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Open AccessArticle

Studies on Possible Ion-Confinement in Nanopore for Enhanced Supercapacitor Performance in 4V EMIBF4 Ionic Liquids

by Jie Deng 1,†, Jing Li 2,†, Zhe Xiao 2, Shuang Song 2 and Luming Li 1,3,*
1
College of Pharmacy and Biological Engineering, Chengdu University, Chengdu 610106, China
2
Department of Chemical Engineering, Sichuan University, Chengdu 610065, China
3
Institute of Advanced Study, Chengdu University, Chengdu 610106, China
*
Author to whom correspondence should be addressed.
These authors contributed equally to this work.
Nanomaterials 2019, 9(12), 1664; https://doi.org/10.3390/nano9121664
Received: 14 October 2019 / Revised: 12 November 2019 / Accepted: 18 November 2019 / Published: 22 November 2019
(This article belongs to the Special Issue Emerging Nanomaterials for Lithium-Sulfur Batteries and Beyond)
Supercapacitors have the rapid charge/discharge kinetics and long stability in comparison with various batteries yet undergo low energy density. Theoretically, square dependence of energy density upon voltage reveals a fruitful but challenging engineering tenet to address this long-standing problem by keeping a large voltage window in the compositionally/structurally fine-tuned electrode/electrolyte systems. Inspired by this, a facile salt-templating enables hierarchically porous biochars for supercapacitors filled by the high-voltage ionic liquids (ILs). Resultant nanostructures possess a coherent/interpenetrated framework of curved atom-thick sidewalls of 0.8-/1.5-nanometer pores to reconcile the pore-size-dependent adlayer structures of ILs in nanopores. Surprisingly, this narrow dual-model pore matches ionic radii of selected ILs to accommodate ions by unique coupled nano-/bi-layer nanoconfinements, augmenting the degree of confinement (DoC). The high DoC efficiently undermines the coulombic ordering networks and induces the local conformational oscillations, thus triggering an anomalous but robust charge separation. This novel bi-/mono-layer nanoconfinement combination mediates harmful overscreening/overcrowding effects to reinforce ion-partitioning, mitigating long-lasting conflicts of power/energy densities. This interesting result differs from a long-held viewpoint regarding the sieving effect that ion-in-pore capacitance peaks only if pore size critically approaches the ion dimension. Optimal biocarbon finally presents a very high/stable operational voltage up to 4 V and specific energy/power rating (88.3 Wh kg−1 at 1 kW kg−1, 47.7 Wh kg−1 albeit at a high battery-accessible specific power density of 20 kW kg−1), overwhelmingly outperforming most hitherto-reported supercapacitors and some batteries. Such attractive charge storage level can preliminarily elucidate an alternative form of a super-ionic-state high-energy storage linked with both the coordination number and coulombic periodism of the few ion-sized mesopores inside carbon electrodes, escalating supercapacitors into a novel criterion of charge delivery. View Full-Text
Keywords: supercapacitor; porous carbon; ionic liquid; confinement effect; coulombic ordering; high energy density supercapacitor; porous carbon; ionic liquid; confinement effect; coulombic ordering; high energy density
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MDPI and ACS Style

Deng, J.; Li, J.; Xiao, Z.; Song, S.; Li, L. Studies on Possible Ion-Confinement in Nanopore for Enhanced Supercapacitor Performance in 4V EMIBF4 Ionic Liquids. Nanomaterials 2019, 9, 1664.

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