What is the reason why lithium-ion batteries cannot be overcharged and over-discharged

Strictly speaking, charging the voltage of a lithium-ion battery above the limit B (4.20V) is overcharge. However, overcharge also has a degree of magnitude. Generally speaking, it is not considered as overcharge if it is below 4.24V. Or it is acceptable. Lithium-ion battery cell manufacturers’ battery specifications generally indicate that the charging limit voltage is 4.20V+/-0.04V. Between the limit A and the limit B (above 4.24V, less than 4.35V) This area can be called medium overcharge. The discharge capacity of lithium-ion batteries in this voltage range will be higher than that of normal lithium-ion batteries. Please take a look at the data of the medium overcharge test. The test condition is the battery cell: GY383450. Nominal capacity: 550mAh. Nominal charging limit voltage 4.20V Charging system: when the voltage is lower than the charging limit voltage, charge at 1C (550mA), after the voltage reaches the charging limit voltage, the voltage remains unchanged, and the current is gradually reduced to 20mA before charging Termination. This is the conventional CC/CV charging mode. Discharge system: 1C (550mA), discharge termination voltage 2.75V. We can try to change the charging limit of the lithium-ion battery voltage and deliberately overcharge this test product. List the test table As follows: the number of cycles charge limit voltage V charge capacity mAh discharge capacity mAh to the ratio of rated capacity% The first time 4.20V570565 This is the rated capacity The second time 4.30V610608108% The third time 4.20V569564100% The fourth time 4.35V633627112% The three times are for comparison, indicating that the rated capacity of the battery has hardly changed after one overcharge. A conclusion can be drawn from this that overcharge can increase the capacity of the battery. But this trend is not consistent. When I put this battery After continuing the overcharge cycle with the charging limit voltage of 4.35V, the conclusion is not so optimistic. After the 50th time, the battery capacity is 480mAh. It is already 85% of the rated capacity. And I used to carry out this type of battery In the normal cycle test, its capacity is still more than 88% of the rated capacity at 150 cycles. It can be seen that overcharge has severely shortened 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 after 50 cycles, the thickness is 4.25mm. Because the voltage above 4.35V will not reach the lithium-ion battery protected by the circuit. All users often encounter moderate This phenomenon of overcharging. Many unqualified egg chargers (often with Ni-MH and Li-ion gears to choose from) are the culprits of overcharging. So what will the overcharging of higher than 4.35V look like. I am doing During the overcharge safety test, the protection circuit of the battery is removed, and then the lithium-ion battery cell is charged with a voltage of 5.0V. As a result, after 3 to 4 hours, the battery is seriously bulging, and some of it is unqualified The battery cell exploded. Under an electron microscope, the negative electrode of a lithium battery has a layered structure, and the positive electrode is a stack of angular crystals, the shape of which varies with the anode material. The important consequence of over-discharge is the laminar 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 solvent oxidation caused by overcharge is much higher than the heat released by the reaction between lithium ions and the solvent in the reversible state): As the temperature of the battery rises, the reactions between metallic lithium and the solvent, and lithium-intercalated carbon and the solvent occur one after another, causing the battery to catch fire and explode. As the electrolyte decomposes, the binding agent can also react with lithium metal. After overcharge, needle-like lithium metal crystals are everywhere on the pole piece, and a micro short circuit will occur when the diaphragm is pierced. In the lighter case, self-discharge is intensified; in the worst case, the crystal branch short-circuit current causes the battery temperature to rise sharply, and the electrolyte decomposes and vaporizes. In this case, no matter if the temperature is too high, the material will burn and explode. The outer shell was broken first, causing the air to enter and oxidize the lithium metal violently, which ended in a combustion and explosion. Disclaimer: Some pictures and content of articles published on this site are from the Internet. If there is any infringement, please contact to delete. Previous article: How many specifications of button batteries are there?