Look specifically for a 2S lipo that is designed for Tx or Rx use. A Tx lipo should have a very low C rating to avoid puffing when left fully charged for long periods of time.
a lipo that comes with an EC3 or XT60 plug will have too high of a C rating for Tx use
GForce 2600mAh 2S LiPO Transmitter TX Pack (Futaba/JR) | Value Hobby
You'll probably need to splice on a Hitec battery connector from your old battery, or did Hitec switch to using a servo connector on the 9X battery?
Pete