What are the hazards of overcharging and overdischarging lithium batteries?

　　Overdischarge refers to the discharge when the voltage reaches the rated voltage and the discharge continues. For example, the rated discharge voltage of a ternary lithium battery is 3.2V. If the discharge voltage is lower than 3.2V, the discharge is continued. What harm does over-discharge have to the battery?

　　26650 lithium battery

　　After the battery has discharged the internally stored power, after the voltage reaches a certain value, continued discharging will cause overdischarge. Overdischarge of the battery may bring catastrophic consequences to the battery, especially high current overdischarge or repeated overdischarge. 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 partially recover, and the capacity will be significantly reduced. .

　　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.

　　Strictly speaking, charging the voltage of a lithium-ion battery above the limit B (4.20V) is an overcharge. However, overcharge also has a degree of magnitude. Generally speaking, it is not considered as an overcharge if it is below 4.24V. Accept the micro-overcharge.

　　When the lithium-ion battery has been continuously overcharged 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. When I tested this type of battery in a normal cycle, its capacity was still more than 88% of the rated capacity after 150 cycles. It can be seen that charging 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 its thickness is 4.25mm after 50 cycles.

　　Because the voltage above 4.35V will not reach the lithium-ion battery under the protected circuit. All users often encounter the phenomenon of medium overcharge. Many unqualified egg chargers (often nickel The hydrogen and lithium ion gear can be selected) is the culprit of overcharge.

　　So what would an overcharge above 4.35V look like?

　　When I did the charging safety test, I removed the battery protection circuit, and then charged the lithium-ion battery cell with a voltage of 5.0V.

　　As a result, after 3 to 4 hours, the battery is severely swollen.

　　And some unqualified batteries 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 whose shape 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.

　　Overcharge 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 of metallic lithium with the solvent and the lithium-intercalated carbon with the solvent occur successively, causing the battery to catch fire and explode. With the decomposition of the electrolyte, the binding agent can also react with lithium metal.

　　After overcharging, needle-like lithium metal crystals are everywhere on the pole piece, and a micro short circuit will occur if 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, all ended in a combustion explosion.

　　The composition of lithium cobalt oxide battery materials, LiCoo2 basically uses the liquid phase synthesis process of layered structure lithium ion secondary battery cathode material (lithium cobalt oxide) in the current commercial lithium ion batteries. It uses polyvinyl alcohol (PVA) ) Or polyethylene glycol (PEG) aqueous solution as the solvent. Lithium salt and cobalt salt are respectively dissolved in PVA or PEG aqueous solution. The mixed solution is heated and concentrated to form a gel. The resulting gel is heated and decomposed. Burning at high temperature, grinding and sieving the burned powder to obtain lithium sulphate powder. Compared with the prior art, the invention has the advantages of low synthesis temperature, high purity of the obtained product, uniform chemical composition and the like.