Over the past decades, Li-ion batteries (LIBs) have been widely used in daily life as the most mature energy storage devices, such as portable electronic devices, large power grids, and electric vehicles [1, 2]. The graphite is the most conventional commercial anode material for LIBs, however, it always cause unstable solid electrolyte interphase (SEI) layer and lithium dendrites, due to its low lithiation voltage (~0.1 V vs. Li⁺/Li). In addition, the rate capability and cycle stability of graphite are not satisfactory [3, 4]. Therefore, it is necessary to develop an anode material with even higher lithiation voltage and rate behaviors. Typically, TiO₂ [5] and Li₄Ti₅O₁₂ [6] can be potential candidates due to their higher operating potential and excellent rate capability. But they are limited by lower theoretical capacities of ~170mAh/g [5, 6]. As an intercalation anode material, Nb₂O₅ has a large theoretical specific capacity of ~200 mAh/g, along with a small volume expansion rate of only ~3% compared to other alloy-type materials, which ensures the structural stability of Nb₂O₅-based electrodes during repeated charge-discharge cycles, and excellent electrochemical properties [7-9].

However, the Nb₂O₅ is limited seriously by its intrinsic modest conductivity. To date, most researches generally focus upon constructing carbon-based Nb₂O₅ materials to increase the electronic conductivity, such as Nb₂O₅@C [8], Nb₂O₅/C nanocomposites [10], Nb₂O₅/graphene [11-14]. In recent years, twodimensional (2D) transition metal carbide (MXene) has emerged, which has high electrical conductivity, stable 2D structure. So, it would be expected as excellent conductive substrates [15-20].

At present, the most studied MXene is the Ti-based MXene. As previously reported, Ti₃C₂T_x/TiO₂ composites can be prepared directly from Ti₃C₂T_x by hydration reaction [21]. And the hybrid Ti₃C₂T_x/TiO₂ shows good electrochemical Li-storage performance. Unfortunately, few reports about partial oxidation of the niobiumbased MXene (denoted as Nb₂CT_x) towards efficient synthesis of Nb₂CT_x-based hybrids for electrochemical charge-storage applications can be retrieved so far.

Herein, the Nb₂O₅ nanorods (NRs) were first in-situ grew on the multi-layered Nb₂CT_x surfaces through a simple one-step mild hydrothermal avenue, where the Nb₂CT_x acted both as the niobium source and conducting medium. This strategy simultaneously stabilized the 2D structure of Nb₂CT_x, and rendered a hierarchical Nb₂CT_x/Nb₂O₅ hybrid. The Nb₂CT_x as a conductive matrix made up for the poor electrical conductivity of the Nb₂O₅, and the retained 2D structure was conducive to the rapid Li⁺ transport. Encouragingly, the hybrid Nb₂CT_x/Nb₂O₅ exhibited excellent rate performance, stable reversible capacity, and long cycle life, highlighting its enormous potential in the practical application of LIBs.

The hybrid Nb₂CT_x/Nb₂O₅ was prepared as follows. Firstly, 2.0 g of Nb₂AlC powder (11 technology Co., Ltd.) was added in 20 mL of HF solution (50 wt%). After stirred at room temperature (RT) for 90 h, the suspension was centrifuged at 3500 rpm and washed with deionized (DI) water until the pH value of the solution went up to 6. And dried at 60 ℃ for 24 h in vacuum, the Nb₂CT_x power was obtained. Secondly, 0.5 g of the resulted Nb₂CT_x was added into 40 mL of DI water. After magnetic stirring for 20 min, the solution was poured into an autoclave (50 mL) and kept at 180 ℃ for 12 h. After washed with DI water and dried at 60 ℃ for 12 h under vacuum, the hybrid Nb₂CT_x/Nb₂O₅ was finally obtained.

Structures and morphologies of samples were investigated by X-ray powder diffraction (XRD, Rigaku Ultima IV), 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.

Electrochemical properties of samples were tested on 2032 coin-type cells. The working electrodes slurry were prepared by mixing as-prepared active materials (80 wt%) with carbon black (10 wt%) and polyvinylidene fluoride in N-methylpyrrolidone. After grinding and coating the mixture on the copper foil current collector, the pole pieces were dried in vacuum at 110 ℃ for 24 h. Metal lithium and a polypropylene film are used as the counter electrode and a separator, along with 1.0 mol/L LiPF₆ in a mixture of ethylene carbonate/diethylene carbonate (1:1, v/v) as the electrolyte. All cells were assembled in an Ar-filled glove box (MBRAUN) with the concentrations of O₂ and H₂O < 0.1 ppm. Galvanostatic charge/discharge tests were conducted with the voltage range of 1.0–3.0 V (vs. Li/Li⁺) by a LAND test system (Land CT2001A). Cyclic voltammetry (CV) measurements were conducted on an IVIUM electrochemical workstation (The Netherlands).

Here, we characterize the morphologies of hybrid Nb₂CT_x/Nb₂O₅ by FESEM and (HR)TEM techniques. Obviously, accordionlike Nb₂CT_x (Fig. 1a) is successfully prepared with the HF acid etching. And the multi-layered MXene Nb₂CT_x still well maintains the 2D structure after hydrothermal reaction (Fig. 1b). One especially notes that there are some cylindrical shape nanorods (NRs) appearing on the surface of the nanosheets (NSs), and some even located between the NSs (Figs. 1b and c). Furthermore, the original smooth NSs become even rougher. Further TEM observation (Fig. 1b) evidences, besides the cylindrical NRs of larger size, the existence of smaller NRs in-situ grown on these NSs, which renders the rough the surface of the Nb₂CT_x NSs. For better revealing such phenomenon, hydrothermal reaction was further extended up to 24 h. These NRs has grown with even larger size appearing on the surface of NSs (Fig. S1 in Supporting information). Further HRTEM examination (Fig. 1d), corresponding to the blue circle region in panel (d), shows well-defined lattice fringes with a spacing of ~0.393 nm (Fig. 1e), which can be well indexed to the (001) crystal plane of Nb₂O₅. According to the principle of minimum energy (Wulff theory) [22], the [001] crystal orientation of Nb₂O₅ is the preferred growth orientation, and the formed Nb₂O₅ crystals will preferentially grow into rod-like architecture along the direction. As is well known, some single-layered Nb₂CT_x are always generated during HF etching.

During the subsequent hydrothermal oxidation of Nb₂CT_x to Nb₂O₅, the single-layered MXene Nb₂CT_x scattered outside lacks the effects of -OH, -O or -F plasma and the constraints between layers, it is therefore much easier to grow into the NRs shape, and the size of the NRs is larger than those in-situ located on the surface of the NSs with a interlayer distance of 1.059 nm (Fig. 1f). The highmagnification (HR)TEM (Figs. 1g and h) further confirms the Nb₂O₅ NRs with discernable (101) crystal plane grown on the Nb₂CT_x NSs. The SAED pattern (the inset in panel g) also visualize the quasisingle crystal nature of these Nb₂O₅ NRs. STEM and corresponding EDS mapping images (Fig. 1i) manifest the uniform elemental distribution throughout the hybrid. Similarly, the Nb, O, and C are also evenly distributed throughout the cylindrical NRs (Fig. S2 in Supporting information).

The crystalline structure of the Nb₂AlC, Nb₂CT_x and Nb₂CT_x/Nb₂O₅ are characterized by the XRD technique. From the XRD pattern (Fig. 2), it can be seen that the characteristic peak of Nb₂AlC at 38.9° disappears in the Nb₂CT_x, and the shift and broadening of the (002) peak at 13.68° with HF acid etched, verifying the successful fabrication of multi-layered Nb₂CT_x, which is consistent with previous report [16]. As examined from the XRD diffraction reflections, it is easy to conclude that the co-existence of Nb₂CT_x and Nb₂O₅ in the product of Nb₂CT_x/Nb₂O₅, revealing the partial oxidation of the Nb₂CT_x into Nb₂O₅. Additionally, the (002) signal of Nb₂CT_x in the hybrid weakens somewhat, which results from the existence of the numerous Nb₂O₅ NRs on their surfaces. The main strong peaks centering at 2θ = 22.3°, 28.5° and 36.3° are well indexed to (001), (100) and (101) crystal planes of the pseudo-hexagonal Nb₂O₅ (JCPDS No. 28-3717) in the hybrid.

Thanks to the unique hybrid structures, the Nb₂CT_x/Nb₂O₅ electrode will be endowed with both the merits of the Nb₂CT_x and Nb₂O₅, and exhibits remarkable lithium-storage properties. Typical CV profiles (0.1 mV/s) recorded in the voltage of 1.0–3.0 V (vs. Li/Li⁺) evidence a pair of cathodic/anodic peak centered at ~1.6 and 1.8 V, which are closely associated with the oxidation/reduction between Nb⁴⁺/Nb⁵⁺ [2]. Fig. 3a demonstrates the selected voltagecapacity plots of the hybrid Nb₂CT_x/Nb₂O₅ anode. Apparently, the initial discharge and charge capacities are estimated as ~456 and ~275 mAh/g respectively, corresponding to the 1^st Coulombic efficiency (CE) value of ~60.3%. The irreversible capacity loss should be ascribed to the electrolyte decomposition and formation of the SEI layer [21]. More strikingly, the anode still can retain reversible discharge/charge capacities as large as ~193.2/~193.3 mAh/g in 20^th cycle, along with a high CE value of ~99.9%. Besides this, the Nb₂CT_x/Nb₂O₅ hybrid also shows very good capacity retention and stability in the process of increasing current densities, as profiled in Fig. 3b. At a current density of 0.1 A/g, the average discharge capacity of the hybrid anode is estimated as approximately 200 mA/g. When it further rises to 0.2, 0.5, 0.1, 2.0, 5.0 and 10 A/g, the hybrid Nb₂CT_x/Nb₂O₅ still can be stabilized as ~180, ~150, ~125, ~100, ~85 and ~65 mAh/g, respectively, that is, the capacity retention of 32.5% with the 50-time increase in current density. More appealingly, with the current density returning back to 0.1 A/g, the specific capacity still can be recovered at about 190 mAh/g, highlighting the exceptional electrochemical reversibility and rate performance of the Nb₂CT_x/Nb₂O₅ electrode. More competitively, the high-rate behaviors of our Nb₂CT_x/Nb₂O₅ are better than other Nb₂O₅-based anodes, for instance, Nb₂O₅ nanofibers (~110 mAh/g at 1 A/g) [23], Nb₂O₅ nanowires/graphene (~80 mAh/g at 5 A/g) [24], T-Nb₂O₅/ GCN (~50 mAh/g at 10 A/g) [25], pure Nb₂CT_x (~50 mAh/g at 1.0 A/g) [26, 27], as summarized in Fig. 3c. Fig. 3d demonstrates the cycle performance of the hybrid at a current density of 100 mA/g. Appealingly, the Nb₂CT_x/Nb₂O₅ electrode shows a small attenuation, and can maintain a high discharge capacity of ~190 mAh/g, much better than that of the commercial Nb₂O₅ (~99.8 mAh/g, Fig. S4 in Supporting information). More encouragingly, under a large current density of 1.0 A/g (Fig. 3e), the hybrid anode still can deliver a discharge as large as ~102 mAh/g over 1000 consecutive charge-discharge cycles, corresponding to a small capacity decay of 0.058 mAh/g per cycle, which indicates that the Nb₂CT_x/Nb₂O₅ anode possess excellent cycle stability even under high current density.

In summary, a simple yet efficient hydrothermal strategy is developed to in-situ convert multi-layered niobium-based MXene (Nb₂CT_x) to hierarchical Nb₂CT_x/Nb₂O₅ composite. Benefitting from the synergistic contribution of 2D conductive Nb₂CT_x and high-capacity Nb₂O₅, the hybrid Nb₂CT_x/Nb₂O₅ was endowed with convenient ionic and electronic diffusion/transport, and exhibited excellent rate performance (~65 mAh/g at 10 A/g) and long cycle performance (~102 mAh/g after 1000 cycles at 1 A/g). More significantly, our research here provides a simple and feasible strategy to construct high-performance Nb₂CT_x-based anodes for advanced LIBs.

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.

The authors acknowledge the financial support from the National Natural Science Foundation of China (Nos. 51772127 and 51772131), Taishan Scholars (No. ts201712050), Major Program of Shandong Province Natural Science Foundation (No. ZR2018ZB0317), Natural Science Doctoral Foundation of Shandong Province (No. ZR2019BEM038), Natural Science Doctoral Foundation of the University of Jinan (No. XBS1830) and Collaborative Innovation Center of Technology and Equipment for Biological Diagnosis and Therapy in Universities of Shandong.

Supplementary material related to this article can befound, in the online version, at doi:https://doi.org/10.1016/j.cclet.2020.03.006.