Which would be a terrible idea. since both energy, and energy storage is a commodity. GM may as well merge with United Fruit Company.

The only two gains I could see would be diversification (But then why not merge with Bank of America? Or Google?), and an integrated logistics system (Except that logistics for electricity and gasoline are long-solved non-problems - and have nothing to do with auto manufacturing.)

It's a brilliant idea if you consider that all of this knowledge work is reusable and necessary on Mars.

The IP for this tech will be only minimally encumbered through a relationship with a friendly entity.

It's worth noting that in practice, the management of GM and EM coordinate so closely that they might as well be a single entity.