Metal-air systems, and lithium-air in particular, are all the rage today, thanks to their theoretical energy density (3,500 Wh/kg), which is fifteen times greater than that of lithium-ion batteries, and their eco-compatibility. However, to make such systems operational, numerous scientific and technological hurdles linked to i) the system's low energy efficiency due to wide polarization, ii) its low rechargeability, iii) the electrolyte's limited compatibility with oxygen and iv) Li's dendritic deposition at the negative electrode - to name but a few - need to be overcome. These issues and the research they have generated to date have been discussed.
The formation of the (O-) ion, a powerful nucleophile, during O2 reduction is problematic, as it leads to the degradation of most electrolytes known to date. Fundamental studies on the mechanics of the formation of superoxide species LiO2 and peroxide Li2O2 have shown that the growth mechanism of the oxide produced depends on the donor number of the solvent (ND). This has important consequences for the performance of Li-air cells with maximum capacity (1,200 mAh/g Li2O2) at high ND electrolytes (DMSO). However, Li2O2 is a poor electrode material due to its insulating properties, resulting in mediocre power performance. To counteract this drawback, the concept of using Li2O2 to store charge and a redox mediator (DiButhylBenzoQuinne, DBBQ) to transfer electrons has been successfully pursued. However, the challenge remains to find another redox mediator that can operate in oxidation and work synergistically with DBBQ. Numerous catalysts (oxides, even metals such as Pd) have been used with limited success to enhance the functioning of the positive electrode (e.g. carbon tissue). Attempts to replace the Li electrode with a Si electrode to overcome dendrite problems have not been much more convincing. This work, extended to other M-air systems (M = Al, Ni, Zn, Co Cd), in non-aqueous or even aqueous media, was full of exciting surprises from a fundamental point of view, but a nightmare in terms of applications. The elegant idea of taking advantage of the addition ofCO2 to the Li-air system to increase system capacity was also quickly disproved.
The lecture concludes with a quote from Toyota, a fervent supporter of Li-air technology since its inception: "Li-air batteries may or may not ever happen, but it's not for lack of trying". This sums up the craze for Li-air batteries, which mobilized scientists, industrialists and financiers, only to be forgotten in the space of five years, giving rise to another fad, the all-solid batteries.
