eBike winter battery power
You're right - a cold battery cannot provide as much power as a room-temperature battery.   

I had an ebike based on SLA batteries, and in summer it would get 25 km range, which dropped to under 5 km in the cold frosty mornings of a winter.

It is possible to get battery heaters, common in places like Canada where its cold enough to freeze the water in a SLA battery and split the casing.  But on a bike you're going to be using battery power to keep the battery warm.  A better solution is to store the bike inside at both ends of your journey.

Perhaps a wrap made from a thermal blanket could help keep heat in, but there are downsides to that too.

Suggest you read To what extent should I insulate an electric bike battery during winter?