Cation Driven Assembly of Two-Dimensional Reduced Graphene Oxide/Bilayered Vanadium Oxide Heterostructures for Li-Ion Batteries

Inorganic two-dimensional (2D) materials can be advantageous for applications that require efficient ionic conductivity and high redox activity. They offer open diffusion pathways for ion intercalation as well as strategies to potentially mitigate structural damage that can occur from the repeated expansion and contraction processes due to the reversible Li⁺ ion intercalation during extended Li⁺ ion battery cycling. 2D bilayered vanadium oxide (BVO or δ-V₂O₅●<i>n</i>H₂O) is an especially promising material for electrochemical applications because it has an open structure with large interlayer spacing and vanadium’s high oxidation state. These properties allow BVO to host a large number of charge carrying ions and undergo several reduction steps. However, the electrochemical performance of this bilayered phase is limited by its poor electronic conductivity and structural instability. In this work, we improved the cyclability and rate performance of BVO through an exfoliation process, the subsequent cation-driven assembly of exfoliated BVO and single-layer graphene oxide (GO) nanoflakes, and heat treatment processing to simultaneously reduce GO and partially dehydrate BVO.<br/><br/>Exfoliation followed by cation-driven assembly is a versatile technique that can be utilized for a variety of metal oxides and different conductive carbon precursors. The cations are used to maximize and stabilize the heterointerface formation between the BVO and GO by pulling these negatively charged nanoflakes together. As a proof of principle, exfoliated BVO and GO nanoflakes were assembled using a concentrated lithium chloride solution. The resulting flocculates were filtered, dried, and heat treated under vacuum at 210 °C to form a 2D heterostructure comprised of δ-V₂O₅●<i>n</i>H₂O and partially reduced graphene oxide as evidenced by cross-sectional scanning electron microscopy images, energy dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, and thermogravimetric analysis. The heat treatment is used to reduce the interlayer water content of BVO for better long-term stability and obtain reduced GO (rGO) to increase its electronic conductivity. The characterization techniques were also used to determine that the graphitic content of the heterostructure could be controlled by changing the initial GO concentration prior to assembly. Moreover, we found that the amount of GO present affected the BVO phase stability and electrochemical behavior.<br/><br/>The electrochemical charge storage properties of heterostructures with 0, 10, 20, and 35 wt% rGO were evaluated in Li⁺ ions cells with non-aqueous electrolyte and exhibited initial capacities of 245, 260, 180, and 150 mAh g^-1, respectively. However, BVO/rGO-0wt% retained only 10% of its initial capacity after 20 cycles at a 20 mA g^-1 specific current. In contrast, the BVO/rGO-10wt%, BVO/rGO-20wt%, and BVO/rGO-35wt% materials retained 80%, 97%, and 100% of their initial capacities after 20 cycles at the same specific current. Further, the BVO/rGO-35% heterostructure retained 75% of its initial capacity (110 mAh g^-1) after increasing the specific current from 20 mA g^-1to 200 mA g^-1, while the BVO/rGO-10 wt% and BVO/rGO-20 wt% samples retained only 18% (50 mAh g^-1) and 67% (120 mAh g^-1) of their initial capacities under the same cycling conditions. The improved capacity retention and rate performance of the materials with higher GO content indicates that the presence of GO inhibits the structural degradation of BVO during cycling as well as improves the overall electronic conductivity of the heterostructure. In summary, the cation driven assembly approach uses Li⁺ ions to induce and stabilize the formation of stacked 2D layers of GO and exfoliated BVO for applications in energy storage devices, and it can be applied in a variety of systems utilizing 2D materials with complementary properties.