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https://doi.org/10.1038/s41467-019-09408-2 OPEN Dynamic imaging of crystalline defects in lithium-manganese oxide electrodes during electrochemical activation to high voltage
Qianqian Li1,2,3, Zhenpeng Yao 3,5, Eungje Lee 4, Yaobin Xu 3, Michael M. Thackeray4, Chris Wolverton3, Vinayak P. Dravid3 & Jinsong Wu1,3
1234567890():,; Crystalline defects are commonly generated in lithium-metal-oxide electrodes during cycling of lithium-ion batteries. Their role in electrochemical reactions is not yet fully understood because, until recently, there has not been an effective operando technique to image dynamic processes at the atomic level. In this study, two types of defects were monitored dynamically during delithiation and concomitant oxidation of oxygen ions by using in situ high-resolution transmission electron microscopy supported by density functional theory calculations. One stacking fault with a fault vector b/6[110] and low mobility contributes minimally to oxygen release from the structure. In contrast, dissociated dislocations with Burgers vector of c/2[001] have high gliding and transverse mobility; they lead to the formation, transport and release subsequently of oxygen related species at the surface of the electrode particles. This work advances the scientific understanding of how oxygen participates and the struc- tural response during the activation process at high potentials.
1 State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan, Hubei 430070, China. 2 Materials Genome Institute, Shanghai University, Shanghai 200444, China. 3 Department of Materials Science and Engineering, NUANCE Center, Northwestern University, Evanston, IL 60208, USA. 4 Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, IL 60439, USA. 5Present address: Department of Chemistry and Chemical Biology, Harvard University, 12 Oxford Street, Cambridge, MA 02138, USA. These authors contributed equally: Qianqian Li, Zhenpeng Yao. Correspondence and requests for materials should be addressed to Q.L. (email: [email protected]) or to V.P.D. (email: [email protected]) or to J.W. (email: [email protected])
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’ ithium-ion batteries are today s dominant electrical energy layer of the Li2MnO3 structure with the speculation that oxygen storage technology; they continue to attract research and diffusion occurs sluggishly throughout the charged Li2MnO3 L fi 35–37 development support to improve their speci c energy, structure before O2 gas is released at the particle surface . power, durability, cycling stability, and safety for emerging Although defects are commonly observed in electrochemically markets such as electric vehicles1,2. Conventional cathode mate- cycled lithium-metal-oxide electrodes38,39, they are often not rials are typically lithium transition metal oxides and phosphates, mentioned when describing reaction mechanisms40. While it 3 4 such as LiCoO2 (LCO) , LiNi1 − x − yMnxCoyO2 (NMC) , is still not clear if there is a connection between crystalline 5 6 LiMn2O4 (LMO) , and LiFePO4 (LFP) , that operate typically by defects and oxygen redox and evolution reactions, defects (de-)intercalation of lithium during charge and discharge when induced into electrochemically cycled Li2MnO3 electrodes have the transition metal ions are oxidized and reduced, respectively, been widely observed38,39. Stacking faults in the Mn-rich to store and release electrical energy. In this case, the specific layers have been detected through X-ray diffraction and TEM 34,41,42 capacity of the cathode, and hence the energy of the cell, is limited measurements in both pristine Li2MnO3 and in partially 39 by the number of electrons per transition metal ion that can delithiated “Li2 − xMnO3” samples . Other crystallographic participate in the redox reactions. Lithium-rich metal-oxide defects, such as partial dislocations, have also been identified 3+ 4+ 40 electrodes that operate by both cationic (e.g., Mn ↔Mn ) and during the charging of Li2MnO3 . While these planar defects anionic (e.g., O2−↔O1−) or hybridized redox reactions are are generated to release mechanical strain and stress, their attractive materials because they have the potential to increase contribution to electrical energy storage and oxygen release in the energy storage capacity of lithium-ion batteries. Examples lithium-ion batteries remains unclear. of materials that operate by anionic electrochemical reactions are In this study, the relationship between crystalline defects and 7,8 9 10 11 Li2Ru1 −ySnyO3 ,Li3NbO4 ,Li3IrO4 ,Li5FeO4 ,Li2Mn1 −y lithium extraction and oxygen evolution reactions in Li2MnO3 12 13 14 MyO2F ,Li4Mn2O5 , and Li4(Mn,M)2O5 . The reversible has been probed in detail. In situ TEM combined with density capacity of these reactions in lithium-rich materials is enabled functional theory (DFT) calculations have been used to study 10 by highly covalent metal-oxygen bonding or by non-bonding the structural evolution of a Li2MnO3 electrode during the oxygen p orbitals generated by local lithium-excess configurations first charge (delithiation) and the mechanism of oxygen loss. around O in the structure11,14–16. The in situ TEM complemented by DFT calculation approach Li2MnO3 has a theoretical capacity of 459 mAh/g, which cor- has proved to be an effective method for observing and responds to the extraction of 2 Li per formula unit, when analyzing the dynamic evolution of microstructure in battery 17 43–46 Li2MnO3 is activated chemically with acid or electrochemically electrodes during lithiation/delithiation cycles .First,it above 4.5 V vs. Li+/Li in lithium cells18. Lithium extraction, allowed us to identify dynamic defects that appear in the hydrogen-ion exchange, and oxygen loss reactions trigger a Li2MnO3 structure during the electrochemical reaction, which conversion of the parent layered structure to one with spinel-like are different to those that exist in the pristine state. Second, the features, which severely compromises the practical capacity, results shed light on the reversibility of oxygen redox reactions electrochemical potential, and cycling stability of the electrode at the atomic scale and the irreversibility of reactions that 17–21 fi and cell . However, when integrated with a LiMO2 compo- are associated with oxygen loss, which have signi cant impli- − fi nent, the resulting xLi2MnO3·(1 x)LiMO2 composite structures cations for lowering the cycling ef ciency of the electrode, deliver a rechargeable capacity of more than 250 mAh/g after particularly on the firstcycle.Giventhenatureofelectro- electrochemical activation of the Li2MnO3 component above chemical lithium extraction reactions, we presume that these 4.5 V22. Unfortunately, structural instabilities and voltage fade of dynamically formed defects result from changes in localized these high capacity electrodes during cycling have thus far pre- lithium-ion concentration. Two types of defects were observed: cluded their use in commercial lithium-ion battery products23–29. One is a stacking defect with a fault vector of b/6[110], which Although pure Li2MnO3 is now viewed as an unrealistic cathode has low activation energy for mobility that we tentatively material for commercial lithium-ion battery applications due to associate with a reversible oxygen redox reaction (i.e., without its rapid degradation, the underlying mechanism of the failure is oxygen loss). The second is a dissociated dislocation with unclear. Here we reported experimental finding and theoretical Burgers vector of c/2[001] that prompts the formation and modeling results, which provides deeper insights on the under- release of O2 at high electrochemical potentials (above 4.5 V), lying failure mechanisms. thereby contributing to capacity loss during the initial charge/ Despite the progress made, a comprehensive understanding of discharge cycle. These discoveries and observations have pos- the complex reaction mechanisms that occur during the electro- sible implications for designing new materials and controlling − chemical activation of structurally integrated xLi2MnO3·(1 x) reversible oxygen redox reactions in high capacity lithium- LiMO2 electrodes is still lacking. Such knowledge is critical if the metal-oxide electrodes, notably those containing a Li2MnO3 limitations of anionic reactions are to be overcome. For this rea- component. son, in situ transmission electron microscopy (TEM) images of a Li2MnO3 electrode were recorded to monitor the dynamic struc- tural changes that occur during the initial charge of the cell. A Results particular objective was to search for clues that might unravel the As synthesized Li2MnO3 and its defects.Li2MnO3 has a layered mechanism by which oxygen is lost from the Li2MnO3 electrode monoclinic structure (space group C2/m), with an atomic con- fi structure, while maintaining the tetravalent oxidation state of the guration, Li[Li1/3Mn2/3]O2, in which layers of lithium (Li) fi manganese ions according to a simpli ed, ideal anodic electro- alternate with layers of lithium and manganese (Li1/3Mn2/3) chemical reaction22,30–33: wherein the Li:Mn ratio is 1:2. (Supplementary Fig. 1). In the þ À manganese-rich layer, the Li and Mn ions are arranged in a Li MnO ! MnO þ 2Li þ 1= O þ 2e : ð1Þ 2 3 2 2 2 honeycomb fashion as illustrated in Supplementary Fig. 2a. Structural changes and oxygen loss that occur during deli- Varying the stacking order of the manganese-rich (Li1/3Mn2/3) fl thiation of Li2MnO3 have already been reported by several layers in uences the crystal symmetry of the system. For example, groups, for example, by Rana et al.30 and Yu et al34. These studies as shown in the Supplementary Fig. 2b, the addition of a second disclose, without specifying a mechanism, that delithiation occurs manganese-rich layer generates “close-packed” AB stacking concurrently from both the lithium layer and the transition metal (note: with reference to the metal cation layers only)47. The
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C lithium-ion deficiencies and resulting crystal strain, making it a c 0 s f 1 Li energetically favorable for the (001) lattice planes to glide during [100] domain O the early stages of delithiation (Fig. 1f–h). C Mn B Stacking faults B 1 0.420.47 nm nmA The in situ TEM studies revealed another defect type, not observed in the pristine Li2MnO3 electrode, but uniquely [110] domain A generated by the delithiation process. It can be described as a C d 301 s g 1 dissociated partial dislocation with Burgers vector c/2[001] with Stacking faults a simultaneous transverse movement or “climbing” of the partial C dislocation. More precisely speaking, the defect is a dissociated C2 0.50 nm C 1 0.45 nmA 2 dislocation consisting of an antiphase boundary (with fault 5 nm vector of 1/2[001]) and the partial dislocation bounded to the A antiphase boundary. The “fault plane” of the antiphase boundary b (1–11) e 408 s h B is in the (100) plane with the atomic structure of the defect (1 – 10) (001) shown in Fig. 2b. Climbing of the dislocation refers to the (021) B movement of the defect across the (100) plane, while gliding 0.52 nm C C2 0.46 nmA 2 refers to the movement in the (100) plane. This information (020) leads us to believe that this active defect motion is largely A responsible for the transport of an oxidized oxygen species within Fig. 1 The exitance form and motion of stacking faults in lithium extraction the Li2MnO3 crystal and the ultimate release of oxygen gas at the process. a Transmission electron microscopy (TEM) image of (001) surface. Experimental (TEM) and computational evidence for this hypothesis is provided in Figs. 2–4. stacking faults in pristine Li2MnO3 with fault vector of b/6[110], which are confirmed by a corresponding electron diffraction (b). During delithiation, As shown in the TEM image (Fig. 2a) and the computer model the gliding of the b/6[110] partial dislocation shears the stacking of the of a slightly delithiated Li2 − xMnO3 −δ structure (Fig. 2b), in which x and δ are both small, defects exist as dissociated (001) plane from ABC1 in pristine Li2MnO3 (c), to AC2C1 (d), and AC2B (e) after 0, 301, and 408 s, respectively; corresponding density functional dislocations, or stacking faults, bounded by two partial disloca- tions. As the c/2[001] Burgers displacement vector is perpendi- theory (DFT) structural models are provided of pristine Li2MnO3 (f) and generated defects (g, h), respectively. The scale bar is 5 nm cular to the (001) lattice plane, the defect cuts the Li2MnO3 crystal into small fractions along the (001) plane. When further lithium ions are electrochemically removed from the structure, 41 addition of the third layer would provide ABC1 and ABC2 the density of defects increases significantly, as indicated by the orderings, corresponding to two basic stacking sequences: growing number of green arrowheads in Fig. 2c–f. Along the fault C2/m (ABC1) and P3112 (ABC2). More complex stacking order- plane (100), contrast of the white spots corresponding to Mn ings (e.g., well-ordered C2/c48 and other faulting arrangements39) columns becomes weak, possibly due to the Mn migration in the can be achieved by combining these two basic sequences in Li layer. It is thus suspected that the Mn migration is related to different ways. formation of the defect, as the migration happens in the core of Stacking faults exist in pristine Li2MnO3 samples when the defect. synthesized at 800 °C in air39,40,42,49. These planar defects can A comparison of an experimental voltage-composition plot of a clearly be seen at the domain boundaries of two orientation Li/Li2MnO3 cell (black line) with the calculated plot of a cell variants, namely the [100] and [110] domains in Fig. 1a; the (colored lines) with a Li MnO electrode containing defects is fi 2 3 stacking fault disorder in pristine Li2MnO3 is con rmed by the provided in Fig. 2g. The b/6[110] defects are believed to be offset and streaking of the diffraction spots in the corresponding associated predominantly with the first step (4.89 V), that is, TEM diffraction pattern shown in Fig. 1b. The defect shown in without oxygen loss, whereas the c/2[001] defects are believed to Fig. 1b is the result of a shear of the (001) layers; this defect can be be associated predominantly with the second step (5.03 V), that described alternatively as a stacking fault bounded by a partial is, with oxygen loss. Without the c/2[001] defects, the second step dislocation with a Burgers vector b/6[110]50. These planar defects was calculated to occur at a slightly higher voltage (5.13 V). release and accommodate strain and stress in the Li2MnO3 Figure 2h illustrates a schematic evolution of the c/2[001] defects crystals. Similar defects have been observed by others in partially and a generalized mechanism by which oxygen species can be 39 delithiated and relithiated samples , implying that these defects transported through a highly defective and faulted Li2MnO3 are active sites during charge and discharge reactions but might electrode structure before being released as fully oxidized O2 gas not participate significantly in oxygen loss reactions. at the surface. Although dislocations that move transversely to a glide plane Dynamic defects generated in delithiation. In situ TEM images in metal-oxide structures are unusual, the c/2[001] dislocation recorded during the initial stages of electrochemical delithiation, formed dynamically during electrochemical delithiation of – that is, after 0, 301, and 408 s, are depicted in Fig. 1c e. A Li2MnO3 can glide and climb with apparent ease. The evolution description of the cell design, which we have used effectively in of these defects (indicated by the green bars) as delithiation 45 – previous studies of lithium insertion electrodes, such as Co3O4 , progresses is highlighted in Fig. 3a d; corresponding Fourier is provided in detail in the Supplementary Information section. filtered images, showing only the (001) lattice planes to The structural changes that occur by the glide of the b/6[110] emphasize the perpendicular movement of the defects relative dislocation during delithiation (Fig. 1c–e) were interpreted with to the (001) planes, are shown in Fig. 3e–h. The dislocations the aid of structural models predicted by DFT calculations glide progressively toward the right surface, as indicated by (Fig. 1f–h). The data show that, on gliding, the stacking sequence the changing position of the blue “half-cross” markers with of a (001) lattice plane in a [100] domain changes from ABC1 in increasing delithiation (reaction time). During this process, pristine Li2MnO3 to an intermediate AC2C1 arrangement and the distance between defects narrows until they merge and subsequently to AC2B. During this process, the (001) lattice become one (Fig. 3d, h). The Li compositional gradient between spacing increases from ~0.47 to ~0.52 nm. The DFT models the surface and the core (and thus the strain caused) could be the show, as expected, that the stacking fault defects are induced by driving force for the climbing and gliding of the defects.
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ab0 s c143 s (001) plane
b
e 254 sf 702 s
O0
O2
g h 6.0 5.5 5.13 V
(V) 4.89 V O2 + 5.0 4.5 5.03 V 4.0 Voltage decrease by 3.5 introducing defects 3.0
Voltage vs. Li/Li 2.5 2.0 0 100 200 300 400
x in Li2–xMnO3 0 Li Mn O2–/1– O
Fig. 2 The form and motion of the second kind of defects upon delithiation. a Dissociated dislocation in Li2MnO3 with Burgers vector c/2[001] formed dynamically in delithiation. b Atomic model of the dislocation containing lithium vacancies in both Li- and Li-Mn layers. c–f Defect density (green arrowheads) of the Li2MnO3 electrode increases as the delithiation process progresses. g Comparison of experimental voltage–composition plot of a Li/Li2MnO3 cell (black) with the calculated plot of cells containing Li2MnO3 electrodes with b/6[110] defects dominating the first step (4.89 V) and c/2[001] defects dominating the second step (5.03 V). Without the c/2[001] defects, the second step would occur at voltage of 5.13 V. h Illustration of the c/2[001] defects and a proposed mechanism by which oxygen is transported in a highly defective Li2MnO3 electrode structure and released at the surface
a 532 s b 562 sc 592 sd 622 s
GlidingG lid in g
5nm e f g h
g in b lim ClimbingC
Fig. 3 Dynamic gliding and transverse movement (“climbing”)ofac/2[001] dislocation during delithiation. a–d Time-lapsed high-resolution episcopic microscopy (HREM) images. e–h Corresponding Fourier-filtered images of a–d, showing only the (001) lattice plane fringes. Gliding, climbing, and merging of the dislocations towards the surface prompts the formation and release of O2 gas. The scale bar is 5 nm
The creation of these defects during electrochemical delithia- (or hybridized) is not yet known. Furthermore, because the tion are dependent on lithium-ion vacancies and, in the absence c/2[001] dislocations are mobile and migrate towards the of oxidized manganese above 4+, on oxidized O2− species, as surface upon further delithiation, “trapped” oxidized O2− species proposed in Fig. 2b and referred to as O0 for simplicity and in these structural defects can be transported from the interior of 2− convenience. The extent to which the O species is oxidized aLi2MnO3 crystal to the surface, where they can either be
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released as O2 gas or, alternatively in the case of a conventional one-half of the lithium ions have been extracted from Li2MnO3, lithium-ion cell, react with an organic liquid electrolyte solvent. thereby providing clues about the composition at which energe- With this information, we tentatively propose that a dislocation- tically favorable defects would form in an inert environment. The assisted electrochemical reaction in which lithium is extracted formation of the c/2[001] defects will promote the O2 release at from Li2MnO3 ultimately with oxygen release could proceed as a earlier stage. Of particular significance, however, is that the follows: calculations indicate that the c/2[001] defect boundary and the lithium-depleted structure (e.g., x=1.875) contains short O-O (i) the formation of c/2[001]-induced defects during lithium bonding distances (~1.5 Å) (Fig. 4c) relative to the non-bonding extraction, distance of ~3.1 Å in pristine Li2MnO3, consistent with earlier (ii) the formation of oxygen vacancies within the defects with 51 2− calculations reported by Benedek et al. , thereby giving credence oxidized O species residing at defect boundaries, to the mechanism suggested in this study. (iii) the gliding and “climbing” of the defects that transport the oxidized O2− species to the crystal surface and, (iv) combination reactions between O0 species and the release Discussion In summary, two types of stacking faults and corresponding of O2 molecules at the surface. partial dislocations, formed during the electrochemical delithia- A computerized schematic of this process is provided in fi tion of Li2MnO3 electrodes have been identi ed by in situ TEM Fig. 2h. Readers are encouraged to watch the videos of the in situ studies supported by DFT calculations. Defects with fault vector TEM experiments provided in the Supplementary Information of b/6[110] appear to have low activation energy and may con- section, in which the evolution and dynamic behavior of defects tribute to reversible oxygen redox behavior. On the other hand, during electrochemical delithiation of Li2MnO3 electrodes can be dissociated dislocations with Burgers vector of c/2[001] are cre- observed in real time. ated at higher voltage (>4.5 V) and assist the transport of oxidized oxygen species to the electrode surface where O2 is formed and Oxygen release confirmed by DFT calculations. The release of released irreversibly. The study reveals an important connection 0 between crystalline defects and the electrochemical behavior of oxygen from Li2MnO3 was also assessed by calculating the O vacancy formation energy as a function of Li removal—a lower lithium-rich metal-oxide materials, which may pave the way for Δ Form further understanding and control of oxygen redox reactions, EVac value implying a more facile O extraction process, while a negative ΔEForm value implies a spontaneous release of oxygen. particularly in high capacity Li2MnO3-stabilized electrodes for Vac lithium-ion batteries. The calculations, mapped graphically in Fig. 4a, show that the calculated oxygen-vacancy formation energy decreases with increasing Li removal, but remains largely positive over the Methods compositional range (0.0 < x < 1.0), suggesting that lithium Synthesis of nanostructured Li2MnO3. All the chemicals used in the work are analytically pure grade. Stoichiometric amounts of Li2CO3 and MnCO3 precursor extraction would have to be charge compensated by a partial powders were thoroughly mixed and fired at 800 °C for 12 h. The heating rate was oxidation of the oxygen ions without any oxygen release. Spon- 2 °C/min and cooling rate was not controlled (furnace cooling). The obtained taneous oxygen release is predicted to occur only after a large powder sample was ground and sieved for the subsequent characterization and electrochemical measurements. amount of lithium has been extracted from an ideal Li2MnO3 structure, that is, Li2 − xMnO3, x > 1.5. Such structural stability seems highly unlikely particularly in a practical lithium cell In situ TEM. The open half-cell was constructed in an in situ electrical probing TEM holder (Nanofactory Instrument). This holder has a dual-probe design, that environment in which the highly oxidizing Li2 − xMnO3 electrode would be in direct contact with a reactive electrolyte solvent. is, one Au rod is used as the sample holder with a small amount of nanostructured Li2MnO3 attached to its tip; on the other side, a STM tungsten (W) probe driven by Figure 4b shows that the energy difference of Li2-xMnO3 with and Piezo-motor capable of 3D positioning with a step size of 1 nm was used to mount without c/2[001] defects becomes negative once approximately Li metal. The W probe tip was scratched by Li metal strip and then affixed on the
a c 3
2 c/2[001] defect brings down E Form (eV per O) 1 Vac 3.2 Å 1.5 Å 2.2 Å Vac Form
E 0
–1 O2 releasing favored
O vacancy formation Li0.25MnO3 Li0.125MnO3 MnO3 energy –2 0.0 0.5 1.0 1.5 2.0
b 0.5 d 0.4 0.3 0.2 0.1 0.0 –0.1 3.1 Å 1.6 Å –0.2 c/2[001] defect formation (eV per atom) favoredc Energy difference –0.3 –0.4 0.0 0.5 1.0 1.5 2.0 Original c/2[001] defect x in Li2−xMnO3
Fig. 4 The impact of defect formation on oxygen loss during delithiation of Li2MnO3. a Calculated O vacancy formation energy as a function of Li removal before and after c/2[001] defects are introduced to the system. b Energy difference between Li2MnO3 structures with and without c/2[001] defects. c Oxygen–oxygen interactions in Li2 − xMnO3 systems without defects. d Oxygen–oxygen interactions in systems with c/2[001] defects
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50. Jarvis, K. A., Deng, Z., Allard, L. F., Manthiram, A. & Ferreira, P. J. Additional information Understanding structural defects in lithium-rich layered oxide cathodes. Supplementary Information accompanies this paper at https://doi.org/10.1038/s41467- J. Mater. Chem. 22, 11550–11555 (2012). 019-09408-2. 51. Benedek, R., Thackeray, M. M. & van de Walle, A. Free energy for protonation reaction in lithium-ion battery cathode materials. Chem. Mater. 20, Competing interests: The authors declare no competing interests. 5485–5490 (2008). Reprints and permission information is available online at http://npg.nature.com/ reprintsandpermissions/ Acknowledgements Q.L., Z.Y, Y.X., E.L., M.M.T., C.W., and V.P.D. were supported as part of the Center for Journal peer review information: Nature Communications thanks Kyeongjae Cho, Electrochemical Energy Science, an Energy Frontier Research Center funded by the US Meng Gu and the other anonymous reviewer(s) for their contribution to the peer review Department of Energy (DOE), Office of Science, Basic Energy Sciences under Award of this work. Peer reviewer reports are available. # DE-AC02-06CH11357. J.W. and V.P.D. were also supported by the Samsung Advanced Institute of Technology (SAIT)’s Global Research Outreach (GRO) Program. This work Publisher’s note: Springer Nature remains neutral with regard to jurisdictional claims in made use of the EPIC facility of Northwestern University’s NUANCE Center, which has published maps and institutional affiliations. received support from the Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF ECCS-1542205); the MRSEC program (NSF DMR-1720139) at the Materials Research Center; the International Institute for Nanotechnology (IIN); the Open Access This article is licensed under a Creative Commons Keck Foundation; and the state of Illinois, through the IIN. We acknowledge the com- Attribution 4.0 International License, which permits use, sharing, puting resources from (i) the National Energy Research Scientific Computing Center, a adaptation, distribution and reproduction in any medium or format, as long as you give DOE Office of Science User Facility supported by the Office of Science of the DOE under appropriate credit to the original author(s) and the source, provide a link to the Creative contract DE-AC02-05CH11231; and (ii) Blues, a high-performance computing cluster Commons license, and indicate if changes were made. The images or other third party operated by the Laboratory Computing Resource Center at Argonne National Labora- ’ tory. Q.L. gratefully acknowledges the supporting of National Natural Science Founda- material in this article are included in the article s Creative Commons license, unless tion of China (Grant No. 51702207). J.W. was supported by the Fundamental Research indicated otherwise in a credit line to the material. If material is not included in the ’ Funds for the Central Universities (WUT: 2019III012GX). article s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/ Author contributions licenses/by/4.0/. Q.L., Z.Y., and J.W. conceived the project. Q.L., Y.X., J.W., and V.P.D. performed the in situ TEM and interpretation). Z.Y. and C.W. conducted DFT simulations. E.L. and M.M.T. conducted materials synthesis, battery measurements, and data interpretation. © The Author(s) 2019 All of the authors contributed to the writing to the manuscript before submission.
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