Carbon nanocomposite as cathode materials for Li-ion batteries

Carbon nanocomposite as cathode materials for Li-ion batteries

Journal of Power Sources 298 (2015) 280e284 Contents lists available at ScienceDirect Journal of Power Sources journal homepage: www.elsevier.com/lo...

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Journal of Power Sources 298 (2015) 280e284

Contents lists available at ScienceDirect

Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour

Synthesis of Li2FeP2O7/Carbon nanocomposite as cathode materials for Li-ion batteries Hiroaki Nagano, Izumi Taniguchi* Department of Chemical Engineering, Graduate School of Science and Engineering, Tokyo Institute of Technology, 12-1, Ookayama-2, Meguro-ku, Tokyo 152-8552, Japan

h i g h l i g h t s  Li2FeP2O7/Carbon nanocomposite was prepared by a novel preparation route.  Li2FeP2O7/Carbon nanocomposite was agglomerates of primary particles.  The carbon was well distributed on the surface of agglomerates.  The nanocomposite cathode delivered 100 mAh g1 at 0.05 C.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 9 June 2015 Received in revised form 5 August 2015 Accepted 18 August 2015 Available online xxx

A Li2FeP2O7/Carbon (C) nanocomposite was successfully synthesized via a combination of spray pyrolysis and wet ball milling followed by annealing from a precursor solution; in which LiNO3, H3PO4 and Fe(NO3)3$9H2O were stoichiometrically dissolved into distilled water. Ascorbic acid was added to the precursor solution as a reduction agent. The peaks of the Li2FeP2O7/C nanocomposite obtained by X-ray diffraction analysis were indexed to the monoclinic structure with the space group P21/c. The Li2FeP2O7/C nanocomposite cathode delivered a first discharge capacity of 100 mAh g1 at 0.05 C, which corresponded to 91% of its theoretical capacity. After various higher discharge rates from 0.05 to 2 C in the cycle performance test, a discharge capacity of 93 mAh g1 was achieved at 0.05 C, which showed an excellent capacity retention (93%) after 29 cycles. © 2015 Elsevier B.V. All rights reserved.

Keywords: Li2FeP2O7 Lithium-ion batteries Nanocomposite Cathode materials Powder technology Spray pyrolysis

1. Introduction Lithium-ion batteries have been widely used in cellular phones, laptop computers and other portable electronic devices owing to their high working voltage, large energy density and long cycle life. They are also considered to be the most promising means of energy storage for practical electric vehicles (EVs) and plug-in hybrid electric vehicles (PHEVs). It is well known that the cathode materials of lithium-ion batteries have a significant impact on the battery capacity, cycle life, safety and cost. Recently, lithium transition-metal pyrophosphates (Li2MP2O7, M]Fe, Mn and Co) [1e7] have been considered as a new promising cathode material of Li-ion batteries owing to their higher theoretical capacities (~220 mAh g1) than other polyanion materials, such as LiMPO4 * Corresponding author. Tel./fax: þ81 3 5734 2155. E-mail address: [email protected] (I. Taniguchi). http://dx.doi.org/10.1016/j.jpowsour.2015.08.068 0378-7753/© 2015 Elsevier B.V. All rights reserved.

(M]Fe and Mn) [8]. Especially, Li2FeP2O7 cathode material [9e13] has attracted more attention during the past several years, due to its high safety, low material cost and environmental friendliness. However, Li2FeP2O7 or Li2FeP2O7/Carbon(C) has been synthesized from a relatively expensive precursor (LiH2PO4) [10e13] or chemically unstable precursors (Fe(CH3COO), FeC2O4 or Fe) [3,7,8,10,12,13]. In our previous studies, we developed a novel synthesis route, i.e., a combination of spray pyrolysis (SP) and wet ball milling (WBM) followed by annealing to prepare LiMPO4/C (M]Fe and Mn) [14,15] and Li2FeSiO4/C [16] nanocomposites to overcome their poor electronic conductivities and slow lithium-ion diffusion. In this study, we have prepared Li2FeP2O7/Carbon(C) nanocomposite from relatively cheap and chemically stable precursors (LiNO3, H3PO4 and Fe(NO3)3$9H2O) by a combination of SP and WBM followed by annealing. Furthermore, its physical and electrochemical properties have been investigated.

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2. Experimental The precursor solution used in this study was prepared by dissolving stoichiometric amounts of LiNO3, H3PO4 and Fe(NO3)3$9H2O in distilled water. Ascoribic acid was added into the precursor solution as a reducing agent. A schematic diagram of the SP setup that we developed has been provided elsewhere [17]. The precursor solution was atomized at a frequency of 1.7 MHz using an ultrasonic nebulizer. The sprayed droplets were transported to a reactor using a 3% H2þN2 gas with a gas flow rate of 4 dm3 min1, heated at 800  C and converted into solid particles. The resulting particles were then milled with 5 wt.% acetylene black (AB) in ethanol by planetary high-energy planetary ball milling (Fritsch, Pulverisette 7) at 200 rpm for 1 h and then annealed at 600  C for 2 h in a 3% H2þN2 atmosphere to obtain the desired material. The crystalline phases of the samples were studied by X-ray diffraction (XRD, Rigaku, Ultima IV with D/teX Ultra) analysis using Cu-Ka radiation. The carbon, iron and phosphor distributions in the sample were observed by a field emission scanning electron microscopy (FE-SEM, Hitachi, SU9000) with energy-dispersive spectroscopy (EDS, Ametech, Genesis-APEX) at 6.0 kV and a transmission electron microscopy (TEM, JEOL, Ltd., JEM-2010F) system equipped with an energy-dispersive spectroscopy (EDS) system operated at 8 kV. The morphology of the obtained samples was also examined by SEM (Keyence YE-8800). The BrunauereEmmetteTeller (BET) specific surface area was determined from the nitrogen absorptionedesorption isotherm using a Micromeritics TriStar-II analyzer. The carbon content of the samples was estimated using an element analyzer (CHNS, Elementar, Vario Micro Cube). The electrochemical performance of the Li2FeP2O7/C nanocomposite was investigated using coin-type cells (CR2032). A 1 mol dm3 LiPF6 solution in a solvent mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) in a 1:1 volume ratio (Tomiyama Pure Chemical Co., Ltd.) was used as the electrolyte. The cathode consisted of 70 wt.% Li2FeP2O7/C, 10 wt.% polyvinylidene fluoride (PVdF) as a binder and 20 wt.% AB. The cells were galvanostatically cycled in a constant current-constant voltage mode at a 0.05 C rate (1 C ¼ 110 mA g1) to 4.3 V, held at 4.3 V until C/100, and then discharged to 2.0 V at different C-rates. 3. Results and discussion Spray pyrolysis synthesis from the precursor solution; in which stoichiometric amounts of LiNO3, H3PO4 and Fe(NO3)3$9H2O were dissolved into distilled water, was firstly carried out for different synthesis temperatures ranging from 600 to 800  C. The XRD patterns of all the samples were identified as Li3Fe2(PO4)3 crystal structure. Only those of the sample prepared at 800  C are shown in Fig. 1a. A further annealing at 600  C for 2 h in 3% H2þN2 atmosphere was performed for the spray pyrolysis samples. However, a final product (Li9Fe3(P2O7)3(PO4)2) included Fe3þ instead of the desired material (Li2FeP2O7) was obtained by the SP followed by annealing (Fig. 1a). This may clearly indicate that a reducing agent should be added to the starting solution. Thus, ascorbic acid was used as a reducing agent. Fig. 1b shows the XRD patterns of the samples prepared from the precursor solution with ascorbic acid additive by SP at 800  C, and then annealed at 600  C for 2 h in 3% H2þN2 atmosphere. While the weak peaks attributed to LiFePO4 crystal structure are observed in the XRD patterns of the SP sample, the XRD patterns of the final sample are identified as a monoclinic structure with space group P21/c [4,10]. The morphology of the Li2FeP2O7 prepared from the precursor solution with ascorbic acid additive by SP with heat treatment is

Fig. 1. XRD patterns of the sample prepared by SP with annealing from precursor solution with (a) and without (b) ascorbic acid additive.

presented in Fig. 2a. The obtained Li2FeP2O7 powders are spherical particles with a size ranging from 1 to 3 mm. Although carbon content in the sample was measured by the element analyzer, there was no carbon in it owing to completely decompose of ascorbic acid in the SP synthesis process at 800  C. The obtained Li2FeP2O7 sample was used as a cathode active material and its battery performance was evaluated at a chargeedischarge rate of 0.05 C, as shown in Fig. 2b. The Li2FeP2O7 electrode delivers 63 mAh g1 at first cycle and 62 mAh g1 at second cycle, corresponding to 57% of its theoretical capacity (110 mAh g1). Also, a narrow potential plateau at approximately 3.4 V in only the discharge profiles, which may attribute to Fe3þ/Fe2þ reduction reaction, and a large potential difference between charge and discharge profiles can be obviously seen in the figure. These may be due to its poor electronic conductivity, which is a feature of many polyanionic systems, and relatively large particle size of the sample, as shown in Fig. 2a [18]. In order to enhance the electrochemical properties of Li2FeP2O7, a novel synthesis route, i.e., a combination of spray pyrolysis and wet ball milling (WBM) followed by annealing was employed to reduce the particle size of Li2FeP2O7 and well mix it with carbon. The sample prepared at 800  C by SP was chosen as the precursor material on the WBM procedure to prepare the Li2FeP2O7/C nanocomposite. Fig. 3a shows the XRD patterns of the sample prepared by a combination of SP and WBM followed by annealing at 600  C for 2 h. The XRD patterns of the Li2FeP2O7 sample prepared by SP with annealing are also shown in the figure as a reference. The XRD

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Fig. 2. SEM image (a) and chargeedischarge profiles (b) of Li2FeP2O7 prepared by SP with annealing.

peaks of the sample prepared by the present synthesis method are also indexed to the monoclinic structure with space group P21/c. However, the peak intensities of the sample markedly reduce in comparison with those of the Li2FeP2O7 sample by SP with annealing. This reason may be due to reduce the particle size of Li2FeP2O7 by the WBM process and inhibit to grow up Li2FeP2O7 particles during the annealing by the carbons attached on them. The morphology and element distribution of the sample prepared by the combination of SP and WBM with annealing are shown in Fig. 3b and c, respectively. By comparison with the SEM image in Fig 2a, it can be clearly seen that a well mixing between carbon and Li2FeP2O7 particles and a reducing the Li2FeP2O7 particle size simultaneously progresses. From the TEM images of the Li2FeP2O7/ C composite sample as shown in Fig. 4, we could also confirm the micro-mixing sate between carbon and Li2FeP2O7 particles that fine carbon particles partially attach on the surface of Li2FeP2O7 agglomerates. The well mixing between carbon and Li2FeP2O7 particles may provide a higher conductive route for the electron motion in chargeedischarge process. Furthermore, the small particle size of Li2FeP2O7 may give rise to a short diffusion distance of Liþ in an electrode. The BET measurement showed that the Li2FeP2O7/C composite had a large specific surface area of 25 m2 g1, which might provide a large effective area between the surface of Li2FeP2O7/C nanocomposite particles and the electrolyte to enhance the electrochemical performance of the electrode. The cell performance test of the Li2FeP2O7/C composite cathode

Fig. 3. XRD patterns (a), SEM image (b) and elemental mapping (c) of Li2FeP2O7/C nanocomposite prepared by a combination of SP and WBM with annealing.

was carried out at various discharge rates from 0.05 to 2 C, in which 5 cycles were carried out at each rate and finally the cell went back to 0.05 C for another 4 cycles. Fig. 5a shows the chargeedischarge profiles of the Li2FeP2O7/C nanocomposite cathode at different discharge rates from 0.05 to 2 C. The Li2FeP2O7/C nanocomposite cathode exhibits a discharge potential plateau at approximately 3.5 V, which corresponds to the Fe3þ/Fe2þ redox reaction [3,4], and delivers a first-discharge capacity of 100 mAh g1 at a discharge rate of 0.05 C, which corresponds to 91% of the theoretical capacity. However, the discharge capacity drastically decreases with increasing C-rate from 0.05 to 1.0 C owing to the partially distributed carbon particles on the surface of Li2FeP2O7 agglomerates.

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Fig. 4. TEM images and EDS spectrum of the Li2FeP2O7/C nanocomposite prepared by a combination of SP and WBM with annealing.

Fig. 5b shows the cycle performance of the Li2FeP2O7 nanocomposite cathode. After various higher discharge rates, a discharge capacity of 93 mAh g1 is achieved at a discharge rate of 0.05 C, which shows an excellent reversible retention (93%) after 29 cycles. The good electrochemical performance, together with low cost and toxicity, makes the Li2FeP2O7/C nanocomposite cathode material a promising candidate for large-scale lithium-ion batteries. Table 1 gives the comparison of electrochemical properties of the Li2FeP2O7/C nanocomposite prepared in this study with those of previous works. The electrochemical performance of the present Li2FeP2O7/C nanocomposite may be approximately in the same level as the electrochemical performance of those synthesized from a relatively expensive precursor (LiH2PO4) [10e13] or

chemically unstable precursors (Fe(CH3COO), FeC2O4 or Fe) [3,7,8,10,12,13]. 4. Conclusions A Li2FeP2O7/C nanocomposite was successfully synthesized by a novel synthesis route, i.e., a combination of SP and WBM followed by annealing. The XRD peaks of the Li2FeP2O7/C nanocomposite were indexed to a monoclinic structure with space group P21/c. The Li2FeP2O7/C nanocomposite cathode delivered a first-discharge capacity of 100 mAh g1 at 0.05 C. After various higher discharge rates from 0.05 to 2 C in the cycle performance test, a discharge capacity of 93 mAh g1 was achieved at 0.05 C, which showed an excellent capacity retention (93%) after 29 cycles.

Acknowledgments This research was partially supported by the Ministry of Education, Culture, Sports, Science and Technology of Japan (Grant No. 24360405). The authors are also grateful to Mr. J. Koki and Mr. A Genseki, staff members of the Center for Advanced Materials Analysis (Tokyo Institute of Technology, Japan), for the FE-SEM with EDS and the TEM with EDS analyses of the samples. References [1] L. Adam, A. Guesdon, B. Raveau, J. Solid State Chem. 181 (2008) 3110. [2] P. Barpanda, S. Nishimura, A. Yamada, Adv. Energy Mater. 2 (2012) 841. [3] H. Zhou, S. Upreti, N.A. Chernova, G. Hautier, G. Ceder, M.S. Whittingham, Chem. Mater. 23 (2011) 293. [4] M. Tamaru, P. Barpanda, Y. Yamada, S. Nishimura, A. Yamada, J. Mater. Chem. 22 (2012) 24526. [5] A. Gutierrez, N.A. Benedek, A. Manthiram, Chem. Mater. 25 (2013) 4010. [6] H. Kim, S. Lee, Y.U. Park, H. Kim, J. Kim, S. Jeon, K. Kang, Chem. Mater. 23 (2011) 3930. [7] N. Furuta, S. Nishimura, P. Barpanda, A. Yamada, Chem. Mater. 24 (2012) 1055. [8] A.K. Padhi, K.S. Nanjundaswamy, J.B. Goodenough, J. Electrochem. Soc. 144 (1997) 1188. [9] L. Tan, S. Zhang, C. Deng, J. Power Sources 275 (2015) 6. [10] J. Du, L. Jiao, Q. Wu, Y. Liu, Y. Zhao, L. Guo, Y. Wang, H. Yuan, Electrochim. Acta 103 (2013) 219. [11] P. Barpanda, T. Ye, S.-C. Chung, Y. Yamada, S. Nishimura, A. Yamada, J. Mater. Chem. 22 (2012) 13455. [12] B. Zhang, X. Ou, J.-C. Zheng, C. Shen, L. Ming, Y.-D. Han, J.-L. Wang, S.-E. Qin, Electrochim. Acta 133 (2014) 1. [13] J.-C. Zheng, X. Ou, B. Zhang, C. Shen, J.-F. Zhang, L. Ming, J. Power Sources 268 (2014) 96. [14] M. Konarova, I. Taniguchi, J. Power Sources 195 (2010) 3661. [15] Z. Bakenov, I. Taniguchi, Electrochem. Commun. 12 (2010) 75. [16] S. Bin, I. Taniguchi, J. Power Sources 199 (2012) 278. [17] I. Taniguchi, C.K. Lim, D. Song, M. Wakihara, Solid State Ion. 146 (2002) 1462. [18] M. Konarova, I. Taniguchi, Mater. Res. Bull. 43 (2008) 3305.

Fig. 5. Electrochemical performance of Li2FeP2O7/C nanocomposite cathode. (a) Chargeedischarge profiles at different C rates. (b) Cycle performance. Table 1 Comparison of electrochemical performance of Li2FeP2O7/C nanocomposites of this study and previous reported data. Authors

Precursors

Discharge capacity

Zhou et al. [3]

Li(CH3COO) Fe(CH3COO) NH4H2PO4 Li2CO3 FeC2O4$2H2O (NH4)2HPO4 LiOH$H2O Fe(NO3)3$9H2O, Fe NH4H2PO4 Li2CO3 FeC2O4$2H2O (NH4)2HPO4 LiH2PO4 Fe(NO3)3$9H2O LiH2PO4 FeC2O4$2H2O Li2CO3, FeC2O4$2H2O, (NH4)2HPO4 LiOH, FeC2O4$2H2O, (NH4)2HPO4 LiF, FeC2O4$2H2O, (NH4)2HPO4 LiH2PO4 FeC2O4$2H2O LiNO3 Fe(NO3)3$9H2O H3PO4

85 mAh g1(2.5 mA g1)

Furuta et al. [7]

Tan et al. [8]

Du et al. [10]

Barpanda et al. [11] Zhang et al. [12] Zhang et al. [12]

Zhnag et al. [12]

Zhang et al. [12]

Zheng et al. [13] This work

100 mAh g1(0.05 C)

107 mAh g1(0.05 C)

103 mAh g1(0.1 C) 75.7 mAh g1(1 C) 100 mAh g1(0.05 C) 103 mAh g1(0.05 C) 80 mAh g1(1 C) 95 mAh g1(0.05 C) 60 mAh g1(1 C) 85 mAh g1(0.05 C) 43 mAh g1(1 C) 70 mAh g1(0.05 C) 38 mAh g1(1 C) 100 mAh g1(0.05 C) 67 mAh g1(1 C) 100 mAh g1(0.05 C) 47 mAh g1(1 C)