Factors Influencing the Electrochemical Properties of High-Voltage

Jun 25, 2013 - Ming Xu , Linfeng Fei , Wei Lu , Zhaoyong Chen , Tao Li , Yan Liu , Guanyin Gao , Yanqing Lai , Zhian Zhang , Peng Wang , Haitao Huang...
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Factors Influencing the Electrochemical Properties of High-Voltage Spinel Cathodes: Relative Impact of Morphology and Cation Ordering Katharine R. Chemelewski, Eun-Sung Lee, Wei Li, and Arumugam Manthiram* Materials Science and Engineering Program, The University of Texas at Austin, Austin, Texas 78712, United States S Supporting Information *

ABSTRACT: In order to achieve consistent electrochemical properties essential for the commercialization of the high-voltage spinel cathode LiMn1.5Ni0.5O4, a deeper fundamental understanding of the factors contributing to capacity fade is required. Specifically, the relationship between cation ordering, impurity phases present, and particle morphology must be elucidated. We present here a comparison of stoichiometric LiMn1.5Ni0.5O4 cathodes with a 3:1 Mn/Ni ratio prepared by different methods with varying morphologies and degrees of cation ordering. Careful structural, chemical, and electrochemical characterizations illuminate the relative influence of the various factors on the electrochemical cycling stability and high-rate performance. It is found that although an increase in the degree of cation ordering decreases the rate capability, the crystallographic planes in contact with the electrolyte have a dominant effect on the electrochemical properties. KEYWORDS: lithium-ion batteries, spinel oxides, morphology, crystal chemistry, cation ordering

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prolonged annealing at 700 °C, followed by a slow cooling rate, the Mn4+ and Ni2+ ions tend to order, resulting in the adoption of the P4332 space group, in which the Mn4+ ions occupy the 12d sites and the Ni2+ ions occupy the 4a sites. The synthesis technique, and especially the annealing conditions, strongly influence where a particular sample will fall on the spectrum of degree of cation ordering.8,14−16 While intensive characterization techniques such as neutron diffraction, nuclear magnetic resonance, magic angle spinning, and magnetic susceptibility have been successfully employed to compare the degree of cation ordering,17,18 there exists another simple yet effective method to compare the relative cation order. Our group proposed the strategy of harnessing the cubictetragonal transition occurring during operation below 3 V to the degree of cation ordering, allowing for direct observation of the lithium insertion reactions at ∼2.7 and 2.0 V.14 The length of the plateau observed at ∼2.7 V increases relative to the length of the plateau at ∼2.0 V with increasing cation ordering. This phenomenon can be attributed to an overall decrease in lattice strain during lithium-ion insertion when the Mn4+ and Ni2+ ions order, leading to an enlargement of the empty 16c octahedral sites and thereby allowing for facile lithium insertion below 3 V.19

INTRODUCTION Energy storage has become one of the greatest challenges to address the growing global demand for energy. Lithium-ion batteries are appealing in this regard, and the high-voltage spinel LiMn1.5Ni0.5O4 is an attractive candidate as a nextgeneration cathode for electric vehicles and grid storage of electricity generated by intermittent sources such as solar and wind energy.1 The spinel structure offers the advantage of a robust cubic close-packed oxygen array with 3-dimensional lithium-ion diffusion, allowing fast insertion/removal of lithium ions during discharge/charge processes.2 Additionally, this stoichiometry permits access to both the Ni2+/3+ and Ni3+/4+ redox couples, which are active in the high voltage range of ∼4.7 V; this provides an improvement over the currently used LiMn2O4 spinel cathode, which only utilizes the Mn3+/4+ couple and operates at a lower voltage of ∼4.0 V.3 However, this highvoltage operation comes at the expense of electrolyte stability, and aggressive side reactions resulting from the solid− electrolyte interaction degrade the active material and lead to irreversible capacity loss.4−6 Moreover, the electrochemical properties of the samples with identical chemical formulas vary widely,7−10 and the root cause of this variability has been contentiously debated. LiMn1.5Ni0.5O4 is known to have two different forms depending on the arrangement of Mn4+ and Ni2+ ions in the lattice.11−13 The disordered phase with the Fd-3m space group is characterized by a random distribution of the Mn4+ and Ni2+ ions in the 16d octahedral sites of the spinel lattice. After © 2013 American Chemical Society

Received: May 6, 2013 Revised: June 23, 2013 Published: June 25, 2013 2890

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sample was annealed at 700 °C for 96 h, followed by a slow cooling rate of 1 °C/min. The samples prepared by firing at 900 °C will be identified by the initial firing temperature, and the samples postannealed for 96 h at 700 °C will be identified by the annealing temperature. The crystal chemistry of the samples was characterized by X-ray diffraction with Cu Kα radiation. The lattice parameter values and the rock salt phase content were determined by the Rietveld refinement with FullProf Suites software. The compositions of the synthesized samples were verified by inductively coupled plasma (ICP) analysis. The morphology was characterized by a scanning transmission electron microscope (SEM) equipped with energy dispersive X-ray spectroscopy (EDX) capability (Hitachi) and transmission electron microscopy (TEM) (JEOL). Electrochemical testing was carried out in CR2032 cells with lithium metal counter electrodes. Cathode slurries were prepared by dispersing the active material, super P conductive carbon, and polyvinylidene difluoride (PVDF) with a wt ratio of 80:10:10 in a N-methyl pyrrolidone (NMP) solvent. This slurry was then cast onto an aluminum foil and dried overnight under vacuum at 100 °C. The resulting electrode was then punched into disks with an area of 1.2 cm2. The active material loading in the electrode was 5−7 mg/cm2. The cells were then assembled with Celgard separators and an electrolyte of 1 M LiPF6 salt dissolved in a 1:1 vol ratio of ethylene carbonate (EC)/diethyl carbonate (DEC). For cyclability testing, the cells were operated in a voltage window of 3.5−5.0 V under a current rate of C/2 based on the specific capacity of the samples. For rate capability testing, the cells were operated in a voltage window of 3.0− 5.0 V with 10 cycles at each of the following current rates: C/5, C/2, 1C, 5C, 10C, and C/10.

Another consideration for analyzing electrochemical properties is the formation of a rock salt impurity phase.3,17,20 This phase is rich in nickel, prompting a partial reduction of Mn4+ to Mn3+ in the main spinel phase, which has been theorized to enhance the electronic conductivity.21,22 Following the annealing treatment at 700 °C to increase the degree of cation ordering, the rock salt phase has been shown to assimilate into the main spinel phase, thereby decreasing the amount of Mn3+ in the spinel phase.23,24 This correlation indicates that rock salt impurity phase is closely linked to the level of cation ordering, further complicating the determination of which factor dominates the control of the electrochemical performances. An emerging finding is that the particle morphology, especially the surface crystallographic planes, influences the electrochemical properties.25−29 Computational studies have indicated that the low-index planes such as (100), (110), and (111) have different surface energies and are prone to formation of different solid−electrolyte interfacial (SEI) layers depending on the arrangement of surface atoms and dangling bonds.30,31 Indeed, recent experimental results have shown that the crystallographic planes present on the surface may have a profound effect on the cycling stability and high-rate performance.25,28,29 However, to present a valid comparison of these complex factors, it is critical to conduct careful characterization of the planes present, degree of cation ordering, and segregation of the rock salt phase. We present here a systematic study to elucidate the relationship among cation ordering, impurity phases, and morphology, and the relative impact of these factors on the electrochemical properties.

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RESULTS AND DISCUSSION The LiMn1.5Ni0.5O4 samples were given different designations (Cubic, Spherical, Octahedral, Truncated) based on the visual appearance of the primary particles. SEM images of the cathodes prepared at 900 °C can be seen in Figure 1a−d, with

EXPERIMENTAL SECTION

The LiMn1.5Ni0.5O4 cathodes were synthesized by four different routes to produce four distinct morphologies. The sample designated here as “Cubic” was synthesized by a hydrothermal process. Stoichiometric amounts of MnSO4·H2O and NiSO4·6H2O were dissolved in deionized water with cetyl trimethylammonium bromide and urea additives and sealed in a hydrothermal autoclave vessel. The mixture was then heated at 160 °C for 12 h with a heating rate of 3 °C/min and a cooling rate of 3 °C/min. The solid product formed was collected and washed by repeated centrifugation and decanting until the filtrate reached a neutral pH. The powder was then dried in an air oven overnight at 100 °C. The resulting mixed metal carbonate was ground with a required amount of LiOH·H2O and fired at 900 °C with a heating rate of 3 °C/min and a cooling rate of 5 °C/min to produce the final spinel material. The sample designated here as “Spherical” was synthesized by a continuously stirred tank reactor (CSTR) method. A 2 M mixed-metal solution containing required amounts of MnSO 4 ·H 2 O and NiSO4·6H2O was prepared. Separately, a 2 M solution of Na2CO3 was prepared containing 0.05 M NH4OH additive. The mixed-metal and carbonate solutions were added simultaneously by peristaltic pumps into the tank reactor apparatus over a 12 h reaction period. The vessel temperature was maintained at a temperature of 60 °C, and the pH was maintained at 8. The resulting mixed-metal carbonate precipitate was washed with deionized water and filtered until the filtrate reached a neutral pH. The powder was then dried in an air oven overnight at 100 °C. The final spinel material was obtained by grinding the mixed-metal carbonate precursor with a required amount of LiOH· H2O and fired at 900 °C with a heating rate of 3 °C/min and a cooling rate of 5 °C/min. The CSTR process for synthesizing the sample designated here as “Octahedral” has been described in detail in a previous report.28 Similarly, the co-precipitation procedure employed for the synthesis of the sample designated as “Truncated” has been previously outlined.14 In order to facilitate maximum cation ordering, a portion of each

Figure 1. SEM images of the morphology of the (a) Cubic, (b) Octahedral, (c) Truncated, and (d) Spherical samples prepared at 900 °C, with the magnified images for surface detail of the (e) Truncated and (f) Spherical samples.

their designations indicated. Magnified insets are included in Figure 1e,f for surface detail of the Truncated and Spherical samples. The Cubic sample comprises a collection of pseudosquare planes growing parallel to one another along the [112] direction, as will be discussed below. It also features many small, irregular surface agglomerations. The Octahedral sample comprises regular, nonporous octahedra. The Truncated sample has the same basic shape as the Octahedral sample, but with large sections of the edges and vertices 2891

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chamfered. The primary particles of the Spherical sample are large, cauliflower-like spheres, but the magnified image shows that the surface is decorated with nanoscale octahedra on the order of 30 nm across one edge. After the annealing treatment at 700 °C, the morphologies of some samples are visibly altered, as seen in Figure 2. The Cubic

Figure 3. Schematic representation of the crystallographic planes present on the surface facets. Figure 2. SEM images of the morphology of the (a) Cubic, (b) Octahedral, (c) Truncated, and (d) Spherical samples after the annealing treatment at 700 °C, with magnified images for surface detail of the (e) Truncated and (f) Spherical samples.

plane. This plane also happens to be perpendicular to the (111) plane. These results indicate that the smooth facets of the cube structures consist of {112} planes, whereas the striated side facets consist of the {111} family of planes. For further reference, the various dhkl interplanar spacings for this structure are listed in Table S1 in the Supporting Information. Another interesting aspect observed on the Cubic sample is the small surface agglomerations, which have been theorized to be rock salt phase impurity particles.17 To confirm this theory, elemental line scans were performed with EDX, as seen in Figure 4e,f. The Cubic sample prepared at 900 °C (Figure 4e) shows distinct nickel-rich and manganese-poor compositions in the agglomerations, supporting the theory that these particles may be rock salt impurity phase. Likewise, after the annealing process (Figure 4f), the agglomerated surface particles show slight manganese depletion. In order to accept the premise that cation ordering changes after the annealing treatment, it is imperative to compare the degree of ordering by experimental techniques such as X-ray diffraction and an examination of the voltage profiles below 3 V corresponding to the lithium insertion reaction into the 16c octahedral sites. Accordingly, the X-ray diffraction patterns collected before and after the 700 °C annealing treatment can be seen in Figure 5. The magnified inset pictured in Figure 5b shows evidence of the rock salt impurity phase before annealing. As seen in Figure 5d, the rock salt phase diminishes after annealing at 700 °C for 96 h, supporting the assumption that the degree of cation ordering has increased and nickel has been assimilated into the main spinel phase. The chemical composition, especially the Mn:Ni ratio, has a profound effect on the electrochemical properties. Even small fluctuations can result in a shortened 5 V plateau and introduction of Mn3+ into the structure.17,23 Therefore, the Li:Mn:Ni ratio for each sample was determined experimentally by the ICP analysis. Those results, along with the Rietveld refined lattice parameters of each phase, are tabulated in Table 1. The chemical formulas of the samples are roughly stoichiometric LiMn1.5Ni0.5O4, within the error bars of the ICP measurement. Additionally, the Rietveld fitting reveals the

sample has roughly the same shape, but the amount of small agglomerated particles on the surface is substantially decreased. Similarly, the Octahedral sample retains the same approximate particle size and shape, but the vertices became somewhat rounded. In contrast, the Truncated sample no longer exhibits a high degree of truncation after the annealing treatment, and the overall particle size has decreased by about half. This major rearrangement indicates that the truncated surfaces do not possess a thermodynamically favorable close-packed arrangement of atoms, and the material has adopted a new morphology to minimize the free energy.30 Like the Cubic and Octahedral samples, the Spherical sample has a similar morphology before and after annealing, and a slight increase in particle size is observed after annealing. As described in previous reports,27−29 the Octahedral sample is known to have single-plane facets consisting of the {111} family of planes. By extension, the nanoscale octahedra decorating the surface of the Spherical sample also consist of the {111} family of planes. It is also known from previous reports and basic geometry that the Truncated planes consist of (111) planes on the faces of the octahedra, (110) planes on the edges, and (100) planes on the corners.25 To aid discussion, schematic drawings depicting the various crystallographic planes present on each facet can be seen in Figure 3. However, to conduct a thorough comparison of the crystallographic planes in contact with the electrolyte, careful TEM visualization was carried out for the Cubic sample, as seen in Figure 4a−d. Figure 4a and b show, respectively, the particle overview and HRTEM image of the lattice fringe on the top (smooth) facet of a particle. The dhkl measured perpendicular to this surface corresponds to the (111) interplanar spacing, indicating that the striated side faces grow along the [111] direction, perpendicular to the top facet. In Figure 4c and d, the (striated) side of the particle was examined, and the HRTEM data reveal that the perpendicular direction has a dhkl of ∼0.33 nm, corresponding to the interplanar spacing for the (112) 2892

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Figure 5. (a) X-ray diffraction patterns of the samples prepared at 900 °C, (b) a magnified 2θ region to illustrate the presence of the LixNi1−xO rock salt phase, (c) X-ray diffraction of patterns of the samples after annealing at 700 °C, and (d) a magnified 2θ region to illustrate the assimilation of the rock salt phase into the spinel lattice after annealing.

and Truncated samples prepared at 900 °C have approximately equal levels of cation ordering. For numerical ease of comparison, the capacities of the various plateaus are listed in Table 2. Now that the nuances in morphology and cation ordering have been established, a comparison of the cycling performance and rate capability of the samples prepared at 900 °C are presented in Figure 7. Although these data were collected at room temperature, it is readily apparent that the samples consisting of a majority of {111} family of planes on the surface exhibit superior capacity retention and rate capability. In fact, as seen in Figure 6, the Octahedral and Truncated samples have similar cation ordering characteristics when prepared at 900 °C, but Figure 7 clearly demonstrates the considerable discrepancy in electrochemical cycling between the two samples. After the annealing treatment, the differences in electrochemical performance are even more pronounced, as seen in Figure 8. The Cubic and Octahedral samples show similar longterm capacity retention before and after annealing at 700 °C. However, the rate capability is slightly lower for these samples after the annealing treatment. Considering the fact that the Cubic and Octahedral samples show only relatively minor changes in morphology, this lower rate capability can likely be

Figure 4. (a) TEM overview of a the top facet of a Cubic particle prepared at 900 °C, (b) HRTEM of the top facet of the Cubic particle with evidence of the (111) interplanar spacing, (c) SEM overview of the striated side face of a Cubic particle prepared at 900 °C, (d) HRTEM of the striated side face of the particle with evidence of the (112) interplanar spacing, (e) EDX line scan of an agglomerated particle on a Cubic surface prepared at 900 °C, and (d) EDX line scan of an agglomerated particle on a Cubic surface after annealing at 700 °C.

presence of rock salt impurity prior to the annealing treatment, which diminishes in most samples after annealing at 700 °C. The relative degree of cation ordering was compared by examining the voltage profiles below 3 V, as seen in Figure 6. As described in the introduction, the relative lengths of the plateaus observed at ∼2.7 and ∼2.0 V indicate the level of cation ordering in the sample. It is clear from Figure 6 that the Cubic sample is the most disordered when fired at 900 °C and the Spherical sample is the most ordered, while the Octahedral 2893

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Table 1. Compositional Analysis Data and Lattice Parameters of the Various LiMn1.5Ni0.5O4 Samples phase composition (wt %) molar ratio

900 °C

lattice parameter (Å)

700 °C

sample

Li

Mn

Ni

900 °C

700 °C

spinel

LixNi1−xO

spinel

LixNi1−xO

Cubic Octahedral Truncated Spherical

0.97 0.98 1.04 1.02

1.51 1.51 1.47 1.48

0.52 0.50 0.50 0.50

8.1787(2) 8.1819(1) 8.1754(2) 8.1654(2)

8.1679(1) 8.1786(1) 8.1675(1) 8.1643(2)

93.9 96.6 95.7 96.5

6.1 3.4 4.3 3.5

97.0 100.0 98.5 100.0

3.0 0.0 1.5 0.0

Figure 6. Discharge from 5.0 to 1.9 V to compare the lithium insertion reaction into the 16c octahedral sites.

attributed to the increase in charge transfer resistance associated with cation ordering and corresponding decrease in the Mn3+ content.21,22 The Spherical sample also has slightly lower rate capability after annealing (along with a higher degree of cation ordering) but actually exhibits better long-term capacity retention than when initially prepared at 900 °C. This could be ascribed to the increase in particle size of the nanoscale octahedra decorating the surface of the large spherical primary particles. Previous studies have indicated that nanoscale spinel particles are more susceptible to reaction with the electrolyte during high-voltage operation, thus degrading the cycle life.32−34 As seen in Figures 1f and 2f, the surface octahedra have dimensions of ∼30 nm before annealing, whereas the particles grow to ∼100 nm after annealing at 700 °C.

Figure 7. Electrochemical properties of the samples prepared at 900 °C: (a) capacity retention at a constant C/2 rate and (b) rate capability.

Table 2. First Discharge Capacity Values at Different Voltage Regions of the Various LiMn1.5Ni0.5O4 Samples discharge capacity (mAh/g) capacity below 3 V

capacity retention (%)

sample

total

capacity above 3 V

total

∼2.7 V

∼2.0 V

after 100 cycles

at 10C rate compared to that at C/10 rate

Cubic 900 °C Octahedral 900 °C Truncated 900 °C Spherical 900 °C Cubic 700 °C Octahedral 700 °C Truncated 700 °C Spherical 700 °C

241 249 229 248 216 223 246 260

126 125 124 122 122 124 126 127

115 124 105 126 94 99 120 133

15 45 47 70 22 50 58 105

100 79 58 56 72 49 62 28

98.5 98.3 69.1 88.7 94.5 95.0 43.6 87.1

88.3 96.2 64.1 90.5 87.5 90.3 25.6 77.9

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Figure 8. Electrochemical properties of the samples after annealing at 700 °C: (a) capacity retention at a constant C/2 rate and (b) rate capability.

Figure 9. (a) Discharge profiles of the Truncated sample annealed at 700 °C and (b) the capacity contribution below ∼3.9 V.

It is noteworthy though that the Truncated sample exhibits such poor electrochemical performance after annealing, even with morphology more closely resembling regular octahedra. In order to determine the origin of this behavior, the discharge profiles at different cycle numbers are shown in Figure 9a. Upon examination of the shape of these discharge profiles, a sloping region can be seen below ∼3.9 V, which is indicative of the presence of a layered phase.35 The percentage of capacity contribution from the slope region is plotted in Figure 9b. It is evident from the voltage profiles in Figure 9a that the spinel capacity degrades with extended cycling, but the capacity contribution below 3.9 V remains constant. This results in an increasing contribution of the capacity below 3.9 V to the total capacity as seen in Figure 9b. To confirm the presence of a separate layered phase, elemental mapping along with a line scan was performed with EDX to identify the Mn- and Ni-rich regions, as seen in Figure 10. The SEM images reveal certain areas that appear as if there is a residue covering the surface of the particles, seen in Figure 10a. The elemental mapping of these regions (Figure 10b) shows a distinct segregation of Mn in the bulk and Ni in the surface residue. The line scan (Figure 10c) also confirms this observation. Additionally, recent studies have indicated that a nickel-rich rock salt phase can be identified by XRD peaks at 2θ values of 37.5° and 43.5°.36,37 As seen in Figure 10d, small

peaks are apparent at both places, indicating that an ordered rock salt phase is present in the sample. Although a transition from spinel to layered structure is typically unfavorable, the combination of the electrochemical discharge analysis, elemental mapping, and expanded XRD views support that a major rearrangement occurs in the Truncated sample during the annealing treatment at 700 °C, resulting in increased cation ordering accompanied by a partial transition to a nickel-rich ordered rock salt phase. Another consequence of the formation of a separate nickel-rich layered phase is the enrichment of the main spinel phase with manganese, which could lead to dynamic Jahn−Teller distortion and increased dissolution of manganese and the active material into the electrolyte. To ensure that all considerations were taken into account, dQ/dV plots of all the samples were compared to explain the differences in cation ordering in terms of intrinsic polarization. Figure 11 shows scans for samples prepared at 900 °C, which illustrate the variations in the degree of cation ordering through a variation in polarization and decrease in the voltage difference between the two peaks at ∼4.7 V with increasing cation order. Specifically, the Cubic sample shows a wide space between the peaks at ∼4.6 and 4.75 V with minimal displacement in the voltage of the cathodic and anodic reactions, indicating a disordered cation arrangement. On the other end of the 2895

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Figure 11. dQ/dV plots of the of samples prepared at 900 °C.

Figure 12. dQ/dV plots of the samples after annealing at 700 °C.

the Truncated and Spherical reactions in the 4 V region, where mainly cathodic peaks appear. Since there is only evidence of a cathodic reaction at 4 V with an absence of a similar anodic reaction (as seen in the insets), especially after the first cycle, the dQ/dV indicates that a small fraction of Mn3+ is present in the structure following a discharge process but is not present for the subsequent charge cycles. This suggests that the manganese ions may have undergone a disproportionation into Mn2+ and Mn4+, and that the resulting Mn2+ ions might have dissolved into the electrolyte. This behavior further supports the capacity fade observed in Figures 7 and 8.

Figure 10. (a) SEM of a representative Truncated particle annealed at 700 °C with surface irregularity, (b) elemental mapping by EDX, (c) line scan showing Mn and Ni segregation, and (d) expanded view of the XRD showing the ordered rock salt phase.

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spectrum, the Spherical sample exhibits slightly closer peak spacing at ∼4.7 and 4.75 V and the anodic reaction takes place at a higher voltage than the corresponding cathodic reaction, which is indicative of increased polarization and a high degree of cation order.24 In line with the results from the samples prepared at 900 °C, the samples annealed at 700 °C exhibit much higher polarization and closer spacing between the reaction peaks, as seen in Figure 12. Another noteworthy point is the decrease in Mn3+ content observed after annealing, which can be seen in the magnified insets. Of particular interest is the asymmetry in

CONCLUSIONS A detailed characterization of the cation ordering and crystallographic planes in contact with the electrolyte shed light on the relationship between these structural aspects and the electrochemical cycling stability. The Octahedral and Truncated samples prepared at 900 °C have nearly identical degrees of cation ordering but are on the extremes in terms of electrochemical cycling performance. Additionally, the Spherical sample is shown to have a high degree of cation ordering when prepared at 900 °C, with an even higher degree of 2896

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ordering after annealing at 700 °C, yet it exhibits acceptable capacity retention and good rate capability as a result of the nanoscale octahedra decorating the surfaces of the particles. As with the cation ordering tendency, the presence or absence of the rock salt phase does not have a profound effect on the electrochemical properties. On the other hand, the spinel-tolayered transition observed in the Truncated sample after annealing at 700 °C is highly detrimental to capacity retention and rate capability, despite the formation of octahedral particles. These results indicate that the crystallographic planes overall play a dominant role in either facilitating or obstructing the electrochemical insertion and removal of lithium from the structure. However, formation of an electrochemically active layered phase also has a profound influence on the cycling performance, regardless of the cation ordering behavior or crystallographic planes.

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(15) Zheng, J.; Xiao, J.; Yu, X.; Kovarik, L.; Gu, M.; Omenya, F.; Chen, X.; Yang, X. Q.; Liu, J.; Graff, G. L.; Whittingham, M. S.; Zhang, J. G. Phys. Chem. Chem. Phys. 2012, 14, 13515. (16) Wang, L.; Li, H.; Huang, X.; Baudrin, E. Solid State Ionics 2011, 193, 32. (17) Cabana, J.; Casas-Cabanas, M.; Omenya, F. O.; Chernova, N. A.; Zeng, D.; Whittingham, M. S.; Grey, C. P. Chem. Mater. 2012, 24, 2952. (18) Moorhead-Rosenberg, Z.; Shin, D. W.; Chemelewski, K. R.; Goodenough, J. B.; Manthiram, A. Appl. Phys. Lett. 2012, 100, 213909. (19) Strobel, P.; Ibarra-Palos, A.; Anne, M.; Poinsignon, C.; Crisci, A. Solid State Sci. 2003, 5, 1009. (20) Yi, T.-F.; Hu, X.-G. J. Power Sources 2007, 167, 185. (21) Lazarraga, M. G.; Pascual, L.; Gadjov, H.; Kovacheva, D.; Petrov, K.; Amarilla, J. M.; Rojas, R. M.; Martin-Luengo, M. A.; Rojo, J. M. J. Mater. Chem. 2004, 14, 1640. (22) Kunduraci, M.; Al-Sharab, J. F.; Amatucci, G. Chem. Mater. 2006, 18, 3585. (23) Song, J.; Shin, D. W.; Lu, Y.; Amos, C. D.; Manthiram, A.; Goodenough, J. B. Chem. Mater. 2012, 24, 3101. (24) Shin, D. W.; Bridges, C. A.; Huq, A.; Paranthaman, M. P.; Manthiram, A. Chem. Mater. 2012, 24, 3720. (25) Kim, J. S.; Kim, K.; Cho, W.; Shin, W. H.; Kanno, R.; Choi, J. W. Nano Lett. 2012, 12, 6358. (26) Cabana, J.; Zheng, H.; Shukla, A. K.; Kim, C.; Battaglia, V. S.; Kunduraci, M. J. Electrochem. Soc. 2011, 158, A997. (27) Chen, Z.; Qiu, S.; Cao, Y.; Ai, X.; Xie, K.; Hong, X.; Yang, H. J. Mater. Chem. 2012, 22, 17768. (28) Chemelewski, K. R.; Shin, D. W.; Li, W.; Manthiram, A. J. Mater. Chem. A 2013, 1, 3347. (29) Hai, B.; Shukla, A. K.; Duncan, H.; Chen, G. J. Mater. Chem. A 2013, 1, 759. (30) Hirayama, M.; Ido, H.; Kim, K. S.; Cho, W.; Tamura, K.; Mizuki, J.; Kanno, R. J. Am. Chem. Soc. 2010, 132, 15268. (31) Leung, K. J. Phys. Chem. C 2012, 116, 9852. (32) Kim, D. K.; Muralidharan, P.; Lee, H. W.; Ruffo, R.; Yang, Y.; Chan, C. K.; Peng, H.; Huggins, R. A.; Cui, Y. Nano Lett. 2008, 8, 3948. (33) Zhao, X.; Reddy, M. V.; Liu, H.; Ramakrishna, S.; Rao, G. V. S.; Chowdari, B. V. R. RSC Adv. 2012, 2, 7462. (34) Qian, Y.; Deng, Y.; Shi, Z.; Zhou, Y.; Zhuang, Q.; Chen, G. Electrochem. Commun. 2013, 27, 92. (35) Lee, E.-S.; Huq, A.; Chang, H.-Y.; Manthiram, A. Chem. Mater. 2012, 24, 600. (36) McCalla, E.; Rowe, A. W.; Shunmugasundaram, R.; Dahn, J. R. Chem. Mater. 2013, 25, 989. (37) McCalla, E.; Dahn, J. R. Solid State Ionics 2013, 242, 1.

ASSOCIATED CONTENT

S Supporting Information *

Table of interplanar spacing for the Cubic sample (PDF). This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: (512) 471-1791. Fax: 512-471-7681. E-mail: manth@ austin.utexas.edu. 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.

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dx.doi.org/10.1021/cm401496k | Chem. Mater. 2013, 25, 2890−2897