Enthalpy of formation of Li_xCoO₂ (0.5 ≤ x ≤ 1.0)

Single-phase 03-type Li_xCoO₂ (0.5 ≤ x ≤ 1.0) samples were synthesized by chemically extracting lithium from LiCoO ₂ with an oxidizer, NO₂BF₄, in acetonitrile medium under an argon atmosphere. The enthalpy of formation of Li _xCoO₂ at 298 K was determined by high-temperature oxide melt solution calorimetry. The enthalpy of formation from binary oxides plus oxygen for Li_xCoO₂ at 298 K varies linearly with x, and Li_xCoO₂ becomes energetically less stable with decreasing x. From the linear trend, the enthalpies of formation for O3-type CoO ₂ and monoclinic Li_0.5CoO₂ were estimated. Monoclinic Li_0.5CoO₂ is energetically more stable than the undistorted O3-type Li_0.5CoO₂. For CoO₂, the energetic stability decreases in the order O1 > P3 > O3.