The spatial and electronic effects of polypyrrole between MnO₂ layers enhance the diffusion ability of Zn²⁺ ions

Abstract

New electrochemical energy storage systems have stringent requirements for energy storage materials, and traditional MnO₂ cannot comply with the requirements because of the problems of electrical conductivity and phase transition. In this work, a novel polypyrrole (PPy) intercalation MnO₂ (MnO₂/PPy-x) material was prepared and proved to be suitable for use in a high performance cathode of aqueous zinc ion batteries (AZIBs). The material characterization results proved that PPy played a key role between MnO₂ layers, and the reduction of Mn and extension of Mn–O bonds inhibited the distortion reaction of MnO₂, resulting in enhanced structural stability and excellent cycle life. In addition, electrochemical analysis revealed the H⁺/Zn²⁺ co-intercalation mechanism, and MnO₂/PPy-1 had high electrical conductivity, and fast reaction kinetics. Density functional theory (DFT) calculation proved the change of electron distribution between the MnO₂ layers. The PPy endowed MnO₂ with excellent electrical conductivity. Moreover, as an interlayer spacer, it hindered charge transfer and decreased the binding ability of Zn²⁺ and MnO₂. As a result, the electrochemical performance of MnO₂/PPy-1 was greatly enhanced. The final results demonstrated that MnO₂/PPy-1, which has a high conductivity and wide layer spacing, offered a superior capacity of 234 mA h g⁻¹ and a long cycle life of 2000 cycles at a current density of 1 A g⁻¹. In addition, according to the test results of pouch batteries, MnO₂/PPy-1 shows great potential for the flexible device market because of its superior flexibility and safety. This work provides a new method and approach for the modification of MnO₂-based materials.

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1. Introduction

Lithium-ion batteries currently dominate the energy storage field because of their high energy density and stable cycle life. However, as society's requirements for environmental protection, economic benefits, and safety increase, the high cost, safety issues brought about by organic electrolytes, and thermal runaway phenomenon of lithium-ion batteries have become the main factors hindering their further development.^1–4 Fortunately, aqueous zinc ion batteries (AZIBs) have attracted the attention of researchers because of abundant zinc reserves, their superior electrochemical energy storage level, and their high safety performance.^5–9 AZIBs are considered suitable lithium-ion battery candidates. In recent years, many types of material such as manganese oxides,^10,11 vanadium oxides,^12,13 Prussian blue analogs,^7,14 and organic compounds^15,16 have been reported as cathode materials for AZIBs. Among them, MnO₂ has a very long history, which can be traced back to the initial zinc–manganese battery system, and the cathode material of the first AZIBs was a MnO₂ material.¹⁷ Therefore, MnO₂ has a relatively mature preparation process and improvement measures, and the advantages of diverse crystal forms, suitable voltage, and high theoretical capacity have become additional advantages for its use as an ideal cathode. Nevertheless, the low conductivity of MnO₂ and the easy phase transition problem during the charge and discharge process are still major challenges to the industrialization of AZIBs.

Generally, MnO₂ with a layered structure is considered to have a positive effect on the inhibition of phase changes during charging and discharging. To obtain a high-performance cathode for AZIBs, previous researchers have chosen to introduce polymers into the interlayers of MnO₂ to play a key role between the layers and prevent structural collapse. In addition, the high conductivity and particular functional groups of the polymers effectively promote the diffusion and transfer of ions in the host materials. Similarly, the intercalation of polymers reduces the average oxidation state of the materials and inhibits the disproportionation reaction of Mn³⁺. For example, Huang et al. introduced polyaniline molecules into the interlayers of δ-MnO₂ to limit the phase change caused by the insertion of hydrated H⁺/Zn²⁺ and the subsequent structural damage.^17,18 Under the action of polyaniline, δ-MnO₂ showed an extremely high utilization rate of 90% (280 mA h g⁻¹) and an extremely long cycle life (5000 times). In conclusion, the application of the intercalation strategy brought new possibilities for high-performance AZIBs.

In previous studies, the intercalation of conductive polymers generally occurred during the polymerization process, which led to overly complex reactions between the intercalated species and the host material, making it unfavorable for researchers to analyze the intercalation mechanism. In response to this, we simplified the previous intercalation method. First, the polypyrrole (PPy) was prepared by an ice bath method, and then it was introduced into the interlayers of MnO₂ by a one-step hydrothermal method to obtain the final product (MnO₂/PPy-x). Characterization results such as X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Raman, scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS) showed that MnO₂/PPy-1 had a widened interlayer spacing and suppressed Mn distortion reaction, which contributed to enhanced ion transport ability and structural stability of the material. Through electrochemical analyses such as cyclic voltammetry (CV), galvanostatic charge and discharge (GCD), galvanostatic intermittent titration technique (GITT), and ex situ XRD, the energy storage mechanism of MnO₂/PPy-1 was proposed to be related to the H⁺/Zn²⁺ insertion/extraction mechanism, and during the charge and discharge process, the appearance/dissolution of by-products Zn₂SO₄(OH)₆·nH₂O (ZSH) and Zn_xMnO(OH)₂ (ZMO) takes place. Density functional theory (DFT) analysis demonstrated that inserting PPy between the layers of MnO₂ will greatly weaken the interaction between Zn²⁺ and the host material, and change the interlayer electronic environment, thereby enhancing the electrochemical reaction and conductivity of MnO₂ (Fig. 1). Finally, the MnO₂/PPy-1 achieved a superior capacity of 234 mA h g⁻¹, and the capacity retention rate is 92% after 2000 cycles. In addition, the operation results of pouch batteries proved that MnO₂/PPy-1 was suitable as a promising substitute for wearable devices.

Fig. 1 Schematic of the mechanism with PPy between the layers of MnO₂.

2. Results and discussion

2.1. Material characterization

In this work, MnO₂-based cathode materials with different proportions of intercalation were prepared, by regulating the content of PPy. In XRD characterization, it can be found that the prepared samples conform to the standard card JCPDS 80–1098 of the layered manganese dioxide phase. Specifically, as shown in Fig. 2a, with the intercalation of the PPy, the diffraction peaks attributed to the layered manganese dioxide phase become wider and weaker, and no new peaks appear, indicating that the introduction of PPy did not change the phase attributed to layered manganese dioxide. It is worth noting that the wide and weak diffraction peaks meant the MnO₂/PPy-x had typical nanocrystal characteristics, which contributed to improving the electrical conductivity and enhancing electrochemical reactions. In addition, with the increase of PPy, a gradually strengthening peak in the range of 20–25° in the XRD pattern emerged, which was attributed to the characteristic peak of PPy and served as important evidence for the successful introduction of PPy.

Fig. 2 (a) XRD of MnO₂/PPy-x. (b) FTIR spectra of MnO₂/PPy-x. (c) Raman spectra of MnO₂/PPy-x. SEM of MnO₂/PPy-1 at (d) 1 μm, and (e) 200 nm. (f) SEM–EDX of MnO₂/PPy-1.

FTIR is a common tool to explore the composition and structure of materials. As shown in Fig. 2b, the infrared absorption peak at 500–800 cm⁻¹ belongs to the stretching vibration of Mn–O bonds in the MnO₆ octahedron.¹⁰ The diffraction peaks at 1594 cm⁻¹ and 3426 cm⁻¹ were attributed to the interlayer water of the material. The diffraction peaks of MnO₂/PPy-x were enhanced because of the successful intercalation of PPy, which increased the interlayer space of MnO₂, thereby resulting in more interlayer water molecules, which was beneficial for the insertion/extraction of Zn²⁺.^11,19 In addition, interlayer water molecules played an important role in stabilizing the material structure and balancing the charge in transition metal oxides. Additionally, the combination of water molecules and confined alkali metal ions achieved high-efficiency charge storage in neutral electrolytes.^20,21 The peaks at 1350 cm⁻¹ and 1387 cm⁻¹ correspond to the stretching vibration of the C–N bonds and the deformation and stretching of the pyrrole ring (C=C).^11,22,23 In particular, vibration peaks related to the C–N bonds and C=C bonds appeared for MnO₂. This may be affected by the elements C and N in the air, which was proved in later elemental mapping. In addition, the stronger vibration peaks in the MnO₂/PPy-x indicated that the introduction of a trace amount of PPy increased the content of elements C and N in the material.

Raman spectra further revealed the changes in the composition and structure of MnO₂ after the introduction of PPy. As shown in Fig. 2c, the vibration peaks of the Mn–O bonds characteristic of MnO₂ (570 cm⁻¹ and 640 cm⁻¹) in MnO₂/PPy-x still existed but had changed, which was the result of PPy intercalation.¹¹ In addition, the regulation of the Mn–O bonds was conducive to enhancing the storage efficiency of Zn²⁺ ions.²⁴ The characteristic peaks at 903 cm⁻¹ and 1083 cm⁻¹ were attributed to the in-plane bending and deformation of the C–H bonds and N–H bonds.²⁵ The vibration peak of the pyrrole ring was located at 1273 cm⁻¹, and the vibration peak at 1486 cm⁻¹ was attributed to the stretching mode of the π-conjugated structure of the polymer main chain. The appearance of these characteristic peaks demonstrated the successful intercalation of PPy.^26,27 In conclusion, the similar results for the FTIR spectra and Raman spectra indicated that PPy had been successfully introduced into the interlayers of MnO₂, effectively expanding the interlayer spacing of the host material and greatly improving the diffusion rate of Zn²⁺.

SEM and energy dispersive X-ray (EDX) spectroscopy were used to analyze and study the morphological structure and elemental composition of MnO₂ and MnO₂/PPy-1. Fig. 2d and e exhibit the morphology of MnO₂/PPy-1 at scales of 1 μm and 100 nm, respectively. They show a typical nanoflower structure composed of nanosheets with a thickness of 12.9 nm, which is consistent with the structure of MnO₂ (Fig. S1†). Compared to MnO₂ (15.1 nm), the intercalation of PPy reduced the thickness of the MnO₂ nanosheet, resulting in the optimization of interlayer space. Meanwhile, PPy particles were observed on the surface of MnO₂/PPy-1 with an uneven distribution (red box), which led to the rapid capacity decay. This finding will be proved in the following text. The EDX analysis of MnO₂ and MnO₂/PPy-1 is shown in Fig. S2† and Fig. 2f. Through comparison, it was found that the increased content of element O and the appearance of an additional 2.65% of element N provided strong evidence for the introduction of PPy. In addition, the appearance of element C in MnO₂ confirmed the appearance of C-related groups in the FTIR spectrum.

The chemical bonds and valence states of the materials were analyzed by XPS. The full XPS spectra of MnO₂ and MnO₂/PPy-1 are shown in Fig. 3a. This shows the energy spectra of elements Mn, O, N, and C. Fig. 3b shows the high-resolution XPS spectrum of Mn 2p. The Mn 2p_3/2 and Mn 2p_1/2 features are shown at 642.3 eV and 654.2 eV, respectively. The spin energy difference of MnO₂/PPy-1 was 11.9 eV, which is typical of an MnO₂ structure and is similar to previous studies.^11,19 In addition, compared with MnO₂ (11.76 eV), the higher spin energy of MnO₂/PPy-1 was attributed to the intercalation of PPy changing the electronic structure of the host material and enhancing its electrical conductivity. The two sets of peaks of Mn 2p can be fitted to Mn³⁺ and Mn⁴⁺. In addition, a characteristic peak attributed to Mn²⁺ was found at 643.0 eV in the MnO₂/PPy-1 material.^28–30 The Mn²⁺ originated from the reduction of the average oxidation state of Mn caused by the intercalation of PPy, which effectively inhibited the distortion reaction of Mn. Therefore, enhanced material structural stability and excellent cycle life were obtained. As shown in Fig. 3c, Mn–O–Mn bonds, Mn–O–H bonds, and H–O–H bonds were detected in the spectrum of O 1s, located at 530.0 eV, 531.9 eV and 533.2 eV, respectively.³¹ Compared with MnO₂, the increased H–O–H bonds in MnO₂/PPy-1 demonstrated the introduction of PPy, effectively expanding the interlayer spacing of MnO₂ and increasing the content of lattice water. The increase in the content of lattice water contributed to the promotion of the diffusion of H⁺/Zn²⁺, reduced the internal resistance of the battery, increased the effective contact area of the electrode, and provided more electrochemical reaction interfaces.³² In addition, the highest binding energy of the H–O–H bonds corresponded to the concentration of adsorbed oxygen, and was an important indicator for evaluating the content of oxygen vacancies.³³ The XPS spectrum of C 1s is shown in Fig. S3.† Notably, 284.8 eV, 285.7 eV, and 288.9 eV correspond to C–C bonds, C–N bonds, and C=O bonds, respectively, which is consistent with the results of FTIR and Raman spectra.^30,34 In addition, element N, with an atomic ratio of 0.51%, was detected in XPS, and the XPS spectrum is shown in Fig. 3d. The characteristic peaks of 398.3 eV and 399.2 eV were attributed to =N– bonds, and –NH– bonds.²⁶ These results demonstrated the successful insertion of PPy.

Fig. 3 (a) XPS full spectrum of MnO₂/PPy-1 and MnO₂. (b) Mn 2p, (c) O 1s, and (d) N 1s XPS spectra of MnO₂/PPy-1 and MnO₂.

2.2. Electrochemical analysis

A CR2032 coin cell was assembled with zinc foil as the anode and a 2 M ZnSO₄ + 0.2 M MnSO₄ solution as the electrolyte to evaluate the electrochemical performance of the MnO₂ and MnO₂/PPy-x cathodes. The working mechanism is shown in Fig. 4a. CV of the materials was performed in the voltage range 0.8–1.8 V to evaluate their cycle stability and reaction kinetics (Fig. 4b). The MnO₂/PPy-1 exhibited two pairs of redox peaks at 1.25/1.57 V and 1.36/1.61 V, corresponding to the two-step insertion mechanism of Zn²⁺/H⁺.³⁵ In addition, the reduced redox potential difference made the charge–discharge process more reversible. In particular, the results of five cycles at a scanning rate of 0.1 mV s⁻¹ verified the reversible performance of the material. Except for the activation in the first cycle, the CV curves of the cycles generally coincide. The GCD of MnO₂/PPy-1 was carried out at a current density of 0.1–5 A g⁻¹ (Fig. 4c), and the results were found to be consistent with the CV results. Two charge–discharge platforms were discovered, corresponding to the insertion/extraction of Zn²⁺/H⁺. Clearly, as the current density was increased, the discharge platform attributed to Zn²⁺ insertion disappeared. The reason for this is that at a high current density, the H⁺ with a smaller ionic radius is more likely to be extracted from the interlayers of MnO₂.^36,37

Fig. 4 (a) Schematic diagram of AZIBs with zinc foil as the anode and MnO₂/PPy-1 as the cathode. (b) CV curves of MnO₂/PPy-1 at a scanning rate of 0.1 mV s⁻¹. (c) GCD curves of MnO₂/PPy-1 at a current density of 0.1–5 A g⁻¹. (d) Rate performance of MnO₂ and MnO₂/PPy-x. (e) EIS of MnO₂ and MnO₂/PPy-x. (f) Cyclic performance of MnO₂ and MnO₂/PPy-1 at a current density of 0.1 A g⁻¹. (g) Cyclic performance of MnO₂ and MnO₂/PPy-1 at a current density of 1 A g⁻¹. (h) Ragone diagram of MnO₂/PPy-1 and other types of cathode materials for AZIBs.

Rate performance is an important indicator of battery performance. The current density was increased from 0.1 A g⁻¹ to 5 A g⁻¹, and the rate capabilities of the MnO₂ and MnO₂/PPy-x cathodes were evaluated during this period, as shown in Fig. 4d. The specific capacities of MnO₂/PPy-1 were 419, 360, 333, 312, 282, 233, and 172 mA h g⁻¹. When the current was restored to 0.1 A g⁻¹, the MnO₂/PPy-1 exhibited an enhanced specific capacity of 587 mA h g⁻¹. The sources of these enhanced capacities will be explained when discussing the cycle performance later. Compared with MnO₂, the MnO₂/PPy-1 clearly exhibited increased rate performance. This result demonstrated the appropriateness of introducing PPy between the layers of MnO₂ to enhance the conductivity and structural stability of the material. To explore the influence of PPy on the electrical conductivity of MnO₂, electrochemical impedance spectroscopy (EIS) was performed. As shown in Fig. 4e, the electrochemical impedance of MnO₂ was 263 Ω, while the charge transfer resistance of MnO₂/PPy-1 was reduced to 37 Ω, much lower than that of MnO₂, indicating the effective improvement of the electrical conductivity of MnO₂ by PPy intercalation.

Benefiting from the high conductivity and interlayer supporting role of the PPy, the MnO₂/PPy-1 exhibited excellent cycling performance. The performance at a current of 0.1 A g⁻¹ is shown in Fig. 4f. The initial capacities of the MnO₂ and MnO₂/PPy-1 samples were 127 mA h g⁻¹ and 257 mA h g⁻¹, respectively. In particular, the capacity of MnO₂/PPy-1 increased significantly during the cycling process, and the highest capacity reached even an astonishing 817 mA h g⁻¹. As found in SEM, in the initial state, the PPy was unevenly coated on the surface of the MnO₂, and the unique structural reorganization of δ-MnO₂ during the charge and discharge process made PPy insert into MnO₂ during the cycling process, resulting in the capacity being increased. However, the MnO₂/PPy-1 had a regrettable capacity decay. This may be because the secondary intercalation of the PPy during the structural reorganization process hindered the dissolution of the ZSH by-products. The deposition of a large amount of ZSH hindered the ion transmission and led to a reduction in cathode utilization.³⁸ Surprisingly, after a short activation process (0.1 A g⁻¹), the MnO₂/PPy-1 retained a superior specific capacity of 234 mA h g⁻¹ and a high capacity retention rate of 92% after 2000 cycles at a current density of 1 A g⁻¹ (Fig. 4g), which was completely different from the performance at a current of 0.1 A g⁻¹. This may be attributed to the short activation process allowing the material to have a standing process after the secondary intercalation of the PPy, so the ZSH by-product had time to dissolve. In addition, the MnO₂ was short-circuited after 1300 cycles, which once again demonstrated the effective improvement of PPy intercalation on the performance of MnO₂. Although there were some defects in this work, upon comparing with some published works, it was found that the MnO₂/PPy-1 samples still had certain advantages^19,39–46 (Fig. 4h and Table S1†).

Wearable devices have strict standards for the flexibility and safety of electronic devices, which is beginning to prompt researchers to pay more attention to the operation of batteries in bent and punctured states.^47,48 We prepared a pouch battery with the MnO₂/PPy-1 electrode as the cathode and observed its operation in normal, bent, and torn states. As shown in Fig. 5a–c, the prepared pouch battery can operate normally in both normal and bent states, and can even work normally after being cut, and no heating or fire phenomena were observed. The performance of the pouch battery under extreme conditions demonstrated that the MnO₂/PPy-1 had excellent flexibility and high safety, reflecting its considerable prospects in the flexible device market.

Fig. 5 Photographs of the light bulb of a pouch battery prepared with MnO₂/PPy-1 as a cathode: (a) normal state, (b) bending state, and (c) puncturing state.

2.3. Mechanism analysis

Reaction mechanism research is an important means to describe the internal connection of electrochemical reactions inside batteries. Therefore, we conducted an in-depth analysis of CV curves at different scanning rates. As shown in Fig. 6a, two pairs of redox peaks corresponded to the two-step insertion mechanism of the MnO₂/PPy-1 electrode. At high scanning rates, the oxidation peak at 1.61 V gradually disappeared, which was attributed to the faster diffusion speed of H⁺, consistent with the GCD results. The value of b was an index for evaluating the capacity contribution in the energy storage process of MnO₂/PPy-1, as shown in eqn (1):^12,37

i = av^b (1)

 eqn (1) can be simplified to eqn (2):

log(i) = b log(v) + log(a) (2)

where a and b are adjustable parameters, and i and v represent the peak current (mA) and scanning rate (mV s⁻¹), respectively. When b = 0.5, the electrode reaction was controlled by diffusion behavior. When b = 1, the capacitance contribution was dominant. Fig. 6b shows the capacitance contribution of MnO₂/PPy-1 at a scanning rate of 1 mV s⁻¹. As shown in Fig. 6c, after linear fitting of the CV results of MnO₂/PPy-1, the b values of peak 1, peak 2, and peak 3 were 0.66896, 0.72901, and 0.54846, respectively. The results demonstrated that the reaction of MnO₂/PPy-1 was controlled by a mixture of diffusion and capacitance behaviors, and the capacitance behavior was dominant. Quantitative analysis of the capacitance contribution was carried out using eqn (3):

i = k₁v + k₂v^1/2 (3)

where k₁v and k₂v^1/2 represent the capacitance and diffusion contribution, respectively. As shown in Fig. 6d, the electrochemical capacity of MnO₂/PPy-1 was mainly contributed to by capacitance behavior, and as the scanning rate increased, the proportion of the capacitance contribution increased. The capacitance behavior dominated by proton diffusion was the source of the high capacity of the MnO₂/PPy-1 cathode.⁴⁹

Fig. 6 (a) CV curves of the MnO₂/PPy-1 cathode at various scan rates. (b) Contribution of capacitance behavior at 1 mV s⁻¹. (c) Corresponding relationship between peak currents and scan rates. (d) The capacitance and diffusion behavior ratio in the total contribution at different scan rates.

Ex situ XRD is an important means to demonstrate the structural evolution and energy storage mechanism of the MnO₂/PPy-1 electrode during the cycling process. As shown in Fig. 7a, the strong peaks at 26.5° and 54.6° of the MnO₂/PPy-1 electrode were attributed to the graphite foil current collector. When charged to 1.4 V, new diffraction peaks appeared at 16.3°, 21.3°, and 58.7° (red boxes), which was attributed to the formation of the by-product ZSH. The embedding of H⁺ into the host material led to an increase in OH⁻ in the electrolyte, which combined with Zn²⁺ detached from the anode to generate ZSH.¹⁹ In addition, the new peaks located at 32°–36° were attributed to ZMO, which was caused by the combination of Mn²⁺ and OH⁻ ions in the electrolyte.⁵⁰ When discharged back to 0.8 V, the peaks related to ZSH and ZMO disappeared, demonstrating the high reversibility of MnO₂/PPy-1, and the specific chemical equations are as follows:

2Zn²⁺ + SO₄²⁻ + 6OH⁻ + nH₂O ↔ Zn₂SO₄(OH)₆·nH₂O (4)

Fig. 7 (a) Ex situ XRD patterns of the MnO₂/PPy-1 cathode in different charge and discharge states. (b) GITT profiles and corresponding diffusion coefficient of H⁺/Zn²⁺ for the MnO₂/PPy-1 cathode.

Further, the GITT technique was used to evaluate the ion diffusion of the MnO₂ and MnO₂/PPy-1. The ion diffusion coefficient (D_Zn²⁺) can be expressed as eqn (6):

D_Zn²⁺ = 4/πτ·(m_BV_M/M_BA)²·(ΔE_S/ΔE_τ)² (6)

where τ is the current pulse time, m_B is the mass of the positive electrode, M_B and V_M are the molecular volume and molar volume, respectively, A is the surface area of the electrode in contact with the electrolyte, and ΔE_s and ΔE_τ represent the voltage changes during two consecutive relaxation periods and the voltage changes during the constant current pulse, respectively. As shown in Fig. 7b and Fig. S7,† the ion diffusion coefficient of MnO₂/PPy-1 was 10^−6.2–10^−9.2 cm² s⁻¹, which was higher than that of MnO₂ (10^−5.5–10⁻¹¹ cm² s⁻¹). The results demonstrated that the introduction of PPy enhanced the ion diffusion ability of MnO₂. In addition, the ion diffusion coefficient exhibited two different stages, which were related to the H⁺/Zn²⁺ insertion mechanism of AZIBs. Notably, the higher diffusion coefficient in the first stage occurred from the faster transmission ability of H⁺.

2.4. Theoretical calculation

Theoretical calculations were used to analyze the structural models, density of states (DOS), interaction with Zn²⁺, and electron density distribution after adsorbing Zn²⁺ for the MnO₂ and MnO₂/PPy-1. As shown in Fig. 8a and b, the intercalation of PPy clearly enhanced the interlayer spacing of the MnO₂, and optimized the diffusion rate of interlayer ions, consistent with the material characterization results. The adsorption capacity of MnO₂ and MnO₂/PPy-1 for Zn²⁺ and their adsorption models are shown in Fig. 8c–e. Compared with MnO₂ (−1.13 eV), the MnO₂/PPy-1 had a lower Zn²⁺ adsorption capacity (−0.65 eV). This result indicated that the PPy layer effectively weakened the interaction between Zn²⁺ and MnO₂ and contributed to increased ion diffusion. Fig. 8f shows the DOS of the MnO₂ and MnO₂/PPy-1. The MnO₂ exhibited a wide band gap of 1.482 eV, demonstrating its poor electrical conductivity, consistent with the electrochemical results. After PPy intercalation, the band gap of MnO₂/PPy-1 is 1.042 eV. The reduced band gap energy makes the electron transition in the material easier, resulting in more electrons jumping from the valence band to the conduction band and enhancing the conductivity of MnO₂. In addition, new DOS appears near the Fermi level, proving that the intercalation of PPy changes the interlayer electronic environment of MnO₂ and greatly improves its conductivity. Notably, the DOS of PPy appeared in the range of −10 to −15 eV, demonstrating the successful intercalation of the PPy. The differential charge densities of the MnO₂ and MnO₂/PPy-1 are shown in Fig. 8g and h. There was a large amount of charge accumulation and dissipation around Zn and O. But for the MnO₂/PPy-1, the existence of the PPy layer blocked the charge transfer behavior, and weakened the electrostatic interaction between Zn²⁺ and MnO₂, consistent with the adsorption energy results. The DFT calculation results demonstrated that the PPy intercalation will optimize the interlayer spacing and electronic environment, and enhance the electrical conductivity of the MnO₂. In addition, the PPy layer blocked the binding between Zn²⁺ and MnO₂, improving the reaction kinetics of Zn²⁺, which contributed to the enhanced electrochemical performance of the MnO₂/PPy-1.

Fig. 8 Diagrammatic figures from DFT calculations: (a) MnO₂, and (b) MnO₂/PPy-1. (c) The adsorption energies of Zn²⁺ in MnO₂ and MnO₂/PPy-1. E_ads of Zn²⁺ in (d) MnO₂ and (e) MnO₂/PPy-1. (f) DOS of MnO₂ and MnO₂/PPy-1. The differential charge density distribution of Zn²⁺ in (g) MnO₂ and (h) MnO₂/PPy-1.

3. Conclusion

In conclusion, a new PPy intercalation method was proposed to improve the electrochemical performance of MnO₂. The SEM results showed that PPy intercalation did not change the structure of MnO₂, but increased its interlayer spacing compared to the original morphology. The FTIR and XPS results demonstrated the increase in the content of interlayer water molecules, which was beneficial for high-efficiency charge storage in the electrolyte. In addition, the reduction of Mn played an important role in inhibiting its distortion reaction. It is worth noting that the unique electrical conductivity of the PPy made a huge contribution to the electrochemical performance of MnO₂/PPy-1. Thanks to this, the MnO₂/PPy-1 showed an extraordinary specific capacity (817 mA h g⁻¹), a long life of 2000 cycles, and an excellent retention rate of 92%. The CV, GCD and GITT results revealed the H⁺/Zn²⁺ co-insertion mechanism of MnO₂/PPy-1. Theoretical calculation results demonstrated that PPy played a role as an interlayer pillar, and the PPy layer as a spacer in MnO₂ effectively weakened its electrostatic interaction, and enhanced the diffusion ability of Zn²⁺. Finally, the good flexibility and satisfactory safety of MnO₂/PPy-1 provided a promising candidate for wearable devices.

Data availability

All relevant data are within the paper.

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

This work was supported the National Natural Science Foundation of China (Grant No. 62101296 and 52303335), the China Postdoctoral Science Foundation (2021M702656 and 2023M730099), the Natural Science Foundation of Shaanxi Province (Grant No. 2021JQ-756 and 2023-JC-QN0577), the Graduate Innovation Fund of the School of Mechanical Engineering, Shaanxi University of Technology (SLGJX202404).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4qi02739f

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