Cell vent

Safety and Hazards. The linear carbonate solvents are highly flammable with flash points usually below 30 °C. When the lithium ion cell is subject to various abuses, thermal runaway occurs and causes safety hazards. Although electrode materials and their state-of-charge play a more important role in deciding the consequences of the hazard, the flammable electrolyte solvents are most certainly responsible for the fire when a lithium ion cell vents. The seriousness of the hazard is proportional to the size of the cell, so flame-retarded or nonflammable lithium ion electrolytes are of special interest for vehicle traction batteries. 

Batteries tend to perform more poorly as the operating temperature is decreased because of decreased conductivity of the electrolyte and slower electrode kinetics. Ultimately, freezing of the electrolyte occurs and the battery fails. Batteries tend to perform better at higher temperatures only up to the point that loss of performance occurs because of cell venting and drying out or parasitic reactions in the cell. Overall, alkaline cells have less performance loss at low and high temperatures than do Ledanchfi cells as can be seen in Figure 8. 

Thermal runaway The cell condition where the internal cell reactions generate more thermal heat than the cell can dissipate. The condition causes cell venting and premature failure. 

A cell vent designed to stop burning discharge from a vent. 

At 58 s into the overcharge (after fiill charge), cells 1 and 2 carried less current leaving 3 and 4 to carry more current at 206 s, cells 1 and 2 carried higher current, and all 4 cells stabilized at 320 s at about 590 s, cell 3 appeared to have its CID activated, followed by the other 3 cells over the next minute at about 640 s, the temperature on cell 3 increased from 109 °C to 120 °C in less than 1 s with runaway to >500 °C and, at this point, the other three cells vented flame as well, probably due to thermal propt ation of the initial event. The test was repeated with fresh Sony Li-ion cells obtained from a battery vendor. Figure 17.13 gives the results obtained that showed that all the cells had their CIDs activated as expected. 

In prismatic cells and in cell designs that do not incorporate a CID, overcharge has resulted in the breakage of the cell header weld or pouch, releasing flammable gases and electrolyte. In some cases, this resulted in disassembly of the cell. For this reason, cell vents or rupture disks that are to be relied upon for a leak before burst design should operate as expected. A minimum of two samples from every new production lot should be tested to confirm that the vents work adequately and as designed for protection. 

Figure 2.20 Schematic representation of a quadrupoie-based iCP-MS instrument equipped with a coiiision/reaction ceii 1, sampiing cone and skimmer 2, extraction iens 3, coiiision/ reaction ceii 4, quadrupoie fiiter 5, detector 6, coiiision or reaction gas 7, mass flow controller 8, getter (to remove traces of O2) 9, cell entrance 10, cell exit 11, cell vent. Reproduced with permission of Elsevier from [82].

Materials must be compatible with the cell chemistry chosen. For example, aluminum reacts with alkaline electrolytes and must be protected where cell venting may occur. 

Safety features The safety features incorporated in the cell and battery will obviously influence handling procedures. These features include cell-venting mechanisms to prevent excessive internal cell pressure, thermal cutoff devices to prevent excessive temperatures, electrical fuses, PTC devices and diode protection. Cells are hermetically or mechanically crimped-sealed, depending on the electrochemical system, to effectively contain cell contents if cell integrity is to be maintained. 

Abusive conditions could adversely affect the performance of the L1/S02 battery and result in cell venting, mpture, explosion, or tire. Preventative measures are discussed in Sec. 14.4. 

The same factors that affect cycle life affect overall battery life. Operation or storage at extreme temperatures, overcharging, cell venting, and abuse will reduce battery life. For optimum life, operation and storage should be as close to normal temperatures (20°C) as possible. Recommended and permissible temperature limits are shown in Table 29.3. 

Since potassium hydroxide readily combines with carbon dioxide in the air to form potassium carbonate, thus reducing conductivity, cell vents are usually covered with a vent cap or a low-pressure relief valve. 

Cell vent or tear away tab allows the safe release of gas if excessive pressure builds up within cells. Vents are typically activated if the internal cell pressure exceeds 10 bar (150psi). 

Limited pulse current capability of coin and low-capacity types makes them robust against temperature rise due to short circuit. However, temperature rise and internal pressure are still a concern, and system-level safety devices such as PTC devices may be required in most applications to make sure batteries stay below dangerous Hmits. High discharge rate, accidental charging, and cell reversal can all lead to cell venting, temperature rise, and possible expulsion of electrolyte. Puncture tests have resulted in hydraulic vent and case temperatures in excess of 100 °C [3]. Crushing showed similar results. 

Figure 8.19 Mass spectra obtained on Elan DRCp instrument for 1 pg/L each of K, Ca, Cr, Mn, Fe, Ni, Cu, Zn, Se and Sr collected with a quadrupole cell vented to a pressure of 10 Torr (a) pressurized with methane and operated in a manner that allows simultaneous stability of precursors and products (at quadrupole stability parameters a = 0 and q = 0.15) and with no postK ell energy discrimination (b) and at the same methane pressure and conditions but at (a, q) = (0,0.7), i.e. with narrower mass bandpass of the quadrupole (c).

Prepare the sample cell and fill with sample to a minimum depth of 3 mm. Provide adequate head space, and if required, a cell vent hole to prevent bowing of the window when testing volatile samples. 

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