Energetics of lithium transition metal oxides applied as cathode materials in lithium ion batteries.

摘要

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 alpha-NaFeO2 structure LiNi1-x COxO2 solid solution, Li1+xMn 2-xO4 spinel, LiMO2 (M = Mn, Fe and Co), and chemically delithiated LixCoO2.; The enthalpies of formation and enthalpies of mixing for LiNi1-x COxO2 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 (Li2O, NiO and CoO) and O2, and the lattice parameters decrease approximately linearly with the Co content x in LiNi1-xCOxO 2. The solid solution deviates slightly positively from ideality.; Lithium substituted spinel Li1+xMn2-xO 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 (Li2O, Mn2O3 and MnO2 ) becomes more exothermic with x in Li1+xMn2-x O4 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 Mn3+ to Mn4+ versus formation of oxygen vacancies).; LiMO2 samples (M = Mn, Fe, and Co) with different structures were synthesized and their enthalpies of formation from oxides (Li2O and M2O3) or oxides (Li2O 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 LiFeO2 were investigated by differential scanning calorimetry and high temperature oxide melt solution calorimetry. The gamma phase is the stable form of LiFeO2 at room temperature and the a phase is stable at high temperature.; Energetics of chemically delithiated LixCoO2 samples (0.5 ≤ x ≤ 1.0) with the alpha-NaFeO2 structure (O 3 type) was studied. The enthalpies of formation from binary oxides (Li2O and CoO) and oxygen vary linearly with x and LixCoO 2 becomes less stable with decreasing x. The enthalpies of formation from oxides and oxygen for monoclinic Li0.5CoO2 and O3-CoO2 were estimated by extrapolation from the linear trend.