Structural Phase Transition from Rhombohedral (R3̅m) to Monoclinic (C2/m) Symmetry in Lithium Overstoichiometric Li1+δCo1−δO2−δ

Abstract

Stoichiometric lithium cobalt oxide LiCoO2 is known to exhibit several structural phase transitions with x in LixCoO2 at ambient temperature (T); e.g., an initial rhombohedral (R3̅m) phase transforms into a monoclinic (C2/m) phase at x ∼ 0.5. In contrast, lithium overstoichiometric (Li)3b[LiδCo1−δ]3aO2−δ with δ ≥ ∼0.02, where δ is the Li+ ions at the 3a (Co) site, maintains the R3̅m symmetry until x ∼ 0.5 in Lix(LiδCo1−δ)O2−δ at ambient T, and this is the reason why such material has been widely used in commercial lithium ion batteries. We performed X-ray diffraction measurements in the T range between 100 and 300 K for the lithium overstoichiometric Lix(Li0.02Co0.98)O1.98 samples with x = 1, 0.56, and 0.51 to understand the factors that govern the structural changes in Lix(LiδCo1−δ)O2−δ with δ ≥ 0. Both x = 0.56 and 0.51 samples exhibit a structural phase transition from the high-T R3̅m phase to the low-T C2/m phase at 250 K (=Ts1). Furthermore, these samples indicate another structural phase transition at 170 K (=Ts2); although their crystal structures still have the C2/m symmetry, the degree of monoclinic distortion starts to decrease below Ts2, associated with a magnetic anomaly and a freezing of the Li+ ions at the 3b site. Because the two structural phase transitions of Ts1 (=330 K) and Ts2 (=150 K) are also observed for the stoichiometric LixCoO2 compound with x ∼ 0.5, the C2/m phase in Lix(LiδCo1−δ)O2−δ is found to appear in the limited x and T ranges. The characteristics and possible origin of Ts1 and Ts2 for both stoichiometric LixCoO2 and lithium overstoichiometric Lix(Li0.02Co0.98)O1.98 samples are discussed.