The Effects of Ru⁴⁺ Doping on LiNi_0.5Mn_1.5O₄ with Two Crystal Structures

Doping of Ru has been used to enhance the performance of LiNi_0.5Mn_1.5O₄ cathode materials. However, the effects of Ru doping on the two types of LiNi_0.5Mn_1.5O₄ are rarely studied. In this study, Ru⁴⁺ with a stoichiometric ratio of 0.05 is introduced into LiNi_0.5Mn_1.5O₄ with different space groups (Fd3¯m, P4₃32). The influence of Ru doping on the properties of LiNi_0.5Mn_1.5O₄ (Fd3¯m, P4₃32) is comprehensively studied using multiple techniques such as XRD, Raman, and SEM methods. Electrochemical tests show that Ru⁴⁺-doped LiNi_0.5Mn_1.5O₄ (P4₃32) delivers the optimal electrochemical performance. Its initial specific capacity reaches 132.8 mAh g^-1, and 97.7% of this is retained after 300 cycles at a 1 C rate at room temperature. Even at a rate of 10 C, the capacity of Ru⁴⁺-LiNi_0.5Mn_1.5O₄ (P4₃32) is still 100.7 mAh g^-1. Raman spectroscopy shows that the Ni/Mn arrangement of Ru⁴⁺-LiNi_0.5Mn_1.5O₄ (Fd3¯m) is not significantly affected by Ru⁴⁺ doping. However, LiNi_0.5Mn_1.5O₄ (P4₃32) is transformed to semi-ordered LiNi_0.5Mn_1.5O₄ after the incorporation of Ru⁴⁺. Ru⁴⁺ doping hinders the ordering process of Ni/Mn during the heat treatment process, to an extent.