LAGP solid electrolyte with optimized interface for dendrite-free solid Li-metal battery

LAGP solid electrolyte with optimized interface for dendrite-free solid Li-metal battery

Accepted Manuscript Novel synergistic coupling composite chelating copolymer/LAGP solid electrolyte with optimized interface for dendrite-free solid L...

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Accepted Manuscript Novel synergistic coupling composite chelating copolymer/LAGP solid electrolyte with optimized interface for dendrite-free solid Li-metal battery Qingpeng Guo, Yu Han, Hui Wang, Shizhao Xiong, Weiwei Sun, Chunman Zheng, Kai Xie PII:

S0013-4686(18)32522-2

DOI:

https://doi.org/10.1016/j.electacta.2018.11.050

Reference:

EA 33051

To appear in:

Electrochimica Acta

Received Date: 10 September 2018 Revised Date:

8 November 2018

Accepted Date: 8 November 2018

Please cite this article as: Q. Guo, Y. Han, H. Wang, S. Xiong, W. Sun, C. Zheng, K. Xie, Novel synergistic coupling composite chelating copolymer/LAGP solid electrolyte with optimized interface for dendrite-free solid Li-metal battery, Electrochimica Acta (2018), doi: https://doi.org/10.1016/ j.electacta.2018.11.050. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. 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.

ACCEPTED MANUSCRIPT Novel synergistic coupling composite chelating copolymer/LAGP solid electrolyte with optimized interface for dendrite-free solid Li-metal battery Qingpeng Guo*, Yu Han*, Hui Wang, Shizhao Xiong, Weiwei Sun, Chunman Zheng and Kai Xie

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College of Aerospace Science and Engineering, National University of Defense Technology, Changsha, Hunan, 410073, China.

ABSTRACT

Solid electrolyte is one of the promising electrolytes to realize safe and high energy density

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rechargeable lithium metal battery. However, in view of the characteristics and defects of the solid inorganic and polymer electrolytes, the high-performance solid electrolyte is still a challenge for

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effective application in batteries. Herein, we report a novel inorganic-polymer flexible composite electrolyte prepared through synthetic of chelating copolymers. With synergistic coupling, the active filler-Li1.5Al0.5Ge1.5(PO4)3 can be uniformly distributed and maintains good composite with the polymer matrix. Thereby, this composite structure promotes the electrolyte with increased electrochemical stability and enhanced thermal stability. In our case, the larger volume fraction of continuous interface between the filler and gel polymer layer is helpful to increase ionic

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conductivity and Li+ ion transference number of composite electrolyte. Specifically, a large amount of fillers as a rigid part can block dendrites and provide effective pathways for lithium ion transfer, meanwhile, the uniform distribution of gel polymer layer on the surface of inorganic

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particles facilitates a soft contact to reduce the interface impedance and asubstantial flexibility to adapt the volume change in electrode. For further illustration, the solid-state lithium battery of

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LiFePO4/Li using this composite electrolyte shows relatively satisfactory performance, indicating the considerable cycling performance and acceptable rate capability. Thus, owing to its easy processing and compelling characteristics, this type of electrolyte make promise to reshape the feasibility of the high safety, stable and high energy density rechargeable solid-state lithium metal batteries at room-temperature. Keywords: Solid composite electrolyte; Flexible membrane; Chelating polymer; Li1.5Al0.5Ge1.5(PO4)3; Compatibility; Lowered interface impedance.

*Corresponding author. Tel.: +86-0731-84573149, fax: +86-0731-84573149. E-mail addresses: [email protected] ( Q, Guo ), [email protected] ( Y, Han ).

ACCEPTED MANUSCRIPT 1. . Introduction Lithium-based batteries as the next-generation rechargeable batteries have been ushered spring with the expanding demand for energy storage.[1-5] Given the fact

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that lithium metal is the lightest among all the metallic elements, it possesses the highest theoretical specific capacity (3860 mAh g-1) and the lowest redox potential (-3.04 V vs. the standard hydrogen electrode).[6,7] The most attractive place is that

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the Li metal anode can be matched with several high-voltage or non-lithiated cathode

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materials to prepare high energy density batteries.[6] However, for practical applications, uncontrollable lithium dendrites or resulting “dead” Li still affect the performance of the rechargeable lithium-based batteries, or even lead to serious and sporadic safety issues including thermal runaway and combustion/explosion of the

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batteries.[7,8]

In terms of both improving safety and effectiveness, inorganic solid electrolyte (ISE) is one of the most promising approaches to match lithium metal electrode.[9-14]

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This is because ISE is just in line with the well-understood effective theory of

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inhibiting the growth of lithium dendrites: the electrolyte ensures high shear modulus and the higher ionic conductivity and Li+ transference number (tLi+).[6,15] For the solid electrolytes, NASICON-framework structure of Li1.5Al0.5Ge1.5(PO4)3 has been of particular interest because of its high room-temperature ionic conductivity, superior thermal stability and universal environmental requirements.[16,17] However, this type of electrolyte is still facing intrinsic challenges such as complicated integrally molding process, extremely brittle and fragile characteristics resulting a relatively

ACCEPTED MANUSCRIPT large solid-solid interfacial resistance between the electrode and electrolyte.[16,18-20] Given these factors, solid-state composite electrolytes that can synergistically combine the beneficial properties of both ISE and solid polymer electrolyte (SPE) are

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recognized as an effective strategy to solve the aforementioned issues.[7,21-25] Moreover, a higher ratio of active ceramic filler will also help to improve the safety and electrochemical stability of the battery.[26] However, after reaching a certain

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filler ratio, the performance of the electrolyte begins to degrade due to the particle

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agglomeration and precipitation at high concentration which lowers the volume fraction of effectively rapid conduction interface between the filler and the polymer.[26]

With this perspective, in order to pursue a high proportion content of active filler

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in composite electrolytes, which bring high electrochemical stability and excellent thermal safety, we have grafted chelating groups on the polymer matrix of PVDF making the polymer gel layer effectively wrapped on each filler-LAGP surface

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through the covalent anchored effect. Also, the improved interfacial adhesion between

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the ceramic phase and polymer matrix alleviates the sedimentation of the fillers and effectively increases the volume fraction of the rapid conduction interface. Owing to this feature, the composite electrolyte demonstrates attractive properties, such as facile processability, the higher ionic conductivity and Li+ ion transference number. Specifically, this type composite electrolyte facilitates a stable interface with small interfacial impedance between electrolyte and Li metal anode, which can guarantee sufficient cross-boundary ion transportation and curb the growth of lithium dendrites.

ACCEPTED MANUSCRIPT With the above merits, the battery of LiFePO4/Li combined with this electrolyte membrane shows excellent performance, including high coulomb efficiency, good cycle life, and acceptable rate performance. Thus, this electrolyte is bound to play an

density, high safety and deformable shapes. 2. Experimental Section

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important role in the new generation energy storage applications with high energy

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Synthesis of copolymer matrix and preparation of composite electrolytes:

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Copolymer synthesis: As illustrated in Figure 1a, there require three major steps for the synthesis of the target product. a. The polymer of poly(vinylidene fluoride) (PVDF, 4 g) was dispersed in 100 ml LiOH solution (5 M) and stirred for 8 h at 60 °C to form a dark brown suspension. Then, the suspension was suction filtered and

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washed with deionized water. Finally, the modified PVDF powder was dried at 60 °C for 24 hours. b. PVDF-GMA was synthesised according to the free radical polymerization. 1 g of the modified PVDF was dissolved in 15 ml of

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N-methylpyrrolidone (NMP) solvent under vigorous stirring at 50 °C. Then, 31.5

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mmol of glyceryl methacrylate (GMA) and 0.23 mmol of the initiator dibenzoyl peroxide were added, and the reaction of the mixture was carried out at 80 °C for 12 hours under argon atmosphere protection. The mixture was precipitated in the methanol and soaked in chloroform for 2 hours, and the resulting product was filtered and dried in vacuum oven at 105 °C for 24 hours. c. The solution-A was prepared by dissolving 4 g of iminodiacetic acid (IDA) in 156 ml of dimethyl sulfoxide solvent (DMSO), and 1.52 g of LiOH was further added for the neutralization. The product

ACCEPTED MANUSCRIPT obtained in process b was once again dissolved in 15ml NMP solvent to obtain solution-B. The solution A was added into B and mixed under vigorous stirring at 65 °C for 10 hours. Afterward, the mixture solution was precipitated, filtered and

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washed. Finally, the target product of PVDF-GMA-IDA was obtained after dried in vacuum oven at 85 °C for 24 hours.

Preparation of composite electrolytes: The different mass ratio of the

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PVDF-GMA-IDA and poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP)

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were dissolved in 1-Methyl-2-pyrrolidone (NMP) through magnetic stirred for 30 min at 50 °C. Then lithium salt lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), ionic liquid 1-methyl-3-ethylimidazolium bis(trifluoromethylsulfonyl)imide (EMITFSI) and Li1.5Al0.5Ge1.5(PO4)3 (LAGP) powder with the appropriate ratios of mpolymer =5:5:7:5 were added to the above solution. Among them,

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matrix:mLiTFSI:mEMITFSI:mLAGP

the LAGP powder were prepared according to our previous studies.[20] A homogeneous solution was finally formed after stirring for 3 h. Then, the resulting

24

h

to

obtain

composite

electrolyte

membrane:

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for

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mixture was cast onto a glass culture dish and subsequently dried at 100 °C in vacuum

(PVDF-HFP)x-(PVDF-GMA-IDA)y-LAGP-EMITFSI-LiTFSI composite electrolyte, referred to as PHxPGIyLEL. At last, the films were cut into circles with diameters of 19 mm and stored in argon atmosphere for further characterizations. Characterization: The surface morphology of the composite electrolyte was characterized by HITACHI S-4800 scan electron microscopy (SEM). The chemistry and crystal phase

ACCEPTED MANUSCRIPT were characterized by Fourier transform infrared (FT-IR) spectra (Bruke V70 spectrometer in the range of 4000-400 cm-1) and X-ray power diffraction (SIEMENS D-500, 2θ=10°~70°), respectively. 1H-NMR spectra was recorded in DMSO-d6 on a

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Fourier transform NMR spectrometer with a measuring range of -4~18 ppm. Electrochemical Measurements:

The blocking type cells with the structure of stainless steel/solid composite

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electrolyte/stainless steel was assembled and the electrochemical impedance

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spectroscopy (EIS) at various temperatures (25~100 °C) were measured with AC amplitude of 10 mV from 0.01 to 105 Hz. The ionic conductivity (σ) of the membrane was calculated by the formula: σ = d/(Rb·S). The lithium-ion transference number (tLi+) was

measured

by

a

combination

measurement

of

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impedance

and

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chronoamperometry. The electrochemical stability of the membranes was measured by cyclic voltammograms (CV) and linear sweep voltammetry (LSV) using prepared Li/PH0.35PGI0.15LEL/SS cells at the scanning rate of 0.1 mV s-1. The CV analysis was

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performed in the potential limit between -0.5 V and 2.5 V and for LSV, the potential

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range fixed from 2.5 V to 6 V. Another way to evaluate interface compatibility of the PH0.35PGI0.15LEL against the lithium electrode was through the symmetric Li/PH0.35PGI0.15LEL/Li analog cells with the different current density from 0.02 to 0.1 mA cm-2, and each cycle with the charge and discharge current of 0.1 mA cm-2 was for 20 min, respectively. Meanwhile, the cycles with the other current density (from 0.02 to 0.08 mA cm-2) was for 1 h, respectively. After cycles, the Li electrode obtained from the disassembled cell, which was as the samples for SEM or XPS tests.

ACCEPTED MANUSCRIPT The solid-state batteries of Li/PH0.35PGI0.15LEL/LiFePO4 coin cells were assembled. The cathode was prepared in the conventional casting process by mixing 80 wt % LiFePO4 powders (carbon-coated LiFePO4 particles with a diameter of 200

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nm-1µm, the SEM image of the LiFePO4 cathode materials were shown in Figure S1), 10 wt % carbon black, and 10 wt % PVDF-GMA-IDA (which was contained in ingredient of PH0.35PGI0.15LEL) in N-methylpyrrolidone (NMP) solvent. The

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composite cathode was dried in a vacuum oven at 105 °C for 12 h. The loading

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density of LiFePO4 cathode was controlled to be 2.05 mg cm-2. The thickness of Li metal was about 0.45 mm, and the battery was assembed by a certain pressure making the composite cathode, electrolyte and Li electrode closely contact in button cell. The cell was cycled at constant current (0.05 C) between 2.7 and 3.85 V at room

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temperature. The rate studies were carried out with charging in the same current and discharging with different current density. 3. Results and Discussion

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3.1 Synthesis and physical characterization of electrolyte

Figure 1.Schematic illustrations for the synthesis procedure of polymer matrix PVDF-GMA-IDA, PHxPGIyLEL and the photograph of the membrane.

Figure 1 shows schematic images about the synthesis procedure of the polymer matrix PVDF-GMA-IDA and the PHxPGIyLEL. First, the copolymers were

ACCEPTED MANUSCRIPT synthesized by free radical copolymerization of PVDF (with double bonds) and GMA, and then the ring opening reaction was happened between PVDF-GMA and IDA, thus, getting the target product of PVDF-GMA-IDA. To confirm the successful modified

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reaction of the polymer matrix, Fourier transform infrared (FTIR) spectra of PVDF, PVDF-GMA and PVDF-GMA-IDA are compared in Figure 2e. Comparing the spectrum 1 and 2, there have new absorption peaks appear at 908 and 1730 cm-1,

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which correspond to the characteristic absorption peaks of epoxy groups and carbonyl

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groups, respectively, indicating that GMA has been grafted to the PVDF matrix. For the spectrum 3, the new absorption peak at 3400 cm-1 is assigned to -N-H and -OH, alternatively, the disappeared epoxy absorption peak at 908 cm-1 indicates that a ring-opening reaction has occurred between PVDF-GMA and IDA, that means the

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PVDF-GMA-IDA has been successfully prepared.

The PHxPGIyLEL membranes were fabricated via conventional solution-casting technique, wherein the different mass ratio of PVDF-GMA-IDA and PVDF-HFP

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should be controlled to ensure the formation of flexible and self-standing polymer

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membrane. As we investigated, when the value of mPVDF-GMA-IDA: mPVDF-HFP is higher than 0.67, the membranes can be easily damaged reflecting the poor film formation (as shown in Figure S2). The PH0.35PGI0.15LEL membrane exhibits great flexibility and yellowish color with about 200 µm thickness (Figure 1). To investigate the effect of IDA on morphology of the membranes, scanning electron microscopy (SEM) was comparatively conducted on the electrolytes as shown in Figure 2a-d. It could be found that each LAGP particle is surrounded by a uniform gel polymer layer, which is

ACCEPTED MANUSCRIPT different from the composite electrolytes PVDF-HFP-LAGP-EMITFSI-LiTFSI (referred to as PHLEL) without the chelating ingredients (Figure S3). This should be ascribed to the chelate bonds between inorganic ceramics and polymer playing an

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active role in the interface chelating,[27] which might conducive to the related performance of the composite electrolytes. In addition, the gold atomic could destroy the polymer layer structure after prolonged vacuum sputtering of the electrolyte, as

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shown in the Figure S4, using this experimental treatment, we further verify that the

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polymer layer indeed coats and acts on the surface of the inorganic ceramic particles.

Figure 2 . (a)-(d) SEM micrographs of the prepared PH0.35PGI0.15LEL with different

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magnifications. (e) FTIR spectrum of the PVDF, PVDF-GMA and PVDF-GMA-IDA. (f) XRD patterns of the polymer matrix and other different electrolyte ingredients.

The XRD patterns of the different components in the electrolytes are presented in

Figure 2f. In general, the pattern of PVDF contains three strong characteristic peaks of crystalline, indicating the semi crystalline nature of PVDF. After polymerization, these characteristic peaks diminish or even disappear, implying the amorphous phase of polymer matrix PVDF, meanwhile, some diffraction peaks from GMA and IDA

ACCEPTED MANUSCRIPT appeared in copolymer. After adding LAGP, obviously, characteristic peaks of solid composite electrolyte PHLEL and PH0.35PGI0.15LEL are almost consistent with pure LAGP characteristic spectrum peak, which implies that LAGP can reduce the degree

become more disordered.[20]

3.2.1 Electrochemical performance analysis

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3.2 Electrochemical performance and spectrum analysis

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of crystallinity of polymer matrix, making neat molecular chains in crystal district

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High ionic conductivity and stable electrochemical window are key prerequisites for electrolytes intended for battery applications.[28] Arrhenius plots of ionic conductivity with different contents of chelating polymer in composite electrolytes as function of temperature are shown Figure 3a. Firstly, PHxPGIyLEL exhibits slight

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higher ionic conductivity than that of other solid composite polymer electrolytes and the composite electrolytes without chelating agent (see Table S1 and Figure S5), the enhanced ionic conductivity may be due to the fact that the improved interfacial

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adhesion between the ceramic phase and gel copolymer with low interaction force and

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the good ionic transport at particle-particle interfaces.[29-31] In addition, PH0.35PGI0.15LEL provides the best ionic conductivity than that of PHxPGIyLEL with other different chelating agents at different temperatures. At ambient temperature, the ionic conductivity of PH0.35PGI0.15LEL was observed to be 5.79×10-4 S cm-1. This result demonstrates that the percolation threshold corresponds to the content of chelating ingredient, with optimal content of chelating agents, the effective lithium ion transfer pathway is formed, including LAGP, gel polymer and the larger volume

ACCEPTED MANUSCRIPT fraction of continuous interface. Namely, the ionic conductivity of this type composite electrolyte strongly relies on the conductive paths for the Li+. Meanwhile, the lithium-ion transference number (tLi+) of the electrolyte is also very important for the

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lithium metal batteries, the lower tLi+ will lead to a strong space charge near the anode and hence dendritic Li deposition.[15] Thus, the tLi+ of the PH0.35PGI0.15LEL was measured by the potentiostatic polarization method, Figure 3b presents the

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chronoamperometry and AC impedance of the Li/PH0.35PGI0.15LEL/Li cell at 25 °C.

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The composite electrolyte also possesses the tLi+ as high as 0.54, indicating that this electrolyte can be effectively used to solve some essential problems in solid LMBs. In this case, the improved tLi+ can be explained by that the main component LAGP with unity theoretical Li-ion transference number was involved in the process of lithium

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ion transport, on the other hand, anions could partially hinder or even fixed in the PH0.35PGI0.15LEL (as proposed in Figure 3c). The electrochemical stability of the PH0.35PGI0.15LEL was studied in the stainless

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steel/SPE/Li cells via cyclic voltammetry (CV) and linear-sweep voltammetry (LSV)

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measurements at room temperature. As illustrated in Figure 3d, no significant oxidation current was observed below 5.0 V, almost identical to that the composite electrolyte displayed electrochemically stable up to 5.0 V. In addition to the negative scan, Li+ plating and stripping current peaks were observed versus Li/Li+ in the different scanning process. Except for the first cycle, CV curves are consistent after many times cycles, indicating that the Li+ plating/striping process is reversible.

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Figure 3. (a) Arrhenius plots of ionic conductivity with different contents of chelating polymer in composite electrolytes as function of temperature. (b) Chronoamperometry profiles of Li/PH0.35PGI0.15LEL/Li with a step potential of 10 mV, and insets indicate the changes of impedance spectra before and after tests for Li-transfer number. (c) The schematic illustration for Li+ migration in the composite electrolyte. (d) CV and LSV curves of PH0.35PGI0.15LEL at 25 °C.

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3.2.2 Spectrum study and possible reaction mechanism in the PH0.35PGI0.15LEL To validate our assumption that the covalent anchored effect between copolymer and ceramic particles could ameliorate the related performance of the composite

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electrolytes. FTIR, and 1H NMR spectroscopy were performed and the results were

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shown in Figure 4.[32] Here, in order to exclude the influence of other components on the spectrum result, we detached the final composite solid-electrolyte and examined whether there had interaction between the chelating agent component and the ceramic particles in the solvent. FTIR spectra were taken for different mixing ratios of IDA, the intensity of the spectral peaks is significantly enhanced when the content of IDA exceeds 0.25 g and the differences are not as great as the content continues to increase (Figure 4a). And compared with the changes of the spectral peaks before and after the

ACCEPTED MANUSCRIPT addition of LAGP, the stretching vibration peaks of the interactions between metal element Al, O and N elements appeared at low frequencies from 400 cm-1 to 700 cm-1, indicating that the metal elements in the nano-ceramic particles were coordinated with

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N and O of IDA, respectively. Specifically, the characteristic absorption peaks at 1660, 1430 and 1400 cm-1 are corresponds to -C=O, -C-N and O=C-O stretching modes of IDA respectively. However, the intensity of the peaks decreases with the addition of

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LAGP, which means that LAGP particles may have the association with the -C-N

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group and O=C-O group in IDA. This phenomenon is further confirmed in the results of 1H NMR measurement. As shown in Figure 4c, the typical peaks of the proton in the -HN- (~4.35 ppm) and -CH2- (~3.42 ppm) are both shift to upfield with the addition of LAGP, which is attributed to the interaction between LAGP and -C-N

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group, O=C-O group in IDA increasing the electron density of these groups. Aforementioned analysis of the FTIR and NMR tests can be described by a possible reaction mechanism in PH0.35PGI0.15LEL which is proposed and displayed in Figure

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4d and e. Not just the polymer matrix and IL have a mutual influence on the side of

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the association with Li+, resulting in a better dissociation of lithium salt as we have confirmed in previous studies,[33] but also, with the covalent anchored effect between LAGP and chelating polymer, the performances of the composite electrolyte have been affected by this design.

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Figure 4. (a) and (b) FTIR spectra of DMSO mixing with different concentrations of IDA and a certain amount of LAGP at the wavenumbers of 400-4000 cm-1. (b) 1H NMR spectra of pure IDA and IDA coupling with LAGP. (d) and (e) The possible reaction schemes between different components in PH0.35PGI0.15LEL.

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3.2.3 Interface compatibility with lithium electrode The stability of the electrode-electrolyte interface is crucial for the cycling

performance of LMBs. One method is to monitor the time dependent alternating current impedance spectra of symmetrical Li/PH0.35PGI0.15LEL/Li cell at room temperature. As shown in the Figure 5a and b, the cells had lower interfacial resistance and the values of Ri were found to continued increasing until the 3rd day and then stayed at a stable value about 220 Ω, implying a stable interface between

ACCEPTED MANUSCRIPT PH0.35PGI0.15LEL and Li metal electrode can be built. In our case, the value of final stable interface impedance is significantly lower than that of the unchelated and modified composite electrolyte (Figure S6), profiting from that the uniformly coated

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gel polymer layer facilitates a soft contact with the Li metal electrode to maintain the smaller interface impedance and adapt the volume change in electrode. The interface stability of Li/Li symmetric cells and Li+ transport capability across the interface were

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further evaluated by galvanostatic Li plating/stripping cycling tests at different current

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density. It is observed that the voltage of the cells gradually increases with the increase of current density from 0.02 mA cm-2 to 0.1 mA cm-2 (Figure 5c), which is mainly due to the polarization voltage that appears as the current increases. When the circulating current is maintained at 0.1 mA cm-2, the Li/Li cell with PH0.35PGI0.15LEL

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still exhibits a smaller overpotential in the voltage hysteresis and longer cycling stability with flat and stable voltage plateau, which is also in good agreement with the slightly decreases of EIS Nyquist plots of the cell before and after cycles (Figure S7).

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Meanwhile, we compared the interface stability of Li/Li symmetric cell with the

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polymer electrolyte without inorganic fillers , as shown in Figure S8, the cell exhibits unstable cycle voltage when the circulating current at 0.1 mA cm-2, the phenomenon of a gradual increase and a sudden drop in voltage reflecting the poor interface compatibility. Therefore, the improved interface stability of PH0.35PGI0.15LEL should be attributed to the excellent properties of the electrolyte with high ionic conductivity and high tLi+ making even ion transport through the interface, furthermore, inorganic ceramic particles possess electrochemically stable may suppress harmful interfacial

ACCEPTED MANUSCRIPT side reactions between electrolyte and electrodes.[17] To clarify this, the surface morphology of the Li electrode after 795 h of cycling was analyzed by SEM. We can see that there exist lithium dendrites on the surface of the cycled Li electrode with the

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polymer electrolyte without inorganic fillers (Figure S9). In comparison, although there have inconsistency and not flat surfaces compared to before cycling, a large amount of fillers in composite electrolytes generally acted as physical barriers to

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restrict the growth of lithium dendrite. Meanwhile, the PH0.35PGI0.15LEL with the low

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interfacial impedance and good compatibility can greatly outperformed the Li metal, making it become a promising candidate for future applications in high-energy

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densities rechargeable LMBs.

Figure 5. (a) Time evolution of the impedance response of Li/PH0.35PGI0.15LEL/Li cell. (b) Time evolution of the specific interfacial resistances (Ri) of the cell for different storage time. (c) Voltage profiles for the Li/PH0.35PGI0.15LEL/Li symmetric cell with various current densities at 25 °C.

3.3 Effect of PH0.35PGI0.15LEL in batteries To demonstrate the feasibility of the electrolyte practical application and

ACCEPTED MANUSCRIPT galvanostatic performance in batteries, solid-state LiFePO4/Li batteries based on PH0.35PGI0.15LEL were assembled. The design of the cell as described in Figure 6a, in order to build effective ion channel and make full use of the active material of cathode,

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active material particles (Figure S10).

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we evenly distribute the appropriate amount of electrolyte components among the

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Figure 6. The performance of PH0.35PGI0.15LEL in solid-state battery of LiFePO4/Li: (a) Schematic diagram representing the solid-state battery of LiFePO4/Li. (b) Cyclic voltammograms of the LiFePO4/PH0.35PGI0.15LEL/Li cell at different scan rate. (c) Cycling performance of LiFePO4/PH0.35PGI0.15LEL/Li (0.05 C), insets indicate charge and discharge profiles. (d) C-rate discharge performance of LiFePO4/Li cells.

Firstly, Figure 6b shows cyclic voltammetry (CV) curves of the LiFePO4/Li cell

at different scan rate. The well-defined redox peaks were observed corresponding to the reversible extraction and insertion of Li+ in the cathode. As with the increase of the scan rate, the redox peaks become broad gradually and the distance between redox peaks has a slight increase but remain narrow, which means the smaller polarization of electrochemical reaction in the cell.[34] In addition, compared with

ACCEPTED MANUSCRIPT LiFePO4/PVDF-HFP-LAGP composite electrolyte without chelating agent/Li cell (Figure S11), the LFP/PH0.35PGI0.15LEL/Li cell shows a superior cycling performance with initial discharge capacity (130.5 mAhg−1) at 0.05 C. However, the capacity of the

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cell increases gradually during the first 5 cycles, perhaps as a result of that the electrode is activated and a stable interfacial film is gradually formed between the electrolyte and the electrode. And there still maintains 135.2 mAh g-1 after the 100th

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cycles reflecting excellent cycle stability (Figure 6c). Note that the coulombic

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efficiency after the first cycle gradually increases and remains stable in the throughout cycling test, which indicates superior electrode/electrolyte interfaces stability during the long-term cycles. More importantly, the battery with PH0.35PGI0.15LEL exhibits nearly no increase of polarization in charge/discharge voltage profiles after cycles,

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verifying the smaller interfacial resistance of the battery. In addition, the composite electrolyte was further subject to cycling at various discharge current densities from 0.05 to 2 C. Figure 6d shows the rate capabilities of LiFePO4/Li cells. The discharge

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capacity of the cells dropped with increasing current density, which could be

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explained by the increase in polarization of the cells. The cell could deliver capacities of 136.5, 130.1, 120.2, 105.5, 80.9 and 31.2 mAh g−1 at the discharging rate of 0.05, 0.1, 0.2, 0.5, 1 and 2 C, respectively. When the discharge current rate returns to 0.05 C at the end, the reversible capacity was almost recovered to 135.6 mAh g-1. It suggests that the PH0.35PGI0.15LEL is still structurally and electrochemically stable. 4. Conclusion To

summarize,

a

novel

synergistic

coupling

composite

chelating

ACCEPTED MANUSCRIPT copolymer/LAGP solid electrolyte was fabricated and effectively used in solid-state Li-metal batteries, exhibiting safe and good electrochemistry. Benefitting from the effect of synergistic coupling between polymer matrix and active fillers, the

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composite electrolyte can maintain high inorganic ceramic content and evenly distribute without sacrificing the relevant electrochemical performance. Thereby, the larger volume fraction of continuous interphase between the filler and polymer gel

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layer helps to increase of ionic conductivity and Li+ ion transference number of the

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solid composite electrolyte. Specifically, the composite structure also promotes the electrolyte with increased electrochemical stability, reflecting a stable interface with small interfacial impedance between electrolyte and Li metal anode and exceptional ability to suppress Li dendrite growth. Thus, with these advantages of

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PH0.35PGI0.15LEL, the symmetric Li/Li cell exhibits an unprecedentedly small overpotential in the voltage hysteresis and longer cycling stability with flat and stable stripping/plating voltage plateau. Moreover, solid Li-metal batteries utilizing

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PH0.35PGI0.15LEL deliver excellent cycling stability with high coulombic efficiency

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and maintain a high discharge capacity after more than 100 cycles. This novel fabrication strategy for the inorganic ceramic-polymer composite electrolyte with rigidity and flexibility presents a promising approach for developing high energy density and high security solid-state lithium metallic batteries. Supporting Information Supporting Information is available from the ACS Online Library or from the author.

ACCEPTED MANUSCRIPT Acknowledgements This work was financially supported by the National Natural Science Foundation of China (No.51702362) and National Postdoctoral Program for Innovative Talents

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(BX201700103). References

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