Molten-salt batteries: pros and cons of a 40-year-old “innovation”
23/11/2022 – All about lithium batteries, Blog
While the future of energy will be renewable, there are no “miracle” solutions and it is important to make things clear.
The episode of LE IENE entitled “Renewables, the storage and battery revolution” generated a great deal of interest in molten salt batteries, which, however, are neither a new nor a perfect technology. Here we analyse how it works, and the pros and cons.
Energy saving and sustainable development are at the heart of a global debate increasingly focused on the energy transition towards the use of renewable energy sources, driven by an increased awareness of the damage caused over time by the intensive exploitation of energy resources and climate change that has convinced countries to adopt green initiatives and policies.
In order to safeguard our and our planet’s health, a radical change in thinking and shared action is absolutely essential, but it is also important to clarify the technologies available today that are truly effective, without falling into the traps generated by false information.
Everyone agrees, our future will be driven by renewable energy, such as wind and solar power, but we must be careful: we are talking about intermittent renewable sources, as they derive from natural flows that are not always available and often difficult to control. We should remember that the sun does not always shine and the wind does not blow every day, so how can their energy be fully exploited?
The introduction of lithium batteries allows us to exploit renewable energies as much as possible through the creation of storage systems, which allow us to store energy and then use it when natural sources fail to deliver it. There are an increasing number of solutions available in the field of energy storage, but not all of them are yet efficient.
The media fuss that was generated after the episode of the well-known Italian TV programme LE IENE on 18 October 2022 entitled “Renewables, the storage and battery revolution” brought the topic of molten salt batteries into the spotlight. This technology is certainly interesting, but neither new nor perfect, as instead it was described. In the Mediaset report, in fact, salt batteries are portrayed as an ideal solution for electrification: high-performance, ecological, safe, almost eternal batteries… But where is the catch?
We talked about it in episode 45 of Battery Weekly 2022, our weekly column on the world of batteries, where our electrification experts Marco Righi, Alan Pastorelli and Daniele Invernizzi shed some light on this much-discussed technology, recounting its origins, uses, advantages and, above all, its many limitations.
A LIVE COLUMN, EVERY MONDAY AT 6PM, ON FLASH BATTERY’S LINKEDIN AND YOUTUBE CHANNELS,
TO TAKE STOCK OF THE LATEST ELECTRIFICATION TRENDS.
So-called “salt” batteries, not to be confused with sodium-ion batteries, are actually sodium metal chloride (SMC) batteries, consisting of a metal-based cathode and a molten sodium anode, enclosed in a steel casing and separated by a ceramic membrane that allows their ions to pass through, but not the electrons, which instead move through an external electrical circuit, during charging or discharging.
Salt batteries consist of many cells that contain a mix of different materials inside them besides salt, such as alumina, iron, sodium or other derivatives such as ferrous chloride and sulphide, nickel chloride, sodium tetrachloroaluminate, etc.
*[Fig.1]
In order to explain how this works, we have taken our cue from the data sheet of the FZSONICK salt battery manufacturer quoted in the report, which illustrates the process in detail, explaining how in the discharge phase, the active materials are sodium chloride and metal powders, predominantly nickel-based, while in the charging phase, these are converted to sodium chloride and metal.
The solid-state electrolyte is ß-alumina, which allows the transport of sodium ions and provides insulation between anode and cathode. The cells must be heated to a temperature of approximately 250 °C to function and are thermally insulated by a special casing.
Many people will ask: but isn’t 250°C too much?
High temperature is precisely one of the characteristic aspects of these batteries. The sodium chloride used must be molten for it to work, which is why the temperature must be kept so high: the salt battery, in fact, only works when the salt is molten and this, like many other salts, melts at extremely high temperatures of 200 to 300 degrees: these are precisely the internal working temperatures of these batteries.
The picture below gives a very clear overview of the composition of a molten salt battery. The core of the battery consists of individual elements (cells) at around 2.6 V each, and around them are several layers of insulation, which act as protection and make up the rest of the battery, to ensure that the core remains at a constant temperature of around 250 °C to 270 °C.
*[Fig.2]
Molten salt batteries have actually existed for over 40 years! It is, in fact, a well-established technology, at a time in history very similar to the one we are experiencing today, when oil prices skyrocketed and led to a real energy crisis where more and more researchers worked to find alternative solutions.
One of the solutions to the energy crisis was the development of the ZEBRA battery (Zeolite Battery Research Africa) based on molten salt, first studied by South African researcher Johan Coetzer at the CSIR (Council for Scientific and Industrial Research) and patented in 1978.
Through the years, this battery has attracted the attention of numerous industrialists, who have further developed and improved it, taking it around Europe, passing it through various hands, from the English AERE Harwell, to the German AEG and the Swiss FAMM, which later became MES-DEA, the first to formalise its small-scale production and also to use it in some vehicles, such as some electric buses.
This company still exists today, under the name FZSONICK, and continues its production of sodium metal chloride batteries, implemented with state-of-the-art systems.
We are certainly talking about a technology with potential, but if it has not developped into large-scale production in all these years, it is surely also because of the various application limitations.
It is true, the technology behind molten salt batteries has never broken through, but like any respectable energy source, it has its pros and cons and is more or less effective depending on the area of application.
Let us look in detail at the advantages and disadvantages of salt batteries.
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.
*[Fig.3]
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.
*[Fig.4]
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.
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:
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.
*[Fig.5]
Many technologies have established themselves over time as environmentally friendly alternatives to polluting fossil fuels, and salt batteries are undoubtedly among them. The signal is therefore positive and there is a growing commitment to a clean energy transition for the benefit of our planet.
But if the word “batteries” is still associated almost exclusively with lithium, the reason is quickly stated.
The high efficiency of lithium-ion batteries makes them undisputed leaders in the field of electrification, with high energy density and the ability to guarantee excellent performance on a continuous basis. In addition, lithium batteries are available in numerous chemistries and utilise different elements and compositions to suit a wide variety of usage situations.
Their widespread use has allowed lithium batteries to achieve a good compromise between price and quality over the years, making them easily accessible, but they also have another great advantage: they are available in different formats and their energy density allows lithium batteries to be sized according to the specific needs of the applications or storage systems on which they are installed.
Lithium batteries are also not to be regarded as unsafe. The battery manufacturer plays an essential role in ensuring their safety and reliability over time starting with the choice of the right chemistry (LFP chemistry, for example, is the safest and most stable on the market), through to correct cell assembly and intelligent control electronics that guarantee stable performance over time.
Finally, it is important to remember that when a lithium-ion battery reaches the end of its useful life on a vehicle, i.e. when its remaining capacity falls below 80%, it will still have numerous possibilities for use in other areas, one of which is energy storage, for powering the utilities of homes and buildings. This makes it possible to improve the operation of electricity grids and, at the same time, make greater use of energy from renewable plants.
To be clear, condemning one technology while extolling another is not the intent of this article, but as we have always liked to emphasise, it is important to provide clarity, especially in a sector such as batteries, which is evolving every day with new and very different technologies.
The future of energy will undoubtedly be renewable, and we are all moving towards sustainable choices, but we must bear in mind that there are no technologies that are simply miraculous: each one has positive and negative aspects and it is up to us to inform ourselves as best we can, weighing up the merits, shortcomings, limitations and opportunities, and clearly assessing the goals we want to achieve.
WHY THE FLASH BATTERY LITHIUM BATTERIES ARE A DIFFERENT BREED FROM THE COMPETITION ONES
SUBSCRIBE TO THE NEWSLETTER TO RECEIVE NEW ARTICLES
AND FLASH BATTERY INSIGHTS
Source Fig. 1: Example of molten-salt battery. Image taken from GeoPop’s article on 19/11/22. bit.ly/3ESXSox
Source Fig. 2: Composition of a salt battery. Image taken from manufacturer’s website FZSONICK on 22/11/22. http://bit.ly/3XmpvNP
Source Fig. 3: Salt battery charging curve with pre-heating phase. Image taken from manufacturer’s website FZSONICK on 22/11/22. http://bit.ly/3XmpvNP
Source Fig. 4: Self-discharge of salt batteries. Image taken from manufacturer’s website FZSONICK on 22/11/22. http://bit.ly/3XmpvNP
Source Fig. 5: Best conditions of use for salt batteries (2 to 10 hours of back up) . Image taken from manufacturer’s website FZSONICK on 22/11/22. http://bit.ly/3XmpvNP