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Novel LiNi0.5Mn1.5O4 porous microellipsoids as high-performance cathode materials for lithium ion batteries
Journal of Power Sources 288 (2015) 353e358
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Short communication
Novel LiNi0.5Mn1.5O4 porous microellipsoids as high-performance cathode materials for lithium ion batteries Jin Pan a, Jianqiu Deng a, b, *, Qingrong Yao a, b, Yongjin Zou a, b, Zhongmin Wang a, b, Huaiying Zhou a, b, Lixian Sun a, b, Guanghui Rao a, b a b
School of Material Science and Engineering, Guilin University of Electronic Technology, Guangxi, Guilin 541004, China Guangxi Key Laboratory of Information Materials, Guilin University of Electronic Technology, Guangxi, Guilin 541004, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
A facile template method is applied to prepare LiNi0.5Mn1.5O4 porous microellipsoids. LiNi0.5Mn1.5O4 porous microellipsoids deliver excellent electrochemical properties. The discharge capacity reaches 124.8 mA h g1 at 5C rate. The capacity retention is 93.7% after 150 cycles at elevated temperature.
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Article history: Received 18 January 2015 Received in revised form 29 March 2015 Accepted 20 April 2015 Available online
We report the outstanding electrochemical performance of LiNi0.5Mn1.5O4 porous microellipsoids with a coreeshell configuration synthesized by a simple self-template method. The structure and electrochemical properties of the products are investigated via X-ray diffraction, scanning electron microscopy, BrunauereEmmetteTeller method and various electrochemical analysis techniques. LiNi0.5Mn1.5O4 porous microellipsoids have a discharge capacity of 110.3 mA h g1 after 400 cycles at a rate of 5C (735 mA g1). The capacity retention ratio is 93.7% of the maximum capacity (142.3 mA h g1) after 150 cycles under 5C rate at elevated temperature (55 C). The porous ellipsoidal product could be a promising cathode material for high-performance lithium ion batteries. © 2015 Elsevier B.V. All rights reserved.
Keywords: Porous microellipsoid Cathode Core-shell configuration High cycling performance
1. Introduction The exhaustion of crude-oil and global environmental concerns have accelerated efforts to develop lithium ion batteries (LIBs) with high energy density, superior rate capabilities and safety features for mobile electronics and electric vehicles [1]. LiNi0.5Mn1.5O4 is considered one of the most promising cathode materials for LIBs * Corresponding author. School of Material Science and Engineering, Guilin University of Electronic Technology, Guangxi, Guilin 541004, China. E-mail address: [email protected] (J. Deng). http://dx.doi.org/10.1016/j.jpowsour.2015.04.133 0378-7753/© 2015 Elsevier B.V. All rights reserved.
because of its high operating voltage, good chemical stability, low cost and environmental friendliness [2e4]. However, the possible factors: low reversible capacities, poor cycling and rate performance at elevated temperature (50e60 C), and the electrolyte decomposition at the high working voltage, impede the practical application of this cathode material for high-power LIBs [5, 6]. To solve these problems, many efforts have been dedicated to surface coating, element doping and nano-sized LiNi0.5Mn1.5O4 with different morphologies. To address the problem of capacity fade of the spinel LiNi0.5Mn1.5O4, cation doping has been tried to improve the cycling
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performance of LiNi0.5Mn1.5O4 [7e11]. Aklalouch et al. [8] have reported that spinel LiMn1.4Cr0.2Ni0.4O4 delivers the high rate capability with capacity retention of 92% at 60C discharge rate and keeps large cycling stability at elevated temperature (55 C). Co substitution can improve the electrical conductivity and structural stability of LiNi0.5Mn1.5O4, thus enhance the electrochemical performances [9, 11]. However, the modifications through too high doping level decrease the capacity of the spinel cathode material due to increment in the polarization and block of migration pathway of electrons in octahedral sites. Meanwhile, researchers have also employed surface modification with stabilizing oxides and phosphates such as ZrO2, AlPO4, LiFePO4 to solve the capacity fading at elevated temperature [12e14]. Unfortunately, surface modification by oxides or phosphates coating weaken the rate capability of LiNi0.5Mn1.5O4. Recently, nano-sized LiNi0.5Mn1.5O4 cathode materials with different morphologies have been synthesized to enhance the highrate performance, such as nanoparticles [15], nanospheres [16], nanotubes [17], nanorods [18]. Nanostructured LiNi0.5Mn1.5O4 has short transport lengths of charged species and larger electrode/ electrolyte contact area, leading to excellent rate capability. However, compared with micro-sized particles, nanostructured active materials have lower tap density and much more surface side reactions. Therefore, micro-/nanohybrid structured LiNi0.5Mn1.5O4 has attracted wide attention, including porous [19], hollow [2, 20] and yolk-shell [21] structure microspheres consisting of nanograins, as the cathode materials for high-power LIBs. The superior electrochemical performance of these cathode materials can be attributed to their unique porous or hollow nano/micro-structure. In addition, micro-sized LiNi0.5Mn1.5O4 particles and spheres with coreeshell structure are proved to demonstrate good cycling stability at high temperature [22, 23]. Here, the high-performance LiNi0.5Mn1.5O4 porous microellipsoids with a coreeshell structure are prepared using a simple self-template method. The prepared products exhibit high specific capacities, good cycling stability and rate capability as a cathode for LIBs.
Fig. 1. Schematic illustration of the preparation of LiNi0.5Mn1.5O4 porous microellipsoids with a core@void@porous-shell configuration.
microscopy (FESEM, FEI Quanta 450). The surface areas of the products were tested by the BrunauereEmmetteTeller (BET, Quantachrome autosorb iQ2) method with N2 used as the adsorption gas. The electrochemical properties of LiNi0.5Mn1.5O4 porous microellipsoids were measured in CR2032 coin-type cells. The
2. Experimental section The precursor MnCO3 microellipsoids were prepared by a hydrothermal method reported in our previous work with minor modifications [24]. In a typical experiment, KMnO4, C6H12O6 and NaOH in molar ratio of 1:1.1:0.33 were dissolved in 80 ml deionized waters. The mixed solution was transferred into an autoclave and maintained at 150 C for 24 h. The precipitates were centrifuged and washed with the deionized water and ethanol for three times, then dried at 120 C to get MnCO3 microellipsoids. The porous LiNi0.5Mn1.5O4 microellipsoids were synthesized in the following procedure. Firstly, the intermediates were prepared by thermal decomposition of the MnCO3 microellipsoids at 350 C for 4 h. Then the as-obtained ellipsoidal intermediates as templates were mixed with LiOH$H2O and C4H6NiO4$4H2O in ethanol solution. The molar ratio of Li:Ni:Mn is equal to 1.05:0.5:1.5. The ethanol was evaporated slowly under stirring to obtain a mixture. Finally, the products were synthesized by grinding the mixture manually for several minutes and then sintering 12 h in air. According to the influence of sintering temperatures on the electrochemical performance of Li1.02Ni0.5Mn1.5O4 spinel cathode materials [25], the sintering temperature was set to be 750 C in this work. The crystal structure of the as-prepared LiNi0.5Mn1.5O4 microellipsoids was determined by powder X-ray diffraction (XRD, PIXcel3D). The morphologies of the precursors, intermediates and final products were analyzed by field emission scanning electron
Fig. 2. (a) FESEM image of MnCO3 precursors, (b) FESEM image of MnCO3/MnO2 intermediate microellipsoids.
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Fig. 3. (a) FESEM image of LiNi0.5Mn1.5O4 porous microellipsoids, (b) magnified view of the single porous microellipsoid, (c) the cross-section image of a broken microellipsoid.
working electrodes were prepared by mixing the active materials, acetylene black and polyvinylidene fluoride (PVDF) dissolved in Nmethyl-2-pyrrolidone (NMP) solution at a weight ratio of 7:2:1, pasting onto a pure Al foil, and then drying at 100 C under vacuum for 12 h. The Al foil was punched into circular sheets with a diameter of 1.4 cm, and the loading density of the active material in the sheet was about 2.9 mg cm2. Pure lithium foil was used as the counter electrode. The electrolyte was a solution of 1.0 M LiPF6 dissolved in ethylene carbonate (EC)/dimethyl carbonate (DMC) (1:1 in volume). The cells were assembled in an argon-filled glove box with the concentrations of moisture and oxygen below 0.5 ppm. The galvanostatic charge/discharge tests of the cells were
Fig. 4. Nitrogen adsorption/desorption isotherms and corresponding pore size distribution of LiNi0.5Mn1.5O4 porous microellipsoids.
carried out using Arbin BT-2000 battery testing system at different current rates in the voltage range of 3.5e4.95 V. The cyclic voltammetry (CV) curve was recorded by using a Modulab (Solartron Analytical) electrochemical workstation. Electrochemical impedance spectroscopy (EIS) technique was performed after five cycles of chargeedischarge testing in the frequency range of 100 kHz to 0.1 Hz with an ac amplitude of 5 mV. 3. Results and discussion The schematic illustration of the preparation of LiNi0.5Mn1.5O4 porous microellipsoids with a coreeshell configuration is shown in Fig. 1. Uniform micro-sized MnCO3 ellipsoids (Fig. 2a) synthesized
Fig. 5. Powder X-ray diffraction pattern of LiNi0.5Mn1.5O4 porous microellipsoids.
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Fig. 6. A typical cyclic voltammogram of LiNi0.5Mn1.5O4 porous microellipsoids at a scan rate of 0.05 mV s1.
by the hydrothermal method are used as the precursors in the preparation process of LiNi0.5Mn1.5O4. The procedure for the formation of LiNi0.5Mn1.5O4 porous microellipsoids is multi-step and complicated. In step 1, the outer layer of MnCO3 microellipsoids is converted into MnO2 by thermal decomposition during precalcination process at 350 C to form a MnCO3/MnO2 coreeshell structure, which has been reported in literature [26]. The ellipsoidal morphology is kept during the pre-calcination process (see Fig. 2b). In step 2, LiNi0.5Mn1.5O4 porous microellipsoids are formed by the introductions of LiOH$H2O and C4H6NiO4$4H2O using an impregnation method, and then the dehydration of LiOH$H2O, decomposition of C4H6NiO4$4H2O and the lithiation process during high temperature calcinations. The total reactions of formation of LiNi0.5Mn1.5O4 can be expressed by the following equations: 2LiOH$H2O þ 3MnO2 þ C4H6NiO4$4H2O þ 4O2 / 2LiNi0.5Mn1.5O4 þ 10H2O þ 4CO2 (shell) and 4LiOH$H2O þ 6MnCO3 þ 2C4H6NiO4$4H2O þ 9O2 / 4LiNi0.5Mn1.5O4 þ 16H2O þ 14CO2 (core). The formation of the void and pores in LiNi0.5Mn1.5O4 porous microellipsoids are ascribed to the release of CO2 during the calcination process and the Kirkendall effect [20]. FESEM images of LiNi0.5Mn1.5O4 porous microellipsoids are shown in Fig. 3. The as-prepared LiNi0.5Mn1.5O4 maintains ellipsoidal morphology with an average diameter of 1.8 mm and an
Fig. 7. The discharge curves of LiNi0.5Mn1.5O4 porous microellipsoids at different rates.
Fig. 8. (a) The charge/discharge curves and (b) cycling performance of LiNi0.5Mn1.5O4 porous microspheriods measured at a 1C charge rate and a 5C discharge rate.
average length of 3.1 mm after the high temperature sintering. From Fig. 3b and c, the coreeshell configuration could be clearly demonstrated. The core and shell consist of nano-sized subunits. The specific surface area of LiNi0.5Mn1.5O4 porous microellipsoids is 15.5 m2 g1 determined by BET method (Fig. 4). The corresponding pore size distribution presented in the illustration demonstrates that the pore sizes of LiNi0.5Mn1.5O4 are in the range of 1.4e31 nm. The BET results reveal the relative high surface area and porosity of the products. The diffraction peaks for LiNi0.5Mn1.5O4 porous microellipsoids can be indexed to a cubic spinel structure with a small amount of impurity (Fig. 5). The main peaks are sharp, indicating high crystallinity of LiNi0.5Mn1.5O4. The feature peaks of the impurity are positioned at 37.6 , 43.7 and 63.5 , which are assigned to LixNi1xO2 [27]. The impure phase is formed during high temperature calcinations. Similar results have also been reported in LiNi0.5Mn1.5O4 hollow microspheres prepared by calcinations at 800 C [20]. The average crystallite size of LiNi0.5Mn1.5O4 microellipsoids was calculated by the Scherrer equation [28] from XRD data. The value of the average crystallite size is 57.6 nm. Fig. 6 shows a typical cyclic voltammogram of LiNi0.5Mn1.5O4 porous microellipsoids at a scan rate of 0.05 mV s1. Two welldefined redox peaks at 4.6e4.8 V are observed during charge and discharge processes, which are attributed to two-stage redox reaction of Ni2þ/Ni4þ couple [8]. The small redox peaks at the 4.1 V correspond to oxidation and reduction of Mn3þ/Mn4þ. The present of Mn3þ ions is due to the oxygen loss during high temperature
J. Pan et al. / Journal of Power Sources 288 (2015) 353e358
Fig. 9. Cycling performance of LiNi0.5Mn1.5O4 microellipsoids under 5C rate at 55 C, The test was conducted at 5C discharge rate after charging at 1C rate within the voltage range of 3.5e4.95 V.
sintering [16]. To evaluate the rate capability of LiNi0.5Mn1.5O4 microellipsoids, the galvanostatic chargeedischarge tests were performed at different charging/discharging rates. The discharge curves are demonstrated in Fig. 7. The discharge curve of at a rate of 0.1C clearly exhibits a well defined discharge plateau located at about 4.7 V vs. Li/Liþ and a smaller plateau about at 4.0 V, which are attributed to Ni2þ/Ni4þ and Mn3þ/Mn4þ, respectively. The discharge plateaus are in accordance with the reduction peaks in the CV curve. Similar discharge plateaus have been reported in previous works [23, 29, 30]. The discharge capacity at 0.1C rate is 136.8 mA h g1, which can be comparable to that of LiNi0.5Mn1.5O4 porous microspheres [31]. Excellent rate capability can be seen. The discharge capacities are 132.5, 126.3, 108.8 and 100.4 mA h g1 at 0.5C, 1C, 5C and 10C, respectively. The rate capability is better than that of LiNi0.5Mn1.5O4 microspheres and nanorods [31, 32]. However, the rate capability needs to be further improved, compared with previously reported LiNi0.5Mn1.5O4 with porous or hollow structures [18, 21, 33]. The porous coreeshell structure plays a dual role on electrochemical performance of LiNi0.5Mn1.5O4 microellipsoids. On the one hand, the pores allow the electrolyte to enter,
Fig. 10. The Nyquist plot of the LiNi0.5Mn1.5O4 electrode.
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and porous structure and nano-sized subunits increase the contact surface area between the active materials and the electrolyte. The nano-sized crystallites can reduce the Li-ion diffusion distance of Li-ions inside active materials. So the relatively high discharge capacity and good rate capability can be obtained. On the other hand, the micro-sized cores impede the rapid diffusion of lithium ions and are detrimental to buffer the lattice stress caused by JahneTeller distortion during cycling, thus deteriorate the electrochemical performance of LiNi0.5Mn1.5O4 microellipsoids. Moreover, the void and pores with larger surface area may increase the hazard of side reactions between the electrode and electrolyte. The cycling performance of LiNi0.5Mn1.5O4 porous microellipsoids were investigated by galvanostatic chargeedischarge measurements conducted at a 1C charge rate and a 5C discharge rate. The charge/discharge curves of LiNi0.5Mn1.5O4 porous microellipsoids at room temperature are presented in Fig. 8a. The cycling performance of LiNi0.5Mn1.5O4 microellipsoids (Fig. 8b) demonstrates that the discharge capacity reaches its maximum of 124.8 mA h g1 at the 43rd cycle. The capacity is higher than that of LiNi0.5Mn1.5O4 microspheres and particles [16, 34]. From Fig. 8b, LiNi0.5Mn1.5O4 porous microellipsoids deliver stable cycling performance. The discharge capacity is 110.3 mA h g1 and keeps 88.4% of the maximum capacity after 400 cycles. The cycling performance is superior to that of LiNi0.5Mn1.5O4 reported in the literature [35e38]. Compared to pure LiNi0.5Mn1.5O4 porous nanorods and yolk-shell-structure microspheres [18, 21], however, the capacity fading is faster. It may be due to the presence of LixNi1xO2 impurity in LiNi0.5Mn1.5O4 porous microellipsoids that deteriorates the electrochemical performance of the spinel cathode materials [39]. The high discharge capacity and relative excellent cycling performance of LiNi0.5Mn1.5O4 microellipsoids can be attributed to the porous coreeshell structure. The void and pores facilitate to enhance the contact between LiNi0.5Mn1.5O4 microellipsoids and the electrolyte, and accommodate the volume changes during charging/discharging, leading to the improved cycling stability. The porous structure also provides a stale structure of LiNi0.5Mn1.5O4 microellipsoids during cycling. Li et al. have investigated the structure stability of LiMn2O4 during cycling by the ex-situ SEM technique, and confirmed that the porous structure with interior space is remained after cycling [40]. The nano-sized subunits are beneficial to rapid Li-ion diffusion due to short transport distance, contributing to high capacity and good rate capability. The Ni substitution can suppress Mn dissolution and JahneTeller distortion by increasing the average oxidation stat of Mn [41], thus effectively retard capacity fading. In addition, the high crystallinity can keep the stability of crystal structure of LiNi0.5Mn1.5O4 microellipsoids. Sano has reported a very small volume change (13.86 Å3) of LiNi0.5Mn1.5O4 during the entire charging process, which was prepared at a calcinations temperature of 750 C [42]. It is well known that one of the main drawbacks of LiNi0.5Mn1.5O4 cathodes is the inferior cycling stability at elevated temperature [6]. In this work, we also investigated the cycling performance of LiNi0.5Mn1.5O4 microellipsoids at 55 C, as shown in Fig. 9. This cathode material display excellent cycling stability at elevated temperature (55 C). The discharge capacity reaches its maximum value of 142.3 mA h g1 at the 4th cycle under 5C discharge rate. After 150 cycles the capacity is 133.4 mA h g1, and the capacity retention is 93.7% of the maximum capacity. The results are better than those of the previous LiNi0.5Mn1.5O4 [23], and LiNi0.5Mn1.5O4 modified by surface coating and ion doping [43e45]. The excellent electrochemical performance of LiNi0.5Mn1.5O4 porous microellipsoids at elevated temperature can be ascribed to the synergistic effect of porous structure, micro-sized core and high crystallinity. The porous structure of the microellipsoids can accommodate the volume change of LiMn1.5Ni0.5O4 during lithium
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insertion/extraction processes and can release the lattice stress caused by JahneTeller distortion during cycling. The micro-sized core can reduce the hazard of undesirable electrode/electrolyte reactions. The high crystallinity provides the good structure stability during the cycling. To understand the kinetic behaviors of lithium ion in LiNi0.5Mn1.5O4 porous microellipsoids, EIS technique was employed. Fig. 10 shows the Nyquist plot of the electrode measured after five chargeedischarge cycles and fitted curve using Zview software. The plot consists of a depressed semicircle at high frequency relating to the charge transfer resistance (Rct) and a sloping line in the low frequency region ascribing to the diffusion of lithium ion in LiNi0.5Mn1.5O4. Similar impedance spectra have been observed on LiNi0.5Mn1.5O4 [10, 46]. The value of Rct fitted by the equivalent circuit inserted in Fig. 10 is 35.1 U, meaning the fast transport of charged species between the electrode and electrolyte. 4. Conclusions The structure and electrochemical properties have been investigated in details for LiNi0.5Mn1.5O4 porous microellipsoids prepared by a simple self-template method. LiNi0.5Mn1.5O4 porous microellipsoids have a coreeshell structure, and deliver high capacities, good rate capability and cycling stability. The maximum discharge capacity reaches 124.8 mA h g1 at 5C rate and the capacity retention is 88.4% after 400 cycles. At elevated temperature (55 C), the capacity and retention ratio are 133.4 mA h g1 and 93.7% after 150 chargeedischarge cycles at 5C rate, respectively. The facile self-template method can be applied to produce cathode materials with complex configuration for high-performance lithium ion batteries. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (No. 21363005, 51371061 and 11164005), the Guangxi Natural Science Foundation (No. 2012GXNSFBA053154 and 2012GXNSFGA060002) and Innovation Project of Guangxi Graduate Education (YCSZ2014139). References [1] W. Liu, J. Liu, K. Chen, S. Ji, Y. Wan, Y. Zhou, D. Xue, P. Hodgson, Y. Li, Chem. Eur. J. 20 (2014) 824. [2] C. Zhu, T. Akiyama, RSC Adv. 4 (2014) 10151. [3] D.I. Choi, H. Lee, D.J. Lee, K.-W. Nam, J.-S. Kim, R.A. Huggins, J.-K. Park, J.W. Choi, J. Mater. Chem. A 1 (2013) 5320. [4] J. Xiao, X.L. Chen, P.V. Sushko, M.L. Sushko, L. Kovarik, J.J. Feng, Z.Q. Deng, J.M. Zheng, G.L. Graff, Z.M. Nie, D. Choi, J. Liu, J.G. Zhang, M.S. Whittingham, Adv. Mater. 24 (2012) 2109. [5] R. Santhanam, B. Rambabu, J. Power Sources 195 (2010) 5442.
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Short communication
Novel LiNi0.5Mn1.5O4 porous microellipsoids as high-performance cathode materials for lithium ion batteries Jin Pan a, Jianqiu Deng a, b, *, Qingrong Yao a, b, Yongjin Zou a, b, Zhongmin Wang a, b, Huaiying Zhou a, b, Lixian Sun a, b, Guanghui Rao a, b a b
School of Material Science and Engineering, Guilin University of Electronic Technology, Guangxi, Guilin 541004, China Guangxi Key Laboratory of Information Materials, Guilin University of Electronic Technology, Guangxi, Guilin 541004, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
A facile template method is applied to prepare LiNi0.5Mn1.5O4 porous microellipsoids. LiNi0.5Mn1.5O4 porous microellipsoids deliver excellent electrochemical properties. The discharge capacity reaches 124.8 mA h g1 at 5C rate. The capacity retention is 93.7% after 150 cycles at elevated temperature.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 January 2015 Received in revised form 29 March 2015 Accepted 20 April 2015 Available online
We report the outstanding electrochemical performance of LiNi0.5Mn1.5O4 porous microellipsoids with a coreeshell configuration synthesized by a simple self-template method. The structure and electrochemical properties of the products are investigated via X-ray diffraction, scanning electron microscopy, BrunauereEmmetteTeller method and various electrochemical analysis techniques. LiNi0.5Mn1.5O4 porous microellipsoids have a discharge capacity of 110.3 mA h g1 after 400 cycles at a rate of 5C (735 mA g1). The capacity retention ratio is 93.7% of the maximum capacity (142.3 mA h g1) after 150 cycles under 5C rate at elevated temperature (55 C). The porous ellipsoidal product could be a promising cathode material for high-performance lithium ion batteries. © 2015 Elsevier B.V. All rights reserved.
Keywords: Porous microellipsoid Cathode Core-shell configuration High cycling performance
1. Introduction The exhaustion of crude-oil and global environmental concerns have accelerated efforts to develop lithium ion batteries (LIBs) with high energy density, superior rate capabilities and safety features for mobile electronics and electric vehicles [1]. LiNi0.5Mn1.5O4 is considered one of the most promising cathode materials for LIBs * Corresponding author. School of Material Science and Engineering, Guilin University of Electronic Technology, Guangxi, Guilin 541004, China. E-mail address: [email protected] (J. Deng). http://dx.doi.org/10.1016/j.jpowsour.2015.04.133 0378-7753/© 2015 Elsevier B.V. All rights reserved.
because of its high operating voltage, good chemical stability, low cost and environmental friendliness [2e4]. However, the possible factors: low reversible capacities, poor cycling and rate performance at elevated temperature (50e60 C), and the electrolyte decomposition at the high working voltage, impede the practical application of this cathode material for high-power LIBs [5, 6]. To solve these problems, many efforts have been dedicated to surface coating, element doping and nano-sized LiNi0.5Mn1.5O4 with different morphologies. To address the problem of capacity fade of the spinel LiNi0.5Mn1.5O4, cation doping has been tried to improve the cycling
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performance of LiNi0.5Mn1.5O4 [7e11]. Aklalouch et al. [8] have reported that spinel LiMn1.4Cr0.2Ni0.4O4 delivers the high rate capability with capacity retention of 92% at 60C discharge rate and keeps large cycling stability at elevated temperature (55 C). Co substitution can improve the electrical conductivity and structural stability of LiNi0.5Mn1.5O4, thus enhance the electrochemical performances [9, 11]. However, the modifications through too high doping level decrease the capacity of the spinel cathode material due to increment in the polarization and block of migration pathway of electrons in octahedral sites. Meanwhile, researchers have also employed surface modification with stabilizing oxides and phosphates such as ZrO2, AlPO4, LiFePO4 to solve the capacity fading at elevated temperature [12e14]. Unfortunately, surface modification by oxides or phosphates coating weaken the rate capability of LiNi0.5Mn1.5O4. Recently, nano-sized LiNi0.5Mn1.5O4 cathode materials with different morphologies have been synthesized to enhance the highrate performance, such as nanoparticles [15], nanospheres [16], nanotubes [17], nanorods [18]. Nanostructured LiNi0.5Mn1.5O4 has short transport lengths of charged species and larger electrode/ electrolyte contact area, leading to excellent rate capability. However, compared with micro-sized particles, nanostructured active materials have lower tap density and much more surface side reactions. Therefore, micro-/nanohybrid structured LiNi0.5Mn1.5O4 has attracted wide attention, including porous [19], hollow [2, 20] and yolk-shell [21] structure microspheres consisting of nanograins, as the cathode materials for high-power LIBs. The superior electrochemical performance of these cathode materials can be attributed to their unique porous or hollow nano/micro-structure. In addition, micro-sized LiNi0.5Mn1.5O4 particles and spheres with coreeshell structure are proved to demonstrate good cycling stability at high temperature [22, 23]. Here, the high-performance LiNi0.5Mn1.5O4 porous microellipsoids with a coreeshell structure are prepared using a simple self-template method. The prepared products exhibit high specific capacities, good cycling stability and rate capability as a cathode for LIBs.
Fig. 1. Schematic illustration of the preparation of LiNi0.5Mn1.5O4 porous microellipsoids with a core@void@porous-shell configuration.
microscopy (FESEM, FEI Quanta 450). The surface areas of the products were tested by the BrunauereEmmetteTeller (BET, Quantachrome autosorb iQ2) method with N2 used as the adsorption gas. The electrochemical properties of LiNi0.5Mn1.5O4 porous microellipsoids were measured in CR2032 coin-type cells. The
2. Experimental section The precursor MnCO3 microellipsoids were prepared by a hydrothermal method reported in our previous work with minor modifications [24]. In a typical experiment, KMnO4, C6H12O6 and NaOH in molar ratio of 1:1.1:0.33 were dissolved in 80 ml deionized waters. The mixed solution was transferred into an autoclave and maintained at 150 C for 24 h. The precipitates were centrifuged and washed with the deionized water and ethanol for three times, then dried at 120 C to get MnCO3 microellipsoids. The porous LiNi0.5Mn1.5O4 microellipsoids were synthesized in the following procedure. Firstly, the intermediates were prepared by thermal decomposition of the MnCO3 microellipsoids at 350 C for 4 h. Then the as-obtained ellipsoidal intermediates as templates were mixed with LiOH$H2O and C4H6NiO4$4H2O in ethanol solution. The molar ratio of Li:Ni:Mn is equal to 1.05:0.5:1.5. The ethanol was evaporated slowly under stirring to obtain a mixture. Finally, the products were synthesized by grinding the mixture manually for several minutes and then sintering 12 h in air. According to the influence of sintering temperatures on the electrochemical performance of Li1.02Ni0.5Mn1.5O4 spinel cathode materials [25], the sintering temperature was set to be 750 C in this work. The crystal structure of the as-prepared LiNi0.5Mn1.5O4 microellipsoids was determined by powder X-ray diffraction (XRD, PIXcel3D). The morphologies of the precursors, intermediates and final products were analyzed by field emission scanning electron
Fig. 2. (a) FESEM image of MnCO3 precursors, (b) FESEM image of MnCO3/MnO2 intermediate microellipsoids.
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Fig. 3. (a) FESEM image of LiNi0.5Mn1.5O4 porous microellipsoids, (b) magnified view of the single porous microellipsoid, (c) the cross-section image of a broken microellipsoid.
working electrodes were prepared by mixing the active materials, acetylene black and polyvinylidene fluoride (PVDF) dissolved in Nmethyl-2-pyrrolidone (NMP) solution at a weight ratio of 7:2:1, pasting onto a pure Al foil, and then drying at 100 C under vacuum for 12 h. The Al foil was punched into circular sheets with a diameter of 1.4 cm, and the loading density of the active material in the sheet was about 2.9 mg cm2. Pure lithium foil was used as the counter electrode. The electrolyte was a solution of 1.0 M LiPF6 dissolved in ethylene carbonate (EC)/dimethyl carbonate (DMC) (1:1 in volume). The cells were assembled in an argon-filled glove box with the concentrations of moisture and oxygen below 0.5 ppm. The galvanostatic charge/discharge tests of the cells were
Fig. 4. Nitrogen adsorption/desorption isotherms and corresponding pore size distribution of LiNi0.5Mn1.5O4 porous microellipsoids.
carried out using Arbin BT-2000 battery testing system at different current rates in the voltage range of 3.5e4.95 V. The cyclic voltammetry (CV) curve was recorded by using a Modulab (Solartron Analytical) electrochemical workstation. Electrochemical impedance spectroscopy (EIS) technique was performed after five cycles of chargeedischarge testing in the frequency range of 100 kHz to 0.1 Hz with an ac amplitude of 5 mV. 3. Results and discussion The schematic illustration of the preparation of LiNi0.5Mn1.5O4 porous microellipsoids with a coreeshell configuration is shown in Fig. 1. Uniform micro-sized MnCO3 ellipsoids (Fig. 2a) synthesized
Fig. 5. Powder X-ray diffraction pattern of LiNi0.5Mn1.5O4 porous microellipsoids.
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Fig. 6. A typical cyclic voltammogram of LiNi0.5Mn1.5O4 porous microellipsoids at a scan rate of 0.05 mV s1.
by the hydrothermal method are used as the precursors in the preparation process of LiNi0.5Mn1.5O4. The procedure for the formation of LiNi0.5Mn1.5O4 porous microellipsoids is multi-step and complicated. In step 1, the outer layer of MnCO3 microellipsoids is converted into MnO2 by thermal decomposition during precalcination process at 350 C to form a MnCO3/MnO2 coreeshell structure, which has been reported in literature [26]. The ellipsoidal morphology is kept during the pre-calcination process (see Fig. 2b). In step 2, LiNi0.5Mn1.5O4 porous microellipsoids are formed by the introductions of LiOH$H2O and C4H6NiO4$4H2O using an impregnation method, and then the dehydration of LiOH$H2O, decomposition of C4H6NiO4$4H2O and the lithiation process during high temperature calcinations. The total reactions of formation of LiNi0.5Mn1.5O4 can be expressed by the following equations: 2LiOH$H2O þ 3MnO2 þ C4H6NiO4$4H2O þ 4O2 / 2LiNi0.5Mn1.5O4 þ 10H2O þ 4CO2 (shell) and 4LiOH$H2O þ 6MnCO3 þ 2C4H6NiO4$4H2O þ 9O2 / 4LiNi0.5Mn1.5O4 þ 16H2O þ 14CO2 (core). The formation of the void and pores in LiNi0.5Mn1.5O4 porous microellipsoids are ascribed to the release of CO2 during the calcination process and the Kirkendall effect [20]. FESEM images of LiNi0.5Mn1.5O4 porous microellipsoids are shown in Fig. 3. The as-prepared LiNi0.5Mn1.5O4 maintains ellipsoidal morphology with an average diameter of 1.8 mm and an
Fig. 7. The discharge curves of LiNi0.5Mn1.5O4 porous microellipsoids at different rates.
Fig. 8. (a) The charge/discharge curves and (b) cycling performance of LiNi0.5Mn1.5O4 porous microspheriods measured at a 1C charge rate and a 5C discharge rate.
average length of 3.1 mm after the high temperature sintering. From Fig. 3b and c, the coreeshell configuration could be clearly demonstrated. The core and shell consist of nano-sized subunits. The specific surface area of LiNi0.5Mn1.5O4 porous microellipsoids is 15.5 m2 g1 determined by BET method (Fig. 4). The corresponding pore size distribution presented in the illustration demonstrates that the pore sizes of LiNi0.5Mn1.5O4 are in the range of 1.4e31 nm. The BET results reveal the relative high surface area and porosity of the products. The diffraction peaks for LiNi0.5Mn1.5O4 porous microellipsoids can be indexed to a cubic spinel structure with a small amount of impurity (Fig. 5). The main peaks are sharp, indicating high crystallinity of LiNi0.5Mn1.5O4. The feature peaks of the impurity are positioned at 37.6 , 43.7 and 63.5 , which are assigned to LixNi1xO2 [27]. The impure phase is formed during high temperature calcinations. Similar results have also been reported in LiNi0.5Mn1.5O4 hollow microspheres prepared by calcinations at 800 C [20]. The average crystallite size of LiNi0.5Mn1.5O4 microellipsoids was calculated by the Scherrer equation [28] from XRD data. The value of the average crystallite size is 57.6 nm. Fig. 6 shows a typical cyclic voltammogram of LiNi0.5Mn1.5O4 porous microellipsoids at a scan rate of 0.05 mV s1. Two welldefined redox peaks at 4.6e4.8 V are observed during charge and discharge processes, which are attributed to two-stage redox reaction of Ni2þ/Ni4þ couple [8]. The small redox peaks at the 4.1 V correspond to oxidation and reduction of Mn3þ/Mn4þ. The present of Mn3þ ions is due to the oxygen loss during high temperature
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Fig. 9. Cycling performance of LiNi0.5Mn1.5O4 microellipsoids under 5C rate at 55 C, The test was conducted at 5C discharge rate after charging at 1C rate within the voltage range of 3.5e4.95 V.
sintering [16]. To evaluate the rate capability of LiNi0.5Mn1.5O4 microellipsoids, the galvanostatic chargeedischarge tests were performed at different charging/discharging rates. The discharge curves are demonstrated in Fig. 7. The discharge curve of at a rate of 0.1C clearly exhibits a well defined discharge plateau located at about 4.7 V vs. Li/Liþ and a smaller plateau about at 4.0 V, which are attributed to Ni2þ/Ni4þ and Mn3þ/Mn4þ, respectively. The discharge plateaus are in accordance with the reduction peaks in the CV curve. Similar discharge plateaus have been reported in previous works [23, 29, 30]. The discharge capacity at 0.1C rate is 136.8 mA h g1, which can be comparable to that of LiNi0.5Mn1.5O4 porous microspheres [31]. Excellent rate capability can be seen. The discharge capacities are 132.5, 126.3, 108.8 and 100.4 mA h g1 at 0.5C, 1C, 5C and 10C, respectively. The rate capability is better than that of LiNi0.5Mn1.5O4 microspheres and nanorods [31, 32]. However, the rate capability needs to be further improved, compared with previously reported LiNi0.5Mn1.5O4 with porous or hollow structures [18, 21, 33]. The porous coreeshell structure plays a dual role on electrochemical performance of LiNi0.5Mn1.5O4 microellipsoids. On the one hand, the pores allow the electrolyte to enter,
Fig. 10. The Nyquist plot of the LiNi0.5Mn1.5O4 electrode.
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and porous structure and nano-sized subunits increase the contact surface area between the active materials and the electrolyte. The nano-sized crystallites can reduce the Li-ion diffusion distance of Li-ions inside active materials. So the relatively high discharge capacity and good rate capability can be obtained. On the other hand, the micro-sized cores impede the rapid diffusion of lithium ions and are detrimental to buffer the lattice stress caused by JahneTeller distortion during cycling, thus deteriorate the electrochemical performance of LiNi0.5Mn1.5O4 microellipsoids. Moreover, the void and pores with larger surface area may increase the hazard of side reactions between the electrode and electrolyte. The cycling performance of LiNi0.5Mn1.5O4 porous microellipsoids were investigated by galvanostatic chargeedischarge measurements conducted at a 1C charge rate and a 5C discharge rate. The charge/discharge curves of LiNi0.5Mn1.5O4 porous microellipsoids at room temperature are presented in Fig. 8a. The cycling performance of LiNi0.5Mn1.5O4 microellipsoids (Fig. 8b) demonstrates that the discharge capacity reaches its maximum of 124.8 mA h g1 at the 43rd cycle. The capacity is higher than that of LiNi0.5Mn1.5O4 microspheres and particles [16, 34]. From Fig. 8b, LiNi0.5Mn1.5O4 porous microellipsoids deliver stable cycling performance. The discharge capacity is 110.3 mA h g1 and keeps 88.4% of the maximum capacity after 400 cycles. The cycling performance is superior to that of LiNi0.5Mn1.5O4 reported in the literature [35e38]. Compared to pure LiNi0.5Mn1.5O4 porous nanorods and yolk-shell-structure microspheres [18, 21], however, the capacity fading is faster. It may be due to the presence of LixNi1xO2 impurity in LiNi0.5Mn1.5O4 porous microellipsoids that deteriorates the electrochemical performance of the spinel cathode materials [39]. The high discharge capacity and relative excellent cycling performance of LiNi0.5Mn1.5O4 microellipsoids can be attributed to the porous coreeshell structure. The void and pores facilitate to enhance the contact between LiNi0.5Mn1.5O4 microellipsoids and the electrolyte, and accommodate the volume changes during charging/discharging, leading to the improved cycling stability. The porous structure also provides a stale structure of LiNi0.5Mn1.5O4 microellipsoids during cycling. Li et al. have investigated the structure stability of LiMn2O4 during cycling by the ex-situ SEM technique, and confirmed that the porous structure with interior space is remained after cycling [40]. The nano-sized subunits are beneficial to rapid Li-ion diffusion due to short transport distance, contributing to high capacity and good rate capability. The Ni substitution can suppress Mn dissolution and JahneTeller distortion by increasing the average oxidation stat of Mn [41], thus effectively retard capacity fading. In addition, the high crystallinity can keep the stability of crystal structure of LiNi0.5Mn1.5O4 microellipsoids. Sano has reported a very small volume change (13.86 Å3) of LiNi0.5Mn1.5O4 during the entire charging process, which was prepared at a calcinations temperature of 750 C [42]. It is well known that one of the main drawbacks of LiNi0.5Mn1.5O4 cathodes is the inferior cycling stability at elevated temperature [6]. In this work, we also investigated the cycling performance of LiNi0.5Mn1.5O4 microellipsoids at 55 C, as shown in Fig. 9. This cathode material display excellent cycling stability at elevated temperature (55 C). The discharge capacity reaches its maximum value of 142.3 mA h g1 at the 4th cycle under 5C discharge rate. After 150 cycles the capacity is 133.4 mA h g1, and the capacity retention is 93.7% of the maximum capacity. The results are better than those of the previous LiNi0.5Mn1.5O4 [23], and LiNi0.5Mn1.5O4 modified by surface coating and ion doping [43e45]. The excellent electrochemical performance of LiNi0.5Mn1.5O4 porous microellipsoids at elevated temperature can be ascribed to the synergistic effect of porous structure, micro-sized core and high crystallinity. The porous structure of the microellipsoids can accommodate the volume change of LiMn1.5Ni0.5O4 during lithium
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insertion/extraction processes and can release the lattice stress caused by JahneTeller distortion during cycling. The micro-sized core can reduce the hazard of undesirable electrode/electrolyte reactions. The high crystallinity provides the good structure stability during the cycling. To understand the kinetic behaviors of lithium ion in LiNi0.5Mn1.5O4 porous microellipsoids, EIS technique was employed. Fig. 10 shows the Nyquist plot of the electrode measured after five chargeedischarge cycles and fitted curve using Zview software. The plot consists of a depressed semicircle at high frequency relating to the charge transfer resistance (Rct) and a sloping line in the low frequency region ascribing to the diffusion of lithium ion in LiNi0.5Mn1.5O4. Similar impedance spectra have been observed on LiNi0.5Mn1.5O4 [10, 46]. The value of Rct fitted by the equivalent circuit inserted in Fig. 10 is 35.1 U, meaning the fast transport of charged species between the electrode and electrolyte. 4. Conclusions The structure and electrochemical properties have been investigated in details for LiNi0.5Mn1.5O4 porous microellipsoids prepared by a simple self-template method. LiNi0.5Mn1.5O4 porous microellipsoids have a coreeshell structure, and deliver high capacities, good rate capability and cycling stability. The maximum discharge capacity reaches 124.8 mA h g1 at 5C rate and the capacity retention is 88.4% after 400 cycles. At elevated temperature (55 C), the capacity and retention ratio are 133.4 mA h g1 and 93.7% after 150 chargeedischarge cycles at 5C rate, respectively. The facile self-template method can be applied to produce cathode materials with complex configuration for high-performance lithium ion batteries. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (No. 21363005, 51371061 and 11164005), the Guangxi Natural Science Foundation (No. 2012GXNSFBA053154 and 2012GXNSFGA060002) and Innovation Project of Guangxi Graduate Education (YCSZ2014139). References [1] W. Liu, J. Liu, K. Chen, S. Ji, Y. Wan, Y. Zhou, D. Xue, P. Hodgson, Y. Li, Chem. Eur. J. 20 (2014) 824. [2] C. Zhu, T. Akiyama, RSC Adv. 4 (2014) 10151. [3] D.I. Choi, H. Lee, D.J. Lee, K.-W. Nam, J.-S. Kim, R.A. Huggins, J.-K. Park, J.W. Choi, J. Mater. Chem. A 1 (2013) 5320. [4] J. Xiao, X.L. Chen, P.V. Sushko, M.L. Sushko, L. Kovarik, J.J. Feng, Z.Q. Deng, J.M. Zheng, G.L. Graff, Z.M. Nie, D. Choi, J. Liu, J.G. Zhang, M.S. Whittingham, Adv. Mater. 24 (2012) 2109. [5] R. Santhanam, B. Rambabu, J. Power Sources 195 (2010) 5442.
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