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carbon textiles as a flexible O2 electrode for Li–O2 batteries
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Hierarchical N-doped carbon nanocages/carbon textiles as a flexible O2 electrode for Li-O2 batteries Jia Liu , Dan Li , Siqi Zhang , Ying Wang , Guiru Sun , Zhao Wang , Haiming Xie , Liqun Sun PII: DOI: Reference:
S2095-4956(19)30881-2 https://doi.org/10.1016/j.jechem.2019.10.024 JECHEM 995
To appear in:
Journal of Energy Chemistry
Received date: Revised date: Accepted date:
23 May 2019 21 October 2019 26 October 2019
Please cite this article as: Jia Liu , Dan Li , Siqi Zhang , Ying Wang , Guiru Sun , Zhao Wang , Haiming Xie , Liqun Sun , Hierarchical N-doped carbon nanocages/carbon textiles as a flexible O2 electrode for Li-O2 batteries, Journal of Energy Chemistry (2019), doi: https://doi.org/10.1016/j.jechem.2019.10.024
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. © 2019 Published by Elsevier B.V. and Science Press on behalf of Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences
Communication
Hierarchical N-doped carbon nanocages/carbon textiles as a flexible O2 electrode for Li-O2 batteries Jia Liu, Dan Li, Siqi Zhang, Ying Wang, Guiru Sun, Zhao Wang, Haiming Xie*, Liqun Sun* Nation & Local United Engineering Laboratory for Power Batteries, Faculty of Chemistry, Northeast Normal University, Changchun 130024, Jilin, China Corresponding authors. *E-mail addresses: [email protected] (H. Xie); [email protected] (L. Sun). Keyword: N-doped carbon nanocages/carbon textiles; Flexible; Binder-free; Li-O2 batteries Supporting Information Placeholder ABSTRACT: The conventional Li-O2 battery (LOB) has hardly been considered as a next-generation flexible electronics thus far, since it is bulk, inflexible and limited by the absence of an adjustable cell configuration. Here, we present a flexible Li-O2 cell using N-doped carbon nanocages grown onto the carbon textiles (NCNs/CTs) as a self-standing and binder-free O2 electrode. The highly flexible NCNs/CTs exhibits an excellent mechanic durability, a promising catalytic activity towards the ORR and OER, a considerable cyclability of more than 70 cycles with an overpotential of 0.36 V on the 1st cycle at a constant current density of 0.2 mA/cm2, a good rate capability, a superior reversibility with formation and decomposition of desired Li2O2, and a highly electrochemical stability even under stringent bending and twisting conditions. Our work represents a promising progress in the material development and architecture design of O2 electrode for flexible LOBs.
Li-O2 batteries (LOBs) have been now attracting increasing attentions and R&D efforts as promising power sources for the electric vehicles (EVs), because they have far higher theoretical energy density than conventional Li-ion batteries (LIBs) [1]. In an ideal system, the net electrochemical reaction is 2Li + O2 ⇄ Li2O2, with a battery discharge described by the forward direction (oxygen reduction reaction (ORR) and a charge described by the reverse direction (oxygen evolution reaction (OER) [2,3]. However, it is now well-known that the real battery chemistry could be considerably more complicated. Besides several issues like stability of electrolyte, poor performance of Li metal negative electrode, etc., one of challenges for LOBs is to develop an O2 positive electrode with highly stability and superior performance [4,5].
Carbonaceous materials have been always employed as fillers to provide the porosity and electronic conductivity for the O2 electrode thus far. Due to the facilitation on the formation of a localized electron-donor state near Fermi level and the enhancement of electron-donor property for the carbon matrixes, nitrogen-doped carbon (N-doped carbon) material can even exhibit the promising catalytic performance towards the electrochemical reactions [6,7]. Many factors of N-doped carbon are believed to be linked to its electrochemical performance, such as the proper pore size and surface area5. In this respect, the hierarchical N-doped carbon nanocages (NCNs) could exhibit better electrochemical performance than one-dimensional N-doped carbon materials [8,9] and two-dimensional N-doped graphene [10,11], due to its higher theoretical specific surface area and unique hollow structure. By now, one possible approach for NCNs’ preparation is an “in situ” generated MgO template method using pyridine as the precursor [12,13], which requires complicate steps, with a high energy consumption and an inevitable difficulty to control the pore size due to secondary microstructure [14]. Owing to the uniform organicinorganic crystal structure and unique tunable cavities, metal-organic frameworks (MOFs) have exhibited a promising potential to act as both self-sacrificial templates and precursors to synthesize porous Ndoped carbon with well-defined pore size distribution [15,16]. Additionally, as s subclass of MOFs, zeolitic imidazolate frameworks (ZIFs) not only display exceptional thermal and chemical stability, but also contain a rich nitrogen source in imidazolate ligands, which makes ZIFs as a promising candidate for the development of N-doped carbon materials [17]. So far, metal embedded NCNs fabricating from ZIFs have been used for LIBs for few times [16,18] and LOBs once [19]. Unfortunately, pure NCNs derived from ZIFs as electrode material have been hardly tested in LOBs. Besides the O2 electrode material, the composition and architecture of O2 electrode are also of significance to battery performance. Recently, the binder-free O2 electrode has been introduced as an appealing alternative for avoiding the negative influence of the binder [20–24]. Equally, in view of the evergrowing demand for electronic devices with flexibility, miniaturization and increased portability, the rapid development for flexible electronics has been driven much in part by the development for the flexible electrode [25,26]. However, flexible LOB is still in its infancy, since the conventional O2 electrode is bulk, inflexible and limited by the absence of an adjustable configuration, which has been reported only a few times thus far [27–31]. Therefore, to obtain a flexible binder-free O2 electrode with highly electrochemical performance for LOBs is still a big challenge. Herein, we report a novel design of a flexible and binder-free O2 electrode (NCNs/CTs) by growth of hierarchical NCNs derived from ZIFs onto the carbon textiles (CTs). The highly flexible NCNs/CTs exhibits an excellent mechanic durability, a promising catalytic activity towards the ORR and OER, a considerable cyclability of more than 70 cycles with an overpotential of 0.36 V on the 1 st cycle at a constant current density of 0.2 mA/cm2, a good rate capability, a superior reversibility with formation and de-
composition of desired Li2O2, and a highly electrochemical stability even under stringent bending and twisting conditions. Our work represents a promising progress in the material development and architecture design of O2 electrode for flexible and wearable LOBs.
Fig. 1. (a) XRD patterns of pristine CTs and as-prepared NCNs/CTs, (b) N2 adsorption/desorption isotherm curves (Inset: the pore size distribution) of NCNs/CTs, (c) C1s and (d) N1s core level spectra of NCNs/CTs.
Fig. 1(a) shows the XRD patterns of pristine CTs and as-prepared NCNs/CTs. The diffraction peaks for both samples at 2θ = 25.3º and 42.7º can be perfectly indexed to (002) and (101) reflections of graphitic carbon (JCPDS No.00-023-0064), respectively [20]. Raman spectrum of NCNs/CTs was also studied, as shown in Fig. S1 in Supporting Information (SI). Fig. 1(b) displays the N2 adsorption/desorption isotherm curves of NCNs/CTs, which exhibits a typical IV-type curve yielding a surface area of 343 m2 g–1. The pore size distribution of as-prepared sample locates in a range of 1–160 nm, indicating the coexistence of mesopores (2–50 nm) and macropores (> 50 nm). Such unique multiscale porous structure is beneficial to achieving fast Li+ migration and O2 diffusion, which has been proved to be favorable for electrochemical reactions [30]. The NCNs/CTs has a pore size distribution mainly in a range of 1–10 nm (Inset of Fig. 1(b)). The survey spectrum (0–800 eV) of NCNs/CTs basically includes C, N, and O without any other impurities (Fig. S2 in SI). The C 1s spectrum of NCNs/CTs, as shown in Fig. 1(c), reveals that the presence of C exists as two different components at binding energies of 284.7 (C–C) and 286.0 eV (C=N) [32]. The corresponding N content of 1.9 wt% was obtained by XPS results. In Fig. 1(d), the high resolution N1s spectrum of NCNs/CTs can be deconvoluted into three types at binding energies of
394.5, 398.0 and 401.2 eV, corresponding to the pyridinic N (N1), pyrrolic N (N2) and pyridine-N-oxide groups (N3), respectively [13]. These several N-containing functional groups can not only provide a direct evidence of a successful N doping into the carbon lattice instead of dangling on carbon surface for NCNs/CTs sample, but also be favorable to promote the electrochemical reactions due to the formation of vacancies and/or defects [33,34].
Fig. 2. (a) SEM image of pristine CTs, (b and c) SEM images of NCNs/CTs, EDS elemental mapping of (d) C, and (e) N in SEM image (c), (f) TEM image of NCN, (g) linear elemental distribution of C element of NCNs corresponding to the green line in (f), and (h) HRTEM image of as-prepared NCNs.
SEM images exhibit that pristine CTs are woven by the carbon fibers with a diameter of 8 μm (Fig. 2(a)) and the NCNs are well-dispersedly grown onto the CTs (Fig. 2(b)). Fig. 2(c) shows that the NCN displays the 3D hierarchical cube-like morphology with a particle diameter of 850 nm. Note that the surface of NCN is quite rough, composing of numerous primary particles with tens of nanometers, indicating that the pores could be formed during calcination process. From the EDS elemental mapping results, as shown in Fig. 2(d and e), it can be seen that the NCN is composed of well-dispersed C and N elements, further indicating that doping N was successful. N content obtained from EDS results (10.7 wt%) was much higher than that obtained from XPS results, suggesting that the doped N was highly distributed in the interior of NCNs instead of the surface. Considering the difficulty for TEM test of selfstanding NCNs/CTs electrode, we prepared the NCNs powder under the similar synthetic method with the only difference of avoiding the CTs involvement. In Fig. 2(f and g), it can be concluded that the NCN displays the hollow interior of nanocage morphology, possessing the cage vacancy size of 550 nm and cage shell of around 100 nm. Such porous 3D hollow structure can serve as the unique tunnels for sufficient penetration of the electrolyte, favorable Li+ migration, and fast O2 diffusion, which is beneficial to the electrochemical reactions for LOBs. The HRTEM image in Fig. 2(h) displays well-defined
lattice fringe with a distance of 3.45 Å, which corresponded to the (002) lattice plane of carbon [35], confirming a successful preparation of NCNs/CTs.
Fig. 3. (a) CVs of Li-O2 cells with pristine CTs and as-prepared NCNs/CTs under O2 atmosphere at a constant scan rate of 0.1 mV/s within a potential range of 2.2–4.5 V vs. Li/Li+, (b) the discharge profiles of Li-O2 cells with different electrodes at a constant current density of 0.1 mA/cm2 with the cut off voltage of 2.3 V vs. Li/Li+, the cycling responses for cells with (c) pristine CTs, (d) NCNs + binder/CTs and (e) NCNs/CTs, and (f) the variations of round-trip efficiency and terminal charge potential for cells with different electrodes at a constant current density of 0.2 mA/cm2 under a capacity limit of 0.5 mAh/cm2 within a potential range of 2.3–4.5 V vs. Li/Li+. Note that the weight ratios of NCNs for NCNs/CTs and NCNs + binder/CTs are around 5 wt%, as seen in Experimental Section in SI.
CV was employed to investigate the catalytic activity of NCNs towards the ORR and OER. In Fig. 3(a), the NCNs/CTs electrode exhibits a higher onset reduction potential and a lower onset evolution potential as well as significantly larger cathodic and anodic currents than those of pristine CTs, demonstrating that the NCNs did enhanced the reaction kinetics. This provides a direct evidence that NCNs can act as a highly active catalyst to promote the electrochemical reactions for LOBs. Note that the delivered capacities were calculated based on the whole area of O2 electrode. In Fig. 3(b), a cell with NCNs/CTs delivers a much higher discharge capacity of 7.0 mAh/cm2 (corresponding to 11051.5 mAh/gNCNs based on
the weight of NCNs on the CTs) than that with the pristine CTs (1.8 mAh/cm2) and the NCNs + binder/CTs (4.9 mAh/cm2), indicating that the NCNs/CTs can significantly promote the ORR for LOBs. Additionally, to further study the influence of NCNs amount on the battery performance, we also prepared the electrodes of 2 wt% NCNs/CTs, 15 wt% NCNs/CTs, 2 wt% NCNs + binder/CTs, and 15 wt% NCNs + binder/CTs by adjusting the dosages of raw materials (see the Experimental Section in SI). The delivered discharge capacities for cells with different electrodes containing various amount of NCNs (2, 5, and 15 wt%) are shown in Fig. S3 in SI. For the electrodes containing the same amount of NCNs, the delivered capacity for a cell with such binder-free designed electrode is higher than that for the electrode with the binder participation. This further confirms that the binder-free architecture can enhance the discharge capacity by avoiding material aggregation and/or presenting the binder-involved parasitic reactions. Interestingly, note that the discharge capacity for a cell with 5 wt% NCNs/CTs (7.0 mAh/cm2) is similar with that for a cell with 15 wt% NCNs/CTs (7.3 mAh/cm2). This could be given an explanation that the NCNs display the obvious agglomerations for the high content NCNs-containing electrode of 15 wt% NCNs/CTs (Fig. S4 in SI), which inevitably decreases the active sites and thus exhibits the similar battery performance with low content but highly dispersed NCNs sample (i.e., 5 wt% NCNs/CTs). Therefore, 5 wt% is the most trade-off decoration amount and we thus select 5 wt% NCNs/CTs (remark “NCNs/CTs” to be short in this work) as the representative sample to the further studies. The cyclabilities of cells with difference electrodes at a constant current density of 0.2 mA/cm2 under a capacity limit of 0.5 mAh/cm2 were evaluated, as shown in Fig. 3(c-f). Note that the cell with NCNs/CTs displays an overpotential of 0.36 V on the 1st cycle, much lower than that of 1.52 V in a cell with pristine CTs and 0.73 V in a cell with NCNs + binder/CTs. In Fig. 3(f), it can be seen that the cell with NCNs/CTs exhibits a better cyclability and higher round-trip efficiency that those for cells with pristine CTs and NCN + binder/CTs. This because that the NCNs decoration on CTs could provide the active sites, which could enhance the reaction kinetics for ORR and OER. Equally, the unique nanocage structure of NCNs can facilitate the easy Li+ migration and fast O2 diffusion and the inter-particle pores can provide a short diffusion pathway for the reactants and products for better electrocatalytic activity. Moreover, binder-free architecture has been proved that it can avoid electrode material aggregation and present the binderinvolved parasitic reactions. The inhibition of side products (e.g., Li2CO3 and CH3OOLi formations) can effectively avoid the electrode passivation and capacity fading [6]. Therefore, the promoting effects on electrochemical reactions for NCNs/CTs including the suppressing of the cell overpotential and improvements of cyclic stability can be attributed to the highly catalytic activity of NCNs, the unique hierarchically porous microstructure of NCNs/CTs, and the 3D binder-free design. The cell with NCNs/CTs also exhibits a good rate performance, as seen in Fig. S5 in SI. Furthermore, the gravimetric energy and power characteristics of cells with pristine CTs, NCNs + binder/CTs, and NCNs/CTs upon the discharge
at a constant current density of 0.1 mA/cm2 with the cut off voltage of 2.3 V vs. Li/Li+ are shown in Fig. S6 in SI. Note that as-prepared NCNs/CTs delivers a higher energy density that those for pristine CTs and NCNs + binder/CTs based on the whole weight of the O2 electrode. In addition, it has been well established that the NCNs/CTs electrode exhibits a considerable potential on practical applications by comparing the gravimetric energy and power densities of free-standing carbon-based O2 electrodes during discharge in previous reports (see Table S1 in SI).
Fig. 4. SEM images of (a) discharged and (b) recharged NCNs/CTs, (c) XRD patterns, and (d) Raman spectra of NCNs/CTs discharged and recharged to 2 mAh/cm2 at a constant current density of 0.1 mA/cm2.
The reversibility of as-prepared NCNs/CTs was studied by tracking the morphology evolution and identificating the discharged and recharged NCNs/CTs (the discharge/charge profiles can be seen Fig. S7 in SI), as shown in Fig. 4. In Fig. 4(a), it can be seen that the toroidal products with a particle size of 500 nm uniformly deposited on the electrode after discharge, which has been reported in the previous work [36,37]. After recharge, as shown in Fig. 4(b), NCNs/CTs has been well recovered with absence of toroids, suggesting the highly reversibility of NCNs/CTs electrode. XRD and Raman results (Fig. 4(c and d)) prove the Li2O2 dominates the discharge produce, and which is then decomposed during recharge. The impedances for the cells with different electrodes of pristine CTs, NCNs + binder/CTs, and NCNs/CTs at different electrochemical states using a constant current density of 0.1 mA/cm2 under a specific capacity limit of 0.5 mAh/cm2 were also studied by EIS, as shown in Fig. S8 in SI. The corresponding impedance values are fitted by the equivalent circuit (Fig. S9 in SI), as shown in Table S2 in SI. After discharge, the impedance increases significantly for all cells, indicating the formation of the
insulating Li2O2 on the cathode surface (Fig. S8 in SI). Note that the impedance is almost recovered for a cell with NCNs/CTs after recharge (Fig. S8(c) in SI), confirming that most Li2O2 is decomposed. However, the impedances still maintain large for cells with pristine CTs and NCNs + binder/CTs after recharge, as seen in Fig. S8(a and b) in SI, indicating that some products remain irreversibly on the cathode surface. Therefore, based on the SEM, XRD, Raman, and EIS results, it can be concluded that the NCNs/CTs electrode exhibits a highly reversibility associated with the formation and decomposition of desired Li2O2, at least for the 1st cycle.
Fig. 5. The images of Li-O2 cells with “coffee bag” setups using NCNs/CTs as the O2 electrode for bending to (a) 0º, (b) 180º, (c) 360º, respectively, and for twisting to (a) 90º, (b) 180º, (c) 360º, respectively. Inset: the corresponding images of NCNs/CTs.
In order to investigate the potential application of NCNs/CTs electrode in foldable and flexible electronic field, the bending and twisting properties of the cells with NCNs/CTs were evaluated by powering a commercial green light-emitting diode with various torsion angles, as presented in Fig. 5. It can be seen that the electrochemical stabilities were perfectly maintained even though the cells were blended and twisted severely. These results represent a promising progress in the development of a low-cost and versatile foldable and flexible O2 electrode for LOBs.
In summary, N-doped carbon nanocages grown onto the carbon textiles (NCNs/CTs) was successfully fabricated following a facile route, which can be employed as a flexible, self-standing, and binder-free O2 electrode for LOBs. A cell with NCNs/CTs displayed a considerable cyclability of more than 70 cycles with an overpotential of 0.36 V on the 1st cycle at a constant current density of 0.2 mA/cm2, a good rate capability, a superior reversibility with formation and decomposition of desired Li2O2, and a highly electrochemical stability even under stringent bending and twisting conditions. We expect that this work can make a contribution to the development of electrode material with highly electrochemical performance and to the design of a self-standing and binder-free electrode architecture for flexible LOBs.
Supporting Information This material is available free of charge via the Internet at http://pubs.acs.org. Experimental sections and additional characterization data. Notes The authors declare no competing financial interests. Acknowledgments This work was supported by National Key R&D Program of China (2016YFB0100500), Special fund of key
technology
research
and
development
projects
(20180201097GX)
(20180201099GX)
(20180201096GX), Jilin province science and technology department. The R&D Program of power batteries with low temperature and high energy, Science and Technology Bureau of Changchun (19SS013). Key Subject Construction of Physical Chemistry of Northeast Normal University; General Financial Grant from the China Postdoctoral Science Foundation (Grant 2016M601363); Fundamental Research Funds for the Central Universities (Grant 2412017QD011); Jilin Scientific and Technological Development Program (Grant 20180520143JH); and National Natural Science Foundation of China (Grant 21805030).
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TOC Being a self-standing and binder-free O2 electrode, N-doped carbon nanocages grown onto the carbon textiles (NCNs/CTs) electrode exhibit a considerable electrochemical performance and a great potential on flexible Li-O2 batteries.
Hierarchical N-doped carbon nanocages/carbon textiles as a flexible O2 electrode for Li-O2 batteries Jia Liu , Dan Li , Siqi Zhang , Ying Wang , Guiru Sun , Zhao Wang , Haiming Xie , Liqun Sun PII: DOI: Reference:
S2095-4956(19)30881-2 https://doi.org/10.1016/j.jechem.2019.10.024 JECHEM 995
To appear in:
Journal of Energy Chemistry
Received date: Revised date: Accepted date:
23 May 2019 21 October 2019 26 October 2019
Please cite this article as: Jia Liu , Dan Li , Siqi Zhang , Ying Wang , Guiru Sun , Zhao Wang , Haiming Xie , Liqun Sun , Hierarchical N-doped carbon nanocages/carbon textiles as a flexible O2 electrode for Li-O2 batteries, Journal of Energy Chemistry (2019), doi: https://doi.org/10.1016/j.jechem.2019.10.024
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. © 2019 Published by Elsevier B.V. and Science Press on behalf of Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences
Communication
Hierarchical N-doped carbon nanocages/carbon textiles as a flexible O2 electrode for Li-O2 batteries Jia Liu, Dan Li, Siqi Zhang, Ying Wang, Guiru Sun, Zhao Wang, Haiming Xie*, Liqun Sun* Nation & Local United Engineering Laboratory for Power Batteries, Faculty of Chemistry, Northeast Normal University, Changchun 130024, Jilin, China Corresponding authors. *E-mail addresses: [email protected] (H. Xie); [email protected] (L. Sun). Keyword: N-doped carbon nanocages/carbon textiles; Flexible; Binder-free; Li-O2 batteries Supporting Information Placeholder ABSTRACT: The conventional Li-O2 battery (LOB) has hardly been considered as a next-generation flexible electronics thus far, since it is bulk, inflexible and limited by the absence of an adjustable cell configuration. Here, we present a flexible Li-O2 cell using N-doped carbon nanocages grown onto the carbon textiles (NCNs/CTs) as a self-standing and binder-free O2 electrode. The highly flexible NCNs/CTs exhibits an excellent mechanic durability, a promising catalytic activity towards the ORR and OER, a considerable cyclability of more than 70 cycles with an overpotential of 0.36 V on the 1st cycle at a constant current density of 0.2 mA/cm2, a good rate capability, a superior reversibility with formation and decomposition of desired Li2O2, and a highly electrochemical stability even under stringent bending and twisting conditions. Our work represents a promising progress in the material development and architecture design of O2 electrode for flexible LOBs.
Li-O2 batteries (LOBs) have been now attracting increasing attentions and R&D efforts as promising power sources for the electric vehicles (EVs), because they have far higher theoretical energy density than conventional Li-ion batteries (LIBs) [1]. In an ideal system, the net electrochemical reaction is 2Li + O2 ⇄ Li2O2, with a battery discharge described by the forward direction (oxygen reduction reaction (ORR) and a charge described by the reverse direction (oxygen evolution reaction (OER) [2,3]. However, it is now well-known that the real battery chemistry could be considerably more complicated. Besides several issues like stability of electrolyte, poor performance of Li metal negative electrode, etc., one of challenges for LOBs is to develop an O2 positive electrode with highly stability and superior performance [4,5].
Carbonaceous materials have been always employed as fillers to provide the porosity and electronic conductivity for the O2 electrode thus far. Due to the facilitation on the formation of a localized electron-donor state near Fermi level and the enhancement of electron-donor property for the carbon matrixes, nitrogen-doped carbon (N-doped carbon) material can even exhibit the promising catalytic performance towards the electrochemical reactions [6,7]. Many factors of N-doped carbon are believed to be linked to its electrochemical performance, such as the proper pore size and surface area5. In this respect, the hierarchical N-doped carbon nanocages (NCNs) could exhibit better electrochemical performance than one-dimensional N-doped carbon materials [8,9] and two-dimensional N-doped graphene [10,11], due to its higher theoretical specific surface area and unique hollow structure. By now, one possible approach for NCNs’ preparation is an “in situ” generated MgO template method using pyridine as the precursor [12,13], which requires complicate steps, with a high energy consumption and an inevitable difficulty to control the pore size due to secondary microstructure [14]. Owing to the uniform organicinorganic crystal structure and unique tunable cavities, metal-organic frameworks (MOFs) have exhibited a promising potential to act as both self-sacrificial templates and precursors to synthesize porous Ndoped carbon with well-defined pore size distribution [15,16]. Additionally, as s subclass of MOFs, zeolitic imidazolate frameworks (ZIFs) not only display exceptional thermal and chemical stability, but also contain a rich nitrogen source in imidazolate ligands, which makes ZIFs as a promising candidate for the development of N-doped carbon materials [17]. So far, metal embedded NCNs fabricating from ZIFs have been used for LIBs for few times [16,18] and LOBs once [19]. Unfortunately, pure NCNs derived from ZIFs as electrode material have been hardly tested in LOBs. Besides the O2 electrode material, the composition and architecture of O2 electrode are also of significance to battery performance. Recently, the binder-free O2 electrode has been introduced as an appealing alternative for avoiding the negative influence of the binder [20–24]. Equally, in view of the evergrowing demand for electronic devices with flexibility, miniaturization and increased portability, the rapid development for flexible electronics has been driven much in part by the development for the flexible electrode [25,26]. However, flexible LOB is still in its infancy, since the conventional O2 electrode is bulk, inflexible and limited by the absence of an adjustable configuration, which has been reported only a few times thus far [27–31]. Therefore, to obtain a flexible binder-free O2 electrode with highly electrochemical performance for LOBs is still a big challenge. Herein, we report a novel design of a flexible and binder-free O2 electrode (NCNs/CTs) by growth of hierarchical NCNs derived from ZIFs onto the carbon textiles (CTs). The highly flexible NCNs/CTs exhibits an excellent mechanic durability, a promising catalytic activity towards the ORR and OER, a considerable cyclability of more than 70 cycles with an overpotential of 0.36 V on the 1 st cycle at a constant current density of 0.2 mA/cm2, a good rate capability, a superior reversibility with formation and de-
composition of desired Li2O2, and a highly electrochemical stability even under stringent bending and twisting conditions. Our work represents a promising progress in the material development and architecture design of O2 electrode for flexible and wearable LOBs.
Fig. 1. (a) XRD patterns of pristine CTs and as-prepared NCNs/CTs, (b) N2 adsorption/desorption isotherm curves (Inset: the pore size distribution) of NCNs/CTs, (c) C1s and (d) N1s core level spectra of NCNs/CTs.
Fig. 1(a) shows the XRD patterns of pristine CTs and as-prepared NCNs/CTs. The diffraction peaks for both samples at 2θ = 25.3º and 42.7º can be perfectly indexed to (002) and (101) reflections of graphitic carbon (JCPDS No.00-023-0064), respectively [20]. Raman spectrum of NCNs/CTs was also studied, as shown in Fig. S1 in Supporting Information (SI). Fig. 1(b) displays the N2 adsorption/desorption isotherm curves of NCNs/CTs, which exhibits a typical IV-type curve yielding a surface area of 343 m2 g–1. The pore size distribution of as-prepared sample locates in a range of 1–160 nm, indicating the coexistence of mesopores (2–50 nm) and macropores (> 50 nm). Such unique multiscale porous structure is beneficial to achieving fast Li+ migration and O2 diffusion, which has been proved to be favorable for electrochemical reactions [30]. The NCNs/CTs has a pore size distribution mainly in a range of 1–10 nm (Inset of Fig. 1(b)). The survey spectrum (0–800 eV) of NCNs/CTs basically includes C, N, and O without any other impurities (Fig. S2 in SI). The C 1s spectrum of NCNs/CTs, as shown in Fig. 1(c), reveals that the presence of C exists as two different components at binding energies of 284.7 (C–C) and 286.0 eV (C=N) [32]. The corresponding N content of 1.9 wt% was obtained by XPS results. In Fig. 1(d), the high resolution N1s spectrum of NCNs/CTs can be deconvoluted into three types at binding energies of
394.5, 398.0 and 401.2 eV, corresponding to the pyridinic N (N1), pyrrolic N (N2) and pyridine-N-oxide groups (N3), respectively [13]. These several N-containing functional groups can not only provide a direct evidence of a successful N doping into the carbon lattice instead of dangling on carbon surface for NCNs/CTs sample, but also be favorable to promote the electrochemical reactions due to the formation of vacancies and/or defects [33,34].
Fig. 2. (a) SEM image of pristine CTs, (b and c) SEM images of NCNs/CTs, EDS elemental mapping of (d) C, and (e) N in SEM image (c), (f) TEM image of NCN, (g) linear elemental distribution of C element of NCNs corresponding to the green line in (f), and (h) HRTEM image of as-prepared NCNs.
SEM images exhibit that pristine CTs are woven by the carbon fibers with a diameter of 8 μm (Fig. 2(a)) and the NCNs are well-dispersedly grown onto the CTs (Fig. 2(b)). Fig. 2(c) shows that the NCN displays the 3D hierarchical cube-like morphology with a particle diameter of 850 nm. Note that the surface of NCN is quite rough, composing of numerous primary particles with tens of nanometers, indicating that the pores could be formed during calcination process. From the EDS elemental mapping results, as shown in Fig. 2(d and e), it can be seen that the NCN is composed of well-dispersed C and N elements, further indicating that doping N was successful. N content obtained from EDS results (10.7 wt%) was much higher than that obtained from XPS results, suggesting that the doped N was highly distributed in the interior of NCNs instead of the surface. Considering the difficulty for TEM test of selfstanding NCNs/CTs electrode, we prepared the NCNs powder under the similar synthetic method with the only difference of avoiding the CTs involvement. In Fig. 2(f and g), it can be concluded that the NCN displays the hollow interior of nanocage morphology, possessing the cage vacancy size of 550 nm and cage shell of around 100 nm. Such porous 3D hollow structure can serve as the unique tunnels for sufficient penetration of the electrolyte, favorable Li+ migration, and fast O2 diffusion, which is beneficial to the electrochemical reactions for LOBs. The HRTEM image in Fig. 2(h) displays well-defined
lattice fringe with a distance of 3.45 Å, which corresponded to the (002) lattice plane of carbon [35], confirming a successful preparation of NCNs/CTs.
Fig. 3. (a) CVs of Li-O2 cells with pristine CTs and as-prepared NCNs/CTs under O2 atmosphere at a constant scan rate of 0.1 mV/s within a potential range of 2.2–4.5 V vs. Li/Li+, (b) the discharge profiles of Li-O2 cells with different electrodes at a constant current density of 0.1 mA/cm2 with the cut off voltage of 2.3 V vs. Li/Li+, the cycling responses for cells with (c) pristine CTs, (d) NCNs + binder/CTs and (e) NCNs/CTs, and (f) the variations of round-trip efficiency and terminal charge potential for cells with different electrodes at a constant current density of 0.2 mA/cm2 under a capacity limit of 0.5 mAh/cm2 within a potential range of 2.3–4.5 V vs. Li/Li+. Note that the weight ratios of NCNs for NCNs/CTs and NCNs + binder/CTs are around 5 wt%, as seen in Experimental Section in SI.
CV was employed to investigate the catalytic activity of NCNs towards the ORR and OER. In Fig. 3(a), the NCNs/CTs electrode exhibits a higher onset reduction potential and a lower onset evolution potential as well as significantly larger cathodic and anodic currents than those of pristine CTs, demonstrating that the NCNs did enhanced the reaction kinetics. This provides a direct evidence that NCNs can act as a highly active catalyst to promote the electrochemical reactions for LOBs. Note that the delivered capacities were calculated based on the whole area of O2 electrode. In Fig. 3(b), a cell with NCNs/CTs delivers a much higher discharge capacity of 7.0 mAh/cm2 (corresponding to 11051.5 mAh/gNCNs based on
the weight of NCNs on the CTs) than that with the pristine CTs (1.8 mAh/cm2) and the NCNs + binder/CTs (4.9 mAh/cm2), indicating that the NCNs/CTs can significantly promote the ORR for LOBs. Additionally, to further study the influence of NCNs amount on the battery performance, we also prepared the electrodes of 2 wt% NCNs/CTs, 15 wt% NCNs/CTs, 2 wt% NCNs + binder/CTs, and 15 wt% NCNs + binder/CTs by adjusting the dosages of raw materials (see the Experimental Section in SI). The delivered discharge capacities for cells with different electrodes containing various amount of NCNs (2, 5, and 15 wt%) are shown in Fig. S3 in SI. For the electrodes containing the same amount of NCNs, the delivered capacity for a cell with such binder-free designed electrode is higher than that for the electrode with the binder participation. This further confirms that the binder-free architecture can enhance the discharge capacity by avoiding material aggregation and/or presenting the binder-involved parasitic reactions. Interestingly, note that the discharge capacity for a cell with 5 wt% NCNs/CTs (7.0 mAh/cm2) is similar with that for a cell with 15 wt% NCNs/CTs (7.3 mAh/cm2). This could be given an explanation that the NCNs display the obvious agglomerations for the high content NCNs-containing electrode of 15 wt% NCNs/CTs (Fig. S4 in SI), which inevitably decreases the active sites and thus exhibits the similar battery performance with low content but highly dispersed NCNs sample (i.e., 5 wt% NCNs/CTs). Therefore, 5 wt% is the most trade-off decoration amount and we thus select 5 wt% NCNs/CTs (remark “NCNs/CTs” to be short in this work) as the representative sample to the further studies. The cyclabilities of cells with difference electrodes at a constant current density of 0.2 mA/cm2 under a capacity limit of 0.5 mAh/cm2 were evaluated, as shown in Fig. 3(c-f). Note that the cell with NCNs/CTs displays an overpotential of 0.36 V on the 1st cycle, much lower than that of 1.52 V in a cell with pristine CTs and 0.73 V in a cell with NCNs + binder/CTs. In Fig. 3(f), it can be seen that the cell with NCNs/CTs exhibits a better cyclability and higher round-trip efficiency that those for cells with pristine CTs and NCN + binder/CTs. This because that the NCNs decoration on CTs could provide the active sites, which could enhance the reaction kinetics for ORR and OER. Equally, the unique nanocage structure of NCNs can facilitate the easy Li+ migration and fast O2 diffusion and the inter-particle pores can provide a short diffusion pathway for the reactants and products for better electrocatalytic activity. Moreover, binder-free architecture has been proved that it can avoid electrode material aggregation and present the binderinvolved parasitic reactions. The inhibition of side products (e.g., Li2CO3 and CH3OOLi formations) can effectively avoid the electrode passivation and capacity fading [6]. Therefore, the promoting effects on electrochemical reactions for NCNs/CTs including the suppressing of the cell overpotential and improvements of cyclic stability can be attributed to the highly catalytic activity of NCNs, the unique hierarchically porous microstructure of NCNs/CTs, and the 3D binder-free design. The cell with NCNs/CTs also exhibits a good rate performance, as seen in Fig. S5 in SI. Furthermore, the gravimetric energy and power characteristics of cells with pristine CTs, NCNs + binder/CTs, and NCNs/CTs upon the discharge
at a constant current density of 0.1 mA/cm2 with the cut off voltage of 2.3 V vs. Li/Li+ are shown in Fig. S6 in SI. Note that as-prepared NCNs/CTs delivers a higher energy density that those for pristine CTs and NCNs + binder/CTs based on the whole weight of the O2 electrode. In addition, it has been well established that the NCNs/CTs electrode exhibits a considerable potential on practical applications by comparing the gravimetric energy and power densities of free-standing carbon-based O2 electrodes during discharge in previous reports (see Table S1 in SI).
Fig. 4. SEM images of (a) discharged and (b) recharged NCNs/CTs, (c) XRD patterns, and (d) Raman spectra of NCNs/CTs discharged and recharged to 2 mAh/cm2 at a constant current density of 0.1 mA/cm2.
The reversibility of as-prepared NCNs/CTs was studied by tracking the morphology evolution and identificating the discharged and recharged NCNs/CTs (the discharge/charge profiles can be seen Fig. S7 in SI), as shown in Fig. 4. In Fig. 4(a), it can be seen that the toroidal products with a particle size of 500 nm uniformly deposited on the electrode after discharge, which has been reported in the previous work [36,37]. After recharge, as shown in Fig. 4(b), NCNs/CTs has been well recovered with absence of toroids, suggesting the highly reversibility of NCNs/CTs electrode. XRD and Raman results (Fig. 4(c and d)) prove the Li2O2 dominates the discharge produce, and which is then decomposed during recharge. The impedances for the cells with different electrodes of pristine CTs, NCNs + binder/CTs, and NCNs/CTs at different electrochemical states using a constant current density of 0.1 mA/cm2 under a specific capacity limit of 0.5 mAh/cm2 were also studied by EIS, as shown in Fig. S8 in SI. The corresponding impedance values are fitted by the equivalent circuit (Fig. S9 in SI), as shown in Table S2 in SI. After discharge, the impedance increases significantly for all cells, indicating the formation of the
insulating Li2O2 on the cathode surface (Fig. S8 in SI). Note that the impedance is almost recovered for a cell with NCNs/CTs after recharge (Fig. S8(c) in SI), confirming that most Li2O2 is decomposed. However, the impedances still maintain large for cells with pristine CTs and NCNs + binder/CTs after recharge, as seen in Fig. S8(a and b) in SI, indicating that some products remain irreversibly on the cathode surface. Therefore, based on the SEM, XRD, Raman, and EIS results, it can be concluded that the NCNs/CTs electrode exhibits a highly reversibility associated with the formation and decomposition of desired Li2O2, at least for the 1st cycle.
Fig. 5. The images of Li-O2 cells with “coffee bag” setups using NCNs/CTs as the O2 electrode for bending to (a) 0º, (b) 180º, (c) 360º, respectively, and for twisting to (a) 90º, (b) 180º, (c) 360º, respectively. Inset: the corresponding images of NCNs/CTs.
In order to investigate the potential application of NCNs/CTs electrode in foldable and flexible electronic field, the bending and twisting properties of the cells with NCNs/CTs were evaluated by powering a commercial green light-emitting diode with various torsion angles, as presented in Fig. 5. It can be seen that the electrochemical stabilities were perfectly maintained even though the cells were blended and twisted severely. These results represent a promising progress in the development of a low-cost and versatile foldable and flexible O2 electrode for LOBs.
In summary, N-doped carbon nanocages grown onto the carbon textiles (NCNs/CTs) was successfully fabricated following a facile route, which can be employed as a flexible, self-standing, and binder-free O2 electrode for LOBs. A cell with NCNs/CTs displayed a considerable cyclability of more than 70 cycles with an overpotential of 0.36 V on the 1st cycle at a constant current density of 0.2 mA/cm2, a good rate capability, a superior reversibility with formation and decomposition of desired Li2O2, and a highly electrochemical stability even under stringent bending and twisting conditions. We expect that this work can make a contribution to the development of electrode material with highly electrochemical performance and to the design of a self-standing and binder-free electrode architecture for flexible LOBs.
Supporting Information This material is available free of charge via the Internet at http://pubs.acs.org. Experimental sections and additional characterization data. Notes The authors declare no competing financial interests. Acknowledgments This work was supported by National Key R&D Program of China (2016YFB0100500), Special fund of key
technology
research
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(20180201097GX)
(20180201099GX)
(20180201096GX), Jilin province science and technology department. The R&D Program of power batteries with low temperature and high energy, Science and Technology Bureau of Changchun (19SS013). Key Subject Construction of Physical Chemistry of Northeast Normal University; General Financial Grant from the China Postdoctoral Science Foundation (Grant 2016M601363); Fundamental Research Funds for the Central Universities (Grant 2412017QD011); Jilin Scientific and Technological Development Program (Grant 20180520143JH); and National Natural Science Foundation of China (Grant 21805030).
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TOC Being a self-standing and binder-free O2 electrode, N-doped carbon nanocages grown onto the carbon textiles (NCNs/CTs) electrode exhibit a considerable electrochemical performance and a great potential on flexible Li-O2 batteries.