In 1997, Padhi et al.  proved the chemical reversible deintercalation of lithium ions from the lithium iron phosphate (LFP) by a wet chemical and an electrochemical oxidation route. Since then, LFP is discussed to be a promising intercalation cathode material for the application in lithium ion batteries (LIB). Compared to the commercial employed intercalation compound Li(Co1/3Mn1/3Ni1/3)O2 , 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.
The insertion of lithium ions into the heterosite iron phosphate (FP), also known as lithiation reaction or discharging of LIB, is a complex process. During the lithiation of FP, at first a homogeneous lithium poor solid solution phase (β-phase) is formed. With an increasing degree of lithiation, the solubility limit is exceeded and a second, lithium rich phase (α-phase) appears. The width of this miscibility gap is known to depend on both temperature  and crystallite size .
This contribution focuses on the determination of the enthalpy of mixing in the system LiFePO4-FePO4 at room temperature for samples with significantly different particle size distributions by means of isothermal titration calorimetry (ITC). The lithiation progress is controlled by stepwise adding of the dissolved reducing agent lithium iodide into a dispersion of FP. The calorimetric results are in good accordance with thermodynamic calculations applying a Redlich-Kister-approach for the excess Gibb’s enthalpy of mixing and a simple model in order to describe the influence of the particle size onto the width of the miscibility gap.
The ITC method appears to be a new promising research tool in order to investigate redox reaction induced phase transition processes of lithium intercalation compounds like LFP. It offers the opportunity to determine the enthalpy of mixing of LFP and FP at 25 °C, which can provide new insights for a better understanding of electrode reaction of LIB.
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