Layered Oxide Cathodes for Li-Ion Batteries: Oxygen Loss and

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Layered Oxide Cathodes for Li-Ion Batteries: Oxygen Loss and Vacancy Evolution Hanlei Zhang, Brian M. May, Fredrick Omenya, M. Stanley Whittingham, Jordi Cabana, and Guangwen Zhou Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.9b03245 • Publication Date (Web): 15 Aug 2019 Downloaded from pubs.acs.org on August 15, 2019

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Chemistry of Materials

Layered Oxide Cathodes for Li-Ion Batteries: Oxygen Loss and Vacancy Evolution Hanlei Zhang1, 2, Brian M. May2, 3, Fredrick Omenya2, M. Stanley Whittingham2, Jordi Cabana2, 3 and Guangwen Zhou1, 2* 1. Materials Science and Engineering Program & Department of Mechanical Engineering, State University of New York, Binghamton, New York 13902, United States 2. NorthEast Center for Chemical Energy Storage, State University of New York, Binghamton, New York 13902, United States 3. Department of Chemistry, University of Illinois at Chicago, Chicago, Illinois 60607, United States

Abstract Loss of oxygen in layered transition metal oxides is a major reason for the structural degradation and thus the fade of electrochemical performance in the cathodes for Li-ion batteries.

Via

in-situ

transmission

electron

microscopy

observations

of

LiNi0.80Co0.15Al0.05O2 (NCA), we found that the oxygen loss in the layered cathode is a two-stage process with distinct release rates. The initial rapid oxygen loss generates a high concentration of oxygen vacancies that results in the formation of an amorphized, vacancy-containing rock-salt layer in the surface. In the second stage, the slower rate of oxygen loss allows recrystallization of this defective phase via coalescing of atomic oxygen vacancies, which results in the formation of a cavity-containing surface layer with a crystalline rock-salt structure over the layered phase in the bulk. Comparison of the insitu results with electrochemically cycled NCA cathodes confirms this two-stage process of oxygen loss. These results provide unprecedented microscopic details regarding the structural degradation of layered oxides arising from oxygen loss and have broader implications in manipulating the oxygen activity in the electrode.

*To whom correspondence should be addressed: [email protected]

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Introduction Layered transition metal oxides attract an intensive research attention as the cathode for lithium ion batteries (LIB) due to their high capacity (>200 mAh/g), energy density, output voltage (~4.3 V) and rate capability.1–5 The R3m structure of the layered oxides allows a high-degree intercalation of Li+ cations into its interlayer octahedral sites.3,6,7 Since this layered structure is the foundation for the outstanding electrochemical properties,8,9 maintaining its integrity thus becomes a major task in preventing fade of the electrochemical performance.4,5,10,11 Oxygen loss during electrochemical cycling damages the face-centered cubic (F.C.C.) oxygen framework of the oxides,12–14 resulting in structural degradations such as phase transformation, cation dissolution, mechanical cracking and lattice dissolution.15–17 Understanding the microscopic process governing the oxygen loss holds critical value on elucidating the electrochemical stability of the layered oxides. Oxygen loss is initiated from the outermost surface of the cathode and propagates toward the inner regions, accompanied by compositional and structural changes extending deeper into the subsurface region. Unfortunately, probing the fast, local oxygen loss dynamics at the atomic scale has been a major challenge. Particularly, monitoring the structural evolution associated with oxygen loss in-situ remains elusive because of the difficulty in spatially and temporally resolving the phenomena occurring at the surface and subsurface regions. Oxygen is a light element and only yields weak signals when measured with electron beams,18,19 making it challenging to image. Probing oxygen with spectroscopy methods

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(e.g., electron energy loss spectroscopy (EELS),13,20,21 X-ray photoelectron spectroscopy (XPS)22,23 and transmission X-ray microscopy (TXM)24,25) can reveal the presence of oxygen, but they lack the proper spatial resolution (< 1 nm or atomic-scale) and are not structurally sensitive.26–29 Additionally, the complex electrochemical conditions in a real battery cell being cycled drastically increase the difficulty to perform the microscopic measurements on oxygen loss dynamics.30 Therefore, it is important to employ in-situ approaches to monitor oxygen loss with the required spatial and temporal resolution in order to capture microscopic details. In this work, we employ in-situ transmission electron microscopy (TEM) to monitor the structural evolution in LiNi0.80Co0.15Al0.05O2 (NCA) induced by oxygen loss at an elevated temperature, in a reducing environment created by the high vacuum in TEM. Through in-situ TEM observations, we demonstrate that the oxygen loss in the NCA cathode occurs via a two-stage process, with an initial stage of rapid loss that transforms the cathode surface layer into an amorphized rock-salt phase enriched with oxygen vacancies, followed by a second, significantly slower stage that is accompanied by recrystallization of the amorphized rock-salt layer via coalescing of the vacancies. The insitu TEM results are corroborated by ex-situ TEM examination of electrochemically cycled NCA cathodes, showing that oxygen loss during cycling follows the same pathway as when induced by heating.

Results and Discussion

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In-situ of observation of oxygen loss. Figure 1 presents time-sequence HRTEM images showing a pristine NCA particle kept at 400 ˚C, in a reducing environment generated by the high vacuum (~10-7 Torr) (see also Supplementary Movie_1. Note that the temperatures utilized for material synthesis are 700 ˚C and 800 ˚C). Before heating (Figure 1(a)), no structural defects can be observed in the particle. After 218 s of heating (Figure 1(b)), a high concentration of vacancy clusters formed from the surface of the primary particle to a depth of ~15 nm. The surface layer with vacancy clusters is marked with orange dashed lines. A magnified view of the vacancy clusters is shown in the inset of Figure 1(b), as marked with yellow arrows. As the heating continues to 685 s (Figure 1(c)), the thickness of the vacancy-containing layer remains almost unchanged, as marked with orange dashed lines. The inset of Figure 1(c) presents a magnified view of the same region as that in the inset of Figure 1(b), which shows that the vacancy clusters disappeared with the continued heating. The in-situ TEM observations indicate that the oxygen loss process is time dependent. It should be noted that the layered phase in the surface region stays unchanged under e-beam illumination up to 30 min at room temperature, as shown in Supplementary Movie_2 and our previous work.17,31 Therefore, the oxygen loss is majorly driven by the high vacuum and elevated temperature, other than from the e-beam effect.

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Figure 1 | In-situ TEM imaging of oxygen loss at 400 C. (a) A pristine particle without oxygen loss. (b) The particle after heating for 218 s, showing the formation of a high concentration of vacancy clusters in the surface layer of ~15 nm. (c) The particle after heating for 685 s, showing no change in the thickness of the vacancy-containing layer. Insets are magnified views showing the evolution of vacancy clusters as a function of heating time, as indicated by the yellow arrows.

Zoom-in HRTEM imaging was performed to reveal the difference in microstructural feature of the particle at the two reaction stages shown in Figures 1(b, c). Figure 2(a) illustrates a scanning transmission electron microscopy high angle annular dark field (STEM-HAADF) view of the pristine particle before heating, consisting of the intact layered structure without vacancy clusters. A thin layer of rock-salt phase forms in the surface due to the slight degradation of the layered phase upon the synthesis.32,33 Figure 2(b) presents an HRTEM view of the particle at stage 1 (Figure 1(b)), from which a threelayer structure can be identified, as marked by the dashed blue lines. The outermost layer is composed of an amorphized phase of ~10 nm thickness (see magnified view in Figure 2(c)), as revealed by the dominant, diffusive halo of its electron diffractogram (Figure 2(d)). Nonetheless, diffraction spots associated with the rock-salt phase could be resolved (marked with green circles), indicating that this surface layer possesses an amorphized rock-salt structure. Quantitative measurement indicates that the spots in Figure 2(d) correspond to a d-spacing of 3.03 Å, which matches well with the (110) planes of the rocksalt phase.34,35 A well-defined rock-salt phase of ~ 2 nm formed underneath the

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amorphized layer (see magnified view in Figure 2(e)). Beyond 12 nm, the original layered phase was found in the bulk (Figure 2(f), see also Section II of Supplementary Materials). The oxygen loss results in anion vacancies in the layered phase and drives its transformation into the chemically disordered rock-salt phase.31,36,37 Oxygen vacancies in metal oxides significantly reduce the anion-cation coordination, thus reducing the crystallinity of metal oxides.38,39 Surfaces are usually the sink for vacancies initially in the bulk. However, the pristine NCA in our experiments is nearly stoichiometric. The oxygen loss from the surface results in a high concentration of oxygen vacancies in the surface region, which diffuse inward and accumulate in the subsurface. Due to the ability of the rock-salt phase to host oxygen vacancies at the cost of its crystallinity,40 a high concentration of oxygen vacancies in the surface rock-salt layer results in a rock-salt-like structure with the amorphous feature. This amorphized phase is based on the welldefined Fm3m rock-salt phase, while its crystallinity is largely reduced, as shown in Figures 2(b-d). We use the term “amorphized rock-salt” to describe this rock-salt phase with a strong amorphous feature.

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Figure 2 | Analysis of the inward diffusion of oxygen vacancies (stage 1). (a) STEM-HAADF image of the pristine particle without heating, showing no oxygen-loss-induced vacancy clusters. (b) HRTEM view of the surface region of the particle heated to 218 s, showing the formation of a threelayer configuration with an amorphized rock-salt-like structure in the surface. (c-f) Magnified analysis of the amorphized phase, well-defined rock-salt structure and layered phase shown in (b), as marked by dashed boxes c, e and f in (b), respectively.

Figure 3 illustrates an in-situ bright-field TEM observation showing more details about oxygen-loss-induced inward migration and clustering of vacancies at 400 C in stage 1. At the beginning of heating (Figure 3(a)), the particle has a relatively uniform image contrast. After heating for ~6 min, the surface rock-salt layer forms and vacancy clusters with a brighter contrast become visible in the subsurface region, as indicated by blue arrows in Figure 3(b). The brighter contrast corresponds to a reduced mass, which results from the formation of oxygen vacancy clusters. Figure 3(c) is a TEM image of the particle after ~15 min of heating, showing more and larger vacancy clusters that develop further

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toward the inner region. Therefore, in this reaction stage, oxygen vacancies continuously diffuse from the surface towards the core of the primary particle.

Figure 3 | Inward diffusion and clustering of oxygen vacancies. (a-c) In-situ HRTEM observation showing the inward diffusion of oxygen vacancy clusters.

Figure 4 presents HRTEM observations of the surface region after further heating of the primary particle at 400 C to stage 2 (Figure 1(c)). The initial three-layer configuration at stage 1 transformed into a two-layer configuration (stage 2), in which the surface layer is taken by a crystalline rock-salt phase and the bulk remains as the layered phase. Figure 4(a) presents an HRTEM image of the surface rock-salt layer, showing that 8


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it is crystalline and contains cavities of 1-5 nm, as marked with yellow and blue arrows. Figure 4(b) presents a magnified view of the cavity in Figure 4(a). A magnified atomic view of the surface rock-salt layer (Figure 4(c)) confirms its good crystallinity, compared with the amorphized rock-salt layer formed in stage 1. Figure 4(d) is a magnified view of the rock-salt surface composed of facets featuring (111)- and (002)-type terraces, which are low surface energy facets of the rock-salt phase.41–43 HRTEM observation of the bulk area (Section III of the Supplementary Materials) indicates the layered phase remains unchanged compared with stage 1. This suggests that the oxygen loss is significantly slowed down and stage 2 is dominated by the recrystallization of the amorphized rocksalt phase, concomitant with the coalescing of oxygen vacancies to form larger cavities.

Figure 4 | Coalescing of oxygen vacancies and crystallization of the amorphized surface rocksalt layer (stage 2) (a) HRTEM image showing the crystalline, cavity-containing surface rock-salt layer after the coalescing of oxygen vacancies. (b) Magnified view of the cavity in (b). (c) Magnified HRTEM image of the surface rock-salt layer, indicating its high crystallinity. (d) Magnified view showing

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the surface configuration of the rock-salt layer, indicating that it is composed of (111)- and (002)-type facets.

Figure 5(a) schematically illustrates the three-layer core-shell morphology of the primary particle resulting from the inward migration of oxygen vacancies (stage 1). A layer of high-vacancy amorphized rock-salt phase forms as the outer surface, and the bulk is composed of the layered phase with a small amount of oxygen vacancies. Between the amorphized rock-salt phase and the layered phase is a thin layer of crystalline rock-salt phase. Figure 5(b) shows the configuration of the two-layer configuration after the coalescing of oxygen vacancies (stage 2), composed a crystalline, cavity-containing rocksalt layer in the surface with the layered phase in the bulk. Figures 5(c-d) schematically summarize the evolution of oxygen vacancies between stages 1 and 2.

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Figure 5 | Schematics showing the two-stage oxygen loss in the layered cathode. (a) The threelayer configuration in stage 1. (b) The two-layer configuration in stage 2. (c) Pristine Layered phase without oxygen loss. (d) Inward diffusion of oxygen vacancies leading to the build-up of a vacancy gradient, with a high-vacancy amorphized rock-salt layer on the surface. (e) Coalescing of oxygen vacancies in the amorphized rock-salt layer, leading to the formation of cavities and crystallization of the amorphized rock-salt phase.

At stage 1, the concentration of oxygen vacancies is highest in the surface and decreases towards the core, imposing the three-layer core-shell configuration shown in Figure 5(a). In the layered phase, the accumulation of oxygen vacancies results in the formation of the rock-salt phase. In the surface region, an overly high concentration of

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oxygen vacancies is accumulated due to the fast loss, which is more than the necessary amount to induce the crystalline rock-salt. The high concentration of oxygen vacancies in the surface rock-salt layer reduces its structural integrity, leading to a highly defective, amorphized phase. On the other hand, the concentration of oxygen vacancies in the layered phase is not high enough to induce the layered  rock-salt phase transformation, so vacancy clusters form in the layered phase. Between the layered phase in the bulk and the outermost amorphized layer is a transient layer, in which the oxygen loss is sufficient to induce the layered to rock-salt phase transformation but not enough to induce amorphization, so a crystalline rock-salt structure is thus generated. In other words, the presence of the amorphized rock-salt phase is evidence of the rapid oxygen loss at stage 1. At stage 2, no amorphized rock-salt phase is generated, indicating that the oxygen loss is largely reduced, which can be attributed to the surface rock-salt formed in stage 1. Compared with the layered phase, the rock-salt phase is oxygen-deficient and thermodynamically more stable in the reducing environment, so it does not undergo further oxygen loss. Also, the formation of the rock-salt shell isolates the layered phase beneath and blocks the outward diffusion pathway of oxygen. The surface rock-salt layer thus serves as a passivating coating in protecting the underlying layered structure. The recrystallization process during stage 2 is driven by the coalescing of atomic oxygen vacancies to form cavities. Both the coalescing of oxygen vacancies and the recrystallization of the amorphized rock-salt phase are thermodynamically favored because the crystalline rock-salt phase is more stable than the amorphous phase. In addition, the clustering of vacancies can significantly reduce the number of dangling

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bonds formed by the atomic vacancies, thereby resulting a more stable structure. As shown in Figure 4(d), the recrystallized surface layer is terminated with energetically favorable (111)- and (002)-facets,32,33,42,43 further confirming that the crystalline rock-salt has a low-energy surface configuration.

Inward diffusion pathways of oxygen vacancies. As shown in Figures 1-3, the loss of oxygen from the layered cathode (stage 1) results in the formation of an amorphized rock-salt layer in the surface. EDS analysis of the amorphized rock-salt phase confirms that it is an oxygen-deficient phase compared with the pristine layered phase (Section IV of the Supplementary Materials). Therefore, the thickness of the surface amorphized rock-salt layer can be considered as an indicator for the degree of oxygen loss. Electron energy loss spectroscopy (EELS) and HRTEM were performed at different surface regions of a primary particle during stage 1. The amorphized layer on the surface of a particle at stage 1 (kept at 400 °C for 7min) is ~8 and 2.5 nm thick along the direction of the lithium channels and on the (003) facet, respectively (Figure 6(a)). The preferred formation of the rock-salt phase along the lithium channels is reproducible (Section V of the Supplementary Materials), indicating that the inward diffusion of oxygen vacancies is preferred within the (003) atomic planes. Diffusion of oxygen vacancies perpendicular to the (003) plane is also possible, but at a much slower rate. This can be attributed to the fact that the layered phase is composed of alternative cationic and anionic planes along the [003] direction.44–46 The diffusion of oxygen vacancies within a single (003) anionic plane that is composed of O2- is easier

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than the diffusion of oxygen vacancies across the cationic and anionic planes along the [003] direction.

Figure 6 | Diffusion pathways of oxygen vacancies. (a) HRTEM view showing the formation of the amorphized rock-salt phase along the direction of the lithium channels and on the (003) surface. (b) STEM image showing an NCA particle heated at 400 C for 10min. (c) EELS spectra showing the OK edge obtained from locations (i), (ii) and (iii), as marked with orange dashed circles in (b). (d) EELS spectra showing the Ni-L edge from locations (i), (ii) and (iii) in (b). (e) Schematic showing the preferred inward diffusion of oxygen vacancies along the direction of the lithium channels.

To precisely determine chemical conditions at different facets of the particle, EELS spectra were obtained from three regions, i) the (003) facet that is parallel to the (003) plane, ii) the 110.4˚ corner region, and iii) the facet facing the Li channels, as marked in Figure 6(b). Figure 6(c) presents the O-K edge spectra from the three locations. The (003)

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facet and the 110.4˚ corner exhibit a clear presence of the pre-peak (marked with a yellow arrow), whereas the pre-peak is absent in the O-K edge from the facet facing the Li channels. The pre-peak of the oxygen K-edge arises from the hybridization between the TM 3d and oxygen 2p orbitals.20,47 Due to the loss of oxygen, the amorphized surface rock-salt layer is enriched with oxygen vacancies. The deficiency of oxygen in the surface rocksalt layer induces reduction of the extent of TM3d - O2p hybridization, leading to a reduced pre-peak intensity. The (003) facet and 110.4˚ corner have a high pre-peak intensity, corresponding with the pristine R3m layered phase with a small amount of the rock-salt, whereas the facets facing the Li channels have the diminished pre-peak intensity due to the faster oxygen loss along this direction. Figure 6(d) presents the Ni-L edge spectra from the three selected locations shown in Figure 6(b). As can be seen, the L3/L2 ratios at the (003) facet and the 110.4˚ corner are almost the same, while it is much lower at the facet facing the Li channels. The L3/L2 ratio is directly related to the oxidation state of Ni, meaning that the Ni at the (003) surface and the 110.4˚ corner has a higher valence compared with the opening of Li channels. This means that Ni is more reduced in the rock-salt layer facing the Li channels due to its higher concentration of oxygen vacancies. Figure 6(e) schematically summarizes the inward diffusion of oxygen vacancies and the formation of the amorphized rock-salt layer on the different facets of a primary particle. The outward diffusion of oxygen is preferred along the Li channels, resulting in a

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subsequently higher concentration of oxygen vacancies in the surface region facing these channels.

Oxygen loss in electrochemically cycled NCA. To confirm that the two-stage oxygen loss process observed in the in-situ TEM heating experiments is genuinely relevant to the electrochemical reactions in a lithium ion battery, we examined electrochemically cycled TODA-NCA (Figure 7, an industrial level cathode composed of large particles), and platelet NCA particles synthesized in our lab (Figure 8). The TODANCA cathodes were cycled between 3.0 - 4.3 V, at a rate of C/10. Figure 7(a) shows the surface morphology of a TODA NCA particle after 30 cycles, revealing a cavity-containing rock-salt shell in the particle surface. Zoom-in HRTEM observations indicate that the rocksalt shell is ~5-6 nm thick (Section VI of the Supplementary Materials). Magnified view of the surface (Figure 7(b)) shows that it is composed of a partially crystallized rock-salt phase with (111)- and (002)-type facets (marked with blue and purple dashed lines, respectively). Magnified views of the rock-salt layer shown in Figure 7(b) show that half of the areas have a well-crystallized rock-salt structure (Figure 7(c)), alternating with domains of an amorphized phase (Figure 7(d)). Therefore, the cathode shown in Figures 7(a-d) has partially undergone the recrystallization in stage 2, with the remaining amorphized rock-salt phase resulting from stage 1. Figure 7(e) presents the HRTEM view of a TODA-NCA particle after 100 cycles, which has completed the crystallization of the rock-salt layer in stage 2.

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Figure 7 | Oxygen loss in electrochemically cycled TODA-NCA cathodes. (a) STEM-HAADF image of a 30-cycle TODA-NCA particle, showing the formation of a porous rock-salt shell. (b) Magnified view showing the surface morphology of the 30-cycle TODA NCA particle, which is a partially crystallized rock-salt phase. (c, d) Magnified views of the rock-salt phase shown in (c), indicating that the surface rock-salt shell is partially crystallized alternating with small amorphous rocksalt domains. (e) HRTEM image of a 100-cycle TODA-NCA particle, showing a fully crystallized rocksalt shell in the surface. (f) Discharge capacity vs cycle number curve of a 300-cycle TODA-NCA cathode.

Figure 7(f) presents a discharge capacity vs cycle number curve of a TODA-NCA cathode cycled between 3.0 - 4.3 V for 300 cycles, at a rate of C/10. Besides the gradual capacity loss as a function of cycling, an abrupt capacity loss occurred around the 50th cycle in the curve. This phenomenon is confirmed by repeated testing of the same cells under the same condition (see Section VI of the Supplementary Materials).

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phenomenon can be attributed to the crystallization of the surface rock-salt shell. As shown in Figures 7(a-d), after 30 cycles the rock-salt shell is partially crystallized. Around 50 cycles, the surface rock-salt shell is fully crystallized and an isolating rock-salt shell forms in the particle surface, which significantly reduces the ionic and electronic conductivity of the particle, resulting in the abrupt loss of capacity. Figure 7 confirms that oxygen loss has decreased to a slow rate at the 30th cycle. This is consistent with the observations of oxygen loss performed via other methods,48,49 which showed that the oxygen loss is most pronounced in the first few cycles. Observation of electrochemically cycled platelet NCA confirms the two-stage oxygen loss over multiple particle morphologies. The platelet NCA cathodes are cycled between 3.0 - 4.3 V, at a rate of C/3. An HRTEM image of the surface region of a platelet NCA particle after 30 cycles (Figure 8(a)) shows the formation of a rock-salt shell, comparable to the TODA-NCA cathode. Magnified views of this rock-salt shell (Figures 8(b, c)) indicate that it is partially crystallized, namely undergoing the crystallization transformation of stage 2. Crystallized rock-salt phase is observed along with oxygen vacancies, as indicated with a red arrow in Figure 8(b). The rest of the surface shell remains amorphous (Figure 8(c)).

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Figure 8 | Oxygen loss in electrochemically cycled platelet NCA cathodes. (a) HRTEM image of a 30-cycle platelet NCA particle, showing a partially crystallized rock-salt shell. (b, c) Magnified views of the rock-salt shell in (a), showing the presence of a crystallized rock-salt phase with vacancies and an amorphous phase. (d) HRTEM image of a 60-cycle platelet NCA particle, showing the formation of a fully crystallized rock-salt shell with cavities, as marked by red arrows. (e) Magnified view showing the crystallized lattice of the surface rock-salt shell in (d). (f) HRTEM image of the layered phase in the bulk of the 60-cycle platelet NCA particle.

Figure 8(d) presents the surface region of a platelet NCA particle after 60 cycles, showing the presence of cavities in the rock-salt shell, as marked with red arrows. A magnified view of the rock-salt phase (Figure 8(e)) shows that it has a well-crystallized lattice, indicating that this particle has completed the crystallization of stage 2. Observation of the bulk region of this particle (Figure 8(f)) confirms that it is composed of a R3m layered structure.

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By comparing the surface configurations as revealed from the in-situ TEM observations in Figures 2 and 4 with the electrochemically cycled samples shown in Figures 7 and 8, a good match can be noted in the phase and microstructure evolution between the two types of the samples. This indicates the close relevance of the in-situ TEM experiments to the electrochemical cycling process. The good match between in-situ heating TEM experiments and electrochemically cycled samples can be attributed to the similar driving effects on electron transfer from lattice oxygen (O2-) towards transition metal cations of 3+. During the charging process, Li+ is first removed from the cathode, driving the oxidation of transition metal cations from 3+ towards 4+. The highly oxidative transition metal cations can obtain electrons from the lattice O2-, resulting in the reduction of transition metals towards 2+ and oxidation of O2- towards O2 with the release of gaseous O2 from the cathode. 50,51 The overall reaction is:

M3+ + O2- → M2+ + O2

(1)

The high vacuum environment in TEM facilitates the loss of gaseous O2, thereby allowing reaction (1) to proceed in the forward direction. By this means, the high vacuum in TEM has a similar effect on driving electron transfer from O2- to M3+ and thereby inducing oxygen loss as electrochemical charging. The observed two-stage process of oxygen loss clarifies the formation of rock-salt shell in the surface. The phase transformation in the surface of electrochemically cycled cathode particles leads to a highly defective rock-salt structure. Oxygen vacancies diffuse into and subsequently coalesce within this shell, enabling its crystallization. This surface rock-salt shell reduces the capacity and ionic conductivity of the cathode. Due to the

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preferred oxygen loss along the lithium channels, the rock-salt layer along the directions of the lithium channels are thicker than the other surfaces. As shown from our TEM observations, cavities and well-crystallized rock-salt phases always coexist at the same time, confirming that their formation is mutually related, other than independent. The high temperature in our in-situ TEM experiments enhances the kinetics of the cavity formation and crystallization of the amorphous rock-salt layer. This is also consistent with the cycled cathodes, in which the crystallization of amorphized rock-salt takes much longer time due to the lower temperature compared to the in-situ heating TEM experiments. This work provides directions toward controlling the oxygen loss, which is a major need in preventing the structural degradations in layered cathodes. Oxygen vacancies represent the most fundamental structural defect induced by the oxygen loss, whose evolution results in the formation of the rock-salt phase and cavities. These defects can further lead to even more detrimental structural degradations, such as the cracking of the cathode17 and surface roughening.52,53 These defects reduce the mass of the electrochemically active layered phase, reducing the capacity of the cathode. They also reduce the ionic conductivity of the cathode, imposing greater overpotentials which ultimately degrade the energy density of the cell. Therefore, it is important to reduce the oxygen loss and prevent the associated structural and electrochemical degradations. As we have shown in this work, the oxygen vacancies are generated in the particle surface. Therefore, stabilizing the particle surface against oxygen loss can cut off the source of oxygen vacancies and prevent the following degradation. Approaches made

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include surface coating with passivated metal oxides such as Al2O354,55 or coating the layered phase with an electrochemically compatible phase that does not easily go through oxygen loss, such as a cobalt- and manganese-rich spinel phase.56,57 Also, our work has shown that the bulk of the particle does not go through significant oxygen loss due to the slow diffusion of oxygen vacancies in the deeper layered lattice. Accordingly, larger particles are preferred in terms of preventing degradation. However, the larger particles may also have slower electrochemical kinetics due to their low ionic and electronic conductivities. This contradiction calls for the optimization of the size of the cathode particles.

Conclusions In conclusion, we have demonstrated that oxygen loss in the layered oxide cathodes for Li-ion batteries follows a two-stage process. First, a rapid loss results in the formation of a three-layer core-shell configuration with an amorphized layer at the surface, an intermediate layer of crystalline rock-salt phase in the subsurface and the layered phase in the bulk. Thereafter, the rate of oxygen loss slows down and allows for the coalescing of oxygen vacancies in the surface layer, driving the recrystallization of the amorphized rock-salt phase, which transforms the three-layer structure into a two-layer core-shell configuration with a cavity-containing crystalline rock-salt shell in the surface. The twostage oxygen loss process observed from the in-situ TEM experiments is further confirmed in electrochemically cycled NCA cathodes. Because the chemical activity of oxygen plays a dominant role in controlling the stability of the layered oxides, our results

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may have a broader impact for manipulating the oxygen loss to affect the cathode particles at different levels from the atomic vacancy formation and migration, to the microstructure evolution induced by oxygen vacancies, and to the macroscale electrochemical performance.

Methods Sample preparation. Two LiNi0.80Co0.15Al0.05O2 (NCA) cathodes were adopted in this work. One NCA material was obtained from TODA America Inc., which is termed as “TODA-NCA” in this work. The other NCA material adopted for the in-situ TEM experiments are platelets (termed as “platelet NCA” in this work), which were hydrothermally synthesized via a 2-step process.58 Ni(NO3)2·6H2O, Co(NO3)2·6H2O, and Al(NO3)3·9H2O were first mixed in water using the appropriate stoichiometric ratio, followed by addition of NaOH using a molar ratio of 1:2, in a total of 70 mL solution. The mixture was hydrothermally treated at 160°C for 12h, using water in a Teflon®-lined stainless vessel, producing the hydroxide precursor. The precursor was washed, dried, and then annealed at 400°C for 4h to decompose the layered double hydroxide structure.59 The resulting oxide was mixed with LiOH·H2O, and subsequently calcined at 800°C for 1h, followed by 700°C for 12h. X-ray diffraction (XRD) results32 are in good agreement with those of the NCA material with the ideal R3m layered structure.60 In-situ TEM heating. The pristine material was dispersed on a copper grid coated with a lacey carbon film for TEM observations, which were performed using a FEI Titan 80-300 microscope with a field emission gun (FEG) and an image aberration corrector,

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operated at an acceleration voltage of 300 kV. In-situ TEM heating was performed using a Gatan® double-tilt heating holder installed in the TEM. Temperature ramping was performed at a rate of 100˚C/min until reaching the required temperature. STEM observation. STEM-HAADF observation was performed using a FEI Titan 80300 microscope with a FEG and a probe aberration corrector, operated at an acceleration voltage of 300 kV. Drawing. Schematics of the crystallographic configurations were drawn using the VESTA crystallographic software.61

Associated Content Supporting Information. Supplementary Movies and description; HRTEM images of the layered phase at stage 1 and 2; EDS and HRTEM observations of the rock-salt phase at stage 1; HRTEM image of the surface of the 30-cycle TODA NCA; Capacity vs cycle number curves of TODA-NCA. This material is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgement This work was supported as part of the NorthEast Center for Chemical Energy Storage (NECCES), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award # DE-SC0012583. The TEM work was carried out at the Center for Functional Nanomaterials, Brookhaven 24


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National Laboratory, which is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract No. DE-AC02-98CH10886.

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Layered Oxide Cathodes for Li-Ion Batteries: Oxygen Loss and

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