Phase-Homogeneous LiFePO 4 Powders with Crystallites Protected by Ferric-Graphite-Graphene Composite

Phase-homogeneous LiFePO 4 powders have been synthesized. The content of impurity crystalline phases was less than 0.1%, according to synchrotron diffractometry (SXRD) data. Anisotropic crystallite sizes L ¯ V h k l were determined by XRD. A low resistance covering layer of mechanically strong ferric-graphite-graphene composite with impregnated ferric (Fe 3+ ) particles < 10 nm in size increases the cycleability compared to industrial cathodes. In accordance with the corrosion model, the destruction of the Fe 3+ -containing protective layer of crystallites predominates at the first stage, and at the second stage Fe escapes into the electrolyte and to the anode. The crystallite size decreases due to amorphization that starts from the surface. The rate capability, Q ( t ), has been studied as a function of L ¯ V h k l , of the correlation coefficients r i k between crystallite sizes, of the Li diffusion coefficient, D , and of the electrical relaxation time, τ el . For the test cathode with a thickness of 8 μm, the values of D = 0.12 nm 2 /s, τ el = 8 s were obtained. To predict the dependence Q ( t ), it is theoretically studied in ranges closest to experimental values: D = 0.5 ÷ 0.03 nm 2 /s, τ el = 8/1 s, average sizes along [010] L ¯ 1 = 90/30 nm, averaged r ¯ = 0/1.