[Paper Review] A general Lewis acidic etching route for preparing MXenes with enhanced electrochemical performance in non-aqueous electrolyte

Youbing Li, Hui Shao+15 more|arXiv (Cornell University)|Sep 29, 2019

MXene and MAX Phase MaterialsMaterials Science19 references19 citations

TL;DR

This paper proposes a general Lewis acidic etching method using molten salts to synthesize MXenes from non-traditional MAX phases (with A-elements Si, Zn, Ga), enabling the production of Ti3C2 MXene with exceptional electrochemical performance in non-aqueous electrolytes. The method achieves a high Li+ storage capacity of 738 C g⁻¹ (205 mAh g⁻¹) with pseudocapacitive behavior and high-rate capability in a 1M LiPF6 carbonate electrolyte.

ABSTRACT

Two-dimensional carbides and nitrides of transition metals, known as MXenes, are a fast-growing family of 2D materials that draw attention as energy storage materials. So far, MXenes are mainly prepared from Al-containing MAX phases (where A = Al) by Al dissolution in F-containing solution, but most other MAX phases have not been explored. Here, a redox-controlled A-site-etching of MAX phases in Lewis acidic melts is proposed and validated by the synthesis of various MXenes from unconventional MAX phase precursors with A elements Si, Zn, and Ga. A negative electrode of Ti3C2 MXene material obtained through this molten salt synthesis method delivers a Li+ storage capacity up to 738 C g-1 (205 mAh g-1) with high-rate performance and pseudocapacitive-like electrochemical signature in 1M LiPF6 carbonate-based electrolyte. MXene prepared from this molten salt synthesis route offer opportunities as high-rate negative electrode material for electrochemical energy storage applications.

Motivation & Objective

• To develop a general synthetic route for MXenes beyond Al-containing MAX phases.
• To enable the synthesis of MXenes from MAX phases with A-elements such as Si, Zn, and Ga.
• To enhance electrochemical performance of MXenes in non-aqueous electrolytes for energy storage applications.
• To demonstrate high-rate lithium-ion storage capability in a carbonate-based electrolyte.
• To establish a redox-controlled etching mechanism using Lewis acidic melts.

Proposed method

• Utilization of molten salt systems with strong Lewis acidity to selectively etch A-elements from MAX phases.
• Employment of redox control to manage the etching process and prevent unwanted side reactions.
• Synthesis of MXenes from MAX phases with A-elements including Si, Zn, and Ga, not previously accessible via conventional HF-based etching.
• Characterization of resulting MXenes using XRD, XPS, SEM, and TEM to confirm phase purity and 2D morphology.
• Electrochemical testing in 1M LiPF6 in EC/DEC carbonate electrolyte to evaluate Li+ storage performance.
• Analysis of charge storage mechanisms via cyclic voltammetry and galvanostatic charge-discharge measurements.

Experimental results

Research questions

• RQ1 Can a general Lewis acidic etching route be developed to access MXenes from non-Al-containing MAX phases?
• RQ2 What is the electrochemical performance of MXenes synthesized via molten salt etching in non-aqueous electrolytes?
• RQ3 How does the redox-controlled etching mechanism in molten salts compare to conventional HF-based etching in terms of selectivity and yield?
• RQ4 To what extent does the resulting MXene exhibit pseudocapacitive behavior in a carbonate-based electrolyte?
• RQ5 Can MXenes derived from Si-, Zn-, or Ga-based MAX phases deliver high-rate lithium storage capacity?

Key findings

• The molten salt etching method successfully produced MXenes from MAX phases with A-elements Si, Zn, and Ga, expanding the scope of accessible MXenes beyond Al-containing precursors.
• The synthesized Ti3C2 MXene achieved a specific capacity of 738 C g⁻¹ (205 mAh g⁻¹) in a 1M LiPF6 carbonate electrolyte, demonstrating high energy storage capability.
• The material exhibited pseudocapacitive-like electrochemical behavior, indicated by a near-rectangular cyclic voltammogram and linear potential dependence of capacitance.
• High-rate performance was confirmed by maintaining significant capacity at high current densities, indicating fast ion and electron transport.
• The method enables selective A-site etching through redox control, minimizing damage to the MXene layers and preserving structural integrity.
• The approach is generalizable, as evidenced by successful synthesis from multiple unconventional MAX phase precursors.