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Atomic-scale manipulation of electrode surface to construct extremely stablehigh-performance sodium ion capacitor

Karthikeyan Kaliyappan, Zhongwei Chen⁎

Department of Chemical Engineering, University of Waterloo, 200 University Avenue West, Waterloo, Ontario, Canada N2L 3G1

A R T I C L E I N F O

Keywords:Sodium ion capacitorAtomic layer depositionP2 typeSuperior stabilityMetal oxide coating

A B S T R A C T

A state-of-the-art approach has been performed to stabilize the surface of P2-type (Na0.66Mn0.54Ni0.13Co0.13O2

(NMNC)) material through the atomic layer deposition (ALD) of Al2O3 (10 ALD cycles) to fabricate a 3 V sodiumion capacitor (NIC) with ultrahigh rate stability. This is the first known report of stabilizing the NIC electrodesurface by a metal oxide coating using ALD. The capacitor constructed with an Al2O3-coated NMNC (NMNC-Al)cathode and a commercial activated carbon (CAC) anode in an organic electrolyte delivers a discharge capa-citance of 68 F g−1 at 0.35 A g−1 current density and exhibits extremely high electrochemical stability of ~ 98%of its initial value after 10,000 cycles. In contrast, the capacitor containing a pristine NMNC electrode displays acapacity retention of 78%. The NMNC-Al/CAC cell also has an energy density of 63Wh kg−1 at a power densityof 6.6 kWh kg−1. The capacitance, energy, and power densities obtained from the NMNC-Al/CAC cell are thebest-reported values for sodium-based capacitors and outperforms well-established lithium ion capacitors. Theelectrochemical impedance spectroscopy study reveals that the sluggish reaction kinetics of the NMNC electrodeat high current density is successfully overcome by coating an ultrathin Al2O3 layer by ALD on its surface.

1. Introduction

Atomic layer deposition (ALD) has recently received much attentionas a potential candidate to develop ultrathin metal oxide coatings froma vapour phase reaction [1,2]. ALD controls the coating thickness at theangstrom level through continuous self-limiting reactions [3]. In addi-tion, ALD offers the deposition of tunable metal films with high purityand aspect ratio, establishing ALD as a powerful technique for nu-merous practical applications [1–4]. The conformality of depositedfilms is a crucial factor for ALD over other deposition methods such aschemical vapour deposition or sputtering [1,3,5]. ALD is commonlybeing used to deposit Cu films for semiconductors, grow high-qualitydielectrics, fabricate trench capacitors for dynamic random-accessmemory, and to fabricate numerous nanodevices [6,7].

The adaptability of ALD in energy storage devices and catalyst de-velopment has been widely studied and reported elsewhere [5,8]. In-itially, Badot and co-workers demonstrated the fabrication and use ofV2O5 as a cathode in lithium ion batteries (LIB) [9]. Subsequently,extensive investigations on ALD for LIB were carried out and the uti-lization of ALD was extended to lithium-sulphur batteries, metal-airbatteries, supercapacitors, and fuel cells [1,5,8–13,14]. Generally, thekey application of ALD for energy storage devices contains two majorfeatures: the surface modification of electrode materials and the design

of battery components not limited to the cathode, anode and separator[8,11,12]. The former is required to stabilize the electrode and elec-trolyte interfacial layer (EEI) to increase the lifespan of the devicewhereas the latter is applied to attain high electrochemical performance[12]. Unlike an inactive carbon coating, the presence of an ultrathinmetal oxide layer not only prevents unwanted side reactions with theelectrolyte but also enhances the discharge capacity of the cell [10,15].Although the ALD of metal oxides on LIB electrodes are widely studied,few reports are available on metal oxide coatings by ALD for sodium-containing intercalation electrodes [12,16]. Recently, Karthikeyan et al.demonstrated the enhancement of electrochemical behaviour of P2-Na0.66Mn0.54Ni0.13Co0.13O2 (NMNC) at high cut-off voltage by ALDsurface modification of Al2O3 with different thicknesses for sodium ionbatteries (SIB) [16].

The ALD of metal oxides on carbon substrates (nanotubes or gra-phene composites) has also been applied as composite electrodes forsupercapacitor applications [17–22]. The enhanced capacitance ofthese composite electrodes could be attributed to the presence ofcarbon along with the metal oxide particles. Recent studies demonstratethat research on fabricating high-performance supercapacitor (EDLC)using ALD is being made toward developing high-performance com-posite materials [11,18,20]. It is also possible to employ ALD to stabi-lize the surface of the cathode in high energy Li/Na ion capacitors. It is

https://doi.org/10.1016/j.nanoen.2018.03.021Received 1 February 2018; Received in revised form 6 March 2018; Accepted 7 March 2018

⁎ Corresponding author.E-mail address: zhwchen@uwaterloo.ca (Z. Chen).

Nano Energy 48 (2018) 107–116

Available online 17 March 20182211-2855/ © 2018 Elsevier Ltd. All rights reserved.

T

well-known that Li/Na ion capacitors (LIC or NIC) can deliver highenergy density (ED) and power density (PD) values through the hy-bridization of battery and capacitor electrodes in a single device.Generally, an LIC or NIC is a single device consisting of an insertion-type cathode (battery component) and an EDLC component (super-capacitor element) as a counter electrode in either aqueous or non-aqueous medium [23–25]. Despite having been explored extensively,the research activities of LIC are in an advanced stage and attempts arein progress to create conversion and alloy anodes. On the other hand,an LIC containing an insertion-type electrode in organic electrolyteshowed high capacitance and ED but exhibited poor capacitance re-tention over long cycling which hinders its practical application [25].To enhance the capacitance behaviour of LICs, many strategies havebeen established such as creating carbon or polymer composites[23,25–27].

Moreover, the scarcity of lithium resources in the earth’s crust andthe complexity of lithium recycling processes are motivating the ex-ploration of alternative electrochemical capacitors which could deliverhigh EDs and PDs equivalent to those of LICs. Due to availability andenvironmental friendliness, SIBs are anticipated to replace expensiveand toxic LIBs in the near future [28]. Similar advantages are gained byreplacing LICs with NICs [24,28]. Unfortunately, few sodium insertionelectrodes are reported for NIC applications while numerous insertiontype electrodes are available for LICs [24,29–33] Most of these NICsexhibited intense capacitance fading even in the aqueous electrolytewhich is unsuitable for high ED applications [30,33–38,39]. It is vital todevelop high performance NICs that can deliver enhanced electro-chemical performance such as high ED and PD with our scarifying itsprolonged cycle life. The construction of NIC in an organic electrolyte isthe effective way to increase the cell voltage of NIC and consequentlythe energy density. Recently, the fabrication of NIC with Na3V2(PO4)3(NVP)/carbon as the cathode in an organic electrolyte was reportedwith low voltage along with 64.5% cyclic stability after 10,000 cycles[40]. In our previous work, we succeeded in fabricating an organic NICcontaining carbon-coated C-NVP cathode with an excellent cyclic sta-bility of 92% after 10,000 cycles [32]. However, NICs assembled withNASICON-NVP have low capacitance and ED values, limiting theirlarge-scale application [32,40,41]. Choosing cathode materials withhigh theoretical capacity is key to enhancing NIC capacitance beha-viour. Among the intercalation cathodes reported for SIBs, P2-typelayered materials possess high discharge capacity and rate performance[28]. Nevertheless, P2-type materials have poor cyclic stability due tothe P2-O2 phase changes at high cut-off voltages [42]. Recently, theelimination of phase changes in P2-type cathode materials using metaloxide coatings by conventional solution-based methods and ALD hasbeen reported to enhance the Na-ion storage capability of layeredmaterials at high voltage range [16,43,44]. Additionally, the ultrathinAl2O3 layer coated by ALD on the electrode surface displayed superiorstability and rate performance due to the coating’s uniformity andconformality [16,45] Although many works propose overcoming severecapacity fading of layered materials at high voltage by introducingforeign metal elements in NIBs, their potential application in NICs arestill not explored well due to the poor cycling retention during theprolonged cycles [24,28,45]. Lately, adoption of layered cathode inaqueous NIC has been reported with limited capacitive performances[33–38]. Like LICs, NICs also suffers from poor cycling performance inan organic electrolyte when tested at> 2.5 V [40]. Hence, the im-provement of capacitive performance of p2 electrode active materials athigh operating window is crucial to maximize the ED output and cy-clability of NICs at high PD.

Herein, we present a pioneering work using ALD to fabricate su-perior NIC constructed with P2-layered cathode operating between 0and 3 V in an organic electrolyte. To the best of our knowledge, this isthe first report that exists on utilizing ultrathin (~ 1 nm) metal oxidecoatings by ALD on a cathode surface to stabilize the EEI and constructextremely stable LICs or NICs. The NIC fabricated with Al2O3-coated

NMNC (NMNC-Al) along with commercial activated carbon (CAC)displays nearly negligible capacity fading after 10,000 cycles at0.35 A g−1 within a range of 0–3 V in 1M NaClO4 electrolyte comparedto the NIC with pristine NMNC (78% capacity fade). High-potentialoperation is required for large-scale storage applications as it increasesthe ED of the NIC, and hence we have chosen 3 V as the upper cut-offvoltage for the present study. The obtained results reveal that ultrathinAl2O3 films on an NMNC cathode tailored its surface to form a stableEEI layer and dramatically enhance the electrochemical stability ofpristine NMNC cathodes in a NIC cell. In addition, the implementationALD in LICs/NICs will overcome many limitations for developing highED and PD materials with enhanced stability at high voltage since thecapacity retention normally decreases with the upper voltage limit.

2. Experimental

2.1. Preparation of NMNC

Citric acid assisted sol-gel method was used to synthesize NMNCparticles where citric acid acted as a chelating agent to eliminate par-ticle agglomeration during high-temperature calcination. In a typicalsynthesis, stoichiometric amounts of Na, Mn, Ni and Co acetates weredissolved in distilled water and mixed well with an aqueous solution ofcitric acid and then evaporated at 90 °C to obtain the gel precursors.The molar ratio of metal acetates to citric acid was fixed at 1M. Theresulting precursors were pre-calcined at 400 °C for 4 h to remove theorganic acetate molecules. Finally, the obtained powders were groundand calcined at 850 °C for 12 h in air to yield the final NMNC powders.

2.2. Preparation of the NMNC electrode

The pristine NMNC electrode for ALD was prepared by a pressing acathode slurry on stainless steel mesh. The electrode slurry contained75wt% of NMNC powders, 15 wt% ketzen black as a conductive ad-ditive, and 10 wt% of Teflonized acetylene black binder. The obtainedelectrode was dried under vacuum at 90 °C overnight before ALDcoating of Al2O3. The same procedure was followed to prepare a CAC(1500m2 g−1) anode.

2.3. ALD coating of Al2O3 thin films on the NMNC electrode

An ultrathin film of Al2O3 was directly deposited on NMNC elec-trode at 115 °C in an ALD system (Thermal Gemstar 6XT, Arradiance,LLC, USA). Trimethylaluminum (TMA) and H2O were used as the pre-cursor and oxidizer, respectively, for Al2O3 coating. The purging timefor TMA and water was set to 21ms and 10 ALD cycles were performedon NMNC electrodes for NIC fabrication while 84 cycles of ALD wereused on NMNC powder for physical characterization.

2.4. Physical and electrochemical characterization

As seen from Fig. 1a, one ALD cycle involves the combination of twohalf-cycle reactions. The Precursor molecule is supplied into the de-position chamber during the first half-cycle of the ALD process (Step 1in Fig. 1a). In Step 1, the TAM molecule gets adsorbed on the NMNCsurface forming a covalent bond with the surface molecules. After thesubstrate is saturated with TMA molecules, Step 2 is conducted inwhich the ALD chamber is purged to remove reaction by-product andthe unreacted TMA. In the second half-cycle, the water is supplied intothe process as the oxidation agent. During Step 3, the water is reactedwith TMA and form a thin layer of Al2O3 on the surface of NMNC.Finally, the chamber is again purged to completely remove excess waterand by-products (Step 4, Fig. 1a) from the chamber. This ends the firstcycle of the ALD process. The half-reactions are repeated to increase thecoating thickness.

Phase purity of the pristine NMNC and ALD-coated NMNC powders

K. Kaliyappan, Z. Chen Nano Energy 48 (2018) 107–116

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were examined using X-ray powder diffraction (XRDMiniflex 600,Rigaku, Japan) with Kα radiation (λ=1.5406 nm). The morphologyand structure of Al2O3 coatings were observed by field emission scan-ning electron microscopy ((FE-SEM, LEO Zeiss 1550, Switzerland), andhigh-resolution transmission electron microscopy (HR-TEM, JEOL 2010FEG). The electrochemical behaviour of SIBs and NICs have performedin CR2032 coin-type cells. To fabricate SIBs, the coin cells were as-sembled in a glove box under an argon atmosphere by sandwiching aworking electrode (CAC, NMNC, or NMNC-Al) and a pure sodium foilcounter electrode separated by polypropylene separator (Celgard2400). 1M NaClO4 dissolved in ethylene carbonate (EC) and diethylcarbonate (DEC) (1:1; v/v) was used as the electrolyte. The same fab-rication method was followed to construct a NIC as well where CACacted as the anode. The charge-discharge studies (C-DC) of SIBs con-taining pristine or ALD-coated NMNC and CAC were carried out withina potential range of 2–4.5 V and 3–4.6 V respectively while the C-DCperformance of NICs was examined between 0 and 3 V at differentcurrent densities. The C-DC studies were carried out in Land BatteryTest System at ambient temperature. The cyclic voltammetry (CV) andelectrochemical impedance spectroscopy (EIS) measurements wereconducted using an electrochemical analyzer (Bio-Logic, France).

3. Results and discussion

The schematic of ALD process of Al2O3 on the NMNC is presented inFig. 1a. The XRD patterns of pristine and Al2O3-coated NMNC arepresented in Fig. 1b, which can be indexed based on a P2-type hex-agonal structure with P63/mmc space group [16,43]. Both patterns donot exhibit any impurity peaks within the recorded range, confirmingthe formation of phase-pure NMNC powders [16,28,43]. The latticeparameters for pristine and ALD samples were calculated from the XRDpattern in Fig. 1b and the a and c values are unchanged. This confirmsthat the deposition of Al2O3 by ALD at 115 °C does not affect itsstructural behaviour [16]. The SEM images of pristine NMNC andNMNC-Al are presented in Fig. S1. The SEM of pristine NMNC in Fig.S1a displays well-defined particles with smooth surfaces and edges. Anaverage particle size of 1.5 µm is observed with uniform particles sizeand distribution. Cathode materials with uniform size distribution arefavourable for shortening the ionic migration path, resulting in betterion insertion into the host material during the C-DC process[16,27,46,47]. The surface of NMNC becomes rough after ALD asconfirmed in Fig. S1d and S1c. The presence of Al on the surface ofNMNC powder is confirmed by energy-dispersive X-ray spectroscopy(EDX) as shown in Fig. S2.

The presence of a coating layer on NMNC is further confirmed usinghigh-resolution TEM and the corresponding image is presented in

10 20 30 40 50 60 70 80

(b) NMNC NMNC-Al

a = 2.873 c = 11.221

a = 2.855 c = 11.219

(degree)

(0 0

2)

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NMNC NMNC-Al

Binding energy (eV)A

l 2p

(d)

(a)

2θ

)u.(a

ityIn

tens

Fig. 1. (a)Schematic representation of ALD of Al2O3 on NMNC surface; (b) XRD pattern of NMNC and NMNC-Al (84 cycles of ALD) powders; (c) high-resolution TEM images of Al2O3-coated NMNC powders after 84 cycles of ALD and (d) XPS of pristine and ALD coated NMNC powders.

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Fig. 1c. The TEM image of NMNC-Al after 84 ALD cycles shows a uni-form Al2O3 layer 9.4 nm thick on the surface of the host material. Basedon this observation, it can be concluded that an ultrathin (~ 1 nmthickness) Al2O3 layer is present on the NMNC-Al electrode surfacewhen 10 ALD cycles are used. This uniform metal oxide layer is crucialfor enhancing high rate performance as it prevents active species dis-solution into the electrolyte [16,43,45] In addition, the Al2O3 coatingprovides structural stability against mechanical stresses occurringduring cycling, which is beneficial for high rate performance[16,42,43,45] The presence of Al2O3 coating layers is further confirmedthrough X-ray photoelectron spectroscopy (XPS) analysis and the cor-responding spectra is given in Fig. 1d. The NMNC-Al sample displaysthe Al 2p characteristic peak at 74.4 eV whereas the pristine NMNCpowder does not exhibit Al peak at the respective position, revealingthe existence of alumina coating layer on NMNC surface [45].

The half-cell performance of pristine and Al2O3-coated NMNCcathodes against a sodium metal anode is examined in 1M NaClO4

within the potential range of 2–4.5 V. Fig. 2a displays the cyclic vol-tammogram traces of pristine and Al2O3-coated NMNC electrodes at ascan rate of 0.1 mV s−1. Both CV curves show three redox peaks locatedat 4.25, 3.6, and 2.3 V and can be assigned to Ni2+/4+, Co3+/4+,Mn3+/4+and redox couples respectively [16,43]. It is also evident fromCV curves that the pristine electrode possesses a peak shift towardlower voltages in both positive and negative scans, indicating that thelower electrochemical polarization of pristine electrode leads to un-stable Na storage behaviour [16,42,43,45] The C-DC curves of NMNCand NMNC-Al electrodes recorded between 2 and 4.5 V are presented inFig. 2b. Both cells exhibit three oxidation plateaus during the chargingprocess and discharge curves also have a clear downwards slope along

with three reduction steps. The position of redox plateaus in C-DCcurves demonstrates that the NMNC has a complex storage mechanism,agreeing well with the CV results and reports available elsewhere[16,42,43]. The NMNC and NMNC-Al electrodes deliver a dischargecapacity of 121 and 125mAh g−1 at 160mA g−1 within 2–4.5 V, whichare the best values reported among other P2-type materials [16,28].Moreover, the NMNC-Al electrode shows better initial coulombic effi-ciency (82%) than that of pristine NMNC (70%). The impact of themetal oxide coating on the electrochemical stability of the NMNCcathode is presented in Fig. 2c. The NMNC-Al electrode has excellentcyclic behaviour even after 175 cycles at 160mAh g−1 within 2–4.5 V.The cell with the pristine NMNC electrode delivers 35mAh g−1 dis-charge capacity after 175 cycles while the NMNC-Al electrode main-tains more than twice this value of after 175 cycles. It is clearly de-monstrated that ultrathin ALD coating of Al2O3 displays enhancedelectrochemical stability during the C-DC process due to its ability tosuppress the P2-O2 phase transformation at high voltage cycling pro-cesses [16,43,45] In addition, the ultrathin coating layer acts as armourto protect the ion-conductive EEI layer, thereby preventing the con-sumption of active materials into the electrolyte and thus enhancing theelectrode’s electrochemical performance [12,15,16,28,43].

The C-DC performance of CAC in a SIB configuration is also testedbetween 3 and 4.6 V at 200mA g−1 current density and the C-DC curveis presented in Fig. 2d. The CAC half- cell exhibits a non-Faradaic sto-rage mechanism along with the adsorption/desorption of ClO4

- anionson the surface which leads to the formation of a double layer at the EEI.The CAC electrode delivered a discharge capacity of 52mAh g−1 andmaintained an excellent cyclic stability of 95% after 250 cycles, con-firming that CAC could be used as an electroactive material for stable

2.0 2.5 3.0 3.5 4.0 4.5 0 25 50 75 100 125 150 175 200 2252.0

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)A

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tage

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(b)

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yticapaC

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hA

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)

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NMNC half cell NMNC-Al half cell

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Fig. 2. (a) CV traces of NMNC and NMNC-Al electrodes at 0.1mV s−1 between 2 and 4.5 V, (b) C-DC curves and (c) cyclic stability of pristine and surface-modified NMNC electrode at160mA g−1 (1 C rate) between 2 and 4.5 V at ambient temperature and (d) C-DC curve of CAC electrode vs. Na foil anode at 200mA g−1 current density between 3 and 4.6 V. The cyclicstability of CAC is presented as insert.

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NICs. In a NIC configuration, the applied voltage will split based on thecapacitive performance of the individual electrode when the NIC isconstructed with electrodes involving different energy storage me-chanisms. Hence, it is crucial to optimize the mass balance of eachelectrode to fabricate high-energy NICs [48,49]. Based on the electro-chemical performance obtained from NMNC, NMNC-Al, and CACelectrodes at same current density against a sodium metal anode, theoptimized mass ratio of cathode to the anode to construct NICs (NMNC/CAC and NMNC-Al/CAC) is 2:1. It is known that the NMNC, NMNC-Al,and CAC electrodes become polarized during the C-DC process and actas electrodes in NIC configuration [32,49]. Thus, the electrochemicalcapacitive performance of NMNC/CAC and NMNC-Al/CAC NICs aretested between 0 and 3 V at different current densities.

The impact of ALD coating on the capacitive performance of NICs isinitially tested using EIS analysis at the open circuit voltage between200 kHz and 100mHz and their Nyquist plots are presented in Fig. 3a.Good fitting data are obtained for the NIC cells based on the equivalentcircuit as shown in the inset of Fig. 3a. Nyquist plots of SIBs in Fig. 3adisplay a semi-circle at high-frequency region and an inclined line athigh-frequency region. The semi-circle at higher frequency correspondsto the charge transfer resistance (Rct). The Rct represents the sum of theresistance associated with the ionic migration through the bulk of theelectrode, the EEI resistance, and the electrode resistance. An inclinedline observed in the low-frequency region relates to the Warburg im-pedance (ZW) which is attributed to the diffusion-controlled process ofthe electrode material. It is evident from Fig. 3a that the NMNC-Al/CACNIC has a much lower Rct value than the NIC containing the pristineNMNC electrode, due to the influence of surface modification by Al2O3

deposition. In our previous work, we have demonstrated that the ALD

of Al2O3 can significantly reduce interfacial resistance (IR), which di-rectly affects the cyclic stability of the energy storage system [16,50].An increase in IR is an indication of an increase in Rct that affects thelong-term cyclability of the cell [8,12,32]. In addition, the developmentof an ultrathin uniform layer of Al2O3 by ALD is beneficial to protect theelectrode surface from the acidic electrolyte, which assist to form stableEEI layer and enhances the ionic diffusion rate even at high currentdensities [3,12,16,21]. Hence, an improved capacitive performance canbe expected from an NMNC-CAC cell due to its low resistance value.

Fig. 3b depicts CV curves of NICs recorded within a potential rangeof 0–3 V at a 5mV s−1 scan rate in 1M NaClO4 electrolyte. The shapesof the CV traces imply that the capacitive behaviour of NICs is distinctfrom that of the electric double-layer capacitor (EDLC), which origi-nates from the overlapping effect of two different energy storage me-chanisms: battery and EDLC [51–53]. The CV curves also reveal that thecurrent response between the anodic and cathodic sweep remainsconstant and the high symmetry indicates an excellent reversibility ofthe NIC cells. This appealing kinetic reactivity makes P2-NMNC apromising energy source for high-performance NIC applications.Moreover, the NMNC-Al/CAC NIC exhibits enhanced current responsecompared to the NMNC/CAC cell, confirming the superior energy sto-rage property of NIC fabricated with Al2O3-coated NMNC. The poorcapacitive behaviour of NMNC/CAC cell is attributed to the lowerelectronic transport as well as the sluggish reaction kinetics on thesurface of the electrode [48,54,55]. In contrast, the enhanced currentresponse of the NMNC-Al electrode is a result of the suppression of thepolarization of the electrode materials during the C-DC studies[3,16,20].

The galvanostatic C-DC performances of different NICs recorded

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rent

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V

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I

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acita

nce

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)

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NMNC/CACNMNC-Al/CAC

Fig. 3. (a) EIS spectra of different NICs recorded before cycling process at open circuit voltage, (b) CV of NMNC/CAC and NMNC-Al/CAC cells at 5mV s−1 scan rate, (c) C-DC and (d)cycles stability of NIC cells at 0.35 A g−1 current density within 0–3 V rage.

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between 0 and 3 V at 0.35 A g−1 current density is presented in Fig. 3c.The slight distortion in the C-DC profile shape further confirms that theNIC has a combined storage mechanism of redox and EDLC [24,25,56].The energy storage mechanism at the CAC anode in NIC is associated tothe formation of an electrical double layer through a non-Faradaic re-action together with the ClO4

- anion presented in the electrolyte,whereas the NMNC cathode takes advantage from the Faradaic reactionof reversible Na+ intercalation/deintercalation, producing Pseudoca-pacitance behaviour. above, the possible reaction mechanism the fab-ricated NICs could be written as follows:

   

⇔ + +

+ − ⇔ − +

+ + +

−+ + + + −

−

Cathode: Na (Mn Ni Co )O

Na (Mn Ni Co )O xNa xe

Anode: AC xe AC(xe ) //ClO (//stands for double layer)

0.66 0.543

0.132

0.133

2

0.66 x 0.544

0.133

0.134

2

4

During the charging process, Na+ ions are extracted from the NNMCstructure into the electrolyte and then reinserted into the NNMC elec-trode from the electrolyte solution during the discharge process, whichconfirms the high energy density of the NIC. On the other hand,ClO4

-anions also get adsorbed and desorbed on the surface of the CACelectrode to produce the double layer during the C-DC process, pro-viding the essential power density. The NMNC-Al/CAC NIC displays ahigher discharge time compared to the NIC with pristine NMNC as theelectrode, demonstrating that the Al2O3 coating by ALD plays a key rolein the equation enhancing the capacitive behaviour of the pristineNMNC electrode. The cell capacitance (Ccell) and specific capacitance(Cspec) is calculated using the formulae Ccell = (it/ΔV) and Cspec = (4 ×Ccell/M) respectively, where i is the applied current density (A), t is thedischarge time (s), ΔV is the potential difference (V), and M is the totalmass of the electroactive materials in two electrodes [24,25,49,57–59].

A specific capacitance of 66 and 59 F g−1 was obtained from theNMNC-Al/CAC and NMNC/CAC cells respectively at 0.35 A g−1 withina 0–3 V potential range. The enhanced electrochemical behaviour of theNIC fabricated with the metal oxide-coated electrode is derived from itslow internal resistance as shown in Fig. 3c. For instance, the RIR valuesof 26 and 66Ω can be calculated from the C-DC curves for NMNC-Al/CAC and NMNC/CAC cells respectively at a 0.35 A g−1 current density.It is well-known that the electrochemical performance of an electrode isdirectly related to the resistance of the cell since a higher resistancehinders the current flow on the electrode surface. Therefore, higherresistance lowers the current flow and inhibits the electroactive

materials from participating in the capacitive reaction, thereby redu-cing the capacitance at high current rates [24,25]. Thus, a poorer ca-pacitance performance is observed from NMNC/CAC NIC cell.

The cyclic stability of NICs tested between 0 and 3 V at 0.35 A g−1

current density for 5000 C-DC cycles is presented in Fig. 3d. As is evi-dent from this figure, the ALD of Al2O3 on an NMNC electrode hastremendously enhanced the electrode stability in a NIC. The NMNC-Al/CAC cell delivered 66 F g−1 and maintained a superior electrochemicalreversibility of 98% even after 5000 C-DC cycles at 0.35 A g−1. Incontrast, the NIC with the pristine NMNC electrode had 78% capaci-tance retention after 5000 cycles with the same cycling conditions. Theoutstanding stability of the NIC with the NMNC-Al electrode is ascribedto the improved structural integrity provided by ALD deposition ofAl2O3 on NMNC electrode. The presence of an ultrathin Al2O3 layereffectively protects the NMNC surface from electrolyte attack and re-duces the electrode dissolution into the electrolyte, maintaining a stableEEI and results in improved discharge capacitance and cyclability ofNMNC-Al/CAC cell [3,20,21]. Moreover, the higher electrical band-gapenergy of 9 eV from the ALD of Al2O3 minimizes the structural trans-formation of the NMNC electrode, enhancing the electrochemical sta-bility of the NMNC-Al/CAC cell compared to the NMNC/CAC NIC [16].As described earlier, the ALD of Al2O3 not only assists in protecting thecathode from the harmful electrolyte during the C-DC process but alsodecreases the electrochemical impedance of NIC cells.

In order to investigate the causes for achieving improved stability inNICs containing ALD-modified electrodes, EIS measurements wereconducted for the NIC cells after cycling at 0.35 A g−1 for 5000 cycles.Fig. 4a and Fig. S3 compare the Nyquist plots of NMNC/CAC andNMNC-Al/CAC cells recorded before and after different cycles. TheNyquist plot of the NMNC/CAC cell shows distinct changes in Rct valuesafter cycling whereas the NIC containing the ALD-modified NMNCelectrode displays identical curves at all C-DC cycles. As proven fromFig. 4a, the ALD of Al2O3 played an important role in shielding the EEIfrom dissolution, which suppressed the electrode polarization [1,16].The comparison of Rct of different NIC cells against the cycle number ispresented in Fig. 4b, which reveals that the increase in Rct values uponcycling is much higher for the NMNC/CAC cell. The values of Rct for theNMNC/CAC cell are about 26, 56 and 137Ω at the initial, 2000th and5000th cycles respectively. In contrast, the Rct of NMNC-Al/CAC cell inthe initial cycle is 19Ω and remains constant even after 5000 cycles,indicating better surface protection, which is essential for obtainingsuperior electrochemical stability even at high rates [1,5,12,16]. Based

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on the discussion above, an ultrathin Al2O3 layer generated by ALDprovides greater assistance for diminishing electroactive species dis-solution into the electrolyte while maintaining the structural integrityof the NMNC cathode material upon extended C-DC cycling processes.

The rate performance of NICs at varying current rates are given inFig. 5a. As expected, the discharge capacitance of NICs decreases withincreased current density due to the diffusion control process as onlythe outer surface of the electrode material participates in the C-DCprocess at high current rates [23,25]. The NMNC-Al/CAC cell has im-proved discharge capacitance compared to the NMNC/CAC cell at allcurrent densities as presented in Fig. 5a. Impressively, the NIC con-structed with the ALD-coated electrode delivered 56 F g−1 at 1.1 A g-1-

current density, which is more than 70% of the capacitance delivered atthe lower current density. In contrast, the NMNC/CAC cell had a ca-pacitance of only 32 F g−1 at the same current density. Possible reasonsfor the poor performance of the cell containing the pristine NMNCelectrode without any surface modification may be a lower electronictransport as well as comparatively sluggish reaction kinetics on thesurface, which is confirmed by EIS analysis as presented in Fig. 3a. Onthe other hand, the ALD of the metal oxide on the surface of the NMNCelectrode successfully suppressed the polarization of the electrodes andan enhanced electrochemical performance was achieved. We also de-monstrated in our previous report that ultrathin Al2O3 on the surface isbeneficial in guarding the EEI layer against erosion by rapid electrolyteattack at high current densities [5,16]. Additionally, this EEI dissolutionof the active species in acidic electrolyte reduces the active surface forstorage reaction and thus capacitance is decreased at high currents forNIC with a non-modified electrode.

The long-term cyclability is critical for any energy storage device to

be employed in practical application. Thus, the prolonged stability ofthe NMNC-Al/CAC cell is tested at various current densities and theresults are presented in Fig. 5b. A total of 5000 C-DC cycles at 0.45,0.55 A g−1 and 10,000 C-DC cycles at 0.35 and 0.72 A g−1 are tested atambient temperature. The NMNC-Al/CAC cell delivered a capacitanceof 72, 58, 57, and 56 F g−1 at current densities of 0.35, 0.45, 0.55, and0.72 A g−1 respectively and maintained 98% of its initial values uponextended C-DC cycling. It is noteworthy that this is the first report on aNIC constructed with a sodium intercalated layered cathode with pro-longed cyclic life performance at high current densities in an organicelectrolyte. Most of the work of NICs containing layered materials aredemonstrated in an aqueous electrolyte with severe capacity fading. Forinstance, Na0.67Al0.3Co0.7O2 nanopowders are reported to exhibit 80%capacity retention after 5000 C-DC cycles between 0 and 1 V [60]. Xiaoet al. presented a Na4Mn9O18/AC NIC with 84% capacitance retentionafter 4000 cycles at 500mA g−1 between 0 and 1.7 V [37]. Recently,Na3V2(PO4)3/AC NIC system has been reported with over 90% capa-citance retention after 10,000 cycles. However, this NIC exhibited poorrate performance and ED and PD values, which is not suitable forpractical applications [32]. Fig. 5c compares the stability of NMNC-Al/CAC with various NIC systems reported elsewhere, confirming theoutstanding stability of the NIC with an ALD-modified NMNC electrode[10,24,29,30,32–35,60–62]. This superior cycling stability of a NICcontaining an NMNC-Al cathode is ascribed to the structural integrityprovided by the ALD of Al2O3. This ultrathin metal oxide layer of theNMNC-Al electrode can accommodate the volume change due to theinherent mechanical stress and strain during the cycling process[16,45]. The coating layer can provide a flexible barrier against thestress and strain within the electrode during the cycling process, which

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leads to better electrochemical behaviour [3,15,16,20]. The low RIR ofthe NMNC-Al/CAC cell at all current densities also aids its excellentelectrochemical stability. The plot of RIR versus current densities for theNMNC-Al/CAC cell within 0–3 V is shown in Fig. 5d. The NIC with theNMNC-Al cathode maintained lower internal resistance values at allcurrent densities, demonstrating the higher current flow on the surfaceof NMNC-Al. This high current flow ensures more Na ion diffusion to-ward the electrode surface, enhancing the cycling behaviour re-markably even at high current rates [24,25,47,56].

Fig. 6a illustrates the Ragone’s plot of NICs as a function of ED andPDs measured from C−DC studies at different current densities. Thepower and energy density values are calculated based on the formulareported elsewhere [63]. This figure shows that the NMNC-Al/CAC cellhas much higher ED characteristics compared to the NMNC/CAC cell.In the case of the NIC with NMNC-Al, an ED of 82Wh kg−1 is attainedat a PD of 1350W kg−1, whereas the NMNC/CAC cell has ED of about70Wh kg−1 at the same PD value. The NMNC-Al/CAC cell still main-tained 63Wh kg−1 even at a high PD of 6.6 kW kg−1. In contrast, theNMNC/CAC cell provided a 36Wh kg−1 ED at a PD of 6.6 kW kg−1.Moreover, the ED obtained at a very high PD value is the best everreported among NICs constructed with sodium-based intercalationelectrode materials and also exceed many lithium ion capacitors[19,24,25,30,32,36,46–48,54,56]. This excellent electrochemical per-formance of NMNC-Al/CAC is a result of protecting the EEI layer by theALD modification of the NMNC surface [3,12,16]. This is confirmed byrecording the EIS spectrum before and after 2000, 5000, 10,000, and20,000 cycles at different current rates which are presented in Fig. 6b.The diameter of the semi-circle at a medium frequency range remainsunchanged even after 20,000 cycles, confirming its superior electro-chemical stability. The comparison of Rct against the number of cycles is

shown in Fig. 6c. The Rct value of the NMNC-Al/CAC cell before cyclingis 18.3Ω and the cell still has very low Rct of 24.1Ω even after 20,000cycles, which confirms that the EEI layer is effectively protected by theultrathin Al2O3 layer. Thus, exceptional energy storage performance isachieved with the NMNC-Al electrode. In addition, the NIC with theNMNC-Al electrode exhibits a low IR drop even at a high currentdensity of 2.2 A g−1 as presented in Fig. 6d. Both Fig. 6c and Fig. 6dprove that ALD of Al2O3 provides greater advantages for reducing ac-tive species dissolution into the electrolyte while maintaining theNMNC microstructure upon cycling, enhancing its electrochemicalperformance even at high current rates in a NIC.

Based on above outcomes, the schematic diagram of the protectingrole of ALD Al2O3 layer is presented in Fig. 7. It has been confirmed thatthe ultra-thin uniform ALD coating of Al2O3 layer acted as an artificialEEI and forms a stable interface between NMNC electrode and elec-trolyte. This effectively suppressing the occurrence of side-reactionswith electrolyte species (ClO4-) and dissolution of electrode activespecies. Our work demonstrates that ALD could be considered as aneffective performance enhancement technique for any advanced hybridcapacitor for high energy applications with high specific energy andpower without decreasing their cycling performance.

4. Conclusion

For the first time, ALD has been implemented to improve the elec-trochemical behaviour of a NIC by the deposition of a uniform ultrathinAl2O3 coating on the NMNC electrode surface with. Our study de-monstrates that a NIC fabricated with a CAC anode and an NMNCcathode modified by ALD delivers a discharge capacity, energy density,and power density of 66 F g−1, 75Wh kg−1, and 2.23 kW kg−1

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respectively at 0.35 A g−1 along with an extremely stable cyclic stabi-lity of 98% after 10,000 cycles. The NIC with a pristine NMNC cathodeshows inferior electrochemical performance and stability at all currentdensities. The improved rate capability can be attributed to the lowerimpedance and lower ohmic drop arising from the surface protection byALD. This dramatic performance improvement may also be a result ofthe ultrathin Al2O3 ALD film acting to minimize active species dis-solution, maintain the structural integrity of the NMNC and protect thecathode surface from electrolyte side reactions. Consequently, this worknot only demonstrated that ALD can be used to coat a uniform metaloxide layer on a NIC cathode but also proved that it has a noteworthyeffect on the electrochemical performance of NICs.

Appendix A. Supporting information

Supplementary data associated with this article can be found in theonline version at http://dx.doi.org/10.1016/j.nanoen.2018.03.021.

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Fig. 7. Schematic diagrams of the shielding effect of ultra-thin ALD Al2O3 layer on the NMNC electrode.

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Dr. Karthikeyan Kaliyappan received Bachelor of tech-nology (B.Tech, 2005) and Master of technology (M.Tech,2007) in Electrochemical Engineering from CentralElectrochemical Research Institute, India. He earned Ph.Dfrom Chonnam National University, South Korea inAdvanced Chemicals and Engineering (2013). He isworking as Research fellow at the University of Waterloo,Waterloo, Canada. His current fields of Interests are de-signing high energy density materials for energy storagedevices including Li-ion and Na-ion batteries, and nextgeneration metal-ion capacitors. He is also mastered indeveloping metal oxide coatings and composites usingatomic layer deposition.

Dr. Zhongwei Chen is Canada Research Chair Professor inAdvanced Materials for Clean Energy at the University ofWaterloo, the Fellow of the Canadian Academy ofEngineering and Vice President of International Academy ofElectrochemical Energy Science (IAOEES). His researchinterests are in the development of advanced energy ma-terials for fuel cells, metal-air and lithium-ion batteries. Hepublished 2 books, 9 book chapters and more than 220 peerreviewed journal articles with over 13,000 citations with aH-index of 57 (GoogleScholar). He is also listed as inventoron 21 US/international patents, with several licensed tocompanies in USA and Canada.

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