Elsevier

Journal of Power Sources

Volume 515, 15 December 2021, 230621
Journal of Power Sources

Gradient architecture to boost the electrochemical capacitance of hard carbon

https://doi.org/10.1016/j.jpowsour.2021.230621Get rights and content

Highlights

  • A third-order graduation polymerization method was developed.

  • The dynamic radical oxidation method was used for ultramicroporous-confined defects.

  • The gradient structure could improve the “three-ultrahigh” performance of carbons.

Abstract

Carbon-based supercapacitors with outstanding electrochemical performances are strongly desired for the development of portable and wearable electronics. From the free radical engineering point of view, this study innovatively developed carbon/carbon gradient-structured microspheres (CCGMs) by the strategy of “third-order graduation polymerization” and “dynamic radical oxidation”. Compared to conventional hybrid materials, for the first time, the gradient structure is proved to be extremely beneficial for improving the capacitive performance. Benefiting its gradient-structured advantages, the CCGMs exhibited “three-ultrahigh” performance with gravimetric, volumetric, and areal capacitances of 408.23 F g−1, 741.11 F cm−3, and 4390 μF cm−2, respectively. In addition, the assembled all-solids-state micro-supercapacitors delivered superior volumetric and areal specific capacitances (7.704 F cm−3 and 9.245 mF cm−2 at 5 μA cm−2), excellent life-span (100% after 12000 cycles), excellent volumetric and areal energy densities (1070 mWh cm−3 and 1284 μWh cm−2). This work paves a new way to develop unique carbon composites for high‐performance energy storage devices.

Introduction

Carbon-based materials as supercapacitor electrodes have attracted considerable interest in both fundamental research and industrial applications, owing to their wide source, low price, high power density and long lifespan [1,2]. Among them, hard carbon (pyrolytic carbon from the polymer) with non-graphitizable at 2500 °C have been a darling in the field of energy storage, which not only have been reported to have enhanced performance as anodic material in lithium-ion batteries (LIBs) and sodium-ion battery but also is being studied as electrode material in supercapacitors [[3], [4], [5]]. In principle, an excess of defects including voids, vacancies, cracks, dislocations, edge sites, grain boundaries, etc. in hard carbon are common but are undesirable as for electrode materials in supercapacitors due to the sluggish kinetic [5,6]. Of particularly interest, several examples have documented that the recently developed defect engineering opens a window to further boost the capacitive performance [[7], [8], [9]]. This is mainly due to the defects have been reported to be very useful in modulating the electronic structures and increasing the pseudocapacitance, delivering an enhanced performance in supercapacitors. As a result, the hard carbons stand out. Nevertheless, most hard carbons are less dense versus conventional graphite carbon which needs the structural modification, thereby hardly satisfy the standards for capacitive performance, particularly for volumetric and areal capacitances. These two new evaluation indexes are inspired by the ever-growing market requirement for miniature and accurate electronics [10]. Based on the evaluation criterion of capacitive performance, gravimetric capacitance relies on porous structure, while the highly developed channels in carbons bring enhanced specific surface area and sacrificed packing density, being opposed to the need of improving areal capacitance and volumetric capacitance [[11], [12], [13], [14]]. Up to now, significant challenges still remain in balancing the trade-off between gravimetric capacitance and areal capacitance as well as volumetric capacitance. Attempting to resolve this contradiction, research efforts have been directed towards designing of hybrid carbon materials as the electrode materials such as transition metal oxides/carbon, conductive polymer/carbon, carbon/carbon [[15], [16], [17], [18]]. However, most of these hybrid materials usually have interphase boundary resulting from discontinuous structure patterns, resulting in insufficient ionic diffusion [19]. Instead, carbon/carbon hybrid materials open new avenues to further improve the overall capacitive performance owing to their architecture adjustability and phase interface compatibility. The common synthesis strategies focus on template methods [20,21]. The time-consuming multistep synthesis procedure results in a structural instability and high preparation cost in largescale production. Therefore, developing one-step cost-effective strategy to synthesis promising composite materials is of great importance.

Inspired by gradient-structured biological tissues in nature, functional gradient material, which has continuous changes in internal composition and microstructure, is considered to be a promising candidate since it has many attractive characteristics which hasn't been found in other hybrid structure such as strong environmental serviceability and robust consecutive interfaces [22,23]. Nowadays, a number of common tactics have been reported, such as physical evaporation, plasma spraying, chemical vapor deposition and precursor infiltration pyrolysis, etc. [[24], [25], [26]], and these functional gradient materials have been employed successfully in astronautics industry, biomedical sciences and electronic engineering [[27], [28], [29]]. But the facts show that these hybrids are seldom obtained in the micro and nano scale range, further impeding being applied in electrochemical energy storage. Based on the above considerations, developing a simple and time-saving strategy for fabricating gradient-structured nanohybrids is highly in demand.

Herein, from the reactivity ratios point of view, we developed a “third-order graduation polymerization” strategy, in which the “dynamic radical oxidation” (DRO) was coupled for the first time to develop confined defects and to implement auto-filter functionality aiming at oxygen functional groups. Based on it, the carbon/carbon gradient-structured microspheres (denoted as CCGMs) were obtained after carbonization with convenient and mild features that are relevant requirements for practical applications. More importantly, the CCGMs with graded distribution along thickness is reflected not only in component and microstructure but also in functional groups. Notably, the gradient structure endows a high packing density, ultra-low specific surface area, and a large inventory of ultramicropore-confined defects, and thus greatly enhanced the overall capacitive performance as electrode materials for supercapacitors, which is higher than that reported for hard carbon-based electrode materials and is the highest capacitance reported for polyacrylonitrile-based carbon materials in volumetric and areal capacitance, as far as it is known.

Section snippets

Experimental section

Materials: acrylonitrile (AN), vinyl acetate (VAC), ethylene glycol dimethacrylate (EGDMA), azodiisobutyronitril (AIBN) ammonium persulfate ((NH4)2S2O8), cyclohexane and butyl acetate were obtained from Shanghai Aladdin Bio-Chem Technology Co. Both nickel foam and acetylene black were obtained from Solvay S.A. Polytetrafluoroethylene (60 wt% PTFE) was bought from Sinopharm Chemical. All chemical reagents were analytical grade and used without further purification.

Preparation of Trilaminar

Results and discussion

The whole fabrication procedure is summarized in Fig. 1a, demonstrating an improved fabrication mechanism of gradient-structured carbons. Here, we demonstrate a new method referred to as “third-order graduation polymerization” for the development of gradient-structured carbon precursor based on an explicit reaction of dynamics of free-base co-polymerization, which is a vital step. The premise is that the selected solvent polarity could yield finely powdered products with a regularly spherical

Conclusion

In summary, we presented a “third-order graduation polymerization” approach and “dynamic radical oxidation” strategy to develop the gradient-structured carbon/carbon hybrid microspheres with tailorable ultra-microporous-defined defects. Benefiting from gradient structure, the obtained CCGM-0.5 exhibited an obvious boost in overall capacitance performance, rate capability and the flexible electronic device. In 6 M KOH electrolyte, CCGM-0.5 showed prominently improved gravimetric, volumetric and

CRediT authorship contribution statement

Qi Liu: Experiment, Investigation, Data curation, Writing – original draft, preparation. Weiqun Gong: Extended experiments. Shengnan Li: Software. Chenyang Zhao: Experiment, Investigation. Dandan Xu: Image Processing. Yujing Liu: Writing – review & editing. Ziyin Yang: Formal analysis, Nuclear Magnetic Resonance analysis. Xiao Bai: Formal analysis, Nuclear Magnetic Resonance analysis. Anguo Ying: Conceptualization, Methodology, and, Writing – review & editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China; China (Grant No. 21978154, 21576176) and the Natural Science Foundation of Shandong Province; China (Grant No. ZR2020QB191).

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