Si with x =0.000, 0.015, 0.030, and 0.050 is successfully doped into layered LiNi0.5Mn0.5-xSixO2 using a sol − gel method. Si doping results in a decrease in lattice constant, Li/Ni mixing, stress, and size of primary particles. Si is distributed at the Mn site probably nonuniformly to cause two different states of Si4+. Si doping exerts no influence on the 2p bands of Ni2+, and however gives rise to a slight splitting of 2p3/2 band of Mn4+, and tends to increase the diffusion activation energy of Li+. The optimum performance is achieved for LiNi0.5Mn0.5-xSixO2 with x =0.015 which has the highest BET surface area. Detailed analysis of structural modifications allows one to conclude that the remarkable alleviation of agglomeration of primary particles other than the optimization of structure related parameters such as the lattice constant, metal − oxygen bond length, ordering, cation mixing, size, and stress is responsible predominantly for the performance improvement. This study suggests that silicon is not an effective dopant for performance improvement of layered LiMO2 simply from the perspective of structural modifications and optimization.