Enthalpies of Formation of Layered LiNixMnxCo1-2xO2 Compounds As Promising Li-Ion Battery Cathode Materials
Authors: Masoumi Maryam , Cupid Damian Marlon , Reichmann Thomas Ludwig , Seifert Hans Jürgen 
Published: February 2020

Layer structured lithium mixed transition metal oxides with the general formula of LiNixMnyCozO2 (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, LiNixMnyCozO2 (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 LiNixMnxCo1-2xO2 (0≤x≤0.5) solid solution series. However, there are very limited thermochemical data reported on these compounds. The enthalpies of formation of LiNi1−xCoxO2 compounds were determined by Wang et al. [1] using high temperature oxide solution calorimetry and the enthalpy of formation of LiNi1/3Mn1/3Co1/3O2 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 LiNixMnxCo1-2xO2(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 LiNixMnxCo1-2xO2 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 LiNixMnxCo1-2xO2 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 LiNixMnxCo1-2xO2 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

Wang, M. & Navrotsky, A. Enthalpy of formation of LiNiO2, LiCoO2 and their solid solution, LiNi1-xCoxO2. Solid State Ionics 166 (2004), 167-173.

Idemoto, Y., & Matsui, T. Thermodynamic stability, crystal structure, and cathodic performance of Lix(Mn1/3Co1/3Ni1/3)O2 depend on the synthetic process and Li content. Solid State Ionics 179 (2008), 625-635.