Redox Comediation with Organopolysulfides in Working Lithium-Sulfur Batteries
A redox comediation strategy is proposed to accelerate the sulfur redox kinetics. Di-t-butyl disulfide (DtbDS) is introduced as a model comediator and realizes highrate, high-sulfur-loading, and lean-electrolyte Li-S batteries. The redox comediation strategy demonstrates a novel approach to promote the sulfur redox kinetics for high-performance Li-S batteries and inspires more redox comediators represented by DtbDS.
INTRODUCTION<!>RESULTS AND DISCUSSION<!>ll<!>Electrochemical Performance of Li-S Batteries<!>EXPERIMENTAL PROCEDURES<!>Materials Availability<!>Data and Code Availability<!>Raw Materials<!>Li2S Nucleation and Growth Tracking<!>Electrochemical Performance of DtbDS<!>Article<!>Theoretical Simulations
<p>The pursuit for high-energy-density battery systems drives grand exploration of emerging chemistries beyond the current energy storage technique based on lithium-ion intercalation. [1][2][3][4] In the past decades, lithium-sulfur (Li-S) batteries have attracted extensive attention due to their ultrahigh theoretical specific energy (2,600 Wh kg À1 ) and advantages of earth abundance, low cost, and environmental friendliness of cathode sulfur. 5,6 Generally, the sulfur redox reactions refer to the conversion from solid sulfur to dissolved lithium polysulfide (Li 2 S n , 3 < n % 8) to solid lithium sulfide (Li 2 S). In the Li-S system operating upon multi-phase and multi-electron conversion, the deposition of Li 2 S from dissolved lithium polysulfides accounts for at least three-quarters of the theoretical capacity, i.e., 1,254 mAh g S À1 . 7,8 However, the sluggish deposition kinetics and film-like two-dimensional (2D) growth of insulating Li 2 S in routine ether electrolyte often result in severe irreversible loss of active sulfur and rapid capacity decay. [9][10][11] Therefore, regulating the deposition and dissolution behavior of Li 2 S is critical to achieve high-capacity, high-rate, and long-cycling Li-S batteries especially under practical conditions.</p><p>Generally, the deposition/dissolution behavior of Li 2 S is regulated by soluble lithium polysulfides. Serving as intrinsic redox mediator (RM), lithium polysulfides participate in the chemical comproportionation and disproportionation processes (referring to the redox mediation process) parallel to their electrochemical reduction and oxidation and directly determine the deposition rate and growth dimension of Li 2 S on conductive substrates. However, the intrinsic RM of lithium polysulfide is restricted to the dynamic battery evolution regarding its species and concentration and therefore fails to execute the mediated process timely and thoroughly,</p><p>The Bigger Picture</p><p>Lithium polysulfide intermediates play a decisive role in the multiphase and multi-electron sulfur redox reactions but are unfortunately insufficient to afford rapid sulfur redox kinetics. Herein, a redox comediation strategy is proposed to accelerate the sulfur redox kinetics. An organopolysulfide (di-t-butyl disulfide, DtbDS) is introduced as a model comediator in working Li-S batteries, which reversibly reacts with lithium polysulfides and provides kinetically favorable pathways for polysulfide conversion. The intervention of DtbDS is demonstrated to facilitate the deposition and dissolution of lithium sulfide (Li 2 S) with reduced polarization and favors three-dimensional Li 2 S deposition with high capacity. The redox comediation strategy demonstrates a novel approach to promote the sulfur redox kinetics for high-performance Li-S batteries and inspires more redox comediators represented by DtbDS.</p><p>Chem 6, 3297-3311, December 3, 2020 ª 2020 Elsevier Inc. 3297 ll especially at the end of charge or discharge (Figure 1A). 12,13 Moreover, the above phenomenon aggravates under conditions, such as high current density, high sulfur loading, and lean electrolyte, wherein the growth of Li 2 S terminates prematurely due to large polarization caused by the sluggish mediation by lithium polysulfide RMs. 11,14,15 To overcome the above issues, numerous heterogeneous mediators, such as metal, 16 metal oxides, 17 sulfides, 18 and nitrides, 19,20 have been designed and introduced to facilitate the deposition/dissolution of Li 2 S via chemisorption and/or surface catalysis of lithium polysulfides. [21][22][23][24][25][26][27] Despite the enhanced battery performance realized by heterogeneous mediators, such a mediation process heavily relies on the conductive surface 28 that the heterogeneous mediators are easily covered by insulating Li 2 S and lose their functions especially at higher depth of dischage. 29 Meanwhile, modulating the solvation structure of the polysulfide anions would relieve the dependence of the regulation strategy on conductive surfaces. 30,31 Typically, high-dimensional Li 2 S deposition on limited surfaces can be achieved via tuning the composition and concentration of electrolyte solvent [32][33][34] and/or supporting salts. 35 Nevertheless, such solution is merely effective at low current densities or high temperature required for smooth lithium polysulfide conversions, reducing the application potentials of practical Li-S batteries. [36][37][38][39] Considering the insufficient redox mediation capability of the intrinsic lithium polysulfide RMs, it is a rational strategy to introduce extrinsic RMs to assist and strengthen the homogeneous redox mediation process. 40,41 Taking the Li 2 S deposition process as an example, the extrinsic RM chemically reduces lithium polysulfides to Li 2 S in the electrolyte to maximize the usage of favorable surfaces and increase the deposition dimensions of Li 2 S with higher discharge specific capacity, while the oxidized RM + is further reduced on conductive substrates with inherent kinetic advantages. 42 Given an appropriate thermodynamic potential and favorable kinetics, the above-mentioned redox mediation scheme is generally effective for other polysulfide-involving processes, including Li 2 S dissolution and elemental sulfur formation. 43 However, in essence, the redox mediation capability of the intrinsic lithium polysulfide RMs remains unchanged and poor. Furthermore, according to the thermodynamic potential sequence, an extrinsic RM can only regulate a single electrochemical process where other processes at different potentials remain kinetically sluggish. In addition, extrinsic RMs often result in severe corrosion of lithium anode through their own shuttling. 42,44 Therefore, to promote the kinetics of the intrinsic lithium polysulfide RMs for higher redox mediation capability is highly regarded to essentially facilitate the multi-phase and multi-electron sulfur redox reactions and improve the Li-S battery performances with higher specific capacity, better rate responses, and longer cycling lifespan.</p><p>Inspired by the mechanism of coenzyme in bio-systems, a redox comediation strategy is proposed to accelerate the sulfur redox kinetics for high-performance Li-S batteries in this contribution. Compared with coenzyme that interacts with corresponding enzyme to promote the catalytic activity, a redox comediator (coRM) chemically modulates the intrinsic lithium polysulfide RMs to promote their redox mediation capability with faster kinetics (Figure 1B). As a proof of concept, an organopolysulfide, di-t-butyl disulfide (DtbDS), is introduced as a coRM. Concretely, chemically reactive DtbDS spontaneously reacts with lithium polysulfides to generate lithium t-butyl polysulfide (LitbPS) species through the reaction indicated in Figure 1C. The terminals of LitbPS are t-butyl and lithium, respectively. The LitbPSs serve as active RMs possess enhanced redox mediation capability compared with the original lithium polysulfide RMs manifested with significantly improved sulfur redox kinetics. In addition, the introduction of DtbDS coRM regulates the deposition of Li 2 S from 2D to three-dimensional (3D) through strengthened chemical decomposition/disproportionation pathway. Consequently, directed by the redox comediation strategy, DtbDS endows Li-S batteries with higher specific, improved rate performance and longer cycling lifespan under harsh conditions of high sulfur loading and lean electrolyte.</p><!><p>Characterization of the Reactions between Lithium Polysulfide RMs and DtbDS CoRM Generally, coRMs are expected to interact with the intrinsic RMs to promote their functions of accelerating sulfur redox kinetics. Therefore, the reaction of DtbDS with lithium polysulfides is investigated in the first place.</p><p>DtbDS and Li 2 S 6 with equimolar amount were mixed in 1,3-dioxolane/1,2-dimethoxyethane (v:v = 1/1) and characterized comprehensively to gain insights into their reactions. Li 2 S 6 was selected because it is recognized as the primary RM among various lithium polysulfides in working Li-S batteries. Ultraviolet-visible (UV-vis) spectroscopy was first conducted to evaluate the lithium polysulfides qualitatively. 45 Both the original DtbDS and Li 2 S 6 exhibited a clear absorption peak at l = 276 nm (Figure 2A). After the reaction, the peaks of the Li 2 S 6 + DtbDS mixture appeared at l = 332 nm, which corresponds to (CH 3 ) 3 C-S n -Li assigned to the LitbPS mentioned above. Similarly, the nuclear magnetic resonance (NMR) spectra of 7 Li also confirm the exchange reaction. The chemical signal of Li 2 S 6 downshifted from 3.3 to 2.8 ppm due to the chemical reaction between DtbDS and Li 2 S 6 (Figure 2B).</p><p>To further determine the composition of the newly formed lithium polysulfides, the reactants were sent for mass spectrometry (MS) (Figure S1). For the LitbPS fragment of (CH 3 ) 3 C-S n À , when n = 8 (that is, m/z = 345), the mixture of Li 2 S 6 + DtbDS Article exhibited a strong signal of about twelve times than the original DtbDS (Figure 2C).</p><p>When n was reduced to 6 corresponding to shorter sulfur chains, the signal intensity of the mixed Li 2 S 6 + DtbDS sample was also significantly higher than that of the unreacted DtbDS. When n was further reduced to 4, the signal intensity of the two is not much different due to the instability of (CH 3 ) 3 C-S 4 À . In contrast, at m/z = 89 corresponding to (CH 3 ) 3 C-S À , the fragment signal was observed in fresh DtbDS but almost disappeared in the mixed Li 2 S 6 + DtbDS sample, proving that DtbDS did react with Li 2 S 6 .</p><p>X-ray photoelectron spectroscopy (XPS) was further conducted to reveal the details on the reaction between DtbDS and Li 2 S 6 . Li 2 S 6 exhibited typical terminal sulfur (S T ) and bridge sulfur (S B ) signals at 163.1 and 161.2 eV in the sulfur 2p spectrum (2p 3/2 , same quotes for other sulfur 2p spectra), respectively (Figure 2D). The emergence of elemental sulfur at 165.8 eV was due to the chemical disproportionation of Li 2 S 6 during drying. 46   ll chemical environment with higher binding energy of 56.2 eV appeared besides the pristine Li-S interaction at 55.5 eV, which is attributed to the newly formed (CH 3 ) 3 C-S n -Li species. All phenomena prove that DtbDS undergoes a spontaneous chemical reaction with Li 2 S 6 to form lithium t-butyl polysulfides, and the newly generated polysulfide species altered the disproportionation of the original lithium polysulfides. Such a reaction can also be found in other organopolysulfides, such as dimethyl disulfide, dimethyl trisulfide, and diphenyl disulfide, which have been recently verified to react spontaneously with lithium polysulfide to form organic lithium polysulfides. [47][48][49] Among them, dimethyl disulfide and dimethyl trisulfide can function as co-solvents for electrolyte in Li-S cells to change the electrochemical reaction pathways of the sulfur cathodes with large additions. [50][51][52] Kinetic Evaluation of DtbDS-Comediated Polysulfide Conversion Considering organopolysulfides can serve as cathode active materials in several cases, electrochemical evaluation on DtbDS itself was performed to exclude its contribution to capacity as an interference. Cells with lithium metal as anode and carbon paper (CP) as cathode were assembled, and the electrolyte contained additional 100 mmol L À1 DtbDS corresponding to a theoretical discharge capacity of about 4% of sulfur in each cell (calculated as sulfur loading of 1.2 mg S cm À2 ). Within the electrochemical window of routine Li-S cells (1.7-2.8 V versus Li/Li + ), no redox current peak belonging to DtbDS appeared in the cyclic voltammetry (CV) curve (Figure S2A). In addition, the discharge capacity of DtbDS was close to 1 mAh g S -1 throughout 200 cycles, which can be virtually negligible as expected (Figure S2B).</p><p>Therefore, DtbDS alone possesses no electrochemical reactivity, and any increase in capacity observed in the following experiments is mainly attributed to the redox comediation mechanism endowed by DtbDS.</p><p>The as-formed lithium t-butyl polysulfides by the introduction of DtbDS coRM are expected to exhibit enhanced redox mediation capability than the original lithium polysulfide RMs. In order to investigate the sulfur redox kinetics with or without DtbDS, potentiostatic intermittent titration technique (PITT) was conducted to determine the actual potential and capacity of Li 2 S deposition and dissolution. Generally, the PITT discharge curves of Li 2 S 8 catholyte can be divided into two regions. 53,54 The first is the liquid-liquid conversion region, corresponding to the electrochemical conversion from long-chain polysulfides to short-chain polysulfides, while the second region undergoes the liquid-solid conversion recognized as the deposition of Li 2 S. The first process continued until 2.04 V for the pristine Li 2 S 8 catholyte but was advanced to 2.06 V with DtbDS (Figure 3A), implying the presence of LitbPS altered the electrochemical equilibrium and promoted the kinetics of the liquid-solid conversion. Specifically, in the first region, the initial current responses increased by 14%-34% at each potentiostatic step with DtbDS added, suggesting improved liquid-liquid conversion kinetics. In the second region, the Li 2 S deposition peak time for the cell with DtbDS was earlier corresponding to faster deposition rate. 29 The Li 2 S deposition capacity was also higher with DtbDS (270 mAh g S À1 ) than the control cell (214 mAh g S À1 ). Through the entire discharging PITT, the cell with DtbDS exhibited a total discharge capacity of 600 mAh g S À1 , which was 1.5 times than that of pure polysulfide (Figure 3C).</p><p>The PITT curves for the charge process were exhibited in Figure 3B to reveal the influence of DtbDS-induced redox comediation on the dissolution of solid  In contrast, the cell with DtbDS achieved a higher peak current of 0.115 mA and an earlier peak time of 2.80 h. Similar to the discharging PITT, higher peak current and earlier peak time implies faster kinetics for the Li 2 S dissolution process. Through the entire charging PITT, the comediation of DtbDS contributed to a total charge capacity of 445.8 mAh g S À1 , 20% higher than that of the pristine cell (Figure 3C). Therefore, the participation of the DtbDS coRM significantly enhanced the conversion kinetics for both Li 2 S deposition and dissolution and greatly increased the capacity of both processes.</p><p>The discharging and charging PITT measurements afford vital evidence on the kinetic promotion for the phase-transition-involved processes, while the kinetics of liquid-liquid conversion between dissolved lithium polysulfides was evaluated using symmetric cell techniques. 55 Symmetric cells with both electrodes being conductive CP and Li 2 S 6 electrolyte were assembled, and DtbDS was added to the experimental group. The electrochemical impedance spectroscopy (EIS) of the pristine Li 2 S 6 symmetric cell exhibited an interfacial charge transfer impedance of 53 U, which was reduced to 20 U with DtbDS added as the coRM (Figure 3D). Such reduced interfacial resistance proves enhanced electrochemical conversion kinetics at the working interfaces. Similar results were demonstrated in the CV profiles (Figure 3E). With the addition of DtbDS, the redox current of the symmetric cell under the polarization of 0.8 V increased from 0.034 to 0.057 mA. Consequently, the liquid-liquid conversion process was also kinetically promoted by the introduction of DtbDS.</p><!><p>The above discrepant kinetic evaluations evidently prove DtbDS as an effective coRM to facilitate the kinetics for Li 2 S deposition, Li 2 S dissolution, and the conversion between lithium polysulfides. First-principle calculations further reveal the possible reason why LitbPSs possess better kinetics than intrinsic lithium polysulfides at the atomic level. Figure S3A exhibits the optimized geometrical structures of different LiPSs and LitbPSs molecules. The presence of tert-butyl induces a charge redistribution on the sulfur atoms and changes the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) energies of the original LiPSs (Figure S3B). The change of the LUMO and HOMO energy levels leads to different redox capabilities of LitbPSs and accordingly improves the redox kinetics.</p><p>Considering the importance of the Li 2 S deposition process toward high-capacity and high-stable Li-S batteries, ex situ morphology analysis was conducted at different depth of discharges to better probe nucleation and growth of Li 2 S comediated by DtbDS. Concretely, the Li-S cells were disassembled, and the cathodes were washed to remove the lithium salts and remaining polysulfides at the specified discharge point marked in Figure 4A. The cells discharged to 0.07 mAh corresponding to point b and c reflect the situation of Li 2 S nucleation at early stage. For the cell without DtbDS, Li 2 S deposited with a thin film layer on the surface of CP fibers (Figure 4B). In contrast, the Li 2 S deposition with DtbDS coRM was thicker and possessed Article a loose and porous morphology (Figure 4C). The rough surface with more active three-phase boundaries is beneficial for the subsequent reduction of polysulfides. 28 To reveal the growth behavior following Li 2 S nucleation, the cells were ceased with higher discharge capacity of 0.14 mAh or the voltage dropped to 1.7 V marked as point d and e in Figure 4A. The discharge process of the cell without DtbDS terminated earlier by the voltage cut due to the dense insulating Li 2 S film grown on the CP fibers that completely covered the conductive surface (Figure 4D). Such 2D deposition of Li 2 S renders lower discharge capacity and rapid loss of active materials in electrolyte. In direct contrast to that, the DtbDS-comediated Li 2 S deposition demonstrated a radial growth on the nuclei into numerous Li 2 S blocks (Figure 4E). Obviously, the intervention of DtbDS coRM changed the Li 2 S deposition from thin conformal 2D film to porous 3D layer, leading to a 5-fold increase in the discharge capacity. The dimensional increase in Li 2 S deposition is mainly afforded by the strengthened chemical deposition pathway considered as the key function of redox mediation. Specifically, electrochemical Li 2 S deposition from short-chain polysulfides mainly leads to 2D growth due to its dependence upon the three-phase interface. 29 On the contrary, chemical Li 2 S deposition goes through lithium polysulfide disproportionation independently on the conductive surface and, therefore, results in 3D growth with higher discharge capacity. 10 Under the redox comediation of DtbDS, it is speculated that the chemical Li 2 S deposition pathway is facilitated by modifying the pristine lithium polysulfides into lithium t-butyl polysulfides, thereby realizing the 3D deposition of Li 2 S and achieving higher capacity.</p><!><p>The introduction of DtbDS benefits the sulfur redox kinetics in model experiments, where the actual performances in Li-S batteries were further examined to evaluate the redox comediation strategy. Figure 5A exhibits the rate performances. At low current densities, the Li-S cells with DtbDS comediation provided the specific capacity of 1,136, 939, 863, and 810 mAh g S À1 at 0.1, 0.2, 0.5, and 1 C (1 C = 1,675 mA g S À1 ), respectively, 14% higher than those of the cells that rely only on intrinsic polysulfide mediation. Such higher specific capacities are probably brought by the 3D deposition of Li 2 S endowed by DtbDS. Under higher discharge rates of 2 C and 4 C, the Li-S cells with DtbDS maintained high discharge capacity of 741 and 566 mAh g S À1 , respectively, whereas the discharge capacity was dramatically decreased in absent of DtbDS. Recalling the previous kinetic studies, DtbDS significantly improved the sulfur redox kinetics regarding both liquid-liquid and liquid-solid conversion, thus enabling Li-S batteries with high-rate performances. DtbDS also endowed Li-S batteries with satisfying cycling stability. The DtbDS-comediated cell still maintained a discharge capacity of approximately 600 mAh g S À1 after 300 cycles at 0.5 C (Figure S4). In contrast, the blank cell suffered violently fluctuating Coulombic efficiency after only about 180 cycles, probably due to anode corrosion and failure caused by too much lithium polysulfides that could not be converted on the cathode timely. The capacity decay rate of the Li-S cells with DtbDS at 4 C achieved 0.25% per cycle throughout 150 cycles (Figure 5C). However, the capacity of the Li-S cells lacking DtbDS decreased to almost 0 mAh g S À1 after 8 cycles due to the sluggish kinetics.</p><p>Galvanostatic discharge-charge profiles further confirmed the above inference (Figure 5B). Although the polarization voltage was similar, the Li-S cells with DtbDS showed longer discharge platforms in both the liquid-liquid transition and the subsequent Li 2 S deposition processes at the current density of 0.2 C. Additionally, DtbDS reduced the overpotential required for Li 2 S nucleation, which can be attributed to the promoted liquid-solid conversion kinetics. When the current density was ll 3304 Chem 6, 3297-3311, December 3, 2020</p><p>Article increased to 4 C, the polarization of the discharge curves deteriorated sharply. Constrained by the sluggish kinetics of Li 2 S nucleation and growth, the intrinsic polysulfide redox mediators could not support the second discharge platform and the discharge process was forced to terminate at the liquid-liquid conversion stage. In contrast, although the polarization has increased to about 1.8 V in the second discharge platform, the Li-S cells with DtbDS exhibited a typical dual-platform discharge profile. Therefore, redox comediation by DtbDS is validated to be effective to improve the rate performances with higher capacity and lower polarization in Li-S batteries.</p><p>To further demonstrate the potential of employing DtbDS coRM under harsh conditions toward applications, Li-S cells with high-sulfur-loading (5 mg S cm À2 ) and low electrolyte/sulfur (E/S) ratio of 5 mL mg S À1 were evaluated. The molar ratio of DtbDS to sulfur was lowered to 1:125 under such conditions. At a relative large current density of 0.2 C (Table S1), DtbDS still enabled the lean-electrolyte Li-S batteries with a high initial discharge capacity of 594 mAh g S À1 (Figure 5D) along with a capacity decay rate of 0.36% per cycle during 50 cycles (Figure S5). However, the control cells Article without DtbDS only delivered an initial discharge capacity of 416 mAh g S À1 and the capacity decay rate was three times than that with DtbDS. Also, the lean-electrolyte Li-S cells almost lost the higher discharge platforms (Figure S6). Nevertheless, the capacity contributed by the liquid-liquid conversion increased from 74.8 to 114.9 mAh g S À1 and that of the liquid-solid conversion also enhanced by 32% with the DtbDS comediator.</p><p>To validate the practicality of DtbDS in Li-S batteries, a 1.5 Ah-level Li-S pouch cell with DtbDS was assembled (Figure 5E), wherein the sulfur loading of the cathode was 4.9 mg cm À2 (single side) and the E/S ratio was controlled at 4.0 mL mg S À1 .</p><p>The DtbDS-comediated pouch cell delivered a discharge capacity of 1,313 mAh g S À1 at 0.05 C corresponding to an energy density of 300 Wh kg À1 at the cell level and maintained stable cycling for 40 cycles with a specific capacity of 1,135 mAh g S À1 (256 Wh kg À1 ). It is worth noting that the above energy density was calculated based on the mass of the entire pouch cell, including current collectors, package, and tabs. Through fabricating large-capacity pouch cells and reducing the mass of the non-active components, the actual energy density can be further improved.</p><p>Overall, the reaction of DtbDS comediator and the intrinsic polysulfide redox mediators generated kinetically favorable lithium t-butyl polysulfides and further improved the performance of Li-S batteries. Similar organopolysulfides are expected to exhibit resemble comediating effects, as long as these molecules: (1) are able to chemically react with lithium polysulfides, (2) afford reactants with higher redox mediation capability to accelerate the sulfur redox kinetics, and (3) are chemically stable but not severely corrosive to metallic lithium. To further reveal the criterions for effective redox comediators, additional organodisulfides were evaluated in Li-S cells, including diphenyl disulfide (DPDS), diallyl disulfide (DADS), and dipropyl disulfide (DprDS). Among the above organodisulfides, the cells with DPDS only provided a discharge capacity of less than 400 mAh g À1 (Figure S7A); the DADS-cell provided higher discharge capacity in the initial cycles, but a rapid decline in capacity and a sharp fluctuation in Coulombic efficiency occurred afterward (Figure S7B); DprDS also exhibited a rapid capacity and Coulombic efficiency degradation after 50 cycles. First-principles calculations were further conducted to reveal the differences among the above-mentioned organodisulfides at the atomic level. Figure S8A shows the optimized geometry of the four organodisulfides and the corresponding charge distribution on the sulfur atoms based on natural bond orbital (NBO) analyses. The LUMO energies of DPDS, DADS, DprDS, and DtbDS increase in turn (Figure S8B), which indicates that DtbDS has the best reductive stability among all while the others are more reactive with reductive components in batteries, such as Li metal anode. The high reactivity of DPDS, DADS, and DprDS with reductive components in the battery results in a quick degradation of the battery capacity and cycling stability. Therefore, only DtbDS can persistently comediate the long-term electrochemical cycling processes to afford desirable battery performances.</p><p>In summary, a redox comediation strategy is proposed to assist the intrinsic lithium polysulfide redox mediators with higher capability to regulate the sulfur redox kinetics. DtbDS, an organopolysulfide as comediator, spontaneously reacts with lithium polysulfides to generate lithium t-butyl polysulfides as active redox mediators with kinetic merits. The DtbDS comediation accelerates the deposition and dissolution of Li 2 S and alters the Li 2 S deposition mode from 2D to 3D with higher discharge capacity. Consequently, the Li-S batteries with DtbDS as comediator exhibit superb rate performances of 566 mAh g S À1 at 4 C, remarkable cycling stability at high sulfur loading of 5 mg S cm À2 , or lean-electrolyte operability at 5.0 mL ll 3306 Chem 6, 3297-3311, December 3, 2020</p><p>Article mg S À1 . The novel redox comediation strategy expands the chemistry of redox mediation for kinetic promotion in multi-electron and multi-phase electrochemical processes and inspires further exploration of redox comediators for advanced energy-related systems.</p><!><p>Resource Availability Lead Contact Further information and requests for resources and reagents should be directed to and will be fulfilled by the Lead Contact, Professor Jia-Qi Huang (jqhuang@bit. edu.cn).</p><!><p>This study did not generate new reagent nor materials. All materials used in this work are commercially available.</p><!><p>The published article includes all datasets generated or analyzed during this study.</p><!><p>All the raw materials, including Li 2 S, sulfur (S), DtbDS, DPDS, diallyl disulfide (DADS), dipropyl disulfide (DprDS), nonwoven CP, 1,2-dimethoxyethane (DOL), 1,2-diamethoxyethane (DME), N-methyl pyrrolidone (NMP), poly(vinylidene fluoride) (PVDF, HSV-300), polypropylene (PP) separator (Celgard 2400), aluminum (Al) foil, lithium foil, lithium bis(trifluoromethanesulfony)imide (LiTFSI), and lithium nitrate (LiNO 3 ) were purchased from commercial sources and used without further purification.</p><p>Characterization of Lithium t-Butyl Polysulfides 100 mmol L À1 Li 2 S 6 and 100 mmol L À1 DtbDS solutions were prepared for spectroscopic tests. Specifically, S and Li 2 S powders were mixed at a stoichiometric molar ratio of 5:1 to obtain the Li 2 S 6 solution. Note that the solvents of the above solutions were DOL/DME (v/v = 1:1) without any additional lithium salt to exclude the effect of lithium salts in characterization. Li 2 S 6 and DtbDS solutions (v/v = 1:1) were mixed and stirred at room temperature for 24 h to obtain the mixed sample (denoted as Li 2 S 6 + DtbDS).</p><p>The solutions were diluted fifty times for UV-vis measurements. Diluted Li 2 S 6 , DtbDS, and Li 2 S 6 + DtbDS solutions were sealed in quartz glass cuvettes in an argon (Ar)-filled glove box and measured using a UV-3600 spectrometer (Shimadzu, Japan). The original Li 2 S 6 and mixed Li 2 S 6 + DtbDS solutions were sealed in NMR tubes to perform the 7 Li NMR spectra on a JNM-ECA6oo NMR spectrometer. The diluted solutions were also encapsulated in Ar atmosphere for MS. The matrix-assisted laser desorption/ionization time of flight MS (MALDI-TOF MS, Shimadzu, Japan) testing required the addition of a stable matrix (2,5-Dihydroxybenzoic acid, DHB), and the background peaks of which were detected to eliminate its influence on the sample spectra.</p><p>The original Li 2 S 6 , DtbDS, and Li 2 S 6 + DtbDS solutions were loaded on PP disks for XPS analysis. Specifically, under Ar atmosphere, the three solutions (10 mL) were added to three PP disks (5 mm in diameter) and dried at room temperature, respectively. Then the loaded PP disks were sealed in a vial and transferred to the chamber of PHI Quantera scanning X-ray microprobe system (ULVAC-PHI, Japan). The binding energies for XPS analysis were calibrated by the carbon 1s peak at 284.6 eV. ) to wet the anode surface. The total sulfur content in each cell was controlled to be around 0.64 mg. PITT was conducted on the Solartron 1470E electrochemical workstation. For discharge PITT evaluation, the above cells were first rested at the open-circuit voltage for 2.0 h, and the cutoff condition for a given step was the current being less than 30 mA or longer than 4.0 h. For charge PITT evaluation, the above cells were first galvanostatically discharged to 1.7 V at 0.05 C and then potentiostatically discharged at 1.7 V for 4 h to ensure that all the electroactive polysulfides were converted into Li 2 S. The subsequent steps are similar to the discharge PITT, except that the set voltages were gradually increased.</p><!><p>Similar to the cells for PITT evaluation, Li | CP/polysulfide cells were assembled to observe the nucleation and growth of Li 2 S. For the cells without DtbDS, the catholyte was consisted of 40 mL 0.5 mol L À1 [S] of Li 2 S 6 in DOL/DME (v/v = 1:1) with 1.0 mol L À1 LiTFSI. For the cells with DtbDS, DtbDS was added to the catholyte with a concentration of 50 mmol L À1 . All the cells were discharged at a current of 0.1 mA. In order to observe the nucleation and subsequent growth behavior of Li 2 S, the cells were terminated when the discharge capacity reached 0.07 and 0.14 mAh (or the voltage dropped to 1.7 V in cases without DtbDS), respectively. After that, the cells were immediately disassembled, and the CPs were taken out and cleaned with DME for scanning electron microscopy (SEM) observation. The SEM was conducted on JSM 7401F (JEOL, Tokyo, Japan) operated at 3.0 kV.</p><!><p>Li | CP/DtbDS cells were assembled using CP as the cathode conductive substrate, lithium foil as the anode, and PP as the separator. Catholyte was composed of 20 mL 100 m mol L À1 DtbDS, 1.0 mol L À1 LiTFSI, and 2 wt % LiNO 3 in DOL/DME (v/v = 1:1). CV tests were conducted on the Solartron 1470E electrochemical workstation at a scanning rate of 0.1 mV s À1 . Galvanostatically cycles were conducted on a Neware multichannel battery cycler under 0.1 C (0.2 mA cm À2 ).</p><!><p>Li | S Cell Assembly and Electrochemical Performance Evaluation Carbon-sulfur composites were prepared by ball-milling. Concretely, 30 mg multi-walled carbon nanotubes (CNTs) and 70 mg S were ball-milled with PVDF binder at a weight ratio of 9:1 in NMP solvent to form a slurry. The slurry was then coated on Al foils, dried at 60 C for 12.0 h, and punched into small disks with a diameter of 13 mm as sulfur cathodes. The areal sulfur loading was 1.2 or 5 mg S cm À2 for the routine and harsh condition measurements. The sulfur content in the composite was 70 wt % (63 wt % within the cathode). Electrochemical measurements were conducted using standard 2025-coin-type cells, where a piece of Li foil with a diameter of 16.0 mm was used as the anode and a piece of PP membrane served as the separator. 1.0 mol L À1 LiTFSI and 2 wt % LiNO 3 dissolved in DOL/DME (v/v = 1:1) solvents was adopted as the control electrolyte. Extra DtbDS was added to the above blank electrolyte with the concentration of 100 mmol L À1 . The E/S ratio was 15.6 and 5 mL electrolyte /mg S for cells with 1.2 and 5 mg S cm À2 , respectively. Similarly, for the electrochemical tests of other organosulfides, the electrolyte consisted of 100 mmol L À1 organosulfides (DPDS, DADS, and DprDS, respectively), 1.0 mol L À1 LiTFSI, and 2 wt % LiNO 3 in DOL/DME (v/v = 1:1). The E/S ratio of cells with organosulfides was 15.6 mL electrolyte /mg S with sulfur loading of 1.2 mg S cm À2 . The cells were galvanostatically cycled between 1.7-2.8 V on the Neware multichannel battery cycler under different C-rates (1 C = 1,672 mA g À1 ).</p><p>For Li-S pouch cell evaluation, the well-mixed slurry was double-side coated on Al foil and then dried at 60 C for 12 h. Then, 4 3 7 cm 2 electrodes with a single-side areal sulfur loading of 4.9 mg cm À2 were cut. The Li anode sheets were also cut with the same size as sulfur cathode. The thickness of the Li anode was 50 mm (single side) on Cu current collector. To assemble a pouch cell, the cathode and anode electrodes were stacked layer by layer with PP separators. All the electrodes were controlled in parallel connection through an electrode lug terminal. The composition of the electrolyte was the same as the coin cells, and the E/S ratio was controlled as 4 mL electrolyte /mg S . The cells were galvanostatically cycled between 1.7-2.55 V on the Neware multichannel battery cycler under 0.05 C (1 C = 1,672 mA g À1 ).</p><!><p>The first-principle calculations were conducted in Gaussian (G09) 56 program with Becke's three-parameter hybrid method using the Lee-Yang-Parr correlation functional (B3LYP) at 6-311G++G(d, p) level. 57 The universal solvation model of SMD 58 was used to describe the solvation effects in DME environment. The dielectric constant and solvent radius were set to be 7.2 and 4.19 A ˚, respectively. Frequency analysis was performed to ensure the ground state of optimized lithium polysulfides clusters. Both linear and cyclic structures were considered for the clusters and the latter has a lower total energy than the former.</p>