Electrostatic Self-assembly of 0D–2D SnO2 Quantum Dots/Ti3C2Tx MXene Hybrids as Anode for Lithium-Ion Batteries

0D–2D SnO2 quantum dots/MXene (SnO2 QDs/MXene) hybrids were synthesized by electrostatic self-assembly. MXene not only provides efficient pathways for fast transport of electrons and Li ions, but also buffers the volume change of SnO2 during charge/discharge process. The 0D–2D SnO2 QDs/MXene hybrids deliver high capacity, excellent cycle and rate performances as anode of lithium-ion batteries. 0D–2D SnO2 quantum dots/MXene (SnO2 QDs/MXene) hybrids were synthesized by electrostatic self-assembly. MXene not only provides efficient pathways for fast transport of electrons and Li ions, but also buffers the volume change of SnO2 during charge/discharge process. The 0D–2D SnO2 QDs/MXene hybrids deliver high capacity, excellent cycle and rate performances as anode of lithium-ion batteries. MXenes, a new family of two-dimensional (2D) materials with excellent electronic conductivity and hydrophilicity, have shown distinctive advantages as a highly conductive matrix material for lithium-ion battery anodes. Herein, a facile electrostatic self-assembly of SnO2 quantum dots (QDs) on Ti3C2Tx MXene sheets is proposed. The as-prepared SnO2/MXene hybrids have a unique 0D–2D structure, in which the 0D SnO2 QDs (~ 4.7 nm) are uniformly distributed over 2D Ti3C2Tx MXene sheets with controllable loading amount. The SnO2 QDs serve as a high capacity provider and the “spacer” to prevent the MXene sheets from restacking; the highly conductive Ti3C2Tx MXene can not only provide efficient pathways for fast transport of electrons and Li ions, but also buffer the volume change of SnO2 during lithiation/delithiation by confining SnO2 QDs between the MXene nanosheets. Therefore, the 0D–2D SnO2 QDs/MXene hybrids deliver superior lithium storage properties with high capacity (887.4 mAh g−1 at 50 mA g−1), stable cycle performance (659.8 mAh g−1 at 100 mA g−1 after 100 cycles with a capacity retention of 91%) and excellent rate performance (364 mAh g−1 at 3 A g−1), making it a promising anode material for lithium-ion batteries.

which the 0D SnO 2 QDs (~ 4.7 nm) are uniformly distributed over 2D Ti 3 C 2 T x MXene sheets with controllable loading amount. The SnO 2 QDs serve as a high capacity provider and the "spacer" to prevent the MXene sheets from restacking; the highly conductive Ti 3 C 2 T x MXene can not only provide efficient pathways for fast transport of electrons and Li ions, but also buffer the volume change of SnO 2 during lithiation/delithiation by confining SnO 2 QDs between the MXene nanosheets. Therefore, the 0D-2D SnO 2 QDs/MXene hybrids deliver superior lithium storage properties with high capacity (887.4 mAh g −1 at 50 mA g −1 ), stable cycle performance (659.8 mAh g −1 at 100 mA g −1 after 100 cycles with a capacity retention of 91%) and excellent rate performance (364 mAh g −1 at 3 A g −1 ), making it a promising anode material for lithium-ion batteries.

Introduction
Lithium-ion batteries (LIBs) are widely used in various portable electronics, electric tools, and electric vehicles due to their high energy density, long cycle life, and environmental friendly [1,2]. Nevertheless, the conventional graphite anode of LIBs with a specific capacity of 372 mAh g −1 can hardly meet the rapidly increasing demand of high energy density. Great efforts have been made to develop promising anode materials with high capacity, such as transition metal oxides [3,4], alloys [5,6], metal oxides/sulfates [7][8][9], and phosphorous [10][11][12]. Among various metal oxides, SnO 2 has attracted a lot of attention due to its relatively high theoretical capacity (790 mAh g −1 , which is twice than the currently used graphite), low average working potential (~ 0.6 V vs. Li + /Li), natural abundance and low price [13][14][15][16]. Unfortunately, its practical application as anode material in LIBs is seriously limited by the poor cycle stability resulting from the severe volume change (> 300 vol%) during the charge/ discharge process. Meanwhile, SnO 2 also suffers from low electrical conductivity, resulting in poor rate capability. To improve the electrochemical performances of SnO 2 , several strategies have been proposed to overcome these issues. One of the effective way is to fabricate nanostructured SnO 2 , such as SnO 2 quantum dots (QDs) [17], SnO 2 hollow spheres [18], or SnO 2 nanowires [19], which could restrain the structure changes during lithium alloying and shorten the ion diffusion lengths [20]. The other way is to combine SnO 2 with various carbon materials possessing high conductivity such as carbon nanotubes, carbon fiber or graphene [21][22][23][24][25][26]. The conductive carbon can not only improve the overall electrical conductivity of the composites but also act as a buffer to slow down the structure collapse of the electrode. MXenes are a newly emerging family of 2D materials with a general formula of M n+1 X n T x , where M represents early transition metal (M = Ti, Sr, V, Cr, Ta, Nb, Zr, Mo, and H f ), X is carbon and/or nitrogen, and T stands for the surface termination groups (-OH, -F=O) [27][28][29][30][31]. Due to their excellent electrical conductivity, tailorable surface chemistries, and mechanical properties, MXenes have recently attracted great interests in energy storage devices such as LIBs, sodium-ion batteries, and supercapacitors [32][33][34][35]. Particularly, as one of the most widely studied MXenes, Ti 3 C 2 T x is a potential anode material for LIBs, which can deliver excellent cycle stability and superior rate performances. However, the theoretical reversible capacity of Ti 3 C 2 T x is only 320 mAh g −1 [36]. On the other hand, MXene can also be used as a substrate to fabricate hybrids with other active materials, including metal oxides, which have higher theoretical capacity but poor conductivity and are prone to large volume change during charge/discharge. In the hybrids, MXene can improve the conductivity and buffer the volume changes of the metal oxide, while the metal oxide provides high capacity. Thus, the MXene/ metal oxide hybrid could achieve high capacity, stable cycle and good rate performance [37][38][39][40][41]. For example, Ti 3 C 2 T x /Fe 2 O 3 nanocomposite was prepared by confining Fe 2 O 3 nanoparticles into Ti 3 C 2 T x nanosheets through ball-milling method, which showed reversible capacities of ~ 203 mAh g −1 at 1 C and 100 mAh g −1 at 10 C [37]. MXene/NiCo 2 O 4 composite with a capacity reaching up to 1330 mAh g −1 was synthesized by spray coating method [38]. Some recent works indicate that MXene is also a good substrate to improve the lithium storage performance of SnO 2 . The hybrids of SnO 2 with MXene prepared by wet hydrothermal approach has shown a high capacity of 1021 mAh g −1 and improved cycle stability [40]. However, a few reports indicate that MXene is unavoidable to be oxidized to titanium dioxide (TiO 2 ) as a by-product during these processes, thereby, influencing the electrochemical performance of the composites. Recently, our group proposed a general route to self-assemble transition metal oxide nanostructures on Ti 3 C 2 T x MXene nanosheets through van der Waals interaction. The proposed method allowed fabrication of hybrids without getting MXene to be oxidized and achieved enhanced cycle and rate performance [15].
In this work, a simple method is proposed to prepare 0D-2D SnO 2 QDs/MXene hybrids by electrostatic selfassembling SnO 2 QDs on the surface of 2D Ti 3 C 2 T x MXene nanosheets under ultrasonication. Since, the synthesis is accomplished in a very mild condition, i.e., ultrasonication treatment of the mixture of negatively charged MXene and positively charged SnO 2 QDs at room temperature, the MXene oxidation is effectively avoided. In the hybrids, 2D nanosheet of MXene acts as a substrate to support 0D SnO 2 QDs, which can not only provide large electrode/electrolyte interface area for fast reversible transport of electrons and ions, but could also inhibit the aggregation of SnO 2 QDs and buffer the volume changes during charge/discharge process. The SnO 2 QDs with ultra-small particle size can effectively maximize the activity and specific capacity and minimize 1 3 the volume change while inhibiting the structure collapse during charge/discharge process and shortening the lithium diffusion pathways. In addition, the SnO 2 QDs acts as a "spacer" to prevent the MXene nanosheets from restacking and thus protecting the Li + migration channels and active sites. These unique features endow the 0D-2D SnO 2 QDs/ MXene hybrids with high lithium storage capacity, excellent cycle stability and superior rate performance, indicating a promising anode for LIBs.

Synthesis of Ti 3 C 2 T x MXene
The Ti 3 C 2 T x MXene was synthesized by etching Ti 3 AlC 2 (400 mesh, purchased from Yiyi Technology Co., Ltd.) with LiF + HCl solution as reported previously [41]. Typically, 1.0 g Ti 3 AlC 2 powder was subjected to a mixture containing LiF (1.0 g) and hydrochloric acid (12 M, 10 mL) under stirring conditions for 24 h at 35 °C. After several times of centrifugation-washing with deionized (DI) water, the product was then dispersed in 50 mL DI water, stored under ultrasound for 30 min. The dark green supernatant was collected by centrifugation at 3500 rpm for 1 h. Finally, the black MXene liquid was obtained and sealed for future use.

Synthesis of SnO 2 QDs
In order to obtain SnO 2 QDs, 4 mmol SnCl 2 ·2H 2 O and 4 mmol thiourea (CH 4 N 2 S) were added to 30 mL DI water and magnetically stirred at room temperature to form a milky suspension. After stirring for 24 h, a clear yellow aqueous solution containing SnO 2 QDs was obtained.

Synthesis of 0D-2D SnO 2 QDs/MXene Hybrids
In a typical synthesis, 5 mL of as-prepared SnO 2 QDs solution was added to 40 mg 0.1 mg mL −1 Ti 3 C 2 T x MXene solution under ultrasonication for 6 h. The SnO 2 QDs were deposited on the Ti 3 C 2 T x layers during this process and solid-liquid separation was observed. Finally, the black precipitate of SnO 2 QDs/MXene hybrids were collected by vacuum filtration, washed with water and dried in a vacuum oven at 80 °C for 6 h. The above hybrid was denoted as SnO 2 QDs/MXene-52. For comparison, we also prepared another SnO 2 QDs/Ti 3 C 2 T x hybrids (SnO 2 QDs/MXene-51) under the same conditions, but the addition amount of MXene in this solution was 20 mg.

Materials Characterization
Scanning electron microscopy (SEM) characterization was conducted using a Gemini SEM 500. Transmission electron microscopy (TEM) characterization was conducted on a JEOL JEM-F200 (HR) equipped with selected area electron diffraction (SAED). X-ray diffraction (XRD) patterns were recorded on a Bruker D8 ADVANCE with monochromatic Cu Kα radiation (λ = 1.54060 Å). Raman spectra were obtained with a LabRAM HR Evolution Raman spectrometer (633 nm). X-ray photoelectron spectroscopy (XPS) analysis was performed using a Thermo Fisher ESCALAB Xi + to analyze the chemical compositions of the samples.

Electrochemical Measurements
The working electrodes were prepared by mixing 80 wt% active material, 10 wt% Super P and 10 wt% carboxymethylated cellulose (CMC) in DI water. After coating the slurry on the copper foil, the electrodes were dried at 60 °C in vacuum oven for 8 h to remove the solvent. To test electrochemical performances, CR2025-type coin cells were assembled in an argon-filled glove box (Mikrouna, H 2 O, O 2 < 0.1 ppm) using Li foil as half-cell counter electrode, and microporous membrane (Celgard 2400) as separator. The electrolyte was 1 M LiPF 6 in a mixture of ethylene carbonate (EC)/diethyl carbonate (DEC)/dimethyl carbonate (DMC) with a volume ratio of 1:1:1. The charge/discharge (GCD) tests were performed using a LAND BT2000 battery tester. The potential window was 0.01-2.5 V versus Li/Li + . The CV traces were recorded on a CS350 electrochemical workstation from 0.01 to 2.5 V. Electrochemical impedance spectroscopy (EIS) measurements were performed on the VSP Bio-Logic SAS at frequencies ranging from 10 mHz to 100 kHz with an applied AC signal amplitude of 10 mV. The capacities of the samples were calculated based on all the components in the active materials, i.e., the mass of SnO 2 QDs/Ti 3 C 2 T x hybrids.

Results and Discussion
The synthesis route for 0D-2D SnO 2 QDs/MXene by electrostatic self-assembly is illustrated in Fig. 1. Firstly, transparent yellow aqueous solution of positively charged SnO 2 QDs is prepared by hydrolysis, dehydration and oxidation of SnCl 2 ·2H 2 O in DI water, where thiourea is added as a promoter and stabilizer (Fig. S1a). The mercaptan group can easily bound to Sn 2+ , and the SnO 2 QDs whose zeta potential is +99.0 mV (Fig. S1b) is surrounded by a positively charged protic amino group (-NH 3 + ), which makes it highly stable and electropositive [42]. The Ti 3 C 2 T x MXene solution with its negatively charged groups (-F, -OH) [43], and zeta potential of − 36.2 mV, is very stable in water (Fig. S1a) due to the hydrophilicity and electrostatic repulsion between neighboring nanosheets. When positively charged SnO 2 QDs are added into the negatively charged Ti 3 C 2 T x colloidal solution, the SnO 2 QDs can easily load onto the surface of the MXene nanosheets. After the continuous ultrasonication of 6 h, the supernatant becomes clear and colorless, and black precipitates are obtained. The positively charged SnO 2 QDs are captured by the negatively charged MXene nanosheets only by ultrasonic treatment without other components and additional treatments, implying an electrostatic self-assembly mechanism for the designed 0D-2D SnO 2 QDs/MXene hybrids. The Ti 3 C 2 T x MXene with 2D layered structure as a conductive matrix facilitates the charge transfer and accommodates the volume change of SnO 2 QDs; the SnO 2 QDs thus prevents the MXene nanosheets from restacking by working as "spacer" and providing channels for fast transfer of Li ion with promising electrochemical performance.
SEM and TEM were employed to characterize the morphology and structure of the as-prepared samples. The pure Ti 3 C 2 T x (Fig. S1c) displays a compact 2D layered structure. After decoration with SnO 2 QDs, the SnO 2 QDs/MXene hybrids exhibit 0D-2D structure, as shown in Fig. 2a. From  Fig. 2b, it can be seen that the spatially dispersed SnO 2 QDs are evenly distributed over the surface of the MXene nanosheets. The particle-size distribution analysis (Fig. S1d) indicates the SnO 2 QDs have an average particle size of ~ 4.5 nm, favoring the electrode/electrolyte interactions. Since the diffusion of Li ions is strongly dependent on the transport length and the active sites, the ultra-small particle size of SnO 2 QDs can not only expose a large number of electrochemical active sites shuttling Li ions in and out, but also shorten the diffusion path of Li ion transport, which are beneficial for improving the capacity and rate performance. The phase structures of the as-prepared SnO 2 QDs/ MXene hybrids were verified by XRD as shown in Fig. 3a. Ti 3 C 2 T x exhibits the major peaks, such as the (002), (006), (008), (0010), and (0012) [44]. Typically, the (002) plane of the Ti 3 C 2 T x MXene is located at 2θ = 6.4°, corresponding to an interplanar spacing of 13.7 Å [45]. In addition, the pure tetragonal phase of crystalline SnO 2 (JCPDS No. 41-1445) is located at 26.5°, 34.1°, and 52.4°, corresponding to (110), (101), and (211). The XRD patterns of the SnO 2 QDs/MXene hybrids consist of Ti 3 C 2 T x and tetragonal phase SnO 2 , indicating that SnO 2 QDs/MXene hybrids have successfully formed. No any extra crystalline phase peaks such as TiO 2 are observed, indicating the electrostatic self-assembly process is mild with no oxidation of MXene. In addition, the (002) peak of the SnO 2 -MXene composite (Fig.  S1e) was downshifted compared to pure MXene, indicating increased interlayer spacing of the MXene in the hybrids.
Raman spectra further confirm the phase characteristics of the SnO 2 QDs/MXene-52 hybrid. As shown in Figs. 3b and S1f, the peaks of pure SnO 2 at 271, 465, and 698 cm −1 can be assigned to the E g and A 1g active mode of SnO 2 , respectively [46]. Moreover, the pure MXene exhibits a peak at 199 cm −1 which corresponds to the A 1g symmetry out-of-plane vibrations of Ti atoms, whereas the peaks at 381 and 622 cm −1 are related to the E g group vibrations, including in-plane (shear) modes of Ti, C, and surface functional group atoms [47]. For the SnO 2 QDs/MXene-52, the spectrum manifests six cognizable Raman-active modes, combining the characteristic Raman peaks of Ti 3 C 2 T x and SnO 2 . In comparison with the pure SnO 2 , the peak intensity of SnO 2 in the hybrid has significantly declined. This suggests that the SnO 2 QDs are well separated by the Ti 3 C 2 T x nanosheets, which weakens the Raman signal from the SnO 2 . It's worth noting that no strong peak for TiO 2 was detected at 144 cm −1 , confirming that MXene has not oxidized during the hybrid formation.
The elemental composition of SnO 2 QDs/MXene hybrid was further analyzed by XPS. For pristine Ti 3 C 2 T x MXene, there are only four main elements, Ti, C, F, and O, whereas for SnO 2 QDs/MXene-52, the Sn element is also detected, indicating the presence of SnO 2 in the hybrid (Fig. S2a). In the high-resolution C 1s spectrum (Fig. 3c), the characteristic peaks correspond to C-Ti (284.2 eV), C-C (288.7 eV), C-O (288.1 eV), C=O (289.5 eV), and O-C=O (291.6 eV) [48]. The particular peak located at 282.1 eV can be assigned to the C-Ti bond in Ti 3 C 2 T x sheets. The XPS spectrum of the Ti 2p (Fig. 3d) reveals that peaks of Ti bound to C, Ti(II), and Ti(III). The Ti-C 2p 3/2 , Ti(II) 2p 3/2 , Ti(III) 2p 3/2 , T-C 2p 1/2 , Ti(II) 2p 1/2 , and Ti(III) 2p 1/2 peaks are detected at binding energies of 455.1, 455.6, 456.8, 458.8, 461.3, and 462.3 eV, respectively. After loading the SnO 2 QDs, no Ti(IV) peak is observed, suggesting no oxidation of MXene in the SnO 2 QDs/MXene-52 (Fig. 3f). Meanwhile, the XPS in Sn 3d region has two peaks at 487.2 and 495.6 eV, which are attributed to Sn 3d 5/2 and Sn 3d 3/2 of SnO 2 , confirming the formation of SnO 2 (Fig. S2b). Besides this, the relative intensities of Ti-C, Ti(II), and Ti(III) peaks in SnO 2 QDs/ MXene-52 are relatively weaker than pure Ti 3 C 2 T x , indicating decreasing Ti 3 C 2 T x signal for the hybrid. Furthermore, the slight shift of the Ti-C component to lower binding energy indicates that the synthesis of SnO 2 QDs/MXene hybrid by electrostatic attraction might have caused shift in the electron density (Fig. 3e). All of these observations demonstrate that SnO 2 QDs have been successfully deposited on the surface of the Ti 3 C 2 T x sheets. In addition, the introduction of SnO 2 QDs on the Ti 3 C 2 T x surface increases the accessible surface area [49], which is evident from the nitrogen adsorption measurements (Fig. S2c). The SnO 2 QDs/MXene-52 shows a typical type I adsorption/desorption isotherms with a BET specific surface area of 184 m 2 g −1 , much higher than that of the pure MXene (19 m 2 g −1 ). The large surface area is beneficial for accelerating electrolyte diffusion and accommodating volume change of the SnO 2 QDs during charge/discharge. Thus, much improved lithium storage performances could be expected.
To evaluate the electrochemical performances of the SnO 2 QDs/MXene hybrids as anode materials in LIBs, coin-type half cells were assembled using lithium as counter electrode. CV curves of Ti 3 C 2 T x MXene and SnO 2 QDs/MXene hybrids were scanned between 0.01 and 2.5 V. For the pure Ti 3 C 2 T x MXene, the broad irreversible reduction peak at around 0.7 V is observed in the first lithiation process, which is attributed to the formation of a solid electrolyte interphase (SEI) generated from the reaction of Ti 3 C 2 T x with Li ion [17]. In the subsequent cycles, the reversible peaks near 0.77 and 1.5 V may be the consequence of the reaction between Li + and the titanium-based compounds (Fig. S3a) [50]. For the pure SnO 2 QDs (Fig. S3b), the SnO 2 is considered to transform into Li x Sn (x ≤ 4.4) and Li 2 O during the initial discharge process (lithium insertion). The cathodic peak at 0.05 V result from the alloying process of Sn to Li x Sn (0 ≤ x ≤ 4.4), and the strong anode peak at about 0.5 V is caused by the de-alloying process of Li x Sn (0 ≤ x ≤ 4.4).  Fig. 3 a XRD patterns, b Raman spectra of MXene, SnO 2 , and SnO 2 QDs/MXene hybrid. XPS spectra of c MXene for high-resolution C 1s, d Ti 2p, e SnO 2 QDs/MXene-52 for high-resolution C 1s, f Ti 2p 1 3 The anode peak at 1.2 V is caused by the partial reversible transformation of Sn to SnO 2 owing to its quantum size [51,52]. However, in the second and third cycles of the pure SnO 2 , the intensities of the cathodic and anodic peaks show continuous and significant decline, indicating rapid capacity degradation because of the structural variation of SnO 2 during lithiation/delithiation processes (Fig. S3b). The SnO 2 QDs/MXene-52 (Fig. 4a) shows CV curves with similar characteristic peaks, but the curves in the 2nd and 3rd cycles almost overlap, indicating good cycle performance. In comparison with the SnO 2 QDs/MXene-51 (Fig. 4b), the SnO 2 QDs/MXene-52 display better overlapping CV curves with the same peak positions, implying excellent reversibility in its conversion reaction.
Furthermore, the GCD profiles (Fig. 4c, d) of the SnO 2 QDs/MXene at 50 mA g −1 coincides well with the CV curves. The GCD profile of the Ti 3 C 2 T x and SnO 2 QDs electrodes were also tested at the same conditions (Fig. S3c,  d). In the initial discharge profile of the SnO 2 QDs/MXene hybrids, the plateaus at 0.7 and 0.05 V correspond to the SEI formation and lithium alloying; while during the initial charge, the plateaus at 0.5 and 1.2 V relate to lithium de-alloying and partial reversible SnO 2 formation, respectively. The reversible capacity of the SnO 2 QDs/MXene-52 is 887.4 mAh g −1 in the first cycle, with initial Coulombic efficiency (CE) of about 51.2%. The low CE might have resulted from the SEI and Li 2 O formation as well as the electrolyte decomposition. In the 5th cycle, the capacity was determined to be about 847.6 mAh g −1 and the corresponding CE reached about 100%. With the SEI film protection, the capacity reaches a stable state. In contrast, the SnO 2 QDs/MXene-51 delivers an initial reversible capacity of 897.5 mAh g −1 with a CE of 43.0% (Fig. 4d). The results indicate that the introduction of more SnO 2 QDs can increase available active sites and the capacity of the hybrid material, but at the same time cause more side reactions and reduce the initial CE, due to its ultra-small particle size and high surface area. Therefore, only appropriate ratio of SnO 2 QDs and MXene sheets could reach an optimum electrochemical performance.  Figure 5a shows the comparison of the charge/discharge cycle performance among Ti 3 C 2 T x MXene, SnO 2 QDs, and SnO 2 QDs/MXene at 100 mA g −1 . The bare Ti 3 C 2 T x MXene has a low initial capacity of 79.2 mAh g −1 with a CE of 34.2%, which is attributed to the restacking of MXene nanosheets [36]. The initial capacity of the pure SnO 2 QDs can reach 835.9 mAh g −1 , but it fades very rapidly due to the severe pulverization of SnO 2 during charge/discharge. After 20 cycles, the capacity remains only 144.6 mAh g −1 with a capacity retention ratio of about 17% of the initial capacity. In contrast, by electrostatic self-assembly of the SnO 2 QDs on the 2D Ti 3 C 2 T x MXene nanosheets, the SnO 2 QDs/ MXene hybrids exhibit much enhanced cycle stability. The capacity of SnO 2 QDs/MXene-52 reaches to 659.8 mAh g −1 with 91% retention of the initial capacity after 100 cycles, which is much higher than that of the pure SnO 2 QDs and Ti 3 C 2 T x MXene. In a comparative evaluation of SnO 2 QDs/ MXene hybrids with different SnO 2 /MXene ratio, it is found that the SnO 2 QDs/MXene-52 with abundant MXene shows better cycle stability than the SnO 2 QDs/MXene-51, indicating the importance of adequate MXene substrate in achieving optimum performance [53].
Another attractive feature of the SnO 2 QDs/MXene electrodes is the excellent rate performance. As shown in Fig. 5b,   [56], and Si@Ti 3 C 2 [57] etc. Electrochemical impedance spectroscopy was employed to compare the reaction kinetics of the SnO 2 QDs/MXene-52 and the bare SnO 2 QDs. The typical Nyquist plots of the two electrodes (Fig. 5d) consist of a compressed semicircle in the intermediate frequency region and a diagonal line in the low frequency range. The semicircle is related to the charge transfer resistance (R ct ), and the oblique line is related to Warburg impedance, suggesting the diffusion of Li ion in the active materials [58]. The R ct values of the SnO 2 QDs/MXene-52 and SnO 2 QDs electrode were calculated to be 75.50 and 100.20 Ω, respectively. Obviously, SnO 2 QDs/MXene-52 possesses much lower R ct value compared to bare SnO 2 QDs. This can be attributed to the high electrical conductivity of the MXene and the fast charge diffusion reaction due to its unique 0D-2D structure. Moreover, the SnO 2 QDs/MXene-52 shows a relatively steep lowfrequency tail, indicative of high Li ion diffusibility, which results from the efficient ion transfer pathways constructed by MXene.
The above excellent electrochemical performances are because of the synergic effect between MXene nanosheets and SnO 2 QDs, and the mechanism is illustrated in Fig. 5e. In the SnO 2 QDs/MXene hybrids, the MXene nanosheets act as 2D substrates for uniform anchoring of SnO 2 QDs. The MXene nanosheets prevent the aggregation of SnO 2 QDs and work as an elastic buffer space to adapt to the volume expansion/contraction of SnO 2 QDs during charging and discharging, thus leading to good cycle stability. Furthermore, MXene nanosheets with good electric conductivity construct effective conductive channels for SnO 2 QDs, facilitating fast charge transport and improving the rate performance. Moreover, the unique 0D-2D structure offers massive electrochemically active sites for high specific capacity, which contribute in improving the electrochemical performance of electrode materials, thus excellent lithium-ion storage performances are obtained for SnO 2 QDs/MXene hybrids.

Conclusions
The 0D-2D SnO 2 QDs/MXene hybrids have been successfully synthesized by an efficient electrostatic selfassembly strategy. The 0D SnO 2 QDs with an average size of 4.7 nm are uniformly distributed on the 2D MXene nanosheets with strong adhesion, acting as structurally stable host for lithium storage. The 2D MXene nanosheets buffer the volume change of SnO 2 QDs during charge/discharge and construct effective channels for charge transport. Besides this, the 0D-2D structure creates additional active sites. The 2D conductive Ti 3 C 2 T x MXene, ultra-small SnO 2 QDs, and unique 0D-2D nanoarchitecture are synergistically responsible for the outstanding electrochemical performances of the SnO 2 QDs/ MXene hybrids. As an anode for LIBs, it exhibits a high specific capacity of 887.4 mAh g −1 at 50 mA g −1 , excellent rate capability (364 mAh g −1 at 3 A g −1 ), and superior cycle stability (659.8 mAh g −1 after 100 cycles with 91% retention). These results indicate that the 0D-2D SnO 2 QDs/MXene is a promising anode material for advanced LIBs. In addition, the electrostatic self-assembly method could be extended to other transition metal oxide/MXene hybrids and will have potential applications in sodium-ion batteries, potassium-ion batteries, and supercapacitors.