What are the dangers of overcharging and overdischarging lithium batteries?
Overdischarge
When discharging, the voltage reaches the rated voltage and continues to discharge. For example, the rated discharge voltage of a ternary lithium battery is 3.2V. If it is lower than 3.2V and continues to discharge, it is over-discharged.
After the battery discharges the internal stored power and the voltage reaches a certain value, continuing to discharge will cause over-discharge. Over-discharge of the battery may bring disastrous consequences to the battery, especially over-discharge with large current, or repeated over-discharge will affect the battery bigger. Generally speaking, over-discharge will increase the internal pressure of the battery, destroy the reversibility of the positive and negative active materials, decompose the electrolyte, deposit lithium on the negative electrode, and increase the resistance. Even if it is charged, it can only be partially recovered, and the capacity will also decline significantly. .
The main consequence of over-discharge is the layered collapse of the negative plate. When recharging, the quantity and convenience of lithium ions embedded in the negative plate are limited. Decreased capacity, increased internal resistance, and shortened life cannot be recovered.
Strictly speaking, charging the voltage of a lithium-ion battery above the limit (4.2V) is overcharging. But there is also a degree of overcharge. Most people think that it is not overcharged below 4.24V. Or it is called "acceptable" micro-overcharge.
When the lithium-ion battery has been charged with a charge limit voltage of 4.35V to continue
After the overcharge cycle. The conclusion is not so optimistic. After the 50th cycle, the battery capacity is 480mAh, which is already 85% of the rated capacity. When I tested this type of battery for normal cycles, its capacity was still above 88% of the rated capacity at 150 cycles. It can be seen that overcharging seriously shortens the cycle life of the battery. In addition, it should be mentioned that the overcharged battery has slightly bulged, the original thickness is 3.84mm, and the thickness after 50 cycles is 4.25mm.
Due to the voltage above 4.35V on the lithium-ion battery of the protected line
For the battery, it will not reach. All users often encounter the phenomenon of moderate overcharging. Many unqualified egg chargers (often have Ni-MH and Li-ion on it)
The gear can be selected) is the culprit of overcharging.
So what would an overcharge above 4.35V look like.
When I did the charging safety test, I always removed the protection circuit of the battery, and then charged the lithium-ion battery core with a voltage of 5.0V.
As a result, after 3 to 4 hours, the battery is seriously swollen,
Moreover, some unqualified batteries exploded.
Under the electron microscope, the negative electrode of the lithium battery has a layered structure, and the positive electrode is a collection of angular crystals, and the shape varies with the anode material.
The main consequence of over-discharge is the layered collapse of the negative plate. When recharging, the quantity and convenience of lithium ions embedded in the negative plate are limited.
Decreased capacity, increased internal resistance, and shortened life cannot be recovered.
Overcharging is even more terrible!
The battery is fully charged. The negative electrode changes from the intercalation reaction of lithium ions to the deposition of lithium metal on the surface of the negative electrode, and the solvent is oxidized (the heat released by the oxidation of the solvent caused by overcharging is much higher than that released by the reaction of lithium ions with the solvent in the reversible state.
heat): As the temperature of the battery rises, the reactions between lithium metal and solvent, lithium-intercalated carbon and solvent occur one after another, and the battery catches fire and explodes. As the electrolyte decomposes, the binder and lithium metal can also react.
After overcharging, needle-shaped lithium metal crystals are everywhere on the pole piece, and a micro-short circuit will occur if the diaphragm is pierced. In light cases, the self-discharge will be aggravated; in severe cases, the short-circuit current of dendrites will cause the battery temperature to rise sharply, and the electrolyte will decompose and gasify.
In this case, whether the temperature is too high to make the material burn and explode. Or the shell was broken first, allowing air to enter and violently oxidize the lithium metal, all of which ended in combustion and explosion.