Conversion-type electrode materials based on transition metal oxides are promising anode materials for next-generation lithium-ion batteries (LIB) due to their high theoretic specific capacities (e.g. Co3O4: 890 mAh/g) compared to extensively used graphite (372 mAh/g). The aim of this work is to combine the experimental thermodynamics with modeling and simulation to better understand the complex mechanism behind the conversion reaction of Co3O4 in the electrochemical half-cell.
The equilibrium phases and coulometric titration curves during lithiation/delithiation can be calculated using CALPHAD-based thermodynamic descriptions of the multi-component material systems. However, the development of the thermodynamic descriptions requires reliable thermodynamic and phase diagram data. Thus, in the first step of this work, key thermochemical experiments and phase diagram investigations were conducted to clarify the inconsistencies in the existing literature data.
The enthalpy of reduction of Co3O4 to CoO was determined using high-temperature oxide-melt and transposed temperature drop calorimetry. Additionally, since reliable heat capacity data are needed to extrapolate the thermodynamic descriptions to temperatures relevant for battery applications, the heat capacity of Co3O4 in the temperature range of 240 to 1150 K was measured using differential scanning calorimetry. The experimental data were then compared to calculations performed using an existing thermodynamic description of the Co-O system to assess its reliability for extrapolations to higher order systems.
The Li-Co phase diagram was investigated from quenching tests performed on several intermetallic samples. The samples were subsequently characterised using powder-XRD and ICP-OES analyses. A thermodynamic description of the Li-Co sub-system was developed based on these results and used for a thermodynamic description of the multi-component Li-Co-O material system.
Electrochemical cycling tests were also carried out in this work using half cells with coin cell geometry. Thick electrodes containing CoO and Co3O4 active materials were prepared by a tape-casting process. The results of the electrochemical testing are discussed in comparison to calculations from the achieved thermodynamic description of the Li-Co-O material system.