Letter www.acsami.org
Improving Ionic Conductivity and Lithium-Ion Transference Number in Lithium-Ion Battery Separators Raphael Zahn, Marie Francine Lagadec, Michael Hess, and Vanessa Wood* Laboratory for Nanoelectronics, Department of Information Technology and Electrical Engineering, Eidgenoessische Technische Hochschule Zurich, 8092 Zurich, Switzerland S Supporting Information *
ABSTRACT: The microstructure of lithium-ion battery separators plays an important role in separator performance; however, here we show that a geometrical analysis falls short in predicting the lithium-ion transport in the electrolyte-filled pore space. By systematically modifying the surface chemistry of a commercial polyethylene separator while keeping its microstructure unchanged, we demonstrate that surface chemistry, which alters separator−electrolyte interactions, influences ionic conductivity and lithium-ion transference number. Changes in separator surface chemistry, particularly those that increase lithium-ion transference numbers can reduce voltage drops across the separator and improve C-rate capability. KEYWORDS: lithium-ion battery, separator, electrolyte conductivity, lithium-ion transference number, surface modification, polyelectrolyte, layer-by-layer quantified microstructure, where ε and τ have been determined by SEM-FIB tomography.11 Building upon previous work where polyolefin separators have been successfully modified with polyelectrolytes,12,13 we functionalize the PE separator with three polyelectrolytes having different surface charge and chemical properties (Table S1), enabling us to influence the interaction of the separator surface with lithium ions and solvent molecules of LIB electrolytes. Because a polyelectrolyte layer thickness is just a few nanometers14−16 whereas the pores of the PE separator are tens of nanometers1 in diameter, polyelectrolytes present an excellent set of model materials for systematically varying the surface chemistry of the separator while leaving the separator microstructure essentially unchanged. Characterizing the physical, chemical, and electrochemical properties of the unmodified and polyelectrolyte-modified separators, we determine that both microstructure and surface chemistry must be considered to obtain a realistic picture of separator performance in LIBs. Polyelectrolyte-modified separators show improved values for σ and t+ and reduce the voltage drop across the separator and enable superior C-rate capability when placed into LIB half-cells. We discuss the origins of these improvements in terms of separator−electrolyte interactions and, with the support of COMSOL simulations, show that improvements in battery performance come predominately from an increased t+. This work highlights that with the push in industry to high cycling rates, the structure and surface chemistry of the separator cannot be neglected.
L
i-ion battery (LIB) separators are porous membranes that electronically isolate the positive and negative electrodes yet allow ionic transport between them.1,2 Most common are polyolefin separators, < 25 μm-thick membranes of polyethylene (PE) and/or polypropylene (PP) having a complex three-dimensional structure with porosities, ε, of ∼40%.2,3 To increase their mechanical, thermal, or electrochemical stability or to improve their ion transport properties, PE and PP separators have been coated with metal oxide or ceramic nanoparticles,4,5 plasma treated,4,6,7 or coated or gelled with micrometer-thick layers of ion-conducting polymers.8 The improvement of the lithium transport through the separator upon polymer and colloid modifications presents a paradox. From purely geometrical arguments, lithium-ion conductivity in the electrolyte-filled pores of the separator, σ, is the bulk electrolyte conductivity, σbulk, scaled by the effective ϵ transport parameter δ = τ where ε is the porosity and τ is the ϵ
tortuosity of the membrane:1 σ = σbulk τ . Because additive modifications to the separator reduce the membrane porosity, ε, one would expect them to also reduce σ. The fact that modifications, which reduce ε, increase both σ and the lithiumion transference number, t+, of the separator,5,9 indicates that not only the separator morphology but also separator surface chemistry has an important effect on lithium transport through the separator and thus its electrochemical performance in a LIB.10 Although previous studies have noted improvements in σ and t+ with separator surface modifications,5,9 none have decoupled the effects of changed surface chemistry from the effects of changed separator microstructure. Here, we do this. We work with a commercially available PE separator having a well© 2016 American Chemical Society
Received: September 22, 2016 Accepted: November 17, 2016 Published: November 17, 2016 32637
DOI: 10.1021/acsami.6b12085 ACS Appl. Mater. Interfaces 2016, 8, 32637−32642
Letter
ACS Applied Materials & Interfaces
Figure 1. (A) Illustration of PE separator functionalization with polyelectrolytes. (B) SEM micrographs, and water contact angle images of an unmodified PE separator and PE separators modified with one or several layers of polyelectrolytes.
Table 1. Properties of Unmodified and Polyelectrolyte Modified PE Separators unmodified −1
Gurley value (s 100 cm ) contact angles (deg)
deionized water LP50 LP30 electrolyte uptake (%) LP50 LP30 ionic conductivity in LP50 (× 10−3 S cm−1) lithium-ion transference number in LP50 activation energy in LP50 (kJ mol−1)
250 119 46 64 70 70 0.53 0.35 12.8
± ± ± ± ± ± ± ± ±
20 4 2 2 2 4 0.12 0.03 1.0
PEI 280 46 14 14 100 120 0.75 0.45 13.2
± ± ± ± ± ± ± ± ±
20 6 2 5 10 10 0.07 0.03 0.3
PEI-PSS 260 29 13 12 110 120 0.64 0.38 12.8
± ± ± ± ± ± ± ± ±
10 6 3 6 10 20 0.05 0.02 0.7
PEI-PSS-PLLgPEG 270 50 12 16 110 110 0.67 0.42 14.9
± ± ± ± ± ± ± ± ±
10 10 3 3 20 10 0.03 0.02 0.7
separator is not influenced by the polyelectrolyte coatings (see Figures S1 and S2). Using potentiostatic electrical impedance spectroscopy, we measure the ionic conductivity, σ, of the electrolyte in the separators (Figure 2A; Table 1). For the unmodified PE separator, we determine a conductivity of 0.53 mS cm−1. Taking σbulk for LP50 at 30 °C is 10.6 mS cm−1,20 we calculate a σ of 20, a factor of 3 higher than McMullin number, NM = bulk σ the McMullin number of 6.8 predicted for this separator using the FIB-SEM tomography.11 This is consistent with previous work reporting that geometric-based models of ionic transport in the separator overestimate the electrolyte conductivity.21 For the polyelectrolyte-modified separators, electrolyte conductivity is increased by 41% (PEI), 21% (PEI-PSS), and 26% (PEI-PSSPLLgPEG) leading to NM of 14, 17, and 16, respectively. These findings show that by modifying the separator surface chemistry while leaving its microstructure unchanged, it is possible to obtain electrolyte conductivities closer to that expected from a geometrical analysis. From measurement of σ at temperatures between 10 and 60 °C, we determine the activation energies for ion conduction. The PEI-PSS-PLLgPEG modified separator shows a slightly higher activation energy (∼15 kJ mol−1) than the ∼13 kJ mol−1 measured for the other separators (Figure 2B, Table 1), suggesting an interaction of the PEG side chains with the lithium ions as described in literature.8 Indeed, the lithium-ion transference number, t+, determined according to Bruce et al.22 as illustrated in Figure 2C (see SI for details), is 0.35 for the unmodified PE separator, in agreement with the literature value for a similar electrolyte.23 For the
We modify a commercially available PE separator membrane (PE16A; porosity ε = 40.8 ± 1.9%;11 Targray Technology International Inc.) with the different polyelectrolytes: the polycation polyethylenimine (PEI), the polyanion polystyrenesulfonate (PSS), and the polycation PLL-g-PEG, which is composed of a poly-L-lysine backbone (PLL) and poly(ethylene glycol) (PEG) side chains. In a layer-by-layer approach,14 we immerse the plasma-activated PE membranes in polyelectrolyte containing solutions as illustrated in Figure 1A (see the Supporting Information for details). PEI is known to adsorb on plasma activated PE membranes17 and to form a monolayer of ∼0.6 nm18 thickness. The thicknesses for the PSS and PLLgPEG layers are typically ∼1 nm18 and ∼2 nm,19 respectively. First, we confirm that the polyelectrolyte modifications leave the microstructure of the separator essentially unchanged while changing its surface chemistry. We characterize the membranes using SEM (Figure 1B) and air permeability (Table 1) and find no appreciable change to the surface morphology or evidence of pore blockage due to the polyelectrolytes. However, compared to pristine membranes, the contact angles for water and two LIB electrolytes (LP30 and LP50 Selectilyte from BASF) are significantly reduced for the polyelectrolytemodified separators (Figure 1B and Table 1) and their electrolyte uptake (Table 1) is increased. These findings indicate that the polyelectrolyte modifications alter the surface chemistry of the separator, but do not change its structure. We next evaluate the effect of the polyelectrolyte modifications on the electrochemical performance of the separator. Prior to performing electrochemical measurements, we confirm that the thermal and electrochemical stability of the 32638
DOI: 10.1021/acsami.6b12085 ACS Appl. Mater. Interfaces 2016, 8, 32637−32642
Letter
ACS Applied Materials & Interfaces Figure 2. continued
taken, (D) the current evolution during DC polarization is recorded followed by a second impedance spectrum (gray line in C).
modified separators, t+ is increased for the PEI (29%) and PEIPSS-PLLgPEG (20%) and to a much lesser extent for PEI-PSS (8.6%). Trends in improvement of σ and t+ as a function of surface modification are explained in detail below. To understand how improvements to σ and t+ influence the electrochemical performance of a cell, the cycling performance of the separator membranes is tested in coin-type half-cells with lithium titanate (Li4Ti5O12, LTO) as the working electrode and metallic lithium as the counter electrode (Figure S3). Figures 3A,B show that, at high rates, polyelectrolyte modifications reduce the cell potentials and increase galvanostatic charge capacity. The decrease in cell potential for 10C at 50% SOC is 22% for PEI, 12% for PEI-PSS, and 15% for PEI-PSSPLLgPEG. The increase in capacity at 10C is 49% (PEI), 17% (PEI-PSS), and 31% (PEI-PSS-PLLgPEG). The experimentally measured cell potentials and lithiation profiles at various C rates are complemented by COMSOL simulations (see the Supporting Information for details), which use the experimentally measured values for σ and t+ as input parameters. Additional parameters are given in Table S2 and Figure S4. The simulations yield lithiation profiles (Figure S5) and cell potentials (Figure 3B) in good agreement with the measured data. This indicates that the changes in the measured electrochemical performance of the half-cells can be attributed to the changes in σ and t+ coming from the separator surface modifications. In the following discussion, we explain how interactions of the separator surface with solvent molecules and ions of the electrolyte can explain the observed increase in σ and t+. We then use COMSOL simulation to examine which parameter (σ or t+) has a larger influence on LIB performance, providing us insight into how to improve separator design. Decreased electrolyte conductivity σ has recently been linked to changes in the microviscosity of the electrolyte near the separator surface, which restricts mobility of ions and solvent molecules within the pores of a separator.10 This effect can be viewed as an effective reduction in separator porosity (Figure 4A). We showed that polyelectrolyte-modified separators exhibit McMullin numbers between 14 and 17 indicating that, although improved from that of the unmodified separator (NM = 20), ion transport in the separator is still slower than what would be anticipated purely from geometry (NM = 6.8). PEI modified separators, which have a larger surface charge compared to PEI-PSS and PEI-PSS-PLLgPEG modified separators (see Table S1 for the corresponding ζ-potentials), exhibit the largest increase in electrolyte conductivity compared to the unmodified separator. These observations support a hypothesis that Coulombic interactions between the charged polyelectrolytes and the ions in the electrolyte weaken the electrostatic ion−ion interactions between anions and cation in the electrolyte, increasing the degree of ion pair dissociation and consequently enabling faster ionic transport and higher electrolyte conductivity. Likewise, we propose that, as for solid electrolytes,8 coating the separator with chemical groups that enhance lithium-ion transport can increase t+ by, for example, partially stripping the lithium-ion solvation shell as shown schematically in Figure 4B. PEI and PEG are known to act as lithium-ion conduc-
Figure 2. (A) Impedance measurements recorded at 25 °C in a stainless steel|separator|stainless steel configuration for unmodified and polyelectrolyte modified PE separators overlap for a large part of the measured frequency range (inset), but differ at high frequencies (main graph). Electrolyte conductivity is calculated from the intercept with the Re(Z) axis. (B) Arrhenius plot of the electrolyte conductivity vs temperature. (C, D) Data for an unmodified PE separator in LP50 electrolyte at 25 °C. The lithium-ion transference number, t+, is determined using a combination of (C) potentiostatic impedance spectroscopy and (D) a DC polarization step in a Li0|separator|Li0 configuration. After a first impedance spectrum (black line in C) is 32639
DOI: 10.1021/acsami.6b12085 ACS Appl. Mater. Interfaces 2016, 8, 32637−32642
Letter
ACS Applied Materials & Interfaces
Figure 3. Electrochemical performance of polyelectrolyte modified PE separators tested in Li0|separator|Li4Ti5O12 configuration. (A) Representative lithiation/delithiation curves recorded at rates of 1C (top) and 7C (bottom). The dashed line indicates 50% SOC. (B) Cell voltages at 50% SOC for (de)lithiation experiments at various C-rates (0.05C to 10C). The dashed lines show data from the corresponding COMSOL simulations. (C) C-rate performance of polyelectrolyte-modified separators is tested for LTO lithiation applying a constant current protocol (top) and a constant current− constant voltage protocol (bottom).
Figure 4. Influence of polyelectrolyte modifications of separator membranes. (A, top) Interactions between the membrane surface and solvent molecules increase electrolyte viscosity in proximity of separator surface (indicated by shading within pores). This corresponds to an effective reduction in the separator porosity and leads to a decrease in electrolyte conductivity. (A, bottom) Polyelectrolyte modified separators (e.g., PEI-PSS modification) increase electrolyte wetting and reduce the electrolyte viscosity, leading to increased electrolyte conductivity. (B, top) In unmodified separators, lithium ions do not interact with the membrane surface resulting in a low t+. This causes ion gradients and therefore voltage drops across the separator. (B, bottom) Coating the separator with certain polyelectrolytes (e.g., PLLgPEG) enhances t+ via interactions between polymer and lithium ions, leading to smaller ion gradients and voltage drops. Polyelectrolytes that enhance t+ usually also increase σ. (C) COMSOL simulations for Li0|separator|Li4Ti5O12 cells showing how t+ and σ influence the voltage drop across the electrolyte in the separator (charging rate 10C; 98% SOC). The gray shaded area indicates the ranges of t+ and σ that are experimentally accessible for PE separators with our polyelectrolyte modifications.
tors,8,24−27 whereas PSS does not specifically interact with lithium ions. In amorphous PEG-based materials, Li+ ions are solvated by ether groups wrapping around them and coordinating them with 5−6 oxygen atoms.28 The Li+ ions hop along the oxygen sites. Such a polymeric solvation shell can partially or completely replace a liquid solvation shell where a Li+ ion is coordinated by several carbonate molecules. For PEI, the mechanism of lithium-ion conduction is not fully
understood; however, recent studies suggest that strong amide to anion interactions facilitate ion pair dissociation and thereby promote Li+ ion transport.26 Accordingly, we found that, for the PSS-modified separator, t+ only slightly increased (from 0.35 to 0.38) and the activation energy remained the same (12.9 kJ mol−1), whereas for the PEI-modified and the PLLgPEG-modified membranes higher values for t+ (0.45 and 0.42) and activation energy (13.2 kJ 32640
DOI: 10.1021/acsami.6b12085 ACS Appl. Mater. Interfaces 2016, 8, 32637−32642
ACS Applied Materials & Interfaces mol−1 and 14.9 kJ mol−1) were measured. These trends support the picture that the separator surface interacts with solvent molecules and ions of the electrolyte. The combination of increased electrolyte conductivity and higher transference number for polyelectrolyte-modified separators significantly reduced cell potentials during cycling of LTO half-cells (Figure 3B) and resulted in a superior C-rate capability (Figure 3C). The COMSOL simulations give us insight into the origins of these improvements. Figure S6 shows that the increased σ and t+ due to polyelectrolyte modifications lead to smaller LiPF6 concentration gradients and voltage drops across the separator (Figure S7), especially at high (dis)charge rates. Parameter sweeps of t+ (Figure S8) and σ (Figure S9) show the extent to which the LiPF6 concentration gradients and smaller voltage drops can be decreased at different C-rates. The information on voltage drop as a function of t+ and σ for 10C is combined in the density plot in Figure 4C, which shows that t+ has a more pronounced effect on the cell potential than σ. For example, for the PEI-modified separator, the changes in σ and t+ are of similar magnitude (Δσ = +41%, Δt+ = +43%); however, the observed reduction in cell potential is higher because of the change in t+ (−135 mV) than because of the change in σ (−100 mV). This suggests that, for LIB applications, separator surface modifications should primarily aim to improve t+. In summary, we demonstrated that unmodified, commercial PE separators show lower electrolyte conductivity than predicted by their geometry. This effect is caused by unfavorable separator−electrolyte interactions and can be partially counteracted by modifying separators with ultrathin polyelectrolyte coatings. Such surface modifications additionally increase the lithium-ion transference number through specific interactions with Li+ ions, and thereby reduce the cell potentials of LIBs at fast cycling rates.
■
■
ACKNOWLEDGMENTS
■
REFERENCES
This work was supported by a grant from ETH Energy Science Center Seed Fund (R.Z.; Research Grant 2-72013-14), an ETH Research Grant (M.F.L.; Research Grant 0-20978-14), and the Swiss National Science Foundation (M.H.; Research Grant 277078-14). The authors thank Dr. Hilmi Buqa from Leclanché for helpful discussions and for providing the LTO material. Prof. Dr. János Vö rö s (Laboratory of Biosensors and Bioelectronics), Michele Zanini (Laboratory for Surface Science and Technology, ETH Zurich), and Dr. Khay Fong (Laboratory of Food and Soft Materials, ETH Zurich) for technical support and use of their laboratory equipment.
(1) Arora, P.; Zhang, Z. J. Battery Separators. Chem. Rev. 2004, 104, 4419−4462. (2) Zhang, S. S. A Review on the Separators of Liquid Electrolyte LiIon Batteries. J. Power Sources 2007, 164, 351−364. (3) Xue, Z.; Zhang, Z.; Zhang, S. S. Manufacture and Surface Modification of Polyolefin Separator. In Rechargeable Batteries: Materials, Technologies and New Trends; Green Energy and Technology; Springer: New York, 2015; pp 337−352; DOI: 10.1007/978-3-319-15458-9_12. (4) Lee, H.; Yanilmaz, M.; Toprakci, O.; Fu, K.; Zhang, X. W. A Review of Recent Developments in Membrane Separators for Rechargeable Lithium-Ion Batteries. Energy Environ. Sci. 2014, 7, 3857−3886. (5) Cheng, Q. H.; He, W.; Zhang, X. D.; Li, M.; Song, X. Recent Advances in Composite Membranes Modified with Inorganic Nanoparticles for High-Performance Lithium Ion Batteries. RSC Adv. 2016, 6, 10250−10265. (6) Huang, C.; Lin, C. C.; Tsai, C. Y.; Juang, R. S. Tailoring Surface Properties of Polymeric Separators for Lithium-Ion Batteries by Cyclonic Atmospheric-Pressure Plasma. Plasma Processes Polym. 2013, 10, 407−415. (7) Kim, J. Y.; Lee, Y.; Lim, D. Y. Plasma-Modified Polyethylene Membrane as a Separator for Lithium-Ion Polymer Battery. Electrochim. Acta 2009, 54, 3714−3719. (8) Stephan, A. M. Review on Gel Polymer Electrolytes for Lithium Batteries. Eur. Polym. J. 2006, 42, 21−42. (9) Nunes-Pereira, J.; Costa, C. M.; Lanceros-Mendez, S. Polymer Composites and Blends for Battery Separators: State of the Art, Challenges and Future Trends. J. Power Sources 2015, 281, 378−398. (10) Saito, Y.; Morimura, W.; Kuratani, R.; Nishikawa, S. Factors Controlling the Ionic Mobility of Lithium Electrolyte Solutions in Separator Membranes. J. Phys. Chem. C 2016, 120, 3619−3624. (11) Lagadec, M. F.; Ebner, M.; Zahn, R.; Wood, V. CommunicationTechnique for Visualization and Quantification of Lithium-Ion Battery Separator Microstructure. J. Electrochem. Soc. 2016, 163, A992−A994. (12) Wang, Z.; Guo, F.; Chen, C.; Shi, L.; Yuan, S.; Sun, L.; Zhu, J. Self-Assembly of Pei/Sio2 on Polyethylene Separators for Li-Ion Batteries with Enhanced Rate Capability. ACS Appl. Mater. Interfaces 2015, 7, 3314−3322. (13) Xu, W.; Wang, Z.; Shi, L.; Ma, Y.; Yuan, S.; Sun, L.; Zhao, Y.; Zhang, M.; Zhu, J. Layer-by-Layer Deposition of Organic-Inorganic Hybrid Multilayer on Microporous Polyethylene Separator to Enhance the Electrochemical Performance of Lithium-Ion Battery. ACS Appl. Mater. Interfaces 2015, 7, 20678−20686. (14) Decher, G. Fuzzy Nanoassemblies: Toward Layered Polymeric Multicomposites. Science 1997, 277, 1232−1237. (15) Schmitt, J.; Gruenewald, T.; Decher, G.; Pershan, P. S.; Kjaer, K.; Loesche, M. Internal Structure of Layer-by-Layer Adsorbed Polyelectrolyte Films: A Neutron and X-Ray Reflectivity Study. Macromolecules 1993, 26, 7058−7063. (16) Ladam, G.; Schaad, P.; Voegel, J. C.; Schaaf, P.; Decher, G.; Cuisinier, F. In Situ Determination of the Structural Properties of
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b12085. Experimental section including details about used materials (Table S1), separator modifications, and separator characterization (Figures S1 and S2); experimental lithiation/delithiation profiles for Li0|separator| Li4Ti5O12 cells (Figure S3); details regarding the electrochemical COMSOL simulations (Table S2 and Figure S4); simulated lithiation profiles (Figure S5), LiPF6 concentration gradients (Figure S6), and voltage drop across the separator (Figure S7) for Li0|separator| Li4Ti5O12 cells; influence of t+ (Figure S8) and σ (Figure S9) on the LiPF6 concentration and the voltage drop across the separator at different C-rates (PDF)
■
Letter
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Vanessa Wood: 0000-0001-6435-0227 Notes
The authors declare no competing financial interest. 32641
DOI: 10.1021/acsami.6b12085 ACS Appl. Mater. Interfaces 2016, 8, 32637−32642
Letter
ACS Applied Materials & Interfaces Initially Deposited Polyelectrolyte Multilayers. Langmuir 2000, 16, 1249−1255. (17) Greene, G.; Tannenbaum, R. Adsorption of Polyelectrolyte Multilayers on Plasma-Modified Porous Polyethylene. Appl. Surf. Sci. 2004, 233, 336−342. (18) Heuberger, R.; Sukhorukov, G.; Vörös, J.; Textor, M.; Möhwald, H. Biofunctional Polyelectrolyte Multilayers and Microcapsules: Control of Non-Specific and Bio-Specific Protein Adsorption. Adv. Funct. Mater. 2005, 15, 357−366. (19) Pasche, S.; Textor, M.; Meagher, L.; Spencer, N. D.; Griesser, H. J. Relationship between Interfacial Forces Measured by Colloid-Probe Atomic Force Microscopy and Protein Resistance of Poly(Ethylene Glycol)-Grafted Poly(L-Lysine) Adlayers on Niobia Surfaces. Langmuir 2005, 21, 6508−6520. (20) Ue, M.; Sasaki, Y.; Tanaka, Y.; Morita, M. Nonaqueous Electrolytes with Advances in Solvents. In Electrolytes for Lithium and Lithium-Ion Batteries; Jow, T. R., Xu, K., Borodin, O., Ue, M., Eds.; Springer: New York, 2014; Chapter 2, pp 93−165. (21) Landesfeind, J.; Hattendorff, J.; Ehrl, A.; Wall, W. A.; Gasteiger, H. A. Tortuosity Determination of Battery Electrodes and Separators by Impedance Spectroscopy. J. Electrochem. Soc. 2016, 163, A1373− A1387. (22) Evans, J.; Vincent, C. A.; Bruce, P. G. Electrochemical Measurement of Transference Numbers in Polymer Electrolytes. Polymer 1987, 28, 2324−2328. (23) Nyman, A.; Behm, M.; Lindbergh, G. Electrochemical Characterisation and Modelling of the Mass Transport Phenomena in Lipf6-EC-EMC Electrolyte. Electrochim. Acta 2008, 53, 6356−6365. (24) Tanaka, R.; Fujita, T.; Nishibayashi, H.; Saito, S. IonicConduction in Poly(Ethylenimine)-Lithium and Poly(N-Methylethylenimine)-Lithium Salt Systems. Solid State Ionics 1993, 60, 119−123. (25) Pehlivan, I. B.; Georen, P.; Marsal, R.; Granqvist, C. G.; Niklasson, G. A. Ion Conduction of Branched PolyethyleneimineLithium Bis(Trifluoromethylsulfonyl) Imide Electrolytes. Electrochim. Acta 2011, 57, 201−206. (26) Doyle, R. P.; Chen, X. R.; Macrae, M.; Srungavarapu, A.; Smith, L. J.; Gopinadhan, M.; Osuji, C. O.; Granados-Focil, S. Poly(Ethylenimine)-Based Polymer Blends as Single-Ion Lithium Conductors. Macromolecules 2014, 47, 3401−3408. (27) Fergus, J. W. Ceramic and Polymeric Solid Electrolytes for Lithium-Ion Batteries. J. Power Sources 2010, 195, 4554−4569. (28) Bruce, P. G.; Vincent, C. A. Polymer Electrolytes. J. Chem. Soc., Faraday Trans. 1993, 89, 3187.
32642
DOI: 10.1021/acsami.6b12085 ACS Appl. Mater. Interfaces 2016, 8, 32637−32642
