Warning: Contains SPOILERS for Spider-Man: Ho Way Home
Iron Man 2 never explains what new element Tony created for it. Beyond being an essential component needed to generate absurd amounts of energy, the specifics are left uncertain. But there are a couple of possibilities from other parts of the MCU.
Released in 2010, Iron Man 2 is now quite far back in MCU history. A major part of the plot revolves around Tony Stark grappling with the grim realization that palladium from the arc reactor in his chest, created during Iron Man to keep him alive, is now slowly killing him. As the story unfolds, Tony discovers a way to create a completely new element to replace it. The only name ever given to this element is "badassium," a name Tony tries and fails to patent it under.
The Iron Man 2 novelization credits this as one of the first MCU vibranium inventions, but the biggest flaw with this idea is the fact that JARVIS refers to it as a "new element" when vibranium is not new to him. After all, it's been known since at least the 1940s, having been used to create Captain America's shield. Instead of being vibranium, one interesting fan theory is that Tony Stark created uru, the mysterious metal used to create Thor's hammer Mjolnir (via Reddit).
As the arc reactor manages to block Loki's attack with the Mind Stone during dwarves of Nidavellir, shown in Avengers: Infinity War, uru is prized for its ability to store magical energy. If it can do the same with electrical energy, it explains how Iron Man suit can absorb an electrical attack from Thor during The Avengers. It also makes more sense that Tony would be able to create a working infinity gauntlet if he's able to use uru in its construction. What's more, this links back to how Howard Stark knew about the element but was unable to create it himself. The nuclear processes inside stars create all kinds of heavy elements, and real-world physics showed in 1925 that elements can be transmuted from one to another. Living through the heyday of nuclear physics, it follows that Howard would have understood all of this, but been unable to use the knowledge in any meaningful way.
But there are still hints that the element is similar to Avengers: Age of Ultron.
With Ironheart set to make her MCU debut in Black Panther: Wakanda Forever, and with Tony Stark slowly fading from view, perhaps it'll never be explained exactly what element was created during Iron Man 2. Or perhaps it's a secret still to be revealed as the MCU continues to build on the things introduced in earlier movies. One way or another, while Tony Stark may be gone, the technology he created is likely to linger for some time yet to come.
