Mass Transfer of Divalent Ions in an Oxide Host: Comparison of Mg²⁺ and Zn²⁺ Diffusion in Hexagonal K_xW₃O₉ Bronze

Abstract

Pure and isovalent cation-substituted potassium hexagonal tungsten bronze (KHTB) have been synthesized and used as a model oxide host to study the electrochemically driven bulk diffusion of divalent Mg 2+ and Zn 2+ ions. For the cation substitution, tungsten has been replaced by 10% Mo or 5% Cr as a strategy to mitigate the slow diffusivities of the divalent ions. Cyclic voltammetry and electrochemical impedance spectroscopy (EIS) studies reveal the superior kinetics of Zn 2+ insertion compared to that of Mg 2+ , majorly due to the sluggish desolvation/electroadsorption of Mg species at the electrode−electrolyte interface in an organohaloaluminate-based electrolyte. For both divalent cations, the insertion/extraction kinetics exhibits a profound improvement with the isovalent cation-substituted KHTB phases due to the synergistic effect of particle size reduction and enhanced ionic diffusivities. Hence, maximum insertion amounts of 0.18 and 0.20 mol, respectively, for Mg 2+ and Zn 2+ per (W 1−x M x ) 3 O 9 formula unit (M = Mo or Cr) have been achieved within the electrolyte stability window, whereas the unsubstituted KHTB is essentially inactive. Further EIS studies in symmetric cells indicated the inherently faster kinetics of Zn 2+ diffusion compared to that of Mg 2+ in identical hosts. Calculations based on density functional theory (DFT) were implemented to probe the local atomic environment of the diffusing ions. The DFT study suggests that the Zn-ion transition state is stabilized by ∼0.4 eV compared to that of Mg 2+ through a host-to-guest charge transfer, a consequence of the tendency of Zn 2+ ions to form covalent bonds compared to Mg 2+ with the oxide anion.

Multivalent-Ion versus Proton Insertion into Battery Electrodes

“…Previously, our work has shown that the transfer of the Mg 2+ ions from the all-phenyl complex (APC) electrolyte to the hexagonal K x W 3 O 9 host involves a rate-limiting electro-adsorption step, based on combined cyclic voltammetry and EIS techniques (Figure ). The slow electro-adsorption step, which is a capacitive process from the EIS perspective, adds ∼300 mV overpotential burden to the charge–discharge processes. While this overpotential was observed for the insertion of Mg 2+ ions from APC electrolyte that contains complex binuclear Mg 2+ ions, it was absent from the Zn-ion electrochemical insertion from the Zn­(Otf) 2 /ACN electrolyte into the K x W 3 O 9 host.…”

“…(a) Cyclic voltammetry of Mg-ion insertion/extraction into/from K x W 3 O 9 and staircase potentiostatic electrochemical impedance spectroscopy (SPEIS) contour plots obtained from (b) the cathodic and (c) anodic scans of the Mg|K x W 3 O 9 cell with the impedance complex phase angle (φ) represented as color-coded contours. The interfacial Mg 2+ (de)­solvation overpotential ranges are indicated with negative φ in panels b and c. Reproduced with permission from ref . Copyright 2019 The American Chemical Society.…”

Calcium Molybdenum Bronze as a Stable High-Capacity Cathode Material for Calcium-Ion Batteries

“…The demand for rechargeable batteries with a higher energy density and lower cost than the prevalent lithium-ion batteries (LIBs) encourages further research efforts for the development of post-LIB technology . Multivalent-ion batteries based on divalent or trivalent ions (Mg 2+ , Ca 2+ , Zn 2+ , Al 3+ ) are one of the promising post-LIB candidates and are expected to go beyond the energy density limit of LIBs. , Among these, calcium-ion batteries (CIBs) have received considerable interest, due to the recent breakthroughs concerning their anodes. − Reversible plating and stripping have been successfully carried out at elevated temperatures (75–100 °C) and at room temperature . Reversible calcium alloying with a tin anode and calcium intercalation into graphite have been reported.…”

Non-passivating Anion Adsorption Enables Reversible Magnesium Redox in Simple Non-nucleophilic Electrolytes

“…Magnesium batteries are considered as one of the most promising “post lithium-ion batteries” owing to the desirable properties of magnesium anodes, including high theoretical capacity (3832 mAh cm –3 ), low reduction potential (−2.37 V vs standard hydrogen electrode, SHE), Earth abundance, and high stability to air and moisture . However, most rechargeable magnesium batteries (RMBs) suffer from insufficient power density (<0.5 kW kg –1 , 0.8 mW cm –2 ) due to severe Mg anode passivation and the sluggish solid-state diffusion in cathodes. , In contrast to the formation of a Li ion-conducting solid electrolyte interface (SEI) for the lithium metal anode, the interface layer induced by electrolyte decomposition in RMBs usually blocks Mg 2+ diffusion; hence, most simple ionic salts (such as Mg­(ClO 4 ) 2 and Mg­(BF 4 ) 2 ) and polar aprotic solvents (such as carbonates and nitriles) that are prone to precipitate a passivation film are not suitable for RMBs. ,, In the past few decades, a series of nucleophilic magnesium organochloro­aluminates in situ synthesized in ethers were found to be compatible with the Mg anode, enabling reversible Mg deposition/dissolution with high Coulombic efficiency close to 100%. , The nucleophilic organometallic species were regarded as critical components for the reversible Mg deposition/dissolution in nucleophilic electrolytes because of their reactive nature to remove impurities (for example, moisture) prone to precipitate a passivation layer on the Mg anode …”

“…Nucleophilic electrolytes are compatible with Mg intercalation cathodes, but Mg 2+ insertion at room temperature is significantly limited by the high energy barrier of Mg 2+ desolvation at the electrode/electrolyte interface ,, and its diffusion in host materials. , Organic polymers and conversion cathodes based on heterogeneous reactions (such as sulfur and iodine) promise to achieve high-power and high-energy-density Mg storage since the former enable facile ion migration with flexible structures, and the latter bypass clumsy solid-state Mg 2+ diffusion via soluble intermediates (such as polysulfides and triiodides). Unfortunately, the nucleophilic components are not suitable for organic polymer electrodes and conversion electrodes (e.g., sulfur, iodine) due to chemical reactions with electrophilic materials. − …”