Introduction

Recent advancements have broadened research possibilities in battery technology, particularly for large-scale energy storage, revealing new avenues for innovation. The discovery of secondary batteries, particularly the rechargeable Li-ion batteries, led to a revolution in portable consumer electronics and electric vehicles1. The discovery of the Li-ion technology proffered batteries withhighcharge capacity, power density, and longer lifespan. However, the fast depletion of Li reserves on the earth’s crust, high cost, and uneven geographic distribution of lithium resources are the challenges in using Li-ion battery technology for energy storage in large-scale. Hence, the need for the development of safer, cost-effective and environment friendly alternatives like, Zn-ion batteries are on the surge2. However, the critical bottleneck of using the Zn-ion battery is the lower ion diffusion rate and the unavailability of high performance cathode materials to achieve reversible intercalation/de-intercalation3.

Two-dimensional layered materials are characterized by strong covalent bonds within the layers and weak van der Waals forces holding the layers together4,5. As a result of this non-bonding nature of interlayer interaction, foreign species can be inserted without heavily distorting the in-plane covalent bonds. Ion intercalation batteries working with a principle of the reversible intercalation/de-intercalation process of ions, making 2D layered materials suitable for energy storage applications. The intercalation of these layered materials with certain molecules like H2O, and CTAB (cetyltrimethylammonium bromide) increases the interlayer separation and results in the formation of ‘bulk monolayers’ which behaves like a monolayer even though existing in a stacked structure6,7. Such materials offer an easier and shorter diffusion pathway for the intercalation/de-intercalation process8. In addition, the high mechanical stability and electrochemical activity also make 2D materials suitable for ion intercalation batteries9.

α-V2O5 is an orthorhombic polymorph with its ab-layers connected along the c-axis through van der Waals interaction. The crystal structure of the V2O5 comprises three chemically inequivalent oxygen atoms; vanadyl (OI), bridge (OII), and chain (OIII) oxygen10. The layers are interconnected through OI and OII connects the double V–OIII chains (along the a-axis) to form the crystal structure along the b-axis. The layered crystal structure of V2O5, low-cost synthesis, less toxicity, and abundance on the earth’s crust make it a popular cathode material for ion intercalation batteries. The advantage of using V-based cathode materials for ion intercalation batteries is their ability for ‘multi electron transfer’ and high charge capacity, making a suitable host material for multi-valent ion intercalation alongside mono-valencies11. In addition, the abundance of this element on earth, different possible compositions, chemical structures, and electrochemical properties are also in favor. V2O5, a layered and stable material is known to exhibit ion intercalation/de-intercalation properties12. However, the development of a V2O5 based battery was challenging due to the phase transition of V2O5 to its polymorphs observed as a result of intercalation by molecules13. Hence, water-intercalated structures were developed forimprovedbattery performance by introducing an easier diffusion channel without inducing polymorphic phase transition14,15. However, owing to the difficulty in retaining the crystal structure during the synthesis of 2D oxides, the potential of 2D materials for ion intercalation battery applications has not been thoroughly researched.

In recent years, hybrid metal-ion batteries have gained attention as a potential replacement for single-ion batteries. Unlike conventional batteries, hybrid ion batteries utilize multiple types of metal-ions, such as Li, Zn, Na, and Mg, during charging and discharging, which sets them apart16,17,18. Generally, in a hybrid ion battery, the multi-valent cation undergoes deposition/dissolution on the anode and mono-valent cation intercalate/de-intercalate the lattice of cathode material16. As a result, hybrid ion batteries use dual salt electrolyteimproving the energy density and cycling stability16,19. Irrespective of the highest theoretical capacity (3860 mAh/g) of Li anode, the safety issues regulate the use of Li as anode material20. In addition, Li anode undergoes dendrite formation on the surface over recycling21. Whereas, Zn has higher volumetric capacity, abundance, smaller standard potential, low cost and toxicity22. But, the sluggish diffusion of Zn2+ into the host lattice generally results in lower discharge capacity and coulombicefficiency23. While using a hybrid ion battery offers the diffusion of lighter ions like Li+, using Zn as the anode, effectively improves the battery performance compared to Zn-ion battery. The hybrid metal-ion battery improvement in the discharge capacity, coulombic efficiency, and cycling stability24. The sole involvement of Li+ in the diffusion process in Zn/Li hybrid metal-ion battery due to the low diffusivity of Zn2+ is proven from the earlier operando X-ray diffraction studies23.While Zn/Li hybrid-ion batteries using porous V2O5 microplates as cathodes have demonstrated superior performance over conventional single-ion batteries19, the potential of ultra-thin 2D V2O5as cathode remains underexplored.The present study seeks to explore the applications of novel bilayer V2O5nanosheets in Zn-ion batteries and Zn/Li hybrid metal-ion battery systems.

Methods

Synthesis and characterization of 2D V2O5

Using chemical exfoliation technique, 2D V2O5 nanosheets were synthesized by controlling V2O5 concentration in the intercalating medium. Formamide is used as the intercalating agent following the previous studies25,26,27. For the synthesis, bulk V2O5 powder (Merck,99.99%) was added to formamide (Merck,99%) with a concentration of 1 mM. Following the insertion of formamide into the interlayer spaces of V2O5, which caused the crystal to swell. The dispersionwas then subjected to ultrasonic agitation at room temperature to exfoliate individual nanosheets from the bulk crystal. The suspension of exfoliated 2D V2O5 nanosheets was drop-coated onto carbon paper (used for creating the battery electrode) and subsequently heated at 100 °C to remove the formamidemolecules25.

The thickness of the exfoliated nanosheets deposited on a SiO2/Si substrate was measured, and the atomic force microscopy (AFM) topographic image was obtained using a multimode scanning probe microscope (NTEGRA, NT-MDT, Russia), operating in tapping mode. Atomic-resolution STEM images were captured using a probe aberration-corrected (Thermo Fisher Scientific, Themis Z) ultrahigh-resolution TEM at an acceleration voltage of 300 kV. X-ray photoelectron spectroscopy (XPS) analysis was conducted using a SPECS Surface Nano Analysis GmbH, Germany, system with a monochromatic Al Kα X-ray source (1486.7 eV). Raman spectroscopy was performed using an InVia instrument (Renishaw, UK) with a 532 nm Nd/YAG solid-state laser as the excitation source in back-scattering geometry. A thermoelectrically cooled charged-coupled device served as the detector, and an 1800 gr/mm grating was used for laser monochromatization. The optical band gap of the sample was determined through ultraviolet-visible (UV-Vis) absorption spectroscopy (Avantes) in absorption mode. Measurements were taken using a 1 cm wide cuvette to hold the sample in a reference medium, with Deuterium and Halogen lamps as the light sources.

Battery studies

A commercial potentiostat (PGSTAT 302 N, Metrohm Autolab e.v.) was used to investigate the battery performance of 2D V2O5 nanosheets at room temperature. The electrochemical cell for the aqueous Zn-ion battery was constructed using Zn metal as the counter electrode. The exfoliated V2O5 nanosheets were deposited onto carbon paper to create the working electrode. The carbon paper, with 2 cm length and 1.5 cm width, was uniformly coated with 3.6 ml of the sample dispersion solution (1 mM). The optical images of the bare carbon paper and 2D V2O5 coated carbon paper are shown in Figure S1a and b, respectively. The FESEM and EDS studies on the V2O5 coated carbon paper, showed a sufficient coating of 2D V2O5 on carbon (see supplementary Figure S2). From the amount of the coated solution, it can be calculated that, 0.648 mg of 2D V2O5 nanosheet sample was coated on the carbon paper. Aqueous solution of ZnSO4 with 1 M concentration is used as the electrolyte. The cyclic voltammogram (CV) was collected in the range of 0–2 V (V vs. Zn electrode). A schematic experimental arrangement is presented in the Fig. 1a.

Fig. 1
figure 1

The schematic diagram of (a) Zn-ion battery set-up (b) Zn/Lihybrid metal-ion battery set-up.

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The Zn/Li hybrid metal-ion battery with Zn metal anodewas made. Exfoliated V2O5 nanosheets on carbon paper was used as the cathode. Typically, a carbon paper with an area of 2 × 1.5 cm2was coated 4.8 ml of sample dispersion solution (1mM). Thus, the carbon paper was loaded with 0.864 mg of 2D V2O5 nanosheet sample. Zn(ClO4)2 with a 0.5 M concentration and LiClO4 with a 1 M concentration are combined to create a hybrid metal-ion electrolyte. The charging-discharging and cyclic voltammogram (CV) recording were performed in the range of 0.1–2.8 V (V versus Zn electrode) voltage range. Figure 1b shows the schematic of the experimental arrangement.

Results and discussion

Crystal tructure and morphology of chemically exfoliated V2O5

Atomic resolution scanning transmission electron microscopy (STEM) images of the exfoliated nanosheets were collected for understanding the atomic structure and chemical composition. Figure 2a and b show dark field (DF) STEM image and atomic resolution integrated differential phase contrast(iDPC)-STEM image of 2D nanosheets. The DF-STEM image (Fig. 2a) shows the unsupported 2D nanosheets. The atomic resolution iDPC image (Fig. 2b) illustrates the arrangement of the VO5 square pyramidal structure in α-V2O5 along the (001) plane.

Fig. 2
figure 2

(a) Dark field STEM image of the 2D V2O5 sample, and (b) the atomic resolution iDPC (Integrated differential phase contrast)-STEM image of 2D nanosheets. (c) Raman spectrum of 2D V2O5 nanosheets. (d) AFM topography of 2D V2O5 nanosheets. The height profiles of the nanosheets obtained from the marked positions in the AFM images and the distribution of nanosheetthicknessesare also revealed in the figure. (e) XPS survey spectrum of the 2D V2O5 nanosheets. (f) Core level spectrum of the V2p state.

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AFM studies were conducted on the exfoliated V2O5 sample to determine the thickness of nanosheets. An AFM image depicting the evenly dispersed layers can be seen in Fig. 2d. The Fig. 2d shows exfoliated nanosheets with lateral dimensions in the range of 100 to 300 nm. The step height is recorded using the experimental set-up with a standard deviation of 0.1 nm. According to the theoretical description of the crystal structure of monolayer, bilayer and, trilayer 2D V2O5, the layer thicknesses are ~ 0.43, 0.87 and, 1.3 nm, respectively25. The height profile of 2D V2O5 nanosheets (Fig. 2d) indicates that the experimentally measured thicknesses of the individual nanosheets fall between 1.0 and 1.5 nm. Figure 2d shows the thickness distribution of the nanosheets and it is evident that the ensemble of nanosheetsis majorly comprised of bilayer V2O5.

The Raman spectrum of the 2D V2O5 is collected to confirm the crystallographic phase of the material and is given in Fig. 2c. The sample exhibited nine vibrational modes corresponding to 2D V2O5. The theoretically predicted optic modes of monolayer V2O5 at the Γ point are, Γoptic = 3Au + 6B1u + 3B2u + 6B3u + 7Ag + 3B1g + 7B2g + 4B3g. Of these 39 vibrational modes, 21 are Raman active modes25. Out of the 21 Raman active modes, we have observed eight vibrational modes at 111 (A1g1), 162 (B1g1), 267 (B3g), 324 (B2g), 493 (A1g2), 539 (A1g3), 711 (B1g2), 1010 (A1g4) cm−1. The A1g1 and B1g1vibrational modes originate from the chain translation along crystallographic c and a-axes. The mode B3g is due to V‒OII‒V bending and deflection of V‒OI along a direction. The deflection of V‒OIII and V‒OIII′ along the c direction causes the vibrational mode B2g1. The origin of the A1g2 mode is the bending of V‒OII‒V in thec direction. The deflection of V‒OIII′ and V‒OIII along a direction results in the vibrational mode B1g2. The highest frequency A1g4 mode corresponds to the stretching vibration of V‒OI along the c-axis. This out-of-plane mode shows a considerable blue-shift in energy from the value corresponds to bulk V2O5, stems from the stiffening of V‒OI bond upon decrease in thickness25. The peak at 620 cm−1 is a surface mode that appears due to the presence of surface oxygen vacancies, leading to a local reduction of surface25,27.

The XPS survey spectrum of bilayer V2O5 is shown in Fig. 2e. The sample showed peaks at 517.4, 524.8, and 530.1 eV corresponding to V2p3/2,V2p1/2,and O1s, respectively, confirming the existence of V2O5phase28. The deconvolution of core level spectrum revealed two peaks at 517.4 and 516.3 eV(Fig. 2f), which correspond to the binding energy of V2p3/2 for V5+ and V4+ states, respectively.Similarly, 524.8 and 523.1 eV are the peaks characteristic of the binding energies of V2p1/2 for the V5+ and V4+ states, respectively28. The observation of the peaks corresponding to V4+ in XPS is an indicative of the presence of oxygen vacancy, which is also evident from the Raman spectrum. The oxygen vacancy defects in bilayer V2O5were calculated using the area under the curve, which is 17%. The surface activity and, hence, the catalytic property of the sample may be enhanced by the presence of these oxygen vacancy sites.

Aqueous Zn-ion battery using bilayer 2D V2O5 as cathode

Figure 3a displays the cyclic voltammogram (CV), recorded from the sample at a scanning rate of 10 mV/sec.The CV curves showed reduction and oxidation peaks at 0.56, 0.9 and1.1, 0.75 V, respectively. These peaks originate from redox reactions following the intercalation/de-intercalation of Zn2+ into the V2O5 nanosheets. When Zn2+ is intercalated into (the battery is discharged) the interlayer spacing of the nanosheets, the V5+ atoms in the crystal are reduced to V4+, whereas, de-intercalation (or charging) causes the reduced V4+ to oxidize to V5+.The half-cell reactions of the Zn-ion battery during the charging and discharging processes are as follows,

Fig. 3
figure 3

(a) The CV diagram of Zn-ion battery with 2D V2O5 nanosheet as cathode. (b) Variation of specific capacity and Coulombic efficiency with charge/discharge cycles number. The cyclic performance is studies for 1000 cycles with a current density of 4 A/g.

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In discharging process, Zn → Zn2++ 2e−(Anode reaction).

V(5+)2O5+ Zn2+ + 2e− → ZnV(4+)2O5(Cathode reaction).

For charging, ZnV(4+)2O5 → V(5+)2O5+ Zn2++ 2e−(Cathode reaction).

Zn2++2e− → Zn (Anode reaction).

If there are ‘z’ number of electrons transferred, the theoretical specific charge capacity (SCC) of a material is,

$$\:Q=\frac{zF}{M}\:$$
(1)

Where, F is the Faraday’s constant and M is the molecular mass of the cathode material. When two electrons are transferred during intercalation, the V5+ reduces to V4+ and the theoretical charge capacity is 295 mAh/g. When further intercalation of ions happens, V5+ undergoes reduction to V3+ (or V2+), and there are four (or six) electrons are transferred during charging/discharging process. The maximum theoretical SCC for V2O5 with z = 4 and z = 6 are 590 mAh/g and 875 mAh/g, respectively. For bulk V2O5 cathode, reduction to V3+ (or V2+) is less likely to occur, due to the possible structural instability. But the stiffening of several in-plane and out-of plane bonds as observed from the Raman spectrum of 2D V2O525 indicates that the structure can be more stable upon redox reactions. The stiffening of the bonds results from the less screening and more localized bonding, providing more structural stability and a possible deeper reduction. In addition to that, a highly recyclable catalytic performance and enhanced gas sensing with excellent recovery by bilayer V2O527,29 also indicates the stability of 2D V2O5 upon redox reactions. Figure 3b shows the corresponding change in the SCC with different current densities. The SCC of the battery is calculated from the charge/discharge profile using the formula,

$$\:Q=\frac{It}{m}\:$$
(2)

WhereI, t, and m are current and time of charging/discharging, and the mass of active material, respectively. The SCC decreases in the first few charge/discharge cycles before stabilizing after several cycles. The reduction in the SCC may result from the detachment of weakly bonded active material from the substrate. The exfoliated 2D V2O5 showed a discharge capacity of 263 mAh/g with a fast charging rate of 4 A/g. According to the existing studies, the Zn-ion battery with V2O5 under high current densities comparable to the present study showed a SCC of less than 200mAh/g. It is understood that the cathode material property determines the enhanced battery performance. The cathode material of 2D V2O5 nanosheets contains bilayer and trilayer nanosheets. The nanosheets thicknesses are in the 1.0-1.5 nm range. This material exhibits an optical band gap of 3.55 eV, as shown in supplementary Figure S3. Irrespective of the band gap value, electric conduction is possible for the material through an electron hopping mechanism in ab- plane of V2O5 with O-vacancy defects. Ion diffusion into the crystal results in the formation of reduced vanadium states to maintain charge neutrality, and as a result, electron-polarons are generated. The electron hopping between V2(+ 4)O4 and V2(+ 5)O5 (Polaron hopping) is least favorable along [010] direction30. Also, it is theoretically proven that the interlayer conductivity is lesser than the intralayer conductivity31,32.XPS studies confirm the existence of O-vacancy defects in the sample. The exfoliated 2D V2O5 shows a high electrochemically active specific surface area of 211 m2/g, which is ~ 30 times larger than that of bulk V2O529,33.

The Fig. 3b exhibits the SCC and columbic efficiency with the number of charge/discharge cycles. The value of coulombic efficiency (η) of a battery (the ratio of discharging and charging specific capacity) quantitatively indicates the number of Zn2+ that remains inside the active material in the duration of de-intercalation process. The Zn-ion battery showed a good η of ~ 96% during the first few cycles. The coulomb efficiency is observed to gradually increase from 96 to 99%, indicating that the active material forms a stable intercalation/de-intercalation characteristics after ~ 300 cycles. The columbic efficiency is observed to increase gradually from 96.7% to ~ 99% over first 300 cycles. From 500th cycle the columbic efficiency stayed consistently in the range of 98.5–100% with an average of 99%, where a capacity retention of ~ 85% is observed even with a lower columbic efficiency of 99%. The Zn2+ is a comparatively heavier ion with an ionic radius of 0.74Å. As a result, some of the intercalated ions form stronger chemical bonds with the host lattice and reside inside the crystal lattice of active cathode material during de-intercalation34. Hence, a lower columbic efficiency and capacity reduction is innate to Zn-ion batteries. A lower value of η is generally observed for Zn-ion batteries in comparison with Li-ion batteries, owing to the slow Zn2+desolvation and diffusion leading to poor Zn2+ kinetics23. Irrespective of this intrinsic limitation bilayer 2D V2O5 cathode showed a good η and cyclic performance as a result of an easier diffusion pathway, offered by the increased interlayer separation. In addition, the observations persuasively indicates the structural stability of the electrode material. The charge-discharge profiles for various cycles of the Zn-ion battery performance is provided in the Figure S4 in the supplementary information.

The useful lifetime of a battery is prevalently determined by its charge capacity retention. Capacity retention is the percentage of initial discharge capacity retained after a number of charge/discharge cycles. In the present study of Zn-ion battery using 2D V2O5, upto 500 cycles, a capacity retention of 53% is observed due to the gradual increase in the columbic efficiency from 96 to 99%. The next 500 cycles, showed an average columbic efficiency of 99% and capacity retention of 84%. The comparatively good capacity retention shown by the 2D V2O5-based Zn-ion battery indicates that the cathode material is highly stable under the intercalation/de-intercalation process.

Zn/Li hybrid metal-ion battery using bilayer 2D V2O5 as cathode

Figure 4a shows the cyclic CV corresponding to the Zn/Li hybrid metal-ion battery with a scanning rate of 10 mV/sec. The CV diagram evinced reduction peaks at 0.6, 0.9, and 1.2 V. The oxidation peaks were shown at 0.56, 1.1, 1.4, and 2.4 V. As, described earlier, the redox peaks are the implication of intercalation/de-intercalation of Li+ into the cathode material. The V5+ atoms in the crystal are reduced to V3+ (when the number of charges involved in intercalation/de-intercalation process, z = 4) or V4+ (for z = 2) when Li+ is intercalated into the crystal. Further, the reduced V3+ or V4+ ions are oxidized to V5+in the duration of de-intercalation process. From these information, one can represent the half-cell reactions of the battery for charging and discharging as33,

Fig. 4
figure 4

(a) The CV diagram of Zn/Li hybrid-ion battery. (b) The charge/discharge profiles of Zn/Li hybrid metal-ion battery and Zn-ion intercalation battery for the first cycle. (c) The specific charge capacity (SCC) and coulomb efficiency of Zn/Li hybrid metal-ion battery.

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In the discharging process, xZn → xZn2++ 2xe̶ (Anode reaction).

V2O5 + 2xLi++2xe̶→ Li2xV((5−x)+)2O5 (Cathode reaction).

For charging, Li2xV((5−x)+)2O5 → V2O5 + 2xLi++2xe ̶. (Cathode reaction).

xZn2++ 2xe ̶ → xZn (Anode reaction).

Figure 4b displays the charge/discharge characteristics for the Zn-ion and Zn/Li hybrid ion batteries. Equation 2 is used to determine the SCC of the battery.Along with the increase in operating voltage, Fig. 4b shows that, for Zn/Li hybrid metal-ion battery, the SCC has approximately doubled from that of the Zn-ion battery. This increase in specific capacity indicates a deeper reduction of V5+ to V3+ during ion intercalation. TheZn/Li hybrid metal-ion battery has a better power density compared to Zn-ion battery due to their higher operating voltage range, which is also a key parameter for characterizing the battery performance. The Fig. 4c shows SCC and ηof the battery as a function of the number of charge/discharge cycles. The augmented battery performance is because of the availability of Li+ for intercalation. Li+ is a lighter ion with an ionic radius of 0.76Å compared to the Zn2+ (ionic radius of Zn2+being 0.74Å). As a result, the ion diffusion into the crystal increases. With respective current densities of 5.7, 5.2, and 0.6 A/g, the specific charge capacities of the battery is determined to be 410.5, 450, and 510 mAh/g using Eq. 2. The SCC of the battery is higher with a lower current density. The observation is due to the enhanced interaction of the ion with the cathode materials at a slow charging rate (or lower current density). The working potential of the Zn/Li hybrid ion battery resembles that of a Li-ion battery with V2O5 cathode (Fig. 4b), which is higher than that of Zn-ion battery. A high working potential is necessary for achieving high energy density. The sole involvement of Li+in the intercalation/de-intercalation is proven by the XPS studies on the ion intercalated cathode material. The XPS survey spectrum shown in Fig. 5a did not show the signature of Zn2+ intercalation in the cathode material. The XPS peak at 56.2 eV in the high resolution spectrum (Fig. 5b) corresponded to the binding energy of Li1s (intercalated) state35, indicating the intercalation of Li-ion in V2O5. The very weak intensity of the peak is due to the low structure factor of Li. The working potential also indicates the intercalation of the Li+ in the cathode, not the Zn2+. The core level spectra of V are also studied to understand the changes in the valence states and the spectra are provided in the Figure S5 in the supplementary information. The analysis showed that the cathode material undergone reduction upon intercalation. The specific capacities from prior literature with comparable current densities (which is a measure of the charging/discharging rate) are compared with the present findings (Fig. 6).The comparison shows the specific capacity of the Zn/Li hybrid metal-ion battery is notably higher comparing to the vanadium oxide-based Zn-ion batteries with a fast charging current density of 0.6 mAh/g.

Fig. 5
figure 5

(a) XPS survey spectrum of the ion intercalated cathode of Zn/Li battery. (b) High resolution XPS spectrum of Li.

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Fig. 6
figure 6

Raman spectra of completely de-intercalated 2D V2O5 cathode after 1000 charge/discharge cycles of a) Zn/Li hybrid-ion and b)Zn- ion battery, respectively.

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The η is calculated as described in the previous section of the Zn-ion battery. Figure 4c shows the values of η with increasing number of charge/discharge cycles. The charging-discharging upto 1000 cycles are carried out with the Zn/Li hybrid ion battery. The charge/discharge process is carried out with a current density of 5.7 A/g. A high ηof around 99% was demonstrated by the battery from the first cycle upto the 1000th cycle. The lighter Li ion generally shows high ηderived from the easier intercalation/deintercalation compared to other metal-ions50. Thus, a hybrid metal-ion battery improves all of specific charge-capacity along with ηand cyclic stability. In addition, as it was evident from the Zn-ion battery study, bilayer V2O5 cathode offers an energetically favorable diffusion pathway improving intercalation/de-intercalation process. An ex-situ structural characterization of the 2D V2O5 coated carbon paper was done using Raman spectroscopy, after 1000 charge/discharge cycles of Zn-ion and Zn/Li hybrid ion battery. The Raman spectra of 2D V2O5 after 1000 charge/discharge cycles of Zn/Li hybrid ion and Zn-ion battery are shown in Fig. 7a and b, respectively. It can be elucidated from the results that the 2D V2O5 exceeds bulk limits due to enhanced kinetics and structural stability. The charge-discharge profiles of the Zn/Li hybrid metal-ion battery for several cycles are shown in Figure S6 in the supplementary information.

Fig. 7
figure 7

Comparison of the present finding with the literature19,34,36,37,38,39,40,41,42,43,44,45,46,47,48,49. The comparison is made with the specific capacity and the operational voltage of the Zn-ion battery and the Zn/Li hybrid metal-ion battery using related materials of V2O5.

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Considering the discharging specific capacity of the Zn/Li hybrid ion battery, it showed an 89% capacity retention up to 14 cycles. After the 15th cycle, up to the 214th cycle, the SCC showed 98% retention and after the 215th cycle until the 1000th cycle the battery showed capacity retention of ~ 100%. A similar decrease in SCC observed in Zn-ion battery is also shown by Zn/Li hybrid metal-ion battery. The observations further supports the high structural stability upon intercalation process, as observed in the case of Zn-ion battery.

Conclusions

The bilayer 2D V2O5 nanosheets were tested for ion intercalation battery application. As the cathode in a Zn-ion battery, the two-dimensional V2O5nanosheets delivered a specific charge capacity of 263 mAh/g and supported fast charging at 4 A/g, showing a capacity retention ~ 50% over 1000 cycles. An increase in the interlayer separation, resulting with the increase in the bilayer thickness formed in the chemical exfoliation process provided an easier conduit for ionic diffusion. The charge transport through the V2O5 cathode is envisaged to be manifested through the polaron hopping mechanism. Additionally, the bilayer V2O5 nanosheets served as cathode material in a hybrid metal-ion battery, where Zn ions participated in dissolution (or deposition) while Li ions were involved in intercalation (or de-intercalation). The Zn/Li hybrid metal-ion battery demonstrated a notable specific capacity of 510 mAh/g at a charging rate of 0.6 A/g. By incorporating lighter Li⁺ ions, the battery exhibited excellent cycling stability, maintaining a high coulombic efficiency (η) of approximately 100% across 1000 charge/discharge cycles. It is observed in the present study that the novel bilayer V2O5 is stable under a deeper reduction and showed high-performance as the cathode material, which has high SCC with fast charging and long battery life.