Revisiting LiCoO₂ Using a State-of-the-Art In Operando Technique

Lithium overstoichiometric cobalt oxide, Li(Li_δCo_1-δ)O_2-δ, still occupies a privileged position as a positive electrode material for lithium-ion batteries. However, despite its widespread applications in commercial lithium-ion batteries, little is known about its reaction mechanisms and the effects of δ on cyclability at deep charge. We herein revisited this material through a recently developed in operando technique, i.e., rapid, alternating measurements of X-ray diffraction and X-ray absorption spectroscopy. The cyclability degraded when the charge cutoff voltage was >4.4 V versus Li⁺/Li, which corresponds to the Li composition exhibiting a minimum (maximum) lattice parameter along the a_h (c_h) axis. Differences in the structural parameters such as lattice parameters and bond distances clearly appeared between the charge and discharge reactions at a capacity below ∼220 mAh g^-1. These changes occurred because deep charge and/or increasing the amount of δ induced a local distortion in the CoO₆ octahedra. We found a critical Li extraction content that satisfied the need for both high capacity and cyclability for Li(Li_δCo_1-δ)O_2-δ, which can be applied to other layered materials.