Studies of Mg-substituted Li4-xMgxTi5O12 spinel electrodes (0 <= x <= 1) for lithium batteries

Magnesium-substituted Li4-xMgxTi5O12 spinel electrodes (0 < x 1) have been investigated as insertion electrodes for lithium batteries. The substitution of divalent Mg ions for monovalent Li ions in the structure necessitates that thr difference in charge must be compensated by a reduction of an equivalent number of Ti cations from Ti4+ to Ti3+. The substitution increases the conductivity of the [Ti5/3Li1/3]O-4 spinel framework by many orders of magnitude, from sigma < 10(-13) S cm(-1) for insulating Li4Ti5O12 (x = 0), in which all the titanium ions are tetravalent, to = 10(-13) S cm(-1) for Li3MgTi5O12 (x = 1.0), in which the average titanium oxidation state is 3.8. The improved conductivity decreases the area specific impedance of Li/Li4-xMgxTi5O12 cells and increases the rate capability of electrodes for small x, typically x = 0.25. The rechargeable capacity of Li4-xMgxTi5O12 electrodes, particularly those with x close to 1 (130 mAh/g), is inferior to that of unsubstituted Li4Ti5O12 electrodes (s = 0, 150 mAh/g): the smaller capacity is attributed to the partial occupation of tetrahedral (8a) sites by Mg ions in the spinel structure. (C) 2000 The Electrochemical Society. S0013-4651(00)05-034-5. All rights reserved.