Chromium-Doped Nickel Cobaltite Nanoneedles as a Cathodic Material for Li-O2Cells: An X-ray Photoemission and Photoabsorption Spectroscopy Investigation

Li-O2 redox chemistry in aprotic electrolytes is promising to boost the performance of secondary batteries, displaying a theoretical energy density more than an order of magnitude higher than the present state-of-the-art Li-ion technology. However, the electrochemical Li2O2 formation and dissolution occur in parallel with the so-called ORR and OER (i.e., oxygen reduction reaction and oxygen evolution reaction, respectively), thus requiring suitable electrocatalysts to promote the redox kinetics both in discharge and charge. Here, we discuss the electronic structure and the surface chemistry of a nanoneedle-structured nickel cobaltite doped with chromium as a heterogeneous electrocatalyst for aprotic Li-O2 cells. A detailed experimental study of the evolution of occupied and unoccupied electronic states of the material from the pristine to a post-mortem condition after operation as a cathode in a Li-O2 cell is undertaken via ex situ X-ray photoemission (X-ray photoelectron spectroscopy, XPS) and photoabsorption (near-edge X-ray absorption fine structure NEXAFS) spectroscopies. This analysis proved the mixed valence state of the transition metals, their coordination environment within the cobaltite matrix, and their evolution after operation in the cell. In particular, spectroscopic fingerprints of deposition/dissolution phenomena due to solvent degradation were found in the C 1s XP spectra after operation in the Li-O2 cell, together with an involvement of Ni2+/3+ centers in the electrocatalytic processes of oxygen reduction and evolution, enhanced in the presence of a Cr(III) dopant.