Nano-Crystalline Manganese Spinel Li[MxMn2-x]O4 Showing Good Cycling Properties in the 3V Range

Abstract

Manganese spinel LiMn2O4 and its derivatives are the most attractively studied cathode materials for lithium secondary batteries because of their three dimensional Li+ diffusion, low cost, abundance, and nontoxicity. In the LixMn2O4 spinel system (0<x<2), the Mn2O4 spinel framework provides a three dimensional interstitial space for lithium ion transport. Lithium extraction from LixMn2O4 (0<x<1) occurs at 4V versus lithium metal. The electrode cycles well over this range because the cubic structure (space group Fd3m) expands and contracts isotopically during lithium insertion and extraction. For 3V region, i.e. 1<x<2, as Li+ ions are inserted into the spinel structure, a two-phase reaction occurs, which is associated with the onset of an anisotropic Jahn-Teller distortion. This crystallographic distortion, which results in a 16% increase in the c/a ration of the unit cell parameters, is too severe for the electrode to maintain its structural integrity during cycling. Consequently, a LixMn2O4 spinel electrode cycles poorly over the range 1<x<2, and the cell suffers a capacity loss. In this study, nano-sized LiMxMn2-xO4 were prepared by the ultrasonic spray pyrolysis. Powder X-ray diffraction (Rigaku, Rint-200) and HRTEM were used to identify the crystalline phase. Charge-discharge cycles were performed in CR2032 coin type cells and used electrolyte was a 1:2 mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) containing 1M LiPF6 by volume. Although, all samples were tested at 3V region (cut-off 2.4-3.5V), prepared powders were shown that the high discharge capacity (over 120 mAh/g) and good cycling properties.