Solid State Synthesis and Properties of Doped LiMnO 2 Cathode Materials
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cathode materials for rechargeable lithium-ion batteries are well known. Manganese oxides offer lower cost and toxicity, and appear to be safer on overcharge. However, the lithiated manganese oxides which have been investigated to date have also shown limitations which have hindered their applications in commercial cells. The spinel LiMn 20 4 has been extensively studied for its 4 V discharge vs. Li/Li+, but its reversible capacity is lower than LiCoO 2 and LiNiO 2, and a rapid capacity fade occurs at elevated temperatures above 50 TC [3]. The trivalent manganese compound LiMnO 2 offers a theoretical capacity twice that of LiMn20 4, particularly in a Li-ion cell where the only source of Li is the metal oxide. However early reports indicated that LiMnO 2, which crystallises with an orthorhombic structure (o-LiMnO 2), showed poor electrochemical properties [4]. Novel synthetic routes to prepare o-LiMnO 2 compounds at low temperature have produced materials with initial charge capacities of up to 230 mAh/g, but subsequent charge/discharge cycles showed lower capacities and rapid capacity fade [5-7]. More recent reports have described preparations of o-LiMnO 2 at high temperature which show specific capacities of ca. 140 mAh/g and improved cyclability [8]. However the o-LiMnO 2 phases are unstable to delithiation and transform irreversibly into spinel-like materials upon electrochemical cycling. o-LiMnO 2 also shows a capacity fade similar to spinel LiMn 20 4 at elevated temperatures [2]. Although LiMnO2 generally crystallises as the orthorhombic phase, layered LiMnO 2 with an a-NaFeO2-related structure has been prepared by soft chemistry methods [9-11]. The 49 Mat. Res. Soc. Symp. Proc. Vol. 575 © 2000 Materials Research Society
material has a structure analogous to LiCoO 2 and LiNiO 2, but shows a monoclinic distortion due to cooperative ordering of Jahn-Teller distorted MnO 6 octahedra, and does not have the same thermodynamic stability as the Co and Ni compounds [10]. The wet chemistry methods of synthesis are difficult to implement at a large scale and, like o-LiMnO 2, the monoclinic material (m-LiMnO 2) transforms to a spinel-related phase during electrochemical cycling [12]. Recently, Chiang et al. [1] reported that Al-doped m-LiMnO 2 may be crystallised at high temperature (ca. 950 QC)from a precursor powder prepared by atomising and freezedrying a suspension of co-precipitated Al and Mn hydroxide in LiOH solution. The Al-doped compounds show relatively high reversible capacities and reduced capacity fade at elevated temperatures [2]. The present paper describes LiAlIMn1 _,O2 materials with 0 < x < 0.1 prepared by an alternative solid state route developed at Pacific Lithium Limited. The compounds show similar properties to those prepared by the precipitated hydroxide method. The effects of Al on crystal structure and microstructure,
and on the electrochemical
properties of the materials, are discussed in detail. EXPERIMENT LiA1_Mn1 -xO2 (0 _< x _•0.1) materials prepared by solid state reaction were supplie
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