Cation-Driven Assembly of Bilayered Vanadium Oxide and Graphene Oxide Nanoflakes to Form Two-Dimensional Heterostructure Electrodes for Li-Ion Batteries

Abstract

Lithium preintercalated bilayered vanadium oxide (LVO or δ-Li x V2O5·nH2O) and graphene oxide (GO) nanoflakes were assembled using a concentrated lithium chloride solution and annealed under vacuum at 200 °C to form two-dimensional (2D) δ-Li x V2O5·nH2O and reduced GO (rGO) heterostructures. We found that the Li+ ions from LiCl enhanced the oxide/carbon heterointerface formation and served as stabilizing ions to improve structural and electrochemical stability. The graphitic content of the heterostructure could be easily controlled by changing the initial GO concentration prior to assembly. We found that increasing the GO content in our heterostructure composition helped inhibit the electrochemical degradation of LVO during cycling and improved the rate capability of the heterostructure. A combination of scanning electron microscopy and X-ray diffraction was used to help confirm that a 2D heterointerface formed between LVO and GO, and the final phase composition was determined using energy-dispersive X-ray spectroscopy and thermogravimetric analysis. Scanning transmission electron microscopy and electron energy-loss spectroscopy were additionally used to examine the heterostructures at high resolution, mapping the orientations of rGO and LVO layers and locally imaging their interlayer spacings. Further, electrochemical cycling of the cation-assembled LVO/rGO heterostructures in Li-ion cells with a non-aqueous electrolyte revealed that increasing the rGO content led to improved cycling stability and rate performance, despite slightly decreased charge storage capacity. The heterostructures with 0, 10, 20, and 35 wt % rGO exhibited capacities of 237, 216, 174, and 150 mAh g–1, respectively. Moreover, the LVO/rGO-35 wt % and LVO/rGO-20 wt % heterostructures retained 75% (110 mAh g–1) and 67% (120 mAh g–1) of their initial capacities after increasing the specific current from 20 to 200 mA g–1, while the LVO/rGO-10 wt % sample retained only 48% (107 mAh g–1) of its initial capacity under the same cycling conditions. In addition, the cation-assembled LVO/rGO electrodes exhibited enhanced electrochemical stability compared to electrodes prepared through physical mixing of LVO and GO nanoflakes in the same ratios as the heterostructure electrodes, further revealing the stabilizing effect of a 2D heterointerface. The cation-driven assembly approach, explored in this work using Li+ cations, was found to induce and stabilize the formation of stacked 2D layers of rGO and exfoliated LVO. The reported assembly methodology can be applied for a variety of systems utilizing 2D materials with complementary properties for applications as electrodes in energy storage devices.

Role of Cations in the Cation-Driven Assembly Process and their Effect on the Charge Storage Properties of Bilayered Vanadium Oxide and Reduced Graphene Oxide Heterostructures in Alkali Ion Systems

“…The sp 2 hybridized graphene component of each heterostructure and rGO reference sample subsequently combusts at temperatures above ∼350 °C. 31 Lastly, weight gain observed above ∼450 °C can be attributed to the filling of oxygen vacancies present in the V 2 O 5 layers. 41,47,48 The TGA curves can also be used to estimate the molar mass of the heterostructures after annealing.…”

“…40,44,45 Prior to annealing, the inflection points in the TGA curves of the heterostructures at ∼165 °C and the GO reference at ∼150 °C delineate the removal of these groups. 31 Simultaneously, the δ-…”

“…However, while the carbon signal does show evidence of these layers, the vanadium signal is more uniform compared to our previous cation-assembled heterostructures. 31 This outcome could be due to improved synthesis processing, which resulted in a better distribution of vanadium oxide and carbon layers and more uniform elemental maps. Therefore, STEM imaging is required to confirm that there is a 2D interface between LVO and rGO nanoflakes.…”

“…A summary of the contributions of the functional groups present in the C 1s spectra and vanadium oxidation states are shown in Table S1 in the Supporting Information, and a more detailed analysis of the XPS spectra can be found in a previous publication. 31 To study the effect that the assembled cation has on the charge storage properties of the heterostructures, each material was initially evaluated using cyclic voltammetry in Li-ion halfcells. The Li-LVO/rGO, Na-LVO/rGO, and K-LVO/rGO CV curves captured using a 0.2 mV s −1 sweep rate (Figure 4a−c) exhibit similar features owing to the bilayered vanadium oxide serving as the active phase in each structure.…”

“…Cation-driven assembly is one promising method that can be used to interface BVO with more conductive 2D materials. This strategy uses positively charged species to combine one or more 2D phases, often to adjust the final properties of the assembled structure. − Our recent work used Li + ions to assemble exfoliated Li x V 2 O 5 · n H 2 O (ex-LVO) and monolayer graphene oxide (GO), resulting in the formation of a heterostructure, a 2D material defined by an intimate face-to-face contact between two chemically dissimilar building blocks (structural layers or nanoflakes). , GO was chosen as the conductive carbon precursor because reduced GO (rGO) is an established current collector and conducting agent for the fabrication of electrodes with transition-metal oxides. − This material exhibited enhanced electrochemical stability and rate capability compared to samples that simply mixed the two components without the use of cations to initiate this interface formation. Cation-driven assembly has also utilized ions beyond lithium.…”