Some of cathode and anode active materials (e.g., LiFePO₄, Li₄Ti₅O₁₂, Li₂FeSiO₄, TiO₂, MoO₂) exhibit inherently low electronic conductivities and low ionic conductivities; thus, these materials typically suffer from poor electrochemical performance. The reduction of particle size to a nanoscale dimension to enhance lithium ion diffusivity and carbon coating to increase electronic conductivity are the most widely used strategies to improve the performance of the potential active materials. Incorporation of too much carbon content, thick carbon layer coating, and non-uniform carbon coating on individual particles can also deteriorate energy density and battery performance. The complete, uniform, and thin carbon layer coverage ensures that the particles receive electrons from all directions and the electrons penetrate easily through the carbon layer, resulting in low polarization and high-rate performance. Herein, the use of liquid carbon dioxide (l-CO₂) for uniform and ultrathin carbon layer coverage on highly porous LiFePO₄ and MoO₂ particles is discussed. The extremely low surface tension and the low viscosity associated with l-CO₂ make it an excellent coating solvent for uniform and ultrathin carbon layer formation on the highly porous active materials. In addition, the carbon content in the range of 1-10 wt% can be easily controlled by adjusting process parameters such as solution concentration or evaporation velocity. The hierarchically porous MoO₂ microspheres are synthesized in supercritical methanol without using any surfactants or structure-directing chemicals. The highly porous LiFePO₄ particles are synthesized using a co-precipitation method. The MoO₂ and LiFePO₄ particles exhibits hierarchically porous nano/micro structure, in which primary particles with 30-65 mn size are loosely aggregated and form secondary spheres with 0.5-10 mm size. Owing to the low viscosity and low surface tension of l-CO₂, the coating solution is penetrated into the pores of the particles and uniform and ultrathin carbon layer is covered on the primary particles. The thickness of carbon layers were in range of 0.8-2.0 nm by adjusting the carbon precursor concentration in l-CO₂. The TEM analysis on the cross-sectional area of the particles shows that the uniform incorporation of the carbon layer on the whole surface of the micron-sized secondary particles. The carbon-coated LiFePO₄ exhibits ~108.6 mAh g^-1 at a high charge-discharge rate of 30 C. In addition, after 1000 cycles of 10 C, ~ 85% of an initial capacity is maintained. The carbon-coated MoO₂ exhibits enhanced high-rate performance and cell stability.