Heterogeneous intergrowth xLi1.5Ni0.25Mn0.75O2.5·(1 − x)Li0.5Ni0.25Mn0.75O2 (0 ≤ x ≤ 1) composites: synergistic effect on electrochemical performance
A series of xLi1.5Ni0.25Mn0.75O2.5·(1 − x)Li0.5Ni0.25Mn0.75O2 (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 xLi1.5Ni0.25Mn0.75O2.5·(1 − x)Li0.5Ni0.25Mn0.75O2 (0.1 ≤ x ≤ 0.9) exhibit better cycling stability than that of pristine spinel Li0.5Ni0.25Mn0.75O2 (x = 0) and layered Li1.5Ni0.25Mn0.75O2.5 (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, Li0.5Ni0.25Mn0.75O2 and Li1.5Ni0.25Mn0.75O2.5 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 0.8Li1.5Ni0.25Mn0.75O2.5·0.2Li0.5Ni0.25Mn0.75O2 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.