Kinetic Growth and Inter-Layer Expansion as Cathode Engineering Strategies for Enhanced Energy and Power Densities in Sustainable Mg-Ion Batteries

Lithium-ion batteries (LIBs) have played a pivotal role in facilitating the widespread adoption of secondary batteries within the electric vehicles (EV) and energy storage systems (ESS). However, the limited energy density, recyclability, and sustainability of lithium-ion batteries have prompted research groups worldwide to explore alternative options known as "post-lithium-ion batteries." Multivalent (MV)-ion intercalation chemistry casts significant promise for multiplying the theoretical energy density of rechargeable batteries, while utilizing the well-established intercalation hosts for LIBs. However, the increase in charge density of multivalent-ions leads to substantial hindrances in interfacial and bulk diffusion kinetics. To overcome the hindrances, it is crucial to engineer the microstructure of the intercalation hosts. Due to the higher redox potential and larger theoretical capacity, oxides have been at the forefront of developing MV-ion batteries cathode, yet the lower polarizability compared to other chalcogenides results in the sluggish kinetics. In this study, we engineer the crystal structure of molybdenum trioxide (MoO₃), a representative layered host in magnesium-ion (Mg²⁺ as a representative MV-ion) batteries, by introducing organic pillars to expand the interlayer spacing to 177.7 % of pristine MoO₃. At the same time, we facilitated kinetic growth of the primary particles to minimize the diffusion length to 2.88 % of commercial bulk MoO₃. The increased interlayer distance enhanced the intrinsic diffusivity, while the shorter diffusion length resulted in superior interfacial and bulk diffusion kinetics. The interlayer-expanded and kinetically-grown MoO₃ exhibited significantly increased specific capacity of 402 mAh/g as a cathode for magnesium-ion batteries, which was 233 % of pristine MoO₃. Moreover, thus developed materials attained highly improved energy density, long-term cyclability, and rate capability with respect to various active ions, including Li⁺, Na⁺, and Mg²⁺.