According to the info posted on the Apollo fusion website http://apollofusion.com/ , they claim to use a propellant blend that offers " 2-3 X more impulse per mass and volume" in comparison towards traditional Xenon based Hall effect thrusters. The first thought that came to my mind is that it could be an Iodine based blend, however they could right-away have mentioned about it like other propulsion companies.So are there any resources that mention the kind of blend they have developed? or could they just be tinkering with other gases into their blend to increase the impulse?
According to a Bloomberg article, it's mercury. This would be the reason they don't mention it. Though fine for interplanetary missions, it's irresponsible for Earth orbit and especially low Earth orbit.