Phase equilibria, thermodynamic and electrochemical properties of cathodes in lithium ion batteries based on the Li–(Co, Ni)–O system

Currently, LiCoO2 is widely used as cathode in lithium ion batteries for “small” applications (e.g., laptops and cell phones). However, the relatively high cost and safety problems limit its applications in electric vehicles. Partial substitution of Co with other transition metals, such as Ni and Mn, can lower costs and improve battery performance and safety. The present dissertation aims at studying the Li–(Co,Ni)–O system using CALPHAD (calculation of phase diagrams) modeling and ab initio calculations. It is found that DFT (density functional theory) cannot be used to obtain accurate enthalpies of formation for transition metal-containing oxides. By probing a series of metal oxides using ab initio calculations, it is proposed that one needs to correct the GGA data with -0.8 and +0.8 eV shifts for oxides of transition metals with valence state of +2 and +4, respectively. This modifies strong correlations of localized and hybridized d transition metal states and provides an alternative to the Hubbard approach or the GW approximation, which enables the prediction of enthalpies of formation for transition metal-containing oxides. In the first part of the dissertation, the proposed corrections have been verified by the available experimental enthalpy data of the lithium cobalt oxides and lithium nickel oxides. With the obtained enthalpies of formation and the empirical entropy data, Gibbs energy functions of the cathodes are determined and the cell voltages of lithium ion batteries are calculated. Compared to the previously calculated cell voltage data, which underestimate experimental data by up to 15%, the calculations reported here are in good agreement with previously published experimental data. Hence, it is evident that the proposed theoretical approaches enable the prediction of lithium ion battery cell voltages and provide reliable thermodynamic data for further CALPHAD modeling. As important cathode materials, the O3-LiCoO2, O2-LiCoO2, O3-LiNiO2 and O3-Li(Co,Ni)O2 phases have been studied in detail. The corresponding systems are O3 and O2 structural LiCoO2–CoO2, O3 structural LiNiO2–NiO2 and Li(Co,Ni)O2–(Co,Ni)O2. In the second and third parts of the dissertation, these systems are investigated using the CALPHAD approach. All the phases are described with appropriate sublattice models and parameters. The calculations on their phase diagrams and thermodynamic properties are in good agreement with literature data and the obtained ab initio results. Using the thermodynamic description derived in this dissertation, the cell voltages of Li/LiCoO2 and Li/O3-Li(Co,Ni)O2 are well reproduced. Furthermore, the phase stability of the component oxides, which is of relevance for lithium ion battery safety issues, is of interest. In the fourth part, the binary Li–O system is assessed using the CALPHAD approach. The phase equilibria and thermodynamic data of this system are critically reviewed. The liquid phase and two stable lithium oxides, Li2O and Li2O2, are modeled. The Li–O phase diagram at 1 bar total pressure is established for the first time. The literature data and present ab initio results are satisfactorily accounted for by the thermodynamic description. A set of self-consistent thermodynamic parameters for the Li–O system was obtained for modeling of further multi-component Li-containing oxide systems. Above all, the present work shows that CALPHAD modeling and ab initio calculations can successfully be used to predict the phase transitions in lithium ion batteries, as well as the cell voltages. This enables the design of future cathode materials with improved stability and efficiency