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Immobilization of Anions on Polymer Matrices for Gel Electrolytes with High Conductivity and Stability in Lithium Ion Batteries Shih-Hong Wang, Yong-Yi Lin, Chiao-Yi Teng, Yen-Ming Chen, Ping-Lin Kuo, Yuh-Lang Lee, Chien-Te Hsieh, and Hsisheng Teng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b01753 • Publication Date (Web): 25 May 2016 Downloaded from http://pubs.acs.org on June 1, 2016

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ACS Applied Materials & Interfaces

Immobilization of Anions on Polymer Matrices for Gel Electrolytes with High Conductivity and Stability in Lithium Ion Batteries

Shih-Hong Wang,† Yong-Yi Lin,† Chiao-Yi Teng,† Yen-Ming Chen,† Ping-Lin Kuo,† Yuh-Lang Lee,† Chien-Te Hsieh,*,‡ and Hsisheng Teng*,†,§

†

Department of Chemical Engineering and Research Center for Energy Technology and Strategy, National Cheng Kung University, Tainan 70101, Taiwan ‡

Department of Chemical Engineering and Materials Science, Yuan Ze Fuel Cell Center, Yuan Ze University, Taoyuan 32023, Taiwan

§

Center for Micro/Nano Science and Technology, National Cheng Kung University, Tainan 70101, Taiwan

*To whom correspondence should be addressed. Hsisheng Teng: (E-mail): [email protected]; (Fax): 886-6-2344496; (Tel): 886-6-2385371 Chien-Te Hsieh: (E-mail): [email protected]

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Abstract

This study reports on a high ionic-conductivity gel polymer electrolyte (GPE), which is supported by a TiO2 nanoparticle-decorated polymer framework comprising poly(acrylonitrile-co-vinyl acetate) blended with poly(methyl methacrylate), i.e., PAVM:TiO2. High conductivity GPE-PAVM:TiO2 is achieved by causing the PAVM:TiO2 polymer framework to swell in 1 M LiPF6 in carbonate solvent. Raman analysis results demonstrate that the poly(acrylonitrile) (PAN) segments and TiO2 nanoparticles strongly adsorb PF6− anions, thereby generating 3D percolative space-charge pathways surrounding the polymer framework for Li+-ion transport. The ionic conductivity of GPE-PAVM:TiO2 is nearly one order of magnitude higher than that of commercial separator-supported liquid electrolyte (SLE). GPE-PAVM:TiO2 has a high Li+ transference number (0.7), indicating that most of the PF6− anions are stationary, which suppresses PF6− decomposition and substantially enlarges the voltage that can be applied to GPE-PAVM:TiO2 (to 6.5 V vs. Li/Li+). Immobilization of PF6− anions also leads to the formation of stable solid-electrolyte interface (SEI) layers in a full-cell graphite|electrolyte|LiFePO4 battery, which exhibits low SEI and overall resistances. The graphite|electrolyte|LiFePO4 battery delivers high capacity of 84 mAh g-1 even at 20 C and presents 90 % and 71 % capacity retention after 100 and 1000 charge-discharge cycles, respectively. This study demonstrates a GPE architecture comprising 3D space charge pathways for Li+ ions and suppresses anion decomposition to improve the stability and lifespan of the resulting LIBs.

Keywords: Gel polymer electrolyte; Lithium ion battery; Poly(acrylonitrile); Space charge regime; Lithium transference number 2


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1. Introduction

The high-energy characteristics of lithium ion batteries (LIBs) make them useful in hybrid electric and plug-in electric vehicles, which have been proposed as the next modes of transportation.1 LIBs also provide an energy source for nearly every type of electronic device.2 However, safety concerns associated with the leakage or evaporation of solvents in the liquid electrolytes limits the applicability of LIBs. Solid-state electrolytes (SSEs) could greatly enhance safety; however, they suffer from low conductivity at room temperature.3 Many studies have focused on the performance of batteries based on gel polymer electrolytes (GPEs),4 which largely overcome the drawbacks of SSEs. Most of the hosting matrices used in GPEs are organic polymers, which entrap solvent molecules to avoid leakage and evaporation, while maintaining the mobility of electrolyte ions.5 Introducing inorganic oxide nanoparticles to the polymer matrices provides soft GPEs with favorable mechanical properties.6 Additionally, selecting host polymers with appropriate functionalities can disperse oxide nanoparticles that facilitate Li+-ion transport in GPEs by forming percolative space charge profiles (cation-rich regime caused by anion adsorption on the oxide surface) surrounding the nanoparticles.7 The synergistic effect of a polymeric framework with oxide nanoparticles could play a critical role in promoting the applicability of GPEs in LIBs. The GPEs used in LIBs contain various functional polymer segments (see Table 1), which include poly(ethylene oxide), poly(vinylidene fluoride), poly(hexaflouropropylene), poly(acrylonitrile) (PAN), poly(methyl methacrylate) (PMMA), poly(vinyl acetate) (PVAc), poly(vinyl alcohol), and polyimide.8-22 The capacity retention and cycle life of GPE-based LIBs are major concerns under 3



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high-rate operations. Among these functionalities, PAN exhibits excellent ion-solvating ability as well as high chemical and thermal stability. The ion-solvating ability could be used to immobilize anions by breaking up the ion pair and generating space charge regimes to promote the motion of Li+ ion along the polymer chains.23-25 However, strong dipole interactions between nitrile groups render PAN weakly soluble in most solvents. PVAc consisting of protruding acrylates interacts strongly with solvent molecules to facilitate swelling of the polymer and ion transport. Poly(acrylonitrile-co-vinyl acetate) (PAV), which consists of both acrylonitrile and vinyl acetate monomers, represents an excellent backbone for the polymer host in GPEs of high ionic conductivity and chemical stability. PMMA, which is an isomer to PVAc, can be used to supplement PVAc to improve the contact at the electrode-electrolyte interface.26 PMMA exhibits excellent affinity for the surface of the electrode due to the fact that the acrylate in PMMA is less obstructive to polymer-chain rotation than is the acetate in PVAc.

Table 1. Literature survey: Capacity values and cycle lifespans of full-cell lithium ion batteries assembled using various electrolytes.

Electrodes

Capacity (mAh g-1)

Cycling retention, cycles/rate

Ref.

PECA-cellulose/ LiPF6-EC-DMC

G|LMO

109 @ 0.5 C

90%, 100/0.5 C

8

PAV-CN/ LiPF6-EC-DMC-EMC

G|LCO

−

82%, 50/0.2 C

9

P(EO-PO)/ LiPF6-EC-DEC-DMC

G|LFP

125 @ 0.5 C 88 @ 5 C 12 @ 17 C

77%, 450/1 C

10

GPE composition: polymer/electrolytea

b

4


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P(HFBMA -PEGMA)-PVDF/ LiPF6-EC-DEC-EMC

G|LMO

120 @ 0.5 C 30 @ 4 C

86%, 100/0.5 C

11

OEGMA- CCMA/ LiPF6-EC-DMC

G|LNCM

134 @ 0.1 C 126 @ 0.5 C 31 @ 5 C

85%, 85/0.1 C

12

P(MA-AN)-PVA/ LiPF6-EC-DEC-DMC

G|LCO

140 @ 0.2C

96%, 50/0.2 C

13

P(MMA-PEGMA)PVDF/ LiPF6-EC-DMC-EMC

G|LFP

130 @ 0.1 C 90 @ 2 C

55.6%, 50/1 C

14

PDMS-g-(PPO-PEO)PVDF/ LiPF6-EC-DMC-EMC

G|LFP

120 @ 1 C

85%, 100/1 C

15

PEGDA-PVDF/ LiPF6-EC-DMC

G|LCO

116 @ 0.1 C 90 @ 5 C

90%, 100/1 C

16

PVDF-CAB-PE/ LiPF6-EC-EMC

G|LCO

140 @ 1 C

93.1%, 300/1 C

17

PI/ LiPF6-EC-EMC

G|LCO

157 @ 0.2 C 110 @ 2 C

85%, 50/0.5 C

18

TMPTMA-LPO/ LiPF6-EC-DMC- EMC

G|LCO

135 @ 0.1 C 110 @ 1 C

83%, 100/0.2 C

19

PMMA-PVDF/ LiPF6-EC-DMC-EMC

G|LCO

−

98%, 100/0.5 C

20

P(VDF-HFP)-SiO2/ LiPF6-EC-DEC

G|LCO

136 @ 0.2 C 67 @ 2 C

93%, 100/0.5 C

21

P(VDF-HFP)-SiO2/ LiPF6-EC-DEC

G|LCO

−

96%, 100/0.5 C

22

5



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P(AN-VAc)-PMMATiO2/ LiPF6-EC-DEC-DMC a

G|LFP

152 @ 0.1 C 122 @ 5 C 84 @ 20 C

90%, 100/20 C 71%, 1000/20 C

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this work

LiPF6, lithium hexafluorophosphate; EC, ethylene carbonate; DMC, dimethyl carbonate; EMC, ethyl methyl carbonate; DEC, diethyl carbonate; PECA, poly(ethyl α-cyanoacrylate); PAV-CN, cyanoethyl polyvinyl alcohol; P(HFBMA-PEGMA), poly(hexafluorobutyl methacrylate-co-poly(ethylene glycol) ethacrylate); PVDF, poly(vinylidene fluoride); OEGMA, oligo(ethylene glycol) methyl ether methacrylate; CCMA, cyclic carbonate methacrylate; P(MA-AN), poly(methyl acrylate-co-acrylonitrile); PVA, poly(vinyl alcohol); P(MMA-PEGMA), poly(methyl methacrylate-co-poly(ethylene glycol) methacrylate); PDMS-g-(PPO-PEO), poly(dimethylsiloxane) graft poly(propylene oxide)-co-poly (ethylene oxide); CAB, cellulose acetate butyrate; PE, polyethylene; PI, polyimide; TMPTMA, trimethylolpropane trimethylacrylate; LPO, lauroyl peroxide; PMMA, poly(methyl methacrylate), P(VDF-HFP),poly(vinylidene fluoride-co-hexaflouropropylene), P(AN-VAc), poly(acrylonitrile-co-vinyl acetate).

b

G, graphite; LMO, LiMn2O4; LCO, LiCoO2; LFP, LiFePO4; LNCM, Li(Ni1/3Co1/3Mn1/3)O2. Inorganic oxide nanoparticles can be used to improve both the mechanical

strength and ionic conductivity of SSEs or GPEs,4,27 Some oxide nanoparticles exhibit negative zeta-potentials in electrolytes due to the absorption of anions on their insulating surface, which results in the formation of space charge layers surrounding the nanoparticles.7,28 Previous studies have used nanoparticles such as TiO2, SiO2, and Al2O3 for incorporation with various electrolytes.29-31 The space charge regime surrounding oxide nanoparticles may interconnect those formed on the polymer chains, thereby forming three-dimensional (3D) percolative space charge regimes surrounding the polymer:oxide matrices to increase the mobility of Li+ ions. Immobilizing anions also suppresses electrode polarization in the formation of 6


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solid-electrolyte interface (SEI) layers capable of promoting charge transfer at the electrode surface. This study used TiO2 nanoparticles as the inorganic oxide filler due to its high dielectric constant of 100 and acidic nature, which could facilitate the dissociation of the LiPF6 salt and adsorb PF6- anions through Lewis acid-base interaction, thereby generating mobile Li+ ions.32-35 This study demonstrates the concept of connecting oxide nanoparticles with the space charge regimes surrounding polymer functionalities to cause the 3D percolation of Li+ ions in a GPE for LIBs. Figure 1a illustrates the distribution of anions and cations in a liquid electrolyte supported by a separator, in which the attraction between the dissociated anions and cations hinders ion transport. Figure 1b presents a conceptual schematic of a PAV-supported GPE (i.e., GPE-PAV), in which the nitrile functional groups adsorb the PF6- anions of the lithium salt to create space charge layers of Li+-ions surrounding the polymer chains. The attraction between the nitrile groups may block the transport of cations in the space charge layer. In this study, we constructed a TiO2-decorated host framework by incorporating PMMA into PAV to form PAVM, which was subsequently blended with TiO2 nanoparticles. Figure 1c presents a schematic diagram of the resulting GPE-PAVM:TiO2, in which the space charge layers surrounding TiO2 nanoparticles connect the space charge layers of the nitrile groups to form a 3D percolation pathway for Li+ ions. This GPE achieved conductivity values of 4.5 × 10-3 and 5.3 × 10-4 S cm-1 at 30 and −40 ºC, respectively. When assembled using GPE-PAVM:TiO2, the resulting full-cell graphite|electrolyte|LiFePO4 battery exhibited a high Li+-ion transference number of 0.7 and capacity values (based on LiFePO4) of 152 and 84 mAh g-1 at 0.1 and 20 C-rates, respectively. The capacity retention was maintained at 71% after 1000 charge-discharge cycles at 20 C. The high Li+-ion transference number indicates the 7



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synergistic effect of PAVM and TiO2 immobilizing the anions to form a high-conductivity space charge regime in the GPE. This resulted in the high-rate performance of the batteries as well as stable SEI layers capable of extending the lifespan of the device as well as the range of operating voltages.

Figure 1. Conceptual illustration of ion distributions in various electrolytes. (a) Liquid electrolyte supported by a separator (i.e., SLE); the attraction between the dissociated anions and cations hinders ion transport. (b) GPE-PAV; the nitrile 8


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functional groups on PAV chains adsorb the PF6- anions of the lithium salt to create space charge layers (i.e., the yellow regimes) of Li+-ions surrounding the polymer chains. (c) GPE-PAVM:TiO2; the space charge layers surrounding TiO2 nanoparticles connect the space charge layers of the PAV chains, which are segregated by PMMA chains, to form a 3D percolation pathway for Li+ ions.

2. Experimental Section

2.1. Preparation of polymer host of GPEs This study prepared polymer host PAVM:TiO2 through the addition of commercially available a TiO2 powder (P25, Degussa; ca. 21 nm in size) and PMMA (Aldrich, USA; weight-average molecular weight (Mw) = 15000 g mol-1) to a solution comprising 7 wt% PAV (Tong-Hwa, Taiwan; Mw = 500000 g mol-1, acrylonitrile/vinyl acetate ratio = 92/8) dissolved in dimethylacetamide. The acrylonitrile/vinyl acetate ratio of 92/8 is optimal for co-polymer PVA to have high solubility in the electrolyte solvent while maintain the ion-solvating ability of the PAN chains. The TiO2 powder, with a surface area of 50 m2 g-1 and a negative zeta-potential, was dried under vacuum at 120 ºC for 12 h before use. The mixture, with a PAV/PMMA/TiO2 mass ratio of 100/10/7, was magnetically stirred at 70 °C for 12 h. Other mixtures were prepared with a PAV/PMMA mass ratio of 100/10 and PAV/TiO2 mass ratio of 100/7 for PAVM and PAV:TiO2. The resulting polymer solutions were used as a feed in the electrospinning synthesis for PAVM:TiO2, PAVM, and PAV:TiO2 on aluminum foil. Electrospinning was conducted using a polymer solution feed rate of 0.7 mL h-1 and an applied voltage of 15 kV, with a 9



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distance of 15 cm between the tip of a nozzle and the collector. The collected electrospun films (average thickness of 50−200 µm) were dried under vacuum on thin aluminum foil at 60 °C for 12 h. The PAVM:TiO2, PAVM, and PAV:TiO2 films were soaked in a liquid electrolyte solution (LE) of 1 M LiPF6 dissolved in ethylene carbonate/dimethyl carbonate/diethyl carbonate (EC/DMC/DEC; 1:1:1 by volume) in an argon environment for 24 h to trap the LE solution in the polymer network for the formation of GPE-PAVM:TiO2, GPE-PAVM and GPE-PAV:TiO2 films, respectively. The electrochemical performance of the GPEs (ca. 50 µm thick) was compared to the separator-supported LE (i.e., SLE), which was obtained by swelling a commercial separator (Celgard M824, USA; 12 µm thick) using LE. The LE uptakes (all saturated in 40 min) were 0.4, 1.9, 2.4, and 2.4 times the mass of the polymer membranes for SLE, GPE-PAV:TiO2, GPE-PAVM, and GPE-PAVM:TiO2, respectively.

2.2. Electrode preparation and cell assembly The cathode comprised 80 wt% LiFePO4 (BTR New Energy Materials, China), 10 wt% PVDF (Aldrich, USA; Mw = 534000 g mol-1), and 10 wt% carbon black super-P (Taiwan Maxwave Co., Taiwan). A slurry of these materials was prepared in N-methyl pyrrolidone (NMP; Aldrich, USA) for use in coating the aluminum foil using a doctor-blade. After evaporating the solvent, disks of 1.327 cm2 in area were obtained by punching the coated foil followed by drying at 80 °C under vacuum for 12 h. The cathodes were then roll pressed to improve particulate contact and foil adhesion. The resulting electrodes had a thickness of 60−70 µm and an apparent density of 1.1 g cm-3. The anode was prepared in the same manner as the cathode, using 92 wt% mesophase pitch-derived microbead graphite (China Steel Chemical 10


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Co., Taiwan), 6 wt% PVDF, and 2 wt% super-P with copper foil as the substrate. The thickness of the anode ranged from 40 to 50 µm with an apparent density of 1.1 g cm-3. Full-cell LIBs were assembled by sandwiching the GPEs and SLE between a graphite anode and a LiFePO4 cathode and then sealing the battery in a coin cell. The charge capacity ratio of anode to cathode for the full-cell LIBs was adjusted to 1.4. Cell assembly was performed in a glove box filled with argon gas of a low water content (< 1 ppm). The LiNi0.5Mn1.5O4 cathode used in high-voltage tests comprised 85 wt% LiNi0.5Mn1.5O4 (Hongda Energy., Taiwan), 7 wt% PVDE, and 8 wt% carbon black. All the electrodes were soaked with the LE solution before cell assembly.

2.3. Measurements The surface morphology of the polymer fibers and membranes and the graphite electrodes were examined using a scanning electron microscope (SEM; JOEL JSM-6700F, Japan), which was equipped with an energy dispersive X-ray spectrometer (EDS, Oxford INCA400, Japan) for elemental analysis. We also used a transmission electron microscope (TEM; Hitachi H-7500, Japan) to assist the morphology analysis. The thermal analysis of the GPEs was carried out in a differential scanning calorimeter (DSC; Shimadzu DSC-60, Japan). Raman spectra of the specimens were recorded at room temperature at a resolution of 4 cm-1 using a Bayspec Raman spectrometer (USA) with a laser line of 780 nm using the Lorentzian function to de-convolute bands into constituent peaks. The ionic conductivity of the GPEs was determined using AC impedance spectroscopy (Zahner-Elecktrik IM6e, Germany) at temperatures of −40 to 90 °C. Two stainless-steel (SS) electrodes were used to sandwich the GPEs and SLE for the measurement of impedance, which was conducted at 0 V with an AC potential amplitude of 5 mV over a frequency range of 11



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0.1 Hz to 100 kHz. The electrochemical stability of the GPEs was analyzed using linear voltage scans on SS|electrolyte|Li cells at 5 mV s-1. Resistance at the electrolyte-electrode interface was measured using the impedance responses of the full-cell batteries at frequencies ranging from 0.1 Hz to 100 kHz under 3.3 V. X-ray photoelectron spectroscopy (XPS; Kratos AXIS Ultra DLD, UK) with Al Ka radiation was used to quantitatively analyze the chemical composition of the SEI layers on the anodes of full-cell graphite|electrolyte|LiFePO4 batteries. Galvanostatic charge and discharge cycling tests were conducted on full-cell graphite|electrolyte|LiFePO4 batteries between 2.0 and 3.8 V using battery test equipment (Acutech System BAT-750, Taiwan). All electrochemical measurements were conducted at 25 °C.

3. Results and Discussion

3.1. Preparation of GPEs Figure 2(a and b) presents top-view photographs of PAVM and PAVM:TiO2 membranes (thickness of 50 µm), which are flexible, white, and completely opaque to light. The insets present SEM images of electrospun PAVM and PAVM:TiO2 membranes, consisting of nanofibers (120 ± 50 nm in diameter), which are interlaid to form a 3D network with fully interconnected interstitial pores between the fibers. The porous membranes enable the efficient trapping of electrolyte solutions and provide connected channels for ion transport. Figure 2(c and d) presents the GPE films (GPE-PAVM and GPE-PAVM:TiO2), which were derived by soaking PAVM and PAVM:TiO2 membranes in the liquid electrolyte LE. The GPEs appear homogeneous, mechanically stable, and translucent. 12


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Figure 2. Top-view photographs of (a) PAVM membrane, (b) PAVM:TiO2 membrane, (c) GPE-PAVM, and (d) GPE-PAVM:TiO2. The insets of panels a and b present SEM images of electrospun polymer fibers constituting PAVM and PAVM:TiO2. We also obtained GPE-PAV:TiO2 from TiO2-blended PAV to examine the effects of PMMA on ion transport. Figure 3(a and b) presents SEM images of the fibers constituting the PAV:TiO2 and PAVM:TiO2 membranes. The PAV:TiO2 fibers exhibit protruding bulges, whereas the PAVM:TiO2 fibers have a smooth surface. SEM analysis with elemental Ti mapping (Figure 3c) revealed the aggregation of TiO2 nanoparticles at the bulges of PAV:TiO2 fibers. PAV fibers without the incorporation with TiO2 have smooth surface (Figure S1a of the Supporting 13



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Information), indicating that the bulges on the PAV:TiO2 fibers are caused by the aggregation of TiO2 nanoparticles. The SEM image of a PAVM:TiO2 fiber in Figure 3d reveals the even distribution of TiO2 nanoparticles, which is supported by a TEM image (Figure S1b). Figure 3 illustrates the role of PMMA in the segregation of PAV fibers to facilitate the dispersion of TiO2 nanoparticles throughout the polymer matrix of GPE-PAVM:TiO2.

Figure 3. SEM images of fibers constituting the supporting membranes of GPEs: (a) PAV:TiO2 fiber; (b) PAVM:TiO2 fiber; (c) focused PAV:TiO2 fiber with elemental Ti mapping (in red spots); (d) focused PAVM:TiO2 fiber with elemental Ti mapping.

Figure 4 presents a comparison of the Raman spectra of the GPEs and Celgard separator-supported liquid electrolyte (i.e., SLE) in distinct wavenumber regimes. Lorentzian curve fitting was used to de-convolute the Raman band ranging from 700−760 cm-1 (Figure 4a) into its constituent peaks (dotted lines), which consist of ring-bending modes of free-EC and Li+-associated-EC (Li+-EC) at 717 and 726 cm-1, 14


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respectively, as well as the symmetric vibration of dissociated-PF6− (dissociated from electrostatically-interacting Li…PF6 pairs in the solution bulk) at 741 cm-1.36 The intensity of EC peaks (free-EC + Li+-EC) was used as a reference for the evaluation of the relative intensities associated with other peaks or bands in the Raman spectra. The (dissociated-PF6−)/(free-EC + Li+-EC) peak-area ratios for GPE-PAVM:TiO2, GPE-PAVM, GPE-PAV:TiO2, and SLE were 1.1, 0.91, 0.67 and 0.59, respectively. The ratios of the GPEs were larger than that of SLE, indicating that the PF6- ions were solvated through their association with polymer chains or TiO2 in the GPEs.35-38 A comparison of the ratios of GPE-PAVM and GPE-PAV:TiO2 revealed the critical role of PMMA in the segregation of PAV fibers for the solvation of anions. TiO2 is less effective than PMMA in promoting anion-solvation, probably due to the aggregation of TiO2 particles in PAV (Figure 3). With the addition of PMMA for the segregation of polymer chains and TiO2 nanoparticles, GPE-PAVM:TiO2 exhibited dissociated-PF6− of the highest intensity.

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Figure 4. Raman spectra (solid lines) of SLE, GPE-PAV:TiO2, GPE-PAVM, and GPE-PAVM:TiO2 and the corresponding peaks obtained after spectrum de-convolution (dotted lines) in two wavenumber regimes: (a) 700−760 cm-1 and (b) 865−955 cm-1. Figure 4b presents the peak de-convolution of the Raman 865−955 cm-1 region, which includes contributions from the symmetric ring breathing of free-EC and Li+-EC at 894 and 904 cm-1, respectively, as well as the O−CH3 stretching of free-DEC/DMC and Li+-DEC/DMC at 920 and 937 cm-1, respectively.36 The Li+ ions are mainly solvated by EC; therefore, we focused on the association between Li+ and 16


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EC. The (Li+-EC)/(free-EC) peak-area ratios for GPE-PAVM:TiO2, GPE-PAVM, GPE-PAV:TiO2, and SLE were 0.7, 1.3, 2.3, and 3.1, respectively. The smaller the ratios are, the smaller the coordination number, n, in the Li+(EC)n complexes. These demonstrate that the adsorption of PF6− ions on TiO2 and polymer chains resulted in the generation of Li+ ions with low solvent coordination in the space charge regimes surrounding TiO2 and polymer chains (see the scheme in Figure 1c). Low coordination Li+ ions have higher mobility than do high coordination Li+ ions, such that the space charge regimes may represent the express pathway for Li+ transport in the GPEs.7,28

3.2. Ionic conductivity of GPEs This study used electrochemical impedance spectroscopy to analyze the ionic conductivity in GPEs and SLE at various temperatures. Figure S2 of the Supporting Information presents the impedance Nyquist plots of GPE-PAVM:TiO2, GPE-PAVM, GPE-PAV:TiO2, and SLE at temperatures ranging from −40 to 90 ºC. The impedance spectra of the GPEs are more vertical than that of SLE, indicating that polymer chains facilitate ion transport toward the electrode surface to form an electric double layer. As indicated by the vertical spectrum of GPE-PAVM:TiO2, the combination of TiO2 and polymer chains substantially improved ion transport at the electrode-electrolyte interface. This effect may be associated with the situation observed in Raman analysis, in which the ion transport pathway that developed on the TiO2-decorated polymer chains devastated the interfacial barrier and thereby enabled the formation of the double layer. The ionic conductivity of the electrolytes was determined using the following:

σ = RI-1 S-1 d,

(1) 17



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where σ represents ionic conductivity, RI is the real axis intercept in the impedance Nyquist plot, S is the geometric area of the electrolyte-electrode interface, and d is the distance between the two electrodes.39 Figure 5a presents a summary of an Arrhenius plot with ionic conductivity in the electrolyte at temperatures between −40 and 90 °C. The GPEs were more ion-conductive than was SLE. For example, we obtained the following ionic conductivity values at 30 °C: 4.5 × 10-3 (GPE-PAVM:TiO2), 3.4 × 10-3 (GPE-PAVM), 1.6 × 10-3 (GPE-PAV:TiO2), and 5.9 × 10-4 S cm-1 (SLE). This is a clear indication that the space charge regimes surrounding the polymer framework in the GPEs must play a crucial role in enhancing the ionic conductivity. The low conductivity of SLE can be attributed to the use of a separator (Celgard-M824), which was not designed to adsorb anions but rather to obstruct the motion of ions in the electrolyte system.40 GPE-PAVM:TiO2 exhibited ionic conductivity higher than that of the other GPEs. SEM and Raman analyses reflect the synergistic effect of PMMA and TiO2 in segregating polymer chains and forming 3D percolative pathway for Li+ ions along the surface of the TiO2-decorated polymer chains. The GPE-PAV:TiO2 exhibited the lowest conductivity among the GPEs, due to the aggregated PAN segments, which also prevented the dispersion of TiO2 nanoparticles in the polymer framework.

18


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Figure 5. Ionic conductivity of electrolytes determined from impedance analysis at temperatures of -40 to 90 °C and presented using (a) Arrhenius plot and (b) VTF plot.

The ranking of the conductivity values of the electrolytes is identical to that of the (dissociated-PF6−)/(free-EC + Li+-EC) peak-area ratios determined by Raman analysis. The dissociated-PF6− ions were not electrostatically interacted with Li+ in the solution bulk and were solvated through their association with polymer chains or TiO2 in the GPEs. The agreement in the rankings supports the supposition that the polymer chains and TiO2 nanoparticles adsorb PF6−anions to create space charge regimes for 19



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Li+-ion transport.7,24,25,28 The schematic diagram in Figure 1b illustrates the formation of space charge regimes surrounding the polymer chains of GPE-PAV resulting from the absorption of anions by polar PAN chains, which broke up the ion-pair clusters (Li…PF6-) that would otherwise have hindered the transport of Li+ ions.24,25,41,42 The polymer framework provides 1D pathways for the percolation of Li+ ions. The introduction of PMMA can prevent the blocking of pathways caused by the aggregation of PAN chains. Figure 1c illustrates that the introduction of TiO2 nanoparticles created interconnecting space charge layers resulting in the formation of 3D percolative pathways for Li+ ions. At low temperatures, GPE-PAVM:TiO2 exhibited ionic conductivity of 5.9 × 10-4 S cm-1 at −40 ºC, which was as high as that of SLE at 30 ºC. The high conductivity at low temperatures reflects the effect of TiO2 in promoting the mobility of ions and suppressing the aggregation of polymer chains. Figure 5a shows how the temperature dependence of ion conductivity in SLE can be divided into two distinct temperature regimes with a transition near 10 °C. The variations in conductivity in both temperature regimes appear to obey the Arrhenius equation (σ = σ0 exp(−Ea/RT), in which σ0 is the frequency factor, Ea is the activation energy, R is the gas constant, and T is the absolute temperature).43,44 The activation energies were 2.9 and 31 kJ mol-1 for the high and low temperature regimes, respectively. The large Ea in the low-temperature regime indicates that crystallization of the carbonate solvents influenced the conductivity of SLE. The conductivity curves of the GPEs did not fit the Arrhenius equation, but rather the Vogel–Tamman–Fulcher (VTF) equation (σ = AT-1/2 exp[−E0/R(T−T0)], in which A is a constant proportional to the number of carrier ions, E0 is the pseudo-activation energy related to the motion of the polymer segments, and T0 is the ideal glass transition temperature).45 Figure 5b presents the temperature dependence of conductivity, based on the 20


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VTF equation. The linear fit shown in the figure confirms that ionic motion in the GPEs is associated with free-volume transport and segmental motion within the polymer.46 Table 2 presents the parameters obtained from fitting. GPE-PAVM:TiO2 exhibited high carrier ion density (i.e., a high A value) due to the ability of TiO2-decorated polymer chains to absorb anions on their surfaces, thereby increasing Li+ ion concentrations in the free volume. The high degree of ion-pair dissociation in GPE-PAVM:TiO2 also resulted in the low temperature dependence of ion transport (i.e., a low E0 value). The T0 values of the GPEs were ranked as follows: GPE-PAVM:TiO2 (191 K) < GPE-PAVM (194 K) < GPE-PAV:TiO2 (197 K), which supports the supposition that strong anion adsorption increased the repulsion between polymer chains in the GPE-PAVM:TiO2, thereby suppressing the tendency for crystallization at low temperatures. The T0 value can be calculated experimentally from the thermodynamic glass transition temperature (Tg) values of GPEs as “Tg – 50 K”.47,48 We have conducted thermal analysis by using DSC to measure the Tg values of the GPEs. GPE-PAVM:TiO2, GPE-PAVM, and GPE-PAV:TiO2 exhibited Tg values of 240, 244, and 249 K, respectively, which result in “Tg – 50 K” values of 190, 194, and 199 K. The small difference between the T0 values (Table 2) and the “Tg – 50 K” values validates the appropriateness of using the VTF equation to simulate the temperature dependence of the GPEs.

Table 2. Parameters obtained from fitting ionic conductivity data at different temperatures to the Vogel–Tamman–Fulcher equation for various electrolytes (Figure 5b).

Electrolyte

A (S cm-1 K1/2)

E0 (kJ mol-1) 21


T0 (K)


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GPE-PAVM:TiO2

0.31

0.96

191

GPE-PAVM

0.22

1.00

194

GPE-PAV:TiO2

0.11

1.22

197

3.3. Electrochemical stability of GPEs The electrochemical stability of the electrolytes was analyzed using linear voltage scans. Figure 6 presents the voltammograms of cells assembled by inserting electrolytes between a stainless-steel working electrode and a Li counter electrode. When anodic polarization was applied to the working electrode at 5 mV s-1, the SLE cell exhibited an abrupt rise in current (above 0.3 µA cm-2) at approximately 4.5 V (vs. Li/Li+), in agreement with previous studies.49,50 The onset of anodic current flow was associated with the decomposition of PF6− anions and carbonate solvents on the surface of the electrode.51,52 The GPEs had current onset voltages exceeding that of SLE and their current values were ranked as follows: GPE-PAV:TiO2 > GPE-PAVM > GPE-PAVM:TiO2. The current for GPE-PAVM:TiO2 was negligible (below 0.3 µA cm-2) even at a high voltage of 6.5 V (vs. Li/Li+). We conducted linear voltage scans at a low scan rate of 0.2 mV s-1 for SLE and GPE-PAVM:TiO2 (Figure S3 of the Supporting Information) and GPE-PAVM:TiO2 still exhibited high electrochemical stability under such a slow voltage scan. We surmise that the high stability of GPE-PAVM:TiO2 can be attributed to the fact that both the PAN chains and TiO2 nanoparticles strongly adsorb PF6− anions in the polymer framework. Additionally, the attachment of the polymer chains and TiO2 nanoparticles on the electrode surface may have hindered the decomposition of the carbonate solvents under anodic polarization. Immobilization of PF6− anions within the electrolytes suppresses the polarization of 22


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electrodes and reduces ionic diffusion resistance at the electrode surface.51,52 Analysis of the Li+-ion transference number (tLi+) is essential to elucidating the influence of TiO2-decorated polymer frameworks on the transport of the anions.

Figure 6. Linear scan (5 mV s-1) voltammograms of cells assembled by inserting an electrolyte (SLE, GPE-PAV:TiO2, GPE-PAVM, or GPE-PAVM:TiO2) between a working stainless-steel electrode and a Li-metal counter electrode.

A stationary anion (i.e., tLi+ = 1) capable of eliminating polarization and anion decomposition is being sought for LIBs. Cells consisting of two Li-metal electrodes sandwiching SLE or GPEs were used to determine the tLi+ value according the following equation:

t Li + =

I ss (∆VDC − I o Rint, 0 ) I o (∆VDC − I ss Rint,ss )

(2)

where I0 and Iss are the initial and steady-state currents respectively (Figure 7a), when polarizing the cell to a low DC voltage of 5 mV (VDC). The Iss values were determined when the current variation rate was smaller than 0.1 µA/min. Rint,0 and Rint,ss are the initial and steady-state resistance values associated with charge transfer at the 23



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Li-metal interfaces, as determined using AC impedance analysis (Figure 7(b−e)).53 The tLi+ values for GPE-PAVM:TiO2, GPE-PAVM, GPE-PAV:TiO2, and SLE were 0.69, 0.60, 0.48, and 0.38, respectively. This trend or the relative values of the tLi+ support the role of PAN chains and TiO2 in the immobilization of the PF6− anions. Along with the low solvent-coordination number of Li+ ions in the GPE-PAVM:TiO2 electrolyte, the high tLi+ number supports the claim that PAN chains and TiO2 nanoparticles act as a role in controlling the space charge regimes. LIBs assembled using GPE-PAVM:TiO2 would be expected to have a low polarization resistance and long lifespan, due to suppressed anion decomposition on the positive electrode.54-56

24


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Figure 7. (a) Current-time curves of Li|electrolyte|Li cells following application of DC voltage (5 mV) to the cell. (b−e) Corresponding Nyquist impedance plots of the cells, GPE-PAVM:TiO2 (b), GPE-PAVM (c), GPE-PAV:TiO2 (d), and SLE (e), which were used to determine the initial and final Rint values.

A wide voltage window of stability can be highly beneficial in the application of GPEs in LIBs, particularly when using high-voltage cathode materials such as LiNi0.5Mn1.5O4 (4.7 V vs. Li/Li+), Li2NiMn3O8 (4.7 V vs. Li/Li+), LiCoMnO4 (5 V vs. Li/Li+), and LiCrMnO4 (4.8 V vs. Li/Li+).57,58 When assembled with conventional liquid electrolytes, these high-voltage materials exhibited large irreversible capacity and severe capacity fading due to electrolyte decomposition at voltages exceeding 4.5 V (vs. Li/Li+).48,59,60 We conducted high-voltage tests for the GPEs by using Li|electrolyte|LiNi0.5Mn1.5O4 batteries. Figure S4(a and b) of the Supporting Information presents the galvanostatic charge−discharge profiles, in which the cells were charged at 0.1 C-rate and discharged at various C-rates between 3.5 and 4.8 V. The GPE-PAVM:TiO2 battery presented excellent performance by producing high capacity values of 139 and 52 mAh g-1 at 0.1 and 5 C-rates, whereas the SLE battery produced capacity values of only 128 and 12 mAh g-1. Figure S4c illustrates variations in the charge and discharge capacities of these two batteries with the number of galvanostatic cycles set at 1 C. The GPE-PAVM:TiO2 battery retained 99% of capacity after 100 cycles, whereas the SLE battery retained only 68% of the capacity. The results presented in Figure S4 demonstrate the applicability of GPE-PAVM:TiO2 in high-voltage LIBs.

3.4. Electrolyte-electrode interface resistance of full-cell LIBs

25



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Figure 8 presents the complex-plane impedance spectra of full-cell graphite|electrolyte|LiFePO4 batteries, in which LiFePO4 and graphite were respectively used as the working and counter electrodes to sandwich different electrolytes. Measurements were conducted at 3.3 V, which is the difference between the potentials for the biphasic Li+ extraction/insertion in LiFePO4 and graphite. The high-frequency semicircle shown in the spectra characterizes the motion of charge across the SEI layer; the semicircle in the middle frequencies presents charge-transfer impedance; and the sloping line corresponds to Warburg impedance. We employed an equivalent circuit (the inset of Figure 8) consisting of bulk solution resistance (Rb), SEI layer resistance (RSEI), constant phase element of SEI (CPESEI), charge transfer resistance (Rct), the constant phase element of the electric double layer (CPEdl), and Warburg impedance (Zw) to simulate the spectra.

Figure 8. Nyquist impedance spectra of full-cell graphite|electrolyte|LiFPO4 batteries obtained at 3.3V. The solid-line curves are the simulations of the impedance data based on the equivalent circuit presented as the figure inset.

Table 3 presents the resistance data obtained from fitting the spectra to the equivalent circuit. The Rb values are ranked as follows: GPE-PAVM:TiO2 < 26


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GPE-PAVM < GPE-PAV:TiO2 < SLE, which accords with the ionic conductivity data in Figure 5. The ranking of the RSEI values is similar to that of the Rb, indicating that a high tLi+ value suppresses electrode polarization and therefore the resistance to the motion of ions across the interface. The Rct values, which depict reaction kinetics, are almost identical and independent of electrolyte type. AC-impedance analysis on full-cell LIBs reveals the superiority of GPE-PAVM:TiO2 in facilitating ion transport in the bulk solution and across the SEI. Determination of how the electrolyte type affects the formation of the SEI layer requires further analysis.

Table 3. Resistance data of full-cell graphite|electrolyte|LiFePO4 batteries obtained by fitting the impedance spectra (Figure 8) to the equivalent circuit presented in the inset of Figure 8. Voltage (V)

Rb (Ω Ω)

RSEI (Ω Ω)

Rct (Ω Ω)

GPE-PAVM:TiO2

2.2

4.2

6.1

GPE-PAVM

2.3

4.9

6.3

GPE-PAV:TiO2

2.6

5.4

6.3

SLE

4.1

5.4

6.1

3.5. Electrode-electrolyte interface analysis Graphite exfoliation accompanied by electrolyte decomposition leads to the formation of SEI layers on the anode of LIBs.61,62 The SEI layer may protect graphite particles from the co-intercalation of solvent molecules and exfoliation.63 However, the thickness and composition of the SEI layer substantially influence ion transport at the electrode-electrolyte interface. Figure S5(a and b) of the Supporting Information presents SEM images of 27



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pristine graphite particles used in the assembly of LIBs. Figure S5(c−f) presents images of the graphite particles in LIBs, which were assembled using various electrolytes and subjected to a series of charge−discharge cycles. Unlike the pristine graphite, the graphite particles from the cycled LIBs exhibited the deposition of SEI layers. Table 4 presents the elemental composition of the SEI layers, as determined by EDS analysis auxiliary to SEM. The F and P elements may be contributed by PF6− anions, compounds from PF6− decomposition (such as LiF and POF3), and binder PVDF. The F and P contents are electrolyte dependent and ranked as follows: GPE-PAVM:TiO2 < GPE-PAVM < GPE-PAV:TiO2 < SLE. These findings support the results of tLi+ analysis in which segregated PAN chains and TiO2 nanoparticles immobilized PF6− anions and in so doing suppressed the decomposition of the anions.

Table 4. Elemental analysis on SEI layers of anodes in LIBs assembled using various electrolytes and subjected to a series of charge−discharge cycles (Figure S5(c−f)).

Electrolyte

Atomic %

GPE-PAVM:TiO2

C 73.8

O 7.4

F 17.9

P 0.9

GPE-PAVM

59.9

11.4

27.0

1.7

GPE-PAV:TiO2

50.6

13.8

33.5

2.1

SLE

38.8

13.7

43.8

3.7

Figure 9 presents the O 1s spectra of XPS analysis on the SEI layers of the graphite electrodes. The spectra were decomposed into the constituent peaks (dotted lines) using Lorentzian curve fitting. The O 1s peak, ranging from 526 to 540 eV in the XPS spectra, comprises peaks contributed by CO3 (i.e., Li2CO3) located at 532.1 eV and CO+COO (i.e., ROLi and ROCO2Li, where R is either CH3 or C2H5) located 28


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at 533.5 eV.64-66

Li2CO3 is the desired component in the SEI layer for effective

electrode passivation because it is ion-conductive and insoluble in solvents. ROLi and ROCO2Li compounds are organic lithium salts that are soluble in solvent or would eventually transform into Li2CO3 following repeated charging of the cell. As shown in Figure 9, the SEI film grown in GPE-PAVM:TiO2 exhibited the highest CO3/(CO+COO) peak-area ratio, indicating that immobilization of PF6− boosted the formation of Li2CO3 to stabilize the SEI film. The C 1s XPS spectra (Figure S6 of the Supporting Information) also depicts the high CO3 content of the SEI film grown in GPE-PAVM:TiO2. The SEM images in Figure S5 and impedance spectra in Figure 8 reveal the formation of a thinner SEI layer on the graphite of the GPE-PAVM:TiO2 LIB.

Figure 9. O 1s XPS spectra of SEI layers formed on anodes in LIBs assembled using various electrolytes and subjected to a series of charge−discharge cycles. The spectra 29



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were decomposed into CO3 and CO+COO peaks (indicated by the dash lines), which were fitted using a Lorentzian function.

In the Li 1s spectra (Figure 10) ranging from 52 to 60 eV, the peaks at 55.5 and 56.5 eV can be attributed to Li2CO3 and LiF, respectively.64,65 LiF is an unwanted compound in the SEI film because of its low Li+-ion conductivity, higher interface impedance, and restriction to Li+ insertion into graphite.66,67 The low LiF/Li2CO3 peak-area ratio of the SEI film in GPE-PAVM:TiO2 confirms the function of this polymer-TiO2 composite in suppressing the decomposition of PF6− anions.

Figure 10. Li 1s XPS spectra of SEI layers formed on anodes in LIBs assembled using various electrolytes and subjected to a series of charge−discharge cycles. The spectra were decomposed into Li2CO3 and LiF peaks (indicated by the dash lines), which were fitted using a Lorentzian function. 30


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3.6. Performance of full-cell LIBs Figure 11 presents the galvanostatic charge−discharge profiles of full-cell graphite|electrolyte|LiFePO4 batteries, which were charged to 3.8 V at 0.1 C, whereupon they were discharged to 2.0 V at various C-rates. This study defined the battery C-rates according to the LiFePO4 cathode, under the assumption that the maximum achievable capacity was 155 mAh g-1 (determined using a half cell) in this full-cell system. At low rates, all of the cells presented plateaus in the charge and discharge voltage close to the difference in thermodynamic potential between the two electrodes, i.e., 3.3 V. The voltage plateaus deviated from 3.3 V following an increase in the discharge rate. The deviation in voltage (∆V) corresponds to the sum of the serial resistance and polarization of the electrodes.10,23,68 Figure 12 illustrates the variation in ∆V with the discharge rate of the batteries. We used the slope of the linear relationship to determine the overall resistance, which produced values of 30, 46, 64, and 71 Ω for GPE-PAVM:TiO2, GPE-PAVM, GPE-PAV:TiO2, and SLE batteries, respectively. The GPEs proved superior to SLE in facilitating ion motion in the batteries. Introducing TiO2 into the PAV and PAVM polymer frameworks resulted in very different overall resistance values (64 Ω vs. 30 Ω), indicating that the segregation of PAN segments by PMMA to accommodate TiO2 nanoparticles played a key role in immobilizing PF6− ions. This created percolative space charge pathways for Li+ ions, suppressed electrode polarization, and produced ion-conductive Li2CO3 SEI layers. Without the addition of TiO2, GPE-PAVM formed 1D space charge pathways for Li+ ions and the resulting battery exhibited an overall resistance of 46 Ω, which is higher than that of the GPE-PAVM:TiO2 battery due to the inclusion of 3D space charge pathways (Figure 1c). 31



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Figure 11. Galvanostatic charge−discharge profiles of full-cell graphite|electrolyte|LiFPO4 batteries, which were charged at 0.1 C-rate and discharged at various C-rates between 2.0 and 3.8 V: (a) graphite|GPE-PAVM:TiO2|LiFPO4; (b) graphite|GPE-PAVM|LiFPO4; (c) graphite|GPE-PAV:TiO2|LiFPO4; (d) graphite|SLE|LiFPO4. The battery C-rates were defined on the basis of the LiFePO4 cathode by assuming a maximal achievable capacity of 155 mAh g-1 in this full-cell system.

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Figure 12. Deviations in the discharge voltage plateaus from the equilibrium state, 3.3 V, (i.e., ∆V) as a function of the discharge current in graphite|electrolyte|LiFePO4 batteries. The ∆V value corresponds to the sum of the battery’s IR drop resulting from serial resistance and polarization of electrodes.

As shown in Figure 11, all of the batteries presented similar capacities of ca. 150 mAh g-1 at 0.1 C; however, differences in battery capacity became prominent at discharge rates exceeding 1 C. Figure S7 of the Supporting Information presents the capacity retention of the LIBs at discharge rates of 0.1−20 C. The GPE batteries exhibited higher capacity retention than did the SLE battery at all discharge rates. The retention of the LIBs at 20 C exhibited the following ranking: GPE-PAVM:TiO2 > GPE-PAVM > GPE-PAV:TiO2 > SLE, which accorded with the expectation that low battery resistance would lead to high capacity retention. The GPE-PAVM:TiO2 battery produced a high discharge capacity value of 84 mAh g-1 at 20 C, which corresponds to the retention of 55 % of the capacity obtained at 0.1 C. The GPE-PAVM, GPE-PAV:TiO2, and SLE batteries produced only 62, and 45, and 40 mAh g-1 at 20 C (or 42 %, 30 %, and 27 % retention), respectively. 33



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Figure 13 presents the charge and discharge capacities of the above batteries as a function of the number of galvanostatic cycles at a high rate of 20 C. The GPE-PAVM:TiO2, GPE-PAVM, GPE-PAV:TiO2, and SLE batteries presented discharge-capacity retention of 71 %, 64 %, 31 %, and 22 %, respectively, after 1000 cycles. The high degree of immobilization of PF6−anions in the GPE-PAVM:TiO2 battery maintained the capacity at a high rate and largely prevented a decay in capacity by suppressing decomposition of PF6− anions and forming robust SEI layers. The GPE-PAVM:TiO2 battery is superior to other full-cell batteries reported in the literature (Table 1) with regard to the rate and cycle life in charge-discharge.

Figure 13. Charge (solid symbols) and discharge (hollow symbols) capacities of full-cell graphite|electrolyte|LiFePO4 batteries following 1000 cycles of charge−discharge at 20 C-rate over a voltage range of 2.0−3.8 V. The capacity retentions at the 1000th cycle were presented in the figure.

We have developed a GPE architecture that includes 3D space charge pathways for Li+ ions to promote ion transport through the bulk electrolyte and facilitates ion transfer at the electrode-electrolyte interface by suppressing battery polarization and 34


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the growth of stable Li2CO3 in the SEI film.

4. Summary and Conclusions

This study developed a GPE (i.e., GPE-PAVM:TiO2) supported by a polymer framework consisting of PAN chains and TiO2 nanoparticles with strong adsorption capacity for PF6−anions, which creates 3D percolative space-charge pathways for Li+ ions. The polymer framework includes PVAc and PMMA chains that are designed to segregate high-polarity PAN chains and improve the dispersion of TiO2 nanoparticles. Raman analysis revealed that the PAVM:TiO2 framework enhances the dissociation of Li+…PF6− pairs by adsorbing PF6− and reduces the solvent-coordination number of Li+ ions in the space charge regimes. These effects substantially increase the ionic conductivity of the resulting GPE. The proposed GPE-PAVM:TiO2 exhibits a high tLi+ value of 0.7, thereby confirming that the polymer framework is able to immobilize most of the PF6− anions. The stationary-PF6− was shown to increase the electrochemical stability and enlarge the voltage range applicable for GPE-PAVM:TiO2. Immobilization of PF6− anions was also shown to suppress LiF formation and boost the formation of Li2CO3 , thereby stabilizing the SEI film to facilitate Li+-ion transport at the electrode-electrolyte interface. A full-cell graphite|GPE-PAVM:TiO2|LiFePO4 battery exhibited a lower RSEI value and a smaller overall resistance than those of other GPE and SLE batteries. The graphite|GPE-PAVM:TiO2|LiFePO4 battery delivers discharge capacities of 152 and 84 mAh g-1 (based on LiFePO4) at 0.1 and 20 C, respectively, thereby outperforming the graphite|SLE|LiFePO4 battery with capacities of 146 and 40 mAh g-1. The 35



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suppression of electrode polarization and inclusion of robust SEI layers stabilizes the charge-discharge cycling performance of the GPE-PAVM:TiO2 battery, which presented 90 % capacity retention after 100 cycles and 71 % after 1000 cycles at 20 C. This study demonstrates an effective strategy for building 3D space-charge pathways to enhance the efficiency of Li+-ion transport in the bulk solution as well as at the electrode-electrolyte interface through the immobilization of PF6− anions in an oxide nanoparticle-decorated polymer framework. This strategy suppresses the decomposition of PF6− anions on electrodes and thereby substantially expands the range of stable voltage for the GPE and extends the cycling life of the resulting LIBs.

Acknowledgements This research was supported by the Ministry of Science and Technology, Taiwan (104-2221-E-006-231-MY3, 104-2221-E-006-234-MY3, 104-3113-E-006-005, and 104-3113-E-006-011-CC2), and by the Ministry of Education, Taiwan, The Aim for the Top University Project to the National Cheng Kung University.

Supporting Information Available: SEM image of PAV fibers, impedance data of electrolytes at various temperatures, Charge−discharge profiles and cycle lifespans of batteries assembled with LiNi0.5Mn1.5O4, SEM images of graphite anodes before and after charge−discharge cycling, and Discharge capacity of graphite|electrolyte|LiFPO4 batteries at various C-rates. The Supporting Information is available free of charge on the ACS Publications website. 36


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