Solvation structure design for stabilizing MXene in transition metal ion solutions

Although MXene has attracted great interest in diverse fields, it is susceptible to oxidation in water (H2O) with transition metal ions such as Co2+, Fe2+, and Cu2+, which is pronounced at high temperatures. This impedes the preparation of MXene‐based composites and their functional applications. Here, this study revealed that Co2+ increases the maximum and average atomic charge of H in H2O to improve the reactivity of H2O, which leads to the fact that Co2+ catalyzes the oxidation of Ti3C2Tx MXene. Furthermore, the addition of N,N‐dimethyl formamide (DMF) reduces the H2O activity and improves the oxidation stability of Ti3C2Tx in the presence of Co2+ via preferentially forming coordination bonds with Co2+. This strategy is also effective in enhancing the oxidation tolerance of Ti3C2Tx to Fe2+ in H2O. Moreover, it is feasible to enhance the oxidation stability of Ti2CTx MXene in H2O with the existence of Co2+. By virtue of these, the CoO/Ti3C2Tx composite was successfully prepared without obvious Ti3C2Tx oxidation, which is desirable to harness the advantages of Ti3C2Tx as the complementary component for lithium‐ion batteries. This work provides a straightforward paradigm to enhance the oxidation resistance of MXene in H2O in the presence of transition metal ions and at high temperatures, which opens a new vista to use MXene for target applications.

purification, 9 and more.However, MXene has not been widely used in large-scale applications, mostly because MXene has poor environmental stability. 10The inadequate stability originates from their inherent structural properties of surface metastable metals and is influenced by environmental factors such as storage media, 11,12 temperature, 8 illumination, 13 pH, 14 chemical reagents, 15 etc.7][18] Noteworthily, Ti 3 C 2 T x MXene can be quickly oxidized within 0.5 h at room temperature in H 2 O in the presence of transition metal ions, 19 such as Co 2+ , Fe 2+ , and Cu 2+ , and the elevated temperatures drastically accelerate Ti 3 C 2 T x oxidation. 20The oxidation resistance of MXene in transition metal ion solutions remains less unexplored while much necessary, which is desirable not only for the applications of MXene for H 2 O purification but also for the preparation of transition metal compounds/MXene composites for target applications by taking advantages of the synergy of both.Improving the stability of MXene can ensure their reliability and longevity in practical applications, ultimately contributing to the sustainability of the technologies.
Early reports indicate that H 2 O is essential for the stability of MXene, 18 based on which replacing H 2 O with nonaqueous polar solvents shows outstanding effect in inhibiting the oxidation of Ti 3 C 2 T x . 11,12Recently, it has been demonstrated that oxidative-free radicals, especially hydroxyl radical (•OH) produced from the H 2 O decomposition, have a greater negative impact on the stability of Ti 3 C 2 T x . 21The decrease in reactivity and increase in stability of H 2 O have significant implications for the stability of MXene in aqueous environments.Thus, approaches to decrease H 2 O reactivity can be effective to the preservation of MXene.By utilizing an aqueous solution of saturated inorganic salts (such as NaCl, LiCl, and CaCl 2 ) as the storage medium, the hydration effect can be leveraged to significantly reduce free H 2 O, thereby significantly extending the shelf life of Ti 3 C 2 T x . 22Furthermore, Wang et al. discovered that the reactivity of H 2 O can be decreased by increasing the number of electrons on the protons through interactions with highly polar chemicals such as dimethylacetamide and trimethyl phosphate. 23Analogously, Cao et al. demonstrated that the H 2 O reduction in dilute aqueous electrolyte can be decreased by incorporating dimethyl sulfoxide (DMSO) into ZnCl 2 -H 2 O, where DMSO replaces the H 2 O in Zn 2+ solvation sheath due to its higher Gutmann donor number (29.8) compared to that of H 2 O (18). 24,25This approach inhibits the dissolution of solvated H 2 O, which occurs due to the preferential solvation of solvents with higher Gutmann donor numbers than that of H 2 O and strong H 2 O-solvent interactions.Inspired by these findings, this study is expected to control the sol-vation structure of metal ions by introducing solvents with varying Gutmann donor numbers.
Herein, a solvation structure design is proposed by incorporating solvents of higher Gutmann donor number into metal ion solutions to enhance the oxidation tolerance of Ti 3 C 2 T x .Based on the fact that H 2 O is the key factor leading to the degradation of MXene, and transition metal ions such as Co 2+ , Fe 2+ , and Cu 2+ can accelerate the oxidation process of MXene, it is obvious that Co 2+ can improve the reactivity of H 2 O. Owing to the higher Gutmann donor number (26.6) of N,N-dimethyl formamide (DMF) compared to that of H 2 O (18), the sheath of the solvated metal ion is preferentially changed from the H 2 O-dominated one to the DMF-dominated one, resulting in partial H 2 O molecules being released from the sheath to reduce the likelihood of the interaction with the metal ion.Moreover, the strong DMF-H 2 O interaction strengthens H 2 O stability, thereby minimizing the decomposition probability.This method is effective to protect Ti 3 C 2 T x from the oxidation in aqueous solutions in the presence of Co 2+ and Fe 2+ , even at the elevated temperature.To exemplify the highlight of the finding, the CoO/Ti 3 C 2 T x composite was prepared by an in situ growth method in H 2 O using Co 2+ as the precursor of CoO without Ti 3 C 2 T x oxidation, which exhibits superior performance as the electrode material for lithium-ion batteries (LIBs).Our strategy to improve the oxidation tolerance of MXene in H 2 O in the presence of transition metal ions expands the scope of the storage and versatile applications of MXene.

RESULTS AND DISCUSSION
As mentioned previously, the presence of H 2 O facilitates the oxidation of Ti 3 C 2 T x , which will speed up in the presence of transition metal ions.To improve the oxidation tolerance of Ti 3 C 2 T x , the solvation structure of metal ions is regulated by using diverse solvents, including    Considering that there are -OH groups on the surface of Ti 3 C 2 T x , the FTIR of DMF with Ti 3 C 2 T x has been tested, the position of C = O was the same as that for pure DMF (1709 cm −1 ), indicating that the -OH of Ti 3 C 2 T x had no effect on DMF.When Ti 3 C 2 T x was added to DMF-H 2 O, there was no peak shift of the C = O bond and no new peak was observed compared to that of DMF-H 2 O.This result suggested that the interaction between DMF and MXene was not the key factor to enhance the stability of Ti 3 C 2 T x .The zeta potentials of samples were further tested.As shown in Figure S3, the zeta potentials of Ti 3 C 2 T x in DMF-H 2 O and H 2 O are −0.26 and −18.1 mV, respectively.The decreased zeta potential of Ti 3 C 2 T x in DMF-H 2 O compared to that in H 2 O is attributed to the lower permittivity of DMF (37.2) than that of H 2 O (78.4). 28After the addition of Co 2+ , which neutralizes the surface charge of Ti 3 C 2 T x , the zeta potentials of Ti 3 C 2 T x in DMF-H 2 O and H 2 O are 1.65 and −8.97 mV, respectively.This change in the zeta potential for Ti 3 C 2 T x in DMF-H 2 O from negative to positive after adding Co 2+ also results from the combination of the proton acceptors, that is, the oxygen and nitrogen atoms in DMF, with Co 2+ and protons, 29 which confirms the coordination of Co 2+ and DMF.The lower the absolute value of the zeta potential, the less stability of the colloid, 30 leading to the coagulation or aggregation.Based on this, the Ti 3 C 2 T x in DMF-H 2 O with Co 2+ exhibits relatively poor colloid stability than that in H 2 O, suggesting that the change in zeta potential caused by DMF is not the main reason for improving the oxidation stability of MXene.Additionally, the defect/oxygen vacancy concentration is directly related to the oxidation of MXene based on previous publications.The oxidation starts at the vacancy and the edge of MXene, and the number and size of the holes increase with the oxidation progress. 31hus, it is necessary to analyze how the DMF influence the defect/oxygen vacancy of MXene.From the electron paramagnetic response (EPR) spectra of our samples at the fresh state (Figure S4), a prominent signal with a g-value of 2.0 is observed, which is indicative of the presence of Ti and O vacancies within the Ti 3 C 2 T x structure.After the addition of DMF, the EPR signal of M/DMF-H 2 O is the same as that of M-H 2 O, suggesting that the DMF has no effect on the vacancy concentrations of MXene.Consequently, there is no interaction between DMF and defects on MXene to enhance the stability of MXene in Co 2+ .
X-ray absorption spectroscopy (XAS) was performed to investigate the impact of DMF on stabilizing Ti 3 C 2 T x in aqueous Co 2+ solutions.In XAS analysis, the edge position is an indicator of the effective charge present on the atom.Normalized absorption edge position shifts toward higher energy indicates more effective charge. 32The Co X-ray absorption near edge structure (XANES) spectra in Figure 3A display a noticeable shift toward lower energy in the edge position with the addition of DMF.This suggests that DMF leads to a reduction in electron transfer between Co and O in H 2 O or DMF, resulting in weakened binding strength between Co 2+ and H 2 O in the Co 2+ sol-vation sheath. 24More details on the solvation structure are provided by the Co K-edge extended X-ray absorption fine structure in Figure 3B.The solvation structure is largely confined to the first shell, corresponding to the first and strongest peak (the Co-O bond).The introduction of DMF leads to an elongation of the Co-O bond, indicating a reduction of the interaction strength between Co and the solvent. 24The weakening of the bond between Co 2+ and H 2 O enhances the stability of Ti 3 C 2 T x by hindering the decomposition of H 2 O.
Furthermore, the effect of DMF on enhancing the oxidation resistance of Ti 3 C 2 T x in H 2 O with Co 2+ at 100 • C for 10 h was examined.As shown in Figure 3C,D, XRD peaks at 25.3 • , 37.9 • , 47.9 • , 53.9 • , 54.9 • , 62.5 • , and 75.0 • , as well as the Raman peak at 159 cm −1 originating from TiO 2 are observed for M/H 2 O, suggesting that Ti 3 C 2 T x is significantly oxidized.Notably, these peaks are absent for M/DMF-H 2 O, which proves that the structure of Ti 3 C 2 T x is well-preserved with the assistance of DMF.These differences demonstrate the superior effect of DMF on the anti-oxidation of Ti 3 C 2 T x in the presence of Co 2+ at a high temperature, which results from the preferential solvation of DMF in Co 2+ solvation structure and the strong DMF-H 2 O interaction to inhibit the H 2 O reactivity (Figure 3E).Moreover, the protection strategy is also effective when using other transition metal ions, such as Fe 2+ .After storing with Fe 2+ for 1 day, the XRD spectra of Ti  g −1 , which increased to 134.4 m 2 g −1 for CoO/Ti 3 C 2 T x .This can be ascribed to the dispersing effect of Ti 3 C 2 T x as the growth platform of CoO.This is confirmed by the pores size analysis based on the Barret-Joyner-Halenda method (Figure S7), revealing that the pore volume of CoO/Ti 3 C 2 T x is higher than that of CoO.
The morphologies of samples have been analyzed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM).It is obviously seen from the SEM images (Figure 4D,E) that CoO/Ti 3 C 2 T x exhibits a large sheet with dispersed small nanosheets, while bare CoO presents a stacked structure of nanosheets (Figure 4F).CoO/Ti 3 C 2 T x with a typical 2D structure embedded with smaller nanosheets is revealed by the TEM image (Figure 4G).The high-resolution TEM image in Figure 4H exhibits the lattice distance of 0.48 and 0.27 nm, which match well with the (004) and (110) facet of Ti 3 C 2 T x . 34,35The lattice distance of 0.22 nm corresponds well to the (200) facet of CoO.Moreover, the energy dispersive X-ray spectra mappings of CoO/Ti 3 C 2 T x also confirmed the uniform distribution of CoO nanosheets on Ti 3 C 2 T x (Figure 4I).
Electrochemical performance of the CoO/Ti 3 C 2 T x electrode has been performed in half coin cells with Li piece as the reference and counter electrode.The cyclic voltammetry (CV) profiles of the CoO/Ti 3 C 2 T x electrode from 0.01 to 3.0 V (vs.Li + /Li) at 0.1 mV s −1 are displayed in Figure 5A.In the initial cycle, clear reduction and oxidation peaks are observed.In the lithiation progress, two reduction peaks located at 0.60 and 1.11 V correspond to the following reactions: CoO + 2Li + + 2e − → Co + Li 2 O. 36 In the following cycles, these cathodic peaks positively move to 0.75 and 1.55 V, respectively, due to the decrease in electrochemical polarization and irreversible reactions such as the formation of a solid electrolyte interphase (SEI) and the decomposition of the electrolyte. 37A small reduction peak appears at approximately 0.01 V, which was associated with the insertion of Li + into Ti 3 C 2 T x .This indicates the capacity contribution of Ti 3 C 2 T x . 38In the delithiation progress, the anodic peaks at 1.36 and 2.21 V arise from the reversible oxidation of Co to CoO, 39 which gradually stabilizes in the following cycles, suggesting good cycling performance.Figure S8 shows the discharge/charge profiles of the CoO/Ti 3 C 2 T x electrode for the first three cycles at a current density of 0.2 A g −1 within the potential window of 0.01-3.0V. Specifically, the discharge curve of the CoO/Ti 3 C 2 T x electrode for the first cycle features two plateaus at 1.70 and 0.75 V, which are attributed to the electrochemical reduction of CoO to Co and the formation of the SEI layer.Additionally, a discharge slope was observed between 0.6 and 0.01 V, indicating the insertion of Li + into the Ti 3 C 2 T x nanosheets.The corresponding charge profiles show voltage plateaus at 1.22 and 2.60 V, indicating the extraction of Li + .The CoO/Ti 3 C 2 T x electrode exhibited an initial discharge and charge capacities of 1238.7 and 685.9 mAh g −1 , respectively, and gradually stabilized in the following cycles.These findings align well with the CV results.In the CV curves and the discharge/charge profiles, there is no characteristic peak corresponding to the electrochemical reaction of DMF, suggesting that the DMF is not directly affect the electrochemical Li-ion storage mechanism.This was also confirmed by the FTIR of DMF, CoO, and CoO/Ti 3 C 2 T x (Figure S9).It can be seen that the distinctive C = O band of DMF at 1671 cm −1 is absent in the CoO/Ti 3 C 2 T x and CoO, demonstrating that there is no DMF residue in the electrode materials.
The cycling stability is shown in Figure 5B, and the CoO/Ti 3 C 2 T x electrode has a reversible capacity of 796.3 mAh g −1 at a current density of 0.2 A g −1 after 100 cycles.In contrast, the CoO electrode has a sharp decrease in capacity from 1324.1 to 427.8 mAh g −1 after 100 cycles.It is clearly seen that Ti 3 C 2 T x can improve the cycling stability of CoO. Figure 5C displays the rate performance at different current densities from 0.2 to 5 A g −1 .Specifically, the CoO/Ti 3 C 2 T x electrode exhibited remarkable capacity retentions of 1.0, 0.95, 0.88, 0.76, and 0.58 at current densities varying from 0.2 to 0.5, 1, 2, and 5 A g −1 , respectively.However, the capacity retention rate of the CoO electrode decreases quickly to 0.30 at 5 A g −1 .
The result suggested that the addition of Ti 3 C 2 T x greatly enhanced the rate performance of the CoO/Ti 3 C 2 T x electrode.Moreover, a table is provided to compare the rate performances of MXene-based materials for lithium-ion storage.Compared to other MXene-based materials for lithium-ion storage in recent literatures (Table S1), the CoO/Ti 3 C 2 T x electrode prepared in the present study also has outstanding advantages.The improved rate performance of the CoO/Ti 3 C 2 T x electrode is attributed to the fact that Ti 3 C 2 T x is well protected from oxidation in the presence of Co 2+ in the synthesis progress via the solvation structure design to maintain its advantages.As shown in Figure 5D, the CoO/Ti 3 C 2 T x electrode outperforms most of the reported CoO-based materials for LIBs, such as CoO/graphene nanocomposites, 40 CoO@Co 3 O 4 @C, 41 porous carbon cuboid/CoO, 42 CoO/NiO/CoNi, 43 SnO 2 -CoO@graphene, 44 and CoO/rGO. 45Furthermore, the long-term cycling stability of CoO/Ti 3 C 2 T x is shown in Figure 5E.After 1000 cycles, an ideal capacity of 618.7 mAh g −1 at 1 A g −1 is achieved for the CoO/Ti 3 C 2 T x electrode.
To further understand the outstanding electrochemical performance of the CoO/Ti 3 C 2 T x electrode, dynamic investigations were conducted.First, the electrochemical impedance spectroscopy (EIS) measurements were carried out at room temperature.Figure 6A shows the Nyquist curves of CoO/Ti 3 C 2 T x and CoO electrodes.The high-frequency semicircle in this plot indicates chargetransfer processes, while the low-frequency inclined line depicts mass transport.The data are fitted using an equivalent circuit in Figure S10 and presented in Table S2.
The R ct value of the CoO/Ti 3 C 2 T x electrode (52.9 Ω) is much smaller than that of the CoO electrode (261.7 Ω), demonstrating the decreased charge-transfer resistance with the addition of Ti 3 C 2 T x .Furthermore, the Nyquist curve of the CoO/Ti 3 C 2 T x electrode exhibited a steeper line in the low-frequency region than that of the CoO electrode, demonstrating an enhanced Li + diffusion due to the mitigation of CoO agglomeration. Figure 6B shows the relationship between Z′ and ω −1/2 , and the lower slop for the CoO/Ti 3 C 2 T x electrode indicates a higher Li + diffusion ability. 46The discrepancy in Li + transfer kinetics can be examined further using the complex model of capacitance [C(ω)] in impedance behaviors: where Z′(ω) and Z″(ω) denote the real and imaginary components of the complex impedance Z(ω), respectively.With an increase in frequency, the C′'(ω) of the CoO/Ti 3 C 2 T x electrode decreases more slowly than that of the CoO electrode, as shown in Figure S11.This indicates the rapid diffusion and transport of electrolyte ions in the CoO/Ti 3 C 2 T x electrode. 47The relaxation time constant (τ 0 , the least time necessary to discharge all energy when efficiency surpasses 50%) of the CoO electrode cannot be determined because of its slow kinetics, while the τ 0 for the CoO/Ti 3 C 2 T x electrode is calculated to be 1.48 s (Figure 6C).This suggests that the CoO/Ti 3 C 2 T x electrode has a faster discharge efficiency than that of the CoO electrode, which is attributed to the fact that the introduction of Ti 3 C 2 T x efficiently mitigates the stacked structure of CoO, thereby facilitating the charge transfer in the CoO/Ti 3 C 2 T x electrode.Temperature-dependent EIS measurements were also performed to obtain the reaction activation energy E a (Figures 6D and S12).The corresponding fitting results of Nyquist plots for CoO/Ti 3 C 2 T x and CoO electrodes are shown in Table S3.As seen in Figure 6E, the decreased E a for the CoO/Ti 3 C 2 T x electrode (62.87 kJ mol −1 ) compared to the CoO electrode (68.63 kJ mol −1 ) indicates the improved Li + diffusion, further showing the advantage of Ti 3 C 2 T x as the platform to provide more accessible Li + diffusion channels.Based on the Butler-Volmer model, the standard rate constant (k 0 ) of the electrochemical reaction is proportional to its exchange current (i 0 ).From the Tafel plots of CoO/Ti 3 C 2 T x and CoO electrodes (Figures 6F and S13), the CoO/Ti 3 C 2 T x electrode exhibits a much higher i 0 value than that of CoO electrode during the anodic scan, suggesting the faster oxidative kinetics for the CoO/Ti 3 C 2 T x electrode.
The galvanostatic intermittent titration technique (GITT) is further used to get a deep understanding of electrochemical kinetics throughout the delithiation and lithiation processes.Figure 6G shows the overall voltage profile and apparent Li + diffusivity ( Li + ).The calculated  Li + values for the CoO/Ti 3 C 2 T x electrode range from 1.0 × 10 −13 to 3.0 × 10 −12 cm −2 s −1 , which are higher than those of the CoO electrode (Figure S14).This result is in consistent with the dynamic analysis mentioned previously.To further understand the lithium storage mechanism of the as-prepared the CoO/Ti 3 C 2 T x electrode, CV analysis has been conducted with the scan rates ranging from 0.2 to 1.0 mV s −1 (Figure S15).A higher b value is observed in the CoO/Ti 3 C 2 T x electrode compared to that of the CoO electrode (Figure S16), which implies that the CoO/Ti 3 C 2 T x electrode exhibits enhanced diffusion kinetics and intercalation pseudocapacitive behavior.As shown in Figures S17, 18, and 6H, the CoO/Ti 3 C 2 T x electrode exhibits a capacitive contribution of 87.1% at 0.2 mV s −1 , which is higher than that of the CoO electrode (58.0%) and increases to 96.1% as the scan rate increases to 1.0 mV s −1 .Based on these data, it can be inferred that the processes of delithiation and lithiation processes of the CoO/Ti 3 C 2 T x electrode are accompanied by a rapid kinetics.The above analysis suggested that the favorable nanosheet structure of CoO in situ grown on 2D Ti 3 C 2 T x can effectively maintain the advantages of Ti 3 C 2 T x , leading to a high stability and fast charge capability for LIBs.

Stability test for MXene colloidal solution with transition metal ions
The oxidation test for Ti 3 C 2 T x in H 2 O with Co 2+ was carried out at room temperature with a Ti 3 C 2 T x concentration of 1 mg mL −1 , and the mass ratio between Co(NO 3 ) 2 ⋅6H 2 O and Ti 3 C 2 T x was 6:1.The anti-oxidation test for Ti 3 C 2 T x colloidal solution with transition metal ions was conducted by adding an organic solvent to H 2 O with the volume ratio of 7:2.These organic solvents included DMF, FA, AC, and ACN.The stability test of Ti 3 C 2 T x with Fe 2+ was the same as that with Co 2+ , except that the mass ratio between FeCl 2 ⋅6H 2 O and Ti 3 C 2 T x was 2:1.The oxidation test for Ti 2 CT x in H 2 O with Co 2+ was carried out at room temperature with a Ti 2 CT x concentration of 1 mg mL −1 , and the mass ratio between Co(NO 3 ) 2 ⋅6H 2 O and Ti 2 CT x was 1:20.

Preparation of the CoO/Ti 3 C 2 T x composite
The CoO/Ti 3 C 2 T x composite was synthesized via hydrothermal and subsequent calcination.In a typical experiment, hexamethylenetetramine (1.12 g) was dispersed in 14 mL of DMF.Then, 1.14 g of Co(NO 3 ) 2 ⋅6H 2 O was added into the aforementioned solution by stirring for 30 min.Next, 0.19 g of Ti 3 C 2 T x colloidal solution (47.5 mg mL −1 ) was added into the above solution and stirred for 30 min.Then, the mixture was transferred to a Teflon-lined stainless-steel autoclave (25 mL) and kept at 100 • C for 10 h.The sample was cooled down to room temperature, repeatedly rinsed with ethanol and deionized H 2 O, and freeze-dried for 36 h, which was calcined with a flow of N 2 (30 mL min −1 ) for 1 h under 300 • C. The synthetic procedure for pure CoO was identical to that for the CoO/Ti 3 C 2 T x composite, but without Ti 3 C 2 T x .The stability test for Ti 3 C 2 T x colloidal solution with Co 2+ at 100 • C was conducted in the same manner as the synthesis of the CoO/Ti 3 C 2 T x composite except that hexamethylenetetramine was not used.

Characterization
The morphology has been analyzed using a JSM-6700F spectrophotometer through field emission scanning electron microscopy (FE-SEM).The study has utilized a JEOL model Jem 2010 EX instrument to acquire TEM images, with an accelerating voltage of 200 eV.XRD analyses have been performed using a Bruker D8 Advance X-ray diffractometer that was outfitted with Ni-filtered Cu Kα radiation and operated at a scan rate of 0.2 • s −1 .XPS (Kratos, Axis Supra) was tested using an ESCALAB 250 spectrometer (PerkinElmer).FTIR has been obtained using the Nicolet iS20 FTIR spectrometer.XANES spectroscopy has been conducted on Table XAFS-500 (Specreation Instruments Co., Ltd.).To exclude the effect of oxygen in the NO 3 − from Co(NO 3 ) 2 ⋅6H 2 O, CoCl 2 ⋅6H 2 O has been used here.Raman spectroscopies have been performed on a Raman system known as LabRM Aramis.NMR spectroscopy analysis has been conducted utilizing a Bruker Avance III 400-MHz NMR spectrometer.The zeta potentials were conducted using a zeta potential analyzer (Zetasizer Nano ZSP).EPR spectra were obtained using a JES-FA300 (JEOL) with a magnetic field ranging from 0 to 2.0 T and a microwave frequency ranging from 8750 to 9650 MHz.

Electrochemical measurements
The electrochemical properties were assessed by means of coin-type cells (CR2025) utilizing lithium slice as a counter electrode on Neware within the voltage range of 0.01-3.0V.The preparation of the working electrodes (CoO/Ti 3 C 2 T x and CoO) involved the combination of the active material, carboxymethylcellulose sodium binder and super P carbon black in deionized H 2 O at a weight ratio of 8:1:1.The resulting mixture was subjected to grinding, spread onto a copper foil, and dried at 60 • C for 24 h under vacuum.The active material was loaded with a mass of approximately 1.0 mg cm −2 .The assembly of the cell was conducted within a glove box filled with argon gas.The separator was Celgard 2400 membrane, while the electrolyte was

F I G U R E 1
Characterization of Ti 3 C 2 T x in different solvents with Co 2+ .Photographs of fresh Ti 3 C 2 T x in different solvents with Co 2+ and aged solutions for 3 days at room temperature: (A) DMF-H 2 O, (B) FA-H 2 O, (C) H 2 O, (D) AC-H 2 O, and (E) ACN-H 2 O. (F) X-ray diffraction (XRD) patterns and (G) Raman spectra of Ti 3 C 2 T x with Co 2+ in DMF-H 2 O, FA-H 2 O, H 2 O, AC-H 2 O, and ACN-H 2 O after 3 days at room temperature.AC, acetone; ACN, acetonitrile; DMF, N,N-dimethyl formamide; FA, formamide.ACN show varying degrees of quantity reduction and color whitening (Figure 1C-E).The Ti 3 C 2 T x in these solvents was further examined after 3 days of storage.As shown in Figure 1F, Ti 3 C 2 T x MXene in H 2 O (M/H 2 O), AC (M/AC-H 2 O), and ACN (M/ACN-H 2 O) with the existence of Co 2+ exhibit X-ray diffraction (XRD) peaks at about 25.3 • and 47.9 • , which are attributed to TiO 2 (JCPDS no.21-1272).For M/DMF-H 2 O and M/FA-H 2 O, the diffraction peak at around 6.0 • corresponds to the (002) peak of Ti 3 C 2 T x , while those oxidation peaks corresponding to TiO 2 are absent.The (002) peak of M/FA-H 2 O shifts to lower angle and is broader and weaker than that of M/DMF-H 2 O, which could be ascribed to the intercalation of FA. 27 The Raman spectra of M/DMF-H 2 O and M/FA-H 2 O in Figure 1G show the characteristic Ti 3 C 2 T x peaks, including A 1g mode for C at 198 cm −1 and E g mode at 379 and 600 cm −1 corresponding to Ti and surface groups.As for M/H 2 O, the absence of characteristic Ti 3 C 2 T x peaks and the appearance of B 1g for TiO 2 at 159 cm −1 are evident, further indicating the extensive transition from Ti 3 C 2 T x to TiO 2 .In addition, the X-ray photoelectron spectroscopy (XPS) was conducted to obtain the surface chemical composition of M/DMF-H 2 O and M/FA-H 2 O.As shown in Figure S1, the Ti-C peaks completely disappear, and only Ti-O bonds remain for M-H 2 O, suggesting that the sur-face of Ti 3 C 2 T x is oxidized to TiO 2 .In contrast, those peaks attributed to the Ti-C bonds are well observed for M/DMF-H 2 O. Thus, the DMF effectively enhances the oxidation resistance of Ti 3 C 2 T x with the existence of Co 2+ .To understand the effect of the solvent on the antioxidation of Ti 3 C 2 T x in H 2 O with Co 2+ , molecular dynamics (MD) simulations have been conducted to ascertain the solvation structure of Co 2+

F I G U R E 2
Solvation structure characterization.(A) Snapshots of molecular dynamics (MD) simulation boxes for Co(NO 3 ) 2 in H 2 O. (B) Radial distribution functions (RDFs) and coordination number among Co 2+ , H 2 O, and NO 3 − .(C) Snapshots in MD simulation boxes for Co(NO 3 ) 2 in DMF-H 2 O. (D) RDFs and coordination number among Co 2+ , H 2 O, NO 3 − , and N,N-dimethyl formamide (DMF).(E) The maximum and average atomic charges of H atoms in various systems.(F) 1 H nuclear magnetic resonance (NMR) spectra for DMF-H 2 O and pure H 2 O.It reveals that Co 2+ increases the M H and A H in H 2 O (Co-H 2 O, 0.60, 0.50) to improve the reactivity of H 2 O, which leads to the accelerated oxidation of MXene.Additionally, the addition of DMF significantly reduces the M H and A H values in Co-DMF-H 2 O (0.57, 0.21), which in turn diminishes the reactivity of H 2 O, thereby enhancing the oxidation stability of MXene.Nuclear magnetic resonance (NMR) spectroscopy has been used to verify the interaction between DMF and H 2 O. Figure 2F demonstrates that pure H 2 O exhibits proton resonance at approximately 4.0 ppm.In the presence of DMF, the proton nuclei of H 2 O exhibit a notable upfield shift to approximately 3.8 ppm, demonstrating that the electron density of proton in DMF-H 2 O increases compared with that in pure H 2 O.The electrondonating moieties in DMF, namely the carbonyl groups (C = O), lead to an augmentation in the electron density of protons upon interaction with H 2 O, thereby inducing the shielding effect of the protons. 23The observed notable change in the resonance frequency of protons indicates a robust dipole-dipole interaction between DMF and H 2 O, which is present in both the solvation envelopes of Co 2+ and the surrounding molecules outside the solvation structure.Fourier-transform infrared spectroscopy (FTIR) results also have confirmed the robust intermolecular interaction between DMF and H 2 O.According to Figure S2, it is observed that DMF exhibits a distinctive stretching mode of the C = O group at approximately 1709 cm −1 .In the DMF-H 2 O mixture, this FTIR peak experiences a red shift to 1700 cm −1 compared with that in pure DMF, which is attributed to the HB between the C = O double bond in DMF and the O-H bond in H 2 O (C = O⋅⋅⋅H-O).
3 C 2 T x MXene in H 2 O (M/H 2 O-Fe) show a broad peak at about 22 • and a small peak at 25.7 • (Figure S5A), which correspond to amorphous C and TiO 2 , respectively.From the Raman spectroscopy of M/H 2 O-Fe shown in Figure S5B, the A 1g peak of Ti 3 C 2 T x at 197 cm −1 decreases and the B 1g peak of TiO 2 at 159 cm −1 appears.However, for Ti 3 C 2 T x MXene in DMF-H 2 O (M/DMF-H 2 O-Fe), it is observed that the structure keeps well, which indicates that the preferential coordination ability of DMF also improves the stability of Ti 3 C 2 T x with Fe 2+ .Furthermore, the feasibility of this strategy to enhance the oxidation stability of Ti 2 CT x with the existence of Co 2+ was explored.As shown in Figure S6A, XRD peaks of samples at about 7 • , 34 • , and 60 • are ascribed to the (002), (101), and (110) peaks of Ti 2 CT x , respectively.It is noticed that a new peak for Ti 2 CT x /H 2 O appears at around 25 • , which belongs to TiO 2 (JCPDS no.21-1272).This peak is absent for Ti 2 CT x /DMF-H 2 O. Furthermore, the Raman spectra of Ti 2 CT x /DMF-H 2 O show the characteristic Ti 2 CT x peaks (Figure S6B), including A 1g mode for C at 209 cm −1 and E g mode at 396 and 634 cm −1 corresponding to Ti and surface groups.As for Ti 2 CT x /H 2 O, the appearance of B 1g for TiO 2 at 155 cm −1 is evident, indicating the oxidation of Ti 2 CT x.Therefore, DMF still effectively protects the unstable Ti 2 CT x in H 2 O in the presence of Co 2+ from oxidation.

F I G U R E 3 2 F
Mechanism analysis of the effect of N,N-dimethyl formamide (DMF) on stabilizing Ti 3 C 2 T x with the existence of Co 2+ .(A) X-ray absorption near edge structure (XANES) and (B) extended X-ray absorption fine structure (EXAFS) spectra of Co 2+ -DMF-H 2 O and Co 2+ -H 2 O. (C) X-ray diffraction (XRD) patterns and (D) Raman spectra of M/DMF-H 2 O and M/H 2 O after heating at 100 • C for 10 h.(E) Schematic of the Co 2+ solvation structure in DMF-H 2 O solution.The high-temperature anti-oxidation of Ti 3 C 2 T x in H 2 O in the presence of Co 2+ is highly desirable to prepare Co-based compounds/Ti 3 C 2 T x functional composites via solvothermal reaction without compromising the attracting physicochemical properties of Ti 3 C 2 T x .Subsequently, CoO/Ti 3 C 2 T x was prepared via solvothermal reaction with the assistance of DMF.As shown in Figure 4A, for CoO/Ti 3 C 2 T x , the co-existence of CoO diffraction peaks and the (002) peak of Ti 3 C 2 T x indicates the successful in situ growth of CoO on Ti 3 C 2 T x .The Raman spectrum of CoO (Figure 4B) exhibits characteristic vibration peaks observed at 476, 527, and 669 cm −1 , which correspond to E g , T 2g , and A 1g modes in Oh symmetry, respectively. 33Similarly, the Raman spectrum of CoO/Ti 3 C 2 T x exhibited the vibration peaks of CoO and the A 1g mode of Ti 3 C 2 T x at 195 cm −1 .Notably, no TiO 2 Raman peak can be found, indicating that the oxidation of Ti 3 C 2 T x is successfully suppressed in the preparation process of CoO/Ti 3 C 2 T x .The specific surface area and pore structure have been investigated by N 2 adsorption/desorption isotherms, which displayed a distinct type-IV hysteresis (Figure 4C).The Brunauer-Emmett-Teller surface area of CoO was 116.6 m I G U R E 4 Characterization of CoO/Ti 3 C 2 T x and bare CoO.(A) X-ray diffraction (XRD) patterns, (B) Raman spectra, and (C) N 2 adsorption/desorption isotherms of CoO/Ti 3 C 2 T x and CoO.Scanning electron microscopy (SEM) images of (D and E) CoO/Ti 3 C 2 T x and (F) CoO.(G) Transmission electron microscopy (TEM), (H) high-resolution TEM (HRTEM), and (I) the corresponding energy dispersive X-ray spectra (EDS) mappings of CoO/Ti 3 C 2 T x .

F I G U R E 5
The electrochemical performance of CoO/Ti 3 C 2 T x and bare CoO electrodes.(A) The cyclic voltammetry (CV) profiles of the CoO/Ti 3 C 2 T x electrode at 0.1 mV s −1 .(B) Cycling performances of CoO/Ti 3 C 2 T x and bare CoO electrodes.(C) The rate performances of CoO/Ti 3 C 2 T x and bare CoO electrodes.(D) Comparison of gravimetric capacity of the CoO/Ti 3 C 2 T x electrode with other CoO-based anode materials.(E) The long-term cycling performance of the CoO/Ti 3 C 2 T x electrode at 1 A g −1 .

F I G U R E 6
Electrochemical kinetics analysis of CoO/Ti 3 C 2 T x and bare CoO electrodes.(A) Nyquist plots, (B) relationship between Z′ and ω −1/2 , (C) bode curves of C″' of CoO/Ti 3 C 2 T x and bare CoO electrodes.(D) Temperature-dependent Nyquist plots for the CoO/Ti 3 C 2 T x electrode.(E) The calculated activation energies, (F) Tafel plots during anodic scan, and (G) the galvanostatic intermittent titration technique (GITT) curve and  Li + of CoO/Ti 3 C 2 T x and bare CoO electrodes.(H) Capacitance-contribution percentage at different scan rates of the CoO/Ti 3 C 2 T x electrode.

of Ti 3 C 2 T x and Ti 2 CT x nanosheets
21,48referential solvation of solvents with higher Gutmann donor number than H 2 O can significantly reduce the H 2 O activity via weakening the Co 2+ -H 2 O bonds and the robust DMF-H 2 O interaction.As a result, the oxidation of Ti 3 C 2 T x with the presence of Co2+and Fe 2+ can be largely suppressed at room tempera-ture and even at elevated temperature.Our strategy is also effective in enhancing the oxidation tolerance of Ti 2 CT x to Co 2+ in H 2 O.In virtue of this strategy, the CoO/Ti 3 C 2 T x functional composite was prepared via solvothermal reaction with the structure of Ti 3 C 2 T x remaining intact, which exhibited a high lithium storage capacity of 618.7 mAh g −1 at 1 A g −1 after 1000 cycles.This investigation is expected to give a boost to the booming development of transition metal compounds/MXene composites and the applications of MXene in aqueous phase with transition metal ions, such as H 2 O purification.Ti 3 C 2 T x and Ti 2 CT x nanosheets were prepared according to the previous report.21,48