Layer structured lithium mixed transition metal oxides with the general formula of LiNi_xMn_yCo_zO₂ (x+y+z=1) and theoretical capacity of approximately 280 mAh/g are considered as promising intercalation type positive electrode materials for Lithium-ion batteries. Owing to their lower costs and higher capacities, LiNi_xMn_yCo_zO₂ (NMCs) are extensively investigated for electric vehicles and grid storage applications as suitable substitutes for LiCoO₂, the most widely used cathode material for Li-ion batteries.

Several studies have been devoted to investigating the structure and electrochemical behavior of different compositions in the LiNi_xMn_xCo_1-2xO₂ (0≤x≤0.5) solid solution series. However, there are very limited thermochemical data reported on these compounds. The enthalpies of formation of LiNi_1−xCo_xO₂ compounds were determined by Wang et al. [1] using high temperature oxide solution calorimetry and the enthalpy of formation of LiNi_1/3Mn_1/3Co_1/3O₂ and its delithiated phases were measured by Idemoto et al. [2] using acid solution calorimetry.  The aim of this work is therefore to determine the enthalpy of formation of selected compositions in LiNi_xMn_xCo_1-2xO₂(0≤x≤0.5) series by calorimetric measurements in order to study the relative thermodynamic stabilities of these phases and further clarify structure–property relationships in these compounds.

In this study, single phase LiNi_xMn_xCo_1-2xO₂ for x=1/6, 1/3, 0.4, 0.5 were synthesized according to the sol-gel method using metal acetates and adipic acid as a chelating agent. The chemical compositions of the samples after the final heat treatment were measured by inductively coupled optical emission spectroscopy (ICP-OES). In addition, powder X-ray diffraction (XRD) was performed to determine phase impurities and lattice parameters of the respective compounds. Subsequently, the standard enthalpies of formation of the compounds were measured by employing high temperature oxide melt drop solution calorimetry using a Setaram AlexSys-1000 Tian–Calvet calorimeter (Setaram Instrumentation, Caluire-et-Cuire, France) with a sodium molybdate (3Na₂O.4MoO₃) solvent maintained at approximately 700°C.

The powder-XRD results show that all compositions were single phase adopting the α-NaFeO₂ type structure (space group: , No. 166). As x in LiNi_xMn_xCo_1-2xO₂ increases, the a and c lattice parameters increase whereas the c/a ratio decreases. This can be explained by the difference in ionic radii between Ni²⁺ and Co³⁺ and the fact that Co³⁺ which is situated on the transition metal layer is continuously substituted by equal amounts of Mn⁴⁺ and Ni²⁺.

The results of our high temperature oxide melt solution calorimetry measurements show that the standard enthalpies of formation of the LiNi_xMn_xCo_1-2xO₂ compounds from the elements become more exothermic with increasing x. Therefore, contrary to the work of Idemoto et at. [2], we show that the investigated NMC compounds are energetically more stable than LiCoO₂. The increasing thermodynamic stability is caused by the substitution of Co³⁺ with equal amounts of Mn⁴⁺ and Ni²⁺cations on the transition metal layer (3b sites).

Keyword: Lithium nickel manganese cobalt oxide, Layered structure, Oxide solution calorimetry, Enthalpy of formation, Lithium-ion battery

References

1. Wang, M. & Navrotsky, A. Enthalpy of formation of LiNiO₂, LiCoO₂ and their solid solution, LiNi_1-xCo_xO₂. Solid State Ionics 166 (2004), 167-173.
2. Idemoto, Y., & Matsui, T. Thermodynamic stability, crystal structure, and cathodic performance of Li_x(Mn_1/3Co_1/3Ni_1/3)O₂ depend on the synthetic process and Li content. Solid State Ionics 179 (2008), 625-635.