The impact of lithium iron ion battery formation process on battery performance
Formation is a crucial step in the production of lithium iron ion batteries. During formation, a passivation layer, or 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 cycle life, stability, self-discharge, safety, and other electrochemical performance, meeting the requirements for sealed and maintenance-free secondary batteries. Different formation processes result in different SEI films, which have significant differences in their impact on the performance of lithium-ion batteries.
Traditional low-current pre-charging methods help to form a stable SEI film, but long-term low-current charging will increase the impedance of the formed SEI film, thereby affecting the rate discharge performance of lithium-ion batteries. The long process time also affects production efficiency.
The formation process of the lithium iron phosphate system is usually selected as follows:
The charging current is 0.05C~0.2C, the cutoff voltage is 3.6V~3.7V, the charging cutoff current is 0.025C~0.05C, and after a period of rest, it discharges from 0.1C~0.2C to 2.5V. Under different charging and discharging mechanisms, different charging currents affect the formation and quality of SEI, while the battery's dormancy time and charging cutoff current affect the battery's formation time.
The formation process of lithium iron phosphate (LFP) battery systems requires the selection of a suitable cutoff voltage. From the perspective of material crystal structure, when the charging voltage exceeds 3.7V, the crystal structure of LFP may be disrupted, thus affecting the battery's cycle performance. Some internal resistance experiments and SEM observations of the plates also confirm the correctness of the following conclusions:
1. Appropriately reducing the formation voltage and shortening the formation time can effectively reduce the formation and evolution of lithium on the negative electrode surface, thereby obtaining a relatively smooth negative electrode surface. This is because when the conversion voltage is high, the gas generation rate inside the battery is fast, and the gas inside the battery cannot be discharged in time, depositing on the film surface and affecting the contact balance between the film and the negative electrode. During the lithium-ion de-embedding process, due to the unbalanced contact between lithium ions, lithium ions are over-embedded in certain areas, resulting in an uneven negative electrode surface and ultimately affecting battery performance.
2. Tests on the internal resistance of lithium iron phosphate batteries revealed that appropriately reducing the formation voltage and shortening the formation time can lower the battery's internal resistance. High internal resistance caused by high formation voltage is also related to the roughness of the negative electrode surface and the formation of white spots. Since white spots are lithium compounds with poor conductivity, they contribute to the battery's higher internal resistance.
3. Appropriately reducing the formation voltage in the formation process design can improve the single-cycle charge-discharge capability of lithium-ion batteries and enhance their cycle performance. Excessively high formation voltage easily leads to the deposition of lithium and its compounds on the negative electrode surface, increasing the irreversible capacity of the lithium-ion battery and inevitably affecting its capacity. Due to the presence of lithium and its compounds, the battery capacity decreases faster and faster during charge-discharge cycles, impacting the battery's cycle life.
Lithium iron phosphate batteries are recommended.
1) Positive electrode discharge specific capacity
2) Battery charge/discharge efficiency
3) Battery voltage platform
4) Constant current and constant voltage charging ratio of lithium-ion batteries
5) Differences in charge and discharge voltage plateaus of lithium-ion batteries
6) Relationship between capacity of 0.2C and capacity of 0.5C
Formation steps of lithium-ion batteries
The formation process of the SEI film in lithium-ion battery formation includes the following four steps:
Step: Electrons are transferred from the conductive agent graphite particles in the collecting liquid to point A of the SEI film to be formed;
Step: Solventized lithium ions, encapsulated in solvent, diffuse from the positive electrode to point B, forming on the surface of the SEI film.
Step: Electrons at point A diffuse to point B through electron tunneling;
Step 1: Jump to point B, electrons and lithium salt react to solvate lithium ions, form a film agent, etc., to continue generating the original SEI film on the SEI film surface, thereby increasing the thickness of the SEI film on the surface of graphite particles, and finally forming a complete SEI film.
Therefore, the entire reaction process of SEI formation can be broken down into the four steps mentioned above. These four steps determine the entire SEI film formation process.
The formation of a lithium iron phosphate battery is the first charging process, activating the active materials within the battery and forming a dense film on the anode surface to protect the entire chemical interface. This formation is also called activation. After manufacturing the lithium-ion battery, specific charging and discharging methods are used to activate the internal positive and negative electrode materials, improving the overall performance of the battery process. The formation of a lithium-ion battery is a very complex process and a crucial factor affecting its performance.
