The challenge of polyanion redox in oxalates

我们最近报道了草酸铁锂Li2Fe(C2O4)2中的双离子氧化还原，其中铁和草酸基似乎都表现出可逆氧化还原活性。1我们随后的结果表明，这种现象并不是这种化合物所特有的，在过渡金属草酸盐中相当普遍，包括钠。这就产生了一个特征的DQ/DV曲线，它由一对大的峰和两个较小的峰组成。这些都是非常尖锐的，意味着涉及草酸根，这可能显示出良好的动力学。草酸阴离子是一种特别多才多艺的物种，可能是单齿、双齿、三齿甚至四齿的，产生了大量可能的化合物和非常丰富的结构化学。重要的是要确定引起多阴离子氧化还原行为的结构要求，并将合成和表征一系列材料。除纯草酸盐外，还可以制备含有混合多阴离子的材料，例如草酸盐/磷酸盐。将对这些进行研究，以确定多阴离子氧化还原活性是否延伸到这类化合物。合成方法将包括圣安德鲁斯的P.莱特福特教授的溶剂热合成(法拉第研究所的新成员)和谢菲尔德的微波合成。新材料将通过我们在蒙彼利埃的合作伙伴广泛使用NEXGENNA先进的表征平台进行研究，包括衍射、XAS和穆斯堡尔光谱。这些材料通常表现出较差的电子传导性，因此该项目的一部分将致力于电极的优化。与多尺度建模项目的互动将为这一不寻常的现象提供更多的见解。由于表现出这一性质的材料不限于钠，而是包括钾和锂，我们可能会产生与这两个阴极项目中的任何一个相关的新化合物。姚文伟，阿姆斯特朗等人。自然界的普通人。(2019)103483

We recently reported dual ion redox in the lithium iron oxalate, Li2Fe(C2O4)2 in which both the iron and the oxalate group appear to exhibit reversible redox activity.1 Our subsequent results suggest that this phenomenon is by no means unique to this compound and is quite widespread among transition metal oxalates, including those of sodium. These give rise to a characteristic dQ/dV plot consisting of a large pair of peaks accompanied by two smaller ones. These are very sharp, implying that an oxalate radical is involved, which may show excellent kinetics. The oxalate anion is a particularly versatile species and may be monodentate, bidentate, tridentate or even tetradendate, giving rise to a huge range of possible compounds and a very rich structural chemistry. It is important to establish the structural requirements that give rise to polyanion redox behaviour and a range materials will be synthesised and characterised. In addition to pure oxalates it is possible to prepare materials with mixed polyanions e.g. oxalate/phosphate. These will be investigated to establish whether polyanionic redox activity extends to this family of compounds.Synthetic methods will include solvothermal with Prof. P. Lightfoot in St Andrews (new to the Faraday Institution) and microwave synthesis at Sheffield. The new materials will be studied by extensive use of the NEXGENNA advanced characterisation platform, including diffraction, XAS, and Mössbauer spectroscopy via our partners in Montpellier. These materials typically exhibit poor electronic conductivity so part of the project will be devoted to electrode optimisation. Interaction with the multiscale modelling project will provide additional insights to this unusual phenomenon. Since materials exhibiting this property are not confined to sodium, but include potassium and lithium we may generate new compounds relevant to either of the cathode projects.1. W. Yao, A. R. Armstrong et al. Nature Commun. (2019) 10, 3483