Non-Aqueous K-Ion Battery Performance of MXene-Derived K-Preintercalated Bilayered Vanadium Oxides Electrodes

In this work, we demonstrate the performance of MXene-derived K-preintercalated bilayered vanadium oxide (MD-KVO) in non-aqueous K-ion energy storage system, focusing on the relationships between the MXene-to-oxide transformation process, structure and composition of the material as well as its electrochemical properties.<br/>By tuning synthetic parameters, we demonstrated control of the confined species in the interlayer region of BVO. Specifically, we obtained MD-KVO by first preparing V₂CT<i>_x</i> MXene using a dilute mixed acid etchant, followed by dissolution in a reaction with hydrogen peroxide, and recrystallization via hydrothermal treatment. The MD-KVO exhibited nanoflower morphologies, in agreement with the previously reported V₂CT<i>_x</i>-derived oxides [1], with wide nanosheet “petals” 0.5 – 2.0 μm in lateral size and 4.0 – 5.0 nm in thickness. Energy-dispersive X-ray spectroscopy (EDS) and thermogravimetric analysis (TGA) measurements were used to estimate the composition of the MD-KVO as K_0.400V₂O₅●0.086H₂O. This compositional data was used to modify a Na_0.56V₂O₅ model for pair distribution function (PDF) refinement. By replacing interlayer Na atoms with the experimentally determined K and H₂O content, we observed an evolution of the <i>c</i> lattice parameter from 8.90 Å in Na_0.56V₂O₅ to 9.57 Å in K_0.400V₂O₅●0.086H₂O after refinement, consistent with the (001) <i>d</i>-spacing of 9.59 Å calculated from synchrotron X-ray diffraction. By including spherical diameter as a refinement constraint, an optimal fit was obtained at a diameter of 55.5Å, indicating the local average crystallographic order. Beyond this distance, structural disorder reduces refinement capability with this model. Initial cycling of this material in non-aqueous K-ion cells in a narrow potential window of 2.0 – 3.7 V showed the best stability. Extending the window below 2.0 V or above 3.7 V showed significant degradation of the material’s charge storage properties within the first 5 cycles. Vacuum drying the MD-KVO at 200°C improved cycling stability when the window is extended, indicating that interlayer water participates in parasitic reactions that hamper stability in K-ion systems. We showed improved capacities of 72.75 mAh/g in a 1.5 – 3.8 V window as compared to 50.81 mAh/g in a 2.0 – 3.7 V window. In this extended window, the MD-KVO electrode exhibited good rate capability, with an average capacity of 41.21 mAh/g at 200 mA/g. These results demonstrate how the structure and composition of the MXene-derived K-preintercalated bilayered vanadium oxide influence its electrochemical performance in non-aqueous K-ion batteries.<br/><br/><i>References</i><br/>1. Ridley, P., et al., <i>MXene-Derived Bilayered Vanadium Oxides with Enhanced Stability in Li-Ion Batteries.</i> ACS Applied Energy Materials, 2020. <b>3</b>(11): p. 10892-10901.