[BurningFrog]

> most other electricity storage mechanisms are stationary installations that can only time-shift, but not space-shift

It would be perfectly possible to space-shift by charging up batteries with Australian solar power, ship them to Japan and then ship back for recharge.

I assume that since it's not done, it's not currently practical.

[pjc50]

Requires a huge capital investment in batteries, which are then idle for the voyage in both directions.

However, we can think this through a bit: what is a battery? It's a pair of redox reactions, involving two electrodes and either one or two electrolytes. The electrolytes are usually liquids. That led to the concept of the "flow battery": https://en.wikipedia.org/wiki/Flow_battery

So, if the electrolytes can be stored in tanks separate from the electrodes, theoretically you could put the tanks on ships and cycle the electrolytes between countries.