Electrical Characterization and Charge Transport in Chemically Exfoliated 2D Li_xCoO₂ Nanoflakes

Abstract

Transition metal oxides often possess complex interactions between charge, spin, lattice, and orbital degrees of freedom, resulting in correlated electronic and magnetic phases. Li x CoO2, one of the most commonly used cathode choices in rechargeable batteries, is a prime example of this, evidencing numerous correlated effects evolving as a function of lithium content (x). Due to the strong electrostatic forces that govern the layered nature of this material, however, most investigations of these behaviors have been on bulk forms (>0.1 mm). In the two-dimensional (2D) limit, correlated effects are more easily tuned and studied; therefore, it is of fundamental importance to develop an experimental basis for investigation. Herein, a two-step process is utilized to chemically delithiate and exfoliate LiCoO2 single crystals and study nanoflakes 10–60 nm thick, with 0.37 < x < 0.8. An initial electrical characterization of this new form reflects bulk conduction properties, verifying the reliability of this new technique: temperature-dependent resistance measurements indicate an insulating 2D variable ranging hopping conduction for samples of x > 0.75, and metallic characteristics with a finite residual conductance for x < 0.75. However, these thin flakes also exhibit correlated characteristics less commonly observed and understood in Li x CoO2. An energy barrier upon contact formation is observed for all conditions, independent of lithium concentration and electrode work function, suggesting enhanced correlated effects due to reduced dimensionality. Additionally, charge-ordering phenomena in the temperature-dependent resistance occur under specific preparation conditions. These anomalies are markedly larger in magnitude than previous accounts in bulk systems and are also found in low lithium ranges of x < 0.5, matching theoretical predictions not commonly observed experimentally. This work utilizes a new approach to gain insight behind the complex transport phenomena inherent in Li x CoO2, providing a new opportunity to understand these correlated effects using a 2D, single-crystal form.

Polymer particles armored with cobalt oxide nanosheets for the catalytic degradation of bisphenol A

“…S1, ESI†): (1) as-received lithium cobalt oxide (LCO) was treated with HCl; then (2) the acid-treated LCO was treated with an aqueous solution of tetramethylammonium hydroxide (TMAOH) to produce TMAOH-treated LCO. 35,43,47–49 The traditional exfoliation method used to produce CONs from LCO yields only a low concentration of nanosheets in highly caustic solution ( i.e. , excess of TMAOH).…”

“…S1, ESI †): (1) as-received lithium cobalt oxide (LCO) was treated with HCl; then (2) the acid-treated LCO was treated with an aqueous solution of tetramethylammonium hydroxide (TMAOH) to produce TMAOH-treated LCO. 35,43,[47][48][49] The traditional exfoliation method used to produce CONs from LCO yields only a low concentration of nanosheets in highly caustic solution (i.e., excess of TMAOH). The modified exfoliation method used here utilized semi-exfoliated TMAOH-treated LCO, rather than the initially exfoliated nanosheets to produce highly concentrated aqueous dispersions of CONs at neutral pH 43 by immersing semi-exfoliated TMAOH-treated LCO particles in DI water and subjecting to bath sonication.…”

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.…”

Liquid‐phase exfoliation method to access cobalt oxide nanosheets in pH‐neutral solutions

“…As CONs can be obtained by further exfoliation of the centrifuged powder by the simple addition of deionized water to the powder slurry and are not mentioned in the exfoliation studies of LCO. 21,[23][24][25][26][27][28][29][30][31][32][33][34][35] Upon addition of deionized water, indications of exfoliation are immediately observed, as supported by the presence of a dark solution that survives low-speed centrifugation. The behavior can be explained by an influx of water molecules into the interstices of the TMA-swelled layered cobalt oxide structure.…”

“…Most commonly exfoliated from lithium cobalt oxide (LCO) 21,[23][24][25][26][27][28][29][30][31][32][33][34][35][36] (with many reports coming from our research group [23][24][25]28,29,34,36 ), CONs have been implemented in Liion batteries, thermoelectric devices, armored polymer particles, and layer-by-layer films. 21,23,27,30,31 CONs have also been synthesized through bottom-up techniques, but are not the focus of this work.…”