Chargers work from 95-230 Volts, so unless some country has recently changed its entire electrical grid, the charger is "compatible" with any grid. The question is whether the plug will fit - British plugs don't work anywhere but Great Britain, Australian plugs are also strange. Make sure it comes with a US plug. (And check the model number [not just G6, G7, but the XT number, then look up the frequencies at
G6 or
G7, to see if it includes the frequencies your carrier needs.)
If it's the European, Indian or Brazilian versions, it may not have the frequencies you need. (And there's usually no US warranty with European or "international" variants.)