Understanding the Influence of Composition and Synthesis

Sep 19, 2014 - Lithium- and Manganese-Rich Oxide Cathode Materials for High-Energy Lithium Ion Batteries. Jun Wang , Xin He , Elie Paillard , Nina ...
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Understanding the Influence of Composition and Synthesis Temperature on Oxygen Loss, Reversible Capacity, and Electrochemical Behavior of xLi2MnO3‑(1 − x)LiCoO2 Cathodes in the First Cycle Xingde Xiang,†,‡ James C. Knight,§ Weishan Li,‡ and Arumugam Manthiram*,†,§ †

Materials Science and Engineering Program, The University of Texas at Austin, Austin, Texas 78712, United States School of Materials Science and Engineering, South China University of Technology, Guangzhou 510640, China § McKetta Department of Chemical Engineering, University of Texas at Austin, Austin, Texas 78712, United States ‡

ABSTRACT: With an aim to broaden the understanding of the role of Co3+ in the lithium-rich layered oxide cathodes for lithium-ion batteries, the influence of composition and synthesis temperature on the oxygen loss, reversible capacity, and electrochemical behavior of the xLi2MnO3-(1 − x)LiCoO2 series in the first cycle has been systematically investigated. The charge capacity first increases with increasing x value from 0.2 to 0.6 due to the enhanced oxygen loss, but then decreases with further increase in x from 0.6 to 0.8. Although the discharge capacity shows a trend similar to the charge capacity, the maximum value appears at x = 0.4−0.5, rather than at x = 0.6 because of its larger irreversible capacity loss. Surprisingly, raising the synthesis temperature from 800 to 1000 °C alters the electrochemical behavior of the first charge. The materials synthesized at 900 and 1000 °C show a three-step reaction process consisting of the oxidation of Co3+, simultaneous oxidation of Co3+ and O2−, and finally the oxidation of O2− only, while those synthesized at 800 °C exhibit a two-step reaction process composed of Co3+ oxidation and O2− oxidation occurring separately.

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INTRODUCTION Since the advent of lithium-ion batteries, tremendous efforts are being made to develop cathode materials with higher capacities, higher operating voltages, or both to meet the energy density demands of portable devices, electric vehicles, and stationary grid storage of electricity produced from renewable sources.1−5 Layered Li2MnO3 has drawn much attention in recent years as it has the potential to exhibit a reversible extraction of two lithium ions involving the oxidation of O2− ions on charging above 4.5 V.6 The oxidation of O2− ions results in a loss of oxygen from the lattice during first charge, followed by the reduction of Mn4+ ions during subsequent discharge.7 However, the capacity is sensitive to synthesis temperature. For example, Li2MnO3 synthesized at 600 °C exhibited a reversible capacity of 170 mAh g−1,8 while that synthesized at 400 °C delivered a discharge capacity of more than 250 mAh g−1.9 Also, Li2MnO3 suffers from severe capacity fade due to structural transformation to a spinel phase.6,9 Interestingly, combining the layered Li2MnO3 with layered LiMO2 (M = Mn, Co, and Ni) to give the lithium-rich layered oxides xLi2MnO3-(1 − x)LiMO2 offers a high reversible capacity with improved cyclic stability.10−16 Their high reversible capacities are due to the irreversible loss of oxygen during first charge and subsequent lowering of the oxidation state of tetravalent manganese during first discharge.17,18 As has been reported previously,8,19 the incorporation of Co3+ ions © 2014 American Chemical Society

into these oxides enhances the oxygen loss from the lattice significantly due to an overlap of the Co3+/4+: t2g band with the top of the O2−: 2p band. In order to better understand the role of cobalt in lithium-rich layered oxides and facilitate the smarter design of lithium-rich layered oxides, a more detailed investigation of the xLi2MnO3-(1 − x)LiCoO2 system without other active transition metal (TM) ions is needed. However, only a few studies have focused on xLi2 MnO 3 -(1 − x)LiCoO2.14,20−22 Accordingly, we present here a more systematic investigation of the role of Co3+ on the electrochemical properties in the first cycle by synthesizing the xLi2MnO3-(1 − x)LiCoO2 series of lithium-rich layered oxides with 0.2 ≤ x ≤ 0.8 at different temperatures.

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EXPERIMENTAL SECTION Synthesis. The lithium-rich layered oxides xLi2MnO3-(1 − x)LiCoO2 (0.2 ≤ x ≤ 0.8) were synthesized by a sol−gel method. Required amounts of manganese acetate (Mn(CH3COO)2·4H2O), cobalt acetate (Co(CH3COO)2·4H2O), and lithium acetate (CH3COOLi·2H2O) were dissolved in an aqueous solution of citric acid. The molar ratio of transition metal ions to citric acid was 1:1. A 3 wt % excess lithium was Received: July 30, 2014 Revised: September 15, 2014 Published: September 19, 2014 23553

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Table 1. ICP Results of the Samples Synthesized at Various Temperatures 800 °C

900 °C

1000 °C

x

Li

Mn

Co

Li

Mn

Co

Li

Mn

Co

0.2 0.3 0.4 0.5 0.6 0.7 0.8

1.081 1.136 1.169 1.197 1.238 1.244 1.283

0.180 0.255 0.326 0.396 0.451 0.528 0.574

0.739 0.609 0.505 0.407 0.311 0.228 0.143

1.105 1.129 1.179 1.191 1.224 1.264 1.301

0.177 0.258 0.323 0.401 0.459 0.504 0.552

0.718 0.613 0.498 0.408 0.317 0.232 0.147

1.087 1.115 1.149 1.178 1.230 1.266 1.279

0.180 0.262 0.336 0.403 0.457 0.507 0.571

0.733 0.623 0.515 0.419 0.313 0.227 0.150

Figure 1. XRD patterns and lattice parameters of the xLi2MnO3-(1 − x)LiCoO2 (0.2 ≤ x ≤ 0.8) samples synthesized at various temperatures: (a, b) 800 °C, (c) 900 °C, and (d) 1000 °C.

taken for 800 °C synthesis and a 5 wt % excess lithium for 900 and 1000 °C synthesis to compensate for the volatilization of lithium during heating. The solution was then evaporated at ∼160 °C, and the dry powder was heat treated at 450 °C for 4 h to remove any residual organics. After grinding, the powder was fired at 800, 900, or 1000 °C for 12 h. The heating and cooling rates were 3 °C min−1. Characterization. The samples were characterized with Xray diffraction (XRD, Philips diffractometer with Cu Kα radiation) and scanning electron microscopy (SEM, JSM-6380, Japan). The metal-ion compositions were examined by inductively coupled plasma (ICP) analysis (Varian 715-ES). For electrochemical measurements, the electrodes were prepared by making a slurry of active material, conductive carbon (Super P), and binder (polyvinylidene fluoride; 8:1:1 wt. ratio) in N-methy-pyrrolidone solvent, coating the slurry onto an aluminum foil, and cutting the foil into round cathodes.

CR2032 coin cells were then assembled in an Ar-filled glovebox with the cathode, lithium metal anode, 1 M LiPF6 in ethylene carbonate/diethyl carbonate (1:1 by volume) electrolyte, and Celgard polypropylene separator. Charge/discharge measurements of the cells were performed at 0.02 C rate (assuming a theoretical capacity of 250 mAh g−1) on an Arbin test system. In the first charge curves, the slope and plateau regions were identified based on the onset potential of the oxygen loss reaction obtained from the dQ/dV curves.

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RESULTS AND DISCUSSION Structure and Morphology. Compositional analysis of the xLi2MnO3-(1 − x)LiCoO2 (0.2 ≤ x ≤ 0.8) samples with ICP indicated that the obtained compositions are nearly identical to the desired nominal values (see Table 1). Figure 1a−d presents the XRD patterns of xLi2MnO3-(1 − x)LiCoO2 (0.2 ≤ x ≤ 0.8) synthesized at 800, 900, and 1000 23554

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Figure 2. SEM images of xLi2MnO3-(1 − x)LiCoO2 (x = 0.5) synthesized at various temperatures: (a) 800, (b) 900, and (c) 1000 °C.

Figure 3. Charge/discharge curves of the samples synthesized at (a) 800, (b) 900, and (c) 1000 °C and (d) capacities of the xLi2MnO3-(1 − x)LiCoO2 (0.2 ≤ x ≤ 0.8) samples synthesized at various temperatures.

°C. All peaks can be indexed based on the α-NaFeO2 structure with R3̅m symmetry, except for the broad peaks in the range of 20−25°. These broad peaks are attributed to superlattice reflections arising from an ordering of Li+ and Mn4+ ions in the TM layers. With decreasing LiCoO2 content (increasing x value), the superlattice peaks become more discernible due to the enhancement in the ordering of Li+ and Mn4+ ions caused by increasing Li2MnO3 content. Also with increasing x values, the R3m ̅ reflections shift to lower angles due to the expansion of the lattice caused by the increased amount of larger Li+ ions (Co3+: 0.545 Å, Mn4+: 0.53 Å, Li+: 0.76 Å).19 As exhibited in Figure 1b, the lattice parameters (a and c) increase almost linearly with decreasing LiCoO2 component as expected. Similar c/3a values for the entire series indicate that the layeredness of the series is not significantly influenced by the composition and synthesis temperature. It can also be observed in the XRD patterns that the peak sharpness increases with

increasing synthesis temperature due to the increase in crystallite size. As presented in Figure 2a−c, the particle size of the x = 0.5 sample increases from ∼100 to ∼800 nm as the synthesis temperature increases from 800 to 1000 °C. First Charge/Discharge Profiles. Figure 3a−c displays the first charge and discharge curves of the xLi2MnO3-(1 − x)LiCoO2 series. They all exhibit charge profiles that are typical of the lithium-rich layered oxides, consisting of a sloping region corresponding to TM oxidation and then a plateau region assigned to oxygen loss.23 The charge capacity initially increases with increasing x value from 0.2 to 0.6, but then decreases as the x value increases further to 0.8. As exhibited in Figure 3d, the discharge capacity exhibits a trend similar to that of the charge capacity, but its maximum value appears at x = 0.4−0.5. On raising the synthesis temperature, the compositional effect on the charge and discharge capacities becomes more pronounced. The electrochemical activity of the Mn-rich 23555

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materials (x > 0.6) decreases dramatically when they are sintered at higher temperatures, which has also been reported for other Mn-rich materials, such as Li2MnO3.6,8 In order to gain further understanding of the changes in the charge and discharge capacities, the influence of the composition and synthesis temperature on the sloping-region capacity, plateau-region capacity, and irreversible capacity (IRC) loss were also investigated. As presented in Figure 4,

Figure 5. Oxidation state of Co in the xLi2MnO3-(1 − x)LiCoO2 (0.2 ≤ x ≤ 0.8) samples when oxygen loss begins to occur.

higher temperatures, possibly due to the larger particle sizes, which inhibit the kinetics of Co oxidation. Furthermore, the materials synthesized at 1000 °C show lower IRC values than those synthesized at 800 and 900 °C, suggesting that the higher firing temperature can suppress TM migration and the corresponding elimination of Li vacancies at the end of the first charge. Electrochemical Behavior. Figure 6a−f presents the differential curves (dQ/dV) of the samples to examine the influence of composition and synthesis temperature on the electrochemical behavior during first charge. For the samples synthesized at 800 °C (Figure 6a), the 3.9 V peak corresponding to Co3+ oxidation decreases with increasing x value due to the decrease in Co content. The other peak at 4.5 V corresponding to oxygen loss shows an initial increase because of the increased Li2MnO3 content and then a decrease because of the higher Mn4+ content. The trends of the two peaks are identical to that found earlier with the sloping-region and plateau-region capacities. This result implies that the samples synthesized at 800 °C undergo a two-step reaction process during first charge, where the Co3+ oxidation and O2− ion oxidation occur separately. On the other hand, for the samples synthesized at 900 °C (Figure 6b), the peak for Co3+ oxidation becomes broader and even shifts to a higher potential with increasing x value. This may be caused by the sluggish kinetics of Co3+ oxidation in the larger particles of samples with lower Co content. Moreover, the peak for the oxygen loss separates into two peaks located at 4.3 and 4.5 V. With a larger x value, the 4.3 V peak first increases and then decreases in intensity, exhibiting an inverse trend to the oxidation state of Co (see Figure 5). Hence, the 4.3 V peak may be related to further oxidation of Co during the oxygen loss process, which has also been demonstrated in LiCoO2 by Dahéron et al.24 Similarly, the materials synthesized at 1000 °C also possess the broadening and shift of the Co oxidation peak and the separation of the oxygen loss peak into two, as illustrated in Figure 6c. It can be concluded that the samples synthesized at 900 and 1000 °C possess a three-step reaction process during first charge, where the second step consists of Co and oxide ions being oxidized concurrently. In order to clearly present the differences in the electrochemical properties of the materials synthesized at different temperatures, Figure 6d−f comparatively present the dQ/dV

Figure 4. Sloping-region capacity, plateau-region capacity, and IRC of xLi2MnO3-(1 − x)LiCoO2 (0.2 ≤ x ≤ 0.8) synthesized at various temperatures.

the sloping-region capacity decreases dramatically with increasing x value due to the decrease in cobalt content, while the plateau-region capacity first increases with increasing x value up to 0.6 and then decreases with further increase in x. The increase in the plateau-region capacity can be attributed to the increase in Li2MnO3 character, which induces greater oxygen loss, and the decrease for x > 0.6 can be attributed to the rise in Mn4+ content, which suppresses the conductivity of the material and the oxygen loss.10 For the IRC, the data generally mirrors that of the plateau-region capacity by initially increasing before decreasing with increasing x value. This is because the IRC is caused by the elimination of Li vacancies at the end of the first charge, which is directly related to the number of oxygen vacancies created during the plateau region, so a larger plateau region leads to a larger IRC. The x = 0.4−0.5 samples show a higher discharge capacity than the x = 0.6 sample because of their lower IRC loss. The synthesis temperature also has a significant influence on the capacity values. For a given Co content, a higher synthesis temperature leads to a decrease in the sloping-region capacity and a change in the oxygen loss mechanism. Since the slopingregion capacity is generated purely by Co3+ oxidation, any change while the Co content is fixed suggests that oxygen loss begins at a different oxidation state of Co. Since the slopingregion capacity is completely derived from delithiation with a corresponding oxidation of Co3+, which has been confirmed in a previous report,14 the average oxidation state of Co in the bulk materials can be calculated according to the amount of charge transfer occurring in the sloping-region. As shown in Figure 5, the oxidation state of Co at the onset of oxygen loss in the samples synthesized at 800 °C is relatively stable at ∼3.7+ regardless of the composition. However, for the samples synthesized at 900 and 1000 °C, the oxidation states at the onset of oxygen loss are below 3.6+. The results reveal a lower electrochemical activity of Co in the materials synthesized at 23556

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Figure 6. dQ/dV curves of the xLi2MnO3-(1 − x)LiCoO2 samples with various 0.2 ≤ x ≤ 0.8 values synthesized at (a) 800, (b) 900, and (c) 1000 °C, and dQ/dV curves of the (d) x = 0.2, (e) x = 0.4, and (f) x = 0.6 samples synthesized at various temperatures.

curves of xLi2MnO3-(1 − x)LiCoO2 (x = 0.2, 0.4, and 0.6) samples synthesized at various temperatures. For x = 0.2, with increasing temperature, the intensity of the Co oxidation peak decreases due to the lower oxidation state of Co achieved before the oxygen loss commences (see Figure 5), and the peak separation for the oxygen loss is enhanced because of more Co3+ ions needing to be further oxidized during the oxygen loss process. For x = 0.4, increasing temperature leads to a sharp Co3+ oxidation peak at 3.9 V turning into a broad peak at 4.0 V, which may be associated with the reduced kinetics in the larger particles, and a dramatic enhancement in the separation of the peak for the oxygen loss because large amounts of Co3+ ions are further oxidized during the oxygen loss process. For x = 0.6, the peak separation cannot be clearly identified due to the low cobalt content in the samples. The results indicate that the differences in the electrochemical behavior of xLi2MnO3-(1 − x)LiCoO2 during first charge are associated with the changes in the oxidation state of Co at which the oxygen loss process begins, which is strongly affected by the synthesis temperature.

discharge capacity also shows an initial increase and a later decrease with increasing x value, the maximum value is found at x = 0.4−0.5, not at x = 0.6 because of its larger irreversible capacity loss during the first cycle. Interestingly, the synthesis temperature shows a large influence on the electrochemical properties of the materials during first charge. The samples synthesized at 800 °C exhibit a two-step reaction process composed of the oxidation of Co3+ and the oxidation of O2−, while those synthesized at 900 and 1000 °C possess a threestep reaction process consisting of the oxidation of Co3+, the simultaneous oxidation of Co3+ and O2−, and the oxidation of O2−.

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AUTHOR INFORMATION

Corresponding Author

*Phone 512-471-1791. Fax: 512-475-8482. E-mail: manth@ austin.utexas.edu. Notes

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The authors declare no competing financial interest.

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CONCLUSIONS The composition and synthesis temperature have been demonstrated to be key factors influencing the oxygen loss during first charge, reversible capacity, and electrochemical behavior of xLi2MnO3-(1 − x)LiCoO2 cathodes in the first cycle. The first charge capacity increases first with increasing x value from 0.2 to 0.6 due to enhanced oxygen loss induced by the increased Li2MnO3 content and then decreases with further increase in x due to the lower cobalt content and decreased oxygen loss resulting from poor conductivity caused by the high Mn4+ content. The compositional effects on the charge capacity are not altered by the synthesis temperature. Although the

ACKNOWLEDGMENTS This work was supported by the office of Vehicle Technologies of the U.S. Department of Energy under Contract DEAC0205CH11231 and the Welch Foundation Grant F-1254. One of the authors (X.X.) was supported as a visiting scholar by the National Natural Science Foundation in China (Grant No. 21273084).

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