Lithium-ion battery drying and storage process
main content
Dry storage is a key post-treatment process in the manufacturing of lithium-ion batteries. By placing the cleaned batteries in an ultra-low humidity environment, the risk of moisture erosion to the electrodes, separators and electrolyte is eliminated. For lithium-ion start-stop batteries on motorcycles, dry storage is directly related to the battery's cycle life, low-temperature performance and shock resistance reliability. It is necessary to achieve microgram-level moisture control through precise humidity regulation, material compatibility and intelligent monitoring.
1. Water erosion pathways and hazards
Electrode oxidation: Moisture reacts with the positive electrode active material (such as NCM811) to form LiOH/Li₂CO₃, resulting in lithium loss (0.5% capacity loss per 500ppm H₂O) and an increase of 1530% in the cycle expansion rate.
Electrolyte decomposition: H₂O reacts with LiPF₃ to form HF (the SEI film dissolves when the concentration is ≥100ppm), accelerating the stripping of graphite at the negative electrode and increasing the internal resistance by 2050mΩ.
Diaphragm pore blockage: Moisture adsorption causes the closed-cell temperature of the diaphragm (such as PE/PP composite membrane) to drop by 510℃. Under high-temperature working conditions, it is prone to cause micro-short circuits.
2. Drying kinetics model
The diffusion of moisture follows Fick's second law: \(\frac{\partial C}{\partial t} = D \frac{\partial^2 C}{\partial x^2} \), where the diffusion coefficient D is exponentially related to the temperature T (D= D₀exp(Ea/RT)). Due to the compact shell of the motorcycle battery (wall thickness 0.61.2mm), it needs to be maintained at 4560℃ for 1224 hours to ensure that the internal water activity (Aw) is ≤0.65.
Ii. Particularity of Dry Storage for motorcycle start-stop Batteries
1. Seismic design adaptability
Structural strengthening: The 6063T6 aluminum alloy shell (yield strength ≥220MPa) is adopted, and the moisture absorption rate of the surface oxide layer is 60% lower than that of 304 stainless steel. The drying time can be shortened to 812 hours.
Gradient drying: Staged heating (25℃→45℃→60℃) combined with vacuum gradient (50kPa→90kPa) to prevent stress cracks in the electrode sheet core due to rapid dehydration (crack rate ≤0.1%).
2. Challenges in low temperature and high humidity environments
Moisture absorption inhibitor: Trimethyl phosphate (TMP, accounting for 23 wt-%) is added to the electrolyte to form a hydrophobic SEI film, making the equilibrium moisture content of the battery cell ≤15ppm (the conventional formula is 3050ppm).
Dew point control: The dew point temperature in the drying room is ≤40℃ (corresponding to a relative humidity of ≤1%RH). A combination of a dual-rotor dehumidifier (such as Monte HG500) and micro-heat regeneration is adopted, reducing energy consumption by 30% compared to traditional solutions.
Iii. Key Process Parameters and Equipment Innovation
1. Precise control of temperature and humidity
Temperature curve: Heating rate ≤2℃/min, fluctuation ±0.5℃ during the constant temperature stage, avoiding deformation of the TAB caused by thermal stress (wave edge ≤0.3mm).
Humidity monitoring: Cold mirror dew point meters (such as Optidew) are linked with impedance sensors, achieving an accuracy of ±0.1℃dp, and real-time feedback is provided to adjust the dehumidification unit.
2. Intelligent drying system
Vacuum pulse technology: Alternating vacuuming (95kPa) and nitrogen backfilling (purity ≥99.999%), the moisture diffusion efficiency is increased by 50%, and the drying time is shortened to two-thirds of the conventional process.
Machine vision monitoring: Integrated with a 5μm linear array CCD to detect condensate water on the shell surface, the AI algorithm triggers the secondary drying process when the water stain area is ≥0.01mm².
1. Moisture content detection
Karl Fischer method: The moisture content of the electrolyte is determined by coulometric method (standard ≤20ppm), and the moisture content of the gas phase inside the shell is analyzed by headspace gas chromatography (GC2030) (≤50ppm).
Weight loss method verification: After baking at 105℃ for 2 hours and weighing, the residual moisture content of the motorcycle battery should be ≤100mg/Ah.
2. Environmental simulation test
High and low temperature cycling: After alternating at 40℃ to 85℃ for 100 times, disassemble and test the moisture content of the diaphragm (requirement ≤0.02wt.%).
Vibration-humidity coupling: According to the SAE J2380 standard, when a random vibration of 10200Hz is applied at a humidity of 90%RH for 21 hours, the humidity change inside the shell is ≤3%RH.
V. Technical Challenges and Frontier Directions
1. Solid-state battery compatibility upgrade
Sulfide electrolyte protection: Al₂O₃ atomic layer deposition (ALD) encapsulation (510nm thickness) is adopted to increase the upper limit of humidity tolerance for dry storage from 10%RH to 30%RH.
Dry electrode process: The solvent-free coating technology ensures that the initial moisture content of the electrode is ≤50ppm, and the drying time can be compressed to 46 hours.
2. Green drying technology
Bio-based desiccants: Modified starch adsorbents (such as corn starch grafted with acrylic acid) have a moisture absorption capacity of up to 300mg/g, the regeneration temperature drops from 120℃ to 80℃, and the comprehensive energy consumption is reduced by 40%.
Waste heat recovery system: The exhaust heat from the drying room is recovered by a heat pump (COP≥3.5) to preheat the intake air, achieving a closed loop of energy consumption.
Conclusion
Dry storage, as the "micro-water defense line" for lithium-ion start-stop batteries in motorcycles, is evolving from extensive moisture control to molecular-level moisture management. Through the collaborative innovation of vacuum pulses, ALD packaging and intelligent monitoring systems, modern drying processes have achieved a leap in moisture control accuracy from the ppm level to the ppb level. In the future, with the popularization of dry manufacturing and solid-state batteries, dry storage will be deeply integrated with material modification and interface regulation, providing a more weather-resistant and stable "energy core" for new energy motorcycles.
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