Sub-micro spinel LiNi0.5-xMn1.5+xO4 (X < 0.1) cathode materials powder was successfully synthesized by the ultrasonic-assisted co-precipitation (UACP) method. The structure and electrochemical performance of this as-prepared powder were characterized by powder XRD, SEM, XPS, CV and the galvanostatic charge-discharge test in detail. XRD shows that there is a small LiyNi1-yO impurity peak placed close to the (4 0 0) line of the spinel LiNi0.5-xMn1.5+xO4 and the powders are well crystallized. XPS exhibits that the Mn oxidation state is between +3 and +4, and Ni oxidation state is +2 in LiNi0.5-xMn1.5+xO4. SEM shows that the prepared powders (UACP) have the uniform and narrow size distribution which is less than 200 nm. Galvanostatic charge-discharge test indicates that the initial discharge capacities for the LiNi0.5-xMn1.5+xO4 (UACP) at C/3, 1C and 2C, are 130.2, 119.0 and 110mAh g(-1), respectively. After 100 cycles, their capacity retentions are 99.8%, 88.2%, and 73.5%, respectively. LiNi0.5-xMn1.5+xO4 (UACP) at C/3 discharge rate exhibits superior capacity retention upon cycling, and it also shows well high current discharge performance. CV curve implies that LiNi0.5-xMn1.5+xO4 (x < 0.1) spinel synthesized by ultrasonic-assisted co-precipitation method has both reversibility and cycle capability because of the ultrasonic-catalysis. (c) 2007 Elsevier B.V. All rights reserved.