Near Heterosite Li0.1FePO4 Phase Formation as Atmospheric Aging Product of LiFePO4/C Composite. Electrochemical, Magnetic and EPR Study

Abstract

Freeze-drying method was employed in order to synthesize a LiFePO 4 /C composite. This method produced phosphate nanoparticles (10-20 nm size) covered in a carbon network. Nanostructuring and homogeneous conductive web around the active material promoted a good electrochemical response, with 141 mAh g -1 at C/1 and a capacity retention of 90 up to 300 cycles. However, XRD measurements showed the presence of an approximately 30 of a near heterosite Li 0.1 FePO 4 intermediate phase in the initial sample, as a product of atmospheric aging. EPR spectrum displayed a very broad signal attributed to a high spin Fe 3 ion magnetically coupled to Fe 2+, confirming the formation of the non stoichiometric Li x FePO 4 compound. Electrochemical measurements indicated the presence of active Fe 3 in the initial composite, in a structural environment close to that of a FePO 4 heterosite system. A post-mortem cathode, stopped at 4 V, was characterized. Metastable Li x FePO 4 solid solution type phase was not noticed. Thus, it would have disappeared during cathode cycling life, and all active material would have evolved to a classic biphasic system. © 2011 The Electrochemical Society.