Bioinspired Fabrication of Strong Self-Standing Egg-Sugarcane Cathodes for Rechargeable Lithium–Oxygen Batteries

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLE1 Jun 2021Bioinspired Fabrication of Strong Self-Standing Egg-Sugarcane Cathodes for Rechargeable Lithium–Oxygen Batteries Xiao-Xue Wang, Shu-Cai Gan, Li-Jun Zheng, Ma-Lin Li and Ji-Jing Xu Xiao-Xue Wang State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012 , Shu-Cai Gan State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012 , Li-Jun Zheng State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012 , Ma-Lin Li State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012 International Center of Future Science, Jilin University, Changchun 130012 and Ji-Jing Xu *Corresponding author: E-mail Address: [email protected] State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012 International Center of Future Science, Jilin University, Changchun 130012 https://doi.org/10.31635/ccschem.020.202000340 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesTrack Citations ShareFacebookTwitterLinked InEmail Lithium–oxygen (Li–O2) batteries have attracted considerable attention due to their high theoretical energy density. However nonrenewable and high-cost electrode materials have limited their progress. Herein, the authors design and fabricate a three-dimensional freestanding bi-biomass egg-sugarcane (Egg-SC) electrode with excellent structure and performance as the cathode for Li–O2 batteries. The open, interconnected microchannels derived from the natural SC can provide sufficient pathways for O2 gas diffusion. The heteroatom-doped hollow carbon spheres (HD-HCS) obtained via biomass egg supply many of the triphase active sites for the formation and decomposition of the discharge products of Li2O2. Benefiting from the unique nature and structure of the cathode, Li–O2 batteries show high-rate capacity of 8.07 mAh cm−2 and superior cycle stability of 294 cycles at a current density of 0.1 mA cm−2. The excellent performance and structure of the bi-biomass cathode possess great application potential in nature-inspired materials design for the cathodes of Li–O2 batteries. Download figure Download PowerPoint Introduction With the dramatically growing dependence on and consumption of nonrenewable fossil fuels such as oil, coal, or gas, controlling greenhouse gas emissions has become one of the great challenges of the past few decades.1–3 In response to this challenge, efficient and sustainable energy materials and devices including solar cells, supercapacitors, Li-ion batteries (LIB), and metal–air batteries have rapidly been developed in recent years.4,5 Among these new energy storage systems, nonaqueous lithium–oxygen (Li–O2) batteries have increasingly attracted attention owing to their high specific energy density (3582 Wh kg−1).6–11 A typical nonaqueous Li–O2 battery is composed of a glass fiber as membrane sandwiched between a porous cathode and a Li metal foil anode.12 During discharge, the O2 molecule is reduced and combined with the Li+ produced by the oxided anode, forming insoluble lithium peroxide (Li2O2) or lithium oxide (Li2O) on the cathode. Gradually, the discharge products accumulate and clog the porous cathode, which results in slow mass transport and ultimately leads to poor cycle life, limited capacity, and low round-trip efficiency of the Li–O2 batteries.13–15 Therefore, it is imperative to achieve a freestanding cathode with tailored hierarchical pore structure and excellent electrocatalytic capability to improve the battery performance. Recently, numerous efforts have been devoted to design and fabricate cathodes of high stability in enhancing the electrochemical performance of Li–O2 batteries.16–22 Carbon materials such as graphene,23 carbon nanotubes,24–26 and other carbonaceous materials27–30 are on the forefront of superior electrode materials due to their good conductivity and tunable porous structure, and they have been extensively investigated as O2 cathodes for Li–O2 batteries. However, numerous fossil fuel-based precursors and toxic reagents used in the preparation of these materials result in less favorable yield for practical applications, including portable electronics and electric vehicles. Consequently, researchers have increasingly turned to abundant, eco-friendly, and renewable natural materials for fabricating optimum electrode architecture. Various low-cost and eco-friendly biomass derivatives, including wood,31–33 seaweed,34 fruit peels,35 eggplant,36 leaves,37 and so forth, have been used as electrode materials for energy storage devices. However, to enhance their electrocatalytic properties, most of these electrode materials need to be further decorated by catalysts such as noble metal, metal oxide, and metal nitride, which are still harmful and costly. Therefore, further efforts need to be devoted to constructing eco-friendly current collectors and catalyst materials as the electrodes for energy storage devices, especially for Li–O2 batteries. To date, there has been very little research on constructing the cathodes for Li–O2 batteries using nature-inspired current collectors and biomass-derived bifunctional electrocatalysts. Sugarcane (SC) has a unique natural intrinsic structure through which the open interconnected microchannels supply sufficient pathways to obtain moisture, ions, and nutrients from the soil.38 Egg as a natural organism containing rich O, N, P, and Fe elements that can be widely used as the doping source of the carbon framework. Herein, a novel, renewable, all-biomass Egg-SC cathode is designed and constructed by a simple process. The three-dimensional (3D) porous structure inspired by natural SC and the heteroatom (N, P, and Fe)-doped hollow carbon spheres (HD-HCS) obtained from the biomass egg provide excellent O2 electrocatalytic performance. Related experiments have proven that the carbonized SC has a large number of open microchannels that can provide sufficient pathways for O2 gas diffusion and supply enough space for the formation and decomposition of the discharge products of Li2O2. Benefiting from the large specific area of mesoporous and microporous materials coexisting in HD-HCS derived from the biomass egg, the cathode exhibits excellent O2 reduction reaction (ORR) and O2 evolution reaction (OER) catalytic performance along with high efficiency and durability. The Li–O2 batteries with the novel binder-free, current collector-free, and freestanding cathode exhibit relatively low overpotential, high specific capacity, and excellent cycle stability. This simple but novel bi-biomass electrode design has promising application prospects for many other energy storage devices. Experimental Methods Fabrication process of the SC cathode matrix The SC was peeled and cut into slices perpendicular to the direction of growth. The slices were freeze-dried and transferred to a tube furnace for carbonization at 800 °C for 2 h with a heating rate of 5 °C min−1 under nitrogen atmosphere. Preparation for Egg-SC cathode The method of synthesizing the HCS derived from biomass egg was based on the description in our previous work.8 About 7.5 mL NH3·H2O solution was added to an ethanol solution containing 100 mL ethanol and 7 mL water and stirred vigorously for 30 min. Then 7.5 mL tetraethylorthosilicate and the prepared SC substrate were added into the solution successively, stirred for 10 h, enabling the silicon spheres (SS) to be deposited perfectly in situ on the substrate. After being dried at 80 °C, the electrode sheet with SS was stirred in the diluted egg solution to obtain the heteroatom-doped carbon spheres, heated at 900 °C in a tube furnace for 2 h under nitrogen atmosphere. The Egg-SC cathode was finally obtained by removing the SiO2 template using hydrofluoric acid. According to the calculation, the loading amount of the HD-HCS on the matrix was nearly 0.8 mg cm−2 per electrode piece. Preparation for Sucrose-SC and single-atom (N/P/Fe)-doped cathode As a comparison, sucrose-doped HCS and single-atom (N/P/Fe)-doped HCS were synthesized through a similar process, except for replacing the egg solution with sucrose, hydrazine hydrate, phosphate, and Ferric nitrate as the source of N, P, and Fe. Results and Discussion As one of the main renewable and natural energy resources around the world, SC possesses vertical stalks (Figure 1a) that provide a large number of microchannels in the up-growing direction (Figure 1b), ideal as a freestanding and binder-free matrix. The top-viewed scanning electron microscopy (SEM) image of the freeze-dried SC shows interconnected macroporous and vertical opening channels (Figure 1c). Amazingly, the perpendicular 3D porous framework can also be perfectly maintained after carbonization ( Supporting Information Figure S1). The synthesis strategy of the bi-biomass Egg-SC cathode is displayed in Figure 1d. Monodisperse SS was deposited in situ on the channel wall of the carbonized SC. Biomass eggs with abundant O, N, P, and Fe elements sourced from protein, vitamins, and minerals were used as the carbon sources to fabricate the HD-HCS. Then the SS template was removed, and the microchannel wall was uniformly covered by numerous HD-HCS, indicating the successful construction of the Egg-SC cathode ( Supporting Information Figure S2). As shown in Figure 1e, the as-prepared binder-free, current collector-free, and self-standing Egg-SC cathode with numerous low-tortuosity, vertical, and open channels shortened the Li-ion transport path, facilitated the O2 diffusion, and supplied enough space for the Li2O2 storage. Meanwhile, owing to the mesoporous and microporous materials coexisting in HD-HCS, the specific surface area and the catalytic activity of the cathode greatly improved. Figure 1 | Scheme for the fabrication and working principle of the Egg-SC cathode. (a) The digital photograph of the natural SC. (b) Schematic representation of the SC-inspired 3D current collector of open channels and interconnected macroporous structure. (c) SEM image of the top surface of the freeze-dried SC. (d) Schematic illustration of synthesis of the egg-derived HD-HCS. (e) The mechanism of the Egg-SC cathode with vertical microchannels. The hierarchically porous, open, and vertical microchannels would shorten the Li-ion transport path and facilitate the O2 diffusion. Download figure Download PowerPoint The Egg-SC cathode was obtained by cutting the natural SC vertically into pieces along the direction of growth, freeze-drying it, and carbonizing it in sequence (Figure 2a). With a thickness of about 400 µm, the unique vertical straight structure of Egg-SC shortened the electron transfer distance and facilitated the permeation of the O2 ( Supporting Information Figure S3). The wettability of the cathode was proven by the contact angle (CA) between the electrode surface and the solution. As shown in Figure 2b, the Tetraethylene glycol dimethyl ether (TEGDME) liquid dropped by the cathode completely spread out, corresponding to a CA of 0°, indicating the superior wettability of the Egg-SC cathode toward the electrolyte. In contrast, the CA of the cathode surface and H2O was 64.7°, far exceeding TEGDME and further proving the high lipophilicity of the Egg-SC cathode. The field emission SEM (FESEM) image in Figure 2c shows diameters of numerous microchannels in a range of 100–200 μm of Pristine-SC. The HD-HCS were deposited in situ onto the porous wall of the SC (Figure 2d), providing more active sites, fast electron conduction, and rapid mass transportation in Li–O2 cell. High-magnification SEM showed HD-HCS with a diameter of approximately 450 nm and abundant surface openings with a diameter of 10–50 nm (Figure 2f). As shown in Figure 2g, the transmission electron microscopy (TEM) image demonstrates the mesoporous and microporous materials on the surface of the HD-HCS, consistent with the nitrogen absorption–desorption isotherms in Figure 2h. The nitrogen adsorption–desorption isotherms revealed that the Egg-SC cathode had a Brunauer–Emmett–Teller (BET) surface area of 166 m2 g−1. The pore size distribution (Figure 2h, inset) demonstrated the existence of two types of mesoporous materials (3–5 and 8–50 nm), attributed to the evaporation of volatile species of the egg extract and removal of the SS template. The synergy of the hierarchical micro–meso–macro pores was beneficial to realize high specific surface area and effective electrolyte diffusion. All these results confirmed that this freestanding and binder-free Egg-SC cathode offers excellent would obviously enhance the ORR with respect to high specific areal capacity, long cycling life, and superior rate capability (vide infra). Figure 2 | Characterizations of the Egg-SC cathode. (a) Photograph of the as-prepared Egg-SC cathode. (b) Digital images demonstrate the cathode wettability of TEGDME and H2O via CA. (c) The FESEM image and photograph (inset) of the Pristine-SC. (d) FESEM image of the as-prepared Egg-SC cathode. (e) Elemental mappings of the Egg-SC cathode. The heteroatom codoped nanospheres are uniformly deposited on the wall of the microchannels. (f) FESEM images of the hierarchically porous structure nanospheres with an average size of 400–500 nm. (g) TEM image of the spheres with the hollow porous characteristic. (h) Nitrogen adsorption–desorption isotherms and pore-size distribution (inset) of the Egg-SC cathode. Download figure Download PowerPoint Elemental mappings of C, N, O, P, and Fe obtained from the egg show homogenous distribution on the Egg-SC cathode (Figure 2e). The X-ray photoelectron spectroscopy (XPS) was applied to investigate the chemical composition of the Egg-SC cathode. The survey results (Figure 3a) showed the elemental content of C (80.37%), O (9.12%), N (7.01%), P (2.24%), and Fe (1.26%). The high-resolution C 1s peak was deconvoluted into three different peaks located at ∼284.6, 285.5, and 286.4 eV (Figure 3b). The prominent C 1s peak at 284.6 eV corresponded to the sp2-graphitic carbon skeleton, which is beneficial to improve the electron conductivity. The C 1s peak located at 285.5 and 286.4 eV were attributed to the C=N/C–O/C–P and C=O/C–N, indicating that heteroatoms were successfully doped into the carbon framework and enhanced the electrochemical OER and ORR activities of the cathode.39 Via the Gaussian fitting method, two peaks appeared at 532 and 532.8 eV in the spectrum of O 1s (Figure 3c), which have signed as O1 and O2, respectively.40 The component O1 was associated with the groups of hydroxyl bonds, corresponding to sufficient contact with the adsorbed water on the surface, which contributed to the superior wettability of the cathode. The O2 peak signal was attributed to a great many active defect sites with lower hypoxic coordination numbers, usually observed in small particles, leading to an excellent OER electrocatalysts performance.41 Figure 3d shows that the N 1s XPS profiles of the cathode, pyridinic N, pyrrolic N, graphitic N, and oxidized pyridinic N, correspond to the binding energy peaks at 398.5, 400, 400.8, and 402.3 eV.42 According to previous studies, the binding energy near 398.6 eV in N 1s was ascribed to the pyridinic N, indicating that carbon materials can expose more edge planes, which is beneficial to reduce the ORR overpotential and provide more active sites.43 With the N-doping to the carbon matrix, the ORR and OER catalytic performance were both significantly improved. P was simultaneously doped in a carbon-containing substrate material with N. The P 2p deconvoluted into two different types of peak at 132.5 and 133.5 eV, attributed to the existence of P–C and P–O bonding, respectively (Figure 3e).44 The spectrum of fitted Fe is shown in Figure 3f where the peaks located at 710.1 and 724.2 eV are correlated to the Fe 2p3/2 and Fe 2p1/2, which have positive impact on the OER catalytic activity.45 Simultaneously, finely dispersive Fe species bonded with the neighboring C or N atoms. The coexisting Fe–N and Fe–C in the cathode acting as active sites were beneficial to achieve high catalytic activity toward ORR.25 In summary, the synergistic effect of N, P, and Fe components specifically lead to improved performance of the carbon materials. Figure 3 | (a) XPS spectrum of Egg-SC cathode. Inset: atomic concentration spectrum of the Egg-SC cathode. (b–f) High-resolution XPS spectrum of C 1s, O 1s, N 1s, P 2p, Fe 2p, respectively. Download figure Download PowerPoint To investigate the electrochemical performance of the Egg-SC, a Li–O2 coin cell was assembled composed of the Egg-SC cathode, a glass fiber separator, a lithium foil anode, and a negative current collector (Figure 4a). For comparison, the Pristine-SC, Sucrose-SC, and single-atom (N/P/Fe)-doped-SC (N-doped-SC, P-doped-SC, and Fe-doped-SC) were also prepared. The structure and composition of these single-atom-doped cathodes were also examined by element mapping, X-ray diffraction (XRD), and XPS analysis ( Supporting Information Figures S4–S10). The area of all cathodes was employed to calculate the specific capacity and current densities. Figure 4b shows the first discharge/charge voltage curves of the Li–O2 cells with Pristine-SC, Sucrose-SC, and Egg-SC cathodes at a current density of 0.1 mA cm−2. The discharge and charge plateau of the Li–O2 cell was dramatically improved with the Egg-SC cathode. The overpotential of the Pristine-SC was much higher than the other two cathodes with HCS, indicating that only Pristine-SC acted as an effective cathode collector that supplied good electron transport and high mass transport. The HCS and HD-HCS were uniformly dispersed on the aligned wall of the SC as an effective electrocatalysts that promoted the formation and decomposition of discharge products (Li2O2). It is worth noting that the round-trip efficiency was enhanced with the Egg-SC cathode, which is vital for electrochemical energy storage devices. Specifically, the discharge voltage of the Li–O2 cell with an Egg-SC cathode was higher than that with Pristine-SC and Sucrose-SC by ∼240 and ∼170 mV, respectively. And the charge voltage was much lower than that with the Pristine-SC and Sucrose-SC by ∼780 and ∼410 mV, respectively. Unsurprisingly, the overpotential of the Li–O2 cell with the Egg-SC cathode was lower than the single-atom (N/P/Fe)-doped cathodes ( Supporting Information Figure S11). The results were further proven by the cyclic voltammograms (CVs) of Li–O2 batteries with these cathodes (Figure 4c). The cell with an Egg-SC cathode showed an earlier reduction peak, lower onset potential, and higher reduction current density, suggesting better ORR electrocatalyst performance of Egg-SC. Specifically, compared with the Li–O2 cells of the single-atom (N/P/Fe)-doped cathodes, the ORR peak potential was higher with an Egg-SC cathode ( Supporting Information Figure S12). These results demonstrated the excellent electrochemical performance of the Egg-SC cathode for the formation and decomposition of Li2O2, which might be ascribed to the synergy of higher catalytic activity of the HD-HCS and more reaction sites by the large surface area for the ORR and OER. And the abundant pores facilitated the continuous O2 diffusion and Li+ transport via the microchannels of the cathode. Figure 4 | Electrochemical performance. (a) Schematic diagram of the battery composed of Egg-SC (cathode), glass fiber (separator), and lithium foil (anode). (b) The first charge–discharge profiles of Li–O2 cells with the Pristine-SC, Sucrose-SC, and Egg-SC cathodes at a current density of 0.1 mA cm−2 and a specific areal capacity limit of 1 mAh cm−2. (c) CV curves of the three types of cathodes in Li–O2 cells at a constant scan rate of 0.05 mV s−1. (d) The rate capability of the Li–O2 cells with three types of cathodes at different current densities. (e) The full range test of the Li–O2 cells with three kinds of cathodes at a constant current density of 0.1 mA cm−2 and the discharge terminal cutoff voltage were limited to 2 V. (f) XRD patterns of the Pristine-SC and Egg-SC cathodes and the pattern of the commercial Li2O2. FESEM images of the first-discharged (g) Pristine-SC, and (h) Egg-SC cathodes at a current density of 0.1 mA cm−2 and limited specific capacity of 1 mAh cm−2. (i) Photograph of 33-green light-emitting diodes powered by one Li–O2 cell. Download figure Download PowerPoint According to the investigation of the rate capability of the Li–O2 cells with Pristine-SC, Sucrose-SC, Egg-SC, and single-atom (N/P/Fe)-doped cathodes, the discharge plateau of the Egg-SC cathode was higher than that of the other cathodes at each current density (Figure 4d and Supporting Information Figure S13). In general, the rate capability of the Li–O2 batteries is usually attributed to the diffusion of the O2 and Li+ and the electron transfer. The Egg-SC's excellent rate capability is ascribed to the numerous porous microchannels and openings that prevent the discharge product from clogging the cathode, and the HD-HCS would obviously enhance the ORR and OER electrochemical performance. The electrochemical impedance spectra (EIS) further proved the fast Li+ diffusion of Egg-SC during discharge ( Supporting Information Figure S14). Unsurprisingly, the specific areal capacity of the Egg-SC cathode achieved an extremely high value of 8.07 mAh cm−2 at a current density of 0.1 mA cm−2, which was higher than those for Pristine-SC of 1.74 mAh cm−2, Sucrose-SC of 4.03 mAh cm−2, N-doped-SC of 5.8 mAh cm−2, P-doped-SC of mAh cm−2, and of mAh cm−2. The of the discharge terminal cutoff voltage to 2 (Figure and Supporting Information Figure with the HCS structure, the cells of the Li–O2 battery with Egg-SC showed a high specific areal capacity, indicating that the doping of heteroatom derived from biomass egg was to the performance. The in the specific areal of the Pristine-SC and Egg-SC cathodes was also ascribed to the different of the discharge product Li2O2. To better the of the Egg-SC cathode, the of the discharge product in the Pristine-SC and Egg-SC cathode was investigated under the discharge Figures and show the SEM images of the Pristine-SC and Egg-SC cathodes at a capacity of 1 mAh cm−2. It can be that the discharge product Li2O2 on the Pristine-SC cathode, on the surface and the leading to poor electrochemical performance. As for the Egg-SC cathode, the discharge products uniformly onto the opening wall of the HCS, for the low overpotential and high of the Li–O2 cell. This uniformly and Li2O2 was favorable to provide sufficient and the active sites on the cathode, the electrochemical reaction during the charge and discharge process, the low overpotential and high capacity of the Li–O2 cell. of the discharge product from the unique of the Egg-SC electrode with and triphase reaction which was beneficial to the distribution of the reaction on the cathode this prevent the reduction of the species and their diffusion into the forming of the discharge products to an enhanced mechanism for Li2O2 formation that the charge transport in Li2O2 surface As shown in Figure the XRD spectra of the discharge Pristine-SC and Egg-SC cathodes that Li2O2 is the only product in the two and and were discharge to 2 mAh cm−2, the in the of the discharge the Li–O2 cells with single-atom (N/P/Fe)-doped cathodes were also via the XRD spectra ( Supporting Information Figure and Li2O2 is the only product in The ( Supporting Information Figure and was used to investigate the yield of Li2O2 on the Egg-SC and Pristine-SC cathodes after being to mAh at 0.05 mA cm−2. In with the theoretical the yield was to be about for Egg-SC and for Pristine-SC, indicating the large amount of Li2O2 during to the of high capacity by the formation and decomposition of Li2O2 and the voltage in of ( Supporting Information Figure enough was to light-emitting diodes (Figure of the Li–O2 cell with the Egg-SC cathode was the cycling stability. These cells were with the widely used constant capacity The typical voltage curves of the Li–O2 cells with Pristine-SC, Sucrose-SC, single-atom and Egg-SC cathodes at a current density of 0.05 mA cm−2 and with a specific capacity of mAh cm−2 (Figure and Supporting Information Figure The voltage obtained at the discharge terminal of the Egg-SC in the Li–O2 cell was for 294 cycles (Figure In contrast, the discharge of the Pristine-SC, Sucrose-SC, P-doped-SC, and to after and respectively. This further confirmed that the Egg-SC cathode possesses excellent and cycling stability. with a specific capacity of and mAh cm−2, Egg-SC also excellent cycle stability for and respectively ( Supporting Information Figure To better the cycle stability of the Egg-SC cathode in Li–O2 the of the cathodes after first and cycles were examined by SEM and After the first the Pristine-SC cathode to the Li2O2 and nearly other was through the spectrum and The for the first cycle were which also proved the of both the Pristine-SC and Egg-SC cathodes ( Supporting Information Figure However, after the the Li2O2 was and and lithium on the surface of the Pristine-SC cathode. These the transportation of the and lithium during the constant finally leading to the earlier Li–O2 cell (Figure In comparison, after the Egg-SC cathode surface was only covered by small particles, attributed to the or products from Li2O2 such as or The HD-HCS structure of the cathode was nearly due to the that the discharge products enough to the electrochemical reaction and facilitate rapid electron pathways during the process (Figure The freestanding structure of the Egg-SC cathode fast electron to the Meanwhile, the spectra of the cathodes after cycles further demonstrated that the were and ( Supporting Information Figure the peak value of the reaction product on the Egg-SC cathode was significantly lower than that on the Pristine-SC which was in with the results in Figure To further the and stability of the cell with Egg-SC cathode, gas

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