Correlating Local Structure and Sodium Storage in Hard Carbon Anodes: Insights from Pair Distribution Function Analysis and Solid-State NMR

Abstract

Hard carbons are the leading candidate anode materials for sodium-ion batteries. However, the sodium-insertion mechanisms remain under debate. Here, employing a novel analysis of operando and ex situ pair distribution function (PDF) analysis of total scattering data, supplemented by information on the local electronic structure provided by operando 23 Na solid-state NMR, we identify the local atomic environments of sodium stored within hard carbon and provide a revised mechanism for sodium storage. The local structure of carbons is well-described by bilayers of curved graphene fragments, with fragment size increasing, and curvature decreasing with increasing pyrolysis temperature. A correlation is observed between the higher-voltage (slope) capacity and the defect concentration inferred from the size and curvature of the fragments. Meanwhile, a larger lower-voltage (plateau) capacity is observed in samples modeled by larger fragment sizes. Operando PDF data on two commercially relevant hard carbons reveal changes at higher-voltages consistent with sodium ions stored close to defective areas of the carbon, with electrons localized in the antibonding π*-orbitals of the carbon. Metallic sodium clusters approximately 13−15 Å in diameter are formed in both carbons at lower voltages, implying that, for these carbons, the lower-voltage capacity is determined by the number of regions suitable for sodium cluster formation, rather than by having microstructures that allow larger clusters to form. Our results reveal that local atomic structure has a definitive role in determining storage capacity, and therefore the effect of synthetic conditions on both the local atomic structure and the microstructure should be considered when engineering hard carbons.

Regulating closed pore structure enables significantly improved sodium storage for hard carbon pyrolyzing at relatively low temperature

“…43 In 23 Na NMR spectra, a positive shift means that the element prefers to exist in a metallic state, whereas a negative shift means that the element tends to exist in an ionic state. 44 While for ChT-1100, the peak at 16 ppm may correspond to the process of Na filling in the pores. In combination with the above analysis, it is sure that the high capacity of ChT-1100 in the low voltage plateau region is attributed to the pore filling of Na in the closed pores.…”

“…The strong peak at −10 ppm in the curve of UnT‐1100 indicates that Na is stored mainly in an ionic state 43 . In 23 Na NMR spectra, a positive shift means that the element prefers to exist in a metallic state, whereas a negative shift means that the element tends to exist in an ionic state 44 . While for ChT‐1100, the peak at 16 ppm may correspond to the process of Na filling in the pores.…”

Structure and function of hard carbon negative electrodes for sodium-ion batteries

“…The larger chemical shift was attributed to the enhanced contribution from the Knight shift and increase of the Na 2 s density of states at the Fermi level which was associated with the formation of metallic sodium during the plateau region. These processes were ascribed to Na + insertion in between the graphene layers and also to the filling of pores with sodium clusters [84,87]. The carbons with higher ratio of plateau/slope capacity (relatively larger contribution from plateau region) were found to display larger 23 Na chemical shift at the end of the discharge.…”

“…The local chemical environment and the atomic structure of a material play a major role in dictating the charge storage mechanism and the resultant overall capacity. Stratford et al used 23 Na ssNMR to propose a charge storage mechanism by examining the electronic structure of the sodium inserted in the hard carbon [84]. In this work, they studied four different hard carbon samples, namely, Carbon A, Carbon B, Carbon 1100 • C, and Carbon 1500 • C. Carbon A and Carbon B are commercially available carbons produced by Kureha Battery Materials Japan Co., Ltd and Faradion Ltd, respectively.…”

Sieving carbons promise practical anodes with extensible low-potential plateaus for sodium batteries

“…Indeed, the slow commercialization of SIBs is largely due to the lack of practical high-energy carbon anodes that can play a similar role to the one played by graphite in LIBs [ 12 , 13 ]. Most recently, it has been proposed that the LPP-related sodium storage process can be attributed to the formation of sodium clusters inside ultrasmall nanopores of non-graphitic carbons, especially the anthracite-derived soft carbons and carbohydrate-derived hard carbons [ 9 , 14 – 16 ], which suggests a way for the design of non-graphitic carbon anodes to produce and further extend the LPP [ 17 – 19 ]. Porous carbons (PCs) with open entrances accessible to gas adsorbates are totally free of any LPP and only deliver sloping charge/discharge curves [ 20 – 22 ].…”