The iPhone X offers a non-removable Lithium-Polymer battery. It supports 5/12-watt and for fast charging use 18-Watt which provides a 50% charge in 30 minutes. Apple claims it can stay in standby mode for 20 hours.

The possible battery issues of the iPhone X which a user may face are:
Charge Drain: Sometimes the battery of the iPhone X drains faster than it should. Or sometimes the battery drains drastically in standby mode. This is a common issue reported by users after some period of prolonged usage.
Sudden Shutdown of Phone when battery percentage reaches 20: Some users report that their phone dies suddenly when the battery reaches 20%. This might be because your battery is old and swollen and need to be replaced.
Battery Health Reduces Quickly: A common issue of Battery problems regarding iPhone X is that the general Battery Health reduces too quickly.
Increase of Battery Thickness: The iPhone X battery gets swollen/ increases in thickness after using it for an extended period of time. When a battery gets swollen, it runs the risk of damaging the phone. It should be replaced immediately because it is also a sign that the battery has run its course.