Lithiophilic Co/Co4N nanoparticles embedded in hollow N-doped carbon nanocubes stabilizing lithium metal anodes for Li–air batteries

Ziyang Guo *ab, Fengmei Wang a, Zijian Li c, Yu Yang a, Andebet Gedamu Tamirat b, Haocheng Qi a, Jishu Han a, Wei Li *b, Lei Wang *a and Shouhua Feng a
aTaishan Scholar Advantage and Characteristic Discipline Team of Eco Chemical Process and Technology, State Key Laboratory of Eco-chemical Engineering, College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology, Qingdao 266042, China. E-mail: zyguo@qust.edu.cn; inorchemwl@126.com; Fax: +86-21-51630318; Tel: +86-21-51630318
bDepartment of Chemistry, Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Fudan University, Shanghai 200433, China. E-mail: weilichem@fudan.edu.cn
cSchool of Materials Science and Engineering, Ocean University of China, Qingdao, Shandong 266100, China

Received 29th May 2018 , Accepted 29th June 2018

First published on 29th June 2018


The metallic Li electrode is considered as the most promising anode candidate for next-generation rechargeable batteries due to its super-high energy density. However, the safety and passivation issue, low coulombic efficiency and short cycle-life derived from uneven Li dendrites and an unstable solid electrolyte interphase (SEI) have hindered the practical application of Li metal batteries, especially Li–air batteries. Herein, we have synthesized Co4N-doped Co nanoparticles embedded into hollow nitrogen (N)-doped carbon nanocubes (Co/Co4N-NC) through the pyrolysis of a metal–organic framework (MOF), and further applied it as the Li plating matrix. The high content of lithiophilic Co4N, pyridinic and pyrrolic N species in Co/Co4N-NC can effectively ensure the uniform distribution of Li. In addition, the intercalation reaction in graphitized carbon coupled with electrodeposition reaction in the porous framework can further enhance the Li storage capacity of Co/Co4N-NC. Moreover, we have also demonstrated the formation of a stable SEI layer on the Co/Co4N-NC electrode during the Li plating process. Hence, the Co/Co4N-NC electrode can significantly suppress dendritic Li and shows a high coulombic efficiency of 98.5% over 300 cycles. More importantly, a Li–air battery using the lithiated Co/Co4N-NC electrode displays better cycle performance than a Li–air cell with a Li-anode in ambient air.


Introduction

The worldwide demand for high-end electronic devices has rapidly aroused a great deal of interest in developing high-energy-density rechargeable batteries.1–3 In this regard, high-capacity electrode materials are recognized as the key components for high-energy-density batteries. Lithium (Li) metal is the ultimate anode for next-generation batteries because of its super-high theoretical capacity (3861 mA h g−1) and most electronegative potential (−3.04 V vs. the standard hydrogen electrode).4–8 Hence, Li–metal batteries (LMBs), especially Li–air batteries, have received much attention since they can exhibit much higher theoretical energy than Li-ion batteries.9–14 However, the pure Li anode used in LMBs is severely impeded by dendritic Li formation during the repeated stripping/plating process. The dendritic growth of Li causes many serious problems, including drastic volume fluctuation, an unstable solid electrolyte interphase (SEI) and uncontrollable penetration, which ultimately make LMBs exhibit limited coulombic efficiency, reduced lifetime, and short-circuit and even cause explosion hazards.15

To circumvent the above problems and realize the practical application of Li metal anodes, extensive efforts have been made in recent years. For instance, a series of new liquid electrolytes (such as ionic liquids) or electrolyte additives have been introduced into LMBs to enhance the stability of the SEI.16–21 However, the modulus of the SEI layer is too low to withstand the volumetric deformation of the Li anode. Subsequently, artificial SEI layers and modified separators have been widely investigated to protect the Li anode because of their good mechanical strength.22–27 Nevertheless, the construction process of these protective layers is complicated and their operation conditions are restricted. In addition, applying high-modulus solid/polymer electrolytes is currently considered as an effective strategy for LMBs since they can improve battery safety and suppress dendritic growth,28–33 but most of them cannot work at room temperature, limiting their practical application greatly. Another new powerful solution for Li anode issues is accommodating Li within a three dimensional (3D) porous conductive matrix.34–39 For example, Guo's group36 and Lu's group38 applied 3D copper current collectors as the host to store Li, both of which effectively improved the performance of Li metal anodes. However, these porous matrixes can only provide a high surface area to plate Li, but cannot control the initial process of Li nucleation and the final size of Li particles. Therefore, only using the 3D porous matrix cannot completely eliminate the problems of dendrite growth and inferior cycle stability. Recently, some researchers demonstrated that the “lithiophilic” matrix which has strong binding energy with Li atoms can determine the sizes and sites of Li nucleation and thus suppress Li dendrite growth.40–43 Especially, Zhang and co-workers found that N-doped porous graphene electrodes with lithiophilic nitrogen-containing functional groups can effectively regulate the initial Li nucleation process and thus realize the dendrite-free Li deposits with impressive electrochemical performance.40 Hence, preparing a porous conductive matrix with uniform lithiophilic sites is vitally important for the development of the Li anode.

Herein, we report the design and fabrication of Co4N-doped Co nanoparticles encapsulated into hollow N-doped carbon nanocubes (Co/Co4N-NC) by reductive carbonization of a Co-based metal–organic-framework (MOF; i.e., ZIF-67) as a lithiophilic matrix for Li electrodeposition. The high-content N-doping (e.g. pyridinic nitrogen, pyrrolic nitrogen and Co4N) in Co/Co4N-NC can ensure the uniform distribution of Li nucleation sites. Apart from the Li deposition reaction in the porous matrix, the Co/Co4N-NC electrode can also store Li based on the intercalation reaction due to the existence of graphitized carbon which further enhances the capacity of Li plating. Therefore, the Co/Co4N-NC based Li metal anode exhibits superior plating/stripping cycling performance without Li dendrite formation. More interestingly, a stable and uniform solid electrolyte interphase (SEI) layer is formed on the surface of the lithiated Co/Co4N-NC electrode, which can act as a protective layer to alleviate air attack on this anode. Due to the aforementioned decisive benefits, a Li–air battery with the Co/Co4N-NC based Li metal anode can continuously work in ambient air and exhibit much better electrochemical performance than a Li–air battery with a pure Li anode.

Experimental section

Co/Co4N-NC preparation

Cubic ZIF-67 was first prepared (the corresponding synthesis is given in the ESI). The as-prepared cubic ZIF-67 was then pyrolyzed by heating to 900 °C at a heating rate of 5 °C min−1 under a N2 gas flow and further treated at 900 °C under N2 for 2 h. After the temperature had reduced to room temperature naturally, Co4N-doped Co nanoparticles encapsulated into porous N-doped carbon nanocubes (abbreviated as Co/Co4N-NC) were collected. In addition, Co nanoparticles encapsulated into porous carbon (Co/C), Co nanoparticles mixed with N-doped porous carbon (Co/NC), ZIF-67-derived material pyrolyzed at 800 °C (Co/Co4N-NC-800) and ZIF-67-derived material pyrolyzed at 1000 °C (Co/Co4N-NC-1000) were also synthesized for comparison (see the Experimental section in the ESI for details). Characterization instrumentation has also been given in the ESI.

Configuration of the half-battery with the Co/Co4N-NC electrode

80 wt% Co/Co4N-NC and 20 wt% polyvinylidene fluoride (PVDF) binder were uniformly dispersed in an N-methyl-2-pyrrolidone (NMP) solution to form a slurry, and the obtained slurry was coated onto a piece of Cu foil, and then heated for 12 h at 100 °C in a vacuum drying oven to remove any residual solvent. The mass loading of the active material in the Co/Co4N-NC electrodes is 1.5–2.0 mg cm−2. The resulting Co/Co4N-NC electrode, a Celgard separator dipped in 1.0 M bis (trifluoromethane) sulfonamide lithium salt (LiTFSI) in 1[thin space (1/6-em)]:[thin space (1/6-em)]1 v/v dimethoxyethane (DME) and 1,3-dioxolane (DOL) with 1.0 wt% lithium nitrate (LiNO3) additive, and the pure Li foil were assembled to form a half-battery with the Co/Co4N-NC electrode in an argon-filled glovebox (the contents of water and oxygen are less than 1 ppm).

Assembly of the Li–air battery with the lithiated Co/Co4N-NC electrode and electrochemical measurements

The Co/Co4N-NC electrode was first lithiated through an electrochemical process. Specifically, the half-cell with the Co/Co4N-NC electrode was discharged at a current density of 1 mA cm−2 within a limited time of 20 hours to form a lithiated Co/Co4N-NC composite electrode. Moreover, the surface of this anode is coated with a stable solid electrolyte interface (SEI) film which is formed due to the decomposition products of ether-based electrolyte (1.0 M LiTFSI in DME/DOL with 1.0 wt% LiNO3). After that, the battery was disassembled in an Ar-filled glovebox and the lithiated Co/Co4N-NC electrode was subsequently taken out. The obtained lithiated Co/Co4N-NC anode was washed with 1.0 M bis (trifluoromethane) sulfonamide lithium salt (LiTFSI) in tetraethylene glycol dimethyl ether (TEGDME) (1.0 M LiTFSI/TEGDME) several times. Finally, the lithiated Co/Co4N-NC anode was coupled with a KB/Ru air cathode (the corresponding preparation of the KB/Ru composite electrode is given in the ESI) in 1.0 M LiTFSI/TEGDME to form a Li–air battery with a lithiated Co/Co4N-NC anode. In addition, a Li–air battery using a pure Li anode was prepared under the same conditions. A LAND cycler (Wuhan Land Electronic Co. Ltd.) was used for electrochemical investigation.

Results and discussion

As shown in Fig. 1, Co(NO3)2·6H2O and 2-methylimidazole were used to synthesize cubic ZIF-67 through a seed-mediated growth process (see the Experimental section in the ESI for details). Due to the molecular level connections between Co species and N,C-containing organic linkers in ZIF-67, the resulting cubic sample was thus used as a precursor to obtain hollow N-doped carbon-based cubes with uniformly embedded Co/Co4N nanoparticles (Co/Co4N-NC) by a simple pyrolysis process. In addition, the as-prepared Co nanoparticles encapsulated into porous carbon (Co/C), the Co nanoparticles mixed with porous N-doped carbon (Co/NC), ZIF-67-derived material pyrolyzed at 800 °C (Co/Co4N-NC-800) and ZIF-67-derived material pyrolyzed at 1000 °C (Co/Co4N-NC-1000) were also synthesized for comparison (see the Experimental section in the ESI for details). The morphological characteristics of Co/Co4N-NC were investigated by field emission scanning electron microscopy (SEM) and transmission electron microscopy (TEM). As shown in Fig. 2a and b, the SEM images of the as-prepared Co/Co4N-NC present a typical nanocube structure with a uniform particle size of ∼410 nm which is consistent with the initial morphology of the ZIF-67 precursor (Fig. S1). This result indicates that the pyrolysis procedure does not destroy the original structure of ZIF-67. TEM images of Co/Co4N-NC (Fig. 2c and d) further exhibit that many Co/Co4N nanoparticles are homogenously embedded in the carbon framework. In addition, it can be deduced from Fig. 2e that some carbon nanotubes (CNTs) are formed on the surface of Co/Co4N-NC. Moreover, the high-resolution (HR) TEM image of Co/Co4N-NC further reveals that both the embedded Co/Co4N nanoparticles and the carbon framework are crystalline (Fig. 2f). Specifically, the distance between two adjacent lattice fringes for the outer carbon framework is found to be ∼0.33 nm which is consistent with the (002) layer of graphitic carbon, while the lattice fringe of the inner Co/Co4N nanoparticles shows an interlayer spacing of ∼0.20 nm which corresponds to the (111) plane of the Co crystal structure. Energy-dispersive X-ray (EDX) mapping was also applied to further determine the composition of Co/Co4N-NC. As shown in Fig. 2g–j, the TEM image of Co/Co4N-NC and the corresponding EDX mapping investigation confirm that the elements Co, N and C are uniformly distributed in this architecture. Fig. S2–6 provide the corresponding characterization of N/C, Co/NC and Co/C, further demonstrating the successful synthesis of the contrast samples.
image file: c8ta05013a-f1.tif
Fig. 1 Schematic illustration of the synthetic process for Co/Co4N-NC and its lithiophilic properties.

image file: c8ta05013a-f2.tif
Fig. 2 (a and b) SEM images and (c–f) TEM images of Co/Co4N-NC with different magnifications. (g) TEM image of Co/Co4N-NC (bottom left) and (h–j) the corresponding EDX mapping images of C, N and Co elements.

The crystalline structure of Co/Co4N-NC and its precursor was further analyzed using the powder X-ray diffraction (XRD) pattern. The XRD pattern of the precursor perfectly matches with the standard diffraction peaks of ZIF-67, which suggests the successful synthesis of ZIF-67 (Fig. S7). It can be observed from Fig. 3a that the XRD pattern of Co/Co4N-NC exhibits a broad peak at 26°, which can be ascribed to the (002) facet of the carbon plane. The other main peaks at around 44° and 52° correspond to metallic Co (JCPDS card: no. 150806) and the Co4N phase (JCPDS card: no. 410943), which is in agreement with previous reports.44,45 The Raman spectrum of Co/Co4N-NC (Fig. 3b) presents two prominent peaks at 1590 and 1330 cm−1 corresponding to the G and D bands, respectively, which also confirm the formation of graphitized carbon in Co/Co4N-NC. However, no carbon signal was detected in the Raman spectrum of ZIF-67 (Fig. S8). X-ray photoelectron spectroscopy (XPS) was also carried out to analyze the surface chemical composition of Co/Co4N-NC and its precursor. It can be found that C, N, O and Co species are present in Co/Co4N-NC and ZIF-67 (Fig. S9 and Table S1). The high-resolution Co 2p3/2 XPS spectrum of Co/Co4N-NC can be deconvoluted into three bands: a sharp metallic Co peak (∼778.5 eV), a peak at 780.0 eV ascribed to Co2+ and a broad peak attributed to Co4N (∼781.0 eV),43,44 indicating the co-existence of metallic Co, Co2+ and Co4N species in Co/Co4N-NC (Fig. 3c). More importantly, the content of N species in Co/Co4N-NC can reach 6.95% (Table S1), which is much higher than that of many reported materials (Table S2). The fitting result from the detailed N1s XPS spectrum of Co/Co4N-NC shows four different types of nitrogen species, corresponding to metal-bonded N (N–Co, 398.5 eV), pyridinic N (N-6, 399.5 eV), pyrrolic N (N-5, 400.9 eV), and graphitic N (N-Q, 402.4 eV) (Fig. 3d).45 It can be found from Fig. 3d that N–Co, N-5 and N-6 atoms are overwhelmingly dominant among these nitrogen species in Co/Co4N-NC. Moreover, a recent report has demonstrated that both N-5 and N-6 species have a strong interaction with Li atoms.40 Hence, Co/Co4N-NC with a high content of the lithiophilic nitrogen-containing functional groups can effectively regulate the nucleation process of metallic Li electrodeposition. The N2-adsorption/desorption isotherm measurements further demonstrate that the as-prepared Co/Co4N-NC has a BET surface area of 338.1 m2 g−1 and also exhibits a typical hierarchical porous nature (Fig. S10 and 11). In addition, a series of characterizations for Co/Co4N-NC, Co/Co4N-NC-800 and Co/Co4N-NC-1000 shown in Fig. 2, 3, S12, 13 and Tables S3 and 4 further confirm that Co/Co4N-NC (pyrolyzed at 900 °C) has the optimized electrical conductivity, N content and Co particle size.


image file: c8ta05013a-f3.tif
Fig. 3 (a) XRD pattern, (b) Raman spectra, and the high-resolution (c) Co 2p3/2 and (d) N 1 s XPS spectra of Co/Co4N-NC.

The electrochemical plating performance of Li metal on the Co/Co4N-NC electrode and reference samples, including a Co/NC electrode, Co/C electrode, Cu electrode, Co/Co4N-NC-800 electrode and Co/Co4N-NC-1000 electrode was investigated in a two-electrode configuration using 1.0 M LiTFSI in 1[thin space (1/6-em)]:[thin space (1/6-em)]1 v/v DOL/DME with 1 wt% LiNO3 electrolyte. Fig. 4a gives the voltage curves of Li deposits on these above electrodes at 0.01 mA cm−2. As shown in Fig. 4a, all these electrodes first exhibit similar voltage decrease trends and then the voltages go up to the relatively stable voltage platform, which corresponds to the Li nucleation procedure and mass-transfer process, respectively. According to Zhang's recent work,40 the difference between the voltage at the bottom tip and the subsequently stable platform is denoted as the nucleation overpotential (μn) which mainly depends on the lithiophilic surface. Hence, it can be calculated from Fig. 4a and S14a that the nucleation μn at 0.01 mA cm−2 is 0.0127 V for the Co/Co4N-NC electrode, 0.013 V for the Co/NC electrode, 0.0242 V for the Co/C electrode, 0.0254 V for the Cu foil electrode, 0.0151 V for the Co/Co4N-NC-800 electrode and 0.0162 V for the Co/Co4N-NC-1000 electrode. The largest μn of the Cu foil electrode can be attributed to the non-lithiophilic surface and the lowest specific surface area. In addition, the nucleation overpotential of the Co/NC electrode is lower than that of the Co/C electrode, which suggests that the N-doping can improve the lithiophilicity of the Co/C electrode. Moreover, the Co/Co4N-NC electrode shows the smallest μn, further demonstrating that Co4N species can also enhance the surface lithiophilicity and is thus beneficial to the uniform Li deposition on the Co/Co4N-NC electrode. Especially, the initial capacity of these carbon-based electrodes above 0 V is much higher than that of the Cu electrode, which is mainly attributed to the intercalation reaction of graphitized carbon in these electrodes. Fig. S15 further confirms that Li+ can be inserted into the bulk of Co/Co4N-NC. The nucleation overpotentials of these electrodes at different current densities are also presented in Fig. 4b. It can be found that the μn of all the electrodes increases with increasing current density. More interestingly, it should be noted that all the nucleation overpotentials on the Co/Co4N-NC electrode are much lower than those on the Co/NC, Co/C and Cu foil-based electrodes in the entire range of current density varying from 0.01 to 1 mA cm−2, further demonstrating that the synergy of high content N-doping and Co4N species can effectively ensure the uniform distribution of Li nucleation sites even at the higher current densities (Fig. S16–18). The cycling stability and coulombic efficiency (CE) of the Co/Co4N-NC, Co/NC, Co/C, Cu foil, Co/Co4N-NC-800 and Co/Co4N-NC-1000-based electrodes were also analyzed using the galvanostatic charge/discharge profiles at 0.5 mA cm−2 with a limited areal capacity of 0.5 mA h cm−2 (Fig. 4c and S14b). As shown in Fig. 4c, the Cu foil-based electrode exhibits a rapid drop after only 70 cycles with a CE of ∼86%, indicating the inferior cycle performance of the Cu foil electrode. In contrast, the Co/Co4N-NC electrode retained a CE of ∼99% after 100 cycles, while the Co/NC, Co/C, Co/Co4N-NC-800 and Co/Co4N-NC-1000 electrodes could only retain a CE of 97.5%, 98%, 97.5% and 97.1% after 100 cycles, respectively (Fig. 4c and S14b). These results suggest the superior Li plating/stripping behavior of the Co/Co4N-NC electrode. In addition, the highest CE (∼99%) of the Co/Co4N-NC electrode also indirectly demonstrates that Co/Co4N-NC is the optimized material for Li plating due to its superior electrical conductivity, the high content of lithiophilic N species and the appropriate Co nanoparticle size. On the other hand, it is well known that a high CE is closely related to the formation of a stable SEI on the electrode.39 According to the XPS spectra and Fourier transform-infrared spectroscopy (FT-IR) data (Fig. S19 and 20), it can be demonstrated that there is a stable SEI layer formed on the surface of the Co/Co4N-NC electrode.


image file: c8ta05013a-f4.tif
Fig. 4 (a) The voltage curves of Li plating on the Co/Co4N-NC, Co/NC, Co/C and Cu foil electrodes at 0.01 mA cm−2 and a duration of 20 h and (b) the corresponding overpotentials during the Li nucleation process on the Co/Co4N-NC, Co/NC, Co/C and Cu foil electrodes at different current densities. (c) Comparison of coulombic efficiencies of Li plating/stripping on the Co/Co4N-NC, Co/NC, Co/C and Cu foil electrodes at 0.5 mA cm−2 with a limited capacity of 0.5 mA h cm−2.

To further substantiate the excellent Li plating/stripping performance and electrochemical stability of the Co/Co4N-NC electrode, CE curves at higher fixed capacity were also obtained. Fig. 5a–c present the variation of CE of the bare Cu and Co/Co4N-NC electrodes over cycles at various current densities with the same limited capacity of 1 mA h cm−2. The Co/Co4N-NC electrode exhibits an excellent CE of 98.5% over 300 cycles at 0.5 mA cm−2, outperforming the recently reported anode substrates (Table S5), confirming the superior Li plating/stripping behavior of the Co/Co4N-NC electrode. Even at relatively higher current densities of 1 mA cm−2 and 2 mA cm−2, the Co/Co4N-NC electrodes still display a CE of 97.5% and 96.9% over 95 cycles, respectively. In contrast, the CE of the bare Cu electrodes rapidly drops to <80% after 60 cycles at 0.5 mA cm−2 and after 40 cycles at 1 mA cm−2, respectively. Especially at the higher current density of 2 mA cm−2, the CE of the bare Cu electrode shows a conspicuous fluctuation below 60% over 55 cycles. In addition, the Li plating/stripping performance of the Co/NC and Co/C electrodes was also tested at the same conditions for comparison (Fig. S21). As shown in Fig. 5 and S21, it can be found that the Co/Co4N-NC electrode has a longer cycle life and higher CE compared with the Co/NC and Co/C electrodes. Moreover, the Co/Co4N-NC electrode also shows a superior rate performance (Fig. S22). When the capacity limit was extended to 3.0 mA h cm−2, the CE of the Co/Co4N-NC electrode could still be maintained at ∼91.1% after more than 90 cycles, while the CE of the Cu electrodes showed a sharp oscillation after 5 cycles (Fig. S23). These results indicate that the Co/Co4N-NC electrode has a superior electrochemical performance for Li electrodeposition, which is related to the typical porous structure and plentiful lithiophilic N-containing groups in Co/Co4N-NC. The charge/discharge behaviour of the bare Cu and Co/Co4N-NC electrodes at different cycles was also investigated and the results are summarized in Fig. 5d. As shown in Fig. 5d, all the charging capacities of the bare Cu electrode are obviously lower than its corresponding discharging capacities at the 10th, 50th and 65th cycles, respectively, suggesting the huge irreversibility of Li plating/stripping on the Cu foil electrode. In contrast, there is almost no clear capacity decay on the Co/Co4N-NC electrode over all these cycles. Moreover, the voltage polarizations of the discharge/charge process for the Co/Co4N-NC electrode can retain only ∼0.05 V over the 10th, 50th and 65th cycles, while the voltage hysteresis for the bare Cu electrode can reach 0.10–0.25 V at these cycles. The smaller voltage hysteresis of the Co/Co4N-NC electrode can be attributed to the high electroactive area, enough lithiophilic sites and typical porous structure of Co/Co4N-NC which can enlarge the Li/electrolyte interface, favor the redox reactions and reduce the charge transfer resistance (Fig. S24). In addition, to further clarify the cycle performance of the Co/Co4N-NC electrode, the Co/Co4N-NC anode was first discharged with a limited capacity of 4.0 mA h cm−2 to form the lithiated Co/Co4N-NC electrode and then operated at 1.0 mA cm−2 with a limited areal capacity of 0.04 mA h cm−2 for 100 cycles. The Cu foil electrode was also treated under the same conditions for comparison (Fig. 5e). The voltage profiles of the lithiated Co/Co4N-NC electrodes are very stable and the corresponding voltage overpotentials are much lower than those of the lithiated Cu foil electrode over 100 cycles. The enhanced performance of the Co/Co4N-NC electrodes stems from the high-content lithiophilic functional groups (e.g. N-5, N-6 and Co4N), the abundant porous structure, and the good Li insertion/extraction and deposition/dissolution performance in the as-prepared Co/Co4N-NC.


image file: c8ta05013a-f5.tif
Fig. 5 Comparison of the coulombic efficiencies of Cu foil and Co/Co4N-NC electrodes at various current densities of (a) 0.5 mA cm−2, (b) 1 mA cm−2 and (c) 2 mA cm−2 with the same areal capacity of 1.0 mA h cm−2. (d) Li plating/stripping profiles of Cu foil and Co/Co4N-NC electrodes at 0.5 mA cm−2 with a limited capacity of 1.0 mA h cm−2 at the 10th, 50th, and 65th cycles. (e) Galvanostatic discharge/charge voltage curves of Cu foil and NG electrodes at 1.0 mA cm−2.

The morphological evolution and distribution of metallic Li on the Co/Co4N-NC and Cu foil electrodes were also investigated to further clarify the effects of Co/Co4N-NC over the Li nucleating process (Fig. 6). Fig. 6a–c show the SEM images after different amounts of Li plating on the Cu foil electrode at 0.1 mA cm−2. Compared with the pristine Cu foil electrode (Fig. 6a), the surface of the Cu foil electrode discharged with a capacity of 1.0 mA h cm−2 is covered with a few Li nucleation particles (Fig. 6b). Some of these deposits are wire-like Li growths with submicrometer-scale length. When the lithiated Cu foil electrode continues to be treated with additional Li plating to a total discharge capacity of 2.0 mA h cm−2 (Fig. 6c), a large quantity of Li dendrites is formed on the Cu foil electrode. Moreover, both the diameter and length size of these dendrites have clearly increased, compared with the Li growth before the additional plating (Fig. 6b). However, irrespective of whether it is at the initial stage of Li plating (Fig. 6e) or at the continuous Li deposition to a total capacity of 2.0 mA h cm−2 (Fig. 6f), the Li metal always showed uniform growth on the Co/Co4N-NC electrode without any protruding filaments. Especially, most of these smooth and flat Li deposits are mainly accommodated into the pores around the Co/Co4N-NC particles. These results further indicate that these N-containing groups (N-5, N-6 and Co4N) can guide the Li nucleation distribution and thus avoid the formation of Li dendrites on the Co/Co4N-NC electrode. Based on all the above Li plating/stripping performances and the corresponding SEM characteristics, we can deduce the possible processes for the Li nucleation and plating on the Co/Co4N-NC-based and Cu foil-based electrodes (Fig. 7). As shown in Fig. 7, small Li dendrites are firstly formed on the Cu current collector at the start of the nucleation stage since the rough and non-lithiophilic surface of the Cu foil can readily lead to the random distribution of Li plating and even the continuous Li accumulation on the outside of the initial Li nucleation sites. Subsequently, lots of huge Li dendrites are gradually formed on the Cu foil due to the synergetic effect of the electric field and the concentration gradient. In contrast, there are many lithiophilic groups (e.g. N-5, N-6 and Co4N) in the Co/Co4N-NC electrode which can provide enough active sites for the uniform distribution of Li plating and thus effectively reduce the overpotential over the nucleation process. Moreover, the porous Co/Co4N-NC can not only store Li in the porous structure, but can also accommodate enough Li+ ions in the bulk. As a result, the Co/Co4N-NC electrode is uniformly covered with a smooth and flat Li layer.


image file: c8ta05013a-f6.tif
Fig. 6 SEM images of the Cu foil electrode (a) before plating and after Li deposition with a limited capacity of (b) 1.0 mA h cm−2 and (c) 2.0 mA h cm−2. The morphology of the Co/Co4N-NC electrode (d) before plating and after Li deposition with a limited capacity of (e) 1.0 mA h cm−2 and (f) 2.0 mA h cm−2.

image file: c8ta05013a-f7.tif
Fig. 7 Illustration of the Li electrochemical nucleation and deposition process on the (a) Co/Co4N-NC electrode and (b) Cu foil electrode.

Due to the superior Li deposition/dissolution performance of the Co/Co4N-NC electrode, the lithiated Co/Co4N-NC electrode was prepared (see the Experimental section for details) and used as the anode for Li–air batteries to demonstrate its practical application. It is well known that batteries which breathe O2 from ambient air to work can be called real “Li–air batteries”. Hence, these Li–air batteries with the lithiated Co/Co4N-NC anodes were operated in ambient air. In addition, all these Li–air batteries applied the KB/Ru composite as the catalytic cathode (the corresponding preparation and characterization have been given in the Experimental section of the ESI and Fig. S25) and 1 M LiTFSI/TEGDME solution as the electrolyte. For comparison, the same amount of fresh Li film was also used as the anode for the Li–air cells at the same conditions. Fig. 8 shows the typical discharge/charge voltage curves and the corresponding terminal voltage variation versus cycle number of the Li–air batteries with the lithiated Co/Co4N-NC anode and the Li–air cells with a pure Li anode at 200 mA g−1 with a limited capacity of 500 mA h g−1. As shown in Fig. 8a, the voltage curves of the Li–air battery with the lithiated Co/Co4N-NC anode are very reproducible for 70 cycles, except for a small voltage increase between the first charge profile and the following cycles. In addition, the discharge terminal voltage of the Li–air battery with the lithiated Co/Co4N-NC anode can maintain ∼2.65 V and the corresponding voltage obtained at the terminal charge could remain very stable (<4.15 V) over 70 cycles (Fig. 8b). Moreover, the ex situ XRD pattern and ex situ FT-IR spectral data of the KB/Ru based electrodes at different discharge/charge stages further demonstrate the generation of Li2O2, LiOH and Li2CO3 at the end of discharge and the subsequent decomposition of Li2O2, LiOH and Li2CO3 after full recharge (Fig. S26). In contrast, the overpotentials between the discharge and charge terminal voltages of the Li–air battery with the pure Li anode obviously rise over 34 cycles (Fig. 8c and d). Especially, the Li–air cell with the pure Li anode displays a limited discharge capacity (∼50 mA h g−1) above 2.0 V at the 34th cycle. Hence, the cycle performance of the Li–air battery with the lithiated Co/Co4N-NC anode is much better than that of the cell with the pure Li anode, which can be attributed to the stable SEI layer and the highly reversible Li dissolution/deposition of the lithiated Co/Co4N-NC anode.


image file: c8ta05013a-f8.tif
Fig. 8 (a) Discharge/charge profiles of the Li–air battery using the lithiated Co/Co4N-NC anode at different cycles and (b) the corresponding discharge/charge terminal voltage versus cycle number. (c) Voltage curves of the Li–air cell using the fresh Li anode under different cycles and (d) voltage at the terminal of discharge/charge profiles over cycles. All the batteries were tested at a current density of 200 mA g−1 with a fixed capacity of 500 mA h g−1.

In summary, Co4N-doped Co nanoparticles encapsulated into N-doped porous carbon nanocubes (Co/Co4N-NC) were prepared and synthesized through the seed-mediated growth of cubic ZIF-67 followed by a thermolysis process, which was then used as the Li deposition support. Apart from the lithiophilicity of pyridinic and pyrrolic N, we have demonstrated that the Co4N species in Co/Co4N-NC are lithiophilic. Due to the abundant lithiophilic functional groups (e.g. N-5, N-6 and Co4N), three dimensional porous structure and high-content of Co nanoparticles, Co/Co4N-NC can not only promote the homogeneous nucleation of Li deposits, but can also store enough Li metal through the intercalation reaction of Co species and the electrodeposition reaction in the porous structure. As a result, the growth of Li-dendrites on the Co/Co4N-NC electrode can be effectively alleviated. Compared with the bare Cu foil electrode, the Co/Co4N-NC electrode also delivers a longer and more stable cycling life (300 cycles) with a higher coulombic efficiency (98.5%) at a current density of 0.5 mA cm−2 with a limited capacity of 1 mA h cm−2. Furthermore, a stable SEI film was also found to be formed on the surface of the Co/Co4N-NC electrode over the Li plating procedure which can suppress the potential contamination attack on the anode. Therefore, when the lithiated Co/Co4N-NC electrode was used as an anode coupled with a KB/Ru cathode in TEGDME-based electrolyte to form a Li–air battery, the cycle performance of this cell is obviously better than that of a Li–air battery using a Li anode in ambient air. Our initial proof of concept investigation presented here sheds new light on the development of Li–air batteries with a highly reversible Li-based anode.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors acknowledge funding support from the National Natural Science Foundation of China (51372125, 51572136 and 21571112), the Taishan scholar advantage and characteristic discipline team of Eco chemical process and technology, the Natural Science Foundation of Shandong Province (ZR2018BB034 and ZR2016BQ28), the Shanghai Science and Technology Committee (2017MCIMKF01) and the Doctoral Fund of QUST (0100229009).

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/c8ta05013a

This journal is © The Royal Society of Chemistry 2018