Anisotropic ionic conductivity of LiMn${}_{1-x}$Fe${}_x$PO${}_4$ ($0\le x\le 1$) single crystals

Abstract

We report AC-impedance studies on a series of high-quality LiMn${}_{1-x}$Fe${}_x$PO${}_4$ single crystals with $0\le x\le 1$. Our results confirm quasi-one-dimensional transport in LiFePO${}_4$ with fast Li-diffusion along the $b$-axis. The conductivities along the crystallographic $b$-, $c$- and $a$-axis differ by a factor of about 10, respectively. Whereas, the activation energy $E_{\mathrm{A}}$ of the effective diffusion process is particularly large for the $b$-axis and smallest for the $a$-axis. Remarkably, the $b$-axis ionic bulk conductivity of LiMn${}_{0.5}$Fe${}_{0.5}$PO${}_4$ is of the same order of magnitude as in undoped LiFePO${}_4$, which implies similarly fast Li-transport even upon 50~\% Mn-doping which, owing to the higher redox potential of the Mn${}^{3+}$/Mn${}^{2+}$-couple, yields enhanced energy density in lithium-ion batteries. The overall results of our impedance studies draw a far more complex picture than it would be expected from a simple one-dimensional ionic conductor. Our results suggest a strong contribution of crystal defects in real materials.