The synthesis of Li- and Mn- rich cathode materials with oxalate co-precipitation method was comprehensively investigated. Thermodynamic calculation arid advanced characterizations were conducted to study the effects of reaction pH and precipitation agent on the morphology and composition of the precipitates. It was found that manganese oxalate has a higher solubility compared to nickel oxalate, especially at low reaction pH. In addition, the different capability of Ni2+ and Mn2+ to form transition metal ammonia complexes led to a composition discrepancy in the precipitation. The desired nickel manganese oxalate precursor composition could only be achieved when these factors were considered, and cathode materials synthesized based on this precursor exhibited satisfactory electrochemical performance (about 220 mAh . g(-1) over 70 cycles under C/10 rate). (C) 2013 The Electrochemical Society.