Though there has been quite a bit of development on lead acid batteries, and the last 10-15 years have actually been quite exciting. The focus is more on the deep cycle/energy storage side of them, as opposed to car batteries (which, as you note, are largely a solved problem). But there have been some very real innovations and developments, mostly related to carbon additives into the plates in various forms.
Cycling lead acid creates sulfation on the plates - it's quite literally how the chemistry works. On most of the pure metal plates (lead or alloyed for strength or other useful attributes), running at partial state of charge for any real length of time tends to turn these soft sulfation points (that dissolve back during charging) into a harder form of sulfation that doesn't easily dissolve back. This leads to a capacity loss, and general loss of function as more and more of the plate area is covered in these hard crystals that block any real function.
Sometimes an equalize charge (high voltage) can break them up, sometimes various pulse chains can help break them free or re-dissolve them, but it's been a long standing problem with lead acid, especially for energy storage (which is more likely to be partial state of charge for long periods of time).
In the past decade or two, people have been experimenting with various carbon additives in the plates, and for reasons I don't understand (and I'm honestly not sure they understand either), these work to help prevent the sulfation from hardening. You see it in most of the solar focused batteries these days (Trojan's name for it is Smart Carbon), and what it means is that even after a few weeks of running at partial state of charge, the battery is more likely to full drive off the sulfation and charge fully when recharged. It improves capacity, and substantially improves lifespan in certain operating modes.
And to touch on car batteries, the various "mild hybrid" systems that adjust alternator output to help improve efficiency (leave alternator load light until braking, then load up the alternator and charge the battery quickly) tend to benefit from this sort of change as well, because they cycle a car battery far deeper than a typical starter only use would do.
Pushing things to the limit of lead acid technology, Firefly Energy (which I believe was spun off Caterpillar - yes, the earth moving equipment company) has an interesting lead-carbon-foam battery that, from everything I've seen, behaves an awful lot more like a lithium battery than anything normal for lead. It provides a lot of current, and it more or less doesn't care what state of charge you keep it at. Charge it fully for a cycle or two, and even after long extended partial state of charge operation, it will recover original capacity. I believe it's also a lot less prone to shedding active material during charging, which is the end case of a lot of lead acid batteries (you literally end up with the bulk of the plate material at the bottom of the cell, and a good deep cycle battery will have a larger well at the bottom to help prevent shorting a cell group).
Despite being 150 years old, there's still very much a lot of new development in lead acid batteries, and they've continued to improve quite a bit in recent years.
Plus, unlike lithium, we actually have incredibly good recycling for them. Something like 98% of lead acid batteries are recycled (at least in some countries, others have low rates), and just about every part can be reused. The lead, the acid, and the casing can all be quite reasonably recycled (smelting, purifying, and grinding into plastic pellets) into new batteries with no real loss of capability.
You can look at the 25Y lead price chart here: https://tradingeconomics.com/commodity/lead
So much of the difference comes from that.
Though there has been quite a bit of development on lead acid batteries, and the last 10-15 years have actually been quite exciting. The focus is more on the deep cycle/energy storage side of them, as opposed to car batteries (which, as you note, are largely a solved problem). But there have been some very real innovations and developments, mostly related to carbon additives into the plates in various forms.
Cycling lead acid creates sulfation on the plates - it's quite literally how the chemistry works. On most of the pure metal plates (lead or alloyed for strength or other useful attributes), running at partial state of charge for any real length of time tends to turn these soft sulfation points (that dissolve back during charging) into a harder form of sulfation that doesn't easily dissolve back. This leads to a capacity loss, and general loss of function as more and more of the plate area is covered in these hard crystals that block any real function.
Sometimes an equalize charge (high voltage) can break them up, sometimes various pulse chains can help break them free or re-dissolve them, but it's been a long standing problem with lead acid, especially for energy storage (which is more likely to be partial state of charge for long periods of time).
In the past decade or two, people have been experimenting with various carbon additives in the plates, and for reasons I don't understand (and I'm honestly not sure they understand either), these work to help prevent the sulfation from hardening. You see it in most of the solar focused batteries these days (Trojan's name for it is Smart Carbon), and what it means is that even after a few weeks of running at partial state of charge, the battery is more likely to full drive off the sulfation and charge fully when recharged. It improves capacity, and substantially improves lifespan in certain operating modes.
And to touch on car batteries, the various "mild hybrid" systems that adjust alternator output to help improve efficiency (leave alternator load light until braking, then load up the alternator and charge the battery quickly) tend to benefit from this sort of change as well, because they cycle a car battery far deeper than a typical starter only use would do.
Pushing things to the limit of lead acid technology, Firefly Energy (which I believe was spun off Caterpillar - yes, the earth moving equipment company) has an interesting lead-carbon-foam battery that, from everything I've seen, behaves an awful lot more like a lithium battery than anything normal for lead. It provides a lot of current, and it more or less doesn't care what state of charge you keep it at. Charge it fully for a cycle or two, and even after long extended partial state of charge operation, it will recover original capacity. I believe it's also a lot less prone to shedding active material during charging, which is the end case of a lot of lead acid batteries (you literally end up with the bulk of the plate material at the bottom of the cell, and a good deep cycle battery will have a larger well at the bottom to help prevent shorting a cell group).
Despite being 150 years old, there's still very much a lot of new development in lead acid batteries, and they've continued to improve quite a bit in recent years.
Plus, unlike lithium, we actually have incredibly good recycling for them. Something like 98% of lead acid batteries are recycled (at least in some countries, others have low rates), and just about every part can be reused. The lead, the acid, and the casing can all be quite reasonably recycled (smelting, purifying, and grinding into plastic pellets) into new batteries with no real loss of capability.