A research team from the Hefei Institutes of Physical Science of the Chinese Academy of Sciences has demonstrated ultrafast and highly reversible all-slope sodium storage using specially engineered hard carbon anodes.

The result was published in ACS Nano.

Sodium-ion batteries are appealing due to the abundance and affordability of sodium. However, their performance is often limited by the anode. Hard carbon is one of the most promising anode materials, but conventional structures suffer from an "interlayer confinement" effect that slows down sodium-ion transport. While introducing defects and surface sites can improve ion mobility, too many defects can trigger excessive electrolyte decomposition, forming a thick solid electrolyte interphase (SEI) and reducing the battery's initial Coulombic efficiency.

In this study, the researchers developed an amino-nitrogen–guided pore-engineering strategy using a gas-phase-assisted pyrolysis method. During pyrolysis, ammonia dissociates to generate amino functional groups that selectively react with unsaturated sites in the carbon skeleton. This reaction converts electrochemically irreversible pyrrolic nitrogen into more stable and reversible pyridinic nitrogen. The process also induces the formation of vertically aligned through-pores, helping sodium ions move more freely and relieving the interlayer confinement effect.

The amino groups contribute to the formation of an ultrathin SEI layer with a graded chemical composition. In this SEI, regions beneath the surface that are rich in fluoride improve interfacial ion transport and effectively reduce irreversible sodium loss.

Due to these structural and interfacial enhancements, the engineered hard carbon anode exhibits high initial efficiency and strong reversible capacity. Even under extremely high current conditions, the anode maintains considerable capacity and demonstrates long-term cycling stability over thousands of charge-discharge cycles.

This study offers a promising approach to improving the performance of sodium-ion batteries, an emerging alternative to lithium-ion technology for large-scale energy storage.

Schematic of through-pore structure in hard carbon for sodium ion batteries (Image by WANG Peiyao)