Materials Chemistry A

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1 Journal of Materials Chemistry A PAPER View Article Online View Journal View Issue Cite this: J. Mater. Chem. A, 2014,2, 1353 Received 21st June 2013 Accepted 9th November 2013 DOI: /c3ta12440a Introduction Due to the toxicity and high cost of cobalt (Co), there is immense interest to search for alternative cathode materials for Li-ion batteries. A wide variety of compositions within the Li[Li 1/3 2x/3 Mn 2/3 x/3 Ni x ]O 2 (0 < x # 1/2) series have been studied extensively due to their high capacities While samples with no excess lithium (x ¼ 0.5) are limited in capacity (200 ma h g 1 ), 4,17 21,24 26,34 samples with excess lithium exhibit high discharge capacities of up to 300 ma h g 1. 1,6,8,12 20,25,29 31,33,36 There have been various explanations as to why the excess Li enhances the capacity of these materials. Yoon et al. 35 suggested that the differences in electrochemical properties may arise from the local and long-range ordering of Ni 2+,Mn 4+, and Li + and the amount of Ni 2+ substitution in the Li layer. Some groups, using powder X-ray diffraction (XRD), have indicated that Ni in the transition metal layer interchanges with Li in the Li layer and that this exchange increases as x increases. 18,20,23,35 We will refer to this exchange as interlayer mixing for the remainder of the paper. The presence of a transition metal (M) Materials Science and Engineering Program, University of Texas at Austin, Austin, TX 78712, USA. ferreira@mail.utexas.edu; Fax: ; Tel: Electronic supplementary information (ESI) available. See DOI: /c3ta12440a The role of composition in the atomic structure, oxygen loss, and capacity of layered Li Mn Ni oxide cathodes Karalee A. Jarvis, Chih-Chieh Wang, Arumugam Manthiram and Paulo J. Ferreira* Lithium-rich layered Li[Li 1/3 2x/3 Mn 2/3 x/3 Ni x ]O 2 (0 < x # 1/2) oxide cathodes show promise as a potential candidate for Li-ion batteries due to their high capacity. However, the intricacies of the role of composition with increasing excess Li on the degree of oxygen loss during the first charge and the discharge capacity in subsequent cycles are not fully understood. With an aim to develop a better fundamental understanding, we present here an in-depth investigation of the Li[Li 1/3 2x/3 Mn 2/3 x/3 Ni x ]O 2 (0 < x # 1/2) series with a range of different excess lithium contents prepared by two different synthesis methods. The oxygen loss from the lattice during the first charge and the discharge capacity in subsequent cycles increase with increasing lithium content. In-depth analysis with a combination of X-ray diffraction, scanning electron microscopy (SEM), aberration-corrected scanning transmission electron microscopy (STEM), diffraction-stem (D-STEM), and energy dispersive X-ray spectroscopy (EDS) reveals that the samples transition from an R3m structure to a C2/m structure with increasing lithium content and decreasing nickel to manganese ratio, for both the synthesis methods, indicating that the maximum oxygen loss and discharge capacity are achieved with a single C2/m phase. We further show that within a single particle, the cation layers of these materials can order on different {111} planes in the basic NaCl structure. in the Li layer can lower the Li + mobility and must be minimized to increase the discharge rate and electrode capacity. 26 Understanding interlayer mixing and cation ordering as a function of the Li content is, therefore, critical to develop high-performance lithium-rich layered cathodes. While the XRD patterns of these materials can provide insight into this interlayer mixing, the results can be ambiguous and lead to incorrect conclusions. 10,36 Hence, it is essential to support the XRD data with additional characterization techniques, such as advanced transmission electron microscopy techniques, that can provide a better understanding of the atomic structure of these materials as a function of Li content. However, a comprehensive correlation of the role of the Li content in Li[Li 1/3 2x/3 Mn 2/3 x/3 Ni x ]O 2 (0 < x # 1/2) with atomic structure and electrochemical properties, employing a combination of various analytical tools, is not available in the existing literature. To address this issue, we examine in this paper four distinct compositions in the series Li[Li 1/3 2x/3 Ni x Mn 2/3 x/3 ]O 2 : x ¼ 0.2 (Li[Li 0.2 Ni 0.2 Mn 0.6 ]O 2 ), x ¼ 0.3 (Li[Li 0.13 Ni 0.3 Mn 0.57 ]O 2 ), x ¼ 0.4 (Li[Li 0.07 Ni 0.4 Mn 0.53 ]O 2 ), and x ¼ 0.5 (Li[Ni 0.5 Mn 0.5 ]O 2 ). To keep charge neutrality, changes in the Li content result in changes in the Mn and Ni compositions. Thus, in this paper, we will consider both the change in Li content as well as the Ni : Mn ratio in our discussion. We use XRD and scanning electron microscopy (SEM) for bulk analysis, while aberrationcorrected high angle annular dark eld (HAADF) scanning This journal is The Royal Society of Chemistry 2014 J. Mater. Chem. A, 2014,2,

2 transmission electron microscopy (STEM), coupled with HAADF-STEM computer simulations, is used for atomic level analysis. To further support the STEM images, we employ a novel technique developed in our lab, D-STEM, 37 to identify thephasespresentwithinasingleparticlewitha1nmto2nm spatial resolution. We also perform energy dispersive X-ray spectroscopy (EDS) analysis to determine the chemical composition of the various particles. Finally, we correlate these results with the electrochemical properties of the four compositions to better understand the relationship among atomic structure, composition, and electrochemical properties. To ensure that the results are not solely a consequence of the synthesis route, we examine the four aforementioned compositions prepared by two common synthesis routes: sol gel (ethylenediaminetetraacetic acid (EDTA)) and co-precipitation (hydroxide) methods. Results X-ray diffraction Fig. 1 shows XRD patterns of the Li[Li 1/3 2x/3 Mn 2/3 x/3 Ni x ]O 2 (0 < x # 1/2) samples prepared by the EDTA and hydroxide coprecipitation methods. The layered structure, which consists of alternating Li and M layers on the cation layer, becomes more ordered with increasing Li content, as indicated by a more Fig. 1 XRD patterns of four compositions of the Li[Li 1/3 2x/3 Mn 2/3 x/3 Ni x ]- O 2 (0 < x # 1/2) series (x ¼ 0.5, 0.4, 0.3 and 0.2) prepared by (a) EDTA and (b) hydroxide precursor methods. pronounced splitting of the peaks around ,39 We also note that the intensities of the peaks in the 20 to 35 range, which result from the ordering of Li and Mn in the transition metal layer, 1,3,17,18,20,30,31 increase with increasing Li content for the samples prepared by both the methods. However, these peaks can broaden due to the presence of planar defects on the transition metal layer 3,36,40 and, therefore, should not be used to determine the amount of lithium-ion ordering in the transition metal layers without the corroboration by more advanced techniques, such as TEM. 10,11,36 We will discuss this issue in greater detail below. Transmission electron microscopy Contrast in HAADF-STEM imaging is directly related to the relative atomic number by a factor of Z 1.7 (Z ¼ atomic number). 41 As a result, brighter spots in HAADF-STEM images correspond to atomic columns having higher average atomic number. This results in atomic columns of Mn and/or Ni appearing bright and Li and/or O atomic columns appearing dark in the Li[Li 1/3 2x/3 Mn 2/3 x/3 Ni x ]O 2 (0 < x # 1/2) samples. Hence, to better interpret the structure, the samples were tilted, such that transition metal planes were parallel to the beam and like atoms stacked on top of each other. Fig. 2 shows the aberration-corrected STEM images obtained from the samples with compositions ranging from x ¼ 0.2 to x ¼ 0.5 and synthesized by the EDTA and hydroxide methods. The group of images in each column (taken under the same beam direction) represents different regions within a single particle of a speci c material composition. Clearly, each particle exhibits several atomic arrangements. These arrangements range in area from about 20 nm 2 to 500 nm 2.At rst, these various arrays can be easily mistaken for multiple phases. However, a careful analysis, as discussed below, reveals that, depending on the composition, only one or two phases may exist in any particle. In order to correctly identify the phases for each of the arrangements and facilitate the discussion, we begin by considering the most basic structure (i.e., an NaCl structure) common to all layered oxides. In this structure, the M and Li atoms reside on alternating Na planes and O atoms reside on the Cl sites. In this cubic system, the structure must be viewed down the <211> NaCl zone axis, such that the beam is parallel to the M layers and like atoms stack down the same column. Without any cation ordering, this zone axis produces images where bright columns of the {111} NaCl planes are spaced about 2.4 Å and the bright columns of the {011} NaCl planes are spaced about 1.4 Å (Fig. S1 ). The contrast of each column for the same <211> NaCl zone axis, on the other hand, will change for a cationordered system depending on where the cations reside. This will be discussed in the following sections. Seven different types of atomic arrangements were found in the Li[Li 1/3 2x/3 Mn 2/3 x/3 Ni x ]O 2 samples (Fig. 2) with some compositions containing many of the arrangements and others comprising only a few. To simplify the discussion, each atomic arrangement (A) will be assigned a number (n). For example, we will denote arrangement 1 as A 1. The seven arrangements are shown in greater detail in Fig. 3 as HAADF-STEM simulations J. Mater. Chem. A, 2014,2, This journal is The Royal Society of Chemistry 2014

3 View Article Online Fig. 2 Aberration-corrected HAADF-STEM images of samples prepared by the EDTA and hydroxide methods for compositions with x ¼ 0.5, 0.4, 0.3 and 0.2, viewed down the <211> NaCl zone axis. For simplicity, the scale bar is only shown in image E11 and is 1 nm for all images. Areas that are blank indicate that the corresponding arrangement was not seen for that composition. To better interpret the images, we classify the various arrangements according to the interlayer spacing between {111}NaCl type planes for which bright atomic columns are observed. On this basis, we can identify three categories of atomic arrangements: (1) the {111}NaCl interlayer spacing is 4.7 A between rows of bright spots (A1 and A2), (2) the {111}NaCl interlayer spacing is 2.4 A between rows of bright spots (A3, A4, A5, and A6) and (3) the {111}NaCl interlayer planes alternate between rows of bright spots and rows of weaker spots separated by 2.4 A (A7). In the rst category, A1 exhibits bright atomic columns spaced apart by 1.4 A lying on every other Na {111}NaCl plane (Fig. 2E11, O11 and 3A1). This matches the expected arrangement m) viewed down the [120]T for a trigonal unit cell (space group R3 zone axis (Fig. S2 ). However, some STEM images (Fig. 2E11, E12, O11, and O12) also show some weak intensity in between the bright layers, which indicates interlayer mixing. Arrangement two (A2) in the rst category is composed of two bright atomic columns separated by 1.4 A, alternating with a Li-rich dark atomic column all lying on every other Na {111}NaCl plane (Fig. 2E14,O14, and 3A2). This arrangement matches the monoclinic (space group C2/m) structure of Li2MnO3 viewed down the 0] zone axes.2,10,11,16 [100], [110], or the [11 In the second category, four different arrangements are found (A3 to A6), where every {111}NaCl plane is now separated by This journal is The Royal Society of Chemistry A. Arrangement three (A3) exhibits bright atomic columns lying on {111}NaCl planes separated by 1.4 A (Fig. 2E21 E24, O21 O24, and 3A3). This arrangement is similar to the NaCl structure 11>NaCl zone axis (Fig. S1 ). Arrangement viewed down the <2 four (A4) is different from A3 in that the distance between the bright atomic columns lying on {111}NaCl planes is twice (2.8 A ) that of A3 (Fig. 2E31 E33, O31 O33, and 3A4). Arrangement ve (A5) is similar to A4 but instead of all the bright atomic columns lying on {111}NaCl planes having equal intensity, there is a weaker intensity for every third column (Fig. 2E34 and 3A5). This reduction in intensity for every third column in E34 is not very clear, due to the small size of the image in Fig. 2; therefore, an intensity pro le (Fig. S3 ) is provided to better interpret arrangement A5. This arrangement looks similar to the projection of Li2MnO3 down the [001]M zone axis (Fig. S4a ) but contains some intensity in the pure Li column of Li2MnO3. This intensity results from Ni atoms being randomly distributed on the Li sites of the M layer of Li1.2Mn0.6Ni0.2O2 (Fig. S4b ).10 Arrangement six (A6) is composed of two bright atomic columns separated by 1.4 A, followed by a dark atomic column on every {111}NaCl plane (Fig. 2E44, O43, O44, and 3A6). Only one arrangement (A7) is found in the third category (Fig. 3A7). In this type, bright {111}NaCl planes separated by 4.7 A alternate with weaker {111}NaCl planes also spaced apart by J. Mater. Chem. A, 2014, 2,

4 View Article Online Fig. 3 Simulated HAADF-STEM image showing the seven types of arrangements (A1 to A7) found for the Li[Li1/3 2x/3Mn2/3 x/3nix]o2 (0 < x NaCl zone axis. A schematic of the # 1/2) series, viewed down the <211> 2-D atomic models is shown at the top right corner. The directions are indicated for the most basic structure of layered oxides (NaCl). The scale bars are all approximately 0.5 nm. 4.7 A. Each bright {111}NaCl plane exhibits a sequence of ve bright atomic columns, separated by 1.4 A, followed by a dark column. The atomic columns of the weaker {111}NaCl planes are either separated by 1.4 A (Fig. 2E54) or 2.8 A (Fig. 2O52 O54, and 3A7). To con rm the results of HAADF-STEM imaging, we also collected D-STEM patterns. This technique helps us verify the phases/arrangements observed in STEM and allows us to probe the thicker areas of the particles which are too dense to obtain atomic-resolution STEM images. The D-STEM diffraction patterns are arranged similar to the STEM images of Fig. 2; each column (taken under the same zone axis) represents different 1356 J. Mater. Chem. A, 2014, 2, regions within a single particle of one speci c composition (Fig. 4). Overall, the D-STEM analysis (Fig. 4) shows seven distinct patterns, which match the simulated electron diffraction patterns (Fig. S5 ) of the seven arrangements found by HAADF-STEM imaging (Fig. 2). It is important to mention that the simulations are only used to show the position of the electron diffraction re ections, not intensities, which is explained in the experimental section. Thus, the size and intensities of the diffraction spots in the simulated diffraction patterns have all been made equal and will not re ect the intensities seen in the actual D-STEM diffraction patterns. The rst row of Fig. 4 shows three types of electron diffrac m) (Fig. 4E11, O11, tion patterns: (1) a pattern that matches A1 (R3 and S5A1 ) with a small amount of Li ordering in the transition metal layer indicated by the weak re ections, (2) a pattern that matches A2 (C2/m) (Fig. 4E14, O14, and S5A2 ), and (3) a pattern that is a combination of A1 and A2 (Fig. 4E12, E13 and O12, O13). Note that the diffraction patterns matching A2 contain multiple spots and exhibit streaking due to the presence of planar defects, as previously reported.2,10,11,16,36,40,42 44 Rows two to four in Fig. 4 show quite different diffraction patterns when compared with row one. This is due to the fact that for A3 to A6, every {111}NaCl plane occupied with cations is now separated by 2.4 A (half of 4.7 A for arrangements A1 to A2), which in reciprocal space leads to twice the distance between {111}NaCl spots (indicated by arrows in Fig. 4E21 E44 and O21 O44). There are further distinctions within rows two to four. Row two shows the simplest diffraction pattern (Fig. 4E21 E24 and O21 O24) and matches the NaCl-like arrangement, A3 (Fig. S5A3 ). In row three, however, the magnitude of the rst 3>NaCl direction has decreased by diffracted spot along the <11 half (indicated by triangles) compared with row two. The patterns of Fig. 4E31 E33 and O31 O33 in row three match the A4 arrangement (Fig. S5A4 ). Fig. 4E34 and O34 also show a decrease by one third in the magnitude of the rst diffracted spot along 1>NaCl direction (indicated by circles), which matches A5 the <01 3>NaCl (Fig. S5A5 ). The rst diffracted spot along the <11 direction has now decreased by one third in row four compared with row two (indicated by squares in Fig. 4E42 E44 and O43, O44). This matches A6 (Fig. S5A6 ). Note that pattern O43 has two sets of these intralayer diffracted spots due to an apparent crystal rotation, which we discuss in more detail below. We are now le with row ve (Fig. 4E52 E54 and O52 O54). In this case, the columns of multiple spots and streaks due to Li ordering have doubled (indicated by asterisks), which matches the simulated pattern of A7 (Fig. S5A7 ). However, the magni 1>NaCl direction has tude of the rst diffracted spot along the <01 also doubled, which is not expected from arrangement A7 (Fig. S5A7 ). This extra set of spots is due to double diffraction. Electrochemistry The rst charge pro les of the Li[Li1/3 2x/3Mn2/3 x/3nix]o2 samples, shown in Fig. 5, contain two regions, a slopping region (A) and a plateau region (B), with the exception of the x ¼ 0.5 sample, which contains only region A. Clearly, as the Ni content decreases, the integrated area under the slopping region of the This journal is The Royal Society of Chemistry 2014

5 Fig. 4 Diffraction-STEM (D-STEM) patterns obtained from samples prepared by EDTA and hydroxide methods for compositions with x ¼ 0.5, 0.4, 0.3 and 0.2, viewed down the < 211> NaCl zone axis. For simplicity, the scale bar is only shown in image E 11 and is 1 nm 1 for all images. The notation N/A indicates that the corresponding atomic arrangement is not found in that composition. charging curve decreases. This is expected as the slopping region of the charge curve results from the oxidation of nickel. 1,17 19,45,46 The plateau region around 4.5 V, evident for compositions x ¼ 0.2, 0.3, and 0.4, has been attributed to the oxidation of oxide ions and loss of oxygen from the layered lattice, which is assisted by the presence of excess lithium in the layered oxide composition. 1,12,17 19,46 As expected, the plateau region increases with increasing Li content. The highest discharge capacity (232 ma h g 1 for the EDTA method and 235 ma h g 1 for the hydroxide method) occurs at x ¼ 0.2 and the lowest capacity during the rst charge (194 ma h g 1 for the EDTA method and 183 ma h g 1 for the hydroxide method) occurs at x ¼ 0.5. While slight variations occur for the charge and discharge capacities between the EDTA and OH methods, both methods exhibit the same increase in oxygen loss during the rst charge and increase in discharge capacity with increasing Li content. Discussion At rst glance, the multiple arrangements seen in HAADF-STEM images and D-STEM patterns in our samples may be misinterpreted as different phases. While the XRD data do not show any peaks other than those related to LiMO 2 and/or Li 2 MnO 3, peaks from new phases may overlap with the peaks from either the LiMO 2 or Li 2 MnO 3 phases. We examined the chemical composition of individual particles to address this issue. Fig. S6 shows that the samples do not contain signi cant variations in Mn or Ni. Furthermore, the contrast in Fig. S6 does not vary signi cantly, suggesting that Li and O areas also evenly distributed within the particles. The uniform composition of the particles indicates that new phases are unlikely on the scales seen here. As a result, we pursued a different explanation for the arrangements observed in the STEM images and D-STEM patterns. In NaCl, all of the {111} NaCl planes are considered equivalent. However, in the case of the layered oxides, the interlayer ordering of the Na sites causes the {111} NaCl planes to be dissimilar. To understand the implications of this ordering on the overall atomic structure we shall rst consider the two compositions at either end of the series, namely x ¼ 0.5 (LiMO 2 space group R3m) and x ¼ 0 (Li 2 MnO 3 space group C2/m). For the LiMO 2 structure, consider rst the case where the M and Li layers can alternate on the (111) NaCl planes (Fig. 6a). Let us denote this type of ordering as O 1. Now consider that M and Li layers instead order on alternating (111) NaCl planes (Fig. 6b), (111) NaCl planes (Fig. 6c) or (111) NaCl planes (Fig. 6d), here de ned as types O 2,O 3, and O 4, respectively. These different types of ordering are signi cant because they produce different HAADF-STEM images when viewed along a common NaCl zone axis. Let us examine the effects of the four aforementioned ordering types on the atomic arrangements viewed along the [112] NaCl zone axis (Fig. 7). In the case of O 1, the projected image matches that of A 1 and is equivalent to a [120] T trigonal zone axis (Fig. 7a). Ordering type two (O 2 ) matches arrangement A 4 and is equivalent to a [121] T trigonal zone axis (Fig. 7a). On the other hand, O 3 and O 4 match A 3 and are equivalent to a [10 8 1] T and [281] T trigonal zone axes, respectively (Fig. 7a). The same three arrangements, A 1,A 3, and A 4, are also produced when viewed down the [211] NaCl and [121] NaCl zone axes (Fig. S7 and S8 ). A full list of the zone axes that will produce the various arrangements for LiMO 2 can be found in Table S1. We can conclude that A 2,A 5,A 6, and A 7 cannot be produced by the LiMO 2 system. Therefore, we will next consider Li 2 MnO 3. For simplicity, we will use the same ordering notations of the four {111} NaCl planes as in the LiMO 2, but we now change each M This journal is The Royal Society of Chemistry 2014 J. Mater. Chem. A, 2014,2,

6 Fig. 5 Charge discharge profiles of four compositions of the Li[Li 1/3 2x/3 Mn 2/3 x/3 Ni x ]O 2 (0 < x # 1/2) series (x ¼ 0.5, 0.4, 0.3 and 0.2) prepared by (a) EDTA and (b) hydroxide precursor methods. The curves were measured with a 0.25 C charge discharge rate. The slopping region is labeled A and the plateau region is labeled B. layer in the Li 2 MnO 3 structure so that it is composed of six Mn atoms surrounding a Li atom (Fig. S9 ). Consequently, when the O 1 -type ordering is viewed along the [112] NaCl zone axis, the HAADF-STEM image now matches A 2, which is equivalent to the [100] M monoclinic zone axis (Fig. 7b). Type two ordering (O 2 ) matches arrangement A 5, which is equivalent to the [001] M monoclinic zone axis (Fig. 7b). Finally, O 3 and O 4 match arrangement A 3, which are equivalent to the [323] M and [323] M monoclinic zone axes, respectively (Fig. 7b). On the other hand, when the Li 2 MnO 3 structure is viewed down the [211] NaCl (Fig. S10 ) zone axis and [121] NaCl zone axis (Fig. S11 ), different arrangements can be observed (Table S2 ). For both beam directions, O 2 -type ordering produces A 6. However, the lithium atomic columns on each {111} NaCl plane of A 6 align along a different diagonal (Fig. S10b and S11b ). This switch in stacking, which appears as a crystal rotation from a [011] M to a [011] M, is responsible for producing the double spots in the D-STEM pattern of Fig. 4O 43. Nevertheless, if we consider a Fig. 6 Three dimensional atomic models of LiMO 2 showing the various NaCl {111}-type planes in which the M and Li layers may reside on (a) (111) NaCl, (b) (11 1) NaCl, (c) (1 11) NaCl, and (d) ( 111) NaCl. defect free material, these two orientations, [011] M and [011] M, should not be seen together as they occur under different beam directions. It is well known, however, that layered oxides with lithium ordering in the transition metal layer contain three orientation variants, [100] M, [110] M and [110] M, 11 that result in planar defects on the {111} NaCl planes. Therefore, in a defectfree crystal, the [011] M orientation and the [011] M orientation can only be observed under a [211] NaCl and a [121] NaCl beam direction, respectively, while in a material with three orientation variants, the [011] M orientation and the [011] M orientation can be both observed under a single {211} NaCl beam direction. In fact, all of the possible arrangements in Li 2 MnO 3,A 2,A 3,A 4, A 5, and A 6, can occur simultaneously under the same {211} NaCl beam direction. Yet, A 7 is still not found in the Li 2 MnO 3 system. We propose that A 7 is the result of crystal overlapping among the four possible ordering systems. To validate this hypothesis, we carried out HAADF-STEM image simulations for 5 nm thick particles where half of the thickness consists of one type of ordering and the other half comprises another type of ordering. While there are many possible outcomes of overlapping two crystals with ordering on different {111} NaCl planes (Fig. S12 ), Fig. 3A 7 and also Fig. S12 show that when an overlap of the O 1 and O 4 -type orderings is viewed down the [211] NaCl direction, the structure observed in A 7 (Fig. 2O 52 O 54 ) is generated. Due to the formation of the three orientation variants, [100] M, [110] M and [110] M, mentioned above, the overlap of arrangements O 1 and O 4 may produce a rectangular pattern instead of a parallelogram when O 1 has a [100] M orientation variant (Fig. S13a ). Fig. 2E 54 shows slightly different stacking, as mentioned in the results section. This is the result of A 2 with the [110] M orientation variant overlapping with A 4, and A 6 with a [110] M orientation variant, along the {211} NaCl beam direction (Fig. S13b ) J. Mater. Chem. A, 2014,2, This journal is The Royal Society of Chemistry 2014

7 impurities. 47,48 While we do not see any NiO peaks in XRD or any signi cant composition variation across the particles (Fig. S14 ), it is prudent to con rm that the A 3 arrangement observed in our samples is not associated with the NaCl-like structure. Hence, we examined a particle of Li[Li 0.2 Mn 0.6 Ni 0.2 ]O 2 prepared by the EDTA method under two different zone axes (Fig. 8). When viewed along the [211] NaCl zone axis, the particles show two distinct regions, namely region 1 with an NaCl-like arrangement (Fig. 8a) and region 2 with a C2/m-like arrangement (Fig. 8b). A er tilting the crystal 34 from the [211] NaCl zone axis to the [121] NaCl zone axis, region 1 now appears C2/m (Fig. 8c) and region 2 now appears NaCl-like (Fig. 8d). Therefore, we can conclude that arrangement A 3 for this composition (x ¼ 0.2) is in fact C2/m and the particle is single-phase. When the same analysis was applied to a particle of Li[Mn 0.5 Ni 0.5 ]O 2 (x ¼ 0.5) prepared by the EDTA method (Fig. S15 ), we notice that arrangement A 3 is indeed R3m with some lithium ordering in the transition metal layer. Thus, the arrangements in the HAADF-STEM images and D-STEM patterns that resemble an NaCl-like phase in the series studied here are in fact either R3m, C2/m, or a mixture of the two phases depending on the sample's composition. The discussion, up to this point, has focused on LiMO 2 and Li 2 MnO 3. However, we can expand this discussion to all four compositions examined in this work. We see from the STEM images (Fig. 2) that the composition with no excess lithium, Li [Mn 0.5 Ni 0.5 ]O 2, has an R3m structure with some interlayer mixing (Fig. 2E 11 and O 11 ). The D-STEM diffraction patterns show very small indications of lithium ordering in the transition Fig. 7 Two dimensional atomic models of (a) LiMO 2 and (b) Li 2 MnO 3 projected down the NaCl [11 2] NaCl zone axis for which the cation layers alternate on the (111) NaCl (O 1 ), (11 1) NaCl (O 2 ), (1 11) NaCl (O 3 ), and (d) ( 111) NaCl (O 4 ) planes as indicated. Thus, A 7 results from an overlap in the beam direction of two or more regions of the same phase but ordered on different {111} NaCl planes. At this point, we can conclude that all of the seven atomic arrangements observed by HAADF-STEM imaging and D-STEM diffraction can be explained as the LiMO 2 and/or Li 2 MnO 3 phases with ordering of the cation layers on different {111} NaCl planes in different regions of the particles. However, it is relevant to discuss the NaCl-like arrangement (A 3 ) further as it has been reported that layered oxides may contain NiO Fig. 8 Aberration-corrected HAADF-STEM images of two different regions within the same particle of Li 1.2 Mn 0.6 Ni 0.2 O 2 viewed down (a and c) the [ 211] NaCl zone axis and (b and d) the [ 121] NaCl zone axis. Images (a) and (b) are from region 1 and (c) and (d) are from region 2. The scale bars equal 5 nm. The insets show the particles at higher magnification and have been deconvoluted to reduce noise. 51 This journal is The Royal Society of Chemistry 2014 J. Mater. Chem. A, 2014,2,

8 metal layer (as indicated by the weak re ections) for both preparation methods (Fig. 4E 11 and O 11 ), most likely due to interlayer mixing. For this sample, we do not see arrangements A 5,A 6,orA 7, which only occur for the C2/m phase. We thus conclude that Li[Mn 0.5 Ni 0.5 ]O 2 is primarily an R3m phase with some interlayer mixing that results in a small amount of lithium ordering in the transition metal layer. As excess lithium is added to the material, the amount of the C2/m symmetry increases. There is already some evidence of C2/m for composition x ¼ 0.4 (Fig. 2E 12,O 12,O 52,4E 12,E 42,E 52,O 12, and O 52 ), although most of the particles exhibit an R3m phase with some interlayer mixing. More evidence of C2/m for both synthesis methods appears for x ¼ 0.3 (Fig. 2E 13,O 13,O 43,O 53,4E 13 E 43, E 53,O 13,O 43, and O 53 ) but no interlayer mixing is observed. This material is primarily C2/m with a small amount of R3m. Finally, the Li[Li 0.2 Mn 0.6 Ni 0.2 ]O 2 composition in both methods contains only the C2/m phase (Fig. 2E 14,E 34,E 44,E 54,O 14,O 44,O 54,4E 14, E 34,E 44,E 54,O 14,O 34,O 44, and O 54 ). These results are quite consistent with the expected phases based on the slow cooled phase diagram provided by McCalla et al. 49 Our compositions from x ¼ 0.5 to x ¼ 0.3 fall on or very near the tie line between the phases McCalla et al. labeled as N and M, which correspond to the R3m and C2/m phases presented in this work, respectively. Based on the aforementioned phase diagram, those compositions should be expected to have both the R3m and C2/m phases and a spinel phase. However, as McCalla et al. pointed out, the layered boundary is expected to shi upwards at higher temperatures. 49 Since our materials were heated to 900 C, the compositions x ¼ 0.5 to x ¼ 0.3 will most likely fall well within the layered region. It is also important to point out that the XRD data obtained by McCalla et al. 49,50 for the Li[Mn 0.5 Ni 0.5 ]O 2 composition heated at 800 C for 5 h in oxygen and slow cooled show two strong peaks around 44, indicating two phases, 50 while the same composition heated at 900 C for 12 h in air and slow cooled shows only one peak, indicating a single phase. 49 While our intermediate samples, x ¼ 0.3 and x ¼ 0.4, heated at 900 C for 12 h in air and slow cooled show only one peak near 44 (Fig. S14 ), our data indicate that it contains some of the C2/m phases as well as the R3m phases (Fig. 2 and 4). Our previous work showed that samples in which the R3m and C2/m phases are intermixed across a few atomic planes only exhibit one peak near Two peaks only appear when the two phases show large segregation. 36 Therefore, even though only one peak is seen near 44 for compositions x ¼ 0.3 and x ¼ 0.4, these two samples exhibit both the R3m and the C2/m phases (Fig. 2 and 4). Overall, these results are important, particularly because these intermediate compositions, x ¼ 0.3 and x ¼ 0.4, have never been reported before but are consistent with the presence of the two layered phases, R3m and C2/m, predicted by the phase diagrams published by McCalla et al. 49 We can now correlate these results with the electrochemical data. While both the EDTA and the hydroxide precursor methods show the same phase(s) for each composition, the EDTA method produces slightly higher charge and discharge capacities than the hydroxide method for x ¼ 0.5 to 0.3 (Fig. 5). This is most likely due to the smaller particle sizes of the EDTA sample (Fig. S16 ). Regardless of the synthesis route, the composition exhibits the same role in structure, oxygen loss, and capacity. We note from XRD (Fig. 1) that the addition of Li and reduction of the Ni : Mn ratio reduce the amount of interlayer mixing, which is con rmed by STEM (Fig. 2) and D-STEM (Fig. 4) and agrees with the previous work in the literature. 18,20,23,35 Furthermore, the structure transitions from an R3m symmetry with a small amount of lithium ordering in the transition metal layer at x ¼ 0.5 to a C2/m symmetry at x ¼ 0.2 with intermediate compositions containing regions of both R3m and C2/m phases (Fig. 2 and 4). We, therefore, conclude that adding Li and decreasing the Ni : Mn ratio both stabilize the layered structure (less interlayer mixing) and promote lithium ordering in the transition metal layer, which result in the C2/m phase. The increase of the C2/m phase correlates with an increase in oxygen loss during the rst charge and an increase in discharge capacity. Experimental The Li[Li 1/3 2x/3 Mn 2/3 x/3 Ni x ]O 2 series of samples were prepared by both EDTA precursor and hydroxide precursor methods. For both methods, excess lithium was added due to the loss of lithium during heating. For the EDTA method, required amounts of lithium acetate, magnesium acetate, and nickel acetate were dissolved in deionized water. EDTA and citric acid, acting as complexing agents, were added to NH 4 OH to form a homogeneous aqueous solution with ph 8. The metal acetate solution was then added to the EDTA citric acid solution. The mole ratio of EDTA : citric acid : metal ion was 1 : 1.5 : 1. The mixtures were then heated on a hot plate at 90 C for 12 h until a gel was formed. This gel was then red at 450 C for 6 h to remove the residual organics. The resulting powder was heated at 900 C for 12 h to obtain the nal layered oxide powder. The samples were cooled at a rate of 5 C min 1. The nal products were analyzed by ICP to determine the nal Li content (Table S3 ). The hydroxide precursors were prepared by adding a solution with required amounts of manganese and nickel acetates into 2 M KOH. The co-precipitated powder was then washed with deionized water and dried in an air oven at 100 C. A required amount of LiOH$H 2 O was then mixed with the hydroxide precursor and subsequently red at 900 C for 12 h in air to obtain the nal product. The samples were cooled at a rate of 5 C min 1. The nal products were analyzed by ICP to determine the nal Li content (Table S3 ). Electrochemical performance was evaluated with 2032-type coin cells and a current of 10 ma g 1 between 2 and 4.8 V. The coin cell consisted of the layered oxide cathode, lithium metal anode, 1 M LiPF 6 in ethylene carbonate diethyl carbonate (v/v 1 : 1) electrolyte, and a Celgard polypropylene separator. To make the cathode, a slurry of 80 wt% active material, 10 wt% super p carbon, and 10 wt% polyvinylidene uoride (PVDF) binder with N-methyl-2-pyrrolidone (NMP) solvent was made and then cast onto aluminum foil. The cathode thus obtained was dried in an air oven at 80 C for 3 h and then under vacuum 1360 J. Mater. Chem. A, 2014,2, This journal is The Royal Society of Chemistry 2014

9 at 120 C for 12 h. Once dried, cathodes, with an area of 0.64 cm 2, were punched out to fabricate the coin cells. For TEM analysis, the powders were suspended in ethanol or methanol and placed on a carbon lacey grid. A er the sample preparation, EDS and D-STEM 37 were performed on a JEOL 2010F TEM, while imaging was carried out on a JEOL ARM200F TEM/STEM, equipped with a CEOS corrector for the illuminating lenses and a HAADF detector. Due to possible beam interactions with the sample, we never focus the sample using a stationary probe on the area we image. Beam interaction with the sample, especially Li, is always a concern in electron microscopy. While we cannot directly image Li, a change in the overall structure of the material is expected if Li changes lattice positions or diffuses out of the material. Therefore, we are very contentious of any structural changes. We have selected microscopy conditions, namely beam current, scanning rate and magni cation, such that no observed structural changes were detected for STEM image acquisitions up to 1 min, even though the STEM images shown in this work were obtained a er only 10 s scans. Furthermore, we image the sample a er acquiring spectroscopy data to ensure that the sample was not damaged. Due to noise from faster scan rates, some of the high resolution STEM images were deconvoluted using the maximum entropy method implemented by Ishizuka. 51 All scale bars were added assuming an interplanar spacing of 4.74 Åof the {111} NaCl planes. The electron diffraction patterns were simulated using the online so ware WebEmaps, 52 which assumes kinematic diffraction. However, the samples analyzed in this work were too thick to assume kinematic diffraction and, therefore, the simulations were used only to show the allowed re ections. As a result, all the allowed re ections in the electron diffraction simulations are given the same intensity. To simulate the STEM images, atomic models were created in Diamond 3.2i and exported to the HREM Simulation Suite, which is a TEM/STEM simulation so ware package based on the FFT Multislice technique. 53 For all the compositions, the multislice simulations were carried out assuming a step scanning size of 0.2 Å (less than the distance to be resolved) and a minimum slice thickness of 1.0 Å. The Debye Waller factor (B), which is related to the mean square of the thermal displacement of an atom from its equilibrium position, was taken from the literature. In particular, the structures and B values for each atomic position were taken from: Strobel et al. 54 for the Li 2 MnO 3 phase, Meng et al. 23 for the Li[Mn 0.5 Ni 0.5 ]O 2 phase, and Thomassen for the NiO phase. 55 No B values were available for the simulations of the atomic arrangements; therefore, we assumed B ¼ 1 for all atomic positions for these models. Conclusions We con rm in this paper that, regardless of the precursor synthesis method, increasing the Li content and decreasing the Ni : Mn ratio in Li-rich layered oxides decrease intermixing between Ni ions in the transition metal layer and Li ions in the Li layer. We also show that increasing the Li content results in more Li ordering in the transition metal layer and that compositions lying between Li[Mn 0.5 Ni 0.5 ]O 2 and Li[Li 1.2 Mn 0.6 Ni 0.2 ]O 2 contain both R3m and C2/m phases. On the other hand, Li[Mn 0.5 Ni 0.5 ]O 2 contains the R3m phase and a small amount of lithium ordering in the transition metal layer due to interlayer mixing between Li and Ni, and Li[Li 1.2 Mn 0.6 Ni 0.2 ]O 2 has a pure C2/m phase. Furthermore, increasing the Li content and decreasing the Ni : Mn ratio lead to an increase in the degree of oxygen loss during the rst charge and discharge capacity. Thus, to maximize the discharge capacity, the samples in the Li[Li 1/3 2x/3 Mn 2/3 x/3 Ni x ]O 2 (0 < x # 1/2) system must contain enough Li (20% in the transition metal layer) to form a single C2/m phase. We also found that, regardless of the composition and the synthesis route, the cation layers order on different sets of {111} NaCl planes within a single particle. This is an unexpected and important result because: (1) great care must be taken when analyzing 2-D HAADF-STEM images and electron diffraction patterns of Li-rich layered oxides and (2) it indicates that Li ions do not diffuse down a single 2-D plane across an entire particle in the Li[Li 1/3 2x/3 Mn 2/3 x/3 Ni x ]O 2 (0 < x # 1/2) series, as previously thought. Future work will focus on understanding the cause and impact of these changes in cation ordering. 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