Lifetime limit of tris(trimethylsilyl) phosphite as electrolyte additive for high voltage lithium ion batteries

Article abstract of DOI:10.1039/c6ra06555d

The tris(trimethylsilyl) phosphite (TMSPi) is considered as an ideal electrolyte additive for lithium ion batteries. In this work, its positive effect as well as its failure mechanism in a LiPF₆ containing electrolyte was studied by means of selected electrochemical, structural and analytical techniques. The LiNi_0.5Co_0.2Mn_0.3O₂/graphite cells with TMSPi as electrolyte additive were cycled between 2.8 and 4.6 V. Thanks to the compact cathode electrolyte interphase formed by the oxidative decomposition of TMSPi in a freshly prepared TMSPi containing electrolyte, both the discharge capacity and the cycling stability of cells were enhanced. However, our results also show that TMSPi actually reacts with LiPF₆ at room temperature. TMSPi is consumed by this spontaneous reaction after aging for certain time. In addition, a part of the fluorophosphates, generated from the hydrolysis of LiPF₆, is bonded to one or two TMS groups, causing a decrease in the fluorophosphate content in the CEI film. Consequently, the cycling stability of the lithium ion cells with aged TMSPi containing electrolyte deteriorates. The obtained results offer important insights into the practical application of TMSPi, which means that TMSPi can only be used as an effective additive in a freshly prepared LiPF₆ containing electrolyte.

Full text of DOI:10.1039/c6ra06555d

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Lifetime limit of tris(trimethylsilyl) phosphite as electrolyte
additive for high voltage lithium ion batteries
Received 00th January 20xx,
Accepted 00th January 20xx
a
b
a
a
a
a
Xin Qi, Liang Tao, Hendrik Hahn, Carola Schultz, Dennis Roman Gallus, Xia Cao, Sascha
Nowak, a Stephan Röser, Jie Li, Isidora Cekic-Laskovic, *a Babak Rezaei Rad *a,c and Martin
a
a
Winter *a,c
DOI: 10.1039/x0xx00000x
www.rsc.org/
The tris(trimethylsilyl) phosphite (TMSPi) is considered as an ideal electrolyte additive for
lithium ion batteries. In this work, its positive effect as well as its failure mechanism in a
LiPF6 containing electrolyte was studied by means of selected electrochemical, structural
and analytical techniques. The LiNi0.5Co0.2Mn0.3O2/graphite cells with TMSPi as electrolyte
additive were cycled between 2.8 and 4.6 V. Thanks to the compact cathode electrolyte
interphase formed by the oxidative decomposition of TMSPi in a freshly prepared TMSPi
containing electrolyte, both discharge capacity and the cycling stability of cells were
enhanced. However, our results also show that TMSPi actually reacts with LiPF6 at room
temperature. TMSPi is consumed by this spontaneous reaction after aging for certain
time. In addition, a part of the fluorophosphates, generated from the hydrolysis of LiPF6,
is bonded to one or two TMS groups, causing a decrease in the fluorophosphate content
in the CEI film. Consequently, the cycling stability of the lithium ion cells with aged TMSPi
containing electrolyte deteriorates. The obtained results offer important insights into the
practical application of TMSPi, which means that TMSPi can only be used as an effective
additive in a freshly prepared LiPF6 containing electrolyte.
electrolyte additives to form
a cathode-electrolyte interphase
8
(CEI),5, stabilize the electrolyte salt9, scavenge the corrosive
impurities (HF, PF5),10 or to remove the O2 gas evolved from the
Introduction
cathode material11
.
To facilitate the robust development of hybrid electric vehicles
(HEVs) and electric vehicles (EVs), lithium ion batteries (LIBs), as one
of the most promising energy supplies, are expected to provide
“high voltage” (> 4.5 - 4.6 V) to enhance their energy density. A
variety of cathode materials with high working potential has been
reported so far. However, they are rarely used in commercial cells,
mainly due to the poor electrochemical oxidation stability of the
traditional LiPF6 containing carbonate based electrolytes at these
Similar to the SEI formed on the graphite anode, the CEI is
considered as an essential protective layer on the cathode surface
that helps to prevent further oxidative decomposition of the
electrolyte12. However, a thick CEI film may deteriorate the cell
performance by increasing the interfacial impedance, in particular
when it contains more highly resistive components13, e.g. LiF. CEI
formation additives are expected to produce a robust and thin CEI
layer to enhance both the cycling stability and rate capability14
.
2
high voltages.1, The oxidative decomposition of the carbonate
solvents has been confirmed to produce H2O and CO2.3 The
generated H2O further promotes the hydrolysis reactions of LiPF6 to
generate HF,4 which induces serious damage of metal oxide
Tris(trimethylsilyl) phosphite (TMSPi, shown in Scheme 1), has
been introduced as an effective bifunctional additive to build a CEI
upon its oxidative decomposition as well as to scavenge the HF
16
generated during cycling15. The cycling stability of LiNi0.5Mn1.5O4
6
cathodes5, and the solid-electrolyte interphase (SEI) on the
15,
17
graphite anodes7. An economic way to prevent the oxidation of the
and LiNi1/3Co1/3Mn1/3O2
cathodes at high potential
(≥ 4.5 V vs. Li/Li+) was significantly improved by 0.5 wt. % TMSPi in
the electrolyte. Yim et al.18 calculated the oxidation potential (OP)
and the reduction potential (RP) based on the Kohn-Sham density
functional theory (DFT): TMSPi has an OP as 4.29 V vs. Li/Li+, lower
than the OP of ethylene carbonate (EC, 6.92 V vs. Li/Li+). On the
other hand, the RP of TMSPi is much lower than EC18. Consequently,
the TMSPi would easily undergo an oxidative decomposition on the
cathode but less probable reductive decomposition on the anode. A
traditional electrolyte formulations at high voltage is to use specific
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CEI film can thus be formed to prevent transition metal ion LiNi0.5Co0.2Mn0.3O2 cathode was first determined in the
dissolution from the cathode and further oxidative decomposition potential range from 2.8 V to 4.6 V vs. Li/Li+ with Li metal as
of the electrolyte15. Simultaneously, the trimethylsilyl (TMS) group counter electrode and a practical capacity of 197.5 mAh g-1
shows high binding affinity towards HF owing to the formation of a was obtained. The same procedure was applied to determine
strong Si-F bond. For this reason, TMSPi is also considered as a HF the practical capacity of the graphite anode between 0.01 and
scavenger.
1.5 V vs. Li/Li+, and a specific capacity of 334.5 mAh g-1 at 1 C
Besides Me3SiF (TMSF), two more decomposition compounds: was obtained. The mass loading of the anode was controlled to
Me3SiOH (TMSOH) and Me3SiOSiMe3 (TMSOTMS) have also been have 25 % extra capacity as the cathode. According to the
observed in the 1H-NMR spectra of the TMSPi-containing mass loading of LiNi0.5Co0.2Mn0.3O2, the total amount of
electrolyte18, indicating that the highly electrophilic TMS group acts graphite on the anode side was ca. 7.3 mg. In the constant
not selectively as a F- receptor, but rather as an receptor for all current charge/discharge cycling analysis, the specific current
anions. Hence, it can be suspected that TMSPi may also directly of 197.5 mA g-1 was defined as 1 C. 200 µL electrolyte was
react with the LiPF6 salt or with the hydrolysis reaction products of added into each cell. For each electrolyte formulation, at least
LiPF6. After a certain aging period, the HF scavenging effect of the three cells were assembled to ensure the reproducibility,
electrolyte additive may vanish as all TMSPi molecules are which required a capacity deviation below 0.5 mAh g-1
continuously consumed by this chemical reaction. In this work, we between the parallel cells.
studied the spontaneous reaction between TMSPi and LiPF6 and we
Electrochemical analysis
could identify the products by various analysis methods, including
gas chromatography - mass spectrometry (GC-MS) and nuclear
magnetic resonance (NMR) spectroscopy. We also investigated the
electrochemical performance of the TMSPi containing electrolyte in
a LiNi0.5Co0.2Mn0.3O2/graphite full cell at high cut-off voltage (4.6 V).
The oxidation stability analysis revealed that the aged TMSPi
containing electrolyte failed to form a protective CEI on the
cathode. A detailed interaction between TMSPi and LiPF6 is shown
in this work.
The cycling stability of the cells was analyzed in a three-
electrode Swagelok® T-type cell. The cells were cycled on a
Maccor® series 4300 battery cycler at 20 °C. After three initial
formation cycles at 0.1 C, the cells were charged/discharged at
0.5 C in the following cycles. The oxidation stability of the
electrolytes was determined by linear sweep voltammetry
(LSV) performed by a VMP potentiostat (Biologic
® Science
Instruments). A platinum disc (Ø 1 mm) was used as working
electrode, whereas the counter and reference electrodes were
both lithium metal discs (Ø 12 mm and Ø 6 mm, respectively).
The scan range was set as 3.0 V - 6.0 V vs. Li/Li+ with a scan
rate of 0.1 mV s-1.
Experimental
Electrolyte and electrode preparation
A 3:7 (weight ratio) mixture of ethylene carbonate (EC) and
diethyl carbonate (DEC) with 1 M LiPF6 was used as baseline
electrolyte (LP47, UBE, H2O content ≤ 10 ppm). 0.5 wt. %
TMSPi was added into the baseline electrolyte in an argon
filled glovebox (Braun, H2O and O2 content ≤ 0.1 ppm). The
“fresh TMSPi containing electrolytes” were prepared shortly
prior to each measurement. The “aged TMSPi containing
electrolytes” were stored in glovebox at 20 °C for different
time periods ranging from 12 hours to 2 months.
Qualitative analysis
Qualitative GC-MS experiments were carried out on a GC-MS-
QP2010 Ultra System from Shimadzu® equipped with the AOC-5000
Plus autosampler from CTC Analytics. Separation was realized by
Rxi®5ms, a nonpolar fused silica column (30 m × 250 µm × 250 µm)
from Restek®. Measurements were performed with helium (purity
6.0) from Westfalen AG as carrier gas. The headspace gas generated
from the reaction between LiPF6 and TMSPi was equilibrated in an
agitator at 120 °C for 120 s at 250 rpm. 1 mL gas was injected by a
syringe at 120 °C and 500 µL s-1. After injection at 250 °C, the
following column oven program was executed and the sample
measured in total for 20 min. The temperature was kept at 35 °C for
2 min. and then increased to 60 °C with a rate of 3 °C min.-1.
Afterwards, the temperature increased to 250 °C with a rate of 31
°C min.-1. The mass spectrometer was operated in the electron
ionization (EI) mode with an m/z scan range of 15-300. The ion
source temperature was set to 250 °C, the filament voltage to 70 V
and the detector voltage relative to the particular tuning result. As
software for compound identification and system control, GC-MS
Real Time Analysis and GC-MS Postrun Analysis, which included the
National Institute of Standards and Technology (NIST) 08 library,
were used.
LiNi0.5Co0.2Mn0.3O2 and graphite were used as cathode and
anode active materials, respectively. Li metal (Rockwood
Lithium®, Lithium battery grade) was used as reference
electrode (Ø 6 mm). The cathode was composed of 97 wt. %
LiNi0.5Co0.2Mn0.3O2 (ATL), 2 wt. % carbon black (Super P Li®,
Amerys) and 1 wt. % polyvinylidene difluoride (PVdF, Solef
5130, Solvay) as binder. The anode was constituted of
95.4 wt. % graphite (ATL), 1.5 wt. % Super P Li® and 3.1 wt. %
binder. This binder was a mixture of styrene butadiene rubber
(SBR, Lipaton SB 5521, Synthomer) and sodium carboxymethyl
cellulose (CMC, Walocel CRT 2000 PPA 12, Dow Wolff
Cellulosics®) with a weight ratio of 25:6. Both the cathode and
anode electrodes were punched into round discs (Ø 12 mm).
The mass loading of the cathode was approximately
9.0 mg cm-2, which meant the total amount of
LiNi0.5Co0.2Mn0.3O2 was ca. 9.9 mg. To balance the
cathode/anode capacity ratio, the practical capacity of the
The water content in the electrolyte was determined by the
Karl-Fischer titration method. The measurement was performed by
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1
a combination of 801 stirrer and 851 titrado, acquired from
Metrohm AG (Herisau, Switzerland).
The H-NMR spectra of two aged TMSPi containing electrolytes,
which had been stored in glovebox for one week and for 45 days
Surface analysis of the LiNi0.5Co0.2Mn0.3O2 cathode was (20 °C), respectively, were measured. Figure 2a shows the spectrum
performed by X-ray photoelectron spectroscopy (XPS, Axis Ultra of pure TMSPi, where CD3CN was used as solvent. Figures 2b and 2c
HAS, Kratos, GB) with a monochromatic Al Kα source (hv = 1486.6 illustrate the 1H-NMR spectra of LP47 electrolyte with 0.5 wt. %
eV) at a 10 mA filament current and a 12 kV filament voltage source TMSPi after one week and 45 days, respectively. In order to avoid
energy. Five positions were measured on each sample and each any side reaction between CD3CN and electrolyte components, an
position had an area of 700 µm × 300 µm. The binding energy (BE) experimental set-up, which is shown in Figure 1, was used. The
of the C 1s peak (C-C, BE = 284.6 eV) was used as an internal TMSPi containing electrolyte LP47 was placed in the FEP tube and
reference to calibrate the spectra. All the cells were stopped after CD3CN was filled in the glass tube. The 1H-NMR spectrum of the
150 cycles in the discharged state. The cathodes were removed out TMSPi containing electrolyte after one week of storage shows both
of the cells in a glovebox and transferred into the XPS chamber signals of TMSPi and TMSF (Figure 2b). The slight shift of the TMSPi
under argon protection.
signal from 0.2 ppm (Figure 2a) to 0.03 ppm (Figures 2b and 2c) was
To analyze the reaction products of TMSPi and LiPF6, NMR caused by using different solvents. As already well known, the
1
spectra were recorded on a Bruker Avance III 400 (400.03 MHz, H; multiplet signal in 19F-NMR spectrum at -157 ppm with 1JFH coupling
376.37 MHz, 19F; 161.93 MHz, 31P) spectrometer. All measurements constant of 7 Hz clearly demonstrates a considerable amount of
were performed at 300 K. Chemical shifts (δ
) were reported in parts dissolved TMSF in the solution19. The aged TMSPi containing
per million (ppm), and coupling constants (J values) were given in electrolyte, which was stored in a glovebox at 20 °C for 45 days,
hertz (Hz). All samples were prepared in an argon filled glovebox showed no presence of TMSPi in the electrolyte and only the TMSF
with pre-dried solvents (H2O content less than 10 ppm). Fluorinated could be observed in 1H-NMR spectrum (Figure 2c).
ethylene propylene (FEP) NMR tubes (Ø 3 mm, Wilmad-labglass)
were used to avoid any side reaction between the residual amount
of HF and glass. The FEP tubes containing 200 to 300 µL of samples
were closed by a polytetrafluorethylene (PTFE) plug and placed in a
glass NMR tube (Ø 5 mm) containing 50 µL of acetonitrile-d3
(CD3CN, 99.8 at. % d3, Sigma Aldrich). In this way, any contact
between the deuterated solvent and reaction mixtures, which could
produce unwanted products, was prohibited. A PTFE NMR cap was
used to close the glass NMR tubes which were sealed by Parafilm®
afterwards (Figure 1).
Figure 1: Schematic plot of the NMR sample set-up to identify the
reaction products of LiPF6 and TMSPi. The LiPF6 and TMSPi were
sealed in the FEP tube; and the CD3CN was filled in the glass tube as
a standard reference.
Results and discussion
As reported by Yim et al.18, the Gibbs free energy of the reaction
between TMSPi and LiF was calculated as -34.1 kcal mol-1. It can
Figure 2: The 1H-NMR spectra of pure TMSPi in CD3CN (a), and
0.5 wt. % TMSPi containing LP47 electrolyte stored in a glovebox for
one week (b) and for 45 days (c).
thus be assumed that TMSPi would influence the equilibrium of the
The 19F-NMR spectrum of the TMSPi containing electrolyte aged
for 45 days (Figure 3a) shows different sets of signals
between -80 ppm to -86 ppm which are related to different
fluorophosphates20. A doublet at -83.0 ppm in 19F-NMR spectrum
reaction: LiPF6
⇌ LiF + PF5 by removing LiF and continuously
promote the decomposition of LiPF6. As a result, TMSPi would be
consumed by this reaction after a certain aging time and would
have no longer beneficial effect on the cell performance.
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(1 and 1’) which correlates with a triplet in 31P-NMR spectrum
at -20.1 ppm (1JPF=929 Hz, Figure 3b) proves the formation of
OPF2(OH). In addition, the 19F-NMR spectrum (Figure 3a) shows two
mass (g)
1.21579
1.13990
(ppm)
44.00
50.90
47.45
mass (g)
1.32239
1.35221
(ppm)
38.80
32.20
35.50
1
2
1
Average
sets of doublets at -84.04 ppm and -84.06 ppm with JPF coupling
constants of 967 (2 and 2`) and 1005 (3 and 3`) Hz respectively
which are assigned to OPF(OMe)2 and OPF(OEt)2. However, due to
the low signal intensity of the 31P-NMR spectrum (Figure 3b), the
related signals could not be determined. In Figure 3a, the singlet
at -78.3 ppm cannot be assigned to any product.
As discussed in the work from Mai et al.15, the improvement in
the cycling stability by using TMSPi was also attributed to a
passivating CEI formed by oxidative electrolyte decomposition. The
onset potential was at ca. 4.1 V vs. Li/Li+ at a platinum electrode.15
In this work, as shown in Figure 2, no TMSPi could be observed after
45 days storage. It is of great importance to find out whether a
stable CEI could still be formed after all the TMSPi was consumed by
the reaction with HF and H2O. Therefore, the oxidation stability of
the baseline LP47 electrolyte, the fresh TMSPi containing LP47
electrolyte and the aged TMSPi containing LP47 electrolyte was
determined with a platinum working electrode against a lithium
counter electrode (shown in Figure 4). To get close to the
equilibrium state of the reaction, a relatively low scan rate of
0.1 mV s-1 (50 times lower than in the work of Mai et al.) was used.
Figure 3: (a) 19F-NMR spectrum of the 0.5 wt. % TMSPi containing
LP47 electrolyte stored in glovebox for 45 days: 1 and 1` OPF2(OH);
2 and 2` (OPF(OMe)2); 3 and 3` (OPF(OEt)2) (b) 31P-NMR spectrum of
the same sample.
Figure 4: Oxidation stability determination of the baseline LP47
electrolyte (black line), the fresh LP47 electrolyte containing TMSPi
The decrease of HF in the TMSPi containing electrolyte has (red line) and the aged LP47 electrolyte containing TMSPi (blue
already been confirmed by 19F-NMR16, 21. The TMSPi was thus line).
considered as an effective HF scavenger. However, the high
electron affinity of the TMS group may also attract other anions
As shown in Figure 4, the oxidative decomposition of all three
beside F-. In this work, a water contaminated LP47 electrolyte was electrolytes started at approximately the same onset potential.
used to evaluate the water scavenging ability of TMSPi. The water Similar to the results obtained by Mai et al., the fresh TMSPi
content in the contaminated LP47 electrolyte was 50 ppm. 0.5 containing electrolyte clearly formed a passivating layer on the
wt. % TMSPi was added and the electrolyte was stored at 20 °C for platinum electrode as the anodic current was much lower than the
12 hours. Afterwards, the water content of both electrolytes with baseline electrolyte above 5.3 V vs. Li/Li+. However, the aged
and without TMSPi was measured by Karl-Fischer titration and electrolyte containing TMSPi did not show this passivation behavior
compared. As summarized in Table 1, the TMSPi-containing and the anodic current had a similar value as in the case of the
electrolyte shows a decreased amount of water, indicating that baseline electrolyte. Obviously, the reaction products of TMSPi with
TMSPi acted as a decontaminating agent to purify the electrolyte by HF or with other compounds, such as water, were not able to form
removing the HF and H2O.
a stable CEI. It is necessary to investigate the electrochemical
performance of the aged TMSPi containing electrolyte in a full
Table 1: Water content in the contaminated LP47 electrolyte with lithium ion cells to know whether the electrolyte additive still has a
and without the addition of 0.5 wt. % TMSPi. The electrolytes were positive effect on the cycling stability.
stored in a glovebox at 20 °C for 12 hours.
LP47 (H2O
The cycling stability of LiNi0.5Co0.2Mn0.3O2/graphite full cells was
investigated in the three electrolytes: the baseline LP47 electrolyte,
the fresh TMSPi containing LP47 electrolyte and the aged TMSPi
containing LP47 electrolyte (Figure 5). The cut-off voltage range was
LP47 (H2O
contaminated) +
0.5 wt. % TMSPi
contaminated)
Measurement
Sample
H2O
Sample
H2O
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set between 2.8 V and 4.6 V. To prove the reproducibility of the cell binding energy (ca. 136.8 eV) corresponded to LiPFx, whereas the
performance, two cells with the aged TMSPi containing electrolyte broad peak at the lower binding energy (ca. 135 eV) originated from
are shown in Figure 5.
LiPFxOy.5, 23, 24
Figure 5: Cycling stability of LiNi0.5Co0.2Mn0.3O2/graphite full cells in
the following electrolytes: baseline LP47 electrolyte (black square);
freshly prepared TMSPi containing electrolyte (red dot); aged TMSPi
containing electrolyte (green triangle and blue inverted triangle).
Cut-off voltage ranging from 2.8 V to 4.6 V.
Figure 6: P 2p spectra of the LiNi0.5Co0.2Mn0.3O2 cathode (after 150
cycles, discharged) with (a) aged TMSPi-containing electrolyte, (b)
fresh TMSPi containing electrolyte, (c) baseline LP47 electrolyte and
(d) a pristine electrode without exposure to the electrolyte.
The reasons behind the fast capacity fading of the
LiNi0.5Co0.2Mn0.3O2/graphite full cells charged up to 4.6 V had been
previously studied. On one hand, the irreversible transformation of
the LiNi0.5Co0.2Mn0.3O2 from the pristine rhombohedral phase to a
mixture of spinel and rock salt phase22 significantly reduced the
conductivity of the material. On the other hand, the carbonate
based electrolytes could not resist the high cathode potential1 and
experienced a continuous oxidative decomposition. In this work, all
investigated cells were charged/discharged within the same cut-off
voltage window, so that the deterioration in the cycling stability
caused by the phase transformation should be similar in each case.
The difference in the cycling performance was thus determined by
the electrolyte decomposition. During the first 70 cycles, the cells
with fresh and aged TMSPi containing electrolytes showed similar
specific discharge capacity (ca. 156 mAh g-1) and capacity fading
rate (Figure 5). The cell with the baseline LP47 electrolyte showed a
specific discharge capacity of 143 mAh g-1 and a faster capacity
fading. This could be explained by the fact that the TMSPi helped to
“purify” the electrolyte by removing the HF and H2O contaminates.
However, after 80 cycles, both cells with aged TMSPi containing
electrolyte suffered from a faster capacity fading rate than the ones
with the fresh TMSPi containing electrolyte. The better cycling
stability of the cells with fresh TMSPi containing electrolyte could
be attributed to the stable CEI layer formed in the presence of
TMSPi. After 120 cycles, the discharge capacity of the cells with the
aged TMSPi containing electrolyte decreased to the same level as
the cell with the baseline electrolyte, showing that the aged TMSPi
containing electrolyte failed to form an effective CEI to prevent
further electrolyte oxidation.
Regarding the fluorophosphates compounds, the binding energy
of the P 2p electrons is highly influenced by the attached elements
-
3- 23, 24
.
or groups25, e.g. PFxOy has higher binding energy than PO4
Thus, the fluorophosphates showed a broad peak in the P 2p
spectra. A comparison between Figures 6a and 6b showed that the
CEI film, which was formed by the oxidative decomposition of
TMSPi in the freshly prepared TMSPi containing electrolyte,
contained more fluorophosphates than the CEI obtained by the
baseline LP47 and the aged TMSPi containing LP47 electrolytes. The
contents of LiPFxOy (atomic ratio) in the CEI films formed in
different electrolytes are listed in Table 2.
Table 2: Thickness of the CEI films on LiNi0.5Co0.2Mn0.3O2 cathodes
cycled in different electrolytes, as well as the atomic ratio of the
LiPFx and LiPFxOy in each CEI film. “Dev.” stands for “Mean
deviation”.
Average
thickness
(Å)
LiPFx
Dev.
LiPFxOy
Dev.
Electrolyte
(at. %) (at. %) (at. %) (at. %)
Baseline LP47
LP47 + TMSPi
(fresh)
31.6
3.0
3.1
0.3
0.1
0.9
6.7
0.4
0.1
10.5
21.4
LP47 + TMSPi
(aged)
2.6
0.2
1.50
0.05
The thickness of the CEI films at the different cathode surfaces
was calculated from the obtained XPS measurement results and by
means of the theory developed by Niehoff et al.23, 26. As shown in
Surface analysis by means of XPS was used to characterize the
CEI formed by investigated electrolytes. In the P 2p spectra of the
LiNi0.5Co0.2Mn0.3O2 cathode (Figure 6), the peak at the higher
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Table 2, the fresh TMSPi containing electrolyte enabled the groups and formed (TMSO)PF2, which had a relatively low boiling
formation of a CEI film of 10.5 Å, which was thinner than the CEI point27 and could be detected by GC-MS.
formed by decomposition of the other two electrolytes. All three
The existence of the (TMSO)PF2 could also be confirmed by the
CEI films contained a similar amount of LiPFx. However, the CEI 19F-NMR spectrum (Figure 8a) of the reaction product of LiPF6 and
formed by baseline LP47 had ca. 0.9 at. % LiPFxOy, while the CEI TMSPi. A doublet at -33.5 ppm in 19F-NMR spectrum and a triplet at
formed by the fresh TMSPi containing electrolyte possessed as high 113.2 ppm in 31P-NMR with JPF coupling constant of 1032 Hz
1
as ca. 6.7 at. % LiPFxOy. Based on the better electrochemical (Figure 8b) were in agreement with previously reported NMR
performance, the CEI film with more fluorophosphates could be data27. Furthermore, two sets of doublets in 19F-NMR spectrum
assumed to be more robust and compact and thus effectively (Figure 8a) at -84.0 ppm (broad signals, 1 and 1´) and -73.3 ppm (2
1
diminished the oxidation of the electrolytes.
and 2`) with JPF coupling constant of 980 Hz and 925 Hz,
The thickness of the CEI formed by the aged TMSPi containing respectively, could be assigned to the formation of (TMSO)POF2 and
electrolyte increased to 21.4 Å, while the amount of (TMSO)2POF28 (structure shown in Figure 8d). TMSF and a very
fluorophosphates decreased to 1.50 at. %. The CEI built by the aged small amount of LiPF6 (3 and 3`) were detectable in the 19F-NMR
TMSPi containing LP47 electrolyte was less stable during long-term spectrum (Figure 8a). The corresponding signals also appeared in
cycling.
the 31P-NMR spectrum (Figure 8b) at -17.8 ppm for (TMSO)POF2 as
Consequently, it is important to investigate the reaction a broad triplet (1, 1` and 1``) and at -27.1 ppm for (TMSO)2POF as a
between TMSPi and LiPF6 to find out why less fluorophosphates doublet (2 and 2`). Besides, the 31P-NMR spectrum confirmed the
remained after aging of the electrolyte. To avoid the side reaction formation of tris(trimethylsilyl) phosphate ((TMSO)3PO) and
with the carbonate solvents, the TMSPi was directly mixed with tetrakis(trimethylsilyl) diphosphate ester ((TMSO)2OPOPO(OTMS)2,
LiPF6 in a molar ratio of 5:3, as it was suspected that the TMSPi structure shown in Figure 8d) as a singlet at -27.3 ppm (Figure 8b,
reacted with PF5. The mixture was sealed in plastic vials signal 3) and -30.5 ppm (Figure 8b, signal 4), respectively29. A small
(polypropylene, Shimadzu®) for the GC-MS measurement and amount of (TMSO)2PHO (Figure 8c, structure shown in Figure 8d)
stored in glovebox for two weeks at 20 °C. The product was was also produced in the reaction and could be assigned to a
1
investigated by NMR. The headspace gas was collected by a syringe doublet in 1H-NMR spectrum at 6.8 ppm (1 and 1`) with a JPH
and then injected into the GC-MS device. The mass spectrum is coupling constant of 714 Hz which was in correlation with a doublet
shown in Figure 7. As the EI mode was applied to charge the in the 31P-NMR spectrum at -13.6 ppm (5 and 5`)29
.
molecule fragments, the obtained m/z values directly indicated the
molecular weight of the gas species.
Figure 7: GC-MS of the headspace gas generated from the reaction
between LiPF6 and TMSPi (molar ratio was 3:5).
Besides TMSF, TMSOH and TMSOTMS18 signals, one more peak
with the m/z value of 158 was observed at the retention time of
2.1 min. (Figure 7). According to the possible reaction routines, this
peak was assigned to trimethylsilyl difluorophosphinite
((TMSO)PF2), which was firstly synthesized and reported by Cavell et
-
al.27 in 1973. In the reaction with LiPF6, the POF2 groups were
obtained from the TMSPi via the exchange of the P-O- bond and the
-
P-F bond (shown in Scheme 1). The POF2 was bound to the TMS
6 | J. Name., 2012, 00, 1-3
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Scheme 1: A proposed mechanism for the reaction of TMSPi with
LiPF6, without the participation of O2 and H2O. The formation of
(TMSO)PF2 and (TMSO)POF2 is taken as example.
As the binding energy of Si-F bond is higher than the Si-O
bond18, the TMS groups in the TMSPi will react with the F- from
LiPF6, resulting in the formation of TMSF and P-O- bond. Afterwards,
an exchange reaction between P-O- bond and P-F bond in PF5
Figure 8: NMR spectra of the reaction products of LiPF6 and TMSPi.
(a) 19F-NMR: 1 and 1` (TMSO)POF2; 2 and 2` (TMSO)2POF. (b) 31P-
NMR: 1, 1` and 1`` (TMSO)POF2;
2 and 2` (TMSO)2POF; 3
1
-
-
(TMSO)3PO; 4 (TMSO)2OPOP(OTMS)2; 5 and 5` (TMSO)2PHO. (c) H-
NMR: 1 and 1` (OTMS)2PHO. (d) The schematic structure of the
compounds found in the NMR analysis.
results in the formation of the highly reactive POF3. PF2O2 (PFxOy )
can be formed by a further exchange reaction between the P-O-
bond and P-F (from POF3). Due to the high anion affinity of the TMS
-
groups, the TMSPi could react with various PFxOy moieties and
Based on the products observed by GC-MS and NMR, it is clearly
shown that various TMS containing fluorophosphates have been
generate the products illustrated in Figure 8, e.g. the (TMSO)POF2.
The major product of the reaction between TMSPi and LiPF6 after
two weeks is TMSOPF2, which has been detected by GC-MS analysis
formed. As
a result, the polarity and reactivity of these
fluorophosphates dramatically decreased and could not take part in
the formation of an effective CEI film. The mechanism of the
reaction between TMSPi and LiPF6 is proposed and presented in
Scheme 1. In order to support the proposed mechanism and
reaction steps, computational and theoretical chemistry
approaches are in frame of our ongoing investigations. Not all the
products are mentioned in Scheme 1, while only the formation of
(TMSO)PF2 and (TMSO)POF2 as two key products are taken in
consideration.
3-
(Figure 7). Therefore, the PO3 group from TMSPi is consumed by
the reaction with LiPF6 and do not take part in the formation of the
CEI. In the aged TMSPi containing electrolyte, the CEI film contains
less fluorophosphate components than the CEI formed in the
freshly prepared TMSPi containing electrolyte due to the formation
of various TMS containing phosphates and fluorophosphates.
Moreover, the concentration of LiPF6 is also diminished by this side
reaction with TMSPi, resulting in the loss of charge carriers in the
electrolyte.
Conclusions
A freshly prepared TMSPi containing LP47 electrolyte (1 M
LiPF6 in a mixture of EC/DEC 3:7 (weight ratio)) shows a
significant improvement to the cycling stability of the
LiNi0.5Co0.2Mn0.3O2/graphite lithium ion cells charged to 4.6 V.
Due to its high anion affinity, the TMS group diminishes the HF
and water content in the investigated electrolyte. A thin and
robust CEI which contains more fluorophosphates can be
formed in presence of TMSPi. From this point of view, TMSPi
can be applied as an effective electrolyte additive.
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However, after aging at 20
Journal Name
°
C for 4 weeks, no TMSPi can be 15. S. W. Mai, M. Q. Xu, X. L. Liao, J. N. Hu, H. B. Lin, L. D. Xing, Y.
detected in the electrolyte anymore. It has been confirmed
P. Liao, X. P. Li and W. S. Li, Electrochim Acta, 2014, 147, 565-
that TMSPi facilitates the decomposition of the electrolyte salt
571.
LiPF6, which reduces the content of the conducting lithium salt 16. Y. M. Song, J. G. Han, S. Park, K. T. Lee and N. S. Choi, J Mater
in the electrolyte. The fluorophosphates generated from the Chem A, 2014, , 9506-9513.
decomposition of LiPF6 are captured by the highly electrophilic 17. N. N. Sinha, J. C. Burns and J. R. Dahn, J Electrochem Soc,
TMS groups, and thus cannot form a stable CEI anymore. 2014, 161, A1084-A1089.
Based on the reaction products between LiPF6 and TMSPi, a 18. Y. K. Han, J. Yoo and T. Yim, J Mater Chem A, 2015,
failure mechanism of the TMSPi in the LiPF6 containing 10909.
carbonate electrolytes has been proposed. In conclusion, 19. E. L. Myers, C. P. Butts and V. K. Aggarwal, Chem Commun,
TMSPi has certain lifetime limit when being used as an additive 2006, 4434-4436.
in the LiPF6 based carbonate electrolytes, which means the 20. C. L. Campion, W. T. Li and B. L. Lucht, J Electrochem Soc,
TMSPi containing electrolyte can only be used within a short 2005, 152, A2327-A2334.
time after the addition of TMSPi. Its beneficial effect of 21. N. S. Choi, J. G. Han, S. Y. Ha, I. Park and C. K. Back, Rsc Adv,
2
3, 10900-
improving the cycling stability of the cells deteriorates upon
aging.
2015,
5, 2732-2748.
22. S. K. Jung, H. Gwon, J. Hong, K. Y. Park, D. H. Seo, H. Kim, J.
Hyun, W. Yang and K. Kang, Adv Energy Mater, 2014,
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Acknowledgements
23. P. Niehoff and M. Winter, Langmuir, 2013, 29, 15813-15821.
24. E. Fluck and D. Weber, Z Naturforsch B, 1974, B 29, 603-607.
The authors would like to thank Dr. John Lin (Amperex
Technology Limited) for the financial funding, as well as for the
support of LiNi0.5Co0.2Mn0.3O2 and graphite samples.
25. M. Pelavin, D. N. Hendrickson, J. M. Hollander and W. L. Jolly,
J Phys Chem, 1970, 74, 1116-1121.
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8 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C6RA06555D
Lifetime limit of tris(trimethylsilyl) phosphite as electrolyte additive for
high voltage lithium ion batteries
Xin Qi, a Liang Tao, b Hendrik Hahn, a Carola Schultz, a Dennis Roman Gallus, a Xia Cao, a Sascha Nowak, a
Stephan Röser, a Jie Li, a Isidora Cekic-Laskovic, *a Babak Rezaei Rad *a,c and Martin Winter *a,c
Abstract
In this work, a failure mechanism of tris(trimethylsilyl) phosphite (TMSPi), as a popular additive in a
LiPF6 containing electrolyte for lithium ion batteries, is proposed and elucidated for the first time.