C composite with hollow coaxial structure for high-capacity and high-rate performance in lithium-ion batteries

Accepted Manuscript Li3V2(PO4)3/C composite with hollow coaxial structure for high-capacity and high-rate performance in lithium-ion batteries Yonghai Li, Kaixiong Xiang, Wei Zhou, Yirong Zhu, Li Xiao, Xianhong Chen, Han Chen PII: DOI: Reference:

S0167-577X(17)31903-1 https://doi.org/10.1016/j.matlet.2017.12.136 MLBLUE 23634

To appear in:

Materials Letters

Received Date: Revised Date: Accepted Date:

7 December 2017 23 December 2017 27 December 2017

Please cite this article as: Y. Li, K. Xiang, W. Zhou, Y. Zhu, L. Xiao, X. Chen, H. Chen, Li3V2(PO4)3/C composite with hollow coaxial structure for high-capacity and high-rate performance in lithium-ion batteries, Materials Letters (2017), doi: https://doi.org/10.1016/j.matlet.2017.12.136

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Li3V2(PO4)3/C composite with hollow coaxial structure for high-capacity and high-rate performance in lithium-ion batteries Yonghai Li, Kaixiong Xiang*, Wei Zhou, Yirong Zhu, Li Xiao, Xianhong Chen, Han Chen* School of Metallurgical and Materials Engineering, Hunan University of Technology, Zhuzhou Hunan 412007, PR China Corresponding author: Han Chen, e-mail: [email protected]; Kaixiong Xiang, e-mail: [email protected]

Abstract The Li3V2(PO4)3/C composite with hollow coaxial structure has been successfully synthesized via a facile repeated immersion method induced by cotton fibers. The morphologies, structures and properties are investigated by X-ray diffractometer, scanning electron microscopy and galvanostatic charge-discharge tests. Li3V2(PO4)3/C composite displays a unique hollow coaxial structure constructed by hollow carbon fibers filled with Li3V2(PO4)3/C particles inside and coated by Li3V2(PO4)3/C layer outside. Li3V2(PO4)3/C composite exhibits good rate performance with a specific capacity of 115.7 mAh g-1 at 15 C rate between 3.0-4.3V. The improved properties can be attributed to the formation for hollow coaxial structure. Keywords: Composite materials; lithium-ion batteries; cotton fibers; hollow coaxial structure; cathode materials

1. Introduction Nowadays, rechargeable Li-ion batteries are the most important power source for modern electronic applications, such as portable devices, electric vehicles (EV) and hybrid electrical vehicles (HEV) [1-3]. Monoclinic Li3V2(PO4)3 has been considered as a promising cathode material for lithium ion batteries due to its good ion mobility, high operating potential (up to 4.8 V), high theoretical capacity (197 mAh g -1), and thermal stability [4, 5]. Unfortunately, Li3V2(PO4)3 has an intrinsic low electronic conductivity, which greatly impedes Li3V2(PO4)3’s practical application [6]. To solve the problems, numerous approaches have been developed, such 1

as doping metal ion [7, 8], controlling particle morphologies [9, 10], decreasing the particle size [11, 12] and coating with carbon [13, 14]. Among them, carbon coating is the most effective to improve the electronic conductivity of Li3V2(PO4)3 and restrict the particle growth of Li3V2(PO4)3 [2, 14]. Many carbon resources have been used to synthesize homogeneous Li3V2(PO4)3/C composite, such as glucose [15], sucrose [16] and grapheme [12]. Recently, we found that cotton fiber is a good carbon source to synthesize homogeneous Li3V2(PO4)3/C composite. Cotton fiber is used as carbon source to improve the electronic conductivity of Li3V2(PO4)3, and it can also restrict the growth of Li3V2(PO4)3. Moreover cotton fiber plays a significantly important role on the construction for the hollow coaxial structure. The formation process, structure and electrochemical performance of coaxial Li3V2(PO4)3/C composite were investigated systemically.

2. Experimental 2.1 Material preparation The Li3V2(PO4)3/C composite was prepared through multiple repeated immersion method. LiH2PO4, NH4VO3 and cotton fiber were used as the starting materials. The typical synthesis process is as follows: Firstly, 6 mmol LiH2PO4, 4 mmol NH4VO3 and 2 mmol citric acid were added to 30 mL distilled water. The mixture was mixed with magnetic stirring at 60 OC to obtain a yellow solution. Secondly, 2.1g cotton fibers were added to the above solution, then took it out after 5days and sintered in argon flow at 350 OC for 5 h, at 750 OC for 10 h (M-5). The above procedures, were repeated another three times for every 5 days to obtain various Li3V2(PO4)3/C composites (the as-obtained samples are respectively noted as M-10, M-15, M-20). Lastly, Li3V2(PO4)3/C composites with various carbon content were obtained. 2.2 Material characterization The crystal structures were characterized by powder X-ray diffraction (XRD, Siemens D5000) at the 2ϴ

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range of 10-60O using Cu Kα radiation. The morphology and microstructures were observed by scanning electron microscope (SEM) (JSM-6700F, JEOL, Tokyo, Japan) and transmission electron microscope (TEM, JEM-3010) technologies. The chemical groups of cotton fibers were characterized by Fourier transform infrared spectroscopy (FT-IR) (Nicolet, NEXUS 470, USA) technologies. 2.3 Electrochemical measures The prepared Li3V2(PO4)3/C composites were mixed with acetylene black and polyvinylidene fluoride (PVDF) in the weight ratio of 80:10:10, then dissolved in N-methylpyrrolidone (NMP) to form homogeneous slurry. After that, the resulting slurries were coated onto aluminum foils and dried at 120 OC under vacuum for 12 h. CR2025 coin-type cells were assembled in an argon-filled glove box, lithium foils are used as counter electrodes and polypropylene microporous films (Celgard2400) as separators. The liquid electrolyte is 1 mol L -1 LiPF6 in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) (1:1, v/v). The galvanostatic charge and discharge tests were carried out on NEWARE testing instrument between 3.0 and 4.3 V at various rates (1 C=133 mA g-1). The cyclic voltammograms (CV) of the cells were measured on a CHI 660E electrochemical work station between 3.0 and 4.3 V at the scan rate of 0.1 mV s −1.

3. Results and discussion Fig. 1a is the FTIR spectrum of cotton fibers. The dominant bands can be observed near 1652, 1534 and 1375cm-1 corresponding to amide I, amide II and amide III of the proteins in cotton fibers, respectively. The band at 2920 cm-1 is derived from the CH2 asymmetry stretching vibration of protein and carbohydrate. The band at 1047 cm-1 is caused by the C-O stretching of carbohydrates. Fig. 1b shows the XRD patterns of the obtained Li3V2(PO4)3 composites after calcination. All the diffraction peaks of the Li3V2(PO4)3/C composites are indexed to space group P21/n (PDF#01-072-7074). No 3

carbon diffraction peaks are detected, indicating that the carbon pyrolyzed from cotton fibers is amorphous and has no influence on the crystal structure of Li3V2(PO4)3. Fig.2 shows the SEM images of the raw materials of cotton and the obtained Li3V2(PO4)3/C composites. Fig. 2a displays the images of the cotton fibers before calcination. The twisted fibers and many folds on their surface can be found (inset figure2a). Fig. b shows the images of the raw cotton fibers after calcination. The fibers after carbonization become obviously thinner, the hollow structure can be clearly seen in the circled region. Fig. 2c, d depict the SEM images of Li3V2(PO4)3/C composite obtained by the first immersion of 5 days. It is difficult to distinguish between M-5 and the cotton fibers after calcination. Many small Li3V2(PO4)3 nanoparticles are uniformly adhered to the surface of fibers (Fig. 2d). Unfortunately it is difficult to observe the interior change of hollow fibers marked with the square. The strong polar groups on the surface of cotton fibers, such as hydroxyls, alkoxies and amides proven in the FTIR analysis, can absorb opposite polar ions to form Li3V2(PO4)3 nanoparticles in the calcination process. The Li3V2(PO4)3 nanoparticles become the seeds of growth for Li3V2(PO4)3 particles during the next immersion and calcination process. Fig. 2e, f illustrate the SEM images of Li3V2(PO4)3/C composite obtained from another 5 days (M-10). Many larger particles are attached to the surface of the fibers (Fig. 2e). The carbon fibers turn rounder than M-5, indicating that the continuous Li3V2(PO4)3/C thin layer are coated on the surface of the fibers. Interestingly, a few Li3V2(PO4)3/C flowers are grown on the surface of the fibers from the magnification image (Fig. 2f). The Li3V2(PO4)3/C nanoparticles grow gradually up and are connected together into thin layer, and exceptional particles are act as the seeds of the growth for Li3V2(PO4)3/C flowers. Fig. 2g, h describe SEM images of Li3V2(PO4)3/C composite obtained from more 5 days (M-15). The carbon fibers are uniformly and entirely wrapped by folded Li3V2(PO4)3/C layer, and no Li3V2(PO4)3/C particles are dissociated from the carbon fibers. Especially, Li3V2(PO4)3/C composites are full of 4

the hollow core to form coaxial co-core structure in the inset figure. Fig. 2i, j represent SEM images of Li3V2(PO4)3/C composite obtained from another more 5 days (M-20). The carbon fibers are embedded by Li3V2(PO4)3/C composite, and the interspaces of the carbon fibers are filled with Li3V2(PO4)3/C composites. The carbon contents for M-5, M-10, M-15 and M-20 are 85.2, 40.3, 6.8, 2.4 %, respectively. Their carbon contents examined by chemical method dramatically drop with the increase of immersing times for every five days. To further investigation of the element distribution of Li3V2(PO4)3/C composite, the energy-dispersive Xray spectroscopy elemental mappings of C, O, P and V were presented in Fig. 2k-l. The elemental mapping images well reveal the homogeneous distribution of C, O, P and V, indicating the formation of coaxial co-core structure for Li3V2(PO4)3/C composites. Fig. 3a shows the initial charge–discharge curves of Li3V2(PO4)3/C composites at 0.5 C rate. Especially mentioned, the charge–discharge capacities are calculated based on the total mass of Li3V2(PO4)3 and carbon. Three charge plateaus around 3.61 V, 3.71 V and 4.12 V can be obviously found on the charge curves. M-15 and M-20 display discharge capacity for 123.1 mAh g-1 and 121.5 mAh g-1 at the density of 65.5 mA g-1, respectively. M-5 and M-10 can only deliver discharge capacity of 18 and 75 mAh g-1, respectively. However the calculated capacities can attain 121.6 mAh g-1 for M-5 and 125.6 mAh g-1 for M-10 based on the Li3V2(PO4)3 mass, and are basically equal to that for M-15 and M-20, indicating their high capacity difference from the massive carbon which can not provide effective capacity for cathode materials. The CV curves of all the Li3V2(PO4)3/C composites are shown in Fig. 3b at a scanning rate of 0.1 mV s-1 between 3.0 and 4.3 V. All the Li3V2(PO4)3/C composites exhibit obviously three redox peaks in the potential ranges of 3.0-4.3 V. The results are well in agreement with the voltage plateaus for charge–discharge curves (Fig. 3a). Furthermore, M-15 displays the sharpest peaks and highest intensity than other composites. Fig. 3c shows the cycling performance tested at 5 C 5

rate in the potential of 3.0-4.3 V. The capacity retention of M-15 is over 99% and the coulombic efficiency is nearly 100%, demonstrating the excellent cycling stability for M-15. Fig. 3d shows the rate cycling performance of all the Li3V2(PO4)3/C composites. M-20 exhibits slightly higher capacity (129.1 mAh g-1 at 0.1 C and 123.5 mAh g-1 at 0.5 C) than M-15 (126.8 mAh g-1 at 0.1 C and 121.1 mAh g-1 at 0.5 C) due to its lower carbon content for M-15. With the rate increased, M-15 (121.4 mAh g-1) and M-20 (116.8 mAh g-1) deliver nearly the same discharge capacity at 1 C rate. Compared with M-20, M-15 reveals the higher capacity over 1 C rate and the much lower capacity decay with the rate increase from 1 C to 15 C. M-15 shows wonderful electrochemical performance, it is attributed to the formation of perfect and unique coaxial structure and the appropriate mass proportion of Li3V2(PO4)3 and carbon. The perfect structure and appropriate carbon content for M-15 can increase the electrochemical reaction surface, improve the electrochemical conductivity and promote lithium ion diffusion. Conclusions A facile immersion method has been developed to synthesize good performance Li3V2(PO4)3/C composite with a perfect coaxial structure which provides rapid pathways for Li ion transport. The coaxial structure of Li3V2(PO4)3/C composites were composed by Li3V2(PO4)3 innermost layer, carbon middle layer, and Li3V2(PO4)3 outer layer . The coaxial structure is a new design for improving the electrochemical performance of Li3V2(PO4)3/C composites and the facile immersion method is an effective way to prepare Li3V2(PO4)3/C cathode material. Acknowledgments This work is supported by the National Nature Science Foundation of China (51572079, 51772090) and the Natural Science Foundation of Hunan Province (2016JJ5008, 2016JJ5041) and Hunan Provincial Education Department (16A055). 6

References [1] G. Wang, C. Lu, X. Zhang, B. Wan, H. Liu, M. Xia, H. Gou, G. Xin, J. Lian, Y. Zhang, Nano Energy 36 (2017) 46-57. [2] Y. Li, B. Xu, H. Xu, H. Duan, X. Lu, S. Xin, W. Zhou, L. Xue, G. Fu, A. Manthiram, J. B. Goodenough, Angew. Chem. 129 (2017) 771. [3] C. Shi, K. Xiang, Y. Zhu, X. Chen, W. Zhou, H. Chen, Electrochim. Acta 246 (2017) 1088-1096. [4] S. Wang, Z. Zhang, A. Deb, C. Yang, Y. Li, S.I. Hirano, Electrochim .Acta 143 (2014) 297-304. [5] Y.Q. Qiao, J.P. Tu, X.L. Wang, D. Zhang, J.Y. Xiang, Y.J. Mai, C.D. Gu, J. Power Sources 196 (2011) 77157720. [6] Q. Wei, Q. An, D. Chen, L. Mai, S. Chen, Y. Zhao, K.M. Hercule, L. Xu, A. Minhaskhan, Q. Zhang, Nano Lett. 14 (2014) 1042-1048. [7] Han D W, Lim S J, Kim Y I, et al. Chem. Mater. 26 (2014) 3644-3650. [8] L. Ye, X. Shi, Z. Zhang, J. Liu, X. Jian, M. Waqas, W. He, Adv. Mater. Interfaces 4 (2017) 1601236. [9] L.Ye, K. Wen, Z. Zhang, F. Yang, Y. Liang, W. Lv, Y. Lin, J. Gu, J.H. Dickerson, W. He, Adv.Energy Mater. 6 (2016) 1502018. [10] H. Liu, C. Cheng, X. Huang, J. Li, Electrochim. Acta 55 (2010) 8461-8465. [11] F. Cheng, Y. Xin, J. Chen, L. Lu, X. Zhang, H. Zhou, J. Mater. Chem. A 1 (2013) 5301-5308. [12] M. Choi, K. Kang, H.S. Kim, Y. Lee, B.S. Jin, RSC Adv 5 (2014) 4872-4879. [13] J. Yan, W. Yuan, H. Xie, Z.Y. Tang, W.F. Mao, L. Ma, Mater. Lett.71 (2012) 1-3. [14] W. Duan, Z. Hu, K. Zhang, F. Cheng, Z. Tao, J. Chen, Nanoscale 5 (2013) 6485-6490. [15] L.L. Zhang, Y. Li, G. Peng, Z.H. Wang, J. Ma, W.X. Zhang, X.L. Hu, Y.H. Huang, J. Alloys Compd. 513 (2012) 414-419. [16] L. Wang, J. Bai, P. Gao, X. Wang, J.P. Looney, F. Wang, Chem. Mater. 27 (2015) 5712-5718.

Figure captions Fig. 1 (a) FTIR spectra of the cotton fibers, (b) XRD pattern of all the Li3V2(PO4)3/C composites calcined at 750 O

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Fig. 2 SEM images of raw materials of cotton before calcination (a) and after calcination (b), SEM images of Li3V2(PO4)3/C composite of M-5 (c, d), M-10 (e, f), M-15 (g, h), M-20 (i, j) and the elemental mapping images of M-15 (k, l) Fig. 3 The initial charge–discharge curves at 0.5 C (a), cyclic voltammetry curves (b), cycling performance at 5 C (c), rate performance (d).

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Highlights 1. The coaxial structure can be constructed on the cotton fibers. 2. The polar groups play a significant role on the formation of coaxial structure. 3. The coaxial Li3V2(PO4)3/C composite exhibits excellent electrochemical performance.

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Graphical abstract

Graphical abstract

The coaxial structure Li3V2(PO4)3/C composite was synthesized by repeated immersion method. The Li3V2(PO4)3/C electrode of M-15 exhibits good electrochemical performances.

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