What Are The Hazards Of Long-term Storage Of Lithium-ion Batteries?

For long-term storage of lithium-ion batteries, such as medical, military and power supplies, it is particularly important that the battery has good long-term storage performance. The inside of a lithium ion battery is a relatively complex electrochemical system. After a long time of storage, the internal balance will gradually change. When it accumulates to a certain extent, the battery will often undergo the following changes:

1. Physical characteristics

According to the actual proof after the time storage of the lithium ion battery, the physical characteristics (appearance, size, weight, etc.) of the battery will undergo certain changes, especially the appearance characteristics. This trend of change is more obvious when the temperature and humidity of the storage environment are not good.

In the case of high humidity, after long-term storage of lithium-ion batteries, its increase is significantly higher than that of batteries placed under low humidity. For example, the steel shell of the battery is prone to rust when the humidity is high, resulting in a slight increase in quality. Rust will not affect the internal state of the battery, but it will directly affect the shipment of the product and may have a negative impact on the electronic components that match it.

2. Electrochemical characteristics

Long-term storage of lithium ion batteries will have some side reactions, such as electrolyte decomposition, active material dissolution, lithium deposition, etc. After being left for a long time, the internal balance of the lithium-ion battery gradually changes. When it accumulates to a certain extent, the battery will undergo more obvious changes, which will be directly reflected in the electrochemical characteristics of the battery.

1) Capacity

The long-term storage capacity changes of lithium-ion batteries are mainly reflected in two points: one is the decrease in battery capacity, which is mainly caused by self-discharge; the other is the increase in irreversible capacity, which mainly depends on the irreversible consumption reaction between the internal chemical system of the battery. Self-discharge is inevitable in all lithium-ion batteries. The capacity loss caused by self-discharge can be divided into two types: reversible and irreversible: reversible refers to the part of the capacity that can be restored when charging a lithium-ion battery, and irreversible loss refers to the capacity that cannot be restored. For battery manufacturers and battery users, it is necessary to reduce battery capacity loss after long-term storage.

2) Internal resistance

The internal resistance of a battery refers to the resistance between the positive and negative ends, and is the sum of the resistance of the current collector, electrode active material, diaphragm, electrolyte, conductive handle, and terminal. For lithium-ion batteries, the smaller the internal resistance, the smaller the voltage occupied when the battery is discharged, and the more energy it can output. But for batteries that are stored for a long time, the resistance tends to increase as the storage time increases. Exceeding a certain resistance will cause the internal battery to exceed the benchmark and be scrapped or degraded. Therefore, it is necessary to pay attention to the resistance change of the battery during long-term storage.

Temperature has a great effect on internal resistance: at 25℃, the internal resistance of lithium-ion batteries will change to 0.57mQ when stored for 32 days; at 50℃, the internal resistance will increase by 2.64mΩ when the battery is stored for 1 month; when the ambient temperature reaches 75C , The battery resistance changes rapidly, and after being left for the same number of days, the resistance increase is 8.18mΩ, which is 14 times that at 25°C.

3) Discharge characteristics

After long-term storage, the discharge characteristics of lithium-ion batteries show a downward trend. The low-temperature performance of batteries stored for a long time is significantly reduced.

In summary, the comprehensive characteristics of lithium-ion batteries after long-term storage show a clear downward trend. In order to reduce the negative impact of long-term storage on all aspects of battery performance, the following aspects should be controlled:

(1) Control the temperature and humidity of the storage environment, and store the battery in a low temperature and dry environment, which is conducive to the long-term maintenance of its appearance and internal performance.

(2) Periodically activate the battery. After a certain period of storage, charge and discharge the battery one or two times with a small current, which is beneficial to reduce the irreversible capacity loss of the battery;

(3) Control the state of charge of the battery in long-term storage. Wu Guoliang’s research shows that controlling the battery's charge capacity at a semi-electric state (40% to 60% of the rated capacity) is conducive to long-term storage of the battery.