Correlating Local Structure and Sodium Storage in Hard Carbon Anodes: Insights from Pair Distribution Function...

Area of Science:

• Materials Science
• Electrochemistry
• Solid-State Chemistry

Background:

• Hard carbons are promising anode materials for sodium-ion batteries.
• The precise mechanisms of sodium ion storage in hard carbons are not fully understood.

Purpose of the Study:

• To elucidate the local atomic environments of sodium storage in hard carbons.
• To revise the mechanism of sodium storage in hard carbon anodes.
• To correlate hard carbon microstructure with sodium storage capacity.

Main Methods:

• Operando and ex situ pair distribution function (PDF) analysis of total scattering data.
• Operando 23Na solid-state Nuclear Magnetic Resonance (NMR) spectroscopy.
• Modeling of local carbon structures based on pyrolysis temperature.

Main Results:

• Hard carbon structure consists of curved graphene bilayers; fragment size and curvature depend on pyrolysis temperature.
• Higher-voltage capacity correlates with defect concentration, while lower-voltage capacity relates to larger fragment sizes.
• Sodium ions store near defective carbon regions at higher voltages, forming metallic sodium clusters (13-15 Å) at lower voltages.

Conclusions:

• Local atomic structure significantly dictates sodium storage capacity in hard carbons.
• The formation of sodium clusters, rather than cluster size, determines lower-voltage capacity.
• Optimizing hard carbon anodes requires careful consideration of synthetic conditions to control local atomic and microstructures.