Although the structural characteristics and electrochemical behavior of many promising compositions in the NMC system have been investigated to some extent, there are very few studies devoted to the thermodynamic properties of this class of materials. The enthalpies of formation of LiNi1−xCoxO2 compounds were determined by Wang and Navrotsky  using high temperature oxide solution calorimetry and the enthalpy of formation of LiNi1/3Mn1/3Co1/3O2 and its chemically delithiated phases were measured by Idemoto and Matsui  using acid solution calorimetry. In the present work, with the aim of investigating the effect of cation substitution on thermodynamic stability of NMC materials, the standard enthalpies of formation of two different series of sol-gel made NMC samples, namely LiNixMnxCo1-2xO2 (0 ≤ x ≤ 0.5) and LiNi0.8-yMnyCo0.2O2 (0 ≤ y ≤ 0.4), are determined by high-temperature oxide-melt drop solution calorimetry. The compositions are selected to include the technologically relevant NMC111, NMC442, NMC532 and NMC622 compounds.
It is shown that LiNixMnyCozO2 with layered structure is a nearly ideal solid solution of LiCoO2, LiNiO2 and LiNi0.5Mn05O2. Our calorimetry results show that the standard enthalpies of formation of the compounds from the elements vary almost linearly with composition in the respective area. Furthermore, based on our experimental heat of formation data and available low temperature heat capacity data for LiCoO2, LiNiO2 and LiNi0.5Mn05O2, a model for the variation of the Gibb’s free energy of formation and equilibrium potentials of the NMCs with composition at room temperature is presented. In addition, key electrochemical tests were performed to establish a relationship between the thermodynamic properties and electrochemical performance of the cathode materials. This work should help clarify the structure–property relationships in these compounds.
- M.Wang, & A. Navrotsky, Solid State Ionics 166 (2004), 167-173.
- Y.Idemoto & T. Matsui, Solid State Ionics 179 (2008), 625-635.