Synthesis and Performance of Li2MnSiO4 as an Electrode Material for Hybrid Supercapacitor...

7/30/2019 Synthesis and Performance of Li2MnSiO4 as an Electrode Material for Hybrid Supercapacitor Applications

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Journal of Electrochemical Science and TechnologyVol. 3, No. 2, 2012, 72-79http://dx.doi.org/10.5229/JECST.2012.3.2.72

72

Journal of Electrochemical Science and Technology

Synthesis and Performance of Li2MnSiO4 as an Electrode Mate-

rial for Hybrid Supercapacitor Applications

K. Karthikeyan, S. Amaresh, J.N. Son, and Y.S. Lee

Faculty of Applied Chemical Engineering, Chonnam National University, Gwangju 500-757, Korea

ABSTRACT:

Li2MnSiO4 was synthesized using the solid-state method under an Ar atmosphere at three different

calcination temperatures (900, 950, and 1000oC). The optimization of the carbon coating was also

carried out using various molar concentrations of adipic acid as the carbon source. The XRD pattern

confirmed that the resulting Li2MnSiO4 particles exhibited an orthorhombic structure with aPmn21space group. Cyclic voltammetry was utilized to investigate the capacitive behavior of Li2MnSiO4along with activated carbon (AC) in a hybrid supercapacitor with a two-electrode cell configuration.

The Li2MnSiO4/AC cell exhibited a high discharge capacitance and energy density of 43.2 Fg1 and

54 Whkg1, respectively, at 1.0 mAcm2. The Li2MnSiO4/AC hybrid supercapacitor exhibited an

excellent cycling stability over 1000 measured cycles with coulombic efficiency over > 99 %. Elec-

trochemical impedance spectroscopy was conducted to corroborate the results that were obtained

and described.

Keywords: Lithium manganese silicate, Activated carbon, Hybrid supercapacitors, Energy density,

Specific capacitance

Received May 10, 2012 : Accepted June 8, 2012

1. Introduction

Recently, an increasing interest has been devoted tothe development of supercapacitors because of theirpotential applications in many fields, such as hybridelectric vehicles (HEV), portable electronic devices,memory back-up systems, etc.1) Supercapacitors are

classified into two main categories based on the stor-age mechanism utilized, namely, electric double-layercapacitors (EDLCs) and pseudo-capacitors. EDLCswith various types of carbon based materials havebeen used as electrodes in order to utilize the double-layer capacitance. On the other hand, pseudo-capaci-tors with transition metal oxides or conducting poly-mers have been used as electrodes in order to utilizethe charge-transfer pseudo-capacitance. EDLCs offer

a high power density, a good reversibility and a longcycle life. Pseudo-capacitors have a much higherenergy density and a lower cycle life than EDLCs. Thehybridization of two types of electrodes to form a newcapacitor called a hybrid supercapacitor is a uniqueapproach that is used to enhance the energy densityand the power density of a single cell. One of the elec-

trodes is an energy source electrode (battery like elec-trodes) and the other terminal contains a power sourceelectrode (either an EDLC or a pseudo capacitor elec-trode). The choice of the energy source electrode isalso important because this electrode enhances the cellvoltage (especially in lithium ion batteries) withoutsacrificing the energy and power densities.1-5)

Numerous Li-ion intercalated host materials, suchas LiFePO4, Li4Ti5O12, LiMn2O4, LiNi1/3Co1/3Mn1/3O2,Li4Mn5O12, and LiCoO2, have been studied as elec-trode materials for hybrid supercapacitor applica-tions.2-10) Of the aforementioned energy source

Corresponding author. Tel.: +82-62-530-1904

E-mail address: [email protected]


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Journal of Electrochemical Science and Technology, Vol. 3, No. 2, 72-79 (2012) 73

electrodes, the Mn based materials have attractedattention because they are environmentally benign andcost effective with higher operating voltages. SpinelLiMn2O4 has been extensively studied as an electrodematerial for hybrid supercapacitors. However, capac-ity fading was encountered in the 3 V regions duringprolonged cycling because of the Jahn-Teller distor-tion that is associated with the average valence ofmanganese, which falls below +3.5 in LiMn2O4. Toovercome these issues, various metal ions, such asLiMxMn2xO4 (M = Ni, Cr, and Co), have been used assubstitutes and these new materials have been investi-gated as the cathodes for hybrid electrochemical

devices.10) Nevertheless, the capacity fading has beenminimized but not effectively prevented. Recently, anew family of lithium insertion hosts, i.e. Li2MSiO4(M = Fe, Mn, Co, and Ni), has attracted the attentionof researchers because of its appealing properties thatinclude cell safety, extraction of more than one Li ionwhich offers the theoretical capacity > 330 mAhg1,environmental friendliness, simple synthesis, and costeffectiveness. Orthosilicate group materials provideexcellent thermal stability through strong Si-O bond-ing.11-15) Due to said advantages, Li2MnSiO4 has beenchosen as the energy source electrode in this study for

the first time.A reversible exchange of up to two lithium ions per

formula unit (possible exploration of Mn2+/Mn3+ andMn3+/Mn4+ redox couples) is feasible, provided thatthe above advantages are exhibited, and the structureof the compound is stable.11-15) Previously, we firstlyreported the silicates based supercapacitors with acti-vated carbon (AC) electrodes in a conventional non-aqueous electrolyte (1 M LiPF6 in ethylene carbonate(EC)/dimethyl carbonate (DMC) (1:1 vol./vol.)),which presents a superior cycle performance between0.0-3.0 V at room temperature.16-17) In this study, we

present the synthethic process and electrochemicalproperties of the Li2MnSiO4 material and cell perfor-mance of the Li2MnSiO4/activated carbon (AC) elec-trode as a hybrid supercapacitor in more detail.

2. Experimental

The conventional solid-state reaction method wasemployed to prepare the Li2MnSiO4 powders. Highpurity LiOH.H2O (Junsei 95 %), MnCO3 (Aldrich,USA, 99.9 %), and SiO2 (Aldrich, USA, 99.9 %) wereused as the starting materials. Adipic acid (Aldrich,

USA, 99 %) was used as the carbon source during thesynthesis in order to improve the electric conductivity

of the Li2MnSiO4particles. Stoichiometric amounts ofthe starting materials were ground with adipic acid andheated at 400 oC for 4 h in air. The intermediate prod-uct was finely ground using a mortar and placed into atubular furnace at different calcination temperatures(900, 950, and 1000oC) for 12 h under an Ar atmo-sphere. The final calcination process was carried out toyield the Li2MnSiO4 powder. A similar procedure wasrepeated for the carbon coating with various concen-trations of adipic acid (0, 0.05, 0.1, and 0.2 mol. adipicacid against the total metal ions present in the com-pound). The graphical representation of the optimized

synthesis process is given in the Fig. 1.The thermogravimetric-differential thermal analy-sis (TG/DTA) was conducted using a thermal ana-lyzer system (STA 1640, Stanton Redcroft Inc., UK) ata rate of 5oCmin1 with a thin Pt plate that was used asthe sample holder. The structural properties ofLi2MnSiO4were studied using an X-ray diffractometer(XRD, Rint 1000, Rigaku, Japan) with a Cu-K radia-tion source. The morphological features were investi-gated using a scanning electron microscope (SEM, S-4700, Hitachi, Japan). The electrochemical profiles ofthe hybrid supercapacitor cell were obtained using a

Fig. 1. Schematic representation of the Synthesis of

Li2MnSiO4 powder.


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74 K. Karthikeyan, S. Amaresh, J.N. Son, and Y.S. Lee

CR 2032 coin cell at room temperature. The negativeelectrode was composed of 70% active materials(Li2MnSiO4), 20% conductive additive (ketjen black),and 10 % binder (teflonized acetylene black). Themixture was pressed on a 200 mm2 stainless steelmesh that served as the current collector at a pressureof 300 kgcm2 and dried at 160oC for 4 h in an oven.The same procedure was adopted for the preparationof the positive electrode, i.e. the AC electrode. Thecell was comprised of Li2MnSiO4 as the anode and anactivated carbon as the cathode, and the two electrodeswere separated by a porous polypropylene film (Cel-gard 3401). The electrolyte solution was 1.0 M LiPF6in an EC/DMC (1:1 v/v, Techno Semichem Co., Ltd,Korea) mixture. The cyclic voltammetry (CV) and theelectrochemical impedance spectroscopy (EIS) studieswere performed using an electrochemical analyzer(SP-150, Bio-Logic, France). The cycling studies werecarried out between 0.0-3.0 V with a current density of1.0 mAcm-2 using a cycle tester (WBCS 3000, Won-A-Tech, Korea) in the galvanostatic mode.

3. Results and Discussion

Fig. 2 shows the thermogravimetric-differential

thermal (TG-DTA) traces of the LiOH.H2O, MnCO3,and SiO2 mixture. The thermal traces were recorded at5oCmin-1 from 25 to 1000oC under an argon atmo-sphere. The thermogram clearly showed that multiplethermal events occurred during the analysis. The DTAcurve showed two endothermic events below 120oC,and the corresponding weight loss was also observed

in the TGA curve. The thermal events around 60o

C and100oC were caused by the removal of moisture duringthe loading of the sample and the dehydration of thewater molecules in the starting materials, respectively.In the TG curve, a sharp weight loss (~20%) wasobserved between 300 and 380oC along with a gradualweight loss up to 720oC. The observed thermal events(weight losses) were probably caused by the decom-position of the carbonate moieties that were present inthe starting materials.17,18) Around 720oC, a smallweight loss (~5%) was observed from the TG traces,corresponding to the formation of the Li2MnSiO4phase.15) Finally, a slow and continuous decrease in

the weight of the mixture in the temperature rangefrom 720-1000oC was ascribed to the solid-state reac-tion process. In this temperature range, the formationof highly crystalline Li2MnSiO4 and the existence ofan inconspicuous thermal event were observed.

Fig. 3 represents the powder X-ray diffraction pat-tern of nanocrystalline Li2MnSiO4 at different calcina-tion temperatures. These XRD patterns showed welldeveloped, sharp, and intense peaks that reflected thecrystalline nature of the synthesized Li2MnSiO4. Thecrystalline peaks were indicative of an orthorhombicstructure with aPmn21 space group. However, impu-

rity phases, such as MnO, were unavoidably present insamples calcined at all three temperatures. Similartypes of impurity phases were also exposed during thesynthesis of Li2MnSiO4 powders using various otherapproaches.11-15,20) Dompablo et al.21) succeeded inpreparing a Li2MnSiO4 material without MnO under

Fig. 2. Thermogravimetric-differential thermal (TG-DTA)

traces of the LiOH.H2O, MnCO3, and SiO2 starting

materials.

Fig. 3. X-ray diffraction pattern of Li2MnSiO4 powders

prepared at three different temperatures: (a) 900, (b) 950,

and (c) 1000oC.


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various pressure conditions. At the same time, the for-mation of Mn2SiO3 was inevitable under these abnor-mal pressure conditions. Previous reports and thepresent study indicate that the preparation of phasepure Li2MnSiO4 powder is complicated. Based on thebattery performance of the Li2MnSiO4 with differentcalcination temperatures, 900oC has been chosen forfurther characterization studies. The well knownDebye-Scherrer formula was adopted for the calcula-tion of the crystallite size at 900oC.

In this equation, is the crystallite size, is the X-ray wavelength ( = 1.54056 ), and is the fullwidth at the half maximum (FWHM) of the XRDpeak. From thi s calculation, the mean s ize of theLi2MnSiO4 powders was about 500 nm.

The X-ray diffraction pattern of carbon coatedLi2MnSiO4 compounds prepared by the solid-statereaction with different molar contents of AA is pre-sented in Fig. 4. The orthosilicate groups were suf-fered from some disadvantages like i nherentconducting properties, for instance Li2MnSiO4 (~10

14

Scm1) when compared to other commonly available

candidates like LiCoO2 (~104

Scm1

), LiNiO2 (~103

Scm1), LiMn2O4 (~106 Scm1) and LiFePO4 (~10

9

Scm1) at room temperature.22) Further, the poor con-ductivity issue restricts the possibility of using ortho-silicates in practical cells. A carbon coating is the

effective way to improve the electrochemical perfor-mance of the of the electrode materials, for exampleLiFePO4.

23,24) The source of carbon is also very impor-tant, particularly carboxylic (-COOH-) functionalgroups containing materials, for example carboxylicacids which act as the capping agent to prevent aggre-gation during synthesis. The choice of using adipicacid in the synthesis process is based on the successivecarbon coating and excellent improvement of electro-chemical performance of LiFePO4 in both solid stateand sol-gel routes by us.23,24) Here, the same strategyhas been applied to improve electrochemical perfor-mance of Li2MnSiO4 and the optimization of AA is

performed based on the total metal ions present in thenative compound. The calcination temperature wasfixed at 900oC, which is based on the temperatureoptimization. The occurrence of broad and well-defined Bragg peaks demonstrated the formation ofsize controlled and highly crystallized Li2MnSiO4. All

0.9

cos---------------=

Fig. 4. XRD patterns of Li2MnSiO4 obtained by various

molar ratios of adipic acid to total metal ions: (a) 0, (b)

0.05, (c) 0.1, and (d) 0.2.

Fig. 5. Scanning electron microscope images of

Li2MnSiO4 at 900oC. The molar ratio of adipic acid to total

metal ions is 0.2.


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XRD patterns of these materials obtained at variousmolar ratios of AA were well developed. Based on theoptimization, 0.2 mol. AA containing Li2MnSiO4exhibited the good lithium storage properties (not pre-sented in this paper) when compared to rest of the con-centrations and it composed of 0.52 wt % of carbonafter the synthesis process. The presence of the impu-rity phase MnO is also noticed along with Li2MnSiO4.Appearances of satellite peaks in the X-ray diffractionpattern are indicat ive of an orthorhombic structure,which is analogous to the Li3PO 4 pattern. TheLi2MnSiO4 can be isostructural to certain forms ofLi3PO4: Mn

2+ ions are present within a [SiO4] anionic

silicate network that replaces [PO4] the anionic phos-phate network, and two Li ions are available in 3Ddimensional channels. Further, we believe, the strongSi-Obond translates into very stable electrochemicaland thermal properties, which are important for safety.

The morphological features of the synthesizedLi2MnSiO4 at 900

oC with 0.2 mol. AA were analyzedusing scanning electron microscopy (SEM) and pre-sented in Fig. 5. The SEM image results signified theuneven size (sub-micrometer) and surface morphol-ogy of the Li2MnSiO4 particles that were synthesizedusing the solid-state reaction method. Furthermore, the

appearance of small-sized particles was also observedalong with the larger-sized particles. The SEM studycorrelated well with the crystallite size that wasobtained from the Debye-Scherrer calculations usingthe XRD measurements.

The electrochemical characterizations were per-formed using the cyclic voltammetry (CV) analysis

method with a two-electrode cell configuration. TheCV was an appropriate tool for estimating the differ-ence the Faradaic and non-Faradaic reaction pro-cesses.25) Furthermore, this technique was also used todetermine the preliminary power and energy densitiesof the supercapacitors. Fig. 6 shows the steady statecyclic voltammograms of the Li2MnSiO4/AC hybridsupercapacitor that were obtained from a potentialwindow of between 0.0-3.0 V at various scanningrates ranging from 1 to 25 mV s1. The shapes of theCV traces showed that the current responses dependedon the scanning rate. Additionally, the voltammo-grams maintained an almost rectangular shape with a

mirror image even at higher scanning rates, which cor-responds to excellent capacitive behavior for theLi2MnSiO4/AC hybrid supercapacitor. The CV tracesoverlapped at increased scanning rates. This overlap-ping effect showed that two different energy storagemechanisms were involved in the Li2MnSiO4/AChybrid supercapacitor.4,26) The surface charge accumu-lated at the electrode/electrolyte interface (activatedcarbon electrodes), and the Faradaic electron transferreaction and reversible phase transformation occurredduring the Li+ ion intercalation reaction (Li2MnSiO4electrode).22)

The cycling performance of the Li2MnSiO4/AChybrid supercapacitor was directly measured using thegalvanostatic mode between 0.0-3.0 V with a constantcurrent rate of 1 mAcm2 at room temperature. Thecapacitance (F) of the hybrid cell was calculated usingthe following equation, , whereIis thecurrent density (mAcm2), tis the discharge time (s),and Ccell is the total capacitance (F). The dischargecapacitance (Fg-1) was calculated using CS = 4Ccell/M,where M is the total weight (g) of the two electrodematerials used except for the current collectors.26,27) InFig. 7(a), the discharge curves of the Li2MnSiO4/AC

hybrid supercapacitor were symmetric with respect totheir charge counterpart, and the slopes of every curvewere almost independent of the potential, with thesame discharge capacity. An ohmic drop occurred dur-ing the discharge process because of the lower elec-tronic conductivity of Li2MnSiO4. This Li2MnSiO4/AChybrid supercapacitor exhibited a discharge capaci-tance (CS) of 43.2 Fg

1 during the first cycle, and analmost equivalent capacity was maintained thereafter.Fig. 7(b) represents the cycling profiles of theLi2MnSiO4/AC hybrid supercapacitor with respect tothe coulombic efficiency at a current rate of 1 mAcm2.

Ccell

I*t V=

Fig. 6. Cyclic voltammogram of the Li2MnSiO4/activated

carbon hybrid supercapacitor at various scanning rates: (a)

1 mVs1, (b) 3 mVs1, (c) 15 mVs1, and (d) 25 mVs1.


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The Li2MnSiO4/AC hybrid supercapacitor exhibitedan excellent cycling stability over 1000 measuredcycles with an excellent coulombic efficiency of over99%. The good cycling stability of the novelLi2MnSiO4 electrode that was utilized in the hybridsupercapacitor complemented the intrinsic stability ofthe AC electrode. Usually, the cycle life of aLi2MnSiO4 electrode mainly depends on the operating

rates and depth-of-discharge, which determine theavailable energy and power densities.25)

Rate performance studies were carried out for theLi2MnSiO4/AC hybrid supercapacitor. Current densi-ties ranging from 1 to 10 mAcm2 were used for theLi2MnSiO4/AC hybrid supercapacitor at room temper-ature. Initially, the Li2MnSiO4/AC hybrid supercapaci-tor was cycled 1000 times at 1 mAcm2 and subjectedto various current rates from 2 to 10 mAcm2 for 250cycles each. The energy and power densities corre-sponding to these current rates are graphically illus-trated in Fig. 8(a). The energy and power densities

were calculated for the initial cycle of each currentrate that was applied. As expected, the power densitylinearly varied with respect to the current rates. How-ever, the discharge capacitance and energy densitygradually decreased from 43.2 Fg1 to 29.6 Fg1 andfrom 54 to 37 Whkg1, respectively, as the currentdensity increased from 1 to 10 mAcm2 (Fig. 8(b)).The variations in the specific capacitance were

ascribed to the large ohmic drop, which led to smallcapacitance values, and the activation and concentra-tion polarizations at higher current densities, whichresulted in the low utilization of the active materi-als.4,26) At high current rates, the lithium ions do nothave enough time to enter into the core of the materialwhich also causes the reduction in discharge capaci-tance values.26)

Electrochemical impedance spectroscopy (EIS) wasemployed in order to obtain information about thesupercapacitor performance. Fig. 9 shows a typicalNyquist plot for the Li2MnSiO4/AC hybrid superca-

Fig. 7. (a) Charge-discharge curves of the Li2MnSiO4/

activated carbon hybrid supercapacitor at 1 mAcm2. (b)

Discharge capacitance and coulombic efficiency as a func-

tion of the cycle number.

Fig. 8. (a) Energy and power densities with respect to

different current densities. (b) Specific capacitance vs.

cycle number of the Li2MnSiO4/activated carbon hybridsupercapacitor (the values in the figures correspond to the

current densities in mAcm2).


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pacitor cell at an open circuit voltage in the frequencyrange of 100 MHz to 100 mHz. From the Nyquist plot,the first intersection point on the real axis of the spec-trum in the high frequency region corresponded to thebulk solution resistance of the electrolyte (Rs). AfterRs, a semicircle appeared with a second intersectionpoint on the real axis at medium frequencies. In thisregion of high frequency to medium frequencies,

called the kinetic control regime, only small amountsof electron transfer overcame the activation energy inorder to migrate with the alternating potential. Further-more, the semicircle developed moving across theHelmholtz plane, and was represented by the interfa-cial contact capacitance (Cc) and charge transfer resis-tance (Rc). In the adequately low frequency range, theconstantly decreasing frequency provided enough timefor the charge-complexes in the diffusion layer toovercome the activation energy and move closer to theHelmholtz plane. These charge-transfers ultimatelyoccupied all of the available surfaces of the AC elec-

trodes and contributed to the electrical double layercapacitance (Cdl). The low frequency range, where Cdlcontributed, is called the diffusion control regime. Acapacitance value of 40 Fg-1 was obtained from theNyquist plot for the hybrid cell using the followingrelationship, , where f, Z'', and m arethe frequency, the impedance of the imaginary compo-nent, and the mass of the active material, respectively.Generally, the EIS measurements give rise to slightlylower capacitance values than the cycling studiesbecause the alternating current penetrates into theelectrode bulk with more hindrance.30)

4. Conclusion

Novel Li2MnSiO4 material was synthesized at threedifferent calcinations temperatures under an Ar atmo-sphere using a solid-state reaction method. The XRDpatterns confirmed that the synthesized Li2MnSiO4particles exhibited an orthorhombic structure with aPmn2 1 space group. The crystallite size of theLi2MnSiO4particles lied in the submicrometer rangeand was correlated with SEM and Scherrer analysis.The capacitive performance of the Li2MnSiO4 elec-trode was substantiated through cyclic voltammetrywith an activated carbon electrode in a single cell. The

cycling stability of the Li2MnSiO4/AC hybrid superca-pacitor was investigated over 2250 cycles galvanostat-ically. The Li2MnSiO4/AC cell exhibited dischargecapacitances of 43.2 and 29.6 Fg1 and energy densi-ties of 54 and 37 Whkg1 at low (1 mAcm2) and high(10 mAcm2) current rates, respectively.

Acknowledgements

This work was supported by the Priority ResearchCenters Program through the National Research Foun-dation of Korea (NRF) funded by the Ministry of Edu-

cation, Science and Technology (2009-0094058).

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