MnCo2S4‐CoS1.097 Heterostructure Nanotubes as High Efficiency Cathode Catalysts for Stable and Long‐Life Lithium‐Oxygen Batteries Under High Current Conditions

Abstract

Constructing the heterostructures is considered to be one of the most effective methods to improve the poor electrical conductivity and insufficient electrocatalytic properties of metal sulfide catalysts. In this work, MnCo₂S₄‐CoS_1.097 nanotubes are successfully prepared via a reflux‐ hydrothermal process. This novel cathode catalyst delivers high discharge/charge specific capacities of 21 765/21 746 mAh g⁻¹ at 200 mA g⁻¹ and good rate capability. In addition, a favorable cycling stability with a fixed specific capacity of 1000 mAh g⁻¹ at high current density of 1000 mA g⁻¹ (167 cycles) and 2000 mA g⁻¹ (57 cycles) are delivered. It is proposed that fast transmission of ions and electrons accelerated by the built‐in electric field, multiple active sites from the heterostructure, and nanotube architecture with large specific surface area are responsible for the superior electrochemical performance. To some extent, the rational design of this heterostructured metal sulfide catalyst provides guidance for the development of the stable and efficient cathode catalysts for Li‐O₂ batteries that can be employed under high current conditions.

Introduction

Lithium-oxygen batteries (LOBs) are expected to be used in the next generation of power sources due to their ultrahigh energy density (3500 Wh kg⁻¹),^[1–3] but there are still some unsolved problems, such as poor cycle stability, inferior rate performance, and high charge/discharge overpotentials.^[4–6] Researchers found that one of the main reasons for the appeal challenge is the slow reaction kinetics in the process of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Therefore, there is an urgent need for high-performance cathode catalysts to promote the electrocatalytic activities.^[7–9]

Precious metals have been found to serve as ideal electrochemical cathode catalyst to alleviate polarization and improve LOB performance,^[10,11] but due to the scarcity and high price have limited their practical application, various alternatives were extensively investigated in LOBs. Such as transition metal oxides,^[12–14] sulfides,^[15–17] nitrides,^[18,19] carbides,^[20,21] carbon composites,^[22–24] and alloys.^[25,26] Among them, metal sulfides have attracted extensive attention of researchers, because they own the following advantages: 1) Metal sulfides are normally inherently unstable, and various crystal defects thus form during the formation process, which is conducive to the production of abundant active sites.^[27] 2) Variable valences of metal cations could promote the catalytic reactions.^[28] However, the metal sulfides catalyst still has some problems that need to be solved urgently. In particular, metal sulfides are usually semiconductors or insulators,^[29] and their low electrical conductivity seriously restrain the reaction kinetics in the ORR/OER processes. Based on this fact, a variety of methods have been used to improve the electrical conductivity of metal sulfides to further exert their catalytic activity, mainly including introducing heterogeneous atoms and adopting carbon matrix materials.^[30,31] However, the doping of heterogeneous atoms usually results in the formation of other substances in the metal sulfides, deteriorating electrocatalytic behaviors. In addition, carbon materials have been involved in unavoidable side reactions with superoxide, leading to a rapid increase in charging overpotentials. In recent years, heterostructure catalysts have developed rapidly, especially in the fields of hydrogen evolution reaction (HER),^[32] photocatalysis,^[33] water splitting,^[34] etc. Nowadays, Heterostructures can be defined as the unique architecture that consists of hetero-interfaces formed by different solid-state materials through physical and chemical combinations. Due to the difference in Fermi energy levels between different materials, the electrons transfer across the hetero-interface from material with high Fermi energy levels to that with lower ones, thereby generating an equilibrium state of equal Fermi energy levels. In order to balance their original electron affinity (ɸ) and work function ([IMAGE OMITTED. SEE PDF.]) constant, a space charge region would be formed. Built-in field plants will appear on both sides of the heterogeneous interface, which can greatly accelerate the transportation of electrons and ions, thereby remarkably boosting the electrocatalytic properties. It is also evident that constructing a heterostructure can also provide more active sites by introducing disordered atomic arrangement.^[35–37]

Herein, we have successfully prepared MnCo₂S₄-CoS_1.097 heterostructure nanotubes by reflux and hydrothermal method. Benefitting from unique structure and architecture with large surface area, enough space could be provided for storing discharge products, and rich heterogeneous interfaces were constructed to promote fast transport of oxygen, ions, and electrons, endowing MnCo₂S₄-CoS_1.097 cathodes large specific capacities, good rate performance, and excellent cycling stability.

Results and Discussion

MnCo₂S₄-CoS_1.097 was synthesized by a reflux- hydrothermal process as shown in Scheme 1. In the typical synthesis route, Mn(OAc)₂ 4H₂O, Co(OAc)₂ 4H₂O and polyvinyl pyrrolidone were uniformly dispersed and mixed in ethanol under magnetic stirring, and Mn-Co precursor was achieved after refluxing treatment. Afterward, MnCo₂S₄-CoS_1.097 was achieved via hydrothermal reaction with the addition of thioacetamide (TAA).

Scheme 1. Schematic illustration for the preparation process of MnCo2S4-CoS1.097 heterostructure nanotubes.

In order to investigate the crystal structure and composition of the prepared material, the samples were analyzed and characterized by XRD. Figure 1a shows the XRD patterns of the as-prepared MnCo₂S₄-CoS_1.097 and CoS_1.097 samples. The peaks at 30.9°, 35.5°, 47.1°, and 54.7° can be assigned to (204), (220), (306), and (330) faces of CoS_1.097 (JCPDS No.19-0366), indicate that the CoS_1.097 sample is composed of β-CoS_1.097 phase with a hexagonal structure in the space group P63/mmc (194), while in the MnCo₂S₄-CoS_1.097 XRD spectrum, the extra peaks at 31.5°, 38.2°, and 50.4° are associated with the (311), (400), and (511) faces of MnCo₂S₄ (JCPDS No.73-1703), which is similar to the previous result.^[38]

Figure 1. a) XRD patterns of different samples; b) XPS survey spectrum of MnCo2S4-CoS1.097; high-resolution XPS spectra of c) Mn 2p, d) Co 2p, and e) S 2p of MnCo2S4-CoS1.097; high-resolution XPS spectra of f) Co 2p and g) S 2p of CoS1.097; h) the nitrogen adsorption–desorption isotherms and pore size distribution curve of MnCo2S4-CoS1.097.

X-ray photoelectron spectroscopy (XPS) measurement was performed to further study the chemical composition and elemental chemical state of the samples. Survey spectra of MnCo₂S₄-CoS_1.097 and CoS_1.097 are shown in Figure 1b and Figure S1, Supporting Information. For MnCo₂S₄-CoS_1.097, the characteristic peaks of the Co, Mn, and S can be detected, while it is found that the CoS_1.097 sample is composed of only Co and S elements. In Figure 1c of the high-resolution spectrum of Mn 2p, these peaks are in good agreement with two spin-orbit doublets and a shaking satellite peak. The fitted peaks with binding energies of 641.8 and 653.5 eV are related to Mn²⁺ signals, and the fitted peaks with binding energies of 643.2 and 655.0 eV are ascribed to the Mn³⁺ signals, respectively. By integrating the areas under different peaks, the corresponding Mn³⁺/Mn²⁺ content can be calculated to be 68%/32%. In a similar way, Co 2p can be fitted into two spin-orbit doublets (Figure 1d,f). The fitted peaks of Co²⁺ signals locate at 780.4 and 795.5 eV, while those of Co³⁺ signals locate at 778.8 and 793.8 eV, respectively. The ratios of Co³⁺/Co²⁺ on MnCo₂S₄-CoS_1.097 and CoS_1.097 surfaces were determined to be 46/54 and 38%/62%, respectively. These results show that in the MnCo₂S₄-CoS_1.097 and CoS_1.097 samples, both valence states of Mn and Co are divalent and trivalent, which could serve as effective multiple active sites for electrocatalytic reactions. The spectrum of S2p (Figure 1e) shows three main peaks. The fitted peaks at 162.3 and 163.5 eV are assigned to S 2p_1/2 and S 2p_3/2, while the peak at 169.4 eV is the typical characteristic of sulfur ions with metal ions, respectively. In order to further determine the chemical compositions of the samples, the inductively coupled plasma measurements were performed on the MnCo₂S₄-CoS_1.097, and CoS_1.097 samples, and the contents of each element are as shown in Figure S2, Supporting Information. For MnCo₂S₄-CoS_1.097, the ratio of Mn/Co/S is 1/3.19/5.34. Through further calculation, it can be concluded that the proportions of MnCo₂S₄ and CoS_1.097 in the composite are 45.3% and 54.7%, respectively. For CoS_1.097, the ratio of Co/S is 1/1.11, which is well consistent with the theoretical value. The N₂ adsorption–desorption studies were also conducted for different samples, as given in Figure 1h and Figure S3, Supporting Information. It is clear that the N₂ adsorption isotherms of MnCo₂S₄-CoS_1.097 present IV type H3 hysteresis loop with the P/P₀ ranging from 0.2 to 1.0, and the hysteresis loop exhibits a saturated adsorption platform, indicating uniform pore formation. The specific surface areas of MnCo₂S₄-CoS_1.097 and CoS_1.097 are 57.8001 and 6.3240 m² g⁻¹, and their total pore volume are 0.24 and 0.012 cm³ g⁻¹, respectively, which demonstrates that MnCo₂S₄-CoS_1.097 is more conducive to the mass transfer and efficient storage of discharge products (Li₂O₂).

Morphology of precursor, MnCo₂S₄-CoS_1.097 and CoS_1.097 were captured by field emission scanning electron microscope, and the data are shown in Figure 2a and Figures S4,S5, Supporting Information. The precursor appears as a typical solid tubes-like structure, and its height and bottom side length are about 2 and 0.8um, respectively. The tubes-shaped structure of CoS_1.097 is more rounded, and its height is only around 400 nm. The TEM image in Figure S6a, Supporting Information shows that the tube-like structure is not hollow, and its cross section seems like ellipse shape. The high-resolution transmission electron microscope(HRTEM) image in Figure S6c, Supporting Information and the intensity profile in Figure S6d, Supporting Information recorded from the corresponding region of CoS_1.097 clearly show the (220) crystal lattice fringes with the spacing of 0.252 nm. Moreover, the pattern of selected area electron diffraction (SAED) of CoS_1.097 in Figure S6b, Supporting Information indicates (204), (220), and (306) faces of CoS_1.097. In contrast, MnCo₂S₄-CoS_1.097 shows a hollow tubes-shaped structure. Its size is similar to that of the precursor, but the surfaces are rough and uneven, resulting in a larger surface area. Moreover, the hollow structure could enable more exposed active sites and abundant three-phase reaction zones, which provide enough spaces for Li₂O₂ storage and diffusion tunnels for reactants (Li⁺, O₂) to the reaction sites.

Figure 2. a) SEM image of the MnCo2S4-CoS1.097; b) TEM image of the MnCo2S4-CoS1.097; c) SAED pattern of the MnCo2S4-CoS1.097; d–f) HRTEM images of the MnCo2S4-CoS1.097; g–j) elemental mapping images of MnCo2S4-CoS1.097.

TEM was used to study the more detailed microstructure characteristics of MnCo₂S₄-CoS_1.097, as shown in Figure 2b, it can be seen that the inside of the tube is hollow, and the surface of the hollow tube is composed of particles with a scale of about 20 nm, which is highly consistent with the scanning electron microscope (SEM) image. As shown in Figure 2c, the diffraction ring in the electron diffraction (SAED) mode of MnCo₂S₄-CoS_1.097 can be respectively indexed as (220), (400), and (440) planes of MnCo₂S₄, and (220), (304), (306), and (413) planes of CoS_1.097, which further proved the successful preparation of MnCo₂S₄-CoS_1.097 composites. HRTEM image of MnCo₂S₄-CoS_1.097 is shown in Figure 2d–f. The clearly defined lattice fringes of 0.332 nm in the Figure 2f can be clearly attributed to (220) plane of MnCo₂S₄, and the lattice fringe distance of 0.252 nm is attributed to the (220) crystal plane of CoS_1.097. The interface area between MnCo₂S₄ and CoS_1.097 species is shown in the orange area of Figure 2d. Under the strong electronic interaction between MnCo₂S₄ and CoS_1.097, the crystal lattice in this region is distorted, which may lead to the increased catalytic sites with the unique electrochemical behaviors. The element mapping diagram of MnCo₂S₄-CoS_1.097 (Figure 2g–j) clearly reveals the uniform distribution of Mn, Co, and S elements, further verifying the successful preparation of unique hollow heterostructure of MnCo₂S₄-CoS_1.097.

Figure 3a shows the potential cut-off constant current discharge–recharge voltage curves of CoS_1.097, and MnCo₂S₄-CoS_1.097 cathodes for LOBs at a current density of 200 mA g⁻¹. For better comparison, the specific capacity of KB and carbon paper cathode was also tested under the same conditions (Figure S7, Supporting Information). The CoS_1.097 and MnCo₂S₄-CoS_1.097 cathodes show similar discharge voltages at 2.74 V, which are 0.12 V higher than that of the KB cathode. It is evident that MnCo₂S₄-CoS_1.097 and CoS_1.097 cathodes show a two-stage charging platform during the charging process. At around 3.5 V, the active sites on the cathode surface absorbed O₂ to reduce it, on which the adsorbed oxygen (O₂*) was combined with Li⁺ to form an adsorbed lithium superoxide (LiO₂*) (Li⁺ + O₂* + e⁻→LiO₂*).^[39,40] At 4.2 V, further reduction of LiO₂* to form adsorbed Li₂O₂* (LiO₂* + Li⁺+ e⁻→Li₂O₂*) was conducted. It is found that the discharge/charge capacities of the MnCo₂S₄-CoS_1.097 and CoS_1.097 cathodes are 21 765/21 746 and 17 302/13 626 mAh g⁻¹, respectively, which are much higher than those of KB cathodes with lower overpotentials. The coulombic efficiencies of KB and CoS_1.097 cathodes are 26% and 79%, while the MnCo₂S₄-CoS_1.097 cathode exhibits the coulombic efficiency of 99%. More importantly, the charging voltage of the MnCo₂S₄-CoS_1.097 cathodes is obviously less than that of the CoS_1.097 cathodes. This indicates that the MnCo₂S₄-CoS_1.097 could deliver superior OER activity.

Figure 3. a) Initial discharge/charge profiles of different cathodes at 200 mA g−1 from 2.35 to 4.35 V; b) comparison of rate performance of different cathodes; c) CV curves of different cathodes at 0.15 mV s−1; d) typical discharge/charge profiles of MnCo2S4-CoS1.097 cathode at 500 mA g−1; e) cycling performance of different cathodes at 1000 mA g−1 under a specific capacity limit of 1000 mAh g−1; f) EIS plots and g) XRD patterns of the MnCo2S4-CoS1.097 cathodes at different stages; h) comparison of the cycling stability of MnCo2S4-CoS1.097 cathode with those of representative and most recently reported cathodes based on metal sulfide catalysts and noble metals.

The battery tested sequentially at current densities ranging from 100 to 800 mA g⁻¹ for each 3 cycles under the specific capacity limit of 1000 mAh g⁻¹. It is clear in Figure 3b that the charge and discharge overpotentials of the two cathodes increased with the increase of the current densities. At each current density, MnCo₂S₄-CoS_1.097 cathode shows lower overpotentials in Figure S8, Supporting Information, which also demonstrated that MnCo₂S₄-CoS_1.097 cathode feature better ORR and OER properties. When the current density was restored to 100 mA g⁻¹, the discharge and charge terminal voltages of the MnCo₂S₄-CoS_1.097 battery dropped sharply, with almost no change compared with that at the initial state. It is much more stable than the CoS_1.097 counterparts, implying that the MnCo₂S₄-CoS_1.097 cathode hold excellent rate capability.

Cyclic voltammetry (CV) curves of three different cathodes were tested at a scan rate of 0.15 mV s⁻¹ (Figure 3c). The first cycle curve shows that the MnCo₂S₄-CoS_1.097 cathode offers a higher current than that of the CoS_1.097 and KB cathodes. In addition, unlike KB cathode, both the MnCo₂S₄-CoS_1.097 and CoS_1.097 cathodes display oxidation peaks around 3.5 and 4.1 V, which is consistent with the charging platforms in Figure 3a. Moreover, the MnCo₂S₄-CoS_1.097 cathode shows the lower OER and higher ORR onset potentials, which also indicates that the MnCo₂S₄-CoS_1.097 cathode own better OER/ORR electrocatalytic properties. The reason for these result may be that the formation of a heterostructure could increases the electrical conductivity of the material, while the hollow architecture with a layered construction reduced the diffusion path length of ions and oxygen, thereby promoting electrocatalytic activity.

Figure 3d and Figure S9, Supporting Information show typical voltage curves of the MnCo₂S₄-CoS_1.097, CoS_1.097, and KB cathodes during cycling at 1000 mA g⁻¹ with a fixed capacity of 1000 mAh g⁻¹. Figure 3e gives the discharge/charge terminal voltage curve diagram of three different cathodes, in which the MnCo₂S₄-CoS_1.097 cathode cycled 167 cycles at 2.0 to 5.0 V, while the CoS_1.097 and KB cathodes only cycled 56 cycles and 7 cycles under the same conditions. It is worth noting that the ORR and OER overpotentials of the MnCo₂S₄-CoS_1.097 cathodes are significantly lower than that of the CoS_1.097 and the KB cathodes. In addition, the cycle performance MnCo₂S₄-CoS_1.097 cathodes under the current conditions of 2000 mA g⁻¹ with cut-off specific capacity of 1000 mAh g⁻¹ were also tested (Figure S10, Supporting Information), and cycled 57 times. This illustrates that the MnCo₂S₄-CoS_1.097 cathodes could exhibit superior stability and reversibility at high current densities.

Figure 3g shows the XRD pattern after 1st discharging. Two new peaks appeared at 32.7° and 34.8°, respectively, which corresponding to the (200) and (201) crystal planes (JCPDS#73)-1640) of Li₂O₂.^[41,42] In addition, SEM and TEM investigations have been conducted to recognize the discharge products. As shown in Figure S11a,c, Supporting Information, the dense package products precipitated on the surfaces and filled the center hole of the MnCo₂S₄-CoS_1.097 cathodes after discharging. HRTEM image in Figure S11d, Supporting Information shows a spacing of 0.258 nm, corresponding to the (201) lattice of Li₂O₂, and the different lattice fringes in the SAED pattern in Figure S11b, Supporting Information well coincided with the crystal lattices of MnCo₂S₄, CoS_1.097, and Li₂O₂. After 1st and 60th recharging, the XRD pattern rarely changed compared at its initial state, which confirms the stable cycle performance of the MnCo₂S₄-CoS_1.097 cathodes. Electrochemical impedance spectroscopy (EIS) data at different stages are shown in Figure 3f with the corresponding equivalent circuit in the inset. Typically, R_ohm represents the Ohmic resistance of the Li-O₂ cell, and R_int1 and R_int2 denote the charge transfer resistance between the cathode/electrolyte and Li anode/electrolyte, respectively. The mass diffusion rate is related to the speed of transporting reaction species. The variations of R_int1 during cycling are associated with the deposition of discharge products.^[22,43] The sudden increase of the R_int1 from 13.5 Ω at fresh stage to 45.9 Ω after discharging is due to the covering of insulating Li₂O₂ on cathode surfaces, which normally decreased the electrical conductivity and increased the charge transfer resistance.^[44,45] After 1st charging, Li₂O₂ decomposed, and the R_int1 became 18.7 Ω, almost equal to that at the pristine state. After 60th charging, the charge transfer resistance slightly increased, indicating the excellent cycling performance of MnCo₂S₄-CoS_1.097 cathodes. In addition, compared with the most of the reported typical sulfides^[46–53] and noble metals based cathodes (Figure 3h),^[54–62] it is worth mentioning that the MnCo₂S₄-CoS_1.097 cathode deliver superior cycling stability under similar testing conditions.

In order to reveal the discharging/charging mechanism during cycling based on MnCo₂S₄-CoS_1.097 cathodes, the high-resolution XPS spectra of Li 1s at various stages (Figure 4a) with limited specific capacity of 1000 mAh g⁻¹ at 1000 mA g⁻¹ were collected and shown in Figure 4c–f. According to previous literature reports, the binding energies at around 55.2 and 56.1 eV in Li 1s spectrum can be ascribed to the Li₂O₂ or Li_{2 −x}O₂ (mixture of LiO₂ and Li₂O₂), respectively.^[63–65] At State I, the proportion of Li_{2 − x}O₂ and Li₂O₂ is 48%/52%, suggesting that in the initial stage of discharging, the following three reactions experienced at the same time: [Image Omitted. See PDF][Image Omitted. See PDF][Image Omitted. See PDF]

Figure 4. a) Discharge/charge curves for the initial cycle at 500 mA g−1 under a cutoff specific capacity of 1000 mAh g−1; b–f) high-resolution Li 1s XPS spectra of MnCo2S4-CoS1.097 cathodes at different states; g) in situ DEMS profiles for MnCo2S4-CoS1.097 cathodes with charge curves at 500 mA g−1.

At State II, the ratio of Li_{2 − x}O₂/Li₂O₂ becomes 24%/76%, indicating that more LiO₂ was oxidized to Li₂O₂. At State III, the content of Li_{2 − x}O₂ increased to 33%, indicating that at the beginning of the charging reaction, part of Li₂O₂ converted to LiO₂ through the delithiation reaction. In order to confirm this conclusion, the gas evolution rates of O₂, CO₂, and H₂O were tested by in situ differential electrochemical mass spectrometry (DEMS) with the galvanostatic charge voltage profile of MnCo₂S₄-CoS_1.097 presented in Figure 4g. The blue dotted line in the Figure 4g represents the amount of electrons passing per second (v(e⁻)), and the calculation result is 3.01 nmol s⁻¹. When the charging specific capacity is 760 mAh g⁻¹, the value of O₂ release was approaching to the maximum, and only the decomposition of LiO₂ happened. Before that, the ratio of the electron transfer rate to the release rate of oxygen v(e⁻):v(O₂) is between 1 and 2, indicating that the decomposition of LiO₂ and Li₂O₂ are simultaneously present in the subsequent charging process. During the initial charging process, the content of LiO₂ increased, which indicates that Li₂O₂ decomposed according to the following steps, and the reaction rate of Equation (4) is faster, demonstrating the accumulation of LiO₂. [Image Omitted. See PDF][Image Omitted. See PDF]

Conclusions

In summary, MnCo₂S₄-CoS_1.097 nanotubes were successfully constructed by simply reflux and hydrothermal process. The hollow structure could not only provide abundant active sites and sufficient storage room for discharge products, but also supply rich heterogeneous interfaces between the MnCo₂S₄ and CoS_1.097 to promote fast transport of oxygen, ions, and electrons, thus exhibiting superior ORR/OER performance in LOBs. The MnCo₂S₄-CoS_1.097 nanotubes cathode shows high discharge/charge specific capacities of 21 765/21 746 mAh g⁻¹ at 200 mA g⁻¹, and it also delivers pleasant rate capability. In addition, outstanding cycling stability was also achieved with a retention capacity of 1000 mAh g⁻¹ after 167 cycles at 1000 mA g⁻¹ and 57 cycles at 2000 mA g⁻¹. This rational synthetic strategy to construct heterogeneous structure could offer a guidance for the application of other sulfide materials in LOBs, which is also expected to be extended to other energy storage and conversion systems.

Experimental Section Preparation of the MnCo₂S₄-CoS_1.097 and CoS_1.097

0.45 g of Mn(OAc)₂⋅4H₂O, 0.9 g of Co(OAc)₂.4H₂O, and 3.0 g of polyvinyl pyrrolidone (PVP, K15, Mw≈10 000) were dispersed in 200 mL ethanol, and a clear pink solution formed, which was then gradually heated to 85 °C and treated under reflux for 2 h. After cooling to room temperature, the resulting precipitate was collected by centrifugation and washed four times with ethanol and deionized water, followed by drying in an oven at 50 °C for 12 h to obtain the Mn-Co precursor. Afterward, 0.08 g of the above-mentioned Mn-Co precursor and 0.120 g of TAA were added to 40 mL ethanol under stirring for 10 min. The solution was then transferred to an autoclave and kept at 120 °C for 6 h. After cooling down to room temperature, the product was collected by centrifugation, washed several times with ethanol and water, followed by drying in an oven at 60 °C for 12 h to gain MnCo₂S₄-CoS_1.097. CoS_1.097 was achieved via the same procedure without adding Mn(OAc)₂⋅4H₂O.

Material Characterizations

X-ray diffractometer (RigakuD/ max 2500, Japan)) was used to study the phase composition. Morphology and elemental composition of the samples were analyzed by a field SEM (Hitachi, S-4800, Japan) with energy dispersive X-ray spectroscopy and a HRTEM (JEM- 2100F, 200 kV). The specific surface area and pore size distribution of the samples were determined by the N₂ adsorption/desorption isotherms of Brunauer–Emmett–Teller with a Micrometritics analyzer (ASAP2020). An XPS (ESCALAB250) was performed to analyze the chemical species and bonding properties of MnCo₂S₄-CoS_1.097 and CoS_1.097.

Electrochemical Measurements

To fabricate the cathode, a slurry containing an active catalyst sample, KB and polytetrafluoroethylene with a mass ratio of 40:40:20 was uniformly dispersed in isopropanol with ultrasonic treatment. The mixture was then sprayed evenly on the carbon paper on a heating base to remove isopropanol. Then, it was held in a vacuum oven at 120 °C for 10 h to achieve the cathode. All batteries were assembled in a glove box (H₂O < 0.1 ppm, O₂ < 0.1 ppm) filled with an argon atmosphere. The batteries consisted of an as-prepared cathode, a glass fiber separator, a Li metal foil as a counter anode, and the electrolyte of 1 m lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in tetraethylene glycol dimethyl ether. The Li-O₂ batteries were tested using a LAND test system (CTA2001A, Wuhan Land Electronics Co., Ltd.) in 2032 coin-type cells in a high-purity oxygen box with a cut-off voltage from 2.35 to 4.35 V (relative to Li⁺/Li). The specific capacity and current were calculated based on the weight of the active catalyst supported on the cathode. CV was performed at a scan rate of 0.15 mV s⁻¹ on an electrochemical workstation (PARSTAT MC2273), on which EIS were also conducted.

Acknowledgements

This work was supported by China Postdoctoral Science Foundation (2020M672054), Natural Science Foundation of Shandong Province (ZR2020QB122), and Young Scholars Program of Shandong University (2019WLJH21).

Conflict of Interest

The authors declare no conflict of interest.

Data Availability Statement

The data that support the findings of this study are openly available in figshare at http://doi.org/10.1002/advs.202103302, reference number advs.202103302R1.