Local structural studies of LiCr_yMn_2-yO₄ cathode materials for Li-Ion batteries

The local crystal structure of a spinel-type solid solution, LiCr_yMn_2-yO₄ has been studied by molecular dynamics (MD) and extended X-ray absorption fine structure (EXAFS), respectively. The MD simulation was carried out using a partially ionic model, and the relevant potential parameters were optimized. The simulated compositional dependence of lattice parameters and thermal expansion coefficients were found to be reproducible with experimental values. From the simulation results, the pair correlation function curve for Mn⁴⁺-O in LiMn³⁺Mn⁴⁺O₄ was broader than that in LiCr³⁺Mn⁴⁺O₄. This indicates that the local distortion of the lattice is suppressed by substituting a Cr³⁺ ion for a Mn³⁺ ion. Similar results were also obtained by EXAFS measurement. The force constant k derived from MD simulation increases by substituting Cr³⁺ ion for Mn³⁺ ion, and this corresponds to the strengthening of the bond between a transition metal cation and an oxide ion.