Modifying Li@Mn6 Superstructure Units by Al Substitution to Enhance the Long-Cycle Performance of Co-Free Li-Rich Cathode

As one of the most promising cathodes for Li-ion batteries, Li-rich layered oxides suffer from low Coulombic efficiency, severe capacity fading, and voltage decay, which are related to the aggregated Li@Mn-6 superstructure units. Herein, a Co-free Li-rich oxide Li[Li1/4Mn1/2Ni1/6Al1/12]O-2 through Al substitution of Co in Li[Li1/4Mn1/2Ni1/6Co1/12]O-2, is designed. Combining the average structural refinement with the detailed local structural/chemical analysis, it is found that the introduced Al ions occupy the Mn sites in Li@Mn-6 superstructure units, which further induces the partial replacement of the central Li ions in Li@Mn-6 units by Ni2+. The modified superstructure units stabilize the anionic framework and suppress structural degradation during long-term cycling. A superior cyclability (a capacity retention of 91.4% after 500 cycles at 1 C) is achieved. This work not only deepens the understanding into the mechanism of Al substitution, but also provides a novel route to design high-performance Li-rich cathodes by modifying the local functional units.