Power Battery Safety Design Series No. 3 - Safety Related Testing

1、 Short circuit

External short circuit: For the national standard test, the external short circuit corresponds to the battery fully charged for testing, external internal resistance less than 5mΩ, short circuit time 10min, observation 1h, the battery is required not to catch fire and not to explode. More stringent is the IEC62133 standard for external short-circuit requirements, need to be carried out at 55 ℃, which undoubtedly increases the difficulty. The failure mechanism is also well understood, when the external short circuit occurs, the instantaneous generation of high current through the internal battery, the temperature rises sharply, the electrolyte decomposition produces a large amount of gas, the internal pressure of the battery increases, eventually leading to burst spray or fire, explosion, battery failure.

How to reduce the chances of battery failure when an external short circuit occurs? For metal-cased batteries, the explosion-proof valve, the FUSE on the battery cover and other safety measures to avoid battery failure, but still need a lot of experiments to explore the conditions and boundaries of battery failure.

Internal short circuit: For the national standard, the internal short circuit corresponds to the test for pinprick and extrusion, but with the increase in energy density, pinprick basically everyone can not pass, so it was canceled, but in some international standards are still required. Extrusion is generally passed by the standard is the battery deformation or the battery force to meet the standard can be. The failure mechanism is currently considered to be the internal short circuit of the battery leading to direct contact between the positive and negative electrodes, which instantly generates a large amount of heat, triggering a chain reaction and eventually leading to failure, involving multiple reactions, which phase corresponds to which reaction is difficult to explain, and it is difficult to simulate the actual situation during the actual test.

There is also a special research institute abroad to simulate the internal short circuit, and it is concluded that the direct short circuit between positive aluminum foil and negative active material is the main cause of thermal failure. So, how to reduce the probability of internal short circuit? This requires the control of raw materials, process technology, testing methods and so on. The raw materials and process technology have already been talked about before, so I am not repeating here, but the detection is throughout the whole battery manufacturing process, how to screen out the risk of internal short circuit from the early stage? This requires some high-precision equipment that can sensitively detect impurities, particles inside the battery, as well as the decline in the electrical properties of the battery caused by this, including each step of the voltage insulation test, battery aging test after the completion of production, internal defects of high-precision electrochemical detection, etc., can reduce the risk of short power battery.

2、Heating

For the national standard, heating is actually the ability of the battery to be placed in a certain temperature for a period of time, and eventually will not fail.

For different components, the temperature of thermal failure is different. For SEI film, the temperature is higher than 130℃ and it starts to decompose gradually.

Different cathode materials have different thermal stability. The thermal stability of high nickel materials is around 200°C. As the temperature rises, the cathode materials start to decompose and release oxygen, which further aggravates the failure.

For electrolyte, the reaction also occurs gradually with the increase of temperature.

Thus, there are two ideas to improve the heating failure, one is to solve the problem from the raw material side, such as improving the thermal stability of the cathode and anode materials, increasing the decomposition temperature of the electrolyte, improving the film-forming additives, etc. On the other hand, the problem needs to be solved from the use side, such as providing good heat dissipation paths and channels, specifying the maximum current and temperature range for the battery, adopting liquid cooling system for the whole pack, etc. Liquid cooling system, etc., can significantly reduce the probability of thermal failure of the battery.

2、Overcharge

Overcharge corresponds to the national standard is reduced year by year, first from 2 times the upper voltage to 1.5 times, the follow-up is reported to be reduced to 1.2 times, but also from the side to reflect the high energy density cells over the difficulty of this test. The failure mechanism is charged to a certain extent, electrolyte decomposition gas production, lithium precipitation on the surface of the cathode, diaphragm thermal contraction and other reflections triggered by internal short circuit, which in turn triggered a chain reaction, resulting in thermal failure, and ultimately the battery fire and explosion.

So, how to effectively reduce the risk of battery overcharge? In fact, it is also necessary from the material, design, use and other aspects to complete this thing. At the raw material end, the doping of materials, improving the thermal stability of raw materials, using non-flammable electrolyte, anti-overcharge additives, etc., at the design end, the explosion-proof valve design on the battery cover, reverse valve design, etc., at the use end, the BMS sampling accuracy control, timely alarm, etc., can effectively reduce the risk of overcharging.