General Strategy To Synthesize Highly Dense Metal Oxide Quantum

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General Strategy to Synthesize Highly Dense Metal Oxide Quantum Dots-Anchored Nitrogen-Rich Graphene Compact Monoliths Enable Fast and High-Stability Volumetric Lithium/Sodium Storage Junlu Zhu, Yunyong Li, Ying Huang, Changzhi Ou, Xingxing Yuan, Liang Yan, Wenwu Li, Haiyan Zhang, and Pei Kang Shen ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.9b00279 • Publication Date (Web): 10 Apr 2019 Downloaded from http://pubs.acs.org on April 11, 2019

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ACS Applied Energy Materials

General Strategy to Synthesize Highly Dense Metal Oxide Quantum Dots-Anchored Nitrogen-Rich Graphene Compact Monoliths Enable Fast and High-Stability Volumetric Lithium/Sodium Storage

Junlu Zhua, Yunyong Lia,*, Ying Huanga, Changzhi Oua, Xingxing Yuana, Liang Yan a, Wenwu Lia, Haiyan Zhanga, and Pei Kang Shenb

aGuangdong

Provincial Key Laboratory of Functional Soft Condensed Matter, School

of Materials and Energy, Guangdong University of Technology, No. 100 Waihuan Xi Road, Guangzhou Higher Education Mega Center, Guangzhou 510006, China E-mail: [email protected] (Y.Y. Li) bCollaborative Innovation Center of Sustainable Energy Materials,

Guangxi University,

Nanning, Guangxi 530004, China

-----------------------------------------------------------------------------------------------------------------*Corresponding author. Tel.: +86-20-39322570, Fax: +86 20 39322570 E-mail addresses: [email protected] (Y.Y. Li).

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ABSTRACT Volumetric performance of a material is more attractive than gravimetric performance in consumer electronics and electric vehicles, but rarely emphasized in earlier studies of lithium-ion batteries (LIBs), especially current sodium-ion batteries (SIBs). Herein, we report a simple and general strategy with the assistance of a small amount of graphene oxide (~10 wt%) as an “assembled binder” to design porous yet highly dense metal oxide quantum dots-anchored nitrogen-rich reduced graphene oxide (denoted as HD-MOx-N-RGO) compact monoliths. Taken TiO2 as a representative, the asfabricated HD-TiO2-N-RGO compact monolith, consisting of well-dispersed and ultrasmall-sized TiO2 quantum dots (~4.0 nm) anchored on N-RGO, exhibits a high electrical conductivity of 343.7 S m-1, high density of 1.8 g cm-3, and porosity, thus both leading to high gravimetric and volumetric capacities without degradation after 100 cycles at 0.1 A g-1 and superior rate capability at 10 or 5 A g-1 as anode in LIBs and SIBs, respectively. More importantly, when the current density is increased to 2.0 A g-1, it still both exhibits a high-stability lifespan with over 91% capacity retention after 1000 cycles in LIBs and SIBs. Detailed analysis of microstructures, composition and electrochemical kinetics reveal that the superior rate and long-cycling performance stem from the ultra-small size of TiO2-QDs and the strong interaction between N-RGO and TiO2, which not only facilitates bulk Li+/Na+ intercalation but also improves the interfacial Li/Na storage. This study demonstrates our strategy is very promising in designing compact energy storage materials with fast and high-stability volumetric performance.

KEYWORDS: Metal oxide, nitrogen-rich graphene, hybrid monolith, volumetric density, lithium/sodium ion batteries 2


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INTRODUCTION Lithium-ion batteries (LIBs) and sodium-ion batteries (SIBs) have been highly valued in electric vehicles, smart power grids and portable mobile devices, because of their high energy density, good cycle performance, light weight and environmental friendliness.1-4 With the acceleration of people’s life rhythm and the improvement of safety awareness, fast, long-cycle and safe electrode materials are urgently required as LIBs and SIBs still display insufficient rate performance and service life in present.5-7 In the past decade, nanoparticulate materials, particularly zero-dimensional quantum dots (QDs), as electrode materials are attracted great interests in improving lithium/sodium storage and rate capability because ultrafine QDs not only greatly shorten ion diffusion distance in the solid phase, but also endow ultrahigh surface-tovolume ratio and abundant active for electrochemical reactions. Nevertheless, these ultra-small QD materials would be extremely easily to cause agglomeration among themselves during cycling due to their high surface energy.8 Therefore, it is very imperative to develop some strategies to prevent the agglomeration of QD materials. Graphene has been regarded as one of most ideal hosts to incorporate with ultrafine nanoparticles to tackle the agglomeration and conductivity issue.9-11 Particularly, nitrogen-doped graphene or nitrogen-rich graphene can create localized sufficient active sites, which can generate strong coupling with metal oxides (e.g., TiO2, Fe3O4, and SnO2, etc.) by chemical bonding and provide fast charge transfer owing to the introduction of electron-rich surface in graphene.12-17 Therefore, the introduction of nitrogen-rich surface or high-content nitrogen doping in graphene would be conducive to providing abundant active sites for chemical binding of ultrafine QD materials as well as good electrode/electrolyte wettability and electrical conductivity for electrochemical reaction, which should be able to give these QD material-based 3



nitrogen-doped graphene nanocomposites fast electron and ion transport in LIBs or SIBs.18 Most recently, volumetric performance has become more increasing attention than gravimetric performance in consumer electronics (e.g. smartphones, ultra-books, etc.) and electric vehicles, because where electrochemical energy storage devices need store as much and fast energy as possible in a very limited space.19-21 Therefore, volumetric performance of a material is a crucial parameter in developing the devices with high volumetric energy density, but rarely emphasized in earlier studies of LIBs, especially current SIBs. Until recently, some scientists just start devoting to fabricating the electrode materials with high volumetric capacity for next-generation rechargeable batteries.19, 22 Transition metal oxides, particularly titanium dioxide (TiO2),23-25 are the most promising anode materials because of their low cost, natural abundance, nontoxicity and high safety. More importantly, their high density could facilitate to achieve high volumetric performance of electrode materials. Nevertheless, owing to the intrinsic low electrical conductivity of metal oxides,26 they need to be electrically conductive, so when used as electrode materials in LIBs or SIBs, metal oxides usually need to be made into nanoparticle form and simultaneously loaded on a conductive carbon support such as above-mentioned graphene or heteroatom-doped graphene. This nanocomposite design has been demonstrated an effective strategy in improving their gravimetric capacities, while leading to the obvious decrease in volumetric capacity because of the fluffy stack among individual nanocomposites,22, 27 In order to achieve high volumetric capacity of nanocomposites, a disassembly-reassembly strategy has been reported in recent, to make the RuO2 nanoparticles uniformly loaded into the compact graphene monolith, and finally achieving a high volumetric performance in supercapacitors.28 As for this strategy, it need experience the disassembly of reduced 4


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graphene oxide (RGO) hydrogels and reassembly of RGO micro-gels with the assist of creating Ru(OH)3 as well as the final heated treatment, which results in a very complex synthesis processes and special forming condition. Hence, exploiting a simple and universal strategy to compactly assemble the individual nanocomposites, particularly above-mentioned QD material-based nitrogen-doped graphene nanocomposites, and simultaneously maintain their high gravimetric capacity, aiming at achieving a fast and high volumetric performance in rechargeable batteries, is still a crucial challenge.29 Herein, we report a general and facile scaling-up strategy for the fabrication of highly dense metal oxide QDs-anchored nitrogen-rich reduced graphene oxide (denoted as HD-MOx-N-RGO) compact monoliths, as shown in Scheme 1. In this synthesis, we adopt graphene oxide (GO) with a small amount (~10 wt%) as an “assembled binder” to make these as-prepared MOx QDs-anchored nitrogen-rich reduced graphene oxide (marked as MOx-N-RGO) hybrid powder assembled together and naturally compressed into dense and strong HD-MOx-N-RGO hybrid monolith in room temperature. The detailed designing considerations are follows: (1) the formation of a nitrogen-rich surface on RGO (denoted as N-RGO) through the in-situ polymerization of cyanamide to provide sufficient coordination sites for good immobilization and homogeneous growth of MOx (e.g. TiO2) nanoparticles meanwhile improve the interfacial stability; (2) in-situ hydrolysis and immobilization of MOx (e.g. TiO2) nanoparticles on the surface of N-RGO at a low-temperature mixed solvothermal condition of 120 oC to obtain an ultra-small size of MOx (e.g. TiO2) nanoparticles anchored on N-RGO, which would drastically shorten the ion diffusion path in the solid phase and provide abundant electrochemical reactive sites, thus giving the hybrid monolith superior rate capability and high lithium/sodium storage; (3) the introduction of GO (~10 wt%) to make MOxN-RGO nanohybrids assembled into MOx-N-RGO hydrogel in the high-temperature 5



hydrothermal condition of 180 oC; (4) the final evaporation-induced drying at room temperature to make the MOx-N-RGO hydrogels naturally compressed into dense HDMOx-N-RGO monoliths, thus achieving a high-volumetric lithium/sodium storage. Taken TiO2 as a representative, we design a porous yet dense HD-TiO2-N-RGO monolith, in which a large number of ultra-small TiO2-QDs (~4.0 nm) are homogeneously anchored onto N-RGO, and the HD-TiO2-N-RGO hybrid monolith displays a good electrical conductivity of 343.7 S m-1 and a high density of 1.8 g cm-3, which is far higher than the tap density of pure TiO2 nanoparticles (0.51 g cm-3), even higher than that of the densifying nano-sized TiO2 solid made by mechanical compression at 40 MPa (1.6 g cm-3). When used as anode in LIBs and SIBs, the HDTiO2-N-RGO monolith electrode both exhibits highly stable gravimetric and volumetric capacities with no degradation after 100 cycles at 0.1 A g-1 (204.1 mA h g-1 and 517.1 mAh cm-3 for LIBs, and 203.4 mA h g-1 and 515.1 mAh cm-3 for SIBs), respectively. Besides, it also both shows superior rate capacity at 10 or 5 A g-1 and long lifespans of 1000 cycles with over 91% capacity retention at 2 A g-1 for LIBs and SIBs, respectively. These results demonstrate that our strategy is very promising in designing compact energy storage materials with fast and high-stability volumetric performance.

RESULTS AND DISCUSSION The formation of HD-TiO2-N-RGO compact hybrid monolith is clarified in Scheme 1 (see experimental details). First, an in-situ polymerization of cyanimide on the GO surface was employed to form a thin layer of nitrogen-rich cyanamide polymers (marked as Pcy) on RGO, thereby obtaining the N-RGO.30-31 In this process, the amino groups of cyanamide would undergo the nucleophilic reaction with oxygen functional groups on the surface of GO. At the same time, the GO is reduced to conductive RGO. 6


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Then, ultra-small TiO2-QDs are uniformly grown onto N-RGO surface by in-situ hydrolysis of Ti4+ precursor ions in ethanol solution after adding a small amount of deionized water in a low-temperature mixed solvothermal condition of 120 oC. In this process, the nitrogen-rich surface of N-RGO can provide sufficient coordination sites for anchoring metal ions, thus giving rise to TiO2-N-RGO nanohybrids. After that, TiO2-N-RGO hydrogels were obtained by the introduction of a small amount of GO (~10 wt%) in the high-temperature hydrothermal condition of 180 oC for 12 h. Finally, the HD-TiO2-N-RGO monoliths were obtained by evaporation-induced drying at room temperature. By comparison, the sample made with the same process of HD-TiO2-NRGO compact hybrid monolith but without the introduction of GO in the Step III was also prepared (marked as TiO2-N-RGO-180, see experimental details). Interestingly, the TiO2-N-RGO-180 sample exhibited scattered powders (powder precipitate), indicating that the TiO2-N-RGO nanohybrids cannot be assembled into a compact bulk solid without the introduction of GO (see Figure S1). The results demonstrate a key role of GO during the assembly process of hydrogel, even high-density monolith. In other words, the GO acts as the “assembled binder”, which can make these TiO2-N-RGO nanohybrids assembled together and form the TiO2-N-RGO hydrogel at the hightemperature hydrothermal condition. Despite the exact mechanism is still vague, we believe that the possible mechanism might be that: first, the fully hydrophilic GO was reduced to partially hydrophobic RGO, and then the partially hydrophobic RGO would spontaneously interconnect with the TiO2-N-RGO nanohybrids with layered structure (see Figure S2) and meanwhile be assembled into the TiO2-N-RGO hydrogel in the high-temperature hydrothermal condition. This should be consistent with the mechanism of reported self-assembled graphene hydrogel.32-34 The morphologies of HD-TiO2-N-RGO compact hybrid monolith and TiO2-N-RGO 7



hydrogel are investigated by SEMAs shown in Figure 1A and Figure S3, the TiO2-NRGO hydrogel shows a 3D self-supporting cylinder with a volume of 5.98 cm3 (Figure 1A, Left) and its freeze-dried microstructure exhibits a typically foam-like macroporous structure (Figure S3). According to the calculation of its weight and volume (Here, the weight is 147.6 mg and the volume is 5.98 cm3), the apparent density of freeze-dried TiO2-N-RGO hydrogel is 0.025 g cm-3. After the evaporation-induced drying, the HD-TiO2-N-RGO hybrid monolith still exhibits a self-supporting cylindrical shape, but its structure becomes stiff and its volume is dramatically shrank and decreased to only 0.082 cm3 (~1/70 of the volume of parent hydrogel) (Figure 1A, Right). According to the calculation, the apparent density of HD-TiO2-N-RGO monolith reaches up to about 1.8 g cm-3, which is far higher than the tap density of pure TiO2 nanoparticles (0.51 g cm-3), even higher than that of the densifying nano-sized TiO2 solid made by mechanical compression at 40 MPa (1.6 g cm-3) (Figure S4 and Experimental section S1 in the Supporting Information). This result further demonstrates the advantage of our strategy in designing high-density compact graphene-based composites. Furthermore, the microstructure of HD-TiO2-N-RGO compact monolith was also confirmed by SEM. Compared with the freeze-dried TiO2N-RGO hydrogel (Figure S3), the HD-TiO2-N-RGO compact monolith (Figure 1 (B and C)) shows totally different microstructure, which exhibits a highly compact structure but without obvious large pores at low magnification. This suggests that the large pores in hydrogel were compressed in large quantities by the evaporation-induced drying, thus forming a high-density monolith. Moreover, the further magnified SEM images (Figure 1 (D to F)) exhibit highly densifying assembly of TiO2-N-RGO nanohybrids containing a great number of ultrafine TiO2-QDs, and abundant mesoporous channels (Figure 1E) were observed in the interspaces among TiO2-QDs, 8


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which was illustrated in Inset in Figure 1E. More pictures see Figure S5 in the Supporting Information. In addition, the microstructures of GO, N-RGO, TiO2-N-RGO nanohybrid and HDTiO2-N-RGO compact monolith were investigated by TEM, respectively. As shown in Figure S6A, the GO exhibited a few-layered 2D sheet structure with a wrinkled surface. After the polymerization of cyanamide on GO surface, the N-RGO (Figure S6B) still maintained 2D sheet-like morphology, but due to the coating of Pcy on RGO, it exhibited a dark contrast. After the anchoring of TiO2-QDs on N-RGO by a lowtemperature mixed solvothermal of 120 oC, the TiO2-N-RGO nanohybrid (Figure S2) still exhibited the similar 2D sheet-like structure as N-RGO, but ~4.0 nm TiO2-QDs were homogeneously anchored on the surface of N-RGO (Figure S2B), indicating that the N-RGO plays a strong role in dispersing and anchoring ultrafine TiO2-QDs. Besides, the selected-area electron diffraction (SAED) pattern corresponding to the (101), (004), (200), and (211) crystalline planes, and the well-defined lattice fringes of (101) planes with an inter-planar spacing of 0.35 nm (Inset in Figure S2B) also further confirmed the formation of anatase TiO2 (PDF No.21-1272) on N-RGO. Surprisingly, after the experiencing of high-temperature hydrothermal condition of 180 oC for 12 h and following evaporation-induced drying, the microstructures of HD-TiO2-N-RGO hybrid monolith (Figure 1 (G and H)) maintained the same 2D layered structure and good dispersion of TiO2-QDs as those in TiO2-N-RGO nanohybrid (Figure 1 (G and H)), suggesting strong interaction between TiO2 and N-RGO well. In addition, the similar SAED pattern (Inset of Figure 1H), and the clear lattice fringes of (101) planes for anatase TiO2 (Inset in Figure 1I), as well as the same particle size of about 4.0 nm for TiO2-QDs were obtained for the HD-TiO2-N-RGO compact monolith, respectively, indicating that the dispersion, chemical structure, and particle size of TiO2-QDs 9



anchored on N-RGO have no obvious change after further undergoing the hightemperature hydrothermal and following evaporation-induced drying. The outcome illustrates the advantages of our strategy in designing the high-density hybrid materials yet with the well dispersion and ultrafine particle size of active materials on conductive carbon matrix. Moreover, the presence and distribution of C, O, N and Ti element were further confirmed by scanning TEM (STEM) elemental mapping of HD-TiO2-N-RGO compact monolith. As shown in Figure 1 (J to M), a large number of monodisperse TiO2-QDs were uniformly dispersed onto the surface of N-RGO and the elemental mapping of carbon (M1), nitrogen (M2), oxygen (M3), and titanium (M4) exhibited homogenous distribution, respectively, indicating the uniform nitrogen doping and good dispersion of TiO2 on N-RGO. Besides, the contents of TiO2 in TiO2-N-RGO nanohybrids and HD-TiO2-N-RGO compact monolith were also determined by TGA, which are around 71.2 wt% and 63.8 wt%, respectively (more details were shown in Figure S7). This result indicates that the introduction of a small amount of GO do not markedly cause the decrease in content of TiO2 for the hybrid monolith. The structural characteristics and composition of HD-TiO2-N-RGO hybrid monolith were also determined by XRD, Raman spectroscopy, and XPS, respectively. Figure 2A gives the XRD patterns of HD-TiO2-N-RGO monolith and the controlled samples of GO, N-RGO, pure TiO2, and TiO2-N-RGO nanohybrids. At 2θ=9.1°, a sharp diffraction peak was observed for GO, implying a high-degree oxidation of GO. After the in-situ polymerization of cyanamide on GO surface, the N-RGO exhibited a broad diffraction peak at 2θ=24.6°, which indicates that the GO is reduced to RGO in high degree after the polymerization of cyanimide. A series of strong, sharp peaks at 25.3°, 37.8°, 48.0° 53.9°, 55.1°, and 62.7° can be observed in the XRD pattern of pure TiO2, corresponding to the (101), (004), (200), (105), (211), and (204) planes of anatase TiO2 (PDF No.2110


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1272), respectively. As for the TiO2-N-RGO nanohybrids and HD-TiO2-N-RGO monolith, all their diffraction peaks are well matched with the pure anatase TiO2, respectively, though they become relatively weak and broad. The results indicated that TiO2 nanoparticles were successfully anchored on N-RGO for the two samples (Noting that the C (002) peak of N-RGO is almost overlapped with the main (101) peak of anatase TiO2). In addition, according to the (200) reflection of TiO2-N-RGO nanohybrids and HD-TiO2-N-RGO monolith, the average particle sizes of TiO2-QDs are both ~4 nm (according to the calculation of Scherrer Equation), which are consistent with the results of HR-TEM of TiO2-N-RGO nanohybrid and HD-TiO2-N-RGO monolith (Figure S2B and Figure 1I). The result further demonstrates that the crystal structure and particle size of TiO2-QDs in the HD-TiO2-N-RGO monolith have no obvious change though the TiO2-N-RGO nanohybrids experience the high-temperature hydrothermal of 180 oC and followed evaporation-induced drying, suggesting strong coupling of TiO2 with N-RGO. The Raman spectra of GO, N-RGO, TiO2-N-RGO nanohybrid, HD-TiO2-N-RGO monolith, and pure TiO2 were shown in Figure 2B. GO, N-RGO, TiO2-N-RGO nanohybrid, and HD-TiO2-N-RGO hybrid monolith exhibited a same D band at 1346 cm-1 and G band at 1590 cm-1 of graphene, suggesting that the four samples contained few-layer graphene. The Raman peaks at 153 (Eg), 200 (Eg), 396 (B1g), 513 (A1g) and 633  cm-1 (Eg) are belong to the characteristic peaks of pure anatase TiO2.35-36 And the three characteristic peaks of Eg Raman modes of anatase TiO2 can be clearly observed in both TiO2-N-RGO nanohybrid and TiO2-N-RGO hybrid monolith, further demonstrating the successful anchoring of TiO2 on N-RGO. Figure 2C exhibited the XPS spectra of GO, N-RGO, and HD-TiO2-N-RGO compact monolith, which were employed to determine the chemical compositions and valence 11



states of elements in the composites. The elemental composition and contents of three samples are listed in Table S1. For GO, there are only two strong peaks of C 1s and O 1s, suggesting abundant oxygen-functional groups in the GO. The further highresolution C 1s spectrum demonstrated that a great amount of O-C=O and C-O functional groups in the GO (Figure 2C1, Top). The functional groups of GO can be seen at the binding energies of 284.6 (C-C/C=C), 286.7 (C-O), and 288.9 (O-C=O) eV 37

As for the N-RGO, except for the peaks of C 1s and O 1s, an additional strong N 1s

peaks was observed (Figure 2C), and in the XPS spectrum of N-RGO, the atomic ratio of N element is 29.05 at% (Table S1), which indicates successful polymerization of cyanimide on GO surface, thus forming a nitrogen-rich surface on RGO. Moreover, the peak intensity of the oxygenated carbon species of N-RGO became obviously weaker than the GO (Figure 2C1), implying that GO is reduced to RGO after the polymerization of cyanimide. Besides, two new peaks at 285.5 and 288.4 eV can be clearly observed in the high-resolution C 1s spectrum (Figure 2C1 middle), which can be ascribed to the Pcy on the surface of N-RGO. The de-convoluted peaks centered at the binding energies of 285.5 and 288.4 eV can be assigned to the C-N and H2N-C=N functional groups of Pcy, respectively.30 As for the HD-TiO2-N-RGO monolith, its spectrum consists of the peaks of C, O, N, and Ti. The peaks of C-N and H2N-C=N could also be observed in the high-resolution C 1s spectra, indicating that the composition of N-RGO would not be destroyed after experiencing the high-temperature hydrothermal condition of 180 oC. The high-resolution N 1s spectra of N-RGO (Figure 2C , top) and HD-TiO -N-RGO 2 2

monolith (Figure 2C2, bottom) both visualize the presence of pyridinic N (398.7 eV), pyrrolic N (400.0 eV), graphitic N (401.3 eV), illustrating that the successful formation of N-rich surface on RGO.30, 38 Furthermore, the binding energy centered at 399.95 eV can also be attributed to anionic N in the O-Ti-N linkages.39 The high-resolution Ti 2p 12


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spectrum of HD-TiO2-N-RGO hybrid monolith and pure TiO2 were given in Figure 2C3. The peaks of Ti 2p1/2 and Ti 2p3/2 are attributed to the peaks of Ti4+ state (their spin energy separation of Ti 2p1/2 and Ti 2p3/2 is 5.8 eV40-41), further demonstrating the formation of TiO2. Moreover, in the Ti 2p spectrum of TiO2-N-RGO monolith, an 0.8 eV shift towards higher binding energy was observed compared to pure TiO2, suggesting a strong electron interaction between N-RGO and TiO2-QDs,42 which may be due to partial formation of oxygen-titanium-nitrogen bonds. As seen in the fourier transform-infrared spectroscopy (FTIR) (Figure S8A), very weak absorption peaks of oxygen-functional groups (1390 cm-1 for -OH, 1725 cm-1 for C=O, 1550-1610 cm -1 for -COOH and 3000-3600 cm-1 for hydroxyl groups) in N-RGO and HD-TiO2-N-RGO monolith can be observed, further revealing the partial reduction of GO. And two new peaks arise at 1170-1510 cm-1 and 3430 cm-1 in the spectra of NRGO should belong to the stretching vibrations of C-N and C=N.43 This results further indicates the successful polymerization of cyanimide on RGO surface. As for the HDTiO2-N-RGO monolith, the peaks of C-N and C=N did not be obviously observed, which may be attributed to the high loading of TiO2 nanoparticles anchored on NRGOmakes these peak signal masked or inactive30. Besides, the electrical conductivities of HD-TiO2-N-RGO monolith, along with the controlled sample of HD-TiO2-RGO monolith (the synthesis and physical characterization of HD-TiO2-RGO monolith, see the Experimental section and Figure S9), were measured by I-V curve using electrochemical workstation (Figure S8B), respectively. The measurement apparatus was also shown in the inset of Figure S8B. According to the calculation from the curves (Figure S8B), the conductivity of HDTiO2-N-RGO compact monolith is 343.7 S m-1, which is far higher than the HD-TiO2RGO monolith (39.5 S m-1). The result indicates that the nitrogen-rich doping can 13



remarkably improve the electrical conductivity of RGO,39, 44-45 even the whole monolith, thus further enhancing the electrochemical performance. Nitrogen adsorption-desorption experiments were also used to study and compare the porosities of different materials. Figure 3A showed the isotherm of TiO2-N-RGO nanohybrids and HD-TiO2-N-RGO compact monolith. The isotherm of TiO2-N-RGO nanohybrids exhibited mixed characteristics of Type I and Type IV with a weak hysteresis loop, and its Brunauer-Emmett-Teller (BET) specific surface area is 312.5 m2 g-1 with a total pore volume of 0.21 cm3 g-1. Compared with the TiO2-N-RGO nanohybrids, the HD-TiO2-N-RGO monolith showed a wide and pronounced hysteresis loop (Type H2a), indicates the existence of a large number of mesopores. Besides, its BET specific surface area has no obvious change compared with the TiO2-N-RGO nanohybrids (See Table S2), suggesting that the introduction of GO and followed evaporation-induced drying do not change the specific surface area of HD-TiO2-NRGO monolith. Moreover, their corresponding density functional theory (DFT) pore size distributions were also shown in Figure 3B. As for the TiO2-N-RGO nanohybrids, it exhibited a great number of 1-4 nm small pores (1-2 nm micropores and 2-4 nm mesopores) and 20-100 nm large pores. However, the HD-TiO2-N-RGO monolith only exhibited a great number of 1-10 nm pores (1-2 nm micropores and 2-10 nm mesopores). These results indicated the appearance of larger mesopores while the disappearance of large pores after the introduction of GO and followed evaporation-induced drying. Therefore, these mesopores allow efficient ion diffusion into the high-density hybrid monolith, which is very important to improve the rate performance in rechargeable batteries.46-47 The well dispersion and ultrafine size of TiO2, plus the high electronic conductivity and volumetric density should enables the compact hybrid monolith electrode to have 14


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promising electrochemical performance in batteries. Therefore, we firstly evaluated the electrochemical performance of HD-TiO2-N-RGO anode in LIBs, and compared it with HD-TiO2-RGO monolith and N-RGO, and pure TiO2 electrodes. Figure 4A shows the cyclic voltammetry (CV) curves of HD-TiO2-N-RGO monolith electrode in the first 5 cycles at a scan rate of 0.2 mV s-1. The reaction equation of TiO2 + xLi+ + e- ↔ LixTiO2 (x = 1) was used to express the insertion and extraction process of Li-ion for anatase TiO2 electrode.48 It is clear to observe a pair of cathodic and anodic peaks at about 2.04 V and 1.65 V, which corresponds the plateaus of Li-ion extraction and insertion in the anatase TiO2, respectively. The galvanostatic charge/discharge measurements was used to assess the Li-ion storage properties of HD-TiO2-N-RGO monolith electrode in a potential range of 1.03.0 V vs Li+/Li at 0.1 A g-1. The total weight of active materials (e.g. HD-TiO2-N-RGO monolith) was used for the calculation of gravimetric capacity. Figure 4B show the typical charge/discharge curves in the 1st, 2nd, 5th, 10th, 20th, 50th, and 100th cycle. The first discharge capacity was about 252.1 mA h g-1, based on the total mass of active materials. In all curves, an obvious charge-discharge potential plateaus can be observed at 2.0 and 1.65 V, which is accordant with the CV curves. Importantly, the peak shapes and plateaus of both charge and discharge curves are unchanged after 100 cycles, demonstrating that the electrochemical reaction was stable and reversible. Figure 4C compares the cyclic performance of HD-TiO2-N-RGO monolith, HDTiO2-RGO monolith, N-RGO and pure TiO2 electrodes at 0.1 A g-1. Among them, the HD-TiO2-N-GRO monolith electrode given the highest gravimetric capacity and the optimal cyclic stability. It displayed a discharge specific capacity of 216.6 mA h g-1 at the 2th cycle and was maintained at 204.1 mA h g-1 with ~94.2% capacity retention and nearly 100% Coulombic efficiency after 100 cycles. As for the HD-TiO2-RGO 15



monolith, it only stabilized at 151.3 mA h g-1 after 100 cycles and its capacity retention was about 72% compared to the second cycle capacity of 210.1 mA h g-1. These results imply the nitrogen-rich surface of N-RGO is conducive to capacity increase and can effectively enhance the cyclestability, which should be attributed to the strong coordination sites of N-RGO for firm anchoring of TiO2-QDs.39, 49 By comparison, a fast capacity degradation was observed and only the capacity of 35 mA h g-1 was retained after 100 cycles (only 21.1% capacity retention) for the pure TiO2 anode, which may be because of its agglomeration and poor electrical conductivity of TiO2 particles during cycling. As for the N-RGO, it showed a very stable capacity of ~64 mA h g-1 within 100 cycles, but its capacity was much lower than the HD-TiO2-N-RGO monolith anode. Besides, the volumetric capacity of HD-TiO2-RGO monolith electrode was also given in Figure 4D, which was calculated based on the total volume of whole electrode materials (including with 5 wt% acetylene black and 10 wt% binder) (The calculation details of volumetric capacity, see Experiment section S4 in the Supporting Information). The volumetric capacity of HD-TiO2-RGO monolith electrode was 548.7 mA h cm-3 at the second cycle and stabilized at 517.1 mA h cm-3 after 100 cycles at 0.1 A g-1. However, the volumetric capacity of TiO2-N-RGO powder electrode was 343.7 mA h cm-3 at the second cycle and stabilized at 335.1 mA h cm-3 after 50 cycles at 0.1 A g-1 (Figure S10 A) The rate performance of HD-TiO2-N-RGO monolith electrode was given at various current densities ranged from 0.1 to 10 A g-1. It can be seen from Figure 4E, the HDTiO2-N-RGO monolith electrode delivered the discharge volumetric capacities of 528.7, 489.4, 402.8, 353.8, 310.9, 276.8 (~52.5% capacity retention), 253.7 mA h cm-3 (~48.2% capacity retention), corresponding to the gravimetric capacities of 208.7, 193.2, 159.0, 139.7, 122.7, 1093, 100.2 mA h g-1 at 0.1, 0.2, 0.5, 1.0, 2.0, 5.0, 10 A g-1, respectively. 16


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When the current densities restored to 0.1 A g-1, the discharge capacity recovered to 526.0 mA h cm-3, indicating an excellent capacity reversibility. The results give a good rate capability of HD-TiO2-N-RGO monolith electrode in LIBs. Besides, the HD-TiO2-N-RGO monolith electrode also showed superior ultra-long lifespan, that is, a highly stable capacity of 306.9 mA h cm-3 (121 mA h g-1) with about 95.0% capacity retention after 1000 cycles at 2.0 A g-1 compared with the value of the 2nd cycle (Figure 4G). Such large capacity, good stability and superior rate capability of the highly dense monolith anode in LIBs draw our attention to insight into the mechanism. First, the EIS was used to evaluate the kinetics of the whole HD-TiO2-N-RGO monolith electrode, which was compared with HD-TiO2-RGO monolith and pure TiO2 electrodes, as revealed in Figure 4F. In the Nyquist plots, the charge transfer through the electrodeelectrolyte interface is obtained from the middle-frequency semicircle, while the steep sloping at low frequency represents the behavior of Li-ion solid-state diffusion in the electrode.50 The modeling AC based on the modified Randles equivalent circuit was adopted to inspect the exact kinetic differences among different materials (Inset in Figure 4F) and the specific data are shown in Table S3. Among them, the smallest charge-transfer resistance (Rct) of 47.1 Ω was obtained for the HD-TiO2-N-RGO monolith anode, which is superior to the HD-TiO2-RGO monolith (71.9 Ω) and also far superior to the pure TiO2 (145.4 Ω). The results suggest that the N-RGO can more effectively reduce the Rct of whole electrode material than the RGO. The results may be attributed to the nitrogen-rich doping of RGO, which can strongly couple with TiO2QDs, and provide good electrode/electrolyte wettability, as well as improve the electrical conductivity of hybrid monolith. It is worth noting that the resistance is significantly reduced compared to the values before electrochemical performance test 17



(Figure S11 A), indicating the activated process of HD-TiO2-N-RGO electrodes, and thus benefit the electrochemical performance. Then, in order to further clarify the kinetics origin of electrode materials towards Li+, CV measurements with different sweep rates were also carried out, and the results were shown in Figure 5. All of the CV shapes are kept well with the increase of scan rate from 0.2 to 5 mV s-1 (Figure 5A), demonstrating that the TiO2-N-RGO monolith anode has a good rate performance. Generally, the peak current (ip) of CV curves and scan rate (v) can be expressed by the relationship of ip = avb, where a and b both are constants.51-53 The b value, which ranges from 0.5 to 1.0, can be determined by the slope of the log(v)-log(i) plots, as shown in Figure 5B. Particularly, the b values of 0.5 and 1.0 suggest total diffusion-controlled behavior and surface pseudo-capacitive process, respectively. In our work, the b values are 0.77 and 0.69 for cathodic and anodic scan, respectively, indicating that the HD-TiO2-N-RGO monolith anode exhibits a combination of diffusion-controlled and pseudo-capacitive behaviors in LIBs. In addition, by separating the speciﬁc contribution from the diffusion-controlled and capacitive behaviors, we can quantify the exact ratio between the two contributions at a certain scan rate (see the description and calculation details in the Supporting Information), and the calculated results based on the bulk intercalative peak current (id) and surface pseudo-capacitive peak current (ic) at different scan rates were given in Figure 5D. When the scan rate increased from 0.2 to 5.0 mV s-1, the surface pseudocapacitive contributions also increased from 32.1% to 70.5%. This results suggest that the bulk lithium insertion storage is main contribution at low scan rate, whereas the surface pseudo-capacitive storage dominated the total storage capacity at high scan rate, that is, an integrated bulk and interfacial lithium storage process for HDTiO2-N-RGO monolith electrode in LIBs. 18


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Moreover, the morphologies and structures of the HD-TiO2-N-RGO monolith anode after experiencing long cycle lifespan with 1000 cycles at 2.0 A g-1 were shown in Figure S12. It is clear to see that plenty of TiO2-QDs still homogenously anchored on N-RGO surface, implying no obvious agglomeration after 1000 cycles. Besides, the lattice fringes of TiO2 particles were clearly displayed in the HR-TEM (Figure S12C), which testified that the crystal structures of TiO2-QDs still maintained well after 1000 cycles. This evidence indicates that the N-RGO can make these TiO2-QDs firmly anchored, thus avoiding the agglomerations even at such rigorous testing conditions. Next, the sodium storage performance of HD-TiO2-N-RGO monolith electrode was investigated in half-cell SIBs. Figure 6A shows the CV curves of HD-TiO2-N-RGO monolith electrode for the first 5 cycles with the voltage range from 0.01 V to 3.0 V vs Na+/Na at a scan rate of 0.1 mV s-1. A pair of cathodic/anodic peaks at 0.72 and 0.83 V corresponds to the de-sodiation and sodiation process, respectively, which is in agreement with the literatures

25, 51, 54.

And the CV curves from first to fifth cycle

overlap well, indicating a good reversibility of the HD-TiO2-N-RGO monolith electrode during charging/discharging processes. Besides, the galvanostatic charge/discharge curves and cycling stability of HD-TiO2-N-RGO monolith electrode at 0.1 A g-1 were also given in Figure 6 (B and C), respectively. Similar to LIBs, the sodium storage behavior of HD-TiO2-N-RGO monolith electrode also exhibited a highly reversible and stable volumetric (gravimetric) capacity, which was 522.5 mA h cm-3 (206.3 mA h g-1) at the second cycle and remained at 515.1 mA h cm-3 (203.4 mA h g-1) after 100 cycles at 0.1 A g-1, with about 98.5% capacity retention and nearly 100% coulombic efficiency (Figure 6C). Moreover, the rate performance of HD-TiO2-N-RGO monolith electrode delivered the discharge volumetric capacities of 521.8, 423.0, 360.7, 329.0, 295.8, 264.8 mA h cm-3 (~50.8% capacity retention), corresponding to the 19



gravimetric capacities of 206.0, 167.0, 142.4, 129.9, 116.8, 104.5 mA h g-1 at 0.1, 0.2, 0.5, 1, 2, 5 A g-1, respectively (Figure 6D). When the current densities restored to 0.1 A g-1, the discharge capacity can recover to 517.6 mA h cm-3, indicating an excellent capacity reversibility. These results demonstrate a superior rate capability of HD-TiO2N-RGO monolith electrode in SIBs. In addition, the long-term cycling performance of HD-TiO2-N-RGO monolith electrode also given a highly stable volumetric capacity of 279.3 mA h cm-3 (110.3 mA h g-1) with about 91.4% capacity retention after 1000 cycles at higher current density of 2.0 A g-1 compared with the value of the 2nd cycle (Figure 6F). Apart from them, the TiO2 nanoparticles anchored on N-RGO in the HD-TiO2-NRGO monolith electrode after experiencing long cycle lifespan with 1000 cycles at 2.0 A g-1 still kept good dispersion and ultra-small size (Figure S13), and the crystal structures of TiO2-QDs still remained well after 1000 cycles (Inset in Figure S13B), which indicated that N-RGO can also tightly hold these TiO2-QDs, even under such strict test conditions of large-size Na+ insertion and extraction for 1000 cycles, further demonstrating our structural design has the unique advantages. From the above results, we know, the HD-TiO2-N-RGO monolith electrode also displayed a fast and high-stability volumetric sodium storage as anode in SIBs. Therefore, the electrochemical kinetics of HD-TiO2-N-RGO monolith anode material towards Na+ were also investigated. First, the CV measurements at various scan rates ranged from 0.1 to 2 mV s-1 were given in Figure 6F. All of the CV shapes are kept well, indicating that the TiO2-N-RGO monolith anode displays a good rate performance in SIBs. The log(i)-log(v) plots for the HD-TiO2-N-RGO monolith electrode was shown in Figure 6G, and the b values are 0.87 and 0.89 for cathodic and anodic scan in SIBs, respectively, suggesting a main surface pseudo-capacitive behavior for the HD-TiO2N-RGO monolith electrode in SIBs, which is different with that in LIBs (see Figure 20


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5B). In addition, the quantified bulk intercalative peak current (id) and surface pseudocapacitive peak current (ic) at various scan rates were also shown in Figure 6I, respectively. Unlike the behaviors in LIBs, the HD-TiO2-N-RGO monolith electrode in SIBs mainly exhibited a surface pseudo-capacitive storage at various scan rates from 0.1 to 2 mV s-1, corresponding to the capacitive contributions from 56.4% to 84.4%. The difference of electrochemical kinetics in LIBs and SIBs should be attributed to the different ionic radius of lithium and sodium ions, which is agreement with previous reports.55-56 Besides, the further electrochemical kinetic of HD-TiO2-N-RGO monolith electrode was also investigated by EIS (Figure S14). A small charge-transfer resistance (Rct) of 54.1 Ω was obtained for the HD-TiO2-N-RGO monolith electrode, which would be beneficial to the high rate performance. Based on the above result analysis, we clearly demonstrate that the HD-TiO2-N-RGO monolith electrode exhibits high-volumetric lithium/sodium storage, and superior rate and long-cycle performance, which should be ascribed to the synergism of N-RGO and ultrasmall-sized TiO2 and the unique compact monolith electrode design, as shown in Figure 7 and described as follows: (1) the nitrogen-rich surface of N-RGO can provide abundant coordination sites for firm anchoring and uniform growth of TiO2 nanoparticles as well as strong interfacial stability, thus giving the hybrid monolith a long-term cycling stability; (2) the ultra-small size of TiO2 anchored on N-RGO can greatly shorten the ion diffusion length in the solid phase and provide more active for electrochemical reactions, thus giving the hybrid monolith superior rate capability and high lithium/sodium storage; (3) the porous, yet compact hybrid monolith can provide the integrated electrode with high compacted density and fast electron/ion transfer, thus achieving a fast and high-volumetric lithium/sodium storage.

21



CONCLUSION In summary, a simple and general strategy with the assistance of a small amount of graphene oxide (~10 wt%) as “assembled binder” to design porous yet compact HDTiO2-N-RGO hybrid monoliths have been successfully achieved. In the compact hybrid monoliths, a large number of ultra-small TiO2-QDs (~4.0 nm) were homogenously anchored onto N-RGO. And the HD-TiO2-N-RGO compact monolith exhibited a high density of 1.8 g cm-3, high conductivity of 343.7 S m-1, and porosity, thus both leading to high gravimetric and volumetric capacities without degradation after 100 cycles at 0.1 A g-1 (204.1 mA h g-1 and 517.0 mAh cm-3 for LIBs, and 203.4 mA h g-1 and 515.1 mAh cm-3 for SIBs) and superior rate capability at 10 or 5 A g-1 as anode in LIBs and SIBs, respectively. More importantly, it both exhibited a high-stability lifespan with over 91% capacity retention after 1000 cycles in LIBs and SIBs at 2.0 A g-1. This study demonstrates our strategy is very promising in designing energy storage materials with fast and high-stability volumetric performance, which also extend to other oxide-based materials, even high-capacity alloy-type storage materials such as Sn, Si, and Sb for fast and high-stability volumetric lithium/sodium/ potassium storage. EXPERIMENTAL SECTION Synthesis of GO, N-RGO, TiO2-N-RGO nanohybrids, HD-TiO2-N-RGO monolith, TiO2-N-RGO-180, pure TiO2, and HD-TiO2-RGO compact monolith GO was obtained by the previously reported method.57-58 As for the synthesis of HDTiO2-N-RGO compact monolith, it was shown in Scheme 1. Briefly, 16 mL 50 wt% cyanamide was firstly mixed with 400 mL GO aqueous suspension (0.5 mg mL-1) at continuous stirring and reaction at 90 oC for 12 h in a glass vessel to make the in-situ polymerization of cyanamide on the GO surface, thus forming the N-RGO. Next, 75 mg as-obtained N-RGO and 6.72 mmol TiCl4 were uniformly dispersed in 400 mL 22


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absolute ethanol by ultrasonic treatment. After that, a certain volume of deionized water (10 mL) was dropwise added into the above solution at continuous stirring, and then they were transferred into a Teflon-lined stainless steel autoclave and treated at 120 oC for 2 h, and finally the black precipitates were collected to obtain the TiO2-N-RGO nanohybrids. Then, 1080 mg as-prepared TiO2-N-RGO powders were well dispersed into 40 mL deionized water, followed by slowly dropped into 40 mL 3.0 mg mL-1 GO aqueous suspension via strongly ultrasonic treatment, and then the mixture solution was treated at 180 oC for 12 h in a 90 mL Teflon-lined stainless steel autoclave, thus obtaining a cylindrical TiO2-N-RGO hydrogel. Noting that the different sizes of hydrogels can be controlled by using different volumes of autoclave. Finally, the cylindrical TiO2-N-RGO hydrogel was naturally shrank by evaporation-induced drying at room temperature for more than 48 h, thus obtaining the HD-TiO2-N-RGO compact monolith. As the controlled samples, the preparation processes of HD-TiO2-RGO compact monolith sample and the TiO2-N-RGO-180 sample are similar to that of HDTiO2-N-RGO compact monolith, but without adding cyanamide and GO, respectively. The pure TiO2 nanoparticles were made the same process as that of TiO2-N-RGO nanohybrids, but without adding N-RGO. Electrode fabrication and electrochemical measurements. Different active materials, such as HD-TiO2-N-RGO monolith, HD-TiO2-RGO, NRGO or pure TiO2, were mixed with acetylene black and poly(vinyl difluoride) (PVDF) in a weight ratio of 8.5:5:10 in N-methylpyrrolidone (NMP) solvent dispersant to prepare the anode slurry. And then the as-prepared slurry was coated on copper oil and dried under vacuum at 120 oC overnight to obtain the anode electrode. Afterwards, the dried materials were cut into 0.78 cm2 (Diameter =1.0 cm) disks, followed by pressing with 4.0 MPa, respectively. And the active materials loading was about 4.0 ± 0.5 mg 23



cm-2. 2032 coin-type cells were assembled in an argon-filled glove box by using lithium or sodium metal as the counter electrode and a microporous polyethylene or GF/F glass fiber as separator, respectively. 1.0 mol L-1 LiPF6 dissolved in an ethylene carbonate and diethyl carbonate (1:1 V/V) or 1.0 mol L−1 NaClO4 dissolved in an ethylene carbonate and ethyl methyl carbonate (1:1 V/V) with 5% fluoroethylene carbonate was used as the electrolytes for LIBs or SIBs, respectively. The charge-discharge performance of the cells was tested on a LAND CT2001A battery tester (Wuhan, China) at different current densities. CV and EIS (in the frequency range of 100 KHz~10 mHz under AC stimulus with 5 mV of amplitude) were conducted on electrochemical workstation (Eco Chemie Autolab B.V. (Metrohm, Switzerland)). ASSOCIATED CONTENT Supporting Information Available: Characterizations, additional results for Experimental section, XRD, TEM images, SEM images, physical characteristics, and comparison of the impedance parameters of different samples. This material is available free of charge via the Internet at http://pubs.acs.org.

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Corresponding Author *E-mail: [email protected]. Fax: (+8620)-2039322570. Tel: (+8620)-39322570. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (51502043), Guangdong Province Universities and Colleges Pearl River Scholar Funded Scheme (2017), Guangdong Natural Science Foundation for Distinguished Young Scholar (2016A030306030), Pearl River S&T Nova Program of Guangzhou (201710010145), Science and Technology Projects of Guangdong Bureau of Quality and Technical Supervision (2018PT05), and Guangdong Province Science and Technology Program (2017B050504004). REFERENCES (1) Huang, Y.; Zheng, Y.; Li, X.; Adams, F.; Luo, W.; Huang, Y.; Hu, L. A Perspective on Electrode Materials of Sodium-Ion Batteries Towards Practical Aplication. ACS Energy Lett. 2018, 3, 1604–1612. (2) Han, J.; Kong, D.; Lv, W.; Tang, D.; Han, D.; Zhang, C.; Liu, D.; Xiao, Z.; Zhang, X.; Xiao, J. Caging Tin Oxide in Three-Dimensional Graphene Networks for Superior Volumetric Lithium Storage. Nat. Commun. 2018, 9, 402. (3) Xu, X.; Liu, Z.; Ji, S.; Wang, Z.; Ni, Z.; Lv, Y.; Liu, J.; Liu, J. Rational Synthesis of Ternary FeS@TiO2@C Nanotubes as Anode for Superior Na-Ion Batteries. Chem. Eng. J. 2019, 359, 765-774. (4) Liu, Z.; Ji, S.; Xu, X.; Hu, R.; Liu, J.; Liu, J. Dramatically Enhanced Li-Ion Storage of ZnO@C Anodes through TiO2 Homogeneous Hybridization. Chem-Eur J. 2019, 25, 582-589. 25



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32


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Scheme 1 Schematic illustration of the fabrication process for the HD-TiO2-N-RGO compact hybrid monolith.

33



Figure 1. Microscopic characterizations of HD-TiO2-N-RGO compact monolith. (A) Digital photographs of TiO2-N-RGO hydrogel (left) and HD-TiO2-N-RGO compact monolith (right). (B-F) SEM images, (G-I) TEM images, and (J-L) STEM images of HD-TiO2-N-RGO compact monolith. (M) STEM image and EDX elemental maps for 34


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C (M1), N (M2), O (M3), and Ti (M4) taken from the orange square in (J) for the HDTiO2-N-RGO compact monolith. The insets in (H) are the SAED pattern of HD-TiO2N-RGO compact monolith (top) and the average size of TiO2 nanoparticles anchored on N-RGO (bottom), respectively. The insets in (E) and in (I) are the schematic diagram of HD-TiO2-N-RGO compact monolith and the enlarged view corresponding to the area outlined by the white square in (I), respectively.

35



(101)

TiO2 (PDF# 21-1272) (211)

TiO2-N-RGO

Intensity (a.u.)

TiO2-N-RGO

Intensity (a.u.)

B

HD-TiO2-N-RGO

(204)

(200)

(105)

A

(004)

HD-TiO2-N-RGO

N-RGO

GO

N-RGO

GO

Eg Pure TiO2

10

Eg

20

30

40

50

60

70

B1g A1g Eg

C

Pure TiO2

500

 degree

1000

1500

2000

-1

Raman shift (cm ) C1

O 1s

C-C/C=C

C1s

C-O

C 1s GO

C=O

N 1s

O 1s

Intensity (a.u.)

Intensity (a.u.)

GO

C 1s

N-RGO

N-RGO

C-C/C=C

H2N-C=N

C-O

C-N

O 1s

HD-TiO2-N-RGO

C-C/C=C

Ti 2p C 1s

HD-TiO2-N-RGO

800

600

C-O

C-N

N 1s

400

200

282

284

Binding energy(eV) C2

286

H2N-C=N

288

290

Binding energy(eV) N 1s

C3

Pyrrolic N 400eV

Ti 2p

3/2 HD-TiO2-N-RGO Pure TiO2

Pyridinic N 398.7eV

Graphitic N 401.3eV

N-RGO

Intensity (a.u.)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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HD-TiO2-N-RGO Pyrrolic N 400eV

Pyridinic N 398.7eV

1/2

5.8eV

Graphitic N 401.3eV

5.6eV

394 396 398 400 402 404 406 408

Binding energy(eV)

468

465

462

459

456

Binding energy (eV)

36


453

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Figure 2. XRD patterns (A), Raman spectra (B) and XPS spectra (C) of various materials. (C1), (C2), and (C3) are the XPS C 1s, N 1s and Ti 2p spectra of various materials, respectively.

37



Figure 3. (A) Nitrogen adsorption/desorption isotherms (A) and (B) DFT pore-size distribution curves of TiO2-N-RGO nanohybrids and HD-TiO2-N-RGO hybrid

1st cycle

.1 1.0

1.5

2.0

2.5

Voltage (V vs. Li/Li ) 800

D HD-TiO2-N-RGO

400 200 0

0

20

40

60

80

Cycle number

100

1st 2nd 5th 10th 20th 50th 100th

2.0 1.5 1.0 0

3.0 +

600

2.5

50

100

150

200

250 -1

Specific capacity (mAh g ) 600

0.1 A g-1

E

HD-TiO2-N-RGO

0.2 A g-1

500

200 HD-TiO2-RGO

150 Pure TiO2

100

N-RGO

50 0

0

2 A g-1 5 A g-1

40

60

F

HD-TiO2-RGO Pure TiO2

0 0

10

20

30

40

0

Cycle number

50

100

150

200

400

100 80 60

300

Discharge Charge

2.0 A g-1 0

200

400

600

Cycle number

38


250

Zre (Ohm) G

200

100

HD-TiO2-N-RGO

300

500

100

80

Cycle number

100

10 A g-1

200

20

200

1 A g-1

300

100

HD-TiO2-N-RGO

0.1 A g-1

0.5 A g-1

400

C

250

800

Coulombic efficiency (%)

0.0

B

Gravimetric capacity (mAh g-1)

5th cycle

Volumetric capacity (mAh cm-3)

Current (mA)

-.1

3.0

-Zim (Ohm)

A

-.2

Voltage (V vs. Li/Li+)

monolith.

-3 -3 Volumetric capacity (mAh cm ) Volumetric capacity (mAh cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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40 20 0 1000

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Figure 4. Electrochemical performances of various materials in LIBs. (A) Typical CV curves at a scan rate of 0.2 mV s-1 and (B) galvanostatic charge/discharge curves at 0.1 A g-1 for the HD-TiO2-N-RGO monolith electrode. (C) Cycling performance of various materials at 0.1 A g-1. (D) Volumetric capacities of HD-TiO2-N-RGO monolith electrode. (E) Volumetric capacities versus current densities. (F) Nyquist plots after rate performance testing in LIBs. (G) Long-term cycling of HD-TiO2-N-RGO monolith electrode in LIBs at current densities of 2.0 A g-1.

39



Figure 5. Electrochemical analysis of the HD-TiO2-N-RGO monolith electrode in LIBs. (A) CV curves at various scan rates, (B) the relationship between log-peak current (ip) and log-scan rate (v) from 0.2 to 5 mV s-1, (C) the plots of ip/v1/2 vs v1/2 used for calculating constants k1 and k2 at cathodic peak potentials, (D) calculated surface pseudo-capacitive and bulk insertion cathodic peak currents at various scan rates.

40


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Figure 6. Electrochemical performances of the HD-TiO2-N-RGO monolith electrode in SIBs. (A) Typical CV curves at a scan rate of 0.1 mV s-1, (B) galvanostatic charge/discharge curves at 0.1 A g-1, (C) cycling performance at 0.1 A g-1, (D) volumetric capacities versus current densities, (E) long-term cycling stability at 2.0 A g-1, (F) CV curves at various scan rates, (G) the relationship between log-peak current (ip) and log-scan rate (v) from 0.1 to 2 mV s-1, (H) the plots of ip/v1/2 vs v1/2 used for calculating constants k1 and k2 at cathodic peak potentials, and (I) calculated surface pseudo-capacitive and bulk insertion cathodic peak currents at various scan rates.

41



Figure 7. Graphical illustration of the structural merits and superior volumetric lithium/sodium storage mechanism of the HD-TiO2-N-RGO compact monolith electrode in battery systems.

42


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Graphical Abstract ion

ion

ion

ion

ion

ion

ion

ion

ion

TiO2-QDs

ion

ion ion

ion

ion

ion

Li+/Na+

3 Capacity (mAh/cm )

e-

+ TiO2 +

LixTiO2/ NaxTiO2

e-

-3 Capacity (mAh cm )

ion

interfacial ion storage

Current collector

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


800 600 400 200

High density TiO2-N-RGO electrode

LIB SIB

0.1 A g-1

0 0

20

40

60

80

100

Cycle Number

500 400 300 200

LIB SIB

2.0 A g-1

100 0

200

400

600

800

1000

Cycle Number

Porous yet highly dense metal oxide quantum dots-anchored nitrogen-rich reduced graphene oxide (HD-MOx-N-RGO) compact monoliths were fabricated, which exhibited high conductivity of 343.7 S/m and high density of 1.8 g/cm3, thus leading to highly stable gravimetric and volumetric capacities without degradation after 100 cycles at 0.1 A/g, and long lifetime of 1000 cycles with >91% retention at 2 A/g in both LIBs and SIBs.


General Strategy To Synthesize Highly Dense Metal Oxide Quantum

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