In situ IR spectroscopy during oxidation process of cobalt Prussian blue analogues.

Cobalt Prussian blue analogues (Co-PBA; NaCo[Fe(CN)]), consisting of cyano-bridged transition metal network, -Fe-CN-Co-NC-Fe-, are promising cathode materials for Na-ion secondary batteries. In the oxidation process, oxidization of Fe and/or Co are compensated by Na deintercalation. Here, we investigated the oxidization process of three Co-PBAs by means of in situ infrared absorption (IR) spectroscopy.

Energy harvesting thermocell with use of phase transition.

A thermocell that consists of cathode and anode materials with different temperature coefficients (α = dV/dT) of the redox potential (V) can convert environmental thermal energy to electric energy via the so-called thermal charging effect. The output voltage V of the current thermocell, however, is still low (several tens mV) and depends on temperature, which are serious drawbacks for practical use of the device as an independent power supply. Here, we report that usage of phase transition material as electrode qualitatively improve the device performance.

The effect of 3d-electron configuration entropy on the temperature coefficient of redox potential in CoMn Prussian blue analogues.

Recently, it was reported that a thermocell can convert temperature into electrical energy by using the difference in the thermal coefficient (α ≡ dV/dT) of the redox potential (V) between the cathode and anode materials. Here, we systematically investigated α of NaxCo1-zMnz[Fe(CN)6]y (Co1-zMnz-PBA) against Mn concentration (z). The z-dependence of α is interpreted in terms of the 3d-electron configuration entropy (ΔS3d) of the redox site.

Thermal efficiency of a thermocell made of Prussian blue analogues.

Recently, it was reported that a thermocell can convert temperature into electric energy by using the difference in the thermal coefficient (α = dV/dT) of the redox potential (V) between the cathode and anode materials. Among battery materials, Prussian blue analogues (PBAs) are promising materials for thermocell, because α changes from approximately -0.3 mV/K in NaMn[Fe(CN)] 3.

Na(+) diffusion kinetics in nanoporous metal-hexacyanoferrates.

Metal-hexacyanoferrates (metal-HCFs) are promising candidates for cathode materials of sodium-ion secondary batteries (SIBs). Here, we systematically investigated Na(+) diffusion constants (D) and the activation energies (Ea) of metal-HCFs against the framework size (= a/2). We found that the magnitude of D (Ea) systematically increases (decreases) with increases in a, indicating that steric hindrance plays a dominant role in Na(+) diffusion.

Fast discharge process of layered cobalt oxides due to high Na⁺ diffusion.

Sodium ion secondary battery (SIB) is a low-cost and ubiquitous secondary battery for next-generation large-scale energy storage. The diffusion process of large Na(+) (ionic radius is 1.12 Å), however, is considered to be slower than that of small Li(+) (0.