High Capacity of Hard Carbon Anode in Na-Ion Batteries Unlocked by PO_x Doping

Abstract

The capacity of hard carbon anodes in Na-ion batteries rarely reaches values beyond 300 mAh/g. We report that doping PO x into local structures of hard carbon increases its reversible capacity from 283 to 359 mAh/g. We confirm that the doped PO x is redox inactive by X-ray adsorption near edge structure measurements, thus not contributing to the higher capacity. We observe two significant changes of hard carbon's local structures caused by doping. First, the (002) d-spacing inside the turbostratic nanodomains is increased, revealed by both laboratory and synchrotron X-ray diffraction. Second, doping turns turbostratic nanodomains more defective along ab planes, indicated by neutron total scattering and the associated pair distribution function studies. The local structural changes of hard carbon are correlated to the higher capacity, where both the plateau and slope regions in the potential profiles are enhanced. Our study demonstrates that Na-ion storage in hard carbon heavily depends on carbon local structures, where such structures, despite being disordered, can be tuned toward unusually high capacities.

Hard Carbons for Sodium‐Ion Battery Anodes: Synthetic Strategies, Material Properties, and Storage Mechanisms

“…[55] Thus, dopings of this nature,w hich can have ad irect impact on capacity and stabilitye nhance-ments through the nanovoid regime, can be and should be implemented to desirable effect. Heteroatoms, such as N, S, and P, increase the electrode-electrolyte wettability,i na ddition to electrode conductivity enhancement through mediation in electron delocalization.…”

“…[49,50] The enlarged interlayer spacinga nd increased defect concentration contribute to the increase in the storage capacityt hrough the intercalation mechanism.T he major gains achievablef rom heteroatom doping strategies have been through the intercalation-deintercalation paths, which have been made facile by increasing the interlayer separations and barrierr eduction for intercalation reactions.T he defects ites developed in the hard carbon structure throughh eteroatom doping also help to improve the electrode-electrolyte interface by facilitating better wetting. [55] Nitrogen-doped hard carbon systems have been welle xplored as anode materials for SIBs. [51][52][53][54] Doping with certain groups,s uch as PO x ,i sn oted to lead to substantial enhancement in capacitive storaget hrough the sodiump ore-filling mechanism,a part from minor enhancements in intercalation capacities.…”

“…More recently,doping with the PO x functional group was implemented by Li et al [55] Such doping in hard carbon was achieved by treating the carbon source with H 3 PO 4 ,b ut to avoid the generationo fm icroporosity,g raphene oxide was added during the pyrolysis reaction.…”

“…[30] Figure 20. [55]. Simulatedc harge distribution for graphene/Nasystems with Na binding energies for graphene with d) monoand e) divacancies and f) large in-planedefects.…”

Na‐Ion Batteries for Large Scale Applications: A Review on Anode Materials and Solid Electrolyte Interphase Formation

“…It has been shown that the composition [25,60] and quality [61] of the electrolyte influence strongly on the amount of charge lost for the SEI formation. Despite recent reports about tuning the porosity through PO x doping, [65] or demonstrating the correlation between pore size and capacity, [43] a deeper understanding of the mechanism of charge storage and its correlation with microstructural features appears necessary. [48] Some strategies have been successfully used to increase the coulombic efficiency by reducing the specific surface area, such as carbon coating [62,63] or the use of a graphene additive mixed with the precursor prior to pyrolysis.…”