Morphology and Electrochemistry of Hydrothermally Prepared LiMnPO 4
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Morphology and Electrochemistry of Hydrothermally Prepared LiMnPO4 Jan L. Allen, T. Richard Jow and Jeff Wolfenstine US Army Research Laboratory Adelphi, MD 20783, U.S.A. ABSTRACT In this study, we explored the synthesis of LiMnPO4 through hydrothermal methods using urea as the hydroxide ion source. The hydrothermally prepared LiMnPO4 was examined through x-ray diffraction, microscopy, surface area and electrochemical measurements. Small crystallites were formed and significant agglomeration of particles was observed. The effect of additives to control nucleation and growth of the LiMnPO4 is reported. None of the attempted additives led to the desired morphology. At a C/5 discharge rate, a capacity of about 53 mAh/g was observed for a carbon coated sample of hydrothermally prepared LiMnPO4. INTRODUCTION The LiMnPO4 phospho-olivine is a potential positive electrode material for Li-ion batteries of interest owing to the intrinsic high thermal stability of phospho-olivines, the abundance and low cost of manganese, the higher lithium intercalation potential, 4.1V, than the now commercialized LiFePO4 phospho-olivine, 3.5 V, with about the same theoretical capacity of 170 mAh/g [1]. So far efforts to use LiMnPO4 as a positive electrode have been slowed by the poor rate capability which has been suggested to result from its low electronic and ionic conductivity [2], the Jahn-Teller effect of Mn3+ in the charged MnPO4 phase [3] and the lattice mismatch between LiMnPO4 and MnPO4 [4]. Approaches to improve the rate performance can build upon the successful approaches taken for the enhancement of rate capability of LiFePO4, such as carbon coating to improve electronic conductivity and synthesis of nanoparticles to improve ionic conductivity. Additionally, recent research to elucidate the functioning of the two phase olivine phase during charge and discharge suggests that control of the particle morphology can enhance rate capability if it is possible to obtain particles with a short b-axis, the axis along which Li+ ions conduct [5-7]. Hydrothermal synthesis is therefore of interest owing to the possibility to achieve a relatively rapid particle morphology equilibration and through control of the solution parameters and temperature some means to alter the particle morphology and size are available. Whittingham et al. have done pioneering research to describe the hydrothermal synthesis of LiFePO4 and other olivines [8,9]. Recently Ellis et al. [10] and Fang et al. [11] have reported on hydrothermal synthesis of LiMnPO4. This work explores the synthesis of LiMnPO4 through hydrothermal methods using urea as the source of hydroxyl ions in place of lithium hydroxide. Urea is decomposed under hydrothermal conditions according to chemical reaction (1): NH2(CO)NH2 + 3 H2O → 2NH4OH + CO2
(1)
Therefore, a solution containing Li+, Mn2+ and PO43- ions treated hydrothermally in the presence of urea, will precipitate LiMnPO4. In many cases, this method has been used to prepare nanoparticles of oxides and often leads to differe
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