Sulfide-Bridged Covalent Quinoxaline Frameworks for Lithium–Organosulfide Batteries

behavior of model compounds mimicking the sulfide linkages of the COFs and operando Raman studies on the framework structure unravels the reversibility of the profound Li-ion–organosulfide interactions. Thus, integrating redox-active organic-framework materials with covalently anchored sulfides enables a stable Li–OrS battery mechanism which shows benefits over a typical Li–S battery


Introduction
The opportunities for the atomically precise design of ordered 2D covalent organic frameworks (2D-COFs) are entirely different from those of amorphous linear polymers, cross-linked polymers, and hyper-branched polymers, allowing for manipulating primary and higher order arrangements constituting heteroatom (N, S, and O, etc.) containing functional groups to an unprecedented level. [1]This emerging class of ordered polymeric materials shows reticular growth of organic subunits interlocked via strong covalent linking (of Schiff bond formation, [2] boroxine linkage, [3] CC bond formation, [4] amide linkage, [5] phenazine linkage, [6] benzothiazole linkage, [7] dioxin, [8] dithiine linkage, [9] etc.) to configure a 3D array by π-π interactions between adjacent layers with good anticipation over composition and properties.The prediction of the structure is The chelating ability of quinoxaline cores and the redox activity of organosulfide bridges in layered covalent organic frameworks (COFs) offer dual active sites for reversible lithium (Li)-storage.The designed COFs combining these properties feature disulfide and polysulfide-bridged networks showcasing an intriguing Li-storage mechanism, which can be considered as a lithium-organosulfide (Li-OrS) battery.The experimental-computational elucidation of three quinoxaline COFs containing systematically enhanced sulfur atoms in sulfide bridging demonstrates fast kinetics during Li interactions with the quinoxaline core.Meanwhile, bilateral covalent bonding of sulfide bridges to the quinoxaline core enables a redox-mediated reversible cleavage of the sulfursulfur bond and the formation of covalently anchored lithium-sulfide chains or clusters during Li-interactions, accompanied by a marked reduction of Li-polysulfide (Li-PS) dissolution into the electrolyte, a frequent drawback of lithium-sulfur (Li-S) batteries.The electrochemical behavior of model compounds mimicking the sulfide linkages of the COFs and operando Raman studies on the framework structure unravels the reversibility of the profound Li-ion-organosulfide interactions.Thus, integrating redox-active organic-framework materials with covalently anchored sulfides enables a stable Li-OrS battery mechanism which shows benefits over a typical Li-S battery.
well advanced due to the crystalline nature of COFs.Predictive theoretical modeling and design of the unit cell of COFs starting from the known monomeric building units, is a key enabler for the design of functional materials. [10]Structural modeling facilitates the prediction of interaction sites of the framework structure with guest molecules or ions.Therefore, COFs are a class of crystalline porous organic polymers, in which systematic interpretation of the structure-property relationships allows a deeper understanding, e.g., in the case of the redox interactions with guest ions as it is relevant for the integration in functional electrode materials for charge storage in batteries and capacitors, an endeavor extremely challenging for amorphous materials. [11,12]The high surface area of porous COFs contributes to the electric double layer capacitor formation.[15] However, the fabrication of battery electrodes with the COF as the active species requires proper redox-type interactions with the functional groups of COFs.The combination of both these features in a COF for designing novel electrode materials is interesting to enhance the charge storage capacity. [16,17]mong many charge storage devices, lithium-sulfur (Li-S) batteries built with a light-weight Li-metal anode, electrolytes of Li-salts, and a low-cost sulfur cathode supply a higher theoretical capacity (1675 mAh g −1 ) than other systems. [18]his charge storage device exhibits multistep redox activity of the Li-ions with sulfur under applied potential to produce lithium-polysulfide (Li-PS) intermediates.[23] Ongoing research by encapsulating the polysulfide (PS) in some porous hosts such as porous carbon, [24,25] porous metal oxides, [26] metal-organic frameworks, [27] and synthetically developed porous organic polymers [28] improves the battery performance and durability incrementally.But the lack of direct covalent connectivity of the embedded PSs to these frameworks inhibits the redox activity of sulfur in the Li-S battery due to the poor electronic/ionic conductivity of LiPS intermediates.Meanwhile, a few organic polymers, such as sulfurized polyacrylonitrile (S-PAN), where the PS is directly integrated into the carbon skeleton, carry the electron flow to activate the anchored PSs. [29]Unfortunately, the S-PAN cathodes contain insufficient sulfur loadings (of less than 55 wt%) due to the lack of porosity of PAN that impedes their use in commercial Li-SPAN batteries owing to extremely low energy density. [29][31] In this context, 2D-COFs represent a new class of crystalline porous polymers.39][40] However, only one side of the polysulfide chain is directly attached to the framework's skeleton, while the other one remains free, leading to some irreversible reactions during the interaction with Li and resulting in a substantial Li-PS dissolution.In this work, a disulfide-bridged monomer has been chosen to construct a novel imine-linked 2D-COF, which is easily transformable to a polysulfide-bridged framework by post-synthetic sulfurization. [41]This sulfurization induces in parallel the well-known topo-chemical conversion of imine to the benzothiazole linkage which interlocks the imine bond to maintain the electronic flow for the redoxactivation of the bridging polysulfides. [42]The strategy of sulfide bridging significantly protects the polysulfide chains by covalently linking both ends with the COF backbone, suppressing the Li-PS dissolution in the fabricated battery where Li-metal acts as a counter electrode.As a result, nearly 84% retention of the specific capacity was obtained after 500 charge-discharge cycles during reversible electrochemical Li-sulfide interactions.Additionally, this polysulfide-bridged framework offers pseudocapacitive chelating sites for Li interaction owing to the presence of a quinoxaline core connecting the sulfide-bridged linkers.The comparative battery studies using an iso-structural quinoxaline-based 2D-COF without disulfide bridges confirm the redox contributions from sulfide bridging.This pseudocapacitive Li interaction of the quinoxaline-containing organic backbone along with the redox activity of the sulfide bridging increases the active mass for Li framework interactions which also corroborates well with the theoretical modeling.These dual active sites for Li framework interactions achieved the specific capacities of 620 and 950 mAh g −1 at 100 mA g −1 normalized to the mass of the COF and sulfur, respectively.The coherence of experimental and theoretical investigations confirms the fast and reversible cleavage of the SS bonds in the sulfide-bridge and the formation of either Li-sulfide chains or clusters which was only possible due to the strong covalent attachment of both sides of the Li-sulfide directly to the polymeric structure of the COFs.This improves the kinetics of the fabricated batteries and results in low overpotential during faster charging since the entropy of dissociated sulfide bridging is low and the ends remain in proximity due to the connection to the polymeric framework structure.Meanwhile, quantitative electrochemical studies on the soluble molecular model compounds mimicking the sulfide bridging of the COFs and operando Raman studies of the sulfide-bridged COFs unravel the potent but reversible redox behavior of the covalently bound disulfide and polysulfide entities.This is the first report of a dual Li-storage mechanism in lithium-organosulfide (Li-OrS) batteries [43,44] derived from a sulfide-bridged quinoxaline framework (Figure 1).

Synthesis and Characterizations
A family of three COFs (DUT-185-S 0 , DUT-185-S 2 , and DUT-185-S n ) with quinoxaline cores varying in sulfur content and corresponding molecular model systems (MC-S 0 , MC-S 2 , and MC-S n ) were synthesized via a rational design strategy (Figure 2A-D and Schemes S1-S3, Supporting Information).Among these, the biphenyl-bridged COF named DUT-185-S 0 was synthesized by reacting the 4,4',4'',4''',4'''',4'''''-(diquinoxalino[2,3-a:2',3'-c] phenazine-2,3,8,9,14,15 hexayl)hexabenzaldehyde linker with 1,4-benzidine following the previously reported procedure by Feng and co-workers. [45]Meanwhile, the novel disulfide-bridged COF named DUT-185-S 2 was constructed by reacting the above-mentioned quinoxaline aldehyde with 4,4'-disulfanediyldianiline by solvothermal condensation (Figure 2A, details in the Supporting Information).This crystalline COF was postsynthetically modified to a sulfur-rich and polysulfide-bridged COF named DUT-185-S n according to the procedures reported by the groups of Fan [41] and Lotsch [42] (Scheme S2, Supporting Information).The pulverization followed by the infiltration of sulfur changes the orange color of DUT-185-S 2 to a dark brown color accompanied by the conversion of imine to benzothiazole and the disulfide to polysulfide bridges.[48][49][50] The molecular model compound of DUT-185-S n , i.e., MC-S n and elemental analysis of this polysulfide-bridged COF strongly suggest the formation of a tetrasulfide bridging between the two phenyl rings of the aminebased linker (Figure 2D and Scheme S3, Figures S12-S18, Tables S2 and S3, Supporting Information).Therefore, the structural differences between these three quinoxaline-based COFs lie in the bridging between the quinoxaline cores (Figure S1A, Supporting Information).After soxhlet extraction and supercritical drying, the obtained COF powder showed, in all cases, moderate porosity (Figure S5, Supporting Information).DUT-185-S 0 and DUT-185-S 2 were obtained as crystalline materials, but DUT-185-S n has an amorphous structure (Figure 2B).The reflections present in the powder X-ray diffraction (PXRD) patterns of the crystalline frameworks allowed for determining the unit cell and aided the modeling of the structure of the novel disulfide-bridged framework (DUT-185-S 2 ) (Figures S19 and S20 and Table S4, Supporting Information).The flexibility of the disulfide bridging (SS) and lack of π-stacking of the benzene ring along this bond in DUT-185-S 2 results in the splitting of the first and second reflection into doublets (3.37° and 3.68°, 5.73°, and 6.60°), which is assigned to a higher degree of freedom of COF layers. [51]Hence, the preliminary geometry and energy-minimized structure of the DUT-185-S 2 with density functional tight-binding (DFTB) method matches well with the lower symmetry (P1) structure of the framework with slipped AA-stacking of the layers, similar to the previously reported DUT-185-S 0 (Figure S2, Supporting Information). [45]he single-crystal structure of the disulfide-bridged model compound of DUT-185-S 2 (i.e., MC-S 2 ) shows significant bending of the -CSS-linkage (up to 105°) in contrast to the complete planner structure of the biphenyl-bridged model compound of DUT-185-S 0 (i.e., MC-S 0 ).Hence, the buckling of the layers of disulfide-bridged COF (i.e., DUT-185-S 2 ) along the C-S-S-C plane is expected after DFTB geometry optimization of this COF.However, the pronounced π-stacking of the quinoxaline core restricts free rotation along the dangling disulfide bridging.Therefore, the ordered structure of DUT-185-S 2 gained appreciable porosity which was confirmed by reversible nitrogen, carbon dioxide (CO 2 ), toluene, and heptane physisorption isotherms (Figure S21, Supporting Information).The hysteresis in the isotherms of CO 2 and toluene indicates the polarizing ability of the disulfide moiety of DUT-185-S 2 . [9]However, the crystallinity shows a marked decrease for DUT-185-S n caused by the low rotational barrier of tetrasulfide bridging and disordering of the layers.This disordering also leads to a decrease in the porosity in comparison to DUT-185-S 2 and DUT-185-S 0 as observed from the N 2 physisorption measurement recorded at 77 K (Figure S5, Supporting Information).

Redox Activities of Organosulfide Linking for Li Storages
The role of different sulfide bridging between the quinoxaline cores of DUT-185-S 2 and DUT-185-S n is investigated both experimentally and computationally to understand the Li + storage mechanism.This is accompanied by control experiments with DUT-185-S 0 where sulfide bridging was replaced by biphenyl bridging between the quinoxaline cores.Since the poor solubility limits the proper understanding of Li-storage abilities of different functional groups of polymeric-COFs, the highly soluble molecular model compounds of these three frameworks (namely, MC-S 0 , MC-S 2 , and MC-S n ) were considered for establishing the electrochemical activity by standard three-electrode cyclic voltammetry (CV) measurements (Figure 3A and Figures S22-S24, Supporting Information). [52]he model compound with biphenyl bridging (MC-S 0 ), when compared to a blank measurement in Li + -containing electrolyte (using 1 m lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) in tetrahydrofuran as supporting electrolyte, a glassy carbon working electrode, a platinum counter electrode, and a nonaqueous Ag/AgNO 3 reference electrode), showed no additional redox peaks, indicating that the imine bonds remain unchanged (Figure 3A and Figure S22, Supporting Information).However, the model compounds with biphenyl disulfide bridging (MC-S 2 ) and biphenyl tetrasulfide bridging (MC-S n ) show an intense redox activity at 1.4 and 1.8 V, respectively (Figure 3A and Figures S23 and S24, Supporting Information).The current density near this voltage enhances from disulfide to tetrasulfide bridging (Figure 3A-ii,iii).Since the imine bonds of MC-S 0 and MC-S 2 and the benzothiazole ring of MC-S n were redox-silent during the electro-reduction within this limited potential window, the extracted areal contribution of the CVs of MC-S 0 and MC-S n indicates a comparable electrochemical activity of covalently linked bridging sulfides.This is also proportional to the amount of Li + interaction and subsequent charge transfer, considering the equimolar concentration of all the model systems (Figures S23 and S24, Supporting Information).Hence, the four times higher electro-activity of MC-S n than MC-S 2 as estimated from the area under the CV curve also justifies its better Li-storage ability (Figure 3B).Each sulfur atom in the polysulfide chains of MC-S n was possibly polarized by multiple Li-ions while the aromatic ring current of benzothiazole maintained the electron flow to these sulfide chains.Interestingly, even after 200 cycles, the completely reversible Li + interactions imply MC-S n is unlikely to segregate into irreversible Li 2 -S n type species (supporting video showing no color change of the electrolyte solution) and reveal the importance of covalent anchoring of the sulfide chains to the organic backbone (Figure 3C).However, the Li + interactions with sulfur (at 1.8 V) follow the typical diffusion-controlled redox mechanism as observed from the linearity of the square root of scan rate versus peak current plot (Figure 3D). [52]The comparison of this result with the theoretical investigations for the Li-organosulfide interaction mechanism emphasizes the rapid redox kinetics of sulfursulfur bond dissociation and recombination (Figure 3E and Figures S25 and S26, Supporting Information).
The calculated lower energy structures (70-90 randomly created structures optimized first with the DFTB GFN2-xTB method and then with density functional theory (DFT) using the Perdew-Burke-Ernzerhof functional) [53][54][55][56][57][58] of lithiated MC-S 2 show favorable insertion of one Li into the SS bond while the second Li also establishes a short bond formation to another sulfur atom.Interestingly, even though the SS bond dissociates, there is still a lithiated-sulfide (Li 2 S 2 ) type cluster (Type 1) and chain formation (Type 2) whereby two sulfurs are connected to one Li, and the CS bond is intact.Therefore, Li presumably detaches since the sulfur atoms are close enough to re-establish their bond during the reverse cycling.While carrying out a similar calculation, 6 Li were coordinated with the tetrasulfides (S 4 2− ) unit of MC-S n at lower energies due to easily dissociable SS bonds (Figure 3F).However, these Li-sulfide interactions were not identical to the conformations like the cluster and linear patterns observed for the lithiated MC-S 2 (Figure S26, Supporting Information).The lowest energy structures have all of the Li ions between the aromatic rings, all SS bonds are broken, and the CS bonds remained intact (Figure S27, Supporting Information).Only at higher energies, a profound interaction of aromatic π electron clouds with Li was revealed (Type 3) for both MC-S 2 and MC-S n systems.
[61][62] The thermodynamic stabilities of the COF units with varying concentrations of Li + were determined by calculating average binding energies (Figure 4A,B and   Figures S28-S36, Supporting Information, more information in Supporting Information).The maximum Li + storage capacity of the molecular units of the COF is considered to correspond to Li + concentration up to which the binding energy is negative (Figure 4B).The very first step of the Li + interaction mechanism in these three COF models is the energetically favorable chelation of 3 Li + with the quinoxaline cores corresponding to a large negative binding energy (−1.46 eV) (Figure 4A-i and Figures S29  and S30, Supporting Information).This chelating ability of the nitrogen centers in the quinoxaline core is facile and kinetically very fast having no bond dissociation associated with it.Any further Li + ion binding at the imine site of the molecular unit of DUT-185-S 0 is unfavorable.In the molecular unit of DUT-185-S 2 , after the occupation of 3 Li + ions at quinoxaline binding sites, additional 2 Li + ions bind at the disulfide site to furnish feasible binding energy of −1.28 eV (Figure 4A-ii and Figures S32  and S33, Supporting Information).From a closer look, Li ions result in the dissociation of disulfide bridges to preferably form a four-membered Li 2 S 2 ring having two Li-sulfide chains (S-Li-S) (detailed discussion in Figure S32C, Supporting Information).Additional attempts to bind Li + ions at available imine sites result in unfavorable interaction in DUT-185-S 2 .A similar Li + interaction behavior is followed in DUT-185-S n (Figure 4A-iii and Figures S35 and S36, Supporting Information).After the saturation of the quinoxaline core with 3 Li + ions (−1.46 eV), the polysulfide (S 4 ) site undergoes redox interactions with 8 Li + ions to furnish binding energy of −0.92 eV and forms a stable Li 8 S 4 cluster which is also covalently connected to the carbon skeleton of the COF and reluctant to dissociate in soluble Li 2 S n species.Further binding of Li + ions with polysulfides leads to an energetically unfavorable redox process.

Operando Raman Studies during Framework Lithiation
In situ spectrochemical studies enable identifying the Li-sulfide interactions in the periodic framework structure of the COFs assisted by computational models based on their molecular units as described before (Figure 5 and Figures S37-S39, Supporting Information).The prominent Raman shifts of the sulfursulfur (SS) connectivities and carbonsulfur (CS) bonds of the sulfide bridging in DUT-185-S 2 and DUT-185-S n bring an effective opportunity to monitor this Li-sulfide redox mechanism under applied potential (Figure S1C, Supporting Information). [63]Since the electrochemical oxidationreduction of the sulfide bridgings happens within the voltage window from +1.0 to −1.5 V as observed from the CVs of DUT-185-S 2 and DUT-185-S n (comparable to MC-S 2 and MC-S n ), the Raman shift of CS and SS bonds was scrutinized after each 0.5 V interval of the chronoamperometry measurements (Figure 5A-i,B-ii). [63,64]Theoretically, there are two possible Li interaction sites in DUT-185-S 2 and DUT-185-S n as observed from the lithiated modeled structure; either at the phenazinecontaining quinoxaline core or sulfide bridging between biphenyl units (Figure 5A-ii,B-ii).Since the Raman shifts of aromatic quinoxaline cores (from 1000 to 1800 cm −1 ) experience a facile Li-chelation and do not undergo any bond dissociation upon lithiation within the electrochemical window investigated, no noticeable change was observed except the appearance of an additional broad signal at 746 cm −1 and two intense signals at 912 and 1033 cm −1 owing to interference of the electrolyte (Figure 5A-iii,B-iii and Figure S37, Supporting Information). [45]eanwhile, the unique Raman signal of this disulfide bond of DUT-185-S 2 splits into two (396 and 348 cm −1 ) due to a mild Li + -coordination to the sulfide bridging upon the addition of the electrolyte (Figure 5A-iii). [41,65]Hence, the appearance of new signals is assigned to the formation of new noncovalent interactions associated with Li to the disulfide bonds.The original signal of the covalently bound disulfides (SS) also is still observable after the addition of electrolyte as heterolytic cleavage of these bonds requires an increase in reduction potential from open circuit potential (Figure 5A-i).The relative intensity of this new signal at 348 cm −1 increases while going to more negative potential (up to −1.5 V), suggesting a stronger degree of covalent Li-S interaction as the SS bond was cleaved through reduction.However, the relative intensity of this signal decreases again during oxidative cycling (from −0.5 to 1 V).The complete disappearance was not possible due to the large polarizing ability of the SS bond in the presence of Li + within the electrolyte.The change of Raman shift was also prominent along the CS linkage of a disulfide bond at variable potentials.At increasing reduction potentials, the Li + begins to coordinate to the reduced S atoms which result in the appearance of a new signal at 631 cm −1 associated with the CS linkage, shifted to higher wavenumbers than the CS linkage at 616 cm −1 (Figure 5A-i).A similar reduction trend was also observed for the polysulfide-bridged DUT-185-S n .Going toward a more reductive voltage, the strong covalent bonding of Li + to the polysulfide chains results in the intensification of a new signal at 738 cm −1 followed by a decrease of the CS signal and sulfide bridging signals at 685 and 489 cm −1 , respectively (Figure 5B-i).The appearance and disappearance of these signals are reversible following the reduction and oxidation of sulfide bridgings under applied potential.This observation indicates that upon reduction of the covalently connected sulfide bridgings at negative potentials, Li + coordination takes place resulting in the formation of covalent Li-sulfide chains (Li 2 S 2 ) and Li-sulfide clusters (Li 8 S 4 ), observed to be reversible on the spectroelectrochemical time scale.This observation strengthened the computational findings too (Figure 5A-ii,B-ii).

The Cathodic Activity of Frameworks in Batteries
These redox active frameworks were then implemented as cathode materials in coin cells, using Li-metal as the counter electrode.The coin cells were subjected to further electrochemical measurements to uncover the Li-storage mechanism for COF electrodes (Figure 6 and Figure S40, Supporting Information).The Li + originating from the electrolyte system [LiTFSI in dioxolane/dimethoxyethane (1:1) with 0.1 m lithium nitrate (LiNO 3 )] forms a rectangular-shaped CV plot within a voltage window from 1.8 to 2.6 V, owing to electrolytic double layer formation with the electron-rich phenazine nitrogen on the quinoxaline core of DUT-185-S 0 (Figure 6A-i).The very broad oxidation and reduction peaks even at a very low sweep rate (0.025 mV s −1 ) indicate pseudocapacitive behavior for the fast Li-chelating ability of this phenazine core of the quinoxaline unit. [45,66]Based on the previous reports the quinoxaline units can also be saturated with 6 Li + followed by further discharging to 1 V. [66] But the restricted potential window of the LiTFSI electrolyte (1.8 to 2.6 V) induces only weak interactions of the Li + with the quinoxalines.On the other hand, the redox-active disulfide bridging of DUT-185-S 2 undergoes prominent electrochemical redox conversion when interacting with the Li + (at 2.1 V) along with the Li-chelating characteristics of the quinoxaline unit (Figure 6B-i).The presence of these dual active sites in this COF was confirmed from the high area under the CV curve.The reversible redox peaks in the CV are associated with disulfide bond dissociation and recombination upon Li interactions which corroborates the computational findings.Interestingly, the current density contribution for this Li-sulfur interaction at 2.1 V was enhanced when the disulfide bridging was replaced by the tetrasulfide bridging of DUT-185-S n (Figure 6C-i).This observation is very similar to the model compound sys-tems (Figure 3A-ii,iii).Hence, the number of redox-active sulfur atoms plays an imperative role in storing more Li + .The conversion of the imine to the benzothiazole linkage in DUT-185-S n also aids the faster electron transfer mechanism to the tetrasulfide bridge.This conducts the electron smoothly throughout the structures and inhibits the irreversible degradation of both sides of the anchored sulfide chains.In this case, the negligible redox contribution from the soluble LiLi-PS was confirmed by the absence of any sharp peak at 2.3 V, commonly observed in sulfur-doped carbonaceous materials (Figure S41, Supporting Information).The cleavage of the SS bond in such organosulfides by Li-sulfide interactions is highly reversible since the formed Li-sulfides chains in DUT-185-S 2 and Li-sulfide cluster in DUT-185-S n are covalently bound to the polymeric backbone of the COFs and remain in close affinity to each other.This underlines the significant role of the polysulfide bridges in COFs for the novel Li-OrS battery mechanism.The interpretation of CVs to understand the lithiation mechanism is in good agreement with the computational studies.The charge density difference plots of the molecular units of the COFs after maximum Li-occupancy display significant charge depletion (cyanoshaded area) around the Li ions and charge accumulation (yellowish green shaded area) near the electron-rich surfaces of quinoxaline cores in all the molecular units of the COFs (Figure 6A-ii,B-ii,C-ii and Figure S42, Supporting Information).Meanwhile, the polarization of disulfide bridging suggests possible Coulombic interactions between Li + ions and DUT-185-S 2 , which results in a strong Lisulfide bonding.The higher charge distribution within the polysulfide chains (-S 4 -) of the lithiated molecular unit of DUT-185-S n determines its stronger Li interaction ability to form a covalently anchored Li-sulfide cluster (Li 8 S 4 ) which results obviously in enhanced Li-storage ability of DUT-185-S n compared to DUT-185-S 0 and DUT-185-S 2 .
The gradual increase of the scan rate from 0.025 to 1.0 mV s −1 brings more insights into the redox process associated with the Li-storage in these COFs (Figure 6A-iii,B-iii,C-iii and Figure S43, Supporting Information).The absence of any strong redox behavior from extremely low to high scan rates in DUT-185-S 0 shows only pseudocapacitive-type Li-chelation with the quinoxaline cores (Figure 6A-iii).Whereas the prominent and reversible redox peaks for Li + interacting with sulfides in DUT-185-S 2 and DUT-185-S n imply additionally faster electrochemical dissociations and recombination of the SS bond at higher scan rates (Figure 6B-iii,C-iii).The covalently anchored Li-sulfide chain formation (-Li 2 S 2 -) in DUT-185-S 2 and Li-sulfur cluster (-Li 8 S 4 -) formations in DUT-185-S n keep the dissociated disulfide units in proximity.This helps to overcome the diffusion limitations of the Li + and maintain the low overpotentials of the oxidation-reduction of sulfides even when the scan rate is high (Figure S43, Supporting Information).Unlike sulfur-doped carbons, the redox peaks for sulfide oxidation-reduction in the sulfide-bridged COFs do not separate strongly from each other and a low overpotential is maintained at high current densities, which could be a reason for the fast electron transfer mechanism on the surface of the sulfide-bridged COFs.The linearity of the logi p (i p is the peak current measured at 2.1 V) versus logν (ν is the scan rate) plot and the extracted slope value close to 0.75 also confirm predominating surface-controlled electron transfer mechanisms during lithiation of the sulfide bridges and quinoxaline cores of the COFs (Figure 6A-iv,B-iv,C-iv). [67]oreover, a fit to the power law, i p = aν b (i P is the peak current) and the derived Cottrell's equation can quantify the percentage of the pseudo-capacitive contribution from the quinoxaline core and redox phenomena of the disulfide bridges of DUT-185-S 2 and DUT-185-S n . [13]A gradual increase in the redox contribution was noticed from DUT-185-S 0 (38%), DUT-185-S 2 (65%) to DUT-185-S n (71%) owing to an increase in the number of redox-active sulfur atoms in the frameworks (the insets of Figure 6A-iv,B-iv,C-iv).Moreover, the quinoxaline core in each of these COFs maintains its faradaic activity during the chelation of the Li + on the COF surface.
The galvanostatic charge-discharge (GCD) process of the derived coin cells also follows a similar trend as observed from the CVs of each of the frameworks (Figure 7A-i ) from the continuous voltage gradient (Figure 7A-i).This proves the fast pseudocapacitive Li-storage mechanism only in the quinoxaline core of this COF.A narrow voltage plateau at 2.1 V at low current densities for DUT-185-S 2 suggests the marked redox activity from the disulfide bridge (Figure 7A-ii).Though the capacity contribution from these redox-active disulfide units gradually decreases with the reduction of the Li + -interaction time (or with the increase of the current densities), the rapid Li + chelating by the quinoxaline core still enables a good balance for gaining the overall capacity (220 mAh g −1 at 100 mA g COF −1 ).The conversion of the disulfide to the tetrasulfide bridging and the topochemical transformation of imine to benzothiazole linkages in DUT-185-S n electrochemically activate more sulfur atoms for surface-induced redox reactions with Li + .
These covalently anchored tetrasulfide moieties of the COF also retain their strong redox activity due to the formation of a covalently bound Li 8 S 4 type cluster at 2.1 V even when the current density is high (Figure 7A-iii).Since the mass percentage of the redox active tetrasulfide bridging in DUT-185-S n is very low in comparison to the overall mass of the COF, the gravimetric capacity considering the COF loading in the electrode can only be maintained by introducing more Li storage functionality to the redox silent counterparts of the framework.In that way, the accumulation of Li per unit cell of the framework will be higher.Here, the Li chelating ability of the quinoxaline cores together with the redox active sulfide chains plays this desired role in achieving overall considerable sp.capacity (620 mAh g COF −1 at 100 mA g COF −1 and 950 mAh g sulfur −1 at 100 mA g sulfur −1 ) (Figure 7B and Figure S44, Supporting Information).Integration of these dual active sites in the framework (which works within the same potential window for charging-discharging) reduces the amount of redox inactive dead mass in the COFderived batteries and enhances the overall sp.capacities.The low applied current while considering only sulfur loading generally led to prolonged lithiation-delithiation.That resulted in the fluctuation of the stability plots and sudden gain or loss of the capacities in intermediate cycles.Considering the entire electrode mass, the applied current density of 75 mA g cathode −1 or 0.13 mA cm −2 cathode was required to achieve the capacity of 620 mAh g cathode −1 . The drop in the second discharge capacity after the completion of the first cycle is correlated with the irreversible destruction of the inherent porosity of the COF-derived electrodes (Figure S44A-C, Supporting Information).Though the porous nature of the COF-derived electrode assists in the facile lithiation via the insertion mechanism during the first discharge cycle and regulates the formation of some irreversible lithiated species in its pore, the poor electronic conductivity of the COF-derived electrodes hinders addressing those species directly for reversible delithiation during the next charging step.From the second cycle onward only the electroactive segments and the redox-active functional groups of the COFs are responsible for achieving the reversible and stabilized capacity.
Building the porous framework structure with sulfide linkages provides a rapid diffusion path for guest Li + ions.This benefits faster redox kinetics and good rate stability for DUT-185-S n (300 mA g COF −1 at 1 A g −1 ) at high current densities (Figure S45, Supporting Information).Interestingly the disulfide and tetrasulfide bridging in DUT-185-S 2 and DUT-185-S n are interlocked by strong covalent linking with the COF backbone which protects the sulfur from dissolution into the electrolyte even when the SS bond cleavage occurs.The sulfide counterparts form a covalently anchored Li-sulfide chain or cluster, being the integrated part of the polymeric backbone of the COF.Therefore, the voltage plateau (near 2.3 V) for the electroactivity of soluble Li-PS in the typical Li-S battery was missing (Figure S41, Supporting Information) and the Li interaction with the sulfides was quite reversible owing to the proximity of the sulfur atoms.
The covalent bridging of these sulfide species to the polymeric structure of DUT-185-S n reduces the dissolution problem during lithiation and benefits the facile reconnection of those bonds to achieve good reversibility.Hence, the cycle stability, specific capacity, and energy density of the DUT-185-S n -derived cathode are significantly higher than most of the high-performing disulfides [68][69][70][71] and also comparable to other polysulfides (e.g., S-BOP) [72] (Table S5, Supporting Information).However, the electronic conductivity of such frameworks needs to be improved to achieve high-rate performance like the vulcanized polyacrylonitrile (S-PAN) [73,74] and sulfurized covalent triazine framework. [37,75]he gradual sloping of the discharge voltage with the increase of the capacity from 2.6 to 2.1 V for DUT-185-S n derived coin cell corresponds to the electrochemical Li-organosulfide bond formation on the electrode surface (Figure 7A-iii).Interestingly, this "on surface" Li-organosulfide battery mechanism protects the short-length sulfur chain from solubilizing in electrolyte and restricts the irreversible lithium-sulfide (Li 2 S) formation.On the other hand, the CV measurements of a coin cell derived from the sulfur-doped carbon as a cathode material with otherwise same configuration show a prominent redoxactivity of soluble Li-PS at 2.28 V which is also reflected well in the strong voltage plateau from 2.4 to 2.2 V in the GCD profile (Figure 7C,D).An extensive shuttle current measurement was performed after both the cells achieved steady states by charging at 2.6 V and then each of the discharge voltages (2.45, 2.35, and 2.25 V) was kept constant for 6 h. [76,77]There the absence of this voltage plateau and the CV peak in the case of DUT-185-S n is attributed to the negligible shuttle current observed at 2.25 V compared to sulfur-doped carbon (Figure 7E and Figure S47, Supporting Information).Furthermore, a symmetrical cell constructed by two identical DUT-185-S n derived electrodes shows capacitive behavior in a low scan rate CV experiment which is in stark contrast to the sulfur-doped carbon material (Figure S48, Supporting Information).The formed Li-PS dissolves into the electrolyte from sulfur-doped carbon and participates in the irreversible electrochemical oxidation of the electrolyte and resulting in the deformation of the symmetrical capacitive characteristics.The covalently bound tetra sulfides in DUT-185-S n are highly protected from this dissolution failure mechanism.The dissociated Li + from the electrolyte experiences reversible Li-organosulfide bond formation on the surface of the tetrasulfide-bridged DUT-185-S n .Since this accumulation of charge density on the surface of symmetrical electrodes balances each other, no redox peak was observed even at the 5 mV s −1 sweep rate of the CV.This experiment indicates covalently bound sulfur's importance in reducing the polysulfide shuttle.The long cycle stability of DUT-185-S n at 100 mAh g −1 current density with 84% retention of sp.capacity also agrees well with the negligible shuttle effect (Figure 7B and Figure S44, Supporting Information).Therefore, the separator of the disassembled coin cells of DUT-185-S n after prolonged cycling showed no color change (Figure S49, Supporting Information).The electrochemical impedance spectroscopy measurements after 200 cycles confirm a huge proliferation of the charge transfer resistivity of the COF in the derived electrodes resulting in gradual capacity fading (Figure S50 and Table S6, Supporting Information).However, this loss of capacity originating from the poor electronic conductivity of the COF is lower than the capacity drop of the sulfur-doped carbon-derived electrode associated with sulfur dissolution.The strong covalent bridging of the redox-active sulfides between the electroactive segments of the COFs reduced the degradation of the derived electrode after prolonged cycling.The long-range stacking of the sulfidebridged COFs resulted in good integrity of the overall morphology of the derived electrodes after 200 cycles as observed from the different magnifications of field-emission scanning electron microscope imaging (Figures S51-S54, Supporting Information).The multiple-time ion percolation through the COF flakes generates holes within the flakes and enhances interparticle separation to maintain the diffusivity of solvated ions (Figure S54, Supporting Information).Unlike other sulfide cathodes in Li-S batteries, the strong covalent bridging of the redox-active polysulfides to the electroactive COF units in DUT-185-S n slows down the decomposition of the entire electrode materials with cycling by reversible breaking and formation of the SS linkages.The chemical integrity of the sulfur entities was again confirmed by ex situ characterization (FT-IR in Figure S55, Supporting Information and Raman studies in Figure S56, Supporting Information) of the derived electrodes after the 200th cycle charge-discharge.However, the ex situ X-ray photoelectron (XPS) studies (XPS in Figures S57 and S58, Supporting Information) on the derived electrode resulted in a huge interference from the excess electrolytes which were strongly bound to the electrode surface.Unfortunately, the intense SO x n− signals from the sulfur-containing electrolyte masked the signals of the SC and SS bonds of the COFs.
Established state-of-the-art sulfur-loaded carbon cathodes are not compatible with the use of carbonate electrolytes used in lithium-ion battery as polysulfides dissolved in the electrolyte-degraded carbonates via nucleophilic attack. [78,79][31] This offers room to achieve the maximum Li storage capacity of the functionalized COF followed by the participation of most of the redox-active groups of the COFs.However, the largely irreversible electro-reduction in the very first cycle of the CV and hence the low capacity even at very low current densities (450 mAh g COF −1 at 100 mA g COF −1 ) indicates that more optimization of the electrode-electrolyte system is required considering the electrochemical stability of the COF.

Conclusion
The covalent atomically precise integration of the highly redox active sulfide chains in the skeleton of COF structures induces "on surface" electrochemical Li-sulfide interactions, inhibiting the dissolution of Li-PS into the electrolyte.This allows for faster redox kinetics and prolonged cycling of the Li-OrS batteries compared to typical Li-S batteries.The systematic elucidation supported by operando Raman studies and computational findings on the model redox-active motifs of the COFs showcase the possibilities of a novel lithium-organosulfide (Li-OrS) battery mechanism.This strategy, combined with the pseudocapacitive behavior of the Li-chelating functional groups of the COFs significantly boosts the overall Li-storage capacity.However, the minimal electronic conductivity of the overall framework restricts the electrochemical activation of the redox functionalities and results in a drop in Li storage ability at high current density.The future design of conducting frameworks decorated with ample organosulfides is envisioned as the next step in developing high-performing Li-OrS batteries.

Figure 1 .
Figure 1.A) Sulfur-doped porous-carbon-derived Li-S battery shows the polysulfide shuttle effect.B) Polysulfide-bridged redox-active COF shows an unprecedented Li-OrS battery mechanism having negligible shuttle effect.

Figure 2 .
Figure 2. A) Linkers and synthesis conditions of quinoxaline-based COFs (DUT-185-S 0 , DUT-185-S 2, and DUT-185-S n ) and their chemical structures.Inset: Photographs of the synthesized COFs powders.B) PXRD pattern of the COF powders.C) The functional building units of the COFs suitable for Li-interactions.D) The structure of synthesized molecular models (MC-S 0 , MC-S 2, and MC-S n ) constituting the identical bridging connections of quinoxaline-COFs.

Figure 3 .
Figure 3. A) CVs and probable Li + interaction mechanism of the structurally related model compounds of: i) DUT-185-S 0 (MC-S 0 ) and ii) DUT-185-S 2 (MC-S 2 ), iii) DUT-185-S n (MC-S n ).B) Bar plots of the area under the CVs show the Li + -storage capacities of each model compound.C) Cycle stability of polysulfide-bridged model compounds (MC-S n ) observed from CV. D) The CVs at variable scan rates show the strong redox activity from polysulfide bridging of MC-S n .(Inset) The linear fit of the peak current of redox-active MC-S n versus scan rates plot shows the Li + diffusion-controlled mechanism.E,F) Energy profile of different possible conformers of lithiated biphenyl-disulfide units and biphenyl-tetrasulfide units.Cyan-highlighted circles show the position of conformers shown as insets in the graph.Hydrogen, carbon, sulfur, and lithium are depicted in white, brown, yellow, and green, respectively.

Figure 4 .
Figure 4. A) i-iii) The stepwise Li incorporations mechanism into the molecular units of COFs.The Li-interacted structures in the square frames show the maximum Li-storage possibilities.B) The variation of binding energy with stepwise Li incorporation into COF units.

Figure 5 .
Figure 5. A-i) and B-i) The redox activity of electron-rich DUT-185-S 2 and DUT-185-S n was obtained from three electrode CV measurements.A-ii) and B-ii) The lowest energy conformer of the lithiated molecular unit of DUT-185-S 2 and DUT-185-S n .Inset: The lowest energy conformer of the lithiated biphenyl-disulfide unit and biphenyl tetrasulfide unit.A-iii) and B-iii) The Raman signals of DUT-185-S 2 and DUT-185-S n were recorded at a low-frequency and high-frequency region during chronoamperometry measurement using spectro-electrochemical feature spectral changes for different types of Li-framework interactions.
-iii and Figure S44, Supporting Information).No prominent voltage plateau was observed for DUT-185-S 0 other than the capacity gain (105 mAh g −1 at 100 mA g COF −1

Figure 6 .
Figure 6.A-i), B-i), and C-i) The redox activity of DUT-185-S 0 , DUT-185-S 2 , and DUT-185-S n -derived coin cells as observed from CV measurements.A-ii), B-ii), and C-ii) The charge density difference plots (XY plane) of maximum Li-ion capacity molecular unit of DUT-185-S 0 , DUT-185-S 2 , and DUT-185-S n , showing the possible redox active sites.The cyan and yellowish-green shaded areas indicate the depletion and accumulation of charge, respectively.A-iii), B-iii), and C-iii) CV measurements data of DUT-185-S 0 , DUT-185-S 2 , and DUT-185-S n derived coin cells at different scan rates.A-iv), B-iv), and C-iv) The linearity of log(peak current) versus log(scan rates) plots of DUT-185-S 0 , DUT-185-S 2 , and DUT-185-S n derived coin cells.The percentage of capacitive contribution and redox activity to the charge storage using Cottrell's equations (inset).

Figure 7 .
Figure 7. A-i-iii) The GCD profiles of DUT-185-S 0 , DUT-185-S 2 , and DUT-185-S n derived coin cells at different current densities.B) The cycling stability of DUT-185-S 0 , DUT-185-S 2 , and DUT-185-S n derived coin cells using GCD techniques.C) The comparison of CV profiles of coin-cells derived from DUT-185-S n and sulfur-doped carbon.D) The comparison of GCD profiles of coin cells derived from DUT-185-S n and sulfur-doped carbon shows different regions of voltage plateaus (E).The comparison of shuttle current of the coin cells derived from DUT-185-S n and sulfur-doped carbon at 2.25 V.