Flash Pyrolyzed Coal Char for Sodium Ion Battery Anodes

Abstract

Present day lithium ion batteries face challenges regarding the sustainability, cost, and natural resource availability of their components. Sodium has been shown to be a complementary energy storage material, providing usable volumetric energy density. In designing sodium ion batteries (SIBs), hard carbon is used at the anode in place of graphite, due to the necessary micro domain structure needed to store sodium ions. Coal char is a low cost material and viable source of hard carbon. The use of coal char in SIBs also provides a high value, secondary use pathway for the abundant char component of coal.In this work, flash pyrolyzation is explored as a novel approach for coal char synthesis for SIB anodes using a Utah local bituminous coal. During flash pyrolyzation, char is heated in a drop-tube furnace at 1000 oC/s vs. the traditional slow pyrolyzation approach where char is heated at 20 oC/min. Flash pyrolyzed (FP) char has a surface structure that is vastly different in comparison to the slow pyrolyzed (SP) char (Figure 1.), larger d-spacing (FP d-spacing = 0.379 nm, SP d-spacing = 0.364 nm) and a smaller closed micropore diameter (FP micropore = 1.52 nm, SP micropore = 1.91 nm), as concluded from X-ray diffraction (XRD) and small angle X-ray scattering (SAXS) analysis, respectively.The SIB performance of FP char vs. SP char is tested and compared in ether-based and ester-based electrolytes. FP char performs better as a SIB anode than SP char in both electrolytes. Additionally, both char anodes achieve higher reversible capacity and better initial coulombic efficiencies (ICE) in an ether-based vs. ester-based electrolyte. When FP char is combined with an ether-based electrolyte, a 10th cycle reversible capacity of 109.4 mAh/g is achieved, whereas a SP char with an ester-based electrolyte cell has a 10th cycle reversible capacity of 72.5 mAh/g, when both cells are cycled at 50 mA/g (sodium metal half-cells). FP ether cells also perform with an initial coulombic efficiency (ICE) of 64%, but SP ester cells only show an ICE value of 52%. The enhanced electrochemical performance of the FP cells is attributed to differences in surface/bulk structure due to the high heating rate. The improvement in char anode performance in ether-based cells is likely due to the formation of a thinner solid-electrolyte interphase and/or improved sodiation due to an ether-based electrolyte. Figure 1