NaMoO2PO4 glass ceramic nanocomposite as a novel cathode material for magnesium-ion batteries

doi:10.21203/rs.3.rs-4946752/v1

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Research Article

NaMoO₂PO₄ glass ceramic nanocomposite as a novel cathode material for magnesium-ion batteries

https://doi.org/10.21203/rs.3.rs-4946752/v1

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Unlocking superior Mg-ion cells with good cycling performance as a future battery candidate is now crucial. However, structural instability is mainly reported in current oxide frameworks. Additionally, poor diffusion kinetics are typical due to the affinity of Mg²⁺ ions to interact with oxide anions. Herein, NMoP-0 glass was obtained according to the molar ratio 20 Na₂S to 40 MoO₃-40 P₂O₅ mol%. NMoP-0 was thermally treated to obtain NMoP-8 and NMoP-12 glass ceramic nanocomposites (GCN) to obtain the desired NaMoO₂PO₄. XRD identified the crystal structure of NMoP-12 to be NaMoO₂PO₄ with a crystallite size of 38 nm. The electrodes were tested by EIS, CV and GCD in three and two electrode systems, both confirming their reversible electrochemical activity. The initial specific capacitance values of NMoP-0, NMoP-8 and NMoP-12 in Mg-ion cells were estimated to be 214, 82 and 130 mAh g^− 1, respectively. Meanwhile, the NMoP-12 cells showed the best capacity retention behavior and a diffusion coefficient ∼ 10⁻¹⁴, which means that Mg²⁺ ions diffusion in NMoP-12 is moderately favorable. This promising performance of NaMoO₂PO₄ GCN suggests its potential as a novel cathode material for magnesium-ion batteries, sparking hope for future advancements in battery technology.

Glass-ceramic

Nanocomposites

Electrochemical

NaMoO2PO4

Mg ion battery

Magnesium ion batteries (MIBs) are a viable alternative to lithium-ion batteries that can prevent the exhaustion of lithium resources. Magnesium has low reduction potential, a high volumetric capacity of 86% over lithium, low reactivity in air, and a high melting point. It is an inexpensive, environmentally green, and widely accessible metal^{1, 2}. Unlike lithium, the Mg²⁺ plating/stripping on the anode surface was regarded as dendrite-free³. However, when certain electrolytes were used, dendritic morphologies were reported⁴. Additionally, the high charge density of Mg²⁺ induces strong coulombic interactions with host material, leading to sluggish kinetics, and hence, the primary challenge delaying the practical orientation of MIBs is finding a suitable cathode material capable of overcoming this disadvantage. Additionally, the cathode material of MIBs must be carefully designed to other significant parameters, including a high specific capacity, and a long cycle life ^{5, 6, 7}.

Amorphous materials are predicted to reach higher specific capacities compared to crystalline materials due to its open structures and lack of grain boundaries⁸. According to Heo et al., amorphous iron fluorosulfate preserved the intercalation abilities despite repetitive cycling with severe structural disordering. The structural changes didn’t have a substantial impact on the intercalation host’s diffusion capabilities, and this can be extended to a variety of amorphous cathode materials such as glasses⁹. There are some amorphous cathode materials for MIBs that have been examined recently such as V₂O₅¹⁰, showing both high voltage and specific capacity. However, some studies suggest that V⁵⁺ may be toxic even at low concentrations.¹¹. Since molybdenum exhibits multiple oxidation states and is nonhazardous, it is expected to be an excellent vanadium alternative. Additionally, MoO₃ functions as a network modifier as well as a network former¹². The introduction of MoO₃ creates MoO₄ units and Mo-O-P bonds, improving the samples' thermal stability and glass-forming ability due to cross-linking strengthening^{13, 14}. Zhao et al. prepared amorphous molybdenum polysulfide using a one-step solvothermal method. They reported that a-MoS_x presented a very high capacity compared to its analogous pure crystalline phase MoS₂¹⁵.

Molybdenum is a polyvalent that can access any oxidation state between 0 to 6 and as a result, forms different types of structures¹⁶ such as polyoxometalates¹⁷ and NASICON^{18, 19}. The transition metal oxide, MoO₃, combines with phosphorus pentoxide, P₂O₅, to make molybdenum phosphate glass. The unpaired d-electron of MoO₃ within such glasses can rapidly move from one oxidation state to a higher state, allowing for the presence of both states in these glasses and showing remarkable electrical conductivities²⁰. As the concentration of MoO₃ increases, the redox ratio of Mo⁵⁺/Mo⁶⁺ also increases²¹. Multivalent ions, such as Mg²⁺, can diffuse more quickly in a host with good electronic conductivity and a transition metal ion (TMO) capable of switching its oxidation state. Multiple oxidation states can efficiently attain electroneutrality and lower Mg²⁺ diffusion barriers, possibly overcoming sluggish kinetics²². Therefore, when MoO₃ is added to the amorphous matrix, it is expected to increase the energy density of the host material significantly.

Divalent Mg²⁺ has stronger polarizability than Li⁺ and Na⁺, resulting in its significantly stronger interaction with the host lattice. This leads to higher intercalation energy barriers and poor Mg-ion diffusion coefficients²³. Given that S ions have a considerably greater polarization ability than O ions, maybe less interaction between the framework and alkali metal ions would occur if oxygen was partially replaced with sulfur²⁴. Owing to the larger ionic radius of than oxygen, the lattice parameters may expand and thus, migrating ion transport channel volume is improved²⁵. Additionally, it was confirmed that TMO ions such as vanadium and iron²⁶ can be reduced by S atoms to produce lower valences, improving the electrochemical performance. This can be expanded to other TMO including Molybdenum²⁷.

The conductivity can be increased with controlled nanocrystallization of the glassy matrix to form a dual microstructure, producing glass-ceramic nanocomposites. The disordered interfacial areas between the nanocrystallites and the glass network have been demonstrated to be associated with an improvement in conductivity due to facilitating conducting paths transporting charge carriers²⁸. Nevertheless, most TMOs undergo significant structural alterations over prolonged cycling, which results in unsatisfactory cycling stability²⁹. This can be overcome by creating complex nanostructures formed from these oxides, allowing for a better contact area between the electrolyte and the electrode along with short transport pathways for ions and electrons^{18, 30}.

To the best of our knowledge, there are no research papers exploring the electrochemical performance of either Na₂S- MoO₃- P₂O₅ ternary glass or NaMoO₂PO₄ glass ceramic nanocomposite (GCN) as cathodes materials although there are some studies on other properties such as structural³¹, thermodynamical³², humidity-sensing and sodium ion conductivity³³.

This work successfully prepared glass of 20 Na₂S – 40 MoO₃ – 40 P₂O₅ molar ratio, and its corresponding glass ceramic nanocomposites (GCNs) were obtained by annealing at 500°C. Differential scanning calorimeter (DSC) and X-ray diffraction (XRD) were used for sample characterization. The scanning electron micrograph (SEM) and energy dispersive X-Ray spectrometry (EDS) were exploited in degradation analysis. In addition, we investigate the electrochemical properties in a three-electrode setup in 1M H₂SO₄. Moreover, we apply the current electrodes as cathodes in a fully assembled magnesium ion cell. The electrochemical charge storage mechanism is examined in both three and two-electrode systems via electrochemical impedance spectroscopy (EIS) measurements, cyclic voltammetry (CV) curves, galvanostatic charge-discharge (GCD) for many cycles and chronoamperometry (CA). The results of the study enable us to predict that they will be helpful for future advancements in battery technology.

2.1 Material preparation and characterization

The molar ratio of 20 Na₂S to 40 MoO₃-40 P₂O₅ mol% was selected to produce a glass sample (labeled as NMoP-0) by melt-quenching technique. All Chemicals were purchased and used as received as follows; Na₂S.H₂O (Sigma Aldrich 98%), MoO₃ (Sigma Aldrich ≥ 99.5%), and P₂O₅ (Sigma Aldrich 99%). The relevant quantities of the reagents were thoroughly mixed and then, covered in a suitable crucible. After that, the crucible was placed in a Nabertherm B 180 electric furnace where the mixture was melted at 1290°C for fifteen minutes. The glass sample was finally acquired by quenching of the molten glass between two stainless-steel discs at 27°C (room temperature) after the batch had wholly melted and all traces of the used chemicals had disappeared. An agate mortar was used to ground the as prepared glass.

DSC analysis was carried out by Linseis TGA PT1000. A Small amount of glass (20 mg) was loaded into an empty alumina crucible, and the data was recorded in the range of 50–650°C, gradually increasing ambient temperature by 10°C/min. The heat treatment (HT) of NMoP-0 was performed at 500°C for 8 and 12 h, labeled as NMoP-8 and NMoP-12, respectively. Amorphous nature confirmation of NMoP-0 matrix and structure characterization of heat-treated samples were enabled using XRD (Rigaku Miniflex 600 diffractometer) at room temperature with CuKα radiation. The JEM-2100 TEM instrument was employed for High-resolution transmission electron microscope (HRTEM) analysis.

2.2 Cell Assembling and electrochemical tests

A mixture (75:15:10 wt%) of active material (NMoP-0, NMoP-8, and NMoP-12): polyvinylidene fluoride (Lanxess 100%): super P carbon (alfa Aesar + 99%) was used to fabricate the working electrode. The mixture was kept on a magnetic stirrer in N-methyl-2-pyrrolidinone NMP (LOBA CHEMIE 98%) for 20 h to ensure the elements were evenly dispersed throughout the slurry. This slurry was coated on titanium sheets then it was dried for 12 h at 70°C.

The electrochemical performance was first evaluated in a three-electrode system, as shown in Fig. 1(a). A counter electrode, platinum wire, and reference electrode, Ag/AgCl, were used in 1 M H₂SO₄ aqueous solution as electrolyte. EIS data was recorded in the frequency range of 10 Hz to 1MHz (analyzed by Ec-lab software). CV curves were obtained using Amel 2551 Potentiostat in the potential range of 0 to 0.7 V at scan rates of 5, 10, 20, 30 and 50 mV s^− 1. GCD studies were conducted using NEWARE BTS4000 battery tester at current densities of 10, 20. 40, 60, 100 and 200 mA g^− 1.

Subsequently, the two-electrode system evaluations were carried out in a Swagelok type cell, as illustrated in Fig. 1(b). An argon glovebox was used to properly test the electrodes as a cathode for a magnesium ion battery with a magnesium metal disk as an anode. The electrolyte was prepared as a solution of 1:2 Tetraethylene glycol dimethyl ether (Alfa Aesar + 98%): acetonitrile (CHEM lab NV 99.9%) containing 1.5 M Mg(NO₃)₂.6H₂O (Alfa Aesar + 98%). Whatman (GF/D) glass microfibre filters were used as separators. All measurements were carried out at a fixed temperature of 35°C. CV was performed versus Mg/Mg²⁺ in 0.1–2.3 V potential window at a scan rate of 10 mV s^− 1. GCD was carried out in 0.1–2.2 V window.

2.3 EX situ analysis

Ex situ XRD and electron microscope (Jeol JMS-700 EDS) were key tools to investigate the storage mechanisms in the magnesium ion cells. Structural alterations and surface morphology changes of working electrodes were carefully examined at successive electrochemical stages.

3.1 Differential scanning calorimeter (DSC)

DSC is performed to examine the thermal response and verify glassy nature. Figure 2 displays the collected DSC signal of the NMoP-0 glass. The DSC pattern exhibits one exothermic peak, positioned according to the corresponding crystallization temperature (Tc) at 482°C. The glass transition temperature (Tg) is evident as a single endothermic peak around 427°C indicating that the sample is highly homogeneous³⁴. This value is greater than previously reported T_g for similar, although sulfur-free, systems by 17°C ³⁵, explained as formation of P-O-Mo linkages instead of Mo-O-Mo connections. Moreover, a similar (20°C) shift was observed for sodium borosilicate glasses after sulfur incorporating up to the maximum possible sulfur loading (2.0 mass%)³⁶. The overall thermal stability factor, the difference between values of T_c and T_g, or ΔT, ³⁷ is calculated to be 55°C which is low compared to literature^{33, 38}. A narrower variation between the two peaks suggests less thermally stable glass. Another factor related to glass thermal stability is Hruby coefficient (H= (ΔT/Tg), is also found to be low (0.13). These results can be explained in terms of the weakening of bonds due to the presence of more non-bridging oxygens after sulfur addition.

3.2 X‑ray diffraction (XRD) and High-resolution transmission electron microscope (HRTEM)

The recorded XRD patterns of NMoP-0 glass, NMoP-8 and NMoP-12 with detailed characterization are displayed in Fig. 3(a). In the as-prepared glass, the absence of strong visual crystallization peaks and the presence of merely broad diffuse scattering suggest the existence of short-range order, which is typical of an amorphous nature. The glass ceramic nanocomposite NMoP-8 sample thermally treated for 8h at 500°C showed sharp and broad diffraction peaks confirming the growth of extremely fine crystallites and corresponding to primary Monoclinic MoO₂ phase (PDF#00-032-0671) with P21/n space group. Its crystal structure is constituted of distorted MoO₆ octahedron units³⁹. The precipitated crystallite size is calculated at θ, Bragg diffraction angle, of the respective XRD peak, by Scherer formula⁴⁰:

$$\:D=\frac{0.9\:\lambda\:}{{\beta\:}_{D}\:\text{c}\text{o}\text{s}\theta\:}$$

Where λ stands for the wavelength of line CuKα (taken as 1.542 Å), and βD (the FWHM) for the corrected full width (that is, in radians) at half maximum. The average precipitated crystallite size is calculated for NMoP-8 to be 53 nm. A second phase was detected and matched to be Monoclinic NaMoO₂PO₄ (PDF#01-076-1626) with P21/n space group. The heat treatment period was extended to 12h at the same temperature until MoO₂ completely disappeared. The glass ceramic nanocomposite NMoP-12 indicated single phase NaMoO₂PO₄ with crystallite size of 38 nm. The NaMoO₂PO₄ structure is illustrated in Fig. 3(b), as generated by VESTA software⁴¹. It is formed from distorted MoO₆ octahedra, similar to that of MoO₂, that are linked to polyhedral NaO₇ and tetrahedral PO₄ groups via corner-sharing with two distinct phosphorus sites having their charges neutralized by Mo and Na as reported by Kierkegaard⁴². The HRTEM images of thermally treated NMoP-8 and NMoP-12 glass ceramic nanocomposites are shown in Fig. 3(c) and (d), respectively, at the 200 nm scale. It reveals the presence of nearly spherical shaped nanoparticles scattered across a glass matrix. Additionally, it provides average size of these nanoparticles. The particle size is verified to be 68 and 51 nm for NMoP-8 and NMoP-12, respectively. This is consistent with the XRD-calculated crystallite size determined according to Debye-Scherrer equation.

3.3 Electrochemical performance

3.3.1 Three-electrode evaluations

3.3.1.1 Electrochemical impedance spectroscopy (EIS)

EIS measurement was employed in a wide frequency window to evaluate the reaction process of current electrodes. The resistance Z' (represented at x-axis) and reactance Z'' (represented at y-axis) are widely used for depicting EIS data. All plots exhibit three components as displayed in Fig. 4 with equivalent circle. R_o is intercepting the x-axis in the high frequency end and is calculated to have almost the same value ($\:\sim$2.8 Ω) for all electrodes. This can be ascribed to electrolyte resistance and electron transfer resistance between the titanium current collector and electrode active material. A semicircle R₁ is related to interfacial charge transfer resistance between the electrode and electrolyte. The overall surface area of each of the active materials is inversely proportional to the crystallite size of constituent crystallites. The contact area is likewise inversely proportional to the particle size, and hence interface resistance, assuming that particles are totally immersed within the electrolyte⁴³. In our case, the particle size ratio of NMoP-8 and NmoP-12 is calculated to be $\:\sim$ 1.3, according to TEM findings, and the experimentally obtained resistance ratio is 1.2, which is nearly the same, confirming that R₁ is a surface-controlled phenomenon. The result further implies that the physical contact at the electrolyte and active electrode interface is uniform. Generally, the presence of molybdenum facilitates electron conductivity across the current collector, the electrode, and the electrolyte⁴⁴. A sloped line is observed at the low-frequency end of all plots, which is regarded as Warburg impedance Z_w associated with ion-limited diffusion process inside the electrode active material. The NmoP-12 electrode showed the minimum resistance values, smaller arc radius and Z_w, compared to other samples. This can be related to the small crystallite size and indicates that heat treatment was an effective method to improve conductivity and overall electrochemical performance. All electrode materials exhibit a slight decrease in resistance values following short cycling (10 cycles), which is attributed to improved diffusion processes and reaction activity⁴⁵.

3.3.1.2 Cyclic voltammetry (CV)

The CV curves of all electrodes at different scan rates are displayed in Fig. 5(a-c). All samples exhibit two redox waves, so the multi-step ion insertion/extraction process is recommended. The redox peaks are nearly symmetrical, and their peak-to-peak separations remain $\:\le\:$100 mV (at 5 mV s⁻¹), suggesting a quite reversible nature of the redox reactions. NMoP-12 electrode displays much higher peak current densities. It also exhibits the lowest ratio of peak current densities at scan rates of 5 to 50 mV s^− 1. The Mo⁵⁺/Mo⁶⁺ redox reactions hence have better reversibility which should improve the energy storage ability⁴⁶. The NMoP-12 performance can be ascribed to larger contact area and more active sites for reaction. As scanning rates increase, CV curves don’t change its shape. However, oxidation and reduction peaks are shifted in opposite directions so that peak to peak separation is increased indicating that the reaction gets less reversible. This can be ascribed to electrode polarization. Also, the scan rate increase leads to accelerated charge transfer kinetics and as a result, surface-controlled capacitance process dominates⁴⁷.

Typically, diffusion- and capacitive-controlled charge storage are regarded as the two different mechanisms controlling the CV plot. Their contribution can be calculated at scan rate $\:v$ as follows⁴⁸:

$$\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:{i}_{p}/{v}^{0.5}={i}_{diff}+{i}_{cap}={k}_{1}{v}^{0.5}+{k}_{2}$$

where $\:{i}_{p}$ stands for peak current, $\:{i}_{diff}\:$and $\:{i}_{cap\:}$ for the diffusion limited- and surface-controlled current contributions, respectively. By using the slope and intercept of $\:{i}_{p}/{v}^{0.5}$ vs. $\:{v}^{0.5}$ as shown in Fig. 5(d-f), k₁ and k₂ can be calculated and hence, diffusion- and capacitive-controlled percentages are obtained. The diffusion-controlled percentages at a scan rate of 5 mV s^− 1 are illustrated as inset Fig. 5(d-f). Additionally, detailed contributions of limited diffusion and capacitance-controlled at all scan rates is shown in Fig. 5(g-i). It was found that only NMoP-12 electrode, among all samples, has more diffusion (84 − 63%) than capacitive-controlled percentage, indicating that it is a battery-type electrode. The capacitive current percentages generally increased with scan rate increase for all electrodes, favoring fast kinetics of charge-transfer and high-rate performance type devices⁴⁸.

3.3.1.3 Galvanostatic charge/discharge

The storage properties of NMoP-0, NMoP-8 and NMoP-12 positive electrodes were highlighted using GCD technique. The GCD curves are presented in Fig. 6(a–c) in the potential window of 0–0.8 V at six different current densities. Figure 6(d) shows GCD plots of the three electrodes at an equal current density of 10 mA g^− 1. The three curves exhibited a typical GCD curve of the battery-type electrode with several voltage plateaus. The following formula is generally used to estimate the specific capacitance C_S (F g^− 1) based on the GCD behavior⁴⁹.

$$\:{C}_{s}=\:\frac{I\:\varDelta\:t}{m\:\varDelta\:V}$$

where I is the fixed discharge current intensity, t denotes the discharge period, m for the mass of the active material, and V is the entire voltage window. The specific capacitance of NMoP-0, NMoP-8 and NMoP-12 electrodes was calculated to be 40, 205 and 1519 F g^− 1 at 10 mA g^− 1, respectively. The NMoP-12 cathode possesses a structure of smaller average crystalline size, resulting in a shorter ion diffusion distance and better electrochemical performance⁵⁰. It is commonly recognized that the electrochemical properties are substantially affected by the crystalline form and size, homogeneity, and porous structure of the active material^{51, 52}. All specific capacitance values considerably decrease as current density increases, reflecting ion exchange process⁴⁴. This decrease in discharge periods is correlated to ion intercalation at the cathode material surface across the cathode/electrolyte interface. In contrast, when a small current is applied, the specific capacitance is high due to sufficient time for the processes of ions intercalation/deintercalation to take place in deeper porosities in the active material⁵⁰.

3.3.2 Testing in full cells

To examine the electrochemical performance of NMoP-0, NMoP-8, and NMoP-12 samples as cathode materials for mg-ion batteries, they were assembled in full cells as positive electrodes and tested as follows.

3.3.2.1 Electrochemical impedance spectroscopy (EIS)

EIS measurements were carried out to investigate the full cells' bulk and interface related impedance elements and calculate the ions' diffusion coefficient. Reactive and resistive elements that form the EIS spectrum function during unique timescales, which implies they can be determined separately⁵³. The EIS spectra of the three electrodes are illustrated in Fig. 7(a) before and after short cycling. The EIS graphs are divided into four parts for all electrodes. Figure 7(a) shows the fitted equivalent circuit as inset. The electronic resistance of the cathodes, liquid electrolyte ionic resistance and connectors resistance are symbolized as R_o, which intercepts the Z' axis at the high frequency end⁵⁴. The NMoP-8 electrode shows a higher R_o value, compared to other electrodes, suggesting a higher internal resistance. The high-frequency R_ct is regarded as the resistance to charge transfer in the electrolyte/ electrode contact interface in addition to the resistance of Mg²⁺ ions transportation across the electrode surface⁵⁵. The electrodes surface polarization is denoted by constant phase element CPE⁵⁶ where CPE is used to account for the distortion of semicircles. The medium frequency R_SEI is related to the solid electrolyte interphase (SEI) passivation layer that restricts ions insertion/desertion across the electrode/electrolyte contact interface and irreversibly consumes ions⁵⁷. Table 1 lists the fitted R_o, R_SEI and R_ct values for all cathodes. It is noteworthy that the NMoP-12 electrode exhibits the lowest values of all these parameters among other samples, the low impedance is ascribed to the excellent contact area between the electrode and electrolyte at interface. This agrees with the findings of the current EIS in three-electrode systems. An increase in semicircle radius, i.e., resistance values, is found for all electrodes after cycling; this can be attributed to SEI layer formation. The side reactions result in an insulating SEI layer on alkali metal surfaces. This leads to electrolyte depletion. The electrolyte is fully consumed before other cell parts. Additionally, after a few cycles, the SEI layer rapidly dries, raising cell impedance and accelerating cell polarization. When cell polarization is sufficient to shut down the redox reactions within the operating window, a rapid capacity drop occurs⁵⁸.

Table 1

EIS parameters and calculated Mg ions diffusion coefficient.
EIS parameters	R_o (Ω)	R_ct (Ω)	CPE₁ (µF. s^(α−1))	R_SEI (Ω)	CPE₂ (µF. s^(α−1))	D_Mg²⁺ (cm²s^− 1)
EIS parameters	R_o (Ω)	R_ct (Ω)	CPE₁ (µF. s^(α−1))	R_SEI (Ω)	CPE₂ (µF. s^(α−1))	Before CV	After CV
NMoP-0	16.26	2.498	2.81	422.2	41.27	3.85E-16	1.96E-16
NMoP-8	34.09	31.23	2.88	451.2	115.0	7.36E-16	3.88E-16
NMoP-12	15.81	1.854	1.85	74.61	18.11	1.23E-14	3.12E-15

The kinetics of the limiting process of Mg²⁺ ion diffusion inside bulk cathode is modelled by Warburg impedance, Z_w at low frequencies. Figure 7(b) displays Z' vs ω^−0.5 before and after cycling. The slope of the plot at low frequency is used as the Warburg factor (A_W). The diffusion coefficient D_Mg²⁺ of Mg²⁺ ions is given by⁵⁹:

$$\:{D}_{{\text{M}\text{g}}^{+2}}=\frac{{R}^{2}{T}^{2}}{2{A}^{2}{n}^{2}{F}^{4}{C}^{2}{\text{A}}_{\text{W}}^{2}}\:$$

Where R denotes the gas constant (8.314 J mol^− 1), T denotes absolute temperature (kelvin degree), A denotes electrode surface area (cm²), F denotes Faraday constant, n denotes the number of transferred electrons (= 2 for Mg²⁺) and C denotes Mg²⁺ concentration. The diffusion coefficients are listed in Table 1. The NMoP-0 and NMoP-8 cathodes exhibit moderately favorable (in the order of 10^− 16). Moreover, the NMoP-12 cathode has 1–2 higher orders of magnitude before and after cycling. This confirms better diffusion rate and kinetics and as a result, enhanced overall electrochemical performance is expected.

3.3.2.2 Cyclic voltammetry (CV) and Chronoamperometry (CA)

Cyclic voltammetry of NMoP-0, NMoP-8, and NMoP-12 containing cells was performed at 10 mV s^− 1 to verify the electrochemical activity and Mg²⁺ storage mechanisms in these electrodes. The CV plots are displayed in Fig. 8(a). No sudden current increase is observed and hence, no electrolyte decomposition is evident up to 2.3 V vs Mg/ Mg²⁺. The presence of redox peaks confirms that the redox reaction is responsible for the cell storage mechanism and that the cell is rechargeable. According to CV plots, the current density is maximum for the cell with NMoP-12 cathode, highlighting the effect of heat treatment on increasing conductivity. The NMoP-0, NMoP-8 cells show a typical cathodic peak at $\:\sim$0.476 V (Mo⁶⁺ reduction) and an anodic peak at $\:\sim$1.558 V vs Mg/ Mg²⁺, as previously reported for a similar Mg ion battery system with MoO₂ as cathode active material⁶⁰. However, for NMoP-12 cell, this cathodic peak exhibited a positive shift towards $\:\sim$0.651 V vs Mg/ Mg²⁺, narrowing the peak split. This suggests that the rate by which the electrochemical reaction takes place is increasing and that the associated redox peaks are able to appear earlier⁶¹. A second broad anodic peak is seen for all cells which can be ascribed to successive steps of Mg²⁺ ion deintercalation from cathode materials in the voltage window of 0.1–2.3 V, i.e., two transfer reactions with one electron each^{62, 63}. It can be attributed to activation of Mo⁴⁺/ Mo⁵⁺ at higher voltages, which is a highly reversible redox couple beneficial for energy storage devices⁶⁴.

The NMoP-12 cell showed the best electrochemical performance, so it was selected for chronoamperometry to study its electrode stability and switching response before and after 50 GCD cycles. Figure 8(b) illustrates the chronoamperometric response of NMoP-12 electrode. The experiment was conducted using a 10s width of pulse between − 1000 and + 1000 mV so that Mg²⁺ ions migrate in and out of the electrode. The current is initially high, but it rapidly drops after that before stabilizing at a nearly fixed value and eventually stops after the potential becomes zero V. That means the electrochemical process ends after 10s and the current rapidly drops to zero. The current densities after cycling are lower compared to data collected before GCD tests, indicating higher impedance values, and confirming EIS analysis. However, equilibrium current densities after cycling are stable with time with no noticeable degradation with scan number like before GCD tests and are achieved faster (in $\:\sim$ 2s), proposing a stable nature of NMoP-12 electrode.

3.3.2.3 Galvanostatic charge/discharge

To investigate cell capacity and cycling stability, GCD cycling curves are presented in Fig. 9(a) for NMoP-0, NMoP-8, and NMoP-12 cells. A 2.3 V cutoff for charging was selected to minimize electrolyte decomposition as much as possible and avoid severe ion extraction from active material of cathode, which could trigger irreversible nanocomposite phase changes. The NMoP-0 and NMoP-12 cells started at the 2.2 V (OCV) open circuit, confirming their electrochemical activity. NMoP-0 cell discharged to 0.01 V with multiple plateaus at 0.2 and 0.4 V, followed by a charge to 2.3 V with a single plateau at 1.9 V, typically attributed to the extraction process of Mg²⁺. NMoP-12 exhibited the same behavior with nearly the same plateaus. The NMoP-8 cell displays the largest voltage hysteresis between the DCh/Ch plateaus which can be related to high cell impedance as suggested by EIS parameters in Table 1. This resistance generates polarization, which reduces the discharge potential and forces the charge potential to rise to be able to drive the chemical processes. The cycling performance of the three cells is shown in Fig. 9(b). The NMoP-0 electrode started with the largest capacity of 214 $\:\text{m}\text{A}\text{h}\:\text{g}$⁻¹. However, this storage ability rapidly decayed with a retention of 11% after 5 cycles. Additionally, NMoP-8 had the minimum (~ 82 mAh g^− 1) specific capacity and a small capacity ~ 19% retention after few cycles. On the other hand, the NMoP-12 electrode initially showed a capacity of 130 $\:\text{m}\text{A}\text{h}\:\text{g}$⁻¹ and retained 40% of this capacity after the 5 cycles. This can be related to the 3D skeleton of NaMoO₂PO₄ and its ability to withstand structural changes compared to 2D structures as MoO₂ Moreover, this is reasonably expected with the high Mg²⁺ diffusion coefficient values of NMoP-12, compared to other electrodes, as validated in EIS study.

3.4 Ex situ XRD, SEM and EDS analysis

Ex situ XRD data of the best electrode (NMoP-12) were recorded at three stages to demonstrate the responsible mechanism for charging and discharging. XRD patterns of NMoP-12 as pristine, discharged, and successively charged are shown in Fig. 10(a). All stages show several peaks that are clearly titanium sheet current collector related, as matched with (PDF#00-044-1294), and a dominant peak at 2θ of 26.4°, which is matched with C (PDF#00-012-0212). The characteristic peak of NaMoO₂PO₄ corresponding to (210) at 2θ of 16.4° is magnified in Fig. 10(b) at all stages. Only very slight shifts are noticed after cycling, rendering a lower possibility that a structural failure throughout the cycling process is responsible for current cell capacity fading. Upon the initial discharge, the zoomed in peak is shifted to become located at a slightly higher 2θ degree (lattice parameter decrease at ions insertion). Notably, it restores its original position for the pristine electrode after further cycling in the following charge process (lattice parameter increase at ions extraction), indicating that NMoP-12 electrodes can reversibly be cycled⁶⁵.

To gain further insight into NMoP-12 electrode morphology and degradation process, SEM scans at different stages of cycling are displayed at 50 µm and 10 µm in Fig. 11. The particles in Fig. 11(a-b) originally form a very rough surface appearance due to their uneven shapes, and the large aggregations. These agglomerations can be related to super P carbon presence and are built up of several tiny particles, thus making it extremely difficult to determine the precise grain size and minimize the contact area between NMoP-12 and the electrolyte. After discharge, some of the separator fibers couldn't be removed, presumably because of volume altering of NMoP-12 as seen in Fig. 11(c-d). This may promote large pressure on the separator, which is adhered to the electrode surface. Figures 11(e-f) demonstrate clearer smooth surfaces of the Dch/Ch electrode and a lower level of aggregation. There were no major cracks in the pristine NMoP-12 cathode. On the contrary, large crack development was apparent with large size after cycling. This electrode surface microcrack evolution confirms the loss of NMoP-12 active material lowering storage amount and participating in cell deterioration. The primary reason for these cracks can be the inadequate flexibility of the binder, PVDF, which makes it unable to follow the periodic volume changes of nanocomposite throughout cycling, resulting in the active materials to exfoliate⁶⁶. No obvious decomposition products were detected after cycling, except for a sheet-like coating of a local area as encircled in Fig. 11(f). This layer is ascribed to metallic magnesium deposition and is due to SEI presence and slowing traffic at current Mg/electrolyte as confirmed by EIS analysis. EDS spectra and elemental mass normalization of NMoP-12 electrodes are graphed in Fig. 12(a-b) for the Pristine NMoP-12 electrode and its respective DCh and DCh/Ch electrodes. EDS is used to identify the specific elemental composition. There are several distinctive bands that match the nanocomposite starting elements O, Na, Mo, P, and S. Other bands related to super P carbon (C), PVDF (F), anode (Mg) and separator (B and Si) were also found. Mg and Si bands only appeared after cycling which is reasonable with no impurity detected. The shifts in Na and Mg mass % are shown as inset in Fig. 12(a) and are in line with electrochemical storage where Mg is adsorbed in NMoP-12 surface upon discharge and departure afterwards in subsequent charge. This decrease in Mg after charge is consistent with minimal metallic magnesium deposition on NMoP-12 surface, as illustrated in SEM images. This ex-situ study contributes to understanding the cycling behavior of the current electrodes.

Our complete study of Sodium phosphate glasses containing molybdenum as a potential cathode material for rechargeable batteries shows significant improvements in electrochemical performance after appropriate heat treatment at 500°C. Redox peaks illustrated in all CV curves is evidence of the reversible nature of confirmed redox reactions. The $\:{D}_{{\text{M}\text{g}}^{+2}}$ values suggests that the glass matrix is carefully modified such that Mg²⁺ ions are moderately able to overcome the energy barrier of diffusion in NMoP-12 cathode. NMoP-12 achieved an acceptable specific capacity with the best cycling stability among other electrodes. This can be related to having its small crystallite size and 3D skeleton, enhancing the overall performance of the cells. Ex-situ analysis proved no phase changes with minimal decomposition products on the NMoP-12 cathode after cycling and confirming crack formation as a primary reason for electrode degradation. This work demonstrates that the current glass ceramic NaMoO₂PO₄ nanocomposites can be explored more thoroughly as suitable electrodes for future magnesium ion batteries, especially with specific electrolyte and binder stability enhancements.

Conflict of interest

The authors declare that there is no conflict of interest in the current article.

Author Contributions Statement

N K Wally and M M El-Desoky wrote the main manuscript, I Sheha and M M El-Desoky review the manuscript, I Morad prepared all figures

Funding

There is no

Author Contribution

M M El-Desoky and N K Wally wrote the main manuscript , I Sheha and M M El-Desoky review the manuscripte, I Morad prepared all figures

Data availability

Data will be made available on request.

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No competing interests reported.

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NaMoO₂PO₄ glass ceramic nanocomposite as a novel cathode material for magnesium-ion batteries

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