Microwave assisted synthesis for ϵ-MnO2 nanostructures on Ni foam as for rechargeable Li–O2 battery applications

Lithium-air batteries exhibits high practical energy densities ranging from 1000 to 4000 Wh Kg−1, rendering them appealing for applications in portable electronic devices and electric vehicles. Nevertheless, they grapple with challenges like low charge–discharge efficiency, limited stability over multiple cycles, and electrode degradation stemming from undesirable side reactions, thus impeding their commercial market. In this study, ϵ-MnO2 petal-like nanostructures were synthesized on Ni foam via simple, microwave-assisted synthesis approach. The resulting ϵ-MnO2/Ni electrode demonstrated storage capacities (1982 mAh g−1 discharge capacity at 200 mA g−1) alongside enhanced cyclability and stability over 100 cycles, independent of discharge depth. This electrochemical performance can be attributed to its 3D morphology, oxygen defects, and the absence of side reactions from carbon-based additives. Overall, ϵ-MnO2/Ni electrode catalysts hold potential for realizing cost-effective Li-O2 based energy storage technologies.

The electrochemical performance of Li-O 2 batteries hinges mainly on the electrocatalyst.However, current research indicates sluggish kinetics at the catalyst surface [16], leads to poor oxygen evolution reactions (OER) during charging and inadequate oxygen reduction reactions (ORR) during discharging.Researchers have explored various catalytic materials to address this issue, including noble metals and their alloys [17], carbonbased materials [18], and transition metal oxides/nitrides [19,20].Among these, manganese oxide (MnO 2 ) have been a focal point of investigation due to their abundance, eco-friendliness, affordability, and ease of preparation [21][22][23].In general, the intrinsically low electronic conductivity of MnO 2 is responsible for reduced electrochemical performances of the devices [24].
To enhance the electronic conductivity of MnO 2 , it can be deposited on conductive substrates or mixed with electronically conducting agents, primarily carbon-based compounds [25,26].However, recent findings have shown that carbon becomes less stable and undergoes oxidation, decomposing to form Li 2 CO 3 when charged at voltages exceeding 3.5 V, particularly in the presence of Li 2 O 2 [27].
A diverse range of MnO 2 polymorphs has emerged as promising candidates.Among these, five notable polymorphs stand out: α-MnO 2 , β-MnO 2 , γ-MnO 2 , δ-MnO 2 , and ò-MnO 2 , each possessing distinct crystal structures and properties.The selection of an optimal cathode material plays a pivotal role in determining the overall performance and feasibility of Li-O 2 batteries [28,29].β-MnO 2 exhibits better oxygen kinetics than α-MnO 2 but faces challenges in achieving high capacity and long cycling stability [28,29].γ-MnO 2 with a layered structure exhibits higher electrochemical activity due to high surface area and porosity but suffers from side reactions and stability issues.δ-MnO 2 , a layered structure, shows promise for ion intercalations but faces challenges with capacity fading and structural degradation [30][31][32][33].Among the polymorphs, ò-MnO 2 stands out as a favorable cathode material for Li-O 2 batteries, thanks to its unique 3D framework with interconnected tunnels, providing high surface area, efficient oxygen diffusion, and good stability during cycling [30].Further, the use of carbonaceous binders or conducting agents may result in the isolation or agglomeration metal oxide (MO) particles.These particles can detach from the current collector due to electrochemical corrosion, leading to reduced electronic conducitivity and capacity fading [33] A promising alternative avenue involves MO on electrically conductive substrates such as Ni/Cu.This approach not only capitalizes on the inherent stability of MO against corrosive agents but also establishes connectivity (binding) between active components, thereby amplifying the electronic conductivity.Notably, this strategy finds instance in the successful deposition of active materials on LIBs materials, showcasing its viability with Ni, Cu, and stainless steel current collectors [2,9,25], and Ni foam in Li-O 2 batteries [34].
In this study, we report binder-free and self-supporting ò-MnO 2 cathode coated on Ni-mesh for rechargeable Li-O 2 batteries.Abundant structural oxygen defects are available in ò-MnO 2 , which helps in enhancing the catalytic characteristics of the electrode [23,35].Further, ò-MnO 2 belongs to the tetragonal crystal system and has a layered structure with well-defined tunnels along the c-axis.The tunnels are formed by interconnected octahedral Mn O6 units, providing a unique three-dimensional (3D) flower framework, facilitating efficient ion diffusion and ion insertion within its lattice during cyclic process [35].Further, the 3D arrangement of atoms in the lattice can result in shorter and direct pathways for ions to move within the material.Shorter ion diffusion paths minimize the resistance encountered by ions during intercalation, leading to lower polarization and enhanced electrochemical performance [35].Thus, the combination of 3D porous ò-MnO 2 nanostructures that resemble flower petals deposied on Ni foam is beneficial for the migration of oxygen or air.We have developed a hybrid material, ò-MnO2/Ni, using a sustainable, affordable, and highly scalable microwave synthesis technique.To the best of our knowledge, this hybrid material synthesised using microwave technique has not been previously studied electrochemically for Li-O 2 batteries.

Materials preparation
Nanostructured ò-MnO 2 was deposited onto 12 mm diameter Ni foams by applying 900 W of microwave power and 10.5 bar of pressure.The deposition took place for 30 min in a 150 °C water bath containing 80 ml of distilled water with 0.1 M manganese acetate and 0.1 M sodium sulphate.Teflon vials were used for the synthesis.Prior to the microwave synthesis, the Ni foam was washed using acetone and distilled water, then dried at 120 °C under vacuum for 6 h [35,36].After deposition, the plated foils were rinsed several times in distilled water and dried in air.The coated electrodes were annealed in air between 350 °C and 450 °C for 10 h as shown in figure 1.The samples were stored in the glove box filled with Argon to avoid any further oxidation [26].

Cell assembly
The Swagelok cell-type cell (nut-lock-type-cell, X2 labware, Singapore) assembly was conducted in an Argon filled glove box, with < 1 ppm O 2 and H 2 O levels.The cell assembly was as follows: a polished lithium (Li) metal foil anode (13 mm dimeter) (0.1 mm thick, 99.5% pure, Honjo metal from Japan) was placed on the negative side of the CR2032 coin cell case, followed by a glass fiber separator (nominal thickness 0.30 mm, porosity 92%-98%, Johns Manville's from USA) soaked in three drops (2 ml) of electrolyte, and finally an ò-MnO 2 /Ni cathode (geometric area, 0.32 cm 2 ) was placed on the positive side along with the spring and spacer for tight sealing.The top positive cover has evenly distributed machine-drilled, four 1.0 mm diameter, holes for the flow oxygen or air [37,38].The electrolyte used was 1 M lithium bis(trifluoromethylsulfonyl)amide (LiTFSI) in tetra (ethylene glycol)-dimethyl-ether (TEGDME).The cell was transferred to a sealed bottle filled with 1 atm of high-purity oxygen after the assembly.We tried to minimize any possible entry of air by purging O 2 a few (3-4) times.The cells were kept in highly pure oxygen for 5 h and batteries were kept in an ambient atmosphere before electrochemical testing [39].

Materials characterization
X-ray diffraction (XRD, PANalytical X'Pert PRO, Cu-Kα radiation) equipped with a fast linear detector (X'Celerator).XRD patterns were collected in the 2θ range 10°-100°with a scan rate of 160 s step −1 and a step size of 0.017°.Rietveld refinements of XRD powder patterns were performed with the Generalized Structure Analysis System (GSAS), along with the graphical user interface EXPGUI [36].The morphology of the prepared samples was examined using scanning electron microscope (SEM) (JEOL JSM-6700F) with 10 kV voltage and In-lens fine view scan mode.An upper secondary electron detector is used.The sample was coated with gold for 30 sec with 35 A current to get the better resolution images and to avoid surface charging effects [37].X-ray photoelectron spectroscopy (XPS) of the prepared samples were recorded using an AXIS ultra DLD spectrometer with monochromatic Al-Kα radiation (Kratos Analytica).The survey spectra were obtained in the range 0-1200 eV.Charge referencing was carried out against carbon (C1s binding energy = 284.6 eV).Casa XPS software was used to analyse XPS spectral data.Raman analysis was conducted with a Raman spectrometer (Model Lab ram HR Evolution, Horiba Scientific) and an Argon Laser (National Laser Model 800AL) with a wavelength of 514 nm and power of 100 mW was used.The experiment was conducted in the ambient air.Thermogravimetric (TG) analysis (Netzsch, STA 449 F3) was performed in the temperature range of 30 °C to 700 °C with rate of heating 2 °C min −1 [38,39].The amount of deposited MnO 2 was measured by weighing the nickel foam substrate before and after electrodeposition using the high-precision balance in TG analyser.The mass ratio of MnO 2 to Ni was nearly 1%.This mass was used to calculate the specific capacity of the Li-O 2 cells in this work.XRD powder patterns of the Li-O 2 sample after 100 cycles was collected and rietveld refinment was done.SEM and Energy Dispersive X-ray Spectroscopy (EDS) mapping was collected for identification of the morphological changes in the air electrode after 100 cycles.

Electrochemical measurements
After the assembled batteries were exposed to an oxygen atmosphere for 5 h at the open-circuit voltage (OCV) (∼3.1 V versus Li/Li + ), they underwent cycling at various charge/discharge current densities within the voltage range of 2.0-4.3V versus Li/Li + .This was performed using an Arbin BT2000 cell test instrument, Netherlands.The specific capacity was determined based on the quantity of ò-MnO 2 , (∼0.2 mg cm −2 ) serving as the electrocatalyst material in a Li-O 2 cell.Cyclic voltammograms (CV) were obtained at a scan rate of 0.1 mV s −1 using Arbin Tester.Electrochemical impedance spectroscopy (EIS) was conducted using a Solartron 1260 connected to a 1296 dielectric interface, covering a frequency range of 15 MHz to 1 Hz at room temperature [ [40][41][42][43][44]. Nyquist plots were analyzed by fitting with equivalent circuits using Z View-Plot software.The equivalent circuit comprised elements R b for bulk resistance, R SEI for solid electrolyte interfacial resistance, and R ct for charge transfer resistance of the Li-O 2 battery.

Results and discussion
The top section of figure 2 displays the scanning electron microscopy (SEM) image of ò-MnO 2 coating on a Ni substrate, achieved through microwave synthesis followed by heating at 350 °C for 10 h.Inset SEM images (including a low-resolution micrograph and a comparative flower) reveal a well-interconnected, flower petallike nanostructure coating on the substrate.The thickness of the deposited petals varies, ranging from 210 nm to 30 nm [45].The void size measures about 326 nm (see figure S1).It is noted that the deposition temperature affects the morphology of the sample.Subsequently, the deposited samples underwent heat treatment at 450 °C.Following ∼10 h of heating at 450 °C, the sample's morphology from microwave synthesis was primarily characterized by agglomerated particles with irregular sizes.Moreover, the main component was Mn 2 O 3 rather than ò-MnO 2 (bottom section of figure 2).The material annealed at 350 °C exhibited higher crystallinity, as demonstrated in the XRD (see figure 3), in comparison to the product heat treated at 450 °C (refer to figure S2).
Figure 4 illustrates the results of thermogravimetric analysis (TGA), showcasing a stepwise weight loss attributed to phase changes in the sample.The TGA plot is annotated with numerals (1-5) and the DTG curve is marked with alphabets (A-B) to signify these transformations.Initially, the TGA plot exhibits a sharp decline, signifying weight loss at approximately 100 °C.This is succeeded by an exothermic peak at 150 °C (labeled as A in DTG), indicating the removal of physically adsorbed and crystalline water.Following a minor increase in weight, subsequent weight loss proceeds smoothly until a sharp exothermic peak emerges around 450 °C (indicated as B).This increase in weight primarily results from oxygen absorption.Beyond this temperature, a constant plateau is observed.The XRD analysis of the sample annealed at 450 °C reveals the presence of the Mn 2 O 3 phase, indicating the transformation of MnO 2 into Mn 2 O 3 upon heating at or above 400 °C.For samples annealed at temperatures below 450 °C, XRD patterns align with ò-MnO 2 (JCPDS card no.89-5171), consistent with Rietveld refinement data.However, annealing at temperatures exceeding 450 °C introduces phases of Mn 2 O 3 (JCPDS 24-0508, observed at 550 °C), underscoring the phase transition during annealing, as confirmed by XRD data.Annealing at 350 °C was instrumental in achieving the H 2 O-free ò-MnO 2 phase, a critical factor in Li-O 2 cell performance.To assess the surface oxidation state of Mn in the formed nanostructures, XPS analysis was conducted (figure 5(b)).For samples annealed for 10 h at 350 °C, the Mn 2p 3/2 peaks appear at 638.5 and 649.4 eV, indicating potential oxidation states of Mn in the sample [41].The presence of doublets in Mn 2p 3/2 and Mn 2p 1/2 arises from a mixed-valent Mn oxides system (Mn 4+ /Mn 3+ ), indicating the existence of oxygen vacancies on the surface of ò-MnO 2 .The O1s XPS spectra (figure 5(b)) display peaks at 529.9 and 531.4 eV, associated with lattice oxygen species and surface-adsorbed oxygen.Surface-adsorbed oxygen is found on the oxygen vacancies of ò-MnO 2 , a known enhancer of electrocatalytic activity in prior studies [41].
Few Li-O 2 cells were prepared using Li/liquid electrolyte/ò-MnO 2 .The open circuit voltage of one of the freshly cells was 3.3 V and it remained stable for 7 days, indicating the low self-discharge of the battery before testing.Figure 6 (top) shows characteristic charge and discharge profiles of the cells prepared with ò-MnO 2 /Ni samples heated/annealed for 10 h at 350 °C.
The discharge curve represents the oxygen reduction, while the charge curve signifies the oxygen evolution process.Notably, two plateaus observed during discharge correspond to the formation of Li 2 CO 3 and Li 2 O 2 , a confirmation corroborated by the Raman spectrum.Charge and discharge data were collected within the voltage range of 2.00-4.30V versus Li/Li + at a current density of 200 mA g −1 .The sample prepared at 350 °C demonstrated the highest discharge capacity (figure 6, bottom).As anticipated, capacity increased while chargedischarge overpotential decreased with lower current densities [46,47].Initially, a discharge specific capacity of 1982 mAh g −1 was observed at a rate of 200 mA g −1 .Battery cyclic performance for the sample annealed at 350 °C, maintained at a constant current density of 200 mA g −1 , was conducted without regulating the depth of  Beyond 50 cycles, the specific capacity stabilized at 433 mAh g −1 .This diminished performance may be attributed to the agglomerated morphology and the formation of Mn 2 O 3 .The cell with ò-MnO 2 /Ni electrodes annealed at 350 °C exhibits one of the best performances when compared with previous reports, as detailed in the table 1.
For the sample prepared at 350 °C, ò-MnO 2 /Ni electrode catalyst was separated from the Li-O 2 battery and analysed after charge and discharge performance at a current density of 200 mA g −1 .The species present in the discharged state was then identified by Raman spectroscopy (figure 7).In line with SEM after discharge (figure S5a) they indicate Li 2 O 2 deposition on an essentially unchanged MnO 2 nanostructure.After recharging the Li 2 O 2 vanishes and the morphology of the sample was observed to be similar to the initial state with flower petal like morphology (figure S5b), i.e. which is similar to freshly prepared ò-MnO 2 coated on Ni foam electrode [48].
Li 2 O 2 peak was observed at ∼820 cm −1 instead of ∼790 cm −1 [48].This observed shifts may arise from two reasons, (i) changes in electronic and lattice states of Li 2 O 2 due to the pressure and chemical interatcions during battery cycling, leading to added oxygenated defects, (ii) the presence of impurties affecting the vibration states of Li 2 O 2 [48].The obtained results underscore the stability of ò-MnO 2 when in contact with liquid electrolyte and diverse lithium oxygen species generated throughout charge and discharge cycles.Impressively, the intricate porous 3D nanostructures of the electrode remained intact, serving as an effective catalyst for the oxidation of Li 2 O 2 during cycling.The agglomeration of the electrode is due to the irreversible reaction at electrode and is responsible for the initial fading of specific capacity.Reduction of the pore size while charge discharge process also may contribute to the capacity fading (Figure S5c).
Figure 8 illustrates the possible reaction mechanism of lithium and O 2 in the presence of ò-MnO 2 as catalyst, which is in line with the previous reports [38,39].At the time of discharge, the oxygen defects from ò -MnO 2 are reacted with O 2 and forms oxyegne ions (O −2 ) by the reduction of oxygen.The O 2− ions then reacts with solvated Li-ion or adsorb Li-ion to form Li + n O 2− .The reaction between these will lead to two Li + n O 2− releasing O 2 and forming Li 2 O 2 , which gets attached to the surface of ò-MnO 2 .During charge, lithium peroxide particles are oxidized in two steps which corresponds to two anodic peaks as shown in the cyclic voltammetry of the battery (figure 9).Both charge-discharge plateau regions and anodic-cathodic peaks are in line with previous report [23].The remarkable catalytic performance of 3D ò-MnO 2 /Ni nanostructure can be ascribed to different  factors.Initially, the availability of rich oxygen vacant sites in ò-MnO 2 offer intrinsic high activity.The high favorable oxygen transport and charge transfer is due to the high specific surface area and high porosity of the 3D flower like morphology of ò-MnO 2 .These redox/oxidation pairs are demonstrated highly reversible, showing no visible decay after three cycles.Even after 30 cycles the characteristic redox peaks retain the shape, further confirming the high stability of these electrochemical reactions.Electrochemical impedance of the assemble battery with ò-MnO 2 nanostructures on Ni foam annealed at 350 °C before, after 1st cycle and after 100 cycles (figure 10).Further, equivalent circuit has been shown inside figure 10.
The bulk resistance of the cell (R b ) is the total resistance of the electrodes, electrolyte, and the separator.The resistance and capacitance of the SEI layer, R SEI , and CPE SEI , respectively, forms the first semicircle in the Nyquist plot and are associated with the deposition of the interfacial layer on the electrode.The second  The total obtained resistance was 620 Ω after 100 cycles, indicating the high resistance formed on the surface of the ò-MnO 2 .The initial increase of the R SEI and R ct may be responsible for the reduction of the specific capacity.The SEI is a combination of both organic and inorganic components, and it requires further in situ atomistic level study to understand its formation and the correlation between various components in the equivalent circuit, which will be focused in our next study.These factors facilitate the rapid and reversible formation and decomposition of the electrically passivating Li 2 O 2 , which plays an important role in the cyclic performance of an organic Li-O 2 battery [49].It was observed that the conductivity further reduced after 100 cycles.This reduction can be attributed to the formation of Li 2 CO 3 in addition to Li 2 O 2 , as confirmed using Raman spectra.
It is expected that the nanostructures of ò-MnO 2 are shielded by the discharge product that appear as overgrown aggregates.The identification of these discharged chemical species was accomplished through Raman spectroscopy (figure 7), with distinct Raman peaks indicating the formation of Li 2 O 2 [50,51].
After 100 cycles, post-mortem analysis of the ò-MnO 2 /Ni was conducted using XRD, SEM, and EDS analysis.Figure S2  The sample annealed at 450 °C for 10 h showed the formation of Mn 2 O 3 (Figure S3) with space group Pbca and lattice parameters a = 9.362(1) Å, b = 9.402(1) Å, and c = 9.457(1) Å along with Ni and NiO impurities.This indicates that the optimum temperature to achieve the targeted compound is 350 °C, which is about 100 °C lower compared to the previous report using electrodeposition [23].XRD of the ò-MnO 2 /Ni after the 1st discharge indicated the possible formation of Li 2 CO 3 and LiF, along with other unknown compounds as shown Impressively, the integrity of the 3D nanostructured electrode catalyst remained intact, showcasing its efficacy in catalyzing the lithium peroxide during the charging phase.This electrode holds better catalytic activities for both ORR and OER than Super P (SP) [52,53].

Conclusions
In conclusion, the 3D nanostructured ò-MnO 2 that resembled flower petals was coated on Ni foam using a simple microwave synthesis method, and it was investigated as the cathode catalyst compounds for rechargeable Li-O 2 batteries.It was found that ò -MnO 2 /Ni had a high initial dis-charge specific capacity of 1982 mAh g −1 at a current density of 200 mA g −1 , with modest capacity fading lasting up to 100 cycles.In this case, we didn't apply any extra conductive agents or binders that would have reduced the weight of the entire cell and increased the gravimetric capacity.The 3D nanostructure of the ò-MnO 2 /Ni electrode, the potential oxygen defects, and the inherent electronic conductivity of the nickel, which supports the coated MnO 2 .All of these, can be credited for the electrode's catalytic activity.The microwave methodology used here may assist in accelerating the largerscale production of electrode catalyst for Li-O 2 and other types of batteries as well.Moreover, there is a compelling need for thorough in situ investigations aimed at identifying the surface charge changes for mitigating the capacity fading.Given the accesibility and abudance of the raw materials (Mn 2 O 3 ) utilized in this study, there is a promising prospect for its evolution into a financially viable energy storage solution.

Figure 2 .
Figure 2. (a)) SEM image of ò-MnO 2 coated on Ni-foam using microwave synthesis followed by annealing at 350 °C.Inside top right is ò-MnO 2 at 2 μm scale range (with X 30.26 magnification) and at the bottom is flower image for comparing morphology.b) SEM image of ò-MnO 2 coated on Ni-foam followed by annealing at 450 °C (with X 70.21 magnification).Inside is Ni-foam at 100 μm scale.

Figure 5 .
Figure 5. XPS data for (a) Mn element (b) Oxygen element of the sample heated at 350 °C for 10 h.

Figure 6 .
Figure 6.Electrochemical performance of Li-O 2 batteries using ò-MnO 2 catalyst on Ni heated at 350 °C in air for 10 h: (top) chargedischarge preformnce for electrodes, in the voltage range of 2.0-4.3V versus Li/Li + with 200 mA/g rate.(bottom) Change of specific capacity of Li-O 2 cell with number of cycles.The current densities and specific capacities are calculated based on the total mass of ò-MnO 2 .

Figure 8 .
Figure 8.(a) Illustration of the reaction mechanism in Li-O 2 batteries on cycling: left and right parts correspond to the discharge and charge processes (b) Demonstration of powering LED with Li-O 2 battery which is stable for more than 10 days.Inside is to show the Ni foam coated with ò-MnO 2 side facing air cathode.
presents the magnified portion of the Rietveld refinement of the powdered XRD, showing compounds annealed at 350 °C for 10 h, indicating ò-MnO 2 alongside Ni.The full range of Rietveld refinement patterns for samples annealed at 450 °C for 10 h, indicating ò-MnO 2 with Ni, is shown in figure S3 for comparison.The XRD patterns for samples prepared at 350 °C and 450 °C are plotted in figure S3 for better distinction between the two synthesized samples.The selected annealing temperatures post microwave treatment were 350 °C and 450 °C to attain the ò-MnO2 phase and remove any surface hydroxyl groups, which are unfavorable to aprotic Li-O 2 cells.All characterization techniques used indicated that the ò-MnO 2 samples were sufficiently dry for electrochemical studies.The lattice parameters for ò-MnO 2 from Rietveld refinements of XRD are a = 2.785(4) Å and c = 4.40(2) Å with space group P63/mmc, and a = 3.52549(6) Å (space group Fm-3m) for the Ni metal.

Figure 10 .
Figure 10.Electrochemical Impedance Spectroscopy (EIS) of Li-O 2 cells with ò-MnO 2 /Ni electrodes anealed for 10 h at 350 °C.Equivalent circuit fitted is in placed on the top.

Table 1 .
Comparison of previously employed MnO 2 cathode variants for Li-O 2 battery.