4 ms·

"Molten salt" would need a bit of heating for sure.

adrian_b 4y ago
Certainly. Nevertheless, this is without doubt one of the most promising news about batteries ever published. It was well known that an aluminum-sulfur battery could have very high performance, but previously nobody was able to make one that could be recharged without damage. The point of this announcement is that at MIT they have discovered a molten salt electrolyte that not only allows recharge without damage, but it also allows the recharge at a very high current. An aluminum electrode is among the best materials from the point of view of the ratio between electrical capacity and mass. It is worse than a lithium electrode only in the ratio 9/7. However, in a lithium-ion battery, the electrode that stores lithium also contains other materials than lithium, so it is much heavier (and also much more expensive, because the other materials usually include nickel and/or cobalt). So an aluminum-sulfur battery should be both lighter and much cheaper. This aluminum-sulfur battery will never be able to replace lithium batteries at small capacities, e.g. in a laptop or mobile phone, because it must be operated at a temperature high enough to melt the electrolyte, so it requires a thick thermal insulation, which would take too much of the device at small volumes. On the other hand, at big capacities, such as for a car, it has extremely good chances of replacing the lithium-ion batteries. The longevity of such a battery might be limited not by the number of recharge cycles, but by the time of operation, because the molten salt might slowly corrode whatever materials are used for making the battery. Nevertheless, there is a lot of experience with making enclosures and electrodes resistant to molten salts. Moreover, this battery uses very low-temperature molten salts, which are heated only a little above the boiling temperature of water. Such a low temperature diminishes a lot the risks of corrosion and it allows the use of reduced thermal insulation.
- jsmcgd 4y ago
  I agree 110 degrees Celsius is probably too much for a mobile phone, but my laptop has approached 100C before. I'm sure if properly designed it could handle the slightly higher temperatures. Any thoughts on what the specific energy might be?
  - adrian_b 4y ago
    Neglecting the additional mass for a heater and for thermal insulation, the specific capacity in ampere-hour per kg should be less than for non-rechargeable lithium batteries (which have a slightly lighter lithium metal electrode and also lighter organic electrolyte), but much higher than for lithium-ion rechargeable batteries. The mass of the heater and of the thermal insulation will diminish the specific capacity only a little for large batteries, but much more for small batteries. To get the specific energy we also need the output voltage, which is mentioned neither in the paper abstract nor in the press release. In any case, the voltage will be much lower than for lithium-ion batteries, so it is likely that despite the higher specific capacity such Al-S batteries will have lower specific energy. Even so, the difference in cost could be so large that this would not matter. It is hard to say which will be the output voltage, because the enthalpy of the reaction can vary a lot in function of the structure of the sulfur deposited on the electrode during charging and on the type of sulfide that forms during discharging. Googling for the enthalpy of formation of aluminum sulfides provides contradictory information, with various values corresponding to output voltages between 0.7 V and 1.4 V. Anyway, it is probable that the output voltage of an Al-S battery would be at most half, but more likely no more than a third of that of a Li-ion battery. Probably the specific energy will be less than for Li-ion, but it is hard to predict if the difference will be significant or negligible. On the other hand, for the energy per volume it is much more likely that it could exceed the energy per volume of the Li-ion batteries.

Certainly. Nevertheless, this is without doubt one of the most promising news about batteries ever published. It was well known that an aluminum-sulfur battery could have very high performance, but previously nobody was able to make one that could be recharged without damage. The point of this announcement is that at MIT they have discovered a molten salt electrolyte that not only allows recharge without damage, but it also allows the recharge at a very high current. An aluminum electrode is among the best materials from the point of view of the ratio between electrical capacity and mass. It is worse than a lithium electrode only in the ratio 9/7. However, in a lithium-ion battery, the electrode that stores lithium also contains other materials than lithium, so it is much heavier (and also much more expensive, because the other materials usually include nickel and/or cobalt). So an aluminum-sulfur battery should be both lighter and much cheaper. This aluminum-sulfur battery will never be able to replace lithium batteries at small capacities, e.g. in a laptop or mobile phone, because it must be operated at a temperature high enough to melt the electrolyte, so it requires a thick thermal insulation, which would take too much of the device at small volumes. On the other hand, at big capacities, such as for a car, it has extremely good chances of replacing the lithium-ion batteries. The longevity of such a battery might be limited not by the number of recharge cycles, but by the time of operation, because the molten salt might slowly corrode whatever materials are used for making the battery. Nevertheless, there is a lot of experience with making enclosures and electrodes resistant to molten salts. Moreover, this battery uses very low-temperature molten salts, which are heated only a little above the boiling temperature of water. Such a low temperature diminishes a lot the risks of corrosion and it allows the use of reduced thermal insulation.

jsmcgd 4y ago
I agree 110 degrees Celsius is probably too much for a mobile phone, but my laptop has approached 100C before. I'm sure if properly designed it could handle the slightly higher temperatures. Any thoughts on what the specific energy might be?
- adrian_b 4y ago
  Neglecting the additional mass for a heater and for thermal insulation, the specific capacity in ampere-hour per kg should be less than for non-rechargeable lithium batteries (which have a slightly lighter lithium metal electrode and also lighter organic electrolyte), but much higher than for lithium-ion rechargeable batteries. The mass of the heater and of the thermal insulation will diminish the specific capacity only a little for large batteries, but much more for small batteries. To get the specific energy we also need the output voltage, which is mentioned neither in the paper abstract nor in the press release. In any case, the voltage will be much lower than for lithium-ion batteries, so it is likely that despite the higher specific capacity such Al-S batteries will have lower specific energy. Even so, the difference in cost could be so large that this would not matter. It is hard to say which will be the output voltage, because the enthalpy of the reaction can vary a lot in function of the structure of the sulfur deposited on the electrode during charging and on the type of sulfide that forms during discharging. Googling for the enthalpy of formation of aluminum sulfides provides contradictory information, with various values corresponding to output voltages between 0.7 V and 1.4 V. Anyway, it is probable that the output voltage of an Al-S battery would be at most half, but more likely no more than a third of that of a Li-ion battery. Probably the specific energy will be less than for Li-ion, but it is hard to predict if the difference will be significant or negligible. On the other hand, for the energy per volume it is much more likely that it could exceed the energy per volume of the Li-ion batteries.