You see, the image comes from a US patent titled "Electric vehicle extended range hybrid battery pack system," that Tesla originally filed December 8, 2010 and was granted June 25, 2013. The abstract explains:
You can read the whole thing in the included gallery but, put simply, what Tesla is proposing is an electric-electric hybrid system that uses two different battery types to create an EV with tremendous range. The lithium-ion battery (or whatever chemistry works best by the time this sees the light of day) would be there to provide electric performance most of the time while the metal-air pack helps move the car on long-distance drives. This makes sense because metal-air batteries have high energy density but low power density, so they're well-suited for long hauls.
A power source comprised of a first battery pack (e.g., a non-metal-air battery pack) and a second battery pack (e.g., a metal-air battery pack) is provided, wherein the second battery pack is only used as required by the state-of-charge (SOC) of the first battery pack or as a result of the user selecting an extended range mode of operation. Minimizing use of the second battery pack prevents it from undergoing unnecessary, and potentially lifetime limiting, charge cycles. The second battery pack may be used to charge the first battery pack or used in combination with the first battery pack to supply operational power to the electric vehicle.
Tesla is not alone in looking for unusual alternatives to today's alternatives. Toyota, for example, has been looking at next-gen solid state batteries and thinks they could arrive around 2020. IBM, too, is working on lithium-air batteries and says they could offer a 500-mile range in an otherwise regular EV. Think of all these efforts as a fight for the Sakichi prize.