zhang et al-advanced materials

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Polymerized Ionic Networks with High Charge Density: Quasi-Solid Electrolytes in Lithium-Metal Batteries

Pengfei Zhang , Mingtao Li ,* Bolun Yang , Youxing Fang , Xueguang Jiang , Gabriel M. Veith , Xiao-Guang Sun , and Sheng Dai*

Dr. P. Zhang, Prof. M. Li, Dr. X.-G. Sun, Prof. S. Dai Chemical Sciences Division Oak Ridge National Laboratory Oak Ridge , TN 37831 , USAE-mail: [email protected]; [email protected] Prof. M. Li, Prof. B. Yang School of Chemical Engineering and Technology Xi’an Jiaotong University Xi’an , Shaanxi 710049 , China Dr. Y. Fang, Dr. X. Jiang, Prof. S. Dai Department of Chemistry University of Tennessee Knoxville , TN 37996 , USA Dr. G. M. Veith Materials Science and Technology Division Oak Ridge National Laboratory Oak Ridge , TN 37831 , USA

DOI: 10.1002/adma.201502855

electrodeposition of Li during repeated charge–discharge cycles leads to the formation and proliferation of dendrites from the anode to the cathode, which could cause catastrophic cell failure with the risk of thermal runaway-induced fi re if a fl am-mable electrolyte is employed. [ 8 ]

To address these issues, various solid electrolytes (e.g., Li nitride, polymer electrolyte) have been widely investigated as alternatives for organic liquid electrolytes. [ 9 ] Polymer electro-lytes are one of the promising classes; however, the popular complexes of an Li salt and polyethylene oxide have very low conductivities at room temperature (<10 −5 S cm −1 ). [ 10 ] In solid polymer electrolytes, ionic conductivities are governed by both the segmental motion of the chains and the number of dissociated carrier ions and their mobility. [ 11 ] As was noted earlier, PILs may have the potential to serve as polymer elec-trolytes for advanced batteries, and the charge density of the PILs is an important factor controlling ionic conductivity. For example, Yang and co-workers illustrated that gel polymer electrolytes based on dicationic PILs showed an ionic conduc-tivity of around 4.6 × 10 −5 S cm −1 at 25 °C, which was superior to the performance of the composite electrolytes containing monocationic PILs (e.g., ≈10 −5 S cm −1 at 25 °C). [ 12 ] Recently, Drockenmuller and co-workers demonstrated the facile syn-thesis of 1,2,3-triazolium-based PIL networks with an anhy-drous ionic conductivity of 2 × 10 −8 S cm −1 at 30 °C. [ 13 ] Unfor-tunately, there is still a large difference between these polymer materials and the state-of-the-art organic liquid electrolytes in Li-ion batteries (around 10 −2 S cm −1 at room temperature) with respect to ionic conductivity. To design high ion conduc-tivity polymers, the density of ion pairs must be increased.

In this contribution, we demonstrate an interesting class of PILs with a high charge density (six ion pairs per repeating unit)—polymerized ionic networks (PINs). Their macro-molecular backbones build on two simple and effi cient path-ways ( Scheme 1 ): (1) a direct nucleophilic substitution reaction between hexakis(bromomethyl)benzene and 4,4-bipyridine, which enables ionization and polymerization in one process, and (2) a free radical polymerization of the ionic monomer bearing six 1-vinylimidazolium cations. Solid-like ternary electrolytes with PINs as the polymer hosts and LiTFSI in 1-ethyl-3-methyl-imidazolium bis(trifl uoromethane)sulfonimide (EMIM-TFSI) (0.5 mol kg −1 ) as the plasticizer exhibit good ionic conductivi-ties (up to 5.32 × 10 −3 S cm −1 at 22 °C), wide electrochemical stability windows (up to 5.6 V), and good interfacial compat-ibility with the electrodes. Moreover, an Li/LiFePO 4 battery assembled with the PIN-based electrolytes possesses an initial high discharge capacity of 146 mA h g −1 at 25 °C.

In the past two decades, poly(ionic liquid)s (PILs), a new family of functional polymers, have attracted increasing interest owing to their promising performance in various fi elds such as mem-brane science, catalysis, analytical chemistry, CO 2 capture, and electrochemistry. [ 1 ] In general, PILs are prepared by conven-tional or controlled radical polymerization of ionic liquid (IL) monomers bearing vinyl groups. [ 2 ] PILs with designable cations and anions afford several unique properties, such as ionic con-ductivity, processability, self-assembly to colloidal nanoparticles, and controllable porosity. [ 3 ] For instance, a family of porous PIL membranes have been fabricated by a template-free strategy. [ 3d,e ] Actually, PILs were originally designed as solid ionic conductors by the Ohno group in 1998. [ 4 ] The rationale behind the above strategy is that the presence of an IL moiety in the repeating unit of the PIL chains can integrate some desirable IL characteristics (e.g., high ionic conductivity) into the polymeric architecture. [ 5 ] Therefore, PILs could be considered potential solid electrolytes, a topic of signifi cant interest in the fi eld of electrochemistry.

Meanwhile, lithium (Li) metal batteries are considered as one of the most attractive candidates for next-generation energy storage systems. [ 6 ] For battery applications, Li metal possesses a number of advantages, such as high theoretical capacity (3860 mA h g −1 ), and the lowest negative electrochemical poten-tial (–3.04 V vs a standard hydrogen electrode). [ 7 ] However, several issues, such as serious safety and/or cell lifetime prob-lems induced by dendrite formation or the instability of the electrolyte (e.g., leakage of liquid electrolytes), continue to obstruct the development of the Li metal batteries for practical applications. It is widely acknowledged that the uneven

Adv. Mater. 2015, DOI: 10.1002/adma.201502855

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The synthesis of PIN-1 involved the cross-linking of a hexakis(bromomethyl)benzene tecton with a 4,4-bipyridine linker via a nucleophilic substitution mechanism, [ 14 ] followed by an ion-exchange process to introduce TFSI anions into the matrix. This pathway coupled alkylation and polymerization in one reaction and proceeded under mild conditions. At 80 °C, the solution became yellow and precipitated in several min-utes. The other route for synthesizing PIN-3-Br proceeded by the polymerization of monomers densely substituted by 1-vinylimidazolium cations, a precursor recently developed by the Poly(ionic liquid)s Group in Germany. [ 15 ] The temperature required for current radical polymerization (120 °C) is higher than that used for the synthesis of polyvinylimidazolium

nanoparticles (65 °C). The current reaction was performed in solvothermal conditions with autogenic pressure, thereby leading to a solid electrolyte material with a higher degree of cross-linking. All the PIN samples are insoluble in organic solvents (e.g., ethanol, tetrahydrofuran, and N , N -dimethylfor-maldehyde) or water, possibly owing to the formation of 3D networks. The anion exchange from Br − to TFSI − was aimed at achieving a wider electrochemical voltage window and better compatibility with EMIM-TFSI and the electrodes. The anion exchange process of PINs was carried out in a 15 wt% LiTFSI aqueous solution at 50 °C for easier accessibility of the Br − anions trapped in the highly cross-linked networks. After the anion exchange, four peaks in the Fourier transform infrared



Scheme 1. The reaction route to obtain polymeric ionic networks (PINs): a) PIN-1. b) PIN-2. c) PIN-3. d) PIN-4.

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(FTIR) spectra appeared (1344, 1177, 1128, and 1050 cm −1 ) that could be assigned to the vibration sorption of TFSI anions ( ν a (SO 2 ), ν a (CF 3 ), ν s (SO 2 ), and ν a (S–N–S), respectively ( Figure 1 a,b).

The radical polymerization of PIN-3-mono was also charac-terized by FTIR, during which the intensity of the characteristic peak for the vinyl group (1654 cm −1 ) signifi cantly decreased (Figure 1 b). Based on the integrated signal areas, the ratio of vinyl group before and after polymerization is 4.43:1, and the degree of polymerization of PIN-3 (the fraction of reacted repeating units) is around 77.4% (Figure 1 c). A higher degree of polymerization was observed (93.1%) in the PIN-4 sample, suggesting that most vinyl moieties had already been incorporated into the cross-linked polymer network (Figures S1 and S2, Supporting Infor-mation). The monomer conversion of PIN-2 was also investi-gated by nuclear magnetic resonance (NMR) measurements (note that the hexakis(bromomethyl)benzene monomer cannot be completely dissolved in N -methyl-2-pyrrolidinone, resulting in the study of PIN-1 synthesis by NMR impossible). The peak at δ = 4.7 ppm (for –CH 2 –Br in 1,3,5-tris(bromomethyl)benzene) shifts to 6.0 ppm after the nucleophilic substitution by 4,4′-bipy-ridine, and the ratio of reacted –CH 2 Br can be used to esti-mate the degree of cross-linking (Figure 1 d and Figure S3–S6,

Supporting Information). The degree of polymerization reached 49.2% in ≈5 h at room temperature, after when a lot of solids were formed in the solution.

The structure of the PINs was investigated at the molecular level by solid-state 13 C cross-polarization magic angle spinning nuclear NMR spectroscopy. [ 14a,b,e ] In PIN-1-Br, the signal at 145.7 ppm (Figure 1 e) is attributed to the pyridinium carbon (C-2, C-4, and C-6 positions) bearing the positive charge, and peak C (128.1 ppm) is assigned to the carbons at the C-3 and C-5 positions. The B peak at 135.5 ppm is ascribed to the substituted phenyl carbons. The peak at 63.7 ppm should be the result of the benzylic carbon atom close to the positive unit. The NMR spec-trum of PIN-3-Br is shown in Figure 1 f. The high-intensity peak at 138.2 ppm is considered the overlapped contribution of the phenyl carbons and the imidazolium carbon at the C-2 position, and the carbon atoms at the C-4 and C-5 positions of the imi-dazolium ring lead to the peak at 124.9 ppm. The broad signal from 22.5 to 64.0 ppm consists of three peaks at 55.4, 49.9, and 40.2 ppm, which can be induced by the benzylic carbon atoms and the 1-ethyl group of the imidazolium ring, respectively.

Figure 2 a illustrates the thermogravimetric analysis curves of the PIN samples, which are thermally stable up to 250 °C, well above the melting point of Li metal. X-ray diffraction



Figure 1. a–c) Fourier transform infrared spectra of PIN samples. d) 1 H nuclear magnetic resonance (NMR) spectra of samples (in D 6 -DMSO) during the synthesis of PIN-2-Br, reaction condition: 1,3,5-tris(bromomethyl)benzene (1 mmol), 4,4′-dipyridyl (1.5 mmol), N -methyl-2-pyrrolidinone (15 mL) solvent, stirring at room temperature for 165 min and at 40 °C for 140 min. e) Solid-state 13 C NMR spectrum of the PIN-1-Br sample. f) Solid-state 13 C NMR spectrum of the PIN-3-Br sample.


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(XRD) measurements of PINs exhibited two broad refl ections around 22° and 40°; thus, these polymers were in an amor-phous state (Figure 2 b). It can be seen in the scanning electron

microscopy (SEM) image that the PIN samples are made of secondary irregular par-ticles with rough surfaces (Figure 2 c,d). The particle size of PIN-1 (≈21.0 µm) is much bigger than that of PIN-3 (≈300–600 nm) (Figure S7, Supporting Information).

Predetermined amounts of the as-pre-pared PINs were directly blended with LiTFSI or LiTFSI in an EMIM-TFSI solution (0.5 mol kg −1 ) to produce hybrid electrolytes ( Figure 3 a, Table S1, Supporting Informa-tion). As shown in the photographs, the PIN-1@LiTFSI sample was processed into a solid electrolyte membrane, which exhibited moderate ionic conductivity at room tempera-ture (≈1.17 × 10 −4 S cm −1 ) (Figure 3 b). The positively charged backbone of the PINs was expected to interact with TFSI anions by elec-trostatic force and thus liberates Li + for faster mobility along the highly charged channels (Figure 3 a). To understand the coupled struc-ture, XRD patterns of PINs@LiTFSI samples were collected. It is interesting that the crys-talline peak of LiTFSI disappeared after it was loaded into the PIN hosts (Figure 3 c). All the PINs@LiTFSI samples afforded extremely

broad peaks, similar to those of amorphous PINs. It seems that the strong electrostatic interaction and the sharing of the same anion between the LiTFSI and the PINs may restructure the



Figure 2. a) Thermogravimetric analysis curves of PIN samples in an N 2 atmosphere; heating rate: 10 °C min −1 . b) X-ray diffraction patterns of PIN samples. c) Scanning electron microscopy (SEM) image of PIN-1. d) SEM image of the PIN-3 sample.

Figure 3. a) Schematic representation of the PIN-1@LiTFSI-EMIMTFSI hybrid electrolyte used in the study. b) A photograph of a PIN-3@LiTFSI solid membrane. c) X-ray diffraction patterns of PIN@LiTFSI samples. d) N1s X-ray photoelectron spectroscopy curves of PIN-3 and controlled samples with LiTFSI (28.7 wt%) or EMIM-TFSI (190 wt%) additives. e) A photograph of PIN-3@LiTFSI-EMIMTFSI.


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ion pair structure of LiTFSI. Because both positive and nega-tive charges are borne on the nitrogen atoms of PINs, N1s X-ray photoelectron spectroscopy (XPS) measurements were then carried out to investigate the electrostatic interaction. The N1s peaks of the PIN samples were broad, suggesting the coexist-ence of different nitrogen species. The peak was fi tted with a Shirley background and Gaussian–Lorenz model functions, and two peaks at ≈399.2 and ≈401.5 eV were obtained, which could be assigned to the pyridinum N + form and N − species in the TFSI anion, respectively (Figure 3 d). [ 16 ] The N1s XPS spectra of PIN-3@LiTFSI (1/0.287, wt/wt) with more TFSI anions afforded a higher intensity of peak for N − atoms; and only one peak corresponding to the N − species in TFSI anion was observed, indicating the homogeneous distribution of LiTFSI anions in the PIN-3 host. Similar results were also collected for the PIN-3@EMIMTFSI sample (1/1.9, wt/wt). Thus, it is con-cluded that PINs with highly charged structures can provide a solid, mechanically strong matrix alone or together with other ionic compounds for the fabrication of solid electrolytes.

In consideration of the interfacial property toward the elec-trode material and the desired ionic conductivity, 0.5 mol kg −1 LiTFSI in EMIM-TFSI was incorporated into PINs to fabricate solid-like electrolytes (Figure 3 e, PIN@LiTFSI-EMIMTFSI). Compared with the ratio of ILs doped in the monocationic PIL (e.g., 65 wt%) or the dicationic PIL (e.g., 95 wt%), a signifi cantly enhanced value was observed in PINs (up to 190 wt%) as the polymer host (Table S1, Supporting Information). [ 12 ] Such a large difference in the IL incorporation capability can be attributed to the stronger intermolecular interaction between ILs and the highly charged PINs, which has also been observed by Yang and co-workers. [ 12a ] Initially, the electrochemical stability of the PIN@LiTFSI-EMIMTFSI electrolytes at 25 °C was studied by linear sweep voltammetry, as shown in Figure 4 a. Except for PIN-3 (≈4.7 V vs Li/Li + ), all other PIN-based electrolytes are stable above 5.0 V versus Li/Li + . In particular, the decomposition of PIN-2 occurs around 5.6 V versus Li/Li + , which makes it suitable for application in high-voltage batteries. Based on the decomposing voltage, it can be concluded that PINs linked by 4,4′-bipyridine (PIN-1 and PIN-2) are more stable than those from the polymer-ized vinyl groups (PIN-3 and PIN-4). It seems reasonable since the backbone with bipyridine linker is more stable than that linked by the C–C bond. It can also be observed that PINs with three substitution groups are more stable than the PINs with six substitutions (PIN-2 > PIN-1, PIN-4 > PIN-3). The steric hin-drance of PIN-1 and PIN-3 with six big substituted groups makes them more decomposable, in comparison with PIN-2 and PIN-4.

Ion-conducting properties of PIN@LiTFSI-EMIMTFSI elec-trolytes under hydrous conditions were assessed by AC imped-ance techniques, and the ionic conductivities as a function of temperature for those samples are presented in Figure 4 b. As expected, the PIN samples with a higher charge density (PIN-1 vs PIN-2 and PIN-3 vs PIN-4) afforded a higher ionic conductivity; the charge density of the polymer might be a factor affecting the transport of Li + ions. It is understandable that the ionic conductivities of the PIN-based samples increase under higher temperatures, roughly following Vogel–Tamman–Fulcher-type behavior. The increase indicates that the ionic conductivity of the electrolytes is closely correlated with the viscosity of the solution, and the ion mobilities in the electrolytes are mainly

determined by the property of the LiTFSI-EMIMTFSI electro-lytes. It is important that all the solid-like electrolytes possess good ionic conductivities on the order of ≈10 −2 −10 −3 S cm −1 at 22 °C, and the optimized sample of PIN-3@LiTFSI-EMIMTFSI exhibits an ionic conductivity of 5.32 × 10 −3 S cm −1 at 22 °C, a very high value for a polymer-based solid electrolyte.

Lithium redox in the PIN@LiTFSI-EMIMTFSI electrolytes was characterized by cyclic voltammograms (CVs) with an Li/electro-lyte/Pt cell, as shown in Figure 4 c,d and in Figure S8 (Supporting Information). The plating and stripping of Li on the nickel elec-trode can be clearly observed, as in the EMIM-TFSI IL electro-lytes. [ 17 ] In the fi rst cycle for the PIN-1-based electrolyte and the PIN-3-based electrolyte, the cathodic peaks corresponding to the plating of Li are about –0.5 and –0.38 V versus Li/Li + , respectively; and in the anodic scan, the peaks corresponding to the stripping of Li are around 0.14 and 0.10 V versus Li/Li + , respectively. It is noteworthy that in case of the Pt electrode, many peaks due to the formation of Li–Pt alloys are observed, such as 0.62 and 0.70 V versus Li/Li + in Figure 4 c,d, respectively. [ 17b,c ] The Li redox in the PIN@LiTFSI-EMIMTFSI electrolytes could be generated by forming a solid electrolyte interface (SEI) on the platinum electrode; the CV curves do not change obviously in the following cycles, suggesting that the SEI fi lm formed is stable. The revers-ible plating and stripping of Li in the electrolytes and the stable SEI fi lm generated indicate the PIN@LiTFSI-EMIMTFSI electro-lytes are suitable for application in Li batteries.

As an example, Li/LiFePO 4 batteries with the electrolyte PIN-1@LiTFSI-EMIMTFSI or PIN-3@LiTFSI-EMIMTFSI were assembled, and their cycling performances were charac-terized at room temperature (Figure 4 e,f). The batteries with PIN-1 and PIN-3 show good initial discharge capacity of 146 and 143 mA h g −1 , respectively, and their corresponding Coulombic effi ciencies reach 97.5% and 96.3%. These results indicate that the PIN-based solid-like electrolytes are promising candidates for use in Li metal batteries. However, subsequently, the battery capacity decreases slowly with cycling (Figure 4 g–h, Figure S9, Supporting Information). It is noted that the solid-like electrolytes are different from the traditional sticky polymer electrolytes. It is found that our battery performance is closely associated with the thickness of the solid-like electrolyte wafer and the wafer-shaped stress. The optimization of the PIN-based electrolytes is ongoing. Anyway, these results indicate that the PIN-based solid-like elec-trolytes are potential candidates for use in Li metal batteries.

In summary, PINs, an interesting class of charged frameworks, were designed using two easy strategies, extending the scope of the ionic polymer library from one charge per unit to densely charged networks. Nucleophilic-substitution-mediated polymerization, particularly, provides a direct, effective, versatile pathway to obtain ionic polymers. Moreover, it has been shown that it is possible to create solid-like polymer electrolytes with acceptable ionic conduc-tivities at room temperature (5.32 × 10 −3 S cm −1 ). The key point of this strategy is that a facile channel for Li + mobility in a solid-like condition can be easily achieved via the introduction of PINs as the electrolyte matrix. The high charge density of PINs provides abun-dant, weakly coordinating sites during Li + movement via electro-static interaction. Meanwhile, their hierarchical and robust back-bone not only provides a rich void for Li + mobility, but also con-tributes to the mechanical strength of the electrolyte membrane. We believe that PIN-based solids or solid-like electrolytes might


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provide an alternative route for safe Li batteries. PILs were unveiled around 17 years ago, but their unexpected potentials in material science are a young topic, full of promise. In principle, the present fi ndings (PINs and nucleophilic-substitution-mediated polymerization) can be extended to a number of ionic organic materials for specifi c tasks by careful selection of monomers.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements P.Z. and X.J. (polymer synthesis and characterization) were supported as part of the Fluid Interface Reactions, Structures and Transport

(FIRST) Center, an Energy Frontier Research Center funded by the US Department of Energy, Offi ce of Science, Offi ce of Basic Energy Sciences. Y.F., G.V., X.S., and S.D. (XPS and battery characterizations) were supported by the U.S. Department of Energy (DOE), Basic Energy Sciences (BES), Materials Sciences and Engineering Division. M.L. appreciates the fi nancial support from the National Natural Science Foundation of China (21303132).

Received: June 13, 2015 Revised: August 7, 2015

Published online:

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Figure 4. a) Linear sweep voltammetry plots of a Li cell with PIN@LiTFSI-EMIMTFSI solid-like electrolytes. b) Temperature dependence of the ionic conductivity of PIN@LiTFSI-EMIMTFSI solid electrolytes. c,d) Cyclic voltammograms for PIN@LiTFSI-EMIMTFSI electrolytes. Working electrode: platinum; counter electrode and reference electrode: lithium; scan rate: 10 mV s −1 . e,f) Charge–discharge curves of Li/PIN@LiTFSI-EMIMTFSI/LiFePO 4 cells at room temperature. g,h) Discharge capacity versus cycle number of the two hybrid electrolyte systems.


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