Well-known since the 18th century,hexacyanoferrate, or “Prussian blue”, is currently getting its “second life” as a promising material for Li-ion batteries and electrochromic devices.
Electrochemical, spectroscopic, and structural measurements were used to characterize the electrochromic behavior and stability of electrodeposited MoO3 thin films. These films are prepared from metal/hydrogen peroxide solution on transparent conductive glass (i.e., ITO). The as-deposited films are partially crystalline and show an improvement in crystallinity after heat-treatment in air at 260~ These films display reversible electrochromic properties from blue to clear, when cycled in a 1 M solution of lithium perchlorate in propylene carbonate between -0.5 and +2.5 V. The observed effects of heat-treatment on the electrochromic properties are discussed and related to x-ray photoelectron spectroscopy analysis. The elemental depth profiles of colored MoO3 films were analyzed by secondary ion mass spectrometry.
Nanostructured lithium metal orthosilicate materials hold a lot of promise as next generation cathodes but their full potential realization is hampered by complex crystal and electrochemical behavior. In this work Li2FeSiO4 crystals are synthesized using organic-assisted precipitation method. By varying the annealing temperature different structures are obtained, namely the monoclinic phase at 400°C, the orthorhombic phase at 900°C, and a mixed phase at 700°C. The three Li2FeSiO4 crystal phases exhibit totally different charge/discharge profiles upon delithiation/lithiation. Thus the 400°C monoclinic nanocrystals exhibit initially one Li extraction via typical solid solution reaction, while the 900°C orthorhombic crystals are characterized by unacceptably high cell polarization. In the meantime the mixed phase Li2FeSiO4 crystals reveal a mixed cycling profile. We have found that the monoclinic nanocrystals undergo phase transition to orthorhombic structure resulting in significant progressive deterioration of the material's Li storage capability. By contrast, we discovered when the monoclinic nanocrystals are cycled initially at higher rate (C/20) and subsequently subjected to low rate (C/50) cycling the material's intercalation performance is stabilized. The discovered rate-dependent electrochemically-induced phase transition and stabilization of lithium metal silicate structure provides a novel and potentially rewarding avenue towards the development of high capacity Li-ion cathodes.
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