We examined the crystal structures of Li2(NixMn1-x)O3(-δ) (x = 0, 1/10, 1/6, and 1/4) to elucidate the relationship between the structure and electrochemical performance of the compounds using neutron and synchrotron X-ray powder diffraction analyses in combination. Our examination revealed that these crystals contain a large number of stacking faults and exhibit significant cation mixing in the transition-metal layers; the cation mixing becomes significant with an increase in the Ni concentration. Charge-discharge measurements showed that the replacement of Mn with Ni lowers the potential of the charge plateau and leads to higher charge-discharge capacities. From a topological point of view with regard to the atomic arrangement in the crystals, it is concluded that substituting Mn in Li2MnO3 with Ni promotes the formation of smooth Li percolation paths, thus increasing the number of active Li ions and improving the charge-discharge capacity.