Batteries are going to improve rapidly over the next decade, and the battery you replace your current LiFePO4 with at end of life is likely to be much better when that time comes.
One good point that was made is that lead acid starts to behave terribly after losing 20% of its capacity, to the point that they fail outright and can no longer hold a charge. Lithium can usually keep working way longer than that, just with a reduced runtime.
Those cycle life numbers look great, but lead acid batteries in practical applications tend not to last the lifespan advertised on brochures. We can gauge that by how confident the manufacturers are:
* The Trojan SIND 06 610 has a 2 year warranty.
* The NSB100FT - 3 years.
* The Battleborn BB10012: 10 years
The Trojan battery weights a whopping 220lbs and holds 600amp hours. It's not even in the same category as the other two. You need a minimum of two as they output 6v. The price for the amount of storage they provide is pretty good though.
> Lead-acid age by cycle count more than elapsed time, so you could have a battery last well longer than 10 years if you treat it well.
Have you ever seen a lead acid battery last this long? Was it able to hold a meaningful amount of charge?
Lead acid batteries may be cheaper but they are unlikely to survive for as long as claimed.
That said - if you want a battery backup that you basically never use, there is a chance that you'll get a good return on investment on deep cycle lead acid (specifically those industrial ones) – if they don't fail for whatever reason and are properly maintained. For the same amount of money you can buy a ridiculous amount of storage for a real emergency.
Good point about the recycle rate. Lithium is theoretically just as recyclable as lead acid, but the logistics are not there yet (but how will that look like in 10 years?).
Do test the cells individually with an acid tester, a simple hydrometer one is easy to take along when you go shopping. One important thing to keep in mind when buying any kind of battery is that for some batteries there are pretty strict requirements as to how they can be installed, what kind of physical isolation you have to use and how you need to vent any gases that they produce. These rules can easily disqualify some battery chemistries or configurations from consideration.
LiFePO4 age by elapsed time more than cycle count, so after 10 years they will be reaching the end of their useful life regardless of how much you have used them. Lead-acid age by cycle count more than elapsed time, so you could have a battery last well longer than 10 years if you treat it well.
I'd note that lead-acid batteries are divided into deep-cycle batteries that have are designed for longevity and basic batteries that are optimised for peak supply current (for vehicle starter motors). Use a deep-cycle battery, otherwise yes it will die very quickly.
I'd argue that if you're in a part of the world that has unreliable sunlight, such that you want to store say 4 days of energy but rarely actually use it all, then lead-acid is a perfect fit, because you just buy lots of storage at a cheaper price than LiFePO4, and that regime automatically treats the batteries well.
If you're in a part of the world where sunlight is reliably every day and you only want to buy enough battery to last a single night, then I can see that LiFePO4 may be a better option. But I'd still be inclined to buy several days of lead-acid instead. If you want to charge your batteries from cheap early morning grid power and effectively cycle twice a day, then LiFePO4 is definitely the better option.
The other thing that is nice about lead-acid batteries is the recycling rate - they are very easily recyclable and very highly recycled. I don't think we're there with LiFePO4 yet.