Gradient Mn-La-Pt Catalysts with Three-layered Structure for Li-O2 battery

Gradient Mn-La-Pt catalysts with three-layered structure of manganese dioxide (MnO2), lanthanum oxide (La2O3), and Platinum (Pt) for Li-O2 battery are prepared in this study. The mass ratio of the catalysts is respectively 5:2:3, 4:2:4, and 3:2:5 (MnO2: La2O3: Pt) which is start from the side of the electrolyte. The relationship between morphology structure and electrochemical performance of gradient catalyst is investigated by energy dispersive spectrometry and constant current charge/discharge test. The Li-O2 battery based on gradient Mn-La-Pt catalysts shows high discharge specific capacity (2707 mAh g−1), specific energy density (8400 Wh kg−1) and long cycle life (56 cycles). The improvement of the Li-O2 battery discharge capacity is attributed to the gradient distribution of MnO2 and Pt and the involvement of La2O3 that can improve the energy density of the battery. More important, this work will also provide new ideas and methods for the research of other metal-air battery.

characteristic peak of the standard card, and three kinds of homemade catalysts are determined as manganese dioxide, lanthanum oxide and carbon supported platinum. The SEM image of the cathode electrode of the Li-O 2 battery with the gradient catalyst is shown in Fig. 1d. Figure 1d shows that the three catalysts are uniformly distributed to surface of this carbon paper. The rod-like structure is considered manganese dioxide (MnO 2 ), the scale-like structure is regarded as lanthanum oxide (La 2 O 3 ) and the granular structure is carbon supported platinum (Pt/C). This condition is in favor of ORR and OER. Figure 2 shows the energy dispersive spectrometry (EDS) images of the gradient cathode material. A comprehensive analysis of the samples is carried out here. The three spectrograms of sample content represent morphology of the first-layer, the second-layer and the third-layer catalyst layer, respectively. It can be seen from the three figures that the content of Mn is increased, but the content of Pt is reduced. In Fig. 2b, the La content is much less than the other two components of Mn and Pt. It indicates that the content of the three elements of each layer is approximately the same as the content of the prepared. So, the sprayed slurry is relatively uniform on the carbon paper.
The sectional scanning images of SEM are shown in Fig. 3. It can be clearly seen that the three layer structures of the cathode from the Fig. 3a. Figure 3b shows the peak intensities of the Mn, La and Pt three elements for the underlined part from left to right. The red line, green line and blue line represent Mn, Pt and La respectively which indicates the trend of peak intensities for three elements. Mn shows a downward trend in the whole trend of Mn and La appears trend of lower in both sides and higher in the middle, but there is no change in the trend of Pt. In the first-layer catalyst layer, peak intensity of Pt is reduced, and there is a possible reason that Pt is actually Pt/C and is close to the carbon paper. Owing to there is much interspace among the carbon paper. Pt is likely to enter the interspace of carbon paper, so the peak intensity of Pt is relatively low.  4:2:4, and 3:2:5, which is start from the side of the electrolyte. In the gradient catalyst structure, MnO 2 plays an important role in the discharge process and Pt/C has great effects on the charging process. The MnO 2 shows good catalysis property for ORR, and Pt is a good catalyst for OER. La 2 O 3 can improve the discharge plateau. In the discharge and charging processes, the Li-O 2 battery using a gradient catalyst can effectively provide a special site for electrochemical reaction. MnO 2 , La 2 O 3 , and Pt/C distribute orderly on the catalyst layer of the cathode that is advantageous to cycle performance and long-time operation of Li-O 2 battery.

Electrochemical test of gradient
AC impedance test is performed on the gradient catalyst and conventional catalyst at the voltage of 2.4 V and 25 °C. Nyquist diagrams of the batteries can be seen from the Fig. 5, the high-frequency region is a half arc and the low-frequency region is almost a straight line. The internal resistance of the battery can be obtained from the intersection of high frequency on the real axis, and the mass transfer resistance of the batteries can be obtained from the low frequency region of the Nyquist diagram, with the increase of the high frequency radius, and the resistance of mass transfer increases. Figure 5 shows that the mass transfer resistance of porous cathode is lower than that of traditional cathode. The Li-O 2 battery using the gradient catalyst is beneficial to the electrochemical reaction and the mass transfer of oxygen. Figure 6a,b show the power and energy density curves of the Li-O 2 batteries with gradient catalyst and conventional catalyst at different current densities at 25 °C. As shown in the Fig. 6a, the power density increases with the increase of the current density, and the power density of the gradient catalyst is slightly higher than that of the traditional catalyst. These results show that the discharge platform of assembled battery is stable and the structure of the cathode has a little influence on the Li-O 2 battery power density. Figure 6b shows the curve graph of the energy density changes with the current density in the batteries. The two curves are undulating at 0.01-0.1 mA cm −2 and the curves decrease with current density at 0.1-0.5 mA cm −2 . The phenomenon is attributed to the high severity of electrode polarization at a high current density. Results show that the energy density of Li-O 2 battery with a three-layered composite gradient catalyst as the cathode was 8400 Wh kg −1 , and the battery with a conventional catalyst is 6300 Wh kg −1 . La 2 O 3 can improve the discharge plateau. The gradient catalyst exhibits better specific energy than the conventional catalyst at different current densities that due to the involvement of La 2 O 3 improve the energy density of the battery.

Experimental Section
Preparation of catalysts and cathode.
(I) MnO 2 was prepared through the following process: 2.5 g of MnSO4·H2O (Sinopharm Chemical Reagent Co., Ltd.) and 1.5 g of KMnO4 (Sinopharm Chemical Reagent Co., Ltd.) were added to 20 mL deionized water to fully dissolve. Firstly, 100 mL deionized water was added to a three-neck flask, and the oil bath was heated to 95 °C. Secondly, the two kinds of solutions were slowly added to the three-neck flask from the two sides and stirred constantly. After adding the two kinds of solution, both sides of the flask were blocked with a ground glass stopper, heated to 95 °C, and continued for 18 h under this temperature until a brown suspension was obtained. Thirdly, the suspension was cooled at room temperature, and static precipitation was allowed to occur for 1 h. The samples were oscillated for 10 min under ultrasonic wave to make the precipitation disperse evenly and for filtration to occur. Water was then added, and several times of dispersion and filtration were implemented. Finally, the resulting catalyst was dried for 12 h at 80 °C in a vacuum. The MnO 2 catalyst was obtained. (II) The detailed La 2 O 3 preparation process was as follows. A total of 2.6 g of lanthanum nitrate (Sinopharm Chemical Reagent Co., Ltd.) and 1.4 g of citric acid (Sinopharm Chemical Reagent Co., Ltd.) were dissolved in 5 mL deionized water. The two kinds of solutions were added to a third beaker slowly at the same time. Then, the 5 mL deionized water was added. Until the pH of the mixed solution was 2.0, the mixed solution was placed in a furnace. The resulting catalyst was calcinated for 1.5 h at 700 °C in a muffle furnace. The La 2 O 3 catalyst was then obtained. (III) The Pt/C catalyst was prepared by impregnation reduction method. Here, the catalyst precursor was hexachloroplatinic acid (H 2 PtCl 6 ) (Shanghai Aladdin Reagent Co., Ltd.) and reducing agent was formaldehyde  (Shanghai Aladdin Reagent Co., Ltd.). The detailed steps of the preparation process were as follows 28 : firstly, the 30 ml of water and 30 ml of isopropanol (Shanghai Aladdin Reagent Co., Ltd.) were added to 50 mg Vulcan XC-72 (Shanghai King Chemical Co., Ltd.), and the suspension was ultrasonically stirred for 1 h. Then, add H 2 PtCl 6 to the suspension and stir ultrasonically for 2 h, the pH value of the suspension was adjusted to 12 by NaOH aqueous solution, then adding excessive formaldehyde to the suspension drop by drop. The mixture was sonicated for 20 min and then heated to 80 °C, it was agitated continuously for 3 h under this temperature. At last, the prepared catalyst was washed by ultra pure water until the chloride ion was not detected. The Pt/C catalyst was dried at 110 °C for 24 h in a vacuum drying oven. The Pt/C catalyst of 40 wt. % was obtained.
The oxygen electrode consists of a gas diffusion layer and three layer catalysts in this article. The preparation process of gas diffusion layer was as follows: The carbon paper (Shanghai River Electric Co., Ltd.) was soaked in Polytetrafluoroethylene emulsion (PTFE) (5 wt. %, DuPont Corporation) for 20 min and allowed to dry naturally. Afterward, carbon paper was dried at 350 °C. The preparation of the catalyst layer was as follows: The three kinds of the homemade catalysts, the super P conductive black and PTFE binder were made into the uniformly slurry and then spray to the gas diffusion layer, and the mass ratio of Super P/catalysts/binder was 70/20/10. The all operation steps were carried out at 100 °C and with the 2 atm. The prepared oxygen electrode was placed in a vacuum drying oven for 24 h. The carbon loading of oxygen electrode was 1.0 mg cm −2 .
The catalyst ratio of the three layer catalysts is different during the preparation of the electrode. The mass ratio of the catalyst near the electrolyte side was 5:2:3 (MnO 2 : La 2 O 3 : Pt/C), that of the catalyst near the cathode side was 3:2:5, and between the two layers was 4:2:4. The ratio of the carbon loading of the three layer catalysts was 2:1:2.
Fabrication of battery with gradient Mn-La-Pt catalysts. Lithium sheet (16 mm diameter, 0.2 mm thick) was used as anode electrode of Li-O 2 battery (Tianjin Lithium Industry Co., Ltd.). The oxygen electrode was cut into a circle with a diameter of 10 mm as cathode electrode. The electrolyte (2 mol L −1 ) was consisted of commercial electrolyte and lithium tetrafluoroborate (the molar ratio of 1:3). The polypropylene microporous membrane was cut into circles with a diameter of 16 mm as separators. The conventional catalyst that mentioned in the text was composed of MnO 2 and Pt/C.
The structure of the battery was shown in Fig. 7. The assembly process of the battery was carried out in a glove box filled with argon atmosphere. The stainless steel on the bottom of the fixture was just as the current collector of the negative electrode, the foam nickel was used as the current collector of the positive electrode. The main function of the diaphragms is to isolate the positive and negative electrode. The effect of the stainless steel spring is to ensure better contact between the internal and external circuits. The battery was allowed to stand for 1 h before testing. Physical characterization of catalysts and cathode. Scanning electron microscopy (SEM) is used to scan the sample surface by clustered electron beam, which uses the interaction between the electron and the sample, collects the secondary electrons and the scattered electron from the surface of the sample, and converts it to the electrical signal, finally displays the image that is synchronized with the emission electron beam. The physical and chemical information can be directly observed on the surface of the sample by SEM, such as morphology, composition, crystal structure and so on. The SEM (Hitachi S-4800) was used to observe the surface morphology of the electrode in this paper. Energy dispersive spectrometry (EDS) can be analyzed for the types and content of elements, because each element has a specific X-ray characteristic wavelength, which is determined by the characteristic energy released during the energy level transition.
X-ray powder diffraction (XRD) is a kind of research method, which is through X-ray diffraction of materials, analysis of the diffraction pattern, and to obtain information about the composition of the materials, the structure or form of internal atoms or molecules. The used instrument was Rigaku Ultima IV (Japan) in this paper.
Electrochemical measurements of Li-O 2 battery. AC impedance test of the Li-O 2 battery was measured at the voltage of 2.4 V for 25 °C by electrochemical analyzer (CHI 660E) which frequency range from 100 kHz to 0.1 Hz. alternating voltage amplitude of AC impedance test was 5 mV. It can be roughly divided into high frequency and low frequency regions.
The Li-O 2 battery was tested in the voltage range of 2 V to 4.5 V by the NEWARE cycler (Shenzhen Neware Electronic Co. Ltd.) at different current densities. All processes of the assembly battery were carried on a glove box with dry and pure oxygen at the constant current density of 0.01 mA cm −2 (0.025, 0.05, 0.075, 0.1, 0.25, and 0.5 mA cm −2 ).