Gel polymer electrolyte with high Li+ transference number enhancing

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Gel polymer electrolyte with high Li+ transference number enhancing cycling stability of lithium anode Yanan Wang, Lixin Fu, Liyi Shi, Zhuyi Wang, Jiefang Zhu, Yin Zhao, and Shuai Yuan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b21352 • Publication Date (Web): 16 Jan 2019 Downloaded from http://pubs.acs.org on January 17, 2019

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ACS Applied Materials & Interfaces

Gel polymer electrolyte with high Li+ transference number enhancing cycling stability of lithium anode Yanan Wanga, Lixin Fua, Liyi Shia, Zhuyi Wanga,*, Jiefang Zhub, Yin Zhaoa, Shuai Yuana,c* a Research

Center of Nanoscience and Nanotechnology, Shanghai University, Shanghai

200444, China b Department

of Chemistry-Ångström Laboratory, Uppsala University, 75121 Uppsala,

Sweden c Emerging

Industries Institute, Shanghai University, Jiaxing, Zhejiang 314006, China

* Corresponding authors. E-mail addresses: [email protected] (Z. Wang), [email protected] (S. Yuan).

Abstract: Lithium anodes suffer severe safety problems in liquid electrolyte systems resulted from unstable Li plating/stripping process and Li-dendrite growth, leading to rapid degradation of Li-metal batteries. A polyethylene (PE)-supported gel polymer electrolyte with excellent electrolyte uptake/retention capability was simply prepared in this paper by the construction of cross-linked polymer networks on the surface of poly(ethylenimine) (PEI)-primed PE separator to stabilize the lithium anode. The highly delocalized negative charge of p-styrene sulfonate groups on polymer networks 1

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plays a role in regulating the Li+ and anions transport, giving rise to high Li+ transference number. This gel polymer electrolyte extended the electrochemical stability to 4.8 V, and improved the stability of interface between electrolyte and lithium metal anode (reduced overpotential and suppressed lithium dendrites) during storage and repeated lithium plating/stripping cycling. The Li metal anode-based battery employing this gel polymer electrolyte exhibits excellent cycling stability and C-rates capability.

Keywords: lithium metal anode; Gel polymer electrolyte; Li+ ion transference number; C-rate capability; Cycling stability.

Introduction Driven by the rapid development of electric vehicles industry, the batteries with high energy density such as Li-Oxygen batteries and Li-Sulfur batteries, as well as insertiontype lithium ion batteries with lithium metal anodes have become a research hotspot in recent years, and therefore Li metal anodes have received special attention.1-2 However, uncontrollable lithium dendrite growth, severe volume effect, and complex lithium/electrolyte interface side effects seriously hinder the application of lithium metal anode.3-5 Various methods including electrolyte additives,6-8 artificial solid electrolyte interface (SEI),9-11 superconcentrated electrolyte12-13 and structured Li anodes, 14-15 etc. have been proposed to address the above-mentioned issues of Li metal anodes. However, these methods cannot essentially solve the safety issues resulted from 2


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the intrinsic thermodynamic instability of lithium anode in liquid electrolyte.16 The substitution of liquid electrolyte with solid polymer electrolyte provides bright opportunities to solve the safety problem of lithium metal anodes, but the practical application of solid electrolytes is restricted by low ionic conductivity and high solid/solid interface resistance.17-19 In this case, the gel polymer electrolytes (GPEs), integrate the virtues of liquid and solid electrolytes such as acceptable ionic conductivity, good interfacial contact and wettability with electrodes and suppressed leakage of organic solvents, have drawn considerable attentions. Conventional GPEs often belong to dual-ion conductors, and their Li+ transference number is generally less than 0.5, which indicates that ionic conductivity is mainly contributed by anion motion.20 The anions accumulated at the electrode surface would result in the formation of concentration gradient, which limits the rate charge/discharge capability of the battery.21 Several strategies have been developed to fabricate single Li+ ion conducting GPEs by immobilizing anions on the polymer skeleton22-24 or adding traps for anions,2527

but often at the expense of ionic conductivity. In this paper, the cross-linked polymer networks with p-styrene sulfonate groups

were constructed on the PEI-primed PE separator surface, which was swelled with the liquid electrolyte to form PE-supported gel polymer electrolyte. The flexibility and highly delocalized negative charge of p-styrene sulfonate groups not only reduce ionpairing with Li+ cations to facilitate the transport of more Li+, but also hinder the migration of anions by the electrostatic shielding effect, leading to high Li+ transference number. This gel polymer electrolyte with high Li+ ion transference number enhanced 3



the stability of interface between lithium electrode and electrolyte during storage and repeated lithium plating/stripping cycling, as well as restrained the polarization effects and the growth of lithium dendrites. The battery with lithium metal anode exhibits excellent cycling stability and C-rates capability by employing this gel polymer electrolyte.

2. Experimental Section 2.1. Materials Sodium p-styrene sulfonate hydrate (C8H7NaO3S·xH2O, 90%), poly (ethylene glycol) dimethacrylate (PEGDMA, average Mw=750 gmol−1), branched-poly(ethylenimine) (PEI, average Mw=25000 gmol−1), lithium hydroxide monohydrate (LiOH·H2O), Nmethylpyrrolidon (NMP) were purchased from Aldrich. Dimethyl sulfoxide (DMSO, AR) was obtained from Shanghai Titan Scientific Co., Ltd. 2,2-Azobis (2methylpropionitrile) (AIBN) with 98% content was obtained from Shanghai Macklin Biochemical Co., Ltd. Graphite was got from Sinopharm chemical reagent Co. Ltd (SCRC). The polyethylene separator (PE, with a thickness of 8 μm and porosity of ~50%) was provided by SK Innovation. Li foils were obtained from Shenzhen Kejingstar Technology LTD. The electrolyte solution of 1 M LiPF6 dissolving in the mixture solvent of ethylene carbonate (EC), ethyl methyl carbonate (EMC) and diethyl carbonate (DMC) (1/1/1, volume ratio) was provide by Guotai Huarong Co., Ltd. The reagents were not purified before use. Deionized (DI) water was produced in our lab.

4


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2.2. Preparation of PE-supported GPE

Scheme 1. Illustration for the fabrication of PE-supported gel polymer electrolyte. The fabrication process of PE-supported gel polymer electrolyte consists of four steps as illustrated in Scheme 1. (1) Fabrication of PEI-primer layer on PE separators The pristine PE separators were treated by plasma for 1 min (80W) and then immersed into 2M PEI aqueous solution for 20 min and subsequently washed by DI water for 5 min to get PEI-primed PE separators. (2) Fabrication of polymer networks (PN) on PEI-primed PE separators 10.0 g monomer (C8H7NaO3S·xH2O), 1.0 g crosslinker (PEGDMA) and 0.4 g thermal initiator (AIBN) were mixed in 30.0 g DMSO and then stirred overnight to get 5



the uniform polymer precursor solution. The PEI-primed PE separators were immersed in this polymer solution for 1 hour to fully absorb it, and then treated in vacuum oven for 12 h at 80℃ to initiate the polymerization. (3) Ion exchange of modified PE separators After thermal polymerization, PE separators were taken out from vacuum oven and cleaned by DI water to remove unreacted monomers, and then immersed in 2M LiOH solution for 12 h to replace the Na+ ions on the sulfonate groups by Li+ ions. Subsequently, the separators were washed once again by DI water to remove the extra LiOH and then dried in vacuum oven for 8 h at 80℃. (4) Formation of PE-supported GPE After the ion exchange, the PN-modified separators were prepared which were then immersed in the electrolyte for 2 hours to get the PE-supported gel polymer electrolyte (PE-supported GPE).

2.3. Characterization The FT-IR spectra were carried out with Nicolet 6700. The XPS spectra were recorded on a PerkinElmer PHI 5000C ESCA spectrometer to confirm the crosslink process of polymer networks (PN) on PE separators. The morphologies of samples were observed on the field emission scanning electron microscope (FE-SEM), which was equipped with an energy dispersive X-ray spectrometer (EDS) . The hydrophilicity of samples was determined by contact angle on DSA100 (KRUSS). The equation (1) and (2) were used to obtain the electrolyte uptake and leakage of separators: 6


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uptake (%) = (Wt -W0) /W0 ×100%

(1)

leakage (%)=(Wt –We)/(Wt -W0) ×100% (2) Where W0 , Wt represents the weight of pristine separator and the separator after absorbing electrolyte sovlent, respectively. The extra electrolyte was removed from the separator surface by using a filter paper. We is the equilibrium weight of separator with soaking electrolyte which was measured by putting it between two pieces of filter paper under a weight of 100 g. The electrolyte-soaked PE separator or PE-supported gel polymer was sandwiched between two stainless steel (SS) electrodes to measure the impedance in the frequency range of 0.01Hz-1MHz (AUTO LAB, PGSTA T302N). The ion conductivity was calculated by the following equation:

 = d/(R×S)

(3)

Where σ, d, R, and S represents ionic conductivity, thickness, bulk resistance, and the area of separator, respectively. The electrochemical impedance spectra (EIS) and chronoamperometry analysis of Li/Li symmetric cells were carried out to obtain the Li+ ion transference number by: tLi+ = Is(ΔV − I0R0)/I0(ΔV − IsRs)

(4)

Where ΔV represents the potential difference (10 mV). I0 and IS represents the initial and steady state current, respectively. R0 and RS represent the interfacial resistance before and after polarization, respectively. The interfacial property between electrolyte and lithium metal was also studied by measuring the static impedance change of Li/Li symmetric cells with time, as well as

7



the lithium plating/stripping dynamic behavior under a current density of 2 mAcm-2 for 30 min. The frequency range for impedance analysis was 0.01 Hz to 1 MHz. The electrochemical performances of assembled coin-type LiCoO2/Li unit cells, such as the C-rate and cycling specific capability, were evaluated by the battery cycle system (Wuhan LAND Electronics Co., LTD, China) between 3.0 V-4.2 V. The preparation process of the cathode is as follows: LiCoO2, acetylene black (conductive agent) and PVDF (binder) (8:1:1, by weight) were mixed in the presence of N-methyl-2pyrrolidone (NMP) to get an uniform slurry. The slurry was coated onto the aluminum foil (thickness of 14μm), and dried at 120 °C for one day. At last the sheet was cutted into small discs with a radius of 5 mm for testing. The resulting LiCoO2 electrodes had a thickness of 150 μm which loads the active mass of 2.55 mg/cm2. The C-rate capacities of the cells were measured in the range of 0.2 C-5 C. The cycling stability of the cells was evaluated by cycling at 0.2 C for 300 cycles. Meanwhile, the EIS spectra of the LiCoO2/Li cells were measured on the AUTO LAB electrochemical workstation ( PGSTA T302N) from 0.01 Hz to1 MHz.

3. Results and Discussion 3.1. Formation of PE-supported GPE

8


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Fig. 1. (a) FT-IR spectra of PEGDMA, C8H7NaO3S·xH2O monomers, pristine PE and polymer networks (PN)-modified PE separators; (b) XPS spectra of PE and polymer networks (PN) modified PE separators.

The FT-IR and XPS spectra (Fig. 1) were used to confirm the formation process of polymer networks (PN) on PE separator. According to Fig.1(a), the PE separator shows the peaks at 2850-3000 cm-1 and 1465 cm-1 belonging to the stretching and bending vibrations of C-H bond, respectively.28-29 In contrast, the polymer networks (PN)modified PE separator exhibits some new characteristic peaks which were ascribed to the C-N bond of PEI at 1650 cm−1,28, 30 the C=O bond of PEGDMA at 1728 cm−1,31-32 and the asymmetric stretching vibrations (1191 cm-1 and 1135 cm-1) and the symmetric stretching vibrations (1040 cm-1 and 1005 cm-1) of SO3- of C8H7NaO3S·xH2O monomers,24, 33 respectively. Similarly, the XPS spectrum of PE separator shows the C1s peaks (Fig. 1(b)). However, the PN-modified PE separator exhibits new N 1s, O 1s, and S 2p peaks originating from PEI, PEGDMA and C8H7NaO3S·xH2O. These results confirm the fabrication of PEI primer layer and polymer networks on the PE 9



separator surface. The disappearance of FT-IR characteristic peaks attributed to C=C bonds for the PN-modified PE separator confirms the occurrence of double bonds thermal polymerization between PEGDMA and C8H7NaO3S·xH2O monomers. The cross section and surface elemental distributions of PN-modified PE separator were analyzed by EDS (Fig. S1). The uniform distribution of N and S throughout the cross section and surface of PN-modified PE separator indicates the successful polymerization of monomer polymers throughout the entire inner and outer surface of PE separator. As shown in Fig. S2, after the fabrication of polymer networks on the PE separator surface, the contacted angel of separator decreases significantly from 121.4° to 42.6° due to the existence of hydrophilic groups, which agrees well with the good affinity of PN-modified PE separator to electrolyte compared with the pristine PE.

10


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Fig. 2. Surface SEM images of (a) pristine PE separator, (b) electrolyte-soaked PE separator, (c) PN-modified PE separator, (d) PE-supported GPE. The microstructure of separators with or without electrolyte was observed by SEM. According to Fig.2, the PE separator remains sub-micron interconnected pores after the electrolyte uptaking. However, the pores of the PN-modified PE separator almost disappear due to the swelling of polymer networks after absorbing the electrolyte, indicating the formation of PE-supported gel polymer electrolyte (PE-supported GPE).

Fig. 3. (a) The electrolyte uptake of PE separator and PE-supported GPE under different wiping time, (b) The electrolyte leakage test of PE separator and PE-supported GPE.

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The electrolyte uptake/retention capability of pristine PE and PE-supported GPE was evaluated (Fig. 3 (a) and (b)). Base on the results, the wiping time has a great effect on the electrolyte uptake, and the electrolyte uptake of pristine PE and PE-supported GPE firstly decreases with the increase of wiping time and then gets stable at 128% and 330%, respectively. Meanwhile, the electrolyte leakage of PE-supported GPE is much lower and slower than the pristine PE. These results show that PE-supported GPE possesses much higher electrolyte uptake/retention capability due to the existence of hydrophilic polymer networks.

3.2. Electrochemical Properties The ion conductivities were analyzed by measuring the Nyquist plots of SS/SS cells ( Fig. S3). The intercept on the X-axis in the high frequency region is corresponding to the bulk resistance Rb, which was significantly decreased for PE-supported GPE. Therefore, the ionic conductivity, which is directly determined by the amount of mobile ion species and the ions mobility rate in the separator/electrolyte system [34], was improved significantly for PE-supported GPE (0.45mS cm-1) compared with the pristine PE separator (0.18mS cm-1). According to Fig. S2 and Fig. 3, the PE-supported GPE possesses excellent electrolyte uptake and retention which can facilitate the ionic conductivity. As well, the dangling chain configuration of p-styrene sulfonate groups on polymer networks can promote the mobility of ions, which synergistically promotes the ionic conductivity. 12


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To improve the Li+ ion transference number can effectively decrease the polarization effect and increases the energy output of battery. As illustrated in Fig. 4, the Li+ ion transference

numbers

of

samples

were obtained

by

combining

the

chronoamperometry and AC impedance analysis. Compared with the pristine PE separator, the PE-supported GPE exhibits extremely high Li+ ion transference number of 0.72, which may be ascribed to its high electrolyte uptake and interaction between the functional groups of the polymer networks and the ions in the electrolyte solution. The p-styrene sulfonate groups possess highly delocalized negative charges. The flexible and delocalized nature not only reduces ion-pairing with Li+ cations (i.e. increases the number of free Li+), but also facilitates the Li+ transport. Moreover, it also could hinder the migration of anions via the electrostatic shielding effect. The ion transport regulation of p-styrene sulfonate groups and the enhanced electrolyte uptake of PE-supported GPE synergistically contribute to the significantly enhanced Li+ ion transference number.

Fig. 4. Chronoamperometry profiles and Nyquist plots of Li /Li symmetric cells with

13



the (a) electrolyte-soaked PE separator and (b) PE-supported GPE.

The electrochemical stabilities of electrolyte-soaked PE separator and PE-supported GPE were evaluated by the linear sweep voltammetry (LSV) plots of Li/SS cells. According to the results (Fig. S4), the PE-supported GPE shows wider electrochemical stability window (4.8 V) than pristine PE separator (4.5V), indicating better compatibility with the cathode. This stability enhancement was owing to the excellent electrolyte retention of PE-supported GPE and the stabilization of anions via the electrostatic effects of p-styrene sulfonate groups on the polymer networks, which reduced the irreversible decomposition of solvent molecules as well as electrolyte anions on the cathode. The interfacial stability between lithium anodes and electrolytes is one key issue affecting the cycle life of lithium metal anode-based cells.35 The Li/electrolyte interface stability was studied by the static impedance change of Li/Li symmetric cells storaged at 25℃ (Fig. 5(a)). A semicircle reflecting the overall interfacial resistance (Rint) can be observed in all the spectra, which is mainly caused by the passivation layer deposited on the lithium metal surface. According to the fitted impedance parameters (Fig. 5(b)) using the equivalent circuit, Rint is extremely larger than the bulk solution resistance Rb, which is the leading factor affecting the resistance of Li/Li cell. The increment of Rint should be ascribed to the growth of passivation layer at the interface of Li/electrolyte. Compared with the cell employing PE separator, the cell with PE-supported GPE exhibits much slower increment of Rint, demonstrating better Li/electrolyte interfacial 14


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stability over storage.

Fig. 5. (a) Nyquist plots of Li/Li symmetric cells with PE separator or PE-supported GPE; (b) The variation of Rb and Rint with the storage time. 15



The Li/electrolyte interfacial stability is further investigated by the lithium plating and stripping test (Fig. S5). The test is carried out under f a constant current density of 2.0 mAcm-2. Compared with the cell assembled with pristine PE separator (control, black), the Li/Li cell with PE-supported GPE shows more stable voltage profile and smaller voltage hysteresis (red), which demonstrates that the PE-supported GPE can slow down the degradation of lithium metals and inhibit the consumption of electrolytes during cycling, which results in more stable lithium metal /electrolyte interface.

3.3. Battery Performance The C-rate capabilities and charge/discharge curves of LiCoO2/Li cells employing PE separator and PE-supported GPE, respectively, are illustrated in Fig. 6. The batteries were charged to 4.2 V at 0.2 C and discharged to 3.0 V at various C- rates. Although the voltage and capacity of both cells are reduced with the increase of C- rates, the cell employing PE-supported GPE exhibits higher capacity under various C-rates. The capacity difference between two cells gets larger at higher C-rates, indicating the effects of polarization (i.e. the drop of IR) be more dominant.36-37 At 5 C-rate, the cell based on PE separator losts most of the capacity, while the PE-supported GPE-employed cell still possesses a discharge capacity of 64.3 mAh g-1, reflecting higher discharge C-rate capabilities and cathode utilization.

16


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Fig. 6. (a) C-rate performance of LiCoO2/Li cells based on PE separator and PEsupported GPE, (b) Deviation (ΔV) between the discharge voltage plateaus and the typical charge/discharge plateaus (3.9 V) vs. C-rates; (c) Discharge profiles of cells based on PE separator and (d) PE-supported GPE. The typical charge/discharge plateaus for LiCoO2/Li unit cells is at 3.9 V under 0.2 C. With increasing C-rates, the discharge profiles gradually deviate from the equilibrium state which indicates the energy loss due to fast ions migration. This voltage plateaus difference (ΔV) corresponds to the IR drop of cells caused by the series resistance Rb and the electrode polarization. Fig. 6(b) shows a linear relationship between ΔV and C-rate. The slope of fitted line gives the sum of Rb and the electrode polarization resistance, which is 197 Ω and 144 Ω for the PE separator and PEsupported GPE, respectively. Since Rb is quite small, the electrode polarization

17



resistance plays a dominating role in the total resistance of cells. According to the results in Fig. 4, the PE-supported GPE displays much higher tLi+ than PE separator due to the ion transport regulation of p-styrene sulfonate groups and the excellent electrolyte uptake, higher Li+ transference number can mitigate the anions accumulation around the electrode/electrolyte interface especially at high C-rates. Consequently, the the electrode polarization originating from anions accumulation and the corresponding polarization resistance is lower (Fig. 6(b)), contributing to higher capacity retention at higher C-rates.

160 140 120 100 80 60 40 20 0

100 80 60 40

pristine PE PE-supported GPE

0

50

100

20 150 200 Cycle Number

250

Coulomb Efficiency / %

Discharge Capacity / (mAh g-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0 300

Fig. 7. Cycle stability and Coulombic efficiency of LiCoO2/Li cells with PE separator and PE-supported GPE at 0.2 C in the voltage range of 3.0-4.2 V.

The cycle performance of LiCoO2/Li cells with PE separator and PE-supported GPE was measured at 0.2 C for 300 cycles, which is shown in Fig. 7. The cell assembled with the PE-supported GPE exhibits higher discharge capacity retention and Coulomb efficiency with cycling. When cycling to the 300th, the PE-supported GPE-employed

18


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cell exhibits a discharge capacity retention of 62.7% (81.5 mAh g-1) with a Coulombic efficiency of 99.0%, far higher than the PE separator-employed cell (33.3%, 40.4 mAh g-1). The AC impedance spectra collected at different cycles and the corresponding fitted results are exhibited in Table 1 and Fig. 8. which gives us more in-depth understanding to the improved cycling performance of the PE-supported GPE-employed LiCoO2/Li cell. Generally, the total internal resistance of cells is composed of the combination resistance Rb, the SEI resistance (RSEI), as well as the charge transfer resistance (Rct) at the interface between electrode and electrolyte, corresponding to the intercept on the xaxis and two semicircles in the Nyquist plots, respectively. A sharp increase of the internal resistance during cycling is usually accompanied by a dramatic capacity decay. As shown in Table 1, Rb is so small that the total internal resistance is mainly dominated by the RSEI and Rct. It should be noted that the cell with the PE-supported GPE shows smaller increase of Rct and RSEI during cycling. RSEI gradually gets stable after 100 cycles. On the contrary, the cell with the PE separator exhibits continuous and rapid growth of Rct and RSEI. The restrained increase of RSEI for the PE-supported GPE demonstrates more stable SEI film on the surface of lithium matal anode. Meanwhile, smaller Rct value illustrates faster charge transfer at the interface between electrolyte and electrode, which can explain well that the PE-supported GPE-employed cell shows higher capacity retention during repeated charge/discharge cycles.

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Table 1. Resistance data of cells. 2nd cycle

Sample

50th cycle

100th cycle

300th cycle

Rb

RSEI

Rct

Rb

RSEI

Rct

Rb

RSEI

Rct

Rb

RSEI

Rct

Pristine PE

1.0

18.8

42.1

2.3

29.6

146.6

2.7

31.9

520.5

5.4

109.5

1145.0

PE-supported GPE

1.3

20.1

82.2

2.1

24.2

104.8

2.7

37.8

339.1

3.1

40.3

593.1

1800

200

(a)

150

-Z'' / Ω

1500

-Z'' / Ω

1200

100

50

0

900

0

50

0

50

100

150

200

100

150

200

Z' / Ω

600 300 0 200

(b) 1500

-Z'' / Ω

150

100

1200

50

-Z'' / Ω

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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900

0

600

Z' / Ω

2nd 100th

50th 300th

300 0

0

300

600

900 1200 Z' / Ω

1500

1800

Fig. 8. Nyquist plots of LiCoO2/Li cells with (a) PE separator, (b) PE-supported GPE at the 2nd, 50th, 100th and 300th cycle, respectively.

20


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The uncontrolled lithium dendrite growth which is initiated by unstable electrochemical reactions at the lithium/electrolyte interface may cause serious damage to the Coulombic efficiencies and cycling life of lithium batteries, and even results in short-circuit and safety problems when piercing the separator. The SEM images of lithium metal anode from the disassembled LiCoO2/Li cells after 300 cycles are shown in Fig.9. It’s obviously that the surface of lithium anode disassembled from the cell with PE-supported GPE remains relatively smooth. While the lithium electrode of PE separator-employed cell exhibits myriad rod-like dendrites. The Li dendrites formation and growth is rooted in the uneven deposition of Li+ ions.38 The suppressed dendrite growth of the PE-supported GPE-employed cell was attributed to the enhanced electrolyte uptake and ionic conductivity, as well as high Li+ transference number. The increased Li+ ion transference number and electrolyte uptake of PE-supported GPE can enhance the Li+ ion transfer rate through the electrolyte/ lithium anode interface, as well as provide more homogeneous Li+ ion flux to enhance the uniformity of lithium deposition.35, 39 In addition, according to the results of modelling and simulations by Newman et al.,40 the high ionic conductivity and Li+ ion transference number are beneficial for the reduction of concentration gradient and extension of the “Sand's time”. As a result, the growth of lithium metal dendrites can be delayed or even stopped.38

21



Fig. 9. Top view SEM images of lithium metal anodes from the disassembled LiCoO2/Li cells based on (a) PE separator, (b) PE-supported GPE after the 300th cycle.

The SEM micrographs and XPS spectra of pristine PE separator and PE-supported GPE disassembled from the LiCoO2/Li cells after 300 cycles were observed and analyzed (Fig. S6 and Fig. S7). According to the SEM images (Fig. S6), the PE separator still retains the pore structure, and the surface of the PE-supported GPE stays smooth and flat without crack and spalling, which confirms that both the PE separator and PE-supported GPE are stable enough during the repeated charge-discharge process. According to the XPS spectra of PE separator and PE-supported GPE (Fig. S7), the pristine PE separator indicates many new peaks attributed to the F 1s, F KLL, O 1s, P 2s and P 2p, and the PE-supported GPE shows new F 1s, F KLL peaks compared with

22


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the XPS results before cycling in Fig. 1(b). The appearance of F and P is due to the electrolyte decomposition, and the much lower abundance of F and P for the PEsupported GPE indicates that the electrolyte side reaction is suppressed in the gel electrolyte system, which is consistent with the improved charge/discharge efficiency and cycle performance of cells.

4. Conclusion A PE-supported GPE with the excellent electrolyte uptake/retention capability and high Li+ transference number was fabricated, which extended the electrochemical stability windows and improved the interfacial stability of lithium matal and electrolyte during storage and repeated lithium plating/stripping cycling. Based on these advantageous characteristics, the LiCoO2/Li unit cell with this PE-supported GPE exhibits reduced electrode polarization at high C-rate and suppressed formation of lithium dendrites, and therefore superior C-rates capability and cycling performance.

Supporting Information EDS analysis of the surface and cross-section of PN-modified PE separator; Water contact angle of PE and PN-modified PE separators; Nyquist plots of SS/SS cells based on PE separator and PE-supported GPE; LSV plots of Li/SS cells based on PE separator and PE-supported GPE; Voltage-time profiles of Li/Li symmetric cells assembled with PE separator and PE-supported GPE; Surface SEM micrographs; XPS spectra and 23



Elemental abundance of PE separator and PE-supported GPE disassembled from the cells after 300 cycles. Acknowledgments The National Natural Science Foundation of China (51711530162, 21503131) is acknowledged. The Swedish Research Council is also acknowledged for its special collaboration project with China.

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Gel polymer electrolyte with high Li+ transference number enhancing

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