Toward understanding of electrical limitations (electronic, ionic) in LiMPO 4 (M = Fe, Mn) electrode materials

To better understand the factors responsible for the poor electrochemical performances of the olivine-type LiMnPO 4, various experiments such as chemical delithiation, galvanostatic charge and discharge, cyclic voltamperometry, and impedance conductivity, were carried out on both LiFePO 4 and LiMnPO 4. Chemical delithiation experiments confirmed a topotactic two-phase electrochemical mechanism between LiMnPO 4 and the fully delithiated phase MnPO 4 (a = 5.909(5) Å, b = 9.64(1) Å, and c = 4.768(6) Å). We conclude that the limiting factor in the MnPO 4/LiMnPO 4 electrochemical reaction is nested mostly in the ionic and/or electronic transport within the LiMnPO 4 particles themselves rather than in charge transfer kinetics or structural instability of the MnPO 4 phase. For instance, the electrical conductivity of LiMnPO 4 (σ ∼ 3.10 -9 S cm -1 at 573 K, ΔE ∼ 1.1 eV) was found to be several orders of magnitude lower than that of LiFePO 4 (σ ∼ 10 -9 S cm -1 at 298 K, ΔE ∼ 0.6 eV). © 2005 The Electrochemical Society. All rights reserved.