A general method for precise modification of –O termination on MXenes by low-pressure flash annealing

Abstract

The physicochemical properties and application performance of MXenes are fundamentally linked to their surface chemistry. Specifically, two-dimensional (2D) MXenes terminated with the –O group exhibit remarkable potential in a wide range of applications, such as energy storage, catalysis, and electronics. However, conventional synthetic techniques, such as prolonged high-temperature annealing in an electric furnace, usually cause irreversible structural damage or undesirable phase transformations of 2D MXene slabs into bulk 3D structures, resulting in a substantial reduction in the surface area and a consequent decline in application performance. Herein, we propose a general method for the precise modification of –O termination on MXenes by employing low-pressure flash annealing (LP-FA) of raw MXenes with complex and diverse terminations, guided by Le Chatelier's principle. The low-pressure environment facilitates gas-generation solid–solid reactions at relatively lower temperatures, thereby promoting the removal of superficial –F terminations and the formation of –O terminations on MXenes. Additionally, the rapid heating rate (10³ K s⁻¹) and short duration (∼5 s) of flash annealing, coupled with a lower peak annealing temperature, effectively prevent structural damage or layer-by-layer stacking of MXene slabs and enhance the potential applications of –O terminated MXenes. As an illustration of practical applications, we have demonstrated that the –O terminated Nb₂CT_x MXene exhibits an exceptional capacity of 420 mA h g⁻¹ at a current density of 50 mA g⁻¹, along with remarkable stability over more than 3000 cycles of Li-ion charge/discharge testing. This performance positions it among the highest-performing MXene anode materials. Consequently, our LP-FA method introduces an additional parameter, beyond the conventional temperature and time factors, to modulate general modifications and advance the industrial application of –O terminated MXene-based advanced functional materials.

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Ye-Chuang Han

Dr Ye-Chuang Han received his PhD from Xiamen University in 2022 and subsequently worked as a postdoctoral researcher with Prof. Zhong-Qun Tian from 2022 to 2024. After completing his postdoctoral training, he joined the Fujian Science & Technology Innovation Laboratory for Energy Materials of China (IKKEM) in 2024 as a titled associate research scientist. His research interest focuses on the synthetic methodology in a non-equilibrium high-temperature environment, advanced solid catalysts, and catalysis under multi-field coupling conditions.

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1 Introduction

MXenes (M_n+1X_nT_x) constitute an extensive family of two-dimensional (2D) transition metal carbides, nitrides, or carbonitrides, where M denotes early transition metals (e.g., Ti, V, and Nb), X represents carbon (C) or nitrogen (N), and T_x signifies various surface terminations (e.g., –F, –Cl, –OH, and –O) that form diverse and complex superficial compositions.^1–3 The phase structure and composition of MXenes are highly tunable, rendering them versatile material platforms for a broad spectrum of industrially significant applications, including energy storage, catalysis, and electronics.^4–9 Importantly, the diverse applications of MXenes are fundamentally connected to their surface chemistry, as the termination groups (T_x) significantly influence the intrinsic physicochemical properties (e.g., stability, conductivity, and hydrophilicity) and the electronic structure of MXenes.^10–13 Among the various types of MXenes, those terminated with –O are particularly noteworthy. Extensive research, predominantly theoretical calculations, has highlighted their distinctive properties and exceptional performance across numerous applications.^14–16 For instance, –O terminated MXenes (e.g., V₂C and Ti₃C₂) exhibit superior thermodynamic stability and higher specific capacity for Li- or Na-ion batteries, significantly outperforming their –F, –Cl, or –OH terminated counterparts.^16–19 Moreover, in the context of one of the most crucial electrocatalytic reactions for achieving carbon neutrality, the hydrogen evolution reaction, the Gibbs free energy for the adsorption of atomic hydrogen on –O terminated MXenes (e.g., Ti₂C, V₂C, and Nb₂C) approaches the ideal value of 0 eV, highlighting their potential as a promising family of 2D alternatives to precious metal catalysts.^20–23 In addition, –O terminated MXenes also display great potential for serving as Schottky-barrier-free metal contacts to 2D semiconductors, thereby addressing the persistent issue of contact resistance in 2D electronics.^15,24 Consequently, the experimental development of general and precise methods for the preparation of –O terminated MXenes is of paramount importance to satisfy the requirements of fundamental research and forthcoming industrial applications.

The general preparation of –O terminated MXenes is essential for exceeding the performance limitations of traditional MXene-based functional materials; however, it remains a formidable challenge. Typically, MXenes are synthesized through a top-down approach, wherein the atomically thin elemental layer A (e.g., Al, Si, and Sn) is selectively etched from parent MAX phases.^25,26 During this process, terminations occur concurrently due to the reaction between the freshly exposed M_n+1X_n slabs and the etchant species (typically fluorine-containing solution), resulting in –F and –OH terminations on the surface of MXenes.²⁷ Therefore, substantial efforts have been devoted to exploring general methodologies for modulating the surface chemistry and subsequently achieving precise modification of –O terminations on MXenes.^28,29 High-temperature annealing is the most commonly employed method for depleting –F terminations from the MXene surface, and –O terminations move to occupy the abandoned sites.³⁰ However, the conventional prolonged annealing process typically induces irreversible structural damage or even destruction of MXenes, accompanying the phase transformation from 2D sheets to a 3D bulk structure, ultimately leading to a significant decline in their electrochemical performance (Fig. 1).^30–32 Molten salts and electrochemical modification are other methods for introducing functional terminations such as –Cl and –O onto the surface. Nevertheless, these methods necessitate complex post-processing steps, which may inadvertently introduce additional terminations and generate substantial amounts of liquid waste, thereby posing environmental pollution risks.^33–36 Consequently, there is an urgent need to develop a general and industrially viable method for the precise modification of –O terminations on MXenes.

Fig. 1 Strategy comparison for the modification of –O terminations on MXenes. Traditional strategy: modification of –O terminated MXenes by increasing the reaction temperature (T↑) and extending the reaction time (t↑) to drive the reaction (ΔG < 0), and the obtained –O terminated MXenes typically show a damaged structure and degraded application performance. This work: modification of –O terminated MXenes by lowering the pressure in a reactor (P↓) to drive the reaction (ΔG < 0), and the obtained –O terminated MXenes well-maintained their 2D M_n+1X_n slab structure and possess superior application performance than typically prepared counterparts.

Herein, we present a general method for the precise modification of –O termination on MXenes by low-pressure flash annealing (LP-FA) guided by Le Chatelier's principle, far surpassing the time and energy efficiency of typical reported methods (Fig. 1). Specifically, the low-pressure environment allows the gas-generation solid–solid reactions to occur at a relatively low temperature by reducing the total Gibbs free energy, leading to the depletion of –F and the formation of stable –O terminations on MXenes. Furthermore, the rapid heating rate (10³ K s⁻¹) and short duration (∼5 s) of thermal treatment help to prevent excessive oxidation and structural collapse of basic M_n+1X_n slabs, thereby preserving the structural integrity of MXenes and are beneficial to their electrochemical performance. As a result, –O terminated MXenes exhibited exceptional performance as active anode materials for lithium-ion batteries. In particular, the –O terminated Ti₃C₂T_x exhibits a high specific capacity of 266 mA h g⁻¹ and maintains robust stability after 3000 cycles at an extremely high current density of 3 A g⁻¹. In addition, Nb₂CT_x, V₂CT_x, and Ta₄C₃T_x with a uniform –O termination were synthesized by LP-FA in 5 seconds, in which Nb₂CT_x MXene exhibits an extraordinary capacity of 420 mA h g⁻¹ at a current density of 50 mA g⁻¹, representing one of the best-performing MXene anode materials. Furthermore, the LP-FA strategy is amenable to the batch processing of surface terminations on MXenes, and for the first time we have achieved comprehensive –O termination coverage on MXene thin films with areas exceeding 100 cm² (∼2 g). This advancement holds promise for general synthesis and facilitates the upcoming industrial applications of –O terminated MXenes-based advanced functional materials.

2 Results and discussion

2.1. Synthesis and characterization of –O terminated MXenes

The synthesis of –O terminated MXenes mainly contains two processes (Fig. 2a). Raw MXene suspensions were first obtained by an etching and a subsequent exfoliation process (Fig. S1–S3†), in which tetramethylammonium hydroxide (TMAOH) or lithium chloride (LiCl) was typically used to intercalate and delaminate MXene nanosheets, and various surface terminations (e.g., –F, –OH, and –O) form on the as-etched MXenes with diverse and complex superficial compositions.²⁵ Second, the freeze-dried MXene powders or films were annealed with a transient high-temperature pulse in a low-pressure environment (Fig. S4†). As a result, four representative –O terminated MXenes (Ti₃C₂, V₂C, Nb₂C, and Ta₄C₃) with different metal elements in M sites or with different atomic layers in M_n+1X_n slabs were generally synthesized by low-pressure flash annealing (LP-FA) at a temperature of 400 °C (V₂C, Nb₂C, and Ta₄C₃) or 500 °C (Ti₃C₂) and a pressure of less than 15 Pa in just 5 seconds (Fig. 2b–e and Table S1†). The Rietveld refinement XRD results indicate that the crystal structure of Ti₃C₂T_x (LP-FA) was retained well after LP-FA treatment (Fig. 2b), and the c-lattice space of post-treated MXenes (LP-FA) decreased compared to freshly prepared raw MXenes (Fig. S1†), because the intercalated H₂O molecule and TMA⁺ intercalants start to de-intercalate and decompose during LP-FA.¹⁰ Significantly, such a high-efficiency LP-FA method represents one of the fastest rates for –O terminated MXene preparation compared to reported literature values (Fig. 2f and Table S2†) and shows great potential for basic and applied research and technology.

Fig. 2 (a) Schematic illustration of the LP-FA route from MAX to –O terminated MXenes. (b–e) Rietveld refinement XRD patterns of Ti₃C₂T_x, Nb₂CT_x, V₂CT_x, and Ta₄C₃T_x after LP-FA treatment, respectively. (f) Comparison of the overall annealing time and peak temperature employed for the O-terminated MXenes between this work and reported literature.

To further characterize the microstructure of the as-obtained –O terminated MXenes, transmission electron microscopy was employed for sample analysis. Taking the most widely studied Ti₃C₂T_x MXene system as an example, a typical nanosheet morphology was obtained after etching the parent Ti₃AlC₂ MAX and subsequent delamination treatment (TEM image, Fig. 3a). The selected area electron diffraction (SAED) technique was employed to analyse the in-plane crystal structure of as-obtained raw Ti₃C₂T_x (inset of Fig. 3a), confirming the typical hexagonal structure with interplanar spacings similar to those reported in previous studies and consistent with XRD results.³⁷ After LP-FA treatment of raw Ti₃C₂T_x MXene (Fig. S5†), no crystallographic structure or 2D morphology change was detected in the post-treated Ti₃C₂T_x MXene (LP-FA) (Fig. 3b), and distinct lattice fringes were also observed in high-resolution TEM images (Fig. S6†). Furthermore, energy dispersive spectroscopy (EDS) mapping of Ti₃C₂T_x (LP-FA) confirmed the uniform distribution of Ti and O, with a significant reduction in –F termination (different colours for the same brightness; Fig. 3c). The specific atomic arrangement of Ti₃C₂T_x (LP-FA) along the c-axis was analysed using scanning transmission electron microscopy (STEM). As shown in Fig. 3d and e, each Ti₃C₂T_x (LP-FA) monolayer is observed. The STEM-EELS line scan reveals that Ti and O atoms are alternately arranged, while the signal from the F element is negligible. To further demonstrate the advantages of the LP-FA strategy, tube furnace annealing (TF) and flash annealing (FA) under an argon atmosphere were employed in the thermal treatment of raw Ti₃C₂T_x. The structure of MXenes begins to partially decompose from Ti₃C₂T_x to TiC_x after undergoing long-term heating, even in an inert atmosphere (Fig. 3f), concordant with previously reported results.³⁸

Fig. 3 TEM images (inset: SAED patterns) of (a) Ti₃C₂T_x and (b) Ti₃C₂T_x (LP-FA). (c) STEM image of Ti₃C₂T_x (LP-FA) and the corresponding EDS mapping images for Ti, O and F. (d and e) Atomic-scale STEM-EELS mapping and intensity line profile of Ti₃C₂T_x (LP-FA). (f) TEM image of Ti₃C₂T_x (TF).

2.2. Mechanism of low-pressure flash annealing

To further explore the in-depth synthetic mechanisms of –O terminated MXenes by LP-FA, mechanistic studies were performed both theoretically and experimentally. Thermodynamics-driven chemical reactions occurred during the annealing process, and key reactions that involve the generation of Ti₃C₂O_x can be described by eqn (1) and (2).³¹ Both reactions produce gas-phase products, and qualitative analysis of Gibbs free energy (ΔG) for eqn (1) and (2) are presented in eqn (3) and (4), respectively. The thermodynamical analysis suggests that temperature (T) and partial pressure of gas-phase products (p_H2O or p_F2) are two core synthetic parameters that determine the elimination of –F and the formation of –O on MXene.

We propose that the pressure parameter that is easily ignored during the annealing of MXenes may pave a facile and highly efficient route for the modification of –O terminations on MXenes. Lowering the partial pressure of gaseous products offers two major advantages: (i) leads to a remarkable decrease in ΔG and allows the reaction to occur at a lower temperature, which obeys Le Chatelier's principle;^39,40 (ii) accelerates the desorption and diffusion of generated F₂ or H₂O molecules (following Gilliland's equation), which are corrosive toward MXene slabs at elevated temperatures.^40,41 Based on the theoretical analyses above, it is reasonable to conclude that the LP-FA method applies to the general modification of –O terminations on MXenes, in which a high-temperature pulse with high heating or cooling rates of up to ∼10³ °C s⁻¹ and a tunable temperature window can be triggered controllably (Fig. S7†).^42,43

To elucidate the relationship between LP-FA and the structure of as-obtained MXenes, the fine composition of as-obtained –O terminated MXenes was further characterized by a combination of techniques such as energy-dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), etc. The results revealed that LP-FA-treated Ti₃C₂T_x possesses unitary –O terminations and negligible –F terminations, and the atomic percentage of F contents decreases progressively from 15 at% to 1 at% (Fig. 4a–c, S8, S9 and Tables S3–S6†). In contrast, Ti₃C₂T_x (TF) and Ti₃C₂T_x (FA) did not completely remove the –F terminations, maintaining F contents of 9 at% and 5 at%, respectively. Furthermore, Nb₂CT_x, V₂CT_x, and Ta₄C₃T_x also exhibit negligible F content variations after LP-FA in which the initial atomic percentages of F decrease from 10 at%, 8 at%, and 8 at% to 0 at%, 3 at%, and 0 at%, respectively (Fig. S10–S12, Tables S7 and S8†), confirming the generality and precision of LP-FA for the preparation of –O terminated MXenes.

Fig. 4 High-resolution XPS spectra of (a) F 1s and (b) O 1s for Ti₃C₂T_x (Raw), Ti₃C₂T_x (TF), Ti₃C₂T_x (FA) and Ti₃C₂T_x (LP-FA). (c) F content of Ti₃C₂T_x (Raw), Ti₃C₂T_x (TF), Ti₃C₂T_x (FA) and Ti₃C₂T_x (LP-FA), calculated based on XPS and EDS analyses. (d) The Ti/F atomic ratio of Ti₃C₂T_x (Raw) and various Ti₃C₂T_x (LP-FA) under different reaction temperature and time conditions, calculated based on the EDS analysis.

Subsequently, to further reveal the effects of temperature and time on F desorption from MXenes during LP-FA, a series of controlled trials were conducted. We observed an increase in the O/Ti atomic ratio with the elevation of LP-FA reaction temperature or the extension of the reaction time (Fig. 4d and Table S9†). Specifically, the results indicate that after LP-FA treatment at 400 °C for 5 seconds, the atomic ratio of Ti/F decreased from initial 3/1.85 to 3/1.1, indicating that F desorption reactions had already occurred at much lower temperatures compared with the typical tedious furnace annealing counterpart (e.g., 500 °C that lasted for hours).¹⁰ This observation further supports the facilitative role of the persistent low-pressure process in the reaction. Upon conducting further experiments at 500 °C for 3 seconds, the atomic ratio of Ti/F was 3/0.6, and substantial removal of F was observed, while at 500 °C for 5 seconds, the F content in the sample was negligible (Ti/F atomic ratio was 3/0.08 ), underscoring the critical importance of both temperature and time. After LP-FA treatment at 600 °C for 5 seconds, the F content of MXene was quite close to that treated at 500 °C for 5 seconds (Ti/F atomic ratio was 3/0.16), thus too high temperatures and excessively long LP-FA duration should be avoided. At the microlevel, across the formation of –O termination on MXenes, the removal of F leaves behind thermodynamically unstable vacancies on the MXene surface.⁴⁴ Then, oxygen species further bind to bare MXenes and result in the formation of a stable –O terminated MXene, which is beneficial for performance improvement.⁴⁵

2.3. Electrochemical characterization

In previous studies, the Li-ion storage capacity of MXenes was found to be strongly dependent on their surface terminations.^16,46 The –F terminations hinder Li⁺ transport, whereas –O terminated MXenes possess superior capacities due to lower adsorption energies than other functional group terminated or bare MXenes.⁴⁷ The uniform –O terminations and an intact laminar structure make Ti₃C₂T_x (LP-FA) a promising candidate for Li-ion storage and other electrochemical applications. The electrochemical performance of –O terminated Ti₃C₂T_x produced via different methods was evaluated for lithium-ion storage by assembling CR2032 coin-type cells with lithium metal as the counter electrode. The cyclic voltammetry (CV) profiles of Ti₃C₂T_x (Raw), Ti₃C₂T_x (TF), Ti₃C₂T_x (FA), and Ti₃C₂T_x (LP-FA) at various scan rates from 0.5 mV s⁻¹ to 100 mV s⁻¹ are shown in Fig. 5b and S13.† Specifically, Ti₃C₂T_x (LP-FA) delivers a capacity of 161 mA h g⁻¹ at a scan rate of 0.5 mV s⁻¹, corresponding to 200 F g⁻¹ (Tables S10–S13†). In comparison, Ti₃C₂T_x (Raw), Ti₃C₂T_x (TF), and Ti₃C₂T_x (FA) exhibited capacities of 85 mA h g⁻¹, 150 mA h g⁻¹ and 139 mA h g⁻¹, respectively. It is observed that the current (i) and the scan rate (v) obey a power-law relationship: i = av^b, where the b-value of 0.86 indicates that Ti₃C₂T_x (LP-FA) exhibited a mixed diffusion-controlled capacitive behavior.^48–50 The capacitive processes account for 86.3% of charge storage at 100 mV s⁻¹, which is advantageous for superior rate and long-cycle performance (Fig. S14†).

Fig. 5 Electrochemical performance of various MXene anodes. (a) Schematic illustration of the process of Li⁺ insert MXenes during lithiation. (b) Cyclic voltammetry profiles at 0.5 mV s⁻¹. (c) Electrochemical impedance measurements. (d) Voltage profiles of the Ti₃C₂T_x (LP-FA) electrode from 0.05 A g⁻¹ to 5 A g⁻¹. (e) Rate performance of Ti₃C₂T_x (Raw), Ti₃C₂T_x (TF), Ti₃C₂T_x (FA), and Ti₃C₂T_x (LP-FA) from 0.05 A g⁻¹ to 5 A g⁻¹. (f) Capacity of Nb₂CT_x, Nb₂CT_x (LP-FA), V₂CT_x, V₂CT_x (LP-FA), Ta₄C₃T_x, and Ta₄C₃T_x (LP-FA) at the current density of 0.05 A g⁻¹. (g) Long-term cycling performance at the current density of 3 A g⁻¹ and the corresponding coulombic efficiency of Ti₃C₂T_x (LP-FA). (h) Comparison of the capacity achieved by MXenes (LP-FA) and other reported Ti₃C₂T_x and graphite (the grey dash line).

Ti₃C₂T_x exhibited excellent rate performance after the LP-FA process. Various Ti₃C₂T_x MXenes at different current densities were tested, ranging from 0.05 A g⁻¹ to 5 A g⁻¹ (Fig. 5d, e and S15†). After the first cycle of lithiation and delithiation, Ti₃C₂T_x (Raw), Ti₃C₂T_x (TF), Ti₃C₂T_x (FA), and Ti₃C₂T_x (LP-FA), respectively, achieved reversible capacities of 185.9 mA h g⁻¹, 195.3 mA h g⁻¹, 215.1 mA h g⁻¹ and 269.1 mA h g⁻¹, respectively, at a current density of 0.05 A g⁻¹. It was observed that Ti₃C₂T_x (LP-FA) exhibited the best rate performance. Even at a high current density of 5 A g⁻¹, it retained a capacity of 86.2 mA h g⁻¹. When the current density returned to 0.05 A g⁻¹ after 40 cycles, a reversible capacity of 241.9 mA h g⁻¹ with nearly 100% coulombic efficiency was still restored. The high capacity and rate capability of –O terminated MXenes can be ascribed to their lower –F content and complete laminar structure, which provide more Li⁺ absorption sites and facilitate faster Li⁺ transport. Specifically, electrochemical impedance spectroscopy (EIS, Fig. 5c and S16†) measurements were also conducted to reveal the different electrochemical reaction kinetics between various MXene electrodes. As shown in Fig. 5c, a charge transfer resistance (R_ct) of 148 Ω was achieved for Ti₃C₂T_x (LP-FA), remarkably lower than Ti₃C₂T_x (Raw, 387 Ω), Ti₃C₂T_x (TF, 335 Ω), and Ti₃C₂T_x (FA, 218 Ω) counterparts, demonstrating its extraordinary Li⁺ diffusion kinetics.

The stability of various Ti₃C₂T_x MXene electrodes after 3000 cycles at a high current density of 3 A g⁻¹ was tested, following the initial two cycles at a relatively low current density of 0.05 A g⁻¹ to sufficiently activate the anode (Fig. 5g). After 3000 cycles, Ti₃C₂T_x (Raw), Ti₃C₂T_x (TF), Ti₃C₂T_x (FA), and Ti₃C₂T_x (LP-FA) delivered the specific capacity of 39 mA h g⁻¹, 137 mA h g⁻¹, 212 mA h g⁻¹, and 266 mA h g⁻¹, respectively. It was observed that Ti₃C₂T_x (TF), Ti₃C₂T_x (FA), and Ti₃C₂T_x (LP-FA) exhibited a gradual increase in capacity during the first 2000 cycles. This can be attributed to improved electrolyte accessibility to active sites, facilitated by increased interlayer spacing during cycling.^51,52 This can be demonstrated by the change in the c-lattice spacing of the Ti₃C₂T_x (LP-FA) electrode before and after cycling. As shown in Fig. S17,† the c-lattice spacing of cycled Ti₃C₂T_x (LP-FA) (14.8 Å) is significantly larger than that of the pristine Ti₃C₂T_x (LP-FA) (10.1 Å).

Furthermore, similar to Ti₃C₂T_x, the electrochemical performance of Nb₂CT_x, V₂CT_x, and Ta₄C₃T_x before and after LP-FA was also tested (Fig. S18–S20†). Compared with their pristine MXenes, Nb₂CT_x (LP-FA), V₂CT_x (LP-FA) and Ta₄C₃T_x (LP-FA) exhibited varying degrees of increasing capacity and improved rate performance (Fig. 5f and S19†). Notably, the Nb₂CT_x (LP-FA) electrode delivers a maximum specific capacity of 420 mA h g⁻¹ at 50 mA g⁻¹. Interestingly, according to previous work and our experiment, V₂CT_x exhibits a higher capacity than Nb₂CT_x; however, Nb₂CT_x (LP-FA) exhibited a higher capacity than V₂CT_x (LP-FA).⁵³ This peculiar phenomenon can be explained by the difference in the content of –F terminations after LP-FA treatment. The V–F bond (590 ± 63 kJ mol⁻¹) is much stronger than the Nb–F bond (452 kJ mol⁻¹), leading to different degrees of –F termination retention.⁵⁴ The comparison of the long-term Li-ion storage capability of MXenes (LP-FA) with other reported MXenes (Fig. 5h and Table S14†) demonstrates their excellent performance, surpassing even that of the graphite anode (depicted by the grey dashed line). The above analysis and tests demonstrate the generality and superior performance of our LP-FA strategy in rapidly and non-destructively achieving uniform –O terminated MXenes.

2.4. Scalable synthesis of –O terminated MXenes

Of particular importance is that scalability is a critical challenge in bridging experimental research and industrial application. Herein, we have designed a scaled-up high-temperature device that rapidly provides plenty of–O terminated MXenes by the LP-FA strategy (Fig. S21†). The scaled-up high-temperature device was also able to maintain a heating rate of 10³ K s⁻¹ by adjusting the current (Fig. S22†). By using the modified blade-coating method,⁵⁵ a free-standing Ti₃C₂T_x (Raw) film with an area exceeding 100 cm² and a mass over 2 g was fabricated (Fig. S23†). Thanks to the high thermal conductivity of both the carbon paper and MXene,⁵⁶ the thermal field distribution remained uniform throughout the heating process. After undergoing the same LP-FA process (500 °C for ∼5 s) as the small-scale setup, Rietveld refinement XRD results indicated that the structure of the MXene remained well-preserved with no formation of carbide or oxide phases, consistent with the original and small-scale samples (Fig. 6a). The surface terminations’ composition of large-area Ti₃C₂T_x after LP-FA (LA-Ti₃C₂T_x (LP-FA)) was investigated by XPS, confirming that the anchored –F terminations decreased from 15 at% to 1 at%, were almost entirely removed during the LP-FA process, leaving the Ti₃C₂T_x MXene with –O terminations and an intact structure (Fig. 6b, S24 and Table S4†). The aforementioned characterization and analysis results demonstrate that the LP-FA process is suitable for scalable production.

Fig. 6 Scalability of the LP-FA strategy for –O terminated MXenes. (a) Rietveld refinement XRD and (b) high resolution XPS F 1s of LA-Ti₃C₂T_x (LP-FA).

3 Conclusions

In conclusion, we present a general low-pressure flash annealing strategy (LP-FA) based on Le Chatelier's principle for the rapid modification of –O terminations on MXenes. Low-pressure reduced Gibbs free energy of gas generation reactions allows and facilitates the removal of superficial –F termination and the formation of –O termination on MXenes at a relatively lower temperature. Combining rapid heating rates (10³ K s⁻¹) and short duration (∼5 s) of flash annealing, at a lower annealing temperature, efficiently prevents excessive oxidation and structural collapse, thereby preserving the structural integrity of MXenes. This strategy is applicable to various MXenes, which include different metal elements in M sites or with different layers in M_n+1X_n slabs. Additionally, Nb₂CT_x (LP-FA), which possesses unitary –O termination and an intact structure, exhibits an extraordinary capacity of 420 mA h g⁻¹ at 50 mA g⁻¹, one of the highest values reported for an MXene electrode; Ti₃C₂T_x (LP-FA) exhibits a high specific capacity of 266 mA h g⁻¹ and maintains robust stability after 3000 cycles at a high current density of 3 A g⁻¹. We also demonstrated that the LP-FA strategy can be scaled up, achieving a yield of more than 100 cm² (∼2 g). Our work underscores the importance of a rational understanding of pressure during the synthesis of modification of –O terminations on MXenes to meet diverse applications in energy storage, electronics, and beyond.

4 Experimental section

4.1. Preparation of various MXenes

(i) ML-MXenes were prepared by selectively etching element A from MAX. Ti₃C₂T_x: 1 g of Ti₃AlC₂ MAX powder (200 mesh; 11 Technology Co., Ltd) was immersed in 17 mL HCl (36–38 wt%) and 3 mL HF (48–50 wt%) and stirred at 35 °C for 24 h. Nb₂CT_x: 1 g of Nb₂AlC MAX powder (200 mesh; 11 Technology Co., Ltd) was immersed in 10 mL HF and stirred at 35 °C for 114 h. V₂CT_x: 1 g of V₂AlC MAX powder (200 mesh; 11 Technology Co., Ltd) was immersed in 8 mL HCl and 12 mL HF and stirred at 50 °C for 72 h. Ta₄C₃T_x: 1 g Ta₄AlC₃ MAX powder (200 mesh; 11 Technology Co., Ltd) was immersed in 10 mL HF and stirred at room temperature for 100 h. The different multilayer MXenes were washed with deionized (DI) water and centrifuged (5000 rpm) to collect the sediment. The washing procedure was repeated 5–6 times until the pH of the supernatant reached 6–7.

(ii) To obtain MXenes (Raw), ML-Ti₃C₂T_x was added to 1 g LiCl and 50 mL deionized H₂O and stirred at 60 °C for 1 h. ML-Nb₂CT_x, ML-V₂CT_x, and ML-Ta₄C₃T_x were added to 30 mL tetramethylammonium hydroxide (TMAOH, 5 wt%) and stirred at 50 °C for 4 h. Subsequently, the supernatant was separated by centrifugation (8000 rpm) twice to acquire clay-like sediment. Finally, the delaminated MXenes were collected by centrifugation (3500 rpm). The supernatant was frozen in liquid nitrogen for 5 minutes and then started to freeze-dry.

4.2. Synthesis of Ti₃C₂T_x (TF), Ti₃C₂T_x (FA) and MXenes (LP-FA)

The as-prepared Ti₃C₂T_x (Raw) was calcined at 500 °C for 4 h in an Ar atmosphere, with a heating rate of 10 °C min⁻¹. Ti₃C₂T_x (FA): Ti₃C₂T_x (Raw) was positioned between two layers of carbon paper, and Joule heating triggered flash annealing was employed to synthesize Ti₃C₂T_x (FA) at 500 °C for 5 s in an Ar atmosphere. Ti₃C₂T_x (LP-FA): similar to Ti₃C₂T_x (FA), Jolue heating triggered thermal flash was also employed at 500 °C for 5 s under continuous vacuum pumping (<15 Pa). Other LP-FA comparative experiments were conducted at 400 °C for 5 s, 500 °C for 3 s and 600 °C for 5 s, respectively. Nb₂CT_x (LP-FA), V₂CT_x (LP-FA), and Ta₄C₃T_x (LP-FA): similar to Ti₃C₂T_x (LP-FA), they were synthesized at 400 °C for 5 s under continuous vacuum pumping (<15 Pa).

4.3. Material characterization

Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) were recorded on a GeminiSEM 500. Transmission electron microscopy (TEM) was performed on a JEOL JEM-2100Plus. X-ray photoelectron spectroscopy (XPS) characterization was performed with an ESCALAB 250Xi system, and the spectra were acquired with a monochromatic Al K_α source. Scanning transmission electron microscopy (STEM), selected area electron diffraction (SAED) and EDS were performed on a JEOL JEM-F200. The local elemental distribution (line scans) was analysed by highly efficient energy dispersive spectroscopy (EDS) at 300 kV with a point-to-point resolution of 0.2 nm and a maximum resolution of 0.06 nm using a high-angle annular dark-field (HAADF) high-resolution scanning transmission electron microscope. The samples for cross-sectional transmission electron microscopy (TEM) were prepared using a FEI HELIOS NanoLab600i Focused Ion Beam (FIB) system.

X-ray diffraction (XRD) patterns were recorded on a RIGAKU Ultima IV with a Cu K_α X-ray source (λ = 1.5406 Å) operating in standard mode at 40 kV and 30 mA, equipped with a D/TEX one-dimensional array detector. Full spectrum fitting (Rietveld refinement) in the 2θ range of 5–65° was carried out using GSAS II software. Refinement included background subtraction of diffraction peaks, sample displacement, and crystallographic parameters. Due to the highly anisotropic nature of MXene samples, preferred orientation and anisotropic stress refinements were also performed. Crystallographic parameters of the synthesized material were obtained through XRD Rietveld refinement.

Raman spectra were obtained using an XploRA PLUS microRaman spectrometer. The samples were excited with a 532 nm laser at a power of 1.5 mW, employing a 100× long working distance objective and a grating of 1200 gr·mm⁻¹.

4.4. Electrochemical measurements

The electrodes were obtained by mixing and grinding the anode material with Super P and 5% PVDF in a mass ratio of 70 : 15 : 15 with an appropriate amount of N-methyl-2-pyrrolidone (NMP) solution. Then, a squeegee was used to coat the electrode slurry onto copper foil. The coated electrode was dried in a vacuum at 100 °C for 10 h. The lithium sheet was utilized as the counter electrode. The electrolyte contained 1 M LiPF₆ in ethylene carbonate (EC) and dimethyl carbonate (DEC) solution (EC/DEC with a ratio of 1 : 1 by volume). The polypropylene membrane (Celgard 2500) was used as the separator. All Li half-cells were assembled in an argon-filled glove box with oxygen and water contents less than 0.1 ppm. The electrodes were assembled into CR2032 coin-type cells. All electrochemical measurements were performed at RT. The rate performance, long-term cycling stability and galvanostatic charge/discharge (GCD) profiles were tested on a LAND test system. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements were performed on a CHI660D electrochemical workstation (Shanghai CH Instrument Company, China); EIS was conducted in a frequency range of 200 kHz–10 mHz and an AC perturbation amplitude of 5 mV. The electrode sheet after cycling was opened in the glove box and cleaned with dimethyl carbonate to wash the surface. Finally, the electrode was put in a vacuum and dried overnight. The specific capacitance (F g⁻¹) and capacity (mA h g⁻¹) of the CV profiles were calculated according to the following formulae:

Q = CV (6)

where C is the gravimetric capacitance (F g⁻¹), V is the voltage window (V), t is the recording time (s), i is the response current (A), m is the mass of the working electrode (g), and Q_c (in C g⁻¹) and Q_m (in mA h g⁻¹) are the gravimetric capacities.

Data availability

All data are provided in this article and the ESI,† and any data deemed relevant can be obtained from the corresponding authors.

Author contributions

S.-H. Y and Y.-C. H. conceived the idea and designed the experiments. Y.-C. H., X. X. and L. Z. supervised the work. S.-H. Y., P.-Y. C. and T. Z. performed experiments and sample characterization. S.-F. H. performed the Rietveld refinement analysis of XRD. M.-Z. Y. and H.-Y. Y. helped revise the manuscript and draw graphs. S.-H. Yin, P.-Y. Cao and T. Zhang contributed equally to this work. S.-H. Y. and Y.-C. H. wrote the manuscript with the contribution of all authors.

Conflicts of interest

Y.-C. H. and S.-H. Y. are inventors on a patent (CN202410898789.0) that was filed on 5th July 2024 by Suzhou XRISE Advanced Material Co. Ltd. The remaining authors declare no competing interests.

Acknowledgements

This work was supported by the National Key Research and Development Program of China (Grant No. 2024YFA1509500 and 2023YFB3811303), the National Natural Science Foundation of China (Grant No. 22301106, 92372101, and 52422205), the Natural Science Foundation of Fujian Province of China (2024J09063), and the China Postdoctoral Science Foundation (Grant No. GZB20230378, 2024T170504, and 2022M722646). We thank Suzhou XRISE Advanced Material Co., Ltd for their great help in the design and development of the HTP equipment. The authors would like to thank Prof. Zhong-Qun Tian for his guidance and discussions.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5ta01646k

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