http://journals.cambridge.org Downloaded: 16 Aug 2016 IP address: 175.159.122.91 Nanostructured high specific capacity C-LiFePO 4 cathode material for lithium-ion batteries Khadije Bazzi, Kulwinder S. Dhindsa, Ambesh Dixit, Moodakere B. Sahana, and Chandran Sudakar Department of Physics and Astronomy, Wayne State University, Detroit, Michigan 48201 Mariam Nazri Applied Sciences Inc., Cedarville, Ohio 45314 Zhixian Zhou Department of Physics and Astronomy, Wayne State University, Detroit, Michigan 48201 Prem Vaishnava Department of Physics, Kettering University, Flint, Michigan 48504 Vaman M. Naik Department of Natural Sciences, University of Michigan-Dearborn, Dearborn, Michigan 48128 Gholam A. Nazri and Ratna Naik a) Department of Physics and Astronomy, Wayne State University, Detroit, Michigan 48201 (Received 1 May 2011; accepted 4 October 2011) We report synthesis of nanosize LiFePO 4 and C-LiFePO 4 powders with a narrow particle size distribution (20–30 nm) by ethanol-based sol–gel method using lauric acid (LA) as a surfactant for high specific capacity lithium-ion battery cathode material. X-ray diffraction measurements demonstrated that the samples were single-phase materials without any impurity phases. The average crystallite size was found to decrease slightly from 29 nm to approximately 23 nm with carbon coating. The ratio of the Raman D-band (;1350 cm À1 ) to G-band (;1590 cm À1 ) intensities (I D /I G ) and electronic conductivity of these materials show strong dependence on the amount of surfactant coverage. Remarkably, cell prepared with carbon-coated LiFePO 4 synthesized using 0.25 M solution of LA showed a very large specific capacity approaching the theoretical limit of 170 mAh/g, in stark contrast to the specific capacity of cell consisting of pure of LiFePO 4 (;75 mAh/g) measured at the same C/2 discharge rate. I. INTRODUCTION LiFePO 4 is a particularly advantageous cathode material for lithium-ion rechargeable batteries. Compared to conven- tional cathode materials, such as LiCoO 2 and LiMn 2 O 4 , it has improved capacity retention versus charge–discharge cycle numbers, particularly at elevated temperatures, and it is less toxic, more affordable, thermally stable, easily synthe- sized, and environmentally safe. 1–3 Particularly, LiFePO 4 is much safer at higher temperatures, owing to the thermal stability of PO 4 3À polyanion. 1 LiFePO 4 has an ordered olivine structure (S.G: Pnma) in which FeO 6 octrahedra share common corners in the b–c plane. The oxygen atoms in the crystal structure of LiFePO 4 are arranged in distorted, hexagonal close-packed arrangement, in which the lithium and the iron atoms occupy octahedral sites, whereas the phosphorous atoms occupy the tetrahedral sites. The strong P–O covalent bonds stabilize the antibonding Fe 31 /Fe 21 state through the Fe–O–X (X 5 P, S, As, Mo, or W) inductive effects to produce high operating potential. This results in a high lithium intercalation voltage of 3.4 V versus lithium metal. LiFePO 4 also has a high theoretical capacity of 170 mAh/g. However, its application as a cathode material has been largely hampered by its poor electrical conductivity (10 À9 S/cm) 4 when compared to that of LiCoO 2 (10 À3 S/cm) 5 and LiMn 2 O 4 (10 À5 S/cm). 6 Suboptimal rate capability and electrochemical performance of LiFePO 4 system are the consequences of its limited electrical conduc- tivity. Extensive research has been performed to enhance the electrical conductivity of LiFePO 4 including the use of dopants, metal dispersions, carbon coating, and the cosyn- thesis of the phosphate with carbon. 7–20 Yamada et al. 21 found that controlling the particle size by varying the annealing temperatures is important for the high perfor- mance of the LiFePO 4 cathode material. On the other hand, Chung and Chiang 22 found that by doping supervalent cations in LiFePO 4 , the electronic conductivity can be enhanced by a factor of 10 6 and capacity can be increased to about 160 mAh/g at C/10 rate. Nonetheless, Nazar et al. attributed such an improvement in electronic conductivity to the formation of an iron phosphide phase during high a) Address all correspondence to this author. e-mail: rnaik@wayne.edu DOI: 10.1557/jmr.2011.353 J. Mater. Res., Vol. 27, No. 2, Jan 28, 2012 Ó Materials Research Society 2011 424