LiMBO 3 (M=Fe, Mn): Potential Cathode for Lithium Ion Batteries
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LiMBO3 (M=Fe, Mn): Potential Cathode for Lithium Ion Batteries Jan L. Allen, Kang Xu, Sam S. Zhang and T. Richard Jow Sensors and Electron Devices Directorate, U.S. Army Research Laboratory Adelphi, MD 20783-1197, U.S.A. ABSTRACT Recently discovered borates, LiMBO3 (M=Fe, Mn), share similarities with LiFe(Mn)PO4. They are polyanion structures, contain extractable lithium and suffer from low electronic conductivity. They are attractive to replace expensive, less abundant redox metals in current use in cathodes with environmentally friendly iron or manganese. Phosphate or borate groups adjacent to the redox active metal increase the voltage of the redox couple through an inductive effect. The LiFeBO3 discharge curve shows a pseudo-plateau around 2.6 V for the Fe(II) / Fe(III) couple. This study brings to bear techniques to improve electrode conductivity to produce LiMBO3 composite electrodes thus allowing access to some of the high, theoretical specific capacity. At low current, up to 70 percent of lithium could be extracted from LiFeBO3 that was prepared in the presence of high surface area, highly electrically conductive carbon black. Attempts to improve the cathode properties of LiMnBO3 were less successful. INTRODUCTION There is a growing interest in polyanion framework structured compounds as cathodes in lithium ion batteries. A polyanion containing a countercation with relatively high electronegativity functions as an electron-withdrawing group, withdrawing electron density from the covalent bonds of the nearest-neighbor redox cations. This inductive effect lowers the redox potential of the metal and raises the voltage of a cell with respect to the anode. Thus, in principle, one could tune the redox potential within a common structure by judicious choice of countercation. In practice, this inductive effect has been explored most intensively in the olivine, LiFePO4 system [1]. The phosphate group raises the redox potential of the Fe(II) / Fe(III) couple to around 3.5 V. An iron-based cathode would have obvious benefits, as iron is cheap, abundant and environmentally benign. There have been only a few reports exploring borate-containing compounds as lithium ion electrode materials [2-3]. Boron is lightweight, relatively abundant, and just slightly less electronegative than phosphorus. Recently, LiMBO3 (M=Mn, Fe, Co) compounds have been reported [2]. Preliminary characterization of lithium extraction / insertion showed very little capacity (less than 0.04 lithium per formula unit) [2]. Recent work on LiFePO4 has shown that careful synthesis to control particle size [4], the preparation of carbon-active nanocomposites [5] and post-synthesis carbon coatings [6] can dramatically improve cathode performance. We have implemented similar techniques to explore improvements in the performance of LiFeBO3 and LiMnBO3 cathodes. We have succeeded in improving initial capacity at low current to about 0.72 Li atoms per LiFeBO3 formula unit, a specific capacity of 160 mAh/g but attempts to improve the cathodic behavior of
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