where should I place thermistor on lithium battery?
Its going to make little difference where you place the thermistor on the battery, however the more material you have between the thermistor and the battery the more thermal resistivity you will have. 

If you can put it next to the foil you will measure the battery temperature better (less thermal resistivity from heatshrink that normally comes on the li-ion packs). Most applications measure battery temperature for thermal runaway reasons so if the battery is getting hot, its going to get hot everywhere.