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Homogeneous lithium electrodeposition with Pyrrolidinium-based ionic liquid electrolytes Lorenzo Grande, Jan von Zamory, Stephan Koch, Julian Kalhoff, Elie Paillard, and Stefano Passerini ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 25 Feb 2015 Downloaded from http://pubs.acs.org on February 25, 2015
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ACS Applied Materials & Interfaces
Homogeneous
lithium
electrodeposition
with
Pyrrolidinium-based ionic liquid electrolytes Lorenzo Grande †,‡,§
†,‡,§
, Jan von Zamory
†,‡,§
, Stephan L. Koch
†,‡,§
, Julian Kalhoff §, Elie Paillard
*, Stefano Passerini †,‡,§*
ABSTRACT: In this article, we report on the electroplating and stripping of lithium in two ionic liquid (IL) based electrolytes, namely N-butyl-N-methylpyrrolidinium bis(fluorosulfonyl) imide (Pyr14FSI) and N-butyl-N-methylbis(trifluoromethanesulfonyl)imide (Pyr14TFSI), and mixtures thereof, both on nickel and lithium electrodes. An improved method to evaluate the Li cycling efficiency confirmed that homogeneous electroplating (and stripping) of Li is possible with TFSI-based ILs. Moreover, the presence of native surface features on lithium, directly observable via SEM imaging, was used to demonstrate the enhanced SEI-forming ability, i.e. fast cathodic reactivity, of this class of electrolytes, as well as the suppressed dendrite growth. Finally, the induced inhomogeneous deposition allowed witnessing the SEI cracking, revealing previously unreported bundled Li fibers below the pre-existing SEI, as well as non-rod-shaped protuberances resulting from Li extrusion.
Keywords: lithium metal; dendrites; ionic liquids; SEI; energy storage; nickel
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1. Introduction Electrochemical energy storage has established itself as an essential technology in several applications, be it intermittent renewable energy sources, laptops, smartphones or electric vehicles. Li-ion batteries have quickly gained the lion’s share of the market thanks to their high specific energy. However, the “rocking chair” concept faces an intrinsic theoretical limitation of 250-300 Wh kg-1, mostly because of restrictions related to the cathode’s maximum specific capacity and voltage
1–3
. Resorting to new battery chemistries is therefore vital if higher energy
densities are to be reached. Lithium has long been proposed as a good anode material for Limetal, Li/S and Li/O2 batteries 2. It sports a high specific capacity (3,861 mAh g-1) and a very negative potential (-3.05 V vs. SHE), but has found only limited commercial applications in secondary batteries due to its high reactivity and tendency to form short circuits. The first phenomenon leads, in aprotic electrolytes, to the formation of an electronically insulating (but ionically conductive) passivation layer on top of lithium, every time that a fresh surface is exposed to the electrolyte. This film, which has been dubbed the “SEI”, or solid-electrolyte interphase 4, consists of a mix of organic and inorganic insoluble compounds that result from native species and electrolyte decomposition products. It is an unavoidable phenomenon and generally regarded as a protective layer. Undesired side reactions can, however, still take place during cell operation, leading to a low coulombic efficiency and irreversible capacity loss. The situation is further exacerbated by inhomogeneous Li deposition, which can induce short circuits through what are commonly known as “dendrites”, whose uncontrolled growth causes the perforation of the separator (or polymer electrolyte) and internal cell failure 5. Several models have been proposed to explain the mechanisms of dendrite onset and/or growth, among which it is worth noting those by Chalzaviel et al.6–8 and Yamaki et al.9. The first
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one predicts that nucleation and growth of dendrites initiate when the applied current density hits the limiting value ilim and triggers the depletion of Li+ ions at the surface of the metal, causing a large, localized electric field to appear at the interface. Dendrite growth can, however, occur even below ilim, due to small inter-electrode distance and surface defects that are responsible for a localized increase in current density
10
. Ways to circumvent the detrimental effect of defects
have been devised, such as applying an external pressure
11
, pulse charging 12, using composite
polymer electrolytes 13, ionic liquids (IL) 14,15, block copolymers 16 and protective films/additives 17–22
, but surface imperfections remain intrinsic and unavoidable. Yamaki et al. stressed the
importance of such defects by describing the conditions under which dendrites can grow. According to them, lithium deposition takes place underneath the SEI, until stress on its surface leads to a disruption of the texture and to the extrusion of lithium from these sites. Deposition can continue at the base of these whisker-like structures until Li+ transport is sufficiently hindered. At this point, deposition starts to take place on the tip and at the whiskers’ kink sites, which are rich in lattice defects. During the reverse process, such whiskers dissolve at the base (where the SEI is fresher and less resistive), ultimately causing the physical detachment of the outgrowths from the substrate 9. Finally, according to the authors, a higher current during stripping is beneficial for the cycle life, because it promotes the dissolution of lithium at the tip and kink sites rather than from the base. However, the assumptions of Yamaki’s model are rather broad and neglect both lithium’s rheological properties and the elasticity of the SEI, which Monroe and Newman 23 have taken into account in an alternative explanation based on Li+ mass transport in solution and surface tension. More recently, Kramer et al. observed a mixed kinktip-base growth behavior via in-situ optical microscopy with carbonate-based electrolytes
24,25
.
They showed the role played by the SEI shell present on lithium whiskers, which prevents
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dendrite shortening during lithium dissolution and favors a thinning mechanism, instead. Furthermore, they also stressed the importance of defect sites (be it SEI irregularities or lattice defects) and ionic mass transport in triggering needle-like dendrites, in accordance with other recent observations
24,26,27
good lithium deposition
. A uniform, low resistance and elastic SEI is paramount to ensure a
28
, and ionic liquids are among the proposed solutions to form an
interphase with such characteristics and mitigate side effects. Basile et al. studied the cyclability of
lithium
after
chemical
interaction
with
N-propyl-N-methylpyrrolidinium
bis(fluorosulfonyl)imide (Pyr13FSI) and lithium tetrafluoroborate (LiBF4) as salt. They followed the changes in morphology through scanning electron microscopy (SEM) and recorded cauliflower-like shapes appearing on the surface of lithium after 18 days of reaction with the electrolyte, which, however, did not impact the cyclability 20. A smooth surface after extensive cycling was recorded by the same group using lithium bis(fluorosulfonyl)imide (LiFSI) as salt in the same IL 29. In both works, a 12h rest period was fixed before each lithium plating/stripping tests, which, in the authors’ view, allowed the FSI- anion to react and build a stable SEI on the metal’s surface, thanks to the cleavage of the S—F bond in FSIshowed
that
lithium
bis(trifluoromethanesulfonyl)imide
30,31
. In our group, we recently
(LiTFSI)
in
N-butyl-N-
methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (Pyr14TFSI) also contributes to an enhancement in cyclability if a rest period of one week is provided to the cell 32, in spite of the higher strength of the anion’s C—F bond. The adoption of a TFSI- anion allows to extend the IL’s electrochemical stability window on the cathodic side
33,34
, thereby increasing its tolerance
to lithium metal and inhibiting electrolyte degradation during cycling. In this paper, we analyzed the interplay between ionic liquid mixtures containing these two anions in terms of lithium nucleation and deposition on nickel and addressed the efficiency
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determination of Li metal cycling when an initial homogeneous plating cannot be reached. Finally, we report on unique observations of the morphological changes underneath the SEI during Li deposition onto Li foil. 2. Experimental section Materials Used Lithium foil (50 µm, 99.999%) was obtained from Rockwood Lithium GmbH, and stored in a dry room (R.H.
