You can have an interposer charge controller. It isn't complicated. Many/most cells in consumer products already have one.

Can they experience a thermal runaway? Can it be triggered by minor deformations or dents?

Battery safety has very little to do with energy density. High energy density can make the outcomes worse for an unsafe battery, but doesn’t tell you anything about the risk of the fire to begin with.

The reason current Li-ion batteries are unsafe is that they contain a flammable electrolyte. There are batteries with higher energy density that have solid or semi-solid electrolytes. They’re so safe you should probably worry a lot more about driving your car than charging your battery.

Worrying about LFP or So-ion is probably also a waste of time.

LFP batteries are pretty safe. Look up some videos of demonstrations. You can cut them in half with a metal knife and nothing happens besides the loss of capacity, and I suppose the knife heats up from the short-circuit current as well.

I don't know what kind of chemistry shenanigans are going on to make that happen, but it's impressive.

A tank of diesel and a generator are a kind of battery that you have to worry about very little.

inherently self-limiting processes are nothing rare or special

It's definitely not that cut and dry. Here's a paper with a good breakdown of safety comparisons: https://www.sciencedirect.com/science/article/pii/S2352152X2...

Tldr: while sodium thermal runaways are less dangerous than lithium, they are more likely to happen, and in particular charging a sodium battery to 100% SOC makes thermal runaway significantly more likely than in LFP batteries.

It's true that things like puncture or short circuit are much much less dangerous than with lithium. That is a big part of why the increased thermal runaway risk gets overlooked.

Discharging is not the issue with LiPo4 or other lithium tech. They can work up to negative temperatures without any issue. They can go down to -20°C (-4°F).

The problem is charging the battery at negative, as this damages the internal structure. So you need to heat up the battery (aka, you discharge by warming up heating elements), until you hit the right point. And then you can charge up, what will create heat and thus allow for non-damaging charging.

Some even have a fun trick, where if your going to a EV station with your car (end point route on your build in sat nav). They can start to pre-drain the battery if its really too cold, to warm up the batteries even more. So by the time your at the charger, the batteries are on temperature. I forgot what brand did it that way, seen that feature years ago.

What is different is that Sodium-Ion batteries can charge from negative (i think it was -10°C), as they do not suffer from the same damaging effect.

There's a yttrium-containing variety of LFP that's supposed to work better at lower temperature (down to -45 C). But yttrium itself is quite expensive ($35/kg).

LFMP variant of LFP with added magnesium mostly solved this. Presently it is not as long-lasting as LFP so is not suitable for home batteries, but it covers typical lifespan of a car and solves the issues with cold climate.

lithium batteries can usually discharge below 0C, it's the charging at sub-zero temperatures that kills them.

Lithium can discharge just fine at -25C without heaters.

Charging it at or around 0C will destroy it extremely rapidly due to electroplating of lithium.

I’m hopeful that sodium batteries are going to come down in cost quickly, but if you can find a provider like Base Power or similar where it’s heavily subsidized, there’s a small upfront fee, and they can provide you the economic benefits until sodium batteries reach favorable economic success criteria, that’s a shim until you can replace that with sodium batteries you own outright at a favorable price. You’d have to model what you can get today with a sodium chemistry versus what you can get on offer from one of these battery service providers.

It’s fundamentally a modeling exercise in the forward economic returns curve in the context of your energy needs. Lithium battery lease today as a power purchase agreement (potentially), and then swapping out for a purchased and owned sodium battery pack when the cost hits the right price. It’s always harder to predict the future the further out you attempt to predict, and this technology state of the art is moving very rapidly.