Impact of the Lithium Cation on the Voltammetry and Spectroscopy of [XVM11O40]n- (X = P, As (n=4), S (n=3); M = Mo, W): Influence of Charge and Addenda and Hetero Atoms

Polyoxometalates (POMs) have been proposed as electromaterials for lithium-based batteries because they provide access to multiple electron transfer reactions coupled to fast lithium ion transport processes and high stability over many redox cycles. Consequently, knowledge of reversible potentials and Li+ cation-POM anion interactions provides a strategic basis for their further development. In this study, detailed cyclic voltammetric studies of a series of [(XVM11O40)-M-V](n-) (XVM11n-) POMs (where X (heteroatom) = P (n = 4), As (n = 4), and S (n = 3) and M (addenda atom) = Mo, W) have been undertaken in CH3CN in the presence of LiClO4, with n-Bu4NPF6 also present when required to keep the ionic strength close to constant value of 0.1 M. An analysis of the data has allowed the impact of the POM charge, and addenda and hetero atoms on the reversible potentials and the interaction between Li+ and the oxidized (XVM11n-)-M-V and reduced (XVM11(n+1)-)-M-IV forms of the V-V/IV redox couple to be determined. The (SVM113-/4-)-M-V/IV process is independent of the Li+ concentration, implying the absence of the association of this cation with either (SVM113-)-M-V or (SVM114-)-M-IV redox levels. However, lithium-ion association constants for both V-V and V-IV redox levels were obtained from a comparison of simulated and experimental cyclic voltammograms for the reduction of the more negatively charged (XVM114-)-M-V (X = P, As; M = Mo, W), since the Li+ interaction with these more negatively charged POMs is much stronger. The interaction between Li+ and the oxidized, (XVM11n-)-M-V, and reduced, (XVM11(n+1)-)-M-IV, forms was also investigated by V-51 NMR and EPR spectroscopy, respectively, and it was confirmed that, due to their lower charge density, (SVM113-)-M-V and (SVM114-)-M-IV interact significantly less strongly with the lithium ion than (XVM114-)-M-V and (XVM115-)-M-IV (X = P, As). The lithium-POM association constants are substantially smaller than the corresponding proton association constants reported previously, which is attributed to a smaller surface charge density. The much stronger impact of Li+ on the W-VI/V- and Mo-VI/V-based reductions that occur at more negative potentials than the V-V/IV process also has been qualitatively evaluated.