Overheating, lithium cells

Freshly made cells of the "lithium-sulfiir dioxide" system are characterized by enhanced intrinsic pressure up to 0.4 MPa. Only by the end of discharge or rather by the time liquid sulfur dioxide is exhausted, this pressure somewhat decreases. Therefore, such cells are produced in rather robust steel nickel-plated cases of a not very large size. As a rule, the maximum capacity of individual cells does not exceed 30 Ah. The cases of sulfur dioxide-lithium cells are usually equipped with relief valves preventing fracture of cases at an increase in pressure (e.g., because of the cell overheating). 

The self-discharge reaction of calcium with calcium chromate is highly exothermic, forming complex chromium(m) oxides. Above about 600°C the selfdischarge reaction accelerates, probably due to the markedly increasing solubility of the chromate in the chloride electrolyte. This acceleration increases the rate of fonnation of calcium-lithium alloy. The resulting thermal runaway is characterized by short battery lives, overheating and cell step-outs, shorts, and noise characteristics of excess alloy. 

The liquid electrolyte generally requires hermetic sealing, which may reduce the energy density. In addition, for safety reasons, lithium ion rechargeable batteries and lithium-metal primary batteries having liquid electrolytes are designed to vent automatically when certain abuse conditions exist, sucb as a substantial increase in internal pressure which can be caused by internal or external overheating. If the cell is not vented under extreme pressure, it can explode because the liquid electrolyte used in liquid Li cells is extremely flammable. 

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