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

Since the reversible deintercalation of lithium ions from lithium iron phosphate (LFP) was proved by Padhi et al. in 1997 [1], this compound is discussed to be one of the most promising lithium transition metal olivine structure for application in lithium ion batteries (LIB) as cathode material. Compared to the currently used Li(Co1/3Mn1/3Ni1/3)O2 [2], LFP combines several benefits like low toxicity, low costs, small volume change during charge/discharge cycles, high thermal stability and a relatively high theoretical specific capacity of 170 mAh/g. Furthermore, batteries which contain LFP as electrode material provide 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. In general, the lithiation of FP starts with the formation of a homogeneous solid solution phase until the saturated phase separates into a lithium rich (α-phase) and a lithium poor phase (β-phase). The width of the miscibility gap depends on temperature [3] as well as on the primary 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, that the formation of meta stable solid solution phases depends significantly on the applied charging rate.

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 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 at the same time. In order to lithiate the dispersed FP powder, lithium iodide dissolved in acetonitrile was deployed as the reducing agent as well as a Li+-source. 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. Supplementary a simple model taking into account the influence of the particle size on the miscibility gap width is included.

In summary, ITC represents a new promising research tool for studying redox reaction induced phase transitions of lithium intercalation compounds e.g. lithium iron phosphate. It offers the opportunity to determine the enthalpy of mixing of the lithium rich and lithium poor 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 and intercalation mechanisms.

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