Defect Engineering of Hexagonal MAB Phase Ti2InB2 as Anode of Lithium‐Ion Battery with Excellent Cycling Stability

Abstract Hexagonal MAB phases （h‐MAB） have attracted attention due to their potential to exfoliate into MBenes, similar to MXenes, which are predicted to be promising for Li‐ion battery applications. However, the high cost of synthesizing MBenes poses challenges for their use in batteries. This study presents a novel approach where a simple ball‐milling treatment is employed to enhance the purity of the h‐MAB phase Ti2InB2 and introduce significant indium defects, resulting in improved conductivity and the creation of abundant active sites. The synthesized Ti2InB2 with indium defects (VIn‐Ti2InB2) exhibits excellent electrochemical properties, particularly exceptional long‐cycle stability at current densities of 5 A g−1 (5000 cycles, average capacity decay of 0.0018%) and 10 A g−1 (15 000 cycles, average capacity decay of 0.093%). The charge storage mechanism of VIn‐Ti2InB2, involving a dual redox reaction, is proposed, where defects promote the In‐Li alloy reaction and a redox reaction with Li in the TiB layer. Finally, a Li‐ion full cell demonstrates cycling stability at 0.5 A g−1 after 350 cycles. This work presents the first accessible and scalable application of VIn‐Ti2InB2 as a Li‐ion anode, unlocking a wealth of possibilities for sustainable electrochemical applications of h‐MAB phases.

the morphology and microstructure of the samples.The instruments were also equipped with energy-dispersive X-ray spectroscopy (EDS) for elemental mapping.X-ray photoelectron spectroscopy (XPS, ESCALAB) analysis was carried out using an Mg-Kα light source.Electron paramagnetic resonance (EPR, Bruker EMXnano) was utilized to detect unpaired electrons present in atoms or molecules within the material.

Electrochemical Tests
CR-2032 type coin-cells were assembled inside an argon-filled glove box with H2O and O2 concentrations kept below 0.1 ppm.For electrode preparation, a slurry was formed by uniformly grinding 70 wt% active material, 20 wt% carbon black, and 10 wt% polyvinylidene fluoride (PVDF) in 1-methyl-2-pyrrolidone (NMP).This slurry was then coated onto copper foil.After vacuum drying at 60 °C for 24 hours, circular electrodes with a diameter of 12 mm and an average mass of active materials approximately 1.0 mg cm -2 were obtained from the electrode tape.The counter electrode, electrolyte, and separator used were lithium foil, 1 M LiPF6 (ethylene carbonate (EC): diethyl carbonate (DEC) = 1:1 v/v), and Cellgard 2400, respectively.

Galvanostatic charge/discharge (GCD) measurements and Galvanostatic Intermittent Titration
Technique (GITT) were performed using the Neware battery test system (Shenzhen, China) at various current densities within a voltage range of 0.01-3.0V. Electrochemical impedance spectroscopy (EIS) from 0.01 Hz to 100 kHz and Cyclic voltammetry (CV) were conducted using an electrochemical workstation (CHI660E).Furthermore, a lithium-ion full cell was constructed with LiFePO4 (LFP) as the cathode and VIn-Ti2InB2 as the anode.The LFP powders were purchased from Shenzhen KeLuDe Technology for the assembly of the VIn-Ti2InB2||LFP full battery.The cathode was prepared by mixing active material (80%), carbon black (10%), and PVDF (10%) to form a homogeneous slurry in NMP, which was then coated on Al foil.The separator and electrolyte used were consistent with those in half cells.To establish a stable solid electrolyte interface layer, the VIn-Ti2InB2 anode was activated at 0.05 A g -1 for 3 cycles in half cells.Additionally, the mass ratio of anode to cathode (N/P) was maintained at 1:1.2, and the specific capacities of the full-cell devices were determined based on the mass of the anode.Considering the potential difference between the cathode and anode, the voltage range for the GCD tests in the full battery was set to 0.5-4.0V.

Calculation Setting
The Vienna Ab-initio Simulation Package (VASP) [2] was utilized in this study for all density functional theory (DFT)-based calculations.The calculations involved the use of the projector augmented plane-wave (PAW) [3] method and the Perdew-Burke-Ernzerhof (PBE) [4] functional within the generalized gradient approximation (GGA) to compute the exchange-correlation interaction energy.For the relaxation and total energy calculations of the slab models, a Monkhorst-Pack k-point mesh with a sampling of 4×4×1 was employed.The cutoff energy of the plane wave basis was set to 520 eV to ensure accurate results.Furthermore, the energy convergence criterion was set to 10 -5 eV to guarantee reliable outcomes.To investigate the electronic structure of Ti2InB2 (001) and ( 010) surfaces, a slab model consisting of six atomic layers with a 3×3 supercell was constructed.These atomic layers were separated from adjacent layers by a vacuum thickness of 20 Å along the Z direction (refer to Figure S1).To assess the impact of the indium vacancy on the electronic structure of Ti2InB2 (001), calculations were performed to determine the charge density and densities of states (DOS) of VIn-Ti2InB2 (001).These calculations aimed to evaluate any changes induced by the presence of the indium vacancy.
The adsorption energies (Eb) of Li atoms on various surfaces were determined using the following equation: where  Li@surf ,  surf and  Li are the calculated energies of Li@surface, bare surface and a Li atom in the most stable Li metal phase, respectively.
The climbing-image nudged elastic band (CI-NEB) approach was employed to calculate the energy barrier associated with the migration of Li + ions [5] .This method is commonly used for studying reaction pathways and transition states.In addition, charge density difference analysis and Bader charge analysis were conducted to investigate the charge distributions within the surface structures [6] .These analyses provide insights into the redistribution of charges and allow for a better understanding of the electronic properties and interactions involved.
The detailed calculation method of theoretical capacity: A 3 × 3 supercell of VIn-Ti2InB2 and Ti2InB2 monolayer were constructed.These atomic layers were separated from adjacent layers by a vacuum thickness of 50 Å along the Z direction (Figure S12(a-d

Supplementary Table
)).According to our calculated adsorption energies of a Li atom at different sites on the (010) and (001) surfaces of VIn-Ti2InB2 and Ti2InB2, we adsorb lithium atoms to the site of In vacancy titanium surface of (001) (FigureS12(ef)).The detailed calculation method is: (1) The average adsorption energy,   , is defined as  =    @ −   − ()   where   represents the energy of VIn-Ti2InB2 and Ti2InB2 monolayer,    @ represents the energy of  Li atoms adsorbed at the VIn-Ti2InB2 and Ti2InB2 monolayer,   represent the energy of each Li atom in bulk, and  is the number of Li atoms adsorbed.When   changes from negative to positive, the adsorbed Li atoms gather to form clusters due to the cohesive energy and further adsorption will stop.(2) The theoretical capacity, C, is defined as  =    +   where  represents the maximum number of atoms adsorbed,  represents the valence of the Li atoms,  represents the Faraday constant (26,801 mAh g −1 ), and Li atoms to the   and   represent respective molar weights of VIn-Ti2InB2/Ti2InB2 monolayer and Li.

Figure S2 :
Figure S2: XRD pattern of Ti2InB2 MAB phase after HCl etching in comparison with simulated

Figure S3 :
Figure S3: (a) Side and (c) Top views of the crystal structure mode of h-MAB phase Ti2InB2.

Figure S5 :
Figure S5: (a) SEM image and (b) particle size distribution of VIn-Ti2InB2 phase after ball

Figure S8 :
Figure S8: (a) the XPS full survey spectrums and (b) The high-resolution XPS spectra of B 1s for

Figure S10 :
Figure S10: Photos of lithium-ion battery assembled with pure VIn-Ti2InB2 powder as electrode

Figure S11 :
Figure S11: CV curves of VIn-Ti2InB2 and Ti2InB2 electrodes at a scan rate of 0.1 mV s -1 .

Figure S14 :
Figure S14: (a) Rate performance of VIn-Ti2InB2 and pristine Ti2InB2 anodes after long cycling at

Figure
Figure S17：(a) EIS measurements of the pristine Ti2InB2 electrode after different cycles; (b)

Figure S18 :
Figure S18: (a) Ex-situ XPS In 3d and (b) C 1s spectra of VIn-Ti2InB2 electrode during the 1 st

Figure S20 :
Figure S20: (a) Ex-situ XRD patterns at different voltages of the second cycle; (b) XRD patterns

Figure S21 :
Figure S21: Cross-section SEM views of VIn-Ti2InB2 electrode (a) before and (b)after 600 cycles

Figure S22 :
Figure S22: Top view of SEM images of VIn-Ti2InB2 electrode after 600 cycles at 1 A g -1 .

Figure S27 :
Figure S27: The possible adsorption sites for Li atom at (a and b) (001) and (c and d) (010)

Figure S28 :
Figure S28: (a) Calculated adsorption energies of Li + ion at different sites on the (010) and (001)

Figure S29 :
Figure S29: The calculation results of charge density difference on the (001) and (010) surface of VIn-Ti2InB2.The yellow and cyan surfaces indicate the charge gain and lost regions, respectively (isovalue, 0.0015).

Figure S30 :
Figure S30: Calculated energy barriers for Li migration along the paths of (a) B-bri to B-bri and (b) In-top to In-top on the (010) surface of VIn-Ti2InB2.

Table S1 .
Element contents of pristine Ti2InB2 and VIn-Ti2InB2 obtained by XPS analysis.

Table S2 .
Comparison of the electrochemical performances of VIn-Ti2InB2 anode with those reported MAX/MAB phase anode for LIBs.