Frankly, I wouldn't worry too much about the longevity of the battery.
https://en.m.wikipedia.org/wiki/Lithium-ion_battery
Typical wear for a Li-ion battery is that it will be reduced to 80% of its original capacity after 500 recharge cycles. It's important to note that they mean full, complete recharge cycles--from 0-100%. If you typically recharge the battery when it reaches, say, 20% remaining power, and you charge it back up to 100%, then that's only 80% of a full discharge cycle.
In my case, on average, I recharge the ZX2 once every 10 days. And by that 10th day, I am indeed at approximately 20% remaining charge.
So that's approximately 36 recharges per year. And realistically, that's only roughly 29 FULL recharge cycles per year, since I'm only recharging 80% of the capacity each time.
Round that up to an even 30 per year, and it becomes obvious that it would take about 16 years for me to reach 500 complete recharge cycles. At which point, the battery would STILL contain 80% of its original capacity.
Of course, the simple march of time would probably speed that number up. But it's still safe to say that one shouldn't notice any appreciable degradation in battery life after two years. And likely, not even after five years. By that time, we'll all have moved on to our solid gold NW-WM1Zs anyway. Amirite?