3.1.DTA Results
All the synthesized Na(Cu)1−x(VO)xPO4 glass and glass ceramic cathode samples are subjected to DTA analysis to monitor changes in glass transition, crystallization, melting, and stability before and after heat treatment at its Tc for 5h. The compositionally dependent characteristic DTA parameters (Tg, Tc, Tm, \(\varDelta\)T: Tc-Tg, and Kgl=(Tc - Tg)/(Tm - Tc)) of all the glass (dotted) and glass-ceramic (solid lines) cathode samples are examined here using DTA curves. The usual temperatures of Tg and Tc are found to be 643 K and 761 K, respectively (Table. 1), according to the endothermic and exothermic dips of the G-NaCu0.9(VO)0.1PO4 glass cathode sample (x = 0.1 mol%), as shown in Fig. 1. Among all the glass cathode samples under investigation, the best glass-forming ability is demonstrated by the G-NaCu0.7(VO)0.3PO4(x = 0.3 mol%) glass cathode sample, which had the lowest endothermic and exothermic peak intensities. All of these glass cathode samples have their stability tested using Hurby’s stability parameter Kgl= \(\frac{(\text{T}\text{c}-\text{T}\text{g} )}{(\text{T}\text{m}- \text{T}\text{c})}\), which says that the higher the value of Kgl, the more stable they are against crystallization[30]. Also, of all the synthetic glass samples, G-NaCu0.5(VO)0.5PO4 had the greatest value of Tg, and \(\varDelta\)Tshowedthe remarkable thermal stability and its ability to vitrify when cooled. The best disorder of the G-NaCu0.7(VO)0.3PO4 glass network may be the reason for the partial substitution of tetrahedral VO4 sites for CuO6 structural sites. Additionally, all of these glass cathode samples are heated in a tube furnace with 5% H2-95% Ar gas flow for 5h at their crystallization temperature (Tc) in order to discover variations in the degree of crystallization. The endothermic and exothermic maximum of each sample of crystallized glass can be seen as solid curves in Fig. 1. This tendency is expected to arise from the selective formation of one or more crystalline phases with residual glassy character.
The Hurby's stability parameter (Kgl), the glass-forming ability parameter (\(\varDelta\)T), and the glass transition temperature(Tg) of a particular glass-ceramic cathode sample are very close to its identical glass cathode sample in Fig. 2 (a&b) (Table. 1). Although mixed polyanion glass cathode networks are predicted to have the best precipitation propensity of a few chosen nanocrystalline phases, GC-NaCu0.7(VO)0.3PO4 (x = 0.3 mol%) glass-ceramic cathode networks are envisaged to give the highest values of both ΔT and Kgl in comparison[16, 30–32]. Despite having a lower Kgl value than the other samples being studied, the GC-NaCu0.7(VO)0.3PO4 (x = 0.3 mol%) glass-ceramic cathode network has emerged as our preferred choice. The reasons for this are twofold: (a) the low glass-transition temperature (618 K), which would lead to good electrical properties; and (b) the ideal degree of crystallization by the precipitation of monocrystalline phases, which would result in a correlation between electrical conductivity and electrochemical performance. Hence, it is highly essential to correlate phase forming ability and its intensity variation and the corresponding EIS performance of all the mixed polyanion GC-NaCu1 − x(VO)xPO4 glass-ceramic cathode samples, which is well discussed in the following section.
Table 1
DTA parameters of the as-quenched and heat treated at crystalline temperature for 5h of all the Na Cu1 − x(VO)xPO4(x = 0.1, 0.3, 0.5, 0.7and 0.9 mol%) mixed polyanion glass and glass-ceramic cathodes
Polyanion glass and glass–ceramic composition | Transition temperature (Tg) K | Crystalline temperature (Tc) K | Melting temperature (Tm) K | Stability (ΔT = Tc-Tg K) | Kgl= \(\frac{(\mathbf{T}\mathbf{c}-\mathbf{T}\mathbf{g}}{(\mathbf{T}\mathbf{m}- \mathbf{T}\mathbf{c})}\) |
G-NaCu0.9(VO)0.1PO4 | 619 | 742 | 1174 | 123 | 0.284 |
G-NaCu0.7(VO)0.3PO4 | 618 | 806 | 1192 | 155 | 0.384 |
G-NaCu0.5(VO)0.5PO4 | 624 | 769 | 1161 | 145 | 0.369 |
G-NaCu0.3(VO)0.7PO4 | 610 | 746 | 1156 | 145 | 0.353 |
G-NaCu0.1(VO)0.9PO4 | 626 | 756 | 1179 | 130 | 0.307 |
Heat treated at Tc for 5hrs |
GC-NaCu0.9(VO)0.1PO4 | 643 | 761 | 1202 | 118 | 0.267 |
GC-NaCu0.7(VO)0.3PO4 | 667 | 822 | 1235 | 155 | 0.373 |
GC-NaCu0.5(VO)0.5PO4 | 650 | 786 | 1209 | 136 | 0.331 |
GC-NaCu0.3(VO)0.7PO4 | 631 | 761 | 1176 | 130 | 0.313 |
GC-NaCu0.1(VO)0.9PO4 | 649 | 777 | 1204 | 128 | 0.299 |
3.2 XRD and SEM studies
The XRD data of present glass and glass-ceramic cathode samples will explore the composition, structure, and compositional-dependent behavioral change. The mixed polyanionG-NaCu0.9(VO)0.1PO4 glass cathode sample lacks of crystalline peaks, as shown by the XRD spectrum and the plane surface of the SEM picture, both of which are indicative of the sample's amorphous nature. But when all samples are considered, the same holds true [Inset of Fig. 3].
When they crystallized at their Tc (glass-ceramic) temperatures, crystallized peaks of all of the mixed polyanion G-NaCu1 − x(VO)xPO4 glass cathode samples revealed phase formation and local structure (Fig. 3). The primary cause of the diffraction patterns observed here is the formation of the conducting NaCu(PO4) (ICSD 581303), Na2Cu(P2O7) (ICSD 556822),NaV2O5(ICSD 760908) indexed with orthorhombic/P2_12_121, monoclinic/P21/C, and Orthorhombic/P2_12_121 crystalline phases, respectively, which are identified as the most stable structures. Also, XRD spectra depict the impure phases of NaV2O5, which exhibit an orthorhombic/Pmmn structure (Fig. 3). The probability that glass samples may crystallize at its Tc rises as x increases up to 0.3 mol% (G-NaCu0.7(VO)0.3PO4), can be shown in Fig. 3. It is also noteworthy to see in Fig. 3 that the partial substitution of CuO6 structural sites with tetrahedral VO4 sites in the glass-ceramic network, which results in a reduction in the length of phosphate chains, is mostly responsible for the intensity variation of as precipitated phases (NaCu(PO4) (ICSD 581303), Na2Cu(P2O7) (ICSD 556822), NaV2O5(ICSD 760908)[33, 34]. This might greatly improve the overall electrochemical performance. Lattice parameters that have been refined experimentally are listed in Table 2, along with a comparison of those values to calculated values. The average crystallite size of all G-NaCu1 − x(VO)xPO4 glass cathode samples is calculated using the Williamson-Hall (W-H) equation, which excluded experimental effects and strain broadening at FWHM (full width of half maximum) of XRD peaks (Fig. 4)[35].
βCosθ = Cλ/ D + 4εsinθ--------------------------- (1)
Where FWHM, Bragg angle, a correction factor (C ~ 1), the crystallite size (nm), lattice strain, and X-ray wavelength (0.1540 nm) are the variables in turn. Average crystallite size for the ideal GC-NaCu0.7(VO)0.3PO4 glass-ceramic cathode network is about 68 nm (Fig. 4). However, it is crucial to realize that during the crystallization, the concentration of copper ions in the + 2 state (Cu2+ tetrahedral units in glass network) is approximately 90%, whereas the same is approximately 10% prior to crystallization. This means that the strength of the glass network is improved to a greater extent without compromising the structure of major crystalline phases.This will help to improve the extended cycle life and rate capabilities to get high voltage after crystallization. Each glass-ceramic sample's crystallization propensity is evaluated using the Jade software; the results are reported in Table 2. The best GC-NaCu0.7(VO)0.3PO4 (x = 0.3 mol%) glass-ceramic sample crystallizes to the highest degree (70.42%) and must have a microstructure that includes both the grains and the thin boundaries separating them, as shown in Table 2. The relationship between the microstructure and crystallizability of the best-devitrified sample will be examined using SEM analysis (Fig. 5).
Table 2
Calculated and experimental Structural parameters of mixed polyanion glass-ceramic GC-NaCu(1−x)(VO)xPO4 (x = 0.1, 0.3, 0.5, 0.7, and 0.9 mol%) cathode powder mixtures
Cathode composition | Major crystalline phase | Ref | a (Å) | b (Å) | c (Å) | α (o) | β (o) | γ (o) | Volume [Å^3] | Crystal system/ Space group |
NaCu0.9 (VO)0.1PO4 | NaCu PO4 | Exp. | ICSD: 581303 | 4.85 | 7.31 | 9.84 | 90 | 90 | 90 | 348.73 | Ortho rhombic/ P2_12_12 1 |
Cal. | This work | 4.81 | 7.29 | 9.83 | 90 | 90 | 90 | 348.70 |
Δ(±) | | 0.04 | 0.02 | 0.01 | 0 | 0 | 0 | 0.03 |
NaCu0.7 (VO)0.3PO4 | Na2Cu P2O7 | Exp. | ICSD: 556822 | 5.07 | 13.35 | 9.87 | 90 | 117.89 | 90 | 590.62 | Monoclinic P21/C |
NaCu0.5 (VO)0.5PO4 | Cal. | This work | 5.06 | 13.32 | 9.85 | 90 | 117.89 | 90 | 590.59 |
NaCu0.3 (VO)0.7PO4 | Δ(±) | | 0.01 | 0.03 | 0.02 | 0 | 0 | 0 | 0.03 |
NaCu0.1 (VO)0.9PO4 | NaV2O5 | Exp. | ICSD: 760908 | 3.62 | 9.75 | 11.84 | 90 | 90 | 90 | 417.61 | Ortho rhombic/P2_12_121 |
| Cal. | This work | 3.59 | 9.71 | 11.77 | 90 | 90 | 90 | 417.59 |
Δ(±) | | 0.03 | 0.04 | 0.07 | 0 | 0 | 0 | 0.02 |
All mixed polyanion NaCu1 − x(VO)xPO4 glass and glass-ceramic cathode samples are pictured using a scanning electron microscope (SEM) to differentiate the variation in the structure of the surface of the sample before and after crystallization when heat treated at their Tc. The absence of grains in the microstructures of glass samples, which is consistent with the XRD findings, provides unmistakable proof of their glassy nature (Inset of Fig. 5). It is anticipated that the partial replacement of the tetrahedral VO4 by the octahedral CuO6 structural sites will be held accountable for the precipitation of crystalline phases such as NaV2O5 (ICSD 760908) NaCu(PO4) (ICSD 581303) and Na2Cu(P2O7) (ICSD 755492). On the other hand, numerous interconnected crystalline grains are visible after the glass sample has crystallized at its Tc. Further, it is extremely clear that the irregular shape of particles progressively transforms into a regular shape and from that point on, particles start to aggregate with a large amount of the remaining glassy phase (Fig. 5). The aggregation of the ideal particle size between 27 nm to 58 nm in the best GC-NaCu0.7(VO)0.3PO4 glass-ceramic sample supports to accommodate sudden volume changes, resulting to trigger the fast diffusion of Na+ ions in the glass-ceramic network leading to fast rate capability and voltage for longer durations[36]. These important findings helped to clarify the challenges and observe the highly ordered particle size distribution of the present mixed polyanion GC-NaCu1 − x(VO)xPO4 glass-ceramic cathode matrix to achieve best electrochemical performance.
3.3. Electronic-Structure Analysis
Structural illustration of major crystalline phases NaCuPO4, and Na2CuP2O7, indexed with Orthorhombic /P2_12_121 and MonoclinicP21/C, as presented in Fig. 6 (a&b). These structures are drawn using VESTA software along three crystallographic axis[37]. Herein, the structure of NaCuPO4, phase is slated with the interlocking between cornerly shared CuO6 octahedral units and tetrahedral PO4 units (Fig. 6a). On the other hand, structure of orthorhombic sodium copper pyrophosphate (Na2CuP2O7) phase is precipitated as a layered structure, by separating [Cu(P2O7)−2] slabs parallel with one another by Na+ ions (Fig. 6b). Wherein, there are two more observed observations are also found; i) ‘O’ atoms are shared between tetrahedral CuO4 units uniformly, ii) tetrahedral PO4 and CuO4 tetrahedral units are alternatively connected by these [Cu(P2O7)−2] layered slabs along [100] direction, and intra layer distance is obtained about 5.18Å offers 2D pathways with large open channels for the Na+ ion migration along b-axis in an ordered layer structure polymorph leading to for the fast Na+ ion migration via glass-ceramic network (Fig. 6b).
3.4 Transport Properties
The electrochemical transport of Na+ ions in the mixed polyanion GC-NaCu1 − x(VO)xPO4 glass-ceramic cathode network is calculated by the EIS investigation utilizing a frequency range of 0.1Hz-106 MHz and voltage (10 mV), recorded at room temperature. Nyquist plots are displayed using a semicircle from the high to mid-frequency domains and an angled line over the low-frequency domain (Fig. 7(a)). Bulk resistance (Rb) is obtained for each glass-ceramic sample by intercepting the Zreal axis at the high-frequency region of the semicircle.
Table 3
Charge transfer resistance (Rct), conductivity (σdc) and diffusion coefficient of the sodium ion (DNa+) of the of all mixed polyanion NaCu1 − x(VO)xPO4 glass-ceramic samples
Mixed polyanion glass-ceramic cathode | Charge transfer resistance (Rct)103Ω | Conductivity(σdc) 10− 4 S/Cm | DNa+ (Cm2S-1) |
GC-NaCu0.9(VO)0.1PO4 | 10.461 | 0.0121 | 9.872x10− 15 |
GC-NaCu0.7(VO)0.3PO4 | 7.086 | 0.179 | 7.62x10− 14 |
GC-NaCu0.5(VO)0.5PO4 | 8.832 | 0.1442 | 5.92 x10− 14 |
GC-NaCu0.3(VO)0.7PO4 | 9.55 | 0.1333 | 1.42 x10− 14 |
GC-NaCu0.1(VO)0.9PO4 | 9.975 | 0.1277 | 2.664x10− 15 |
At the electrode/electrolyte interface, the mid-frequency region of the semicircle displays charge-transfer resistance (Rct) (Table 3). Although the low-frequency incline line reveals the diffusion of Na+ ions in the solid electrode, the bulk conductivity (σb) of each glass-ceramic sample is calculated (σb = (t/A)*(1/R)), taking into consideration of dimensions as(area and thickness)[38, 39]. The Fig. 7(b) illustrates the charge transfer resistance (Rct) and electrical conductivity (σb) as a function of composition. The highest conducting GC-NaCu0.7(VO)0.3PO4 glass-ceramic sample (σb = 0.179x10− 4 S/cm)and had the lowest charge-transfer resistance (Rct = 7.086x103 Ω) and the strongest inverse correlation between Rct and σb (Fig. 7b). It is also crucial to understand that the Rct values are decreasing as a result of delays in the insertion or removal of the Na+ ion in the wider active centers of the glass-ceramic network, which typically happen when the vanadium concentration is raised from 0.1 to 0.3 mol%. Further it is evident that the stable NaV2O5, NaCu(PO4), and Na2Cu(P2O7) structures, when paired with increased electrical conductivity, would cause Na+ ion diffusion, hence enhancing the electron movement in the GC-NaCu0.7(VO)0.3PO4 glass-ceramic network[23](Fig. 8).
It is often established that XPS is a useful method for researching elemental surface states in oxide glass materials[40, 41]. Here, XPS spectra are used to analyse the elemental state of the best conducting glass-ceramic GC-NaCu0.7(VO)0.3PO4 cathode network, as illustrated in Fig. 8a. as shown in Fig. 8b the 2P3/2 and 2P1/2 states of Cu have binding energies of 952 eV and 932 eV, respectively; these peaks correspond to the Cu+ and Cu2+[42]. There are additional features that are located at energies of 941 eV. This is reasonably close for a Cu doublet as well, although I suspect that these could be paramagnetic satellites associated with the same species giving rise to the features at 933 and 952. Another possible interpretation is the presence of a second Cu species with a different oxidation state. Furthermore, by calculating ratios of normalized area (the area of peak divided by the sensitivity of instrument (0.48 eV)) of 2P1/2 and 2P3/2 states of Cu, we were able to determine the Cu2+/Cu+ molar concentration ratio. As demonstrated by the largest Cu+/Cu+ 2 ratio (0.98) in the XPS study (Fig. 8c), it is evident that there is every chance to create the strongest inductive effect of the mixed polyanions in the best conducting GC-NaCu0.7(VO)0.3PO4 glass-ceramic network during the process of heat treatment under ideal conditions at its Tc. For the optimum concentration of CuO (0.7 mol%), tetrahedral PO4 sites tend to replace partially with octahedral CuO6 sites, resulting to see the alteration in the density due to heavier density of CuO (6.4 g/cm3) than P2O5 (2.4 g/cm3) which will decrease the ‘Tg’ (glass transition temperature) and increase the modifying action of copper ions in the Na-VO-P glass-ceramic cathode network (Table 1 &Table 3). This is resulted to form, exceptionally flexible and wider active centers to trigger the fast intercalation and de-intercalation processes and optimum reduction potential due to mixed Cu+ 2/Cu+ states without compromising the stability of network[43] (Fig. 8c).
3.5. Electrochemical analysis
EIS is a vital technique for monitoring the redox reaction during the charge and discharge mechanism in both crystalline amorphous material networks. The charge and discharge patterns of all the mixed polyanion GC-NaCu1 − x(VO)xPO4 glass-ceramic cathode half cells are shown in Fig. 9(a to e), recorded at 0.1C. It is evident that all charge and discharge profiles include a plateau zone with a slope between 0 and 4.0 V, when measured at 0.1C, the first discharge and charge capacities of each glass-ceramic cathode sample recorded about 70.1, 85.5, 76.2, 72.5, and 66.3 mAhg− 1, and 49.8, 82.5, 75.2, 71.3 and 65.1 mAhg− 1, respectively. Similarly, corresponding discharge and charge capacities for the 100th cycle are recorded as 54.5, 82.2, 74.8, 64.8, and 59.5 mAhg− 1 and 45.70, 82.6, 70.4, 60.5, and 59.7 mAhg− 1, respectively. However, the discharge and charge capacities of the best-conducting glass-ceramic cathode sample are delivered to be 85.5, 82.2, 79.9, and 78.6 mAhg− 1, and 92.7, 82.6, 81.2, and 74.3 mAhg− 1 respectively, after 1st and 1000 cycles which are very close to theoretical limit (Table 4) (Fig. 10(a)).
Table 4
Summary of the charge/discharge capacity and reversible efficiency of all the mixed polyanion glass-ceramic cathodes
Mixed polyanion glass-ceramic cathode | First discharge capacity (mAh g− 1) | First charge capacity (mAh g− 1) | Capacity loss in the first cycle (mAh g− 1) | Reversible efficiency in the first cycle (%) | Discharge capacity at 100th cycle (mAh g− 1) | Charge capacity at 100th cycle (mAh g− 1) | Capacity loss after 100th cycle (mAh g− 1) | Reversible efficiency for 100th cycle (%) |
GC-NaCu0.9(VO)0.1PO4 | 70.1 | 49.8 | 20.3 | 59.3 | 54.5 | 45.7 | 8.8 | 83.9 |
GC-NaCu0.7(VO)0.3PO4 | 85.5 | 82.5 | 3 | 96.37 | 82.2 | 82.6 | 0.6 | 99.3 |
GC-NaCu0.5(VO)0.5PO4 | 76.2 | 75.2 | 1.0 | 98.7 | 74.8 | 70.4 | 4.4 | 93.8 |
GC-NaCu0.3(VO)0.7PO4 | 72.5 | 71.3 | 1.2 | 97.0 | 64.8 | 60.5 | 4.3 | 92.9 |
GC- NaCu0.1(VO)0.9PO4 | 66.3 | 65.1 | 1.2 | 98.2 | 59.5 | 59.7 | 3.3 | 96.7 |
Table 5
Summary of discharge capacity of all mixed polyanion glass-ceramic cathodes.
Mixed polyanion glass-ceramic cathode | Theoretical discharge capacity (mAhg− 1) | Exp. Discharge capacity(mAhg− 1) |
1st cycle(0.1C rate) | 500th cycle (0.1C rate) | Discharge capacity retention % | 1st cycle 5Crate | 500th cycle5Crate | Discharge capacity retention % |
GC-NaCu0.9(VO)0.1PO4 | 149 | 87.96 | 87.78 | 99.8 | 56.94 | 57.29 | 99.39 |
GC-NaCu0.7(VO)0.3PO4 | 148.2 | 90 | 82.08 | 91.2 | 61.73 | 61.86 | 99.79 |
GC-NaCu0.5(VO)0.5PO4 | 147.5 | 86.09 | 85.99 | 99.89 | 60.96 | 60.78 | 99.71 |
GC-NaCu0.3(VO)0.7PO4 | 145.4 | 84.03 | 83.76 | 99.68 | 53.094 | 52.918 | 99.67 |
GC-NaCu0.1(VO)0.9PO4 | 143.2 | 83.14 | 82.96 | 99.78 | 51.84 | 52.02 | 99.66 |
A crucial feature of this family of networks is the exceptionally low discharge capacity loss (4 mAhg− 1) even up to 3000 cycles (Fig. 10b). The four-channel migration routes of Na+ ions across the (NaV2O5, NaCu(PO4), and Na2Cu(P2O7)) phases are projected to reduce the path length for electrons, improving the electrical conductivity, for GC-NaCu0.7(VO)0.3PO4 best conducting glass-ceramic cathode network. As shown in Fig. 11(a, b) which charts the change in discharge capacity against various current rates, the discharge capacity of the best GC-NaCu0.7(VO)0.3PO4 glass-ceramic sample declines slowly from 0.1C to 10C rates, and almost recovered when the current rate bring back to 0.1C. Additionally, it has been shown that at higher current rates (10C), the best-conducting glass-ceramic cathode sample gives the highest retention of discharge capacity (99.79%) (Table 5). This might be due to the presence of state and conductive crystalline phases (NaCu(PO4), and Na2Cu(P2O7)) of the GC-NaCu0.7(VO)0.3PO4 glass-ceramic cathode network, which leads to the formation of 2D Na+ ion migration paths. The Significant capacity retention can be observed when the current rate is decreased from 10C to 0.1C, enhancing the cycle life stability and rate capability more than other crystalline compounds (Fig. 11(b)). However, the low electrical conductivity of the remaining glass-ceramic cathode samples may be directly related to the level of redox process purity, which ranges from lower (0.1C) to higher current rates (10C). At their middle-frequency range, the Nyquist curves of the best-conducting sample in Fig. 11(c) show only one semicircle, which is followed by a spike lasting even up to 1000 cycles, as determined at 0.1C. The diameter of the semicircles in Fig. 11(c) is used to obtain the lowest charge transfer resistance (Rct) from 1st to 1000 cycles which determines its best electro chemical performance. Figure 11(d) displays the cyclic voltammogram of GC-NaCu0.7(VO)0.3PO4 half-cell with a voltage range of 0.2 to 4 V at 100 V/s cycles. Note that the difference between the oxidation peak positions (2.10, 2.15, 2.19, 2.12, 2.20 V) and the reduction peak positions (1.69, 1.81, 1.83, 1.61, 1.67 V) is what causes cell polarization up to 1000 cycles[44]. The most cycle-life-capable and was found to have the lowest polarization even up to 1000 cycles results for its short Na+ ion diffusion migration via this cathode network (Table 6).
Table 6
Polarization for all the glass-ceramic samples
Mixed polyanion glass-ceramic cathode | Anodic Voltage (V) | Cathodic Voltage (V) | Polarization (V) |
GC-NaCu0.9(VO)0.1PO4 | 2.10 | 1.69 | 0.41 |
GC-NaCu0.7(VO)0.3PO4 | 2.15 | 1.81 | 0.34 |
GC-NaCu0.5(VO)0.5PO4 | 2.19 | 1.83 | 0.36 |
GC-NaCu0.3(VO)0.7PO4 | 2.12 | 1.61 | 0.51 |
GC-NaCu0.1(VO)0.9PO4 | 2.20 | 1.67 | 0.53 |
The existence of the previously mentioned conductive crystalline phases (NaCu(PO4) and Na2Cu(P2O7)) should be verified with the aid of XRD spectra up to 3000 cycles in order to confirm the cause for the long-term cycle stability and capacity of the GC-NaCu0.3(VO)0.7PO4 glass-ceramic cathode half-cell up to 3000 cycles (Fig. 12). It is clear from Fig. 12 that even against an amorphous background, both of these conductive crystalline phases NaCu(PO4) and Na2Cu(P2O7) remain present for up to 3000 cycles. As stated in the structural representation of the main crystalline phases in Fig. 6, these stable and conducting phases generously accommodate larger Na+ ions through their wider active centers without altering their original structure. This is further demonstrated by combining the structure and EIS studies that the best conducting GC-NaCu0.7(VO)0.3PO4 glass-ceramic cathode half-cell achieved the highest ionic diffusivity for longer durations (3000 cycles), which indicates its potential ability to store the electrical energy in significant quantities.