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A Nitrogen-Doped Perovskite as A Bifunctional Cathode Catalyst for Rechargeable Lithium-Oxygen Batteries Jinbo Zhang, Chaofeng Zhang, Wei Li, Qi Guo, Hongcai Gao, Ya You, Yutao Li, Zhiming Cui, Ke-Cheng Jiang, Huijin Long, Dawei Zhang, and Sen Xin ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b17289 • Publication Date (Web): 17 Jan 2018 Downloaded from http://pubs.acs.org on January 17, 2018

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ACS Applied Materials & Interfaces

A Nitrogen-Doped Perovskite as A Bifunctional Cathode Catalyst for Rechargeable Lithium-Oxygen Batteries Jinbo Zhang,a Chaofeng Zhang,a Wei Li,b Qi Guo,a Hongcai Gao,c Ya You,c Yutao Li,c Zhiming Cui,c Ke-Cheng Jiang,d Huijin Long,d Dawei Zhang*a and Sen Xin*c a

School of Chemistry and Chemical Engineering, Hefei University of Technology, Hefei,

230009, P. R. China. b

Department of Chemistry & Biochemistry, Utah State University, Logan, Utah, 84322, United

States. c

Department of Mechanical Engineering, the University of Texas at Austin, Austin, Texas

78712, United States. d

Jiangsu TAFEL New Energy Technology Inc., Nanjing, Jiangsu, 211113, P. R. China.

* Corresponding authors. E-mail addresses: [email protected]; [email protected]. KEYWORDS: nitrogen-doped LaNiO3, oxygen vacancies, oxygen reduction reaction, oxygen evolution reaction, rechargeable lithium-oxygen batteries.

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ABSTRACT: In this work, nitrogen-doped LaNiO3 perovskite was prepared and studied, for the first time, as a bifunctional electrocatalyst for oxygen cathode in a rechargeable lithium-oxygen battery. N doping was found to significantly increase the Ni3+ contents and oxygen vacancies on the bulk surface of the perovskite, which helped to promote the oxygen reduction reaction and oxygen evolution reaction of the cathode and therefore, enabled reversible Li2O2 formation and decomposition on the cathode surface. As a result, the oxygen cathodes loaded with the N-doped LaNiO3 catalyst showed improved electrochemical performance in terms of discharge capacity and cycling stability to promise practical Li-O2 batteries.


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INTRODUCTION

Rechargeable lithium-oxygen (Li-O2) battery has been considered as a superior energy storage technology over the traditional lithium-ion battery, and is especially promising for use in electric vehicles (EVs) since it can output an ultrahigh theoretical specific energy of 3500 W h kg-1 (including O2) with Li2O2 as the final discharge product.1-10 However, there are still obstacles in practical application of Li-O2 battery, including poor cycle life, high overpotentials, and instability of electrolyte.11-13 In a non-aqueous Li-O2 battery, oxygen is reduced to Li2O2 upon discharge (i.e., oxygen reduction reaction or ORR); during the reverse charge process, Li2O2 decomposes via oxygen evolution reaction (OER) to release the oxygen. Nevertheless, Li2O2 is insoluble in non-aqueous electrolyte, which may cover the cathode and block the migration channels for electrolyte and oxygen. Hence, apart from the lithium anode and electrolyte, the reversible formation and decomposition of Li2O2 on the oxygen cathode also have a determining effect on the battery performance.14-16 Over the past years, many catalysts for promoting the ORR/OER of oxygen cathode have been reported,17-20 such as noble metals, metal oxides and carbon materials. While certain improvements were observed on cathode performance, these catalysts still suffer from many disadvantages including high material costs and poor chemical stability, which hinder their battery use. Perovskite oxides (ABO3, A: rare earth metal; B: transition metal) have attracted more attentions as catalysts for fuel cells and metal-O2 batteries because of their low cost, defective structure and high electronic and ionic conductivity.21 Zhang developed effective perovskite electrocatalysts and observed morphology-dependent improvement in battery performance.22,23 Suntivich and coworkers found that LaNiO3 exhibited favorable electrocatalytic activity and


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related it to the eg1 (eg = 1) electron configuration of transition metal atoms in B-sites and the covalency of B-O bond.24-26 It is known that doping of metal cations with different valences in A or B-sites could induce generation of structure defects or ionic defects and thereby affect the electrocatalytic activity. 27,28 Particularly, the metal dopant could introduce oxygen vacancies into the material, which not only promotes the formation of B-O bond, but provides active reaction sites to facilitate the migrations of both the reactants and the products (including the intermediate products) during the catalytic process.29-31 On the other hand, nitrogen, which has a similar atomic size to oxygen and a smaller ionization energy, has also been proved a favorable nonmetal dopant for metal oxides.32 The N doping in metal oxides could also induce the formation of oxygen vacancies as a result of charge balancing.33,34 In addition, it also helps to narrow the band gap because of the lower electronegativity of N, which is beneficial for facilitating the electron transport in metal oxides.35,36 However, to the best of our knowledge, the current studies about N-doped perovskite are mainly focused on the photocatalytic performance, and there is barely any research about the effect of N doping on the electrocatalytic performance of perovskite. Herein, the N-doped LaNiO3 was prepared and studied, for the first time, as a bifunctional electrocatalyst for oxygen cathode in a rechargeable Li-O2 battery. Through changing the nitridation time, we have synthesized LNONs with different doped contents of N, and found that the doped N significantly increased the contents of Ni3+ and oxygen vacancies on the surface of the LNON, which helped to promote the electrocatalytic activity for ORR/OER in both aqueous alkaline electrolyte and non-aqueous electrolyte. Compared to pure LaNiO3 (LNO), an oxygen cathode based on the LNON catalyst showed a higher discharge specific capacity and rate capability, lower overpotential and better cycling stability in a rechargeable Li-O2 battery.


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EXPERIMENTAL SECTION

Synthesis of LNO and nitrogen-doped LNO. All reagents used in the experiment were of analytic grade and used without further purification. The LNO were prepared by the sol-gel method, using metal nitrates (as metallic precursors), citric acid (as chelating agent) and ethylene glycol (as binding agent) with the molar ratio of 1:2:4. The synthesis began with the dissolution of stoichiometric amounts of La(NO3)3•xH2O and Ni(NO3)2•H2O (Sinopharm) in deionized water. After that, citric acid (Sinopharm) and ethylene glycol (Sinopharm) were dropwisely added into the above solution, and the mixture was stirred at 80 °C for several hours to form the gel. Subsequently, the gel was heated at 350 °C to generate the amorphous citrate precursor, which was then sintered at 750 °C (heating rate: 2 °C min-1) in oxygen for 4 h and to yield the LNO. The LNON was synthesized by nitriding the LNO. In a typical synthesis, the prepared LNO was calcinated in a tubular furnace under continuous NH3 flow at 450 °C for 2 hours, 4 hours and 6 hours, respectively, to yield the LNON. To remove the any NH3 residue on the sample surface, the product was firstly washed by deionized water and then purified by centrifugation. The three LNON catalysts prepared with different nitridation times were separately denoted as LNON/2h, LNON/4h and LNON/6h. Material characterization. SEM images were collected on a field-emission scanning electron microscope (FESEM, SU8020, Hitachi). TEM and HRTEM images were recorded on a Hitachi JEM-2100F transmission electron microscope. XRD patterns were collected on a CMA X'Pert


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PRO MPD X-ray diffractometer with Cu Kα1 radiation (λ = 1.5405 Å). XPS analysis was performed on a Rigaku D/MAX2500V X-ray photoelectron spectrometer with an exciting source of Al Ka (1486.6 eV). The N2 adsorption-desorption isotherms of the samples were collected on a Quantachrome NovaWin instrument at 77 K. The specific surface area was calculated by the Brunauer-Emmett-Teller (BET) method, and the pore volume/width was calculated by the Density Functional Theory (DFT). Electrochemical measurements. The ORR and OER activities of the as-prepared catalysts were tested by using a rotating-disk electrode (RDE, ATA-1B), which was a three-electrode system consisting of a platinum electrode as the auxiliary electrode, a saturated calomel electrode as the reference electrode and a glassy carbon electrode as the working electrode. A 0.1 M KOH aqueous solution was used as the aqueous alkaline electrolyte. The cathode ink was prepared by dispersing 10 mg of catalyst powder, 2.5 mg of Vulcan XC-72 carbon black and 60 µL of 5 wt% Nafion in 2 mL of 3:1 (v:v) water/isopropanol solution and sonicating the mixture for 1 h. The glassy carbon disk electrodes (diameter: 3 mm) were polished on a chamois leather with 1.0 µm γ-alumina powder. After that, 4.0 µL of the ink suspension was pipetted and coated onto the polished glass carbon, forming a catalytic layer after drying. The electrochemical tests were performed on an Autolab electrochemical workstation, during which the ORR polarization profiles were measured within a voltage range of from 0 to 0.65 V, and the OER polarization profiles were measured from 0 V to 1.0 V. To test the electrocatalytic activity of the LNO and LNON catalysts in non-aqueous Li-O2 batteries, 2032-type coin cells were assembled in an argon-filled glove box with oxygen and moisture contents lower than 0.1 ppm. The cell consisted of a lithium metal anode, a glass fiber separator (Whatman grade GF/D), an oxygen cathode and two pieces of nickel foam (thickness:


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1 mm) as the cathode current collector. An ether electrolyte consisting of 1 M lithium bis(trifluoromethane) sulfonamide in triethylene glycol dimethyl ether (TEGDME) was used. To prepare the oxygen cathode, the as-prepared catalysts, Vulcan XC-72 and polyvinylidene fluoride (PVDF) with a mass ratio of 3:6:1 were first thoroughly mixed and dispersed in Nmethyl-2-pyrrolidone, yielding a homogeneous slurry. The slurry was then pasted onto the carbon paper, dried at 80 °C for 12 h, and cut into slices (diameter: 12 mm) to obtain the cathode. The as-assembled cell was sealed into an oxygen-filled bottles, and tested on a LAND CT2001A multichannel battery testing system. The galvanostatic discharge-charge test was performed within a voltage window of 2.0-4.5 V (versus Li+/Li), the initial discharge profiles were collected at a current density of 50 mA gcat−1 and the cycling tests were carried out at 250 mA gcat−1. The current density and specific capacity of cathode were calculated based on the mass of cathode catalyst. For the ex-situ SEM and XRD characterization of cathode, the batteries were disassembled in the glove box, in which the cathode was taken out and immersed in TEGDMG for 12h to remove any electrolyte residue on the its surface, and finally dried in vacuum oven.

RESULTS AND DISCUSSIONS

As shown in Figure 1a, the pure LNO and the as-synthesized LNON samples with different nitridation times, namely, LNON/2h, LNON/4h and LNON/6h were characterized by X-ray diffraction (XRD). The diffraction patterns of the LNON/2h and LNON/4h are consistent with the standard powder diffraction file (PDF#33-0711) of LNO, a rhombohedral structure with space group R-3m (166), indicating that LNON/2h and LNON/4h maintain the same crystal structure as the pure LNO after nitridation. The enlarged XRD patterns of LNON/4h shifts


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slightly toward a higher 2θ value compared to that of LNO (Figure S1), which is consistent with Jing's work on N-doped LaFeO3 and suggest the incorporation of nitrogen atoms into the crystal lattice to partially substitute oxygen.37 However, it should be noted that the LNON/6h exhibits a new diffraction peak at around 33° because of the lattice defects, which is likely due to the excess content of doped N in LNO.34 The LNON/4h and the LNO were further characterized by the scanning electron microscopy (SEM). Both materials showed a particle-like morphology with an average diameter of c.a. 150 nm (Figure 1b and S2). The N2 adsorption/desorption isotherms also demonstrated a negligible change in Brunauer-Emmett-Teller specific surface area of LNON/4h (4.474 m2 g-1) compared with that of the LNO (4.912 m2 g-1) (see Figure S3 and Table S1). All the above results prove that the nitridation process does not significantly alter the morphology or structure of the perovskite. According to the high-resolution transmission electron microscopy (HRTEM) image in Figure S4, the LNON/4h shows an interplanar spacing of 0.274 nm, which corresponds well with the (110) plane of LaNiO3 perovskite. The energy dispersive X-ray (EDX) elemental mappings obtained by TEM demonstrate a uniform N distribution in the N-doped perovskite (Figure 1c), with an atomic ratio of N calculated to be 1.27% for the LNON/4h. The doping of N into the crystal lattice of LNO was also confirmed by X-ray photoelectron spectroscopy (XPS). Referring to the N 1s spectra in Figure 1d, the LNON/4h shows a strong peak at 399.5 eV. This peak, according to Jing et al, is more likely attributed to the N atoms that substitutes the oxygen at the anionic lattice sites rather than residue N species adsorbed on the surface of the LNO, such as NH3 (398.8 eV), NO or NO2 (≥400 eV).37-39


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Owing to charge balancing in the LNON, the incorporated N3- may result in either generation of oxygen vacancies or oxidation of the cation in B site. To clarify this point, we also collected the core-level O 1s spectra from the LNO and the LNON/4h during the XPS test, and presented them in Figure 1e. A peak splitting was performed to yield four peaks that represent four different oxygen species. The peaks at lower binding energy (528.8 and 529.2 eV) are assigned to the lattice oxides from lanthanum oxide (Olat (La)) and nickel oxide (Olat (Ni)). A significant decrease in strength of the Olat (Ni) peak is observed, which agrees with the above XPS finding in the N 1s spectra and together they confirm the substitution of Ni-O bond by Ni-N bond in LaNiO3 after nitridation. The other two higher peaks at 531.4 and 532.0 eV are assigned to the adsorbed oxygen species from lanthanum hydroxides (Oads (La)) and nickel hydroxides (Oads (Ni)) on the surface of catalysts (Oads), which are bound up with the oxygen vacancies.40,41 The above two Oads peaks are significantly enhanced after nitridation in comparison to LNO, indicating that the N doping in perovskite leads to an elevated content of oxygen vacancy. According to Hardin et al, the oxygen from the hydroxyl group (OH-) on perovskite particle surface can promote the formation of O-O bond in OOH-, which is the rate-determining step for OER.42 Moreover, a stronger B-O bond induced by the adsorbed oxygen could not only promote the rate of O22-/OH- displacement and OH- regeneration to improve the ORR performance, but enhance the OER catalytic activity as well.42 Because of the partial overlap between the Ni 2p3/2 and La 3d3/2 spectra (Figure S5), a subtraction was made to obtain the Ni valence on the surface of the LNO and the LNON/4h, and the results are two Ni 2p3/2 peaks with different chemical states (854 eV for Ni2+ and 856 eV for Ni3+, see Figure 1f).43 While the Ni2+ signal almost vanishes after nitridation, the intensity of Ni3+ peak is notably enhanced, suggesting that the doped N favors the generation of Ni3+, and this


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result matches well with the N 1s XPS analysis. Since Ni3+ has a higher covalence to bond oxygen on the perovskite surface than other transition metal cations, the formation of Ni3+ may help to improve the catalytic activity for both ORR and OER.24-26 The electrocatalytic activities of catalysts toward ORR and OER were evaluated in O2saturated 0.1 M KOH using a rotating disk electrode (RDE). Figure 2(a) shows the ORR polarization curves on glassy carbon-supported perovskite-type catalysts and the Vulcan XC-72 reference at 1600 rpm. Compared with others, the LNON/4h with a most positive ORR onset potential (~0.27 V vs SCE) was observed. Moreover, at -0.65 V vs SCE, the limiting diffusion current density of LNON/4h (3.75 mA cm-2) is highest among that of pure LNO (2.88 mA cm-2), LNON/6h (3.47 mA cm-2) and LNON/2h (3.32 mA cm-2), even almost twice than that of Vulcan XC-72 (1.93 mA cm-2). Figure 2(b) shows the Koutecky-Levich plots for glassy carbonsupported LNO, LNON/2h, LNON/4h, LNON/6h and Vulcan XC-72 based on ORR polarization curves (Supporting Information). All plots exhibited linear feature, suggesting a first-order kinetics characteristics of the ORR. The electron transfer numbers of these samples were calculated and summarized inTable S2. The n value of LNON/4h is about 3.91, which is the most close to a four-electrons process (O2 + 2H2O + 4e— → 4OH—) of the ORR in alkaline solution. Thus, the LNON/4h has the best ORR activity and kinetics according to the above discussions, and the high electrocatalytic activity is due to the higher concentration of Ni3+ and the improved covalency of Ni-O bond on the surface of LNON/4h in the alkaline electrolyte.24-26 We also tested the OER activity by RDE in O2-saturated 0.1 M KOH solution from 0.0 V to 1.0 V at 1600 rpm. As shown in Figure 2(c), the onset potential of LNON/4h (0.612 V) are also better than that of LNO (0.7 V). And the limiting diffusion current density of LNON/4h (23.6 mA cm-2) is more than two times higher than that of LNO (11.4 mA cm-2) at 1.0 V vs SCE. The


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enhanced adsorption of hydroxyl tied up with the oxygen vacancies on the surface of LNON/4h played a dominant role in promoting the OER activity.42 Non-aqueous Li-O2 batteries were also assembled for an all-around evaluation of catalytic performance (Scheme 1a). According to Scheme 1b, the doping of N into the LNO during the nitridation process also introduces more oxygen vacancies into the perovskite, which could improve the formation/decomposition of Li2O2 on the oxygen cathode and thereby promote the catalyst activities.30,44 During the discharge process, the oxygen molecule (O2) firstly binds with the oxygen vacancy and then reduced to a superoxide radical (O2-), which finally binds with Li+ to form LiO2 (Scheme 1c).44 The as-formed LiO2 is thermodynamically unstable and tends to react with another LiO2 through the disproportionation reaction, yielding Li2O2. During the charge process, one Li ion is extracted from Li2O2 to generate LiO2, which binds with the oxygen vacancy and gets rid of Li+/e- to release O2 (Scheme 1c).44 Figure 3(a-c) shows the exsitu SEM images obtained from the oxygen cathode containing LNON/4h at different operational states of battery. At fully discharged state, toroidal-like Li2O2 forms and aggregates on the cathode surface.45 After charging, the Li2O2 almost completely disappears and the cathode restores its pristine morphology. The finding agrees with periodic appearance/disappearance of Li2O2 peaks in the ex-situ XRD patterns collected during the discharge/charge of battery (Figure 3d), which demonstrates an excellent reversibility of cathode reaction catalyzed by the N-doped perovskite. Figure 4a shows the initial discharge/charge profiles of cathodes supported with the LNON/4h and LNO catalysts, and without any catalyst (sole use of Vulcan XC-72 carbon black) under a current density of 50 mA gcat-1. The LNON/4h-supported cathode delivers an initial discharge specific capacity of 5910 mA h gcat-1, which is significantly higher than the capacities delivered


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by the LNO-supported cathode (4130 mA h gcat-1) and the catalyst-free one (3200 mA h gcat-1). Besides, the discharge and charge platforms are obviously improved with the help of LNON/4h catalyst in comparison to that of LNO and Vulcan XC-72. In detail, the discharge platforms of LNON/4h, LNO and Vulcan XC-72 are 2.8, 2.75 and 2.7 V. Moreover, two plateaus appear in the charge profile of the LNON/4h-supported cathode, which correspond to the stepped oxidation from Li2O2 to LiO2 (~3.4 V) and then to O2 (~4.1 V) and fit well with the OER mechanism in Scheme 1c.44 The LNON/4h-supported cathode also shows the lowest overpotential, suggesting much improved kinetics of cathode reaction and brings the highest round-trip efficiency of Li-O2 battery. Although the recent study have demonstrated a better rate capability based on the LiOH product than that based on Li2O216, the LNON/4h-supported cathode still displays improved rate performance in battery benefited from the fast kinetics. Upon increasing the current density to 100, 200 and 400 mA gcat-1, the LNON/4h-supported cathode delivers specific capacities of 4400, 3701 and 3084 mA h gcat-1, respectively, which are much higher than the values delivered by the LNO-supported cathode (Figure 4b). Besides, the LNON/4h-supported cathode also shows higher capacity retention rates than the LNO-supported cathode under the above current densities (inset of Figure 4b). The LNON/4h-supported cathode also enables a Li-O2 battery with stable cycling performance. When tested under a current density of 250 mA gcat-1 and a cut-off capacity of 500 mA h gcat-1, the LNON/4h-supported cathode can maintain stable cycling for 50 cycles within a voltage window of 2.6-4.7 V, while the LNO-supported cathode and the catalyst-free cathode merely last for 32 and 25 cycles, respectively (Figure 4c, 4d and S6). Using the ex-situ SEM, we further investigated the morphological evolution on the surface of the above cathodes during the discharge-charge process. Before cycling, the three cathodes show similar surface morphology


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(Figure 5a, 5d and 5g). The cathode morphology does not change notably with no apparent accumulation of Li2O2 on the surface after the initial discharge (Figure 5b, 5e and 5h), which is probably due to the limited discharge capacity and high current density.46 After 15 cycles, only the LNON/4h-supported cathode shows clear surface morphology while the two controls are completely covered by Li2O2 (Figure 5c, 5f and 5i). Besides, the LNON/4h-supported cathode with more oxygen vacancies promotes the decomposition of the Li2O2 on the cathode surface, which is also helpful to inhibiting the side reactions between cathode and electrolyte.30 Since the blocking of carbon pores by Li2O2 is the main reason of cathode failure, it is reasonable for the LNON/4h-supported cathode to have the longest cycle life.

CONCLUSION

In summary, a nitrogen-doped LaNiO3 was fabricated and served as a bifunctional cathode catalyst for rechargeable Li-O2 battery. The introduction of N into the perovskite lattice introduces additional oxygen vacancies to facilitate the formation/decomposition of Li2O2 and enable the N-doped catalyst with improved ORR/OER catalytic activity over the nondoped one. Cathodes supported with the LNON/4h catalyst were able to deliver a high initial discharge capacity of 5910 mA h gcat-1 and excellent cycling/rate performance in a Li-O2 battery. The results obtained from this study validates the feasibility of using non-metal dopant to improve the electrocatalytic activity of perovskite, which will shed lights on rational design of cathode catalyst for Li-O2 and other metal-O2 batteries. ASSOCIATED CONTENT


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Supporting Information. Supplementary materials including the calculations and results of Koutecky-Levich plots and electron transfer numbers, the partially enlarged XRD and nitrogen adsorption/desorption isotherms patterns of the LNO and the LNON/4h, SEM image of the LNO, HRTEM image of the LNON/4h, La 3d and Ni 2p3/2 spectra of the LNO and the LNON/4h, cycling performance of Li-O2 battery based on Vulcan XC-72 cathode are provided. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding author *Email: [email protected]; [email protected]. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. J.Z., C.Z. and W.L. contributed equally. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work is supported by National Natural Science Foundation of China (Grant No. 51472070, 21403050, U1361110), and Anhui Provincial Natural Science Foundation (Grant No. 1608085MB32).

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Figure 1. (a) XRD patterns of LNO and LNON with different nitridation time, (b) SEM image of (b) LNON/4h, (c) EDX elemental mapping of LNON/4h obtained by TEM, and XPS spectra for (d) N 1s, (e) O 1s and (f) Ni 2p of LNO and LNON/4h.


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Figure 2. (a) ORR polarization curves on glassy carbon-supported LNO, LNON catalysts and Vulcan XC-72 at 1600 rpm. (b) Koutecky-Levich plots for glassy carbon-supported LNO, LNON catalysts and Vulcan XC-72. (c) OER polarization curves on glassy carbon-supported LNO, LNON catalysts and Vulcan XC-72 at 1600 rpm.


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Figure 3. Ex-situ SEM images of the LNON/4h-supported cathode (a) in pristine state; (b) after initial discharge and (c) after initial charge. (d) Ex-situ XRD patterns collected from the LNON/4h-supported cathode in pristine state (XRD 1), after initial discharge (XRD 2) and after initial charge (XRD 3) under a current density of 50 mA gcat-1.


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Figure 4. Electrochemical performances of Li-O2 batteries. (a) initial discharge/charge profiles of Li-O2 batteries assembled with LNON/4h-supported, LNO-supported and catalyst-free (Vulcan XC-72) cathodes under a current density of 50 mA gcat-1; (b) rate capability and capacity retention rate (inset) of Li-O2 batteries assembled with the LNON/4h-supported and LNOsupported cathodes; (c) and (d) show cycling performance of the above batteries under a current density of 250 mA gcat-1 and a cut-off capacity of 500 mA h gcat-1.


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Figure 5. SEM images showing the morphological evolution on the surface of the cathodes supported with (a-c) LNON/4h, (d-f) LNO and (g-i) Vulcan XC-72 during the discharge-charge process. The batteries were cycled under a current density of 250 mA gcat-1 and a fixed cut-off capacity of 500 mA h gcat-1.


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Scheme 1 Illustrations of (a) the Li-O2 battery; (b) the process of nitridation and (c) the formation and decomposition of Li2O2 during discharge-charge process.


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