Article pubs.acs.org/cm
Improved Cycle Life and Stability of Lithium Metal Anodes through Ultrathin Atomic Layer Deposition Surface Treatments Eric Kazyak,†,¶ Kevin N. Wood,†,‡,¶ and Neil P. Dasgupta*,†,‡ †
Department of Mechanical Engineering, University of Michigan, Ann Arbor, Michigan 48109, United States Joint Center for Energy Storage Research, University of Michigan, Ann Arbor, Michigan 48109, United States
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Downloaded by UNIV OF CALIFORNIA SAN DIEGO on September 14, 2015 | http://pubs.acs.org Publication Date (Web): September 11, 2015 | doi: 10.1021/acs.chemmater.5b02789
S Supporting Information *
ABSTRACT: Improving the cycle life and failure resistance of lithium metal anodes is critical for next-generation rechargeable batteries. Here, we show that treating Li metal foil electrodes with ultrathin (∼2 nm) Al2O3 layers using atomic layer deposition (ALD) without air exposure can prevent dendrite formation upon cycling at a current density of 1 mA/cm2. This has the effect of doubling the lifetime of the anode before failure both under galvanostatic deep discharge conditions and cyclic plating/stripping of symmetric Li−Li cells. The ALD treated electrodes can be cycled for 1259 cycles before failure occurs, which is attributed to improved electrode morphology resulting from homogeneous Li ion flux across the electrode/electrolyte interface.
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which forms as dendrites become electrically isolated from the bulk electrode, leading to capacity fade of the cell over time.9 Unfortunately, dendrite growth is a positive feedback phenomenon; as cycling proceeds, deposition will preferentially occur on existing dendrites, exacerbating the problem.8 Work in the field of Li-ion batteries has previously shown that nanoscale coatings can significantly improve the cyclability and rate capability of a variety of anode and cathode materials.10−13 Several of these studies have shown that ultrathin Al2O3 layers can suppress SEI formation after electrolyte exposure, leading to reduced capacity fade and improved cyclability.11,14,15 Recent work has also attempted to stabilize the surface of Li metal anodes either through electrolyte modification or by interfacial layers to improve the homogeneity of Li electrodeposition and stripping.2,4,16−19 However, the majority of these papers have not explored direct coatings on the surface of bulk Li metal electrodes due to challenges associated with air instability and the low melting temperature of Li. Investigation of controlled Li electrodeposition on Cu current collectors has demonstrated the importance of improved homogeneity of Li flux to avoid dendrite formation.4,17 Therefore, it is valuable to extend this knowledge to facilitate homogeneous Li flux on bulk Li metal electrodes to prevent dendrite formation and enable their use in battery manufacturing. One promising approach for atomically precise modification of electrode/electrolyte interfaces is atomic layer deposition (ALD). ALD is a modified chemical vapor deposition process
atteries with longer lives, higher capacities, and reduced safety concerns are needed for many applications including electric vehicles, grid storage, and consumer electronics. For many years, lithium (Li) metal has been considered the “ideal” anode material because of its ability to store lightweight Li in the metallic form without the need for an inactive host material and/or conductive scaffold. This provides a capacity of 3680 mAh/g and the lowest theoretical anode potential, making it an enabling technology for next-generation battery systems including Li−sulfur and Li−air.1 Unfortunately, stability issues resulting from electrode/electrolyte interactions prevent extended cycling of Li metal.2 These interactions lead to the formation of porous dendritic structures that cause a reduction in Coulombic efficiency and eventual failure.3,4 This type of failure not only shortens battery life but can also cause safety hazards as a result of gas evolution and possible ignition of the flammable electrolyte. A natural solid electrolyte interphase (SEI) is known to form as a result of Li metal interacting with the electrolyte.5 This layer acts as an ionic conductor and electronic insulator and evolves to form a complex, multilayer surface coating.6 However, as metallic Li is plated or stripped, uneven current distributions resulting from surface inhomogeneities lead to localized “hot spots” where Li preferentially nucleates, resulting in the fracture of the SEI layer due to localized stresses. This exposes the underlying Li metal leading to dendrite growth and further deleterious and potentially dangerous side reactions.6 The effect of these reactions is 3-fold: (1) dendrite growth can lead to short-circuiting,7 (2) rapid consumption of the electrolyte causes high overpotentials and thick SEI layers that consume previously active Li and can lead to cell failure due to insufficient electrolyte,8 and (3) inactive or “dead Li”, © XXXX American Chemical Society
Received: July 21, 2015 Revised: September 4, 2015
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DOI: 10.1021/acs.chemmater.5b02789 Chem. Mater. XXXX, XXX, XXX−XXX
Article
Downloaded by UNIV OF CALIFORNIA SAN DIEGO on September 14, 2015 | http://pubs.acs.org Publication Date (Web): September 11, 2015 | doi: 10.1021/acs.chemmater.5b02789
Chemistry of Materials in which precursors react with the substrate in self-limiting halfreactions, pulsed cyclically to build up coatings one atomic layer at a time.20 Owing to the vapor phase precursors and selflimiting reactions, ALD can conformally coat a wide range of materials with submonolayer precision of film thickness. Additionally, many ALD chemistries can be deposited at low temperatures in an inert environment, allowing deposition on sensitive substrates such as Li metal. ALD has been recognized as a potential means of engineering the SEI in battery systems.11,15,21−24 Despite these promising results, direct ALD on the surface of Li metal has only been reported very recently in one other study, demonstrating that 14 nm Al2O3 layers (∼117 ALD cycles based on a reported growth rate of 1.2 Å/cycle) were able to delay corrosion in a variety of environments including air, organic solvents, and polysulfides. Significantly improved capacity retention was observed for this approach in Li−S cells after the first 100 charge−discharge cycles.25 However, questions remain on the effects of ALD treatments upon extended cycling and eventual electrode failure. In particular, the effects of ALD on the morphological evolution of dendrites through failure, as well as the improvement of total cycle life, have not been investigated. In this work, we present a study of ultrathin (
