On the thermal behavior of Li bis(oxalato)borate LiBOB

Article abstract of DOI:10.1016/j.tca.2007.03.001

The lithium salt, bis(oxalato)borate, LiBC₄O₈ (LiBOB), is one of the most important Li salts which are explored today in connection with R & D of novel, high energy density rechargeable Li batteries. The thermal stability of this salt in the temperature range of 40-350 °C was rigorously studied by accelerating rate calorimetry (ARC), differential scanning calorimetry (DSC), and thermal gravimetric analysis (TGA). XRD, FTIR, SEM, and ICP were used to analyze the products of its thermal decomposition reactions. Studies by DSC and pressure measurements during ARC experiments with LiBOB detected an endothermic reaction with an onset at ～293 °C, involving a complete irreversible decomposition of the LiBOB in which gaseous products are formed. In the first stage, Li₂C₂O₄ (crystalline), B₂O₃ (glass), CO and CO₂ gases are formed in both confined and open volumes. The next (final) stage in the thermal reactions is an irreversible formation of lithium triborate LiB₃O₅ (glass).

Full text of DOI:10.1016/j.tca.2007.03.001

Thermochimica Acta 457 (2007) 64–69
On the thermal behavior of Li bis(oxalato)borate LiBOB
Ella Zinigrad, Liraz Larush-Asraf, Gregory Salitra, Milon Sprecher, Doron Aurbach∗
Bar-Ilan University, Ramat-Gan 52900, Israel
Received 27 November 2006; received in revised form 26 February 2007; accepted 1 March 2007
Available online 6 March 2007
Abstract
The lithium salt, bis(oxalato)borate, LiBC4O8 (LiBOB), is one of the most important Li salts which are explored today in connection with R &
D of novel, high energy density rechargeable Li batteries. The thermal stability of this salt in the temperature range of 40–350 ◦C was rigorously
studied by accelerating rate calorimetry (ARC), differential scanning calorimetry (DSC), and thermal gravimetric analysis (TGA). XRD, FTIR,
SEM, and ICP were used to analyze the products of its thermal decomposition reactions. Studies by DSC and pressure measurements during
ARC experiments with LiBOB detected an endothermic reaction with an onset at ∼293 ◦C, involving a complete irreversible decomposition of
the LiBOB in which gaseous products are formed. In the first stage, Li2C2O4 (crystalline), B2O3 (glass), CO and CO2 gases are formed in both
confined and open volumes. The next (final) stage in the thermal reactions is an irreversible formation of lithium triborate LiB3O5 (glass).
© 2007 Elsevier B.V. All rights reserved.
Keywords: ARC; DSC; TGA; Thermal decomposition; LiBOB
ric analysis (DTA–TGA), using a scanning rate of 10 ◦C min−1
,
1. Introduction
showed that LiBOB is stable up to 302 ◦C. Beyond this tempera-
ture, it decomposes, rather than melts [2]. This salt is chemically
stable in organic solutions, but is reported to slowly decompose
by hydrolysis to LiBO2 and LiOOCCOOH [2]. Amine et al.
[4] presented the thermogravimetric curve for LiBOB in the
temperature range of 20–600 ◦C. Yu et al. [5] reported on the
stability of LiBOB in the temperature range of 240–300 ◦C, and
on LiBOB decomposition to Li2CO3, B2O3, CO2, and CO when
the temperature is higher than 300 ◦C. They observed a weight
loss related to two processes at temperatures in the region of
320 ◦C and 450 ◦C. It was reported that the decomposition of
LiBOB at 302 ◦C produces B2O3 and CO2 [6].
The new lithium salt, bis(oxalato)borate, was recently pro-
posed by Lischka et al. [1] and Xu and Angell [2] as a very
promising and advantageous electrolyte for rechargeable Li ion
batteries, instead of the problematic LiPF6, which is the com-
monly used Li salt in electrolyte solutions for commercial Li ion
batteries.
During the last ten years, a number of papers appeared in
the literature demonstrating the good performance of graphite
anodes and transition metal cathodes in solutions containing
LiBOB (compared to their behavior in solutions based on
LiPF6). It was also demonstrated that the thermal stability of
electrolyte solutions for Li ion batteries based on LiBOB is much
higher than those based on other Li salts (e.g., LiPF6, LiBF4) [3].
Hence, it is clear that the intrinsic thermal behavior of LiBOB is
interesting and important regards with R & D of novel and safer
Li ion batteries. Indeed, in recent years some reports on the ther-
mal studies of LiBOB appeared in the literature, as summarized
below. According to thermogravimetric experiments, lithium-
bis(oxalato)borate is fully stable up to approximately 300 ◦C [1].
Differential thermal analysis combined with thermogravimet-
Yet despite past studies of LiBOB and its solutions regards
both electrochemical performance and thermal behavior, there is
stillnotacomprehensivepictureoftheintrinsicthermalbehavior
of this salt nor a clear determination of all the products of its
thermal reactions. The aim of the present work is to remedy said
differences, and achieve a comprehensive understanding of the
calorimetric response of LiBOB and a rigorous analysis of its
decomposition products.
2. Experimental
∗
Highly purified LiBOB salt was obtained from Chemetall
Inc. and was used as received. The pristine salt and its thermal
Corresponding author. Tel.: +972 3 5318309; fax: +972 3 5351250.
E-mail address: aurbach@mail.biu.ac.il (D. Aurbach).
0040-6031/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.tca.2007.03.001

E. Zinigrad et al. / Thermochimica Acta 457 (2007) 64–69
65
dissociation products were characterized by Fourier transform
infrared (FTIR) spectroscopy (Magna 860 Spectrometer from
Nicolet Inc., placed in a glove box under H2O- and CO2-
free atmosphere), elemental analysis by an inductively coupled
plasma (ICP) spectrometer (Ultima 2, Jobon Yvon Horiba),
optical microscopy (AX70/AX70A, Olympus Inc.), scanning
electron microscopy (SEM) (JEOL JSM 840), and X-ray diffrac-
tion using a D8 Advance System from Brucker Inc. XRD
˚
patterns were obtained using Cu K␣ radiation (λ = 1.54 A) at
40 mA and 40 kV. We used a cell which enabled the isolation
of the sensitive samples from atmospheric gases by a polyester
Mylar-type film, 90 ␮m thick, transparent to X-ray radiation. An
accelerating rate calorimeter (ARC) from Arthur D Little Inc.,
Model 2000, a differential scanning calorimeter (DSC) from
Mettler Toledo Inc., Model DSC 822, and thermogravimetry
(TG, DTA)(SDT2960, TAinstrument)wereusedforthethermal
analysis of LiBOB.
In the ARC measurements, about 1 g of salt was loaded in a
titanium flask (8.2 ml volume) in an argon-filled glove box (VAC
Inc.) and transferred to the ARC under purified Ar atmosphere.
In the search for self-heating, the salt was heated from 40 to
300 ◦C in 5 ◦C increments at the rate of 2 ◦C min−1 at a sensitiv-
ity threshold of 0.02 ◦C min−1. The controller was programmed
to wait 15 min for the sample and the calorimeter temperatures
to equilibrate, and then to search during 20 min for a temper-
ature increase of 0.02 ◦C min−1. After the ARC experiments,
the titanium reaction vessel were cooled with liquid nitrogen
until the pressure was close to the atmospheric pressure. The
gaseous products were released through a specially designed
high-pressure valve.
TGA–DTA measurements were conducted under pure nitro-
gen atmosphere in alumina crucibles, 70 ␮l in volume. DSC
measurements in a confined volume were conducted in her-
metically sealed, gold-plated stainless steel crucibles, 30 ␮l
in volume (Mettler Toledo Inc.) over a temperature range of
40–350 ◦C. The crucibles were filled with about 10 mg of the
salt, and then sealed in a glove box under argon atmosphere.
Fig. 1. (a) A typical ARC response during heating LiBOB. (b) The chart empha-
sizes the behavior in the temperature range 240–300 ◦C.
related to LiBOB decomposition. The weight loss calculated
from the TGA and ARC measurements for the complete process
is 61.2 0.5%.
Fig. 3 shows typical DSC heating curves obtained for LiBOB
(a) and mixtures of Li oxalate–B2O3, 1:3 mol/mol (b) in closed
crucibles. One endothermic process is to be seen in the tem-
perature range 210–390 ◦C (Fig. 3a). It is identified as LiBOB
The heating/cooling rate was 1 ◦C min−1
.
3. Results
Fig. 1 shows temperature and pressure changes (with time)
measured during ARC experiments on 1.0 g of LiBOB heated
up to 300 ◦C. There is no evidence of an exotherm in this
temperature range. The significant pressure increase with an
onset of 265 ◦C (Fig. 1a) and the negative slope in the tem-
perature steps appearing at about 268 ◦C (Fig. 1b), may be
explained by an endothermic decomposition of LiBOB. The
gaseous products of this reaction were collected and analyzed
(see below).
Fig. 2 presents TGA and DTA curves for LiBOB and
Li oxalate samples. DTA curve b of LiBOB, shows two
endothermic peaks at about 100 ◦C and 310 ◦C, respectively,
accompanied by weight loss (TGA curve a). The first endotherm
is attributed to the loss of adsorbed water, which amounts to
∼0.06% of the sample as calculated from the TGA results. The
second endotherm with an onset at about 293 ◦C (Fig. 2) is
Fig. 2. TGA (a and c) and DTA (b and d) curves obtained by heating LiBOB
samples at a rate of 1 ◦C min−1 (14.830 mg) (a and b), and Li oxalate (5.255 mg)
(c and d).

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E. Zinigrad et al. / Thermochimica Acta 457 (2007) 64–69
Table 1
Parameters related to LiBOB decomposition, appearing in the DSC curves at a heating rate of 1 ◦C min−1
No.
Weight (mg)
(␮mol)
Temperature interval (◦C)
Onset temperature (◦C)
Peak temperature (◦C)
ꢀUr of decomposition (kJ mol−1
)
1
3.587
4.379
4.987
5.095
6.842
8.037
10.195
18.49
22.57
25.71
26.26
35.27
41.43
52.55
280.4–328.7
261.5–331.4
283.0–340.0
266.0–344.0
260.2–335.1
258.8–336.4
268.5–344.5
297.04
286.07
296.00
290.00
298.24
295.06
290.93
309.80
304.50
322.89
314.77
315.32
315.54
313.76
41.10
45.98
33.42
33.58
38.71
37.80
37.31
2*
3
4
5
6
7
Average
293.33 3.72
313.80 3.81
38.27 3.14
*
An open crucible.
Fig. 3b showing the thermal behavior of a Li oxalate–B2O3
mixture, 1:3 mol/mol reflects an endothermic reaction between
these two species in the temperature range that overlaps the
range of LiBOB decomposition, with a reaction heat around
67.9 kJ mol−1 of reacting LiB3O5. This datum is very important
for the discussion below.
Figs. 4 and 5 show SEM and optical micrographs respec-
tively of pristine and heated LiBOB powder. The pristine white
powder consists of elongated round particles with a length of
0.1–0.5 mm and a diameter of 0.05–0.25 mm. Surprisingly, the
particles’ shape and size did not change noticeably after heating
to 350 ◦C (Fig. 4a and b) in spite of the weight loss of about
60%. They are non transparent under transmitted light (Fig. 5a)
and do not demonstrate non homogeneity under polarized light
(Fig. 5b).
Fig. 6 shows XRD patterns of the pristine salt and the solid
decomposition product (collected after heating). The XRD pat-
terns of the pristine LiBOB correlates well with literature data
[7]. No observation of pristine LiBOB peaks after heating to
350 ◦C indicates a complete LiBOB decomposition. These pow-
ders, after heating in a closed volume in an ARC bomb, in DSC
crucibles, and in opened alumina crucibles in TGA tests, show
Fig. 3. DSC curves obtained by heating: (a) LiBOB sample, 8.037 mg; and (b)
mixture Li2C2O4–B2O3, 1:3 mol/mol, 1.717 mg. Heating rate 1 ◦C min−1
.
irreversible decomposition since DSC cooling curves did not
show any peaks. The results of 7 different DSC tests at a heat-
ing rate of 1 ◦C min−1 are summarized in Table 1. The onset of
the endothermic dissociation of LiBOB is at about 293 ◦C. The
decomposition heat is about 38 kJ mol−1 (196 Jg−1).
Fig. 4. SEM micrographs of: (a) pristine LiBOB samples; (b) LiBOB after heating to 350 ◦C. A scale appears in each micrograph.

E. Zinigrad et al. / Thermochimica Acta 457 (2007) 64–69
67
Fig. 7. FTIR spectra of: (a) pristine LiBOB powder [9]; (b) the powder obtained
by heating LiBOB to 350 ◦C; and (c) Li oxalate powder is shown for comparison.
The spectra were obtained by KBr pellet method.
identical XRD patterns. All peaks correspond to monoclinic
lithium oxalate [8]. Storing of this product for 10 days at 300 ◦C
did not cause any structural changes (confirmed by XRD and
FTIR).
There is good agreement between the XRD measurements
and the FTIR spectra (Fig. 7). As seen in Fig. 7, the IR bands of
pristine LiBOB are not detected after heating to 350 ◦C, but all
the expected IR peaks of Li oxalate can be seen in the spectrum
of the thermal product.
Fig. 8 compares FTIR spectrum of the gaseous product of
LiBOB thermal decomposition to the spectra of CO2 and CO.
From these studies, it is clear that the gaseous product of LiBOB
decomposition comprises both CO2 and CO. The discrepancy
between FTIR signals of these gases occurs due to the fact that
the absorbent coefficient of CO is near 7 times smaller than
Fig. 5. Photomicrographs from optical microscope of the powder obtained after
heating of LiBOB samples to 350 ◦C: (a) transmission (×5); (b) polarized light
(×10).
Fig. 6. XRD patterns of: (a) a pristine LiBOB powder; (b) a powder obtained by heating LiBOB to 350 ◦C; (c) same as (b) after storage at 300 ◦C for 10 days. Line
marks of Li oxalate are shown for comparison.

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E. Zinigrad et al. / Thermochimica Acta 457 (2007) 64–69
All the B2O3 produced by the first step (Eq. (1)) is con-
sumed in the second (Eq. (2)). However, this second reaction
consumes only 1/3 of the Li oxalate formed in the first. This
correlates well with the fact that the final products of LiBOB
decomposition contain Li oxalate, as evidenced by both XRD
and FTIR (Figs. 6 and 7). The calculated weight loss for decom-
position equation (3) is 61.6%. This matches the experimental
result (DTA–TGA, Fig. 2) of ca. 62%. The Li triborate (LiB3O5)
is formed as a non crystalline glass phase and cannot therefore be
identified by X-ray diffraction. However, Fig. 7 presents FTIR
spectra of pristine LiBOB, of a sample after heating to 350 ◦C,
and of Li oxalate for comparison. The spectrum of the decom-
posed sample clearly shows oxalate bands at 449, 516, 775,
1330, and 1665 cm−1, and three new bands at 1087, 1382 and
866 cm−1 (not related to oxalate). These peaks can be assigned
to the strong bands of LiB3O5 at 1372, 1087 and 860 cm−1 [13].
The ICP analysis of the solid product after heating LiBOB to
350 ◦C showed the presence of 9.3% Li and 13.9% B. This fits
well the composition expected for solid product suggested by
Eq. (3), namely a mixture of Li2C2O4 + LiB3O5 (9.4% Li and
14.5% B).
Fig. 8. FTIR spectrum of a gas phase that is formed in the ARC calorimetric
bomb due to a thermal decomposition of LiBOB (a). Spectra of CO2 (b) and CO
(c) are presented for comparison.
that of CO2 at room temperature [10]. The solid product of
LiBOB thermal decomposition was dissolved in distilled water
and analyzed by ICP and found to contain 9.3% Li and 13.9%
B.
The following experiment was carried out to confirm the
occurrence of the reaction postulated in Eq. (2). Mixtures con-
taining Li2C2O4:B2O3 1:3 mol/mol were thoroughly ground
in an agate mortar and pestle and were then subjected to
DSC in closed crucibles up to 500 ◦C. A pronounced broad
endothermic peak appears in the temperature range that over-
laps the range in which LiBOB decomposes (Fig. 3). The
onset of this peak is near 210 ◦C that is ca. 80 ◦C lower
than the onset of the LiBOB decomposition peak (Eq. (1)).
This temperature is far for the oxalate decomposition (above
400 ◦C) and the endotherm is attributed only to reaction (2).
Hence, reaction (2) begins immediately upon the formation
of Li2C2O4 and B2O3 by LiBOB decomposition (Eq. (1)).
The heat of the endothermic reaction between Li oxalate and
B2O3 was found to be about 67.9 kJ mol−1 of the product
LiB3O5. According to Eq. (3), only 1/3 mol of LiB3O5 is formed
by the decomposition of 1 mol of LiBOB. Therefore, about
67.9/3 ≈ 22.6 kJ mol−1 of LiBOB decomposed is absorbed for
theformationof1/3 molofLiB3O5. Thisvalueispartoftheover-
all heat of decomposition LiBOB, of namely about 38 kJ mol−1
(see Table 1). Consequently, the heat of the first decomposition
step of LiBOB, Eq. (1), is near 38 − 23 = 15 kJ mol−1. XRD
measurements of the product of heating the Li2C2O4–B2O3
mixture (up to 350 ◦C) showed no evidence of a crystalline
phase.
A mixture Li2O–B2O3 containing 75 mol% B2O3 should
undergo slow crystallization upon storage at elevated temper-
atures, above 500 ◦C [14]. The crystals of LiB3O5 were grown
on a seed from the solution-melt using the method of tempera-
ture lowering [15]. The end product of LiBOB decomposition
was stored at 300 ◦C during 10 days by further cooling at a rate
of 1 ◦C min−1, followed by XRD measurements. (This is the
highest temperature at which a mixture containing Li oxalate
can be heated and remains stable). No evidence for the forma-
tion of any new crystalline phase, except the existing Li oxalate,
was observed by X-ray diffraction (Fig. 6c).
4. Discussion
Based on the above results and as detailed below, it is sug-
gested that the decomposition of LiBOB at ∼300 ◦C occurs as
follows:
6LiBC4O8(cryst) → 3Li2C2O4(cryst) + 3B2O3(glass)
+ 9CO(gas) + 9CO2(gas);
(1)
(2)
(3)
Li2C2O4(cryst) + 3B2O3(glass)
→ 2LiB3O5(glass) + CO(gas) + CO2(gas).
The overall thermal decomposition process of LiBOB is:
6LiBC4O8(cryst) → 2Li2C2O4(cryst) + 2LiB3O5(glass)
+ 10CO(gas) + 10CO2(gas).
The calculated weight loss for decomposition of LiBOB
according to Eq. (1) is 55.7%. However, the TGA and ARC
experiments show a greater weight loss upon thermal decompo-
sition. The TGA data (Fig. 2) show a weight loss of about 62%.
Clearly, an additional reaction involving gas evolution is tak-
ing place. This cannot be the decomposition of lithium oxalate
to carbonate, because to our TGA–DTA findings (Fig. 2) as
well as literature data [12] this occurs only above 400 ◦C. Fur-
thermore, FTIR spectra of the LiBOB samples after heating to
350 ◦C indicated the absence of B2O3 as a final decomposition
product (Fig. 7). The IR spectrum of the vitreous B2O3 contains
a stronger absorption band at ∼1265 cm−1, a weak absorption
band at ∼720 cm−1, and also a shoulder situated at ∼1400 cm−1
[11].
Hence, we suggest reaction (2) above as a follow up process
that well explains both the weight loss observed and the absence
of B2O3 in the end product.

E. Zinigrad et al. / Thermochimica Acta 457 (2007) 64–69
69
5. Conclusions
[2] W. Xu, C.A. Angel, Electrochem. Solid-State Lett. 4 (2001) E1–
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State Lett. 5 (2002) A26–A29.
[4] K. Amine, J. Liu, S. Kang, I. Belharouak, Y. Hyung, D. Vissers, G. Heriksen,
J. Power Sources 129 (2004) 14–19.
[5] B.-T. Yu, W.-H. Qui, F.-S. Li, G.-X. Xu, Electrochem. Solid-State Lett. 9
(2006) A1–A4.
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[7] P.Y. Zavalij, S. Yang, M.S. Whittingham, Acta Cryst. B59 (2003) 753–
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[8] JCPDS-ICDD, 00-024-0646.
[9] L. Larush-Asraf, M. Biton, H. Teller, E. Zinigrad, D. Aurbach, J. Power
Sources, in press.
The study of LiBOB decomposition by ARC, DSC and TGA
at elevated temperatures, the analysis of the decomposition prod-
ucts by XRD, FTIR and element analysis (ICP), and further
thermal studies of mixtures of Li oxalate and B2O3 showed that
LiBOB decomposes in an irreversible endothermic reaction at
an onset temperature of 293 4 ◦C according to the following
equation:
6LiBC4O8(cryst) → 2Li2C2O4(cryst) + 2LiB3O5(glass)
+ 10CO(gas) + 10CO2(gas).
[10] K. Wakatsuki, S.P. Fuss, A. Hamins, M.R. Nyden, Proceedings of the
Combustion Institute 30, 2005, pp. 1565–1573.
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303.
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1175–1179.
[13] U. Moryc, W.S. Ptak, J. Mol. Struct. 511–512 (1999) 241–
249.
[14] B.S.R. Sastry, F.A. Hummel, J. Am. Ceram. Soc. 41 (1958) 7–17.
[15] Yu.F. Shepelev, R.S. Bubnova, S.K. Filatov, N.A. Sennova, N.A. Pilneva,
J. Solid State Chem. 178 (2005) 2987–2997.
This reaction occurs in two stages in the same temperature
range:
Stage 1: 6LiBOB(cryst) → 3Li2C2O4(cryst) + 3B2O3(glass)
+ 9CO(gas) + 9CO2(gas);
Stage 2: Li2C2O4(cryst) + 3B2O3(glass) → 2LiB3O5(glass)
+ CO(gas) + CO2(gas).
References
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1999.