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 = 5.909͑5͒ Å, b = 9.64͑1͒ Å, and c = 4.768͑6͒ Å͒. 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 ͑ ϳ 3.10 −9 S cm −1 at 573 K, ⌬E ϳ 1.1 eV͒ was found to be several orders of magnitude lower than that of LiFePO 4 ͑ ϳ 10 −9 S cm −1 at 298 K, ⌬E ϳ 0.6 eV͒.

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Solid-state electrode materials for Li-ion batteries are of considerable interest worldwide. Along with the intensively studied transition-metal oxides Li x MO 2 (M ) Co, Ni, and Mn) and Li x V 2 O 5 , polyanion structures built of corner-sharing MO 6 octahedra (M ) Fe, Ti, V, and Nb) and (XO 4 ) ntetrahedra (X ) S, P, As, Mo, and W) 1-11 have garnered much attention. Seminal studies on these materials focused on Fe 2 (XO 4 ) 3 (X ) S, 1 Mo, 2 and W 3 ). Materials such as Li x FePO 4 , 4 Li x MM′(XO 4 ) 3 , 5-10 Li x FeP 2 O 7 , 11 and Li x VOXO 4 12-14 were recently identified as good hosts for the extraction/intercalation of Li between 2.5 and 4 V vs Li/Li + . 6,15 Because of the lower covalence of the M-O bonds in these polyanion structures, the Fe 3+ /Fe 2+ and V 4+ /V 3+ redox couples lie at more useful potentials than in the simple oxides.The NASICON framework [MM′(XO 4 ) 3 ] ∞ , 16 which allows for extensive substitution on the octahedral (M and M′) and tetrahedral (X) sites, is particularly attrac-* To whom correspondence should be addressed.