Use of Tween Polymer To Enhance the Compatibility of the Li

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The Use of Tween Polymer to Enhance the Compatibility of Li/Electrolyte Interface for High Performance and Safety Quasi-Solid-State Lithium Sulfur Battery jie liu, Tao Qian, Mengfan Wang, Jinqiu Zhou, Na Xu, and Chenglin Yan Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b01882 • Publication Date (Web): 01 Jun 2018 Downloaded from http://pubs.acs.org on June 1, 2018

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The Use of Tween Polymer to Enhance the Compatibility of Li/Electrolyte

Interface

for

High

Performance

and

Safety

Quasi-Solid-State Lithium Sulfur Battery Jie Liu, Tao Qian, Mengfan Wang, Jinqiu Zhou, Na Xu, and Chenglin Yan*

Soochow Institute for Energy and Materials InnovationS, College of Physics, Optoelectronics and Energy & Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou 215006, China. Key Laboratory of Advanced Carbon Materials and Wearable Energy Technologies of Jiangsu Province, Soochow University, Suzhou 215006, China.

Abstract: Lithium metal batteries attract increasing attention recently due to its particular advantages in energy density. However, as for practical application, the development of solid-state lithium metal batteries is restricted because of the poor Li/electrolyte interface, low Li-ion conductivity and irregular growth of Li dendrites. To address above issues, we herein report a high Li-ion conductivity and compatible polymeric interfacial layer by grafting tween-20 on active lithium metal. Sequential oxyethylene groups in tween-grafted Li (TG-Li) improve the ion conductivity and the compatibility of Li/electrolyte interface, which enables low overpotentials and stable performance over 1000 cycles. Consequently, the poly(ethylene oxide)-based solid-state lithium sulfur battery with TG-Li exhibits a high reversible capacity of 1051.2 mAh g-1 at 0.2 C (1 C = 1675 mAh g-1) and excellent stability for 500 cycles at 2 C. The decreasing concentration of sulfur atom with increasing Ar+ sputtering depth indicates that the polymer interfacial layer works well in suppressing polysulfide reduction to Li2S/Li2S2 on metallic Li surface even after long time cycles. Keywords: Quasi-Solid-State Li-S Battery, Li/electrolyte interface, in-situ XRD, polysulfide diffusion

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Lithium metal batteries (LMBs) are considered as one of the optimal energy storage systems with high energy density because Li metal anode has ultrahigh specific capacity of 3,860 mAh g−1, a very low redox potential (-3.040 V versus standard hydrogen electrode) and a small gravimetric density of 0.534 g cm-3.1-4 However, the poor cyclability and potential safety hazard always seriously constrained the development of LMBs.5,6 It is well-known that the solid electrolyte interphase (SEI) layer on lithium surface, resulting from the spontaneous reaction between reactive lithium metal and most organic electrolyte, always continuously generate and break during repeated cycling.7-9 As a result, both lithium metal and electrolyte suffer from a constant loss, which lead to low Coulombic efficiency and rapid capacity decay. Meanwhile, there are always potential safety issues resulting from the instability and flammability of organic electrolytes, as well as the terrible growth of lithium dendrites.7,10,11 As a potentially practical application of LMBs, the solid-state LMBs partly solve the above obstacles, especially the security issue, and thus the development of solid-state LMBs draws increasing

attention

in

recent

years.

Due

to

high

ionic

conductivity,

good

chemical/electrochemical stability, high mechanical strength and close adhesion to electrodes, gel polymer electrolytes (GPEs) are extensively investigated in solid-state LMBs.12-15 GPEs can be obtained by immobilizing the organic solvents such as EC, DEC, DME and DOL, etc. in a polymer network. The polymer network ensures the mechanical stability, and the organic electrolyte make contributions to high ionic conductivity.14 As a stable polymer with sequential oxyethylene groups (-CH2CH2O-), PEO exhibits good compatibility with electrodes and is considered suitable as the polymer matrix of GPEs.16 The oxyethylene groups can coordinate with lithium ions and help to dissociate and dissolve Li salts, which further enhance the ionic conductivity of solid-state battery.14,17 However, although PEO-based GPEs are extensively researched in solid-state battery, their progress is decelerated due to some remaining tough problems. The embedded organic electrolyte improves the conductivity of GPEs but also threatens the battery security due to the inevitable decomposition of organic electrolyte after long-term cycling.18,19 The continuous generation and break of SEI on bare Li lead to a tough interphase.7,20 Moreover, the issue on Li-dendrite is still retained, which resulted in safety hazard and limited long-lifespan cycle ability for these batteries matching with metallic lithium. These negative effects are magnified 2


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in varying degrees when GPEs are used in lithium-sulfur (Li-S) battery. Due to its high theoretical specific capacity (1675 mAh g-1) and energy density (2600 Wh kg-1) relative to conventional lithium ion batteries, Li-S battery attracts lots of attentions nowadays.21-25 Li-S battery based on PEO system is expected to perform the advantage of GPEs and improve the safety of Li-S battery. But the unexpected deposition of electron/ion-insulated Li2S on Li anode resulting from the dissolution and diffusion of lithium polysulfides decreases the ion conductivity of the resultant Li-S batteries and deteriorates the electrolyte-anode interface.26-28 The worsening compatibility between electrolyte and Li anodes renders low reversible capacities and poor cyclability to the batteries. Herein, we fabricated a compatible polymer interfacial layer on the surface of lithium metal to achieve high Li-ion conductivity and the resistance to polysulfides. This “smart” layer makes good interfacial connection and helps to improve the compatibility between Li anode and electrolyte. Tween 20, an organic polymer with sequential oxyethylene groups and alkyl groups, is employed to complete this novel pattern. The -CH2CH2O- structure is expected to facilitate the continuous ion transport and long alkyl chain in tween molecular can effectively damped the polysulfides diffusion, which was revealed by density functional theory (DFT) calculations. As a result, this polymer layer can efficiently keep polysulfides from frequent reduction by active lithium so that a reliable and compatible interface between anode and electrolyte is obtained as exhibited in Figure 1a. The symmetric cell test elucidates the as-prepared TG-Li has low overpotential and can be stable for 1000 cycles. In-situ XRD and pressure measurement demonstrate that TG-Li can efficiently prevent the decomposition of electrolyte and thus improve the safety of batteries. When using TG-Li as anode, the PEO-based solid-state Li-S battery shows a high reversible capacity of 1051.2 mAh g-1 at 0.2 C. Importantly, the battery still delivers a high specific capacity of 552.1 mAh g-1 when the current rate is increased to 4 C, which is attributed to the high ionic conductivity of the interfacial layer. The prominent cycle performance can be demonstrated by the remarkable capacity retention of 98.4% at 1 C over 150 cycles and the long-term cycling for 500 cycles with a low decrement rate of 0.058% per cycle based on the initial discharge capacity. X-ray photoelectron spectroscopy (XPS) of TG-Li at different depths of interfacial layer evidenced that the polymer layer worked well in obstructing polysulfides diffusion in resultant 3


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PEO-based Li-S battery. Undoubtedly, this new strategy achieves high Li ion conductivity and remarkable compatibility between solid electrolyte and anode, as well as a long-term and stable interface via inhibiting the shuttle effect, which provides a promising prospect in advanced solid-state Li-S battery system. In order to fabricate uniform polymer interfacial layer on lithium surface, the lithium wafer was immersed into tween/anhydrous THF solution. Then the lithium plate was then taken out and rinsed by THF to move out the residual tween, followed by drying at room temperature. To exclude the impact of solvent, we put bare Li wafer into anhydrous tetrahydrofuran (THF) solution (denoted to THF-Li) and almost no morphology change can be seen, as shown in Figure S1a. In contrast, after immersing bare Li in tween-20 for the same time, the Li surface presents an enormous change (Figure S1b), which indicates that in the competing reaction system of tween-20/THF solution, the inactive reaction between THF and Li metal can be ignored. All of the above processes were carried out in a glovebox (Ar atmosphere, O2 < 0.1 ppm and H2O < 0.1 ppm). The cross-section scanning electron microscopy (SEM) images of TG-Li (Figure S2a, S2b) and the elemental mapping (Figure S3), showing the homogeneous distribution of carbon that derives from tween molecular, indicate the polymer interfacial layer forms on the metal surface with an average thickness of ~5 µm, which is significant to keep the polysulfides away from metallic Li. XPS results in Figure S4a demonstrate that tween molecule reacted with lithium metal by giving the compositional analysis of TG-Li. The binding energies are calibrated with respect to the C 1s peak at 284.8 eV. The results confirm the chemical composition of the polymer layer with the presence of C and O. Besides the strong hydrocarbon peak (284.6 eV), the high-resolution C 1s spectra shows two main C peaks at 289.6 and 286.1 eV, corresponding to -O(C=O)- and C-O, respectively, as shown in Figure S4b.29,30 DFT calculations were used to evaluate the resistance of alkyl chains to polysulfides and the results are shown in Figure 1b. The affinity between carbon chain and Li2S8 is as low as -0.12 eV, which is much lower than that of the common electrolyte solvents 1,3-Dioxolane (DOL) and dimethoxyethane (DME) to Li2S8 (-0.75 eV and -0.60 eV, respectively). The binding energy between alkyl chains and Li2S6 (-0.13 eV) were also computed to manifest the weak interaction between alkyl chains and Li2S6. Figure 2a simulates the different procedures 4


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when polysulfides shuttle and gets close to bare Li and TG-Li surface. It is well-known that a SEI layer forms when lithium reacts with most organic electrolyte solvents.7,31 However, the SEI layer is easily broken when Li unevenly plates/strips. What’s worse, the shuttled polysulfides will be irreversibly reduced to lithium sulfide on the metallic Li surface. Both of that eventually result in the formation of dendrites and “dead Li”, which results in low Coulombic efficiency and rapid capacity degeneration, as well as potential safety hazards.32 After being grafted with tween molecular, the lithium surface is full of alkyl chains, which sufficiently suppress the contact and reaction between polysulfides and lithium metal. To reveal the suppressed redox, we directly put TG-Li and bare Li into 2 mM Li2S8 solution and then characterize their surface by XPS. Li2Sx (1 ≤ x ≤ 4) signals with high intensities are observed on bare Li surface, which is attributed to the active reaction between metallic Li and Li2S8 (Figure S5a). In comparison, the TG-Li surface shows weak signal of S 2p spectra, attributing to the residual Li2S8 that blocked in the interfacial layer (Figure S5b). The barely visible Li2Sx (1 ≤ x ≤ 4) signals indicate that the polymer layer can efficiently keep polysulfides from frequent reduction by active Li metal.33 We further conducted the UV/Vis spectroscopy investigation to observe the reflection variation of Li2S8 solution before and after immersing bare Li or TG-Li (Figure S6). The UV/Vis spectroscopy of TG-Li is almost overlapped, which means that Li2S8 is not involved in any redox reaction. In comparison, the reflection curves of bare Li shows a noticeable reflection change and transformation to Li2Sx (1 ≤ x ≤ 6), indicating that Li2S8 is easily reduced by active bare Li.25,34 Sequential oxyethylene groups on TG-Li impel the rapid delivery of Li-ion, which can facilitate even Li plating/stripping. To estimate the electrochemical performance of TG-Li, quasi-solid-state symmetrical cells were assembled with two identical TG-Li or bare Li foil. Figure S7 plots the dependence of the conductivity on 1/T for the PEO-based electrolyte in the temperature range from 25 °C to 60 °C, showing the conductivity of 5.01 × 10-5 S cm-1 at room temperature, and the transference number of PEO-based electrolyte was ~0.32. Figure 2b exhibits the voltage hysteresis of two symmetrical cells at a current density of 0.5 mA cm−2. For the symmetrical bare Li cell, the crude oxide layer and spontaneously formed SEI hinder the transport of Li-ion, which results in a difficult Li plating/stripping and contributes to the high overpotential. The continuous generation of new SEI layer leads to the rough change of 5


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overpotential. A significant voltage increase was found in the bare Li symmetrical cell after 200 hours plating/stripping, which is ascribed to the inherent instability of spontaneously formed SEI during discharging and side reactions. A same situation was observed in THF-Li symmetrical cell, which exhibits an increasing overpotential after several plating/stripping cycles, indicating that THF-Li cannot maintain the stable and fast Li+ transfer (Figure S8). The TG-Li symmetrical cell exhibits stable voltage profiles at a low overpotential of ∼17 mV and the flat voltage plateaus can be maintained for prolonged lifetime over 1000 h without an obvious “bump” during cycling. This result indicates a lower energy barrier for both its nucleation and stripping processes, which is attributed to the durability of polymer interfacial layer and the prominent Li-ion conductivity arose from sequential oxyethylene groups. The overpotential at a current density of 1.0 mA cm−2 is shown in Figure S9, which also exhibits a low overpotential and better stability of TG-Li. After discharging/charging 200 h, the symmetrical cells are disassembled. It can be found that the surface of bare Li became more rugged and the bulk dead Li was deposited on it (Figure 2c, 2d), which seriously decrease the ion conductivity and deteriorate the battery stability. However, TG-Li has little change in morphology (Figure 2e, 2f), indicating the splendid Li-ion transport capacity of TG-Li. In-situ XRD was performed to directly monitor the variation of lithium anode in PEO-based Li-S battery during the charge-discharge process (Figure 3a). Figure 3b and 3c show the contour maps of in-situ XRD patterns collected during the initial cycle of Li-S cells using TG-Li (noted as TG-Li//S) and bare Li foil (noted as bare Li//S) as the anodes, respectively, at a rate of 0.3 C, where blue color represents high intensity. It is obviously seen that the XRD peaks at 21.2° and 23.4° appear in bare Li//S battery, which are identified as the Li2CO3 phase decomposing from the ether-based electrolyte,35,36 but cannot be detected in TG-Li//S battery. This result convinces us that TG-Li can efficiently prevent the decomposition of electrolyte, which can be further evidenced by subsequent pressure measurements. Besides, after immersing the TG-Li and bare Li in the electrolyte for 24 h, we directly characterized their surface. The XRD profiles of TG-Li show similar peaks before and after immersing and there is no new signal appeared (Figure S10). However, after dipping bare Li into the electrolyte for same time, the signals of Li2CO3, Li2SOx (3 ≤ x ≤ 7) in corresponding XRD pattern can be observed,10, 35, 36 which were ascribed to the decomposition products of 6


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electrolytes. It is well known that some by-products, such as CH4, H2 and CO2, etc, would form upon battery cycling, especially during charge, due to the decomposition of electrolyte when it comes into contact with the fresh lithium.37 To eliminate interference, diglyme using lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) as the electrolyte salt was chosen to replace the more commonly used DOL and DME solvent due to its lower volatility. Figure 3f shows the pressure trend when the battery using bare Li and TG-Li as the anode, respectively, are cycled at 0.1 C. For the battery with bare Li as anode, the pressure always increases and the tendency goes up faster after cycling 18 h. The pressure increases by 0.8 psi after 50 h. In contrast, the pressure-time curve of TG-Li//S battery shows a much smaller slope, demonstrating much lower pressure increase during 40 h cycling. This result suggests that the long-term and stable interface of TG-Li anode can evidently prevent the decomposition reactions of electrolyte, which is quite significant for battery security. Due to the shuttle effect, polysulfides will diffuse from cathode to anode and eventually reduce to Li2S when come into contact with lithium metal. For bare Li//S battery, the Li2S phase evolution can be obviously seen at ~13.6° as shown in Figure 3d and the ascending intensity indicates that Li2S deposits on lithium surface ceaselessly.38 However, no peak related to Li2S is observed during TG-Li//S battery cycles as shown in Figure 3e, revealing that the Li2S cannot be deposited on lithium anode, which can be further confirmed by the comparison of Li2SOx (3 ≤ x ≤ 7) phase at 17.4° and 25.6° as shown in the XRD pattern.10,36 It is clearly seen that the Li2SOx (3 ≤ x ≤ 7) phase in bare Li//S battery (Figure 3b), resulting from the oxidation of Li2S and Li2S2 by LiNO3, cannot be observed in TG-Li battery (Figure 3c) during the discharge-charge procedure, which is due to the fact that the polymer interfacial layer of TG-Li can effectively suppress the reduction reaction between polysulfides and lithium metal. Using the stable TG-Li as anode, we further demonstrate their prominent electrochemical performances in PEO-based solid-state Li-S batteries. Figure 4a clearly exhibits the rate performance differences between solid-state TG-Li//S and bare Li//S cells. For TG-Li//S cell, the specific capacity of 1,051.2 mAh g-1 can be obtained at 0.2 C, and the capacity still remains over 578.6 mAh g-1 even when the current rate is up to 4 C. In contrast, much lower specific capacity retention is observed for the bare Li//S cell as the capacity dropped 7


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significantly from ∼982.6 mAh g-1 at 0.2C to less than 285.1 mAh g-1 at 4C, which is attributed to the high resistance and the difficult Li+ transfer especially at high current densities. Figure 4b and Figure S11 show the rate capability differences of TG-Li anode tested in liquid ether-based electrolyte and PEO-based electrolyte, respectively. The brilliant rate capacity indicates the excellent compatibility between TG-Li and gel electrolyte. The greatly improved battery reversible capacity of PEO-based cell with TG-Li as anode is found to be closely associated with Li-ion transport in the electrode. Cyclic voltammetry (CV) analysis is implemented to estimate the Li+-ion diffusion coefficients (DLi) of the cathodes according to Randles–Sevcik equation as following: Ip= (2.69×105) n3/2ADLi1/2CLiν1/2

(1)

where Ip is the peak current, n is the number of reaction electrons (n = 2 for Li-S battery), A is the area of electrode (A = 1.13 cm2), DLi is the lithium ion diffusion coefficient, CLi is the concentration of Li+ (CLi = 1.16 × 10-3 mol cm-3) and ν is the scanning rate.39,40 This equation reveals that the electrochemical process of a cell is affected by Li+ diffusion. Figure 4c shows the CV curves of TG-Li at different scan rates from 0.1 to 0.5 mV s-1. From the linear relationship of Ip and ν1/2 (Ip-A = 0.3672ν1/2 – 3.01E-4, Ip-C = -0.2668ν1/2 – 2.93E-4) as shown in Figure 4d, the lithium ion diffusion coefficient (DLi+-C: cathodic peak at ~2.05 V, and DLi+-A: anodic peak at ~2.30 V) are obtained, where DLi+-C = 7.16 × 10-8 cm2 s-1 and DLi+-A = 1.40 × 10-7 cm2 s-1. In contrast, the bare Li foil exhibits lower diffusion coefficient (DLi+-C = 3.36 × 10-8 cm2 s-1 and DLi+-A = 7.53 × 10-8 cm2 s-1) as shown in Figure S12a and S12b. This result suggests that the polymer layer of TG-Li can facilitate Li+ transport and its conductivity is even better than the spontaneous SEI, which is of quite significance to figure out the poor interface issue between GPE and bare Li metal. The electrochemical impedance spectroscopy (EIS) measurements of PEO-based TG-Li//S and Li//S cells before and after 50 cycles were carried out as shown in Figure 4e. Previous studies showed that the semicircle in the low-frequency range could reflect the charge transfer process, while the semicircle in the high-frequency region could be assigned to the surface film resistance, which is associated with SEI and insulating layer of solid Li2S/Li2S2.39,41 It is clearly seen from Figure 4e that before cycling, TG-Li//S cell has a considerably lower charge transfer resistance (~ 20.4 Ω) compared to bare Li//S cell (~ 40.3 8


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Ω), which indicates the poor Li/electrolyte interface in bare Li//S cell and the fast lithium ion diffusion in TG-Li//S cell.39 After 50 cycles, TG-Li//S cell shows much lower interfacial resistance value in high-frequency region (6.8 Ω) than bare Li//S cell (14.2 Ω). This is because the compatible interfacial layer of TG-Li can suppress the deposition of Li2S/Li2S2 on the metallic lithium surface. This result is in good agreement with the analysis of the lithium ion diffusion coefficients. Furthermore, we characterize the impedance of both TG-Li//S and bare Li//S cells in liquid electrolyte before cycling as shown in Figure S13a and S13b. Both bare Li//S and TG-Li//S cell in PEO-based electrolyte have larger interfacial resistance than that in liquid electrolyte. But, the interfacial resistance of TG-Li//S cell is smaller than that in bare Li//S cell with PEO-based electrolyte, which means TG-Li//S cell achieves compatible and high ion conductivity interface in PEO-based electrolyte. Figure 4f compares the cycling performances between TG-Li//S cell and bare Li//S cell at the current rate of 1C for 150 cycles. The TG-Li//S cell delivers high initial discharge capacity of 814.7 mAh g-1. After 150 cycles of charge/discharge, a reversible capacity of 801.3 mAh g-1 can be obtained (capacity retention of 98.4%). In contrast, the bare Li//S cell shows a relatively faster decline of capacity, decreasing from 731.0 mAh g-1 to 385.2 mAh g-1 (corresponding to a capacity retention of 52.6%). The fast capacity decay can also be observed in THF-Li//S battery and the inferior performance demonstrates that the THF-Li made no impact on the improvement of battery cycling (Figure S14). The TG-Li//S cell exhibits a high initial capacity of 1002.1 mAh g-1 when the sulfur loading is 2.8 mg cm-2, and it still keeps excellent stability even the loading is as high as 4.9 mg cm-2 as shown in Figure 4g. Long-term performance of TG-Li//S demonstrates the excellent stability and Coulombic efficiency, as well as high specific capacity, for 500 cycles at 2C with a low decrement rate of 0.058% per cycle (Figure 4h). Figure S15 shows the long-term cycling of the TG-Li//S batteries without LiNO3 additive in the electrolyte, which demonstrates the similar stability with that using LiNO3 additive. A relatively higher Coulombic efficiency in the cell with LiNO3 additive is because LiNO3 can catalyze the conversion of highly soluble polysulfides to slightly soluble elemental sulfur on the cathode.42,43 Additionally, TG-Li is proved to be also outstanding in improving the electrochemical performance in commercial LiFePO4 (LFP) battery. As shown in Figure S16, the TG-Li//LFP cell maintains the capacity over 132.5 mAh 9


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g-1 after 200 cycles at the rate of 1C (1C = 178 mAh g-1) and keeps the Coulombic efficiency at around 99.4% all the time, while the cell using bare Li as anode presents an obviously decreased capacity and efficiency. To get the insights into restriction of TG-Li to polysulfides in long-term, XPS investigation of TG-Li was applied to investigate the ratio variation of different elements at the different depth of polymer interfacial layer after the anode is discharged/charged 100 times, which can be achieved by etching the polymer layer with Ar+-ion sputtering (Figure 5a). Figure 5b shows the XPS depth profiles of Li 1s, where the curves go from green to red corresponding to increasing detection depths. The Li-S peak at ~55.6 eV is detected at the early stage of sputtering.44 As the sputtering goes on, the detection region goes deep into the polymer layer. As a result, the Li-S peak gradually shifts to Li-O at ∼54.4 eV, which indicates the polysulfides are on the decline as the detection depth increases.45 The signal of Li0 at ~52.7 eV arises when the detection region is near the surface of the metallic Li foil underneath the polymer layer. In order to further illustrate the resistance of polymer layer to polysulfides diffusion, the high-resolution S 2p spectra of TG-Li at different etching depth is exhibited as shown in Figure 5c. The strong peaks at 160.2 eV, 161.5 eV, 162.7 eV, corresponding to Li2S, Li2S2 and Li2Sx (4 ≤ x ≤ 8) respectively, are observed when the etch depth is 0 nm, which is attributed to the diffusion of highly soluble polysulfides (Figure 5d). The peak intensities decreased significantly when the etching depth is up to 1500 nm and 3000 nm (Figure 5e and 5f), demonstrating the very low level of lithium sulfides near metallic lithium. The decreasing concentration of sulfur atomic as the etching depth increasing is shown in Figure 5g and 5h, indicating that the polymer interfacial layer plays in important role in restricting polysulfides. In conclusion, we constructed a stable polymer interfacial layer with high ion conductivity on active lithium metal, which can achieve excellent compatibility between Li anode and solid electrolyte. The alkyl chains in polymer layer refuses polysulfides to go near the active metallic lithium and thus prevent the reaction between polysulfides and metallic lithium in Li-S battery as confirmed by DFT calculations. This interfacial layer efficiently suppresses the growth of Li dendrite and induces TG-Li anode to a low overpotential. It is noted that TG-Li can improve the battery security by inhibiting the decomposition of electrolyte as demonstrated by in-situ XRD and pressure measurements. The high conductivity of polymer 10


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layer and compatible Li/electrolyte interface guarantee the solid-state Li-S battery a high reversible capacity of 1051.2 mAh g-1 at 0.2 C. The prominent cycle performance can be demonstrated by the remarkable capacity retention of 98.4% at 1 C over 150 cycles and the long-term cycling for 500 cycles with a low decrement rate of 0.058% per cycle. After discharge/charge 100 times, the TG-Li was used to investigate the S 2p spectra at different depth of interfacial layer by Ar+-ion sputtering. The decreasing sulfur atom concentration with increasing Ar+ sputtering depth well indicates that the polymer layer is sufficient in resisting the diffusion of polysulfides to the surface of the metallic lithium.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: nl-2018-01882e. Experimental

Section,

computational

method,

SEM

results,

XPS

results,

UV/Vis

spectroscopy of Li2S8 solution, and additional details on solid-state battery. This material is available free of charge via the Internet at http://pubs.acs.org

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

Author Contributions J.L. and T.Q. contributed equally to this work.

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS We acknowledge the support from the National Natural Science Foundation of China (No. 51622208 and No. 21703149) and Natural Science Foundation of Jiangsu Province (No. 11


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BK20150338).

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Figure 1. a) Schematic diagram of tween-grafted lithium metal. The polymer interfacial layer can provide high ion conductivity in PEO-based Li-S battery and efficiently keep lithium polysulfides away from active metallic lithium and isolate their frequent reduction. Moreover, this polymer layer can improve the compatibility between Li electrode and electrolyte due to the polymer structure of tween. b) DFT calculations on the affinity between the alkyl chain and Li2S8, as well as Li2S6, to evaluate the resistance of alkyl chains to polysulfides.

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Figure 2. (a) Schematic exhibits the mechanisms of the Li stripping/plating behaviors of bare Li and TG-Li. (b) The voltage profiles of bare Li foil symmetric cells and TG-Li symmetric cells at a current density of 0.5 mA cm-2, and all the electrochemical characterizations were carried out at 25 °C. The cross section SEM images of (c) Bare Li and (e) TG-Li before stripping/plating and (d) (f) Their changes in morphologies after tripping/plating 50 times. Scale bar 20 um. All the electrochemical characterizations were carried out at 25 °C.

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Figure 3. (a) The schematic of in-situ XRD investigation. The contour maps of in-situ XRD patterns collected during the initial cycle of (b) bare Li//S and (c) TG-Li//S battery at a rate of 0.3 C. The Li2S phase evolution in (d) Bare Li//S and (e) TG-Li//S battery, demonstrating that Li2S cannot deposit on metallic Li surface of TG-Li. (f) The pressure measurement exhibits much smaller pressure change in TG-Li//S battery, which indicates TG-Li can efficiently improve the safety of electrolyte.

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Figure 4. (a) Rate performances of TG-Li//S and bare Li//S cells. (b) Rate capability of TG-Li anode tested in PEO-based electrolyte. (c) Representative voltammograms of the TG-Li obtained at different scan rates and (d) the linear relationship of Ip and ν1/2. (e) Nyquist plots of the impedance spectra of the TG-Li//S and bare Li//S cells before/after 50 cycles at a current rate of 1C. (f) Cycling performance comparison between TG-Li//S cell and bare Li//S cell at current rate of 1C for 150 cycles. (g) Cycling performance of TG-Li//S cell with high sulfur loading of 2.8 mg cm−2 and 4.9 mg cm−2 at a current density of 500 µA cm−2. (h) The specific capacity and Coulombic efficiency of the TG-Li//S cell at 2C rate. All the electrochemical characterizations were carried out at 25 °C.

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Figure 5. (a) The schematic of XPS analysis at the different depths achieved by etching the TG-Li anode with Ar+-ion sputtering. (b) XPS depth profiles of Li 1s, where the curves go from green to red corresponding to increasing detection depths (0 ~ 3000 nm). The Li-S peak gradually shifts to Li-O as the detection depth increases, indicating the polysulfides are on the decline. (c) XPS spectra of S 2p at different etching depth. The high-resolution S 2p spectra of TG-Li at the etching depth of (d) 0 nm, (e) 1500 nm and (f) 3000 nm, demonstrating the efficiently rejection of polymer layer to polysulfides. (g) Summary of the atomic concentration of Li, S and C on TG-Li anode with the etching depth increasing. (h) The zoomed image of sulfur atomic concentration in (g), the decreasing trend indicates that the interfacial layer plays a significant role in restricting polysulfides.

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In-situ XRD patterns of lithium metal anodes for quasi-solid-state lithium sulfur batteries 359x166mm (96 x 96 DPI)


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Use of Tween Polymer To Enhance the Compatibility of the Li

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