Thermodynamic Investigations and Modeling of Co 3 O 4 -Based Conversion Anode Materials

Conversion-type electrodes based on transition metal oxides are promising anode materials for the next-generation lithium-ion batteries (LIB) due to their high theoretic specific capacities, compared to the commercially used graphite anodes (372 mAh/g). Cobalt oxide based anodes exhibit theoretic specific capacities of up to 890 mAh/g (Co 3 O 4 ). However, conversion materials need to overcome several drawbacks, including significant capacity losses after the first discharge, relatively low cycling stabilities, and a large potential hysteresis between charge and discharge. According to Larcher et al. [1] , the charge / discharge reactions at very low current densities can be written as: 10 Li + + Co 3 O 4 + 10 e - ⇌ Li 2 O + 3 CoO + 8 Li + + 8 e - ⇌ 3 Co + 4 Li 2 O. (1) For higher discharge current densities, the formation of the Li x Co 3 O 4 metastable intermediate phase, was reported [1] . x Li + + Co 3 O 4 + x e - → Li x Co 3 O 4 . (2) Since the intermediate reactions taking place during the conversion mechanism are only partially understood, a combined experimental, modelling and simulation approach can be used to clarify the thermodynamics of the first discharge of Co 3 O 4 -based conversion anodes. Phase formation and open circuit voltages during lithiation / delithiation can be calculated using CALPHAD-based models and simulations of the multi-component systems. However, reliable thermodynamic and phase diagram data are essential pre-requisites to be able to model the Gibbs free energies of the phases for generation of self-consistent thermodynamic descriptions. Therefore, key thermochemical and phase diagram investigations were performed in this work to clarify inconsistencies in the existing literature data. Further, electrochemical tests were conducted to validate the results of the simulations as well as to generate new data for the thermodynamic modelling. First, the enthalpy of reduction of Co 3 O 4 to CoO was determined using high-temperature oxide melt and transposed temperature drop calorimetry. This reaction is of particular importance because it is an intermediate step in the conversion reaction (see equation (1)). Secondly, since reliable heat capacity data are needed to extrapolate the thermodynamic descriptions to temperatures relevant for battery applications, the heat capacity of Co 3 O 4 was measured in the temperature range of 200 to 1150 K using differential scanning calorimetry. The experimental data were then compared to calcul