Solvent-Controlled Charge Storage Mechanisms of Spinel Oxide Electrodes in Mg Organohaloaluminate Electrolytes

Phenylphosphonate surface functionalisation of MgMn2O4 with 3D open-channel nanostructures for composite slurry-coated cathodes of rechargeable magnesium batteries operated at room temperature

Spinel-type MgMn₂O₄, prepared by a propylene-oxide-driven sol–gel method, has a high surface area and structured bimodal macro- and mesopores, and exhibits good electrochemical properties as a cathode active material for rechargeable magnesium batteries. However, because of its hydrophilicity and significant water adsorption properties, macroscopic aggregates are formed in composite slurry-coated cathodes when 1-methyl-2-pyrrolidone (NMP) is used as a non-aqueous solvent. Functionalising the surface with phenylphosphonate groups was found to be an easy and effective technique to render the structured MgMn₂O₄ hydrophobic and suppress aggregate formation in NMP-based slurries. This surface functionalisation also reduced side reactions during charging, while maintaining the discharge capacity, and significantly improved the coulombic efficiency. Uniform slurry-coated cathodes with active material fractions as high as 93 wt% can be produced on Al foils by this technique employing carbon nanotubes as an electrically conductive support. A coin-type full cell consisting of this slurry-coated cathode and a magnesium alloy anode delivered an initial discharge capacity of ∼100 mA h g⁻¹ at 25 °C.

Phenylphosphonate functionalisation is an easy, highly effective strategy to fabricate slurry-coated nanostructured MgMn₂O₄ cathodes for rechargeable magnesium batteries at active material fractions up to 93 wt% for rechargeable magnesium batteries cycled at 25 °C.

Structure Design of Long-Life Spinel-Oxide Cathode Materials for Magnesium Rechargeable Batteries

Development of metal-anode rechargeable batteries is a challenging issue. Especially, magnesium rechargeable batteries are promising in that Mg metal can be free from dendrite formation upon charging. However, in case of oxide cathode materials, inserted magnesium tends to form MgO-like rocksalt clusters in a parent phase even with another structure, which causes poor cyclability. Here, a design concept of high-performance cathode materials is shown, based on: i) selecting an element to destabilize the rocksalt-type structure and ii) utilizing the defect-spinel-type structure both to avoid the spinel-to-rocksalt reaction and to secure the migration path of Mg cations. This theoretical and experimental work substantiates that a defect-spinel-type ZnMnO₃ meets the above criteria and shows excellent cycle performance exceeding 100 cycles upon Mg insertion/extraction with high potential (≈2.5 V vs Mg²⁺ /Mg) and capacity (≈100 mAh g^-1 ). Thus, this work would provide a design guideline of cathode materials for various multivalent rechargeable batteries.

Rational Design of Mixed Solvent and Porous Graphene-Supported Spinel Oxide Electrodes for High-Rate and Long Cycle-Life Mg Batteries

The development of Mg batteries based on the interfacial charge storage mechanism, where the capacity originates from capacitive processes and the solvent-related interfacial reactions, could efficiently circumvent the challenge of intercalation-based Mg batteries with sluggish kinetics. In this work, the proposed Mg organohaloaluminate mixture electrolyte is reported to improve the charge storage performance of the graphene-supported cathodes, resulting in both high cycling stability (91% capacity retention after 2000 cycles) and high rate capability (51% capacity retention when the current density increases by 100 times). The experimental and computational studies have revealed that the exceptional cell performance originates from the optimized electrode/electrolyte interface, where the highly reversible interfacial reactions occur with the 1,2-dimethoxyethane additive in the typical all-phenyl complex electrolyte. The fast charge-transfer kinetics along the surface of highly porous and conductive graphene-supported electrodes have also been observed.

Effects of Particle Size on Mg2+ Ion Intercalation into λ-MnO2 Cathode Materials

An emergent theme in mono- and multivalent ion batteries is to utilize nanoparticles (NPs) as electrode materials based on the phenomenological observations that their short ion diffusion length and large electrode-electrolyte interface can lead to improved ion insertion kinetics compared to their bulk counterparts. However, the understanding of how the NP size fundamentally relates to their electrochemical behaviors (e.g., charge storage mechanism, phase transition associated with ion insertion) is still primitive. Here, we employ spinel λ-MnO₂ particles as a model cathode material, which have effective Mg²⁺ ion intercalation but with their size effect poorly understood to investigate their operating mechanism via a suite of electrochemical and structural characterizations. We prepare two differently sized samples, the small nanoscopic λ-MnO₂ particles (81 ± 25 nm) and big micron-sized ones (814 ± 207 nm) via postsynthesis size-selection. Analysis of the charge storage mechanisms shows that the stored charge from Mg²⁺ ion intercalation dominates in both systems and is ∼10 times higher in small particles than that in the big ones. From both X-ray diffraction and atomic-resolution scanning transmission electron microscopy imaging, we reveal a fundamental difference in phase transition of the differently sized particles during Mg²⁺ ion intercalation: the small NPs undergo a solid-solution-like phase transition which minimizes lattice mismatch and energy penalty for accommodating new phases, whereas the big particles follow conventional multiphase transformation. We show that this pathway difference is related to the improved electrochemical performance (e.g., rate capability, cycling performance) of small particles over the big ones which provides important insights in encoding within the particle dimension, that is, the single-phase transition pathway in high-performance electrode materials for multivalent ion batteries.

Highly Branched VS4 Nanodendrites with 1D Atomic-Chain Structure as a Promising Cathode Material for Long-Cycling Magnesium Batteries

Rechargeable magnesium batteries have attracted increasing attention due to the high theoretical volumetric capacities, dendrite formation-free characteristic and low cost of Mg metal anodes. However, the development of magnesium batteries is seriously hindered by the lack of capable cathode materials with long cycling life and fast solid-state diffusion kinetics for highly-polarized divalent Mg²⁺ ions. Herein, vanadium tetrasulfide (VS₄ ) with special one-dimensional atomic-chain structure is reported to be able to serve as a favorable cathode material for high-performance magnesium batteries. Through a surfactant-assisted solution-phase process, sea-urchin-like VS₄ nanodendrites are controllably prepared. Benefiting from the chain-like crystalline structure of VS₄ , the S₂^2- dimers in the VS₄ nanodendrites provide abundant sites for Mg²⁺ insertion. Moreover, the VS₄ atomic-chains bonded by weak van der Waals forces are beneficial to the diffusion kinetics of Mg²⁺ ions inside the open channels of VS₄ . Through a series of systematic ex situ characterizations and density functional theory calculations, the magnesiation/demagnesiation mechanism of VS₄ are elucidated. The VS₄ nanodendrites present remarkable performance for Mg²⁺ storage among existing cathode materials, exhibiting a remarkable initial discharge capacity of 251 mAh g^-1 at 100 mA g^-1 and an impressive long-term cyclability at large current density of 500 mA g^-1 (74 mAh g^-1 after 800 cycles).