Electrochemical properties of LiFePO4-multiwalled carbon nanotubes compositecathode materials for lithium polymer batteryBo Jina,b, En Mei Jina, Kyung-Hee Parka, Hal-Bon Gua,*aDepartment of Electrical Engineering, Chonnam National University, 300 Yongbong-dong, Gwangju 500-757, South KoreabCollege of Materials Science and Engineering, Jilin University, Changchun 130025, Chinaa r t i c l ei n f oArticle history:Received 7 June 2008Received in revised form 19 July 2008Accepted 2 August 2008Available online 11 August 2008Keywords:OlivineLiFePO4-MWCNTs compositeOrthorhombicHydrothermal methoda b s t r a c tLiFePO4-multiwalled carbon nanotubes (MWCNTs) composites were prepared by a hydrothermal methodfollowed by ball-milling and heat treating. Cyclic voltammetry, ac impedance and galvanostatic charge/discharge testing results indicate that LiFePO4-MWCNTs composite exhibits higher discharge capacityand rate capability than pure LiFePO4at high-rate at room temperature. It is demonstrated that the addedMWCNTs not only increase the electronic conductivity and lithium-ion diffusion coefficient but alsodecrease crystallite size and charge transfer resistance of LiFePO4-MWCNTs composite.? 2008 Elsevier B.V. All rights reserved.1. IntroductionRecently, lithium iron phosphate with an ordered olivine-typestructure, LiFePO4, has attracted extensive attention due to lowcost, safety and high compatibility with environment [1–9]. How-ever, it is difficult to attain the full capacity because the electronicconductivity (?10-9S/cm) is very low, which leads to initial capac-ity loss and poor rate capability, and diffusion of Li+ion across theLiFePO4/FePO4boundary is slow due to its intrinsic character [1].Many researchers have suggested solutions to this problem as fol-lows: (i) coating with a conductive layer around the particles[10,11]; (ii) ionic substitution to enhance the electrochemicalproperties [12,13]; and (iii) synthesis of particles with well-definedmorphology [14].Li et al. [15] demonstrated that LiFePO4/MWCNTs compositecathode displayed the initial discharge capacity of 155 mAh/g at0.1 C rate and the gradual decrease in discharge capacity upon cy-cling. Whittingham et al. [16] indicated that the added MWCNTs inpure LiFePO4 enhanced the electronic conductivity of the finalproduct. Sakamoto et al. [17] suggested that V2O5/singlewalledcarbon nanotubes composite electrode exhibited specific capaci-ties in excess of 400 mAh/g at high discharge rates and retainedthis level of capacity on cycling. All the above papers used the li-quid electrolyte. Zaghib et al. [18] reported the electrochemicalperformance of natural graphite-fibers/polyethylene oxide (PEO)-based gel electrolyte/LiFePO4batteries. Appetecchi et al. [19] sug-gested that Li/LiFePO4polymer cells were capable of deliveringcapacities exceeding 100 mAh/g at temperatures above 90 ?C evenat moderately high rates using PEO as a polymer matrix. However,up to now, there is no report on the electrochemical performanceof LiFePO4-MWCNTs composite using polymer electrolyte espe-cially at room temperature.In this study, MWCNTs were added to improve the electronicconductivity of pure LiFePO4. For the first time, we used 25PVDFLi-ClO4EC10PC10as solid polymer electrolyte (SPE) to analyze the elec-trochemical properties of LiFePO4-MWCNTs composite by cyclicvoltammetry (CV), ac impedance and galvanostatic charge/dis-charge tests at room temperature.2. ExperimentalThe preparation of pure LiFePO4was described in detail previ-ously [20]. Five weight percentage of MWCNTs were added intothe solution of LiFePO4hydrothermally synthesized at 170 ?C andN-methyl-2-pyrrolidone (NMP), the mixture was ball-milled for10h using a shaker type of ball mill (Planetary Mono Mill). Afterdrying at 90 ?C for 12h, the powders were pelletized and furtherheated at 500 ?C for 1h in nitrogen atmosphere. After cooling toroom temperature, the mixture of NMP and LiFePO4-MWCNTscomposite was ball-milled again for 10h. Finally, the mixturewas dried at 90 ?C for 12h. For comparison, pure LiFePO4withoutMWCNTs was synthesized by the same ball-milling and heatingtemperature.The composite electrodes were prepared by mixing pure LiFe-PO4or LiFePO4-MWCNTs composite with carbon black and polyvi-nylidene fluoride (PVDF) in a weight ratio of 70:25:5 in NMP. The1388-2481/$ - see front matter ? 2008 Elsevier B.V. All rights reserved.doi:10.1016/j.elecom.2008.08.001* Corresponding author. Tel.: +82 62 530 0740; fax: +82 62 530 0077.E-mail address: hbgu@chonnam.ac.kr (H.-B. Gu).Electrochemistry Communications 10 (2008) 1537–1540Contents lists available at ScienceDirectElectrochemistry Communicationsjournal homepage: www.elsevier.com/locate/elecom
