Arduino pro mini 3.3V + solar panel - one 18650 3.7V or three 1.2V AA batteries?

Projects General Guidance
1

I am planning on building a simple weather station which is going to have an outside unit (sender) and a receiver inside.

In the outside unit I would like to use a 3.3V Pro Mini. I also have a 6V 100mAh solar panel laying around, I know it is not much, but I want to install it somewhere. I also have a bunch of 1.2V AA Ni-Mh batteries, with around 7-800 mAh capacity, and some TP4056 18650 charger unit with protection circuits.

The question is which method should I use:
a) Hook three AA batteries in serial and connect the solar panel directly to them, with a diode of course.
b) Or would it be better to buy a 18650 Li-Ion cell and connect it to the solar panel with a TP4056?

I also want to program the Arduino to put itself in some sort of sleep or power saving mode, and just wake up in every 2-3 minutes to measure and send data. Any suggestions or experiences about this? Is the built in LowPowerDeepSleep mode would be enough?

2

Go with the NiMH batteries, as they are much more tolerant of abuse than Li-ion cells, and don't require a charge controller with a small solar panel.