Tunable Zn²⁺ de-solvation behavior in MnO₂ cathodes via self-assembled phytic acid monolayers for stable aqueous Zn-ion batteries

Abstract

Sluggish ion diffusion kinetics at the electrode/electrolyte interface leads to insufficient rate capability and poor structural reversibility, which are mainly attributed to the hydrated Zn²⁺ migration process being inhibited due to its huge de-solvation energy barriers. Herein, a self-assembly method is proposed in which a multifunctional monolayer with phytic acid (PA) is coated on the surface of MnO₂ strongly attaching to the substrate with the formation of chemical bonding to effectively prevent the dissolution of PA in the electrolyte. Due to the negative charge and inherent ultra-hydrophilicity of PA, modified MnO₂ demonstrates stronger adsorption of positive ions and captures reactive water molecules, easily accelerating the de-solvation process of interfacial hydrated Zn²⁺, efficiently achieving reversible Zn²⁺ insertion/extraction. Meanwhile, the coating layers can protect the substrate from attack by active water molecules, thus inhibiting cathode dissolution during battery cycling. Experimental characterization studies reveal that the protected MnO₂ cathode exhibits a remarkable specific capacity of 273 mA h g⁻¹ with a zinc intercalation capacity contribution of more than 60% at a current density of 0.1 A g⁻¹. Additionally, even at a high current density of 1 A g⁻¹, it maintains a capacity of 197 mA h g⁻¹, far exceeding that of pure MnO₂. Furthermore, the Zn-ion pouch cell, serving as a proof of concept, achieves an impressive energy density of 300 W h kg⁻¹ and exhibits remarkable capacity retention of 77% even after 100 cycles. This work offers a universal strategy for expediting the de-solvation process of hydrated Zn²⁺ on the surface of manganese-based cathodes in aqueous zinc-ion batteries.

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Ke Lu

Ke Lu is currently a full professor at Anhui University, Hefei, China. He received his Ph.D. from Shandong University in 2018. During 2017–2018, he worked as a visiting PhD student at the Institute of New Energy for Vehicles at Tongji University, under the supervision of Prof. Yunhui Huang. Prior to his current position, Ke worked as a postdoctoral researcher at Northern Illinois University and Argonne National Laboratory (2018–2020). He has published more than 90 peer-reviewed papers with an H-index of 34. His group works in electrochemistry and related applications.

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Introduction

Low cost and improved safety have gradually become the primary considerations in the field of energy storage; in this context, aqueous zinc-ion batteries stand out as the most promising alternative technology.^1–6 Various cathode materials have been developed for aqueous Zn batteries, including manganese-based oxides, vanadium-based oxides, Prussian blue analogs, and organic compounds.^7–11 Among them, α-MnO₂ with a 2 × 2 tunnel structure, serving as a non-toxic and versatile cathode, holds significant potential.^12–16 Additionally, the theoretical energy density of the Zn/MnO₂ system exceeds 250 W h kg⁻¹, comparable to the theoretical energy density of commercial lithium-ion batteries (200–300 W h kg⁻¹).^17–21

However, severe capacity degradation, low discharge capacity, and poor cycling reversibility hamper the advancement of Zn/MnO₂ batteries.^22–25 In particular, the low proportion of the capacity contributed to by zinc-ion (de)intercalation during the cycling process hinders the full exploitation of the Zn/MnO₂ cell's performance.^26–29 Furthermore, zinc ions in solution typically exist in the form of hexa-coordinated complexes with water molecules.^30–32 During discharge, the insertion of zinc ions with water sheaths into the cathode electrode material can easily disrupt the structure of the cathode electrode material, reducing battery lifespan.^33–35 Moreover, the introduction of water molecules can lead to irreversible side reactions, resulting in the loss of active materials and causing significant capacity decay.^36–38 To overcome these obstacles, surface-coating strategies have been proposed to alter the electrode–electrolyte interface chemistry as a way of facilitating the transfer of electrons and ions at the electrode surface and prohibiting side reactions.^39–41 By adjusting suitable hydrophilic or hydrophobic interfacial layers, the de-solvation behavior of zinc ions can be accelerated, promoting faster migration rates of water-sheath-free zinc ions and facilitating zinc intercalation, thus enhancing stability.^42,43

In this study, we present a novel approach to modify the interface of MnO₂ by utilizing self-assembled phytic acid. Through chemical bonding, the PA molecules strongly attach to the substrate, preventing PA from dissolving in the electrolyte. The modified layer can enhance the adsorption of positive ions and facilitate the de-solvation process of interfacial hydrated Zn²⁺. This modification not only enables efficient reversible Zn²⁺ insertion/extraction but also protects against the attack of active water molecules, thereby preventing cathode dissolution during battery cycling. Experimental characterization studies demonstrate that the protected MnO₂ cathode exhibits an impressive specific capacity of 273 mA h g⁻¹, with the zinc intercalation capacity contributing more than 60% at a current density of 0.1 A g⁻¹. Additionally, the proof-of-concept Zn-ion pouch cell attains a high energy density of 300 W h kg⁻¹ and retains 77% of its capacity after 100 cycles.

Results and discussion

Structural characterization of self-assembled MnO₂@PA nanowires

As illustrated in Fig. 1a, pristine MnO₂ nanofibers were fabricated through a hydrothermal reaction. The MnO₂ was then immersed in phytic acid (PA) solution and the PA functional film was chemically attached to the MnO₂ surface via spontaneous completion of a self-assembly process. The ESI (Fig. S1 and S2†) provides details on the determination of PA concentration and dosage. The effect of the coating layer on MnO₂ was further elucidated through scanning electron microscopy (SEM) and X-ray diffraction (XRD) (Fig. S3 and S4†). These analyses revealed that the macroscopic structure of the nanorods remains discernible, and all diffraction peaks can be accurately indexed to the phase-pure α-MnO₂ (standard JCPDS 44-0141). This result proved that the flexible film coating did not damage its crystal structure. The microstructure of the sample was revealed through transmission electron microscopy (TEM) imaging (Fig. 1b and c). In comparison with MnO₂ samples (Fig. 1b) with clear lattice stripes, the surface of MnO₂@PA (Fig. 1c) appeared blurred, indicating that the rod-like manganese oxide surface was coated with a thin film. Moreover, the thickness of the layer was determined by the linear energy dispersive X-ray (EDX) spectroscopy scan (Fig. 1d), yielding a measurement of approximately 3–4 nm. Subsequently, the energy dispersive X-ray (EDX) spectroscopy image is shown in Fig. 1e, which confirmed the presence of elemental P on the MnO₂@PA material. Fig. 1f further demonstrates the uniform distribution of manganese (Mn), oxygen (O), and phosphorus (P) elements through transmission electron microscopy elemental mapping. Additionally, the distribution of the P element along the primary structure of the nanorods indicated the effective encapsulation of the film around the MnO₂. In order to obtain the bound state between the phytic acid layer and the MnO₂ matrix, the X-ray photoelectron spectroscopy (XPS) survey spectra of the pristine MnO₂ and MnO₂@PA were recorded. As depicted in Fig. 1g, the 2p orbital of phosphorus (P) in MnO₂@PA exhibits a diffraction peak at a binding energy of 132.9 eV, indicating the presence of a P–O–Mn bond on the surface of the MnO₂@PA nanowires. Meanwhile, Fig. 1h presents the high-resolution O 1s XPS spectra, where the appearance of a peak for P–O–Mn (531.2 eV) in the O 1s spectrum of MnO₂@PA further substantiated that the PA molecule exists in a chemically bonded state with MnO₂. Predictably, this film has excellent stability due to the presence of a P–O–Mn chemical bond. This result was further verified by an electrode immersion experiment (Fig. S5†) without major fluctuations in pH value after 3 days of the MnO₂@PA electrode being immersed in 2 M ZnSO₄ electrolyte.

Fig. 1 Structural and morphological characteristics of MnO₂@PA nanofibers. (a) Schematic illustration of the synthesis of MnO₂@PA; transmission electron microscopy images of (b) pure MnO₂ and (c) MnO₂@PA; (d) linear EDX scan of the elements and (e) EDX image of MnO₂@PA; (f) HAADF-STEM and corresponding elemental mappings of MnO₂@PA; XPS spectra of (g) P 2p and (h) O 1s for MnO₂@PA and pristine MnO₂.

The strong adsorption and the de-solvation process of zinc ions at the electrolyte/cathode interface

The investigation of zeta potential can provide valuable insights into the unique adsorption characteristics of various ions at the interface. As demonstrated in Fig. 2a, the pristine MnO₂ sample reveals a negatively charged surface with a zeta potential of −4.2 mV. Obviously, the self-assembly of PA with its strong negative charge on the MnO₂ substrate resulted in a significant negative shift in zeta potential (−6.3 mV), implying that the interface of MnO₂@PA can adsorb positive ions more strongly and facilitate a more rapid storage process. Furthermore, additional charges that were adsorbed at the interface may lead to an increase in the interfacial capacitance, which was further investigated by the analysis of differential capacitance, as depicted in Fig. 2b. In particular, a notable positive shift in the potential of zero charge (PZC) is observed on the MnO₂@PA surface.³⁹ These results can be attributed to the strong specific adsorption of cations at the interface, further confirming that the PA coating could enhance the attraction of the interface to insertion ions.

Fig. 2 Adsorption and de-solvation behavior of zinc ions at the interface layer. (a) Zeta potential of MnO₂ and MnO₂@PA cathodes. (b) The differential capacitance–potential curves of MnO₂@PA electrodes. (c) Nyquist plots for MnO₂@PA at various temperatures. (d) The calculated Zn²⁺ de-solvation energy of bare MnO₂ and MnO₂@PA electrodes. (e) Fourier transform infrared (FT-IR) spectra of H-bonds in MnO₂ and MnO₂@PA immersed in 2M ZnSO₄. (f) Raman spectroscopy of SO₄²⁻ in MnO₂ and MnO₂@PA immersed in 2 M ZnSO₄. Schematic diagram of de-solvation behavior in (g) MnO₂ and (h) MnO₂@PA electrodes.

Moreover, the contact angle of the loaded active substance cathode sheet was tested (Fig. S6†). It is clear that the contact angle of the MnO₂@PA cathode was significantly smaller than that of the pristine MnO₂, demonstrating that the modified layer could improve the cathodic hydrophilicity. Thus, the modified film is expected to trap water molecules, thereby facilitating the de-solvation process of Zn(H₂O)₆²⁺ at the interface.^44–47 To validate this assumption, the enhanced Zn²⁺ de-solvation of MnO₂@PA cathodes can be effectively assessed by evaluating the activation energy (E_a) from Nyquist plots at various temperatures, as shown in Fig. 2c. In addition, the E_a profiles at various voltages for the MnO₂ and MnO₂@PA cathodes are shown in Fig. 2d. On the whole, E_a of MnO₂@PA was lower than that of MnO₂ throughout the discharge phase, suggesting that this modified film is beneficial for reducing the energy barrier of ion transfer. In particular, in the Zn²⁺ interpolation-dominated section (about 1.4–1.0 V), the pristine MnO₂ cathode had a very high de-solvation barrier and energy, in contrast to that with the coating of the PA layer, which considerably alleviates the de-solvation potential. In summary, the outcome demonstrated a notable acceleration in the de-solvation process of hydrated Zn²⁺ and an improvement in the insertion kinetics of Zn²⁺ by the PA layer.

Additionally, the strong interaction between the water sheath and the PA layer alters the original chemical force. To support the Zn²⁺ pre-de-solvation effect of the PA layer, Fourier transform infrared (FT-IR) spectra were employed. As shown in Fig. 2e, after immersing the MnO₂@PA cathode in 2 M ZnSO₄, the H-bond wavenumber value decreased, indicating that the strong affinity of the PA interface for the water sheath elongates the H-bond length. Additionally, Raman spectroscopy was used to examine the solvation structure of Zn²⁺. As depicted in Fig. 2f, the peak position of the v-SO₄²⁻ band shifted to a higher frequency within the PA interface, suggesting that Zn²⁺ is tightly bound to SO₄²⁻ anions due to the removal of the water sheath. Fig. 2g and h intuitively display the de-solvation behavior of zinc ions at the interface layer. The gradual approach of hydrated zinc ions towards the positive electrode surface, owing to the negative charge potential of the cathode, prompted some of these ions to undergo de-solvation. The incorporation of the PA layer notably diminishes the de-solvation barrier, facilitating the intercalation of more zinc ions. Additionally, this modified layer exhibited exceptional hydrophilicity, effectively shielding the MnO₂ matrix from the corrosive effects of activated water post-de-solvation. In conclusion, the modification layers can promote the de-solvation of hydrated zinc ions, accelerate the transport kinetics of zinc ions, and increase the electrochemical capacity.

The functional coated layer directs stable Zn²⁺ (de)insertion

To study the effect of the modification layers on electrochemical efficiency, the performance of a modified MnO₂ cathode material was evaluated through a series of electrochemical tests on MnO₂@PA and MnO₂ electrodes. The charge and discharge curves are depicted in Fig. 3a, revealing that the MnO₂@PA cathode has a preferable specific capacity of 273 mA h g⁻¹, significantly outperforming the MnO₂ cathode (158 mA h g⁻¹) under a low current density of 0.1 A g⁻¹. Impressively, the pure MnO₂ exhibits an obvious potential difference with respect to electrochemical behavior when changing from insertion of H⁺ to the insertion of Zn²⁺ (Fig. 3b). Previous studies have indicated that the large molecular size of hydrated zinc ions hinders their reversible insertion into the manganese dioxide lattice, and thus this phenomenon results in a reduced intercalation capacity during the second phase of the pristine MnO₂ as well as to a large potential difference during the transition of the dominant intercalation ions. In contrast, the interfacially modified MnO₂ exhibits smooth electrochemical transition behavior, suggesting that the modified layer significantly improved the intercalation kinetics at the Zn²⁺ insertion (region II). To determine the capacity contribution of Zn²⁺ insertion, it is essential to precisely quantify the proportion of H⁺ intercalation capacity. For this purpose, coin batteries were assembled using 0.2 M MnSO₄ as the electrolyte. Since there is no Zn²⁺ in the electrolyte, the H⁺ intercalation capacity can be quantitatively obtained. Fig. 3c and d show that the capacities of the insertion of H⁺ in MnO₂ and MnO₂@PA cathodes are 87.6 and 101 mA h g⁻¹ at a current density of 0.1 A g⁻¹, respectively. In other words, the capacity contribution of Zn²⁺ insertion in the MnO₂@PA cathode is 63.2%, much higher than the capacity share of 44.5% in the MnO₂ cathode. Moreover, the capacity contributions of Zn²⁺ insertion in the MnO₂@PA cathode at all current densities are collected (Fig. 3e). In particular, the Zn intercalation capacity of the modified cathode decays by only 13.2% when the current density is increased by a factor of 10 and the charge/discharge curves are evident, demonstrating fast interfacial ion transport/reaction kinetics. Fig. 3f illustrates that the MnO₂@PA cathode demonstrates a significant Zn²⁺ insertion capacity exceeding 50% at all current densities. This observation indicates that the modified MnO₂@PA cathode plays a crucial role in enhancing the intercalation capacity of Zn²⁺, ultimately leading to a higher overall capacity being achieved.

Fig. 3 The electrochemical performances of Zn-MnO₂ batteries. (a) Charge–discharge curves of MnO₂@PA and pure MnO₂ cathodes at 0.1 A g⁻¹. (b) The highlighted activation behaviors for the reduction shown in (a) are magnified. The discharge curves of (c) MnO₂ and (d) MnO₂@PA at 0.1 A g⁻¹ in different electrolytes. (e) Charge–discharge profiles of MnO₂@PA collected at different current densities. (f) The capacity of Zn²⁺ insertion of MnO₂@PA and MnO₂ at all current densities.

Optimized cation intercalation kinetics

The improved redox kinetics in altered MnO₂ electrodes was additionally verified through cyclic voltammetry (CV) analysis (Fig. 4a). It is worth mentioning that MnO₂ cathodes modified with PA displayed increased redox currents and narrower cathodic–anodic peak separations in comparison with the original MnO₂ cathode. This indicates that the surface modification has resulted in enhanced ion storage capacity and more robust kinetics. In particular, the height and area of the second reduction peak (1.29 V) for MnO₂@PA far exceed those for pure MnO₂, which corresponds to the Zn insertion process. This difference could be attributed to the smaller ionic volume of de-solvated Zn ions, which is very favorable for their intercalation process and is consistent with the significantly enhanced Zn insertion capacity in the constant-current charge/discharge test. The ion diffusion dynamics in different cathodes was quantified by CV measurements at increasing scan rates (Fig. 4b and S7†). As depicted in Fig. 4c, utilizing analysis of the Randles–Sevcik relationship, a linear correlation exists between the square root of the scan rate and the anodic peak current, and this linear correlation indicates a diffusion-limited electrochemical process, with the slope directly related to the ion diffusion coefficient.³³ The slope of the modified MnO₂ consistently exceeds that of pure MnO₂, demonstrating that the modification layer can enhance ion transport efficiency. To further quantify the ion diffusion kinetics of the samples, galvanostatic intermittent titration technique (GITT) measurements were conducted. Fig. 4d and e display the voltage profiles of MnO₂ and MnO₂@PA recorded during the GITT evaluation. The results presented in Fig. 4f and S8† indicate a higher diffusion coefficient for the coated MnO₂ sample during battery operation, elucidating the accelerated ion diffusion in MnO₂@PA. Notably, the ion diffusion coefficient has a significant decrease when the pure MnO₂ discharge process is converted from H-insertion to Zn-insertion dominance in the second loop discharge stage. In contrast, the modified positive electrode has a higher D value for the full discharge process. This result can be attributed to the de-solvated Zn²⁺ entering the manganese dioxide lattice more easily, leading to a faster ion transport rate during the reduction process.

Fig. 4 Electrochemical reaction kinetics of different cathodes. (a) CV curves for MnO₂ and MnO₂@PA. (b) CV curves of MnO₂@PA at various sweep rates. (c) The dependence of log (peak current) on log (scan rate) for obtaining b values. Charge/discharge profiles of (d) MnO₂@PA and (e) MnO₂ during the GITT measurements. (f) The ion diffusion coefficient vs. voltage for MnO₂@PA and MnO₂ during the 2nd discharge.

Investigations of the degradation mechanism and cycling performance

We further investigated the decay mechanism of the battery and characterized the electrode after cycling. The cathodic degradation mechanism is delineated in Fig. 5a and b. Post-cycling tests, as depicted in Fig. 5a, reveal that the morphology of the modified MnO₂ cathodes remains intact even after 100 cycles, indicating that the coated layer confers high structural integrity upon the treated substrate. In stark contrast, the structure of MnO₂ evolves from its original nanofibers to a composite mixture of nanoparticle aggregations accompanied by structural pulverization (Fig. 5b). As shown in the schematic, this transformation is attributed to the regulatory effect of the PA interlayer. On the one hand, the thin modifier film could accelerate the de-solvation of hydrated zinc ions and facilitate efficient and continuous ion diffusion. On the other hand, the PA layer could shield the MnO₂ matrix from attacks by active water molecules, minimizing the loss of active species and structural collapse to the utmost extent. The results of post-cycling electrode XRD tests further proved that the modified MnO₂ cathode exhibits better cycling performance (Fig. S9†). It is clear that the area of the peak of the by-product Zn₄SO₄(OH)₆·4H₂O (ZHS) is extensive and the MnO₂ peak is not obvious after cycling of the MnO₂ cathode, which means that the ZHS has become the main loading on the electrode, and the active substance has dissolved in large quantities. In contrast, scanning of the MnO₂@PA cathode shows that the peak belonging to MnO₂ is still prominent and the intensity of the ZHS peak is relatively small, confirming that the modified MnO₂ has satisfactory stability. Leveraging the modulated interfacial charge transfer and structural protective coating, the Zn-MnO₂@PA cell demonstrates exceptional energy and cycling performances, as evidenced in Fig. 5c. Notably, the MnO₂@PA cathode maintains a capacity retention of approximately 73% after 500 charging/discharging cycles at 1 A g⁻¹. In contrast, the capacity retention of the unmodified MnO₂ cathode is only 14%. ICP-MS test results further demonstrate the excellent protective effect of the PA layer, as shown in Table S1.† In the electrolyte of the Zn-MnO₂ beaker battery after cycling, a notable presence of the manganese element is evident, suggesting that the electrode material undergoes partial dissolution during the cycling process, which can significantly impair its performance. Conversely, the post-cycling Zn-MnO₂@PA beaker cell showed no substantial detection of phosphorus and manganese elements, highlighting the effective protective role of the PA layer. Similarly, an in-depth examination of the positive electrode is of paramount importance. Therefore, the cycling performance of MnO₂@PA was further evaluated with different N/P ratios, as shown in Fig. S10.† It is evident that the N/P ratio of 1.2 showed the optimum electrochemical performance. It is particularly important to note that the effect of the N/P ratio on cycling stability is significant. Scanning electron microscopy images of the positive electrodes were then obtained after cycling the Zn-MnO₂ and Zn-MnO₂@PA cells, as shown in Fig. S11.† The unmodified Zn-MnO₂ anode showed obvious cracking and inhomogeneity after 200 cycles, in contrast to the modified Zn-MnO₂@PA battery anode, which remained chemically stable throughout. This is related to the good adsorption of active water in the water shell by the cathode-modified layer, which reduces the erosion of active water on the anode surface and provides a stable environment for the stripping and deposition of zinc ions. As a proof of concept, a pouch cell employing the MnO₂@PA cathode was prepared and its cycling stability is depicted in Fig. 5d. As anticipated, the pouch cell showcases a high energy density of 300 W h kg⁻¹ (136 W h kg⁻¹ based on the total weight of electrode materials, as illustrated in Fig. S12†) along with high-capacity retention after 100 cycles. The pouch cell also exhibits the capability to power a red LED (Fig. 5d, inset), emphasizing the potential for future applications of these modified cathodes.

Fig. 5 Degradation mechanism and cycling stability of Zn-MnO₂ cells assembled with modified MnO₂ cathodes. TEM images of cycled (a) MnO₂@PA and (b) MnO₂ cathodes; the schematic of the structural evolution of MnO₂ cathodes after a long cycling test, where the formation and aggregation of ZHS accelerate the degradation of metal oxide cathodes. (c) Long-term cycling performance after various cycling numbers. (d) Cycling stability of the flexible Zn-MnO₂ cell, and (inset) a red LED powered by the tandem battery device.

Conclusion

In summary, we introduce a simplified approach to interfacial engineering that enables the in situ construction of a multifunctional modification layer on the surface of an MnO₂ cathode. The phytic acid layer is wrapped around the substrate with strong chemical bonds, which can inhibit the dissolution of phytic acid in the electrolyte, while stabilizing the structure of MnO₂ to reduce the dissolution of Mn²⁺, enhancing cathode stability during cycling. Moreover, the enhanced adsorption of positive ions and accelerated de-solvation process of interfacial hydrated Zn²⁺ are achieved by the PA layer with its intrinsic negative charge and superhydrophilicity, which effectively increased the diffusion rate of Zn²⁺, improved its intercalation capacity and achieved a smooth transition during the capacity-dominated intercalation ion transition, effectively reducing the potential difference during the transition. Our experimental results highlight the effectiveness of the modified interface in creating a solvation structure with low water content for Zn²⁺. As a result, this Zn/MnO₂@PA cell achieves a specific capacity of 273 mA h g⁻¹ at a current density of 0.1 A g⁻¹, with zinc intercalation capacity exceeding 60%, and retains a specific capacity of 197 mA h g⁻¹ even when the current density is increased by 10 times. Additionally, a proof-of-concept Zn-ion pouch cell demonstrates an impressive energy density of 300 W h kg⁻¹ and maintains 77% capacity retention after 100 cycles. This study provides a facile method to accelerate the de-solvation process of hydrated Zn²⁺ on the surface of manganese dioxide, thus overcoming its slow kinetics and low capacity in aqueous zinc-ion batteries.

Data availability

The data that support the findings of this study are included in the published article and its ESI.† These data are also available from the corresponding authors upon request.

Additional information

Supplementary InformationESI accompanies this paper in the online version of the paper.

Conflicts of interest

The authors declare no competing financial interests.

Acknowledgements

This work was financially supported by the Natural Science Foundation of China (22208335, 22109001), the Opening Project of the State Key Laboratory of Polymer Materials Engineering (Sichuan University) (sklpme2024-1-08), the Postdoctoral Fellowship Program of CPSF (GZB20230950), and the China Postdoctoral Science Foundation (2024M754163).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4nr03635b

‡ These authors contributed equally to this work.

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