I'm pretty sure the phone will only draw the amps it needs even if you use a higher capacity charger. There are a few threads floating around that talk about this.
As far as using faster versus slower charging goes, for lithium ion batteries if the charging rate is kept below 1C (2300mA for Nexus 5) and even better kept below .7C (1610mA for Nexus 5), than according to Battery University there should be no measurable adverse effect on long-term battery capacity from relatively faster charging rates. Also according to Anand Tech (
AnandTech | Google Nexus 5 Review), the Nexus 5 should automatically restrict charging rates to a maximum of 1500mA, so using 2A or 2.1A chargers would not exceed the recommended .7C charging rate.
There are two main things that can be done to protect longer-term battery capacity: avoid deep discharge and avoid exposure to very high temperatures.
My understanding of deep discharge is when the battery charge goes below around 10%. It's optimal on a regular basis not to let the charge go below 20%. With typical usage definitely charge the phone whenever warned that it's getting low at around 15%. Occasionally, if needed to recalibrate battery monitors, it's ok to discharge from 100% to 5%. There is a Joe the Android Guy youtube video on this
( youtube.com/watch?v=rOi1NF7Ipek ) and also Battery University articles
(
http://batteryuniversity.com/learn/a...o_charge_table ).
Maybe the most important thing to protect long-term battery capacity is to avoid exposing the phone to very high temperatures. Battery University warns not to charge lithium ion batteries at temperature greater than 50 degrees centigrade (122 Fahrenheit). It is to be expected that a battery may get lukewarm toward the end of a charge cycle.