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 TiO₂ into MXene and engineer its surface chemistry without multi-step synthesis and other chemical additions. It has been successfully established that anatase TiO₂, structural 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 TiO₂ particle formation and the surface functional groups. Excessive oxidation at higher temperatures results in predominant TiO₂ formation, blocking interlayer spacing, and reducing the specific surface area. The specific capacitance (Cs) of MXene TiO₂ at 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 TiO₂ 350°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 TiO₂ 350 °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.

HIGHLIGHTS

• A Novel Scalable Strategy: Introduces controlled thermal oxidation in air as a simple and scalable annealing method to engineer the surface chemistry and structure of MXene strategically.
• Temperature-Dependent Optimization: Reveals that annealing temperature precisely controls the anatase TiO₂ crystallization and functional group composition, with 350 °C identified as the optimal condition to balance TiO₂ formation and prevent pore blockage.
• Transition to Battery-Type Behavior: Successfully transforms the Mxene’s charge storage mechanism from capacitive to dominant diffusion-controlled, battery-type behavior, enhancing faradaic activity.
• Superior MXene TiO₂  Performance: MXene TiO₂ 350 °C single electrode enhancing the electrochemical performance by approximately ~13% (from 254.98 to 58 F/g).
• Outstanding Supercapattery Performance and Stability: AC//MXene TiO₂ 350 °C exhibits excellent performance (Cs 42 F/g, ED 32.88 Wh/kg, and PD 700.38 W/kg) and stability (capacity retention remains 92% after 5,000 cycles).

GRAPHICAL ABSTRACT