Detailing the Self-Discharge of a Cathode Based on a Prussian Blue Analogue

Abstract

Prussian Blue analogues (PBAs) are a promising class of electrode active materials for batteries. Among them, copper nitroprusside, Cu[Fe(CN)5NO], has recently been investigated for its peculiar redox system, which also involves the nitrosyl ligand as a non-innocent ligand, in addition to the electroactivity of the metal sites, Cu and Fe. This paper studies the dynamics of the electrode, employing surface sensitive X-ray Photoelectron spectroscopy (XPS) and bulk sensitive X-ray absorption spectroscopy (XAS) techniques. XPS provided chemical information on the layers formed on electrode surfaces following the self-discharge process of the cathode material in the presence of the electrolyte. These layers consist mainly of electrolyte degradation products, such as LiF, LixPOyFz and LixPFy. Moreover, as evidenced by XAS and XPS, reduction at both metal sites takes place in the bulk and in the surface of the material, clearly evidencing that a self-discharge process is occurring. We observed faster processes and higher amounts of reduced species and decomposition products in the case of samples with a higher amount of coordination water.

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“…The former can be ascribed to the NO peak, while the latter can be attributed to the oxygen peak arising from the crystalline water. The binding energies of C≡N, NO, and oxygen from water at the C 1s, N 1s, and O 1s spectra are close to the reported values of similar other compounds such as copper nitroprusside and SNP [ 44 , 46 ]. The core-level spectra of Fe 2p exhibited two spin-orbit doublets peaks of Fe 2p 3/2 and Fe 2p 1/2 with Fe 2+ low spin at the binding energies of 710.60 and 720.60 eV, respectively, originated from the Fe 2+ species, [Fe(CN) 5 NO], in NNP [ 44 , 47 ].…”

“…The binding energies of C≡N, NO, and oxygen from water at the C 1s, N 1s, and O 1s spectra are close to the reported values of similar other compounds such as copper nitroprusside and SNP [ 44 , 46 ]. The core-level spectra of Fe 2p exhibited two spin-orbit doublets peaks of Fe 2p 3/2 and Fe 2p 1/2 with Fe 2+ low spin at the binding energies of 710.60 and 720.60 eV, respectively, originated from the Fe 2+ species, [Fe(CN) 5 NO], in NNP [ 44 , 47 ]. The spectrum also showed the contribution of two additional low-intensity peaks at 708.20 and 721.20.…”

“…The C 1s spectrum exhibited only a C≡N peak at a binding energy of 284.85 eV without the presence of any impurity peak. The deconvoluted and fitted N 1s spectra showed the C≡N and NO peaks at 398.10 and 402.90 eV, respectively, as expected from the compound formula [ 44 , 45 ]. The O 1s spectra comprise two-component peaks at the binding energies of 531.50 and 534.85 eV.…”

“…The compositions and the oxidation states of elements of NNP were analyzed by XPS. Figure 3 a shows the survey spectra of NNP, which revealed the presence of C 1s, N 1s, O1s, Fe 2p, and Ni 2p in their respective reported binding energy values [ 44 , 45 ]. Figure 3 b–f display the high-resolution XPS spectra of C 1s, N 1s, O 1s, Fe 2p, and Ni 2p, respectively, in NNP, and the binding energies of characteristics peaks are summarized in Table S3 .…”

“…The spectrum also showed the contribution of two additional low-intensity peaks at 708.20 and 721.20. These can be attributed to the spin-orbit doublets peaks of Fe 2p 3/2 and Fe 2p 1/2 , respectively, with Fe 2+ high spin, formed by the partial degradation of NNP during the XPS experiment by altering the C≡N ligand bonds to the Fe atom (from Fe-CN to Fe-NC) [ 44 , 47 ]. This is further evidenced by the appearance of a weak satellite peak for the high spin Fe 2+ at 714.65 eV [ 47 ].…”