Hydrothermal synthesis of corn cob-like LiFePO 4 /C as high performance cathode material for lithium ion batteries

Transcription

1 Advances in Science and Technology Vol. 93 (2014) pp (2014) Trans Tech Publications, Switzerland doi: / Hydrothermal synthesis of corn cob-like LiFePO 4 /C as high performance cathode material for lithium ion batteries LIN Mei a, WU Xiao b, CHEN Bo Lei c and YUAN Ji Kang d * Department of Applied Physics and Materials Research Centre The Hong Kong Polytechnic University, Kowloon, Hong Kong, China a r@connect.polyu.hk, b r@connect.polyu.hk, c cbl0115@hotmail.com, d *apjkyuan@polyu.edu.hk Keywords: LiFePO 4, Hydrothermal, Morphology, Lithium Batteries. Abstract. Corn cob-like LiFePO 4 (LFP) cathode material was simply synthesized through hydrothermal method using block copolymer (PEG-PPG-PEG) as the surfactant. The influence of ph value and reaction time on the morphology of LFP has been briefly investigated. The presence of copolymer plays an important role in the construction of the hierarchically microstructures. By adjusting the ph value and reaction time, platelet-like, hexagram-like, porous spindle-like and corn cob-like LFP microstructures were obtained. To gain the cell performance of the synthesized LFP, galvanostatic charging-discharging measurement on the as-prepared samples were performed. The porous spindle-like LFP/C shows unexpected electrochemical performance since the spindle-like LFP have large structure which prevents access for the liquid electrolyte. Corn cob-like LFP/C exhibits the best electrochemical performance, discharge specific capacities of 120 mah g -1 after 100 cycles with capacity retention ratios of 80% at 0.1 C. This work also provides the possibility for further investigation into the shape-dependent electrochemical performance of other materials by optimizing the experimental parameters during hydrothermal synthesis. Introduction Since it was first proposed by Goodenough and co-workers in 1997 [1], olivine-structured lithium iron phosphate (LiFePO 4 abbreviated as LFP) has been extensively and intensively studied as one of the most promising cathode material for lithium batteries. It possesses many particular virtues, including low cost, environmental benignity, high safety and the high reversible theoretical capacity (170 mah g -1 ) as well as the acceptable operating voltage (3.4 V vs. Li + /Li) [2-5]. However, the LFP suffers from a poor ionic-electronic conductivity and limited lithium ion diffusion channel, which lead to the low rate capability of Li batteries [6, 7]. Accordingly, a lot of considerable efforts have been made to overcome the limitations of electronic conductivity and Li + diffusion of the LFP. During the past decade, many researches focused on solving these drawbacks by internal cationic doping, particle-size minimized or/and conductive coating [8, 9]. Some nanostructured electrodes are indeed efficient for enhancing Li-ion diffusion, whereas the high surface area of nanostructure always leading to undesirable electrode/electrolyte reactions [10]. Hence, synthesis of micro size LFP with nanostructure is highly desired. One main promising and useful strategy to prepare excellent electrochemical performance LFP materials is accomplished by incorporating nanosize building block and micrometer-sized assemblies. Because that the micro self-assembled structures have combined the advantages of enhanced kinetics of nanometer-sized building blocks and good stability of micrometer-sized assemblies [11]. It was demonstrated that materials with self-assembled nano/micro hierarchical structures is one of the most suitable structures to improve the electrochemical properties [12, 13]. Up to now, several chief methods have been developed to synthesize nanostructured LFP, like hydrothermal, solvothermal, sol-gel, and so on [14-16]. Due to its low working temperature and good morphology control, hydrothermal synthetic method is most widely used to prepare nanostructured LFP. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, (ID: /10/14,14:42:34)

2 Advances in Science and Technology Vol Herein, we report a simple hydrothermal approach combined with high-temperature calcinations of carbon coating to synthesize microstructure LFP with porous spindle-like morphology and self-assembled corn cob-like consisting of nanosize building blocks. Our results suggest that ph value and reaction time play an important role in controlling the formation of morphology. Experiment All LFP samples were synthesized by a simple hydrothermal method. The stoichiometric mixture of lithium hydroxide monohydrate (LiOH H 2 O), iron sulfate heptahydrate (FeSO 4 7H 2 O), and phosphoric acid (H 3 PO 4 ) is in 3:1:1 molar ratio. In a typical process, block copolymer (PEG-PPG-PEG, MW: 2900) was dispersed in deionized (DI) water with constant stirring for 6 h to obtain a homogeneous and transparent solution. FeSO 4 7H 2 O and H 3 PO 4 were dissolved in DI water simultaneously, then poured into the block copolymer solution. Subsequently, LiOH H 2 O solution was added into the above mixed solution dropwise. After stirring 10 min, ph value of the precursor suspension was adjusted by adding appropriate amount of ammonia solution (NH 3 nh 2 O). Finally, a blue-green suspension was obtained after vigorously stirred for 20 min, then transferred into a teflon-lined stainless steel autoclave and heated at 190 C for 12 or 24 h. The autoclave was then cooled to room temperature and the resulting powder was washed, filtered, and dried at 60 C overnight. Aniline (An) monomer was dissolved into moderate diluted HCl solution and kept stirring for 10 min at room temperature. Then the obtained LFP powder were added and sonicated for 10 min. A precooled APS aqueous solution was poured into the above suspension. The molar ratios of aniline to APS is 3. The mixture was reacted for 15 min at 0-5 ºC. The obtained black precipitate was filtration and then washed several times with deionized water, and finally dried overnight in a vacuum at 60 ºC. The powder was annealed in a tube furnace at 650 C for 2 h (heating rate of 5 C/min) under a mixed atmosphere of N 2 (95%) and H 2 (5%) to get the LFP/C composites. The phase purity and crystalline structure of the as-obtained samples were characterized by X-ray powder diffraction (XRD) (Rigaku SmartLab). The morphology of LFP samples was observed by scanning electron microscope (SEM, JEOL Model JSM-6490). The microstructure of LFP were analyzed by transmission electron microscope (TEM, JEOL JEM-2100F). Electrochemical measurements were executed using 2032 coin-type half-cells assembled in an argon-filled glovebox. The working electrodes were prepared by grinding a mixture of LFP/RGO composites, acetylene black, and poly (vinyl difluoride) (PVDF) with the mass ratio of 70:20:10 in NMP (N-methylpyrrolidone). The slurry was uniformly pasted onto Al foil and dried at 60 C for 2 h followed by roll-pressing and further dried at 60 C for 12 h in a vacuum oven before assembly. A polypropylene film (Cellgard 2400) was used as a separator, and pure lithium foil was employed as the anode. The electrolyte was 1 M LiPF 6 dissolved in a 1:1 mixture of ethylene carbonate (EC) dimethyl carbonate (DMC) (Shenzhen Kejing Instrument Co. Ltd.). Galvanostatic charge/discharge measurement of assembled cells was performed on a Land Battery Test System (CT2001A, Wuhan Land Electronic Co. Ltd., China) in the voltage range of V with various current densities at room temperature. Results and discussion Fig. 1 shows the SEM images of LFP samples obtained from precursor with ph value of 8 and 9 under 190 ºC for 12 h. When ph = 8, Fig. 1a exhibit the platelet-like LFP with the size of ~ 10 µm. Fig. 1b shows the high-resolution image of platelet-like LFP, exhibiting smooth surface and density structure. While for ph = 9, the platelet-like grains accumulate with each other to form the symmetrical hexagram-like LFP, with the size of about 25 µm (Fig. 1c, 1d). It is funny that by prolonging the reaction time to 24 h (Fig. 2), the LFP samples express quite distinct morphologies. The platelet-like structure transforms to porous spindle one with a large number of holes on the surface of LFP sample, as shown in Fig. 2a and Fig. 2b. When the reaction condition is ph = 9 for 24

3 154 6th Forum on New Materials - Part A h (Fig. 2c, 2d), the LFP sample shows the corn cob-like microstructure with the average length of 3 µm for each grain. The SEM studies indicate that ph condition and reaction time remarkably influence both the morphologies and grain size of LFP samples. High ph value and long reaction time would favor self-assembled nanostructure. Fig. 1. SEM images of LFP samples from ph value of (a,b) ph = 8, (c,d) ph = 9 for 12h. Fig. 2. SEM images of LFP samples from ph value of (a,b) ph = 8, (c,d) ph = 9 for 24h. Fig. 3. XRD patterns of LFP with various morphologies. The XRD patterns of the LFP synthesized from the precursor at different ph values are shown in Fig. 3. All the samples can be well indexed to the orthorhombic olivine-type structure (JCPDS No ). In the XRD patterns of platelet-like LFP, there are two peaks from Fe2PO5 (JCPDS No ), as marked in Fig. 3. A single phase of orthorhombic LFP was obtained from the other three samples, which suggests that high ph conditions and long reaction time would favor the precipitation of LFP. In order to investigate the crystal growth mechanism of LFP microstructures, typical TEM, HRTEM corresponding selected area electron diffraction (SAED) images of LFP/C with morphologies of porous spindle-like and corn cob-like were performed in Fig. 4. The TEM images (Fig. 4a, 4e) illustrate that the morphologies of the obtained LFP are well consistent with the SEM

4 Advances in Science and Technology Vol images. The HRTEM image (Fig. 4b) of cube clusters LFP exhibits clear lattice fringes indicating a well-crystallized structure of the orthorhombic LFP product. The d-spacing values of the adjacent lattice planes are 0.51 nm, corresponding to (200) planes of orthorhombic LFP. The SAED patterns correspond to the (200), (211) and (011) planes. Local energy dispersive X-ray spectroscopy (EDS) analysis was also conducted for LFP/C (Fig. 4d). Besides the Cu signal coming from the TEM grid, C, N, O, Fe and P were detected. Fig. 3f shows crystal lattices of corn cob-like LFP/C, with the d-spacing 0.37 nm and 0.52 nm, corresponding to the (011) and (110) crystal faces. The diffraction spots (Fig. 4g) are determined to be (101), (011), and (110) planes, respectively. The clear and regularly arrayed spots in the SAED pattern suggest that the prepared corn cob-like LFP is highly crystallization. A typical EDX spectrum of cob-like LFP/C is shown in Fig. 4h. Four peaks at 0.51, 2, 6.4 and 7 kev correspond to Fe/O, P, and Fe, respectively. The peaks at 0.28 and 0.49 kev are associated with C and N. The above results are well coincided with the XRD patterns. Fig. 4. Typical TEM images, HRTEM images and SAED patterns of the as-synthesized LFP/C. (a, b, c) porous spindle-like, (d) EDX for porous spindle-like LFP/C, (e, f, g) corn cob-like, (h) EDX for corn cob-like LFP/C. Fig. 5. Electrochemical characterizations of the LFP/C in the voltage range of V (vs Li + /Li). (a) Cycling profiles tested at a current density of 0.1 C. (b) Glavanostatic discharge voltage profiles at 0.1 C at various rates for corn cob-like LFP/C. (c) Rate performance of corn cob-like LFP/C. To assess the electrochemical properties, galvanostatic charging-discharging measurement on the obtained samples was performed. Fig. 5a shows the typical the long cycling performance curves of the as-prepared LFP/C composites at a current rate of 0.1 C between 2.2 and 4.2 V. As expected, the

5 156 6th Forum on New Materials - Part A cathodes made from corn cob-like LFP/C exhibit higher discharge capacities than porous spindle-like LFP/C. For the porous spindle-like sample, the discharge capacity fade significantly after 30 cycles, in the 100 th cycle is only 80 ma h g -1. No obvious fade was observed in the discharge capacities after 100 charge-discharge cycles for corn cob-like LFP/C. The initial discharge capacity delivery was 150 ma h g -1, and a capacity fade of 20% was observed after 100 cycles. In order to further understand the electrochemical performance of corn cob-like LFP/C, the rate performance cycled at different current densities was observed. Fig. 5b shows that the corn cob-like LFP/C can exhibit a superior rate performance, capacities as high as 150, 130, 107, 86, and even 65 mah g 1 could be obtained at 0.1, 0.2, 0.5, 1, and 2 C, respectively. Another attractive property of the corn cob-like LFP/C sample is the superior rate capability (Fig. 5c). The reversible capacity of electrode regains to 150 ma h g 1 when the current rate returns to 0.1 C after 45 cycles, indicating that corn cob-like LFP/C has excellent stability even after working long time cycles at high rates. Summary In summary, LFP samples with novel hierarchical microstructures were successfully synthesized by hydrothermal method. By controlling the ph values of the precursor between 8 and 9 and prolonging the reaction time, LFP exhibit different morphologies, such as platelet-like, hexagram-like, porous spindle-like and corn cob-like shaped microstructures. The results indicated that ph values and reaction time play an important role in the construction of the hierarchically microstructures. After subsequent heat treatment, SEM images show no obvious variation for the morphology of LFP samples and XRD patterns display single phase of orthorhombic LFP without impurity phase. The corn cob-like LFP/C exhibits the charge/discharge capacity of 150 ma h g -1 at 0.1 C charge/discharge rate and the electrochemical performance has potential for further improvement. The methodology is hopeful to pioneer a new path to prepare other phospholivines LiMPO 4 (M = Mn, Co, Ni, etc.). References [1] A.K. Padhi, K.S. Nanjundaswamy, J.B. Goodenough, Phospho-olivines as positive-electrode materials for rechargeable lithium batteries, J. Electrochem. Soc., 144, (1997) [2] S.L. Yang, M.J. Hu, L.J. Xi, R.G Ma, Y.C. Dong, C.Y. Chung, Solvothermal synthesis of monodisperse LiFePO 4 micro hollow spheres as high performance cathode material for lithium ion batteries, Appl. Mater. Interfaces, 5, (2013) [3] Y.Z. Zhang, L. Chen, J.K. Ou, J. Wang, B.Z. Zheng, H.Y. Yuan, Y. Guo, D. Xiao, Improving the performance of a LiFePO 4 cathode based on electrochemically cleaved graphite oxides with high hydrophilicity and good conductivity, J. Mater. Chem. A, 1, (2013) [4] Y.H. Huang, J.B. Goodenough, High-rate LiFePO 4 lithium rechargeable battery promoted by electrochemically active polymers, Chem. Mater., 20, (2008) [5] C.B. Zhu, Y.Yu, L. Gu, K. Weichert, J. Maier, Electrospinning of highly electroactive carbon-coated single-crystalline LiFePO 4 nanowires, Angew. Chem., Int. Ed. 50, (2011) [6] J.L. Yang, J.J. Wang, Y.J. Tang, D.N. Wang, X.F. Li, Y.H. Hu, R.Y. Li, G.X. Liang, T.K. Shamb, X.L. Sun, LiFePO 4 -graphene as a superior cathode material for rechargeable lithium batteries: impact of stacked graphene and unfolded grapheme, Energy Environ. Sci., 6, (2013) [7] Y.S. Hu, Y.G. Guo, R. Dominko, M. Gaberscek, J. Jamnik, J. Maier, Improved electrode performance of porous LiFePO 4 using RuO 2 as an oxidic nanoscale interconnect, Adv. Mater. 2007, 19,

6 Advances in Science and Technology Vol [8] G. Yang, C.Y. Jiang, X.M. He, J.R. Ying, F.P. Cai, Preparation of V-LiFePO 4 cathode material for Li-ion batteries, Ionics, 18, (2012) [9] J. Jiang, W. Liu, J.T. Chen, Y.L. Hou, LiFePO 4 Nanocrystals: liquid-phase reduction synthesis and their electrochemical performance, Appl. Mater. Interfaces, 4, (2012) [10] B. Kang, G. Ceder, Battery materials for ultrafast charging and discharging, Nature, 458 (2009) [11] J. Su, X.L. Wu, C.P. Yang, J.S. Lee, J. Kim, Y.G. Guo, Self-assembled LiFePO 4 /C nano/microspheres by using phytic acid as phosphorus source, J. Phys. Chem. C, 116, (2012) [12] A.M. Cao, J.S. Hu, H.P. Liang, L.J. Wan, Self-assembled vanadium pentoxide (V 2 O 5 ) hollow microspheres from nanorods and their application in lithium-ion batteries, Angew. Chem. 117, (2005) [13] F.F. Cao, Y.G. Guo, L.J. Wan, Better lithium-ion batteries with nanocable-like electrode materials, Energy Environ. Sci., 4, (2011) [14] X.Q. Ou, L. Pan, H.C. Gu, Y.C. Wu, J.W. Lu, Temperature-dependent crystallinity and morphology of LiFePO 4 prepared by hydrothermal synthesis, J. Mater. Chem., 22, (2012) [15] K.F. Hsu, S.Y. Tsay, B.J. Hwang, Synthesis and characterization of nano-sized LiFePO 4 cathode materials prepared by a citric acid-based sol-gel route, J. Mater. Chem., 14, (2004) [16] C.W. Sun, S. Rajasekhara, J.B. Goodenough, F. Zhou, Monodisperse porous LiFePO 4 microspheres for a high power li-ion battery cathode, J. Am. Chem. Soc. 133, (2011)