High-density sodium and lithium ion battery anodes from banana peels.

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 (19-217 m(2) g(-1)) and a relatively high electrode packing density (0.75 g cm(-3) vs ∼1 g cm(-3) for graphite). Tested against Na, BPPG delivers a gravimetric (and volumetric) capacity of 355 mAh g(-1) (by active material ∼700 mAh cm(-3), by electrode volume ∼270 mAh cm(-3)) after 10 cycles at 50 mA g(-1). A nearly flat ∼200 mAh g(-1) 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 g(-1) at 500 mA g(-1) is degraded by 7% after 600 cycles, while a capacity of 336 mAh g(-1) at 100 mAg(-1) is degraded by 11% after 300 cycles, in both cases with ∼100% cycling Coulombic efficiency. For LIB applications BPPG offers a gravimetric (volumetric) capacity of 1090 mAh g(-1) (by material ∼2200 mAh cm(-3), by electrode ∼900 mAh cm(-3)) at 50 mA g(-1). 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.