Suppression of Polysulfide Dissolution and Shuttling with Diphenyl Disulfide Electrolyte for Lithium Sulfur Batteries

: Lithium-sulfur (Li-S) batteries have been rising potential competitors in new battery technologies beyond lithium-ion battery rea, but the dissolution and shuttle of lithium polysulfide seriously deteriorate the cycling performance. Herein, we proposed diphenyl disulfide (DPDS) as the electrolyte additive for Li-S batteries. DPDS can interact with the discharge intermediate S2-4/S2-6 ions, changing the stable form of lithium polysulfide in the electrolyte, inhibiting the shuttle of lithium polysulfide, and protecting the lithium anode. The initial discharge specific capacity of a Li-S battery with conventional electrolyte at 0.5 C is 1352.6 mAh/g, and the specific capacity of a Li-S battery with 200 cycles is 790.5 mAh/g. The initial discharge specific capacity of the battery containing 5.0 wt.% DPDS electrolyte at 0.5 C was 1296.3 mAh/g, and the capacity after 200 cycles was 899.9 mAh/g.


Introduction
At present, lithium-sulfur (Li-S) battery has become one of the most promising new high-energy secondary batteries due to its high theoretical specific capacity (1675 mAh/g) and high theoretical energy density (2600 Wh/kg) [1][2][3].Since the pioneering work by Nazar's group in 2009, proposing that confining S into a porous carbon host can improve the utilization and enhance the cycle life, the research on Li-S batteries has achieved rapid progress [4].However, a suite of troublesome problems, severe polysulfide shuttle and sluggish redox kinetics, hinder the commercialization of Li-S batteries.The dissolution and shuttle of polysulfide not only cause the loss of active electrode materials, shorten the cycling life, but also enables the battery to have a high selfdischarge rate [5][6][7].
In order to suppress the shuttle effect of lithium polysulfide, a large number of effective methods have been used, such as the design of porous cathode materials, the interlayer of blocks of lithium polysulfide, and the functional additive for electrolyte [8][9][10].Wu et al. [11,12] proved that the mercaptan-based additive biphenyl-4,4'-dithiol (BPD) could form a BPD-Sn complex with polysulfides, reducing the dissolution of polysulfides and inhibiting the shuttle effect.Gu et al. [13] found that adding carbon disulfide (CS 2 ) to the electrolyte could form a complex with polysulfide, which inhibits the migration of long-chain polysulfide to the Li anode.Yang et al. [14] added bis(4-nitrophenyl) carbonate (BNC) to the electrolyte, it can react with soluble polysulfide to generate insoluble polysulfides and by-products, which further react with lithium metal negative electrode to generate a passivation layer with good lithium conductivity, and the low impedance of the passivation layer is conducive to improving the electrochemical performance of Li-S batteries.In addition, cobalt metallocene (Co(η 5 C 5 H 5 ) 2 ), [15] dithiothreitol (DTT) [16], N-methyl-N-ethylpyrrolidine (MEP+) cation [17], amino aniline trimer (ACAT) [18], glutamic acid [19], AlPO4 [20] are functional additives that can effectively inhibit polysulfide shuttle.
The previous research of our research group found that the -S-R structure in sulfur-containing ether structure vulcanizing accelerator can interact with the S-S bond of lithium polysulfide, the high solubility lithium polysulfide can be transformed into S8 and Li 2 S/Li 2 S 2 , which can reduce the sulfur content in the electrolyte and inhibit the shuttle effect [21].DPDS contains -S-R structure, which is a common vulcanization accelerator, but its role as an electrolyte additive is not clear.Herein, we propose to use diphenyl disulfide (DPDS) as a functional additive to restrain the shuttling of lithium polysulfide, study the impact on the cycle and rate performance, and analyze its chemical reaction with lithium polysulfide and its action mechanism for Li-S battery.

Results and discussion
Figure 1(a) and Table S1 show the cycling performance at 0.5C and Coulombic efficiency (CE) of Li-S batteries with and without DPDS additive.The discharge specific capacity of different amounts (1.0 wt.%, 2.0 wt.%, 5.0 wt.%, 8.0 wt.%) of DPDS additives increased, especially the Li-S battery with 8.0 wt.% DPDS additive electrolyte shows an initial discharge specific capacity of 1474.7 mAh/g, and the second cycle still maintains a high specific capacity of 1402.0 mAh/g.The initial specific capacity of conventional electrolyte is also relatively high (1352.6mAh/g), but the second cycle is reduced to 1117.7 mAh/g.The specific capacity of Li-S battery with conventional electrolyte drops rapidly in the first 10 cycles, and the capacity decay reaches 29.3%, the battery with 8.0 wt.% DPDS electrolyte attenuation rate is only 12.2%, and 5.0 wt.% DPDS electrolyte attenuation rate is 16.7%.After 180 cycles, the specific capacity 5.0 wt.% DPDS battery still maintains 985.5 mAh/g, while the conventional electrolyte battery only has 785.8 mAh/g.8.0 wt.% DPDS battery has the highest specific capacity at the first cycle, but its capacity declines rapidly in the subsequent cycles.Considering comprehensively, we choose the 5.0 wt.% DPDS battery as the object of follow-up research.As the DPDS additive contains sulfur, it may provide some capacity, in order to deduct the capacity of the DPDS additive and show the true capacity of the battery, we use sulfur-free carbon as the cathode and 5.0 wt.% DPDS additive as the electrolyte to assemble the battery, its cycle performance as shown in Figure 1(b).It can be seen that DPDS can provide 70-130 mAh/g capacity, but it is unstable and decays quickly, which indicates that the S-S bond in DPDS can participate in the electrochemical reaction.Figure 1(c) shows the cycling performance of 5.0 wt.% DPDS battery after deducting the capacity of DPDS additive, DPDS additive can significantly improve the stability of the Li-S battery, the specific capacity is still 899.9mAh/g after 200 cycles and the capacity retention rate is 69.4%, while that of conventional electrolyte is 789.9 mAh/g and 58.4%, respectively.DPDS shows excellent electrochemical performance even at 1 C (as shown in Figure 1(d)), the specific capacity remained 1018.4mAh/g after 100 cycles, while the conventional electrolyte was only 827.6 mAh/g.Figure 2(a) shows the CV curves of Li-S batteries with different electrolytes.The CV curves of Li-S battery with conventional electrolyte consist of two reduction peaks and two oxidation peaks, corresponding to the discharge and charging processes respectively.In the first cycle, the reduction peak at 2.25 V corresponds to the reduction of sulfur to long-chain polysulfide, while the reduction peak at 2.0 V is due to the formation of shortchain Li 2 S 2 /Li 2 S [22,23].The two oxidation peaks of 2.42V and 2.47V formed in the anodic oxidation process correspond to the oxidation process of Li 2 S 2 /Li 2 S to intermediate lithium polysulfide and the conversion process of polysulfide to sulfur [16].Compared to conventional electrolyte batteries, 5.0 wt.% DPDS electrolyte battery appears a new shoulder peak at 2.17 V, and the reduction of DPDS to polysulfide lithium at 2.09 V (as shown in Figure 2(b)).There is no simple superposition of the two reduction peaks, which indicates that DPDS is not a separate redox reduction but affects the entire electrochemical discharge progress.CV curves of 5.0 wt.% DPDS electrolyte batteries have a high coincidence at the first three cycles, it shows good reversibility compared to the conventional electrolyte battery.In addition, the areas of the oxidation peak and reduction peak are in direct proportion to the specific capacity, which means that the capacity contributed by DPDS as an active material is negligible compared with the entire specific capacity of Li-S batteries.Therefore, DPDS could reduce polarization and improve cycle reversibility by acting on the discharge process of Li-S batteries.
Figure 2(c, d) shows the charge and discharge curves of conventional and 5.0 wt.% electrolyte batteries.The first high voltage platform (2.3-2.4V)during the discharge progress represents the transformation of S to lithium polysulfide, and the second low platform (2.0-2.1V)represents the transformation of lithium polysulfide to Li 2 S 2 /Li 2 S, and the voltage difference (ΔE) between the charge and discharge platforms represents the polarization voltage of the Li-S batteries.Compared with the polarization voltage of 204 mV of conventional electrolyte battery, the polarization voltage of 5.0 wt.% DPDS electrolyte battery is only 164 mV, which means that the DPDS additive can alleviate the polarization.In addition, DPDS reduced the Li 2 S oxidation potential from 2.32 V to 2.13 V during the first cycle charge progress, accelerated the Li 2 S oxidation reaction kinetics, and improved the reversibility of redox.Figure 2(e) shows the rate performance of Li-S batteries in conventional electrolyte and 5.0 wt.% DPDS electrolyte, the 5.0 wt.% DPDS electrolyte battery has better rate performance and reversibility.Figure 2(f) shows the Nyquist plots of Li-S batteries in conventional electrolyte and 5.0 wt.% DPDS electrolyte.The semicircle diameter of the 5.0 wt.% DPDS electrolyte battery is smaller, showing relatively small interface impedance, reflecting high interface compatibility between electrode and electrolyte.After 100 cycles, the interface impedance of the 5.0 wt.% DPDS electrolyte battery is relatively high, which may be due to the poor conductivity of the generation of discharge products in the electrochemical reaction.
To study the reaction mechanism of DPDS in Li-S battery, we have made the discharge visualization battery devices to observe the color change of electrolyte, the visualization experiment result as shown in Figure 3.In the conventional electrolyte (Figure 3(a, c)), with the increase of the discharge time at the high discharge voltage, the elemental S is continuously converted into lithium polysulfide, and the polysulfide in the electrolyte is continuously accumulated, so the color of the process from A to C is gradually deepened, showing a dark brownred color; At the later stage of discharge, the color of electrolyte gradually becomes light and brown-yellow due to the transformation of polysulfide into insoluble Li2S/Li2S2.In the discharge process of the battery containing DPDS additive (Figure 3 (b, d)), the color also experienced a process from dark to light (photo B-F in Figure 3(d)), but compared with the conventional electrolyte, the color of DPDS electrolyte is lighter, lighter brown and light yellow, indicating that the concentration of lithium polysulfide in the DPDS electrolyte was lower than that of the conventional electrolyte battery during the discharge process.It can be seen that the DPDS additive could prevent the dissolution of lithium polysulfide in the electrolyte during the discharge of the Li-S battery, which is beneficial to inhibit the shuttle effect.We believe that the reaction of DPDS with lithium polysulfide changes the stable form of lithium polysulfide in the electrolyte, thereby reducing the concentration of lithium polysulfide during battery discharge.To verify this conjecture, we add an equal amount of 20 mM Li2S6 to the conventional electrolyte and the 5 wt.%DPDS respectively to simulate the electrolyte environment during the discharge process, and observe the color change of the electrolytes, the pictures as shown in Figure 4((a  To explore the influence of DPDS additive on the anode, we tested SEM images in Li anode after 100 cycles in conventional electrolyte Figure S3((a) and (b)) and 5 wt.%DPDS electrolyte Figure S3((c) and (d)).In the conventional electrolyte, due to the dissolution and shuttle of polysulfides, side reactions occur on the Li anode, making the Li anode surface porous and loose, accompanied by the scattered accumulation of non-conduction species [11,26].However, in the 5 wt.%DPDS electrolyte, the surface of the Li anode is still smooth and dense.In addition, an XPS test was carried out on the Li anode cycling in conventional electrolyte and in the 5 wt.%DPDS electrolyte after 100 cycles, the results as shown in Figure S3(c).In conventional electrolytes, the peak at 169.0 eV (S 2p3/2) represents the S-O bond in − SO2CF3, which is the product of the decomposition of LiTFSI salts [27,28].The peak at 167.0 eV (S 2p3/2) represents thiosulfate, which is the product of lithium polysulfide oxidized by LiNO3 [21,29].The two strong peaks at 161.7 eV and 160.5 eV indicate that the Li anode surface is covered with a large amount of irreversible Li2S2 and Li2S [30], which is due to the polysulfides shuttle to the Li anode and have a side reaction with metallic Li.When 5.0 wt.% DPDS is added, the peaks at 161.7 eV and 160.5 eV drop sharply, which means that the amount of insoluble Li2S2 and Li2S decreases.SEM images and XPS results show that the addition of DPDS additive effectively inhibits the shuttle effect of polysulfide.

Conclusions
In summary, we provide an effective additive to improve the cycle performance of Li-S batteries.DPDS additive can interact with discharge intermediate S 2- 4 / S 2- 6 , change the stable form of polysulfide in the electrolyte, reducing the content of S 2- 4 / S 2- 6 significantly, and inhibiting the dissolution shuttle of lithium polysulfide.DPDS also reacts with metallic Li and generates insoluble substances such as Li2S2/Li2S through its reaction with polysulfide, for this reason, it can reduce the side reaction of Li anode and the accumulation of non-conductive species, to improve the stability of the Li anode.DPDS additive can increase the cycling performance of the Li-S battery, the initial specific capacity of the battery with conventional electrolyte at 0.5C is 1352.6 mAh/g, after 200 cycles the capacity is 790.5 mAh/g, and the capacity retention rate is 58.4%.By contrast, the initial specific capacity of the battery contains 5.0 wt.% DPDS at 0.5 C was 1296.3 mAh/g, after 200 cycles is 899.9 mAh/g, and the capacity retention rate was 69.4%, which was 11.0% higher than that of conventional electrolyte.mAh/cm2 deposition capacity.The assembled Li-S batteries were allowed to stand for 12 h, after that they were charged and discharged at a potential of 1.7-2.8V (vs.Li/Li+) on a LANDAN test system (Wuhan, China).Cyclic voltammetry (CV) measurements were performed on an electrochemical workstation (Solartron 1470 E) in a voltage range of 1.5-3.0V (vs.Li/Li+) with a scan rate of 0.05 mV/s.Electrochemical impendence spectroscopy (EIS) measurements were conducted on an electrochemical workstation (Solartron 1470E), the frequency range operated 0.01Hz to 1 MHz and a voltage amplitude of 5 mV.All tests were accomplished at 25℃.

Characterization
Li anodes were collected from recycled Li-S or Li-Cu coin cells, washed with fresh DME solvent to remove Li salts and dried in a glove box full of Ar.SEM observations and elements characterizations were obtained using scanning electron microscopy (SEM, FEI Nova Nano SEM) and energy dispersive spectrometer (EDS, attached to scanning electron microscopy).An FEI Tecnai TF20, Jeol 2100 equipped with an energy dispersive spectroscopy detector was used for the transmission electron microscopy (TEM) and elemental distribution analyses.The X-ray photoelectron spectroscopy (XPS) spectra were obtained by Thermo ESCALAB 250 spectrometer.The ultraviolet spectrum was characterized by U-4100 ultraviolet-visible near-infrared spectrophotometer (Hitachi, Japan)

Figure 1 .
Figure 1.Cycling performance of Li-S batteries: (a) The batteries with different mass fractions of DPDS additive at 0.5 C; (b) The battery with 5.0 wt.% DPDS as electrolyte and sulfur-free carbon as cathode; (c) the batteries with conventional electrolyte and with 5.0 wt.% DPDS electrolyte deduct the capacity of additive at 0.5 C; (d) the batteries with conventional electrolyte and with 5.0 wt.% DPDS electrolyte at 1 C.

Figure 2 .
Figure 2. Cyclic voltammograms of Li-S batteries in the voltage range of 1.5-3.0V at a scanning rate of 0.05 mV/s with (a) conventional electrolyte with S/C cathode, (b) 5.0 wt.% DPDS electrolyte with S/C cathode and S-free/C cathode; charge-discharge voltage profiles of Li-S batteries in (c) conventional electrolyte and (d) 5.0 wt.% DPDS additive electrolyte; (e) rate performance of Li-S batteries in conventional electrolyte and 5.0 wt.% DPDS electrolyte; (f) Nyquist plots of Li-S batteries in conventional electrolyte and 5.0 wt.% DPDS electrolyte.

Figure 3 .
Figure 3. Visualization experiment of Li-S batteries at different discharge stages: discharge curves of batteries with (a) conventional electrolyte and (b) 5.0 wt.% DPDS electrolyte; color change of (c) conventional electrolyte and (d) 5.0 wt.% DPDS electrolyte at different discharge levels.
) and (b)).Both the unreacted electrolytes are colorless and transparent (Figure 4(a)), after adding the Li2S6 solution (Figure 4(b)), the color of 5 wt.%DPDS electrolyte on the right side is lighter, indicating the chemical reaction of DPDS with Li2S6.We tested the ultraviolet spectrum of Li2S6 without and with DPDS additive, the results are shown in Figure 4(c).Li2S6 solution has a shoulder peak at about 280 nm, corresponding to S 2- 4 /S 2- 6 ions[24, 25], the shoulder peak at about 280 nm of Li2S6 solution with DPDS disappears and a new shoulder peak appears at about 230 nm, which is different from the characteristic peak of DPDS (at about 240 nm).It indicates that DPDS reacts with S 2- 4 /S 2- 6 ions, reducing the content of S 2- 4 /S 2- 6 ions, and producing new substances.

Figure S1 .
Figure S1.SEM images of (a) C and (b) S/C; TEM images of (c) C and (d) S/C.

Table S1 .
Cycling data of Li-S batteries with different mass fractions of DPDS additive at 0.5 C.