How to efficiently heat a lithium-ion battery so that it can be charged in sub-zero environment?
Could you simply located the battery buried down a couple meters (below the frost line)? Then it would be warm in the winter and cool in the summer, without wasting energy heating it. 

Otherwise, there's a trade-off between keeping the battery warm (you want to have insulation to minimize the wasted energy) and keeping it cool in the summer (you want to get any heat out, especially when charging). 

It's not hard to find something to do what you're asking for.. maybe look for "heat tracing" that will run off of 12V. They're self-limiting PTC resistance cables used to prevent pipes from freezing.