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.
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_x 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_x-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 c 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) d-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.

References
1. Ridley, P., et al., MXene-Derived Bilayered Vanadium Oxides with Enhanced Stability in Li-Ion Batteries. ACS Applied Energy Materials, 2020. 3(11): p. 10892-10901.