C cathode material prepared by solvothermal method

Solid State Communications 150 (2010) 81–85

Contents lists available at ScienceDirect

Solid State Communications journal homepage: www.elsevier.com/locate/ssc

Enhanced electrochemical performance of unique morphological LiMnPO4 /C cathode material prepared by solvothermal method Yourong Wang, Yifu Yang ∗ , Yanbo Yang, Huixia Shao College of Chemistry and Molecular Science, Wuhan University, Wuhan 430072, PR China

article

info

Article history: Received 24 January 2009 Received in revised form 13 April 2009 Accepted 30 September 2009 by M. Wang Available online 4 October 2009 PACS: 81.20-n 84.60 Dn Keywords: A. Li-ion battery A. LiMnPO4 D. Electrochemical performance E. Solvothermal method

abstract The LiMnPO4 /C composite material with ordered olivine structure was synthesized in 1:1(v/v) enthanol–water mixed solvent in the presence of cetyltrimethylammonium bromide (CTAB) at 240 ◦ C. Rod-like particle morphology of the resulting LiMnPO4 /C powder with a uniform particle dimension of 150 × 600 nm was observed by using scanning electron microscope and the amount of carbon coated on the particle surface was evaluated as 2.2wt% by thermogravimetric analysis, which is reported for the first time to date for LiMnPO4 /C composite. The measurement of the electrochemical performance of the material used in rechargeable lithium ion battery shows that the LiMnPO4 /C sample delivers an initial discharge capacity of 126.5 mA h g−1 at a constant current of 0.01 C, which is 74% of the theoretical value of 170 mA h g−1 . The electrode shows good rated discharge capability and high electrochemical reversibility when compared with the reported results, which is verified further by the evaluation of the Li ion diffusion coefficient of 5.056 × 10−14 cm2 /s in LiMnPO4 /C. © 2009 Elsevier Ltd. All rights reserved.

1. Introduction Olivine structured lithium transition-metal (ortho) phosphates have recently attracted much attention as potential Li-ion battery cathode materials due to their lower toxicity, lower cost, better thermal and chemical stability [1,2]. LiMnPO4 offers an equilibrium redox voltage of 4.1 V versus Li+ /Li, which is considered to be compatible with most liquid electrolytes presently used in Liion batteries [3]. However, some disadvantages such as poor rate performance and poor conductivity have limited their practical application in high power batteries [4–6]. In order to overcome these limitations, much effort has been paid. Carbon coating has been verified to be an efficient way to increase the electrochemical performance of these materials [7,8]. However, the carbon coating method obviously helps nothing in the lattice electronic conductivity or chemical diffusion coefficient of lithium within the crystal. In recent years, two ways have been adopted for this improvement: one is that the metal ion doping was involved in enhancing the intrinsic electronic conductivity of the material [9,10] and the other one is where the synthesis processes were optimized to minimize the particle size and obtain uniform particle size distribution and

∗

Corresponding author. Tel.: +86 27 87218624; fax: +86 27 68754067. E-mail address: [email protected] (Y. Yang).

0038-1098/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2009.09.046

unique morphology, thereby reducing the diffusion path length for lithium ions in the cathode material [11,12] and establishing a large contact area with conductive additives such as carbon. Kwon et al. [4] reported that the cathode material of LiMnPO4 with a size of 130 nm could be obtained by sol–gel synthesis with a reversible capacity of 134 mA h g−1 at 0.1 C. By the precipitation of LiMnPO4 from aqueous solution, the resulting particle size was as small as 100 nm, and a reversible capacity of 70 mA h g−1 at 0.05 C was observed [13]. Recently, the platelet-like LiMnPO4 material with a thickness of 35 nm was successfully synthesized by polyol method with a demonstrated capacity of >50 mA h g−1 even at 5 C and with good capacity retention upon cycling [14]. Based on these results, it is evident that the particle size and morphology are critical in determining the usable capacity and charge/discharge rate capability of lithium ion batteries. The traditional synthesis method of lithium ion battery materials was restricted in direct solid state reaction of precursors at high temperature for a long time [15]. But in recent years, wet methods are selected more widely that can offer more advantages such as better homogeneity, more regular morphology, sub-micron sized particles, and larger specific surface area [16,17]. A considerable improvement in the performance of cathode materials has been accomplished with this method. In this article, we report LiMnPO4 /C prepared by solvothermal method with unique particle morphology and uniform size distribution at different current densities.

82

Y. Wang et al. / Solid State Communications 150 (2010) 81–85

3. Results and discussion 3.1. Structure and morphology of LiMnPO4 /C The results from the XRD analysis showed in Fig. 1 indicate that all the samples are in good agreement with the standard LiMnPO4 with an ordered olivine structure indexed by orthorhombic Pnmb space group. No impurity phase (as Li3 PO4 reported in the literature) was found. FTIR analysis was performed on a carbon-coated LiMnPO4 . The absorbance spectra of the materials showed in Fig. 2 manifests that all the bands belongs to the characteristic of LiMnPO4 [18] to the exclusion of a no-recognized, low-intensity band, which we think is due to a negligible amount of impurity present. Generally speaking, the carbon layer coated on the LiMnPO4 particle surface is beneficial to the improvement of the electronic

20

30 2θ/degree

40

Fig. 1. XRD patterns of samples: (A) as-prepared LiMnPO4 /C, (B) LiMnPO4 after TGA of LiMnPO4 /C, and (C) standard LiMnPO4 (Card 33-804).

0.8 0.7 0.6 Absorbance units

The precursors used were LiOH.H2 O (Hengxin Chemicals Co. Ltd., Shanghai), H3 PO4 (Tianda Chemicals Factory, Tianjin), MnSO4 .H2 O (Shanghai Chemicals Factory) and cetyltrimethylammonium bromide (CTAB, Kermel Chemicals Co. Ltd., Tianjin). The stoichiometric amount of the precursors was dissolved in 1:1 (v/v) ethanol–water mixed solvent. The obtained solution was transferred into stainless steel autoclaves and heated at 240 ◦ C for 12 h, after which, it was cooled slowly to ambient temperature. The offwhite precipitate was filtrated, rinsed several times with ethanol and finally dried at 100 ◦ C. The as-prepared LiMnPO4 was mixed with a certain amount of dextrose as carbon source, and finally it was calcined at 700 ◦ C for 5 h under Ar to yield LiMnPO4 /C. The structural characterization was detected using X-ray powder diffraction (XRD, shimadzu XRD-6000) with Cu Kα radiation. The diffraction patterns were analyzed by Rietveld method using Fullprof software. The morphology of the samples was examined by a field emission scanning electron microscopy (FESEM, Hitachi S-300). A classical fitting procedure of the Fourier transform infrared (FTIR) spectra ranging from 400 to 1400 cm−1 was done to characterize the spectral features of the LiMnPO4 /C cathode materials. In order to determine the exact amount of carbon coated on the olivine particles, thermogravimetric analysis (TGA; Seiko 6300 TGADTA) was carried out from room temperature to 700 ◦ C at a heating rate of 10 ◦ C min−1 in O2 flux. Electrochemical testing of the LiMnPO4 cathode materials was performed by using coin-type 2016 cells with a lithium metal anode. The cathode materials were made by mixing LiMnPO4 /C, acetylene black, PTFE in a weight ratio of 80:15:5, respectively. The electrolyte was prepared with 1 mol L−1 LiPF6 dissolved in ethylene r carbonate–dimethyl carbonate (1:1, v/v), and a Celguard 2325 microporous membrane (American) was used as the separator. The cells were assembled in a glove box filled with pure argon. The cathode performance of the electrode made with the sample materials was examined by a program-controlled Battery r Test System (Land , Wuhan, China). The charge and discharge characteristics of the cathodes were evaluated in the voltage range of 2.4–4.8 V vs. Li+ /Li0 at room temperature. The electrochemical capacity of the samples was evaluated on account of the active material. Both electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) were performed in a three-electrode cell with lithium foil as counter and reference electrodes by using a CHI 660B Electrochemical Workstation (Chenghua, Shanghai, China) at room temperature. The CV tests were carried out at a potential scan rate of 0.01 mV s−1 and 0.2 mV s−1 , respectively. In the EIS measurement, the excitation voltage applied to the cells was 5 mV and the frequency was in the range of 100 kHz to 0.05 Hz.

Intensity/a.u.

2. Experimental

0.5 0.4 0.3 0.2 0.1 0.0 -0.1

400

600

800 1000 Wavenumber(cm-1)

1200

1400

Fig. 2. FTIR spectra of LiMnPO4 /C.

conductivity of the materials. However, the added carbon will impact the volumetric energy density of the LiMnPO4 /C composites which could restrict the practical application. So, a control on carbon amount to an appropriate level is very important for the preparation of the LiMnPO4 /C cathode material.The amount of carbon in the carbon-coated materials is generally evaluated by TGA analysis as in Ref. [19]. Thereby, the TGA performed under flowing oxgen is applied to determine the amount of carbon coated on the LiMnPO4 . Fig. 3 shows the typical result of TGA analysis. From Fig. 3, it is found that while the sample is heated under O2 flux up to 700 ◦ C, LiMnPO4 is still stable which is different from LiFePO4 which could be completely oxidized under such conditions [20], as evident from Fig. 1(B) which is the XRD of LiMnPO4 after TGA experiment of the LiMnPO4 /C sample. No more weight loss was found when the temperature was rised to more than 700 ◦ C, whereas carbon has been lost gradually by combustion in the sample under flowing oxgen and a weight loss of 2.2% indicates a carbon content of this level in the LiMnPO4 /C sample. As compared with LiFePO4 /C [21], the carbon content of 2.2% has no severe effect on the tap density and the volumetric energy density of the LiMnPO4 /C composites. The lattice parameters of the typical samples calculated from a Rietveld refinement are shown in Table 1. It is obvious that the parameters a, b and c are almost the same with those of LiMnPO4 , which are also close to that in the literature [6,11]. This clearly indicates that the existence of the coated amorphous carbon has no influence on the crystalline structure of LiMnPO4 . The morphology of the LiMnPO4 /C sample is shown in Fig. 4. The SEM micrograph shows that the sample has small and uniform particle dimensions, the product particles are rod-like and the

Y. Wang et al. / Solid State Communications 150 (2010) 81–85

Fig. 3. TGA curves of LiMnPO4 /C.

83

Fig. 5. Galvonostatic charge and discharge curves of LiMnPO4 /C. Table 1 Calculated crystal parameters of LiMnPO4 /C.

Fig. 4. SEM image of the prepared LiMnPO4 /C sample.

average size is about 150 × 600 nm. This unique morphology and small size of the particle is tuned by changing the ratio of enthanol and water in the mixed solvent, which is the first reported to date for LiMnPO4 /C. Moreover, it was found in our primary experiment that these rod-like particles are easy to congregate without the surfactant CTAB (not shown here). Thus, the surfactant CTAB is used to act as a dispersant to disperse the rod-like particles. The narrow size distribution and unique morphology make the materials possible to achieve better electrochemical performance. 3.2. Electrochemical properties of LiMnPO4 /C For potential battery application, the electrochemical performance of the LiMnPO4 /C composite was examined. Coin cells constructed with a LiMnPO4 /C cathode and lithium anode were galvanostatically charged and discharge between 2.4 and 4.8 V. Fig. 5 shows the charge and discharge curves of a cell for the first three cycles at a rate of 0.01 C. It can be seen from Fig. 5 that the plateaus are obvious in charge and discharge curves at potential of 4.1 V (versus Li+ /Li). This behavior corresponds to the solid-state redox of Mn3+ /Mn2+ in LiMnPO4 accompanied with Li+ ion extraction and insertion [22]. The initial discharge capacity has reached 126.5 mA hg−1 , which is 74% of the theoretical values (170 mA h g−1 ) and is the better value reported to date for this material. This may be attributed to the unique rod-like morphology and small and uniform size of this material, which can further result in an improved electronic conductivity of LiMnPO4 and the possibly faster diffusion of Li+ ion in the olivine structure. The smaller particle size, which is helpful for the accessibility of the redox centers, is preferable to achieve larger capacity. In the following two cycles, the reversible capacities are 123.5 mA hg−1 and

Samples

a (Å)

b (Å)

c (Å)

A B

10.4367 10.4430

6.0951 6.1044

4.7448 4.7466

120.7 mA hg−1 , respectively. It seems that the discharge capacity decreases slightly with cycle, which is expected to be attributed to the loss of charge capacity because the ploarization of the charge curve has become larger with the increase in the cycle. As shown in Fig. 5, the upper cut-off potential of 4.8 V (vs. Li+ /Li) is reached earlier, but the capacity retentiveness is still better than the reported results. The more remarkable advantage of this material is its rate capability. Fig. 6 shows the voltage profiles of LiMnPO4 /C as a function of specific capacity with discharge rates of 0.1 C and 0.01 C. All the charge processes of the cell were fulfilled with the same current of 0.01 C to insure the identical initial conditions for each discharge. The electrode is able to deliver a specific capacity of 126.5 mA h g−1 at a rate of 0.01 C, with a voltage plateau at about 4 V versus Li/Li+ . The capacity reached 113.6 mA h g−1 at a rate of 0.1 C. A good voltage plateau still remained above 3.7 V. The rated discharge performance indicates that the LiMnPO4 /C composite synthesized by the method introduced in this work is promising to be used as a cathode material for high-power lithium batteries. The rate capability could be attributed to the high phase purity, narrower particle size distribution, unique rod-like morphology and improved conductivity through carbon connection. As is well known, olivine LiMnPO4 material is characterized by poor electronic conductivity and limited lithium-ion diffusion. The character of small particle size of LiMnPO4 /C ensures a large surface area of the reaction phase and the reduction of the diffusion length of lithium ion inside, and results in faster reaction and diffusion kinetics. The CV measurements were performed on LiMnPO4 /C electrode to characterize its electrochemical reactions in Li-ion cells. Fig. 7 shows the typical cyclic voltammograms of the LiMnPO4 /C electrodes. The reduction and oxidation peak positions are located at 3.93 V and 4.25 V (versus Li+ /Li), respectively. This indicates that the as-prepared LiMnPO4 /C has a reversibility of its electrochemical reaction. Additionally, it is worth noting that there is an extra oxidation peak with peak potential higher than that of LiMnPO4 /C, which is ascribed to the oxidation of electrolyte [12]. So, the upper potential limit for the charge should be suitably set to avoid the oxidation of electrolyte, otherwise, parts of the electrons exchanged are actually towards the oxidation of electrolyte rather than the active material itself during the process of charge–discharge; this is

84

Y. Wang et al. / Solid State Communications 150 (2010) 81–85

5.0

700 600 500

4.0 Z''/ Ω

E(V) vs. Li/Li+

4.5

3.5 3.0

400 300 200 100

2.5

0 2.0 -20

0

20

40 60 80 Capacity (mA h g-1)

100

120

0

140

I/mA

0.05

400

600 Z'/ Ω

800

1000

Fig. 8. Impedance spectra for LiMnPO4 /C electrode.

Fig. 6. Discharge curves at different discharge current densities.

0.10

200

3 2 1

0.00 -0.05 3 2 1 -0.15 3.2 3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.8 E /V -0.10

Fig. 7. CV of the LiMnPO4/C composite at a scan rate of 0.01 mV s−1 . Insert: at a scan rate of 0.2 mV s−1 for the first three cycles.

adverse to the cycling performance of the cell. The insert in Fig. 7 shows the good overlap of the CV curves for the first three cycles. This clearly demonstrates further the good life cycle of the as-prepared LiMnPO4 /C powder materials. To gain further insight on the good electrochemical performance, EIS in the fully discharged state is used to evaluate the Li-ion diffusion coefficient in LiMnPO4 /C. Fig. 8 shows the typical Nyquist plot of the LiMnPO4 /C composite electrodes. The profile exhibits a semicircle in the high frequency region and a straight line in the low frequency region. An intercept at the Z0 axis in high frequency corresponds to the ohmic resistance (Re ), which represented the resistance of the electrolyte. The semicircle in the middle frequency range is attributed to the charge transfer process, whose diameter on the Z0 axis is approximately equal to the charge transfer resistance (Rct ). The inclined line in the low frequency represented the Warburg impedance (Zw ), which was associated with lithium-ion diffusion in the active cathode material. Thus, the diffusion coefficient of lithium ions could be calculated from the low frequency spots according to the following equation [23] D=

R2 T 2

(1) 2n2 A2 F 4 C 2 σ 2 where R is the gas constant, T is the absolute temperature, A is the surface area of the cathode, n is the number of transferred electrons per molecule during oxidization, F is the Faraday constant, C is the lithium ion concentration in electrode material, and σ is the Warburg factor which is related to Z00 , as shown in Eq. (2). Z 00 = σ ω1/2 .

(2)

Fig. 9. Relationship between Z00 and square root of frequency (ω−1/2 ) in the low frequency region.

The relationship between Z00 and reciprocal square root of frequency (ω−1/2 ) in the low frequency region is shown in Fig. 9. According to Figs. 8 and 9, the diffusion coefficient of lithium ions in the LiMnPO4 /C is calculated as 5.056 × 10−14 cm2 /s. This numerical value was comparable with that of the reported LiFePO4 /C, which are currently used in commercial Li-ion batteries [24]. This clearly proves that the diffusion of Li-ion in LiMnPO4 /C has been ameliorated significantly. 4. Conclusions The LiMnPO4 /C nanocomposite material is synthesized and optimized by solvothermal method and is successfully dispersed by the surfactant CTAB. The resulting product is well crystallized and the particles are in rod-like form. This unique morphology is reported for the first time for LiMnPO4 . Measurements by TGA and XRD show that LiMnPO4 is stable and the appreciable amount of carbon with 2.2 wt% is coated on LiMnPO4 particle surface. When the as-prepared LiMnPO4 /C nanocomposite is used as a cathode material for the rechargeable lithium ion battery, it delivers an initial discharge capacity of 126.7 mA h g−1 at a rate of 0.01 C and exhibits good discharge capability and high electrochemical reversibility when compared with the reported results of LiMnPO4 /C. The calculated Li-ion diffusion coefficient in the LiMnPO4 /C further demonstrate that the good electrochemical performance is attributed to the ameliorated diffusion path due to the small size and unique morphology of the particles. Therefore, it can be concluded that the solvothermal method is of potential

Y. Wang et al. / Solid State Communications 150 (2010) 81–85

use in tuning the size and morphology of the material to improve its function. Acknowledgements The financial support by the 863 National Research and Development Project Foundation of China (grant No. 2006AA11A152) is gratefully acknowledged. References [1] J.M. Tarascon, M. Armand, Nature 414 (2001) 359. [2] A.K. Padhi, K.S. Nanjundaswamy, J.B. Goodenough, J. Electrochem. Soc. 114 (1997) 1188. [3] A. Yamada, S.C. Chung, J. Electrochem. Soc. 148 (8) (2001) A960–A967. [4] N.H. Kwon, T. Drezen, I. Exnar, I. Teerlinck, M. Isono, M. Graetzel, Electrochem. Solid-State Lett. 9 (2007) A277. [5] H.S. Fang, L.P. Li, G.S. Li, Chem. Lett. 36 (2007) 436. [6] T.R. Kim, D.H. Kim, H.W. Ryu, J.H. Moon, J.H. Lee, S. Boo, J. Kim, J. Phys. Chem. Solids. 68 (2007) 1203. [7] B. Jin, H.-B. Gu, K.-W. Kim, J. Solid State Electrochem. 12 (2008) 105.

85

[8] H. Gabrisch, J.D. Wilcox, M.M. Doeff, Electrochem. Solid-State Lett. 9 (2006) A360. [9] C.A.J. Fisher, V.M.H. Prieto, M.S. Islam, Chem. Mater. 20 (2008) 5907. [10] J. Xu, G. Chen, Y.-J. Teng, B. Zhang, Solid State Commun. 147 (2008) 414. [11] M. Yonemura, A. Yamada, Y. Takei, N. Sonoyama, R. Kanno, J. Electrochem. Soc. 151 (2004) A1352. [12] C. Delacourt, L. Laffont, R. Bouchet, C. Wurm, J.B. Leriche, M. Morcrette, J.M. Tarascon, C. Masquelier, J. Electrochem. Soc. 152 (2005) A913. [13] C. Delacourt, P. Poizot, M. Morcrette, J.M. Tarascon, C. Masquelier, Chem. Mater. 16 (2004) 93. [14] I. Exnar, J. Miners, Nanotechnology solutions enabling industrial battery applications: LiMnPO4 synthesised via Polyol, olivines in lithium batteries, Caltech, January 17–18, 2007. [15] A. Yamada, M. Hosoya, S.-Ch. Chung, Y. Kudo, K. Hinokuma, K.-Y. Liu, Y. Nishi, J. Power Sources 119/121 (2003) 232. [16] T. Drezen, N.-H. Kwon, P. Bowen, I. Teerlinck, M. Isono, I. Exnar, J. Power Sources. 174 (2007) 949. [17] N.N. Bramnik, H. Ehrenberg, J. Alloys Compd. 464 (2008) 259. [18] C.M. Burba, R. Frech, J. Power Sources. 172 (2007) 870. [19] S. Beninati, L. Damen, M. Mastragostino, J. Power Sources. 180 (2008) 875. [20] I. Belharouak, C. Johnson, K. Amine, Electrochem. Commun. 7 (2005) 983. [21] H. Huang, S.C. Yin, L.F. Nazar, Electrochem. Solid-State Lett. 4 (2001) A170. [22] G. Li, H. Azuma, M. Tohda, Electrochem. Solid-State Lett. 149 (2002) A743. [23] A.J. Bard, J.R. Faulkner, Electrochemical methods, 2nd ed., Wiley, 2001, 231. [24] F. Gao, Z. Tang, Electrochim. Acta. 53 (2008) 5071.