A LiNi0.5Mn1.5O4 material for use as a 5 V spinel cathode for lithium-ion batteries was synthesized for the first time using a hydrothermal approach. The effects of the LiOH concentration and hydrothermal temperature on the hydrothermal products were investigated systematically, where the LiOH concentration (e.g., 1.1 M) was proven critical to successfully synthesize the 5 V spinel. At low LiOH concentrations (e.g., 0.8-1.0 M), the obtained products may be MnOOH, alpha-MnO2, and 4 V spinel, while high concentrations (e.g., > 1.8 M) favor the Li2MnO3 phase. The particle size of the as-prepared 5 V spinel is ca. 30 nm, and its formation is associated with the dissolution-recrystallization mechanism from an amorphous precursor. Cell tests show that the as-prepared LiNi0.5Mn1.5O4 material obtained at 180 degrees C exhibits a capacity of ca. 100 mAh g(-1) at a current density of 28 mA g(-1), where ca. 60 mAh g(-1) above 4.2 V. Such a 5 V material shows an excellent rate capability with a capacity of ca. 73 mAh g(-1) delivered at 2800 mA g(-1), 73% of that obtained at 28 mA g(-1). (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3521292] All rights reserved.