Yuki Takei

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

A huge kinetic difference in olivine Li x MPO 4 (M = Fe,Mn) is demonstrated in a quantitative manner. Galvanostatic discharge profiles and the current relaxation to the stepwise anodic overvoltage (chronoamperometry) 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 3+ sites are suggested as an important rate-limiting factor. In spite of the higher open-circuit voltage of Mn 3+ /Mn 2+ (4.05 V) compared to that of Fe 3+ /Fe 2+ (3.45 V) in the olivine framework, the abnormally large polarization may eliminate pure LiMnP0 4 as a practical lithium battery cathode due to much lower effective energy density than LiFePO 4 .

Fast Charging LiFePO4

An excellent fast charging ability for olivine LiFePO 4 is demonstrated with respect to its advantageous open-circuit voltage of 3.4 V vs. Li/Li + with an appropriately large overvoltage margin to the electrolyte oxidation, structural integrity under the nonequilibrium state induced by the large overpotential, and moderate electrochemical activity easily enhanced to the practical level by efficient nanocomposite formation. The current relaxation to the applied stepwise voltage of 4.2 V vs. Li/Li + for the Li x FePO 4 was fast enough to consume most of the theoretical capacity within 10-20 min and the speed is competitive with that of the conventional electrode composite of Li x Mn 2 O 4 , while the capacity consumption of Li x MnPO 4 under the identical experimental conditions was almost null.

Magnetic properties of Li(MnyFe1−y)PO4 and its delithiated phases

A series of synthetic (MnyFe1−y)PO4 (y<0.8) phases with negligible disorder and impurities were obtained by chemical oxidation of crystallized isotypic Li(MnyFe1−y)PO4 under optimized conditions. This enabled us to perform the comparative magnetic study along the two solid-solution lines based on inherent susceptibility data with no parasitic magnetic signals. All samples exhibit antiferromagnetism but the decrease in Neel temperature is more than seven times larger in (MnyFe1−y)PO4 than in Li(MnyFe1−y)PO4. This was discussed in terms of the asymmetric superexchange interaction by an unusual synergetic effect of Jahn-Teller distortion and edge-shared interconnection with PO4 tetrahedra.