Li MnSiO /carbon nanofiber cathodes for Li-ion batterieslfnazar/publications/... ·...

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ORIGINAL PAPER Li 2 MnSiO 4 /carbon nanofiber cathodes for Li-ion batteries Hyunjung Park & Taeseup Song & Rajesh Tripathi & Linda F. Nazar & Ungyu Paik Received: 8 December 2013 /Revised: 25 February 2014 /Accepted: 5 March 2014 /Published online: 2 April 2014 # Springer-Verlag Berlin Heidelberg 2014 Abstract Pure single-phase Li 2 MnSiO 4 nanoparticle- embedded carbon nanofibers have been prepared for the first time via a simple sol-gel and electrospinning technique. They exhibit an improved electrochemical performance over con- ventional carbon-coated Li 2 MnSiO 4 nanoparticle electrodes, including a high discharge capacity of 200 mAh g 1 , at a C/20 rate, with the retention of 77 % over 20 cycles and a 1.6- fold higher discharge capacity at a 1 C rate. Keywords Li-ion batteries . Cathodes . Electrochemical characterizations . Charging/discharging Introduction Lithium-ion batteries (LIBs) are the most promising of all energy storage devices for their many applications from mo- bile electronics to electric vehicles due to their stable cycle performance and high energy efficiency [16]. While several negative electrode materials with a higher capacity have been developed, positive electrode (cathode) materials have not yet witnessed such improvements [79]. A balanced development of both electrodes is essential to achieve an increase in specific energy. The silicates (Li 2 MSiO 4 ; M=Mn, Fe, Ni, Co) are considered highly promising cathode candidates due to their high natural abundance leading to a low cost and their envi- ronmentally benign nature and promising electrochemical properties [1013]. Among these, Li 2 MnSiO 4 holds particular interest due to the high feasibility of attaining double oxida- tion of the metal within the electrolyte stability window (i.e., (de)intercalating two Li + ions), which is unlikely for the other transition metals, including Fe [1419]. However, structural instability associated with Jahn-Teller distortion at the Mn site and inherently low electronic conductivity (10 16 S cm 1 at room temperature and 3×10 14 S cm 1 at 60 °C) [10, 20] results in large irreversible capacity, capacity fading, and poor rate capability, which prohibits its practical use [21]. To over- come these challenges, Li 2 MnSiO 4 particles coated with car- bon layers and Li 2 MnSiO 4 particle-carbon composite mate- rials have been explored [2226]. These electrode systems show a considerable improvement in the kinetics associated with the electronic and ionic conductivities compared to Li 2 MnSiO 4 alone, although the other problems still remain to be resolved. Recently, Rangappa et al. demonstrated that a Li 2 MnSiO 4 nanostructure with the two-dimensional (2D) leaf- let morphology improves the reversible capacity and cycle retention [27]. The ultrathin layer enables facile accommoda- tion of the lattice stress caused by Jahn-Teller distortion oc- curring during cycling and a reduction in lithium diffusion path. This work has been furthered using supercritical solvothermal chemistry to prepare ultrafine particles with hierarchical structures [28] and organic-inorganic hybrid poly- mers. These inspiring studies lead one to conclude that there are still many novel approaches that can be developed to overcome the inherently poor conductivity of Li 2 MnSiO 4 using nanostructuring [29]. Here, we utilize electrospinninga well-known technique, utilized for silicon fibers for example [9], but which has proven unsuccessful for Li 2 MnSiO 4 to fabricate crystallites Hyunjung Park and Taeseup Song contributed equally to this work H. Park : U. Paik (*) WCU Department of Energy Engineering, Hanyang University, Seoul 133-791, South Korea e-mail: [email protected] T. Song Department of Materials Science & Engineering, Hanyang University, Seoul 133-791, South Korea T. Song : R. Tripathi : L. F. Nazar (*) Department of Chemistry, University of Waterloo, Waterloo, ON N2L 3G1, Canada e-mail: [email protected] Ionics (2014) 20:13511359 DOI 10.1007/s11581-014-1105-4

Transcript of Li MnSiO /carbon nanofiber cathodes for Li-ion batterieslfnazar/publications/... ·...

Page 1: Li MnSiO /carbon nanofiber cathodes for Li-ion batterieslfnazar/publications/... · crystallite-carbon nanofiber matrix are also beneficial to the structural stability. These materials

ORIGINAL PAPER

Li2MnSiO4/carbon nanofiber cathodes for Li-ion batteries

Hyunjung Park & Taeseup Song & Rajesh Tripathi &Linda F. Nazar & Ungyu Paik

Received: 8 December 2013 /Revised: 25 February 2014 /Accepted: 5 March 2014 /Published online: 2 April 2014# Springer-Verlag Berlin Heidelberg 2014

Abstract Pure single-phase Li2MnSiO4 nanoparticle-embedded carbon nanofibers have been prepared for the firsttime via a simple sol-gel and electrospinning technique. Theyexhibit an improved electrochemical performance over con-ventional carbon-coated Li2MnSiO4 nanoparticle electrodes,including a high discharge capacity of ∼200 mAh g−1, at aC/20 rate, with the retention of 77 % over 20 cycles and a 1.6-fold higher discharge capacity at a 1 C rate.

Keywords Li-ion batteries . Cathodes . Electrochemicalcharacterizations . Charging/discharging

Introduction

Lithium-ion batteries (LIBs) are the most promising of allenergy storage devices for their many applications from mo-bile electronics to electric vehicles due to their stable cycleperformance and high energy efficiency [1–6]. While severalnegative electrode materials with a higher capacity have beendeveloped, positive electrode (cathode) materials have not yetwitnessed such improvements [7–9]. A balanced developmentof both electrodes is essential to achieve an increase in specific

energy. The silicates (Li2MSiO4; M=Mn, Fe, Ni, Co) areconsidered highly promising cathode candidates due to theirhigh natural abundance leading to a low cost and their envi-ronmentally benign nature and promising electrochemicalproperties [10–13]. Among these, Li2MnSiO4 holds particularinterest due to the high feasibility of attaining double oxida-tion of the metal within the electrolyte stability window (i.e.,(de)intercalating two Li+ ions), which is unlikely for the othertransition metals, including Fe [14–19]. However, structuralinstability associated with Jahn-Teller distortion at the Mn siteand inherently low electronic conductivity (∼10−16 S cm−1 atroom temperature and ∼3×10−14 S cm−1 at 60 °C) [10, 20]results in large irreversible capacity, capacity fading, and poorrate capability, which prohibits its practical use [21]. To over-come these challenges, Li2MnSiO4 particles coated with car-bon layers and Li2MnSiO4 particle-carbon composite mate-rials have been explored [22–26]. These electrode systemsshow a considerable improvement in the kinetics associatedwith the electronic and ionic conductivities compared toLi2MnSiO4 alone, although the other problems still remainto be resolved. Recently, Rangappa et al. demonstrated that aLi2MnSiO4 nanostructure with the two-dimensional (2D) leaf-let morphology improves the reversible capacity and cycleretention [27]. The ultrathin layer enables facile accommoda-tion of the lattice stress caused by Jahn-Teller distortion oc-curring during cycling and a reduction in lithium diffusionpath. This work has been furthered using supercriticalsolvothermal chemistry to prepare ultrafine particles withhierarchical structures [28] and organic-inorganic hybrid poly-mers. These inspiring studies lead one to conclude that thereare still many novel approaches that can be developed toovercome the inherently poor conductivity of Li2MnSiO4

using nanostructuring [29].Here, we utilize electrospinning—a well-known technique,

utilized for silicon fibers for example [9], but which hasproven unsuccessful for Li2MnSiO4—to fabricate crystallites

Hyunjung Park and Taeseup Song contributed equally to this work

H. Park :U. Paik (*)WCU Department of Energy Engineering, Hanyang University,Seoul 133-791, South Koreae-mail: [email protected]

T. SongDepartment of Materials Science & Engineering, HanyangUniversity, Seoul 133-791, South Korea

T. Song : R. Tripathi : L. F. Nazar (*)Department of Chemistry, University of Waterloo, Waterloo,ON N2L 3G1, Canadae-mail: [email protected]

Ionics (2014) 20:1351–1359DOI 10.1007/s11581-014-1105-4

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embedded in a thin conductive carbon matrix (Li2MnSiO4/CNFs) [30]. These provide a significantly improved electro-chemical performance compared to nanoparticles. OurLi2MnSiO4/C nanofibers were synthesized via a facile tech-nique from precursors that creates single phase, nanocrystal-line Li2MnSiO4 with a uniformly coated carbon layer in onestep. This is significant, since synthesis of Li2MnSiO4 by sucha route is very difficult without producing very significantamounts ofMnO that reduce the activemass [30]. Also uniqueto our process is that carbonization and crystallization occursimultaneously, which result in a homogeneous distribution ofactive material in the fiber. The process simply relies on astoichiometric mixture of lithium and manganese acetates in asolution combined with tetraethyl orthosilicate and poly(eth-ylene oxide)(PEO) as an electrospinning agent. Thenanocrystallites are connected through continuous electricallyconducting one-dimensional (1D) fibers, ensuring efficientpathways for electron transport.

Experimental

Synthesis of Li2MnSiO4/C nanofibers

For the synthesis of the Li2MnSiO4/C nanofibers, 0.135-glithium acetate and 0.177-g manganese acetate were mixedwith a solution containing a 1-ml ethanol and 1-ml deion-ized water for 1 h. Tetraethyl orthosilicate 0.213 g wasadded into the solution and stirred for another 1 h. At thesame time, a polymeric solution consisting of a ∼0.1-gPEO (Mw=600,000, Aldrich), 3-ml ethanol, and deionizedwater (v/v=1:1) was also prepared by stirring. Both thesolutions were mixed together and magnetically stirredovernight. The final solution was loaded into a plasticsyringe that was equipped with a syringe pump. A metallicneedle with a diameter of 1.0 mm was connected to the tipof the syringe and a piece of alumina foil as a fibercollector was placed 15 cm away from the needle. Thefeeding rate was set at 0.5 ml/h. This nozzle was connectedto a high-voltage supply. A high voltage of ∼20 kV wasexerted on the needle and foil to collect the electrospunnanofibers. The electrospinning process was conducted atroom temperature in air. To prepare Li2MnSiO4/C nanopar-ticles, the same solution for the Li2MnSiO4/C nanofiberswas used, and the synthesis procedure was the same but noPEO was added in the solution. And then, two kinds ofsolutions for nanoparticles were put into the oven at 50 °Cfor gelation by evaporating solvents. Finally, theelectrospun nanofibers and two kinds of solutions in gela-tion were loaded into each alumina crucible, and they wereplaced at the center of a tube furnace. The Li2MnSiO4

nanopart ic les , Li2MnSiO4/C nanopart ic les , andLi2MnSiO4/C nanofibers were obtained by heating at a rate

of 5 °C min−1 and calcination at 700 °C in 10 % H2/Ar gasfor 10 h.

Material characterization

As-prepared Li2MnSiO4 active materials were characterizedas follows. The microscope images were obtained using afield emission scanning electron microscope (FE-SEM,JEOL JSM07600F), field emission transmission electron mi-croscope (FE-TEM, JEOL JEM-2,100 F), and high-resolutiontransmission electron microscope (HR-TEM, JEOL JEM-2,100 F). X-ray diffraction patterns of samples were obtainedusing a Rigaku D/MAX RINT-2000 diffractometer at a scanrate of 5o/min. The weight fraction of carbon in the activematerials was determined by a simultaneous TGA/DTA/DSCanalyzer.

Electrochemical characterization

Working electrodes were fabricated using Li2MnSiO4 NPs,Li2MnSiO4 C NFs, and Li2MnSiO4/C nanofibers as activematerials; Kejten black™ (EC-300 J, Mitsubishi Chem)which has a surface area of 800 m2/g as a conducting agent;and polyvinylidene fluoride (PVdF) as a binder in a weightratio of 70:20:10. All the samples were fabricated using a2032R-type coin cell comprising the three types of samplesmentioned above as working electrodes, a pure lithium metalfoil as a counter electrode, 1 M LiPF6 in ethylene carbonate/diethylene carbonate (EC/DEC, 1:1 vol%, PANAX StarLyte,S. Korea) as a electrolyte, and a polypropylene (PP) separator.All the electrochemical tests were conducted by using a bat-tery cycle tester (TOSCAT 3000, Toyo System, Tokyo, Ja-pan). Impedance spectra were gained using an impedanceanalyzer (PARSTAT 2273, Princeton Applied Research).

Results and discussion

Scheme 1 shows the electrode configuration with the 1D fibergeometry, where the carbon matrix leads to a more efficientelectron transport compared to that of nanoparticle electrodeswith or without a carbon coating layer. This results in im-proved electrochemical performance. The confined morphol-ogy and the nano-sized dimensions of the Li2MnSiO4

crystallite-carbon nanofiber matrix are also beneficial to thestructural stability. These materials exhibit better electrodekinetics associated with both lithium ion and electron migra-tion. We demonstrate this by showing a comparison of theelectrochemistry of the carbon-coated nanoparticles andcrystallite-embedded nanofibers using impedance and galva-nostatic intermittent titration techniques to probe the kineticsand conductivity.

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Figure 1 shows electron microscopy images of the pristineLi2MnSiO4/C nanofibers (NFs). Figure 1a displays the scan-ning electron microscopy (SEM) image of electrospunLi2MnSiO4/C NFs on the metal collector after heat treatment.The nanofibers have an average diameter of ∼400 nm andcontinuous 1D geometry. Transmission electron microscopy(TEM) was employed to further investigate their

microstructures. The low-magnification TEM image in(Fig. 1b) reveals that Li2MnSiO4 crystallites with a diameterof ∼30 nm are embedded in the carbon fiber matrix. Thisunique microstructure of the nanofiber is more clearly ob-served in a bright field scanning TEM (STEM) image(Fig. 1c). The lattice-resolved TEM image (Fig. 1d) indicatesthat the carbon matrix and the Li2MnSiO4 particles are in their

Scheme 1 Schematic illustration of a a Li2MnSiO4 nanoparticle electrode, b a Li2MnSiO4/C nanoparticle electrode, and (c) a Li2MnSiO4/C nanofiberelectrode; the diagram shows the difference in electron pathways between the three types of electrodes

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amorphous and crystalline phases, respectively. A 2-nm thickcarbon layer is also visible, uniformly coated on the nanofibersurface. A lattice spacing of 2.7 Å corresponding to the (210)plane of the Li2MnSiO4 was clearly observed in the high-resolution TEM image (HR-TEM) in Fig. 1e. The selectedarea electron diffraction (SAED) pattern (inset, Fig. 1e) con-firms that the Li2MnSiO4 NFs are of a highly crystallinenature. The compositional line profile in Fig. 1f based onenergy dispersive X-ray analysis (EDX) reveals that Mn, Si,O, and C elements are uniformly distributed in the nanofiber.The weak signal of carbon is attributed to its low weightfraction in the nanofiber. Figure 2 shows the SEM images ofLi2MnSiO4 nanoparticles (NPs) and carbon-coatedLi2MnSiO4 (Li2MnSiO4/C) nanoparticles (NPs). TheLi2MnSiO4 NPs and Li2MnSiO4/C NPs are composed ofagglomerated particles with a primary particle size of ∼50 nm.

Thermogravimetric (TG) analysis (Fig. 3a) indicates a car-bon mass fraction of ∼6 wt% in Li2MnSiO4/C NPs andLi2MnSiO4/C NFs. X-ray diffraction (XRD) patterns(Fig. 3b) exhibit predominant sharp diffraction reflections thatwere indexed to the orthorhombic unit cell of Li2MnSiO4 withspace group Pmn21 [31]. Impurities such asMnO and Li2SiO3

were barely detectable [20, 21]; their relative intensities arenegligible (<1 %). The lattice parameters of theLi2MnSiO4 determined from indexing the XRD patternare a=6.2984 Å, b=5.3846 Å, and c=4.9678 Å, whichare similar to those of previous reports [21, 31]. Fromthe line broadening defined by the Scherrer’s equation, thecoherence length of the Li2MnSiO4 crystals in the nanofiberswere estimated to be ∼58 nm (010), ∼32 nm (110), and∼37 nm (210), respectively, which is in good agreement withthe TEM results.

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The electrochemical performance of electrodes com-prised of either Li2MnSiO4 NPs, Li2MnSiO4/C NPs, orLi2MnSiO4/C NFs as active materials were evaluatedover the potential window of 1.5∼4.8 V vs. Li/Li+ atroom temperature (25 °C) and at a rate of 0.05 C(16.5 mA g−1; 1 C=330 mA g−1). Figure 4a showscharge-discharge profiles of the three electrodes on thefirst cycle. The first charge capacities of Li2MnSiO4

NPs, Li2MnSiO4/C NPs, and Li2MnSiO4/C NF

electrodes are 105 mAh g−1, 192 mAh g−1, and314 mAh g−1 , respect ively. In par t icular, theLi2MnSiO4/C NF electrode exhibits a long voltage pla-teau over the potential window from ∼4.0 to 4.8 V. Thisis attributed to extraction of Li+ ions from the intersti-tial sites in the unit cell of Li2MnSiO4 and the oxida-tion of Mn2+ and Mn3+ and represents a considerableimprovement over electrodes comprised of the nanopar-ticle or carbon-nanoparticle materials that we use forcomparison. Although Li2MnSiO4/C NFs exhibit a highspecific capacity, it is less than theoretical for the sili-cate (∼330 mAh g−1). The structural degradation duringcycling and insufficient electrical conductivity could bepossible factors. Balaya et al. have recently confirmedthat the large capacity on the first oxidation sweep ofcarbon-coated nanostructured Li2MnSiO4 is accompa-nied by a complete oxidation of Mn2+ to Mn4+ on thesurface by using ex-situ XPS to monitor the valencechange [32]. It is undoubtedly accompanied by someelectrolyte oxidation as well. We observe thatLi2MnSiO4 NPs, Li2MnSiO4/C NPs, and Li2MnSiO4/CNF electrodes deliver 63 mAh g−1, 132 mAh g−1, and197 mAh g−1 reversible discharge capacities, respective-ly. These lower reversible capacities result from anamorphization of the Li2MnSiO4 active material afterLi extraction, as previously reported [22]. The reversiblecapacity of 197 mAh g−1 at C/20 is very similar to thatobtained for a 15-nm Li2MnSiO4 nanocrystallites coatedwith poly(3,4-ethylenedioxythiophene) (PEDOT) pre-pared by supercritical solvothermal reaction, on initialdischarge cycles of electrodes run at room temperature(192 mAh g−1 ) [33], and also similar to the firstdischarge capacity of Li2MnSiO4 formed in an ultrathin“nanoleaflet” morphology (220 mAh g−1) [27]. Howev-er, the latter was collected at a slower rate of C/50where even higher capacities would be expected, andfurthermore, it dropped to 130 mAh g−1 on the secondcycle. The Li2MnSiO4/C NFs, by contrast, exhibit bettercycling behavior than the others (Li2MnSiO4/C NPs andLi2MnSiO4 NPs) at room temperature, as shown inFig. 4b. The capacity retention of the Li2MnSiO4 NPs,Li2MnSiO4/C NPs, and Li2MnSiO4/C NF electrodeswere somewhat similar (between 63 and 77 % over20 cycles, with the highest value being for theLi2MnSiO4/C NFs), reflecting the fact that theamorphization which governs the capacity fade is simi-lar for all three. A capacity of over 160 mAh/g wasretained at the 20th cycle at room temperature, which issignificantly greater than that exhibited by Li2MnSiO4/PEDOT (140 mAh g−1) [27] at the same current densityand temperature, although the latter contains over twicethe carbon content in the silicate composite (i.e., 13 %carbon) and the same conductive additive fraction in the

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electrode. We note that cell temperature and currentdensity rate play an important role in the electrochem-ical performance of Li2MnSiO4 owing to the slow ki-netic response of the material (as discussed below), andso these factors are vital in any comparison. The con-fined geometry of Li2MnSiO4 particles in the carbonmatrix as well as the nano-sized dimension of theLi2MnSiO4 particles enables favorable mechanics duringcycling. The Li2MnSiO4/C NFs showed a much im-proved electrochemical performance compared to previ-ously reported carbon-coated Li2MnSiO4 nanoparticles[31]. The rate capabilities of these three electrodes werealso evaluated. The Li2MnSiO4 NPs, Li2MnSiO4/C NPs,and Li2MnSiO4/C NF electrodes deliver capacities (rel-ative to theoretical) of 25, 33, and 40 % at a 1 C rate,respectively (Fig. 4c). In I-V characteristic results(Fig. 5a), the Li2MnSiO4/C NF electrode shows a muchhigher electronic conductivity compared to those of bothNP electrodes. Although Li2MnSiO4/C NPs exhibited aslightly higher electronic conductivity than that ofLi2MnSiO4 NPs, the improvement is negligible. Theimpedance spectroscopy results also show the same

trend. These results support our contention that the 1Dgeometry and uniform carbon layer in the Li2MnSiO4/CNFs enable efficient and fast electron transport, resultingin a significant improvement in rate capability.

The electrochemical properties of Li2MnSiO4/C NFelectrodes were further investigated at high temperatureand various voltage windows (Fig. 6). The Li2MnSiO4/CNF electrode exhibits first charge and discharge capacities,respectively, of ∼305 mAh g−1 and ∼209 mAh g−1 at anelevated temperature (50 °C) and at a 0.05 C rate over thevoltage window of 1.5∼4.6 V vs. Li/Li+ (Fig. 6a). Thisrepresents only a slight increase in capacity owing toenhanced kinetics associated with both Li ion and electrontransport at the elevated temperature. This is in interestingcontrast to nanoleaflet or PEDOT composites ofLi2MnSiO4, where a much improved capaci ty(350 mAh g−1) has been reported at 45 °C, even higherthan theoretical (333 mAh g−1) for the two electron redoxcouple [27]. Whether this can be attributed to the “beaker-type cell” employed in the latter studies, which contain anexcess of electrolyte compared to a coin cell, or theirsignificantly slower current, density of C/50 is not clear.

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In our studies, the temperature certainly plays a role inlowering the polarization; however, the voltage plateau,which is over 4.2 V during charge at room temperature,is significantly lower at 50 °C. Despite the improvementin kinetics at the elevated temperature, Li2MnSiO4/C NFelectrode shows a poor cycling performance compared tothat at room temperature, which might be attributed to thedegradation of an active material and accelerated electro-lyte decomposition reactions expected at the highertemperatures.

The effect of the voltage window on the electrochem-ical performance was also monitored since the voltagerange is closely related to the degree of amorphization inLi2MnSiO4 and the electrolyte decomposition (Fig. 7).Charge and discharge capacities of Li2MnSiO4/C NFsincrease with the cut-off charge voltage. However, boththe cycling performance and the Columbic efficiency arenoticeably degraded with the increase of cut-off chargevoltage. These results imply that the improvement in theelectrochemical performance of the Li2MnSiO4-based elec-trode could be achieved by diminishing the electrolytedegradation and further improving the kinetics associatedwith carrier transport.

The galvanostatic intermittent titration technique(GITT) was employed to gain insight into the origin

of the enhanced electrochemical kinetics and to betterunderstand the effects of the morphology control of theactive material on the electrochemical properties.Figure 8a shows the overall transient voltage profilesof the three types of electrodes as a function of lithiumion content during the first cycle. Among the threetypes of electrodes, the Li2MnSiO4/C NF electrodeshows the greatest degree of lithium insertion/extraction. Figure 8b shows the enlarged voltage pro-files of the region marked in yellow dotted line inFig. 8a. It is evident that the Li2MnSiO4/C NF electrodeexhibits a much lower voltage plateau of ∼4.12 V vs.Li/Li+ than those of the other electrodes (Li2MnSiO4/CNPs ∼4.25 V and Li2MnSiO4 NPs ∼4.5 V). The totalchange in the cell voltage (ΔEτ) of the electrodesduring Li extraction for time τ was obtained bysubtracting the IR drop. The calculated ΔEτ ofLi2MnSiO4 NPs, Li2MnSiO4/C NPs, and Li2MnSiO4/CNF electrodes are 0.25 V, 0.31 V, and 0.33 V, respec-tively. A low value of ΔEτ indicates an improvedlithium chemical diffusion coefficient in the electrode.Internal resistances of the three samples were obtainedfrom the difference between the quasi-open-circuit volt-age (QOCV) and the closed circuit voltage (CCV) ineach transient step (Fig. 8c). Upon charging, the

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Cut-off voltage:2.0 ~ 4.8V2.0 ~ 4.7V2.0 ~ 4.6V

Cycle number

Cap

acity

(mA

h g-1

)

40

50

60

70

80

90

100

Cou

lom

bic

effic

ienc

y (%

)

0 50 100 150 200 250 300

1.8

2.4

3.0

3.6

4.2

4.8

Cut-off Voltage1.5 - 4.8V1.5 - 4.7V1.5 - 4.6V

Vol

tage

(V)

Li/L

i+

Capacity(mAh/g)

(a) (b)

(c) (d)

Fig. 7 a The first voltage profiles and b cycle performance measured over 1.5 V∼4.6, 4.7, and 4.8 V and c the first voltage profiles and d cycleperformance examined in the window 2.0 V∼4.6, 4.7, and 4.8 V for Li2MnSiO4/C NFs at a rate of 0.05 C and 25ºC

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Li2MnSiO4/C NF electrode shows a much lower resis-tance compared to those of the two NP electrodes.Electrochemical performance and GITT/impedance re-sults support our conclusion that the improvement inboth the mechanics and kinetics of the Li2MnSiO4 can

be achieved by the structural engineering of theelectrode.

Conclusion

In conclusion, Li2MnSiO4/C NFs were synthesized via asimplified electrospinning technique that coats the poly-mer solution on the mix of inorganic precursors,allowing the crystallization of pure Li2MnSiO4 silicate,concurrent with carbon matrix formation. The electronmicroscope images confirm the fibrous morphology andcarbon distribution in the fiber matrix as well as at thesurface. These unique features in the morphology ofelectrospun fibers lead to efficient electron transportalong the carbon-coated 1D geometry of Li2MnSiO4/CNF electrodes. I-V and impedance measurements con-firm facile transport properties of Li2MnSiO4/C NFs,compared to carbon-coated nanoparticles of the samematerial and show—confirming previous speculations—that this feature is uniquely responsible for improvingthe electrochemical properties of the nanofiber material.Our electrode design strategy could be applied to otherpolyanion cathode materials such as LiFePO4F,LiFePO4, and Li2FeSiO4 to achieve enhanced electro-chemical performance, which is currently underway inour laboratories.

Acknowledgment This work was financially supported by theNational Research Foundation of Korea (NRF) through GrantNo. K20704000003TA050000310, the Global Research Laboratory(GRL) Program provided by Ministry of Science, ICT (Informa-tion and Communication Technologies) and Future Planning in2013, the International Cooperation program of the Korea Insituteof Energy Technology Evaluation and Planning(KETEP) grantfunded by the Korea government Ministry of Trade, Industryand Energy (No. 2011 T100100369).

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