Abstract
Banana peel pseudographite (BPPG) offers superb dual functionality for sodium ion battery (NIB) and lithium ion battery (LIB) anodes. The materials possess low surface areas (19â217 m2 gâ1) and a relatively high electrode packing density (0.75 g cmâ3 vs â¼1 g cmâ3 for graphite). Tested against Na, BPPG delivers a gravimetric (and volumetric) capacity of 355 mAh gâ1 (by active material â¼700 mAh cmâ3, by electrode volume â¼270 mAh cmâ3) after 10 cycles at 50 mA gâ1. A nearly flat â¼200 mAh gâ1 plateau that is below 0.1 V and a minimal charge/discharge voltage hysteresis make BPPG a direct electrochemical analogue to graphite but with Na. A charge capacity of 221 mAh gâ1 at 500 mA gâ1 is degraded by 7% after 600 cycles, while a capacity of 336 mAh gâ1 at 100 mAgâ1 is degraded by 11% after 300 cycles, in both cases with â¼100% cycling Coulombic efficiency. For LIB applications BPPG offers a gravimetric (volumetric) capacity of 1090 mAh gâ1 (by material â¼2200 mAh cmâ3, by electrode â¼900 mAh cmâ3) at 50 mA gâ1. The reason that BPPG works so well for both NIBs and LIBs is that it uniquely contains three essential features: (a) dilated intergraphene spacing for Na intercalation at low voltages; (b) highly accessible near-surface nanopores for Li metal filling at low voltages; and (c) substantial defect content in the graphene planes for Li adsorption at higher voltages. The <0.1 V charge storage mechanism is fundamentally different for Na versus for Li. A combination of XRD and XPS demonstrates highly reversible Na intercalation rather than metal underpotential deposition. By contrast, the same analysis proves the presence of metallic Li in the pores, with intercalation being much less pronounced.
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