Origin of fast charging in hard carbon anodes
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Currently, although hard carbon anodes have potential in fast charging performance, there are relatively few reports on sodium-ion battery research based on ampere-hour scale batteries, which can support high-current rapid charging while maintaining good cycling stability.
Recently, researchers including Hu Yongsheng and Associate Researcher Lu Yaxiang from the Institute of Physics, Chinese Academy of Sciences/State Key Laboratory of Condensed Matter Physics, in collaboration with Professor Marnix Wagemaker from Delft University of Technology in the Netherlands, have made progress in the study of the fast charging mechanism of ampere-hour scale sodium-ion batteries based on hard carbon anodes.
The research involves the preparation of hard carbon spherical materials with different microstructures by controlling the carbonization temperature, followed by evaluation in ampere-hour scale cylindrical cells. The 26700 cylindrical sodium-ion battery developed in the study demonstrates rapid charging and discharging capabilities at 6.5C, with approximately 83% capacity charged in about 9 minutes. It can achieve 3000 consecutive charge-discharge cycles (at 100% depth of discharge), with the peak surface temperature of the fully charged battery reaching only around 44.3°C. The hard carbon anode used in this battery, designed with precise nano-wedge-shaped pores (~1 nm aperture), exhibits excellent fast charging performance and high areal capacity (approximately 2.2 mAh/cm²), while avoiding the issue of sodium metal deposition.
The research introduces lithium as a "probe" to compare the lithium and sodium storage behaviors of hard carbon. Through over-discharge experiments, it obtains capacity previously not fully recognized before the nucleation overpotential, revealing similarities and differences in lithium-sodium electrochemical behaviors. Both lithium and sodium exhibit hidden plateau capacities below 0 V, with lithium showing more hidden capacity. This is attributed to the slightly positive plateau potential of approximately 30-40 mV for sodium compared to lithium. Furthermore, the study explores the lithium and sodium storage behaviors in the sloping and plateau regions.
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For the sloping region, the research finds a lithium-to-sodium sloping capacity ratio of 1.8, consistent with literature statistics. Additionally, it notes that the projected area ratio of sodium ions to lithium ions is exactly 1.8, suggesting that lithium and sodium share the same storage active sites in the sloping region. Through DFT simulations of Li/Na adsorption at the graphite layer edge, the study discovers that both lithium and sodium tend to choose the same adsorption sites. Due to the smaller size of lithium ions, more can be accommodated in the same surface area. Furthermore, the study quantifies carbon defect concentrations using temperature-programmed desorption combined with mass spectrometry and confirms that the contribution capacity of the sloping region calculated based on the active surface area aligns with experimental data. This indicates that the sloping region capacity corresponds to the adsorption of Li/Na at the same defect sites.
The research demonstrates the fast charging characteristics of ampere-hour scale sodium-ion batteries based on amorphous carbon anodes, elucidating the storage positions, states, and diffusion modes of sodium in amorphous carbon, laying the groundwork for the design and preparation of low-cost, high-performance carbon anode materials for sodium-ion batteries.
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