I found this battery, but I can not confirm that is the right one, I haven't opened my mug yet.
https://batteryclerk.com/products/ember-mug-2-smart-mug-replacement-battery?srsltid=AfmBOopBc1_lVKU8ROvTHKvU-rrEMF18d4qjAQ28369woNMABmfBljk1
***SKU*** CS-EMB200SL
== ***Ember Mug 2 Smart Mug Replacement Battery*** ==
***INFO***
* This is a replacement compatible with a Ember Smart Mug Battery
* Volts: 7.4V (7.4 Volts)
* Capacity: 3500mAh (3.5Ah)
* Type: Li-ion
* Color: Blue
* Ember Mug 2 Replacement Batteries.
* Mfgr Part # INR18650 MH1-2S1P
* Dimensions: 2.61 in. L x 1.46 in. W x 0.78 H in.
* Weight: 0.3 lb
* Warranty: 1 Year
Disclaimer: Our products are not affiliated with or authorized by Ember.