The easiest way is to put a voltage divider with high value resistors; 1M Omh each. And a ceramic capacitor of 100nF from the middle point to GND, to have enough current for the short readings.
This way you will convert the voltage range of the battery from about 2.5V - 3.7V to half: 1.25V - 1.85V
This is also better for accuracy, because the ADC is not very precise in the extremes of the range, but in the middle is good enough.
This way it works fine.