Nowadays, electrochemical energy storage plays a major societal role due to its widespread technological applications. Active insertion electrode materials have a crystal structure with insertion sites, channels and/or interlayer spacings allowing the rapid insertion and extraction of lithium ions with generally little lattice strain. A typical class of such active electrode materials is represented by Prussian blue analogues (PBAs).
Copper nitroprusside Cu[Fe(CN)5(NO)], a Prussian blue analogue, has been investigated for its potentially interesting performances as cathodic active material, since the nitrosyl group is a non-innocent ligand and may theoretically act as a third redox centre, beyond metals’ ones.
A scalable and reproducible synthesis of copper nitroprusside has been successfully carried out. Characterization revealed the crystallinity of the compound and a I4mm tetragonal structure.Electrochemical tests have been performed both on coin cells, defining different redox processes in the selected potential window.
Operando characterization [1, 2] has been achieved through different techniques (XAFS, XRD, FTIR) and focused mainly on the first discharge process, since the material clearly undergoes a deep modification during the early stages of reaction.
Both Cu and Fe are electroactive at the beginning of cycling, as corroborated by operando XAFS measurements. Aclose inspection of the Cu K-edge XANES evidences a progressively decrease of the edge energy position and reveals a pre-edge peak increasing during discharge which is well known fingerprint of the occurrence of reduction to Cu(I).Moreover, a partial reduction of Fe(II + δ) to Fe(II) also occurs.
Nitrosyl ligand takes part in the first discharge process, as highlighted by operando FTIR and periodic density functional theory (DFT) calculations (VASP). Vibrational frequencies were calculated and agreed with operando IR measurements. Operando IR and XRD confirmed the material undergoes a gradual and partially irreversible transformation during the first cycle, displaying in XRD patterns an increase in amorphisation and a reduction in peaks’ intensity.
The reduction of isonitrosyl group is responsible for an extra-capacity during the first discharge, giving rise to a remarkable specific capacity, fairly above the average among PBAs. Outstandingly, it represents a unique case in Prussian Blue analogue materials where a ligand is electrochemically active, beyond the metals’ redox centres.
 M.Giorgetti and L. Stievano (2017). “X-Ray Absorption Spectroscopy Study of Battery Materials”, in X-ray Characterization of Nanostructured Energy Materials by Synchrotron Radiation, Dr. M. Khodaei, L. Petaccia (Ed.), InTech, DOI: 10.5772/66868.
 G. Aquilanti, M. Giorgetti, R. Dominko, L. Stievano, I. Arčon, N. Novello and L. Olivi, J. Phys D: Applied Physics, 2017, 50:074001.