The commercialization of large-scale energy storage systems has renewed the scientific interest on electrode materials for sodium-ion batteries (SIBs). Polyanionic sodium orthosilicates [Na₂M(SiO₄)] are an interesting group of SIB cathode materials with high redox potentials via a two-electron redox process, and their three-dimensional framework. However, studies on these materials are limited due to difficulties in synthesizing them in stable and electrochemically desirable phase. Exhibiting rich polymorphism, orthosilicates undergo irreversible structural changes during processing and/or cycling. Here, we report a solvothermal process, which yields in situ carbon-coated Na₂CoSiO₄ with monoclinic structure. Besides physical characterization, we discuss its electrochemical performance in coin-cell configuration vs. Na anode. Na₂CoSiO₄ exhibits significantly lower polarization (0.15 V) than Na₂FeSiO₄ and Na₂MnSiO₄. As polyanionic compounds are poor electronic conductors with sluggish redox kinetics, we observe that these limitations could be overcome in Na₂CoSiO₄ by incorporating functionalized multi-walled carbon nanotubes (MWCNTs). The Na₂CoSiO₄/MWCNT composite cathode reversibly delivers 125 mAh g⁻¹ at C/20 rate, almost 50% more than pristine Na₂CoSiO₄. We also provide insights into the Na⁺ diffusion and exchange current by applying electrochemical impedance spectroscopy (EIS), revealing that the Na⁺ diffusion coefficient increases by one order while the exchange current doubles when functionalized MWCNTs are added to Na₂CoSiO₄.