Although there is no risk of overcharging with the use of a high-quality charger, the battery should not remain connected to the charger for more than 24 hours. The one thing that all experts agree on is that smartphones are smart enough not to allow an overload to occur. The additional protection chips inside ensure that can't happen on a tablet, smartphone, or even a laptop. Once the internal lithium-ion battery reaches 100% capacity, charging stops.
If you leave your smartphone plugged in overnight, it will consume a little energy constantly, giving new energy to the battery every time it drops to 99%. That's eating up the phone's lifespan (see below). The internal battery of my phone is 2460 mAh and I connected a 15000 mAh power bank thinking that would give me days of energy. This means that the external battery will not continue to charge the entire battery, which avoids the concern of whether the phone is safe when connected to a power bank.
A power bank is a portable device that can supply power from its built-in batteries through a USB port. Download the definitive guide to flying with a portable charger, which includes a list of airlines and their battery regulations.