I can't remember the specific year that Pokemon stopped releasing the energy cards specific to the sets. At some point, they only released them as generic cards throughout that year's releases. This means that the energy cards in 2013 were released in four releases that year, Plasma Storm (2/6/2013), Plasma Freeze (5/8/2013), Plasma Blast (8/14/2013) and Legendary Treasures (11/6/2013). In addition, since they were released throughout all of the releases, they were un-numbered and did not have the icon to tell what set they were included with. The only information that is on them is the year's copyright.
One other thing to note, this only applies to the nine basic energy cards. (Grass, Fire, Water, Lightning, Psychic, Fighting, Darkness, Metal, and Fairy)
The special energy cards mostly were included with the sets with propoer numbering and the release icon. There are 78 of those.
I hope this helps.
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