Even though the use of renewable resources, such as wind, solar and tidal energy has grown in the last years and is foreseen to allow providing most of the energy consumption by 2030 [1
], most of our energy consumption still derives from fossil fuels. In fact, the decrease of this proportion would require an increase of stationary energy storage capabilities, as most renewable sources provide electricity in an intermittent way, incompatible with the electric network needs, as well as a switch to electric transportation. For both purposes, larger battery packs, as compared to portable electronics, are needed, raising larger safety concerns.
Indeed, if secondary Li-ion batteries offer among the highest energy densities, with values up to 800 Wh·L−1
for the latest generation of 18,650 cells, they are based on a system which is not thermodynamically stable. Indeed, lithiated graphite, generally used as anode, is unstable toward most chemicals and its operation is made possible by the kinetic passivation of its surface by electrolyte degradation products, forming a protective layer commonly called “Solid Electrolyte Interphase” (SEI) [4
] by analogy with Li metal [5
]. At the same time, layered oxide cathodes typically used in Li-ion cells, such as LiCoO2
, are known to release oxygen in case of overcharge or overheating [6
In addition, the electrolytes are made of a mixture of cyclic alkyl carbonates, usually ethylene carbonate (EC), and volatile linear alkyl carbonates with low flash points such as dimethyl carbonate (TF(DMC)
= 17 °C) or diethyl carbonate (TF(DEC)
= 25 °C) and the use of the low thermal stability [8
] salt LiPF6
brings the additional concern of generating HF in case of contact with the atmosphere or water traces above 60 °C [10
]. For all these reasons, the operation of Li-ion batteries above 50 °C is problematic, especially as high temperatures can induce the failure of the SEI this resulting in the direct exothermic reaction of lithiated graphite with the electrolyte [12
], possibly chain reactions, releasing rapidly the stored energy in what is generally referred to as “thermal runaway”.
On the other hand, ionic liquids (ILs) have attracted a growing attention as electrolytes for Li-based batteries, due to their non-flammability, negligible vapor pressure, high thermal and electrochemical stabilities [13
]. Indeed, if the use of graphite requires, for most IL-based electrolytes, the addition of additives, such as VC, for forming the SEI [18
], some ILs based on the bis(fluorosulfonyl)imide (FSI−
) anion possess intrinsic SEI forming properties [20
], as well as higher conductivities [20
] as compared to the more commonly used bis(trifluoromethanesulfonyl)imide (TFSI−
) based ILs. These electrolytes, in addition to allowing higher operation temperatures for Li-ion batteries, and thus easier heat management for large packs, also allow good reversibility of the Li metal electrode both for TFSI−
] and FSI−
-based electrolytes [26
], which enables their use in emerging Li-metal battery technologies, such as Li–metal oxide, Li–air, and Li–S [17
Besides the electrolyte, the separator plays an important safety role in lithium batteries by preventing the physical contact between the electrodes, while allowing the Li+
ions transport through its pores, filled with the electrolyte. Tri-layered polyolefin separators including a polyethylene (PE) layer sandwiched between two polypropylene (PP) layers are commonly used in Li-ion batteries due to their chemical inertia and to the safety feature they offer, the so-called “shut-down effect” as, in case of overheating, the PE layer melts, loosing its porosity (i.e
., mechanically blocking the Li+
ion movement), while the PP layer prevents large dimensional changes until its own melting, thus preventing short-circuits [29
]. Nevertheless, these separators still suffer from rather limited temperature range, with an onset for shrinkage around 100 °C [32
]. Thus, in recent years, separators with “zero shrinkage”, made of polymer coated with inorganic particles have been introduced, such as Separion®
, allowing higher safety [33
]. As Li-ion electrolytes are hydrophilic, these separators also allow for a better wetting as compared with polyolefin separators, which makes them especially suitable for large battery cells. Other materials, chemically less inert, such as cellulose have also been investigated [34
] as cheaper alternatives to dry-stretched or wet processed commercial Li-ion separators.
However, in most reports on ionic liquids for Li-ion or Li-metal batteries, glass fiber (GF) separators are used, while their thickness of ca. 300 µm, versus ca. 25 µm for commercial separators is not adapted for commercial applications. If the ionic liquid family is wide, the most promising ILs comprise the TFSI− and FSI− anions, which confer them hydrophobic properties, which a priori, makes them more compatible with polyolefin separators as compared with hydrophilic conventional electrolytes, even though their higher viscosity, raises questions. Nevertheless, no study can be found concerning the use of separators for IL-based electrolytes in Li-ion or Li-metal batteries.
If for Li-ion batteries the contact between the porous electrodes and the separator is limited to a small fraction of the tridimensional composite electrode total surface, which limits the interfacial reactivity, a different situation occurs in Li-metal batteries as the separator is in contact with a larger fraction of the total electrode surface, making the study of the interfacial reactivity more crucial. Indeed, the pre-existing “native” SEI, originating from the passivation of Li in dry air, might crack at the contact with the separator, as it is the case for semi-crystalline solid polymer electrolytes [36
], which induces increased reactivity and electrolyte consumption for SEI “self-repair”. Moreover, the main drawback of the Li electrode is the deposition of Li under the form of dendrites, which prevented so far the successful commercialization of liquid electrolyte-based, Li-metal secondary batteries, while commercial Li-metal polymer batteries do not include any separator. The formation of dendrites is not only related to the total current density [37
], but, also, presumably, to the local variation of current densities linked to inhomogeneities within the SEI [39
], which could also originate from the presence of a separator. Thus, separator’s morphology and chemical composition are both likely to influence the properties of the Li/(electrolyte + separator) interface.
In this work, we investigated combinations of four different hydrophobic IL-based electrolytes with eight separators in terms of wettability, conductivity and behavior versus Li metal. Two commercial polyolefin microporous separators (Celgard®), as well as a ceramic-coated membrane (Separion®), were used. In addition, polyamide and cellulose-based separators were tested in order to investigate the effect of the chemistry on the ensemble (separator + electrolyte). For comparison, high porosity separators (non woven mats) including two glass fiber separators, as well as a PP separator, were also investigated.
3.1. Preparation of Ionic Liquid Based Electrolytes
Four ionic liquids (ILs) made of combinations of the N
) and N
), paired with bis(fluorosulfonyl)imide (FSI−
) and bis(trifluoromethanesulfonyl)imide (TFSI−
), shown in Figure 16
. The ILs were synthesized as previously reported [46
], using N
-methylpyrrolidine (97%), 1-bromobutane (99%) and ethylacetate (ACS grade, >99.5 wt %) as received. 2-bromoethyl methyl ether (>85 wt %) was distilled shortly before use. All these chemicals were from Sigma-Aldrich, St. Louis, MO, USA. Potassium bis(fluorosulfonyl)imide (KFSI, Dai-Ichi Kogyo Seiyaku Co., Kyoto, Japan) and LiTFSI (3M, St. Paul, MN, USA) were used as received for IL syntheses. The resulting ILs were all colorless liquids at room temperature. Their purity was checked by 1
H NMR for organics while ICP-OES showed halide content below 20 ppm. The ILs including PYR12O1+
) were dried at 80 °C (resp. 100 °C) at 10−3
mBar for 24 h and then at p
mBar for 24 h. The electrolytes were prepared by stirring (under vacuum < 10−7
mBar) at 60 °C) the pre-dried ILs and LiTFSI (dried for 24 h at p
mBar at 120 °C) in a 9:1 molar ratio. The IL-based electrolytes were then stored in glass vessels in a desiccator inside a dry room with relative humidity below 0.1% at 21 °C.
Molecular formulae of PYR14+, PYR12O1+, FSI− and TFSI−.
Molecular formulae of PYR14+, PYR12O1+, FSI− and TFSI−.
The ionic conductivities of the IL based electrolytes, measured using a MMates AC conductimeter are listed in Table 3
Conductivity of the electrolytes at 20 °C.
Conductivity of the electrolytes at 20 °C.
|Electrolyte||σ (20 °C)/S·cm−1|
|PYR14FSI:LiTFSI (9:1)||2.23 × 10−3|
|PYR14TFSI:LiTFSI (9:1)||1.07 × 10−3|
|PYR12O1FSI:LiTFSI (9:1)||3.94 × 10−3|
|PYR12O1TFSI:LiTFSI (9:1)||2.09 × 10−3|
3.2. Separators Characterizations
The separators were dried under vacuum for at least 48 h, at 70 °C excepted for the glass fiber separators and Separion®, which were dried at 100 °C.
The separators thicknesses, listed in Table 1
, were measured using a Mitutoyo micrometer thickness gauge (average of 5 measurements) while the Gurley numbers were measured using a 4110N GENUINE GURLEY Densometer (Gurley Precision Instruments, Troy, NY, USA). The measurements were repeated ten times.
Except for the air permeability measurements and SEM imaging, all measurements and cell preparations were performed inside a dry room with humidity below 0.1% at 21 °C.
The wettability of the separators was assessed by following the contact angles of electrolyte drop deposited on the surface of the separators using a Drop Shape Analysis system (DSA100S, Krüss, Hamburg, Germany). The measurements were repeated three times.
3.3. Electrochemical Measurements
The ionic conductivity (McMullin numbers) was measured as follow: The Celgard®
membranes were soaked in the electrolyte for 12 h while the other separators required only 15 min wetting prior to the conductivity measurements. Stacks from 1 to 5 separators were then placed between two stainless-steel electrodes (12 mm diameter) and their resistance was measured by electrochemical impedance spectroscopy (EIS) using a Solartron 1260 Frequency Response Analyser (Solartron Analytical, Farnborough, UK). The McMullin numbers (ratio of the resistivity of the separator soaked with electrolyte to the resistivity of the electrolyte itself, N
m) were then calculated [48
Symmetrical Li/(electrolyte + separator)/Li pouch cells were assembled using two 50 µm Li foils (99.999%, Rockwood Lithium GmbH, Frankfurt am Main, Germany), nickel current collectors and sealed under vacuum in pouch bag cells in the dry-room. The cells were then stored for three days at 21 °C and their impedance were monitored by EIS.
Galvanostatic Li plating/stripping tests were performed at 40 °C in a climatic chamber (MK53, Binder GmbH, Tuttlingen, Germany). A current density of 0.1 mA·cm−2 was applied using a S4000 battery cycler (Maccor Inc., Tulsa, OK, USA), reversing the polarity every hour. Electrochemical impedance spectroscopy (EIS) measurements were then acquired at 21 °C, after the 10th, 20th, 30th, 40th, 60th, 80th, and 100th cycles. The cycling of the cells was resumed after temperature equilibration for at least one hour after each impedance measurement.
3.4. Scanning Electron Microscopy (SEM) Imaging
SEM images of the separators were taken using a SEM Auriga (Carl Zeiss, Jena, Germany) after sputtering with gold. The images of the Li electrode after cycling were taken after disassembling the cells in the dry room and rinsing of the Li electrode using dimethyl carbonate. The electrodes were then transferred to the SEM chamber using a sealed cell.
The systematic study of separators for IL-based electrolyte ensembles allowed finding alternatives to GF separators for safe Li-ion batteries. It was shown that Celgard 2500 is a valid practical alternative to glass fiber separator for TFSI-based ILs while FSI-based IL only poorly wet polyolefin separators. Separion® (or other hydrophilic separators) is thus more versatile choice for Li-ion batteries than non-functionalized polyolefin for all tested hydrophobic ILs, considering that the SEI building on graphite would not “a priori” be much influenced much by the separator morphology, given the limited surface area in contact and the lack of pre-existing SEI prior to the first charge. Moreover, studying the (separator + electrolyte) properties toward Li metal and Li metal cycling allowed drawing the following conclusions:
A scale for SEI-forming properties, based on the 0.1 LiTFSI-0.9 IL electrolytes behaviors with different separators could be proposed and the electrolyte with the highest conductivity, PYR12O1FSI, allowed the best performance. As a consequence, Separion®, which is not favorable for Li metal cycling when combined with TFSI-based electrolytes, due to its rough ceramic surface inducing extended damages within the “native” SEI, led to the best Li cycling results when combined with PYR12O1FSI, probably thanks to the formation of an effective SEI as a combination of initial SEI damage and evolution in contact with the electrolyte.
The morphology of the separator is of particular importance: Given the presence of a pre-existing SEI, the roughness of the separator influence the evolution the SEI after the initial contact, which then evolves, depending on the reactivity of the (electrolyte + separator) combination. SEM images confirmed the electrochemical results and evidenced that, in PYR14TFSI, a thicker SEI is deposited close to where significant damage of the SEI occurs while, in less damaged areas, the original surface morphology is barely modified.