English

• Lithium-oxygen (Li-O₂) batteries now have captured a world-wide attention due to the extremely high theoretical energy density of 3 505 Wh·kg^-1, showing promising applications in electric vehicles^[1-10]. In nona-queous Li-O₂ batteries, during discharge, oxygen is reduced to O^2- which reacts with Li⁺ to form Li₂O₂ at cathode. Upon the subsequent recharge, Li₂O₂ can be electrochemically decomposed to release oxygen and Li^{+ [4]}. Unlike the shuttle mechanism in Li-ion batteries, the deposition of insulating/insoluble Li₂O₂ will cause sluggish oxygen reduction/evolution reaction (ORR/OER) kinetics, leading to high polarization, low yieldable capacity and poor cycling stability^[11-12]. It is widely acknowledged that the electrode kinetics can be enhanced by using efficient catalysts. Besides the components, the architecture of catalytic electrode should also be specially designed to adapt the Li₂O₂ deposition without drastically changing the structure of the electrodes^[13-17].

  Carbon materials have been widely used as the catalysts for Li-O₂ batteries due to the low cost, high electronic conductivity, light weight and easy mani-pulation of the porosity^[18-23]. However, carbon materials suffer from decomposition chemically or electrochemi-cally in contact with Li₂O₂ or LiO₂^[24-25]. In addition, the catalytic performance of carbon materials for OER is not satisfactory. Although noble metals provide the best catalytic activity for both ORR and OER^[26-33], the high cost limits their large-scale applications. Transi-tion metals oxides, such as MnO₂, Co₃O₄, NiO, and CoOOH, are suitable due to low cost, structural stability and relatively high catalytic activity for ORR/OER^[34-42]. Thus, a compromise can be made by combining transition metals oxides and noble metals. However, unlike carbon materials, it is difficult to allocate free space for Li₂O₂ deposition in oxides.

  Array-type electrode prepared by direct growth route is desirable since the voids between the arrays could be used to store Li₂O₂^{[37, 41, 43-46]}. Cui et al.^[37] first report a Co₃O₄-array-catalyzed Li-O₂ cell with low polarization and high capacity. The work by Chang et al.^[43] show that Li-O₂ cell could sustain stable cycling up to 300 times at 500 mAh·g^-1 when using a carbon/binder-free RuO_x /TiN nanotube arrays cathode. Recent report by Liu et al.^[44] show that Li-O₂ batteries with TiO₂-array cathode demonstrate long cycle life, superior rate capability, high round-trip efficiency, and good recoverability of the catalytic electrode. For the array-type electrode, realizing conformal growth of Li₂O₂ on the arrays is desirable to retain the voids between the arrays and keep intimate contact between Li₂O₂ and catalyst. Nevertheless, as previously reported, conformal growth of insulating Li₂O₂ will easily deactivate the catalyst with low capacity due to blocked electron transport^[47-50].

  In this work, we design a unique core-shell Co₃O₄@δ-MnO₂/Pt arrays-type electrode on nickel foam by a controllable, facile route as binder-free electrodes for Li-O₂ cells, where the Co₃O₄ acts as the "core" and ultrathin δ-MnO₂ nanosheets with Pt as the "shell". MnO₂ was selected because of its high catalytic activity for ORR/OER^{[34-36, 51]}. Co₃O₄ acts both as the catalytically active component and as the substrate for MnO₂ deposition. The decoration of Pt not only enhances the catalytic activity but also guides conformal growth of Li₂O₂. The advantages of this electrode design include: (1) the array-type structure facilitates the electrode wetting by electrolyte and O₂ transport and supplies large room for Li₂O₂ loading; (2) the conformal growth of thin-layered Li₂O₂ on Co₃O₄@δ-MnO₂/Pt enables its easy decomposition upon charge; (3) the porous structure of δ-MnO₂ assures high Li₂O₂ loading despite the conformal growth mode; (4) the side reactions related to binder and conductive agent are totally precluded or largely reduced due to binder- and conductive-agent-free electrode configura-tion. As a result, Li-O₂ cells with core-shell Co₃O₄@δ-MnO₂/Pt arrays exhibit high capacity and long cycle life.

  1.   Experimental

  1.1   Electrode preparation

  Co₃O₄@δ-MnO₂/Pt arrays-type electrodes were synthesized by three steps. First, Co₃O₄ nanowire arrays on Ni foam substrate were prepared by a facile hydrothermal route. Co(NO₃)₂·6H₂O (1.2 mmol), NH₄F (1.2 mmol), and urea (3 mmol) were dissolved into 50 mL of deionized (DI) water under vigorous stirring. The solution was then transferred into a Teflon-lined stainless steel autoclave with a piece of Ni foam immersed. The autoclave was sealed and heated in an electric oven at 120 ℃ for 5 h. After being cooled to room temperature, the Ni foam piece with a pink deposit was collected, washed with DI water and absolute ethanol several times, and dried at 60 ℃ in air overnight. The product was further heated at 400 ℃ for 2 h in air to obtain Ni-supported Co₃O₄ (Co₃O₄/Ni). For the growth of δ-MnO₂ in the second step, a piece of Co₃O₄/Ni was first immersed into a 0.04 mol·L^-1 aqueous solution of glucose for 24 h, followed by carbonization at 450 ℃ in Ar for 2 h. After that, 80 mg KMnO₄ (99.5%, Sinopharm Chemical Reagent Co., Ltd.) was dissolved in 60 mL of DI water with vigorous stirring to form a homogeneous solution. The carbon modified Co₃O₄/Ni was soaked in the above solution for 1.5 h. Afterwards, the mixture was transferred into a Teflon-lined stainless steel autoclave and heated at 85 ℃ for 2.5 h. After cooling down to room tempera-ture naturally, the Ni foam with a brown deposit was collected and washed repeatedly with DI water and absolute ethanol. The electrode was then dried at 60 ℃ in air overnight followed by heating at 300 ℃ in Ar for 2 h to obtain Ni-supported core-shell Co₃O₄/δ-MnO₂. Finally, for the platinum deposition, H₂PtCl₆·6H₂O was dispersed in DI water at a concentration of 0.24 mg·mL^-1 under stirring, a piece of Ni-supported Co₃O₄@δ-MnO₂ was soaked in the above solution overnight. The electrode was then dried at 60 ℃ in air for 5 h followed by heating at 300 ℃ in Ar for 2 h to get Ni-supported Co₃O₄@δ-MnO₂/Pt. The total loading of Co₃O₄@δ-MnO₂/Pt on Ni substrate is around 1.4 mg·cm^-2 and Pt loading is around 0.2 mg·cm^-2.

  1.2   Electrode characterization

  X-ray diffraction (XRD) patterns of the electrodes were acquired using a Rigaku D/Max-2550pc powder diffractometer equipped with Cu Kα radiation (λ=0.154 1 nm). The operating voltage and current were 40 kV and 250 mA, respectively, and 2θ=10°~80°. The morphologies of the pristine and cycled electrodes were observed by field-emission scanning electron microscopy (SEM) using an S-4800 microscope (Hitachi, Japan) at an accelerating voltage of 5 kV. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) were conducted on a JEM 2100F microscope at an accelerating voltage of 200 kV. X-ray photoelectron spectra (XPS) of the discharged and charged electrodes were collected on a KRATOS AXIS ULTRA-DLD spectrometer with Al Kα radiation (hν=1 486.6 eV). To analyze the cycled elec-trodes by SEM, TEM and XPS, the electrodes or electrode components were carefully handled before the various ex-situ characterizations^[52].

  1.3   Electrochemical measurements

  Coin-type Li-O₂ cells were assembled in the Ar-filled glovebox using lithium foil as the anode, Ni-supported Co₃O₄@δ-MnO₂/Pt as the cathode, and Celgard C480 membrane as the separator. The electro-lyte was 1 mol·L^-1 LiClO₄ (≥99.99%, Sigma-Aldrich) in tetraethylene glycol dimethyl ether (TEGDME). The cathodes were dried at 80 ℃ in vacuum overnight before cell assembly. The assembled cells were purged with pure O₂ for 20 min and stayed at open voltage circuit (OCV) for 5 h before the electrochemical tests. Charge and discharge cycling was performed on a Neware battery cycler (Shenzhen, China) in a voltage window of 2.0~4.5 V (vs Li/Li⁺). The specific capacity (mAh·g^-1) and current density (mA·g^-1) of the cells were calculated based on the weight of Co₃O₄@δ-MnO₂/Pt. For the cells using Co₃O₄ and Co₃O₄@δ-MnO₂ catalysts, the specific capacity and current density were calculated based on the weight of Co₃O₄ and Co₃O₄@δ-MnO₂, respectively. Electrochemical imped-ance spectroscopy (EIS) measurements were conducted on the VersaSTAT3 electrochemistry workstation by applying an AC signal of 5 mV amplitude over the frequency range 10^-2 to 10⁵ Hz. All of the electro-chemical tests were performed at 25 ℃.

  2.   Results and discussion

  Fig. 1a and the enlarged view (Fig. 1b) show the XRD patterns of Co₃O₄ and Co₃O₄@δ-MnO₂/Pt supported on Ni foam. The three strong diffraction peaks at 44.5°, 51.8°, and 76.4° (2θ) are from the nickel subs-trate, and the presence of Co₃O₄ and Co₃O₄@δ-MnO₂/Pt is confirmed by XRD patterns in Fig. 1b, while the presence of Pt is not detected in XRD due to the low content. The XRD shows that the MnO₂ is δ-MnO₂. In order to further confirm the existence of Pt, XPS analysis was performed. The survey spectrum in Fig. 1c reveals the presence of the expected O, Co, Mn and Pt elements. In Fig. 1d, the bands at 69.3 and 72.7 eV correspond to the binding energy of Pt4f_7/2 and Pt4f_5/2, respectively^[53]. In Fig.S1a, the peaks at 778.6 and 794.1 eV are the Co2p spectra of Co³⁺, and the peaks at 780.3 and 796.0 eV are Co2p of Co^{2+ [54]}. The bands at 639.8 and 651.4 eV (Fig.S1b) correspond to the binding energy of Mn2p_3/2 and Mn2p_1/2, in good accordance with the previous report^[55].

  图 1

  

  图 1  (a) XRD patterns of Co₃O₄ and Co₃O₄@δ-MnO₂/Pt on Ni foam; (b) Enlarged view in (a); (c) XPS survey spectrum and (d) Pt4f XPS spectrum of Co₃O₄@δ-MnO₂/Pt

  Figure 1.  (a) XRD patterns of Co₃O₄ and Co₃O₄@δ-MnO₂/Pt on Ni foam; (b) Enlarged view in (a); (c) XPS survey spectrum and (d) Pt4f XPS spectrum of Co₃O₄@δ-MnO₂/Pt

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  The morphology and structural features of the prepared electrodes were characterized by SEM and TEM. Fig. 2 shows the SEM images of Co₃O₄, Co₃O₄@δ-MnO₂, and Co₃O₄@δ-MnO₂/Pt at different magnifica-tions. Note that Co₃O₄ nanowires grow uniformly on the skeletons of the Ni foam. The Co₃O₄ nanowires have a diameter below 200 nm and a length of several microns (Fig. 2(a, b)). After the MnO₂ growth, thin MnO₂ nanosheets are uniformly covered on the whole surface of the Co₃O₄ nanowires, forming a core-shell porous structure as shown in Fig. 2(c, d). As seen in Fig. 2(e, f), the introduction of Pt does not change the morphology of core-shell Co₃O₄@δ-MnO₂ obviously with the array structure maintained. The pores in Pt/δ-MnO₂ and the voids between the arrays are beneficial to the electr-olyte infiltration and oxygen gas transportation and provide the space for Li₂O₂ deposition. The Co₃O₄@δ-MnO₂/Pt was further characterized by TEM, HRTEM and energy dispersive X-ray spectroscopy (EDS) mapping as shown in Fig. 3. The results confirm that Co₃O₄, δ-MnO₂ and Pt construct a uniform electrode, where Co₃O₄ exhibits a polycrystalline nanowire struc-ture, δ-MnO₂ has a sheet-like shape and Pd nanocry-stals have a small size below 20 nm. In Fig. 3f, the lattice spacings of 0.46, 0.22 and 0.21 nm correspond to the planes of Co₃O₄ (111), Pt (111) and δ-MnO₂ (112), respectively. The above characterizations confirm the formation of Co₃O₄@δ-MnO₂/Pt.

  图 2

  

  图 2  SEM images of (a, b) Co₃O₄, (c, d) Co₃O₄@δ-MnO₂, and (e, f) Co₃O₄@δ-MnO₂/Pt

  Figure 2.  SEM images of (a, b) Co₃O₄, (c, d) Co₃O₄@δ-MnO₂, and (e, f) Co₃O₄@δ-MnO₂/Pt

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  图 3

  

  图 3  (a) Dark-field TEM image, (b~e) EDS mappings in the selected area and (f) HRTEM images of Co₃O₄@δ-MnO₂/Pt

  Figure 3.  (a) Dark-field TEM image, (b~e) EDS mappings in the selected area and (f) HRTEM images of Co₃O₄@δ-MnO₂/Pt

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  The electrocatalytic activity of Co₃O₄@δ-MnO₂/Pt was investigated in Li-O₂ cells and compared with that of Co₃O₄ and Co₃O₄@δ-MnO₂. The current density and specific capacity were calculated by the total mass of catalyst on the Ni substrate. The cycling performance of the Co₃O₄, Co₃O₄@δ-MnO₂ and Co₃O₄@δ-MnO₂/Pt-catalyzed Li-O₂ cells was evaluated by galvanostatic cycling at 200 mA·g^-1 between 2.0~4.5 V with a limited capacity of 500 mAh·g^-1. As shown in Fig. 4(a, b), the cell with Co₃O₄@δ-MnO₂/Pt catalyst can sustain stable cycling for 65 cycles. While for the cells with Co₃O₄ and Co₃O₄@δ-MnO₂ catalytic cathodes, the cycling can last only 10 cycles and 28 cycles, respectively (Fig. 4a and Fig.S2). To demonstrate the high catalytic activity of Co₃O₄@δ-MnO₂/Pt, the cell was also cycled at a higher limited capacity of 1 000 mAh·g^-1 at a current density of 200 mA·g^-1. In this case, the stable cycling can still last 32 cycles (Fig.S3). It should noted that the electrode has a relatively high catalyst loading of about 1.4 mg·cm^-2. The cycling performance of the cell is better than or comparable with those with similar catalysts and same limited capacity^[56-57]. The better cycle performance of the Li-O₂ cell with Co₃O₄@ δ-MnO₂/Pt catalyst can be attributed to the unique microstructure and components of the cathode. Pt nanoparticles are highly efficient in catalyzing the ORR/OER in the air cathode^[58], leading to enhanced formation/decomposition of Li₂O₂ and thereby long cycle life. In addition, the binder-free electrode design also contributes to good cycling stability of the cell with Co₃O₄@δ-MnO₂/Pt catalyst. Of note is that, the cell shows performance degradation after 60 cycles. This may result from factors such as decomposition of electrolyte or Li corrosion. A previous report show that Pt shows no selectivity in its catalytic activity toward Li₂O₂ oxidation reaction and electrolyte decomposition^[31], which is also partly responsible for the limited cycle life.

  图 4

  

  图 4  (a) Terminal voltages of Co₃O₄, Co₃O₄@δ-MnO₂ and Co₃O₄@δ-MnO₂/Pt-catalyzed Li-O₂ cells at a limited capacity of 500 mAh·g^-1; (b) Voltage profiles of Co₃O₄@δ-MnO₂/Pt-catalyzed Li-O₂ cell; Discharge profiles of (c) Co₃O₄@δ-MnO₂ and (d) Co₃O₄@δ-MnO₂/Pt-catalyzed Li-O₂ cells at various current densities Cells were discharged and charged to 500 mAh·g^-1 at 200 mA·g^-1

  Figure 4.  (a) Terminal voltages of Co₃O₄, Co₃O₄@δ-MnO₂ and Co₃O₄@δ-MnO₂/Pt-catalyzed Li-O₂ cells at a limited capacity of 500 mAh·g^-1; (b) Voltage profiles of Co₃O₄@δ-MnO₂/Pt-catalyzed Li-O₂ cell; Discharge profiles of (c) Co₃O₄@δ-MnO₂ and (d) Co₃O₄@δ-MnO₂/Pt-catalyzed Li-O₂ cells at various current densities Cells were discharged and charged to 500 mAh·g^-1 at 200 mA·g^-1

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  Fig. 4(c, d) show the discharge profiles of the Co₃O₄ @δ-MnO₂ and Co₃O₄@δ-MnO₂/Pt-catalyzed Li-O₂ cells at various current densities. In the figures, E_rev is the reversible potential of Li-O₂ cells, namely, 2.96 V vs Li/Li⁺. At 100 mA·g^-1, the cell with Co₃O₄@δ-MnO₂/Pt yields a capacity of 2 480 mAh·g^-1 and exhibits a high discharge plateau of 2.71 V. When the current density increases from 100 to 800 mA·g^-1, the discharge capacity and plateaus decrease gradually. But even at 800 mA·g^-1, the cell could still deliver a capacity of 492 mAh·g^-1 due to the excellent catalytic activity of Co₃O₄@δ-MnO₂/Pt for ORR. In comparison, the cell with Co₃O₄@δ-MnO₂ catalyst shows a lower capacity of 1 693 mAh·g^-1 and a lower discharge plateau of 2.59 V at 100 mA·g^-1. The discharge capacity decreases rapidly to 83 mAh·g^-1 when the current density increases to 800 mA·g^-1. The low capacity of the Co₃O₄@δ-MnO₂-catalyzed cell indicates its poor ORR catalytic activity. The obviously enha-nced catalytic activity of Co₃O₄@δ-MnO₂/Pt is closely related to the introduction of Pt. It is expected that the presence of Pt will alter the crystallization behavior of Li₂O₂ by supplying catalytically active sites.

  To clarify the superior catalytic activity of Co₃O₄ @δ-MnO₂/Pt and the role that it plays in increasing the cycle performance of the cell, the electrodes after discharge were observed by SEM and TEM. The loading of the SEM/TEM holders to the chamber was finished as soon as possible to minimize the exposure of the samples to air. As shown in Fig. 5(a, b), no large Li₂O₂ particles can be found on the discharged Co₃O₄ @δ-MnO₂/Pt electrode, and the pristine array-type structure of Co₃O₄@δ-MnO₂/Pt was generally preserved after discharge to 500 mAh·g^-1. To reveal the growth position of Li₂O₂ on the electrode, the morphologies were observed and compared to the original Co₃O₄@δ-MnO₂/Pt. In the discharged electrodes, it seems that the Li₂O₂ forms inside the porous δ-MnO₂ and a fluffy substance grows conformally along the surface of the Co₃O₄@δ-MnO₂/Pt arrays, where the original array structure with voids is clearly visible. Namely, the pores in the pristine Co₃O₄@δ-MnO₂/Pt were filled by the Li₂O₂. This form of Li₂O₂ is usually rich in defects and poorly crystallized and thus easily to be decomp-osed^[59]. The fluffy substance is further characterized by TEM (Fig. 5c) and is confirmed to be Li₂O₂ by selected area electron diffraction (SAED, Fig. 5d). From these results, it can be concluded that Co₃O₄@δ-MnO₂/Pt arrays catalyzes the conformal growth of thin-layered Li₂O₂ along the surface of electrode.

  图 5

  

  图 5  (a, b) SEM images, (c) TEM image and (d) SAED pattern of Co₃O₄@δ-MnO₂/Pt after the first discharge; (e, f) SEM images of Co₃O₄@δ-MnO₂/Pt after the first charge

  Figure 5.  (a, b) SEM images, (c) TEM image and (d) SAED pattern of Co₃O₄@δ-MnO₂/Pt after the first discharge; (e, f) SEM images of Co₃O₄@δ-MnO₂/Pt after the first charge

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  For the Co₃O₄@δ-MnO₂/Pt electrode, after recharge, the porous structure is visible again, indi-cative of sufficient decomposition of the discharge product (Fig. 5e). Besides, the array-type structure of Co₃O₄@δ-MnO₂/Pt remains intact (Fig. 5f). The rever-sible formation/decomposition of Li₂O₂ is confirmed by Li1s XPS (Fig. 6a) and further supported by EIS mea-surements (Fig. 6b and Table 1). In the Nyquist plots, the fitted curves are obtained by using the equivalent circuit in the inset, where R_e represents ohm resi-stance of cell components, R_f and Q₁ represent surface film resistance and relaxation capacitance, R_ct and Q₂ correspond to the charge transfer resistance and double-layer capacitance, and Z_w is associated with the bulk diffusion of Li ions. In the table, Y is admittance response of constant phase element Q₁ and Q₂ and n is index of the angular frequency^[60]. The remarkable increase in R_ct (from 331.9 to 767.1 Ω) after discharge indicates the deposition of insulating Li₂O₂ which passivates the electrode, whereas the decrease in R_ct (from 767.1 to 301.6 Ω) denotes the sufficient removal of Li₂O₂ after recharge. From these results, it can be seen that the Ni-supported Co₃O₄@δ-MnO₂/Pt electrode is highly efficient in catalyzing ORR/OER by controlling the Li₂O₂ growth, which can explain the high discharge capacity and long cycle life of the cells.

  表 1

  

  表 1  Fitting results of the Nyquist plots using the equivalent circuit

  Table 1.  Fitting results of the Nyquist plots using the equivalent circuit

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  Sample	Re / Ω	Rf / Ω	Q1		Rct / Ω	Q2
  			Y	n		Y	n
  Initial	81.9	135.0	3.4×10-5	0.73	331.9	1.8×10-5	0.84
  After discharge	89.1	178.1	3.0×10-6	0.61	767.1	3.1×10-6	0.89
  After recharge	114.8	162.8	6.7×10-5	0.28	301.6	5.9×10-6	0.92

  图 6

  

  图 6  (a) Li1s XPS after the first cycle and (b) Nyquist plots and corresponding fittings of Li-O₂ cells with Co₃O₄@δ-MnO₂/Pt electrode at the initial state and discharged/recharged to 2.2 V/4.3 V

  Figure 6.  (a) Li1s XPS after the first cycle and (b) Nyquist plots and corresponding fittings of Li-O₂ cells with Co₃O₄@δ-MnO₂/Pt electrode at the initial state and discharged/recharged to 2.2 V/4.3 V

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  3.   Conclusions

  In summary, we propose a unique design of a core-shell Co₃O₄@δ-MnO₂/Pt arrays-type electrode with a controllable, facile route. In this design, the array-type structure facilitates the electrode wetting and oxygen gas transport and supplies free volume for Li₂O₂ loading. The presence of Pt supplies the catalytically active centers and guides the conformal growth of thin-layered Li₂O₂. The conformal, thin-layered growth of Li₂O₂ on Co₃O₄@δ-MnO₂/Pt enables its easy decomposition upon charge. The porous structure of δ-MnO₂ makes high Li₂O₂ loading possible even though it has a conformal growth mode. As a result, Li-O₂ cell catalyzed by Co₃O₄@δ-MnO₂/Pt arrays delivers high discharge capacity (2 480 mAh·g^-1 at 100 mA·g^-1) and shows good cycling stability (65 cycles at 200 mA·g^-1 with a limited capacity of 500 mAh·g^-1). This work provides a new design of efficient catalytic cathode for Li-O₂ cells.

  Supporting information is available at http://www.wjhxxb.cn

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  Figure 1  (a) XRD patterns of Co₃O₄ and Co₃O₄@δ-MnO₂/Pt on Ni foam; (b) Enlarged view in (a); (c) XPS survey spectrum and (d) Pt4f XPS spectrum of Co₃O₄@δ-MnO₂/Pt

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  Figure 2  SEM images of (a, b) Co₃O₄, (c, d) Co₃O₄@δ-MnO₂, and (e, f) Co₃O₄@δ-MnO₂/Pt

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  Figure 3  (a) Dark-field TEM image, (b~e) EDS mappings in the selected area and (f) HRTEM images of Co₃O₄@δ-MnO₂/Pt

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  Figure 4  (a) Terminal voltages of Co₃O₄, Co₃O₄@δ-MnO₂ and Co₃O₄@δ-MnO₂/Pt-catalyzed Li-O₂ cells at a limited capacity of 500 mAh·g^-1; (b) Voltage profiles of Co₃O₄@δ-MnO₂/Pt-catalyzed Li-O₂ cell; Discharge profiles of (c) Co₃O₄@δ-MnO₂ and (d) Co₃O₄@δ-MnO₂/Pt-catalyzed Li-O₂ cells at various current densities Cells were discharged and charged to 500 mAh·g^-1 at 200 mA·g^-1

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  Figure 5  (a, b) SEM images, (c) TEM image and (d) SAED pattern of Co₃O₄@δ-MnO₂/Pt after the first discharge; (e, f) SEM images of Co₃O₄@δ-MnO₂/Pt after the first charge

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  Figure 6  (a) Li1s XPS after the first cycle and (b) Nyquist plots and corresponding fittings of Li-O₂ cells with Co₃O₄@δ-MnO₂/Pt electrode at the initial state and discharged/recharged to 2.2 V/4.3 V

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  Table 1.  Fitting results of the Nyquist plots using the equivalent circuit

  Sample	Re / Ω	Rf / Ω	Q1		Rct / Ω	Q2
  			Y	n		Y	n
  Initial	81.9	135.0	3.4×10-5	0.73	331.9	1.8×10-5	0.84
  After discharge	89.1	178.1	3.0×10-6	0.61	767.1	3.1×10-6	0.89
  After recharge	114.8	162.8	6.7×10-5	0.28	301.6	5.9×10-6	0.92

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