Chromium-Doped Nickel Cobaltite Nanoneedles as a Cathodic Material for Li-O2 Cells: An X-ray Photoemission and Photoabsorption Spectroscopy Investigation
Abstract
Li-O-2 redox chemistry in aprotic electrolytesis promising to boost the performance of secondary batteries, displayinga theoretical energy density more than an order of magnitude higherthan the present state-of-the-art Li-ion technology. However, theelectrochemical Li2O2 formation and dissolutionoccur in parallel with the so-called ORR and OER (i.e., oxygen reductionreaction and oxygen evolution reaction, respectively), thus requiringsuitable electrocatalysts to promote the redox kinetics both in dischargeand charge.
Here, we discuss the electronic structure and the surfacechemistry of a nanoneedle-structured nickel cobaltite doped with chromiumas a heterogeneous electrocatalyst for aprotic Li-O-2 cells. A detailed experimental study of the evolution of occupiedand unoccupied electronic states of the material from the pristineto a post-mortem condition after operation as a cathode in a Li-O-2 cell is undertaken via ex situ X-ray photoemission (X-rayphotoelectron spectroscopy, XPS) and photoabsorption (near-edge X-rayabsorption fine structure NEXAFS) spectroscopies.
This analysis provedthe mixed valence state of the transition metals, their coordinationenvironment within the cobaltite matrix, and their evolution afteroperation in the cell. In particular, spectroscopic fingerprints ofdeposition/dissolution phenomena due to solvent degradation were foundin the C 1s XP spectra after operation in the Li-O-2 cell, together with an involvement of Ni2+/3+ centersin the electrocatalytic processes of oxygen reduction and evolution,enhanced in the presence of a Cr(III) dopant.