- Email: [email protected]
Synthesis of spinel LiNi0.5Mn1.5O4 cathode material with excellent cycle stability using urea-based sol–gel method
Materials Letters 89 (2012) 251–253
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Synthesis of spinel LiNi0.5Mn1.5O4 cathode material with excellent cycle stability using urea-based sol–gel method Ou Sha a,n, Shaoliang Wang a, Zhi Qiao a, Wei Yuan a, Zhiyuan Tang a, Qiang Xu a, Yanjun Su b a b
Department of Applied Chemistry, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China Mcnair Technology Co., Ltd., Dongguan, Guangdong 523700, China
a r t i c l e i n f o
abstract
Article history: Received 9 June 2012 Accepted 28 August 2012 Available online 5 September 2012
The spinel LiNi0.5Mn1.5O4 cathode material has been successfully prepared employing a simple and novel urea-based sol–gel method. The material exhibits excellent cycle stability to reach 132, 123 and 113 mA h g 1 at 1, 3 and 5 C (25 1C), respectively. When cycled at 55 1C, 105 mA h g 1 could be achieved at 5 C with only 8.6% capacity fading after 150 cycles. In addition, the satisfactory high rate (20 C) cycle performance at 25 1C has been shown to reach 97.8 mA h g 1, retaining 87.5% after 200 cycles. The results indicate that urea should be a universal and low-cost chelating agent for the synthesis of LiNi0.5Mn1.5O4 cathode material with excellent electrochemical properties. & 2012 Elsevier B.V. All rights reserved.
Keywords: LiNi0.5Mn1.5O4 spinel Urea Sol–gel preparation Cycle stability Diffusion
1. Introduction With the increased requirements of high power energy storage systems in hybrid electric vehicle (HEV) and electric vehicle (EV) applications, spinel LiNi0.5Mn1.5O4 (LNMS) cathode material for lithium ion batteries has drawn considerable attention based on its high operating voltage (4.7 V), high theoretical capacity (147 mA h g 1) and unique 3-dimensional Li þ diffusion channels. However, the undesired interfacial side reaction between highvoltage charged LiNi0.5Mn1.5O4 and electrolyte, along with the inevitable presence of LixNi1 xO as a second phase, provokes serious capacity fading during cycling, especially at high rate and elevated temperature (55 1C). To overcome these drawbacks, several approaches [1–5] have been proposed, including the optimization of synthesis conditions. Sol–gel technology is regarded as an effective synthesized route in homogeneously mixing all reagents at atomic or molecular level and carefully controlling the particle size in a narrow distribution. The selection of chelating agents such as citric acid [6], malic acid [7], ethylene glycol [8], cellulose, ascorbic acid and resorcinol/formaldehyde composite [9], is a key factor for the synthesis of LiNi0.5Mn1.5O4 by the sol–gel method. As a simple and low-cost organic compound, urea has been reported as precipitation agent [10], combustion agent [11] or shape-controlled agent [12], but few works on chelating agent have been reported in LiNi0.5Mn1.5O4 system. Therefore, in this
n
Corresponding author. Tel.: þ86 769 83017180; fax: þ86 769 83195372. E-mail address: [email protected] (O. Sha).
0167-577X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2012.08.126
work, we have synthesized spinel LiNi0.5Mn1.5O4 cathode material employing a novel urea-based sol–gel method for the first time. The cycle performances of LiNi0.5Mn1.5O4 at different rates and elevated temperature were discussed in detail.
2. Experimental The as-prepared LiNi0.5Mn1.5O4 was synthesized via a simplified sol–gel method employing acetates as raw materials and urea as a chelating agent. Stoichiometric amounts of all acetates were dissolved in de-ionized water and stirred at 50 1C. The urea solution (the mole ratio of urea and all metal ions is 1:1) was added with continuous stirring. After stirring constantly for 5 h, the temperature was raised to 80 1C for the evaporation of water. The brown transparent gel was obtained in a vacuum oven at 120 1C overnight. To decompose the organic constituents, the ground gel was pre-sintered at 450 1C in air. After being ground again, the powder was sintered at 800 1C for 12 h followed by an annealing treatment at 650 1C for 10 h in air. The structural and crystalline phase analyses were conducted using powder X-ray diffraction (XRD, D/MAX-2500, Japan) employing Cu Ka radiation at the scanning speed of 41 min 1. The morphology of the material was observed by scanning electron microscopy (SEM, Nanosem 430, America), 10 kV. Electrochemical tests were carried out on CR2032 coin-type cells. The cathodes were fabricated by mixing the experimental active material with acetylene black and polyvinylidene difluoride (PVDF) at a weight ratio of 8:1:1. The active material mass loading and electrode surface were 2.3 ( 70.1) mg cm 2 and
252
O. Sha et al. / Materials Letters 89 (2012) 251–253
˚ which is well consistent with by an MDI Jade Software is 8.171 A, the values reported previously [5,8]. As the SEM image (inset) and the corresponding particle size histogram shown in Fig. 1(b), the sample consists of small particles in the range of 250–350 nm mainly. The reduction of the particle growth should be ascribed to the large amount of gas release during the heat treatment process. The cycle performance of LiNi0.5Mn1.5O4 has been investigated by cycling the cells at different rates (25 1C), and the results have been plotted in Fig. 2(a). As shown, the capacities gradually rise to a steady state in the first few cycles, due to the gradual infiltration of electrolyte with the active material particles. The maximum attainable discharge capacities of 132, 123 and 113 mA h g 1 can be obtained at 1, 3 and 5 C, respectively. After 200 cycles, the corresponding capacity retention rates are 91.7%, 95.4% and 87.1%. The inset of Fig. 2(a) reveals the charge– discharge curves of the material at different rates. All the curves represent one main charge plateau at about 4.8 V and one main
0.785 cm2, respectively. Using lithium metal as anode, 1 M LiPF6 in a 2:3 (by volume) mixture of ethylene carbonate (EC)/diethyl carbonate (DEC) as electrolyte, the cells were assembled in an argon-filled glove box. The galvanostatic charged/discharged tests were measured at various C-rates (1 C¼147 mA g 1) at ambient temperature (25 1C) or elevated temperature (55 1C) between 3.5 V and 5 V. Cyclic voltammetry (CV) tests were carried out at different scan rates (0.05, 0.10, 0.20 and 0.40 mV s 1) from 3.5 to 5 V.
3. Results and discussion Fig. 1(a) shows the XRD pattern of the as-prepared LiNi0.5Mn1.5O4 material. All the diffraction peaks can be indexed based on a well-defined spinel cubic structure, along with minor traces of LixNi1 xO impurity phase close to the characteristic peaks of (311), (400) and (440). The lattice parameter calculated (111)
60 ∗ LixNi1-xO
10
20
30
∗
∗
∗
40
50
60
Frequency / % (533) (622)
(511)
(440) (531)
(400) (331)
(222)
(220)
(311)
Intensity / a.u.
50
70
40 30 20 10 0 100
80
2θ ⁄ °
200
300
400 500 600 Particle size / nm
700
800
900
Fig. 1. (a) X-ray diffraction pattern of LiNi0.5Mn1.5O4 and (b) SEM image (inset) and the corresponding particle size histogram of LiNi0.5Mn1.5O4.
150
150
90 60 30 0
0
5.0 4.8 4.6 4.4 4.2 4.0 3.8 3.6 3.4 3.2 0
1C 3C 5C 20
50
40
Discharge capacity / mAhg-1
120
Potential vs. (Li/Li ) / V
Discharge capacity / mAh g-1
25 °C
5C 3C1C 60
80
100
100
120
55 °C
Charge rate: 5 C Discharge rate: 5 C
120
90
60
30
140
150
200
0
0
30
Cycle number
60
90
120
150
Cycle number
Discharge capacity / mAh g-1
150
25 °C
Charge rate: 1 C Discharge rate: 20 C
120
90
60
30
0
0
50
100 Cycle number
150
200
Fig. 2. (a) Cycle performance and charge–discharge curves (inset) of LiNi0.5Mn1.5O4 at different rates (1C, 3C and 5C; 25 1C); (b) cycle performance of LiNi0.5Mn1.5O4 at 5C rate (55 1C); and (c) high rate cycle performance of LiNi0.5Mn1.5O4 at 20C discharge rate (25 1C).
O. Sha et al. / Materials Letters 89 (2012) 251–253
2.0
4. Conclusions
1.5
0.40 mV s-1 0.20 mV s-1 0.10 mV s-1 0.05 mV s-1
Current / mA
1.0 0.5
High pure spinel LiNi0.5Mn1.5O4 has been prepared employing a simple and low-cost urea-based sol–gel method. The results demonstrate that the material with small particles exhibits excellent cycle stability at 55 1C and 94.3 mA h g 1 can be achieved at 5 C after 150 cycles (capacity fading rate: 8.6%). Besides, the satisfactory high rate (20 C) cycle stability has been shown to reach 97.8 mA h g 1 at 25 1C, retaining 87.5% after 200 cycles. Using the inexpensive urea as a chelating agent should be an efficient and convenient approach for the synthesis of LiNi0.5Mn1.5O4 cathode material which is expected to be a potential power sources for high power lithium ion batteries.
υ
0.0 -0.5 -1.0 -1.5 -2.0
253
ip 3.5
4.0 4.5 Potential vs.(Li/Li+) / V
5.0
Fig. 3. Cyclic voltammograms for LiNi0.5Mn1.5O4 electrode at different potential sweeping rates.
discharge plateau at around 4.7 V, corresponding to the Ni2 þ /Ni3 þ and Ni3 þ /Ni4 þ redox couples. The 4.0 V plateau characteristic of the Mn3 þ /Mn4 þ redox couple is almost invisible, which is indicative of the highly pure product. Nevertheless, due to the ohmic polarization and electrochemical polarization, the voltage platform differences between charge and discharge become larger at higher rates. Normally, the cycle stability of the LNMS-based spinels at 55 1C was reported at low rates [2,3]. In this work, we have assessed the material’s cycle performance at a higher rate (5 C, 55 1C). The result in Fig. 2(b) suggests that it still exhibits wonderful cycle stability to reach 105 mA h g 1 at maximum and retain 94.3 mA h g 1 at the 150th cycle, with only 8.6% capacity fading. On the other hand, the high rate capability of LNMS-based spinels also is an important parameter for practical applications [13]. The cycle performance at the high rate of LiNi0.5Mn1.5O4 has been shown in Fig. 2(c), when discharged at 20 C (25 1C, charge rate: 1 C), 97.8 mA h g 1 could be achieved. After 200 cycles, it decreases to 85.6 mA h g 1 (87.5% of the initial discharge capacity), revealing satisfactory high rate cycle performance. Fig. 3 displays the cyclic voltammograms for LiNi0.5Mn1.5O4 recorded at different potential sweeping rates. The peak current (ip) increased with the increase of the scan rate (u). Assuming that the intercalation reaction is controlled by the solid-state diffusion of Li þ , the dependence of the peak current on the square root of the scan rate (u1/2) can be applied to determine the lithium diffusion coefficient (DLi) according to the following equation: ip ¼ 2:69 105 n3=2 ADLi 1=2 n1=2 C 0Li where n is the number of electrons involved in the electrochemical reaction (n¼ 2), A is the surface area of the electrode, and C0Li is the bulk concentration of Li þ in the electrode (0.0244 mol cm 3) [1,14], u is the potential sweeping rate (V s 1). The calculated DLi in this case is 3.2 10 11 cm2 s 1 which is identical with the values reported previously and even better than some [1,15], indicating higher lithium ion diffusion kinetics in the material. Therefore, the stable structure and fast lithium diffusion speed afforded by the formation of small particles could explain the excellent cycle stability and satisfactory rate capability in our material.
Acknowledgments This work was supported by the National Natural Science Foundation of China (no. 20973124) and Ph.D. Programs Foundation of Ministry of Education of China (no. 20070056138).
References [1] Yang TY, Zhang NQ, Lang Y, Sun KN. Enhanced rate performance of carboncoated LiNi0.5Mn1.5O4 cathode material for lithium ion batteries. Electrochim Acta 2011;56:4058–64. [2] Arrebola JC, Caballero A, Hernan L, Morales J. Re-examining the effect of ZnO on nanosized 5 V LiNi0.5Mn1.5O4 spinel: an effective procedure for enhancing its rate capability at room and high temperatures. J Power Sources 2010;195:4278–84. [3] Shin DW, Manthiram A. Surface-segregated, high-voltage spinel LiMn1.5Ni0.42Ga0.08O4 cathodes with superior high-temperature cyclability for lithium-ion batteries. Electrochem Commun 2011;13:1213–6. [4] Lin CY, Duh JG, Hsu CH, Chen JM. LiNi0.5Mn1.5O4 cathode material by lowtemperature solid-state method with excellent cycleability in lithium ion battery. Mater Lett 2010;64:2328–30. [5] Jo M, Lee YK, Kim KM, Cho J. Nanoparticle–nanorod core–shell LiNi0.5Mn1.5O4 spinel cathodes with high energy density for Li-ion batteries. J Electrochem Soc 2010;157(7):A841–5. [6] Hwang BJ, Wu YW, Venkateswarlu M, Cheng MY, Santhanam R. Influence of synthesis conditions on electrochemical properties of high-voltage Li1.02Ni0.5Mn1.5O4 spinel cathode material. J Power Sources 2009;193: 828–33. [7] Liu GQ, Wen L, Liu GY, Tian YW. Rate capability of spinel LiCr0.1Ni0.4Mn1.5O4. J Alloys Compd 2010;501:233–5. [8] Duncan H, Abu-Lebdeh Y, Davidson IJ. Study of the cathode–electrolyte interface of LiMn1.5Ni0.5O4 synthesized by a sol–gel method for Li-ion batteries. J Electrochem Soc 2010;157(4):A528–35. [9] Chi LH, Dinh NN, Brutti S, Scrosati B. Synthesis, characterization and electrochemical properties of 4.8 V LiNi0.5Mn1.5O4 cathode material in lithium-ion batteries. Electrochim Acta 2010;55:5110–6. [10] Xiao L, Liu XJ, Zhao X, Liang HX, Liu HX. Preparation of LiNi0.5Mn0.5O2 cathode materials by urea hydrolysis coprecipitation. Solid State Ionics 2011;192(1): 335–8. [11] Sharma Y, Sharma N, Rao Subba GV, Chowdari BVR. Li-storage and cyclability of urea combustion derived ZnFe2O4 as anode for Li-ion batteries. Electrochim Acta 2008;53:2380–5. [12] Cao AM, Manthiram A. Shape-controlled synthesis of high tap density cathode oxides for lithium ion batteries. Phys Chem Chem Phys 2012;14: 6724–8. [13] Aklalouch M, Amarilla JM, Rojas RM, Saadoune I. Sub-micrometric LiCr0.2Ni0.4Mn1.4O4 spinel as 5 V-cathode material exhibiting huge rate capability at 25 and 55 1C. Electrochem Commun 2010;12:548–52. [14] Chung MD, Seo JH, Zhang XC, Sastry AM. Implementing realistic geometry and measured diffusion coefficients into single particle electrode modeling based on experiments with single LiMn2O4 spinel particles. J Electrochem Soc 2011;158(4):A371–8. [15] Zhang XL, Cheng FY, Zhang K, Liang YL, Yang SQ, Liang J, et al. Facile polymerassisted synthesis of LiNi0.5Mn1.5O4 with hierarchical micro–nano structure and high rate capability. RSC Adv 2012;2(13):5669–75.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Synthesis of spinel LiNi0.5Mn1.5O4 cathode material with excellent cycle stability using urea-based sol–gel method Ou Sha a,n, Shaoliang Wang a, Zhi Qiao a, Wei Yuan a, Zhiyuan Tang a, Qiang Xu a, Yanjun Su b a b
Department of Applied Chemistry, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China Mcnair Technology Co., Ltd., Dongguan, Guangdong 523700, China
a r t i c l e i n f o
abstract
Article history: Received 9 June 2012 Accepted 28 August 2012 Available online 5 September 2012
The spinel LiNi0.5Mn1.5O4 cathode material has been successfully prepared employing a simple and novel urea-based sol–gel method. The material exhibits excellent cycle stability to reach 132, 123 and 113 mA h g 1 at 1, 3 and 5 C (25 1C), respectively. When cycled at 55 1C, 105 mA h g 1 could be achieved at 5 C with only 8.6% capacity fading after 150 cycles. In addition, the satisfactory high rate (20 C) cycle performance at 25 1C has been shown to reach 97.8 mA h g 1, retaining 87.5% after 200 cycles. The results indicate that urea should be a universal and low-cost chelating agent for the synthesis of LiNi0.5Mn1.5O4 cathode material with excellent electrochemical properties. & 2012 Elsevier B.V. All rights reserved.
Keywords: LiNi0.5Mn1.5O4 spinel Urea Sol–gel preparation Cycle stability Diffusion
1. Introduction With the increased requirements of high power energy storage systems in hybrid electric vehicle (HEV) and electric vehicle (EV) applications, spinel LiNi0.5Mn1.5O4 (LNMS) cathode material for lithium ion batteries has drawn considerable attention based on its high operating voltage (4.7 V), high theoretical capacity (147 mA h g 1) and unique 3-dimensional Li þ diffusion channels. However, the undesired interfacial side reaction between highvoltage charged LiNi0.5Mn1.5O4 and electrolyte, along with the inevitable presence of LixNi1 xO as a second phase, provokes serious capacity fading during cycling, especially at high rate and elevated temperature (55 1C). To overcome these drawbacks, several approaches [1–5] have been proposed, including the optimization of synthesis conditions. Sol–gel technology is regarded as an effective synthesized route in homogeneously mixing all reagents at atomic or molecular level and carefully controlling the particle size in a narrow distribution. The selection of chelating agents such as citric acid [6], malic acid [7], ethylene glycol [8], cellulose, ascorbic acid and resorcinol/formaldehyde composite [9], is a key factor for the synthesis of LiNi0.5Mn1.5O4 by the sol–gel method. As a simple and low-cost organic compound, urea has been reported as precipitation agent [10], combustion agent [11] or shape-controlled agent [12], but few works on chelating agent have been reported in LiNi0.5Mn1.5O4 system. Therefore, in this
n
Corresponding author. Tel.: þ86 769 83017180; fax: þ86 769 83195372. E-mail address: [email protected] (O. Sha).
0167-577X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2012.08.126
work, we have synthesized spinel LiNi0.5Mn1.5O4 cathode material employing a novel urea-based sol–gel method for the first time. The cycle performances of LiNi0.5Mn1.5O4 at different rates and elevated temperature were discussed in detail.
2. Experimental The as-prepared LiNi0.5Mn1.5O4 was synthesized via a simplified sol–gel method employing acetates as raw materials and urea as a chelating agent. Stoichiometric amounts of all acetates were dissolved in de-ionized water and stirred at 50 1C. The urea solution (the mole ratio of urea and all metal ions is 1:1) was added with continuous stirring. After stirring constantly for 5 h, the temperature was raised to 80 1C for the evaporation of water. The brown transparent gel was obtained in a vacuum oven at 120 1C overnight. To decompose the organic constituents, the ground gel was pre-sintered at 450 1C in air. After being ground again, the powder was sintered at 800 1C for 12 h followed by an annealing treatment at 650 1C for 10 h in air. The structural and crystalline phase analyses were conducted using powder X-ray diffraction (XRD, D/MAX-2500, Japan) employing Cu Ka radiation at the scanning speed of 41 min 1. The morphology of the material was observed by scanning electron microscopy (SEM, Nanosem 430, America), 10 kV. Electrochemical tests were carried out on CR2032 coin-type cells. The cathodes were fabricated by mixing the experimental active material with acetylene black and polyvinylidene difluoride (PVDF) at a weight ratio of 8:1:1. The active material mass loading and electrode surface were 2.3 ( 70.1) mg cm 2 and
252
O. Sha et al. / Materials Letters 89 (2012) 251–253
˚ which is well consistent with by an MDI Jade Software is 8.171 A, the values reported previously [5,8]. As the SEM image (inset) and the corresponding particle size histogram shown in Fig. 1(b), the sample consists of small particles in the range of 250–350 nm mainly. The reduction of the particle growth should be ascribed to the large amount of gas release during the heat treatment process. The cycle performance of LiNi0.5Mn1.5O4 has been investigated by cycling the cells at different rates (25 1C), and the results have been plotted in Fig. 2(a). As shown, the capacities gradually rise to a steady state in the first few cycles, due to the gradual infiltration of electrolyte with the active material particles. The maximum attainable discharge capacities of 132, 123 and 113 mA h g 1 can be obtained at 1, 3 and 5 C, respectively. After 200 cycles, the corresponding capacity retention rates are 91.7%, 95.4% and 87.1%. The inset of Fig. 2(a) reveals the charge– discharge curves of the material at different rates. All the curves represent one main charge plateau at about 4.8 V and one main
0.785 cm2, respectively. Using lithium metal as anode, 1 M LiPF6 in a 2:3 (by volume) mixture of ethylene carbonate (EC)/diethyl carbonate (DEC) as electrolyte, the cells were assembled in an argon-filled glove box. The galvanostatic charged/discharged tests were measured at various C-rates (1 C¼147 mA g 1) at ambient temperature (25 1C) or elevated temperature (55 1C) between 3.5 V and 5 V. Cyclic voltammetry (CV) tests were carried out at different scan rates (0.05, 0.10, 0.20 and 0.40 mV s 1) from 3.5 to 5 V.
3. Results and discussion Fig. 1(a) shows the XRD pattern of the as-prepared LiNi0.5Mn1.5O4 material. All the diffraction peaks can be indexed based on a well-defined spinel cubic structure, along with minor traces of LixNi1 xO impurity phase close to the characteristic peaks of (311), (400) and (440). The lattice parameter calculated (111)
60 ∗ LixNi1-xO
10
20
30
∗
∗
∗
40
50
60
Frequency / % (533) (622)
(511)
(440) (531)
(400) (331)
(222)
(220)
(311)
Intensity / a.u.
50
70
40 30 20 10 0 100
80
2θ ⁄ °
200
300
400 500 600 Particle size / nm
700
800
900
Fig. 1. (a) X-ray diffraction pattern of LiNi0.5Mn1.5O4 and (b) SEM image (inset) and the corresponding particle size histogram of LiNi0.5Mn1.5O4.
150
150
90 60 30 0
0
5.0 4.8 4.6 4.4 4.2 4.0 3.8 3.6 3.4 3.2 0
1C 3C 5C 20
50
40
Discharge capacity / mAhg-1
120
Potential vs. (Li/Li ) / V
Discharge capacity / mAh g-1
25 °C
5C 3C1C 60
80
100
100
120
55 °C
Charge rate: 5 C Discharge rate: 5 C
120
90
60
30
140
150
200
0
0
30
Cycle number
60
90
120
150
Cycle number
Discharge capacity / mAh g-1
150
25 °C
Charge rate: 1 C Discharge rate: 20 C
120
90
60
30
0
0
50
100 Cycle number
150
200
Fig. 2. (a) Cycle performance and charge–discharge curves (inset) of LiNi0.5Mn1.5O4 at different rates (1C, 3C and 5C; 25 1C); (b) cycle performance of LiNi0.5Mn1.5O4 at 5C rate (55 1C); and (c) high rate cycle performance of LiNi0.5Mn1.5O4 at 20C discharge rate (25 1C).
O. Sha et al. / Materials Letters 89 (2012) 251–253
2.0
4. Conclusions
1.5
0.40 mV s-1 0.20 mV s-1 0.10 mV s-1 0.05 mV s-1
Current / mA
1.0 0.5
High pure spinel LiNi0.5Mn1.5O4 has been prepared employing a simple and low-cost urea-based sol–gel method. The results demonstrate that the material with small particles exhibits excellent cycle stability at 55 1C and 94.3 mA h g 1 can be achieved at 5 C after 150 cycles (capacity fading rate: 8.6%). Besides, the satisfactory high rate (20 C) cycle stability has been shown to reach 97.8 mA h g 1 at 25 1C, retaining 87.5% after 200 cycles. Using the inexpensive urea as a chelating agent should be an efficient and convenient approach for the synthesis of LiNi0.5Mn1.5O4 cathode material which is expected to be a potential power sources for high power lithium ion batteries.
υ
0.0 -0.5 -1.0 -1.5 -2.0
253
ip 3.5
4.0 4.5 Potential vs.(Li/Li+) / V
5.0
Fig. 3. Cyclic voltammograms for LiNi0.5Mn1.5O4 electrode at different potential sweeping rates.
discharge plateau at around 4.7 V, corresponding to the Ni2 þ /Ni3 þ and Ni3 þ /Ni4 þ redox couples. The 4.0 V plateau characteristic of the Mn3 þ /Mn4 þ redox couple is almost invisible, which is indicative of the highly pure product. Nevertheless, due to the ohmic polarization and electrochemical polarization, the voltage platform differences between charge and discharge become larger at higher rates. Normally, the cycle stability of the LNMS-based spinels at 55 1C was reported at low rates [2,3]. In this work, we have assessed the material’s cycle performance at a higher rate (5 C, 55 1C). The result in Fig. 2(b) suggests that it still exhibits wonderful cycle stability to reach 105 mA h g 1 at maximum and retain 94.3 mA h g 1 at the 150th cycle, with only 8.6% capacity fading. On the other hand, the high rate capability of LNMS-based spinels also is an important parameter for practical applications [13]. The cycle performance at the high rate of LiNi0.5Mn1.5O4 has been shown in Fig. 2(c), when discharged at 20 C (25 1C, charge rate: 1 C), 97.8 mA h g 1 could be achieved. After 200 cycles, it decreases to 85.6 mA h g 1 (87.5% of the initial discharge capacity), revealing satisfactory high rate cycle performance. Fig. 3 displays the cyclic voltammograms for LiNi0.5Mn1.5O4 recorded at different potential sweeping rates. The peak current (ip) increased with the increase of the scan rate (u). Assuming that the intercalation reaction is controlled by the solid-state diffusion of Li þ , the dependence of the peak current on the square root of the scan rate (u1/2) can be applied to determine the lithium diffusion coefficient (DLi) according to the following equation: ip ¼ 2:69 105 n3=2 ADLi 1=2 n1=2 C 0Li where n is the number of electrons involved in the electrochemical reaction (n¼ 2), A is the surface area of the electrode, and C0Li is the bulk concentration of Li þ in the electrode (0.0244 mol cm 3) [1,14], u is the potential sweeping rate (V s 1). The calculated DLi in this case is 3.2 10 11 cm2 s 1 which is identical with the values reported previously and even better than some [1,15], indicating higher lithium ion diffusion kinetics in the material. Therefore, the stable structure and fast lithium diffusion speed afforded by the formation of small particles could explain the excellent cycle stability and satisfactory rate capability in our material.
Acknowledgments This work was supported by the National Natural Science Foundation of China (no. 20973124) and Ph.D. Programs Foundation of Ministry of Education of China (no. 20070056138).
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