Intercalation of solvated Na-ions into graphite

Abstract

The reversible intercalation of solvated Na-ions into graphite and the concomitant formation of ternary Na–graphite intercalation compounds (GICs) are studied using several in operando techniques, such as X-ray-diffraction (XRD), electrochemical scanning tunnelling microscopy (EC-STM) and electrochemical quartz crystal microbalance techniques (EQCM). Linear ethylene glycol dimethyl ether homologues (“glymes”) G_x with x + 1 O-atoms were used as solvents, where x is 1–4. The intercalation mechanism of Na⁺(G_x)_y-complexes was investigated with a focus on the phase transitions and diffusion rates of the Na⁺(G_x)_y-complexes inside the graphite lattice. For the four shortest glymes (G₁ to G₄), it is found using XRD that an intermediate stage 2 Na–GIC (NaC₄₈) is formed upon the partial sodiation of the graphite electrode. At full sodiation stage 1 Na–GIC (NaC₁₈, 112 mA h g⁻¹) is obtained for G₁, G₂ and G₄, while the G₃ system also forms a stage 1 Na–GIC but with less Na incorporated (NaC₃₀, 70 mA h g⁻¹). The phase transitions of a battery electrode upon ion-intercalation are visualised using STM on the atomic scale for the first time. In addition, the local diffusion rates of the intercalated species inside the electrode were determined, a unique approach for determining kinetic effects in batteries on the atomic scale. The formation of a solid electrolyte interphase (SEI) is observed with STM as well as with an EQCM, while the latter technique is used for novel in situ hydrodynamic spectroscopy, giving further insight into the intercalation mechanism.