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May 15 2025To deal with NMP residues in lithium iron phosphate batteries, it is necessary to establish a five-dimensional prevention and control system of "drying-environment-monitoring-maintenance-recycling" based on process details.
In the manufacturing of lithium-ion batteries, NMP (N-methylpyrrolidone) is a key solvent, and its residue problem is becoming a core hidden danger affecting the performance of battery cells. Especially for lithium iron phosphate battery systems that pursue high stability, NMP residue may cause chain reactions such as SEI membrane abnormalities and electrolyte decomposition, ultimately leading to capacity decay and safety hazards.
1.The harm of NMP residue to lithium-ion batteries
In the production of lithium-ion batteries, NMP is mainly used to disperse positive electrode materials, but its residual content exceeding the standard will directly impact the performance of the battery cell. Taking lithium iron phosphate batteries as an example, its electrolyte system is more sensitive to impurities, and NMP residue may cause the following effects:
(1)Electrochemical performance degradation
The residual NMP that is not completely dried during the coating process is prone to side reactions with the electrolyte, directly leading to a decrease in the first cycle coulombic efficiency. In addition, excessive residual NMP may cause battery cell expansion and bloating, posing potential safety risks.
(2)Abnormal pole piece structure
In the later stage of drying, if NMP is not completely volatilized, it may re-condense on the coating surface, causing wrinkles on the electrode surface or uneven microscopic holes, further hindering the transmission channel of the lithium-ion battery inside the electrode and affecting the rate performance of the battery cell.
(3)Disordered binder distribution
NMP residues may also cause the aggregation of binders such as PVDF, leading to local delamination of the electrode, resulting in a drop in capacity and an increase in internal resistance.
(4)Abnormal SEI film formation
Too much NMP residue will disturb the negative electrode SEI film, and its side reaction products will corrode the negative electrode surface, causing the SEI film to be loose and porous, increasing the risk of lithium dendrite growth, and further aggravating the capacity loss of the battery cell.
(1)Electrochemical performance degradation
The residual NMP that is not completely dried during the coating process is prone to side reactions with the electrolyte, directly leading to a decrease in the first cycle coulombic efficiency. In addition, excessive residual NMP may cause battery cell expansion and bloating, posing potential safety risks.
(2)Abnormal pole piece structure
In the later stage of drying, if NMP is not completely volatilized, it may re-condense on the coating surface, causing wrinkles on the electrode surface or uneven microscopic holes, further hindering the transmission channel of the lithium-ion battery inside the electrode and affecting the rate performance of the battery cell.
(3)Disordered binder distribution
NMP residues may also cause the aggregation of binders such as PVDF, leading to local delamination of the electrode, resulting in a drop in capacity and an increase in internal resistance.
(4)Abnormal SEI film formation
Too much NMP residue will disturb the negative electrode SEI film, and its side reaction products will corrode the negative electrode surface, causing the SEI film to be loose and porous, increasing the risk of lithium dendrite growth, and further aggravating the capacity loss of the battery cell.
2. NMP process control measures in the production process
(1)Drying process optimization
Temperature gradient design: The cathode coating oven needs to be set up with three stages of "low temperature preheating-high temperature volatilization-slow cooling and shaping" to ensure that the residual NMP content is less than 500ppm.
Wind speed and vacuum coordination: The wind speed in the lithium iron phosphate battery coating area must be ≥1.5m/s, combined with a negative pressure of -15kPa to accelerate the escape of residual solvents.
(2)Environmental humidity management
Lithium iron phosphate batteries are extremely sensitive to moisture. The humidity of the mixing and coating environment needs to be ≤20%RH to prevent NMP from forming an acidic complex with moisture after absorbing moisture, which will aggravate the microscopic hole defects in the electrode.
(3)Online process monitoring
Infrared thermal imager: Real-time monitoring of the temperature field distribution at the oven outlet to locate the high-risk area for NMP condensation.
Gas chromatography early warning: When the NMP concentration reaches 25% LEL, an audible and visual alarm is triggered, and 50% LEL automatically shuts down to avoid batch contamination.
(4)Equipment cleaning and maintenance
Clean the inside of the oven and the air duct regularly to prevent NMP condensate from flowing back to the electrode surface and causing secondary contamination, and ensure a dry and clean environment.
(5)Solvent recovery system construction
Improve the efficiency of solvent recovery and ensure that the NMP recovery rate is increased to more than 85% to meet the high-precision coating requirements of lithium iron phosphate batteries. The recovered NMP must be treated with molecular sieves or other effective methods before it can be recycled.
Temperature gradient design: The cathode coating oven needs to be set up with three stages of "low temperature preheating-high temperature volatilization-slow cooling and shaping" to ensure that the residual NMP content is less than 500ppm.
Wind speed and vacuum coordination: The wind speed in the lithium iron phosphate battery coating area must be ≥1.5m/s, combined with a negative pressure of -15kPa to accelerate the escape of residual solvents.
(2)Environmental humidity management
Lithium iron phosphate batteries are extremely sensitive to moisture. The humidity of the mixing and coating environment needs to be ≤20%RH to prevent NMP from forming an acidic complex with moisture after absorbing moisture, which will aggravate the microscopic hole defects in the electrode.
(3)Online process monitoring
Infrared thermal imager: Real-time monitoring of the temperature field distribution at the oven outlet to locate the high-risk area for NMP condensation.
Gas chromatography early warning: When the NMP concentration reaches 25% LEL, an audible and visual alarm is triggered, and 50% LEL automatically shuts down to avoid batch contamination.
(4)Equipment cleaning and maintenance
Clean the inside of the oven and the air duct regularly to prevent NMP condensate from flowing back to the electrode surface and causing secondary contamination, and ensure a dry and clean environment.
(5)Solvent recovery system construction
Improve the efficiency of solvent recovery and ensure that the NMP recovery rate is increased to more than 85% to meet the high-precision coating requirements of lithium iron phosphate batteries. The recovered NMP must be treated with molecular sieves or other effective methods before it can be recycled.
