LAAN-A-TA-E010
A pplication News
Ther ma l An a lysis
N o . T146
Measurement of Separator in Lithium-Ion Battery
■ Introduction
The lithium-ion battery is widely used as a power
supply for portable electronic devices. It is believed
that its future applications will be even further
expanded to include electric vehicles and largescale equipment.
A fine porous polyolefin film is generally used as the
separator in a lithium-ion battery, and serves to
isolate the anode and cathode while preventing
electrons from flowing directly between the anode
and cathode, as well as to allow the electrolyte or
ions to pass through. Accordingly, these properties
are important with respect to the lithium-ion
battery’s performance as well as safety. Here we
introduce the physical properties test results using
both DSC and TMA measurement.
■ Measurement by DSC
The separators were removed from three different
kinds of lithium-ion batteries for cellular phone, and
were measured by DSC. An endothermic peak caused
by the melting of polyethylene was measured in the
region of 100 – 150 ˚C for all separators. The melting
peak temperature becomes higher in the order of
➀<➂<➁. It is believed that the separator contracts at
around this temperature, but when the battery
generates an abnormal amount of heat, it is obviously
DSC-60
TMA-60
preferable from the standpoint of safety that
contraction of the separator occurs at a higher, rather
than lower temperature. A small peak is observed at
around 160 ˚C in sample ➁, which may be caused by
the melting of polypropylene at trace levels in this
separator. Since the heat of fusion is proportional to
the degree of crystallinity, the degree of crystallinity of
these separators increases in the order of ➀<➁<➂.
DSC
mW
4.00
2.00
➀
➁
➂
Heat of fusion -130.18 J/g
2.00 mg
160.4 ˚C
1.80 mg Heat of fusion -164.97 J/g
1.98 mg Heat of fusion -182.80 J/g
128.8 ˚C
0.00
-2.00
135.1 ˚C
134.4 ˚C
-4.00
50.00
100.00
Temp [˚C]
Fig. 1 DSC Measurement Results
150.00
No.T146
■ Measurement by TMA
The changes in dimension that occurred when heating
the separators were measured by TMA. (➀ and ➁
correspond to the same samples as those in DSC
measurement.) Fig. 2 shows the results of film
measured by TMA in elongation mode in the MD
direction; 1 g force of tensile load was applied during
heating to keep the film straightened. It is clear that
each sample begins to contract at around 100 ˚C, and
exhibits expansion beyond 150 ˚C. When separator
film begins to melt, it contracts. This corresponds to
the fact that when the film stretches during the film
forming process, it crystallizes. By comparing the
results for samples ➀ and ➁ , it is seen that sample
contracts at a higher temperature, which agrees with
the DSC measurement results.
TMA
um
2000
[Temperature Program]
Heating Rate Hold Temperature Hold Time
[˚C/min ]
[ ˚C ]
[ min ]
10.00
200.0
0
0
➁
MD
-2000
➀
-4000
TD
50
100
Temp [˚C]
150
Fig. 2 TMA Measurement Data (MD)
Fig. 3 shows the results of film measured by TMA in
elongation mode in the TD direction. Both samples
exhibit contraction, although the amount of contraction
is less than that in the MD direction, and sample ➀
shows less contraction than sample ➁. From the
standpoint of safety, it is a drawback that ➀ contracts
at a lower temperature. Yet this is offset by the
advantage that its amount of contraction in the TD
direction is smaller than that of sample ➁ .
TMA
um
2000
1000
[Temperature Program]
Heating Rate Hold Temperature Hold Time
[˚C/min ]
[ ˚C ]
[ min ]
10.00
200.0
0
➀
0
-1000
➁
-2000
-3000
50
100
Temp [˚C]
150
Fig. 3 TMA Measurement Data (TD)
Stress
direction, and ➁ in the TD direction. In the TD direction,
sample ➀ shows higher strain up to the point of
breaking and higher elasticity. This, together with the
smaller amount of contraction seen during heating in
the TMA testing of Fig 3, leads to the expectation that
elongation of ➀ will be smaller in the TD direction
during forming of the film than sample ➁ .
Stress
■ Measurement by Testing Machine
We compared the tensile strength of separators ➀ and
➁ using the AG-X universal testing machine. Fig. 4 and
Fig. 5 show the results of measurement in the MD and
TD direction, respectively. The results for the MD
direction were ➀175.6 N/mm2 and ➁129.5 N/mm2, and
in the TD direction, ➀36.9 N/mm2 and ➁78.2 N/mm2.
Higher tensile strength is seen for ➀ in the MD
Strain
Fig. 4 Stress - Strain Curves (MD)
Strain
Fig. 5 Stress - Strain Curves (TD)
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