Investigating the Kinetic Effect on Structural Evolution of LixNi0.8Co0

Feb 27, 2017 - (25) The final Li contents in the materials are not same at the end of discharge. ..... Even though XRD results in Figure 2 represent t...
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Investigating the Kinetic Effect on Structural Evolution of LixNi0.8Co0.15Al0.05O2 Cathode Materials during the Initial Charge/ Discharge Eunmi Jo,†,‡ Sooyeon Hwang,†,∥ Seung Min Kim,§ and Wonyoung Chang*,†,‡ †

Center for Energy Convergence, Korea Institute of Science and Technology (KIST), Seoul 02792, Republic of Korea Energy Conversion Technology, Korea University of Science and Technology, Daejeon 34113, Republic of Korea § Carbon Composite Materials Research Centre, Institute of Advanced Composite Materials, Korea Institute of Science and Technology (KIST), Wanju-gun 55324, Republic of Korea ‡

S Supporting Information *

ABSTRACT: In this work, we investigate the structural evolution and reaction kinetics of Li x Ni 0.8 Co 0.15 Al 0.05 O 2 (NCA) cathode materials induced by the initial charge/ discharge as a function of the state of charge (SOC 50 and 90%) and C-rates (0.1−10C), with a combination of highresolution transmission electron microscopy (HRTEM) imaging, selected area electron diffraction (SAED), and electron energy loss spectroscopy (EELS). During initial charging, the effects of C-rates on the structural modifications of NCA cathode materials are strongly dependent on how much the lithium is extracted from the pristine NCA. The structural modifications become more substantial as the extent of the charge increases, particularly at higher C-rates. In the highly delithiated state (90% SOC), even the particles charged at the same C-rate show significant variations in the degree of the structural modifications. The changes in the crystallographic and electronic structures at the subsurface scales, which were induced by the initial charging to 90% SOC at the rate of 0.1C, are nearly recovered during the initial discharge, except for the NCA discharged at the rate of 10C. To quantify the extent of the irreversible phase transition at the nanoscale, we have utilized HRTEM imaging and scanning transmission electron microscopy (STEM)− EELS line scanning techniques, which enable us to draw complementary results. This comparative analysis provides valuable information that is useful not only for obtaining a complete understanding of the mechanisms by which the degradation is initiated, but also for improving and designing Ni-rich layered cathode materials with better charging and discharging kinetics.

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INTRODUCTION Lithium-ion batteries (LIBs) have been widely utilized as power sources in portable devices such as laptops and cellular phones. Recently, LIBs have gained much attention for large-scale applications, particularly electric vehicles (EVs). Compared to LIBs for small portable devices, EVs additionally require LIBs to have high specific power and long cycle life of over 10 years.1−4 High specific power of LIBs not only allows fast acceleration to a certain speed in EVs, but also facilitates their fast charging in a shorter time.5,6 Longer cycle life is required to operate EVs without replacing the LIB pack during their lifetime. However, on the basis of the current technology, it is possible to utilize only a small portion of the charge capacity of LIB electrode materials to meet the requirement of both a high rate capability and long cycle life for EVs.6,7 This means that EVs would go to longer distances after a single charging if a larger portion of the charge capacity of the present electrode materials is used. Therefore, it is critical to understand the effects of the C-rate and state of charge (SOC) on the degradation or capacity fading of LIBs and to improve the rate © 2017 American Chemical Society

capability and cycle life while utilizing a larger portion of the charge capacity.6,7 LiNi0.8Co0.15Al0.05O2 (henceforth referred to as NCA) is considered as one of the most promising cathode candidates for EV applications because of its high discharge capacity (200 mAhg−1). However, NCA cathode materials have a disadvantage of structural instability, which results in a drastic capacity fade and impedance rise with cycling.8−10 These drawbacks originate from the unstable Ni4+ ions in the charged state, which are readily reduced to more thermodynamically favorable valences accompanied by a series of phase transitions.11−13 So far, most of the studies have focused on investigating the degradation mechanisms depending on the SOC14−17 or after the electrochemical cycle9,18,19 at relatively low C-rates via Xray based techniques such as X-ray diffraction (XRD)9,20−22 and X-ray absorption spectroscopy (XAS).9,14,18 BalasubramaReceived: August 8, 2016 Revised: February 27, 2017 Published: February 27, 2017 2708

DOI: 10.1021/acs.chemmater.6b03282 Chem. Mater. 2017, 29, 2708−2716

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Chemistry of Materials

and 10C) using a galvanostatic condition (i.e., constant current). For the initial discharging experiments, the NCA cathode materials were first charged to either SOC 50% or 90% at the rate of 0.1C and then lithiated (discharged) to 2.0 V at different C-rates (0.1 and 10C). The a m o u n t o f l i t h i u m r e m a i n i n g i n t he c a t h o de ( x in LixNi0.8Co0.15Al0.05O2) was estimated by the charge applied to the cell and the mass of the active material based on the theoretical capacity of 280 mAhg−1 assuming 100% columbic efficiency. Materials Characterization. The XRD patterns were acquired with a D-max 2500/PC X-ray diffractometer (Rigaku) using a Cu Kα radiation over a 2θ range of 10−70° at a scan rate of 2°/min. The XRD patterns were used to analyze the bulk structure of the NCA particles in the mixed slurries. For the TEM observations, the NCA particles were abraded gently from the Al foil, and then the particles in a small vial of pure DMC were sonicated for an adequate dispersion before the solution was dropped onto a lacey-carbon TEM grid. The sample preparation and loading into the TEM sample holder were performed either in a dry room or in an Ar-filled glovebox to minimize the exposure of the sample to air and moisture. The bright field (BF) images and EEL spectra were acquired with a Tecnai G2 F20 (FEI) TEM operating at a relatively low accelerating voltage of 120 kV to minimize the electron beam effect and a Quantum 963 GIF system (Gatan). The background of the spectra was subtracted by using the power-law method embedded within the TEM Imaging and Analysis (TIA) software (FEI). The energy resolution, which was determined by the full-width at the half-maximum of the zero-loss peak, was approximately 1.0 eV.

nian et al. performed in situ XRD experiments on LiNi0.85Co0.15O2 cathode materials charged at several different rates and showed that there exist changes in the lattice parameters of LiNi0.85Co0.15O2.23 Yoon et al. investigated the structural changes of mixed LiMn2O4−LiNi1/3Co1/3Mn1/3O2 composite cathode materials at different discharge rates via an in situ time-resolved XRD technique for evaluating lithium diffusion kinetics in mixed phases.24 However, because of the averaging nature of X-ray techniques, the detailed information about how differently the structural modifications of Ni-rich cathode materials originate and propagate depending on different charging/discharging rates at the microscopic level is quite limited. To fully understand the performance of a system, it is sometimes more important to investigate the evolution of surfaces rather than the bulk and to consider the variations among the particles rather than the average properties because the surface is usually more vulnerable to the reaction conditions, and a few highly damaged particles may lead to catastrophic failure of the system. In this regard, a transmission electron microscopy (TEM) investigation combined with an electron energy loss spectroscopy (EELS) on local structural evolutions in terms of the charging/discharging rates is crucial as a complementary study to the X-ray tool based works. In this study, we investigate the degradation mechanism of a NCA cathode as a function of the SOC and C-rate after the initial charge and discharge. To examine the effects of the Crate and SOC during the initial charge, we analyzed the structural evolutions of NCA materials after charging to 50 and 90% SOC at the charging rates of 0.1, 1, and 10C. For the initial discharging experiments, the NCA cathode materials charged to 50 and 90% SOC at 0.1C rate were discharged down to 2 V at 0.1 and 10C rates, respectively. This set of discharging experiments allows us to understand the effects of the discharging rates on how much the structural modifications induced by the initial charging process can be recovered during the initial discharge process. Throughout this study, we show that the structural degradation even after the initial charge critically depends on the SOC and C-rate in the surface region. Also, the NCA materials discharged from 90% SOC at 10C rate exhibit severe particle-to-particle variations in the degree of the degradation even though the NCA materials discharged from 90% SOC at 0.1C are almost recovered to the original state. Thus, it is important to thoroughly understand the local structural evolutions depending on the SOC and C-rates. This study opens up new methodology for developing new cathode materials for EV applications.

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RESULTS AND DISCUSSION Figure 1 shows the voltage profiles of the LixNi0.8Co0.15Al0.05O2 (NCA) cathode materials for initial charge/discharge experiments. To independently evaluate the effects of the C-rates

EXPERIMENTAL SECTION

Electrochemical Measurements. Positive electrodes (LixNi0.8Co0.15Al0.05O2; commercial product) were prepared by forming a mixed slurry of 90 wt % of active NCA material, 6 wt % of conducting materials (Denka black), and 4 wt % of polyvinylidene fluoride (PVDF) binder in n-methyl pyrrolidone (NMP) solvent. The mixed slurry was doctor-bladed onto an Al foil, which acted as a current collector, and was pressed at 80 °C for improving the adhesive property between the slurry and the Al foil. The cathodes were then dried at 80 °C in a vacuum oven at least overnight. The 2032-type coin cells were assembled with a cathode part, which included a Li metal for the anode, a Celgard separator, and an electrolyte of 1 M LiPF6 dissolved in ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) (1:1:1 by volume). For the initial charging experiments, the NCA cathode materials were electrochemically delithiated (charged) until they reached the lithium compositions of x = 0.5 (SOC 50%) or 0.1 (SOC 90%) at different C-rates (0.1, 1,

Figure 1. (a) Initial charge profiles of the LixNi0.8Co0.15Al0.05O2 (x = 0.5 and 0.1) at various charge current rates (0.1, 1, and 10C). (b) Galvanostatic curves of NCA electrodes for the initial discharge experiment. NCA electrodes are initially charged to the SOC of 50% or 90% at the rate of 0.1C and then discharged to 2.0 V at various discharge current rates (0.1 and 10C). 2709

DOI: 10.1021/acs.chemmater.6b03282 Chem. Mater. 2017, 29, 2708−2716

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Chemistry of Materials during the initial charge or discharge, we utilize two electrochemical conditions. For initial charge experiments as shown in Figure 1a, the NCA was electrochemically delithiated at different C-rates (0.1, 1, and 10C) until the composition of the remaining lithium reached a value of x = 0.5 and 0.1. We designed the charge experiments in which the same amount of lithium was extracted from the NCA cathode materials regardless of the C-rates. As the charging current increases from 0.1 to 10C, the cell voltage increases more rapidly because of the overvoltage induced by the internal resistance of the cell. Faster charging leads to more polarization, in particular, in the highly delithiated state (90% SOC). For initial discharge experiments as shown in Figure 1b, the NCA cathode materials were first charged to an SOC of either 50 or 90% at the rate of 0.1C and then discharged to 2.0 V at different C-rates (0.1 and 10C) for evaluating the reaction kinetics during the lithiation process. When higher current is applied, more capacity loss is observed because higher current induces larger voltage drop during the discharge process, which leads the cell voltage to reach the cutoff voltage of 2 V much faster.25 The final Li contents in the materials are not same at the end of discharge. The structural changes induced in the bulk NCA cathode materials after the initial charge/discharge were analyzed with the XRD patterns shown in Figure 2a (blue area, initial charge; yellow area, initial discharge). All the XRD patterns of the charged and discharged cathode materials are indexed to have the hexagonal system with a space group of R3̅m, which is the same as the pristine material. The XRD result in Figure 2a reveals that the original crystal structure of bulk cathode materials is well maintained during initial charge/discharge

process regardless of SOC and charging/discharge rates. However, a shift in the positions of diffraction peaks is perceptible, which indicates that the lattice parameters of the NCA materials are modified. During the initial charge, the XRD patterns obtained from the sample at the SOC of 50% are almost similar to those of the pristine with only slight peak shift. On the other hand, in case of the sample charged at the SOC of 90%, the peak for (108) reflection is absent, and the (107) peak shifts significantly to higher angles as compared to that of the pristine. These changes correspond to the existence of spinel-like structures.26 Therefore, 90% of lithium extraction from the original structure results in the phase transition observed even at the bulk scale, which indicates that the SOC is a critical factor affecting the phase transition during the initial charge. As Li ions are reintercalated after the initial discharge, most of the XRD peaks are recovered back at the similar level of the pristine state regardless of discharging rates and initial charged states. Figure 2b shows the quantitative changes in the lattice parameters and the corresponding changes in the volume of the unit cell. Significant changes are observed in the lattice parameters and the unit cell volumes after the initial charge. However, the parameters are all restored to the level of the pristine state after the discharge. During the initial charge, the parameter “a” shows the decrease with the decrease in the Li content because of the oxidation of transition metal (TM) ions, especially Ni ions (rNi3+ = 0.56 Å and rNi4+ = 0.48 Å).27 The caxis is expanded first owing to the electrostatic repulsion between the TMO2 layers when charged to 50% SOC; however, it decreases at the SOC of 90%. The average charge of O ions is expected to decrease when the lithium content is decreased, so the repulsion of the O layer also decreases. This induces the reduction in the c-axis.22 After the discharge, all the parameters are recovered to the original values when Li ions are reintercalated in the lattice. However, it seems to have no significant correlation between the bulk structural changes and the charging/discharging rates. It is well-known that the fast charging possibly induces steep Li concentration gradients and large inhomogeneities from the surface to the center of the particles. From Figure 1a, we can see that for the NCA charged to the SOC of 90%, the voltage measured in the 10C charge state (∼5.4 V) is much more polarized than that in the 0.1C charge test (∼4.7 V). This means that while the total amount of Li extraction is the same, locally the surface of the NCA charged at 10C is much more delithiated than that of the NCA charged at 0.1C at the expense of lower delithiation level inside the bulk. However, our XRD results shown in Figure 2 provide only a slight indication of structural transition to a disordered spinel regardless of the Crates. The XRD analysis does not provide any insightful information on the degree of structural transformation caused by the different C-rates during the initial charge. Since the surface of a cathode material is subjected to higher overpotential than the bulk, we take advantage of TEM techniques, which enable us to identify local structural inhomogeneities induced by the initial charge/discharge with various conditions. In Figure 3, we investigate the effects of SOC and charge/ discharge rates on the evolution of the crystal structures at the subsurface areas of the NCA particles by obtaining the SAED patterns. In our previous work, we have studied the SAED patterns of the NCAs charged to the SOCs of 50 and 90% at the mild charging rate (0.1C). For the half-charged (SOC 50%) NCA, the SAED patterns represent the existence of only the layered R3̅m structure or the mixed phases having both the

Figure 2. (a) XRD patterns of NCA at the pristine, the initial charge, and discharge state. Blue area, initial charge; yellow area, initial cycle. After 30 degrees, reflections are magnified on the basis of (104) and (110) peaks. (b) Quantitative changes in the lattice parameters and the corresponding changes in the volume of the unit cell with various electrochemical conditions. 2710

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Figure 3. SAED patterns of NCA after (a−d) initial charge and (e−h) initial discharge with various conditions.

Figure 4. O K-edges and Ni L2,3-edges of the EEL spectra acquired (a) during the initial charge state and (b) initial discharge with various SOCs and current rates. Each zero point of O K and Ni L2,3 spectra was designated based on the O main peak and Ni L2 peak at the highest value, respectively. Orange-colored regions represent the ΔE ranges of pristine NCA particles. The two spectra (10 C-1 and 10 C-2) show the variations in the chemical state when NCA was discharged from 90% SOC at a rate of 10C.

layered and spinel structures depending upon the locations of the particles. In case of the overcharged (SOC 90%) NCA, the SAED patterns exhibit diffraction spots from both the layered and spinel structures.16 Here, we further evaluate the effects of higher charging rates (1 and 10C) to the SOCs of 50 and 90%. A mixture of the layered and spinel crystal structures is observed from the sample with 50% of Li ions removed (Figure 3a,b). The SAED patterns in Figure 3c and d show that the original layered structure mostly transforms into the spinel structure at the subsurface area for the sample charged at 90% SOC. Consequently, during the initial charge, the degree of structural phase transition at the subsurface area clearly depends on the SOC. There does exist the difference in the degree of phase transition between 0.1C and 10C, but it is not

quite distinct between 1C and 10C. When NCA charged at 50% or 90% SOCs is discharged down to 2.0 V, the transformed phase at the subsurface area is mostly recovered to the original layered structure except for the sample discharged from 90% SOC at the rate of 10C. The SAED pattern in Figure 3h still shows the existence of the spinel structure. This indicates that not only the SOCs, but also the charging/discharging rates affect the phase evolution at the subsurface area even during initial charge/discharge process. Additional SAED patterns are shown in Figures S1 and S2 in the Supporting Information. In addition to the SAED study, we perform EELS to further understand the effects of the SOCs and the charging/ discharging rates on the structural evolutions in more 2711

DOI: 10.1021/acs.chemmater.6b03282 Chem. Mater. 2017, 29, 2708−2716

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ions occurs and the degree of the Ni4+ reduction gets much more severe with the increase of the charge current rate when lithium is extracted to 90% of the original lithium contents. Figure 4b shows the EEL spectra of the O K- and Ni L2,3-edges after the initial discharge. The NCA SOC of either 50 or 90% was at the same rate of 0.1C and then discharged to 2.0 V at different C-rates (0.1 and 10C). All the pre-edges of the O Kand Ni L2,3-edges are well recovered to the level of the pristine after the discharge from 50% SOC regardless of the discharge current rate. However, when the NCA is discharged from 90% SOC at a rate of 10C, ΔE does not recover to its original value, and the relative intensity of the L3 edge does not decrease, which implies that the electronic structural changes induced during the initial charge process cannot be fully recovered to the original state during the high-rate discharge. Overall, the variations in the electronic structure (as a function of the SOC and C-rate) are well matched with the changes in crystallographic structures (Figure 3), but it should be noted that the evolution in the electronic and crystallographic structures by an electrochemical reaction is reflected in EEL spectra (Figure 4) more sensitively than in SAED patterns (Figure 3). For example, the SAED patterns in Figure 3c and d do not show a clear difference in the degradation of NCA between 1C and 10C charging, but EEL spectra in Figure 4a clearly show the difference. To evaluate the particle to particle variations, Figure 5 shows the changes in the ΔE of the O K-edge and the L3/L2 relative

quantitative manner. Modification in EEL spectra not only reflects the changes in electronic structures and phase transition, but also can be analyzed more quantitatively than SAED pattern. Modifications in the electronic structure of the NCA are distinctly reflected in the O K- and Ni L2,3-edges in the EEL spectra. The changes in the EEL spectra of O and Ni are tracked by defining two indices: the first is the ΔE of the O K-edge, and the second is the L3/L2 edge intensity ratio of Ni. The O K-edge EEL spectrum of a LiTMO2 material with a layered structure consists of two distinct peaks: a pre-edge peak and the following main peak. The ΔE of the O K-edge is the energy-loss difference between the main and the pre-edge peaks at their highest intensities. The pre-edge peak includes information on the transition of electrons from the 1s core state to the unoccupied 2p states that are hybridized with 3d states of the TMs.4,28 When the pre-edge peak moves to a lower energy loss (higher ΔE), this entails the oxidation of the TM linked with O. In contrast, when the pre-edge peak shifts to a higher energy loss (lower ΔE), this indicates that the TM ions bound to O are reduced. Additionally, the changes in the chemical state of the TM ions are also directly related to the changes in the TM L edges. The L2,3 edge of the TMs is attributed to the electronic transitions from the 2p levels to the unfilled levels of d-type symmetry, and the energy splitting of the 2p1/2 and 2p3/2 levels, which occurs as a result of the spin− orbit coupling and causes the splitting of the L2,3 edge.29 The white-line ratio (L3/L2) decreases with oxidation; however, it increases with the reduction of the TM ions.8,30 Thus, the L3/ L2 intensity ratio is a good measure of the TM valence state. Figure 4 shows the changes in the electronic structures of NCA cathode materials induced by initial charge (Figure 4a)/ discharge (Figure 4b) process at various C-rates (0.1−10C) and SOCs (50−90%). This figure clearly shows how both the O K and Ni L2,3 edge EEL spectra near the subsurface region change depending on the electrochemical charging or discharging conditions. Each of the O K and Ni L2,3 edge EEL spectrum is replotted by ΔE from the position of the main peak of O K-edge and L2-peak of the pristine (before charging) NCA as the reference positions, respectively. The O K-edge EEL spectra from the pristine NCA and NiO are shown as references. We use the O K-edge of NiO, which has a rock-salt structure, as an indicator of the irreversible structural modification since phase transformation to rock-salt structure is accompanied by substantial oxygen loss. The irreversible structural modification Figure 4a shows that the pre-edge of the O K is slightly shifted to lower energy after 50% of the lithium is extracted from the pristine state at all rates, but all the preedge peaks are located at almost the similar position of the O K pre-edge of the pristine considering the inherent energy resolution of the electron beam (∼1 eV) itself. In addition, when the spectra are normalized to the intensity of the Ni L2 edges, the white-line ratios (L3/L2) of all samples charged at 50% SOC show the similar level to that of the pristine regardless of the charge current rates. On the other hand, when the NCA is charged to the SOC of 90%, the pre-edge peak of the O K-edge gradually shifts to a higher energy loss (lower ΔE), and the intensity of the pre-edge decreases considerably with an increase in the C-rate. In particular, after charging to 90% SOC at 10 C, O K of NCA becomes similar to that of NiO, indicating that irreversible phase transformation partially takes place at local area. Additionally, the relative intensity of Ni L3 peak becomes stronger with higher C-rates. Thus, our EELS results indicate that the undesirable reduction of Ni4+ to Ni2+

Figure 5. Modifications in the ΔE of the O K-edge and the L3/L2 relative ratio of the Ni L2,3-edges obtained from several NCA particles during the (a, b) charge/(c, d) discharge depending on the C-rates (0.1−10C) and SOCs (50−90%). The orange- and blue-colored areas represent the ranges of the ΔE of the O K and the L3/L2 ratio of the Ni L2,3-edges obtained from the pristine NCA particles.

ratio of the Ni L2,3-edges obtained from several NCA particles during the charge/discharge depending on the C-rates (0.1− 10C) and SOCs (50−90%). The orange- and blue-colored areas represent the ranges of the ΔE of the O K and the L3/L2 ratio of the Ni L2,3-edges obtained from the pristine NCA particles. When charged to the SOC of 50%, the ΔE of the O K increases slightly, and the L3/L2 ratios have the similar levels to the pristine sample regardless of charging rates. Also, the ΔE of the O K and the L3/L2 ratios of the Ni are recovered to the state of untreated NCA after discharged from the SOC of 50% regardless of the discharging rates. However, when charged to the SOC of 90%, the charging rates play a critical role in the 2712

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Chemistry of Materials variations in the values of the ΔE of the O K and the L3/L2 ratios of the Ni. This is similarly the case for the discharging experiments. There are little variations among 5 ΔE values to the pristine samples when the material is discharged at 0.1C (Figure 5c). On the other hand, ΔE of the NCA at 10C discharge rate from the SOC of 90% has the particle-to-particle variations among the particles (Figure 5c). As shown in Figure 5d, the L3/L2 ratios of Ni for the samples discharged at 10C from 90% SOC also have larger variations among the particles than those for the sample discharged at 0.1C from 90% SOC. When the NCA is discharged from SOC 90% at a rate of 10C, ΔE and L3/L2 ratio of some particles do not fully recover to the original value, which implies that the electronic structural changes induced by the initial charge do not completely return to the original state during the high rate discharging. We identify that there are considerable variations in terms of ΔE of the O K-edge and L3/L2 ratio of Ni in case of 10C discharge sample from SOC 90% by EELS analysis. In that case, the L3/L2 ratios of some particles increase compared to that of the pristine sample, indicating the reduction of the Ni ions. To neutralize the charge imbalance due to the undesirable Ni reduction, other TMs in the NCA should be oxidized, or O2 gas should be released. The changes in the L3/L2 intensity ratio of Co are almost similar to that of the pristine, indicating that Co ions rarely participate in charge compensation during initial charge and discharge as shown in Figure S3 in the Supporting Information. Thus, we can assume that O is likely to leave the original structure for charge compensation because Co and Al ions hardly play a role for neutralizing charge imbalance. We selected the two samples from Figure 5c to check the porosity induced by O2 release. Figure 6 presents BF images of two

Figure 7. Evolution of ΔE of the O K-edges and L3/L2 intensity ratio of the Ni L2,3-edges at (a, b) initial charge state, (c, d) initial discharge state depending on the SOC and C-rate. Orange area corresponds to the ΔE of O K-edge of pristine, and blue area corresponds to the L3/ L2 intensity ratio of pristine. In case of the discharged state from SOC 90%, the standard deviation is shown in the graph to indicate the particle to particle variations.

sample initially charged to 90% SOC at the 10C rate. In addition, the sample discharged from 90% SOC at 10C shows the significant variations in the ΔE of the O K-edge and L3/L2 ratio of Ni L2,3-edges even though the average value almost returns to the range of the pristine sample, indicating incomplete recovery from initial charged state. These EELS results clearly represent the implication about how NCA particles composing the positive electrode degrade upon the fast charging and discharging. It does not seem that the whole electrode deteriorates simultaneously. Instead, some of particles start to degrade severer than others. Even though we fabricate the electrodes as perfectly as possible, there should be a variation depending on the location of the electrode in terms of the dispersion of various components (active materials, conductive agents, and binders), the degree of impregnation of the electrolytes, the thicknesses of the electrodes, and so on. All these factors cause the difference in internal resistances depending on the location, and the fast charging and discharging rates make even larger difference in the overpotential acting on different parts of the electrode. This results in severer particle-to-particle variations. Therefore, to improve the rate capability of the LIB, the development of not only new cathode materials with high rate capability, but also new processes to minimize variations of the electrode is necessary. Since we have selected so far the areas of interest for EELS investigation by the SAED aperture in the TEM mode, we acquire the averaged EELS signal from subsurface region (∼50 nm). To examine the phase transition occurring at the surface region, we perform HRTEM imaging for the samples after

Figure 6. Bright-field TEM images from the surface area after the initial 10C discharge from the SOC of 90%, (a) NCA particle #1 in Figure 5c with the pristine level of ΔE of O K-edge, (b) NCA particle #2 in Figure 5c with the lowest ΔE of O K-edge.

NCA particles with the pristine level of ΔE and the lowest ΔE after the initial 10C discharge from the SOC of 90%, respectively. Compared to the morphology of NCA particle #1 with the pristine level of ΔE in Figure 6a, the outer area of NCA particle #2 having the lowest ΔE becomes more porous and rough in Figure 6b. It is obvious in Figure 6 that the morphology at the surface area can be substantially modified for some portions of NCA particles depending on the charging/ discharging rates even after the initial cycle. As the summary of the EELS experiment in Figure 5, Figure 7 shows the plots of the changes in the ΔE of the O K-edge and L3/L2 ratio of Ni L2,3-edges depending on C-rates with different SOCs during the initial charge and discharge. Figure 7 clearly shows that the structural evolutions do not severely occur in case of low SOC and C-rate, but there is the significant Ni reduction for the 2713

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Chemistry of Materials initial charge to 90% SOC at 1C and 10C rates, respectively. In Figure 8, both NCA samples have the rock-salt structure at the

Figure 9. HRTEM images acquired at the surface of NCA particles after the initial discharge from 90% SOC charged state at the discharging rates of (a) 10C and (b) 0.1C until 2 V. (a-1−b-2) Fast Fourier transformations of the indicated areas in panels a and b are also included. S and R represent spinel and rock-salt structures, respectively.

Figure 8. HRTEM images acquired at the surface of NCA particles after the initial charge to 90% SOC at (a) 1C and (b) 10C rates. (a-1− b-2) Fast Fourier transformations of the indicated areas in panels a and b are also included. S and R represent spinel and rock-salt structures, respectively. (a-1-1−b-2-1) Inverse FFT images acquire with the contribution of circled points in FFT image.

modifications occur at the nanoscale. Figure 10 shows the O K EEL spectra taken from the NCA particle discharged from the SOC of 90% at different C-rates using the ΔE index. Since the ΔE value reflects the chemical status of the correlated effects of the TMs in Ni-based layered cathode materials, the ΔE of the O K-edge can be used as an indicator of the extent of the surface modification zone. As an electron probe moves from the

edges, even though SAED patterns in Figure 3 do not show the existence of the rock-salt structure. By acquiring FFT from the whole image, selecting and masking the reflection spots only coming from the rock-salt structure or spinel structure, and then performing inverse FFT, we can get the information about how much area of the sample in the TEM image contains the rock-salt or spinel structure. In Figure 8, it is obvious that the NCA charged to 90% SOC at 10C rate has larger area consisting of the rock-salt and spinel structures at the surface than the NCA charged at 1C rate. We also acquire the HRTEM images for the samples discharged from 90% SOC at the rates of 0.1C and 10C, respectively. The HRTEM image and FFT patterns in Figure 9a show that the sample discharged at 10C rate from 90% SOC still contains the rock-salt structure at the surface and relatively large near surface area with the spinel structure. On the other hand, in Figure 9b, the sample discharged at the rate of 0.1C from 90% SOC mostly consist of the spinel and layered structures in the near surface. The appearance of the rock-salt structure at the edges of NCA particles is of critical importance to the performance of LIB cell: the development of electrochemically inactive and insulating rock-salt phase leads to the capacity loss and the increase in internal cell resistance. Even though XRD results in Figure 2 represent the complete recovery to the original structure after discharging even at 10C, the HRTEM results in Figure 9 clearly show the existence of the rock-salt structure after discharging at 10C. Finally, we acquired the EELS data by scanning a subnanometer sized electron probe from the edge of the surface to the center of the particles. High spatial resolution of the STEM-EELS technique compared to EELS in the TEM mode enables us to quantify the extent to which the structural

Figure 10. Series of O K-edge EEL spectra acquired along the line from edge to the depth of 20 nm by STEM−EELS line-scanning technique. NCA particles are discharged from the 90% SOC charged state at the rate of (a) 0.1C and (b) 10C (red line, EEL spectra with the absence of pre-edge; orange, EEL spectra with ΔE, which is at least 1 eV smaller than ΔE of the pristine; black, EEL spectra with ΔE comparable to ΔE of the pristine). (a-1, b-1) High angle annular dark field images showing STEM−EELS line-scanning regions. (c) Evolution of ΔE from the edge to the interior of the NCA particles representing the degradation depths of the samples discharged at 0.1C and 10C discharge rates. 2714

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Chemistry of Materials edge to the inner region of a particle, the ΔE of the O K edge increases, in other words, the low ΔE values near the surface indicate that the TM, in particular Ni, is reduced at the surface. In the case of the NCA discharged at the rate of 0.1C, the depth to which the pre-edge peak of the O K edge clearly shows up is around 3 nm from the edge of the particle; however, the degradation depth becomes almost the twice (6−7 nm) in the case of the NCA discharged at 10C from an SOC of 90%. The degradation depths of the NCA discharged at 0.1 and 10C are 3−4 and 6−7 nm, respectively. The degradation depth is defined as the distance from the edge to the point where ΔE becomes comparable to that of the pristine NCA. In Figure 2, the discharging at 0.1C from 90% SOC results in the capacity of 225 mAhg−1, while the discharging at 10C leads to 180 mAhg−1. Therefore, there is about 20% difference in Li contents. However, the difference in the degradation zone (the sizes of particles are roughly 500 nm) is below 1%. This is the reason why the analytical tools to measure averaged properties such as XRD are hard to detect the evolutions and the TEM based techniques including the STEM−EELS are highly required. The small structural evolutions in NCA particles can be an initiation point for severe degradation and make a big difference in LIB cell performances. Additional examples are shown in Figure S4 in the Supporting Information. It might be imaginable that a fast and deep charge may induce severer structural changes and a fast and deep discharge may impede the structural recovery of cathode materials. However, a detailed and thorough investigation of the degree of structural transformation caused by the different C-rates and SOCs during the initial charge and discharge is of significance for the development of next generation cathode materials for high-power LIBs. In this study, we investigated the structural modifications induced by not only harsh experimental conditions (10C, 90% SOC), but also mild conditions (0.1C, 1C and 50% SOC) at different levels of scale (bulk/subsurface/ surface) using both the XRD and various TEM-based techniques as the complementary tools. Only subtle changes occurred during the initial charge/discharge under mild conditions. Even though an average crystal structure of the deeply charged NCA cathode returns completely to the original structure after the initial discharge regardless of the discharging rates (Figure 2), the HRTEM (Figures 8 and 9) and STEM− EELS (Figure 10) results show that the phase transitions occurred during the initial charge and the crystallographic and electronic structures at the surface of the NCA particles were not fully recovered even after a mild discharge. This provides an insightful information on the degree of recovery at varying discharging rates from different charged states. Using a STEM− EELS technique, we successfully discerned and quantified the nascent material degradation at the nanoscale instigated by the initial charge/discharge. In addition, TEM also allowed us to investigate the particle-to-particle variations. This is important because a few particles undergoing severer structural modifications may initiate material degradation or thermal runaway. By taking advantage of the high-spatial resolution and sitespecific analysis of TEM, we could even detect the subtle and sporadic changes occurring under mild conditions and thus provide a complete understanding of the kinetic effects during the initial charge/discharge.

charge/discharge as functions of the SOC and C-rate. When the 50% of Li ions are delithiated from the lattice or Li ions are lithiated to the lattice from the 50% delithiated state, the NCA cathode materials undergo only exiguous crystallographic phase transition to the spinel structure with the majority of the layered structures during the charging process, and all structural evolutions are recovered during the discharging process from the 50% SOC irrespective of the induced current rates. However, in the highly delithiated state (90% SOC), the degree of the structural evolutions is highly dependent on the induced current rates. The ΔE of the O K-edge decreases and the L3/L2 ratio of Ni increases rapidly as higher C-rates are induced, indicating a severe reduction of Ni ions. After the discharge from SOC 90% at 10C rate, there are large particleto-particle variations in the crystallographic and electronic structures, even though the average ΔE of the O K-edge and the L3/L2 ratio of Ni from five different particles return to the ΔE and the L3/L2 ranges of the pristine NCA. The in-depth investigation using HRTEM imaging and STEM−EELS line profile reveals that the surface areas with a few nanometer-scale undergo the irreversible phase transition and remain unrestored even after the first charge/discharge. The structural instability caused by the reduction of the Ni ions results in a loss of oxygen for maintaining the charge neutrality and thus the formation of pores at the surface. This study clearly shows that the degree of the structural instability of NCA cathode materials is highly dependent upon SOC and C-rates and demonstrates how TEM-based techniques can be utilized for investigating the effects of SOC and C-rates on the local structural evolutions of the cathode materials even during initial charge/discharge.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b03282. Additional SAED patterns of NCA; Co L2,3 edges of EEL spectra; additional series of O K edge EEL spectra (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Wonyoung Chang: 0000-0003-2216-9002 Present Address ∥

(S.H.) Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973, United States. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Korea Institute of Science and Technology (KIST) Institutional Programs (Project Nos. 2E27062 and 2E27090). This research was partially supported by the National Research Foundation of Korea Grant funded by the Korean Government (NRF-2011-C1AAA001-0030538).

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CONCLUSION In this work, we investigate the crystallographic and electronic structural changes in NCA cathode materials during the initial

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