li-ion battery separators, mechanical integrity and failure mechanisms leading to soft and hard internal shorts - li ion battery cell
The integrity of the separator is an important factor in preventing internal short circuit in lithiumion batteries.
Local penetration test (
DingTalk or cone punch)
Usually sporadic results are produced in completely similar cells and test sets
Ups one battery has a heat out of control while the other has a very small reaction.
We conducted an experimental study of the separator under mechanical load and found two different deformation and failure mechanisms, which can explain the difference in short circuit properties of other similar tests.
In addition, through the study of failure patterns, we provide a hypothesis about the process of forming a local "soft short circuit" in cells that cannot detect failure.
Finally, we propose the standard to predict the soft and short start from the experimental data.
The separator was purchased from MTI company and the specifications are given in.
One-way tensile tests were carried out with 5 strip samples of different widths: 5mm, 10mm, 15mm, 20mm and 25mm.
Sharp razor for specimen (
No visible tear along the cutting edge)
Repeat five times in each direction along MD, TD and DD.
According to the ASTM D882 of the film, the separator shall cut the uniform width in the range of 5mm to 25. 4u2009mm.
With the limited height of the original separator roller, the total length of the Strip sample is fixed at 60mm and the specification length is 35mm.
For Polymer separators that usually do not have obvious local neck shrinkage before failure in one-way tensile tests, fracture determines the range of material elasticity and plastic curves.
To eliminate edge damage, the separator is clipped to 5-
Mm Descartes graphic paper for cutting.
Strip samples were tested on the Instron 5944 one-way stretch machine with a 100 N load cell and nominal strain was obtained from digital image correlation (DIC)method (Vic 2D, 2009).
The loading speed is 25mm/min.
For compression testing, cut the sample with a 16mm diameter hammer
Drive the punch once from the folded partition to make sure all layers have the same direction.
Stacked samples were tested on MTS single-axis testing machine and tested with a 200 kN load cell at a strain rate of 0.
012/s in order to have the same strain rate as the single-axis tensile test.
The two-way punch head test was carried out with a punch size of 1/8 to 1 inch.
In order to minimize contact friction, the punch is processed with Teflon bulk material.
Use a hammer with a diameter of 45mm to drive the hole cutter to cut the sample.
Sample Test diameter (See figure A1 B)was 32u2009mm.
Each type of separator was tested with four different punches.
Repeat five times per test scenario.
The loading speed is selected at 12 mm/min to ensure that the strain rate at the start of the fracture is comparable to that of the single-axis tensile test.
The fixture is shown in the figure.