Sodium Storage Mechanism Investigations through Structural Changes in Hard Carbons

Abstract

Hard carbons, due to their relatively low cost and good electrochemical performance, are considered the most promising anode materials for Na-ion batteries. Despite the many reported structures of hard carbon, the practical use of hard carbon anodes is largely limited by low initial Coulombic efficiency (ICE) and the sodium storage mechanism still remains elusive. A better understanding of the sodium ion behaviour in hard carbon anodes is crucial to develop more efficient sodium ion batteries. Here, a series of hard carbon materials with tailored morphology and surface functionality was synthesized via hydrothermal carbonisation and subsequent pyrolysis from 1000 to 1900 °C. Electrochemical results revealed different sodiation-desodiation trends in the galvanostatic potential profiles and varying ICE, and were compared with theoretical studies to understand the effect of the varying hard carbon structure on the sodium storage process at different voltages. Furthermore, electrode expansion during cycling was investigated by in-situ dilatometry; to the best of our knowledge, this is the first time the technique has been applied to hard carbons for ion storage mechanism investigation in Na-ion batteries. Combining experimental and theoretical results, we propose a model for sodium storage in our hard carbons that consists of Na-ion storage at defect sites and by intercalation in the high voltage slope region and via pore-filling in the low voltage plateau

Defects in Hard Carbon: Where Are They Located and How Does the Location Affect Alkaline Metal Storage?

“…While the magnitude of the drop in is larger for V C , the absolute is 0.02 eV, which is much lower than 3.24 eV for the V C at the same position. This suggests that at these chemical potentials, the HC surface is unstable to oxidation, in good agreement with experiment, [ 7,36 ] and one of the vectors of surface oxidation could be expected to occur via the formation of the O C .…”

“…[ 7,38,39,74 ] The effect of surface, interlayer distance, pore size, carbon morphology, defects, and heteroatoms will be dependent on pyrolysis/carbonization temperature (with less surface area, fewer defects, and more graphitic character observed with increasing pyrolysis temperature) and the initial reagents (where oxygen and nitrogen dopants can be introduced to tune material properties). [ 21,27,36,39,75 ] Additionally, the main contribution to Li storage in HC comes from intercalation, with initial contribution from surface adsorption as confirmed by in situ Raman analysis of Li in HCs. [ 33,65,76–79 ] For KIBs, the graphitic stacks’ interlayer distances accessible to Na + and Li + storage can be inaccessible, with greater interlayer distances required.…”

“…From experimental evidence, carbon‐based anodes have both curved and planar motifs, with surfaces and their defects having a particular influence on electrolyte breakdown and the initial metal adsorption. [ 7,11,36–39 ] These curved pores have been observed both from pair distribution analysis and transmission electron microscopy imaging. [ 7,11,36–38 ] In this study, we investigate the effect of these morphologies on defect formation as a function of lattice position.…”

“…[ 7,11,36–39 ] These curved pores have been observed both from pair distribution analysis and transmission electron microscopy imaging. [ 7,11,36–38 ] In this study, we investigate the effect of these morphologies on defect formation as a function of lattice position. Based on our previous investigation of defect formation on graphene, [ 32,34 ] the following defects are considered: carbon vacancy (V C ), nitrogen substitutional or graphitic nitrogen defect (N C ), nitrogen substitutional defect and carbon vacancy or pyridinic nitrogen defect (N C V C ), oxygen substitutional defect (O C ), double oxygen substitutional defect (2O C ), oxygen substitutional defect and carbon vacancy (O C V C ), triple oxygen substitutional defect (3O C ), nitrogen and oxygen substitutional defect (N C O C ), and double oxygen single nitrogen substitutional defect (N C 2O C ).…”

“…[ 50 ] The surface adsorption can also be followed by studying the anode expansion, due to the lower volume expansion associated with adsorption when compared to intercalation. [ 36,50 ] The volumetric change of a HC NIB anode was studied by Wang et al. showing that the initial sodiation process took place mainly through surface adsorption (due to the stable volume), whereas intercalation occurred after 300 s where there was a sharp rise in volume.…”