In situ small-angle X-ray scattering reveals solution phase discharge of Li–O ₂ batteries with weakly solvating electrolytes

Abstract

Electrodepositing insulating lithium peroxide (Li2O2) is the key process during discharge of aprotic Li–O2 batteries and determines rate, capacity, and reversibility. Current understanding states that the partition between surface adsorbed and dissolved lithium superoxide governs whether Li2O2 grows as a conformal surface film or larger particles, leading to low or high capacities, respectively. However, better understanding governing factors for Li2O2 packing density and capacity requires structural sensitive in situ metrologies. Here, we establish in situ small- and wide-angle X-ray scattering (SAXS/WAXS) as a suitable method to record the Li2O2 phase evolution with atomic to submicrometer resolution during cycling a custom-built in situ Li–O2 cell. Combined with sophisticated data analysis, SAXS allows retrieving rich quantitative structural information from complex multiphase systems. Surprisingly, we find that features are absent that would point at a Li2O2 surface film formed via two consecutive electron transfers, even in poorly solvating electrolytes thought to be prototypical for surface growth. All scattering data can be modeled by stacks of thin Li2O2 platelets potentially forming large toroidal particles. Li2O2 solution growth is further justified by rotating ring-disk electrode measurements and electron microscopy. Higher discharge overpotentials lead to smaller Li2O2 particles, but there is no transition to an electronically passivating, conformal Li2O2 coating. Hence, mass transport of reactive species rather than electronic transport through a Li2O2 film limits the discharge capacity. Provided that species mobilities and carbon surface areas are high, this allows for high discharge capacities even in weakly solvating electrolytes. The currently accepted Li–O2 reaction mechanism ought to be reconsidered.

Exclusive Solution Discharge in Li–O2 Batteries?

“… 16 , 17 , 20 − 23 In a recent study with operando small- and wide-angle X-ray scattering (SAXS/WAXS), we found that Li 2 O 2 structures indicating surface growth are absent even in weakly LiO 2 -solvating electrolytes and at high overpotentials. 10 This is in line with large Li 2 O 2 particles imaged via electron microscopy in weakly solvating electrolytes at practical current densities and raises questions about the surface mechanism occurring. 24 − 27 Consequently, truly capacity-limiting factors as well as measures and governing factors for Li 2 O 2 packing density are still obscure.…”

“…(ii) LiO 2 is soluble and mobile even in weakly solvating electrolytes ( Figure 2 ). (iii) Li 2 O 2 forms to the widest extent via solution-mediated DISP ( Figure 2 and a recent operando SAXS/WAXS study 10 ). (iv) Li 2 O 2 particles become smaller and more numerous with increasing current ( operando SAXS/WAXS 10 and refs ( 8 ), 14 , ( 36 ), and ( 37 )).…”

“…(iii) Li 2 O 2 forms to the widest extent via solution-mediated DISP ( Figure 2 and a recent operando SAXS/WAXS study 10 ). (iv) Li 2 O 2 particles become smaller and more numerous with increasing current ( operando SAXS/WAXS 10 and refs ( 8 ), 14 , ( 36 ), and ( 37 )). (v) Li 2 O 2 particles become larger and less numerous with increasing LiO 2 dissociation ( operando SAXS/WAXS 10 and refs ( 8 ), ( 12 ), and ( 14 )).…”

“… 2 − 6 Capacity, deposit structure, and battery lifetime are intrinsically linked to the underlying physicochemical mechanisms. 5 , 7 − 10 …”

Understanding Lithium-Mediated Oxygen Reactions at the Au|DMSO interface: Are We There?

“…Consistent with the above results, a very recent study from Prehal et al reported that higher species mobilities and electrode surface area, namely lower overpotentials, can facilitate the solution-mediated growth of the Li 2 O 2 and a high capacity even in the low DN solvent. 61 Therefore, they proposed a unified ORR mechanism, which includes the dependence of dissociation of solvated LiO 2 , species mobilities and areal current densities.…”