Spinel cathode LiNi0.5Mn1.5O4 (LNMO) has attracted high interest owing to its high-energy-density, cobalt-free, and high-voltage operation. For further application, the solid-state method as the simpler parameter control is favorable for scalable synthesis of LNMO cathode materials. However, the solid-state method has the disadvantage of producing irregular particle shapes that form the non-uniform cathode-electrolyte interphase, leading to the low cycle stability of the batteries. Here, we investigate the effective synthesis route of LNMO cathode materials via a solid-state method that can produce a regular truncated octahedral shape. The route of adding a Li-source (i.e., LiOH) to the transition metal in the oxide phase after the first heat treatment is proven to effectively produce a regular truncated octahedral shape that affects high specific capacity, long-term stability, and high-rate performance. The LNMO produced via this route synthesis showed a specific capacity of 128.53 mA h/g at a current density of 0.1 C. After 250 cycles, the truncated octahedral shape LNMO still possessed excellent cycling performance, 90.32% 3–4.8 V at 0.5 C. These results showed that this route is an effective way for the scalable and easy solid-state synthesis of the truncated octahedral shape of high-performance spinel LNMO cathode materials.