Polyoxometalate‐Bridged Synthesis of Superstructured Mesoporous Polymers and Their Derivatives for Sodium–Iodine Batteries

Abstract Despite the impressive progress in mesoporous materials over past decades, for those precursors having no well‐matched interactions with soft templates, there are still obstacles to be guided for mesoporous structure via soft‐template strategies. Here, a polyoxometalate‐assisted co‐assembly route is proposed for controllable construction of superstructured mesoporous materials by introducing polyoxometalates as bifunctional bridge units, which weakens the self‐nucleation tendency of the precursor through coordination interactions and simultaneously connects the template through the induced dipole–dipole interaction. By this strategy, a series of meso‐structured polymers, featuring highly open radial mesopores and dendritic pore walls composed of continuous interwoven nanosheets can be facilely obtained. Further carbonization gave rise to nitrogen‐doped hierarchical mesoporous carbon decorated uniformly with ultrafine γ‐Mo2N nanoparticles. Density functional theory proves that nitrogen‐doped carbon and γ‐Mo2N can strongly adsorb polyiodide ions, which effectively alleviate polyiodide dissolving in organic electrolytes. Thereby, as the cathode materials for sodium–iodine batteries, the I2‐loaded carbonaceous composite shows a high specific capacity (235 mA h g−1 at 0.5 A g−1), excellent rate performance, and cycle stability. This work will open a new venue for controllable synthesis of new hierarchical mesoporous functional materials, and thus promote their applications toward diverse fields.

Then 200 μL of toluene was added to forming an emulsion solution. Afterwards, 1 mL of PD solution (0.1M in aqueous solution) and 500 μL of AMT solution (0.08M in aqueous solution) were sequentially added to the mixed solution and kept stirring for 2 h. Then 500 μL of APS (1M in aqueous solution) was added to initiate PD monomer polymerization. After reacting for 8 hours, MoOx 2-/mesoPPD particles were obtained through centrifugation and washed with DI water and ethanol for several times. The final products were dried under vacuum at 60 °C for 12 h. Using the same procedure, H4[Si(W3O10)4] ·xH2O, Na3O40PW12·xH2O, and H3PO4 ·12MoO3 were selected to replace AMT at the same concentration to obtain the corresponding products.
The nonporous Mo-PPD samples were synthesized under the same procedure without adding toluene.

Synthesis of NC sample.
100 mg PD monomer was dissolved into 5 mL DI water, and 500 mg APS was added subsequently to induce the polymerization of PD. The obtained product of poly(p-phenyldiamine) was washed by water and ethanol for at least 3 times. Then corresponding NC was obtained under the same heat treatment procedure.

Synthesis of I2@γ-Mo2N/mNC and I2@γ-Mo2N/NC samples.
The preparation of I2@γ-Mo2N/ mNC and I2@γ-Mo2N/NC through a same sublimation diffusion method. I2 (400 mg) was spread on the bottom of a sealed reactor, and then 50 mg powders in a 5 mL glass vial were placed in the sealed reactor. The sealed reactor was heated to 180 °C for 12 h. The specific I2 content of the composite was calculated based on the mass difference of samples before and after I2 sublimation diffusion.

Electrochemical Measurements
The working electrodes were prepared by mixing the obtained active material samples, PVDF binder and Super P carbon black (Nanjing XFNANO Materials Tech Co., Ltd) with a mass ratio of 7:2:1 using NMP solvent. The above slurry was evenly casted on carbon cloth with a diameter of 12 mm and then dried at 60°C for 12 h. The mass loading of active materials on the electrodes is in the range of 0.7-1.5 mg cm -2 . The 2032-type coin cells were assembled in an Ar-filled glove box (O2 < 0.1 ppm and H2O < 0.1 ppm). 1M NaClO4 in ethylene carbonate and diethyl carbonate (EC/DEC) (1:1 by volume) with 5% 4-Fluoroethylene Carbonate (FEC) was used as electrolyte (DoDoChem, Suzhou, China). Na foil was utilized as reference/counter electrode and glass fiber membranes (Whatman® GF/C) was used as the separator. Cyclic voltammetry (CV) tests were performed on an electrochemical workstation (CHI760E, Chenhua, Shanghai). Galvanostatic charge/discharge measurements were carried out at various current densities using a battery testing system (LAND CT2001A) at a voltage range of 1.5-3.3 V (vs Na + /Na). All specific capacities and current densities are normalized to the mass of iodine.

Density Functional Theory (DFT) Calculations
The crystals were performed using the Cambridge Sequential Total Energy Package (CASTEP)1 based on the pseudopotential plane wave (PPW) method. [1] Electron-ion interactions were described using the ultrasoft             With the molar ratio of PD/AMT decreases to 12:1, the morphology of mesoporous nanospheres appeared. While the molar ratio comes to 3:1, the nanosphere transformed into irregular particles, which can be ascribed to strong coordination between PD and AMT, destroying nano-emulsion assembly.