The role of the local structural properties in the electrochemical characteristics of Na_1-xFe_1-yNi_yO₂ cathodes

The natural abundance of sodium makes the Na-ion batteries (SIBs) attractive devices in the framework of a global economy change toward net zero CO₂ emission. SIBs naturally deliver relatively lower energy density with respect to the Li-ion counterparts (LiBs); however, their lower cost and fast charge/discharge ability make them a promising competitor to LiBs to load level the intermittent power from renewable energy sources for smart grids or renewable power stations. The O3-type NaFeO₂ is a promising candidate for SIBs cathodes, even if the irreversible structural transition occurring during Na-ion extraction/insertion seriously hinders its practical application. Partial replacement of Fe by Ni significantly improves its electrochemical properties. The possible reasons of such improvement are here investigated accessing the details on the Fe and Ni local electronic and structural properties by means of x-ray absorption spectroscopy and spin-polarized density functional theory (DFT) calculations. Different Ni concentrations and charge states have been analyzed. The results support the stability of the electronic properties of Fe and Ni as a function of cycling in a partially substituted system. Instead, the local structure is affected by the Fe substitution as well by the charge/discharge cycling. In particular, the decrease of Fe–O covalency and the local disorder by partial substitution of Fe with Ni seem at the origin of the improved performances.