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Facile synthesis of micrometer Li1.05Mn1.95O4 and its low temperature performance for high power lithium ion batteries
Electrochimica Acta 81 (2012) 191–196
Contents lists available at SciVerse ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Facile synthesis of micrometer Li1.05 Mn1.95 O4 and its low temperature performance for high power lithium ion batteries Si-Rong Li a , Yu Qiao a , Yi Sun a,b , Si-Yuan Ge a , Yi-Meng Chen a , Ingo Lieberwirth b , Yan Yu a , Chun-Hua Chen a,∗ a CAS Key Laboratory of Materials for Energy Conversion, Department of Materials Science and Engineering, University of Science and Technology of China, Anhui Hefei 230026, China b Max Planck Institute for Polymer Research, Mainz 55128, Germany
a r t i c l e
i n f o
Article history: Received 11 June 2012 Received in revised form 24 July 2012 Accepted 24 July 2012 Available online 28 July 2012 Keywords: Lithium manganese oxide Low temperature performance Diffusion coefficient Lithium ion batteries
a b s t r a c t Micrometer Li1.05 Mn1.95 O4 has been synthesized by solid state reactions with the nano-Mn3 O4 as a precursor. X-ray diffraction, scanning electron microscopy and laser particle sizer are employed to investigate the structures, morphologies and particle size distributions of the powder. The micrometer Li1.05 Mn1.95 O4 exhibits good rate performance at room temperature with a specific capacity of 98.4 mAh g−1 at 5 C. The Li1.05 Mn1.95 O4 /Li half cell also shows good cycling performance at elevated temperature with 90.5% of its initial capacity retained after 100 cycles at 1 C. At −20 ◦ C, the Li1.05 Mn1.95 O4 delivers a stable cycling performance with a specific capacity of 84.5 mAh g−1 , being 84.1% of the capacity at room temperature. The cyclic voltammetry (CV) and rate performance measurements illustrate an increasing polarization with decreasing the temperature. In addition, the diffusion coefficients of lithium ions (DLi+ ) in Li1.05 Mn1.95 O4 at various temperatures (25, 0, −10 and −20 ◦ C) are determined to be in the magnitude of 10−10 to 10−12 cm2 s−1 by cyclic voltammetry (CV) method. © 2012 Elsevier Ltd. All rights reserved.
1. Introduction Spinel structure lithium manganese oxide (LiMn2 O4 ) and olivine structure lithium ion phosphate (LiFePO4 ) have been considered to be the optimum cathode materials in lithium ion batteries (LIBs) for electric vehicles (EVs) and hybrid electric vehicles (HEVs) due to their low cost and high safety. Recently, Park et al. [1] have made a comparison of spinel LiMn2 O4 and olivine LiFePO4 for the application in EVs and HEVs. For LiFePO4 , both of its electronic conductivity (10−9 to 10−8 S cm−1 ) and ionic conductivity (10−11 to 10−9 S cm−1 ) are much lower than those of spinel LiMn2 O4 (10−4 S cm−1 and 10−6 S cm−1 respectively) at room temperature. Thus, there will be a challenge for the rate performance of LiFePO4 , especially when the EVs and HEVs are used in the winter or other cold conditions. Furthermore, LiFePO4 needs to be synthesized under an inert atmosphere, which makes it more difficult to produce LiFePO4 powders with consistent quality from batch to batch compared with the production of LiMn2 O4 powders in air. Although spinel LiMn2 O4 exhibit these intrinsic advantages compared to olivine LiFePO4 , it has still some drawbacks which limit its practical applications in large-scale for the EVs and HEVs: (1) the dissolution of manganese from the active material into the electrolyte; (2) Jahn–Teller
∗ Corresponding author. Tel.: +86 551 3606971; fax: +86 551 3601592. E-mail address: [email protected] (C.-H. Chen). 0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2012.07.086
distortion of the spinel LiMn2 O4 ; (3) the instability of the electrolyte at high voltage; (4) the phase transition from cubic to unstable tetragonal. These problems have caused poor long term cycling performance of spinel LiMn2 O4 , especially at elevated temperatures. Many attempts have been tried to solve these problems such as cationic doping with Al [2], Cr, V [3] and Ti [4] to stabilize the spinel structure and surface coatings [5–8] to separate the contact of active materials and electrolyte. Besides, excess lithium in the spinel (Li1+x Mn2−x O4 ) is proved to be effective to improve the cycling performance for enhanced stability of spinel structure [9–11]. On the other hand, nanometer particles are believed to decrease the diffusion length of Li+ and improve the rate performance during the charge and discharge process. However, they can also cause more side reactions on the large surface of the nanometer particles as well as the dissolution of Mn2+ into the electrolyte. More importantly, nanometer particles usually leads to low packing density which is undesirable to fabricate the electrode laminate with a high loading density in practical batteries for the HEVs and EVs. In addition, LIBs need to work efficiently both at low temperatures and elevated temperatures. In literature, there are very few studies on the electrochemical performance of spinel LiMn2 O4 at low temperatures [12]. Thus, in this paper, we try to synthesize micrometer Li1.05 Mn1.95 O4 powders and much attention is paid to their electrochemical performance at low temperatures (e.g. −20, −10 and 0 ◦ C).
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2. Experimental All the chemical reagents were analytical grade. Firstly, a nanometer-sized Mn3 O4 powder was prepared in an oil-bath synthesis process as reported in our previous work [13]. In brief, 2.45 g Mn(Ac)2 ·4H2 O was dissolved into 100 ml diethylene glycol (DEG) at the temperature of 80 ◦ C. Then, the solution was heated to 160 ◦ C in an oil bath and then kept there for 8 h under continuous stirring. After centrifugation and washing, the Mn3 O4 powder can be collected. Then the Mn3 O4 powder was mixed with LiCH3 COO·2H2 O (5 mol% excess lithium source) and grinded for 30 min. The mixtures were sintered at different temperatures (700, 800 and 900 ◦ C) for 10 h and cooled down to room temperature in air to obtain three spinel Li1.05 Mn1.95 O4 powders (referred as B700, B800 and B900 for the samples sintered at 700, 800 and 900 ◦ C, respectively). Besides, a contrast sample was prepared directly from a milled mixtures of stoichiometric Mn(CH3 COO)2 ·4H2 O and LiCH3 COO·2H2 O heating at 800 ◦ C for 10 h (referred as A800). The particle morphology of the powders was studied under a scanning electron microscope (SEM, JSM-6390LA). The crystalline structures were analyzed by X-ray diffraction (Philips X’Pert Pro Super, Cu K␣ radiation) in the 2 range of 10–70◦ at room temperature. The particle size distribution of the samples was determined by a laser particle sizer (Rise-2008). The electrochemical properties of the Li1.05 Mn1.95 O4 samples were characterized using CR2032 coin cells. The electrode laminates were prepared with 80 wt% Li1.05 Mn1.95 O4 , 10 wt% acetylene black (AB) as a conductive additive and 10 wt% poly(vinylidene difluoride) (PVDF) as a binder. N-methyl-2-pyrrolidone (NMP) was used as the solvent to make slurries. The slurries were uniformly coated on an aluminum foil and dried at 70 ◦ C for several hours. Then the laminates were punched into round discs with a diameter of 14 mm which were further dried in vacuum at 70 ◦ C for half an hour to completely remove NMP and moisture. The loading density of the active material in the laminates was about 2.6 mg cm−2 . Then CR2032 coin-cells were assembled in an argon-filled glove box (MBRAUN LABMASTER 130) with both the moisture content and oxygen levels less than 1 ppm. The Li1.05 Mn1.95 O4 electrodes were used as the working electrodes and lithium metal as the counter
Fig. 1. X-ray diffraction patterns of the Li1.05 Mn1.95 O4 powders: (a) A800, (b) B700, (c) B800 and (d) B900.
electrode. The electrolyte was 1.0 M LiPF6 in ethylene carbonate/dimethyl carbonate (1:1, v/v). The cells were galvanostatically cycled on a multi-channel battery cycler (NEWWARE BTS-610) in the voltage range of 3.30–4.35 V. The cyclic voltammograms (CV) of the cells were measured with an electrochemical work station (CHI 604b) between 3.30 and 4.35 V at different scan rates (0.1, 0.2, 0.5 and 1 mV s−1 ). The impedance spectra of the cells were also measured with the CHI 604b in the frequency range from 0.01 Hz to 100 kHz with the AC amplitude of 5.0 mV. 3. Results and discussion The XRD patterns of B700, B800, B900 and A800 are shown in Fig. 1. All the diffraction peaks can be indexed to a cubic phase of LiMn2 O4 (JCPDS 35-0782). However, an impurity phase of Mn2 O3 is observed in the pattern of A800. A very tiny diffraction peak from likely Mn2 O3 is also detected at 2 of about 32◦ for B900. This is reasonable for the materials synthesized at high temperature. Anyway, this result reveals that a pure phase Li1.05 Mn1.95 O4
Fig. 2. SEM images of the Li1.05 Mn1.95 O4 powders: (a) B700, (b) B800, (c) B900 and (d) A800.
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Fig. 5. The electrochemical impedance spectra of four Li1.05 Mn1.95 O4 /Li cells measured at fully charge state after 5 cycles at room temperature. Fig. 3. Particle size distribution curves of four Li1.05 Mn1.95 O4 samples.
can be easily prepared with the nanometer Mn3 O4 as the precursor. Fig. 2 gives the SEM images of the four Li1.05 Mn1.95 O4 samples after sintering at high temperatures. It can be seen that the particle size is less than 1 m for B700 with some degree of agglomeration. With increasing the sintering temperature, the particle size increases significantly to 1–2, 1–3 and 1–3 m for B800, B900 and A800, respectively. However, a particle growth is observed for both B900 and A800. On the other hand, the particle size distributions of the four samples are shown in Fig. 3. It can be seen that B700, B800 and A800 show a narrow distribution with D50 (the value of particle size at 50% cumulative population) of 1.33, 1.80 and 1.80 m, respectively. Nevertheless, B900 with the powder sintered at 900 ◦ C exhibits two broad peaks at 2 and 11 m, respectively, which is likely caused by the high sintering temperature. Thus, through the solid state reactions with Mn3 O4 and LiCH3 COO·2H2 O as the precursors, we can successfully prepare homogeneously distributed Li1.05 Mn1.95 O4 with particle size of 1–2 m. Fig. 4a shows the cycling performance of the Li1.05 Mn1.95 O4 samples in the voltage range of 3.30–4.35 V at 0.2 C at room temperature. Obviously, B800 shows the best cycling performance with an initial specific capacity of 104 mAh g−1 and about 94% or 98 mAh g−1 of capacity retention after 56 cycles. Note that, the theoretical specific capacity of Li1.05 Mn1.95 O4 is lower than LiMn2 O4 due to partial occupation of excess lithium ions in 16c sites in the spinel structure [1]. That leads to a theoretical
Fig. 6. The cycling performance of B800 and A800 at the rate of 1 C at 55 ◦ C.
capacity of 127.5 mAh g−1 corresponding to the reversible reaction: Li1.05 Mn1.95 O4 ↔ Li0.2 Mn1.95 O4 + 0.85 Li. Since the bottlenecks of high power cells are their power density, cycling performance and thermal stability, a small capacity sacrifice is acceptable for the application of high power cells. However, both B700 and B900 show a rapid capacity fading with only 77 mAh g−1 and 86 mAh g−1 retained after 56 cycles, or 74.5% and 75.7% of their initial capacities, respectively. The poor cycling performances of B700 can be ascribed to the poor crystallinity of Li1.05 Mn1.95 O4 . While for B900, a small amount of the impurity of Mn2 O3 (Fig. 1) might be the
Fig. 4. The electrochemical performance of four Li1.05 Mn1.95 O4 samples at room temperature: (a) cycling performance at 0.2 C and (b) rate performance charged at 0.2 C and discharged from 0.5 to 5 C.
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Fig. 7. The cycling performance of B800 at 0.2 C at low temperatures (25, 0, −10 and −20 ◦ C).
main factor leading to the fast capacity fading. For A800 synthesized at the same sintering temperature of 800 ◦ C as for B800 but with a different manganese precursor (Mn(CH3 COO)2 ·4H2 O instead of Mn3 O4 ), a rapid capacity fading is also observed with only 86 mAh g−1 retained after 56 cycles mainly due to the Mn2 O3 impurity in the electrode. Fig. 4b shows the rate performance of the half-cells Li/LiMn2 O4 charged at 0.2 C and discharged from 0.5 C to 5 C. Due to the narrow particle size distribution and good crystallinity of Li1.05 Mn1.95 O4 , B800 shows the best rate performance that a specific capacity of 103.1, 100.5, 99.1, 98.4 mAh g−1 can be obtained at 1 C, 2 C, 3 C
Fig. 8. The CV curves of B800 in the second cycle at low temperatures (25, 0, −10 and −20 ◦ C) with a scan rate of 0.1 mV s−1 .
and 5 C, respectively. On the other hand, considerably poorer rate capability is observed for other three samples, especially at high current densities. A specific capacity of 87.5, 84.2 and 70.9 mAh g−1 is obtained at 5 C for B700, A800 and B900, respectively. Such a good rate performance of B800 makes it competitive for the micrometer Li1.05 Mn1.95 O4 to be practically used in EVs and HEVs. The AC impedance spectra of the four samples charged to 4.35 V after 5 cycles are given in the Fig. 5. It is noticed that, B800 shows the smallest impedance of about 110 while much higher impedance of around 300 is measured for the samples B700, B900 and A800.
Fig. 9. Rate performance of B800 at various low temperatures: (a) 25 ◦ C, (b) 0 ◦ C, (c) −10 ◦ C and (d) −20 ◦ C.
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Fig. 10. The CV curves of B800 with different scan rates of 0.1, 0.2, 0.5 and 1 mV s−1 at temperatures of (a) 25 ◦ C, (b) 0 ◦ C, (c) −10 ◦ C and (d) −20 ◦ C.
This result confirms that B800 with a narrow particle size distribution and good crystallinity exhibits the best rate performance among the four samples. The cycling performance at elevated temperatures for LiMn2 O4 is considered to be one of the biggest challenges for the practical applications. Thus, a relative comparison of the samples A800 and B800 cycled at 55 ◦ C is given in Fig. 6. It can be seen that B800 shows much more stable cycling performance than A800. For B800, a specific capacity of 86.9 mAh g−1 can be obtained after 100 cycles which is 90.5% of its initial capacity. For A800, much faster rapid capacity fading is observed because only 58.8 mAh g−1 is retained after 100 cycles, which is less than 60% of its initial capacity. In fact, in comparison with some literature data, such a good cycling performance at elevated temperatures for B800 is even much better than the LiMn2 O4 modified by surface coating with MgO [7], FePO4 [5], TiO2 [6] or polymer [8]. It is worth noting that LIBs should work effectively not only at elevated temperature but also at low temperatures, especially for the practical application of EVs and HEVs in winter or high latitudinal areas. Fig. 7 illustrates the cycling performance of B800 under a rate of 0.2 C at −20, −10, 0 and 25 ◦ C, respectively. It can be seen that all the cells at low temperatures exhibit an excellent cycling stability with almost no capacity fading. During 30 cycles, an average discharge capacity of 96.4, 90.7 and 84.5 mAh g−1 can be obtained at 0 ◦ C, −10 ◦ C, −20 ◦ C, respectively. Besides, with temperature dropping, there is a slightly decrease of the capacity, holding 95.7% (0 ◦ C), 87.5% (−10 ◦ C) and 84.1% (−20 ◦ C) of the capacity measured at 25 ◦ C. This result shows that the Li1.05 Mn1.95 O4 /Li cells exhibit superior electrochemical performance at low temperatures, indicating that the electrochemical properties of Li1.05 Mn1.95 O4 spinel
is less sensitive to temperature change compared with LiFePO4 olivine [14]. Fig. 8 shows the cyclic voltammetry (CV) curves of the Li1.05 Mn1.95 O4 /Li cells in the second cycle at various low temperatures under a scanning rate of 0.2 mV s−1 , in which the voltage difference (V) between the coupled anodic and cathodic peaks at around 4.1 V can be easily measured (inset of Fig. 8). For the Li1.05 Mn1.95 O4 /Li cells, in association with the Mn3+ /Mn4+ redox couples during the charge and discharge steps, there are two cathodic or oxidation peaks and two corresponding anodic or reduction peaks at room temperature. With decreasing the temperature, there is an apparent rightward shift of the cathodic peaks as well as a leftward shift of the anodic peaks, indicating a gradual increase in the polarization of the Li1.05 Mn1.95 O4 /Li cells. Besides, one broad peak turns up at both −10 and −20 ◦ C instead of two cathodic peaks at 0 and 25 ◦ C, respectively, which further confirms the large polarization at low temperatures. The V values for Li1.05 Mn1.95 O4 /Li cells at −20, −10, 0 and 25 ◦ C are 0.546, 0.311, 0.173 and 0.091 V, respectively. This is mainly due to the decrease in the ionic conductivities of both the electrolyte and electrodes as well as a deceleration of electrochemical reactions for the cells at low temperatures. To further investigate the electrochemical performance at low temperatures, the rate performance of the sample B800 from −20 ◦ C to 25 ◦ C is shown in Fig. 9. At 25 ◦ C, B800 shows a good rate performance that a specific capacity of 98.6 mAh g−1 can be obtained at 4 C as well as a small polarization for the redox processes. When the temperature drops to 0 ◦ C, the capacity decreases to 84.8 mAh g−1 at 4 C and a much greater polarization can be observed with increasing the current density. Much low
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4. Conclusions
Fig. 11. The relationship between the anodic peak current (ip ) and the square root of scan rate (1/2 ).
temperatures (e.g. −20 and −10 ◦ C) can bring up an enhanced polarization that the voltage profiles turn to be sloping lines rather than well-defined S-curves at high temperatures (e.g. 0 and 25 ◦ C). For example, the specific capacity of the sample B800 at −10 ◦ C is only 85.9, 83.3, 79.9 and 44.8 mAh g−1 at 0.3, 1, 2 and 4 C, respectively. Meanwhile, these values decrease to 77.8, 71.6, 64.6 and 41.7 mAh g−1 , respectively, at −20 ◦ C. To further study the electrode kinetics at various low temperatures, the apparent diffusion coefficients of lithium ion from −20 ◦ C to 25 ◦ C are determined by means of a conventional CV method [14–17]. The anodic peaks used to calculate the lithium ion diffusion coefficients are those marked in Fig. 10, which are corresponding to the 0.5 lithium intercalation into Li0.5 Mn2 O4 to form LiMn2 O4 . The CV curves of the Li1.05 Mn1.95 O4 /Li cells between 3.30 and 4.35 V at different scan rates (0.1, 0.2, 0.5 and 1 mV s−1 ) are shown in Fig. 10. Because the charge transfer between the interfaces of the electrode and the liquid electrolyte is fast enough, the lithium ion diffusion within the electrode can be considered to be the rate limiting step. In this case, the linear relationship between the CV scan rate and the peak current can be written as: 1/2 ∗ 1/2 C Li+ Li
ip = (4.64 × 106 )n3/2 ST −1/2 D
(1)
where ip is the peak current (in unit: A), n is the charge concentration in Li1.05 Mn1.95 O4 , S is the contact area between electrode and electrolyte (in unit: cm2 . Here the geometric area of electrode, 1.54 cm2 , is used for simplicity), T is the temperature (K), C∗ Li is the bulk concentration of lithium in electrode (0.0238 mol cm−3 , calculated from LiMn2 O4 ), and (V s−1 ) is the scan rate. Based on the Eq. (1), the peak current has a linear relationship with the square root of scan rate, as shown in Fig. 11. Thus, the values of DLi+ at various temperatures are determined and tabled in the inset of Fig. 11. It can be found that the DLi+ at room temperature is in a magnitude of 10−10 cm2 s−1 , which is similar to the results reported in literature [18,19]. In addition, with decreasing the temperature, the value of DLi+ decreases sharply because a magnitude of 10−11 cm2 s−1 is obtained at 0 and −10 ◦ C, respectively. The value even falls down to 2.31 × 10−12 cm2 s−1 at −20 ◦ C, indicating a sluggish electrode kinetics at low temperatures especially at −20 ◦ C.
With a nanometer Mn3 O4 powder as the precursor, micrometer sized Li1.05 Mn1.95 O4 powders of about 1–2 m with a narrow particle size distribution have been successfully synthesized by a solid state reaction method. The Li1.05 Mn1.95 O4 /Li cells exhibit a high rate performance with a specific capacity of 98.4 mAh g−1 at 5 C at room temperature. A stable cycling performance is observed at 55 ◦ C that a specific capacity of 86.9 mAh g−1 can be obtained after 100 cycles at 1 C, which is 90.5% of its initial capacity. Furthermore, the Li1.05 Mn1.95 O4 sample also shows much stable cycling performance at low temperatures with 96.4, 90.7 and 84.5 mAh g−1 at 0, −10 and −20 ◦ C, respectively. With decreasing the temperature, an increase in the cell polarization can be observed, especially at high current densities. A specific capacity of 100.2, 90.6, 79.9 and 64.7 mAh g−1 can be obtained at 2 C for the cells at −25, 0, −10 and −20 ◦ C, respectively. The diffusion coefficients of lithium ion measured by CV method show a drop from 10−10 cm2 s−1 at 25 ◦ C to 10−12 cm2 s−1 at −20 ◦ C. Acknowledgments This study was supported by National Science Foundation of China (grant nos. 20971117 and 10979049) and Education Department of Anhui Province (grant no. KJ2009A142). We are also grateful to the Solar Energy Operation Plan of Academia Sinica. References [1] O.K. Park, Y. Cho, S. Lee, H.C. Yoo, H.K. Song, J. Cho, Energy & Environmental Science 4 (2011) 1621. [2] Y.L. Ding, J. Xie, G.S. Cao, T.J. Zhu, H.M. Yu, X.B. Zhao, Journal of Physical Chemistry C 115 (2011) 9821. [3] N. Jayaprakash, N. Kalaiselvi, D. Bhuvaneswari Gangulibabu, J. Solid State Electrochemistry 15 (2011) 1243. [4] L.L. Xiong, Y.L. Xu, C. Zhang, Z.W. Zhang, J.B. Li, Journal of Solid State Electrochemistry 15 (2011) 1263. [5] C.B. Qing, Y. Bai, J.M. Yang, W.F. Zhang, Electrochimica Acta 56 (2011) 6612. [6] L.H. Yu, X.P. Qiu, J.Y. Xi, W.T. Zhu, L.Q. Chen, Electrochimica Acta 51 (2006) 6406. [7] J.S. Gnanaraj, V.G. Pol, A. Gedanken, D. Aurbach, Electrochemistry Communications 5 (2003) 940. [8] G.H. Hu, X.B. Wang, F. Chen, J.Y. Zhou, R.G. Li, Z.H. Deng, Electrochemistry Communications 7 (2005) 383. [9] K. Amine, J. Liu, S. Kang, I. Belharouak, Y. Hyung, D. Vissers, G. Henriksen, Journal of Power Sources 129 (2004) 14. [10] K. Amine, J. Liu, I. Belharouak, S.H. Kang, I. Bloom, D. Vissers, G. Henriksen, Journal of Power Sources 146 (2005) 111. [11] C. Bellitto, M.G. DiMarco, W.R. Branford, M.A. Green, D.A. Neumann, Solid State Ionics 140 (2001) 77. [12] Y. Ein-Eli, R.C. Urian, W. Wen, S. Mukerjee, Electrochimica Acta 50 (2005) 1931. [13] S.R. Li, Y. Sun, S.Y. Ge, Y. Qiao, Y.M. Chen, I. Lieberwirth, Y. Yu, C.H. Chen, Chemical Engineering Journal 192 (2012) 226. [14] X.H. Rui, Y. Jin, X.Y. Feng, L.C. Zhang, C.H. Chen, Journal of Power Sources 196 (2011) 2109. [15] S.R. Li, S.Y. Ge, Y. Qiao, Y.M. Chen, X.Y. Feng, J.F. Zhu, C.H. Chen, Electrochimica Acta 64 (2012) 81. [16] X.H. Rui, N. Ding, J. Liu, C. Li, C.H. Chen, Electrochimica Acta 55 (2010) 2384. [17] K. Wang, R. Cai, T. Yuan, X. Yu, R. Ran, Z.P. Shao, Electrochimica Acta 54 (2009) 2861. [18] Y.H. Rho, K. Dokko, K. Kanamura, Journal of Power Sources 157 (2006) 471. [19] M.D. Chung, J.H. Seo, X.C. Zhang, A.M. Sastry, Journal of the Electrochemical Society 158 (2011) A371.
Contents lists available at SciVerse ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Facile synthesis of micrometer Li1.05 Mn1.95 O4 and its low temperature performance for high power lithium ion batteries Si-Rong Li a , Yu Qiao a , Yi Sun a,b , Si-Yuan Ge a , Yi-Meng Chen a , Ingo Lieberwirth b , Yan Yu a , Chun-Hua Chen a,∗ a CAS Key Laboratory of Materials for Energy Conversion, Department of Materials Science and Engineering, University of Science and Technology of China, Anhui Hefei 230026, China b Max Planck Institute for Polymer Research, Mainz 55128, Germany
a r t i c l e
i n f o
Article history: Received 11 June 2012 Received in revised form 24 July 2012 Accepted 24 July 2012 Available online 28 July 2012 Keywords: Lithium manganese oxide Low temperature performance Diffusion coefficient Lithium ion batteries
a b s t r a c t Micrometer Li1.05 Mn1.95 O4 has been synthesized by solid state reactions with the nano-Mn3 O4 as a precursor. X-ray diffraction, scanning electron microscopy and laser particle sizer are employed to investigate the structures, morphologies and particle size distributions of the powder. The micrometer Li1.05 Mn1.95 O4 exhibits good rate performance at room temperature with a specific capacity of 98.4 mAh g−1 at 5 C. The Li1.05 Mn1.95 O4 /Li half cell also shows good cycling performance at elevated temperature with 90.5% of its initial capacity retained after 100 cycles at 1 C. At −20 ◦ C, the Li1.05 Mn1.95 O4 delivers a stable cycling performance with a specific capacity of 84.5 mAh g−1 , being 84.1% of the capacity at room temperature. The cyclic voltammetry (CV) and rate performance measurements illustrate an increasing polarization with decreasing the temperature. In addition, the diffusion coefficients of lithium ions (DLi+ ) in Li1.05 Mn1.95 O4 at various temperatures (25, 0, −10 and −20 ◦ C) are determined to be in the magnitude of 10−10 to 10−12 cm2 s−1 by cyclic voltammetry (CV) method. © 2012 Elsevier Ltd. All rights reserved.
1. Introduction Spinel structure lithium manganese oxide (LiMn2 O4 ) and olivine structure lithium ion phosphate (LiFePO4 ) have been considered to be the optimum cathode materials in lithium ion batteries (LIBs) for electric vehicles (EVs) and hybrid electric vehicles (HEVs) due to their low cost and high safety. Recently, Park et al. [1] have made a comparison of spinel LiMn2 O4 and olivine LiFePO4 for the application in EVs and HEVs. For LiFePO4 , both of its electronic conductivity (10−9 to 10−8 S cm−1 ) and ionic conductivity (10−11 to 10−9 S cm−1 ) are much lower than those of spinel LiMn2 O4 (10−4 S cm−1 and 10−6 S cm−1 respectively) at room temperature. Thus, there will be a challenge for the rate performance of LiFePO4 , especially when the EVs and HEVs are used in the winter or other cold conditions. Furthermore, LiFePO4 needs to be synthesized under an inert atmosphere, which makes it more difficult to produce LiFePO4 powders with consistent quality from batch to batch compared with the production of LiMn2 O4 powders in air. Although spinel LiMn2 O4 exhibit these intrinsic advantages compared to olivine LiFePO4 , it has still some drawbacks which limit its practical applications in large-scale for the EVs and HEVs: (1) the dissolution of manganese from the active material into the electrolyte; (2) Jahn–Teller
∗ Corresponding author. Tel.: +86 551 3606971; fax: +86 551 3601592. E-mail address: [email protected] (C.-H. Chen). 0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2012.07.086
distortion of the spinel LiMn2 O4 ; (3) the instability of the electrolyte at high voltage; (4) the phase transition from cubic to unstable tetragonal. These problems have caused poor long term cycling performance of spinel LiMn2 O4 , especially at elevated temperatures. Many attempts have been tried to solve these problems such as cationic doping with Al [2], Cr, V [3] and Ti [4] to stabilize the spinel structure and surface coatings [5–8] to separate the contact of active materials and electrolyte. Besides, excess lithium in the spinel (Li1+x Mn2−x O4 ) is proved to be effective to improve the cycling performance for enhanced stability of spinel structure [9–11]. On the other hand, nanometer particles are believed to decrease the diffusion length of Li+ and improve the rate performance during the charge and discharge process. However, they can also cause more side reactions on the large surface of the nanometer particles as well as the dissolution of Mn2+ into the electrolyte. More importantly, nanometer particles usually leads to low packing density which is undesirable to fabricate the electrode laminate with a high loading density in practical batteries for the HEVs and EVs. In addition, LIBs need to work efficiently both at low temperatures and elevated temperatures. In literature, there are very few studies on the electrochemical performance of spinel LiMn2 O4 at low temperatures [12]. Thus, in this paper, we try to synthesize micrometer Li1.05 Mn1.95 O4 powders and much attention is paid to their electrochemical performance at low temperatures (e.g. −20, −10 and 0 ◦ C).
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2. Experimental All the chemical reagents were analytical grade. Firstly, a nanometer-sized Mn3 O4 powder was prepared in an oil-bath synthesis process as reported in our previous work [13]. In brief, 2.45 g Mn(Ac)2 ·4H2 O was dissolved into 100 ml diethylene glycol (DEG) at the temperature of 80 ◦ C. Then, the solution was heated to 160 ◦ C in an oil bath and then kept there for 8 h under continuous stirring. After centrifugation and washing, the Mn3 O4 powder can be collected. Then the Mn3 O4 powder was mixed with LiCH3 COO·2H2 O (5 mol% excess lithium source) and grinded for 30 min. The mixtures were sintered at different temperatures (700, 800 and 900 ◦ C) for 10 h and cooled down to room temperature in air to obtain three spinel Li1.05 Mn1.95 O4 powders (referred as B700, B800 and B900 for the samples sintered at 700, 800 and 900 ◦ C, respectively). Besides, a contrast sample was prepared directly from a milled mixtures of stoichiometric Mn(CH3 COO)2 ·4H2 O and LiCH3 COO·2H2 O heating at 800 ◦ C for 10 h (referred as A800). The particle morphology of the powders was studied under a scanning electron microscope (SEM, JSM-6390LA). The crystalline structures were analyzed by X-ray diffraction (Philips X’Pert Pro Super, Cu K␣ radiation) in the 2 range of 10–70◦ at room temperature. The particle size distribution of the samples was determined by a laser particle sizer (Rise-2008). The electrochemical properties of the Li1.05 Mn1.95 O4 samples were characterized using CR2032 coin cells. The electrode laminates were prepared with 80 wt% Li1.05 Mn1.95 O4 , 10 wt% acetylene black (AB) as a conductive additive and 10 wt% poly(vinylidene difluoride) (PVDF) as a binder. N-methyl-2-pyrrolidone (NMP) was used as the solvent to make slurries. The slurries were uniformly coated on an aluminum foil and dried at 70 ◦ C for several hours. Then the laminates were punched into round discs with a diameter of 14 mm which were further dried in vacuum at 70 ◦ C for half an hour to completely remove NMP and moisture. The loading density of the active material in the laminates was about 2.6 mg cm−2 . Then CR2032 coin-cells were assembled in an argon-filled glove box (MBRAUN LABMASTER 130) with both the moisture content and oxygen levels less than 1 ppm. The Li1.05 Mn1.95 O4 electrodes were used as the working electrodes and lithium metal as the counter
Fig. 1. X-ray diffraction patterns of the Li1.05 Mn1.95 O4 powders: (a) A800, (b) B700, (c) B800 and (d) B900.
electrode. The electrolyte was 1.0 M LiPF6 in ethylene carbonate/dimethyl carbonate (1:1, v/v). The cells were galvanostatically cycled on a multi-channel battery cycler (NEWWARE BTS-610) in the voltage range of 3.30–4.35 V. The cyclic voltammograms (CV) of the cells were measured with an electrochemical work station (CHI 604b) between 3.30 and 4.35 V at different scan rates (0.1, 0.2, 0.5 and 1 mV s−1 ). The impedance spectra of the cells were also measured with the CHI 604b in the frequency range from 0.01 Hz to 100 kHz with the AC amplitude of 5.0 mV. 3. Results and discussion The XRD patterns of B700, B800, B900 and A800 are shown in Fig. 1. All the diffraction peaks can be indexed to a cubic phase of LiMn2 O4 (JCPDS 35-0782). However, an impurity phase of Mn2 O3 is observed in the pattern of A800. A very tiny diffraction peak from likely Mn2 O3 is also detected at 2 of about 32◦ for B900. This is reasonable for the materials synthesized at high temperature. Anyway, this result reveals that a pure phase Li1.05 Mn1.95 O4
Fig. 2. SEM images of the Li1.05 Mn1.95 O4 powders: (a) B700, (b) B800, (c) B900 and (d) A800.
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Fig. 5. The electrochemical impedance spectra of four Li1.05 Mn1.95 O4 /Li cells measured at fully charge state after 5 cycles at room temperature. Fig. 3. Particle size distribution curves of four Li1.05 Mn1.95 O4 samples.
can be easily prepared with the nanometer Mn3 O4 as the precursor. Fig. 2 gives the SEM images of the four Li1.05 Mn1.95 O4 samples after sintering at high temperatures. It can be seen that the particle size is less than 1 m for B700 with some degree of agglomeration. With increasing the sintering temperature, the particle size increases significantly to 1–2, 1–3 and 1–3 m for B800, B900 and A800, respectively. However, a particle growth is observed for both B900 and A800. On the other hand, the particle size distributions of the four samples are shown in Fig. 3. It can be seen that B700, B800 and A800 show a narrow distribution with D50 (the value of particle size at 50% cumulative population) of 1.33, 1.80 and 1.80 m, respectively. Nevertheless, B900 with the powder sintered at 900 ◦ C exhibits two broad peaks at 2 and 11 m, respectively, which is likely caused by the high sintering temperature. Thus, through the solid state reactions with Mn3 O4 and LiCH3 COO·2H2 O as the precursors, we can successfully prepare homogeneously distributed Li1.05 Mn1.95 O4 with particle size of 1–2 m. Fig. 4a shows the cycling performance of the Li1.05 Mn1.95 O4 samples in the voltage range of 3.30–4.35 V at 0.2 C at room temperature. Obviously, B800 shows the best cycling performance with an initial specific capacity of 104 mAh g−1 and about 94% or 98 mAh g−1 of capacity retention after 56 cycles. Note that, the theoretical specific capacity of Li1.05 Mn1.95 O4 is lower than LiMn2 O4 due to partial occupation of excess lithium ions in 16c sites in the spinel structure [1]. That leads to a theoretical
Fig. 6. The cycling performance of B800 and A800 at the rate of 1 C at 55 ◦ C.
capacity of 127.5 mAh g−1 corresponding to the reversible reaction: Li1.05 Mn1.95 O4 ↔ Li0.2 Mn1.95 O4 + 0.85 Li. Since the bottlenecks of high power cells are their power density, cycling performance and thermal stability, a small capacity sacrifice is acceptable for the application of high power cells. However, both B700 and B900 show a rapid capacity fading with only 77 mAh g−1 and 86 mAh g−1 retained after 56 cycles, or 74.5% and 75.7% of their initial capacities, respectively. The poor cycling performances of B700 can be ascribed to the poor crystallinity of Li1.05 Mn1.95 O4 . While for B900, a small amount of the impurity of Mn2 O3 (Fig. 1) might be the
Fig. 4. The electrochemical performance of four Li1.05 Mn1.95 O4 samples at room temperature: (a) cycling performance at 0.2 C and (b) rate performance charged at 0.2 C and discharged from 0.5 to 5 C.
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Fig. 7. The cycling performance of B800 at 0.2 C at low temperatures (25, 0, −10 and −20 ◦ C).
main factor leading to the fast capacity fading. For A800 synthesized at the same sintering temperature of 800 ◦ C as for B800 but with a different manganese precursor (Mn(CH3 COO)2 ·4H2 O instead of Mn3 O4 ), a rapid capacity fading is also observed with only 86 mAh g−1 retained after 56 cycles mainly due to the Mn2 O3 impurity in the electrode. Fig. 4b shows the rate performance of the half-cells Li/LiMn2 O4 charged at 0.2 C and discharged from 0.5 C to 5 C. Due to the narrow particle size distribution and good crystallinity of Li1.05 Mn1.95 O4 , B800 shows the best rate performance that a specific capacity of 103.1, 100.5, 99.1, 98.4 mAh g−1 can be obtained at 1 C, 2 C, 3 C
Fig. 8. The CV curves of B800 in the second cycle at low temperatures (25, 0, −10 and −20 ◦ C) with a scan rate of 0.1 mV s−1 .
and 5 C, respectively. On the other hand, considerably poorer rate capability is observed for other three samples, especially at high current densities. A specific capacity of 87.5, 84.2 and 70.9 mAh g−1 is obtained at 5 C for B700, A800 and B900, respectively. Such a good rate performance of B800 makes it competitive for the micrometer Li1.05 Mn1.95 O4 to be practically used in EVs and HEVs. The AC impedance spectra of the four samples charged to 4.35 V after 5 cycles are given in the Fig. 5. It is noticed that, B800 shows the smallest impedance of about 110 while much higher impedance of around 300 is measured for the samples B700, B900 and A800.
Fig. 9. Rate performance of B800 at various low temperatures: (a) 25 ◦ C, (b) 0 ◦ C, (c) −10 ◦ C and (d) −20 ◦ C.
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Fig. 10. The CV curves of B800 with different scan rates of 0.1, 0.2, 0.5 and 1 mV s−1 at temperatures of (a) 25 ◦ C, (b) 0 ◦ C, (c) −10 ◦ C and (d) −20 ◦ C.
This result confirms that B800 with a narrow particle size distribution and good crystallinity exhibits the best rate performance among the four samples. The cycling performance at elevated temperatures for LiMn2 O4 is considered to be one of the biggest challenges for the practical applications. Thus, a relative comparison of the samples A800 and B800 cycled at 55 ◦ C is given in Fig. 6. It can be seen that B800 shows much more stable cycling performance than A800. For B800, a specific capacity of 86.9 mAh g−1 can be obtained after 100 cycles which is 90.5% of its initial capacity. For A800, much faster rapid capacity fading is observed because only 58.8 mAh g−1 is retained after 100 cycles, which is less than 60% of its initial capacity. In fact, in comparison with some literature data, such a good cycling performance at elevated temperatures for B800 is even much better than the LiMn2 O4 modified by surface coating with MgO [7], FePO4 [5], TiO2 [6] or polymer [8]. It is worth noting that LIBs should work effectively not only at elevated temperature but also at low temperatures, especially for the practical application of EVs and HEVs in winter or high latitudinal areas. Fig. 7 illustrates the cycling performance of B800 under a rate of 0.2 C at −20, −10, 0 and 25 ◦ C, respectively. It can be seen that all the cells at low temperatures exhibit an excellent cycling stability with almost no capacity fading. During 30 cycles, an average discharge capacity of 96.4, 90.7 and 84.5 mAh g−1 can be obtained at 0 ◦ C, −10 ◦ C, −20 ◦ C, respectively. Besides, with temperature dropping, there is a slightly decrease of the capacity, holding 95.7% (0 ◦ C), 87.5% (−10 ◦ C) and 84.1% (−20 ◦ C) of the capacity measured at 25 ◦ C. This result shows that the Li1.05 Mn1.95 O4 /Li cells exhibit superior electrochemical performance at low temperatures, indicating that the electrochemical properties of Li1.05 Mn1.95 O4 spinel
is less sensitive to temperature change compared with LiFePO4 olivine [14]. Fig. 8 shows the cyclic voltammetry (CV) curves of the Li1.05 Mn1.95 O4 /Li cells in the second cycle at various low temperatures under a scanning rate of 0.2 mV s−1 , in which the voltage difference (V) between the coupled anodic and cathodic peaks at around 4.1 V can be easily measured (inset of Fig. 8). For the Li1.05 Mn1.95 O4 /Li cells, in association with the Mn3+ /Mn4+ redox couples during the charge and discharge steps, there are two cathodic or oxidation peaks and two corresponding anodic or reduction peaks at room temperature. With decreasing the temperature, there is an apparent rightward shift of the cathodic peaks as well as a leftward shift of the anodic peaks, indicating a gradual increase in the polarization of the Li1.05 Mn1.95 O4 /Li cells. Besides, one broad peak turns up at both −10 and −20 ◦ C instead of two cathodic peaks at 0 and 25 ◦ C, respectively, which further confirms the large polarization at low temperatures. The V values for Li1.05 Mn1.95 O4 /Li cells at −20, −10, 0 and 25 ◦ C are 0.546, 0.311, 0.173 and 0.091 V, respectively. This is mainly due to the decrease in the ionic conductivities of both the electrolyte and electrodes as well as a deceleration of electrochemical reactions for the cells at low temperatures. To further investigate the electrochemical performance at low temperatures, the rate performance of the sample B800 from −20 ◦ C to 25 ◦ C is shown in Fig. 9. At 25 ◦ C, B800 shows a good rate performance that a specific capacity of 98.6 mAh g−1 can be obtained at 4 C as well as a small polarization for the redox processes. When the temperature drops to 0 ◦ C, the capacity decreases to 84.8 mAh g−1 at 4 C and a much greater polarization can be observed with increasing the current density. Much low
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4. Conclusions
Fig. 11. The relationship between the anodic peak current (ip ) and the square root of scan rate (1/2 ).
temperatures (e.g. −20 and −10 ◦ C) can bring up an enhanced polarization that the voltage profiles turn to be sloping lines rather than well-defined S-curves at high temperatures (e.g. 0 and 25 ◦ C). For example, the specific capacity of the sample B800 at −10 ◦ C is only 85.9, 83.3, 79.9 and 44.8 mAh g−1 at 0.3, 1, 2 and 4 C, respectively. Meanwhile, these values decrease to 77.8, 71.6, 64.6 and 41.7 mAh g−1 , respectively, at −20 ◦ C. To further study the electrode kinetics at various low temperatures, the apparent diffusion coefficients of lithium ion from −20 ◦ C to 25 ◦ C are determined by means of a conventional CV method [14–17]. The anodic peaks used to calculate the lithium ion diffusion coefficients are those marked in Fig. 10, which are corresponding to the 0.5 lithium intercalation into Li0.5 Mn2 O4 to form LiMn2 O4 . The CV curves of the Li1.05 Mn1.95 O4 /Li cells between 3.30 and 4.35 V at different scan rates (0.1, 0.2, 0.5 and 1 mV s−1 ) are shown in Fig. 10. Because the charge transfer between the interfaces of the electrode and the liquid electrolyte is fast enough, the lithium ion diffusion within the electrode can be considered to be the rate limiting step. In this case, the linear relationship between the CV scan rate and the peak current can be written as: 1/2 ∗ 1/2 C Li+ Li
ip = (4.64 × 106 )n3/2 ST −1/2 D
(1)
where ip is the peak current (in unit: A), n is the charge concentration in Li1.05 Mn1.95 O4 , S is the contact area between electrode and electrolyte (in unit: cm2 . Here the geometric area of electrode, 1.54 cm2 , is used for simplicity), T is the temperature (K), C∗ Li is the bulk concentration of lithium in electrode (0.0238 mol cm−3 , calculated from LiMn2 O4 ), and (V s−1 ) is the scan rate. Based on the Eq. (1), the peak current has a linear relationship with the square root of scan rate, as shown in Fig. 11. Thus, the values of DLi+ at various temperatures are determined and tabled in the inset of Fig. 11. It can be found that the DLi+ at room temperature is in a magnitude of 10−10 cm2 s−1 , which is similar to the results reported in literature [18,19]. In addition, with decreasing the temperature, the value of DLi+ decreases sharply because a magnitude of 10−11 cm2 s−1 is obtained at 0 and −10 ◦ C, respectively. The value even falls down to 2.31 × 10−12 cm2 s−1 at −20 ◦ C, indicating a sluggish electrode kinetics at low temperatures especially at −20 ◦ C.
With a nanometer Mn3 O4 powder as the precursor, micrometer sized Li1.05 Mn1.95 O4 powders of about 1–2 m with a narrow particle size distribution have been successfully synthesized by a solid state reaction method. The Li1.05 Mn1.95 O4 /Li cells exhibit a high rate performance with a specific capacity of 98.4 mAh g−1 at 5 C at room temperature. A stable cycling performance is observed at 55 ◦ C that a specific capacity of 86.9 mAh g−1 can be obtained after 100 cycles at 1 C, which is 90.5% of its initial capacity. Furthermore, the Li1.05 Mn1.95 O4 sample also shows much stable cycling performance at low temperatures with 96.4, 90.7 and 84.5 mAh g−1 at 0, −10 and −20 ◦ C, respectively. With decreasing the temperature, an increase in the cell polarization can be observed, especially at high current densities. A specific capacity of 100.2, 90.6, 79.9 and 64.7 mAh g−1 can be obtained at 2 C for the cells at −25, 0, −10 and −20 ◦ C, respectively. The diffusion coefficients of lithium ion measured by CV method show a drop from 10−10 cm2 s−1 at 25 ◦ C to 10−12 cm2 s−1 at −20 ◦ C. Acknowledgments This study was supported by National Science Foundation of China (grant nos. 20971117 and 10979049) and Education Department of Anhui Province (grant no. KJ2009A142). We are also grateful to the Solar Energy Operation Plan of Academia Sinica. References [1] O.K. Park, Y. Cho, S. Lee, H.C. Yoo, H.K. Song, J. Cho, Energy & Environmental Science 4 (2011) 1621. [2] Y.L. Ding, J. Xie, G.S. Cao, T.J. Zhu, H.M. Yu, X.B. Zhao, Journal of Physical Chemistry C 115 (2011) 9821. [3] N. Jayaprakash, N. Kalaiselvi, D. Bhuvaneswari Gangulibabu, J. Solid State Electrochemistry 15 (2011) 1243. [4] L.L. Xiong, Y.L. Xu, C. Zhang, Z.W. Zhang, J.B. Li, Journal of Solid State Electrochemistry 15 (2011) 1263. [5] C.B. Qing, Y. Bai, J.M. Yang, W.F. Zhang, Electrochimica Acta 56 (2011) 6612. [6] L.H. Yu, X.P. Qiu, J.Y. Xi, W.T. Zhu, L.Q. Chen, Electrochimica Acta 51 (2006) 6406. [7] J.S. Gnanaraj, V.G. Pol, A. Gedanken, D. Aurbach, Electrochemistry Communications 5 (2003) 940. [8] G.H. Hu, X.B. Wang, F. Chen, J.Y. Zhou, R.G. Li, Z.H. Deng, Electrochemistry Communications 7 (2005) 383. [9] K. Amine, J. Liu, S. Kang, I. Belharouak, Y. Hyung, D. Vissers, G. Henriksen, Journal of Power Sources 129 (2004) 14. [10] K. Amine, J. Liu, I. Belharouak, S.H. Kang, I. Bloom, D. Vissers, G. Henriksen, Journal of Power Sources 146 (2005) 111. [11] C. Bellitto, M.G. DiMarco, W.R. Branford, M.A. Green, D.A. Neumann, Solid State Ionics 140 (2001) 77. [12] Y. Ein-Eli, R.C. Urian, W. Wen, S. Mukerjee, Electrochimica Acta 50 (2005) 1931. [13] S.R. Li, Y. Sun, S.Y. Ge, Y. Qiao, Y.M. Chen, I. Lieberwirth, Y. Yu, C.H. Chen, Chemical Engineering Journal 192 (2012) 226. [14] X.H. Rui, Y. Jin, X.Y. Feng, L.C. Zhang, C.H. Chen, Journal of Power Sources 196 (2011) 2109. [15] S.R. Li, S.Y. Ge, Y. Qiao, Y.M. Chen, X.Y. Feng, J.F. Zhu, C.H. Chen, Electrochimica Acta 64 (2012) 81. [16] X.H. Rui, N. Ding, J. Liu, C. Li, C.H. Chen, Electrochimica Acta 55 (2010) 2384. [17] K. Wang, R. Cai, T. Yuan, X. Yu, R. Ran, Z.P. Shao, Electrochimica Acta 54 (2009) 2861. [18] Y.H. Rho, K. Dokko, K. Kanamura, Journal of Power Sources 157 (2006) 471. [19] M.D. Chung, J.H. Seo, X.C. Zhang, A.M. Sastry, Journal of the Electrochemical Society 158 (2011) A371.