不同烘烤温度下聚偏氟乙烯黏结剂结晶行为及其对正极剥离强度的影响OA

Adhesive integrity of the cathode is crucial for the structural stability of lithium-ion batteries.Peeling strength stands as a key indicator that directly reflects the interfacial bonding between active materials and current collectors.During electrode fabrication,baking temperature serves as a critical process parameter not only for removing solvents and moisture but also for influencing the microstructure and mechanical properties of the binder.Polyvinylidene fluoride(PVDF),a predominant cathode binder,is widely used due to its superior electrochemical stability and mechanical strength.Notably,the crystallization behavior of PVDF is highly sensitive to the thermal treatment.Crystalline perfection,grain size and phase state directly determine the flexibility and adhesion force of electrodes.Moderate crystallinity favors stable interfacial bonding,whereas excessive crystallization leads to PVDF rigidification and severe degradation of binding performance.However,the fundamental mechanisms linking baking temperature to PVDF crystalline behavior as well as the corresponding cathode electrode peeling force have not been systematically elucidated.To address this issue,systematic investigation was performed in this work based on the lithium iron phosphate(LFP)cathode electrodes and PVDF binders treated at different baking temperatures(25℃,120℃,130℃,140℃,145℃and 160℃).A series of characterization methods including X-ray diffraction(XRD),Fourier Transform Infrared Spectroscopy(FTIR),gel permeation chromatography(GPC),differential scanning calorimetry(DSC),and dynamic mechanical analysis(DMA)and tensile testing were conducted to reveal PVDF crystalline behavior evolution,phase state transition and mechanical property changes.In addition,these properties were further correlated with the peeling force of the LFP cathodes.The results indicate that the applied baking temperature does not change the intrinsic α-phase crystal structure,molecular weight or molecular weight distribution of PVDF,but significantly affect its crystalline perfection and phase state.With increasing baking temperature,PVDF undergoes three distinct crystallization stages:1)low-temperature annealing(25-130℃),where PVDF grain retains its inherent anisotropic sheet-like crystal structure;2)medium-temperature ripening(130-145℃),during which the PVDF molecular chains diffuse from small grains to larger ones,leading to the continued growth of preferentially oriented grains;3)high-temperature partial melting-recrystallization(145-160℃),where temperatures approaching the PVDF melting point drive the partial melting and recrystallization of small grains,promoting significant lamellar thickening and the formation of a more perfected crystal structure.Further investigation indicates that when the baking temperature is≤130℃,PVDF maintains an elastic state without obvious crystalline behavior.This allows the PVDF to retain a high mechanical strength,ensuring stable interfacial bonding and maintaining the cathode peeling strength at relative high level.As a result,the peeling force of the electrode decreases slowly from 6.64 mN/mm at 25℃to 3.53 mN/mm at 130℃.Conversely,when the baking temperature is higher than 140℃,the low-temperature melting peak shifts to a higher temperature.The melting enthalpy increases from 2.9 J/g at 130℃to 5.74 J/g at 140℃,indicating a sharp increase in PVDF crystallinity and the thickening of lamellar crystals.Simultaneously,PVDF transforms from an elastic state to a viscous flow state,leading to a significant decrease in storage modulus,tensile stress,and strain.Consequently,the cathode peeling force decreased to 3.33 mN/mm at 140℃and then plummeted to 1.93 mN/mm and 1.91 mN/mm at 145℃and 160℃,respectively.Consequently,this research reveals the microscopic mechanism by which excessive baking temperatures induce electrode adhesion failure.Specifically,this failure is driven by crystalline behavior changes within the binder,despite the fact that elevated temperatures facilitate the thorough removal of residual moisture and solvents.These findings offer essential theoretical guidance for optimizing the baking process of lithium-ion battery electrodes and the rational selection of PVDF binders.Ultimately,this work contributes to enhancing the structural stability and cycling performance of batteries while reducing production costs in industrial applications.