Keep it at a cool-ish room temperature. Never let it freeze, never let it get hot. Use it little a few times a year. It will likely be 70% as new after 6 years. The chemistry is similar to humans basically.
As cells age naturally, they age faster if not used, and age even faster if used often, they call for to get the lowest cost one just powerful enough to do the job and use them regularly for maximum benefit in terms of $/use.
I’m still regularly using a 6700 Powercore II, it’s in my summer EDC kit. Invoice says > 3 years old. If I had paid extra $ for it to be larger, then the excess capacity would still be aging from non-use.
The oddity in all the Powercore I bought is the very basic Powercore 10000. I bought two, gave one to one person, another to someone different, I see them in use regularly, I ask how they are doing and feedback seems they are as-new, a few scuff marks is all that’s apparent. That product was well designed and built.