Exfoliation of Bilayered Vanadium Oxide and Oxide/MXene Heterostructures for Energy Storage
Authors:   , Zhang Raymond , Pomerantseva Ekaterina 
Published: November 2022

The bilayered phase of vanadium oxide ([delta]-V₂O₅·nH₂O or BVO) synthesized via sol-gel synthesis method is an attractive candidate to be used as a cathode material for Li-ion batteries (LIBs) due to its expanded interlayer spacing, and high theoretical capacity. However, poor electrical conductivity and rapid capacity fading present challenges for achieving high-rate performance and good cycle life during cycling in LIBs. A viable solution to mitigating these drawbacks is to fabricate two-dimensional (2D) heterostructures comprised of vanadium oxide and an additional selected 2D material. Titanium carbide (Ti₃C₂T_x) MXenes are particularly of interest for their high electronic conductivity (10³ S·cm⁻¹) and well developed delamination protocols. Herein, we present a wet chemistry based approach for assembling BVO and Ti₃C₂T_x MXene nanoflakes into 2D heterostructures in an aqueous suspension. In this study, we first demonstrate a simple liquid phase exfoliation technique utilizing probe ultrasonication to exfoliate bulk [delta]-Li_xV₂O₅·nH₂O (LVO). Previous experiments from our group showed the exfoliation technique in n-methyl-2-pyrrolidone, an organic solvent that is toxic, flammable, and resulted in low yield during exfoliation. Here, for the first time, we have successfully performed exfoliation of LVO in aqueous media and obtained few layered nanoflakes with high yield after centrifugation. Despite the partial solubility of vanadium oxide in water, these nanoflakes suspended in aqueous media maintained chemical stability and readily assembled into a free-standing film using vacuum filtration. The LVO nanoflake films have a 2D layered morphology as confirmed by SEM and maintain the bilayered structure as confirmed by XRD. Due to the hydrated nature of LVO, we also highlight the importance of controlling interlayer water content with vacuum drying for achieving better cycling stability. The comparison of a 105°C and 200°C vacuum drying temperatures and corresponding interlayer water contents was carried out using XRD, TGA, and Raman spectroscopy characterization methods. The 200°C vacuum dried LVO nanoflake cathode exhibited an initial ion storage capacity of 212 mAh·g⁻¹ which was 32.5% higher than the sample dried at 105°C. In addition, galvanostatic cycling experiments conducted for the 200°C vacuum dried LVO nanoflake cathode show there were significant improvements in capacity retention by ~35%, compared to the 105°C dried sample, after 100 cycles at a current density of 20 mA·g⁻¹. Rate capability experiments with cycling conditions of 20, 50, 100, 200, and 20 mA·g⁻¹ in 10 cycle intervals demonstrate that the 200°C dried LVO cathode maintains a higher capacity retention of 29.14% at 200 mA·g⁻¹ compared to the sample dried at 105°C (19.41%). After returning to the lowest current density of 20 mA·g⁻¹ (cycle 41-50), minimal changes in capacity retentions (71.47% and 74.44% for 105°C and 200°C dried LVO) suggest that structural water content does not greatly affect the LVO electrode's tolerance to high current densities. Subsequently, dispersions of the LVO and Ti₃C₂T_x nanoflakes in water were combined in different weight ratios of 9:1, 4:1, and 1:1 LVO to Ti₃C₂T_x. The 2D heterostructure electrostatic assembly was facilitated by the introduction of cationic species into the mixed suspensions to induce flocculation, and free-standing flocculate films (f-LVO/Ti₃C₂T_x) were obtained through vacuum filtration. Resistivity measurements showed that heterostructure films exhibited electronic conductivities that were ~10⁵ higher than the film composed of pristine LVO nanoflakes. Following similar drying protocols for pristine LVO, the f-LVO/Ti₃C₂T_x films were ground and vacuum dried at 200°C prior to electrode fabrication. Through galvanostatic cycling at a current density of 20 mA·g⁻¹ and potential window of 2-4V, the 9:1 f-LVO/Ti₃C₂T_x cathodes delivered the highest ion storage capacity of 167 mAh·g⁻¹. Lower capacities of 121.13 and 121.74 mAh·g⁻¹ for 4:1 and 1:1 ratio f-LVO/Ti₃C₂T_x are attributed to higher Ti₃C₂T_x content and its inactivity within the tested potential window. Higher capacity retentions of 9:1 (90.28%), 4:1 (90.62%), and 1:1 (84.32%) f-LVO/Ti₃C₂T_x electrodes relative to pristine LVO (78.68%) suggests that hybridization of LVO and Ti₃C₂T_x leads to improved cycling stability. Furthermore, rate capability experiments (same conditions as pristine LVO) demonstrate that the 9:1, 4:1, and 1:1 f-LVO/Ti₃C₂T_x samples maintain a low capacity retention of 38.57%, 34.61%, and 21.25% respectively at a current density of 200 mA·g⁻¹, indicating that optimization of the assembly and electrode fabrication process are needed. However, all weight ratio f-LVO/Ti₃C₂T_x electrodes have a high capacity retention of above 92% after being cycled at increasing current densities and returning to the initial current density of 20 mA·g⁻¹ and suggest that the coupling Ti₃C₂T_x with LVO enables greater tolerance to high current densities. In this work we also show that increased capacities of 237.40 and 248.40 mAh·g⁻¹ for 4:1 and 1:1 heterostructure electrodes can be acquired upon the extension of the potential window to 1-4V covering the region where Ti₃C₂T_x exhibits redox activity. These results demonstrate an environmentally friendly and safe approach to obtaining 2D LVO nanoflakes and offers pathways to constructing novel vanadium oxide based 2D heterostructures for improving electrochemical performance in energy storage devices.