Factors controlling the fluorine content and the electrochemical performance of spinel oxyfluoride cathodes

An investigation of spinel LiMn2-y-zLiyMzO4-eta F eta (M=Ti and Ni, 0 <=eta <= 0.29) cathodes synthesized by firing the cation-substituted LiMn2-y-zLiyMzO4 oxide analogs with the fluorinating agent NH4HF2 at 450 degrees C reveals that the amount of fluorine that could be incorporated into the spinel lattice is limited by the Mn valence in the spinel lattice; it is difficult to incorporate fluorine as the Mn valence lowers to around 3.5+. Although fluorine substitution increases the capacity, spinel cathodes with a Mn valence of < 3.6+ encounters a larger lattice mismatch between the two cubic phases formed during the charge-discharge process and higher Mn dissolution, resulting in a degradation of the electrochemical properties. For example, LiMn1.8Li0.1Ni0.1O3.8F0.2 with an initial Mn valence of 3.61+ exhibits a combination of high capacity, excellent cyclability with graphite anode at 60 degrees C, good storage characteristics, high rate capability, and low irreversible capacity compared to LiMn1.8Li0.1Ni0.1O3.71F0.29. (c) 2007 The Electrochemical Society.