Controlled Thermal Oxidation of TiO2-MXene: A Scalable Annealing Strategy for High-Performance Asymmetric Supercapatteries

Abstract

Enhancing the faradaic storage mechanism, stability, high power density, and energy density of energy storage requires further development of MXene, an oxidation-prone material, while preserving its interlayer structure and stability from restacking and excessive oxidation. This work presents a novel controlled thermal oxidation strategy, a scalable single-step annealing method to incorporate TiO2into MXene and engineer its surface chemistry without multi-step synthesis and other chemical additions. It has been successfully established that anatase TiO2structural defects, and oxide functional groups, which are critical for enhancing the redox reaction kinetics formed by the annealing treatment in the temperature range of 350-650 °C under air conditions. Among the annealed samples, TiO2-modified MXene treated at 350 °C exhibited the most promising electrochemical performance. Its effective redox reactions demonstrated a battery-type behavior, dominated by diffusion-controlled charge storage, instead of the typical capacitive behavior driven by surface area. This was owing to an optimal balance between TiO2particle formation and the surface functional groups. Excessive oxidation at higher temperatures results in predominant TiO2formation, blocking interlayer spacing, and reducing the specific surface area. The specific capacitance (Cs) of MXene TiO2at 350 °C increases by up to 13% from the Cs of MXene reached 289.58 F/g at a scan rate of 20 mV/s, indicating superior electrochemical performance. Furthermore, an asymmetric supercapattery was manufactured by pairing MXene TiO2350°C with activated carbon (AC) as a practical approach to improving the energy density without sacrificing power density or capacity retention of the device. AC//MXene TiO2350 °C supercapattery demonstrates remarkable performance with a Cs of 49.42 F/g, energy density (ED) of 32.88 Wh/kg, and power density (PD) of 700.38 W/kg. Furthermore, it also demonstrated exceptional cycle stability (retaining 92% of its capacity after 5,000 cycles), and reduced resistance (equivalent series resistance (ESR), charge transfer resistance (Rct), and ion diffusion resistance), outperforming traditional AC-based supercapatteries. These findings highlight the novelty of controlled thermal oxidation as a simple yet effective route for engineering high-performance TiO2-MXene-based electrodes for advanced energy storage systems.