Key Takeaways & Executive Findings
- •• • Low (80 °C) and high (260 °C) softening point asphalts achieve high oxygen incorporation (20–25%) during pre-oxidation, forming deep cross-linking that suppresses graphitization, yielding hard carbon with large interlayer spacing and abundant closed pores, resulting in initial charge capacities of 314.7 and 306.6 mA·h/g, respectively. • • Medium softening point (160 °C) asphalt exhibits insufficient cross-linking with only ~5% oxygen content, leading to a dense structure with smaller interlayer spacing (3.46 Å) and a significantly reduced capacity of 173.2 mA·h/g, highlighting the nonlinear effect of softening point. • • First-cycle coulombic efficiencies of 81.3% (EPOC-80) and 79.3% (EPOC-260) demonstrate that optimized pre-oxidation pathways enhance reversible sodium storage, critical for practical anode efficiency. • • Both EPOC-80 and EPOC-260 show excellent cycling stability with capacity retention >81% after 100 cycles, indicating robust structural integrity for long-term battery operation.
Abstract
This study elucidates the nonlinear relationship between the softening point of coating asphalt and its oxidative cross-linking behavior, as well as the sodium storage performance of the derived hard carbon. Comparative analysis of asphalts with low (80 °C), medium (160 °C), and high (260 °C) softening points revealed that both the 80 and 260 °C asphalts incorporated a higher oxygen content (20%–25%) during oxidation, leading to the formation of a deeply cross-linked structure dominated by anhydride and ester groups. This effectively suppressed graphitization during carbonization, yielding hard carbon with large interlayer spacing, high disorder, and abundant closed pores. The derived hard carbon exhibited superior sodium storage performance: the initial charge capacities of EPOC-80 and EPOC-260 reached 314.7 and 306.6 mA·h/g, with first-cycle coulombic efficiencies of 81.3% and 79.3%, respectively, along with excellent cycling stability and rate capability. In contrast, the medium softening point asphalt (160 °C) showed limited oxygen incorporation (~5%) and insufficient cross-linking after oxidation, resulting in a densely packed hard carbon with smaller interlayer spacing (3.46 Å) and restricted sodium storage sites, which led to a significantly reduced capacity of 173.2 mA·h/g. This work provides new design principles and theoretical support for optimizing hard carbon anode structures through precise control of the precursor softening point.
1. Introduction
Sodium-ion batteries (SIBs) are emerging as a cost-effective alternative to lithium-ion systems for large-scale energy storage, yet their commercialization hinges on the development of high-performance anode materials. Hard carbon, with its suitable sodium intercalation potential and high theoretical capacity, stands out as a leading candidate. However, the performance of hard carbon is intrinsically linked to its microstructure—interlayer spacing, defect density, and closed pore volume—which must be optimized to facilitate sodium ion storage. Petroleum asphalt, abundant and carbon-rich, offers a promising precursor, but its conversion to high-quality hard carbon requires precise control over the carbonization process. Pre-oxidation is a known strategy to stabilize the carbon framework by introducing oxygen cross-links, but the influence of the precursor's intrinsic properties, particularly its softening point, on the oxidation pathway and final carbon structure has been largely overlooked.
This study addresses this gap by systematically investigating three coating asphalts with distinct softening points (80, 160, and 260 °C) derived from the same source. The results reveal a nonlinear relationship: both low and high softening point asphalts undergo deep oxidation (20–25% oxygen) leading to highly disordered carbons with large interlayer spacing and abundant closed pores, whereas the medium softening point asphalt exhibits insufficient cross-linking (~5% oxygen), resulting in a denser, less effective structure. These findings establish the softening point as a critical design parameter for tailoring hard carbon anodes, offering a rational approach to precursor selection and process optimization for high-performance sodium-ion batteries.
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LÜ Xiaojun, DIAO Jingjing, ZHAO Qingbo, CAI Tianfeng, HAN Dongyun, YANG Zhanxu, CAO Zubin (2026). Effect of Coating Asphalt Softening Point on Pre-Oxidation Pathway and Sodium Storage Performance of Derived Hard Carbon. Journal of Fuel Chemistry and Technology. https://doi.org/10.3724/2097-213X.2025.JFCT.0032
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Frequently Asked Questions
What is the mechanistic basis for the nonlinear effect of softening point on the pre-oxidation pathway and resulting hard carbon structure?
The nonlinearity arises from the balance between molecular mobility and reactivity. Low softening point (80 °C) asphalt has high fluidity and reactivity, allowing deep oxygen incorporation (20–25%) and extensive cross-linking. High softening point (260 °C) asphalt, though less mobile, possesses a more condensed aromatic structure that facilitates stable cross-link formation. In contrast, medium softening point (160 °C) asphalt exhibits intermediate behavior, resulting in insufficient cross-linking (~5% oxygen) and a more ordered carbon structure with smaller interlayer spacing (3.46 Å) and fewer closed pores.
How do the electrochemical performance metrics of EPOC-80 and EPOC-260 compare to those of EPOC-160, and what are the implications for practical sodium-ion battery anodes?
EPOC-80 and EPOC-260 deliver initial charge capacities of 314.7 and 306.6 mA·h/g, respectively, with first-cycle coulombic efficiencies of 81.3% and 79.3%, and capacity retention >81% after 100 cycles. In contrast, EPOC-160 shows a significantly lower capacity of 173.2 mA·h/g. These results indicate that optimizing the softening point can yield hard carbons with high reversible capacity and stable cycling, essential for meeting the energy density and longevity requirements of commercial SIBs.
What structural parameters (interlayer spacing, closed pore volume) are critical for sodium storage, and how do they vary across the three samples?
Large interlayer spacing (>0.37 nm) and abundant closed pores are known to enhance sodium storage via intercalation and pore filling. EPOC-80 and EPOC-260 exhibit expanded interlayer spacing and high closed pore volume, facilitating high capacities. EPOC-160, with a smaller interlayer spacing of 3.46 Å and fewer closed pores, relies primarily on surface adsorption, leading to lower capacity.
What are the potential scalability and cost implications of using different softening point asphalts for industrial production of hard carbon anodes?
The study demonstrates that both low and high softening point asphalts can produce high-performance hard carbon, offering flexibility in precursor selection based on availability and cost. However, the pre-oxidation step requires careful control of oxygen content and temperature, which may impact process economics. The improved capacity and cycling stability could offset these costs by enhancing battery performance and lifespan.
How does the pre-oxidation temperature and atmosphere affect the oxygen content and cross-linking degree, and what are the optimal conditions?
The study does not specify exact pre-oxidation conditions, but it indicates that oxygen content of 20–25% is achieved for low and high softening point asphalts, while only ~5% for medium. This suggests that the softening point dictates the optimal oxidation conditions. Further optimization of temperature, time, and atmosphere could enhance cross-linking efficiency, but the current results provide a baseline for process design.
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