In this study, a one-pot method of polymer-mediating process is proposed to synthesize high voltage spinels (LiNi1/2Mn3/2O4) with tunable particle size and high-crystallization. The primary particle size can be controlled from nanometer to micrometer via changing different type of polymer additives. Relationships between particle sizes and electrochemical performances at wide temperature ranges (−20 °C–55 °C) are investigated. Nano-sized spinel delivers a good rate capability and higher capacitance at both room and low temperatures, while the micro-sized spinel performs better in capacity retention, especially at elevated temperature. The correlations among the particle size, temperature durability and cycling stability are demonstrated. Besides, the influence of polymer-mediation on the morphology transformation of spinel is also probed and discussed.