High Capacity, Temperature‐Stable Lithium Aluminum Manganese Oxide Cathodes for Rechargeable Batteries

Manganese oxides are of great interest as low cost and environmentally sound intercalation cathodes for rechargeable lithium ba tteries, but have suffered from limited capacity and instability upon cycling at the moderately high temperatures (50-70°C) encountered in many applications. Here, we show that Li xAl0.05Mn0.95O2 of both the monoclinic and orthorhombic ordered rock salt structures exhibit stable cycling and high discharge capacities at elevated temperatures, after an initial transient associated with a spinel like phase t ransformation. In cells utilizing Li anodes tested at 55°C, rechargeable capacities of 150 mAh/g for the orthorhombic and 200 mAh/g for t he monoclinic phase and energy densities ~500 Wh/kg were achieved over more than 100 cycles (2.0-4.4 V). At low current densities, cha rge capacities approached the theoretical limit. The temperature stability and excellent electrochemical performance, combined with nontoxicity and low raw materials cost, make these compounds attractive cathodes for advanced lithium batteries. © 1999 The Electrochemical Society. S1099-0062(98)07-062-1. All rights reserved. Manuscript submitted July 15, 1998; revised manuscript received October 12, 1998. Available electronically January 11, 1999. Lithium-ion batteries are presently the power source of choice for portable electronics due to their reliability, safety, and high energy density on a volume or weight basis. Commercial applications of batteries based on LiCoO 2 intercalation cathodes have undergone enormous growth since 1995, and provided that performance and cost can reach accepted goals, implementation in larger scale applications such as electric vehicles is anticipated. An intensive search for new electrode materials has been driven by the need for higher energy density at lower cost. Lithium manganese spinels have been the focus of many cathode studies,1-4 in large part due to their low cost and toxicity compared to LiCoO 2 and LiNiO 2 . However, LiMn 2 O 4 spinel exhibits lower rechargeable capacity than LiCoO 2 (115-120 vs. 120-135 mAh/g),5 and rapid capacity fade at elevated temperatures in the range of 5070°C.6-9 Several recent studies have focused on metastable or disordered manganese oxides. Monoclinic LiMnO 2 (m-LiMnO 2 ) with the layered rock salt structure ( α-NaFeO 2 type) has been synthesized by ion exchange of lithium salts with NaMnO 2 10 and by hydrothermal reaction,11 but capacity is low and significant fade has been reported in room-temperature tests to date. Orthorhombic LiMnO 2 (o-LiMnO 2 ) of the ordered rock salt structure described by Hoppe et al. 12 has also been studied by several groups, 13-15 and has been prepared with varying degrees of crystallographic disorder. Increased capacity (initial values at room temperature exceeding 200 mAh/g) is seen in the more disordered forms of this material, but is also accompanied by greater capacity fade.15 As we show, the cycling performance of o-LiMnO 2 at elevated temperatures is also poor. Amorphous manganese oxyiodide16 and manganese oxide17 cathodes have recently been synthesized, in which high rechargeable capacities (260-278 mAh/g) are achievable upon insertion to lower voltages (< 2 V). The elevated temperature properties of these materials have not been reported. An attractive cathode material would meet or exceed the electrochemical performance and elevated temperature stability of LiCoO 2 , while retaining the low cost of the manganese oxides. The aluminumdoped manganese oxides of this work appear to meet this goal. We originally focused on Li x Al y Mn 1-y O 2 solid solutions in order to investigate the effect of aluminum doping on intercalation voltage, extending a study in which aluminum doping was found to increase voltage in LiCoO 2 .18 m-LiMnO 2 (C2/m) has the same layered cation ordering as LiCoO 2 (R3 _ m space group, α-NaFeO 2 structure type). In a previous paper we showed that Li x Al y Mn 1-y O 2 solid solutions can be stabilized in this structure under reducing synthesis conditions. 19 As discussed later, the solid solubility of Al in LiMnO 2 is too low for a meaningful test of voltage effects in this system. However, the monoclinic phase of composition LiAl 0.25 Mn 0.75 O 2 showed discharge capacity as high as 180 mAh/g at room temperature in short term cycling. 19 Here we