Liquid carbon dioxide-based coating of a uniform carbon layer on hierarchical porous MoO₂ microspheres and assessment of their electrochemical performance

Hierarchical mesoporous MoO₂ microspheres coated with a uniform and ultrathin carbon layer were synthesized via a supercritical methanol (scMeOH) route followed by high-pressure free-meniscus coating (hFMC) using liquid carbon dioxide (l-CO₂) as a coating solution. The MoO₂ particles synthesized in scMeOH without surfactants or structure-directing chemicals exhibited nanosized primary particles of 30-80 nm diameter, whose loose aggregation produced microsized secondary particles of 0.5-4.8 μm diameter. The MoO₂ particles had a highly porous structure with an average pore diameter of 10.5 nm and a porosity of 52.7%. An ultrathin, uniform, and conformal carbon layer with thickness in the range of 1.5-2.0 nm was coated on the primary MoO₂ particles using l-CO₂ as low-viscosity and low-surface tension solvent. When tested as anode in lithium ion batteries, the carbon-coated, hierarchical porous MoO₂ (C-MoO₂) particles exhibited excellent electrochemical performance. C-MoO₂ with a carbon content of 6.6 wt% delivered a reversible discharge capacity of 451.5 mAh g^-1 at 83.8 mA g^-1 after 100 cycles. C-MoO₂ with a carbon content of 14.1 wt% exhibited 120.9 mAh g^-1 of discharge capacity at the fast charge-discharge current of 3352 mA g^-1. The enhanced electrochemical performance of C-MoO₂ was attributed to enhanced charge transfer kinetics to the electrode surface and increased Li ion diffusivity to the MoO₂ phase during the charge/discharge process, as confirmed by electrochemical impedance spectroscopy.