iPhone 6 has Li-ion batteries with intercalated lithium compound material more effective compared to the metallic lithium used in a non-rechargeable lithium battery.
iPhone 6 battery and logic board are built in with dual overcharge protection,
Useful Tips for a healthy and long-lasting battery:
We highly recommend not to keep your iPhone charging overnight, disconnect the iPhone once it is fully charged.
Do not use the cheap charging cable.
Turn-on Auto brightness.
Keep Auto Lock time short.
Disable Unneeded vibrations.
Avoid keeping you iPhone in moisture or in hot car in parking.