Thermal reactions of lithiated graphite anode in LiPF6-based electrolyte

Article abstract of DOI:10.1016/j.tca.2008.09.017

The thermal reactions of a lithiated graphite anode with and without 1.3 M lithium hexafluorophosphate (LiPF₆) in a solvent mixture of ethylene carbonate (EC) and ethylmethyl carbonate (EMC) were investigated by means of differential scanning calorimetry (DSC). The products of the thermal decomposition occurring on the lithiated graphite anode were characterized by Fourier transform infrared (FT-IR) analysis. The lithiated graphite anode showed two broad exothermic peaks at 270 and 325 °C, respectively, in the absence of electrolyte. It was demonstrated that the first peak could be assigned to the thermal reactions of PF₅ with various linear alkyl carbonates in the solid electrolyte interphase (SEI) and that the second peak was closely related to the thermal decomposition of the polyvinylidene fluoride (PVdF) binder. In the presence of electrolyte, the lithiated graphite anode showed the onset of an additional exothermic peak at 90 °C associated with the thermal decomposition reactions of the SEI layer with the organic solvents.

Full text of DOI:10.1016/j.tca.2008.09.017

Thermochimica Acta 480 (2008) 10–14
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Thermal reactions of lithiated graphite anode in LiPF6-based electrolyte
Nam-Soon Choi∗, Irina A. Profatilova,
Sung-Soo Kim, Eui-Hwan Song
Energy Laboratory, Corporate R&D Center, Samsung SDI Co. Ltd., 575 Shin-dong, Yeongtong-gu,
Suwon-si, Gyeonggi-do 443-731, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
The thermal reactions of a lithiated graphite anode with and without 1.3 M lithium hexafluorophosphate
(LiPF6) in a solvent mixture of ethylene carbonate (EC) and ethylmethyl carbonate (EMC) were investigated
by means of differential scanning calorimetry (DSC). The products of the thermal decomposition occur-
ring on the lithiated graphite anode were characterized by Fourier transform infrared (FT-IR) analysis.
The lithiated graphite anode showed two broad exothermic peaks at 270 and 325 ◦C, respectively, in the
absence of electrolyte. It was demonstrated that the first peak could be assigned to the thermal reactions
of PF5 with various linear alkyl carbonates in the solid electrolyte interphase (SEI) and that the second
peak was closely related to the thermal decomposition of the polyvinylidene fluoride (PVdF) binder. In the
presence of electrolyte, the lithiated graphite anode showed the onset of an additional exothermic peak
at 90 ◦C associated with the thermal decomposition reactions of the SEI layer with the organic solvents.
© 2008 Elsevier B.V. All rights reserved.
Received 30 June 2008
Received in revised form
22 September 2008
Accepted 23 September 2008
Available online 30 September 2008
Keywords:
Thermal decomposition
Lithiated graphite anode
LiPF6
FT-IR
DSC
1. Introduction
perature peak, heat evolution) of various electrolytes [6], lithiated
anodes [7–9], and delithiated cathodes [10]. In order to investi-
The safety of Li-ion cells is mainly governed by the thermal
reactivity of the battery components [1]. Understanding the ther-
mal decomposition reactions of electrolytes, anodes, and cathodes
at high temperatures is essential for the design of safe and high
performance Li-ion batteries. It is well known that the thermally
unstable lithiated graphite anodes and LiPF6-based electrolytes can
lead to Li-ion cell safety issues [2–4]. The decomposition of the
solid electrolyte interphase (SEI) layer that results from the elec-
trochemically reductive decompositions of the electrolyte on the
graphite anode can initialize some exothermic reactions between
the lithiated graphite anode and the electrolyte [5]. When a cell
is heated above a certain temperature, exothermic chemical reac-
tions between the electrodes and the electrolyte can take place and
lead to an increase of the cell internal temperature. If the generated
heat is greater than the energy that can be dissipated, exother-
mic processes will occur and the cell temperature will increase
rapidly. This temperature growth will accelerate chemical reac-
tions and lead to the production of even more heat, eventually
resulting in the thermal runaway of batteries. Most studies have,
so far, focused on the thermal behavior (temperature onset, tem-
gate the exothermic peaks observed in thermogravimetry (TG) and
derivative thermal analysis (DTA) profiles, Yang et al. monitored
the evolution of the gas products as a function of the temperature
and weight variation and they proposed different possible reac-
tion pathways [11]. Howard et al. investigated thermal reactions of
lithiated graphite anode and delithiated cathode with and with-
out an electrolyte using ARC and DSC-TG. Their results are very
informative to understand thermal behaviors of cell components
(anode, cathode, and electrolyte). However, the characterization for
decomposition products at elevated temperatures was not carried
out to explain the mechanism of battery thermal runaway in detail
[12] There is no rigorous Fourier transform infrared (FT-IR) analysis
available to characterize each exothermic peak on the differential
scanning calorimetry (DSC) thermograms. There are some possi-
ble exothermic reactions that trigger thermal runaway, including
the thermal decompositions of the electrolyte, the reductions of
the electrolyte by the anode, the oxidations of the electrolyte by
the cathode, the thermal decompositions of the anode and cath-
ode, the melting of the polyolefin separator, and the consequent
internal short.
The present work aims to understand the thermal decompo-
sition reactions of lithiated graphite anode with and without an
electrolyte solution. We have conducted a detailed investigation of
the thermal decomposition products by means of vibrational spec-
∗
Corresponding author. Tel.: +82 31 210 7575; fax: +82 31 210 7555.
E-mail address: ns75.choi@samsung.com (N.-S. Choi).
0040-6031/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.tca.2008.09.017

N.-S. Choi et al. / Thermochimica Acta 480 (2008) 10–14
11
troscopy because of the amount of structural information that can
be readily obtained using this technique.
2. Experimental
The electrolyte solution is composed of a commercially available
1.3 M lithium hexafluoro phosphate (LiPF6) dissolved in a solvent
mixture of ethylene carbonate (EC) and ethylmethyl carbonate
(EMC) with a 3:7 volume ratio. To evaluate the thermal properties
of the lithiated graphite anodes, a slurry was prepared by mixing
97 wt.% of graphite particles constituting the active material and
3 wt.% of a polyvinylidene fluoride (PVdF) binder dissolved in anhy-
drous N-methyl-2-pyrrolidinone (NMP). The resulting slurry was
cast on a copper foil. The composite electrode was then dried in a
convection oven at 110 ◦C for 2 h. The electrode was next pressed
and its thickness was around 40 ␮m.
Thermal analyses of lithiated graphite anodes and electrolytes
were conducted by using a differential scanning calorimetry tech-
nique (DuPont TA 2000 DSC). Each sample was sealed in a hermetic
stainless steel pan and scanned at a heating rate of 10 ◦C min−1
within an appropriate temperature range under a nitrogen atmo-
sphere.
FT-IR spectra of samples obtained from lithiated graphite anodes
heated to various temperatures, electrolytes, organic solvents, and
LiPF6 salt were recorded from transmission measurements in a
Nicolet NEXUX 870 spectrometer with a spectral resolution of
4 cm−1 under a nitrogen atmosphere. After the cells were charged
to 0.01 V vs. Li/Li+, they were carefully opened in a dry room and the
electrodes were subsequently rinsed in a dimethyl carbonate (DMC)
solvent in order to remove the residual electrolyte. They were then
dried under vacuum.
A coin-type half cell with a graphite anode and a Li counter
electrode was used in all experiments and assembled in an Ar-
filled glove box with less than 1 ppm of both oxygen and moisture.
Charge experiments were galvanostatically performed at a rate of
0.1 ◦C using a computer-controlled battery measurement system
(TOSCAT 3000 U).
Fig. 1. DSC heating curves of (a) LiPF6, (b) the fully lithiated graphite anode, (c)
the fully lithiated graphite anode with 10 wt.% LiPF6, (d) the fully lithiated graphite
anode with 30 wt.% LiPF6, (e) the enlarged profile of (a) and (f) the enlarged profile
of (b).
3. Results and discussion
charged to 0.01 V are shown in Fig. 2. The pronounced peaks at
1455 and 856 cm−1 in the lithiated graphite anode before heating
are originated from the Li2CO3 generated by a direct two-electron
reduction of ethylene carbonate (EC). The absorption peaks at
1630 and 1051 cm−1 are associated with lithium alkylcarbonates
(ROCO2Li), which are formed on the anode surface by a single elec-
tron electrochemical reduction of EC [15,16]. It is well known that
the solid electrolyte interphase (SEI) layer is mainly composed of
Li2CO3 and various lithium alkylcarbonates (ROCO2Li) as well as
small amounts of LiF, Li2O, and LiOH formed by the electrolyte
decomposition [17]. When the lithiated graphite anode (LiC6 + PVdF
binder) without an electrolyte is heated in a DSC cell, five possi-
ble decomposition reactions generating the exothermic heat are as
follows:
Fig. 1 presents the DSC heating curves for a LiPF6 salt, a lithiated
graphite anode powder, and a lithiated graphite anode with LiPF6.
After the cells were charged in EC/EMC (3/7) with 1.3 M LiPF6, they
were carefully opened in a dry room. The lithiated graphite anodes
were subsequently rinsed in a dimethyl carbonate (DMC) solvent
to remove the residual electrolyte and they were finally dried. The
thermal properties of the resulting anodes were measured. The spe-
cific charge capacity of the graphite anode was approximately of
370 mAh g−1 at a charge cut-off potential of 0.01 V vs. Li/Li+. LiPF6
enclosed in a sealed stainless steel pan shows a sharp endother-
mic peak at around 197 ◦C and a broad endothermic peak with a
peak temperature of 290 ◦C, as shown in Fig. 1(a). It is clear that the
first peak and the second endothermic peak are attributed to the
melting and the thermal decomposition of LiPF6 (LiPF6 (s) ↔ LiF
(s) + PF5 (g)), respectively [13]. An additional small endothermic
peak related to the hydrogen fluoride (HF) removal from LiPF6
appears at about 64 ◦C in Fig. 1(e). Yang et al. reported that the
HF free acid is inevitably left over during the salt production and
the amount of acid depends on the production technology [14].
The DSC curve (b) for a lithiated graphite anode without electrolyte
shows two broad exothermic peaks at 270 and 325 ◦C. In an attempt
to characterize these two exothermic peaks, FT-IR analyses were
performed at various temperatures (before heating, 200, 275 and
400 ◦C). The ex situ FT-IR spectra of the lithiated graphite anode
•
Thermal decompositions of the meta-stable compounds
like lithium alkyl carbonate in the presence of the lithi-
ated graphite anode (2Li + (CH2OCO2Li)2 → 2Li2CO3 + C2H4,
ROCO2Li → R• + H• + Li2CO3, and R• + O2 → ROO• + H• → ROOH).
•
Thermal reaction of PF5 (generated from LiPF6 decomposition)
with the intercalated Li ions (Li + PF5 → Li1−xPF5−x + xLiF).
•
Thermal reactions of OPF3 and PF5 (generated from LiPF6
decomposition) with organic compounds in the SEI layer
(OPF3 + ROCO2Li → OP(R)1−xFx + Li2CO3).

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N.-S. Choi et al. / Thermochimica Acta 480 (2008) 10–14
Fig. 2. FT-IR spectra of (a) non-cycled graphite anode, (b) the fully lithiated graphite
anode before heating, heated to (c) 200, (d) 275 and (e) 400 ◦C.
Fig. 3. DSC heating curves of (a) the fully lithiated graphite anode with EC:EMC
(3:7)/1.3 M LiPF6, (b) LiF with EC:EMC (3:7)/1.3 M LiPF6, (c) Li2C2O4 with EC:EMC
(3:7)/1.3 M LiPF6, (d) Li2CO3 with EC:EMC (3:7)/1.3 M LiPF6, (e) EC:EMC (3:7)/1.3 M
LiPF6 and (f) Li metal with EC:EMC (3:7)/1.3 M LiPF6.
•
Thermal reaction of HF (generated from LiPF6 decomposition)
with Li2CO3 in the SEI layer (HF + Li2CO3 → LiF + H2CO3).
•
Thermal reaction of HF generated from
a PVdF binder
with intercalated Li ions (–CH2–CF2– → –CH CF– + HF,
HF + Li → LiF + 1/2H2).
agreement with work by Tarascon’s group [21]. The carbon–carbon
triple bonds are formed by elimination of HF from the –CH2–CF2–
bond in the PVdF binder in the presence of lithiated graphite at
high temperatures. We think that the reaction of HF produced via
the dehydrohalogenation of a PVdF binder with intercalated Li ions
contributes to the exothermic heat around 325 ◦C.
New peaks assigned to the P–O–C antisymmetric stretching and
the P–OH stretching in the phosphorous compounds appeared at
1031 and 943 cm−1 in the FT-IR spectrum obtained after heating the
lithiated graphite anode up to 200 ◦C (Fig. 2(c)) [18]. The intensity
of the peak assigned to the phosphorous compounds (1031 cm−1
)
Fig. 3 shows the DSC heating curves for EC/EMC (3/7)/1.3 M
LiPF6, for a lithiated graphite anode, a Li metal, LiF, Li2C2O4 (lithium
oxalate), and Li2CO3 in presence of an electrolyte solution. The DSC
curves for the lithiated graphite anode with and without electrolyte
are quite different as shown in Figs. 1(b) and 3(a), respectively. It can
clearly be seen that results for the lithiated graphite anode with-
out EC/EMC/1.3 M LiPF6 do not indicate the onset of an exothermic
peak at 90 ◦C. Yamaki et al. reported that the first exothermic peak
around 140 ◦C is caused by the reaction (SEI formation) of an elec-
trolyte and a lithiated graphite, whose surface is covered by PVdF
binder without formation of SEI. DSC measurements were carried
out for Li metal with EC/EMC 1 M LiPF6 to identify this point. There
is no peak around 140 ◦C, as shown in Fig. 3(f). The exothermic
heat generation began at 140 ◦C and two peak temperatures were
190 and 215 ◦C. This indicates that these two exothermic peaks are
ascribed to the thermal reactions (SEI formation) of the electrolyte
and the Li metal. It should be noted that the reactivity of Li metal is
analogous with lithiated graphite. From this result, we can say that
the first exothermic peak at 120 ◦C in Fig. 3(a) is not produced by
the reaction of an electrolyte with a lithiated graphite. In order to
characterize this exothermic peak, a FT-IR spectrum of the lithiated
graphite with electrolyte heated up to 200 ◦C was obtained. It is
difficult to accurately identify the ROCO2Li peak because ROCO2Li
actually represents a series of lithium alkyl carbonates. It should be
noted that their peak positions depend on the R group and are deter-
mined by the SEI layer structure. The relative intensities of the peaks
assigned to lithium alkyl carbonates (ROCO2Li) (1630–1635, 1434,
increased while heating the lithiated graphite anode up to 275 ◦C
(Fig. 2(d)). It was reported that PF5 (g) formed via the thermal
decomposition of LiPF6 reacts with protic impurities like ROH and
water to generate phosphorous oxyfluoride (OPF3) [19]. OPF3 can
react with carbonate-containing compounds to produce P–O–C and
P–OH bonds. Indeed, the lithiated graphite anodes after adding 10
and 30 wt.% LiPF6 showed distinct and large exothermic peaks at
258 and 266 ◦C, respectively, as illustrated in Fig. 1(c) and (d). The
increase of LiPF6 amount led to the argumentation of the peak
intensity. The LiPF6 trapped in the SEI layer during the charging
process thermally decomposes into LiF (s) and PF5 (g) at around
200 ◦C. The PF5 (g) generated in a sealed DSC pan simultaneously
produces OPF3 and HF by reaction with water traces. It is plau-
sible that highly reactive compounds (such as PF5, OPF3, and HF)
react simultaneously with the metastable decomposition products
in the SEI layer and thereby amplify the exothermic heat. A small
exothermic peak around 310 ◦C in Fig. 1(c) seems to correspond to
the thermal decomposition of the PVdF binder and this point could
be confirmed from the FT-IR results. The C–F stretching peak of the
PVdF binder (1160 cm−1) almost disappeared after heating the lithi-
ated graphite anode up to 400 ◦C. In addition, new peaks assigned
to the C C, C O, and C C stretching were observed at 1677, 1778,
and 2010 cm−1, respectively, in Fig. 2(e). The formation of unsatu-
rated carbon–carbon bonds in halogenated hydrocarbons occurs by
dehydrohalogenation, namely the elimination of a hydrogen and a
halogen atom from two adjacent carbon atoms [20]. This result is in

N.-S. Choi et al. / Thermochimica Acta 480 (2008) 10–14
13
Fig. 4. FT-IR spectra of the fully lithiated graphite anode with EC:EMC (3:7)/1.3 M
Fig. 5. FT-IR spectra of (a) the fully lithiated graphite anode with EC:EMC (3:7)/1.3 M
LiPF6 (heated up to 200 ◦C), (b) EMC, (c) EMC/1.3 M LiPF6, (d) EC, (e) EC/1.3 M LiPF6
and (f) LiPF6.
LiPF6 (a) before heating (no electrolyte), heated to (b) 200 and (c) 400 ◦C.
The two broad exothermic peaks between 180 and 300 ◦C
seem to be the result of the thermal decompositions of EC/EMC
(3/7)/1.3 M LiPF6 as shown in Fig. 3(a) and (e). Finally, in order
to characterize the onset of the large exothermic peak at 300 ◦C,
FT-IR measurements were performed for the lithiated graphite
anode with electrolyte heated at 400 ◦C. The C–F stretching peak
of the PVdF binder (1160 cm−1) was not observed in Fig. 4(c)
and this result was in accordance with that of an electrolyte-free
lithiated graphite. In addition, the peaks assigned to lithium alkyl-
carbonates (ROCO2Li) (1635, 1434, 1317 and 1051 cm−1) and Li2CO3
peaks (1488 and 863 cm−1) prominently increased. After two large
exothermic peaks with the onset around 140 ◦C appear, the broad
exothermic peak around 300 ◦C is shown in Fig. 3(f). This indicates
that SEI formation begins at 140 ◦C via the thermal reactions of an
electrolyte with a Li metal and continues until 380 ◦C. It is believed
1317 and 1051 cm−1), the lithium oxalate (Li2C2O4) peaks (1630,
1320 and 770–780 cm−1), and the Li2CO3 peaks (1455, 856 cm−1
)
decreased remarkably, as shown in Fig. 4(a) and (b). This means
that various lithium alkyl carbonates, Li2C2O4, and Li2CO3 present
in the SEI layer thermally decompose upon heating up to 200 ◦C
and then form new compounds. Free radicals (R•, H•) are produced
by the thermal decomposition reactions of lithium alkyl carbonates
(ROCO2Li in the SEI layer → R• + H• + Li2CO3) at high temperatures
and a recombination of these radicals with protic compounds (EC,
EMC, water trace, etc.) in the electrolyte occurs, resulting in the
production of exothermic heat. To clarify the thermal reactions
between SEI and an electrolyte, DSC measurements for main com-
ponents in SEI with an electrolyte were carried out. Fig. 3(c) and
(d) exhibit that Li2C2O4 and Li2CO3 reported as important com-
ponents in SEI [22] thermally decompose around 195 and 105 ◦C,
respectively. This result can successfully rationalize that Li2C2O4
and Li2CO3 react with HF generated from LiPF6 in an electrolyte
(Li2C2O4 + 2HF → 2LiF + H2C2O4, Li2CO3 + 2HF → 2LiF + H2CO3) and
their peaks in Fig. 4(a) and (b) are reduced via these reactions.
LiF formed by the electrochemical decomposition of LiPF6 did not
show significant exothermic peaks except for two broad exothermic
peaks produced by the electrolyte decomposition in Fig. 3(b). The
comparison of Fig. 3(a) and (d) manifests that the first exothermic
peak around 120 ◦C is attributed to the thermal reactions between
Li2CO3 in SEI and an electrolyte in presence of a lithiated graphite
anode. We think that the schematic model for the thermal reactions
of SEI with an electrolyte proposed by Dahn’s group is appropriate
to explain the exothermic peak around 120 ◦C [23].
Fig. 5 shows the FT-IR spectra for the organic solvents, the elec-
trolyte solutions, and LiPF6. There are no peaks related to EMC in
the lithiated graphite anode with electrolyte heated at 200 ◦C. Sharp
peaks in Fig. 4(b) are assigned to the solvated EC interacting with Li+
ions (1806, 1774, 1481, 1400, 1164, 902, 777 and 715 cm−1) and the
LiPF6 salt (835 and 557 cm−1). This implies that the EMC solvent,
having a low boiling point of 108 ◦C, is evaporated from the lithiated
graphite anode with electrolyte upon the heating up to 200 ◦C and
disappears after the disassembling of the DSC cells. Fig. 6 shows
the FT-IR spectra for the region of the EC ring-breathing mode. It is
found that a peak corresponding to free EC (893 cm−1) was absent in
the lithiated graphite anode with electrolyte heated at 200 ◦C. This
indicates that all the EC molecules participate in the dissociation of
LiPF6 due to the evaporation of EMC.
Fig. 6. FT-IR spectra for the ring breathing mode of EC in EC/1.3 M LiPF6 and charged
graphite anode with EC:EMC (3:7)/1.3 M LiPF6 heated to 200 ◦C.

14
N.-S. Choi et al. / Thermochimica Acta 480 (2008) 10–14
that the thermal decomposition reactions of organic solvents and
LiPF6 with a lithiated graphite continue from 200 to 400 ◦C and
that a large amount of ROCO2Li and Li2CO3 is thereby produced.
It can be concluded from these results that the exothermic peak
around 350 ◦C is caused by the SEI formation (lithium alkylcarbon-
ates, Li2CO3) and the thermal decomposition reactions of the PVdF
binder with the lithiated graphite.
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The thermal and spectral studies of a lithiated graphite anode,
with and without electrolyte, clearly showed that the SEI layer,
the LiPF6-based electrolyte, and the PVdF binder on the lithiated
graphite anode thermally decompose at high temperatures. More-
over, it could be confirmed that the PF5 formed by the thermal
decomposition of LiPF6 reacts with ROH in the SEI layer and that
this reaction leads to the formation of phosphorous compounds on
the lithiated graphite anode surface at around 200 ◦C. The presence
of the electrolyte led to thermal decomposition reactions of the SEI
layer on the lithiated graphite anode at a low onset temperature
around 90 ◦C.
Acknowledgement
The authors would like to thank Samsung SDI Corporation for
the support of this work.
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