In Situ and Operando Neutron Powder Diffraction of Na x NiO 2 : A Case Study for Sodium-Ion Batteries

Abstract

Neutron powder diffraction (NPD) is a non-destructive technique uniquely suited to investigating industrially relevant sodium-ion cathode materials. Unlike X-ray diffraction, it offers excellent contrast between neighbouring transition metal ions and is sensitive to lighter elements such as O/Na, which are key to revealing superstructural orderings characteristic of sodium cathodes. However, in situ/operando NPD battery studies are hindered by sample quantity requirements, and lack of suitable sample environments. Using a cell developed at ISIS, we observed no detrimental effects from vacuum exposure required for Time-of-Flight NPD. We collect/analyse data in two modes: operando, wherein electrochemical structural evolution is tracked, and in situ, wherein higher signal-to-noise data is collected during voltage holds, for phase-pure samples at voltage cutoffs. Using Na x NiO 2 as a model system, we present methodology to account for the complex background profile characteristic of electrochemical environments enabling, for the first time, quantitative Rietveld refinements against in situ electrochemical NPD, and providing a systematic framework for future studies. Structural models are refined against in situ data and benchmarked against combined refinement of ex situ SXRD/NPD measurements. This work unlocks potential for previously inaccessible insights into structure-property relationships and degradation, facilitating development of next-generation cathodes with enhanced useful lifetime, capacity, and performance.