CoO2, The End Member of the Li x CoO2 Solid Solution

TLDR

Lithium‑ion battery cathode LiₓCoO₂ has been extensively studied, yet the existence of an end‑member phase at complete lithium deintercalation remains uncertain. An in‑situ X‑ray diffraction cell built with dry‑plastic lithium‑ion battery technology enabled structural characterization of LiₓCoO₂ at and near x = 0 for the first time. The study isolated a hexagonal single‑layered O1 end‑member phase with a = 2.822 Å and c = 4.29 Å that shows increased crystallographic quality at x ≈ 0, reverts to a three‑layer O3 phase upon lithium reinsertion, and allows 95 % lithium re‑insertion with reversible cycling to 4.2 V.

Abstract

While has been widely studied in the past 15 years as a promising positive electrode material in lithium‐ion batteries, suprisingly, many questions are still unanswered concerning the electrochemical characteristics of the lithium intercalation material. Among these is the existence of an end member phase on complete lithium deintercalation. The use of dry plastic lithium‐ion battery technology has allowed the construction of an in situ x‐ray diffraction cell which allows structural characterization of at x values at and close to 0 for the first time. Instead of the expected destruction of the core structure of by a drastic increase in structural disorder, an increase in crystallographic quality occurred as x approached 0. For the first time, the end member phase was isolated. This phase is a hexagonal single‐layered phase (O1) believed to be isostructural with and has lattice parameters of a = 2.822 Å and c = 4.29 Å. The phase converted immediately back to a three‐layer (O3) delithiated type phase on lithium reinsertion. Electrochemical studies show that 95% of lithium can be reinserted back into the structure on complete delithiation and reversible cycling properties are maintained when cycled back to 4.2 V.