What makes lithium-ion batteries degrade if not used?
Inside the lithium battery is a relatively complex electrochemical system. After a long period of shelving, the internal balance gradually changed. When accumulated to a certain extent, batteries tend to change in several ways:
Firstly, physical properties
According to the actual proof of lithium battery storage over time, the physical characteristics (appearance, size, weight, etc.) of the battery will change to a certain extent, especially the appearance characteristics. This trend is more pronounced when the temperature and humidity of the storage environment are not good.
In the case of high humidity, the gain of lithium batteries after a long time is significantly higher than that of batteries placed in low humidity conditions. For example, the steel shell of the battery is prone to rust in high humidity, resulting in a slight improvement in quality. Rust will not affect the internal state of the battery, but will directly affect the shipment of the product and may have a negative impact on the accompanying electronic components.
Secodly, electrochemical performance
Long-term storage of lithium battery will produce some side reactions, such as electrolyte decomposition, dissolution of active substances, lithium deposition, etc., after a long time of shelving, the internal balance state of lithium battery gradually changes. When the accumulation reaches a certain degree, the battery will change significantly, which is directly reflected in the electrochemical characteristics of the battery.
1) capacity
The capacity change of lithium battery is mainly reflected in two points: first, the battery retention capacity is reduced, which is mainly caused by self-discharge; The other is the increase in irreversible capacity, which depends mainly on the irreversible consumption reaction between the chemical systems inside the battery. Self-discharge is inevitable in all lithium batteries. The capacity loss caused by self-discharge can be divided into two categories: reversible and irreversible: reversible refers to the lithium battery charging can recover part of the capacity, irreversible loss refers to the capacity can not be recovered. Battery manufacturers and battery users need to reduce capacity loss after long-term storage.
2) the internal resistance
The INTERNAL RESISTANCE OF a battery IS THE RESISTANCE between the positive and negative electrodes, and is the sum of the resistance where the fluid, electrode active material, diaphragm, electrolyte, conducting handle and terminal are located. For lithium batteries, the smaller the internal resistance, the lower the voltage required to discharge the battery, and the more energy it can output. However, for long-term storage batteries, their resistance will increase with the increase of storage time. Exceeding a certain resistance will cause the internal battery to exceed the baseline and become obsolete or degenerate. Therefore, it is necessary to pay attention to the resistance changes of the battery during long-term storage.
Temperature has a great influence on the internal resistance: lithium battery stored at 25℃ for 32 days, the internal resistance becomes 0.57MQ; When the battery is stored at 50℃ for one month, the internal resistance increases by 2.64mω; When the ambient temperature reaches 75℃, the internal resistance of the battery changes rapidly. Within the same days, the resistance increases by 8.18 Mω, which is 14 times of that at 25℃.
3) Discharge characteristics
The discharge characteristics of lithium batteries show a downward trend after long-term storage. The low temperature performance of the battery is significantly reduced after long-term storage.
Therefore, the comprehensive characteristics of lithium batteries show a significant downward trend after long-term storage.