Methods of producing functionalized carbon nanosheets capable of use as electrode materials in electrochemical energy storage cells, electrodes and electrode materials formed thereby, and electrochemical energy storage cells of sodium-ion batteries that utilize such electrodes and electrode materials. Such a method of producing functionalized carbon nanosheets includes preparing a solution containing dissolved glucose, mixing a quantity of NaCl crystals with the solution to form a mixture, drying the mixture to form a gel comprising NaCl crystals each having a layer of glucose thereon, heating the gel in an inert atmosphere to a processing temperature and for a duration sufficient to cause carbonization of the glucose and in situ functionalization with oxygen-containing functional groups, and removing the NaCl crystals to yield the functionalized carbon nanosheets.