THE EFFECT OF MIXED Mn VALENCES ON Li MIGRATION IN LiMn₂O₄:

MOLECULAR DYNAMICS SIMULATIONS

Kenji Tateishi, Douglas du Boulay, and Nobuo Ishizawa

Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta Midori Yokohama 226-8503, Japan (tateishi@r3401.msl.titech.ac.jp)

Lithium manganese spinels are attractive candidates for cathode materials of rechargeable lithium ion batteries and have been studied extensively. LiMn₂O₄ presents a first order structural phase transition at 290 K. Rodríquez-Carvajal et al [1] solved the structure at 230 K and concluded that the transition results from partial charge ordering on the Mn sites. Verhoeven et al [2] showed that lithium ions exist at both 8a and 16c interstices of the Fd-3m structure and that lithium exchange between those sites begins at around 285K and correlates with the Mn charge ordering. In our previous x-ray study, it was revealed that lithium and oxygen atoms are distributed statistically about their ideal positions at room temperature, an effect which we attributed primarily to mixed Mn valences.

Here, molecular dynamics simulations were undertaken to study the correlation between local structural disorder and lithium ion migration in stoichiometric LiMn₂O₄. The displacements of oxygen atoms from ideal positions were strongly governed by the arrangement of neighboring Mn³⁺ and Mn⁴⁺ valences. Consequently, changes of Mn valences with respect to time directly involve time-dependent LiO₄ tetrahedral distortions. Simulations were made assuming two distinct models: a) a model in which the arrangement of Mn valences do not change with respect to time and, b) a model in which the arrangement of Mn valences change in time keeping but preserving overall charge neutrality. In the model a), although lithium atoms occupy both 8a and 16c interstices, continuous lithium ion migrations were not observed. On the other hand using model b), continuous lithium migrations were observed. If we adopt the charge ordering viewpoint, then these results are quite consistent with Verhoeven’s report because the lithium exchange commenced near the reported phase transition temperature. That is, we have strong evidence that lithium migration is indirectly associated with the hopping of 3d electron between Mn³⁺ and Mn⁴⁺ ions via the displacements of the coordinated oxygen atoms.

References

1 Rodríquez-Carvajal, J., et al (1998) Physical Review Letters 81, 4660-4663.

2 Verhoeven, V. W. J., et al (2001) Physical Review Letters 86, 4314-4317.