We studied the phase stability of P4₃32-type LiNi_0.5Mn_1.5O_4-d phase through the substitution of transition-metals by using ab initio DFT calculations as well as flux growth of their single crystals. Computational studies reveal that small quantities of Cu²⁺ substitution into Mn²⁺ site stabilizes LiNi_0.5Mn_1.5O_4-d  crystalized into P4₃32 lattice as compared to that of Fd-3m. Furthermore, Cu²⁺ substitution enhances its electronic conductivity through the d-electron doping effect and reduces activation energy for the Li⁺ transfer. As predict computationally, flux grown LiNi_0.49Cu_0.01Mn_1.49O_3.62 single crystalline particle-based composite electrodes show flat plateau in their charge-discharge curves at around 4.7V (vs Li⁺/Li) arose from the Ni²⁺/Ni⁴⁺ redox couples, and showed higher specific capacity of ca. 140 mAh/g under operation with current density of 0.2 C. And cyclic voltammetry, Raman spectroscopy, TEM coupled with SAED, and Rietvelt confinement reveals the LiNi_0.49Cu_0.01Mn_1.49O_3.62 particles are crystallized into highly ordered P4₃32 lattice. Interestingly, C rate and high voltage capability are also enhanced significantly with no any assistance of additives. More than 70% of original discharge capacity is retained under operation with current density of 10C. And 96 % of original discharge capacity is retained after 100 cycle charge-discharge test with cut-off voltage of 4.8-3.5V.