Conditions Extrêmes et Matériaux : Haute Température et Irradiation
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2016

ACL
doi
OpenAccess

L.Lutz, W.Yin, A.Grimaud, D.Alves Dalla Corte, M.Tang, L.Johnson, E.Azaceta, V.Sarou-Kanian, A.J.Naylor, S.Hamad, J.A.Anta, E.Salager, R.Tena-Zaera, P.G.Bruce, J-M.Tarascon, 'High Capacity NaO2 Batteries – Key Parameters for Solution - Mediated Discharge', J. Phys. Chem. C 120 20068-20076 (2016) doi:10.1021/acs.jpcc.6b07659

The Na-O2 battery offers an interesting alternative to the Li-O2 battery, which is still the source of a number of unsolved scientific questions. In spite of both being alkali metal-O2 batteries, they display significant differences. For instance, Li-O2 batteries form Li2O2 as the discharge product at the cathode, whereas Na-O2 batteries usually form NaO2. A very important question that affects the performance of the Na-O2 cell concerns the key parameters governing the growth mechanism of the large NaO2 cubes formed upon reduction, which are a requirement of viable capacities and high performance. By comparing glyme-ethers of various chain lengths we show that, the choice of solvent has a tremendous effect on the battery performances. In contrast to the Li-O2 system, high solubilities of the NaO2 discharge product do not necessarily lead to increased capacities. Herein we report the profound effect of the Na+ ion solvent shell structure on the NaO2 growth mechanism. Strong solvent-solute interactions in long-chain ethers shift the formation of NaO2 towards a surface process resulting in submicrometric crystallites and very low capacities (ca. 0,2 mAh/cm2(geom)). In contrast, short-chains, which facilitate desolvation and solution-precipitation, promote the formation of large cubic crystals (ca. 10 um), enabling high capacities (ca. 7.5 mAh/cm2(geom)). This work provides a new way to look at the key role that solvents play in the metal-air system.