TiNbC MXene cathode for high-performance aluminum-ion batteries

Abstract

Al-ion batteries (AIBs) have emerged as a promising energy storage technology due to their high theoretical capacity, cost-effectiveness, and superior safety. However, the lack of stable and efficient cathode materials capable of reversible Al-complex ion (e.g., [AlCl₄]⁻) insertion/extraction remains a critical challenge. In this work, we developed TiNbCT_x MXene as a high-performance cathode material for AIBs, achieving remarkable capacity and cycling stability. Unlike symmetric-structured Ti₂CT_x, the TiNbCT_x cathode leverages synergistic Ti–Nb bimetallic effects to enhance the electronic conductivity and electrochemical activity. Here we show, TiNbCT_x delivers a high reversible capacity of 194 mAh·g⁻¹ at 0.2 A·g⁻¹ with 800-cycle stability. Through combined experimental characterization and density functional theory (DFT) calculations, we elucidate the kinetic mechanisms of energy storage, offering fundamental insights for the rational design of advanced cathode materials in AIBs.

Introduction

Developing large-scale energy storage systems is strongly required to ensure effective power dispatch and load management of renewable energy sources such as wind, hydro, and solar power due to their intermittency and volatility^1,2,3. Al-ion batteries (AIBs), as an emerging large-scale energy storage technology, have recently gained significant attention due to their high energy density, low cost, and exceptional safety profile^4,5,6,7. The development of durable, high-capacity cathode materials for efficient and stable intercalation of Al-complex ions remains a priority. Transition metal oxides/sulfides, such as V₂O₅, Mo₆S₈, and CuS, suffer from rapid capacity decay and structural degradation under extended cycling^8,9,10. Structural modifications have shown limited success in improving cycle life and capacity retention of above mentioned cathode materials. Carbon-based materials with layered structures have shown promise for accommodating Al-complex ions while improving cycle stability, but suffered from low specific capacity¹¹. Studies on two dimensional V₂CT_x and Nb₂CT_x MXenes in AIBs indicate that MXenes can maintain stable capacities over hundreds of cycles due to their unique layered structure and tunable interlayer spacing, providing new avenues for cathode optimization in AIBs^12,13. Continued exploration of MXene materials, along with structural modifications, could pave the way to more reliable and efficient AIBs suitable for large-scale energy storage solutions. MAX phases are layered ternary carbides/nitrides that serve as precursors for MXenes, which are produced by selectively etching the “A” layer (typically Al or Si) from the MAX phase to yield two-dimensional transition metal carbides/nitrides. Two-dimensional (2D) transition metal carbides and nitrides (MXenes) have a formula of M_n+1X_nT_x (n = 1–4)^14,15, where M is an early transition metal, X is carbon and/or nitrogen, and T_x represents the surface termination groups, such as –OH, –O, and –F. MXenes offer an unusual combination of metallic conductivity and hydrophilicity, exhibiting attractive electrochemical properties^16,17,18,19. In addition, their ability of intercalating ions and small organic molecules led to promising performance in energy storage applications^20,21,22,23. However, almost all the studies have been on as-synthesized mono-transition metal MXenes, such as Ti₃C₂T_x, V₂CT_x, and Nb₂CT_x^24,25,26.

In this study, we fabricated dual-transition metal TiNbCT_x MXene as a cathode material in AIBs by selectively removing Al atomic layers from the TiNbAlC MAX phase through HF acid etching and followed delamination (see “Methods” section for details). The synthesized TiNbCT_x MXene incorporated with surface terminations (–OH, –O, and –F) exhibited effective ion storage and high specific capacity. The Nb incorporation in TiNbCT_x architecture facilitated the [AlCl₄]⁻/Al³⁺ adsorption compared with the mono-elemental Ti₂CT_x MXene. The rechargeable AIB featured with TiNbCT_x MXene as the cathode, Al foil as the anode, and [EMIm]Cl/AlCl₃ as the electrolyte, demonstrated a high specific capacity of 194 mAh g⁻¹ at a current density of 200 mA g⁻¹ after 800 cycles. The energy storage mechanisms of the TiNbCT_x MXene cathode were investigated by ex-situ characterizations alongside complementary density functional theory calculations.

Results and discussion

Figure 1a schematically illustrates the synthesizing process of TiNbCT_x MXene from the TiNbAlC precursor through hydrofluoric acid (HF) etching. Bulk TiNbCT_x MXene with an accordion-like structure (Fig. 1b) was obtained after selectively removal of Al layers in the TiNbAlC MAX phase (Figs. S1, S2a–c). Figure 1c shows the scanning electron microscopy (SEM) image of TiNbCT_x MXene flakes after Tetramethylammonium hydroxide (TMAOH) delamination. A high resolution TEM image of as synthesized TiNbCT_x MXene (Fig. 1d) indicates the hexagonal symmetry of the Ti and Nb atoms. Figure S2d–f reveal the morphology of monolayer TiNbCT_x MXene, indicating a high degree of exfoliation. Figure S3 shows the EDX mapping of TiNbCT_x MXene flake, presenting a uniform elemental distribution of Ti, Nb, and C. Ti₂CT_x MXene was fabricated to be a control group through a reported method²⁷. Figure 1e–g presents the X-ray diffraction (XRD) patterns of the synthesized TiNbCT_x and Ti₂CT_x MXenes and their precursors TiNbAlC and Ti₂AlC MAX phases. After the selective etching and delamination, the (002) characteristic diffraction peak of TiNbAlC shifted from 2θ = 12.66° to 2θ = 6.82°, while that of Ti₂AlC shifted from 2θ = 13.02° to 2θ = 7.47°, indicating an enlarged c-axis lattice constant and interlayer spacing due to the removal of Al atomic layers from the MAX precursors^28,29,30. The enlarged interlayer spacing of MXenes facilitates the transport of Al-complex ions during the charge/discharge process in AIBs, thereby improving their electrochemical performance. Figure S4a presents the atomic force microscopy (AFM) image of few-layer TiNbCT_x MXene flakes, while Fig. S4b shows the height profile along the trace line in Fig. S4a. The profile indicates that the prepared TiNbCT_x MXene flakes have a thickness of ~4.8 nm, which corresponds to three molecular layers (with a monolayer thickness of ~1.6 nm). Figure 1i shows the wide-range X-ray photoelectron spectroscopy (XPS) spectra of as-synthesized TiNbCT_x and Ti₂CT_x MXene, indicating the existence of −F and −O terminations. Figure S5 displays the high-resolution XPS spectra of TiNbCT_x, where the C1s high-resolution XPS spectrum shows two strong peaks at 284.38 eV and 284.88 eV, corresponding to C-Ti/C-Nb and C-C bonds in TiNbCT_x MXene, respectively²⁸. Additionally, three weak peaks at 285.98 eV, 286.98 eV, and 290.28 eV correspond to C-O, O = C-O, and C-F bonds, indicating the presence of surface functional groups such as −O, −F, and −OH on the TiNbCT_x MXene successfully prepared by HF acid etching³⁰.

Fig. 1: Synthesis pathway, morphological features, and compositional characterization of TiNbCT_x MXene.

a Schematically illustration of the synthesizing process of TiNbCT_x MXene from the TiNbAlC precursor through hydrofluoric acid (HF) etching. b SEM image of bulk TiNbCT_x MXene, and c) delaminated TiNbCT_x MXene flake. d High-resolution TEM image of TiNbCT_x MXene. e XRD patterns of Ti₂AlC MAX, TiNbAlC MAX, Ti₂CT_x MXene, and TiNbCT_x MXene. f Localized zoom around (002) characteristic peak of the XRD pattern of delaminated Ti₂CT_x MXene and TiNbCT_x MXene. g Wide-range X-ray photoelectron spectroscopy (XPS) spectra of as-synthesized TiNbCT_x and Ti₂CT_x MXene.

Using a combination of X-ray diffraction (XRD), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS), we systematically investigated the structural evolution and electrochemical behavior of the TiNbCT_x cathode material during charge/discharge cycles. Figure 3a presents the ex-situ XRD spectra of the TiNbCT_x cathode before and after cycling. In the initial state, the (002) characteristic peak of d-TiNbCT_x appears at 7.52°, while after full charging, this peak shifts to a lower angle at 5.82°, directly confirming the successful insertion of the [AlCl₄]⁻ and the resulting increase in interlayer spacing. After fully discharging, the (002) peak shifts back towards a higher angle, approaching its original position (around 7.4°), indicating that most of the Al-complex ions have been extracted from the interlayer, although some remain chemically adsorbed on the MXene surface. This finding is consistent with the material’s reversible charge/discharge behavior. Figure 3b shows the incremental changes in the Raman spectra of d-TiNbCT_x during the charge/discharge process, further revealing changes in the electronic structure of the d-TiNbCT_x cathode. During charging, the D peak (1355 cm⁻¹) shifts to a lower frequency, while the G peak (1580 cm⁻¹) shifts to a higher frequency. Both weakened peaks lead to a decrease in the I_D/I_G ratio from 0.837 to 0.821. This change is attributed to the electronic band structure adjustment caused by the insertion of the [AlCl₄]⁻, which occupies empty states and reduces the generation of defect-induced electron-hole pairs³¹. During discharge, both the D and G peaks return to their original positions, and their intensities increase, further confirming the highly reversible nature of the charge/discharge process. Figure 3c presents the XPS spectra of the Ti 2p core levels before and after cycling. After fully charging, the binding energy of the Ti-C-T_x component shifts from 460.38 to 460.88 eV, with a notable increase in peak intensity³⁰. This indicates a strong chemical adsorption interaction between the TiNbCT_x MXene surface and the inserted Al-complex ions. The chemical adsorption of these Al-complex ions leads to electron release, further increasing the effective positive charge in the MXene cathode. Figure 2d, e show the XPS spectra of Al 2p and Cl 2p core levels before and after charge/discharge. In the initial state, no signals for Al 2p or Cl 2p were detected in the TiNbCT_x cathode. However, after charging to 2.3 V, the XPS peaks for Al 2p and Cl 2p are significantly enhanced due to the insertion of [AlCl₄]⁻ anions. After discharging to 0.01 V, although the Al 2p and Cl 2p signals are still present, they are much weaker, suggesting that the extraction of the [AlCl₄]⁻ is not complete. This is consistent with the chemical adsorption interaction between the MXene and the inserted ions, as well as the XRD results. These findings, supported by XRD and Raman spectroscopy data, collectively reveal that the specific capacity of the TiNbCT_x MXene electrode primarily arises from the insertion/extraction of interlayer Al-complex ions and the chemical adsorption effects associated with this process.

Fig. 2: Ex-situ characterizations of the TiNbCT_x MXene cathode in AIBs.

a XRD pattern of TiNbCT_x MXene before the test and after fully (dis)-charging. b Raman spectra of TiNbCT_x MXene measured during the (dis)-charge process. c Ti 2p core level XPS spectra of TiNbCT_x MXene before the test and after fully (dis)-charging. d Al 2p, and (e) Cl 2p core level XPS spectra of TiNbCT_x MXene before the test and after fully (dis)-charging.

To investigate the energy storage properties of TiNbCT_x and Ti₂CT_x cathode materials in AIBs, we recorded cyclic voltammetry (CV) curves at different scan rates (Fig. 3a, b). Figure 3a shows the CV curves of TiNbCT_x and Ti₂CT_x cathodes in the voltage range of 0.1 V to 2.3 V at a scan rate of 1 mV·s⁻¹. Compared to the Ti₂CT_x cathode, the TiNbCT_x electrode exhibits more prominent oxidation and reduction peaks, as well as a larger integrated curve area. This indicates more active ion insertion and extraction during the charge/discharge process, reflecting more significant redox reactions. This high electrochemical activity grants the TiNbCT_x electrode higher efficiency in charge storage and release, resulting in superior electrochemical performance. Further, Fig. 3b shows the CV curves of the TiNbCT_x electrode during the second to fifth cycles at a scan rate of 0.5 mV·s⁻¹. In the second cycle, two distinct oxidation peaks are observed at 1.27 V and 2.21 V, along with reduction peaks at 0.967 V and 1.949 V. These characteristic peaks directly confirm the occurrence of ion insertion and extraction (i.e., redox reactions). Notably, the subsequent scans exhibit excellent reproducibility, confirming both the reversibility of the redox reactions in the TiNbCT_x electrode and its high stability during electrochemical cycling. Next, constant current charge/discharge tests were conducted on both Ti₂CT_x and TiNbCT_x electrodes. Figure 3c displays the voltage-specific capacity curves of both electrodes at a current density of 0.1 A g⁻¹. The results show that, during the first and fifth cycles, TiNbCT_x exhibited specific capacities of 678 mAh g⁻¹ and 571 mAh g⁻¹, respectively, which are significantly higher than the 347 mAh g⁻¹ and 176 mAh g⁻¹ observed for Ti₂CT_x. The observed reduction in initial cycle capacity from 678 mAh g⁻¹ to 571 mAh g⁻¹ is primarily attributed to electrolyte decomposition and interfacial side reactions. Even after 100 and 200 cycles, TiNbCT_x maintained a specific capacity of over 200 mAh g⁻¹, far surpassing 118 mAh g⁻¹ of Ti₂CT_x. Figure 3d presents the cycling stability of the TiNbCT_x electrode at a higher current density of 0.2 A g⁻¹. Due to electrode activation, TiNbCT_x achieved a specific capacity of 194 mAh g⁻¹ after 800 cycles, demonstrating excellent cycling stability. Figure 3e shows the capacity retention rates of Ti₂CT_x and TiNbCT_x after cycling at different current densities. The TiNbCT_x electrode demonstrated a capacity retention rate of 34.76%, which is superior to 28.97% of Ti₂CT_x. Rate performance tests further confirm the advantages of TiNbCT_x as a cathode material. Figure 3g shows the rate performance of TiNbCT_x at different current densities. At current densities of 0.1, 0.2, 0.5, 1, and 0.1 A g⁻¹, the TiNbCT_x electrode provided reversible capacities of 189, 127, 83, 61, and 269 mAh g⁻¹, respectively, which are significantly higher than the 62, 40, 26, 20, and 43 mAh g⁻¹ provided by the Ti₂CT_x electrode. The limitations in high-rate performance are predominantly governed by the restricted diffusion kinetics of aluminum ions. Notably, after 20 cycles, the TiNbCT_x electrode showed an increase in specific capacity at 0.1 A g⁻¹ compared to the first five cycles, consistent with the earlier described electrode activation phenomenon. Figure 3h demonstrates the cycling performance of TiNbCT_x and Ti₂CT_x at a current density of 0.1 A g⁻¹. The TiNbCT_x electrode consistently exhibited significantly higher specific capacity throughout the entire test cycle compared to Ti₂CT_x. These excellent electrochemical performances are attributed to the unique Janus structure of TiNbCT_x, which greatly facilitates ion and electron transport within the electrode, thereby imparting superior rate capability. The Nyquist plot in Fig. 3f reveals the redox reaction kinetics and charge transfer resistance of the TiNbCT_x cathode. The TiNbCT_x cathode shows much lower charge transfer resistance (R_ct = 29 Ω) in the mid-to-high-frequency region, indicating outstanding performance in redox reaction kinetics and electronic conductivity. In the low-frequency region, the diffusion response curves of TiNbCT_x and Ti₂CT_x are shown in Figure S6, illustrating the relationship between impedance and phase angle (θ). The Warburg factor (θ₂) of TiNbCT_x is much smaller than that of Ti₂CT_x (θ₁), which corresponds to a higher diffusion coefficient of [AlCl₄]⁻ in the TiNbCT_x electrode. This explains its superior rate performance, further supporting the earlier conclusions.

Fig. 3: Electrochemical features of Ti₂CT_x MXene and TiNbCT_x MXene in AIBs.

a CV curves of Ti₂CT_x MXene and TiNbCT_x MXene at a scan rate of 1.0 mV s⁻¹. b The 2nd to 5th CV curves of TiNbCT_x MXene at a scan rate of 0.5 mV s⁻¹. c (Dis)-charge profiles of Ti₂CT_x MXene and TiNbCT_x MXene at a current density of 0.1 A g⁻¹. d Selected (dis)-charge profiles of TiNbCT_x MXene at a current density of 0.2 A g⁻¹. e Capacity retention of Ti₂CT_x MXene and TiNbCT_x MXene at different current densities. f EIS spectra of Ti₂CT_x MXene and TiNbCT_x MXene. g Rate performance of Ti₂CT_x MXene and TiNbCT_x MXene. h Cyclic performance of Ti₂CT_x MXene and TiNbCT_x MXene at a current density of 0.1 A g⁻¹. The active mass loading of TiNbCT_x and Ti₂CT_x electrodes was standardized at 1.2 mg cm⁻² with an error margin of ±0.1 mg cm⁻².

To gain a deeper understanding of the reaction kinetics and mechanisms of TiNbCT_x and Ti₂CT_x, we recorded CV curves at different scan rates (Fig. 4a, d). The results show that, at the same scan rate, TiNbCT_x exhibits sharper and more distinct oxidation and reduction peaks, indicating more significant ion insertion and extraction processes. To better quantify the contribution of capacitive behavior, we calculated the b values for the TiNbCT_x MXene electrode at various scan rates. These values were determined based on the relationship between the peak current and scan rate from the CV curves. For the oxidation and reduction peaks, the b values were 0.83 and 0.66, respectively (Fig. S7). These values are close to 1, suggesting that pseudocapacitance is the dominant contributor to the overall capacitance of the TiNbCT_x electrode. Figure 4b, e illustrate how the contribution of capacitive behavior changes as the scan rate increases. Compared to Ti₂CT_x, TiNbCT_x shows a higher contribution from non-capacitive behavior, indicating more pronounced battery-type characteristics. Typically, intercalation-based materials provide higher specific capacity and energy density because they utilize the entire volume of the material for energy storage. Moreover, as the scan rate increases from 0.3 to 10 mV s⁻¹, the pseudocapacitive contribution of TiNbCT_x gradually increases. At a scan rate of 10 mV s⁻¹, the capacitive contribution reaches 84%, demonstrating clear pseudocapacitive behavior and contributing to superior cycling stability. Figure S8 provides a visual representation of the ratio of capacitive behavior during the rapid diffusion of Al ions at a scan rate of 1 mV s⁻¹, showing the typical voltage distribution of capacitive current relative to the total current. At this rate, the intercalation mechanism contributes 46%, while the adsorption mechanism accounts for 54%. Figure 4c, f, respectively, show the self-discharge curves of the Ti₂CT_x and TiNbCT_x MXene cathode at a charge rate of 200 mA g⁻¹, indicative of a more stable discharging plateau of the TiNbCT_x compared to the Ti₂CT_x cathode.

Fig. 4: Capacitive contribution of Ti₂CT_x MXene and TiNbCT_x MXene.

a CV curves of Ti₂CT_x MXene at different scan rates from 0.3 to 5 mV s⁻¹. b Contribution of capacitive behavior changes as the scan rate increases. c Self-discharge curve of the Ti₂CT_x MXene cathode at a charge rate of 200 mA g⁻¹. d CV curves of TiNbCT_x MXene at different scan rates from 0.3 to 10 mV s⁻¹. e Contribution of capacitive behavior changes as the scan rate increases. f Self-discharge curve of the TiNbCT_x MXene cathode at a charge rate of 200 mA g⁻¹.

First-principles calculations are used to comprehend the impact of Nb incorporation on the electrochemical performance of Ti₂CO₂. The optimized structure of the Ti₂CO₂ monolayer is shown in Fig. 5a. As our experiments suggest that Nb forms a solid solution with Ti₂CO₂, so we study a TiNbCO₂ monolayer in which the Nb and Ti are distributed evenly, as shown in Fig. 5b. The obtained band structures in Fig. 1a, b show that the Nb incorporation results in a transition from semiconducting to metallic behavior, which benefits fast electron transport during charging and discharging. As our experiments suggest that [AlCl₄]⁻ and Al³⁺ ions participate in the charging and discharging processes, we study the interaction of AlCl₄ and Al with the Ti₂CO₂ and TiNbCO₂ monolayers. The adsorption energy is calculated as E_{Ti2CO2/TiNbCO2+AlCl4/Al} – E_{Ti2CO2/TiNbCO2} – E_AlCl4/Al, where the three terms are the total energies of the Ti₂CO₂/TiNbCO₂ monolayer with one adsorbed AlCl₄/Al, the pristine Ti₂CO₂/TiNbCO₂ monolayer, and AlCl₄/Al, respectively. A more negative adsorption energy indicates stronger adsorption. We find for both the Ti₂CO₂ and TiNbCO₂ monolayers that AlCl₄ (Fig. 5c, d) and Al (Fig. 5e, f) absorb preferentially on top of a C atom, with adsorption energies of −1.35 and −1.54 eV in the case of AlCl₄ and −3.98 and −4.44 eV in the case of Al, respectively. Therefore, the Nb incorporation enhances the adsorption of AlCl₄ and Al.

Fig. 5: Geometric configuration, electronic properties, and adsorption behavior of Ti₂CO₂ and TiNbCO₂ monolayers.

a Top and side views of the Ti₂CO₂ monolayer and the corresponding band structure. b Top and side views of the TiNbCO₂ monolayer and the corresponding band structure. Top and side views of AlCl₄ adsorbed on the (c) Ti₂CO₂ and (d) TiNbCO₂ monolayers. Top and side views of Al adsorbed on the (e) Ti₂CO₂ and (f) TiNbCO₂ monolayers. Blue spheres = Ti, green spheres = Nb, gray spheres = C, red spheres = O, pink spheres = Al, and yellow spheres = Cl.

Conclusions

In this study, we synthesized TiNbCT_x MXene, a dual-transition-metal material, as a high-performance cathode for aluminum-ion batteries (AIBs). The asymmetric Ti–Nb configuration optimizes electronic properties, reduces ion migration barriers, and enhances conductivity and electrochemical activity—yielding more active sites than single-metal Ti₂CT_x MXene. TiNbCT_x exhibits exceptional electrochemical performance, delivering an initial discharge capacity of 678 mAh g⁻¹ at 0.1 A g⁻¹ and retaining a stable capacity of ~200 mAh g⁻¹ after 800 cycles at 0.2 A g⁻¹. Cyclic voltammetry confirms highly reversible redox reactions and substantial [AlCl₄]⁻ intercalation/extraction. Combined XRD, XPS, and Raman spectroscopy analyses reveal that the synergistic interplay between [AlCl₄]⁻ intercalation and surface adsorption governs the performance. Notably, Nb incorporation expands the interlayer spacing while preserving the crystal structure, facilitating rapid ion transport. At low scan rates, TiNbCT_x operates as a battery-type material, achieving high capacity and energy density, whereas capacitive behavior dominates at higher rates, ensuring remarkable cycling stability. This work not only advances a promising cathode for AIBs but also provides fundamental insights into the rational design of MXene-based materials for energy storage.

Methods

Materials and characterization

Preparation of MAX

Ti (99.7 wt% pure, 300 mesh), Nb powder (99.7 wt% pure, 300 mesh), Al powder (99.7 wt% pure, 300 mesh), C powder (99.7 wt% pure, 300 mesh), C₄H₁₃NO·5H₂O (99.7 wt% pure), and HF acid were purchased from Aladdin. In this study, the raw materials were accurately weighed according to a molar ratio of Ti:Nb:Al:C = 1:1:1.2:1. The weighed powders were mechanically ground in an agate mortar until a uniform mixture with fine particle size was achieved. The mixed powder was pre-pressed into pellets (13 mm diameter, 2 cm thickness) under 10 MPa uniaxial pressure for 5 min. This processing step enhances the precursor density, thereby facilitating homogeneous reactions during subsequent high-temperature sintering. The pellets were transferred into a crucible and subjected to calcination in a tube furnace under an argon atmosphere. The heating program was set to increase the temperature at a rate of 10 °C/min to 1000 °C, then at a rate of 5 °C/min to 1600 °C, where it was held for 1 h. After calcination, the controlled cooling rate under Ar atmosphere to 600 °C was implemented to mitigate potential thermal shock-induced damage to the tubular furnace. Furthermore, the sintered TiNbAlC MAX phase underwent mechanical fragmentation and ball-milling, yielding particles with an approximate size of 200 mesh.

Preparation of MXenes

The synthesized TiNbAlC MAX phase powder was gradually and repeatedly added to a mixed solution of 48% HF acid and 12 M concentrated hydrochloric acid in a 6:1 ratio. The mixture was then placed in a hydrothermal reactor and subjected to a 48 h etching process at 80 °C. After the etching, the resulting mixture was washed by repeated centrifugation (5000 rpm, 5 min) with deionized water to remove residual HF acid and impurities, until the pH of the supernatant reached approximately 6. The material was then treated using freeze-vacuum drying to obtain TiNbCT_x MXene bulk material with an accordion-like structure. To achieve delamination, 1 g of the exfoliated multilayer TiNbCT_x was mixed with 20 ml of 1 M TMAOH solution. After 15 h of intercalation, a water suspension of single- or few-layer MXene was successfully prepared. The suspension was then washed several times with deionized water and centrifuged to collect the single- or few-layer TiNbCT_x MXene flakes. Finally, after freeze-vacuum drying, pure single- or few-layer TiNbCT_x MXene powder was obtained.

Materials characterization

For TiNbAlC MAX, TiNbCTx MXene, and the cathode materials, we employed X-ray diffraction (XRD) to analyze their crystal structures. The microstructure and morphology of the materials were observed using scanning electron microscopy (SEM). Energy dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) were further utilized to investigate the chemical composition in detail. To accurately measure the thickness of the films, atomic force microscopy (AFM) was used. Finally, transmission electron microscopy (TEM) was employed to observe the fine morphology of the materials, and its electron diffraction capabilities were used to analyze the internal structure.

Assembly of Al−ion batteries and electrochemical measurements

The preparation of the cathode material for the AIB involves mixing TiNbCT_x MXene, carbon black powder, and polyvinylidene fluoride (PVDF) binder in a 7:2:1 mass ratio, using N-methyl-2-pyrrolidone (NMP, 99%, Alfa Aesar) as the solvent. The resulting slurry is then drop-cast directly onto a molybdenum foil current collector to form the cathode material, ensuring that the effective active material loading for each cell is 1.0–1.5 mg·cm⁻². The chloroaluminate ionic liquid electrolyte is prepared by mixing 1-ethyl-3-methylimidazolium chloride ([EMIm]Cl, 99.9%) and anhydrous high-purity aluminum chloride (AlCl₃, 99.99%) in a 1:1.3 molar ratio. Finally, the MXene cathode, aluminum (Al) anode, AlCl₃/[EMIm]Cl electrolyte, and Whatman GF/C microporous membrane separator are assembled together to complete the Swagelok-type Al battery in a glove box under an inert atmosphere. Constant current charge/discharge curves were recorded using a Neware battery cycler. Cyclic voltammetry (CV) tests were performed in a voltage range of 0.1 to 2.3 V using an EC-Lab system (Biologic VMP-3) electrochemical workstation, at various scan rates (0.1, 0.3, 0.5, 1, 3, 5, 10 mV s⁻¹). Additionally, electrochemical impedance spectroscopy (EIS) measurements were conducted over a frequency range from 1 MHz to 0.01 Hz.

Ex situ characterization of the MXene cathode

The cathode was disassembled from the Swagelok–type cells and placed in an Ar-filled glovebox, and washed with tetrahydrofuran, sonicated in dimethoxyethane for 1 h, and then dried in vacuum for 12 h. The dried cathode was then characterized via ex-situ X-ray photoelectron spectroscopy (XPS), and FTIR spectroscopy. For the AIB cathode materials during charge/discharge cycling, XRD, XPS, and Raman spectroscopy were used to reveal key information regarding the structural changes, chemical state variations, ion transport mechanisms, and electronic conduction mechanisms of TiNbCT_x MXene throughout the cycling process.

Computational method

Computational method: First-principles calculations based on density functional theory are performed by the Vienna ab-initio simulation package³², using the generalized gradient approximation of Perdew-Burke-Ernzerhof for the exchange-correlation functional. The van der Waals interaction is taken into account by means of the DFT-D3 method³³. The plane wave cutoff energy is set to 500 eV, the total energy convergence threshold is set to 10⁻⁵eV, the atomic force threshold is set to 0.01 eV/Å, and a Monkhorst-Pack k-sampling with 0.018 Å⁻¹ spacing is used. The thickness of the vacuum layer in the slab models exceeds 15 Å to avoid unphysical interaction.