It is well-known that some surface modifiers or additives can effectively improve the electrode properties of Li-storage materials. But, some of them may be harmful to the finished battery. The coupling and the interaction between the cathode, separator, electrolyte and anode should be considered when selecting the additives. One example was presented in this work. The electrochemistry of LiFe0.4Mn0.6PO4/C+Ag composite lithium ion battery cathode has been investigated. It was found from the results that the Ag additive can effectively improve the high-rate discharge-ability of LiFe0.4Mn0.6PO4/C composite cathodes and increase the reversibility. But the further intensive investigation has revealed that Ag is not suitable to use as the additive in cathode materials if the operation voltage of cathode electrode is more than 3.7V vs. Li/Li+.