THE STRUCTURE OF THE LITHIUM ION INSERTION SPINEL Li_1+xMn_2-xO4. Helena Berg^l, Josh Thomas^l & Erik Kelder^2, Institute of Chemistry, Uppsala University, Uppsala, Sweden¹. Dept. of Chemistry, Delft University of Technology, Delft, The Netherlands².

Various forms of the lithium insertion compound LiMn₂O₄ are currently being studied as potential cathode materials for lithium/polymer batteries. In this context, it is of the utmost interest to extend the capacity range of the material. This, in practice, means increasing the number of lithium ions which can be inserted and extracted reversibly from the material. One route has been to probe the possibility of preparing stoichiometric compounds with the general formulation Li_1+xMn_2-xO₄ for O>x>0.5, and thereafter examining their electrochemical properties. This is currently seen as a viable means of suppressing the cooperative Jahn-Teller distortion, which causes the spinel structure to pass through a cubic to tetragonal phase transition, with accompanying loss of lithium insertion capacity.

In this study, a neuton structure determination of a powder sample containing a two-phase mixture of the target compound Li_1+xMn_2-xO₄ and another impurity phase Li₂MnO₃ have been studied. The two phases were refined simultaniously in a multiphase refinement mode, resulting in the effective formulation Li_l.17Mn_l.83O₄. This involves the occupation of an octahedral site hithero not found to be occupied by lithium ions in any spinel, and is clearly related to the incidence of oxygen vacancies in the present structure. Unfortunately from a battery standpoint, the impurity phase, Li₂MnO₃, is not electrochemically active, and is therefore a troublesome source of capacity loss in any cell constructed using this mixture.

This research is supported by NFR and NUTEK in Sweden, and the EU(Joulell) Non-Nuclear Energy Programme.