Design and Performance of a New Zn0.5Mg0.5FeMnO4 Porous Spinel as Anode Material for Li-Ion Batteries

Due to the low capacity, low working potential, and lithium coating at fast charging rates of graphite material as an anode for Li-ion batteries (LIBs), it is necessary to develop novel anode materials for LIBs with higher capacity, excellent electrochemical stability, and good safety. Among different transition-metal oxides, AB2O4 spinel oxides are promising anode materials for LIBs due to their high theoretical capacities, environmental friendliness, high abundance, and low cost. In this work, a novel, porous Zn0.5Mg0.5FeMnO4 spinel oxide was successfully prepared via the sol–gel method and then studied as an anode material for Li-ion batteries (LIBs). Its crystal structure, morphology, and electrochemical properties were, respectively, analyzed through X-ray diffraction, high-resolution scanning electron microscopy, and cyclic voltammetry/galvanostatic discharge/charge measurements. From the X-ray diffraction, Zn0.5Mg0.5FeMnO4 spinel oxide was found to crystallize in the cubic structure with Fd3¯m symmetry. However, the Zn0.5Mg0.5FeMnO4 spinel oxide exhibited a porous morphology formed by interconnected 3D nanoparticles. The porous Zn0.5Mg0.5FeMnO4 anode showed good cycling stability in its capacity during the initial 40 cycles with a retention capacity of 484.1 mAh g−1 after 40 cycles at a current density of 150 mA g−1, followed by a gradual decrease in the range of 40–80 cycles, which led to reaching a specific capacity close to 300.0 mAh g−1 after 80 cycles. The electrochemical reactions of the lithiation/delithiation processes and the lithium-ion storage mechanism are discussed and extracted from the cyclic voltammetry curves.


Introduction
Recently, the extensive demand for consumer electronics and electric vehicles has led to a significant increase in the demand for rechargeable lithium-ion batteries (LIBs) owing to their high energy density, long cycle life, environmental friendliness, and no memory effect [1][2][3].Since the 1990s, LIBs have been regarded as one of the systems with the most energy storage and have gained widespread applications in mobile electronic devices such as handheld computers, tablets, and mobile phones.LIBs with higher energy density and fast-chargeable capability are requested for many energy storage systems, especially for electric vehicles (EV) and stationary energy storage devices [4][5][6].
In commercialized, traditional LIBs, graphite has been the most extensively used anode material to date due to its low cost and long cycle life [7,8].Due to the low capacity of graphite material (only 372 mAh g −1 ), its low working potential (<0.2 V vs. Li + /Li), and its lithium coating at fast charging rates, graphite-based LIBs are limited and cannot meet the demand for fast-chargeable batteries with high energy density [9,10].Therefore, developing novel anode materials for LIBs with higher capacity, low cost, nontoxicity, excellent electrochemical stability, and good safety has recently become an important research issue.In this regard, alloys and transition-metal oxides have appeared as the most popular possibilities to substitute the graphite material in LIBs [11][12][13].
Recently, nanostructured spinel ferrites have received tremendous interest for application in LIBs [39].Among nanostructured spinel ferrites, magnesium ferrite MgFe 2 O 4 nanoparticles prepared through the sol-gel method, as anode materials for LIBs, have shown a specific capacity near 474 mAh g −1 after 50 cycles under a current density of 90 mA g −1 [40].In this phase, Mg was used as an electrochemical inactive pillar to enhance the cyclability of MgFe 2 O 4 spinel [30].The manganese ferrite MnFe 2 O 4 with mesoporous microsphere morphology has been reported by Z. Zhang et al. [41].It has shown specific capacities of 712.2 and 552.2 mA h g −1 at 0.2 C and 0.8 C, respectively, after 50 cycles, suggesting good performance for fast charging.Another spinel ferrite, zinc ferrite ZnFe 2 O 4 , has gained much attention as an anode material for LIBs due to its great electrochemical performance with different morphologies, such as hollow spherical [42], microsized capsules [43], macroporous [44], porous nanospheres [45], and fiber-in-tube [46] morphologies, among others.
Additionally, manganese-based AB 2 O 4 spinel oxides have also attracted much interest as alternative electrode materials for LIBs owing to their excellent electrochemical performance [47,48].Among these manganites, ZnMn 2 O 4 microsheets were synthesized by K. Cai et al. using a microwave-assisted hydrothermal route [49].From their electrochemical properties, they showed a capacity of 337.3 mAh g −1 after 40 cycles at a rate of 0.1 C. Another spinel manganite, the MgMn 2 O 4 spinel, was investigated by Z. Wang et al. [50].The MgMn 2 O 4 nanoparticles with particle sizes ranging from 20 to 30 nm prepared using the sol-gel method have shown a discharge capacity of 483.6 mAh g −1 after 100 cycles.
In recent years, Co-, Ni-, Zn-, Fe-, Cr-, Mn-, and Li-based multi-component spinel oxides have attracted great attention as high-performance anodes for LIBs and long-life lithium-ion batteries [51,52].L. Dong et al. [51] reported a multi-component transitionmetal oxide (Ni,Co,Mn)Fe 2 O 4−x with oxygen vacancies as an anode material for LIBs, which has shown a specific capacity of 650.5 mAh g −1 at 2 A g −1 after 1200 cycles, i.e., excellent reversible capacity and good cycling stability.K. Tian et al. [52] found that spinel oxides (CoNiZnXMnLi) 3 O 4 (X = Fe, Cr) are high-performance anodes for LIBs.On the other hand, Nb-based oxides such as Nb 2 O 5 , Nb 12 O 29 , TiNb 2 O 7 , Ti 2 Nb 10 O 29 , etc., have attracted much attention as anode materials for high-rate lithium-ion energy storage [53].
From the above, to simultaneously improve the stability, energy density, and high-rate capability of anode materials for LIBs, a novel, 3D interconnected structure belonging to transition-metal-oxide-type spinels is urgently needed.In this work, we report a facile route to synthesize 3D porous Zn 0.5 Mg 0.5 FeMnO 4 spinel oxide using sol-gel method with a theoretical capacity of 976.3 mAh g −1 as an anode electrode for LIBs.Herein, Mg is used as an inactive stabilized pillar, which can produce good cycling stability.The synthesized active material is characterized by different techniques such as X-ray diffraction, thermogravimetry analysis, scanning electron microscopy, and the nitrogen adsorptiondesorption isotherm to report, respectively, its crystallographic structure, thermal stability, morphology, and specific surface area.The electrochemical performance of the prepared porous Zn 0.5 Mg 0.5 FeMnO 4 spinel oxide is tested by cyclic voltammetry and galvanostatic discharge/charge measurements.

X-ray Diffraction, Rietveld Refinement, and Crystal Structure
The X-ray powder diffraction (XRD) technique was first employed to analyze the crystallographic structure, formation, and phase purity of the as-synthesized Zn 0.5 Mg 0.5 FeMnO 4 sample.Figure 1a shows the wide-angle XRD pattern of the Zn 0.5 Mg 0.5 FeMnO 4 sample acquired at room temperature.From this figure, the XRD data reveal the formation of the AB 2 O 4 -type spinel phase as all the experimental XRD peaks observed at different 2θ values are well matching with standard peaks of the MgFe 2 O 4 spinel phase that crystallizes in the cubic Fd3m structure with a lattice parameter of a = 8.3827 Å (JCPDS card: 88-1936) [54], which indicates that the as-synthesized Zn 0.5 Mg 0.5 FeMnO 4 sample exhibits a single spinel phase with the cubic Fd3m system.However, no additional peaks from any impurities could be identified in the XRD pattern, revealing a high purity of the synthesized spinel phase and confirming that the organic species of CA have been completely removed.In addition, the synthesized spinel shows sharp XRD peaks with small full width at half maximum (FWHM), suggesting good crystallization of the synthesized Zn 0.5 Mg 0.5 FeMnO 4 spinel oxide.Besides, the widening of the obtained XRD peaks provides proof of the existence of nanoscale-dimension spinel particles.From the obtained structural parameters by Rietveld refinement, the crystal structure of Zn0.5Mg0.5FeMnO4spinel oxide was drawn.Figure 2 displays the crystal structure of Zn0.5Mg0.5FeMnO4spinel oxide with the cubic structure in Fd3 m symmetry.As clearly For further deep analysis of structural parameters, the XRD pattern of the synthesized Zn 0.5 Mg 0.5 FeMnO 4 spinel ferrite was refined using the Rietveld method [55,56].The structural refinement was carried out with the cubic structure and Fd3m as a space group.The Rietveld refinement plot of the synthesized Zn 0.5 Mg 0.5 FeMnO 4 spinel is displayed in Figure 1b, where the Bragg reflections are revealed by the green vertical bars, the observed XRD (Y obs ) pattern is shown by open black circles, the simulated XRD (Y cal ) pattern by the Rietveld method is represented by red solid lines, and the difference curve (Y obs -Y cal ) is indicated by pink solid lines.From this figure, it can be seen that the observed XRD pattern shows great agreement with the calculated XRD pattern, revealing the success of a realized structural refinement for the synthesized Zn 0.5 Mg 0.5 FeMnO 4 spinel ferrite.Moreover, the quality of the structural refinement was evaluated by the consideration of the different R-factors such as structure factor (R F ), Bragg factor (R B ), profile factor (R P ), expected factor (R exp ), and profile factor (R wp ) [57][58][59][60].The results of the powder X-ray diffraction of the synthesized Zn 0.5 Mg 0.5 FeMnO 4 spinel oxide in the Rietveld refinement such as structural parameters, profile parameters, and different reliability R-factors are summarized in Table 1.According to the obtained values of different R-factors, the synthesized Zn 0.5 Mg 0.5 FeMnO 4 spinel oxide crystallizes in the cubic Fd3m structure with a lattice parameter of a = 8.4265 (2) Å.From the obtained structural parameters by Rietveld refinement, the crystal structure of Zn 0.5 Mg 0.5 FeMnO 4 spinel oxide was drawn.Figure 2 displays the crystal structure of Zn 0.5 Mg 0.5 FeMnO 4 spinel oxide with the cubic structure in Fd3m symmetry.As clearly shown in the figure, the crystal structure of Zn 0.5 Mg 0.5 FeMnO 4 spinel oxide is constructed by 3D interconnected tetrahedral and octahedral polyhedra, where Zn 2+ and Mg 2+ cations are located in the tetrahedral sites (green balls), Fe 3+ and Mn 3+ are located in the octahedral sites (blue balls), and the red balls represent the O 2− oxygen anions.Moreover, this structure shows three Wyckoff sites-the 8a, 16d, and 32e sites, which correspond to the tetrahedral, octahedral, and oxygen sites, respectively.Only 1/8 of the tetrahedral sites are occupied by Zn 2+ /Mg 2+ cations, 1/2 of the octahedral sites are occupied by Fe 3+ /Mn 3+ cations, and the 32e sites are totally occupied by oxygen anions.Therefore, the spinel structure exhibits 7/8 of the tetrahedral sites and 1/2 of the octahedral sites that are empty.During the first discharge of electrochemical cells, the lithium ions can be inserted into the unit cell to occupy the lacunar sites.From the obtained structural parameters by Rietveld refinement, the crystal structure of Zn0.5Mg0.5FeMnO4spinel oxide was drawn.Figure 2 displays the crystal structure of Zn0.5Mg0.5FeMnO4spinel oxide with the cubic structure in Fd3 m symmetry.As clearly shown in the figure, the crystal structure of Zn0.5Mg0.5FeMnO4spinel oxide is constructed by 3D interconnected tetrahedral and octahedral polyhedra, where Zn 2+ and Mg 2+ cations are located in the tetrahedral sites (green balls), Fe 3+ and Mn 3+ are located in the octahedral sites (blue balls), and the red balls represent the O 2-oxygen anions.Moreover, this structure shows three Wyckoff sites-the 8a, 16d, and 32e sites, which correspond to the tetrahedral, octahedral, and oxygen sites, respectively.Only 1/8 of the tetrahedral sites are occupied by Zn 2+ /Mg 2+ cations, 1/2 of the octahedral sites are occupied by Fe 3+ /Mn 3+ cations, and the 32e sites are totally occupied by oxygen anions.Therefore, the spinel structure exhibits 7/8 of the tetrahedral sites and 1/2 of the octahedral sites that are empty.During the first discharge of electrochemical cells, the lithium ions can be inserted into the unit cell to occupy the lacunar sites.

Thermal Analysis
In order to understand the mechanism of the calcination of the synthesized xerogel, the xerogel sample was characterized by thermogravimetric analysis (TGA) in a wide temperature range of 20-700 • C. Figure 3 depicts the thermogravimetric curves (TG) of the synthesized xerogel and the calcined sample of Zn 0.5 Mg 0.5 FeMnO 4 spinel.In the case of the xerogel sample (red curve), two weight losses take place in the temperature range from room temperature to 350 • C. The first loss is observed before 140 • C, which is due to the evaporation of the water used as a solvent during the synthesis and the physically adsorbed water on the surface.The second weight loss observed in the temperature range of 140-350 • C is attributed to the decomposition of the organic species of the citric acid (CA) associated with the elimination of nitrogen dioxide (NO 2 ↑ ).Further, no weight loss is present above 350 • C.Moreover, the thermal stability of the synthesized spinel was examined by thermogravimetric analysis (TGA).For the calcined sample at 700 • C (blue curve), its TG curve demonstrates no weight loss in the whole temperature range of 20-700 • C, indicating good thermal stability of the synthesized porous Zn 0.5 Mg 0.5 FeMnO 4 spinel oxide and confirming that the organic molecules of the citric acid (CA) have been completely removed after calcination at 700 • C.
Molecules 2023, 28, x FOR PEER REVIEW 5 of 18 cations are located in the octahedral sites (blue balls), and the red balls represent the O 2− oxygen anions.

Thermal Analysis
In order to understand the mechanism of the calcination of the synthesized xerogel, the xerogel sample was characterized by thermogravimetric analysis (TGA) in a wide temperature range of 20-700 °C. Figure 3 depicts the thermogravimetric curves (TG) of the synthesized xerogel and the calcined sample of Zn0.5Mg0.5FeMnO4spinel.In the case of the xerogel sample (red curve), two weight losses take place in the temperature range from room temperature to 350 °C.The first loss is observed before 140 °C, which is due to the evaporation of the water used as a solvent during the synthesis and the physically adsorbed water on the surface.The second weight loss observed in the temperature range of 140-350 °C is attributed to the decomposition of the organic species of the citric acid (CA) associated with the elimination of nitrogen dioxide (NO2 ↑ ).Further, no weight loss is present above 350 °C.Moreover, the thermal stability of the synthesized spinel was examined by thermogravimetric analysis (TGA).For the calcined sample at 700 °C (blue curve), its TG curve demonstrates no weight loss in the whole temperature range of 20-700 °C, indicating good thermal stability of the synthesized porous Zn0.5Mg0.5FeMnO4spinel oxide and confirming that the organic molecules of the citric acid (CA) have been completely removed after calcination at 700 °C.

Morphology, Purity, and Specific Surface
The microstructural morphology of the synthesized spinel was revealed by scanning electron microscopy (SEM).Figure 4a shows the SEM image of the synthesized Zn0.5Mg0.5FeMnO4spinel oxide, which demonstrates its surface morphology.From the figure, it can be seen that the prepared spinel shows a microsized porous morphology with different pore sizes ranging from 2 to 5 µm.Moreover, the porous morphology can make the electrolyte used in the cells of LIBs penetrate easily, which leads to the acceleration of lithium ions diffusion, enhancing the electrochemical performance.For further deep analysis of the surface morphology, the synthesized spinel was characterized by high-resolution scanning electron microscopy (HR-SEM).The obtained HR-SEM image is presented in Figure 4b.It can be seen from this figure that the porous Zn0.5Mg0.5FeMnO4spinel is composed of interconnected nanosized spherical particles with sizes ranging from 50 to 100 nm.

Morphology, Purity, and Specific Surface
The microstructural morphology of the synthesized spinel was revealed by scanning electron microscopy (SEM).Figure 4a shows the SEM image of the synthesized Zn 0.5 Mg 0.5 FeMnO 4 spinel oxide, which demonstrates its surface morphology.From the figure, it can be seen that the prepared spinel shows a microsized porous morphology with different pore sizes ranging from 2 to 5 µm.Moreover, the porous morphology can make the electrolyte used in the cells of LIBs penetrate easily, which leads to the acceleration of lithium ions diffusion, enhancing the electrochemical performance.For further deep analysis of the surface morphology, the synthesized spinel was characterized by high-resolution scanning electron microscopy (HR-SEM).The obtained HR-SEM image is presented in Figure 4b.It can be seen from this figure that the porous Zn 0.5 Mg 0.5 FeMnO 4 spinel is composed of interconnected nanosized spherical particles with sizes ranging from 50 to 100 nm.To carry out an investigation on the purity of the synthesized porous Zn0.5Mg0.5FeMnO4spinel oxide, an energy-dispersive X-ray (EDX) spectrometer was utilized.The results of this investigation are displayed in Figure 5.As clearly shown in the figure, only Zn, Mg, Fe, Mn, and O peaks could be recorded, which suggests the existence of only Zn, Mg, Fe, Mn, and O elements in the product.No additional element was recorded.The EDX results confirm approximately the atomic ratio 0.5:0.5:1:1:4 of Zn, Mg, Fe, Mn, and O elements, respectively, which gives further proof of the high purity of the synthesized spinel.Besides, the SEM/EDX instrument was also used to provide different element distribution maps of the 3D metal oxide network.The surface of the synthesized spinel and its EDX element mapping results are shown in Figure 6.From these maps, it is clearly demonstrated that the Zn, Mg, Fe, Mn, and O elements are uniformly distributed across the whole sample.To carry out an investigation on the purity of the synthesized porous Zn 0.5 Mg 0.5 FeMnO 4 spinel oxide, an energy-dispersive X-ray (EDX) spectrometer was utilized.The results of this investigation are displayed in Figure 5.As clearly shown in the figure, only Zn, Mg, Fe, Mn, and O peaks could be recorded, which suggests the existence of only Zn, Mg, Fe, Mn, and O elements in the product.No additional element was recorded.The EDX results confirm approximately the atomic ratio 0.5:0.5:1:1:4 of Zn, Mg, Fe, Mn, and O elements, respectively, which gives further proof of the high purity of the synthesized spinel.Besides, the SEM/EDX instrument was also used to provide different element distribution maps of the 3D metal oxide network.The surface of the synthesized spinel and its EDX element mapping results are shown in Figure 6.From these maps, it is clearly demonstrated that the Zn, Mg, Fe, Mn, and O elements are uniformly distributed across the whole sample.To carry out an investigation on the purity of the synthesized porous Zn0.5Mg0.5FeMnO4spinel oxide, an energy-dispersive X-ray (EDX) spectrometer was utilized.The results of this investigation are displayed in Figure 5.As clearly shown in the figure, only Zn, Mg, Fe, Mn, and O peaks could be recorded, which suggests the existence of only Zn, Mg, Fe, Mn, and O elements in the product.No additional element was recorded.The EDX results confirm approximately the atomic ratio 0.5:0.5:1:1:4 of Zn, Mg, Fe, Mn, and O elements, respectively, which gives further proof of the high purity of the synthesized spinel.Besides, the SEM/EDX instrument was also used to provide different element distribution maps of the 3D metal oxide network.The surface of the synthesized spinel and its EDX element mapping results are shown in Figure 6.From these maps, it is clearly demonstrated that the Zn, Mg, Fe, Mn, and O elements are uniformly distributed across the whole sample.To carry out an investigation on the purity of the synthesized porous Zn0.5Mg0.5FeMnO4spinel oxide, an energy-dispersive X-ray (EDX) spectrometer was utilized.The results of this investigation are displayed in Figure 5.As clearly shown in the figure, only Zn, Mg, Fe, Mn, and O peaks could be recorded, which suggests the existence of only Zn, Mg, Fe, Mn, and O elements in the product.No additional element was recorded.The EDX results confirm approximately the atomic ratio 0.5:0.5:1:1:4 of Zn, Mg, Fe, Mn, and O elements, respectively, which gives further proof of the high purity of the synthesized spinel.Besides, the SEM/EDX instrument was also used to provide different element distribution maps of the 3D metal oxide network.The surface of the synthesized spinel and its EDX element mapping results are shown in Figure 6.From these maps, it is clearly demonstrated that the Zn, Mg, Fe, Mn, and O elements are uniformly distributed across the whole sample.Figure 7 shows the nitrogen adsorption-desorption isotherm of the synthesized porous Zn 0.5 Mg 0.5 FeMnO 4 spinel oxide collected at a liquid nitrogen temperature of 77 K.The analysis of the nitrogen adsorption-desorption isotherm was performed to obtain the specific surface area of the synthesized porous Zn 0.5 Mg 0.5 FeMnO 4 spinel oxide using the Brunauer-Emmett-Teller (BET) method.From the nitrogen adsorption-desorption isotherm, the porous spinel shows a type IV nitrogen sorption isotherm with a small hysteresis loop at high partial pressures (P/P 0 ), suggesting a characteristic of a mesoporous material.Using the Brunauer-Emmett-Teller (BET) method, the specific surface area of the synthesized porous Zn 0.5 Mg 0.5 FeMnO 4 spinel oxide was found to be 13.54 m 2 g −1 .The obtained small specific surface area is probably due to the high aggregation of nanoparticles, as shown in the HR-SEM image (Figure 4b).Moreover, the synthesized porous Zn 0.5 Mg 0.5 FeMnO 4 spinel oxide has a larger specific surface area than that of graphite powder, which is 6.89 m 2 g −1 [61].
Molecules 2023, 28, x FOR PEER REVIEW 7 of 18 Figure 7 shows the nitrogen adsorption-desorption isotherm of the synthesized porous Zn0.5Mg0.5FeMnO4spinel oxide collected at a liquid nitrogen temperature of 77 K.The analysis of the nitrogen adsorption-desorption isotherm was performed to obtain the specific surface area of the synthesized porous Zn0.5Mg0.5FeMnO4spinel oxide using the Brunauer-Emmett-Teller (BET) method.From the nitrogen adsorption-desorption isotherm, the porous spinel shows a type IV nitrogen sorption isotherm with a small hysteresis loop at high partial pressures (P/P0), suggesting a characteristic of a mesoporous material.Using the Brunauer-Emmett-Teller (BET) method, the specific surface area of the synthesized porous Zn0.5Mg0.5FeMnO4spinel oxide was found to be 13.54 m 2 g −1 .The obtained small specific surface area is probably due to the high aggregation of nanoparticles, as shown in the HR-SEM image (Figure 4b).Moreover, the synthesized porous Zn0.5Mg0.5FeMnO4spinel oxide has a larger specific surface area than that of graphite powder, which is 6.89 m 2 g −1 [61].

Electrochemical Results
In order to further investigate the beneficial effects of the Zn0.5Mg0.5FeMnO4spinel oxide with a porous morphology as anode for LIBs, its electrochemical performance was analyzed in a half-cell configuration.The cyclic voltammetry (CV) measurements were conducted over the 0.01-3.00V voltage range vs. Li + /Li under ambient conditions.Figure 8 shows the initial five CV curves of the synthesized porous Zn0.5Mg0.5FeMnO4spinel oxide collected at a sweep rate of 0.05 mV s −1 .Specifically, in the initial cathodic sweep (discharge, lithiation), two dominant peaks at 0.35 and 0.7 V are observed in the first cycle.In agreement with previous studies [62,63], the cathodic peak that appeared at 0.7 V is attributed to the reversible reduction of Zn 2+ , Fe 3+ , and Mn 3+ ions together, which are incorporated in Zn0.5Mg0.5FeMnO4spinel phase to their metallic states Zn, Fe, and Mn associated with the formation of MgO and amorphous Li2O, as demonstrated in Equation (1).After the reduction of Zn0.5Mg0.5FeMnO4, the formed Zn metal can react with Li + ions, giving the reversible formation of LiZn alloys (Equation ( 2)) [46].The other intense peak observed at 0.35 V corresponds to electrolyte decomposition, which leads to the irreversible formation of a solid electrolyte interface (SEI) [64,65].For the subsequent cathodic sweeps, only one peak is obtained near 0.67 V, which is assigned to the reduction of ZnO, Fe2O3, and MnO to Zn, Fe, and Mn, respectively, as previously reported [66,67].The electrochemical reactions of the lithiation process (discharge) can be shown, as reported in [68,69].

Electrochemical Results
In order to further investigate the beneficial effects of the Zn 0.5 Mg 0.5 FeMnO 4 spinel oxide with a porous morphology as anode for LIBs, its electrochemical performance was analyzed in a half-cell configuration.The cyclic voltammetry (CV) measurements were conducted over the 0.01-3.00V voltage range vs. Li + /Li under ambient conditions.Figure 8 shows the initial five CV curves of the synthesized porous Zn 0.5 Mg 0.5 FeMnO 4 spinel oxide collected at a sweep rate of 0.05 mV s −1 .Specifically, in the initial cathodic sweep (discharge, lithiation), two dominant peaks at 0.35 and 0.7 V are observed in the first cycle.In agreement with previous studies [62,63], the cathodic peak that appeared at 0.7 V is attributed to the reversible reduction of Zn 2+ , Fe 3+ , and Mn 3+ ions together, which are incorporated in Zn 0.5 Mg 0.5 FeMnO 4 spinel phase to their metallic states Zn, Fe, and Mn associated with the formation of MgO and amorphous Li 2 O, as demonstrated in Equation (1).After the reduction of Zn 0.5 Mg 0.5 FeMnO 4 , the formed Zn metal can react with Li + ions, giving the reversible formation of LiZn alloys (Equation ( 2)) [46].The other intense peak observed at 0.35 V corresponds to electrolyte decomposition, which leads to the irreversible formation of a solid electrolyte interface (SEI) [64,65].For the subsequent cathodic sweeps, only one peak is obtained near 0.67 V, which is assigned to the reduction of ZnO, Fe 2 O 3 , and MnO to Zn, Fe, and Mn, respectively, as previously reported [66,67].The electrochemical reactions of the lithiation process (discharge) can be shown, as reported in [68,69].
On the other hand, the first cycle is easily distinct from the subsequent cycles.Besides, the difference between the redox peaks in the first and subsequent cycles is due to the capacity loss of the anode spinel during the first cycle.From the third cycle, the CV curves are similar and the redox peaks remain consistent, revealing good reversibility of the Zn0.5Mg0.5FeMnO4anode.Subsequently, the results of the galvanostatic discharge/charge cycling of the synthesized porous Zn0.5Mg0.5FeMnO4spinel oxide acquired at a current density of 150 mA g −1 over the potential region of 0.01-3.00V (vs.Li + /Li) for various cycles (1st, 2nd, 5th, 10th, and 20th) are shown in Figure 9a.In accordance with previous reports, the profile of the potential versus capacity plot of the studied porous Zn0.5Mg0.5FeMnO4spinel oxide is similar to those of ZnFe2O4 and MgFe2O4 [73].It is worth recalling that the Zn0.5Mg0.5FeMnO4spinel oxide has a theoretical capacity of 976.3 mAh g −1 as an anode electrode for LIBs.From Figure 9a, the initial discharge and charge capacities are 1132.9and 676.4 mA h g −1 , respectively.Note that the initial discharge capacity is higher than the theoretical capacity.This can be explained by the lithium-ion storage through a surface faradic redox reaction (pseudocapacitive).The irreversible capacity loss between discharge and charge of the In the charge process (delithiation), the distinct anodic peaks are between 1.6 V and 1.8 V for all cycles, which probably contribute to the various oxidations of metallic Zn, Fe, and Mn to their oxide states Zn 2+ (ZnO), Fe 3+ (Fe 2 O 3 ), and Mn 2+ (MnO), respectively, as represented in Equation (4) [39,70].Therefore, the related reactions of the charge process (delithiation) for all cycles can be summarized as follows [71,72]: 0.5 Zn + Fe + Mn + 3 Li 2 O → 0.5 Zno + 0.5 Fe 2 O 3 + MnO + 6 Li + + 6 e − (4) On the other hand, the first cycle is easily distinct from the subsequent cycles.Besides, the difference between the redox peaks in the first and subsequent cycles is due to the capacity loss of the anode spinel during the first cycle.From the third cycle, the CV curves are similar and the redox peaks remain consistent, revealing good reversibility of the Zn 0.5 Mg 0.5 FeMnO 4 anode.
Subsequently, the results of the galvanostatic discharge/charge cycling of the synthesized porous Zn 0.5 Mg 0.5 FeMnO 4 spinel oxide acquired at a current density of 150 mA g −1 over the potential region of 0.01-3.00V (vs.Li + /Li) for various cycles (1st, 2nd, 5th, 10th, and 20th) are shown in Figure 9a.In accordance with previous reports, the profile of the potential versus capacity plot of the studied porous Zn 0.5 Mg 0.5 FeMnO 4 spinel oxide is similar to those of ZnFe 2 O 4 and MgFe 2 O 4 [73].It is worth recalling that the Zn 0.5 Mg 0.5 FeMnO 4 spinel oxide has a theoretical capacity of 976.3 mAh g −1 as an anode electrode for LIBs.From Figure 9a, the initial discharge and charge capacities are 1132.9and 676.4 mA h g −1 , respectively.Note that the initial discharge capacity is higher than the theoretical capacity.This can be explained by the lithium-ion storage through a surface faradic redox reaction (pseudocapacitive).The irreversible capacity loss between discharge and charge of the first cycle is normally due to the formation of SEI film [74,75].The studied spinel displays an extended voltage plateau at about 0.5 V followed by a gradual slope for the first discharge.The potential plateau is probably due to the conversion from the reduction of Zn 2+ , Fe 3+ , and Mn 3+ transition metals to metallic states of Zn, Fe, and Mn, respectively [76,77].During discharge/charge cycling, the discharge and charge capacities gradually decrease with increasing cycle numbers, which is probably attributed to the aggregation and pulverization of Zn 0.5 Mg 0.5 FeMnO 4 nanoparticles, leading to poor electrical contacts between Zn 0.5 Mg 0.5 FeMnO 4 nanoparticles and that between the active material and current collector [40].The cycling performance of the Zn 0.5 Mg 0.5 FeMnO 4 anode at the current density of 150 mA g −1 during 80 cycles is displayed in Figure 9b.It is clearly seen that the Zn 0.5 Mg 0.5 FeMnO 4 anode shows good cycling stability in its capacity during the initial 40 cycles, with a specific capacity of 484.1 mAh g −1 after 40 cycles followed by a gradual decrease in the range of 40-80 cycles.Moreover, the Zn 0.5 Mg 0.5 FeMnO 4 anode exhibits a specific capacity close to 300.0 mAh g −1 after 80 cycles at an applied current density of 150 mA g −1 .For the first cycle, the anode gives a Coulombic efficiency of 60.2%, which is owing to the abovementioned capacity loss in the first cycle.From the second cycle, the anode provides a stable value of Coulombic efficiency, which is more than 98% during 80 cycles.In order to compare the obtained performance with the literature, Table 2 compares the results of the electrochemical performance of the elaborated Zn 0.5 Mg 0.5 FeMnO 4 spinel oxide with previously studied anodes.
Molecules 2023, 28, x FOR PEER REVIEW 9 of 18 first cycle is normally due to the formation of SEI film [74,75].The studied spinel displays an extended voltage plateau at about 0.5 V followed by a gradual slope for the first discharge.The potential plateau is probably due to the conversion from the reduction of Zn 2+ , Fe 3+ , and Mn 3+ transition metals to metallic states of Zn, Fe, and Mn, respectively [76,77].
During discharge/charge cycling, the discharge and charge capacities gradually decrease with increasing cycle numbers, which is probably attributed to the aggregation and pulverization of Zn0.5Mg0.5FeMnO4nanoparticles, leading to poor electrical contacts between Zn0.5Mg0.5FeMnO4nanoparticles and that between the active material and current collector [40].The cycling performance of the Zn0.5Mg0.5FeMnO4anode at the current density of 150 mA g −1 during 80 cycles is displayed in Figure 9b.It is clearly seen that the Zn0.5Mg0.5FeMnO4anode shows good cycling stability in its capacity during the initial 40 cycles, with a specific capacity of 484.1 mAh g −1 after 40 cycles followed by a gradual decrease in the range of 40-80 cycles.Moreover, the Zn0.5Mg0.5FeMnO4anode exhibits a specific capacity close to 300.0 mAh g −1 after 80 cycles at an applied current density of 150 mA g −1 .For the first cycle, the anode gives a Coulombic efficiency of 60.2%, which is owing to the abovementioned capacity loss in the first cycle.From the second cycle, the anode provides a stable value of Coulombic efficiency, which is more than 98% during 80 cycles.In order to compare the obtained performance with the literature, Table 2 compares the results of the electrochemical performance of the elaborated Zn0.5Mg0.5FeMnO4spinel oxide with previously studied anodes.As presented in Figure 10, the rate performance of the studied Zn0.5Mg0.5FeMnO4anode is examined by increasing the scan rates from 150 to 400 mA g −1 and then back to 150 mA g −1 .For the first current density (150 mA g −1 ), the Zn0.5Mg0.5FeMnO4anode reaches a As presented in Figure 10, the rate performance of the studied Zn 0.5 Mg 0.5 FeMnO 4 anode is examined by increasing the scan rates from 150 to 400 mA g −1 and then back to 150 mA g −1 .For the first current density (150 mA g −1 ), the Zn 0.5 Mg 0.5 FeMnO 4 anode reaches a specific capacity of 569.1 mAh g −1 after 10 cycles.When gradually increasing the current density, the specific capacity of the Zn 0.5 Mg 0.5 FeMnO 4 anode gradually decreases and reaches a specific capacity of 533.2 mAh g −1 after 20 cycles at a current density of 200 mA g −1 , and a specific capacity of 474.6 mAh g −1 after 30 cycles at a current density of 300 mA g −1 , with good cycling stability.For the subsequent 10 cycles, the Zn 0.5 Mg 0.5 FeMnO 4 anode exhibits a specific capacity of 365.3 mAh g −1 after 40 cycles at a current density of 400 mA g −1 .When the current density is switched back to 150 mA g −1 , the specific capacity of the studied anode increased rapidly to reach 481.8 mAh g −1 .In the range of 40-80 cycles, the Zn 0.5 Mg 0.5 FeMnO 4 anode was found to show a gradual decrease during discharge/charge cycling and reached a specific capacity close to 300.0 mAh g −1 after 80 cycles at an applied current density of 150 mA g −1 .specific capacity of 569.1 mAh g −1 after 10 cycles.When gradually increasing the current density, the specific capacity of the Zn0.5Mg0.5FeMnO4anode gradually decreases and reaches a specific capacity of 533.2 mAh g −1 after 20 cycles at a current density of 200 mA g −1 , and a specific capacity of 474.6 mAh g −1 after 30 cycles at a current density of 300 mA g −1 , with good cycling stability.For the subsequent 10 cycles, the Zn0.5Mg0.5FeMnO4anode exhibits a specific capacity of 365.3 mAh g −1 after 40 cycles at a current density of 400 mA g −1 .When the current density is switched back to 150 mA g −1 , the specific capacity of the studied anode increased rapidly to reach 481.8 mAh g −1 .In the range of 40-80 cycles, the Zn0.5Mg0.5FeMnO4anode was found to show a gradual decrease during discharge/charge cycling and reached a specific capacity close to 300.0 mAh g −1 after 80 cycles at an applied current density of 150 mA g −1 .
For further deep analysis of the kinetics of electrochemical lithiation/delithiation processes, the CV curves of the studied Zn0.5Mg0.5FeMnO4anode were acquired at various scan rates ranging from 0.05 to 0.40 mV s −1 , as shown in Figure 11a.The measured peak current (i) for the cathodic and anodic sweeps at various scan rates (v) was used to analyze the lithium-ion intercalation kinetics of the studied Zn0.5Mg0.5FeMnO4anode.For the cathodic and anodic sweeps, the peak current (i, A) and the scan rates (v, mV s −1 ) can be related according to the Dunn relation, as expressed in Equation ( 6) [79,80]: Both sides of Equation ( 6) were taken in the logarithm function, which can be written as In these expressions, a and b are adjustable parameters.In general, three lithium-ion intercalation kinetics can be distinguished depending on the b-value.In particular, when the b-value approaches 0.5, the lithium-ion storage mechanism is controlled by the diffusion process.When the b-value is close to 1, the lithium-ion storage mechanism is through a surface faradic redox reaction (pseudocapacitive); whereas, when the b-value is between 0.5 and 1, the lithium-ion storage mechanism is a combination of a pseudocapacitive process and diffusion-controlled insertion [81].The b-value was determined by plotting Log(i) against Log(v) for cathodic and anodic peaks, as shown in Figure 11b.The b-value for the cathodic and anodic sweeps corresponds to the slope of linear fitting of the peak currents For further deep analysis of the kinetics of electrochemical lithiation/delithiation processes, the CV curves of the studied Zn 0.5 Mg 0.5 FeMnO 4 anode were acquired at various scan rates ranging from 0.05 to 0.40 mV s −1 , as shown in Figure 11a.The measured peak current (i) for the cathodic and anodic sweeps at various scan rates (v) was used to analyze the lithium-ion intercalation kinetics of the studied Zn 0.5 Mg 0.5 FeMnO 4 anode.For the cathodic and anodic sweeps, the peak current (i, A) and the scan rates (v, mV s −1 ) can be related according to the Dunn relation, as expressed in Equation ( 6) [79,80]: Both sides of Equation ( 6) were taken in the logarithm function, which can be written as In these expressions, a and b are adjustable parameters.In general, three lithium-ion intercalation kinetics can be distinguished depending on the b-value.In particular, when the b-value approaches 0.5, the lithium-ion storage mechanism is controlled by the diffusion process.When the b-value is close to 1, the lithium-ion storage mechanism is through a surface faradic redox reaction (pseudocapacitive); whereas, when the b-value is between 0.5 and 1, the lithium-ion storage mechanism is a combination of a pseudocapacitive process and diffusion-controlled insertion [81].The b-value was determined by plotting Log(i) against Log(v) for cathodic and anodic peaks, as shown in Figure 11b.The b-value for the cathodic and anodic sweeps corresponds to the slope of linear fitting of the peak currents versus scan rates.According to the estimated slopes (Figure 11b), the lithium-ion storage mechanism of the studied Zn 0.5 Mg 0.5 FeMnO 4 anode is a combination of diffusioncontrolled intercalation and pseudocapacitance behavior.versus scan rates.According to the estimated slopes (Figure 11b), the lithium-ion storage mechanism of the studied Zn0.5Mg0.5FeMnO4anode is a combination of diffusion-controlled intercalation and pseudocapacitance behavior.For further study of the changes in the structure and morphology of the electrode material after cycling, and to reveal the lithium storage mechanism, ex situ XRD and SEM characterizations of the uncycled and cycled electrodes at a current density of 0.15 A g −1 were explored.Figure 12 shows the ex-XRD (20-80°) patterns of the synthesized Zn0.5Mg0.5FeMnO4electrodes in different states: before cycling, discharged, and charged after 80 cycles.From this figure, the XRD pattern of the synthesized Zn0.5Mg0.5FeMnO4electrode before cycling confirms the presence of the spinel phase, Cu foil, and the used CMC as a binder.After the discharge process, all the diffraction peaks of the Zn0.5Mg0.5FeMnO4spinel phase disappear, revealing that the Zn0.5Mg0.5FeMnO4spinel became an amorphous phase.Subsequently, there is no added peak observed after the charge process, revealing that it still maintains an amorphous phase under continuous lithiation/delithiation processes, which was an interesting discovery.Similar results have been obtained in several previous works that studied transition-metal-based spinels as anodes for LIBs [82,83].According to C. Duan et al. [84], the amorphous phase formed after the first discharge is beneficial to buffer the volume change of the electrode and maintain the stability and integrity of the electrode.For further study of the changes in the structure and morphology of the electrode material after cycling, and to reveal the lithium storage mechanism, ex situ XRD and SEM characterizations of the uncycled and cycled electrodes at a current density of 0.15 A g −1 were explored.Figure 12 shows the ex-XRD (20-80 • ) patterns of the synthesized Zn 0.5 Mg 0.5 FeMnO 4 electrodes in different states: before cycling, discharged, and charged after 80 cycles.From this figure, the XRD pattern of the synthesized Zn 0.5 Mg 0.5 FeMnO 4 electrode before cycling confirms the presence of the spinel phase, Cu foil, and the used CMC as a binder.After the discharge process, all the diffraction peaks of the Zn 0.5 Mg 0.5 FeMnO 4 spinel phase disappear, revealing that the Zn 0.5 Mg 0.5 FeMnO 4 spinel became an amorphous phase.Subsequently, there is no added peak observed after the charge process, revealing that it still maintains an amorphous phase under continuous lithiation/delithiation processes, which was an interesting discovery.Similar results have been obtained in several previous works that studied transition-metal-based spinels as anodes for LIBs [82,83].According to C. Duan et al. [84], the amorphous phase formed after the first discharge is beneficial to buffer the volume change of the electrode and maintain the stability and integrity of the electrode.versus scan rates.According to the estimated slopes (Figure 11b), the lithium-ion storage mechanism of the studied Zn0.5Mg0.5FeMnO4anode is a combination of diffusion-controlled intercalation and pseudocapacitance behavior.For further study of the changes in the structure and morphology of the electrode material after cycling, and to reveal the lithium storage mechanism, ex situ XRD and SEM characterizations of the uncycled and cycled electrodes at a current density of 0.15 A g −1 were explored.Figure 12 shows the ex-XRD (20-80°) patterns of the synthesized Zn0.5Mg0.5FeMnO4electrodes in different states: before cycling, discharged, and charged after 80 cycles.From this figure, the XRD pattern of the synthesized Zn0.5Mg0.5FeMnO4electrode before cycling confirms the presence of the spinel phase, Cu foil, and the used CMC as a binder.After the discharge process, all the diffraction peaks of the Zn0.5Mg0.5FeMnO4spinel phase disappear, revealing that the Zn0.5Mg0.5FeMnO4spinel became an amorphous phase.Subsequently, there is no added peak observed after the charge process, revealing that it still maintains an amorphous phase under continuous lithiation/delithiation processes, which was an interesting discovery.Similar results have been obtained in several previous works that studied transition-metal-based spinels as anodes for LIBs [82,83].According to C. Duan et al. [84], the amorphous phase formed after the first discharge is beneficial to buffer the volume change of the electrode and maintain the stability and integrity of the electrode.To demonstrate the microstructural change in the studied electrode after cycling, the SEM images of the synthesized Zn 0.5 Mg 0.5 FeMnO 4 electrodes in different states-before cycling, discharged, and charged after 80 cycles-are displayed in Figure 13.Compared to the state before cycling, the Zn 0.5 Mg 0.5 FeMnO 4 electrode shows significant spherical porosity constructed by 3D interconnected nanoparticles.After 80 cycles, it can be seen that the porosity of the synthesized Zn 0.5 Mg 0.5 FeMnO 4 spinel is diminished and the surface becomes dense, whereas the size of the nanoparticles increased obviously due to the huge volume change during continuous lithiation/delithiation processes.This confirms the aggregation of Zn 0.5 Mg 0.5 FeMnO 4 nanoparticles, which led to a reduction in the specific surface area.This reduction in the specific surface area can explain the gradual decrease and poor stability of the specific capacity during discharge/charge processes with cycling.To demonstrate the microstructural change in the studied electrode after cycling, the SEM images of the synthesized Zn0.5Mg0.5FeMnO4electrodes in different states-before cycling, discharged, and charged after 80 cycles-are displayed in Figure 13.Compared to the state before cycling, the Zn0.5Mg0.5FeMnO4electrode shows significant spherical porosity constructed by 3D interconnected nanoparticles.After 80 cycles, it can be seen that the porosity of the synthesized Zn0.5Mg0.5FeMnO4spinel is diminished and the surface becomes dense, whereas the size of the nanoparticles increased obviously due to the huge volume change during continuous lithiation/delithiation processes.This confirms the aggregation of Zn0.5Mg0.5FeMnO4nanoparticles, which led to a reduction in the specific surface area.This reduction in the specific surface area can explain the gradual decrease and poor stability of the specific capacity during discharge/charge processes with cycling.

Materials
Zinc chloride ZnCl2, magnesium nitrate hexahydrate Mg(NO3)2•6H2O, iron nitrate nonahydrate Fe(NO3)3•9H2O, manganese nitrate tetrahydrate Mn(NO3)2•4H2O, and citric acid monohydrate C6H8O7•H2O of analytical grade were purchased from Sigma Aldrich.All purchased reagents were directly used as received without further purification.The porous Zn 0.5 Mg 0.5 FeMnO 4 spinel oxide was synthesized using a facile sol-gel method.Initially, specific molar ratios of ZnCl 2 , Mg(NO 3 ) 2 •6H 2 O, Fe(NO 3 ) 3 •9H 2 O, and Mn(NO 3 ) 2 •4H 2 O were dissolved together in deionized water under vigorous magnetic stirring for 30 min to obtain a homogeneous solution.To this homogeneous solution, citric acid (CA) dissociated in deionized water was added at a molar ratio of CA/cations = 2.5.Then, the solution was kept on the hot plate at 90 • C until a viscous gel formed followed by cooling slowly down to room temperature.Subsequently, the resulting gel was dried in an oven at 120 • C for 12 h to obtain the xerogel powder.Finally, the as-synthesized powder was annealed in the furnace at 700 • C for 12 h at a heating rate of 3 • C min −1 in air to obtain the porous Zn 0.5 Mg 0.5 FeMnO 4 spinel oxide.During this synthesis, the Zn 0.5 Mg 0.5 FeMnO 4 spinel oxide was formed from the reagents according to the following chemical reaction:

Characterization Techniques
For the structural analysis and the phase identification, the synthesized porous Zn 0.5 Mg 0.5 FeMnO 4 spinel was firstly characterized by powder X-ray diffraction (XRD, Bruker D8 Advance) with Cu-K α (λ 1 = 1.54056Å and λ 2 = 1.54439Å) radiations.The XRD analysis was carried out at room temperature over a 2 theta range from 10 • to 80 • at a scan rate of 0.02 • /s.The structural refinement of the prepared sample was carried out by the Rietveld method using Full-prof software (version 7.95) [85].Thermal stability of the synthesized xerogel and calcined sample was investigated by thermogravimetric analysis (TGA) performed on a Discovery TGA analyzer in the temperature range of 20-700 • C at a heating rate of 10 • C min −1 under a constant air purging flow.The surface morphology and surface elemental composition of the prepared powder spinel were evaluated by employing a scanning electron microscope interconnected with an energy-dispersive X-ray spectrometer (SEM/EDX, Zeiss Evo 10).For further deep analysis of the surface morphology, a high-resolution scanning electron microscopy (HR-SEM) image was obtained on a JEOL JSM-IT500HR instrument.To obtain the Brunauer-Emmett-Teller (BET) specific surface area of the synthesized porous spinel, the nitrogen adsorption-desorption isotherm was collected at a liquid nitrogen temperature of 77 K using a Micromeritics Instrument.

Electrochemical Measurements
To make the working electrodes of Zn 0.5 Mg 0.5 FeMnO 4 spinel, dry powders of the synthesized spinel (active material), carbon black, and carboxymethyl cellulose (CMC) binder were mixed at a weight ratio of 80:10:10, respectively.To form a homogeneous slurry, the mixture was dispersed in ultrapure water and mixed for 2 h under vigorous magnetic stirring.Subsequently, the above slurry mixture was cast on a copper foil current collector with a thickness of 0.1 mm and a mass loading of 1~1.2 mg cm −2 using the doctor-blade technique.The loaded copper foil was dried at 60 • C under vacuum for 4 h to eliminate the water used as a solvent and then cut into disks of 11 mm in diameter.The produced electrodes were dried at 80 • C under vacuum for 12 h.All the electrochemical measurements of Zn 0.5 Mg 0.5 FeMnO 4 spinel as anode material for LIBs were performed at room temperature using CR2032 coin cells in a half-cell configuration.Zn 0.5 Mg 0.5 FeMnO 4 //Li coin cells were assembled in an Ar (99.999%)-filled glove box (Jacomex, France, H 2 O and O 2 contents ≤ 1.0 ppm) with high-purity metallic lithium disks as the counter and reference electrodes, Whatman as a separator, and a solution of LiPF 6 (2 mol L −1 ) dissolved in EMC as the electrolyte.Their electrochemical performance was examined using a BioLogic Science multi-channel battery cycler for testing cyclic voltammetry (CV), galvanostatic discharge/charge, long-term cycling, and rate capability over the 0.01-3.00V voltage range (vs.Li + /Li).The cyclic voltammetry (CV) data were measured at various scan rates: 0.05, 0.10, 0.20, 0.30, and 0.40 mV s −1 .The galvanostatic discharge/charge and long-term cycling of the studied spinel were performed at a current density of 150 mA g −1 .Moreover, the rate capability analysis was realized at different current densities of 150, 200, 300, and 400 mA g −1 .The assembled coin cells were set still for 7 h before testing.In this work, the discharging and charging are considered as the lithiation and delithiation processes, respectively.

Conclusions
In this work, the electrochemical performance of a porous Zn 0.5 Mg 0.5 FeMnO 4 spinel oxide elaborated by a facile sol-gel route as anode material for LIBs was investigated in detail.As demonstrated by the XRD, Rietveld method, SEM, and EDX results, the product has a high-purity phase with the desired chemical composition.From the structural analysis, the formation of a single AB 2 O 4 -type spinel phase of the synthesized sample was confirmed.Rietveld refinement evidenced the cubic spinel with Fd3m symmetry.Under flowing air, the synthesized porous Zn 0.5 Mg 0.5 FeMnO 4 spinel oxide showed a good thermal stability as the temperature increased to 700 • C. The SEM image confirmed the successful formation of porous morphology with pore sizes ranging from 2 to 5 µm.From the HR-SEM image, the 3D interconnection of the nanosized spherical particles led to a porous morphology.According to the BET method, the specific surface area of the synthesized porous Zn 0.5 Mg 0.5 FeMnO 4 spinel oxide was found to be 13.54 m 2 g −1 .For the electrochemical performance, the Zn 0.5 Mg 0.5 FeMnO 4 anode showed good cycling stability in its capacity during the initial 40 cycles, with a retention capacity of 484.1 mAh g −1 after 40 cycles at a current density of 150 mA g −1 , followed by a gradual decrease in the range of 40-80 cycles, which led to reaching a specific capacity close to 300.0 mAh g −1 after 80 cycles at a current density of 150 mA g −1 .Besides, the electrochemical reactions of the lithiation/delithiation processes were extracted from the CV curves.For the lithium-ion storage mechanism, a combination of diffusion-controlled intercalation and pseudocapacitance behavior was found.In general, this work is a way forward to develop inexpensive, highly efficient, and eco-friendly transition-metal oxide as a promising anode material for LIBs.

Figure 1 .
Figure 1.(a) X-ray powder diffraction pattern of the elaborated Zn 0.5 Mg 0.5 FeMnO 4 spinel oxide compared with JCPDS card no.88-1936 of the MgFe 2 O 4 phase.(b) Rietveld refinement of the XRD pattern for the elaborated Zn 0.5 Mg 0.5 FeMnO 4 spinel oxide with the cubic Fd3m structure.

Figure 2 .
Figure 2. Three-dimensional crystal structure of Zn0.5Mg0.5FeMnO4spinel oxide with the cubic Fd3 m structure.The Zn 2+ and Mg 2+ cations are located in the tetrahedral sites (green balls), Fe 3+ and Mn 3+

Figure 2 .
Figure 2. Three-dimensional crystal structure of Zn 0.5 Mg 0.5 FeMnO 4 spinel oxide with the cubic Fd3m structure.The Zn 2+ and Mg 2+ cations are located in the tetrahedral sites (green balls), Fe 3+ and Mn 3+ cations are located in the octahedral sites (blue balls), and the red balls represent the O 2− oxygen anions.

Figure 6 .
Figure 6.The SEM image and EDX element mapping results of Zn, Mg, Fe, Mn, and O of the synthesized porous Zn0.5Mg0.5FeMnO4spinel oxide.Scale bar, 5 µm.

Figure 6 .
Figure 6.The SEM image and EDX element mapping results of Zn, Mg, Fe, Mn, and O of the synthesized porous Zn0.5Mg0.5FeMnO4spinel oxide.Scale bar, 5 µm.

Figure 6 .
Figure 6.The SEM image and EDX element mapping results of Zn, Mg, Fe, Mn, and O of the synthesized porous Zn0.5Mg0.5FeMnO4spinel oxide.Scale bar, 5 µm.

Figure 6 .
Figure 6.The SEM image and EDX element mapping results of Zn, Mg, Fe, Mn, and O of the synthesized porous Zn 0.5 Mg 0.5 FeMnO 4 spinel oxide.Scale bar, 5 µm.

Figure 8 .
Figure 8. Cyclic voltammetry (CV) curves of the synthesized porous Zn0.5Mg0.5FeMnO4spinel oxide measured over the 0.01-3.00V voltage range (vs.Li + /Li) at a scan rate of 0.05 mV s −1 for the initial five cycles.

Figure 11 .
Figure 11.(a) Cyclic voltammetry (CV) curves of the synthesized porous Zn0.5Mg0.5FeMnO4spinel oxide at scan rates of 0.05, 0.1, 0.2, 0.3, and 0.4 mV s −1 .(b) The determination of the b-value of the synthesized porous Zn0.5Mg0.5FeMnO4spinel oxide using the linear fitting of the peak currents versus scan rates.

Figure 11 .
Figure 11.(a) Cyclic voltammetry (CV) curves of the synthesized porous Zn 0.5 Mg 0.5 FeMnO 4 spinel oxide at scan rates of 0.05, 0.1, 0.2, 0.3, and 0.4 mV s −1 .(b) The determination of the b-value of the synthesized porous Zn 0.5 Mg 0.5 4 spinel oxide using the linear fitting of the peak currents versus scan rates.

Figure 11 .
Figure 11.(a) Cyclic voltammetry (CV) curves of the synthesized porous Zn0.5Mg0.5FeMnO4spinel oxide at scan rates of 0.05, 0.1, 0.2, 0.3, and 0.4 mV s −1 .(b) The determination of the b-value of the synthesized porous Zn0.5Mg0.5FeMnO4spinel oxide using the linear fitting of the peak currents versus scan rates.

Figure 12 .
Figure 12.Ex-XRD patterns of the synthesized Zn 0.5 Mg 0.5 FeMnO 4 electrodes in different states: before cycling, discharged, and charged after 80 cycles.

Table 1 .
The results of the Rietveld structural refinement for structural parameters, profile parameters, and various reliability R-factors of the elaborated Zn 0.5 Mg 0.5 FeMnO 4 spinel oxide.

Table 1 .
The results of the Rietveld structural refinement for structural parameters, profile parameters, and various reliability R-factors of the elaborated Zn0.5Mg0.5FeMnO4spinel oxide.

Table 2 .
The results of the electrochemical performance of the elaborated Zn0.5Mg0.5FeMnO4spinel oxide compared with previously studied anodes.

Table 2 .
The results of the electrochemical performance of the elaborated Zn 0.5 Mg 0.5 FeMnO 4 spinel oxide compared with previously studied anodes.