Structural Evolution of LixNiyMnzCo1-y-zO2 Cathode Materials during

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Structural Evolution of LixNiyMnzCo1-y-zO2 Cathode Materials During High-Rate Charge and Discharge Sooyeon Hwang, Eunmi Jo, Kyung Yoon Chung, Kyo Seon Hwang, Seung Min Kim, and Wonyoung Chang J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b02579 • Publication Date (Web): 08 Nov 2017 Downloaded from http://pubs.acs.org on November 9, 2017

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The Journal of Physical Chemistry Letters

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Structural Evolution of LixNiyMnzCo1-y-zO2 Cathode

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Materials during High-Rate Charge and Discharge

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Sooyeon Hwang1,2, Eunmi Jo1, Kyung Yoon Chung1, Kyo Seon Hwang3, Seung Min Kim4,*,

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Wonyoung Chang1,*

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1

Center for Energy Convergence, Korea Institute of Science and Technology (KIST), Seoul

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02792, Republic of Korea 2

Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York

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11973, United States 3

Department of Clinical Pharmacology and Therapeutics, College of Medicine, Kyung Hee

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University, Seoul 02447, Republic of Korea 4

Carbon Composite Materials Research Centre, Institute of Advanced Composite Materials,

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KIST, Wanju-gun 55324, Republic of Korea

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AUTHOR INFORMATION

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Corresponding Author

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*S. M. Kim ([email protected]) *W. Chang ([email protected])

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ACS Paragon Plus Environment

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Ni-rich lithium transition metal oxides have received significant attention due to their high

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capacities and rate capabilities determined via theoretical calculations. Although the structural

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properties of these materials are strongly correlated with the electrochemical performance, their

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structural stability during the high-rate electrochemical reactions has not been fully evaluated

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yet. In this work, transmission electron microscopy is used to investigate the crystallographic and

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electronic structural modifications of Ni-based cathode materials at a high charge/discharge rate

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of 10 C. It is found that the high-rate electrochemical reactions induce structural inhomogeneity

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near the surface of Ni-rich cathode materials, which limits Li transport and reduces their

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capacities. This study establishes a correlation between the high-rate electrochemical

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performance of the Ni-based materials and their structural evolution, which can provide

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profound insights for designing novel cathode materials having both high energy and power

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densities.

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TOC GRAPHICS

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KEYWORDS lithium ion batteries, high rate, cathode, structural evolution, transmission

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electron microscopy


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After the successful utilization of lithium ion batteries (LIBs) for small and portable devices,

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their demand for larger scale applications (such as electric vehicles and smart grids) has been

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steadily increasing.1,2 In particular, the use of LIBs in electric vehicles requires both high energy

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density and power density, which would enable prolonged driving on a single battery charge,

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sufficient acceleration, and relatively short charging times. High energy density can be achieved

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by increasing the capacity of each battery cell or widening the operating voltage range. Since the

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power density of LIBs is related to their ability to charge and discharge quickly, the transport of

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Li ions and electrons during the electrochemical process should be fast enough for reaching a

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satisfactory level of power density. Thus, to incorporate LIBs into electric vehicles, electrode

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materials with both high energy density and Li ion diffusivity must be developed.

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To improve the figure of merits of cathode materials, lithium transition metal oxides (described

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by the formula LiTMO2, where TM is a transition metal such as Mn, Co, or Ni) with a layered

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structure (space group R 3ത m) have received significant attention. During the extraction or

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insertion of lithium ions from/into cathode materials, TMs are participating the redox for charge

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neutrality; thus, controlling the TM content of LiTMO2 is critical for the optimization of their

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properties. Generally, Ni incorporation can be used to increase the capacity by modifying the

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electronic structure.3-5 In addition, the activation barrier of the Li-ion movement near Ni2+ or

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Ni3+ ions is lower than that near Co3+ or Mn4+ ions;6-8 hence, Li transport is facilitated in the Ni-

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rich environment. However, it is well known that Ni-rich cathode materials exhibit significant

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structural instability and related safety issues.3-5,9-12 Nevertheless, Ni-rich LiTMO2 compounds

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still remain highly promising cathode materials with potentially high energy density and power

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density, which can be realized by improving their structural stability during electrochemical


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processes. For this purpose, a detailed analysis of the structural evolution of Ni-rich LiTMO2

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materials must be performed particularly at high-rates.

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In this work, various transmission electron microscopy (TEM) techniques were used to

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investigate the structural modification of Ni-based cathode materials (LiNi0.8Mn0.1Co0.1O2 and

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LiNi0.4Mn0.3Co0.3O2, which are further referred to as NMC811 and NMC433, respectively) after

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the charge and discharge processes conducted at a high rate of 10 C. In previous studies, the

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reversibility of the structural modifications of NMC cathode materials was observed at a

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moderate charge/discharge rate of 0.1 C;13 however, the structural changes of cathode materials

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at high currents have not been investigated in sufficient detail. Yang et al. reported the formation

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of an intermediate phase and structural modifications of Li1−xNi1/3Co1/3Mn1/3O2 after charging at

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various rates ranging from 0.1 C to 60 C.14 In the real batteries, the discharge process is also

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important for their successful operation; thereby, structural analysis must be performed both after

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the charge and discharge steps. Here, by examining the changes in the crystallographic and

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electronic structures of NMC433 and NMC811 materials observed both in the bulk and on their

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surfaces at high charge/discharge rates, it is possible to determine the mechanism of their

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structural modifications induced by electrochemical reactions, which can be subsequently used

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for designing novel cathode materials for high-power applications.

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The structural evolutions of NMC433 and NMC811 materials induced by their charge at a high

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rate of 10 C are investigated by acquiring selected area electron diffraction (SAED) patterns for

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five different particles (see Figure S1 in the Supporting Information section). We set a cutoff

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voltage of 4.8 V to investigate the structural modification at a highly delithiated state. Because of

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the high potential increase induced by a high charging rate at the beginning of the charging

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process (see Figure S2 in Supporting Information), it was practically difficult to set a cutoff


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voltage below 4.5 V, especially for NMC811. High-rate electrochemical testing may lead to

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inhomogeneous distribution of charges inside the active particles; thus, we specify the area of

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interest as the bulk (the inside area) and the near surface (the outer area) with a selected area

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aperture (size of 160 nm). The charge profiles of the studied NMC433 and NMC811 samples are

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shown in Figure S2 of the Supporting Information section. Figures 1a and b show the SAED

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patterns and electron energy loss spectra (EELS) of the representative samples obtained for each

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composition. Despite the initial charge to the cutoff voltage of 4.8 V, the charging process with a

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high current results in the modifications of the crystalline and electronic structures of NMC811

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even at the bulk area. EELS, which demonstrates the unoccupied density of states,15 is employed

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to characterize the chemical state of NMC cathode materials. Pre-edge of oxygen K-edge (~530

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eV) is sensitive to the oxidation state of binding transition metals: it moves to higher energy loss

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when the metal is reduced, and vice versa.13 Transition metal L2,3 edge itself also changes with

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valence: the onset energy shifts to lower energy loss, and L3/L2 intensity ratio becomes higher

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when the metal ion is reduced.13,16 Near the surface of NMC811, the SAED is similar to that

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acquired in the bulk area, but we could notice a significant change in EELS, particularly for the

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O K-edge, indicating a considerable reduction in transition metals and formation of the rocksalt

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phase. More Detailed explanations are available at elsewhere.17 The presence of the diffraction

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spots originated from the spinel structure indicates that structural modifications also occur near

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the NMC433 surface. (See Figure S3 in the Supporting Information for detailed indexing of

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SAEDs) Figure 1c summarizes the structural evolution observed at NMC433 and NMC811

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particles after their initial high-rate charge to 4.8 V. To see the trend for structural changes after

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electrochemical lithium extraction, information acquired from five different particles are

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summarized in the Venn diagram in Figure 1c. Each box represents layered, spinel, and rocksalt


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structures after charge, and the overlapped areas indicate mixed phases. Considering the phase

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transformation takes place from the original layered R 3ത m structure to a disordered spinel

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structure and eventually to the rock-salt structure, more severe structural modifications are

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observed near the NMC811 surface as compared to those of NMC433 and in the bulk region.

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Therefore, the high-rate initial charge leads to a structural evolution of NMC cathode materials,

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and their degree of deformation depends on the TM composition. It is interesting to note that the

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charge capacity of NMC811 is much smaller than that of NMC433 at high charging rates, despite

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the well-known ability of Ni-rich compounds to exhibit relatively high capacities.

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Since the operation of LIBs is based on the insertion and removal of lithium ions between the

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cathode and the anode, structural changes of electrode materials observed during the discharge

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process are as important as those detected during charging. To investigate the structural changes

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that occur during the high-rate (10 C) discharge, the coin cells with NMC cathodes were first

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charged to 4.3 V or 4.8 V at a rate of 0.1 C, then discharged at a 100 times higher current down

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to 2.0 V. Structural issues of cathode materials under normal and abused conditions are of

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practical importance; hence, we employed two different charged states of 4.3 V (maximum

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normal operating potential) and 4.8 V (abused condition) for subsequence discharging

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experiments down to 2.0 V. Figure S4 in the Supporting Information section displays the charge

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and discharge curves obtained for the NMC433 and NMC811 cathode materials. Similar to the

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charge capacity (Figure S2), the discharge capacity of NMC811 from the charged state to 4.8 V

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is far lower than that of NMC433 measured at a high discharge rate of 10 C. Although the

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NMC811 delivers higher capacity during a charge to 4.3 V than NMC433, discharge capacities

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of these materials from the charged state to 4.3 V are comparable, indicating inferior rate

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capability of NMC811 compared to NMC433. The SAED patterns acquired for the bulk and near


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surface areas of the five particles with different compositions are presented in Figures S5 and S6

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of the Supporting Information section. Figures 2a and b display typical SAED patterns and EELS

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of the NMC433 and NMC811 samples obtained after their high-rate discharge from 4.8 V. The

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SAED patterns and EELS indicate that the near surface of NMC433 and both bulk and near

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surface of NMC811 have mixed layered and spinel structures. The contribution of the spinel

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structure to the near surface region is greater than that to the bulk region regardless of the TM

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composition. In our previous work, it was demonstrated that partial phase transformations could

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occur near the surfaces of NMC433 and NMC811 even after the initial charge to 4.8 V at a rate

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of 0.1 C; however, the lithium insertion to the structures of these materials conducted at a slow

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rate of 0.1 C can recover these structural deformations both in the bulk and near the surface

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area.13 Thus, in contrast to the low or moderate discharge rates, high discharge rates are

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unfavorable for recovering the charge-induced structural inhomogeneity. Figures 2c, d

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summarize all the SAED patterns obtained for the NMC433 and NMC811 samples at a high

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discharge rate in the Venn diagram, indicating that the inhomogeneity of their crystalline

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structures still exists after the lithiation process regardless of the cut-off voltage. In addition, the

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most significant particle-to-particle variations are observed near the surface of NMC811 where

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the most severe structural changes occur during the delithiation process.

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The structural and chemical modifications of the NMC433 and NMC811 surfaces after the

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high-rate discharge were further investigated via high-resolution electron microscopy (HREM)

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and scanning transmission electron microscopy (STEM)-EELS. Figure 3 shows the HREM

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images and fast Fourier transform (FFT) results obtained for their selected regions and the

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changes in the L3/L2 intensity ratios of the Ni, Co, and Mn L2,3 edges along the scanning paths of

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NMC433 and NMC811 after the high-rate discharge from the charged state to 4.8 V. In the case


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of NMC811 compound, the L2,3 Mn and Co edges cannot be clearly distinguished in the inner

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particle regions because of high sample thickness and relatively small contents of Mn and Co

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elements; thus, the L3/L2 ratios of Mn and Co are partially plotted in Figure 3b. The

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corresponding raw EELS containing the Ni, Co, and Mn L2,3 edges are presented in Figure S7 of

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the Supporting Information section. The L2,3 edge features of TMs such as the L3/L2 intensity

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ratio are sensitive to their oxidation states;18,19 thus, in this work, the L3/L2 intensity ratio was

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utilized to track the changes in the TM chemical state along the scanning route. Previously, it

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was found that the L3/L2 ratios of the L2,3 edges of Ni, Mn, and Co increased during reduction

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and decreased during oxidation.20-23 The HREM images and corresponding FFT results shown in

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Figure 3 demonstrate that both the NMC433 and NMC811 samples undergo structural

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transformations, leading to the formation of the spinel and rock-salt structures on their surfaces.

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In particular, the presence of the rock-salt structure indicates that more severe phase

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transformations occur at the edge of the NMC433 and NMC811 specimens rather than near their

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surfaces, as indicated by the SAED patterns depicted in Figures 2, S5, and S6. These structural

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modifications observed at a couple of tens of nanometers from the sample edge become

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irreversible after a high-rate discharge. Since the phase transformations from the layered

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structure to the spinel and rock-salt structures are accompanied by the reduction of TMs inside

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the studied materials,13,24 the variations of the L3/L2 intensity ratio were also examined. During

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the electron probe scanning from the edge to the inner area of the particles along a line, the

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changes in the L3/L2 ratio become more distinct: a higher white line ratio of Mn on the NMC433

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surface but higher L3/L2 ratio of Ni in the NMC811. The EELS results demonstrate that the

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reduction of Mn ions is the main reason for local structural modifications of NMC433, while the

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reduction of Ni ions drives the phase transformations on the NMC811 surface. It is interesting to


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note that in our previous work, 20 charge and discharge cycles performed at a rate 0.1 C induced

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similar chemical evolutions corresponding to Mn and Ni reduction on the surfaces of NMC433

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and NMC811, respectively.25 Therefore, it can be assumed that the mechanism of the surface

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degradation at a high rate of 10 C is similar to that observed at a moderate rate of 0.1 C;

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however, the high-rate discharge does not allow the surface of NMC cathode materials to recover

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from structural and chemical modifications due to insufficient time, which results in more severe

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structural degradation even after the first discharge process.

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A comparison of the SAED patterns and EELS spectra recorded for the NMC cathode

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materials after the charge and discharge processes (Reference 13, Figures 2, 3) shows that the

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structural deformations induced by charging at various rates and cut-off voltages are usually

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recovered during discharge. However, it is noteworthy that the high-rate discharge can also

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induce partial structural variations of NMC811 particles. Figure 4 shows the SAED patterns of

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the NMC811 particles obtained after high-rate discharge (they are marked with the symbols +

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and # in Figure 2d). For these particles, the contribution of the diffraction intensity from the

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spinel structure to the SAED patterns (indicated by white arrows in Figure 4) is much stronger in

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the bulk area than near the surface region (regardless of the cut-off voltage). These SAED

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patterns indicate that the original layered structure of NMC811 is partially recovered during the

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high-rate discharge only in the areas close to the specimen edges while the transformed spinel

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structure is retained in the bulk. Thus, it is possible that the discharge of some NMC811 samples

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which a high current can be an additional factor contributing to their local structural evolution.

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Pan et al. calculated the diffusion coefficients of NMC cathode materials via ab initio

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simulations, which showed that the Li-ion diffusion coefficient generally tended to increase with

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the Ni content.7 The diffusivity of Li species also strongly depends on the size of the Li layer


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along the c-axis since the larger space reduces the activation energy of Li movement.6 Cation

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mixing in a layered structure significantly reduces the Li slab opening, which limits Li-ion

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diffusion. Since further Li/TM mixing causes phase transitions from the layered to the spinel and

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even rock-salt structures, local phase transformations may considerably degrade the kinetic

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properties of NMC materials. Ni-rich cathode materials (such as NMC811) may be inherently

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favorable for fast lithium diffusion; however, a significant degree of cation mixing and the

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subsequent phase transformations limit their lithium ion diffusivity. During the high-rate

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discharge, the presence of spinel and rock-salt structures near the surface may impede the

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diffusion of lithium ions to the particle bulk, resulting in only partial structural recovery of the

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NMC811 surface. In addition, since it is hard for lithium ions to reach the bulk of active

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materials, only a fraction of the NMC811 sample can participate in electrochemical reactions,

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resulting in a considerable capacity loss during the high-rate discharge. Meanwhile, the cathode

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materials with smaller Ni contents (such as NMC433) may originally exhibit lower Li ion bulk

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diffusivity; however, their better structural stability ensures Li movement to the bulk area even at

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high discharge rates, thus preventing additional local structural evolution and capacity loss. The

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larger potential drop in the beginning of the high-rate discharge process observed for NMC811

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material (as compared to that of NMC433) in Figure S4 of the Supporting Information section

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results from its higher resistance, which is most likely caused by the structural transition during

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the initial charge process.

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In this work, we demonstrate that the high-rate charge-discharge performance of NMC811 is

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inferior to that of NMC433: the high-rate charge and discharge capacities of NMC811 are much

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lower than those measured at moderate rates, owing to the severe structural inhomogeneity. It

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can be concluded that the structural imperfectness and instability of NMC811 observed during


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electrochemical reactions at high charge/discharge rates are the origin of the discrepancy

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between the theoretical and experimental results. This study also reveals that the structural

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degradation of Ni-rich cathode materials observed during the high-rate electrochemical processes

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is mainly caused by the reduction of Ni ions, while the degradation of Mn-rich materials (with a

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smaller Ni content) is caused by the reduction of Mn ions. In addition, slower Li diffusion inside

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NMC811 due to the severe structural degradation during the high-rate discharge may lead to a

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lower degree of structural recovery in the bulk area as compared to that near the material surface.

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Therefore, the development of various methods for preventing surface phase transformations and

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reducing the electrode resistance must be conducted to achieve both the high energy density and

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power density of Ni-rich cathode materials.


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FIGURES

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Figure 1. Bright field image, SAEDs and EELS of (a) NMC433 and (b) NMC811 after charging

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to 4.8V at a rate of 10C. Circles with the dotted line denote the contribution from the spinel

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structure in case of mixed phase. (c) Summary of structural evolutions of five different NMC

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particles after high-rate charge to 4.8V.


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Figure 2. Bright field image, SAEDs and EELS of (a) NMC433 and (b) NMC811 after a

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discharge at a rate of 10C from 4.8 V. Circles with the dotted line denotes the contribution from

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the spinel structure in case of mixed phase. Summary of structural evolutions of five different (c)

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NMC433 and (d) NMC811 particles after high-rate discharge from 4.3V or 4.8V to 2V. Signs of

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+ and # are used to indicate the identical particles in bulk and near the surface, respectively.

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These particles are shown in Figure 4 in more details.


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Figure 3. HREM images, fast Fourier transformation results acquired from designated areas, and

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L3/L2 ratios of EELS along scanning path of (a) NMC433 and (b) NMC811 after 10C discharge

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from 4.8 V.


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Figure 4. Bright field images and SAEDs of NMC811 after high-rate discharge from (a) 4.3 V

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and (b) 4.8 V (white arrows indicate the positions of diffraction spots from the spinel structure).

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ACKNOWLEDGMENT

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This work was supported by the Korea Institute of Science and Technology (KIST)

7

Institutional Programs (Project Nos. 2E27062). This research was also supported by the

8

Technology Development Program to Solve Climate Changes of the National Research

9

Foundation

(NRF)

funded

by

the

Ministry

of

Science

&

ICT

(grant

number:

10

2017M1A2A2044482). S.H. acknowledges the support from the Center for Functional

11

Nanomaterials, Brookhaven National Laboratory, which is supported by the U.S. Department of

12

Energy, Office of Basic Energy Sciences, under Contract No. DE- SC0012704.

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ASSOCIATED CONTENT

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Supporting Information. Experimental details, charge-discharge profiles, and further data

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regarding SAEDs, and STEM-EELS line scans.

17

AUTHOR INFORMATION


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1

Notes

2

The authors declare no competing financial interests.

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(4)

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(6) (7)

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(10)

(11)

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Figure 1. Bright field image, SAEDs and EELS of (a) NMC433 and (b) NMC811 after a charge to 4.8V at a rate of 10C. Circles with dotted line denotes the contribution from spinel structure at mixed phase. (c) Summary of structural evolutions of five different NMC particles after high-rate charge to 4.8V.

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Figure 2. Bright field image, SAEDs and EELS of (a) NMC433 and (b) NMC811 after a discharge at a rate of 10C from 4.8 V. Circles with dotted line denotes the contribution from spinel structure at mixed phase. Summary of structural evolutions of five different (c) NMC433 and (d) NMC811 particles after high-rate discharge from 4.3V or 4.8V to 2V. Signs of + and # is used for indicating an identical particle at bulk and at near surface.

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Figure 3. HREM images, fast Fourier transformation results acquired from designated areas, and L3/L2 ratios of EELS along scanning path of (a) NMC433 and (b) NMC811 after 10C discharge from 4.8 V.

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Figure 4. Bright field images and SAEDs of NMC811 after high-rate discharge from (a) 4.3 V and (b) 4.8 V (white arrows indicate the positions of diffraction spots from the spinel structure).

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Structural Evolution of LixNiyMnzCo1-y-zO2 Cathode Materials during

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