Stabilization of xLi2MnO3•(1-x)LiMO2 electrode surfaces (M = Mn, Ni, Co) with mildly acidic, fluorinated solutions

It has been demonstrated previously that Li2MnO3-stabilized LiMO2 electrodes [xLi(2)MnO(3)center dot(1 - x)LiMO2, M = Mn, Ni, Co] can provide anomalously high electrochemical capacities (similar to 250 mAh/g) if charged to high potentials (>4.6 V). High- voltage charging results in an irreversible capacity loss on the initial charge/discharge cycle; it also damages the electrode surface, leading to a high cell impedance. In this paper, we report that preconditioning 0.1Li(2)MnO(3)center dot 0.9LiMn(0.256)Ni(0.372)Co(0.372)O(2) electrode powders [alternatively Li-1.048(Mn0.333Ni0.333Co0.333)(0.952)O-2 in Li1+xM1-xO2 notation] with extremely mild acidic solutions of NH4PF6, (NH4)(3)AlF6, and NH4BF4 salts in water and methanol (pH 6-6.5) leads to remarkable cycling stability of both lithium half- cells and full lithium-ion cells when repeatedly charged to high voltages (>= 4.5 V). The enhanced electrochemical performance is attributed to stabilized electrode surfaces that are etched and passivated by fluorinated species. The low- temperature behavior of unconditioned and preconditioned electrodes is presented. (c) 2008 The Electrochemical Society.