Article pubs.acs.org/JPCC
Role of Li Concentration and the SEI Layer in Enabling High Performance Li Metal Electrodes Using a Phosphonium Bis(fluorosulfonyl)imide Ionic Liquid Gaetan M. A. Girard,† Matthias Hilder,† Donato Nucciarone,§ Kristina Whitbread,§ Serguei Zavorine,§ Michael Moser,§ Maria Forsyth,† Douglas R. MacFarlane,‡ and Patrick C. Howlett*,† †
Institute for Frontier Materials (IFM), Deakin University, Waurn Ponds VIC 3216, Australia School of Chemistry, Monash University, Clayton VIC 3800, Australia § Cytec Solvay Group, Niagara Falls, Canada ‡
S Supporting Information *
ABSTRACT: In this study the performance of the lithium (Li) anode is characterized in two alternative ionic liquid electrolytes: (i) a solution of 0.5 mol·kg −1 of lithium bis(fluorosulfonyl)imide (LiFSI) in trimethyl(isobutyl)phosphonium FSI (P111i4FSI) and (ii) an equimolar mixture of these two salts, effectively an inorganic−organic mixture IL. We have investigated the formation of the solid electrolyte interphase (SEI) at the lithium electrode and its influence on the polarization potential, the electrode surface impedance and deposition morphologies. Lithium metal cycling is revealed to be significantly more stable in the electrolyte with high lithium salt concentration due to the creation of a more uniform SEI. Stable and effective cycling was demonstrated at high applied currents (up to 12 mA·cm−2) with large areal capacities being transferred with each polarization cycle (up to 6 mAh·cm−2 at 50 °C). An average Coulombic efficiency of not less than 99.2% was demonstrated under these conditions and SEM observations of the cycled electrode surfaces show a uniform and compact deposit. Combined with spectroscopic characterization of the electrolyte and electrode surface, these observations indicate a role for the speciation and transport properties of these high concentration ionic liquid electrolytes in modifiying the physicochemical properties of the SEI which result in enhanced cycling performance of the Li metal electrode. interphase (SEI) film. Excessive growth of the thickness and impedance of this film during cycling are known to be critical factors contributing to cell failure.9−11 The SEI layer is formed from decomposition products of the electrolyte and must accommodate rapid changes in the morphology of plated Li metal during deposition/dissolution cycling.11 Organic carbonate solvents and LiPF6 salts have commonly been used in Liion batteries, in particular due to their compatibility with intercalation electrodes as well as their high conductivity which supports relatively rapid charge/discharge rates. However, it is well established that Li metal cycling efficiency in organic carbonate electrolytes is very poor and results in dendritic or mossy Li metal deposits and low efficiency (CE) < 80%.12 Therefore, in the context of rechargeable batteries, the use of Li metal requires alternative solvents. Electrolytes based on Ionic Liquids (ILs) have recently been the subject of great interest due to their low reactivity with Li metal resulting in a lower
1. INTRODUCTION Lithium (Li) metal is an ideal candidate as an anode material for rechargeable Li batteries. It possesses a very high theoretical specific capacity (3861 mAh·g−1), 10× higher than that of a graphite intercalation electrode, a low density (0.534 g·cm−3) and the most negative potential of all metallic anode materials (−3.04 V vs standard hydrogen electrode).1,2 However, problems such as dendritic Li growth, limited cycling efficiency during subsequent Li plating/stripping processes, and incompatibility of lithium metal with most organic solvent electrolytes have prevented the use of Li metal in rechargeable devices for practical applications. 3,4 Since the release of the first commercial Li-ion battery in 19915 with graphite as an alternative anode material, the interest in using Li metal electrodes has only recently started to attract attention for high energy density rechargeable Li batteries (such as Li−air, Li−S batteries).2,6−8 The electrolyte is one of the critical components that dictates the overall performance and cycling stability of Li metal anodes. Reactions that occur between the electrolyte components and Li metal result in the formation of the so-called solid-electrolyte © 2017 American Chemical Society
Received: February 27, 2017 Revised: August 20, 2017 Published: August 23, 2017 21087
DOI: 10.1021/acs.jpcc.7b01929 J. Phys. Chem. C 2017, 121, 21087−21095
Article
The Journal of Physical Chemistry C
between 3 and 6 mAh·cm−2).9 Advanced Li−S or Li−O2 cells could be expected to require even larger amounts of Li for practical operation.7 Recently, several studies have reported significant differences between the SEI properties and cell performance for dilute and highly concentrated IL electrolytes.13,15,31,32 Thus, novel electrolytes that can support high rate and stable cycling of Li metal anodes, for example, recently highlighted “solvent-in-salt” electrolytes,33 are needed to further develop practical rechargeable Li metal batteries. In this study we demonstrate that the use of a highly concentrated IL electrolyte based on the P111i4FSI IL with high LiFSI salt concentration results in dendrite-free plating of Li metal on a Li substrate at high rates and capacities. For the first time, this is demonstrated at capacities required for practical battery application. We also show that this exceptional performance is made possible by the formation of a stable, compact and low resistance SEI layer at the Li metal anode.
propensity for dendritic Li morphology and improved Coulombic efficiency for Li plating/stripping.10,13−16 However, these features are not retained during long-term cycling or when high current densities are applied.14,17 Nevertheless, some ILs have been targeted as potential alternative electrolytes for lithium batteries.18,19 They have numerous features that should lend themselves for use in high-energy lithium batteries, including negligible volatility and flammability, high ionic conductivity, and wide electrochemical windows.20 Interfacial studies of ILs on Li metal originally started with ammonium-based ILs. For example, Howlett et al. demonstrated that 0.5 mol·kg−1 of LiTFSI dissolved in the ionic liquid N-propyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (C3mpyrTFSI) supported high cycling efficiency (>99%) on copper at low current densities.10,14 More recently, Basile et al. reported the cycleability of Li in the bis(fluorosulfonyl)imide (FSI) based ammonium counterpart (same cation with a smaller anion).2122 They noticed the formation of a smooth and stable SEI on Li metal after extensive cycling, accredited to the lithium bis(fluorosulfonyl)imide (LiFSI) salt. In their work, unstable cycling behavior of Li|Li cells (over 5000 cycles at 0.1 mA· cm−2) was attributed to a surface reorganization process. However, no evidence for lithium dendrite formation was observed; SEM imaging showed morphology changes that occurred during cycling, but no evidence of needle-like dendrite features was observed. In these previous works, and the majority reported in the literature, the Li salt concentration used was typically around 0.5 to 1.0 mol·kg−1 (Li+ molar ratio = 0.05). Interestingly, Yoon et al. reported that high rate cycling performance (at higher than 3C charge and discharge rates) could be achieved in Li| C3mpyrFSI|LiCoO2 cells with high LiFSI salt concentration (3.2 mol·kg−1 Li salt, that is, a 1:1 molar ratio of salt and IL) in spite of the reduced ionic conductivity that occurs with increasing Li salt concentration.13 This study revealed that a highly concentrated IL electrolyte can, in principle, outperform an organic liquid electrolyte (1 M LiPF6 in EC:DMC). More recently, novel phosphonium-based ILs have demonstrated superior transport properties compared to their ammoniumbased counterparts.21−27 Lin et. al have shown that a hydrophobic phosphonium-based IL electrolyte could operate a Li|LiCoO2 metal battery at elevated temperature (100 °C).26 The authors also found that an increase in the Li salt concentration (up to 1.6 M Li salt in the IL) led to a higher capacity retention (90%) than that obtained with lower salt concentrations. One of our recent studies revealed that trimethyl(isobutyl)phosphonium FSI IL (P111i4FSI) readily dissolved high amounts of LiFSI salt and the highly concentrated electrolyte (3.8 mol·kg−1, i.e., a 1:1.2 molar ratio of IL/salt) supported reversible and efficient Li cycling even at switching potentials as negative as −1.1 V versus Li/Li+, indicating high electrolyte stability for Li batteries.28,29 In the majority of studies reported to date, the applied current densities used have been limited to relatively low values (usually between 0.1 and 1.0 mA·cm−2),13,17,21,30 which are insufficient to meet the typical requirements of many applications (>3 mA·cm−2).1,23 Perhaps more importantly, very few studies report the deposition and dissolution of substantial quantities of Li metal, rarely exceeding 0.1 mAh.cm−2.6,13,29 However, for practical batteries the transfer of much larger amounts of charge is required (i.e., commercial Li-ion cells typically incorporate cathode loadings which cycle
2. EXPERIMENTAL SECTION 2.1. Materials Used. The ionic liquid trimethyl(isobutyl)phosphonium bis(fluorosulfonyl)imide (P111i4FSI, Figure 1)
Figure 1. Chemical structure of P111i4FSI.
was provided by Cytec Canada Inc. with >99.5% purity. The structure and purity were con- firmed by 1H, 13C, 19F, and 31P NMR, MS, and FT-IR spectroscopy. LiFSI (Nippon Shokubai, Japan) was used without further purification. The organic carbonate electrolyte (1 M LiPF6 in EC/DMC (1:1)) was provided by Solvionic (France). The electrolytes were prepared by dissolving the desired amount of salt into the ionic liquid as previously reported.27 In this work, the solution of 3.8 mol·kg−1 of LiFSI in P111i4FSI is referred to as the highly concentrated IL electrolyte (1:1.2 P+/Li+, i.e., 55 mol % Li+), whereas the solution of 0.5 mol·kg−1 of LiFSI in P111i4FSI is referred to as the dilute concentrated IL electrolyte (1:0.15 P+/Li+, i.e., 15 mol % Li+). Lithium foil (375 μm, 99.9%) was obtained from Sigma-Aldrich, and stored in an argon (Ar) filled glovebox (
