A Dithiin‐Linked Covalent Organic Polymer for Ultrahigh Capacity Half‐Cell and Symmetric Full‐Cell Sodium‐Ion Batteries

Abstract Sodium ion‐batteries (SIBs) are considered as a class of promising alternatives to lithium‐ion batteries (LIBs) to overcome their drawbacks of limited sources and safety problems. However, the lack of high‐performance electrode materials hinders the wide‐range commercialization of SIBs. Comparing to inorganic counterparts, organic electrode materials, which are benefitted from flexibly designable structures, low cost, environmental friendliness, and high theoretical gravimetric capacities, should be a prior choice. Here, a covalent organic polymer (COP) based material (denoted as CityU‐9) is designed and synthesized by integrating multiple redox motifs (benzoquinone and thioether), improved conductivity (sulfur induction), and intrinsic insolubility (rigid skeleton). The half‐cell SIBs exhibit ultrahigh specific capacity of 1009 mAh g−1 and nearly no capacity drop after 650 cycles. The first all‐COP symmetric full‐cell shows high specific capacity of 90 mAh g−1 and excellent rate capability. This work can extend the selection of redox‐active moieties and provide a rational design strategy of high‐performance novel organic electrode materials.


Materials and solvents
Unless other mentioned, all the materials and solvents used in the experiments were purchased from TCI chemical, Aladdin Biochemical, Zhengzhou Alfa, J&K Chemical, and Anaqua without further purification.

Measurements
Fourier Transform infrared (FT-IR) spectra were recorded on a on a PerkinElmer Spectrum Two FTIR Spectrometer.Solid-state 13 C cross polarization-total sideband suppression nuclear magnetic resonance (CP-TOSS NMR) spectrum was tested on a Bruker Avance NEO 400WB.XPS patterns were measured using a Thermo Fisher ESCALAB Xi + X-ray Photoelectron Spectrometer (Al Kα: λ = 0.834 nm) under ambient conditions.The thermogravimetric curves were measured on a PerkinElmer STA 6000 using ceramic pan as the container with a heating rate of 10°C•min -1 and a nitrogen flow rate of 20 cm 3 •min -1 .Diffuse reflectance spectrum (DRS) was recorded on a Hitachi UH4150 UV-VIS-NIR Spectrophotometer using an integrating sphere.Cyclic voltammetry (CV) tests were carried out on an IviumStat electrochemical workstation with a voltage range of 0.1-3.5 V and a scan rate of 0.1 mV s -1 .The galvanostatic charge/discharge measurements were performed on a NEWARE test system (NEWARE, CT4008) at current densities of 20, 100, 200, 400, 500, 800, 1000, 1500, 2000, 3000 and 5000 mA g -1 .Electrochemical impedance spectroscopy (EIS) was obtained by applying a sine wave with an amplitude of 10 mV in the frequency range from 100 kHz to 100 mHz on an IviumStat electrochemical workstation.

Preparation of CityU-9-based cathode.
CityU-9, Ketjen Black, and polyvinylidene fluoride (PVDF) were mixed in N-methyl-2pyrrolidone (NMP) with a mass ratio of 5:4:1.The obtained slurry was pasted onto an Al foil and dried at 80°C for 12 h in a vacuum oven.The loaded mass of the active material is about 0.12 mg cm -2 .

Fabrication of a half SIB coin cell with the CityU-9-based cathode.
A sodium disk was first pressed to the stainless steel spacer.Then an O-ring was placed on the smaller case and pressed against the case.Next, a spring, the assembled stainless steel spacer and sodium disk were placed on the O-ring sequentially, with the sodium disk facing upwards.
After that, a separator (glass fiber) was placed on top of the sodium disk as centered as possible.200 μL of 1M sodium hexafluorophosphate (NaPF6) in diglyme as the electrolyte was then dropped onto the separator.Next, the former obtained CityU-9-based cathode was placed on top, with the cast film facing the sodium disk and centered as much as possible with the sodium disk to avoid uneven current densities.Finally the larger case was placed on top and the coin cell was packed with 0.8 kPa pressure.
CityU-9, Ketjen Black, and polyvinylidene fluoride (PVDF) were mixed in N-methyl-2pyrrolidone (NMP) with a mass ratio of 5:4:1.The obtained slurry was pasted onto an Al foil and dried at 80°C for 12 h in a vacuum oven, and thereby the (pristine) CityU-9-based anode is obtained.The obtained (pristine) CityU-9-based anode was combined with a Na electrode to fabricate a half cell battery.After performing a discharging process at 50 mA g -1 for about 10 hours, the cell was disassembled in the glovebox to isolate the presodiated CityU-9-based electrode as the anode of the full cell.

Fabrication of the half SIB coin cell with the CityU-9-based anode.
An O-ring was placed on a smaller cap and pressed against the case.Next, a spring, a stainless steel spacer and the (pristine) CityU-9-based anode were placed on the o-ring sequentially, with the cast film of the anode facing upwards.After that, a separator (glass fiber) was placed on top of the anode as centered as possible.200 μL of 1M sodium hexafluorophosphate (NaPF6) in diglyme as the electrolyte was then dropped onto the separator.Next, a sodium disc cathode was placed on top, with the cast film facing the anode and centered as much as possible with the anode to avoid uneven current densities.Finally the larger case was placed on top and the coin cell was packed with 0.8 kPa pressure.

CityU-9-based anode.
An O-ring was placed on a smaller cap and pressed against the case.Next, a spring, a stainless steel spacer and the presodiated CityU-9-based anode were placed on the O-ring sequentially, with the cast film of the anode facing upwards.After that, a separator (glass fiber) was placed on top of the anode as centered as possible.200 μL of 1M sodium hexafluorophosphate (NaPF6) in diglyme as the electrolyte was then dropped onto the separator.For pre-sodiation of anode, we discharged the BHT-BQ anode to a discharge capacity of 158 mAh g -1 for the 2 nd cycle in a voltage window of 0.01-2.0V.There should be 5.0 mol Na + in CityU-9 anode after presodiation process, which can ensure that the full cell has sufficient sodium source.Next, the

Figure S4 .
Figure S4.Interaction mode of Na ion with one oxygen atom and two sulfur atoms.

Figure S5 .
Figure S5.The structure and ESP distribution of CityU-9 after adding 3 Na ions.

Figure S6 .
Figure S6.The structure and ESP distribution of CityU-9 after adding 9 Na ions.

Figure S8 .
Figure S8.Bader charge analysis of CityU-9 after adding different numbers of Na ions.

Figure S13 .
Figure S13.(a) Charging and discharging curves of CityU-9 cathode-based half-cell SIB of specific cycles.(b) Cycling stability of CityU-9 cathode-based half-cell SIB at current density of 0.05 A•g −1 .

Figure S15 .
Figure S15.Cycling stability of symmetric full-cell SIB based on CityU-9 cathode and presodiated anode at current density of 0.1 A•g −1 .