Effects of methanesulfonic acid (MSA) and aminomethylsulfonic acid (AMSA) as additives for positive electrolyte on thermal stability and electrochemical performance are investigated. Both additives can improve the thermal stability of V(V) electrolyte, and AMSA has better effect, especially. The electrochemical results show that V(IV)/V(V) exhibits superior electrochemical activity and reversibility with additives, and the diffusion coefficient of V(IV) species, exchange current density and reaction rate constant become larger with additives in positive electrolyte. Among the two additives, AMSA has better effect for improvement of electrochemical activity and kinetics. The cell using positive electrolyte with additive of AMSA was assembled and the charge-discharge performance was evaluated. The assembled cell using AMSA as positive electrolyte additive shows good cycling performance, with higher energy efficiency (81.5%) and larger discharge capacity retention (40 cycles: 82.7%). The improved electrochemical performance may be ascribed to more active sites provided by -NH2 group and the enhanced hydrophilicity of the electrode provided by -NH2 and -SO3H groups. (C) 2013 Published by Elsevier Ltd.