Nonflammable hybrid solid electrolyte membrane for a solid-state lithium battery compatible with conventional porous electrodes

Journal Pre-proof Nonflammable hybrid solid electrolyte membrane for a solid-state lithium battery compatible with conventional porous electrodes Xin Zhou, Hao Jiang, Hao Zheng, Yi Sun, Xin Liang, Hongfa Xiang PII:

S0376-7388(19)33552-5

DOI:

https://doi.org/10.1016/j.memsci.2020.117820

Reference:

MEMSCI 117820

To appear in:

Journal of Membrane Science

Received Date: 21 November 2019 Revised Date:

1 January 2020

Accepted Date: 3 January 2020

Please cite this article as: X. Zhou, H. Jiang, H. Zheng, Y. Sun, X. Liang, H. Xiang, Nonflammable hybrid solid electrolyte membrane for a solid-state lithium battery compatible with conventional porous electrodes, Journal of Membrane Science (2020), doi: https://doi.org/10.1016/j.memsci.2020.117820. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.

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Nonflammable hybrid solid electrolyte membrane for a solid-state lithium battery compatible with conventional porous electrodes Xin Zhou, Hao Jiang, Hao Zheng, Yi Sun, Xin Liang, Hongfa Xiang* School of Materials Science and Engineering, Anhui Provincial Key Laboratory of Advanced Functional Materials and Devices, Hefei University of Technology, Hefei 230009, PR China.

Abstract: Solid-state lithium batteries are highly promising for energy storage applications because of their high energy density and good safety characteristics. To promote the commercial progress of solid-state lithium batteries, solutions are desirable for two limitations of the technology: the low ionic conductivity of solid electrolytes and interfacial compatibility between electrolyte and electrodes. Here, we develop a hybrid solid electrolyte composed of Li7La3Zr2O12, ionic liquid, LiTFSI, and PVDF-HFP. Because its components are nonflammable, the hybrid solid electrolyte is likewise completely nonflammable. After pressing, the thickness of the hybrid solid electrolyte can be adjusted. The phase conversion of β-phase PVDF-HFP reduces the barrier of ionic conduction because of enhanced organic– inorganic interfacial ion transfer. More importantly, the ionic liquid stored in the hybrid solid electrolyte not only enhances the conductivity of the electrolyte but also solves the interfacial compatibility issue between solid electrolytes and conventional porous electrodes.

Keywords: hybrid solid electrolyte; Li7La3Zr2O12; ionic liquid; solid battery; safety

1.

Introduction In the past dozen years, lithium (Li) ion batteries have been widely used in

communications, computers, consumer electronics, and the automotive industry because of 1

their high energy density and long lifetime.[1-4] In the development history of rechargeable Li batteries, one recurring safety issue has been the use of flammable organic electrolytes.[5-6] Recently, solid-state Li batteries with nonflammable solid electrolytes have proven a direct and effective solution for the safety issues.[7-9] High-performance solid electrolytes have considerable potential over traditional liquid electrolytes, and can increase the energy density of batteries while having good safety characteristics. At present, the applications of various solid electrolytes, including inorganic ceramic electrolytes and solid polymer electrolytes, are mainly impeded by their low ionic conductivity (10-3–10-6 S cm-1) at room temperature.[10-13] Composite solid electrolytes composed of ceramic fillers and polymer matrix can exhibit the increased ionic conductivity, especially for the active ceramic fillers for Li conduction such as Li1.3Al0.3Ti1.7(PO4)3, Li0.3La0.557TiO3 and Li7La3Zr2O12 (LLZO).[14-17] Garnet-type LLZO has high room-temperature ionic conductivity of 10-3–10-4 S cm-1 and good compatibility with Li metal, and thus has been the preferable ceramic component for composite solid electrolytes.[18-21] Zhang et. al. prepared poly(vinylidene fluoride) (PVDF) based composite electrolyte incorporating Li6.75La3Zr1.75Ta0.25O12 (LLZTO) ceramics. The ionic conductivity of optimal PVDF/LLZTO composite solid electrolyte membranes can reach a maximum value of 5 × 10-4 S cm-1 at 25 °C.[22] Recently, Li and coworkers developed a composite polymer electrolyte composed of polyvinylidene fluoride-co-hexafluoropropylene (PVDF-HFP), Li bis(trifluoromethanesulfonyl) imide (LiTFSI) and LLZO. Benefiting from the higher lithium salt-dissolving ability and electrochemical stability of PVDF-HFP, the composite polymer electrolyte exhibited enhanced conductivity of 7.63 × 10-4 S cm-1 at high temperature, and a wide electrochemical window of 5.3 V.[5] In order to further enhance the ionic conductivity of composite solid electrolytes, introduction of some liquid components is an effective strategy. As an ionic liquid (IL), N-methyl-N-propylpiperidinium bis(trifluoromethanesulfonyl)imide (PP13TFSI) has favorable characteristics of non-volatilization and non-combustion, and is widely used in solid polymer electrolytes. Pan et. al. prepared an ionic liquid gel polymer 2

electrolyte (denoted as ILGPE) composed of 20% PVDF-HFP, 20% LiTFSI and 60% PP13TFSI, with a high ionic conductivity of 1.3 × 10-3 S cm-1 at 23 °C. [23] Nevertheless, compromises must be made between the leakage concern of the high content of the liquid component and a low ionic conductivity of the reduced content of PP13TFSI, e.g., 5.9 × 10-4 S cm-1 of the ILGPE with 50% PP13TFSI. Hybrid solid electrolytes (HSEs) are highly promising for commercial applications because of the combined advantages between inorganic solid components (e.g., LLZO), organic solid components (e.g., PVDF-HFP) and liquid components (e.g., PP13TFSI IL). Another major challenge confronting solid electrolyte technology is the interface compatibility between the electrolyte and the electrode material.[24-27] Insufficient contact between the solid electrolyte and commonly used porous electrodes causes high solid-solid interface resistance and electrochemical polarization.

[28-30]

One popular strategy to minimize

the interface resistance is to replace the binder with a polymer electrode during electrode preparation. However, this method requires major modification of current battery fabrication processes, especially in the electrode manufacturing process. For commercial applications, solid battery assembly processes should ideally be based on existing state-of-the-art battery manufacturing techniques. Recently, Wang et. al. fabricated a cathode-supported solid electrolyte by directly casting PEO-based electrolyte onto the commonly used LiFePO4 cathode electrode, providing enhanced interfacial adhesion. The as-fabricated solid-state battery can deliver an initial discharge capacity of 125 mAh g-1 at 0.1 C at room temperature.[31] Additionally, the liquid components stored in the HSEs can be released under pressure to change the solid–solid electrode/electrolyte interface into a solid–liquid–solid interface with a reduced impedance. The HSE membranes under appropriate pressures can meet the requirements of high energy density for practical applications, which has great potential for applications in solid-state Li batteries.

3

In this work, an HSE membrane is fabricated from LiTFSI salt, LLZO, PP13TFSI IL, and PVDF-HFP matrix. As mentioned above

[10]

, PVDF and LLZO are chosen because of

their wide electrochemical windows and the high conductivity of PVDF/LLZO composite solid electrolyte. Another reason for choosing PVDF polymer is its good adhesive ability and utility as a binder in the cathode electrode. This allows the electrolyte to integrate as a stable structure with the cathode via infiltration of PVDF between the two components. The dispersion of LLZO powders in the electrolyte can improve the mechanical properties of the membrane, promote Li+ transport through the presence of a fast ionic conductor, reduce the TFSI- transport, and thereby increase the ionic conductivity and ionic transfer number. PP13TFSI IL as a liquid component is beneficial to the high ionic conductivity of the solid electrolyte at room temperature. Furthermore, the PP13TFSI stored in the solid electrolyte can be released under pressure to change the solid–solid interface between the electrolyte and the porous electrode into a solid–liquid–solid interface between the IL component and the electrode/electrolyte solids.[32] Most importantly, PP13TFSI has extremely low vapor pressure and excellent flame retardance, giving the cell system good safety characteristics.[33] 2. Experimental section 2.1 Materials and preparation of LLZO PVDF-HFP (average Mw ~400,000) was purchased from Sigma-Aldrich. PP13TFSI was received from Shanghai Chengjie Chem. Co, Ltd. LiTFSI was bought from Guotai Chaowei Co, Ltd. LLZO was prepared by a traditional solid-state reaction. Stoichiometric amounts of LiOH·H2O, La2O3, and ZrO2 received from Aladdin were ball milled in a planetary mill with zirconia balls in 2-propanol at 400 rpm for 12 h . Then the mixed powders were dried in a 70 °C vacuum oven for 12 h. Subsequently, the dried powders were calcined at 900 °C for 12 h in a muffle furnace to obtain the cubic phase LLZO powder. 2.2 Preparation of HSE membranes

4

Typically, hybrid solid electrolyte (HSE) membranes are prepared as follows: 0.8 g PVDF-HFP, 0.4 g LiTFSI, and 0.8 g PP13TFSI (for 40% IL, with 1.2 g, and 1.8 g PP13TFSI for 50% IL and 60% IL, respectively) were dissolved in 20 ml N, N-dimethyl formamide with stirring for 3 h at 50 °C. Then, 0, 0.1 and 0.2 g of LLZO were added into the mixed polymer solution to fabricate HSE membranes with 0, 5% and 10% LLZO, respectively. The resultant solutions were poured into clean Teflon dishes and oven dried at 110 °C for 3 h, in order to evaporate the solvent. 2.3 Characterization of the HSE membranes The morphologies of LLZO particles and HSE membranes were examined on a field emission scanning electron microscope (FESEM, Hitachi SU8020) and a transmission electron microscope (TEM, HT7700). Fourier transform infrared spectrometer (FTIR, Nicolet-6700) spectra were collected from 3200 cm-1 to 650 cm-1. The ionic conductivities were determined by electrochemical impedance spectroscopy (EIS) over a range from 100 kHz to 0.1 Hz. EIS measurements were carried out in the temperature range from 30 °C to 100 °C on a CHI660 electrochemical workstation (Shanghai Chenhua) with stainless steel (SS)|HSE|SS cells under no pressure. In these cells, the HSE membrane was sandwiched into two parallel stainless steel (SS) electrodes. The ionic conductivity (σ) was calculated from the equation (1):

σ=

∗ (1)

where Rb is the bulk resistance from ac impedance, and L and A are the thickness and effective area of the electrode, respectively.[34,35] The linear sweep voltammograms (LSV) tests of Li|HSE|SS batteries under normal pressure were performed in a CHI660 electrochemical workstation (Shanghai Chenhua) to

5

evaluate the electrochemical stability of the HSE membranes. The scan rate and the potential range are set up to 1 mV s−1 and 3–6 V, respectively. In order to measure the Li ion transference number (tLi+) of each HSE membrane, ac impedance and dc polarization of Li|HSE|Li symmetrical cells were carried out on the above electrochemical station. The transference number was calculated using the following equation (2) [36]:

=

(∆ − (∆ −

) )

(2)

The initial (I0) and steady (Iss) currents flowing through the cell were recorded from the dc polarization test (10 mV). R0 and RSS were obtained from the ac impedance measurement at frequencies between 100 kHz and 0.01 Hz. These represent the interface impedance values before and after the polarization test. Ea is a parameter reflecting the temperature dependence of the electrolytes’ ionic conductivity. For all samples, the conductivity variation with temperature follows the Arrhenius equation (3):

−

σ=

(3) In the Arrhenius equation, Ea is the activation energy, σ0 is the pre-exponential factor, R and T represent the molar gas constant and the absolute temperature, respectively.[37] The electrochemical properties of the solid-state batteries were tested with a 2032-type coin cell of Li|HSE|LiFePO4 between 2.5 and 3.8 V on a LAND battery cycler. The LiFePO4 cathode was composed with 80% LiFePO4 (supplied by Amperex Technology Limited Company), 10% conductive carbon additive (Super-P, TIMCAL), and 10% PVDF (Solvay

6

5130) on an aluminum foil substrate (Φ 14 mm). The mass loading of LiFePO4 active material was controlled at around 3 mg cm−2. 3. Results and discussion The HSE membrane was designed to consist of PVDF-HFP, LLZO, PP13TFSI, and LiTFSI. The high molecular weight PVDF-HFP was chosen as the polymer matrix, because of its wide electrochemical window, and because it allows multiple ion transport in its amorphous regions. Additionally, its strong adhesive ability helps form a stable structure integrated with the cathode, by using PVDF as binder. The LLZO-containing HSE membranes colored light brown (Fig. 1a). Garnet-type LLZO has a pure cubic structure (Fig. S1a) with a size of around 1–3 µm (Fig. S1b), with a homogeneous distribution (Fig. 1b). According to the ac impedance (Fig. S1c), the ionic conductivity of pure LLZO is 1.6×10-4 S cm-1. SEM imaging (Fig. 1c) indicates that the HSE membrane mainly consists of microspheres in size of 5–6 µm, without signs of naked LLZO particles (1–3 µm). This suggests that the microspheres are a PVDF-wrapping LLZO shell-core structure. The adhesive nature of PVDF tightly bonds the microspheres, along with pores formed to accommodate the liquid component, PP13TFSI (the structural formula is shown in Fig. S1d). The thickness of the HSE membrane is about 120 µm (Fig. S1e), and pores clearly exist in the surface of the membrane. Ideally, this should be avoided or eliminated to minimize resistance in the cells.

7

Fig. 1. Photograph of HSE membranes with various amounts of LLZO and 40% PP13TFSI (a); SEM images of the LLZO powders (b) and the surface with 5% LLZO (c); Photographs of 5% LLZO-containing HSE membranes with 40% IL, 50% IL, and 60% IL, before and after standing on a paper for 5 min at room temperature (d); SEM images of the cross-section of the HSE membrane under no pressure (e) and 15 MPa normal pressure (f); TEM image of βphase PVDF-HFP in the HSE membrane under no pressure (g) and normal pressure (h).

In an ideal solid-state electrolyte, the quantity of the liquid component (PP13TFSI) should be minimized. Of the percentages studied here, 40% PP13TFSI in HSE containing 5% LLZO was found to be optimal. Higher contents of 50% and 60% PP13TFSI induce leakage (Fig. 1d). The TGA curves (Fig. S2) show that no weight loss appears below 200 °C in any of 8

the three membranes, indicating the good thermal stability of the HSE membranes. As reported previously [33], PP13TFSI experienced near total thermal decomposition from 342 °C to 484 °C, with a main endothermic peak at 453 °C. Typically, thermal decomposition of LiTFSI/PVDF-HFP electrolyte will start at around 350 °C.[5] Therefore, the performance difference between the HSE membranes containing 40% PP13TFSI and higher levels (50% and 60%) at 300 °C and 400 °C may be caused by the strong interaction of components in the former (40%), and by the existence of free PP13TFSI or weaker interaction between PP13TFSI and other components in the 50% and 60% PP13TFSI membranes. Furthermore, comparison of electrochemical performance, shown in the Supporting Information (Figs. S3 and S4), indicates negligible performance difference between HSE membranes containing 40% IL and those with more IL. Since adding more PP13TFSI does not improve performance and does interfere with safety and stability, a level of 40% PP13TFSI is considered to be optimal. Therefore, all the HSE membranes discussed below contain 40% PP13TFSI except when specially mentioned otherwise. In Fig. 1e, the SEM image of the cross-section of the 5% LLZO HSE membrane shows a sphere network structure, which is typical for PVDF-HFP/LiTFSI hybrid solid electrolytes.[38] Before pressure is applied, the LLZO is wrapped in the PVDF-HFP spherical network structure, and there are many voids between the microspheres. After applying pressure, the HSE membrane becomes less porous and the irregular flocculent polymer morphology increases. The arrangement of LLZO particles tends to be orderly and tight under a normal pressure of 15 MPa. As shown in the Supporting Information (Fig. S5), under pressure, the diffraction peaks of LLZO in the HSE membrane shift 0.3° to the right, and the peaks become more intense. Observation shows that under normal pressure, the size of LLZO crystal faces in HSE decreases and the arrangement of LLZO particles tends to be orderly and tight. The tightly arranged LLZO can effectively transfer Li+ and block the transmission of TFSI-, explaining the increased ion transfer number after the addition of LLZO. The EDS mapping 9

images of the elements Zr and S (Fig. S6) indicate that the cubic LLZO particles are dispersed homogeneously in the PVDF-HFP matrix, and that LLZO is naked in the electrolyte matrix after pressing. TEM imagery (Fig. 1g) of the HSE membrane also shows that the β-phase PVDF-HFP formed spherical structures that link to each other. The TEM image of the HSE membrane after pressure was applied (Fig. 1h) shows that the LLZO particles are effectively connected to each other by the amorphous PVDF-HFP polymer, which is further supported by FTIR results. In Fig. 2a, the characteristic absorption bands of PVDF-HFP, CH2 wagging, antisymmetric CF2 stretch, and CF3 out-of-plane vibrations absorb IR radiation at 1404.91 cm1

, 1183.59 cm-1, and 1049.53 cm-1, respectively. After pressure was applied, the absorption

bands at 1347.99 cm-1 and 1130.05 cm-1, which are attributed to the TFSI- anion, show the increased intensities. This is believed to be caused by the exudation of IL. The absorption bands at 877.52 cm-1 and 836.95 cm-1 belong to the β-phase of PVDF-HFP. The absorption bands shown in Fig. 2b at 2946.95 cm-1 and 2913.48 cm-1 are associated with the C-H stretching vibrations of PVDF-HFP. The band at 1662.82 cm-1 increased, indicating the enhanced interaction between LiTFSI and the PVDF-HFP polymer chain. As pressure was applied, the β-phase gradually transformed into the amorphous phase

[39]

, and

thus the intensities of the related bands weaken. The phase conversion of β-phase PVDF-HFP during pressing can reduce barriers to ionic conduction, because of the accelerated organicinorganic interfacial ion transfer on the naked LLZO surface.

10

Fig. 2. FTIR spectra of the LLZO particles, ionic liquid (IL), LLZO solution in IL, PVDF-HFP electrolyte with IL, PVDF-HFP electrolyte with IL and LiTFSI, and PVDF-HFP electrolyte with 40% IL, 20% LiTFSI and 5% LLZO (a); FTIR spectra of the HSE membranes under no pressure and normal pressure (b).

In our design for the HSE membrane, the stored PP13TFSI is released and transferred into the pores of the conventional electrode during cell assembly under pressure. Fig. 3 illustrates the interface wetting effect of IL in the HSE membrane under the influence of external pressure. This process serves to wet the porous electrode surfaces, improving the interface between porous electrodes and electrolytes, as shown in Fig. S7a and S7b in the Supporting Information. Therefore, the interface contact between the HSE membrane and the

11

electrode material changes from Solid-Solid to Solid-Liquid-Solid, reducing the interface impedance compared to traditional all-solid batteries. Similarly, we can see the phase transition of PVDF-HFP (the polymer matrix of HSE) during the application of pressure. This reduced the gap between the matrices and enhanced ion transport. The normal pressure during cell assembly (15 MPa, as calculated in Fig. S7c of the Supporting Information) reduced the thickness of the HSE membrane from its initial value of 120 µm to 100 µm, the true thickness after battery assembly. Higher pressure will reduce membrane thickness further. We also investigated the relationship between the thickness of the HSE membrane and the applied pressure (as shown in Fig. S8 in Supporting Information). The thickness change of the membrane under pressure indicates that 10 MPa is the actual press which the HSE membrane experienced during cell sealing. In commercial pouch cells, a normal pressure of 30 MPa may reduce the thickness of HSE membrane as low as 30–50 µm, enhancing the energy density of solid-state batteries. This also indicates that the HSE membrane has good flexibility and thickness adjustability. Additionally, the thickness of the HSE membrane will not recover after the pressure is released, because the phase transition of PVDF-HFP in the HSE membrane is unidirectional.

Fig. 3. Schematic illustration of the HSE membrane in Li|HSE|LiFePO4 cell under press.

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Fig. 4a shows the temperature dependence of the ionic conductivities of HSE membranes that include 0–10% LLZO under no pressure. The ionic conductivity of the membrane with 5% LLZO reaches 1.12 mS cm−1, higher than that of membrane without LLZO (0.98 mS cm−1) at 30 °C. The difference in ionic conductivity between 5% LLZO and 10% LLZO (1.14 mS cm-1) is negligible. The activation energy Ea of the HSE membrane increases with increasing LLZO content, which suggests that LLZO particles not only decrease the crystallinity of the polymer matrix, but also form ion transfer pathways by organic–inorganic interfacial ion transfer on the naked LLZO surface. [40] The LSV curves in Fig. 4b show the electrochemical stability of the HSE membranes. The HSE membrane without LLZO begins to decompose at ~4.75 V (vs. Li+/Li), while the stable oxidation potential of an HSE membrane with 5% LLZO reaches ~4.92 V. When the loading of the LLZO particles increases to 10%, the HSE membrane decomposes at ~4.60 V. This is believed to be because the excess LLZO powder takes up too much space and only provides a limited interfacial area for ionic transport, as reported previously.[2] From the dc polarization and ac impedance shown in Fig. 4c and 4d, tLi+ can be calculated from the equation of tLi+ = Iss (ΔV- I0R0)/ I0(ΔV- ISSRSS) as proposed by Vincent. [41]

The calculated tLi+ value of 5% LLZO HSE membrane is around 0.37, while the values of

HSE membranes with 0% and 10% LLZO are 0.34 and 0.36, respectively (Supporting Information, Fig. S9). These are comparable with the values of conventional liquid electrolytes (0.20–0.40).

[36,42]

Introduction of appropriate LLZO can limit the TFSI- transfer

and not affect the Li+ transfer, since LLZO is a good Li+ conductor. The electrochemical impedance results (Fig. 4e and 4f) present a great decrease on interface impedance (Rf) and charge transfer impedance (Rct) after appropriate pressure is applied.

13

Fig. 4. Temperature dependence of the ionic conductivities of the HSE membranes. The interpolation table shows the activation energy of the HSE membranes (a); LSV curves of the HSE membranes with 0~10% LLZO (b); Current time profile of a symmetrical Li|HSE|Li cell with 5% LLZO (c); The Nyquist impedance spectra of the Li|HSE|Li cell with 5% LLZO before and after polarization (d); Electrochemical impedance spectra of the Li|HSE|LiFePO4 cell under no pressure (e) and 15 MPa normal pressure (f).

14

The fire resistant of the HSE membrane with 5% LLZO is compared with that of conventional liquid electrolyte-separator composition in Fig. 5. In Fig. 5a, when the PE separator with liquid electrolyte is close to fire, it immediately shrinks and combusts in approximately 1 s, and burns out 5 s later. By contrast, the HSE membrane is totally nonflammable, does not shrink and exhibits fire-resistant properties (Fig. 5b). The fire selfextinguishes as soon as the ignition source is removed, and only slight scorch marks appear on the edges of the membrane. These results indicate that the HSE membrane is highly nonflammable, and suitable for high-safety batteries.

Fig. 5. Flammability tests of traditional electrolyte of 1-M LiPF6/EC+DEC (1:1) and PE membrane (a); flammability test of HSE membrane with 5% LLZO (b).

Cell performance of Li||LiFePO4 cells with HSE membranes with 0–10% LLZO and 40% PP13TFSI was investigated as shown in Fig. 6. The good cycling stability of the cells using the LLZO-containing HSE membranes was also evaluated at 0.1 C (Fig. 6a). The coulombic efficiency of the cell is almost 100% in 50 cycles (Fig. 6b). Fig. 6c shows that the initial discharge capacity of Li||LiFePO4 cell using 5% LLZO-containing HSE membrane is 15

158 mAh g−1 at 0.1 C, which is higher than that obtained using HSE membrane without LLZO (142 mAh g−1), but lower than that using 10% LLZO (163 mAh g−1). In general, as the content of LLZO increases, the HSE membrane exhibits higher ionic conductivity and electrochemical stability.

Fig. 6. Li||LiFePO4 CR2032-type coin cells‘ cycling performance at 0.1 C at room temperature, using three HSE membranes of different LLZO concentrations (a) and (b); Charge–discharge performance (c); rate capability (d); Long cycling performance using the HSE membrane with 5% LLZO (e).

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Fig. 6d shows the rate performance of the Li||LiFePO4 cells with various HSE membranes. When 5% LLZO was introduced, the cell delivered reversible discharge capacity of 140 mAh g−1 at 1 C. With 10% LLZO-containing HSE membrane, the cell shows lower reversible discharge capacities of 70 mAh g−1 at 1 C, indicating that excess LLZO particles can limit the transport of Li+ ions in the amorphous region of the polymer. The long cycle battery performance is shown in Fig. 6e, which evaluates the cycling stability of the battery system.

4. Conclusion A new type of nonflammable and thickness-adjustable hybrid solid electrolyte (HSE) membrane, which is compatible with conventional porous electrodes, has been fabricated via a solution casting method. The composition of the HSE allows the electrolyte to seep out IL to wet the porous electrodes under pressure during cell assembly. This permits good electrolyteelectrode interfacial compatibility. The actual thickness of the HSE membranes under various pressures can be adjusted to meet the requirement of high energy density in practical applications. Benefiting from the nonflammability of its components, the HSE membrane is completely nonflammable. The optimal HSE membrane with 5% LLZO and 40% PP13TFSI exhibits excellent electrochemical properties, including high ionic conductivity of 1.12 mS cm-1, high oxidation potential of 4.95 V, a Li+ transfer number of 0.37 to ensure ion transport, and good thermodynamic stability. In a solid-state Li battery using a traditional LiFePO4 porous cathode, that HSE membrane can also deliver superior rate capability of 140 mAh g−1 at 1.0 C and stable cycling performance without capacity loss after 50 cycles at 0.1 C at room temperature. Considering the excellent safety characteristics and electrochemical performance, the hybrid solid electrolyte has great potential for applications in solid-state Li batteries.

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Acknowledgments This study was supported by the National Natural Science Foundation of China (Grant Nos. 21676067 and 21606065), Anhui Provincial Natural Science Foundation (Grant No. 1908085QE178), and the Fundamental Research Funds for the Central Universities (Grant No. JZ2017YYPY0253).

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HIGHLIGHTS Hybrid solid electrolyte membrane is completely nonflammable. Hybrid solid electrolyte membrane is compatible with common porous electrode. Electrolyte membrane in Li||LiFePO4 cells delivers 140 mAh g−1 at 1.0 C.

Conflict of interest The authors declare no conflict of interest.

Author statement

X. Zhou and H. Xiang designed the research. X. Zhou and H. Jiang conducted the fabrication and electrochemical characterization of the hybrid solid electrolyte membrane. X. Zhou and H. Zheng did cell assembly and performance measurements of the Li||LiFePO4 CR2032-type coin cells. X. Zhou, Y. Sun, X. Liang and H. Xiang wrote the manuscript. All authors contributed to the discussion of the manuscript.