Air carrier Captain reported a passenger's Lithium Ion battery powered thermos experienced a thermal runaway during boarding. The Captain removed the thermos to the ramp where the fire department took command of the situation.

Narrative

During boarding the number one Flight Attendant reported smoke in the cabin. I could smell an electrical odor and shut off the recirc fans and the packs. I also closed the bleeds engine and APU just in case that was the source. Flight Attendant and Passengers were starting to panic from the smoke as I exited the cockpit. The cause of the smoke was a lithium ion battery attached to a thermos. The owner was coming forward with it. I quickly grabbed it and exited the aircraft. I proceeded out of the jetbridge to a place on the ramp that wasn't close to anything. When I exited the aircraft and told the Flight Attendant to deplane the aircraft thru the forward door. There was a fire extinguisher on the ramp I uncovered and monitored the situation. The fire department arrived and took command of the situation.The number one Flight Attendant came to the cockpit to see if we had a containment bag. Theirs is in the middle of the cabin and was not able to be reached because of the people in the aisle. A bag in the forward galley would be helpful. She was hesitant to grab the thermos and exit the aircraft. In my opinion this may have been from the FAA and companies' policy of the Flight Attendants are not allowed to exit the aircraft with Passengers on board. It was a high stress situation; and we are briefing them to not leave the aircraft when the truth is action needs to be taken and that was removing the threat. It all worked out and will be hard to prevent. I do think Passengers are going to carry Lithium-ion batteries on the aircraft in things we are not expecting. A containment kit in the forward galley would be helpful. The Passenger doesn't even understand the accidental danger of what they are carrying.