English

• It is common knowledge that rechargeable lithium-ion batteries (LIBs), one of the most important energy carriers, have been widely used in our daily life. However, the further and large-scale applications of LIBs are hindered by shortage of lithium resources, high production coast, and inherent safety problems. In this regard, more and more researchers have made an effort to develop other rechargeable metal-ion batteries [1-3]. Benefitting from the abundant reserves and low cost of sodium sources, sodium-ion batteries (SIBs) have promising applications in energy storage [4-6]. Similar to LIBs, the electrochemical behaviors of SIBs are highly dependent on the properties of the electrode materials. Unfortunately, due to the larger ionic radius of Na⁺ (1.06 Å) in comparison with Li⁺ (0.76 Å) [7, 8], most of the anode materials suitable for LIBs cannot be directly used by SIBs, which will cause severe losses in energy capacity and cycle stability.

  To date, many works focus on developing suitable anode candidates for SIBs, such as metal oxides/sulfides [9, 10], porous carbon materials [11, 12], metallic alloys [13, 14]. Besides, previous studies showed that sodium titanate compounds (Na_xTi_yO_z) couldbe deemed as a promising class of SIB anode materials due to their high theoretical specific capacity of 178 mAh/g and lowest charge/ discharge voltage plateau of 0.3 V vs. Na⁺/Na [15, 16]. Up to now, various smart nanostructured Na_xTi_yO_z such as nanowires [17, 18], nanosheets (NSs) [19], nanorods [20], were fabricated for shortening the Na⁺-diffusion distance and increasing reaction interfaces. For example, recently, using Ti₃C₂T_x (T for F, OH and O) as titanium source, Dong et al. prepared Ti₃C₂ MXene-derived NaTi_1.5O_8.3 nanoribbons by a simultaneous oxidation and alkalization processes, which offered an enhanced reversible capacity [21]. Unfortunately, the synthetic method in this work destroyed the texture of MXene entirely, which make the intrinsic advantages of the Ti₃C₂T_x MXene cannot be utilized. As is known to all, the Ti₃C₂T_x material could be an excellent natural matrix candidate in energy storage materials if two-dimensional (2D) robust structure, high conductivity and unique electronic properties of Ti₃C₂T_x MXene could be fully utilized and synergistically interplayed with excellent components [22-27]. Besides, the inherent poor electrical conductivity and instable structure of Na_xTi_yO_z, which can cause sluggish kinetics and large capacity decay during cycle process thus poor electrochemical performance, can be addressedif themerits of Ti₃C₂T_x were preserved.

  Herein, the ultrathin NaTi₈O₁₃/NaTiO₂ (NTO) nanoribbons were in-situ grew on the multi-layered Ti₃C₂ surfaces through a facile yet effective two-step mild hydrothermal method, where the Ti₃C₂T_x acts both as the titanium source and conducting medium. This strategy simultaneously stabilizes the structure of MXene and obtains the NTO/Ti₃C₂ hybrids. The well-preserved 2D structure can effectively guarantee the larger electrode-electrolyte contacting area and facilitate the Na-ion migration. Meanwhile, the high conductivity originating from the Ti₃C₂ matrix leads to fast charge transmission. Moreover, the robust interface contact between NTO and Ti₃C₂ layer, and the excellent mechanical flexibility of Ti₃C₂ not only buffers the volume change during repeated charging/discharging but remits the peeling off of NTO, inhibiting the drop in capacity during cycling. Encouragingly, the NTO/Ti₃C₂ demonstrates large reversible capacities, excellent rate capability and ultralong lifespan, exhibiting fascinating potential in advanced SIBs.

  The TiO₂/Ti₃C₂ hybrids were firstly prepared according to our previous work [25]. Then, 0.2 g of TiO₂/Ti₃C₂ powder was added in 40 mL of 10 mol/L NaOH solution. After sonication for 1 h, the mixture was poured into an autoclave (50 mL) and kept at 200 ℃ for 48 h. After washed with de-ionized (DI) water and dried under vacuum, the NTO/Ti₃C₂ hybrid was finally obtained.

  Structures and morphologies of samples were investigated by X-ray powder diffraction(XRD, RigakuUltimaIV, Japan), field-emission scanning electron microscopy (FESEM, JEOL-6300 F, 15 kV), transmission electron microscopy (TEM), scanning TEM (STEM), selected area electron diffraction (SAED) and high-resolution TEM (HRTEM) (JEOL JEM 2100 system) with an energy dispersive X-ray spectroscopy (EDS) system. The determination of surface elements was conducted by X-ray photoelectron spectroscopy (XPS, Thermo, Escalab 250xi) with Al Ka monochromatic X-ray source (hv =1486.6 eV). Raman spectrum was collected on Lab RAM HR Raman spectroscopy. The nitrogen adsorption/desorption isotherms were examined on NOVA 2000, Quantachrome.

  The electrochemical performances of samples were tested on 2032 coin-type cells with the sodium metal foil as the counter/ reference electrodes. The working electrodes slurry were prepared by mixing as-prepared active materials (70 wt%) with carbon black (20 wt%) and carboxymethyl cellulose (10 wt%) in DI water. Then, the slurry was pasting uniformly onto Copper foil substrate and dried at 110 ℃ for 11 h in vacuum. The glass fibers acted as the separator. 1.0 M NaClO4 in a mixture of ethylene carbonate (EC)/ propylene carbonate (PC) (1:1, v/v) with 5 wt% fluoroethylene carbonate (FEC) was used as the electrolyte. All cells were assembled in an argon-filled glove box (MBRAUN, Germany) with the concentrations of O₂ and H₂O < 0.1 ppm. Galvanostatic charge/ discharge tests were conducted with the voltage range of 0.01–3.00 V (vs. Na/Na⁺) by a LAND test system (Land CT2001A, China). The cyclic voltammetry (CV) tests in the potential range of 0.01–3.00 V were recorded by an electrochemical workstation (IviumStat.h, the Netherlands).

  The whole synthetic strategy of layered NTO/Ti₃C₂ hybrid material is illustrated in Fig. 1a. Firstly, the Ti₃AlC₂ powder is etched fully by HF solution, forming the Ti₃C₂T_x MXene. It is well known that the Ti₃C₂T_x MXene tends to transform into stable oxide in dissolved oxygen under high temperatures due to the high portion of exposed Ti atoms on the surface. Accordingly, the TiO₂/ Ti₃C₂ hybrid can be fabricated through a simple and friendly oxidation progress in DI water by hydration treatment. As a result, Ti₃C₂T_x is partially oxidized, and Ti atoms on the surface of Ti₃C₂T_x are in-situ transformed into TiO₂ nanoparticles (NPs). The NTO/Ti₃C₂ composite is further prepared by a hydrothermal alkalization process, in which the transformation of the TiO₂ NPs into the NTO nanoribbons occurs. Strikingly, the involved oxidation process we devised here is more moderate, and avoids the high-temperature annealing procedure during facile synthesis of the NTO. More importantly, the unique advantages of MXene itself can be greatly preserved in our case.

  Figure 1

  

  Figure 1.  (a) Schematic illustration for synthesizing the NTO/Ti₃C₂ composite. (b) XRD patterns of the NTO/Ti₃C₂ and Ti₃C₂T_x (the upright inset in panel b).

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  The structure of pure-phase Ti₃C₂T_x and NTO/Ti₃C₂ composites (Fig. 1b) are characterized by typical XRD technique. Obviously, besides the signal (2θ = 7.2°) for the Ti₃C₂, those reflections centered at 14.6°, 22.9°, 29.9°, and 44.0° correspond to the (101), (110), (11-3), (12-4) planes of the NaTi₈O₁₃ (JCPDS No. 80-1284) with a space group R-3 (148), respectively, and the peaks centered at 40.7° and 60.6° are related to the (104) and (110) planes of the NaTiO₂ (JCPDS No. 89-0802) with space group R-3m (166), which indicates the co-existence of NaTi₈O₁₃ and NaTiO₂ phases in the resulted NTO/Ti₃C₂ specimen. Unexpectedly, the (11-3) peak of the NaTi₈O₁₃ has the highest diffraction intensity in XRD patterns. To be specific, the NaTi₈O₁₃ in the co-existence preferentially grows along the (11-3) plane. From this result, it is clear that the exists of the Ti₃C₂ matrix influences the driving force for NaTi₈O₁₃ growth, such as enhance the nucleation by creating disorder in the incorporation of adatoms into the lattice [28], thus induce a preferred growth in the (11-3) direction of the NaTi₈O₁₃. Notably, the characteristic (002) peak attributed to the Ti₃C₂T_x shifts from 2θ = 9.8° (the inset in Fig. 1b) to 2θ = 7.2°, suggesting the d-spacing value of the Ti₃C₂ matrix increases from 0.902 nm to 1.228 nm after the in-situ growth of the hexagonal NTO. All the above analyses confirm that the NTO phases have been successfully formed into the interlayers of the conductive Ti₃C₂ matrix.

  Specific morphologies and micro-structures of the Ti₃C₂T_x, TiO₂/Ti₃C₂T_x and NTO/Ti₃C₂ were characterized by FESEM and (HR) TEM measurements in detail. A panoramic view (Fig. 2a) reveals that the accordion-like multilayer feature of the Ti₃C₂T_x obtained after acid etching from the bulk Ti₃AlC₂. Further closer examination (Fig. 2b) clearly shows the interlayer space of < 60 nm existing in the pristine Ti₃C₂T_x with a smooth surface, which benefits more exposed electroactive sites. After the hydration process, the morphology becomes even rougher due to the in-situ formation of TiO₂ NPs on the Ti₃C₂ surface (Figs. 2c and d). As shown in Figs. 2e and f, the anchored phases change from NPs to connected thin flats coating on the Ti₃C₂ NSs, suggesting the transformation of TiO₂ into the hybrid NTO after the following alkalization process. As we intended, both TiO₂/Ti₃C₂, and even NTO/Ti₃C₂ almost preserve the pristine 2D open structure, which reveals that the formation of NTO is under very mild condition, and do not cause the damage of intrinsic properties, e.g., conductivity, intensity and ion transport characteristic, of the MXene.

  Figure 2

  

  Figure 2.  FESEM images of (a, b) Ti₃C₂T_x, (c, d) TiO₂/Ti₃C₂ and (e, f) NTO/Ti₃C₂. (g-j) TEM/(HR)TEM, SAED pattern (the inset in panel h), (k) STEM and corresponding element (O, Sn and Ti) mapping images of the NTO/Ti₃C₂.

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  The TEM image (Fig. 2g) clearly reveals the layered structure of NTO/Ti₃C₂ hybrids. The apparent lattice distance of 1.22 nm, as visualized in Fig. 2h, corresponds to the (002) crystalline plane of the Ti₃C₂ in the hybrid, which is in good line with the above XRD analysis. The dotted-line pattern in SAED (the inset in the Fig. 2h) manifests the crystalline structure of the hybrid. Under HRTEM observation (Fig. 2i), the lattice fringes are measured to be 0.22 nm and 0.29 nm, which are in good agreement with the (104) plane of the NaTiO₂ and (11-3) plane of the NaTi₈O₁₃, proving the successful in-situ growth of the NTO phases on the Ti₃C₂ surface. Expectedly, the in-situ formed NTO layer consists of well isolated elongated nanoribbons (Fig. 2j). Apparently, the NTO nanoribbons are about 5 nm in the thickness and interconnect with the Ti₃C₂ matrix through the disordered carbon "binder". The perfect combination of the two components suppresses the volume change and peeling off of NTO under sodiation/desodiation process, and enhances the conductivity of the hybrid. The STEM image and corresponding elemental (Ti, C, O and Na) EDS mapping images (Fig. 2k) of the hybrid materials verify the uniform attachment of the NTO upon the Ti₃C₂ marix.

  The chemical composition and surface valance state of the NTO/ Ti₃C₂ are conducted by XPS analysis. The XPS survey spectrum (Fig. 3a) shows the co-existence of Al, C, Ti, O, F and Na elements in the hybrid material, in which the Na KLL (497 eV), F KLL (565/835 eV) and O KLL (976 eV) represent the Auger peaks for Na, F, K elements respectively [29]. The weak Al and F peaks, are derived from the raw material, and the fluorine functional groups on the surface of MXene due to the HF etching progress, respectively. The high resolution Ti 2p XPS spectrum (Fig. 3b) can be deconvolution into four pairs peaks, corresponding to Ti-C (455.3/461.1 eV), Ti²⁺ (456.6/462.8 eV), Ti³⁺ (458.3/464.1 eV) and Ti⁴⁺ (458.8/465.0 eV), respectively [26, 27]. The large proportion of Ti-C peaks and low chemical state of Ti peaks indicate that the Ti₃C₂ matrix are well preserved after the oxidation and alkalization processes, which allows the merits of Ti₃C₂ to be fully utilized to enhance the electrochemical performance for sodium storage. Furthermore, the visual increasing Ti³⁺ and Ti⁴⁺ contribution, compared to the pristine MXene [25], reveals the successful formation of NTO in hybrid, along with the enhanced peak intense of elemental Na in the survey spectrum. The C 1s XPS spectrum of the hybrid (Fig. 3c) shows three kinds of C orbitals in NTO/Ti₃C₂, namely C-Ti (281.7 eV), C—C (284.8 eV, 288.7 eV), and C—O (286.1 eV), respectively [26]. The proportion of C—C peak increases obviously compared to pristine MXene [25], which can be attributed to carbon layers because of the precipitation of Ti atoms from Ti₃C₂ crystal during hydrothermal process. Further Raman observation of the NTO/Ti₃C₂ (Fig. S1 in Supporting information) shows the two broad peaks located at 1376.20 and 1595.93 cm^-1, which are associated with the disorderly induced characteristic D-band and the sp2 hybridized graphitized G-band of carbon, respectively [30]. The ratio of D- and G- band intensity (I_D/I_G), is ~4.78, suggesting the appearance of disordered (amorphous) carbon layers in hybrid due to the precipitation and oxidation of the outermost Ti atoms in the MXene structure. Finally, the high-resolution O 1s spectrum (Fig. 3d) shows three components. Specifically, the main peak at 530.0 eV is attributed to the Ti—O bonds, and two broad peaks at the binding energy of 531.5 and 532.8 eV can be indexed well to the O—C and—OH groups, respectively [27].

  Figure 3

  

  Figure 3.  XPS spectra of (a) survey spectrum, and high-resolution spectra of (b) Ti 2p, (c) C 1s and (d) O 1s for the NTO/Ti₃C₂ composite.

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  One advantage of this architecture is that the NTO forms not only on the surface of MXene sheets, but also in the layers of Ti₃C₂, as evidenced by XRD discussion above, which is also supported by the nitrogen sorption isotherms (Fig. S2 in Supporting information). Obviously, the isotherms of the Ti₃C₂T_x, TiO₂/Ti₃C₂ and NTO/Ti₃C₂ (Figs. S2a–c) exhibit typical-IV behaviors, indicating the main existence of mesopores in these samples. The BrunauerEmmett-Teller (BET) surface area of the NTO/Ti₃C₂ is estimated as ~13.3 m²/g, higher than those of Ti₃C₂T_x (~9.5 m²/g) and TiO₂/ Ti₃C₂ (~11.6 m²/g). The increased specific surface area for the hybrid benefits from the opening or swelling of the layers during oxidation and alkalization, which allows the adsorbates to penetrate between layers. Corresponding pore volume and average pore size (D) of the samples are calculated by using Barret-JoynerHalenda (BJH) method (Table S1 in Supporting information). The D value of the NTO/Ti₃C₂ is ~9.8 nm, which is higher than those of Ti₃C₂T_x (~9.0 nm) and TiO₂/Ti₃C₂ (~9.5 nm). The large pore size indicates the increased number of large mesopores in the hybrid NTO/Ti₃C₂, leading to the largest pore volume of ~0.045 cm³/g for the NTO/Ti₃C₂, as collected in Table S1. The NTO/Ti₃C₂ hybrids with high BET surface area and porous structure could provide a large sur-/interfaces for the rapid penetration of electrolyte ions, hence achieving remarkably electrochemical sodium storage capacities.

  Electrochemical properties of the NTO/Ti₃C₂ are firstly elucidated by cyclic voltammetry (CV) measurements. Fig. 4a shows the first three CV curves at a scan rate of 0.5 mV/s within a voltage window of 0.01–3.0 V vs. Na/Na⁺. The cathodic curves exhibit great decay between the initial sweep and following scans, which arises from the irreversible formation of solid electrolyte interphase (SEI) layer between the electrode and electrolyte. Significantly, two pairs of peaks are observed in the subsequent cycles, located at 2.59/2.82 V and 2.50/2.73 V, which mainly attribute to the insertion/extraction of Na⁺. It is worth noting that those cathodic and anodic peaks are almost overlapping in the second and third scanning cycles, demonstrating fairly high reversibility and stability of the NTO/ Ti₃C₂ hybrid. Fig. 4b shows the first three discharge-charge curves of the NTO/Ti₃C₂ anode at a current density of 200 mAh/g. The initial discharge and charge capacity are ~162 and ~125 mAh/g respectively, corresponding to the initial Coulombic efficiency (ICE) of ~77.2%, which are higher than those of pristine Ti₃C₂T_x in our previous work (~156 and ~77 mAh/g, corresponding to the ICE of ~59%) [25] due to the decrease of surface functional groups. It should be pointed out that the plateaus in discharge-charge curves correspond well with redox peaks in CV profiles. To evaluate the capacity and stability of the NTO/Ti₃C₂, the cycling properties of bare Ti₃C₂T_x, TiO₂/Ti₃C₂ and the NTO/Ti₃C₂ hybrids were comparatively conducted at 200 mAh/g for 500 consecutive cycles (Fig. 4c). A more attractive result is that the NTO/Ti₃C₂ anode has the highest specific capacity of ~118 mAh/g compared to the Ti₃C₂ (~75 mAh/g) and TiO₂/Ti₃C₂ (~101 mAh/g) after cycles. In addition, the NTO/Ti₃C₂ anode possesses the excellent rate performance with specific capacities of ~157, ~143, ~118, ~105, ~82 and ~78 mAh/g at 50, 100, 200, 500, 1000 and 2000 mA/g, respectively (Fig. 4d), which can be attributed to efficient ions and electrons transport in the electrode. Remarkably, when the current density is relaxed to 50 mA/g, the capacity can quickly returns to ~152 mAh/g. To fully prove the excellent stability of the NTO/Ti₃C₂ electrode, the high-rate cycling performance was tested (Fig. 4e), and impressively, the composite deliver a reversible capacity of 82 mAh/g at 2000 mA/g ever after 1900 cycles, exhibiting the preeminent stability of the hybrid anode.

  Figure 4

  

  Figure 4.  (a) The 1^st, 2^nd and 3^rd CV curves at 0.5 mV/s and (b) galvanostatic chargedischarge plots at 200 mA/g of NTO/Ti₃C₂ sample. (c) Cycle performance for all specimens at 200 mA/g. (d) Rate performance of the TiO₂/Ti₃C₂ and NTO/Ti₃C₂ anodes at various current densities ranging from 100 mA/g to 2000 mA/g, along with the CE data of the NTO/Ti₃C₂. (e) Long-term cycling performance and CE data of the NTO/Ti₃C₂ anode at 2000 mA/g.

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  In summary, a novel NTO/Ti₃C₂ hierarchical layered composite, in which the NTO nanoribbons in-situ growth on the surface of 2D Ti₃C₂ MXene, was successfully prepared by a mild two-step hydrothermal method. The well-preserved 2D structure, high conductivity originating from Ti₃C₂ matrix, and the synergistic effect between the NTO and the Ti₃C₂ hugely benefit to achievement of high-rate long cycle life and sodium-storage properties. Impressively, the NTO/Ti₃C₂ delivered a reversible and stable capacity of ~82 mAh/g even after 1900 cycles at 2000 mA/g for SIBs. More significantly, our investigation here provides a viable methodology to develop active and durable MXene-based hybrids for long-cycle-life SIBs in the near future.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  Acknowledgments

  The authors acknowledge the financial support from National Natural Science Foundation of China (Nos. 51772127 and 51772131), Taishan Scholars (No. ts201712050), Major Program of Shandong Province Natural Science Foundation (No. ZR2018ZB0317) and Collaborative Innovation Center of Technology and Equipment for Biological Diagnosis and Therapy in Universities of Shandong.

  Appendix A. Supplementary data

  Supplementary material related to this article canbefound, in the online version, at doi:https://doi.org/10.1016/j.cclet.2020.02.016.

  
  
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  Figure 1  (a) Schematic illustration for synthesizing the NTO/Ti₃C₂ composite. (b) XRD patterns of the NTO/Ti₃C₂ and Ti₃C₂T_x (the upright inset in panel b).

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  Figure 2  FESEM images of (a, b) Ti₃C₂T_x, (c, d) TiO₂/Ti₃C₂ and (e, f) NTO/Ti₃C₂. (g-j) TEM/(HR)TEM, SAED pattern (the inset in panel h), (k) STEM and corresponding element (O, Sn and Ti) mapping images of the NTO/Ti₃C₂.

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  Figure 3  XPS spectra of (a) survey spectrum, and high-resolution spectra of (b) Ti 2p, (c) C 1s and (d) O 1s for the NTO/Ti₃C₂ composite.

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  Figure 4  (a) The 1^st, 2^nd and 3^rd CV curves at 0.5 mV/s and (b) galvanostatic chargedischarge plots at 200 mA/g of NTO/Ti₃C₂ sample. (c) Cycle performance for all specimens at 200 mA/g. (d) Rate performance of the TiO₂/Ti₃C₂ and NTO/Ti₃C₂ anodes at various current densities ranging from 100 mA/g to 2000 mA/g, along with the CE data of the NTO/Ti₃C₂. (e) Long-term cycling performance and CE data of the NTO/Ti₃C₂ anode at 2000 mA/g.

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