I am designing a circuit that will charge and discharge Li-on batteries in controlled manner. Although "design" is somewhat an overstatement: currently it is in basic concept stage, whose soundness is to be verified by simulation.

I would need a model for a battery in ADIsimPE, preferably Li-ion, but any low-voltage battery model would do?

Yes, a simple battery model can be created to test your circuits. There are more complicated circuit models that can be found by searching around, but I have found this simple model adequate for most of my simulations. It consists of a voltage source(battery) representing the completely discharged voltage, a non-linear capacitor to show the nonlinear charge/discharge curve, and a resistor for the internal resistance of the battery.

The PWL capacitor can be found in the SIMPLIS Primitives category in the "Place" Menu item. Below is a simple example of a PWL capacitor definition for a small lithium ion battery and its charge curve with a 20A charge current. By adjusting the charge values in the PWL cap definition window you can adjust the curve to match your battery.

When simulating charging set the PWL cap initial condition to 0, and when discharging set the initial condition above 0 to represent the amount of charge in the battery.

I hope this simple model meets your needs.