To reduce the interfacial resistance between the electrodes and electrolyte in all solid-state battery, we have recently suggested the idea of a single-phase all-solid-state battery that is made from a single material with the three roles of electrolyte, positive electrode and negative electrode. In this research, the effects of Na3BO3 addition into Na3V2(PO4)3 were investigated with respect to the electrochemical properties. Na3BO3 addition improved the relative density of Na3V2(PO4)3 by acting as a flux to fill the voids of the sintered body, as confirmed by scanning electron microscopy (SEM) observations. The electrical conductivity is increased by the addition of Na3BO3, although the activation energy is also increased. Charge/discharge measurement revealed that Na3V2(PO4)3+Na3BO3 pellets exhibit improved cyclability and optical observation indicated a different mechanism for the in-situ formation of the electrode from a Na3V2(PO4)3 pellet.