Journal of Alloys and Compounds 588 (2014) 25–29

Contents lists available at ScienceDirect

Journal of Alloys and Compounds

journal homepage: www.elsevier .com/locate / ja lcom

Letter

High rate capability of Li3V2(PO4)3/C composites prepared via aTPP-assisted carbothermal method and its application inLi3V2(PO4)3||Li4Ti5O12

0925-8388/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.jallcom.2013.11.081

⇑ Corresponding author. Tel: +86 769 8301 7180; fax: +86 769 8319 5372.E-mail address: wenfengmao123@gmail.com (W.-f. Mao).

1 These authors contributed equally to this work.

Wen-feng Mao ⇑,1, Nuo-nuo Zhang 1, Zhi-yuan Tang, Ya-qing Feng, Chen-xiang MaDepartment of Applied Chemistry, School of Chemical and Engineering, Tianjin University, Tianjin 300072, PR China

a r t i c l e i n f o

Article history:Received 29 October 2013Received in revised form 13 November 2013Accepted 13 November 2013Available online 21 November 2013

Keywords:Lithium vanadium phosphateMeso-tetraphenyl porphyrinElectrode materialsFull batterySolid state reactions

a b s t r a c t

High rate Li3V2(PO4)3/C composite with excellent electrochemical performance has been successfullysynthesized via a sample carbon thermal method using meso-tetraphenyl porphyrin (TPP) as carbonsource. The as prepared Li3V2(PO4)3/C composite delivers a reversible capacity of 103.71 mA h g�1 at20 C charge/discharge rate. The Li+ diffusion coefficient ranges from 10�9 to 10�10 cm2 s�1 based on dif-ferent scanning rates. The excellent high rate performance may attribute to the small particle size of theLVP/C and the carbon derived from thermal decomposition of TPP. In addition, its application in the full-cell, Li3V2(PO4)3kLi4Ti5O12, is also presented and discussed.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

Given the merits of high safety, high power density and long cy-cle life, polyanion-type cathode materials exhibit great potential tobe used in high power LIBs (lithium-ion batteries) [1,2]. Especiallymonoclinic lithium vanadium phosphate (LVP) with a three-dimensional net structure has attracted significant attention dueto its safety, stable structure, acceptable average voltage and highpower density [3–8]. The theoretical capacity of LVP is133 mA h g�1 by extracting two lithium ions in the potential rangeof 3.0–4.3 V based on the V3+/V4+ redox couple, and the averageoperating voltage is as high as 3.85 V [9–13].

However, poor intrinsic electronic conductivity of LVP, resultingfrom the separation of VO6 octahedra by PO4 tetrahedra, limits itspractical application in LIBs [11,14–17]. Various strategies havebeen adopted to solve the problem [8,18–20], such as carbon andconductive metal (Cu, Ag) coating [7,19,21,22], metal ions doping(Na+, Mg2+, Cl� etc.) [20,23–26], surface modification (LiFePO4,Li4Ti5O12, etc.) [11,18] and reducing particle size [27].

It is reported that LiFePO4–Li3V2(PO4)3/C composite synthesizedvia a modified solid-state method exhibits excellent electrochemi-cal performance [18] and solid-state synthesize method has at-tracted much attention due to its promising industrialized

potential for synthesizing LVP [18,28]. Moreover, the introductionof carbon cannot only enhance the electronic conductivity but alsorestrict the growth of LVP primary particle. Zhang et al. synthe-sized LVP@C/graphene composite and they believe that the coatingcarbon can greatly improve the performance of LVP. Liu et al. alsoprepared carbon coated LVP composite [29] and they found thatthe LVP/C sample prepared with 10 wt% glucose has a uniform car-bon layer about 10 nm on the surfaces, and it also presented excel-lent electrochemical performance [19].

Porphyrins are a large class of natural occurring intensely col-ored, red or purple, macrocyclic pigmented compounds, which pre-sents great potential in many fields such as molecular wires,photosensitizers, solar cells, electrochemical and optic sensors.TPP as a major one of porphyrins, its basic structure has in commona substituted aromatic macrocyclic ring consisting of four pyrrole-type rings linked together by four methine bridges. And its interac-tion with different metal ions have been investigated [30,31].However, to our knowledge, there is no report on using TPP forsynthesizing LVP/C composite.

In this paper, TPP is firstly introduced as carbon source to syn-thesize LVP/C composite via a simple carbon thermal method. Thewell-crystallized LVP/C presents excellent electrochemical perfor-mance at high charge/discharge rates, and the Li+ diffusion behav-ior of the LVP/C is investigated. In addition, its application in thefull-cell, Li3V2(PO4)3kLi4Ti5O12 (LVPkLTO) is also presented anddiscussed.

26 W.-f. Mao et al. / Journal of Alloys and Compounds 588 (2014) 25–29

2. Experimental

The carbon coated Li3V2(PO4)3/C composite was prepared by a carbon thermalreduction method using V2O5, Li2CO3, NH4H2PO4 and TPP as raw materials. Stoichi-ometric amounts of V2O5, Li2CO3 and NH4H2PO4 were ball-milled with suitableamounts of TPP (10 wt%). Then the obtained precursors were pre-heated at350 �C for 4 h and then sintered at 800 �C for 8 h in N2 atmosphere to obtain theLVP/C composite. Commercial LTO/C was obtained from BTR Corporation, China.

Power X-ray diffraction (XRD, Rigaku D/max 2500v/pc, Cu Ka radiation) mea-surement was performed with LVP/C composite. The morphologies were observedusing JSM-6380LA scanning electron microscope (SEM) and high-resolution trans-mission electron microscopy (TEM Tecnai G2 F20, Philips). The electronic conductiv-ity of the composite was measured on a four-point probes resistivity measurementsystem.

The electrochemical tests were performed using 2032 coin-type cells. The elec-trode was fabricated by mixing 85% active material (LVP or LTO) with 10% acetyleneblack and 5% polyvinylidene difluoride (PVDF). The active material loadings wereabout 4.0 mg cm�2 and the diameter of electrode was 10 mm. The LVPkLi coin-typecell was composed of the LVP/C cathode, lithium metal anode, a celgard2300 sepa-rator, and LiPF6 in 1:1 ethylene carbonate (EC) and diethyl carbonate (DEC) as theelectrolyte. The LTOkLi and LVPkLTO coin-type cells were fabricated use the samemethod as that of LVPkLi. All cells were assembled in a N2 filled glove box.

Cyclic voltammetry (CV) was carried out on a Gamry PCI4–750 electrochemicalworking station at a scan rate of 0.05 mV s�1.

3. Results and discussion

Fig. 1(a) shows the XRD pattern of the prepared LVP/C compos-ite. The diffraction pattern matches well with the monoclinic LVPwith the space group of P21/n; a small amount of Li3PO4

(PDF#:15-0760) detected at about 2h = 22.5 [10]. The similar dif-fraction peak of impurity is also detected in the recent literaturesand the appearance of Li3PO4 does not severely affect the electro-chemical performance of LVP/C [9,32,33]. Obtained by refinement,the lattice parameters for the LVP/C composite are a = 8.535(5),b = 8.591(2), c = 12.023(0), b = 89.38(2), which are consistent withprevious reports [10,23]. No evidence of carbon is detected, sug-gesting the amorphous of residual carbon. As shown in the SEM

Fig. 1. XRD (a), SEM (b) and TEM (c, d)

image in Fig. 1(b), the LVP/C composite has loose porousmorphology with the particle size ranging from 0.3 to 1.0 lm,although some agglomerated particles can be observed. To furtherinvestigate the inner microstructure of prepared LVP/C composite,TEM images (Fig. 1(c) and (d)) are displayed. It can be found thatthe particle size of LVP/C is about 200 nm and the particles arewrapped with the carbon decomposed from TPP. In addition, theelectronic conductivity of LVP/C is measured to be 8.0 � 10�6 -S cm�1, which is much than that of pristine LVP, i.e. 9.0 � 10�8 -S cm�1 [34]. It is believed that the carbon layer can not onlyinhibits the growth of LVP particles but also provides good electri-cal contact between the particles, beneficial for the high rate elec-trochemical performance.

Fig. 2(a) and (b) shows the charge/discharge curve and corre-sponding cycle performance of the LVP/C composite in LVPkLihalf-cell with different charge/discharge rates from 0.5 C to 1 C,5 C, 10 C and then reset to 0.5 C. For the 0.5 C charge curve, threecharge plateaus at 3.60, 3.68 and 4.08 V are observed, correspond-ing to the sequence phase transition processes from Li3V2(PO4)3 toLi2.5V2(PO4)3, Li2V2(PO4)3 and LiV2(PO4)3, respectively. The first Li+

is extracted in two steps because of the existence of an orderedphase Li2.5V2(PO4)3. Three corresponding discharge plateaus at4.05, 3.65 and 3.56 V are signed as the reinsertion of the two lith-ium ions that accompanied the phase transition from LiV2(PO4)3 toLi2V2(PO4)3, Li2.5V2(PO4)3 and Li3V2(PO4)3, respectively. When thecells are operated at 0.5 C, 1 C, 5 C and 10 C charge/discharge rates,the discharge capacities are 126.34 mA h g�1, 126.13 mA h g�1,123.46 mA h g�1and 118.16 mAh g�1, respectively. Additionally,as shown in Fig. 2(c), the LVP/C composite shows no capacity fad-ing even after 200 cycles at 1 C charge/discharge rate, indicatingthe excellent stability of LVP/C. Fig. 2(d) shows the charge/dis-charge curve and corresponding cycle performance of preparedLVP/C composite at 20 C charge/discharge rate. The largest specificcapacity of 103.71 mA h g�1 is obtained at 16th cycles, which may

of the prepared LVP/C composite.

Fig. 2. Electrochemical performances of the synthesized LVP/C composite in LVPkLi half-cell at different discharge rates in the potential of 3.0–4.3 V. (a) and (b) Charge/discharge curves and the corresponding cycling performances of the LVP/C composite by varying the charge/discharge rate; (c) and (d) The charge/discharge curves andcycling performance of LVP/C at 1 C and 20 C charge/discharge rate.

Fig. 3. (a) CV curves of the LVP/C sample in LVPkLi half-cell at various scan rates in the potential of 3.0–4.5 V. (b) the corresponding linear relationship between the peakcurrent (ip) and the square root of scan rate (m1/2).

Table 1The diffusion coefficients of Li+ in the LVP/C composite calculated from CV.

State Peaks Ds (cm2 s�1)

Charge A1 3.632 � 10�10

A2 1.005 � 10�9

A3 1.886 � 10�9

Discharge B3 8.863 � 10�10

B2 5.867 � 10�10

B1 8.393 � 10�10

W.-f. Mao et al. / Journal of Alloys and Compounds 588 (2014) 25–29 27

be responsible for the activation of the electrode [28], and 90% ofcapacity retention is maintained after 92 cycles compared withthe highest capacity. The excellent high rate performance exhibitspromising potential for high power LIBs.

To further investigate the electrochemical performance of theLVP/C, CV curves at various scan rates were measured. InFig. 3(a), three redox peaks corresponding to the V3+/V4+ redox cou-ple are presented, which is in agreement with the three charge/dis-charge plateaus in Fig. 2(a). The Li+ diffusion coefficient (Ds) can bealso calculated based on CV results. As shown in Fig. 3(b), each re-dox peak current (ip) has a linear relationship with the square rootof scan rate (m1/2), and the Randles–Sevchik equation for a semi infinite diffusion of Li+ into LVP cathode can be applied.

ip ¼ ð2:69� 105Þn1:5ADLiþCLiþm0:5; ð1Þ

where ip is the peak current (A), n is the charge-transfer number, A isthe contact area between LVP and electrolyte (0.785 cm2), CLi+ is the

Fig. 4. (a) Charge/discharge curve and cycling performance of LTOkLi half-cell at 1 C rate in the potential of 1.0–3.0 V. (b) and (c) Charge/discharge curve and thecorresponding cycling performance of LVPkLTO full-cell at 0.5 C rate in the potential of 1.0–2.8 V. (d) CV curve of LVPkLTO full-cell at a scan rate of 0.05 mV s�1 in the potentialof 1.0–2.8 V.

28 W.-f. Mao et al. / Journal of Alloys and Compounds 588 (2014) 25–29

concentration of lithium ions in the cathode (3.7 � 10�3 mol cm�3),and m is the potential scan rate (V s�1). The Li+ diffusion coefficient(Table 1) ranges from 10�9 to 10�10 cm2 s�1 and they are very closeto or higher than the ones reported by other groups [11,35].

It is well known that the performance in full cell is importantfor LVP practical application. Here we demonstrate theperformance of such cathode in full-cell assembly with LTO elec-trode. Fig. 4(a) presents the charge/discharge profile and corre-sponding cycle performance of LTOkLi cells. The observed voltageprofile of the LTO electrode evolves around 1.55 V vs. Li+/Li. Andthis plateau belongs to a coexistence of two active phases(Li4Ti5O12 and Li7Ti5O12) in the electrode. The initial capacity ofLTO/C is 157.16 mA h g�1, and still remains 156.25 mA h g�1 after200 cycles, with capacity retention of 99.42%, indicating the excel-lent cycling stability.

We have employed a LVP to LTO capacity balance of 0.9:1.0 tothe assembling and to the test of a full LVPkLTO lithium-ionbattery. The cell is expected to be characterized by three voltageplateaus since the LTO electrode evolve on two-phase redoxprocess (Li4Ti5O12 and Li7Ti5O12) while LVP electrode evolvesfour-phase redox process (LiV2(PO4)3, Li2V2(PO4)3, Li2.5V2(PO4)3

and Li3V2(PO4)3). Fig. 4(b) and (c) presents the second charge/dis-charge curve and corresponding cycle performance of the LVPkLTOfull-cell at 0.5 C rate in the potential of 1.5–2.8 V. As expected bythe intrinsic features of the two electrodes, three charge/dischargeplateaus at 2.07/1.97 V, 2.15/2.05 V and 2.56/2.46 V can be ob-served. The average charge and discharge voltage plateaus arearound 2.335 and 2.235 V. The total reaction of the full cell canbe explained by:

3Li3V2ðPO4Þ3 þ 2Li4Ti5O12 $ 3LiV2ðPO4Þ3 þ 2Li7Ti5O12: ð2Þ

To further confirm our analysis of the full cell, the CV profilewas tested. As can be seen from Fig. 4(c), three pair of peaks at2.06 V/1.98 V, 2.16 V/2.04 V and 2.59 V/2.42 V can be observed,which are in good agreement with the charge/discharge curves.

Recently, LVP/C composite synthesized by other groups[9,29]exhibited good high-rate performance. However, the adoptedmethods were too complex for the industrialization. In this work,the LVP/C composite is synthesized by a simple carbon thermalmethod and it exhibits excellent rate capability with a specificcapacity of 103.71 mA h g�1 at 20 C charge/discharge rate. More-over, its presents expect electrochemical performance in theLVPkLTO full cell. The excellent electrochemical performance ofthe LVP/C composite would be attributed to (1) the excellent elec-tronic conductivity carbon network derived from the decomposi-tion of TPP, so the growth of the LVP/C particles could be greatlyinhibited and (2) the nanoscale particle shortening the diffusiondistance of lithium ion and increasing the contacted area betweenelectrode and electrolyte.

4. Conclusions

In this paper, The LVP/C composite with extraordinary dischargecapability is synthesized via a simple carbon thermal method usingTPP as carbon source. The prepared LVP/C composite exhibits excel-lent rate capability and cyclic performance, which may attribute tothe reduced particle size and the existence of conductive carbon de-rived from the decomposition of TPP. Moreover, its application in anew type of lithium ion battery, Li4Ti5O12kLi3V2(PO4)3, is also inves-tigated. The results of our study indicate that LVP can be a potentialcandidate for LIBs.

References

[1] C. Masquelier, L. Croguennec, Chem. Rev. 113 (2013) 6552–6591.[2] V. Aravindan, J. Gnanaraj, Y.-S. Lee, S. Madhavi, J. Mater. Chem. A 1 (2013)

3518–3539.[3] S.C. Yin, H. Grondey, P. Strobel, M. Anne, L.F. Nazar, J. Am. Chem. Soc. 125

(2003) 10402–10411.[4] Y.-Z. Li, Z. Zhou, M.-M. Ren, X.-P. Gao, J. Yan, Mater. Lett. 61 (2007) 4562–4564.[5] Y. Li, Z. Zhou, X. Gao, J. Yan, Electrochim. Acta 52 (2007) 4922–4926.[6] Y. Li, Z. Zhou, M. Ren, X. Gao, J. Yan, Electrochim. Acta 51 (2006) 6498–6502.

W.-f. Mao et al. / Journal of Alloys and Compounds 588 (2014) 25–29 29

[7] S. Wang, Z. Zhang, S. Fang, L. Yang, C. Yang, S.-I. Hirano, Electrochim. Acta 111(2013) 685–690.

[8] J. Yoon, S. Muhammad, D. Jang, N. Sivakumar, J. Kim, W.-H. Jang, Y.-S. Lee, Y.-U.Park, K. Kang, W.-S. Yoon, J. Alloys Comp. 569 (2013) 76–81.

[9] W.-F. Mao, J. Yan, H. Xie, Z.-Y. Tang, Q. Xu, J. Power Sour. 237 (2013) 167–171.[10] W.-F. Mao, J. Yan, H. Xie, Z.-Y. Tang, Q. Xu, Electrochim. Acta 88 (2013) 429–

435.[11] L. Wang, X. Li, Z. Tang, X. Zhang, Electrochem. Commun. 22 (2012) 73–76.[12] M. Hu, J. Wei, L. Xing, Z. Zhou, J. Power Sour. 222 (2013) 373–378.[13] L. Su, Y. Jing, Z. Zhou, Nanoscale 3 (2011) 3967–3983.[14] B. Huang, X. Fan, X. Zheng, M. Lu, J. Alloys Comp. 509 (2011) 4765–4768.[15] Y.Q. Qiao, J.P. Tu, Y.J. Mai, L.J. Cheng, X.L. Wang, C.D. Gu, J. Alloys Comp. 509

(2011) 7181–7185.[16] F. Wu, F. Wang, C. Wu, Y. Bai, J. Alloys Comp. 53 (2011) 236–241.[17] L.-L. Zhang, Y. Li, G. Peng, Z.-H. Wang, J. Ma, W.-X. Zhang, X.-L. Hu, Y.-H. Huang,

J. Alloys Comp. 513 (2011) 414–419.[18] Y. Guo, Y. Huang, D. Jia, X. Wang, N. Sharma, Z. Guo, X. Tang, J. Power Sour. 246

(2014) 912–917.[19] Q. Liu, F. Yang, S. Wang, L. Feng, W. Zhang, H. Wei, Electrochim. Acta 111

(2013) 903–908.[20] R. Wang, S. Xiao, X. Li, J. Wang, H. Guo, F. Zhong, J. Alloys Comp. 575 (2013)

268–272.[21] W. Hao, H. Zhan, J. Yu, Mater. Lett. 83 (2012) 121–123.[22] J. Yan, W. Yuan, H. Xie, Z.-Y. Tang, W.-F. Mao, L. Ma, Mater. Lett. 71 (2012) 1–3.

[23] W.-L. Wu, J. Liang, J. Yan, W.-F. Mao, J. Solid State Electrochem. 17 (2013)2027–2033.

[24] M. Ren, Z. Zhou, Y. Li, X.P. Gao, J. Yan, J. Power Sour. 162 (2006) 1357–1362.[25] M.M. Ren, Z. Zhou, X.P. Gao, W.X. Peng, J.P. Wei, J. Phys. Chem. C 112 (2008)

5689–5693.[26] J. Yan, W. Yuan, Z.-Y. Tang, H. Xie, W.-F. Mao, L. Ma, J. Power Sour. 209 (2012)

251–256.[27] B. Pei, Z. Jiang, W. Zhang, Z. Yang, A. Manthiram, J. Power Sour. 239 (2013)

475–482.[28] W.-F. Mao, J. Yan, H. Xie, Y. Wu, Z.-Y. Tang, Q. Xu, Mater. Res. Bull. 47 (2012)

4527–4530.[29] L. Zhang, S. Wang, D. Cai, P. Lian, X. Zhu, W. Yang, H. Wang, Electrochim. Acta

91 (2013) 108–113.[30] M.K. Panda, K. Ladomenou, A.G. Coutsolelos, Coord. Chem. Rev. 256 (2012)

2601–2627.[31] J.F. van Staden, R.I. Stefan-van, Staden, Talanta 80 (2010) 1598–1605.[32] L. Zhang, H. Xiang, Z. Li, H. Wang, J. Power Sour. 203 (2012) 121–125.[33] J. Yan, W.-F. Mao, H. Xie, Z.-Y. Tang, W. Yuan, X.-C. Chen, Q. Xu, L. Ma, Mater.

Res. Bull. 47 (2012) 1609–1612.[34] S.C. Yin, P.S. Strobel, H. Grondey, L.F. Nazar, Chem. Mater. 16 (2004) 1456–

1465.[35] X.H. Rui, N. Ding, J. Liu, C. Li, C.H. Chen, Electrochim. Acta 55 (2010) 2384–

2390.