Propellants get expelled as plasma. Most of them are gas at room-temperature, or at least subject to sublimation at the pressures afforded in deep space. Maybe we could store, say, mercury as a solid in the shade beyond 1 AU; but we ain't using that in Earth orbit. Peeking in at Handmer's place, here's something on propellant boiloff.
This isn't for microcargoes, for which we're doing just fine with Hall thrusters. For longer missions I'd add that interplanetary journeys should be using fusion, which will lower the amount of propellant the ship needs to schlep. And once said ship gets to Deimos or some C-type near asteroid they'll be able to extract more propellant thence and just use that when needed. As we learn in Agile and JIT, inventory is waste.
All well and good: but we cannot get away from inventory in orbit, and anyway fusion propellant is mad pricey. We need a gas station, in short.
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