EMB-145 Captain reported his personal phone expanded in size during cruise. As a precaution the phone was secured in the thermal bag and continued to a safe landing.

Narrative

In the cruise on the way to ZZZ I reached for my phone to check something in my logbook app; I noticed my phone was at 10% battery so I removed my personal spare battery (Same brand but different model as the company supplied backup battery) and went to connect it to my phone when I realized it had expanded in size about 50%. It's normally about half an inch thick and was now about 3/4 of an inch thick. It was cool to touch at least so I wasn't concerned about it immediately catching fire but also didn't want it to develop into a worse situation at a less convenient time so I showed the FO (First Officer) and explained I intended to get the FA (Flight Attendant) to put the battery in the thermal containment bag and asked him to take the radios. I Made and a Call to the FA; and asked her to fetch the 'thermal bag'; it must have caught her by surprise because it took a moment for her to understand what I was asking for her to do. I didn't bother using the company format as I didn't want her to waste time writing anything down at that stage I just wanted her to get the bag. She went to get the bag and I got out of my seat and ready to give it to her and also so I could reach the gloves in case the battery started heating up which it didn't. The FA called me at the door with the bag shortly later and I spotted her through the door opened the door and let her deal with it from there. I gave her a minute or two to do what she needed then called her to check how she was with it and she confirmed she'd dowsed it in ice and water and stowed it in the steel container. I left the seatbelt sign on for a while after this to ensure passengers moving through the cabin wouldn't interfere with her if she needed to do something. Quickly reviewed the QRH on my Ipad from the EFB Fire concern page; seeing that it flowed to the smoke fire fumes checklist I agreed with the FO that this did not really apply. I took the radios back and sent an ACARS message to dispatch to see what would be necessary on arrival in ZZZ to be legal to return to ZZZ1 in regards to MELs and what not. I briefly chatted with the FA at some point I think just after landing. We had a quick casual debrief of the event after the passengers were off and none of us could identify any major issues with what had happened; we asked the rampers if we could have the bag after they'd dealt with the battery to return the bag to ZZZ1 so maintenance could re-fit it if necessary. They obliged.My only concern from the event was that given the battery does have electrical components that may not be water proof I was worried that dowsing them in water may short out the battery and initiate a thermal runaway. That may have otherwise been prevented. Not sure if this is something to be looked into in terms of the guidance our FAs receive.