Guest Commentary pubs.acs.org/JPCL
Fundamental Challenges Facing Next-Generation Li Ion Batteries
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the same time. Despite over 4 decades of research, the understanding of the composition and properties of the EEI layer is limited. Gauthier et al. highlight a critical need for understanding the reactions with electrolyte in light of the recent developments in high-capacity positive electrodes. While the importance of the EEI layers for battery performance is widely acknowledged, the exact mechanisms of how they form still remain poorly understood. Among these, Gauthier et al. point out that the mosaic model for the SEI layer on lithium and graphite is widely accepted; however, this has not been fully verified experimentally. In this Perspective, Gauthier et al. advocate for the development of synchrotron and in situ techniques to help identify the missing pieces of the SEI puzzle [Gauthier, M. J. Phys. Chem. Lett. 2015, 6, 4653−4672]. The development of high-capacity positive electrode materials leads to the generation of highly reactive oxygen species (superoxide or peroxo-like) on the EEI layer, and the high nucleophilicity of the oxygen species on the EEI layer can decompose electrolyte solvent molecules. A similar issue has been reported in the case of Li−air batteries, where the solution-mediated discharge growth of lithium peroxide proceeds via a superoxide intermediate that causes solvent degradation via nucleophilic attack.6 This highlights an urgent need for preparing model electrodes based on pellets or thin films that allow for a thorough investigation of the interaction between the electrolyte and the active surface facet. Many-next generation anodes, such as Sn and Si, are being actively pursued, but they suffer from large volume expansion and contraction during charging and discharging, respectively. Electrochemical cycling induces mechanical failure, which is a major source of inefficiency that limits the life of Li ion batteries.7 The development of a systematic framework that links electrochemistry and solid mechanics for Li ion batteries still remains in its infancy. In the third Perspective in this issue, Kusoglu and Weber discuss the electrochemical−mechanical coupling in the context of ion-conducting soft materials and provide a comprehensive discussion of the various types of defects that can form in solid−electrolyte membranes or at its interfaces [Kusoglu, A.; Weber, A. Z. J. Phys. Chem. Lett. 2015, 6, 4547−4552]. A grand challenge that faces solid-state electrolytes is to address simultaneously the performance and durability demands, that is, to reduce transport resistances without compromising mechanical stability. The Perspective by Kusoglu and Weber points out that identifying the origins and impacts of electrochemical phenomena on mechano-chemistry could allow one to exploit and control reactions. This provides the opportunity to identify, optimize, and harness mechanical and electrochemical phenomena that could synergistically go beyond what can be achieved independently. An electrochemical−mechanical coupling approach has been demonstrated recently where compressive strain was used to tune the electrocatalysis of platinum films the for oxygen reduction reaction.8
move toward electrification of road transportation is becoming a societal goal of vital importance. There are several factors driving this: the need to shift away from oil, a need to curb CO2 emissions, and the urgent need to improve air quality in densely populated cities. The issues associated with air quality have become so alarming that there is a major push toward electric vehicle (EV) adoption in densely populated cities to improve the quality of breathable air and limit particulate matter levels. The current state of EVs is well-summarized by a quote in the Washington Post, “Prices on electric cars will continue to drop until they’re within the reach of the average family.”1 This quote was featured in the Post in 1915, and a century later, we are still facing the same cost issue. EV adoption is limited primarily due to the cost and inadequate storage capacity of today’s Li ion batteries powering EVs. New battery chemistries are being explored as possible high-energy alternatives to current Li ion batteries (the so-called “Beyond Li Ion” (BLI) chemistries). Among these BLI chemistries, there has been intense research activity into lithium−air and lithium−sulfur over the last 5 years.2,3 More recently, solid-state Li ion batteries have attracted commercial interest notably with the acquisitions of two startups, Seeo and Sakti3, by Bosch and Dyson, respectively.4,5 This issue of JPC Lett. includes three Perspectives that cover various issues associated with these BLI chemistries. In this issue of JPC Lett., McCloskey presents a comprehensive discussion of the practically achievable energy density and cost for a lithium−sulfur battery [McCloskey, B. D. J. Phys. Chem. Lett. 2015, 6, 4581−4588]. On the basis of simple faradaic calculations, he argues that an electrolyte to sulfur (E/S) ratio of 11.1 and 4.0 is the point where an “idealized” Li−S battery will surpass a traditional Li ion battery on a specific energy (Wh/kg) and energy density (Wh/L) basis. He makes a strong recommendation that all future Li−S studies focus on E/S ratios less than 11 and more realistically around ∼5. Enabling Li−S batteries requires enabling lithium metal anodes, and this also enables a battery where the Li metal anode can be coupled with an advanced intercalation-based cathode material. He shows that it is very unlikely that a practical Li−S battery can compete favorably on a volumetric energy density basis with such a battery consisting of a Li metal anode and an advanced intercalation cathode material. Finally, to enable Li−S batteries requires a solid separator that can suppress dendrite formation and reduce parasitic reactions between Li and electrolyte constituents. He identifies a cost target of ∼$10/m2 for the separator in order for a Li−S cell to be a cost-competitive alternative to Li ion batteries. These targets are indeed daunting and require some innovative solutions. A direction of research that could achieve the low E/ S ratios is to identify encapsulating materials for S that possess high Li insertion potentials. Stable and reliable functioning of Li ion batteries hinges on the formation of a stable electrode−electrolyte interface (EEI) layer that is conductive to Li+ and is electronically insulating at © 2015 American Chemical Society
Published: November 19, 2015 4673
DOI: 10.1021/acs.jpclett.5b02411 J. Phys. Chem. Lett. 2015, 6, 4673−4674
Guest Commentary
The Journal of Physical Chemistry Letters The development of next-generation energy-dense, longlasting batteries involves overcoming fundamental challenges in engineering interfaces. As illustrated by these Perspectives, the engineering of these interfaces will require filling the gaps in our current understanding of the chemical, electrical, mechanical, and thermal properties of the interface and, more importantly, an understanding of the coupling between these aspects.
Venkatasubramanian Viswanathan*
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Department of Mechanical Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, United States
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected].
RELATED READINGS
(1) A Million Electric Vehicles. http://www.nationalreview.com/ article/268815/million-electric-vehicles-robert-bryce (June 7, 2011). (2) Luntz, A. C.; McCloskey, B. D. Nonaqueous Li−air batteries: a status report. Chem. Rev. 2014, 114, 11721−11750. (3) Manthiram, A.; Fu, Y.; Su, Y.-S. Challenges and prospects of lithium−sulfur batteries. Acc. Chem. Res. 2013, 46, 1125−1134. (4) LeVine, S. Vacuum cleaner-maker Dyson is buying experimental battery startup Sakti3. Quartz; 2015; http://qz.com/525623. (5) LeVine, S. Bosch is acquiring the advanced battery-tech start-up Seeo. Quartz; 2015; http://qz.com/489123. (6) Khetan, A.; Luntz, A.; Viswanathan, V. Trade-Offs in Capacity and Rechargeability in Nonaqueous Li-O2 Batteries: Solution-Driven Growth versus Nucleophilic Stability. J. Phys. Chem. Lett. 2015, 6, 1254−1259. (7) Woodford, W. H.; Carter, W. C.; Chiang, Y.-M. Design criteria for electrochemical shock resistant battery electrodes. Energy Environ. Sci. 2012, 5, 8014−8024. (8) Sethuraman, V. A.; Vairavapandian, D.; Lafouresse, M. C.; Maark, T. A.; Karan, N.; Sun, S.; Bertocci, U.; Peterson, A. A.; Stafford, G. R.; Guduru, P. R. Role of Elastic Strain on Electrocatalysis of Oxygen Reduction Reaction on Pt. J. Phys. Chem. C 2015, 119, 19042−19052.
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DOI: 10.1021/acs.jpclett.5b02411 J. Phys. Chem. Lett. 2015, 6, 4673−4674
