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.
We demonstrate that peat moss, a wild plant that covers 3% of the earth's surface, serves as an ideal precursor to create sodium ion battery (NIB) anodes with some of the most attractive electrochemical properties ever reported for carbonaceous materials. By inheriting the unique cellular structure of peat moss leaves, the resultant materials are composed of three-dimensional macroporous interconnected networks of carbon nanosheets (as thin as 60 nm). The peat moss tissue is highly cross-linked, being rich in lignin and hemicellulose, suppressing the nucleation of equilibrium graphite even at 1100 °C. Rather, the carbons form highly ordered pseudographitic arrays with substantially larger intergraphene spacing (0.388 nm) than graphite (c/2 = 0.3354 nm). XRD analysis demonstrates that this allows for significant Na intercalation to occur even below 0.2 V vs Na/Na(+). By also incorporating a mild (300 °C) air activation step, we introduce hierarchical micro- and mesoporosity that tremendously improves the high rate performance through facile electrolyte access and further reduced Na ion diffusion distances. The optimized structures (carbonization at 1100 °C + activation) result in a stable cycling capacity of 298 mAh g(-1) (after 10 cycles, 50 mA g(-1)), with ∼150 mAh g(-1) of charge accumulating between 0.1 and 0.001 V with negligible voltage hysteresis in that region, nearly 100% cycling Coulombic efficiency, and superb cycling retention and high rate capacity (255 mAh g(-1) at the 210th cycle, stable capacity of 203 mAh g(-1) at 500 mA g(-1)).
This is the first report of a hybrid sodium ion capacitor (NIC) with the active materials in both the anode and the cathode being derived entirely from a single precursor: peanut shells, which are a green and highly economical waste globally generated at over 6 million tons per year. The electrodes push the envelope of performance, delivering among the most promising sodiation capacity -rate capability -cycling retention combinations reported in literature for each materials 4 materials being desirable. Previously, researchers have primarily focused on improving the power capability of the anode in order to catch up with the fast kinetics of the capacitive cathode. 39,42,44,46 NIC devices have been recently fabricated using the following anode-cathode combinations: V 2 O 5 /CNT//AC, 38 Na x H 2-x Ti 3 O 7 //AC 43 , with AC meaning conventional activated carbon. This creates a necessity to include excess mass (i.e. volume), generally several times more than that of the anode, in order to achieve the charge balance between the two electrodes. 39,43,44 The Na ion insertion processes into the bulk of the negative electrodes are known to be substantially more kinetically sluggish than those for Li, 4,49,50 posing a secondary major challenge to achieving attractive Na ion -based hybrid devices.An inexpensive carbon-based negative electrode with a Na redox potential near Na/Na + would not only provide a cost advantage over the inherently more costly inorganic materials but would also maximize the device energy density. 11,40,49,51,52 Ideally such electrode materials would also be truly green, 7,8,20,26,30,31,33,47,53,54,55,56,57,58,59,60,61 being derived from organic waste products that otherwise possess no economic value. Peanuts are a globally cultivated legume food staple, with the peanut shells having only limited commercial end-use as filler in animal feed or as charcoal. 62 In 2010 the peanut plant was cultivated on 21 million hectares worldwide, 63 producing approximately 20 million tons, with an estimated value of 9 billion USD. 64 This produces roughly 6 million tons of peanut shell waste.Researchers have prepared activated carbons from peanut shells and explored their 5 applications in environmental science (e.g. sorbents for organic and metal pollutants removal 65,66 ) and energy storage (e.g. supercapacitor, 67,68 lithium ion battery 69,70 ).These "classical" activated carbons were prepared by direct pyrolysis followed by high temperature activation. 62,67,71,72 In terms of the synthesis methodology and by the resultant structure and performance, such ACs are analogous to commercial products, which are micro-scale particulates with tortuous 3D pore networks. In terms of the synthesis methodology and by the resultant structure and performance, such ACs are analogous to commercial products, which are micro-scale particulates with tortuous 3D pore networks. In this work we take an alternative approach: We tailor the synthesis process to take full advantage of the unique structure of the peanut shell and actuall...
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