Lithium-ion battery formation process
main content
Formation is the core process in the manufacturing of lithium-ion batteries. Through the first charge and discharge, the active substances of the positive and negative electrodes are activated to build a stable electrochemical system. For lithium-ion start-stop batteries of motorcycles, the formation process directly determines the battery's low-temperature starting ability, cycle life and reliability under high-frequency vibration conditions. This paper conducts an analysis from the dimensions of formation mechanism, the particularity of motorcycle application, process parameter optimization and quality control.
1. SEI film formation mechanism
Electrochemical activation: During the first charge, lithium ions deintercalate from the positive electrode (such as NCM811, LFP) and intercalate between the layers of the graphite negative electrode. Along with the electrolyte (EC/DMC/EMC system), an irreversible reaction occurs on the surface of the negative electrode, generating a solid electrolyte interface film (SEI film). This film is approximately 100 to 120nm thick. Its main components are Li₂CO₃, LiF and organic lithium compounds (such as ROCO₂Li). It has ionic conductivity but electronic insulation properties.
Functional value: The SEI film prevents the continuous decomposition of the electrolyte, reduces the loss of active lithium, and enhances the cycle stability. Poor-quality SEI films can lead to increased internal resistance (a 0.5% increase in capacity loss for every 500ppm of moisture) and intensified self-discharge.
2. Gas generation kinetics
Gas generation type: When the voltage is lower than 2.5V, mainly H₂ and CO₂ are generated (electrolyte hydrolysis reaction); The 3.03.5V interval is dominated by C₂H₄ (EC decomposition); Above 3.8V, CH₄, C₂H₆ (DMC/EMC decomposition) is produced.
Control strategy: Due to the vibration scenario, the gas production of the motorcycle battery needs to be controlled (≤3% volume expansion). The side reactions are reduced through a stepped current (0.05C→0.15C→0.2C), and vacuum pulse exhaust (pressure 95kPa) is combined to lower the risk of gas expansion.
Ii. the particularity of battery formation for starting and stopping motorcycles
1. High-frequency vibration adaptability
Structural reinforcement: The 6063T6 aluminum alloy shell (yield strength ≥220MPa) is adopted. The moisture absorption rate of the surface oxide layer is 60% lower than that of stainless steel, reducing the risk of SEI film rupture caused by moisture invasion.
Anti-vibration process: Pre-pressed corrugated TAB design (peak height 0.10.2mm) enhances the interfacial bonding force. After vibration testing, the impedance fluctuation of the SEI film is ≤5%.
2. Optimization of low-temperature start-up performance
Electrolyte formula: Add trimethyl phosphate (TMP, 23 wT. %) to enhance the low-temperature ionic conductivity (conductivity ≥2mS/cm at 30℃), and combine it with the negative electrode pre-lithiation process to ensure that the discharge capacity retention rate at 20℃ is ≥85%.
Formation parameter adjustment: In a low-temperature environment, 0.03C ultra-low current pre-charging is adopted (the duration is extended to 8 hours) to reduce crystal defects in the SEI film and enhance interface stability.
Iii. Key Process Parameters and Equipment Innovation
1. Current and voltage control
Stepwise charging and discharging
Pre-charging stage: 0.05C constant current charging (25 minutes) to activate the microstructure of the material;
Main charging stage: 0.15C pulse charging (pulse width 20ms, intermittent 10ms) accelerates lithium-ion migration;
Constant voltage stage: Maintain a constant voltage of 4.2V until the cut-off current is ≤0.01C to ensure the density of the SEI film.
Dynamic parameter adjustment system: Based on the digital twin model, it adjusts the power curve in real time. For instance, when the shell temperature is detected to be ≥45℃, it automatically reduces the current by 0.02C to prevent thermal runaway.
2. Technological breakthroughs in equipment
Laser cleaning integration: Before formation, a 1064nm laser is used to clean the oxide on the surface of the tabs, increasing the welding strength by 40% (≥12N), which is suitable for the high current impact requirements of motorcycle start-stop batteries (starting current ≥300A).
Intelligent monitoring: Embedded with LN8541 dew point sensor (accuracy ±0.1℃dp) to monitor the humidity in the drying room (≤1%RH), and synchronized to the MES system via 5G module to achieve process stability with CPK≥1.67.
Iv. Quality Control and Industry Standards
1. SEI film detection technology
Electrochemical impedance spectroscopy (EIS) : The test frequency range is 10 MHZ to 100 KHZ, and the fitting RSEI (SEI film resistance) is ≤50mΩ as the qualified standard.
X-ray photoelectron spectroscopy (XPS) : Analyze the LiF content in the SEI film (≥15% is considered high-quality film), and trigger an automatic repair mechanism for non-conforming products.
2. Special standards for motorcycles
QC/T 10942018: It is stipulated that the start-stop battery needs to pass the 10℃ low-temperature start-up test (5s voltage ≥10.56V) and the capacity retention rate ≥80% after 1000 start-stop cycles.
Thermal diffusion protection: Referring to GB 380312020, it is required that the battery pack should have no open flame within 30 minutes after the thermal runaway of a single cell. This is achieved through the design of double-layer explosion-proof valves (burst pressure 1.21.5MPa).
V. Technological Trends and Challenges
1. Solid-state battery compatibility
Sulfide electrolyte encapsulation: A 5nm Al₂O₃ layer was deposited by ALD, which expanded the temperature window for SEI film formation from 10%RH to 30%RH and reduced the drying energy consumption by 40%.
Dry electrode process: The solvent-free coating technology ensures that the initial moisture content of the electrode sheet is ≤50ppm, and the formation time is compressed to 46 hours.
2. Green manufacturing innovation
Bio-based solvents: Modified polyurethane with soybean oil replaces NMP, VOCs emissions ≤10mg/m³, and combined with microwave-assisted cleaning technology, the comprehensive energy consumption is reduced by 30%.
Conclusion
Technological breakthroughs in formation processes are driving the iteration of lithium-ion start-stop batteries for motorcycles towards high reliability and long lifespan. Through precise regulation of SEI films, intelligent parameter optimization and the application of environmentally friendly materials, modern battery activation technology has broken through the traditional bottleneck. In the future, with the popularization of 46 series large cylindrical batteries and breakthroughs in solid-state electrolyte technology, the formation process will be deeply integrated with thermal management and energy transmission functions, providing a more efficient "power heart" for new energy motorcycles.
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