Na⁺ ion migration on the surface of reduced graphene oxide

The increasing demand of sodium ion batteries (SIBs) remarkably accelerates the study of solid-state sodium ion conductors due to their potential application as solid-state electrolytes in SIBs. In the present work, the sodium ion is attached to reduced graphene oxide (rGO) to realize a sodium ion conductor. Tuning the activation energy of migration for Na⁺ and Li⁺ ions on rGO surface is investigated by varying the concentration of both ions. The lowest values of activation energies for Na⁺ and Li⁺ conduction are found to be 0.28 eV and 0.37 eV, respectively. It is seen that the activation energy of migration of the Na⁺ ion is smaller than that of the Li⁺ ion. The lower positive charge density of Na⁺ compared to Li⁺ causes this lowering of activation energy in Na⁺ due to the comparatively weak cation-π interaction between the Na⁺ ion and the carbon hexagon. From the relaxation study, the relaxation exponent (β) value of the Na⁺ ion is found to be smaller than that of the Li⁺ ion. This deviation from Debye-type relaxation behavior of the Na⁺ ion also agrees well with the decreasing value of activation energy as mentioned above. We hope that this study will aid the design of ion conductors for solid-state SIBs.