Pseudo-solid State Batteries See the Light

Abstract

Dunn and colleagues design and synthesize a thin film pseudo-solid state electrolyte for next-generation lithium batteries based on solution-processed ionogels. They first demonstrate 125 mAh/g capacity for 150 cycles with minimal capacity loss in lithium iron phosphate and lithium metal full cell devices at C/2 rate. Next, they demonstrate the ability to photo-pattern the ionogel, and thus enable versatile and scalable fabrication and processing routes, while also retaining similar excellent electrochemical performance. Dunn and colleagues design and synthesize a thin film pseudo-solid state electrolyte for next-generation lithium batteries based on solution-processed ionogels. They first demonstrate 125 mAh/g capacity for 150 cycles with minimal capacity loss in lithium iron phosphate and lithium metal full cell devices at C/2 rate. Next, they demonstrate the ability to photo-pattern the ionogel, and thus enable versatile and scalable fabrication and processing routes, while also retaining similar excellent electrochemical performance. In the human struggle to address climate change while also meeting anticipated increases in energy demand associated with rising world living standards and population, prominent voices have singled out advances in energy storage, specifically battery technology, as necessary.1Deutch J. Joule. 2017; 1: 3-5Abstract Full Text Full Text PDF Scopus (38) Google Scholar, 2McKibben, B. “It’s Time to Stand Up for the Climate—and for Civilization.” Wired. January 18, 2017. https://www.wired.com/2017/01/stand-up-for-the-climate-and-civilization/.Google Scholar Lithium ion (Li-ion) batteries are the current gold standard for energy storage—today they power billions of mobile electronic devices, and in the projected near future they are expected to replace gasoline in vehicular transport, even without subsidy.3“Electric Cars: The Death of the Internal Combustion Engine.” The Economist. August 12, 2017. https://www.economist.com/news/leaders/21726071-it-had-good-run-end-sight-machine-changed-world-death.Google Scholar While the decrease in energy storage cost (evaluated in $/kWh) associated with Li-ion has actually outpaced expectations following an accelerated learning curve,4Kittner N. Lill F. Kammen D.M. Nat. Energy. 2017; 2: 17125Crossref Scopus (493) Google Scholar we are still fast approaching fundamental electrochemical performance limits in terms of energy density that can be achieved with canonical Li-ion battery chemistries and cell architecture.5Thackeray M.M. Wolverton C. Isaacs E.D. Energy Environ. Sci. 2012; 5: 7854-7863Crossref Scopus (1922) Google Scholar In recent years, this has motivated a substantial research push in so-called “post Li-ion” technologies, which look to novel chemistries or cell designs (or both).6Goodenough J.B. Park K.-S. J. Am. Chem. Soc. 2013; 135: 1167-1176Crossref PubMed Scopus (6323) Google Scholar One of the most promising approaches involves shifting to an all solid-state architecture by replacing the traditional liquid non-aqueous electrolyte with a suitable solid Li-ion conductor. With this strategy, there are new and realistic opportunities to outperform traditional Li-ion batteries in a number of important aspects: a functioning solid-state electrolyte could eliminate safety concerns that typically arise with flammable liquid electrolytes, mitigate the formation of Li dendrites and thus enable the use of high energy density Li metal anodes, and reduce parasitic side reactions that occur in liquid electrolyte batteries, which reduce operational lifetime. Moreover, employing a solid-state electrolyte relaxes the requirement for a physical non-electrochemically active separator between anode and cathode (to avoid short-circuit). Significantly, it removes the restriction of constructing cells in a traditional planar architecture, enabling three-dimensional battery designs that allow more energy to be stored in a smaller package, accessible at shorter charge and discharge times. These unique features suggest that solid-state batteries could first make their way to market as “microbatteries”7Oudenhoven J.F.M. Baggetto L. Notten P.H.L. Adv. Energy Mater. 2011; 1: 10-33Crossref Scopus (615) Google Scholar that provide on-board power supply to wireless microelectronic devices, giving rise to the growingly anticipated “Internet of Things.” At the same time, Toyota very recently made a bold announcement to sell cars in the early 2020s powered by solid-state batteries.8McLain, S. “Toyota Nears Technological Breakthrough in Electrical-Car Batteries.” The Wall Street Journal. July 24, 2017. https://www.wsj.com/articles/toyota-nears-major-technological-breakthrough-in-electric-car-batteries-1500985883.Google Scholar Despite showing great promise, there are significant scientific and engineering obstacles that prevent practical and widespread commercial adoption of solid-state electrolytes.9Janek J. Zeier W.G. Nat. Energy. 2016; 1: 16141Crossref Scopus (1619) Google Scholar The defining challenge is obtaining stable interfaces between solid electrolytes and electrodes,10Tian Y. Shi T. Richards W.D. Li J. Kim J.C. Bo S.-H. Ceder G. Energy Environ. Sci. 2017; 10: 1150-1166Crossref Google Scholar which remains a very active area of research. To navigate around the challenges posed by incorporating a solid-state electrolyte into Li-ion batteries, one clever strategy is to design an electrolyte that seeks the benefits of both solid-state and traditional liquid electrolytes.11Vioux A. Coasne B. Adv. Energy Mater. 2017; (Published online September 4, 2017)https://doi.org/10.1002/aenm.201700883Crossref PubMed Scopus (31) Google Scholar In this issue of Joule, Dr. Bruce Dunn and colleagues make two important strides forward in developing high-performance and scalable pseudo-solid state electrolytes based on the concept of ionogels.12Ashby D.S. Deblock R.H. Lai C.-H. Choi C.S. Dunn B. Joule. 2017; 1 (this issue): 344-358Abstract Full Text Full Text PDF Scopus (44) Google Scholar They first demonstrate the ability to synthesize a high-quality ionogel thin film less than 1 micron thick, thinner than previous reported works (typically hundreds of microns), and then implement in high-power density full cells. Second, they impressively demonstrate the capability of templating the ionogel electrolyte in regular geometries using a photo-patterning procedure, which highlights the potential for high-throughput fabrication and processing, a critical feature required for scale-up and large-scale commercialization. Using a low-temperature sol-gel synthesis procedure, Dr. Dunn and colleagues are able to produce a dense, macroscopically rigid material comprised of an ionic liquid solvent (here 0.5M lithium bis(trifluoromethanesulfonyl)imide, or LiTFSI) that penetrates into the pores of a mesoporous inorganic solid structural framework (here a silica, SiO2, network). The ionic liquid then remains physically confined due to capillary forces. Because the synthesis procedure occurs in two main stages, beginning first as a low-viscosity solution and then later undergoing an irreversible sol-gel transition to the pseudo-solid state, the electrolyte in liquid form can achieve good wetting of the electrode material and establish a high-fidelity interface between electrode and pseudo-solid electrolyte, which ensures both high capacity and rate capability. Indeed, in full cells comprised of a Li metal anode, ionogel electrolyte, and lithium iron phosphate (LiFePO4) cathode, the authors were able to obtain capacities of 145 mAh/g in a 10 hr charge and discharge time, matching the state-of-the-art performance with ionic liquid electrolytes, and even more impressively maintained capacities of 125 mAh/g in half hour charge and discharge intervals, with only 4% loss of capacity over 150 cycles and coulombic efficiency above 99%. Next, Dunn and colleagues adapted their processing procedure such that the crosslinking process associated with producing the pseudo-solid electrolyte could be triggered upon UV exposure. Patterning using photolithography is probably most well known in the semiconductor industry, where it is used at industrial scale for microfabrication of complex integrated circuits. By adding this functionality to battery design and processing, Dunn and colleagues are able to translate this technology for the first time to ionogels with a crosslinked inorganic matrix. With this new system, the authors were also able to achieve electrochemical performance similar to that of their previous full cells. In summary, this work is a shining example of the critical importance of pursuing energy research not only for the purpose of achieving optimized lab-scale performance, but also for realizing commercial scale-up. As we have witnessed recently with traditional Li-ion batteries and silicon photovoltaics, production in high volumes is one of the key drivers of plummeting prices and improved performance, and as such it makes perfect sense to incorporate these considerations into the research process at the earliest stages. With the advances in synthesis and processing of ionogel electrolytes made in this work, we inch closer to pseudo-solid state batteries seeing the light. Patternable, Solution-Processed Ionogels for Thin-Film Lithium-Ion ElectrolytesAshby et al.JouleSeptember 27, 2017In BriefSolid electrolytes have the potential to be safer alternatives to liquid electrolytes for lithium-ion batteries while being effectively configurable for powering small electronics. However, solid electrolytes typically exhibit poor interfaces and low ionic conductivity. Ionogel electrolytes, consisting of ionic liquid trapped inside a mesoporous solid, mitigate these limitations by maintaining a nanoscale fluidic state while behaving macroscopically solid. Adapting the synthesis process to achieve photo-patterning enables ionogels to be utilized in a variety of device architectures. Full-Text PDF Open Archive