http://pubs.acs.org/journal/aelccp
Formation of an Anti-Core−Shell Structure in Layered Oxide Cathodes for Li-Ion Batteries Hanlei Zhang,†,‡ Fredrick Omenya,‡ 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: The layered → rock-salt phase transformation in the layered dioxide cathodes for Li-ion batteries is believed to result in a “core−shell” structure of the primary particles, in which the core region remains as the layered phase while the surface region undergoes a phase transformation to the rock-salt phase. Using transmission electron microscopy, here we demonstrate the formation of an “anti-core−shell” structure in cycled primary particles with a formula of LiNi0.80Co0.15Al0.05O2, in which the surface and subsurface regions remain as the layered structure while the rock-salt phase forms as domains in the bulk with a thin layer of the spinel phase between the rock-salt core and the skin of the layered phase. Formation of this anti-core−shell structure is attributed to oxygen loss at the surface that drives the migration of oxygen from the bulk to the surface, thereby resulting in localized areas of significantly reduced oxygen levels in the bulk of the particle, which subsequently undergoes phase transformation to the rock-salt domains. The formation of the anti-core−shell rock-salt domains is responsible for the reduced capacity, discharge voltage, and ionic conductivity in cycled cathodes.
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chemically active, reducing the energy barrier for rock-salt transformation.2,15 After the formation of the surface rock-salt shell, phase transformation in the bulk is deferred due to the isolating effect of the rock-salt shell that separates the layered phase in the bulk from the degrading environment.16 The core−shell model has been widely acknowledged because it has been experimentally confirmed from transmission electron microscopy (TEM) observation of the thin edge area of the primary particles.8,17,18 However, most primary particles have a potato-like morphology with sizes ranging from hundreds of nanometers to a few tens of micrometers. The large size of the primary particles prohibits microscopic observation of the core area, for which only the thin edge area is possible for TEM analysis. The microstructural feature and phase transformation (if any) deep in the bulk cannot be fully observed in the local subsurface region of only ∼10 nm from the outermost surface. TEM observation of the bulk area of the primary particles (usually defined as over tens of nanometers away from the outermost surface) is very challenging because its large thickness (thicker than hundreds of nanometers) prohibits penetration of the electron beam
he so-called core−shell structure is widely acknowledged as the structural model for understanding the layered → spinel → rock-salt phase transformation in layered dioxide cathodes for lithium ion batteries during electrochemical cycling.1−4 In this model, the spinel and rocksalt phases nucleate in the surface of a primary particle and grow inward, forming a spinel/rock-salt shell in the surface with the core area of the particle remaining as the layered phase.5−8 Thermodynamically, the phase transformation pathway (layered → spinel → rock-salt) is energetically favorable, which means that the layered phase will eventually transform to the disordered rock-salt phase if proper conditions like high temperature, a high cutoff voltage, or a degrading electrochemical environment are given.9−11 Therefore, the core−shell model is a kinetic approach for understanding the pathway of the phase transformation, for which the rock-salt transformation occurs first in the surface region of the primary particle. This is because the degrading conditions for the rocksalt transformation are more readily present in the surface region, leading to the preferred rock-salt formation. For instance, in the surface, the O2− anions in the cathode dioxide can be readily reduced to oxygen molecules that thus escape into the environment, leading to the formation of the rock-salt phase in the surface region.12−14 Also, the electrolyte in contact with the particle surface makes the surface material more © 2017 American Chemical Society
Received: September 24, 2017 Accepted: October 19, 2017 Published: October 19, 2017 2598
DOI: 10.1021/acsenergylett.7b00921 ACS Energy Lett. 2017, 2, 2598−2606
Letter
Cite This: ACS Energy Lett. 2017, 2, 2598-2606
Letter
ACS Energy Letters
the formation of the rock-salt phase in the bulk, which is in contrast to the widely reported core−shell structure in which the rock-salt phase is present in the surface region.7,21,22 Diffraction patterns and HRTEM images shown in Figure S1 confirm that the pristine cathode particles have a layered structure without any rock-salt domains formed in the bulk. By contrast, the formation of the rock-salt domains in the bulk has been repeatedly observed in cycled primary particles, as shown in Figure S2. A very thin layer of spinel-like phase is observed between the skin of the layered phase and the rock-salt domain, which will be analyzed in detail in Figure 2. Observation of rock-salt domains in the bulk as shown in Figure 1 is made possible by TEM specimen preparation using the ultramicrotome method. Otherwise, HRTEM imaging is restricted to a subsurface region ∼10 nm deep from the outermost surface of potato-like primary particles. As can be seen from Figure 1a, the surface region remains as the layered phase with a thickness of ∼20 nm and a large rocksalt domain is present in the bulk region away from the surface. The Fm3m ̅ structure of the rock-salt domain and the R3m ̅ layered structure of the skin of the layered phase are confirmed by their diffractograms in Figure S3. Figure 1b is a representative zoom-in view from the rock-salt domain along the [110] zone axis, and Figure 1c schematically illustrates the corresponding [110] projection view of the rock-salt structure. Figure 1d is a representative zoom-in view of the layered phase in the surface, which confirms that it has the typical R3̅m layered structure with alternating transition metal (TM)/Li slabs stacking along the c-axis, as schematically shown in Figure 1e. The bright atomic columns in Figure 1d correspond to the TM cations, with lithium cations existing between these TM slabs. Because the lithium cations contribute very little to the contrast of the HRTEM image,23,24 they are invisible and show up as dark contrast between the TM slabs. More details about the imaging conditions of the HRTEM images in Figure 1 can be found in Figure S4. Figure 2a is a zoom-out TEM view of a primary particle, and Figure 2b is a zoom-in HRTEM view obtained ∼50 nm from the outermost surface, showing the boundary of a rock-salt domain formed in the bulk of the primary particle, as marked with a purple dashed line. Observation of this deeper area of the particle is made possible with the microtome sectioning. A magnified view from the region outside of and next to the domain boundary (Figure 2c) shows that it has the Fd3̅m spinel-like structure with the presence of Li-site TM cations. The TM lattice also shows some distortion compared with the
through the bulk region. While other techniques, such as X-ray diffraction, can provide structural information on the layered cathode,1,19 they lack the spatial resolution and are prone to generate an average result, yielding experimental difficulties in resolving the structural features in the core region. Plate-shaped particles with a thickness of 20−40 nm may be good enough for TEM observation,20,21 but the entire region of the particle may be close to the surfaces, making it have no real bulk area. Therefore, revealing the intrinsic behavior of the bulk region cannot be ensured if the characterization is performed with the highly selective regions that are limited to the surface and subsurface areas. To overcome these issues, we employ an ultramicrotome method to section large primary particles into thin slices with a nominal thickness of
