Engineering Catalysis at Solid–Solid Interfaces Using Non-Precious Mixed Metal Oxides for Energy Storage in Next Generation Metal-Air Batteries
Authors: Deshpande Siddharth , Samira Samji , Roberts Charles , Nikolla Eranda , Greeley Jeffrey 
Published: December 2020

Advancing our understanding of solid–solid interfacial electrocatalysis is central for engineering the next generation energy storage technologies. Li-O2 batteries provide the highest energy density among all battery technologies, making them attractive for the widespread electrification of the transportation (>500 miles) and the aviation sectors.1 These regenerative systems rely on the reversible redox chemistry between metallic Li and molecular O2 leading to the formation and dissociation of solid Li
x
O2species (xLi+ + O2 + xe– -> Li
x
O2; E0 = 3.0 V vs. Li/Li+; 1=< x =< 2). Although promising, these systems suffer from large overpotential losses potentially stemming from challenges in electron transport through Li
x
O2 species, consequently resulting in reduced round-trip efficiencies (55-60%).1 Various catalysts have been used to overcome these losses. However, lack of a fundamental understanding on the interfacial factors that dictate selective formation of Li
x
O2 species on an electrocatalyst surface, has hindered systematic optimization of the energetics for these processes. Therefore, development of a framework to investigate the atomistic interactions between an electrocatalyst surface and the formed Li
x
O2 solid products is key to enhancing their overall performance.
In this contribution, atomically-controlled synthesis, detailed electrochemical and characterization studies, along with periodic density functional theory (DFT) calculations are combined to showcase the importance of the surface structure in tailoring the solid–solid interfacial catalysis for enhanced cell performance.2 This is demonstrated through the incorporation of non-precious 3d metal-based mixed metal oxide cathode electrocatalysts belonging to the first-series Ruddlesden-Popper (R-P) phase of the general form (A = La, Ca, Sr; B = Mn, Fe, Co, Ni).3 The flexibility in the A- and B-site compositions, can be explored to tune the geometric and the electronic structure of the catalysts, thus making them appealing candidates.4-5 Initially, the experimental and theoretical calculations are benchmarked using La2NiO4 (LNO) as the catalyst. A significant enhancement in the overall performance (>0.7 V) is observed upon the incorporation of catalytically active (001) NiO terminated LNO. The discharge products formed on these surfaces are characterized using numerous techniques, including Raman spectroscopy, chemical titration and mass spectroscopy. These studies indicate that the enhanced performance of LNO stems from its ability to effectively stabilize electronically conductive lithium deficient LixO2 (x<2) films during discharge, which oxidizes at a lower potential than the conventionally observed insulating Li2O2. The developed combinatorial framework for LNO, is extended to various A- and B-site systems to identify the geometric and electronic factors that aid in selective perturbation of film formation energetics, that leads to enhanced performance. A framework for tuning solid–solid interfacial catalysis on these systems is devised; knowledge that is critical for enhancing the efficiency of next generation energy storage technologies.

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