One should pay attention to certain things when buying a power bank. Let me list out a few. -
Dont Go for Cheaper Ones
Cheaper power banks normally use refurbished batteries. As stated in the above response, go for a branded device with warranty.
Capacity You Need
Dont just go for higher capacity power banks. They may even not be suitable for your device. Buy the one that meets your requirements.
Using connectors that are not original may damage your charging point on the phone. It may also cause short circuit. Using original connector will ensure stable output from the power bank.
Ensure the power bank has some features that are safe for your device. Make sure the power bank has Short circuit protection, Over charging protection and temperature protection.
The power bank you sholud have UL certification, or any kind of quality certification that applies to your region. It will ensure that the device you are buying is of standard quality.
If you follow all these tips and take extreme care while selecting a power bank, it will not damage your device.
Live....and Let Live!