The particle-size effects on the thermodynamic properties and kinetic behavior of a Li(x)FePO(4) electrode have a direct influence on the electrode properties. Thus, the development of high-performance Li-ion batteries containing a Li(x)FePO(4) cathode requires a complete understanding of the reaction mechanism at the atomic/nano/meso scale. In this work, we report electrochemical calorimetric and potentiometric studies on Li(x)FePO(4) electrodes with different particle sizes and clarify the particle-size effect on the reaction mechanism based on the entropy change of (de)lithiation. Electrochemical calorimetry results show that a reduction in particle size shrinks the miscibility gap of Li(x)FePO(4) while potentiometric measurements demonstrate that the Li(x)FePO(4) particles equilibrate into either a kinetically metastable state or a thermodynamically stable state depending on the particle size.

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 Na 3 V 2 (PO 4 ) 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+2x V 2 (PO 4 ) 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+2x V 2 (PO 4 ) 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.