From the historical NASA document [Space Handbook: Astronautics and Its Applications](https://history.nasa.gov/conghand/spcover.htm), by Robert W. Buchheim (1959).

[Super high-energy bipropellants](https://history.nasa.gov/conghand/propelnt.htm)
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300 to 385, ISP
    
 - Fluorine-Hydrogen    
 - Fluorine-Ammonia    
 - Ozone-Hydrogen    
 - Fluorine-Diborane

But fluorine is very difficult to handle. Doing a combustion experiment back in the mid 1960s, I had a regulator burn up that was apparently not properly passivated that released a whole bottle of fluorine to the atmosphere.  No injuries, luckily.