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

• Hierarchical porous nano/micron MoO2 spheres synthesized in supercritical methanol.
• Liquid CO2 (l-CO2) was used for ultrathin and uniform carbon layer on MoO2.
• Carbon layer <2 nm was coated on the nanosized primary particles of MoO2 spheres.
• Carbon-coated MoO2 exhibited 120.9 mAh g−1 at the fast current of 3352 mA g−1.
• l-CO2 coating enhanced charge transfer kinetics and Li ion diffusivity.

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.