Fluoride-Free Synthesis of Surface-Modulated MXene via Molten Salt Etching for High-Performance Sodium-Ion Batteries

Abstract

Conventional fluoride-based etching methods for MXene synthesis pose environmental hazards and introduce electrochemically inert terminal groups, which limit their performance in energy storage applications. Herein, a sustainable fluoride-free strategy is employed to synthesize a modified MXene (denoted as S-MX) through molten salt etching. This approach not only mitigates safety concerns but also yields a material with an open three-dimensional (3D) architecture and a uniquely tailored surface chemistry, characterized by the presence of S-terminations and a notable absence of F-groups. When evaluated as an anode material for sodium-ion batteries (SIBs), the S-MX electrode exhibits a significantly enhanced reversible capacity of approximately 320 mAh g⁻¹ at 100 mA g⁻¹, and superior rate capability (retaining 137.2 mAh g⁻¹ at 5 A g⁻¹). Comprehensive electrochemical analysis reveals that the performance enhancement stems from the synergistic effects of an expanded interlayer spacing, which facilitates Na⁺ ion diffusion, and a surface chemistry that promotes capacitive-dominated storage. This is evidenced by a high capacitive contribution of 86 % and a reduced charge-transfer resistance. This work provides a foundational strategy for the eco-friendly fabrication of high-performance MXene-based electrodes.