According to the filing, the focus is on "A method for charging a metal-air battery pack at the maximum possible rate while maintaining an ambient oxygen concentration below a preset concentration is provided, thereby minimizing the risks associated with generating oxygen during the charging cycle."

Lithium Air Tops the Chart – Image From 2013

Tesla had previously hinted at the use of metal-air battery technology, possibly as a range extender of sorts, but nothing concrete ever came to light. Patents filed by Tesla on metal-air date back to 2010, but to date we haven't seen the technology employed in a Tesla product. That could change if the Tesla sorts out what appears to be a charging complication linked to the new tech.