One of the most interesting aspects of this technology, especially nowadays, where the unstable international geopolitical situation has generated several problems related to the supply of materials, is its composition. Molten salt batteries are made of raw materials readily available and found in nature, such as simple table salt, nickel, iron and ceramics, and are also easy to dispose of.
Moreover, they guarantee long life cycles: the FZSONICK data sheet that we have analysed mentions more than 4,500 charge and discharge cycles at 80%: an outstanding number which, if it were true for all uses, would equal the life cycles of lithium batteries with LFP chemistry.
Ultimately, the salt battery guarantees high safety standards, as its intrinsic composition can neither burn nor explode.
We are therefore talking about safe, durable and sustainable salt batteries. So why is it that in 40 years they have never taken the place of lithium batteries?
The real Achilles’ heel of molten salt batteries has always been the fact that, in order to work, they require a very high constant temperature, in the range of 250-300 °C, because only at such temperatures can the salt melt. This aspect brings with it several issues, let us take, for example, a 48V, 200Ah battery delivering 9.6 kWh of energy and analyse what happens in a particular charging phase.
In the graph, it is evident that the charging phase only begins when the internal temperature reaches 270°C. If we pay attention to the lower axis indicating time, we see that it takes between 10.5 and 11 hours just to reach the melting temperature that allows the battery to function. Given the long warm-up times, it is obvious why this technology has never been extended for vehicle use.
Another very important aspect related to temperature is self-discharge. If we start from a 100% SOC, the battery will discharge to zero in 80 hours to keep itself at temperature.
The graph above shows what happens when you disconnect the battery from the charger and leave it standing: the internal BMS will use the stored energy to keep itself at operating temperature, but in doing so it will self-consume its own energy. Therefore, as long as it has energy, the pack will be able to maintain a constant temperature, but it will discharge very quickly. In fact, we see that in 80 hours, the SOC reaches zero, which means that in 24 hours, the battery will use 30 per cent of its energy just to keep itself operational (thus, in a 9.6 kWh battery pack, 3 kWh per day will be wasted just to keep the battery at temperature).
When the battery is discharged to zero, at which point it will start to cool down and will not be able to wotk after another warm-up phase.
This is a major issue, let’s take a concrete example:
Let’s imagine a moving car whose molten salt battery pack works perfectly. The driver, however, reaches his destination at some point and turns off the car, leaving it parked for an unspecified time. What happens if the battery pack cools down? The battery is discharged and the car no longer starts. This is precisely one of the main limitations: Salt batteries consume a lot of energy and should be left permanently attached to the charging post to keep them in operation.
Uses and applications: where does it make sense to use a salt battery?
Self-discharge is not intrinsic to the cell itself, but depends on the thermal insulation used. Obviously, however, the more you isolate it by limiting heat dissipation in the environment, the more it will become untolerant of intense work phases, in which energy is also generated by its internal resistance, at which point it will no longer be able to dissipate heat, becoming overheated.
Molten salt batteries are therefore not suitable for use in the automotive or industrial vehicle segments, which require fast recharging, high discharge power and the possibility of prolonged shutdowns without, however, losing autonomy.
The ideal cycle for these batteries should be:
- Frequent with discharge times of 2 to 10 hours
- With an intermediate power, to aid heating without, however, risking overheating
- With on-grid applications where the battery can remain connected at all times
The following graph shows precisely how the best use is 2 to 10 hours of total backup time and this, concretely, translates into the field use of energy storage, which is ideal particularly in summer, when photovoltaic production is very high. In winter, however, when production is low, it must still be understood that there will be a constant loss of energy to keep it going.