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

TL;DR

This study investigates the anisotropic ionic conductivity in LiMn$_{1-x}$Fe$_x$PO$_4$ single crystals, revealing directional differences, the impact of doping, and the role of crystal defects, with implications for lithium-ion battery performance.

Contribution

It provides detailed impedance measurements showing anisotropic ionic transport and the effects of Mn doping on conductivity in LiMn$_{1-x}$Fe$_x$PO$_4$ single crystals, highlighting complex conduction mechanisms.

Findings

Conductivity varies by a factor of 10 along different crystallographic axes.

LiMn$_{0.5}$Fe$_{0.5}$PO$_4$ exhibits similar b-axis conductivity to LiFePO$_4$.

Crystal defects significantly influence ionic transport.

Abstract

We report AC-impedance studies on a series of high-quality LiMn$_{1-x}$Fe$_x$PO$_4$ single crystals with $0\leq x \leq 1$. 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 $E_{\rm A}$ 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.