N-Butyllithium-Treated Ti3C2Tx MXene with Excellent

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N-Butylithium Treated TiCT MXene with Excellent Pseudocapacitor Performance Xifan Chen, Yuanzhi Zhu, Miao Zhang, Jinyi Sui, Wenchao Peng, Yang Li, Guoliang Zhang, Fengbao Zhang, and Xiaobin Fan ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.9b04301 • Publication Date (Web): 02 Aug 2019 Downloaded from pubs.acs.org on August 4, 2019

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N-Butylithium Treated Ti3C2Tx MXene with Excellent Pseudocapacitor Performance Xifan Chen,† Yuanzhi Zhu, Miao Zhang,† Jinyi Sui,† Wenchao Peng,† Yang Li,† GuoLiang Zhang,† Fengbao Zhang,† and Xiaobin Fan*,† † School of School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China Faculty of Chemical Engineering, Kunming University of Science and Technology, Kunming 6 50500, China.

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ABSTRACT

Mxenes, a family of two-dimensional (2D) transition-metal carbide and nitride materials, are supposed to be the promising pseudocapacitive materials because of their high electronic conductivity and hydrophilic surfaces. MXenes, prepared by removing the “A” elements of their corresponding MAX phases by hydrofluoric acid (HF) or LiF–HCl etching, possess abundant terminal groups like –F, –OH and –O groups. It has been proven that the MXenes with fewer –F terminal groups and more –O groups showed higher pseudocapacitor performance. And in organic reactions, –OH and –X (X = halogen) groups could turn to ether groups in strong nucleophilic reagent. Inspired by that, herein, we report an n-Butylithium treated method to turn the –F and – OH terminal groups to –O groups on the Ti3C2Tx MXenes. Two types of Ti3C2Tx MXenes prepared by either HF or LiF–HCl etching were systematically investigated, and comparison with the traditional KOH/NaOH/LiOH treated method was also carried out. It is found that most the –F terminal groups on the Ti3C2Tx MXenes can be successfully removed by n-Butylithium, and abundant –O terminal groups were formed. The n-Butylithium treated Ti3C2Tx MXenes show promising application in high performance pseudocapacitors. A record high capacitance of 523 F g–1 at 2 mV s-1 was obtained for the n-Butylithium treated Ti3C2Tx MXenes, and 96% capacity can be remained even after 10000 cycles.

KEYWORDS. MXenes, n-Butylithium, surface modification, few structure damage, high pseudocapacitance


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Supercapacitors are being considered as the next generation energy storage devices for their high power density, fast charge discharge ability, safe operation and long cycling life.1-5 Supercapacitors are generally divided into electrochemical double-layer capacitors and pseudocapacitors. Compared with double-layer capacitors, pseudocapacitors achieved more attention because of their higher energy density.6-10 Pseudocapcitive materials, usually the metal oxides and conducting polymers, are the most important part for pseudocapacitor.3, 11 Recently, MXenes are supposed to be a promising candidate for their high capacitance, excellent electronic conductivity, hydrophilic surfaces and long cycling life.1, 12-15 Mxenes, a family of two-dimensional (2D) transition-metal carbide and nitride materials, have a layered structure of stacked Mn+1XnTx nanosheets (M = Ti, V, Cr, Zr, Nb, Mo, etc.; X = C, N; T = F, OH, O, etc.), such as Ti3C2Tx.16 Normally, MXenes were prepared by removing the “A” elements (A = Al, Si, Sn, etc.) of their corresponding MAX phases by hydrofluoric acid (HF) or LiF–HCl etching.1,

17, 18

The obtained MXenes could be applied in microwave absorption,19

membrane separation,20 electrocatalysis and energy storage,21-24 attributed to their unique physical and chemical properties.19, 25, 26 It should be noted that the MXenes prepared by direct HF etching are usually multilayer, while those from the LiF–HCl method are few-layer nanosheets.1, 18 These two kinds of MXenes are different not only in layer number and surface terminal groups,27 but also in electrochemical properties. For example, the LiF–HCl prepared MXenes could achieve high capacitance of 245 F g–1 in supercapacitor, but the capacitance of the HF prepared MXenes is lower than 100 F g–1.1, 28 This discrepancy may be ascribed to their different interlayer distance and surface terminal groups.1 In recent reports, many researchers demonstrated that the properties of MXenes depend on the surface terminal groups.21, 28-32 Xiaohui Wang et al. reported that the – O terminal group of MXene is involved in bonding with hydronium in the H2SO4 electrolyte,


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giving rise to the pseudocapacitance in the acidic electrolyte.33 The pseudocapacitor charge storage mechanism in Ti3C2Tx in H2SO4 electrolyte is generally recognized as the followed reaction:34, 35 Ti3C2Ox(OH)yFz +δe– +δH+ = Ti3C2Ox–δ(OH)y+δFz

(1)

It is clear that bonding between oxygen functional groups and hydronium ion from H2SO4 electrolyte occurs during the charging process, and the change of Ti oxidation state lead to the high pseudocapacitance.34 However, the F terminations of MXene is a detriment to charge storage and F– is not known to participate in any pseudocapacitive energy storage processes.28 According to this mechanism, the more –O terminal groups, or the fewer –F terminal groups, the higher performance of MXene in pseudocapacitor can be expected. Therefore, significant advances have been made in MXene surface modification over the years.28, 30, 36 For example, annealing is a common method to remove the surface functional groups, and annealed Ti3C2Tx in Ar demonstrated the improved capacitance of 270 F g–1 due to fewer –F groups and larger interlayer distance.30 However, MXene materials are easily oxidized during the annealing process even with inert gas protection, and the oxidation products like TiO2 could affect the electrochemical behavior. On the other hand, the surface F terminations can be also replaced by alkali treatment.37 It is found that KOH/NaOH/LiOH modified Ti3C2Tx achieved a high capacitance in supercapacitor.28,

30, 36

Although the alkalization treatment is convenient, the alkaline aqueous

solution cannot eliminate the F terminations, while abundant MXene nanoribbons derived from the MXene nanosheets were usually found.36, 38, 39 A more efficient method for tuning the –O and –F terminal groups on MXene is highly desired.


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Usually, the –OH and –X (X = halogen) groups could turn to ether groups in nucleophilic reagent, in organic reactions. Inspired by that, we develop a strategy to modify the surface terminal groups of MXene. Herein, n-Butylithium (n-BuLi), as a strong nucleophilic reagent and reducing reagent,40, 41 is used to replace the –F with –O terminal groups (Figure 1). Compared with the common KOH/NaOH/LiOH modification method,28, 30, 36 the strategy here is more efficient in replacing the –F, while no damage of the MXene structure was found. More importantly, the MXene (including the multilayer Ti3C2Tx and few-layer Ti3C2Tx) after the n-BuLi modification show significantly increased capacitance in pseudocapacitor, and a record high capacitance of 523 F g–1 at 2 mV s-1 was obtained.

Figure 1. The schematic of the n-BuLi modified and alkali modified MXene synthesis.


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RESULTS and DISCUSSION Similar to previous studies, the multilayer Ti3C2Tx and few-layer Ti3C2Tx precursors were prepared by etching with 40 wt% HF and the LiF-HCl mixture, respectively.18 Both the multilayer and fewlayer Ti3C2Tx were then treated with 2.5 M n-BuLi in hexane and 2.5 M aqueous LiOH (for comparison). In brief, the n-BuLi modified MXene was prepared by treating the Ti3C2Tx with nBuLi at 50 °C for 10 days, and then the sample was washed with hexane for 4 times, followed by washed with water for 3 times. For convenience, the multilayer Ti3C2Tx and its derivatives after modified by n-BuLi and LiOH are tagged as M-Ti3C2Tx, n-M-Ti3C2Tx and L-M-Ti3C2Tx, respectively. While the FTi3C2Tx, n-F-Ti3C2Tx and L-F-Ti3C2Tx are referred to the few-layer Ti3C2Tx and its two derivatives from n-BuLi and LiOH treatment. X-ray diffraction (XRD) patterns of all these samples were shown in Figure 2. The main (104) diffraction (2θ of 39°) disappeared in M-Ti3C2Tx, compared with the Ti3AlC2 MAX (Figure 2a). This result suggests that the Al element was successfully removed after HF etching process. Compared with the M-Ti3C2Tx, the (002) peaks of both the LM-Ti3C2Tx and n-M-Ti3C2Tx shift from 8.6° to a lower angle of 7.13°, which corresponds to a dspacing of 12.38 Å. The increased d-spacing was attributed to the presence of intercalated Li+ ions between the Ti3C2Tx layers. On the other hand, no observable change can be found in the XRD patterns of F-Ti3C2Tx before and modification (Figure 2b), because of the presence of intercalation Li+ ions in F-Ti3C2Tx during the etching of Ti3AlC2 MAX.


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Figure 2. (a) XRD patterns of Ti3AlC2 MAX, M-Ti3C2Tx, L-M-Ti3C2Tx and n-M-Ti3C2Tx. (b) XRD patterns of F-Ti3C2Tx, L-F-Ti3C2Tx and n-F-Ti3C2Tx. To characterize the morphology, the scanning electron microscopy (SEM) was first employed, and the typical images of different samples were shown in Figure 3. In line with previous reports, layer structure and abundant open edges could be clearly observed in M-Ti3C2Tx (Figure 3a, Figure S1a), indicating that multilayer Ti3C2Tx was successfully prepared. After treated with LiOH, however, obvious corrosion of the edges and surface of the M-Ti3C2Tx can be readily observed (Figure 3b). In line with previous study, abundant nanoribbons can be clearly seen for prolonged (10 days) treatment (inset in Figure 3b). Interestingly, we cannot observe any change in the sample


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treated by n-Butylithium after react for 10 days. Corrosion of the few-layered counterpart (FTi3C2Tx) was also observed in the sample treated with LiOH, especially for prolonged treatment (Figure 3e, Figure S2d), whereas no corrosion can be found for the n-F-Ti3C2Tx (Figure 3f). Nitrogen gas sorption analysis also has been carried out, and the results are show in Figure S3. The Brunauer−Emmett−Teller (BET) surface areas of M-Ti3C2Tx, L-M-Ti3C2Tx, n-M-Ti3C2Tx, FTi3C2Tx, L-F-Ti3C2Tx and n-F-Ti3C2Tx are 2.5, 7.4, 13.7, 6.3, 9.7 and 22.4 m3 g–1, respectively. The n-BuLi modified MXene has lager surface area than the other samples, which may help their performance in energy storage.

Figure 3. (a-f) Typical SEM images of M-Ti3C2Tx, L-M-Ti3C2Tx, n-M-Ti3C2Tx, F-Ti3C2Tx, L-FTi3C2Tx and n-F-Ti3C2Tx, the inset images are the corresponding SEM images after 10 days alkalization.


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To explain the morphology change of the M-Ti3C2Tx and F-Ti3C2Tx after modification, systematical X-ray photoelectron spectroscopy (XPS) analysis was carried out (Figure 4). In comparison with the M-Ti3C2Tx precursor, the survey spectra (Figure 4a,c) of corresponding samples after the n-BuLi and LiOH treatment shows obvious decrease in element F and increase in element O. Specifically, the F content in M-Ti3C2Tx is estimated to be 16%, whereas the F content in L-M-Ti3C2Tx and n-M-Ti3C2Tx are only 8% and 2%, respectively (Figure 4i). Meanwhile, the O content significantly increased from 17% (M-Ti3C2Tx) to 33% and 36% after treatment by LiOH and n-BuLi, respectively, indicating the substitution of the –F by –O/–OH terminal groups. Interestingly, high-resolution C 1s spectra (Figure 4b) reveal that the C–Ti bond (peak at 281.7 eV) disappears in the sample (L-M-Ti3C2Tx) after treatment with LiOH, while that of the n-M-Ti3C2Tx remains the same with the M-Ti3C2Tx precursor. In concert with this result, the high-resolution Ti 2p spectra (Figure 4c) show much stronger TiO2 and Ti3+ 2p signal and much weaker C–Ti peak in L-M-Ti3C2Tx, when compared with M-Ti3C2Tx and n-M-Ti3C2Tx. This result suggests that abundant C–Ti bonds in M-Ti3C2Tx will be destroyed by LiOH treatment, while the C–Ti bonds of the counterpart treated with n-BuLi remains intact. Moreover, in Figure 4c, the high-resolution O 1s spectra of the n-M-Ti3C2Tx possessed the highest Ti–O signal but minimum TiO2 peak, conforming that the –O terminal could be formed in the n-BuLi treating method. This result is also supported by the electrical conductivities of different samples. The conductivities of the pristine M-Ti3C2Tx, L-M-Ti3C2Tx and n-M-Ti3C2Tx are 21000 S m–1, 2700 S m–1 and 8100 S m–1, respectively. The decrease in the conductivity of the L-M-Ti3C2Tx and n-M-Ti3C2Tx after treatment can be attributed to the changes in the surface functional groups, as well as the damage of the MXene basal structure. Compared with the LiOH treated counterpart, a relatively smaller loss in the conductivity was observed in n-M-Ti3C2Tx, because of the less damage in the MXene


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structure. For the treatment of F-Ti3C2Tx (Figure 4e-f), however, limited reduction of element F was observed by treating with LiOH, while the F content of the sample (n-F-Ti3C2Tx) treated by n-BuLi significantly reduces from 12% to 2%. Considering XPS is surface-sensitive, energy dispersive spectroscopy (EDS) analysis was also carried out to reveal the change of element content (Figure 3j). In line with the XPS results above, the F contents in M-Ti3C2Tx, L-M-Ti3C2Tx and n-M-Ti3C2Tx are 19%, 11% and 5%, respectively. Meanwhile, the O contents in M-Ti3C2Tx, L-M-Ti3C2Tx and n-M-Ti3C2Tx are 13%, 19% and 28%, respectively. Similar trend can be also observed in the treatment of F-Ti3C2Tx. To probe the mechanism for the reduction of F, the intermediate sample (n-M-Ti3C2Tx without the last water washing step) was also characterized. We found that the measured F to Ti atomic ratios in the M-Ti3C2Tx, the final n-M-Ti3C2Tx product and the n-M-Ti3C2Tx sample without the water washing step are 0.52, 0.13 and 0.32, respectively. Similar trend can be also observed during the treatment of F-Ti3C2Tx. Based on these results, we can conclude that n-BuLi can remove some –F and turn some –OH into –O in the first step. Then, the residue –F terminal groups can be further removed/substituted by –O/–OH terminal groups in the washing step. These results demonstrate the high efficiency and universal applicability of the n-BuLi treating method in changing the –F to –O terminal groups for Ti3C2Tx.


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Figure 4. (a) XPS survey spectra of M-Ti3C2Tx, L-M-Ti3C2Tx and n-M-Ti3C2Tx. High-resolution C 1s (b), O 1s (c) and Ti 2p (d) XPS spectra F-Ti3C2Tx, L-F-Ti3C2Tx and n-F-Ti3C2Tx. (e) XPS survey spectra of F-Ti3C2Tx, L-F-Ti3C2Tx and n-F-Ti3C2Tx. High-resolution C 1s (f), O 1s (g) and Ti 2p (h) XPS spectra F-Ti3C2Tx, L-F-Ti3C2Tx and n-F-Ti3C2Tx.(i) Element distribution of the samples were measured by XPS. (j) Element distribution of the samples were measured by EDS. The many lithium oxide and residue organic molecules can absorb on the surface of the n-BuLi treated MXene without the washing step, leading the obvious increase of O content compared with other samples.

To evaluate the capacitor performance of different samples, we conducted cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) experiments in the three-electrode system (Figure 5) in 1 M H2SO4 electrolyte. Platinum electrode acted as a counter electrode, and the


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reference electrode consisted of Ag/AgCl in 3 M KCl. Figure 5a showed the CVs of M-Ti3C2Tx, L-M-Ti3C2Tx and n-M-Ti3C2Tx electrode in the potential range of –0.4 to 0.2 V at 2 mV/s, and the GCD curves of these samples (at 1 A g–1) were shown in Figure 5b. Noted that no clearly separated oxidative and reductive peaks could be observed in the CV loops. According to previous report,33 reversible bonding/debonding of hydronium to/from the terminal –O are usually observed in the MXene electrode. In specific, hydronium in the H2SO4 electrolyte will bond with the terminal –O upon discharging near –0.4 V. And the CV curve of n-M-Ti3C2Tx showed sharper slope near –0.4 V, which proved the increase of –O terminal groups in the obtained n-M-Ti3C2Tx. Meanwhile, the GCD curves of these samples are nonlinear, but no plateaus appeared. According to previous report,42 the shapes of these CV loops and GCD curves conform the characteristics of pseudocapacitive materials. To provide further insight, according to the transport and charge storage mechanism,4, 13 the relationship between current (i) and scan rate (v) could be assumed as the following equation: i = avb

(2)

Where a and b are variables obtained by plotting log i and log v. The b value provides important information on the charge storage kinetics. When b = 1, the current is capacitive, whereas b = 0.5 suggests the current is diffusion limited. The CVs of n-M-Ti3C2Tx at different scan rate were obtained in Figure 5c, and the relationship between peak currents and scan rates is shown in Figure 5d. In Figure 5d, the b values for the peak current of L-M-Ti3C2Tx and n-M-Ti3C2Tx are 0.95 and 0.97, respectively, supporting the capacitor type charge storage mechanism of modified MTi3C2Tx. Thus, the gravimetric capacitance could be calculated from the CV curves according to the following equation:33


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Cg =

∫𝑖𝑑𝑉

(3)

𝑣𝑚𝑉

Where i is the current, v the voltage scan rate, m the mass of the samples in working electrodes, and V the voltage windows. From these CV curves, the M-Ti3C2Tx, L-M-Ti3C2Tx and n-M-Ti3C2Tx achieved the capacitances of 42, 197 and 354 F g–1 at 2mV s-1, respectively. The capacitance of M-Ti3C2Tx is coincide with previous study.28 The highest capacitance of n-M-Ti3C2Tx suggested the important role of the –F to –O conversion. And the relationship between the capacitance of Ti3C2Tx and n-BuLi treatment time is shown in Figure S4. Note that the n-M-Ti3C2Tx electrode can withstand 10000 cycles with more than 90% of the capacitance remained (Figure 5e,f), conforming its good cycle stability. To further investigate the energy storage mechanism, we quantified the capacitance contribution to the total current using the following reaction:29 i = 𝑘1𝑣 + 𝑘2𝑣0.5

(4)

The current (i) at a fixed potential is the sum of the two processes: capacitive (𝑘1𝑣, v is the voltage scan rate) and diffusion controlled (𝑘2𝑣0.5). By finding the values of 𝑘1 and 𝑘2, we quantified the fraction of the current originating from the capacitive and diffusion-controlled process. Accordingly, the shaded portions in Figure 5g represent the capacitive components. In Figure 5h, the capacitive components of the capacitance of n-M-Ti3C2Tx at 2, 5, 10, 20 and 50 mV s–1 are 68%, 80%, 85%, 87% and 90%, respectively. These result indicate that the main capacitance is contributed from the capacitive capacitance. According to reaction (1) and previous reports,34, 35 the capacitive capacitance include the conversion from –O to –OH, as well as the electrochemical double layer capacitance.


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For the few layered Ti3C2Tx prepared by etching with LiF–HCl, the electrochemical test of FTi3C2Tx, L-F-Ti3C2Tx and n-F-Ti3C2Tx was also carried out (Figure 5i-o). The CV loops (Figure 5i) and GCD curves (Figure 5j) also conform the characteristics of pseudocapacitive materials. And the CVs of n-F-Ti3C2Tx at different scan rate are shown in Figure 5k, and the responding relationship between peak currents and scan rates was also summarized (Figure 5l). It is revealed that the b value of n-F-Ti3C2Tx is approaching 1, implying that the energy storage mechanism is capacitor type. The capacitance of 255 F g–1 at 2mV s-1 was calculated from the CV curve of FTi3C2Tx, which is much higher than that of the M-Ti3C2Tx prepared by HF method. This result is in consisted with previous reports.1 After treated with n-BuLi, the capacitance was further increased, and an excellent capacitance of 523 F g–1 at 2 mV s-1 was achieved in the obtained n-FTi3C2Tx. Note that this capacitance is the highest value reported for surface modified MXene electrode in 1 M H2SO4 electrolyte (Table s1), demonstrating the outstanding performance of nBuLi treated method. On the contrary, however, only limited increase of the capacitance of the LiOH treated sample (L-F-Ti3C2Tx) was observed (259 F g–1 at 2mV s-1), and it can be hardly improved by long time alkalization (Figure S5). This result is in line with the XPS analysis. Electrochemical impendance spectroscopy (EIS) test also has been carried out (Figure S6). As expected, the Nyquist plots of all these samples showed negligible semicircles and a very small charge transfer resistance (Rct), in concert with the good conductivity of these samples. Note that the slope of the n-BuLi treated MXene (both n-M-Ti3C2Tx and n-F-Ti3C2Tx) in low frequencies is nearly vertical, indicating the smaller ion diffusion resistance. Moreover, the life cycle test (Figure 5m,n) demonstrates the capacitance retention of n-F-Ti3C2Tx electrode is 96% after 10000 cycles. The capacitance contribution of n-F-Ti3C2Tx was analyzed (Figure 5o and Figure S7). And the capacitive components of the capacitance of n-F-Ti3C2Tx at 2, 5, 10, 20 and 50 mV s–1 are 82%,


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84%, 86%, 89% and 91%, respectively, which suggesting that the main capacitance is contributed from the capacitive capacitance. Otherwise, the comparison of the rate performance was summarized in Figure 5p.28, 33, 43 The n-M-Ti3C2Tx and n-F-Ti3C2Tx in this study shows the stateof-the-art performance. Note that the decline in capacitance at higher scan rates is common for pseudocapacitive electrodes, since the rate of electrochemical reaction is controlled by electrolyte ion diffusion at high scan rates. KOH and NaOH treated Ti3C2Tx show similar capacitor performance with LiOH treated Ti3C2Tx, and the detail is shown in Figure S5. Note that the nBuLi treated Ti3C2Tx also show significant improvement in lithium-ion battery (Figure S6). All these results demonstrate the n-BuLi treated method here is a protective, universal and highly efficient strategy to modify the surface group of MXenes, as well as to improve the electrochemical property.


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Figure 5. (a-g) The electrochemical tests of M-Ti3C2Tx, L-M-Ti3C2Tx and n-M-Ti3C2Tx. (a) CVs in 1 M H2SO4 (at 2 mV s–1). (b) GCD curves in 1 M H2SO4 (at 1 A g–1). (c) CV curves of n-MTi3C2Tx at different scan rate. (d) The relationships between peak currents and scan rates of L-MTi3C2Tx and n-M-Ti3C2Tx. (e) Capacitance retention test of n-M-Ti3C2Tx electrode at 50 mV s–1. (f) The GCD curves of n-M-Ti3C2Tx for the 1st cycle and 10000th cycle. (g) CV of n-M-Ti3C2Tx at mV s–1, showing the capacitive (in shade) and diffusion-controlled process. (h) Capacitive and diffusion-controlled capacitance contribution of n-M-Ti3C2Tx at different scan rate. (i-o) The electrochemical tests of F-Ti3C2Tx, L-F-Ti3C2Tx and n-F-Ti3C2Tx. (i) CVs in 1 M H2SO4 (at 2 mV s–1). (j) GCD curves in 1 M H2SO4 (at 1 A g–1). (k) CV curves of n-F-Ti3C2Tx at different scan rate. (l) The relationships between peak currents and scan rates of L-F-Ti3C2Tx and n-F-Ti3C2Tx. (m) Capacitance retention test of n-M-Ti3C2Tx electrode at 50 mV s–1. (n) The GCD curves of nF-Ti3C2Tx for the 1st cycle and 10000th cycle. (o) CV of n-F-Ti3C2Tx at mV s–1, showing the capacitive (in shade) and diffusion-controlled process. (p) Comparison of rate performance reported in this work and previously for Ti3C2Tx.


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CONCLUSIONS In summary, we report here a strategy to form the –O by the –F terminal groups on Ti3C2Tx MXene, in order to improve its pseudocapacitive performance. Compared with traditional alkalization treated method, the n-BuLi modified Ti3C2Tx possessed much more –O and limited –F terminal groups without damage on the two-dimensional structure. Systematical study on the electrochemical tests of two types of Ti3C2Tx (M-Ti3C2Tx and F-Ti3C2Tx) demonstrated that nBuLi treated Ti3C2Tx shows much better performance than the LiOH treated counterpart. A record high capacitance of 523 F g–1 at 2 mV s–1 could be obtained for the n-Butylithium treated Ti3C2Tx MXenes, and 96% capacity can be remained even after 10000 cycles. The strategy here may provide a universal strategy for the modification of the MXenes family. METHODS AND EXPRIMENTALS Materials Ti3AlC2 was purchased from 11 Technology Co. Ltd, China, hydrofluoric acid (HF, ≥40 %) was purchased from Aladdin Co. Ltd (product number k1628044), n-Butylithium (2.5 M solution in hexanes) was purchased from SPC scientific Co. Ltd (product number H12001), LiF was purchased from Macklin Inc (product number L812327), LiOH was purchased from Aladdin Co. Ltd (product number A1626026), Polytetrafluoroethylene (PTFE) was purchased from Lizhiyuan Co. Ltd (product number D210C), Super p Li was purchased from Canrd Co. Ltd (product number EL-GCC-001).


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Preparation of Ti3C2Tx Multilayer Ti3C2Tx was synthesized by immersing 1g Ti3AlC2 in 10 mL 40% aqueous HF solution at room temperature (RT) for 20 h. The resulting suspensions were washed six times with deionized water and centrifuged to separate the powder until the pH of the liquid reached ~ 5. The samples were vacuum freeze dried at -50 °C for 48 h, collected carefully for further experiments. Few-layer Ti3C2Tx was prepared by LiF–HCl method. Specifically, 1 g LiF was added into 10 mL 9 M HCl aqueous solution in 100 mL Teflon container, with stirring for 20 min. Then, 1 g Ti3AlC2 was slowly added into the mixture solution, followed by a stirring at 35 °C for 24 h. After that, the mixture was washed several times with deionized water via centrifugation (5min per cycle at 10000 rpm). These washing cycles were repeated until pH 5 was achieved. After the last centrifugation cycle, the as-prepared Ti3C2Tx sediment was dispersed with deionized water, followed by sonicating at room temperature for 1 h, and centrifuging for 0.5 h at 3500 rpm. The supernatant was vacuum freeze dried at -50 °C for 48 h, collected carefully for further experiments. Modification of Ti3C2Tx 1 g multilayer/few-layer Ti3C2Tx was modified by adding 20 ml n-BuLi/hexane (2.5 M) under Ar atmosphere at 50 °C for 10 days. The samples were washed four times with hexane to remove excess n-BuLi and organic residues. n-BuLi modified Ti3C2Tx was achieved by immediately sonicating with deionized water for 2 h, and the samples were purified by centrifugation. The samples were vacuum freeze dried at –50 °C for 48 h, collected carefully for further experiments. To achieve LiOH modified Ti3C2Tx, 1 g multilayer/few-layer Ti3C2Tx was added in 20 ml aqueous solution of LiOH (2.5 M). Then, the mixture was sonicating for 2 h, and the samples were purified


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by centrifugation. The samples were vacuum freeze dried at –50 °C for 48 h, collected carefully for further experiments. Physical characterization The samples were characterized by scanning electron microscopy (SEM, S-4800, HITACHI), Xray photoelectron spectroscopy (PHI5000 Versa Probe), Nitrogen gas adsorption/desorption measurements (BJBulider SSA‐7000), four-point probe meter (RST-9 4 PROBES TECH) and Xray diffraction (XRD, D8-Focus, Bruker Axs). Electrochemical testing The n-BuLi modified Ti3C2Tx, LiOH modified Ti3C2Tx and Ti3C2Tx were made into electrode by mixing them with Super p Li carbon black, and PTFE binder in ethanol with a weight ratio of 8:1:1. The mixture was coated onto the conductive carbon cloth (WOS1002, CeTech Co., Ltd.) substrate, which was followed by drying at 80 °C for 12 h in a vacuum oven. Each fabricated supercapacitor electrode contained ∼3 mg of active material. Supercapacitor electrochemical measurements were performed in three-electrode cells, in which n-BuLi modified Ti3C2Tx, LiOH modified Ti3C2Tx and Ti3C2Tx served as the working electrode, platinum was used as the counter electrode, and Ag/AgCl in saturated KCl was the reference electrode. The electrolytes were H2SO4 (1 mol/L). Cyclic voltammograms (CV), galvanostatic charge-discharge (GCD) and Electrochemical impedance spectroscopy (EIS) measurements using a CHI760C Electrochemical Workstation (Shanghai Chenhua, China).


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ASSOCIATED CONTENT Supporting Information. Analytical data and the Li-ion batteries performance are included. The Supporting Information is available free of charge on the ACS Publications website. AUTHOR INFORMATION Corresponding Author: Xiaobin Fan * [email protected]. ORCID Xiaobin Fan: 0000-0002-9615-3866 ACKNOWLEDGMENT This study is supported by the National Natural Science Funds (No. 21878226). REFERENCES (1)

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Table of Content

High pseudocapacitor performance achieved by n-Butylithium modified Ti3C2Tx with few defects


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N-Butyllithium-Treated Ti3C2Tx MXene with Excellent

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