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

Joshua M Stratford,Seyyed Shayan Meysami,Dean S Keeble,Philip A Chater,Maria-Magdalena Titirici,Annette K Kleppe,Jerry Barker,Clare P Grey,Phoebe K Allan,Christopher J Wright

doi:10.1021/jacs.1c06058

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 23Na 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.

Highlights

Sodium-ion batteries are widely considered to be an attractive future technology for large-scale energy storage owing to the low cost and high abundance of the raw materials.[1]
We present highly consistent data sets obtained from operando pair distribution function (PDF) experiments performed during thecharge processes for two commercially relevant hard carbons with similar average pore diameters; these give new insights into the sodiuminsertion processes and direct observation of the size of sodium clusters
A number of glucose-derived hard carbons are studied; these are referred to by their pyrolysis temperatures e.g., 1100 °C refers to a hard carbon produced by pyrolysis of glucose at 1100 °C

Summary

Introduction

Sodium-ion batteries are widely considered to be an attractive future technology for large-scale energy storage owing to the low cost and high abundance of the raw materials.[1]. Hard carbons have attracted the most interest of all the potential materials, due to their low cost and good reversible capacities.[2] details of their structures and the mechanisms underlying sodium storage within hard carbons remain the subject of debate. The total capacity, and the proportion of this capacity observed on each of these two processes, varies between carbons synthesized under different conditions.[3−6] A number of models have been proposed to explain these electrochemical processes, the majority of which concentrate on the possibility of sodium storage in three aspects of the carbon structure: (1) graphene−graphene interlayer gallery spacing, (2) defects and graphene fragment edge sites, and (3) micropores

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Discussion

Conclusion