Date: 2026-07-17 hits: 101
Edge voltage is a key quality control indicator in the manufacturing process of pouch cells, directly reflecting the insulation integrity of the aluminum-plastic film encapsulation structure. The industry typically uses 1V as a standard control threshold; cells exceeding this threshold pose risks of aluminum layer corrosion, gas expansion, and leakage during subsequent storage and cycling. Most industry practitioners only manage edge voltage at the level of fine-tuning process parameters, lacking a systematic understanding of its underlying failure mechanisms. The following analysis, starting from the electrochemical essence and combining process and material dimensions, dissects the core causes of high edge voltage.
I. The Electrochemical Essence of Edge Voltage: Micro-galvanic Cell Effect in the Encapsulation Area
Ideally, the inner polypropylene (PP) layer of the aluminum-plastic film is a complete insulating layer, with no conductive path between the positive electrode tab and the aluminum layer in the middle of the aluminum-plastic film; the edge voltage should be 0V. When a micron-level defect occurs in the PP layer, the electrolyte penetrates along the defect and contacts the aluminum layer, simultaneously forming an electronic pathway. The sealed area then constitutes a micro-galvanic cell system.
The electrolyte acts as an ion conductor, providing a migration channel for lithium ions; the electronic pathway at the defect point forms the external circuit. Under the combined action of lithium ions and electrons, the aluminum layer undergoes an aluminum-lithium alloying reaction, resulting in a measurable potential difference between its two ends, i.e., the edge voltage. The industry-standard 1V control line essentially corresponds to the formation potential range of the aluminum-lithium alloy; exceeding this threshold signifies that aluminum layer corrosion has progressed from a potential risk to a substantial reaction stage.
II. Failure Cause Analysis
1. Mismatch in Hot-Pressure Packaging Parameters is the Primary Cause
The vast majority of cases of abnormal edge voltage stem from improper control of hot-pressing parameters in the top-sealing and side-sealing processes. The fusion bonding of the PP layer is the result of the coupling of temperature, pressure, and time; any parameter deviating from the reasonable range will directly damage the integrity of the insulation layer.
When the heat-sealing temperature is too high, the PP layer over-melts and is extruded, resulting in a sharp reduction in local thickness or even complete loss, directly exposing the aluminum layer to the electrolyte side and forming an ion pathway; insufficient temperature leads to incomplete PP fusion, resulting in micro-gaps within the seal edge, allowing the electrolyte to slowly penetrate along the interface. The logic of the effects of pressure and time is consistent with this: excessive pressure causes the polypropylene (PP) layer to thin and break, while insufficient pressure results in inadequate interfacial bonding. Prolonged hot-pressing time leads to thermal aging and embrittlement of the PP, making it prone to cracking during subsequent bending; insufficient time results in inadequate fusion depth. It is worth noting that acceptable peel strength at room temperature does not guarantee reliable long-term insulation. Some cells initially exhibit normal edge voltages, but the values rise significantly after one week of high-temperature storage, a typical manifestation of the slow deterioration of the PP layer under thermal stress and electrolyte wetting.
2. Mechanical Stress in the Folding Process Causes Insulation Damage
While hot pressing leads to material-level failure, the folding process directly damages the packaging structure through mechanical stress. After the soft-pack battery cell is packaged, it needs to be folded. The aluminum-plastic film in the bending area bears tensile strain, and the inner PP layer is the most vulnerable structure.
The larger the folding angle and the smaller the bending radius, the higher the tensile strain at the dome, and the probability of micro-cracks in the PP layer increases exponentially. Using a double or even triple folding design, each dome is a fatigue test for the PP layer. Furthermore, insufficient smoothness of the folding tool, folding position misalignment, and excessive stamping speed can all create micro-cracks invisible to the naked eye inside the seal, providing channels for electrolyte penetration, ultimately resulting in a gradual increase in edge voltage over time. Some production lines excessively compress the folding size to reduce battery cell thickness, often at the cost of sacrificing edge voltage yield.
3. Intrinsic Material Defects of Aluminum-Plastic Film and Tab Adhesive
No matter how strict the process control is, it cannot compensate for the inherent defects of the materials themselves. The quality fluctuation of the aluminum foil film is another key variable contributing to abnormal edge voltage.
The uniformity of the P-layer thickness of the aluminum-plastic film, pinhole defects in the aluminum foil substrate, and the interlayer adhesion between the nylon and aluminum layers all directly affect encapsulation reliability. Low-priced aluminum-plastic films often exhibit uneven P-layer thickness, with thinner areas breaking first after hot pressing; some aluminum foils contain micron-sized pinholes, allowing electrolyte to slowly penetrate after injection, causing the edge voltage to rise linearly with storage time. Furthermore, the melt compatibility between the tab adhesive and the aluminum-plastic film's P-layer is equally critical. When the melt flow index differs significantly, microscopic gaps form at the sealing boundary, becoming a high-risk area for ion pathways. Therefore, when changing aluminum-plastic film or tab adhesive suppliers, it is essential to conduct edge voltage tracking tests after high-temperature and high-humidity storage, rather than simply assessing initial peel strength.
4. Encapsulation Failure Mechanism in the Tab Area
The tab lead-out location is a high-risk area for abnormal edge voltage. During the top sealing process, the tab passes through the middle of the sealing edge; this area has a complex structure, making it difficult to control encapsulation consistency.
When the top sealing temperature is too high, the adhesive layer on the tab surface melts excessively and runs off, exposing the tab's metal body, which then directly contacts the aluminum layer of the aluminum-plastic film, forming a stable electronic pathway. Another common scenario is that the tab is misaligned, with the metal portion extending beyond the adhesive coverage area, directly conducting to the aluminum layer after encapsulation. The failure type can usually be initially determined by the voltage amplitude: in the 0.8V~2V range, ion pathways are dominant, and the corrosion process is relatively slow; when the voltage exceeds 3V, a complete electronic pathway has likely formed, and the corrosion rate accelerates significantly, indicating a high-risk level.
5. Data Interference Introduced by the Testing Method
When analyzing abnormal edge voltage, the influence of the testing method itself cannot be ignored. Edge voltage exhibits typical decay characteristics: the value is highest at the moment of probe contact, then rapidly decreases with the polarization process, and stabilizes after a few seconds.
The sampling rate of the testing equipment, the probe contact pressure, the selection of test points, and the ambient humidity all affect the final reading. It is not uncommon for the same cell to show a difference of more than 0.5V in the results tested by different personnel. Therefore, the production line must clearly define the testing conditions, including the waiting time after contact, probe specifications, and test points, to avoid misjudging test errors as product failures.
III. Summary
The core essence of high edge voltage is the failure of the P-layer insulation in the aluminum-plastic film, which leads to the formation of ion or electron pathways in the encapsulation area, constituting a micro-galvanic cell system. The mismatch of hot-pressing encapsulation parameters is the most significant contributing factor. Mechanical damage from edge folding, intrinsic material defects, and encapsulation failure in the tab area are common failure paths. The testing method can also introduce data fluctuations.
The core approach to engineering management is to implement full-process protection centered on the integrity of the PP layer: optimizing the hot-pressing process window, maintaining reasonable folding allowance, strictly inspecting incoming materials, and strengthening the control of electrode tab packaging. In terms of risk classification, values below 0.8V are considered safe, values between 0.8V and 2V require close monitoring, and cells exceeding 3V are recommended for direct isolation to prevent them from flowing into downstream processes.
