Beyond Oxygen Redox Strategy in Designing Cathode Material for Batteries: Dynamics of a Prussian Blue-like Cathode Revealed by Operando XRD and XAFS and by Theoretical Approach

The dynamics of the lithiation/delithiation process and the nitrosyl electroactivity in copper nitroprusside were studied by operando X-ray diffraction (XRD) and operando X-ray absorption fine structure (XAFS). Data were interpreted based on a joint study performed by means of density functional theory (DFT)calculations. This approach allows retrieving the relevant structural and electronic information from the measured scattering and absorption data and therefore to untangle the lithiation mechanism in copper nitroprusside, which occurs with the reduction of both metals generating a lattice basal plane contraction and an axial elongation. An increase of the Debye-Waller factors for Cu-N bonds and a decreasing trend for the Cu-NC-Fe linear chains along with lithium insertion reveal a general increase in the Cu local disorder, which is thought to be the main cause of the rapid capacity fading observed during cycling. The ligand electroactivity of the nitrogen atom, detected by following vibrational frequencies, delivers an extra capacity and represents an alternative path to cationic and oxygen redox.