Not illegal. But the airline I traveled on (not one of those listed) wouldn't let me carry any in checkin baggage, and no disconnected LiON battery in carry-on (only in, as part of, a "device"). Although in practice they didn't stop devices or spare device batteries in check-in or carry-on, I wasn't game to carry on the larger disconnected battery I wished to transport.
Biting the hand that feeds IT
