Thermodynamic Investigation of Cobalt-Oxide Based Material Systems for Lithium Ion Batteries

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. Co 3 O 4 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 Co 3 O 4 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 Co 3 O 4 . 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 + + Co 3 O 4 + 10 e - ⇌ Li 2 O + 3 CoO + 8 Li + + 8 e - ⇌ 3 Co + 4 Li 2 O. (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 Co 3 O 4 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 Co 3 O 4 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 orde