Constructing 1D/2D NiCo‐LDH Nanowire/MXene Composites for Efficient And Stable Lithium Storage

Bimetallic layered double hydroxide (LDH) delivers competitive performance in lithium‐ion battery (LIB) because of the unique layered structure. However, the electrochemical performance of LDH is hindered by the low conductivity and the huge structure change upon cycling. To face this challenge, the 1D/2D NiCo‐LDH nanowire/MXene composites (NiCo‐LDH/MXene) are dedicatedly constructed via a simple one‐step hydrothermal method. Owing to the robust conductive skeleton is enabled by MXene nanosheets and abundant electroactive sites provided by NiCo‐LDH nanowires, the NiCo‐LDH/MXene anode exhibits rapid ion diffusion, exceptional redox reaction, and superior structural stability. Consequently, it offers a high specific capacitance of 1081 mAh g−1 after 100 cycles at the current density of 100 mA g−1 and impressive rate performance, i.e., 439 mAh g−1 at the current density of 1000 mA g−1. These results will motivate the exploration of advanced electrode materials for application in energy storage devices.


Introduction
Lithium-ion battery (LIB) has been widely implemented in the energy-driven field due to its high energy density, low selfdischarge, and long cycle life. [1,2]Unfortunately, the traditional graphite anode suffering from low specific capacity cannot satisfy the ever-growing demand for high-power electronics. [3,4,5]Hence, constructing electrode materials that can effectively accommodate lithium ions is highly and urgent ly required.
As an emerging 2D material, MXene exhibits high potential in energy storage, water purification, and electrocatalysis et al., which is attributed to its good structural stability, hydrophilic surface properties, and excellent electrical conductivity. [6,7]Inevitably, similar to most 2D materials, MXene nanosheets are easy to accumulate due to the strong intermolecular DOI: 10.1002/admi.202300962interaction between the adjacent nanosheets, which severely limits their application. [8,9][12][13] In addition to preventing the aggregation of individual nanomaterials, it has been demonstrated that the unique structure and the synergistic effect for MXene-based composites are favorable to significantly enhance the electrochemical performance.In spite of this, the reported MXene-based composites are mainly based on physical mixing or electrostatic adsorption, resulting in weak bonding between materials and slow electron transport.
Recently, bimetallic layered double hydroxide (LDH) has emerged as a promising anode material for LIB due to its unique layered structure, which can supply a large number of electroactive sites and facilitate the diffusion of lithium ions. [14]Nevertheless, the inherent shortcomings associated with LDH are low conductivity and poor structure stability. [15]Meanwhile, the volume expansion of LDH is pretty large during the Li + insertion/extraction, leading to the rapid decline of the capacity upon the cycling. [16]One of the most effective strategies to address the above challenges is the in situ growth of LDH on robust and conductive substrate to obtain a composite anode for LIB. [17]In this case, the highly strengthened LDH composite electrode not only promises fast electron transfer but also restricts the deterioration of the LDH structure.For instance, Yang et al. [18] synthesized the composite of NiCo-LDH nanoparticles and reduced graphene nanosheets as anode for LIB, which exhibits a capacitance retention of 88% after 3000 cycles.In addition, it is well known that materials with 1D structure could provide a high surface-to-volume ratio and greatly shorten the ion transport distance.However, to the best of our knowledge, few reports are concerned about the rational design of sturdy heterostructure based on 1D LDH, especially in combination with 2D MXene nanosheet, which is believed to have outstanding performance in LIB.
In this work, the 1D/2D NiCo-LDH nanowires/MXene (NiCo-LDH/MXene) composite is developed by a one-step hydrothermal method with the in-situ growth of NiCo-LDH nanowires on MXene nanosheets.On one hand, NiCo-LDH nanowires with 1D nanowire structure and abundant redox reaction sites could not only be used as a spacer to prevent the restacking of MXene flakes but also ensure the fast electron transfer.On the other hand, MXene nanosheets could offer an interconnected steady conductive network and reduce the volume expansion of NiCo-LDH.Thanks to the strong synergistic effect offered by this hierarchical material, the high-performance LIB with the specific capacity of 1081 mAh g −1 after 100 cycles at the current density of 100 mA g −1 has been achieved.Our research confirms the importance of structural design of electrode materials and will provide new guidance for engineering strategies of 1D/2D heterostructures.

Results and Discussion
The schematical synthesis of the NiCo-LDH/MXene composite is drawn in Figure 1.As can be seen, the few-layered MXene is initially prepared by etching the Al Layers from Ti 3 AlC 2 with LiF/HCl and then delaminated using the sonication treatment.Afterward, the NiCo-LDH nanowires are uniformly grown on the interlayer of MXene by the hydrothermal reaction, forming the unique 1D/2D heterostructure.This novel nanoarchitecture not only could reduce the agglomeration of the respective NiCo-LDH and MXene, but also provide the robust basis to connect NiCo-LDH and MXene, leading to the accelerated electron transfer and Li + diffusion.
The morphologies of the NiCo-LDH/MXene are observed by scanning electron microscopy (SEM) and the resulting images are shown in Figure 2. It can be seen that the MXene (Figure 2a) exhibits the wrinkled 2D laminar structure, which provides a good basis to grow LDH.The SEM images of NiCo-LDH/MXene with different magnifications are revealed in Figure 2b,c, where the dense 1D NiCo-LDH nanowires are well-grown on the surface of MXene with random orientation.It is obvious that this heterostructure can efficiently prevent the aggregation of MXene layers, providing more feasible active sites for fast kinetics.Meanwhile, the interconnected NiCo-LDH nanowires could offer numerous diffusion paths, further facilitating the rapid transfer of electrons.Moreover, the MXene matrix with a robust structure is beneficial for alleviating the large volume change of NiCo-LDH.TEM images further demonstrate the growth of NiCo-LDH nanowires on the surface of the MXene flakes, as shown in Figure 2d.The lattice fringe corresponding to the (012) plane of NiCo-LDH is marked, [19] which is 0.26 nm.Besides, diffraction rings representing the (113), (104), and (015) crystal planes of LDH are captured from the FFT pattern of NiCo-LDH/MXene, demonstrating the highly polycrystalline characteristic. [20]Figure 2e shows the HAADF and the corresponding EDS mapping image of NiCo-LDH/MXene, further revealing the successful preparation of heterostructure due to the uniform distribution of Ni and Co on C and Ti.
Figure 3a presents the X-ray diffraction (XRD) patterns of MXene, NiCo-LDH, and NiCo-LDH/MXene.All the XRD patterns of NiCo-LDH and NiCo-LDH/MXene are the same except for a peak presented at 5.71°, which belongs to MXene. [21]he main diffraction peaks located at 11.8°, 19.16°, 21.9°, 34.7°, 37.5°, 39.6°, 59.98°, and 62.25°can be readily assigned to the (003), (002), (006), (012), ( 104), (015), ( 110) and (113) planes of NiCo-LDH. [22]In comparison to pure MXene, the peak related to the (002) plane shifts from 7.41°to 5.71°in NiCo-LDH/MXene, reflecting that the layer spacing of MXene increases from 1.18 to 1.52 nm.This proves that the restacking of MXene layers is effectively suppressed by introducing NiCo-LDH nanowires.Figure 3b shows the Fourier transform infrared spectrum (FTIR) of MXene, NiCo-LDH, and NiCo-LDH/MXene, which features absorption bands at 3443 cm −1 , 1630 cm −1, and 1387 cm −1 , corresponding to the O-H stretching vibration of water molecules in the interlayer, hydrogenbonded OH groups and N-O stretching vibration from NO 3 −1 , respectively.Furthermore, the absorption peak at 829 cm −1 is indexed to the metal-oxygen stretching or bending modes in the brucite-like crystal lattice of LDH.The results of FTIR clearly indicate the successful preparation of NiCo-LDH/MXene nanocomposites [23] [24,25] Besides, the peak at 289.78 eV corresponding to O-C = O emerges in NiCo-LDH/MXene, which enables to improve the binding strength between NiCo-LDH and MXene.Figure 4c exhibits the high-resolution Ti 2p XPS spectrum.The peaks at 454.89 and 460.24 eV can be assigned to Ti-C (2p 3/2 and 2p 1/2 ), while the peaks at 457.85 and 463.09 eV correspond to the Ti-O (2p 3/2 and 2p 1/2 ). [26]The high-resolution Ni 2p XPS spectrum (Figure 4d) reveals two strong peaks at 872.71 and 855.4 eV with satellite peaks at 879.62 and 860.88 eV, which are attributed to Ni 2p 1/2 and Ni 2p 3/2 . [27,28]It is noteworthy that the characteristic peak of Ni 2p of NiCo-LDH/MXene is shifted toward lower binding energy than that of NiCo-LDH, manifesting the presence of charge transfer at the interface of MXene and NiCo-LDH.In Figure 4e, the high-resolution Co 2p XPS spectrum can be divided into Co 2p 1/2 and Co 2p 3/2.Two characteristic peaks at 780.62 and 796.44 eV accompanied by two satellite peaks at 785.04 and 801.82 eV prove the presence of Co 2+ . [29,30]ikewise, the charge transfer at MXene and NiCo-LDH interface is also found due to the shift of characteristic peaks related to Co 2p.In Figure 4f, the high-resolution O 1s spectrum of NiCo-LDH/MXene demonstrates the existence of peaks located at 529.71, 531.02, and 535.02 eV, which can be assigned to Ti-O, M-OH, and H 2 O adsorbed between layers, respectively.
Figure 5a shows the cyclic voltammetry (CV) curves of the NiCo-LDH/MXene electrode measured at 0.1 mV s −1 from 0.001 to 3 V.In the first cycle, the reduction peak appearing at 0.66 V is attributed to the formation of a solid electrolyte interface (SEI) film. [31]Besides, the reduction peak located at 1.59 V is caused by the conversion of Co (Ni) ions to metal Co (Ni), while the oxidation peaks at 1.32 and 2.28 V are result from Co and Ni being oxidized to Co 2+ and Ni 2+ , respectively. [32,33]In the subsequent cycles, all curves are almost coinciding, which illustrates the excellent cycling reversibility of the NiCo-LDH/MXene electrode.Figure 5b draws the EIS (0.01-10 5 Hz) of NiCo-LDH/MXene, NiCo-LDH, and MXene electrode.It can be observed that all curves exhibit an arc in the high-frequency region followed by a linear shape in the low-frequency region.As known, the diameter of semicircles, the intercept on the real axis, and the slope of the straight line correspond to the charge-transfer resistance (R ct ), the equivalent series resistance (R s ), and the Warburg impedance (Z w ), respectively. [34]The fitted results illuminate that the R s of NiCo-LDH, MXene, and NiCo-LDH/MXene electrode is 9.68, 8.72, and 7.66 Ω, respectively.Moreover, the R ct of NiCo-LDH/MXene electrode (195 Ω) is lower than that of NiCo-LDH (203.6 Ω), featuring the advances of NiCo-LDH/MXene.Figure 5c shows the constant discharge/charge (GCD) test between 0.01 and 3 V at the current density of 100 mA g −1 .The initial discharge and charge capacities of the NiCo-LDH/MXene electrode are 1747 and 1194 mAh g −1 , respectively.As a result, the initial Coulombic efficiency (CE) is 61.3%.However, the CE increases to 95.3% after five cycles and then stabilizes at ≈98%.This signifies a fast stabilization of the SEI layer and a high reversibility of the conversion reaction. [35]Figure 5d shows the rate capability of NiCo-LDH/MXene, NiCo-LDH, and MXene electrode at different current densities.Taking NiCo-LDH/MXene electrode for example, under the current density of 0.1, 1, and 2 A g −1 , the capacitance reaches 1063, 490.1, 212.6 mAh g −1 , respectively.When the current density returns to 0.1 A g −1 , the capacitance still remains at 981.2 mAh g −1 .Figure 5e illustrates the cyclic performance of three electrodes.It can be seen that the capacitance of MXene, NiCo-LDH, and NiCo-LDH/MXene keeps at 177.4, 238.7, and 1008 mAh g −1 , respectively, after 100 cycles at 0.1 A g −1 .There is no obvious lithium dendrite in the scanned image of NiCo-LDH/MXene after 100 cycles.The prominent LIB performance based on NiCo-LDH/MXene anode is therefore demonstrated, which mainly resulted from the synergistic effect between the 1D NiCo-LDH nanowires and 2D MXene nanosheets.
Figure S2 (Supporting information) displays the highresolution XPS spectrum of NiCo-LDH/MXene electrodes before and after 100 cycles.It can be observed that the C-Ti bond at 281.53 eV moved toward the left along the X-axis, and meanwhile, the peak area became larger, which is presumably due to the deintercalation of Li + from MXene. [36]Further, the M-O bond relating to O 1s XPS spectrum shifted toward the low binding energy.Significantly, the peak intensity of Ni 2p decreased obviously.By normalizing the peak area, it is found that the relative area of Ni 2p 3/2 increases from 53% to 56%, which features that Ni 2+ and Ni 3+ continuously undergo redox reactions upon cycling.Based on the above discussion, we can deduce the electrochemical reaction of the MXene/NiCo-LDH/MXene electrode: The Li + storage mechanism of NiCo-LDH/MXene electrode can be studied by performing CV curves at different scan rates.Figure 6a shows the CV curves at scan speed ranging from 0.1 to 2 mV s −1 .All curves have the similar shape, implying the stable electrochemical reaction.Generally, the relationship between peak current (i) and scanning rate (v) follows the power law as given below. [37]= v b (3) where a and b are adjustable parameters.If the value of b is ≈1, capacitive behavior plays a dominant role.If the value of b is ≈0.5, it means that the diffusion-controlled process plays a major role. [38,39]In Figure 6b, the b value of NiCo-LDH/MXene is 0.52 and 0.68 for anodic and cathodic process, respectively, which shows that the main Li + storage of NiCo-LDH/MXene is a diffusion-controlled process.Through the following equation regarding the current response at a fixed potential, the contribution ratio of the capacitive component to the total capacity can be further quantified.It is expressed as the combination of two mentioned mechanisms: the capacitive effects (k 1 v) and diffusioncontrolled processes (k 2 v 1/2 ). [40] As shown in Figure 6c, the orange and green areas separately represent the diffusion-controlled and capacitive-controlled processes.At the scan rate of 0.5 mV s −1 , the diffusion-controlled capacitance occupies 67% of the area.In Figure 6d, the capacitivecontrolled process gradually dominates upon the scan speed increases. [41]When the scan speed is 2 mV s −1 , the capacitivecontrolled process reaches 57%.Besides, the galvanostatic intermittent titration technique (GITT) is used to evaluate Li + transport characteristics of NiCo-LDH and NiCo-LDH/MXene electrodes as shown in Figure 6e.The Li + diffusion coefficient (D Li + ) can be calculated by the following equation [42][43][44] : where  is the discharge/charge time, m B , V M , M B, and S represent the active material mass, molar volume, molecular mass, and effective area, respectively, ΔE s is the voltage change caused by a single pulse, ΔE t is the total transient voltage variation during a GCD process.As shown in Figure 6f, the D Li+ value is between 2.74 × 10 −9 and 2.43 × 10 −8 cm 2 s −1 , which is much higher than that of NiCo-LDH (1.69 × 10 −11 -1.25 × 10 −10 cm 2 s −1 ), demonstrating the faster Li + diffusion in NiCo-LDH/MXene.To further investigate the interaction between the MXene and NiCo-LDH, the first-principles calculations with density functional theory (DFT) methods are applied. [45,46] S3 and S4 (Supporting Information).The corresponding relative energy with respect to the migration coordinate is given in Figure 7e.It is shown that the diffusion barrier is 0.133 eV for NiCo-LDH/MXene, which is lower than that of NiCo-LDH (0.164 eV).This suggests that the construction of 1D/2D heterostructure makes the migration of Li + easier. [47]oreover, the density of states (DOS) and projected density of state (PDOS) for NiCo-LDH and NiCo-LDH/MXene are further calculated and listed in Figure 7f,g, respectively.The DOS of NiCo-LDH/MXene at the Fermi level is markedly increased, which means better electrical conductivity with improved charge transfer kinetics. [48]This high conductivity of NiCo-LDH/MXene promises the rapid migration of Li + during the charge/discharge processes.Figure 7h shows the charge density difference of NiCo-LDH/MXene, where the electron depletion and accumulation are separately described in cyan and yellow color.The results show that a large number of electrons in NiCo-LDH are exhausted, while a large number of electrons gather near MXene, leading to the electron transfer from NiCo-LDH to MXene.This 1D/2D heterointerface ensures rapid Li + transport and stable storage as well as the superior performance of LIBs.

Conclusions
In summary, the 1D/2D NiCo-LDH/MXene anode for LIBs has been successfully developed by the facile growth NiCo-LDH nanowires on the surface of laminated MXene nanosheets.This novel nanoarchitecture not only efficiently relieves the agglomeration of MXene layers, but also significantly promotes Li + diffusion and electron transfer.Benefiting from the synergistic effect, the composite anode delivers a much higher specific capacitance than that of pristine NiCo-LDH and MXene as well as the outstanding rate capability and cycle stability.This work features the advantage of microstructure regulation of electrode materials for high-performance rechargeable batteries.

Experimental Section
Preparation of MXene: First, 1.0 g LiF was added to 20 mL HCl solution with stirring for 30 min.Afterward, 1.0 g Ti 3 AlC 2 powder was slowly added to the above solution and stirred at 40 °C for 96 h.Second, the washed product was repeatedly centrifuged with deionized water (DI) until the pH of the supernatant approached 7, and then the clayey precipitate was obtained.Subsequently, the precipitate was dispersed in 30 mL DI for 2 h, and then centrifuged at 3500 rpm for 1 h.After that, the ex-MXene suspension with highly dark green dispersibility was collected.Finally, the layered MXene was obtained after freeze-drying.
Preparation of NiCo-LDH/MXene: The NiCo-LDH/MXene nanocomposite was produced via a simple hydrothermal method.Initially, 100 mg MXene, 1.744 g Ni(NO3)2•6H2O, 1.164 g Co(NO3)2•6H2O, and 1.892 g urea were dispersed in a mixed solvent containing 30 mL methanol and 30 mL ultrapure water, and then the mixture was magnetically stirred in room temperature.Successfully, the mixture was transferred into the Teflon-lined autoclave undergoing hydrothermal reaction at 150 °C for 12 h.Finally, the product (NiCo-LDH/MXene) was dried at 60 °C in a vacuum oven after washing with centrifugation by ethanol and DI several times.For comparison, the pure NiCo-LDH was prepared by the same method without involving MXene.
Materials Characterization: Scanning electron microscopy (SEM; Hitachi SU5000, Japan) with an operation voltage of 3 kV was used to analyze the morphology and size of the samples.Transmission electron microscopy (TEM; FEI Talos200s, U.S.A.) was triggered at 200 kV to investigate the microstructure of the nanocomposite material.X-ray diffraction (XRD; Smart Lab, Japan) was employed to characterize the crystallization of crystals by using Cu K radiation ( = 1.5406Å) in the 2 range from 5°to 80°(Bruker D8 Advance).The experimental data was analyzed by the Jade 6.0 program.X-ray photoelectron spectroscopy (XPS; Thermo ESCALAB 250Xi) was conducted with a monochromatic Al K source (hv = 1486.6eV).Advantage software was used for spectral peak fitting and the binding energies were corrected using the peak of C 1s at 284.80 eV as an internal standard.The functional group of the specimens was examined by the Fourier transform infrared spectrum (FTIR; spectrum Two03040404, U.S.A.).The Brunauer-Emmett-Teller (BET; JW-BK200C, China) was conducted to test the specific surface area and pore size of composite materials in the liquid nitrogen environment at 77.53 K.
Electrochemical Measurements: The electrochemical characteristics of NiCo-LDH/MXene and NiCo-LDH were studied by making the CR2032 type LIB cells.Initially, the 70% NiCo-LDH/MXene or NiCo-LDH, 20% PVDF, and 10% acetylene black were mixed homogenously, and then ground with N-methyl-2-pyrrolidone (NMP).After that, the slurry was evenly pasted on the copper foil and dried at 70 °C for 12 h to remove the NMP and other impurities.Subsequently, it was punched into disks with a diameter of 12 mm.The coins-typed LIB was assembled in a glove box filled with argon.The electrolyte was 1 M LiPF 6 dissolved in ethylene carbonate/dimethyl carbonate (1:1 by Vol.), and Celgard 2500 was used as the separator.The battery testing system (CT2001A, LANHE, China) was utilized to realize the constant discharge/charge (GCD) performance of the NiCo-LDH/MXene or NiCo-LDH.Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were performed on a PARSTATMC electrochemical station.
Computational Methods: The first principles were employed to perform all spin-polarization density functional theory (DFT) calculations within the generalized gradient approximation (GGA) using the Perdew-Burke-Ernzerhof (PBE) formulation. [49]The ion core was expressed by applying the projection augmented wave (PAW) potential and taking valence electrons into account using a plane wave basis set with a kinetic energy cutoff of 450 eV. [50]The partial occupancies of the Kohn-Sham orbital were allowed by using the Gaussian smear method with a width setting of 0.05 eV.The electronic energy was considered self-consistent when the energy change was smaller than 10 −4 eV.Geometric optimization was based on the judgment of convergence when the force changes less than 0.03 eV Å −1 .The dispersion interaction was described by Grimme's DFT-D3 method.Finally, the adsorption energies (E ads ) were calculated as E ads = E ad/sub -E ad -E sub , where E ad/sub , E ad, and E sub were the total energies of the optimized adsorbate/substrate system, the adsorbate in the structure and the clean substrate, respectively.

Figure 1 .
Figure 1.The schematic illustration of the synthesis of NiCo-LDH/MXene.
Figure S1 (Supporting Information) shows the zeta potential of NiCo-LDH, MXene, and NiCo-LDH/MXene (DLS, Malvern Nano ZS90).Obviously, the pure NiCo-LDH and MXene realize a positive and negative potential of 10.2 and −16.5 mV, respectively, which provide the basis of the coupling between the NiCo-LDH and MXene due to the strong electrostatic interaction.As expected, the zeta potential of the NiCo-LDH/MXene shifted toward the negative direction relative to the NiCo-LDH, which again proves the successful construction of the

Figure 4
shows the X-ray photoelectron spectroscopy (XPS) of NiCo-LDH, Mxene, and NiCo-LDH/MXene.The Ni, Co, O, Ti, and C elements are verified in the survey spectrum in Figure 4a.
Figure 4b shows the high-resolution C 1s XPS spectrum of MXene and NiCo-LDH/MXene.The strong peaks at 284.80 and 286.03 eV correspond to the C = C/C-C and C = O, respectively.Most significantly, the peak at 282.78 eV representing the C-Ti-T x of MXene is completely replaced by the C-Ti (281.53 eV) of NiCo-LDH/MXene, illuminating the active reaction between the MXene and NiCo-LDH.

Figure 5 .
Figure 5. a) CV curves of the NiCo-LDH/MXene electrode at 0.1 mV s −1 , b) EIS, c) GCD curves, d) rate capability at various current densities ranging from 0.1 to 5 A g −1 , e) cycling stability at 0.1 A g −1 for 100 cycles.

Figure 6 .
Figure 6.a) CV curves of NiCo-LDH/MXene at different scanning speeds, b) relationships between peak currents and sweep rates for determining the b value of cathodic peaks and anodic, c) capacitive current contribution (green region), d) contribution ratio of capacitive and diffusion-controlled currents at different scan rates, e) the GITT curves of NiCo-LDH and NiCo-LDH/MXene at 0.1 A g −1 , f) the diffusion coefficient of Li + during the discharge process.
Figure 7a,b shows the constructed structure of NiCo-LDH and NiCo-LDH/MXene.Figure 7c,d draw the modeling of migration paths for Li + diffusion on NiCo-LDH and NiCo-LDH/MXene, respectively.The initial state, transition state, and final state models of lithiumion migration in NiCo-LDH/MXene and NiCo-LDH are shown in Figures