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Electrochemical performance of symmetric supercapacitor based on Li4Mn5O12 electrode in Li2SO4 electrolyte
Materials Chemistry and Physics 126 (2011) 432–436
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Electrochemical performance of symmetric supercapacitor based on Li4 Mn5 O12 electrode in Li2 SO4 electrolyte Yan Jing Hao, Qiong Yu Lai ∗ , Xiao Yun Xu, Ling Wang College of Chemistry, Sichuan University, Chengdu 610064, China
a r t i c l e
i n f o
Article history: Received 23 January 2010 Received in revised form 13 October 2010 Accepted 23 October 2010 Keywords: Inorganic compounds Annealing Electrochemical techniques Electrochemical properties
a b s t r a c t In this paper, spinel Li4 Mn5 O12 electrode material was prepared by a citric-acid-assisted combustion method. The structure and morphology of Li4 Mn5 O12 were characterized by X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDAX), scanning electron microscopy (SEM) and electron tunneling electron microscopy (TEM). The results showed that the Li4 Mn5 O12 powders were nano-sized particles. XRD Rietveld refinement gives the crystal structure of prepared sample. Electrochemical performance of Li4 Mn5 O12 electrode was studied by cyclic voltammetry, electrochemical impedance spectroscopy and galvanostatic charge/discharge measurements. Li4 Mn5 O12 /Li4 Mn5 O12 symmetric supercapacitor was fabricated at the first time. The maximum specific capacitance was 69 F g−1 in the potential range of 0–1.4 V in 1 mol L−1 Li2 SO4 electrolyte. The energy density was 67.6 W h kg−1 at a power density of 493.8 W kg−1 based on the total weight of the active electrode materials. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Supercapacitor is a promising candidate for power sources of electric vehicles due to its high power density. However, its energy density is low (3–6 W h kg−1 ). Recently, symmetric and asymmetric capacitors were explored by using lithium insertion materials with high energy density [1–4]. At the same time, neutral aqueous electrolytes in supercapacitors have many advantages including high ionic conductivity, low-cost, nonflammability, and no specific requirements for battery assembly and good safety [5,6]. Previous studies on the electrochemical performance of activated carbon (AC) in Li2 SO4 , Na2 SO4 and K2 SO4 solutions show that Li2 SO4 based electrolyte exhibited the best rate behavior owning to the smallest hydrated ionic radius of Li+ , weakest solvation interaction between Li+ ion and H2 O molecules and highest ionic conductivity of Li+ [7–10]. Therefore, exploration on the electrochemical active materials that could undergo reversible Li+ intercalation/deintercalation would be favorable for novel asymmetric or symmetric supercapacitor in lithium salt solution. Lithium insertion materials for lithium ion batteries such as Li4 Ti5 O12 , Li2 Ti3 O7 , LiCoO2 and LiNi0.5 Mn1.5 O4 have been reported as electrode materials for supercapacitors in organic electrolyte [11–14]. Xia and co-workers [7–10] developed some
∗ Corresponding author. Tel.: +86 28 85416969; fax: +86 28 85416969. E-mail address: [email protected] (Q.Y. Lai). 0254-0584/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2010.10.043
new hybrid electrochemical supercapacitor such as AC/LiMn2 O4 , LiTi2 (PO4 )3 /AC and LiTi2 (PO4 )3 /MnO2 in aqueous electrolyte. These capacitors showed good electrochemical properties. Among all of the lithium ion intercalated materials, spinel-type substances are promising electrode materials because of the high intercalation potential, good cyclability and rate capability for supercapacitor and lithium ion battery [15,16]. The [M2 ]O4 framework of Li[M2 ]O4 spinel is an attractive host structure for lithium insertion–extraction reactions because it provides a threedimensional network of face-sharing tetrahedra and octahedra for lithium ion diffusion. Spinel Li4 Mn5 O12 (or Li [Li0.33 Mn1.67 ]O4 ), which is the end member of the Li1+x Mn2−x O4 solid solution with x = 0.33 and an oxidation state of 4+ for Mn, has become an attractive candidate for 3V cells in organic electrolyte. It has been found that the cubic symmetry can be preserved x = 2.5 lithium ions in Li4+x Mn5 O12 formula in the 3V region without Jahn–Teller distortion [17,18]. This can provide high specific capacitance for supercapacitor based on Li4 Mn5 O12 material. In our previously work [19–21], Li4 Mn5 O12 was used as electrode material in hybrid supercapacitors such as Li4 Mn5 O12 /AC capacitor and Li4 Mn5 O12 /MnO2 -AC capacitor. In these hybrid supercapacitors, spinel Li4 Mn5 O12 was used as the cathode material, and activated carbon (AC) or carbon coated MnO2 was used as the anode material. They delivered good electrochemical properties in Li2 SO4 electrolyte. In the present work, Li4 Mn5 O12 /Li4 Mn5 O12 symmetric supercapacitor was fabricated at the first time. The electrochemical performance of the capacitor was investigated systematically and a possible mechanism of charge and discharge was proposed.
Y.J. Hao et al. / Materials Chemistry and Physics 126 (2011) 432–436 2. Experimental Li4 Mn5 O12 was prepared by citric-acid-assisted combustion method. All the chemicals were provided by Chengdu KeLong Industries Co., China. Stoichiometric amount of Li(CH3 COO)2 ·2H2 O (AR, 99%, 1.0306 g), Mn(CH3 COO)2 ·4H2 O (AR, 99%, 3.0946 g) (the molar ratio of Li:Mn = 4:5) and a certain amount of citric acid (the molar ratio of citric acid to total metal ions was 1:1) were mixed together. The obtained mixture was ground sufficiently in a ceramic mortar for about 1 h until the mixture changed to a light pink viscous complex gel. Then the obtained viscous complex mixture was dried at 75 ◦ C under vacuum for 24 h to obtain a dry precursor. After ground sufficiently in mortar, the precursor powder was preheated at 300 ◦ C in air for 2 h in a furnace and then calcined at 300 ◦ C in air for 6 h. The powders were cooled in the furnace to room temperature to obtain final product. The samples were characterized by XRD, using a D/max-rA diffractometer with Cu K␣ radiation operated at 40 kV and 100 mA. Data were collected in the range 10–70◦ ( = 0.15418 nm). The morphology of prepared particles was characterized by scanning electron microscopy (SEM) in a Hitachi-S-450 scanning microscope and Japan electron tunneling electron microscopy (TEM, Jeol JEM-2010). The chemical composition of the compound was confirmed by a field emission scanning electron microscope (FESEM, JSM6700F) equipped with an energy dispersive X-ray spectroscope (EDAX, Oxford INCA). The electrode was prepared by a casting technique. 65 wt% of the Li4 Mn5 O12 active material, 30 wt% acetylene black and 5 wt% polyvinylidene fluoride (PVDF) were mixed together. Then a certain amount of N-methy1-2-pyrrolidine (NMP) was dropped into the mixture to form a slurry. The slurry was sufficiently blended for about 1 h. The final obtained slurry was subsequently brush-coated onto a stainless steel grid that served as a current collector (active material area is 0.5 cm2 ). Then the electrode was dried at 80 ◦ C for about 2 h to obtain the Li4 Mn5 O12 electrode. The typical mass load of Li4 Mn5 O12 electrode material was 1.6 mg. The electrolyte was 1 mol L−1 Li2 SO4 solution. Capacitor test was performed using a two-electrode glass cell consisted of Li4 Mn5 O12 positive electrode and Li4 Mn5 O12 negative electrode with Neware Battery Program-control Testing System. The distance between two electrodes was 1 cm. The cyclic voltammetry was characterized using a three-electrode glass cell, in which a stainless steel grid coated with Li4 Mn5 O12 was used as the working electrode, platinum and saturated calomel electrode (SCE, 0.242 V) as counter and reference electrode respectively. The measurements were performed on a LK2005 electrochemical workstation system with a voltage scan rate ranging from 3 to 20 mV s−1 . The electrochemical impedance spectroscopy experiment was also carried out by using a three electrode cell, in which a stainless steel grid coated with Li4 Mn5 O12 was used as the working electrode, platinum and saturated calomel electrode (SCE, 0.242 V) as counter and reference electrode respectively. It was performed by Potentiosta/Galvanostat IM6ex (ZHANER Elektrik; Germany) instrument at 0.5 V. The frequency limits were typically set between 1000 kHz and 0.01 Hz.
3. Results and discussion 3.1. XRD and EDAX analysis Fig. 1(a) shows the X-ray diffraction patterns of Li4 Mn5 O12 powders. To determine the structure, XRD pattern was analyzed with the FULLFROF program [22]. The calculated unit parameter of a = 8.14565 is very close to the previous results obtained by Takada
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et al. [23]. In addition, well-fitted refinement profiles in Fig. 1(b) also confirmed that all the diffraction peaks can be indexed in the ¯ (JCPDS No. 46-0810) of a cubic spinel, which space group Fd3m indicates that in the final products, partial lithium ions are located at the tetrahedral 8a sites, manganese ions and rest lithium ions are randomly distributed at the octahedral 16d sites with the ratio of 1:5, the oxygen ions occupy the 32e sites. Thus, Li4 Mn5 O12 can be described as [Li]8a [Li1/3 Mn5/3 ]16d [O4 ]32e [24]. The crystal structure of spinel Li4 Mn5 O12 as well as important bond lengths and bond angles is depicted in Fig. 2. The MO6 octahedra share edges with six octahedral-site neighbors, two of which are located in the same plane and the remaining four are above and below the plane. The LiO4 tetrahedral sites share corners with octahedral sites. The three-dimensional structure could provide access channel for lithium ions during charge/discharge process. To estimate the quantity of the elements present in the synthesized Li4 Mn5 O12 product, energy dispersive X-ray analysis (EDAX) of the sample is given and depicted in Fig. 3. The peaks around 0.52 and 5.92 kV clearly show the presence of Mn and O elements. The molar ratio of O:Mn in the prepared Li4 Mn5 O12 product is 2.31, which is very close to the theoretical value 2.40. 3.2. SEM and TEM analysis The SEM images of Li4 Mn5 O12 powders are shown in Fig. 4(a). As can be seen in the figure, the sample powders were composed of some irregular nano-particles with average particle size of about 100 nm. The large particles of Li4 Mn5 O12 may be seen as being consisted of aggregates of Li4 Mn5 O12 primary particles. In this work, urea is used as a complexation agent in the formation of a homogeneous gel matrix. During subsequent heat treatment, the precursor carbonizes and, simultaneously, releases some gases that leave sufficient porosity to form nano-particles product. Urea plays a key role in forming the Li4 Mn5 O12 nano-particle to increase the surface area. The distribution of smaller particles may be more advantageous in the enhancement of the specific surface area and the specific capacitance. This is because it increases the probability of conducting pathways in the electrode/electrolyte interface. From the TEM image shown in Fig. 4(b), it is clear that the nanoparticles of Li4 Mn5 O12 are irregular spherical structure with particle size of about 50–100 nm. 3.3. Electrochemical analysis The cyclic voltammogram (CV) curves of the Li4 Mn5 O12 electrode are given in Fig. 5. As shown in Fig. 5, the CV curves are
Fig. 1. (a) XRD pattern of Li4 Mn5 O12 sample and (b) its Rietveld refinements.
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¯ Fig. 2. (a and b) Crystal structures of Li4 Mn5 O12 with space group of Fd3m. The interatomic distances (Å) and bond angles (◦ ) are obtained from the refinement of XRD data at room temperature.
roughly rectangular in shape and exhibit near mirror-image current response on voltage reversal. This indicated a capacitive behavior of the present sample in the potential range of 0–1.4 V. At low scan rate, two broad current peaks can be observed, i.e., an anodic peak appearing at about 0.5 V and a cathodic peak occurring at ca. 1.1 V (vs. SCE). The oxidation/reduction peak-potential of the Li4 Mn5 O12 electrode in Li2 SO4 electrolyte at low scan rate of 5 mV s−1 suggest that the Li+ insertion/extraction process should be viewed as a quasi-reversible reaction. It is inferred that the reaction occurred on the electrode is the redox process of Mn(IV) ↔ Mn (III). The useful potential window for the electrode is around 0.3–1.3 V vs. SCE (as seen from the curves at 3 and 5 mV s−1 rate). There is almost no H2 and O2 evolution observed in this potential range. The specific capacitance of the single Li4 Mn5 O12 electrode was calculated to be 265 and 245 F g−1 according to the following equation [5]:
C=
Q 2m V
(1)
where Q refers to the charge integrated from the cathodic sweep and V and m refer to the difference in the voltage window and weight of the single Li4 Mn5 O12 electrode respectively. When the scan rate increased from 3 mV s−1 to 20 mV s−1 , the two redox peaks for Li+ ions insertion/extraction exhibited unconspicuous gradually. For comparison, the specific capacitance change of Li4 Mn5 O12 electrode in the potential range of 0–1.4 V is also listed. The discharge specific capacitances of the Li4 Mn5 O12 electrode were calculated to be 293, 274, 239, and 126 F g−1 at the scan rates of 3 mV s−1 , 5 mV s−1 , 10 mV s−1 and 20 mV s−1 respectively. The specific capacitance decreased with increasing of the scan rate. This indicated that the insertion/extraction of Li-ions occur not only on the surface of the Li4 Mn5 O12 powders but also into the inner lattice of the material. At high sweep rate, the diffusion of Li-ions is almost limited to the surface of the Li4 Mn5 O12 particles. The charge storage mechanism of Li4 Mn5 O12 is based on the concept of intercalation of Li+ in the electrode during reduction and deintercalation upon oxidation. During the charge process, Li-ions in Li2 SO4 electrolyte solution insert into spinel Li4 Mn5 O12 to form Li4+x Mn5 O12 .
Fig. 3. Energy-dispersive X-ray spectroscopy of nano-Li4 Mn5 O12 sample.
Y.J. Hao et al. / Materials Chemistry and Physics 126 (2011) 432–436
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Li4 Mn5 O12 . The process could be described as follows: Li4 Mn5 O12 + xLi+ + xe− ⇔ Li4+x Mn5 O12 Moreover, adsorption of electrolyte cations such as Li+ on the manganese oxides surface contributes to part of the capacitance of the capacitor, too. In the present study, the redox process is mainly governed by the insertion and deinsertion of Li+ from the electrolyte into the nanostructured Li4 Mn5 O12 matrix. Increasing the scan rate has a direct impact on the diffusion of Li+ ions into the Li4 Mn5 O12 matrix. In other words, the faster scan rate will lead to the Li+ ions reaching only the outer surface of the electrode and not the interior of Li4 Mn5 O12 matrix. This greatly reduces the available capacity from 293 to 126 F g−1 when the scan rate is increased from 3 to 20 mV s−1 . Hence, it is clear that when the scan rate is higher the participation of the Li4 Mn5 O12 electrode is limited only to the outer surface and not the interior metrix, thus leading to a lower specific capacitance. Fig. 6 shows the typical charge–discharge curve and cycling behavior of the symmetric supercapacitor in the potential range of 0–1.4 V in 1 mol L−1 Li2 SO4 solution at a current rate of 100 mA g−1 . The discharge curve is almost linear and the sudden drop of potential at the beginning of charge and discharge is associated to the internal resistance of the supercapacitor. The average internal resistance of the capacitor can be calculated according to R = (Vcharge − Vdischarge )/(2I), where Vcharge and Vdischarge are the voltage at the end of charge and at the beginning of the discharge, respectively, and I the is absolute value of the current which is the same during the charge and the discharge. This gives an average value of 21 , reflecting the non-optimized contacts between the electrodes and the current collectors, the solution resistance. The discharge specific capacitance of the supercapacitor was calculated as follows [4]: C=
Fig. 4. (a) SEM and (b) TEM graph of Li4 Mn5 O12 particles.
According to the literature report [25], Li4 Mn5 O12 can accommodate 3 Li ions per formula to form Li7 Mn5 O12 ; this can provide a high capacitance for the supercapacitor. During the discharge process, Li-ions in spinel Li4+x Mn5 O12 material were deinserted to form
Fig. 5. CV curves of the Li4 Mn5 O12 electrode at different scan rates from 3 to 20 mV s−1 in 1 mol L−1 Li2 SO4 .
I × t V × m
(2)
where I is the discharge current, V is the discharge voltage in t, t is the discharge time, and m is the total active material mass of positive and negative electrode in the symmetric supercapacitor. According to Eq. (2), the specific capacitance of the supercapacitor was calculated to be 69 F g−1 . Furthermore, the capacitor delivers an energy density of 67.6 W h kg−1 at a power density of 493.8 W kg−1 based on the total weight of the active electrode materials. The coulomb efficiency () of the electrode during charge/discharge is also shown in Fig. 6(b), which is calculated according to = (tD /tC ) × 100, tD and tC are the expressions of discharge and charge times. It can be found that the coulomb efficiency increases with growth of cycle numbers and remained about 98% after 100 cycles, implying that the reversibility of charge/discharge increases. The coulomb efficiency loss during charge/discharge process is mainly due to over charge/discharge and decomposition of the aqueous electrolyte. According to results shown in Fig. 5, the Li4 Mn5 O12 electrode shows nearly typical capacitive behavior, and hydrogen and oxygen evolution reactions are hardly observed in the potential range of ca. 0.3–1.3 V versus SCE. This is the reason that the capacitor showed the high coulomb efficiency close to 100% even in the voltage range of 0–1.4 V. The result reveals that the Li4 Mn5 O12 electrode has a stable coulomb efficiency, which means good capacitance properties and high electrochemical stability and reversibility for Li4 Mn5 O12 material. Fig. 7(a) shows the impedance spectra of the Li4 Mn5 O12 electrode in 1 mol L−1 Li2 SO4 solution The Nyquist diagram for Li4 Mn5 O12 electrode includes a small semicircle at high frequency intercept of the real axis, followed by a straight line. The straight line with a slope has relation to the diffusion control process and the semicircle part observed at high frequency corresponds to the charge transfer process at the compound–electrolyte interface and
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Fig. 6. (a) Typical charge/discharge curve and (b) cycling performance of Li4 Mn5 O12 /Li4 Mn5 O12 capacitor.
Fig. 7. (a) Impedance spectra and (b) the Bode plots of the Li4 Mn5 O12 electrode in 1 mol L−1 Li2 SO4 solution.
is known as Faradaic resistance [26,27]. The Bode plots for the Li4 Mn5 O12 electrode are shown in Fig. 7(b). The total impedance increased with decreasing in the frequency. In the lower frequency (0.01 Hz), the negative values of the phase angle reach −65◦ for the Li4 Mn5 O12 electrode. The result of phase angle indicates that the electrode material has good capacitive behavior. 4. Conclusion A novel symmetric supercapacitor in which Li4 Mn5 O12 is used both as a positive electrode and a negative electrode in 1 M Li2 SO4 mild aqueous electrolyte has been introduced. Spinel Li4 Mn5 O12 electrode material with the average particle size of about 100 nm for supercapacitor has been synthesized by a citric-acid-assisted combustion method, and characterized by XRD, EDAX, SEM, and TEM observation. The symmetric Li4 Mn5 O12 /Li4 Mn5 O12 capacitor showed a good pseudocapacitive behavior with a specific capacitance of 69 F g−1 at a current rate of 100 mA g−1 . The energy density and power density were 67.6 W h kg−1 and 493.8 W kg−1 respectively based on the total weight of the active electrode materials. Cyclic voltammetrical results showed that specific capacitance of single Li4 Mn5 O12 electrode can reach 293 F g−1 at the scan rates of 3 mV s−1 . The mechanism of charge storage in Li4 Mn5 O12 is proposed based on the concept of intercalation of lithium ion in the electrode. EIS impedance result demonstrated that the charge transfer resistance for the Li4 Mn5 O12 electrode is small and the electrode material has good capacitive behavior. Acknowledgement The authors acknowledge the financial support from the National Natural Science Foundation of China (no. 20701029).
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Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Electrochemical performance of symmetric supercapacitor based on Li4 Mn5 O12 electrode in Li2 SO4 electrolyte Yan Jing Hao, Qiong Yu Lai ∗ , Xiao Yun Xu, Ling Wang College of Chemistry, Sichuan University, Chengdu 610064, China
a r t i c l e
i n f o
Article history: Received 23 January 2010 Received in revised form 13 October 2010 Accepted 23 October 2010 Keywords: Inorganic compounds Annealing Electrochemical techniques Electrochemical properties
a b s t r a c t In this paper, spinel Li4 Mn5 O12 electrode material was prepared by a citric-acid-assisted combustion method. The structure and morphology of Li4 Mn5 O12 were characterized by X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDAX), scanning electron microscopy (SEM) and electron tunneling electron microscopy (TEM). The results showed that the Li4 Mn5 O12 powders were nano-sized particles. XRD Rietveld refinement gives the crystal structure of prepared sample. Electrochemical performance of Li4 Mn5 O12 electrode was studied by cyclic voltammetry, electrochemical impedance spectroscopy and galvanostatic charge/discharge measurements. Li4 Mn5 O12 /Li4 Mn5 O12 symmetric supercapacitor was fabricated at the first time. The maximum specific capacitance was 69 F g−1 in the potential range of 0–1.4 V in 1 mol L−1 Li2 SO4 electrolyte. The energy density was 67.6 W h kg−1 at a power density of 493.8 W kg−1 based on the total weight of the active electrode materials. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Supercapacitor is a promising candidate for power sources of electric vehicles due to its high power density. However, its energy density is low (3–6 W h kg−1 ). Recently, symmetric and asymmetric capacitors were explored by using lithium insertion materials with high energy density [1–4]. At the same time, neutral aqueous electrolytes in supercapacitors have many advantages including high ionic conductivity, low-cost, nonflammability, and no specific requirements for battery assembly and good safety [5,6]. Previous studies on the electrochemical performance of activated carbon (AC) in Li2 SO4 , Na2 SO4 and K2 SO4 solutions show that Li2 SO4 based electrolyte exhibited the best rate behavior owning to the smallest hydrated ionic radius of Li+ , weakest solvation interaction between Li+ ion and H2 O molecules and highest ionic conductivity of Li+ [7–10]. Therefore, exploration on the electrochemical active materials that could undergo reversible Li+ intercalation/deintercalation would be favorable for novel asymmetric or symmetric supercapacitor in lithium salt solution. Lithium insertion materials for lithium ion batteries such as Li4 Ti5 O12 , Li2 Ti3 O7 , LiCoO2 and LiNi0.5 Mn1.5 O4 have been reported as electrode materials for supercapacitors in organic electrolyte [11–14]. Xia and co-workers [7–10] developed some
∗ Corresponding author. Tel.: +86 28 85416969; fax: +86 28 85416969. E-mail address: [email protected] (Q.Y. Lai). 0254-0584/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2010.10.043
new hybrid electrochemical supercapacitor such as AC/LiMn2 O4 , LiTi2 (PO4 )3 /AC and LiTi2 (PO4 )3 /MnO2 in aqueous electrolyte. These capacitors showed good electrochemical properties. Among all of the lithium ion intercalated materials, spinel-type substances are promising electrode materials because of the high intercalation potential, good cyclability and rate capability for supercapacitor and lithium ion battery [15,16]. The [M2 ]O4 framework of Li[M2 ]O4 spinel is an attractive host structure for lithium insertion–extraction reactions because it provides a threedimensional network of face-sharing tetrahedra and octahedra for lithium ion diffusion. Spinel Li4 Mn5 O12 (or Li [Li0.33 Mn1.67 ]O4 ), which is the end member of the Li1+x Mn2−x O4 solid solution with x = 0.33 and an oxidation state of 4+ for Mn, has become an attractive candidate for 3V cells in organic electrolyte. It has been found that the cubic symmetry can be preserved x = 2.5 lithium ions in Li4+x Mn5 O12 formula in the 3V region without Jahn–Teller distortion [17,18]. This can provide high specific capacitance for supercapacitor based on Li4 Mn5 O12 material. In our previously work [19–21], Li4 Mn5 O12 was used as electrode material in hybrid supercapacitors such as Li4 Mn5 O12 /AC capacitor and Li4 Mn5 O12 /MnO2 -AC capacitor. In these hybrid supercapacitors, spinel Li4 Mn5 O12 was used as the cathode material, and activated carbon (AC) or carbon coated MnO2 was used as the anode material. They delivered good electrochemical properties in Li2 SO4 electrolyte. In the present work, Li4 Mn5 O12 /Li4 Mn5 O12 symmetric supercapacitor was fabricated at the first time. The electrochemical performance of the capacitor was investigated systematically and a possible mechanism of charge and discharge was proposed.
Y.J. Hao et al. / Materials Chemistry and Physics 126 (2011) 432–436 2. Experimental Li4 Mn5 O12 was prepared by citric-acid-assisted combustion method. All the chemicals were provided by Chengdu KeLong Industries Co., China. Stoichiometric amount of Li(CH3 COO)2 ·2H2 O (AR, 99%, 1.0306 g), Mn(CH3 COO)2 ·4H2 O (AR, 99%, 3.0946 g) (the molar ratio of Li:Mn = 4:5) and a certain amount of citric acid (the molar ratio of citric acid to total metal ions was 1:1) were mixed together. The obtained mixture was ground sufficiently in a ceramic mortar for about 1 h until the mixture changed to a light pink viscous complex gel. Then the obtained viscous complex mixture was dried at 75 ◦ C under vacuum for 24 h to obtain a dry precursor. After ground sufficiently in mortar, the precursor powder was preheated at 300 ◦ C in air for 2 h in a furnace and then calcined at 300 ◦ C in air for 6 h. The powders were cooled in the furnace to room temperature to obtain final product. The samples were characterized by XRD, using a D/max-rA diffractometer with Cu K␣ radiation operated at 40 kV and 100 mA. Data were collected in the range 10–70◦ ( = 0.15418 nm). The morphology of prepared particles was characterized by scanning electron microscopy (SEM) in a Hitachi-S-450 scanning microscope and Japan electron tunneling electron microscopy (TEM, Jeol JEM-2010). The chemical composition of the compound was confirmed by a field emission scanning electron microscope (FESEM, JSM6700F) equipped with an energy dispersive X-ray spectroscope (EDAX, Oxford INCA). The electrode was prepared by a casting technique. 65 wt% of the Li4 Mn5 O12 active material, 30 wt% acetylene black and 5 wt% polyvinylidene fluoride (PVDF) were mixed together. Then a certain amount of N-methy1-2-pyrrolidine (NMP) was dropped into the mixture to form a slurry. The slurry was sufficiently blended for about 1 h. The final obtained slurry was subsequently brush-coated onto a stainless steel grid that served as a current collector (active material area is 0.5 cm2 ). Then the electrode was dried at 80 ◦ C for about 2 h to obtain the Li4 Mn5 O12 electrode. The typical mass load of Li4 Mn5 O12 electrode material was 1.6 mg. The electrolyte was 1 mol L−1 Li2 SO4 solution. Capacitor test was performed using a two-electrode glass cell consisted of Li4 Mn5 O12 positive electrode and Li4 Mn5 O12 negative electrode with Neware Battery Program-control Testing System. The distance between two electrodes was 1 cm. The cyclic voltammetry was characterized using a three-electrode glass cell, in which a stainless steel grid coated with Li4 Mn5 O12 was used as the working electrode, platinum and saturated calomel electrode (SCE, 0.242 V) as counter and reference electrode respectively. The measurements were performed on a LK2005 electrochemical workstation system with a voltage scan rate ranging from 3 to 20 mV s−1 . The electrochemical impedance spectroscopy experiment was also carried out by using a three electrode cell, in which a stainless steel grid coated with Li4 Mn5 O12 was used as the working electrode, platinum and saturated calomel electrode (SCE, 0.242 V) as counter and reference electrode respectively. It was performed by Potentiosta/Galvanostat IM6ex (ZHANER Elektrik; Germany) instrument at 0.5 V. The frequency limits were typically set between 1000 kHz and 0.01 Hz.
3. Results and discussion 3.1. XRD and EDAX analysis Fig. 1(a) shows the X-ray diffraction patterns of Li4 Mn5 O12 powders. To determine the structure, XRD pattern was analyzed with the FULLFROF program [22]. The calculated unit parameter of a = 8.14565 is very close to the previous results obtained by Takada
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et al. [23]. In addition, well-fitted refinement profiles in Fig. 1(b) also confirmed that all the diffraction peaks can be indexed in the ¯ (JCPDS No. 46-0810) of a cubic spinel, which space group Fd3m indicates that in the final products, partial lithium ions are located at the tetrahedral 8a sites, manganese ions and rest lithium ions are randomly distributed at the octahedral 16d sites with the ratio of 1:5, the oxygen ions occupy the 32e sites. Thus, Li4 Mn5 O12 can be described as [Li]8a [Li1/3 Mn5/3 ]16d [O4 ]32e [24]. The crystal structure of spinel Li4 Mn5 O12 as well as important bond lengths and bond angles is depicted in Fig. 2. The MO6 octahedra share edges with six octahedral-site neighbors, two of which are located in the same plane and the remaining four are above and below the plane. The LiO4 tetrahedral sites share corners with octahedral sites. The three-dimensional structure could provide access channel for lithium ions during charge/discharge process. To estimate the quantity of the elements present in the synthesized Li4 Mn5 O12 product, energy dispersive X-ray analysis (EDAX) of the sample is given and depicted in Fig. 3. The peaks around 0.52 and 5.92 kV clearly show the presence of Mn and O elements. The molar ratio of O:Mn in the prepared Li4 Mn5 O12 product is 2.31, which is very close to the theoretical value 2.40. 3.2. SEM and TEM analysis The SEM images of Li4 Mn5 O12 powders are shown in Fig. 4(a). As can be seen in the figure, the sample powders were composed of some irregular nano-particles with average particle size of about 100 nm. The large particles of Li4 Mn5 O12 may be seen as being consisted of aggregates of Li4 Mn5 O12 primary particles. In this work, urea is used as a complexation agent in the formation of a homogeneous gel matrix. During subsequent heat treatment, the precursor carbonizes and, simultaneously, releases some gases that leave sufficient porosity to form nano-particles product. Urea plays a key role in forming the Li4 Mn5 O12 nano-particle to increase the surface area. The distribution of smaller particles may be more advantageous in the enhancement of the specific surface area and the specific capacitance. This is because it increases the probability of conducting pathways in the electrode/electrolyte interface. From the TEM image shown in Fig. 4(b), it is clear that the nanoparticles of Li4 Mn5 O12 are irregular spherical structure with particle size of about 50–100 nm. 3.3. Electrochemical analysis The cyclic voltammogram (CV) curves of the Li4 Mn5 O12 electrode are given in Fig. 5. As shown in Fig. 5, the CV curves are
Fig. 1. (a) XRD pattern of Li4 Mn5 O12 sample and (b) its Rietveld refinements.
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¯ Fig. 2. (a and b) Crystal structures of Li4 Mn5 O12 with space group of Fd3m. The interatomic distances (Å) and bond angles (◦ ) are obtained from the refinement of XRD data at room temperature.
roughly rectangular in shape and exhibit near mirror-image current response on voltage reversal. This indicated a capacitive behavior of the present sample in the potential range of 0–1.4 V. At low scan rate, two broad current peaks can be observed, i.e., an anodic peak appearing at about 0.5 V and a cathodic peak occurring at ca. 1.1 V (vs. SCE). The oxidation/reduction peak-potential of the Li4 Mn5 O12 electrode in Li2 SO4 electrolyte at low scan rate of 5 mV s−1 suggest that the Li+ insertion/extraction process should be viewed as a quasi-reversible reaction. It is inferred that the reaction occurred on the electrode is the redox process of Mn(IV) ↔ Mn (III). The useful potential window for the electrode is around 0.3–1.3 V vs. SCE (as seen from the curves at 3 and 5 mV s−1 rate). There is almost no H2 and O2 evolution observed in this potential range. The specific capacitance of the single Li4 Mn5 O12 electrode was calculated to be 265 and 245 F g−1 according to the following equation [5]:
C=
Q 2m V
(1)
where Q refers to the charge integrated from the cathodic sweep and V and m refer to the difference in the voltage window and weight of the single Li4 Mn5 O12 electrode respectively. When the scan rate increased from 3 mV s−1 to 20 mV s−1 , the two redox peaks for Li+ ions insertion/extraction exhibited unconspicuous gradually. For comparison, the specific capacitance change of Li4 Mn5 O12 electrode in the potential range of 0–1.4 V is also listed. The discharge specific capacitances of the Li4 Mn5 O12 electrode were calculated to be 293, 274, 239, and 126 F g−1 at the scan rates of 3 mV s−1 , 5 mV s−1 , 10 mV s−1 and 20 mV s−1 respectively. The specific capacitance decreased with increasing of the scan rate. This indicated that the insertion/extraction of Li-ions occur not only on the surface of the Li4 Mn5 O12 powders but also into the inner lattice of the material. At high sweep rate, the diffusion of Li-ions is almost limited to the surface of the Li4 Mn5 O12 particles. The charge storage mechanism of Li4 Mn5 O12 is based on the concept of intercalation of Li+ in the electrode during reduction and deintercalation upon oxidation. During the charge process, Li-ions in Li2 SO4 electrolyte solution insert into spinel Li4 Mn5 O12 to form Li4+x Mn5 O12 .
Fig. 3. Energy-dispersive X-ray spectroscopy of nano-Li4 Mn5 O12 sample.
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Li4 Mn5 O12 . The process could be described as follows: Li4 Mn5 O12 + xLi+ + xe− ⇔ Li4+x Mn5 O12 Moreover, adsorption of electrolyte cations such as Li+ on the manganese oxides surface contributes to part of the capacitance of the capacitor, too. In the present study, the redox process is mainly governed by the insertion and deinsertion of Li+ from the electrolyte into the nanostructured Li4 Mn5 O12 matrix. Increasing the scan rate has a direct impact on the diffusion of Li+ ions into the Li4 Mn5 O12 matrix. In other words, the faster scan rate will lead to the Li+ ions reaching only the outer surface of the electrode and not the interior of Li4 Mn5 O12 matrix. This greatly reduces the available capacity from 293 to 126 F g−1 when the scan rate is increased from 3 to 20 mV s−1 . Hence, it is clear that when the scan rate is higher the participation of the Li4 Mn5 O12 electrode is limited only to the outer surface and not the interior metrix, thus leading to a lower specific capacitance. Fig. 6 shows the typical charge–discharge curve and cycling behavior of the symmetric supercapacitor in the potential range of 0–1.4 V in 1 mol L−1 Li2 SO4 solution at a current rate of 100 mA g−1 . The discharge curve is almost linear and the sudden drop of potential at the beginning of charge and discharge is associated to the internal resistance of the supercapacitor. The average internal resistance of the capacitor can be calculated according to R = (Vcharge − Vdischarge )/(2I), where Vcharge and Vdischarge are the voltage at the end of charge and at the beginning of the discharge, respectively, and I the is absolute value of the current which is the same during the charge and the discharge. This gives an average value of 21 , reflecting the non-optimized contacts between the electrodes and the current collectors, the solution resistance. The discharge specific capacitance of the supercapacitor was calculated as follows [4]: C=
Fig. 4. (a) SEM and (b) TEM graph of Li4 Mn5 O12 particles.
According to the literature report [25], Li4 Mn5 O12 can accommodate 3 Li ions per formula to form Li7 Mn5 O12 ; this can provide a high capacitance for the supercapacitor. During the discharge process, Li-ions in spinel Li4+x Mn5 O12 material were deinserted to form
Fig. 5. CV curves of the Li4 Mn5 O12 electrode at different scan rates from 3 to 20 mV s−1 in 1 mol L−1 Li2 SO4 .
I × t V × m
(2)
where I is the discharge current, V is the discharge voltage in t, t is the discharge time, and m is the total active material mass of positive and negative electrode in the symmetric supercapacitor. According to Eq. (2), the specific capacitance of the supercapacitor was calculated to be 69 F g−1 . Furthermore, the capacitor delivers an energy density of 67.6 W h kg−1 at a power density of 493.8 W kg−1 based on the total weight of the active electrode materials. The coulomb efficiency () of the electrode during charge/discharge is also shown in Fig. 6(b), which is calculated according to = (tD /tC ) × 100, tD and tC are the expressions of discharge and charge times. It can be found that the coulomb efficiency increases with growth of cycle numbers and remained about 98% after 100 cycles, implying that the reversibility of charge/discharge increases. The coulomb efficiency loss during charge/discharge process is mainly due to over charge/discharge and decomposition of the aqueous electrolyte. According to results shown in Fig. 5, the Li4 Mn5 O12 electrode shows nearly typical capacitive behavior, and hydrogen and oxygen evolution reactions are hardly observed in the potential range of ca. 0.3–1.3 V versus SCE. This is the reason that the capacitor showed the high coulomb efficiency close to 100% even in the voltage range of 0–1.4 V. The result reveals that the Li4 Mn5 O12 electrode has a stable coulomb efficiency, which means good capacitance properties and high electrochemical stability and reversibility for Li4 Mn5 O12 material. Fig. 7(a) shows the impedance spectra of the Li4 Mn5 O12 electrode in 1 mol L−1 Li2 SO4 solution The Nyquist diagram for Li4 Mn5 O12 electrode includes a small semicircle at high frequency intercept of the real axis, followed by a straight line. The straight line with a slope has relation to the diffusion control process and the semicircle part observed at high frequency corresponds to the charge transfer process at the compound–electrolyte interface and
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Fig. 6. (a) Typical charge/discharge curve and (b) cycling performance of Li4 Mn5 O12 /Li4 Mn5 O12 capacitor.
Fig. 7. (a) Impedance spectra and (b) the Bode plots of the Li4 Mn5 O12 electrode in 1 mol L−1 Li2 SO4 solution.
is known as Faradaic resistance [26,27]. The Bode plots for the Li4 Mn5 O12 electrode are shown in Fig. 7(b). The total impedance increased with decreasing in the frequency. In the lower frequency (0.01 Hz), the negative values of the phase angle reach −65◦ for the Li4 Mn5 O12 electrode. The result of phase angle indicates that the electrode material has good capacitive behavior. 4. Conclusion A novel symmetric supercapacitor in which Li4 Mn5 O12 is used both as a positive electrode and a negative electrode in 1 M Li2 SO4 mild aqueous electrolyte has been introduced. Spinel Li4 Mn5 O12 electrode material with the average particle size of about 100 nm for supercapacitor has been synthesized by a citric-acid-assisted combustion method, and characterized by XRD, EDAX, SEM, and TEM observation. The symmetric Li4 Mn5 O12 /Li4 Mn5 O12 capacitor showed a good pseudocapacitive behavior with a specific capacitance of 69 F g−1 at a current rate of 100 mA g−1 . The energy density and power density were 67.6 W h kg−1 and 493.8 W kg−1 respectively based on the total weight of the active electrode materials. Cyclic voltammetrical results showed that specific capacitance of single Li4 Mn5 O12 electrode can reach 293 F g−1 at the scan rates of 3 mV s−1 . The mechanism of charge storage in Li4 Mn5 O12 is proposed based on the concept of intercalation of lithium ion in the electrode. EIS impedance result demonstrated that the charge transfer resistance for the Li4 Mn5 O12 electrode is small and the electrode material has good capacitive behavior. Acknowledgement The authors acknowledge the financial support from the National Natural Science Foundation of China (no. 20701029).
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