It is the wrong technology for grid storage and absent a law-of-physics breakthrough in battery technology there will never be enough excess battery capacity on vehicles such that any person would be willing to give up any of those electrons.
This is a fanciful idea that is utterly disconnected from the real world. The storage systems that serious people are considering for the grid are large-scale, scalable systems like compressed air, pumped water, flywheels, and flow batteries. Yes, there are a few examples where Li-Ion has been used in grid applications, but this will never be widespread because the economics are all wrong, not to mention the safety.
Here are some companies to look at.
http://www.eosenergystorage.com/ does zinc-air.
http://www.capstoneturbine.com/ makes systems that can be spun up literally in seconds to add scalable peak capacity driven by natural gas. The better you handle the peaks, the less important storage becomes.
This article talks about developments in flow-battery technology. If you are going to use large-scale chemical storage on the grid, it will be something like this. http://www.csmonitor.com/Environment/Energy-Voices/2014/0109/Clean-energy-storage-on-the-cheap-in-new-flow-battery
If Musk thinks he will sell a lot of lithium batteries for the grid, he is a fool. And I don't think he is a fool. If he can build a good, economical lithium-ion battery, there will be plenty of demand for that in the transportation world at least for the nest 8-10 years. Worst case, he will have a dependable supply of batteries for Tesla production. But that is the point of the OP article. Do we have any good reason to believe Musk can do Li-Ion better that all the others who have been trying (with mostly disappointing results) to take this technology forward? Not really, but again, I wouldn't bet against Musk.
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