Crystalline Grain Interior Configuration Affects Lithium Migration

Publication Date (Web): April 18, 2016 ... Utilizing advanced microscopy techniques, the interior configuration of a single grain, multiple monocrysta...
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Crystalline Grain Interior Configuration Affects Lithium Migration Kinetics in Li-Rich Layered Oxide Haijun Yu,†,∥ Yeong-Gi So,‡ Akihide Kuwabara,§ Eita Tochigi,‡ Naoya Shibata,‡ Tetsuichi Kudo,†,‡ Haoshen Zhou,*,† and Yuichi Ikuhara*,‡,§ †

Energy Technology Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Umezono 1-1-1, Tsukuba 305-8568, Japan ‡ Institute of Engineering Innovation, School of Engineering, The University of Tokyo, Tokyo 113-8656, Japan § Nanostructures Research Laboratory, Japan Fine Ceramics Center, Mutsuno, Atsuta, Nagoya 456-8587, Japan ∥ College of Materials Science and Engineering, Beijing University of Technology, Pingleyuan #100, Chaoyang District, Beijing 100124, China S Supporting Information *

ABSTRACT: The electrode kinetics of Li-ion batteries, which are important for battery utilization in electric vehicles, are affected by the grain size, crystal orientation, and surface structure of electrode materials. However, the kinetic influences of the grain interior structure and element segregation are poorly understood, especially for Li-rich layered oxides with complex crystalline structures and unclear electrochemical phenomena. In this work, cross-sectional thin transmission electron microscopy specimens are “anatomized” from pristine Li1.2Mn0.567Ni0.167Co0.067O2 powders using a new argon ion slicer technique. Utilizing advanced microscopy techniques, the interior configuration of a single grain, multiple monocrystal-like domains, and nickel-segregated domain boundaries are clearly revealed; furthermore, a randomly distributed atomic-resolution Li2MnO3-like with an intergrown LiTMO2 (TM = transitional metals) “twin domain” is demonstrated to exist in each domain. Further theoretical calculations based on the Li2MnO3-like crystal domain boundary model reveal that Li+ migration in the Li2MnO3-like structure with domain boundaries is sluggish, especially when the nickel is segregated in domain boundaries. Our work uncovers the complex configuration of the crystalline grain interior and provides a conceptual advance in our understanding of the electrochemical performance of several compounds for Li-ion batteries. KEYWORDS: Atomic-resolution, twin domain, domain boundary, theoretical calculation, Li+ migration, lithium-rich layered oxides

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structure evolution that accompanies variation of the lithium concentration.32 Multiple studies of LLOs have been performed, especially to clarify the kinetics-controlled reaction processes associated with Li+ extraction/insertion behavior from/into the crystalline grains under different voltage ranges.11,13−17,25,33−35 Both electrochemical and kinetics studies have revealed that the Li+ diffusion behavior in LLOs is clearly divided into several processes, thereby indicating possible inhomogeneous crystalline structures.27,28,36 Further studies that used in situ X-ray absorption spectroscopy have indicated that a Li2MnO3-like structure distributed in LLOs is a key factor that affects the rate capability.28 However, researchers do not yet understand the cause of the poor electrode kinetics of LLOs, which is challenging primarily because of their pristine structural complexities.

i-ion batteries (LIBs), which operate through extraction/ insertion of Li+ from/into electroactive host materials, are currently one of the most important energy storage devices.1−6 Investigation of electrode materials such as stoichiometric LiMnxNiyCo1−x−yO27,8 and Li-rich layered oxides (LLOs)9−11 is highly important because they represent the major cathode materials for LIBs. In particular, LLOs with almost twice the energy density of already largely commercial LiCoO2 are considered to be the most promising cathode materials for next-generation LIBs and have recently been advancing energetically.10−24 However, there is much debate regarding these compounds, and their electrochemical performance in terms of rate capability and cycle stability needs to be greatly improved before practical applications can be realized.11−13,25−31 The electrochemical performance of electrode materials is largely controlled by the pathways of charge carriers in and out of a crystal and their kinetic barriers, which are determined by their crystal/electronic structures in their pristine states and the © XXXX American Chemical Society

Received: September 28, 2015 Revised: March 17, 2016

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Figure 1. Multiple monocrystal-like domains in CSTTs-LLOs. (a) BF-TEM image of the studied grain. (b) ED image of the whole grain in (a). (c) Simulated ED image based on two structures projected along different crystallographic directions. (d−g) DF-TEM images associated with the different ED patterns of A−D that are circled by red dotted lines in (b). (h) Three-dimensional schematic of the relationships among the domains A−D in panels d−g. (i) Schematic of the crystalline grain projected along the [100]mon direction for domains A−D. The white dotted lines in (a,d,e,f,g) indicate the outline of the studied crystalline grain in domains A, B, C and D. The green arrows in (b,c) indicate the ED diffractions from the Li2MnO3-like structure projected along the [001̅]mon direction, the yellow arrows indicate the overlapped ED diffractions from both the Li2MnO3-like and LiTMO2 structures, and the yellow circles in (b) indicate multiple reflection by both the Li2MnO3-like and LiTMO2 structures. The simulated ED patterns with white, red, green, and blue colors in (c) originate from the two-structure diffractions associated with domains A−D. The blue plates in (h,i) indicate the LiMn2-like layers of the Li2MnO3-like structure.

materials, including direct observation of the Li2MnO3-like structure unit distribution in crystalline grain, structure-, or composition-coordination environments, and the relationship with the Li+ migration behavior are still unclear, thus preventing further in-depth understanding of their electrode kinetics, structure evolution, cycle stability, and reaction mechanisms. Undoubtedly, it is imperative to gain a clear understanding of the Li2MnO3-like structure of a crystalline grain because it is the basis for understanding these compounds and improving their electrochemical performance. Here, based on cross-sectional thin transmission electron microscopy specimens (CSTTs) “anatomized” from Li1.2Mn0.567Ni0.166Co0.067O2 powders, the

Several studies that have investigated the structure of LLOs have been conducted.4,22,26,37−39 Delmas and Tarascon have revealed that the complicated local structure of LLOs can be attributed to rotation of the LiMn2-like layer in the Li2MnO3like structure,40,41 which has also been demonstrated in our work.38 Furthermore, we have identified the coexistence of two structure units (rhombohedral LiTMO2 structure with R3̅m space group and monoclinic Li2MnO3-like structure with C2/m space group, which are shown in Figure S1a) on the grain edge using advanced microscopy technology,38 which has been well validated by refining the synchrotron X-ray diffraction patterns.35 However, the interior configuration of LLO B

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Figure 2. Atomic-scale Li2MnO3-like crystal domain distribution in domain A. (a) The superlarge and atomic-resolution HAADF STEM images in domain A of Figure 1d. (b,c) HAADF and ABF STEM images of the enlarged two-structure coexistence regions highlighted in yellow in image (a). The red and blue dotted lines in (a) and (b) indicate the rhombohedral LiTMO2 and monoclinic Li2MnO3-like crystal domains, respectively. (d) Line intensity profiles from point 1 to 2 and from point 3 to 4 in (b). The green arrows in (d) indicate the periodic difference between these two structures. Rh and Mon are used to represent the rhombohedral LiTMO2 and monoclinic Li2MnO3-like structures, respectively. (e) Representational HAADF STEM image of the monoclinic Li2MnO3-like crystal domain with several stacking faults in domain A, which are linked by the red lines. (f) Schematic of the monoclinic Li2MnO3-like structure with stacking faults in (e).

atomic-scale Li2MnO3-like crystal domain distribution within multiple monocrystal-like domains and domain boundaries (DBs) in a single crystalline grain was revealed for the first time using a variety of microscopy techniques and computer simulations. The Li+ migration in the Li2MnO3-like crystal domain along different directions with or without DBs and the effect of implantation of element segregation were thoroughly investigated using density functional theory. Results and Discussion. Large and Smooth CSTTs-LLOs. In previous (scanning) transmission electron microscopy ((S)TEM) observations of as-synthesized LLOs with grain sizes of 200−500 nm (Figure S1a in Supplementary Section I), we could only focus on the edges of grains because the electron beam could not transmit through the thick specimens.38,42 As shown in conventional bright-field (BF)-TEM images (Figure S1c,d), we could not observe and reveal the interior structure of the grains due to their large thicknesses; the region that could be observed was limited to a very small area. In this work, to obtain a large thin area for (S)TEM observations and reveal both the interior and exterior structures of these LLOs, pristine LLOs were “anatomized” using a new technique (the argon ion slicer (ArIS) method, which is based on a shadowing technique) to yield suitable CSTTs (Figure S1b) in which large and homogeneous cross sections of the powders are thin enough (about 20−50 nm) for electron transparency. The BFTEM images of these CSTTs-LLOs shown in Figure S1e,f indicate obviously transparent color compared with the assynthesized LLOs (Figure S1c,d), and the smooth electron transparent area in the CSTTs-LLOs has been estimated to be approximately 1.0 × 105 nm2, which is larger by several orders of magnitude than that of the as-synthesized LLOs. On the basis of these CSTTs images, not only the edges but also the interior structures of these crystalline grains can be clearly investigated, and the confusions caused by structure over-

lapping of thick specimen for STEM observation can well avoided. Multiple Monocrystal-Like Domains in CSTTs-LLOs. The structures of many CSTTs-LLO crystalline grains were investigated in this study, and a BF-TEM image of a representative grain is shown in Figure 1a. As indicated by the dashed line in Figure 1a, we observed DBs in the grain, which suggested that the interior structure of the grain was not monocrystal but instead consisted of multidomains. The size of each domain was approximately 100 nm. Figure 1b shows an electron diffraction (ED) pattern taken from the whole grain, which includes spotted streaks from the Li2MnO3-like (C2/m) structure and diffraction spots (indicated by the yellow arrows) from LiTMO2 (R3̅m) or Li2MnO3-like (C2/m) structures, similar to our previously reported ED patterns obtained from the grain edge of LLOs.38 However, some other diffraction spots (indicated by the green arrows in Figure 1b) were also detected. To reveal the relationship between the ED patterns and the interior structure of the grain, we took conventional dark-field TEM (DF-TEM) images using the diffraction spots of A−D in Figure 1b. Figure 1d−g shows the DF-TEM images using spots A through D shown in Figure 1b. The domains A through D are separately visible in the DF-TEM images. Therefore, it can be concluded that the grain is composed of domains A through D, as shown in Figure 1a. On the basis of the DF-TEM imaging, the domains A through D included a monoclinic Li2MnO3-like structure because the diffraction spots A through D came from the Li2MnO3-like structure (see Figures S2−S4 in Supporting Information Section II). The rhombohedral LiTMO2 structure could not be individually shown by DF-TEM imaging because all of the ED patterns of the LiTMO2 structure were included in those of the Li2MnO3-like structure (Supporting Information Section II). However, as shown in the following atomicresolution images, the existence of the LiTMO2 structure was C

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Figure 3. Atomic-scale domain boundaries in the grain. (a−c) Atomic-scale HAADF STEM images of the existence of the A, B domains (a), A, D domains (b) and D, B domains (c). The yellow lines in (a) indicate the transitional region between domains A and D. The Fourier transforms of domains B and D are presented in the insets of images (a,b), respectively; the patterns circled in yellow highlight the ED patterns of the Li2MnO3like structure, whereas the patterns circled in red denote the overlapped ED patterns reflected by both the LiTMO2 and Li2MnO3-like structures. (d,e) Enlarged HAADF/ABF STEM images of the region marked in green in images (a,c), respectively. Simulated HAADF and ABF STEM images are shown in (d,e), respectively. (f,g) HAADF STEM and line intensity profile (from point 1 and 2) of the enlarged domain boundaries between domains A and B in (a). The yellow arrows in (g) denote the transition metal layer of domain B in the transitional region between domains A and B. (h,i) Line intensity profiles and enlarged one-row HAADF STEM images between points 1 and 3 in domain A and between points 2 and 4 in domain B, respectively, are shown in image (b). These demonstrate the transition between the Li2MnO3-like or LiTMO2 structure in domains A and D. (j) Line intensity profiles and enlarged one-row HAADF STEM images between point 5 in domain D and point 6 in domain B are shown in image (c). These demonstrate the transition between domains D and B. (k) Schematic of the Li2MnO3-like and LiTMO2 “twin domains” distribution in the grain with multiple monocrystal-like domains.

at the position of spot D (Figure S5 in Supporting Information Section II). However, this structure was determined to be unlikely to form based on the atomic-scale STEM analysis presented in the following section. The ED spots circled with yellow dotted lines in Figure 1b are considered to be related to

directly evidenced. Note that bright and dark band contrasts are observed in domains A−C, which imply the existence of stacking faults in the Li2MnO3-like structure. In this sample, a spinel LiMn2O4-like structure could also be formed from a point in the composition and caused a reflection D

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Figure 2e, the LiMn2-like layers with different stacking configurations along the [103]mon-axis are revealed, as indicated by the red line. The stacking configurations of the LiMn2-like layers can be interpreted as the formation of three structural units projected along the [100]mon, [1̅10]mon, and [1̅1̅0]mon directions, where the three structures are related by rotating approximately 120° with each other along the [103]mon axis (see Figure S2). The three observed structures are thin planner crystals, and they caused the streaks along the [103]mon axis in the ED pattern shown in Figure 1b and the band contrasts in the DF-TEM images shown in Figure 1d. A schematic of the orientation relationship between the LiMn2-like layers is presented in Figure 2f. All of the atomic-scale images are consistent with the ED analysis. Figure 3a−c shows HAADF-STEM images that demonstrates the existence of domains A, B, and D and their DBs. In domains B and D, the simultaneously recorded HAADF/ABFSTEM images are shown in Figure 3d,e, respectively, with a larger magnification of the region indicated by a green square. The observed features of the HAADF/ABF-STEM images in domains B and D are well reproduced by the image simulations shown in Figure 3d,e, respectively. In Figure 3d, the simulated images of the monoclinic Li2MnO3-like structure projected along the [011]mon, [3̅2̅3]mon, [61̅3]mon directions and the rhombohedral LiTMO2 structure projected along the [8101]rh direction overlap with each other (Figure S9). The orientation relationship between the [011]mon, [3̅2̅3]mon, and [61̅3]mon directions is associated with 120° of rotation along the [103]mon axis, equivalent to the relationship between the [100]mon, [1̅10]mon, and [1̅1̅0]mon directions, as shown in domain A. Therefore, the crystal domain distribution in domain B was most similar to that in domain A, and the “twin domains” were distributed randomly in this domain. In Figure 3e, the simulated HAADF/ABF-STEM images originated from the overlapping of the simulated images of the monoclinic Li2MnO3-like structure projected along the [001̅]mon, [3̅16̅]mon, and [3̅1̅6]mon directions and the rhombohedral LiTMO2 structure projected along the [11̅1]̅ rh direction (Figure S10). Therefore, the “twin domains” distribution in domain D was most similar to those in domains A, B, and C. Note that the LiMn2O4-like spinel structure is not proposed to exist in the domain D because of the disagreement between experimental HAADF/ABF-STEM images and image simulations (Figure S11). The DBs between domains A and B, A and D, and B and D are clearly observed in Figure 3a−c, respectively, and are indicated with red dotted lines. The enlarged HAADF-STEM image and the intensity profile across the DB from point 1 to 2 are shown in Figure 3f,g, respectively. Figure 3f,g shows that domain A was connected smoothly with domain B and that the HAADF-STEM intensity of domain A gradually changed to that of domain B in the transitional region. The transitions from the LiTMO2 and Li2MnO3-like crystal domains in domain A to those crystal domains in domain D are shown clearly, with the line intensity profiles and the enlarged one-row HAADF-STEM images in Figure 3h and 3i, respectively; the transition between the two crystal domains in domains B and D is evidenced by the line intensity profile and the corresponding one-row HAADF-STEM image shown in Figure 3j. The domains are connected smoothly with each other, similar to Figure 3f. Therefore, through atomicresolution HAADF/ABF-STEM observations and computer simulations, in combination with ED and BF/DF-TEM techniques, all of the detailed structures in domains A, B, C

multiple reflection of the monoclinic Li2MnO3-like structure or rhombohedral LiTMO2 structure, although these patterns could also be generated from the spinel LiMn2O4-like structure (see the details presented in Supporting Information Section II, Figure S5e). All of the experimental ED spots in Figure 1b agree well with the simulated ED pattern in Figure 1c. The three-dimensional sketch map of these domain relationships is shown in Figure 1h, where the structure of each domain is represented by stacking the LiTM2-like layers along the [103]mon direction. When the LiTM2-like layers in domain A were observed along the [100]mon direction, the LiTM2-like layers in domains B−D were observed along the [011]mon, [011̅ ]mon, and [001̅]mon directions (vice versa), respectively, as shown in Figure 1i. Although the domain structure of the grain seems to be complicated, the crystallographic environment of the domains is equivalent. We observed many grains with various zone axes and found that all of the grains consisted of multiple monocrystal-like domains with a similar orientation of the domain relationships as in Figure 1i. The existence of multidomains and relative DBs in a grain will surely affect the Li+ diffusion behavior, which will be discussed in detail in the last section. Atomic-Scale Li2MnO3-Like Crystal Domain Distribution in CSTTs-LLOs. On the basis of the large and smooth CSTTs-LLO specimens, the atomic-scale structures of different domains (A−D) and DBs were successfully revealed using high-angle annular dark-field (HAADF) and annular bright-field (ABF) STEM imaging. Figure 2a shows a clear HAADF-STEM image of domain A with a large area of 430 nm2. Two types of atomic arrangements, successive bright-dot (indicated by the red dotted lines) and periodic dark-bright-bright-dot (unlabeled dots) patterns, are clearly observed in the image. The simultaneously recorded HAADF- and ABF-STEM images, including both types of atomic arrangements, are shown in Figure 2b,c, respectively. In the HAADF-STEM image (Figure 2b), only TM atoms are visualized as bright spots, whereas in the ABF-STEM image (Figure 2c), all of the TM, oxygen, and even lithium atoms (between oxygen layers or between TMs within LiMn2-like layers) can be identified as dark dot contrasts. Here, “twin domain” of LiTMO2 and Li2MnO3-like crystal domain can be distinguished unambiguously by detection of the two characteristic periodic arrangements on the TM layers: TM−TM−TM for the LiTMO2 structure and Li−TM−TM for the Li2MnO3-like structure (Figures S6−S8 in Supporting Information Section III). Figure 2a−c clearly shows both TM−TM−TM and Li−TM−TM arrangements over the entire region, which proves that “twin domain” coexisted and were randomly distributed in domain A. Some Li−TM−TM arrangements with weak dot contrasts are considered as Li/ TM−TM−TM arrangements, supposing that a few Li atomic sites in the LiMn2-like layer of Li2MnO3-like structure units are occupied by TM atoms.38 The shortest TM−TM spacing of both structures in Figure 2b,c is 0.14 nm (see Figure S8), as confirmed by the line intensity profiles shown in Figure 2d, which were obtained from point 1 to 2 and from point 3 to 4 in Figure 2b (Figure 2d). These results strongly suggest the nonsolid solution of LLOs and thus the intergrowth of LiTMO2 and Li2MnO3-like “twin domain” in the LLO grains. Note that domain A included not only “twin domains” but also abundant stacking faults in the Li2MnO3-like crystal domain, as mentioned in the ED analysis (Figure 1). Figure 2e shows one of the representative HAADF-STEM images of domain A. In E

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Figure 4. DFT of Li+ migration in LLOs. (a,b) Optimized DB calculation models projected along different directions based on the relationship shown in Figure 3h (Li2MnO3-like structure projected along the [11̅ 0]mon direction versus Li2MnO3-like structure projected along the [001]̅ mon). The calculated interface energy is only 0.42 J/m2. (c,d) Li+ migration formation energy perpendicular and parallel to the DB, respectively. (e,f) Li+ diffusion trajectories (yellow dotted lines) perpendicular and parallel to the DB, respectively.

EDB+vacancy and EDB are the calculated total energies of the DB model with and without a Li vacancy, respectively. ELi is the total energy of a Li metal obtained by the density functional theory calculations. In principle, the ΔEf in the domain A (or D) model and that in the bulk-like regions of the DB model should have the same values. As shown in Figure 4c, however, there was a difference of approximately 0.5 eV between them in the present calculations. This difference arose because the DB model was not large enough to discount the influence of the DB in the bulklike region. It should be noted that short-range Li+ migration energy barriers in the bulklike region of the DB model were similar to those of the domain interior models. The relative energy states during the migration process were reproduced well. The energy profiles of long-range Li+ migration through the DB were clearly different from those of the bulk. The highest energy barrier in the Li⊥DB direction increased from 0.88 to 1.01 eV and from 0.83 to 1.13 eV in domains A and D, respectively. The highest energy barrier of Li+ migration in the Li//DB direction was 0.90 eV, similar to that of 0.87 eV in the bulk. Figure 4e and f show the Li+ migration trajectories of the Li⊥DB and Li//DB modes, respectively. These calculations suggest that the DB should reduce the Li+ mobility; however, only one DB was considered in this model. Multiple DBs exist in real grain, as shown in Figure 1a; thus, the Li+ migration energy barriers in grains will greatly increase in actual LLOs. Furthermore, the DB model shown in Figure 4 is only a

and D of CSTTs-LLOs and the domain transitions were clearly observed. The spatial distribution map of the Li2MnO3-like and intergrown LiTMO2 “twin domains” in the entire crystalline grain with multidomains and DBs are presented in the schematic shown in Figure 3k. Multiple monocrystal-like domains in a single grain increase the Li+ migration interface number from the grain interior to exterior and can influence the electrode kinetic behavior of LLOs. To understand the relationship between the crystal structure and electrochemical performance, first-principles density functional theory on Li+ migration in a Li2MnO3-like crystal domain with or without DBs was thoroughly investigated in this work. Figure 4a,b shows the optimized DB calculation models based on the relationship shown in Figure 3h (the process models are detailed in Supporting Information Section V). Li+ migration with a vacancy mechanism was introduced in this DB model and is described in detail in Supporting Information Section VI. Two different paths for Li+ migration were considered: one was perpendicular to the DB (noted Li⊥DB), and the other as parallel to the DB (noted Li//DB). Figure 4c,d shows the energy barrier comparison of Li+ migration perpendicular and parallel to the DB, respectively with and without the DB. The formation energies (ΔEf) represented in the vertical axes of Figure 4c,d were evaluated using the following equation ΔEf = (E DB + vacancy + E Li) − E DB F

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Figure 5. Composition distribution of the grain and its effect on Li+ migration. (a) HAADF-STEM image of CSTTs-LLOs for composition investigation. (b−e) Energy dispersive spectrometer maps of oxygen, manganese, nickel, and cobalt, respectively, of the grain shown in (a). (f,g) Li+ migration formation energy perpendicular and parallel to the DB, respectively, with nickel segregation at DBs. (h,i) Li+ diffusion trajectories perpendicular and parallel to the DB, respectively, with nickel segregation at DBs. In the DB model with nickel segregation, only one nickel ion in the TM layer is considered, and nickel at a DB is marked in green.

structure model, and the influence of composition on Li+ migration was not considered. One of the representational energy dispersive spectrometer maps of TMs and oxygen is shown in Figure 5b−e, which is based on the HAADF-STEM image shown in Figure 5a. It is clear that oxygen and manganese were uniformly distributed in the grain, whereas the distributions of nickel and cobalt were uneven. In particular, nickel was segregated not only at the grain surface but also at the DBs. On the basis of LiMn0.42Ni0.42Co0.16O2 electrodes for LIBs, our previous research clearly demonstrated that the greatly increased lattice stress and degraded rate performance are caused by nickel ion migration from the TM layer to the Li layer.43 Thus, the nickel segregated in the Li layer of DBs will increase the Li+ migration energy barriers. In this study, a Li2MnO3-like structure model with nickel segregation in the TM layer of DBs was established (see Supporting Information Section VII for details), and the influence of nickel segregation at DBs on the Li+ migration kinetics was revealed. As shown in Figure 5f,g, nickel segregation clearly changed the Li+ migration behavior near the DB. The relative Li+ migration trajectories in the Li⊥DB and Li//DB direction are shown in Figure 5h,i, respectively. The energy barriers of the Li⊥DB path just over the DB increased by approximately 0.20 eV in comparison with Li⊥DB without nickel segregation. In this region, Li+ moves near the

nickel atom. The influence of the nickel atom becomes negligible in regions far from the nickel atom. Although the nickel segregation slightly changed the barrier heights of longrange migration with Li⊥DB, as shown in Figure 5f, it should be noted that the energy barriers of long-range migration with Li⊥DB with nickel segregation were still greater than those of the bulk. In contrast, Li+ migration with Li//DB was greatly affected by nickel segregation. Because of this segregation, the highest energy barrier of Li+ migration with Li//DB increased from 0.90 (Figure 4d) to 1.55 eV (Figure 5g). Therefore, the stochastic distribution of DBs caused by the Li2MnO3-like crystal domain with rotating stack and the nickel segregation at DBs in the grains will largely impede the Li+ extraction/insertion from/into the crystals, which influences the poor rate performance of LLOs. In addition to determining an unambiguous spatial structure using a variety of advanced microscopy techniques and understanding the structureperformance relationship of these high-energy electrodes for LIBs, this work is also extremely important in establishing the basis of their voltage degradation mechanism and mysterious highly reversible capacity through further credible structureevolution investigation of these electrodes during electrochemical cycling. The present findings emphasize that the structure of these materials is not a simple-solid solution; nanosized “twin domains” separation and Li2MnO3-like crystal G

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Nano Letters

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domain stacking fault in the multidomains of crystalline grains create complexity in understanding their actual structures. Moreover, atomic-resolution microscopy structure investigations on large and smooth CSTTs (with an area of approximately 100 000 nm2) will largely eliminate the possibly biased conclusions that have come before. Because LLOs are one of the electrode materials that possess a complex crystal structure and unexplained electrochemical phenomena, this work should motivate future studies to map the crystalline grain interior structure of many other compounds for LIBs and to reveal the relationship between the electrochemical performance and interior configuration.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.5b03933. ED simulation of the whole crystal particle, “twin domain” distinguish and separation, HAADF/ABF STEM image simulation, domain boundary models of Li2MnO3-like structure, Li+ vacancy migration mechanism, and Ni-segregated domain boundary model. (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*(H.Z.) E-mail: [email protected]. *(Y.I.) E-mail: [email protected]. Author Contributions

H.Y., Y.-G.S., and A.K. contributed equally to this work. Dr. Haijun Yu prepared the material, designed the experiments, analyzed the STEM and electron diffraction pattern data, and wrote the manuscript. Dr. Yeong-Gi So observed and analyzed the STEM and electron diffraction pattern. Dr. Akihide Kuwabara constructed the DB models of the Li2MnO3-like structures and calculated the Li+ migration energy barriers. Dr. Eita Tochigi discussed and revised the manuscript. Professor Haoshen Zhou and Professor Yuichi Ikuhara designed the experiments and directed the entire study. Professor Tetsuichi Kudo and Dr. Naoya Shibata discussed the STEM and electron diffraction patterns. We also want to thank Professor Lin Gu for valuable discussion regarding the STEM and electron diffraction patterns. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Part of this work was supported financially by the Funding Program for World-Leading Innovative R&D on Science and Technology (FIRST Program), a Grant-in-Aid for Scientific Research on Innovative Areas, “Nano Infomatics” (Grants 25106003 and 25106008) from JSPS, the National Natural Science Foundation of China (Grants U1507107 and 21503009) and the Funding Projects for “Thousand Youth Talents Plan” and “Returned High-level Talents of Beijing”.

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REFERENCES

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DOI: 10.1021/acs.nanolett.5b03933 Nano Lett. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.nanolett.5b03933 Nano Lett. XXXX, XXX, XXX−XXX