it’s still a Li-ion (like most current rechargeables) but the electrolyte - the medium that transfers the ions from the anode to the cathode - (the + and -) inside the battery is made of a solid material instead of the current gen liquid ones. The benefits are less weight/size (as liquids take up more space than solids) and a more stable composition - the liquid electrolyte can’t leak - the way batteries get gunky and corroded if left for a while