Samsung mobile phone battery explosion part of the bad thing on the isolation film, bare cell insulation detection can not be ignored

Lithium battery, the full name of lithium-ion battery, when discharging, lithium ions from the negative electrode to the positive electrode, and finally embedded in the positive electrode; when charging, lithium ions from the cathode to the anode, and finally embedded in the negative graphite.
The lithium ions oscillate back and forth, and the positive and negative electrodes are always in contact with each other, and are connected through an external circuit. At the same time as the lithium ions migrate, the corresponding electronic movement of the external circuit generates a current. This is the basic meaning of the battery, rechargeable discharge. If the positive and negative electrodes are in direct contact, causing a short circuit, and the electric energy is converted into heat energy, it will cause the temperature to rise out of control in a short time, and then burn and explode. Preventing the internal anode and cathode of the battery from short-circuiting to ensure the safe use of the battery is the second meaning of the battery. In the battery, the component that isolates the anode and cathode is called a separator (plate), which is a porous polymer polyolefin film with a certain strength, that is, a layer of porous plastic.
During long-term use of the battery, the microstructural changes of the positive electrode material during charging and discharging are irreversible, resulting in an increase in the volume of the entire cell system, especially for high energy density and fast charging batteries. The high-energy-density cell design uses a positive and negative material with a higher gram capacity (amount of charge per unit weight). One of the characteristics of these materials is that the volume is increased even more when lithium ions are inserted. The expansion of the battery will cause an extrusion/stretching effect on the separator, and the separator has a greater risk of breakage. The Samsung note7 battery burned this time, in the current mainstream mobile phone manufacturers/models, the energy density is ** (see the table below, the data comes from the official website), and also use the 1.2C fast charge. The risk is not high. According to Samsung's previous analysis, there are some reasons why the SDI battery is on fire.

One is that the battery isolation film Overhang is not enough. One is that when the battery is charged and discharged, the separator is squeezed due to the foreign matter particles, causing the isolation diaphragm to be damaged. Battery design and manufacturing practitioners should identify and prevent this high-risk defect from the outset. Especially for power batteries, tens or even hundreds of times the monomer capacity, once a short circuit occurs, the resulting uncontrolled release of energy is more serious.
Overhang refers to the portion of the separator that is outside the positive and negative pole pieces. It is generally specified according to the width of the battery pole piece plus the corresponding design margin. Overhang is designed to take into account the dimensional changes in the cell during the cell manufacturing process and during use.

For example, the existing lithium battery system requires stricter water content inside the battery core, because moisture causes the battery to generate more side reactions during charging and discharging, consumes electrolyte, damages the active surface of the positive and negative electrodes, and changes the structure of the active material. To reduce the charge and discharge performance of the battery and shorten the battery life.
Therefore, the battery will have a high-temperature water removal process. At this time, the polyolefin separator will cause heat shrinkage due to the thermal memory effect, resulting in a reduction in size and an increased risk of contact between the anode and the anode! Also talk about foreign particles. During the whole manufacturing process, the battery core is very easy to bring in foreign particles, such as: foreign matter in the isolation film, introduction of the separator film, falling of the positive electrode particles, metal particles generated by machine wear, air dust, and introduction of employee operations. These particles have different risks of squeezing the separator due to electrical conductivity, particle size, particle hardness, particle surface geometry, and the like. A battery with a large amount of foreign particles tends to have a faster voltage drop and a shorter service life. As the battery life increases, the risk of short-circuiting of such a cell is greater. Is there a way to identify such a large short circuit risk?
Referring to the insulation test method of the electrical and electronic industry, most battery manufacturers now conduct insulation tests on batteries. American Standard / National Standard Insulation test for electrical appliances is a breakdown test, adding a high voltage to both ends of the device under test. If the insulation isolation unit has defects or insufficient insulation performance, leakage current will occur and it will be regarded as conduction. , record the breakdown voltage. Generally, the breakdown voltage of an insulated cable is required to be 2000V or more. When the cell is tested for insulation, it is tested for withstand voltage. Usually, a voltage is specified and applied to both ends of the cell. If the cell resistance is less than a certain value, it is considered as a short circuit.
The short-circuit resistance at this time is generally below the kohm level in the case where the separator is defective or because the overhang is insufficient to cause direct contact between the anode and the cathode. For the presence of foreign particles, particularly metal particles with a good conductivity/high risk factor, the detection resistance is generally close to the Mohm level. The withstand voltage test is generally considered to be a non-destructive test. The test process can be equivalent to a capacitor charging process. After the test voltage is removed, the carrying charge will quickly dissipate. However, in the presence of conductive particles, a distorted electric field is formed, a tip discharge is generated, and the isolation film is broken down. The higher the voltage, the more obvious the performance, and the higher the probability of particle detection.
The safety of the battery is so big that it cannot be ignored. The insulation of the bare cell cannot be ignored.

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