Structural Isomers: Small Change with Big Difference in Anion Storage

Highlights The effect of small changes in isomers on electrochemical performance have not been focused in batteries and two isomers are reported for Zn-ion batteries here. The isomer tetrathiafulvalene (TTF) could store two monovalent anions reversibly, showing remarkable performance that outperformed most of the reported organic electrode materials for zinc-ion batteries. The isomer tetrathianaphthalene (TTN) could only reversibly store one monovalent anion and upon further oxidation, it would undergo an irreversible solid-state molecular rearrangement to TTF. Supplementary Information The online version contains supplementary material available at 10.1007/s40820-023-01239-7.


S1 Precursor Preparation
Scheme S1 Synthetic process of TTN S2 Preparation of Tetrabutylammonium Bis(1,3-dithiole-2-thione-4,5dithiol) Zincate tetrabutylammonium bis(1,3-dithiole-2-thione-4,5-dithiol) zincate was prepared according to previous report with slight modification [S1].Under inert gas protection, a 1 L three necked flask was charged with 4.6 g of sodium (200 mmol) and placed in an ice-water bath.Then, 36 mL of carbon disulfide (600 mmol) and 60 mL of dimethylformamide were slowly added dropwise over 45 min.After the addition, the reaction mixture was allowed to warm to 25 °C and stir 12 h, during which time it becomes increasingly deep red/violet.To remove the unreacted sodium, slowly drop 20 mL of methanol in an ice-water bath.Afterwards, added the mixed solution of 80 mL methanol and 100 mL deionized water into the three necked flask quickly.A solution of 4.1 g of ZnCl2 (30 mmol) in 150 mL of concentrated aqueous ammonium hydroxide and 100 mL of methanol was slowly added dropwise to the reaction mixture with vigorous stirring.A solution of tetrabutylammonium bromide (10.5g,50 mmol) in deionized water (50 mL) was added dropwise at least 1 h, and the solution was stirred overnight.The precipitate was collected by filtration, washed with deionized water, isopropanol and ether, and dried in vacuum to obtain 14 g of red powder in 78% yield. 13C NMR (600 MHz, CDCl3, 300 K) δ/ppm 208.86, 136.18, 58.98, 24.13, 19.86, 13.80.

S3 Preparation of 4,5-dibenzoylthio-1,3-dithiole-1-thione
According to the literature [S1], in a 1 L round-bottomed flask, 16 g of tetrabutylammonium bis(1,3-dithiole-2-thione-4,5-dithio) zincate was dissolved in 400 mL acetone.Then, 40 mL of benzoyl chloride was added via a dropping funnel over 4 h.After stirring for 12 h, the resulting yellow-light brown precipitate was filtered and Nano-Micro Letters S3/S27 washed with water and acetone.These crude products were air-dried and dissolved in 350 mL of chloroform.Afterwards, 0.5 g activated carbon was added and stirred for 10 min under reflux.The mixture was filtered while hot, and the filter cake was washed with 50 mL of hot chloroform.The combined chloroform solution was concentrated to 150 mL, and then 50 mL of methanol was added dropwise at 60°C.The solution was left in the refrigerator overnight to recrystallize and the resulting precipitate was collected and air-dried, affording yellow crystals (8.1 g, 41%). 13C NMR (600 MHz, CDCl3, 300 K) δ/ppm 212.36, 185.47, 134.90, 134.82, 133.68, 129.15, 128.00.
Therefore, the redox potential of TTN in the high-concentration Zn(ClO4)2•6H2O aqueous electrolyte should shift downward more than that in the low-concentration one, as indicated by equation ( 2), owing to the increased activity of the anion (α ClO 4 -), which was proportional to the mole concentration.In addition, a higher concentration of electrolyte could also inhibit the evolution of oxygen, which was beneficial to expand the electrochemical window of the electrolyte.electrolyte and the voltage range of 0.5-2.2V. d Ex-situ 1 H NMR spectra of DBTTN 2.2.e The proposed process of solid-state molecular rearrangement of DBTTN to DBTTF at high voltage If the conversion process from TTN to TTF was the same as that of base induce deprotonation rearrangement, the designed fused-TTN (DBTTN) should not experience isomerization at high voltage (or further oxidation), due to the absence of hydrogen atoms at the ethene bridges.The synthesis process of DBTTN was shown in Fig. 27a, and the 1 H NMR spectra of DBTTN was consistent with the literature [S2], proving the accurate synthesis of the target product (Fig. S27b).We first assembled DBTTN//Zn cell with 10 m Zn(ClO4)2•6H2O as electrolyte, and carried out cyclic voltammetry test on it in the voltage range of 0.5-2.2V.As shown in Fig. S27c, the DBTTN electrode exhibited two strong oxidation peaks at 1.99 V and 2.09 V when first charged.However, during subsequent charging, two new peaks at 1.44 V and 1.82 V, respectively, appeared, in place of the two peaks observed during the first charging.In addition, in each discharge process, there were three reduction peaks at 1.39 V, 0.99 V and 0.85 V, which had a large polarization relative to the oxidation peak of the first charged.This redox behavior was very similar to that of TTN electrodes, so we performed 1 H NMR test on DBTTN after the first charge and then discharge (defined as DBTTN 2.2).As shown in Fig. S27d, compared with the pristine DBTTN, the DBTTN 2.2 exhibited three sets of different chemical shift signals.Among them, the chemical shifts in the range of 7.39-7.36ppm were consistent with the pristine DBTTN, indicating that the pristine state of DBTTN still existed.However, some additional peaks appeared in the range of 7.14-7.11ppm and 7.28-7.26ppm, which belonged to fused-TTF (DBTTF) [S3].This result indicated that the isomerization process from TTN to TTF under high voltage (or further oxidation) was different from the process of base induced deprotonation rearrangement.
The proposed isomerization process may be more reasonable (Fig. S27e).

Scheme S3
The proposed process of solid-state molecular rearrangement of TTN to TTF Fig. S28 Cycling stability of TTF electrode at a current density of 2 C when Super P was used as conductive additive.TTF electrode exhibited low initial specific capacity and poor cycling stability Fig. S29 In-situ UV-vis spectra of TTF electrode for a Super P as conductive additive and b rGO as conductive additive.The insets were the images of the cuvette cell after 10 cycles.It could be clearly observed that when Super P was used as conductive additive, several peaks belonging to TTF derivatives increases significantly with cycling and the color of electrolyte solution changed to orange, indicating that TTF derivatives were dissolved in the electrolyte.However, when rGO was used as conductive additive, the adsorption peak of TTF derivatives were extremely weak and the color of electrolyte showed insignificant change  Although there were some reports that carbon materials such as graphite had the ability to store anions, high voltage (>2 V) was required [S4, S5].In this work, Super P had almost no capacity contribution

Fig. S8 aFig. S9 a
Fig. S8 a Perspective view of a TTF unit cell observed from a axis.b Perspective view of a TTF unit cell observed from b axis.c Perspective view of a TTF unit cell observed from c axis

Fig. S15 Fig. S26 a
Fig. S15 Electronic band structure of a TTF and b TTN by using the crystal structures.The calculated (conduction band minimum) CBM energy and (Valence Band Maximum) VBM energy of TTF and TTN were 0.05 eV, -1.21 eV, -0.19 eV and -2.38 eV, respectively, and the band gap of TTF (1.26 eV) was narrower than that of TTN (2.19 eV).The trends of these results were the same as those exhibited by the calculated HOMO/LUMO energy levels

Fig. S30 a
Fig. S30 a SEM image of the mixture of TTF and rGO.b-e Corresponding elemental C and S mapping.The uniform mixing of TTF and rGO could be proved by SEM images and corresponding element mapping, which was conducive to the transport of ions and electrons and could effectively inhibit the dissolution of intermediates

Fig. S31 aFig. S32 a
Fig. S31 a CV curves of rGO electrode at 1 mV s -1 in 3 m Zn(ClO4)2•6H2O electrolyte.b charge-discharge profile of rGO electrode at 0.53 A g -1 .c the cycling performance of rGO electrode at 0.53 A g -1

Fig. S34 a
Fig. S34 a XPS spectra of TTF electrode in pristine, completely charged and completely discharged state, respectively.b High-resolution Cl 2p XPS spectra of TTF electrode in

Fig.
Fig. S35 a-b CV curves of TTF and TTN electrode at different scan rates from 0.2 to 10 mV s -1 .c-d Plots of log (i, mA) versus log (v, mV s -1 ) by using the corresponding peak currents in the CV curves of TTF and TTN electrode at the different scan rates

Fig. S36 a
Fig. S36 a Nyquist plots of TTF electrode and b Partial enlargement after different numbers of cycles.c Nyquist plots of TTN electrode after different numbers of cycles.The resistance of TTF and TTN electrode after discharged ten cycles was ~20 Ω and ~170 Ω, respectively.The TTF electrode showed lower resistance, further confirming its faster charge transfer for achieving high-rate performance

Table S1
The electrical conductivity for two isomers

Table S2
Electrochemical performance comparison of reported organic electrodes for aqueous rechargeable zinc batteries