Towards the understanding of coatings on rate performance of LiFePO4

Stoichiometric and non-stoichiometric LiFePO4 nanoparticles (sub 100 nm) were synthesized and compared to carbon coated LiFePO4 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 LiFePO4 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 LiFePO4 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 LiFePO4. 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.

► Stoichiometric, non-stoichiometric and carbon coated LiFePO4were prepared.
► The electronic conductivities of the three materials are measured.
► Rate performance of the three materials with different amount of AB was tested.
► The Li+ diffusion pathways of the three materials are elucidated.
► The coating effect on rate performance of the three is interpreted.