I doubt you need worry, all these e-car batteries heve been greatly exceeding both warranty periods and expectations. The waranty periods were just guesswork anyway since none had been around lomg enough to actually know. Nissan first guessed 5 years for bought batteries, their partner Renault guessed 4 years if they were rented. When they exceeded that both companies changed to saying 8 years, thus setting the standard, but many of them have been lasting beyond 10 years.
Key thing though is to avoid the rapid and ultra rapid chargers for the longest life. If your wife only uses the fast home charger it will last longest, it has sufficient range for her round trip anyway with a third of the charge to spare.
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