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

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.


Electrochemical evaluation
The electrode slurry was prepared by mixing 80 wt % active materials, 10 wt % acetylene black, and 10 wt % PVDF binder in NMP solvent. Then, the slurry was loaded on the round-disk Ni foam (1 mm in thickness, 1.1 cm in diameter) with a mass loading of about 3 mg cm 2 . After vacuum-drying and compression under 10 MPa for 30 s, the working electrodes were prepared.
Test in EMIBF4 electrolyte: A symmetric two-electrode coin cell was assembled in pure Ar glove boxes with concentrations of both oxygen and moisture lower than 0.1 ppm. A Whatman membrane (680 µ m in thickness), made from glass microfiber (type: GF/D1823-047), was placed between two electrode sheets with the same loading, and all of them were compressed together and sealed in a 2025-type coin cell. Electrochemical performances including CV, GCD, and EIS were evaluated. The voltage range was 0-4 V in the CV test with different scan rates ranging from 20 to 200 mVs −1 . GCD tests under various current densities of 0.5-10 Ag −1 were also performed between 0 and 4 V. The electrochemical impedance spectroscopy measurement was carried out with an electrochemical analyzer within a frequency range of 10 5 -0.01 Hz.
The specific capacitance (Celectrode, Fg −1 ) based on each electrode was calculated by the formula: where I, Δt, m, and V are the constant current (mA), discharge time (s), total mass of both carbon electrodes (mg), and voltage window (V). The specific energy density (E, Whkg −1 ) was calculated on the basis of the equation: E = CcellV 2 /7.2 = CelectrodeV 2 /28.8 The specific power density (P, Wkg −1 ) was obtained according to the formula:

Material characterization
The morphology and structure of the samples were characterized by a scanning electron microscope (SEM, JSM 7401F, JEOL Ltd., Tokyo, Japan) operated at 3.0 kV and a transmission electron microscope (TEM, JEM 2010, JEOL Ltd., Tokyo, Japan) operated at 120.0 kV. X-ray photoelectron spectroscopy (XPS) measurements were carried out on Escalab 250xi (Waltham, MA, USA). All XPS spectra were calibrated at 284.8 eV using C 1s line, and the raw data were fitted by XPSPEAK program (XPS peak 4.1, Taiwan). The N2 adsorption/desorption isotherm was recorded by an Autosorb-IQ2-MP-C system (Boynton Beach, FL, USA). The specific surface area (SSA) was obtained using the multipoint Brunauer-Emmett-Teller (BET) method, and the pore-size distribution (PSD) was obtained via the Quenched Solid Density Function Theory (QSDFT) and the equilibrium model. Figure S1. N2 adsorption-desorption isotherm curves.   To demonstrate the cyclic lifespan, a 5000-time continuous charge-discharge process was carried out at a very high current density of 10 A g −1 , as seen in Figure S5. Finally, a satisfactory capacitance retention of 83.3% (only 0.33% decay per cycle) was attained for C-0.75, in sharp contrast with the counterparts of constant 100% for C-0.25 and 87.0% for C-0.5. This stability evolution can be understood considering the structural motifs of the samples. In C-0.75 compared to C-0.25, both porosity and hierarchy highly increased, with considerably higher mesoporosity contribution ( Table  1 within the manuscript). To be specific, C-0.25 was purely microporous, while C-0.75 was characterized by a mixture of mesopores and micropores. Hence, under a rather demanding cyclic condition of 10 A g −1 , destruction of the electrode material could occur, thereby causing capacitance degeneration. The opposite trend between lifespan and porosity in essence reveals a great challenge to simultaneously accomplish exceptional capacitance and stability by modulating the porosity. However, the porosity (mainly the pore size distribution, as fully explained in the main manuscript) imposes the main impact on capacitance improvement. Worse still, structural collapse will substantially exacerbate in the presence of heteroatom dopants. This is because the decomposition of unstable heteroatoms can deteriorate the structural fluctuation during repeated redox reactions. C-0.75 did contain a small quantity of N atoms (1.68%), although the pseudocapacitance of the N surface functional groups does not absolutely govern the capacitance contribution (as analyzed in the manuscript). The unstable N-5-and N-6-type bonding will undergo irreversible conversion and therefore fuel textural variation. Such signature often sparks off a dilemma for carbon-based materials in ion liquid-wetted supercapacitors, as has been also widely showcased in the literature.

Supplementary data
Noteworthy, in spite of the poorer cyclability of C-0.75 relative to the other two samples, its capacitance retention was still acceptable. As a matter of fact, capacitance fading of highly electroactive porous nanocarbons has been ubiquitously observed in the literature, [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17], and it was found that over 5000-10,000 cycles, their capacitance retention is commonly stabilized within a range of 80%-90 %, very similar to the case in our study. A thorough comparison between our data and the hitherto published lifetimes of some cutting-edge carbon/ionic liquid systems was made and is summarized in Table S1. Factoring in the important repertoire regarding synthetic tractability, sustainability, and inexpensiveness, C-0.75 could split the difference between capacitance and durability, leading to a commendable supercapacitive performance for ion liquid-filled porous carbons (Table S1). From Table S1, it can also be clearly seen that hetero-atom-doped nanoporous carbons usually present a relatively lower capacitance retention than non-doped porous carbons, evidencing that the dopant or surface functionalities do commonly play a detrimental role in cyclic stability. On the other hand, our results also indicate that manipulating both the morphologic tenability and the purity of exquisitely tailor-made porous carbon nanomaterials is crucial so as to ensure a prolonged cyclic life, while maintaining reinforced specific capacitance, particularly when operating in high-voltage electrolytes. Our further work will concentrate on how to enhance the textural robustness/tolerance of our optimized materials (C-0.75).