A three‐way electrolyte with ternary solvents for high‐energy‐density and long‐cycling lithium–sulfur pouch cells

Lithium–sulfur (Li–S) batteries promise high‐energy‐density potential to exceed the commercialized lithium‐ion batteries but suffer from limited cycling lifespan due to the side reactions between lithium polysulfides (LiPSs) and Li metal anodes. Herein, a three‐way electrolyte with ternary solvents is proposed to enable high‐energy‐density and long‐cycling Li–S pouch cells. Concretely, ternary solvents composed of 1,2‐dimethoxyethane, di‐isopropyl sulfide, and 1,3,5‐trioxane are employed to guarantee smooth cathode kinetics, inhibit the parasitic reactions, and construct a robust solid electrolyte interphase, respectively. The cycling lifespan of Li–S coin cells with 50 µm Li anodes and 4.0 mg cm−2 sulfur cathodes is prolonged from 88 to 222 cycles using the three‐way electrolyte. Nano‐heterogeneous solvation structure of LiPSs and organic‐rich solid electrolyte interphase are identified to improve the cycling stability of Li metal anodes. Consequently, a 3.0 Ah‐level Li–S pouch cell with the three‐way electrolyte realizes a high energy density of 405 Wh kg−1 and undergoes 27 cycles. This work affords a three‐way electrolyte recipe for suppressing the side reactions of LiPSs and inspires rational electrolyte design for practical high‐energy‐density and long‐cycling Li–S batteries.


INTRODUCTION
Low-carbon and sustainable life puts forward strong requirements for safe, clean, and affordable energy storage. 1,2High-energy-density and long-cycling rechargeable batteries are urgently demanded to meet the increasing energy storage demands. 3Lithium-sulfur (Li-S) batteries are regarded as promising next-generation rechargeable batteries owing to their ultrahigh theoretical energy density (2600 Wh kg −1 ) and environmental friendliness. 4,55][16] High energy density at 700 Wh kg −1 level has been achieved in pouch cells very recently, which is significantly superior to the current Li-ion batteries. 17evertheless, current Li-S batteries suffer from limited cycling lifespan compared with commercial Li-ion batteries, which severely restricts the practical applications of Li-S batteries. 18he cycling lifespan of Li-S batteries is critically restricted by the parasitic reactions between lithium polysulfides (LiPSs) and Li metal anodes. 19,20Li-S batteries generally undergo a precipitation-dissolution conversion process. 21LiPSs are generated as inherent intermediates, dissolve in the electrolyte, and diffuse to the anode side during charge and discharge processes. 22The highly active and dissolved LiPSs chemically react with Li metal anodes to form low-valence sulfides such as Li 2 S or Li 2 S 2 . 23The undesirable side products participate in the formation of solid electrolyte interphase (SEI) and induce uneven Li plating/stripping. 24Massive Li dendrites are formed during Li plating due to the unfavorable SEI.6][27][28] Meanwhile, SEI repeatedly cracks and reconstructs along with Li plating and stripping, thus continuously exposing fresh active Li. 29,30The exposed active Li continuously reacts with LiPSs parasitically to deplete limited Li inventory, which induces rapid failure of Li metal anodes and limited cycling lifespan of Li-S batteries. 31Furthermore, the LiPS side reactions are exacerbated under demanding conditions of using high-sulfur-loading cathodes (>4.0 mg cm −2 ), ultrathin Li anodes (<50 µm), and a low electrolyte to sulfur (E/S) ratio (<4.0 µL mg −1 ) in practical Li-S pouch cells. 18,32It is highly challenging to achieve long cycling lifespan under the prerequisite of high energy density (>400 Wh kg −1 ).Therefore, it is essential to suppress the parasitic reactions between LiPSs and Li metal anodes for achieving high-energy-density and long-cycling Li-S batteries.
The working electrolyte in a battery plays a vital role in inhibiting the parasitic reactions between LiPSs and Li metal anodes. 33The electrolyte of Li-S batteries determines the solvation structure and reactivity of LiPSs with Li metal anode.Meanwhile, the electrolyte components participate in the formation of SEI to potentially protect Li metal anodes. 34,35A large number of electrolyte design strategies have been reported to mitigate the LiPS side reactions and improve the cycling performance of Li-S batteries. 36Highly solvating electrolyte with high-donor-number solvents promotes fast cathode conversion kinetics but induces rapid anode failure due to high LiPS solubility and reactivity. 37,38Conversely, sparingly solvating electrolyte inhibits the dissolution of LiPSs and suppresses the LiPS parasitic reactions, 39 including ionic liquid electrolytes, 40 solvated ionic liquid electrolytes, 41 high-concentration electrolytes, 42 and localized high-concentration electrolytes. 43However, severely sluggish cathode kinetics in the sparingly solvating electrolytes limits the realization of high energy density in practical Li-S batteries. 44Encapsulating LiPS electrolyte was recently proposed to balance the mitigation of the LiPS side reactions and smooth cathode conversion kinetics for high-energy-density and long-cycling Li-S batteries. 45,468][49] Various electrolyte additives have been proposed to fabricate beneficial SEI and inhibit the LiPS side reactions, such as LiNO 3 , 50 Li 2 SO 4 , 51 and SnI 2 . 524][55] Based on these considerations, rationally integrating multifunctional electrolyte components to mitigate the side reactions, construct a robust SEI, and maintain the cathode kinetics is of great importance toward Li metal anode protection in practical Li-S batteries.A multifunctional electrolyte design is expected to simultaneously realize high-energy-density and long-cycling Li-S batteries.
In this contribution, a three-way electrolyte (TWE) with ternary solvents is proposed to enable high-energy-density and long-cycling Li-S pouch cells.The ternary solvents composed of 1,2-dimethoxyethane (DME), di-isopropyl sulfide (DIPS), and 1,3,5-trioxane (TXA) are employed to guarantee smooth cathode conversion, mitigate LiPS parasitic reactions, and construct a robust organic-rich SEI, respectively.Satisfactory cathode conversion kinetics and stabilized Li metal anodes with suppressed LiPS parasitic reactions are simultaneously realized.The cycling lifespan of Li-S coin cells with 50 µm Li anodes and 4.0 mg cm −2 sulfur cathodes is prolonged from 88 to 222 cycles.Concretely, a nano-heterogeneous solvation structure with encapsulated LiPSs is constructed to mitigate the LiPS side reactions.Moreover, an organic-rich SEI is fabricated to suppress the LiPS side reactions and enable uniform Li deposition.Consequently, a 3.0 Ah-level Li-S pouch cell employing the TWE realizes a high initial energy density of 405 Wh kg −1 and undergoes 27 cycles.Post-analysis of the cycled pouch cell is further conducted to verify the effectiveness of the electrolyte design.

The concept of TWE design
A TWE with ternary solvents for Li-S batteries is expected to simultaneously suppress the LiPS parasitic reactions, construct a robust organic-rich SEI, and maintain satisfactory cathode redox kinetics.The as-proposed TWE for high-energy-density and long-cycling Li-S batteries is composed of DME, DIPS, and TXA with 1.0 M lithium bis-(trifluoromethanesulfonyl) imide (LiTFSI).Specifically, DME is employed to maintain high LiPS solubility and smooth cathode conversion kinetics due to its strong sol-vating power.DIPS is expected to encapsulate LiPSs in a heterogeneous solvation structure due to its weak solvating power.TXA is expected to construct an organic-rich SEI by reductive decomposition.The above ternary solvents are expected to inhibit the parasitic reactions between LiPSs and Li metal anodes while maintaining smooth cathode redox kinetics.The conventional electrolyte (denoted as CVE) composed of 1,3-dioxolane (DOL) and DME with 1.0 M LiTFSI is employed as the control electrolyte.

The electrochemical performance of TWE in model cells
The electrochemical performances of the as-designed TWE was firstly evaluated.The cathode conversion kinetics was evaluated by Li 2 S 6 symmetric cells.According to the electrochemical impedance spectroscopy spectra and quantitative fitting results, similar resistances are observed in the cells with both CVE and TWE, indicating similar cathode conversion kinetics in both electrolytes (Figures 1a,  S1, and S2).Cyclic voltammetry (CV) was further conducted and similar current responses are displayed in the cells with CVE and TWE (Figure S3).Accordingly, smooth cathode conversion is ensured in TWE so that the high-energy-density advantage of Li-S batteries can be maintained.
The Li metal anode cycling stability and the LiPS parasitic reactions were further investigated.Li | Li symmetric cells were assembled with 0.5 M [S] Li 2 S 8 and evaluated at a current density of 1.0 mA cm −2 and an areal capacity of 4.0 mAh cm −2 .The symmetric cell with TWE maintains stable for more than 500 h while that with CVE displays gradually increasing voltage polarization after 300 h due to the failure of Li metal anodes (Figure 1B), indicating stable Li metal anodes with TWE.To further quantitatively evaluate the mitigation of the LiPS side reactions, shuttle currents were measured as a quantitative indicator.Low shuttle currents (<0.07 mA cm −2 ) are observed in the cell with TWE at different voltages from 2.20 to 2.60 V (Figure S4).Comparatively, the shuttle currents in the cell with CVE are about 0.18 mA cm −2 , indicating that the LiPS parasitic reactions are mitigated significantly by TWE.Visual experiments were conducted to intuitively monitor the side reactions of LiPSs with Li metal anodes.An amount of 2.5 mM [S] Li 2 S 8 was dissolved in CVE and TWE, respectively, and a Li foil was immersed into the electrolytes.Both CVE and TWE appear light green before soaking (Figure S5a).However, CVE becomes almost colorless after 4 h while TWE still maintains light green (Figure S5b), indicating LiPSs do not fully react with Li metal in TWE.Therefore, TWE can mitigate the LiPS side reactions and protect Li metal anodes more effectively than CVE.The inhibited LiPS side reactions were further validated in Li-S coin cells without LiNO 3 .The initial Coulombic efficiency (CE) in the cell with TWE is 77.8% compared with 47.4% of that with CVE (Figure 1C).The CE in the cell with TWE gradually rises during cycling and ultimately reaches about 92.0%, while that with CVE only remains at about 74.0%, indicating suppressed LiPS parasitic reactions with TWE (Figure S6).
Furthermore, the Li metal anodes after 8 cycles were disassembled to observe the morphology of cycled Li anodes (Figure S7).According to the scanning electron microscopy (SEM) images, loose and porous Li deposits with massive cracks and dendrites are observed on the cycled Li metal anode with CVE (Figure 1D).Conversely, the cycled Li metal anode with TWE exhibits a dense and homogeneous Li deposition morphology, indicating stabilized Li metal anodes by TWE (Figure 1E).Therefore, the LiPS parasitic reactions are suppressed in TWE to stabilize Li metal anodes and achieve long-cycling Li-S batteries.

The Li-S full cell performance of TWE
The effect of the as-proposed electrolyte at full cell level was further evaluated in Li-S coin cells.The Li-S coin cells were assembled under demanding conditions of using 4.0 mg cm −2 cathodes, 50 µm Li metal anodes, and a low E/S ratio of 6.6 µL mg −1 .The Li-S coin cells were cycled at 0.1 C (1 C = 1672 mA g −1 ) after activation at 0.05 C. The Li-S cell with TWE exhibits an initial discharge specific capacity of 855 mAh g −1 and maintains 535 mAh g −1 with 62.6% capacity retention after 221 cycles (Figures 2A and S8).In comparison, the Li-S cell with CVE maintains only 529 mAh g −1 with a capacity retention of 51.1% after 88 cycles, indicating a prolonged cycling lifespan of Li-S full cells with TWE (Figure S9).Moreover, the Li-S cell with TWE delivers a high initial CE near 100% and remains steady throughout the cycling process due to the suppressed LiPS parasitic reactions.Specifically, the voltage-specific capacity profiles of the Li-S cell with TWE are almost coincident at the 10 th and the 85 th cycles (Figure 2B).The voltage polarization remains unchanged even within 200 cycles (Figure S10).However, a sloping and shortened second discharge plateau is observed with enlarged voltage polarization at the 85 th cycle compared with that at the 10 th cycle in the Li-S cell with CVE, which originates from the failure of Li metal anodes with insufficient Li reservoir and accumulated inactive Li.
Li-S coin cells with ultrahigh sulfur loading of 7.9 mg cm −2 were further assembled and cycled at 0.1 C. The Li-S cell with TWE exhibits an initial areal capacity of 5.5 mAh cm −2 and stably undergoes 69 cycles with a remaining areal capacity of 2.1 mAh cm −2 (Figure 2C).In contrast, the areal capacity of the Li-S cell with CVE rapidly decays to 2.3 mAh cm −2 after 35 cycles due to Li anode failure.The Li-S cell with TWE also exhibits more stable CE during cycling compared with that with CVE.Similar results of voltage-specific capacity profiles are observed compared with the Li-S cells using 4.0 mg cm −2 sulfur cathodes.The voltage polarization of the Li-S cell with TWE remains stable from the 10 th cycle to the 30 th cycle while that with CVE deteriorates rapidly during cycling (Figure S11).The prolonged cycling lifespan of Li-S batteries and inhibited polarization increment indicate protected Li metal anodes and inhibited LiPS side reactions.
In order to further verify the inhibition of the LiPS side reactions in Li-S full cells, the cycled Li metal anodes were examined by post-analyses.X-ray photoelectron spectroscopy (XPS) was conducted to analyze the SEI composition on the cycled Li metal anodes after 50 cycles.
The wide-scan XPS spectra of the Li metal anode surface derived in CVE or TWE are shown in Figure S12.Similar species distributions are observed including Li 2 SO 4 , Li 2 SO 3 , Li 2 S 2 , and Li 2 S in the SEI with both CVE and TWE (Figure S13).However, the sulfur atomic content in the SEI with TWE is lower than that with CVE (1.61% vs. 2.58%, 1.73% vs. 2.57%, 1.31% vs. 1.82%, and 1.19% vs. 1.63% at 0, 90, 180, and 270 s sputtering time, respectively, Figure 2D).Therefore, the parasitic reactions between LiPSs and Li metal anodes in Li-S batteries are indeed mitigated by TWE.
Moreover, the Li deposition morphologies were observed using SEM.The Li metal anodes at the 10 th cycle were analyzed (Figure S14).More uniform Li deposition is observed in TWE compared with that in CVE (Figure S15).Moreover, denser Li deposition with a thickness of 62 µm is observed in TWE than that of 73 µm in CVE (Figure S16).The Li metal anodes at the 50 th cycle were further analyzed.The cycled Li metal anode in CVE exhibits a loose morphology with exposed Cu foils, while that in TWE maintains a complete Li morphology (Figure S17).According to the SEM images, the Li deposition in CVE is loose and porous with numerous cracks and cavities due to the serious parasitic reactions on working Li metal anodes (Figure 2E).In contrast, homogenous and dense Li deposition is observed in TWE (Figure 2F).Moreover, the thickness of deposited Li in TWE is 79 µm, which is much smaller than that of 102 µm in CVE, indicating denser Li deposition in TWE (Figure 2G,H).The dense and uniform Li deposition in TWE suggests that the as-proposed TWE effectively suppresses the LiPS side reactions and protects Li metal anodes in Li-S batteries.

The LiPS solvation structure in TWE
In order to gain insights into the Li metal anode protection effect of TWE, the solvation structure of LiPSs was probed.Molecular dynamics simulations were first conducted in an electrolyte system with 1.0 M [S] Li 2 S 8 (Figure S18).S 8 2− tightly interacts with Li + at the shortest distance of around 2.5 Å based on the centers of mass in both CVE and TWE, indicating strong interaction between Li + and Li 2 S 8 (Figure 3A,C).In CVE, DME emerges at the position of 4.5 Å from the center of S 8 2− through the S 8 2− -Li + -solvent interactions and constitutes the inner solvation shell (shell 1).DOL appears at the position of 6.5 Å and forms the outer solvation shell (shell 2).The coordination number of Li + , DME, and DOL starts to increase at 2.1, 4.1, and 4.9 Å, respectively, whose order is consistent with the peak positions of the radial distribution function (g(r)) due to the difference in solvating power of the solvents (Figure S19a).In contrast, DME also appears in the inner solvation shell (shell 1, 4.5 Å) while DIPS emerges only in the outer solvation shell (shell 2, 6.7 Å) in TWE due to the weak solvating power of DIPS (Figure 3B,D).The coordination number of DME also starts to increase at about 4.1 Å, while that of DIPS begins to increase at a position farther than 6.0 Å (Figure S19b).Therefore, the inner solvation shell of LiPSs remains unchanged to maintain satisfactory cathode conversion kinetics.The introduction of DIPS into TWE mainly changes the outer solvation shell of LiPSs to inhibit the LiPS side reactions.
The solvation structure of LiPSs was further experimentally verified by nuclear magnetic resonance (NMR) measurements.Considering the strong interaction of Li + with S 8 2− and solvents, 7 Li-NMR was employed to infer the distribution of solvents around LiPSs.To separately investigate the influence of DIPS instead of TXA or DOL, the mixed solvent of DME and TXA was selected as the control sample.Compared with 0.5 M [S] Li 2 S 8 in TXA/DME, the chemical shift of 7 Li moves downfield from 3.03 to 3.35 ppm after the introduction of DIPS due to the deshielding effect on Li + (Figure 3E).The deshielding effect on Li + , namely reduced electron cloud density around Li + , originates from weaker Li + -S electrostatic interaction in DIPS than Li + -O interaction in DME.Therefore, a heterogeneous solvation structure is constructed by the double-layer solvation shells to encapsulate LiPSs.The inner solvation shell mainly consists of solvents with strong solvating power such as DME, while DIPS with weak solvating power is distributed only in the outer solvation shell (Figure 3F).The as-constructed solvation structure of LiPSs enables mitigated LiPS parasitic reactions with satisfactory cathode redox kinetics maintained in Li-S batteries.

The formation of organic-rich SEI in TWE
The formation of SEI by TXA was investigated by postanalysis on the cycled Li metal anodes.XPS was firstly employed to analyze the SEI components on the cycled Li metal anode after 8 cycles.The wide-scan XPS spectra of the Li metal anode surface derived in CVE or TWE are shown in Figure S20.The SEI in TWE exhibits much higher carbon atomic content compared with that in CVE (64.7% vs. 62.4%, 43.5% vs. 31.9%,36.2% vs. 26.4%,31.9% vs. 22.0%, and 28.1% vs. 19.2% at 0, 30, 60, 90, and 120 s sputtering time, respectively), indicating more organic components provided by TWE (Figure 4A).The carboncontaining species observed in the SEI with TWE include C-C, C-O, C-SO x , C-F, and C-Li compounds (Figure 4B).The SEI in CVE exhibits a similar species composition as that in TWE (Figure S21).Specifically, it is generally considered that C-C and C-O mainly come from the organic components while the other species mainly come from the inorganic components in SEI.The percentages of the different species in the SEI constructed by CVE or TWE are displayed in Figure 4C.The ratio of organic components in the SEI derived from TWE is 72.5% in total including 65.2% of C-C and 7.3% of C-O, while that derived from CVE is much lower with 65.0% in total including 55.6% of C-C and 9.4% of C-O.Therefore, more organic components are generated in the SEI derived from TWE to fabricate an organic-rich SEI, which is considered to be mainly provided by the TXA co-solvent.Comparison with previously reported Li-S pouch cells regarding battery configuration and performance. 46,56,57[58][59][60][61] The organic-rich SEI formed in TWE was further verified by time-of-flight secondary ion mass spectrometry.C 2 H 2 O − fragments were selected as the representative component to investigate the organic components in SEI (Figure S22).The intensity of the C 2 H 2 O − fragment in the SEI derived from TWE is slightly higher than that derived from CVE on the surface (Figure 4D).However, the intensity of the C 2 H 2 O − fragment in the SEI derived from TWE keeps a high level during sputtering while that derived from CVE declines rapidly.After 400 s sputtering, the intensity of the C 2 H 2 O − fragment in the SEI derived from TWE is 2.5 times as that derived from CVE, indicating richer organic components are formed in the SEI derived from TWE. Visualization results are obtained through three-dimensional views of the spatial distribution of C 2 H 2 O − in SEI across a 100 × 100 µm 2 window.A high level of the C 2 H 2 O − fragment content is observed in the SEI derived from TWE throughout the whole sputtering process, while that derived from CVE reduces rapidly to a relatively low level at half of the sputtering time (Figure 4E,F).Therefore, organic-rich SEI is constructed in TWE afforded by TXA.

2.6
The performance of Li-S pouch cells using TWE The feasibility of TWE to protect Li metal anodes under practical high-energy-density working conditions was further verified in Li-S pouch cells.A 2.5 Ah-level Li-S pouch cell was assembled and evaluated.The Li-S pouch cell was assembled by stacking double-sided Li metal anodes and sulfur cathodes with a size of 4 × 7 cm 2 .The thickness of a single-side Li metal anode was 75 µm, the areal loading of a single-side sulfur cathode was 7.8 mg cm −2 , and the E/S ratio was 3.5 g g S −1 .The Li-S pouch cell with TWE exhibits an initial discharge specific capacity of 1021 mAh g −1 and the highest discharge specific capacity of 1132 mAh g −1 at the 12 th cycle after activation (Figure S23).The pouch cell with TWE delivers stably 39 cycles, and the voltage polarization remains steady during cycling (Figure S24).The energy density is 332 Wh kg −1 based on the pouch cell total mass (Table S1).In comparison, the Li-S pouch cell with CVE only cycles stably for 19 cycles under the same conditions (Figure S25).The voltage polarization of the second discharge plateau increases gradually during cycling, indicating severe LiPS parasitic reactions taking place (Figure S26).The working voltage reduces to below the cut-off voltage at the 20 th cycle, inducing a sudden drop in the discharge specific capacity and the failure of the pouch cell.Therefore, the Li-S pouch cell cycles more stable with TWE compared with CVE due to the suppressed LiPS side reactions and stabilized Li metal anodes.Furthermore, a 3.0 Ah-level Li-S pouch cell was assembled and evaluated to realize higher energy density and verify the effectiveness of TWE.Similar conditions with the 2.5 Ah-level pouch cell were employed except that the single-side sulfur loading was 7.9 mg cm −2 and the E/S ratio was 3.0 g g S −1 .The initial energy density of the Li-S pouch cell with TWE is up to 405 Wh kg −1 based on the pouch cell total mass (Table S2).The Li-S pouch cell with TWE exhibits an initial discharge specific capacity of 1292 mAh g −1 and undergoes stably 27 cycles due to mitigated side reactions of LiPSs (Figure 5A).The voltage polarization remains almost unchanged from the 1 st to the 27 th cycle (Figure 5B).In comparison, the Li-S pouch cell with CVE under the same conditions only cycles stably for 15 cycles and exhibits extreme voltage polarization without the second discharge plateau at the 16 th cycle due to the serious LiPS parasitic reactions (Figures S27 and S28).
Compared with recent research, more demanding condi-tions are employed in this work and more outstanding performances are realized by using TWE (Figure 5C). 46,56,577][58][59][60][61] The Li-S pouch cells using the as-proposed TWE realize superior overall performances with regard to high energy density and long cycling lifespan in contrast with the other reported works.
The cycled Li-S pouch cells at the 400 Wh kg −1 level were disassembled to evaluate the parasitic reactions on the Li metal anode.Extensively exposed Cu current collector and fragmented deposited Li are observed in the pouch cell with CVE (Figure 6A) and a large amount of inactive Li remains on the separator (Figure 6C), indicating severe parasitic reactions and unstable Li metal anodes in CVE.Conversely, more complete Li metal anodes and little inactive Li on the separator are observed in the pouch cell with TWE (Figure 6B,D), suggesting that the Li metal anodes are effectively stabilized by the inhibited LiPS parasitic reactions in TWE.The deposition morphologies of the cycled Li metal anodes were further investigated by SEM.Uneven and broken deposited Li is observed in CVE with large cracks and holes (Figure 6E).Specifically, chunky Li and dendritic Li exist simultaneously on the Li metal anode in CVE (Figure S29).However, uniform and dense Li deposition is observed in TWE (Figure 6F) and chunky Li piles up compactly (Figure S30).Furthermore, the sulfur-containing species on the cycled Li metal anodes were detected by XPS.High-valence sulfur species including Li 2 SO 4 , Li 2 SO 3 , and Li 2 S 2 O 3 mainly exist on the surface of the SEI derived from TWE, while low-valence sulfur species including Li 2 S and Li 2 S 2 exist throughout the SEI during 160 s sputtering (Figure 6G).The SEI in CVE exhibits a similar species distribution to that in TWE (Figure S31).Based on the analysis of the cycled Li metal anodes in pouch cells, TWE is proved to effectively suppress the LiPS parasitic reactions and stabilize Li metal anodes in Li-S pouch cells.As a result, the cycling lifespan of high-energy-density Li-S pouch cells is significantly prolonged with TWE.

CONCLUSION
A TWE with ternary solvents is proposed to enable high-energy-density and long-cycling Li-S batteries.DME is employed to maintain moderate LiPS solubility and smooth cathode conversion kinetics due to its strong solvating power.A nano-heterogeneous solvation structure with encapsulated LiPSs is constructed by introducing the DIPS co-solvent to enable mitigated the LiPS parasitic reactions with satisfactory cathode redox kinetics maintained.Furthermore, an organic-rich SEI is produced by the TXA co-solvent to enable uniform Li deposition.Consequently, the cycling lifespan of Li-S coin cells with 50 µm Li anodes and 4.0 mg cm −2 sulfur cathodes is prolonged from 88 to 222 cycles due to the mitigated LiPS side reactions and protected Li metal anodes.Furthermore, the TWE enables the Li-S pouch cell to achieve 39 cycles with an initial energy density of 332 Wh kg −1 and 27 cycles with an initial energy density of 405 Wh kg −1 .This work affords a TWE recipe for the LiPS side reaction suppression and inspires rational electrolyte design for practical high-energy-density and long-cycling Li-S batteries.

F I G U R E 1
The electrochemical performance of TWE.(A) The electrochemical impedance spectroscopy (EIS) spectra of Li 2 S 6 symmetric cells with CVE or TWE.(B) The cycling performance of Li | Li symmetric cells at 1.0 mA cm −2 and 4.0 mAh cm −2 .(C) The voltage-specific capacity profiles of Li-S coin cells without LiNO 3 .SEM images of deposited Li with (D) CVE and (E) TWE after 8 cycles.

F I G U R E 2
The performance evaluation of Li-S coin cells.(A) The cycling performance of Li-S coin cells with a high-areal-loading sulfur cathode of 4.0 mg cm −2 , an ultrathin Li metal anode of 50 µm, and a low E/S ratio of 6.6 µL mg −1 at 0.1 C. (B) The corresponding voltage-specific capacity profiles at the 10 th and the 85 th cycle.(C) The cycling performance of Li-S coin cells with an ultrahigh-areal-loading sulfur cathode of 7.9 mg cm −2 and an ultrathin Li metal anode of 50 µm at 0.1 C. (D) Sulfur atomic content of the SEI at different sputtering times after 50 cycles.SEM images of deposited Li with (E) CVE and (F) TWE and cross-sectional SEM images of Li metal anodes with (G) CVE and (H) TWE after 50 cycles.

F I G U R E 3
The solvation structure of LiPSs in electrolyte.The radial distribution function (g(r)) around S 8 2− in (A) CVE and (C) TWE.Snapshots of the molecular distributions around S 8 2− in (B) CVE and (D) TWE.The green, yellow, gray, and white spheres represent Li, S, C, and H atoms, respectively.The DOL molecules are shown in the wireframe form while the DME and TFSI − molecules are omitted for clarity.(E) The 7 Li-NMR spectra of 0.5 M [S] Li 2 S 8 in electrolyte with or without DIPS.(F) The schematic diagram of the solvation structure of LiPSs in TWE.

F I G U R E 4
The evaluation of the SEI derived from TWE. (A) Carbon atomic content of the SEI at different sputtering times after 8 cycles.(B) C 1s XPS spectra of the SEI derived from TWE after 60 s sputtering.(C) Species ratio of different C species at different sputtering times in the SEI derived from CVE or TWE.(D) Depth sputtering profiles of C 2 H 2 O -by time-of-flight secondary ion mass spectrometry (ToF-SIMS).The corresponding three-dimensional distribution views of C 2 H 2 O − in the SEI derived from (E) CVE and (F) TWE.

F I G U R E 5
The electrochemical performance of Li-S pouch cells with TWE.(A) The cycling performance of the 3.0 Ah-level Li-S pouch cell with TWE.The inset is the optical image of the Li-S pouch cell.(B) The corresponding charge-discharge profiles at different cycles.(C)

F I G U R E 6
The failure analysis of the cycled pouch cells.Optical images of the Li metal anodes after cycling with (A) CVE and (B) TWE.Optical images of the separators after cycling with (C) CVE and (D) TWE.SEM images of deposited Li after cycling with (E) CVE and (F) TWE.(G) S 2p XPS spectra of the SEI derived from TWE at different sputtering times.
This work was supported by the Beijing Municipal Natural Science Foundation (Z200011), the National Key Research and Development Program (2021YFB2500300 and 2021YFB2400300), the National Natural Science Foundation of China (22061132002, 22379013, and T2322015), the Seed Fund of Shanxi Research Institute for Clean Energy (SXKYJF015), the S&T Program of Hebei Province (22344402D), the Tsinghua-Jiangyin Innovation Special Fund (TJISF), the Tsinghua-Toyota Joint Research Fund, the Institute of Strategic Research, Huawei Technologies Co., Ltd., and the Ordos-Tsinghua Innovative & Collaborative Research Program in Carbon Neutrality.The authors acknowledged the support from Tsinghua National Laboratory for Information Science and Technology for theoretical simulations.The authors thank Qian-Kui Zhang, Yu-Chen Gao, Zi-Xian Chen, Yun-Wei Song, Chang-Xin Zhao, Shu-Yu Sun, and Prof. Jia-Qi Huang for helpful discussion.C O N F L I C T O F I N T E R E S T S TAT E M E N T The authors declare no conflict of interest.O R C I D Qiang Zhang https://orcid.org/0000-0002-3929-1541R E F E R E N C E S