Page 428 - Handbook of Battery Materials

400 13 Rechargeable Lithium Anodes

13.8.2
Safety Test Results
13.8.2.1 External Short
We experienced no safety problems during the external short tests because of the
Polyswitch inside the cell. We confirmed that even if the Polyswitch fails to operate,
the short-circuit current stops flowing before thermal runaway occurs because
◦
the micropores are closed by the polyethylene separator, which melts at 125 C
(‘separator shutdown’).

13.8.2.2 Overcharge
In the overcharge tests we carried out, there was no fire or explosion. The cell
impedance increased suddenly in every test. This was due to the oxidation of the
electrolyte with a low charging current, or to the separator melting with a high
charging current. In practical applications, an electronic device should be used to
provide overcharge protection and ensure complete safety.

13.8.2.3 Nail Penetration
The nail penetration test is very important and is considered to simulate an internal
short in a cell. No electronic device can protect against an internal short, so the cell
itself should pass this test. Our cell did not ignite or explode.

13.8.2.4 Crush
The cell should also be able to survive a crush test because an electronic device
cannot provide protection in this case either. Our test cell remained safe in crush
tests, both with a bar and with a flat plate.
13.8.2.5 Heating
◦
The heating test is carried out by increasing the temperature at a rate of 5 C
min −1 and then holding it constant at least until the maximum cell temperature
induced by the internal exothermic reactions starts to decrease. If the thermal
stability decreases after cycling, we have to be careful when estimating the safety.
The thermal stability of our cell is defined by the maximum temperature at which
◦
it can be ensured that no fire will occur. For our cell, this is 130 C before cycling.
The thermal stability limit becomes even higher after cycling. These results are
considered to be closely related to the increase in the thermal stability of a lithium
anode with an increase in the number of charge–discharge cycles as the result of
the formation of a special lithium surface film containing vanadium.

13.9
Conclusion
It is worthwhile attempting to develop a rechargeable lithium metal anode. This
anode should have a high lithium cycling efficiency and be very safe. These