Catholyte engineering to release the capacity of iodide for high‐energy‐density iodine‐based redox flow batteries

Due to the high solubility, high reversibility, and low cost of iodide, iodine‐based redox flow batteries (RFBs) are considered to have great potential for upscaling energy storage. However, their further development has been limited by the low capacity of I− as one‐third of the I− is used to form I3− (I2I−) during the charging process. Herein, we have demonstrated that the pseudohalide ion, thiocyanate (SCN−), is a promising complexing agent for catholyte of iodine‐based RFBs to free up the I− by forming iodine‐thiocyanate ions ([I2SCN]−) instead of I3−, unlocking the capacity of iodide. Applying this strategy, we have demonstrated iodine‐based RFBs with full utilization of iodide to achieve high capacity and high energy density. Both the zinc/iodine RFB and polysulfide/iodine RFB with SCN− complex agent achieve their theoretical capacity of around 160 A h Lposolyte−1 (6.0 M I− in catholyte). Therefore, the zinc/iodine RFB delivers a high energy density of 221.34 W h Lposolyte−1, and the polysulfide/iodine RFB achieves a highenergy density of 165.62 W h Lposolyte−1. It is believed that this effective catholyte engineering can be further generalized to other iodine‐based RFBs, offering new opportunities to unlock the capacity of iodide and achieve high energy density for energy storage.

aqueous RFBs. Nonaqueous RFBs have the advantages of a wide voltage window and a wide operating temperature range, but they are limited by the low solubility and high cost of redox species. [11][12][13][14][15][16] Vanadium RFBs (VRFBs), the most representative aqueous RFBs, have a relatively mature preparation technology and are currently in the commercial demonstration stage. However, VRFBs have some limitations such as low energy density, high materials costs, narrow temperature adaptability, and strong corrosivity of the electrolytes, which hinder their widespread deployment. [17][18][19] In addition, the performance assessment of RFBs is not consistent in the reported works, making it challenging to evaluate the potential of various RFBs for practical application. Therefore, Lu et al. summarized and reported standard testing protocols including evaluating device performance of both symmetric and asymmetric cells. Such a comprehensive understanding of the assessment methods is believed to promote the benchmarking and further development of RFBs. 20 Based on the standard testing methods, they recently reported using a multi-electron heteropoly acid (H 6 P 2 W 18 O 62 , HPOM) as the negolyte in aqueous RFBs. The low freezing point, fast redox kinetics, and high electron solubility of HPOM allow the application of a high-power-density RFB at low temperatures. The corresponding HPOM-VRFBs show a high-power density (282.4 mW cm −2 ) at −20 • C. 21 Other aqueous RFBs, such as organic RFBs, [22][23][24][25][26][27] zinc-based RFBs, [28][29][30] and polysulfide-based RFBs, [31][32][33][34] also suffer from challenges like high chemical costs, low energy density, and insufficient cycle life. [35][36][37][38] It is therefore of an urgent need to develop new strategies to boost the energy density with low-cost materials.
The high solubility and high reversibility of iodide make iodine-based RFBs have great development potential. 39 For example, the solubility of I − can be increased to 8.5 mol L −1 with Li salts. 40 The zinc/iodine RFBs (ZIRFBs) with a 5.0 M ZnI 2 electrolyte have a high-energy density of around 167 W h L posolyte −1 , 41 which is significantly higher than VRFBs of 50 W h L posolyte −1 . 42 However, the iodine-based RFBs are still hindered by several critical challenges, such as the capacity attenuation due to the shuttle effect of polyiodide and the low utilization of iodide especially. [43][44][45][46] During charging, the iodide anions (I − ) in the electrolyte are oxidized to iodine (I 2 ) which is not stable and tends to interact with the residual I − , forming I 3 − . Therefore, only two-thirds of the I − in the electrolyte can be utilized, limiting the energy density of iodine-based RFBs. In addition, I 3 − at a high state of charge (SOC) can be further oxidized to I 2 precipitation distributed at the surface of the electrode, causing high polarization and low energy density.
Although device engineering such as electrode modification and membrane designing have been demonstrated to achieve high performance, 47-50 the one-third capacity of iodide is still suppressed by forming I − /I 3 − redox species. Recent studies have shown that electrode and separator design can improve the performance of iodine-based RFBs effectively, but one-third capacity of iodide is still suppressed by forming I − /I 3 − redox species.
To free up the I − for high capacity, electrolyte additive engineering has been applied to stabilize the oxidized I 2 during the charging process. Chloride ions (Cl − ) and bromide ions (Br − ) have been employed as additives in the electrolyte to complex with oxidized I 2 by forming iodinechloride ions (I 2 Cl − ) or iodine-bromide ions (I 2 Br − ) and instead of I 3 − . For example, Mousavi et al. 51 54 and polyvinyl pyrrolidone, 55 can be used as complexing agents because the bonding between the oxygen functional groups with the I 2 is stronger than that between I − and I 2 . Therefore, finding a material with a strong affinity to bond with I 2 is an effective way to unlock the capacity of iodine-based RFBs. Inspired by the aforementioned works and a known complex ([I 2 SCN] − ), 56,57 we intend to investigate the effect of pseudohalide, thiocyanate anion (SCN − ), as a complexing agent on the capacity improvement of iodine-based RFBs. First, the calculation results indicate that the SCN − has a stronger bonding with I 2 than Cl − , Br − , and I − . Then, our experimental results show that the SCN − additives increase the dissolution of I 2 precipitation during charging by the complexing interactions between SCN − and I 2 , forming [I 2 SCN] − which is dissolvable in the electrolyte. Such [I 2 SCN] − formation during the charging process enables decent reversibility of the battery reactions and boosts the device capacity by freeing up the one-third I − . Thanks to such catholyte engineering with KSCN additives for the complexing reaction to stabilize I 2 , we achieve high capacity and high-energy density in both zinc/iodine and polysulfide/iodine RFBs.

RESULTS AND DISCUSSION
We first sought to investigate the bonding between SCN − and I 2 and further prove the feasibility of SCN − to free up the one-third I − in iodide-based RFBs. Figure 1 shows the density functional theory (DFT) calculation results in which we can see the complexing between halide ions and I 2 by forming I 3 − , I 2 Cl − , and I 2 Br − , respectively. The dotted lines in Figure 1A-C indicate the formation of I-I bond, Cl-I bond, and Br-I bond. The structure of the SCN − and I 2 molecules is shown in Figure 1D. From the electrostatic potential (ESP) plot between I 2 and SCN − , it can be seen that there is the lowest ESP energy between sulfur and iodine. The I-S bond is easily formed in regions of high charge density between S and I. Furthermore, Figure 1F shows The solubility test of I 2 with KSCN was then conducted to investigate their complex with I 2 . As shown in Figure 2A, the SCN − additive can promote the dissolution of insoluble I 2 in deionized water.
The effect of KSCN on the reversibility and reaction kinetics of iodide was investigated by cyclic voltammetry (CV) test ( Figure S1). Apparently, with the increase of KSCN, the potential difference between oxidation peak and reduction peak decreased; obviously, the continuous addition of KSCN can continuously reduce the potential difference of I − , indicating that the KSCN can improve redox reversibility of I − . In addition, after the addition of KSCN, the peak current of the I − was effectively increased, implying that KSCN could enhance the reaction kinetic process of I − ( Figure 2B). CV tests were performed in KI and KI + KSCN solutions at different scan rates. As shown in Figure S2A,B, the peak current increased linearly with the square root of the scan rate with a coefficient of determination (R 2 ) close to 1, confirming the diffusion limitation process of iodide. According to the Butler-Volmer equation, the kinetic rate constant of I − /[I 2 SCN] − in 0.1 M KI with KSCN additive is 3.81 × 10 −3 cm s −1 , which is higher than the diffusion coefficient of I −  Figure S2C presents the galvanostatic intermittent titration technique (GITT) results of iodine/iodine symmetric RFB with KSCN (IIRFB-SCN) and iodine/iodine symmetric RFB (IIRFB) under 4 min intermittent charging/discharging processes. At the current density of 20 mA cm −2 , the time of a GITT cycle consumed by the IIRFB-SCN is 341 min, which is twice longer than that of the IIRFB (156 min), proving that more energy can be stored in the IIRFB-SCN. In addition, the voltage drop of the IIRFB (42.70 mv) is larger than that of the IIRFB-SCN (36.00 mv), which may be due to the suppression of the formation of I 2 with KSCN at high SOC. As shown in Figure S2D, the linear sweep voltammetry (LSV) test exhibits a lower onset overpotential of 0.1 M KI + 3.0 M KSCN than the 0.1 M KI, which further proved that KSCN can enhance the kinetics of the redox reactions.
Therefore, it is both theoretically and experimentally evidenced that SCN − is a promising complexing agent to free up the I − and boost the device capacity of iodide-based RFBs. The capability of KSCN as the complex agent to release the capacity of I − using is investigated using symmetric RFBs. Figure 2C shows that the volume capacity of IIRFB-SCN is increased by 58.66% compared to IIRFB. Combined with the above analysis, SCN − can complexe with I 2 to form [I 2 SCN] − during charging, releasing onethird of I − . In addition, Figure S3 shows the capability of KSCN to release the capacity of I − is higher than that of the KCl and KBr with an order of KSCN > KBr > KCl, which agrees well with the calculation results. Furthermore, after 100% SOC, the positive carbon felt (CF) of IIRFB-SCN has no I 2 deposition, whereas the positive CF of IIRFB, iodine/iodine symmetric RFB with KCl (IIRFB-Cl), and iodine/iodine symmetric RFB with KBr (IIRFB-Br) have obvious I 2 deposition ( Figure S4). Based on the above phenomena, it can be concluded that SCN − is an effective complex agent to free up the one-third I − capacity of iodine-based RFBs and eliminate the formation of I 2 even at high SOC to allow high reversibility. It is worth noting that the IIRFB-SCN with 0.5 M I − maintains a stable cycle performance with an average discharge capacity of 133.69 A h L −1 and a relatively high average coulombic efficiency (CE) of 99.77% without obvious degradations over 180 cycles, which suggests an excellent reversibility and stability of the IIRFB-SCN ( Figure S5). It is worth noting that the IIRFB-SCN with 0.5 M I − shows a high CE (average 99.77%) without noticeable fluctuations. What is more, IIRFB-SCN exhibits a high volume capacity (average 133.69 A h L −1 ) with no significant attenuation over 180 cycles, which suggests excellent reversibility and stability of the IIRFB-SCN ( Figure S5).
The X-ray diffraction system (XRD) result in Figure 2D  To demonstrate the capability of releasing I − capacity using KSCN complex agent in battery systems, a ZIRFB-SCN cell was assembled with a structure as shown in Figure 3A. CV measurements were conducted to verify the reaction reversibility in ZIRFB-SCN. For the catholyte, a solution of 0.1 M KI with 3.0 M KSCN was scanned at the 20 mV s −1 . As shown in Figure 3B, one pair of redox peaks with high reversibility was revealed. The redox reaction of iodide is as follows: The redox reaction between Zn and Zn 2+ is: The equilibrium potential difference of the ZIRFB-SCN is 1.39 V. Both the redox peak current densities of [I 2 SCN] − /I − and Zn 2+ /Zn have a linear relationship with the square root of sweeping rates, indicating reactions are controlled by diffusion at room temperature ( Figures  S2A,B and S6A,B).
ZIRFB-SCN cells with 6.0 M KI and various concentrations of KSCN as catholytes were prepared and tested to find the optimized condition. According to the stoichiometric ratio, 6.0 M I − requires 3.0 M KSCN to form [I 2 SCN] − . As shown in Figure S7, the charge capacity of ZIRFB-SCN reaches the theoretical value (1608.0 mA h), and the discharge capacity reaches 1592.4 mA h with KSCN increases to 3.0 M, indicating that the capacity of I − has been fully released. Therefore, the concentration of KSCN was kept at 3.0 M in the following battery tests.
To further verify the capability of KSCN to unlock the capacity of I − , ZIRFBs and ZIRFB-SCNs with 1.0-6.0 M KI are tested. Figure 3C and Figure Figure 3E and Figure  S9A-D show the galvanostatic cycling profiles of the ZIRFB-SCNs with different concentrations of KI. The ZIRFB-SCN maintains a stable cycle performance with high capacity retention and CE. It is worth noting that the ZIRFB-SCN with 0.5 M KI achieves capacity retention of 99.63% and an average CE of 98.78% after 76 cycles (74.9 h). Furthermore, the electrolyte utilization of the ZIRFB-SCN with 1.0 M KI can exceed 99.53% under different current densities. On the contrary, the highest electrolyte utilization rate of ZIRFB with 1.0 M KI is 63.06% at 20 mA cm −2 , which is much lower than that of the ZIRFB-SCN system ( Figure 3F).
As shown in Figure 3G, the CE of ZIRFB-SCN remains stable at around 99.62% at different current densities. The energy efficiency (EE) and voltage efficiency (VE) decrease with the increase of current density, which is due to the serious concentration polarization and ohmic polarization of the device at high current density. The ZIRFB-SCN has a high EE of 56.34% when the current density increases to 120 mA cm −2 . In addition, the EE and VE can recover to their original value after the current density is back to 20 mA cm −2 . Figure 3H shows the stability of the ZIRFB-SCN device, where the capacity has a negligible change after 37 cycles. The decent rate performance of the battery proves the stability and reliability of the ZIRFB-SCN. Meanwhile, the ZIRFB-SCN shows a maximum power density of 206.30 mW cm −2 at a current density of 302.21 mA cm −2 ( Figure 3I). To demonstrate the versatility of KSCN in different battery systems, K 2 S is further used as the negative active material to couple with KI + KSCN and form a PSIRFB-SCN cell. The K 2 S solution was configured for CV test to verify the redox reversibility of the polysulfide (Figure 4A), and its redox reaction is as follows: The anolyte is a mixture of polysulfide phases (e.g., S 2− , S 2 2− , S 3 2− , and S 4 2− ), and the average oxidation state is determined as S 2 2− in order to better represent the redox reaction. Combined with the redox reaction of I − /[I 2 SCN] − , the potential difference of PSIRFB-SCN is 1.04 V. CV curves of the two redox pairs at different sweeping rates were recorded. (Figures S2A and S10A). There is a linear relationship between the peak current and the square root of the scan rate, indicating that the reaction is controlled by diffusion. (Figures S2B and S10B).
Electrochemical tests were performed to investigate the effect of the KSCN complexing agent on the performance of PSIRFB-SCN. Figure 4B shows the charging/discharging curves of polysulfide/iodine RFB and PSIRFB-SCN with 6.0 M KI. It can be seen that KSCN can significantly increase the discharge capacity of the battery, from 108.06 to 159.25 A h L −1 , which is an increase of 47.37% compared to the traditional PSIRFB. The PSIRFB-SCN cells using 2.0, 3.0, 4.0, and 5.0 M KI all free up the one-third iodide capacity (Figure S11A-F) and achieve a high capacity of around 98% of their theoretical values ( Figure 4C). Similarly, the increase in the device capacity is due to the formation of [I 2 SCN] − in the catholyte so that all the I − can involve in the charging/discharging reactions.
In addition, it can be seen that the PSIRFB exhibits a high charging voltage platform oabout 1.13 V, however, after adding KSCN, the charging voltage is reduced by about 0.19 V. Similarly, the discharge voltage plateau (1.06 V) of the PSIRFB-SCN is significantly higher than the PSIRFB (0.94 V). The reason for the decrease in overpotential is the increase in the conductivity of the electrode and the enhancement of the kinetic reaction. The formation of the [I 2 SCN] − prevents the I 2 precipitation from blocking the pore structure of electrode, reducing the polarization and enhancing the stability of PSIRFB-SCN. As shown in Figure 4D, the PSIRFB-SCN with 0.5 M KI allows stable charging/discharging cycles about 202 cycles (199.1 h) with high capacity retention (97.40% after 202 cycles) and high CE (average 99.60%). When the I − concentration increases to 1.0, 2.0, 3.0, 4.0, 5.0, and 6.0 M ( Figure S12A-F), the voltage profiles of the PSIRFB-SCN remain a high and stable CE. Figure 4E shows the rate performance of PSIRFB-SCN at 10-50 mA cm −2 where the CE remains stable at around 99% regardless of the change in current density. On the one hand, the EE and VE decreased from around 90% to around 56% when the current density is increased from 10 to 50 mA cm −2 . On the other hand, both the EE and VE are back to their initial value of around 90% when the current density is suddenly reduced to 10 mA cm −2 . As shown in Figure 4F, the electrolyte utilization of the device is over 97% at all the current densities. However, the electrolyte utilization of the PSIRFB is only 62.92% at 10 mA cm −2 , which is much lower than that of the PSIRFB-SCN. Figure 4G shows the polarization results of PSIRFB-SCN at 100% SOC, where the PSIRFB-SCN achieves the highest power density (146.34 mW cm −2 ) at 322.39 mA cm −2 . Based on the above analysis, it is clearly shown that the KSCN can also significantly improve the performance of the PSIRFB.
To verify the usefulness of KSCN in large-scale applications, ZIRFB-SCN and PSIRFB-SCN stacks with three single cells are designed and investigated ( Figure 5A,C). The stack shows a high CE of 97.06% and a stable discharge capacity (the capacity retention of 100.00% after 44 cycles [72.7 h]) at 20 mA cm −2 ( Figure 5B). The excellent battery performance of the ZIRFB-SCN stack proves the scalability of the ZIRFB-SCN system. When switching to an 80% capacity-controlled charge mode, the PSIRFB-SCN with 1.0 M I − can continuously run for 128 charging/discharging cycles (169.0 h) with a high capacity retention (100.00% after 128 cycles) and a stable CE of 97.47% without obvious degradations at 20 mA cm −2 ( Figure 5D).
The excellent stability of the PSIRFB-SCN stack indicates that the PSIRFB-SCN system has great potential for practical applications. Figure 5E shows the unlock capacity rate of I − substantially surpasses other iodinebased catholytes, while the energy density of ZIRFB-SCN is the highest. On the one hand, PSIRFB-SCN and ZIRFB-SCN have better performance than other reported iodine-based RFBs. On the other hand, the cathode electrolyte is also the lowest cost, further confirming the practicality of the PSIRFB-SCN and ZIRFB-SCN systems ( Figure 5F).
It is worth noting that although KSCN can effectively improve the performance of iodine-based RFBs, the cycle life of iodine-based RFBs is still an important challenge for further development. The shuttle effect of polyiodide and the strong corrosion of iodine may be important reasons for reducing the battery life of iodine-based RFBs. In further exploration, the development of highly corrosion-resistant battery materials (such as electrodes, membranes, and sealing materials) is necessary for the commercialization of the iodine-based RFBs.

CONCLUSION
In

Materials
Potassium iodide (KI, 99.9%), potassium thiocyanate (KSCN), potassium sulfide (K 2 S), zinc bromide (ZnBr 2 ), ammonium bromide (NH 4 Br), potassium bromide (KBr), iodine (I 2 ), and potassium chloride (KCl) were obtained from Sinopharm Chemical Regent. In addition, all the chemicals were analytically pure and used without further purification. CF (5.5 mm, carbon >98.8%, bulk density 0.08-0.11 g cm −3 ) was purchased from Liaoning JinGu Carbon Materials. The Nafion 212 membrane was supplied by DuPont Company. Prior to use, the Nafion 212 membrane was immersed in 1.0 M KOH solution at 80 • C for 1.5 h to obtain K + -type ion exchange membrane and then soaked in deionized water for use.

Characterization
CV, LSV, and CA measurements were performed on the VSP BioLogic workstation using a three-electrode electrochemical system with a gold disk as the working electrode (2.0 mm in diameter), a platinum mesh (1.0 cm × 1.0 cm) as the counter electrode, and a Hg/Hg 2 Cl 2 as the reference electrode.
XRD (Bruker D8A A25 X) was performed using Cu K α radiation ranging from 5 • to 90 • . ESI-MS was conducted on an Agilent qtof6550.
IIRFB was assembled and performed with the charging/discharging process. Taking out the CF after the first discharge process for the XRD test. At 100 SOC% states, taking out the catholyte samples and then 10 6 -fold diluted for the ESI-MS tests.
The charging/discharging cycle performance tests were conducted on a LANHE (CT20001A) or a NEWARE Battery Testing System (CT-4008-5V6A-S1) within the voltage window of −0.5 to 0.5 V for IIRFB and IIRFB-SCN, 0.0-1.6 V for PSIRFB, PSIRFB-SCN, ZIRFB, and ZIRFB-SCN at current density of 20 mA cm −2 . In addition, the theoretical capacity of the battery is set as a protection condition during the charging test. The rate performance tests were conducted on a NEWARE Battery Testing System (CT-4008-5V6A-S1). Rate performance tests were carried out under different current densities. For each applied current density, the cell ran 6 charging/discharging cycles and CE, EE, and VE. The polarization curves were conducted on an Arbin Instruments (BT-I) at 100% SOC and the current was gradually increased from 270 mA by 10 mA s −1 for 600 s. The charging/discharging cycle performance tests for the cell stack were conducted on the LANHE (CT20001A) within the voltage window of 0.0-4.8 V for PSIRFB-SCN and ZIRFB-SCN at 20 mA cm −2 . The theoretical capacity of the battery is set as a protection condition during the charging test.

Theoretical calculation
Theoretical calculations were carried out based on DFT using the Gaussian 16 package. The structure was first optimized using the Perdew-Burke-Ernzerhof exchange and correlation functional at the 6-31G level, and an ESP map was generated to indicate the distribution of charge density.

Cost calculation
The cost of each of the chemicals is referred to the website (https://www.reagent.com.cn/). The cost of the iodine-based catholyte can be estimated according to the following equations: where in the catholyte, the P a is the price of active material ($ mol −1 ), C a is the concentration of the active material (mol L −1 ), P a,s is the cost of the supporting materials ($ mol −1 ), C a is the concentration of active material (mol L −1 ), C a,s is the concentration of supporting electrolyte (mol L −1 ), and Cap a are the capacity at the given active material (A h L −1 ). The results of cost calculation are shown in Tables S7-S17.