Synthesis of Nitrogen‐Doped Mesoporous Structures from Metal–Organic Frameworks and Their Utilization Enabling High Performances in Hybrid Sodium‐Ion Energy Storages

Abstract Sodium‐ion energy storage is of the most attractive candidate for commercialization adoption due to the safety and cost demands of large‐scale energy storage systems, but its low energy density, slow charging capability, and poor cycle stability are yet to be overcome. Here, a strategy is reported to realize high‐performance sodium‐ion energy storage using battery‐type anode and capacitor‐type cathode materials. First, nitrogen‐doped mesoporous titanium dioxide (NMTiO2) structures are synthesized via the controlled pyrolysis of metal–organic frameworks. They exhibit interconnected open mesopores allowing fast ion transport and robust cycle stability with nearly 100% coulombic efficiency, along with rich redox‐reactive sites allowing high capacity even at a high rate of ≈90 C. Moreover, assembling the NMTiO2 anode with the nitrogen‐doped graphene (NG) cathode in an asymmetric full cell shows a high energy density exceeding its counterpart symmetric cell by more than threefold as well as robust cycle stability over 10 000 cycles. Additionally, it gives a high‐power density close to 26 000 W kg−1 outperforming that of a conventional sodium‐ion battery by several hundred fold, so that full cells can be charged within a few tens of seconds by the flexible photovoltaic charging and universal serial bus charging modules.

20 °C, where is was maintained for 15 m, and then it was stirred for 4 h under 40 °C. After then, DI water (100 ml) was slowly added into the mixture, where it was maintained for 1 h.
The mixture was cooled to room temperature and the hydrogen peroxide (2 ml, H 2 O 2 ) was added into the mixture. The mixture was washed via vacuum filtration with Hydrochloric acid (HCl), Acetone and DI water, respectively. The product was dried via freeze drying for further use. Finally, the GO solution (1mg/ml) was chemically reduced with hydrazine (1µg/ml) as a reduction agent in oil bath at 80 °C. Then, the product was filtered and dried following the same method as for a GO product.

S1.4. Synthesis of NG
The RGO solution was drop casted onto a glass, and put into the plasma enhanced chemical vapor deposition (PECVD) chamber. Then, hydrogen and nitrogen gas plasma were sequentially applied for 3m and 10 m, respectively.

S2.1. TEM (Transmission electron microscopy)
For the TEM observation (Tecnai F20 produced by Philips / JEM-ARM200F produced by JEOL / Tiatan cubed G2 produced by FEI company), the samples were dispersed with the acetone solvent and put on a Cu mesh grid. An energy dispersive spectrometer (EDS) attached to the TEM was used to obtain the local elemental information, the line elemental profile and the elemental mapping.

S2.2. SEM (Scanning electron microscope)
For the SEM observation ((Hitachi, SU-5000), the samples were dispersed with the acetone solvent and dropped on a small piece of a silicon wafer.

S2.3. XPS (X-ray photoelectron spectroscopy)
The XPS spectra were obtained using a Sigma Probe of Thermo VG Scientific, which is equipped by a 350 W Al anode x-ray source along with a multi-anode, a pulse counting, and a hemispherical analyzer. The spectra were collected using an incident photon energy of 1486.6 eV and were also corrected for the detector's work function.

S2.4. XRD (X-ray diffraction spectroscopy)
The powder X-ray data were collected using a SmartLab θ-2θ diffractometer in the reflectance Bragg-Brentano geometry employing a Johansson type Ge (111) monochromator filtered Cu Kα1 radiation at the 1200W (40 KV, 30 mA) power and equipped with a high speed 1D detector (D/teX Ultra). The powders of the sample structures were held in a holder stage and scanned by the scan speed of 2 °/min in a continuous mode.

S2.5. FT-IR (Fourier Transport-Infrared Spectroscopy)
The chemical bonding information of the functional groups present in samples was analyzed by using a FTIR spectroscopy (FT/IR-6100, JASCO). The NMTiO 2 and reference samples were ground with KBr using mortar and pestle in the ratio of 1:100 in weight and then the mixture was pressurized by the hand-operated pressure to the thin pellet with an width of 6 mm. The spectra were obtained at 2 cm -1 with 50 scans per spectra in the range of 500 to 4000 cm -1 .

S2.6. Surface area and pore size analyzer
The N 2 adsorption and desorption isotherms were determined by a Quantachrome Instruments Autosorb-1c apparatus at 77 K. The samples were outgassed at 333 K and for 24 hours before measurements.

S2.7. Thermal oxidation behavior analysis.
The thermal behavior of NH2-MIL-125 (Ti) during the thermal oxidation procedure was investigated by thermogravimetric analysis (NETZCH TG 209 F1 Libra) in a range of 20 to 700 ℃ under 5 ℃ per min heating rate and air flow conditions.

S2.8. UV-visable absorption spectra.
To investigate the changes on nitrogen doping effects, the diffused apsorption spectra were optained by a VARIAN Cary-300 UVVis spectrophotometer using powder samples as prepared.

S3.1. Electrochemical half cells
The electrochemical properties of the NMTiO 2 were characterized by using the 2032 type coin cells in which Celgard 2400 and Na foil were used as separators and counter/reference electrodes, respectively. On the sample preparation, the active material, super P, and polyvinylidene fluoride (PVDF) (80:10:10 in weight) were dispersed in N-methyl-2pyroolidinone (NMP) to form a slurry. Then, the slurry was cast onto the Cu or Al foil using the doctor blade technique. The cast electrodes were dried in a vacuum oven at 80 ˚C overnight. We used the standard organic electrolyte in which the 1M NaClO 4 is dissolved in ethylene carbonate (EC) and diethyl carbonate (DEC) (EC : DEC = 1 : 1 in volume). The entire cell preparation steps were conducted in an argon-filled glove box with the moisture content and oxygen levels less than 1 ppm. All of the electrochemical performance measurements were done at room temperature using a potentiostat/galvanostat/EIS system (VSP, Bio-Logic). Moreover, electrochemical analyses of the half-cells were measured in an operating potential range of 1 to 3 V (anode) and 3 to 4.5 V (cathode) versus. Na/Na + , respectively. The impedance analysis was conducted in a frequency range of 0.01 Hz to 1000 kHz with the amplitude of 5 mV. Before the measurements, the pre-sodiation procedure was conducted by attaching the NMTiO 2 electrode with the Na foil during 1 hour. The GITT measurements were carried out at ~0.1 C in a period of 1 minute. The mass loading was measured by the ultra-microbalance (XP2U, Mettler Toledo, d= 0.1 µg).

S3.2. Hybrid full cells
Hybrid sodium-ion energy storage full cell devices were assembled using the NMTiO 2 composite as the anode and the NG as the cathode, respectively. The NG cathode was prepared in the same manner as the anode. All the electrochemical analyses were performed using the same methods used in the half cell.
The electrochemical performances of full cell devices were determined on the mass loading of 3 to 12 mg cm -2 for the total mass of anode and cathode electrodes. The overall cell energy/power densities were calculated based on the total active mass of cathode and anode materials. The specific capacitance (C s ) is calculated by using the following equations [S2] of where i is the applied current (A), t is the discharge time (s), m is the total mass (g) of active materials in both the anode and cathode, and ΔV is the potential difference (V). The power density (P, W kg -1 ) and the energy density (E, Wh kg -1 ) were calculated using the following equations [S3], as described by where V max and V min are the potentials at the beginning and the end of the discharge (V).

S4.1 Effects on the nitrogen doping of NMTiO 2
We investigated the color changes on nitrogen-doped TiO 2 NPs by UV-vis absorption spectroscopy as shown in Supplementary Figure S1a. The shift in absoprtion specra is indicated by the nitrogen-doping on balance bands. We also found that the bandgap was reduced by about 0.54 eV. Depending on this band structure difference, the NMTiO 2 sample shows a light-yellow color (Supplementary Figure S1b).

S4.2 Morphology and size of NMTiO 2
The morphology and size of NH 2 -MIL-125 (Ti) and NMTiO 2 samples were observed by the SEM and TEM analyses, as shown in Figs. S1 and S2. The top images correspond to those for the bare NH 2 - . This product appears in the rectangular shape with round edges and its average size is about 1 um width and 0.2 um thickness. Likewise, the size and shape of the NMTiO 2 (bottom images in Figures S1 and S2) synthesized from the NH 2 -MIL-125 (Ti) are quite similar. We have observed the edge shape sharpened during the annealing procedure. Figure S2 also shows that the width and thickness of the NMTiO 2 were shrink to 0.5 um and 0.1 um, respectively. Also, mesopores were determined to be introduced into the interior parts of the products, which play to give the fast transport channels for redox ions.

S4.3. Chemical information of NMTiO 2 samples
The constituents of the NMTiO 2 , which are mainly composed of Ti and O species including the small amount of nitrogen and residual carbon atoms, are confirmed from STEM-EDS, XPS and EELS. As shown in Figure S3, the EDS mapping image presents each element for Ti, O, N, and C atoms.. We also find from the XPS spectra ( Figure S4) that the each peak for Ti 2p, O 1s, C 1s, N 1s is detected at the corresponding position.

S4.4. Chemical information of NG samples
The chemical information of the nitrogen-doped graphene (NG) is observed from XPS and STEM-EDS ananlyses. As shown in the EDS mapping image (Figure S5), the elements of C, O, N are confirmed and the nitrogen is clearly shown in the mapping image and graph. Also, the XPS analysis ( Figure S6) show clearly the peaks for C 1s, O 1s, and N 1s.

S4.5. Crystal structure and surface area analysis of NH 2 -MIL-125 (Ti)
The crystallographic informations on NMTiO 2 and NH 2 -MIL-125(Ti) were collected through the XRD anaylsis ( Figure S7). We find that the diffraction patterns of the NMTiO 2 match well with those of the anatase phase (JCPDS card No. 21-1272). [S2] However, the diffraction peaks of the rutile pahse having the broad shape and low intensity have not been deteced.
Additionally, the high crystalline NH 2 -MIL-125 (Ti) showed the sharp diffraction peaks and the N 2 adsorption/desortion ananlysis demonstrated that the NH 2 -MIL-125 (Ti) has a surface area of ~ 1356 m 2 g -1 with the typical hysteresis of a microporous material.

S4.6. Chemical binding information for nitrogen dopants in the crystal lattice of NMTiO 2
The chemical information of nitrogen species doped in the NMTiO 2 crystals was investigated by the FT-IR analysis (Figures 2i and S8). The sample was prepared in the form of a pellet by the pressurization method. The NMTiO 2 powder samples were mixed with KBr in the ratio of 1 : 100 in weight in mortar and pestle. Then, the mixture was pressurized by the hand-  The thermal oxidation behavior of NH2-MIL-125 (Ti) under the air flow conditions was analyzed through the thermogravimetric analysis (TGA). In the first range below ~335 o C, we find that small amounts of surface absorbed water molecules and impurities were removed.
After then, the crystallization of titanium ions and the combustion of carbon species were observed to be occurring up to ~ 550 o C. These results support that the thermal oxidation temperature could be proceeded above 350 o C.

Figure S10. The TG analysis of the NH 2 -MIL-125 (Ti) confirming thermal oxidation and crystallization behaviors
procedures, we find that the particle and pore size with the pore distribution were clearly changed. The particle and pore size became smaller and the more mesopores were introduced in the products. The pore size analysis ( Figure S9) shows clearly that the mesopores with an average size of ~ 4 nm are signicantly increased under the pyrolysis condition at 350 o C. Figure S11. The pore size distribution at different processing temperatures.
To understand the diffusion kinetics of sodium-ion in the NMTiO 2 anode, the galvanostatic intermittent titration technique (GITT) measurements were analyzed using the NMTiO 2 halfcells. Figure S12a shows the time-voltage profile of NMTiO 2 during charging/discharging and the voltage plateaus for phase transition by Na + insertion were clearly observed during charging and discharging reactions. The difference of discharging behaviors was also shown in Figure S12b and c. The discharging time of the 350 ℃ sample exhibits to be much longer than that of the 500 ℃ sample. Moreover, the 350 ℃ sample was found to accommodate the higher concentration of Na + ions up to 1 mole fraction. ) [S4] where m and M are the mass and the molecular weight of the electrode material, respectively; V m is the molar volume; S is the active area of the electrode; L is the thickness of the electrode.
These results imply that the nitrogen-doped mesoporous TiO 2 synthesized at the low annealing temperature can give the facilitated Na-ion diffusion between the surface and the electrolyte. The plots of potential vesus τ 1/2 ( Figure S12 f and g) were also found to fit into a straight line.

S4.10. Performance for the NG half cell
The performance for the half-cell of a NG sample as the cathode can be shown in Figure S10.
The cathode was fabricated with sodium metal foil as an anode. And also, cathode slurry has a mass ratio of 8:1:1 (NG:PVDF:Super P) and the slurry was coated on aluminum foil by the doctor blading method. The average weight of the total cathode is 2~3.5 mg. The highest capacitance of this electrode is ~78 mAh g -1 from the cyclic voltammetry (CV) and galvanic potential measurements. This result represents relatively high capacity.

S4.11. Performance for the NG//NG full cell
We investigated the performance of the NG//NG full cell device ( Figure S11). The anode and cathode electrodes have been also fabricated on copper and aluminum foils as fabricated before. It is notable that the average weight of the total cathode is 4~7 mg. The highest specific capacitance of this full cell is ~ 85.4 F g -1 at a current density of 0.1 A g -1 , which is determined through the cyclic voltammetry (CV) and galvanic potential measurements.

S4.12. Cycle performance and capacities of full cell devices with with high mass loadings
We find that the delivered capacities of the full cell devices with high mass loadings exhibit stable performance without large capacity loss. In addition, we carried out the analysis for the stability of the full cell energy storage. The NMTiO 2 //NG full cell device exhibits an excellent stability over 10,000 cycles at a current density of 3 A g -1 with the high capacity retention and nearly 100% coulombic efficiency. Figure S15. The high mass loading charge-discharge profiles and specific capacities along with cycle stability. a) The charge-discharge profiles on the total mass loading of both electrodes (mg cm -2 ), b) The capacities of full cell devices with high mass loadings at a current density of 1 A g -1 and c) the cycle performance of the NMTiO 2 //NG full cell at a current density of 3 A g -1 .

S4.13. Structural stability of NMTiO2 during the charge-discharge reactions.
We carried out the analysis for the structural stability of the NMTiO 2 during the chargedischarge reactions through STEM measurements. The results show that the porosity and crystallinity of NMTiO 2 were well maintained even after the 10,000 cycles, while the pore and nanocrystal size was slightly increased.

S4.14. Demonstration of an ultrafast charging module
For realization of high-performance energy delivery using the NMTiO 2 //NG energy storage full cell devices, we fabricated the USB chargeable LED module ( Figure S13). These modules can be charged in the very fast charging time within 20 sec, and we find that the capacity is sufficient to operate the two blue and red led ramps. Figure S17. The USB chargeable led device demonstrating the ultrafast charging capability within 20 seconds.