Rock-salt Growth Induced (003) Cracking in Layered Positive

Oct 20, 2017 - Using transmission electron microscopy techniques, here we show that the propagation and fracturing of platelet-like rock-salt phase al...
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Rock-Salt Growth-Induced (003) Cracking in a Layered Positive Electrode for Li-Ion Batteries Hanlei Zhang,†,‡ Fredrick Omenya,‡ Pengfei Yan,§ Langli Luo,§ M. Stanley Whittingham,‡ Chongmin Wang,*,§ and Guangwen Zhou*,†,‡ †

Materials Science and Engineering Program & Department of Mechanical Engineering, State University of New York, Binghamton, New York 13902, United States ‡ NorthEast Center for Chemical Energy Storage, State University of New York, Binghamton, New York 13902, United States § Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, Washington 99352, United States S Supporting Information *

ABSTRACT: For the first time, (003) cracking is observed and determined to be the major cracking mechanism for the primary particles of Ni-rich layered dioxides as the positive electrode for Li-ion batteries. Using transmission electron microscopy techniques, here we show that the propagation and fracturing of platelet-like rock-salt phase along the (003) plane of the layered oxide are the leading cause for the cracking of primary particles. The fracturing of the rock-salt platelet is induced by the stress discontinuity between the parent layered oxide and the rock-salt phase. The high nickel content is considered to be the key factor for the formation of the rock-salt platelet and thus the (003) cracking. The (003)-type cracking can be a major factor for the structural degradation and associated capacity fade of the layered positive electrode. the positive electrode,11 accelerating other degradation mechanisms such as the phase transformation12 and causing more severe structural degradation in subsequent cycles.13 Traditionally, cracking in the positive electrode is considered as a function of cycling,13 which is a complicated electrochemical process with a large number of side reactions,14,15 including phase transformation, structural disordering, dissolution of transition metals (TMs), and microcracking. All of these side reactions have potential influence on cracking,8 making the real leading cause for cracking largely unresolved. Also, the phenomena of cracking involve many subcomponents, including material peel-off in the surface layer, microcracking, intergrain cracking, and intragrain cracking, which are highly interwoven and should be separated for observation and analysis using electron microscopy tools. Besides the interwoven cracking phenomena, another daunting difficulty prohibiting the detailed observation of cracking is the large size of the primary particles. Most primary particles have a potato-like morphology, with the size ranging from 100 nm to 10 μm. The large size of the primary particles prohibits microscopic observation of the core area, for which only the thin edge area is possible for clear transmission

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ayered dioxide positive electrodes play a critical role in the modern lithium-ion battery (LIB) industry, particularly in next-generation electric vehicles (EVs) and electric energy storage.1−3 Besides the requirements for a higher capacity and rate capability,4,5 the structural stability is another concern affecting the performance of the layered positive electrodes.6 Cracking of the primary particles leads to significant structural degradation and thus deteriorates the overall electrochemical performance of the positive electrode.7,8 Upon cracking, fresh surfaces without the solid−electrolyte interface (SEI) are generated, which exhibits distinctive electrochemical properties compared with the old surfaces with SEI. Also, the electrolyte is allowed into the deep area of the positive electrode, making the original core region of the particles vulnerable to surface-related degradations. Because cracking significantly changes the morphology of the positive electrode, exploring the cracking process is thus critical for understanding the electrochemical degradations, including first-cycle columbic inefficiency, capacity fade, and voltage fade. Compared with other structural degradations in localized regions like surface reconstruction, phase transformation, and microstructural defects (e.g., dislocations and planar defects),9,10 cracking generates immediate and severe large-scale damage on the layered structure, leading to a much more significant reduction of the electrochemical performance. Cracking also temporarily increases the chemical reactivity of © XXXX American Chemical Society

Received: September 21, 2017 Accepted: October 20, 2017 Published: October 20, 2017 2607

DOI: 10.1021/acsenergylett.7b00907 ACS Energy Lett. 2017, 2, 2607−2615

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http://pubs.acs.org/journal/aelccp Cite This: ACS Energy Lett. 2017, 2, 2607-2615

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ACS Energy Letters

Figure 1. (a) Bright-field TEM image of a one-cycle primary particle with a (003) crack that breaks the particle into two pieces. (b) STEM view of a 30-cycle particle that is fractured into multiple parallel platelet-like fragments (c) Schematic showing the parallel platelet-like fragments shown in (b). (d) Electron diffraction pattern from region 1 in (a), corresponding to the rock-salt phase. (e) Electron diffraction pattern from region 2 in (a), in which the dominating spots are from the rock-salt phase with the presence of extra spots (indicated by the arrows) associated with the layered phase. (f) Diffraction pattern from region 3 in (a), indicating the coexistence of the layered and rock-salt phases similar to (e), but with more layered phase. (g) Electron diffraction pattern from region 4 in (a), corresponding to the rock-salt phase. (h) Electron diffraction pattern from region 5 in (a), indicating a mixture of the layered and rock-salt phases, similar to (e).

electron microscopy (TEM) analysis.16−18 Fractures as well as other extended defects (e.g., grain boundaries and cavities) are deep in the bulk and cannot be fully observed in the local subsurface region, which is only a few tens of nanometers from the outermost surface. Even when a crack is large enough to be observed at low magnification, the large thickness in the bulk region of the particle can still block the cracking feature or reduce the TEM image contrast along the e-beam direction, making the microscopic observation very challenging. The agglomeration of the primary particles is another difficulty. Overlapping of agglomerated primary particles prohibits clear TEM observation, and only well-separated particles are suitable for observation, for which revealing the intrinsic behavior in the bulk region cannot be ensured because the characterization is performed in the highly selective regions that are limited to the surface areas and well-separated particles. To overcome the interwoven phenomena and large-particle issues, we employ an ultramicrotome method to section agglomerated primary particles into thin slices with a nominal thickness of 300) of the sliced TEM specimens of the 1-cycle and 30cycle samples, we can find that the presence of cracking becomes more significant upon increasing the number of cycles. Figure 1a shows a low-magnification TEM image of a primary particle from the one-cycle sample, which is fragmented into two pieces by a long, straight crack. The lower fragment is platelet-like with two parallel surfaces. As known from electron diffractogram and high-resolution TEM (HRTEM) analysis in the later part of this work, these platelet-like fragments have two parallel facets that are parallel to the (003) plane of the layered phase. Figure 1b illustrates a scanning TEM (STEM) view of a 30-cycle particle, which shows that the entire particle is fully fractured into relatively parallel (003)-type platelets that are similar in thickness. Figure 1c is a schematic showing the parallel fragmented platelets in Figure 1b. Electron diffraction patterns are obtained from five regions near the crack of the one-cycle particle, as marked in Figure 1a. Regions 1−3 are located on the upper, larger fragment, while locations 4 and 5 are on the lower, smaller platelet-like fragment. These diffraction patterns are precisely indexed using electron diffraction simulation, which can be found in Figure S2. Location 1 (Figure 1d) is confirmed to be the rock-salt phase. The electron diffraction pattern from region 2 (Figure 2608

DOI: 10.1021/acsenergylett.7b00907 ACS Energy Lett. 2017, 2, 2607−2615

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ACS Energy Letters

parallel to the (003) plane of the layered phase (Figure S3). This preferred growth of the rock-salt platelet along the (003) plane of the layered structure coincides with the (003) cracking of the primary particle, suggesting that the (003) cracking of the primary particles is induced by the extensive (003) growth of the rock-salt platelet into the primary particle. The development of the (003) cracking is through the gradual fracturing of the rock-salt platelet, nucleating in the surface and progressively developing into the bulk of the primary particle (Figure S4). Figure 3 shows a representative TEM observation illustrating the development of (003) cracking. Figure 3a is a low-magnification TEM view of the one-cycle sample with a partly developed (003) crack (highlighted by a green dashed line). The crack develops from the top-right end of the green line into the depth of ∼300 nm, and the rest (the lower-left end) of this crack remains undeveloped. Due to the nature of the (003) cracking, two sharp, flat crack faces parallel to the (003) plane are partly exposed at the top-right end of the developing crack. Figure 3b is a schematic illustrating the key microstructural features of the cracking primary particle shown in Figure 3a. Figure 3c is a HRTEM image obtained from the lower-left end of the developing (003) crack, which corresponds to the unfractured area in front of the crack tip, as marked by a red dashed square in Figure 3a. The HRTEM image indicates the presence of the rock-salt phase in front of the crack tip with the rock-salt/layered interface parallel to the (003) plane of the layered structure, confirming that a rock-salt platelet exists in the undeveloped part of the crack. The crystallographic relationship on the interface can be identified as (111)R// (003)L and [220̅ ]R//[110]L (a magnified HRTEM view of the rock-salt phase can be found in Figure S5a). In front of the crack tip, the rock-salt/layered interface extends in the bulk region for over 500 nm to form a thin rock-salt platelet along the (003) plane (Figures S5b), demonstrating that the (003) rock-salt platelet serves as the nucleus for the (003) cracking. By examining a large number of the primary particles, the (111)R//(003)L rock-salt/layered interfaces have been repeatedly observed in front of the crack tips of developing (003) cracks, confirming that the fracturing of the rock-salt platelet is the reason for the (003) cracking (Figure S5c,d). In Figure 3c, partial fracturing in the rock-salt region is clearly visible (the boundary between the fractured and intact rock-salt regions is marked by orange dashed lines), while the rock-salt/layered interface remains adherent, indicating that the cracking occurs inside of the rock-salt platelet rather than along the rock-salt/layered interface. The cleavage plane of the rocksalt phase is determined to be the (200)-type plane (Figure 3c, marked by orange dashed lines), consistent with the wideacknowledged cleavage plane of the rock-salt phase.19,20 The cleavage of the rock-salt phase leads to fracturing of the entire rock-salt platelet, as schematically shown in Figures 3d,e. The originally intact rock-salt platelet (Figure 3d) cleaves along its (200)-type planes, which are inclined with the (003) rock-salt/ layered interface. Accordingly, the (200) cleavage facet of the rock-salt phase cannot extend all the way through the platelet. Instead, many short segmented {200} facets are generated to break up the entire rock-salt platelet, forming a zigzag crack face, as schematically shown in Figure 3e. Therefore, the (003) crack face has two very different morphologies at the macroand microscales: at the macroscale, the crack face is roughly straight along the (003) plane of the layered phase, while at the

1e) is similar to that of region 1 with the strong bright spots from the rock-salt phase, as well as some extra weak spots (pointed out by the red arrows in Figure 1e) from the layered phase, indicating the dominant presence of the rock-salt phase with a small amount of the layered phase in this region. Figure 1f is the diffraction pattern from a larger region 3 that contains region 2 (i.e., region 3 contains more bulk compared with region 2). The diffraction pattern is almost the same as that from region 2 (Figure 1e), but the spots associated with the layered phase have a much stronger intensity, implying that the rock-salt phase forms only in the region close to the crack face and the region deeper away from the crack face remains as the layered phase. Figure 1g,h shows diffraction patterns obtained from the smaller platelet-like fragment, which confirms that the rock-salt phase is only formed on the crack face, the same as the larger fragment (Figure 1d−f). The diffraction from the crack face (Figure 1g) is exclusively the rock-salt phase, while the diffraction obtained from the region slightly deeper from the crack face (Figure 1h) is a composite pattern with spots from the rock-salt and layered phases. Therefore, the rock-salt phase is only present on both faces of the crack while the rest of the particle remains as the layered phase. The observed rock-salt phase at the two faces of the crack also serves as clear evidence that the crack is a real microstructural feature induced by the growth and fracturing of a rock-salt platelet into the primary particle rather than an artifact from the TEM specimen preparation process: the rock-salt phase develops into a thin platelet across the primary particle along the (003) plane of the layered phase and then fractures along the planes parallel to the (003) plane of the layered phase. Upon fracturing, the rock-salt platelet breaks into multiple smaller grains that tilt against each other, leading to the differently oriented rock-salt grains along the cracked surfaces (Figure 1d−h). Observation of unfractured rock-salt platelets confirms the rock-salt-induced (003) cracking, as shown in Figure 2. Figure

Figure 2. (a) STEM view of a 30-cycle particle with multiple parallel (003) rock-salt platelets formed across the particle (marked by orange arrows). (b) Magnified HRTEM view of a 30-cycle particle with an unfractured (003) rock-salt platelet developed into the bulk of the layered phase.

2a is a 30-cycle particle with a high concentration of parallel (003) platelets (showing as slightly dark strips, as marked by orange arrows) but no obvious crack formation. Figure 2b is a magnified HRTEM view of such a platelet in the 30-cycle particle, which shows the growth of the thin platelet deep into the bulk of the primary particle but not causing cracking yet. The intragranular platelet is confirmed to be the rock-salt phase by HRTEM imaging and electron diffraction analysis (Figure S3). The growth of the rock-salt platelet into the layered particle results in the rock-salt/layered interfaces that are 2609

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Figure 3. (a) TEM view of a one-cycle particle. A (003) crack has partially developed across the particle, as marked with a green dashed line. Its fully cracked right end results in the exposed crack faces that are parallel to the (003) plane of the layered phase. (b) Schematic illustrating the key microstructural features of the cracking primary particle shown in (a), where the white dashed line denotes the unfractured part of the intragranular platelet of the rock-salt phase. (c) HRTEM view from the undeveloped lower-left end of the developing crack (i.e., the red square in (a)), which confirms the intragranular growth of the rock-salt platelet. A partial fracturing of the rock-salt phase is observed with the cleavage plane being the {200}-type planes of the rock-salt phase, as indicated by the orange dashed lines. (d) Schematic showing a rock-salt platelet formed along the (003) plane across the primary particle. (e) Schematic showing the fracturing of the rock-salt platelet along its microscale (200)-type facets, leading to the macroscale (003) cracking of the primary particle. (f) Schematic showing the macroscale/ microscale morphologies of (003) cracking, in which the macroscale crack face is along the (003) plane of the layered phase while in the microscale the crack face is composed of many short (200)-type rock-salt facets. (g) Magnified view of the orange rectangle in (a), where the development of a surface rock-salt platelet results in a stepped structure and an exposed (003) face. (h,i) Schematics showing the fracturing of the surface rock-salt platelet, which results in the stepped structure in (g), where (h) corresponds to the formation of a rock-salt platelet on the (003) surface of the primary particle and (i) illustrates the formation of a stepped structure resulting from the fracturing of the surface rock-salt platelet.

induced stepped surface feature is repeatedly observed in the cycled samples, as shown in Figure S5e,f. These observations indicate that the cleavage of the surface rock-salt phase only results in the stepped surface morphology, while the cleavage of the rock-salt platelet penetrating the bulk leads to the (003) cracking of the primary particles. The loss of oxygen drives the formation of the rock-salt phase, which is enriched with TM cations compared with the layered phase. When the loss of oxygen occurs in the surface, the rock-salt phase nucleates and propagates in the surface; however, when oxygen in the bulk diffuses out through the surface, oxygen-deficient areas in the bulk is generated and the rock-salt phase nucleates in the bulk. In this work, the rock-salt platelet is observed to develop along the (003) plane. This is because the (003) plane is a kinetically favorable diffusion channel for oxygen due to its role as the diffusion channel for lithium. The formation of rock-salt platelets along the (003) plane can therefore be attributed to the preferential oxygen loss from the (003) plane that starts from the surface and

microscale, it exhibits a zigzag morphology containing multiple {200}-type rock-salt facets, as schematically shown in Figure 3f. If the rock-salt platelet forms on the (003) surface of the primary particle instead of penetrating the bulk, stepped surface structures will be generated other than (003) cracking, as shown in Figure 3g−i. Figure 3g is the magnified view of a typical cracked surface with a large stepped structure formed by surface fracturing. As schematically shown in Figure 3h, a rocksalt platelet forms on the (003) surface of the primary particle. This relatively thick rock-salt platelet has only partially propagated along the (003) planes, and the {200}-type cleavage/fracturing of the surface rock-salt platelet results in a stepped structure on the (003) surface of the layered phase, as schematically shown in Figures 3i. Because the cleavage is limited inside of the rock-salt platelet rather than on the rocksalt/layered interface, some residue rock-salt phase is still present on the fractured faces of the primary particle (Figure 3g,i), similar to the residual rock-salt facets resulting from the intragranular (003) cracking (Figure 3e,f). Such a facture2610

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Figure 4. Schematic showing the cracking pathways for the primary particle of the layered phase, induced by the (003) rock-salt platelet. (a) Intact particle with the layered structure. (b) (003) cracking due to the formation and fracturing of the rock-salt platelet in the bulk of the primary particle, which results in the platelet-shaped (003) fragments. (c) Development and fracturing of the rock-salt platelet on the (003) surface, leading to the stepped structure. (d) Development and fracturing of multiple parallel rock-salt platelets in the bulk, breaking the primary particle into multiple parallel platelet-shaped (003) fragments.

progressively develops toward the bulk. The layered → rocksalt transformation is also featured by TM migration through the lithium channel parallel to the (003) plane.21,22 Migration of TM cations across the TM slabs (perpendicular to the lithium channel), although theoretically possible, is crystallographically and energetically unfavorable.23,24 The octahedral TM cations in the TM layers result in large energy barriers for the diffusion of TM cations perpendicular to the TM slabs,25,26 which slows down the layered → rock-salt transformation along the [003] direction. This explains why the layered → rock-salt transformation preferably develops along the lithium channels of the (003) planes, resulting in formation of the rock-salt platelets with flat (111)R//(003)L rock-salt/layered phase boundaries. Figure 4 schematically summarizes the rock-salt plateletinduced (003) cracking. The original structure of the primary particle is the intact layered phase (Figure 4a). The rock-salt phase can develop along the (003) plane to form a rock-salt platelet, and the fracturing of this platelet results in the plateshaped (003) fragments (Figure 4b). Alternatively, if the rocksalt platelet develops on the (003) surface, its fracturing will lead to stepped structures on the (003) surface, as schematically shown in Figure 4c. Development and fracturing of multiple parallel rock-salt platelets break the primary particle into parallel plate-shaped (003) fragments, as shown in Figure 4d and the STEM image in Figure 1b. Knowing the role of the rock-salt platelet in (003) intragranular cracking, two natural questions arise: (1) what is the structure of cracking upon its nucleation in the rock-salt platelets during cycling? (2) What is the driving force for the fracturing of the rock-salt platelets? Our following TEM analysis provides further insight into these questions. Mechanical Cracking of the Rock-Salt Platelet. As shown above, the rock-salt platelet growth is considered as the major reason for (003) cracking. In the Ni-rich layered dioxides, the effect of the spinel phase is insignificant due to its low content. Ni3+ and Ni4+ are actively oxidative,11,27 and they can be easily reduced to Ni2+. Therefore, the spinel phase containing Ni3+ and Ni4+ is much less stable than the rock-salt phase containing Ni2+ (majorly NiO), which explains the little spinel phase observed in NCA. It is known that the rock-salt phase is much more fragile than the layered and the spinel phases.28−30 However, the brittleness of the rock-salt phase is not the only necessary condition for the (003) intragranular cracking. As shown in Figures 3 and 4, the position of rock-salt platelets is critical for

(003) cracking: rock-salt platelets formed on the surface can only lead to surface stepped structures. The sandwich structure generated by the rock-salt platelet growth into the bulk of the primary particle is the key factor for (003) cracking. The polycrystalline (average) Young’s modulus of the highNi rock-salt phases is around 170 GPa at room temperature,31,32 similar to that of the layered phases (e.g., the Young’s modulus of LiCoO2 is 171 GPa).33 However, due to the structural anisotropy of the layered phase, its modulus in the (003)L/(111)R phase interface is much smaller than that along its c-axis.34 In other words, in the (003)L/(111)R interface, the rock-salt phase has a much higher modulus compared with the layered phase. Because the a-axis of the layered phase is in the rock-salt/layered interface, changing of the a-axis generates inplane strain that directly applies to the fragile rock-salt platelet, leading to its fracturing. The expansion and contraction of the a-axis of the layered phase during cycling apply continuous cyclic strain shocks onto the rock-salt plate and are the major reasons for its fracturing. Once a rock-salt platelet is formed, its lattice parameters remain unchanged during the subsequent cycling because the rock-salt phase is electrochemically inactive. Conversely, the lattice parameter of the surrounding layered structure keeps changing along with the electrochemical cycling, which applies cyclic strain shocks on the sandwiched rock-salt platelets. The layered phase undergoes ∼0.5% expansion in the a-axis during the lithiation process and the same amount of extraction during delithiation.25,35 According to the (111)R//(003)L rocksalt/layered interfacial relationship, the change in the aparameter of the layered lattice generates in-plane strain on the rock-salt platelet. The retraction of the a-axis upon delithiation generates a tensile strain field concentrated in the surface region of the rock-salt platelet (Figure 5a; see details in Figure S6), and the expansion of the a-axis during lithiation releases the rock-salt plate from the high-tension state, bringing a fatigue-fracturing effect on the rock-salt platelet. The electrochemical lithiation−delithiation cycling imposes cyclic tensile strain shocks on the surface region of the rock-salt platelet, causing its fracturing and thereby crack nucleation when beyond the fatigue endurance. During lattice retraction of the layered phase, the rock-salt lattice parameters remain almost unchanged; therefore, the lattice planes in the layered phase are bent, as shown in Figure 5b, confirming that the cycling induces a tensile strain field in the rock-salt platelet. The tensile field in the rock-salt platelet is the key factor for its 2611

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surface region of the platelet and a crack tip is thereby generated. HRTEM mapping is performed to visualize the atomic structure of the crack tip, as presented in Figure 6. Figure 6a shows a one-cycle particle with a rock-salt plate where a crack tip has just initiated. The crack tip is too small to be observed in the low-magnification view of Figure 6a, but a magnified HRTEM view (Figure 6b) clearly shows its morphology. The developing crack tip is highlighted with a green arrow in Figure 6b, and a high concentration of rock-salt domains and defects is observed around the crack tip (these structures are determined to be the rock-salt phase by their diffractograms in Figure S7). A magnified HRTEM view of the crack tip (Figure 6c) shows that it is a (111)-type rock-salt facet fracturing along {200}-type planes (demonstrated by its diffractogram and corresponding diffraction simulation in Figure S7a,b). On the (111)-type facet, a groove (i.e., the opening tip) with a small {002}-type facet is present (Figure 6c), which is schematically illustrated in Figure 6d. The (002)-type facet is the cleavage plane for the rock-salt structure38 because fracturing of the rock-salt platelet along the (002)-type plane is both energetically and structurally favorable.38,39 Therefore, Figure 6c confirms that the fracturing of the rock-slat platelet is via the cleavage of the {002}-type rock-salt lattice planes (schematically shown in Figure 6d),

Figure 5. (a) Schematic showing the tensile strain concentrated in the surface region of the rock-salt platelet, generated by retraction of the a-parameter of the layered phase upon delithiation. (b) HRTEM view of the rock-salt/layered interface from a one-cycle particle, showing the bending of the lattice planes in the layered phase caused by the retraction of its a-axis.

fracturing. Although the compression strain field can be also present during cycling, the rock-salt phase tolerates compression strain better than tensile strain,36,37 meaning that the compression strain cannot be the reason for the fracturing of the rock-salt platelet. Deformation of the Rock-Salt Platelet and Nucleation of the Crack Tip. As discussed above, after the formation of the rocksalt platelet, cyclic tensile strain shocks are concentrated in the

Figure 6. (a) One-cycle particle with the presence of a rock-salt platelet in which a crack has just initiated. (b) Magnified HRTEM view of the just-initiated crack, obtained from the selected region in (a). A high concentration of domains and planar defects is observed around the crack tip. Two areas with different distances from the crack tip are selected for zoom-in observation. (c) Magnified HRTEM view right from the crack tip, which proves to be a (111)-type rock-salt facet cracking along a (200)-type plane. (d) Schematic showing the (002)-type cleavage at the atomic level observed in (c). (e) Magnified view of a region ∼3 nm away from the crack tip, which contains planar defects including a twin interface (marked out by purple/yellow dots). (f) Schematic showing the configuration near the crack tip, where the concentrated strain field in the crack tip leads to a deformed zone around the crack tip. (g) Capacity vs cycling count curve of NCA cycled between 3.0 and 4.3 V (30 cycles), showing a capacity bump in the first few cycles. 2612

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the first electrochemical cycle, only one capacity bumping shows up in the first few cycles. Although (003) cracking can lead to a sudden increase in the capacity, it also allows the electrolyte to penetrate into deeper regions of the primary particle. The development of a new SEI on the cracked surfaces not only accelerates the decomposition of the electrolyte, which thus decreases the amount of electrolyte, but also permanently reduces the Li+ and electronic conductivity of the primary particles. In the long-term view, the (003) cracking largely degrades the layered structure of the particle and decreases the capacity. Because the formation of the rock-salt platelets is highly dependent on the high level of nickel content, the layered oxides with a lower Ni content predictably undergoes more spinel transformation rather than the rock-salt transformation. Thus, in low-Ni-containing layered oxides, the rock-salt platelet is less likely to form, meaning that reducing the nickel content is a potential approach to prevent (003) cracking. This prediction is in line with the electrochemistry measurements showing that the capacity bumping for the nickel−manganese− cobalt layered dioxides (NMC) with lower nickel contents is much less compared with the Ni-rich layered oxides,48,49,51,52 suggesting less (003) intragranular cracking in low-Ni dioxides. The (003) intragranular cracking is shown as a major cracking mechanism in high-Ni-containing layered dioxides as the positive electrode of Li-ion batteries. The rock-salt phase is observed to develop along the (003) plane to form rock-salt platelets, and fracturing of the rock-salt platelets leads to (003) cracking of the primary particles. The stress discontinuity between the electrochemically active layered phase and the inactive rock-salt phase induces cyclic mechanical shocks onto the rock-salt platelets, which leads to tearing/fracturing of the rock-salt lattice. Tensile stress concentrates in the surface region of rock-salt platelets, leading to nucleation and inward movement of the crack tip, which finally propagates into a (003) crack across the primary particle. Although the intragranular cracking results in a temporary capacity increase in the first several cycles, it accelerates the structural degradation and reduces the electrochemical performance of the positive electrode in the long run.

consistent with the results in Figure 3. The (111)-type facet in Figure 6c does not directly participate in the fracturing. In vacuum, the (111)-type facet is electrostatically polar, meaning that its formation increases the Gibbs free energy.39 However, the cycled sample is immersed in an ionic environment where the electrostatic polarity of the surface can be compensated by the environment, making the (111)-type facet energetically and structurally favorable in this environment. Figure 6e is extracted from a region that is ∼3 nm away from the crack tip, which exhibits a high concentration of planar defects and interfaces. A set of rock-salt twins is present, as marked by purple and yellow dots, with the twin boundary identified to be the {111}-type plane (see their diffractograms in Figure S7c,d), as highlighted by a red arrow. Besides the twin boundary, the rest of the planar defects in Figure 6e are also indexed to be along {111}-type planes. Because the (111) plane is the sliding plane of the cubic structure,40 the {111}-type planar defects are thus evidence of ductile deformation around the crack tip, resulting from the tensile stress field present around the crack tip.37,41 In other words, the existence of the deformed zone around the crack tip region confirms that an intensive strain field is present around the crack tip, as mentioned in Figure 5. Due to the fragility of the rock-salt phase, this deformed zone only extends to a small range of ∼10 nm. Figure 6f schematically summarizes the structural features around the crack tip. As discussed in Figure 5, the change of the lattice parameters of the layered phase applies cyclic tensile strain shocks on the rock-salt platelet. The concentration of the tensile stress in the surface region of the rock-salt platelet leads to nucleation of the crack tip and formation of a deformed area around the crack tip. When the stress in the deformed area exceeds the tensile limit of the rock-salt phase, the crack tip fractures into pieces and the crack thereby propagates forward, which finally develops into a (003) crack. Cracking and Electrochemistry. The (003) cracking of the primary particles affect the capacity of the positive electrode, as shown in Figure 6g, which presents the relationship between capacity and the number of cycles. In the general trend, the capacity decreases with increasing number of cycles. An interesting phenomenon is that a capacity bump shows up in the first few cycles (1−10 cycles). In some other similar Ni-rich layered dioxides, the bumping is also present,2,42−50 although the bump can happen in either earlier or later cycles. Analysis of over 300 primary particles from both 1-cycle and 30-cycle materials (Figures S9 and S10) shows that ∼3.0% of the 1-cycle particles and ∼4.3% of the 30-cycle particles have developed (003)-type cracking, suggesting that (003)-type cracking develops mainly in the first electrochemical cycle, consistent with the presence of the capacity bumping in the first few cycles. Therefore, this capacity bumping can be attributed to (003) cracking: once the (003) cracking happens, it generates fresh surfaces without SEI, which has higher Li+ and electronic conductivities. Due to the new, fresh surfaces, a higher volume percentage of the primary particle is electrochemically activated, and the capacity thus temporarily increases. It has been shown that the height in capacity bump increases with the Coulombic rate (C-rate).2,46,47,51 A higher Crate can increase the rock-salt transformation and thus induce more cracking, thereby generating more fresh surfaces and making a higher capacity bump, which supports that intragranular cracking is a reason for the short-term capacity increase (bump). Because (003) cracking preferably develops in

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* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.7b00907. Experimental details, TEM images, electron diffraction and simulation, and schematics (PDF) Moive of the layered phase kept in the electron beam (MP4)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (C.W.). *E-mail: [email protected] (G.Z.). ORCID

Hanlei Zhang: 0000-0001-6540-0556 Langli Luo: 0000-0002-6311-051X M. Stanley Whittingham: 0000-0002-5039-9334 Chongmin Wang: 0000-0003-3327-0958 Guangwen Zhou: 0000-0002-9243-293X 2613

DOI: 10.1021/acsenergylett.7b00907 ACS Energy Lett. 2017, 2, 2607−2615

Letter

ACS Energy Letters Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS 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 # DESC0012583. 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.

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DOI: 10.1021/acsenergylett.7b00907 ACS Energy Lett. 2017, 2, 2607−2615