A POM@CNT hybrid nanostructure enabling fast kinetics and high capacity in lithium-ion batteries†

Eman Gul ^ab, Zeeshan Haider ^ac, Tanveer Hussain Bokhari ^c, Mashkoor Ahmad ^a, Gul Rahman *^b and Amjad Nisar *^a
aNanomaterials Research Group, PD, PINSTECH, Islamabad, 44000, Pakistan. E-mail: chempk@gmail.com
^bInstitute of Chemical Sciences, University of Peshawar, Peshawar, 25000, Pakistan. E-mail: gul_rahman47@uop.edu.pk
^cDepartment of Chemistry, Government College University, Faisalabad, 38000, Pakistan

First published on 25th June 2025

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Introduction

Lithium-ion batteries (LIBs) have emerged as the cornerstone of modern energy storage systems dominating the market, owing to their high energy density, excellent cycling stability, and reliability.^1,2 With the continually rising demand for advanced portable electronics, renewable energy systems, and electric vehicles, the typical graphite anode, with a limited specific capacity (372 mAh g⁻¹) and slow lithium-ion (Li⁺) intercalation/deintercalation kinetics, struggles to keep pace with the ever-growing energy requirements.^3–7 Therefore, there is an imperative need for innovative materials to elevate LIBs’ performance.

Molecular clusters are becoming attractive options to achieve high capacity and stable performance in LIBs.^8–10 Among molecular clusters, polyoxometalates (POMs) are promising electrode materials for LIBs.^11–13 POMs act like electron sponges, enabling the reversible transfer of multiple electrons, and functioning as a semiporous molecular capacitor, making them excellent for energy storage.^14–16 Despite these advantages, POMs face several challenges, such as low conductivity between clusters and their high solubility in electrolytes during Li⁺ intercalation/deintercalation processes. To address these challenges, the integration of POMs with conductive substrates has proven to be a promising strategy, effectively enhancing their electrochemical performance while mitigating inherent drawbacks.^17–20 This approach holds significant potential for advancing the application of POM-based materials in next-generation energy storage systems.^21,22

In recent years, researchers have increasingly explored the synergy between POMs and nanocarbons to develop molecular composites aimed at tackling global energy challenges.²³ In particular, carbon nanotubes (CNTs) hold great promise as conductive frameworks due to their exceptional electrical conductivity, remarkable mechanical strength, and large surface area.²⁴ Consequently, the molecular dispersion of redox-active POMs onto CNTs offers a compelling strategy for designing high-performance anode materials with enhanced capacity and structural stability during Li⁺ intercalation/deintercalation. As a result, numerous studies have paid attention to hybridizing POMs with CNTs to obtain high-performance anode materials.¹⁸ For example, Kawasaki et al. have introduced the concept of molecular cluster batteries (MCBs) for electrochemical energy storage systems.²⁵ They reported the attachment of POM clusters with single-walled carbon nanotubes (SWNTs) by electrostatic interactions. The resulting material, used as a cathode, exhibited a specific capacity of 320 mAh g⁻¹. Ma et al. reported a covalently tethered SiW₁₁–pyrene hybrid combined with SWNTs via π–π interaction. The obtained SWNT/Py–SiW₁₁ composite showed a discharge capacity of 580 mAh g⁻¹.²⁶ Song et al. non-covalently anchored POM clusters onto CNTs and evaluated the resulting composites as anode materials for lithium-ion batteries (LIBs). The composites exhibited enhanced performance compared to their individual components, owing to synergistic effects.^27,28 Hu et al. reported a composite prepared by in situ polymerization of polyaniline (PANI) on multi-walled carbon nanotubes (MWCNTs). The redox-active (PMo₁₂) was then immobilized on PANI/MWCNT.²⁹ The PMo₁₂/PANI/MWNT nanocomposite showed an excellent discharge capacity of 1000 mAh g⁻¹ for over 100 cycles. Iqbal et al. prepared POM-based ionic crystals, which then interacted with SWNTs resulting in an SWNT nanocomposite.³⁰ The prepared composite delivers a discharge capacity of 1020 mAh g⁻¹ for 100 cycles. Among various POMs, H₃PMo₁₂O₄₀ (PMo) is an effective material for applications such as energy storage, due to its high redox activity, and electrochemical tunability.

Herein, we report an amphiphilic polyoxometalate–carbon nanotube hybrid nanostructure (POM@CNT), which is successfully synthesized via a simple and facile method based on electrostatic interaction between PMo Keggin nanoclusters and functionalized CNTs for high-performance lithium-ion batteries (LIBs), as illustrated in Fig. 1. The amphiphilic nature arises from the combination of hydrophilic PMo nanoclusters and hydrophobic long-chain surfactant molecules, which assemble on the functionalized CNT surface. This unique configuration significantly enhances the electrochemical activity, delivering a high initial discharge capacity of 2100 mAh g⁻¹. The nanostructure exhibits outstanding electrochemical performance as a LIB anode, demonstrating its potential for advanced energy storage applications.

Results and discussion

The FTIR and Raman spectra of POM@CNT, PMo, and CNTs confirm the successful immobilization and integration of PMo with functionalized CNTs (Fig. 2a). In the FTIR spectrum, CNTs exhibit characteristic peaks, including a broad band at ∼3400 cm⁻¹ (O–H stretching) and a peak at ∼1700 cm⁻¹ (CO stretching), confirming their chemical modification.²⁹ For PMo, the distinctive Keggin structure is evidenced by peaks at 1064 cm⁻¹ (P–O asymmetric stretching), 992 cm⁻¹ (MoO stretching), 872 cm⁻¹ (Mo–O_s–Mo, corner-sharing), and 792 cm⁻¹ (Mo–O_e–Mo, edge-sharing).^31–33 POM@CNT retains these characteristic peaks, confirming the presence of PMo. Additionally, shifts in the functional group peaks of CNTs were observed, which may be attributed to their interaction or bonding with PMo. The stretching vibrational absorption bands of C–H at 2852 and 2923 cm⁻¹, along with the C–N absorption band at 1487 cm⁻¹, are attributed to the presence of cetyltrimethylammonium bromide (CTAB).^34,35 These spectral features, together with the observed shifts, confirm the successful integration of PMo onto the functionalized CNTs. The detailed peak assignments are listed in Table S1 (ESI†).

The Raman spectra provide significant insight into the structural and chemical features of the materials (Fig. 2b). The CNTs exhibit a prominent peak at approximately 1373 cm⁻¹ (D-band) that signifies defects during functionalization, while the peak at 1605 cm⁻¹ (G-band) corresponds to the graphitic lattice vibrations.^36,37 The characteristic peaks observed at 822 and 992 cm⁻¹ are attributed to M–O bond vibrations in the PMo structure.³⁸ These peaks confirm the Keggin structure of PMo. In the prepared POM@CNT hybrid structure, the bands of both CNTs and PMo are retained, confirming the effective integration and synergistic interactions between PMo and CNTs. The Raman spectrum complements the FTIR findings by highlighting vibrational modes associated with graphitic structures and functionalized groups.

The XRD measurements illustrate the characteristic patterns of POM@CNT, CNTs, CPMo, and PMo (Fig. S1, ESI†). The CNTs exhibit a prominent peak around 2θ ≈ 26.2° corresponding to the (002) plane of graphitic carbon. Plane (100) at 2θ ≈ 42° corresponds to the in-plane structural order of carbon atoms within each graphene layer. This peak represents the interlayer spacing of graphene sheets, indicating the crystallinity of the CNT structure.³⁹ For PMo, distinct sharp peaks are observed at specific 2θ, indicating its unique crystalline structure. These peaks arise from the ordered atomic arrangement in PMo, indicating its crystallinity.³³ The amphiphilic compound (CTA)₃PMo₁₂O₄₀ (CPMo), formed by the interaction of the cationic surfactant CTAB with PMo, exhibits an amorphous nature, as confirmed by the absence of characteristic PMo XRD peaks. This suggests effective dispersion of PMo within the surfactant matrix. Similarly, POM@CNT also displays an amorphous structure, attributed to the presence of CTA, which facilitates uniform dispersion of POMs on the CNT surface and prevents aggregation of the active material. The homogeneous distribution enhances interfacial contact between the POMs and CNTs, facilitating efficient charge transfer and improving electrochemical performance. Additionally, the amorphous structure provides greater structural flexibility, enabling better accommodation of volume changes and mechanical strain during repeated charge–discharge cycles, thereby enhancing lithium-ion storage capability. For POM@CNT, the (002) peak of the CNTs becomes broader and less intense, implying a disruption in CNT crystallinity due to interaction with PMo. Hence, the XRD results also support FTIR and Raman data, indicating structural changes caused by the strong interaction between PMo and CNTs.

A detailed characterization of the POM@CNT hybrid nanostructures is shown in Fig. 3. The TEM images (Fig. 3a and b) of POM@CNT reveal a uniform dispersion of PMo on the surface of functionalized CNTs. This also indicates the successful functionalization of CNTs, as their tubular structure is visible, while PMo appears as discrete clusters. The SEM images (Fig. 3c and d) of the POM@CNT nanostructures show the rough surface indicating a uniform dispersion of PMo on CNTs. SEM supports the TEM results, indicating a strong interaction between PMo and CNTs. Fig. 3e shows an SEM image of pristine CNTs. CNTs appear in an agglomerated and tangled form. Fig. 3f shows an SEM image of acid-treated CNTs. The CNTs exhibit clear, smooth, and detangled surfaces. The EDX spectra coupled with TEM and SEM provide elemental composition analysis of the POM@CNT composite. Peaks corresponding to Mo, O, P, and C further validate the presence and successful integration of PMo with the CNTs (Fig. 3g and h).

The X-ray photoelectron spectroscopy (XPS) analysis of the POM@CNT composite provides valuable insights into its elemental valence state and chemical bonding. The survey scan of the POM@CNT composite confirms the presence of Mo(3d), P(2p), C(1s), and O(1s) in the expected structure of the prepared composite, as illustrated in Fig. 4 and Fig. S2 (ESI†). To further investigate the oxidation states, the narrow scan of the Mo(3d), P(2p), C(1s), and O(1s) has been collected, as shown in Fig. 4(b–d) and Fig. S2 (ESI†).⁴⁰ The deconvoluted spectrum of the Mo 3d region reveals the two distinct peaks corresponding to Mo⁶⁺ oxidation states at binding energies around 236 eV (3d_5/2) and 233 eV (3d_3/2), indicating the presence of Mo oxides. Similarly, the C(1s) spectrum exhibits peaks near 284–290 eV, which can be attributed to various carbon bonds, i.e., sp²-hybridized CC, sp³-hybridized C–C bonds, and carbonyl (CO) groups from CNTs, highlighting the functionalization of CNTs or interaction with the POM structure.⁴⁰ The O(1s) scan further supports these observations, showing peaks at 532–534 eV corresponding to MoO and Mo–O bonds from Mo oxides and C–O bonds, likely arising from functional groups on the CNTs. These features confirm the integration of PMo onto CNTs. Additionally, the high-resolution spectrum of the P(2p) region shows a peak in binding energy at around 133 eV, Fig. S2 (ESI†). This confirms the presence of the P within the composite, which is a component of the POM structure, which further supports well-defined bonding environments for the constituent elements. These results are in good agreement with complementary XRD (Fig. S1, ESI†) measurements, which reinforce the structural integrity and successful synthesis of the composite.

Following the detailed structural characterization, the POM@CNT composite was explored as a potential anode material for LIBs. Fig. 5a presents the cyclic voltammetry (CV) profiles of the POM@CNT anode for the first four cycles, measured between 0.1 and 3.0 V (vs. Li⁺/Li) at a scan rate of 0.3 mV s⁻¹. The redox peaks at 1.1 and 1.56 V correspond to the reduction and oxidation of PMo clusters in the composite. The broad reduction peak near 1.1 V in the first cycle is attributed to the solid electrolyte interphase (SEI) formation, which stabilizes the electrode surface and diminishes in subsequent cycles. Notably, the overlapping CV curves of the 2nd, 3rd, and 4th cycles demonstrate excellent electrochemical reversibility and structural stability of POM@CNT during Li⁺ ion insertion/extraction. During the 1st cathodic scan at around 1.1 V, Li⁺ ions are inserted into the PMo clusters, forming the Li_xPMo₁₂O₄₀ complex, as shown in eqn (1).

PMo12O40 + xLi+ + xe− → LixPMo12O40	(1)

During the anodic peak at around 1.56 V, Li⁺ ions are extracted from the PMo clusters, restoring PMo to its original state, as shown in eqn (2).

LixPMo12O40 → PMo12O40 + xLi+ + xe−	(2)

The galvanostatic charge–discharge (GCD) profile of the POM@CNT composite was collected in the voltage range of 0.1–3.0 V (versus Li⁺/Li). Fig. 5b demonstrates the GCD curves for the 1st, 2nd, and 3rd cycles of POM@CNT hybrid at a current density of 0.1 A g⁻¹. The curves display a sloping voltage profile, which is ascribed to the intrinsic lithium storage mechanisms of the hybrid structure. Unlike conventional graphite, which shows a flat plateau due to a well-defined two-phase intercalation reaction, the PMo stores lithium via complex multi-step redox reactions. These redox processes occur over a range of potentials, resulting in a sloping curve. Additionally, the CNT framework contributes lithium storage through surface adsorption and interlayer interactions, which also exhibit non-plateau behavior. The POM@CNT composite shows in the 1st cycle a high initial discharge capacity of 2100 mAh g⁻¹ and charge capacity of 902 mAh g⁻¹, with coulombic efficiency of 43%. The low initial coulombic efficiency (43%) is primarily due to irreversible processes during the first cycle, such as solid electrolyte interphase (SEI) formation and electrolyte decomposition catalyzed by the redox-active PMo clusters, which irreversibly consume lithium ions. After the 1st cycle, the POM@CNT composite exhibited reversible capacities of 968 mAh g⁻¹. The synergistic interaction between CNTs and PMo promotes the intercalation and deintercalation of Li-ions and electrons. Fig. S3 and S4 (ESI†) show the galvanostatic charge/discharge profiles for the 1st, 2nd, and 3rd cycle of POMs and CNTs at a current density of 0.1 A g⁻¹. The PMo electrode shows in the 1st cycle an initial discharge capacity of 1300 mAh g⁻¹ and charge capacity of 610 mAh g⁻¹, while CNTs show an initial discharge capacity of 1065 mAh g⁻¹ and charge capacity of 420 mAh g⁻¹.

Fig. 5c evaluates the rate performance of POM@CNT, PMo, and CNT electrodes at current densities of 0.1, 0.2, 0.3, 0.5, 1, 2, and 3 A g⁻¹. The reversible capacities decrease with an increase in current densities. In comparison to PMo and POM@CNT, the CNT electrode shows insignificant reversible capacities of 441, 311, and 265 mAh g⁻¹ at current densities of 0.1, 0.2, and 0.3 A g⁻¹, respectively. The PMo electrode reversible capacities are also shown in Fig. 5c. The POM@CNT hybrid nanostructures exhibit a stable and superior capacity of 880 mAh g⁻¹ at a low current density of 0.1 A g⁻¹. When the current density increases to 3 A g⁻¹, the hybrid structure shows the reversible capacity of 608 with 63% capacity retention. This high capacity retention indicates the excellent rate capabilities of the POM@CNT electrode, highlighting its high Li⁺ diffusion and effective electron transport within the electrode matrix, which can be attributed to an interaction between PMo with CNTs. When the current density was reduced back to 0.2 A g⁻¹, the POM@CNT composite nearly regained its original reversible capacity. Fig. S5 (ESI†) shows the cycling performance of POM@CNT, PMo, and CNTs at a current density of 0.2 A g⁻¹, which is the extended data of Fig. 5c. Among all the electrodes, POM@CNTs still possess and maintain their specific capacity of 820 mAh g⁻¹ at a current density of 0.2 A g⁻¹, followed by PMo and CNTs, which display lower capacities. POM@CNTs maintained their cycling stability over repeated cycles compared to PMo and CNTs.

To assess the long-term cycling stability of the POM@CNT electrode, GCD measurements were carried out for 500 cycles at a high current density of 1 A g⁻¹ (Fig. 5d). The electrode delivered an initial discharge capacity of 1850 mAh g⁻¹, demonstrating its high lithium-ion storage capability. The specific capacity stabilized at 570 mAh g⁻¹ by the 15th cycle but then increased to 950 mAh g⁻¹ after 250 cycles. The initial decrease in capacity during the first 15 cycles can be attributed to the formation of the solid electrolyte interphase (SEI) layer, electrolyte decomposition, and structural stabilization of the POM@CNT hybrid nanostructure electrode. Moreover, incomplete wetting of the electrode also contributes to the capacity loss. However, from cycle 15 to around cycle 250, a gradual increase in capacity suggests an electrochemical activation process, likely due to enhanced Li⁺ ion diffusion and improved electrolyte penetration into the porous structure of the POM@CNT hybrid. The structural synergy between the redox-active PMo and conductive CNTs contributes to better charge transport and increased utilization of active sites. This activation behavior highlights the robust structural integrity and adaptability of the hybrid electrode under prolonged cycling. Beyond 250 cycles, however, the capacity gradually declined, reaching 281 mAh g⁻¹ after 500 cycles, likely due to progressive electrode degradation under prolonged cycling conditions. It is noteworthy that although the POM@CNT electrode exhibits a relatively low initial coulombic efficiency, it demonstrates stable efficiency and reversible capacity in subsequent cycles, indicating strong potential for long-term electrochemical performance.

To investigate the electrochemical kinetics of the fabricated electrodes (POM@CNT, PMo, and CNTs), electrochemical impedance spectroscopy (EIS) was conducted over a frequency range of 0.01 Hz to 100 kHz. Fig. 6 presents the Nyquist plots for all three samples. The impedance spectra exhibit a depressed semicircular arc in the high-frequency region, corresponding to the charge transfer resistance (R_ct) and electrolyte resistance (R_s), followed by a ∼45° inclined line attributed to Warburg impedance (Z_w), which reflects solid-state diffusion of lithium ions in the active material. Notably, the POM@CNT exhibits a smaller semicircle radius compared to PMo and CNTs, indicating lower R_ct and enhanced charge transfer kinetics. The constant phase element (CPE) was used to account for the non-ideal capacitive behaviour, and the capacitance was calculated using the relation:

C = R(1−n)/n(CPE)1/n

where C is capacitance, R is resistance, and n represents the deviation from ideal Debye behaviour. The equivalent circuit was used to model the lithium coin cell (inset, Fig. 6). The fitted circuit elements confirm that the experimental data align well with this model.

The corresponding fitting parameters for all three electrodes, as obtained from EIS measurements, are summarized in Table S2 (ESI†). The smaller semicircle diameter of the POM@CNT electrode indicates a low R_ct value (53.6 Ω), suggesting more efficient Li⁺ ion transport during electrochemical reactions. These results demonstrate that the synergistic interaction between PMo and CNTs facilitates faster Li⁺ ions diffusion during the charge/discharge processes, which significantly improves the electrochemical kinetics and overall performance of the anode material in LIBs.

To understand the experimental results, we have also performed DFT calculations. Fig. 7 depicts energy level alignment at the CNT/Keggin (POM@CNT) interface. The work function of multiwall CNTs is much lower than the lowest unoccupied molecular orbital (LUMO) of the Keggin molecule. Owing to favourable energy alignment, charge transfer is expected from the CNTs to the Keggin molecule (Fig. 7). To further understand the impact of interfacial charge transfer on redox reaction kinetics, Fig. 8 shows the charge density at the LUMO of the Keggin molecule. Interestingly, the charge is delocalized over the whole of the Keggin molecule. Therefore, owing to the enhanced charge density of the Keggin molecule due to charge transfer and the delocalized nature of its LUMO, redox reaction kinetics will be superior for the CNT-Keggin heterostructure as compared to either the pristine Keggin molecule or functionalized CNTs. This is indeed confirmed by impedance spectroscopy (Table S2, ESI†) where the CNT-Keggin heterostructure has the lowest charge transfer resistance as compared to either the functionalized CNTs or Keggin molecule. We further studied the adsorption of Li on Keggin and the CNTs. The adsorption energy of Li on the CNTs is found to be ∼1.50 eV. In contrast, the adsorption energy of Li on Keggin is significantly larger (∼4.50 eV) for the optimum Mo–O quadrate position. Besides, calculations showed that 3 H atoms of Keggin molecules were easily replaced by Li atoms with an energy gain of 0.6 eV per H atom.

These results highlight the synergetic impact of at CNT/Keggin heterostructure. Since Li adsorption is more favourable on Keggin as compared to CNT, thus the Li adsorption capacity of the CNT/Keggin heterostructure will be superior as compared to the pristine CNTs. This is indeed confirmed by specific capacity measurements (Fig. 5c) where the POM@CNT heterostructure provides the highest values of specific capacity. Moreover, interfacial charge transfers from CNTs in the delocalized frontier orbital of the Keggin molecule will improve the redox reaction kinetics of the CNT/Keggin heterostructure as compared to either pristine Keggin or CNTs. Overall, these results are in good agreement with the experimental findings.

The electrochemical performance of various POM-based materials reported in the literature is compared with the POM@CNT hybrid synthesized in this work (Table 1). The POM@CNT hybrid surpasses most reported POM-based materials, exhibiting a higher initial discharge, excellent cycling stability (645 mAh g⁻¹ over 350 cycles at 1.0 A g⁻¹), and superior rate capability (880 mAh g⁻¹ at 0.1 A g⁻¹). This enhanced performance stems from strong electrostatic interactions between POMs and functionalized CNTs, ensuring uniform dispersion, efficient charge transport, and structural stability, leading to improved capacity and cycling durability. These synergistic effects result in superior capacity, rate performance, and cyclic stability.

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Experimental

Chemicals and reagents

Phosphomolybdic acid (H₃PMo₁₂O₄₀) was purchased from Panreac. Nitric acid (HNO₃), sulfuric acid (H₂SO₄), and cetyltrimethylammonium bromide (CTAB) were brought from AVONCHEM. Chloroform and ethanol were purchased from AnalaR. Multiwall CNTs were made by Tsinghua University, Beijing, China. All materials were of analytical grade and were used without further refinement. De-ionized (DI) water was used throughout the experimentation.

Functionalization of carbon nanotubes

Purified multiwall carbon nanotubes (CNTs, 100 mg) were dispersed in 40 mL of a mixed acid solution consisting of concentrated H₂SO₄ (98%) and HNO₃ (65%) in a 1:3 (v/v) ratio. The mixture was first sonicated for 15 min to ensure uniform dispersion and initial functionalization. Subsequently, the reaction mixture was refluxed at 100 °C for 2 h under continuous magnetic stirring to promote oxidative surface modification of the CNTs. After the reaction, the system was cooled to room temperature (∼25 °C), and the acid-treated CNTs were repeatedly washed with deionized water via centrifugation until the supernatant reached a neutral pH (∼7.0), ensuring complete removal of residual acids and by products. Finally, the functionalized CNTs were dried in a vacuum oven at 80 °C for 2 h to eliminate any remaining moisture.⁴⁷

POM amphiphile formation

The amphiphilic compound (CTA)₃PMo₁₂O₄₀ (CPMo) was synthesized through a two-phase reaction between cetyltrimethylammonium bromide (CTAB) and phosphomolybdic acid (H₃PMo₁₂O₄₀). In a typical procedure, 1.09 g of CTAB was dissolved in 15 mL of chloroform (solution A) while 1.9 g of H₃PMo₁₂O₄₀ was dissolved in 15 mL of deionized water (solution B). Solution B was then added dropwise over 10 minutes to the magnetically stirred (500–700 rpm) solution A at room temperature (25 °C). As the reaction progressed, the initially transparent biphasic mixture gradually transformed into an opaque yellow-milky emulsion, indicating the successful formation of the amphiphilic CPMo complex at the organic–aqueous interface through ionic interaction between CTA⁺ cations and PMo₁₂O₄₀³⁻ anions. The reaction was allowed to proceed under continuous stirring for 1 hour to ensure completion. After phase separation, the colourless aqueous supernatant was decanted, and the chloroform phase containing the product was left to evaporate under ambient conditions for 24 hours, yielding greenish crystalline solids. The final product was vacuum-dried overnight at 50 °C.

POM@CNT hybrid nanostructure preparation

In a typical synthesis, 30 mg of functionalized CNTs was dispersed in 20 mL of ethanol via ultrasonication for 1 hour (D1). Separately, 45 mg of (CTA)₃PMo₁₂O₄₀ was dispersed in 20 mL of ethanol using ultrasonication for 30 minutes (D2). The D2 dispersion was then added to D1 under continuous stirring, which was maintained for an additional 1 hour. The resulting CNT/(CTA)₃PMo₁₂O₄₀ hybrid nanostructure (POM@CNT) was collected by centrifugation and washed twice with ethanol. The product was first dried at 50 °C for 24 hours in a hot air oven, followed by overnight drying at 50 °C in a vacuum oven.

Material characterization

X-ray powder diffraction (XRD) patterns were recorded using a Rigaku D/MAX 2500 diffractometer equipped with Cu Kα radiation (λ = 1.5406 Å) over a 2θ range of 20° to 80°. The morphology and microstructure of the samples were examined by field-emission scanning electron microscopy (FE-SEM) using a TESCAN MIRA A3 system equipped with energy-dispersive X-ray spectroscopy (EDX). Transmission electron microscopy (TEM) was carried out on a JEOL JEM-2100F microscope operated at 200 kV. Fourier transform infrared (FTIR) spectra were collected using a Nicolet iS50 spectrometer in the range of 400–4000 cm⁻¹. Raman spectra were acquired using a HORIBA Scientific XploRA PLUS spectrometer coupled to an Olympus BX43 confocal microscope. X-ray photoelectron spectroscopy (XPS) measurements were conducted on a Thermo Scientific ESCALAB 250Xi instrument using a monochromatic Al Kα radiation source.

Fabrication of electrodes

The working electrodes were fabricated by mixing 70 wt% active material, 20 wt% carbon black, and 10 wt% polyvinylidene fluoride (PVDF) binder in N-methylpyrrolidone (NMP) to form a slurry. The mixture was stirred for 24 hours to ensure homogeneity. The resulting slurry was uniformly coated on copper foil and dried in a vacuum oven at 50 °C for 24 hours. The dried electrodes were then punched into 15 mm diameter disks.

Preparation of cells and electrochemical measurements

The electrochemical behavior of the working electrode was investigated using CR2025 coin-type half-cells assembled under an argon atmosphere in a glovebox. The working electrode served as the anode, while a lithium metal chip was used as the counter and reference electrode. The electrolyte consisted of 1 M LiPF₆ dissolved in a 1:1 (v/v) mixture of ethylene carbonate (EC) and diethyl carbonate (DEC). A polypropylene film (Celgard-2400) was used as the separator. Galvanostatic charge–discharge measurements were performed at various current densities using an MTI-BSTA-MA (USA) battery analyzer in a voltage range of 0.1–3.0 V (vs. Li⁺/Li) at room temperature. Cyclic voltammetry (CV) measurements were performed using a CHI660C electrochemical workstation (Chenhua, Shanghai, China) at a scan rate of 0.5 mV s⁻¹ to investigate the electrochemical kinetics of the cells. Electrochemical impedance spectroscopy (EIS) measurements were performed from 0.01 Hz to 100 kHz with a 5 mV amplitude.

DFT calculations

Density functional theory (DFT) calculations were carried out using the plane-wave pseudopotential method as implemented in the VASP suite.⁴⁸ For the exchange–correlation functional, the generalized gradient approximation (GGA) with Perdew–Burke–Ernzerhof (PBE) parameterization was employed.⁴⁹ It should be noted that modelling the experimentally functionalized CNTs is challenging due to the unknown nature and positions of the functional groups. Therefore, a metallic (4,4)@(8,8)@(12,12) three-walled carbon nanotube was employed to represent the multi-walled carbon nanotube (CNT). It should be pointed out that CNTs may have a small bandgap similar to reduced graphene,⁵⁰ due to the interaction with various functional groups. Nevertheless, this should not qualitatively change the energy level alignment as discussed in the main text considering a very large offset between the LUMO and Fermi levels of Keggin and CNT, respectively (Fig. 7). Specifically, to study energy level alignment at the interface, the energy levels of both the Keggin cluster and the CNT were calculated separately using the HSE06 functional.⁵¹ This approach is reasonable since only a weak interaction is expected between CNT and Keggin owing to their non-planar geometries.⁵²

The adsorption energy of a Li atom on a CNT or the Keggin cluster was calculated using the following expression:

Eads = Ex + ELi − Ex/Li

where E_x, E_Li and E_x/Li are the total energies of the CNT or Keggin, Li atom, and Li adsorbed CNT or Keggin molecule. As per the above definition, Li adsorption is stable (unstable) for a positive (negative) value of E_ads.

Conclusions

In this study, an amphiphilic POM@CNT hybrid nanostructure was synthesized through electrostatic interaction between Keggin-type PMo nanoclusters and functionalized CNTs. The hybrid structure served as an anode material for lithium-ion batteries (LIBs), where PMo clusters contributed to capacity primarily via redox reactions, while CNTs enhanced electrical conductivity. The results demonstrate improved reversibility, elevated rate capability, and stable cycling performance. At a current density of 0.1 A g⁻¹, the POM@CNT electrode exhibited a high initial discharge capacity of 2100 mAh g⁻¹. The results confirm that the POM@CNT hybrid nanostructured developed in this work exhibits a competitive combination of high initial capacity, good cycling stability, and favourable rate performance, making it a promising candidate for high-performance lithium-ion battery anodes.

Author contributions

Eman Gul: experimentation, investigation, writing – original draft, data curation. Zeeshan Haider: experiments, material synthesis, investigation. Tanveer Hussain Bokhari: methodology, resources. Mashkoor Ahmad: methodology, formal analysis, manuscript writing. Gul Rahman: supervision, formal analysis, validation. Amjad Nisar: conceptualization, validation, formal analysis, funding acquisition, supervision, project administration.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability

All investigations done during this study are included in this published article and its ESI.† There are no hidden or unpublished data.

Acknowledgements

This work was supported by PINSTECH, Islamabad, Pakistan. The authors acknowledge Dr Saqib Javaid (TPD, PINSTECH) for conducting the DFT analysis and Dr Amina Zafar (CAFD, PINSTECH) for performing the EIS studies.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: XRD patterns (Fig. S1), high-resolution XPS spectra (Fig. S2), galvanostatic charge/discharge profiles (Fig. S3 and S4), cycling performance (Fig. S5), the assignment of FTIR spectra (Table S1), and fitting parameters (Table S2). See DOI: https://doi.org/10.1039/d5qm00376h

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