Nano-Sized Lithium Manganese Phosphate/Carbon Nanotube Composites with Enhanced Electrochemical Activity for Lithium-Ion
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Nano-Sized Lithium Manganese Phosphate/Carbon Nanotube Composites with Enhanced Electrochemical Activity for Lithium-Ion Batteries Satoru Tsumeda1, Scott D. Korlann1, Shunzo Suematsu1, and Kenji Tamamitsu2 1 United Chemi-Con, Inc. (Nippon Chemi-Con Corp. Group), 625 Columbia Street, Brea, CA 92821, U.S.A. 2 Nippon Chemi-Con Corporation, 363 Arakawa, Takahagi, Ibaraki 318-8505, Japan ABSTRACT Olivine lithium manganese phosphate, LiMnPO4 is a promising cathode material for high energy and safe lithium ion batteries. However, LiMnPO4 possesses excessively poor electrochemical activity, compared to conventional cathode materials. To enhance the electrochemical activity, we have synthesized LiMnPO4/multi-walled carbon nanotube (MWCNT) composites by employing an in-situ sol-gel method. The LiMnPO4/MWCNT composites were investigated by utilizing X-ray diffraction, thermogravimetric analysis, scanning electron microscope, transmission electron microscope, and galvanostatic chargedischarge cycling. The LiMnPO4 showed a particle size of ca. 50 nm and capacity of 102 mAh/g at 0.1 C without C.V. charging mode. This study demonstrated that the electrochemical activity of LiMnPO4 was significantly affected by not only pH and the amount of a chelating agent but also unreacted Mn2+. This is the first report analyzing the existence and effects of unreacted Mn2+ in LiMnPO4 synthesized by a sol-gel method. INTRODUCTION In 1997, Padhi et al. first demonstrated that LiFePO4 (LFP) exhibited reversible lithium insertion-extraction [1]. Since this time, olivine-type LiMPO4 (M=Mn, Fe, Co, Ni) have been actively studied as promising cathode materials for lithium ion batteries (LIBs) as they have a relatively high theoretical capacity (~ 170 mAh/g) and are highly safe. This exceptional safety is ascribed to the strong covalent bond between the oxygen and the P5+ ion, which is able to prevent O2 release from the crystal lattice under abusive conditions. Lithium metal oxides such as LiCoO2 used as cathode materials in commercial LIBs can release O2 at elevated temperatures (under abusive condition) and occasionally initiate perilous conditions such as fire and explosion of the LIBs [2, 3]. Among olivine-type LiMPO4, LiMnPO4 (LMP) has garnered a great deal of attention due to its high redox potential of 4.1 V vs. Li/Li+, which is compatible with common electrolytes and is capable of delivering higher energy than the commercial LIBs utilizing LFP (3.5 V vs. Li/Li+). However, LMP suffers from excessively poor electrochemical activity due to its low ionic, electronic conductivity (< 10-10 S cm-1), which is much smaller than LFP (1.8 x 10-8 S cm-1), and the large lattice misfit between LMP and MnPO4 phases arising from the JahnTeller deformation [4, 5]. Hence, many research groups have been aggressively working on enhancing the electrochemical activity by utilizing the same techniques found effective in improving LFP, which include minimization of particle size to reduce Li+ and electron diffusion path lengths, carbon coating and Mn-site substitutio
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