Nickel oxide nanoparticles decorated highly conductive Ti3C2 MXene as cathode catalyst for rechargeable Li–O2 battery

Journal of Alloys and Compounds 824 (2020) 153803

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Nickel oxide nanoparticles decorated highly conductive Ti3C2 MXene as cathode catalyst for rechargeable LieO2 battery Xingyu Li , Caiying Wen , Mengwei Yuan , Zemin Sun , Yingying Wei , Luyao Ma , Huifeng Li *, Genban Sun ** Beijing Key Laboratory of Energy Conversion and Storage Materials, College of Chemistry, Beijing Normal University, Beijing, 100875, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 10 July 2019 Received in revised form 1 January 2020 Accepted 9 January 2020 Available online 13 January 2020

With a remarkably high theoretical energy storage capacity, a rechargeable lithium oxygen battery has attracted enormous attention. However, inert kinetics of oxygen evolution reaction and oxygen reduction reaction process generate low round-trip efficiency and poor cyclability. NiO materials are recognized as efficient and low-cost catalysts for LieO2 battery. Here, we report a controllable approach to synthesize metal oxide decorated highly conductive Ti3C2 composite as cathode catalyst for rechargeable LieO2 battery. In this composite, multi-layered Ti3C2 MXene enacts a superior host to load NiO nanoparticles on account of the open layered structure, the good electronic conductivity and the excellent chemical stability. Serving as LieO2 battery cathode catalyst, NiO/Ti3C2 nanomaterials deliver a high initial capacity of 13350 mAh g
1 and good cycling performance of over 90 rounds at a current density of 100 mA g
1 and 500 mA g
1, respectively. Such properties of the prepared composite are attributed to the excellent conductivity of MXene and the high catalytic activity of NiO. As far as we know, this is the prior report that MXenes based materials are made into LieO2 battery cathodes catalyst and proved to have a potential application in cathode materials of LieO2 battery. © 2020 Elsevier B.V. All rights reserved.

Keywords: NiO/Ti3C2 nanomaterials Ti3C2 MXene Cathode catalyst LieO2 battery

1. Introduction The growing demand for future energy storage technologies has been driving rapid development in battery research [1e5]. Rechargeable lithium-oxygen (LieO2) batteries are widely studied for their extremely high gravimetric energy densities, which may meet the driving mileages need of electric vehicles [6e9]. Nevertheless, many challenges need to be addressed before they are put into practical use. In non-aqueous LieO2 battery, the discharge reaction is the process of oxygen reduction that oxygen extracted from outside atmosphere reacts with lithium ion from electrolyte to form insoluble Li2O2 in air cathode. The charging reaction is the oxygen evolution involving insoluble Li2O2 decomposing into oxygen and lithium ion. One of the major issue in LieO2 battery is the inert kinetics of oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) process in air cathode, which generates low round-trip efficiency and poor cyclability [10e14]. To alleviate

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (H. Li), [email protected] (G. Sun). https://doi.org/10.1016/j.jallcom.2020.153803 0925-8388/© 2020 Elsevier B.V. All rights reserved.

these problems, researchers proposed a useful strategy of adding efficient catalyst to air cathode. Therefore, various cathode catalysts were designed for ORR and OER process, including noble metals [15,16], alloys [17e19], transition metal oxides [20e23] and a number of carbonaceous materials [24e27]. Among these catalysts, Ni-based materials have drawn much attention due to their efficiently electrocatalytic activity and abundant availability [28e32]. While the intrinsic poor conductivity and being easy to aggregate still severely suppress the application of NiO. In order to address the above issues, increasing endeavors have been devoted to develop highly efficient and stable NiO-based electrocatalyst. Zhao et al designed urchin-like NiOeNiCo2O4 heterostructure microsphere as cathode catalyst of LieO2 battery which exhibited a high specific capacity of 8406 mA h g
1 and an enhanced cycle life of 80 cycles in contrast to the super P electrodes [20]. Zhu et al synthesized NiOgraphene foam as cathode catalyst exhibited extremely high discharge specific capacity of 25986 mA h g
1 and long-term stability of 34 rounds [33]. Combining transition metal oxide with two dimensional materials for instance graphene is a resultful strategy to improve catalyst activity for cathode materials. Recently, as a new member of the two-dimension materials family with unique electronic and

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structure properties, MXenes have attached a great deal of attention [34]. As the result of the combination of terminal transition metal of the surface with a metallically conductive carbide core, MXene is considered as a promising electrode material for energy storage devices. Ti3C2, also denoted as Ti3C2Tx (Tx denotes oxygen or fluorine surface groups), is one of the extensively researched and promising members in MXenes group [35e37]. Xue et al reported Mn3O4/Ti3C2 nanocomposite exhibits high electrochemical activity of ORR reaction owing to the conductivity and hydrophilicity of Ti3C2 MXene [38]. Zou et al designed a novel hierarchical porous NieCo-mixed metal sulfide on Ti3C2 MXene (denoted as NiCoS/ Ti3C2) which shows superior activity toward oxygen evolution reactions [39]. Wang et al synthesized ZnO/Ti3C2 composite acting as a supercapacitor electrode which exhibits an lifted specific capacitance and outstanding cycling stability [40]. Wang et al reported Fe3O4/Ti3C2 for lithium-ion batteries shows high reversible capacities and long cycle life [41]. Moreover, to maximize the advantages of Ti3C2, a variety of Ti3C2-based composite materials are synthesized, such as CNTs@Ti3C2 [42], SnSeTi3C2 [43], Ti3C2@g-C3N4 [44] and so on. The application in many other fields of MXenes materials have been reported, but little research has been done in the lithium-oxygen system. As far as we know, only Lee investigated the potential of diverse transition-metal MXene nitrides as resultful cathode materials for lithium-oxygen battery chemistry from the theoretical perspective [45]. Herein, we developed a facile ultrasonication way to introduce NiO nanoparticles into multi-layered Ti3C2 MXene and form a series of NiO/Ti3C2 nanocomposite as cathode catalyst of LieO2 battery. NiO nanoparticles homogeneously dispersed on the surface and interlayer of Ti3C2 MXene in NiO/Ti3C2 composites, thus enhancing the exposure of catalytic active sites. In addition, the high metallic conductivity of Ti3C2 substrate acts as conductive network, which facilitates electron transporting through the whole electrode. As expected, the NiO/Ti3C2 composite, synthesized with a weight ratio of 2:5, exhibits a high specific capacities up to 13350 mAh g
1 and an excellent cycle performance over 90 rounds at a current density of 100 and 500 mA g
1, respectively. Hence, it is believed that the composites of transition metal oxide and two-dimensional layered nanomaterials (TMO/MXenes) have potential application in LieO2 battery cathode catalyst.

was heated at 400  C for 2 h in air by the heating rate of 5  C/min. The as-formed product was NiO nanoparticles.

2. Experimental section

The air electrodes of LieO2 battery were fabricated with mixing active materials (NiO/Ti3C2, Ti3C2, NiO), Ketjenblack (KB), and polyvinylidene fluoride (PVDF) binder in N-methyl-2-pyrrolidinone (NMP) with the weight ratio of 45 : 45: 10, respectively. The homogeneous slurry was pasted on the carbon papers (purchased from Toray) and then dried at 80  C overnight in vacuum oven to obtain air electrodes. The slurry was pasted on the carbon paper with a loading about 0.80 mg as a result the loading of active material and KB were about 0.36 mg. As a comparison, the mass of the pure KB electrode was about 0.4 mg. The current density and specific gravimetric capacity of all electrodes were calculated based on the mass of the KB. The LieO2 battery composed of a Li foil anode, a glass fiber filter separator, an electrolyte and an air cathode, were assembled in an argon-filled glove box (MBraun, Germany) with an environment of water and oxygen levels less than 0.5 ppm. The electrolyte was composed of 100 mL 1 M bis-(trifluoromethane) sulfonamide lithium salt (LiTFSI, NanJie-Scientific, 99.95%) in the tetraethylene glycol dimethyl ether (TEGDME, NanJie-Scientific, 99%) with the water concentration at less than 10 ppm. All tests were conducted in CR2032 coin cells with porous channel to get oxygen in. Galvanostatic discharge-charge test was performed on a NEWARE (Shenzhen China) multichannel battery test system confined the voltage between 2.0 and 4.5 V. Cyclic

2.1. Preparation of Ti3C2 MXene and NiO nanoparticles The 2.5 g of as-purchased Ti3AlC2 (98%, Beijing Forsman Technology Company) powders were blended with 30 mL ethanol and ball-milled for 8 h with 50 r/min to get a homogeneous slurry. The acquired slurry was separated by centrifuging and drying in a vacuum oven at 60  C for 24 h. Then residue was ground down to fine powder. Next, the 2 g of Ti3AlC2 fine powders were mixed with 30 mL HF solutions (40 wt %) with magnetically stirring at 60  C for 24 h to selective etching of the Al layers completely. After that the resulting suspension was centrifuged and washed with distilled water for several times to remove HF until the pH tuned to neutral. The obtained product was Ti3C2 powder and dried under vacuum oven at 80  C for 24 h. NiO nanoparticles were synthesized using hydrothermal method followed with heat-treatment process. First, 0.45 g CO(NH2)2 and 0.43 g Ni(NO3)2$6H2O were dissolved in 80 mL H2O and magnetically stirred for 30 min to obtain a homogeneous solution. Next, the homogeneous solution was poured into a 100 mL Teflon-lined autoclave and maintained 140  C for 12 h to get precursor. After centrifuged and rinsed with distilled water for three times, the mixture was dried at 100  C for 24 h. Then the percursor

2.2. Preparation of NiO/Ti3C2 nanomaterials 100 mg of Ti3C2 powder and 40 mg of NiO nanoparticles were mixed in 80 mL and 20 mL distilled water and treated by ultrasonication for 1 h respectively. Then, two acquired suspension were blended together next with long ultrasonication process retained 6 h. After that, this suspension was separated by centrifugation and washed with distilled water and ethanol to get the NiO/Ti3C2 composite. Lastly, the as-formed composite was dried in vacuum oven at 60  C for 24 h. Other composites with different mass ratios of NiO and Ti3C2 were synthesized under the same conditions. The prepared NiO/Ti3C2 nanomaterials are denoted as NiO/Ti3C2-1, NiO/ Ti3C2-2, and NiO/Ti3C2-3 with the weight ratios of NiOeTi3C2 being 1:5, 2:5 and 3:5, respectively. 2.3. Materials characterization The crystalline structure of all samples were characterized by Xray powder diffraction (XRD) using X’pert ProMPD diffractometer (CuKa,l ¼ 1.54056 Å). The morphology and size of all the synthesized nanomaterials were analyzed with the field-emission scanning electron microscopy (FESEM, S-8010, Hitachi) and the highresolution transmission electron microscopy (HRTEM, FEI, Techai G2 F20). X-ray photoelectron spectroscopy (XPS) was measured to investigate the surface composition of the samples using an ESCALAB 250Xi spectrometer (Thermo Fisher) with Al Ka radiation as the X-ray source for excitation. Nitrogen adsorption-desorption test were conducted by Quadrasorb SI analyzer 77 K. BrunauereEmmette-Teller (BET) surface area was calculated using experimental points at a relative pressure of P/P0 ¼ 0.05e1.00. The pore size distribution was obtained from the Barrett-Joyner-Halenda (BJH) method. Inductively coupled plasma-atomic emission spectrometry (ICP-AES) measured by Jarrel-ASH, ICAP-9000 was used to characterize the composition of all sample. 10 mg samples were dissolved in 5 mL aqua regia and heated for fully dissolution. 2.4. Electrochemical measurements

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voltammetry (CV) were carried out via an electrochemical workstation (Zennium IM6 station, Germany). The electrochemical impedance spectrum was tested by AC impedance over the frequency range from 10 mHz to 100 kHz with an amplitude of 5 mV. The lithium foils were served as the reference electrode and counter electrode, and the air cathode was served as the working electrode. 3. Result and discussion As illustrated in Fig. 1a, the NiO/Ti3C2 MXene composites were synthesized via the ultrasonication of Ti3C2 MXene and NiO nanoparticles. Firstly, accordion-like Ti3C2 MXene (Fig. 1c) nanosheets possessing flat surface and large lateral size were manufactured by selectively etching out the Al layers in pristine Ti3AlC2 (Fig. 1b). The followed step was further exfoliation and hybridization by ultrasonication treatment. As shown in Fig. S2f, NiO nanoparticles with a very small size (about 8 nm) could be easily moved into the interlayer space of the multi-layered Ti3C2 or adsorbed on its surface by van der Waals interactions. NiO nanoparticle can act as the spacer in the NiO/Ti3C2 composite to prevent the Ti3C2 nanosheet from restacking. Fig. 1def reveal the morphology of the NiO/Ti3C2 composites with different mass ratio of NiO nanoparticles. As is shown in Fig. 1def and Fig. S2, the area covered by NiO nanoparticles on the surface of Ti3C2 MXene increases as enhancing the mass ratio. However, it is apparent that NiO nanoparticles got aggregation for the NiO/Ti3C2-3 composite (Fig. 1f) which can be

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explained that the restricted multi-layered Ti3C2 MXene surface could not load the excess NiO nanoparticles with good dispersibility. The crystalline structures and morphologies of all the products were investigated via X-ray powder diffraction. Fig. 1g shows the XRD patterns of Ti3C2 and NiO/Ti3C2 nanomaterials. The diffraction peaks obtained from the XRD spectra of the as-prepared Ti3C2 MXene make a perfect match with the characteristic structure of the etched Ti3C2 MXene [34,46]. After the HF treatment, the diffraction peak at 2q ¼ 39 belonging to the Ti3AlC2’s (104) plane vanished, and the (002) peak attributed to layered Ti3C2 shifted to small degree (Fig. S1). Both results manifest that the Al layers have been selectively etched then the Ti3AlC2 phase are transformed into layered Ti3C2 MXene. As for the XRD pattern of NiO/Ti3C2, extra five peaks at 37.2 , 43.3 , 62.9 , 75.4 and 79.4 are observed, which correspond to the (111), (200), (220), (311) and (222) crystal faces of cubic NiO phase (JCPDS card no. 47-1049), respectively. Besides, the diffraction peak at 62.9 grow stronger meaning the loading mass of NiO increases in NiO/Ti3C2 composites. Since the XRD patterns of as-prepared NiO/Ti3C2 composites perfectly agree with the ones of the layered Ti3C2 and NiO crystal, it can be concluded that the NiO/ Ti3C2 composites are successfully synthesized. TEM, HRTEM and SAED were performed to further study the crystalline structures and morphologies of Ti3C2 MXene and NiO/ Ti3C2 nanomaterials, as shown in Fig. 2. TEM image of Ti3C2 (Fig. 2a) displays that Ti3C2 is a layered structure after HF etching, which agrees with the Naguib’s experiment results [34]. As shown in the high-resolution transmission electron microscopy (HRTEM) image,

Fig. 1. (a) Schematic describing the synthesis process of NiO/Ti3C2 nanomaterials; SEM images of (b) bulk Ti3AlC2, (c) accordion-like Ti3C2 MXene, (d) NiO/Ti3C2-1, (e) NiO/Ti3C2-2 and (f) NiO/Ti3C2-3 samples; (g) XRD patterns of Ti3C2 and NiO/Ti3C2 nanomaterials.

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Fig. 2. (a) TEM image and (b) high-resolution TEM image of Ti3C2 MXene, inset shows the SAED pattern of Ti3C2; (c) TEM image and (d) high-resolution TEM image of the NiO/Ti3C22 sample, inset shows the SAED pattern of NiO; (e) SEM image and individual Ti, C, Ni and O element mapping images for NiO/Ti3C2-2 sample.

the as-prepared Ti3C2 MXene is only several layers thick and the layer thickness is about 1.0 nm. The inset of Fig. 2b shows the select area electron diffraction of Ti3C2 MXene which indicates the hexagonal symmetry of the planes [47]. Fig. 2c reveals that NiO nanoparticles, with a narrow size distribution between 5 nm and 10 nm (Fig. S2(f)), completely covered the surface of Ti3C2. Fig. 2d shows the lattice fringes of NiO/Ti3C2 composite, where the interplanar spacing of 0.24 nm should be assigned to the (111) planes of NiO. The SAED pattern (inset in Fig. 2d) of NiO/Ti3C2 composite shows the identifiable diffraction circles corresponding to (111) and (200) planes of NiO, which agrees with the XRD result. As shown in Fig. 2e, the elements distribution of Ti, C, Ni and O can be clearly observed in mapping images of the NiO/Ti3C2-2 sample, which illustrates that the NiO nanoparticles are anchored on the Ti3C2 matrix. XPS was carried out to obtain the detailed chemical composition and bond structure of Ti3C2 and NiO/Ti3C2 composites. As depicted in Fig. 3a, the XPS survey spectrum of Ti3C2 MXene presented the characteristic peaks of Ti 2p, C 1s, O 1s and F1s while no singal for Al emerged, revealing successfully getting rid of Al layers and introduction of oxygen and fluorine group during HF treatment. The XPS survey spectrum of NiO/Ti3C2-2 indicates the existence of Ni element in the composite and it can be seen as a proof of NiO covering on Ti3C2 MXene.The high resolution XPS spectra of Ti 2p core levels of Ti3C2 and NiO/Ti3C2 samples are shown in Fig. 3b. The Ti 2p of Ti3C2 spectra reveals the presence of TieC, Ti2þ and Ti3þ bonds at 455.1 eV (461.4 eV), 455.8 eV (461.4 eV) and 457.2 eV (462.9 eV), respectively. Moreover, two peaks at 458.6 eV (sp3) and 464.2 eV (sp1) are corresponding to the TieO bonds. In the case of Ti 2p for NiO/Ti3C2 spectra, it can be seen that the peaks related to TieO bonds are obviously stronger compared with pristine Ti3C2; meanwhile the peak corresponding to TieC bonds become weaker than Ti 2p of Ti3C2 spectra. The reason for the difference in the Ti 2p profile of Ti3C2 and NiO/Ti3C2 is that the surface of Ti3C2 is uniformly covered with NiO nanoparticles and the surface of Ti3C2 is partially oxidized during the synthesis process of NiO/Ti3C2 composite. However, the TiO2 phase is not detected by XRD, indicating that the oxidation only partially occurred on the surface [47e49]. As for the high resolution XPS spectra of C 1s spectra in Fig. 3c, C]O at 288.5 eV, CeO at 286.3 eV, and CeC at 284.6 eV are clearly demonstrated [50,51]. In particular, the peak at 281.5 eV indicates the presence of CeTi bonds which implied the structure of Ti3C2

MXene is retained. For further investigation, the high resolution XPS spectra Ni 2p core levels of both samples are shown in Fig. 3d. The Ni 2p spectra of both samples are well fitted with two spinorbit doublets and two shake-up satellites (denoted as Sat.). The above results demonstrate that the NiO nanoparticles anchored on Ti3C2 are successfully synthesized by the facile ultrasonication method. The BET tests and Barrett-Joyner-Halenda (BJH) model were used to obtain the surface areas and pore size distribution of Ti3C2 and NiO/Ti3C2-2 composites. As presented in Fig. 4a, the NiO/Ti3C22 composite displays a hysteresis loop, meaning the presence of the meso-porous structure. The specific surface and porous property of all samples are listed in Table S1. The corresponding BET surface areas of NiO/Ti3C2-2 is about 58.899 m2g-1 which is much more than 7.345 m2g-1 of Ti3C2 MXene. The higher surface area of the NiO/Ti3C2-2 nanomaterial is explained by the uniform distribution of NiO nanoparticles on Ti3C2 surface as well as the increasing interlayer spacing of the Ti3C2 sheets. As shown in Fig. 4b, it is exhibited that the mean size of a pore is around 10 nm for Ti3C2 and NiO/Ti3C2-2 composite. The NiO/Ti3C2-2 nanomaterial with mesoporous structure and high surface area offer enough space to accommodate discharged product. To examine the rate and the cyclic performance, the lithiumoxygen batteries are further tested by galvanostatic chargedischarge measurements. Fig. 5a displays the first full dischargecharge curve for NiO/Ti3C2, pure NiO and Ti3C2 at a current density of 100 mAh g
1. The initial specific discharge capacity of NiO/ Ti3C2-2 is 13350 mAhg
1, which is higher than other catalyst. Fig. 5b shows the rate capability of the LieO2 battery with NiO/Ti3C2-2, evaluated at different current densities (100, 200, and 500 mA g
1). With current density increasing from 100 to 500 mA g
1, the initial discharge capacity decreases from 13350 mAh g
1 to 5790 mAh g
1. Fig. 5c demonstrates the first discharge-charge curve of LieO2 battery with Ti3C2, NiO and NiO/Ti3C2 electrode at 500 mA g
1 with a limited capacity to 500 mA h g
1. The NiO/Ti3C2-2 electrode exhibits the lowest discharge and charge over-potentials of 0.21 V and 0.64 V, respectively, thus leading to obtain a high round-trip efficiency of 76.4%. The cyclability of the LieO2 battery are investigated by a capacity-limited strategy to restrict the depth of the discharge at 500 mA g
1. As can be seen from Fig. 5dee and Fig. S3, the NiO/ Ti3C2-2 sample can steadily cycle over 90 rounds which is much more than pure Ti3C2, NiO, Ketjenblack (KB) and other NiO/Ti3C2

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Fig. 3. (a) XPS profiles of the Ti3C2 and NiO/Ti3C2-2 samples (b) Ti 2p spectra and (c) C 1s spectra of the Ti3C2 MXene and NiO/Ti3C2-2 samples. (d) Ni 2p spectra of the NiO and NiO/ Ti3C2-2 samples.

Fig. 4. (a) Nitrogen adsorption-desorption isotherms and (b) pore size distributions of Ti3C2 and NiO/Ti3C2-2 samples.

composites. In the case of the NiO/Ti3C2 composites, it could be noticed that the loading mass of NiO shows a significant impact on the electrochemical performance. With the increase of loading mass of NiO in NiO/Ti3C2 composites (Table S2), the capacity and

cycle performance are both first strengthened and then weakened. For the NiO/Ti3C2-1, its capacities and cycling stability are higher than pure Ti3C2 because of the introduction of NiO with catalytic activity. As for NiO/Ti3C2-3 hybrid, the specific capacity is much

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Fig. 5. (a) First full dischargeecharge profiles of Ti3C2, NiO and NiO/Ti3C2 based cathodes measured at 100 mAh g
1. (b) Specific capacity of NiO/Ti3C2-2 based cathodes operated at different current densities. (c) Dischargeecharge curves and (d) The cycle performance of Ti3C2, NiO and NiO/Ti3C2 based cathode with a capacity limitation of 500 mAh g
1 at 500 mA g
1.(e) The charge and discharge voltage profiles at different cycles of NiO/Ti3C2-2 at a current density of 500 mA g
1 limited of 500 mAh g
1. (f) CV curves of LieO2 batteries with Ti3C2, NiO and NiO/Ti3C2 nanocomposite at a sweep rate of 5 mV s
1. (g) EIS before the first discharge of the LieO2 battery with Ti3C2, NiO and NiO/Ti3C2 nanocomposite. (h) Charge-transfer mechanism.

higher than pure Ti3C2 and NiO/Ti3C2-1, but lower than NiO/Ti3C2-2 due to its excess aggregation of NiO in the beginning that leads to the reduction of electrocatalytic activity sites. Obviously, NiO/Ti3C22 with a moderate loading mass of NiO shows the best performance at diverse current rates from 100 to 500 mA g
1 and long charge/ discharge cycles among the investigated samples. In order to investigate the electrocatalytic activity of Ti3C2, NiO and NiO/Ti3C2, CV is performed at a sweep rate of 5 mV s
1. As can be seen from Fig. 5f, the onset reduction potential and peak current of NiO/Ti3C2-2 are higher than other samples, which illustrate high activity for ORR. The two broad OER peaks of NiO/Ti3C2 are preliminarily assigned to the deintercalation process from the outer part of Li2O2 for the peak at low potential and the oxidation of the bulk of Li2O2 for the peak at high potential, respectively [52]. The NiO/Ti3C2-2 also presented a much higher peak current for OER than pure NiO and Ti3C2, suggesting high OER catalytic activity. In addition, there is obviously shift of the position of oxidation and reduction peak after the combination of NiO nanoparticles and Ti3C2. It can be ascribed to the layer MXene’s favorable electronic

conductivity as well as the high catalytic activity of NiO nanoparticles in the NiO/Ti3C2 composite. Fig. 5g shows the EIS spectrum of the LieO2 battery with the Ti3C2, NiO and NiO/Ti3C2 electrodes. The inset in Fig. 5g is the corresponding equivalent circuit, where RS, Rct, W, and CPE stand for the solution resistance, charge-transfer resistance, mass transport component (Warburg impedance), and double-layer capacitance, respectively. The Ti3C2 MXene with excellent electronic conductivity can act as a conductive network thus promote the transport of electron/ion in the electrode. As presented in the fitting results of Table S3, the Nyquist semicircle (RS) of the NiO/Ti3C2 composites cathode is lower than pure the NiO, indicting higher electronic conductivity and lower chargetransfer resistance on account of the introduction of Ti3C2. The charge-transfer mechanism of NiO/Ti3C2 composites in LieO2 battery are shown in Fig. 5h. The unique structure of NiO nanoparticles anchored on layer-structured Ti3C2 possesses following three advantages. First, the high specific surface area of the Ti3C2 layered structure facilitates electrolyte and oxygen infiltration effectively. Second, the homodisperse of NiO on Ti3C2 MXene is beneficial to

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increase electrochemical active sites and shorten pathway for Liþ diffusion during the electrochemical process. Third, the highly conductive Ti3C2 MXene and highly electrocatalytic NiO nanoparticles contact close that ensure the effective electrons transport. From the above results, we can conclude that the NiO/Ti3C2 MXene composite shows significant advantages compared to the pure MXene and NiO nanoparticles. Furthermore, Table S4 concluded the LieO2 batteries performance of the NiO/Ti3C2 catalyst in comparison with some representative catalysts according to previous literature. It clearly demonstrates that the NiO/Ti3C2 composite electrode shows a better performance than most of the other electrodes under similar testing conditions, especially in terms of capacity and cycling stability, which makes the NiO/Ti3C2 composite a promising material for advanced LieO2 batteries. The XRD measurements was used to investigate the discharged and charged products of NiO/Ti3C2 cathode. As shown in Fig. 6a, there are three emerging diffraction peaks at 32.8, 34.8 and 58.7 for the discharged cathode, which correspond to diffraction peaks of the (100), (101) and (110) planes of the discharge product Li2O2 (JCPDS 73-1640), respectively. No other diffractions peak appeared, especially the diffraction peak of Li2CO3, suggesting that Li2O2 is the dominating product. After the charge process, the Li2O2 diffraction peak almost vanished, indicating that Li2O2 was almost completely decomposed. To further characterize the morphology and reversible formation and decomposition of Li2O2, SEM measurements were carried out. Fig. 6b, c and d display of the SEM images of the NiO/Ti3C2 electrode before discharge, after discharge and after charge respectively. As can be observed in Fig. 6c, a large amount of

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the discharged product Li2O2 have a toroidal shape [53] and the size of the toroidal particles ranges from 300 to 400 nm. When the lithium-oxygen battery is fully charged, all the discharged product Li2O2 particles disappear. This indicates that the NiO/Ti3C2 sample has a stable structure in the catalytic process.

4. Conclusion To summarize, a facile ultrasonication way to construct NiO/ Ti3C2 nanomaterials have been developed and primarily taken as cathode catalyst for LieO2 battery. The composition materials of NiO nanoparticles and Ti3C2 MXene nanosheet exhibit excellent LieO2 battery performance. It profits from the MXene’s favorable electronic conductivity as well as the interaction of layered Ti3C2 and NiO nanoparticles. To be specific, the Ti3C2 MXene ensures fast electron transfer and the NiO nanoparticles provides enough catalytic active sites. The good dispersion of the NiO nanocrystals on the Ti3C2 tremendously enhances the concentration of the electrocatalyst active sites of the NiO/Ti3C2, thus improving the catalytic activity of the NiO/Ti3C2 nanomaterials. Among all the NiO/Ti3C2 nanomaterials, the NiO/Ti3C2-2 cathode displays highest initial discharge capacity up to 13350 mAh g
1 as well as lowest overpotentials for both OER and ORR at 500 mA g
1. More importantly, the NiO/Ti3C2-2 cathode material exhibits outstanding cycle performance of more than 90 cycles at the current density of 500 mA g
1. These excellent electrochemical characteristics of NiO/ Ti3C2 indicate it can serve as a promising candidate for highperformance LieO2 battery cathode catalyst. In addition, the

Fig. 6. (a) XRD patterns of the NiO/Ti3C2 based cathode in different states. SEM images of the NiO/Ti3C2 based cathode in different: (b) before discharge, (c) after discharge, (d) after recharge.

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synthetic method is facile and universal, thus can be applied to fabricate other TMO/MXenes nanomaterials used in energy storage. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement Xingyu Li: Conceptualization, Methodology, Software, Investigation, Writing - original draft. Caiying Wen: Validation, Formal analysis, Visualization, Software. Mengwei Yuan: Validation, Formal analysis, Visualization. Zemin Sun: Resources, Writing review & editing, Supervision, Data curation. Yingying Wei: Resources, Investigation, Data curation. Luyao Ma: Resources, Investigation, Data curation. Huifeng Li: Writing - review & editing, Project administration, Funding acquisition. Genban Sun: Writing review & editing, Funding acquisition. Acknowledgements This work was supported by the National Natural Science Foundation of China of China (grant number: 21871028, 21471020 and 21771024). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2020.153803. References [1] M. Liu, Z. Yang, H. Sun, C. Lai, X. Zhao, H. Peng, T. Liu, A hybrid carbon aerogel with both aligned and interconnected pores as interlayer for highperformance lithium-sulfur batteries, Nano. Res. 9 (2016) 3735e3746. [2] M. Idrees, S. Batool, J. Kong, Q. Zhuang, H. Liu, Q. Shao, N. Lu, Y. Feng, E.K. Wujcik, Q. Gao, T. Ding, R. Wei, Z. Guo, Polyborosilazane derived ceramicsNitrogen sulfur dual doped graphene nanocomposite anode for enhanced lithium-ion batteries, Electrochim. Acta 296 (2019) 925e937. [3] X. Lou, C. Lin, Q. Luo, J. Zhao, B. Wang, J. Li, Q. Shao, X. Guo, N. Wang, Z. Guo, Crystal structure modification enhanced FeNb11O29 anodes for lithium-ion batteries, ChemElectroChem 4 (2017) 3171e3180. [4] K. Le, M. Gao, W. Liu, J. Liu, Z. Wang, F. Wang, V. Murugadoss, S. Wu, T. Ding, Z. Guo, MOF-derived hierarchical core-shell hollow iron-cobalt sulfides nanoarrays on Ni foam with enhanced electrochemical properties for high energy density asymmetric supercapacitors, Electrochim. Acta 323 (2019), 134826. [5] B. Kirubasankar, V. Murugadoss, J. Lin, T. Ding, M. Dong, H. Liu, J. Zhang, T. Li, N. Wang, Z. Guo, S. Angaiah, In situ grown nickel selenide on graphene nanohybrid electrodes for high energy density asymmetric supercapacitors, Nanoscale 10 (2018) 20414e20425. [6] Z. Jiang, K.M. Abraham, A polymer electrolyte-based rechargeable lithium/ oxygen battery, J. Electrochem. Soc. 143 (1996) 1e5. [7] P.G. Bruce, S.A. Freunberger, L.J. Hardwick, J.-M. Tarascon, Li-O2 and Li-S batteries with high energy storage, Nat. Mater. 11 (2011) 19. bart, M. Holzapfel, P. Nova k, P.G. Bruce, Rechargeable Li2O2 [8] T. Ogasawara, A. De electrode for lithium batteries, J. Am. Chem. Soc. 128 (2006) 1390e1393. [9] M. Park, H. Sun, H. Lee, J. Lee, J. Cho, Lithium-air batteries: survey on the current status and perspectives towards automotive applications from a battery industry standpoint, Adv. Energy. Mater. 2 (2012) 780e800. [10] F. Cheng, J. Chen, Metal-air batteries: from oxygen reduction electrochemistry to cathode catalysts, Chem. Soc. Rev. 41 (2012) 2172e2192. [11] N. Feng, P. He, H. Zhou, Critical challenges in rechargeable aprotic Li-O2 batteries, Adv. Energy. Mater. 6 (2016), 1502303. [12] F. Li, T. Zhang, H. Zhou, Challenges of non-aqueous Li-O2 batteries: electrolytes, catalysts, and anodes, Energy. Environ. Sci. 6 (2013) 1125e1141. [13] J. Wang, Y. Li, X. Sun, Challenges and opportunities of nanostructured materials for aprotic rechargeable lithium-air batteries, Nano Energy 2 (2013) 443e467. [14] R. Padbury, X. Zhang, Lithium-oxygen batteries-Limiting factors that affect performance, J. Power Sources 196 (2011) 4436e4444. [15] X. Xin, K. Ito, Y. Kubo, Electrochemical behavior of Ru nanoparticles as

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