Tailoring Electrolyte Additives with Synergistic Functional Moieties for Silicon Negative Electrode-Based Lithium Ion Batteries: A Case Study on Lactic Acid O-Carboxyanhydride

Article abstract of DOI:10.1021/acs.chemmater.9b03173

Silicon (Si) has attracted much attention to be applied as a negative electrode (N) material for lithium ion batteries (LIBs) with increased energy density. However, the huge volume changes during (de-)lithiation of the Si, accompanied with the breakdown of the initially formed solid electrolyte interphase (SEI), result in the gradual consumption of active lithium and electrolyte and, hence, a poor cycling performance of LIBs with Si-based N. The addition of various electrolyte additives was proven to be able to reduce the active lithium consumption by the formation of a more effective/flexible and, therefore, better protecting SEI on the Si. Within this study, we synthesize the new electrolyte additive lactic acid O-carboxyanhydride (lacOCA), which is designed to incorporate two different moieties within its structure, that both show to function as effective SEI additives. The addition of small amounts of 2 wt % of lacOCA to the baseline electrolyte significantly improves the electrochemical performance of NMC-111||Si full cells in terms of discharge capacity retention and Coulombic efficiency. The lacOCA also outperforms the comparable additives lactide and diethyl dicarbonate, which are chosen to individually represent the moieties incorporated within the lacOCA structure, proving the synergistic effect of the two different moieties, when in one molecule. Ex situ investigations of the SEI by means of X-ray photoelectron spectroscopy and attenuated total reflectance Fourier transform infrared spectroscopy reveal that the SEI formed by lacOCA is mainly composed of poly(lactic acid) and lithium carbonate, which enables a significant reduced consumption of active lithium during charge/discharge cycling of the NMC-111||Si full cells.

Full text of DOI:10.1021/acs.chemmater.9b03173

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Article
Tailoring Electrolyte Additives with Synergistic Functional
Moieties for Silicon Negative Electrode-Based Lithium Ion
Batteries: A Case Study on Lactic Acid O-Carboxyanhydride
Roman Nölle, Jan-Patrick Schmiegel, Martin Winter, and Tobias Placke
Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.9b03173 • Publication Date (Web): 03 Dec 2019
Downloaded from pubs.acs.org on December 7, 2019
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Tailoring Electrolyte Additives with Synergistic Functional Moieties for
Silicon Negative Electrode-Based Lithium Ion Batteries:
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A Case Study on Lactic Acid O-Carboxyanhydride
Roman Nölle 1, Jan-Patrick Schmiegel 1, Martin Winter 1,2, **, Tobias Placke 1, *
1 University of Münster, MEET Battery Research Center, Institute of Physical Chemistry,
Corrensstr. 46, 48149 Münster, Germany
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2 Helmholtz Institute Münster, IEK-12, Forschungszentrum Jülich GmbH,
Corrensstr. 46, 48149 Münster, Germany
Abstract
Silicon (Si) has attracted much attention to be applied as negative electrode (N) material for
lithium ion batteries (LIBs) with increased energy density. However, the huge volume changes
during (de-)lithiation of the Si, accompanied with the breakdown of the initially formed solid
electrolyte interphase (SEI), result in the gradual consumption of active lithium and electrolyte
and, hence, a poor cycling performance of LIBs with Si-based N. The addition of various
electrolyte additives was proven to be able to reduce the active lithium consumption by the
formation of a more effective/flexible and, therefore, better protecting SEI on the Si. Within
this study, we synthesize the new electrolyte additive lactic acid O-carboxyanhydride
(lacOCA), which is designed to incorporate two different moieties within its structure, that both
show to function as effective SEI additives. The addition of small amounts of 2 wt.% of lacOCA
to the baseline electrolyte significantly improves the electrochemical performance of
NMC-111 || Si full cells in terms of discharge capacity retention and Coulombic efficiency. The
lacOCA also outperforms the comparable additives lactide (LAC) and diethyl dicarbonate
(DEDC), which are chosen to individually represent the moieties incorporated within the
lacOCA structure, proving the synergistic effect of the two different moieties, when in one
molecule. Ex-situ investigations of the SEI by means of X-ray photoelectron spectroscopy
(XPS) and attenuated total reflectance Fourier-transform infrared spectroscopy (ATR-FTIR)
reveal that the SEI formed by lacOCA is mainly composed of poly lactic acid (PLA) and lithium
carbonate, which enables a significant reduced consumption of active lithium during
charge/discharge cycling of the NMC-111 || Si full cells.
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*Corresponding author:
Dr. Tobias Placke
**Co-Corresponding author:
Prof. Dr. Martin Winter
tobias.placke@uni-muenster.de
martin.winter@uni-muenster.de
m.winter@fz-juelich.de
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TOC (Graphical Abstract)
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1. Introduction
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The demand for lithium ion batteries (LIBs) has grown significantly over the last few
years, mainly driven by the automotive sector related to the development towards electro
mobility. Although millions of electric vehicles (EVs), including plug-in hybrid electric
vehicles (PHEVs) and battery-electric vehicles (BEVs), are already in use worldwide, there is
still a need for improvements mainly in terms of energy density (>500 Wh L-1 at pack level)
and cost (<125 US$ kWh-1 at pack level) for LIBs to enable an extensive mass-market
penetration of EVs.1-3 Despite optimizations regarding the battery cell and pack design (e.g.
cell housing, inactive materials, etc.), the development and inclusion of advanced active
materials possessing higher energy densities than present state-of-the-art active materials, is
believed to be the only option to achieve the aforementioned values for energy density and costs
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for future LIBs.4-6 Apart from carbon/graphite as standard negative electrode (N) material ,
mainly alloying-type negative electrode materials, and in particular silicon (Si) are considered
to be the most promising candidates to further increase the energy density of N.4, 8-10 Although
already small amounts of silicon (very often as SiOx) are added to the carbon-based N within
some commercial LIBs 1, 11, 12, the incorporation of higher amounts of Si, which would enable
a strong boost in terms of energy density, is still hampered by the insufficient cycle life of such
cells. This issue is essentially related to huge volume variations of Si upon (de-)lithiation, going
along with structural instabilities of N.10, 13, 14 Mainly the breakdown and in consequence the
continuous (re-)formation15, 16 of the solid electrolyte interphase (SEI)17, 18 at the alloy surface
consumes significant amounts of active lithium (Li) and electrolyte, consequently resulting in
a poor cycling performance of LIBs incorporating Si-based N.19-23 Various approaches to
address those issues are reported in the literature, including nanostructuring of the Si24-28, using
intermetallic29, 30 ,or carbon-based composite materials31, 32, the application of artificial
protection layers33, 34 or including pre-lithiation techniques35-37 for compensating active Li
losses.
A straightforward approach, i.e. simple to incorporate within actual LIB cell production
processes, is the addition of small amounts of appropriate additives to the base electrolyte
consisting of LiPF6 and organic carbonate solvents38, 39, which has proven to drastically improve
the performance of Si-based N related to the formation of a more stable SEI. Doubtlessly,
fluoroethylene carbonate (FEC) is the commonly used electrolyte additive when studying Si-
based N.40-42 However, various different additives including vinylene carbonate (VC)43, 44
,
borate-based molecules45-47, isocyanates20 or anhydrides48, 49 have also been reported to
beneficially effect the performance of Si negative electrodes. Furthermore, carbon dioxide
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(CO2), already investigated with Li metal and graphite N more than 20 years ago50-52, was
recently reported to remarkably improve the performance of LIBs containing a Si-based N.53-55
In a recent publication, Gasteiger and co-workers revealed that the addition/presence of CO2 in
the electrolyte resulted mainly in the formation of Li2CO3 as SEI component, whereas they
proposed different mechanism for the formation of Li2CO3 by CO2.56 Besides an
electrochemical reduction, CO2 can also act as reaction-type additive to scavenge reactive
components/impurities of the electrolyte (e.g. water, alkoxides etc.) and, hence, reduces or
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inhibits electrolyte degradation, like alkyl carbonate trans-esterification or LiPF6 hydrolysis.55-
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Although all these electrolyte additives are reported to have a beneficial effect, there is
still a debate on the exact working principle of those additives and how they influence the Si
electrodes. Studies on the additives FEC and VC revealed the formation of polymeric species
incorporated within the SEI, which are believed to be more compatible with the huge volume
variations of Si.20, 41, 42, 60-62 In contrast to that, various publications revealed that the inorganic
lithium carbonate (Li2CO3) is the main SEI component when CO2 is used as electrolyte additive,
which cannot not be considered to be flexible.52, 56 Hence, one need to keep in mind that the
SEI is not a simple one-component system, but rather a complex interphase containing various
different compounds, including salt decomposition products (e.g. LiF, LixPOyFz for LiPF6-
based electrolytes) and solvent (e.g. lithium ethylene dicarbonate (LEDC) for EC-based
electrolytes) or additive (e.g. poly(VC) for VC- or FEC-based electrolytes) decomposition
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products.18,
Most likely, not solely the molecular composition but also the formed
(nanostructured) morphology may determine the properties of the SEI.64 Furthermore,
synergistic effects have been observed for electrolytes containing more than one additive,
indicating that an appropriate interplay between different compounds and structures may be the
key to an optimized SEI for Si-based N.55, 65, 66
Considering the gained insights into the composition/structure-property relationships of
the SEI, cyclic diesters like lactide (LAC) might be suitable candidates as electrolyte additives
for Si-based N, as they are commonly known to be able to undergo ring opening polymerization
(ROP) to form poly(lactic acid) (PLA) (see Scheme 1). Despite those cyclic diesters, O-
carboxyanhydrides (OCA) are also able to form polyesters by a catalyst (e.g. nucleophilic or
metal-based) induced ROP, including the release of one equivalent carbon dioxide (CO2).67, 68
The ROP of lactic acid O-carboxyanhydride (lacOCA) to PLA is exemplarily depicted in
Scheme 1. Actually, Jeon et al. reported the use of various cyclic diesters (including LAC) as
SEI-forming electrolyte additive for LiCoO2 || graphite LIBs, attributing their effectiveness to
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the formation of a polymeric SEI at the graphite N by ROP of the cyclic diesters.69 However,
to best of our knowledge there is no scientific report using OCA molecules as electrolyte
additives for LIBs. Due to the additional release of CO2 during the ROP and, therefore, the
incorporation of two moieties within one molecule, this material class might be even more
interesting for application as electrolyte additive for Si-based N, as the simultaneous formation
of polymeric (i.e. PLA) and inorganic species (i.e. Li2CO3) may allow for synergistic effects on
the properties of the formed SEI.
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O
O
O
O
O
n
2n
O
O
O
-2CO2
2n
O
O
lactide (LAC)
poly(lactic acid) (PLA)
lactic acid O-carboxyanhydride (lacOCA)
Scheme 1: Ring opening polymerization of lactide (LAC) to poly(lactic acid) (PLA) and lactic acid O-carboxyanhydride
(lacOCA) to poly(lactic acid) (PLA) including release of carbon dioxide (CO2).
As revealed in several scientific reports, a drastic decay in capacity of full cells
incorporating a Si-based N is essentially triggered by the continuous (re-)formation of the SEI
due to its breakoff during the huge volume changes of the Si. These volume variations impede
an effective protection of the electrode by the SEI, as fresh surface of the Si is exposed to the
electrolyte at potentials below its electrochemical stability window in every cycle, leading to
reductive decomposition of the electrolyte going along with the continuous consumption of the
electrolyte as well as active Li. Since the amount of active Li is restricted to the capacity of
used positive electrode (P) material (i.e., NMC in this study), in contrast to active Li-excessed
“half cells” (i.e., using a Li metal electrode in large capacity excess70), the described continuous
SEI formation immediately shows up in capacity decrease of Si-based full cells.21 It was
reported for different additives (e.g. FEC and VC) that the formation of a more effective and,
therefore, better protecting SEI by the additives can significantly decrease the mentioned
consumption of active Li in full cells.21, 71 Therefore, we strongly recommend to perform
investigation of electrolyte additives for LIBs directly in full cells, as results achieved in half
cells or Li metal cells may not directly be transferred to full cells.23, 72 Thus, we recently reported
on how to choose the appropriate cell setup according to the intended aim of the investigation,
including studies of electrolytes and additives.70
Within this work, lacOCA was synthesized via carbonylation of lactic acid using
triphosgene as carbonylating agent. The as-synthesized molecule was investigated as electrolyte
additive within NMC-111 || Si full cells and thoroughly compared with the additives LAC and
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54,
diethyl dicarbonate (DEDC), while the latter one undergoes decomposition to DEC and CO2
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73, 74, therefore is considered as CO2-releasing additive. The performance of the Si-based LIB
full cells is significantly improved by the addition of only 2 wt.% of lacOCA to the baseline
electrolyte and also outperformed the comparative additives LAC and DEDC, proving the
synergistic effect of both moieties incorporated within one molecule. Post-cycling investigation
of the Si N by means of X-ray photoelectron spectroscopy (XPS) and attenuated total
reflectance Fourier-transform infrared spectroscopy (ATR-FTIR) confirmed the formation of
an SEI incorporating both PLA and Li2CO3, related to the reductive decomposition of the
lacOCA additive.
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2. Experimental
2.1 Synthesis of lactic acid O-carboxyanhydride (lacOCA, 5-methyl-1,3-dioxolane-2,4-dione)
LacOCA was synthesized based on the methods reported by Sun et al.75, as depicted in
Scheme 2. The synthesis of this air- and moisture-sensitive compound was performed under
dry argon atmosphere using standard Schlenk techniques. THF (Sigma Aldrich, purity:
≥99.9%), hexane (Merck, purity: ≥99%) and diethyl ether (Fisher Scientific, purity: ≥99%)
were dried with the appropriate drying agent and stored under argon atmosphere, while all other
chemicals were used as-received.
A flame-dried Schlenk flask was filled with a suspension of lactic acid lithium salt
(0.90 g, 10 mmol, 1.0 eq., Acros Organics, purity: 99%) and activated charcoal (~30 mg) in
20 mL anhydrous THF. Triphosgene (3.56 g, 12 mmol, 1.2 eq., Sigma Aldrich, purity: 98%)
was dissolved in 40 mL anhydrous THF and was slowly added to the cooled (0 °C) suspension
by a dropping funnel. The reaction mixture was warmed up to room temperature and
subsequently stirred for further 18 h. (Caution: Evolving gases were trapped with a wash-battle
containing a mixture of ethanol, water and ammonia solution (25%). The solvent and any
unreacted triphosgene or phosgene gas were removed under reduced pressure and trapped with
liquid nitrogen. The latter solution was carefully neutralized with the ammonia solution.)
The residue was washed with THF and filtrated using a filtration cannula in order to remove
the activated charcoal. The solvent was removed under reduced pressure and subsequent
recrystallization of the residue from dry diethyl ether and hexane at -25 °C afforded colorless,
needlelike crystals.
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O
THF
O
activated charcoal
Cl
O
Cl
Li
Cl
Cl
Cl
Cl
+
O
O
O
0°C to RT, 18 h
O
O
OH
O
Scheme 2: Synthesis route of lactic acid O-carboxyanhydride (lacOCA) from lactic acid lithium salt and triphosgene.
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NMR: δ: 1H (400 MHz, CDCl3, TMS): δ = 5.14 (q, 1H), 1.72 (d, 3H); 13C (400 MHz, CDCl3,
TMS): δ = 167.59, 148.03, 76.13, 16.48.
The analytical data is in accordance with the reported values.75 The NMR spectra are depicted
in the supporting information (see Figure S1 and Figure S2). Furthermore, the amount of
residual chloride ions was determined by ion chromatography and found to be ≈34 ppm in the
final electrolyte. The cycled Si electrodes were also analyzed via XPS, however, no chloride
species were found, so that any significant impact of residual chloride impurities on the
electrochemical performance and/or SEI formation can be excluded.
2.2 Electrochemical investigations
Pure Si thin film electrodes (≈750 nm, ≈0.17 mg cm-2, for detailed description of
preparation see ref.20) were paired with NMC-111 electrodes (Ø12 mm, 0.53 mAh cm-2 for at
cut-off potential of 4.3 V vs. Li|Li+, 90% active material content, Customcells Itzehoe) to
NMC-111 || Si full cells with a N/P ratio of ≈1.13 (considering a capacity of 0.60 mAh cm-2 for
the Si electrodes, based on the active material content and a theoretical capacity of
3,579 mAh g-1 for Si). Electrochemical investigations were carried out in three-electrode
Swagelok-type T-cells assembled in an argon-filled glovebox (MBraun). High-purity Li metal
foil (Albemarle Corporation, battery grade) was used for the reference electrode (Ø6 mm) and
Celgard 2500 (polypropylene, one layer, Ø13 mm and Ø10 mm for reference) was applied as
separator. An overall amount of 120 µL electrolyte was added to the cell. The “baseline”
electrolyte contained 1 M LiPF6 in EC/DEC 3:7 (w/w) (all components from BASF SE, battery
grade) to which the investigated additives lacOCA (synthesized, 99.25 mol-% as determined
via differential scanning calorimetry, see Figure S3), LAC (Alfa Aesar, 98+%) and diethyl
dicarbonate (DEDC) (Alfa Aesar, 97%) were added in amounts of 2 wt.%. For further
comparison to the lacOCA additive, the well-known additives vinylene carbonate (VC; BASF
SE, battery grade) and fluoroethylene carbonate (FEC; BASF SE, battery grade) were also
studied as “reference” electrolytes and each added in amounts of 2 wt.% to the baseline
electrolyte. It should be noted, that a higher amount of electrolyte is used in these laboratory
three-electrode T-cells in comparison to commercial cells. Therefore, the percentage of
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electrolyte additive or the actual amount of additive molecules probably needs to be adjusted
within commercial cell set-ups.
Charge/discharge cycling of the NMC-111 || Si cells was performed in a voltage range
of 3.0 – 4.3 V at 20 °C on a Maccor series 4000 battery tester. The cycling protocol started by
charging the cell up to 4.3 V with a current of 0.05C (1C = 150 mA g-1), followed by
discharging down to 3.0 V at 0.1C. Two additional charge/discharge steps at 0.1C completed
the formation process of the NMC-111 || Si full cells. Afterwards, the cells were cycled for 100
cycles at a constant charge/discharge current of 1C, additionally applying a constant voltage
step at the charge and discharge cut-off voltage until the current dropped below 0.05C. The
specific capacities shown in this report are related to the active material weight of the NMC-111
electrode. The presented data are average values of three cells for each electrolyte formulation,
demonstrating reproducibility of the obtained results. Note, that the electrochemical results of
the “baseline” and “reference” electrolytes shown in this manuscript (i.e., baseline, baseline +
2 wt.% VC, baseline + 2 wt.% FEC) have already been reported in our previous publication.20
Linear sweep voltammetry measurements were performed at a VMP3 (Biologic Science
Instruments) with a scan-rate of 25 µV s-1 at 25 °C up to a potential of 6 V (vs. Li|Li+).
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2.3 Post-mortem investigations of Si negative electrodes
The Si thin film electrodes and NMC-111 electrodes were harvested from the cycled
cells in an argon-filled glovebox and washed two times via dipping the cycled electrodes in
200 µL DMC (BASF, battery grade) prior to surface analysis, in order to remove electrolyte
residues and in particular LiPF6 salt.
X-ray photoelectron spectroscopy (XPS) was performed to analyze the SEI formed on
the Si thin film electrodes in different electrolyte solutions after 103 cycles. The samples were
transferred to the XPS device (Axis Ultra DLD, Kratos) without exposure to ambient air in a
sealed container. Monochromatic Al Kα X-rays (hν = 1486.6 eV) with a 10 mA emission
current and 12 kV accelerating voltage was applied. A charge neutralizer was used to
compensate for charging of the samples. The measurement was performed at a 0° angle of
emission and a pass energy of 20 eV. The fitting was carried out with CasaXPS. Calibration of
the binding energy (BE) of the recorded spectra was performed by using the C 1s C-H/C-C peak
(BE = 284.5 eV) as an internal reference. To guarantee reproducibility, three different
measurement spots per sample were investigated.
Attenuated total reflectance Fourier-transform infrared spectroscopy (ATR-FTIR) of the
cycled and washed Si electrodes was performed using a Vertex 70 IR spectrometer from Bruker
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Optics. The measurements were conducted with 256 scans and a spectral resolution of 2 cm-1
within a sample chamber, which was continuously purged with argon to minimize influences
of ambient conditions.
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3. Results and discussion
3.1 Electrochemical performance of NMC-111 || Si cells
The effect of the synthesized lacOCA as SEI-forming electrolyte additive is investigated
within NMC-111 || Si full cells. Amorphous, pure Si thin film electrodes, prepared via
magnetron-sputtering, serve as negative electrode (N) within those full cells, as the absence of
binder and conductive additives will enable a direct correlation of the SEI formed by the
different electrolytes/additives on the performance of the Si-based N. The synthesized additive
lacOCA was added in an amount of 2 wt.% to the baseline electrolyte, (i.e. 1M LiPF6 in
EC:DEC (3:7, by wt.)). Furthermore, lacOCA is compared to the electrolyte additive LAC,
which was proven to form an polymeric SEI on a graphite-based N by reductive
decomposition.69 For a comparison to CO2 as electrolyte additive, DEDC was additionally
investigated as electrolyte additive, as it was revealed by Chevrier et al. that the addition of this
CO2-liberating molecule to the electrolyte has the same effect on the performance of Si-based
LIB cells like the addition of pure CO2 to the cell via dry ice.54
To investigate the tendency of the different electrolytes and additives towards reductive
decomposition, i.e. forming an SEI on the Si electrode, the differential capacity vs. the Si
electrode potential plots of the NMC-111 || Si full cells during the first charge process for the
four different electrolyte solutions are depicted in Figure 1. Since the lithiation of the used
amorphous Si thin film electrodes starts below potentials of 0.3 V (vs. Li|Li+)20, any current
flow at higher potentials can be attributed to parasitic reactions, i.e. the reductive decomposition
of the electrolyte at the surface of the Si electrode. Note that the peak at an electrode potential
of ≈0.55 V (vs. Li|Li+), observed for all four electrolyte solutions, is related to the formation of
Li2O from the oxide surface layer on the Si electrodes.76
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Figure 1: Differential capacity vs. electrode potential plots of Si negative electrodes within NMC-111 || Si full cells during the
first charging process using different electrolytes, focussing the region of reductive electrolyte decomposition on the Si
electrode. Addtionally, the chemical structures of the investigated additives lactic acid O-carboxyanhydride (lacOCA), lactide
(LAC) and diethyl dicarbonate (DEDC) are depicted.
For the baseline electrolyte no distinct peak, despite of the already mentioned one at
≈0.55 V (vs. Li|Li+), in the differential capacity plot can be observed, however at an electrode
potential of ≈1.1 V (vs. Li|Li+) a start of parasitic current flow can be noted, which can be
attributed to different reductive decomposition reactions of the electrolyte components like the
solvents EC and DEC and also the conductive salt LiPF6.77 When lacOCA is added to the
baseline electrolyte, a distinct peak at 1.0 V (vs. Li|Li+) is observed, proving the reductive
decomposition of the synthesized additive lacOCA at the Si electrode. When the additives LAC
or DEDC are added to the baseline electrolyte, peaks at ≈0.9 V (vs. Li|Li+) are observed for
both electrolytes, which is in good agreement to reports using these or similar additives.69, 73
Hence, the additive lacOCA exhibits a tendency to be reductively decomposed at slightly higher
potentials than the comparative additives LAC and DEDC, whereas for all three additives a
decrease of current flow below electrode potentials of 0.6 V (vs. Li|Li+) is observed, indicating
that the additive-derived SEI layer decreases parasitic reactions at lower potentials.
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To analyze the oxidative stability of the various electrolyte solutions, linear sweep
voltammetry measurements were performed on LiMn2O4 electrodes78. As can be seen in Figure
S4, all electrolytes display a sufficient stability (>5 V vs. Li|Li+) towards oxidative
decomposition, with the VC-based electrolyte being oxidatively decomposed already at
≈4.7 V (vs. Li|Li+). Hence, the developed additive lacOCA and also the investigated additives
LAC and DEDC are not believed to undergo a significant oxidative decomposition at the P
within the NMC-111 || Si full cells.
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To investigate the impact of the synthesized lacOCA and the two additional additives
LAC and DEDC on the cycling performance of Si-based LIBs, NMC-111 || Si full cells have
been assembled and charged/discharged for 103 consecutive cycles. The specific discharge
capacities as well as Coulombic efficiencies (CEs) vs. the cycle number of the cells containing
the four different electrolytes are depicted in Figure 2, exact values thereof are provided in
Table 1 for selected cycles.
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Cycle number
Figure 2: Specific discharge capacities (a) and Coulombic efficiencies (b) of NMC-111 || Si full cells cycled in the voltage
range of 3.0 – 4.3 V at 20 °C with four different electrolyte solutions.
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Table 1: Specific discharge capacities and Coulombic efficiencies of selected cycles of NMC-111 || Si full cells for the four
different electrolyte solutions.
5
6
7
Coulombic
Discharge
Discharge
Capacity
efficiency / %
8
9
Electrolyte
capacity (1st
capacity (103rd retention (103rd
1st
cycle
5th
103rd
cycle
cycle) / mAh g-1 cycle) / mAh g-1
cycle) / %
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cycle
72.9 ± 98.7 ± 95.4 ±
1.5 0.2 0.6
75.7 ± 99.2 ± 99.1 ±
1.9 0.1 0.1
68.2 ± 98.3 ± 98.2 ±
1.8 0.4 0.4
69.0 ± 98.6 ± 97.9 ±
2.1 0.2 0.4
Baseline
131 ± 3
136 ± 3
126 ± 3
129 ± 4
16 ± 3
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2% lacOCA
Baseline +
2% LAC
Baseline +
2% DEDC
The NMC-111 || Si full cells possess a discharge capacity of 131 mAh g-1, accompanied
with a CE of 71.7% in first cycle, when applying the baseline electrolyte without any electrolyte
additive. Upon continuous charge/discharge cycling, these cells show a strong decrease in
discharge capacity, offering only a capacity of 15 mAh g-1 in the 103rd cycle, associated with
steadily declining CEs leading to a value of only 95.5% in the 103rd cycle. It has to be kept in
mind, that pure Si electrodes are used for this study, therefore, the performance of these Si-
based LIB cells cannot be directly compared to the cycling performance of Si/carbon-based
negative electrodes. To check whether the SEI derived by the one of three different additives
is capable of reducing the loss of active Li upon charge/discharge cycling, the NMC-111 || Si
full cells are investigated with the baseline electrolyte comprising 2 wt.% of the additives.
When lacOCA is added to the electrolyte, the discharge capacity as well as CE in the first cycle
is increased to 136 mAh g-1 and 75.7%, respectively. Furthermore, the capacity retention of the
cells is significantly increased, namely 67 mAh g-1 can be achieved during discharge in the
103rd cycle, corresponding to more than the fourfold of the cells with the baseline electrolyte.
These huge improvements can also be observed in terms of the CE, which reaches a value
higher than 99% in the 5th cycle, which is maintained until the last cycle. The addition of the
two additional additives LAC and DEDC also results in an improvement of the full cells
compared to the baseline electrolyte, as discharge capacities of 37 mAh g-1 for LAC-based and
48 mAh g-1 for DEDC-based electrolytes can be achieved after 103 cycles (Table 1). Whereas
the addition of these two additives can enhance the performance related to discharge capacity
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retention and CEs upon continuous charge/discharge cycling compared to the baseline
electrolyte without an additive, both are not able to reach the performance of full cells
containing lacOCA as electrolyte additive. As the structure of lacOCA was designed to combine
both moieties, i.e. the liberation of CO2 of DEDC and the ROP polymerization of LAC, within
one molecule, we believe that this synergy is responsible for the effectiveness of the lacOCA
additive. To verify this assumption, the additives DEDC (2 wt.%) and LAC (1 wt.%) are
combined within one electrolyte (see Figure S5 and Table S1), enabling an increased discharge
capacity of 60 mAh g-1 in the 103rd cycle compared to the cells cycled with only one of the
additives LAC or DEDC. However, the improvement achieved by the addition of lacOCA is
not reached, what might be related to inclusion of both moieties within one molecule, rather
than combining them separately. This is most likely also related to the different potentials at
which the additives are reduced, with lacOCA exhibiting a higher reduction potential of
≈1.0 V (vs. Li|Li+) compared to the slightly lower potential of ≈0.9 V (vs. Li|Li+) for DEDC and
LAC. Hence, the mechanism of SEI formation and, consequently the composition and
morphology of the formed SEI might be different for lacOCA compared to the combination of
LAC and DEDC which was intended to mimic the additive lacOCA. Nevertheless, it also has
to be kept in mind that a further optimization of the electrolyte amount and additive amount in
large-scale (e.g. pouch bag) cells is mandatory to verify this assumption. Recently, Lucht and
co-workers revealed such a synergetic behavior for the salt lithium difluoro(oxalate)borate
(LiDFOB), which in turn represents a combination of lithium bis(oxalato)borate (LiBOB) and
lithium tetrafluoroborate (LiBF4). Despite an identical chemical composition by the
combination of LiBOB and LiBF4, the LiDFOB-based electrolyte enabled significantly
improved performance of the investigated Li metal electrodes formed on a copper current
collector within LiFeO4 || Cu cells, which was proven to be related to the nanostructured SEI
formed by LiDFOB.64 To assess the effectiveness of the lacOCA as additive for Si-based LIBs,
a comparison to the standard additives FEC and VC is also performed (see Figure S6 and
Table S1), in which the lacOCA-based outperforms the FEC-based electrolyte, identifying it
as a highly interesting electrolyte additive for Si-based N. A more detailed comparison of the
additives VC and FEC in the same cell setup can be found in our previous publication, in which
the VC-based electrolyte showed an even better performance than the FEC-based electrolyte.20
Note, that the type of active material (i.e. pure Si, Si/carbon composites etc.) and especially the
cell setup (half-cell setup vs. full-cell setup) may have a significant influence on the results.70
In Figure 3 the accumulated Coulombic inefficiencies (ACIEs) for the NMC-111 || Si
full cells cycled with the four different electrolytes are depicted. As for full cells comprising a
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Si-based N the Coulombic inefficiency is mainly attributed to SEI formation, the ACIE, which
corresponds to the sum of the Coulombic inefficiencies over a number of cycles, can therefore
be used as an indicator for the active Li loss related to SEI formation.19 As already highlighted
in Table 1, the cells containing the lacOCA additive exhibit the highest CE and, therefore, the
lowest ACIE in the 1st cycle. Whereas for all electrolytes a steady increase of the ACIE can be
observed, the slope for all additive containing cells is decreased compared to cells comprising
the baseline electrolyte, with lacOCA showing the by far lowest slope. These findings are in
good agreement with the results of the remaining discharge capacities (see Figure 2 and Table
1) after 103 consecutive charge/discharge cycles, showing that the additives are able to decrease
parasitic reactions, particularly the consumption of active Li by the formation of an SEI with
improved protecting properties. As already observed in terms of capacity retention, the
combination of both additives LAC and DEDC is not able to reach the low ACIE observed for
lacOCA-based electrolyte (see Figure S7). A comparison to the prominent additives FEC and
VC is also given, proving the promising results achieved for the lacOCA additive.
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Figure 3: Accumulated Coulombic inefficiencies (ACIEs) of NMC-111 || Si full cells over 103 charge/discharge cycles for
the four different electrolyte solutions.
All electrochemical experiments within this report are performed within a full cell setup,
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meaning that the cell is operated via control of the cell voltage. However due to the
incorporation of a reference electrode (Li|Li+) within the applied three-electrode configuration,
the individual electrode potentials of the P (NMC-111) and N (Si), can be simultaneously
monitored (NOT controlled!) upon cycling. As there is a distinct relation between the
lithiation/de-lithiation degree and the electrode potential for both P and N70, 79, there should be
a correlation between the development of the discharge capacities (i.e. capacity decrease related
to the loss of active Li) and the electrode potentials of the NMC-111 || Si full cells during
cycling. It was revealed that the loss of active Li, for example related to the SEI formation at
N, can have a significant effect on the development of the electrode potentials within cell-
voltage controlled full cells80, and this effect is particularly pronounced when a Si-based N is
used.21 The development of the individual electrode potentials (P: NMC-111 and N: Si) of the
NMC-111 || Si full cells cycled with the baseline or lacOCA-based electrolyte is depicted in
Figure 4, precise values for the electrode potentials at the end of charge/discharge are
summarized in Table 2. Note, that any shift of electrode potentials of P or N at the end of
charge/discharge are strictly interconnected, since the cut-off voltages are fixed and the cell
voltage is defined as the difference of the electrode potentials of P and N.80 In the first cycle,
only small differences between both electrolytes can be observed. At the end of charge, both, P
and N reach an electrode potential of ≈4.4 V (vs. Li|Li+) and ≈0.1 V (vs. Li|Li+), respectively,
whereas at the end of discharge, P and N reach an electrode potential of ≈3.7 V (vs. Li|Li+) and
≈0.7 V (vs. Li|Li+), respectively. Upon continuous charge/discharge cycling of the full cells, the
electrode potentials of P and N at the end of charge/discharge are gradually shifting towards
higher values for both electrolytes, whereas these shifts are clearly increased for the baseline
electrolyte. Related to the steady increase of the electrode potential of the N at the end of charge,
also the P is forced to cycle at higher potentials, hence, to higher Li extraction ratios. Within
the last cycles (>180 h) of the cell with baseline electrolyte, the P reaches values of higher than
4.6 V (vs. Li|Li+) with additional, abnormal spikes, indicating a strong overcharge of the NMC-
111 going along with structural instabilities.81 Furthermore, after the last discharge the N
reaches a potential of ≈1.3 V (vs. Li|Li+), proving the complete de-lithiation of the Si, connected
with a potential of ≈4.3 V (vs. Li|Li+) for P, demonstrating that the NMC-111 is re-lithiated to
only a very low extent after 103 cycles in the baseline electrolyte. The observed strong shifts
of the electrode potentials confirm that the continuous formation of the SEI on the Si gradually
consumes active Li and subsequently result in poor performance of full cells. When lacOCA is
added to the electrolyte, these shifts are strongly delayed to larger cycle numbers as can be seen
for the electrode potentials after the last discharge cycle, namely ≈0.8 V (vs. Li|Li+) for N and
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≈3.8 V (vs. Li|Li+) for P. The addition of only small amounts of 2 wt.% of the additive thus can
drastically reduce the consumption of active Li, most likely due to the formation of an SEI with
improved protection behavior. A similar, but slightly reduced effect is observed for the
comparative additives LAC and DEDC (see Figure S8).
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Figure 4: Development of the individual electrode potentials of the NMC-111 positive electrode and the Si negative electrode
upon 103 charge/discharge cycles of NMC-111 || Si full cells within baseline or lacOCA-based electrolyte in the voltage range
of 3.0 – 4.3 V.
Table 2: Electrode potentials at the end of charge and end of discharge of the Si and NMC-111 electrodes during the 1st and
103rd cycle for NMC-111 || Si full cells cycled in the baseline or lacOCA-based electrolyte.
Electrode potential / V vs. Li|Li+
103rd
charge
4.53
103rd
discharge
4.27
Electrolyte
Baseline
Electrode
1st charge
1st discharge
P (NMC-111)
N (Si)
4.40
0.10
4.37
0.07
3.67
0.67
3.67
0.67
0.23
1.27
P (NMC-111)
N (Si)
4.46
3.78
Baseline + 2%
lacOCA
0.16
0.78
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In addition to the individual electrode potentials, also the development of the difference
between the average charge and discharge voltage (ΔV) of the full cells cycled with the different
electrolytes is investigated, which displays the polarization of the cell and, hence, gives an
indication on the cell impedance.82, 83 As depicted in Figure 5, ΔV is very similar for all
electrolytes at the beginning of cycling, indicating that the different electrolyte additives have
no significant impact on the overall impedance of the cell in the initial cycles. Whereas ΔV
increases over cycling for all different electrolytes, cells with the baseline electrolyte exhibit a
stronger increase of ΔV after ≈60 cycles compared to the additive-based cells, which all show
the same increase over 103 cycles. Hence, all investigated additives are able to reduce the
impedance increase of the NMC-111 || Si full cells during prolonged charge/discharge cycling.
The polarization of the full cells is also investigated for the additives VC and FEC as can be
seen in Figure S9. Whereas the cells cycled with the FEC-based electrolyte display a very
similar increase of ΔV compared to lacOCA, the VC-based cells show a slightly reduced
polarization increase. Hence, the impact of (new) electrolyte additives on the polarization of
Si-based full cells might be an aspect to be studied in more detail in following studies. In general,
this effect is likely to a great extent related to the lower consumption of active Li for the
additive-based cells, which results in lower de-lithiation degrees and, therefore, lower upper
cut-off potentials of the NMC-111 positive electrodes. This again will lead to less parasitic
reactions and structural instabilities at/of the NMC-111 P, respectively, which both are known
to result in an impedance increase of the cell.84 The phenomenon of the steady increase of ΔV
during cycling of full cells with a Si-containing N was recently also reported by Louli et al.,
which was not observed for full cells based on a graphitic negative electrode.85 The values for
ΔV observed within our study are significantly higher compared to their ones, what is most
likely related to completely different experimental conditions, including active materials, cell
setup, cycling conditions etc. A study of Petibon et al. also revealed that ΔV increased more
rapidly for Si-based full cells after the applied electrolyte additive FEC was completely
consumed, which also coincided with a more severe capacity loss of the cells.86 These results
underlines that electrolyte additives can have a severe impact on various parameters of LIB
cells, including capacity retention but also cell polarization.
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Figure 5: Development of the difference between average charge and discharge cell voltage (ΔV) over 103 charge/discharge
cycles of the NMC-111 || Si full cells within the four different electrolytes.
3.2 SEI investigations by means of XPS and ATR-FTIR
Ex situ surface investigations of the cycled Si electrodes are performed to elucidate and
confirm the beneficial impact of the addition of the electrolyte additives on the performance of
the NMC-111 || Si full cells related to the SEI formation on the Si. ATR-FTIR spectra of the Si
electrodes cycled in the four different electrolyte solutions and additional reference spectra of
Li2CO3 and PLA are presented in Figure 6. The Si electrode cycled within the baseline
electrolyte shows strong absorptions at 1652, 1317 and 825 cm-1, which can be related to
lithium alkyl carbonates, particularly lithium ethylene dicarbonate (LEDC)66, 87-89, and
additional absorptions at 1480, 1413 and 860 cm-1 related to Li2CO3, showing that the SEI in
the baseline electrolyte is mainly formed by reductive decomposition of EC.88 When lacOCA
is added to the electrolyte, the cycled Si electrode clearly display characteristic absorptions
related to C=O and C-O vibration bands at 1750 and 1082 cm-1, most likely arising from PLA
as evidenced by the reference spectra. Absorptions at similar, slightly higher wave numbers
have also been correlated to polycarbonates like poly(VC) formed by reductive decomposition
of VC and also FEC.62, 90 Additionally small peaks related to Li2CO3 (1480, 1413 and 860 cm-1)
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can be observed, proving the presence of both PLA and Li2CO3 within SEI derived by lacOCA
as supposed in Scheme 1. However, it should be noted that the observed Li2CO3 (or at least a
fraction of that) may also originate from reductive decomposition of EC, as EC reduction might
not be fully prevented by the lacOCA additive. For Si electrodes cycled within the LAC-based
electrolyte, also the absorptions at 1750 and 1082 cm-1 corresponding to PLA can be observed,
although with reduced intensities. Additional small absorptions related to LEDC (1652, 1317
and 825 cm-1) and also Li2CO3 (1480, 1413 and 860 cm-1) can be observed, indicating that the
LAC additive is not able to completely prevent the reductive decomposition of EC. When
DEDC is added to the electrolyte, the spectra are strongly dominated by absorptions at 1480,
1413 and 860 cm-1 indicating that the SEI is mainly composed of Li2CO3, which was already
reported when CO2 is used as electrolyte additive.52, 56
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Li2CO3, x0.3
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Figure 6: ATR-FTIR spectra of the Si negative electrodes after charge/discharge cycling in NMC-111 || Si full cells in the four
different electrolyte solutions for 103 cycles. Additionally, reference spectra of pure lithium carbonate (Li2CO3) and poly(lactic
acid) are depicted.
The cycled Si electrodes are also examined via XPS. The F 1s, O 1s and C 1s core
spectra of the Si electrodes cycled in the four different electrolytes are provided in Figure 7.
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The electrodes cycled in the baseline electrolyte display a strong peak in the F 1s spectra at
685 eV related to the presence of LiF 42, 91, including a small shoulder at 687 eV, typically
attributed to the presence of LixPOyFz and/or LixPFy 22, 92, which are arising from decomposition
of the conductive salt LiPF6. When lacOCA is added to the electrolyte these two peaks display
a very low intensity, indicating that the addition of the additive significantly reduces the
decomposition of the conductive salt to be incorporated within the SEI. In contrast, the
electrodes cycled in the LAC- and DEDC-based electrolytes exhibit both peaks related to LiF
and LixPOyFz / LixPFy, whereas the intensities of the latter are significantly higher and the
former slightly lower compared to the baseline electrolyte, indicating that the ratio of LixPOyFz
/ LixPF6 species to LiF within the SEI is increased by the addition of LAC or DEDC to the
baseline electrolyte.
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In the O 1s spectra, the Si electrode cycled in the baseline electrolyte displays a peak
centered at 531.4 eV (C=O) and an additional shoulder at 533.6 eV (C-O), which can be
assigned to the presence of lithium alkyl carbonates, like LEDC, and Li2CO3 62, 93, which is also
in good agreement with the results of the ATR-FTIR investigations (Figure 6). For the lacOCA,
both peaks show an increased intensity and particularly the ratio of C-O to C=O bonds is
94
increased. Considering pure PLA, the ratio of C-O to C=O should exactly account for 1:1 ,
however, the slightly increased contribution of C=O for the Si electrode cycled with lacOCA
additive may be attributed to the existence of Li2CO3 besides PLA as already revealed via
ATR-FTIR. Besides that, also some additional species like alkyl carbonates, formed by a
possible EC and/or DEC reduction, might also influence the ratio of C-O to C=O bonds. For
the LAC-based electrolyte, the O 1s spectra displays a similar distribution as for the lacOCA
sample with a decreased intensity of the peak at 533.6 eV (C-O), which might be related to the
presence of significant amounts of LEDC and/or Li2CO3, related to EC/DEC reduction, besides
PLA formed by the reduction of LAC. The Si electrode investigated with the DEDC additive
shows a strong peak centered at 531.4 eV with only a small shoulder at 533.6 eV, indicating
that the SEI formed in this electrolyte is dominated by Li2CO3 accompanied with small amounts
of lithium alkyl carbonates.93
In the C 1s spectrum for the baseline electrolyte, the main contribution is related to
hydrocarbons (C-C/C-H) at a binding energy of 284.5 eV, including small additional peaks at
286.7 eV (C-O) and 289.9 eV (O-(C=O)-O / CO3). These findings are consistent with the
presence of Li2CO3 and high amounts of LEDC 87, 93, as already observed in the O 1s and also
ATR-FTIR spectra. When lacOCA is added to the electrolyte, the spectrum is dominated by
peaks related to C-C/C-H bonds (284.5 eV), C-O bonds (286.7 eV) and C=O bonds/CO2
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environments (289.0 eV). Furthermore, the intensities of the C-O and C=O peaks are very
similar, matching the chemical structure of PLA (see Scheme 1), which was also reported in
literature for PLA samples.94, 95 Additionally, a small contribution at higher binding energies of
289.9 eV might be related to the presence of Li2CO3 besides PLA. The Si electrode cycled in
the LAC-based electrolyte displays a very similar spectra, with a slightly increased contribution
of C-O environments, which could indicate higher amounts of lithium alkyl carbonates present
in the SEI compared to lacOCA. For the DEDC-based sample, peaks at 284.5 (C-C/C-H), 286.7
(C-O) and 289.9 eV (O-(C=O)-O / CO3) are observed, consistent with the presence of high
amounts of carbonates, i.e. Li2CO3 and lithium alkyl carbonates like LEDC.
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Similar results were also reported for the most commonly used additives for Si-based
negative electrodes, VC and FEC. The formation of partially crosslinked polymeric species
(like poly(VC)) within the SEI has been revealed to be the key for the effectiveness of those
additives. 61, 62, 96
F 1s
O 1s
C 1s
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C=O
LiF
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LixPOyFz
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Binding Energy / eV
Binding Energy / eV
Binding Energy / eV
Figure 7: XPS core spectra (F 1s, O 1s and C 1s) of the Si negative electrodes after cycling in NMC-111 || Si full cells in the
four different electrolyte solutions for 103 cycles.
The relative atomic concentrations of the surface of the different electrodes are provided
in Figure 8. Note that these ratios only provide information about the composition of the top
surface of the Si electrodes (information depth of ~10 nm of XPS) and, therefore, about the
elemental composition of the measured SEI but not absolute amounts of the whole width of the
electrode or SEI. Consistent with the observations from the XPS core spectra, the addition of
lacOCA to the baseline electrolyte strongly reduced the amount of fluorine species in the SEI
layer formed at the Si electrode, demonstrating that the decomposition of LiPF6 is diminished,
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what is likely related to a superior protection of the Si electrolyte by lacOCA reducing the
decomposition of LiPF6 at the Si N. At the same time, the relative amounts of carbon and
oxygen are significantly increased, matching the findings of mainly PLA and additionally
Li2CO3 to be incorporated within the SEI formed by lacOCA. Furthermore, a lower amount of
Li is found within the measured top layer of the SEI for the lacOCA-based electrolyte, which
might indicate that the additive reduces the consumption of active Li via improved SEI
formation. However, the relative concentration of Li measured via XPS cannot be directly
related to the capacity loss (=loss of active Li) observed in the long-term cycling experiments
of the NMC-111 || Si full cells, as only ≈10 nm of the SEI are investigated and statements on
the thickness or the amount of the different SEI layers are not possible. As an example, the
relative amounts of Li measured by XPS are very similar for the baseline and the DEDC-based
electrolyte, although the DEDC additive significantly reduced the capacity fading of the full
cells. Most likely, this is related to the formation of less SEI in cells comprising the additive,
related to a better protection behavior of the SEI formed by the additive. Therefore, only the
combination of various electrochemical investigations as described above and analytical
methods (e.g. ATR-FTIR and XPS) allows for revealing the working principle of an electrolyte
additive to improve the performance of LIBs.
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Overall the data from ATR-FTIR and XPS reveal that the addition of the different
electrolyte additives has a significant impact on SEI formation at the Si negative electrode,
hence, could verified to be the reason for improving the electrochemical performance of the Si-
based full cells. Moreover, the simultaneous formation of PLA and Li2CO3 within the SEI for
the lacOCA-based electrolyte could be proven, which underlines the synergistic effect of both
species. Both species are likely formed by lacOCA, however, a contribution of reductive EC
and/or DEC decomposition to the Li2CO3 formation cannot be fully excluded. Further insights
into the SEI formation mechanism at the Si electrodes might be achieved by cryo-SEM/TEM
studies97, which however, is beyond the scope of this work.
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F
O
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Figure 8: Relative atomic concentrations of the surface of the Si electrodes after 103 charge/discharge cycles in NMC-111 || Si
full cells within the four different electrolytes.
To analyze if the developed electrolyte additive lacOCA also influences the cathode
electrolyte interphase (CEI) formation, the NMC-111 electrodes are investigated via XPS. The
corresponding spectra can be seen in Figure 9. The pristine NMC-111 electrode displays a
distinct peak at 688.2 eV in the F 1s spectra, which is related to the C-F bonds of the
polyvinylidene difluoride (PVdF) binder used for the NMC-111 electrodes. After cycling in the
baseline electrolyte, the intensity of this peak is reduced and further displays a shoulder to
slightly lower binding energies, which can be related to a film covering the surface of the
electrode and additional species like LixPFy and/or LixPOyFz found at the surface of the
NMC-111 electrode, respectively. Furthermore, a strong peak related to LiF can be observed
for the baseline electrolyte, whereas only a small shoulder is detected for the lacOCA-based
electrolyte, showing that the addition of the additive significantly reduces the amount of LiF
within the CEI layer. In the O 1s spectra, two distinct peaks at 529.4 eV and ≈532 eV can be
observed for the pristine electrode, which can be attributed to metal oxides of the NMC (M-O)
and residuals like Li2CO3 or surface groups of the conductive carbon, respectively.66, 92 After
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cycling in the baseline electrolyte, the peak attributed to metal oxides is vanished, suggesting
that the active material of the P is covered with a surface film. If lacOCA is added to the
electrolyte, this peak is still visible with a slightly reduced intensity compared to the pristine
electrode. This likely indicates that the formed CEI is thinner for the lacOCA-based electrolyte
in comparison to the baseline electrolyte. The C 1s spectrum of the pristine NMC-111 electrode
displays a main peak at 284.5 eV related to the conductive carbon of the composite electrode,
and two additional peaks at 286.5 eV and 291.1 eV, which both can be attributed to the PVdF
binder.92 The electrode cycled in the baseline electrolyte does not show the peaks related to the
binder, instead a significant shoulder related to C-O environments can be observed, referring to
decomposition products of the electrolyte. For the lacOCA additive, the peaks attributed to
PVdF still can be observed, however, also additional contributions of CO2 and CO3
environments are also present at the surface of the NMC-111 electrode. This points on the
formation of a CEI layer mainly composed of organic species by the addition of lacOCA to the
electrolyte. These observations are also supported by the P 2p spectra, in which a significant
peak related to oxyfluorophosphate compounds at ≈135 eV can be seen for the baseline
electrolyte, whereas only a very small shoulder at ≈136.5 eV related to residues of the
conducting salt is observed for the electrode cycled in the lacOCA-based electrolyte. Overall,
these results suggest that there is less electrolyte decomposition taking place at the NMC-111
P in the full cells containing the lacOCA additive. Hence, the formed CEI layer is likely to be
thinner compared to the one formed in the baseline electrolyte. Furthermore, the latter is
composed of both, inorganic and organic species, whereas the CEI formed in the lacOCA-based
electrolyte mainly contains organic species. Considering the insights gained from Figure 4,
namely the stronger potential shift of the NMC-111 electrode to higher values in the baseline
electrolyte, the difference in the CEI may be also strongly related to the different upper cut-off
potentials reached for the different electrolytes. Several publications have proven that higher
cut-off potentials applied for NMC-based electrodes result in an increased electrolyte
decomposition at the P. This phenomenon was mainly attributed to oxygen release from the
NMC subsequently oxidizing the electrolyte, or the formation of highly reactive Ni4+ at the
surface of the NMC, which both are induced by high de-lithiation degrees (at high cut-off
potentials) of the NMC active material.98-101 Hence, the reduced consumption of active Li
within the NMC-111 || Si full cells by the addition of the lacOCA additive may also indirectly
impact the CEI formation at the P, by lowering the effective cut-off potential of the NMC-111
electrode. To check if the lacOCA also directly impacts the formation and the composition of
the CEI, the NMC-111 electrodes would need to be cycled at a fixed cut-off potential in a half
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cell configuration for both electrolytes, what is, however, beyond the scope of this work.
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Figure 9: XPS core spectra (F 1s, O 1s , C 1s and P 2p) of the NMC-111 positive electrodes, pristine and after cycling in
NMC-111 || Si full cells in the baseline or lacOCA-based electrolyte for 103 cycles.
4. Conclusion
Within this study, the specifically tailored molecule 5-methyl-1,3-dioxolane-2,4-dione
(lacOCA) was successfully synthesized and investigated as electrolyte additive for Si-based
electrodes within NMC-111 || Si full cells. The addition of only 2 wt.% of the additive to the
carbonate-based baseline electrolyte significantly improved the electrochemical performance
of the full cells in terms of capacity retention after 103 consecutive charge/discharge cycles and
also in terms of Coulombic efficiency (CE) throughout the whole cycling. The synthesized
additive lacOCA was compared to the two literature-known additives diethyl dicarbonate
(DEDC) and lactide (LAC), as the lacOCA was designed to combine the beneficial effects on
the SEI formation of both, i.e. the formation of Li2CO3 by DEDC and of poly(lactic acid) (PLA)
by LAC (see Scheme 1 and graphical abstract).
Both additives DEDC and LAC, used for comparison, helped to improve the
performance of the full cells compared to the baseline electrolyte, but were not as effective as
the synthesized molecule lacOCA. Trying to mimic the effect of the lacOCA via combination
of DEDC and LAC within one electrolyte improved the performance of the cell compared to
the single additives, however did not reach the improvements by the addition of lacOCA. These
results suggest that the combination of two functional moieties within one molecule allows for
better synergistic effects than the combination of two different molecules. This finding is
confirmed by a recent literature report.64 By the combination of different electrochemical and
analytical investigations including XPS and ATR-FTIR surface analyses, the beneficial effect
of the additive is suggested to be related to formation of a SEI layer composed mainly of
reduction products of lacOCA (and possibly to some extent also of EC and/or DEC) , namely
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PLA and Li2CO3, which was able to significantly reduce the consumption of active Li upon
charge/discharge cycling of the NMC-111 || Si full cells. The formation of an effective SEI
layer by an electrolyte based on the lacOCA additive in comparison to an EC-based electrolyte
without additive(s) is schematically depicted in Figure 10.
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Figure 10: Schematic illustration of the SEI formation at the Si negative electrode by the EC-based baseline electrolyte (a) and
the lacOCA-based electrolyte (b). The addition of the lacOCA additive to the baseline electrolyte results in the formation of a
more effective SEI at the Si electrode, reducing the consumption of active lihium and electrolyte.
We anticipate that (new) electrolyte additives including two or more different functional
moieties within their structure will boost the performance of LIB cells via synergistic effects of
the different moieties. We have shown, that the beneficial effect of such additives can be well
elucidated in LIB full cells based on Si/carbon negative electrodes. Furthermore, a
comprehensive future study with respect to the optimization of the electrolyte amount and
additive amount in large-scale (e.g. pouch bag) cells is mandatory to verify the practical usage
of this novel additive, e.g. also with respect to gas formation during the formation cycles.102, 103
Additionally, the incorporation of an additional functional moiety within the structure, e.g. a
moiety including fluorine atom104, might be interesting to further investigate the aspect of
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synergistic effects of electrolyte additives.
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Supporting Information
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The Supporting Information is available free of charge on the ACS Publications website at
http://pubs.acs.org.
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NMR spectra, differential scanning calorimetry, additional discharge capacity and Coulombic
efficiency vs. cycle number plots, additional electrode potential vs. time plots, ΔV plots.
Acknowledgements
The authors wish to thank the German Federal Ministry for Economic Affairs and
Energy (BMWi) for funding this work in the project “Go3” (03ETE002D). Dr. Vassilios
Siozios is acknowledged for performing DSC measurements, Johannes Thienenkamp for the
NMR measurements and Andre Bar for the graphical support.
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