Effect of activation process on microstructure and electrochemical

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Energy, Environmental, and Catalysis Applications

Effect of activation process on microstructure and electrochemical properties of N-doped carbon cathode in Li-O batteries 2

Suhe Li, Meiling Wang, Ying Yao, Tuo Zhao, Lei Yang, and Feng Wu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b12691 • Publication Date (Web): 30 Aug 2019 Downloaded from pubs.acs.org on August 30, 2019

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Effect of Activation Process on Microstructure and Electrochemical Properties of Ndoped Carbon Cathode in Li-O2 Batteries

Suhe Li1, Meiling Wang1, Ying Yao1, 2 *, Tuo Zhao1, Lei Yang1, Feng Wu1, 2

1. Beijing Key Laboratory of Environmental Science and Engineering, School of Materials Science &Engineering, Beijing Institute of Technology, Beijing 100081, China 2. National Development Center of High Technology Green Materials, Beijing 100081, China

__________________________ Corresponding Author *Dr. Y. Yao. Email: [email protected]. Tel: 86-10-68918766.

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Abstract Lithium-oxygen (Li-O2) batteries have the potential to provide high energy densities, however, which suffer from low actual specific capacity and poor cycle performance. Hence, it is urgent to design a satisfactory oxygen electrode for Li-O2 battery. In this study, carbonaceous materials, denominated CA, CB and CC, from chitin were prepared by three activators of H3PO4, KOH and KHCO3 as oxygen electrodes materials for Li-O2 batteries. The different carbon structural characteristics from the same precursor were regulated and controlled by different chemical reagents. Finally, the spherical particle cluster structure of CA has high specific surface area, rich N doping, good connectivity and uniform surface chemistry, so that CA acts as an oxygen electrode presenting excellent electron conductivity, providing sufficient and stable electrochemical activity sites for ORR and storing abundant discharge products. Electrochemical measurements indicate that at the current density of 0.02 mA/cm2, CA-based battery delivers a high specific capacity of 16600 mA h/g and a stable cycle performance of 210 cycles. This study proposes a functional carbonaceous material from chitin as cathode oxygen electrode, which provides an economical and sustainable way for the improvement of oxygen electrode and application of Li-O2 battery.

Keywords: chitin-derived carbon; activation process; microstructure; oxygen electrode; lithium-oxygen battery


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1. Introduction The expansion of green and sustainable energy storage and conversion technologies have become very important factor attributable to the depletion of fossil fuels and the associated environmental problems.1-3 The rechargeable lithium-oxygen (Li-O2) battery has the highest theoretical energy density (11682 W h/kg) compared with the state-of-theart batteries making it an ideal candidate for advanced high-energy application.4 However, the actual property of nonaqueous Li-O2 batteries concerning

discharge specific capacity,

cyclability and coulombic efficiency is far from theoretical value, mainly due to the sluggish cathode kinetic associated with charge transfer during discharge and charging.5-7 In order to give full play to the advantage of energy density of Li-O2 batteries, it is necessary to design an oxygen cathode with reasonable pore structure and surface chemistry property.8-10 In addition to non-carbon based catalyst (Pt, Pd, Au, MnO2, CO3O4 and so on), carbon materials have been widely used as the oxygen electrode/catalyst especially for nonaqueous Li-O2 batteries as a consequence of their high electric conductivity, tailorable surface chemistry and structure and low cost.4 To further improve the electrocatalytic properties of carbon-based electrodes, nitrogen doping is considered as a feasible strategy to improve reaction kinetics, O2 and Li+ transfer and discharge products accommodation.11, 12 Not only the chemisorption mode of O2 can be delocalized by the Ndoped charge, from the Pauling model to the Yeager model, but also the adjacent nitrogen dopants affect the spin density and charge the arrangement of carbon atoms on surface of



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carbon materials.13-15 The defective active site can promote the oxygen reduction and lithium ion binding process.13-16 Therefore, nitrogen doping is considered to be a booster for ORR in Li-O2 battery. Inspired by this concept, several methods for applying N-doped carbon as oxygen electrode catalytic material have been tried. Recently, polydopaminederived N-doped graphitic, graphene and nanotube etc. have been studied for oxygen reduction, presenting an enhanced oxygen reduction catalytic activity.17-19 However, due to the complicated preparation process and high cost of these N-doped materials, now research has turned to seek renewable natural resources as N-doped carbon precursors.11 Biochar functionalized materials attract concern on account of their economic feasibility , self-rich N element, adjustable surface chemistry and fast electron transport capability, and can be directly used as nitrogen doping precursors. Nonetheless, the application of it to prepare functionalized N-doped carbon materials for Li-O2 battery cathode is rare. At present, only nori and poplar inflorescences have been used as nitrogendoped oxygen electrode materials in Li-O2 battery.20, 21 More importantly, the preparation methods of N-doped biochar materials are particularly noteworthy. Different preparation ways with various chemical reagents are effective means of adjusting the surface chemistry, catalytic activity and microstructure of biochar by a series of crosslinked polycondensation reactions,22, 23 but there is almost no research about this aspect. Therefore, we demonstrate functionalized carbon materials derived from chitin that is a nitrogen-containing polysaccharide with the second abundant biological polymer in nature which is cheaper and easier to obtain than other commercial carbon materials.24, 25


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In this study, we focus on the different processes that three different activators H3PO4, KOH and KHCO3 are used to develop different N-doped carbonaceous materials. By comparing their various microstructures and electrochemical performances as oxygen electrode for Li-O2 batteries, reasons for reinforcing the property of Li-O2 batteries are analyzed, and an excellent nitrogen doping carbon material as cathode catalytic materials is proposed. So far, there is no report proposed to discuss the application of chitin prepared by different chemical activation methods as effective cathode electrodes materials for LiO2 batteries.

2. Materials and methods 2.1 Synthesis of carbonaceous materials All chemicals and reagents used were analytical grade. Firstly, 100 mL of H3PO4 was added to 10 g chitin, which was stirred well at 25 ˚C for 12 h. Secondly, this solution was pre-carbonized in an air-circulating oven for 4 h at 250 ˚C. Thereafter, the mixture was put in to the tubular furnace with a N2 flow rate of 80 mL/min and heated up to 600 ˚C at a rate of 10 ˚C/min then maintain pyrolysis temperature for 1 h. Finally, the carbonized material was washed by DI water to a neutral PH. After drying in the oven at 80 ˚C, the obtained material was designated CA. When KOH was used as an activator, the mass ratio of chitin to KOH is 5:2. Initially, the raw material was immersed in a saturated potassium hydroxide solution , later dried in an oven at 80 °C for 24 hours, came after pyrolysis in a tubular furnace (heating rate of 10



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° C/min, pyrolysis temperature of 800 °C for 1 h, N2 flow rate of 100 ml/min). After pyrolysis, excess dilute HCl was added to the material to remove the activator, then washed them by DI water and dried in an oven to get CB. The preparation of CC was similar to that of CB, the activator became KHCO3. Chitin and KHCO3 mass ratio set to 1:4. CC was then prepared by the same pyrolysis process as CB. 2.2 Characterizations Using Jobin-Yvon HR800 confocal Raman spectrometer to analyze Raman spectroscopy information of samples. X-ray diffraction (XRD) patterns were recorded on an XRD analyzer (Rigaku Ultima IV, Rigaku, Japan) using Cu Kα radiation (λ=1.5406 Å) and data were collected in the 2 θ range of 10-90° at a scan rate of 8 °/min to characterize the crystal texture of carbons and discharge products. Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH)analysis were performed by a Tristar 3000 surface area to make certain the specific surface area and pore-size distribution. Obtaining morphologies of the samples using a scanning electron microscope (SEM) instrument (JSM-6400, JEOL, Japan). X-ray photoelectron spectroscopic (XPS) measurements were conducted with a PHI Quantera II system (UlvacPHI Inc., Japan) to ensure the basic composition and valence of the element. 2.3 Oxygen electrode preparation and battery assembly In the preparation of oxygen electrode, the mass ratio of carbonaceous material and binder polyvinylidene fluoride (PVDF) was 8:2. Then, N-methyl-2-pyrrolidone (NMP) solvent was added to the mixture and ground into a paste. After that, brush the paste evenly


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on the carbon paper (TGP-H-060 carbon paper, Torray) cut with a 1.1 cm diameter. Eventually, the cathodes were baked in the oven at 80 °C for 12 h to remove the remaining NMP solvent. Swagelok batteries with an oxygen electrode surface area of 0.5 cm2 were used for battery tests. All batteries were accomplished in an argon-filled glovebox (m-braum, Labstar (1950/780)) where the H2O and O2 level remained below 0.5 PPM and 1 PPM, respectively. The anode of the battery is lithium metal sheet (2.2 mm thick). Tetraethylene glycol dimethyl ether (TEGDME) and 1M lithium bis-trifluoromethane sulfonimide (LiTFSI) were used as electrolyte. The glass fiber membrane (GF/D, Whatman) as the separator was fully soaked in the electrolyte. After the battery was assembly, the sealed battery was removed from the glove box and connected to a high purity O2 channel with high-purity oxygen (99.999%). Gas connections were carefully controlled to minimize outside air pollution and maintained oxygen pressure at 1 atm for 24 h. 2.4 Electrochemical Measurements. In this experiment, the LAND-CT2001A battery test system (Wuhan, China) was used to galvanostatic discharge and charge test to observe the electrochemical performances of the batteries. The electrochemical window was 2.0 to 4.8 V versus Li/Li+ at different current densities.

3. Results and discussion SEM test were carried out to investigate the microstructure and morphology of the



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three materials CA, CB and CC. Figure 1(a), (b) and (c) show that CA has an open cluster channel structure piled up by small spheres about 50 nm in diameter. The special spherical cluster structure of CA is mainly related to the activation process: During the precarbonization process at 250 ˚C, the intermolecular hydrogen bonds of chitin macromolecules were broken, making the macromolecular structure of chitin loose and scatter into small fragments, which were further cross-linked to form pellets. During the carbonization heating process, the content of aliphatic hydrocarbon in the reactant decreased progressively, while the aromatic ring increased steadily.26 This process cause CA to form an initial spherical cluster structure. When the pyrolysis temperature reached to 600 ˚C, the carbonization was dominated by dehydrogenation, and there are aromatics structures with high condensation degree in carbonaceous material.27 At the same time, H3PO4 also have an oxidation eroding effect on the formed carbon. Some studies have shown that H 3 PO 4 has potentiality to inhibit the unstable volatilization of carbon materials. 35 This ability is conducive to the formation of high yield about 13% and stable structure of carbonaceous materials. Therefore, the surface morphology of the sphere is characterized by many concavity and convexity, and these sharp parts have relatively high surface energy to provide active defect sites for ORR and OER reaction. Meanwhile, compared with other two-dimensional materials such as carbon nanosheets, the spherical particle cluster structure creates abundant channels, allowing oxygen and lithium ions to pass freely. 28-30 As can be seen from Figure 1(d)-(i), CB and CC have a honeycomb


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macropores structure rather than a spherical cluster structure. This is due to the pore formation mechanism of KOH and KHCO 3 as activators is similar, and it is very different from the formation mechanism of H 3 PO 4 . As shown in Figure 1 (d), (e) and (f), CB has a large number of macropores with a diameter about 1 μm. Interestingly, CC has a more uniform macropores nanosheet structure with well-developed large pore size and thin pore walls about 500 nm and the nanosheet is as thin as several nanometers (Figure 1 (g), (h) and (i)). The possible reason is that in the activation process, when KOH was used as the activation agent, according to researches of Otawa et al., the raw material and KOH always undergo a solid-solid reaction when the temperature is lower than melting point (380 ˚C) of KOH.23,

31, 32

In the erosion process of KOH, it is easy to have

heterogeneous reaction due to the solid phase reaction. While the activator changed to KHCO3 that is easily decomposed by heat, so the decomposition reaction of KHCO3 came up firstly.22 The large amount of gas generated by the reaction expands in the pyrolysis to make uniform pores inside the material. Therefore, extremely consistent pore structures of CC can be seen. The activation process explains why CC have a different macropore distribution structure from CB. Whether it is CB or CC, this macropores structure acts as a solid host cathode for Li-O2 battery, providing sufficient space to accommodate discharge products. X-Ray diffraction (XRD) and Raman measurements of the specimens were carried out to determine the structure characteristic of samples. Figure 2 (a) shows that the wideangle XRD patterns of all samples exhibit two broad peaks centered at 23.8° and 43.8° of



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amorphous carbon, consistent to typical (002) and (100) lattice planes, respectively.33, 34 According to Bragg Diffraction Equation: d002 decreases, and the degree of graphitization of the material increase.35, 36 Therefore, the graphitization degree of the three materials is CA > CB > CC. CA has higher degree of graphitization and fewer lattice defects, which reduces the migration resistance of electrons, and its kinetic performance in Li-O2 battery as cathode material can be improved. The same regularity can be proved on Raman measurement results in Figure 2 (b). The two distinguished characteristic D and G bands of the carbonaceous materials are located at peaks of 1345 cm-1 and 1595 cm-1, respectively, where in the D band represents a disordered graphite structure, and the G band represents a crystalline graphite carbon structure, and the intensity ratio of the D and G bands (ID/IG) can be used to determine the extent of disordering within the specimens.37 From the spectra, the ID/IG ratio of CA, CB and CC is 0.89, 0.96 and 0.99, respectively. Those indicate an obviously defect structure of three materials, which is benefit to provide active sites for electrochemical reactions in Li-O2 battery. Besides, CA has the lowest ID/IG ratio among three materials, which means there are more sp2 in-plane vibration carbon molecules, thus has better conductivity. In order to further understand the distinguished pore structure of CA, N2 adsorption-desorption curve and the pore size distribution of CA are shown in Figure 2 (c). The specific surface area of CA is 753.74 m2/g, and the average pore diameter is 6.37 nm. According to IUPAC, the CA isotherms is a typical feature of mesoporous materials, which can be classified as type IV. 38, 39 The pore size distribution of CA


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displays that the pore size distribution is mainly concentrated in the size range of 10~40 nm. Due to the distinction of pore size, the transport of O2 and Li+ in various types of pores and the tolerance of discharge products are also very different. The micropores can produce a relatively large surface area, but the pore size is small so that the micropores are quickly blocked by Li2O2 at the beginning of the discharge, resulting in a large amount of internal space that cannot continue to participate in the electrochemical reaction.40 Therefore, the utilization of the pore channels for micropores is very low, and the contribution to the cathode specific capacity is insufficient.41 For macropores, the pore size is large and not easy to be blocked, so the pore space can be fully utilized. However, since the specific surface area in these pores are small, the contribution of providing reaction sites for ORR is also meagre.42 Opportunely, for mesopores, they have both sufficiently large pore sizes to promotes the transport of O2 and Li+ and high specific area to provide sufficient reaction sites.28, 43 Hence, the spherical cluster structure in CA can not only provide a large tri-phase (solid–liquid–gas) regions, but also accommodate a large number of discharge products, which can promote the improvement of specific capacity and cycling performance of the Li-O 2 battery. After the successful design and synthesis of the various carbonaceous materials with abundant pore structures and high specific surface areas, electro-chemical properties measurement was carried out to illustrate the potential applications of these chitin carbonaceous materials as cathodes oxygen electrodes in Li-O2 batteries. Figure 3 (a) shows galvanostatic discharge tests of Li-O2 batteries assembled with CA,



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CB and CC as oxygen electrodes at different current densities. At a current density of 0.02 mA/cm 2 , the initial discharge capacity of CA is 16600 mA h/g with a termination discharge voltage of 2.0 V, which is the highest among three carbon-based oxygen electrodes (CB is 5200 mA h/g, and CC is 4000 mA h/g in the same conditions) and better than other researches such as Super P of 3462 mA h/g, Ketjenblack carbon of 5180 mA h/g, carbon nanofiber of 7280 mA/g and N-doped graphene aerogels of 1000 mA h/g.42, 44

And the initial discharge voltage platform of CA, CB, CC is about 2.76 V, 2.71 V and

2.70 V, respectively, which is higher than most commercial carbon materials. When the current density turned to 0.2 mA/cm2, the specific capacity of CA is 8400 mA h/g, CB is 1150 mA h/g and CC is 1500 mA h/g, respectively. This proves that CA has excellent rate specific capacity performance. Simultaneously, the discharge platform of CA is still the highest of 2.62 V compared with CB and CC about 2.50 V. In the case of high current, although the discharge platforms of the three electrodes are decreased, CA still shows better rate performance and excellent discharge specific capacity. The main possible reasons are: Firstly, different activators cause the three carbonaceous materials to have different microstructures, which leads to different electrocatalytic properties. The spherical cluster packing structure makes CA have better connection integrity and higher conductivity, and has more tri-phase contact regions and reactive sites, which is conducive to achieving high discharge specific capacity of Li-O2 battery. Besides, there are more macropores in CB and CC, resulting in loose structure, low pore volume utilization. At the same time, lacking of mesopores which play a vital role in catalytic reaction will affect the


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electronic conductivity of the material, so the catalysis performance of CB and CC is not as good as CA. Finally, the electrode reaction is dependent on electron transfer in the LiO2 battery, and the electronic conductivity of the material is closely related to its electrochemical properties as an oxygen electrode. For CB and CC, their honeycomb porous structure has more defects, but there is no sp2 hybrid conductive graphite carbon in CA, which causes their electron conductivity to be inferior to CA. Based on the above analysis, the different structures of the materials lead to the diversity specific capacity performance of Li-O2 batteries. For the purpose of further studying the better performing electroactive sites of CAbased oxygen electrode, the morphology and structure of CA electrode after discharge are analyzed, which are shown in Figure 3(b) and (c). It is obvious that the surface of the material is covered with a layer of round pancake particles with a diameter of about 500 nm. Relevant studies have shown that this pancake particle structure is a general property of electrochemically formed Li2O2.21, 45 XRD and Raman spectroscopy are used to analyze the chemical composition of the oxygen electrode at different stages of electrochemical charge and discharge with a specific capacity limited to 5000 mA h/g in first cycle and the results are shown in Figure 3(d) and (e). In Figure 3(d), compared with the XRD pattern of the initial cathode, new diffraction peaks emerge, which can be reasonably assigned to the (100) and (101) peaks of crystalline Li2O2.46 The curve of the Raman test of Figure 3(e) shows obvious peaks at 1093 cm-1, 1126 cm-1 and 1508 cm-1, corresponding to Li2O2, LiO2 and LiO2-C, respectively. Although LiO2 is not the main discharge products, relevant



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studies show that the existence of LiO2 can improve the performance of the battery.10 The dense accumulation of particles indicates that the spherical particles can be used as ideal reaction sites which have better catalytic activity for electrochemical reaction, especially for ORR process, besides, the pores among the particles allow enough space in the material to accommodate the discharge product Li2O2 without being blocked, thus improving the cycling performance of the Li-O2 battery. The cycle performance of the batteries is another practical application worthy of attention. The cycling test of carbon-based Li−O2 batteries are carried out under different current densities with a discharge depth of 500 mA h/g as shown in Figure 4. It can be seen from Figure 4 (a) that when the current density is 0.02 mA/cm2, the CA-based Li-O2 battery could maintain more than 99.99% cycle efficiency after 70 cycles (about 350 h). When the current density raises to 0.2 mA/cm2 (Figure 4 (d)), the battery can circulate 200 cycles stably without capacity loss, which is better than the performance of rechargeable Li-O2 batteries. Especially in the case of low current density, the discharge voltage platform of CA can still maintain about 2.5 V after 70 cycles of discharge, consequently yielding an enhanced cycle stability. At the same current density, the cycling performance of CA is significantly better than that of CB and CC. Figure 4 (b) shows that when the current density is 0.02 mA/cm2, the CB-based battery could maintains 50 cycles, and with the current density becoming 0.2 mA/cm2 (Figure 4 (e)), the battery can circulate 80 cycles. However, the coulomb efficiency decreases and capacity loss occurred in the later stage of cycles. The cycling test results indicate that the performance of CC-based battery is


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between that of CA and CB. CC-based batteries could circulate steadily for 50 cycles and 150 cycles under 0.02 mA/cm2 and 0.2 mA/cm2 current density, respectively (Figure 4 (c) and (f)). From the above cycle curves, a unified phenomenon can be found. As the number of cycles increases, the difference between the battery’s charge and discharge voltage platform gradually increases. The charge-discharge voltage polarization difference of the first 1 to 10 cycles is small, and the energy conversion efficiency of the battery is high. As the cycle progresses further, the discharge voltage platform drops slightly and the charging voltage platform rises. This phenomenon is mainly due to the fact that as the battery charge and discharge reaction progresses, the discharge product accumulates more and more on the surface of the electrode, causing channel blockage, affecting the transport of Li+ and O2, thereby increasing the polarization of the ORR/OER. The charging voltage is a common charging potential of a carbon-based electrode material. Overall, the electrochemical properties of the three carbon-based batteries performed well among similar carbonaceous materials. It may be related to the fact that chitin contains elements N and O, so that the prepared carbonaceous oxygen electrode materials contain double doping of N and O, especially N doping. Relevant studies have shown that N-doped can show better ORR catalytic activity.19 The surface chemistry and component of CA, CB and CC are further characterized by XPS. As can be seen from Figure 5 (a), the content of element N in CA is the highest, which exhibit higher reversible specific capacity, rate capacity and cycling performance. Figure 5 (b)



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further shows that N elements are doped in the form of Oxidized-N, Graphitic-N, PyrrolicN and Pyridinic-N. The content of the pyrrolic nitrogen in CA was slightly lower than that of CB, but pyridine nitrogen was higher than that of CB. Some theoretical calculations have shown that pyrrolic-N, pyridinic-N and graphitic-N can affect the adsorption and desorption of Li+ and O2 on carbon-based materials, thus improving the catalytic activity of the materials.47-49 Figure 5(c) shows different contents of nitrogen functional groups of CA, CB and CC. The data are obtained from the high-resolution spectrum of N 1s after peak fitting. The graphitic-N is the main form in both CA and CC, accounting for 48.4% and 52.6%, respectively, while the contents of pyridinic-N in the two materials are 29.8% and 14.6%, respectively. On the contrary, the graphitic-N and pyridinic-N contents in CB are only 5.0% and 12.1%, while pyrrolic-N accounts for 80.2%. The possible reason for the difference in nitrogen contents is the effects of activators. Alkaline activators are more susceptible to damage the edge of the five-membered ring structure to form the pyrrolic-N, and the acid activator is easier to form graphitic-N, which is replaced by the carbon atom in the graphite layer.37,50 In the graphite layer, the structure is more stable.51 It is apparent that the nitrogen in the form of graphite and pyridine nitrogen have more obvious effects to induce active site to participate in the destruction of the ortho-bond of the oxygen molecule.47 Thus, suitable N-doping promotes the combination of Li+ and O2 to form Li2O2 discharge products that cause Li-O2 battery to show better electrochemical properties. On the other hand, the three materials also contain O-containing functional groups


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(Figure S1). O-doped also changes the surface chemistry of the carbonaceous material, thereby affecting the electrochemical performance of the Li-O2 battery.51-53 On the one hand, the defects of carbonaceous materials caused by O doping will increase the active sites of the discharge reaction, which leads to the increase of discharge specific capacity of the battery and the decrease of overpotential. On the other hand, the introduction of O element can accelerate the decomposition of electrolyte and the oxidation corrosion of carbon-based materials, which is not conducive to improving the cycling performance of the battery. Correspondingly, compared with the N element, the meaning of the O element doping has not been clearly explained, and further research is needed.

Conclusion In this study, chitin used as a precursor and carbon materials with different microstructures were prepared by different chemical methods using three activators of H3PO4, KOH and KHCO3. The three materials have spherical particle cluster channel structure, uneven honeycomb macropores structure, and uniform honeycomb nanosheet macropores structure, respectively. Among them, the open spherical particle cluster channel of CA increases the nucleation sites of Li2O2 compared to the internally etched structure of CB and CC. At the same time, high N doping makes CA have better catalytic activity as a ORR catalyst, so CA prepared by using H3PO4 had an ultra-high specific capacity of 16600 mA h/g (at current density of 0.02 mA/cm2) and 8400 mA h/g (at current density of 0.2 mA/cm2) as an oxygen electrode of a Li-O2 battery, and could stably circulate



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more than 200 cycles at high current density. This study demonstrates a cheap and environment-friendly method for adjusting and customizing the skeleton of chitin-based carbon materials, and provides a new method for biochar as a high-capacity and long-cycle life oxygen cathode in Li-O2 batteries.

Acknowledgments This work was supported by the National Key R&D Program of China through Grant 2018YFC1900102. The use of Swagelok cell was supported by Cunzhong Zhang at Beijing Institute of Technology.

Supporting information High-resolution C 1s and O 1s XPS scan of three carbon materials CA, CB and CC.

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Figure Captions Figure 1. SEM images of CA (a, b, c), CB (d, e, f) and CC (g, h, i) at different magnifications. Figure 2. XRD patterns (a) and Raman spectra (b) of CA, CB and CC; Nitrogen adsorption−desorption isotherms and pore-size distributions (insets) of CA (c). Figure 3. Initial full discharge voltage profiles of Li‖CA−O2 battery, Li‖CB−O2 battery, and Li‖CC−O2 battery at a current density of 0.02 and 0.2 mA/cm2 (a); SEM images of the CA as the oxygen electrode catalyst discharged until the discharge voltage below 2.0 V (b) and (c); XRD (d) and Raman (e) characterizations of CA-based oxygen electrode in the first cycle at 0.02 mA/cm2 when limiting the specific capacity to 5000 mA h/g. Figure 4. Cycling tests of Li‖CA−O2 battery, Li‖CB−O2 battery, and Li‖CC−O2 at 0.02 mA/cm2 (a) (b) (c) and 0.2 mA/cm2 (d) (e) (f) when limiting the discharge specific capacity to 500 mA h/g. The insets in are specific capacity efficiency trends vs cycle number. Figure 5. Full-scan XPS (a) and High-resolution N 1s XPS scan (b) of CA, CB and CC; Nitrogen contents on the surface of three materials CA, CB and CC (c).


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Figure 1.

Figure 2.



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Figure 3.


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Figure 4.



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Figure 5.


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Graphical abstract


Effect of activation process on microstructure and electrochemical

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