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Interdigitated Eutectic Alloy Foil Anodes for Rechargeable Batteries Karl J. Kreder, III, Brian T. Heligman, and Arumugam Manthiram* Materials Science and Engineering Program & Texas Materials Institute, The University of Texas at Austin, Austin, Texas 78712, United States S Supporting Information *
active layer have shown diminished rate performance and an increased propensity for lithium plating.13 As a metallic foil, the IdEA anode does not require an external current collector, significantly reducing the fraction of inactive anode components. The IdEA anode investigated here offers a full realized capacity of 250 mA h g−1 for more than 150 cycles, a significant improvement upon the graphite/copper composite anode. When the energy density of the AZT IdEA anodes is compared to that of the graphite-based systems, the higher average discharge voltage of ∼0.4 V leads to a 10% decrease in the operating voltage. This energy penalty is more than compensated by the >50% increase in the gravimetric capacity. While this initial exploration of the IdEA anode framework utilized aluminum, zinc, and tin with lithium, future embodiments of the IdEA anode framework may utilize alternative alloy systems or other working ions such as Na and Mg; the concept can radically change the battery industry. The various compositions investigated in this work are given in Table S1. The morphology of both alumimum−tin (AT) and aluminum−zinc−tin (AZT) alloys with 50−70 wt % Sn were characterized with scanning electron microscopy (SEM) as well as elemental maps generated with energy dispersive X-ray spectroscopy (EDX). Prior to the analysis, both the as-cast ingots as well as the foils were sectioned, mounted, and polished with traditional metallographic preparation techniques. The SEM micrographs of AT ingots reveal an archetypical binary eutectic alloy, shown in Figure 1b−g. Primary aluminum dendrites are surrounded by tin with no observed aluminum−tin miscibility. The size scale of the aluminum and tin features seen in the micrographs of the as-cast ingots are on the order of tens of micrometers. The as-cast ingots were cold-rolled anisotropically, such that there was an ∼100-times elongation along the x-axis, a corresponding ∼100-times reduction along the z-axis, and little change along the y-axis, as seen in Figure 1b,h−m. The macroscopic deformation of the ingots into foils induced similar changes in the microstructure of the foils. Most importantly, the aluminum/tin feature sizes in the rolled foils were reduced to ∼200 nm along the z-axis, which is critical in mitigating electrochemical milling and allowing reversible alloying with lithium.14 SEM micrographs and EDX maps of the AZT ingots and foils can be seen in Figure S1. In the AZT foils, the zinc does not appear as its own separate phase or in a eutectic
ABSTRACT: An interdigitated eutectic alloy (IdEA) foil is presented as a framework for the development of alloy anodes with a capacity that is significantly higher than that of the traditional graphite/copper assembly. It is a simple, low-cost approach that can be applied to a broad range of alloy systems with various working ions such as Li, Na, or Mg.
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hile alloy anodes offer high charge-storage capacities, the huge volume changes along with high irreversible capacity loss result in catastrophic capacity fade. Attempts to accommodate the volume changes by nanostructuring the material and buffering with inactive components often result in diminished active-material loading and high processing cost.1−4 An alternative processing strategy utilizes an eutectic alloy microstructure to accommodate the volume change.5−7 We demonstrate here that the casting and anisotropic cold rolling of a binary eutectic alloy offers a simple, scalable framework for the production of a high-performance, interdigitated eutectic alloy (IdEA) foil anode. Previous work on electrochemically active alloying foils have had limited impact due to the inability of the macroscopic material to accommodate the large volume change.8−10 The aluminum−tin IdEA foil anode presented here as an example has nanosized electrochemically active tin domains surrounded by an electrically conductive aluminum network, enabling stable cycling. To fully understand the benefits of the IdEA anode strategy, the practically realized energy density of graphite/copper composite anodes must be understood. Graphite has a theoretical capacity of 370 mA h g−1, but the realized capacity is only ∼150 mA h g−1 when the weights of the copper foil, conductive carbon, and binder are included.11,12 Efforts to improve the energy density of graphite/copper composites by increasing the thickness of the © 2017 American Chemical Society
Received: September 5, 2017 Accepted: September 19, 2017 Published: September 19, 2017 2422
DOI: 10.1021/acsenergylett.7b00844 ACS Energy Lett. 2017, 2, 2422−2423
Energy Express
Cite This: ACS Energy Lett. 2017, 2, 2422-2423
Energy Express
ACS Energy Letters
much of the “loss” during the first cycle proved reversible, and lithium recovery during the subsequent 15 cycles showed the irreversible first cycle losses to be 12.8%. With a capacity limit of 200 mA h g−1, the AZT70 alloy was able to achieve more than 200 cycles (Figure 2h), while with a capacity limit of 250 mA h g−1, the AZT70 alloy achieved more than 150 cycles at an average Coulombic efficiency of 97.9%, shown in Figure 2g. When the AZT70 alloy was cycled at a higher capacity of 300 mA h g−1, it was able to cycle for 83 cycles before abruptly dying. The difference in performance between the AT and AZT anodes is attributed to increased accessibility of tin in the zinc doped system. The tin accessibility may be higher because of an intrinsic change in the ionic conductivity of the tin−zinc solid solution, or because of the introduction of new unobserved phase boundaries between eutectic tin−zinc. Optimization of alloy composition, processing, and cycling parameters are expected to result in further improvements in Coulombic efficiency. The present work demonstrated aluminum−zinc−tin eutectic foils as an interdigitated eutectic alloy foil anode for lithium-ion batteries. All metrics of this first iteration of the AT/AZT IdEA system, including capacity, cost, Coulombic efficiency, average voltage, effective first cycle losses, and rate capability demonstrate the practical viability of a battery concept with a broad range of potential alloy systems and working ions.
Figure 1. (a) Illustration of the cast ingot, rolled into a foil, and subsequent lithiation of the tin portion of the foil. SEM micrographs and EDX elemental maps of both AT alloys before and after rolling. Specifically, SEM micrographs and EDX elemental maps of the (b, e) AT50, (c, f) AT60, (d, g) AT70, and (h, k) alloys as cast ingots. SEM micrographs and EDX elemental maps of the (h, k) AT50, (i, l) AT60, (j, m) AT70 alloys after anisotropic cold-rolling into foils. Tin is shown as red and aluminum as green in EDX elemental maps.
microstructure, rather according to EDX, zinc is spread evenly in both the aluminum and tin regions with a slight preference for the tin domains (Figure S2). The AT and AZT alloys were assembled into lithium half-cells and tested over several capacity-limited discharge ranges with a 1.0 V cutoff limit on charge. Cyclability and capacity limits tested of selected compositions are summarized in Table S2. The foils were highly dense with areal loadings between 10−15 mg cm−2, and the total foil weight was used when computing the gravimetric energy densities. The majority of the cycled capacity can be attributed to the first three redox peaks of Sn at 0.70, 0.57, and 0.45 V, shown in Figure 2d, rather than aluminum. When the
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.7b00844. Additional characterization of the IdEA anodes (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Arumugam Manthiram: 0000-0003-0237-9563 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Award Number DE-SC0005397.
Figure 2. Color images of AZT70 anode (a) before cycling (b) and after 40 cycles at C/10 rate at 250 mA h g−1. SEM image of (c) AZT70 anode after cycling with corresponding EDX elemental map. (d) Cyclic voltammogram of AZT70 performed from 0.3 to 1.0 V. (e) Charge−discharge profiles of AZT70 cycled at 250 mA h g−1. Cyclability of the AZT70 alloy, limited to (f) 300, (g) 250, and (h) 200 mA h g−1 capacity ranges in lithium half-cell.
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REFERENCES
(1) Derrien, B. G.; et al. Adv. Mater. 2007, 19, 2336−2340. (2) Cao, F. F.; et al. Adv. Mater. 2011, 23, 4415−4220. (3) Kreder, K. J.; Manthiram, A. Chem. Commun. 2017, 53, 865−868. (4) Xia, X.; et al. ACS Energy Lett. 2016, 1, 492−499. (5) Hu, R.; et al. J. Phys. Chem. C 2009, 113, 18953−18961. (6) Wang, J.; et al. J. Alloys Compd. 2014, 610, 308−314. (7) Yang, J.; et al. Solid State Ionics 1996, 90, 281−287. (8) Kuksenko, S. P. Russ. J. Electrochem. 2013, 49, 67−75. (9) Nyholm, L.; et al. J. Power Sources 2014, 269, 266−273. (10) Ji, B.; et al. Adv. Mater. 2017, 29, 1604219. (11) Yazami, R.; Touzain, Ph. J. Power Sources 1983, 9, 365−371. (12) Wood, D. L.; et al. J. Power Sources 2015, 275, 234−242. (13) Singh, M.; et al. J. Electrochem. Soc. 2015, 162, A1196−A1201. (14) Lee, S. W.; et al. Proc. Natl. Acad. Sci. U. S. A. 2012, 109, 4080− 4085.
lithiation of aluminum is minimized, occurring around 0.3 V, the formation of unstable, brittle, AlLi intermetallics is prevented, ensuring the integrity of the IdEA anode. While aluminum plateaus are present during the first several charge−discharge cycles, the subsequent delamination of the Al−Sn layers seen in Figure 2a−c caused a drastic increase in the amount of electrochemically active tin. For the AZT70 cell cycled at 300 mA h g−1, this resulted in an increase in tin capacity from from 5 mA h g−1 on cycle 1 to 190 mA h g−1 on the 10th cycle. No aluminum platueaus were observed for the bulk of the cycling, shown in Figure 2e. The first cycle Coulombic efficiency of AZT70 that was limited to 300 mA h g−1 was 72.9%. However, 2423
DOI: 10.1021/acsenergylett.7b00844 ACS Energy Lett. 2017, 2, 2422−2423
