(673g) Mechanistic Insights Into the Electrocatalytic Lithium-O2 Reduction and Evolution Reactions

The lithium-air battery can far exceed the specific energy of lithium-ion batteries, but prototypes suffer from poor round-trip efficiency, low current density, and capacity fading.  Unlike the O₂ reduction reaction in hydrogen fuel cells, which forms water that is the same as the solvent and can be vented to the atmosphere, the Li analog (Li-ORR) is usually carried out in aprotic organic solvents, resulting in two peculiarities: 1) The reduction products readily precipitate into solid phases; 2) organic solvents are susceptible to attack by reactive oxygen, making it difficult to assess the microscopic reaction mechanisms.  There has been conflicting experimental evidence regarding the efficacy of electrocatalysts for this reaction.  Nonetheless, the sluggishness of the Li-ORR/OER will need to be addressed before the rechargeable Li-air battery becomes a reality.  We use DFT calculation-based modeling techniques to interrogate the intrinsic activity of metals, metal oxides, and carbon for the Li-ORR.  We propose that on unreactive surfaces the reaction proceeds associatively through superoxide formation, whereas on reactive surfaces the initial reduction occurs between Li and dissociated O instead.  The predicted Li-ORR activities form a volcano trend with respect to oxygen binding energy, in close agreement with Lu et al.  The OER activities are evaluated in a similar framework, with the predictions broadly in line with experimental evidence.  Our findings demonstrate that the intrinsic electrocatalytic activity toward Li-ORR/OER differs considerably for different substances and can be reasonably reflected theoretically.