Study on Etching and Electrochemical Properties of Cr₂AlB₂

The development of advanced anode materials for lithium-ion batteries (LIBs) is critical to meet the growing demand for high-energy-density and long-cycle-life energy storage systems. Among various emerging materials, MBenes, two-dimensional (2D) derivatives of ternary layered MAB phases, have drawn significant attention, due to their metallic conductivity, tunable surface chemistry and structural flexibility. Cr₂AlB₂, a representative MAB phase, holds promise for synthesizing 2D-CrB (a type of MBene) through selective etching of the Al layer. However, challenges persist in optimizing etching parameters, understanding the structural evolution and validating the electrochemical performance of 2D-CrB. Previous studies were primarily focused on theoretical predictions or preliminary synthesis, leaving gaps in systematic experimental validation and mechanistic insights. This study was aimed to establish a controllable HCl etching protocol for Cr₂AlB₂ to produce high-purity 2D-CrB and elucidate the structural advantages of the 2D architecture in enhancing lithium storage performance.

The Al layer in Cr₂AlB₂ was selectively etched using a wet chemical approach with HCl. Key parameters, including HCl concentration, temperature and etching duration, were systematically optimized. Structural and morphological evolution of the etched products was characterized by using X-ray diffraction (XRD), scanning electron microscopy (SEM) and Brunauer-Emmett-Teller(BET) surface area analysis. Electrochemical evaluations were conducted by assembling CR2032 coin cells with 2D-CrB or Cr₂AlB₂ as the working electrode, lithium foil as the counter/reference electrode and 1 M LiPF₆ in EC/DMC/EMC (1:1:1 v/v) as the electrolyte. Cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and galvanostatic charge-discharge tests at various rates (0.1-1.6 C) were performed to assess lithium storage behavior.

Optimal etching conditions (3 mol·L^-1 HCl, 20 ℃, 6 h) enabled efficient Al layer removal, yielding 2D-CrB with a porous multilayered structure. XRD results confirmed the complete disappearance of Cr₂AlB₂ peaks and the formation of 2D-CrB as the dominant phase. SEM results revealed a transition from dense lamellar Cr₂AlB₂ to exfoliated 2D nanosheets with uniform thickness and exposed edges. BET analysis demonstrated a 12-fold increase in specific surface area (from 14.77 to 183.02 cm³·g^-1) and a broader pore size distribution centered at 3 nm, facilitating electrolyte infiltration and ion diffusion. Electrochemically, 2D-CrB outperformed Cr₂AlB₂ with an initial discharge capacity of 347.1 mAh·g^-1 at 0.1 C, retaining 65.4% capacity after 100 cycles. At 0.2 C, 2D-CrB maintained a stable capacity of 157 mAh·g^-1 over 100 cycles, with Coulombic efficiency > 98%, while Cr₂AlB₂ exhibited rapid decay to 59 mAh·g^-1. EIS and CV analyses suggested this enhancement to reduced charge transfer resistance, improved Li⁺ diffusion kinetics and effective buffering of volume strain owing to the 2D porous framework.

Controlled HCl etching enabled transformation of Cr₂AlB₂ into high-performance 2D-CrB with tailored porosity and enhanced electrochemical properties. The optimized 2D structure exhibited abundant active sites, rapid ion transport pathways and mechanical resilience against cycling-induced stress, thus addressing the critical limitations of bulk MAB phases. The findings validate 2D-CrB as a promising anode candidate for LIBs and offer a scalable synthesis strategy for MBene materials. Furthermore, the mechanistic insights into structure-property relationships paved a way for designing advanced 2D materials for energy storage applications.