Don't get excited just yet.
The technology already exists to make rechargeable lithium metal batteries (these were the first kind to be produced). But lithium metal is rather reactive, and the batteries had a tendency to catch fire or explode. So lithium metal batteries were replaced by lithium ion batteries, which are the kind we have today. Lithium ion batteries contain no metallic lithium, so are much safer.
In this experiment, they propose to have not only lithium metal, but also possibly sodium metal! Sodium metal is vastly more reactive than lithium metal, so the thought of putting that in a battery is scary.
In short, an interesting experiment perhaps, but a very long way from being a safe, reliable and practical design.
Another point is that if you store 6x the energy inside a battery, it will take 6x longer to charge, and also if it fails it will release 6x the energy in unwanted heat, fire and flames. The big problem with electric batteries is you only have to accidentally short circuit them to let the energy out. The more energy they are storing, the worse the outcome.