## 3. Results and Discussion

Figure 1 schematically illustrates the preparation process of 3D porous Ti

_{3}C

_{2}/NiCo-MOF composites via self-assembly induced by hydrogen bonding. First, 2D NiCo-MOF nanosheets are synthesized at room temperature through coordination interaction between bimetallic ions (Ni

^{2+}, Co

^{2+}) and BDC ligands, in which both Ni and Co atoms are coordinated octahedrally by six O atoms for the generation of 2D bimetal layers separated by BDC molecules [

26], leading to plenty of −COOH groups on the MOF nanosheets. Then, multilayered Ti

_{3}C

_{2} MXene with enlarged interlayer spacing is obtained by etching Ti

_{3}AlC

_{2} with HF solution followed by alkalization, and subsequent exfoliation through sonication results in the formation of Ti

_{3}C

_{2} nanosheets whose surface is anchored by large amount of terminal groups (−F, −O, and −OH). Thus, when NiCo-MOF was added into Ti

_{3}C

_{2} nanosheets solution, the Ti

_{3}C

_{2}/NiCo-MOF composite film with interconnected porous structure was naturally constructed owing to the interlayer hydrogen bonds between MXene nanosheets and MOF nanosheets upon vacuum-assisted filtration.

X-ray diffraction (XRD) patterns of Ti

_{3}AlC

_{2}, Ti

_{3}C

_{2}, alk-Ti

_{3}C

_{2}, NiCo-MOF, and Ti

_{3}C

_{2}/NiCo- MOF-0.4 are illustrated in

Figure 2. Clearly, typical diffraction peaks corresponding to Ti

_{3}AlC

_{2} phase ((JCPDS) card No. 52-0875) can be observed in

Figure 2a. After HF etching of Ti

_{3}AlC

_{2}, the sharp (104) diffraction peak at around 39° almost vanishes, suggesting a successful transformation from Ti

_{3}AlC

_{2} to Ti

_{3}C

_{2} caused by the removal of etched Al layers [

28]. Simultaneously, a strong (002) peak shifts from 9.52° for Ti

_{3}AlC

_{2} to 8.87° for Ti

_{3}C

_{2} MXene with an increasing of the interlayer spacing from 0.93 to 1.0 nm. Recent studies have indicated that the interlayer spacing can be further enlarged after treatment of Ti

_{3}C

_{2} with strong alkaline solution [

29]. Therefore, when using LiOH as an alkalizer in our experiment, the distance between the layers of Ti

_{3}C

_{2} is efficiently expanded, as evidenced by the XRD pattern of alk-Ti

_{3}C

_{2} in which the (002) peak shifts to 7.27° (

d = 1.22 nm). The alkalization process facilitates the delamination of Ti

_{3}C

_{2} and the intercalation of Li

^{+}.

Figure 2b depicts the crystal features of NiCo-MOF and Ti

_{3}C

_{2}/NiCo-MOF. NiCo-MOF exhibits three main peaks at 8.8°, 15.5°, and 18.2° that are indexed to (200), (001), and (201) planes, respectively, which are typical characteristics of NiCo-MOF ultrathin nanosheets synthesized with BDC ligands [

26,

30]. After combination with Ti

_{3}C

_{2} nanosheets, the diffraction peaks generated by the Ti

_{3}C

_{2}/NiCo-MOF composite are similar to those from pure NiCo-MOF and Ti

_{3}C

_{2} MXene, indicating that the presence of the MXene nanosheets has little effect on the crystal structure of the NiCo-MOF. However, the (002) peak for Ti

_{3}C

_{2}/NiCo-MOF composite shifts to lower angle 6.3° compared with that of alk-Ti

_{3}C

_{2} in

Figure 2a. The further increased interlayer spacing strongly suggests that MOF nanosheets interleave Ti

_{3}C

_{2} layers to effectively overcome the self-restacking of MOF or Ti

_{3}C

_{2} flakes, which in turn provides easy access for electrolyte ions during electrochemical reaction and guarantees high-rate capability.

The morphology and microstructure of Ti

_{3}C

_{2}, alk-Ti

_{3}C

_{2}, exfoliated Ti

_{3}C

_{2}, NiCo-MOF, and Ti

_{3}C

_{2}/NiCo-MOF-0.4 were characterized by SEM. As shown in

Figure 3a, the etched Ti

_{3}C

_{2} MXene exhibits an accordion-like multilayered architecture composed of individual nanoflakes, with the spacing between flakes ranging from tens of nanometers to hundreds of nanometers. After the treatment with LiOH solution, the obtained alk-Ti

_{3}C

_{2} still preserves a multilayered structure (

Figure 2b), but the alkalization process enlarges the interlayer spacing remarkably and is very favorable for the subsequent delamination. As the suspension of alk-Ti

_{3}C

_{2} subjected to ultrasonication, 2D lamellar structures of Ti

_{3}C

_{2} nearly disappear and ultrathin nanosheets with a lateral size of several micrometers can be clearly observed in

Figure 3c, demonstrating a successful exfoliation of layered Ti

_{3}C

_{2}. A similar ultrathin morphology is also detected for the as-synthesized NiCo-MOF, which has a smaller lateral dimension than Ti

_{3}C

_{2} sheets (

Figure 3d). However, after coupling with Ti

_{3}C

_{2} nanoflakes induced by the hydrogen-bond interaction, the Ti

_{3}C

_{2}/NiCo-MOF composite exhibits 3D hierarchical architectures assembled by NiCo-MOF and Ti

_{3}C

_{2} nanosheets (

Figure 3e). The interlaced nanosheets are tightly attached to form interconnected porous networks for fast charge storage and also prevent the self-restacking of both sheets. Moreover, the cross-sectional SEM image (

Figure 3f) confirms the layered structure of the composite film with alternating Ti

_{3}C

_{2} and MOF nanosheets layers.

The detailed microstructure of the Ti

_{3}C

_{2}/NiCo-MOF-0.4 composite is further investigated by TEM. As shown in

Figure 4, Ti

_{3}C

_{2} MXene has a larger lateral size than NiCo-MOF, which is in agreement with the SEM observation in

Figure 3c,d. Small-sized NiCo-MOF sheets adhere to the surface of large MXene flakes to form a hierarchical structure, as verified by a sharp contrast between the both components (

Figure 4a). Meanwhile, the lattice fringes are very visible in high resolution transmission electron microscopy (HRTEM) image and lattice spacing assigned to (103) plane is about 0.247 nm (

Figure 4b), which is in accordance with that of MXene phase [

28]. Meanwhile, NiCo-MOF does not show distinct crystal lattice as expectation owing to its low crystallinity. The selected area electron diffraction (SAED) pattern (inset in

Figure 4b) reveals the high crystallinity of Ti

_{3}C

_{2}. The EDS elemental mapping (

Figure 4c) indicates the homogeneous distribution of Ti, C, O, Co, and Ni elements in the Ti

_{3}C

_{2}/NiCo-MOF nanocomposites, which demonstrates that NiCo-MOF sheets uniformly integrate with Ti

_{3}C

_{2} sheets. In addition, to confirm the structural advantages of the porous Ti

_{3}C

_{2}/NiCo-MOF composite, N

_{2} isotherm was employed to measure the specific surface area. The Ti

_{3}C

_{2}/NiCo-MOF-0.4 composite possesses a higher BET surface area of 60.3 m

^{2} g

^{−1} than that of MXene (23.5 m

^{2} g

^{−1}) or NiCo-MOF (37.1 m

^{2} g

^{−1}) [

31]. The increased surface area caused by 3D porous structures and expanded interlayer spacing between Ti

_{3}C

_{2} sheets could offer rapid infiltration of electrolyte and more active sites for electrochemical reaction.

Raman spectra were also performed to verify the surface structure of Ti

_{3}C

_{2}, NiCo-MOF, and Ti

_{3}C

_{2}/NiCo-MOF-0.4. As can be found from

Figure 5a, pure Ti

_{3}C

_{2} MXene exhibits typical Raman peaks. In particular, the peaks at 203, 575, and 719 cm

^{−1} are attributed to A1g symmetry out-of-plane vibrations of Ti and C atoms, respectively, while those at 282, 365, and 622 cm

^{−1} correspond to the Eg group vibrations, including in-plane (shear) modes of Ti, C, and surface functional group atoms [

32]. While for pristine NiCo-MOF, the peak at 415 cm

^{−1} is assigned to Ni−O, and the two peaks at 526 and 630 cm

^{−1} correspond to Co−O stretching vibration [

33,

34], as well as others at 1423 and 1607 cm

^{−1} corresponding to C−C and C=O vibration (

Figure 5b), respectively. In addition, the peaks at 860, 1136, and 1175 cm

^{−1} can be ascribed to the deformation modes of the C−H groups, which are also the characteristics peaks of the NiCo-MOF [

35]. Note that the aforementioned peaks of Ti

_{3}C

_{2} and NiCo-MOF appear in the Ti

_{3}C

_{2}/NiCo-MOF-0.4, indicating the co-existence of NiCo-MOF and Ti

_{3}C

_{2} in the composite.

XPS measurement was carried out to investigate the surface electronic states of Ti

_{3}C

_{2}/NiCo-MOF-0.4. The XPS survey spectrum presented in

Figure 6a suggests the presence of C, Ti, O, F, Co, and Ni elements in the composite, in which F, Ti, and C elements come from Ti

_{3}C

_{2} after HF etching, while Co, Ni, and O elements originate from NiCo-MOF. High-resolution XPS spectra of Ti 2p (

Figure 6b) can be deconvoluted into four pairs of doublets for Ti−C (455.4/461 eV), Ti

^{2+} (456/461.3 eV), Ti

^{3+} (458.1/463.4 eV), and TiO

_{2} (458.9/464.5 eV) [

36]. As shown in

Figure 6c, seven peaks of O 1s XPS spectra attributed to the surface Ni−O, Ti−O

_{2−x}, O

_{sa} (surface active oxygen), C−Ti−O

_{x}, Co−O, C−Ti−(OH)

_{x} (or O=C−O), and H

_{2}O

_{ads} (adsorbed water) species are centered at 529.6, 530.1, 530.6, 531.2, 531.4, 532.1, and 533.3 eV, respectively [

37,

38]. The Ti 2p and O 1s results indicate that the Ti

_{3}C

_{2} is partially oxidized to TiO

_{2} owing to more defective and large exposed surface of nanosheets. However, considering the fact that no obvious peaks related to TiO

_{2} can be detected in the XRD pattern of Ti

_{3}C

_{2}/NiCo-MOF-0.4, this partial oxidation reaction possibly only takes place on the contact surface of Ti

_{3}C

_{2} and NiCo-MOF in view of an effective surface analysis method of XPS measurement. As for the C 1s core level spectra (

Figure 6d), it can be fitted with four peaks located at 281.6 (C−Ti), 284.6 (C−C), 286.1 (C−O), and 288.3 eV (O=C−O), respectively. For Ni 2p and Co 2p spectra in

Figure 6e, f, two peaks of Co 2p

_{1/2} (796.8 eV) and Co 2p

_{3/2} (780.2 eV) along with satellite peaks at 785.1 and 802.6 eV are observed, while the major peaks at 856.1 and 873.6 eV are assigned to Ni 2p

_{1/2} and Ni 2p

_{3/2}, respectively, which coincide with the reported value [

39,

40].

In order to further evaluate the lithium storage performance of the prepared composites, the CV profiles of Ti

_{3}C

_{2}/NiCo-MOF-0.4 as anode material for LIBs are given in

Figure 7a. It can be found that, in the first turn of the CV curve, two peaks are easily observed—the first peak at around 1.12 V corresponds to the irreversible solid electrolyte interphase (SEI) formation [

41], while another peak around 0.6 V can be attributed to the trapping of Li

^{+} between Ti

_{3}C

_{2} and NiCo-MOF nanosheets [

19]. During the subsequent cycles, a broad oxidation reversible peaks located at 1.24 V may be caused by the extraction of Li

^{+} from Ti

_{3}C

_{2} and NiCo-MOF nanosheets. In the second discharge cycle, the two cathodic peaks at 0.81 and 1.41 V are related to the reduction of Co

^{2+} and Ni

^{2+} to metallic Co and Ni, respectively [

38], as well as partial insertion of Li

^{+} in Ti

_{3}C

_{2}, and the peak shift is the result of the irreversible reaction during the first charge-discharge cycle. The CV curves of the second and third circles are highly coincident, indicating that the Ti

_{3}C

_{2}/NiCo-MOF-0.4 electrode is highly reversible in the electrochemical reaction process.

Figure 7b shows the charge/discharge curves for the first three cycles of the Ti

_{3}C

_{2}/NiCo-MOF-0.4 at 0.1 A g

^{−1}. During the first cycle, Ti

_{3}C

_{2}/NiCo-MOF-0.4 delivers a high discharge and charge capacity of 603.6 and 428.8 mAh g

^{−1}, respectively. The capacity loss is the result of the formation of SEI film, which is in agreement with CV results. Nevertheless, the almost overlapped discharge and charge curves of Ti

_{3}C

_{2}/NiCo-MOF-0.4 in the second and third cycles demonstrate the good reversibility and stability of the composite electrode.

The rate performance of Ti

_{3}C

_{2}/NiCo-MOF composites and Ti

_{3}C

_{2} electrodes is presented in

Figure 7c. It is obvious that the Ti

_{3}C

_{2} electrode exhibits a charge capacity of 141, 116, 85, and 67 mAh g

^{−1} at a current density of 0.1, 0.2, 0.5, and 1 A g

^{−1}, respectively. The relatively low capacity results from the slow Li ions diffusion limited by compact stacking of multi-layer Ti

_{3}C

_{2}. In the case of Ti

_{3}C

_{2}/NiCo-MOF composites, their electrochemical performance depends largely on the NiCo-MOF loading in the composite. The specific capacity of the Ti

_{3}C

_{2}/NiCo-MOF composites increases to the maximum and then decreases with the increase of NiCo-MOF content, that is, Ti

_{3}C

_{2}/NiCo-MOF-0.4 exhibits the highest capacity at the same current density compared with Ti

_{3}C

_{2} and other composite electrodes. Specially, for the Ti

_{3}C

_{2}/NiCo-MOF-0.4 electrode, a discharge capacity of 402, 366, 303, and 256 mAh g

^{−1} can be obtained at 0.1, 0.2, 0.5, and 1 A g

^{−1}, respectively. As the current density goes back to 0.1 A g

^{−1}, the capacity almost recovers its initial value, indicating excellent rate performance of the Ti

_{3}C

_{2}/NiCo-MOF-0.4 electrode.

The cycling performances of Ti

_{3}C

_{2}/NiCo-MOF composites and Ti

_{3}C

_{2} at a current density of 0.1 A g

^{−1} are summarized in

Figure 7d. Apparently, the capacity of Ti

_{3}C

_{2} shows a downward trend in the initial 30 cycles and remains at around 110 mAh g

^{−1} in 300 cycles. In contrast, that of the Ti

_{3}C

_{2}/NiCo-MOF composites decreases rapidly before 20 cycles, and then increases gradually and remains stable in the following cycling process. An initial decline in the capacity could be associated with the irreversible reaction between Ti

_{3}C

_{2}/NiCo-MOF nanosheets and the electrolyte and lithiation-induced mechanical degradation, while a subsequent increase in the capacity reveals a significant lithium-induced reactivation of the composite electrodes [

42]. As expected, Ti

_{3}C

_{2}/NiCo-MOF-0.4 exhibits superior cycling stability. It delivers a discharge capacity of 609 mAh g

^{−1} and charge capacity of 440 mAh g

^{−1} in the first cycle, respectively. The corresponding coulombic efficiency is 72.2% and nearly reaches 100% afterwards. After 300 cycles, the Ti

_{3}C

_{2}/NiCo-MOF-0.4 electrode achieves a capacity of 402 mAh g

^{−1}, much higher than all electrodes. The above results clearly indicate enhanced Li storage of Ti

_{3}C

_{2}/NiCo-MOF-0.4 in the aspect of both high capacity and excellent cycling performance, which may be connected with the 3D porous interpenetrating frameworks and enhanced electrical conductivity aroused by the coupling effect of NiCo-MOF and Ti

_{3}C

_{2}. As far as we know, the surface of Ti

_{3}C

_{2} nanosheets anchored by −F, −O, and −OH groups after HF etching and alkalization actually impedes Li

^{+} transport and reduces the conductivity of Ti

_{3}C

_{2}. After integrating with NiCo-MOF flakes, these groups from Ti

_{3}C

_{2} could be bonded to hydrogen atom in −COOH from MOF to construct 3D porous Ti

_{3}C

_{2}/NiCo-MOF composites, facilitating accessibility of composite nanosheets to the electrolyte ions. Moreover, with increasing mass ratio of NiCo-MOF to Ti

_{3}C

_{2} MXene (e.g., from 0.1 to 0.4), the specific surface area, and the interlayer spacing of composites also increase, which undoubtedly improve active sites for electrochemical reaction and speed up diffusion and transport of ions, thus leading to superior electrochemical performance of Ti

_{3}C

_{2}/NiCo-MOF-0.4 composite. As the mass ratio is further increased up to 0.5, the excessive MOF sheets with poor conductivity increase the internal resistance of composite electrode and result in performance degradation of Ti

_{3}C

_{2}/NiCo-MOF-0.5. Therefore, appropriate NiCo-MOF content in the composites is essential to achieve the optimal electrochemical performance.

To highlight the role of NiCo-MOF in acquiring enhanced Li storage performance of Ti

_{3}C

_{2}/NiCo-MOF composite electrodes, the long-term stability of the Ti

_{3}C

_{2}/NiCo-MOF-0.4 electrode at a high current density of 1 A g

^{−1} is explored. As illustrated in

Figure 8a, a high discharge capacity of 504.5 mAh g

^{−1} can be reached at the first cycle. Then, the Ti

_{3}C

_{2}/NiCo-MOF-0.4 electrode exhibits a slightly increased capacity after the initial 30 cycles and remains at a relatively high capacity of 240 mAh g

^{−1} along with a capacity retention of 85.7% even after 400 cycles, verifying its excellent long cycling life at a high rate as well. Incorporation of NiCo-MOF nanosheets into the interlayers of Ti

_{3}C

_{2} MXene results in the formation of a porous interconnected architecture, which endows the Ti

_{3}C

_{2}/NiCo-MOF-0.4 electrode with robust structural integrity to withstand the volume changes during the fast charge-discharge process, thus ensuring its high-rate capability and long cycling durability.

The prominent performance of Ti

_{3}C

_{2}/NiCo-MOF-0.4 electrode materials for LIBs can be confirmed by EIS measurement.

Figure 8b displays the Nyquist plots of the Ti

_{3}C

_{2} and Ti

_{3}C

_{2}/NiCo-MOF electrodes. The inset in

Figure 8b is an equivalent circuit model that includes bulk electrolyte resistance (

R_{s}), the charge transfer resistance

R_{ct}, and the Warburg resistance (

W_{s}) related to Li

^{+} ions diffusion in the bulk electrode [

43]. As shown in the Nyquist plots, the depressed semicircle in the medium-to-high frequency region represents

R_{ct}, and an inclined line in the low frequency range corresponds to

W_{s}. The two points that the semicircle intersects the real axis are

R_{s} and

R_{s} +

R_{ct}. All the electrodes exhibit similar

R_{s}, while the

R_{ct} of Ti

_{3}C

_{2}/NiCo-MOF-x (x = 0.2, 0.3, 0.4, and 0.5) and Ti

_{3}C

_{2} electrodes is calculated to be 58.3, 39.2, 28.1, 41.9, and 446.2 Ω, respectively. It is evident that Ti

_{3}C

_{2}/NiCo-MOF-0.4 shows the lowest charge transfer resistance, demonstrating that the synergistic effect of the NiCo-MOF and Ti

_{3}C

_{2} significantly improves the charge transfer ability of the composite electrode. Additionally, a larger slope of Ti

_{3}C

_{2}/NiCo-MOF-0.4 in the low frequency region suggests the greatly reduced Li

^{+} diffusion impedance. Both rapid electron and ion transport at the interface and fast Li

^{+} diffusion rate into electrode lead to better electrochemical performance of the Ti

_{3}C

_{2}/NiCo-MOF-0.4 electrode.

To get a better understanding of the electrochemical reaction process, the storage mechanism of the Ti

_{3}C

_{2}/NiCo-MOF-0.4 electrode is also analyzed. The CV curves of the Ti

_{3}C

_{2}/NiCo-MOF-0.4 electrode recorded at various scan rates from 0.2 to 1.0 mV s

^{−1} are plotted in

Figure 9a. In general, the variation of current (

i) with scan rate (

ν) is represented by the power law of

i =

aν^{b}, where

a and

b are adjustable parameters [

44]. The

b value of 0.5 or 1.0 corresponds to diffusion-controlled process or capacitive behavior, respectively, and it can be calculated by the slope of fitted line of log

i versus log

ν.

Figure 9b depicts the relationship between log

i and log

ν from 0.2 to 1.0 mV s

^{−1}. The Ti

_{3}C

_{2}/NiCo-MOF-0.4 composite possesses

a,

b values of 0.61 and 0.54 for the anodic and cathodic peaks, respectively, suggesting that the charge storage process is dominated by diffusion-controlled process, which leads to high capacity of Ti

_{3}C

_{2}/NiCo-MOF-0.4 via electron involved redox reaction, as aforementioned [

37]. In order to quantitatively determine the ratio of diffusion-controlled and capacitive contribution, a formula of

i =

k_{1}ν +

k_{2}ν^{1/2} is applied, where

k_{1}ν and

k_{2}ν stand for capacitive and diffusion-controlled contributions [

45]. On the basis of the analysis of CV curves using this equation, about 42.8% of capacitive contribution can be achieved at the scan rate of 0.2 mV s

^{−1} (

Figure 9c). Furthermore, it can be found that the capacitive contribution increases from 42.8% to 54.8% with the scan rate from 0.2 to 1.0 mV s

^{−1} in

Figure 9d, indicating that the Ti

_{3}C

_{2}/NiCo-MOF-0.4 composite displays enhanced rate performance.