superior replacement battery for solar outdoor spotlight?

Dunno. You can somewhat predict the useful life of the battery in days by estimating the number of charger/discharge cycles. At 90F, my guess(tm) is that you're probably good for about 300 charge/discharge cycles, which is about a year.

While only one factor, trying to charge and remain charged at 100% capacity while hot is the magic formula for killing LiIon batteries. I suggest using a larger capacity LiIon battery, and that will never fully charge, thus extending the battery life. Also, make an effort to keep it cool, such as painting it white or reflective. You'll never use or need the full capacity of the battery, so your lights should work fairly normally.

Another idea is to use supercaps. You'll need a fairly large number depending on load. They're also rather expensive and bulky. However, they last for millions of charge/discharge cycles.