Article pubs.acs.org/JPCC
Preparation, Comparative Energy Storage Properties, and Impedance Spectroscopy Studies of Environmentally Friendly Cathode, Li(MMn11/6)O4 (M = Mn1/6, Co1/6, (Co1/12Cr1/12)) M. V. Reddy,*,† A. Sakunthala,‡ S. SelvashekaraPandian,§ and B. V. R. Chowdari† †
Department of Physics, Solid State Ionics & Advanced Batteries Lab, National University of Singapore, 117542, Singapore Department of Physics, Karunya University, Coimbatore-641114, Tamilnadu, India § Department of Nano Sciences and Technology, Karunya University, Coimbatore-641 114, Tamil Nadu, India ‡
S Supporting Information *
ABSTRACT: Undoped and doped lithium manganese oxides, Li(MMn11/6)O4 (M = Mn1/6, Co1/6, and (Co1/12Cr1/12)), were prepared by the simple polymer precursor route at different temperatures. The compounds were characterized by X-ray diffraction, SEM, density, BET surface area, and electroanalytical techniques. The energy storage performance of the compounds differs with the preparation temperature and doped elements, such as Co and Cr. The compound, Li(Co1/6Mn11/6)O4, was found to give a better energy storage performance than LiMn2O4 and Li((Co1/12Cr1/12)Mn11/6)O4. For a high current density of 600 mA g−1, Li(Co1/6Mn11/6)O4 prepared at 650−850 °C delivered a discharge capacity of 65−100 (±3) mAh g−1 at the end of 200−1000 cycles. Excellent capacity retention was noted in all cases. The comparative impedance spectroscopy studies were made and the mechanisms were discussed.
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INTRODUCTION Compared with other secondary batteries, such as Ni-MH, NiCd, and lead acid batteries, lithium-ion batteries (LIBs) are found to be the best source of energy for all the electronic products, such as mobile phones, cameras, iPods, etc. This is mainly because of the high energy density, compactness, long cycle life, and lower self-discharge rate of the LIBs.1−8 Recently, advancements in electronic products demands more power density from the LIB. For example, as the functions of mobile handsets become more advanced, the power consumed by running applications, such as mobile TV or Internet access, is increasing and demands a high current for a long time. When devices run on high current, the stored energy will rapidly deplete due to the internal resistances of the battery. Preparing cathode materials with good quality in terms of purity and suitable morphology is one way to achieve balanced energy and power density. Among the different cathode materials used in LIBs, the compound LiMn2O 4 is best known for its environmentally friendly characteristics and its abundancy in nature.5,9 It has a cubic close packing (ccp) of oxygen atoms with Mn occupying half of the octahedral (16d), and Li an eighth of the tetrahedral sites (8a). Half the Li ions are occupied in the tetrahedral sites where Li−Li interaction takes place, and the other half are occupied at sites with no Li−Li interaction. In addition to these occupied sites, empty 16c © 2013 American Chemical Society
octahedral sites and 48f and 8b tetrahedral sites are also present in the crystal for Li ions to get intercalated and deintercalated. LiMn2O4 undergoes capacity fading during electrochemical cycling; reasons for capacity fading and its structural studies are nicely reviewed by the group of Tarascon.9,10 In the literature, various efforts have been made to improve the cycling performance of LiMn2O4 by partial replacement of Mn in LiMn2O4 by Li, Co, Cr, Ni, Al, Ru, etc.11−22 and the surface coating23 to improve Li cycling. Previously, we studied the preliminary electrochemical performance of the compounds, Li(MMn 11/6 )O 4 (M = Mn 1/6 , Co 1/6 , (Co 1/12 Cr 1/12 ), (Co1/12Al1/12), (Cr1/12Al1/12)) prepared at 750 °C.17 For academic and applied aspects, here, we report comparative studies of compounds Li(MMn11/6)O4 (M = Mn1/6, Co1/6, (Co1/12Cr1/12)), prepared at different temperatures of 650−850 °C, and structural and energy storage performances studies.
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EXPERIMENTAL SECTION Material Preparation. The compounds, Li(MMn11/6)O4 (M = Mn1/6, Co1/6, (Co1/12Cr1/12)) were prepared by the polymer precursor method, using the polymer, polyvinylpyrroReceived: September 15, 2012 Revised: April 2, 2013 Published: April 4, 2013 9056
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lidone (PVP). Depending on the solid solution, stoichiometric amounts of lithium acetate (0.103 mol, purity 99.9%, Alfa Aesar), manganese acetate (purity 99%, Acros), cobalt acetate (purity 99%, Fisher Scientific), and ammonium dichromate (purity 99%, Fisher Scientific) were mixed with the polymer polyvinylpyrrolidone (PVP K40 Aldrich; molecular weight, 40 000) dissolved in 100 mL of distilled water, then refluxed at 100 °C for a 2 h duration. PVP acts as a capping agent. The 0.2 mol of manganese acetate was used for preparation of the compound LiMn2O4. For the compounds Li(Co1/6Mn11/6)O4 and Li((Co1/12Cr1/12)Mn11/6)O4, 0.18 mol of manganese acetate and 0.02 mol of the corresponding raw materials of the doping element were taken. The metal ion-to-polymer molar ratio was kept as 1:1.5. The resultant solution after 2 h of reflux was evaporated on a hot plate, and the dry residue was further heat-treated at the required temperature for about 6 h in air, at a heating rate of 3 °C min−1. Two sets of samples were prepared, one at 650 °C and the other at 850 °C. The chemicals used and the preparation steps followed were similar to those for the compounds prepared at 750 °C.17 The samples prepared at a low temperature, 550 °C, were found to have phases other than the original phase of the compounds and are not discussed here. Material Characterization and Electrode Fabrication. The samples were characterized by X-ray diffraction analysis (XRD) (X’pert MPD, Panlytical), and scanning electron microscopy (SEM) (JEOL JSM-6700F) instruments. The doped elemental analysis of the prepared samples was determined by energy-dispersive X-ray spectroscopy using a Jeol JED 2300 system. The electrode and coin cell fabrications were made similar to the procedures reported. The composite electrodes for the electrochemical studies were made in the ratio of 70:15:15 (in wt %) with active material, Super P conductive carbon black, and Kynar 2801 as binder, respectively. N-methyl-pyrrolidnone (NMP) was used as the solvent. Etched Al foil (20 μm) was used as the current collector. The thickness of the electrodes was ∼20 μm, and the geometrical area of the electrode used was 2 cm2. The active material mass was around 3−5 mg. The coin cells of type CR2016 were made in an Ar-filled glovebox using lithium metal foil as the counter and the reference electrodes. The electrolyte used was LiPF6 in ethylene carbonate + dimethyl carbonate in the ratio of 1:1 by volume (Merck, Selectipur LP40). Cyclic voltammetry (CV) studies at a scan rate of 0.058 mV sec−1 and galvanostatic cycling studies at 1C (120 mA g−1) and 5C (600 mA g−1) rates were made in the voltage range of 3.5−4.3 V vs Li, cycled at room temperature. The C rates (current rates) were calculated based on the attainable capacity (1C = 120 mA g−1) of LiMn2O4 in the voltage range of 3.5 to 4.3 V.24 Impedance spectroscopy studies were carried out in the frequency range of 0.18 MHz to 3 mHz with an ac signal amplitude of 10 mV by using a Solartron Impedance/gainphase analyzer (model SI 1255) coupled to a potentiostat (SI 1268) at room temperature (RT). Further details on instrumentation and data fitting are given in our previous studies.25,26
Figure 1. Rietveld refined XRD patterns of the compounds Li(MMn11/6)O4 (M = Mn1/6, Co1/6, (Co1/12Cr1/12)) (a) prepared at 650 °C for 6 h in air and (b) prepared at 850 °C for 6 h in air.
positional parameters. The compounds were found to be 100% pure phase, with no impurities. Comparison of the lattice parameters of the compounds prepared at 650, 750, and 850 °C are summarized in Table 1. The lattice parameter values were found to decrease with the decrease in the preparation temperature, which indicates an increase in Mn4+ ions compared to Mn3+ ions. The reason is that the compound Table 1. Comparative Structural, Density (experimental), Surface Area, and Particle Size Data of the Compounds Li(MMn11/6)O4 (M = Mn1/6, Co1/6, (Co1/12Cr1/12))a M in Li(MMn11/6)O4
a (Å) (±0.002) density (g/cm3) surface area (m2/g) particle size range (in nm) calculated using SEM images
650 °C
750 °C17
850 °C
LiMn2O4 8.225 4.04 1.76 50−100
8.234 4.14 1.15 150−500(*)
8.240 4.21 0.54 1−2 μm
Li(Co1/6Mn11/6)O4 a (Å) (±0.002) 8.191 8.210 density (g/cm3) 4.21 4.29 3.75 2.10 surface area (m2/g) particle size range (in nm) ∼50 60−100(*) calculated using SEM images Li(Co1/12Cr1/12Mn11/6)O4 a (Å) (±0.002) 8.202 8.212 density (g/cm3) 4.16 4.24 3.24 1.7 surface area (m2/g) particle size range (in nm) ∼50 60−100(*) calculated using SEM images
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RESULTS AND DISCUSSION Structural Analysis. The Rietveld-refined XRD pattern of the compounds, Li(MMn 11/6 )O 4 (M = Mn 1/6 , Co 1/6 , (Co1/12Cr1/12)), prepared at 650 and 850 °C are shown in Figure 1a,b, respectively. XRD patterns were refined using space group Fd3̅m, with the reported cubic structure and
a
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8.213 4.28 0.52 ∼1 μm
8.216 4.30 1.21 ∼1 μm
Note: (*) from ref 17 using TEM analysis. dx.doi.org/10.1021/jp309180k | J. Phys. Chem. C 2013, 117, 9056−9064
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Figure 2. SEM images of the compounds Li(MMn11/6)O4: (a, d) M = Mn1/6, (b, e) M = Co1/6, (c, f) M = (Co1/12Cr1/12)). (a, b, c) Compounds prepared at 650 °C. (d, e, f) Compounds prepared at 850 °C. Scale bar: 100 nm (×30K).
LiMn2O4 has half the manganese in the 3+ oxidation state and the other half in the 4+ oxidation state, Li(Mn3+Mn4+)O4. Here, the lattice parameter values are related to the Mn3+/Mn4+ ratio in the compound with Mn4+ ions having a smaller ionic size of 0.53 Å when compared to the Mn3+ ions with an ionic size of 0.65 Å. Irrespective of the preparation temperature, the lattice parameter values were found to be lower for the doped compounds (Table 1). The decrease of the lattice parameter is due to differences in the ionic radii of Co3+ (0.545 Å) and Cr3+ (0.615 Å), in comparison to that of Mn3+ (0.645 Å).27 We note that, for any solid solution materials preparation, we must consider the doping elements charge balance and similar ionic radius. Co and Cr are in the 3+ oxidation state. The present samples are prepared in air; in principle, Co and Cr will go stable +3 oxidation state. Also, XRD patterns clearly show the absence of Cr2O3 or Co3O4 phases. A similar decrease of lattice parameters was noted with doped LiMn2O4 compounds.12,16,28 Energy-dispersive X-ray spectroscopy studies clearly show the presence of Co, Cr, Mn, and O elements, and EDX plots are shown in the Supporting Information. Further studies, such as neutron diffraction, synchrotron X-ray diffraction, X-ray absorption spectroscopy, X-ray photoelectron microscopy,
and Auger electron spectroscopy studies, are needed for further detailed information about the crystal and electronic structure of the above compounds. Morphology Analysis. The SEM images of the compounds, Li(MMn11/6)O4 (M = Mn1/6, Co1/6, (Co1/12Cr1/12)), prepared at 650 and 850 °C are shown in panels a−c and d−f of Figure 2, respectively. The SEM analysis shows the particles to be nanoparticles of less than 100 nm for all the compounds synthesized at 650 °C; 1−2 μm sized particles were found for the higher calcination temperature of 850 °C. The particle size, surface area, and density values are given in Table 1. The values corresponding to compounds prepared at 750 °C are given for comparison.17 The surface area values were found to decrease with the increase in the synthesis temperature. Irrespective of preparation temperature, it was found to be higher for the doped compounds than the undoped compound. The density of the compounds at different preparation temperatures was found to be ∼4.32 g/cm3 and agrees with the calculated density (Table 1). Electrochemical Performance. Cyclic Voltammetry Studies. The mechanism of lithium intercalation and deintercalation and information about redox couples were 9058
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known through CV analysis.21,24,29−32 The second cycle cyclic voltammograms of the compounds, Li(MMn11/6)O4 (M = Mn1/6, Co1/6, (Co1/12Cr1/12), prepared at 650 °C, and cycled till 4.3 and 5 V, are shown in Figure 3a,b, respectively. The CV plot
Figure 4. Charge/discharge curves of the compounds prepared at 650 °C, (a) LiMn2O4, (b) Li(Co1/6Mn11/6)O4, and (c) Li((Co1/12Cr1/12)Mn11/6)O4, at different cycle numbers, for the 1C rate (120 mA/g). (d) Discharge capacity vs cycle number plots for compound LiMn2O4 (green symbol) at (a) 1C and (d) 5C rates, Li(Co1/6Mn11/6)O4 (blue symbol) at (b) 1C and (e) 5C rates, and Li((Co1/12Cr1/12)Mn11/6)O4 (black symbol) at (c) 1C and (f) 5C rates.
spinels, a high current value (y axis) was observed at the lower voltage region. The reason is as follows: The doped compounds have the lower lattice parameter, and hence, the number of Mn4+ ions will be higher than the Mn3+ ions in the 16d sites, as discussed in XRD analysis. This resulted in a higher Mn−O bond strength, which, in turn, decreased the Li−O interactions. As a result, more Li ions were extracted even at a lower voltage in the case of doped compounds, resulting in a higher current in the lower voltage region.36 Increased Mn−O strength also prevents the Mn ion migration from the 16d to 16c sites and keeps the structure stable on repeated cycling. This could be understood by the absence or suppression of the double hexagonal phase transitions at 4.5 V, which will otherwise appear due to the migration of Mn3+ ion.10,21 We note the double hexagonal phase transition leads to drastic capacity fading when we cycle above 4.5 V.21 The above phase transition was suppressed in the case of doped compounds (Figure 3b), but appeared for the updoped compound LiMn2O4. The second cycle cyclic voltammograms of the compounds, Li(MMn11/6)O4 (M = Mn1/6, Co1/6, (Co1/12Cr1/12)), prepared at 850 °C, cycled till 4.3 and 5 V, are shown in Figure 3c,d, respectively. As in the case of the 650 °C sample, for the undoped compound, a high current value was observed at the higher voltage region, whereas for doped compounds, a high current value was observed at the lower voltage region. Also, the double hexagonal phase transition at 4.5 V was suppressed only in the case of doped compounds (Figure 3d). Similar observations were made in the case of compounds prepared at 750 °C.17 From the comparative study, it was understood that the double hexagonal phase transformation at 4.5 V was suppressed for the doped compounds irrespective of the synthesis temperature. The comparative CV of the compound LiMn2O4 synthesized at different temperatures is shown in Figure 3e. It was observed that, for the sample LiMn2O4 prepared at 650 °C, the double hexagonal phase transformation was well suppressed
Figure 3. Second cycle CV of the compounds Li(MMn11/6)O4 (M = Mn1/6, Co1/6, (Co1/12Cr1/12)) prepared (a, b) at 650 °C and (c, d) at 850 °C. (e) 2nd cycle CV (3.5−5 V) of LiMn2O4 prepared at 650 °C (black line), 750 °C (red line), and 850 °C (blue line). (f) Enlarged view of (e) between 4.3 and 5 V.
shows two peaks during Li intercalation and deintercalation, which was reflected as a two-stage region in the galvanostatic charge and discharge cycling in Figure 4a−c. During anodic scan (charge cycle), the lithium (x) deintercalation was between 1 ≤ x ≤ 0.5 at ∼4.06 V (deintercalated from highenergy site where Li−Li interaction exists) and between 0 ≤ x ≤ 0.5 at ∼4.18 V (deintercalated from low-energy site where no Li−Li interaction exists),33−35 as in eqs 1 and 2. LiMn2O4 → Li 0.5Mn2O4 + 0.5Li + 0.5e−
Li 0.5Mn2O4 → λ ‐ MnO2 + 0.5Li + 0.5e
−
(1) (2)
The lower the energy of the site occupied by Li ions and higher voltage are required for deintercalation. During the cathodic scan (discharge cycle), the lithium intercalation was between 0 ≤ x ≤ 0.5 at ∼4.07 V and between 1 ≤ x ≤ 0.5 at ∼3.94 V. A site with a lower energy level will be occupied first. By comparing CV results of all the three compounds in Figure 3a,b, it was observed that, during the anodic scan, for the undoped compound LiMn2O4, a high current value (y axis) was observed at the higher voltage region, whereas for substituted 9059
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Table 2. Electrochemical Data of the Compounds Li(MMn11/6)O4 (M = Mn1/6, Co1/6, (Co1/12Cr1/12)) Prepared at 650 °C M in Li(MMn11/6)O4-650 °C (Mn1/6) total integrated cathodic and anodic area by using CV studies:
(Co1/6)
136
(Co1/12Cr1/12)
120
105
current rate:
1C
5C
1C
5C
1C
5C
1st charge capacity (±3) mAh g−1 1st discharge capacity (±3) mAh g−1 irreversible capacity loss (ICL) 2nd discharge capacity (±3) mAh g−1 200th cycle discharge capacity (±3) mAh g−1 capacity retention (%); (2−200 cycles) 500th cycle discharge capacity (±3) mAh g−1 capacity retention (%); (2−500 cycles) 1000th cycle discharge capacity (±3) mAh g−1 capacity retention (%); (2−1000 cycles)
139 104 35 103 90 88 84 82 77 75
106 86 20 83 76 93 70 85
126 87 39 87 86 99 85 98 82 94
78 67 11 63 67 100 63 100
102 81 21 82 81 100 78 96
88 64 24 62 64 100 59 95
compared to samples prepared at 750 and 850 °C. This could be seen clearly in Figure 3f. This is due to the difference in lattice parameter values for the same compound when prepared at different preparation temperatures (Table 1). Therefore, a low temperature of preparation is found to be better for the undoped compound LiMn2O4. Galvanostatic Charge/Discharge Studies for the Samples Prepared at 650 °C. The charge/discharge plateaus at the 1C rate of the compounds Li(MMn11/6)O4 (M = Mn1/6, Co1/6, (Co1/12Cr1/12)), prepared at 650 °C, are shown in Figure 4a−c. A well-defined two-stage region was observed for all the compounds under study. The flat plateau regions were maintained even at the 500th and 1000th cycles. The discharge capacity versus cycle number plots at 1C and 5 C rates, for the above compounds, are shown in Figure 4d. The compound LiMn2O4 was found to give a higher initial discharge capacity value when compared to the doped compounds (Table 2). It was also reflected by the CV studies, where the total integrated peak area was higher for the compound LiMn2O4 than the doped ones (Table 2). Its cycling stability of 82% and 85% at the end of the 500 cycles for 1C and 5 C rates, respectively, with good discharge capacity values (Table 2) were found to be better compared to other literature reports.37 Table 2 also points out that, for the 1C rate, at the 200th and the 500th cycles, the discharge capacity of LiMn2O4 was higher than that of the doped compounds. After a long cycling at the 1000th cycle, the discharge capacity of LiMn2O4 decreased, indicating its capacity retention to be low compared to that of the doped compound (Li(Co1/6Mn11/6)O4). This shows the importance of particle size and doping in improving the capacity and cycle life, respectively. In addition to particle size, other reasons, such as conductivity differences between bare and doped compounds also play an important role. We note that further conductivity studies are needed to explain the exact mechanisms. Galvanostatic Charge/Discharge Studies for the Samples Prepared at 850 °C. The charge/discharge plateau at the 1C rate for the compounds Li(MMn11/6)O4 (M = Mn1/6, Co1/6, (Co1/12Cr1/12)) prepared at 850 °C are shown in Figure 5a−c. A well-defined two-stage region was observed for the doped compounds. The discharge capacity versus cycle number plots at 1C and 5 C rates, for the above compounds, are shown in Figure 5d. The cycling data are given in Table 3, and the 200th cycle data suggest the poor performance of the bare compound when compared to the doped compounds. Li(Co1/6Mn11/6)O4
Figure 5. Charge/discharge curves of the compounds prepared at 850 °C, (a) LiMn2O4, (b) Li(Co1/6Mn11/6)O4, and (c) Li((Co1/12Cr1/12)Mn11/6)O4, at different cycle numbers, for the 1C rate (120 mA/g). (d) Discharge capacity vs cycle number plots for compound LiMn2O4 (green symbol) at (a) 1C and (d) 5 C rates, Li(Co1/6Mn11/6)O4 (blue symbol) at (b) 1C and (e) 5C rates, and Li((Co1/12Cr1/12)Mn11/6)O4 (black symbol) at (c) 1C and (f) 5C rates.
gives better capacity and stability than the compound Li(Co1/12Cr1/12Mn11/6)O4. Comparative Analysis. The discharge capacity data of all the compounds at the 200th cycle for the 1C and 5C rates are compared in Figure 6a−c. The data corresponding to samples prepared at 750 °C are included here for better comparison, and the following conclusions were made.17 LiMn2O4. High capacity (Figure 6a) and stability (Figure 6d) was obtained for the sample prepared at 650 °C, followed by 750 and 850 °C samples. The high capacity for the 650 °C sample was due to the nanoparticles around 100 nm. Another probable reason for its better stability is its lattice parameter value close to the lattice parameter value of 8.23 Å. The capacity and cycle stability were found to decrease with the increase in sample preparation temperature, due to an increase in the lattice parameter and the particle size values. Further 9060
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Table 3. Electrochemical Data of the Compounds Li(MMn11/6)O4 (M = Mn1/6, Co1/6, (Co1/12Cr1/12)) Prepared at 850 °Ca M in Li(MMn11/6)O4-850 °C (Mn1/6) total cathodic and anodic area by using CV studies:
a
(Co1/6)
130
(Co1/12Cr1/12)
125
130
current rate:
1C
5C
1C
5C
1C
5C
1st charge capacity (±3) mAh g−1 1st discharge capacity (±3) mAh g−1 irreversible capacity loss (ICL) 2nd discharge capacity (±3) mAh g−1 200th cycle discharge capacity (±3) mAh g−1 capacity retention (%); (2−200 cycles)
120 88 32 85 57(*) 67
126 57 69 56 43(*) 78
111 84 27 86 85 100
103 71 32 70 70 100
96 80 16 81 82 100
116 58 58 61 66 100
Note: (*) at the 150th cycle.
retention was found to be 98−100% irrespective of preparation conditions, but for the bare compound, it was less than 90%. Comparative Electrochemical Impedance Spectroscopy Studies. Electrochemical impedance spectroscopy (EIS) is one of the most informative among the different electroanalytical techniques to understand the kinetics of the Li+ ion insertion/ deinsertion process in an intercalation compound.25,38−42 The impedance results of all the compounds corresponding to 4.0 V measured during the charged state of the first cycle were compared as in Figure 7i−iii and Table 4. The spectra were fit using the equivalent circuit as in Figure 7iv. R(sf+ct) stands for the combined surface film and charge transfer resistance. CPE(sf+dl) is the corresponding constant phase element due to surface film capacitance and double layer capacitance. The surface film resistance and charge transfer resistance values were fit separately in the case of LiMn2O4 prepared at 650 and 750 °C. The small semicircle at the low-frequency end corresponding to the bulk resistance (Rb) was noticed mostly for all the samples. CPEb is the corresponding constant phase element due to bulk capacitance. The physical meaning and reasons for variation of Rsf, Rct, and Rb and CPEsf, cdl, and b are clearly explained in previous reports.40,43−45 In brief, surface film resistance gives information about the solid electrolyte interface; charge transfer resistance arises at the interface between electrode and electrolyte. Bulk resistance arises due to electronic resistivity of the active material and ionic conductivity in the pores of the composite electrode (active material + conducting carbon + binder) filled with the electrolyte. Usually, the capacitance values CPE(sf+dl) is of the order of micro-farads and bulk capacitance is of the order in milli-farads. As seen in Table 4, irrespective of preparation temperature, a low impedance value was noticed for the compound Li(Co1/6Mn11/6)O4. The surface film resistance and charge transfer resistance were found to be higher in the case of LiMn2O4 prepared at 650 and 750 °C, indicating controlled surface film formation in the case of doped compounds. Comparing LiMn2O4 prepared at different temperatures, impedance was lower for the sample prepared at 650 °C, having good correlation with electrochemical performance.
Figure 6. Comparative 200th cycle discharge capacity values (y axis) ((±3) mAh g−1) for samples prepared at different temperatures (x axis) (dark colored bar: 1C rate) and (gray colored bar: 5C rate) represented as a chart: (a) LiMn2O4, (b) Li(Co1/6Mn11/6)O4, and (c) Li(Co1/12Cr1/12Mn11/6)O4. (d) The capacity retention values at the end of 200 cycles for the compound LiMn2O4 prepared at different temperatures (Note: in the case of (d), the capacity retention data corresponding to the sample prepared at 750 °C are for the 150th cycle).
complementary careful structural, in situ microscopy and spectroscopy studies are needed to explain the above observed behavior. Li(Co1/6Mn11/6)O4 and Li(Co1/12Cr1/12Mn11/6)O4. High reversible capacity was seen in the case of the sample prepared at 750 °C, followed by the 850 and 650 °C samples, with excellent performance in the case of the compound Li(Co1/6Mn11/6)O4 (Figure 6b,c). The discharge capacity values were found to overlap for the 850 and 650 °C samples. The better performance of the 750 °C sample was due to the nanoparticles less than 100 nm. The particles were submicrometer in the case of the 850 °C samples, which leads to long diffusion paths and a reduced capacity. In the case of the 650 °C samples, the too low values of lattice parameters (Table 1) suggest more Mn4+ ions during the low-temperature synthesis, which will not contribute to capacity. This is because Mn4+ is inactive and only Mn3+ is active during charge/discharge. Also, the surface reactions, such as manganese dissolution, were expected to be more due to the too low particle size of less than 60 nm, resulting in reduced capacity values. In the case of doped compounds, the capacity
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CONCLUSIONS A novel, simple, and cost-effective method was used for the preparation of nano/submicrometer-sized particles of Li(MMn11/6)O4 (M = Mn1/6, Co1/6, (Co1/12Cr1/12)). The polymer PVP, being biocompatible, nontoxic, and cheap, can be considered as a best chelating/combustion agent. A decrease in the trend in the storage values was noted with an increase of preparation temperature of the undoped LiMn2O4 material. 9061
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Figure 7. Comparative Nyquist plots (Z′ vs −Z″) for the compounds prepared at (i) 650, (ii) 750, and (iii) 850 °C. Charged state to 4.0 V. (a) LiMn2O4, (b) Li(Co1/6Mn11/6)O4, and (c) Li(Co1/12Cr1/12Mn11/6)O4. Symbols represent experimental spectra; continuous lines are fitted data using equivalent circuit used to fit the impedance spectra. (iv) Equivalent electrical circuit used to fit the above impedance sepectra. Note, some cases, Rsf/sf and Rct/CPEdl used separately.
Table 4. Electrochemical Impedance Data for Undoped and Doped LiMn2O4 Compounds during Charged State at 4.0 V for the First Cycle M in Li(MMn11/6)O4
Re ohm
R(sf) ohm
R(ct) ohm
(Mn1/6)-650 °C (Co1/6)-650 °C (Co1/12Cr1/12)-650 °C (Mn1/6)-750 °C (Co1/6)-750 °C (Co1/12Cr1/12)-750 °C (Mn1/6)-850 °C (Co1/6)-850 °C (Co1/12Cr1/12)-850 °C
2.8 3.2 3.6 2 2.6 1.9 2.4 2.8 2.9
30
45
R(sf+ct) ohm 79 83
47
Rb ohm
CPE(sf) μF
CPE(dl) μF
68
110
11 55
142
33 51 112 122 61 101
46 49 26 88
The optimized synthesis temperature was concluded as 650 °C for bare LiMn2O4 and 750 °C for the doped compounds, Li(Co1/6Mn11/6)O4 and Li((Co1/12Cr1/12)Mn11/6)O4. The absence or suppression of the double hexagonal phase transitions at 4.5 V was noted with doped compounds. Irrespective of the preparation temperature, a better cycle life was noticed for the doped compounds when compared to the undoped compound. The observed energy storage performance and mechanisms were discussed based on various analytical techniques. This study shows the importance of optimizing the preparation conditions apart from doping the parent compound with different metal ions. Further careful physical, structural studies and morphology of the undoped and doped LiMn2O4 compounds are in progress.
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CPEb mF
CPE(sf+dl) μF
31 25
92 65
19 32 51 7
34 59 40 41 18
Ws (±10) ohm
89
71 334 47 560 214 285 26 243 50
ASSOCIATED CONTENT
S Supporting Information *
Further details of EDX analysis SI.1. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected],
[email protected]. Telephone: +65-65162607. Fax: 65-67776126. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank the NUS-India Research Initiative (IRI), Singapore for the SIPIS 2009 programme for support, and part of the work is supported by the Defence Advanced Research 9062
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Projects Agency (DARPA), USA (Grant no. R-144-000-226597). Thanks are due to anonymous referees and the editor for constructive comments and helpful suggestions on our manuscript.
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