A novel carbonophosphate, Na 3 MnPO 4 CO 3 , was synthesized as a cathode material using a mechanical ball milling method with starting materials of MnCO 3 and Na 3 PO 4 without washing or drying. Duo to the formation of nano-size particles and good dispersion of the obtained Na 3 MnPO 4 CO 3 , the initial discharge capacity in an organic electrolyte of 1 M NaPF 6 /ethylene carbonate (EC): dimethyl carbonate (DMC) (1:1 v / v ) was 135 mAh∙g −1 and 116 mAh∙g −1 at 1/30 C and 1/10 C, respectively. We also investigated the cathode properties of Na 3 MnPO 4 CO 3 in an aqueous electrolyte of 17 m NaClO 4 . This is the first investigation of the electrochemical performance of Na 3 MnPO 4 CO 3 with aqueous electrolyte. Na 3 MnPO 4 CO 3 achieved a discharge capacity as large as 134 mAh g −1 even at a high current density of 2 mA cm −2 (0.5 C), because of the high ionic conductivity of the aqueous electrolyte of 17 m NaClO 4 .