Relationship between the crystal structures of LiMn_1.5Ni_0.5O₄ and LiMn_1.5Ni_0.45Fe_0.05O₄ and their internal resistances as cathode materials for lithium ion batteries

It has been reported that the partial substitution of Fe for Ni in LiMn_1.5Ni_0.5O₄ improves the rate capability of batteries wherein it is used as a cathode material. To understand the mechanism of this improvement, LiMn_1.5Ni_0.5O₄ and LiMn_1.5Ni_0.45Fe_0.05O₄ samples were prepared to compare their crystal structures and the internal resistances of batteries fabricated using them. Electrochemical measurements showed that resistance of LiMn_1.5Ni_0.45Fe_0.05O₄ to the diffusion of Li⁺ ions into bulk is lower than that for LiMn_1.5Ni_0.5O₄. The crystal structures of LiMn_1.5Ni_0.5O₄ and LiMn_1.5Ni_0.45Fe_0.05O₄ were examined by neutron diffraction and transmission electron microscopy. The mass percentage of the ordered P4₃32 phase in LiMn_1.5Ni_0.45Fe_0.05O₄ was found to be smaller than that in LiMn_1.5Ni_0.5O₄, and the coexistence of an ordered P4₃32 phase and a disordered Fd3 ¯ m phase leads to the formation of boundaries in the primary particles of the samples. From these results, it was proposed that the reason for the internal resistance improvement was that the boundaries between the P4₃32 and Fd3 ¯ m phase impeded the diffusion of Li⁺ ions. Therefore, LiMn_1.5Ni_0.45Fe_0.05O₄ exhibited lower internal resistance because it contained a lower amount of P4₃32 phase and consequently, a lower occurrence of boundaries.