Thermodynamics of LiFePO₄ Solid-Phase Synthesis Using Iron(II) Oxalate and Ammonium Dihydrophosphate as Precursors

Abstract

A detailed thermodynamic analysis and an experimental study of the thermolysis mechanism of Li2CO3 + NH4H2PO4 + FeC2O4 blends were performed from the viewpoint of the usage of these compounds for the LiFePO4 solid-phase synthesis. Thermodynamic calculations of the assumed chemical reactions have been done within a practically important (for LiFePO4 synthesis) temperature range of (25 to 900) °C. The thermodynamic parameters of the related compounds (Li2O, Li3PO4, (NH4PO3)4, NaFePO4, FePO4 2H2O, FePO4, and LiFePO4) were determined more precisely than earlier. The temperature dependences of changes in the standard Gibbs energy, standard enthalpy, standard entropy, and standard molar heat capacity for the chemical reactions in LiFePO4 synthesis were calculated. Various paths of oxalate decomposition may well proceed concurrently with the predomination of this or that path under slight changes in the experimental conditions. The formation of orthorhombic lithium orthophosphate Li3PO4 was detected just in a blend grinded at room temperature. Heating up to 360 °C results in full destruction of the reaction mixture; Li3PO4 in its activated state is a crystal component at this synthesis stage. Lithium orthophosphate structurally belonging to the same spatial Pnma group as the target product LiFePO4 is the basis of its synthesis.

Heat capacity (C) and entropy of olivine-type LiFePO4 in the temperature range (2 to 773) K

“…The molar standard entropy of LFP at 298.15 K has been determined to be 130.95 J Á K À1 Á mol À1 , which is about 6 J Á K À1 Á mol À1 lower than the value calculated from the oxides [5] and 7 J Á K À1 -Á mol À1 higher than the standard entropy reported by Shang et al based on the DFT phonon calculations. The lower value from the DFT calculation can be explained by the missing magnetic entropy contribution whereas the difference of our value to the standard entropy derived from the oxides may be understood in terms of the crystallite size effect on the magnetic entropy, as discussed above, and an overestimation of the oxide entropy contribution.…”

“…Particularly, reliable experimental heat capacity data reaching from low to high temperatures and entropies are missing. To the best of our knowledge, there is only one source of experimental heat capacity data covering the temperature range from (150 to 773) K ([4] also cited by [5]). Unfortunately, the accuracy of these data is not specified.…”

Facile Synthesis and Electrochemical Investigation of Li9V3(P2O7)3(PO4)2as High Voltage Cathode for Li-Ion Batteries

“…Similar reaction pathway has been reported for LiFePO 4 . [38][39][40] In the temperature range between 300 and 700…”

E-pH Diagrams for the Li-Fe-P-H2O System from 298 to 473 K: Thermodynamic Analysis and Application to the Wet Chemical Processes of the LiFePO4 Cathode Material

“…Through the in-depth researches on LFP in recent years, thermodynamic parameters of LFP (e.g., heat capacity, entropy, and Gibbs free energy of formation) have been obtained. , In this work, a series of E-pH diagrams for the Li-Fe-P-H 2 O system were plotted at room and high temperatures (from 298.15 to 473.15 K). These E-pH diagrams provide a new way to explain the current processes of LFP’s hydrothermal synthesis and hydrometallurgical recovery.…”