Effects of Chemical versus Electrochemical Delithiation on the Oxygen

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Effects of Chemical vs. Electrochemical Delithiation on the Oxygen Evolution Reaction Activity of Nickel-rich Layered LiMO

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Veronica Augustyn, and Arumugam Manthiram J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.5b01538 • Publication Date (Web): 08 Sep 2015 Downloaded from http://pubs.acs.org on September 8, 2015

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The Journal of Physical Chemistry Letters

Effects of Chemical vs. Electrochemical Delithiation on the Oxygen Evolution Reaction Activity of Nickel-rich Layered LiMO2 Veronica Augustyn and Arumugam Manthiram* Materials Science and Engineering Program & Texas Materials Institute The University of Texas at Austin Austin, TX 78712, USA *

Corresponding Author: [email protected]

1 ACS Paragon Plus Environment


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Abstract: Nickel-rich layered LiMO2 (M = transition metal) oxides doped with iron exhibit high oxygen evolution reaction (OER) activity in alkaline electrolytes. The LiMO2 oxides offer the possibility of investigating the influence of the number of d electrons on OER by tuning the oxidation state of M via chemical or electrochemical delithiation. Accordingly, we investigate here the electrocatalytic behavior of LiNi0.7Co0.3O2 and LiNi0.7Co0.2Fe0.1O2 before and after chemical delithiation. In addition to varying the oxidation state of the transition-metal ions, we find that chemical delithiation also affects the local chemical environment and morphology. The electrochemical response differs depending on whether the delithiation occurred ex situ chemically or in situ during the electrocatalysis. The results point to the important role of in situ transformation in LiMO2 in alkaline electrolytes during electrocatalytic cycling.

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The electrocatalysis of the 4-electron oxygen evolution reaction (OER) requires significant overpotentials and leads to large inefficiencies for electrochemical hydrogen production and rechargeable metal-air batteries. The lowest overpotentials for the OER in alkaline electrolytes are achieved with the noble-metal oxides IrO2 and RuO2.1–3 Recent work has identified high activity with nickel and cobalt oxides and hydroxides, particularly those doped with iron,4–6 some that can achieve mass activities similar to or lower than that of IrO2.7 Besides the identification of materials with low overpotentials, research in OER electrocatalysis has also focused on the correlation of materials characteristics to electrocatalytic activity8,9 in order to understand the mechanism of the OER. Recently, layered lithium metal oxides (LiMO2, M = transition metal) have been investigated for the OER in alkaline solutions.10–13 These compounds are interesting for electrocatalysis because they can be synthesized with a variety of transition-metal ions and the mobility of Li+ allows for the tuning of the oxidation state of the transition-metal ions. Therefore, they can be useful for correlating the material characteristics with electrocatalytic activity. Li+ can be extracted via either chemical or electrochemical oxidation. Electrochemical delithiation of layered LiCoO2 to Li0.5CoO2 in a non-aqueous electrolyte has resulted in higher OER mass activity.12 Chemical delithiation of layered LiCoO2 with NO2BF4 to Li0.64CoO2, Li0.34CoO2, and Li0.09CoO2 led to similar mass activities but lower specific activities than the assynthesized sample.14 These findings also identified that during chemical delithiation, the surface area of the LiCoO2 increased 10 times over that of the as-synthesized material. Moreover, ex situ Raman spectroscopy showed that chemically delithiated layered LiCoO2 contained spinel Co3O4 on the surface, a phase not evident by X-ray diffraction (XRD). The as-



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synthesized LiCoO2 transformed to spinel Co3O4 after 10 cyclic voltammetry cycles. Our recent results identified that among the various LiMO2, Fe-doped nickel-rich compounds exhibit the highest electrocatalytic activity for the alkaline OER.15 The objective of this study is to determine whether transition-metal oxidation state is the only parameter varied during the delithiation of layered lithium transition-metal oxides. Here, we investigate the effects of ex situ chemical and in situ electrochemical delithiation on the OER activity of LiNi0.7Co0.3O2 and LiNi0.7Co0.2Fe0.1O2. The chemical compositions and XRD patterns of the as-synthesized and chemically delithiated LiNi0.7Co0.3O2 (LNCO) and LiNi0.7Co0.2Fe0.1O2 (LNCFO) are shown, respectively, in Table 1 and Figure 1a and c. In the as-synthesized compounds, the Li content is ~ 1. Layered LiMO2 oxides crystallize in the O3 structure, where Li and M are arranged in between the oxygen layers, which have an ..ABCABC... stacking sequence. In an ideal structure, the transition metal M is in a +3 oxidation state and both Li and M are in an octahedral coordination. LiNiO2, where Ni is in the 3+ oxidation state, is very difficult to synthesize even with excess Li and an oxidizing atmosphere,16 resulting in the formation of Li1-xNi1+xO2.17 This Li-deficient compound contains Ni2+ which, due to its ionic size similar to that of Li+, easily mixes with Li+ in the lithium planes, resulting in a cation-mixed material. Our prior results showed that increased cation mixing leads to poor OER activity.15 In LNCO and LNCFO, the presence of Co3+ greatly stabilizes Ni3+ and reduces cation mixing. The degree of cation mixing can be estimated from the intensity ratio of the (003) to (104) XRD peaks (I(003)/I(104)) as shown in Table 1. I(003)/I(104) values > 1.2 for LNCO and LNCFO indicate good cation order.18 With an Li content of ~ 1 in the as-synthesized material, the nominal transition-metal oxidation state will be +3 for LNCO and LNCFO.


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LNCO and LNCFO were chemically delithiated to Li contents of, respectively, 0.6 and 0.5 as verified by the inductively coupled plasma – optical emission spectroscopy (ICP-OES) analysis in Table 1. Chemical delithiation will increase the nominal oxidation state of the transition-metal ions to 3.4 (Li0.6NCO), 3.5 (Li0.5NCO), 3.4 (Li0.6NCFO), and 3.5 (Li0.5NCFO). Chemical delithiation to these values does not lead to a phase transformation, as observed in the XRD patterns, consistent with previous reports.19 The I(003)/I(104) ratio remains > 1.2, indicating good cation ordering in the chemically delithiated samples.

Table 1. Chemical compositions and XRD intensity ratios of the (003) to (104) peaks for the assynthesized and chemically delithiated LiNi0.7Co0.3-xFexO2 _____________________________________________________________________________ Sample

ICP-OES Composition

I(003)/I(104)

LNCO

Li0.99Ni0.69Co0.31O2

1.5

Li0.6NCO

Li0.57Ni0.68Co0.32O2

1.3

Li0.5NCO

Li0.48Ni0.68Co0.32O2

1.6

LNCFO

Li1.01Ni0.68Co0.21Fe0.10O2

1.6

Li0.6NCFO

Li0.63Ni0.68Co0.22Fe0.11O2

1.5

Li0.5NCFO

Li0.51Ni0.68Co0.21Fe0.11O2

1.8

__________________________________________________________________________________________________________________

Micro Raman spectroscopy was performed on the as-synthesized and chemically delithiated LNCO and LNCFO samples (Figure 1b and d) as this technique is more sensitive to the local chemical environment than XRD. The O3 structure (R-3m space group) has two Raman-active modes, Eg and A1g, located at 400 - 650 cm-1 and corresponding to the polarization of the oxygen atoms either parallel or perpendicular to the c-axis.20 The Raman spectra of the assynthesized materials consist of broad peaks of low intensity. Such spectra have been observed 5 ACS Paragon Plus Environment


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for Ni-rich compounds and they are attributed to the high electronic conductivity of Ni-rich LiMO2, which decreases the optical skin depth.21 The chemically delithiated compounds exhibit two broad peaks in the same spectral region but there is now a noticeable peak separation. This type of peak separation has been observed during in situ micro Raman of LiNi0.80Co0.15Al0.05O2 during electrochemical delithiation in a non-aqueous Li+ electrolyte,22 and has been attributed to changes in the a and c lattice parameters with Li content.23 Therefore, while XRD does not indicate the presence of structural differences between as-synthesized and chemically delithiated LNCO and LNCFO, the ex situ Raman results demonstrate that the local chemical environment changes upon chemical delithiation.


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Figure 1. XRD patterns and micro Raman spectra of (a), (b) LNCO and chemically delithiated LNCO and (c), (d) LNCFO and chemically delithiated LNCFO. The O3 structure is maintained in both the as-synthesized and chemically delithiated samples.

Scanning electron microscopy (SEM) images of LNCO and LNCFO before and after chemical delithiation to Li = 0.5 are shown in Figure 2. The morphology of the as-synthesized LNCO and LNCFO consists of large aggregates of several microns made up of spherical particles that are several hundreds of nanometers in size. The surface area of LNCO, determined via nitrogen adsorption and Brunauer-Emmett-Teller (BET) theory, was 1 m2 g-1, as expected for the high-temperature solid state synthesis. Upon chemical delithiation, the sample morphology changes to aggregates composed of smaller spherical particles than the as-synthesized material. The BET surface area of Li0.5NCO was 7 m2 g-1, larger than that of LNCO and consistent with the SEM observations. The ICP-OES, Raman, SEM, and BET results indicate that chemical delithiation of LNCO and LNCFO leads to changes in chemical composition, transition-metal oxidation state, local chemical environment, and morphology.



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Figure 2. SEM images of (a) LNCO, (b) Li0.5NCO, (c) LNCFO, and (d) Li0.5NCFO. After chemical delithiation, the particle size decreases due to the prolonged stirring and an oxidizing chemical environment. The BET surface area of LNCO was 1 m2 g-1 whereas that of Li0.5NCO was 7 m2 g-1. Scale bar = 2.5 µm.

The electrocatalytic activity for the OER in 0.1 M KOH for LNCO, Li0.5NCO, LNCFO, and Li0.5NCFO is shown in Figure 3 on the basis of mass and surface area after the 3rd CV cycle. As shown previously,15 the Fe-doped materials exhibit superior electrocatalytic activity. Chemical delithiation to Li = 0.5 does not lead to an increase in the mass catalytic activity for LNCO and LNCFO. The SEM results in Figure 2 and BET of LNCO and Li0.5NCO indicate that the morphology and surface area change after chemical delithiation. Normalizing the electrocatalytic activity to the BET surface area indicates that the chemically delithiated samples


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exhibit lower activity than the as-synthesized compounds, while both Fe-doped samples maintain higher electrocatalytic activity than LNCO and Li0.5NCO.

Figure 3. OER activity at a 379 mV overpotential in 0.1 M KOH for as-synthesized and chemically delithiated LNCO and LNCFO, after the 3rd CV cycle: (a) mass activity and (b) specific activity. By both mass and surface area, the Fe-containing samples exhibit superior electrocatalytic activity. Normalization to the BET surface area indicates that the chemically delithiated samples exhibit lower specific activity than the as-synthesized ones.

The electrochemical results indicate that on the basis of the BET surface area, the chemically delithiated samples exhibit lower activity than the as-synthesized samples. The difference between the as-synthesized and chemically delithiated samples may be due to the changes that occur between the first and subsequent cyclic voltammogram (CV) cycles, as shown in Figure 4a and b for LNCFO and Li0.5NCFO. In the first cycle, LNCFO exhibits a single cathodic peak on the reverse sweep while no peaks are present in Li0.5NCFO. On the third cycle, LNCFO exhibits two prominent anodic peaks and a single cathodic peak (shifted to more positive potentials from the first sweep). These peaks likely correspond to the reversible insertion/extraction of cations from the electrolyte into the structure. Li0.5NCFO again does not 9 ACS Paragon Plus Environment


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contain any redox peaks; only the double-layer charging current is present. The insets of Figure 4a and b show the entire CV potential range. During the first cycle, the OER mass activity of LNCFO is lower than that of Li0.5NCFO while on the third cycle, the OER mass activity is the same. The CVs of LNCO and Li0.5NCO are shown in Figure S1. During the first cycle, LNCO exhibits a prominent irreversible anodic peak while only the double layer charging current is present in Li0.5NCO. Between the first and third CV cycles, the mass activities of the chemically delithiated materials stayed the same (within experimental error) while those of the as-synthesized materials increased, as shown in Table S1. Since Raman spectroscopy is sensitive to the local chemical and structural environments, we performed ex situ experiments after three CV cycles (stopping the scans at 0.2 V vs. SCE). The results are shown in Figure 4c and d for, respectively, LNCFO and Li0.5NCFO. Three separate areas were selected to ensure reproducibility of the results. In both materials, the Raman spectrum shows peaks attributed to the O3 layered structure indicating that no phase transformation occurred within the sampling depth of the Raman method. In the case of layered LiCoO2 (which has a larger optical skin depth than LiNiO2), Raman peaks attributed to spinel Co3O4 were readily visible within three CV cycles. Chemically delithiated Li0.5NiO2 was found to form a spinel-like phase by heating at 200°C; further heating led to the formation of LiNiO2 and NiO.24 NiO, which exhibits a prominent Raman peak at ~ 1101 cm-1,25 was not observed in the Raman spectrum of LNCFO and Li0.5NCFO after cycling.


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Figure 4. RDE CVs at 10 mV s-1 and 1600 rpm in 0.1 M KOH for LNCFO and Li0.5NCFO for (a) the first cycle and (b) the third cycle (the insets show the entire potential region) and ex situ micro Raman spectra after 3 OER cycles of (c) LNCFO and (d) Li0.5NCFO.

The ex situ Raman spectra (Figures 4c and d) also show that the Li content of LNCFO is not affected by electrochemical cycling since LNCFO does not exhibit increased peak splitting after cycling. In order to investigate whether Li+ was extracted electrochemically during the OER in LNCFO, we performed CVs in a neutral pH electrolyte (Figure S2) since the OER onset shifts to higher potentials at lower pH values. These results show that at 1 mV s-1, LNCFO exhibits a large, mostly irreversible anodic peak that is attributed to electrochemical delithiation. It corresponds to the extraction of ~ 0.2 mol of Li+ from the structure. The position of the



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delithiation peak may be insensitive to the pH, and with a maximum at 0.67 V (vs. SCE), it occurs at the same time as the OER in 0.1 M KOH. On the other hand, Li0.5NCFO does not exhibit any redox peaks and only the double-layer charging current is present. Subsequent cycling in the neutral pH electrolyte of LNCFO and Li0.5NCFO shows a similar CV shape for both compounds. These results indicate that in LNCFO, (i) Li+ is extracted in situ during OER in 0.1 M KOH and (ii) that Li+ extraction in the alkaline electrolyte during OER leads to the formation of an electrochemical surface with activity different from that of Li0.5NCFO. That only ~ 0.2 Li+/LNCFO are removed at a slow sweep rate of 1 mV s-1 indicates why no peak separation is observed in the ex situ Raman results after cycling at 10 mV s-1 in 0.1 M KOH. Furthermore, the fact that Li+ does not continue to be electrochemically removed from LNCFO indicates that the material may form a surface structure that prohibits further Li+ diffusion after the first cycle. In conclusion, we investigated the effects of ex situ chemical and in situ electrochemical delithiation on the OER activity of LNCO and LNCFO. Chemically delithiated samples had a different composition, transition-metal oxidation state, local chemical environment, and morphology compared to the as-synthesized material. The as-synthesized LNCO and LNCFO undergo electrochemical delithiation during the first OER cycle. The mass-activity of assynthesized and chemically delithiated compounds is similar, but the specific activity based on the BET surface area indicates that the as-synthesized materials exhibit superior activity. Assynthesized LNCFO exhibits prominent pre-OER redox peaks that appear after the first cycle irreversible delithiation and only in an alkaline electrolyte. The in situ electrochemical delithiation during the OER leads to unique surface activity in LNCFO. The results presented here demonstrate that while tuning of the transition-metal oxidation state with delithiation for


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electrocatalysis is tantalizing, it also involves chemical, morphological, and structural changes that can also affect the OER.

ACKNOWLEDGEMENTS This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, under award number DE-SC0005397. We thank Dr. Richard Piner for assistance with Raman spectroscopy and Dr. Longjun Li for SEM imaging.

ASSOCIATED CONTENT Supporting Information. Experimental methods, CVs of LNCO and Li0.5NCO in 0.1 M KOH and, CVs of LNCFO and Li0.5NCFO in a neutral-pH electrolyte are included in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.



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Effects of Chemical versus Electrochemical Delithiation on the Oxygen

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