An ideal diode Integrated Circuit (IC) acts as a very low ohmic switch when the polarity is correct and protects the device from reverse polarity.
An ideal diode can also be used for switching between batteries when the device has two batteries in parallel. If the ideal diode is placed in series with each battery, it prevents the current flowing between them if they have different voltages.
The supply voltage range typically starts at 1.5 V, so ideal diodes are suitable only for 3 V batteries. Also, the quiescent current of such a device can be significantly higher than the nPM2100 PMIC's current consumption.