Proton Intercalation of Mo3Nb2O14 Anode for Aqueous Batteries
Authors: Kawai Kosuke , Igarashi Yuta , Jang Seon Joon , Yazawa Koji , Gotoh Kazuma , Kikkawa Jun , Yamada Atsuo , Tateyama Yoshitaka , Okubo Masashi 
Published: December 2024

Energy storage systems are indispensable to power electric devices such as mobile phones, electric vehicles, and smart grids. Lithium-ion battery dominates the largest segment of the energy storage system market owing to its high energy density and long cycle life. However, flammable organic liquid electrolytes are used in lithium-ion batteries, which is an essential risk of fire and explosion accidents. Substituting water solvent for organic solvent directly approaches the safety issue. In particular, proton is the cation with the smallest ionic radius, which enables host materials to be densely protonated with minimal lattice distortion during charging. Therefore, aqueous proton battery is enthusiastically developed as an alternative to lithium-ion batteries.

In general, acidic electrolyte shows intense hydrogen evolution reaction (HER) just below 0 V vs SHE because of its high activity of proton. To suppress HER in aqueous rechargeable batteries, molecular crowding electrolytes can be used, where mixed crowding agents (macromolecules or small hydrophilic molecules) reduce activities of H2O/H3O+ at electrode-electrolyte interfaces.1,2 Meanwhile, active materials should also possess high durability against physical and chemical change upon charge/discharge. Especially, large volume expansion/shrinkage usually raise intraparticle cracking on active materials, which exacerbates parasitic reactions at electrode-electrolyte interfaces. For example, layered α-MoO3 exhibits monoclinic distortion and undergoes a significant volume change (ΔV/V = 5.5%) upon H+ intercalation, resulting in severe intraparticle cracking.3 Other H+-intercalation hosts (V2O3 and h-WO3 0.6H2O) also exhibit large volume changes (ΔV/V = 2.4% and 2.5%, respectively) upon charge/discharge.4,5 The microscopic origins of these structural transformation involve either expansion/contraction of the interlayer distance between loosely stacked layers or the local distortion of MO
n
polyhedra upon H+ intercalation. Strain-free H+ intercalation, which is required for stable operation of an aqueous proton battery, has rarely been reported.

In this presentation, we report strain-free H+ intercalation in the Mo3Nb2O14 host structure. Mo3Nb2O14 is categorized as a bronze phase with the general MO3−x
(M = Mo, Nb, W) formula; edge- and corner-sharing Mo/NbOn (n = 6 and 7) polyhedra form dense oxide-ion arrays with open tunnels that may relieve local structural distortion upon H+ intercalation.

Mo3Nb2O14 was synthesized by sintering a mixture of MoO3 and Nb2O5 at 700 °C for 12 h in the air. Galvanostatic charge/discharge measurements show that Mo3Nb2O14 exhibits reversible five-proton intercalation (193 mAh g−1) in an acidic molecular crowding electrolyte (4.2 mol L−1 H2SO4/10H2O–90PEG + 5 wt% HFE). The strain-free nature of Mo3Nb2O14 upon charge/discharge per the formula unit (ΔV/V < 0.5%) is confirmed by 1H solid-state nuclear magnetic resonance spectroscopy, X-ray absorption spectroscopy, X-ray diffraction (XRD), and density functional theory calculations. Rietveld refinement for the ex situ XRD patterns and atomic-resolution scanning transmission electron microscopy reveal that reversible shrinkage and rotation of the open tunnel in the Mo3Nb2O14 lattice relieve the volume changes of Mo/NbOn (n = 6 and 7) upon redox reactions. Consequently, intraparticle cracking is suppressed on Mo3Nb2O14 particles, which minimizes HER to achieve high Coulombic efficiencies (> 99.7%). Finally, a full-cell that consists of the Mo3Nb2O14 anode, the prussian blue analogue vanadium hexacyanoferrate cathode 6, and the acidic molecular crowding electrolyte retains 98.5% of the initial specific capacity after 1000 cycles.

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Figure 1