Chlorine-based batteries based on Cl0-Cl-redox reaction (CLRR) have broad application prospects in the field of high-performance energy storage. However, the poor gas-liquid conversion characteristics and CL fixation ability of CLRR will lead to Cl2 leakage, reduce the reversibility of the battery and increase the safety risk. Based on this,Prof. Chi Chun-yi and Assistant Prof. Li Nan (corresponding author) of City University of Hong KongThe co-use of the selenium-based organic molecule diphenadiene (Di-PH-SE) as a CL anchoring agent achieves atomic-level Cl immobilization through sulfur-halogen coordination chemistry, resulting in a highly reversible CLRR with ultra-low Cl2 emissions and significantly high discharge voltages (187 vvs zn2+/zn）。The elevation of Cl fixation (two oxidized Cl0s anchored to a single pH-Se) and the multivalence state conversion of Se facilitated the six-electron conversion process, resulting in a significant increase in discharge capacity up to 507 mAh·g-1 with an average voltage of 151 V, coulombic efficiency up to 993%, with an energy density of up to 665 wh·kg-1. Based on the excellent reversibility of the electrode, the electrode exhibited good rate performance (205 mAh·g-1 at 5 a·g-1) and cycling performance (capacity retention rate of 77 after 500 cycles).3%）。In addition, this pouch battery has an average capacity of up to 687 mAh·cm-2, and has extraordinary self-discharge performance, showing great potential for practical applications. This study provides a new idea between selenium electrode and chlorine electrode through sulfur-halogen coordination chemistry, and provides a new idea for the development of reversible and efficient batteries with halogen redox reactions. The chlorine redox reaction (CLRR) has a promising role in advanced electrochemical energy storage systems due to its high reaction potential, excellent theoretical weight capacity, rapid mass transfer, and high natural abundance of Cl. An early prototype of the Cl2-based battery was first proposed in 1884 as a ZN Cl2 battery system. However, the development of CLRR electrodes has been hampered over the past few decades, mainly due to the poor reversibility of CL2-based batteries and severe CL2 leakage. Therefore, improving CL fixation is a necessary condition for the manufacture of reversible and environmentally friendly CL2 batteries. Although the physical adsorption of Cl2 on carbon-based materials can delay the release of toxic Cl2 gas to a certain extent, it cannot completely eliminate the precipitation of Cl2, thereby reducing the Coulombic efficiency (CE) of Clrr. **Reading guide
Figure 1Development strategy of a unique Cl anchoring agent based on coordination chemistry for efficient fixation of Cl. As shown in the figure, among these IV, V, and VI atoms, selenium (Se) and Cl atoms can form Se-Cl covalent bonds, and the bond length is close to that of Cl-Cl covalent bonds. This indicates that the binding energy of the Cl-Se bond is close to that of the Cl-Cl bond. In addition, the Se-Cl bond can potentially output a high voltage comparable to that of the Cl-Cl bond, making SE a promising high-performance Cl0 anchoring agent. In particular, the intrinsic valence diversity of selenium favors the redox kinetics during charging and discharging. 
Figure 2Electrochemical performance of the di-ph-s se te electrode in reaction with chlororedox. In order to get rid of the severe capacity decay of elemental Se electrodes, the authors explored an organic molecule with a chlorine group as a CL anchoring agent, which is expected to exhibit superior cycling stability than elemental chlorine. Diphenyl organic molecules linked to chalcogenide atoms (S, Se, or Te) were selected as anchoring agents for CLRR, denoted as Di-Ph-S, Di-Ph-Se, and Di-Ph-Te, respectively. The authors then used Raman spectroscopy and Fourier transform infrared spectroscopy (FTIR) to characterize the active species. The results showed that the organic Di-Ph-S Sete TE molecule was well bound in the prepared organic electrode. In principle, the normalized organic sulfur ions can be coordinated with Cl-carriers to achieve atomic-level adsorption of Cl. The author then increased the operating voltage to 20 V to trigger the CLRR in the ZN Di-PH-S SETE battery. As expected, new redox peaks corresponding to CLRR were detected in all cells. It is worth noting that the CLRR anode peak of the Zn Di-Ph-SE battery (178 V) is sharper than the other two cells, indicating that the Di-Ph-Se electrode has a higher promotion efficiency for CLRR compared to the Di-Ph-S and Di-Ph-Te electrodes. This result was further confirmed by the GCD curve, where a long, flat CLRR platform was detected in the Zn Di-Ph-Se cell (187 V) with a discharge capacity of up to 509 mAh·g-1. In order to confirm the CL fixation efficiency of the Di-PH-S SETE electrode, the charging process was monitored using in-situ gaseous Cl2 analysis (20 V constant voltage charge for 3 h) Generation of CL2. For Di-Ph-S and Di-Ph-Te electrodes, at 2Cl2 above 45 ppm was detected at 0 V, while for the Di-Ph-SE electrode, only a small amount of Cl2 was recorded during the entire 3-hour charge (27 at 0 v4 ppm)。It can be seen that the Di-Ph-SE electrode can effectively capture oxidized Cl0 and significantly reduce the formation of gaseous Cl2, which is essential to improve the reversibility of CLRR batteries. It is worth noting that the reversible capacity of the ZN Di-PH-SE battery after 100 cycles remains at 423 mAh·g-1 and has 834% high capacity retention and 96More than 5% high CE. This good cycling performance, which is superior to most reported CLRR electrodes, may be due to the high structural stability of the diphenyl framework and the efficient immobilization of the Se atom to the Cl. 
Figure 3The transformation process and Cl anchoring mechanism of Di-Ph-SE electrode. To study the redox process of the Di-PH-SE cathode, the authors first used off-situ XPS in different states of charge and discharge. Simply put, when discharged, at 55A new peak of SE 3D was detected at 1 EV, indicating a transition from (PH-SE)0 to (PH-SE)-1. During charging (Charge to 14 v), the ph-se)-1 peak disappears, indicating a reversible transition from (ph-se)0 to (ph-se)-1. In addition, the authors also studied the XPS spectra of Cl2P to further verify the interaction mode of Di-Ph-Se with Cl. After charging to 19 v and 2At 0 V, the Cl2p peaks shifted to 202., respectively3 ev and 2005 EV, with a Cl2p peak of ionic Cl- (200.)1 ev and 1989 EV), indicating that Cl- oxidation produces organic Cl0. The authors further performed in situ Raman characterization to reveal the products of the redox process. ZN Di-PH-SE battery at 0The GCD curve at 5 A·G-1 shows that after the end of the discharge process (A, B, C), a new peak appears at 257 cm-1 (point C), which may be due to the formation of (ph-se)-, unlike the (ph-se)0 peak at 243 cm-1 (point A). When the Zn Di-Ph-Se battery is charged, the decline of the (ph-Se)- peak and the reappearance of (Ph-Se)0 indicate a reversible transition from (Ph-Se)- back to (Ph-Se)0. When the charging voltage increases to 1At 8 V (point I), a new peak was detected at 195 cm-1, which was caused by oxidation products (pH-Se)3+. Combining electrochemical and spectroscopic results, the authors deduced that the redox conversion of I-PH-SE may potentially promote the reversibility of CLRR by forming a reversible valence bond with oxidized CL, thereby enhancing the fixation of CLRR. Density functional theory (DFT) simulations further confirm the above conclusions. 
Figure 4Electrochemical properties of ZN Di-PH-SE batteries in reaction with chlorine redox. Next, the authors focused on the reaction kinetics of the designed Zn Di-Ph-SE battery with CLRR. With the increase of the scanning rate, the CV curve showed that the electrochemical polarization was weak and there was a slight peak shift, indicating that the conversion kinetics of Di-Ph-SE were fast during the electrochemical process. This may be attributed to the properties of the small molecule, which can accommodate the rapid diffusion of ions in the electrode, as well as the inherently rapid transformation kinetics between Cl- and Cl0. The GCD curve showed that the capacity of the Di-PH-SE electrode was 441 mAh·g-1, and the CE was as high as 99%. In 187 V and 1Two distinct plateaus were observed at 44 V, attributed to Cl0 CL- transformation and (ph-Se)3+ (pH-Se)0 transformation. Notably, thanks to the Di-Ph-Se electrode-boosted CLRR and the resulting flat discharge platform, the ZN Di-Ph-Se battery has an output energy density of up to 665 Wh·kg-1 (calculated based on the active material) and an average output voltage of 151 V, much higher than the other reported ZIBS cathodes. In addition, the rate performance of the Zn Di-PH-SE cell was evaluated by GCD curves at different current densities, which gradually decreased to 05 a·g-1, the capacity has almost returned to its original value. The calculated results of the capacitance contribution of Cl and Se under different current densities show that with the increase of current density, the capacitance contribution of Cl decreases gradually, and the capacitance contribution of Se increases gradually. Even at a high current density of 5 a·g-1, the conversion reaction of Cl still accounts for 18 of the total capacity1%, which indicates the efficient fixation of Cl in the Di-Ph-Se electrode and the fast reaction kinetics of this organic electrode structure. 
Figure 5Evaluate the stability and utility of Zn Di-PH-SE batteries. To evaluate the long-term stability of such ZN Di-PH-SE cells, the authors collected long-term cycling performance at 3 A·G-1. After 500 cycles, the reversible capacity remains at 2162 mAh·g-1 with a capacity retention rate of up to 773%。Compared with the CE and cycle life of other reported CL-based batteries, the cycle stability of the ZN Di-PH-SE cells designed by the authors was significantly improved, with a high CE (991%)) and long cycle life, which indicates that the Di-Ph-SE electrode has superior CL anchoring performance, achieving excellent cycle stability in the cell with a capacity retention rate of 612% (106 mAh after 180 cycles). In addition, the pouch battery provides 6The high areal capacity of 87 mAh·cm-2 exceeds all previously reported ZIBS, indicating that the developed ZN Di-PH-SE battery has great potential for practical application. In addition, the converted ZN Di-PH-SE battery has excellent self-discharge performance and a low self-discharge rate, and the capacity of the battery remains at 72 after 5 days of standing5% or so, and the average discharge voltage is 12 V, Di-Ph-Se cathode with efficient CL anchoring has excellent stability. chen, z., hou, y., wang, y., wei, z., chen, a., li, p., huang, z., li, n., zhi, c., selenium-anchored chlorine redox chemistry in aqueous zinc dual-ion batteries.adv. mater