There is a label on the cell phone battery about the battery capacity, which is usually expressed in milliampere hours or mAh. This capacity is related to the service time of the cell phone battery, but this capacity only represents the theoretical value of the maximum battery capacity. With the use of the battery, the actual battery capacity will gradually shrink. Why is this?

In an ideal lithium battery, the capacity balance will not change during its cycle, and the initial capacity in each cycle is a certain value. However, there are some differences between theory and practice. Side reactions, such as electrolyte decomposition, active substance dissolution, lithium metal deposition, etc., will continue to occur inside the lithium battery, and these side reactions are irreversible and will affect the performance of the battery.

Overcharge

When the battery is overcharged, both the positive and negative electrodes of the battery will cause capacity loss. In the negative electrode, lithium ion is easily reduced and deposited on the surface of the negative electrode, blocking the insertion of lithium. The discharge efficiency is reduced and the capacity is lost. At the positive pole, when the ratio of positive active substance to negative active substance is too low, positive overcharge is easy to occur. The capacity loss caused by positive overcharge is mainly due to the electrochemical inert substances, which have destroyed the capacity balance between electrodes, and its capacity loss is irreversible.

In addition, oxidation reaction will also occur in the electrolyte during overcharge. When the pressure is higher than 4.5V  The electrolyte will oxidize to form insoluble substances and gases, which will block the micropores of the electrode and hinder the migration of lithium ions, resulting in capacity loss during the cycle. In addition, the generated oxide gas will increase the electrolyte concentration, reduce the stability of electrolyte, and ultimately affect the battery capacity. Assuming that a small amount of electrolyte is consumed during each charge, more electrolyte will be needed during battery assembly. For a constant container, this means that a smaller amount of active substance is loaded, which will result in a decrease in the initial capacity. In addition, if solid products are produced, a passive film will be formed on the electrode surface, which will cause the polarization of the battery to increase and reduce the output voltage of the battery.

2、 Electrolyte decomposition

Electrolyte is composed of solvent and supporting electrolyte. Insoluble products, such as Li2Co3 and LiF, are usually formed after the positive pole decomposes. The capacity of the battery is reduced by blocking the pores of the electrode. The reduction reaction of electrolyte will have adverse effects on the capacity and cycle life of the battery. In addition, during the initial charge and discharge, the electrolyte decomposition will form a passive film on the electrode surface, and the formation of the passive film will consume lithium ions, which will lead to the capacity imbalance between the two poles and reduce the specific capacity of the entire battery.

3、 Self discharge

Self discharge refers to the natural loss of capacitance of the battery when it is not in use. There are two cases of capacity loss caused by self discharge of lithium ion battery: one is reversible capacity loss; The second is the loss of irreversible capacity. Reversible capacity loss means that the lost capacity can be recovered during charging, while irreversible capacity loss is the opposite. The positive and negative electrodes may act with the electrolyte in the charging state, causing lithium ion embedding and de embedding. The lithium ion embedded and de embedded in the positive and negative electrodes is only related to the lithium ion in the electrolyte, so the capacity of the positive and negative electrodes is unbalanced, and this part of capacity loss cannot be recovered during charging

4、 Electrode instability

The positive active material will oxidize the electrolyte to decompose and cause capacity loss when charged. The factors affecting the dissolution of cathode materials are: structural defects of cathode active materials; The charging potential is too high; Carbon black content in cathode material.

5、 Collecting fluid

Copper and aluminum are commonly used materials for negative and positive collector respectively. The aluminum foil is easy to form oxide film on the surface whether in air or in electrolyte. At the same time, the overall corrosion and local corrosion (such as pitting) on the surface of the collector, poor adhesion and other reasons will increase the electrode reaction resistance, increase the internal resistance of the battery, and lead to capacity loss and lower discharge efficiency. A layer of insulation corrosion product film is formed by corrosion of copper collector during use. As a result, the internal resistance of the battery increases and the discharge efficiency decreases during the cycle, resulting in capacity loss.

Therefore, the loss of battery capacity is related to the quality of the battery itself. At the same time, attention should be paid to avoid using low-quality chargers or working in high temperature environment when charging. However, the mobile phone is being updated so quickly that in most cases your mobile phone has been replaced before your battery has any problems, so you don't need to pay special attention when using it.