Guest Commentary pubs.acs.org/JPCL
Expanding the Ragone Plot: Pushing the Limits of Energy Storage
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as well as understanding how to enhance the charge stored in electrode materials at a given applied electrostatic potential.4−6 For batteries, many directions are currently being pursued to provide higher energy density and lower cost than current stateof-the-art Li-ion batteries. These directions focus on improving materials for Li-ion batteries, including the development of high-energy electrode materials such as silicon anodes7 and high capacity Li-rich cathodes,8 and new, high-energy battery chemistries, such as lithium−sulfur,9 lithium−air,10 and magnesium-ion.11 It is worth noting that Li-ion batteries remain the dominant high-energy rechargeable battery technology because they provide all necessary requirements for a commercial battery: they have reasonably long service lifetimes (∼2−10 years), are relatively cost-effective ($250− 400/kWh using various estimates), have high gravimetric and volumetric energy densities (250−300 Wh/kg and 600−700 Wh/L on a cell-level), high power capabilities (as anyone who has driven a Tesla Model S can attest), and are sufficiently safe to power portable electronics and plug-in electric vehicles.12,13 Unfortunately, all “beyond Li-ion” batteries currently suffer from deficiencies in at least one of these categories, although they do hold promise to surpass Li-ion in others (typically energy density and affordability). One of these batteries, the Mg-ion battery, is highlighted in the excellent Perspective by Bucur et al (Bucur, C. B.; et al. Confession of a Magnesium Battery. J. Phys. Chem. Lett. 2015, 6, 10.1021/acs.jpclett.5b01219). Mg-ion batteries potentially have a few advantages over Li-ion batteries. First, although magnesium has a higher molecular weight than lithium, it ionizes to a divalent ion, providing twice as much charge per atom as Li. Mg’s higher gravimetric density than Li also enhances its advantage in volumetric storage capacity. Furthermore, unlike Li metal, Mg metal has been observed to strip and plate without dendrite formation at high Coulombic efficiency in certain organic electrolyte compositions. However, Mg-ion battery development is currently inhibited by challenges in identifying practical electrolyte and cathode compositions. Many of the electrolytes in which efficient Mg stripping-plating is observed are either pyrophoric or have low anodic stability, disallowing them to be used in the presence of a high voltage cathode. Furthermore, high voltage cathode materials for Mg-ion batteries that exhibit reversible Mg-ion insertion/deinsertion are scarce, and most have low gravimetric capacities, even compared to typical Li-ion battery cathodes. Bucur et al. provide an important overview of the current understanding of these challenges and potential research directions to solve them. Although capacitors are not viable for large-scale, high-energy storage, they have found commercial use in applications that need fast, pulsed power (for example, lasers) and leveling of current fluctuations in control electronics. In fact, capacitors have clear advantages over batteries in low-energy applications, as they are typically cost-effective, can be charged significantly
ortable energy storage devices are prevalent in our everyday lives, from powering laptops and cell phones, to serving as a backup energy supply in numerous electronic applications, including those in military operations, automobiles, satellites, and remote sensors. Yet, emerging markets and technologies will continue to increase the importance of lightweight, affordable, long-life energy storage. For example, as they become an increasingly viable option for residential and commercial use, distributed renewable energy sources present a potentially enormous market for energy storage; Tesla recently announced that it will start selling a ∼ 10 kWh battery pack, the Powerwall, for this purpose.1 Furthermore, electric vehicles, which demand high-power, high-energy storage, are gaining a small foothold in the light vehicle market, although inadequate energy storage has been cited as the primary bottleneck limiting their widespread adoption.2 To this end, capacitors and batteries are both used in various energy storage applications, but for different purposes. Traditional capacitors store energy through electrostatic charging at their electrode−electrolyte interfaces under an applied potential. This process allows capacitors to deliver energy very rapidly, but only in small amounts. In contrast, batteries store energy through electrochemical reactions that typically occur throughout the entire bulk of their electrode active materials, thereby allowing comparatively large amounts of energy to be stored, although without being as quickly delivered as in capacitors. This power/energy trade-off is captured in the so-called Ragone plot, shown in Figure 1.
Figure 1. Ragone plot comparing various electrochemical energy storage devices. In electric vehicles, increasing specific energy would increase charge-to-charge range, whereas increasing specific power would enhance the vehicle’s acceleration. Courtesy of Venkat Srinivasan.3
Energy storage research generally focuses on moving every device’s performance closer to the upper right-hand corner of this plot. For capacitors, increasing specific energy is crucial and remains a limitation impeding them from being implemented in large-scale energy storage systems. To increase energy density, researchers are actively pursuing new electrolytes that will potentially provide a large operating potential stability window, © 2015 American Chemical Society
Published: September 17, 2015 3592
DOI: 10.1021/acs.jpclett.5b01813 J. Phys. Chem. Lett. 2015, 6, 3592−3593
Guest Commentary
The Journal of Physical Chemistry Letters
(10) Girishkumar, G.; McCloskey, B.; Luntz, A. C.; Swanson, S.; Wilcke, W. Lithium−air battery: promise and challenges. J. Phys. Chem. Lett. 2010, 1, 2193−2203. (11) Muldoon, J.; Bucur, C. B.; Gregory, T. Quest for nonaqueous multivalent secondary batteries: magnesium and beyond. Chem. Rev. 2014, 114, 11683−11720. (12) Manthiram, A. Materials challenges and opportunities of lithium ion batteries. J. Phys. Chem. Lett. 2011, 2, 176−184. (13) Abraham, K. M. Prospects and limits of energy storage in batteries. J. Phys. Chem. Lett. 2015, 6, 830−844. (14) Cao, J.; Emadi, A. A new battery/ultracapacitor hybrid energy storage system for electric, hybrid, and plug-in hybrid electric vehicles. Power Electronics, IEEE Transactions on 2012, 27, 122−132.
faster, and have much longer lifetimes, the latter two being a result of no electrochemical reactions occurring in appropriately designed capacitors. Another interesting direction of research includes hybrid capacitor/battery systems, which are being considered for large-scale applications such as electric vehicles, where the capacitive storage propels the car during acceleration or stores energy from regenerative braking, when high-power operation is necessary, whereas battery storage is used during lower-power cruising.14 Vatamanu and Bedrov provide an interesting perspective on materials challenges facing capacitors. They emphasize that understanding electrode−electrolyte interactions will be critical to identify new materials to enhance capacitor energy density. They particularly highlight the use of electrolytes employing room temperature ionic liquids (RTILs), which can exhibit high potential stability windows, among other useful properties. Applying theoretical molecular dynamics calculations to the behavior of RTILs and other promising electrolytes under confined conditions prevalent in highly porous solid electrode materials may also provide important insight into optimize capacitor performance. In conclusion, it is an exciting time for research in energy storage. Our community’s goal will be to ensure batteries and capacitors continuously improve to power new, demanding applications at reasonable costs and service lifetimes. There’s also no question that expanding the Ragone plot into the highenergy and high-power regions will be critical in our pursuit of powering extremely important emerging green technologies, such as electric vehicles and distributed renewable energy.
Bryan D. McCloskey
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Department of Chemical and Biomolecular Engineering, University of California, Berkeley, California 94720, United States Environmental Energy Technologies Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
REFERENCES
(1) Tesla Powerwall. http://www.teslamotors.com/powerwall (accessed July 2015). (2) Wagner, F. T.; Lakshmanan, B.; Mathias, M. F. Electrochemistry and the future of the automobile. J. Phys. Chem. Lett. 2010, 1, 2204− 2219. (3) The Three Laws of Batteries (and a Bonus Zeroth Law). https:// gigaom.com/2011/03/18/the-three-laws-of-batteries-and-a-bonuszeroth-law/ (accessed July 2015). (4) Chen, J.; Li, C.; Shi, G. Graphene materials for electrochemical capacitors. J. Phys. Chem. Lett. 2013, 4, 1244−1253. (5) Jiang, D.-e.; Wu, J. Microscopic insights into the electrochemical behavior of nonaqueous electrolytes in electric double-layer capacitors. J. Phys. Chem. Lett. 2013, 4, 1260−1267. (6) Richey, F. W.; Elabd, Y. A. In situ molecular level measurements of ion dynamics in an electrochemical capacitor. J. Phys. Chem. Lett. 2012, 3, 3297−3301. (7) Chan, C. K.; Peng, H.; Liu, G.; McIlwrath, K.; Zhang, X. F.; Huggins, R. A.; Cui, Y. High-performance lithium battery anodes using silicon nanowires. Nat. Nanotechnol. 2008, 3, 31−35. (8) Erickson, E. M.; Ghanty, C.; Aurbach, D. New horizons for conventional lithium ion battery technology. J. Phys. Chem. Lett. 2014, 5, 3313−3324. (9) Pascal, T. A.; Wujcik, K. H.; Velasco-Velez, J.; Wu, C.; Teran, A. A.; Kapilashrami, M.; Cabana, J.; Guo, J.; Salmeron, M.; Balsara, N.; et al. X-ray absorption spectra of dissolved polysulfides in lithium− sulfur batteries from first-principles. J. Phys. Chem. Lett. 2014, 5, 1547− 1551. 3593
DOI: 10.1021/acs.jpclett.5b01813 J. Phys. Chem. Lett. 2015, 6, 3592−3593
