What is lithium battery separation at low temperature and high temperature

2020-04-08 143

  When the lithium-ion battery is charging, the lithium ion is removed from the positive electrode and embedded in the negative electrode; however, when some abnormal conditions occur, such as insufficient space for lithium-ion to be embedded in the negative electrode, too large resistance for lithium-ion to be embedded in the negative electrode, and lithium-ion that cannot be embedded in the negative electrode can only get electrons on the surface of the negative electrode, and then form silver white lithium metal Simple substance, which is often called "lithium analysis".

  1. Lithium evolution due to poor electrolyte moistening

  As the channel of lithium ion conduction, if the amount of electrolyte is small or not enough to moisten the electrode, lithium will be separated.

  Less liquid injection and lithium evolution: when the liquid injection is less, the migration path of lithium ion between the positive and negative poles is blocked, and then a fine-grained area without lithium insertion or lithium evolution area is formed.

  Large liquid loss and lithium evolution: even if the liquid injection quantity is ensured to meet, there is still a risk of lithium evolution due to insufficient electrolyte in the electric core. It is difficult to absorb liquid, aging time is not good after injecting liquid, pressure of clamp is too high, and vacuum is too strong.

  Poor moisture and lithium evolution in the center of the pole piece: when the core sucks liquid, the electrolyte generally penetrates into the center of the pole piece from the head and tail of the core. If the given electrolyte moisture time is insufficient, the center of the pole piece may not be fully moistened by the electrolyte. Lithium ions come to the center of the negative pole piece, because there are not enough conduction channels, lithium evolution occurs.

  Anode dead pressure + large loss of liquid lithium analysis: pure anode dead pressure or loss of liquid will constitute lithium analysis, the principle has been mentioned above. Large negative compaction will also reduce the liquid retention of the cell. If both occur at the same time, it will form a very severe compression + liquid retention low lithium evolution.

  2. Water content hyperbranched lithium

  Too much water will react with the lithium salt (LiPF6) in the electrolyte, and then reduce the capacity of the cell and cause gas production. There are two main sources of water content: electrolyte water content overrun and electrode water content overrun before liquid injection.

  Excessive electrolyte water content for lithium analysis: after the electrolyte expires or the storage conditions are improper, the excessive water will react with LiPF6 and generate LIF, then consume lithium ion in the electrolyte and reduce the cell capacity. Because of the high reactive activity in the middle part of the cell and the low surrounding area, the electrode with excessive electrolyte water content can not be fully lithium embedded because of the differentiation of lithium salt.

  The reaction principle is the same as that of electrolyte water content, but the interface is more complex than that of electrolyte water content: not only there are insufficient areas of lithium intercalation around the electrode, but also there are irregular areas of non lithium intercalation or even lithium intercalation in the center of the electrode. This shows that the overspended water content in the electrode is not a "uniform" reaction with lithium salt, with a greater degree of response, more lithium consumption, and more simple appearance of the middle of the electrode without lithium.

  3. Preparation and analysis of lithium by abnormal charging

  Formation is the initial charging process of lithium-ion battery, and lithium separation is caused by lithium ion can not be embedded in the negative electrode, only occurs in the charging process. Therefore, abnormal formation process is easy to lead to lithium evolution.

  High current formation and lithium analysis: during normal temperature formation, stable and low impedance SEI film can be formed only when the current is small. If the current is too large, the surface of the negative electrode will form a high impedance and uneven by-product, which will affect the insertion of lithium ions and form lithium analysis.

  Direct separation and volume analysis of lithium without conversion: its principle is basically the same as that of large-scale conversion, and both lithium analysis and non embedded lithium composed of converted gas will occur.

  Lithium evolution due to poor formation and touch: the cell is very fragile at the time of formation. At this time, the SEL film protecting the negative electrode has not yet been formed, and the interface cannot ensure excellent touch due to continuous gas production. Therefore, if the gas between the electrodes is not completely discharged before the formation, or the gas produced during the formation is too large and not discharged, the poor touch between the electrodes will be formed, which is also an important reason for lithium formation and analysis.

  Lithium analysis without clamping plate: high clamping temperature can promote the formation of SEI film. The clamping plate gives pressure to the electric core and then ensures that the formed gas can be discharged in time. However, if the clamp fails to be put on or the pressure is not added to the clamp, the formed gas will stay between the electrodes and cannot be discharged, and the insufficient area of brown lithium inlay or even lithium analysis will occur in the corresponding direction.

  Lithium is not separated by hot and cold pressing before battery formation: for the cell formed by unconditional hot pressing of fixture, hot and cold pressing or fixture bagging shall be continued before battery formation. The gravity of the thin cell is small, and the pole piece is simple and does not fit tightly. If the above process is not carried out before the formation, lithium analysis caused by poor touch is very simple.

  Before formation, the gas between the electrodes is not exhausted and lithium is analyzed: after the cell is filled with liquid, we expect that the electrodes are filled with electrolyte completely and there is no gas before liquid injection. However, if the vacuumizing effect after liquid injection is not safe or the way of standing before formation is not appropriate, there will be a trace of gas between the electrodes, and then lead to lithium evolution.

  After formation, small bubble like black coagulation: when the area of the electric core is relatively large and thin, the formed gas may be difficult to be discharged, and the negative plate corresponding to the foaming direction between the electrodes cannot be lithium embedded, and black spots occur.

  4. Analysis of lithium caused by capacity separation of lithium battery

  Volume separation itself is not easy to be the cause of lithium analysis attack, but some abnormalities in the previous process will be reflected in the volume separation. For the core with large thickness or too tight internal coil, it will simply deform after volume separation and form poor touch of the electrode. The bad touch area will be filled by the gas inside the core and then lose the lithium ion migration channel. Finally, it forms a strip like area without lithium intercalation, and may be accompanied by lithium evolution.

  5. Lithium evolution due to core deformation: the abnormal reason is the same as that in the above example: it is caused by the deformation of coiled core with thick or large internal stress. The reason why lithium evolution exists in this example is that the gas between the electrodes has been completely exhausted, and lithium ions can flow between the positive and negative electrodes, but the lithium evolution is due to poor formation and large distance between the positive and negative electrodes.

  6. Overcharge analysis of lithium: for lithium dilaurate and ternary, in order to ensure the stability of the data, the design capacity is far lower than the theoretical capacity, that is to say, even in the state of full charge, there are still many lithium ions in lithium dilaurate and ternary that are not embedded out, and after overcharging, these lithium ions outside the "braided brush" reach the embedded space that the negative electrode does not meet, so it is necessary to Lithium is analyzed. Correspondingly, the practical capacity of lithium iron phosphate is close to the theoretical capacity. Even if it is overcharged, it can not release too many lithium ions, so it is difficult to form lithium analysis.

  7. Lithium separation by low temperature charging: at low temperature, the ionic conductivity of the electrolyte will decrease, the impedance of lithium ion from the positive electrode and embedded in the negative electrode will be greatly increased, and the added range of embedded negative electrode impedance will be larger, and then lead to lithium separation.

  8. Lithium evolution after gas production by high temperature storage

  After the high temperature storage of the cell, the gas will be produced simply, and the attack gas will exist between the electrodes. At this time, the cell will be charged and discharged. The direction of gas production is because the lithium ion transmission path is isolated, and the negative electrode will attack the black area without lithium, and lithium may occur around the black area.

  9. Lithium evolution after circulation

  After a long cycle, the interface of any cell must be abnormal. From the data point of view, the premature consumption of electrolyte, the premature decay of positive electrode life and negative electrode life will lead to different lithium evolution phenomena. In the process of circulation, if the path of lithium ion embedded in the negative electrode is blocked, it will cause severe lithium evolution of the negative electrode and overall expansion of the appearance of the cell.

  Zhongshan lithium battery manufacturer http://www.bolaberita.com/