In recent years, rechargeable lithium ion batteries have become significant power sources for portable electronics, electric vehicles and large-scale grid storage as a response to an increasing worldwide demand for high energy density batteries. As one of the most important components in a battery, cathode materials have been highly investigated in recent years.
The layered lithium mixed transition metal oxides with the general formula of LiNixMnyCozO2 (x+y+z=1) are promising intercalation type active materials for lithium ion batteries, with a theoretical capacity of approximately 280 mAh/g. These so-called NMC materials offer higher capacities, lower cost, and improved safety compared to LiCoO2 and are particularly attractive for applications in electric vehicles and grid storage.
Although the electrochemical behavior of many promising compositions in the NMC system is to some extent investigated, there are not many studies devoted to the measurement of their thermochemical properties. The enthalpies of formation of LiNi1−xCoxO2 compounds were determined by Wang et al.  using high temperature oxide solution calorimetry and the enthalpy of formation of LixNi1/3Mn1/3Co1/3O2 and its delithiated phases were measured by Idemoto et al.  using acid solution calorimetry. The aim of the present study is therefore to determine the enthalpy of formation of selected compositions in the NMC system, including the technologically relevant NMC111 and NMC442 compositions, by high temperature oxide drop solution calorimetry.
High temperature oxide melt drop solution calorimetry was performed on samples synthesized by the sol-gel method. In addition, the thermal stabilities of different compositions were studied using simultaneous differential thermal analysis/ thermogravimetric analysis to investigate possible decomposition reactions. The chemical compositions of the NMC samples were measured by ICP-OES (inductively coupled optical emission spectroscopy). Powder X-ray diffraction (XRD) was also performed to determine phase impurities and lattice parameters of the respective compounds.
The thermochemical data obtained in this work could be used as valuable input data for the development of CALPHAD-based thermodynamic descriptions of the Li–Ni-Mn-Co–O system, which can then be used to calculate open circuit voltages and coulometric titration curves based on the Gibbs free energy descriptions of the assessed phases in the multi-component systems.