Facet-Dependent Rock-Salt Reconstruction on the Surface of Layered

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Facet-dependent Rock-salt Reconstruction on the Surface of Layered Oxide Cathodes Hanlei Zhang, Brian M. May, Jon Serrano-Sevillano, Montse CasasCabanas, Jordi Cabana, Chongmin Wang, and Guangwen Zhou Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b03901 • Publication Date (Web): 02 Jan 2018 Downloaded from http://pubs.acs.org on January 2, 2018

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Chemistry of Materials

Facet-dependent Rock-salt Reconstruction on the Surface of Layered Oxide Cathodes

Hanlei Zhang

1, 2

2, 3

, Brian M. May

4

4

2, 3

, Jon Serrano-Sevillano , Montse Casas-Cabanas , Jordi Cabana 5

1, 2

Chongmin Wang *, Guangwen Zhou

,

*

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 4. CIC Energigune, Albert Einstein 48, Parque Tecnológico de Álava, Miñano 01510, Spain 5. Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, Washington 99352, United States

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

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Abstract The surface configuration of pristine layered oxide cathode particles for Li-ion batteries significantly affects the electrochemical behavior, which is generally considered to be a thin rock-salt layer in the surface. Unfortunately, aside from its thin nature and spatial location on the surface, the true structural nature of this surface rock-salt layer remains largely unknown, creating the need to understand its configuration and the underlying mechanisms of formation. Using scanning transmission electron microscopy, we have found a correlation between the surface rock-salt formation and the crystal facets on pristine LiNi0.80Co0.15Al0.05O2 primary particles. It is found that the originally (014) and (003) surfaces of the layered phase result in two kinds of rock-salt reconstructions: the (002) and (111) rock-salt surfaces, respectively. Stepped surface configurations are generated for both reconstructions. The (002) configuration is relatively flat with monoatomic steps while the (111) configuration shows significant surface roughening. Both reconstructions reduce the ionic and electronic conductivity of the cathode, leading to a reduced electrochemical performance.

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Introduction Layered oxide cathode materials for lithium ion batteries attract wide attention due to their outstanding capacity and rate capability1, 2. However, their application is limited by their structural instability3, 4, which results in reduced long-term electrochemical performance5-8. This instability is largely attributed to the layered → rock-salt phase transformation occurring in the surface of the primary particles, resulting from its high chemical reactivity towards the environment7, 9. Even the surface of the pristine particles without electrochemical cycling is easily degraded to a rock-salt layer with a thickness of a few atomic layers10-12. This surface rock-salt layer protects the bulk of the particle from further degradation9, 13, but it also exhibits a reduced ionic and electronic conductivity, which compromises lithium transport into the primary particles5, 11, 14. This surface rocksalt layer on the pristine particles also serves as the nucleus for a continued layered → rock-salt phase transformation during the electrochemical cycling15-19. Therefore, an indepth understanding of the morphology and atomic configuration of the rock-salt layer on the surface of pristine particles is essential for comprehending the surface degradation and the associated fade of electrochemical performance as a function of cycling. There is currently a great lack of knowledge regarding the configuration and formation process of the surface rock-salt layer on the pristine particles, except for its thin nature and spatial location on the surface11,

20

. The rock-salt layer is simply

assumed to uniformly distribute on surface13, 15, 21. Thus, the layered → rock-salt phase transformation is considered a simple diffusional process22, in which the detailed structure and morphology of the surface rock-salt layer are not taken into 3



consideration13. However, here we show that the surface rock-salt growth formation depends on the crystalline facets of the primary particles of LiNi0.80Co0.15Al0.05O2 (NCA), rather than being a random process. With probe-aberration-corrected scanning transmission electron microscopy (STEM) techniques and thin-plate NCA particles, the facet-dependent rock-salt formation is identified at the atomic level. The structural anisotropy of the layered phase leads to the anisotropic growth of the surface rock-salt layer: along the orientations in the (003) plane of the layered phase, the diffusional and structural properties are sufficiently different compared with those along the c-axis, which is perpendicular to the (003) plane. Accordingly, the surface configuration of the rock-salt layer is found dependent on the crystal facets of the primary particles. The influence of the facet-dependent surface reconstructions of the rock-salt phase on the electrochemical performance of cathode is thus discussed.

Experimental Section Sample preparation. LiNi0.80Co0.15Al0.05O2 platelets were synthesized in a 2-step process for observation. Ni(NO3)26H2O, Co(NO3)26H2O, and Al(NO3)39H2O 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 of 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 structure23. The 4


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resulting oxide was mixed with LiOHH2O, and subsequently calcined at 800°C for 1h, followed by 700°C for 12h. STEM imaging, energy dispersive spectroscopy (EDS) and electron energy loss spectroscopy (EELS) were obtained from the as-synthesized material (Figs. S1(a-c)), which are in good agreement with those of stoichiometric LiNi0.80Co0.15Al0.05O2 cathodes used industrially13,

24

. High-resolution X-ray diffraction (XRD) data (Fig. S1(d)) were

collected using 11 BM beamline at the Advanced Photon Source of Argonne National Laboratory in transmission geometry at 295 K with a calibrated wavelength of λ = 0.414540 Å, a step size of 0.001º and a time per scan of 0.1 seconds, which yields results that are in good agreement with those of the NCA material with the ideal R3m layered structure25. Rietveld refinement of the high-resolution XRD pattern shows that the as-synthesized material has parameters a = b = 2.86627(2) Å, c = 14.1909(1) Å and c/a = 4.95, as well as occ Ni3a = 0.027(3) with χ2 = 4.117. Li/Ni exchange gives a compound

with

the

following

unit

formula

unit:

[Li0.973Ni0.027]3a[Ni0.773Li0.027Co0.15Al0.05]3bO2. The Rietveld refinement results are also in good agreement with LiNi0.80Co0.15Al0.05O2 materials with an ideal R3m layered structure and stoichiometry26, 27. Details of the Rietveld refinement can be found in Fig. S1(e) and Table S2. Transmission electron microscopy (TEM) imaging. The pristine material was dispersed on a copper grid coated with a lacey carbon film for scanning transmission electron microscopy (STEM) high angle annular dark field (HAADF) and high-resolution transmission electron microscopy (HRTEM) observations, which were performed using the following microscopes, respectively: 1) FEI Titan 80-300 microscope equipped with 5



a field emission gun (FEG) and a probe aberration corrector, operated at an acceleration voltage of 300 kV. 2) FEI Titan 80-300 microscope with a FEG and an image aberration corrector, operated at an acceleration voltage of 300 kV. STEM simulation. STEM-HAADF simulation in this work was performed using the JEMS TEM simulation software. The microscopic parameters used for the simulation were taken from the Titan 80-300 microscope that was used for the experimental measurements.

Results Macroscale morphology of pristine primary particles. STEM-HAADF analysis confirmed that the pristine primary particle shows a layered structure (space group R3m), covered by a thin surface layer of the rock-salt phase (space group Fm3m) with a thickness of ~5 atomic planes (Fig. 1(a) and Fig. S2). HRTEM observation of the same material (Fig. S3) confirms the presence of the surface rock-salt layer, rather than artifacts generated by the STEM observation. This rock-salt layer is a minor structural degradation occurring on the particle surface upon the synthesis process, likely due to reduction of the layered phase upon treatment at high temperature. A magnified view of the layered phase (Fig. 1(b)) shows that it is alternating TM and lithium slabs stacking along the [003] direction, as schematically shown in Fig. 1(c). Due to the stacking of (003) slabs, the (003) surface of the layered phase is well shaped and straight. 6


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Differently, along the [120] direction shown in Fig. 1(c), the crystal configuration is more complicated and unlikely to form a well-defined straight surface; a surface with a relatively random shape would form instead.

Fig. 1: (a) Atomic STEM-HAADF view of a pristine primary particle, which possesses the R3m layered structure with a thin layer of the Fm3m rock-salt phase formed on the surface (~5 atomic layers thick). (b) Magnified view of the layered phase in (a). (c) Schematic showing the stacking of TM and lithium slabs along the [003] direction. (d) Low-magnification STEM image of a representative pristine primary particle showing the (003)-type and non-(003) surfaces. (e) Schematic showing the (003) and non-(003) surfaces of a representative primary particle. In the degrading environment, the (003) and non-(003) facets develop differently configurated rock-salt reconstructions due to the structural anisotropy of the layered phase.

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Due to the structural anisotropy of the layered structure (Figs. 1(b, c)), the pristine primary particle develops two very different kinds of macroscale facets: one pair of long and straight facets that are parallel to the (003) plane of the layered lattice, different from the set of facets that are nonparallel to the (003) plane of the layered phase, as experimentally and schematically shown Figs. 1(d, e). Surface reconstructions of these two kinds of facets are analyzed, as shown in the following.

Atomic-level morphologies of rock-salt reconstructions on the non-(003) and (003) surfaces. Fig. 2(a) is an STEM view showing the rock-salt reconstruction on a non-(003) facet, where a rock-salt layer of ~5 atomic planes is present. The surface reconstruction features relatively long and straight (002) rock-salt facets (as identified by the diffractogram of Fig. 2(a) in Fig. S4) with atomic-height steps, as marked with orange dashed lines in Fig. 2(a). A zoom-in view of the stepped region (Fig. 2(b)) confirms that the steps have a height of 1 atomic plane, with the riser between two steps being a (111)-type facet (Figs. 2(b) and (c)).

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Fig. 2: (a) STEM-HAADF view of a reconstructed (002) rock-salt facet containing a sequence of monoatomic steps. (b) Zoom-in view showing the (002) stepped surface of the rock-salt layer with the step riser being the (111) plane. (c) Schematic showing the crystallographic features of the stepped surface. (d) Crystallographic configuration of the stepped surface on the (002) rock-salt layer (showing the TM cations only), with the surface TM cations marked in purple, the bulk cations marked in grey and the missing cations indicated by dashed circles.

Previous work has shown that the rock-salt phase can form in the surface region via inward diffusion of TM cations along lithium channels10, 28, and Fig. 2 indicates that this inward diffusion leads to (002) rock-salt reconstruction. Crystallographically, the stepped structure forms via missing half of the (002) atomic plane on the surface, as 9



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schematically shown in Fig. 2(d) (the missing TM cations are represented by dashed circles). Since the (002)-type plane is the cleavage plane of the rock-salt structure29-31, the formation of the stepped surface by exposing the (002)-type planes of the rock-salt lattice is both energetically and structurally favorable. Unlike the relatively flat rock-salt reconstruction on the non-(003) surface shown in Fig. 2, the reconstruction on the (003)-type facet of the layered particle has a much rougher morphology, as shown in Fig. 3. The magnified STEM view in Fig. 3(a) shows a pyramid-like (stepped) structure of the rock-salt reconstruction, which is more clearly shown in its zoom-out view (Fig. 3(c), marked with a dashed line). Compared with the (002) reconstruction in Fig. 2, this reconstruction leads to significant roughness with a high concentration of monoatomic steps making up pyramid-like structures (Figs. 3(a, c)). The topmost atomic layer has the smallest terrace width, and the layers beneath progressively develop into wider terraces, as schematically illustrated in Fig. 3(b). The terraces are (111) planes of the rock-salt phase and are separated by (002) microfacets. The high concentration of the pyramid-like structures transforms the surface into a hilland-valley structure, as shown in Fig. 3(d). A magnified HRTEM view of a peak structure (Fig. 3(e)) clearly shows its Fm3m nature of the rock-salt phase. As shown in Fig. 3(d), some regions in the (111) reconstruction show an amorphous-like structure, which can be induced by lattice distortion and even a loss of crystallinity15,

20, 32

,

indicating that the (111) reconstruction is much more defective compared with the (002) reconstruction.

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Fig. 3: (a) STEM view of the rock-salt reconstruction on a (003)-type surface with pyramid-like structures. (b) Schematic showing the atomic configuration of the pyramid-like structure. (c) Zoomout STEM image showing the rough surface resulting from the (111) rock-salt reconstruction. (d) Further zoom-out HRTEM view clearly showing the hill-and-valley surface of the (111) rock-salt reconstruction. (e) Magnified HRTEM view of a protruded area (hill) on the rough (111) reconstruction surface shown in (d).

According to the observations above, rock-salt surfaces, with (002) and (111) orientations, are dominantly present on the primary particles (Figs. 2 and 3). The atomic configurations of the exposed facets are largely dependent on their electrostatic properties, which determine whether their presence is energetically favorable. To 11



simplify the discussion, here we assume that the rock-salt phase in the current work has the formula of NiO, although, in reality, it contains a small amount of Li, Co and Al cations as well33, 34. In this model rock-salt system, the (002)-type surface is composed of 50% of Ni2+ cations and 50% of O2- anions, as schematically shown in Fig. 4(a). The equal number of cations and anions on the (002) surface leads to its electrostatic neutrality, suggesting that its exposure is energetically favorable35, as observed in Fig. 2. Conversely, the (111)-type planes of the rock-salt phase are composed of alternating anion/cation sheets stacking along the [111] direction (Fig. 4(b)), so the exposed (111) facet is either positively or negatively charged, increasing the electrostatic energy and thus energetically unfavorable. Fig. 4(c) schematically shows the electrostatically positive (111) surface with the Ni2+ cations on top (marked in purple). Therefore, the (111) surface tends to reconstruct to form the so-called “octopolar” structures on the (111) plane36, generating an electrostatically neutral and stable (111) surface.

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Fig. 4: (a) Projection view of the neutral (002) surface of the NiO rock-salt structure, which is composed of 50% of cations and 50% of anions. (b) 3-D view of the rock-salt structure showing the alternating stacking of the (111) anion/cation planes. In (a, b) the oxygen anions are represented by smaller red circles to emphasize the TM cations. (c) Projection view of the cation arrangement on the (111) surface of the rock-salt phase before reconstruction, where the surface cations are highlighted in purple with the bulk cations marked in grey. (d) Schematic showing the reconstructed (111) surface cations with an ordered arrangement, as marked in purple. (e) Schematic showing the reconstructed (111) surface cations with a disordered arrangement, as marked in purple. (f) Schematic showing side view ([[110] direction) of the loose, disordered (111) surface cations induced by the surface reconstruction, as shown in (e).

The electrostatic polarity of the (111) surface is eliminated by the absence of half of the Ni2+ cations on the surface, with the oxygen anions underneath thereby becoming 13



surface anions. After this reconstruction, the surface is composed of 50% of cations and 50% of anions, which is electrostatically neutral. A surface cations and their neighboring anions compose an octopolar structure, as described in the literature36. If the removal of the surface cations follows an ordered fashion that does not change the lattice symmetry of the (111) plane, a (111)-p(2×2) surface is then generated, as schematically shown in Fig. 4(d). However, as shown from its [110] side view (see details in Fig. S5), this surface exhibits two vacant columns for every three atomic columns, which is inconsistent with experimental results as shown in Fig. 3. Therefore, the (111) rock-salt surface cations must exhibit a disordered configuration, as shown in Fig. 4(e). The [110] side view of the disordered rock-salt surface cations (Fig. 4(f)) has no vacant columns, while the lower concentration of the surface cations compared to the bulk leads to their reduced contrast, consistent with the experimental data in Fig. 3.

Discussion Reconstructing mechanisms of the (002) and (111) rock-salt surfaces. As aforementioned, two different facets of the rock-salt layer are present on the pristine primary particle: (002) and (111). Details of the atomic processes leading to the two surface configurations are presented below. Fig. 5 indicates that the (002) rock-salt reconstruction forms due to gradual migration of TM cations from the surface region into the subsurface area, using the 14


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lithium ion channels (marked in Fig. 5(a)) as the migration pathway due to the small size of Li+ and its loose attachment to the lattice6, 37, 38. Magnified STEM views of the Lichannels obtained from the bulk region to the surface clearly illustrate the migration pathway of TM cations (Figs. 5(b-d)). The bulk of the particle maintains the layered structure with no TM cations on the Li-sites, as demonstrated in Fig. 5(b). Li+ cations contribute very little to the HADDF contrast39, 40, so the Li sites exhibit a black contrast. In the subsurface region (Fig. 5(c)), Li sites exhibit a dim contrast due to a partial presence of TM cations on the Li sites. The contrast of the Li and TM sites becomes the same in a disordered rock-salt structure. Therefore, the subsurface area still preserves the layered structure, with rock-salt features due to the presence of TM cations on Li sites38, 41. An equal occupancy of all sites by TM cations leads to the formation of a completely disordered rock-salt phase on the particle surface, as shown in Fig. 5(d). The depth-dependent phase distribution shown in Figs. 5(b-d) indicates that the rocksalt transformation is a function of the inward diffusion of TM cations. Details of the relationship between the layered → rock-salt transformation and the STEM-HAADF contrast are presented in Fig. S6.

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Fig. 5: (a) STEM view showing a (002) rock-salt reconstruction on a pristine primary particle. The TM cations migrate along the lithium channels towards the bulk of the particle, inducing the formation of the rock-salt phase. (b) Magnified view of the bulk area in (a), which is the typical layered phase with no Li-site TM cations. (c) Magnified view of the subsurface area in (a), which shows a slight contrast of Li-site TM cations. (d) Magnified view of the surface area in (a), where the Li-site and TM-site TM cations have almost the same contrast, confirming that it has a rock-salt structure.

The

layered

→

rock-salt

phase

transformation

determines

the

(002)

reconstruction shown in Fig. 5. In the NCA cathode, the rock-salt phase has the formula of MO, in which M = 70% ~ 100% Ni2+ cations42, along with a small amount of Li, Co, Al cations33. In other words, the rock-salt phase is enriched with TM cations (nickel in this case) compared with the layered phase. The inward migration of TM cations increases their concentration, providing the necessary condition for the rock-salt transformation. Accompanying the enrichment of TM cations, lithium and oxygen are partially removed 16


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from the surface area, escaping into the environment or forming lithium salts such as hydroxide or carbonate41. Knowing from Fig. 5 that the (002) reconstruction develops via the inward migration of TM cations onto the Li sites, a detailed pathway for the (002) rock-salt reconstruction is proposed, as demonstrated in Fig. 6. The (002) plane of the rock-salt phase corresponds to the (014) plane of the layered phase, on which the (002) rock-salt reconstruction takes place. HRTEM observation of a clear (014) surface of the layered phase without rock-salt reconstruction shows a rough morphology (Figs. 6(a) and S7). As schematically shown in Fig. 6(b), this layered surface is atomically rough with multiple protruding TM cations, where the TM cations in the surface have fewer nearest neighbors and thus have dangling bonds exposed to the surface. These surface dangling bonds produce an inwardly directed force that drives the migration of the surface atoms into Li sites in the subsurface region, as shown in Fig. 6(c). This inward migration leads to the enrichment of TM cations in the surface and subsurface region, transforming the rough (014) surface of the layered phase into the flat (002) surface of the rock-salt phase. During this process, the oxygen and lithium ions in the surface and subsurface regions are partially removed to enable the rock-salt transformation induced by the inward migration of TM cations. The high instability of the rough (014) surface accelerates the inward diffusion of the TM cations, and, by extension, the (002) rock-salt reconstruction. After the (002) reconstruction, any individually exposed TM cations on the surface, as schematically shown in Fig. 6(d), render it unstable through the formation of uncompensated charge. These defects tend to be eliminated to minimize the surface energy via the formation of stepped structures.

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Fig. 6: (a) HRTEM view of the atomically rough (014) surface of the layered phase before the (002) rock-salt reconstruction. (b) Schematic showing the atomically rough (014) surface of the layered structure without the (002) rock-salt reconstruction, comparable to (a). (c) Schematic showing the interior migration of the protruding TM cations on the (014) surface of the layered phase leading to the (002) rock-salt reconstruction. (d) Schematic showing the reconstructed (002) rock-salt surface with random, positively charged TM cations remaining on the surface. (e) Schematic showing the unstable, exposed TM cations migrating into the subsurface region of the particle. (f, g) Schematics showing the merging of the exposed TM cations on the (002) rock-salt surface to form a stepped structure, minimizing the total free surface area of the exposed cations. The positively charged free surfaces in (f, g) are marked with orange dashed lines. 18


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Theoretically, there are two possible reconstruction pathways to eliminate the unstable, exposed TM cations on the (002) reconstruction to minimize the surface free energy, as schematically shown in Figs. 6(e-g). The first one is through inward migration of exposed cations into the subsurface region of the parent particle (the interior of the (002) reconstruction), leading to the inward growth of the rock-salt layer, as schematically illustrated in Fig. 6(e). The other pathway involves the aggregation of the highly exposed cations on the (002) surface, as illustrated demonstrated in Figs. 6(f, g) and S8. The aggregation of the exposed cations results in the formation of atomic steps on the (002) surface of the rock-salt layer, consistent with the experimental observations. This mechanism lowers the total polar surface area of exposed cations (see orange dashed lines in Figs. 6(f, g)), and reduces electrostatic energy. Unlike the growth of the (002) reconstruction along the lithium channels, the (111) reconstruction propagates inwards along the c-axis of the layered phase, which is perpendicular to the lithium channels (Fig. 7(a)). The topmost area is fully transformed into the rock-salt phase, the subsurface is a partially developed rock-salt phase and the bulk preserves a rather defect-free layered framework. In the absence of lithium channels, this (111) reconstruction develops via the interlayer mixing of the lithium and TM cations, as schematically shown in Fig. 7(b). Li/Ni interlayer mixing is facile in the Ni-rich cathode studied here7, 9, 11, 16, 38, which transforms the R3m layered structure into the Fm3m rock-salt structure with a much lower Gibbs free energy38, 43.

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Fig. 7: (a) STEM image showing the inward development of the (111) rock-salt reconstruction along the c-axis of the layered phase, forming a straight reconstruction surface. (b) Interlayer mixing which transforms the Li+ cations onto the (003) surface of the layered phase. (c) Schematic showing partial interlayer mixing of the Li/TM cations occurring on the (003) surface of the primary particle, leading to the (111) rock-salt reconstruction with a flat surface. (d) Complete interlayer mixing in which Li+ detach from the (003) surface and TM cations migrate along the Li channels, leading to the (111) reconstruction. (e) Stepped surface resulting from the (111) reconstruction. (f) Schematic showing the zig-zag surface resulting from the (111) rock-salt reconstruction.

The roughening effect of the (111) reconstruction depends on the development of the interlayer mixing. When a small amount of the TM cations exchanges the position with Li+ (Fig. 7(c)), the framework of the layered phase is maintained and the (111) reconstruction exhibits a straight surface (Fig. 7(a)). When the TM and Li cations completely shift their positions, the Li cations become the outermost surface atomic layer while the TM cations occupies the second atomic layer (Fig. 7(d)). The Li+ cations 20


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in the surface are highly unstable due to their increased dangling bonds, more open structure, and lower bonding energy with the oxygen anions7, 9, favoring their extraction from the primary particle. The subsurface TM cations can migrate along the lithium channels towards the regions without interlayer-mixing (Fig. 7(d)), leading to the formation of the rock-salt phase in these regions. As a result, both lithium and TM cations in the interlayer-mixing region diffuse away, leaving behind a (111) rock-salt stepped surface structure, as schematically illustrated in Fig. 7(e). Such an atom migration process shown in Figs. 7(d, e) results in the hill-and-valley surface morphology of the (111) reconstruction of the rock-salt phase, as shown in Figs. 3 and 7(f). In other words, the macroscale roughness of the (111) reconstruction is attributed to its roughness at the atomic level (Figs. 7(d, e)). This hill-and-valley surface configuration formed via lattice dissociation results in lattice distortion or even amorphization in some regions of the rock-salt layer, as shown in Fig. 3(d).

Rock-salt reconstruction and electrochemistry. As shown below, the surface layered → rock-salt phase transformation depends on the structural anisotropy of the layered phase, for which the different facets of the primary particles lead to significantly different features of the surface rock-salt layer, including the growth pathways and the atomic configurations. A shared feature of the (002) and (111) rock-salt reconstructions is the loss of lithium and oxygen, leading to the enrichment of TM cations in the surface region. The ionic radius of Ni2+ cations makes them energetically favored to occupy the Li octahedral sites, leading to the formation of the rock-salt phase. In contrast, Ni3+ is 21



much less likely to appear in interlayer defects7 because of the size mismatch with Li+. Therefore, the loss of O2- from the structure is accompanied by the reduction of the Ni3+ cations and the formation of Ni2+, which is the essential component for interlayer mixing and thus formation of the rock-salt phase. Surface rock-salt reconstructions are electrochemically inactive because they reduce the electronic and, especially, ionic conductivity of the cathode through the elimination of percolation channels for charge carriers compared to the layered phase37. Unfortunately, the high chemical activity of the layered phase makes it very facile to transform to the rock-salt phase during heating or in contact with the gaseous environment7, 9. The limited thickness of both surface reconstructions in pristine materials (~5 atomic layers) seems to be insufficient to shut down transport, as this oxide is highly electrochemically active44. However, when the cycle number increases, the rock-salt reconstruction layer is found to grow into larger electrochemically inactive domains, which eventually degrades performance10, 45. In terms of atomic configuration, the (111) reconstruction is much rougher compared with the (002) reconstruction. The (002) reconstruction generates a flat, well-structured facet, while the (111) reconstruction forms an etched, loosely reconstructed surface. In the corroded areas (valley-like structures) of the (111) reconstruction, the topmost surface of the layered phase is completely etched out and the layered structure beneath is exposed to the degrading environment. This newly exposed layered phase transforms to the rock-salt phase via the (111) reconstruction, which leads to more etching and the formation of more exposed layered phase. The chain reaction continues until the entire (003) surface of the layered phase is transformed to the (111) reconstruction, which 22


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exhibits a zig-zag surface morphology resulting from the surface etching. On the contrary, the (002) reconstruction results from inward diffusion of TM cations, followed by the stabilization of the surface reconstruction. Therefore, the (111) reconstruction results in a continuous formation of the rock-salt phase with a defective lattice, while the (002) reconstruction leads to the formation of a well-structured rock-salt layer with a relatively intact lattice that has improved passivation property and provides protection to the underlying layered phase. The zig-zag (111) reconstruction surface significantly increases the surface area, which can lead to more cation dissolution compared with the intact, flat (002) rock-salt surface during electrochemical cycling. In other words, the different configurations of (014 ) and (003) planes lead to (002) and (111) rock-salt reconstructions which should show significantly different chemical reactivity. From

the

perspective

of

electrochemical

properties,

the

(002)

rock-salt

reconstruction blocks the lithium channels, which reduces the Li+ and electronic conductivity of the particle, an undesirable outcome. However, as mentioned above this (002) reconstruction needs the Li+ cations to be removed for the inward diffusion of TM cations. Once the (002) reconstruction forms, the loss of Li cations becomes difficult, prohibiting the further development of the reconstruction. Conversely, the (111) reconstruction does not require the removal of Li+ cations first. The interlayer mixing of lithium and TM cations expose the lithium cations onto the surface and leads to its detachment. As the (111) reconstruction takes place, fresh layered phase is continuously exposed in the surface (Fig. 7(e)), which maintains the progression of (111) reconstruction. Therefore, the (111) reconstruction develops faster than the (002)

23



reconstruction, resulting in larger electrochemically inactive rock-salt domains that reduces the ionic and electronic conductivity of the cathode.

Conclusion Two kinds of rock-salt induced surface reconstructions, (002)-type and (111)-type, are observed on Ni-rich layered primary particles. Both reconstructions feature stepped structures, but the (002) stepping is significantly flatter compared with the (111) surface that shows a highly rough hill-and-valley morphology. The (002) reconstruction is induced by inward migration of surface TM cation along lithium channels, while the (111) reconstruction occurs due to the interlayer mixing of Li/TM cations. Both reconstructions are potential sources for structural degradation during electrochemical cycling: the (002) reconstruction reduces the ionic and electronic conductivity. In turn, the (111) reconstruction leads to significant roughing of the (003) surface which largely increases the surface area and may induce more surface etching and cation dissolution by the electrolyte.

Acknowledgement

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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. This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. CMW thanks the support of the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the U. S. Department of Energy under Contract No. DE-AC02-05CH11231, Subcontract No. 18769 and No. 6951379 under the Advanced Battery Materials Research (BMR) program. The TEM work was conducted in the William R. Wiley Environmental Molecular Sciences Laboratory (EMSL), a national scientific user facility sponsored by DOE’s Office of Biological and Environmental Research and located at PNNL. PNNL is operated by Battelle for the Department of Energy under Contract DE-AC05-76RLO1830.

Supporting Information Chemistry of the pristine sample; Repeated STEM/HRTEM observations of the surface and bulk; STEM simulations; Diffractograms; Schematics showing the atomic processes;

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Facet-Dependent Rock-Salt Reconstruction on the Surface of Layered

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