Heteroatom-Induced Electronic Structure Modulation of Vertically

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Heteroatom Induced Electronic Structure Modulation of Vertically Oriented Oxygen Vacancy-rich NiFe Layered Double Oxide Nanoflakes to Boost Bifunctional Catalytic Activity in Li-O2 Battery Jiabao Li, Chaozhu Shu, Zhiqun Ran, Minglu Li, Ruixin Zheng, and Jianping Long ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b08184 • Publication Date (Web): 29 Jul 2019 Downloaded from pubs.acs.org on July 29, 2019

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ACS Applied Materials & Interfaces

Heteroatom Induced Electronic Structure Modulation of Vertically Oriented Oxygen Vacancy-rich NiFe Layered Double Oxide Nanoflakes to Boost Bifunctional Catalytic Activity in Li-O2 Battery Jiabao Li, Chaozhu Shu*, Zhiqun Ran, Minglu Li, Ruixin Zheng, Jianping Long*

College of Materials and Chemistry & Chemical Engineering, Chengdu University of Technology, 1#, Dongsanlu, Erxianqiao, Chengdu 610059, Sichuan, P. R. China

KEYWORDS: Co-doped NiFe LDO, oxygen vacancy, electronic structure, free-standing electrode, Li – O2 battery.

ABSTRACT: NiFe-based transition metal oxide (NiFe-TMO) has been excavated as effective electro-catalyst for Lithium-Oxygen (Li-O2) batteries due to its superior catalytic activity for oxygen evolution reaction. Ameliorating the bifunctional catalytic ability of NiFe-TMO is essential for the further performance improvement of Li-O2 batteries. Herein, we regulated the electronic structure of free-standing NiFe LDO nanosheets array via introducing foreign Co ion to improve its bifunctional catalytic activity in Li-O2 batteries. Combining with well-designed electrode architecture and the deliberately modified surface electronic structure, this strategy markedly alleviates polarization problem in terms of low overpotential (0.98 V) and 1 ACS Paragon Plus Environment

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the discharge voltage within 110 cycles remains stable at 2.89 V without significant attenuation. This study illustrates an intimate connection between electronic structure engineering and catalytic activity optimization that is critical for rational design of Li-O2 batteries.

1. Introduction Li-O2 battery is considered as one of the most promising power sources for next generation energy storage and conversion technologies by virtue of its highest theoretical specific energy of nearly 3500 Wh kg-1 among secondary batteries1. When the cell discharges, Oxygen diffuses through the porous cathode to the surface of catalyst, losing electrons and then combining with lithium ion, resulting in the generation of insoluble lithium preoxide (Li2O2), which is called the oxygen reduction reaction (ORR)2. In turn, the oxidation of Li2O2 to O2 occurs during charge process (oxygen evolution reaction, OER). Although the Li-O2 batteries have desirable energy density, the sluggish ORR/OER kinetics is the dominant factor hampering its further application, which endows the development of efficient ORR/OER catalysts top priority3. Up to now, commercial electrocatalysts generally depend on Pt, Ru, Ir and other noble metal elements due to their superb activity4. However, the scarcity and high costs of these noble metals restrict their universal utilization in Li-O2 batteries. Therefore, the exploitation of earth-abundant, high activity and cost-effective catalysts as a substitute for noble metal catalysts is imperative. The transition metal oxide (TMO), especially the NiFe-containing transition metal oxide (NiFe-TMO)

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arouses wide concern since it simultaneously meets the demand of low cost and high catalytic activity5. The feature of the outstanding decomposition ability of Li2O2 and the synergy effect between the Ni2+and Fe3+makes it extremely appealing in the field of OER catalysis 6,7,8. Despite NiFe-TMO has attractive activity, it still suffer from the low electron conductivity, which is also a significant barrier to overcome for their further development 9. Lots of efforts have been devoted to improve NiFe-TMO conductivity, of which combining NiFe-TMO with conductive carbon matrix, such as CNT, Graphene, has been accepted as an effective approach. Unfortunately, this lagging coupling strategy will pose adverse impacts including but not limited to the production of unwanted byproducts and the reduction of active sits that caused by the introduction of carbon-based conductive agent and organic binder

10

. Therefore, it is

imminent to design an elegant cathode without carbon and polymer binder which could avoid the above mentioned disadvantages. Besides the consideration of cathode structure, the homogeneous distribution of Ni2+/Fe3+ at the atomic level is another key factor for the superior cycling performance and conductivity. The undesirable phase separation could jeopardize the interaction between active cations, resulting in unsatisfied catalytic activity. Based on above consideration, Layered double hydroxide (LDH) is an ideal precursor to prepare TMO based electrodes with uniform ion distribution. LDH has a brucite-like structure in which divalent ion (M2+) is coordinated by hydroxyl groups octahedron, and a part of M2+ is substituted by trivalence ion (M3+) to form a positively charged layer, and anion is usually inserted 3 ACS Paragon Plus Environment


between the layers11. This edge-sharing layered structure imparts LDH not only a orientation arrangement of M2+/ M3+ in the hydroxide layer which will expose a large number of active sites and regulate the active ion distribution, but also the highly tunable property that benefits to the introduction of multiple valences cations12. These attractive features will be inherited after calcinations, making LDH a promising precursor candidate for ameliorating the degree of Ni2+/Fe3+ homogeneity and electrocatalysis performance. Nevertheless, apart from these two essential factors that facilitate the achievements of high catalytic and cycling stability electrode mentioned above, the disadvantage of single OER catalytic activity still restricts NiFe-TMO application in Li-O2 batteries. So the bifunctional catalysis ability must be further taken into momentous deliberation. As is known to all, Co is a greatly potential alternative bifunctional catalysis site, which attributes to its optimal adsorption strength between oxygen intermediates and active Co cations13. Furthermore, it is proven that Ni2+/Ni3+ and Co2+/Co3+ is favorable redox couple that is helpful to provide donor–acceptor chemisorption sites for oxygen, which results in the promotion of both ORR and OER14-15. Moreover, substantial studies have confirmed that incorporating Co into NiFe-based compounds can effectively boost its OER activity because the doped Co sites with variable valence states can significantly enhance electron interaction thereby modify the electronic structure of NiFe-based compounds and meanwhile induce the generation of oxygen vacancies which could decrease the adsorption ennergy of oxygen-contained intermediate16. Hence, it is


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logically rational to deduce that the incorporation of the Co ion into NiFe-TMO is an advisable strategy to boost its bifunctional electrocatalysis activity. Inspired by the perspective discussed above, we synthesis ternary TMO, namely, CoNiFe layered double oxide (CoNiFe LDO) which vertically grows onto Ni foam (denoted as CoNiFe LDO@Ni foam) by two facile steps: CoNiFe LDH@Ni foam precursor is firstly prepared via hydrothermal method and then calcined to obtain CoNiFe LDO@Ni foam. The as prepared electrode was directly used as the cathode of the Li-O2 batteries, and the subsequent study on the free-standing cathode substantiates its impressive performance. Compared with the carbon based electrode, the battery based on CoNiFe LDO@Ni foam demonstrates a lower overpotential of 0.98 V at current density of 250 mA g-1, excellent rate performance and superior cycling stability with no attenuation of terminal discharge voltage in 110 cycles. 2. Experimental details 2.1 Synthesis of CoNiFe LDH@Ni foam precursor

CoNiFe LDH@Ni foam was synthesized by one-pot hydrothermal method as illustrated in Scheme1. The specific steps are as follows: First, Ni(NO3)2 ▪6H2O, Co(NO3)2 ▪6H2O, Fe(NO3)3 ▪9H2O (all the reagents used here were the analytical grade) were uniformly mixed in a molar ratio of 2:1:1. Afterward, urea (700 mg) and NH4F (150 mg) were added into the mixture, then the mixture were dissolved in 70 mL of distilled water and stirred until no precipitation can be found. At the same time, the nickel foam with an area of 4*4 cm square was ultrasonically cleaned in 0.5 M hydrochloric acid, deionized water and absolute ethanol in sequence. After that, the 5 ACS Paragon Plus Environment


well dispersed solution and the washed nickel foam were transferred into a stainless-steel Teflon-lined autoclave and maintained at 120 oC for 16 hours to obtain CoNiFe LDH@Ni foam. 2.2 Synthesis of CoNiFe LDO@Ni foam.

CoNiFe LDO@Ni foam was synthesized by calcinating CoNiFe LDH@Ni foam in air with a heating rate of 5 ºC min-1 and maintain at 450 ºС for 3 h before it cooled down to room temperature. 2.3 Synthesis of NiFe LDO@Ni foam

The preparation of NiFe LDO@Ni foam is analogous to that of the CoNiFe LDO@Ni foam but only that it is not necessary to add cobalt nitrate in the first step. 2.4 Structure and morphology characterization

X-ray diffraction (XRD, D/MAX-IIIC, Japan) was applied with monochromatized Cu-Kα radiation in the 2θ range of 10-80o for phase analysis. Scanning electron microscopy (SEM, JSM-6700F, Japan) is used to study the microstructure. In order to investigate the valence state, the X-ray photoelectron spectroscopy (XPS, ESZALB 250XL) is employed. The transmission electron microscope (TEM, JEOL 2100F) attached with energy dispersive X-ray spectroscopy (EDS) were performed. The pore size distribution and surface area image is studied by using nitrogen adsorption apparatus (ASAP, 2010M). The information about oxygen vacancy was obtained by Transient-state photoluminescence (PL) spectra (Hltachi F 4600) at room temperature


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(295K). UV-Vis spectroscopy (UV-vis2000) was employed here to quantitative analysis the yield of Li2O2. 2.4 Battery assembly and electrochemical Tests

The freestanding CoNiFe LDO@Ni foam was used directly as Li-O2 batteries cathode, as well as the freestanding control cathode NiFe LDO@Ni foam. The cathode was digested in aqua regia and subjected to Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES). The calculated loading mass of CoNiFe LDO and NiFe LDO on Ni foam is ~1mg cm-2. As the comparison, the CoNiFe LDO powder electrode and the super p electrode were prepared as follow: the CoNiFe LDO powder/super p powder mixed with polyvinylidene fluoride (PVDF) in a weight ratio of

9:1

under

magnetic

stirring

with

the

presence

of

the

solvent

N-methyl-2-pyrrolidinone (NMP). Then the mixture was uniformly coated on the cleaning Ni foam, and dried at 80 oC overnight. The Li-O2 batteries were assembled in glovebox which filled with high-purity Ar gas. The Li-O2 batteries consist of Li pellet anode, glass fiber separator (GF/D, Whatman) and electrolyte that composed of 1M lithium bis (trifluoromethanesulfonyl) imide (LiTFSI) dissolved in tetraethylene glycol dimethylether (TEGDEM). The exact amount of electrolyte added to the battery is 150 μL and the water level in the electrolyte is 18.2 ppm. Electrochemical measurements were carried out by using RST 5000F electrochemical workstation and LAND CT 2001A electronic load. Cyclic voltammetry (CV) tests were conducted at a scan rate of 0.1 mV s-1 with the voltage range of 2-4.5 V. The electrochemical impedance spectroscopy (EIS) analysis was carried out at the frequency range of 10 7 ACS Paragon Plus Environment


mHz and 100 kHz and at a perturbation amplitude of 5 mV. The discharge/charge curves were tested in the voltage range of 2.0-4.5 V at different current density. All electrochemical measurement results were normalized based on the total weight of the cathode catalysts. 3 Results and Discussion 3.1 Characterization of CoNiFe LDHs and LDOs

The X-ray diffraction (XRD) patterns of CoNiFe LDH and CoNiFe LDO are shown in Figure S1 and Figure 1 respectively. The diffraction peak of CoNiFe LDH is distinguished sharp, indicating satisfied crystalline. The typical LDH structure is confirmed because the domain planes of the as-prepared composite are manifested and precisely matches with that of NiFe-LDH (JCPDS no. 51-0463). Intriguingly, the perfect agreement between the Bragg reflections of CoNiFe LDH and NiFe LDH reveals the incorporation of Co ion changes the layered structure of NiFe LDH scarcely. Such feature is inherited to CoNiFe LDO. CoNiFe LDO peak (black line in Figure 1) and NiFe LDO peak (blue line in Figure 1) match well in terms of the same identical spinel-like structure , of which diffraction peak located at 35o, 43o, 63o assigning to (311), (400), (440) , respectively, of NiFe2O4 with space group of Fd-3m (PDF 74-2081). The LDO peaks are relatively weak because the introduction of Co may cause long range disorder in NiFe LDO17. Although there is a conspicuous change in major phase after LDH converting to LDO, the sheet-like morphology of LDH is retained. As revealed by the scanning electron microscopic (SEM) images, the ultra-thin CoNiFe LDO sheets vertically grow on the surface of 3D-Ni foam skeleton 8 ACS Paragon Plus Environment

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(Figure 2a and b), as well as the CoNiFe LDH (Figure 2d). The porous architecture formed by the vertically aligned LDO nanosheets can be vividly seen in Figure 2c. This unique vertical distribution of ultra-thin nanosheets with three-dimensional configuration not only exposes a large quantity of edges with open space that could be effectively used as the active ORR/OER sites for boosting reaction that are difficult on bulk materials, but also forms hierarchical porous that supply discharge product with better accommodation and build short pathway for electronic transportation and oxygen diffusion. To some extent, this hierarchical structure superiority is attributed to the combination of hexagonal shape LDH precursor and 3D Ni skeleton.

Scheme1. Schematic illustration of the preparation of CoNiFe LDO@Ni foam

Figure 1. the XRD pattern of CoNiFe LDO 9 ACS Paragon Plus Environment


Figure 2. SEM images of samples: a, c) Co doping NiFe LDO growing on Ni foam; b, d) CoNiFe LDH free-standing cathode

In addition, transmission electron microscopic (TEM) is conducted here to further demonstrates CoNiFe LDO/LDH porous structure and ultra-thin nature. As shown in Figure 3a, the CoNiFe LDH nanosheets exhibit bending, crimpling morphology due to the enormous disparity between the thickness and lateral size, which also indicates its ultrathin property. After calcinated at 450 oC, CoNiFe LDH converts to CoNiFe LDO with the characteristic of transparent sheets containing spots that are unevenly distributed on the nanosheets (Figure 3b), indicating the porosity and ultra-thin nature of CoNiFe LDO18. It is believed that the formation of the pore which can be observed in CoNiFe LDO originates from the removal of the anions and other guest molecules, such as Co32-, in the interlayer region and the collapse of the layer during the thermal treatment19. The N2 adsorption/desorption isotherm and pore size distribution curves 10 ACS Paragon Plus Environment

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of CoNiFe LDH and CoNiFe LDO are compared in Figure S2a and b. It can be seen that the CoNiFe LDO sample demonstrates type-IV hysteresis loop, indicating that there is a large number of mesopores in CoNiFe LDO which is particularly desirable as pathways for efficient ion transportation, thereby making huge contribution to improve rate capability and specific capacity of Li-O2 batteries. The BET surface area of LDO is 43.152 m² g-1, which is much higher than that of LDH (9.684 m² g-1), caused by the formation of abundant pores. The pore size distribution of LDO and LDH can be observed in the inset of Figure S2a and b respectively. The majority pore size of LDO distributes in the range of 5~15 nm, suggesting the mesoporous structure of CoNiFe LDO. The high-resolution TEM (HRTEM) images of CoNiFe LDH and CoNiFe LDO are displayed in Figure 3c and d. The lattice fringes of LDH and LDO are quite distinct from each other. Lattice spacing of 0.25 nm and 0.29 nm can be fairly indexed to the domain plane (012) of CoNiFe LDH and (422) of CoNiFe LDO, respectively. The selected area electron diffraction (SAED) patterns of CoNiFe LDH and CoNiFe LDO are displayed in Figure 3e and f, showing the transition from the hexagonal LDH to face-centered cubic spinel platelet20. Both of them exhibits six fold-like point pattern which served as a sufficient corroboration of hexagonal shape of the nanoflakes, consisting with the SEM results. This also implies that there is minimal reorganization of the Co/Ni/Fe atoms in the crystal15. Combining with the XRD results, it can be seen that the spots in SEAD diffraction pattern of LDO are sharper while the XRD peak is broader. One of the factors that contribute to this difference is different 11 ACS Paragon Plus Environment


probing directions of XRD and SAED, that is, SAED probes in-plane crystallinity only, whereas X-ray peak widths include symmetry-equivalent cross-plane reflections21. Figure S3 is the commensurable elemental mapping image of CoNiFe LDO, which demonstrates the homogeneous distribution of Ni, Fe and Co elements without any obvious segregation. These attractive properties of CoNiFe LDO are expected to favor the ORR/OER activity by virtue of the combination of ultrathin porous nanoflakes with homogeneous cations dispersion and vertically growth on 3D nickel skeleton that can increase the contact area between the electrolyte and catalytic surface, shorten the path of ion migration, conducing much to the massive exposure of active sites as well.

Figure 3. a)TEM image of CoNiFe LDH; b)TEM image of CoNiFe LDO; c, d) HRTEM images of CoNiFe LDH and CoNiFe LDO; e, f) selected area electron diffraction (SAED) images of CoNiFe LDH and CoNiFe LDO 12 ACS Paragon Plus Environment

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The X-ray photoelectron spectroscopy (XPS) were carried out to obtain deeper understanding of the catalytic activity of different LDO samples and detailed information of valence state, the corresponding spectrum is shown in Figure 4 and S4. It can be observed from Figure 4a that the Ni 2p spectrum in CoNiFe LDO@Ni foam is deconvoluted into four peaks, among which, two main peaks of Ni 2p3/2 and Ni 2p1/2 spin–orbital coupling are observed at 855.5 and 872.9 eV, and the rest are two shakeup satellites (denoted as sat.). The peaks centred at 855.0 and 872.5 eV can be ascribed to Ni2+, while those at 855.9 and 873.9 eV should be attributed to Ni3+ 22, unveiling the mixed-valence state nature of Ni in CoNiFe LDO. For the core-level Fe 2p spectra of CoNiFe LDO in Figure 4b, the peak located at 710.9 eV and 723.9 eV should be indexed to the existence of Fe3+, evidenced by the satellite peak of Fe3+ at 717.5 eV23. The Co 2p spectrum displays Co 2p3/2 and Co 2p1/2 spin–orbital coupling with biding energy of 780.1 eV and 795.1 eV, respectively, in Figure 4c. To be specific, The Co 2p3/2 peak can be ascribed to Co2+ and Co3+ with peak position at 780.6 and 782.3 eV, while the Co 2p1/2 peak can be assigned to Co2+ and Co3+ with the peak centered at 797.6 and 798.2 eV24. The multivalent state characteristic of Co was further verified by two shakeup satellite peaks located at around 782.1 and 786.2 eV between the two main peaks. The co-existence of high valence state and low valence state of Co and Ni in Co doped LDO layer is expected to demonstrate advantage in ORR because it could delivers donor–acceptor chemisorption sites for oxygen. At the same time, the multivalent state also enhances the electron interactions within the metal oxide matrix which further boosts the synergy between catalytic sites 13 ACS Paragon Plus Environment


toward OER, therefore improving the bifunctional catalytic activity25. In order to explore the influence of Co doping on the electronic structure of nickel and iron, XPS patterns of the Co doped LDO and the pristine NiFe LDO were compared (Figure 4 e and S4). Both Figure 4e and Figure S4 vividly reflect a significant negative shift in Ni/Fe spectra after the incorporation of Co, which indicates there is an increase in the electron density in Ni and Fe, leading to the formation of electron-rich structure of both Ni and Fe. The lower valence states and richer-electron-structure of Ni and Fe sites, illustrating the electron transfer between Co and Ni/Fe sites, is benefits to lower the over-potential of OER26. The binding energy evolution of O site in CoNiFe LDO is shown in Figure 4d, in which the O 1s spectra is deconvoluted into three peaks. O1 peak with binding energy of 529.2 eV can be assigned to the metal-oxygen bond in metal oxides. A typical oxygen peak (O2) in OH groups can be observed at 529.4 eV. Peak O3 located at 531.8 eV, revealing the formation of defect sites with a low oxygen coordination number in CoNiFe LDO, disappears in NiFe LDO as shown in Figure 4f. The formation of defect sites should be attributed to the increasing concentration of oxygen vacancies caused by the increase in the electron density of Ni/Fe site which needs fewer –O to coordinate with17. The existence of oxygen vacancies are expected to accelerate the interaction between oxygen-contained intermediate and active sites and thus improve the catalytic activity of CoNiFe LDO for oxygen involved reactions27. The existence of oxygen vacancy in CoNiFe LDO@Ni foam is further evidenced by the Transient-state photoluminescence (PL) spectra, which is sensitive in probing and 14 ACS Paragon Plus Environment

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detecting the concentration of vacancies. As shown in Figure 4f, the CoNiFe LDO@Ni foam exhibits not only higher luminescence intensity indicating higher concentration of oxygen vacancy but also the appearance of a peak located at ~410 nm which is caused by the recombination of holes with two-electron trapped oxygen vacancies. This result is in agreement with the above XPS analysis. Oxygen defects can lead to unsaturated coordination of the surrounding metal, which in turn enhances the delocalization of electrons thus accelerating the charge transfer and making the surface of LDO more prone to activation28. In addition, many studies have shown that oxygen vacancies can not only effectively bond oxygen and lithium peroxide, but also accelerate the transport of lithium ions, thereby achieving synergistic improvement of the ORR and OER processes and improving the overall performance of the battery29-30.



Figure 4.XPS spectra of CoNiFe LDO: a) Ni 2p; b)Fe 2p;c) Co 2p; d) O 1s XPS spectra of CoNiFe LDO; e) comparison of Fe 2p XPS spectra in CoNiFe LDO and pristine NiFe LDO; f) PL spectra of CoNiFe LDO@Ni foam and NiFe LDO

3.2 Electrochemical Performance of CoNiFe LDO in LOB

The superior architecture and surface properties of the as-prepared free-standing CoNiFe LDO encourage us to investigate its electrochemical performances as an 16 ACS Paragon Plus Environment

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oygen electrode for Li-O2 batteries. Commercial super p electrodes, NiFe LDO and CoNiFe LDO powder based electrodes are also studied for comparison. The first charge and discharge curves are shown in Figure 5a, the super p electrode exhibits a voltage platform of 4.25 V during charging process and a voltage platform of 2.63 V during discharge process at a current dnsity of 250 mA g-1. The potential gap of super p is 1.62 V, indicating its poor catalytic activity, especially the poor OER activity. By contrast, Li-O2 battery based on CoNiFe LDO elecrtode fabricated in traditional methode shows steady plateau at 2.75 V in discharge profile and 3.87 V in charge profile. The increase of discharge voltage and the decrease of charge voltage are attributed to the more effective catalytic activity of CoNiFe LDO for ORR and OER than super p. This improvement trend is more pronounced in the charge-discharge diagram of free-standing CoNiFe LDO electrode based Li-O2 battery. At the same current density, the discharge plateau of CoNiFe LDO@Ni Foam cathode is close to 2.9 V and the charge voltage is 3.68 V. Thus, charge potential of CoNiFe LDO@Ni Foam is dramaticly reduced and it possesses relatively low overpotentials of 0.78 V, demonstrating the outstanding bifuctional catalytic activity of CoNiFe LDO@Ni Foam. The decreased overpotential is partly attributed to the structural advantage of the freestanding CoNiFe LDO@Ni Foam electrode. In addition, to clarify the significance of cobalt incorparation, we compared the first charge and discharge profile of NiFe LDO@Ni Foam and CoNiFe LDO@Ni Foam under the same current density in Figure S6, it can be observed that without the introduction of Co, the discharg voltage is lower, the overpotential is higher and the specific capacity is also 17 ACS Paragon Plus Environment


smaller. Hence, such disparity of electrochemical performance leads to the reasonable conclusion that regulation of electronic structure caused by introducing Co is the vital factor for optimizing the catalytic activity for OER/ORR, thus determining the overall performance of Li-O2 battery. In order to further eliminate the contribution of the substrate (Ni foam) to the catalytic ORR/OER, thereby highlighting the excellent catalytic activity of the CoNiFe LDO@Ni foam catalyst itself, the first discharge and charge profile of Ni Foam based Li-O2 battery is presented in Figure S7. As demonstrated in Figure S7, the cell based on Ni foam electrode exhibits small discharge capacity with only 30 mAh g-1 and extremely high overpotential of 1.6 V. It can be indicated that the catalytic activity of Ni foam towards OER/ORR is very poor. The full discharge capacity tests for Li-O2 batteries based on both CoNiFe LDO and super p electrode was also performed and the result was shown in Figure S8. As is displayed in Figure S8, the CoNiFe LDO@Ni foam outperformed super p cathode in two ways: firstly, the CoNiFe LDO@Ni foam cathode exhibits a discharge platform of 2.82 V which is higher than that of super p electrode; secondly, the discharge capacity of CoNiFe LDO@Ni foam is much larger than that of super p where the former possesses discharge capacity of 7006.3 mAh g-1 and the discharge capacity of super p is only 4090.8 mAh g-1. In order to further illustrate the improved catalytic activity of CoNiFe LDO@Ni Foam cathode caused by heteroatom incorporation and structrue design, the cyclic voltammetry curves of four samples are shown in Figure 5b. The cathode peak is assigned to the oxygen reduction reaction and the anode peak is ascribed to the Li2O2 18 ACS Paragon Plus Environment

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decomposition reaction (Li2O2O2+2e-+2Li+)31. Under the same scanning rate of 1 mV s-1, the cathode peak of CoNiFe LDO @Ni Foam electrode (2.7 V, red line) is more positive than that of super p electrode (2.5 V, blue line), revealing the better ORR activity of CoNiFe LDO@Ni Foam electrode. Meanwhile, the anode peak potential of Li2O2 decomposition accurs at 3.81 V, which is the lowest value among super p ,CoNiFe LDO, NiFe LDO@Ni Foam, indicating the superb OER catalytic activity of CoNiFe LDO@Ni Foam. Furthermore, the peak current of CoNiFe LDO@Ni Foam is the highest among these four samples, further suggesting its outstanding catalytic activity. Thus, the alleviated polaration in Li-O2 system should attributed to the introduction of heteroatom Co, which reduces the activation barrier required for Li2O2 formation/decomposition, and the advanced free-standing structure that improves electron tansportation. In fact, in addition to the well designed electrode structure which can improve charge transfer, the doping of Co also ameliorates the electron electron migration kinetic by incorporating oxygen vacancy in CoNiFe LDO. The electrochemical impedance spectroscopy (EIS) results of as prepared electrodes confirm this view point. As presented in Figure S9, the semcircle of Nyquist plot in middle frequency region represents the charge transfer resistance. The smaller the diameter of the semicircle, the superior the charge transfer. Compared with CoNiFe LDO and NiFe LDO@Ni Foam, the semicircle of the CoNiFe LDO@Ni Foam is reduced significantly, indicating the fastest charge transfer, which thereby leds to an optimized catalytic activity. The huge disparity of the charge transfer rate of cobalt-doped 19 ACS Paragon Plus Environment


electrodes and undoped cobalt electrodes indicates that cobalt incorporation exerts important role in reducing the electron transfer resistance by means of tunning the electronic structure and inducing the formation of oxygen vacancy, which is in consonance with above discussed XPS results. Similar examples of enhanced electron conduction caused by oxygen vacancies have also appeared in other studies29, 32. The rate capability of CoNiFe LDO@Ni Foam cathode is further examined with a cut-off capacity of 500 mAh g−1 (Figure 5c). As the current density increases from 50 mA g-1 to 200 or even 400 mA g-1, the discharge voltage platform of CoNiFe LDO@Ni Foam based cell basically maintains above 2.9 V, which is close to the theoretical nominal battery voltage of 2.96 V33. In contrast with the rate performance of CoNiFe LDO cathode (Figure 5e) under the same condition, the free-standing CoNiFe LDO cathode shows improved performance in terms of elevated discharge voltage and decreased charge voltage. The rate capability of super p cathode is also shown in Figure 5d. When the current density increases from 50 mA g-1 to 400 mA g-1, the super p cathode shows steady discharge platform while the polarization values during charging is higher than that of troditional CoNiFe LDO cathode, unveiling the intrinsic superior ability of CoNiFe LDO to relieving the polarization poblem. In addition, the overpotential of NiFe LDO@Ni Foam is the highest among all samples as shown in Figure 5f. This unsatifactory performance is another concrete evidence that the introdution of Co, which creates optimal electronic structure and oxygen defect to adsorb intermediate species, is significant for improving slugguish ORR/OER kinetics. 20 ACS Paragon Plus Environment

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Figure 5.a) first discharge and charge profiles of CoNiFe LDO@Ni foam, CoNiFe LDO and super p cathodes; b) the CV curve of free-standing CoNiFe LDO@Ni foam, CoNiFe LDO, NiFe LDO@Ni Foam, super p; c) the rate capacity of CoNiFe LDO@Ni Foam; d) the rate capacity of super p; e) CoNiFe LDO rate performance; f) NiFe LDO@Ni Foam rate capacity; g) the cycling performance of CoNiFe LDO@Ni Foam, at a current density of 500 mA g−1 with a limited capacity of 1000 mAh g−1; h) the cycling profile of super p at a current density of 500 mA g−1 with a limited capacity of 1000 mAh g−1; i) Comparison of the cycling stability of CoNiFe LDO@Ni foam and super p.

The cycling performance is another important indicator for practical applications. The long-term stability of CoNiFe LDO@Ni Foam and commercial super p electrode was tested at 500 mA g-1 with limited capacity of 1000 mAh g-1. The charge and discharge profiles of CoNiFe LDO@Ni foam cathode and super p electrode at different cycles were shown in Figure 5g and h. It can be seen that the discharge platform of the cell catalyzed by CoNiFe LDO@Ni foam maintains above 2.8 V within 110 cycles and the overpotential is only 0.87 V at 110 cycles. When cycled to 90 cycles, the overpotential of battery based on super p electrode has increased to 1.56 V and the capacity decay occurs around the 80th lap. As depicted in Figure 5i, the terminal voltage of CoNiFe LDO@Ni Foam maintains stably at around 2.89 V in 110 cycles. In contrast, the super p electrode demonstrates a lower discharge terminal voltage than CoNiFe LDO@Ni Foam in initial stage and the voltage is continuously attenuated to about 2 V as the cycle number increasing to 90. 22 ACS Paragon Plus Environment

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To further elaborate the improvement of electrocatalytic activity and the excellent cycling stability of CoNiFe LDO, the charge and discharge products of Li-O2 batteries based on different cathodes were analyezed. The ex situ SEM of super p and CoNiFe LDO@Ni Foam cathode under different charge/discharge stage are presented in Figure 6. After being discharged to 2 V at current density of 250 mA g-1, the super p electrode was overwhelmingly coated by toroid-like particles as presented in figure 6b, part of which still exist after charging to 4.5 V (Figure 6c). By contrast, the CoNiFe LDO@Ni Foam electrode exhibites better cycling performance under same condtion. As revealed in Figure 6e, the film-like discharge product is loosely covered on the vertical nanosheet array or filled in the pores, thus the edge of the nanosheet with high catalytic activity can still be observed faintly. As shown in Figure 6f, after charging to 4.5 V, the film-like product disappeared completely, and the edges of the CoNiFe LDO nanosheets were fully recovered and exposed. Unlike super p, of which active sites were flooded by small toroid-like particle, the film-like discharge product of CoNiFe LDO@Ni Foam sparsely distributed on the edges or in the pore guaranteeing effective exposure of catalytically active sites and intimate contact between the product and the active surface, which resulting in superior cycling performance. The outstanding decomposition ability of discharge product on CoNiFe LDO@Ni Foam motivates us to characterize the chemical composition of the discharge product. Here, Raman spectroscopic study on both super p and CoNiFe LDO@Ni Foam was carried out. The two Raman bands located at 1352 cm-1, 1580 cm-1 in Figure 7a can be assigned to super p34. The peak at 788 cm-1 suggestes the 23 ACS Paragon Plus Environment


formation of Li2O2 after discharging, which is commonly accepted as typical discharge product of Li-O2 battery35. Meanwhile, the characteristic peak of Li2CO3, which possesses poor conductivity, appears at 743 cm-1 and 1088 cm-1 indicating the parasitic reaction occurs during discharging36. Both Li2O2 and Li2CO3 Raman signals remain after charging, implying unqualified discharge product decomposition ability of super p, which is consistant with the XRD results in Figure S10a, where peaks assigned to both Li2O2 and Li2CO3 can still be observed after being charged to 4.5 V. Neverthless, in Figure 7b, it is found that only the Raman band at 788 cm-1 assigning to Li2O2 can be observed after discharging on CoNiFe LDO@Ni Foam, which disappears in charge process. The corresponding XRD pattern shows no obvious signal of Li2O2 peak except the characteristic peaks of CoNiFe LDO (Figure S10b). Furthermore, the XPS results of Li1s and C1s after CoNiFe LDO@Ni foam based cell discharged and charged were also presented in Figure 7c and Figure 7d. As Figure 7c displayed, the character peak sited at 54.7 eV should attributed to the Li-O bond in Li2O2 which disappeared after charging37, demonstrating a reversible process of generating/decomposing Li2O2. The only peak in Figure 7d with binding energy of 284.6 eV which is associated with C=C further confirms the absence of Li2CO3 during discharging since that the character signal of Li2CO3 usually locates at 289.8 eV. Combining Raman, XPS and XRD results, it is sensible to speculate that the discharge product on CoNiFe LDO@Ni Foam cathode is amorphous Li2O2. Accroding to previous study, the film-like amorphous Li2O2 is more desirable than crystalline Li2O2 due to its more optimistic electrochemical decomposition kinetics 24 ACS Paragon Plus Environment

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which should be ascribed to improved charge transport.33 This result is in consistence with the ultra-low OER overpotential of CoNiFe LDO@Ni Foam cathode. In addition, EIS was conducted to study the charge transportation of CoNiFe LDO@Ni Foam and super p cathode at different charge/discharge state. As presented in Figure S11a, the diameter of semi circle at middle frequency region for super p electrode is almost twice that of the CoNiFe LDO@Ni Foam electrode after discharge, demonstrating huge charge transfer resisitance of super p cathode after discharging, which may be ascribed to the generation of insulating byproduct Li2CO3 and the crystalline Li2O2. The superior cycling stability of CoNiFe LDO@Ni Foam based Li-O2 battery is further comfirmed by the substantial recovery of the semicircular diameter during charge in Figure S11b. The UV-Vis spectroscopy combined with TiOSO4 titration was performed here to quantitative analysis the yield of Li2O2. When the discharged electrode was immersed into TiOSO4 solution, Li2O2 reacts with H2O immediately forming H2O2. The formed H2O2 then reacts with TiOSO4 via the following reaction: TiOSO4+H2O2+H2SO4→ H2[Ti(O2)(SO4)2]+H2O. The color of titration solution changes from transparent to yellow or orange as the reaction proceeding, which can be ascribed to the generation of H2[Ti(O2)(SO4)2]38. The color change of the titration solution makes TiOSO4/H2SO4 solution a sensitive system to detect H2O2 generated from Li2O2. The UV-Vis test can be used to quantitatively analyze the amount of Li2O2 based on the linear relationship between absorbance and discharge capacity. As depicted in the inset of Figure 7e, the color of the titration solution becomes darker when the 25 ACS Paragon Plus Environment


discharge capacity grows from 1 mAh g-1 to 2.5 mAh g-1 and the absorbance of each sample also increases to a high value. The corresponding Lambert-Beer plot which normalizes the extinction of TiOSO4 under different state to the theoretical discharge capacity is presented in Figure 7f, according to which we can quantify the discharge product Li2O2 on the cathode. By comparing the obtained weight of Li2O2 from UV-Vis result with the theoretical weight that calculated based on the discharge capacity, the yield of the Li2O2 was then obtained and summarized in Table S1. As discharge capacity increased, the yield of Li2O2 was maintained at ~80%, which further indicates that the major discharge product of CoNiFe LDO based Li-O2 battery is Li2O2. Furthermore, the photograph pictures of CoNiFe LDO@Ni foam electrode and super p electrode immersed in TiOSO4 solution were displayed in Figure S12 for comparison. The more the Li2O2 generated, the darker the solution is. When discharge capacity increase from 1 mAh g-1 to 2.5 mAh g-1, the color of the solution containing both CoNiFe LDO@Ni foam and super p electrode grows darker but the solution color containing super p is lighter than that containing CoNiFe LDO@Ni foam, demonstrating the yield of Li2O2 is lower on super p cathode which may be due to the formation of byproduct instead of Li2O2 on super p electrode.


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Figure 6. The SEM image of super p: a) pristine super p; b) after discharge to 2 V; c) after charge to 4.5 V; the SEM image of CoNiFe LDO: d) pristine LDO; e) after discharge to 2 V; f) after charge to 4.5 V.



Figure7. a) the Raman spectrum of super p after charge and discharge; b) the Raman spectrum of CoNiFe LDO after charge and discharge; c) the XPS pattern of CoNiFe LDO@Ni foam after charge and discharge; d) the XPS pattern of CoNiFe LDO after discharge; e) the UV-Vis spectra of titration solution at different discharge capacity and the corresponding photograph; f) the Lambert-Beer data plot of CoNiFe LDO@Ni faom cathode.

Based on the above results, several appealing features of CoNiFe LDO@Ni Foam electrode that benifits to the large capacity and low overpotential are manifested as 28 ACS Paragon Plus Environment

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follow: (1) the deliberate design of electrode structure. The ultrathin mesoporous CoNiFe LDO nanosheets vertically grow on the porous structure of the Ni foam, which constitutes a large number of microreactor with plenty of CoNiFe LDO nanosheet’s edges, ensuring the perfect infiltration of electrolyte and oxygen to each reactors. This well designd architecture also facilitates the fast electronic transport between current collector and electrocatalytic materials as well as guarantee sufficient exposure of active sits to some extent. (2) the introduction of heteroatom Co results in the reduced overpotentials and improved capacity during cycling. This desirable property can be attributed to favorable donor–acceptor chemisorption sites generated by Ni2+/Ni3+ and Co2+/Co3+, which therefore motivate the catalysis kinetics. In addition, the evolution of electronic structure at Ni/Fe site also accelerates the catalytic reaction significantly by reducing the resistance. It should be noted that the existence of oxygen vacancies originating from Co incorporation also play important role in enhancing the electron interactions between oxygen and active sites, hence improving the intrinsic catalysis activity of CoNiFe LDO. 4.Conclusion In summary, a high-performance cathode has been developed for Li-O2 battery by introducting Co into NiFe LDO nanosheets. The Co dopant can effectively modulate electronic structure of Ni/Fe site thereby boosting the bi-functional catalytic activity of CoNiFe LDO. At the same time, the well designed electrode structure with ultra-thin porous nanosheets grown upright on the 3D porous foam nickel skeleton endows the cathode with good cycle stability. The superiority of this deliberately 29 ACS Paragon Plus Environment


designed electrode was further validated by significantly reduced overpotential at different current density (only 0.5 V at 50 mA g-1) and improved cycle stability (over 110 cycles). This heteroatom incorporation strategy has emerged as an effective approach to address the issues such as large polarization and limited capacity in Li-O2 batteries. Therefore, this work shed light on the rational design of electrode materials for Li-O2 batteries. ASSOCIATED CONTENT

Supporting Information. Supplementary materials including N2 sorption isotherms and the pore size distribution curve (inset) of CoNiFe LDO and CoNiFe LDH, EDS picture of CoNiFe LDO, XPS pattern of O1s spectrum in NiFe LDO, First charge discharge profile of super p, CoNiFe LDO, CoNiFe LDO@Ni foam, NiFe LDO@Ni foam, electrochemical impedance spectroscopy of CoNiFe LDO@Ni foam, CoNiFe LDO, NiFe LDO@Ni Foam, electrochemical impedance spectroscopy of super p after charge and discharge and CoNiFe LDO after charge and discharge.

AUTHOR INFORMATION Corresponding Author * Email: [email protected] (Chaozhu Shu)

* Email: [email protected]. (Jianping Long) Notes There are no conflicts to declare.


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ACKNOWLEDGMENT

This work was financially supported by Science and Technology Department of Sichuan Province (Grant No. 2019YJ0503) and the Cultivating Program of Chengdu University of Technology Middle-Aged Key Teachers (KYGG201709).

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Well-Dispersed

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Heteroatom-Induced Electronic Structure Modulation of Vertically

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