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A Composite Gel Polymer Electrolyte Based on Poly(vinylidene fluoride-hexafluoropropylene) (PVDF-HFP) with Modified Aluminum-Doped Lithium Lanthanum Titanate (A-LLTO) for High-Performance Lithium Rechargeable Batteries Hang T. T. Le, Duc Tung Ngo, Ramchandra S Kalubarme, Guozhong Cao, Choong-Nyeon Park, and Chan-Jin Park ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b05301 • Publication Date (Web): 27 Jul 2016 Downloaded from http://pubs.acs.org on August 2, 2016

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ACS Applied Materials & Interfaces

A Composite Gel Polymer Electrolyte Based on Poly(vinylidene fluoride-hexafluoropropylene) (PVDF-HFP) with Modified Aluminum-Doped Lithium Lanthanum Titanate (A-LLTO) for High-Performance Lithium Rechargeable Batteries

Hang T. T. Le,† Duc Tung Ngo,† Ramchandra S. Kalubarme,† Guozhong Cao,‡ Choong-Nyeon Park,† and Chan-Jin Park †,* †

Department of Materials Science and Engineering, Chonnam National University, 77,

Yongbongro, Bukgu, Gwangju 500-757, South Korea ‡

Department of Materials Science and Engineering, University of Washington, Seattle, WA

98195-2120, United States * Corresponding author. Tel.: +82-62-530-1704; Fax: +82-62-530-1699. E-mail address: [email protected] (C.J. Park)

1 ACS Paragon Plus Environment


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KEYWORDS: solid electrolyte, gel polymer electrolyte, ionic conductivity, stability, Li dendrites

ABSTRACT: A composite gel polymer electrolyte (CGPE) based on poly(vinylidene fluoridehexafluoropropylene) (PVDF-HFP) polymer that includes Al-doped Li0.33La0.56TiO3 (A-LLTO) particles covered with a modified SiO2 (m-SiO2) layer was fabricated through a simple solutioncasting method followed by activation in a liquid electrolyte. The obtained CGPE possessed high ionic conductivity, a large electrochemical stability window, and interfacial stability- all superior to that of the pure gel polymer electrolyte (GPE). In addition, under a highly polarized condition, the CGPE effectively suppressed the growth of Li dendrites due to the improved hardness of the GPE by the addition of inorganic A- LLTO/m-SiO2 particles. Accordingly, the Li-ion polymer and Li-O2 cells employing the CGPE exhibited remarkably improved cyclability compared to cells without CGPE. In particular, the CGPE as a protection layer for the Li metal electrode in a Li-O2 cell was effective in blocking the contamination of the Li electrode by oxygen gas or impurities diffused from the cathode side, while suppressing the Li dendrites.

INTRODUCTION Lithium (Li) metal has been considered to be an ideal anode material for rechargeable batteries due to its high theoretical specific capacity (3860 mAh g-1), low density (0.59 g cm-3) and low electrode potential (-3.04 V vs. SHE).1-2 Furthermore, in improved lithium ion batteries (LIBs) with higher energy densities such as lithium metal-based batteries (LMBs), lithium-air (Li-air) batteries and lithium-sulfur (Li-S) batteries, Li metal has continued to be favored as an anode material.1, 3-6 Despite the incomparable advantages, the practical application of Li metal


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anodes in commercial rechargeable batteries has not progressed yet due to poor cycling stability and safety issues.1, 7 Its inherent problems such as uncontrollable Li dendritic growth and low coulombic efficiency during Li deposition/stripping should be addressed. To date, in order to protect Li metal electrodes from contamination in the open type batteries such as Li-air batteries or to enhance the electrochemical performance of most batteries employing Li metal, numerous methods have been suggested. One approach is to modify liquid electrolytes, which can lead to the formation of thin, uniform and protective SEI (solid electrolyte interphase) layers by optimizing the solvent, salts and additives.8-9 Through this approach, the growth of Li dendrites can be effectively suppressed. However, the safety issues related to the active Li metal in damaged batteries remain. In addition, for the Li-air battery where the cathode side is open to the environment, the Li metal can be degraded by water and/or contaminants which can diffuse in from the outside.10-12 A second approach is to employ solid electrolyte membranes in order to protect the Li metal from oxygen, water and other gaseous impurities,13-14 and suppress the growth of Li dendrite.15 Currently, two available major categories of solid electrolyte are solid polymer electrolytes (SPEs)16-18 and solid ceramic electrolytes (SCEs)13, 19-20 having various chemical compositions and structures. However, these organic and inorganic solid electrolytes pose their own drawbacks. For instance, the insufficient hardness of SPEs results in inefficient suppression for Li dendrite growth, although their fabrication and handling procedures are much easier than those of the SCEs. In addition, in their current state, these solid electrolytes still exhibit problems such as insufficient Li ion conductivity at room temperature, chemical degradation when in contact with Li metal21-22 and the formation of reaction products during the charge-discharge process.23 Consequently it is necessary to design a new solid electrolyte material that can exhibit the advantages of both SPE



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and SCE such as high ionic conductivity, hardness, a stable interface and good electrochemical performance. Even though most SPEs exhibited poor wettability and restricted ionic conductivities (< 10-5-10-6 S cm-1 at room temperatures),24-26 the enhanced gel polymer electrolytes (GPEs) based on various host polymer matrices have been developed remarkably.27-30 Among them, the GPEs based on a copolymer host of poly vinylidiene fluoride-hexafluoropropylene (PVDF-HFP) have attracted significant interest due to their high ionic conductivity, stable interfacial properties and electrochemical performance.14,

31-34

To further improve the ionic conductivity of GPEs,

inorganic fillers in the form of nanoparticles, such as SiO2, ZnO, Al2O3 and molecule sieves, have been introduced into the polymer matrix.31-33, 35-36 Nevertheless, high ionic conductivity could be obtained only when a very low amount of filler was added.36-37 In contrast, the better hardness of GPEs can normally be obtained with higher inorganic filler content.38 This indicates that it is important to find the optimum content of inorganic filler. Notwithstanding these imperatives, the role of the inorganic phase in suppressing the growth of Li dendrites has hardly been studied. In particular, it may be helpful if the growth or inhibition mechanism of Li dendrites during the operation of batteries can be visualized properly. Further, the reported Li ion conductivity of the prepared composite electrolytes was still low, remaining of the order of 10-4 S cm-1.14, 34 In this study, in anticipation of garnering the advantages of both organic and inorganic solid electrolytes, a free-standing composite gel polymer electrolyte (CGPE) membrane with high Li ion conductivity, good processability and efficient suppression of Li dendrite growth was proposed and implemented through the incorporation of perovskite structured aluminum-doped lithium lanthanum titanate (A-LLTO) solid electrolyte powder into a PVDF-HFP polymer


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matrix. Herein, A-LLTO solid electrolyte was used as a dispersed phase with its critical advantages such as high Li ion conductivity at room temperature (2.99×10-3 S cm-1)39 high lithium diffusion coefficient,40 high thermal stability and large electrochemical window.41 However, A-LLTO is chemically unstable in contact with Li metal.39 Thus, to address the problem, A-LLTO powders were designed to be encapsulated by an ultra-thin outer shell of modified SiO2 (m-SiO2), which is chemically stable when in contact with Li metal and thin enough not to hinder the Li ion transport.42-43 The surface of the outer m-SiO2 layer also can contribute toward enhancing adhesion between inorganic particles and the polymer matrix in the resultant CGPE.31-32, 44 In addition, the growth of Li dendrites was monitored within a transparent cell employing the CGPE. Finally, the feasibility of application of the CGPE as a solid electrolyte in a Li-ion polymer battery and a protective layer for the Li metal in a Li-air battery was investigated.

EXPERIMENTAL SECTION Preparation of core shell structured A-LLTO/m-SiO2 particles Micro-sized

A-LLTO

powder

with

the

nominal

composition

of

(Li0.33La0.56)1.005Ti0.99Al0.01O3 and 20 mol% excess Li2O was fabricated by a two-step process. An A-LLTO pellet was prepared exactly by a process reported elsewhere.13 The A-LLTO pellets obtained after sintering at 1350 oC were continuously ground using a mortar and pestle, and then further ball-milled using zirconia balls for 12 h to achieve micro-sized A-LLTO particles. Subsequently, modified SiO2 (m-SiO2) was coated on the A-LLTO particles by a wet chemical method.45 In the particular process, 2 g of micro-sized A-LLTO powder was predispersed in 100 ml ethanol by ultrasonication. Then, 5 g of polyvinylpyrrolidone (PVP, K30)



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was dissolved in deionized (DI) water and ultrasonically vibrated for 30 min to achieve complete dissolution. A mixture of A-LLTO and PVP in ethanol and DI water was obtained by mixing the above two suspensions under constant stirring for 8 h at room temperature. The resultant suspension was centrifuged to remove the supernatant and attain the surfactant-stabilized ALLTO particles. The surfactant-stabilized A-LLTO particles were then dispersed in 150 ml ethanol followed by the addition of 12 ml of DI water and 2 ml of NH3 (28 vol%), to form a suspension. In the next step, the temperature of the resultant suspension was maintained at 40 oC, and then the solution of 20 µl tetraethoxysilane (TEOS) in 5 ml ethanol was added dropwise under stirring for 5 min, to complete the hydrolysis reaction. Then 10 µl of 3-Glycidoxypropyl trimethoxysilane (GPTMS) was subsequently added to control the thickness of the resultant SiO2. After vigorous stirring for 5 h, the resultant precursors were separated by centrifuge and washed three times with DI water and ethanol. Finally, the obtained powder was dried in air at 120 oC for 24 h to obtain core-shell structured A-LLTO/m-SiO2 particles.

Preparation of composite gel polymer electrolyte (CGPE) A composite membrane was prepared by mixing PVDF-HFP, A-LLTO/m-SiO2 powders and dibutyl phthalate (DBP) in weight ratios of 2:8:3, respectively, in acetone with stirring at 50 oC to form a homogenous mixture, followed by casting on a glass plate with a doctor blade. After drying in a vacuum oven at 50 oC for 4 h, the wet films became dry and translucent. Further, the films were immersed in 500 ml methanol for 4 h to remove DBP and then dried in vacuum oven at 70 oC. Finally, the free-standing composite membranes based on PVDF-HFP with ALLTO/m-SiO2 were cut into circular shapes before being activated in the glove box. Then, the composite membrane was activated by soaking in the liquid electrolyte of 1 M LiPF6 in EC/EMC


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(50/50 in vol%) or the electrolyte of 1 M lithium bis(trifluoromethane sulfonyl)imide (LiTFSI) in tetraethylene glycol dimethyl ether (TEGDME), depending on the cell type, for longer than 2 h to obtain desired composite gel polymer electrolytes (CGPEs).

Preparation of electrodes and cell assembly In the present work, two kinds of cathodes were fabricated, a LiCoO2 cathode for a Li-ion polymer cell and a carbon free MnO2-based oxygen electrode for a Li-O2 cell. For the Li-ion polymer cell, a LiCoO2 electrode was prepared by coating a slurry containing commercial LiCoO2 (Sigma Aldrich 99.8%), Super P Carbon and polyvinylidene fluoride (PVFF) (8:1:1) in N-Methyl-2-pyrrolidone (NMP) solvent onto an Al foil and drying at 120 oC in a vacuum oven for 12 h. The mass loading was controlled at 8 mg cm-2. Li-ion polymer cells were assembled in the form of Li/CGPE/LiCoO2 by sandwiching the prepared composite gel polymer electrolyte (CGPE) between the Li metal anode and the LiCoO2 cathode. For comparison, a Li-ion cell containing a separator (glass microfiber, Whatman, GF/F, 150) soaked in the same liquid electrolyte (1 M LiPF6 in EC/EMC (50/50 in vol%)), which was also used for the activation process of CGPE, was also prepared. For the Li-O2 cell, a carbon- and binder-free MnO2-based oxygen electrode was fabricated using a hydrothermal method that has been specified elsewhere.46 The MnO2 catalysts were grown on a clean Ni foam sheet at 150 oC for 5 h in an autoclave containing 500 ml solution of 20 mM KMnO4, 10 mM MnSO4 and few drops of NH3 (28 vol%). After the reaction, the Ni foam, covered with MnO2 catalysts, was taken out and rinsed with DI water to remove impurities. Finally, the Ni foam covered with MnO2 was annealed at 450 oC for 5 h, followed by punching the oxygen electrode of Ni foam covered with MnO2 (MnO2@Ni) into several discs having a



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diameter of 16 mm. From the negative electrode to the positive electrode side, the Li-O2 cell consisted of a Li metal electrode, a composite gel polymer electrolyte (CGPE), a glass fiber separator soaked in 1 M LiTFSI in TEGDME, and a MnO2@Ni oxygen electrode- to form the configuration of Li/CGPE/separator/MnO2@Ni. The CGPE was also activated in the solution of 1 M LiTFSI in TEGDME before assembling the cell. Substantially unlike the Li-ion polymer cell with the configuration of Li/CGPE/LiCoO2, the Li-O2 cell purposely employed a separator to separate CGPE membrane from the MnO2@Ni oxygen electrode, due to the chemical instability of PVDF-HFP polymer in contact with Li2O2 which can be formed on the surface of MnO2 catalysts during the discharge process.23 Both Li-ion polymer and Li-O2 cells were assembled into CR-2032 type coin cells as shown in Figure S1 in the Supporting Information. The positive pans of the Li-O2 coin cells had several holes drilled in them to allow oxygen gas to penetrate. These holes were covered with an oxygen-permeable polytetrafluoroethylene (PTFE) film to prevent the evaporation of electrolyte from the cells. The Li-O2 cells were put into a small box filled fully with pure oxygen gas at a pressure of 1 atm.

Microstructural and electrochemical characterization The crystal structure of materials was characterized by X-ray diffractometer (XRD, Rigaku DIII ultima with Cu Kα radiation). The microstructure of materials was also observed by fieldemission scanning electron microscopy (FE-SEM, S-4700 Hitachi) and transmission electron microscopy (TEM, Tecnai G2, Philips). In addition, the chemical structure of synthesized powder was analyzed using Fourier transform infrared spectroscopy (FTIR, Prestige-21 Shimadzu). The Shore D hardness values of the composite membranes including pure PVDF-


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HFP were analyzed by measuring the penetration depth of a Durometer indenter with a Shore hardness meter (GS-702N Hardmatic Mitutoyo). The electrical conductivity of composite gel polymer electrolytes (CGPEs) was analyzed using sandwich cells of SS (stainless steel)/CGPE/SS. In addition, SS/Li/CGPE/Li/SS sandwich coin cells were assembled to investigate the interfacial resistance between Li and CGPE and the electrochemical phenomena during the deposition and striping of Li. In order to monitor the growth process of Li dendrites using an optical microscope (Leica ICC50 HD), cells composed of Li/gel electrolyte (GPE)/Li, with two Li foil electrodes and GPE were placed and sealed between two transparent lamellas, were assembled in an Ar-filled dry glove box before the polarization test. An electrochemical impedance spectroscopy (EIS) measurement was performed using a ZIVE SP2 instrument in the frequency range of 0.1 Hz - 1 MHz with a voltage amplitude of 10 mV. The charge-discharge behaviors of the Li-ion polymer and Li-O2 cells were investigated using a Wonatech WMPG-1000 battery cycler.

RESULTS AND DISCUSSION Figure 1a-d show the SEM and TEM images of core-shell structured A-LLTO/m-SiO2 particles which were prepared by chemical methods, at various magnifications. The core particles of A-LLTO having an average size ranging from 1 to 2 µm (Figure 1a, b) were covered with a thin, modified SiO2 (m-SiO2) shell as shown in Figure 1b, c. Furthermore, the HR-TEM image (Figure 1d) clearly shows the interface between crystalline A-LLTO and the amorphous m-SiO2 layer. The thickness of the m-SiO2 layer was estimated to be ~5 nm. In addition, the inter-planar spacing measured from the HR-TEM image was ~0.35 nm corresponding to the



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(101) planes of the tetragonal A-LLTO lattice. The crystal structure of the A-LLTO/m-SiO2 powder was investigated using XRD. For comparison, single m-SiO2 powder was also prepared. In Figure 1e, all discrete peaks observed in the XRD pattern for A-LLTO/m-SiO2 are well matched to those of the tetragonal phase of Li0.33La0.56TiO3 (JCPDS card No. 01- 087- 0935), confirming the perovskite structure of the A-LLTO core. Interestingly, no additional peaks corresponding to SiO2 or other silicon compounds were observed. Similarly, the XRD pattern for m-SiO2 exhibited only a broadened peak with relatively weak intensity, demonstrating that the synthesized m-SiO2 had an amorphous phase. To elucidate the nature of the surface of the m-SiO2 shell layer, A-LLTO/m-SiO2 and ALLTO/SiO2 powders (with the SiO2 shell unmodified) were analyzed using FTIR. As shown in Figure 1f, the FTIR spectrum for the A-LLTO/SiO2 sample clearly illustrated the appearance of absorption peaks at 473, 798, 1099 and 949 cm-1 which respectively corresponded to the bending vibration, symmetric stretching vibration, asymmetric stretching vibration of Si-O-Si and bending vibration of Si-OH.47 The absorption peaks observed at 1651 and 3440 cm-1 are related to bending and stretching vibrations of -OH, which was generated in the hydrolysis process of TEOS. The double shoulder peak appearing at 2363 cm-1 we ascribe to the stretching vibration of carbon dioxide.33 Note that after the modification process for SiO2 layer, the absorption intensities of -OH group observed at 1651 and 3440 cm-1 for the A-LLTO/m-SiO2 sample were smaller than those of the A-LLTO/SiO2 (unmodified). This implies that surface -OH groups of SiO2 were substituted by GPTMS. Concurrently, the appearance of the peak at 2927 cm-1 in the spectrum for the A-LLTO/m-SiO2 sample, corresponding to the stretching vibration of -CH group, also confirmed the presence of chemically adsorbed GPTMS on the surface of the precipitated SiO2.48 In contrast, physically adsorbed GPTMS and reaction byproducts on the


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precipitated SiO2 were completely cleaned after centrifugation and rinsing several times with DI water and ethanol. Due to the presence of the GPTMS modifier on the surface of m-SiO2, the thickness of the SiO2 shell covering the A-LLTO particles was controlled to within a few nanometers. The agglomeration of SiO2 particles was also hindered during the precipitation process. Moreover, the partly formed –OH group on the SiO2 surface in the A-LLTO/m-SiO2 can facilitate capturing water impurities, and the existing epoxy group can contribute toward improving the ionic conductivity and the stability of the phase boundary.31 Accordingly, the interfacial interaction between inorganic particles and polymer matrix can be reinforced.49-51 Thus, the employment of A-LLTO/m-SiO2 particles fabricated through a surface modification process in the CGPE is expected to improve both the ionic conductivity of the CGPE and block the water contaminant which can possibly diffuse from the cathode side in Li-air batteries.11 Actually, the function of inorganic particles of A-LLTO/m-SiO2 is to not only properly maintain high ionic conductivity but also enhance the mechanical hardness of the CGPE. However, as the content of A-LLTO/m-SiO2 increased, the ionic conductivity of the CGPE decreased due to the decrease in volume fraction of Li-ion conducting polymer phase (Figure S2 and Table S1 in the Supporting Information). The optimum content of A-LLTO/m-SiO2 was determined based on the cycling stability52 in preliminary cycle test for Li-LiCoO2 cells employing the CGPE with various content of A-LLTO/m-SiO2 (Figure S3 in the Supporting Information). With 80 wt.% of A-LLTO/m-SiO2 in the CGPE, the Li-LiCoO2 cell showed the highest capacity retention during 100 cycles of charge and discharge. Therefore, the “2:8” mass ratio of PVDF-HFP to A-LLTO/m-SiO2 was selected to further tests. Henceforth, the “CGPE” is referred as a composite gel polymer electrolyte based on PVDF-HFP and A-LLTO/m-SiO2 with the mass ratio of 2:8.



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Figure 2 presents SEM images of pure PVDF-HFP and composite (PVDF-HFP : ALLTO/m-SiO2 = 2:8) membranes before and after activation process. To observe the microstructure of the obtained composite membranes of PVDF-HFP and A-LLTO/m-SiO2, the membranes were frozen in liquid nitrogen and then coated with platinum by sputtering before the observation. The pure polymer membrane based on the PVDF-HFP before the addition of ALLTO/m-SiO2 powder exhibited a highly porous structure with numerous well-distributed macro and mesoscale pores (Figure 2a). These macro and mesoscale pores in the membrane are essential for the efficient uptake of the liquid electrolyte during the activation process and the gelation of the macro-porous membrane.33 After activation in the solution of 1 M LiPF6 in EC/EMC (50/50 in vol%), the membrane was totally swelled. The pores disappeared, bringing about a smoother surface (Figure 2c), and accordingly the resultant gel polymer electrolyte (GPE) became almost transparent (Figure S4a in the Supporting Information). The ionic conductivity of the pure GPE based on the PVDF-HFP was measured to be 1.56 × 10-3 S cm-1 (Table S1 in the Supporting Information). This value is comparable to or higher than that reported in previous studies.31-32, 53 In contrast, with the addition of A-LLTO/m-SiO2 particles into the PVDF-HFP, the surface of the composite membrane became rougher (Figure 2b) and showed a uniform distribution of the particles in PVDF-HFP polymer matrix (inset in Figure 2b). The macro-pores did not appear, illustrating the strong adhesive framework between inorganic A-LLTO/m-SiO2 particles and the polymer matrix in the composite membrane. Even after activation in the liquid electrolyte, the CGPE remained initial opaque white due to the scattering of light by the particles (Figure S4b in the Supporting Information). Figure 3 shows the XRD pattern of the composite membrane. For comparison, the XRD patterns of the pure polymer (PVDF-HFP) membrane and A-LLTO/m-SiO2 powder are


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displayed as well. Obviously, the peak intensity for the composite membrane decreased remarkably compared to that of the A-LLTO/m-SiO2 powder, due to the presence of the PVDFHFP polymer matrix. It is can be seen that the characteristic peak observed at 20.4 o for the pure PVDF-HFP membrane completely disappeared in the XRD pattern for the composite membrane. This suggests that the added microscopic A-LLTO/m-SiO2 particles might lower the crystallinity of the host polymer. Accordingly, the composite membrane possessed more amorphous domains, which can be beneficial for the absorption and uptake of liquid electrolyte and Li-ion transport. As a result, the holding capacity for liquid electrolyte and the ionic conductivity of CGPE after activation of the composite membrane in liquid electrolyte can be improved.32-33 Interestingly, the ionic conductivity of the CGPE based on PVDF-HFP and A-LLTO/m-SiO2 was measured to be 1.22 × 10-3 S cm-1 (Table S1), which was slightly lower than that of the pure GPE based on pure PVDF-HFP (1.56 × 10-3 S cm-1). As reported in our previous study,13 the intrinsic ionic conductivity of the interior A-LLTO in A-LLTO/m-SiO2 is fairly high, 2.99 × 10-3 S cm-1. However, the presence of the outer m-SiO2 shell can reduce the conductivity of the A-LLTO/mSiO2 to 1.05 × 10-4 S cm-1 (Figure S5 in the Supporting Information). Accordingly, the increase in ionic conductivity owing to the increased amorphous domain in CGPE may be lessened by the inclusion of electrically-resistant SiO2. Nevertheless, the ionic conductivity of the CGPE was comparable to that of the pure polymer electrolyte and still larger than that reported for other CGPEs.14, 54-55 Further, to investigate the compatibility of the CGPE with a Li metal electrode, a symmetric cell composed of Li/CGPE/Li was galvanostatically cycled at the current density of 0.48 mA cm-2 for each charge-discharge duration of 2 h, which corresponds to the chargedischarge test condition for the Li-ion polymer cell employing the CGPE at the rate of 0.5C. In



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addition, for comparison, symmetric cells containing a glass fiber separator with the liquid electrolyte and pure gel polymer electrolyte (GPE) were also fabricated. Herein, the solution of 1 M LiFP6 in EC/EMC (50/50 in vol%) was employed as a reference liquid electrolyte and an electrolyte for the activation of CGPE. As shown in Figure 4, the cell employing the CGPE showed a relatively large overpotential during the initial few cycles. However, the overpotential for the CGPE cell gradually decreased and stabilized with the cycling. This phenomenon is presumably attributed to the surface activation of the electrodes after several charge-discharge cycles. After 1094 h, the cell employing the reference liquid electrolyte failed. In addition, the cell employing the pure GPE failed after the slightly longer duration of 1194 h, suggesting that the pure GPE is not sufficiently effective at suppressing the growth of Li dendrite and the undesirable reaction between Li metal and the electrolyte during the cycling process. In contrast, the cell employing the CGPE operated for the much longer duration of 1922 h, which was nearly double the cycle life of the other cells employing the liquid electrolyte or the pure GPE. This result indicates that the CGPE is very stable in the presence of the Li metal electrode, which is also in good agreement with the impedance data for the cells with the rest time showing the superior stability of the CGPE (Figure S6 in the Supporting Information). In order to visualize further the suppressive effect of CGPE on the growth of Li dendrites, Li/GPE/Li symmetric cells employing the pure GPE and CGPE were kept polarized at the current density of 0.5 mA cm-2. The growth process of Li dendrites was monitored by using an optical microscope. For the cell employing the pure GPE, Li dendrites grew vigorously with polarization time as shown in Figure 5a. Immediately after cell assembly, an inhomogeneous SEI layer formed on the surface of the reactive Li electrode. During the charge process, the Li plating concentrated preferentially at places where the SEI layer was thin and/or at defects, to


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allow a high Li ion flux, leading to triggering the growth of Li dendrites. With the growth of Li dendrites at these spots (as indicated using white arrows in Figure 5a), the Li surface became enlarged. After 14 h, the Li dendrite could grow up to 0.2 mm in the length. In contrast, for the cell employing the CGPE, the polarization time needed to have a similar Li dendrite length was 40 h (Figure 5b). In addition, for the cell employing the pure GPE, after 80 h the Li dendrites spread out of the area selected for observation. On the contrary, it took 210 h for the cell employing CGPE to spread the Li dendrites out of the observation area. This implies that CGPE is superior to pure GPE in suppressing Li dendrites. With the presence of micro-sized ALLTO/m-SiO2 particles in the CGPE leading to the increased hardness of the gel polymer, the growth of Li dendrite could be effectively hindered similarly as previously reported.14, 34 Further, the applicability of the CGPE in the rechargeable Li metal-based batteries such as a Li-ion polymer cell composed of Li-LiCoO2 and a Li-O2 cell was investigated. These cells could operate at up to 4.5 V when using the CGPE, which has an electrochemically stable window up to 5 V vs. Li/Li+ as shown in Figure S7 in the Supporting Information. Sometimes, for the Liion polymer cells employing Li metal anode, the cells have been charged at a much smaller Crate than the C-rate used for discharge, to minimize the effect of the Li dendritic growth on cyclability of the cell. For instance, the charge rate was five times lower than discharge rate.31, 5657

However, in this work, to clearly elucidate the role of the CGPE on the improvement of

cyclability of cell by the suppression of Li dendrite, the Li-ion polymer cell in the form of LiLiCoO2 was galvanostatically cycled at the same charge-discharge rate of 0.5C for a long-term duration of 500 cycles in the cell potential range of 3.5 - 4.2 V. For comparison, two reference cells employing a glass fiber separator containing the liquid electrolyte of 1 M LiPF6 in EC/EMC and the pure GPE were also tested. The Li-LiCoO2 cell employing the CGPE exhibited superior



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electrochemical performance to the cell employing either the pure GPE or the glass fiber separator containing the relevant liquid electrolyte as shown in Figure 6a. After 500 cycles of charge-discharge, the cell employing the CGPE maintained capacity corresponding to 80.5% of the initial discharge capacity and coulombic efficiency higher than 99.2% (Figure S8 in the Supporting Information). In contrast, the capacity of both reference cells employing the pure GPE and separator containing the liquid electrolyte faded significantly. After 500 cycles, the capacity retention of the reference cell of glass fiber separator was found to be about 32.6% of the original discharge capacity and the fluctuating coulombic efficiency was around 96%. Similarly, the reference cell of the pure GPE only remained 47.4% of the initial capacity with coulombic efficiency of 98.1% after 500 cycles. However, in the first cycle, the specific discharge capacity of the cell containing CGPE (153 mAh g-1) was lower than that of the cell using the pure GPE (157 mAh g-1) and that of the cell employing the glass fiber separator containing the liquid electrolyte (166 mAh g-1). This can be explained using electrochemical impedance data as shown in Figure 6b. Before the cyclic charge-discharge tests, the cell employing the CGPE had a larger impedance than that of both the cell employing the pure GPE and the cell employing glass fiber separator. Interestingly, after the next 12 cycles, the capacity of the cell with CGPE increased and achieved a maximum value of 165 mAh g-1. Accordingly, the first 12 cycles can be considered as the duration for activation of the cell, which is quite consistent with the cycling results obtained for the symmetric Li/Li cells, as aforementioned (Figure 4). It is noticeable that within the first 100 cycles the difference in the cyclability among the cells with and without the CGPE was not large. However, after the 100th cycle, the cells without the CGPE exhibited prominent capacity fading. This resulted from the significantly increased cell impedance. As seen in Figure 6c, contrary to the impedance data for


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the cells obtained before the cycling test (Figure 6b), the impedance for the cells without CGPE became much larger than that of the cell employing the CGPE after cycling for 500 cycles. In order to clarify effect of the CGPE on the changes in the EIS spectra of the Li-LiCoO2 cells in detail, equivalent circuits, viz., model 1 (inset in Figure 6b) and model 2 (inset in Figure 6c) were used to analyze the impedance spectra for the cells obtained before and after cycling test, respectively. As for the as-assembled cells, total impedance could be regarded as a simple combination of electrolyte resistance Re in series with a parallel set of double-layer capacitance Cdl with the charge transfer resistance Rct and Warburg impedance Zw, accompanied with the intercalation capacitance CL.58, 59 After cycling test, due to the formation of passivating film on the surface of the electrodes, the shape of the impedance spectra changed totally. Two semicircles were observed instead of one semicircle as observed for the as-assembled cells. One semicircle observed in the high-to-medium frequency region originates from the resistance of the electrode surface films (Rf) covering the LiCoO2 cathode and Li anode, and the other appearing in the low frequency region represents charge transfer resistance (Rct). Therefore, impedance spectra for the cells after cycling were referred as a combination of (Rf //CPEf), (RctW//CPEct) and CL together with electrolyte resistance Re.60 The Warburg component in both models reflects the diffusion of Li-ion in the electrolyte, in the SEI and from intercalation between the LiCoO2 particles.61-63 The fitting results are enumerated in Table 1. As seen in Table 1, the electrolyte resistance (Re = 5.41 Ω) and charge transfer resistance (Rct = 39.9 Ω) of the cell with the CGPE were slightly higher than those of the cells employing the pure GPE (Re = 5.14 Ω, Rct = 33.97 Ω) and the glass fiber (Re = 4.95 Ω, Rct = 28.76 Ω). Thus, the cell with the CGPE revealed the smaller capacity than that of the others in the first cycle as aforementioned. After cycling, the appearance of Rf component along with the increase in the charge transfer resistance contributed



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toward increasing the total impedance of the cells. As a result, the capacity of the tested cells faded with the cycling time (Figure 6a). It is noticeable that after 500 cycles, the change in electrolyte resistance of the cells was negligible. This implies the stability of the electrolytes during cycling. In addition, the interfacial resistance (Ri) estimated from the summation of Rf and Rct of the cell with the CGPE was 223.98 Ω, which was much smaller compared with that of the other cells without CGPE. Meanwhile, the difference in the interfacial resistance between the rest cells was found to be quite small; Ri = 299.26 Ω for the cell employing glass fiber (liquid electrolyte) and Ri = 279.02 Ω for the cell employing pure GPE. This demonstrates that the addition of inorganic filler of A-LLTO/m-SiO2 into the CGPE decreases the electrode interfacial resistance.64-65 Accordingly, the cyclability of the cell with the CGPE was improved significantly. Further, the Li-O2 cells where the CGPE was placed as a protection layer for Li metal between the Li metal and the glass fiber separator were charge-discharge tested at the current density of 0.1 mA cm-2 under the limited capacity mode of 1000 mAh g-1. The CGPE was activated in a solution of 1 M LiTFSI in TEGDME before assembling the cell. The Li-O2 cell without the CGPE was also tested for comparison. The capacity of the Li-O2 cell was calculated based on the weight of the MnO2 catalyst formed on the Ni-foam. Here, carbon-free flake-like αMnO2 grown on Ni-foam as shown in Figure S9 in the Supporting Information was used as the oxygen electrode. As depicted in Figure 7a, the Li-O2 cell employing the CGPE as a protection layer for the Li metal electrode could operate up to 71 cycles when considering the cut-off potentials of 2 V and 4.5 V for discharge and charge, respectively, which have been considered to be potential limits.13,

66

During charge-discharge cycling, the terminal cell potential for

discharge for the cell without CGPE decreased gradually with cycling. In contrast, the Li-O2 cell


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without the CGPE posed a cycle life of only 47 cycles within the potential range of 2.0 - 4.5 V, as shown in Figure 7b. In particular, after 45 cycles the terminal discharge potential decreased dramatically, and the terminal discharge potential quickly dropped below 2.0 V before accomplishing the capacity of 1000 mAh g-1 in the 48th cycle (inset of Figure 7b). On the whole, the Li-O2 cell employing the CGPE protection layer dominated over the cell containing only liquid electrolyte without the CGPE. The remarkable improvement in cyclability achieved for the Li-O2 cell employing the CGPE is attributed to the combined effect of the hardness of the inorganic A-LLTO/m-SiO2 particles and the high viscosity of the PVDF-HFP polymer gel. In particular, due to the enhanced hardness of GPE by the addition of the A-LLTO/m-SiO2 particles, the dendritic growth of Li metal could be effectively suppressed. In addition, the PVDF-HFP polymer matrix also prevented the diffusion of trace water impurities and other by-products formed by eventual degradation of the oxygen electrode during cycling, from the cathode side to the anode.10-11 In particular, owing to the surface modification process, the SiO2 surface of ALLTO/m-SiO2 particles was functionalized. Accordingly, the ability of the remaining silanol groups to trap trace impurities such as water and the adhesion between the A-LLTO/m-SiO2 particles and the polymer matrix were enhanced.31, 67 As a result, the deterioration of the Li metal electrode could be suppressed, and thereby the long-term stability of the Li-O2 cell was improved remarkably. Even though the rigidity of the CGPE is incomparable to that of relevant pure A-LLTO inorganic solid electrolyte, the CGPE possesses enough hardness to suppress the growth of Li dendrites. To demonstrate this, the Li-O2 cells with and without the CGPE were disassembled in the glove box after cycling under the limited capacity mode, and the surfaces of the Li metal electrodes were analyzed by SEM. As shown in Figure 7c-d, the Li metal electrode detached



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from the cell employing the CGPE showed a relatively smooth and flat surface. In contrast, the Li electrode from the cell without the CGPE had a porous and rough surface with numerous Li dendrite bushes despite the shorter duration of charge and discharge (Figure 7e-f). Nevertheless, at a higher magnification, the formation of a few dendrites, which are indicated using the redcolored arrows in Figure 7c, was also observed on the surface of the Li electrode even for the Li-O2 cell employing the CGPE protection layer. Certainly, it is difficult to fully avoid the growth of Li dendrites due to the relative softness of the PVDF-HFP polymer matrix contained in the CGPE. But, due to presence of the polymer matrix the CGPE can exhibit flexibility and easy handling, which cannot be easily obtained for the ceramic A-LLTO solid electrolyte. Moreover, the CGPE has enough hardness to effectively suppress the growth Li dendrite and the desirable high ionic conductivity to enable good cell operation.

CONCLUSIONS Composite gel polymer electrolyte (CGPE) based on poly(vinylidene fluoridehexafluoropropylene) (PVDF-HFP) polymer that included Al-doped Li0.33La0.56TiO3 (A-LLTO) particles covered with a modified SiO2 (m-SiO2) layer was prepared using a simple casting method followed by activation in a liquid electrolyte. The resultant CGPE exhibited a high ionic conductivity of 1.22 × 10-3 S cm-1 and an electrochemical stability window up to 5 V vs. Li/Li+. In particular, the use of CGPE in Li metal-based batteries was feasibly demonstrated via two accesses: (i) as a gel polymer electrolyte for Li-ion polymer battery, which has been preferred due to its safety; and (ii), as a protection layer for the Li metal anode of a Li-air battery, which can deliver extremely high specific energy. The charge-discharge cycling data for the Li/Li symmetric cells and corresponding observations of the Li surface evidenced the effective


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suppression of the Li dendritic growth by employing the CGPE in the cell. In presence of the CGPE, the Li-LiCoO2 polymer cell offered 80.5% capacity retention after 500 charge-discharge cycles at 0.5C, which was nearly three times higher than the outcome for the cell without the CGPE. Furthermore, the Li-O2 cell employing the CGPE as a protection layer for Li metal exhibited a longer cycle life of 72 cycles under the limited capacity mode of 1000 mAh g-1, which was larger by 25 cycles than with the cell without the CGPE. We attribute the outstanding performance of the Li metal-based cells employing the CGPE to the effective suppression of the Li dendrites, the prevention of electrolyte decomposition and the diffusion of O2 gas and contaminants from the outside environment that would otherwise deteriorate the Li metal electrode.

ASSOCIATED CONTENT Supporting Information Schematic for the configuration of Li-LiCoO2 and Li-O2 cells; data and discussion of how the optimized CGPE was obtained; digital photo of pure GPE and CGPE; results for measuring the ionic conductivity of A-LLTO/m-SiO2; impedance spectra of the symmetric cell using the pure GPE and CGPE; linear voltammogram for the cell of stainless steel/CGPE/Li; columbic efficiency and cell potential profiles for the Li-LiCoO2 cells employing the GPE, the CGPE and the glass fiber separator soaked in the liquid electrolyte under charge-discharge rate of 0.5C for 500 cycles; morphology and structure characterizations of carbon-free MnO2@Ni oxygen electrode for Li-O2 cell such as SEM image, XRD pattern, EDS spectra. This material is available free of charge via the Internet at http://pubs.acs.org.



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AUTHOR INFORMATION Corresponding Author *Tel.: +82-62-530-1704. Fax: +82-62-530-1699. E-mail address: [email protected] (C.J. Park) ACKNOWLEDGMENT This research was supported by the National Research Foundation (NRF) of Korea grant (No. 2015R1D1A3A01019399 and No. 2016R1A2B4015883).

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TABLE

Table 1. Various impedance parameters evaluated using the equivalent circuit model for Li-LiCoO2 cells with the glass fiber separator soaked in a solution of 1 M LiPF6 in EC/EMC, the pure GPE, and the CGPE

Sample

Asassembled

After 500 cycles

Re

Rf

CPEf

Rct

CPEdl

W

CL

-1 1/2

(Ω)

(Ω)

Cf (F)

n

(Ω)

Cdl (F)

n

(Ω s )

(F)

Glass fibre

4.95

-

-

-

28.76

2.27×10-4

0.728

8.57×10-2

1.89×10-2

Pure GPE

5.14

-

-

-

33.97

2.64×10-5

0.829

4.26×10-2

1.28×10-2

CGPE

5.41

-

-

-

39.90

1.84×10-5

0.812

9.52×10-3

1.89×10-3

Glass fibre

5.15

210.88

1.99×10-4

0.905

88.38

5.81×10-6

0.711

2.60×10-2

9.41×10-2

Pure GPE

5.16

199.36

1.90×10-3

0.933

79.66

4.68×10-5

0.686

7.31×10-2

2.577

CGPE

5.50

152.50

2.83×10-3

0.938

71.48

7.72×10-5

0.640

1.99×10-1

3.500



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FIGURES

Figure 1. (a) SEM, (b) TEM images of A-LLTO/m-SiO2 core-shell structured particles and (c-d) the magnified TEM images corresponding to the marked regions; (e) XRD patterns and (f) FTIR spectra for A-LLTO/m-SiO2 and A-LLTO/SiO2 particles.


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Figure 2. SEM images of (a) pure PVDF-HFP membrane, (b) composite (PVDF-HFP + ALLTO/m-SiO2) membrane, (c) gel polymer electrolyte (GPE) based on the pure PVDF-HFP after activation in the liquid electrolyte, and (d) composite gel polymer electrolyte (CGPE) based on the PVDF-HFP + A-LLTO/m-SiO2 after activation. Insets shown in (a-d) are the corresponding magnified images.



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Figure 3. XRD patterns of A-LLTO/m-SiO2 powder, pure PVDF-HFP and composite (PVDFHFP + A-LLTO/m-SiO2) membranes.


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Figure 4. Galvanostatic cycling at 0.48 mA cm-2 for Li/ electrolyte/Li cells containing different electrolytes: the reference liquid electrolyte of 1 M LiPF6 in EC/EMC (50/50 in vol%) impregnated into glass fiber separator; and the CGPE and pure GPE activated in the same reference liquid electrolyte.



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Figure 5. The growth of Li dendrite with time in the Li/GPE/Li symmetric cells employing (a) the pure GPE and (b) the CGPE, under the current density of 0.5 mA cm-2. The GPEs were prepared after activation in a solution of 1 M LiTFSI in TEGDME.


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Figure 6. Electrochemical performance of the Li-ion cells in the form of Li-LiCoO2 employing CGPE, GPE, and a glass fiber separator, soaked in a solution of 1 M LiPF6 in EC/EMC: (a) cyclability under the charge-discharge rate of 0.5C for 500 cycles; impedance spectra and relevant equivalent circuits for the cells (b) before and (c) after 500 cycles of charge-discharge.



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Figure 7. Cyclability and the corresponding cell potential profiles (inset) for the Li-O2 cells (a) with and (b) without the CGPE, which was activated in a solution of 1 M LiTFSI in TEGDME. SEM images of the Li metal electrodes detached from the Li-O2 cells after charge-discharge cycling tests under limited capacity of 1000 mAh g-1: (c, d) with and (e, f) without the CGPE.


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GRAPHICAL ABSTRACT


PVDF-HFP - ACS Publications - American Chemical Society

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