Nanostructured high specific capacity C-LiFePO 4 cathode material for lithium-ion batteries
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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 Naika) Department of Physics and Astronomy, Wayne State University, Detroit, Michigan 48201 (Received 1 May 2011; accepted 4 October 2011)
We report synthesis of nanosize LiFePO4 and C-LiFePO4 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 1) to G-band (;1590 cm 1) intensities (ID/IG) and electronic conductivity of these materials show strong dependence on the amount of surfactant coverage. Remarkably, cell prepared with carbon-coated LiFePO4 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 LiFePO4 (;75 mAh/g) measured at the same C/2 discharge rate.
I. INTRODUCTION
LiFePO4 is a particularly advantageous cathode material for lithium-ion rechargeable batteries. Compared to conventional cathode materials, such as LiCoO2 and LiMn2O4, it has improved capacity retention versus charge–discharge cycle numbers, particularly at elevated temperatures, and it is less toxic, more affordable, thermally stable, easily synthesized, and environmentally safe.1–3 Particularly, LiFePO4 is much safer at higher temperatures, owing to the thermal stability of PO43 polyanion.1 LiFePO4 has an ordered olivine structure (S.G: Pnma) in which FeO6 octrahedra share common corners in the b–c plane. The oxygen atoms in the crystal structure of LiFePO4 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 Fe31/Fe21 state through the Fe–O–X (X 5 P, S, As, Mo, a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2011.353 424
J. Mater. Res., Vol. 27, No. 2, Jan 28, 2012
or W) inductive effects to produce high operating potential. This results in a high lithium intercalation voltage of 3.4 V versus lithium metal. LiFePO4 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 9 S/cm)4 when compared to that of LiCoO2 (10 3S/cm)5 and LiMn2O4 (10 5 S/cm).6 Suboptimal rate capability and electrochemic
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