Correlations between preparation methods, structural features and electrochemical Li-storage behavior of reduced...

Wide differences in the structural features of graphenic carbon, especially in the case of reduced graphene oxides (rGO), are expected to have considerable impacts on the properties, thus leading to significant scatter and poor understanding/prediction of their performances for various applications, including as electrode materials for electrochemical Li-storage. In this context, the present work develops a comprehensive understanding (via thorough experimentation, including in situ X-ray diffraction studies, and analysis) on the effects of graphene oxide (GO) reduction methods/conditions on the structural features (mainly 'graphenic' ordering) and concomitant influences of the same on electrochemical Li-storage behavior. 'Moderately oxidized' GO (O/C ∼0.41) was reduced via three different methods, viz., (i) using hydrazine hydrate vapor at room temperature (rGO-H; O/C ∼0.23), (ii) thermal reduction by annealing at just 500 °C (rGO-A; O/C ∼0.20) and (iii) hydrazine treatment, followed by the same annealing treatment (rGO-HA; O/C ∼0.17). Raman spectroscopy, in situ X-ray diffraction recorded during annealing and high resolution TEM imaging indicate that while GO and rGO-H had considerable defect contents [I(D)/I(G) ∼1.4 for rGO-H], including a very non-uniform interlayer spacing (varying between 3.1 and 3.6 Å), the 500 °C annealed rGO-A and rGO-HA had significantly reduced defect contents [I(D)/I(G) ∼0.6] and near-perfect 'graphenic' ordering with a uniform interlayer spacing of ∼3.35 Å. Despite the nanoscaled dimensions, defect structures, especially the non-uniform interlayer spacing, resulted in relatively poor reversible Li-capacity and rate capability for the non-annealed rGO-H, even in comparison to the bulk graphitic carbon. By contrast, the annealed rGOs, especially the rGO-HA, not only possessed a superior reversible Li-capacity of ∼450 mA h g^-1 (at C/20), but also exhibited a significantly improved rate capability (even compared to most rGOs reported in the literature), retaining ∼120 mA h g^-1 along with flat potential profile (below ∼0.2 V against Li/Li⁺) even at 10C (as possibly never reported before with graphitic/graphenic carbons).