Enhanced Lithium-Ion Storage through Anchoring Nanocrystalline MoO2/C Microspheres in rGO Nanosheets: Boosting Pseudocapacitance and Facilitating Rapid Conversion

Abstract

Both crystalline and amorphous MoO₂ exhibit distinct advantages for lithium-ion battery applications, with the former favoring lithium-ion intercalation and the latter undergoing complete lithiation via a conversion reaction. However, their sluggish lithium-ion insertion rates and inadequate charge transfer kinetics hinder their full potential. To address these challenges, we have developed a controllable approach that integrates liquid-phase dispersion of the H_xMoO₃/C and graphene oxides (GO) precursors followed by freeze-drying and low-temperature calcinations, aiming to merit the ionic conductivity of MoO₂/C nanocrystallite and the electronic conductivity of reduce graphene oxides (rGO), respectively. This facile method yields bubble-sheet-like MoO₂/C@rGO composites, where the quantitatively strategic incorporation of rGO can effectively mitigate recrystallization and surface oxidation of MoO₂ nanocrystals. Furthermore, the approximately 70% volume shrinkage of H_xMoO_3/C precursors into MoO₂/C can create a flexible void space between the microspheres and the rGO coating for better accommodating volume variations during lithiation. Electrochemical measurements show that MoO₂/C@rGO delivers high initial coulombic efficiency (ICE, e.g., up to 71.3% at 100 mA g⁻¹), impressive rate performance (e.g., achieving 60 mAh g⁻¹ at 1 A g⁻¹, and 34.1% retention from 0.1 to 2 A g⁻¹) and excellent cyclability (e.g., retaining 98.9% of its capacity after 200 cycles) when employed as an intercalation-type anode material above 1.00 V (vs. Li/Li⁺). Remarkably, electrochemical analysis indicates that the capacitive contribution is dominant during high-rate applications (e.g., up to 73.06% ratio at a scan rate of 0.50 mV s⁻¹), exhibiting a pseudocapacitive behavior. Additionally, MoO₂/C@rGO exhibits enhanced ICE (e.g., up to 76.9% at 100 mA g⁻¹), accelerated activation (e.g., achieving peak performance within 10 cycles at 100 mA g⁻¹), superior rate performance (e.g., achieving 446 mAh g⁻¹ at 2 A g⁻¹), and remarkable cyclability (e.g., maintaining 510 mAh g⁻¹ with 88.9% capacity retention over 600 cycles at 1 A g⁻¹) when applied as a conversion-type anode material at between 1.00 to 3.00 V (vs. Li/Li⁺). To elucidate the mechanisms underlying the high-rate performance and the increased capacity, ex-situ XRD, ex-situ SEM, ex-situ TEM, and electrochemical analysis were carried out. No evident phase transition or material pulverization can be observed upon cycling above 1.00 V; therefore, the structural evolution is entirely single-phase or solid-solution, which accompanies with pseudocapacitive characteristic. However, the diffraction peaks downshift, the eminent of Li_xMoO_2+δ, the pulverization, and the gradual amorphization of MoO₂ can be observed upon cycling, indicating an sequential intercalation-to-conversion activation from a crystalline state to an amorphous state.