The hazards of overcharging and discharging in lithium batteries

Overdischarge refers to the discharge of a ternary lithium battery when the voltage reaches the rated voltage and continues to discharge. For example, if the rated discharge voltage of a ternary lithium battery is 3.2V and continues to discharge below 3.2V, it is called overdischarge. What are the hazards of excessive discharge to batteries?

After discharging the stored energy inside the battery, if the voltage reaches a certain value and continues to discharge, it will cause over discharge. Over discharge of the battery may bring catastrophic consequences, especially high current over discharge or repeated over discharge, which have a greater impact on the battery. Generally speaking, over discharge can increase the internal pressure of the battery, disrupt the reversibility of positive and negative active substances, decompose the electrolyte, deposit lithium on the negative electrode, increase resistance, and even if charged, only partially recover, resulting in a significant decrease in capacity.

The important consequence of over discharge is the layered collapse of the negative electrode plate. When recharging, it limits the number and convenience of lithium ions embedded in the negative electrode plate. A decrease in capacity, an increase in internal resistance, and a shortened lifespan cannot be restored.
Strictly speaking, overcharging a lithium-ion battery to a voltage higher than the threshold B (4.20V) is considered overcharging. However, overcharging can also be of varying degrees. Generally, it is not considered overcharging when the voltage is below 4.24V, which is also known as "acceptable" micro overcharging.

The conclusion is not so optimistic when the lithium-ion battery continues to overcharge at a limiting voltage of 4.35V. After the 50th cycle, the battery capacity was 480mAh, which was already 85% of the rated capacity. However, when I conducted normal cycle tests on this type of battery before, its capacity was still above 88% of the rated capacity after 150 cycles. It can be seen that overcharging has seriously shortened the battery's cycle life. Additionally, it should be mentioned that the overcharged battery has slightly bulged, with an original thickness of 3.84mm and a thickness of 4.25mm after 50 cycles

Due to the fact that a voltage above 4.35V is not achievable for lithium-ion batteries with protected circuits, all users often encounter moderate overcharging. Many unqualified egg chargers (often with nickel hydrogen and lithium-ion levels to choose from) are the culprits of overcharging.

So what would an overcharge above 4.35V look like?
When I conducted charging safety tests, I removed the protective circuit of the battery and charged the lithium-ion battery cells with a voltage of 5.0V.
The result is that after 3-4 hours, the battery becomes severely swollen,
And some unqualified battery cells have experienced explosions.
Under an electron microscope, the negative electrode of a lithium battery is a layered structure, while the positive electrode is a pile of angular crystals with different shapes depending on the anode material.

The important consequence of over discharge is the layered collapse of the negative electrode plate. When recharging, it limits the number and convenience of lithium ions embedded in the negative electrode plate.
Capacity reduction, increased internal resistance, and shortened lifespan cannot be restored.
Overcharging is even more terrifying!

The battery reaches a fully charged state. The negative electrode undergoes a reaction of lithium ion insertion, which leads to the deposition of lithium metal on the surface of the negative electrode. The solvent is oxidized (the heat released by solvent oxidation caused by overcharging is much higher than the heat released by the reaction between lithium ion and solvent in a reversible state). As the battery temperature increases, the reactions between metallic lithium and solvent, as well as between lithium embedded carbon and solvent, occur successively, causing the battery to ignite and explode. As the electrolyte decomposes, the binding agent can also react with lithium metal.

After overcharging, needle shaped lithium metal crystals are everywhere on the electrode, which can cause micro short circuits when puncturing the diaphragm. Mildly, exacerbate self discharge; The short-circuit current of crystal branches causes a sharp increase in battery temperature, and the electrolyte decomposes and gasifies. In this situation, whether it is the material burning and exploding due to excessive temperature, or the shell being stretched out first, causing air to enter and undergo intense oxidation with lithium metal, it all ends with combustion and explosion.