Potentiometric Study to Reveal Reaction Entropy Behavior of Biphasic Na_1+2xV₂(PO₄)₃ Electrodes

Sodium-ion batteries potentially provide the opportunities to realize the energy storage system beyond the stateof- the-art lithium-ion batteries, though at present their performance is limited partly due to lack of suitable positive electrode materials. NASICON-type Na₃V₂(PO₄)₃ is a promising candidate for the positive electrode materials because of its high capacity and high operating potential, however, the electrode reaction of Na_1+2xV₂(PO₄)₃ (0 : x : 1) including a biphasic region is not yet fully understood. Here, in order to clarify the microscopic mechanism of the biphasic reaction, the reaction entropy of the electrochemical cell including the Na_1+2xV₂(PO₄)₃ positive electrode was measured using the potentiometric method. The temperature-dependent open-circuitvoltage reveals that the reaction entropy is almost constant for 0.1 : x : 0.9. The constant reaction entropy of the electrochemical cell suggests that the electrode reaction proceeds through the boundary migration between the Na-rich and -poor phases without substantial change in the configurational entropy.