Elucidating the Lithium-Ion Storage Mechanism in Insertion-Available Hard Carbon with Abundant Closed Porosity: Toward a Practical Material for Batteries
Authors: Tseng Liang-Chieh , Tai Chen-Wei , Hu Chi-Chang 
Published: December 2024

In the past two decades, there has been rapid development in high-performance batteries to address the issue of energy shortage. While graphite has served as the negative electrode material for commercial lithium-ion batteries (LIBs), its small theoretical capacity of 372 mAh g−1 and slow kinetics of intercalation/de-intercalation reactions have posed limitations. Thus, there is a quest for alternative materials for further advancement. Hard carbon (HC) has emerged as promising materials for both lithium-ion and sodium-ion batteries (SIBs). HC not only possesses a larger interlayer spacing that facilitates ion diffusion but also features a unique closed pore structure that enables it to achieve a larger theoretical capacity than graphite. These advantages make HC a promising candidate for overcoming the limitations associated with traditional graphite-based electrodes in LIBs. However, recent research has primarily focused on biomass-derived HC and heteroatom-doping HC for LIBs. These HCs, rich in heteroatoms (e.g., N, P, S), often exhibit low initial coulombic efficiencies due to electrolyte consumption of SEI layer which form in the first cycle of lithiation. Additionally, most HC delivers non-faradaic reactions with completely sloping galvanostatic charge-discharge (GCD) curves, indicating pure capacitive responses. This behavior will limit the nominal cell voltage in LIBs. Herein, we propose solutions by introducing phenolic-formaldehyde resin-derived HC microbeads with tunable microporous structures ,aiming to establish them as practical material for the negative electrode in LIBs.
The carbon porosity is well manipulated by tailoring chemically cross-linked environments via microwave-induced polymerization of phenolic formaldehyde resin. HC samples, prepared at various calcination temperatures under argon, show increased crystallinity with higher calcination temperatures. On the other hand, the diverse molecular properties of gas molecules enable us to differentiate between various pore structures in HCs. N2 adsorption effectively characterizes open pore structures, while O2 adsorption is suitable for identifying closed pore structures. Additionally, CO2 adsorption is employed to distinguish ultra-micropore structures which are smaller than 0.7 nm. By combining the results of multiple gas adsorption isotherms with in situ X-ray diffraction, a solid correlation between porous properties and electrochemical responses has been successfully established. The results indicate that porosity in HC can change with different calcination temperature. With increasing calcination temperatures, open pores are initially transformed into closed pores, and ultimately, non-porous structure are detected. Furthermore, closed ultra-micropores, which play the same role as the graphitic interlayer spacing in graphite, are termed as “active closed pores”, because they can facilitate the insertion of lithium ions and mitigate substantial initial irreversible capacity by blocking the penetration of solvents. Our proposed equation shows that HC calcinated at 900 degrees (HC-900) exhibits the closed pore volume of 0.336 cm3g−1.The HC-900 with abundant active closed pores can deliver a reversible capacity of 550 mAh g−1 at 50 mA g−1 in operating window of 0.001–1.5 V (vs. Li+/Li), with an ultrahigh low-voltage plateau capacity of 230 mAh g−1 contributed by the faradic reaction of lithium-ion insertion in active closed pores. Unlike some reported methods, such as holding at a constant voltage, charging/discharging at a very small current density, or overcharging to negative voltage regions, the insertion of active closed pores is easily unlocked at a current density of 50 mA g−1. Moreover, the HC-900 also demonstrates excellent rate capability (210 mAh g−1 at 2 A g−1) and cycling stability (90% capacity retention after 200 cycles at 0.1 A g−1 and 86% after 1500 cycles at 1 A g−1), ensuring it as an ideal negative electrode material for LIBs. In summary, we have successfully synthesized and tuned the structure of active closed pore-rich HC. Additionally, we have developed an effective method and equation to evaluate the amount of active closed pores, enabling us to achieve a reversible capacity of 550 mAh g-1 for the negative electrode in LIBs.

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