Most manufacturers warranty hybrid/electric batteries for 8-10 years or 160,000 km, so 5-6 years of normal use shouldn't be a big problem if the car's been treated well. Obviously it depends on how it's been driven and charged, but generally modern batteries are pretty robust - I know people with hybrids that are 7-8 years old without major performance drops. If you're buying used, I'd get the battery condition checked by a specialized mechanic before you sign anything, that way at least you know exactly what condition it's in.