Battery performance mainly includes battery cycle performance, voltage, internal resistance and high temperature storage performance. This article analyzes the effects of lithium battery formation process, battery aging and battery internal resistance on the lithium battery performance.
1. Effect of battery formation process on lithium battery performance
A key factor affecting the lithium-ion battery performance is the solid electrolyte film (SEI) formed by the decomposition of the electrolyte on the surface of the anode in lithium-ion batteries. The SEI film is formed by charging and discharging for the first time during the battery formation process.
A stable SEI film can protect the anode from being consumed during the subsequent decomposition of the electrolyte and prevent graphite from falling off, so the formation process is an important process in the manufacturing process of lithium-ion batteries. The state of the SEI film formed by different formation processes is different, and different SEI film states have different effects on battery performance. Therefore, different formation processes have different effects on the lithium-ion battery performance.
① Effect of formation charge and discharge current on battery performance
The formation charge and discharge current mainly includes the first part of the open charging current, the second part of the closed charging current and the fourth part of the closed discharge current. The first part of the opening formation is mainly small current charging, the purpose is to form a stable and dense SEI film, so that the gas generated by the additive reaction in the electrolyte can be discharged, and the impact on battery performance such as battery cycle performance and rate performance can be reduced. Moreover, the type and quantity of the electrolyte additive, the reaction potential and the time are different, and the charging rate required for the reaction is different.
Therefore, the charging at this stage mainly chooses the ladder charging mode, that is, the first step is low-current charging, and the subsequent steps increase current charging on the basis of the previous step. The second part of closed formation is mainly to increase the charging current on the basis of the first part. In the first part, part of the additives in the electrolyte has reacted and a dense SEI film has been formed. However, excessive density of the SEI film will affect the transport of lithium ions during the reaction, so it is necessary to gradually increase the current to make the formed SEI film satisfy the transition from dense to loose.
In addition, increasing the charging current will also shorten the battery charging time and improve production efficiency. However, if the charging current is too large, the temperature of the batteries will rise, the SEI film will be destroyed, and it will be dissolved and reorganized. Battery capacity attenuation, poor cycle performance, and even cause safety accidents. The fourth part of closed discharge is to discharge the fully charged battery for the first time, so as to complete the entire activation process of the battery.
Before discharging, the SEI film on the surface of the anode has been basically formed, so the discharge current of this part can be equal to or slightly greater than the charging current of the second part, but the current should not be too large, which will lead to serious polarization of the batteries and rapid temperature rise of the batteries. In addition, in order to ensure the consistency of the battery, a part of small current discharge should be performed after high current discharge.
② Influence of formation charging and discharging time on battery performance
The formation charging and discharging time mainly includes the above-mentioned first part of the opening charging time, the second part of the closing charging time and the fourth part of the closing discharging time. The first part of the open charging time is the low current charging time, which should not be too long, because long-term low current charging will increase the impedance of the formed SEI film and increase the internal resistance of the batteries. The second part is the closed charging time.
If there is no voltage limit, charging for a long time will cause the batteries to overcharge, and charging for a short time will cause the active material of the electrode inside the batteries to not be fully activated, and the SEI film will not be dense and incomplete, which will affect the battery performance. Therefore, this part of the charging time should be controlled jointly with the charging cut-off voltage. The fourth part is that the closed discharge time is related to the discharge depth of the battery. Without the limit of the discharge cut-off voltage, the longer the battery discharge time, the deeper the battery discharge depth will lead to over-discharge of the battery and shorten the life of the battery, thus affecting the battery performance.
2. Effect of aging time and temperature on battery performance
The aging time is the interval between the first charge and the first discharge. After the lithium-ion battery is fully charged for the first time, it needs a certain period of rest to remove the internal polarization of the battery, which will have a significant impact on the capacity and impedance of the battery. The impact of temperature on battery performance is mainly reflected in the increase of temperature, the acceleration of electrolyte and additive decomposition, the thickening of SEI film on the surface of anode, and the increase of battery internal resistance.
LiPF6 is the main component of the lithium ion battery electrolyte. LiPF6 will be thermally decomposed at too high a temperature to generate PF5. PF5 will further hydrolyze with the water in the electrolyte to generate HF. HF is an important reason for the dissolution of metallic iron in cathode materials.
3. Influence of lithium battery internal resistance on battery performance
Internal resistance is one of the key characteristics of lithium batteries. Generally speaking, battery internal resistance is divided into ohmic internal resistance and polarization internal resistance, which affect the power and charge and discharge efficiency of the battery. In short, all factors that hinder the movement of lithium ions and electrons in the lithium battery from one pole to the other constitute the internal resistance of the lithium battery.
The smaller the internal resistance of the battery, the better, because high internal resistance will increase its own heat loss, and it will not be able to discharge with a large current. In addition, when the internal resistance is large, the battery will heat up during use. If the temperature is too high, the working voltage of the battery will be reduced, and the discharge time will be shortened, which will seriously affect the battery performance and battery life, and even cause the risk of spontaneous combustion.
Therefore, the development of low internal resistance batteries can effectively improve the battery performance and prolong the service life of the battery. The industry mainly reduces the internal resistance of the battery by improving the adhesion and adhesion of the active material and the current collector, reducing polarization, and protecting the current collector.