Thermochemical Study of the Phase Transition Process in the LiFePO4-FePO4 System at 25°C
Authors: Thomas Christian , Hüttl Regina , Seidel Jürgen , Bohmhammel Klaus , Mertens Florian 
Published: February 2020

Since the proof of reversible deintercalation of lithium ions from lithium iron phosphate (LFP) by Padhi et al. 1997 [1], this compound is discussed to be a promising cathode material for application in lithium ion batteries (LIB). Compared to the currently used Li(Co1/3Mn1/3Ni1/3)O2
[2], LFP combines several benefits like low costs, low toxicity, small volume change, higher thermal stability and a relatively high theoretical specific capacity of 170 mAh/g. Furthermore, batteries which contain LFP as electrode material offer a constant cell potential over a broad range of the charge/discharge cycle.
The insertion of lithium into heterosite ferric phosphate (FP), also known as lithiation reaction, is a complex process. The lithiation of FP starts with the formation of a homogeneous solid solution phase until the phase separates into a lithium rich (α-phase) and a lithium poor phase (β-phase). The width of the miscibility gap is not only a function of temperature [3], but also depends on the crystallite size [4]. Based on in operando x-ray diffraction (XRD) studies of LIB’s, Wang et al.
[5] and Orisaka et al.
[6]revealed the formation of meta stable solid solution phases during fast charging.
This contribution is focused on the determination of the enthalpy of mixing of the lithium rich and poor phase for samples with different particle size distributions at room temperature. For this purpose, we applied the method of isothermal titration calorimetry (ITC) which enables us to control the composition of the dispersed solid by stepwise adding of the dissolved reactant and to measure directly the heat flux generated by the lithiation reaction. In our case lithium iodide dissolved in acetonitrile was applied as the reducing agent as well as a Li+-source in order to lithiate the dispersed FP powder. The experimental findings are completed by thermodynamic calculations with respect to the phase equilibria. The formation of the miscibility gap is simulated by means of a Redlich-Kister-approach for the excess Gibbs energy of mixing including a simple model that accounts for the influence of the particle size on the width of the miscibility gap.
In summary, ITC represents a new promising research tool for studying redox reaction induced phase transitions of lithium intercalation compounds like lithium iron phosphate. It offers the opportunity to determine the enthalpy of mixing of the α- and β-phases, which can provide important thermodynamic data for the temperature management of lithium ion batteries as well as for a better understanding of the electrode reactions.
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