App note from Littelfuse on PolySwitch PPTC for lithium cells protection. Link here (PDF)
Primary lithium cells (i.e., AA and 2/3A) and rechargeable lithium cells (i.e., 18650, 17500, and prismatic) are used in many portable electronics applications, such as laptop or notebook computers. Their popularity is based on the fact that they offer high energy density, high capacity and long cycle life, with no memory effect.
Lithium cells are sensitive to faults caused by overcurrent and overtemperature conditions that may result from accidental shorting of the cell terminals and, in the case of rechargeable lithium cells, abusive charging or charger failure. For this reason, lithium cells generally require individual protection.