Electrochemical Mechanism of ε-Keggin-Type POMs for Li-Ion and K-Ion Batteries

摘要

Polyoxometalates (POMs) are a class of materials, which find application in many fields. Properties such as ion conductivity, redox activity, good thermal stability, and ability to form complex with a multitude of cations make them interesting in many applications, including catalysts, coatings, membranes, films, electrochemical devices, pigments, analytical reagents, etc. [1]. Among different type of POMs, the Keggin type structure is one which found application in Li-ion batteries. Materials like (NH4)1.5H8.5(Zn3Mo12O40).6H2O (Mo-Zn-O) and (NH4)2.1H7.5(Mn2.2Mo12O40).4H2O (Mo-Mn-O) have been used as cathode, in the potential region 1.5-4V Li/Li+, displaying large capacities of 270 mAh/g and 269 mAh/g, respectivery[2]. In this work the hydrothermal method has been used to obtain cubic (NH4)2H1.5(Fe3Mo12O40).5.25H2O (Mo-Fe-O) POM with a=b=c=19.042 ? 0.001? (S.G: Fd-3m), which is isostructural to the above mentioned Mo-Mn-O. The samples Mo-Fe-0 have been tested in a 3-electrode cell configuration, where metallic Li or K are applied as a reference and counter electrodes. In the low potential region (0.01-3V), this material can reach a lithiation/delithiation capacity of 1355 mAh/g, 1638 mAh/g, 1392 mAh/g and 1334 mAh/g at the current densities of 200 mA/g, 300 mA/g, 500 mA/g and 1000 mAg, respectively. In potassium system, the first discharge capacity is much lower: at the current densities of 200 mA/g, 300 mA/g, 500 mAg and 1000 mA/g the capacity is 345 mAh/g, 410 mAh/g, 292 mAh/g and 357 mAh/g, respectively, and cycling stability is also worse. In both Li-ion and K-ion systems the degradation of the initial cubic structure is observed. The amorphisation of the initial structure during lithiation (potassiation) is accompanied by the conversion reaction, with production of α-LiFeO2 and metallic Mo in case of Li-ion half-cell and metallic Fe - in case of K-ion half-cell. The mechanism was investigated using in situ synchrotron radiation diffraction (ALBA, Barcelona) and in situ XAS (PETRAIII, Hamburg). This study gives understanding on what happens inside the rechargeable cell during the electrochemical reaction. Mo-Fe-O POM shows a mixed intercalation-conversion mechanism in a Li-half cell in the potential region 0.01 - 3 V vs. Li/Li+.