Double transition metal MXene (TixTa4−xC3) 2D materials as anodes for Li-ion batteries

A bi-metallic titanium–tantalum carbide MXene, TixTa(4−x)C3 is successfully prepared via etching of Al atoms from parent TixTa(4−x)AlC3 MAX phase for the first time. X-ray diffractometer and Raman spectroscopic analysis proved the crystalline phase evolution from the MAX phase to the lamellar MXene arrangements. Also, the X-ray photoelectron spectroscopy (XPS) study confirmed that the synthesized MXene is free from Al after hydro fluoric acid (HF) etching process as well as partial oxidation of Ti and Ta. Moreover, the FE-SEM and TEM characterizations demonstrate the exfoliation process tailored by the TixTa(4−x)C3 MXene after the Al atoms from its corresponding MAX TixTa(4−x)AlC3 phase, promoting its structural delamination with an expanded interlayer d-spacing, which can allow an effective reversible Li-ion storage. The lamellar TixTa(4−x)C3 MXene demonstrated a reversible specific discharge capacity of 459 mAhg−1 at an applied C-rate of 0.5 °C with a capacity retention of 97% over 200 cycles. An excellent electrochemical redox performance is attributed to the formation of a stable, promising bi-metallic MXene material, which stores Li-ions on the surface of its layers. Furthermore, the TixTa(4−x)C3 MXene anode demonstrate a high rate capability as a result of its good electron and Li-ion transport, suggesting that it is a promising candidate as Li-ion anode material.

www.nature.com/scientificreports/ used the formula M n+1 AX n (n = 1-4) that later evolved to the term MAX phase where " M "is an early transition metal (Sc, Ti, V, Cr, Zr, Nb, Mo, Hf, Ta); A is group13 or 14 elements (Al, Si, P, S, Ga, Ge, As, Cd, Ln, Sn, Tl, Pb) and X is Carbon and/or Nitrogen 31,32 (iii) MXene material, Ti 3 C 2 , was successfully prepared in 2011 by Barsoum et al. 33,34 through the immersion and exfoliation processes of Ti 3 AlC 2 (MAX phase) in hydrofluoric acid. As the acid treatment completely removed the aluminum (A element of MAX phase), forming an exfoliated 2D lamellar crystal similar to graphene, the suffix "ene" was added to the remaining MX phase, leading to the term MXene 33,34 .
Especially, Bi-metallic MXenes has attracted great attention because they present unique synergistic effects on their mechanical, textural and physicochemical properties 11,16,[35][36][37][38][39] . Titanium and tantalum alloys are well known for their unique combination and exceptional set of properties in the field of powder metallurgy, nuclear industry, reprocessing applications for storing nitric acid 40 and biomedical applications [41][42][43] . Titanium carbide MXenes are the most reported material to date and tantalum carbide is gaining attention in the field of theranostics, cancer therapy and few other electrochemical applications. A combination of these two MXenes offers interesting properties and a unique set of chemistry. MXenes have an advantage in comparison with graphene in tunability of material properties. In the case of graphene, the only way to tune the material properties is by its functionalization, whereas in the case of MXenes, these can be done by alloying, processing and functionalization [44][45][46][47] . One more key factor governing the properties of the material is defects because they have a huge role in controlling conductivity 48 , mechanical strength and metallic or semiconductor behaviors 47,49 . For example, single layer graphene with zero defects is known as a semi-metal, whereas inducing defects into its sheet generates a semiconductor behavior. Lattice imperfections are unavoidable in the case of solid-state physics and/or chemistry. Thermal defects in titanium have been recently reported 47,49 . Carbon vacancies are unavoidable in the MAX phases so as the MXene. Although defect-free MXenes are proving its potential in various applications, it is still unclear whether these defects would improve or deteriorate the material performance. When tantalum and titanium form an alloy compound, Ti atoms will occupy the outer layer, while Ta atoms will prefer the middle layer 50 . In this case, these defects become more interesting because of Ta, which has a self-oxidizing behaviour that passivates its surface, thereby providing new physicochemical properties.
MXene materials have gained great attention in battery applications for its extraordinary electrochemical properties to store Li-ions between its layered structure 51 and thus these materials offer a chemically stable surface 51,52 . Moreover, MXene materials have been utilized as conductive support of conversion 53,54 and alloying 55,56 materials, which is due to its excellent mechanical and electronic properties 57 . Huang et al. 54 have reported sandwich like Na 0.23 TiO 2 /Ti 3 C 2 MXene composite. This unique sandwich morphology can relieve the strain of the electrode on cycling and deliver an enhanced carrier transport mechanism to prevent aggregation of active material. Xia et al. 58 used an interfacial assembly strategy to assemble Si porous nanospheres on titanium carbide MXene sheets; improving the electrode's electron transport and stability. The MXene surface groups enable strong interaction with Si porous nanoparticles and develop pseudocapacitive behaviour, which can be advantageous for Li-ion storage. The Si-MXene assembly delivered an 1154 mAh/g at 0.2 A/g with good cycling stability. Meng et al. 59 have explored black phosphorous quantum dots with Ti 3 C 2 MXene as battery electrodes with enhanced pseudocapacitive capability. This interface enables high electrical conductivity, relieving stress on cycling with enhanced charge adsorption, and efficient interfacial electron transfer. The MXene composite electrode delivered 910 mAh/g at 100 mA/g with long cycling stability over 2400 cycles. Here, we report the preparation of a lamellar bi-metallic titanium-tantalum carbide MXene, Ti x Ta (4−x) C 3 (where x = 2), with expanded interlayer spacing, as promising Li-ion host material with high capacity, excellent rate capability and long-term cyclability. The raw Ti x Ta (4−x) C 3 MXene was synthesized through an etching process with HF, in which Al atoms were extracted from Ti x Ta (4−x) AlC 3 MAX phase (where x = 2), promoting its structure delamination concurrently. The layered Ti x Ta (4−x) C 3 MXene displayed an expanded interlayer d-spacing of 3.37 Å with a layer thickness of 0.325 nm, which allows reversible Li-ion storage between its laminates. Therefore, Ti x Ta (4−x) C 3 delivered a remarkable reversible high specific discharge capacity of about 459 mAhg −1 at an applied C-rate of 0.5 °C with a coulombic efficiency of around 99% after 200 cycles, as well as an excellent rate capability.

Materials characterization
X-ray diffractograms were obtained using a Bruker D8 XRD operated in Bragg-Brentano geometry with fixed slits (at room temperature). The vibrational modes were recorded using a Raman spectrometer (Horiba Scientific) with a 532 nm green laser. Morphological pictograms were recorded using supra-55 FE-SEM (Carl Zeiss) and a G2-20 TWIN TEM (FEI-TECNAI), respectively. High-resolution TEM images were recorded at 200 kV accelerating voltage integrated with a Gatan Orius CCD camera. Elemental composition was analyzed using a MultiLab 2000 XPS spectrometer (Thermo Scientific). All the elemental spectra were referenced with respect to the carbon (C1s-284.6 eV). Thermogravimetric Analysis was carried out using TA Instruments SDTQ600.
Electrochemical evaluation. The bi-metal Ti x Ta (4−x) C 3 MXene nanoparticles' electrochemical performance as Li-ion host material was analyzed using potentiostatic cyclic voltammetry and galvanostatic chargedischarge cycling as per our earlier reports 6,60,61 . First, Ti x Ta (4−x) C3 MXene powder was mixed with conductive additive (super P carbon) and binder (PVDF) with a mass ratio of 80:10:10 in solvent (NMP). The resultant anode slurry was uniformly coated on a Cu foil and dried in vacuum at 80 °C for 24 h. The resultant anode films was punched out (12 mm of diameter).The electrochemical testing was carried out using coin-type cells 2032 in half-cell configuration with a Li and Celgard 2500 disks as counter electrode and separator, respectively, and 1.0 M LiPF6 electrolyte (EC: DEC-1:1).

Results and discussion
MXene compounds are traditionally prepared via a three-step process. First, a ball-milling is carried out to mix all the components efficiently. Then, a hot press treatment at a high temperature is performed to obtain a homogeneous MAX phase. Finally, an etching process with hydrofluoric acid is implemented to remove the "A" component of the MAX phase, giving a lamellar material, MXene. However, the preparation of the bi-metallic Ti x Ta (4−x) C 3 MXene involves an extra-initial step that consists in the formation of a TiTa alloy through ball-milling as given in (Fig. 1a). The bi-metallic Ti x Ta (4−x) C 3 MXene was used as active materials of the working electrode in a half-cell battery ( Fig. 1b) that gives an open-circuit voltage (OCV) > 3.0 V vs. Li/Li + after resting 24 h (Fig. 1c), which demonstrated its potential as anodic electrode turning on a white led (Fig. 1d). Figure 2 shows the XRD pattern of alloyed titanium and tantalum (TiTa), which were found out to be in the hexagonal phase matching with the JCPDS card number of 00-044-1294. The XRD pattern of the TiTa alloys, are narrow/sharp confirming micron sized particles. The major peaks in the system are martensitic α″ and minor being β (austenite phase, JCPDS card number of 00-044-1288), where α″ is inversely proportional to the particle size and β is directly proportional i.e. the particles with a smaller size, the XRD pattern will have dominated martensitic α″ peaks and minor β peaks 62 . In our case, there is also a major α″ for the alloyed TiTa. The powder XRD pattern of the Ti x Ta (4−x) AlC 3 MAX phase is shown in Fig. 3. The XRD peaks of the synthesized sample are mainly ascribed to tantalum-titanium aluminum carbide with small traces of AlTa 3 , Ta 2 O, and TiO 2. The obtained XRD pattern was in phase with the earlier reports matching with the tantalum aluminium carbide (ICSD-156383) 27,32 . A shift in 2θ was also observed because of the double ordered metal atoms. The synthesised samples were in hexagonal crystal system, which can also be seen in SAED pattern obtained from the TEM analysis as given in Fig. S3. Furthermore, by alloying Ti with Ta, the elemental Ti has a lower melting point than the Ta but as the particle is in micron size, they can be sintered together. In case of lower temperatures, Ti and Ta rich zones can appear in the material, but as MAX phases require a high temperature of 1500 °C, at this condition an interdiffusion of Ti and Ta atoms occurs at their interphase due to Kirkendall effect 63,64 . As there is a difference between Ti and Ta diffusion coefficients, there is an improper transport of atoms from one side of the interphase to the other, resulting in a partial diffusion and the formation of defects as vacancies or porosity in the zone, where the atoms move out 64 . Consequently, there is a downshift of the XRD peaks in the case of the MXene phase in comparison with its parental MAX, as a result of an increase in the C lattice parameter from after HF etching 32,39,65 . The minor reflections between 5° and 25° disappeared in the case of MXene phase because of the non-standard orientation of the sample (vertical orientation assuming based on the experimental data by Ghidiu et al. 66 ). Also, the contamination peaks 66 ascribed to AlTa 3 and Ta 2 O are not observed as they are removed during the etching and washing processes. www.nature.com/scientificreports/ The Raman spectrum of the synthesized bi-metallic Ti x Ta (4-x) AlC 3 MAX phase and its corresponding Al etched Ti x Ta (4−x) C 3 MXene phase are given in (Fig. S1). The Raman active modes are given from ω 3 to ω 10 because of TiTa, Al, and C 32,67-69 . The modes ω 3 , ω 6 , ω 9 are due to the intermediate layer element "Aluminum", which are suppressed due to the exchange of Al with its lower atoms (O, F, OH) or its removal during the HF etching processes.
The vibrational modes ω 4 and ω 7 have undergone a complete shift to lower wavenumbers due to Al etching as the lattice parameter "c" suffers an increment of its value. Also, there are few unidentified active modes in the spectrum, which could be assigned to the interaction of Ti and Ta atoms in the MXene phase and out-of-plane vibrations 70 . Furthermore, XPS elemental analysis was carried out to investigate the oxidation state and chemical composition of the synthesized MXene sample, which is given in (Fig. 4). Distinct peaks due to the presence of elements of titanium (Ti), tantalum (Ta), aluminum (Al) and oxygen (O) were visible in the full scan survey. The binding energies of tantalum and other elements are in phase with the existing reports 27,32,71 . The full scan spectrum of Ti x Ta (4−x) C 3 MXene sheets in the Al region confirm that the synthesized materials were aluminumfree after HF etching processes (Fig. 4a). The spectrum of the Ta 4f. region was fitted by following 4f5/2 and 4f7/2 components that correspond to elemental Ta metals and its oxides as TaO 2 (red) that are the two main species. The Ta 4f5/2 (black) and Ta 4f7/2 (green) binding energies are around 25.9 and 23.8 eV indicating that tantalum is in carbide environment (Fig. 4b). TaO 2 has weak peaks probably that arise from the surface oxidation of the   (Fig. 4d). FE-SEM images of the synthesized bi-metal Ti x Ta (4−x) AlC 3 MAX phase and MXene samples are given in Fig. 5. The images show that the morphology of the synthesized MAX phase are in a layered solid structure (Fig. 5a-c). A small amount of nano layered structure was also observed, which can be attributed to the presence of other impurity phases in the sample. whereas the MXene sample shows that the layers are exfoliated during the etching processes (Fig. 5d-f). In case of both MAX phase and MXenes, at higher magnification of 10 µm both morphology looks similar. When the magnification is reduced to 5 µm and 2 µm, there is a clear difference in as the MAX phase with layered solid structure and etched MXene has an exfoliated layered structure with surface roughness because of etching processes. Furthermore, TEM micrographs of both the MAX phase and MXene are given in Fig. 6a-l, in which both the MAX phase and MXene exhibit layered structure and a thin arrangement enough to be electronically transparent; as the number of layers increases, this moved to a darker shade. Notably, the d-spacing of MXene increased from 0.329 nm to 0.337 nm at (004) plane due to the removal of Al atoms during the HF etching, which is in good agreement with that d-spacing calculated from the XRD data. As seen in (Fig. 6e,k), in comparision with the MAX phase, surface of the MXene sample is rough as a result of the surface functalization during thr etching processes. Understanding dislocations and kink bands were quite difficult until Morgiel et al. described them in their publication "Microstructure of Ti 3 SiC 2 based ceramics in 1996" 72 . The dipole formed by the two dislocation segments is quite common in layered solids and other materials as MAX phases 73 and mica 74 . It is evident from the FE-SEM and TEM images of both MAX phases and MXene that samples have a kink band (Fig. 5i). Since kinking is a type of buckling, porosity will enhance in the kinking effect 75,76 . The kink band favors electrochemical performance to a certain extent 77,78 . As seen in (Fig. 6f,i) in case of MAX phase, the HRTEM images were in darker shades because of thre layered solid behaviour, but it can be clearly distingushable in case of the MXene sample as there are different shades because of difference in thickness and spacing obtained due to removal of Al during the etchign processes.
In order to understand the thermal behavior of the sample, thermogravimetric analysis was carried out from RT to 1200 °C. The analysis results are given in (Fig. 7), which shows a heavyweight loss for the MAX phase due to the loss of other impurities/mixed phases, whereas on the other hand in the case of MXene sample, the weight loss is less. In MAX phases, samples has the same behavior till 650 °C followed by huge weight loss from 650 to 900 °C, which might be because of TiO 2 decomposition and/or due to formation of nanocrystals of TaC The lamellar bi-metallic Ti x Ta (4−x) C 3 MXene was further evaluated using potentiostatic and galvanostatic methods to understand its electrochemical properties and redox mechanisms as Li-host anode material (Fig. S2) displays cyclic voltammograms recorded at scan rate of 0.2 mV s −1 from 0.01 to 3.0 V vs Li/Li + . During the initial cathodic cycle, irreversible broad peaks appear around 0.99 and 0.53 V vs Li/Li +54 , which are related to the solid electrolyte interface (SEI) layer formation and Li-ion adsorption on the MXene pores 53 . The intercalation reaction between Ti x Ta (4−x) C 3 and Li-ions occurs around 0.5 V vs Li/Li + , which can be expressed as: Ti x Ta (4−x) C 3 + xLi + + xe − → Li x Ti x Ta (4−x) C 3 53-55 . The subsequent CV profiles reveal a stable cathodic electrochemical process and suggest a two-step lithiation process as described in the previous literature 39 . During the anodic scans, a broad peak centered at 0.31 V vs Li/Li + arises that is associated to Li-ion extraction process: Li x Ti x Ta (4−x) C 3 → Ti x Ta (4−x) C 3 + xLi + + xe −53-55 . Also, the constant charge capacity beyond 0.5 V vs. Li/Li + can be ascribed to the reversible redox Li-ion desorption processes on the active material surface or pores.
The cyclability and rate capability of the layered Ti x Ta (4−x) C 3 MXene were evaluated using galvanostatic (dis) charge measurements (Fig. 8a). The capacity delivered by the electrodes was estimated based on the mass of Ti x Ta (4−x) C 3 and the applied C-rate was 1.0 °C = 372 mA g −1 . Figure 7a shows the charge-discharge profiles of the  www.nature.com/scientificreports/ capacity was caused by the formation of the SEI layer that occurs during the 1st discharge cycle and might also be due to the formation of double Li layers between MXene layers 39 .
Remarkably, the MXene anode delivers a high specific discharge capacity of about 459 mAh g −1 at a C-rate 0.5 °C with a capacity retention of about 97% after the activation process (cycle 3) and a coulombic efficiency of around 99% after 200 cycles. Figure 8b displays the cycle performance of the layered Ti x Ta (4−x) C 3 MXene anode, which shows a stable cycling behavior with a slight increase of capacity during some cycles, reaching a discharge capacity of 476 mAh g −1 at a 0.5 °C-rate after 100 cycles, which decay until 459 mAh g −1 after 200 cycles. This loss of capacity could be associated with the disconnection of the active particles from the electrode as a result of the formation of a thick SEI between the MXene layers or might also be due to the restacking of the layered MXene material 84 , which can generate mechanical stress, cracks, and fractures inside the anodic electrode 85 . Figure 9a presents the Nyquist plot of fresh and cycled bi-metallic Ti x Ta (4−x) C 3 MXene anode, in which the charge transfer resistance of the cycled electrode increases during cycling, indicating the formation and growth of a thick SEI layer as the number of cycles increases.
The rate capability of the bi-metallic Ti x Ta (4−x) C 3 MXene anode was further evaluated under various C-rates during 10 D-C cycles per applied C-rate (Fig. 9b). The MXene anode exhibits an average of specific discharge    51 . Results show that the synthesized MXene achieved an excellent electrochemical redox performance compared to the known MXenes (Table 1), which could be attributed to the formation of a stable, promising bi-metallic MXene material, which store Li-ions on the surface of its layers.

Conclusions
In conclusion, this is the first report, where bi-metallic titanium-tantalum carbide MXene Ti x Ta (4−x) C 3 was successfully prepared through a conventional etching process of TixTa (4−x) AlC 3 MAX phase. XRD and Raman analysis proved that the MAX phase was successfully prepared via a consecutive ball milling and heat treatment route. The lamellar MXene structural arrangement remains after the HF etching process, which is Al free according to  www.nature.com/scientificreports/ the XPS analysis. FE-SEM and TEM characterizations exposed the exfoliated nature of the Ti x Ta (4−x) C 3 MXene after removing the Al atoms from its corresponding MAX Ti x Ta (4−x) AlC 3 phase. It also revealed a delaminated MXene structure with an expanded interlayer d-spacing of 3.37 Å. The lamellar Ti x Ta (4-x) C 3 MXene was used as Li-ion host anode, giving a remarkably high reversible high specific discharge capacity of 459 mAh g −1 at of 0.5 °C rate with a coulombic efficiency of around 99% after 200 cycles and a capacity retention of about 97%. Also, the MXene anode presented a high rate capability, recovering 98% of the delivered capacity of 482 mAh g −1 at a C-rate of 0.1 °C after being cycled at various C-rate conditions. This new, electrochemically active Ti x Ta (4−x) C 3 MXene provides a new approach to design high-capacity and stable Li-host anode materials for the next generation of rechargeable batteries.

Experimental part
Titanium (Ti), tantalum (Ta), aluminum (Al), graphite (C) and hydrofluoric acid (HF) with high purity were acquired from Alfa Aesar, while toluene was purchased from Sigma-Aldrich. All the chemicals were used without any further purification. Also, double distilled water was used to wash the resultant MXene material.
TiTa alloy preparation. The titanium-tantalum alloy (hereby referred to as TiTa) was synthesized by the powder metallurgy method using high purity Ti and Ta elements. During the alloy preparation stage, both elements were mixed with an atomic ratio of 1:1 through a ball milling process for 12 h at 250 rpm with a charge ratio of 1:10 under the presence of toluene. Then, the resultant sample was centrifuged and dried at 80 °C for 24 h, obtaining the TiTa alloy powder.

Synthesis of the MAX phase.
To obtain Ti x Ta (4−x) AlC 3 MAX phase, the pre-alloyed TiTa powder, aluminum and graphite in the proper mass amount were ball milled for 10 h with a charge ratio of 10:1. Then, the resultant mixture was loaded on to a graphite punching die and hot pressed at 1500 °C during 3 h under 1 Ton of pressure and a vacuum of 2 × 10 −5 mbar.
Synthesis of the MXene. To convert the synthesized Ti x Ta (4−x) AlC 3 MAX phase into Ti x Ta (4−x) C 3 MXene, 1 g of the raw material was immersed in 20 mL of 40 v/v% HF solution and stirred for 4 days. The resultant dispersion was washed with double distilled water, vacuum filtered and dried at 80 °C for 24 h, obtaining the Ti x Ta (4−x) C 3 MXene.