Activating layered LiNi0.5Co0.2Mn0.3O2 as a host for Mg intercalation in rechargeable Mg batteries

Accepted Manuscript Title: Activating layered LiNi0.5 Co0.2 Mn0.3 O2 as a host for Mg intercalation in rechargeable Mg batteries Authors: Yongbeom Cho, Hyungsub Kim, Kyojin Ku, Gabin Yoon, Sung-Kyun Jung, Byungju Lee, Kisuk Kang PII: DOI: Reference:

S0025-5408(17)30151-4 http://dx.doi.org/doi:10.1016/j.materresbull.2017.04.047 MRB 9308

To appear in:

MRB

Received date: Revised date: Accepted date:

11-1-2017 18-4-2017 25-4-2017

Please cite this article as: Yongbeom Cho, Hyungsub Kim, Kyojin Ku, Gabin Yoon, Sung-Kyun Jung, Byungju Lee, Kisuk Kang, Activating layered LiNi0.5Co0.2Mn0.3O2 as a host for Mg intercalation in rechargeable Mg batteries, Materials Research Bulletinhttp://dx.doi.org/10.1016/j.materresbull.2017.04.047 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Activating layered LiNi0.5Co0.2Mn0.3O2 as a host for Mg intercalation in rechargeable Mg batteries Yongbeom Cho, Hyungsub Kim, Kyojin Ku, Gabin Yoon, Sung-Kyun Jung, Byungju Lee, Kisuk Kang* Y. Cho, H. Kim, K. Ku, G. Yoon, S.-K. Jung, B. Lee, Prof. K. Kang Department of Materials Science and Engineering, Research Institute of Advanced Materials (RIAM), Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 151-742, Republic of Korea E-mail: [email protected]

Graphical abstract

The avenue to utilize layered oxides into Mg insertion host through electrochemical synthesis is investigated using aqueous Mg electrolytes. The results indicate that the waterintercalated NCM523 is synthesized during electrochemically oxidation. The magnesiation reaction accompanied with deintercalation of water leads to structural transformation of P3 structure to wrinkled layered structure during magnesiation. Structural transformation of amorphous into P3 phase occurs during demagnesiation. Highlights 

The first demonstration of the Ni multi-redox reaction combined with multi-valent Mg2+ intercalation in a rechargeable battery system.



Water intercalated P3-NCM523 phase is capable of delivering a capacity of ~190 mAh g-1 at a voltage of 3.1 V vs. Mg/Mg2+



Insertion of Mg2+ ions is accompanied by the extraction of water molecules during the two-phase reaction, leading to the formation of a wrinkled layered structure

Layered crystal structures are some of the most intensively studied intercalation hosts for guest ion insertion. For Mg insertion, layered transition metal sulfides or selenides have been 1

used for reversible Mg intercalation; however, far less intercalation hosts have been found for layered oxides most likely because of the strong interaction between Mg2+ and the oxide host. Here, we demonstrate that layered LixNi0.5Co0.2Mn0.3O2 (NCM523), which is an important commercial electrode for Li-ion batteries but has been regarded as electrochemically inactive in rechargeable Mg batteries, can function as a reversible host for Mg2+ if water opens up the layers and screens the electrostatic interaction between Mg2+ and the host. Upon the formation of water-intercalated NCM523, the discharge capacity dramatically increases utilizing the multi-redox reaction of Ni2+/Ni3+/Ni4+, which exhibits an average voltage of ~3.1 V (vs. Mg/Mg2+) in rechargeable Mg batteries with an energy density of 589 Wh kg-1 in the first discharge. Keywords: Magnesium; layered oxide; water; P3 phase; LiNi0.5Co0.2Mn0.3O2

Introduction To address environmental issues and the ever-growing demands for wireless electronic devices, the development of efficient and high-performance energy storage systems is indispensible.1,2 Thus, considerable attention has been focused on advanced rechargeable batteries.3 Li-ion batteries (LIBs), which utilize graphite and layered lithium transition metal oxides as the anode and cathode, respectively, display the most reliable electrochemical performance among various rechargeable battery systems and have thus been successfully commercialized for small portable electronics and mid-sized power applications.4, 5 However, the energy density of LIBs remains insufficient for new large-scale applications such as fullsized electric vehicles because of the limitation of conventional cathode materials, whose theoretical capacities are limited by the number of interstitial sites for Li guest ions in their crystal structure and corresponding one electron charge stored per interstitial site. Identifying a new intercalation host whose theoretical capacity can exceed those of the well-known layered oxides is challenging. One possible approach is to explore a new chemistry that can 2

carry multi-electron guest ions and store multiple charges per intercalation site even in a known host.6 The Mg rechargeable battery system is one potential candidate, where a Mg guest ion can deliver two electrons per interstitial site in the host structure, possibly doubling the capacity of a given intercalation compound used in LIBs when combined with a multiredox element.7 However, the intercalation of divalent guest ions generally exhibits unacceptably sluggish kinetics in conventional host materials, which is attributable to the strong Coulombic interaction between divalent guest ions and the host framework, delaying the development of Mg rechargeable batteries.8-9 Various materials have been previously investigated as possible intercalation hosts for Mg2+ ions.10-18 Among them, sulfides and selenides including Chevrel phases (Mo6T8, T = S, Se) have been the most intensively studied group of electrodes that exhibited an excellent electrochemical activity for reversible Mg insertion and extraction.19-21 The relatively fast kinetics of Mg2+ ions in these groups was attributed to the open structure that reduces the Coulombic interactions between Mg2+ and host ions. The large framework consisting of S and Se ions could provide a sufficient space for Mg intercalation. However, sulfides and selenides usually exhibit low energy density because of the low polarity and heavy weight of S2- and Se2- anions, which reduce the voltage and specific capacity, respectively.22-24 Moreover, the open host framework based on the large anions is disadvantageous for the volumetric energy density. Accordingly, recent attempts have been made to utilize oxide compounds as Mg intercalation hosts to increase the voltage and specific capacity. Among these compounds, several nano-sized layered oxides such as V2O5 and MoO3 displayed electrochemical activity with Mg, delivering slightly higher energy density than Chevrel phases.25 More recent studies by Cabana et al. reported that a Mn-based oxide could serve as a high-voltage Mg intercalation electrode with a voltage of ~2.9 V (vs. Mg/Mg2+).26 In addition, several other groups have explored a number of Mn-based oxides as potential Mg insertion hosts.27-29 3

Nevertheless, the energy density of reported oxides has not been sufficiently high to compete with that of LIBs. Utilizing Ni as a redox center can be a promising strategy to achieve a high energy density in oxides because of the high redox potential of Ni. Moreover, its capability of being a multi redox center would be advantageous in accommodating multi-valent Mg2+ carrier ions and exploiting the corresponding extended capacity.30-32 The unique feature of the Ni+2/Ni+4 redox reaction is attributed to the relative instability of Ni3+, which contains unpaired spins in the octahedral environments in the layered oxides.4,33 Thus, Ni-based layered oxides such as LiNixCoyMnzO2 (NCM) have been extensively studied as intercalation hosts and demonstrated as high-energy-density electrodes in LIBs.34,35 However, to date, Mg intercalation in Ni-based layered materials is considered impossible because of the high migration barrier of Mg2+ ions in the layered framework.

Herein, we demonstrate for the first time that layered NCM

electrodes allow the insertion and extraction of Mg2+ ions if water opens up the layers and screens the electrostatic interaction between Mg2+ and the host. A water-intercalated NCM523 electrode is capable of delivering a discharge capacity of 190 mAh g-1 at ~3.1 V (vs. Mg/Mg2+), which is the highest average voltage reported in rechargeable Mg batteries, by utilizing the multi-redox reaction of Ni2+/Ni4+. It is also revealed that a reversible structural transformation can occur between the water-intercalated P3 phase and the wrinkled layered phase during magnesiation and demagnesiation.

Results and Discussion Considering that electrode materials containing water molecules (e.g., MnO2·xH2O and V2O5·xH2O) can effectively screen the electrostatic interaction between guest and host ions, the insertion of water molecules into a crystal can be an efficient approach to overcome the sluggish kinetics of high-valent guest ions in oxide-based electrodes.36-42 In addition, the 4

synthesis of water-intercalated layered oxides was demonstrated in the work of C. Delmas et al. using the chemical oxidation method.41 Inspired by these previous findings, we devised a simple electrochemical setup that can simultaneously synthesize water-intercalated NCM523 and evaluate its electrochemical properties by exploiting aqueous Mg electrolytes (see experimental section for details). Figure 1 shows the series of galvanostatic experiments performed on the NCM523 electrode in the Mg electrochemical cell. NCM523 became electrochemically active with Mg under a certain charging protocol, showing a significant increase of the discharge capacity. When the charging of NCM523 was performed up to 120 or 150 mAh g-1, only a negligible amount (~15 or 30 mAh g-1) of the capacity was obtained during the first discharge (Figure 1a and b), indicative of the absence of Mg uptake in the structure. Note that the electrochemical charge profile up to 150 mAh g-1 is consistent with the previously known profile of a NCM523 electrode in conventional LIBs, implying that the typical Li extraction reaction from the structure occurs in the Mg cell.32 However, as the electrode is further delithiated in the aqueous medium, the discharge capacity gradually increases with a plateau at ~ 3 V (vs. Mg/Mg2+). Interestingly, after a critical point of charge capacity (~150 mAh g-1), the discharge capacity increases dramatically, as observed in Figure 1c and d. When we charged the NCM523 electrode up to 220 mAh g-1, the discharge capacity reached 86% of the charge capacity (190 mAh g-1) with an average voltage of 3.1 V vs. Mg/Mg2+, which corresponds to an energy density of 589 Wh kg-1. Note that the charge profile over the critical point of the capacity (150 mAh g-1) deviates substantially from that observed for the NCM523 electrode in conventional LIBs, exhibiting a voltage plateau at ~3.5 V (vs. Mg/Mg2+) (see Figure S1 in Supporting Information). This finding strongly implies that the electrochemical reaction of NCM523 in an aqueous Mg cell is not simply the Li+ extraction reaction from the host for the corresponding capacity region. Ex situ X-ray diffraction (XRD) analysis of the electrode was conducted at different states of charge to monitor the structural transformation in the Mg cell. The structural 5

evolution of the NCM523 electrode is shown in Figure 2a (right) as a function of the charging capacity (left) during the first charging step. The crystal structure of the as-prepared electrode is well matched with the O3-type NCM523 layered structure (Supporting Information, Figure S2). However, slight shifts of the peaks such as (003) occur from point A to E (~ 125 mAh g1

), which agrees with previous reports on the charging behavior of NCM523 electrodes in

LIBs. 43 This result confirms the occurrence of a typical Li extraction reaction in this region. Interestingly, subsequent charging to 150 mAh g-1 (point F) induces a significant reduction of the (003) peak intensity with notable broadening. Moreover, with further charging corresponding to the extraction of approximately 0.8 Li ion (points G and H), a new peak arises at a significantly lower angle, approximately 12.8o, and the characteristic (003) peak from the O3 layered structure disappears in the XRD patterns. The characteristic evolution of the major XRD peaks is apparent in Figure 2b (left) and indicates that a new phase with a substantially larger c-lattice parameter than that of the O3 phase is formed at the expense of the original O3 phase. Note that the observed structural transformation begins at point F, which coincides with the critical point of the capacity (150 mAh g-1) in Figure 1. The new peak near 12.8o corresponds to a d-spacing of 6.9 Å in the layered structure, and this unusually large value is not generally observed in known layered transition metal oxides. However, considering the cases of hydrated or solvent- cointercalated layered transition metal oxides, it is likely to originate from the cointercalation of water into the NCM523 layered structure.44,45 Indeed, we observed that the new phase matches with the well-known p-type transition metal oxyhydroxide in a full-pattern matching even though a precise refinement was not possible because of the broad nature of the peaks (Supporting Information, Figure S3).42 We propose that the observed new phase can be assigned as a P3 phase because (i) the slab space of ~6.9 Å is generally observable in P3 layered phases and (ii) the O3 phase can easily transform into the P3 phase without breaking TM–O bonds based on the gliding mechanism of [Ni0.5Co0.2Mn0.3]O2 slabs (Figure 2b (right)).46 The broadening 6

of the main peak during the transformation is most likely due to the inhomogeneously distributed slab space resulting from the bending or incomplete sliding of slabs during the extraction of Li+ ions and insertion of water molecules. In the highly charged state of the electrode, water molecules and remaining Li+ ions are expected to share the Li slab space, forming the relatively well-ordered P3 structure. In layered lithium transition metal oxides, the deficient Li content in the Li layer generally causes an unscreened repulsion between opposing oxygen layers, resulting in an unstable layered phase.47 To compensate for the structural instability of Li-deficient layered oxides, oxygen evolution and/or TM migration from the TM slab to the Li slab tend to occur, which induce the partial phase transformation to the spinel or rock salt phase.48,49 In our case, it is speculated that the Li-deficient NCM523 beyond the critical amount of Li extraction could be stabilized by the water-induced O3–P3 phase transformation in the aqueous system instead of the oxygen evolution or transition metal migration. The structural evolution of P3-NCM523 was further probed during a subsequent discharge and recharge, as shown in Figure 3. A close examination of the discharge from A to F reveals that the discharge reaction can be roughly divided into two steps. In the first stage, slight shifts of the (003), (006), and (015) peaks are observed from A to B (inset of Figure 3), indicating that a single-phase electrode reaction of the P3 phase occurs up to a discharge capacity of 30 mAh g-1. In this solid-solution region, the c-lattice parameter decreases, whereas the a- and b-lattice parameters exhibit slight increases according to the full-pattern matching in Table S1 (Supporting Information). In the subsequent discharging to 2.6 V (vs. Mg/Mg2+), all the peaks of the P3 phase gradually decrease in intensity and disappear at the end of the discharge. The gradual reduction of the peak intensity implies the loss of the longrange ordering in the P3 phase during the second discharge step, which will be further discussed in the next section. However, the original P3 structure is recovered when it is

7

recharged (as observed in Figure 3) even though the peak intensity is not fully recovered, as denoted by the dotted circle in the figure. To directly monitor the structural evolution of NCM523 during the electrochemical cycle, high-resolution transmission electron microscopy (HR-TEM) was used to visualize the local structure of the fully charged and discharged NCM523 electrodes. Figure 4a shows the charged NCM523 electrode and clearly reveals the well-ordered lattice fringe of the layered structure. The d-spacing of the charged phase was approximately 6.7 Å (3.4 nm divided by 5) estimated from the lattice image, which agrees well with that observed in the XRD pattern in Figure 2 and is comparable to those of the (0 0 3) planes of typical P3 hydrated layered oxides.38,42 However, after discharging the electrode, the TM slabs become significantly waved and wrinkled, and the interslab distance decreases to 4.9–6.9 Å, as observed in Figure 4b. This result indicates that the long-range ordering has been lost, which would result in the significant reduction in the XRD peak intensity observed in Figure 3. Nevertheless, this finding confirms that the overall layered framework is maintained, which supports the feasible restoration of the P3 layered phase after the charging demonstrated in Figure 3. The decreased c-lattice parameter after the discharge is attributed to a strong attraction between the inserted Mg2+ and O2- ions, which squeezes water molecules out of the structure, resulting in the decreased interslab distances and waved TM slabs. We conducted thermal gravimetric analysis (TGA) and Fourier-transform infrared spectrometry (FTIR) to further understand the structural transformation mechanism involving the water molecules during charge and discharge. Although the TGA curve of the pristine electrode does not exhibit any signature of water, that of the charged sample shows a continuous loss of weight below 250°C. Moreover, a sharp drop of weight occurs between 120 and 190°C (Figure 5a). According to previous reports, this rapid drop of weight provides clear evidence of the elimination of the water inserted in the crystal; however, the gradual decrease of the weight below 250°C is attributed to the loss of the surface-absorbed water.50,51 8

The total water content is estimated to be approximately 0.24 H2O molecules per chemical formula unit of NCM523. For the discharged sample, no apparent drop of the weight was observed between 120 and 190°C; simply a monotonous decrease was observed until 250°C, and the overall water content was substantially decreased. This finding indicates that a considerable amount of crystal water was lost after the discharge, which agrees with our structural data from the XRD and TEM analyses. The ex situ FTIR spectra in Figure 5b also confirm the presence of water molecules in the NCM electrode. Although the IR absorption peaks in the 650–400 cm-1 range are characteristic for layered oxides corresponding to the O– M–O bending vibration and asymmetric stretching modes of the MO6 group, the absorption peaks at approximately 1617 and 3400 cm-1 are attributed to the O–H stretching and H–O–H bending modes of crystalline water. 52-54 The series of FTIR results indicate that the water is gradually inserted after a critical capacity of 150 mAh g-1 in the charging reaction, which is also remarkably consistent with the observed critical capacity in Figure 1. As the electrode is discharged, the intensities of the peaks at approximately 1617 and 3400 cm-1 decrease, which supports the extraction of water molecules out of the P3 phase upon magnesiation, leading to the formation of a wrinkled layered structure. The reversible insertion and extraction of Mg2+ ions during the electrochemical cycle were supported from TEM and SEM energy-dispersive X-ray spectroscopy (EDS) mapping analysis. The chemical mapping of the pristine electrode in Figure 5c reveals that Mg ions are not observable in the NCM523 particles. However, Mg ions clearly appear throughout the electrode particles after the discharge in Figure 5d, indicating the presence of Mg2+ ions in the discharged P3-NCM523 phase. This finding is also observed in the SEM-EDS mapping in the cross-sectional image of the NCM523 particles, confirming that Mg2+ ions were incorporated in the NCM523 structure (Supporting Information, Figure S4). The ex situ X-ray photoelectron spectroscopy (XPS) results presented in Figure S5 a, b, and c further confirm the reversible Mg2+ insertion and extraction in NCM523. The Mg 2p at ~50.3 eV could be 9

clearly detected with the discharged electrode, which disappears with the recharged electrode along with the Mg KLL Auger peak at approximately 308 eV.56-58 The redox reaction of NCM523 with Mg2+ ion de/insertion was analyzed using X-ray absorption spectroscopy (XAS) to monitor the oxidation states of Ni, Co, and Mn. Figure 6a, c, and e present X-ray absorption near edge structure (XANES) spectra of the electrodes during the first charge process. The Ni K-edge XANES spectrum shows a clear rightward shift up to the charge capacity of 210 mAh/g, which is indicative of the oxidation of Ni from +2 to +3 and then to +4. The Co and Mn K-edge XANES spectra, however, show little change of the oxidation states from the initial Mn4+ and Co3+ states during charging. However, the overall shapes of the spectra were slightly altered, which is attributed to the structural transformation to the P3 phase that occurs during the first charging process, as suggested in the previous sections. During the discharge in Figure 6b, the Ni K-edge shifted back to the initial valence state, indicating the reversible Ni redox reaction. The Ni K-edge spectrum gradually changed, leading to an isosbestic point at 8354 eV during the discharge process in Figure 6b, which generally appears in two-phase electrode materials.59,60 The two-phase-like change of the spectrum in the discharge is also consistent with the phase transformation from the P3 phase to the disordered layered structure in the previous section. The Co and Mn Kedge XANES spectra display little change in valence states from Co3+ and Mn4+ during discharge, confirming that the primary redox reaction occurs in the Ni in agreement with the redox reaction of NCM523 in Li rechargeable systems. This finding supports the idea that the multi-valent Mg2+ intercalation can be successfully accompanied by the multi-redox reaction in Ni.

Conclusion Reversible Mg intercalation in a commercial NCM523 electrode was successfully demonstrated by incorporating water molecules in the layer space, forming a novel P3 phase 10

NCM523 material. The P3-NCM523 phase is capable of delivering a capacity of ~190 mAh g1

at a voltage of 3.1 V vs. Mg/Mg2+ through the multi-redox reaction of Ni from +2 to +4. The

discharge mechanism of the P3 NCM523 consists of the solid-solution reaction of the P3 phase followed by the two-phase reaction between the P3 and disordered layered phase. The insertion of Mg2+ ions is accompanied by the extraction of water molecules during the twophase reaction, leading to the formation of a wrinkled layered structure. The strong attraction between inserted Mg2+ and O2- ions most likely squeezed out water molecules, forming the wrinkled layered structure. This work is the first successful demonstration that the Ni multiredox reaction can be combined with the multi-valent Mg2+ intercalation in a rechargeable battery system. The Ni2+/Ni3+/Ni4+ redox potential of 3.1 V vs. Mg/Mg2+ is one of the highest values reported among Mg battery cathodes. We believe that this strategy, which enables layered oxides to insert and extract Mg2+ ions through the formation of a water-intercalated phase, is applicable to various Ni-based layered oxides and hints at a path toward the development of promising cathode materials for Mg rechargeable batteries.

Experimental Section Characterization of LiNi0.5Co0.2Mn0.3O2: The LiNi0.5Co0.2Mn0.3O2 sample was supplied by Samsung Fine Chemicals (Daejeon, Korea). The powder sample was analyzed using a X-ray diffractometry (XRD, D2 PHASER, Bruker, Bremen, Germany) equipped with Cu-Ka 0.2o min-1 in the 2theta range of 10-70o. Electrochemical characterization: Tested working electrodes were prepared by the following sequence. A slurry of 70wt% active materials, 20 wt% carbon black (Super-P; Timcal, Bodio, Switzerland), and 10 wt% polyvinylidene fluoride (PvdF) binder, dissolved in N-methyl-1,2-pyrrolidone (NMP, 99.5%; Sigma-Aldrich, St. Louis, Mo, USA), was pasted onto Ti-mesh current collector. NMP was evaporated overnight at 70 °C in a vacuum oven. 11

The individual cells were assembled in the sequence of counter electrode (Pt coil), reference electrode (Ag/AgCl, MF-2052; Bioanalytical Systems, Inc., Germany), working electrode in a 3 electrode beaker-type cell. The electrolyte consisted of 1M magnesium sulfate (MgSO4) in DI water. The electrochemical performances of all these cells were measured using a potentiogalvanostat (WBCS 3000, WonA Tech, Korea). Ex situ analysis: For the structural characterization, field-emission scanning electron microscopy (FE-SEM, MERLIN Compact; ZEISS) X-Ray diffractometry, high-resolution transmission electron microscopy (HR-TEM, JEM-2100F; JEOL, USA) were performed. Xray photoelectron spectroscopy, TEM-EDS, SEM-EDS analysis were conducted to estimate existence of magnesium in the particle. Samples were further characterized for identifying insertion of water molecules in the structure using Thermal gravimetric analysis (TGA) and Fourier transform infrared (FT-IR) microscope (Hyper ion 3000) by KBR pellet analysis. Ex situ X-ray absorption spectroscopy (XAS) measurements (Ni K-edge, Co K-edge, Mn Kedge) were performed at beam line 8C at the Pohang Accelerator Laboratory (PAL).

Acknowledgements This work was supported by the World Premier Materials grant funded by the Korea government Ministry of Trade, Industry and Energy and by the National Research Foundation of Korea(NRF) grand funded by the Korea government(MSIP) (No. 2015R1A2A1A10055991).

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Figure 1. Electrochemical activation of LiNi0.5Co0.2Mn0.3O2 electrode in an aqueous Mg cell. Capacity-voltage profiles of NCM – Pt (Ag/AgCl reference electrode) 3 electrode cells charged up to (a) 120 mAh g-1, (b) 150 mAh g-1, (c) 180 mAh g-1, and (d) 220 mAh g-1.

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Figure 2. Mechanism of the electrochemical activation of LiNi0.5Co0.2Mn0.3O2 electrode. (a) Ex-situ XRD patterns of NCM523 in a 3-electrode cell during the first charging process. (b) Amplified XRD patterns from E to H in the 2θ range 10-22° and a schematic representation of the structural evolution at E and H. 19

Figure 3. Ex-situ XRD patterns of LiNi0.5Co0.2Mn0.3O2 in a 3-electrode cell during the discharge and charge process.

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Figure 4. High magnification lattice images of (a) fully charged NCM523 and (b) fully discharged NCM523, respectively. The lattice images were taken from the region corresponding to the box in the right low magnification HR-TEM image. The insets in the left figures plot the intensity of lattice images denoted with the black rectangle as a function of the distance.

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Figure 5. (a) TGA curves of NCM523 electrode at as-prepared, fully charged and fully discharged states. The TGA test was conducted at a heating rate of 5 °C min-1 under N2 flow. (b) IR spectra of NCM523 electrode during charging and discharging. Peaks at 1500-600 cm-1 range are attributable to PvdF binder.55 TEM-EDS mapping of (c) as-prepared and (d) discharged NCM523.

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Figure 6. XANES spectra of Ni K-edge during first (a) charging and (b) discharging, Co Kedge during (c) charging and (d) discharging and Mn K-edge during (e) charging and (f) discharging.

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