Formation is a crucial step in lithium battery production. During formation, a passivation layer, known as the solid electrolyte interphase (SEI) film, is formed on the surface of the negative electrode. The quality of the SEI film directly affects the battery's electrochemical performance, including cycle life, stability, self-discharge, and safety, in order to meet the "maintenance-free" requirement for rechargeable batteries. Different formation processes result in different SEI films, which have significantly different impacts on battery performance.
Traditional low-current pre-charging methods facilitate stable SEI film formation; however, prolonged low-current charging increases the impedance of the formed SEI film, thus affecting the rate discharge performance of lithium-ion batteries. The long process time also impacts production efficiency. Different lithium battery systems require different formation processes; this article focuses on the lithium iron phosphate battery system for analysis.
The formation process for lithium iron phosphate systems is typically selected as follows:
The charging current is 0.05C~0.2C, the cutoff voltage is 3.6~3.7V, the charging cutoff current is 0.025C~0.05C, and after a period of rest (10-20 min), the battery is discharged at 0.1~0.2C to 2.5V, and then rested for a period of time (20-60 min). Under different charging and discharging mechanisms, the different charging currents affect the formation and quality of SEI, while the resting time and charging cutoff current affect the battery formation process time.
The formation process of lithium iron phosphate (LFP) batteries requires the selection of a suitable cutoff voltage. From a material crystal structure perspective, charging voltages exceeding 3.7V may damage the crystal structure of LFP, thus affecting the battery's cycle performance. Some internal resistance experiments and electrode SEM observations also confirm the following conclusions:
1. Appropriately reducing the formation voltage and shortening the formation time can effectively reduce lithium plating on the negative electrode surface, resulting in a smoother negative electrode sheet. This is because when the formation voltage is high, the gas generation rate inside the battery is faster, causing the gas inside the battery to be unable to escape in time, depositing on the separator surface and affecting the contact balance between the separator and the negative electrode. During the lithium-ion insertion/extraction process, the imbalance in contact between the two causes excessive lithium-ion insertion in some areas, resulting in an uneven negative electrode surface and ultimately affecting battery performance.
2. After formation, the battery's internal resistance was tested, and it was found that appropriately reducing the formation voltage and time could lower the battery's internal resistance. The high internal resistance caused by high formation voltage is also related to the rough surface of the negative electrode and the formation of white spots. Since white spots are lithium compounds, they have poor conductivity, resulting in higher battery internal resistance.
3. Appropriately reducing the formation voltage in the formation process design can improve the battery's initial charge-discharge capacity and cycle performance. Excessively high formation voltage can easily cause lithium and its compounds to deposit on the negative electrode surface, increasing the battery's irreversible capacity and inevitably affecting its overall capacity. Due to the presence of lithium and its compounds, the battery's capacity decays faster and faster during charge-discharge cycles, impacting its cycle life.
