J. Phys. Chem. C 2008, 112, 14665–14671
14665
Comparison of Microwave Assisted Solvothermal and Hydrothermal Syntheses of LiFePO4/C Nanocomposite Cathodes for Lithium Ion Batteries A. Vadivel Murugan, T. Muraliganth, and A. Manthiram* Electrochemical Energy Laboratory and Materials Science and Engineering Program, The UniVersity of Texas at Austin, Austin, Texas 78712 ReceiVed: June 16, 2008; ReVised Manuscript ReceiVed: July 17, 2008
Highly crystalline LiFePO4 nanorods have been synthesized within a short reaction time of 5-15 min at 3 V). Following this, the phospho-olivine LiFePO4 was identified as a promising cathode by Padhi et al.3 in 1997. The low cost and environmentally benign nature of Fe as well as the excellent chemical stability and safety imparted by the covalently bonded PO4 groups have drawn much attention toward LiFePO4 in recent years. However, the low electronic conductivity (ca. 10-9-10-10 S cm-1) as well as the slow one-dimension lithium ion diffusion in LiFePO4 is an impediment to realize high rate capability,3,4 a parameter critical for high power applications. These difficulties are now being overcome by coating a thin layer of a conductive material like carbon onto the surface of LiFePO4 particles5-8 and by keeping the particle size of LiFePO4 small at the nanoscale through novel synthesis approaches.9-14 Among the various synthesis methods adopted, hydrothermal15-20 methods have been particularly successful in offering high performance LiFePO4. However, they require either long reaction times in an autoclave at 140-200 °C for 5-24 h or further post heat treatment at temperatures as high as 800 °C * To whom correspondence should be addressed. Phone: 512-471-1791. Fax: 512-471-7681. E-mail:
[email protected].
for 0.5-12 h in an inert atmosphere to achieve a high degree of crystallinity and phase pure material.15-20 The hydrothermal method also gives larger submicron size particles with a wider distribution of particle size. In this regard, microwave assisted synthesis processes are appealing as they can offer the product rapidly within a short time with a high degree of control of particle size and morphology. Recently, Beninati et al.21 and Wang et al. 22 have reported the heating of the solid-state precursors with carbon or carbonaceous substances in a conventional microwave oven to obtain LiFePO4. However, the initial discharge capacity was much lower (125 mAh/g) than the theoretical capacity (170 mAh/g), and they were unable to control the particle size with the solid state precursors employed. We present here a novel microwave-assisted hydrothermal (employing water as a solvent) and solvothermal (employing tetraethyleneglycol as a solvent) synthesis approach that offers highly crystalline LiFePO4 with a controlled, smaller particle size within a short reaction time (5-15 min) at
