pubs.acs.org/JPCC
Article
In Situ UV−Vis Analysis of Polysulfide Shuttling in Ionic LiquidBased Li-FeS2 Batteries
Aliya S. Lapp, Grace Whang, Austin Bhandarkar, Igor V. Kolesnichenko, Bruce S. Dunn,
Timothy N. Lambert, and A. Alec Talin*
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ABSTRACT: Herein, a combination of in situ and ex situ UV−Vis spectroscopy is
used to study the formation and shuttling of polysulfides (PSs) in Li-FeS2 batteries.
We find three key results. First, PS shuttling significantly decreases (∼4×) when an
ionic liquid electrolyte (1.0 M LiFSI in PYR14TFSI; ILE) is used in place of a
conventional organic electrolyte (1.0 M LiTFSI in 1:1 DOL:DME). Furthermore,
when the Li salt anion in the ILE is changed from FSI− to TFSI−, PS shuttling is
effectively suppressed. Second, the total amount of PSs formed using the ILE is
approximately double the amount shuttled. This result has implications for battery
aging as solubility of PSs could conceivably increase upon extended storage. Third, the
majority of PS shuttling occurs during the first cycle. Together, these findings provide
important insights for understanding and thus preventing PS shuttling in Li-FeS2
batteries.
■
TFSI−-based ionic liquid electrolytes (ILEs) are weak Lewis
bases (weak electron donors).1 This property has shown
promise as a strategy for PS mitigation.1,20,21 For example, Lee
and coworkers have shown that using a TFSI−-based ILE (0.6
M LiTFSI in PYR13TFSI) for Li-FeS2 batteries leads to >4×
improved capacity retention compared to a conventional
organic electrolyte (1.0 M LiPF6 in EC/DEC).1 In addition to
enhancing PS suppression, ILEs have multiple advantages over
classical organic electrolytes for battery applications, including
low vapor pressure, a wide electrochemical window, thermal
stability over a large range of temperatures, and nonflammability.22−24
Although Lee and coworkers attributed improved capacity
retention in a TFSI−-based ILE to decreased PS shuttling, the
presence of PSs was inferred indirectly (e.g., by impedance),
was exclusively ex situ, and lacked mechanistic insight.1 The
most direct PS detection method used by these authors was
energy dispersive spectroscopy (EDS). However, it is well
known (but often overlooked) that EDS is essentially a
qualitative technique.25,26 Additionally, it has previously been
documented that elemental sulfur is unstable under the
conditions used for electron microscopy,26−28 due to
sublimation,27,28 and is subject to swift sulfur redistribution27
INTRODUCTION
FeS2 is a promising conversion cathode material due to its rich
natural availability, high specific capacity (894 mAh/g), low
toxicity, low cost, and high energy density.1−5 These attributes,
along with its established commercial use in primary Li
batteries, make FeS2 an attractive cathode material for
secondary Li batteries.1−5 Nevertheless, FeS2 suffers rapid
capacity fade due to several proposed mechanisms that include
pulverization,6,7 low electronic conductivity of intermediate
species formed during cycling, 7 and polysulfide (PS)
shuttling.1,4,7 Here, we specifically focus on the latter aspect,
which refers to the cascade of reactions that occur when
dissolved PS species diffuse between the cathode and anode of
a battery during electrochemical cycling.8−10 In addition to
capacity fade,8,9,11,12 this parasitic phenomenon leads to anode
corrosion,9,10 self-discharge,9,10,12,13 and low Coulombic
efficiency.8−12 Indeed, suppression of PS shuttling is believed
to be one of the biggest hurdles for Li-S batteries.8−12
PS formation in sulfur-containing batteries is a complex
interplay of electrochemical redox reactions and chemical
disproportionation reactions.9,10,12 It has been well established
that both Li2S and S are generated during electrochemical
cycling of Li-FeS2 batteries.3,5,14,15 Both of these species are
capable of inducing PS shuttling.9−12,16 Despite the propensity
to form and shuttle PSs in Li-FeS2 batteries, in-depth
mechanistic analysis and quantification of PSs in Li-FeS2 is
lacking.17
The electrolyte plays a key role in PS shuttling.18,19 The
most critical factor for determining PS solubility is believed to
be Lewis basicity (e.g., donor strength of the solvent).18
© 2022 American Chemical Society
Received: November 25, 2021
Revised: March 1, 2022
Published: March 15, 2022
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glovebox (Mbraun, Germany) having oxygen and water levels
of <1.0 ppm (each).
Reduced graphene oxide (rGO) was acquired from
Graphenea (Cambridge, MA). Stainless steel 316 coin cell
cases (CR2032), wave springs, spacers (0.2−0.5 mm thick),
Timcal Super P (carbon black), and C-coated Al foil (>99.9%,
16 μm) were obtained from MTI Corp. (Richmond, CA). The
glass microfiber separators (grade GF/C) used in the coin cells
were obtained from Whatman and hole-punched to a 5/8″
diameter before use. Polypropylene separators (40 μm pore
size) were from Targray Technology, Inc. (Kirkland, Canada).
The separators were baked under vacuum at ∼150 °C for at
least 4 h prior to use and then transferred to the glovebox for
storage.
Quartz cuvettes, each with a 1.0 cm path length, were
purchased from Starna Cells, Inc. (Atascadero, CA). The
deuterium/tungsten light source (DH-2000-BAL), SQUARE
ONE cuvette holder, 200−400 μm UV−Vis optical fibers,
USB2000 miniature fiber optic spectrometer, and OceanView
2.0 UV−Vis software were acquired from Ocean Insight
(Orlando, FL).
FeS2 Cathode Preparation. Commercial FeS2 powder
was ball-milled according to a previous procedure from the
literature.36 More specifically, 5.0−5.6 g of 3.0 mm stainless
steel Fritsch media and an equal mass of FeS2 were added into
the 20 mL grinding bowl of a Fritsch Pulverisette 7 Premium
Line Planetary Micromill. Milling was carried out at 1000 rpm
over the course of 6 h, with either (1) no rest period or (2) a
brief (5−10 min) pause at the midway point (3 h).
Subsequently, nickel mesh was used to separate the FeS2
powder from the stainless steel media. Finally, the FeS2 was
transferred to a glass scintillation vial and stored in the
glovebox.
FeS2 slurries (80% ball-milled FeS2, 10% carbon powder
(1:1 Super P: rGO), and 10% PVDF, by mass) were prepared
in two steps. First, the solid components (FeS2, Super P, and
rGO) were ground in a mortar and pestle for 10 min. Second,
PVDF (10%) dissolved in NMP (15 g/L) was added, and the
resulting mixture was stirred overnight.
For coin cell measurements, the as prepared FeS2 slurry
(20−30 μL) was drop-cast onto a 1.6 cm2 disk of C-coated Al
foil and allowed to dry overnight. Once dry, the cathodes were
baked in a vacuum oven at 150 °C (≥ 4 h) to remove residual
water. After baking, the cathodes were transferred to the
glovebox for storage.
For flooded cell UV−Vis measurements, the FeS2 slurry was
drop-cast onto a ∼0.7−1.0 cm2 area of a 3.0 cm × 1.0 cm
(length × width) rectangular strip of C-coated Al foil. An ∼1.0
cm2 area of bare foil on the back side of the cathode was used
for electrical connection. Besides these two regions, the foil
(both front and back) was covered in Kapton tape for electrical
insulation. All additional cathode preparation steps were
identical to the previous paragraph.
FeS2 Characterization. The FeS2 was characterized by
scanning electron microscopy (SEM), Raman spectroscopy,
and X-ray photoelectron spectroscopy (XPS). The SEM of the
FeS2 powder (after ball-milling) was acquired using a Zeiss
GeminiSEM 500 variable-pressure field emission SEM at an
operating voltage of 5−10 kV.
Raman spectroscopy was performed on a representative FeS2
cathode using a Renishaw inVia Raman microscope equipped
with a 532 nm laser. Spectra were collected from 58−1839
cm−1 with an acquisition time of 5.0 s, 1.0% laser power, and
and contamination of subsequent samples (which poses an
issue if multiple sulfur-containing samples are analyzed).27
UV−Vis is uniquely suited for direct, in situ analysis of PS
shuttling in Li-FeS2 batteries for several reasons. First, UV−Vis
is specific for shuttled PSs as they are the only soluble Li-FeS2
cycling products4,14,29,30 and any contribution from the
electrolyte is easily subtracted. This is in contrast to Raman
spectroscopy and X-ray absorption spectroscopy (XAS), which
measure both the solid and liquid phases;31−33 this feature can
complicate analyses if peaks overlap. Additionally, given that
XAS is an ensemble technique,32 the contribution of PSs to the
S-edge are likely to be overshadowed by the contribution of
sulfur from majority species (e.g., FeSx=1−2 and PYR14TFSI)
unless shuttling is severe. Second, a fiber optic UV−Vis setup,
like the one used in the present report, is simple to operate and
portable. The latter enables in situ measurements inside a
glovebox, alleviating concerns of air contamination. Air-free
conditions are important for studying PSs because they oxidize
readily.10 Third, UV−Vis can be used for quantitative analysis,
allowing for estimation of the extent of PS shuttling.
Few publications have studied PSs in PYR14TFSI-based ILEs
using UV−Vis.34,35 Furthermore, to the best of our knowledge,
no prior reports have analyzed the UV−Vis of PSs generated
upon cycling Li-FeS2 batteries in PYR14TFSI-based ILEs.
In the present report, we complement the prior work of Lee
and coworkers1 by using in situ UV−Vis to directly assess PS
formation and shuttling upon cycling Li-FeS2 in both a TFSI−based ILE (PYR14TFSI) and a conventional organic electrolyte. Our findings reveal three important insights. First, in situ
UV−Vis shows that PS shuttling decreases ∼4× when 1.0 M
LiFSI in PYR14TFSI is used as the electrolyte as compared to
1.0 M LiTFSI in 1:1 DOL:DME. Furthermore, PSs are
effectively suppressed when the Li salt anion is changed from
FSI− to TFSI−. Second, ex situ extraction of PSs from cycled
coin cells reveals that the amount of PSs formed in 1.0 M
LiFSI PYR14TFSI is approximately double the amount
solubilized. The latter point holds significance for battery
aging and/or high temperature operation, as PSs are expected
to become more soluble in those cases, leading to increased
shuttling. Third, we find that ≥60% of PS shuttling occurs
during the first cycle and becomes effectively constant by the
second cycle. Finally, we discuss the implications of our
findings on the mechanism of PS shuttling in Li-FeS2 batteries.
■
EXPERIMENTAL SECTION
Chemicals and Materials. Lithium bis(fluorosulfonyl)imide (LiFSI; >98%) was obtained from TCI Chemicals
(Portland, OR). Lithium bis(trifluoromethanesulfonyl)imide
(LiTFSI; 99.95%), anhydrous 1,3-dioxolane (DOL; 99.8%
with 75 ppm BHT inhibitor, ≤0.003% H2O), anhydrous 1,2dimethoxyethane (DME; 99.5%, ≤0.003% H2O), sulfur
powder (99.998%), Li foil (99.9%, 0.7 mm thick), iron
disulfide (99.8%, 325 mesh), anhydrous hexanes, and a 1.6 M
solution of n-butyl lithium (nBuLi) in hexanes were purchased
from Sigma-Aldrich (St. Louis, MO). Prior to use, the 1.6 M
nBuLi was diluted in anhydrous hexanes to 0.53 M. The ionic
liquid (IL), 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR14TFSI; 99.9%, ≤20
ppm H2O), was from Solvionic (Toulouse, France). The ILbased electrolytes were heated at 80−100 °C for ≥2 h before
use to remove dissolved gases and residual water. Unless
otherwise indicated, all sample preparation, sample storage,
and UV−Vis measurements were performed in an argon-filled
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4.5 mL, and a 40 μm polypropylene separator was placed
between the working and counter electrodes.
Chemical Lithiation of FeS2. Chemical lithiation of FeS2
was initiated by adding 3.0 mL of 0.53 M n-butyl lithium
(nBuLi; 1.59 mmol) in anhydrous hexanes to 48 mg of pristine
FeS2 powder (0.40 mmol) inside a quartz cuvette. UV−Vis
spectra were set to automatically record (every 30 s) just
before the nBuLi was added to the cuvette. The spectra were
recorded continuously for the first 24 h. Spectra were
additionally obtained at 48 and 72 h. Background subtraction
of these spectra was automatically performed using a
prerecorded blank spectrum of the chemical lithiation agent
(0.53 M nBuLi in anhydrous hexanes). As with the foregoing
UV−Vis experiments, all steps were performed inside the
glovebox.
200 accumulations. WiRE 4.2 software was used for spectral
analysis and baseline subtraction.
XPS spectra (S 2p and Fe 2p) of the pristine FeS2 powder
were obtained using a Kratos Axis Ultra XPS. The XPS was
equipped with a monochromatic aluminum X-ray source
having a voltage of 10 kV and an emission current of 10 mA.
The powders were loaded inside the glovebox and transferred
into the XPS using an air-free transfer carrier to minimize
exposure to air. The data was processed in CasaXPS software
using a peak calibration of 284.8 eV for adventitious carbon
and a Shirley background for the fitting.
UV−Visible Spectroscopy. The fiber optic setup
employed for UV−Vis spectroscopy was assembled as follows.
A 400 μm UV−Vis illumination fiber was used to direct
incoming light from the deuterium/tungsten light source to the
cuvette holder and a 200 μm UV−Vis read fiber directed the
outgoing light from the cuvette holder to a fiber optic
spectrometer. The fiber optic spectrometer contained a 2048
pixel linear CCD array detector and an asymmetric cross
Czerny−Turner optical bench. All UV−Vis spectra were
obtained under inert conditions (inside an argon-filled
glovebox) in a 1.0 cm path length quartz cuvette. Integration
time for the UV−Vis measurements was electrolyte-dependent
but ranged from 8 to 11 ms. A total of 400−500 scans were
averaged for each spectrum (total time per scan: ∼3−6 s). All
UV−Vis spectra were background-subtracted for the pristine
electrolyte contribution (recorded as a blank and automatically
subtracted from subsequent spectra) prior to the initiation of
any electrochemical or chemical reactions.
Electrochemical Cycling. Electrochemical cycling of LiFeS2 was either performed in a coin cell or a flooded cell. For
all cell types, the mass loading of FeS2 on C-coated Al foil was
1−2 mg/cm2, and three electrochemical cycles were recorded
between 3.0 and 1.0 V (vs Li/Li+) at a rate of C/10. Li foil
electrodes were cleaned by abrasion between two polypropylene blocks to remove the native oxide layer prior to use.
Coin cells were implemented for basic electrochemical
characterization as well as ex situ UV−Vis analysis of total PS
formation (more details on this are provided later). In either
case, the coin cells contained a 1.6 cm2 FeS2 cathode and a 1.3
cm2 Li anode, shielded from one another via a 2.0 cm2 glass
microfiber separator (grade GF/C). Sufficient electrolyte (100
μL of either 1.0 M LiTFSI in 1:1 DOL:DME, 1.0 M LiFSI in
PYR14TFSI (ILE-1), or 1.0 M LiTFSI in PYR14TFSI (ILE-2))
was added on top of the Li anode to fully wet the separator. A
0.5 mm stainless steel spacer was positioned below the anode,
while a 0.2 mm stainless steel spacer was placed on top of the
cathode. A wavespring was added on top of the 0.2 mm spacer.
The assembled battery stack was loaded inside the CR2032
coin cases and crimped at a pressure of 0.75 ton.
Electrochemical cycling in a flooded cell was performed
either in a round-bottom flask (rbf) (for ex situ UV−Vis
measurements) or a cuvette (for in situ measurements). For
the former, the electrochemical cell consisted of a 25 mL threeneck rbf filled with 7.0 mL of electrolyte. The working
electrode was FeS2. The counter and reference electrodes were
rectangular strips of polished Li foil, each cut to be
approximately the same size as the working electrode.
Electrodes were separated from one another using the three
necks of the rbf.
The setup for the flooded cuvette cell was similar to the rbf
cell, with the following exceptions: the electrolyte volume was
■
RESULTS AND DISCUSSION
FeS2 Characterization. Figure S1a,b shows representative
scanning electron microscopy (SEM) images of the pristine
FeS2 powder after ball-milling. As shown, particle sizes ranged
from ∼20 nm to 20 μm. Figure S1c,d illustrates that ballmilling does not appreciably affect the phase purity or
crystallinity of the FeS2 powder. Figure S2a shows a Raman
spectrum of a representative FeS2 cathode. Peaks at 338, 376,
and 426 cm−1 correspond to the Eg, Ag, and Tg (3) modes of
FeS2, respectively.37 The remaining peaks, at 1348 and 1596
cm−1, are due to the D and G bands of carbon, respectively.38
Importantly, there is no indication of S8, which has
characteristic peaks expected at ∼150, 200, and 470 cm−1.39
The absence of species other than FeS2 and carbon suggests a
lack of significant impurities. Nevertheless, the XPS spectra of
the pristine FeS2 powder (Figure S2b−d and Tables S1 and
S2) show the presence of a small amount of SO42−, FeO, and
Fe2O3 on the cathode surface despite storage of the powder in
the glovebox and air-free transfer to the XPS.
Electrochemical Characterization. Our primary focus in
the present report is the direct analysis of polysulfides (PSs)
using UV−Vis spectroscopy rather than the assessment of
electrochemical performance. With that caveat in mind, we
briefly discuss the basic electrochemical characterization of LiFeS2. A detailed description of the procedure used for
electrochemical cycling is provided in the Experimental
Section. Coin cells were used for electrochemical characterization rather than flooded cells because the internal pressure
in coin cells helps prevent cycling artifacts due to delamination
(e.g., Section S1 and Figure S3). Additionally, coin cells are
more practically relevant.
Figure 1 compares the first through third discharge
capacities (at a rate of C/10) for Li-FeS2 coin cells cycled
with three different electrolytes: a conventional organic
electrolyte (1.0 M LiTFSI in 1:1 DOL:DME; hereafter
referred to as “DOL/DME”) and two ionic liquid electrolytes
(1.0 M LiFSI in PYR14TFSI (“ILE-1”) and 1.0 M LiTFSI in
PYR14TFSI (“ILE-2”). As shown, although the initial discharge
capacity for DOL/DME (832 mAh/g) is higher than for ILE-1
(696 mAh/g), ILE-1 achieves better capacity retention
between cycles. Indeed, the capacity loss between the first
and third discharge is ∼2× higher for DOL/DME (26%) than
it is for ILE-1 (11%).
As we will show later, PS shuttling is found to be ∼4× lower
in ILE-1 than it is in DOL/DME after three cycles.
Accordingly, although PS shuttling is important, our results
support previous findings6,7 that PSs are not the sole factor
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Figure 2. Electrochemical characterization of the first cycle in 1.0 M
LiTFSI in 1:1 DOL:DME (DOL/DME; black trace) and 1.0 M LiFSI
in PYR14TFSI (ILE-1; red trace). Cycling was performed at a rate of
C/10. Each coin cell contained a 1.6 cm2 FeS2 cathode with a mass
loading of 1.8−1.9 mg/cm2, 100 μL of electrolyte, and a 1.3 cm2 Li
anode.
window in Figure S4, short-chain PSs are implicated in both
electrolytes. Long-chain PSs are additionally implicated as
evidenced by prominent plateaus at ∼2.4−2.5 V (Figure 2).
Overall, the charge/discharge profiles for the first cycle display
features that are consistent with prior results involving PS
formation in Li-S batteries.9,10 This correspondence suggests
that (1) PSs form in both electrolytes and (2) that PSs are
more pronounced in DOL/DME. Evidence for the latter point
comes from the fact that the plateau at ∼2.4 V in DOL/DME
is much more extensive than it is for ILE-1. These points are
further elucidated in the sections that follow.
Ex Situ UV−Vis Spectroscopy. Ex situ UV−Vis measurements were recorded after cycling Li-FeS2 three times between
3.0 and 1.0 V (vs Li/Li+), at a rate of C/10. As alluded to in
the Introduction, the fiber optic UV−Vis setup used here was
portable, facilitating its use in the glovebox and ensuring inert
conditions for both ex situ and in situ UV−Vis measurements.
The black trace in Figure 3 represents the ex situ UV−Vis
spectrum for DOL/DME. Three peaks are observed, centered
Figure 1. Comparison of Li-FeS2 capacity retention for three different
electrolytes: (a) 1.0 M LiTFSI in 1:1 DOL:DME (DOL/DME), (b)
1.0 M LiFSI in PYR14TFSI (ILE-1), and (c) 1.0 M LiTFSI in
PYR14TFSI (ILE-2). The first (black trace), second (blue trace), and
third (red trace) discharge profiles are shown, each at a rate of C/10.
In each coin cell, 100 μL of electrolyte was used, the mass loading of
FeS2 was 1.8−1.9 mg/cm2 (over a 1.6 cm2 area), and the Li anode
was 1.3 cm2.
governing capacity loss in Li-FeS2 batteries. While this case
holds true for bulk Li metal (used here), we note that PSs gain
importance in applications using thin Li anodes. Although the
difference in capacity loss between the two electrolytes (∼2×)
is less than the corresponding gap in the extent of PS shuttling
(∼4×), we cautiously mention that it approximately matches
the difference in PS formation between the two electrolytes
(∼2×; vide infra).
As we show later, in situ UV−Vis experiments indicate that
PS shuttling begins during the first cycle. To further support
this finding, we wish to discuss the electrochemical features of
the first cycle. As shown in Figure 2, the first discharge has a
plateau at ∼1.4−1.5 V in both electrolytes. Although the exact
mechanism of Li-FeS2 batteries at room temperature is still
debated, this first discharge plateau is consistent with previous
reports1−3,14 and has been ascribed to eq 1.1−3,14
FeS2 + 4Li+ + 4e− → Fe 0 + 2Li 2S
Figure 3. Ex situ UV−Vis spectra obtained after cycling Li-FeS2 three
times in 1.0 M LiTFSI in 1:1 DOL:DME (DOL/DME; black trace),
1.0 M LiFSI in PYR14TFSI (ILE-1; blue trace), and 1.0 M LiTFSI in
PYR14TFSI (ILE-2; red trace). Each UV−Vis spectrum was
background-subtracted for the relevant uncycled electrolyte. Spectra
were recorded in a 1.0 cm path length quartz cuvette. Electrochemical
cycling was performed at a rate of C/10.
(1)
The mechanism for the first charge is controversial.
As a
result, we focus on the two potential windows relevant to PSs:
∼2.1−2.2 and ∼2.4−2.5 V. In Li-S batteries, the former and
latter regimes have previously been assigned to short-chain
(e.g., Sx=2−42−) and long-chain (e.g., Sx=5−82−) PSs, respectively.9,10 Above ∼2.4 V, long-chain PSs are oxidized to S8.3,10
As shown by the dQ/dV analysis of the former potential
4,30
at ∼270, 300, and ∼420 nm. The peak at ∼270 nm matches
that expected for solubilized S8, while peaks at ∼300 and 420
nm correspond to S42−.19,40−42
As shown by the blue trace in Figure 3, similar peaks are
observed for ILE-1. Specifically, the main peaks are centered at
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Table 1. Quantification of the Total Amount of S Lost to Polysulfides (PSs) in Three Different Electrolytes
g
DOL/DME
ILE-1h
ILE-2i
Ve (mL)a
mFeS2 (mg)b
mS0 (mg)c
mSlost (mg)d
[S]lost (mM)e
%Slostf
5±1
4±2
4.5
1.0 ± 0.3
1.4 ± 0.3
1.4 ± 0.2
0.5 ± 0.2
0.8 ± 0.2
0.7 ± 0.1
0.24 ± 0.09
0.09 ± 0.02
0.019 ± 0.009
1.6 ± 0.4
0.8 ± 0.3
0.13 ± 0.06
48 ± 10
11 ± 4
2.7 ± 0.8
a
Volume of electrolyte. bMass loading of FeS2. cInitial mass of S for a given FeS2 mass loading. dTotal mass of S lost to PSs. eTotal concentration of
dissolved S. fRatio of mSlost to mS0 multiplied by 100%. g1.0 M LiTFSI in 1:1 DOL:DME (N = 3 trials). h1.0 M LiFSI in PYR14TFSI (N = 4 trials).
i
1.0 M LiTFSI in PYR14TFSI (N = 3 trials).
∼256 and 320 nm, with shoulders at ∼275, 350, and 420 nm.
An expanded view of these peaks is provided in Figure S5. The
peaks at ∼256 and 275 nm match those expected for S8 in the
IL (PYR14TFSI),34 whereas the peaks at 320 and 420 nm
correspond to S42−.19,34,40−42 Although the presence of two
peaks for S8 differs from DOL/DME, two S8 peaks have also
been observed for acetonitrile, DME, TEGDME, and THF.34
The shoulder at 350 nm matches that expected for
S62−.18,19,34,41,42
Due to the fact that S8 is not soluble in all battery
electrolytes,43 we sought to confirm that the peaks at ∼256 and
275 nm in Figure 3 truly represent solubilized S8. To this end,
we dissolved sulfur powder in ILE-1 and performed UV−Vis
spectroscopy (Figure S6). The resulting peaks (at 256 and 276
nm) match those assigned as solubilized S8 in Figure 3. The
presence of solubilized S8 in ILE-1 agrees well with previous
findings from Bieker and coworkers that PYR14TFSI solvates S8
formed from PS disproportionation reactions.34
Solubilized S8 in each electrolyte has two probable sources:
(1) electrochemical cycling1,3,4,15 and (2) chemical disproportionation.10,40,44 In the first case, S8 is expected to form
during the initial charge step,3,4 owing to oxidation of Li2S,3 or
the decomposition of Li2‑x FeS2 (a hypothetical intermediate
electrochemical species) to yield (2 − x)Li+ + FeSy + (2 −
y)S.4,15 In the second case, the spontaneous disproportionation
of long-chain PSs (Sx=5−82−) can generate S8 as a byproduct
(e.g., eq 2).10,40,44
Sx = 5 − 82 − → Sy = 2 − 6 2 − + zS8 (z = (x − y)/8)
measure the diffusion of PS species, it is capable of determining
the concentration of dissolved PS species and their buildup
over time (vide infra). Given the direct proportionality
between UV−Vis absorbance and concentration (via Beer’s
Law) and the fact that UV−Vis was strictly performed on the
electrolyte, the absorbance values of the UV−Vis peaks in
Figure 3 can be used to estimate the extent of PS shuttling.
Accordingly, the trends in the UV−Vis absorbance values in
Figure 3 suggest that the extent of PS shuttling is significantly
decreased in ILE-1 relative to DOL/DME.
Second, despite significant suppression, our results show that
PSs are still generated when Li-FeS2 is cycled in ILE-1. This
finding demonstrates that a more aggressive approach is
needed to completely eliminate PS shuttling in Li-FeS2
batteries. Comparison of the blue and red traces in Figure 3
shows that changing the Li counterion from FSI− (ILE-1; 1.0
M LiFSI in PYR14TFSI) to TFSI− (ILE-2; 1.0 M LiTFSI in
PYR14TFSI) leads to nearly complete suppression of PSs. This
result is not entirely surprising as PS suppression is expected to
be better for pure TFSI−-based electrolytes than FSI−-based
electrolytes.1,46
There are several interrelated factors that influence PS
solubility, including charge delocalization,1 Gutmann donor
number (Lewis basicity),18,47 dielectric constant (a measure of
polarizability),18,19,47 and coordination ability of the cation.18,48 Of these factors, donor number is believed to be
paramount.18 TFSI− is expected to be more charge-delocalized
than FSI− because the -CF3 terminal groups in the former are
more electron-withdrawing than the -F terminal groups in the
latter. Greater charge delocalization translates to weaker Lewis
basicity.18 Thus, ILE-2 (which is purely TFSI−-based) is
expected to have lower PS solubility than ILE-1 (which
contains an FSI−/TFSI− mixture). In addition to having lower
PS solubility, pure TFSI−-based ILEs are significantly more
viscous than pure FSI−-based ILEs.46,49 Higher viscosity leads
to slower ion diffusion, which in turn hinders PS migration
(and thus, PS shuttling).21
Although pure TFSI−-based ILEs (e.g., ILE-2) have PS
suppression characteristics that are superior to pure FSI−-based
ILEs, higher viscosity can lead to increased charge transfer
resistance and polarization. Additionally, pure TFSI−-based
ILEs are less conductive.46,49,50 For example, Nadherna and
coworkers found that the conductivity of a pure TFSI−-based
ILE (0.7 m (or 0.97 M) LiTFSI in PYR14TFSI) was ∼3× lower
than an FSI−/TFSI− mixture (0.97 M LiFSI in PYR14TFSI).50
Importantly, the compositions of these two electrolytes match
those used in the present report for ILE-2 (1.0 M LiTFSI in
PYR14TFSI) and ILE-1 (1.0 M LiFSI in PYR14TFSI),
respectively. Indeed, the higher viscosity and lower conductivity of ILE-2 lead to much poorer cycling at C/10 (e.g.,
Figure 1c vs Figure 1b). Overall, we find that the FSI−/TFSI−
mixture (23%/77%) in ILE-1 makes it a more suitable
electrolyte than ILE-2 (100% TFSI−) for balancing PS
(2)
Therefore, the S8 detected by ex situ UV−Vis in Figure 3
likely originates from a combination of electrochemical and
chemical reactions.
We note, however, that the concentration of solubilized S8
observed in UV−Vis may not represent the total fraction of S8
formed (electrochemical plus chemical) due to its limited
solubility in the electrolyte. PSs are expected to be more
soluble in DOL/DME than in a TFSI−-based ILE.34,43,45
Therefore, it is possible that the same amount of electrochemically generated S8 forms in both electrolytes, but more of
it dissolves in DOL/DME. We investigate the quantification of
total PS formation in the next section.
Overall, Figure 3 reveals two important insights. First, the
UV−Vis absorbance values obtained upon cycling Li-FeS2 in
ILE-1 are markedly lower than those generated in a
conventional organic electrolyte (DOL/DME), indicating
better PS suppression. Our results corroborate the findings
from Lee and coworkers that using a TFSI−-based ILE
significantly mitigates PSs in Li-FeS2 batteries.1
Up until this point, we have not explicitly mentioned the
connection between dissolved PS species and PS shuttling. PS
shuttling involves the diffusion of dissolved PSs between the
cathode and anode.8−10 Although UV−Vis does not directly
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that ILE-1 suppresses PS shuttling ∼4× compared to an etherbased electrolyte (DOL/DME). Additionally, the average %
Slost for ILE-2 (2.7 ± 0.8%) is ∼4× lower than ILE-1.
We hypothesize that lessened shuttling in the ILEs (relative
to DOL/DME) is caused by lower PS solubility. Indeed, we
find that S8 dissolution in ILE-1 is saturated above ∼0.86 mM
(or ∼6.9 mM S), even after heating at ∼80 °C for 7 days,
whereas the expected room temperature solubility of S8 in
DOL/DME is ∼5 mM (or ∼40 mM S).43 Given that the
cycling experiments were run at room temperature and all
three cycles were completed in a maximum of 60 h (2.5 days),
the solubility of S8 in ILE-1 is expected to be even lower than
0.86 mM (or 6.9 mM S). Furthermore, Watanabe and
coworkers showed that total S solubility at 30 °C in 0.5 M
LiTFSI in PYR14TFSI was ∼1−3 mM.20 We note that the
[S]lost that we report for ILE-1 (0.8 ± 0.3 mM; Table 1) is very
close to this limit.
Analysis up until this point has dealt solely with PS shuttling
(dissolved species) and has not addressed adsorbed PSs. In
other words, we have not yet discussed the total amount of PSs
formed (adsorbed plus shuttled). Previous work has indicated
that both FeS2 and one of its proposed cycling products (FeS)
strongly adsorb PSs.29 Therefore, if low solubility conditions
apply in the electrolyte, the amount of PSs shuttled would
conceivably underestimate the total amount of PSs formed. To
quantify PS formation, an Li-FeS2 coin cell was cycled three
times in ILE-1 and then decrimped for ex situ UV−Vis analysis
(Figure S7). Extraction was performed on all components of
the coin cell to best capture total PS formation. To this end,
the cathode and separator were soaked together in 4.5 mL of
neat DOL for 24 h. The anode was soaked in a separate vial
using an identical procedure to avoid contact with the cathode.
Using the UV−Vis spectra shown in Figure S7 and the
calculation method described earlier, the %Slost due to PS
formation after three cycles is 25%. Thus, the amount of PSs
formed (adsorbed plus shuttled) is approximately double the
amount from shuttling alone (%Slost = 11%; Table 1). Bearing
this discrepancy in mind, although the amount of PSs shuttled
in DOL/DME is ∼4× greater than ILE-1, the amount of PSs
formed in DOL/DME is only ∼2× greater (%Slost(DOL/
DME)/%Slost (ILE-1) = 48%/25%) than in ILE-1. We reiterate
that these estimates do not account for rigorously insoluble S
species (e.g., Li2S formed on the Li surface) and should thus be
taken as approximations.
To elucidate the quantitative relationship between PS losses
and capacity fade, we sought to compare two measures of PS
losses: (1) the percentage of dissolved (shuttled) PSs
determined from UV−Vis (Table 1; %Slost) and (2) the
theoretical PS losses (%Slost,theo.) expected if all of the observed
capacity fade in Figure 1 is due to PSs. The latter calculations
(provided in the SI) reveal that the average %Slost from PS
shuttling for DOL/DME (48%) is nearly an exact match for %
Slost,theo calculated from the observed capacity fade (49%). In
other words, PS losses account for the vast majority of Li-FeS2
capacity fade between the first and third cycles in DOL/DME.
For ILE-1, on the other hand, %S lost,theo (21%) is
approximately 2× greater than the average %Slost from PS
shuttling (11%). Therefore, only approximately half of the
observed capacity fade in ILE-1 is attributable to PS shuttling.
As discussed earlier, this finding is consistent with previous
reports showing that capacity fade in Li-FeS2 can originate
from multiple sources.6,7 We note that when total PS
formation is considered (shuttled plus adsorbed, vide supra),
solubility and cyclability in Li-FeS2 batteries at the rate studied.
More specifically, the percentage of TFSI− anions in ILE-1 is
high enough to preserve PS suppression but low enough
(through dilution with FSI−) to be adequately conductive for
cycling at C/10.
Quantification of Polysulfide Shuttling and Formation. In the previous section, we showed that the use of ILE-1
in Li-FeS2 batteries significantly decreased PS dissolution (and
thus, shuttling) relative to DOL/DME. In the present section,
we quantify those differences for both PS shuttling and PS
formation. The former is more practically relevant for the
current set of conditions (room temperature and limited
storage time). The latter (total PS formation) bears
importance for battery aging and high-temperature applications as PS solubility is expected to increase over time and at
elevated temperatures.
To get a quantitative estimate of the extent of PS shuttling in
each electrolyte, we calculated the percentage of S in FeS2 that
is converted to PSs for each electrolyte (%Slost; Table 1).
Details regarding this calculation are provided in the SI
(Section S2). In summary, we calculated the concentrations of
individual PS species using Beer’s Law (A = εbc), multiplied
them by their stoichiometries, and then summed them to yield
the total concentration of S lost to PSs ([S]lost). The total mass
of S lost to PSs (mSlost) was then computed by multiplying
[S]lost by the volume of electrolyte (Ve) and the molar weight
of S. Finally, %Slost was calculated as the ratio of mSlost to the
initial mass of S present in FeS2 (mS0; determined based on
mass loading).
Before proceeding, we mention several caveats. First, the
estimates below strictly represent S loss due to dissolved PSs
and therefore do not include any adsorbed or insoluble S
species (e.g., Li2S). Second, we used values from the literature
for the molar absorptivity of individual PS species due to the
fact that low solubility of Li2S in ILE-1 led to long dissolution
times for reference solutions made from Li2S/S8 mixtures (>3
weeks at 80 ° C). Therefore, the quantities presented here are
estimates rather than absolute. Further discussion on this topic
is provided in the SI.
The average absorbances and concentrations of S8, S62−, and
2−
S4 obtained after Li-FeS2 is cycled three times in each
electrolyte are shown in Table S3. As alluded to earlier, PS
disproportionation reactions are facile. Interconversion of PS
species (e.g., eqs 2 and 3)10,40,44 can lead to variation in
concentrations of individual species between trials and for
different conditions (e.g., ex situ vs in situ).
2Sx 2 − → Sy 2 −(y > x) + Sz 2 −(z < x), where y + z = 2x
(3)
Regardless of whether or not the PSs interconvert, the total
amount of S lost (the sum of the contributions from individual
PS species) is fixed. We therefore focus on the total losses of S
([S]lost, mSlost, and %Slost; Table 1) rather than the losses from
individual PS species (Tables S3 and S4). Among these three
quantities, %Slost (= 100% × mSlost/mS 0 ) is the most
representative because it is calculated relative to the initial
mass loading of S. The relative, rather than absolute, nature of
this quantity minimizes the influence of variations in Ve and
mS0 between the data sets collected for the three electrolytes.
As shown in Table 1, the average %Slost is 11 ± 4% for ILE-1
and 48 ± 10% for DOL/DME. Therefore, our results indicate
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disassembly, and the absence of an applied electrochemical
current.
The in situ UV−Vis spectra recorded in ILE-1 (Figure 4b)
show peaks corresponding to the presence of S8 (256 and 274
nm), S42− (298 and 420 nm), and S62− (343 nm). A magnified
view of these peaks is shown in Figure S8. These peaks are
similar to those obtained in DOL/DME (Figure 4a). Overall,
the peak positions in Figure 4 are in good agreement with
those observed in the ex situ UV−Vis spectra (Figure 3).
Analogous to the scenario with DOL/DME, the differences in
relative peak heights between Figures 3 and 4b are likely due to
enhanced disproportionation reactions (eqs 2 and 3) in the ex
situ experiments. As discussed in the previous section, we
account for variability caused by disproportionation reactions
by calculating the total amount of S lost to dissolved (PS)
species (rather than individual contributions).
There are three key results evident in Figure 4. First, the
majority of PS shuttling occurs during the first cycle. The
absorbance values for S42− (at 414 nm), S62− (at 348 nm), and
S8 (at 273 nm) observed after the first charge in DOL/DME,
respectively, represent 64, 59, and 81% of the corresponding
absorbance values recorded after the third charge. This
situation is even more apparent when Li-FeS2 is cycled in
ILE-1; the absorbance values for S42− (at 420 nm), S62− (at 343
nm), and S8 (at 274 nm) after the first charge represent 81, 84,
and 80%, respectively, of the corresponding values for the third
charge. Second, growth in PS shuttling stagnates after the
second cycle. This result is supported by the fact that the UV−
Vis spectra for the second (gray trace) and third (purple trace)
charges are nearly identical.
Third, the results in Figure 4 show that PS shuttling begins
during the first discharge (blue traces) in both electrolytes.
Although the mechanism of Li-FeS2 electrochemical cycling is
debated,1,3−5,14 S8 is expected to form during the first
charge,1,3,4,51 and PSs (e.g., Sx = 2−82−) are expected to form
either en route to S8 in the first charge4 or through reduction
of S8 in the second discharge.1,30,51 As we demonstrate in the
next section, this surprising result is supported by chemical
lithiation experiments of pristine FeS2 powder, which represent
the chemical analogue of the first electrochemical discharge.
Chemical Lithiation of FeS2. Chemical lithiation of
pristine FeS2 powder with n-butyl lithium (nBuLi) was carried
out with accompanying in situ UV−Vis measurements to
confirm the surprising finding (from the previous section) that
PSs form during the first electrochemical discharge in Li-FeS2
batteries. The spontaneous lithiation reaction of nBuLi with
FeS2 serves as a chemical analogue of the first electrochemical
discharge of Li-FeS2 batteries.52 Comparing UV−Vis for the
initial chemical and electrochemical lithiation reactions isolates
the role of FeS2 in PS formation and eliminates the influence
and/or artifacts of/from the electrolyte, anode, C-based
cathode additives, and/or the electrochemical cycling
procedure.
Chemical lithiation is described in depth in the Experimental
Section. Briefly, FeS2 powder (0.40 mmol) was reacted with
nBuLi in hexanes (1.59 mmol) in a cuvette while in situ UV−
Vis spectra were simultaneously recorded. The molar ratio of
nBuLi: FeS2 (4:1) was selected to match the Li: FeS2 reaction
stoichiometry of the first electrochemical discharge, (e.g., eq
1).1,4,14 We note, however, that previous studies have indicated
that only ∼2 equiv of nBuLi reacts with FeS2.52,53 Importantly,
Jones and coworkers showed that when electrochemical
lithiation was carried out to the same extent as the nBuLi-
the experimental %Slost (25%) is a much closer match for %
Slost,theo. On this basis, we hypothesize that adsorbed PSs might
contribute to Li-FeS2 capacity fade (e.g., by blocking active
sites). We mention the possibility that a fraction of the capacity
fade could originate from dissolved Fe species (e.g., Fe3+/4+ and
FexSy). This line of inquiry will be explored in due course.
In Situ UV−Vis Spectroscopy. While ex situ UV−Vis is
useful for identifying PS species and comparing the extent of
PS shuttling for the two electrolytes, it is not informative as to
when shuttling begins and how it progresses over time. To gain
temporal and mechanistic insight, in situ UV−Vis was
performed for both electrochemical lithiation and delithiation
(discussed here) and chemical lithiation (discussed in the next
section). As described in the Experimental Section, the setup
used for in situ UV−Vis measurements was essentially the
same as the ex situ flooded cell experiments with the following
exceptions: cycling was performed in a cuvette, the electrolyte
volume was decreased (4.5 mL) to match the size of the
cuvette, and the close proximity of the working and counter
electrodes required the use of a separator (40 μm
polypropylene).
Figure 4a shows the in situ UV−Vis spectra recorded over
the course of three cycles in DOL/DME. The same four peaks
Figure 4. In situ UV−Vis spectra recorded at key points during
electrochemical cycling of Li-FeS2 in (a) 1.0 M LiTFSI in 1:1
DOL:DME (DOL/DME) and (b) 1.0 M LiFSI in PYR14TFSI (ILE1). Each UV−Vis spectrum was background-subtracted for the
uncycled electrolyte (black traces). The mass loading of FeS2 was 1−2
mg/cm2, and electrochemical cycling was performed in a 1.0 cm
quartz cuvette containing 4.5 mL of electrolyte at a rate of C/10.
are observed after each complete discharge and charge step:
273, 310, 348, and ∼414 nm. As discussed in the previous
section, these peaks correspond to S8 (273 nm), S42− (310 and
414 nm), and S62− (348 nm). We note that although S62− is
evident in the in situ UV−Vis spectrum for DOL/DME
(Figure 4a), it is absent in the corresponding ex situ UV−Vis
spectrum (Figure 3). This discrepancy is likely due to
enhanced disproportionation in the latter (e.g., eqs 2 and 3)
facilitated by greater electrolyte volume (7.0 mL ex situ vs 4.5
mL in situ), the increased time associated with cell
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peaks at 350, 500, and 621 nm are identified as S62−, S82−, and
S3.‑, respectively. It is tempting to assign the two remaining
peaks (331 and 380 nm) to S42− as peaks for that species are
expected at ∼300−325 and ∼420 nm.19,40,41 However, careful
analysis of the UV−Vis spectra collected from 1−6 h (Figure
5a,c) shows that the peak at 380 nm grows independently of
the peak at 331 nm, suggesting that the two peaks represent
unique species. Alternatively, the peak at 380 nm could
correspond to multiple species (e.g., the convolution of S42−
plus S52−, both of which are expected at ∼420 nm40,41,54).
If the peaks at 331 and 380 nm do indeed represent unique
species, we note that Zaghib and coworkers reported two
independent peaks at similar wavelengths (325 and 385 nm),
which were assigned to S22− and S32−, respectively.55 Overall,
the assignment of these two peaks is not straightforward due to
conflicting peak assignments in the literature40,41,55 as well as
the possibility of convolved peaks40,41,54 and/or a radical
mechanism.54
Regardless of the exact speciation of the peaks at 331 (S22−or
2−
S4 ) and 380 nm (S32− or S42−), the relevant possibilities are
primarily short chain PSs (Sx=2−42−). Thus, the peaks observed
within the first 30 s of measurement (331, 380, and 621 nm
(S3·‑)) collectively suggest that the initial products are shortchain PSs. Long-chain PSs (S62− and S82−) only become
apparent at longer time scales (e.g., after 1 min). On this basis,
we propose that the initial short-chain PS species disproportionate to longer-chain PSs, which are then stabilized by the
highly electron-donating chemical environment provided by
nBuLi (which is a strong base). This hypothesis is supported
by previous literature showing that (1) short-chain PSs
(Sx=2−42−) can disproportionate to form S2− plus long-chain
PSs (Sx=4−82−)40,56 and (2) chemical environments that are
highly electron-donating stabilize long-chain PSs.19 We note
that differences in donor strength help explain why the PS
species in Figure 5 are not an exact match for those in Figures
3 and 4. More specifically, S82− and radical species (e.g., S3.‑)
are unstable in lower donor number chemical environments,
such as those used in Figures 3 and 4.18,19
Although an in-depth analysis of the chemical lithiation data
(Figure 5) is beyond the scope of the present report, there are
three major takeaways. First, PSs form during both the
chemical lithiation (Figure 5) and initial electrochemical
lithiation of FeS2 (Figure 4). As mentioned earlier, this finding
is contrary to previously established knowledge about FeS2,
where S8 is expected to form during the first charge and PSs
are either expected during the first charge (from oxidation of
Li2S to form S8) or the second discharge.1,3,4,51 Although we
do not have a complete understanding of the underlying
mechanism, a tentative mechanism is discussed in the SI
(Section S3).
Second, PS formation (and/or shuttling) during lithiation of
FeS2 is rapid. More specifically, Figure 5b shows that soluble
PSs appear in the first 30 s (out of a total reaction time of
several days).
Third, the absence of S8 during chemical lithiation suggests
that the presence of this species during the initial electrochemical lithiation (Figure 4) originates from disproportionation reactions (of long-chain PS species) in bulk solution (e.g.,
eq 2) rather than as an electrochemical cycling product. This
finding corroborates previous results indicating that the S8
cycling product forms during the first charge.1,3,4,30
As a final point, we note that it is possible that surface
species such as SOx2− from slight surface oxidation of the FeS2
based chemical lithiation, nearly identical solid phase products
formed.53 Thus, even if the lithiation reaction between nBuLi
and FeS2 does not go to completion, it is still representative of
the early stages of lithiation (and can therefore be used to
identify the temporal onset of PS formation). Total reaction
time (72 h) was chosen to be consistent with Whittingham and
coworkers, who indicated that chemical lithiation of FeS2 with
nBuLi takes several days to complete.52
Only the first 25 h are shown in Figure 5a due to the fact
that this time interval captures the features we wish to discuss.
Figure 5. In situ UV−Vis spectra of the chemical lithiation of FeS2
with n-butyl lithium (nBuLi) for two different time intervals: (a) 1−
25 h and (b) 0.5 to 2.0 m. The quartz cuvette used for analysis had a
path length of 1.0 cm. Each UV−Vis spectrum was backgroundsubtracted for the solvent (0.53 M nBuLi in hexanes). (c) Plot of
absorbance versus time for each peak shown in (a). Peak assignments
are indicated above the plot.
When the nBuLi was added to the FeS2 powder, the powder
dispersed throughout the solution, leading to noise in the UV−
Vis spectra and a baseline greater than zero. Baseline
subtraction of the spectra in Figure 5 was performed to
account for this artifact. These effects decreased after
approximately 1 h as the FeS2 powder settled to the bottom
of the cuvette.
Figure 5a shows four main peaks: 331, 350, 380 and 500 nm.
As shown in Figure 5b, peaks at 331, 380, and 621 nm appear
within the first 30 s, whereas the peaks at 350 and 500 nm
appear later (by 1 min). Based on the literature,18,19,34,41 the
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that remain after baking under vacuum at 150 °C (e.g., Figure
S2b,c) played a role in PS formation.
the ex situ UV−Vis spectrum obtained in 1.0 M LiFSI in
PYR14TFSI (ILE-1); calibration curve of S8 in ILE-1;
calculation of sulfur losses from UV−Vis; ex situ UV−
Vis spectra for determining total polysulfide (PS)
formation; calculation of sulfur losses from capacity
fade; magnified view of the in situ UV−Vis spectra
obtained in ILE-1; tentative mechanism of PS formation
(PDF)
■
SUMMARY AND CONCLUSIONS
Our results reveal several important insights relating to PS
formation and shuttling in Li-FeS2 batteries. First, cycling LiFeS2 in a PYR14TFSI-based ionic liquid electrolyte (1.0 M
LiFSI in PYR14TFSI) leads to a significant abatement of PS
shuttling. Specifically, the average total amount of sulfur lost to
shuttling in 1.0 M LiFSI in PYR14TFSI (11%) is a factor of
∼4× lower than in a conventional organic electrolyte (48% for
1.0 M LiTFSI in 1:1 DOL:DME). Furthermore, when the
electrolyte anion is changed to LiTFSI (1.0 M LiTFSI in
PYR14TFSI), loss of sulfur to PSs becomes minimal (∼3%).
Thus, we find that careful choice of electrolyte is key to
controlling the PS shuttling in Li-FeS2 batteries. We note that
the present report analyzed PS formation and shuttling for just
two compositions: 23% FSI−/77% TFSI− (1.0 M LiFSI in
PYR14TFSI) and 100% TFSI− (1.0 M LiTFSI in PYR14TFSI).
Although the latter had superior PS suppression characteristics
to the former, it had inferior cycling performance. Future work
will be aimed at determining the optimal FSI−:TFSI− ratio for
maximizing both PS suppression and cyclability.
Second, our results suggest that suppression of PS shuttling
in a PYR14TFSI-based ionic liquid electrolyte is primarily
related to decreased solubility rather than decreased PS
formation. Although it has previously been shown that FeS2
can serve as a PS trap (through adsorption),29 the success of
this strategy can be limited if PS solubility is sufficiently high.
In particular, we find that when using 1.0 M LiFSI in
PYR14TFSI, the amount of PS formation is approximately
double the amount shuttled. This finding underscores the
importance of analyzing both adsorbed and dissolved PSs to
get an accurate representation of PS formation. Our results are
relevant for both Li-S and Li-FeS2 batteries as one can envision
that PS solubility may increase over time and/or at elevated
temperature.
Third, we show that the majority of PS shuttling (≥60%)
occurs during the first cycle and stagnates by the third cycle.
Furthermore, our findings suggest that PS shuttling begins
during the first discharge. This surprising finding diverges from
the traditional mechanism proposed in the literature. In situ
UV−Vis of the chemical lithiation of FeS2 reveals that PS
formation during FeS2 lithiation is rapid (beginning in just 30 s
out of the 72 h total reaction time). The results from our
mechanistic exploration suggest that the oft-cited reaction 4Li+
+ FeS2 +4e‑ → Fe0 + 2Li2S for the initial lithiation might be an
oversimplification.
Although our results indicate that dissolved PSs are not the
sole contributor to capacity loss, we note that the conditions
used here involve bulk Li foil. The effects of PSs generated by
Li-FeS2 batteries on capacity retention likely gain importance
in battery applications requiring thin Li.
■
Article
■
AUTHOR INFORMATION
Corresponding Author
A. Alec Talin − Materials Physics Department, Sandia
National Laboratories, Livermore, California 94550, United
States; orcid.org/0000-0002-1102-680X;
Email: aatalin@sandia.gov
Authors
Aliya S. Lapp − Materials Physics Department, Sandia
National Laboratories, Livermore, California 94550, United
States
Grace Whang − Department of Materials Science and
Engineering, University of California Los Angeles, Los
Angeles, California 90095, United States
Austin Bhandarkar − Materials Physics Department, Sandia
National Laboratories, Livermore, California 94550, United
States
Igor V. Kolesnichenko − Photovoltaics and Materials
Technology Department, Sandia National Laboratories,
Albuquerque, New Mexico 87123, United States;
orcid.org/0000-0002-9786-4053
Bruce S. Dunn − Department of Materials Science and
Engineering, University of California Los Angeles, Los
Angeles, California 90095, United States; orcid.org/
0000-0001-5669-4740
Timothy N. Lambert − Photovoltaics and Materials
Technology Department, Sandia National Laboratories,
Albuquerque, New Mexico 87123, United States;
orcid.org/0000-0002-2359-2876
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.jpcc.1c10074
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
We thank Dr. Bryan R. Wygant for his assistance in
characterizing the phase purity of the commercial, off-theshelf (COTS) FeS2 powder prior to ball milling. This paper
describes objective technical results and analysis. Any
subjective views or opinions that might be expressed in the
paper do not necessarily represent the views of the U.S.
Department of Energy or the United States Government. This
work was supported by the Laboratory Directed Research and
Development program at Sandia National Laboratories, a
multimission laboratory managed and operated by National
Technology and Engineering Solutions of Sandia, LLC., a
wholly owned subsidiary of Honeywell International, Inc., for
the U.S. Department of Energy’s National Nuclear Security
Administration under contract DE-NA-0003525.
ASSOCIATED CONTENT
sı Supporting Information
*
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.jpcc.1c10074.
Physical characterization (SEM, TGA, XRD, Raman,
and XPS) of the FeS2 used for analysis; electrochemical
characterization of the flooded cells; electrochemical
dQ/dV analysis of Li-FeS2 coin cells; magnified views of
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(21) Ma, G.; Wen, Z.; Jin, J.; Wu, M.; Zhang, G.; Wu, X.; Zhang, J.
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