Thermodynamic properties study of polyoxometalate Na₇[H₂PV₁₄O₄₂]

Lithium-ion batteries (LIBs) are widely used in electronic applications because of their high voltage, high specific energy, long lifespan, and other characteristics. Electrode materials have garnered interest as an indispensable component of LIBs. Na₇[H₂PV₁₄O₄₂] is used as an electrode material because of its excellent properties. In this study, Na₇[H₂PV₁₄O₄₂] was synthesized by the water bath method using NaVO₃ as the raw material, experimentally characterized, and its thermodynamic data were measured using the Neumann-Kopp rule from 298.15 to 843 K, a Physical Property Measurement System from 15 to 309 K, and the MHTC 96 line from 473 to 773 K. The data were fitted to the Debye-Einstein heat capacity equation at low temperatures and a polynomial function at higher temperatures. The heat capacity equation of Na₇[H₂PV₁₄O₄₂] was obtained from the fitted curves. The corresponding enthalpy (▵_298.15 ^TH_m), entropy (▵_298.15 ^TS_m), and Gibbs energy (▵_298.15 ^TG_m) (from 298.15 to 800 K) were calculated according to the heat capacity equation. The obtained heat capacity of Na₇[H₂PV₁₄O₄₂], as a function of temperature, was modeled as Cp = 1502.30 + 0.27T - 2.44E7T^-2 J mol^-1 K^-1 (473-773 K). This study can compensate for the thermodynamic deficiency of Na₇[H₂PV₁₄O₄₂].