At only 0.375mm thick, these lithium-ceramic batteries are awesome for prototyping wearables and other other ultra-thin devices. They are extremely flexible and are designed to withstand a lot of abuse, including cuts and punctures(!). Check out the technology overview video below; it's kind of ridiculous.

I've successfully used them with standard lithium charging ICs.

SPECS:

• Capacity: 105mAh
• Nominal Voltage: 3.75V
• Dimensions: 50x100mm
• Thickness: 0.375mm
• Part Number: 050100
• More info here

NOTES:

• These batteries do not have a protection circuit.
• Although these batteries can keep working even after being severely damaged, I would not recommend relying upon a damaged battery to continue functioning to spec in any application.
• I'm not affiliated with Prologium in any way; I have a few of these left over after a project so I'm putting them up here to see if anybody else is interested in messing with them.
• These are really hard to source in low volumes, so get 'em while they last!