Basal Planes Unlocking and Interlayer Engineering Endows Proton Doped-MoO_2.8F_0.2 with Fast and Stable Magnesium Storage

Abstract

Molybdenum trioxide has served as a promising cathode material of rechargeable magnesium batteries (RMBs), because of its rich valence states and high theoretical capacity; yet, it still suffers from sluggish (de)­intercalation kinetics and inreversible structure change for highly polarized Mg2+ in the interlayer and intralayer of structure. Herein, F– substitutional and H+ interstitial doping is proposed for α-MoO3 materials (denoted HMoOF) by the intralayer/interlayer engineering strategy to boost the performance of RMBs. F– substitutional doping generates molybdenum vacancies along the Mo–O-□ or Mo–F-□ configurations (where □ represents the cationic vacancy) for unlocking the inactive basal plane of the layered crystal structure, and it further accelerates Mg2+ diffusion along the b-axis. Interstitial-doped H+ can expand interlayer spacing for reducing Mg2+ energy barrier along the ac plane and serve as a “pillar” to stabilize the interlayer structure. Moreover, anion and cation dual doping trigger shallow impurity levels (acceptors levels and donor levels), which helps to easily acquire the electrons from the valence band and donate the electrons to the conduction band. Consequently, the HMoOF electrode exhibits a high reversible capacity (241 mA h g–1 at 0.1 A g–1), an excellent rate capability (137.4 mAh g–1 at 2 A g–1), and a long cycling stability (capacity retention of 98% after 800 cycles at 1 A g–1) in RMBs. This work affords meaningful insights in layered materials for developing high-kinetics and long-life RMBs.

Enabling Ultrafast Single Mg2+ Insertion Kinetics of Magnesium-Ion Batteries via In Situ Dynamic Catalysis and Re-equilibration Effects

“…The reducing of Mo makes it partially oxidized. 4,18 The morphology of MQDs is characterized by transmission electron microscopy (TEM), as shown in Figure 2a. The particle size of MQDs is less than 10 nm with a typical Tyndall effect (Figure S2).…”

“…Characteristic peaks at 229.5 (3d 3/2 ) and 232.9 (3d 1/2 ) eV in the spectrum of Mo 3d originate from tetravalent Mo and those at 232.2 (3d 3/2 ) and 235.8 eV (3d 1/2 ) correspond to hexavalent Mo (Figure d). The reducing of Mo makes it partially oxidized. , …”

“…To realize the carbon peaking and carbon neutrality on schedule, development of large-scale sustainable energy storage technologies is significantly important nowadays. Rechargeable magnesium-ion batteries (MIBs) have emerged as the most favorable candidates in the era of post-lithium-ion batteries (LIBs) due to their low reduction potential (−2.37 V vs SHE), high volumetric capacity (3883 mA h cm –3 ), abundant reserves, and cost-effectiveness. − Most importantly, the essentially dendrite-free property of magnesium during electrochemical Mg 2+ plating has aroused intensive attention from all over the world, − which can create high intrinsic safety batteries . However, a strong electrostatic interaction between divalent Mg 2+ ions and cathode lattices directly affects its intercalation kinetics, which seriously restricts the specific capacity and rate capability of the cathode materials. − Another bottleneck challenge is the strong solvation ability of Mg 2+ , which will tightly interact with solvent molecules to form solvated ions with larger radius size.…”

Cathode Materials and Chemistries for Magnesium Batteries: Challenges and Opportunities

“…Similar observations were made on MoO 3 , where F − substitution generated Mo vacancies that offered additional basal plane diffusion pathways for fast Mg 2+ migration. [65] Due to the high tendency of cation disordering with Mg, [49] cation vacancies in TM compounds may also provide a thermodynamically favorable driving force for Mg 2+ insertion. Ideally, highly concentrated vacancies with homogeneous distribution among the cathode structure could build a percolating network, serving as Mg diffusion highways.…”

“…Similar observations were made on MoO 3 , where F − substitution generated Mo vacancies that offered additional basal plane diffusion pathways for fast Mg 2+ migration. [ 65 ]…”