Aqueous Li-Ion Batteries: Now in Striking Distance

Abstract

In the inaugural issue of Joule, Chunsheng Wang, Kang Xu, and colleagues demonstrate a groundbreaking advance in Li-ion battery electrolytes with their design and demonstration of 4 V aqueous Li-ion batteries, which in theory could eliminate several environmental, safety, and manufacturing restrictions and concerns raised by non-aqueous electrolytes. In the inaugural issue of Joule, Chunsheng Wang, Kang Xu, and colleagues demonstrate a groundbreaking advance in Li-ion battery electrolytes with their design and demonstration of 4 V aqueous Li-ion batteries, which in theory could eliminate several environmental, safety, and manufacturing restrictions and concerns raised by non-aqueous electrolytes. It’s now uncontroversial to include the invention of lithium ion (Li-ion) battery technology, and its rapid progress over the last 30 or so years, in the pantheon of landmark human technological achievements. Commercially introduced in 1991 by Sony in their video cameras, Li-ion batteries now power billions of mobile electronic devices and therefore support unprecedented levels of human communication and interconnection. The comparison extends further into the space of sustainable energy. Following a so-called “technology learning curve,” constant improvement in Li-ion battery technology is facilitating the imminent electrification of vehicular transport and displacing a significant source of Earth’s greenhouse gas emissions. Advances especially in engineering, scale-up, and manufacturing have driven and continue to drive down costs of electric vehicle battery packs,1Nykvist B. Nilsson M. Nat. Clim. Chang. 2015; 5: 329-332Crossref Scopus (1199) Google Scholar and cost parity with petroleum-fueled vehicles is projected in as soon as a decade.2Plumer, B. “When Will Electric Cars Go Mainstream? It May Be Sooner Than You Think.” New York Times. July 8, 2017. https://www.nytimes.com/2017/07/08/climate/electric-cars-batteries.html.Google Scholar, 3“Electrifying everything: After electric cars, what more will it take for batteries to change the face of energy?” The Economist. August 12, 2017. https://www.economist.com/news/briefing/21726069-no-need-subsidies-higher-volumes-and-better-chemistry-are-causing-costs-plummet-after.Google Scholar For all the value they offer us, batteries follow a deceptively simple design concept with just three active operating components—two electrodes (anode and cathode) separated by electrolyte—that together allow the reversible interconversion of chemical and electrical energy. In reality, materials design for batteries is a painstaking balancing act, where several performance metrics such as energy and power density, safety, cycle life, and cost must be simultaneously optimized through the judicious selection and integration of just these three components. The electrolyte plays an obvious critical role acting as an electrochemically stable and strictly ion-conducting medium that forces electrons to flow between electrodes via an external circuit during battery charge and discharge. Very early on in its development, commercial Li-ion electrolyte chemistry settled on a narrow class of Li-salts dissolved in non-aqueous (organic) solvents (most commonly LiPF6 dissolved in a mix of ethylene and dimethyl carbonate, EC/DMC).4Blomgren G.E. J. Electrochem. Soc. 2017; 164: A5019-A5025Crossref Scopus (1070) Google Scholar In large part due to its ability to remain electrochemically stable across a 4 V threshold (supporting the large Li chemical potential difference between anode and cathode necessary to achieve high energy density), this class of electrolytes remains the workhorse in state-of-the-art Li-ion cells. However, the highly flammable organic solvents and toxic lithium salt anion used also bring various safety and environmental concerns. Furthermore, the moisture-sensitive nature of these electrolytes also applies strict constraints on manufacturing and is responsible for the rigid form-factors of Li-ion batteries due to the requirement of a hermetic seal. In the inaugural issue of Joule, Chunsheng Wang, Kang Xu, and colleagues demonstrate a groundbreaking advance in Li-ion battery electrolytes with their design and demonstration of 4 V aqueous Li-ion batteries,5Yang C. Chen J. Qing T. Fan X. Sun W. von Cresce A. Ding M.S. Borodin O. Vatamanu J. Schroeder M.A. et al.Joule. 2017; 1 (this issue): 122-132Abstract Full Text Full Text PDF Scopus (350) Google Scholar which, in theory, could eliminate all these restrictions and concerns raised by the non-aqueous electrolytes. Although billions of Li-ion batteries have been manufactured, most of which safely operate as designed thanks to significant engineering effort and ingenuity in mitigating these concerns, infrequent but high-profile and catastrophic battery failures in recent memory continue to highlight one of the foremost challenges in battery design: safety. A water-based electrolyte is appealing from an inherent safety and potential cost angle, on the other hand. Unfortunately, above a potential difference of just 1.23 V, pure water experiences the thermodynamic driving force to decompose into hydrogen and oxygen gas, severely limiting the achievable energy density. As a useful analogy, the challenge to produce a 4 V stable aqueous electrolyte is comparable to stabilizing water in liquid form well above its boiling point. For this main reason, aqueous electrolyte chemistries have been left out of the commercial Li-ion battery mainstream, only used in low energy density nickel-metal hydride and lead acid batteries, but they remain an active area of fundamental research.6Kim H. Hong J. Park K.Y. Kim H. Kim S.W. Kang K. Chem. Rev. 2014; 114: 11788-11827Crossref PubMed Scopus (999) Google Scholar As early as 1994, Jeff Dahn and colleagues engineered a reversible aqueous cell capable of sustaining an average voltage of 1.5 V with a LiMn2O4 cathode, VO2 anode, and aqueous LiNO3 electrolyte (with a very small amount of LiOH).7Li W. Dahn J.R. Wainwright D.S. Science. 1994; 264: 1115-1118Crossref PubMed Scopus (978) Google Scholar By 2015, Dr. Wang and Dr. Xu developed a class of “water-in-salt” electrolytes capable of a 3 V electrolyte stability window and demonstrated its use in a 2.3 V full cell with LiMn2O4 cathode and Mo6S8 anode.8Suo L. Borodin O. Gao T. Olguin M. Ho J. Fan X. Luo C. Wang C. Xu K. Science. 2015; 350: 938-943Crossref PubMed Scopus (1925) Google Scholar In that work, the authors used a very high concentration of a complex Li-salt dissolved in water (more than 20 molal of LiTFSI, lithium bis[trifluoromethaneselfonyl]imide). In their current Joule paper, Dr. Wang and Dr. Xu have now crossed the 4 V mark and managed to bridge the long-standing chasm between aqueous and non-aqueous electrolyte stability. By tapping into knowledge accumulated in the battery community over many years of non-aqueous electrolyte design, characterization, and optimization, Xu and Wang were able to borrow key concepts and successfully apply them to water. The thermodynamic electrochemical stability window of modern non-aqueous Li-ion battery electrolytes also falls short of 4 V, just not to the same extent as water-based electrolytes. Rather, the 4 V electrochemical window in modern Li-ion batteries is realized through the electrolyte’s kinetic protection by formation of a passivating layer, which translates broadly to reducing the rate of destabilizing reactions (ideally to near zero). The most significant stability challenge resides at the battery’s anode at low applied voltage (with respect to Li metal), which is overcome in non-aqueous batteries by the additional kinetic stability afforded by the spontaneous formation in the early charge cycles of a thin interfacial layer comprised of reduced electrolyte products. This solid electrolyte interphase (SEI)9Peled E. Golodnitsky D. Ardel G. J. Electrochem. Soc. 1997; 144: L208-L210Crossref Scopus (762) Google Scholar, 10Verma P. Maire P. Novák P. Electrochim. Acta. 2010; 55: 6332-6341Crossref Scopus (2087) Google Scholar stabilizes prolonged cycling by facilitating Li ion conduction while blocking electron transport and consequently prevents further reduction of the electrolyte in ensuing cycles (ideally preventing H2 evolution when applied to aqueous electrolytes). Over the years, chemical and structural characterization of the SEI has revealed that it is truly a complex mosaic of different inorganic and polymeric organic species. Drawing inspiration from the SEI concept underpinning the operation of modern 4 V non-aqueous batteries, Dr. Wang and Dr. Xu adopted an “inhomogeneous SEI strategy,” where a strongly hydrophobic thin organic precursor coating designed for its SEI-forming ability was applied to the anode material (either graphite or Li metal) in cell assembly. Adding small amounts of preferentially reducing chemical species or “electrolyte additives” to the bulk electrolyte to ensure the formation of a better SEI layer is now common practice in non-aqueous Li-ion battery electrolyte design. However, in applying the approach to aqueous electrolytes, there is the additional difficulty of simultaneously preventing water molecules from approaching the negatively charged electrode surface before the SEI is in place and functioning. This cathodic preference for water molecules defines the ultimate challenge for a 4 V class aqueous electrolyte, as highlighted in Yang et al.’s Figure 1 and the accompanying Figure360 (see the figure legend at http://dx.doi.org/10.1016/j.joule.2017.08.009).5Yang C. Chen J. Qing T. Fan X. Sun W. von Cresce A. Ding M.S. Borodin O. Vatamanu J. Schroeder M.A. et al.Joule. 2017; 1 (this issue): 122-132Abstract Full Text Full Text PDF Scopus (350) Google Scholar The authors went through an exhaustive screening process to select a highly fluorinated ether additive completely immiscible with their “water-in-salt” electrolyte that overcomes the issue. During the first charge, this thin precursor layer is reduced to form a robust fluorine-containing SEI, as in standard organic electrolytes, without interference of H2 gas from water decomposition, because its strong hydrophobic nature expels water molecules from the inner-Helmholtz layer of the electrode. With this cell design, they assembled several 4 V aqueous batteries, cycled with LiVPO4F and LiMn2O4 cathodes against Li metal and graphite anodes, and demonstrated impressive coulombic efficiencies between 98% and 99.5% over the first 50 cycles—not quite ready for commercial deployment, but within striking distance. The authors’ aqueous battery design also boasts promising safety features with the potential for improved thermal and chemical stability against abuse or with ambient environment, as reflected in differential scanning calorimetry (DSC) experiments, compared to cells constructed with conventional non-aqueous electrolytes. Additionally, in dramatic physical abuse tests where a charged cell is punctured with a nail to initiate a short-circuit, no fire or smoke is observed with the aqueous cell design, but the opposite is often the case in conventional Li-ion cells. Most interestingly, the ambient-stable aqueous electrolyte allows the cell to graciously fail over a long time frame (hours as compared to seconds in non-aqueous Li-ion cells), as seen in Yang et al.’s Movie S3.5Yang C. Chen J. Qing T. Fan X. Sun W. von Cresce A. Ding M.S. Borodin O. Vatamanu J. Schroeder M.A. et al.Joule. 2017; 1 (this issue): 122-132Abstract Full Text Full Text PDF Scopus (350) Google Scholar In summary, Li-ion batteries have served humanity admirably, and the outlook for reliable renewable energy storage appears as bright as ever. If costs can continue down their historic negative trajectory, there is the realistic opportunity to integrate storage with intermittent energy sources, such as wind and solar, and fully realize a reliable electrical grid powered by renewable energy.11Dunn B. Kamath H. Tarascon J.-M. Science. 2011; 334: 928-935Crossref PubMed Scopus (10075) Google Scholar Ultimately, the enormous scale of this energy transition critically depends on constantly improving upon existing technology and discovering new and viable options, and this development of 4 V aqueous Li-ion batteries represents a significant step forward on both fronts. 4.0 V Aqueous Li-Ion BatteriesYang et al.JouleSeptember 06, 2017In Brief4.0 V aqueous LIBs of both high energy density and high safety are made possible by a new interphase formed from an “inhomogeneous additive” approach that effectively stabilizes graphite or lithium-metal anode materials. Full-Text PDF Open Archive