An idea from the Cold War times – mercury as a spacecraft fuel, used by startup from Silicone Valley.
Recently, Apollo Fusion, a startup led by Mike Cassidy (ex VP of Google) was purchased by Astra for $50 million. The company is specialised in electric propulsion systems. Currently, gases such as xenon or Krypton are used to power spacecrafts, but according to Apollo Fusion, mercury gives new opportunities.
Mercury is a much heavier alternative from used gases, which gives the possibility of obtaining a larger power from such fuel, based on the research, it could be up to 3 times more powerful than the strongest engine known to us today system, powered by xenon.
Mercury would allow you to give a much larger and heavier cosmic vehicles, which would accelerate the investigation of space.
Sources:
https://apollofusion.com
https://spacenews.com/astra-to-acquire-spacecraft-propulsion-company-apollo-fusion/
https://www.popularmechanics.com/space/rockets/a25242578/apollo-fusion-mercury/
https://www.bloomberg.com/news/articles/2018-11-19/this-space-startup-could-lace-the-atmosphere-with-toxic-mercury