Electron delocalization in cyanide-bridged coordination polymer electrodes for Li-ion batteries studied by soft x-ray absorption spectroscopy

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The electronic structure change during the reversible Li-ion storage reaction in a bimetallic MnFe-Prussian blue analogue (LiₓK0.14Mn1.43[Fe(CN)₆] · 6H₂O) was investigated by soft x-ray absorption spectroscopy. The Mn L2,3-edge spectra revealed the unchanged Mn²⁺ high-spin state regardless of Li-ion concentration (x). On the other hand, the Fe L2,3-edge spectra clearly revealed a reversible redox behavior as Fe³⁺↔Fe²⁺ states with Li-ion insertion/extraction. Experimental findings suggested strong metal-to-ligand charge transfer in conjunction with the ligand-to-metal one. The resulting charge delocalization between the Fe and CN is considered to contribute to the high reversibility of the Li-ion storage process.