Experimental Thermodynamic Studies of Cobalt-Induced Suppression of Li+/Ni2+ Cation Disorder in Pristine Layered Cathode Materials

Abstract

Different compositions of lithium transition-metal oxide ternary solid solutions, LiNi0.95–xCoxAl0.05O2 and LiNi0.8Co0.2–xAlxO2, are systematically engineered by sol–gel synthesis. The varying amounts of elemental doping at the transition-metal sites induce a corresponding variation in Coulombic and steric interactions that impact the thermodynamic stability of the layered structure. The formation enthalpies measured using high-temperature oxide-melt solution calorimetry helped us to gain insights into the thermodynamic stability. Our findings, based on X-ray diffraction and X-ray absorption spectroscopy, reveal that cobalt incorporation creates a synergistic steric and Coulombic interaction that simultaneously suppresses Li+/Ni2+ cation disorder while increasing lithium and transition-metal size mismatch. As a result, the enthalpic driving force for the formation of the layered structure relative to the oxides increases with an increasing cobalt content in LiNi0.95–xCoxAl0.05O2. The interplay of steric and Coulombic interactions in LiNi0.8Co0.2–xAlxO2 differs from LiNi0.95–xCoxAl0.05O2, so the enthalpies of formation relative to the oxides become slightly less exothermic with increasing aluminum content. However, the enthalpies of formation from the elements become progressively more exothermic with increasing cobalt and aluminum contents in LiNi0.95–xCoxAl0.05O2 and LiNi0.8Co0.2–xAlxO2, respectively. Also, the mixing enthalpies relative to the endmembers are overall negative in the whole cobalt and aluminum composition range.