In this work, high temperature oxide melt solution calorimetry was employed to investigate the thermodynamic properties of several representative lithium transition metal oxide systems applied as cathode materials in lithium ion batteries: layered α-NaFeO 2 structure LiNi 1-x CO x O 2 solid solution, Li 1+x Mn 2-x O 4 spinel, LiMO 2 (M = Mn, Fe and Co), and chemically delithiated Li x CoO 2 .
The enthalpies of formation and enthalpies of mixing for LiNi 1-x CO x O 2 solid solution series (0 ≤ x ≤ 1) were determined using high temperature oxide melt solution calorimetry. The enthalpies of formation at 298 K from binary oxides (Li 2 O, NiO and CoO) and O 2 , and the lattice parameters decrease approximately linearly with the Co content x in LiNi 1-x CO x O 2 . The solid solution deviates slightly positively from ideality.
Lithium substituted spinel Li 1+x Mn 2-x O 4 (0 ≤ x ≤ 1/3) samples synthesized are stoichiometric at x < 0.25, and oxygen deficient at x ≥ 0.25. The enthalpy of formation from binary oxides (Li 2 O, Mn 2 O 3 and MnO 2 ) becomes more exothermic with x in Li 1+x Mn 2-x O 4 for stoichiometric compounds, and deviates endothermically from this trend for oxygen deficient compounds. This energetic trend is related to two competing substitution mechanisms of lithium for manganese (oxidation of Mn 3+ to Mn 4+ versus formation of oxygen vacancies).
LiMO 2 samples (M = Mn, Fe, and Co) with different structures were synthesized and their enthalpies of formation from oxides (Li 2 O and M 2 O 3 ) or oxides (Li 2 O and MO) plus oxygen at 298 K were determined. The relative stability of the polymorphs of LiMO 2 was established based on their enthalpies of formation. Phase transformations in LiFeO 2 were investigated by differential scanning calorimetry and high temperature oxide melt solution calorimetry. The γ phase is the stable form of LiFeO 2 at room temperature and the a phase is stable at high temperature.
Energetics of chemically delithiated Li x CoO 2 samples (0.5 ≤ x ≤ 1.0) with the α-NaFeO 2 structure (O 3 type) was studied. The enthalpies of formation from binary oxides (Li 2 O and CoO) and oxygen vary linearly with x and Li x CoO 2 becomes less stable with decreasing x. The enthalpies of formation from oxides and oxygen for monoclinic Li 0.5 CoO 2 and O3-CoO 2 were estimated by extrapolation from the linear trend.
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