Structural Evolution of NaNiO2 Via Operando Neutron Diffraction

We propose to measure operando NPD of NaNiO2 vs. Na metal using POLARIS, to study the dynamic crystallographic structure of NaNiO2 as it charges from OCV to 4.5 V, and discharges to 1.25 V. Operando experiments will capture evolution of the superstructure lattice in Na1-xNiO2 through tracking of emergence, shifting, and disappearance of peaks in the low Q-range (~1.4 – 2.2 Å-1). Bragg diffraction will be analysed via Rietveld co-refinement with SRXD data from I11, using TOPAS 7. Data will be used along with complementary ex-situ NPD, ssNMR, XPDF, SQUID, and DFT to elucidate the novel phases bulk and local structures, and their formation mechanisms. Following discussions with instrument scientists, the POLARIS instrument is believed to ideally suit the requirements of operando experiments due to its high neutron flux, minimising collection time, maximising measurements taken per cycle, and improving signal-to-noise (aiding in capturing low-intensity superstructure peaks). NPD provides excellent contrast between the three elements present (bNa= 3.63 fm, bNi = 10.3 fm, bO = 5.80 fm), with enhanced sensitivity towards lighter elements Na and O), providing high confidence in fitting the O atomic positions via Rietveld refinement. Additionally, POLARIS’ wide Q-range may facilitate collection of total scattering data for NPDF analysis, offering insight into the dynamic evolution of the JT-distortion in NaNiO2, through operando tracking of Ni-O bond lengths for the first time.