Interphases of Polymer Electrolytes

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Interphases of Polymer Electrolytes Qing Zhao1,* Lithium sulfur batteries are endowed with intriguingly high energy density but also suffer from complicated interfacial parasitic reactions, especially toward the anode side. In this issue of Joule, Zhang et al. design a new LiN(SO2CF2H) (SO2CF3)(LiDFTFSI)/poly(ethylene oxide) solid-state polymer electrolyte that is compatible with polysulfide chains and can regulate the interfacial chemistry of lithium anode.

Lithium sulfur (Li-S) batteries that share similar configurations of commercial Li-ion batteries have been regarded as promising next-generation energy storage and conversion systems due to their high theoretical energy density (2,600 Wh/Kg) and intrinsically lowcost sulfur cathode.1–3 The conventional Li-S batteries using liquid electrolytes suffer from the active material loss caused by the dissolution of polysulfides, and afterward the seriously parasitic reactions with Li anode on charging process, resulting in the shuttle reaction and low Coulombic efficiency. Solid-state electrolytes (SSEs) have been regarded as reliable approaches to build Li-S batteries with high safety and long life, in which solid-state polymer electrolytes (SPEs) take advantage of easy fabrication, low density, and applicable for largescale roll-to-roll manufacturing. The study of poly(ethylene oxide) (PEO)-based SPEs started from 1973, and they were demonstrated as possible electrolytes for batteries by Armand et al. in 1978.2 Degott tried to use LiClO4/PEO SPEs for Li-S batteries in 1986.4 Through over 40 years’ development, the ion-transport mechanism in bulk SPEs, the strategies to improve ionic conductivity, thermodynamic activity, and mechanical strength have been well understood. In fact, the

SPEs have already been applied in electric ‘‘Bollore´ Bluecar’’ that is equipped with a minimum 30 kWh battery to guarantee a driven distance of 250 km.5 Despite these progresses, the insights of unstable interphases of SPEs toward lithium-metal anode and high-voltage cathode are stilled limited. It is known that the oxidation of PEO happens below 4.0 V, resulting in thick cathode electrolyte interphase (CEI) and fading of the capacity. When facing Li anode, PEO can also be reduced to polyanionic species that lead to thick solid electrolyte interphase (SEI) on Li anode.6 The situation becomes even worse when SPEs are operated on raised temperature above the melting points (70 C) in order to reach a desired conductivity; the increased mobility of polymer segments will at the same time enhance the solubility of polysulfides, reduce the mechanical strength, and increase the dendrite risky. In this issue of Joule, Zhang et al. rationally design the anions of Li-salt to fabricate LiDFTFSI/PEO SPEs, which can regulate the properties of both the bulk phase and interphases.2 Through replacing one –CF3 group in LiN(SO2CF3)2 (LiTFSI) to –CF2H, the fabricated LiDFTFSI/PEO SPEs possess several merits. First, the hydrogen bond generated from CF2H and PEO chain restricts the mobility of anion

and leads to high cation conductivity and high cation transfer number. Second, DFTFSI– with asymmetric structure is easily reduced/decomposed on anode side to form a dense, mechanically strong (LiF rich) and ionic conductive (LiH rich) SEI. Combined with the feasibility of large-scale production, LiDTFSI points out a new strategy to build advanced electrolytes for lithium-metal batteries. Similar with liquid electrolyte, Li salt is considered to be dissolved in polymer matrix; thus, both cations and anions are mobile in SPEs.7 However, unlike cation (Li+), there is usually no electrode to accommodate the anion. Therefore, the movement of anion by electric field can lead to the concentration gradients, further result in the inhomogeneous decomposition of anions, and finally enlarge the polarization and fail the batteries. In a recent paper by Zhang et al. in this issue of Joule, the hydrogen-bond between DFTFSI and O atom in PEO chains constrains the movement of anion, like a single-ion polymer conductor but more mobile.2 Meanwhile, the SEI decomposed from DTFSI is favorable for dense and dendrite-free lithium deposition, as shown in Figures 2 and 3 in the paper.2 Moreover, UV-vis and DFT calculation demonstrate the good chemical stability of both LiDFTFSI and LiTFSI toward polysulfide species, unlike LiN(SO2F)2 (LiFSI), which is favorable for lithiummetal but reactive with polysulfide. As a result, the all-solid-state Li-S batteries assembled with LiDFTFSI/PEO SPEs show long cycle life (>1,300) and high Coulombic efficiency (100%) at an optimized sulfur loading (1 mg/cm2).

1Robert

Frederick Smith School of Chemical and Biomolecular Engineering, Cornell University, Ithaca, NY, USA *Correspondence: [email protected] https://doi.org/10.1016/j.joule.2019.06.004

Joule 3, 1569–1577, July 17, 2019 ª 2019 Elsevier Inc. 1569

Figure 1. Design of Solid-State Polymer Electrolytes for Rechargeable Metal Batteries

The reported work sheds light on the significance to design the interphases of polymer electrolytes and also provides a possible strategy to characterize the highly viscous and blurry electrolyte/electrode interphases through using the liquid analog. However, the polymer is usually more chemically inert than their liquid analog or monomer, which is one of the reasons to develop polymer electrolytes. Therefore, more advanced technologies are still needed to directly investigate the interphases. Compared with solid ceramic electrolytes, the interfacial resistance of SPEs toward plane electrodes such as lithium and sodium are much smaller due to the good viscoelasticity of polymer. However, for most cases, the interfacial resistance between SSEs and porous cathodes is unacceptable. The design of in situ SPEs provides a future route to create lower interfacial resistance. Zhao et al. recently demonstrated that Li-S batteries using in situ formed

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LiTFSI/polyDOL SPEs derived from the liquid dioxolane (DOL) monomer exhibited high capacity and coulombic efficiency.8 The liquid electrolytes can wet both the plane and porous interphase first and then gradually polymerize to form SPEs. Due to the versatile polymer structures by designing the chemical reactions, more SPEs with favorable interfacial contact and chemical compositions are expected to be discovered. An ideal SPE should be not only able to form favorable interphase toward both plane anode and porous cathode, but also thin and mechanically strong with high cation conductivity (Figure 1). For a practical Li-S battery, electrolyte-tosulfur ratio values below 3 mL electrolyte/ mgsulfur, a sulfur loading of at least 5 mg/cm2, and an excess lithium lower than 100% are suggested to expect a total energy density of over 500 Wh/kg.9 SPEs take advantage of low density (1.2 g/cm3 for LiTFSI/ PEO), which is much lower than inor-

ganic SSEs (2 g/cm3 for Li10GeP2S12, >5 g/cm3 for Li7La3Zr2O12 [LLZO]) and can also be fabricated as a thin layer (<50 mm). However, an even thinner film is needed (<15 mm) to reach a high energy density. Meanwhile, the thin film is also beneficial for low area resistance of batteries. Very recently, Wan et al. reported on the fabrication of thin film (8.6 mm) polyimide filled with PEO/LiTFSI for all solid-state batteries, pointing out a new way to prepare mechanical, strong, thin SPEs for high-energy-density lithium-metal batteries.10 In summary, SPEs with merits of being highly flexible, high safety, easy scaleup, light weight, and inhibiting the dissolution of electrochemical active spices have provided a promising opportunity for next-generation lithiummetal batteries. The work by Zhang et al. shows that both the bulk transport and interfacial chemistry of SPEs can be adjusted though designing the anion of Li salts.2 With increasing efforts on

developing thin and strong SPEs that are compatible for both anode and cathode with low interfacial resistance, more advanced high-energy-density energy storage and conversion devices can be expected for applications. 1. Manthiram, A., Fu, Y., Chung, S.H., Zu, C., and Su, Y.S. (2014). Rechargeable lithium-sulfur batteries. Chem. Rev. 114, 11751–11787. 2. Zhang, H., Oteo, U., Judez, X., Eshetu, G.G., Martinez-Iban˜ez, M., Carrasco, J., Li, C., and Armand, M. (2019). Designer Anion Enabling Solid-State Lithium-Sulfur Batteries. Joule 3, this issue, 1689–1702. 3. Pang, Q., Kwok, C.Y., Kundu, D., Liang, X., and Nazar, L.F. (2019). Lightweight Metallic

MgB2 Mediates Polysulfide Redox and Promises High-Energy-Density Lithium-Sulfur Batteries. Joule 3, 136–148. 4. Degott, P. (1986). Polymere Carbone-Soufre Synthese et Proprietes Electrochimiques. PhD thesis (l’Institut National Polytechnique de Grenoble). 5. https://www.bluecar.fr/sites/bluecar/files/ medias/PDF/2_bluecar_20_p.pdf. 6. Zhao, Q., Chen, P., Li, S., Liu, X., and Archer, L.A. (2019). Solid-state polymer electrolytes stabilized by task-specific salt additives. J. Mater. Chem. A Mater. Energy Sustain. 7, 7823–7830. 7. Ma, Q., Zhang, H., Zhou, C., Zheng, L., Cheng, P., Nie, J., Feng, W., Hu, Y.S., Li, H., Huang, X., et al. (2016). Single Lithium-Ion Conducting Polymer Electrolytes Based on a

Super-Delocalized Polyanion. Angew. Chem. Int. Ed. Engl. 55, 2521–2525. 8. Zhao, Q., Liu, X., Stalin, S., Khan, K., and Archer, L.A. (2019). Solid-state polymer electrolytes with in-built fast interfacial transport for secondary lithium batteries. Nat. Energy 4, 365–373. 9. Zhao, Q., Zheng, J., and Archer, L. (2018). Interphases in Lithium–Sulfur Batteries: Toward Deployable Devices with Competitive Energy Density and Stability. ACS Energy Lett. 3, 2104–2113. 10. Wan, J., Xie, J., Kong, X., Liu, Z., Liu, K., Shi, F., Pei, A., Chen, H., Chen, W., Chen, J., et al. (2019). Ultrathin, flexible, solid polymer composite electrolyte enabled with aligned nanoporous host for lithium batteries. Nat. Nanotechnol. Published online May 27, 2019. https://doi.org/10.1038/s41565-0190465-3.

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Pricing CO2 Direct Air Capture Brandon R. Sutherland1,* Negative emissions technologies such as direct air capture systems are an important tool to impede climate change. Recently in Applied Energy, Azarabadi and Lackner reported a generalized cost model for direct air capture of CO2. Their findings emphasize the importance of sorbent cycle duration and stability in minimizing total system cost. The human-caused component of carbon dioxide emissions has grown exponentially starting from the early 1800s, doubling roughly every 30 years.1 This has resulted in an anthropogenic-driven climate change that has increased global average temperatures greater than 1 C beyond the pre-industrial level. To curtail adverse effects associated with a warmer Earth, the Paris Climate Change Agreement has set a target of reducing the temperature increase this century to well below 2 C of this level.2 Realizing this requires reducing all greenhouse gas (GHG) emissions to zero by mid21st century. The world’s ingrained dependence on fossil fuels to produce electricity, to control heating and cooling, and as a transportation fuel make a carbon-

free transition immensely challenging— especially considering the rapid timescales needed to meet climate targets. The energy infrastructure needs to shift toward renewable power sources and on-site capture, storage, and utilization of GHG exhaust streams. A lack of competitive economics for such technology to displace traditional fossil fuels has greatly impeded progress on this front. Still, society does have the tools to fight climate change through technology, government intervention, and public education. It is a complex problem shackled by economic influence that will require an open mind for new proposed solutions and long-term risk management. Beyond simply stopping the emission of GHGs, an idea that is only growing in

importance is taking CO2 out of the biosphere and putting it back in the geosphere using negative emissions technologies (NETs). While it is more cost-effective to reduce GHG emissions toward the zero limit, the further off track the world is from meeting climate goals the more NETs become necessary. Indeed, the Intergovernmental Panel on Climate Change predicts that a slow carbonneutral transition alone is insufficient and NETs are needed curb global average temperature rises.3 One category of NETs consists of CO2absorbing biomass. More trees can be planted (afforestation) or biomass can be farmed, combusted, or broken down through other means and then replanted. The carbon emissions from this process can be captured and sequestered or used. This is termed bioenergy with carbon capture and storage, BECCS. An alternative approach is to take atmospheric CO2 directly from the air and store it underground.4 A consistent problem with each of these methods

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Cell Press, 50 Hampshire Street, 5th Floor, Cambridge, MA 02139, USA *Correspondence: [email protected] https://doi.org/10.1016/j.joule.2019.06.025

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