C nanocomposites as cathode materials for Li-ion batteries

C nanocomposites as cathode materials for Li-ion batteries

Electrochimica Acta 103 (2013) 219–225 Contents lists available at SciVerse ScienceDirect Electrochimica Acta journal homepage: www.elsevier.com/loc...

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Electrochimica Acta 103 (2013) 219–225

Contents lists available at SciVerse ScienceDirect

Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

Synthesis and characterization of Li2 FeP2 O7 /C nanocomposites as cathode materials for Li-ion batteries Juan Du, Lifang Jiao ∗ , Qiong Wu, Yongchang Liu, Yanping Zhao, Lijing Guo, Yijing Wang, Huatang Yuan Institute of New Energy Material Chemistry, Key Laboratory of Advanced Energy Materials Chemistry (MOE), Tianjin Key Lab of Metal and Molecule-based Material Chemistry, Nankai University, Tianjin 300071, PR China

a r t i c l e

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Article history: Received 22 February 2013 Received in revised form 29 March 2013 Accepted 2 April 2013 Available online 16 April 2013 Keywords: Solid-state reaction Lithium iron pyrophosphate/carbon nanocomposites Cathode materials Lithium ion batteries

a b s t r a c t The pristine Li2 FeP2 O7 and Li2 FeP2 O7 /C nanocomposites with different content of carbon have been successfully synthesized via a simple solid-state reaction, using cheap glucose as carbon source. XRD and EDS patterns demonstrate the high purity of the products. SEM images exhibit that the size of the particles is about 50–500 nm. Electrochemical measurements reveal that carbon coating and reducing particle size significantly enhance the electrochemical performances of Li2 FeP2 O7 . Particularly, the Li2 FeP2 O7 /C sample with a carbon content of 4.88 wt.% displays the best performance with a specific discharge capacity of 103.1 mAh g−1 at 0.1 C, which is 93.7% of its one-electron theoretical capacity, meaning 110 mAh g−1 . Meanwhile, it shows favorable cycling stability and excellent rate performance, indicating its potential applicability in Li-ion batteries in the long term. © 2013 Elsevier Ltd. All rights reserved.

1. Introduction The threats arising from limited oil storage and environmental contamination have forced us to look for alternative energy storage and conversion systems. Since the commercialization of LiCoO2 by SONY in 1991, Li-ion batteries have attracted considerable attention in recent years with the increasing demand in the portable electronics market and emerging applications in electric vehicles (EVs) and hybrid electric vehicles (HEVs). It is well known that the cathode materials have a significant impact on battery capacity, cycle life, safety and cost [1,2]. Among those cathode materials, the olivine structure lithium iron phosphate (LiFePO4 ) has been spotlighted as a potential candidate to replace LiCoO2 and LiNiO2 due to its numerous appealing merits such as low cost, low toxicity, high thermal stability and relatively high theoretical specific capacity [3–5]. However, its poor high-rate performance of this material restricts its use in large-scale application owing to its low electronic conductivity and slow lithium-ion diffusion [6–8]. Consequently, it is necessary to explore new cathode materials with elevated efficiency, superior storage capacity, better gravimetric energy density and longer cycle life. Currently, a great deal of interest to new phosphate materials built by polyanions (P2 O7 )4− is driven by their thermal stability and improved safety [9–12]. Scoring over LiFePO4 , the pyrophosphate

∗ Corresponding author. Tel.: +86 22 23498089; fax: +86 22 23502604. E-mail address: [email protected] (L. Jiao). 0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2013.04.036

system offers several positive attributes such as a two-dimensional channel for Li-diffusion vs one-dimensional channel for LiFePO4 , and the possibility of a 2-electron redox reaction. Meanwhile, it can be synthesized by simple solid-state reaction at 600 ◦ C without any further optimization. One of the key aspects of this material is the major structural anomalies, such as anisotropic deformation, significant loss of long-range order, local phase segregation and clustering impurities, which strongly depend on the preparation conditions [13]. Another important factor is the low valence configuration of the transition metal in the discharged state, i.e., Fe2+ for iron ions that allows oxidation at potential higher than 3 V versus Li+ /Li0 . It is expected that lithium iron pyrophosphate (Li2 FeP2 O7 ) could be a candidate as a positive electrode material for Li-ion batteries providing a theoretical capacity of 110 mAh g−1 when one electron is transferred. Since Li2 FeP2 O7 initially reported by Nishimura et al. [14], it arouse researcher’s attention gradually. Nishimura et al. [14] and Zhou et al. [9] found that Li2 FeP2 O7 is a promising 3.5 V class cathode material for Li-ion batteries and can safely deliver highenergy density. Kim et al. [15] reported the detailed atomic sites and occupancies of Li2−x FeP2 O7 determined by Rietveld refinements of neuron diffraction (ND) and XRD. Barpanda et al. [16] synthesized Li2 FeP2 O7 by means of eco-efficient splash combustion method. However, the reported capacity is poor owing to the poor conductivity and large particle size [14]. Carbon coating has been proved to be an effective method for improving the electronic conductivity of the electrode materials [17]. By forming a conductive network in them, the electron transport efficiency is significantly enhanced. On the other hand, many

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other studies have been conducted to mitigate lithium diffusion limitation by reducing particle size which can significantly shorten the diffusion distance and thus lower the ionic diffusion resistance [18]. Therefore, a combination of carbon coating and reducing particle size may be a feasible way to improve electrochemical performance of Li2 FeP2 O7 . To the best of our knowledge, the study of carbon coating and reducing particle size to improve the performance of Li2 FeP2 O7 has not been reported. Moreover, the detailed study about the electrochemical properties of Li2 FeP2 O7 is scarce. In this paper, carbon coating and reducing particle size are adopted to improve the electrochemical property of Li2 FeP2 O7 , and we report our recent work on the synthesis, characterization and electrochemical properties of Li2 FeP2 O7 . The results demonstrate its latent application in Li-ion batteries. 2. Experimental methods 2.1. Preparation of Li2 FeP2 O7 /C The Li2 FeP2 O7 /C material was prepared using a conventional solid-state method. The starting materials, containing stoichiometric amounts of Li2 CO3 , FeC2 O4 ·2H2 O and (NH4 )2 HPO4 , were dispersed in an acetone medium and milled by a planetary highenergy ball-mill for 5 h under Ar protection. The mass ratio of ball-to-powder was 20:1 and the rotation speed was 450 rpm. After the evaporation of acetone, the mixture was sintered in Ar atmosphere at 300 ◦ C for 6 h. The obtained powder was reground and then subsequently sintered at 600 ◦ C for 12 h under an Ar gas flow. For the carbon coating, different amounts of glucose (0, 6.25, 18.75, 31.25 wt.% vs the precursor) were added to the yielded precursor, ball-milled and thermal treated again at 600 ◦ C for 12 h under Ar gas. The products obtained were designated as S1, S2, S3 and S4 for the sake of simplicity. 2.2. Characterizations of samples X-ray diffraction (XRD) was carried out on a Rigaku.D/Max˚ to identify 2500 diffractometer with Cu K␣ radiation ( = 1.5418 A) the crystalline phases. The valence of Fe was obtained using X-ray photoelectron spectra (XPS, PHI 5000) equipped with monochromatic Al K␣ radiation (h = 1486.6 eV). The chemical bondings in the Li2 FeP2 O7 were measured by Fourier transform infrared (FTIR). The morphology was observed through a HITACHI S-4800 scanning electron microscopy (SEM). The S-4800 SEM is equipped with EDS, which is used to analyze the elemental composition. The inner microstructure was tested with a Tecnai 20 transmission electron microscope (TEM). The carbon content was analyzed by a PerkinElmer 2400 Series II CHNS/O elemental analyzer.

rate of 0.1 mV s−1 . Galvanostatic charge–discharge tests were performed between 2.5 V and 4.2 V under a Land (CT2001) automatic Battery tester at ambient temperature. 3. Results and discussion 3.1. The TG/DTA analysis of the precursor Fig. 1 is the TG/DTA curve of the precursors at a heating rate of 10 ◦ C/min from the room temperature to 700 ◦ C under a Nitrogen atmosphere. The TG curve present three evident steps of weight loss and DTA curve displays corresponding exothermic peaks. The initial weight loss (about 3%) below 100 ◦ C is due to the loss of physically adsorbed water. The main weight loss (about 35%) between 100 ◦ C and 350 ◦ C is ascribed to the loss of crystal water and the decomposition of reactants including the decomposition of ammonium, oxalate, carbonate and glucose into NO2 , CO2 , CO and H2 O. The final small weight loss (about 7%) from 350 ◦ C to 550 ◦ C may correspond to the crystallizaton of the pyrophosphate. Above 550 ◦ C, there is nearly no weight loss on TG curve and no exothermic peaks on DTA curve, which indicates the complete crystallization of Li2 FeP2 O7 . Therefore, it is suitable for sintering the precursor above 550 ◦ C to gain Li2 FeP2 O7 /C composites. 600 ◦ C is chosen as the final sintering temperature due to the product sintered at this temperature has better crystallization and electrochemical performance. 3.2. Sample characterization The XRD patterns of the pristine Li2 FeP2 O7 and Li2 FeP2 O7 /C composites are shown in Fig. 2a. As shown in Fig. 2a, all the peaks are sharp and no reflections are detected, indicating that the composites are well crystallized, which are in good accordance with the literature values [9]. The typical diffraction peaks corresponding to carbon yielded from the decomposition of glucose are not visible due to its amorphous phase and the high pack of Li2 FeP2 O7 nanoparticles in the established carbon structure. Elemental analysis displays that the carbon content of S2 to S4 is 1.99, 4.88 and 6.73 wt.%, respectively. Moreover, the EDS results of Fig. 2b unambiguously confirm that the particles in the Li2 FeP2 O7 only consist of Fe, P, and O elements and the atomic ratio of Fe, P, and O is 1:2.14:7.6, which well coincides with the designated stoichiometry of Li2 FeP2 O7 , thus confirming the formation of pure phase (Li2 FeP2 O7 ). It is not possible to detect Li for the obvious reason that the X-ray fluorescence yield is extremely low for elements Li. From the above analysis, we can deduce that the obtained Li2 FeP2 O7 is pure and well crystallized.

2.3. Cell assembly Electrochemical performances of Li2 FeP2 O7 /C composites were evaluated in Li test cell with two-electrode. The working electrode was fabricated by mixing 85 wt.% active materials with 10 wt.% acetylene black and 5 wt.% polyvinylidene fluoride (PVDF) in an N-methyl-2-pyrrolidone (NMP) solution. The resulting slurry was coated on an aluminum foil of 10 mm in diameter. The circular strips were dried at 80 ◦ C for 12 h in a vacuum oven. The lithium metal was used as the counter and reference electrode. The electrolyte was composed of 1.0 mol L−1 LiPF6 dissolved in ethylene carbonate (EC), ethylene methyl carbonate (EMC), dimethyl carbonate (DMC) with a volume ratio of 1:1:1. Celgard 2320 was used as the separator. The cell was assembled in an argon filled glove box with density of H2 O and O2 below 5 ppm. Cyclic voltammetry (CV) was performed under a CHI600B electrochemical workstation at a

Fig. 1. TG/DTA curve of the precursor under flowing nitrogen.

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Fig. 2. (a) X-ray diffraction patterns of Li2 FeP2 O7 /C composites (S1 to S4), (b) EDS spectra of Li2 FeP2 O7 powders (The insert shows the element compositions of Li2 FeP2 O7 ).

Fig. 3. SEM images of the pristine Li2 FeP2 O7 and Li2 FeP2 O7 /C composites (S1 to S4).

Fig. 4. Typical SEM and TEM images of S3. (a) SEM, (b) TEM.

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and 533.5 eV with a shift EO 1s = 1.7 eV (Fig. 6d). Sherwood and coworkers [25–27] have preformed extensive studies on several P–O compounds and attributed the overlapping two-peak structure of O 1s to terminal (lower BE component) and bridging (higher BE component) oxygens. Thus we attribute the peak at 531.8 eV to the terminal oxygen while the compound at 533.5 eV to the bridging oxygen. The peak of the P 2p is at 133.8 eV and the full width at half-maximum (fwhm) is 1.65 eV (Fig. 6e), which can be assigned to a PO4 3− [28]. 3.3. Electrochemical properties

Fig. 5. FTIR spectra of Li2 FeP2 O7 .

SEM images shown in Fig. 3 illustrate that the pristine Li2 FeP2 O7 and Li2 FeP2 O7 /C composites are composed of nanoparticles with an average size of 50–500 nm. The small particle size may give rise to high diffusion coefficient of Li+ in olivine composite cathode. The morphology of the Li2 FeP2 O7 /C has not changed greatly compared with the pristine Li2 FeP2 O7 . Nevertheless, the agglomeration degree of Li2 FeP2 O7 /C composites is different in every picture, and the particle size tends to reduce slightly with increasing carbon content. So, carbon coating can suppress particle growth, and the particle size of Li2 FeP2 O7 can be easily controlled [19,20]. Compared to S1, S2 and S4, sample S3 with homogeneous particle size has no obvious agglomerate. As shown in Fig. 4a, S3 is composed of well-distributed nanoparticles with the particle size of 50–200 nm, which implies that it may have good electrochemical properties. To further examine the particles of S3, TEM (Fig. 4b) investigation is also conducted. It can be seen that the inner Li2 FeP2 O7 material (black) is uniformly coated with a carbon layer (gray), and they are connected with a carbon layer originated from carbonization of glucose. As revealed by the elemental analyzer, the carbon content is about 4.88 wt.%. Fig. 5 shows the FTIR spectra of S3, which provides accurate information of the local structure and chemical bonding in Li2 FeP2 O7 . The FTIR absorption spectra recorded in the range of 400–1200 cm−1 can be divided in two groups corresponding to the internal and external modes of the pyrophosphate structure [15]. Commonly, the vibrational response of phosphate-based compounds exhibits similar spectra with dominant features in the spectral range of 400–1200 cm−1 due to the strongly covalent PO4 groups. Thus the infrared spectra of Li2 FeP2 O7 derive from the fundamental modes of PO4 . The absorption bands at 744.9 cm−1 and 941.9 cm−1 are attributed to asymmetric and symmetric vibrations of P–O–P respectively, which are the typical vibration peaks of pyrophosphate group. The absorption bands around 400–680 cm−1 are assigned to O–P–O bending and 1000–1300 cm−1 corresponding to P–O stretching in the PO4 . To further identify the chemical state of Fe and the other elements in the sample, XPS spectra of every individual element in S3 are acquired and reported in Fig. 6. Fig. 6a shows the XPS spectrum of C element on the surface of the sample. The peak observed at 285.0 eV is attributed to the C 1s obtained by glucose decomposition. The Fe 2p peak with a spin-orbit splitting component of 2p3/2 near 711.6 eV and 2p1/2 near 725.0 eV can be seen in Fig. 6b, which confirm the presence of Fe as Fe2+ in Li2 FeP2 O7 [21]. The spectrum in Fig. 6c indicates the main peak of Li 1s is at 55.3 eV. Comparison of the binding energy (BE) of Li 1s with that reported for various compounds confirms the existence of monovalent Li+ in Li2 FeP2 O7 [22–24]. The shape of O 1s is an overlap of two peaks at about 531.8

Fig. 7 depicts the initial discharge curves of S1–S4 at 0.1 C rate at room temperature. The emersion voltage of the second lithium ion is so high that the electrolyte is easy to decompose (It will be detailed discussed in the following CV measurements), so the range of test voltage is set as 2.5–4.2 V. It can be seen that carbon coating has great effect on the electrochemical performance of the composite. The pristine Li2 FeP2 O7 shows the lowest capacity of 48.5 mAh g−1 . With the carbon content increased from 6.25 to 18.75 wt.%, the discharge capacity increased from 87.3 mAh g−1 to 103.1 mAh g−1 . However, the initial discharge capacity of the sample with 31.25 wt.% glucose is only 92.0 mAh g−1 . The results suggest that proper amount of carbon can effectively improve the electrochemical properties of Li2 FeP2 O7 /C via enhancing the electronic conduction. However, excessive carbon leads to the particle size agglomerate severely, which may result in poor electrochemical properties. Sample S3 exhibits a highest specific discharge capacity of 103.1 mAh g−1 , which is superior to the reported [14]. The excellent performance of S3 is attributed to its uniform morphology and small particles with a thin and efficient carbon layer on the surface of Li2 FeP2 O7 (shown in Fig. 4). The rate properties of S1–S4 measured between 2.5 and 4.2 V are shown in Fig. 8. It can be seen that all the four samples exhibit rather reproducible capacities during 10-cycle test at each current rate. And the capacity can be retrieved when the lower current rate (0.1 C) is applied again. This result confirms that this synthesis method is reliable to produce stable Li2 FeP2 O7 /C composites. Furthermore, it is obvious that the discharge capacity, capacity retention and rate capability of S3 are higher than those of other samples. The specific discharge capacity of S3 is 103.1 mAh g−1 , 75.7 mAh g−1 , 54.4 mAh g−1 and 45.9 mAh g−1 at the rate of 0.1 C, 1 C, 5 C and 10 C, respectively. After 10 cycles, the discharge capacity still remain at 99.7 mAh g−1 , 73.8 mAh g−1 , 52.1 mAh g−1 , and 45.3 mAh g−1 , with the capacity retention of 96.7%, 97.5%, 95.8%, and 98.7%, respectively. On the basis of Figs. 7 and 8, we conclude that the sample of S3 yields the best electrochemical performance which could be attributed to both the carbon coating and the uniform morphology with nanosized particles which enhance the electronic conductivity and Li-ion diffusivity. 3.4. CV measurements The CV curve of S3 in the potential range of 2.5–4.9 V at a scan rate of 0.1 mV s−1 [0] is shown in Fig. 9(a). It can be obviously seen that there are two anodic peaks and one cathodic peak. During the cathodic scan, an intensity peak is obtained at 3.3 V, corresponding to the insertion of lithium ions into the bulk. During the anodic scan, the CV shows two peaks at 3.5 V and 3.8 V followed by a weak peak at 4.7 V. The peaks at 3.5 V and 3.8 V correspond to Fe2+ /Fe3+ redox couple [9]. It may be indexed to the transformation of Li2 FeP2 O7 to Lix FeP2 O7 (1 < x < 2) and LiFeP2 O7 respectively. Voltage curves for Li2 FeP2 O7 /Li half cells shown in Fig. 9(b) illustrates the steps involved in the extraction and insertion reaction between 2.5 V

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Fig. 6. XPS spectra of C 1s, Fe 2p, Li 1s, O 1s and P 2p core level of Li2 FeP2 O7 sample synthesized by solid-state method.

Fig. 7. Initial discharge curves of Li2 FeP2 O7 /C composites (S1 to S4) at 0.1 C between 2.5 and 4.2 V.

Fig. 8. Cyclic performances of Li2 FeP2 O7 /C composites (S1 to S4) at different current densities.

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Fig. 9. (a) Cyclic voltammetry (CV) of S3 at a scan rate of 0.1 mV s−1 between 2.5 V and 4.9 V, (b) Voltage curves for Li2 FeP2 O7 /Li half cells of S3 at 0.1 C rate between 2.5 V and 4.2 V.

and 4.2 V at 0.1 C. As shown in Fig. 9(b), there are two charge platforms at 3.5 V and 3.8 V, which coincide with the CV curves. Thus we induce that the first lithium is extracted in two steps, i.e. 3.5 V and 3.8 V versus Li/Li+ . The weak peak at 4.7 V is very controversial, which may come from the Fe3+ /Fe4+ redox couple or the oxidation of electrolyte at that high potential [9]. And this part need further research. 4. Conclusions Well crystallized Li2 FeP2 O7 /C composites with uniformly distributed particles have been prepared via a solid-state method and systematically characterized by XRD, EDS, SEM, TEM, FTIR, XPS, CV and galvanostatic charge/discharge. The results of charge/discharge tests turn out that the sample with a carbon content of 4.88 wt.% shows the best electrochemical properties. The initial discharge capacity is 103.1 mAh g−1 , which is 93.7% of the theoretical capacity of 110 mAh g−1 for 1 e− transferred. Meanwhile, the material displays a good cyclic stability at different current densities, which could be attributed to the high electronic conductivity and improving Li-ion diffusivity resulting from simultaneous treatment of carbon coating and reducing particle size. These findings demonstrate that Li2 FeP2 O7 is a potential cathode material for Li-ion battery. Acknowledgements This work was financially supported by 973 program (2010CB631303), NSFC (21073100, 51231003). TSTC (10JCYBJC08000, 11JCYBJC07200, 10SYSYJC27600) and 111 Project (B12015). References [1] A.S. Arico, P. Bruce, B. Scrosati, J.M. Tarascon, W.V. Schalkwijik, Conditions for fabrication of ideally ordered anodic porous alumina using pretextured Al, Nature Materials 4 (2005) 366–377. [2] J.M. Tarascon, M. Armand, Issues and challenges facing rechargeable lithium batteries, Nature 414 (2001) 359–496. [3] A.K. Padhi, K.S. Nanjundaswamy, J.B. Goodenough, Cathode materials for secondary (rechargeable) lithium batteries, Journal of the Electrochemical Society 144 (1997) 1188–1194. [4] A. Yamada, S.C. Chung, K.J. Hinokuma, Nanostructured materials for advanced energy conversion and storage devices, Electrochemical Society 148 (2001) A224–A229. [5] L.X. Yuan, Z.H. Wang, W.X. Zhang, X.L. Hu, J.T. Chen, Y.H. Huang, J.B. Goodenough, Development and challenges of LiFePO4 cathode material for lithium-ion batteries, Energy & Environmental Science 4 (2011) 269–284.

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