The MnO₂ nanosheet electrode was prepared with a simple electrodeposition method. Manganese acetate (C₄H₆MnO₄·4H₂O), chromium nitrate (Cr(NO₃)₃·9H₂O), and sodium sulfate (Na₂SO₄) were purchased from Aladdin, China. Stainless steel (SS) felt substrates were first cleaned in an H₂SO₄ solution, rinsed in distilled (DI) water, and air-dried at 60°C. The SS felt substrate was then immersed in a solution containing 0.1 M Na₂SO₄ and C₄H₆MnO₄ with an anodic current density of 0.25 mA·cm⁻² to obtain an electrodeposited layer of MnO₂ nanosheets. Then, the as-obtained deposition was rinsed in DI water, dried at 90°C for 12 h, and was used as the MnO₂ electrode. The loading of MnO₂ was controlled to achieve 0.5 mg·cm⁻² by adjusting the electrodepositing time and weighing.

After electrodeposition, the as-prepared MnO₂ electrode was immersed in a 0.1 M (NH₄)₂CrO₄ solution for 6 h to achieve the full adsorption of Cr ions. After the above treatment, the electrode was taken out of the solution, and the remaining liquid on the electrode surface was wiped dry by filter paper. The electrode was dried at 90°C, and calcined at 350 °C for 3 h, under N₂ atmosphere. The resulting electrode was named Cr₂O₃@MnO_2-x. For comparison, the MnO₂-SS electrode, without chromium adsorption, was also calcined under the same conditions and named as MnO_2-x.

The as-prepared electrodes were cut into a 14 mm discs and used as the cathode in non-aqueous lithium-oxygen batteries. The battery performance was tested in a lithium-oxygen battery developed in-house, with a lithium metal anode, a separator (GF/C, Whatman), 200 μl 1.0 M Lithium bis(trifluoromethanesulphonyl)imide (LiTFSI)-tetraethylene glycol dimethyl ether (TEGDME) electrolyte, and a cathode. The battery was assembled in a glove box and then tested in an O₂ atmosphere, at a pressure of 1.25 atm.

The electrochemical performance of the electrode was first tested through electrochemical impedance spectra (EIS) and cyclic voltammetry (CV). All the tests were carried out in an electrochemical workstation (CHI660, Shanghai Chenhua). The discharge-charge tests were conducted on a battery testing system (CT-4008, Neware) at current densities of 200, 400, and 800 mA·g⁻¹. The cycle stability was tested in a homemade Li⁺-oxygen batteries at the current density of 400 mA·g⁻¹, with a fixed specific capacity of 1,000 mAh·g⁻¹. A LiFePO₄ electrode was used as the reference electrode and counter electrode in the Li⁺-oxygen batteries and the electrode was prepared as follows: lithium iron phosphate (LiFePO₄, MTI Corporation), acetylene Black (AB, Canrd) and Poly tetra fluoroethylene (PTFE, Canrd) (85:5:10 wt%) were thoroughly mixed and pressed on a stainless steel mesh (16 mm diameter) and then dried under vacuum at 120°C for 12 h. To ensure the voltage stability, a ∼10-fold excess of LiFePO₄ was applied in the Li⁺-oxygen batteries. All tests were carried out at room temperature (25 ± 1°C).

The crystal structures of different samples were tested with an X-ray diffraction system (XRD, D/max2500/PC). The morphologies were obtained by scanning electron microscopy (SEM, S-4700) and scanning transmission electron microscopy (STEM, FEI TECNAI G²F20S-TWIN). The valence states of Mn and Cr were characterized by X-ray photoelectron spectroscopy (XPS) on an Axis Ultra spectrometer. The composition of the as-prepared manganese oxide was tested by the iodometry method. The discharge product was analyzed by XPS and Raman spectroscopy (RENISHAW in Via, wave length 532 nm). The morphology of the product was observed by SEM.

The SEM images of different electrodes were shown in Figure 1. From these images, we can clearly find that the MnO₂ prepared by electrodeposition exhibits an irregular nanosheet structure, with a thickness of ∼50 nm. These nanosheets are expected to provide a high surface area for chromium adsorption and electrochemical reactions in lithium-oxygen batteries. After the simple calcination or adsorption-calcination process, the electrode morphology was characterized, as shown in Figures 1C–F, respectively. The results show that after the calcination or adsorption-calcination process, the surface morphology of the electrode remains unchanged. Therefore, the improvement in the charge-discharge performance of the electrode can be attributed to the change of the surface state.

XRD and XPS were measured to investigate the composition and valence state of the elements of the different electrodes, as shown in Figure 2. To avoid the influence of the SS substrate, before the test, the electrode materials on the electrode surface were peeled away by ultrasonication and collected. XRD showed that these patterns exhibited a group of diffraction peaks at 37.1°, 42.4°, 56.0°, and 66.7°, which matched well with the diffraction of the (100), (101), (102), and (110) planes of the ε-MnO₂ (akhtenskite, Joint Committee on Powder Diffraction Standards no. 33-0820). This result indicates that the as-prepared MnO₂ was mainly ε-MnO₂ and that after the adsorption-calcination process, the crystal structure did not change significantly.

XPS was used to identify the valence states of Mn, Cr, and O their species in the different samples. Figure 2B shows the deconvoluted Mn 2p_3/2 peak of the different electrodes, where the peaks at 642.4 and 641.8 eV correspond to Mn (IV) and Mn (III), respectively. The results show that after the calcination/adsorption-calcination, the peak intensity of Mn (III) increased while the peak intensity of Mn (IV) decreased, revealing that the oxygen vacancies and the associated Mn (III) were generated during the heat treatment. These oxygen vacancies are expected to increase the catalytic oxygen reduction activity of the MnO₂-based materials, thereby achieving high discharge capacity.

The existence of Cr on MnO_2-x nanosheets was studied by XPS, as shown in Figure 2C. The test results show that Cr mainly exists in the mixed form of Cr (III) and Cr (VI):The peaks located at 576.8 and 586.4 eV are corresponding to Cr (III) and the peaks located at 578 and 587.8 eV can be attributed to the Cr (VI) within the Cr₂O₃ (Yao et al., 2014; Gan et al., 2016). The mixed states of Cr in Cr₂O₃/MnO_2-x are expected to have high catalytic activity in promoting charge process. The O 1s spectrum, in Figure 2D, shows two peaks at 529.8, and 531.5 eV, which are correlated to the normal lattice oxygen (O_latt), and adsorbed oxygen (O_abs), respectively. Since the surface adsorbed oxygen mainly occurs at the oxygen vacancy, the oxygen vacancy content can be known through the analysis of O_abs (Li et al., 2020b; Mo et al., 2020). Through the O 1s spectrum, it can be found that the proportion of O_abs increases after the calcination and adsorption-calcination process, indicating that the process can introduce more oxygen vacancies in the MnO₂ structure. The increase in oxygen vacancies is consistent with the Mn 2p spectrum and is expected to be beneficial for ORR. Iodometry and Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES) were also used to determine the composition of the different electrode materials. The composition of MnO₂, MnO_2-x and Cr₂O₃/MnO_2-x was found to be MnO_1.99, MnO_1.96, and 0.004Cr₂O₃/MnO_1.96, respectively.

To investigate the distribution of Cr₂O₃ on the MnO_2-x nanosheets after adsorption-calcination, STEM-EDS mapping was carried out, as shown in Figure 3. The TEM images (Figure 3A) show that the electrodeposited MnO_2-x material is in the form of nanosheets, and the diameter of the Cr₂O₃/MnO_2-x nanosheet is consistent with that obtained from the SEM results. EDS mapping (Figures 3B–D) shows that chromium is evenly distributed on the MnO_2-x nanosheets. The surface Cr/Mn ratio of the Cr₂O₃/MnO_2-x was calculated to be 0.153 from the elements abundance test from STEM-EDS mapping (Supplementary Figure S1). This uniform dispersion can be attributed to the uniform adsorption process on the MnO₂ surface. Thus, chromium can be uniformly loaded on the surface of MnO_2-x under low loading, which has two advantages: 1) the low Cr₂O₃ loading can minimize the inhibition effect to MnO_2-x for the discharge process of lithium-oxygen batteries; and 2) uniform chromium distribution is conducive to achieve uniform contact with the discharge product, Li₂O₂, so as to achieve high catalytic effect in the charging process.

From the SEM, XRD, XPS, iodometry and ICP-OES and TEM results, it can be concluded that: 1) a MnO₂ film can be grown on the surface of the substrate by electrodeposition; 2) the adsorption process can adsorb chromium species on the surface of the MnO₂; and 3) the subsequent calcination process introduces a large amount of oxygen vacancies and Mn (III) into manganese dioxide, and at the same time, converts chromium species to Cr₂O₃. The above results indicate that the Cr₂O₃ decorated MnO_2-x nanosheet electrode can be prepared by the combined electrodeposition-adsorption-calcination method. The electrode has a high oxygen vacancy content, low Cr₂O₃ loading, and uniform Cr₂O₃ distribution, and can deliver high discharge capacity and low charge voltage.

Before the charge and discharge test, the impedance characteristics of different electrodes in lithium-oxygen batteries were studied by AC impedance and fitted with a simple equivalent circuit mode, and the results are shown in Figure 4A. In this model, R_Ω corresponds to the ohmic resistance of electrolyte and electrode materials, R_ct represents charge transfer resistance for ORR/OER on the cathode-electrolyte interface, CPE is the constant phase element corresponds to the cathode-electrolyte interfaces and W is the finite length Warburg contribution. The fitting results show that the ohmic resistance of MnO₂, MnO_2-x and Cr₂O₃/MnO_2-x electrode was calculated to be 15.1, 13.7 and 13.9 Ω, respectively. At the same time, the charge transport resistance of MnO₂, MnO_2-x and Cr₂O₃/MnO_2-x electrode was calculated to be 43.4, 36.6 and 38.6 Ω, respectively. This result can be attributed to three reasons: 1) oxygen defects and Mn (III) generated during the calcination and adsorption-calcination process can increase the electronic conductivity of the MnO_2-x nanosheets, thereby reducing the ohmic resistance of the electrode material; 2) oxygen vacancies can improve the adsorption of oxygen species on the surface of the MnO₂-based material, thereby obtaining good catalytic activity for oxygen reduction and lower charge transfer resistance; 3) the small amount of Cr₂O₃ is uniformly distributed on the MnO_2-x nanosheets, thereby reducing the inhibitory effect of Cr₂O₃ on oxygen reduction to a minimum. Therefore, the charge transfer impedances of the MnO_2-x electrode and the Cr₂O₃/MnO_2-x electrode are close in value.

Figure 4B shows the galvanostatic discharge/charge performance of the three electrodes in non-aqueous lithium-oxygen batteries, under a current density of 200 mA·g⁻¹. The MnO₂ electrode delivers a discharge capacity of 4,801 mAh·g⁻¹ with a terminal charge voltage of 4.14 V. Under the same conditions, the MnO_2-x electrode and Cr₂O₃/MnO_2-x electrode exhibit specific discharge capacities of 6,854 mAh·g⁻¹ and 6,779 mAh·g⁻¹, respectively. More importantly, the Cr₂O₃/MnO_2-x electrode delivers a reduced terminal charging voltage of 3.84 V. The energy efficiencies of the MnO₂, MnO_2-x, and Cr₂O₃/MnO_2-x electrodes were calculated to be 66%, 65%, and 78%, respectively. This result suggests that the Cr₂O₃/MnO_2-x electrode exhibits a high specific capacity, the lowest charge voltage, and the highest energy efficiency.

MnO₂-based materials can work as anode active materials in lithium-ion batteries through the lithiation and delithiation reactions, thus showing discharge-charge capacity. In order to exclude the capacity of lithiation and delithiation capacity, the specific capacities of different electrodes were tested in sealed button cells under the same current density. The results (Supplementary Figure S2) show that the lithiation and delithiation capacities of the MnO₂, MnO_2-x and Cr₂O₃/MnO_2-x electrodes were about 225–250 mAh·g⁻¹, which can be ignored compared with the capacity in lithium-oxygen batteries.

The rate capability of the as-prepared MnO₂, MnO_2-x and Cr₂O₃/MnO_2-x electrodes are tested under different current densities, and the results are shown in Supplementary Figure S3, and Figures 4C,D, respectively. The Cr₂O₃/MnO_2-x electrode delivers specific discharge capacities of 6,779, 5,033, and 2,627 mAh·g⁻¹ with the energy efficiencies 78%, 74%, and 72%, under the current densities 200, 400, 800 mA g⁻¹, respectively (Figures 4C,D). At the same time, the MnO₂ and MnO_2-x cathodes can deliver specific capacities of 4,801/6,854, 3,712/5,147, and 2,083/2,706 mAh· g⁻¹ with energy efficiencies, 66/65%, 63/62%, and 62/61, respectively.

The discharge products of Cr₂O₃/MnO_2-x were characterized by SEM, XPS, and Raman spectroscopy, as shown in Figure 5. The SEM image (Figure 5A) shows that the surface of the nanosheets is covered by film-like products after discharge. This film-like product can achieve sufficient contact with the catalyst surface, ensuring good catalytic activity during charging. After charging, the SEM image (Figure 5B) shows that the film-like product completely disappears and that the electrode surface recovers to the same as before discharge. Li 1s XPS spectra (Figure 5C) were obtained from the electrode after cycling to analyze the composition of the discharge product. In the discharged electrode, the Li 1s peak was mainly composed of Li₂O₂ and a small amount of li-based impurities which come from contamination during testing or parasitic reactions, indicating that Li₂O₂ had decomposed on the catalyst during the charging process. Raman spectroscopy (Figure 5D) was also used to study the existence of discharge products on the electrode surface after cycling. The results show that the product after discharge was mainly in the form of Li₂O₂ and that the Li₂O₂ peak disappeared after charging, which is consistent with the XPS results.

The stability of the Cr₂O₃/MnO_2-x electrode was tested in the in-house Li⁺-oxygen battery, using LiFePO₄ as the counter electrode to eliminate the influence of lithium electrode on the cycle performance. The capacity of the battery was limited to 1,000 mAh·g⁻¹, and the battery voltage and the cycle performance are shown in Figure 6A. This experimental result shows that the battery can stably cycle for 100 cycles in the voltage range of 2.0–4.5 V without capacity degradation, but its discharge terminal voltages decrease from 2.693 to 2.530 V with the charge terminal voltages increase from 3.985 to 4.022 V. This voltage change can be attribute to the accumulation of minor by-products rather than by material degradation. To verify this hypothesis, the cycled battery was disassembled, cleaned, and reassembled with a fresh lithium anode, separator, and electrolyte for testing. The discharge/charge performance in Figure 6B shows that the reassembled battery could achieve the same charge and discharge performance as that of a fresh battery, indicating good stability of the Cr₂O₃/MnO_2-x electrode.