Correlating Local Structure and Sodium Storage in Hard Carbon Anodes: Insights from Pair Distribution Function Analysis and Solid-State NMR
[1];
[2];
[4];
[6]
- Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge, CB2 1EW, U.K., School of Chemistry, University of Birmingham, Edgbaston, Birmingham, B15 2TT, U.K.
- Diamond Light Source Ltd., Harwell Science and Innovation Campus, Didcot, OX11 0DE, U.K.
- Faradion Limited, The Innovation Centre, 217 Portobello, Sheffield, S1 4DP, U.K.
- Department of Chemical Engineering, Imperial College London, London, SW7 2AZ, U.K.
- School of Chemistry, University of Birmingham, Edgbaston, Birmingham, B15 2TT, U.K.
- Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge, CB2 1EW, U.K.
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. Furthermore, 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.
- Research Organization:
- Univ. of Cambridge (United Kingdom)
- Sponsoring Organization:
- USDOE
- Grant/Contract Number:
- AC02-05CH11231
- OSTI ID:
- 1814865
- Alternate ID(s):
- OSTI ID: 1819272
- Journal Information:
- Journal of the American Chemical Society, Journal Name: Journal of the American Chemical Society Vol. 143 Journal Issue: 35; ISSN 0002-7863
- Publisher:
- American Chemical SocietyCopyright Statement
- Country of Publication:
- United States
- Language:
- English
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