Lawrence Berkeley National Laboratory

Carbon coating has proven to be a successful approach to improve the rate capability of LiFePO4 used in rechargeable Li-ion batteries. Investigations of the microstructure of carbon coated LiFePO4 after charge discharge cycling shows that the carbon surface layer remains intact over 100 cycles. We find micro cracks in the cycled material that extend parallel to low indexed lattice planes. Our observations differ from observations made by other authors. However the differences between the orientations of crack surfaces in both studies can be reconciled considering the location of weak bonds in the unit cell and specimen geometry as well as elastic stress fields of dislocation.