Hi
I have an iPhone charger by my bed and in my living room as well as in my bag to keep my iPhone 7 charged and ready to go.
The one by the bed is the original charger that came with the phone. The two others are iPad chargers. I have bought and use those because they simply charge the iPhone much, much faster.
But I recently heard that using those might do damage to the iPhones battery long-term. Is that true?
Thanks 
I wouldn’t worry about using an iPad charger from Apple (or some other reliable brand) to charge an iPhone. They scale the power level up and down based on how much the connected device requests.
My understanding of it is that the faster the battery charges the warmer it gets, the warmer it gets the more it degrades the long term performance of the battery. While the phone will only pull as much power as it can handle I think that’s a short term consideration (i.e.: it won’t errrr shouldn’t blow itself up). I recall reading about a way to calculate a “rating” based on the charger power / battery capacity, can’t find where I originally read about it but did find this article: https://www.forbes.com/sites/eliseackerman/2014/12/19/will-rapid-charging-damage-your-iphone-6-or-6-plus/#4403e7f03aaa (see the section on C-rate about half way through).
As far as I can tell from my research, you’re not going to fry your device in the short term but for best long term results slower=better. We’ve got a few iPhones & an iPad in the house so we keep some of these chargers around which so far have done well: https://www.amazon.co.uk/Syncwire-Plug-Travel-Adaptor-Interchangeable-6-8A-34W/dp/B07HYYQ962/ - they’ve got both 1amp & 2.4amp outputs. I did have to buy a replacement one once though, because the girlfriend lost one in a hotel somewhere! 
I wouldn’t worry about it. Apple has a lot of people working on battery tech so we don’t have to think about these things. Chargers are smart, batteries are smart, thankfully the days of NiCads are over.
I charge both iPad and phone by the bed at night. I use a Belkin charger that has two USB ports, one high power for the iPad and one low power for the iPhone. The iPad one is marked by a dot of red nail polish on the lightning connector and on the USB end as well so that I can tell the two lightning connectors apart. Works a treat. I plug the phone in to the plain one and the iPad into the one with the red dot.
If I want to give the phone a quick burst of charge I sometimes plug it in to the higher powered one, but on the whole it seems safer to charge it on the power rating it’s made for. But I could be just being over-cautious.
My iPhone 6 is going MUCH better since I had the battery replaced, too! No more annoying shutdowns.
Judy
I use my MacBook charger with a USB-C to lightning cable all the time. Never had any issues.