Origin of Site Disorder and Oxygen Nonstoichiometry in LiMn1.5Ni0.5

30 May 2013 - ABSTRACT: The high-voltage LiMn1.5Ni0.5O4 spinel cathodes offer high power capability but are plagued by capacity fade, particularly at ...
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Origin of Site Disorder and Oxygen Nonstoichiometry in LiMn1.5Ni0.5−xMxO4 (M = Cu and Zn) Cathodes with Divalent Dopant Ions Katharine R. Chemelewski and Arumugam Manthiram* Materials Science and Engineering Program, The University of Texas at Austin, Austin, Texas 78712, United States S Supporting Information *

ABSTRACT: The high-voltage LiMn1.5Ni0.5O4 spinel cathodes offer high power capability but are plagued by capacity fade, particularly at elevated temperatures. With an aim to develop a better understanding of the factors influencing the electrochemical properties, a systematic investigation of LiMn1.5Ni0.5−xMxO4 (M = Cu and Zn and x = 0.08 and 0.12), in which Ni2+ ions are substituted by divalent Cu2+ and Zn2+ ions, are presented. The effects of dopant ion content, their site occupancy, degree of cation ordering, and oxygen nonstoichiometry have been investigated with respect to the electrochemical properties by cycling at room temperature and elevated temperatures. It is found that although both Zn and Cu are divalent with ionic radii similar to those of Ni2+, they behave quite differently with respect to cation ordering and site occupancy, and higher levels of doping lead to distinct differences in cycling and rate performances. The understanding of the role of site preferences of the cations and the degree of cation ordering could help to develop high-performance high-voltage spinel cathode compositions.

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INTRODUCTION

As global energy demand increases, researchers continue to explore solutions to large-scale energy storage. An appealing candidate for next-generation lithium-ion batteries is the highvoltage spinel cathode LiMn1.5Ni0.5O4 because it provides facile, 3-D lithium-ion diffusion with excellent structural stability and high power capability.1−3 However, its electrochemical properties are sensitive to synthesis conditions,4−8 and it is prone to severe capacity fade, particularly at elevated temperatures.9,10 Additionally, there is ample evidence that ordering between the Mn4+ and Ni2+ ions decreases the electronic conductivity and increases internal resistance because active Ni2+/3+/4+ redox centers are isolated amidst inactive Mn4+ ions.11−13 One strategy that has been successfully employed to combat this issue has been to partially substitute other metal ions for nickel, which has been shown to disrupt cation ordering and stabilize the cycling performance.12 Whereas much effort has been devoted to understanding the influence of doping trivalent ions such as Fe, Co, and Ga,12−20 relatively little investigation has been carried out on the doping of divalent ions such as Zn2+ and Cu2+. In this work, we present a detailed study of the structure and electrochemical properties of the undoped LiMn1.5Ni0.5O4 and the divalent-ion doped LiMn1.5Ni0.42M0.08O4 and LiMn1.5Ni0.38M0.12O4 (M = Cu and Zn) with two levels of doping (0.08 and 0.12). In particular, the role of site disorder and oxygen content on elevatedtemperature cycling performance is elucidated. © XXXX American Chemical Society

EXPERIMENTAL SECTION

Material Preparation. The spinel cathode materials were synthesized by a modified coprecipitation reaction in a continuously stirred tank reactor (CSTR). For the undoped sample, a 2 M mixed metal solution was prepared by dissolving stoichiometric amounts of MnSO4·H2O (ACROS) and NiSO4·6H2O (Alfa Aesar) in deionized water. For Zn- and Cu-doped samples, the appropriate amount of Zn(NO3)2·6H2O (ACROS) and Cu(NO3)2·H2O (ACROS) was substituted for nickel sulfate in the mixed-metal solution. A separate 2 M basic solution of NaOH (Fisher) containing 0.05 M NH4OH (Fisher) was also prepared. These two solutions were then added at a constant feed rate over 12 h to a continuously stirred reaction vessel maintained at a constant temperature of 60 °C and pH of 10. Nitrogen was sparged through the reaction mixture for the duration of the synthesis to prevent oxidation of the transition-metal ions during the reaction. The resulting mixed-metal hydroxide precipitate was filtered and washed with deionized water until the filtrate reached a neutral pH. The hydroxide was then dried in an air oven at 100 °C overnight. The metal content in the hydroxide was determined by thermogravimetric analysis (TGA). An appropriate amount of LiOH·H2O (Alfa Aesar) was then ground with the mixed-metal hydroxide precursor and heated to 900 °C for 15 h with a cooling rate of 1 °C/min to produce Received: May 6, 2013 Revised: May 24, 2013

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Figure 1. (a) X-ray diffraction patterns of LiMn1.5Ni0.5−xMxO4 (M = Ni, Zn, and Cu and x = 0, 0.08, and 0.12) and (b) expanded 2θ region of the pattern.

the final spinel sample. A portion of the final material was postannealed at 700 °C for 48 h with a cooling rate of 1 °C/ min to improve cation ordering. Characterization. The crystal chemistry of the cathode powders was characterized by X-ray diffraction (XRD). The lattice parameters were determined by FullProf Suites Rietveld refinement. Compositional characterization was carried out with inductively coupled plasma (ICP) analysis. Oxygen content was determined by a redox titration. The sample was dissolved in a 0.05 N solution of sodium oxalate, whereby all transition metals present were reduced to the 2+ oxidation state. The resulting solution was titrated with a 0.05 N solution of potassium permanganate, and the oxygen content was calculated based on the amount of oxalate consumed by the sample using the charge neutrality principle. Electrochemical testing was carried out with CR 2023 coin cells with lithium metal counter electrodes. Cathode slurries were prepared by mixing the active material, conductive carbon, and polyvinyledene fluoride (PVDF) in an 80:10:10 wt ratio in 1-methyl, 3-pyrrolidone (NMP) solvent. These slurries were then cast onto an aluminum foil current collector and dried in an air oven at 110 °C. Cathode disks were punched out of this film with a uniform area of 1.2 cm2. The cells were fabricated with Celgard separators, Li anodes, and nickel foam negative current collectors. The electrolyte was a 1 M solution of LiPF6 dissolved in a 1:1 vol ratio of ethylene carbonate and diethyl carbonate.

were collected and carefully analyzed for rocksalt impurity, ordered superstructure reflections, and cation disorder, as seen in Figure 1. As has been discussed in previous reports,1−3,7 the undoped sample shows evidence of a nickel-rich rocksalt impurity phase when prepared at 900 °C, and the main spinel phase has the Fd-3m space group. Annealing the undoped sample at 700 °C causes the Mn4+ and Ni2+ ions in the lattice to order, leading to a decrease in symmetry and transition to the P4332 space group.1−3 This corresponds to an increase in the solubility of nickel in the spinel phase and eliminates the rocksalt impurity. Interestingly, the Cu-doped sample LiMn1.5Ni0.42Cu0.08O4 shows evidence of a high degree of cation ordering with prominent P4 332 superstructure reflections even when prepared at 900 °C. This behavior can be explained by considering the ionic radii of the cations in the structure. Because six-coordinated Cu2+ has an ionic radius (0.86 nm) similar to that of Ni2+ (0.83 nm),21 the Cu2+ ions substitute readily for Ni2+ in the larger 4a sites of the P4332 structure. With an extended annealing time at 700 °C, the cations tend to order and occupy the lowest-energy positions to minimize strain resulting from the size difference between the smaller Mn4+ and larger Ni2+ ions. Because the Cu2+ ion is even larger than Ni2+, the substitution of Cu2+ provides increased incentive for the formation of the P4332 structure. In contrast, the Zn-doped sample LiMn1.5Ni0.42Zn0.08O4 does not show any sign of cation ordering or rocksalt impurity. Instead, an unusually high intensity is observed for the reflection at 2θ ≈ 31°, as seen in Figure 1b. From the Rietveld refinement, the higher intensity is attributed to the occupancy of the tetrahedral sites by the Zn2+ ions, which is consistent with the strong preference of Zn2+ ions with a 3d10configuration

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RESULTS AND DISCUSSION Careful characterization of the crystal chemistry is critical to determine the cation site occupancy behavior in the undoped and divalent-cation-doped samples. Slow-scan XRD patterns B

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samples was unanticipated based on their marked similarities in terms of ionic radius and valence. This discrepancy can be traced to the respective site preferences of the ions and supports the theory that Cu2+ substitutes directly for Ni2+ in the 4a sites while Zn2+ occupies the tetrahedral sites, inducing site disorder and possible oxygen nonstoichiometry. In fact, a closer examination of the charge/discharge profiles illuminates the site disorder by virtue of the presence or absence of an Mn3+ plateau at 4 V. In theory, because Zn2+ and Cu2+ are divalent and doping disrupts the formation of a rocksalt impurity phase, there should not be evidence of an Mn3+ plateau at 4 V. Figure 3 shows the first charge/discharge

for tetrahedral coordination. Rietveld refinement reveals that the best fit is achieved when the Zn2+ ions occupy the tetrahedral sites. The details of this fitting analysis can be seen in the Supporting Information Table S1. This analysis is supported by previous findings that the Zn2+ ions tend to occupy the tetrahedral sites in the 4 V manganese oxide spinels.22−24 Surprisingly, the samples with higher levels of doping LiMn1.5Ni0.38M0.12O4 do not show any sign of cation ordering behavior, even in the case of the Cu-rich sample. In fact, the samples appear further disordered. Additionally, no trace of rocksalt phase can be detected in these samples. These results suggest that higher levels of doping with larger divalent cations may increase the internal lattice strain such that it can no longer support an ordered structure. To compare the relative level of cation ordering in a spinel material, it is useful to operate the cathode outside the usual voltage window of 5.0−3.0 V; for instance, operating at 5.0−2.0 V gives rise to two distinct plateaus at ∼2.7 and ∼2.0 V. As discussed in previous reports,7,11,19 the length of the plateau at ∼2.7 V is governed by the lithium insertion reaction into the 16c octahedral sites, which are larger when the sample is ordered, resulting in lower lattice strain and a longer 2.7 V plateau. The cation ordering tendencies of all samples are compared in Figure 2. The lower ∼2.0 V plateau of the

Figure 3. Voltage profiles of the first charge/discharge cycles of (a) Zn-doped samples and (b) Cu-doped samples.

process for the Zn- and Cu-doped samples prepared at 900 °C and annealed at 700 °C. The low-doped Cu sample shows a trace amount of Mn3+ shoulder at 4.0 V, which is eliminated after annealing, much like an ordered undoped sample. Also, present in the Cu-doped voltage profile is a capacity shoulder at ∼4.3 V, corresponding to the Cu2+/3+ redox couple, consistent with the previous reports.25−27 In contrast, both the Zn-doped sample prepared at 900 °C and the sample postannealed at 700 °C show evidence of a sizable plateau at 4 V, as seen in Figure 3a. Assuming full oxygen occupancy, the presence of Mn3+ is not consistent with the X-ray data, which confirms that there is no rocksalt impurity present in the samples. These data indicate that there may be oxygen nonstoichiometry present in the samples. Similarly, small peaks are observed in the XRD patterns of the Cu-rich samples at 31°, indicative of tetrahedral site disorder. While Cu is expected to exist in the 2+ valence state, with a large excess of copper and high-temperature firing at 900 °C, it is possible to form a small amount of Cu+, which may occupy the 8a

Figure 2. Discharge behavior below 2 V to compare the degree of cation ordering based on the ratio of the plateau lengths at ∼2.7 and ∼2.0 V.

undoped sample decreases after annealing at 700 °C, indicating an increase in the degree of cation ordering. This result is consistent with the expected outcome of the annealing treatment. Additionally, the Cu-doped sample prepared at 900 °C begins with a high degree of cation order, which increases further after annealing at 700 °C. This result is supported by the presence of the superstructure reflections in Figure 1b. However, both of the Zn-doped samples do not change after annealing and remain highly disordered. For ease of comparison, the plateau lengths at different voltage regions are tabulated in Table S2 in the Supporting Information. The change in cation ordering for the undoped sample has been demonstrated before,1−3,7,11,16 but the disparity in ordering behavior between the Zn-doped and Cu-doped C

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tetrahedral sites by displacing a corresponding amount of Li+ into the 16d octahedral sites. This coupled with the presence of a redox shoulder at ∼4 V suggests the formation of Mn3+ in these samples. To confirm this hypothesis, redox titration was performed, as described in the Experimental Section. By quantifying the amount of oxygen nonstoichiometry through titration, it was possible to compare the result to the oxygen content expected based on the amount of Mn3+ present in the electrochemical charge/discharge curves. The content of oxygen per formula unit determined by these two techniques is shown in Figure 4a.

the Cu-doped sample (Figure 5c), the Cu2+ ion substitutes directly for nickel in the 4a site. In contrast, as seen in Figure 5b, Zn2+ ions occupy the 8a tetrahedral sites displacing equal amounts of Li+ ions into the 16d octahedral sites, and the larger Zn2+ ions induces a nearby oxygen vacancy due to the compressive strain on the metal−oxygen bonds near large ions.28 Additionally, the presence of Li+ ions in the 16d octahedral sites promotes cation disorder between Mn4+ and Ni2+. With a firm understanding of the structural aspects of these materials, the effects on the electrochemical cycling performance may be analyzed. The room-temperature cycle life and rate capability of the Zn-doped samples can be seen in Figure 6. By

Figure 4. Correlation of the (a) measured oxygen nonstoichiometry from electrochemical capacity data and redox titration and (b) variation of lattice parameters with Mn3+ content obtained from electrochemical data.

From Figure 4a it can be concluded that the amount of oxygen nonstoichiometry has a direct relationship with the Mn3+ content, but these factors also influence the unit cell lattice parameter, as seen in Figure 4b. The lattice parameter determined from Rietveld refinement becomes correspondingly larger with increasing oxygen nonstoichiometry due to the reduction of smaller Mn4+ to larger Mn3+ ions in the lattice. In addition, the lattice parameter of the low-doped Cu samples is affected by the cation ordering tendency, causing a contraction in cell volume to minimize the overall lattice strain, accompanied by a reduction in the number of Mn3+ ions present. Atomic structural models of these proposed structures are presented in Figure 5. The ordered undoped sample (Figure 5a) shows the arrangement of Mn4+ and Ni2+ ions in, respectively, the 12d and 4a octahedral sites. In the case of

Figure 6. Electrochemical properties of Zn-doped samples: (a) longterm cycle performance at C/2 rate and (b) discharge at various C rates.

Figure 5. Schematic illustration of (a) ordered LiMn1.5Ni0.5O4 structure, (b) ordered LiMn1.5Ni0.42Cu0.08O4 structure, and (c) disordered LiMn1.5Ni0.42Zn0.08O4 structure. D

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a cursory inspection, it is immediately clear that the samples with a high level of doping exhibit steep capacity fade and perform poorly under high-rate discharge conditions. This could be due to fewer nickel ions available for redox reaction in the structure, along with an increased internal resistance resulting from the significant amount of inactive Li+ ions present in the octahedral sites. The low-doped sample shows excellent electrochemical performance, which can be largely attributed to the stabilization of the disordered Fd-3m phase and the presence of Mn3+ carriers to enhance the electrical conductivity.13 The room-temperature electrochemical properties of the Cudoped samples are compared in Figure 7. The low-doped

Figure 8. Elevated temperature cycling at 55 °C with C/2 rate: (a) Zndoped samples and (b) Cu-doped samples.

ble samples prepared at 900 °C. For the low-doped Zn sample, this behavior can be attributed to the fact that the annealed sample shows increased oxygen nonstoichiometry from the titration results; indeed, a higher level of oxygen deficiency has been correlated to improved cyclability in other studies.30−32 This supports the conclusion that small amounts of Mn3+ and oxygen deficiency enhance cycle stability and especially high rate performance. In the case of the Cu-doped samples, because the copper ions are electrochemically active in this window, the replacement of Ni 2+ by Cu 2+ enhances the electrical conductivity, much in the same way that Mn3+ has been linked to enhanced high rate performance. Also, it helps stabilize the long-term cycling performance at elevated temperatures by mildly suppressing cation disorder and strengthening the MO6 framework. This could explain the sharp capacity fade in the highly disordered Zn-rich sample, which does not have the edge-shared octahedral framework with all transition-metal ions as the Cu-doped samples have.33 This discussion would be remiss without considering the effects of particle morphology. Recent studies suggest that the crystallographic planes on the surface of the particle have a profound effect on the electrochemical performance and should be considered in the analysis of high-voltage spinel cathodes.7,34,35 The morphologies of the divalent doped samples can be seen in Figure 9. As previously reported, octahedral morphology features a majority of {111} family of planes on the surface and contributes significantly to fast, stable electrochemical performance. Along those lines, the samples

Figure 7. Electrochemical properties of Cu-doped samples: (a) longterm cycle performance at C/2 rate and (b) discharge at various C rates.

sample shows capacity fade and sluggish performance at high current rates. This result has generally been attributed to poor electronic conductivity with high degrees of cation ordering as nickel ions do not have nickel nearest-neighbors in the structure to facilitate electron transfer.29 Unexpectedly, the Cu-doped samples with high levels of doping show improved electrochemical properties over those with lower doping. As seen in Figure 3, the Cu2+/3+ redox couple is active in this voltage range (∼4.3 V), and the excess copper in the structure provides additional capacity to enhance cycling performance. Additionally, the high level of doping induces cation disorder, which further improves long-term stability. Similarly, the elevated-temperature cycling characteristics can be seen in Figure 8. Once again, it is noteworthy that with the exception of the Zn-rich sample, all doped annealed samples exhibit better electrochemical performance than the comparaE

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complex, which is known to have a square-planar coordination.36 This is distinct from the octahedral and tetrahedral metal−ligand complexes formed in solution with ammonia by all other cations studied here. The other driving force contributing to growth of plates is that with lower levels of doping, the Cu ions may not reduce to monovalent Cu+ upon firing at 900 °C (as seen in Figure 1), thereby stabilizing the unique planes on the surfaces and the plate-like morphology. In the plate-like morphology, the predominant crystallographic plane present on the surface was found to be (112) by TEM analysis (Figure S1 in the Supporting Information), which is consistent with the previous findings.34 However, the Zn-rich sample also forms in octahedral morphology, as seen in Figure 9c,d. This indicates that morphology alone does not dictate the electrochemical performance, and other factors such as site disorder and conductivity also contribute in a meaningful way.

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CONCLUSIONS

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ASSOCIATED CONTENT

Zn- and Cu-substituted high-voltage spinel cathode materials were synthesized via a modified coprecipitation route. It was found that low levels of Zn2+ doping effectively disrupt the ordering tendency between Mn4+ and Ni2+ ions as the Zn2+ ions occupy the 8a tetrahedral sites, displacing equal amounts of Li+ ions into the 16d octahedral sites and introducing slight oxygen nonstoichiometry. However, higher levels of Zn doping causes rapid capacity fade due to the decreasing active Ni redox centers and significant amount of inactive Li+ ions in the octahedral sites, despite the octahedral particle morphology. Substitution of low levels of Cu2+ ions facilitates cation ordering with direct replacement of Ni2+ by Cu2+, but the plate-like morphology likely contributes to capacity fade and poor high rate performance. By substituting higher amounts of Cu2+, the cation ordering tendency is destroyed, and octahedral morphology is achieved, thereby providing conditions for excellent capacity retention. These results indicate that cation disorder coupled to small quantities of carriers such as Mn3+/4+ and Cu2+/3+ improve the electrochemical properties by increasing the electronic conductivity. Additionally, particle morphology plays an important role in the cycling behavior but cannot compensate for other factors such as poor conductivity and structural instability, as in the case of the Zn-rich sample. Figure 9. SEM of (a) LiMn1.5Ni0.42Zn0.08O4 prepared at 900 °C and (b) annealed at 700 °C, (c) LiMn1.5Ni0.38Zn0.12O4 prepared at 900 °C and (d) annealed at 700 °C, (e) LiMn1.5Ni0.42Cu0.08O4 prepared at 900 °C and (f) annealed at 700 °C, and (g) LiMn1.5Ni0.38Cu0.12O4 prepared at 900 °C and (h) annealed at 700 °C.

S Supporting Information *

Supporting Information is available free of charge via the Internet at http://pubs.acs.org.

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showing the best electrochemical properties (i.e., the LiMn1.5Ni0.42Zn0.08O4 and LiMn1.5Ni0.38Cu0.12O4 samples) have octahedral morphology when prepared at 900 °C, and the morphology is retained after annealing at 700 °C. Additionally, the low-doped Cu sample does not have octahedral morphology, as seen in Figure 9e,f. This could contribute to capacity fade and explain why it shows poor cycle stability compared with the Cu-rich sample, which has octahedral morphology (Figure 9g,h). The formation of platelike morphology with low levels of Cu doping can be attributed to two main factors. The first can be generalized to any level of Cu doping in that the wet-chemical synthesis technique employed in this investigation forms a soluble [Cu(NH3)4]2+

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: (512) 471-1791. Fax: 512-471-7681. 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 No. DEAC02-05CH11231 and Welch Foundation grant F-1254. F

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