The safety of batteries is determined by several parameters, primarily the potential difference between the two electrodes – the anode and cathode. The higher this difference, the more dangerous the system. The second important factor is the presence or absence of organic components in the battery. For example, aqueous electrolytes do not burn, unlike organic ones, and cannot withstand a potential higher than 1.3–1.4 volts. The energy capacity of batteries depends on the potential difference. The higher it is, the more energy per unit mass or volume. The third parameter is the release of combustible gases from the system. During side reactions, oxygen or hydrogen can be formed and accumulated. In this case, the slightest spark will cause the battery to explode.
Lead-acid batteries are the most common these days. They have a low energy density and a lot of weight, so they are more suitable for energy storage. Often they are used in cars to start an internal combustion engine. Lithium-ion batteries are best suited for electric vehicles and mobile devices. Many people think that such batteries contain lithium metal, but this is not the case. Lithium is in them in an ionized form.
In lithium-ion batteries, there is a layer of organic electrolyte between the anode and cathode. The most common electrolyte is a mixture of organic carbonates containing LiPF6 – active and thermally unstable compound. This is the most dangerous part of the battery. At temperatures above 60°C, the electrolyte begins to slowly decompose. In this case, the safety of the battery depends on two factors: firstly, on the difference in the potentials of the cathode and anode, and secondly, on the chemical nature of the cathode material itself. There are fundamentally hazardous materials that are chemically unstable and can release oxygen with slight heating or a change in chemical potential. Lithium cobaltate belongs to this type of compounds. Most accidents, fires and explosions occur on this type of battery.
An explosion may occur due to the unsuccessful design of the device. Samsung was forced to write off a whole series of phones due to improper battery placement . An explosion is also possible in a situation where the cathode is connected to the anode. A short circuit occurs inside the system – a large amount of heat is released. This can happen as a result of a manufacturing defect. The electrode particles peel off and can connect the two electrodes together. In this case, overheating, evaporation and decomposition of the electrolyte and, accordingly, gas evolution and explosion will occur.
Another possible cause of a battery explosion is incorrect charging. If the battery is discharged or not charged very quickly, the lithium will not completely enter the carbon. When the battery is drained very quickly, dendrites can grow – thin threads of lithium that stretch from one electrode to another.
The battery may explode due to mechanical impact. If the battery walls are not strong enough, the cathode and anode will close together and an explosion will occur. In all cases of connecting the cathode to the anode, a short circuit occurs, heating and decomposition of the electrolyte. After that, in the best case, evaporation occurs. Some systems have special valves that release excess electrolyte so that it does not explode. The most important thing in large systems is heat dissipation between individual cells. If there are many such cells, they are close to each other and there is no heat sink between them, then they heat up.
The battery should be charged and discharged only in normal mode. In modern large batteries, this is monitored by electronics and prevents violation of the regime. There is another system that is being introduced inside powerful batteries, in which the electrolyte is in a separator – a porous film.
When a manufacturer releases a battery, it goes through two installation cycles. During this time, a thin protective film forms on the anode. It is she who prevents the germination of lithium. If it is not violated and the temperature is not raised, lithium will not be able to germinate. The manufacturer provides this in the first cycle.
Now work is underway to create completely safe batteries. Scientists are developing a system in which a liquid organic electrolyte will be replaced by a glassy or ceramic one. But so far this system has not been brought to a device that would demonstrate its characteristics.
