18650 lithium battery cells

　　What are the failure modes of 18650 lithium battery cells and what should we pay attention to?

　　When using 18650 lithium battery cells, attention should be paid to issues such as connecting wires, battery parameters, and electrolyte density; the failure modes of 18650 lithium battery cells include corrosion deformation of the positive plate, shedding of the active material of the positive plate, softening, and irreversible sulfation, as follows. Let’s introduce it in detail.

　　1. Precautions for using and maintaining 18650 lithium battery cells

　　1. The wiring should be correct and the connection should be firm. In order to prevent the battery from being damaged if the wrench is grounded, the negative pole should be connected first during installation, then the connecting wire between the two batteries, and finally the ground wire. When removing the battery, proceed in reverse order.

　　2. Check the battery parameters once a week. The electrolyte level should always be 10-15mm higher than the plate. If you find that the electrolyte level has dropped, add distilled water in time. Do not expose the electrolyte to the liquid level, otherwise the plate will be damaged. When the electrolyte is insufficient, only distilled water can be added. It is strictly prohibited to use river water, well water, or tap water. It is strictly prohibited to add concentrated sulfuric acid, otherwise the battery will be damaged due to excessive electrolyte density.

　　3. The electrolyte density should be adjusted in time according to regional and temperature changes. In areas with higher temperatures, electrolytes with lower density should be used; in colder areas, the density of the electrolyte should be higher to prevent freezing.

　　4. Always observe whether the battery shell is cracked, whether the installation is firm, and whether the wiring is tight. Timely remove dirt and oil stains on the battery surface, wipe off the electrolyte on the battery cover, remove the oxide layer on the pole posts and wire connectors, and keep the battery surface clean and dry. If the surface of the battery is too dirty, it will cause slow discharge between the electrodes and damage the battery. The battery poles should be protected by applying petroleum jelly to prevent oxidation and rust. The filling hole cap should be tightened and the vent hole on the cap should be unclogged.

　　5. When the voltage of a single battery is lower than 1.8V or the density of the electrolyte is lower than 1.15g/cm3, do not continue to use it and charge it in time. Each charge must be sufficient to prevent undercharging. During use, you should try to increase charging opportunities as much as possible, and always keep the battery working with sufficient power. A fully discharged battery should be charged within 24 hours.

　　2. Failure modes of 18650 lithium battery cells

　　1. Corrosion deformation of positive plate

　　There are three types of alloys currently used in production: traditional lead-antimony alloys, with an antimony content of 4% to 7% by mass; low antimony or ultra-low antimony alloys, with an antimony content of 2% by mass or less than 1% by mass. The lead-calcium series is actually a lead-calcium-tin-aluminum quaternary alloy with a calcium content of 0.06% to 0.1% mass fraction. The positive electrode grid made of the above alloy will be oxidized into lead sulfate and lead dioxide during the battery charging process, which will eventually lead to the loss of the role of supporting active materials and cause the battery to fail; or due to the formation of a lead dioxide corrosion layer, the lead The alloy generates stress, causing the grid to grow and deform. When this deformation exceeds 4%, the entire plate will be damaged. The active material will fall off due to poor contact with the grid, or short circuit at the busbar.

　　2. The active material of the positive plate falls off and softens

　　In addition to the shedding of active materials caused by the growth of the grid, as the charge and discharge are repeated, the bonds between the lead dioxide particles also relax, soften, and fall off from the grid. A series of factors such as the manufacturing of the grid, the tightness of the assembly, and the charging and discharging conditions all have an impact on the softening and shedding of the positive plate active material.

　　3. Irreversible sulfation

　　When a battery is overdischarged and stored in a discharged state for a long time, the negative electrode will form a coarse lead sulfate crystal that is difficult to accept charging. This phenomenon is called irreversible sulfation. Slight irreversible sulfation can still be restored by some methods. In severe cases, the electrode will fail and cannot be charged.

　　4. Premature loss of capacity

　　When low antimony or lead-calcium is used as a grid alloy, a sudden decrease in capacity will occur in the early stages of battery use (about 20 cycles), causing the battery to fail.

　　5. Severe accumulation of antimony on active substances

　　The antimony on the positive electrode grid is partially transferred to the surface of the active material of the negative electrode plate with the circulation. Since the overpotential of H+ reduction on antimony is about 200mV lower than that on lead, the charging voltage decreases when antimony accumulates. Part of the current is used for water decomposition, and the battery cannot be charged properly and therefore fails. The antimony content of the negative active material of the lead-acid battery that failed when the charging voltage was only 2.30V was tested. It was found that the antimony content in the surface layer of the negative active material reached 0.12% to 0.19% mass fraction. For some batteries, such as submarine batteries, there are certain restrictions on the hydrogen evolution efficiency of the battery. The negative active materials of batteries whose hydrogen evolution exceeded the standard were tested and the average antimony content reached 0.4% mass fraction.

　　6. Thermal failure

　　For low-maintenance batteries, the charging voltage is required not to exceed a single cell of 2.4V. In actual use, such as in a car, the voltage regulating device may be out of control, the charging voltage is too high, and the charging current is too large. The heat generated will increase the temperature of the battery electrolyte, causing the battery's internal resistance to drop; the drop in internal resistance will also The charging current is enhanced. The temperature rise and excessive current of the battery reinforce each other and eventually become uncontrollable, causing the battery to deform, crack and fail. Although thermal runaway is not a common failure mode for 18650 lithium battery cells, it is not uncommon. When using, attention should be paid to the phenomenon that the charging voltage is too high and the battery is hot.

　　7. Corrosion of negative electrode bus

　　Under normal circumstances, there is no corrosion problem on the negative electrode grid and bus bar. However, in a valve-regulated sealed battery, when the oxygen cycle is established, the upper space of the battery is basically filled with oxygen, and the bus bar is more or less the electrolyte in the diaphragm along the electrode. Climb up the ear to the busbar. The alloy of the bus bar will be oxidized and further form lead sulfate. If the alloy of the bus bar electrode is not selected properly, the bus bar will have slag inclusions and gaps. The corrosion will deepen along these gaps, causing the tabs to separate from the bus bar and the negative plate to fail.

　　8. Perforation of the diaphragm causes short circuit

　　Some types of separators, such as PP (polypropylene) separators, have larger pore sizes, and the PP fuse will be displaced during use, resulting in large pores. Active materials can pass through the large pores during charging and discharging, causing micro short circuits. , causing the battery to fail.

　　The above are the failure modes and precautions of 18650 lithium battery cells. If you have any questions, please feel free to contact us.

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