The synthesis of triazine–thiophene–thiophene conjugated porous polymers and their composites with carbon as anode materials in lithium-ion batteries

The polymers based on thiophene armed triazine and different thiophene derivatives including thiophene (Th), thieno[3,2-b]thiophene (TT), dithieno[3,2-b:2′,3′-d]thiophene (DTT) or thieno[2′,3':4,5]thieno[3,2-b]thieno[2,3-d]thiophene (TTTT) are synthesized through a Stille coupling reaction. By introducing thiophene derivatives with increasing sizes as the linkage units (from thiophene, DT to DTT, TTTT), the band gaps (Eg) of the resultant polymers decrease continuously. Then the composite materials (polymer@C) between polymers and Vulcan XC-72 carbon are prepared by in situ polymerization to test their electrochemical performances in lithium ion batteries. The synthesized composites show distinct morphologies due to the different linkage units of thiophene or fused cyclothiophene derivatives and the cross-linked structure can be found in composites with the longer thiophene derivatives (bridging molecules) like PTT-3@C and PTT-4@C, which are expected to be beneficial to improve the performances of the electrode materials. The specific capacities of the composites are 495 mA h g−1, 671 mA h g−1, 707 mA h g−1, and 772 mA h g−1 for PTT-1@C, PTT-2@C, PTT-3@C and PTT-4@C at a current density of 100 mA g−1, respectively. In particular, benefiting from the enlarged conjugation length and planarity of the linkage units, the conjugated microporous polymers could deliver continuously improved capacities.

The composite material PTT-1@C was also obtained from the same procedures described above, with additional 558.8 mg VulcanXC-27 carbon powders adding into the mixtures during the reaction process. It was calculated that the mass ratio of the polymer PTT-1 was 30% in the PTT-1@C composite.

Structural characterizations:
The Fourier Transform Infrared spectroscopy were recorded on a Nicolet Avatar 360 FT-IR spectrometer with KBr pellets. The UV-Vis absorption spectroscopy were tested via Shimadzu UV-2550 spectrophotometer. The morphologies of the samples were observed through Hitachi Su-70 scanning electron microscopy (SEM, Hitachi Inc., Tokyo, Japan). The transmission electron microscopy (TEM) was examined to investigate the structural characterization using JEM-2100. The specific surface areas and porosity properties was examined by Nitrogen isotherm adsorptiondesorption at 77.3 K using ASAP 2460-3 (Micromeritics) volumetric adsorption analyzer. X-ray photoelectron spectroscopy (XPS) was conducted with ESCALAB 250Xi spectrometer. X-ray diffraction (XRD) was carried out with the 2θ range from 5 to 800 using Kigaka D/max 2500 X-ray advance diffractometer with a Cu-Ka radiation, and a step scan mode was adopted with a scanning step of 0.02. The thermogravimetric analysis of the samples were conducted on a Netzsch STA449C TG/DSC thermal analyzer under nitrogen atmosphere between 20 °C and 800 °C.

Electrochemical measurements:
The electrochemical performances of the anode composites were tested with CR2032-type coin cells. A mixture is obtained by mixing the active material, acetylene black and polyvinylidene fluoride (PVDF) at a mass ratio of 6: 2.5: 1.5. The right amount of NMP was added to the mixture, 5 be grinded thoroughly to form a homogeneous slurry, and the slurry was then coated on copper foils.
The coated copper foil was dried at 60 °C for 24 h, and then be cut into slices as the working electrode (anode for LIBs). The slices are further dried in vacuum drier at 120 °C for 8 hours, the constant weight of a slice was about 9 mg with the diameter of 12 nm. Then the as-prepared

Cyclic voltammogram of the polymers
Cyclic voltammetry (CV) measurements of the polymers were carried out on a CHI660E (Chenhua, Shanghai) electrochemical workstation in a three-electrode-cell system, drop-casting Ptdisk electrodes covered by the polymers were used as the working electrode with Ag-wire as the reference electrode, platinum as the counter electrode. The sample was prepared by mixing 1.6 mg PTT-1 (PTT-2, PTT-3 or PTT-4), 1.6 mg acetylene black, 0.32 mg PVDF and 1 mL NMP to a 1.5 mL centrifuge tube, then the mixture was sonicated for 30 min until a well-dispersed solution was formed. Finally, 4 μL of the solution was dropped onto the electrode, and dried at room temperature for the CV test. 0.2 mol/L tetrabutylammonium hexafluorophosphate in acetonitrile solution (TBAPF6/CAN) was used as the electrolyte, with a potential window between -2V-2V (vs. pseudo Ag wire reference electrode), and the sweep rate was 100 mV s -1 . All measurements were calibrated against an internal standard of ferrocene (Fc), the ionization potential (IP) value of which is -4.8 eV for the Fc/Fc + redox system.
E g was obtained from the UV-vis absorption spectra. The onset oxidation potential (E onset ) vs.
Ag wire were obtained from CV curves.