An external diode can be used for shunting the reverse current through the external diode instead of the nPM2100 PMIC. Depending on the forward voltage, the current is divided between the external diode and the nPM2100 PMIC.
The diode's forward voltage should be as low as possible. This is to make sure that most of the current goes through the diode instead of the nPM2100 PMIC. This means that a Schottky diode is a good option, though Schottky diodes usually have a relatively high leakage current in the reverse direction. This leakage can impact, for example, the quiescent current for normal operation and ship mode. The selected diode should have low forward voltage and low reverse leakage and be able to withstand high battery current.