Initial issue solved by changing the build of the battery by its manufacturer using an enhanced cell build process. The manufacturer redesigned their packs in a narrower voltage range to reduce thermal signatures. A new charging system was developed by Boeing and the battery manufacturer to more effectively prevent overcharging of the battery manufacturers design. Problem solved. The general understanding as to the initial causes in the incidents is still unclear to this day.
Boeing established teams to retrofit the new battery design. This was 7 years ago.
Li-ion design has progressed significantly since 2013, and they are used in the A350 as of 2016 after Airbus initially dropped them for NiCd after the 787 drama.
Today it is a non issue.