This is a rechargeable, 18650 3.7v 3500mah 15A High Drain PCB Protected Wired Lithium Ion Battery from Keeppower. It has a "button" style top, PCB.
 
Everybody will tell you in the community, these batteries are amoung the best for neopixel lightsabers. Pictures may differ from actual products.
 
*The pictures show 2 batteries but this is not a lot, when you place an order for 1 battery, you will receive 1 battery. 
 
• Protected Circuit Built-in against over charge, over discharge, over current and short-circuit
• Size : 18,8 x 69,0 mm (DxL)
• Weight : 48g
• Nominal Capacity : 3500 mAh / Minimal Capacity : 3350mAh
• Standard Charge Current: 1000mA
• Voltage ; 3.7V
• Discharge Cut-off Voltage: : 2,5V
• Standard Charge Voltage: 4,2V
• Max. Constant Discharge Current: 8A
• Standard Charging Time: 240 minutes
• Certification: CE
• Cell & IC made in Japan / Assembled in China