Cation deficiency associated with the chemical exfoliation of lithium cobalt oxide

Abstract

Compositional changes associated with the chemical exfoliation of lithium cobalt oxide, a layered transition metal oxide, are discussed. Starting from a layered bulk structure, lithium cobalt oxide can undergo chemical exfoliation through a two‐step method: treatment with a protic acid, then treatment with tetramethylammonium hydroxide (this intercalates the layered structure and yields exfoliated nanosheets). This work provides an in‐depth analysis of compositional and structural changes occurring to the powder upon the first step to exfoliation, treatment with acid, revealing variations in vacancies and valence changes depending on the conditions used. Through coupled analysis of X‐ray photoelectron spectroscopy, X‐ray diffraction, UV‐Vis absorption spectroscopy, and inductively coupled plasma‐optical emission spectroscopy data, we illustrate that both lithium and cobalt ions are diffusing out the structure along with the dissolution of full unit cells. As such, nanosheets accessed from the bulk by this exfoliation process should not be considered simply as divisions of the original unit cell. This work provides fundamental insights on the stability of LiCoO2 and the exfoliation of layered transition metal oxides, beyond the access of individual nanosheets, and is vital to determining structure‐property relationships of chemically exfoliated nanosheets (eg, changes in valency which dictate catalytic activity, magnetic susceptibility, etc).

Evaluating the chemical exfoliation of lithium cobalt oxide using UV-Vis spectroscopy

“…Exfoliation of LiCoO 2 (LCO) using "standard" conditions The exfoliation of LCO was accomplished using a two-step wetchemical exfoliation method rst reported by Kim et al 20 and improved in our recent work. 27,37 Aer the second step of the reaction was completed, centrifugation of the resulting slurry allowed for the separation of larger particles (pellet) from colloidally stabilized nanosheet suspensions (supernatant). SEM characterization of the centrifuged pellet showed significant morphological changes under known exfoliation conditions with tetramethylammonium hydroxide (TMAOH) (i.e.…”

“…2. Additional reactions Table 1 The extent of alkali metal extraction and proton replacement, as reported 17,21,27,[29][30][31] Layered TMO Alkali metal extraction (%) Proton replacement (%) Resulting stoichiometry…”

“…1, a layered TMO (typically an alternatively layered structure of transition metal and alkali metal octahedra) can be exfoliated to individual TMO nanosheets using a two-step chemical exfoliation method. 19,20,27,28 Step one is described by the layered TMO reacting with a protic acid replacing the alkali ions in the interstices of the TMO layers with protons. This can be thought of as two concurrent processes: alkali cation extraction and proton replacement in the interlayer.…”

“…This can be thought of as two concurrent processes: alkali cation extraction and proton replacement in the interlayer. While alkali cation extraction is incomplete for some layered TMOs, such as LCO, as proven by elemental analysis, 20,27 acid treatment of commonly exfoliated TMOs such as Cs…”

“…MO 2 ) forms as a result of this incomplete cation exchange reaction. Changes in valence are most commonly associated with the transition metal 20,27,30 but are also known to occur for oxygen. [32][33][34] Although changes in stoichiometry and valence are occurring, proton replacement is the key driving force for step two, intercalation, exfoliation, and osmotic swelling in the powder.…”