Palladium-Filled Carbon Nanotubes Cathode for Improved Electrolyte Stability and Cyclability Performance of Li-O2 Batteries

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Li–oxygen (Li-O2) cathodes using palladium-coated and palladium-filled carbon nanotubes (CNTs) were investigated for their battery performance. The full discharge of batteries in the 2–4.5 V range showed 6-fold increase in the first discharge cycle
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   Journal of The Electrochemical Society ,  164  (1) A6303-A6307 (2017) A6303 F  OCUS  I SSUE OF   S ELECTED  P  APERS FROM  IMLB 2016  WITH  I NVITED  P  APERS  C ELEBRATING  25 Y  EARS OF   L ITHIUM  I ON  B  ATTERIES Palladium-Filled Carbon Nanotubes Cathode for ImprovedElectrolyte Stability and Cyclability Performance of Li-O 2 Batteries Neha Chawla, = , ∗ Amir Chamaani, = , ∗ Meer Safa, ∗ and Bilal El-Zahab z  Department of Mechanical and Materials Engineering, Florida International University, Miami, Florida 33174, USA Li–oxygen (Li-O 2 ) cathodes using palladium-coated and palladium-filled carbon nanotubes (CNTs) were investigated for theirbattery performance. The full discharge of batteries in the 2–4.5 V range showed 6-fold increase in the first discharge cycle of the Pd-filled over the pristine CNTs and 35% increase over their Pd-coated counterparts. The Pd-filled also exhibited improvedcyclability with 58 full cycles of 500 mAh · g − 1 at current density of 250 mA · g − 1 versus 35 and 43 cycles for pristine and Pd-coatedCNTs, respectively. In this work, the effect of encapsulating the Pd catalysts inside the CNTs proved to increase the stability of theelectrolyte during both discharging and charging. Voltammetry, Raman spectroscopy, FTIR, XRD, UV/Vis spectroscopy and visualinspection of the discharge products using scanning electron microscopy confirmed the improved stability of the electrolyte due tothis encapsulation and suggest that this approach could lead increasing the Li-O 2  battery capacity and cyclability performance.© The Author(s) 2016. Published by ECS. This is an open access article distributed under the terms of the Creative CommonsAttribution Non-Commercial No Derivatives 4.0 License (CC BY-NC-ND, http://creativecommons.org/licenses/by-nc-nd/4.0/ ), which permits non-commercialreuse, distribution, and reproduction in any medium, providedthe srcinal work is not changed in anyway and is properly cited. For permission for commercial reuse, please email: oa@electrochem.org. [DOI: 10.1149/2.0491701jes] All rights reserved.Manuscript submitted September 30, 2016; revised manuscript received December 14, 2016. Published December 31, 2016. Thiswas Paper 390 presented at the Chicago, Illinois, Meeting of the IMLB, June 19–24, 2016.  This paper is part of the Focus Issue of Selected Papers from IMLB 2016 with Invited Papers Celebrating 25 Years of Lithium Ion Batteries. High energy density batteries have garnered much attention in re-cent years due to their demand in electric vehicles. Lithium oxygen(Li-O 2 )batterieshavenearly10timesthetheoreticalspecificenergyof common lithium-ion batteries and in that respect have been regardedas the batteries of the future. 1–3 A typical Li-O 2  battery consists of Li anode, porous cathode open to, oxygen and Li + ion conductingelectrolyte separating the electrodes. Li-O 2  battery stores energy viaa simple electrochemical reaction (2Li  +  O 2  ↔  Li 2 O 2 ) in whichLi 2 O 2  is deposited on the surface of cathode via the forward reaction(oxygen reduction reaction, ORR) during discharge and backwardreaction (oxygen evolution reaction, OER) takes place during charg-ing to decompose Li 2 O 2  on the surface of cathode. 2 Since the maindischarge product (Li 2 O 2 ) and other discharge/charge byproducts inLi-O 2  batteries are electrically insulating and not soluble in elec-trolytes, the structure and electronic conductivity of cathode materialshavebeencriticalfactorsindeterminingthelimitingcapacityofLi-O 2 batteries. 4,5 Carbonaceous materials such as carbon nanoparticles, 6,7 carbon nanofibers, 8,9 carbon nanotubes, 7,8,10 graphene platelets, 11,12 and other forms of carbons 13,14 have been commonly used as cathodematerials in Li-O 2  batteries. Among carbon-based materials, carbonnanotubes (CNTs) have been widely used in Li-O 2  cathodes due totheirhighspecificsurfacearea,goodchemicalstability,highelectricalconductivity, and large accessibility of active sites. 15,16 Zhang et al.reported one of the early uses of CNT (single-walled) as cathode ma-terialsinLi-O 2  batteriesinwhichdischargespecificcapacitiesashighas 2540 mAh · g − 1 were obtained at 0.1 mA · cm − 2 discharge currentdensity. 8 Although many research studies have been done to improvethe performance metrics of Li-O 2  batteries, they are still in their earlystagesandmanytechnicalchallengeshavetobeaddressedbeforetheirpractical applications. 17,18 The most common problems impeding thedevelopment of Li-O 2  batteries have been low rate capability, poorrecyclability and low round-trip efficiency. 19,3 All of these issues aresrcinally stemmed from sluggish kinetics and irreversible character-istic of the OER and ORR reactions which causes high overpotentialsin discharging/charging process. Hence, increasing the efficiency of OER/ORR reactions and minimizing the overpotentials during the = These authors contributed equally to this work. ∗ Electrochemical Society Student Member. z E-mail: belzahab@fiu.edu discharging/charging process have been regarded as a meaningful ap-proach to overcome the aforementioned problems in Li-O 2  batteries.Various additives have been explored to remedy this problem includ-ing the use of redox mediators. 20 Redox mediators minimize chargepolarization by acting as charge carriers between the cathode andLi 2 O 2  surface. 21 Alternatively, the use of different noble metals andmetaloxidecatalystshas alsobeenintegratedinthecathodes ofLi-O 2 batteries. 22–25 The catalyst may influence the performance of Li-O 2 batteries by destabilizing the oxidizing species which decreases thecharging overpotential. 26,27 They may also increase the surface ac-tive sites and facilitate charge transport from oxidized reactants tothe electrode which could also lead to formation of nanocrystallineLi 2 O 2 . 14 However, it has been recently shown that the catalyst on theoxygencathodeinLi-O 2  batteriesiseasilydeactivatedduetocontinu-ous accumulation of discharge and charge products upon cycling. 28,29 It also has been reported that coarsening and agglomeration of cat-alyst upon charging/discharging reduces the efficiency of catalyst inLi-O 2  batteries. 23,30 Besides the catalyst deactivation/agglomerationupon cycling, Gittleson et al. have also shown the impact of catalyst-electrolyte compatibility on Li-O 2  battery performance. 31,32 Accord-ing to the report, platinum (Pt) and palladium (Pd) catalysts promoteLi 2 O 2  oxidation at low potentials but also cause electrolyte decompo-sition resulting in the formation of Li 2 CO 3  and thus deactivating thecatalysts. Advanced approaches such as dispersing catalysts in poly-meric membrane over the oxygen electrode, chemically binding thecatalysts on the surface of CNTs by atomic layer deposition, and en-capsulation of catalysts inside carbon cathodes have been developedto overcome the deactivation and agglomeration of catalysts in Li-O 2 batteries. 33–37 Inthis study,we reportthe effectof filling CNTwith Pdnanoparti-cle catalysts to assist the ORR/OER reactions in Li-O 2  batteries whileminimally destabilizing the electrolyte. To assess the effectivenessof this approach, various electrochemical, spectroscopic, and micro-scopic techniques have been used. Experimental  Materials.— Palladium (II) chloride (PdCl 2 , 59% Pd) was pur-chased from ACROS organics. Bis (trifluoromethane) sulfonamide(LiTFSI, purity  >  99.95%), tetraethylene glycol dimethyl ether ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see  50.128.214.93 Downloaded on 2017-01-01 to IP   A6304  Journal of The Electrochemical Society ,  164  (1) A6303-A6307 (2017)(TEGDME, purity  >  99.00%), N-Methylpyrrolidine (NMP, purity > 97.00%), multi-walled carbon nanotubes (MWCNT, D = 5–20 nm,L = 5 µ m, purity > 95.00% carbon basis), Titanium (IV) oxysulfate(TiOSO4) ( ≥ 29% Ti (as TiO 2 ) basis) and Lithium Peroxide (Li 2 O 2 )werepurchasedfromSigma-Aldrich.Carbonclothgasdiffusionlayer(CCGDL, thickness ∼ 300 µ m) was purchased from Fuel Cell Earth.Lithiumfoilchips(purity > 99.90%)andCelgardpolypropylenesepa-rator(thickness ∼ 25 µ m)werepurchasedfromMTICorp.Polyvinyli-dene fluoride (PVDF) was purchased from Alfa Aesar.  Electrode preparation.— Pd-filled MWCNTs were prepared fol-lowing the procedure detailed elsewhere. 38 Briefly, capped MWCNTswere first decapped by nitric acid solution treatment and then 1 mMaqueous solution of PdCl 2  was used to swell 100 mg of decappedMWCNTs until a slurry was formed. Pd-coated CNTs were alsoprepared following the same procedure on untreated capped MWC-NTs. Both slurries of Pd-coated and Pd-filled MWCNTs were driedovernight at room temperature and calcinated in air at 350 ◦ C for 2hours. Corresponding particles were then hydrogenated in an ovenunder hydrogen gas to yield  ∼ 5 wt% Pd nanoparticles. 38 Cathodeswere prepared by coating a slurry of MWCNT (Pristine, Pd-filledand Pd-coated)/PVDF (90/10 Wt% in NMP) on 0.5” diameter carboncloth gas diffusion layer (CCGDL) followed by drying at 120 ◦ C for12 hours. The cathodes were then stored in an Ar-filled glove box tobe used later. The typical loading of MWCNT was 0.5  ±  0.01 mg.All reported capacities in this manuscript are reported per total massof active cathode (CNTs and catalyst).  Electrolyte and battery test assembly.— The electrolyte was pre-pared by adding 1 mol.kg − 1 of LiTFSI salt into TEGDME solvent.Li-O 2  batteries were assembled using a Swagelok type cell with stain-lesssteelrodontheanodesideandastainlesssteeltubeonthecathodeside. Lithium metal disc was used as anode, covered by electrolyte-soaked Celgard 2400 separator, MWCNT-CCGDL and a stainlesssteel mesh as a current collector. Li-O 2  batteries were rested insideAr-filled glove box overnight before electrochemical tests. All elec-trolyte preparation and cell assembly were performed inside Ar-filledglove box ( < 1 ppm O 2  and < 0.1 ppm H 2 O). Characterization.— TheLi-O 2  batterieswereremovedfromargonglove box and placed in the gastight desiccator filled with ultra-highpurity oxygen gas (Airgas, purity  >  99.994%). The batteries wererested under oxygen for 5 hours before testing. Solartron 1470 batterytester was used for galvanostatic discharge/charge tests within a volt-age range of 2.0–4.5 V at a current density of 250 mA · g − 1 . Voltam-metry measurements were performed by an electrochemical worksta-tion (Gamry reference 600) at the rate of 1 mV · s − 1 in the range of 2.0–4.5 V to investigate the catalytic behavior of oxygen electrodes.All charge/discharge and electrochemical tests were measured in atemperature controlled environment at 25 ◦ C. After charge/dischargecycling, the oxygen cathodes were recovered from the batteries inthe Ar-filled glove box, rinsed with acetonitrile and dried under vac-uum. Cathodes were investigated by Raman spectroscopy (BaySpec’sNomadic, excitation wavelength of 532 nm), Fourier transform in-frared (FTIR) spectroscopy (JASCO FT-IR 4100), and Scanning elec-tron microscopy (SEM) (JEOL 6330F). Bruker GADDS/D8 X-raypowder diffraction (XRD) with MacSci rotating Molybdenum anode( λ = 0.71073)operatedat50kVgeneratorand20mAcurrentwasalsoused to collect the diffraction patterns. A parallel X-ray beam in sizeof 100  µ m diameter was directed on to the samples and diffractionintensities were recorded on large 2D image plate during exposuretime. Li 2 O 2  was quantified in the cathodes after discharge using acolorimetric method previously reported by Schwenke et al. 39 Briefly,discharged cathodes were first immersed in water then aliquots weretaken and added to 2% aqueous solution of TiOSO 4 . Instantaneouslya color change occurred and the absorbance spectra of the solutionswere collected using a UV-Vis spectrophotometer (Gamry UV/VisSpectro-115E). The peak intensity at 408 nm was calibrated againstsolutionswithknownconcentrationsofLi 2 O 2 ,intherangeof0.1to10 Figure 1.  Transmission electron micrographs of Pd-coated CNTs (a) and Pd-filled CNTs (b). (c) Raman spectra of Pd-filled and Pd-coated CNTs. mg/ml and linear calibration curve was obtained. Transmission Elec-tron Microscopy (Phillips CM-200 200 kV) was also used to inspectthe carbon nanotubes. Results and Discussion ThecathodesusedinthisstudywerecomposedofMWCNTs(pris-tine, Pd-coated and Pd-filled) coated on the woven carbon cloth gasdiffusion layer (CCGDL). Homogenous three-dimensional networksof carbon nanotubes over CCGDL yields high surface area with anopen structure which improves the electronic contact during chargingand discharging processes. 40 Figures 1a and 1b show TEM images of Pd-coated and Pd-filled CNTs, respectively. In Pd-filled cathodes,Pd nanocatalysts are formed in the inner tubular region of decappedCNTs. 41 Figure 1c shows the Raman spectra of the Pd-coated and Pd-filled CNTs. D and G bands of Pd-filled and Pd-coated CNTs wereidentical in location and intensity ratios [I(G)/I(D)  ∼ 1.2] indicatingthat the decapped CNTs does not have high density of defects on theirsurface.The first discharge and charge behaviors of Pd-coated, Pd-filledand pristine CNTs batteries using 1 M LiTFSI in TEGDME elec-trolyte in the voltage window of 2.0–4.5 vs Li/Li + at the constant Figure 2.  First discharge/charge capacity of pristine CNTs, Pd-coated CNTsand Pd-filled CNTs at a constant current density of 250 mAh · g − 1 between2.0–4.5 V. ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see  50.128.214.93 Downloaded on 2017-01-01 to IP    Journal of The Electrochemical Society ,  164  (1) A6303-A6307 (2017) A6305 Figure 3.  Cyclic voltammetry of pristine, Pd-filled, and Pd-coated CNTsin range of 2–4.5 V vs Li/Li + . All scans rates 1 mV.s − 1 under O 2  or Aratmospheres. current density of 250 mA · g − 1 are shown in Figure 2. The pristine CNTs show a first discharge capacity of 1980 mAh · g − 1 compared to8197 mAh · g − 1 and 11,152 mAh · g − 1 for the Pd-coated and Pd-filledCNTs, respectively. The ∼ 6-fold discharge capacity improvement of Pd-coated CNTs over pristine CNTs resulted from improved ORRand increased surface sites for lithium discharge product depositiondue to the presence of Pd nanocatalysts. Cyclic voltammetry (CV)measurements (Figure 3) confirm higher ORR and OER currents for Pd containing CNTs over pristine CNTs. This also confirms retainedcatalytic activity of Pd when encapsulated inside the CNTs, consis-tent with previously observations. 36,42,35 In addition, the presence of PdnanocatalystsinbothPd-filledandPd-coatedCNTsshiftstheonsetof ORR peak of pristine CNTs from 2.8 V to 2.9 V, showing enhancedcathodic activity. 43 The Pd-filled CNTs also demonstrated 36% in-crease in first discharge capacity over Pd-coated. In the Figure 3, anoxidationpeakat ∼ 3.3VisattributedtotheOERandwasshowntobemore pronounced for the Pd-filled compared to Pd-coated. It is con-sideredthatthepresenceofPdinsidetheCNTsstrengthensthe π elec-tron density on the CNT surface yielding homogeneously distributednucleation sites for Pd-filled CNTs compared to heterogeneously dis-tributed nucleation sites for Pd-coated CNTs. 35 This delocalizationof Li 2 O 2  seeding sites contributes to intimate contact between Li 2 O 2 and CNTs and helps promote the formation of high surface dischargeproducts. At voltage exceeding 3.7 V vs Li/Li + Pd-coated CNTsshows the highest rate of oxidation of electrolyte and electrolyte de-composition products. 31,32,44–46 The subtle OER activity in CV waspreviously reported by Gittleson et al. using TEGDME electrolytein the presence of the noble metal catalysts. 31 Raman spectroscopiccharacterization on discharged cathodes revealed Li 2 O 2  formation at ∼ 790 cm − 1 Raman shift 47 for all discharged cathodes (Figure 4a).However, the Pd-coated CNTs cathode showed a pronounced Ramanshift peak at 1080 cm − 1 , which corresponds to the electrolyte decom-positionproductLi 2 CO 3 . 47,48 ThispeakwasabsentfrompristineCNTcathodes. This behavior confirms the observation from CV, indicatingreduced electrolyte stability due to the presence of Pd on Pd-coatedCNTs and enhanced electrolyte stability for Pd-filled compared toPd-coated CNTs. Following charging, Raman spectroscopic analyseson the cathodes revealed efficient removal of the Li 2 O 2  peaks from allcathodes, and a decrease in peak intensity of Li 2 CO 3  peak for the Pdcontaining CNTs. The decrease in Li 2 CO 3  was previously reportedto be enabled in catalyst-containing cathodes. 49 Li 2 O 2  content in thecathode was quantified using a colorimetric approach previously re-ported by Schwenke et al. 39 The amounts of Li 2 O 2  in the cathodewere determined to be 11.6, 21.4, and 35.4  µ mols for pristine, Pd-coated, and Pd-filled CNT cathodes, respectively. These amounts cor-responded to capacity yields of approximately 88%, 23%, and 37% of the experimental capacities recorded by the pristine, Pd-coated, andPd-filled CNT cathodes, respectively. In order to determine the molarratio of Li 2 O 2  in the discharged cathodes, the cathodes were analyzedusing FTIR following the method reported by Qiao and Ye. 50 Usingpeakintensitiesratioat600cm − 1 (Li 2 O 2 )and862cm − 1 (Li 2 CO 3 ),Pd-coated and Pd-filled cathodes had 19.3% and 33.2% Li 2 O 2  by mole,respectively. By only considering the Li 2 O 2  and Li 2 CO 3  dischargespecies, this observation is in agreement with the UV-Vis quantifica-tion and further confirms the stabilizing effect of the encapsulationof Pd inside the CNTs compared to coating the CNTs. The CV andRaman data also back up these claims, indicating that the electrolyteundergoes more decomposition in cells with Pd-coated CNTs cath-odes. The presence of Li 2 O 2  and Li 2 CO 3  was additionally confirmedby XRD (Figure 4b).The formation of discharge products was visually confirmed us-ing scanning electron microscopy on discharged cathodes shown inFigure 5. The discharge products (Li 2 O 2 ) of pristine CNTs (Figure5a) were conformal around the cathode in rod-like structures withlow porosity. The growth of layers in this morphology often yieldsto passivation and blockage of oxygen to the cathode and eventuallylimits the discharge capacity and cycle life of the battery. 51 The Pd-coated CNTs cathode (Figure 5b) show some platelet-shaped Li 2 O 2 buried by thick conformal layer, suspected to be Li 2 CO 3  as shownin Raman and FTIR measurements. In contrast, the Pd-filled CNTs Figure 4.  (a) Raman spectra after discharging and charging the batteries for pristine, Pd-coated, and Pd-filled CNTs. (b) X-ray diffraction patterns confirming thepresence of Li 2 O 2  and Li 2 CO 3  in Pd-coated and Pd-filled discharged cathodes. ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see  50.128.214.93 Downloaded on 2017-01-01 to IP   A6306  Journal of The Electrochemical Society ,  164  (1) A6303-A6307 (2017) Figure 5.  Scanning electron micrographs of cathodes after discharge for pris-tine CNTs (a), Pd-coated CNTs (b), and Pd-filled CNTs (c). The scale bars are1 µ m. show nano-thin platelets of Li 2 O 2  covering the cathode (Figure 5c).These platelets of Li 2 O 2  yield high surface porosity which in turn donot block the access to the CNTs and enhance the performance.InordertoidentifythesynergyofelectrolyteandPdnanocatalysts,the oxidation stability limit of the electrolyte was determined usinga chronopotentiometric stability test and linear sweep voltammetryunder oxygen atmosphere. Batteries using Pd-coated, Pd-filled andpristine CNTs were assembled and charged without prior discharg-ing at constant current density of 250 mA · s − 1 up to cutoff voltageof 4.5 V. Figure 6 shows higher capacity for Pd-coated comparedto Pd-filled and pristine. Since no discharge products existed, thecapacities obtained are attributed to electrolyte and cathode undesir-able reactions. Again, this supports the claim that Pd-filled promotebetter stability than Pd-coated CNT cathodes. This observation wasalso confirmed using linear sweep voltammetry following a dischargeshowing increased OER peak intensity at 3.3–3.4 V and improvedelectrolyte stability above 3.7 V vs Li/Li + for Pd-filled vs Pd-coatedCNTs (Figure 6b). These results again support previous observations that encapsulating the Pd nanocatalyst helped improve the electrolytestability during the operation of the Li-O 2  battery.The cycling stability of Li-O 2  batteries based on Pd-filled, Pd-coated,andpristineCNTshavebeeninvestigatedandshowninFigure7. Galvanostatic discharge/charge cycling of Li-O 2  batteries at a cur-rent density of 250 mA · g − 1 at a limited capacity of 500 mAh · g − 1 in Figure 7.  Cyclability of the Li-O 2  batteries for fixed cycle capacities of 500mAh · g − 1 atacurrentdensityof250mA · g − 1 andwithvoltagecutoffsof2.0–4.5 V for pristine CNTs, Pd-coated CNTs, and Pd-filled CNTs. The markers ◦ denote discharge capacity and ● denote charge capacity. a voltage window of 2.0–4.5 V vs Li/Li + were conducted. Li-O 2  cellsusingPd-filledCNTsshowthehighestdischargecyclingperformanceof 58 cycles compared to 43 cycles for Pd-coated, and 35 for pristineCNTs. The cycling stability improvement in the case of Pd-coatedand Pd-filled CNTs is a result of previously confirmed OER/ORRimprovement due to the Pd nanocatalysts. The cycling stability im-provementofPd-filledCNTsoverthePd-coatedCNTswasultimatelycredited to the decrease in undesirable discharge/charge products for-mation, e.g. Li 2 CO 3 , afforded by the encapsulation approach. Conclusions The inclusion of Pd catalyst in CNTs for use as cathode materi-als in Li-O 2  batteries has been demonstrated. Using two modes of CNT loading, inside (Pd-filled CNTs) and outside (Pd-coated CNTs),showed that both approaches yielded significant improvement in fulldischargecapacitiesofthebatterieswhilethePd-filledcycledfor35%more cycles of 500 mAh · g − 1 at current density of 250 mA · g − 1 . Theencapsulation of nanocatalyst inside the CNTs improved the stability Figure 6.  (a) Chronopotentiometric test at 250 mA · g − 1 from OCV to 4.5 V for pristine, Pd-coated, and Pd-filled CNTs. (b) Linear sweep voltammetry of pre-discharged Pd-coated and Pd-filled CNTs under oxygen between OCV and 4.5 V vs Li/Li + at scan rate of 1 mV.s − 1 . ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see  50.128.214.93 Downloaded on 2017-01-01 to IP    Journal of The Electrochemical Society ,  164  (1) A6303-A6307 (2017) A6307of the electrolyte by decreasing the formation of Li 2 CO 3  comparedto nanocatalyst-coated CNTs. These observations were confirmed byvoltammetry, Raman and UV/Vis spectroscopy, FTIR, chronopoten-tiometry, electron microscopy, and charge/discharge cycling. Acknowledgments The authors acknowledge the staff of the Advance Materials En-gineering Research Institute (AMERI) at FIU for the assistance inelectron microscopy. 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