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Preparation and characterization of LiAl0.23Mn1.77O4 for supercapacitor electrodes
Materials Chemistry and Physics 110 (2008) 486–489
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Preparation and characterization of LiAl0.23 Mn1.77 O4 for supercapacitor electrodes Yun Xue, Ye Chen ∗ , Mi-Lin Zhang College of Materials Science and Chemical Engineering, Harbin Engineering University, Harbin 150001, PR China
a r t i c l e
i n f o
Article history: Received 14 December 2006 Received in revised form 11 August 2007 Accepted 5 March 2008 Keywords: Composite materials Electrochemical properties EDS
a b s t r a c t LiAl0.23 Mn1.77 O4 was synthesized by high temperature solid-state reaction. The structure and morphology of LiAl0.23 Mn1.77 O4 were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDS). The supercapacitive performances of LiAl0.23 Mn1.77 O4 materials were studied using galvanostatic charge/discharge measurements, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) methods in 2 mol L−1 (NH4 )2 SO4 solution. The results show that the LiAl0.23 Mn1.77 O4 electrode exhibits typical supercapacitive characteristics in aqueous (NH4 )2 SO4 electrolyte. The specific capacitance is up to 185 F g−1 at current density of 2 mA cm−2 . The ohmic resistance (Rsol ) is only 0.22 . Besides, the electrodes showed a stable cycle life in the potential range of 0–1.0 V and retained 93% of initial specific capacitance over 100 cycles. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Supercapacitors are unique energy storage devices between traditional electrostatic capacitors and rechargeable batteries, which attract growing attention for higher power density, longer cycle life than batteries and higher specific energy than conventional capacitors [1–3]. Carbon materials with high surface area are widely used for double layer capacitors [4,5]. Redox supercapacitors utilize many transition and noble metal oxides as electrode materials, such as RuO2 , NiO and MnO2 , which are considered to be excellent materials with their charge-storage mechanisms based on pseudocapacitance [6–11]. Recently, metal composite oxides for supercapacitors have been developed. For example, the electrochemical properties of Lix RuO2+0.5x ·nH2 O with a similar layered structure as H0.2 RuO2.1 ·nH2 O are investigated in lithium-rich electrolyte solution. The specific capacitance of 391 F g−1 is observed at 1 mA charge/discharge current [12]. Though it has high specific capacitance, the cost is relatively high. SrCoO2.5 is prepared as a candidate electrode for the supercapacitor. The specific capacitance is 168.5 F g−1 in the range of 0.1–0.7 V [13]. Kuo and Wu reported that some crystalline ferrite oxides, particularly MnFe2 O4 and CoFe2 O4 , also exhibited pseudocapacitance in NaCl solution. The specific capacitance they gained is higher than 100 F g−1 [14]. Spinel LiMn2 O4 is widely used as the positive electrode (cathode) of secondary lithium battery [15–17]. Al-substituted LiMn2 O4
∗ Corresponding author. Tel.: +86 451 82518792; fax: +86 451 82533026. E-mail address: [email protected] (Y. Chen). 0254-0584/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2008.03.008
could significantly improve battery performance of the LiMn2 O4 cathode in non-aqueous solutions [18–20]. There are few reports about Al-doped LiMn2 O4 used for supercapacitors, especially about its capacitance properties in aqueous electrolytes. In this paper, the electrochemical characteristics of LiAl0.23 Mn1.77 O4 electrode material for supercapacitors in 2 M (NH4 )2 SO4 aqueous solution were investigated. Cyclic voltammetry (CV), charge/discharge analysis and the electrochemical impedance spectroscopy (EIS) methods have been employed. The LiAl0.23 Mn1.77 O4 electrode material is expected to provide both high energy and high power capability for various applications of supercapacitors. 2. Experimental 2.1. Synthesis of LiAl0.23 Mn1.77 O4 LiAl0.23 Mn1.77 O4 spinel material was synthesized by high temperature solidstate reaction. Stoichiometric amounts of lithium carbonate (Li2 CO3 ), electrolytic manganese dioxide (EMD) and aluminum nitrate (Al(NO3 )3 ·9H2 O) were ground with 5 mL ethanol as dispersant in an agate mortar. This mixture was extensively ground and then calcined at 800 ◦ C for 4 h after the ethanol volatilized. Afterwards, the product was cooled down at room temperature and ground again. Thus, Al-doped sample of LiAl0.23 Mn1.77 O4 was obtained. 2.2. Electrode preparation The electrochemical behaviors of LiAl0.23 Mn1.77 O4 were characterized employing a three-electrode cell, using platinum (approximately 4 cm2 ) and saturated calomel electrode (SCE) as counter and reference electrodes, respectively. The positive composite electrode was prepared as the following steps. The mixture containing 70 wt.% LiAl0.23 Mn1.77 O4 , 15 wt.% graphite, 10 wt.% acetylene black (AB) and 5 wt.% polytetrafluoroethylene (PTFE) were well mixed. Then the composite materials were mixed with pure ethanol until a paste was obtained, and it was pasted
Y. Xue et al. / Materials Chemistry and Physics 110 (2008) 486–489
487
Table 1 Elements content of LiAl0.23 Mn1.77 O4 obtained from EDS analysis
Fig. 1. X-ray diffraction (XRD) pattern of LiAl0.23 Mn1.77 O4 sample.
on foam nickel and dried in air. The used electrolyte was 2 M (NH4 )2 SO4 aqueous solution.
Element
Weight (%)
Atomic (%)
O Al Mn
32.11 3.87 61.97
58.20 4.16 32.71
phase of cubic spinel structure with a space group Fd3m where the lithium ions occupy the tetrahedral (8a) sites, manganese and aluminium ions occupy the octahedral (16d) sites. This fact may indicate that the Mn site in LiMn2 O4 is substituted by Al and no other phase is formed. Al substitution does not change the basic structure of LiMn2 O4 [21,22]. The morphology of the Al-substituted spinel LiAl0.23 Mn1.77 O4 was examined by scanning electron microscopy (SEM). SEM micrograph (Fig. 2a) indicates that the particle size is about 0.5 m. The size of the particles is uniform. Fig. 2b shows the Energy dispersive spectrometers (EDS) spectrum of LiAl0.23 Mn1.77 O4 and the results are listed in Table 1. There is an obvious Al peak observed besides Mn and O in Fig. 2b which corresponds to the Al content of 4.16% atom in Table 1. This implies that the Al has been compounded into the sample. And the obtained sample is LiAl0.23 Mn1.77 O4 .
2.3. Characterization and electrochemical tests
3.2. Cyclic voltammetry X-ray diffraction (XRD) pattern was recorded using a D/Max-III A diffractometer with Cu K␣ radiation ( = 0.15418 nm). The data were collected in the 2 range from ◦ 10 to 80◦ with a scan rate of 0.1◦ s−1 . The beam voltage was 30 kV, and the beam current was 20 mA. SEM image was obtained at an accelerating voltage of 20 kV with a JSM-6480A scanning electron microscope (SEM). The electrochemical tests were performed using Im6eX electrochemical workstation.
3. Results and discussion 3.1. XRD analysis and the morphology The X-ray diffraction pattern of LiAl0.23 Mn1.77 O4 is shown in Fig. 1. The diffraction peak of the sample corresponds to a single
Cyclic voltammetry is a useful method to evaluate the intrinsic electrochemical capacitor properties of active materials. Fig. 3 shows the cycle voltammetric behavior of LiAl0.23 Mn1.77 O4 in 2 mol L−1 (NH4 )2 SO4 solution in the potential interval 0–1.0 V recorded with different scan rates. The LiAl0.23 Mn1.77 O4 electrode exhibits typical capacitive behavior. The CV curves obtained at different scan rates show rectangular mirror images with respect to the zero-current line. The obtained results indicate that LiAl0.23 Mn1.77 O4 electrode behaves as an ideal capacitor within the potential window of 0–1.0 V. Few reports are performed on the electrochemical behavior of solid lithium manganate in aqueous
Fig. 2. (a) SEM image of LiAl0.23 Mn1.77 O4 sample and (b) EDS spectrum of LiAl0.23 Mn1.77 O4 sample.
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Y. Xue et al. / Materials Chemistry and Physics 110 (2008) 486–489
Fig. 3. Cyclic voltammograms of LiAl0.23 Mn1.77 O4 electrodes at different scan rates in 2 mol L−1 (NH4 )2 SO4 aqueous solution.
solutions. Jayalakshmi et al. [23] found that LiMn2 O4 was shown to undergo reversible insertion of K+ , Li+ , and NH4 + ions from neutral electrolyte solutions. In our research, we also believe that the capacitance mainly arises from pseudocapacitance caused by lithium ions insertion/extraction into/out of the LiAl0.23 Mn1.77 O4 electrode which takes place at the electrode surface and expandable proton conducting hydrous layer.
Fig. 5. The discharge specific capacitance of LiAl0.23 Mn1.77 O4 electrodes at various discharge current density.
of the charge/discharge, V; m is the amount of LiAl0.23 Mn1.77 O4 in the electrode, g. The specific capacitances of LiAl0.23 Mn1.77 O4 electrodes estimated from charge/discharge curves at different currents are shown in Fig. 5. The specific capacitance decreases slightly with increasing current density. The maximum specific capacitance of the LiAl0.23 Mn1.77 O4 is 185 F g−1 at current density of 2 mA cm−2 .
3.3. Galvanostatic charge–discharge analysis 3.4. Electrochemical impedance spectroscopy analysis Fig. 4 illustrates typical charging/discharging cycles of LiAl0.23 Mn1.77 O4 electrodes recorded at current density of 10 mA cm−2 . The charge/discharge curves were measured in 2 mol L−1 (NH4 )2 SO4 aqueous solution from 0 to 1.0 V. The potential varies linearly and symmetrically with time during both charging and discharging processes. This implies that LiAl0.23 Mn1.77 O4 electrodes have excellent electrochemical reversibility and capacitive characteristics. The specific capacitance of the electrode can be calculated from charge/discharge curves according to the following Eq. (1):
where Cm is the specific capacitance, F g −1 ; I is the current of charge/discharge, A; t is the time of discharge, s; V is the range
In order to understand the resistance associated with LiAl0.23 Mn1.77 O4 electrodes, the electrochemical impedance spectroscopic spectrum was obtained in 2 mol L−1 (NH4 )2 SO4 solution at open circuit voltage. The spectrum in the typical form of Nyquist plots (Z vs. Z ) is shown in Fig. 6. The impedance diagram of LiAl0.23 Mn1.77 O4 electrodes shows a distorted semi-circle in the high-frequency region and an inclined line in the low-frequency region. The high-frequency intercept of the semi-circle on the real axis represents the ohmic resistance (Rsol ). The diameter of the distorted semi-circle provides the charge-transfer resistance (Rct ) representing the sum of the resistance offered to Li migration through the bulk of the electrode, the resistances of the electrolyte/LiAl0.23 Mn1.77 O4 electrode interface and the resistances of electrode itself. The inclined line in the low frequency corre-
Fig. 4. Charge–discharge curves of LiAl0.23 Mn1.77 O4 electrodes at currents density of 10 mA cm−2 .
Fig. 6. AC impedance spectrum of the LiAl0.23 Mn1.77 O4 electrode in 2 mol L−1 (NH4 )2 SO4 aqueous solution.
Cm =
C Q I × t = = m mV m × V
(1)
Y. Xue et al. / Materials Chemistry and Physics 110 (2008) 486–489
489
were characterized by various electrochemical techniques carried out in 2 mol L−1 (NH4 )2 SO4 aqueous solution. The results show that LiAl0.23 Mn1.77 O4 has stable electrochemical capacitor properties in the potential range of 0–1.0 V with a maximum specific capacitance of 185 F g−1 at current density of 2 mA cm−2 in 2 mol L−1 (NH4 )2 SO4 aqueous solution. After 100 charge/discharge cycles, the synthesized material shows high efficiency and stability. Hence, the LiAl0.23 Mn1.77 O4 material is considered as a promising material for supercapacitors. Acknowledgement
Fig. 7. The curves of coulombic efficiency and discharge specific capacity as a function of cycle numbers.
sponds to the diffusion of lithium ions in the samples. Seen from Fig. 6, the Rsol of LiAl0.23 Mn1.77 O4 electrodes is 0.22 , and the Rct resistance is less than 1 . This indicates that the LiAl0.23 Mn1.77 O4 electrodes in 2 mol L−1 (NH4 )2 SO4 solution has high ionic conductivity. In the low frequency range, a line close to 90◦ is attributed to a perfect capacitive characteristic. 3.5. Cyclic life of LiAl0.23 Mn1.77 O4 electrode Fig. 7 shows the charge/discharge cyclic stability of the LiAl0.23 Mn1.77 O4 electrode at a current density of 20 mA cm−2 between 0 and 1.0 V in 2 mol L−1 (NH4 )2 SO4 solution. As indicated in Fig. 7, a decrease of less than 7% of the specific capacitance was observed after 100 cycles. There is a sudden decrease in the specific capacitance value for the first 10 cycles, and the specific capacitance remains almost constant during the following 90 cycles. The variation of the Coulombic efficiency with the cycling number is also shown in Fig. 7. The Coulombic efficiency remains close to 100% in all cycles. 4. Conclusions The LiAl0.23 Mn1.77 O4 material has been prepared and used as supercapacitor electrode in this study. By means of X-ray diffraction analysis, SEM study and EDS examination, it has been demonstrated that the chemical composition of the spinel particle is LiAl0.23 Mn1.77 O4 . Electrochemical properties of LiAl0.23 Mn1.77 O4
This work was supported by A Foundation for the Doctoral Program of Higher Education of China (No. 20050217019). And this work was financially supported by Key Laboratory of Superlight Materials and Surface Technology of Ministry of Education and Harbin Engineering University. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]
A. Burke, J. Power Sources 91 (2000) 37. M.R.J. Brodd, Chem. Rev. 104 (2004) 4245. B.E. Conway, J. Electrochem. Soc. 138 (1991) 1539. C. Portet, P.L. Taberna, P. Simon, E. Flahaut, C. Laberty-Robert, Electrochim. Acta 50 (2005) 4174. J.H. Kim, A.K. Sharma, Y.S. Lee, Mater. Lett. 60 (2006) 1697. G.Y. Yu, W.X. Chen, Y.F. Zheng, J. Zhao, X. Li, Z.D. Xu, Mater. Lett. 60 (2006) 2453. T. Liu, W.G. Pell, B.E. Conway, Electrochim. Acta 42 (1997) 3541. J. Li, X. Wang, Q. Huang, S. Gamboa, P.J. Sebastian, J. Power Sources 160 (2006) 1501. W.C. Fang, J.H. Huang, L.C. Chen, Y.L. Su, K.H. Chen, J. Power Sources 160 (2006) 1506. V. Subramanian, H. Zhu, B. Wei, J. Power Sources 159 (2006) 361. V. Ganesh, S. Pitchumani, V. Lakshminarayanan, J. Power Sources 158 (2006) 1523. Y.Q. Zhao, G.Q. Zhang, H.L. Li, Solid State Ionics 177 (2006) 1335. F. Xiao, X. Zhang, F. Hu, J. Zhang, Mater. Chem. Phys. 94 (2005) 221. S.L. Kuo, N.L. Wu, Electrochem. Solid-State Lett. 8 (2005) A495. G.T. Fey, Y.D. Cho, K.T. Prem, Mater. Chem. Phys. 87 (2004) 275. J. Marzec, K. Swierczek, J. Przewoznik, J. Molenda, D.R. Simon, E.M. Kelder, J. Schoonman, Solid State Ionic 146 (2002) 225. A. Paolone, R. Cantelli, B. Scrosati, P. Reale, M. Ferretti, C. Masquelier, Mater. Sci. Eng. A 442 (2006) 220. B.J. Hwang, R. Santhanam, D.G. Liu, Y.W. Tsai, J. Power Sources 102 (2001) 326. A. Eftekhari, Solid State Ionic 167 (2004) 237. D. Capsoni, M. Bini, G. Chiodelli, V. Massarotti, P. Mustarelli, L. Linati, M.C. Mozzati, C.B. Azzoni, Solid State Commun. 126 (2003) 169. B.J. Hwang, Y.W. Tsai, R. Santhanam, S.K. Hu, H.S. Sheu, J. Power Sources 119–121 (2003) 727. Z. Bakenov, I. Taniguchi, Solid State Ionic 176 (2005) 1027. M. Jayalakshmi, R.M. Mohan, F. Scholz, Langumuir 19 (2003) 8403.
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Preparation and characterization of LiAl0.23 Mn1.77 O4 for supercapacitor electrodes Yun Xue, Ye Chen ∗ , Mi-Lin Zhang College of Materials Science and Chemical Engineering, Harbin Engineering University, Harbin 150001, PR China
a r t i c l e
i n f o
Article history: Received 14 December 2006 Received in revised form 11 August 2007 Accepted 5 March 2008 Keywords: Composite materials Electrochemical properties EDS
a b s t r a c t LiAl0.23 Mn1.77 O4 was synthesized by high temperature solid-state reaction. The structure and morphology of LiAl0.23 Mn1.77 O4 were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDS). The supercapacitive performances of LiAl0.23 Mn1.77 O4 materials were studied using galvanostatic charge/discharge measurements, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) methods in 2 mol L−1 (NH4 )2 SO4 solution. The results show that the LiAl0.23 Mn1.77 O4 electrode exhibits typical supercapacitive characteristics in aqueous (NH4 )2 SO4 electrolyte. The specific capacitance is up to 185 F g−1 at current density of 2 mA cm−2 . The ohmic resistance (Rsol ) is only 0.22 . Besides, the electrodes showed a stable cycle life in the potential range of 0–1.0 V and retained 93% of initial specific capacitance over 100 cycles. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Supercapacitors are unique energy storage devices between traditional electrostatic capacitors and rechargeable batteries, which attract growing attention for higher power density, longer cycle life than batteries and higher specific energy than conventional capacitors [1–3]. Carbon materials with high surface area are widely used for double layer capacitors [4,5]. Redox supercapacitors utilize many transition and noble metal oxides as electrode materials, such as RuO2 , NiO and MnO2 , which are considered to be excellent materials with their charge-storage mechanisms based on pseudocapacitance [6–11]. Recently, metal composite oxides for supercapacitors have been developed. For example, the electrochemical properties of Lix RuO2+0.5x ·nH2 O with a similar layered structure as H0.2 RuO2.1 ·nH2 O are investigated in lithium-rich electrolyte solution. The specific capacitance of 391 F g−1 is observed at 1 mA charge/discharge current [12]. Though it has high specific capacitance, the cost is relatively high. SrCoO2.5 is prepared as a candidate electrode for the supercapacitor. The specific capacitance is 168.5 F g−1 in the range of 0.1–0.7 V [13]. Kuo and Wu reported that some crystalline ferrite oxides, particularly MnFe2 O4 and CoFe2 O4 , also exhibited pseudocapacitance in NaCl solution. The specific capacitance they gained is higher than 100 F g−1 [14]. Spinel LiMn2 O4 is widely used as the positive electrode (cathode) of secondary lithium battery [15–17]. Al-substituted LiMn2 O4
∗ Corresponding author. Tel.: +86 451 82518792; fax: +86 451 82533026. E-mail address: [email protected] (Y. Chen). 0254-0584/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2008.03.008
could significantly improve battery performance of the LiMn2 O4 cathode in non-aqueous solutions [18–20]. There are few reports about Al-doped LiMn2 O4 used for supercapacitors, especially about its capacitance properties in aqueous electrolytes. In this paper, the electrochemical characteristics of LiAl0.23 Mn1.77 O4 electrode material for supercapacitors in 2 M (NH4 )2 SO4 aqueous solution were investigated. Cyclic voltammetry (CV), charge/discharge analysis and the electrochemical impedance spectroscopy (EIS) methods have been employed. The LiAl0.23 Mn1.77 O4 electrode material is expected to provide both high energy and high power capability for various applications of supercapacitors. 2. Experimental 2.1. Synthesis of LiAl0.23 Mn1.77 O4 LiAl0.23 Mn1.77 O4 spinel material was synthesized by high temperature solidstate reaction. Stoichiometric amounts of lithium carbonate (Li2 CO3 ), electrolytic manganese dioxide (EMD) and aluminum nitrate (Al(NO3 )3 ·9H2 O) were ground with 5 mL ethanol as dispersant in an agate mortar. This mixture was extensively ground and then calcined at 800 ◦ C for 4 h after the ethanol volatilized. Afterwards, the product was cooled down at room temperature and ground again. Thus, Al-doped sample of LiAl0.23 Mn1.77 O4 was obtained. 2.2. Electrode preparation The electrochemical behaviors of LiAl0.23 Mn1.77 O4 were characterized employing a three-electrode cell, using platinum (approximately 4 cm2 ) and saturated calomel electrode (SCE) as counter and reference electrodes, respectively. The positive composite electrode was prepared as the following steps. The mixture containing 70 wt.% LiAl0.23 Mn1.77 O4 , 15 wt.% graphite, 10 wt.% acetylene black (AB) and 5 wt.% polytetrafluoroethylene (PTFE) were well mixed. Then the composite materials were mixed with pure ethanol until a paste was obtained, and it was pasted
Y. Xue et al. / Materials Chemistry and Physics 110 (2008) 486–489
487
Table 1 Elements content of LiAl0.23 Mn1.77 O4 obtained from EDS analysis
Fig. 1. X-ray diffraction (XRD) pattern of LiAl0.23 Mn1.77 O4 sample.
on foam nickel and dried in air. The used electrolyte was 2 M (NH4 )2 SO4 aqueous solution.
Element
Weight (%)
Atomic (%)
O Al Mn
32.11 3.87 61.97
58.20 4.16 32.71
phase of cubic spinel structure with a space group Fd3m where the lithium ions occupy the tetrahedral (8a) sites, manganese and aluminium ions occupy the octahedral (16d) sites. This fact may indicate that the Mn site in LiMn2 O4 is substituted by Al and no other phase is formed. Al substitution does not change the basic structure of LiMn2 O4 [21,22]. The morphology of the Al-substituted spinel LiAl0.23 Mn1.77 O4 was examined by scanning electron microscopy (SEM). SEM micrograph (Fig. 2a) indicates that the particle size is about 0.5 m. The size of the particles is uniform. Fig. 2b shows the Energy dispersive spectrometers (EDS) spectrum of LiAl0.23 Mn1.77 O4 and the results are listed in Table 1. There is an obvious Al peak observed besides Mn and O in Fig. 2b which corresponds to the Al content of 4.16% atom in Table 1. This implies that the Al has been compounded into the sample. And the obtained sample is LiAl0.23 Mn1.77 O4 .
2.3. Characterization and electrochemical tests
3.2. Cyclic voltammetry X-ray diffraction (XRD) pattern was recorded using a D/Max-III A diffractometer with Cu K␣ radiation ( = 0.15418 nm). The data were collected in the 2 range from ◦ 10 to 80◦ with a scan rate of 0.1◦ s−1 . The beam voltage was 30 kV, and the beam current was 20 mA. SEM image was obtained at an accelerating voltage of 20 kV with a JSM-6480A scanning electron microscope (SEM). The electrochemical tests were performed using Im6eX electrochemical workstation.
3. Results and discussion 3.1. XRD analysis and the morphology The X-ray diffraction pattern of LiAl0.23 Mn1.77 O4 is shown in Fig. 1. The diffraction peak of the sample corresponds to a single
Cyclic voltammetry is a useful method to evaluate the intrinsic electrochemical capacitor properties of active materials. Fig. 3 shows the cycle voltammetric behavior of LiAl0.23 Mn1.77 O4 in 2 mol L−1 (NH4 )2 SO4 solution in the potential interval 0–1.0 V recorded with different scan rates. The LiAl0.23 Mn1.77 O4 electrode exhibits typical capacitive behavior. The CV curves obtained at different scan rates show rectangular mirror images with respect to the zero-current line. The obtained results indicate that LiAl0.23 Mn1.77 O4 electrode behaves as an ideal capacitor within the potential window of 0–1.0 V. Few reports are performed on the electrochemical behavior of solid lithium manganate in aqueous
Fig. 2. (a) SEM image of LiAl0.23 Mn1.77 O4 sample and (b) EDS spectrum of LiAl0.23 Mn1.77 O4 sample.
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Y. Xue et al. / Materials Chemistry and Physics 110 (2008) 486–489
Fig. 3. Cyclic voltammograms of LiAl0.23 Mn1.77 O4 electrodes at different scan rates in 2 mol L−1 (NH4 )2 SO4 aqueous solution.
solutions. Jayalakshmi et al. [23] found that LiMn2 O4 was shown to undergo reversible insertion of K+ , Li+ , and NH4 + ions from neutral electrolyte solutions. In our research, we also believe that the capacitance mainly arises from pseudocapacitance caused by lithium ions insertion/extraction into/out of the LiAl0.23 Mn1.77 O4 electrode which takes place at the electrode surface and expandable proton conducting hydrous layer.
Fig. 5. The discharge specific capacitance of LiAl0.23 Mn1.77 O4 electrodes at various discharge current density.
of the charge/discharge, V; m is the amount of LiAl0.23 Mn1.77 O4 in the electrode, g. The specific capacitances of LiAl0.23 Mn1.77 O4 electrodes estimated from charge/discharge curves at different currents are shown in Fig. 5. The specific capacitance decreases slightly with increasing current density. The maximum specific capacitance of the LiAl0.23 Mn1.77 O4 is 185 F g−1 at current density of 2 mA cm−2 .
3.3. Galvanostatic charge–discharge analysis 3.4. Electrochemical impedance spectroscopy analysis Fig. 4 illustrates typical charging/discharging cycles of LiAl0.23 Mn1.77 O4 electrodes recorded at current density of 10 mA cm−2 . The charge/discharge curves were measured in 2 mol L−1 (NH4 )2 SO4 aqueous solution from 0 to 1.0 V. The potential varies linearly and symmetrically with time during both charging and discharging processes. This implies that LiAl0.23 Mn1.77 O4 electrodes have excellent electrochemical reversibility and capacitive characteristics. The specific capacitance of the electrode can be calculated from charge/discharge curves according to the following Eq. (1):
where Cm is the specific capacitance, F g −1 ; I is the current of charge/discharge, A; t is the time of discharge, s; V is the range
In order to understand the resistance associated with LiAl0.23 Mn1.77 O4 electrodes, the electrochemical impedance spectroscopic spectrum was obtained in 2 mol L−1 (NH4 )2 SO4 solution at open circuit voltage. The spectrum in the typical form of Nyquist plots (Z vs. Z ) is shown in Fig. 6. The impedance diagram of LiAl0.23 Mn1.77 O4 electrodes shows a distorted semi-circle in the high-frequency region and an inclined line in the low-frequency region. The high-frequency intercept of the semi-circle on the real axis represents the ohmic resistance (Rsol ). The diameter of the distorted semi-circle provides the charge-transfer resistance (Rct ) representing the sum of the resistance offered to Li migration through the bulk of the electrode, the resistances of the electrolyte/LiAl0.23 Mn1.77 O4 electrode interface and the resistances of electrode itself. The inclined line in the low frequency corre-
Fig. 4. Charge–discharge curves of LiAl0.23 Mn1.77 O4 electrodes at currents density of 10 mA cm−2 .
Fig. 6. AC impedance spectrum of the LiAl0.23 Mn1.77 O4 electrode in 2 mol L−1 (NH4 )2 SO4 aqueous solution.
Cm =
C Q I × t = = m mV m × V
(1)
Y. Xue et al. / Materials Chemistry and Physics 110 (2008) 486–489
489
were characterized by various electrochemical techniques carried out in 2 mol L−1 (NH4 )2 SO4 aqueous solution. The results show that LiAl0.23 Mn1.77 O4 has stable electrochemical capacitor properties in the potential range of 0–1.0 V with a maximum specific capacitance of 185 F g−1 at current density of 2 mA cm−2 in 2 mol L−1 (NH4 )2 SO4 aqueous solution. After 100 charge/discharge cycles, the synthesized material shows high efficiency and stability. Hence, the LiAl0.23 Mn1.77 O4 material is considered as a promising material for supercapacitors. Acknowledgement
Fig. 7. The curves of coulombic efficiency and discharge specific capacity as a function of cycle numbers.
sponds to the diffusion of lithium ions in the samples. Seen from Fig. 6, the Rsol of LiAl0.23 Mn1.77 O4 electrodes is 0.22 , and the Rct resistance is less than 1 . This indicates that the LiAl0.23 Mn1.77 O4 electrodes in 2 mol L−1 (NH4 )2 SO4 solution has high ionic conductivity. In the low frequency range, a line close to 90◦ is attributed to a perfect capacitive characteristic. 3.5. Cyclic life of LiAl0.23 Mn1.77 O4 electrode Fig. 7 shows the charge/discharge cyclic stability of the LiAl0.23 Mn1.77 O4 electrode at a current density of 20 mA cm−2 between 0 and 1.0 V in 2 mol L−1 (NH4 )2 SO4 solution. As indicated in Fig. 7, a decrease of less than 7% of the specific capacitance was observed after 100 cycles. There is a sudden decrease in the specific capacitance value for the first 10 cycles, and the specific capacitance remains almost constant during the following 90 cycles. The variation of the Coulombic efficiency with the cycling number is also shown in Fig. 7. The Coulombic efficiency remains close to 100% in all cycles. 4. Conclusions The LiAl0.23 Mn1.77 O4 material has been prepared and used as supercapacitor electrode in this study. By means of X-ray diffraction analysis, SEM study and EDS examination, it has been demonstrated that the chemical composition of the spinel particle is LiAl0.23 Mn1.77 O4 . Electrochemical properties of LiAl0.23 Mn1.77 O4
This work was supported by A Foundation for the Doctoral Program of Higher Education of China (No. 20050217019). And this work was financially supported by Key Laboratory of Superlight Materials and Surface Technology of Ministry of Education and Harbin Engineering University. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]
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