Hu Kunhuang ¹, Liao Zhentong ¹, Yan Zhuanlong ¹, Chen Yuying ¹, Deng Yuxiao ¹, Yan Ming ^{1, 2, 3}

(1. College of Materials and Chemical Engineering, Hubei University of Technology, Wuhan 430068, Hubei, China; 2. Hubei Provincial Key Laboratory of Green Materials for Light Industry, Wuhan 430068, Hubei, China; 3. Collaborative Innovation Center of Green Light-weight Materials and Processing, Hubei University of Technology, Wuhan 430068, Hubei, China)

Extended abstract:

[Background and purposes] MAX phase materials, ternary layered carbides or nitrides with the general formula Mn+1AXn (n=1–3, M=transition metal, A=group ⅢA/ⅣA element, X=C or N), can be selectively etched to produce two-dimensional MXenes. Among them, V₂CT_x MXene derived from the V₂AlC MAX phase has attracted significant attention in lithium-ion battery anodes, due to its high electrical conductivity, layered structure and tunable surface chemistry. However, conventional V₂CT_x suffers from nanosheet restacking and limited interlayer spacing, which restrict its rate performance and cycling stability. A-site doping provides an effective strategy to regulate lattice parameters and interlayer spacing of MAX phases, thereby improving the structural and electrochemical properties of MXenes. This study was aimed to systematically clarify the effects of Sn and Si A-site doping on V₂AlC MAX phase synthesis, etching behavior and MXene interlayer structure, providing a theoretical and experimental basis for developing high-performance MXene-based anodes.

[Methods] High-purity V, Al, C, Sn and Si powders were utilized to synthesize V₂(Al₁−_xA_x)C (A=Sn, Si) solid solutions through pressureless sintering at 1400 ℃ for 2 h in Ar, with molar ratios of V:Al:Sn/Si:C=2.0:(1.5−x):x:0.9. Wet chemical etching of V₂(AlSn_0.2)C and V2(AlSi_0.2)C was conducted using 12 M HCl, 22.4 M HF or HCl/HF mixture (2:3 volume ratio), at varying water bath temperatures for different durations. Phase structures, morphologies and pore characteristics of the products were comprehensively evaluated using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Brunauer-Emmett-Teller (BET) methods.

[Results] Optimal synthesis parameters for pure V₂AlC included V:Al:C molar ratio of 2.0:1.5:0.9, sintering temperature of 1400 ℃ and time of 2 h. For the doped counterparts, the optimal A-site solid solubility limit for both Sn and Si was determined to be x=0.2, above which excessive doping yielded elemental or intermetallic impurities, while both solid solutions maintained a typical layered structure. Selective wet-chemical etching using 22.4 mol·L⁻¹ HF for V₂(AlSn_0.2)C and HCl/HF mixture for V₂(AlSi_0.2)C at 60 ℃ for 5 d yielded structurally integrated MXene nanosheets. Notably, V2(Sn_0.2)CT_x exhibited a larger interlayer spacing and looser stacking, whereas V₂(Si_0.2)CTx displayed thinner layers with prominent bending wrinkles induced by stress release. Quantitative analyses results confirmed that the (002) interlayer spacing followed the sequence of V2(Sn_0.2)CT_x (1.062 nm) >V2(Si_0.2)CT_x (0.968 nm) >V₂CT_x (0.956 nm), with corresponding specific surface areas increasing to 31.429 m²∙g⁻¹, 26.828 m²∙g⁻¹ and 20.071 m²∙g⁻¹, demonstrating that Sn/Si doping effectively modulates interlayer architecture and expands specific surface areas.

[Conclusions] Controlled pressureless sintering and subsequent wet-chemical etching successfully enabled the fabrication of high-purity Sn- and Si-doped V₂AlC solid solutions and their derived V₂CT_x MXene nanosheets. A-site doping proved to be an effective strategy for tailoring the interlayer architecture and surface morphologies of the MXenes. Specifically, Sn doping expanded interlayer spacing and promoted looser stacking, whereas Si doping yielded thinner nanosheets with stress-released bending wrinkles, both enhancing specific surface areas. These structural modifications effectively addressed the critical restacking limitations of V₂CT_x, providing expanded ion transport pathways and abundant exposed active sites.

Key words: V₂AlC; V₂CT_x MXene; A-site doping; wet chemical etching; interlayer spacing; specific surface area