Enhanced rate performance and cycling stability of a CoCO3–polypyrrole composite for lithium ion battery anodes

Journal ofMaterials Chemistry A

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Key Laboratory for Liquid-Solid Structural E

of Education), Shandong University, Jinan, 2

com; Fax: +86 53188392315; Tel: +86 1362

† Electronic supplementary information (Nyquist plots, and subsequent detailedelectrodes aer cycling. See DOI: 10.1039/

Cite this: DOI: 10.1039/c3ta12227a

Received 8th June 2013Accepted 12th July 2013

DOI: 10.1039/c3ta12227a

www.rsc.org/MaterialsA

This journal is ª The Royal Society of

Enhanced rate performance and cycling stability of aCoCO3–polypyrrole composite for lithium ion batteryanodes†

Zhaojun Ding, Bin Yao, Jinkui Feng and Jianxin Zhang*

A CoCO3–polypyrrole composite (CC–PPy) for lithium ion battery anodes was prepared by first synthesizing

urchin-like CoCO3 microspheres (CC) via a hydrothermal route and further modifying them with a PPy

coating. The resulting CC–PPy exhibits excellent cycling stability, outstanding rate performance and a

great recovery capability compared to CC, delivering a reversible capacity of 1070.7, 811.2, 737.6, 518.7,

504.5 and 559 mA h g�1 after 100 cycles at 0.1, 1, 2, 3, 4 and 5 C, respectively, and a recovery capacity

of up to 1787 mA h g�1 after 500 cycles from 1 to 5 C. A more comprehensive lithium storage

mechanism of CoCO3 has been proposed to support the experimental data, which includes two-step

conversion reactions with a total theoretical value of 7 Li per CoCO3. The ‘first-order’ reaction involves

reduction of CoCO3 to metallic Co and the formation of Li2CO3, and the second reaction involves the

further reduction of Li2CO3 to LixC2 (x ¼ 0, 1, 2), along with the formation of Li2O. The lithiation and

delithiation processes of CC and CC–PPy have been compared based on their potential profiles and CV

curves, which show clear two-order character. The kinetic factors for the superior performance of

CC–PPy are analyzed based on the Nyquist plots. Furthermore, the transition from CoCO3 to Li2CO3 to

Li2O and its reversibility is confirmed by ex situ IR spectra recorded at the different discharge–charge

states of CC–PPy.

1 Introduction

With the ourishing development of lithium ion batteries (LIBs)in areas from portable electronic devices to electric vehicles,there is high demand on their portability and larger energydensity, which can not be reached by the current commercialgraphite anode, with a theoretical capacity of only 372 mA h g�1

and low density. Accordingly, transition metal oxides, such asFe3O4 and Co3O4,1,2 have been intensely investigated ascompetitive anodes due to their high theoretical capacity (650–1050 mA h g�1) and density (three times that of carbon). Incontrast to the intercalation mechanism of graphite, transitionmetal oxides are reversibly reduced in a conversion reaction tometal nanoparticles (1–5 nm), with the formation of lithiumoxide.3

The electrochemical conversion reactions have beensuccessfully extended to other compounds, such as suldes,4

nitrides,5 phosphides6 and uorides,7 as well as metal carbon-ates, in recent years. Chowdari reported the cycling behavior of

volution Processing of Materials (Ministry

50061, China. E-mail: jianxinsdu@gmail.

5313936

ESI) available: TEM images of CC–PPy,discussion, of the CC and CC–PPy

c3ta12227a

Chemistry 2013

Nano-(Cd1–3Co1–3Zn1–3)CO3, and rst showed the reversibleconversion reaction of carbonates, which involves the forma-tion and decomposition of Li2CO3, accompanying the reductionand oxidation of metal nanoparticles.8 Later, Tirado studied theelectrochemical performance of MnCO3 and Mn1�xCoxCO3

electrodes.9 The latter displayed slightly better cycling stabilityat high rates.

Similar to most conversion anodes, metal carbonates havepoor electronic conductivity and may suffer from severe volumevariation and particle aggregation with cycling, thus leading tothe disintegration of the electrode and rapid capacity fading.Carbonaceous materials and conductive polymer coatings havebeen reported to be effective ways to overcome these prob-lems.10–12 Recently, Su effectively enhanced the cycling stabilityof the CoCO3 anode by mixing it with graphene nanosheets(GNS).13 The CoCO3–GNS composite delivered a capacity of930 mA g�1 aer 40 cycles at 50 mA g�1, and the value decreasedto 680 mA h g�1 at 500 mA g�1. Remarkably, the author rstattributed the extra capacity over the theoretical capacity(�450 mA h g�1 for 2 Li storage) to the further reduction of C4+

in CO32� to C0 or other low-valence C in the catalysis of newly-

generated Co nanoparticles.It is clear that modied CoCO3 is a potential anode material

for LIBs. However, there are no reports of its cycling perfor-mance for more than 100 cycles and its capacity retention at

J. Mater. Chem. A

http://dx.doi.org/10.1039/c3ta12227a

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Journal of Materials Chemistry A Paper

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current densities beyond 1000 mA g�1. Moreover, its lithiumstorage mechanism needs a more detailed investigation. In thiswork, urchin-like CoCO3 microspheres were rst prepared by ahydrothermal method and further modied by a polypyrrole(PPy) coating. The CoCO3–PPy composite exhibits signicantlyenhanced cycling stability and rate performance (from 1 to 5 C)and great recovery capability relative to the pristine sample,and appears to be a very promising anode material for LIBs.Furthermore, a more comprehensive lithium storage mecha-nism of CoCO3 was proposed to support the excellent electro-chemical data, which consists of two-order conversion reactionswith a total theoretical value of 7 Li per CoCO3.

2 Experimental2.1 Synthesis of urchin-like CoCO3 microspheres

Urchin-like CoCO3 microspheres (abbr. CC) were prepared via ahydrothermal route. CO(NH2)2 (3.003 g) and CTAB (0.1 g) wereultrasonically dissolved in 60 mL of DI water. 20 mL of 0.5 MCoSO4$7H2O solution was then added dropwise to the ureasolution. Aer magnetic stirring for 10 min, the solution wastransferred to a 100 mL Teon-lined stainless-steel autoclaveandmaintained at 120 �C for 12 h. Aer cooling down naturally,the product was ltered and successively washed with water,isopropanol and ethanol. Finally, rose-red CoCO3 powder wasobtained by vacuum drying at 50 �C for 12 h. All of the chem-icals were AR grade and were supplied by Sinopharm ChemicalReagent Co., Ltd without further purication.

2.2 Synthesis of PPy-coated CoCO3 composite

The PPy-coated CoCO3 composite (abbr. CC–PPy) was preparedusing a chemical polymerization method. Briey, 0.04 g SDS(sodium dodecyl sulfate) was rst dissolved in 400 mL of DIwater. The obtained CC power (0.2 g) was then dispersed in theSDS solution by ultrasonication for 5 min and vigorous stirringfor 2 h. Then, pyrrole monomer (0.4 g) was added into thesuspension and stirred for 1 h. Polymerization was initiated byadding 6 mL of 0.2 M FeCl3 solution dropwise, and was allowedto proceed for 12 h at room temperature under stirring. Theblack CC–PPy precipitate was collected by ltration and rinsedwith DI water and ethanol several times. The nal product wasdried at 50 �C under vacuum for 12 h.

Fig. 1 (a) XRD patterns of CC and CC–PPy. (b) FTIR spectra of the prepared PPyand CC–PPy, and commercial CoCO3$xH2O.

2.3 Characterization

The crystal phases of the samples were identied by X-raydiffraction analysis (XRD) on a Rigaku Dmax-rc diffractometerwith Cu Ka radiation (l ¼ 0.154 nm). The morphologies werecharacterized by eld-emission scanning electron microscopy(FESEM; Hitachi SU-70) equipped with energy dispersive X-rayspectroscopy (EDX; Oxford), and high-resolution transmissionelectron microscopy (HRTEM; JEM-2100, 200 kV). Fouriertransform infrared (FTIR) spectra were recorded usingKBr pellets on a Bruker Vector 22 spectrophotometer.The thermogravimetric analysis (TGA) was performed on aPerkin-Elmer TGA-7 Thermo Analyzer under air ow at a rateof 10 �C min�1.

J. Mater. Chem. A

2.4 Electrochemical measurements

Coin-type cells (2025) were assembled in an argon-lled glove-box (Vigor, China) using lithium foil as the counter and refer-ence electrodes, Celgard 2325 as the separators, and a 1.0 Msolution of LiPF6 in ethylene carbonate (EC)–dimethylcarbonate (DMC)–ethylene methyl carbonate (EMC) (1 : 1 : 1 involume) as the electrolyte. The working electrodes were fabri-cated by mixing the sample powders, carbon black and poly-vinylidene uoride (PVDF) in a weight ratio of 65 : 20 : 15 in aN-methyl-2-pyrrolidinone (NMP) solvent. The resulting slurrywas pasted on the copper foil and vacuum dried at 120 �C for 9h. The active material loading density of the electrodes was0.2–0.4 mg cm�2. The cells were galvanostatically dischargedand charged between 0.01 and 3.0 V on a land CT2001A batterytester, with the current density ranging from 100 to 5000 mAg�1. Cyclic voltammograms (CV) and electrochemical imped-ance spectroscopy (EIS) measurements were performed on anAmetek PARSTAT 2273 electrochemical workstation. The CVswere recorded in the range of 0.01–3.0 V at a scan rate of 0.1 mVs�1 while EIS was carried out in the frequency range of 100 kHzto 0.01 Hz with an excitation voltage of 10 mV.

3 Results and discussion3.1 Phase and morphology analysis of CC and CC–PPy

The phase composition of CC and CC–PPy was analyzed by XRDand FTIR spectra. The main diffraction peaks of CC and all ofthe peaks of CC–PPy can be indexed to rhombohedral CoCO3

This journal is ª The Royal Society of Chemistry 2013


Paper Journal of Materials Chemistry A

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(JCPDS no. 78-0209), as shown in Fig. 1a. In addition, a smallamount of Co(CO3)0.5(OH)$0.11H2O (JCPDS no. 48-0083) isobserved as theminor phase of CC, which disappears in CC–PPyprobably due to its sensitivity to the slightly acidic environmentof the polymerization system. For comparison, pure PPy nano-particles were prepared in a similar manner to that describedabove for CC–PPy but without the addition of CC. The IR spectraof the prepared PPy, CC–PPy and commercial CoCO3$xH2O(Aladdin, 99%) are given in Fig. 1b. The spectrum of PPy showsthe typical bands of polypyrrole,14 containing pyrrole ringvibrations at 1474 and 1552 cm�1, ]C–H in-plane vibrationsat 1038, 1197 and 1300 cm�1, and out-of-plane vibrations at795 and 920 cm�1. Meanwhile, the spectrum of commercialCoCO3$xH2O exhibits clear ngerprint peaks of CO3

2� at 742(v4(E0 0)), 862 (v2(A

0 01 )), 1072 (v1(A

01)) and 1404 (v3(E0)) cm�1.15–18 As

marked in Fig. 1b, the CC–PPy spectrum shows characteristicpeaks of both CoCO3 and PPy, providing direct evidence for thesuccessful formation of the CoCO3–polypyrrole composite.

SEM images of CC and CC–PPy are shown in Fig. 2. CCexhibits an urchin-like structure, in which each microsphere(3–5 mm) is composed of numerous radial nanorods withdiameters in the range of 20–50 nm (Fig. 2a and d). The subtlemicro–nanostructure is favorable for lithium diffusion due to ahigh electrode–electrolyte contact area and a short transportpath. However, the urchin-like structure could barely beobserved aer the PPy coating. As shown in Fig. 2b, c and e,CC–PPy is composed of microsized spheroids, whose surfacesare closely wrapped with PPy nanoparticles, forming a CoCO3–

PPy core–shell structure. Close observation of the rectangulararea in Fig. 2c reveals that the inner PPy particles closer to thecore have a smaller diameter (<50 nm) than the outer particles(�100 nm) (Fig. 2f). Due to the deep coating layer, it is difficultto observe the clear microstructure of the CoCO3 core from theTEM images (Fig. S1, ESI†). Even so, a rarely-observed micro-sphere with a bare CoCO3 core (the inset of Fig. 2e) reveals thatthe inner core still retains a nanorod structure. The rods nolonger radiate outwards but instead stack together. Signs of the

Fig. 2 (a and d) Low-magnification and enlarged SEM images of CC. (b, c and e) Lowarea in (c). The inset in (e) shows the SEM image of an individual CC–PPy microsphe


nanorods of the core could also be observed from the peripheryof the spheroids, as shown in Fig. S1d, ESI.† Moreover, the PPycoating layer can be visibly observed in Fig. S1b and c, ESI,†where the inset SAED patterns indicate the amorphous char-acter of the peripheral regions of the composite particle inFig. S1a, ESI.† The preserved micro–nanostructure and the PPyshell may well facilitate the transport of Li-ions and electrons inthe CC–PPy electrode and contribute to its enhanced electro-chemical performance, as discussed later.

3.2 EDX and TGA analysis of CC–PPy

Fig. 3 shows element maps (b–e) and the EDX spectrum (g) ofCC–PPy corresponding to the rectangular area in (a). Theelemental mapping shows the concordance of the distributionof C, O, Co and N, the peaks of which are clearly identied inFig. 3g. The peak for Pt is due to the coated conductive Pt layerfor SEM observation. Sparsely distributed N originates from thePPy layer, while equally and densely distributed Co and Oconrms the existence of CoCO3. Note that there is a cleardensity contrast of C between the periphery and center of thedesignated semisphere. This again strongly supports theCoCO3–PPy core–shell structure since both the CoCO3 core andPPy layer contains C but the content of the latter is higher.Fig. 3f presents the TGA curve of CC–PPy to quantify the amountof PPy. The rst weight loss (2.51%) is due to the elimination ofadsorbed water. The second weight loss is 36.84%, whichresults from both the decomposition of CoCO3 to Co3O4 and theoxidation of PPy.12,16 Based on the theoretical weight loss of32.5% from CoCO3 to Co3O4, the amount of PPy can be calcu-lated to be �7.84 wt%.

3.3 Cycling and rate performance of CC and CC–PPy

The cycling and rate performance of CC and CC–PPy arecompared in Fig. 4a and b, respectively. The rst discharge–charge capacities of CC and CC–PPy are 1816.6/1235.9 and2300.3/1382.6 mA h g�1, with coulombic efficiencies of 68% and

-magnification SEM images of CC–PPy. (f) Enlarged SEM image of the rectangularre with a bare CoCO3 core.

J. Mater. Chem. A


Fig. 3 (a) SEM image of CC–PPy. Element maps for (b) C, (c) O, (d) Co and (e) N, and (g) EDX spectrum of the rectangular area in (a). (f) TGA curve of CC–PPy.


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60.1%, respectively (Fig. 4a). The larger irreversible capacity ofCC–PPy (917.7 mA h g�1) over CC (580.7 mA h g�1) may beattributed to increased lithium consumption for SEI formation(solid electrolyte interphase) due to the PPy coating, as well assome irreversible capacitive Li storage in the initial discharge,as discussed later. The low efficiency of CC is limited to the 1st

cycle, and the 2nd cycle efficiency rises to 98.9%. The 2nd and 3rd

discharge capacities are 1237.2 and 1263.7 mA h g�1, respec-tively. Unlike the rapidly increased efficiency of CC, the effi-ciency of CC–PPy increases gradually to 95.6% in the rst 8cycles. Accordingly, the discharge capacity decreases graduallyto 1067.7 mA h g�1 by the 8th cycle, which is lower than thecorresponding value of CC (1175.9 mA h g�1). However, thecapacity of CC drops rapidly from the 8th cycle, while that ofCC–PPy tends to increase and rises up to 1281.6 mA h g�1 at the45th cycle, indicating the gradual inltration of the electrolyteinto the inner active matter due to the PPy coating. Theremaining values of CC and CC–PPy at the 100th cycle are 361.2and 1070.7 mA h g�1, respectively. Obviously, CC–PPy exhibits amuch better cycling stability compared to CC. The improvementmainly results from the tightly packed PPy layer. It acts as notonly a good electronic conductor, but also a exible cushion,which effectively accommodates the volume change during the

J. Mater. Chem. A

repeated cycling and avoids the disintegration of the electrode,thus ensuring the stable capacity of CC–PPy.

The rate capability of CC–PPy also signicantly exceeds thatof CC (Fig. 4b). Aer two activation cycles, CC–PPy shows alower rst efficiency (81.9%) relative to CC (93.2%) at 0.2 C.However, its capacity drops slightly at 0.2 C (from 1024.6 to936.8 mA h g�1) while that of CC drops rapidly (from 1171.4 to876.1 mA h g�1). The rst efficiencies of CC–PPy and CC at thefollowing rates of 0.4, 0.6, 0.8, 1 and 2 C are 90.8/80.4, 97/87.2,96.9/93, 97.6/94.6 and 89.7/70.6%, respectively. Clearly, CC–PPypresents excellent capacity retention at each rate step relative toCC. Accordingly, its capacity from 0.4 to 1 C changes a little andstabilizes at �800 mA h g�1, whereas that of CC decreasesgradually from 486.6 to 351.4 mA h g�1. The remaining valuesfor CC–PPy and CC at 2 C are 671.1 and 187.8 mA h g�1,respectively. As the current rate is reduced to 0.1 C, the capacityof CC–PPy recovers to its initial level at 0.1 C (1191.9 mA h g�1)while that of CC recovers to the level at the end of 0.2 C (866.3mA h g�1), conrming the superior electrode integrity ofCC–PPy with cycling. As expected, CC–PPy shows better cycla-bility than CC in the subsequent 58 cycles of 0.1 C, with anal value of 1170.5 mA h g�1, while the nal value for CC is701.1 mA h g�1. Surprisingly, compared to the galvanostatic



Fig. 4 (a) Cycling performance of CC and CC–PPy at 0.1 C (1 C¼ 1000mA g�1) for 100 cycles. (b) Cycling behavior of the electrodes at various rates from 0.1 C to 2 C. (c)The discharge–charge performance of CC–PPy at high rates from 1 C to 5 C (100 cycles per rate) and its recovering capability on returning to 0.1 C.


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performance in Fig. 4a, CC exhibits an exceptional recoverycapability and better cycling stability at the recovered 0.1 C,which is probably due to less damage to the electrode integritycaused by high-rate cycles than the low-rate cycles, and thestabilization effect of a complete SEI lm formed in therepeated cycling, as discussed in the ESI.†

To further investigate the high-rate performance of CC–PPy,the electrode was rstly activated at rates from 0.1 to 0.8 C in 14cycles, then tested at rates from 1 C to 5 C for 500 cycles (100cycles per rate), followed by a return to 0.1 C for 24 cycles(Fig. 4c). Note that the rst efficiency at 2, 3, 4 and 5 C (89.7,92.6, 95.2 and 96.7%, respectively) becomes higher and higher.With the gradual inltration of the electrolyte, the capacityincreases from 582.4 to 811.2 mA h g�1 while cycling at 1 C for100 cycles. The initial and nal discharge capacities at 2, 3,4 and 5 C are 767.2/737.6, 672/518.7, 475.2/504.5 and 458.6/


559 mA h g�1, respectively. Except for a certain decay at 3 C, thecapacity at the other rates is stable overall and even rises withcycling. Even when the current density reaches 5000mA g�1, theelectrode still delivers a reversible capacity of 559 mA h g�1,which is much higher than the theoretical value of commercialgraphite (372 mA h g�1). Moreover, CC–PPy exhibits a greatrecovery capability as the rate returns to 0.1 C. The 2nd dischargecapacity at the recovered 0.1 C is 1571.9 mA h g�1, the highestvalue is 1787 mA h g�1 and the nal value is 1606 mA h g�1.

3.4 Analysis of the Li-storage mechanism of CC and CC–PPybased on the potential proles and CV curves

The possible conversion reactions of CC and CC–PPy based onthe conventional mechanism are shown in eqn (1) and (2).Considering the fraction of CoCO3 in CC and CC–PPy is �70 at

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% (as discussed later) and �92 wt% respectively, the theoreticalcapacity of CC and CC–PPy for 2 Li storage is �463.6 and415.3 mA h g�1, respectively. However, the largest galvanostaticcapacity of CC (1263.7 mA h g�1) and CC–PPy (1281.6 mA h g�1)is about three times higher than the theoretical value (Fig. 4a),and the largest recovery capacity of CC–PPy (1787 mA h g�1) ismore than four times higher than the theoretical value (Fig. 4c).Obviously, the conversion mechanism of the two samples is notlimited to eqn (1) and (2) but involves certain subsequentreactions. Su attributed the extra capacity to the furtherconversion of Li2CO3 to low-valence C, along with the formationof Li2O.13 In addition, Eshkenazi suggested eqn (3) to explainwhy the highest Li2O and Li2C2 content in the SEI was accom-panied by the lowest carbonate concentration.19 Additionally,LiC2 was always observed in the lithiated graphene anode.20,21

Based on the previous work, we propose a new conversionmechanism of Li2CO3 in eqn (4), where the theoretical Listorage per Li2CO3 is 4, 4.5 and 5 for x¼ 0, 1 and 2, respectively.The theoretical capacity of CC and CC–PPy is thus 1448.7 (for6.25 Li storage) and 1451.2 mA h g�1 (for 7 Li storage). Here, eqn(1) and (2) are named as ‘rst-order’ reactions while eqn (4) are‘second-order’ reactions. As shown in Fig. 4c, the recoverycapacity of CC–PPy far exceeds its initial value at 0.1 C, and evenexceeds the theoretical value for 7 Li storage. The enhancementprobably results from electrode activation via multiple cycling,which triggers a further degree of ‘second-order’ reactions witha larger assignment of x. The extra capacity may be ascribed toreversible capacitive lithium storage as discussed later.

CoCO3 + 2Li+ + 2e� 4 Co + Li2CO3 (1)

Co(CO3)0.5(OH) + 2Li+ + 2e� 4 Co + 0.5Li2CO3 + LiOH (2)

CO32� + 5e� + 7Li+ 4 3Li2O + 0.5Li2C2 (ref. 19) (3)

Li2CO3 + (4 + 0.5x)Li+ + (4 + 0.5x)e� 4

3Li2O + 0.5(LixC2) (x ¼ 0, 1, 2) (4)

Cyclic voltammetry (CV) is a useful tool for investigatingelectrochemical redox reactions. As shown in Fig. 5, the CVcurves of the two samples agree well with their discharge–charge proles, showing good coincidence with the above-mentioned two-order reactions. To help illustrate the reactionmechanisms, the discharge–charge proles were roughlydivided into several regions based on the integral area ratio ofthe CV peaks.

Three cathodic and two anodic peaks are observed in the rstscan of CC (Fig. 5a), labeled with 1 (0.96 V), 2 (0.64 V), 3 (0.01 V),4 (1.39 V) and 5 (2.15 V), and corresponding to regions ab, bc,cd, fg and gh in Fig. 5c, respectively. Peak 1 mainly resultsfrom ‘rst-order’ reactions. The initial slightly-rising platform(�140 mA h g�1) in region ab probably corresponds to theconversion of Co(CO3)0.5(OH), while the remaining part corre-sponds to that of CoCO3, whose fraction in CC was thus esti-mated as �70 at%. Peak 2 and 3 (region bd) can be mainlyattributed to ‘second-order’ reactions and SEI formation,which both contribute to the plateau in region bc (�500 mA hg�1). However, a larger portion of ‘second-order’ reactions is

J. Mater. Chem. A

considered to be below the plateau. Besides, peak 3 alsoinvolves some capacitive Li storage, which is reversible in theinitial region of the following charge process (region ef in Fig. 5cand e0f0 in Fig. 5a), including pseudocapacitance behavior onthe surface of the polymeric gel lm and interfacial Li-storagewithin the Co–Li2O–LixC2 nanocomposite.22,23 The two anodicpeaks correspond to ‘second-order’ and ‘rst-order’ reversereactions. According to the area ratio, the ‘second-order’ reversereactions probably not only lie in peak 4 (region fg) but alsoinvolve peak 5 (gh), in which ‘rst-order’ reverse reactionsoccur, and the product may be a mixture of CoCO3,Co(CO3)0.5(OH) and Co(OH)2. Namely, the ‘second-order’reverse reactions possibly occurred in two steps, with a largeproportion in peak 4 and the remainder in peak 5.

In the second scan of CC, the ‘rst-order’ peak shis rightand splits into two peaks (1.53 and 1.2 V). This agrees well withthe increased discharge potential and the two small slopedplateaus observed in region ij (Fig. 5c). The former is typical forconversion anodes due to the nano-sized and pseudo-amor-phous character of the electrode created during the rstdischarge,3 while the latter probably results from the higherconversion potential of Co(OH)2 and Co(CO3)0.5(OH) thanCoCO3. Because the SEI lm of CC is mainly formed in the rstcycle, the ‘second-order’ peaks clearly shrink in the secondscan, and were again marked as peak 6 (0.77 V) and 7 (0.01 V),approximately corresponding to regions jk and kl. The anodicpeak potentials appeared to be nearly invariable in the secondscan. However, the peak area shrinks clearly, while a new peakappears at 3.0 V. The trend becomes more obvious in the twosubsequent scans, indicating the second and rst order oxida-tion potentials all shi right with the scanning, probably withmore ‘second-order’ reverse reactions occurring in peak 5 andmore ‘rst-order’ reactions occurring in the new peak. Corre-spondingly, the ‘rst-order’ reduction peaks shi le with thescanning. The results show an obvious electrode polarization ofCC with cycling. This is particularly apparent in the inset ofFig. 5c, where the discharge capacities at the 3rd, 10th, 20th, 50th,and 100th cycle are 1263.7, 1088.7, 770.2, 631.4 and 361.2 mA hg�1, respectively.

Compared to CC, very different CV curves and discharge–charge proles were obtained for CC–PPy. As shown in Fig. 5b,its rst reduction scan consists of a sloped region at 2.8–1.58 V(A0B0), a weak and broad band at�1.3 V (i), a strong peak at 0.6 V(ii) and an end peak at 0.01 V (iii), corresponding to the AB, BC,CD and DE regions in Fig. 5d, respectively. Region A0B0 mayoriginate from another type of capacitive lithium deposition (onthe surface of nano-sized PPy or between the interface of the PPyshell and the CoCO3 core) with poor reversibility, which shrinksin the following scans. Peak i may result from lithium interca-lation in nanostructured CoCO3 before the ‘rst-order’ conver-sion reactions.24 Peaks ii and iii (region CE) could be ascribed to‘rst-order’ and ‘second-order’ conversion reactions and SEIformation, while the latter involves some reversible interfacialdeposition. Similar to CC, ‘second-order’ reactions and SEIformation probably both exist above and below the plateausimultaneously. Clearly, due to the elimination ofCo(CO3)0.5(OH)$0.11H2O, the two-order reduction potentials



Fig. 5 CV curves of (a) CC and (b) CC–PPy at a scan rate of 0.1 mV s�1. Discharge–charge profiles of (c) CC and (d) CC–PPy at 0.1 C.


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of CC–PPy become closer to each other relative to CC, tending tocombine into one strong peak or one plateau, rather than twopeaks, in the rst discharge. In the second scan, peak ii shisright and splits into two peaks (1.07 and 0.7 V), which could beassociated with ‘rst-order’ and ‘second-order’ reactionsrespectively, corresponding to region FG in Fig. 5d, andbecoming one peak at 0.8 V in the following scans.

The two anodic peaks (iv and v) of CC–PPy are much closerthan those of CC in the rst scan, probably with a larger ratio of‘second-order’ reverse reactions occurring in peak v. It appearsthat the two peaks become closer to each other in the subse-quent scans, without the polarization behavior observed for CC(Fig. 5a). This indicates a lower resistance, and thus the stablecycling performance of CC–PPy, which can be more clearlyobserved in the inset of Fig. 5d. The discharge–charge prolesof CC–PPy in the 3rd, 10th, 20th, 50th and 100th cycles are verysimilar to each other, with discharge capacities of 1201.2,1089.9, 1079.6, 1233.9 and 1070.8 mA h g�1, respectively,showing superior reversibility. Furthermore, more facile elec-trode kinetics of CC–PPy relative to CC were demonstrated bythe Nyquist plots of the two electrodes (Fig. S2, ESI†). A detaileddiscussion of the Nyquist plots is given in the ESI.†

3.5 IR analysis of the CC–PPy electrodes at differentdischarge–charge states

To further demonstrate the above lithiation mechanism ofCoCO3 and its reversibility, the phase evolution of the CC–PPyelectrode along with the rst discharge–charge process wasinvestigated by ex situ IR spectroscopy (Fig. 6). In addition, the


formation and variation of the SEI components were alsoanalyzed.

It is obvious that the IR spectrum of the CC–PPy electrodeaer discharge to 0.96 V (Fig. 6a) is very different from that ofthe pure sample (dashed line). Almost all of the typical bands ofPPy (marked with ‘|’) can not be identied (except the band at1044 cm�1), while the bands of CO3

2� are preserved, includingthe bands at 745, 866, 1072–1082 and 1400–1510 cm�1. BothCoCO3 and Li2CO3 contain CO3

2�, however there are distinctdifferences in their IR spectra, especially the peak shapes andrelative absorption intensities in the 400–700, 745–866, and1400–1510 cm�1 regions. Firstly, the Co–O vibration of CoCO3

(SDBS (spectral database for organic compounds) no. 16176)is centered below 400 cm�1 and returns to the baseline at�500 cm�1. The absorption intensity at 400 cm�1 is much lessthan that at 866 cm�1.25 In contrast, the characteristic peaksplitting at 420 and 498 cm�1 for Li–O vibration of Li2CO3 (SDBSno. 12694) appears in the 400–700 cm�1 region. The intensity ofthe band at 498 cm�1 is much higher than the band at866 cm�1, being just lower than the band at around 1400–1510 cm�1. Secondly, due to the larger absorption at 745 cm�1

but lower absorption at 866 cm�1 for CoCO3 than Li2CO3, theband of CoCO3 at 745 cm

�1 is always clearly observed, while thatof Li2CO3 is barely observed. Thirdly, the spectrum of CoCO3

displays a strong and broad band at �1453 cm�1, while thecorresponding band of Li2CO3 splits into two peaks at1432 cm�1 and 1489 cm�1 (SIOC (Shanghai Institute of OrganicChemistry) chemical database). The previously reported IR dataof CoCO3 (ref. 15–18) and Li2CO3 (powder25,26 or SEI compo-nents26–28) agrees well with the above observations.

J. Mater. Chem. A


Fig. 6 IR spectra of the CC–PPy electrode after the first discharge to (a) 0.96 V, (b) 0.71 V and (c) 0.005 V, and subsequent charge to (d) 1.0 V, (e) 1.9 V and (f) 3.0 V. Forthe sake of comparison, the IR spectrum of pristine CC–PPy (dashed line) is also added.


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Except for some broadening of the band at around 400 cm�1

(400–550 cm�1), the spectrum of pure CC–PPy shows the typicalfeatures of CoCO3, with much weaker absorption at 400 cm�1

than at 866 cm�1, a clear band at 745 cm�1 and one broad bandat 1416–1465 cm�1 without splitting (Fig. 6a). Aer discharge to0.96 V, the band of the electrode at around 400 cm�1 becomesbroader (400–580 cm�1) and stronger relative to that of the puresample, showing signs of peak splitting at 420 and 472 cm�1. Aclear peak splitting is observed in the 1400–1510 cm�1 region.In addition to the intense peak at 1418 cm�1, a hump appearsat 1430 cm�1 and a shoulder appears between 1480 and1510 cm�1. These results clearly indicate the formation ofLi2CO3, which is always observed in the following spectra as aconversion reaction product or SEI component. However, theexistence of CoCO3 is conrmed by the weaker absorption at400 cm�1 than at 866 cm�1, a visible peak at 745 cm�1, as well asan absorption maximum at 1418 cm�1, which is consistent withthe corresponding peak of pure CC–PPy (1417 cm�1). The IRresults agree well with the above lithiation mechanism,according to which the ‘rst-order’ reaction is underway at thedischarge to 0.96 V (the inset of Fig. 6). The active material ofthe electrode is then a mixture of CoCO3 and Li2CO3, and thespectrum shows composite features of the two components.Additionally, the bands at 1173, 1233 and 1275 cm�1 and theshoulders at 840 and 1400 cm�1 can be assigned to the PVDFadditive.29,30 The band between 1570 and 1700 cm�1 probablyresults from carbon black (1600 or 1635 cm�1) and absorbedwater (H–O–H bending vibration at 1640 cm�1),31–33 and theband at around 3305 cm�1 is attributed to the stretchingvibration of the O–H group of surface molecular water inter-acting with CO3

2�.16–18

The band at around 400 cm�1 becomes more intense aerthe discharge to 0.71 V (Fig. 6b), with an intensity much higherthan that at 866 cm�1 and similar to that of the main region(1400–1510 cm�1), whose maximum locates at 1430 cm�1 ratherthan 1418 cm�1. The band at 745 cm�1 is barely observed and

J. Mater. Chem. A

the peak at 420 cm�1 becomes more dened. The spectrumshows the typical features of Li2CO3, while those of CoCO3 cannot be recognized. In addition, the electrochemical discharge to0.71 V induces new bands at 1316 and 826 cm�1 and visibleintensity enhancement of the bands at 1650, 1400 and 1072–1082 cm�1. Judging from the IR studies of previous lithiatedanodes,26,27,34–36 these bands can be attributed to lithium alkylcarbonate, (CH2OCO2Li)2, indicating effective SEI formation.According to the above lithiation mechanism, the ‘rst-order’reactions nish and the second reactions are underway on thedischarge to 0.71 V, accompanied with SEI formation. Corre-spondingly, Fig. 6b presents the typical bands of (CH2OCO2Li)2(minor phase) and Li2CO3 (major phase), which may be amixture of non-reduced ‘rst-order’ products and SEIcomponents.

Further lithiation down to 0.005 V (Fig. 6c) leads toremarkable broadening and strengthening of the band ataround 400 cm�1, which shows more than twice the intensity ofthat of the main region and is certainly too strong to beattributed to the Li–O vibration of Li2CO3, but coincides wellwith that of Li2O.25,26 Moreover, the bands at 420, 445 and3673 cm�1 indicate the existence of LiOH,25,37,38 which is prob-ably the reaction product of Li2O with absorbed water duringthe KBr pellet preparation. The results clearly show the forma-tion of Li2O on the discharge from 0.71 to 0.005 V, providingstrong IR evidence for the ‘second-order’ reactions (eqn (4)). Inaddition, the typical bands of both Li2CO3 and (CH2OCO2Li)2are enhanced, indicating further SEI formation in this process.

Corresponding to the spectra recorded at different dischargestates, the specrea obtained along with the charge process canprove the reversibility of the second and rst order reactions.Characteristic bands of LiOH$H2O (SDBS no. 40012) appear at418, 445, 674, 630, 846, 1000, 1507, 1577 and 3566 cm�1 aercharging to 1.0 V (Fig. 6d), instead of bands of LiOH as in Fig. 6c.This mainly results from the further reaction of LiOH withabsorbed water.38 However, the clear absorption superiority at




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around 400 cm�1 indicates the preserved main phase of Li2O.This is reasonable since the electrochemical charge below 1.0 Vmainly involves the extraction of interfacial lithium. In addition,a decrease in the bands at 1650, 1316, 1073 cm�1 is observedsimultaneously with the growth of the bands at 866, 1432 and1498 cm�1. This is probably due to the further reaction of(ROCO2Li)2 with trace water to form Li2CO3.34 Similar effectshave been observed for CoFe2O4 anodes.26

While charged to 1.9 V, the band at around 400 cm�1 shrinksa lot, with a slightly higher intensity than that of the mainregion (Fig. 6e). The peak at 445 cm�1 shis le to 467 cm�1,being consistent with the Li–O variation of Li2CO3 in Fig. 6a. Forsome reason, an unexpected enhancement occurs in the1000–1220 cm�1 region. However, the sharp peak at 1084 cm�1

clearly shows the formation of Li2CO3. All of the results indicatethe recovery of the active material from Li2O to Li2CO3 oncharging from 1.0 to 1.9 V. Correspondingly, the typical bandsof LiOH$H2O distinctly declined.

Aer delithiation up to 3.0 V (Fig. 6f), the band at around400 cm�1 further shrinks and the intensity decreases to be lowerthan that of the main region, whose maximum changesfrom the peak at 1430 cm�1 (Fig. 6e) to a plateau from 1419 to1430 cm�1. The peak splitting at around 400 cm�1 becomesweaker and the typical peaks of LiOH$H2O nearly disappear. Aband at 1072 cm�1 appears instead of the sharp peak at1084 cm�1 (Fig. 6e). All of this suggests the reversible formationof CoCO3 from Li2CO3. Moreover, the peaks at 1650 and1316 cm�1 change a little from 1.0 to 3.0 V, indicating that theSEI component of (ROCO2Li)2 is relatively stable in this process.

4 Conclusions

Urchin-like CoCO3 microspheres (CC) were rst prepared by ahydrothermal method and further modied with a PPy coating.The CoCO3–PPy composite (CC–PPy) exhibits signicantlyenhanced cycling stability relative to CC, as well as excellent rateperformance (from 1 C to 5 C) and a great recovery capability,appearing to be a very promising anode material for LIBs. Amore comprehensive lithium storage mechanism of CoCO3 hasbeen proposed to support the observed experimental data,which consists of two steps of conversion reactions with a totaltheoretical value of 7 Li per CoCO3. The potential proles andCV curves of CC and CC–PPy show clear two-order character,and ex situ IR spectra recorded at different discharge–chargestates of CC–PPy provide strong evidence for the transition fromCoCO3 to Li2CO3 to Li2O, and its reversibility.

Acknowledgements

This work was supported by the National Natural Science Fundsfor Distinguished Young Scholars (Grant no. 51025211).

Notes and references

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Enhanced rate performance and cycling stability of a CoCO3–polypyrrole composite for lithium ion battery anodes