In this paper we would like to show a new approach to an explanation of the nature of the discharge-charge curve of Na/Na(+)/NaxCoO2-y batteries, which can justify the existence of the step-like characteristics. This is still an open problem, which until now had no proper description in the literature. On the basis of comprehensive experimental studies of physicochemical properties of NaxCoO2-y cathode material (XRD, electrical conductivity, thermoelectric power, electronic specific heat) supported by calculations performed using the DFT method with accounting for chemical disorder, it has been shown that the observed step-like character of the discharge curve reflects the variation of the chemical potential of electrons (Fermi level) in the density of states of NaxCoO2-y, which is anomalously perturbed by the presence of the oxygen vacancy defects and sodium ordering. Our studies of structural, electronic and thermal properties of NaxCoO2-y cathode material as a function of concentration of electrochemically intercalated sodium document strong and step-like shift of the position of the Fermi level during introduction of electrons in this process. This effect is coherently supported by the shape of calculated density of states (DOS) of NaxCoO2-y having included oxygen defects and sodium ordering.

Prussian blue (PB) and Prussian blue analogues (PBAs) are commonly synthesized by conventional methods, such as chemical precipitation, thermal decomposition, and electrochemical deposition. Herein, we have successfully synthesized Prussian blue by oxidative print light synthesis (PLS) with a cubic Fe 4 [Fe(CN) 6 ] 3 phase, as confirmed by XRD compared to pure Prussian blue. Furthermore, UV−vis, FT-IR, Raman, and XPS measurements also present experimental evidence of PB formation from the Potassium hexacyanoferrate(II) trihydrate precursor by PLS. STEM images display aggregated PB particles of ca. 500 nm with a homogeneous distribution of Fe, N, C, and K throughout the sample. The electrochemical characterization provides excellent electrocatalytic performances during the charge and discharge processes, with oxidation/reduction reactions of high-and low-spin iron, which is already known as the interconversion of Prussian white to Prussian blue (PW ⇄ PB) and Prussian blue to Prussian green (PB ⇄ PG), respectively. In particular, PLS has been successfully employed as a smart and low-cost protocol to synthesize thin Prussian blue films, and possibly other PBAs, for applications in energy storage devices such as K, Na, and Mg ion batteries.

Sodium chromium hexacyanoferrate (NaCrHCF) is obtained here using a facile co-precipitation method at room temperature.

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Sodium-ion (Na-ion) batteries have been attracting great interest because of a wide range of potential applications and sodium abundance. Yet, the performance of Na-ion electrode materials needs improvement to be able to compete with lithiumion electrode materials. Here, sodium−vanadium hexacyanoferrate (NaVHCF) is investigated as a cathode active material in rechargeable Na-ion batteries. We explore the electrochemical performance of NaVHCF in a liquid organic electrolyte and demonstrate a high and very stable working potential of around 3.3 V vs Na + /Na, achieving excellent capacity retention of 73% after 200 cycles. Vanadium substitution in a Prussian blue crystal structure can improve the cycle life by acting as a pillar in interstitial spaces. These results represent a step forward in the development of cathode materials for Na-ion batteries.

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