Electrochemical and structural study of the 3.3 V reduction step in defective LixMn2O4 and LiMn2O(4-y)Fy compounds

Non-stoichiometric or fluorine-substituted LixMn2O4 are known to present a reversible redox step at 4.5 V as well as a reduction step at 3.3 V appeari ng to the expense of the usual 4.0-4.15 V. We present results of very slow stepwise potentiodynamic studies which allowed to localise the oxidation st ep associated to the 3.3 V reduction level close to 3.95 V. We propose the cubic spinel lattice parameter value as the key parameter for knowing a pri ori if these redox states will be present or not, with 8.215 Angstrom as th reshold value. In situ XRD studies performed across the 3.3 < - > 3.95 V re dox step indicate that these additional states are probably associated to s tructural features. (C) 1999 Elsevier Science S.A. All rights reserved.