Analysis of the cause of battery capacity loss? (1)

Date:2020-12-07
Summary:

Overcharge The so-called overcharging is the process of […]

Overcharge
The so-called overcharging is the process of continuing to charge after exceeding the specified charge termination voltage (usually 4.2V). In the case of overcharge, the battery capacity will be attenuated, mainly due to the following factors: ① the overcharge reaction of the graphite negative electrode; ② the positive electrode overcharge reaction; ③ the electrolyte oxidation reaction during overcharge. When the battery is overcharged, lithium ions are easily reduced and deposited on the surface of the negative electrode: Li++e→Li(s)
The deposited lithium coats the surface of the negative electrode, blocking the insertion of lithium. The reasons for the decrease in discharge efficiency and capacity loss are: ①The amount of recyclable lithium is reduced; ②The deposited metallic lithium reacts with the solvent or supporting electrolyte to form Li2CO3, LiF or other products; ③The metallic lithium is usually formed between the negative electrode and the separator, which may Blocking the pores of the separator increases the internal resistance of the battery. Fast charging, excessive current density, serious polarization of the negative electrode, and more obvious lithium deposition. The capacity loss caused by overcharge of the positive electrode is mainly due to the production of electrochemical inert substances (such as Co3O4, Mn2O3, etc.), which destroy the capacity balance between the electrodes, and the capacity loss is irreversible.
LiyCoO2→(1-y)/3[Co3O4+O2(g)]+yLiCoO2 y<0.4
At the same time, the oxygen generated by the decomposition of the cathode material in the sealed lithium-ion battery does not have a recombination reaction (such as the generation of H2O) and the flammable gas generated by the decomposition of the electrolyte accumulates at the same time, and the consequences will be unimaginable. Overcharge will also cause the oxidation reaction of the electrolyte. The oxidation rate has a lot to do 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 and surface area of ​​the carbon black are also An important factor affecting the oxidation of electrolyte, the larger its surface area, the easier it is for the solvent to oxidize on the surface. When the pressure is higher than 4.5V, the electrolyte will oxidize to generate insolubles (such as Li2Co3) and gas. These insolubles will block the micropores of the electrode and hinder the migration of lithium ions, causing capacity loss during the cycle.
2. Electrolyte decomposition
The electrolyte is composed of a solvent and a supporting electrolyte. After the positive electrode is decomposed, insoluble products such as Li2Co3 and LiF are usually formed. The battery capacity is reduced by blocking the pores of the electrode. The reduction reaction of the electrolyte will have an adverse effect on the battery capacity and cycle life. The gas generated by the reduction will increase the internal pressure of the battery, causing safety problems. The electrolyte is not stable on graphite and other lithium-intercalated carbon negative electrodes, 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 negative electrode and prevent further decomposition of the electrolyte. Thereby maintaining the structural stability of the carbon negative electrode. Under ideal conditions, the reduction of the electrolyte is limited to the formation stage of the passivation film, and this process does not occur after the cycle is stable. The reduction of the electrolyte salt participates in the formation of the passivation film, which is conducive to the stabilization of the passivation film, but the insolubles produced by the reduction will have an adverse effect on the solvent reduction products, and the concentration of the electrolyte decreases during the reduction of the electrolyte salt, which will eventually lead to Battery capacity loss (LiPF6 reduction produces LiF, LixPF5-x, PF3O and PF3). At the same time, the formation of the passivation film consumes lithium ions, which will cause the capacity imbalance between the two electrodes and reduce the specific capacity of the entire alkaline battery . The type of carbon used in the process, the composition of the electrolyte, and the additives in the electrode or electrolyte are all factors that affect the loss of film formation capacity. The electrolyte often contains substances such as oxygen, water and carbon dioxide. A small amount of water has no effect on the performance of the graphite electrode, but too high water content will generate LiOH(s) and Li2O deposits, which are not conducive to lithium ion insertion and cause irreversible capacity loss: H2O+e→OH-+1/2H222 OH-+Li+→LiOH( s) LiOH+Li++e→Li2O(s)+1/2H2
The CO2 in the solvent can be reduced to CO and LiCO3(s) on the negative electrode: 2CO2+2e+2Li+→Li2CO3+CO
CO will increase the internal pressure of the battery, while Li2CO3(s) will increase the internal resistance of the battery and affect the battery performance.