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The so-called overcharging is the process of continuing charging beyond the specified charge termination voltage (generally 4.2V). In the case of overcharging, the battery capacity will be attenuated, mainly due to the following factors: ①overcharging reaction of graphite anode; ②overcharging reaction of positive electrode; ③oxidation reaction of electrolyte during overcharging When the battery is overcharged, lithium ions are easily reduced and deposited on the surface of the negative electrode: Li + + e → Li (s)
Samsung 21700 Battery
The deposited lithium coats the surface of the negative electrode, blocking the insertion of lithium. The reasons for the reduction of discharge efficiency and capacity loss are: ①reduced amount of recyclable lithium; ②reaction of deposited metal lithium with a solvent or supporting electrolyte to form Li2CO3, LiF or other products; ③metal lithium is usually formed between the negative electrode and the separator, possibly Blocking the pores of the separator increases the internal resistance of the battery. Rapid charging, the current density is too large, the negative electrode is severely polarized, and the deposition of lithium will be more obvious. The capacity loss caused by overcharge of the positive electrode is mainly due to the generation of electrochemically inert substances (such as Co3O4, Mn2O3, etc.), which disrupts the capacity balance between the electrodes, and its capacity loss is irreversible.
At the same time, the oxygen generated by the decomposition of the cathode material in the sealed lithium-ion battery will accumulate at the same time due to the absence of recombination reactions (such as the generation of H2O) and the flammable gas generated by the decomposition of the electrolyte. The consequences will be unimaginable. Overcharge will also lead to the oxidation reaction of the electrolyte. The oxidation rate has a great relationship with the surface area of the positive electrode material, the current collector material and the added conductive agent (carbon black, etc.). At the same time, the type of carbon black and the surface area are also An important factor affecting the oxidation of the electrolyte, the larger the surface area, the easier the solvent to oxidize on the surface. When the pressure is higher than 4.5V, the electrolyte will oxidize to form insolubles (such as Li2Co3) and gas. These insolubles will block the pores of the electrode to hinder the migration of lithium ions and cause capacity loss during cycling.
2. Decomposition of electrolyte
The electrolyte is composed of a solvent and a supporting electrolyte. After the cathode is decomposed, insoluble products Li2Co3 and LiF are usually formed. The battery capacity is reduced by blocking the pores of the electrode. The gas generated by the reduction will increase the internal pressure of the battery, resulting in safety problems. The electrolyte is not stable on graphite and other lithium-embedded carbon anodes, and it is easy to react to produce irreversible capacity. The decomposition of the electrolyte during the initial charge and discharge will form a passivation film on the surface of the electrode. The passivation film can separate the electrolyte from the carbon anode and prevent further decomposition of the electrolyte. Thus maintaining the structural stability of the carbon anode. Under ideal conditions, the reduction of the electrolyte is limited to the formation of the passivation film, and this process no longer occurs when the cycle is stable.
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