Healable, Highly Conductive, Flexible, and Nonflammable

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Healable, Highly Conductive, Flexible and Nonflammable Supramolecular Ionogel Electrolytes for Lithium Ion Batteries Panlong Guo, Anyu Su, Yingjin Wei, Xiaokong Liu, Yang Li, Feifan Guo, Jian Li, Zhenyuan Hu, and Junqi Sun ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 06 May 2019 Downloaded from http://pubs.acs.org on May 6, 2019

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ACS Applied Materials & Interfaces

Healable,

Highly

Conductive,

Flexible

and

Nonflammable Supramolecular Ionogel Electrolytes for Lithium Ion Batteries Panlong Guo,† Anyu Su,‡ Yingjin Wei,‡ Xiaokong Liu,† Yang Li,† Feifan Guo,§ Jian Li,† Zhenyuan Hu,† and Junqi Sun*,† †State

Key Laboratory of Supramolecular Structure and Materials College of Chemistry,

Jilin University, Changchun, Jilin 130012, P. R. China ‡Key

Laboratory of Physics and Technology for Advanced Batteries (Ministry of

Education), College of Physics, Jilin University, Changchun, Jilin 130012, P. R. China §State

Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of

Chemistry Jilin University, Changchun, Jilin 130012, P. R. China KEYWORDS: ionogels, lithium ion batteries, poly(ionic liquids), self-healing materials, solid-state electrolytes ABSTRACT. High-performance solid-state electrolytes with healability to repair mechanical damages are important for the fabrication of Li ion batteries (LIBs) with enhanced safety and prolonged service life. In this study, we present the fabrication of

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healable, highly conductive, flexible and nonflammable ionogel electrolytes for use in LIBs by loading ionic liquids and Li salts within a hydrogen-bonded supramolecular poly(ionic liquid) (PIL) copolymer network. The ionogel electrolytes exhibit ionic conductivities as high as 10-3 S/cm, which is comparable to the conventional liquid electrolytes. The Li/LiFePO4 battery assembled with the ionogel membrane exhibits excellent cycling performance and delivers a steady high discharge capacity of 147.5 mAh g-1 and Coulombic efficiency of 99.7% after 120 cycles at the charge/discharge rate of 0.2 C. Importantly, the ionogel membranes can heal damages outside or inside a battery due to the reversible nature of the supramolecular interactions between the components. The damaged ionogel membranes after being healed can effectively restore the original performance of the LIBs. INTRODUCTION Since the announcement of lithium ion batterie (LIBs) in 1991 by Sony Corporation, LIBs have become the fastest growing rechargeable batteries in the market. However, with increasing usage of LIBs in a wide range of products and a variety of environments, the safety issue is becoming the biggest concern for LIBs.1-4 There have been numbers


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of reported accidents of fires and explosions of LIBs on different consumer products, such as the battery fire on a crashed Tesla electric car and the Samsung Note 7 battery explosion. The safety hazard of LIBs mainly originates from the overheating in the battery and the subsequently resulted catastrophic combustion of the liquid electrolytes that are generally composed of highly flammable organic carbonates.5-8 Various circumstances can account for the overheating events of LIBs, such as overcharging, exposure to high temperature, external or internal shorts.1-8 The internal short circuit is the predominant cause of the overheating of LIBs, which can result from the intrinsic defect of the separator or its damage by outside compressive shock, Li-dendrite penetration or welding burrs inside the battery.1-8 The development of nonflammable solid state electrolytes that serve as both electrolytes and separators is deemed to be an ultimate solution to the safety issue of LIBs.1-10 Compared with inorganic solid-state electrolytes that generally suffer from high interfacial resistance and unstable electrolyte/electrode interface, solid polymer electrolytes (SPEs) are more promising because of their superior processability.11 The

convenient dimensional and spatial

controllability of SPEs can simplify the assembly of LIBs and facilitate the fabrication of


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flexible LIBs that are highly demanded nowadays on flexible and wearable electronic devices.11-15 Currently, the primary challenge for SPEs is the relatively low ion conductivity at room temperature, which significantly compromises the battery performance. The requirement of ion conductivity for a satisfactory SPE should be in the range of 10-4~10-2 S/cm.16 Furthermore, the thickness of SPEs is required to strictly restrict within dozens to hundreds of micrometers to lower the internal resistance, such thin SPEs are prone to be damaged outside the LIBs during the assembly process or inside the LIBs resulted from the outside pressure. We believe that self-healing, highly conductive and flexible SPEs that can autonomously heal damages can effectively eliminate the safety hazard and significantly enhance the reliability of LIBs. Self-healing/healable polymer materials can repair physical damage and restore original functions to enhance their reliability and extend their service life.17-22 Healability of polymer materials can be realized through the reversibility of noncovalent interactions or dynamic covalent bonds that act as cross-linkers in polymer composites.17-28 Selfhealing/healable polymer composites integrating with novel functions are currently the focus of the research in this area.29-35 Ionic liquids (ILs) have been regarded as a


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superior alternative to conventional organic liquid electrolytes for LIBs because ILs exhibit many advantageous properties including high ionic conductivity, robust thermal and electrochemical stability and fire-retardancy.36,37 ILs have also been incorporated into covalently cross-linked polymer networks to fabricate gel polymer electrolytes for LIBs.38,39 As the polymeric form of ILs, polymeric ionic liquids (PILs) not only inherit the beneficial properties of ILs, but also exhibit superior processability and spatial controllability.34,40-43 Because of the good compatibility and strong electrostatic interactions between ILs and PILs,41,43 gel polymer electrolytes for LIBs can also be fabricated based on PILs-ILs composites.44 However, highly conductive gel polymer electrolytes with capability of healing physical damages have been rarely developed based on ILs and PILs. In this work, healable and flexible ionogel electrolytes with high ion conductivity are fabricated by immobilizing the ILs of 1,2-dimethyl-3-ethoxyethyl imidazolium bis(trifluoromethanesulfonyl) imide (denoted as DE-IM/TFSI) into a hydrogen-bonded network of PIL copolymers bearing ureido-pyrimidinone (UPy) pendant groups (denoted as PIL-UPy). The quadruple hydrogen bonds of UPy moieties and the electrostatic interactions between the DE-IM/TFSI and the PIL-UPy endow the


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ionogel electrolytes with satisfactory mechanical strength even at the loading content of DE-IM/TFSI as high as 67.3 wt%, resulting in ionogels with high ionic conductivity of >10-3 S/cm. Moreover, the multiple noncovalent interactions between the components of the ionogels endow them with capability of healing physical damages. For the proofof-concept, the as-fabricated ionogels are assembled into Li metal batteries (LMBs) in the configuration of Li|Ionogel|LiFePO4. The ionogel is adhesive to the Li anode and LiFePO4 cathode to ensure the intimate and conformal contact between the ionogel electrolyte and electrodes, thus significantly lowering the interfacial resistance and suppressing the Li dendritic growth. The Li|Ionogel|LiFePO4 battery exhibits excellent cycling performance and notable resistance to Li dendritic growth, which delivers a steady high discharge capacity of 147.5 mAh g-1 and Coulombic efficiency of 99.7% after 120 cycles at the current rate of 0.2 C. More importantly, the ionogel electrolyte can autonomously heal damages outside or inside the battery to restore the battery performance even at the circumstance of electrolyte breakage and thus significantly enhance the reliability of the battery.


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RESULT AND DISCUSSIONS Figure 1a shows the synthesis route of the PIL-UPy copolymers that serve as the supporting framework of the ionogels. The PIL-UPy copolymers were synthesized by random copolymerization of two types of pre-synthesized imidazolium-based ionic liquid monomers in which one bears a UPy group (denoted as the IL-UPy monomer) and the other bears an ethoxyethyl group (denoted as the IL-Ether monomer). The structures of the IL-UPy and the IL-Ether monomers were verified via the 1H NMR spectroscopy (Figure S1 and S2, Supporting Information). The PIL-UPy copolymers containing the ILUPy units with molar fractions of 2.0% and 4.1% (denoted as PIL-UPy-2.0% and PILUPy-4.1%) were synthesized and the halide counterions (Brˉ and Clˉ) were finally replaced with bis(trifluoromethylsulfonyl) imide (TFSIˉ) via an ion-exchange process. The structure and compositions of the PIL-UPy copolymers were testified via 1H NMR and Fourier-transform infrared spectroscopy (Figure S3-S5, Supporting Information). Figure 1b illustrates the preparative process of the ionogels derived from the PIL-UPy2.0% copolymers. Ionic liquids of DE-IM/TFSI (Figure S6, Supporting Information) and the LiTFSI salts were dissolved in acetone, and then mixed with the PIL-UPy-2.0%


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copolymers. After solvent volatilization, the mixture of PIL-UPy-2.0% copolymers, DEIM/TFSI and LiTFSI were hot-pressed at 120 C to generate the ionogel membranes. The mass fraction ratios between the DE-IM/TFSI ionic liquids and the PIL-UPy copolymers are varied from 3.0:1, 3.5:1 to 4.0:1 while the mass fraction ratio between the DE-IM/TFSI ionic liquids and LiTFSI salts is kept constant at 5.0:1.0. After the hotpressing process, the PIL-UPy copolymers were cross-linked via the quadruple hydrogen bonds between the UPy moieties to form a supporting network. The ionic liquids of DE-IM/TFSI and LiTFSI salts were immobilized into the supporting network because of their electrostatic interactions with the PIL-UPy-2.0% copolymers. Finally, a piece of free-standing, homogeneous and highly flexible solid-state ionogel membrane was obtained. It was found that the use of the PIL-UPy-4.1% copolymers with a higher molar fraction of the IL-UPy unit results in inhomogeneous ionogel membranes (Figure S7a, Supporting Information). Moreover, the use of the PIL homopolymer without the UPy moieties generates viscous paste-like composites because of the lack of hydrogenbonding cross-linking (Figure S7b, Supporting Information). Accordingly, the ionogels prepared from the PIL-UPy-2.0% copolymers are exclusively studied in this work.


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Figure 1. (a,b) Schematic illustration of the synthesis of the PIL-UPy copolymers (a) and the preparative process of the ionogel membranes (b). (c) Photographs of the flexible Ionogel-3.5 membrane that can be repeatedly folded and unfolded. (d) Schematic illustration of the configuration of the Li|Ionogel|LiFePO4 cell. The ionogel membranes prepared from the PIL-UPy-2.0% copolymers with the mass fraction ratios of DE-IM/TFSI to PIL-UPy copolymers being 3.0:1, 3.5:1 and 4.0:1 are denoted as Ionogel-3.0, Ionogel-3.5 and Ionogel-4.0 membranes, respectively. As


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indicated in Figure 1c, the Ionogel-3.5 membrane with a thickness of 800 m is highly flexible and can be repeatedly folded and unfolded while maintaining its integrity. The mechanical properties of the ionogel membranes are measured by tensile tests at a stretching speed of 10 mm/min, and the ionogel membranes exhibit elasticity despite their strain at break being low. The tensile strength and Young’s modulus of the ionogel membranes decrease with increasing the contents of DE-IM/TFSI, while their strain at break follows the opposite trend (Figure 2a and Table 1). These results can be explained by the fact that the DE-IM/TFSI ionic liquids act as plasticizers to decrease the mechanical strength of the ionogels but improve their deformability.45-47 Specifically, the tensile strength, Young’s modulus and strain at break of the Ionogel-3.5 membranes are 8.8 kPa, 46.9 kPa and 101%, respectively. The elasticity of the Ionogel-3.5 membranes is further testified via cyclic tensile tests involving successive stretchrelease processes at a 25% strain (Figure S8, Supporting Information). The Ionogel-3.5 membrane undergone a stretch-release process can restore to its original stress-strain performance if a 120-min rest is applied, verifying its satisfactory elasticity. Moreover, the ionogel-3.5 membrane exhibits a high adhesion toward the Li metal anode. The


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Ionogel-3.5 membrane that adheres on a Li metal anode with a contact area of ca. 1.3 cm2 can hold a weight of 25 g (inset of Figure 2a). The adhesion between the Ionogel3.5 membrane and Li metal with a contact area of ca. 0.2 cm2 was further studied via the tensile test. It was found that the adhesion strength is even stronger than the mechanical strength of the Ionogel-3.5 membrane (Figure S9, Supporting Information). It was also found that the ionogel-3.5 membrane shows a similar adhesion toward the LiFePO4 cathode. The adhesion of the ionogel-3.5 membranes toward the Li and LiFePO4 electrodes originates from the coordination interactions of Li/Li+ with the ether groups in the ionogels and the satisfactory elasticity of the ionogels enables their intimate contact with Li and LiFePO4 electrodes in an assembled battery (Figure 1d).


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Figure 2. (a) Stress-strain curves of the Ionogel-3.0, Ionogel-3.5 and Ionogel-4.0 membranes, which were obtained via the tensile tests at the stretching speed of 10 mm/min. The dimensions of the membranes between the two clamps used for stretching in the tensile tests were 10 × 5 × 0.8 mm3. Inset is a photograph showing that the Ionogel-3.5 membrane can adhere to the Li foil to hold a weight of 25 g. The contact area between the membrane and the Li foil is ca. 1.3 cm2. (b) Ionic conductivities of the Ionogel-3.0, Ionogel-3.5 and Ionogel-4.0 membranes as a function of temperature. (c,d) Photographs showing an Ionogel-3.5 membrane (c) and a commercial Celgard


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membrane soaked with liquid electrolytes (d) that are exposed to the flame. (e) TGA curves of the Ionogel-3.0, Ionogel-3.5 and Ionogel-4.0 membranes. Table 1. Summary of the Composition and Properties of the Ionogels. Sample

Composition (wt%)

Properties Strain

DE-

PIL-

IM/TFSI

UPy

Ionogels

LiTFSI

at break

Tensile Strength

Young’s Modulus

Ionic conductivity

(%)

(kPa)

(kPa)

(S/cm, 25 °C)

Ionogel-3.0

65.2%

21.7%

13.1%

81%

9.91

71.32

9.15×10-4

Ionogel-3.5

67.3%

19.2%

13.5%

101%

8.84

46.97

1.41×10-3

Ionogel-4.0

69.0%

17.2%

13.8%

135%

5.41

29.51

1.57×10-3

To be used as electrolytes for LIBs, the ion conductivity, electrochemical and thermal stability, and flammability of the as-prepared ionogel membranes are the crucial parameters to be investigated. Figure 2b depicts the temperature-dependent ionic conductivity of various ionogel membranes. The ionic conductivity of all the ionogel membranes exponentially increases with temperature because of the accelerated ion transportation at a higher temperature. The ionic conductivities of the Ionogel-4.0 and Ionogel-3.5 membranes are much higher than that of the Ionogel-3.0 membrane at a


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fixed temperature, whereas the Ionogel-4.0 membrane only show a slightly higher ionic conductivity than the Ionogel-3.5 membrane. The ionogels with higher contents of ionic liquids facilitate the ion transportation, thereby resulting in higher ionic conductivities. It is notable that the Ionogel-4.0 and Ionogel-3.5 membranes show quite high ionic conductivities of 1.57×10-3 S/cm and 1.41×10-3 S/cm at room temperature, respectively, which are comparable to those of the typical liquid electrolytes.48 The electrochemical stability of the ionogel membranes was further evaluated via linear sweep voltammetry using a typical prototype cell configuration of Li|ionogel|stainless-steel (Figure S10, Supporting Information). All these three types of ionogel membranes exhibit a wide electrochemical stability window up to ca. 5 V versus Li+/Li, which is sufficiently stable for their application in LIBs. Nonflammability of the electrolytes is a pivotal property that promotes the safety of LIBs. The intrinsic fire-retardant property of the ionic liquids and the

PIL-UPy

copolymers

endows

the

as-prepared

ionogel

membranes

with

nonflammability.49,50 It can be observed that no combustion occurs when exposing the Ionogel-3.5 membrane to the flame for more than 5 s and the shape and dimension of the membrane can be well maintained after the flaming test (Figure 2c). In a sharp


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contrast, a commercial Celgard membrane soaked with conventional liquid electrolytes (i.e., 1M LiPF6 in ethylene carbonate/dimethyl carbonate, denoted as LiPF6-EC/DMC) immediately catches fire once being exposed to flame (Figure 2d). Meanwhile, the Celgard membrane soaked with the same ionic liquid electrolytes (i.e., 0.5 mol/kg LiTFSI in the DE-IM/TFSI ionic liquids) as that immobilized in the ionogel membranes shrinks dramatically once being exposed to flame, though no combustion is observed (Figure S11, Supporting Information). The thermal stability of the ionogel membranes was further evaluated via thermal gravimetric analysis (TGA, Figure 2e). The decomposition temperatures of all these three kinds of ionogels are around 395 °C and only 5% weight losses are detected below the decomposition temperature, indicating excellent thermal stability of the ionogels. As an electrolyte in a LIB, the good thermal stability of the ionogels can further reduce the safety hazard resulted from battery overheat. Considering that the Ionogel-3.5 and Ionogel-4.0 membranes exhibit comparable ionic conductivities but the Ionogel-3.5 membranes possess a much higher mechanical strength, the Ionogel-3.5 membrane will be investigated as the SPE in LMBs henceforth. The Li+ transference number (tLi + ) of Ionogel-3.5 membrane was


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calculated by Bruce–Vincent–Evans equation using a Li|Ionogel-3.5|Li symmetric cell. The tLi +

of Ionogel-3.5 electrolyte was 0.26 according to the results of

chronoamperometry and AC impedance (Figure S12, Supporting Information). Such a value of tLi + is similar to those measured for some previously reported ionogel electrolytes.51-53 For usage as SPEs in LMBs, the stability of the Ionogel-3.5 membranes against the Li metal electrodes was evaluated via a polarization test of a symmetric Li metal cell (Li|Ionogel-3.5|Li). Figure S13a displays the time-dependent voltage response of the Li|Ionogel-3.5|Li cell that is charged/discharged for 32 min per cycle at a current density of 0.1 mA/cm2. The deposition/dissolution of Li in the Li|Ionogel-3.5|Li cell occurs at a steady overpotential around 0.1 V up to cycling for 610 h (i.e., 1140 cycles) without random fluctuation or short circuit detected. The polarization test of the cell conducted at a higher current density of 0.5 mA/cm2 also reveals a steady overpotential at around 0.32 V up to 310 h of cycling (Figure S14, Supporting Information). The higher overpotential at a higher current density is due to the more intensive polarization effect compared to the cell cycling at a lower current density. In contrast, the symmetric Li


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metal cell using a commercial Celgard membrane socked with the ionic liquid electrolytes (i.e., 0.5 mol/kg LiTFSI in the DE-IM/TFSI ionic liquids) undergoes large voltage fluctuations after cycling for 125 h and short circuit occurs near 150 h (Figure S13b, Supporting Information). Accordingly, the Ionogel-3.5 membrane is highly conductive to facilitate the Li deposition/dissolution in the symmetric Li metal cell and confer superior cycling stability. The performance of a full LMB using the Ionogel-3.5 membrane as an electrolyte was further examined in the cell configuration of Li|Ionogel3.5|LiFePO4 (Figure 1d). Figure 3a depicts the cycling performance of the Li|Ionogel3.5|LiFePO4 cell measured at a charge/discharge rate of 0.2 C at room temperature. The cell delivers a discharge capacity of 141.1 mAh g-1 and a Coulombic efficiency of 93.4% at the first cycle, which respectively increase to 147.9 mAh g-1 and 99.9% at the 15th cycle due to the optimization of the electrolyte/electrode interface. Notably, the Li|Ionogel-3.5|LiFePO4 cell exhibits excellent cycling stability up to 120 cycles with the discharge capacity (147.5 mAh g-1) and Coulombic efficiency (99.7%) negligibly changed compared to those measured at the 15th cycle. The Li metal electrode and the Ionogel-3.5 membrane still exhibit good adhesion after 120 charge/discharge cycles


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(Figure S15, Supporting Information). As a comparison, the LMBs using commercial Celgard membranes (Celgard 2320) soaked with conventional liquid electrolytes (i.e., 1 M LiPF6-EC/DMC) or the ionic liquid electrolytes (i.e., 0.5 mol/kg LiTFSI in DE-IM/TFSI) exhibit gradually decreased discharge capacity and Coulombic efficiency with increasing the number of charge/discharge cycles (Figure 3a). Specifically, the discharge capacities of the Li|Celgard-liquid electrolytes|LiFePO4 and the Li|Celgardionic liquids|LiFePO4 cells only retain 92.6% and 82.2% of their maximum values after 120 cycles, respectively. Rate performance of the Li|Ionogel-3.5|LiFePO4 cell was further tested at different rates that were gradually increased from 0.1 C to 1 C and then decreased back to 0.1 C with 10 cycles for each current rate. The cell can achieve a stable discharge capacity of 153.9, 146.3, 119.4 and 102.3 mAh g-1 at 0.1, 0.2, 0.5 and 1 C, respectively. When the current rate was returned to 0.1 C after 50 cycles, the discharge capacity (154.2 mAh g-1) hardly changed as compared to that measured in the initial 10 cycles at 0.1 C (e.g., 155.3 mAh g-1 for the 10th cycle) (Figure 3b), indicating good cycling performance of the cell at different rates.


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Figure 3. (a) Cycling performances (0.2 C, 25 °C) of the Li|Ionogel-3.5|LiFePO4 cell and the Li/LiFePO4 cells assembled with commercial Celgard membranes soaked with conventional liquid electrolytes (i.e., 1 M LiPF6-EC/DMC) or the ionic liquid electrolytes (i.e., 0.5 mol/kg LiTFSI in DE-IM/TFSI). (b) Rate performance of the Li|Ionogel-3.5|LiFePO4 cell at 25 °C. (c-e) SEM images of the pristine Li metal (c), the lithium anodes obtained from the Li|Ionogel3.5|LiFePO4 (d) and Li|Separator-liquid electrolytes|LiFePO4 (e) cells that underwent 100 charge/discharge cycles at 0.05 mA/cm2. The Li metal anode shown in (d) was measured after removing the ionogel electrolyte and then being rinsed with dimethyl carbonate. To understand the superior cycling performance of the Li|Ionogel-3.5|LiFePO4 cell, the electrochemical kinetic properties of the cell after 1, 10, 50 and 100 cycles at 0.2 C were studied via electrochemical impedance spectroscopy (EIS) (Figure S16, Supporting Information). The Nyquist plots of the cell were simulated via an equivalent circuit model


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to obtain their bulk resistance (Rb) and interface resistance (Rf), which are summarized in Table S1. It can be found that Rb of the cell undergoes a negligible change from 54.5 Ω for the 1st cycle to 55.8 Ω for the 100th cycle, revealing high stability of the Ionogel3.5 membrane over the cycling test.

Besides, Rf of the cell only exhibits a slight

increase from 318.2 Ω for the 1st cycle to 394.1 Ω for the 100th cycle, indicating good interfacial properties between the Ionogel-3.5 membrane and the electrodes due to the excellent interfacial adhesion of Ionogel-3.5 with Li metal and lower reactivity. The superior cycling performance and low impedance change of the Li|Ionogel-3.5|LiFePO4 cell over cycling imply that the Ionogel-3.5 membrane can effectively suppress the dendritic growth on the Li anode. To verify this deduction, the surface morphology of a pristine Li foil and the Li anode in the Li|Ionogel-3.5|LiFePO4 cell after 100 charge/discharge cycles at 0.2 C was examined by scanning electron microscopy (SEM) (Figure 3c and 3d). The Li anode in the Li/LiFePO4 cell using the Calgard separator soaked with liquid electrolytes (i.e., 1 M LiPF6-EC/DMC) was taken as a control (Figure 3e). The pristine Li foil shows a quite neat and smooth surface (Figure 3c and Figure S17a and 17b, Supporting Information). Comparatively, the Li anode in


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the Li|Ionogel-3.5|LiFePO4 cell shows a slightly altered surface morphology after 100 charge/discharge cycles at 0.05 mA/cm2 due to the formation of a solid electrolyte interface (SEI) layer, while its surface is still quite smooth and compact without apparent Li dendrites observed (Figure 3d and Figure S17c and 17d, Supporting Information). In striking contrast, conspicuous and severe Li dendrites are observed on the Li anode in the Li/LiFePO4 cell using the separator/liquid electrolytes after 100 cycles at 0.05 mA/cm2 (Figure 3e, Figure S17e and 17f, Supporting Information). The significant suppression of the Li dendrites by the Ionogel-3.5 membrane benefited from its combined properties of good flexibility, elasticity and strong adhesion toward the Li anode. All these properties can ensure the intimate and conformal contact between the ionogel membrane and the Li anode and enable the Ionogel-3.5 membrane to become adaptive to the morphology change of the Li anode surface in the process of Li stripping/deposition at the Li anode surface. All the above-mentioned factors can facilitate the formation of a homogeneous SEI layer on the Li anode surface, resulting in the uniform Li deposition and the suppression of Li dendritic growth.


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Our previous study indicates that the electrostatic interactions between charged imidazolium and TFSIˉ groups can enable healing of mechanical damage in PIL copolymers.34 The as-prepared Ionogel-3.5 membranes contain quadruple hydrogen bonds between the UPy moieties and electrostatic interactions between the charged imidazolium and TFSIˉ groups. The reversible and dynamic nature of the hydrogen bonds and electrostatic interactions endow the Ionogel-3.5 membrane with the ability to heal damage. As shown in Figure 4a and 4b, the cut Ionogel-3.5 membrane can be healed together outside or inside a Li/LiFePO4 button cell at 55 °C for 1 h. The rheological tests on the Ionogel-3.5 sample at both 25°C and 55 °C indicate that the ionogel can keep solid gel state at both temperatures. (Figure S18, Supporting Information). Figure 4c shows the typical stress-strain curves of an intact and a cuthealed Ionogel-3.5 membrane. The tensile strength and strain at break of the cut-healed Ionogel-3.5 membrane can reach 70.5% and 79.3% of the corresponding values for the intact membrane. The optical microscope image in the inset of Figure 4c indicates that the cut Ionogel-3.5 membrane is finely healed. It is noteworthy that the cut-healed Ionogel-3.5 membranes, no matter healed inside or outside the button cell, can be


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further used as SPEs in the Li/LiFePO4 cells. The Li/LiFePO4 cells using the healed Ionogel-3.5 membrane as SPEs still exhibit good cycling stability (Figure 4d and Figure S19, Supporting Information). Figure 4d shows the cycling performance of the Li/LiFePO4 cell using a cut Ionogel-3.5 membrane that was healed inside the cell at 55 °C for 1 h. The discharge capacity and Coulombic efficiency of the cell using the healed Ionogel-3.5 membrane are quite steady during 50 charge/discharge cycles. Specifically, the discharge capacity and Coulombic efficiency of the cell after 50 cycles at 0.2 C are 137.1 mAh g-1 and 98.9%, respectively. These values are only subtly decreased compared with those measured for the Li/LiFePO4 cell using the as-prepared Ionogel3.5 membrane at the 50th cycle (147.2 mAh g-1 and 99.4%). In contrast, the Li/LiFePO4 cell assembled with the cut Ionogel-3.5 membrane without healing undergoes a quick failure after only 10 cycles, as evidenced by the sharp drop of the discharge capacity and Coulombic efficiency (Figure 4d). Note that the two parts of the cut Ionogel-3.5 membrane still partially contact with each other (Figure S20, Supporting Information) in the cell due to the confined space and pressure applied when assembling the cell (This mimics the situation where the solid state polyelectrolyte membrane is partially broken).


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In this case, Li+ can still be conducted in the thickness direction around the cutting area, while the conductance will be significantly suppressed. Subsequently, the cut of the Ionogel-3.5 membrane will induce significant inhomogeneous Li deposition/stripping on the Li metal anode, which in turn results in preferential Li dendrite growth on the Li anode around the cutting area due to the weak mechanical strength of the cutting area (Figure S20 and Figure S21a, Supporting Information). In contrast, the healed electrolyte after damage can still effectively prevent the Li dendrite growth and ensure the electrochemical performance of the battery (Figure S21b, Supporting Information). Therefore, when used as SPEs in LMBs, the healing capability of the Ionogel-3.5 membranes derived from the PIL-UPy polymers can significantly enhance the reliability and reduce the safety hazard of the battery.


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Figure 4 (a,b) Photos of the Ionogel-3.5 membranes that were cut into two pieces and then healed outside (a) or inside (b) a Li/LiFePO4 button cell at 55 °C for 1h. (c) Stress-strain curves of the intact and cut-healed Ionogel-3.5 membranes. Inset is the optical microscope images of the Ionogel-3.5 membrane that was cut into two pieces (left) and then healed (right) at 55 °C for 1h. (d) Cycling performances (0.2 C, 25 °C) of Li/Ionogel-3.5/LiFePO4 cells, one of which was assembled with an Ionogel-3.5 membrane that was cut into two pieces and the other was


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assembled with a cut-healed Ionogel-3.5 membrane that was previously cut into two pieces and then healed inside the cell at 55 °C for 1h. CONCLUSIONS In summary, we have demonstrated the fabrication of healable, highly conductive, nonflammable and flexible ionogel electrolytes for use in durable LIBs by immobilizing high contents of ionic liquids and Li salts within hydrogen-bonded PIL-UPy supramolecular polymer networks. The Ionogel-3.5 membranes are healable, nonflammable, highly ionic conductive (>10-3 S/cm) and capable of suppressing the growth of Li dendrites in Li|Ionogel-3.5|LiFePO4 cells. All these favorable merits make the Ionogel-3.5 membranes to be a kind of outstanding solid-state electrolytes for the fabrication of durable LIBs with enhanced safety and reliability. The quadruple hydrogen bonds between the UPy moieties endow the Ionogel-3.5 membranes with the capability of healing physical damages outside or inside a LIB. The healability of the Ionogel-3.5 membranes can effectively reduce the risk of battery failure resulted from their accidental breakage and prolong the service life of the batteries. Because the flexible and healable Ionogel-3.5 membranes can serve as electrolytes and separators at the


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same time, they show great promise for the development of next-generation flexible and healable LIBs with highly enhanced reliability and elongated service life. This study paves a new avenue for the fabrication of healable solid polymer electrolytes for use in durable and reliable LIBs.

EXPERIMENTAL SECTION Preparation of Ionogel Membranes. The ionogel membranes were prepared via dissolving PIL-UPy-2.0% copolymers, DE-IM/TFSI ionic liquids and LiTFSI in acetone, followed by evaporation of acetone and a hot-pressing process. The mass fraction of the LiTFSI component in the ionogels was kept as 13%, whereas the mass fraction ratios between the DE-IM/TFSI and the PIL-UPy-2.0% copolymers were varied from 3.0:1, 3.5:1 to 4.0:1. The as-prepared ionogels are accordingly referred to as Ionogel-3.0, Ionogel-3.5 and Ionogel-4.0, respectively. Taking the preparation of the Ionogel-3.5 membrane as an example, the PIL-UPy-2.0% powder (0.06 g), DE-IM/TFSI (0.21 g) and LiTFSI (0.042 g) were dissolved in acetone (1 mL), and the mixture solution was then transferred into a round-shaped Teflon mold with a diameter of 18 mm. To evaporate acetone, the mixture solution was incubated in an oven at 40 °C for 48 h, followed by drying the resultant concentrated solution at 100 °C for 36 h in a vacuum oven. The as-obtained gel-like material was further hot-pressed in a Teflon mold (height 0.8 mm, diameter 18 mm) at 120 °C under the pressure of 5 MPa for 10 min to generate the final ionogel membrane. The as-


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prepared ionogel membrane was stored in an argon-filled glove box (H2O < 0.1 ppm, O2 < 0.1 ppm) for at least 72 h before its assembly into a LMB. Cell Assembly. LiFePO4-based cathode was prepared by pasting a mixture of LiFePO4 powders, carbon black (Super-P) and PVDF as binder at a weight ratio of 80:10:10 on Al foil. The electrodes were dried in a vacuum oven at 100 °C for 36 h and subsequently cut into circles (D = 11 mm) with the loading content of the LiFePO4 being ~1.5 mg cm-2. The lithium metal foil (battery grade) was used as the anode. To assemble a Li|Ionogel-3.5|LiFePO4 cell, the Ionogel3.5 membrane was sandwiched between the LiFePO4 cathode and the Li anode and then packaged into a CR2025 coin cell. Similarly, the commercial Celgard separators soaked with conventional liquid electrolytes (1 M LiPF6 in EC/DMC =1/1, v/v) or the DE-IM ionic liquid electrolytes (i.e., 0.5 mol/kg LiTFSI in DE-IM/TFSI) were sandwiched between the LiFePO4 cathode and the Li anode, and then packaged into a CR2025 coin cell. The symmetric Li|Ionogel3.5|Li was assembled by sandwiching the Ionogel-3.5 membrane between two Li metal foils followed by packaging in a CR2025 coin cell. The whole process for the cell assembly was conducted in an argon-filled UNILAB glove box. Instruments and Characterizations. 1H-NMR (500 MHz) spectra were recorded on a Bruker AVANCE III spectrometer. The Fourier transform infrared (FTIR) analysis was carried out on a VERTEX 80V FTIR spectrometer (Brucker, USA) under vacuum. The TGA measurements were conducted on the TA Instruments Q5000 in the temperature range of 30900 °C under N2 atmosphere at the heating rate of 10 °C min-1. The tensile tests were conducted on a universal testing machine (Shimadzu AG-I 4 N) under ambient conditions. The morphology of Li anodes and pristine Li foils were characterized by XL30 ESEM FEG scanning electron microscope. The ionic conductivities (σ) of the ionogels were measured using a Biologic (VMP3)


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electrochemical workstation. The data were collected in the frequency range from 100 mHz to 0.1 MHz with an amplitude of 5 mV at an open circuit potential from 25 °C to 80 °C. The ionic conductivities of the ionogels were calculated using the following equation: σ = L/RS, where R is the bulk ohmic resistance of the ionogels, S and L are the area and thickness of the ionogel membrane, respectively. Electrochemical impedance spectra were recorded on a Biologic (VMP3) electrochemical workstation in the frequency range from 1 MHz to 100 mHz with a potential static signal amplitude of 1 mV. The impedance data were analyzed with ZViewImpedance Software. The cycling tests on various cells were performed on a CT2001A cell test instrument (LAND Electronic Co., Ltd.) at 25 °C. The measured lithium ion transference number (tLi + ) was measured by the Bruce and Vincent method,54 a polarization voltage of 10 mV was applied to the symmetrical Li|Ionogel-3.5|Li, and the initial current I0 and the steady-state current Is were measured at 25 °C. Electrochemical impedance spectroscopy was carried out before and after polarization to obtain the initial and the steady-state interfacial resistances, R0 and Rs. The lithium ion transference number (tLi + ) was calculated by the following Bruce–Vincent–Evans equation: tLi + =

𝐼𝑠 △ 𝑉 ― 𝐼0𝑅0 𝐼0 △ 𝑉 ― 𝐼𝑠𝑅𝑠

(1)

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website.


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Details for synthesis, 1H NMR spectra of 3-chloropropyl-UPy, IL-UPy monomer, ILEther monomer, PIL-UPy copolymers and DE-IM/TFSI ionic liquids; FTIR spectra of the PIL-UPy-2.0% copolymers; Photographs of the ionogels prepared from the PIL-UPy-4.1% copolymers and the PIL homopolymer without the UPy moieties; Stress−strain curves of the Ionogel-3.5 membrane; Linear sweep voltammograms using different ionogels; Photograph of commercial Celgard separator soaked with ionic liquid exposed to flame; DC polarization curve and EIS under initial and steady-state current conditions; Voltage profiles of the Li|Ionogel-3.5|Li cell and Li|Separator-ionic liquid|Li cell; Adhesion of Ionogel-3.5 membranetoward Li metal electrode; Experimental and simulated EIS curves of the Li|Ionogel-3.5| LiFePO4 cell; Summary of the Rb and Rf of the Li|Ionogel3.5| LiFePO4 cell; Rheological tests of the Ionogel-3.5 membrane and cycling performance of the Li/LiFePO4 cell with a cut-healed Ionogel-3.5 membrane. SEM images the Li metal anodes obtained from the Li|Ionogel-3.5|LiFePO4. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]


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Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (NSFC grant No. 21225419) and the Ministry of Science and Technology of China (No. 2015CB251103).

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Table of Contents Graphic


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Healable, Highly Conductive, Flexible, and Nonflammable

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