Phosphorus vacancies enriched Ni2P nanosheets as efficient electrocatalyst for high-performance Li–O2 batteries

Journal Pre-proof Phosphorus vacancies enriched Ni2P nanosheets as efficient electrocatalyst for highperformance Li–O2 batteries Zhiqun Ran, Chaozhu Shu, Zhiqian Hou, Peng Hei, Tingshuai Yang, Ranxi Liang, Jiabao Li, Jianping Long PII:

S0013-4686(20)30187-0

DOI:

https://doi.org/10.1016/j.electacta.2020.135795

Reference:

EA 135795

To appear in:

Electrochimica Acta

Received Date: 10 November 2019 Revised Date:

12 January 2020

Accepted Date: 26 January 2020

Please cite this article as: Z. Ran, C. Shu, Z. Hou, P. Hei, T. Yang, R. Liang, J. Li, J. Long, Phosphorus vacancies enriched Ni2P nanosheets as efficient electrocatalyst for high-performance Li–O2 batteries, Electrochimica Acta (2020), doi: https://doi.org/10.1016/j.electacta.2020.135795. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.

Zhiqun Ran: Methodology, Software, Data curation, Writing- Original draft preparation, Writing- Reviewing and Editing. Chaozhu Shu: Conceptualization, Validation,

Visualization

Preparation,

Investigation,

Supervision,

Project

administration, Funding acquisition. Zhiqian Hou: Supervision, Resources, Formal analysis, Investigation. Peng Hei: Formal analysis, Supervision, Software. Tingshuai Yang: Supervision, Resources, Formal analysis. Ranxi Liang: Validation, Supervision, Software. Jiabao Li: Supervision, Software, Resources. Jianping Long: Visualization Preparation, Project administration, Funding acquisition.

Phosphorus Vacancies Enriched Ni2P Nanosheets as Efficient Electrocatalyst for High-Performance Li-O2 Batteries Zhiqun Ran, Chaozhu Shu*, Zhiqian Hou, Peng Hei, Tingshuai Yang, Ranxi Liang, Jiabao Li, and Jianping Long* College of Materials and Chemistry & Chemical Engineering, Chengdu University of Technology, 1# Dongsanlu, Erxianqiao, Chengdu 610059, Sichuan, P. R. China1, E-mail: [email protected] (C.Shu), [email protected] (J. Long). Abstract Compared with the lithium-ion batteries (LIBs), lithium-oxygen batteries (LOBs) demonstrate ultra-high theoretical energy density (≈3505 W h kg-1), which arouse tremendous research interest worldwide. However, the serious challenges facing LOBs are the sluggish oxygen redox kinetics during the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) process, resulting in poor specific capacity and inferior cyclability. Herein, we fabricate phosphorus vacancies enriched Ni2P nanosheets on carbon cloth (denoted as Vp-Ni2P@CC) via a simple NaBH4 reduction strategy as oxygen electrode for high-performance LOBs. Interestingly, the Vp-Ni2P@CC based LOBs exhibit outstanding performance including large specific capacity (10 796 mA h g-1@500 mA g-1) and excellent cyclability (235 cycles@500 mA g-1) compared with LOBs with the Ni2P@CC electrode. It is found that the abundant phosphorus vacancies in the Vp-Ni2P@CC not only act as active sites for

* Corresponding authors at: College of Materials and Chemistry & Chemical Engineering, Chengdu University of Technology, 1# Dongsanlu, Erxianqiao, Chengdu 610059, Sichuan, P. R. China. E-mail address: [email protected] (C.Shu), [email protected] (J. Long)

1

oxygen electrode reactions but also enhance the mobility of electron/Li+, simultaneously promoting the improvement of ORR/OER kinetics. Moreover, the restrained electrons around Ni-P bonds are delocalized due to the presence of phosphorus vacancies, which narrow down the band gap in Vp-Ni2P@CC, eventually contributing to high electrical conductivity and excellent electrocatalytic activity. The study provides a new orientation for further developing oxygen electrode catalysts for LOBs. Keywords: Li-O2 batteries, oxygen electrodes, electrocatalysts, Ni2P, phosphorus vacancies 1. Introduction LOBs with ultra-high theoretical energy densities are considered as the most potential energy storage devices which have been rapidly developed over the past decade [1-4]. However, a series of formidable challenges, such as large overpotential, low discharge capacity and short cycle life, which seriously hinder their large-scale practical application must be overcome [5-8]. These issues are considered to be caused by the sluggish oxygen electrode kinetics during discharge and charge [9]. As a result, it’s highly emphasized that the reasonable design of oxygen electrode materials is extremely important for high performance LOBs. To date, precious metals such as Pt, Ir, or Ru based catalysts possess high electrocatalytic activity, which have been widely used in electrocatalytic energy storage and conversion fields [10-14]. However, the high cost and scarcity of noble metals hinder their practical development. Hence, exploring efficient alternative

2

catalysts that are inexpensive and naturally abundant will be extremely urgent. Transition metal phosphides (MxPy, where M=Ni, Co, Fe, Mn or Cu, etc) [15] with both metalloid characteristics and intrinsic metallic nature which favors superior electrical conductivity and fast electron transfer at the electrode/electrolyte interface are quite promising alternative oxygen electrode materials for LOBs [16, 17]. Among transition metal phosphides, Ni2P have been identified as the functional materials for photocatalysis, supercapacitor and electrocatalysis [16, 18-20]. Moreover, Ni2P possesses abundant Ni-Ni bonds and Ni-P bonds, which demonstrates more excellent electron conductivity, thermal, and chemical stability than that of the phosphorus rich NiPx (NiP2 or NiP3) with plenty of P-P bonds [21]. However, the issues of slow kinetics caused by the deposition of the phase during the cycle process, the pulverization and aggregation of the active substance, as well as the loss of the active substance caused by electrical isolation during the circulation, are presented at the Ni2P electrode [22]. In order to overcome the above-mentioned problems, different strategies have been developed over the past several years. For instance, the synergistic effect between Ni2P and carbon substrate can create abundant active sites and improve stability, thus synergistically enhancing the electrocatalytic performance of Ni2P [23]. In addition, previous studies have found that the surface electronic structure of catalysts have a significant influence on its catalytic activity [24-28]. Thus, creating surface defects on Ni2P to modulate the surface electronic structure is of great significance for improving its catalytic activity. Herein, a phosphorus vacancies enriched Ni2P nanosheet array directly grown on

3

carbon cloth (Vp-Ni2P@CC) electrode was fabricated for improving ORR and OER kinetics in LOBs. The abundant phosphorus vacancies are capable of delocalizing the restrained electrons around the Ni-P bonds owing to generation of new gap band and thus modulating the surface electronic structure of Vp-Ni2P@CC electrode, result in the outstanding electrocatalytic activity [29]. Furthermore, the existence of phosphorus vacancies could increase the mobility of electron and Li+, which significantly boosts the electrical conductivity of Vp-Ni2P@CC [30]. In addition, the synergistic effect between Ni2P nanosheets and carbon substrate with large specific surface areas can provide rich active sites, which are favorable for improving the rate capability of Vp-Ni2P@CC [31, 32]. This work provides a new guidance for the development of oxygen electrode materials with high catalytic activity for Li-O2 batteries. 2. Experimental section 2.1 Treatment of the carbon cloth (CC) Carbon cloth (30 mm×10 mm×0.5 mm) was ultrasonically cleaned with 3.0 M HCl aqueous solution for 30 min. Then, it was washed three times with ethanol and deionized water, respectively, and dried in a vacuum oven at 60 °C for 12 h. 2.2 Synthesis of the Precursor Ni(OH)2@CC In a typical synthesis strategy, the above treated carbon cloth was vertically placed into the 50 mL Teflon-lined stainless autoclave. NiCl2 ▪ 6H2O (0.1200 g), CO(NH)2 (0.2000g) and NH4F (0.0600g) were dispersed into 30 mL deionized water under continuously stirring for 30 min. The mixture was poured into the autoclave and

4

then the autoclave was heated at 160 °C for 12 h. After completing the reaction, the autoclave was cooled down to room temperature naturally. Then, the obtained precursor was taken out of the Teflon-lined autoclave and washed with ethanol and deionized water for three times, respectively, and dried at 60 °C to obtain Ni(OH)2@CC precursor. 2.3 Synthesis of the Ni2P@CC The

Ni2P@CC

was

obtained

via

phosphorization

reaction

between

Ni(OH)2@CC precursor and NaH2PO2 powder at 300 °C for 120 min under Ar atmosphere in a tube furnace with a rate of 2 °C min-1. 2.4 Synthesis of the Vp-Ni2P@CC To fabricate phosphorus vacancies in the Ni2P nanosheets, the Ni2P@CC nanosheets were treated with 1M NaBH4 solution for 1 h. Then the electrode was washed three time with ethanol and deionized water, respectively. Finally, the Vp-Ni2P@CC was obtained by drying the NaBH4 treated Ni2P@CC at 60 °C for 12 h. 2.5 Material characterization X-ray diffraction (XRD) patterns were recorded by using a D/MAX-IIIC (Japan) with Cu Kα radiation with scanning range from 30° to 80°. X-ray photo electron spectra (XPS) were conducted by using an ESZALB 250XL spectrometer with a Al K Alpha X-ray source. Raman spectra was carried out on a Thermo DXR. The photoluminescence (PL) spectra was recorded by using Hltachi F 4600 Spectrophotometer at the excitation wavelength of 320 nm. Electron Paramagnetic Resonance (EPR) measurements were obtained on the JEOL JESX320 at room

5

temperature. Scanning electron microscopy (SEM) was recorded by using a JSM-6700F (Japan). Transmission electron microscopy (TEM) attached with energy dispersive X-ray spectroscopy (EDS) was recorded by using a JEOL 2100F microscope. In order to quantify the amount of Li2O2 on the discharged electrodes, a quantitative analysis was conducted by combining the titration measurement and the Ultraviolet-visible (UV-vis) spectrometry analysis. 2.6 Battery assembly and Electrochemical tests The LOBs were assembled by using Li sheets as anode, Vp-Ni2P@CC, Ni2P@CC or pristine CC as oxygen electrode, and 1 M LITFSI/TEGDME as electrolyte. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were tested by using CHI 660E electrochemical workstation. The EIS was measured in the frequency range of 0.010 Hz~1.0×105 Hz with an amplitude voltage of 5 mV. Galvanization charge/discharge and cycle stability tests were performed by using LAND CT2001A multi-channel battery testing system. 3. Results and discussions

6

Fig. 1. Fabricating procedure and structure diagram of Ni2P@CC and Vp-Ni2P@CC nanosheets. The fabricating procedure and structure diagram of the Ni2P@CC and Vp-Ni2P@CC nanosheets was schematically illustrated in Fig. 1. Ni(OH)2@CC was used as the precursor to synthesize Ni2P@CC and Vp-Ni2P@CC via phosphorization and NaH2PO2 acts as the phosphorus source [33]. Specifically, the synthesized Ni(OH)2@CC precursor was treated with NaH2PO2 at 300 oC under Ar atmosphere to produce Ni2P@CC. NaH2PO2 can be quickly decomposed to PH3 at 300 oC, which can further react with the Ni(OH)2 precursor to generate Ni2P. The reaction equations can be expressed as follows: 2NaH2PO2→Na2HPO4+PH3↑, PH3+Ni(OH)2→Ni2P [34]. The Ni2P@CC sample was then reduced with NaBH4 solution to create phosphorus vacancies in Vp-Ni2P@CC. The X-ray diffraction (XRD) pattern of the as-synthesized precursor is shown in

7

Fig. S1 and it can be seen that all diffraction peaks are well attributed to Ni(OH)2 (JCPDS No. 73-1520). Fig. 2a exhibits the XRD patterns of Ni2P@CC and Vp-Ni2P@CC, and all diffraction peak were consistent with the (111), (021), (210) and (300) planes of the Ni2P standard card (No. 65-9706). The above result proves that the crystal structure of Ni2P can be maintained after NaBH4 treatment. XPS was used to further confirm the chemical composition of Ni2P@CC and Vp-Ni2P@CC. The XPS survey spectrum of the as-synthesized Vp-Ni2P@CC and Ni2P@CC nanosheets indicates the obvious existence of Ni, P, C and O elements (Fig. S2). As shown in Fig. 2b, the shift of P 2p to a lower bonding energy in Vp-Ni2P@CC as compared to Ni2P@CC in the high-resolution P 2p spectrum reveals the presence of phosphorus vacancies [25]. In addition, it can be found that the P-O peak (133.8 and 134.7 eV) exists in the P 2p due to the oxidation of surface P on the Ni2P by air [35]. Obviously, the intensity of P-O peak in Vp-Ni2P@CC is much lower than that in Ni2P@CC, indicating the significantly reduced POxn- on the Vp-Ni2P@CC surface after NaBH4 reduction [25]. It is an important strategy to create abundant phosphorus vacancies on the Ni2P via removing the oxidized POxn- by NaBH4 reduction process. The high-resolution Ni 2p spectrum verifies the existence of Ni species (Fig. 2c). Two major peaks were observed at approximately 856.6 eV and 874.8 eV, which can be attributed to Ni 2p3/2 and Ni 2p1/2 of both Ni2P@CC and Vp-Ni2P@CC, respectively [36, 37]. Meanwhile, two satellite peaks at 861.3 and 879.3 eV were also observed. Importantly, the Ni 2p peak of Vp-Ni2P@CC shifts to a lower bonding energy as compared to that of Ni2P@CC, again indicating the formation of phosphorus

8

vacancies [25, 38, 39]. The Raman spectra of the Ni2P@CC and Vp-Ni2P@CC are shown in Fig. 2d, displaying obviously the D and G bands of carbon at around 1344 and 1597 cm-1, respectively [22]. Compared with Ni2P@CC, the increase of the ID/IG ratio and the blue shift of Raman peaks in Vp-Ni2P@CC indicates that the sp2 hybrid orbital of the carbon is partly damaged by the reduction process, which some phosphorus vacancies can be generated on material surface. As observed in photoluminescence (PL) spectra (Fig. 2e), the Vp-Ni2P@CC exhibits much higher emission intensities than Ni2P@CC. The increasd PL intensity in Vp-Ni2P@CC as compared to Ni2P@CC indicates the high concentration of vacancies in Vp-Ni2P@CC nanosheets [40]. Phosphorus vacancies on Ni2P@CC are capable of promoting the generation and transfer of holes and electrons, which will increase its conductivity and enhance electron transport and thus improve the performance of Li-O2 batteries (vide infra) [29]. Moreover, it is reported that vacancies can not only effectively bond with O2 and Li2O2, but also accelerate the transport of electrons/Li+ during the reaction, thereby accelerating ORR and OER processes in the LOBs [40, 41]. In addition, EPR spectra (Fig. 2f) shows the g value of ~2.08 for Vp-Ni2P@CC, which can be attributed to the appearance of unpaired electrons in the Vp-Ni2P@CC catalyst, further indicating that phosphorus vacancies are successfully created in Vp-Ni2P@CC [25].

9

Fig. 2. (a) XRD pattern of Vp-Ni2P@CC and Ni2P@CC; (b) P 2p and (c) Ni 2p XPS spectrum of Vp-Ni2P@CC and Ni2P@CC; (d) Raman spectra, (e) Photoluminescence (PL) spectra and (f) EPR spectra of Vp-Ni2P@CC and Ni2P@CC. SEM images of Ni2P@CC and Vp-Ni2P@CC were shown in Fig. 3a and b. Ni2P@CC shows the nanosheet morphology with a rough surface, and the morphology of the Vp-Ni2P@CC maintain nearly unchanged. SEM patterns of the

10

Ni(OH)2@CC precursor was shown in Fig. S1b, in which the nanosheets is more smooth than that in Ni2P@CC (Fig. S1c) and Vp-Ni2P@CC (Fig. S1d). As shown in Fig. S3, nanosheets were observed in the TEM images of both pristine Ni2P@CC and Vp-Ni2P@CC. From HRTEM images shown in Fig. 3c and d, the distinct lattice fringe of 0.22 nm can be assigned to the crystal planes (111) of Ni2P nanosheets, which is consistent with the results of XRD. Interestingly, the lattice defects and distortion of Vp-Ni2P@CC (expressed in red circles) can be distinctly found in Fig. 3d, which may be induced by phosphorus vacancies [42, 43]. The element mapping shows the uniform distribution of Ni, P, and C elements on the surface of Ni2P and Vp-Ni2P nanosheets (Fig. 3e and f). The above results indicate the successful synthesis of phosphorus vacancies enriched Ni2P nanosheets on carbon cloth.

11

Fig. 3. SEM images of (a) Ni2P@CC and (b) Vp-Ni2P@CC; HRTEM images of (c) Ni2P@CC and (d) Vp-Ni2P@CC and elemental mapping of (e) Ni2P@CC and (f) Vp-Ni2P@CC. The performance of Li-O2 batteries based on different oxygen electrodes were studied to explore the catalytic activity of various materials. The initial

12

discharge/charge plots of Li-O2 batteries (2.0 V to 4.5 V) were shown in Fig. 4a. Obviously, the initial discharge/charge capacity of the Vp-Ni2P@CC based electrode are much higher than that of Ni2P@CC and pristine CC electrodes. Specifically, the discharge capacity of the Vp-Ni2P@CC based electrode is up to 10796.2 mA h g-1 and the charge capacity reaches 9955.6 mA h g-1 at cutoff voltage of 4.5 V. By contrast, the discharge/charge capacity of Ni2P@CC based batteries are 8860.3 mA h g-1 and 7984.4 mA h g-1, respectively. The improved discharge/charge capacity of Vp-Ni2P@CC electrode further confirms the advantages of the phosphorus vacancies enriched Ni2P nanosheets, which can be attributed to the inherently superior electrocatalytic activities of the Vp-Ni2P@CC electrode with prolific phosphorus vacancies. Moreover, it is obvious that Vp-Ni2P@CC based battery shows a lower overpotential compared with the Ni2P@CC and pristine CC based batteries in Fig. 4b, revealing that phosphorus vacancies enriched Ni2P@CC nanosheets as oxygen electrode catalysts have a much better ORR/OER electrocatalytic activity in LOBs. In addition, the deliberately designed self-supporting nanosheets are capable of providing sufficient specific surface area for deposition of discharge products, ultimately inducing a much improved discharge/charge capacity. In order to further investigate the promotion of the phosphorus vacancies on Ni2P surface on sluggish oxygen redox kinetics in LOBs, the CV measurements were performed at a scan rate of 20 mV s-1 between the 2.0-4.5 V (Fig. 4c). Two typical ORR and OER peaks were observed for different electrodes. Interestingly, the Vp-Ni2P@CC electrode exhibits higher reduction voltage and lower oxidation voltage

13

than Ni2P@CC and pristine CC based electrodes, indicating a lower overpotential of oxygen redox reactions on the Vp-Ni2P@CC based electrode. Therefore, phosphorus vacancies enriched Ni2P@CC electrode is capable of promoting the OER and ORR in LOBs. EIS was carried out to further study the role of the phosphorus vacancies on the performance improvement of LOBs. Fig. 4d shows the EIS plots of Vp-Ni2P@CC, Ni2P@CC and pristine CC based batteries. The diameter of the semicircle at the middle frequency region is evidently smaller for Vp-Ni2P@CC based battery than that of Ni2P@CC or pristine CC based batteries, indicating the lower charge transfer resistance of oxygen electrode reactions on the Vp-Ni2P@CC. Meanwhile, it can be seen that the ohmic resistance of Vp-Ni2P@CC electrode (the high-frequency intercept of the semicircle with the x-axis) is smaller than that of Ni2P and pristine CC electrodes. The electrical conductivity is inversely related to ohmic resistance, so the smaller ohmic resistance, the higher electrical conductivity and thereby the faster the mobility of electron. Therefore, it can be concluded that the existence of phosphorus vacancies can accelerate the mobility of electron. According to Lu Yi-Chun’s study on the working mechanism of solid catalysts in decomposing solid discharged product [44], it is found that the solid catalysts effectively promote Li2O2 oxidation kinetics to evolve oxygen at the surface of product and Li+ at the interface between discharged product and electrode via solid-solid interaction. Their findings highlighted that the overpotential, especially the charge overpotential is closely related to the mobility of lithium ion on the catalyst surface. The low charge overpotential indicates the high mobility of lithium ion on the surface of catalyst on the premise that other conditions

14

remain unchanged. Thus, according to Lu Yi-Chun’ results, it can be concluded that the low charge overpotential of phosphorus vacancy-rich Vp-Ni2P@CC based electrode at the initial charge state (activation polarization region) in our study indicates the high mobility of lithium ion on the surface of phosphorus vacancy-rich Vp-Ni2P@CC as compared to that of pristine Ni2P@CC. Xie Yi et al. found that the high mobility of Li+ on vacancy-rich surface can be attributed to the local built-in electric field caused by the imbalanced charge distribution emerging around the vacancies [45]. Actually, previous study conducted by Liu Xiangfeng et al. also confirmed that vacancies are favorable for enhancing the mobility of lithium ions [46]. Therefore, the phosphorus vacancies in Vp-Ni2P@CC not only act as active sites for adsorption and desorption of O2/Li2O2 but also enhance the mobility of electron/Li+, which simultaneously promote the improvement of ORR/OER activity.

15

Fig. 4. (a) The full discharge/charge plots of the Vp-Ni2P@CC, Ni2P@CC and pristine CC based LOBs at 500 mA g-1 current density with a voltage window of 2.0-4.5 V; (b) The first discharge/charge curve of the Vp-Ni2P@CC, Ni2P@CC and pristine CC based LOBs with a limited capacity of 1000 mA h g-1 at 500 mA g-1; (c) CV profiles of the Vp-Ni2P@CC, Ni2P@CC and pristine CC based LOBs at the scan rate of 20 mV s-1; (d) The EIS plots of the LOBs based on different oxygen electrodes. The rate performance of different oxygen electrodes were studied at different current densities with the cutoff capacity of 1000 mA h g-1 (Fig. 5a and b). The discharge voltage of the Vp-Ni2P@CC based electrode keeps almost unchanged while the charge voltage declines marginally with the increase of current density, as shown in Fig. 5a. In contrast, an obvious increase of charge voltage plateau can be seen in the Ni2P@CC and pristine CC based LOBs (Fig. 5b and Fig. S4). The above results imply that the Vp-Ni2P@CC based LOBs exhibits superior rate capability, mainly ascribing to the existence of abundant phosphorus vacancies. The abundant phosphorus vacancies are capable of effectively improving electron/Li+ transfer throughout the electrode and between electrode/electrolyte interface. In addition, the self-supported structure effectively avoids additional parasitic reactions owing to the absence of polymer binders and the complete coverage of carbon substrate in Vp-Ni2P@CC [47], thus decreasing the interface impedance between catalyst and substrate due to the absence of insulated side products. The cycle performance is another significant issue in practical applications of LOBs. The long-term stability of the different electrodes were studied at current

16

density of 100 mA g-1 with limited capacity of 1000 mA h g-1. The discharge/charge profiles of Vp-Ni2P@CC and Ni2P@CC based Li-O2 batteries at different cycles were shown in Fig. 5c and d. It can be seen that the discharge voltage plateau of the Vp-Ni2P@CC based Li-O2 battery maintains at about 2.76 V and possesses a lower overpotential than the Ni2P@CC electrode at 140 cycles. Interestingly, the discharge terminal voltage of the pristine CC electrode based Li-O2 battery is continuously attenuated to about 2.0 V and the overpotential is up to 2.5 V after 20 cycles (Fig. S5). As depicted in Fig. 5e, the Vp-Ni2P@CC based Li-O2 battery achieves a high cycle life of 235 cycles before the terminal discharge voltage decreasing below 2.0 V. In contrast, the pristine CC and Ni2P@CC based LOBs can only run for about 23 and 140 cycles, respectively. This phenomenon corroborates that phosphorus vacancies enriched Ni2P nanosheets are favorable for improving the cycling stability of LOBs.

17

Fig. 5. The rate capability of the (a) Vp-Ni2P@CC and (b) Ni2P@CC based Li-O2 batteries with a limited capacity of 1000 mA h g-1; The discharge/charge profiles of the (c) Vp-Ni2P@CC and (d) Ni2P@CC based Li-O2 batteries at 100 mA g-1 with a limited capacity of 1000 mA h g-1; (e) The variation of the terminal discharge voltages over cycle number for the Li-O2 batteries with the Vp-Ni2P@CC, Ni2P@CC and pristine CC electrodes. It is well accepted that the phase composition and morphological evolution of the oxygen electrode at different stages have significant influence on the battery performance. The electrodes were thus analyzed by using SEM (Fig. 6). At the pristine state, Vp-Ni2P@CC and Ni2P@CC electrodes demonstrate the ultra-thin

18

nanosheet morphology (Fig. 6a, d). As shown in Fig. 6b, the flower-like discharge products deposit on the Vp-Ni2P nanosheets surface during the first discharge to 5000 mA h g-1 at the current density of 500 mA g-1. The discharge products gradually decompose and the nanosheet morphology is recovered after the fully charging processes (Fig. 6c). In contrast, a bulk of irregular solid discharge products cover on the surface of Ni2P nanosheets after discharging (Fig. 6e). Moreover, the morphology of Ni2P@CC nanosheets is irreversible during the first fully charging process as shown in Fig. 6f. For Vp-Ni2P@CC electrode, the flower-like discharged products with large specific surface area are beneficial to enhancing the contact area between discharged product and electrode, leading to the facile decomposition of discharged product during charge process [48, 49]. In addition, previous study found that the edge site of the discharged product demonstrates a certain degree of amorphous characterization, which is favorable for promoting its decomposition [50]. Thus, the edge-enriched flower-like Li2O2 nanosheets formed on the surface of Vp-Ni2P@CC electrode can be easily decomposed. After charging, the discharged product on the Vp-Ni2P@CC electrode were totally decomposed and the electrode restored its nanosheet morphology, indicating the reversible morphological evolution of Vp-Ni2P@CC electrode. By contrast, due to the accumulation of irregular solid insulated discharged product with large size, the contact between the discharged product and electrode surface is inferior and the accessibility of the electrodes for electrolyte is poor [48, 51, 52]. In addition, it can be clearly seen that byproducts were formed on the surface of Ni2P@CC electrode (Figure 7). Thus, the inferior contact between the insulated discharged product and electrode surface and the formation of byproducts led to the unfavorable recovery of Ni2P@CC electrode in the course of charging. As a result, the nanosheet morphology of the Ni2P@CC electrode has been almost destroyed and the irreversible morphological evolution can be observed after charging. Therefore, it can be understood that the battery voltage rapidly increases and the electrochemical performance of the battery will thus deteriorate, finally leading to the battery failure.

19

Fig. 6. SEM patterns of (a~c) Vp-Ni2P@CC and (d~f) Ni2P@CC electrodes at different states. (a, d) the pristine state; (b, e) after the first discharge to 5000 mA h g-1 at the current density of 500 mA g-1; (c, f) after the first fully recharge process at the current density of 500 mA g-1. The XRD patterns of the Vp-Ni2P@CC electrode at different discharge/charge

20

stages are shown in Fig. 7a. It can be verified that the flower-like discharge product is Li2O2. Fig. S6 and 7 indicate that the discharge products on Ni2P@CC and pristine CC are also Li2O2. The XRD pattern of the fully charged Vp-Ni2P@CC electrode manifests the high reversibility of the oxygen electrode reactions on Vp-Ni2P@CC because the discharge product was completely decomposed after charging. The above results also demonstrate that the morphology of the discharge products can be modified by the electrodes with different surface properties and the phosphorus vacancies enriched Ni2P nanosheets induce the formation of the flower-like discharge product, which is benefit to the decomposition of Li2O2 because of the large reaction area of the flower-like morphology [49]. By contrast, the bulk irregular solid aggregations are formed on Ni2P@CC electrode after discharging, which are generally in poor contact with the active spots on the surface of electrode and thus are difficult to decompose, leading to a slow charging kinetics and a high charge voltage plateau. The EIS plots of Vp-Ni2P@CC based Li-O2 batteries at different states were shown in Fig. 7b. It is found that the battery exhibits large charge transfer resistance after discharging as compared to the initial state, which may be ascribed to the generation of the discharge product on the surface of electrode. However, the charge transfer resistance decreases during the charge process, indicating the excellent recovery characteristics of Vp-Ni2P@CC electrode during cycling. XPS spectra further confirms the outstanding rechargeability of Vp-Ni2P@CC electrode. As shown in Fig. 7c and d, the evolution of Li 1s and C 1s peaks in XPS spectra reveal the formation/decomposition of discharge products on Vp-Ni2P@CC electrode. The

21

characteristic peaks of 54.7 eV can be ascribed to the Li-O bond of Li2O2 after the first discharging, and Li2O2 peaks completely vanishes after the first fully recharging (Fig. 7c) [53, 54]. Meanwhile, there is no characteristic peak of Li2CO3 in C 1s XPS of Vp-Ni2P@CC (Fig. 7d). Combining XPS and XRD results, it is sensible to deduce that the discharge product on Vp-Ni2P@CC electrode is mainly crystalline Li2O2 and the crystalline Li2O2 can be decomposed during charge process, proving the superior rechargeability of the Vp-Ni2P@CC based LOBs. Li 1s and C 1s peaks of the Ni2P@CC electrode were depicted in Fig. 7e and f. Obviously, Li2CO3 peaks at around 55.5 eV can be discovered in Li 1s region of discharged Ni2P@CC electrode, Li2O2 and Li2CO3 still remain after charging process (Fig. 7e) [55]. Correspondingly, C 1s peak at around 290.2 eV associated with Li2CO3 can also be detected in XPS spectra of recharged Ni2P@CC electrode (Fig. 7f) [56]. The formation of Li2CO3 in Ni2P@CC electrode can be attributed to the decomposition of carbon and electrolyte. Since the presence of the abundant phosphorus vacancies in Vp-Ni2P@CC electrode, LOBs can be charged at a relatively low voltage and thus reduce by-products due to the decomposition of the carbon substrate and electrolyte [56].

22

Fig. 7. (a) XRD patterns of Vp-Ni2P@CC electrode at different states; (b) EIS plots of the Vp-Ni2P@CC electrode at different stages; (c) Li 1s and (d) C 1s XPS spectra of Vp-Ni2P@CC electrode after discharging and recharging; (e) Li 1s and (f) C 1s XPS spectra of Ni2P@CC electrode after discharging and recharging. To quantify the reversible formation and decomposition of Li2O2, titration analysis combining with UV-vis spectroscopy was performed to directly and

23

selectively assess the amount of Li2O2. It was previously reported that the discharged product Li2O2 reacts with water in the TiOSO4 solution to form H2O2, and the reaction formula is described as follows: Li2O2 + 2H2O → 2LiOH + H2O2 [57-59]. Once H2O2 is formed in the TiOSO4 solution, the color of the solution will turn yellow or orange relying on the amount of H2O2 due to the formation of the [TiO22+] complex (TiOSO4 + H2O2 + H2SO4 = H2[Ti(O2)(SO4)2] + H2O), of which the wavelength of maximum absorption is 408 nm [60, 61]. In order to quantitatively analyze the amount of Li2O2, the standard absorption spectrum and Lambert-Beer-type curve was obtained via investigating commercial Li2O2 solutions with known concentrations, as displayed in Fig. S8. Fig. 8a and b exhibit the photographs of the Vp-Ni2P@CC and the Ni2P@CC electrodes at different discharge states immersed in the 2% TiOSO4 solution. At the same discharges state, it can be observed that the color of the solution immersed with Vp-Ni2P@CC electrode is darker than that with the Ni2P@CC electrode, qualitatively indicating the larger amount of Li2O2 on Vp-Ni2P@CC as compared to that on Ni2P@CC. Moreover, the UV-vis spectrum and the inset Lambert-Beer-type curve (Fig. 8c) confirm that the intensity of the absorption peak of the solution immersed with Vp-Ni2P@CC electrode is higher than that immersed with Ni2P@CC electrode. According to the experimental results of the titration test, the amount of discharged products on different electrodes were quantitatively calculated and shown in Table 1. For Ni2P@CC electrodes, a relatively low amount of Li2O2 (about 73%) can be detected via UV-vis titration measurement, indicating more side reactions during the discharge process [62]. In contrast, the amount of Li2O2 deposited on the

24

Vp-Ni2P@CC electrode was about 83%, further confirming that Li2O2 is the major discharge product on the Vp-Ni2P@CC electrode. After charging, the titration solution immersed with the Vp-Ni2P@CC electrode became colorless, and the corresponding characteristic absorption peak is negligible. Thus, it can be concluded that the Vp-Ni2P@CC electrode is capable of improving the reversible formation and decomposition of Li2O2.

Fig. 8 Photographs of the TiOSO4 solution with (a) Vp-Ni2P@CC and (b) Ni2P@CC electrodes at different discharge/charge states. (c) UV-vis measurements of the TiOSO4 solution with different electrodes at different states. Inset in Fig. 8c is the Lambert-Beer-type curve at the maximum absorption wavelength of the TiOSO4 solution with different electrodes at different states.

25

Table 1 The yield of discharged product Li2O2 (YLi2O2) on different electrodes at different discharge capacities. Discharge

Theoritical

Experimental Li2O2 YLi2O2 (%) weight (mg)

capacity

Li2O2

(mA h)

weight (mg)

Vp-Ni2P@CC

Ni2P@CC

Vp-Ni2P@CC

Ni2P@CC

0.5

0.43

0.34

0.32

79.07

74.42

1.0

0.86

0.70

0.63

81.40

73.26

1.5

1.29

1.08

0.95

83.72

73.64

4. Conclusion The phosphorus vacancies enriched Ni2P nanosheets on carbon cloth (Vp-Ni2P@CC) have been successfully synthesized via a facile NaBH4 reduction strategy. Phosphorus vacancies can modify the electronic structure of Ni2P in Vp-Ni2P@CC, leading to the improvement of both conductivity and catalytic activity of Vp-Ni2P@CC. In addition, it is capable of modulating the formation of flower-like discharge product which is easy to decompose, further promoting the oxygen electrode reactions in LOBs. The above merits make Vp-Ni2P@CC an excellent oxygen electrode material for LOBs. The Vp-Ni2P@CC based LOBs shows high discharge capacity of 10796.2 mA h g-1 at 500 mA g-1, superior rate performance, and remarkable cycling stability (235 cycles @500 mA g-1). This study offers new orientation into designing advanced oxygen electrode materials for future LOBs and other metal-oxygen batteries. Acknowledgements

26

This work was financially supported by the National Natural Science Foundation of China (Grant No. 21905033), the Science and Technology Department of Sichuan Province (Grant No. 2019YJ0503) and the Cultivating Program of Middle Aged Key Teachers of Chengdu University of Technology (Grant No. KYGG201709). Conflict of Interest The authors declare no conflict of interest. Reference [1]

B. Liu, P. Yan, W. Xu, J. Zheng, B. He, Electrochemically formed ultrafine metal oxide nanocatalysts for high-performance lithium-oxygen batteries, Nano Lett. 16 (2016) 4932-4939.

[2]

A. Eftekhari, B. Ramanujam, In pursuit of catalytic cathodes for lithium-oxygen batteries, J. Mater. Chem. A 5 (2017) 7710-7731.

[3]

C. Shu, J. Wang, J. Long, H. K. Liu, S. X. Dou, Understanding the Reaction Chemistry during Charging in Aprotic Lithium-Oxygen Batteries: Existing Problems and Solutions, Adv. Mater. 31 (2019) 1804587.

[4]

A. Hu, C. Shu, C. Xu, R. Liang, J. Li, R. Zheng, Design strategies toward catalytic materials and cathode structure for emerging Li-CO2 batteries, J. Mater. Chem. A 7 (2019) 21605-21633.

[5]

Z. Hou, J. Long, C. Shu, R. Liang, J. Li, X. Liao, Two-dimensional spinel CuCo2S4 nanosheets as high efficiency cathode catalyst for lithium-oxygen batteries, J. Alloys Compd. 798 (2019) 560-567.

[6]

A. Hu, J. Long, C. Shu, R. Liang, J. Li, Three-Dimensional Interconnected Network Architecture with Homogeneously Dispersed Carbon Nanotubes and

27

Layered MoS2 as a Highly Efficient Cathode Catalyst for Lithium-Oxygen Battery, ACS Appl. Mater. Interfaces. 10 (2018) 34077-34086. [7]

P. Zhang, Y. Zhao, X. Zhang, Functional and stability orientation synthesis of materials and structures in aprotic Li-O2 batteries, Chem. Soc. Rev. 47 (2018) 2921-3004.

[8]

R. Liang, A. Hu, M. Li, Z. Ran, C. Shu, J. Long, Defect regulation of heterogeneous nickel-based oxides via interfacial engineering for long-life lithium-oxygen batteries, Electrochim. Acta. 321 (2019) 134716.

[9]

Z. Ma, X. Yuan, L. Li, Z. F. Ma, D. P. Wilkinson, L. Zhang, J. Zhang, A review of cathode materials and structures for rechargeable lithium-air batteries, Energy Environ. Sci. 8 (2015) 2144-2198.

[10]

F.

Cheng,

J.

Chen,

Metal-air

batteries:

from

oxygen

reduction

electrochemistry to cathode catalysts, Chem. Soc. Rev. 41 (2012) 2172-2192. [11]

M.A. Rahman, X. Wang, C. Wen, High energy density metal-air batteries: a review, J. Electrochem. Soc. 160 (2013) A1759-A1771.

[12]

J. S. Lee, S. Tai Kim, R. Cao, N. S. Choi, M. Liu, K. T. Lee, J. Cho, Metal-air batteries with high energy density: Li-air versus Zn-air, Adv. Energy Mater. 1 (2011) 34-50.

[13]

Y. Liang, C. Z. Zhao, H. Yuan, Y. Chen, W. Zhang, J. Q. Huang, and H. Yu, A review of rechargeable batteries for portable electronic devices, InfoMat, 1 (2019) 6-32.

[14]

X. Li, and J. Wang, One-dimensional and two-dimensional synergized nanostructures for high-performing energy storage and conversion, InfoMat, 2019.

28

[15]

A. Panneerselvam, M. A. Malik, M. Afzaal, P. O'Brien, M. Helliwell, The chemical vapor deposition of nickel phosphide or selenide thin films from a single precursor, J. Am. Chem. Soc. 130 (2008) 2420-2421.

[16]

S. Xie, J. Gou, Facile synthesis of Ni2P/Ni12P5 composite as long-life electrode material for hybrid supercapacitor, J. Alloys Compd. 713 (2017) 10-17.

[17]

S. Liu, J. Feng, X. Bian, J. Liu, H. Xu, Electroless deposition of Ni3P-Ni arrays on 3-D nickel foam as a high performance anode for lithium-ion batteries, RSC Adv. 5 (2015) 60870-60875.

[18]

L. A. Stern, L. Feng, F. Song, X. Hu, Ni2P as a Janus catalyst for water splitting: the oxygen evolution activity of Ni2P nanoparticles, Energy Environ. Sci. 8 (2015) 2347-2351.

[19]

H. Guo, C. Chen, K. Chen, H. Cai, X. Chang, S. Liu, High performance carbon-coated hollow Ni12P5 nanocrystals decorated on GNS as advanced anodes for lithium and sodium storage, J. Mater. Chem. A 5 (2017) 22316-22324.

[20]

Y. Chen, Z. Qin, General applicability of nanocrystalline Ni2P as a noble-metal-free cocatalyst to boost photocatalytic hydrogen generation, Catal. Sci. Technol. 6 (2016) 8212-8221.

[21]

J. F. Callejas, C. G. Read, C. W. Roske, N. S. Lewis, R. E. Schaak, Synthesis, characterization, and properties of metal phosphide catalysts for the hydrogen-evolution reaction, Chem Mater. 28 (2016) 6017-6044.

[22]

C. Wu, P. Kopold, P. A. van Aken, J. Maier, Y. Yu, High Performance Graphene/Ni2P Hybrid Anodes for Lithium and Sodium Storage through 3D Yolk-Shell-Like Nanostructural Design, Adv. Mater. 29 (2017) 1604015.

29

[23]

J. Lin, P. Wang, H. Wang, C. Li, X. Si, J. Qi, Defect-Rich Heterogeneous MoS2/NiS2 Nanosheets Electrocatalysts for Efficient Overall Water Splitting, Adv. Sci. (2019) 1900246.

[24]

K. N. Dinh, X. Sun, Z. Dai, Y. Zheng, P. Zheng, J. Yang, O2 plasma and cation tuned nickel phosphide nanosheets for highly efficient overall water splitting, Nano Energy 54 (2018) 82-90.

[25]

X. Zhou, H. Gao, Y. Wang, Z. Liu, J. Lin, Y. Ding, P vacancies-enriched 3D hierarchical reduced cobalt phosphide as a precursor template for defect engineering for efficient water oxidation, J. Mater. Chem. A 6 (2018) 14939-14948.

[26]

M. Guan, C. Xiao, J. Zhang, S. Fan, R. An, Q. Cheng, Vacancy associates promoting

solar-driven

photocatalytic

activity

of

ultrathin

bismuth

oxychloride nanosheets, J. Am. Chem. Soc. 135 (2013) 10411-10417. [27]

L. Feng, H. Xue, Advances in Transition-Metal Phosphide Applications in Electrochemical Energy Storage and Catalysis, ChemElectroChem 4 (2017) 20-34.

[28]

A. Hu, C. Shu, C. Xu, J. Li, R. Liang, R. Zheng, Interface-engineered metallic 1T-MoS2 nanosheet array induced via palladium doping enabling catalysis enhancement for lithium-oxygen battery, Chem. Eng. J. 382 (2020) 122854.

[29]

Y. Wang, T. Zhou, K. Jiang, P. Da, Z. Peng, J. Tang, Reduced mesoporous Co3O4

nanowires

as

efficient

water

oxidation

electrocatalysts

and

supercapacitor electrodes, Adv. Energy Mater. 4 (2014) 1400696. [30]

R. Gao, Z. Shang, L. Zheng, J. Wang, L. Sun, Z. Hu, X. Liu, Enhancing the Catalytic Activity of Co3O4 Nanosheets for Li-O2 Batteries by the

30

Incoporation of Oxygen Vacancy with Hydrazine Hydrate Reduction, Inorg. Chem. 58 (2019) 4989-4996. [31]

Z. Wu, L. Huang, H. Liu, and H. Wang, Element-Specific Restructuring of Anion-and

Cation-Substituted

Cobalt

Phosphide

Nanoparticles

under

Electrochemical Water-Splitting Conditions, ACS Catal. 9 (2019) 2956-2961. [32]

Y. Yang, M. Luo, Y. Xing, S. Wang, W. Zhang, F. Lv, A Universal Strategy for Intimately Coupled Carbon Nanosheets/MoM Nanocrystals (M= P, S, C, and O) Hierarchical Hollow Nanospheres for Hydrogen Evolution Catalysis and Sodium-Ion Storage, Adv. Mater. 30 (2018) 1706085.

[33]

Y. Shi, B. Zhang, Recent advances in transition metal phosphide nanomaterials: synthesis and applications in hydrogen evolution reaction, Chem. Soc. Rev. 45 (2016) 1529-1541.

[34]

P. Yu, F. Wang, T. A. Shifa, X. Zhan, X. Lou, F. Xia, J. He, Earth abundant materials

beyond

transition

metal

dichalcogenides:

a

focus

on

electrocatalyzing hydrogen evolution reaction, Nano Energy (2019). [35]

C. Tang, R. Zhang, W. Lu, L. He, X. Jiang, A. M. Asiri, X. Sun, Fe-doped CoP nanoarray: a monolithic multifunctional catalyst for highly efficient hydrogen generation, Adv. Mater. 29 (2017) 1602441.

[36]

X. Y. Yu, Y. Feng, B. Guan, X. W. D. Lou, U. Paik, Carbon coated porous nickel phosphides nanoplates for highly efficient oxygen evolution reaction, Energy Environ. Sci. 9 (2016) 1246-1250.

[37]

Z. Li, D. Zhao, C. Xu, J. Ning, Y. Zhong, Z. Zhang, Reduced CoNi2S4 nanosheets with enhanced conductivity for high-performance supercapacitors, Electrochim. Acta, 278 (2018) 33-41.

31

[38]

Y. Sun, K. Xu, Z. Wei, H. Li, T. Zhang, X. Li, Strong Electronic Interaction in Dual-Cation-Incorporated NiSe2 Nanosheets with Lattice Distortion for Highly Efficient Overall Water Splitting, Adv. Mater. 30 (2018) 1802121.

[39]

Y. Yan, J. Lin, J. Cao, S. Guo, X. Zheng, J. C. Feng, J. Qi, Activating and optimizing activity of NiCoP nanosheets for electrocatalytic alkaline water splitting through V doping effect enhanced by P vacancies, J. Mater. Chem. A (2019).

[40]

J. Li, C. Shu, Z. Ran, M. Li, R. Zheng, J. Long, 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, ACS Appl. Mater. Interfaces. 11 (2019) 29868-29878.

[41]

F. Cheng, J. Shen, B. Peng, Y. Pan, Z. Tao, J. Chen, Rapid room-temperature synthesis of nanocrystalline spinels as oxygen reduction and evolution electrocatalysts, Nature Chem. 3 (2011) 79.

[42]

D. Yan, Y. Li, J. Huo, R. Chen, L. Dai, S. Wang, Defect chemistry of nonprecious-metal electrocatalysts for oxygen reactions, Adv. Mater. 29 (2017) 1606459.

[43]

J. J. Zhang, H. H. Wang, T. J. Zhao, K. X. Zhang, X. Wei, Z. D. Jiang, Oxygen vacancy engineering of Co3O4 nanocrystals through coupling with metal support for water oxidation, ChemSusChem 10 (2017) 2875-2879.

[44]

Y. Wang, Z. Liang, Q. Zou, G. Cong, and Y. C. Lu, Mechanistic insights into catalyst-assisted nonaqueous oxygen evolution reaction in lithium-oxygen batteries, J. Phys. Chem. C, 120 (2016) 6459-6466.

32

[45]

Y. Liu, T. Zhou, Y. Zheng, Z. He, C. Xiao, W. K. Pang, and Y. Xie, Local electric field facilitates high-performance Li-ion batteries. ACS nano, 11 (2017) 8519-8526.

[46]

R. Gao, Z. Li, X. Zhang, J. Zhang, Z. Hu, and X. Liu, Carbon-dotted defective CoO with oxygen vacancies: a synergetic design of bifunctional cathode catalyst for Li-O2 batteries. ACS catal. 6 (2015) 400-406.

[47]

A. Hu, J. Long, C. Shu, C. Xu, T. Yang, R. Liang, J. Li, NiCo2S4 Nanorod Arrays Supported on Carbon Textile as a Free-Standing Electrode for Stable and Long-Life Lithium-Oxygen Batteries, ChemElectroChem 6 (2019) 349-358.

[48]

S. Hyun, B. Son, H. Kim, J. Sanetuntikul, and S. Shanmugam, The Synergistic Effect of Nickel Cobalt Sulfide Nanoflakes and Sulfur-doped Porous Carboneous Nanostructure as Bifunctional Electrocatalyst for Enhanced Rechargeable Li-O2 Batteries, Appl. Catal., B 263 (2020) 118283.

[49]

J. Lu, L. Cheng, K. C. Lau, E. Tyo, X. Luo, J. Wen, Effect of the size-selective

silver

clusters

on

lithium

peroxide

morphology

in

lithium-oxygen batteries, Nat. Commun. 5 (2014) 4895. [50]

R. Gao, Z. Yang, L. Zheng, L. Gu, L. Liu, Y. Lee, Enhancing the Catalytic Activity

of

Co3O4

for

Li-O2

Battery

through

the

Synergy

of

Surface/Interface/Doping Engineering, ACS Catal. 8 (2018), 1955-1963. [51]

L. Feng, Y. Li, L. Sun, H. Mi, X. Ren, and P. Zhang, Heterostructured CoO-Co3O4 nanoparticles anchored on nitrogen-doped hollow carbon spheres as cathode catalysts for Li-O2 batteries, Nanoscale, 11(2019) 14769-14776.

33

[52]

F. Tu, J. Xie, S. Zhang, G. Cao, T. Zhu, and X. Zhao, Mushroom-like Au/NiCo2O4 nanohybrids as high-performance binder-free catalytic cathodes for lithium-oxygen batteries, J. Mater. Chem. A, 3(2015) 5714-5721.

[53]

Y. Qin, J. Lu, P. Du, Z. Chen, Y. Ren, T. Wu, In situ fabrication of porous-carbon-supported α-MnO2 nanorods at room temperature: application for rechargeable Li-O2 batteries, Energy Environ. Sci. 6 (2013) 519-531.

[54]

J. Long, Z. Hou, C. Shu, C. Han, W. Li, R. Huang, J. Wang, Free-Standing Three-Dimensional CuCo2S4 Nanosheet Array with High Catalytic Activity as an Efficient Oxygen Electrode for Lithium-Oxygen Batteries, ACS Appl. Mater. Interfaces. 11 (2019) 3834-3842.

[55]

P. Wang, C. Li, S. Dong, X. Ge, P. Zhang, X. Miao, Hierarchical NiCo2S4@NiO Core-Shell Heterostructures as Catalytic Cathode for Long-Life Li-O2 Batteries, Adv. Energy Mater. (2019) 1900788.

[56]

R. Gao, X. Liang, P. Yin, J. Wang, Y. L. Lee, Z. Hu, X. Liu, An Amorphous LiO2-Based Li-O2 Battery with Low Overpotential and High Rate Capability, Nano Energy 41 (2017) 535-542.

[57]

Y. J. Lee, W. J. Kwak, Y. K. Sun, and Y. J. Lee, Clarification of Solvent Effects on Discharge Products in Li-O2 Batteries through a Titration Method, ACS Appl. Mater. Interfaces. 10 (2017) 526-533.

[58]

X. Gao, Y. Chen, L. Johnson, and P. G. Bruce, Promoting solution phase discharge in Li-O2 batteries containing weakly solvating electrolyte solutions, Nat. Mater. 15 (2016) 882.

[59]

H. H. Wang, Y. J. Lee, R. S. Assary, C. Zhang, X. Luo, P. C. Redfern, and E. Indacochea, Lithium Superoxide Hydrolysis and Relevance to Li-O2 Batteries, J. Phys. Chem. C 121 (2017) 9657-9661.

34

[60]

R.M. Sellers, Spectrophotometric determination of hydrogen peroxide using potassium titanium (IV) oxalate, Analyst 105 (1980) 950-954.

[61]

D.W. O'Sullivan, and M. Tyree, The kinetics of complex formation between Ti (IV) and hydrogen peroxide, Int. J. Chem. Kinet. 39 (2007) 457-461.

[62]

W. Yu, H. Wang, J. Hu, W. Yang, L. Qin, R. Liu, and F. Kang, Molecular Sieve Induced Solution Growth of Li2O2 in the Li-O2 Battery with Largely Enhanced Discharge Capacity, ACS Appl. Mater. Interfaces. 10 (2018) 7989-7995.

35

The authors declare no conflict of interest.