283
Polypyrrole Coated LiMn0.15Fe0.85 PO4 Olivine As High Capacity Cathode Electrode for the Lithium-Ion Battery
Tuesday, 10 June 2014
Cernobbio Wing (Villa Erba)
X. Gao (Institue for superconducting and electronic materials, university of wollongong), J. Wang, H. K. Liu, S. Chou (University of Wollongong), G. Chen (Fok Ying Tung Graduate School, The hong kong university of science and technology, Hong Kong, China), Y. Deng (The key laboratory of fuel cell techology of Guangdong Province, school of chemicstry and chemical engineering, Sounth China University of Technology, China), and Z. Shi (Fok Ying Tung Graduate School, The Hong Kong University of Science and Technology, Hong Kong, China)
Lithium iron phosphate has become the subject of great interest as storage cathodes for rechargeable lithium-ion batteries. Unfortunately, its intrinsic low electronic conductivity and low energy density still prevent it from practical commercialization [1]. In the past decade, Padhi et al [2] pioneered interest in the LiFe
1-yMn
yPO
4 olivine system, showing the occupation of some percentage of octahedral 4c sites of LiFePO
4 by Mn combines the advantages of the low toxicity of LiFePO
4 and the relatively high operating voltage of LiMnPO
4 (~4.1 V). This ability to increase the energy density of conventional LiFePO
4 has been the driving force for extensive research on LiFe
1-XMn
xPO
4 [3]. Nevertheless, besides the fundamentally low electronic conductivity of olivine, the Mn
3+ doping may lead to another drawback, which is Mn
3+ dissolution. It is generally accepted that Mn
3+ causes serious mechanical stress and its disproportionation results in Mn
2+dissolution from the surface by Jahn-Teller distortion [4]. Therefore, there have been many efforts to overcome these obstacles, for instance by decreasing the crystallite size and changing the morphology or providing a conductive matrix (e.g. carbon).
In this work, we present the application of a composite LiMn0.15Fe0.85PO4 coated with conductive polypyrrole (PPy) as cathode electrode for the Li-ion battery. The effects of the PPy coating on the physicochemical and electrochemical properties have been extensively studied. SEM and TEM images show that the PPy is uniformly coated on the surface of LiMn0.15Fe0.85PO4. The LiMn0.15Fe0.85PO4 electrode with 9 wt. % PPy coating exhibits a high discharge capacity and good cycle life at room temperature. This electrode delivers a discharge capacity of 135 mAh/g at 1/2 C, and after 100 cycles, the discharge capacity still reaches 126 mAh/g. In contrast, the discharge capacity of the pristine LiMn0.15Fe0.85PO4 is only around 128 mAh/g under the same conditions, and 114 mAh/g remains after 100 cycles, as shown in Fig. 1a. Electrochemical impedance spectroscopy verified the improved electrochemical conductivity of LiMn0.15Fe0.85PO4/PPy electrode. In addition, analysis of the counter electrode (Li film) confirmed that the PPy coating can protect the electrode from HF attack, and thus lead to reduction of Mn3+ dissolution in the LiMn0.15Fe0.85PO4-PPy electrode (Fig. 1b).
[1] A. Yamada, S. C. Chung, K. Hinokuma, Journal of the Electrochemical Society, 148 (2001) A224-A229.
[2] A. K. Padhi, K. S. Nanjundaswamy, J. B. Goodenough, Journal of the Electrochemical Society, 144 (1997) 1188-1194.
[3] S. Y. Chung, J. T. Bloking, Y. M. Chiang, Nat. Mater., 1 (2002) 123-128.
[4] R. Von Hagen, H. Lorrmann, K. C. Möller, S. Mathur, Advanced Energy Materials, 2 (2012) 553-559.
