Reduced Graphene Oxide Paper Electrode: Opposing Effect of Thermal Annealing on Li and Na Cyclability

We study long-term electrochemical sodium and lithium cycling, and tensile testing behavior of thermally reduced graphene oxide (rGO) paper electrodes. We find strong dependence of annealing temperature and gas environment on the electrical conductivity, electrochemical capacity, and rate capability of the electrodes. The effect, however, was opposing for the two cell types. Lithium charge capacity increased with increasing annealing temperatures reaching a stable value of ∼325 mAh·ganode–1 (∼100 mAh·cm–3anode at ∼48 μA·cm–2 with respect to total volume of the electrode) for specimen annealed at 900 °C, while a sharp decline in Na charge capacity was noted for rGO annealed above 500 °C. Maximum sodium charge capacity of ∼140 mAh·g–1anode at 100 mA·g–1anode (∼98 mAh·cmanode–3 at ∼70 μA·cm–2) was realized for specimen reduced at 500 °C. These values are the highest reported for GO paper electrodes. More important, annealing of GO in NH3 environment resulted in a complete shutdown of its Na-ion cyclability showing near-zero charge capacity. On the contrary, NH3 annealing only improved the electrode's Li-ion cycling efficiency and rate capability. This behavior is attributed to the increased level of ordering in graphene sheets and decreased interlayer spacing with increasing annealing temperatures in Ar or reduction at moderate temperatures in NH3 atmosphere. Further, uniaxial tensile tests and videography highlighted the superior elasticity and high strain to failure of crumpled paper electrodes. The present work provides new insights toward the optimization and design of Li and other larger metal-ion battery electrodes where graphene is utilized as an active material, conductive agent, or a flexible mechanical support.