Cite this: RSC Advances, 2013, 3,2812

Tuning the morphology of ZnMn2O4 lithium ion batteryanodes by electrospinning and its effect onelectrochemical performance3

Received 19th November 2012,Accepted 13th December 2012

DOI: 10.1039/c2ra22943a

www.rsc.org/advances

Pei Fen Teh,a Yogesh Sharma,b Yah Wen Ko,a Stevin Snellius Pramanaad

and Madhavi Srinivasan*ac

ZnMn2O4 structures of various morphologies (nanorods, nanofibers, nanowebs) have been prepared via a

facile electrospinning technique by a simple variation of the sintering profile, and have subsequently been

employed as anodes in lithium ion battery applications. After the sintering process, as-spun nanofibers

with high aspect ratio have broken into short segments of ZnMn2O4 nanorods (ZMO-NR). Incorporating an

intermediate carbonization step has strengthened the mechanical integrity of as-spun nanofibers,

resulting in the formation of sintered nanofibers (ZMO-NF) and nanowebs (ZMO-NW). On the basis of

FESEM, HRTEM and XRD studies, the formation mechanism of nanostructures consisting of hierarchically

self-assembled ZnMn2O4 nanocrystals is discussed. Particle size distribution is computed by Rietveld

refinement and HRTEM micrographs, while the valence states are confirmed by XPS. The initial discharge

of ZMO-NF and ZMO-NW demonstrated a high capacity of y1469 mA h g21 and 1526 mA h g21,

respectively, in the voltage ranges of 0.005 V and 3.0 V versus Li/Li+ at 60 mA g21, associated with

reversible capacities of y705 mA h g21 and 530 mA h g21 after 50 cycles. Morphology tuning of anodes

and the importance of interconnected nanoparticulate pathways for lithium ion diffusion are elucidated.

Introduction

Fossil fuel depletion and dependence have raised environ-mental concerns, including air pollution and global warming.1

Renewable energy resources (e.g. solar and wind power2,3),along with suitable energy storage devices, offer strategies toalleviate our reliance on fossil fuels and their environmentaleffects. Since the invention of the first graphite/lithium cobaltoxide (C/LiCoO2) rocking-chair cell in 1985 by Asahi KaseiCorporation,3 lithium ion batteries (LIBs) have been exten-sively utilized in portable electronic gadgets. In spite of this,large volume applications such as renewable energy storagesystems, electric vehicles (EVs) and hybrid electric vehicles(HEVs)2 require advanced LIBs that can provide higher energy/power density. Hence, there has been concerted worldwide

research effort into high capacity anode/cathode materials andsafe electrolytes for future LIBs.4

A graphite anode is presently used in LIBs with a theoreticalcapacity of only 372 mA h g21 due to limited hosts for lithiumion intercalation/de-intercalation in its layered structure.5

Nowadays, transition metal oxides (TMOs) have been widelyinvestigated as high-capacity anodes for LIBs in view of theirhigh theoretical capacity (y400–1000 mA h g21), which is dueto their ability to reversibly react with more than one lithiumion through the conversion reaction.6 TMO was assumed to be‘inert’ to lithium ion insertion/de-insertion until the researchwork by Poizot et al. in 2000.6 CoO is a rocksalt structure thathas no vacancy for lithium intercalation and de-intercalation.Poizot et al. successfully demonstrated the reduction ofnanosized CoO to Co/Li2O during the first discharge, whilenanocrystalline CoO was reversibly formed upon lithium ionextraction. It is worth noting that nanoscaling of TMOs foranode application is very crucial,2,7 as some novel physical andchemical properties are observed only when materials arescaled down to nanodimensions, including the conversionreaction. Nanomaterials provide a higher surface area tovolume ratio and a shorter diffusion pathway for lithium ionsand electron transportation, so the battery kinetics aresignificantly enhanced as compared to their micro-sized orbulk counterparts.8 Hence, the reduction of CoO to extremelytiny grain sizes of Co (2–5 nm), well dispersed in a Li2O matrix,

aSchool of Materials Science and Engineering, Nanyang Technological University,

Nanyang Avenue, Singapore, Singapore 639798bPhysics Section, Department of Paper Technology, Indian Institute of Technology,

Roorkee Saharnapur Campus, 247001, IndiacEnergy Research Institute @ NTU (ERIAN), Research TechnoPlaza, 50 Nanyang

Drive, Singapore, Singapore 637553. E-mail: madhavi@ntu.edu.sg; Fax: +65 6790

9081; Tel: +65 6790 4606dFacility for Analysis, Characterization Testing and Simulation, Nanyang

Technological University, 50 Nanyang Avenue, Singapore

3 Electronic supplementary information (ESI) available. See DOI: 10.1039/c2ra22943a

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2812 | RSC Adv., 2013, 3, 2812–2821 This journal is � The Royal Society of Chemistry 2013

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is kinetically reversible.9,10 Since then, many reports have beenpublished to employ nanosized TMO as anodes by conversionreaction.11–14

Although cobalt-based TMOs exhibit excellent cyclabilityand capacity, they are toxic and not cost effective.15 Thus,cobalt oxides have been substituted by some environmentallybenign oxides, such as iron oxides16,17 and manganeseoxides18,19 prepared by different synthesis methods. Of parti-cular interest is that a combination of more than one TMO,crystallizing in the spinel structure, has shown good capacityvalues on cycling, such as ZnFe2O4,20,21 ZnMn2O4

22,23–25 andZnCo2O4.26,27 By mixing two TMOs, it is possible to consolidatetheir competitive advantages, such as the flexibility to alter thetheoretical capacity and control the working voltage.11 Amongthe ZnB2O4 series (B = Fe, Mn or Co), ZnMn2O4 has captured themost interest, as Mn is cheap and abundant, with the capabilityto perform at lower working voltages than the other two spinels.Nanosized ZnMn2O4 synthesized by different techniques wasreported in several publications.23–25,28–30 NanocrystallineZnMn2O4 by the polymer pyrolysis method25 and nanoparticlesby hydrothermal synthesis31 have shown a capacity retention of569 mA h g21 and 430 mA h g21 after prolonged cycling.However, nanoparticles have a greater tendency to agglomerate,due to their high specific surface area and excessive surfaceenergy,8,13 which inhibit their electrochemical performance. Toprevent the existence of inactive regions by agglomeration andretain the extraordinary properties brought by nanomaterials,extensive study of the engineering of nanomaterials has beenreported.2,7 The nano/micro hierarchical electrode materialscombine the competitive advantages of both micro andnanomaterials,8,32 wherein the synthesis method is tailored tofabricate a self-assembled parent micro-sized structure madeup of numerous building blocks on the nanoscale. For example,the formation of ZnMn2O4 hollow microspheres by ZnMn2O4

nanosized building blocks, demonstrated by L. Zhou et al.33

Each building block of ZnMn2O4 provides the benefits ofnanometer-sized materials during prolonged cycling, while themicro-sized assembly ensures better structural stability andcyclability as an anodic material for LIBs.8

In the present work, such nano/micro ZnMn2O4 nanofibershave been fabricated by a relatively simple and cost effectiveelectrospinning method. By changing the sintering tempera-ture profiles, we have successfully demonstrated the formationof ZnMn2O4 nanorods (ZMO-NR), nanofibers (ZMO-NF) andnanowebs (ZMO-NW). The effect of morphology on thephysical and electrochemical properties of ZnMn2O4 ispresented. One of the key issues of electrospun oxides is therupturing of nanofibers during heat treatment that is essentialfor oxide crystallization.21,34 We report a feasible way to tacklethis issue by introducing an intermediate stage in the sinteringprofile, which promotes the carbonization of polymer con-stituent in the as-spun nanofibers, resulting in carbon–carbonbond formation, which enhances the mechanical integrity offibers to prevent breakage of the fibrous morphology.

Results and discussion

Materials characterization of ZMO-NR, ZMO-NF and ZMO-NW

By electrospinning, high aspect ratio as-spun nanofibers withuniform diameter in the range y150–220 nm were prepared(Fig. 1). Poly(vinylpyrrolidone), (PVP) dissolved in absoluteethanol serves as a polymer backbone to contain the metallicprecursor. A strong electric field was applied between theelectrospinning solution and the collector plate (Fig. S1, ESI3),so long nanofibers were extracted from the needle tip andaccumulated on the collector. Subsequently, a sintering stepwas required for the as-spun fibers to remove the solvent, PVP,as well as to convert the metallic precursors into theirrespective oxides before battery application. Consequentlythermogravimetric analysis (TGA) was conducted on as-spunnanofibers to understand the chemical compositional changeas a function of temperature (Fig. 2). The weight loss of 2% at98 uC is related to the evaporation of volatile species, possiblymoisture, residual ethanol and acetic acid. The second

Fig. 1 High aspect ratio as-spun nanofibers with diameters in the range 150–220 nm.

Fig. 2 TGA of as-spun nanofibers was conducted in air from room temperatureto 650 uC.

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decomposition peak starting at 301 uC corresponds to thedecomposition of the metallic precursors, namely manganeseacetylacetonate and zinc acetylacetonate. The prominentdecline observed at 389 uC is attributed to the removal of thePVP backbone, which is the main constituent in as-spunnanofibers. There is no significant weight change (stabilizes aty25%) observed after y460 uC, indicating completion ofchemical reactions. TGA analysis reflects the decompositionprofiles of each chemical species, providing an importantguideline to design the sintering temperature profile, in which400 uC was chosen as the maximum sintering temperature.Three different sintering conditions have been applied to tunethe morphology and mechanical integrity of sintered products,leading to the formation of ZnMn2O4 nanorods (ZMO-NR),nanofibers (ZMO-NF) and nanowebs (ZMO-NW) (Fig. 3(d)–(f)).

Typically, 1-step sintering (Fig. 3(a)) is commonly used afterelectrospinning to remove polymer backbones such as PVP,21

PVA35 etc., and it is commonly seen that the nanofibersexperience severe destruction of their fibrous morphology afterthe sintering, breaking down into nanorods, nanoparticles orirregular shapes.34,36 This is due to the rapid and excessivevolume contraction within as-spun nanofibers after removal ofthe polymer backbone. Thus, metallic precursors lose contactwith each other, causing rupturing of the fibrous morphology.To prevent the destruction, an extremely low heating ratemight be useful to accommodate the volume contraction ofnanofibers, at the expense of extra energy cost and time.34,36

A similar phenomenon was also noticed in this report.Fig. 3(d) shows that the as-spun nanofibers have broken downinto shorter segments and formed nanorods (ZMO-NR) withnon-uniform diameter after a direct sintering at 400 uC for 2 h.As-spun nanofibers have disassembled into irregular struc-

tures as they are unable to withstand the sudden volumetricchange upon removal of the polymer backbone (Scheme 1(a)).We introduced a 2-step sintering profile (schematicallydisplayed in Fig. 3(b)) to prohibit the loss of fibrousmorphology during heat treatment. The as-spun nanofiberswere calcined at 350 uC for 3 h to enable carbonization of PVP.The selected calcination temperature, 350 uC, is lower than thedecomposition temperature of PVP (389 uC). The carbonizationprocess leads to shrinkage of the fiber diameter to y100–150

Fig. 3 Temperature profiles used in the sintering of as-spun nanofibers are illustrated in (a), (b) and (c). FESEM micrographs clearly indicate the influence of sinteringtemperature on the morphology of sintered products. 1-step sintering (a) has resulted in the formation of (d) ZMO-NR, 2-step sintering has resulted in (e) ZMO-NF.Excessive carbonization step resulted in (f) ZMO-NW.

Scheme 1 Schematic representation of (a) 1-step sintering to produce ZMO-NR,(b) 2-step sintering with the addition of carbonization at 350 uC to yield ZMO-NF, and (c) if longer carbonization duration is applied, as-spun nanofibers mergeand form ZMO-NW.

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nm (Fig. S2, ESI3), which facilitated the reactivity of metallicprecursors within the fibers (Scheme 1(b)). Later on, furtherheating to 400 uC at 0.5 uC min21 allows the conversion fromPVP to carbonaceous products. This carbonization step hasretained its fibrous morphology after sintering because ithelps to reduce the distance between metallic particles, so theyare able to maintain the fiber mechanical integrity with lengthof several microns to form ZMO-NF (Fig. 3(e)) after sintering. Itis worth noting that the heating rate applied to ZMO-NF (1 uCmin21) is double the rate applied to ZMO-NR (0.5 uC min21),without destroying the fibrous morphology and saving energycosts. The diameters of sintered fibers are thinner (80–130 nm)than as-spun fibers (Fig. 1), due to the removal of the organiccomponent. Individual nanofibers are relaxed and interwovenwith one another to form a network with open pores, whichcould not be observed in the relatively short ZMO-NR.Extending the duration of carbonization to 4 h promoted themerging of as-spun fibers into interlinked nanostructures(Scheme 1(c)), forming nanowebs (ZMO-NW) (Fig. 3(f)). Ascompared to ZMO-NF, ZMO-NW has relatively few open poresdue to the coalescence. FESEM images with lower magnifica-tion are shown in Fig. S3, ESI,3 with the purpose of showingthe uniformity of nanostructures obtained. Further increasingthe sintering temperature or extending the carbonizationduration causes the breakage of nanofibers and/or diminish-ment of open pores. Our previous study has emphasized thathigh aspect ratio nanofibers are able to enhance the cyclabilityof the anode, and interconnected fibers render an effectivediffusion pathway for lithium ions and electrons, while theopen pores are beneficial in amplifying the electrolyte/electrode contact area.21 In this report, all three samples wereemployed as anodes for LIB application, and the importance ofproviding a 1D electronic pathway will be further discussed inthe next section.

In order to understand the transformation of functionalgroups before and after sintering under different conditions,Fourier transform infrared (FTIR) spectroscopy was used todetect the vibrational modes for as-spun nanofibers andsintered products. The FTIR spectra of PVP and as-spunnanofibers (Fig. 4(a)) are very similar, showing that the majorconstituent in as-spun fibers is the PVP backbone.21 A broadband at y3439 cm21 correlates to the stretching vibration ofthe O–H bond of either adsorbed water molecules or moisturefrom the atmosphere. Asymmetric stretching of CH2 and C–Hstretching of PVP are observed, due to the presence of twoadjacent peaks at y2959 cm21 and y2874 cm21.Subsequently, the vibration band at 1663 cm21 is attributedto either C–N or CLO functional groups. CH2 scissoring andwagging appears at 1452 cm21 and 1267 cm21, respectively. Allthese functional groups are contributed by the presence ofPVP, as they were almost eliminated after being heated up to350 uC and dwelling for 3 h (Fig. 4(b)). Additionally, some newvibration bands appear between 1650–400 cm21 after thecarbonization step, and these peaks are also found consis-tently in ZMO-NR, ZMO-NF and ZMO-NW (Fig. 4(c)–(e)). Thenew peaks are assumed to correspond to organic ligandsconverted from residual PVP after carbonization, as they arealmost negligible when the as-spun nanofibers are heated upto 650 uC (Fig. 4(f)). As a consequence, the weak peak at y1629

cm21 may be assigned to either CLC or carbonyl derivative (x–CLO–y) with a specific coordination environment (x and y:different functional groups).37,38 Another peak at y1130 cm21

can be interpreted as the C–O stretching presented in allsintered samples.37 Vibration bands at y630 cm21 and y516cm21, corresponding to the metal–oxygen stretching bond,increase in intensity with increasing temperature (to 650 uC),indicating the formation of ZnMn2O4 oxide.

From the FTIR study, some PVP residue was found inZnMn2O4 samples after sintering (Fig. 4(c)–(e)) as expected,because the chosen sintering temperature (400 uC) is not highenough to remove PVP thoroughly. As this residue maycontribute to battery capacitive storage during cycling, soTGA of sintered samples were conducted (ZMO-NR, ZMO-NFand ZMO-NW, see ESI3 Fig. S4) at atmospheric conditions fromroom temperature to 650 uC. It was verified that the residualcontent in all scenarios is below 4 wt%. Hence, theparticipation of PVP residue embedded inside ZnMn2O4 onbattery performance can be neglected.

X-ray diffraction patterns (XRD, Fig. 5) verify the formationof single phase ZnMn2O4 with space group I41/amd (ICSD15305). Rietveld refinement indicates that ZnMn2O4 crystal-lizes in a tetragonal spinel structure with the latticeparameters listed in Table 1. There is a slight variation inthe lattice parameters, probably due to different sinteringconditions. Since the spinel crystal structure is destroyed andthe ZnMn2O4 is amorphized into the nanocrystalline phaseafter the first discharge (lithium insertion), we may assumethat the initial lattice parameters of spinel have a negligibleimpact on long term cycling. Furthermore, crystallite sizes arecalculated by Rietveld refinements (Table 1). The resultsconfirm that all three samples have comparable size and thisis aligned with the measurement of nanocrystals by HRTEMimages (Fig. S5, ESI3).

As mentioned previously, the hierarchically self-assemblednanostructure provides unique properties in battery applica-tions. By HRTEM analysis, ZMO-NR, ZMO-NF and ZMO-NW

Fig. 4 FTIR of (a) as-spun nanofibers, (b) as-spun nanofibers heated to 350 uCand dwelled for 3 h, (c) ZMO-NR, (d) ZMO-NF, (e) ZMO-NW and (f) nanofibersheated to 650 uC. FTIR of PVP is also included.

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consisted of numerous nanocrystals in the ranges of y9(2)nm, 9(2) nm and 9(1) nm, respectively (Table 1). XPS chemicalstate analysis of pristine powders of three morphologies (Fig.S6, ESI3) depicts that the starting powders have Zn and Mn inthe same oxidation states, i.e. Zn2+ and Mn3+.

Application of ZMO-NR, ZMO-NF and ZMO-NW as anodes in LIBs

ZMO-NR, ZMO-NF and ZMO-NW were prepared as anodes forLIB in the 2016 coin cell configuration. To understand thereaction mechanism of ZnMn2O4 with lithium ions, cyclicvoltammetry (CV) was conducted between 0.005–3.0 V at 0.1mV s21 for 5 cycles (Fig. 6). Overall, the reduction andoxidation peaks are similar in three different morphologies,and their first cycles (Fig. 6(a)) are evidently distinguishablefrom the remaining cycles, indicating the change of chemicalreaction mechanism towards lithium ions after the first cycle.In the first cathodic sweep, a very shallow reduction peak isobserved at y1.4 V, which may relate to the intercalation oflithium ions into the spinel structure. This is commonlyobserved in ZnFe2O4 spinel anodes.20,21,39 However, this is thefirst time that we are reporting such a mechanism in ZnMn2O4

spinel. Another small hump at y0.7 V correlates to theirreversible decomposition of electrolyte to form a solidelectrolyte interface (SEI) layer.24,25 Furthermore, the mostintense peak at y0.25 V is attributed to the conversion

reaction of ZnMn2O4 spinel into Zn and Mn metallicnanograins dispersed in a Li2O matrix.25,30 From this pointonwards, ZnMn2O4 spinel experiences the conversion reactionand is thoroughly amorphized; thus the intercalation at y1.4V is not observed in the reverse scans (Fig. 6(b) and (c)). At the

Fig. 5 Rietveld refinements of ZMO-NR, ZMO-NF and ZMO-NW.

Table 1 Lattice parameters and crystallite sizes computed by Rietveld refine-ment and the actual nanocrystal sizes measured by HRTEM

Sample

Lattice parametersCrystallitesize (nm)

Nanocrystalsize (nm)a c

ZMO-NR 5.719(2) 9.171(5) 6.7(1) 9(2)ZMO-NF 5.726(2) 9.186(3) 6.5(1) 9(2)ZMO-NW 5.733(2) 9.193(4) 6.5(1) 9(1)

Fig. 6 Cyclic voltammetry of ZMO-NR, ZMO-NF and ZMO-NW at 0.1 mV s21, (a)1st cycle, (b) 2nd cycle and (c) 5th cycle.

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first anodic sweep (Fig. 6(a)), two oxidation peaks are observedat y1.2 V and y1.6 V, respectively. The peak at y1.2 V can beascribed to the reversible oxidation of Mn to MnO, whereas Znis oxidized to ZnO at y1.6 V.24,25,40

In the second cathodic cycle (Fig. 6 (b)), two peaks areobserved as the coupling reduction of ZnO and MnO at y0.65V and y0.5 V12 for ZMO-NF and ZMO-NW. Correspondingoxidation peaks are also observed at y1.3 V and y1.6 V,respectively. However, these redox peaks are not recognizablein ZMO-NR, as there is only a single redox pair at y0.6 V andy1.3 V at the 2nd cycle (Fig. 6(b)). It is suggested that thediffusion of lithium ions is less efficient in ZMO-NR than inZMO-NF and ZMO-NW, so the chemical reactions are sluggishand merely a single redox peak is found, indicating that theefficiency of battery cycling is highly dependent on electrodemorphology. At the 5th cycle CV of ZMO-NF and ZMO-NW, thereduction peaks of ZnO and MnO merge and appear as a singlepeak at y0.6 V, but its oxidation peaks are still resolvable. Bycomparing the CV at the 1st, 2nd and 5th cycles, ZMO-NR hasa shallower area underneath the curves, so the nanofibers andnanowebs have rendered a better diffusion pathway forlithium ions during electrochemical measurement.

Their electrochemical performances were also investigatedby galvanostatic discharging/charging at a current density of60 mA g21 with a voltage cut-off of 0.005–3.0 V versus Li+/Li.The discharging/charging curves of the 1st cycle are displayedfor comparison (Fig. 7). The first discharge (lithium insertion)capacities of ZMO-NF and ZMO-NW are similar, i.e. 1469 mA hg21 and 1526 mA h g21, respectively, corresponding to y13moles of lithium ions per formula unit of ZnMn2O4. Amongthe three morphologies, ZMO-NR has the worst performanceas its first discharge capacity of 1257 mA h g21 (y11 moles oflithium ions). However, by looking closely into the firstdischarge curve, it was found that all three samples expresssimilar discharge behaviours, especially in the capacity range0–200 mA h g21 (y1.8 moles of lithium ions). A short plateauat y1.4 V is considered as the intercalation of lithium ionsinto the spinels, whereas electrolyte decomposition is seen at

y0.75 V. In the subsequent discharge, ZMO-NF and ZMO-NWtake in y2 moles lithium ions more than ZMO-NR at thedischarge plateau of y0.25 V, indicating that the efficiency ofthe conversion reaction depends on the morphology of thehost materials, while ZMO-NF and ZMO-NW outperform ZMO-NR, which matches with our observation in their CV (Fig. 6). Atthe first charge, the capacities of ZMO-NR, ZMO-NF and ZMO-NW rapidly levelled off to y680 mA h g21, y716 mA h g21 and891 mA h g21, mainly explained by the irreversible reactions ofSEI formation21,26 and the conversion of spinel (ZnMn2O4)into binary oxides (ZnO, MnO and Mn2O3).24,25,28,31,33

After a prolonged cycling (Fig. 8), ZMO-NR exhibits theworst storage ability among the three products. Its retention at50 cycles is y318 mA h g21 (y2.8 moles lithium ions). This issimilar to our previous study on ZnFe2O4 nanorods;21 thecapacity fades severely due to the lack of electronic wiring forlithium ion diffusion during discharging/charging in nanor-ods. The cyclability of ZMO-NW surpasses ZMO-NF at theinitial 25 cycles due to its interconnected nanostructures,which promotes a facile transportation channel for lithiumions (Fig. 8). However, the capacity declines to y530 mA h g21

(y4.7 moles of lithium ions) at the 50th cycle, possibly due tothe closure of pores in nanowebs by volumetric expansion inthe conversion reaction, leading to the loss of its uniquenanostructure. ZMO-NF demonstrates the best performanceamong the three morphologies. It can be maintained at y705mA h g21 (y6.3 moles of lithium ions) after 50 cycles. Hence,the efficiency of ZMO-NF is the best out of all the samples, as ithas demonstrated similar capacity to its 1st charge.Furthermore, ZMO-NF performs better than the ZnMn2O4

nanowires reported by Kim et al.,40 indicating the importanceof electronic wiring.

Since three samples are formed by numerous nanocrystalsof similar size (see Fig. S5, ESI3), we initially assume they shallperform comparably in LIB applications. Nevertheless, whenthe nanocrystals are hierarchically constructed into a specificmorphology, i.e. nanorods, nanofibers or nanowebs, we areable to observe significantly distinguishable battery perfor-mances, especially after a long term cycling. ZnMn2O4

Fig. 7 First galvanostatic cycling of (a) ZMO-NR, (b) ZMO-NF and (c) ZMO-NW at60 mA g21.

Fig. 8 Cycling performance of ZnMn2O4 with different morphologies at 60 mA g21.

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nanocrystals provide the competitive advantage of nanomater-ials if they are specially arranged to form a secondarymorphology. Besides, it is crucial to supply a connectedpathway between all the nanocrystals to serve as diffusionchannels for lithium ions during cycling. Consequently, itexplains why ZMO-NF and ZMO-NW are better than ZMO-NRwith poor electronic wiring, whereas ZMO-NW performs badlyafter the pore closure, as it has lost the characteristic ofnanomaterials. This hypothesis is investigated by electroche-mical impedance studies (EIS) to understand the kinetic ofelectrodes and the factors that are responsible for capacitydegradation of an electrode.41–46

Impedance measurements were carried out on the bareelectrodes with different morphologies of ZnMn2O4 to theworking electrodes versus lithium foil. Besides, the cells werealso galvanostatically cycled at 60 mA g21 and hold 3.0 V(charged state) for 2 h before recording the EIS. The data isshown in the form of Nyquist plots (Z9 vs. 2Z99, where Z9 andZ99 are the real and imaginary parts of complex impedance).Fig. 9 shows the EIS of bare electrodes, as well as the chargedelectrodes at the 2nd, 5th and 10th cycles. The experimentaldata are analyzed by fitting with an equivalent electrical circuitconsisting of resistors and capacitors as shown in Fig. 10,which is similar to the model employed for battery dataanalysis elsewhere in the literature.41–43 The circuit elementsare denoted as electrolyte resistance (Re), the inseparablesurface film (sf) and charge transfer (ct) impedance (R(sf+ct)),the constant phase elements (CPE(sf+dl) (dl: double layer)), thebulk impedance (Rb) and the Warburg impedance (W). CPEsare used instead of pure capacitors, due to the occurrence ofdepressed semi-circles in the spectra, which is an indication ofdeviation from the ideal behaviour of a capacitor.

Results showed that the electrolyte resistance (Re) remainedalmost constant at 6(¡0.5) V, irrespective of cycle number,and the morphologies of ZnMn2O4, as presumed. On the otherhand, the R(sf+ct) and Rb and the associated CPE(sf+dl) and CPEb

are sensitive to the morphology of active materials and varyupon cycling. The Nyquist plots of all the bare electrodesunder OCV (Fig. 9) comprise a depressed semi-circle at highfrequencies, followed by Warburg-type behaviour in the lowfrequency region. The spectra was basically fitted to a singlesemicircle (both R||CPE circuit elements), attributed bysurface film formed when the electrode is in contact withthe electrolyte. However, the contribution by charge transferbetween electrolyte and electrode has also been considered infitting, hence the overall resistance and the CPE arerepresented as R(sf+ct) and CPE(sf+dl), respectively. The fittedvalues are tabulated in Table 2. As seen in Table 2 and Fig. 9,the impedance values of the bare electrodes were noticed to behighest in ZMO-NW, possibly due to the higher surface area ofZMO-NW (BET surface area = y47 m2 g21). The intercon-nected nanostructure of ZMO-NW offers an excessive surfacearea that can be accessed by the electrolyte, so it has asubstantially larger reaction site on the electrodes. Penetrationof the electrolyte into the bulk electrodes through open poresembedded in the nanowebs has also led to greater surface filmformation, and thereby high surface resistance. On thecontrary, surface film formation was found to be lower inZMO-NF (BET surface area = y44 m2 g21) and ZMO-NR (BETsurface area = y40 m2 g21), due to their lower surface area ascompared to ZMO-NW. These results are in tune with thegalvanostatic cycling, whereas ZMO-NW has demonstrated thehighest first discharge capacity. In bare electrodes, the majorconstituent of the surface film is salt in the electrolyte,whereas the chemical composition changes upon cycling. Afterthe first discharge, SEI formation and electrolyte decomposi-tion are essentially to be taken into account for EIS fitting,causing the changes in R(sf+ct) values in subsequent cycles.41

In general, the formation and integration of electrodestakes place in the first few cycles of the conversionreaction,47,48 therefore the associated impedance values arenot consistent at the initial stage of cycling. Besides, thevolumetric contraction and expansion encountered by theelectrode during cycling may cause pulverization of the

Fig. 9 Nyquist plots for (a) ZMO-NR, (b) ZMO-NF and (c) ZMO-NW.

Fig. 10 The equivalent circuit which is used to fit the experimental data in Fig. 9.

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electrode from the current collector or the agglomeration ofactive materials. This phenomenon is reflected as inconsistentimpedance values upon cycling in EIS study, as well as thedegradation of capacity values in galvonostatic cycling.44–46 Inthe present work, impedance of all ZnMn2O4 after 2, 5 and 10complete discharging/charging cycles were testified. Thelargest variation of impedance values is noticed in ZMO-NR.More precisely, the radii of the semicircle in the Nyquist plot(Fig. 9(a)) have spread significantly, so it explains the poorcyclability exhibited by ZMO-NR, which lacks of electronicwiring for lithium ion diffusion. As a result, there might be thepresence of inaccessible regions within the bulk electrode,since nanorods are weakly connected, and hence ZMO-NRshows poor electrical contact with the current collector.

Similar unstable impedance was also found in ZMO-NW,even though the fluctuation is smaller. The increment ofcharge transfer and bulk impedances upon cycling may becaused by the closure of open pores embedded in nanowebs byvolumetric expansion/contraction or electrochemical grinding.In this case, unique morphology of ZMO-NW has diminished;thus a steady capacity loss is also observed. It is worth noticingthat the impedance values derived for ZMO-NF stay relativelyconsistent as compared to the other two samples. Besides, itsimpedance values are the lowest among the three samples. Itwas believed that the enhanced excellent capacity retention ofZMO-NF is contributed to by the continuous framework ofnanofibers with open pores, resulting in good electronicconduction pathway for lithium ions and electrons. Overall,EIS has interpreted that the capacity stabilization is sensitiveto the morphology and integrity of active materials, especiallyat prolonged cycling.

Experimental section

The ZnMn2O4 nanorods (ZMO-NR), nanofibers (ZMO-NF) andnanowebs (ZMO-NW) were prepared by electrospinning.Stoichiometric amounts of zinc acetylacetonate hydrate[Zn(C5H7O2)2?xH2O (Sigma Aldrich)] and manganese acetyla-

cetonate [Mn(C5H7O2)3 (Sigma Aldrich)] were mixed in 3 mlacetic acid (Tedia) with vigorous stirring.Poly(vinylpyrrolidone) (PVP; Fluka) (molecular weight of360 000) was dissolved in absolute ethanol (Sigma Aldrich)to form a 9.5 wt% solution. Subsequently, PVP/ethanolsolution was added to the metallic precursor/acetic acidsolution and vigorously stirred to obtain a homogenoussolution.

The precursor solution was connected to a high voltagepower supply (Gamma High Voltage Research DC PowerSupply). A positive voltage of 12 kV was applied between theneedle tip and the grounded aluminium foil collector, with adistance of 12 cm. The solution was ejected at a constant flowrate of 1 ml h21 by computer-controlled syringe pump (KdScientific). The as-spun fibers were heated in ambient atmo-sphere at different sintering temperature profiles (ramp rate,duration and maximum temperature) to acquire ZnMn2O4

with different morphologies. ZnMn2O4 nanorods (ZMO-NR)were obtained when the as-spun fibers were heated directly to400 uC for 2 h at a ramp rate of 0.5 uC min21. ZnMn2O4

nanofibers (ZMO-NF) were prepared by heating the as-spunfibers to 350 uC at 1 uC min21 and dwelled for 3 h, then weresubsequently ramped up again to 400 uC at 0.5 uC min21, witha dwelling time of 15 min before cooling down to roomtemperature at 1 uC min21. When the dwell duration at 350 uCwas increased from 3 h to 4 h, ZnMn2O4 nanowebs (ZMO-NW)were formed.

Thermogravimetric analysis (TGA) was performed using aTA Instrument Q500 by heating as-spun nanofibers from roomtemperature (r. t.) to 650 uC under air at a ramp rate of 10 uCmin21. Fourier transform infrared spectroscopy (FTIR) wascarried out on the samples in KBr pellets using a Perkin ElmerSpectrum GX at a resolution of 1 cm21. Besides, the surfacearea of ZMO-NR, ZMO-NF and ZMO-NW were recorded ondegassed samples (120 uC/6 h) using Brunauer-Emmett-Teller(BET, Micromeritics, ASAP 2020) by N2 adsorption–desorptionisotherms at 77 K. The structural information was character-ized by powder X-ray diffraction (Bruker D8 Advance, Cu-Karadiation, l = 1.54 Å) with a step size of 0.05u over 10u to 80u.

Table 2 Fitted parameters extracted from EIS data using the circuit in Fig. 10

Cycle number R(sf+ct) (¡5) (V) CPE(sf+dl) (¡5) (mF) Rb (¡5) (V) CPEb (¡0.005) (mF)

ZMO-NR0 (Bare) 57 13 — —2 496 13 125 0.0065 246 12 314 0.01110 118 218 160 0.009ZMO-NF0 (Bare) 80 17 — —2 46 38 10 0.0125 29 37 49 0.01310 19 58 39 0.019ZMO-NW0 (Bare) 184 18 — —2 34 68 206 0.0135 71 20 148 0.01310 115 29 213 0.014

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Rietveld refinement was performed using fundamental para-meters peak shape profile49 implemented in TOPAS.50 A five-coefficient Chebychev polynomial background, a zero error,unit cell parameters, scale factor and crystallite size weresequentially refined. The morphology of the samples wasexamined by field emission scanning electron microscope(FESEM, JEOL 7600F) and high resolution transmissionelectron microscope (HRTEM, JEOL 2100F). The chemicalstates of ZnMn2O4 were analysed by X-ray photoelectronspectroscopy (XPS) by using a Kratos AXIS Ultra spectrometerequipped with Al-Ka monochromatic radiation. Survey scanswere conducted between 0 and 1200 keV calibrated againstadventitious carbon. Deconvolution and peak fitting wereperformed with computer assisted surface analysis (CASA)software using a Voigt function with 30% Lorentzian characterand a Shirley background.

The electrochemical performances of ZMO-NR, ZMO-NFand ZMO-NW were evaluated in a 2016-coin cell. Theelectrodes were prepared by mixing 60 wt% active materials,20 wt% binder (Kynar) and 20 wt% Super P (Timcal) in solvent,N-methyl-pyrrolidone (NMP, Sigma Aldrich) to form a homo-genous slurry. The slurry was coated on etched copper foil bydoctor blade technique (thickness y25 mm) and dried at 80 uCin a vacuum oven for 12 h. Subsequently, the coating waspressed between twin rollers to improve the adhesion betweenthe copper foil and the active materials. The electrodes werecut into circular disks with a diameter of 16 mm, each diskcontaining y2–3 mg of active materials. The electrodes wereassembled in an argon filled glovebox (MBraun, Germany)with oxygen and water content of less than 1 ppm. Lithiummetal served as a counter electrode, Celgard 2400 as theseparator and 1 M solution of LiPF6 dissolved in ethylenecarbonate/diethylene carbonate (EC : DEC = 1 : 1, CharlstonTechnologies Pte Ltd.) was used as the electrolyte.

The lithium storage properties of ZnMn2O4 were examinedby galvanostatic cycling (Multichannel Battery Tester, NewareTechnology Limited) and cyclic voltammetry (CV, Solartron1470E) measurements. These coin cells were galvanostaticallycharged and discharged in the voltage window 0.005–3.0 V atroom temperature and at 60 mA g21. CV testing was studied ina similar voltage range at a constant scanning rate of 0.1 mVs21. Electrochemical impedance spectroscopy (EIS) was mea-sured in the range 100–0.1 Hz by applying a bias voltage of 10mV, and the data were shown as a Nyquist plot.

Conclusion

ZMO-NR, ZMO-NF and ZMO-NW were synthesized by electro-spinning technique, followed by heat treatment with differentsintering profiles. 1-step sintering has created excessivevolumetric change within the as-spun nanofibers, causingthe breakdown of fibers into short nanorods (ZMO-NR). In thisreport, a carbonization process is first introduced as theintermediate step during heat treatment, with the purpose ofimproving the mechanical integrity during sintering and

forming ZMO-NF and ZMO-NW. Three samples are similarlycomprised by numerous nanocrystals of y9 nm, whereasbattery performance is greatly influenced by the hierarchicalnanostructure formed by nanocrystals. By having a highlyconnected nanofiber framework with the presence of openpores, a facile transportation route for lithium ions ispreserved. Hence, ZMO-NF outperforms ZMO-NR and ZMO-NW to have the best cycling stability.

Acknowledgements

This work was supported by funding from the NationalResearch Foundation, Clean Energy Research Project grantnumber NRF2009EWT-CERP001-036. The authors alsoacknowledge Timcal for gratis providing Super P LiTM

Carbon black.

Notes and references

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