(Keynote) Are Chemistries Other Than Lithium Viable?
Authors: Armand Michel 
Published: February 2020

The battery industry totals nearly 100 MWh of storable capacity, and increases at ≈ 10% per year. The largest fraction by far is LIBs.

The realization that Li is a relatively rare element (20 ppm of the earth crust) and spread in a small number of countries raised fear of a shortage, possibly preventing the full deployment of LiBs in the EV sector and grid storage, foreseen in the coming years. However, the first limitation seem to be in the cathode materials, with a shortage of cobalt already felt, considering also that this metal is mined in conflict-torn countries; Co is borderline to be considered a conflict element. There is a drive to reduce the Co fraction in electrode materials, for instance in NMC 8.1.1. containing only 10% of cobalt, but at the expenses of safety.

LiFePO4@C (LFP) is likely to be widely used in gris storage batteries, where only the lithium (and the copper current collector) have sustainability problem. > 10 years lifetime are now expected, and the round trip efficiency of a Graphite/LFP battery is higher that that of higher voltages oxides which require mort stringent cooling. EV types batteries using LFP suffer from a lower energy density than NMC ones, but are to be adopted massively for bus propulsion in cities.

Lithium metal polymer batteries have been successfully developped in one instance, and they can be seen a giving the same energy density as NMC or NCA ones, with only LFP. All-extrusion technology give this systems a cost advantage. The actively thought-for polymers with a lower operating temperature may be around the corner with polyesters, that also are stable to high voltages, though the Li° operation seems problematic

Sodium had been saved from oblivion @ 10 years ago, considering the Li supply. Research on this chemistry is very active, with start-ups lining for future commercialisation. The likely materials contenders are hard carbon and Co-free lamellar oxides or vanadium fluorophosphates. However the other proposed negative electrode materials (Sb, Sn) or even the V fluorophosphates only displace the problem as they use rare elements, which is not the philosophy of Na Batteries. As a rules, these batteries suffer from a very high first cycle loss, which can be compensated by the use of sacrificial salts.

Magnesium has drawn attention also recently and after many attempts, smooth magnesium plating/ dissolution was obtained for instance with the use of carborane (B11CH12)- magnesium salt. However, one must remember that B is scarcer the lithium and the development of more sustainable salts is needed.

The main problem with Mg is its lack of suitable positive electrode material. Mg++ is quasi immobile in the oxides where it intercalates (≠ Li). Sulfides fare a little better at the expenses of the voltage, and the possible intercalation of the MgCl+ species is also a handicap on the energy density. The use of S electrode would be in principle easier than for Li, the MgSn species being polymers, but the electrochemistry proved up to now to be disappointing.

Aluminum is also a contender. The reversible plating/dissolution of Al has even proven in AlCl4
-/Al2Cl7
- melts that have been at the origin of the development of room-temperature ionic liquids that are intrinsically inexpensive. There is however no intercalation of Al3+, this ion being more a scaffolding part in solids with total immobility. Anion intercalation in graphite has been shown to be reversible and fast, but the calculated energy density is very modest. All the oxide material that have been tested (V2O5…) are dissolving in the highly corrosive melts. Thus, the Al electrochemistry will have to use Cl-- (or F-)- exchanging electron material, i.e. using the electrolyte as a reservoir for Al and Cl species. This would of course be operated at the expense of the energy density. Another problem is the finding of a suitable + electrode current collector, again considering the corrosiveness of this media.

The pro and cons of those different chemistries will be reviewed and discussed in the perspective of an exponential demand for energy storage devices in the forthcoming years