# N- and F-Co-Doped Carbon Quantum Dots Coated on a Ni Foam Substrate as Current Collector for Highly Stable Li-Air Batteries

Authors: Yuqi Ma; Ki-Wook Sung; Hyo-Jin Ahn
Journal: International Journal of Energy Research
 (2023)

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## Abstract

Because of their high theoretical density, Li-air batteries (LABs) are expected to be critical components of future energy storage devices. However, the commercial use of LABs is limited by their low charge-discharge efficiency, high overpotential, and low cyclic stability. Therefore, to improve the cyclic stability of LABs, we manufactured N- and F-co-doped carbon quantum dots (NF-CQDs), thinly coated them on the surface of a Ni foam current collector via ultrasonic spray coating, and used the as-fabricated NF-CQD/Ni as the current collector of an LAB cathode. The NF-CQD/Ni-based cathode maintained remarkable cycling stability over 42 cycles (with a limited capacity was 1000 mAh/g) and exhibited a superior specific discharge capacity of approximately 5121.91 mAh/g (at 100 mA/g). NF-CQD/Ni-based cathode exhibits superior electrochemical properties, including its high cycling stability and high discharge capacity, primarily contributed by the NF-CQD coating layer, which prevented the oxidation of the Ni foam substrate and increased the electrical conductivity and electrocatalytic activity of the cathode. Therefore, NF-CQD/Ni can serve as an excellent current collector for LAB cathodes.


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## Body

## 1. Introduction

The greenhouse emitted from burning fossil fuel is one of the primary reasons for global warming becoming increasingly serious. Therefore, considerable efforts have been dedicated to replacing fossil fuels with renewable energy sources [1–3]. Moreover, to meet the need for electric devices, the demand for energy storage devices with excellent safety, stable performance, and high energy density has increased [4, 5]. Li-air batteries (LABs) present an excellent theoretical energy density (11400 Wh/kg) comparable to gasoline (13200 Wh/kg) and are environmentally friendly; therefore, they are expected to be remarkable energy storage systems [6–9]. However, LABs exhibit critical drawbacks, for instance, a high overpotential and low cycling stability, which restrict their practical application [10, 11].Typical LABs were comprised an electrolyte, a cathode, an anode, and a separator. Discharge-charge process of LABs can be described as follows [12–14]:
(1)Dischargeprocess:oxygenreductionreactionORR;2e‐+2Li++O2⟶Li2O2Chargeprocess:oxygenevolutionreactionOER;Li2O2⟶2e‐+2Li++O2In particular, the low cycling stability of LABs is ascribed to the incomplete decomposition and accumulation of Li2O2 at the cathode hindering the transfer of O2 gas to the cathode. Therefore, to improve the cycling stability of LABs, the overpotential during charging should be decreased by improving the electrical conductivity and electrocatalytic activity of the electrode. Various studies have focused on increasing the electrocatalytic activity of LAB cathodes by using cathode materials with high electrical conductivity, such as carbonaceous active materials [15–17], metallic active materials [18, 19], and electrolytes [20, 21]. However, although the current collector plays a critical role by conferring stability and improving the conductivity and adhesion of the cathode, studies on current collectors have been scarce [22, 23]. Therefore, surface engineering of current collectors to prevent oxidation and increase conductivity is critical for fabricating highly stable LABs with excellent cycling stability.In this context, we coated a Ni foam substrate with carbon quantum dots codoped with N and F (NF-CQDs) and used the fabricated sample as the current collector of an LAB cathode. In addition, we estimated the effect of surface engineering on the cycling performances of LABs. Especially, the fabricated NF-CQDs presented large specific surface area, small molecular weight and particle size, high electrical conductivity, and remarkable chemical stability and were synthesized using a simple method. Furthermore, heteroatom doping of CQDs can increase the electrical conductivity and electrocatalytic activity of LAB cathodes. Moreover, the current collector fabricated by depositing a NF-CQD coating layer on the surface of a Ni foam current collector can enhance the cycling stability of LABs by preventing the oxidation of the Ni foam by the water vapor or O2 in air. In addition, because the ultrasonic spray coating method generates fine droplets via the ultrasonic vibration of the spray nozzle, uniform, dense, and thin coating layers can be deposited on the surfaces of current collectors, unlike the coating layers deposited using other coating methods, such as conventional spray coating or dip coating [24–26].


## 2. Experimental

NF-CQDs were fabricated using a hydrothermal method. Specifically, 4 wt% of urea (Aldrich), 0.6 wt% of sodium fluoride (Aldrich), and 9 wt% of citric acid (Aldrich) were dissolved in deionized water, then moved to a Teflon-lined hydrothermal synthesis autoclave, and heated at 180°C for 6 h. The prepared solution was centrifuged three times at 10000 rpm during 25 min and then dialyzed using a 6–8 kD molecular weight cut-off membrane for 24 h to obtain uniform small-sized particles. The dialysate was dried in a 50°C oven overnight to obtain NF-CQD particles. Furthermore, to confirm the effect of F doping, N-doped CQDs (N-CQDs) were fabricated using the same procedure in the absence of sodium fluoride. Ni foam was coated with NF-CQDs using an ultrasonic spray coating method and a 1 wt% NF-CQD dispersion in N-methyl-2-pyrrolidinone (NMP). The feeding rate of the dispersion, spraying time, and frequency were set at 1 mL/h, 2 min, and 130 kHz, respectively. Pristine Ni foam, N-CQD-coated Ni foam, and NF-CQD-coated Ni foam (with a radius of 0.65 cm) are hereafter denoted as bare Ni, N-CQD/Ni, and NF-CQD/Ni, respectively.The element distributions, morphology, and structure of the NF-CQDs were determined by energy-dispersive spectroscopy (EDS; NEO ARM, JEOL) apparatus and a transmission electron microscopy (TEM) instrument. Moreover, the absorbance, crystal structure, chemical bonding structure, and surface functional groups of the N-CQDs and NF-CQDs were evaluated using an X-ray diffraction (XRD) instrument, an ultraviolet-visible (UV-vis) spectrophotometer, a Fourier-transform infrared (FT-IR) spectrometer, and an X-ray photoelectron spectroscopy (XPS; VG Multilab2000m) device, respectively. The thickness of the NF-CQD layer coated on the Ni foam substrate was measured using a scanning transmission electron microscopy (STEM; NEO ARM, JEOL) instrument; the morphologies and structure of the cathode material were analyzed using field emission scanning electron microscope (FESEM, EVO10, Carl Zeiss).To estimate the electrochemical performance of the LAB cathode, we use coin cells (Hohsen Corporation, CR2032) comprising O2-electrode as a cathode and porous polypropylene membrane (Welcose, Celgard 2400) as the separator; tetraethylene glycol dimethyl ether (TEGDME) dissolves lithium bis(trifluoromethanesulfonyl)imide (LiTESI) as the electrolyte [27] and Li foil (Honjo Chemical) as the anode. The cathodes were fabricated by polyvinylidene difluoride (20 wt%, PVDF; Arkema) as the binder and Ketjen black (80 wt%, KB; ECP-600JD, Infochems Inc.) as the cathode active material. A mixture of KB and PVDF was dispersed in NMP (Aldrich) and then spray-coated onto bare Ni, N-CQD/Ni, and NF-CQD/Ni, which were used as current collectors, followed by drying in a 50°C oven overnight. The loading of KB was 0.08±0.01mg. The assembling of cells was carried out in an Ar-atmosphere glove box (99.999%), and the O2 and H2O contents were maintained below 10 ppm. The assembled battery was aged for 24 h in an O2-filled battery jig; discharge-charge tests were conducted using a cycler system (WBCS3000M1) in the voltage range of 2.0–4.5 V (vs. Li/Li+) at 25°C. Furthermore, cycling stability tests were performed with a specific capacity of 1000 mAh/g at a current density of 100 mA/g. Moreover, we used FESEM of bare Ni-, N-CQD/Ni-, and NF-CQD/Ni-based cathodes after the cycling testing to confirm the stability.


## 3. Results and Discussion

Figure1(a) shows the schematic of fabrication of NF-CQDs using hydrothermal, and Figure 1(b) shows the ultrasonic spray coating process to preparation of the NF-CQDs coated on Ni foam. The CQDs were synthesized by hydrothermal method, including citric acid as a carbon source and urea as a nitrogen source, as shown in Figure S1. When heated, citric acid reacted with urea to form citric acid amide; then, further heating leads to deamination, dehydration, and dehydrocondensation of citric acid [28]. Sodium fluoride as a fluorine source and F atoms were incorporated into the honeycomb structure of carbon atoms [29]. Ultrasonic spray coating is an efficient method that can be used to produce fine droplets via ultrasonic vibration and deposit uniform and thin coating layers on various substrates [30]. In this study, an NF-CQD dispersion was finely coated on a Ni foam substrate via ultrasonic spray coating. Because the NF-CQDs contained numerous functional groups, the thin and uniform NF-CQD coating layer enhanced the interfacial adhesion between current collector (Ni foam) and active material (KB) [23, 31]. Therefore, after the generating and decomposition of Li2O2 during repeated discharging-charging, KB, which served as the active electrode material, maintained the combined structure of the current collector. Moreover, during cycling, Ni foam was oxidized by the water vapor and O2 in air and corroded by by-products (LiOH or Li2(CO3)), especially at high potentials (>4 V vs. Li/Li+), which decreased the electrical conductivity of the current collector [32, 33]. However, the NF-CQD coating layer efficiently prevented the oxidation and corrosion of the Ni foam substrate, thereby enhancing the stability of the KB–Ni foam interface [34, 35].Figure 1
Scematics of (a) Fabrication of NF-CQDs using hydrothermal, (b) ultrasonic spray coating preocess, and (c) fabrocated NF-CQDs coated Ni foam by ultrasonic spray coating process.
(a)(b)(c)The morphology and element distribution of the NF-CQDs were examined using TEM and EDS mapping. The low- and high-resolution TEM results of NF-CQDs (Figures2(a) and 2(b)) revealed that the NF-CQDs were ~4.5 nm in diameter and their lattice fringe space was ~0.32 nm, which corresponded to the (002) plane of graphite. To confirm the content distributions of C, O, N, and F, the TEM-EDS mappings of the NF-CQDs were obtained (Figure 2(c)). The EDS data suggested that N and F were present in the NF-CQDs. The UV-vis spectrum and photographs of the NF-CQD dispersion are shown in Figure 2(d) and its inset, respectively. The NF-CQD suspension was grayish blue under natural light (left) and presented royal blue fluoresce under 365 nm UV light irradiation (right). UV-vis absorbance spectra of NF-CQDs included two feature peaks at 333 and 233 nm, which corresponded to n–π∗ transitions of the C=N and C=C bonds and the π–π∗ transitions of the aromatic sp2 C atoms of the core domain, respectively [36, 37]. By comparing the UV-vis spectrum of the N-CQDs and NF-CQDs, it was determined that F doping did not change the characteristic absorption peaks of the CQDs (Figure S2). These results confirmed that we have successfully synthesized NF-CQDs by hydrothermal method.Figure 2
(a) Low- and (b) high-resolution TEM images; (c) EDS mapping results of C, N, and F elements of NF-CQDs; (d) UV-vis absorbance spectra with photographs of NF-CQD solution.
(a)(b)(c)(d)The XRD was used to examine the structures and phases of N-CQDs and NF-CQDs. The broad peak at 26° in the XRD patterns of N-CQDs and NF-CQDs (Figure3(a)) was ascribed to the (002) lattice plane of amorphous graphite (JCPDS 65-6212), and these results were consistent with the TEM results [38]. In the XRD results, it was confirmed that the crystal structure of the CQDs was maintained after F doping. Furthermore, the chemical bonding structures of N-CQDs and NF-CQDs were analyzed by XPS, and the results were calibrated with 284.5 eV C 1s binding energy (Figures 3(b)–3(f)). Figure 3(b) displays the XPS survey spectrum of N-CQDs and NF-CQDs having peaks at 282–292, 396–404, 528–535, and 682–690 eV which correspond to the binding energies of C 1s, N 1s, O 1s, and F 1s, indicating that N and F atoms have been successfully doped to CQDs. The C 1s XPS profile of the NF-CQDs included the characteristic peaks of C–F groups (~291.6 eV), O=C groups (~288.6 eV), C–N/C–O groups (~286.7 eV), and C–C groups (~284.8 eV) (Figure 3(c)). In contrast, the characteristic peak of C–F was absent from the C 1s XPS profile of the N-CQDs, indicating that the F atoms were only present in the NF-CQDs. The N 1s XPS profiles of N-CQDs and NF-CQDs (Figure 3(d)) included the characteristic peaks of pyridinic-N (~398.7 eV), pyrrolic-N (~399.9 eV), and graphitic-N (~401.4 eV). The presence of these groups was ascribed to the use of urea, a precursor of the N-CQDs, as the N source. The pyridinic- and pyrrolic-N presented two p-electrons, which originated from their aromatic π-systems and increased the electrical conductivity of the N-CQDs [39, 40]. The O 1s XPS profiles of N-CQDs and NF-CQDs (Figure 3(e)) included the characteristic peaks of C–O (~530.1 eV), C=O (~531.2 eV), O–C–O (~532.1 eV), and O–C=O (~533.3 eV). Furthermore, the F 1s XPS profiles presented two peaks at ~687.2 and~685.9 eV, which corresponded to the covalent and semi-ionic bonds of C–F (Figure 3(f)). These groups were not observed in the F 1s XPS profile of the N-CQDs, indicating that F atoms were only present in the NF-CQDs. The electronegativity of F (4.0) is higher than that of C (2.5); accordingly, the electron/ion conductivity of the NF-CQDs increases because the polar C–F bonds presented a dipole moment [23]. Therefore, owing to F doping, the C atoms of the NF-CQDs became positively charged, and the activity of the π-electrons of the NF-CQDs was higher than that of the π-electrons of the N-CQDs, causing the higher electrocatalytic activity of NF-CQDs than N-CQDs [39]. Consequently, the NF-CQD coating layer increased the electrical conductivity and electrocatalytic activity of the LAB cathode, thereby promoting the decomposition and formation of Li2O2 during charging and discharging [41].Figure 3
(a) XRD patterns and (b) XPS full spectra, and XPS spectra of (c) C 1s, (d) N 1s, (e) O 1s, and (f) F 1s of N-CQDs and NF-CQDs.
(a)(b)(c)(d)(e)(f)Chemical properties of functional groups on the surface of N-CQDs and NF-CQDs were determined by FT-IR spectrum (Figure4(a)). The characteristic peaks at 776, 1180, 1398, 1448, 1555, 1660, 1718, 3213, and 3445 cm-1 in the FT-IR spectrum of N-CQDs and NF-CQDs were attributed to the telescopic vibrations of O=C–O, C–O, C–OH, C–N, C=C, C=N, C=O, N–H/C–H, and O–H bonds and functional groups, respectively. The characteristic peaks at 1020 and 1104 cm-1 in the FT-IR spectrum of NF-CQDs were attributed to the telescopic vibrations of C–F and C–F2, respectively [42–44]. The characteristic peak of the C–N bonds in the FT-IR spectroscopy of the NF-CQDs confirmed that the CQDs were doped with N atoms. Moreover, the presence of the characteristic peaks of the C=N and C=C bonds indicated that the hydrothermally synthesized NF-CQDs presented a hetero-polyaromatic structure. The characteristic peaks of C–F2 and C–F in the FT-IR spectrum of the NF-CQDs suggested that the CQDs were doped with F atoms. These results were consistent with the aforementioned XPS spectra. Furthermore, STEM results demonstrated that the NF-CQD coating layer was uniform and thin (26.5–28.5 nm) (Figure 4(b)). Because the NF-CQDs surface exist a great quantity of oxygen containing functional groups, the NF-CQD coating layer strongly interacted with the Ni foam substrate, enhancing the adhesion between KB and Ni foam. Furthermore, the morphology of the Ni foam substrate did not change after N-CQD and NF-CQD coating via ultrasonic spraying (Figure S3). Moreover, the uniform and thin CQD coating layers effectively prevented the oxidation of Ni foam during cycling. Therefore, these findings confirmed that the NF-CQD coating layer enhanced the cycling stability of LABs.Figure 4
(a) FT-IR spectrum of N-CQDs and NF-CQDs and (b) cross-sectional STEM image of NF-CQD/Ni.
(a)(b)To assess the electrochemical catalytic activities of the bare Ni, N-CQD/Ni, and NF-CQD/Ni electrodes, LSV measurements were carried out (Figure5). LAB cathodes involve two key reactions: ORR and OER. They all involve multistep electron transfer processes with inherently slow kinetics [45]. In the ORR polarization curve region, the onset potential of NF-CQD/Ni (0.887 V) was higher than that of other electrodes (N-CQD/Ni: 0.81 V and bare Ni: 0.806 V), implying that the NF-CQD/Ni has high ORR catalytic activity. The limiting current density of NF-CQD/Ni-based cathode (-4.755 mA cm-2) was superior then that of bare Ni and N-CQD/Ni-based electrodes (-2.256 and -3.985 mA cm-2, respectively) as shown in Figure 5(b). This result was ascribed to that F atoms can improve the ionic/conductivity of the interfacial layer. In the OER polarization curve region, the NF-CQD/Ni- and N-CQD/Ni-based electrodes showed the superior negative Eonset, exhibiting excellent OER catalytic performance. During the discharge-charge process of LABs, the ORR and OER electrochemical kinetics are crucial; they are generally evaluated by the potential gap (EOER−ORR) between OER (at 10 mA cm-2) and ORR (at -2 mA cm-2) [46]. The NF-CQD/Ni-based and N-CQD/Ni-based electrodes showed a potential difference of 1.07 V, which may be due to the efficient electron transport due to the oxygen reaction promoted by N doping.Figure 5
(a, b) LSV curves of ORR and OER electrocatalytic activity for bare Ni, N-CQD/Ni, and NF-CQD/Ni-based cathode.
(a)(b)To assess the cathode electrochemical performance, we fabricated cathodes by coating KB as the active material on bare Ni, N-CQD/Ni, and NF-CQD/Ni current collectors. The initial constant current discharge-charge curves of the fabricated cathodes are displayed on Figure6 over a current density of 100 mA/g in the voltage range of 2.0–4.5 V. The initial specific capacities of the bare Ni-based cathode (2917.97 mAh/g) were lower than that of the N-CQD/Ni- and NF-CQD/Ni-based cathodes (3963.52 and 5121.91 mAh/g, respectively). Furthermore, the discharge-charge voltage gap of the NF-CQD/Ni-based cathode (1.603 V at 2000 mAh/g) was narrower than that of the bare Ni-based cathode (1.771 V at 2000 mAh/g). This result was ascribed to the higher ORR and OER electrocatalytic activities of the NF-CQD/Ni-based cathode than the bare Ni-based cathode during charging, which promoted the efficient decomposition of Li2O2. The cycling stability of the fabricated cathodes was assessed at a limited capacity of 1000 mAh/g, at a current density of 100 mA/g, and in the voltage range of 2.0–4.5 V (Figure 6(b)). The cycling stability of the NF-CQD/Ni-based cathode (42 cycles) was superior to those of the bare Ni- and N-CQD/Ni-based cathodes (13 and 31 cycles, respectively). During the discharging-charging test, Li2O2 formed continuously as the product of the discharging reaction and decomposed at the cathode surface, thereby lowering the cycling stability of the LAB. In particular, the cycling stability of the LABs featuring the NF-CQD-based cathode was higher than those of the other LABs because F doping caused the electrochemical catalytic activity and electrical conductivity of the electrode to increase, thereby inducing the efficient decomposition of Li2O2. For the N-CQD/Ni- and NF-CQD/Ni-based cathodes, the CQD coating layers suppressed the oxidation of the Ni foam substrates, which improved the cycling stabilities of the corresponding LABs. Moreover, compared with other reported electrodes without carbon coating, the NF-CQD/Ni-based cathode exhibits excellent performance in terms of specific capacity and cycle stability (Table S1) [47–50].Figure 6
(a) Initial galvanostatic discharging/charging curves at the current density of 100 mA/g and (b) cycling stability at a limited capacity of 1000 mAh/g and a current density of 100 mA/g of bare Ni, N-CQD/Ni, and NF-CQD/Ni-based cathode.
(a)(b)The discharge-charge voltage curve of bare Ni-, N-CQD/Ni-, and NF-CQD/Ni-based cathodes are shown in Figure7. The bare Ni-based cathode exhibited a stable discharging-charging voltage gap during the first five cycles (Figure 7(a)). However, after 10 cycles, the charging overpotential of the bare Ni-based cathode increased slowly, causing a decrease in discharge capacity. In contrast, the N-CQD/Ni-based cathode presented a stable voltage gap for 25 cycles. This was attributed by the increase in the conductivity of the N-CQD coating layer induced by N doping, which promoted the ORR and OER at the cathode. Furthermore, the NF-CQD/Ni-based cathode presented excellent cycling stability and a stable voltage gap for 30 cycles. These results were ascribed to the F doping of the NF-CQD coating layer; the electrocatalytic activity of the cathode materials is increased by increasing the activity of the π-electrons of N-CQDs. Therefore, for the bare Ni-based cathode with a low electrocatalytic activity, Li2O2 was only partially decomposed, and it accumulated at the surface of the cathode, causing a decrease in its electrical conductivity. Thereby, the ORR and OER at the bare Ni-based cathode were disturbed, and the cycling stability of the cathode was poor. However, the NF-CQD coating layer prevented the oxidation of the Ni foam substrate and promoted the efficient decomposition of Li2O2 by increasing catalytic activity and conductivity efficiency of electrodes, thereby increasing the cycling capability of the cathode [51, 52].Figure 7
Specific capacity-limited discharging/charging profiles of first and every five cycles at a limited capacity of 1000 mAh/g and a current density of 100 mA/g of (a) bare Ni, (b) N-CQD/Ni, and (c) NF-CQD/Ni-based cathode.
(a)(b)(c)To compare the cycling stabilities of the electrodes after the discharge-charge tests, we analyzed the FESEM of the bare Ni-, N-CQD/Ni-, and NF-CQD/Ni-based cathodes before first discharging, after first discharging and charging, and after the cycling stability tests (Figure8). Before first discharging, bare Ni-, N-CQD/Ni-, and NF-CQD/Ni-based cathodes showed uniform KB particles (Figures 8(a), 8(e), and 8(i)). The products which were formed at first discharging process on the bare Ni cathode were peony flower-like (Figure 8(b)), but they were not completely decomposed after charging (Figure 8(c)). Compared with bare/Ni, the particles on the surface of N-CQD/Ni- and NF-CQD/Ni-based cathodes did not exhibit peony flower-like morphology after first discharging, and they showed the decomposed discharge products and decreased particle size after first charging (Figures 8(f), 8(g), 8(j), and 8(k)). Also, the particles on the surface of N-CQD/Ni- and NF-CQD/Ni-based electrodes were sticky compared to bare Ni electrodes after charge-discharge tests because the N-CQDs and NF-CQDs coated on Ni foam enhance cycling stability by the strong connection between the current collector and electrode. The discharge product of LABs is Li2O2; however, the X-ray energy of Li (52 eV) is too small to be distinguished by EDS, so we analyzed the percentages of the O element to confirm the discharge product content on the surface of electrodes (Table 1). After the first discharge, the atomic percent of O element in the NF-CQD/Ni (49.35 at%) was smaller than others (74.35 at% of bare Ni and 54.95 at% of N-CQD/Ni), but after charging, the atomic percentage of NF-CQD/Ni was only 23.74 at% (65.22 at% of bare Ni and 41.53 at% of N-CQD/Ni). According to the top-view FESEM image of the bare Ni-based cathode, the discharge-charge product comprised peony flower-like Li2O2 particles with a 2-D sharp sheet morphology (Figure 8(d)). It has been reported that Li2O2 grew heterogeneously on the surfaces of cathodes with low ORR and OER activities. Moreover, the incomplete decomposition of Li2O2 caused the accumulation of dense and sharp 2-D Li2O2 sheets on the surfaces of cathodes during discharging-charging, thereby interrupting the gas transfer at the cathodes. In contrast, Li2O2 grew homogeneously on the surfaces of cathodes with high ORR and OER activities, and the small and spherical Li2O2 particles that accumulated at the cathode surfaces were less disruptive for the gas transfer at the cathodes [53, 54]. Therefore, owing to the poor ORR and OER activities of the bare Ni-based cathode, the peony flower-like Li2O2 particles grown on its surface caused its cycling stability to decrease by blocking its pores and decreasing its active sites. The Li2O2 particles that accumulated at the N-CQD/Ni-based cathode surfaces also presented a peony flower-like structure; however, the 2-D Li2O2 sheets were shorter and smaller than those grown on the surface of the bare Ni-based cathode (Figure 8(h)). Moreover, the Li2O2 particles deposited on the NF-CQD/Ni-based cathode were spherical with bump surface (Figure 8(l)). The contents of discharge products on surface of electrodes were determined by analyzing the atomic percentage of O element. As shown in Table 1, the atomic percentage of O in bare Ni-based cathode (82.91 at%) was higher than other cathodes after the cycle test (N-CQD/Ni: 80.87 at% and NF-CQD/Ni: 70.03 at%). These results indicated that codoping with N and F caused the ORR and OER activities of NF-CQD/Ni-based cathode to increase, leading to the uniform growth and efficient decomposition of Li2O2. Moreover, the hindering effect of the spherical Li2O2 particles on the gas transfer at the cathode was lower than that of the 2-D Li2O2 sheets; therefore, the NF-CQD coating layer enhanced the cycling stability of the NF-CQD-based cathode.Figure 8
FESEM images of cathode fabricated with (a–d) bare Ni, (e–h) N-CQD/Ni, and (i–l) NF-CQD/Ni primeval, after 1st discharging, 1st recharging, and cycling test.Table 1
EDS element images for the cathode of bare Ni, N-CQD/Ni, and NF-CQD/Ni primeval, after 1st discharging, 1st recharging, and cycling test.
StatusCathodeChemical composition (at%)COPrimevalBare Ni1000N-CQD/Ni1000NF-CQD/Ni10001st dischargeBare Ni25.6574.35N-CQD/Ni45.0554.95NF-CQD/Ni50.6549.351st chargeBare Ni34.7865.22N-CQD/Ni58.4741.53NF-CQD/Ni76.2623.74After cyclingBare Ni17.0982.91N-CQD/Ni19.1380.87NF-CQD/Ni29.9770.03Therefore, the NF-CQD/Ni-based cathode exhibits highly stable electrochemical performance, which is ascribed to the NF-CQD layer coated on the Ni foam substrate. The NF-CQD/Ni-based cathode exhibits remarkable cycling stability due to the following: (a) the NF-CQD coating layer prevented the oxidation of the Ni foam substrate during discharging–charging, and (b) the NF-CQDs increased the electrical conductivity and electrocatalytic activity of the cathode, facilitating the efficient decomposition of Li2O2.


## 4. Conclusion

In this study, we fabricated a NF-CQD/Ni-based cathode for highly stable LABs using a hydrothermal method followed by ultrasonic spray coating. The NF-CQD/Ni-based cathode displayed an excellent initial discharge capacity of 5121.91 mAh/g at a current density of 100 mA/g, a low discharge-charge voltage gap of 1.603 V at 2000 mAh/g, and outstanding cycling stability over 42 cycles (with a limited capacity of 1000 mAh/g). These results are attributed to the effect of the NF-CQD coating on the Ni foam substrate as follows: (i) the NF-CQD coating layer prevented the oxidation of the Ni foam matrix and (ii) co-doping with N and F improved the electrocatalytic activity and electrical conductivity of the cathode. We believe that coating NF-CQD layers on current collectors can be used to fabricate highly stable LABs.



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*Source: 5310171-2023-02-14.xml*