What are the important reasons for the capacity degradation of lithium-ion batteries?

Reason 1: Overcharge 1. Overcharge reaction of graphite negative electrode: When the battery is overcharged, lithium ions are easily reduced and deposited on the surface of the negative electrode: 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 metal lithium reacts with the solvent or supporting electrolyte to form Li2CO3, LiF or other products; ③The metal lithium is usually formed between the negative electrode and the separator, which may Blocking the pores of the diaphragm increases the internal resistance of the battery; ④Because of the active nature of lithium, it is easy to react with the electrolyte and consume the electrolyte. This leads to a decrease in discharge efficiency and a loss of capacity. Fast charging, excessive current density, serious polarization of the negative electrode, and lithium deposition will be more obvious. This situation tends to occur when the positive electrode active material is excessive relative to the negative electrode active material. However, in the case of a high charging rate, even if the ratio of the positive and negative active materials is normal, the deposition of metallic lithium may occur. 2. Positive electrode overcharge reaction When the ratio of positive electrode active material to negative electrode active material is too low, positive electrode overcharge is prone to occur. The capacity loss caused by overcharge of the positive electrode is mainly due to the appearance of electrochemical inert substances (such as Co3O4, Mn2O3, etc.), which destroy the capacity balance between the electrodes, and the capacity loss is irreversible. (1) LiyCoO2LiyCoO2→(1-y)/3[Co3O4＋O2(g)]＋yLiCoO2y<0.4 At the same time, the positive electrode material decomposes in the sealed lithium-ion battery. The oxygen that appears due to the absence of recombination reaction (such as the formation of H2O) decomposes with the electrolyte The flammable gas that appears will accumulate at the same time, and the consequences will be disastrous. (2) The λ-MnO2 lithium manganese reaction occurs when the lithium manganese oxide is completely delithiated: λ-MnO2→Mn2O3+O2(g)3, the electrolyte is overcharged, and the oxidation reaction occurs when the pressure is higher than 4.5. It will be oxidized to generate insolubles (such as Li2Co3) and gas. These insolubles will block the micropores of the electrode and hinder the migration of lithium ions, resulting in capacity loss during the cycle. Factors affecting the oxidation rate: the surface area of u200bu200bthe positive electrode material. The type and surface area of u200bu200bthe conductive agent (carbon black, etc.) added to the current collector material. Among the more commonly used electrolytes, EC/DMC is considered to have the highest oxidation resistance. . The electrochemical oxidation process of the solution is generally expressed as: solution → oxidation product (gas, solution and solid matter) + ne- oxidation of any solvent will increase the electrolyte concentration, decrease the stability of the electrolyte, and ultimately affect the battery capacity. Assuming that a small amount of electrolyte is consumed every time it is charged, then more electrolyte is required when the battery is assembled. Regarding a constant container, this means loading a smaller amount of active material, which will cause the initial capacity to drop. In addition, if a solid product appears, a passivation film will be formed on the surface of the electrode, which will increase the polarization of the battery and reduce the output voltage of the battery. Reason two: electrolyte decomposition (reduction) I decompose on the electrode 1. Electrolyte decomposes on the positive electrode: The electrolyte is composed of solvent and supporting electrolyte. After the positive electrode is decomposed, insoluble products such as Li2Co3 and LiF are usually formed, which are caused by blocking the pores of the electrode. Reduce the battery capacity, the electrolyte reduction reaction will have an adverse effect on the battery capacity and cycle life, and due to the reduction of gas, the internal pressure of the battery will increase, which leads to safety problems. The positive electrode decomposition voltage is usually greater than 4.5V (related to Li/Li+), so they are not easily decomposed on the positive electrode. On the contrary, the electrolyte is easier to decompose in the negative electrode. 2. Electrolyte decomposes on the negative electrode: The electrolyte is not stable on graphite and other lithium-intercalated carbon negative electrodes, and it is easy to react with 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. 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 post: Do you know the important difference between lithium ion batteries and lithium batteries?