Understanding the Effect of Co3+ Substitution on the Electrochemical

Aug 29, 2014 - Lithium-ion batteries are being widely used in portable electronic devices, such as cameras, laptops, and cell phones, due to their hig...
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Understanding the Effect of Co3+ Substitution on the Electrochemical Properties of Lithium-Rich Layered Oxide Cathodes for Lithium-Ion Batteries 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: The effect of the substitution of Co3+ for equal amounts of Mn4+ and Ni2+ in the lithium-rich layered oxide cathode Li[Li0.2Ni0.2−x/2Mn0.6−x/2Cox]O2 (0 ≤ x ≤ 0.24) has been investigated systematically. Electrochemical charge/discharge measurements in lithium-ion cells indicate that the oxygen loss from the lattice during the first charge is a kinetically slow process, but it can be improved by the substitution of Co3+ due to an overlap of the Co3+/4+:t2g band with the top of the O2−:2p band and an increase in metal− oxygen covalence. Although the increased oxygen loss with increasing Co3+ substitution leads to a higher reversible capacity, the first cycle discharge voltage decreases slightly due to the higher concentration of Mn3+ ions created during the first discharge. However, both the capacity fade and the voltage decay increase during extended cycling with increasing Co3+ substitution due to an increasing migration of the transition-metal ions to the lithium layer, which is promoted by the higher number of oxide-ion vacancies in the lattice, to form a spinel-like phase.

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Fe3+, Cr3+, Co3+) for [Mn0.54+Ni0.52+], and M2+ (M = Mg2+, Cu2+) for Ni2+. The influence is thought to be a result of changes in the covalence, metal−oxygen binding, and electron delocalization of the metal−oxygen bonds.12,22−24 Especially, the substitution of Co3+ for [Mn0.54+Ni0.52+] greatly enhances the oxygen loss and thereby the charge/discharge capacities due to an overlap of the Co3+/4+:t2g band with the top of the O2−:2p band. Accordingly, Co3+ substituted LLOs are the most commonly investigated systems.17,25−28 However, the greater oxygen loss catalyzes the transformation to a spinel-like phase during cycling due to enhanced migration of both Li and TM ions, which causes capacity fade and voltage decay.29−32 Since the Co3+ substitution increases the oxygen loss from the lattice and the reversible capacity, it is important to understand the overall effect of Co3+ substitution on other performance parameters. Accordingly, we present here a systematic investigation of the effects of the substitution of Co3+ for equal amounts of Mn4+ and Ni2+ in Li[Li0.2Ni0.2−x/2Mn0.6−x/2Cox]O2 (0 ≤ x ≤ 0.24) on the oxygen loss, reversible capacity, and discharge voltage in the first cycle and the capacity fade and voltage decay during extended cycling.

INTRODUCTION Lithium-ion batteries are being widely used in portable electronic devices, such as cameras, laptops, and cell phones, due to their higher energy density compared to other rechargeable systems. They are also being pursued intensively as power sources for electric vehicles and stationary storage of electricity generated by renewable energy sources such as solar and wind.1−3 However, the available cathode materials such as layered LiCoO2, spinel LiMn2O4, and olivine LiFePO4 all possess low practical capacities, thus severely limiting the energy density of the batteries.4−6 In this regard, lithium-rich layered oxides (LLOs), such as Li[Li1/3−2x/3Mn2/3−x/3Nix]O2, have drawn much interest as they exhibit high capacities of ∼250 mAh g−1 with lower cost and better safety compared to LiCoO2.7−14 The LLO is usually considered a solid solution of layered Li2MnO3 with C2/m symmetry and layered LiNi1/2Mn1/2O2 with R3̅m symmetry.15−18 During the first charge, the LLO initially exhibits a sloping region corresponding to the oxidation of transition-metal (TM) ions, followed by a plateau region arising from the oxidation of oxide ions and an irreversible loss of oxygen from the lattice. The loss of oxygen from the lattice during cycling results in a reduction of manganese ions at the end of the subsequent discharge, leading to a high reversible capacity. Clearly, the degree of oxygen loss plays a pivotal role in promoting the reversible capacity of LLOs.19−21 Previous studies have demonstrated that the oxygen loss is significantly influenced by cationic doping, such as the substitution of M4+ (M = Ti4+, Ru4+) for Mn4+, M3+ (M = © 2014 American Chemical Society

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EXPERIMENTAL SECTION Material Preparation. The LLOs were prepared by a coprecipitation method. Required amounts (total amount of

Received: July 6, 2014 Revised: August 25, 2014 Published: August 29, 2014 21826

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TM ions was 0.078 M) of manganese sulfate (MnSO4·H2O, >99%), nickel sulfate (NiSO4·6H2O, >98%), and cobalt nitrate (Co(NO3)2·6H2O, >98%) were dissolved in 400 mL of deionized water, and then 10 mL of ammonium hydroxide (29.8%) was added into it. After stirring at 60 °C for 10 min, 100 mL of sodium hydroxide solution (1.6 M) was added dropwise to precipitate the mixed-metal hydroxide particles. The reaction mixture was stirred overnight, and then the hydroxide particles were filtered, washed with deionized water, and dried overnight at 100 °C in an air oven. The collected precipitate was mixed with 5 wt % excess lithium carbonate (Li2CO3, 98%) and fired at 1000 °C for 12 h. Characterization. X-ray diffraction (XRD) patterns of the samples were recorded with Cu Kα radiation with a step of 0.02°. The compositions of the samples were measured with inductively coupled plasma (ICP) analysis (Varian 715-ES), and their morphology was observed by scanning electron microscopy (SEM, JEOL JSM-5610). For electrochemical characterization, a uniform slurry was first made by mixing the active material, conductive agent (Super P carbon), and the binder (polyvinylidene fluoride) at a weight ratio of 8:1:1 in N-methyl-2-pyrrolidone. The cathode was fabricated by coating the obtained slurry onto an aluminum foil and punching out 1.2 cm diameter circular electrodes. CR2032 coin cells were assembled in an Ar-filled glovebox with the fabricated cathode, lithium metal anode, 1 M LiPF6 in ethylene carbonate/diethyl carbonate (v/v 1:1) electrolyte, and a Celgard polypropylene separator. All electrochemical measurements were performed on an Arbin testing system. The cells were cycled at 4.8−2.0 V at a rate of C/20 with the theoretical capacity defined as 263 mAh g−1. When analyzing the charge curves, the shift from the slope region to the plateau region was defined as the onset potential of the oxygen loss reaction, which was obtained from the dQ/dV data. Galvanostatic intermittent titration technique (GITT) testing was carried out with alternating C/10 charges of 10 min, followed by a 40 min rest, to probe the open-circuit potential (OCP) of the samples at different depths of charge.

much larger Ni2+ (0.69 Å) ions and slightly smaller Mn4+ (0.53 Å) ions.12 For instance, the (104) reflection shifts from 44.59° for x = 0 to 44.93° for x = 0.24. This is further confirmed by the decreasing R3̅m lattice parameters with increasing Co content, as seen in Figure 1b. Moreover, the increasing c/3a ratio from 1.660 at x = 0 to 1.667 at x = 0.24 in Figure 1b suggests an increase in the degree of layeredness with increasing Co content. As seen in the SEM images in Figure 2, all of the samples consist of similarly shaped particles in the size range of 600−700 nm, implying that Co3+ substitution does not significantly influence the particle size or morphology. Additionally, their surface areas have been confirmed by Brunauer− Emmett−Teller (BET) measurements to be similar, consistent with our previous report.12 Effect of Co3+ Substitution during the First Cycle. Figure 3 presents the first charge/discharge curves of the Li[Li0.2Ni0.2−x/2Mn0.6−x/2Cox]O2 (0 ≤ x ≤ 0.24) samples cycled at a C/20 rate. All of the samples initially show a sloping region, followed by a plateau region at ∼4.5 V during the first charge. As seen in Figure 3, the charge capacity increases from 306 mAh g−1 at x = 0 to 349 mAh g−1 at x = 0.24. The increase in charge capacity arises from the enhanced oxygen loss (plateau region) caused by Co3+ substitution, as shown in Figure 4b. There is always a large irreversible capacity loss in the first cycle for LLOs, which is caused by the partial elimination of the oxide-ion vacancies and, consequently, a corresponding number of lithium vacancies at the end of the first charge.10,19,20,33 As presented in Figure 3, the discharge capacity of the samples increases dramatically from 217 mAh g−1 at x = 0 to 267 mAh g−1 at x = 0.24, which leads to an increase in Coulombic efficiency from 72% to 78%, respectively, as seen in Figure 4c. These results indicate that Co3+ substitution greatly enhances the reversible capacity of LLOs due to the promotion of oxygen loss and possibly the suppression of lithium vacancy elimination. One factor that determines the energy density of batteries is the average discharge voltage, which has been estimated here by dividing the discharge energy (Wh) by the discharge capacity (Ah). Figure 4d displays a slight decrease in average discharge voltage with increasing Co3+ substitution that is caused by an increase in Mn3+ content that will be described later. For example, the discharge voltage is 3.71 V at x = 0 and 3.65 V at x = 0.24. With an aim to understand the origin of the enhanced reversible capacity and decreased average discharge voltage, the TM redox reactions in the sloping region and the oxygen loss reaction in the plateau region were investigated. As presented in Figure 4a and Table 2, the amount of Li extracted in the sloping region initially increases and then decreases with increasing Co3+ substitution. The capacities in Table 2 were converted to the units of moles of Li in order to make the relationships between the values clearer. According to the amount of Li extracted in the sloping region, it is calculated that the Ni2+ in Li[Li0.2Ni0.2Mn0.6]O2 is oxidized only to 3.53+ before oxygen loss occurs, rather than to the expected value of 4+. This is possibly caused by the slow kinetics of Li extraction, which is supported by the GITT results presented below. Nonetheless, the Ni ions would be further oxidized to 4+ during additional charging through the oxygen loss plateau, which is supported by a previous report.33 The initial increase in the amount of Li extracted in the sloping region with increasing Co content may be attributed to the improvement in electrochemical kinetics caused by the increase in metal−oxygen covalence due to the overlap of the Co3+/4+:t2g band with the top of the O2−:2p band.

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RESULTS AND DISCUSSION Composition and Structure. The chemical compositions of the Li[Li0.2Ni0.2−x/2Mn0.6−x/2Cox]O2 (0 ≤ x ≤ 0.24) samples were confirmed by ICP analysis to be close to their nominal compositions, as summarized in Table 1. Table 1. ICP Results of the Synthesized Li[Li0.2Ni0.2−x/2Mn0.6−x/2Cox]O2 Materials x

designed composition

ICP results

0 0.08 0.13 0.16 0.20 0.24

Li1.20Ni0.20Mn0.60O2 Li1.20Ni0.16Mn0.56Co0.08O2 Li1.20Ni0.13Mn0.54Co0.13O2 Li1.20Ni0.12Mn0.52Co0.16O2 Li1.20Ni0.10Mn0.50Co0.20O2 Li1.20Ni0.08Mn0.48Co0.24O2

Li1.194Ni0.201Mn0.605O2 Li1.194Ni0.161Mn0.563Co0.082O2 Li1.196Ni0.134Mn0.534Co0.136O2 Li1.213Ni0.117Mn0.511Co0.159O2 Li1.208Ni0.099Mn0.492Co0.201O2 Li1.203Ni0.080Mn0.478Co0.239O2

The XRD patterns of the samples are shown in Figure 1a. All peaks could be indexed with a hexagonal structure with R3̅m symmetry, except the broad peaks at 20−25°, which are attributed to superlattice reflections resulting from the ordering of Li+ and TM ions in the TM layers.15,17 As the amount of Co3+ substitution increases, the peaks shift to higher angles due to the substitution of smaller Co3+ ions (Co3+ 0.545 Å) for 21827

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Figure 1. (a) XRD patterns and (b) lattice parameter variations with Co content of Li[Li0.2Ni0.2−x/2Mn0.6−x/2Cox]O2 (0 ≤ x ≤ 0.24).

ions are removed for each O atom removed during the plateau region, so the amount of oxygen vacancies created is half of the plateau capacity. In the x = 0, 0.08, and 0.13 samples, the capacity required to further oxidize the Ni ions to 4+ was subtracted from the plateau capacity before calculating the number of oxygen vacancies. Additionally, if we take the first cycle irreversible capacity loss and assume that the elimination of two oxygen vacancies is accompanied by the elimination of one Li layer site and one TM layer site,11 a significant amount of oxygen vacancies would remain in the material after the first cycle, as given in Table 2, with the amount increasing with Co3+ substitution. Hence, the structural rearrangement or transformation could continue to occur after the first cycle due to the presence of increased oxygen vacancies. Figure 5 presents the dQ/dV curves of Li[Li0.2Ni0.2−x/2Mn0.6−x/2Cox]O2 (0 ≤ x ≤ 0.24) during the first cycle. The positive peak at ∼3.9 V is attributed to the oxidation of the TM ions, and the peak at ∼4.5 V is assigned to the oxidation of oxide ions and the loss of oxygen. The negative peaks between 3.7 and 4.5 V correspond to the reduction of the Ni2+/4+ and Co3+/4+ couples, while the peak at ∼3.5 V is assigned to the reduction of Mn4+ ions to Mn3+. Clearly, relative to the undoped sample, the doped samples show a positive shift of the TM oxidation peak and a negative shift of the O2− oxidation peak. Generally, two factors can cause the shift in redox potentials. The first is due to a change in the intrinsic redox potentials determined by the relative energy position vs Li/Li+, and the other is due to the changes in the electrochemical polarization. In order to confirm the causes of these shifts in the redox peaks, GITT testing was utilized to estimate the intrinsic redox potentials of the electrochemical reactions.34,35 A large spike or drop during the resting period represents a large polarization, while the lowest potential of the spike signifies the open-circuit potential (OCP) of the redox couples at different stages of Li deintercalation. Here, the lowest potential of the first spike in the sloping region of charge is taken as the intrinsic potential of the TM redox couples. As depicted in Figure 6a, the OCP of the TM redox couples increases from ∼3.7 V for the x = 0 sample to ∼3.85 V for the Co-doped samples, suggesting that the positive shift of the dQ/dV peak position is caused by a decrease in the energy position of the TM redox couples with Co3+ substitution. This may be related to the eg vs t2g bands as redox bands associated with Ni2+/3+ and Co3+/4+. On the other hand, the OCP of the oxygen loss process is not affected by Co3+ substitution, as it remains at

Figure 2. SEM images of Li[Li0.2Ni0.2−x/2Mn0.6−x/2Cox]O2: x = (a) 0, (b) 0.08, (c) 0.13, (d) 0.16, (e) 0.20, and (f) 0.24.

Figure 3. First cycle charge/discharge curves of Li[Li0.2Ni0.2−x/2Mn0.6−x/2Cox]O2 (0 ≤ x ≤ 0.24).

The subsequent decrease in the amount of Li extracted in the sloping region at higher Co content is possibly due to the fact that Co can only be oxidized to ∼3.6+ before oxygen loss begins due to an overlap of the Co3+/4+:t2g band with the top of the O2−:2p band, while Ni2+ can be oxidized all the way to ∼4+ before oxygen loss begins. Interestingly, the amount of Li extracted in the plateau region increases with Co3+ substitution from x = 0 to x = 0.24, which is mainly caused by the enhanced oxygen loss arising from the increased band overlap discussed previously. During the sloping region, the Co3+ ions were fully oxidized to ∼3.6+ in all samples, but the Ni ions were not fully oxidized to 4+ for the x = 0, 0.08, and 0.13 samples. Two Li+ 21828

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Figure 4. (a) Capacity in the sloping region, (b) capacity in the plateau region, (c) Coulombic efficiency, and (d) average discharge voltage of Li[Li0.2Ni0.2−x/2Mn0.6−x/2Cox]O2 (0 ≤ x ≤ 0.24) during the first cycle.

Table 2. Amount of Lithium Extracted and Oxygen Lost during the First Charge in Li[Li0.2Ni0.2−x/2Mn0.6−x/2Cox]O2 amount of Li extracted (mol Li)

oxidation state at the end of sloping region

amount of oxygen vacancies (mol vacancies)

x

total

slope

plateau

Ni

Co

created

eliminated

remaining

0 0.08 0.13 0.16 0.20 0.24

0.972 1.025 1.059 1.079 1.105 1.115

0.305 0.325 0.335 0.338 0.329 0.313

0.667 0.710 0.724 0.759 0.776 0.802

3.53 3.73 3.98 4.00 4.00 4.00

3.60 3.60 3.61 3.64 3.64

0.286 0.333 0.361 0.380 0.388 0.401

0.283 0.306 0.290 0.292 0.264 0.262

0.003 0.027 0.071 0.088 0.124 0.139

Figure 5. dQ/dV curves of Li[Li0.2Ni0.2−x/2Mn0.6−x/2Cox]O2 (0 ≤ x ≤ 0.24) for the (a) first charge and (b) first discharge cycles.

∼4.45 V for all the samples, while the spike length obviously does decrease with increasing Co3+ substitution, as shown in Figure 6b. This indicates that the negative shift in the oxygen loss peak is attributed to an improvement in the electrochemical kinetics. Additionally, as seen in Figure 5b, the reduction reactions of Ni2+/4+ and Co3+/4+ at ∼3.7 and ∼4.5 V slightly weaken in intensity, while the reduction reaction of Mn3+/4+ is greatly enhanced by increasing Co3+ substitution. Obviously, the growth of the low-voltage Mn reduction is the

origin of the reduction in average discharge voltage with increasing Co3+ substitution seen in Figure 4d. Effect of Co3+ Substitution after the First Cycle. After the first charge, LLOs exhibit a charge profile without an oxygen loss plateau at 4.5 V and a discharge curve with an extended sloping region below 3.7 V caused by the redox reaction of Mn3+/4+, as shown in Figure 7. The Mn reduction peak increases during extended cycling. Figure 8a presents the cyclability data of these materials, and although they have only run for 30 cycles, a gradual degradation of capacity can clearly 21829

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Figure 6. GITT curves of Li[Li0.2Ni0.2−x/2Mn0.6−x/2Cox]O2 (0 ≤ x ≤ 0.24) in the (a) sloping and (b) plateau regions of charging.

Figure 7. Charge/discharge curves of Li[Li0.2Ni0.2−x/2Mn0.6−x/2Cox]O2 (0 ≤ x ≤ 0.24) at various cycles.

changes are occurring during cycling. The reduction peak intensities of the Ni2+/4+ and Co3+/4+ redox couples at ∼3.7 and ∼4.5 V are weakening, while the intensity of the Mn3+/4+ redox peaks at ∼3.0 V appears constant when comparing the area under the curve. Hence, the capacity fade is mainly attributed to the diminishing Ni2+/4+ and Co3+/4+ redox reactions, possibly due to their dissolution or inactivation due to structural changes. The voltage decay can be explained as the negative shift of the redox potentials and the decrease in the redox reactions at higher potentials. With increasing degree of Co3+ substitution, the intensity reduction and potential shift of the Ni2+/4+ and Co3+/4+ couples are markedly enhanced. Simultaneously, the negative shift of the Mn3+/4+ redox peaks is accelerated, suggesting that Co3+ substitution enhances the

be seen with these samples. Obviously, the capacity fade of Li[Li0.2Ni0.2−x/2Mn0.6−x/2Cox]O2 increases with increasing degree of Co3+ substitution, as seen in Figure 8b. As seen in Figure 8c,d, the average discharge voltages of these materials decay almost linearly upon extended cycling, and the voltage decay is enhanced with increasing Co3+ substitution. For instance, Li[Li0.2Ni0.2−x/2Mn0.6−x/2Cox]O2 (x = 0) possesses a voltage decay of 0.16 V, while Li[Li0.2Ni0.2−x/2Mn0.6−x/2Cox]O2 (x = 0.24) exhibits a voltage decay of 0.31 V in 30 cycles. In order to better elucidate the source of capacity fade and voltage decay, dQ/dV curves at different cycle numbers were generated (Figure 9) to track the changes in the redox reactions during extended cycling. Clearly, the redox potentials of the Ni2+/4+, Co3+/4+, and Mn3+/4+ couples are shifting to lower voltages at higher cycle numbers, indicating that structural 21830

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Figure 8. Plots of the (a) discharge capacity, (b) faded discharge capacity, (c) average discharge voltage, and (d) decayed discharge voltage values after 30 cycles in Li[Li0.2Ni0.2−x/2Mn0.6−x/2Cox]O2 (0 ≤ x ≤ 0.24).

Figure 9. dQ/dV curves of Li[Li0.2Ni0.2−x/2Mn0.6−x/2Cox]O2 (0 ≤ x ≤ 0.24) at various cycles.

the number of oxygen vacancies and the binding energy of the TM−O bonds. According to the electrochemical data collected in this study, the oxygen vacancies are not completely eliminated at the end of the first charge. Because of the continuing presence of oxygen vacancies, the TM ions could migrate continually upon extended cycling. Furthermore, the increased plateau-region capacity brought about by increased Co3+ doping causes more Li ions to migrate from the TM layer to the Li layer during deintercalation, which promotes the

structural transformation upon extended cycling due to the increased oxygen loss plateau in the first cycle. Mechanism of Capacity Fade and Voltage Decay. After the first cycle oxygen loss, the stable octahedral coordination of the TM ions may be destroyed due to the presence of oxygen vacancies. The TM ions could then stabilize by migrating through the neighboring tetrahedral sites into the octahedralsite Li vacancies in the Li layer, which has been demonstrated in previous reports.19,20,36 Such a migration could be affected by 21831

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(6) Kannan, A. M.; Rabenberg, L.; Manthiram, A. High Capacity Surface-Modified LiCoO2 Cathodes for Lithium-Ion Batteries. Electrochem. Solid-State Lett. 2003, 6, A16−A18. (7) Manthiram, A. Materials Challenges and Opportunities of Lithium Ion Batteries. J. Phys. Chem. Lett. 2011, 2, 176−184. (8) Thackeray, M. M.; Kang, S.-H.; Johnson, C. S.; Vaughey, J. T.; Benedek, R.; Hackney, S. A. Li2MnO3-Stabilized LiMO2 (M = Mn, Ni, Co) Electrodes for Lithium-Ion Batteries. J. Mater. Chem. 2007, 17, 3112−3125. (9) Yu, H.; Zhou, H. High-Energy Cathode Materials (Li2MnO3− LiMO2) for Lithium-Ion Batteries. J. Phys. Chem. Lett. 2013, 4, 1268− 1280. (10) Arinkumar, T. A.; Wu, Y.; Manthiram, A. Factors Influencing the Irreversible Oxygen Loss and Reversible Capacity in Layered Li[Li1/3 Mn2/3 ]O2−Li[M]O2 (M = Mn(0.5−y) Ni (0.5−y) Co2y ) and Ni1−yCoy) Solid Solutions. Chem. Mater. 2007, 19, 3067−3073. (11) Wu, Y.; Manthiram, A. Effect of Surface Modifications on the Layered Solid Solution Cathodes (1 − z) Li[Li1/3Mn2/3]O2−(z) Li[Mn0.5−yNi0.5−yCo2y]O2. Solid State Ionics 2009, 180, 50−56. (12) Wang, C.-C.; Manthiram, A. Influence of Cationic Substitutions on the First Charge and Reversible Capacities of Lithium-Rich Layered Oxide Cathodes. J. Mater. Chem. A 2013, 1, 10209−10217. (13) Wang, C.-C.; Jarvis, K. A.; Ferreira, P. J.; Manthiram, A. Effect of Synthesis Conditions on the First Charge and Reversible Capacities of Lithium-Rich Layered Oxide Cathodes. Chem. Mater. 2013, 25, 3267− 3275. (14) Fell, C. R.; Carroll, K. J.; Chi, M.; Meng, Y. S. Synthesis− Structure−Property Relations in Layered, “Li-Excess” Oxides Electrode Materials Li[Li1/3−2x/3NixMn2/3−x/3]O2 (x = 1/3, 1/4, and 1/5). J. Electrochem. Soc. 2010, 157, A1202−A1211. (15) Lu, Z.; Beaulieu, L. Y.; Donaberger, R. A.; Thomas, C. L.; Dahn, J. R. Synthesis, Structure, and Electrochemical Behavior of Li[NixLi1/3−2x/3Mn2/3−x/3]O2. J. Electrochem. Soc. 2002, 149, A778− A791. (16) Xiang, X.; Li, X.; Li, W. Preparation and Characterization of Size-Uniform Li[Li0.131Ni0.304Mn0.565]O2 Particles as Cathode Materials for High Energy Lithium Ion Battery. J. Power Sources 2013, 230, 89− 95. (17) Johnson, C. S.; Li, N.; Lefief, C.; Thackeray, M. M. Anomalous Capacity and Cycling Stability of xLi2MnO3·(1 − x)LiMO2 Electrodes (M = Mn, Ni, Co) in Lithium Batteries at 50 °C. Electrochem. Commun. 2007, 9, 787−795. (18) Wang, Y.; Bie, X.; Nikolowski, K.; Ehrenberg, H.; Du, F.; Hinterstein, M.; Wang, C.; Chen, G.; Wei, Y. Relationships between Structural Changes and Electrochemical Kinetics of Li-Excess Li1.13Ni0.3Mn0.57O2 during the First Charge. J. Phys. Chem. C 2013, 117, 3279−3286. (19) Armstrong, A. R.; Holzapfel, M.; Novak, P.; Johnson, C. S.; Kang, S. H.; Thackeray, M. M.; Bruce, P. G. Demonstrating Oxygen Loss and Associated Structural Reorganization in the Lithium Battery Cathode Li[Ni0.2Li0.2Mn0.6]O2. J. Am. Chem. Soc. 2006, 128, 8694− 8698. (20) Yabuuchi, N.; Yoshii, K.; Myung, S. T.; Nakai, I.; Komaba, S. Detailed Studies of a High-Capacity Electrode Material for Rechargeable Batteries, Li2MnO3−LiCo1/3Ni1/3Mn1/3O2. J. Am. Chem. Soc. 2011, 133, 4404−4419. (21) Hong, J.; Lim, H. D.; Lee, M.; Kim, S. W.; Kim, H.; Oh, S. T.; Chung, G. C.; Kang, K. Critical Role of Oxygen Evolved from Layered Li-Excess Metal Oxides in Lithium Rechargeable Batteries. Chem. Mater. 2012, 24, 2692−2697. (22) Yu, H. J.; Zhou, H. S. Initial Coulombic Efficiency Improvement of the Li1.2Mn0.567Ni0.166Co0.067O2 Lithium-Rich Material by Ruthenium Substitution for Manganese. J. Mater. Chem. 2012, 22, 15507− 15510. (23) Deng, Z. Q.; Manthiram, A. Influence of Cationic Substitutions on the Oxygen Loss and Reversible Capacity of Lithium-Rich Layered Oxide Cathodes. J. Phys. Chem. C 2011, 115, 7097−7103.

layered to spinel-like phase transformation by allowing for more dumbbell defects to form.32 Additionally, Song et al.31 have reported the presence of Ni4+ and Co4+ in their discharged samples, suggesting electrochemical deactivation of Ni and Co ions. Moreover, because of the decrease in the amount of the reduction of Ni and Co ions during discharge, more Mn ions may be reduced to balance the charge, resulting in a further lowering of the average oxidation state of Mn ions. When the average oxidation state of Mn is lower than 3.5+, Jahn−Teller distortion could occur,37,38 thus leading to capacity fade. Hence, the capacity fade and voltage decay can be attributed to deactivation of Ni and Co ions at higher voltages and further activation of Mn ions at lower voltages. Additionally, the capacity fade and voltage decay may also be associated with the dissolution of TM ions at higher operating voltages.31,39

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CONCLUSIONS The effects of substituting Co3+ for equal amounts of Mn4+ and Ni2+ in the LLO series Li[Li0.2Ni0.2−x/2Mn0.6−x/2Cox]O2 have been investigated systematically. The oxygen loss from the lattice is a kinetically slow process, which is enhanced by Co3+ substitution due to the increased metal−oxygen covalence and electron delocalization. Although higher reversible capacities can be achieved with increasing Co3+ substitution due to the increased oxygen loss, the average discharge voltage decreases during cycling because of the increased conversion to the spinel-like phase. Because of this enhanced phase conversion due to severe migration of Li and TM ions caused by the lengthened plateau region, the capacity fade and voltage decay upon extended cycling are severely intensified with Co3+ substitution. The enhanced migration is related to the increase in Li movement and the increased presence of oxygen vacancies.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: (512) 471-1791. Fax: (512) 471-7681 (A.M.). Notes

The authors declare no competing financial interest.

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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. X.X. was supported as a visiting scholar by the National Natural Science Foundation in China (Grant No. 21273084).

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