Synthesis and Processing of High Capacity, High Cycle life and High Discharge Rate Defective Manganospinel films for Rec
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Synthesis and Processing of High Capacity, High Cycle life and High Discharge Rate Defective Manganospinel films for Rechargeable batteries Deepika Singh*, Heinrich Hofmann*, Won-Seok Kim†, Valentin Craciun†, Rajiv K. Singh† *Powder Technology Laboratory (LTP), Department of Materials Science, Swiss Federal Institute of Technology (EPFL), CH 1015 Lausanne, Switzerland. †Department of Materials Science and Engineering, University of Florida, Gainesville, Florida 32611, USA ABSTRACT Significant effort to develop a robust rechargeable battery has been put in the past two decades. The efforts were mainly focused on developing rechargeable battery systems which exhibit high capacity, long cycle life and high discharge rate capabilities. LiMn2O4 based cathodes have been researched extensively as they are not only economical but also environmentally desirable. Research includes composition and doping variation, formation of novel phases and microstructural tailoring, but none of the material modifications have successfully satisfied all the above mentioned performance criteria. In this paper we show a correlation between processing parameters, microstructure and electrochemical performance of Li-Mn-O cathode films. In addition we discuss the formation of metastable oxygen-rich lithium manganospinels, using a unique ultraviolet assisted deposition process. These defective films exhibit high capacity (> 230 mAh/gm), long cycle life (less than 0.05 % capacity loss per cycle for the first 700 cycles), and high discharge rates (> 25 C for 25 % capacity loss). The long cycle life and high capacity was attributed to the ability to cycle the Mn+ valence to less than 3.5 without onset of Jahn-Teller structural transformation, while the high discharge rate was attributed to the extremely high diffusivity of Li+ in the defective Li1-δMn2-2δO4 phase. INTRODUCTION Efforts have been focused on replacing the conventional LiCoO2 positive electrodes with cheaper, safer and environmentally friendly materials such as LiMn2O4 and related compounds17 . In LiMn2O4 phase, the extraction of a Li+ ion from the tetrahedral sites takes place in two closely spaced steps at approximately 3.9 ~ 4.2 V vs. Li / Li+ (LiMn2O4 → Mn2O4 (λ-MnO2)), whereas the insertion of a Li+ ion into the octahedral sites occurs at approximately 3 V vs Li / Li+ (LiMn2O4 → Li2Mn2O4). The insertion of lithium into LiMn2O4 is naturally accomplished by a reduction of the average oxidation state of manganese from 3.5 to 3. The presence of more than 50 % of Jahn-Teller ions (Mn3+) in the host structure introduces a cubic to tetragonal distortion (from c/a = 1 to c/a = 1.16), which upon repeated cycles is believed to deteriorate the electrical contact and decrease the capacity of the cathode8, 9 . Thus, the maximum usable capacity of LiMn2O4 is limited to 0.5 Li atom per Mn atom, which translates to the maximum useable capacities of 120 ~ 140 mAh/gm. The cycle life (defined by 75% reduction in capacity) is typically in the range of 200 ~ 400 cycles, whereas the maximum di
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