Great answers already, I’d like to add a few things: Lithium batteries like to be charged at 1C, meaning that if they have a capacity of 1.6 Ah they like to be charged at 1.6 A and will (theoretically) be fully charged in one hour (this is wrong in practice though). You can charge most of them at a higher rate (e.g. at 3.2 A), but they should be rated for it. If you don’t better, stick to a lower amperage. Too low of an amperage should not be an issue but I’m not sure - it will take ages though.

The voltage has to fit! Batteries change their voltage over the course of charging/discharging. This is more pronounced in older NiMH or NiCdH (discard those) batteries but still relevant. A 12V charger can charge your batteries to 12V. Most chargers can handle different voltages and will usually select the correct voltage depending on the battery. This is important if you are charging LiIons or LiPos (two types of Lithium batteries), as overcharging them is NOT advised. They generally peak at around 4.2V per cell I think with a working charge (don’t know the term) of 3.7V per cell. Some chargers require you to select the number of cells. Charging a 2-cell LiPo with 12-13 volt is a bad idea I wager and charging a 3-cell LiPo with 9.something volt won’t work (it won’t drain it, but the minimal voltage is higher than what the charger provides - current would like to flow the other way if It could).

TL,DR: If you can adjust the charging parameters manually or you are doing something the manual states you shouldn’t be doing, educate yourself. If however you only plug in your phone (with a painfully small battery at 2 Ah) and only provide a 1.5A*5V=7.5W charger, you are perfectly fine.