Abstract: In this Backstory, Yet-Ming Chiang and colleagues explain how their cost-focused approach led to the discovery of an affordable battery technology based on readily available materials. Their findings appear in the October 2017 issue of Joule (http://www.cell.com/joule/fulltext/S2542-4351(17)30032-6). Can batteries ever reach the cost of pumped hydroelectric storage, which today constitutes more than 99% of the deployed energy storage in the world, but has limited growth potential due to its geographical and environmental constraints? This was the overarching question that motivated our project team, which had a 5-year mandate from the U.S. Department of Energy’s Joint Center for Energy Storage Research (JCESR) to answer such challenges. If achieved, such storage could scale in a way that allows intermittent renewable generation (wind and solar) to truly compete with fossil fuel-based electricity generation. Of JCESR’s audacious goals captured by their motto “5-5-5” (meaning 5 times reduction in cost, 5 times increase in energy density, accomplished in 5 years), for grid storage the authors considered the cost challenge to be the most important by far, since almost any electrochemical approach would have a hundred times or higher energy density compared to pumped hydro. Energy density was mainly important insofar as it enabled more compact systems of lower cost. Thus, we obsessed about the cost-per-stored-energy metric, US$/kWh, although other attributes such as lifetime, safety, and toxicity of components would naturally also be important.For low-cost grid storage, however, it was clear that a different set of search criteria would have to apply for new chemistries. Battery economics were not new to our group, from which laboratory research had previously spawned two Li-ion battery companies. In order to guide the group’s efforts, an initial version of Figure 1 in the manuscript, which is a chronological plot tracking the chemical cost of stored energy for various battery chemistries, had been developed for internal use starting in 2012. The chemical cost of stored energy (US$/kWh) is based on the cost of cathode, anode, and electrolyte as finished components and sets a floor for the cost of the final battery system. The obvious and surprising outcome in this analysis was the steeply rising cost (plotted here on a log scale) of battery electrochemistries developed over the past six decades. For applications such as electric vehicles and portable devices, this trend could be excused as the result of a collective, single-minded pursuit of higher energy density. For low-cost grid storage, however, it was clear that a different set of search criteria would have to apply for new chemistries. Within JCESR, techno-economic (TE) modeling of battery systems had come to be fully embraced. It might be said that JCESR legitimized the use of techno-economic analysis as a tool to guide basic research, an objective that notably included identifying which research directions to not pursue. We made the link early on that TWh storage (i.e., capability to support the grid) had to begin with materials that minimized the cost per electron stored. Therefore, it had to be sulfur, which has a US$/Ah cost that is over an order of magnitude lower than any other electroactive material, aside from water and air. We were especially motivated by images of the sulfur pyramids being constructed in Alberta, Canada from the waste from tar sands refineries; one existing stockpile of 4 million cubic meters, if turned into sulfur-based batteries, would store several times the entire pumped hydroelectric capacity in the world today (∼1.6 TWh). Furthermore, the TE modeling suggested that there were two viable paths to low US$/kWh cost storage: nonaqueous systems of higher electrolyte cost (the numerator effect) but also higher voltage and energy density (the denominator effect), or aqueous systems of lower electrolyte cost but also lower energy. Thinking of scalability to TWh, we bet on the aqueous path. Sulfur electrochemistry was familiar to JECSR; a vibrant program on high energy density lithium-sulfur batteries was underway, and the very first JCESR publication (from Yi Cui’s group at Stanford) had demonstrated a lithium-sulfur semi-flow battery in which a nonaqeuous polysulfide solution was the flowing electrode. Our group added to the flow battery concepts the idea of enhancing reversible precipitation of insulating solids (like Li2S) by incorporating a percolating network of nanoscale carbon, in effect creating a fluid electronic conductor. Several other flow battery approaches were also simultaneously being studied within JCESR, all of which were unified by system-level TE modeling. External to JCESR, a different kind of energy-system modeling was also taking place. An MIT and 24M colleague, Marco Ferrara, was modeling the characteristics of storage required to shape the intermittent output of wind and solar farms into output that a utility could directly substitute for current fossil-fuel power plants of various types. An intriguing early finding was that the duration of storage needed would likely be much longer than what is today considered “long duration,” namely many days, rather than hours long. If the concept was to use a sulfur electrode in an aqueous flow cell (this part was settled), it would have to serve as the negative electrode (anode) so that whichever positive electrode (cathode) was used would be stable in water. This is in contrast to the well-known nonaqeuous Li-S battery in which sulfur is the cathode. But what material could serve as the cathode? If it were too costly, it would dominate the cost of chemistry and the key advantage of sulfur would be lost. Zheng Li, who held the position of Research Scientist (and goes by his surname), led the search and constructed an extensive list of cathode candidates. Based on cost and performance, however, there were really only a few options that made sense. We decided to simultaneously screen several of these, one of which was potassium permanganate (KMnO4), a compound commonly used in high school chemistry labs. We expected that upon discharging (reduction), the permanganate solution would precipitate MnO2, a process that is normally irreversible. However, by employing the percolating electronic conductor concept, we hoped that the precipitation reaction might be rendered reversible. Li prepared an electrochemical cell comprised of a suspension of carbon in dissolved Li2S as the anode and a suspension of carbon in dissolved KMnO4 as the cathode, separated by a LiSICON (Li-conducting) solid electrolyte membrane. He found the cell charged and discharged reversibly and had a high charge-storing capacity (300 mAh/g-S). However, given that mistakes in electrochemical interpretation are not unheard of, we decided to verify the cathode reaction product using X-ray diffraction. The analysis showed that the permanganate cathode had indeed precipitated MnO2, but surprisingly, the process was irreversible—the precipitate did not re-dissolve upon charging. Therefore, the experiment had failed. But the result left unanswered a new puzzle: what was the source of the high reversible capacity? Upon further investigation, Li had the answer: the cathode was instead undergoing reversible oxygen reduction and oxygen evolution due to the charge imbalance created as Li+ ions were added to and removed from the cathode solution, respectively. The “ah ha!” moment occurred during our weekly project meeting in Yet-Ming’s office: the “failed” experiment uncovered an aqueous sulfur-based reaction couple for which no cathode compound was required—the ultimate in low-cost battery electrochemistry!One of several laboratory cell designs used in the research. Polysulfide solutions range from pale yellow to dark orange in appearance.View Large Image Figure ViewerDownload Hi-res image Download (PPT)Stephanie Eiler and Andres Badel carrying out rotating electrode experiments.View Large Image Figure ViewerDownload Hi-res image Download (PPT)… [T]he “failed” experiment uncovered an aqueous sulfur based reaction couple for which no cathode compound was required—the ultimate in low-cost battery electrochemistry! From this point, our research team quickly grew to include Fikile Brushett, who joined JCESR while a postdoc at Argonne but was now an assistant professor in MIT’s chemical engineering department. Fikile had co-authored the original flow battery TE modeling work from JCESR and had extensive expertise in polysulfide chemistry as well as flow batteries. Both Liang Su, who later succeeded Zheng Li as Research Scientist when Li left for a faculty position at Virginia Tech, and undergraduate Andres Badel came from Fikile’s group. Andres held the unusual MIT title of “SuperUROP” (“UROP” standing for Undergraduate Research Opportunities Program), an honor awarded to undergraduate students who perform at an especially high level in research. Within Yet-Ming’s group, graduate student Sam Pan, who holds degrees in materials science and computer science, was recruited to the project. Sam immediately pronounced the ongoing pace of experiments to be just like a hackathon. Graduate students Ping-Chun Tsai, and later Kai Xiang, were recruited to the project. Rounding out the research team were undergraduates Joey Valle and Stephanie Eiler. Stephanie is the youngest member of the team and joined the project in her first year at MIT. Upon arriving on campus, she had the good fortune to draw Susan Hockfield, MIT’s former president, as her academic advisor. When asked what she was interested in, she replied “chemistry and energy,” whereupon Susan immediately referred her to Yet-Ming. Serendipity also played a role in bringing this paper to Joule. One day this spring, Philip Earis (Joule Editor-in-Chief) had plans to meet Tonio Buonassisi of MIT’s mechanical engineering department for lunch at a Cambridge restaurant frequented by MIT faculty. Tonio arrived a few minutes early and found Yet-Ming sitting by the door, waiting for a lunch guest who, it turned out, had not shown up due to a scheduling mishap. Tonio and Philip invited Yet-Ming to instead join them for lunch, during which Philip introduced Joule and explained its approach of spanning scales of research and linking fundamental breakthrough science to real-world impact and applications. Upon hearing this, Yet-Ming opened his laptop and showed Philip draft versions of Figures 1 and 10—results which Yet-Ming felt were out of scope for most technical journals in the energy field. The alignment of interests was immediately apparent to both, and Philip promptly invited Yet-Ming to submit the paper to Joule. Since the inception of this work, the concept of long-duration energy storage has gained momentum. At the October 2017 Electrochemical Society Meeting, DOE’s Advanced Research Project Agency – Energy (ARPA-E) will present “Beyond the Hour and the Day: The Need and Technologies for Long-Duration Electrical Storage” (https://ecs.confex.com/ecs/232/webprogram/Paper105326.html), an announcement that is sure to invigorate the research community. Within JCESR, work on the topic has also broadened, with additional PIs joining the research effort in the area of membranes (Brett Helms, LBNL) and aqueous polysulfide chemistry (Linda Nazar, Waterloo, CA). Use-case modeling of long-duration storage has also gained traction in the form of new collaborative work underway at MIT with Marco Ferrara and others. Two new startup companies in long-duration storage are known to have been founded in the U.S. this fall. While it is still early in the game, these developments suggest that long-duration energy storage is on the cusp of breaking out as a new tool to enable low-carbon electricity. Air-Breathing Aqueous Sulfur Flow Battery for Ultralow-Cost Long-Duration Electrical StorageLi et al.JouleOctober 11, 2017In BriefThe dropping cost of wind and solar power intensifies the need for low-cost, efficient energy storage, which together with renewables can displace fossil fuels. While batteries for transportation and portable devices emphasize energy density as a primary consideration, here, low-cost, ultra-abundant reactants deployable at massive (TWh) scale are essential. An air-breathing aqueous sulfur flow battery approach with ultralow energy cost is demonstrated at laboratory scale and shown to have economics similar to pumped hydroelectric storage without its geographical and environmental limitations. Full-Text PDF Open Archive