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1-19-2018
Reduction Reactions of Electrolyte Salts for Lithium Ion Batteries: Reduction Reactions of Electrolyte Salts for Lithium Ion Batteries:
LiPF6, LiBF4, LiDFOB, LiBOB, and LiTFSI LiPF6, LiBF4, LiDFOB, LiBOB, and LiTFSI
Bharathy S. Parimalam
University of Rhode Island
Brett L. Lucht
University of Rhode Island
, blucht@uri.edu
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Citation/Publisher Attribution Citation/Publisher Attribution
Parimalam, B. S., & Lucht, B. L. (2018). Reduction Reactions of Electrolyte Salts for Lithium Ion Batteries:
LiPF6, LiBF4, LiDFOB, LiBOB, and LiTFSI.
Journal of the Electrochemical Society
,
165
, A251-A255. doi:
10.1149/2.0901802jes
Available at: http://dx.doi.org/10.1149/2.0901802jes
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Journal of The Electrochemical Society, 165 (2) A251-A255 (2018) A251
Reduction Reactions of Electrolyte Salts for Lithium Ion Batteries:
LiPF
6
,LiBF
4
, LiDFOB, LiBOB, and LiTFSI
Bharathy S. Parimalam and Brett L. Lucht
∗ ,z
Department of Chemistry, University of Rhode Island, Kingston, Rhode Island 02881, USA
The reduction products of common lithium salts for lithium ion battery electrolytes, LiPF
6
,LiBF
4
, lithium bisoxalato borate
(LiBOB), lithium difluorooxalato borate (LiDFOB), and lithium trifluorosulfonylimide (LiTFSI), have been investigated. The
solution phase reduction of different lithium salts via reaction with the one electron reducing agent, lithium naphthalenide, results
in near quantitative reactions. Analysis of the solution phase and head space gasses suggests that all of the reduction products are
precipitated as insoluble solids. The solids obtained through reduction were analyzed with solution NMR, IR-ATR and XPS. All
fluorine containing salts generate LiF upon reduction while all oxalate containing salts generate lithium oxalate. In addition, depending
upon the salt other species including, Li
x
PF
y
O
z
,Li
x
BF
y
, oligomeric borates, and lithium bis[N-(trifluoromethylsulfonylimino)]
trifluoromethanesulfonate are observed.
© The Author(s) 2018. Published by ECS. This is an open access article distributed under the terms of the Creative Commons
Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any
medium, provided the original work is properly cited. [DOI: 10.1149/2.0901802jes]
Manuscript submitted December 7, 2017; revised manuscript received January 4, 2018. Published January 19, 2018.
A typical lithium-ion battery contains a graphite anode, a lithiated
transition metal oxide cathode, and an electrolyte solution composed
of inorganic lithium salts dissolved in a mixture of organic carbonate
solvents which frequently includes electrolyte additives.
1
The long-
term cyclability of the lithium-ion battery is dependent upon the anode
solid electrolyte interphase (SEI), formed due to the electrochemical
reduction of the electrolyte solution.
2
Understanding the mechanisms
of the reduction reactions along with the products of the reactions
is essential for the development of better lithium-ion batteries. The
SEI has been proposed to contain lithium alkyl carbonates, lithium
carbonate, lithium oxalate, lithium alkoxides, and lithium oxide from
the carbonate solvents and LiF, lithium fluorophosphates, lithium flu-
oroborates, and lithium oxalate from the reduction of electrolyte salts,
depending upon the salt utilitzed.
3–21
Electrolyte additives have also
been used to tailor the properties of the SEI through preferential re-
duction on anode.
1
Despite significant effort over the last two decades,
the formation mechanism of the SEI is not well understood. One dif-
ficulty in understanding the composition of the SEI is that the SEI
is a complicated mixture of compounds, which results from multiple
simultaneous and competing reduction reactions. In addition, since
the SEI is very thin (∼ 50 nm) and unstable in the presence of oxy-
gen or water, characterization is very difficult. We have reported a
detailed analysis of binder free graphitic anodes cycled in simplified
electrolytes which suggest that the initial reduction reaction of the car-
bonates generate lithium alkyl carbonates and LiF as the predominant
components of the anode SEI.
20,21
Synthesis of initial SEI compo-
nents from carbonate solvents in high yield through reduction of the
solvents with lithium naphthalenide has been reported. The unique
advantage of this reduction technique is the generation and isolation
of SEI constituents from individual electrolyte components in high
yield without competing reduction reactions. Reduction of ethylene
carbonate results in generation of lithium ethylene dicarbonate and
ethylene, while the reduction of dialkyl carbonates result in lithium
alkyl carbonates and alkanes.
22
As an expansion of these investiga-
tions, the reduction of some of the most common electrolyte salts with
lithium naphthalenide has been investigated. All reduction reactions
result in precipitation. The precipitates have been analyzed by solution
Nuclear Magnetic Resonance (NMR) Spectroscopy, solid-state Infra-
Red spectroscopy with Attenuated Total Reflectance (IR-ATR) and
X-ray Photoelectron Spectroscopy (XPS). The results provide insight
into the formation mechanism of the anode SEI.
∗
Electrochemical Society Member.
z
E-mail: blucht@chm.uri.edu
Experimental
Battery-grade lithium hexafluorophosphate (LiPF
6
), lithium
tetrafluoroborate (LiBF
4
), lithium bis(oxalato)borate (Li-
BOB), lithium difluoro(oxalato)borate (LiDFOB), and lithium
bis(trifluoromethylsulfonyl)imide (LiTFSI) (Figure 1) were obtained
from BASF. Diethyl ether (Et
2
O), tetrahydrofuran (THF), and
naphthalene were purchased from Sigma-Aldrich. Lithium discs were
obtained from MTI Corporation. All the reagents were stored in an
argon filled glove box at room temperature and used without further
purification. Lithium naphthalenide solution (Li[NAP]) in THF or
Et
2
O was prepared with 10 mol% excess naphthalene. Ethereal
solvents were used since the solvents do not react with Li[NAP]. The
solvent is not expected to significantly alter the reduction products
of the salts. Lithium foils were added to naphthalene solution of
either THF or Et
2
O and stirred for 3 days at room temperature. The
solution turned green or purple, respectively, in a few minutes after
the addition of lithium metal and became darkly colored after stirring
for 3 days, as previously reported.
23
Number of equivalents of reducing agent required for the complete
reduction of electrolyte salts was evaluated by NMR analysis. Elec-
trolyte salts were dissolved in either THF or Et
2
O and reacted with
different molar equivalents of Li[NAP], a one electron reducing agent,
at room temperature overnight. The resulting reaction mixtures were
transferred into clean dry NMR tubes along with a capillary tube filled
with deuterated DMSO and an internal standard. The internal stan-
dards, LiTFSI; hexafluoro benzene; or LiBF
4
, were chosen carefully
to avoid any overlapping peaks with the starting materials or products.
The samples were analyzed with
19
Fand
11
B NMR spectroscopy and
the concentrations of the unreacted electrolyte salts were estimated in
reference to the internal standard.
Electrolyte salts (LiPF
6
, LiBF
4
,LiBOB,LiDFOB,&LiTFSI)were
dissolved in Et
2
O and reduced with appropriate molar equivalents
of Li[NAP] in larger scale. The evolved gasses and volatiles in the
reaction mixtures were analyzed with GC-MS. The solid residues were
washed with Et
2
O three times, dried overnight at room temperature,
and analyzed with IR-ATR, solution NMR and XPS. All the reactions
were conducted inside a nitrogen filled glovebox. XPS and IR-ATR
analyses were conducted with no exposure to air. NMR, GC-MS were
conducted with minimal exposure to air.
GC-MS analyses were conducted on an Agilent 6890-5973N GC
equipped with an G973N mass selective detector. Liquid s amples were
diluted with dichloromethane, mixed with distilled water to remove
the residual electrolyte salts and non-volatile inorganic components,
and the organic phases were utilized for the analyses. Helium was
used as carrier gas at a flow rate of 24 mL/min. The initial column
temperature was 40
◦
C and the temperature was ramped at 10
◦
C/min
to 200
◦
C and held at that temperature for 2 minutes with the total
) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 131.128.197.119Downloaded on 2018-02-28 to IP
A252 Journal of The Electrochemical Society, 165 (2) A251-A255 (2018)
Figure 1. Structures of the electrolyte salts.
run time of 18 minutes. The mass spectra obtained were compared
to the NIST library to determine their molecular structures. THF,
Et
2
O (solvents) and naphthalene (starting material) and were the only
volatile components present in the reaction mixtures. The gas analyses
were performed by sampling the head spaces of the r eaction mixtures
in RB flasks with a 10 μL GC syringe. Helium was used as the carrier
gas at a flow rate of 1.5 mL/min. The initial column temperature was
set to 40
◦
C, and the temperature was ramped at 1
◦
C/minto43
◦
C
and held at that temperature for 2 min with the total run time of 5
min. The mass spectra obtained were compared to the NIST library
to determine their molecular structures.
IR-ATR spectra of the dried solid residues were acquired on a
Bruker Tensor 27 spectrometer equipped with a germanium crystal
in attenuated total reflectance (IR-ATR) mode. Samples were trans-
ferred using air-tight vials and the spectrometer was operated inside
a nitrogen filled glovebox to avoid air exposure. Each spectrum was
acquired with 128 scans from 700 cm
−1
to 4000 cm
−1
at the spectral
resolution of 4 cm
−1
. The data were processed and analyzed using the
OPUS and Originlab software.
NMR spectra of the samples were collected with a B ruker Avance
III 300 MHz NMR spectrometer at room temperature. The solids
were dissolved in D
2
O in the nitrogen filled glovebox and
19
F,
31
P,
11
B, &
13
C NMR spectra of the solutions were acquired.
19
FNMR
spectra are referenced to LiF at −122.0 ppm, and
11
B NMR spectra are
referenced to residual salts: LiBF
4
, LiBOB, or LiDFOB at −1.5, 7.4,
or 2.9 ppm, respectively. The spectra were processed and analyzed
using MestReNova 10.0.2.
XPS spectra of the dried precipitates were acquired using a Thermo
Scientific K-alpha XPS. Samples were made into circular pellets with
a press or stuck on a conductive carbon tape as a thin layer and
transferred from the glovebox to the XPS chamber using a vacuum
transfer module without exposure to air. An argon flood gun was used
to avoid surface charge accumulation during sample analysis. The
binding energy was corrected based on the C 1s of hydrocarbon at
284.8 eV. The data were processed and analyzed using the Thermo
Avantage, XPS Peak 4.1 and the Originlab software.
Results and Discussion
Reduction of electrolyte salts.—The number of electrons required
for the complete reduction of electrolyte salts was investigated by
NMR analysis. Electrolyte salts dissolved in either THF or Et
2
O
were reduced with different molar equivalents of Li[NAP] at room
temperature overnight. Addition of one molar equivalent of Li[NAP]
to LiBOB, LiDFOB and LiTFSI solutions results in immediate dis-
coloration of Li[NAP] and precipitation of solid products, however
discoloration in LiPF
6
and LiBF
4
samples takes roughly an hour, the
color change is due to the consumption of Li[NAP] in the reduction
of the electrolyte salts. Upon incorporation of higher concentrations
of Li[NAP], > 1 molar equivalent, similar discoloration is observed.
However, for samples where color retention is observed for more than
24 hours, the quantity of Li[NAP] required to completely reduce the
salt has been exceeded, thus allowing determination of the approxi-
mate number of equivalents of reducing agent. The reaction mixtures
were transferred into NMR tubes and a capillary, filled DMSO-d
6
and
an internal standard, was added into each tube. The samples were an-
alyzed with
19
Fand
11
B NMR spectroscopy and the concentration of
the remaining electrolyte salts were determined via integration of the
NMR peaks compared to the internal standard, hexafluoro benzene or
LiBF
4
. Reduction of LiBF
4
with 1, 2, and 3 equivalents of Li[NAP]
results in consumption of approximately 40, 69, and 96 ± 4% of the
LiBF
4
, respectively, suggesting 3 e
−
are required for quantitative re-
duction. Similarly, numbers of equivalents of Li[NAP] required for the
reduction of LiBOB, LiDFOB, and LiTFSI were estimated to be 2 e
−
,
2e
−
, and 12 e
−
, respectively. The number of equivalents of Li[NAP]
required for complete reduction of LiPF
6
could not be measured reli-
ably by NMR spectroscopy. However, in all cases low concentrations
of residual salt are observed after the reduction reactions and some of
the reduction products may precipitate prior to complete reduction,
so the number of electrons required for reduction of the different salts
should be viewed as approximate.
The electrolyte salts were then treated with a sufficient quantity
of Li[NAP] to fully reduce the salt. All reactions result in a signif-
icant quantity of precipitate. The remaining solution was analyzed
by GC-MS and NMR spectroscopy. The only component remaining
in solution is a low concentration of the unreacted salt. In addition,
analysis of the headspace of the samples detected no gaseous products
resulting from the reduction reactions. The results suggest that all of
the reduction products of the lithium salts are insoluble. Thus, the
Li[NAP] reduction of all lithium salts investigated results in quantita-
tive conversion to organic solvent insoluble components.
NMR analysis of the solids.—The residual organic solvent insol-
uble solids have been analyzed via a combination of solution NMR
spectroscopy in D
2
O, Infrared spectroscopy with attenuated total re-
flectance (IR-ATR), and X-ray photo electron spectroscopy (XPS).
The residual solids have been dissolved in D
2
O for NMR analysis.
While most of the residual solids dissolve in D
2
O, some of the solid
does not readily dissolve. In addition, some of the reduction products
may react with water to generate subsequent hydrolysis products. The
dissolved solids were analyzed via a combination of
11
B,
13
C,
19
F, and
31
P NMR spectroscopy. Representative NMR spectra of the solids are
provided in Figure 2.
The
19
F NMR spectrum of the reduction product from LiPF
6
dis-
plays a strong singlet corresponding to LiF at −122 and a medium
singlet at −128.5 ppm corresponding to HF.
24
While LiF is a fre-
quently reported as a product of the reduction of LiPF
6
, HF is most
likely generated from the hydrolysis of unreacted LiPF
6
in D
2
O. In
addition, a doublet is observed at −81.3 ppm in the
19
F NMR spec-
trum which has a corresponding triplet at −15.7 ppm in the
31
PNMR
spectrum and a coupling constant of 962 Hz characteristic of LiPO
2
F
2
.
The presence of LiPO
2
F
2
likely results from the hydrolysis of LiPF
2
upon addition of the residual solid to D
2
O, since no extractable oxygen
is present in the reaction media. The XPS data, as discussed below,
provides further support for this assignment.
The
19
F NMR spectrum of the residual solids from the reduction
of LiBF
4
contains a strong singlet at −122 ppm characteristic of LiF.
In addition, the sample exhibits a weak set of peaks at −149 ppm in
the
19
F NMR spectrum characteristic of residual LiBF
4
. A single peak
is observed in the
11
B NMR spectrum peak at −1.5 ppm characteristic
of residual LiBF
4
.
The
13
C NMR spectrum of the residual solids obtained from reduc-
tion of LiBOB displays a strong singlet at 173.2 ppm characteristic of
in lithium oxalate. The other peaks observed in the
13
C NMR spectra
are characteristic of residual solvents, THF and Et
2
O, used for the re-
duction reaction. The peaks at 67.8 and 25.0 ppm are characteristic of
residual THF while the peaks at 66.0 and 14.1 ppm are characteristic
of residual Et
2
O. There are no peaks observed for residual LiBOB
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Journal of The Electrochemical Society, 165 (2) A251-A255 (2018) A253
19
F NMR
LiBF
4
LiDFOB
LiPF
6
LiTFSI
13
C NMR
LiBOB
LiDFOB
11
B NMR
LiBF
4
LiDFOB
LiBOB
31
P NMR
LiPF
6
Figure 2. Solution NMR spectra of the solids from the Li[NAP] reduction of electrolyte salts.
in either the
11
Bor
13
C NMR spectra consistent with quantitative
reduction of LiBOB under the reaction conditions.
The NMR spectrum of the residual solid from the reduction of
LiDFOB is similar to a combination of the reduction products of LiBF
4
and LiBOB. The
19
F NMR spectrum is dominated by LiF at −122
ppm, but also contains small sets of peaks at −147 and −149 ppm
characteristic of residual LiDFOB and LiBF
4
, respectively. The corre-
sponding peaks characteristic of LiDFOB and LiBF
4
are observed in
the
11
B NMR spectra at 2.9 ppm and −1.5 ppm, respectively. The
13
C
NMR s pectrum contains a strong peak at 173.2 ppm characteristic of
lithium oxalate, along with peaks characteristic of residual THF and
Et
2
O. However, unlike LiBOB some residual LiDFOB is observed at
161.1 ppm.
The
19
F NMR spectrum of the solids from the reduction of LiTFSI
shows a strong singlet corresponding to LiF. In addition, two strong
peaks at −75.6 ppm and −72.7 ppm with peak areas in 2:1 r atio.
The peak integrations have a 2:1 ratio which is independent of the
quantity of Li[NAP] added suggesting that they arise from a single
molecular species. The spectral data is consistent with the generation
of lithium bis[N-(trifluoromethylsulfonylimino)] trifluoromethanesul-
fonate (LiOS(CF
3
)(NSO
2
CF
3
)
2
as previously reported.
25
No residual
LiTFSI is observed at −79.4 ppm in the
19
F NMR spectrum.
FTIR analysis of the solids.—In an effort to further understand
the composition of the solids obtained from reduction, the reduction
products of the salts have been analyzed with IR-ATR. The IR-ATR
spectra of the solids generated from the reduction of LiBOB and LiD-
FOB are provided in Figure 3. IR-ATR spectra of the residual solids
for the other salts were also acquired, but the spectra were dominated
by residual solvent and naphthalene since the decomposition prod-
ucts do not contain any functional groups which strongly absorb IR
radiation, consistent with the observation of LiF as the predominant
component by NMR.
The reduction product of LiBOB exhibits strong absorptions
around 1670, 1330 and 780 cm
−1
characteristic of lithium oxalate.
The peaks at 1805 and 1770 cm
−1
are characteristic of -CO
2
-B-CO
2
-
oscillations and the peak at 1250 cm
−1
corresponds to combination
80012001600
LiDFOB
LiBOB
Wavenumber
(
cm
-1
)
x2
Figure 3. FTIR spectra of the solids from the Li[NAP] reduction of LiBOB
and LiDFOB.
of O-C-C asymmetric stretching and O-B-O bending, suggesting the
presence of a combination of LiBOB and crosslinked oligomeric bo-
rates, as previously reported.
26,27
A weak broad absorption is also
observed between 1400 and 1500 cm
−1
, consistent with the presence
of Li
2
CO
3
. In addition to the reduction products, absorptions corre-
sponding to residual THF at 1070 and 910 cm
−1
are also observed. The
reduction product of LiDFOB displays IR absorptions very similar to
the solids from LiBOB consistent with the presence of lithium oxalate,
crosslinked oligomeric borates, and Li
2
CO
3
, except the intensity of
the broad absorption characteristic of Li
2
CO
3
is increased.
X-ray photoelectron spectroscopy of the solids.—The solids gen-
erated from the reduction of LiPF
6
,andLiBF
4
were analyzed with
XPS and the spectra are displayed in Figure 4. XPS analysis of the
other residual solids was attempted, but the insoluble reduction prod-
ucts contain residual solvent and naphthalene which cannot be re-
moved which resulted in contamination of the XPS analysis chamber
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A254 Journal of The Electrochemical Society, 165 (2) A251-A255 (2018)
680685690695
Li
x
BF
y
Li
x
PF
y
x5
LiBF
4
LiPF
6
B.E.
(
eV
)
F1s
LiF
5254565860
B.E. (eV)
Li1s
LiPF
6
LiBF
4
128132136140
LiPF
6
Li
x
PF
y
Li
x
PO
y
F
z
B.E. (eV)
P2p
188192196200
Li
x
BF
y
LiBF
4
LiBF
4
B.E.(eV)
B1s
Figure 4. XPS spectra of the solids from the Li[NAP] reduction of LiPF
6
,LiBF
4
, and LiTFSI.
for several months. Thus, we were unable to obtain XPS spectra of
the other reduction products.
The F1s spectrum of the residual solid from the reduction of LiPF
6
is dominated by a peak at 685 eV characteristic of LiF. The shoulder
at 688.3 eV is characteristic of P-F species in Li
x
PF
y
and Li
x
PF
y
O
z
.
The P2p spectrum contains a strong peak at 130.0 eV corresponding
to Li
x
PF
y
species and the small peak at 136.0 eV characteristic of
Li
x
PF
y
O
z
. The low concentration of Li
x
PF
y
O
z
most likely results from
reaction of Li
x
PF
y
with trace oxygen or moisture. The Li1s spectrum
exhibits a broad peak around 56.3 eV corresponding to combination
of LiF, Li
x
PF
y
,andLi
x
PF
y
O
z
.NoresidualLiPF
6
(F1s, 687.6 eV; P2p,
137.8 eV) is observed.
The F1s spectrum of the residual solids from the reduction of
LiBF
4
is dominated by a peak at 685 eV characteristic of LiF. A
shoulder is observed at 687.5 characteristic of B-F species in Li
x
BF
y
and residual LiBF
4
. The B1s spectrum is dominated by a peak at 190.5
eV corresponding to Li
x
BF
y
species with a small peak at 195.7 eV is
characteristic of residual LiBF
4
. The Li1s spectrum exhibits a broad
peak around 56.3 eV corresponding to combination of LiF, residual
LiBF
4
and Li
x
BF
y
.
Discussion
The reduction products of some of the most common electrolyte
salts have been investigated via a combination of NMR, GC-MS, IR-
ATR, and XPS. Upon reduction, all fluorine containing salts generate
LiF and all oxalate containing salts generate lithium oxalate, in ad-
dition to other components which are dependent upon the structure
of the salt. While the proposed equations provide estimates for the
stoichiometries of the reactions as obtained from experimental re-
sults, due to the insolubility and hydrolytic instability of many of the
reduction products quantitative analysis is difficult.
Reduction of LiPF
6
yields LiF and Li
x
PF
y
(Eq. 1). LiF is the pre-
dominant species observed by
19
F NMR spectroscopy and F1s XPS.
The primary phosphorous containing species observed in the P2p XPS
spectrum is Li
x
PF
y
with additional low concentrations of Li
x
PF
y
O
z
.
The presence of Li
x
PF
y
O
z
likely results from the reaction of Li
x
PF
y
with trace water or oxygen (Eq. 2). Upon preparation of the samples
for NMR analysis via dissolution in D
2
OtheLi
x
PF
y
is converted
to Li
x
PO
y
F
z
via hydrolysis or oxidation. The reduction products of
LiBF
4
are very s imilar to the reduction products of LiPF
6
. The pri-
mary products observed by XPS are LiF and Li
x
BF
y
. Analysis of the
LiBF
4
reduction product by solution NMR spectroscopy r eveals only
LiF suggesting that Li
x
BF
y
and the Li
x
BF
y
hydrolysis or oxidation
products are not soluble in D
2
O. The observations are consistent with
previous reports on the reduction products of LiPF
6
and LiBF
4
.
28,29
LiPF
6
4e
-
4Li
+
4LiF+Li
x
PF
y
[1]
Li
x
PF
y
[O]
Li
x
PO
y
F
z
[2]
LiBF
4
3e
-
3Li
+
3LiF+Li
x
BF
y
[3]
The primary reduction product of LiBOB is lithium oxalate al-
though low concentrations of Li
2
CO
3
are also observed by IR spec-
troscopy (Eq. 4). In addition, crosslinked oligomeric borates are
observed consistent with previous reports.
30
Reduction of LiDFOB
results in the observation of very similar products, lithium ox-
alate, Li
2
CO
3
, and crosslinked oligomeric borates, along with LiF
(Eq. 5). While CO
2
is not observed by GC-MS analysis for either
LiBOB or LiDFOB, the presence of Li
2
CO
3
in the solid residue
likely results from CO
2
reduction by the excess Li[Nap], as previously
reported.
31
B
O
OO
O
O
O
O
O
Li
xe
-
OLi
LiO
O
O
crosslinked
oligomeric borates
+Li
2
CO
3
+
xLi
+
[4]
B
O
O
F
F
O
O
Li
xe
-
OLi
OLi
O
O
xLi
+
+LiF+Li
2
CO
3
+
Crosslinked
oligomeric borates
[5]
Reduction of LiTFSI results in the generation of two water soluble
components, LiF and lithium bis[N-(trifluoromethylsulfonylimino)]
trifluoromethanesulfonate as observed by NMR spectroscopy.
Lithium bis[N-(trifluoromethylsulfonylimino)] trifluoromethanesul-
fonate is likely be formed through reductive cleavage of N-S bond,
followed by insertion of the lithium imide into the S = O bond
(Eqs. 6–7). This mechanism is consistent with a combination of previ-
ously reported computational and experimental results.
14,15,25
Reduc-
tion of LiTFSI requires a large excess of Li[Nap] (∼12) per equivalent
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![Figure 4. XPS spectra of the solids from the Li[NAP] reduction of LiPF6, LiBF4, and LiTFSI.](/figures/figure-4-xps-spectra-of-the-solids-from-the-li-nap-reduction-pb8k27kc.webp)
![Figure 2. Solution NMR spectra of the solids from the Li[NAP] reduction of electrolyte salts.](/figures/figure-2-solution-nmr-spectra-of-the-solids-from-the-li-nap-3tpi6t3c.webp)