Heterogeneous intergrowth xLi_1.5Ni_0.25Mn_0.75O_2.5·(1 - X)Li_0.5Ni_0.25Mn_0.75O₂ (0 ≤ x ≤ 1) composites: Synergistic effect on electrochemical performance

A series of xLi_1.5Ni_0.25Mn_0.75O_2.5·(1 - x)Li_0.5Ni_0.25Mn_0.75O₂ (0 ≤ x ≤ 1) cathode materials have been synthesized. These compounds exhibit dramatic differences in structure, morphology and charge/discharge characteristics. As the x increases, the morphology shows an amazing trend: starting with an octahedral shape (x = 0), transforming to an octahedral/plate shape (0.1 ≤ x ≤ 0.9) in which both the spinel phase and the layered phase can be indexed in the XRD patterns, and ending up with a plate shape (x = 1.0). The particular layered-spinel composites xLi_1.5Ni_0.25Mn_0.75O_2.5·(1 - x)Li_0.5Ni_0.25Mn_0.75O₂ (0.1 ≤ x ≤ 0.9) exhibit better cycling stability than that of pristine spinel Li_0.5Ni_0.25Mn_0.75O₂ (x = 0) and layered Li_1.5Ni_0.25Mn_0.75O_2.5 (x = 1.0) materials. This improved cycling performance of these layered-spinel composites can be ascribed to the heterogeneous intergrowth of some layered phases and spinel phases in the parent structure as detected by TEM. Among these materials, Li_0.5Ni_0.25Mn_0.75O₂ and Li_1.5Ni_0.25Mn_0.75O_2.5 barely deliver the specific capacities of 90 mA h g^-1 and 117 mA h g^-1 at 5 C and show the capacity retentions of about 83% and 86% at 0.2 C after 50 cycles, respectively, while the layered-spinel 0.8Li_1.5Ni_0.25Mn_0.75O_2.5·0.2Li_0.5Ni_0.25Mn_0.75O₂ cathode shows the best rate capability of 162 mA h g^-1 at 5 C and the best cycling stability of 98% after 50 cycles at 0.2 C.