Battery capacity is one of the important performance indicators to measure the performance of lithium batteries. There is a large part of the reason for capacity reduction and the principle of lithium batteries. The capacity reduction of lithium battery packs, in addition to the aging and decline of the battery itself, is more common. , The more important factor is the different self-discharge rate, resulting in unbalanced battery cells in series, which will eventually reduce the capacity of the lithium battery and become less durable.

What is the reason for the low capacity of lithium batteries?

1. Structural changes of cathode materials

Cathode materials are the main source of lithium-ion batteries. When lithium ions are removed from the positive electrode, in order to maintain the material's neutral state, the metal elements will inevitably be oxidized to a high oxidation state, which is accompanied by a change in composition. The transformation of components easily leads to phase transition and changes in bulk structure. The phase transition of the electrode material can cause the change of the lattice parameters and the lattice mismatch. The induced stress caused by the resultant breakage of the crystal grains and the propagation of cracks, causing mechanical damage to the structure of the material, which in turn leads to the electrochemical performance of the lithium battery attenuation.
2. Anode material structure

The commonly used anode materials for commercial lithium batteries include carbon materials, lithium titanate, etc. This article uses typical anode graphite for analysis. The decay of the lithium battery capacity occurs for the first time in the formation stage, during this stage SEI will be formed on the surface of the negative electrode, consuming part of the lithium ions.

With the use of lithium batteries, changes in the graphite structure will also cause the battery capacity to decrease. The study found that although the carbon material after the cycle maintains the morphology and structure of graphite, the half-height width of its (002) crystal plane becomes larger, resulting in a smaller crystal grain size in the c-axis direction, and the change in the crystal structure leads to cracks in the carbon material. , Thereby destroying the SEI film on the surface of the negative electrode and promoting the repair of the SEI film. The excessive growth of the SEI film consumes active lithium, thus causing the irreversible capacity degradation of the lithium battery.

3. Oxidative decomposition of electrolyte

The nature of the electrolyte significantly affects the specific capacity, life, rate charge and discharge performance, operating temperature range and safety performance of lithium batteries. The electrolyte mainly includes three parts: solvent, electrolyte and additives. The decomposition of the solvent and the decomposition of the electrolyte will cause the loss of battery capacity. The decomposition and side reactions of the electrolyte are the main factors for the capacity degradation of lithium batteries. Regardless of the positive and negative materials and processes used, as the lithium battery is recycled, the decomposition of the electrolyte and the interface between the positive and negative materials will occur. Reactions will cause capacity attenuation.

4. Fast charge and discharge

During fast charging, the current density is too large, the negative electrode is severely polarized, and the deposition of the lithium battery will be more obvious, which will make the copper foil at the boundary between the copper foil and the carbon active material brittle, and it is easy to produce cracks. The spontaneous winding of the cell is limited by the fixed space, and the copper foil cannot stretch freely to generate pressure. Under the action of the pressure, the original cracks spread and grow, and the copper foil breaks due to insufficient expansion space.

5. Long-term deep charge and discharge

Discharge should be transferred to the internal structure. First, it will cause excessive volatilization of the electrolyte, and second, the excessive reaction of the negative electrode of the lithium battery will cause the dielectric film to change, which will cause the deintercalation capacity to decrease and form a permanent loss of capacity;

Charging is mainly from the voltage stability and the voltage rise of the power grid in the middle of the night. The charger that has stopped charging will continue to charge after the voltage rises. This will cause the battery to overcharge and cause the change in the structure of the positive electrode material. The violent oxidation reaction then burns and explodes; the electrolyte organic solvent/electrolyte lithium salt decomposes; the lithium evolution of the negative electrode may cause the copper collector of the negative electrode to dissolve and the positive electrode to form copper dendrites.

6. Temperature factor

Temperature is definitely one of the key factors affecting the life of lithium batteries. Too high or too low temperatures will cause a decrease in active lithium ion content, thereby reducing the life of lithium batteries.

Reasons for the decrease in battery capacity:

1. Batteries are aging and declining.

2. The self-discharge rate is different, resulting in unbalanced battery cells in series, which ultimately leads to a decrease in the capacity of the lithium battery and its durability.

3. In the process of battery use and charging and discharging, the phenomenon of "vulcanization" will affect battery capacity.

The lithium battery is unbalanced, how to return to normal?

1. First charge the entire lithium battery pack, and then float for 2 to 3 hours after turning the light. If the battery pack is left under power for a long time and cannot be charged, you can directly charge it for 10 minutes (using the discharge port), and then charge it normally.

2. Unplug the cable of the protection board. Before unplugging, please make a mark. For the board with 2 cables, the cable cannot be reversed. Measure the voltage of the adjacent pins on the cable. If it is 48V, there are 16 voltages, and 60V is 20 voltages. The first string of voltage from the negative pole is the voltage from the negative pole of the battery pack to the first cable, and so on. Find the single string with voltage lower than 3.50V, judge the positive and negative poles, and mark them.

3. Use a 3.6v charger to charge a single string with a voltage lower than 3.50v to 3.60 to 3.70V. However, someone needs to be on duty to prevent the lithium battery from being scrapped due to overcharging.

4. Insert the flat cable back in the original order, pay attention not to reverse the connection, install the battery, and then put it into use.

With continuous use, the capacity of lithium batteries will naturally decay, which is manifested in voltage reduction and insufficient capacity. This is related to the material characteristics of lithium batteries. The simple understanding is that the activity of lithium ions will continue to decrease with use, until the final The capacity is all attenuated.

In order to extend the life of the lithium battery, it is generally recommended to use the lithium battery in a shallow charge and shallow discharge mode. Do not overuse the lithium battery or charge it when it is almost out of power. This is detrimental to the life of the lithium battery.

The nature of the attenuation that affects the decrease in the capacity of lithium batteries is the decrease in the content of lithium ions that can be extracted. The main factors are the structural damage or inactivation of the positive and negative materials, the decomposition of the electrolyte, and the abuse of lithium batteries. Normally, the lithium battery must be used correctly to extend the life of the lithium battery.