Batteries – Nanocarbon Laboratory

We report on the ability of fullerene C₆₀ and hydrogenated fullerene C₆₀H_x (x∼39) to operate as negative electrodes in novel Na-ion batteries. Building upon the known solubility of C₆₀ in common organic electrolytes used in batteries, we developed a suitably optimized solid-state Na-(polyethylene oxide) electrolyte for this application. Electrochemical and structural properties of the fullerene electrodes were investigated through cyclic voltammetry, fixed-current charge/discharge of the electrodes, impedance spectroscopy and powder X-ray diffraction. Both C₆₀ and hydrogenated C₆₀ have been electrochemically intercalated with sodium. Specific capacities after the first cycle are 250 mAh g⁻¹ and 230 mAh g⁻¹ for C₆₀ and C₆₀H_x respectively. However, C₆₀ electrode shows a strong irreversible character after the first discharge, probably due to the formation of stable polymeric Na_xC₆₀ phases, where Na⁺ ions diffusion is hindered. On the contrary, C₆₀H_x displays better reversibility, suggesting that hydrogenation of the buckyball could be effective to preserve sufficiently large interstitial pathways for Na⁺ diffusion upon intercalation.

Two chemically synthesized defective graphene materials with distinctly contrasting extended structures and surface chemistry are used to prepare sodium-ion battery electrodes. The difference in electrode performance between the chemically prepared graphene materials is qualified based on correlations with intrinsic structural and chemical dissimilarities. The overall effects of the materials physical and chemical discrepancies are quantified by measuring the electrode capacities after repeated charge/discharge cycles. Solvothermal synthesized graphene (STSG) electrodes produce capacities of 92 mAh/g in sodium-ion batteries after 50 cycles at 10 mA/g, while thermally exfoliated graphite oxide (TEGO) electrodes produce capacities of 248 mAh/g after 50 cycles at 100 mA/g. Solid-state ²³Na nuclear magnetic resonance spectroscopy is employed to locally probe distinct sodium environments on and between the surface of the graphene layers after charge/discharge cycles that are responsible for the variations in electrode capacities. Multiple distinct sodium environments of which at least 3 are mobile during the charge–discharge cycle are found in both cases, but the majority of Na is predominantly located in an immobile site, assigned to the solid electrolyte interface (SEI) layer. Mechanisms of sodium insertion and extraction on and between the defective graphene surfaces are proposed and discussed in relation to electrode performance. This work provides a direct account of the chemical and structural environments on the surface of graphene that govern the feasibility of graphene materials for use as sodium-ion battery electrodes.

Categoria: Batteries

Electrochemical intercalation of fullerene and hydrofullerene with sodium

Mechanisms of Sodium Insertion/Extraction on the Surface of Defective Graphenes