High-Surface-Area and Porous Co2P Nanosheets as Cost-Effective

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High Surface Area and Porous Co2P Nanosheets as Cost-effective Cathode Catalysts for Li-O2 Batteries Hong-bo Huang, Shao-hua Luo, Cai-ling Liu, Ting-Feng Yi, and Yu-chun Zhai ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b03736 • Publication Date (Web): 06 Jun 2018 Downloaded from http://pubs.acs.org on June 6, 2018

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ACS Applied Materials & Interfaces

High Surface Area and Porous Co2P Nanosheets as Cost-effective Cathode Catalysts for Li-O2 Batteries Hong-bo Huang a,c, Shao-hua Luo b,c,d*, Cai-ling Liu a,c, Ting-feng Yi b,c,d*, Yu-chun Zhai b a

b

School of Metallurgy, Northeastern University, Shenyang 110819, People’s Republic of China

School of Resources and Materials, Northeastern University at Qinhuangdao, Qinhuangdao 066004, People’s Republic of China

c

Key Laboratory of Dielectric and Electrolyte Functional Material Hebei Province, Qinhuangdao 066004, People’s Republic of China d

School of Materials Science and Engineering, Northeastern University, Shenyang 110819, People’s Republic of China

ABSTRACT：To enable lithium-oxygen batteries for practical applications, the design and efficient synthesis of non-precious metal catalysts with high activity and stable structural properties are demanded. The objective is to accelerate the sluggish kinetics of both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) by facilitating electronic/ionic transport and improving oxygen diffusion in a porous structure. In this study, high surface area and porous cobalt phosphide (Co2P) nanosheets are synthesized via an environmentally safe hydrothermal method, where red phosphorous is used as the phosphorous source. It was found that the as-prepared Co2P/acetylene black (AB) composite delivered enhanced electrochemical performances, such as high capacities of 2551 mA h g-1 (based on the total weight of Co2P and AB) or 5102 mA h g-1 (based on the weight of Co2P or AB), and good cycle life of more than 1800 h (132 cycles) in lithium-oxygen battery. The rational design of the Co2P/AB porous oxygen electrode structure provides sufficient accessible reaction sites and a short diffusion path for electrolyte penetration and diffusion of O2.

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KEYWORDS: lithium-oxygen battery, porous, cobalt phosphide (Co2P) nanosheets, oxygen electrode, electrocatalyst 1. INTRODUCTION Rechargeable nonaqueous Li-O2 batteries have been extensively investigated in the past decade due to its high energy density (3860 mA h g-1).1-3 However, there are still several challenges to be addressed, such as poor efficiency, low reaction rate, and low cycle life.4-6 Researchers have made several types of highly active catalysts to increase the efficiency and selectivity of the ORR and OER. As far as we know, Pt,7-9 Ru,10, 11 Pd,12-14 Au15, 16 and RuO217, 18 have been broadly investigated in Li-O2 batteries. Nevertheless, their high cost has motivated dozens of researchers to search more inexpensive and earth abundant alternatives for catalyzing either of the two reactions. Transition metal oxides,18-23 perovskite metal oxides,24, 25 transition metal carbides and nitrides26-29 are believed to be promising alternatives for noble metal catalyst. Currently, transition metal phosphide (TMPs) based electrocatalysts have been widely applied for hydrogen evolution reaction (HER), ORR and other electrocatalytic reactions,30, 31 especially cobalt phosphide and its derivatives.32-34 Zhaoyu Jin et al.32 have synthesized Co2P nanowires and found that they were promising bi-functional electrocatalysts for the overall water-splitting. Juan F. Callejas et al.35 compared the Co2P nanoparticles with morphologically equivalent CoP electrocatalyst for the HER, and they found that Co2P was a highly active HER electrocatalyst. Co2P has been demonstrated with high redox reactivity in an alkaline system. This is due to the fact that metal phosphides have metalloid properties. Their high electrical conductivity makes them cosset in the fields of electrocatalysis and electrochemical energy storage.36 To date, the traditional method of synthesis of metal phosphides is based on the direct reaction between phosphines or phosphorus

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pentachloride and metals or metal salts at high temperatures in an inert atmosphere or under vacuum.37 In order to prepare transition metal phosphide as a cathode catalyst for Li-O2 batteries, several problems should be solved. First, a safe and environmental friendly synthesis process of metal phosphides is particularly important, since the high-temperature solid-state reactions can generate highly reactive, toxic and pyrophoric byproducts (e.g., P4, PH3). Second, the temperature of phosphating process is relatively high (>900 °C), however, the high temperature process is not good for obtaining nano materials. Nanomaterials possess much greater surface areas than similar masses of larger-scale materials, as well as more active catalytic sites.38 Therefore, transition metal phosphides with high surface area, such as nanoparticles, nanowires and nanosheets, should be obtained by a safe method. With this in mind, we propose a generally applied hydrothermal method for preparing the high surface area and porous cobalt phosphide (Co2P) nanosheets. Then, the mixture of cobalt phosphide and acetylene black (AB) was applied as a cathode for Li-O2 batteries. Two-dimensional (2D) Co2P nanosheets with different sizes offer a skeleton for a homogeneous distribution of AB. The rational design of the Co2P/AB porous oxygen electrode structure endures the volume change and provides enough interspaces for rapid diffusion of oxygen and electrolyte to prevent coverage of the exposed active sites. Therefore, it can provide a short diffusion pathway and abundant active sites. In addition, the Co2P/AB electrode delivers high discharge capacities of 2551 mA h g-1 (based on the total weight of Co2P and AB) or 5102 mA h g-1 (based on the weight of Co2P or AB). The Li-O2 batteries with the Co2P/AB porous oxygen electrode can be last for more than 1800 h. Because of its high electrical conductivity, unique micro/nanoarchitecture, and numerous active sites, Co2P/AB electrode is deemed to be one of the competitively active and cost-effective cathode catalysts for Li-O2 batteries.

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2. EXPERIMENT SECTION 2.1. Material Synthesis Red phosphorous was used as the phosphorous source, which was pretreated to micrometer-scale by grinding for 4 h with 300 r/min. Co2P was prepared by a simple hydrothermal method. Firstly, 3 mmol Co(NO3)2·6H2O and 50 mg CTAB were mixed in deionized water (40 ml) via sonication for 10 min. Then, a certain amount of red phosphorus powder (the molar ratio of Co:P= 1:4.5, 1:5.0, 1:5.5, 1:6.0), which have been pretreated to microscale by grinding, was slowly added in the above solution with vigorous agitation. The solution formed a red suspension. Subsequently, the above suspension was transferred into a stainless steel vessel (interior volume 50 ml) and heated at 200 ℃ for 48 h in a self-designed rotary heating furnace. Finally, the solid product was washed with 3 M HCl twice and distilled water equal times to remove trace impurities. Then, the obtained black product was vacuum filtered and dried at 80 ℃ for 8 h. 2.2. Characterization The phase of the samples was analyzed by X-ray powder diffraction (XRD, Rigaku SmartLab, Japan). The morphology of the samples was characterized by scanning electron microscope (SEM, ZEISS SUPRA55, Germany) and transmission electron microscopy (TEM, Hatchie HT-7700). High-resolution TEM (HRTEM) images were recorded on Tecnai G2 F20s-Twin instrument. The surfaces of the samples were analyzed by the X-ray photoelectron spectroscope (XPS, Thermo Scientific

Escala

250Xi,

USA).

N2

adsorption/desorption

was

characterized

by

Brunauer-Emmett-Teller (BET) analysis (SSA-4300, China). Atomic force microscope (AFM) analysis was performed on a Veeco Multimode atomic force microscope (MultiMode 8, Bruker, USA).

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2.3.Electrochemical Performance Measurement Rotating disk electrode experiments were performed on a 3-electrode cell using a CHI660C electrochemical workstation (Shanghai CH Instruments, China). The glassy carbon disk, platinum foil, and saturated calomel electrode served as the working, auxiliary and reference electrodes, respectively. The electrolyte was 0.1 M KOH. 10 µL of prepared slurry was drop-casted onto the working electrode (0.196 cm2). The slurry (6 mg mL-1) was made by sonicating the 3 mg of AB (50 wt%) and 3 mg of Co2P (50 wt%) mixture, or only 6 mg of catalyst were dispersed in dimethylformamide (DMF) with Nafion. 2.4. Preparation of Electrodes for Li-O2 Battery Measurements The Co2P electrode consisted of the as-prepared Co2P 45 wt%, AB 45 wt% and polyvinylidene fluoride (PVDF) binder 10 wt%. AB electrode: AB 90 wt% and PVDF binder 10 wt%. The mixture was dispersed in N-methyl-2-pyrrolidone (NMP), and the formed slurry was coated onto a carbon paper. The total mass loading density for cycling tests was about 1.0 mg cm-2. Button-type CR2025 batteries were assembled in an Ar-filled glovebox, using a lithium metal anode, a glass fiber separator (GF/D, Whatman), the prepared electrode and electrolyte (1 M LiCF3SO3 in TEGDME). The electrochemical tests were carried out at room temperature on a Land CT2001A battery test system in an oxygen atmosphere (about 1 atm). Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were recorded using the computerized potentiostatic instrument (model CHI660C) and Solatron 1260 Electrochemical Interface (Solatron Metrology, UK), respectively. 3. RESULTS AND DISCUSSION

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Scheme 1. Schematic of the preparation and application of Co2P nanosheets.

The Co2P nanosheets were prepared according to Scheme 1. In the first step, red phosphorus was reduced in size by wet ball milling treatment. After the ball milling, red phosphorus particles with an average particle size of several micrometers to several tens of micrometers were obtained (Figure S1). The rate of reaction can be raised by increasing the surface area of red phosphorus particles. In the second step, Co(NO3)2·6H2O, CTAB and red phosphorous powder reacted under high-pressure, high-temperature condition. When the solution heated at 200 °C, the content of H3PO3 and PH3 increased gradually due to the disproportionation and decomposition (equation(1)-(3)). With the increase of the PH3 content, Co2P was generated and agglomerated on the red phosphorus particles surface for the low solubility product constant value (Ksp) (equation(4)). It means that PH3 gas is constantly generated and consumed in the system. That is why porous Co2P nanosheets can be obtained on the red phosphorous surface. Some impurities (Co3(PO4)2(OH), Co2(OH)(PO4)) can be

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removed by washing with 3 M HCl. The reactions for the formation of Co2P nanosheets are as follows. 8P + 12H 2O → 3H 3PO 4 + 5PH 3

(1)

4P + 6H 3PO 4 + 6H 2O → 10H 3PO3

(2)

4H 3 PO 3 → PH 3 + 3H 3 PO 4

(3)

6Co 2+ +4PH 3 → 3Co 2 P + 12H + +P

(4)

Figure 1a shows the XRD pattern of the as-prepared Co2P. All the peaks match well with the orthorhombic Co2P phase (JCPDS card # 32-0306), showing that the high-purity product was obtained. The obvious broadening of all the diffraction peaks of the as-prepared Co2P distinctly indicates the nano-crystallite nature of as-prepared samples. And the two strong diffraction peaks at 40.7° and 40.9° could be well indexed to the (121) and (201) peaks. Moreover, precise control of the molar ratio of Co:P plays a crucial role in the synthesis of pure-phase Co2P. XRD patterns of the samples with different molar ratios of Co:P under 200 ℃ for 24 h were shown in Figure S2. The obtained amount of final product (Co2P) with the molar ratio of Co:P (1:4.5 or 1:5.0) is much fewer than that of Co:P (1:5.5). This is mainly because that the amount of red phosphorus added to the system is too small, resulting in less PH3, which is used to reduce Co2+ ion to get Co2P. As the amount of red phosphorus increases, the Co2P with the molar ratio of Co:P (1:6.0) shows some impurity peaks (red phosphorus) at about 15°. These results indicate that the preferable molar ratio of Co:P is 1:5.5. Figure 1b and c present the SEM images of the product. A large number of Co2P nanosheets with irregular size of about 1-3 um can be observed in Figure 1b. The thickness of Co2P nanosheets is 3-11 nm. A further magnified image shows that these nanosheets built up of nanoparticles can be clearly seen. And the size of the nanoparticles is about 15 nm (see Figure 1c).

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Figure 1. XRD pattern (a), SEM image (b), the corresponding enlarged SEM image (c), TEM image (d), SAED (e), HRTEM image (f), Nitrogen adsorption-desorption isotherm (g), and pore size distribution (h) of the Co2P nanosheets.

The material was further characterized by TEM (Figure 1d) and AFM (Figure S3). Some nanoparticles, which are homogeneous with the size of 5-15 nm, overlapping each other can be easily seen; this could enlarge the active surface area and thus improve the catalytic ability of the

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sample. Notably, the micro- and mesoporous are evenly distributed on the nanosheets, which would offer more edges to expose more surface sites. This can be further confirmed by the nitrogen adsorption/desorption isotherm curves of Co2P nanosheets (Figure 1g), which features an H3 hysteresis loop typical of type IV mesoporous structure. The Co2P nanosheets possess a large specific surface area of 219 m2 g-1, which is much larger than recent state-of-the-art Co2P-based nanostructure, such as Co2P nanorods (25 m2 g-1), Co2P nanoflowers (29 m2 g-1),39 Co2P/g-C3N4 (41.2 m2 g-1),40 Co2P@N, P-C/C (192.42 m2 g-1),41 and Co2P@CoNPG-900 (93.8 m2 g-1),42 etc. Furthermore, the pore size distribution calculated using BJH (Barrett-Joyner-Halenda) method (Figure 1h) indicates that the primary pore size is centered at 2.55 nm. And the average pore size of 1.8 to 7 nm confirms the micro- and mesoporous structure of the material. The porous nanostructure and large surface area will provide not only sufficient catalytic sites but also mass transport pathways. The observed signals of the selected area electron diffraction (SAED) are assigned to the (121), (201), (002) and (310) crystalline planes of Co2P, indicating the formation of crystalline Co2P nanosheets (Figure 1e). Figure 1f is an HRTEM image of the Co2P nanosheets. According to our calculation from the HRTEM image, the d spacings of the lattice fringes are 0.221, 0.209 and 0.215 nm, corresponding to the (121), (211) and (220) planes of Co2P, respectively.

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Figure 2. (a) ORR and (b) OER polarization curves of Co2P/AB, Co2P and pure AB at 1600 rpm in 0.1 M KOH solution with a sweep rate of 5 mV s-1. (c) ORR polarization profiles of Co2P/AB in 0.1 M KOH with a sweep rate of 5 mV s-1 at various rotation rates. (inset: the Koutecky-Levich curves of Co2P/AB based on the ORR polarization profiles at 0.45, 0.50, and 0.55 V) (d) The initial cycling voltage profiles of Li-O2 batteries with pure AB and Co2P/AB electrodes at 0.1 mA cm-2. (e) CV curves of AB and Co2P/AB electrodes within the potential region of 2.0-4.5 V at 0.01 mV s-1. (f) The initial discharge-charge profiles of Li-O2 batteries with Co2P/AB electrodes at various densities.

Rotating disk electrode (RDE) techniques were performed to examine the ORR and OER intrinsic catalytic activities of catalysts. Figure 2a displays the ORR polarization profiles on glassy carbon-supported Co2P/AB, Co2P and pure AB. As shown in the figure, the polarization curves of Co2P/AB and AB exhibit positively shifted onset potentials, compared with the separately tested Co2P catalyst. Therefore, it can be concluded that the incorporation of AB leads to an increase in the ORR onset and half-wave potential, which is consistent with the data reported in literature.43 In the half-wave potential of ORR, it is followed to order Co2P < AB < Co2P/AB. Among the investigated samples, Co2P/AB shows the highest half-wave values (E1/2 = 0.67 V), which is consistent with the 10


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corresponding onset potential of the catalysts. This is most like due to the fact that the conductivity of the metal phosphide is dramatically increased by incorporation of AB. Indeed, Co2P/AB displays the highest limiting current density (J = 3.7 mA cm-2). Figure 2b presents the OER polarization profiles on glassy carbon-supported Co2P/AB, Co2P and pure AB. In the limited current density, it is followed to order Co2P < Co2P/AB < AB. Contrarily to ORR, Co2P shows an excellent performance as an OER electrocatalyst, and the performance displayed by the individually tested AB sample is poor. It can be concluded that the incorporation of AB seems to lead to a decrease in the limited current density. Surprisingly, the OER onset potential of Co2P/AB (1.56 V versus RHE) is lower than that of the Co3O4 catalyst (1.58 V versus RHE), which is the OER catalysts based on non-noble metals of choice in many current Li-O2 studies.43, 44 These results demonstrate that the as-synthesized Co2P/AB is an active and stable bifunctional oxygen electrocatalyst, exhibiting both ORR and OER catalytic performance. Additionally, for the ORR test, the as-prepared Co2P/AB slurry coated on glassy carbon electrode was tested at rotation speeds of 400, 625, 900, 1225, 1600 and 2025 rpm, respectively (Figure 2c). The inset shows the Koutecky-Levich plots of Co2P/AB based on ORR polarization profiles at 0.45, 0.50, and 0.55 V. All plots displayed linear feature, confirming a first-order kinetics characteristic of the ORR. The transferred electron number (n) is about 3.7 per O2 molecule for the Co2P/AB, which is mostly close to a 4-electrons process (O2 + 2H2O + 4e- → 4OH-) of the ORR in alkaline media. The electrochemical properties of the Co2P/AB based Li-O2 battery were examined by using 2025 coin battery. Figure 2d presents the first full discharge and charge performance at 0.1 mA cm-2. The battery delivers a high discharge capacity of 2551 mA h g-1 (based on the total weight of Co2P and AB) or 5102 mA h g-1 (based on the weight of Co2P or AB). For comparison, the pure AB

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electrode exhibits a capacity of 1632 mA h g-1, which is almost half of the value of Co2P/AB cathode. And the overpotential of the Co2P/AB cathode is about 100 mV (ORR) and 220 mV (OER) lower than that of pure AB cathode, suggesting the highly electrochemical catalytic activity of porous Co2P nanosheets. Moreover, the first discharge/charge curves of Co2P/AB electrodes for Li-O2 batteries with different molar ratios of Co:P at 0.1 mA cm-2 are shown in Figure S4. The above results can be further identified by the CV (Figure 2e). The Co2P/AB oxygen electrode clearly exhibits larger ORR and OER currents and lower OER potentials, in comparison with pure AB, indicating that the Co2P/AB oxygen electrode offers bi-functional catalyst during both the anodic and cathodic scans. These results imply that the Co2P/AB oxygen electrode has a higher ORR/OER activity. The enhanced ORR and OER kinetics could improve the discharge/recharge characteristics and the round-trip efficiency of the Li-O2 batteries. In addition, the higher peak current density in the cathodic and anodic scan processes may also contribute to the high catalytic activity of the Co2P/AB electrode. Figure 2f displays the first discharge and charge profiles for these Li-O2 batteries with Co2P/AB electrodes at various current densities of 0.1, 0.2 and 0.4 mA cm-2 in the voltage range from 2.1 to 4.5 V. It shows a superior rate capability, with approximately 75% energy efficiency when the rate increased from 0.1 to 0.4 mA cm-2. As current rate increased to 0.2 and 0.4 mA cm-2, the Co2P/AB electrode still displayed a high discharge capacity. Such an excellent electrochemical performance of Co2P/AB electrode can be largely attributed to the enhanced ORR and OER kinetics. This enhanced ORR and OER kinetics could be reasonably ascribed to the synergistic effect of the high electrical conductivity of AB and the high surface area and porous structure of the Co2P nanosheets.

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Figure 3. (a) and (b) The discharge/charge profiles, (c) and (d) Cyclic performance and corresponding Coulombic efficiency of Li-O2 batteries with Co2P/AB and bare AB electrodes at 0.1 mA cm-2, respectively.

Cycling tests of the Co2P/AB electrode and the pure AB electrode were performed in a voltage range of 2.1-4.5 V vs. Li/Li+ for five full cycles (Figure 3a and b). The first capacities of the two kinds of cathodes are all high, but they decay rapidly in subsequent cycles. The inferior reversibility of our Li-O2 battery could be impacted by the possible electrolyte decomposition;45 on the other hand, the gas-solid state conversion during the reaction may also affect electrode performance.46 Furthermore, as is clearly seen in the cyclic performance and corresponding Coulombic efficiency profiles (Figure 3c and d), the Co2P/AB electrode exhibits much better cycling stability compared to the bare AB electrode. The Co2P/AB electrode delivers a high first discharge capacity of 2551 mA h g-1. After 5 charge/discharge cycles, its discharge capacity still retains 1371 mA h g-1. The cell also exhibits capacity retention of 53.7% with a 90-100% Coulombic efficiency. By contrast, the bare AB electrode presents the first discharge capacity of 1632 mA h g-1, after 5 charge/discharge cycles, its

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discharge capacity decreases to 578 mA h g-1. It exhibits capacity retention of barely 35.4% with a 70-80% Coulombic efficiency. The superior electrochemical and rate performance of Co2P/AB electrode is mainly attributed to the rational design of the electrode structure. The large surface area and ultrathin porous nanostructure of Co2P can provide sufficient accessible reaction sites and a short diffusion path for electrolyte penetration and diffusion of O2. And the incorporation of AB increased the electrical conductivity of the Co2P/AB electrode.

Figure 4. Cycling performance of Li-O2 batteries with Co2P/AB electrode (a) and pure AB electrode (b) at 0.1 mA cm-2 with a limited specific capacity of 500 mA h g-1. (c) Cycling performance of Co2P/AB based Li-O2 batteries at 0.1 mA cm-2 at a limited capacity of 500 mA h gCo2P+AB-1.

A comprehensive study on the electrochemical performance is tested using capacity-limited mode. Figure 4a and b evaluate the cycling performance of the Co2P/AB composite and pure AB oxygen electrodes at 0.1 mA cm-2 with a limited specific capacity of 500 mA h g-1. As shown in 14


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Figure 4a, the overpotential of the Co2P/AB catalyst is 1.4 V, which is a bit smaller than that of the bare AB electrode. The pure AB can also be used as cathode catalysts for Li-O2 batteries, but it demonstrates undesirable electrochemical performance (failed at the 30th cycle). Compared with pure AB, the discharge terminal potential of the Co2P/AB electrode remains above 2.5 V, whereas its charge cutoff voltage is below 4.6 V for 130 cycles. The excellent cycleability of the Co2P/AB oxygen electrode is mainly attributed to the porous oxygen electrode structure, using porous Co2P nanosheets, which have a high catalytic activity, composited with high conductive AB as a cathode for Li-O2 batteries. 2D Co2P nanosheets with various sizes provide a skeleton for a homogeneous distribution of AB. The porous oxygen electrode structure endures the volume change, providing a super conductive path for ion and electron transport and possessing abundant active sites. The corresponding discharge-charge curves (Figure 4c) further show that the Li-O2 batteries with the Co2P/AB porous oxygen electrode can be last for more than 1800 h without degradation, indicating the superior catalysis effect of Co2P. In addition, compared with the previous research results (Table S1), it can be found that the Co2P/AB porous oxygen electrode is a competitive electrochemical catalyst among the reported non-noble-metal-based catalysts for Li-O2 batteries.

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Figure 5. SEM images of the Co2P/AB electrode (a) before discharge, (b) after full discharge and (c) after full recharge. (d) XRD patterns of the Co2P/AB electrode at different discharge-charge states.

The morphology of the Co2P/AB electrode after discharge and recharge was characterized by SEM, the result is shown in Figure 5. The SEM image of Co2P/AB electrode after the first discharge to 2.1 V at 0.1 mA cm-2 is shown in Figure 5b, the electrode is covered with typical toroid-shaped particles with sizes ranging from 100 to 500 nm in diameter. These large Li2O2 toroids can lead to a higher capacity compared to the quasi-amorphous films.47 The morphology of discharge products is consistent with the observation by other researchers.48,

49

These products disappeared and the

electrode surface generally recovered after being fully charged (Figure 5c), indicating that the Co2P/AB electrode has good structural stability over cycling. Furthermore, the discharge products were characterized by XRD. Three diffraction peaks at 34.8°, 40.4°, and 58.4° corresponding to Li2O2 were obtained (Figure 5d), and nearly no Li2CO3 or LiOH was detected. Interestingly, the

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diffraction peaks of Li2O2 are almost invisible after the charging process, confirming that the reversible discharge and charge capacities are mainly attributed to the desired Li2O2 formation /decomposition. Thus the formation/decomposition of Li2O2 is reversible in the Co2P/AB electrode, in accordance with SEM results. This can be further confirmed by the results of the electrochemical impedance spectra (EIS) of the Li-O2 batteries with Co2P/AB and pure AB electrodes. As shown in Figure S5, the impedance of both Li-O2 batteries increased significantly due to the poor electronic conductive discharge products (Li2O2) generated on the discharge process. After the charging process, the Co2P/AB based Li-O2 batteries could almost recover the initial impedance. The results indicate that Co2P/AB has lower impedance and the electrode can be recovered after charging. On the other hand, carbon-based oxygen electrode is unstable and lithium carbonate will appear during the discharge process.50, 51 Li2CO3 may be undetected by XRD because of its low concentration or poor crystallinity,52 the discharge product composition was also analyzed by X-ray photoelectron spectroscopy (XPS), of which the detailed result is shown in Figure S6. It can be noted that Li2O2 is the major discharge product and almost disappears during charge process. 4. CONCLUSIONS In summary, porous Co2P/AB composites with a large surface area (219 m2 g-1) were synthesized and used as cathode for Li-O2 batteries. The porous structure provides interspaces for diffusion of O2 and electrolyte and enhances Li+ diffusion. 2D Co2P nanosheets with different sizes offer a skeleton for a homogeneous distribution of AB. The specific structure can improve the OER performance and their tight contact ensures a super electrical conducting path. The Co2P/AB catalyst based Li-O2 battery possesses a higher discharge platform (~100 mV) and a lower charge platform (~220 mV) compared to pure AB electrode. Furthermore, it presented a high capacity of 2551 mA h g-1, which was almost

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twice as high as that of the AB electrode. And it exhibited a good cyclability of 132 cycles. Besides, the formation/decomposition of Li2O2 was identified as the dominating reactions in the Co2P/AB catalyst based Li-O2 batteries. These results revealed the potential applications of Co2P/AB porous oxygen electrode in Li-O2 batteries. ASSOCIATED CONTENT Supporting Information. The calculations of Koutecky-Levich plots and electron transfer numbers; SEM image of red phosphorus particles after ball milling; XRD patterns of the samples with different molar ratios of Co:P under 200 ℃ for 24h; AFM images of the porous Co2P nanosheets; First discharge/charge curves of Co2P/AB electrodes for Li-O2 batteries with different molar ratios of Co:P at a current density of 0.1 mA cm-2; Electrochemical impedance of Li-O2 batteries with Co2P/AB and pure AB electrodes in three different discharge-charge process stages; XPS spectra (Li 1s and O 1s spectra) of the first discharged and charged Co2P/AB cathode; Comparison of electrochemical performance of the non-noble metal catalysts reported in previous literature and present work. AUTHOR INFORMATION Corresponding author： *E-mail: [email protected] (Shao-hua Luo) *E-mail: [email protected] (Ting-feng Yi) ORCID Shao-hua Luo: 0000-0002-4517-3728

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NOTES The authors declare no conflict of interest. ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (No. 51674068), Natural Science Foundation of Hebei Province (No. E2018501091), The Science and Technology Project of Hebei Province (No. 15271302D), The Training Foundation for Scientific Research of Talents Project, Hebei Province (No. A2016005004), The Basic Scientific Research of Central Colleges, Northeastern University (No. N172304001). REFERENCES (1) Lee, J. S.; Kim, S. T.; Cao, R.; Choi, N. S.; Liu, M.; Lee, K. T.; Cho, J. Metal-Air Batteries with High Energy Density: Li-Air versus Zn-Air. Adv. Energy Mater. 2011, 1, 34-50. (2) Shao, Y. Y.; Ding, F.; Xiao, J.; Zhang, J.; Xu, W.; Park, S.; Zhang, J. G.; Wang, Y.; Liu, J. Making Li-Air Batteries Rechargeable: Material Challenges. Adv. Funct. Mater. 2013, 23, 987-1004. (3) Manthiram, A.; Li, L. J. Hybrid and Aqueous Lithium-Air Batteries. Adv. Energy Mater. 2015, 5, 1401302. (4) Bruce, P. G.; Freunberger, S. A.; Hardwick, L. J.; Tarascon, J. M. Li-O2 and Li-S Batteries with High Energy Storage. Nature Mater. 2012, 11, 19-29. (5) Mahne, N.; Fontaine, O.; Thotiyl, M. O.; Wilkening, M.; Freunberger, S. A. Mechanism and Performance of Lithium-Oxygen Batteries - a Perspective. Chem.

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High-Surface-Area and Porous Co2P Nanosheets as Cost-Effective

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