A simple analysis of the causes of capacity loss in lithium-ion batteries

Analysis of the causes of battery capacity loss: Overcharging refers to the process of continuing to charge beyond the specified charging termination voltage (usually 4.2V). Overcharging can cause a decrease in battery capacity, mainly due to the following factors: ① Overcharging reaction of graphite negative electrode; ② Positive overcharge reaction; ③ Electrolyte undergoes oxidation reaction during overcharging. When the battery is overcharged, lithium ions are easily reduced and deposited on the negative electrode surface: the lithium deposited by Li++e → Li (s) covers the negative electrode surface, blocking the insertion of lithium. The reasons for the decrease in discharge efficiency and capacity loss include: ① reduction in the amount of recyclable lithium; ② The deposited metallic lithium reacts with solvents or supporting electrolytes to form Li2CO3, LiF, or other products; ③ Lithium metal is usually formed between the negative electrode and the separator, which may block the pores of the separator and increase the internal resistance of the battery. Fast charging, excessive current density, severe polarization of the negative electrode, and more pronounced lithium deposition. The capacity loss caused by positive electrode overcharging is mainly due to the generation of electrochemical inert substances (such as Co3O4, Mn2O3, etc.), which disrupt the capacity balance between the electrodes, and the capacity loss is irreversible.

→ (1-y)/3 [Co3O4+O2 (g)]+At the same time, the oxygen generated by the decomposition of the positive electrode material in a sealed lithium-ion battery will accumulate simultaneously with the combustible gas produced by the decomposition of the electrolyte due to the absence of recombination reactions (such as the generation of H2O), and the consequences will be unimaginable. Overcharging can also lead to the oxidation reaction of the electrolyte, and its oxidation rate is closely related to the surface area of the positive electrode material, the current collector material, and the added conductive agent (such as carbon black). At the same time, the type and surface area of carbon black are also important factors affecting the oxidation of the electrolyte. The larger the 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 form insoluble substances (such as Li2Co3) and gases, which will block the micropores of the electrode and hinder the migration of lithium ions, resulting in capacity loss during the cycling process.

The electrolyte decomposition electrolyte is composed of a solvent and a supporting electrolyte. After the decomposition of the positive electrode, insoluble products such as Li2Co3 and LiF are usually formed, which reduce the battery capacity by blocking the pores of the electrode. The reduction reaction of the electrolyte can have adverse effects on the capacity and cycle life of the battery, and the gas generated by the reduction can increase the internal pressure of the battery, leading to safety issues. The stability of electrolyte on graphite and other lithium embedded carbon negative electrodes is not high, and it is easy to react and produce irreversible capacity. During the initial charge and discharge, electrolyte decomposition will form a passivation film on the electrode surface, which 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 no longer occurs when the cycle stabilizes. The reduction of electrolyte salts participates in the formation of passivation films, which is beneficial for the stabilization of passivation films. However, the insoluble substances generated by reduction have adverse effects on the solvent reduction products. Moreover, the concentration of electrolyte decreases during the reduction of electrolyte salts, ultimately leading to the loss of battery capacity (LiPF6 reduction produces LiF, LixPF5-x, PF3O, and PF3). At the same time, the formation of passivation films consumes lithium ions, which can cause capacity imbalance between the two poles and result in a decrease in the specific capacity of the entire battery. The type of carbon used in the process, electrolyte composition, and additives in the electrode or electrolyte are all factors that affect the loss of film-forming capacity. Electrolytes often contain substances such as oxygen, water, and carbon dioxide. Trace amounts of water have no effect on the performance of graphite electrodes, but excessive water content can generate LiOH (s) and Li2O deposition layers, 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)+CO2 in the solvent can be reduced to CO and LiCO3 (s) on the negative electrode: 2CO2+2e+2Li+→ Li2CO3+will increase the internal pressure of the battery, while Li2CO3 (s) will increase the internal resistance of the battery and affect its performance.

Self discharge refers to the natural loss of capacitance of a battery when it is not in use. There are two types of capacity loss caused by self discharge of lithium-ion batteries: reversible capacity loss; The second is the irreversible loss of capacity. Reversible capacity loss refers to the ability of the lost capacity to be restored during charging, while irreversible capacity loss is the opposite. For example, lithium manganese oxide positive electrode and solvent will undergo micro battery interaction, resulting in self discharge and irreversible capacity loss. The degree of self discharge is influenced by factors such as the positive electrode material, battery manufacturing process, electrolyte properties, temperature, and duration. The self discharge rate is mainly controlled by the solvent oxidation rate, so the stability of the solvent affects the storage life of the battery. If the negative electrode is in a fully charged state and the positive electrode undergoes self discharge, the capacity balance inside the battery will be disrupted, resulting in permanent capacity loss. During prolonged or frequent self discharge, lithium may deposit on carbon, increasing the degree of capacity imbalance between the two stages. Pistoia et al. believe that the oxidation products of self discharge block the micropores on the electrode material, making it difficult to insert and remove lithium, increasing internal resistance and reducing discharge efficiency, resulting in irreversible capacity loss.