Conversion-type electrode materials based on transition metal oxides are promising anode materials for next-generation lithium-ion batteries (LIB) because of their high theoretic specific capacities. However, conversion materials generally suffer from relatively low cycling-stabilities due to the loss of inter-particle contacts which occurs as a result of the conversion reaction. Co₃O₄ is an interesting conversion material since it exhibits a much higher theoretical specific capacity (890 mAh/g) compared to extensively used graphite (372 mAh/g). In addition, mixtures of Co₃O₄ with CuO have been shown to lead to further improvements in cycling stability and specific capacity^[1]. Since the conversion reaction mechanism is still not well understood, the aim of this work is to use experimental thermodynamics combined with modeling and simulation to better understand the electrochemistry of the conversion reaction for Co₃O₄.

During electrochemical cycling of LIBs, compositional changes and phase transformations of the electrodes take place when lithium ions are added or removed according to the equation:

10 Li⁺ + Co₃O₄ + 10 e^- ⇌ Li₂O + 3 CoO + 8 Li⁺ + 8 e^- ⇌ 3 Co + 4 Li₂O.                            (1)

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 CALPHAD-based 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 performed to clarify 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. This reaction is of particular importance for conversion electrodes because it is an intermediate step in the conversion reaction (see equation (1)). 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^[2] to assess its reliability for extrapolations to higher order systems.

Since phase diagram data in the Li-Co system are limited, several intermetallic samples were prepared, heat treated at different temperatures and quenched to room temperature. The samples were subsequently investigated using SEM, powder XRD and ICP-OES. A thermodynamic description of the Li-Co sub-system was developed based on these results and used for a preliminary thermodynamic description of the multi-component Li-Co-O material system.

Calculations using the preliminary thermodynamic description were compared to experimentally determined emf and phase diagram data at 673 K from Godshall et al.^[3]. Finally, results of electrochemical cycling tests performed in this work using half cells with coin cell geometry were discussed in comparison to calculations from the thermodynamic description of the Li-Co-O material system.

[1]    D. Wadewitz, W. Gruner, M. Herklotz, M. Klose, L. Giebeler, A. Voss, J. Thomas, T. Gemming, J. Eckert, H. Ehrenberg, J. Electrochem. Soc. 2013, 160, A1333-A1339.

[2]    M. Chen, B. Hallstedt, L. J. Gauckler, J. Phase Equilib. 2003, 24, 212.

[3]    N. A. Godshall, I. D. Raistrick, R. A. Huggins, Mat. Res. Bull. 1980, 15, 561.