Comparative Kinetic Study of Olivine Li[sub x]MPO[sub 4] (M=Fe, Mn)

Abstract

A huge kinetic difference in olivine Li x MPO 4 (M ϭ Fe,Mn) is demonstrated in a quantitative manner. Galvanostatic discharge profiles and the current relaxation to the stepwise anodic overvoltage ͑chronoamperometry͒ are comparatively measured for the Li x FePO 4 and Li x MnPO 4 under identical extrinsic conditions, which are carefully controlled and confirmed using Rietveld refinement for the X-ray diffraction profiles, direct texture observation by scanning electron microscope, Brunauer-Emmett-Teller surface area measurements, and tap density measurements. The current durability for Li x MnPO 4 is orders-of-magnitude inferior to that of Li x FePO 4 , the origin of which is clearly attributed to their intrinsic crystallographic and transport property differences. Heavy polaronic holes localized on the Mn 3ϩ sites are suggested as an important rate-limiting factor. In spite of the higher open-circuit voltage of Mn 3ϩ /Mn 2ϩ ͑4.05 V͒ compared to that of Fe 3ϩ /Fe 2ϩ ͑3.45 V͒ in the olivine framework, the abnormally large polarization may eliminate pure LiMnPO 4 as a practical lithium battery cathode due to much lower effective energy density than LiFePO 4 .

Enhanced electrochemical performance of <30 nm thin LiMnPO4 nanorods with a reduced amount of carbon as a cathode for lithium ion batteries

“…In order to allow the Mn 2+/3+ redox reaction in a LiMnPO 4 cathode, the material needs to be both ionically and electronically conductive for Li + diffusion and electron transport, respectively. However, pure LiMnPO 4 material suffers from poor intrinsic electronic conductivity [10,11]. To improve the electrochemical kinetics, several groups, including previous work by us, reported the enhanced electrochemical properties, combining both, reduced particle size of LiMnPO 4 and carbon coating [12][13][14][15].…”

Improved kinetics and stabilities in Mg-substituted LiMnPO4

“…Despite its nearly ideal potential of 4.1 V vs. Li/Li + , lithium manganese phosphate has been found to be an inferior cathode material compared to LiFePO 4 . 2,3 The much slower kinetics during Li extraction and insertion are directly related to fundamental differences, including lower electronic and ionic conductivities in LiMnPO 4 , the Jahn-Teller effect in Mn 3+ , larger interface strain due to the larger volume change between LiMnPO 4 and MnPO 4 , and the metastable nature of the delithiated phase.…”

Rapid Synthesis and Charge–Discharge Properties of LiMnPO4 Nanocrystallite-embedded Porous Carbons

“…4,5 It was reported that the poor rate capability of LiMnPO 4 is ascribable to a large kinetic barrier at the mismatched interface of MnPO 4 / LiMnPO 4 during Li insertion/extraction processes and that down sizing the olivine particle to nanometer level would be effective to lower the kinetic barrier. 6 Since LiMnPO 4 is expected to have higher energy density than LiFePO 4 due to its higher redox potential of 4.1 V vs. Li/Li + (3.5 V vs. Li/Li + for LiFePO 4 ), various methods to synthesize nanosized LiMn-PO 4 particles have been developed in recent years for achieving reasonable capacity and rate. However, there is still a problem of poor electronic conductivity in the electrode because strong agglomeration of nanosized particles prevents homogeneous mixing with a conductive additive such as carbon black.…”