The influence of formation process parameters (current, time, cut-off voltage, etc.) on battery performance!

A key factor affecting these properties of lithium-ion batteries is the solid electrolyte membrane (SEI) formed by the decomposition of electrolytes on the surface of the negative electrode in lithium-ion batteries. The SEI membrane is formed during the first charge and discharge of the battery during the formation process. The stable SEI membrane can protect the negative electrode from being consumed during the subsequent decomposition of the electrolyte and prevent graphite from falling off. Therefore, the formation process is an important process in the manufacturing process of lithium-ion batteries.

The formation process is the process of charging and discharging the qualified battery after the injection for the first time to form an SEI membrane on the surface of the negative electrode. The battery formation process mainly includes four parts: the first part is open charging (pre-charging or exhausting), the second part is closed charging, the third part is closed aging, and the fourth part is closed discharge. Different SEI membrane states formed by different formation processes have different effects on the performance of the battery. Therefore, different formation processes have different effects on the performance of lithium-ion batteries. Different formation processes mainly include different formation charge and discharge currents, different formation charge and discharge times, different formation charge and discharge cut-off voltages, and different formation aging times and temperatures. Battery performance mainly includes battery cycle performance, voltage, internal resistance and high-temperature storage performance.

1. The influence of formation charge and discharge current on battery performance

The formation charge and discharge current mainly includes the first part of the open charge (exhaust) current, the second part of the closed charge current and the fourth part of the closed discharge current.

The first part of the open formation (pre-charge or exhaust) is mainly a small current charge, the purpose is to form a stable and dense SEI film, so that the gas generated by the reaction of the additives in the electrolyte is discharged, and the impact on the battery cycle performance and rate performance is reduced. Moreover, the type and quantity of electrolyte additives, reaction potential and time are different, and the charging rate required for the reaction is different. Therefore, the charging in this stage mainly selects the step charging mode, that is, the first step is a small current charge, and the subsequent steps increase the current charge on the basis of the previous step.

By studying the pre-charging method of power batteries with lithium iron phosphate as the positive electrode and graphite as the negative electrode, it was found that the thickness, high-temperature storage performance and cycle performance of the process battery with the first step of pre-charging of 0.05C charging current, charging time of 25 min, and the second step of charging current of 0.15C and charging time of 55min were better than the other two pre-charging processes. However, if the charging current in the first step is too small, the battery cycle performance will be reduced. For example, the pre-charging process of power batteries with lithium iron phosphate as the positive electrode, graphite as the negative electrode and a capacity of 202Ah studied by this project team found that the battery cycle performance (800 times @ 94.90%) of the pre-charging process with the first step of pre-charging of 0.05C charging current, charging time of 60min, and the second step of charging current of 0.15C and charging time of 60min was better than the battery (800 times @ 93.80%) of the pre-charging process with the first step of pre-charging of 0.03C charging current, charging time of 100min, and the second step of charging current of 0.15C and charging time of 60min.

The second part of closed-end formation is mainly to increase the charging current based on the first part. In the first part, some additives in the electrolyte have reacted and a dense SEI film has been formed, but the excessive density of the SEI film will affect the transmission of lithium ions during the reaction. Therefore, it is necessary to gradually increase the current to make the formed SEI film meet 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 battery temperature will rise, the SEI film will be damaged, and it will be dissolved and reorganized. The battery capacity will decay, the cycle performance will deteriorate, and even cause safety accidents.

The temperature rise characteristics of lithium-ion batteries at different charge and discharge rates were studied and it was found that except for the electrode surface, the temperature rise at other positions of the battery was consistent with the temperature rise on the battery surface, and a battery ion thermal model was established to simulate the temperature change of the battery at different charge and discharge rates. Based on this model, the temperature rise was taken as a constraint condition, the boundary charging current curve was established, and the optimal charging current scheme for selecting lithium-ion batteries was formulated. In addition, if the charging current is too large, a large number of lithium ions released from the positive electrode cannot be quickly embedded in the negative electrode, resulting in lithium deposition on the surface of the negative electrode, which reduces the battery capacity and deteriorates the cycle performance. The formed lithium dendrites may pierce the diaphragm and cause the battery to short-circuit, which is dangerous.

The fourth part of the closed discharge is to discharge the fully charged battery for the first time, thereby completing the entire activation process of the battery. Before the discharge, the SEI film on the surface of the negative electrode 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 cause serious battery polarization and excessive battery temperature rise. In addition, in order to ensure the consistency of the battery, a part of small current discharge should be performed after the large current discharge.

2. The influence of formation charge and discharge time on battery performance

The formation charge and discharge time mainly includes the first part of the open charge (pre-charge or exhaust) time mentioned above, the second part of the closed charge time and the fourth part of the closed discharge time.

The first part of the open charge (pre-charge or exhaust) time is the small current charging time, which should not be too long, because long-term small current charging will increase the impedance of the formed SEI film and the internal resistance of the battery. Zhang Yanjiang et al. studied the effect of formation charging time on battery performance of lithium iron phosphate positive electrode and graphite negative electrode power battery, and found that appropriately reducing the formation time under the same charging current is beneficial to the formation of SEI film on the surface of the battery negative electrode. The negative electrode surface using this charging method is smooth, which can effectively improve the battery internal resistance, cycle performance and high-temperature storage performance.

The second part of the closed-mouth charging time, if there is no voltage limit, long-term charging will cause the battery to overcharge, and short-term charging will cause the internal electrode active materials of the battery to not be fully activated, and the SEI film is not dense and incomplete, which affects the battery performance. Therefore, this part of the charging time should be controlled in conjunction with the charging cut-off voltage.

The fourth part of the closed-mouth discharge time is related to the discharge depth of the battery. In the absence of the limit of the discharge cut-off voltage, the longer the battery discharge time, the deeper the battery discharge depth, resulting in over-discharge of the battery and shortened life.

3. The effect of formation charge and discharge cut-off voltage on battery performance

The first part of the open charge (pre-formation) cut-off voltage is the cut-off voltage after the battery is pre-charged. The purpose of pre-formation is to remove impurities and form SEI film. The impurities include water, trace elements and trace metal impurities. The formation cut-off voltage affects the reaction path of SEI film formation. An et al. studied the SEI film and its relationship with the cycle and found that different electrolyte additives have different reaction potentials. By controlling the pre-charge cut-off voltage to control the reaction of the additives in the electrolyte and control the formation of the SEI film, an SEI film with excellent performance was obtained.

The second part of the closed-mouth charge cut-off voltage is the cut-off voltage when the battery is fully charged. If the voltage is too high, the battery will be overcharged, causing excessive lithium ions to be released from the positive electrode active material and deposited on the negative electrode surface to form lithium dendrites. Overcharging will also decompose the positive electrode and release oxygen, which is a catalyst for electrolyte decomposition. In addition, the electrolyte solvent will react with the active lithium deposited on the negative electrode surface, causing the loss of positive electrode active material and battery capacity decay.

By studying the effects of different charging cut-off voltages on the voltage, capacity and other properties of lithium manganese oxide lithium ion batteries, it was found that the discharge capacity of lithium manganese oxide lithium ion batteries increases with the increase of charging cut-off voltage, but too high charging cut-off voltage will cause lithium deposition on the negative electrode surface, battery capacity decay, and lithium dendrites formed by lithium deposition on the negative electrode will pierce the diaphragm and cause a short circuit in the battery, affecting the safety performance of the battery. Fan Xiaoping et al. studied the effect of charging cut-off voltage on the cycle performance of lithium ion batteries using soft-pack lithium iron phosphate lithium ion batteries as the research object, and found that the discharge capacity of the battery increased slightly by increasing the charging cut-off voltage from 3.65V to 4.00V. By comparing the 600 cycle data of the above two charging cut-off voltages, it was found that the battery capacity retention rate with a charging cut-off voltage of 4.0V was 97.5%, and the battery capacity retention rate with a charging cut-off voltage of 3.65V was 99.0%. Cao Zheng studied the effect of charging cut-off voltage on the performance of single crystal NCM523/graphite system batteries and found that as the charging cut-off voltage increased, the battery capacity, voltage platform and energy density increased, but the rate performance, high and low temperature discharge and storage performance deteriorated. The cycle performance was not significantly affected in the first 800 times, but after 800 times, the battery with a higher charging cut-off voltage had the fastest cycle life decay rate.

The fourth part of the closed discharge cut-off voltage is the control voltage for the first discharge of the battery. If the voltage is too low, the battery will be over-discharged, the negative electrode current collector will corrode, the SEI film on the negative electrode surface will be damaged and decomposed, the reorganized SEI film will have poor performance, the battery impedance will increase, and the polarization at the end of charging and discharging will increase, resulting in reduced battery charging and discharging efficiency and poor cycle performance. Ouyang et al. conducted an experimental study on the thermal performance of SONY18650 lithium-ion batteries under overcharge and overdischarge conditions and found that the battery voltage dropped rapidly when entering the overdischarge stage, and the battery surface temperature continued to rise to 41°C. After about 250s, the battery voltage and current almost dropped to 0V and 0mA. This is a self-protection mechanism of the battery to prevent overdischarge and overheating. Wu et al. studied battery overcharge and overdischarge using a square lithium-ion battery with a capacity of 40Ah as the research object. By discharging the battery to different cut-off voltages at a 1C discharge rate, it was found that the battery temperature rise did not increase significantly when the discharge voltage dropped from 3.5V to 1.5V. However, when the voltage dropped below 1.5V, the battery voltage, temperature and impedance changed dramatically, with a maximum temperature rise rate of 20°C/s, a voltage drop of 0V, and a battery failure.

4. Effect of aging time and temperature on battery performance

Aging time is the interval between the first charge and the first discharge. After the first full charge, lithium-ion batteries need a certain amount of rest time to remove the internal polarization of the battery, which will have a significant impact on the battery capacity and impedance. Reichert et al. used 18650 lithium-ion batteries to study the effect of rest time on the cycle performance of lithium-ion batteries and found that rest time had a significant effect on the battery performance. There was no significant difference in cycle performance and impedance between batteries with rest time ≤2h and no rest time. This project team studied the effect of aging time on battery performance of a power battery with lithium iron phosphate as the positive electrode and graphite as the negative electrode and a capacity of 24Ah. It was found that with the increase or decrease of aging days, the electrical cycle life increased, 2d@744 [email protected]%, 3d@744 [email protected]%, 4d@744 [email protected]% and 14d@744 [email protected]% respectively.

The effect of temperature on battery performance is mainly manifested in the increase of temperature, the accelerated decomposition of electrolyte and additives, the thickening of SEI film on the negative electrode surface, and the increase of battery internal resistance. At present, the electrolyte component of lithium-ion batteries is mainly LiPF6. At too high a temperature, LiPF6 will undergo thermal decomposition to generate PF5, which will further react with the water in the electrolyte to generate HF. HF is an important cause of the dissolution of metallic iron in the positive electrode material. Coron et al. used 18650 lithium-ion batteries as the research object and conducted aging experiments at 0 and 25°C, respectively. It was found that the life of the battery aged at 25°C was at least twice that of the battery aged at 0°C. Rodrigues et al. studied the high-temperature solid electrolyte membrane (SEI) in graphite electrodes and found that the thermal brittleness of the SEI membrane was the main cause of the decline in the performance of the graphite negative electrode. The experiment proved that increasing the temperature during the formation process can increase the strength of the SEI membrane. The SEI membrane formed at 90°C showed excellent thermal stability and reduced the self-discharge rate of the battery.

In order to improve the high-temperature cycle performance of lithium-ion batteries, methylene disulfonate (MMDS) additives were added to the electrolyte. The MMDS additive can well improve the room-temperature and high-temperature cycle performance of the battery, and as the amount of additives increases, the battery cycle stability is enhanced. However, the additive is sensitive to temperature. Using and storing it at high temperatures will increase the color and acidity of the additive, affecting battery performance. Therefore, the storage temperature of the electrolyte, the static temperature of the battery after filling, and the exhaust temperature of the battery should be strictly controlled to prevent MMDS from failing.