Towards the understanding of coatings on rate performance of LiFePO 4

Abstract

Stoichiometric and non-stoichiometric LiFePO 4 nanoparticles (sub 100 nm) were synthesized and compared to carbon coated LiFePO 4 particles (super 100 nm) of similar electrode composition for rate performance. The materials were characterized by X-ray diffraction (XRD) and high resolution transition electron microscopy (HRTEM) where amorphous layers (<5 nm) were observed on both the non-stoichiometric and carbon-coated LiFePO 4 primary particles. Secondary particles of stoichiometric and non-stoichiometric sub 100 nm particles were confirmed by TEM and scanning electron microscopy (SEM). All three materials display similar particle size distributions as measured with a particle size analyzer and an SEM, indicating that the sub 100 nm particles form secondary particles of approximately the same size as the carbon coated super 100 nm particles. The electronic conductivity measurements of each material indicate that the non-stoichiometric LiFePO 4 measures between 4.5 × 10 −5 and 2.18 × 10 −4 S cm −1, 8 orders of magnitude lower than the conductivity of the carbon coated super 100 nm LiFePO 4. Rate tests of the electrodes demonstrate faster charge and discharge capability as the level of conductive additive in the electrodes approaches 15 wt%. All three materials demonstrate solid state diffusion limitations: electrodes of the non-stoichiometric and the carbon coated material both show extreme high rate performance with the addition of 15% carbon additive. A simple calculation indicates that this is the level of carbon additive needed to completely coat particles of 100 nm in size.