Carbon Cloth Anode

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A durable sodium battery with a flexible Na3Zr2Si2PO12-PVDFHFP composite electrolyte and sodium/carbon cloth anode Qiang Yi, Wenqiang Zhang, Shaoqing Li, Xinyuan Li, and Chunwen Sun ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 21 Sep 2018 Downloaded from http://pubs.acs.org on September 21, 2018

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ACS Applied Materials & Interfaces

A durable sodium battery with a flexible Na3Zr2Si2PO12-PVDF-HFP composite electrolyte and sodium/carbon cloth anode Qiang Yi,1,2 Wenqiang Zhang1,2 Shaoqing Li1,2 Xinyuan Li1,2 and Chunwen Sun1,2,3*

1

CAS Center for Excellence in Nanoscience, Beijing Institute of Nanoenergy and Nanosystems, C hinese Academy of Sciences, Beijing 100083, P. R. China 2 School of Nanoscience and Technology, University of Chinese Academy of Sciences, Beijing 10 0049, P. R. China 3 Center on Nanoenergy Research, School of Physical Science and Technology, Guangxi University, Nanning 530004, P. R. China *

Corresponding authors.

Tel.: +86-10-82854648, fax: +86-10-82854648. Email: [email protected] (C. Sun)

ABSTRACT High-performance electrolytes and electrode materials play a critical role in advanced sodium-ion batteries with higher energy densities. In this work, we prepared a polymethylmethacrylate (PMMA)-filled composite electrolyte (named as GHSE) by in situ polymerization of methylmethacrylate

(MMA)

in

the

porous

Na3Zr2Si2PO12-polymer

vinylidene

fluoride-hexafluoropropylene (NZSPO-PVDF-HFP) composite membrane for the first time. The GHSE membrane exhibits a high ionic conductivity (2.78 ×10-3 S cm-1), wide electrochemical window (~4.9 V), high Na+ ion transference number (~0.63), good thermal stability and flexibility as well as smaller interfacial resistance. Moreover, a composite Na/C anode was prepared, which shows good dendrite suppression ability. The full cell Na0.67Ni0.23Mg0.1Mn0.67O2 |GHSE|Na/C exhibits excellent rate capability with an initial discharge capacity of 96 mAh g-1 even at a higher current density of 192 mA g-1 and excellent cyclability for 600 cycles. These results suggest that the GHSE and Na/C anode are promising electrolyte and anode materials for Na-ion batteries, respectively.

Keywords: Sodium batteries, composite electrolyte membrane, in situ polymerization, PMMA, sodium/carbon cloth anode

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1. Introduction Sodium-ion batteries (SIBs) have recently been extensively studied as power sources for electric energy storage applications due to the low-cost and abundant sodium resources of sodium compared with lithium.1-3 The insertion-compound cathode for rechargeable SIBs should reversibly insert/extract of Na-ion with a bigger radius (1.02 Å).4 P2-type layered transition-metal oxides have a sodium ion diffusion pathway between two face-sharing trigonal prismatic sites.5,6 In particular, P2-type Na0.67Ni0.33Mn0.67O2 (NNM) has become an attractive cathode material due to its high theoretical capacity and operating voltage (>3.5 V vs. Na/Na+), which is endowed by the Ni2+/Ni4+ redox couple.7,8 However, sodium-ion batteries with NNM cathode and organic liquid electrolyte usually show poor electrochemical properties, such as poor rate and cycling performance, which may be ascribed to the disadvantageous transformation from P2 structure to O2 structure at higher charge voltages. Moreover, the leakage of flammable liquid electrolyte causes great safe issues. Therefore, in order to address these concerns, the safe alternative electrolytes are highly desired.9-11 The NASICON-structured Na3Zr2Si2PO12 (NZSPO) has been proved to be a promising electrolyte for solid-state sodium batteries due to its high ionic conductivity (1.2×10-3 S cm-1 at 60oC), wide electrochemical window and superior cation transference number (t≈1). But the large interfacial resistance existing between the electrode and ceramic electrolyte as well as the poor plasticity of inorganic electrolyte are the main obstacle for their practical application on a large scale.12-14 Solid polymer electrolytes, e.g., poly(ethylene oxide) (PEO), exhibit superior flexibility, but their low ionic conductivity at room temperature (99%), nickel nitrate hexahydrate (Ni(NO3)·6H2O), 99.99%) and citric Acid (>99.5%) were purchased from Aladdin, Shanghai, China. Zirconyl nitrate (ZrO(NO3)2, 99.0%) was purchased from Macklin, Shanghai, China. Manganese (II) nitrate tetrahydrate (Mn(NO3)2·4H2O, >98.0%) and magnesium nitrate hexahydrate (Mg(NO3)2·6H2O , >99.0%) were purchased from Adamas, Shanghai, China. Poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP, Mw 3



400000 g·mol-1) was purchased from Sigma-Aldrich, Shanghai, China. Acetone, dimethylacetamide (DMAC) and N-methyl-2-pyrrolidone (NMP) were analytical grade.

2.2 Synthesis of the Na3Zr2Si2PO12 and Na0.67Ni0.23Mg0.1Mn0.67O2 powders Na3Zr2Si2PO12 was synthesized by a sol-gel method. Firstly, tetraethyl orthosilicate was dissolved in a mixture solution of water and ethanol, and then stoichiometric amounts of citric acid was added to adjust the pH to 1~1.5 under magnetic stirring for 1h at 60 oC, then stoichiometric amounts of NaNO3, ZrO(NO3)2 and (NH4)2HPO4 were added into the above solution to obtain a precursor solution. The precursor solution was heated at 80 °C to vaporize solvent under magnetic stirring. The obtained sol was dried at 120oC for 24h and then a gel formed. The product was collected, grounded and calcined at 950 °C for 12 h in air. The Na0.67Ni0.23Mg0.1Mn0.67O2 (NNMM) was prepared by a sol-gel method, as reported previously.11 2.3 Preparation of the GHSE membrane GHSE was prepared through two steps. First, a porous composite membrane was prepared by electrospinning. The uniform suspension solution was prepared by dissolving 1.5 g PVDF-HFP into 3 mL dimethylacetamide (DMAC), 6 mL acetone and 1.5 g ball-milled Na3Zr2Si2PO12. A porous membrane was obtained by electrospinning the above suspension solution. The electrospinning speed and electrospinning time were 0.5 mL/h and 1.5 h, respectively. The preparation procedures are similar to the literatures reported previously.21-28 Second, a composite gel-electrolyte in thickness of 0.15 mm was prepared by in situ polymerizing a mixture containing 2.5 mL of methyl methacrylate (MMA)，0.008 g dibenzoyl peroxide (BPO) and 1mL of liquid electrolyte (1 M NaPF6 in EC/PC (1:1)) in the above porous membrane placed in a glass container at 60oC for 6 hours. The controlled gel-electrolyte of PVDF-HFP-in situ polymerized-PMMA (~0.15 mm in thickness) was prepared by the same method except no addition of the Na3Zr2Si2PO12 4


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powder at the same condition. The glass fiber impregnated with the liquid electrolyte was also prepared. All these processes were conducted in an argon-filled glove box with water and oxygen levels of less than 0.5 ppm. The mass ratio of PMMA gel in the GHSE was determined by the equation as follows: % =

× 100%

(1)

Where w is the mass of GHSE, w0 is the mass of composite electrolyte before loading the gel, is the mass ratio of PMMA based gel. The calculated loading of PMMA gel in the GHSE is about 36.7%. 2.4 Characterization of the Na3Zr2Si2PO12 and GHSE membrane The morphology of the porous membrane and GHSE were characterized by a field emission scanning electron microscope (SEM, Hatchi SU8020). The crystal structure of the Na3Zr2Si2PO12 and Na0.67Ni0.23Mg0.1Mn0.67O2 were characterized by X-ray diffraction (XRD) measurements on Panalytical instrument X’Pert 3 Powder with Cu Kα radiation in the 2θ range of 10°–80°. Attenuated total reflection Fourier transformed infrared (FT-IR) spectroscopy was conducted on Thermo Mattson Infinity Gold FT-IR. Thermo-gravimetric analysis (TGA) and differential scanning calorimeter

(DSC)

were

performed

under

Ar

atmosphere

in

Thermogravimetric Analyzer (TGA/DSC 1) at a scan rate of 10

a

Mettler

o

C min−1.

Electrochemical impedance spectroscopy (EIS) was conducted by sandwiching electrolyte membrane between two stainless-steel electrodes with a ZENNIUM electrochemical workstation (Zahner, Germany) in 106 ~10-2 Hz frequency range with a voltage perturbation of 10 mV. 2.5 Electrochemical measurements and cells assembly The linear sweep voltammogram (LSV) was performed with a working electrode of stainless steel and a counter electrode of sodium metal foil performed on CHI604E at a scan rate of 1 mV s-1 in the potential range from 2V to 6 V at room temperature. Sodium-ion transference number (tNa+) of the electrolytes was measured with the 5



same method as reported in the literature.29 tNa+ was calculated by the following equation:

=

(2)

Where IS and I0 are the initial and steady currents, respectively. R0 and Rs correspond to the initial and steady total resistances, respectively. V corresponds to the polarization potential. The electrochemical impedance spectra (EIS) of the symmetric cell were tested in the frequency range from 106 Hz to 10-2 Hz with AC amplitude of 10 mV. The ionic conductivity (σ) of the electrolyte was calculated by the following equation: =

(3)

Where L is the thickness of the GHSE, S is the area of the electrode, and R is the bulk resistance of the GHSE. The NNMM cathode was prepared by mixing NNMM powder, Super P and PVDF at a mass ratio of 8:1:1 in N-methyl-2-pyrrolidone (NMP); then the mixture slurry was cast on an aluminum foil followed by evaporating the solvent at room temperature. The obtained electrodes were further dried at 120oC in a vacuum oven to obtain the final Na0.67Ni0.23Mg0.1Mn0.67O2 cathode. The mass loading of NNMM cathode was ~2 mg cm-2. All the batteries were assembled and tested with the same method except thatsodium foil was used as anode and the GHSE membrane was used as both electrolyte and separator.

Galvanostatic charge/discharge tests of all coin

cells were performed at a voltage window of 2-4.5V on a LAND battery test system.

3. Results and discussion 3.1 The PMMA-plasticized composite electrolyte As described in the experimental section, the GHSE membrane was mainly fabricated by two steps. In the first step, a porous composite membrane was prepared by electrospinning the mixture of NZSPO powder and PVDF-HFP (1:1 wt %)19 in a mixed solvent. The similar method was reported in previous literatures.30,31 In the second step, a plasticized composite electrolyte membrane was obtained by in situ 6


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polymerization of MMA with 28.6% 1 M NaPF6 in EC/PC (1:1, v/v) liquid electrolyte, as shown in Figure 1a. Figure S1 (Supporting Information) shows XRD patterns of Na0.67Ni0.23Mg0.1MnO0.67 and Na3Zr2Si2PO12 powders. Compared with those standard PDF files for Na0.67Ni0.23Mg0.1MnO0.67 and Na3Zr2Si2PO12, Na0.67Ni0.23Mg0.1MnO0.67 shows pure phase (PDF No. 54-0894) while Na3Zr2Si2PO12 show a little amount of impurity of ZrO2 besides the main phase NZSPO (PDF No. 01-084-1190).

Figure 1. (a) Schematics of the preparation process of GHSE. SEM images of the composite membrane without PMMA (b,c) and with PMMA (d,e).

The morphologies of the composite electrolyte membrane and the GHSE were examined by SEM. As shown in Figure 1b and 1c, it can be seen that the microstructure of the porous composite electrolyte membrane is similar to that of the glass-fiber (Figure S2 a, b) and the NZSPO particles are evenly distributed in the porous membrane (Figure S2c, d, e, f). The membrane has a lot of uneven pores with a size of tens of micrometers, which is beneficial for accommodating the PMMA-based gel. The average diameter of the membrane prepared by electrospinning process is about 4 µm (Figure S2b), which is similar to the previous 7



reports.24,25 After in situ polymerization of MMA in the porous membrane and evaporating solvent, the NZSPO particles are coated with polymers and the polymer fibers become stronger than the pristine ones (Figure 1d, e), which means that the MMA monomers were successfully polymerized and even dispersed in the porous membrane. The polymerization of MMA monomer was also characterized by FT-IR spectra (Figure S3). The cross-linked micro-structure of GHSE enhances its mechanical properties. In addition, the micro-pores (~200 nm in diameter) resulted from evaporating of solvent are evenly distributed in the GHSE (Figure 1d, e). The GHSE shows superior flexibility, which can be twisted, bent and rolled up (Figure S4). The porous membrane provides a flexible fiber matrix for the MMA monomer to polymerize and enable GHSE with flexibility while the PMMA-based gel does not show these properties. Figure 2 b shows the mechanical properties of the GSHE. The maximum stress of GHSE is 6.2 MPa, much higher than those of the PVDF-HFP-in situ polymerized -PMMA of 5.3 MPa and the glass fiber of 0.9 MPa (Figure S5). This higher stress of GHSE may be ascribed to the addition of NZSPO powders to the electrospun membranes, which enhances the mechanical properties of the PVDF-HFP fibers. But the lower strain of GHSE (110%) compared with PVDF-HFP-in situ polymerized -PMMA (118%) may be attributed to the poor mobility of polymer chains that is hampered by the NZSPO powders under external stress. The flexibility of GHSE enables it to be suitable for flexible energy storage devices. Thermal stability is an important parameter for electrolytes. As shown in Figure 2c, the DSC curve shows that the melt temperature of GHSE is about 100 oC. From the TGA curves of GHSE, we can see the decomposition temperature begins to appear around 120 oC (Figure 2c), and the decomposed temperature appears at 184.3 o

C, higher than that of PVDF-HFP-in situ polymerized-PMMA (125.1 oC), as shown

in Figure S6. The corresponding parameters are listed in Table S2. And the GHSE exhibits good dimensional stability, as shown in Figure S7, there is not apparent structure changes when the temperature is increased to 120 oC. All these results reveal that the flexible GHSE membrane has a good thermal stability. 8


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Figure 2. (a) Temperature dependence of the Na-ion conductivity of the GHSE and PVDF-HFP/PMMA gel electrolyte. (b) Stress-strain curve of the GHSE. (c) DTA-DSC curves of the GHSE at a heating rate of 10 oC min-1. (d) The Nyquist impedance spectra of the liquid electrolyte and GHSE, the inset shows the equivalent circuit for fitting. (e) Linear sweep voltammograms of the GHSE sandwiched between metal sodium foil and stainless-steel sheet in a potential range from 2.0 to 6.0 V at a sweep rate of 1 mV s-1. (f) Current-time profile of a symmetrical Na/GHSE/Na cell under a polarization voltage of 10 mV. The Nyquist impedance spectra of the cell before and after polarization are shown in the inset. 9



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The ionic conductivity of GHSE and PVDF-HFP-in situ polymerized-PMMA gel electrolyte were measured by electrochemical impedance spectroscopy (EIS) from 30 o

C to 60 oC at a frequency range of 106 ~10-2 Hz. The calculated ionic conductivity (σ)

of GHSE and PVDF-HFP-in situ polymerized-PMMA gel electrolyte are 2.78 ×10-3 S cm-1 and 1.69 ×10-3 S cm-1 respectively at 30oC ( Table S1). From the Arrhenius equation:

= −

(3)

Where σo, Ea, k and T are pre-exponential factor, activation energy, Boltzmann constant and temperature. Log (σ) versus T-1 shows a linear relationship.32,33 , From the slopes of the lines (Figure 2a), the calculated activation energy of GHSE and PVDF-HFP-in situ polymerized-PMMA gel electrolyte are 0.10 eV and 0.08 eV, respectively, which are similar to that of PMMA-based gel electrolyte (~0.12 eV),34,35 but much smaller than those of PEO-based polymer electrolyte (~0.46 eV),

36

Na3Zr2Si2PO12 (~0.23 eV) 37 and NZSPO-based composite electrolyte (~0.99 eV).19 The interfacial resistances of liquid electrolyte, GHSE and composite electrolyte assembled with NNMM cathode and Na anode were also examined by EIS. As shown in Figure 2d, the total resistance (Rt), including bulk resistance (Rb) and interfacial resistance (Ri), corresponds to the intercept of the semi-circle with the real axis at the lower frequency. The equivalent circuit is modeled by two parts: one is the fitted bulk resistance (Rb) of electrolyte; the other includes the double layer capacitance (Cdl), the liquid electrolyte resistance (R1), the charge transfer resistance (Rct), the Warburg impendence (Zw) and the constant phase element (CPE),38,39 The equivalent circuit model used for fitting is shown in the inset of Figure 2d. The Zw shows the behavior of ion diffusion process.39 So, the effect of pores existed in the GHSE on enhancing the ion transport would be reflected in the value of Zw. As shown in Figure 2d and Figure S8, the Rb of the liquid electrolyte, GHSE and composite electrolyte are 205, 662 and 10000 Ω, respectively. It indicates that the interfacial resistance of GHSE is comparable to the liquid electrolyte, but far smaller than the composite electrolyte. Therefore, the interfacial resistance of GHSE is significantly improved by addition of 10


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PMMA-based gel to the composite electrolyte. The reduced resistance may be attributed to the plasticized PMMA-based gel, which not only links the NZSPO powder and polymer matrix (Figure 1d, e), but also is well compatible with electrode,20 thus leading to a reduced interfacial resistance. The electrochemical stability window of the GHSE and liquid electrolyte were evaluated with a cell NaGHSESS by linear sweep voltammograms (LSV) at a scan rate of 0.1 mV s-1 in the potential range from 2 to 6 V at room temperature. As shown in Figure 2e, no obvious oxidation current is observed until 4.9 V for GHSE, which is higher than that of the glass fiber impregnated with liquid electrolyte (4.6 V) (Figure S9). The wide electrochemical window of GHSE may be ascribed to high oxidation potential of PMMA, high electrochemical window of NZSPO powder (> 5 V) as well as strong interactions between liquid electrolytes and polymer chains.36,40Thus, this high electrochemical window makes the GHSE to be a promising electrolyte for solid-state sodium batteries. The transference number (tNa+) of GHSE was measured with a method described by Vincent et al on Na/Na symmetrical cells by chronoamperometry.41 From Figure 2f, the calculated tNa+ of GHSE is 0.63, which is higher than that of PVDF-HFP- in situ polymerized-PMMA gel electrolyte (0.37), as calculated from Figure S10, but lower than that of composite electrolyte (0.92).19 Compared to the PVDF-HFP- in situ polymerized-PMMA gel electrolyte without NZSPO, the high cation transference number of GHSE may be attributed to the Na-ion conducting NZSPO (t≈1) and anion trapping. The interaction between anion and polymer matrix impedes the movement of anions; and the decreased crystallinity of polymer due to addition of NZSPO particles increases the movement of polymer segment, which facilitates the transfer of Na+ ion. As a result, these two main factors promote the migration of cations. A comparison of the electrochemical parameters including Ea, tNa+, σ and stable electrochemical voltage window of various electrolytes with different polymers and fillers are listed in Table S3. Half cells were assembled with NNMM cathode and metal Na anode. All the cells are tested in 2.0-4.5 V at room temperature. The charge/discharge profiles of the cells 11



with liquid electrolyte and GHSE are shown in Figure 3a and Figure 3c, respectively. The cell with GHSE exhibits smaller polarization compared with the cell with liquid electrolyte. The initial charge capacity of the cell with liquid electrolyte is 100.4 mAh g-1 at 192 mA g-1, slightly larger than that of thee initial discharge capacity of 94.8 mAh g-1. This reduced capacity may be due to the formation of solid-electrolyte interface (SEI) on the anode and the decomposition of electrolyte.42 Compared with the cell with liquid electrolyte, the initial charge/discharge capacities of the cell with GHSE are 107 mAh g-1 and 102 mAh g-1, respectively, indicating a good reversibility, which is consistent with the CV results (Figure S11). As shown in Figure 3c, d, both cells show good Coulombic efficiency during the following cycling tests, which mean excellent reversibility. As shown in Figure S12, the EIS measurements results show that the Rt of the symmetric cell with GHSE and liquid electrolyte are 899 Ω and 1154 Ω respectively after 100 cycles. Compared with the initial values before cycles (Figure 2d), the cell with GHSE only shows an increase of 237 Ω while the resistance of the cell with liquid electrolyte increases 949 Ω, which means a more stable interface forming in the cell with GHSE membrane. After 100 cycles, the discharge capacity of the cell with liquid electrolyte is reduced to 69.8 mA h g-1 while the discharge capacity of the cell with GHSE keeps 94.6 mA h g-1 after 100 cycles. The improved electrochemical performances may be ascribed to the following reasons. Besides the high ionic conductivity and Na-ion transference number, the PMMA filled composite electrolyte is also tolerant to volume changes of electrodes during cycling, which can reduce the capacity decay in the following cycling process. As shown in Figure S13, compared with the glass-fiber based liquid electrolyte, there is no morphology changes for the GHSE membrane. In addition, the superior adhesion of PMMA-based gel to electrodes surface results in a reduced interfacial resistance and a close contact between electrolyte and electrode.

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Figure 3. Electrochemical performances of the sodium batteries with the NNMM cathode and metal Na anode: (a) Charge/discharge profiles of the cell with liquid electrolyte at a current density of 192 mA g-1. (b) Cycling performance of the cell with liquid electrolyte at a current density of 192 mA g-1. (c) Charge/discharge profiles of the cell with the GHSE-based electrolyte at a current density of 192 mA g-1. (d) Cycling performance of the cell with GHSE-based electrolyte at a current density of 192 mA g-1.

3.2 The Na/C anode and full cell Na dendrite is a big concern for sodium batteries. On one hand, the perpetual growth of Na dendrite brings about continuous consumption of Na and electrolyte; on the other hand, the volume fluctuation of the “hostless” Na during cycling may cause infinite volumetric change of metal sodium. Both factors lead to a rapid capacity decay and the serious safety problem.43,44 In order to confine the Na and suppress the growth of Na dendrite, a Na/C composite anode was prepared by dissolving metallic sodium into porous carbon felt, which is demonstrated as an effective strategy to suppress the growth of Na dendrite.45 By a similar method, we prepared a new type of 13



Na/C anode consisting of low-cost carbon cloth and metal sodium. As shown in Figure 4a, the Na/C composite anode was prepared by sandwiching a metallic sodium foil with two carbon cloth under a certain pressure. It can be seen that the Na metal was successfully inserted into the pores of carbon cloth. The average areal loading of the Na/C is about 4.038 g cm-2.

In order to study the ability of the Na/C anode to suppress sodium dendrite, symmetric cells were assembled with two Na/C composite and GHSE electrolyte. For comparison, symmetric cells with two Na metal foils and GHSE were also assembled and tested under the same condition. Figure 4 b and 4 c show the electrochemical impedance spectra of the symmetric cells with Na/C composite electrodes and metal sodium foils after the 1st, 20th and 40th cycles, respectively. The Na/C symmetric cell shows a resistance of 130 Ω after the first cycle, which is a little bit larger than that of the Na symmetric cell (120 Ω) due to the activation of the Na/C surface during the first cycle process. However, after the 20th and 40th cycles, the interfacial resistance of the symmetric Na/C cells only increases to 160 Ω and 270 Ω, respectively. In contrast, the interfacial resistances of the symmetric cell with the Na foil electrodes increase to 520 Ω and 800 Ω after the 20th and 40th cycles, respectively. The larger interfacial resistance with cycling may be ascribed to the gradual formation of SEI film.33 Thus, the symmetric cell with Na/C composite electrode has better interfacial stability than that of the cell with Na electrode. Figure 4d shows the voltage profiles of the sodium plating/stripping cycling in the symmetrical Na/CGHSENa/C at various current densities. It shows that the overpotentials for plating/stripping of metal Na in Na/C|GHSE|Na/C cell are about 5.5, 11.3, 36.5 and 59.5 mV and without obvious fluctuation during 900 hours at current densities of 0.5, 1, 3 and 5 mA cm-2, respectively. In comparison with it, the overpotential of the symmetric cell of Na|GHSE|Na is about 100 mV at a current density of 0.5 mA cm-2. The Na|GHSE|Na cell shows a large and irreversible voltage drop after 96 hours (Figure S14), which may be ascribed to the short circuits induced by the dendrite.46 These results demonstrate that the Na/C composite anode shows 14


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better capability to suppress the dendrite compared to that of the Na metal anode.

Figure 4. (a) Schematics of the fabricating process of the Na/C composite electrode. (b) EIS of the symmetric cell with GHSE sandwiched by two Na/C composite electrodes after 1st, 20th and 40th cycles, respectively. (c) EIS of the symmetric cell with a GHSE membrane sandwiched by two Na foils after 1st, 20th and 40th cycles, respectively. (d) Voltage profile of the symmetrical cell Na/CGHSENa/C at current densities of 0.5, 1, 3 and 5 mA cm-2 and capacities of 0.5, 1, 1.25 and 1.5 mAh cm-2, respectively, showing sodium plating/stripping cycling.

Figure 5a shows the charge/discharge profiles of the full cell NNMM|GHSE|Na/C. The cell delivers a reversible capacity of 124 mAh g-1 at a current density of 24 mA g-1. Figure 5b shows the rate capability of the full cell. The discharge capacity can still reaches 77 mAh g-1 even at a high current density of 768 mA g-1. The good rate capability of the full cell may be due to the superior ionic conductivity of GHSE and 15



improved interfacial resistance of the cell. Figure 5 c shows the cycling performance of the full cell. The initial discharge capacity of the NNMM|GHSE|Na/C full cell is 96 mAh g-1, which is comparable to that of the cell with NNMM cathode, liquid electrolyte and Na/C anode of 95 mAh g-1 at the same current density. The capacity of the full cell NNMM|GHSE|Na/C slightly increases in the first few cycles, which may be attributed to the activation of the interface of the GHSE/electrode. The capacity of the NNMM|GHSE|Na/C cell does not show any decay at a current density of 192 mA g-1 even after 600 cycles. However, the NNMM|liquid electrolyte|Na cell only shows a capacity of 32 mAh g-1 after 600 cycles, which is 32.6% of the initial discharge capacity, as shown in Figure S15. The improved electrochemical performance of the NNMM|GHSE|Na/C cell may be attributed to the low interfacial resistance, close contact of the GHSE with NNMM cathode and the effective suppression of sodium dendrite by the Na/C composite anode. The first Coulombic efficiency of NNMM|GHSE|Na/C cell reaches about 95%. This irreversible capacity may be ascribed to the formation of SEI and decomposition of electrolyte.47 However, during the following cycles, the Coulombic efficiency gradually reaches ~100% (Figure 5 c), indicating a good reversibility of the NNMM|GHSE|Na/C cell. For the cell with the liquid electrolyte, the initial Coulombic efficiency is only 91.6% (Figure S15) and it remains at a lower Coulombic efficiency of ~98% during the following cycles due to the continuous decomposition of liquid electrolyte and the dendrite growth in the anode.

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Figure 5. (a) Typical charge/discharge curves of the full cell NNMM|GHSE|Na/C at various current densities. (b) Rate capability of the full cell. (c) Cycling performance of the full cell at a current density of 192 mA g-1.

4. Conclusions In summary, we have prepared a PMMA-filled composite electrolyte (GHSE) by in situ polymerization of MMA in the porous NZSPO-PVDF-HFP composite membrane for the first time. The GHSE membrane exhibits high ionic conductivity, wide electrochemical window (~4.9 V), high Na+ ion transference number (~0.63), good thermal stability and flexibility as well as smaller interfacial resistance. Furthermore, a Na/C composite anode was prepared. The full cell NNMM|GHSE|Na/C shows excellent rate capability with an initial discharge capacity of 96 mAh g-1 even at a higher current density of 192 mA g-1 and excellent cyclability for 600 cycles. These results suggest that the GHSE and Na/C anode are promising materials for Na-ion batteries.

ASSOCIATED CONTENT Supporting Information 17



The Supporting Information is available free of charge on the ACS Publications website. SEM images, FT-IR spectra, Photographs of the GHSE membrane under different deformation states, XRD patterns and SEM images of NZSPO and NNMM powders, Stress-strain curve of the glass-fiber membrane and PVDF-HFP-in situ polymerized-PMMA gel electrolyte, TGA-DSC curve of PVDF-HFP-in situ polymerized-PMMA gel electrolyte and the physical parameters of GHSE and PVDF-HFP-in situ polymerized-PMMA gel electrolytes, Photographs of GHSE membranes at different temperatures showing thermal shrinkage behavior. The Nyquist impedance spectrum of the composite electrolyte, The ionic conductivity of PVDF-HFP-in situ polymerized-PMMA gel and GHSE membrane at different temperatures, Linear sweep voltammograms of the liquid electrolyte, Current-time profile of a symmetrical Na/PVDF-HFP- in situ polymerized-PMMA/Na cell, The electrochemical parameters of different composite electrolytes, The impedance spectra of the liquid electrolyte and GHSE membrane after 100 cycles, Photographs of the cathode side of GHSE and glass-fiber after 100 cycles, CV curves of the NNMM|liquid electrolyte|Na, NNMM|GHSE|Na and NNMM|GHSE|Na/C cells, Voltage profile of the symmetrical NaGHSENa cell, and Cycling performance of the liquid electrolyte with Na anode and NNMM cathode.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] 18


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Acknowledgements The authors acknowledge the financial support of the National Science Foundation of China (Nos. 51672029 and 51372271) and the National Key R & D Project from Ministry of Science and Technology, China (2016YFA0202702). This work was also supported by and the Thousands Talents Program for the pioneer researcher and his innovation team in China.

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Table of Contents (TOC) The composite electrolyte of GHSE and Na/C composite anode are promising materials for application in sodium ion batteries. The GHSE shows high ionic conductivity, low interfacial resistance and interfacial stability. Moreover, the Na/C composite anode shows good suppression ability to dendrite. The assembled full cell NNMM|GHSE|Na/C exhibits superior rate capacity and cycling performance.

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Carbon Cloth Anode

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