Guest Commentary pubs.acs.org/JPCL
Nanomaterials Developments for Higher-Performance Lithium Ion Batteries
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exhibited different levels of expansion and contraction during lithiation/delithiation that eventually led to degradation. The next generation of materials involved the creation of particles with a controlled composition gradient, thus alleviating the abrupt interface between materials with different amounts of expansion and contraction. Further improvements in the cathode material were obtained by creating rod-shaped particles instead of spheres. Each of these impressive advances in materials synthesis led to improvements in capacity, cycle stability, and safety. On the anode side, Si is closing in on commercial viability as an alternative to graphite as a LIB electrode material. Si has nearly 10 times the charge storage capacity of graphite, and advances in Si materials chemistry and battery formulation have enabled many demonstrations now of exceptional capacities, greater than 1000 mA h g−1 for more than hundreds of cycles.3,6 Still, there are many challenges facing commercial use of Si anodes in LIBs, including current matching between the cathode and anode, as well as making Si materials costs competitive with graphite. Perhaps the most significant challenge, however, remains in attaining long-term, stable battery performance at reasonably fast charge/discharge rates. To fully lithiate Si to the saturated Li−Si phase, Li15Si4, Si must expand by nearly 300%; it is like a sponge for Li. The Si “crystal”7 must itself tolerate this expansion and not fall to pieces; furthermore, the entire battery must be designed to accommodate these volume changes. Many of the advances in Si nanomaterials design for LIBs have focused on creating structures that are mechanically robust under strenuous conditions of large volume expansions and contractions, and many very creative concepts have emerged. As described by Song, et al., 5 these include the creation of porous nanostructures and nanostructures with mechanically strengthening and chemically protective coatings. Many of these nanomaterials design criteria have evolved from the combination of new synthetic capability with new understanding due especially to powerful in situ electron microscopy techniques that allow direct visualization of individual nanostructures as they undergo lithiation and delithiation.5−8 New phenomena have been observed, such as reversible mesoscopic pore formation9 and unanticipated degradation pathways, like those arising from spatially anisotropic expansion and contraction.5 Similar to the approach taken for cathodes described by Myung, et al.4 of integrating materials with complementary positive attributes to make up for their shortcomings, anode materials have been made with combinations of carbon, Si, and germanium (Ge). Si has the highest Li storage capacity of these three materials but has the poorest electrical conductivity. Battery cycling occurs by electrochemical oxidation and reduction, and electrical current needs to move through the electrodes. Intrinsic Si has such poor
ithium ion batteries (LIBs) provide portable power for a variety of applications, ranging from personal electronic devices to electric vehicles. They are fully rechargeable, have high energy density, high operating voltage, and low selfdischarge, and require little maintenance; yet, future applications require more advanced batteries.1 Electric vehicles and large-scale utility energy storage applications need batteries that can operate safely under widely varying environmental conditions with significantly higher energy density than is currently possible. Future needs, as in the integration of electronic devices with living systems (i.e., humans), require entirely new battery concepts: batteries must be very lightweight, mechanically flexible, conformable, and even stretchable, to enable seamless integration with active components, such as sensors and delivery devices, and those that harvest energy, such as microsolar cells or piezoelectric devices.2 The LIB consists of two electrodes, the cathode and the anode, each hosting the Li that cycles reversibly between these materials. The charge storage capacity of the battery depends on the amount of Li that each electrode can hold in the charged and discharged state, and there is an intense search for electrode materials that can hold more Li. The cathode is a Li metal oxide, such as LiCoO2, and the anode is graphite, with charge storage capacities of 137 (for LiCoO2) and 372 mA h g−1. To advance LIB capability, new cathode and anode materials are needed. On the anode side, there are several alternatives to graphite that have significantly higher charge storage capacity, and of these, silicon has the highest possible charge storage capacity of 3579 mA h g−1.3 Cathode replacements with extremely high storage capacity are harder to find, but there is an active search for more advanced cathodes as well.1,4 This issue of J. Phys. Chem. Lett. contains two Perspectives that highlight recent progress toward higherperformance LIB electrode materials. One paper by Song, Hu, and Paik highlights the creation of Si nanostructures with unique morphologies that can improve performance (especially, cycle stability) in LIBs.5 Another paper by Seung-Taek Myung, Hyung-Joo Noh, and co-workers focuses on advanced cathode materials with increased storage capacity combined with improved safety, which is a major challenge.4 Myung et al.4 describe their research group’s progress over the past 10 years as they have advanced toward higher-capacity cathode materials with improved safety. Their approach is to combine the positive attributes of higher storage materials (which turn out to be more dangerous) with lower storage materials that are safer. Their first generation of cathode materials with these attributes were core−shell particles with high capacity (200 mA h g−1) Li[Ni0.8Co0.1Mn0.1]O2 in the core coated with a protective shell of lower capacity (140 mA h g−1), Li[Ni0.5Mn0.5]O2. The core material has poor thermal stability associated with exothermic oxygen evolution at temperatures above 200 °C, whereas the shell material is much more stable. The problem with these materials was that the core and shell © 2014 American Chemical Society
Published: February 20, 2014 749
dx.doi.org/10.1021/jz5002242 | J. Phys. Chem. Lett. 2014, 5, 749−750
The Journal of Physical Chemistry Letters
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electrical conductivity that it can only cycle at very slow rates or when it is in intimate contact with the current collector. The addition of much more electrically conductive carbon and Ge to the Si nanostructures greatly improves performance of the nanowire electrodes by speeding the charge/discharge rate capability.5,10 The addition of coatings, such as graphene, can also mechanically stabilize the materials while improving other aspects in the electrode like the electrical conductivity.11 These developments, however, must be implemented in electrode layers with sufficient loading or “areal density” to provide useful amounts of current to power devices. Very thin electrode layers are not sufficient for real applications, and large amounts of Si nanomaterials are needed at a cost that is reasonably competitive with that of graphite. Thicker electrode films composed of nanoparticles or nanowires require a polymeric binder to hold the electrode material intact during electrode layer deposition and processing. The binder also keeps the anode layer intact as the Si expands and contracts, and the binder chemistry can dramatically influence the capacity and cycle stability.12 The other important factor in the battery is the electrolyte. Graphite undergoes a surface reaction with the electrolyte to form a chemically passivating solid−electrolyte interphase (SEI) layer, which is crucial to the performance of the LIB. Reactions involved in SEI layer formation on Si are different than those for graphite, and new electrolyte formulations are needed; for example, small additions of fluoroethylene carbonate (FEC) to the typical carbonate electrolyte formulation provides significant stability to Si anodes.13 Clearly, the future use of Si anodes in LIBs will rely on morphologically and compositionally advanced nanostructures as the active storage material formulated into electrode layers using subtle, yet distinct, modifications of the chemistry that has been so well developed for graphite. As new cathode and anode materials are developed, ultimately, the two materials must be combined into the same battery. This is not a trivial exercise. Si anodes have an order of magnitude larger charge storage capacity than the most advanced cathode materials. This means that batteries must be formulated using either very little anode material or massive amounts of cathode material. Neither approach is adequate at the moment, although progress is being made toward prelithiation of Si nanomaterials to help alleviate this problem.14 The batteries must also be extremely stable with long calendar life and operate safely. For applications like electric vehicles, safety is of the utmost importance and is a major challenge facing very high energy density LIBs. Perhaps one of the application areas most suited for next generation ultrahigh energy density lightweight LIBs is in medicine, where there is no current technology that can satisfy the need for portable power seamlessly integrated with medical devices interfaced with the human body. These applications require not only high storage capacity materials but materials with unique combinations of mechanical properties, such as those found in nanowires. The work described by Myung, et al.4 and Song, et al.5 provide examples of the rapid progress being made towards the developments of such batteries.
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REFERENCES
(1) Goodenough, J. B.; Kim, Y. Challenges for Rechargeable Li Batteries. Chem. Mater. 2010, 22, 587−603. (2) Lee, Y.-H.; Kim, J.-S.; Noh, J.; Lee, I.; Kim, H. J.; Choi, S.; Seo, J.; Jeon, S.; Kim, T.-S.; Lee, J.-Y.; Choi, J. W. Wearable Textile Battery Rechargeable by Solar Energy. Nano Lett. 2013, 13, 5753−5761. (3) Bogart, T. D.; Chockla, A. M.; Korgel, B. A. High Capacity Lithium Ion Battery Anodes of Silicon and Germanium. Curr. Opin. Chem. Eng. 2013, 2, 1−8. (4) Myung, S.-T.; Noh, H.-J.; Yoon, S.-J.; Lee, E.-J.; Sun, T.-K. Progress in High-Capacity Core−Shell Cathode Materials for Rechargeable Lithium Batteries. J. Phys. Chem. Lett. 2014, 4, 671−679. (5) Song, T.; Hu, L.; Paik, U. One-Dimensional Silicon Nanostructures for Li Ion Batteries. J. Phys. Chem. Lett. 2014, 4, 720−731. (6) Bogart, T. D.; Oka, D.; Lu, X.; Gu, M.; Wang, C.; Korgel, B. A. Lithium Ion Battery Performance of Silicon Nanowires with Carbon Skin. ACS Nano 2014, 8, 915−922. (7) Si becomes amorphous after lithiation. See, for example: McDowell, M. T.; Lee, S. W.; Harris, J. T.; Korgel, B. A.; Wang, C.; Nix, W. D.; Cui, Y. In Situ TEM of Two-Phase Lithiation of Amorphous Silicon Nanospheres. Nano Lett. 2013, 13, 758−764. (8) Santhanagopalan, D.; Qian, D.; McGilvray, T.; Wang, Z.; Wang, F.; Camino, F.; Graetz, J.; Dudney, N.; Meng, Y. S. Interface Limited Lithium Transport in Solid-State Batteries. J. Phys. Chem. Lett. 2014, 5, 298−303. (9) Choi, J. W.; McDonough, J.; Jeong, S.; Yoo, J. S.; Chan, C. K.; Cui, Y. Stepwise Nanopore Evolution in One-Dimensional Nanostructures. Nano Lett. 2010, 10, 1409−1413. (10) Chockla, A. M.; Harris, J. T.; Akhavan, V. A.; Bogart, T. D.; Holmberg, V. C.; Steinhagen, C.; Mullins, C. B.; Stevenson, K. J.; Korgel, B. A. Silicon Nanowire Fabric as a Lithium Ion Battery Electrode Material. J. Am. Chem. Soc. 2011, 133, 20914−20921. (11) Luo, J.; Zhao, X.; Wu, J.; Jang, H. D.; Kung, H. H.; Huang, J. Crumpled Graphene-Encapsulated Si Nanoparticles for Lithium Ion Battery Anodes. J. Phys. Chem. Lett. 2012, 3, 1824−1829. (12) Kovalenko, I.; Zdyrko, B.; Magasinski, A.; Hertzberg, B.; Milicev, Z.; Burtovyy, R.; Luzinov, I.; Yushin, G. A Major Constituent of Brown Algae for Use in High-Capacity Li-Ion Batteries. Science 2011, 334, 75−79. (13) Chockla, A. M.; Bogart, T. D.; Hessel, C. M.; Klavetter, K. C.; Mullins, C. B.; Korgel, B. A. Influences of Gold, Binder and Electrolyte on Silicon Nanowire Performance in Li-Ion Batteries. J. Phys. Chem. C 2012, 116, 18079−18086. (14) Ma, R.; Liu, Y.; He, Y.; Gao, M.; Pan, H. Chemical Preinsertion of Lithium: An Approach to Improve the Intrinsic Capacity Retention of Bulk Si Anodes for Li-Ion Batteries. J. Phys. Chem. Lett. 2012, 3, 3555−3558.
Brian A. Korgel*
McKetta Department of Chemical Engineering, Texas Materials Institute, Center for Nano- and Molecular Science and Technology, The University of Texas at Austin, Austin, Texas 78712, United States 750
dx.doi.org/10.1021/jz5002242 | J. Phys. Chem. Lett. 2014, 5, 749−750
