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

Toru Konishi; Keisuke Kodani; Takuya Hasegawa; Shuhei Ogo; Si-Xuan Guo; John F Boas; Jie Zhang; Alan M Bond; Tadaharu Ueda

doi:10.1021/acs.inorgchem.0c00876

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

Inorg Chem. 2020 Aug 3;59(15):10522-10531. doi: 10.1021/acs.inorgchem.0c00876. Epub 2020 Jun 17.

Authors

Toru Konishi¹, Keisuke Kodani¹, Takuya Hasegawa², Shuhei Ogo³, Si-Xuan Guo⁴, John F Boas⁵, Jie Zhang⁴, Alan M Bond⁴, Tadaharu Ueda^{3

6}

Affiliations

¹ Department of Applied Science, Faculty of Science, Kochi University, Kochi 780-8520, Japan.
² Institute of Multidisciplinary Research for Advanced Materials (IMRAM), Tohoku University, Sendai, Miyagi 980-8577, Japan.
³ Department of Marine Resources Science, Faculty of Agriculture and Marine Science, Kochi University, Nankoku, Kochi 783-8502, Japan.
⁴ School of Chemistry, and ARC Centre of Excellence for Electromaterials Science, Monash University, Clayton, Victoria 3800, Australia.
⁵ School of Physics and Astronomy, Monash University, Clayton, Victoria 3800, Australia.
⁶ Center for Advanced Marine Core Research, Kochi University, Nankoku, Kochi 783-8502, Japan.

PMID: 32786655
DOI: 10.1021/acs.inorgchem.0c00876

Abstract

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 [XV^VM₁₁O₄₀]^n- (XVM₁₁^n-) 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 XV^VM₁₁^n- and reduced XV^IVM₁₁^(n+1)- forms of the V^V/IV redox couple to be determined. The SV^V/IVM₁₁^3-/4- process is independent of the Li⁺ concentration, implying the absence of the association of this cation with either SV^VM₁₁^3- or SV^IVM₁₁^4- 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 XV^VM₁₁^4- (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, XV^VM₁₁^n-, and reduced, XV^IVM₁₁^(n+1)-, forms was also investigated by ⁵¹V NMR and EPR spectroscopy, respectively, and it was confirmed that, due to their lower charge density, SV^VM₁₁^3- and SV^IVM₁₁^4- interact significantly less strongly with the lithium ion than XV^VM₁₁^4- and XV^IVM₁₁^5- (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.