The Japanese website Nokishita reported on Twitter that Nikon will soon announce a new EN-EL18b battery. It is not clear which existing or new cameras will be compatible with this new battery, but the current EN-EL18a model is used in the top-of-the-line Nikon D5, D4s and D4 DSLRs.
Update #1: the new battery is already listed here.
Update #2: the EN-EL18b battery is already listed on Nikon Europe website. Still not clear why this new version was introduced.
Update #3: the new battery is scheduled to be released on March 10th.