In Situ Atomic-Scale Observation of Electrochemical Delithiation

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In Situ Atomic-Scale Observation of Electrochemical Delithiation Induced Structure Evolution of LiCoO2 Cathode in a Working AllSolid-State Battery Yue Gong,†,#,∥ Jienan Zhang,†,∥ Liwei Jiang,†,∥ Jin-An Shi,†,∥ Qinghua Zhang,† Zhenzhong Yang,† Dongli Zou,∇ Jiangyong Wang,‡ Xiqian Yu,† Ruijuan Xiao,*,† Yong-Sheng Hu,*,† Lin Gu,*,†,‡,⊥,# Hong Li,*,† and Liquan Chen†

J. Am. Chem. Soc. 2017.139:4274-4277. Downloaded from pubs.acs.org by MIDWESTERN UNIV on 01/27/19. For personal use only.

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Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China ‡ Department of Physics, Shantou University, Shantou, Guangdong 515063, China ⊥ Collaborative Innovation Center of Quantum Matter, Beijing 100190, China # School of Physical Sciences, University of Chinese Academy of Sciences, Beijing 100190, China ∇ Department of Mechanical, Materials and Manufacturing Engineering, The University of Nottingham Ningbo China, Ningbo, Zhejiang 315100, China S Supporting Information *

anode and high-voltage cathode material at the same time. However, the low ionic conductivity of solid-state electrolytes, their poor contact between electrode and electrolyte, and the grain boundaries within each component of all-solid-state LIB affect battery performance severely. Despite the high scientific and technological importance, and many promising applications of all-solid-state LIBs, few studies focus on the origin of the battery performance on an atomic scale. The structural change of LiCoO2 cathodes during the electrochemical cycle has been studied carefully, with both in situ experiments and ex situ experiments. In traditional liquid LIB, the LiCoO2 cathode undergoes a series of phase transition from layer structure to spinel structure and rock salt structure etc.3 However, so far, few studies have focused on the structure evolution of LiCoO2 in an all-solid-state LIB and the structure at high voltage condition. Newly developed aberrationcorrected scanning transmission electron microscopy (S/ TEM) can help one investigate materials at an atomic scale, unlike X-ray diffraction or neutron diffraction, which can only provide information on a larger scale. Moreover, S/TEM can directly obtain structure information on an atomic scale, and in situ S/TEM has the capability to probe reaction kinetics in realtime. However, traditional probe based in situ TEM holders are difficult to tilt and to keep stable for long periods of time.4 For the first time, we use a state-of-the-art chip based in situ TEM holder, in combination with focused ion beam (FIB) milling, to prepare a working all-solid-state LIB sample. We applied an electrical field and observed the structural changes on an atomic scale with a TEM. In this Communication, we focus on the structural evolution of a layered LiCoO2 cathode in an all-solidstate battery during the delithiation process. To reveal the different LiCoO2 cathode behavior in the all-solid-state LIB, we used both state-of-the-art atomic scale in situ annular brightfield (ABF) STEM, and high-angle annular dark-field

ABSTRACT: We report a method for in situ atomic-scale observation of electrochemical delithiation in a working all-solid-state battery using a state-of-the-art chip based in situ transmission electron microscopy (TEM) holder and focused ion beam milling to prepare an all-solid-state lithium-ion battery sample. A battery consisting of LiCoO2 cathode, LLZO solid state electrolyte and gold anode was constructed, delithiated and observed in an aberration corrected scanning transmission electron microscope at atomic scale. We found that the pristine single crystal LiCoO2 became nanosized polycrystal connected by coherent twin boundaries and antiphase domain boundaries after high voltage delithiation. This is different from liquid electrolyte batteries, where a series of phase transitions take place at LiCoO 2 cathode during delithiation. Both grain boundaries become more energy favorable along with extraction of lithium ions through theoretical calculation. We also proposed a lithium migration pathway before and after polycrystallization. This new methodology could stimulate atomic scale in situ scanning/TEM studies of battery materials and provide important mechanistic insight for designing better all-solidstate battery. he LiCoO2 was first reported by Mizushima et al. as a cathode material for batteries with high volumetric energy density. LiCoO2 was the first commercially used cathode material and it still dominates the portable device market today.1 Lithium-ion batteries (LIB) with better safety property, higher energy density, and higher power density are urgently needed because of the increasing demands of mobile intelligent terminals, electrical vehicles, and the smart grid.2 All-solid-state batteries with solid electrolytes are one approach to address the safety problems associated with flammable organic liquid electrolytes. They also make it possible for lithium to be the

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© 2017 American Chemical Society

Received: December 29, 2016 Published: March 8, 2017 4274

DOI: 10.1021/jacs.6b13344 J. Am. Chem. Soc. 2017, 139, 4274−4277

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Journal of the American Chemical Society (HAADF) STEM imaging, which can simultaneously resolve light and heavy element columns, respectively. This method makes it possible to reveal the origin of the performance of a working all-solid-state battery on an atomic scale.5 We have constructed an all-solid-state LIB with a gold anode, a LiCoO 2 cathode, and Y and Ta doped LLZO (Li6.75La2.84Y0.16Zr1.75Ta0.25O12) as the solid-state electrolyte (SSE) on a micro-electro-mechanical system (MEMS) device nanochip using FIB milling.6 The FIB fabricated sample was prepared using optimized procedures and parameters.7 See the details in the sample preparation section of Supporting Information. The finished all-solid-state LIB device for in situ observation is shown in Figure 1A, and a corresponding

Figure 2. (A) HAADF micrograph of the delithiated LiCoO2 cathode colored using the GPA method. HAADF micrograph colored in blue, and two orientations colored with green and red. Panels B and C are zoomed-in micrographs of the yellow, dashed-line, rectangular area. Panels D and E are zoomed-in micrographs of the pink, dashed-line, rectangular area. For both boundaries, we can see a contrast in the lithium layer in both the HAADF and the ABF micrograph, which suggests heavy atoms are present in the lithium layer. In this case, it is cobalt ions because of the phase transition in the layer structure, spinel, and rock salt. Furthermore, in panels B and C we can see that the basal planes of the two crystals meet at an angle of 112°. The angle is 109.5° in pristine LiCoO2.

antiphase domain boundaries in the [010] zone axis of LiCoO2 after delithiation, see Figure 2B−E. This means that only two kinds of crystal orientations exist in the delithiated LiCoO2 cathode. Therefore, to emphasize the grain size and orientation in the delithiated LiCoO2 cathode, geographical phase analysis (GPA) was used to indicate the orientation and size of each grain area after delithiation in Figure 2A.8 Three colored images overlap together in Figure 2A. They are, respectively, original STEM images colored in blue, and two different orientations colored yellow and red. Hence, the boundaries between different color areas are coherent twin boundaries, while boundaries between similar color areas are antiphase domain boundaries. Figure 2B−E shows zoomed in STEM-ABF and HAADF micrographs of a coherent twin boundary and an antiphase domain boundary. The original 109.5° coherent twin boundary in the pristine LiCoO2 cathode increased to 112° due to the extraction of lithium ions (see the coherent twin-boundary micrographs in Figure 2B,C). The contrast in lithium layer of ABF image becomes weaker comparing with the pristine ABF image, which means the extraction of lithium ions. There is also a 2.7% (±0.4%) layer-spacing expansion, which is induced by the extraction of lithium ions and probably the accumulation of lithium or cobalt ions at domain boundaries.3,9 In addition, we found strong contrasting dots in the lithium layer of LiCoO2 near the boundary for the coherent twin boundary, which can also be seen in the corresponding HAADF micrograph (see the dashed line circles in Figure 2B,C). This indicates that these are high Z atoms, which were cobalt ions in this experiment. A similar phenomenon, such as layer-spacing expansion and cobalt ions in the lithium-ion layer, can also be seen at the antiphase domain boundary (see the ABF and HAADF micrographs in Figure 2D,E). These phenomena are probably due to cobalt ions migrating into lithium-ion layers during electrochemical delithiation, which induces a phase transition from the layer structure to the spinel and rock salt structures.3 These two kinds of boundaries were reported previously during pristine LiCoO2 epitaxial growth, and both boundaries contribute negative ion transportation energy according to first principles calculations.10 In addition to the weaker contrast

Figure 1. (A) SEM image of the FIB fabricated battery on the nanochip to apply the electric field, and corresponding schematic (B). In panel A, the LLZO electrolyte and electron transparent area of the LiCoO2 cathode are highlighted with a red dashed line and a yellow dashed line trapezoid. Panels C and D are pristine LiCoO2 ABF and HAADF micrographs with the corresponding line profile acquired at the red dashed line rectangular zone shown in panel C with both lithium and oxygen contrast. In panels B and C, green, purple, and cyan balls are lithium, oxygen, and cobalt ions, respectively.

schematic in Figure 1B. The left and right sides are gold wires, which were connected with negative and positive electrodes of the external power source, respectively. As for the battery, from left to right are the gold anode, LLZO SSE, and LiCoO2 cathode. The whole battery is connected with gold wires in the nanochip by Pt deposition. The thickness of the cathode and anode materials is approximately 2 μm, and a large SSE is used to make sure both cathode and anode materials are in good contact with the SSE. The electron transparent area for the STEM observation in the LiCoO2 cathode is 50 nm thick. All three parts of the all-solid-state battery were fabricated using FIB milling, and the phase purity and morphology of LiCoO2 and the LLZO particles are confirmed with X-ray diffraction (XRD) patterns and scanning electron microscopy (SEM) images, see Figure S1, S2. Figure 1C,D shows ABF and corresponding HAADF images of a pristine LiCoO2 cathode, which confirms the original layered structure of the LiCoO2 cathode. In the ABF micrograph, both lithium and oxygen contrast can be distinguished clearly, as indicated by the overlying line profile. Figure 2 shows the structure of LiCoO2 cathode after high voltage delithiation. See details of the information about the method of delithiation (voltage applied, time period, and current measurement) in Supporting Information. The single crystal LiCoO2 became polycrystalline with grain sizes of approximately 5−15 nm, forming coherent twin boundaries and 4275

DOI: 10.1021/jacs.6b13344 J. Am. Chem. Soc. 2017, 139, 4274−4277

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Journal of the American Chemical Society of lithium ions, the contrast of oxygen ions also became weaker. This phenomenon probably results from the formation of intermediate phase during delithiation and the movement of oxygen ions of LiCoO2 cathode at high voltage.11 To understand the formation of these two kinds of boundaries during the delithiation process in terms of energy, three models with lithium concentrations ranging from 1 to 0.5 were built to study the observed structural change. The calculated interface energies, which are defined as the energy difference between the boundary model and the bulk model, are shown in Figure 3. The monotonous drop of interface

Figure 4. Schematic of the pathway of lithium ions along the grain boundaries, and the boundary structure obtained using theoretical calculations.

different from a typical liquid electrolyte battery. An atomicscale structural-evolution picture of other important electrodes and solid−solid interfaces in an all-solid-state battery could be observed directly via in situ S/TEM, which provides atomicscale structure information for designing a better all-solid-state battery. The new methodology combines powerful in situ TEM equipment with FIB milling, which should stimulate atomic scale in situ S/TEM studies of not only battery materials but also other promising compounds.

Figure 3. Energy difference between two boundaries and the bulk.

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energy, combined with the decrease of the lithium concentration, suggests that the polycrystallization in the LiCoO2 cathode takes place more likely during the delithiation process. Furthermore, the interface energy of the antiphase domain boundary is higher than that of the coherent twin boundary. This indicates that the antiphase domain boundary is less likely formed than the coherent twin boundary during the delithiation process. This could explain the different quantities of these two boundaries in Figure 2A. On the basis of the in situ STEM results and the theoretical calculations, we can infer why this nanopolycrystallization phenomenon occurs: In an all-solid-state battery, the contact between electrode and electrolyte is typically a physical contact or sometimes a point contact, on a micro or nano scale. LiCoO2 is a typical two-dimensional lithium ionic conduction-material. If this is the case, only the area at the contact point can supply lithium ions during the delithiation process. Together with the delithiation process, the contact point is like the seed, and the grain boundaries are roots grown from the seed. They then form the nanosized domain, which may have a connection with the lithium-extraction induced phase-transition and the low lithium-ion diffusion energy-barrier at the grain boundary (Figure 4).12 Therefore, it is crucial to make the solid−solid interface contact condition good enough to make performance of all-solid-state LIB equal to or better than that of liquid LIB. In summary, we designed a working all-solid-state lithium-ion battery in TEM and directly observed the formation of nanopolycrystallization on an atomic scale by using aberration-corrected STEM, for the first time. All nanosized domains are connected with each other through a coherent twinboundary and an antiphase domain-boundary, which are more easily formed in combination with the extraction of lithium ions according to theoretical calculations. Further theoretical and experimental clarification are needed to clarify the formation mechanism in such a layered structure material. Nevertheless, the current results show that lithium extraction from a wellstudied cathode material in an all-solid-state battery could be

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.6b13344. Sample preparation; structural characterization; theoretical calculation; SEM images and XRD spectra reveal the morphology and purity of pristine LLZO and LiCoO2 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*[email protected] *[email protected] *[email protected] *[email protected] ORCID

Yue Gong: 0000-0002-5764-3117 Lin Gu: 0000-0002-7504-031X Author Contributions ∥

Y.G., J.Z., L.J., and J.-A.S. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Program on Key Basic Research Project (2014CB921002), The Strategic Priority Research Program of Chinese Academy of Sciences (Grant No. XDB07030200), The Key Research Program of Frontier Sciences, CAS (Grant No. QYZDB-SSW-JSC035) and National Natural Science Foundation of China (51522212, 51421002, 51672307).

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