Percolation-tunneling modeling for the study of the electric conductivity in LiFePO_4 based Li-ion battery cathodes

摘要

In this work a percolation-tunneling based model is developed and used to study the electrical con-ductivity of LiFePO_4 composite Li-ion battery cathodes. The active and conductive additive particles are explicitly represented using a random hybrid geometric-mechanical packing algorithm, while the inter-particle electric transport is achieved by including electron tunneling effects. The model is adjusted to the experimental data of a PVDF/C composite with different mixing ratios. The performed study aims to capture the variation of the conductivity of the LiFePO_4 cathode with particle sizes, carbon black particles v/t.% and carbon coating wt.%. It is found that ultra fine carbon-free nanosized particles (~50 nm), which are favorable for improved diffusion, would require a relatively high amount of carbon black (15wt.%) putting at risk the gravimetric capacity of the cell. On the other hand, particles with 1 wt.% continuous carbon coating delivers already sufficient conductivity for all particle sizes without any additives. The further addition of conductive phases is at the risk of redundancy in view of conductivity enhancements. Although continuous carbon coating with loading as low as 1 wt.% is thought to be the most efficient way to achieve electric conductivity, its manufacturability and effect on Li ion diffusion remain to be assessed.