what temperatures should lipo batteries be stored? room temperature would probably be the best, but that's not always possible. is +5°C enough? can you also charge at that temperature?
I would think 15C and up would be much better. I've not seen any testing to see how well LiPo batteries store at various temperatures. I have however seen comparison testing by running a drone with a cold LiPo vs a room temperature LiPo, and the warm one did better if I remember correctly. It's been years since I saw that testing, so I could be misremembering. I am pretty sure the response and run time were worse on the LiPo that was left outside in the cold before being ran in the drone. This was a DJI mini video that I watched where they did that testing. Nothing too scientific, but it was a noticable difference.
Here is a LINK to an article about battery temps for lithium batteries in general and here is a LINK to one written by Gens-Ace about LiPos specifically. Its probably the better article since its aimed at LiPos specifically but its written in Chingrish so there are certain parts you have to figure out what word or phrase they are trying to actually say.
In the past I have experimented with storing my batteries in a fridge, what I discovered was this caused excess condensation to build up when going back and forth between extreme temp changes and my internal wiring would corrode and fail within months of use. I went back to room temp for storage and got far longer usage of my packs without failure.