Advancing Sodium-Ion Battery Cathode Analysis With Cryogenic Ag-coated APT Specimens

Layered oxide cathodes have attracted wide research interest due to their controllable synthesis, tuneability, and high energy density in sodium-ion batteries (SIBs). However, in layered oxide cathodes, capacity retention is unsatisfactory due to structural changes, and the severity of capacity fading increases at higher voltages. Chemical heterogeneity and concentration gradient lead to the co-existence of multiple phases with lattice mismatch and strain development. To achieve the practical usage of high-density and low-cost layered oxide cathodes for SIBs, it is very important to develop an atomic-scale understanding of the compositional changes in the multi-component cathode. Atom probe tomography (APT) is a very promising technique to analyze chemical composition and heterogeneity in three dimensions (3D) with high spatial resolution and brings insights into possible property- or lifetime-limiting factors. However, APT is underpinned by an intense electric field that can drive preferential alkali metal outward migration and cause in situ de-intercalation of alkali metals that makes APT analysis challenging. As a first report, we show that silver (Ag) coating on SIB cathode APT specimens, deposited inside the focused-ion beam (FIB) at cryogenic temperature, allows for analysis of the compositional heterogeneity in air-sensitive sodium-ion-layered oxide cathode material.