent with the XRD data because the crystallinity of LVP 12 phase in the samples is not high. With the increase of temperature, the crystallinity of LVP phase gradually increases. The characteristic platform of LVP in the charge discharge curve is also gradually obvious. The cycle performance of each sample after 50 charge discharge cycles at a constant magnification of 0.1 C is shown in Figure 4(b). It can be seen from the fig- ure that with the increase of calcination temperature, the discharge specific capacity of LVP samples grad- ually increases, and the cycle stability also improves. After 50 cycles, specific discharge capacity of LVP at 900 oC, 800 oC, 700 oC retains 132 mAh/g, 119 mAh/g, and 92 mAh/g respectively, which are 90%, 91% and 76% of the initial discharge specific capac- ity respectively. According to the charge discharge test results at small magnification (0.1 C), the LVP samples treated at 700 oC have shown poor capacity and cycle stability. In order to further study the effect of 800 oC and 900 oC on the performance of LVP, the variable rate charge and discharge performance at high voltage was tested. Figure 4. First circle charge and discharge curve and cycle per- formance diagram of the LVP in the 3.0–4.3 V voltage range ob- tained at different calcination temperatures, and the charge and discharge ratio is 0.1 C. Figure 5 is the first charge discharge curve and cycle performance diagram of each electrode mate- rial under the charge discharge ratio of 0.1 C and 50 cycles in the voltage range of 3.0–4.8 V. When the battery is charged to 4.8 V, three Li+ are completely separated from the LVP, with two-phase electro- chemical platforms of 3.59, 3.68, 4.09 and 4.56 V respectively. The platforms of LVP charge curve obtained by calcination at 700 oC are not obvious, and in the discharge curve, the platforms of the three samples at about 4.0 V are inclined because of the irreversibility of the reaction. Two platforms can be seen in the discharge process, and the three discharge platforms are LVP electrodes. According to the cycle performance diagram in Figure 5(b), the capaci- ty of each sample decreases because the electrode material may dissolve in the electrolyte under high voltage, resulting in the instability of the LVP frame structure. After 50 cycles, the discharge specific ca- pacities of the three samples are 114.2 mAh/g, 134.7 mAh/g, 151.6 mAh/g respectively, 74%, 73% and 77% of the specific discharge capacity of the first cy- cle are retained. Figure 5. First cycle charge discharge curve and cycle perfor- mance diagram of LVP obtained at different calcination tem- peratures in the voltage range of 3.0–4.8 V. The charge discharge ratio is 0.1 C. Similarly, we studied the variable rate perfor- mance of LVP samples calcined at 800 and 900 oC. As shown in Figure 6, the two samples were charged at constant current rate of 0.1 C and discharged at different discharge rates of 0.1 C, 1 C, 2 C, 5 C, 10 C and 20 C. It can be seen that with the increase of dis- charge ratio, the platform of charge discharge curve decreases gradually, and the first discharge specific capacity at each ratio decreases. LVP electrode cal- cined at 900 oC. Under the variable rate performance test, although the coulomb efficiency is less than 800 oC, the material shows good electrochemical performance. Even at higher rates, such as 10 C and 20 C, its specific discharge capacity can still main- tain 88 mAh/g and 50 mAh/g. LVP nanoparticles are attached to carbon nanofibers with excellent conduc- tivity. The composite structure effectively improves the electrochemical properties of LVP particles. 4. Conclusion LVP nanofibers were prepared by a simple and feasible electrospinning method. LVP nanofibers with different morphologies were obtained after treatment at different calcination temperatures for 13 4 h. LVP particles can be embedded on the surface of nanofibers at higher temperatures. The sintering temperature plays an important role in the phase formation of LVP. The higher the temperature, the more LVP phase is formed, Through the test of elec- trochemical properties, the LVP calcined at 900 oC has good cycle performance and rate performance. Therefore, the LVP phase with high electrochemical activity is the most at this temperature. LVP nanopar- ticles are embedded on polymer fibers, and the parti- cles are connected through fibers. Through compari- son, the optimal sintering temperature is 900 oC. The first discharge capacities of LVP samples calcined at 800 oC and 900 oC are 134 mAh/g and 135 mAh/ g in the voltage range of 3.0–4.3 V, reaching the the- oretical capacity of Li3V2(PO4)3, which is 133 mAh/ g. At high magnification, the LVP/C fiber composite sample still has excellent electrochemical properties. Conflict of interest The authors declare that they have no conflict of interest. Acknowledgements This work was supported by the Jiangsu Natural Science Foundation “Synthesis and Electrochemi- cal Properties of Cathode Material Lithium Nickel Manganite of High Performance Lithium Batteries” (BK20141229). References 1. Huang K, Wang Z, Liu S. Principle and key technol- ogy of lithium batteries (in Chinese). Beijing: Chem- ical Industry Press; 2007. p. 12. 2. Yang G, Liu H, Ji H, et al. Temperature-controlled microwave solid-state synthesis of Li3V2(PO4)3 as at cathode materials for lithium batteries. Journal of Power Sources 2010; 195: 5374–5378. 3. Zhang X, Wang K, Wei X, et al. Carbon-coated V2O5 Nanocrystals as high performance cathode materi- als for lithium ion batteries. Chemistry of Materials 2011; 23: 5290–5292. 4. Son J, Kim G, Kim M, et al. Carbon coated NASI- CON type Li3V2–xMx(PO4)3 (M=Mn, Fe and Al) ma- terials with enhanced cyclability for Li-ion batteries. Journal of the Electrochemical Society 2013; 160: A87–A92. 5. Sun C, Rajasekhara S, Dong Y, et al. Hydrothermal synthesis and electrochemical properties of Li3V2(- PO4)3/C-based composites for Lithium batteries. ACS Applied Materials & Interfaces 2011; 3: 3772–3776. Figure 6. Magnification performance diagram of LVP/CNF obtained at 800 oC and 900 oC. 14 6. Kuang Q, Zhao Y, Liang Z. Synthesis and electro- chemical properties of Na-doped Li3V2(PO4)3 cath- ode materials for Li-ion batteries. Journal of Power Sources 2011; 196: 10169–10175. 7. Chen L, Yan B, Xu J, et al. Bicontinuous structure of Li3V2(PO4)3 clustered via carbon nanofiber as high-performance cathode material of Li-ion batter- ies. ACS Applied Materials & Interfaces 2015; 7: 13934–13943. 8. Liu H, Yang D, Zhang X, et al. Kinetics of conven- tional carbon coated-Li3V2(PO4)3 and nanocomposite Li3V2(PO4)3/graphene as cathode materials for power lithium ion batteries. Journal of Materials Chemistry 2012; 22: 11039–11048. 9. Wang H, Li Y, Huang C, et al. High-rate capability of Li3V2(PO4)3/C composites prepared via a polyvi- nylpyrrolidone-assisted sol-gel method. Journal of Power Sources 2012; 208: 282–287. 10. Yang Y, Wang H, Zhou Q, et al. Improved lithium storage properties of electrospun TiO2 with tunable morphology: from porous anatase to necklace rutile. Nanoscale 2013; 5: 10267–10274. 11. Hagen RV, Lorrmann H, Moller KC, et al. Electro- spun LiFe1–yMnyPO4/C Nanofiber Composites as Self-Supporting Cathodes in Li-ion batteries. Ad- vanced Energy Materials 2012; 2: 553–559. 12. Damen L, Giorgio F D, Monaco S, et al. Synthesis and characterization of carbon-coated LiMnPO4 and LiMn1–xFexPO4 (x=0.2, 0.3) materials for lithium-ion batteries. Journal of Power Sources 2012; 218: 250–253.