Lithium ion conductivity in single crystal LiFePO4

Although extensively studied as a potential Li rechargeable battery cathode in its powder form, very little is known about the anisotropy of the ionic and electronic transport properties in LiFePO4 as it may be manifested in single crystal studies. Here, we report on the conductivity of lithium ions along three principal axis directions in single crystal LiFePO4 as a function of temperature by AC impedance spectroscopy. Despite the apparent quasi-two dimensional nature of the crystal structure, suggestive of facilitated inplane diffusion, we show that Li diffusion in LiFePO4 is, to a large extent, confined to one dimension through tunnels along b-axis (using the Pnma symmetry group notation), implying oriented powders in batteries may improve the performance of this material as a cathode in rechargeable batteries. Our results may also explain the numerous failed attempts to enhance the ionic conductivity by introducing divalent and trivalent substitutions to Li+ that, although produce vacancies in the Li sheets, may concurrently impede the diffusion in the tunnels.