I highly recommend the GensAce TATTU series of Lipos. All TATTU batteries have passed the UN 38.3 battery test which is the highest quality standard test for lithium polymer batteries. These batteries are a bit more expensive that other brands.

Your specs are quite a challenge...! Are you referring to 3.5 kg as take-off weight (incl. battery) with additional 1 kg as payload (e.g. camera) ? In general I'd also opt for the TATTU LiPos like the 6S 16000 mAh. However, most of the high energy LiPos have a weight of 1 to 2 kg...  Hope this helps!?

You probably won't be able to do this with a copter. One hour flight time without payload is challenging, but with such high payload it is in my opinion almost impossible, especially with a copter below 3.5kgs MTOW. My copters fly up to 30mins with a payload of about 600g at a takeoff weight of 3.5kgs.

You ca refer to " Introduction to Multicopter Design and Control " chapter 4 or flyeval.com

Try using Li ion 18650 battery packs for better energy to weight ratio

Li ion battries have the potential u seek, but i didn't use them before

Lipo 2700 mAH