Architected Macroporous Polyelectrolytes That Suppress Dendrite

Sep 21, 2018 - Short cycle life is also observed when lithium deposits with a “mossy” morphology; the high surface area of mossy deposits increase...
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Architected Macroporous Polyelectrolytes That Suppress Dendrite Formation during High-Rate Lithium Metal Electrodeposition Longjun Li,† Lin Ma,† and Brett A. Helms*,†,‡,§ Joint Center for Energy Storage Research, ‡The Molecular Foundry, and §Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States

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ABSTRACT: Batteries assembled with lithium metal anodes and high-capacity cathodesincluding air, sulfur, and lithium-rich transition metal oxideshave higher energy density than conventional Li-ion counterparts. Unfortunately, the lifetime of lithium metal cells is typically short, owing to the formation of dendrites on charging, which eventually shorts the cells. Short cycle life is also observed when lithium deposits with a “mossy” morphology; the high surface area of mossy deposits increases the rate of electrolyte degradation, eventually drying out the cells. Here we show that a lithium-ion-conducting, architected macroporous polyelectrolyte (AMP-1) serves as a longlasting host for uniform and dense lithium−metal electrodeposits. High Coulombic efficiencies indicate the low occurrence of parasitic reactions with the electrolyte. Galvanostatic discharge experiments indicate that AMP-1 suppresses dendrite formation, extending over 2-fold the short-circuit time at high current density. Our success opens new directions for lithium anode development for commercial cells.

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INTRODUCTION Lithium−metal batteries assembled with air, sulfur, and lithium-rich transition metal oxide cathodes are well positioned to overtake conventional Li-ion batteries in the marketplace for mobile devices, electric vehicles, aviation, and grid stabilization on account of their higher energy density.1−4 Delaying their entry, however, is ongoing concern over their safety and longevity.5 At the heart of this concern is the stability of the lithium metal anode on cycling in liquid electrolyte, particularly at high current density (i.e., J > 1 mA cm−2).6−11 Electrodeposition of lithium metal from liquid electrolytes is nonuniform: low-density “mossy” morphologies are typical, as are lithium−metal dendrites, which extend into the electrolyte. Whereas high-surface-area lithium deposits are undesirable as they increase the rate of electrolyte degradation, lithium metal dendrites are particularly worrisome as their continued growth eventually shorts the cell, which may initiate thermal runaway. If the temperature on thermal runaway exceeds the flash point of the electrolyte, a fire ensues. To avoid this catastrophe, new materials are needed that direct the electrodeposition of lithium metal from liquid electrolytes to yield dense films; it is also desirable that these materials restrict the electrolyte’s access to the lithium metal surface. Here we show that a lithium-ion-conducting polymer poly(N,N-diallyl-N,N-dimethylammonium bis(trifluoromethylsulfonylimide))when processed as an architected macroporous polyelectrolyte film (AMP-1), promotes the electrodeposition of lithium metal at a high rate with a dense morphology (Figure 1). While the high Li-ion conductivity of AMP-1 aids in minimizing the construct’s internal resistance and overpotential during Li-metal electrodeposition,12−15 it is © XXXX American Chemical Society

Figure 1. Schematic illustration of the AMP-1 laminate suppressing dendrite growth on lithium metal anodes. (a, b) Growth of lithium dendrites on bare copper foil. (c, d) Suppression of lithium dendrites by the AMP-1 laminate. (e, f) The AMP-1 is flat as deposited and flexible during lithium deposition.

the macro-architected character of the film, accessed directly using a simple solvent-casting scheme, that is deterministic in the long-term dimensional stability of the anode. Specifically, the smooth undergrowth of AMP-1 by Li metal throughout the Received: June 4, 2018 Revised: September 5, 2018

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DOI: 10.1021/acs.macromol.8b01188 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

Structural Characterization. The morphology of AMP-1 was observed under a Zeiss Gemini Ultra-55 analytical field emission scanning electron microscope. Electrochemical Characterization. Li−Cu cells were fabricated as CR2032-type coin cells. The Cu disk (uncoated, or coated with AMP-1) and lithium metal foil were sandwiched with a single Celgard 2400 separator in between. The electrolyte used was 1.0 M LiTFSI and 0.2 M LiNO3 dissolved in 1,3-dioxolane/1,2-dimethoxyethane (DOL/DME, 1:1 v/v). Approximately 30 μL of the electrolyte was added to each cell. Electrochemical impedance spectroscopy (EIS) was measured at open-circuit voltage (OCV) with a Biologic VMP3 potentiostat from 1 MHz to 1 Hz with an ac voltage amplitude of 10 mV. Cyclic voltammetry (CV) data were collected in the voltage range of 0−5.0 V at a scan rate of 0.1 mV s−1 with Al foil replacing Cu foil as the current collector. For galvanostatic discharge experiments, Li−Cu cells were discharged at a constant current density of 1 mA cm−2 until short circuit was observed. Galvanostatic cycling tests were performed at 0.1 mA cm−2 for the first five cycles (i.e., to establish an interface/interphase between Li metal and AMP-1) and then at 1.0 mA cm−2 for the remainder of the cycling scheme. The discharge step was limited to 1 h, and the charge step was controlled by setting the cutoff voltage to 0.5 V.

charge maintains a low interfacial area between the two. As a result, the occurrence of parasitic reactions with the electrolyte is low, as evidenced by >95% Coulombic efficiency that is sustainable for at least 80 cycles in asymmetric Li−Cu cells. Additionally, the short-circuit time for these cells at a current density of 1 mA cm−2 increases 2-fold over that observed in the absence of AMP-1, indicating substantial dendrite suppression. The advance reported here builds on growing support in the community that the rectifying character of porous materials at anode surface is advantageous to electrode stabilitythe additional gain here being that such a construct might best be fabricated from an ionically conductive polymer. Our work challenges the conventional wisdom that lithium− metal protection requires complete coverage of the anode surface.16−19 Nonflammable inorganic solid-ion conductors are attractive for anode protection, in principle, offering high shear modulus for dendrite-blocking and compatibility with highvoltage cathodes.20−22 However, in practice, their polycrystalline nature fails to prevent dendrite growth at fragile grain boundaries.23−29 Many also have prohibitively low ionic conductivity, cannot be processed as thin layers, and experience large interfacial charge-transfer resistance once assembled in Li-metal cells, leading to poor cell performance in demanding applications.21,30,31 Artificial solid−electrolyte interphases (SEIs) have also been proposed; they are typically generated in situ via chemical transformation or crystallization of electrolyte additives (e.g., nitrates,32 halides,6,33−35 Cs+ and Rb+,36 ionic liquids,7,37,38 and others8) at the Li-metal surface. These electrolyte additives are typically consumed when fulfilling their roles; thus, long-term stability can be a concern for manufacturers and consumers alike. Preformed ionconducting laminates are longer-lasting in their protective effects. For example, hollow carbon spheres by Zheng et al. suppress lithium dendrites from forming in extended cycling.9 Depending on the specific approach, scalability can be an issue. Furthermore, as is the case of carbon spheres and related designs, their mechanical strength may not be well matched to this application. Polymer laminates are preferred in that regard. Using them, conformal coatings on the lithium surface are possible: e.g., Wei et al. have used a polyimide-coating layer with vertical nanoscale channels leads to uniform Li nucleation and deposition,14 and Lu et al. have also implemented crosslinked polymer electrolytes for dendrite suppression.39 Despite these advances, we see as crucial to polymer laminate development for anode stabilization the coupling of polyelectrolytes for higher ionic conductivity with advanced processing strategies yielding porous architectures for ionic rectification. As such, we see exciting new directions for Li metal batteries with AMP-1 and other architected macroporous polyelectrolyte analogues in place.

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RESULTS AND DISCUSSION In the absence of anode protection schemes, the ramified character of electrolyte-coupled lithium metal electrodeposits evolves over time (i.e., cycling history), typically toward higher surface area morphologies that extend farther and farther from the current collector with each successive cycle. Once the separator is penetrated and dendrites make contact with the cathode, the cell shorts. In advance of that, parasitic side reactions can occur between highly reactive dendrite surfaces and the electrolyte, which increases cell impedance and in some cases dries the cell out, particularly when operating under electrolyte-lean conditions. We hypothesized that the ion current could be more homogeneously distributed across the electrode surface, rectifying it if possible as heterogeneities arise, if AMP-1 were in place due to its Li-ion transport properties and its hierarchically porous structure. The polyelectrolyte comprising AMP-1 is readily available in a variety of molecular weights (and alternative counterions) after carrying out anion metathesis of poly(N,N-diallyl-N,Ndimethylammonium) chloride (MW = 400−500 kg mol−1) with LiTFSI. While poly(N,N-diallyl-N,N-dimethylammonium bis(trifluoromethylsulfonylimide)) has been used in a variety of battery applicationse.g., as a binder, as a gel−polymer electrolyte, and as a solid−electrolyte40−42to demonstrate the principle of our concept, we needed to develop a strategy to architect the polyelectrolyte into a macroporous host for Limetal electrodeposition. Ultimately, we were successful in accessing AMP-1 using evaporation-induced self-assembly (EISA)43−45 directly onto metal supports (e.g., 1.2 cm disks of Cu) from inks of poly(N,N-diallyl-N,N-dimethylammonium bis(trifluoromethylsulfonylimide)) in acetone (e.g., 20 mg mL−1). As acetone evaporates, the associated endotherm leads to condensation of adventitious water vapor, which coalesces into droplets. Owing to the surface tension in the system, polymers gather around water droplets, which then assemble into regular hexagonal patterns. After the first layer of droplets forms at the surface and the droplets assemble, subsequent droplet formation and ordering subsurface are not as facile due to mass transfer limitations for water in the system. This geometry-blocking effect results in less pore regularity, increasingly smaller pores, and lower porosity from surface to substrate; the substrate is

MATERIALS AND METHODS

Preparation of AMP-1 Films via Evaporation-Induced SelfAssembly. Poly(N,N-diallyl-N,N-dimethylammonium bis(trifluoromethylsulfonylimide)) was synthesized by anion exchange as reported elsewhere.40,41 To process the polyelectrolyte as an architected macroporous film (i.e., AMP-1), we cast it from acetone (20 mg mL−1) onto various supports. For example, clean copper (Cu) foil substrates were cut into disks (diameter ∼1.2 cm) and loaded with the ink (30 μL), which was allowed to dry to yield a porous film that appears white due to the scattering of visible light. Coated Cu disks were dried overnight under vacuum prior to cell assembly. B

DOI: 10.1021/acs.macromol.8b01188 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules also influential in this regard. By use of this casting strategy, the resulting morphology of AMP-1 was homogeneous over the entire substrate and macro-architected as intended (Figure 2).

Figure 3. (a, b) Electrochemical impedance spectroscopy (EIS) for Li−Cu cells with or without AMP-1 laminate. (c) CV showing the large stable voltage window of AMP-1 laminate at a scan rate of 0.1 mV s−1. (d) Galvanostatic discharge of Li−Cu cells with or without AMP-1 laminate at a current density of 1 mA cm−2.

Figure 2. Morphology of AMP-1 on copper foil before cycling tests. (a−c) Low- and high-magnification top-down SEM images. The photograph of AMP-1 on the copper electrode is shown as the inset in (a). (d) Cross-section SEM image. Scale bars in panels a, b, c, and d are 40, 10, 4, and 2 μm, respectively.

cells with bare Cu electrode, consistent with the impedance data in Figure 3a,b. The small increase in resistance suggests a homogeneous ion current across the electrode surface is attainable.18 The low resistance, homogeneous ion current, and dendritesuppressing ability of AMP-1 imply the prevalence of side reactions between the organic electrolyte and mossy or dendritic Li deposits are mitigated. These side reactions can be monitored via the cell’s Coulombic efficiency (Figure 4). Without AMP-1, Li−Cu cells were stable for ∼50 cycles (Figure 4a). After cycle 50, these unadorned Li−Cu cells suffered from seemingly random Coulombic efficiency. This is attributed to the growth of micro dendrites, which lead to

To the best of our knowledge, this is the first instance of polyelectrolytes rendered hierarchically porous through EISA strategies. As such, AMP-1 presents an exciting new direction for battery technology development, as the component architecture as well as the composition dictates key performance outcomes in Li-metal electroplating as noted below. Whereas nonporous polyelectrolyte films abound for lithium metal passivation and protection, many are too resistive for high current density operation. AMP-1 alleviates the resistive character of conventional polyelectrolyte laminates by incorporating porosity into its architecture. The low internal resistance of AMP-1 on lithium was demonstrated using Li− Cu cells, which were assembled with Li foil as the anode and AMP-1-coated Cu foil as the cathode. Electrochemical impedance spectroscopy (EIS) (Figure 3a,b) indicated that the impedance is only slightly increased compared with bare Cu electrode: cells with laminated or bare Cu electrodes have very similar EIS profiles, with about only 2 Ω increase in overall resistance. From the cyclic voltammetry (CV) in Figure 3C, there are no significant redox reactions observable over a voltage window of 0−5 V. Other than the double-layer capacitance, the electrolyte may undergo minor reduction and oxidation respectively at the lower and higher voltage limit. In fact, the stability of the same polyelectrolyte has been proven to be stable with lithium metal in previous reports.42 The stabilizing influence of AMP-1 on lithium deposition was evaluated by carrying out the galvanostatic discharge of Li−Cu cells at a current density of 1 mA cm−2. Cells assembled with bare Cu electrodes typically lasted only 35 h before an abrupt drop in voltage was observed (Figure 3d), indicative of a short circuit by dendrites. On the contrary, cells assembled with AMP-1-coated Cu electrodes lasted twice as long (∼70 h). The dendrite-suppressing character of AMP-1 is striking and bodes well for improving the safety of lithium metal batteries. Furthermore, we noted that the overpotential of cells with AMP-1 in place was only slightly higher (∼10 mV) than

Figure 4. Cycling Coulombic efficiency of the copper foil in Li−Cu cells without (a) or with (b) AMP-1 laminate at a current density of 0.1 mA cm−2 in the first five cycles and 1 mA cm−2 in the following cycles for 2 h each cycle. C

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Macromolecules multiple soft short-circuit events in the cell. Specifically, during lithium deposition, lithium needle growth is initiated from the substrate surface or lithium particles. The growth of the needle occurs at the tip behind the SEI layer, which enhances the electrical field and leads to self-amplified growth. On the contrary, with AMP-1 in place on the Cu electrode, Li−Cu cells last for >80 cycles without significant degradation in cell performance (Figure 4b). Their Coulombic efficiencies were ∼96.5% after cycle 5 and stable throughout, supporting the positive role played by AMP-1 during battery cycling as hypothesized. The reported cycle life of Li−Cu cells varies based on experimental conditions; our cells show impressive performance among state-of-art research in the literature, in terms of cycle life, area capacity, and current density. For example, Zhang et al.11 reported a glass fiber cloths modified Li metal anode with a cycle life of ∼70 cycles with an area capacity of 0.5 mAh cm−2 and a current density of 1 mA cm−2. Zhu et al.46 described a poly(dimethylsiloxane) coating film on lithium metal anode with a cycle life of ∼100 cycles with an area capacity of 1.0 mAh cm−2 and a current density of 1 mA cm−2. Cui et al.14 demonstrated a polyimide-coating layer with vertical nanoscale channels protecting lithium metal anode for ∼150 cycles with an area capacity of 0.5 mAh cm−2 and a current density of 2 mA cm−2. In that AMP-1 on Cu is conformal, hierarchically porous, and flexible, it follows that AMP-1 should be able to accommodate the volume change associated with reversible lithium electrodeposition.46,47 Accordingly, AMP-1 should prevent the formation of surface defects, such as cracks and pinholes, which would serve as the hot spots for dendrite growth.18 Furthermore, the hierarchically porous character of the film, particularly the nanochannels at the surface of the electrode, should dampen any perturbations in ion flux during the lithium deposition process to yield a smooth film.14 To understand whether this behavior was indeed imparted by the architected character of AMP-1, we investigated the morphology of lithium metal electrodeposits upon cycling. Without AMP-1, dendritic and mossy lithium deposits were pervasive over the Cu surface, which if left unchecked could easily penetrate the separator and result in a short circuit (Figure 5a,b). However, with AMP-1 in place, the plated lithium surface is flat and smooth; no dendrites were observable (Figure 5c,d). Furthermore, AMP-1 appeared elastic and flexible on sample preparation and subsequent analysis by SEM, desirable to accommodate the volume change associated with lithium metal deposits underneath. These data indicate that cohesive electrical contact between deposited lithium and the current collector is maintained during cycling. In contrast, in cells without AMP-1, mossy lithium deposits detached easily from the Cu current collector and in turn have lost electrical contact with it, consistent with the observed lower Coulombic efficiency of those cells.

Figure 5. Morphology of copper foil after 80 cycle tests. (a, b) Growth of mossy lithium on bare copper foil (top-down view). Without the protection of the deposited lithium, lithium dendrites are easily formed. (c, d) Protected copper surface with AMP-1 laminate (top-down view). With AMP-1 protection, the dendrite growth on copper foil has been suppressed. The copper surface remains smooth after the cycling test. Scale bars in panel a, b, c, and d are 5, 2, 5, and 5 μm, respectively.

supports, obviating the use of surfactants or other templates for porosity. AMP-1 not only guides uniform current distribution on the lithium surface but also aids in maintaining the electrical contact between deposited lithium and the current collector. As a result, Li−Cu cells with AMP-1 achieved significantly longer cycle life than those without. We view architected polyelectrolytes as attractive, scalable, at-the-ready options for protective laminates for lithium metal anodes to suppress dendrite growth, reduce overpotential, and improve cycle life.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (B.A.H.). ORCID

Brett A. Helms: 0000-0003-3925-4174 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Joint Center for Energy Storage Research, an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences. Portions of the workincluding polymer synthesis and characterizationwere performed as a user project at the Molecular Foundry, which is supported by the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract DE-AC02-05CH11231. Ying Wang is thanked for helpful discussions.

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CONCLUSIONS In summary, we have successfully developed a new type of architected macroporous polyelectrolyte, AMP-1, with high intrinsic lithium-ion conductivity for dendrite-free lithium metal anodes. Although polymers abound as protective laminates for lithium metal anodes, they usually involve multistep chemical synthesis, impede lithium-ion transport, or lack architectural features allowing them to serve as ion-current rectifiers. On the other hand, AMP-1 is readily accessible using low-cost and large-area solvent processing onto a variety of

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ABBREVIATIONS AMP-1, architected macroporous polyelectrolyte; SEI, solid− electrolyte interphase; Li, lithium; Cu, copper; SEM, scanning electron microscope; EIS, electrochemical impedance spectroscopy; DOL, 1,3-dioxolane; DME, 1,2-dimethoxyethane; OCV, open circuit voltage; CV, cyclic voltammetry; LiTFSI, lithium bis(trifluoromethylsulfonyl)amide; EISA, evaporationinduced self-assembly. D

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DOI: 10.1021/acs.macromol.8b01188 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.8b01188 Macromolecules XXXX, XXX, XXX−XXX