Yeah, I'm quite curious myself as to why it's more difficult. My chemistry knowledge is chem1 level so all I know is that sodium atoms are larger and the energy levels for state change are slightly different

Well, sure, but you're asking me about something I saw in passing half a decade ago. Basically, the main difficulties with Na Ion Batteries and especially the "Glass Batteries" were the lack of proper cathode with which to create current and also maintain the structure against the naturally occurring atomic reshuffling. In particular there was controversy over John B. Goodenough's research because other battery scientists noted the electrodes both ends contained anode materials which should theoretically produce no electrochemical potential and therefor no cell voltage.

More modern attempts appear to use Graphene structures, which is promising in a lot of different ways: structural stability, durability, current, and material availability.

BTW Rest in Peace John, your good deeds outweigh the bad: a true scientist worth remembering for all time.