PopSci As always, context is important 
Last week, when Toyota told the press that it was working on a magnesium-based electric-car battery that could beat or even replace lithium-ion, the news dashed across the Internet. But anyone inclined to start calling magnesium the "new lithium" needs to keep a few important things in mind.
One...there's no way of knowing exactly what kind of battery Toyota is talking about. Masaki Matsui, lead scientist on the project, says the only thing he can say is that the battery's carrier ion is magnesium (Mg2+).
Two... Nearly every big automaker, battery maker, national lab and university materials-science department in the world has a team of PhDs whose lives consist of testing countless combinations of elements in an effort to build a better battery. Why would Toyota start talking about this particular project-which it admits is at least nine years away from yielding anything useful?
Toyota also has a long history of talking trash on lithium-ion batteries to cast doubt on the worth of electrified cars from GM, Nissan, and others. Undermining lithium-ion technology-by teasing the Next New Thing, and by arguing that even a lithium-ion battery with a theoretically maximal energy capacity is "still not enough to really make a very competitive battery that's necessary for future plug-in, electric and hybrid-electric vehicles," as Toyota's Jeffrey Makarewicz told Bloomberg last week-serves Toyota's competitive purposes.
Three...Any time we hear Toyota bemoan the state of lithium-ion technology, it's important to remember that Toyota has in fact built an extensive supply chain for lithium-ion batteries over the past few years... Which is interesting, because Toyota's suppliers at Panasonic are set to release a laptop-battery-size lithium-ion cell next year that uses a silicon-based anode for a 30-percent jump in energy capacity over today's cells of the same type.
Read full at PopSci