Constructing a stable interface between the sulfide electrolyte and the Li metal anode via a Li+-conductive gel polymer interlayer

Ya-Hui Wang ab, Junpei Yue b, Wen-Peng Wang b, Wan-Ping Chen bc, Ying Zhang b, Yu-Guo Yang a, Juan Zhang bc, Ya-Xia Yin bc, Xing Zhang *bc, Sen Xin *bc and Yu-Guo Guo *bc
aSchool of Science, Beijing Jiaotong University, Beijing 100044, China
bCAS Key Laboratory of Molecular Nanostructure and Nanotechnology, CAS Research/Education Center for Excellence in Molecular Sciences, Beijing National Laboratory for Molecular Sciences (BNLMS), Institute of Chemistry, Chinese Academy of Sciences (CAS), Beijing 100190, China. E-mail: ygguo@iccas.ac.cn; xinsen08@iccas.ac.cn; zhangxing@iccas.ac.cn
cUniversity of Chinese Academy of Sciences, Beijing 100049, China

Received 12th March 2021 , Accepted 5th May 2021

First published on 11th May 2021


Abstract

Due to high ionic conductivity, favorable mechanical plasticity, and non-flammable properties, inorganic sulfide solid electrolytes bring opportunities to the practical realization of rechargeable lithium–metal batteries with high energy, yet their use was impeded by an electrochemically unstable Li–electrolyte interface. Herein, we propose to address the issue via a Li+-conductive gel polymer interlayer, which is derived in situ from a conventional liquid ether electrolyte during the cell fabrication process. The gel polymer interlayer not only enables intimate solid–solid contact and uniform Li-ion flux at the heterointerface but also effectively inhibits interfacial reactions and Li dendrite growth. With improved interfacial stability, a Li–Li symmetric cell with the gel polymer interlayer demonstrates an ultra-stable Li plating/stripping performance of over 1300 hours at 0.1 mA cm−2 and 350 hours at 0.5 mA cm−2 at room temperature, and a high critical current density of >5 mA cm−2. This work offers general insights into a reasonable design of an anode/electrolyte interface for high-energy rechargeable Li–metal batteries.


Introduction

Along with the rapid development of electric vehicles, distributed energy systems and smart electrical grids, batteries with high energy density and safety are in great demand.1–6 In all battery technologies, all-solid-state batteries (ASSBs) based on solid-state electrolytes (SSEs) have been proposed to satisfy these requirements by combining lithium (Li) metals with high-voltage and chalcogen cathodes.7–12 However, the unfavourable conductivity of SSEs and the large interfacial resistance between the Li anode and SSEs make it difficult to realize practical application for ASSBs.13–17

Lately, sulfide electrolytes (SEs) are being considered as one of the most promising solid electrolytes due to their excellent mechanical property (can be densified simply by cold pressing) and high ionic conductivity.18,19 Among them, Li10GeP2S12 (LGPS) with an ionic conductivity of 1.2 × 10−2 S cm−1 comparable with that of the liquid electrolytes and sufficient mechanical strength for resisting Li dendrite growth, exhibits the potential application in ASSBs.20,21 Nevertheless, suffering from the poor compatibility between LGPS and the Li anode, the interfacial resistance could gradually increase and the Li dendrite could grow through the grain boundary or voids in SSEs, causing degradation of the battery performance.14,22–28 In particular, LGPS will be reduced by metallic Li to form an electronic-ion mixed conductor interface (MCI) containing Li2S, Li3P, Li–Ge alloy, etc., results in continuous interfacial parasitic reactions.28–31 In the past few years, much effort has been devoted to improving the interfacial stability between SSEs and the Li anode by heteroatoms doping for LGPS, alloying of the Li metal and employing protective layers between SSEs and the Li anode.32–38 Sun et al. reported a solid-state plastic crystal electrolyte (PCE) as an interlayer for a stable Li–LGPS interface.39 Moreover, an inorganic Li3N–LiF layer was employed by Wang et al. to achieve a dendrite-free solid-state electrolyte, where the highly ionic conductive Li3N reduces the Li plating/stripping overpotential, and LiF with high interface energy plays a role in suppressing dendrites’ growth.40 Although many efforts have been carried out to stabilize the interface between the SSEs and the Li anode, building an in situ uniform and sustained protective layer between the interface of SSEs and the Li anode still remains challenging to achieve.

In this work, we demonstrated that a stable Li anode–SEs interface can be constructed through an in situ gelation strategy. An elastic gel polymer interlayer (GPI) was built to stabilize the interface of the Li anode and LGPS using in situ polymerization of 1,3-dioxolane (DOL) monomers with lithium hexafluorophosphate (LiPF6) as an initiator.41 Compared with the bare Li–SEs interface (Fig. 1a), the GPI introduced by in situ gelation strategy takes advantage of the beneficial properties of liquid precursors, ensuring good interfacial contact with interfaces, and then builds well-connected pathways that enable uniform Li+ flux to restrain dendrite growth (Fig. 1b). The subsequent polymerization transforms the above liquid-state material to a quasi-solid material, which produces an elastic buffer layer to prevent side reaction at the interface. In addition, the low electronic conductivity of the GPI also facilitates dendrite suppression. Consequently, such in situ polymerized elastic GPI strategy provides a universal and facile method for steadying the Li–SEs interface and improving the cycling performance of Li metal batteries.


image file: d1qm00395j-f1.tif
Fig. 1 Graphical illustration of the (a) unstable and (b) stable Li anode–SEs interface.

Result and discussion

First, the chemical compatibility between LGPS and solvents was investigated. Fig. 2a and Fig. S1a (ESI) present the X-ray diffraction (XRD) patterns of LGPS powders immersed in different solvents. After immersing LGPS powder in DOL, 1,2-dimethoxyethane (DME) and DOL/DME (v[thin space (1/6-em)]:[thin space (1/6-em)]v, 1[thin space (1/6-em)]:[thin space (1/6-em)]1) mixed solvent for 24 hours, respectively, the characteristic peaks of the products are consistent with the pristine LGPS powder and no new peaks are generated, indicating good compatibility between LGPS and the above solvents. Furthermore, as shown in the electrochemical impedance spectrum (EIS) (Fig. 2b and Fig. S1b, ESI), the high ionic conductivity (approx. 1.5 mS cm−1) of the solvent-immersed LGPS is well-preserved compared with the pristine LGPS (Table S1 and Fig. S2, ESI), in accord with ref.39. Such excellent stability together with the high ionic conductivity of LGPS was expected to exhibit great electrochemical performance towards efficient battery application. Subsequently, an elastic GPI was prepared by in situ polymerization of the precursor solution of 2 mol L−1 LiPF6 and 1 mol L−1 bis(trifluoromethane) sulfonimide lithium salt (LiTFSI) into a DOL/DME (v[thin space (1/6-em)]:[thin space (1/6-em)]v, 1[thin space (1/6-em)]:[thin space (1/6-em)]1) mixed solvent. Meanwhile, the existence and morphology of GPI are investigated by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) elemental mapping. Fig. 2c shows the side view of the Li anode, GPI and LGPS after in situ polymerization. The thickness of the GPI is about 20 μm, and the EDS mapping confirms the elemental distribution S and P predominately located on the upper part of the cross-sectional view, while C and F are highly concentrated between Li and LGPS, suggesting that the GPI is covered uniformly on the surface of the Li anode. Moreover, it is imperative that the sandwich structure retains high ionic conductivity as shown in Fig. S3 (ESI), implying the admirable electrochemical property of the structure.
image file: d1qm00395j-f2.tif
Fig. 2 (a) XRD pattern and (b) EIS spectrum of pristine LGPS, LGPS in DOL and DME after 24 hours, and (c) side-view SEM and EDS elemental mapping images of Li, GPI and LGPS.

Encouraged by the high ionic conductivity and stability of the sandwich structure, the symmetrical Li|LGPS|Li cell and the Li|GPI–LGPS–GPI|Li cell are assembled to investigate the interfacial stability of LGPS against the Li anode during cycling. First of all, the critical current density is examined at room temperature in symmetric Li|LGPS|Li and Li|GPI–LGPS–GPI|Li cells by gradually increasing currents from 0.1 to 5 mA cm−2 at the fixed capacity of 0.1 mA h cm−2. As shown in Fig. 3a, the overpotential of Li|LGPS|Li increases significantly with the increase of current density, demonstrating that the interface of LGPS against the Li anode is unstable due to the serious interfacial reaction. Conversely, the overpotential of Li|GPI–LGPS–GPI|Li shows a finite increase with the increment of the current density.


image file: d1qm00395j-f3.tif
Fig. 3 Li plating/stripping overpotential profiles in the (a) Li|LGPS|Li cell and (b) Li|GPI–LGPS–GPI|Li cell at step-increased current densities. Comparison of the cycling stability of Li|LGPS|Li and Li|GPI–LGPS–GPI|Li symmetrical cells at room temperature with the current densities of (c) 0.1 mA cm−2 for 0.1 mA h cm−2 and (d) 0.5 mA cm−2 for 0.5 mA h cm−2. Insets, detailed voltage profiles.

At 5 mA cm−2, the overpotential of Li|LGPS|Li is over 4 V, while the overpotential of Li|GPI–LGPS–GPI|Li is only about 0.35 V, confirming that the introduction of GPI increases the critical current density and suppresses the parasitic reactions at the interface between the Li anode and LGPS (Fig. 3b). After that, we further performed a “Li strip/plate test” to estimate the dynamical interface stability and Li ion transport capability across the LGPS SE and Li metal interface. Fig. 3c compares the Li+ plating/striping voltage profiles of the Li|LGPS|Li and Li|GPI–LGPS–GPI|Li cells within 1300 h at 0.1 mA cm−2 for 0.1 mA h cm−2. In the first cycle, both cells showed a similar Li plating/stripping overpotential (approx. 40 mV). After 100 cycles, the overpotential of Li|LGPS|Li symmetric cells has exceeded 4 V, and the overpotential of Li|GPI–LGPS–GPI|Li cells remains at 40 mV. Meanwhile, the cell exhibits a flat and extraordinary stable plating/stripping profile even after prolonged 1300 h, indicating an increase in cycling stability because of the stable interface between LGPS and the Li anode. In addition, the Li|GPI–LGPS–GPI|Li symmetrical battery also exhibits a stable cycling for 350 h as the current density increases to 0.5 mA cm−2 (Fig. 3d), whereas the Li|LGPS|Li cell shows a considerable overpotential with prolonged time (Fig. S4, ESI), further declaring the excellent Li plating/stripping of the GPI modified Li anode.

To better understand the interface stability and Li dendrite prevention ability of the GPI, the surface morphology of the Li anode after Li plating/stripping cycles is visually analysed using optical and SEM images, and the EIS of symmetric Li cells before and after cycles are also recorded. Fig. 4a and Fig. S5 (ESI) show that substantial chunks composed of S, P, and Ge elements are generated on the surface of the pristine Li anode due to the serious parasitic reactions, while the GPI modified Li anode presents a smooth and flat surface (Fig. 4b), suggesting the successful protection from the GPI. These results are consistent with the performance of the Li symmetric batteries as shown in Fig. 3, namely, the introduction of the GPI layer significantly inhibits the side reaction of the Li–LGPS interface and depresses the growth of Li dendrites. Moreover, the electrochemical impedance spectroscopy (EIS) tests of Li symmetric cells before and after cycles exactly coincide with the earlier discussion. In accordance with the interfacial structure, the EIS could be considered as four distinct frequency regions, which correspond to the electrolyte bulk (RSSE-b), grain boundary resistance (RSSE-gb) and the interface resistance (RSUM), and the electrochemical transfer polarization resistance of Li plating/stripping (RLGPI-CT) (Fig. S6, ESI).33,42 Among them, the interphase resistance (RSUM) could be subdivided into the Li–LGPS interphase resistance (RLi–LGPS) or the combined effect of Li–GPI interphase resistance (RLi–GPI) and GPI–LGPS interphase resistance (RGPI-LGPS) for Li|LGPS|Li and Li|GPI–LGPS–GPI|Li cells, respectively. The interface of Li–LGPS during electrochemical operation continuously deteriorates due to the large volume variation and severe side reactions. The deteriorated interface with the enlarged surface area and poor ionic conductivity results in an increased interface resistance RSUM with spreading frequency regions and the buried grain boundary resistance RSSE-gb.43,44 As shown in Fig. 4c, after 30 cycles, the RSUM of the Li|LGPS|Li cell (70 kΩ) is greatly increased compared with that before cycle (Table S2, ESI), indicating that a large number of poor ionic conductors were continuously produced on the Li–LGPS interface.28,45,46 Conversely, the RSUM of the modified Li|GPI–LGPS–GPI|Li cell remains unchanged after cycling (Table S2, ESI), again demonstrating that the GPI as the interlayer between the Li metal and LGPS can block significant interfacial reactions between the Li metal and LGPS, thus promising a higher electrochemical stability (Fig. 4d). Furthermore, the unchanged RSSE-b and RSSE-gb at a high-frequency region (105 Hz) before and after cycles establish that there is no obvious decomposition of LGPS on the surface of the Li|GPI–LGPS–GPI|Li cell (Fig. 4d and Table S2, ESI), which is in accord with the above conclusion.42


image file: d1qm00395j-f4.tif
Fig. 4 SEM and optical images of the Li anode in the (a) Li|LGPS|Li and (b) Li|GPI–LGPS–GPI|Li system after 30 cycles, respectively. Nyquist plots and optical images of (c) Li|LGPS|Li and (d) Li|GPI–LGPS–GPI|Li cells before and after 30 cycles at the current density of 0.1 mA cm−2 for 0.1 mA h cm−2 accordingly.

In order to further explore an insight into the interphase structure change during Li plating/stripping, X-ray photoelectron spectroscopy (XPS) was used to characterize the composition and structure of Li–LGPS, Li–GPI and GPI–LGPS interfaces, respectively. As shown in Fig. 5a, no distinguishable peaks are observed for pure LGPS, which is consistent with the XRD result. Compared with pristine LGPS, Li2S, Li3P and reduced Ge components are detected on the LGPS–Li interface after cycling, where the peaks with binding energies of 159.66 (S 2p3/2), 125.28 (P 2p3/2) and 28.85 (Ge 3d5/2) eV in the XPS spectrum of the Li–LGPS interface can be assigned to Li2S, Li3P and reduced Ge, respectively (Fig. 5b and Table S3, ESI).37,39,40 Accordingly, Li|LGPS|Li exhibits poor Li plating/stripping performance because of the increasing interfacial resistance caused by poor ionic conductors of Li2S and Li3P components, consuming an electrolyte and deteriorating the interface continuously by reduced Ge.31Fig. 5c shows that there is no reduced Ge and Li3P generated on the Li–GPI interface, while the peak at 30.3 (F 2s) and 159.66 (S 2p3/2) eV can be assigned to LiF and Li2S, respectively (Fig. S7 and Table S4, ESI), which could be derived from the decomposition of LiTFSI in the GPI, impressing that the GPI is chemically stable against the Li anode during cycling and is thus strong enough to protect the Li anode.36,47 For the GPI–LGPS interface shown in Fig. 5d and Fig. S8 (ESI), except for the presence of GPI components such as LiF and PF5,41 there is no significant change in the component and structure of cycled LGPS compared with pristine LGPS, indicating that a steady interface is constructed between the GPI and LGPS. The results exhibit that the as-obtained GPI layer could inhibit the decomposition of LGPS, suppress interfacial reactions and render a long lifespan of the Li metal anode and SEs.


image file: d1qm00395j-f5.tif
Fig. 5 Graphical illustrations and X-ray photoelectron spectroscopy (XPS) curves of the interfaces for (a) pristine LGPS, (b) Li–LGPS, (c) Li–GPI and (d) GPI–LGPS after 30 cycles at the current density of 0.1 mA cm−2 for 0.1 mA h cm−2 at room temperature.

Conclusions

In summary, we demonstrated an ingenious approach to construct an artificial elastic gel polymer interlayer (GPI) to stabilize the Li anode–SEs interface, where such a layer is in situ built via polymerization of DOL initiated by LiPF6. The as-obtained GPI combines flexibility with stiffness, and enables sufficient solid–solid contact, avoiding current accumulation caused by point-to-point contact. Moreover, its high ionic conductivity and high Li+ transference number (reported in our previous study) in combination with the mechanically stable interface uniform Li+ flux at the interface. As a result, the Li|GPI–LGPS–GPI|Li symmetric cell demonstrates extra-stable Li plating/stripping over 1300 hours at 0.1 mA cm−2 and 350 hours at 0.5 mA cm−2 at room temperature. The present in situ gelation approach potentially provides a universal and efficient strategy to resolve the intrinsic interfacial issue toward the Li metal anode and SEs, and then paves the way for the next-generation high-energy solid-state Li metal batteries.

Author contributions

Ya-Hui Wang: conceptualization, methodology, investigation, data curation, visualization, writing – original draft, writing – review and editing. Jun-Pei Yue: conceptualization, writing – original draft, funding acquisition. Wen-Peng Wang: writing – original draft, visualization. Wan-Ping Chen: data curation, visualization. Ying Zhang: writing – review and editing. Yu-Guo Yang: writing – original draft, writing – review and editing. Juan Zhang: writing – review and editing. Ya-Xia Yin: writing – review and editing. Xing Zhang: conceptualization, writing – review and editing, supervision, project administration, funding acquisition. Sen Xin: conceptualization, writing – review and editing, supervision, project administration, funding acquisition. Yu-Guo Guo: conceptualization, writing – review and editing, supervision, project administration, funding acquisition.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

This work was supported by the National Key R&D Program of China (Grant No. 2016YFA0202500 and 2019YFA0705700), the Basic Science Center Project of National Natural Science Foundation of China (Grant No. 51788104), the National Natural Science Foundation of China (Grant No. 21975266, 21773264, 21703257, 22005313 and 22005316), and the “Transformational Technologies for Clean Energy and Demonstration,” Strategic Priority Research Program of the Chinese Academy of Sciences under Grant No. XDA 21070300. S. X. acknowledges financial support from the start-up funds from the CAS. J. Yue acknowledges the China Postdoctoral Science Foundation Grant (No. 2019T120135). The authors thank Dr Z. J. Z., Dr X. Y. Z. and Dr Y. S. at the Center for Analysis and Testing, ICCAS for their help for the XPS and XRD analysis.

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Footnote

Electronic supplementary information (ESI) available: Experimental section, materials characterization, electrochemical measurements, the specific values of resistances and the calculation result of ionic conductivity of Fig. 2b, Fig. S1b, detailed equivalent circuit of Fig. 2b and 4c, d, EIS spectrum of GPI-LGPS-GPI composite electrolytes, the EDS elemental mapping images of Li anode in Li|LGPS|Li system after 30 cycles, Li plating/stripping profiles of Li|LGPS|Li cells of 0.5 mA cm−2 for 0.5 mA h cm−2 at room temperature, F 1s, C 1s XPS spectrum of the cycled Li anode and LGPS pellet recovered from Li|GPI–LGPS–GPI|Li battery, and XPS binding energies with attributed species shown in Fig. 5 and Fig. S7, S8. See DOI: 10.1039/d1qm00395j

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