In situ TEM study of lithiation into a PPy coated α-MnO₂/graphene foam freestanding electrode

Abstract

Even with the many desirable properties, natural abundance and low cost of α-MnO₂, its application as the anode in lithium-ion batteries has been limited because of its low intrinsic electrical conductivity and large volume expansion occurring during charge/discharge cycles. In this work, a ternary composite electrode consisting of MnO₂-polypyrrole (PPy) core–shell arrays is grown on graphene foam (GF) to address the above critical issues. The freestanding MnO₂–PPy/GF electrode exhibits a high reversible capacity of 945 mA h g⁻¹ at 0.1 A g⁻¹ after 150 cycles with a coulombic efficiency of over 98%, far better than 550 mA h g⁻¹ for the uncoated counterpart. An in situ TEM examination reveals several functional features of the PPy coating that ameliorate the MnO₂ conversion reaction kinetics, and thus the electrochemical performance of the electrode. The PPy coated MnO₂ nanowires have a lithiation speed three times faster than that of the uncoated MnO₂ along with improved electronic conduction and a stable structure against volume expansion. Such a rational design of an electroactive core and a highly conductive polymer shell on a GF conductive substrate offers a potential solution to developing novel MnO₂-based electrodes with enhanced electrochemical performance.

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1. Introduction

Li-ion batteries (LIBs) are one of the most promising energy storage devices with a wide range of applications ranging from portable electronics to electric vehicles. There have been extensive studies towards developing electrode materials with high capacities to improve the energy density as well as the cycle life of LIBs. A variety of active materials with capacities several times higher than that of graphite have been investigated as anodes of devices.^1–3 Among different metal oxides, such as FeO_x,⁴ CoO_x,⁵ NiO_x⁶ and MnO₂,⁷ in which energy storage occurs by a conversion reaction, MnO₂ with a very high theoretical capacity of 1230 mA h g⁻¹ is an attractive choice because of its natural abundance, environmental benignity, low discharge voltage plateau and low cost. However, the application of MnO₂ as the anode in LIBs is challenging due to its intrinsic limitations of very low electrical conductivity and large volume change upon lithiation/delithiation. Although undesirable volume changes of the active material can be mitigated by designing nanoscale MnO₂, the nanoparticles tend to be severely agglomerated during synthesis, which may adversely affect the microstructure of the electrode, leading to degradation of the electrochemical performance.^7–9

Many efforts have been made to overcome the above mentioned drawbacks. In particular, highly conductive nanocarbon materials, such as graphene and carbon nanotubes (CNTs), with different microstructures have shown effective accommodation of the volume changes of MnO₂. For instance, graphene nanoribbons were developed to prevent large volume expansion, leading to enhanced electrochemical stability of the electrode,⁷ while CNTs were used as the conductive support to enhance the electronic conductivity of MnO₂ composites.¹⁰ A sandwich-like structure consisting of nanosized MnO₂ filler inserted between graphene face materials is a successful example to buffer the volume expansion leading to cyclic stability and capacity enhancement up to 100% in comparison with the electrode consisting of MnO₂ grown on graphene without the second protective graphene layer.¹¹ In the other proposed microstructural design, a 3D MnO₂/CNT framework was constructed by uniform coating of conductive 1D CNTs around and within the MnO₂ microspheres to relieve the strain of the active material during lithiation/delithiation. The composite enhanced the specific capacity to over 1000 mA h g⁻¹ at 0.1 A g⁻¹ after 70 cycles.¹²

Apart from the above mentioned nanocarbon materials, conductive polymers are effective in accommodating structural deformations of MnO₂ as well as in enhancing the electronic conductivity. Among various conducting polymers, polypyrrole (PPy) has been well developed for application in energy storage devices such as rechargeable batteries and supercapacitors owing to its electrochemical properties, easy synthesis and cost effectiveness.^13–15 The combination of MnO₂ and PPy has been studied for supercapacitors, because of both materials have unique pseudocapacitive characteristics, in addition to the above properties of PPy.^16–18 There are also studies of MnO₂–PPy composite anodes for LIBs,^19–21 reporting their high electrochemical performance. However, the influence of PPy on the lithiation behavior and the microstructural integrity of MnO₂ have yet to be fully explored.

Therefore, this work is devoted to elucidating the conversion reaction of MnO₂ as the major mechanism responsible for Li storage and the influence of PPy coating on the electrochemical behavior of the electrode by in situ microscopy. 3D cellular graphene foam (GF) was used as the substrate to grow MnO₂ nanowires via a controlled hydrothermal process. The highly conductive GF with a 3D microstructure is an effective substrate to provide nucleation sites for the growth of MnO₂ nanowires while preventing nanowire agglomeration. A facile chemical polymerization method was adopted to coat a thin layer of PPy on the MnO₂ nanowires to mitigate their volume changes during the lithiation/delithiation cycles. The flexible, conductive coating gave rise to significant enhancements in the reversible specific capacity as well as cyclic stability and rate capability of the electrode. The in situ transmission electron microscopy (TEM) analysis revealed that the PPy coating effectively buffered the volume expansion upon lithiation while offering electronic conduction pathways throughout the active materials.

2. Experimental section

2.1. Synthesis of GFs

GFs were synthesized by chemical vapor deposition (CVD) similar to previous studies.^22,23 In brief, a piece of clean nickel foam template of less than 2 mm in thickness was placed inside a quartz tube which was initially maintained at a base pressure of 5 mTorr and was filled with 500 ml min⁻¹ Ar and 200 ml min⁻¹ H₂. After increasing the temperature to 1000 °C at a rate of 20 °C min⁻¹ and annealing the Ni template for 10 min, CH₄ gas was introduced as the carbon precursor into the tube at a flow rate of 20 ml min⁻¹ for 10 min at atmospheric pressure. Then, the CH₄ supply was stopped and the furnace was cooled down to room temperature to obtain graphene coated Ni foam. Freestanding GF with an intact 3D microstructure was obtained after removing the nickel substrate using FeCl₃/HCl etchant.

2.2. Synthesis of MnO₂/GF and MnO₂–PPy/GF electrodes

MnO₂ nanowire arrays were synthesized by a hydrothermal method. KMnO₄ (2 mmol) and MnSO₄·1H₂O (2 mmol) were dissolved in 30 mL of de-ionized (DI) water and stirred for 20 min. GF was placed in the solution after fixing on a piece of glass, which was transferred to an autoclave and heated at 140 °C for 3 h, followed by cooling to room temperature. The product was rinsed several times with DI water and ethanol, and dried at 60 °C for 12 h to obtain brown colored MnO₂/GF.

A facile chemical polymerization method was employed to coat the MnO₂ nanowires with PPy. 1.5 mmol of sodium p-toluenesulfonate (p-TSS, C₇H₇SO₃Na) dopant was mixed with 10 mL of DI water and 10 mL of ethanol. 120 μL of pyrrole monomer was added into the mixture and stirred for 30 min at room temperature to prepare the first solution. The second solution was prepared using 4 mmol of ammonium persulfate ((NH₄)₂S₂O₈) as the oxidizing agent which was mixed with 30 mL of DI water, after vigorously stirring for 20 min. The MnO₂/GF hybrid was placed in a clean Petri dish, and the first solution was slowly poured into the sample for complete infiltration. Subsequently, the second solution was drop-wisely added in the same manner, and the mixture container was placed in an ice bath in the dark for 1 h for polymerization to take place. The final product was rinsed with DI water and methanol several times and dried in a vacuum oven at 60 °C for 12 h. All the aforementioned processing parameters and materials, including the amounts of chemicals and the conditions for polymerization, have been optimized after extensive trial and error experiments to obtain freestanding electrodes with an optimized uniform microstructure. Fig. 1 schematically illustrates the processing steps as well as the materials obtained after each step.

Fig. 1 Schematic of the synthesis process of MnO₂–PPy/GF.

2.3. Materials characterization

The structure of the electrode was studied using a powder X-ray diffraction (XRD) system (PW1830, Philips) with Cu Kα radiation from 10° to 80°. The microstructural features and morphologies were examined by scanning electron microscopy (SEM, JEOL 6700), field emission transmission electron microscopy (FETEM, JEOL 2010), selected area electron diffraction (SAED) analysis and energy-dispersive X-ray spectroscopy (EDS). The PPy coating was characterized using Fourier transform infrared spectroscopy (FTIR, Perkin Elmer Spectrum One) in the 400–1800 cm⁻¹ wavelength range. Both MnO₂ and MnO₂–PPy nanowires were mixed with potassium bromide (KBr) and pressed to prepare pellets. X-ray photoelectron spectroscopy (XPS, Surface analysis PHI5600, Physical Electronics) was used to evaluate the surface elemental compositions of the materials using the Al Kα line as the excitation source. A TGA/DTA 92 Setaram II testing system was utilized to perform the thermogravimetric analysis (TGA) in air over a temperature range of 50–800 °C at a heating rate of 10 °C min⁻¹.

2.4. Electrochemical measurements

CR2032 button cells were assembled in an Ar-filled glovebox with Li foil as the counter electrode, LiPF₆ (1 M) in ethyl carbonate (EC)/dimethyl carbonate (DMC) (1 : 1 by volume) as the electrolyte and a microporous polyethylene film (Celgard 2400) as the separator. The MnO₂–PPy/GF composites synthesized above with an areal mass loading of 2 mg cm⁻² were used directly as the positive electrode in a half-cell configuration. The cells were tested between 0 and 3 V at different current rates on a LAND 2001 CT battery tester. A CHI660 electrochemical workstation was used to conduct the cyclic voltammetry (CV) tests in the same potential window at a scan rate of 0.2 mV s⁻¹. Electrochemical impedance spectroscopy (EIS) was performed in the frequency range from 0.01 Hz to 100 kHz on the same electrochemical workstation.

2.5. In situ TEM examination

The lithiation mechanism and the structural evolution of MnO₂ nanowires as well as the influence of the PPy conductive coating on the lithiation behavior were studied in real time by in situ electrochemical experiments.^24,25 The nanowires were loaded on a Cu wire tip as the working electrode while the Li metal was attached onto another Cu wire tip as the counter electrode on a scanning tunneling microscopy (STM) holder (Nanofactory®). The holder with the above nanocell was transferred to a TEM column (JEOL 2010) under the protection of an Ar gas flow. During the transfer, a Li₂O film was formed on the counter electrode surface as a solid electrolyte interface (SEI) layer. To drive the diffusion of Li ions to the active material, a negative bias of −2 V was applied to the working electrode when it was in physical contact with the counter electrode, and the lithiation process was recorded using a high speed camera.

3. Results and discussion

3.1. Structure and morphology

Fig. 2a shows the XRD patterns of MnO₂/GF and MnO₂–PPy/GF in comparison with neat MnO₂, which was prepared under the same processing conditions. Two new peaks appeared at 2θ ≈ 26.5 and 54.7° in the spectra of the composites, which correspond to the reflections of the graphitic carbon (002) and (004) in the GF substrate, respectively,^22,26 as indicated by the SEM and TEM images (Fig. S1, ESI†). All the other peaks for the composites are common with the neat MnO₂ and are well fitted with pure α-MnO₂ (JCPDS no. 00-044-0141): the peaks at ∼12.7, 18.1, 28.8, 37.5, 42, 49.9, 60.3 and 69.7° correspond to the (110), (200), (310), (211), (301), (411), (521) and (541) reflections, respectively. The SEM images of the MnO₂/GF composites are shown in Fig. 2b and its inset, indicating a very uniform deposition of MnO₂ nanowires on the surface of GF. They signify the desirable surface characteristics of GF that provided the sites required for nucleation and growth of MnO₂ nanowires without agglomeration during the hydrothermal process. The TEM images in Fig. 2c and its inset present MnO₂ nanowires with several micrometers in length and 20–50 nm in diameter, which are uniform from top to bottom. The HRTEM image and the fast Fourier transform (FFT) patterns in Fig. 2d present lattice fringes running along the wire direction with a reflection corresponding to the (200) plane and a d-spacing of 0.489 nm, typical of α-MnO₂ single crystals, consistent with the XRD result. The α-MnO₂ single crystal nanowires were formed by the redox reaction of KMnO₄ in an acidic environment during the hydrothermal process according to the below equation:^27–29

3MnSO₄ + 2KMnO₄ + 2H₂O → 5MnO₂ + K₂SO₄ + 2H₂SO₄ (1)

Among different polymorphic forms of MnO₂, α-MnO₂ with a tetragonal crystal structure was constructed from the double chains of [MnO₆] octahedra to form 2 × 2 tunnels.^27,30 Such a tunneling structure is favorable for Li ion storage because the large tunnels existing in the crystalline lattice can promote the accommodation and transportation of Li ions.^31,32 A series of hydrothermal syntheses was performed under different processing conditions to reveal a multistep formation mechanism of 1D α-MnO₂. MnO_x units appeared in the solution, followed by the formation of metastable δ-MnO₂ with a layered structure through a condensation reaction. Due to the moderate concentration of the existing cations, K⁺, the layered structure collapsed and transformed to a tunneling structure of α-MnO₂ nanowires.³² As the reaction proceeded, the phase transformation came to an end and long nanowires grew while the short ones were dissolved into the solution.²⁶ Apart from the tunneling structure of α-MnO₂ nanowires, the unique 3D cellular structure of MnO₂/GF composites with an ample open space between the cell walls provided sufficient active sites for electrochemical reactions to take place assisted by fast electron transfer and ion diffusion.

Fig. 2 (a) XRD patterns of pure MnO₂, MnO₂/GF and MnO₂–PPy/GF; (b) SEM image of MnO₂/GF with a high magnification image in the inset; (c) TEM image of MnO₂ nanowires with a low magnification image in the inset; (d) HRTEM image of a single crystal MnO₂ nanowire with a FFT pattern of crystallographic planes in the inset; (e) SEM and (f) TEM image of the MnO₂–PPy/GF composite.

The SEM and TEM images of the MnO₂–PPy/GF composite after PPy coating are shown in Fig. 2e and f. The TEM image reveals the PPy coating with a uniform thickness of ∼20 nm on the MnO₂ nanowire surface. Because of the low temperature polymerization of PPy,³³ the original microstructural features of MnO₂ nanowires were maintained intact. The low temperature polymerization process is unique, facile and time-saving, which is different from the conventional carbon coating methods,^24,34 where a further post-heat treatment was necessary for carbonization of the conductive polymer precursor at an elevated temperature, inevitably leading to phase transformation of the core material. The elemental maps and the energy-dispersive X-ray (EDS) spectrum present the distributions of Mn, O and C atoms (Fig. S2, ESI†), also confirming the core–shell structure of MnO₂–PPy.

To determine the compositions of the PPy coating, MnO₂ and GF in the composites, thermogravimetric analysis (TGA) was carried out in air, as shown in Fig. 3a. GF started to decompose at ∼600 °C,³⁵ while PPy started to decompose at ∼200 °C in the PPy coated sample. According to the two curves, the weight fractions of PPy coating, MnO₂ wires and GF in the MnO₂–PPy/GF composite were estimated to be 12.6, 26.4 and 61 wt%, respectively. The FTIR spectra in Fig. 3b exhibit three prominent peaks located at 461, 519 and 617 cm⁻¹ common to both MnO₂ and MnO₂–PPy nanowires, which are characteristic of the Mn–O vibrations of the MnO₆ octahedra in the α-MnO₂ crystals.^36,37 Three new absorption bands appeared at 930, 1100 and 1191 cm⁻¹, corresponding to the stretching vibrations of PPy in the MnO₂–PPy composite.³⁸ The peaks located at 785 and 966 cm⁻¹ indicate the presence of polymerized pyrrole.³⁹ The band at 1050 cm⁻¹ is attributed to the C–H and N–H deforming vibrations, and the band located at 1559 cm⁻¹ corresponds to the C=C/C–C stretching vibration of PPy rings.³⁸

Fig. 3 (a) TGA curves of MnO₂/GF and MnO₂–PPy/GF composites; and (b) FTIR spectra of MnO₂ and MnO₂–PPy.

The XPS spectra were used to analyze the surface chemistries of MnO₂/GF and MnO₂–PPy/GF composites. Fig. 4a shows the deconvoluted XPS spectra of Mn 2p with two well-defined peaks at ∼642 and ∼654 eV in both composites, corresponding to the binding energies of Mn 2p_1/2 and Mn 2p_3/2, respectively.³⁸ The deconvoluted O 1s XPS spectra of MnO₂–PPy/GF in Fig. 4b show prominent peaks at 529.6, 531.2 and 532.5 eV which are attributed to the oxygen bonds in Mn–O–Mn, Mn–O–H and H–O–H (residual water), respectively.⁴⁰ The extra peak at 533.8 eV corresponds to the C=O bonds in PPy.⁴¹ All these peaks, except that for Mn–O–Mn, were negligible in MnO₂/GF. The C 1s spectrum of MnO₂/GF in Fig. 4c shows the characteristic peaks for GF at 284.4 and 285.6 eV, corresponding to the binding energies of C–C and C–H, respectively. The MnO₂–PPy/GF composite exhibits four peaks at 284.4, 285.6, 287.5 and 288.9 eV attributed to the C–C, C–H, C=N and C=O bonds in PPy, respectively.⁴² The N 1s spectrum of MnO₂–PPy/GF in Fig. 4d exhibits three peaks at 398.4, 400.3 and 401.1 eV, representing the imine (–C=N–), neutral and amine (–NH–), and positively charged structures (–NH⁺–), respectively, confirming the presence of the PPy coating.⁴³

Fig. 4 Deconvoluted XPS spectra of (a) Mn 2p, (b) O 1s and (c) C 1s for MnO₂/GF (bottom curves in red) and MnO₂–PPy/GF (top curves in blue) and (d) N 1s for the MnO₂–PPy/GF composite.

3.2. Electrochemical performance

Fig. 5a shows that the specific capacity of the MnO₂–PPy/GF electrode after 150 cycles at a current density of 100 mA g⁻¹ was 945 mA h g⁻¹, which was significantly higher than 550 mA h g⁻¹ for the MnO₂/GF electrode. The corresponding charge/discharge profiles at different cycle numbers for the PPy coated electrode are shown in Fig. 5b, indicating an obvious plateau at a low potential of ∼0.35 V upon the first discharge with a specific capacity of 1249 mA h g⁻¹. Although there was a large decrease in the 2nd charge capacity, the electrode became gradually stable in the following cycles to achieve an excellent specific capacity and a coulombic efficiency of over 98% for the entire 150 cycles. This observation signifies that the contribution of the PPy coating to the overall capacity was negligible in the first cycle, followed by gradual activation and kinetic stability of the electrode in the ensuing cycles. The rate performances of the two electrodes are shown in Fig. 5c. The MnO₂–PPy/GF electrode exhibits specific capacities of 736, 593 and 430 mA h g⁻¹ when tested at current densities of 0.2, 0.5 and 1 A g⁻¹, respectively. Once the current density was reverted to 0.1 A g⁻¹, it exhibited a high capacity of over 1000 mA h g⁻¹ at the 100th cycle. Excellent stability was maintained for the rest of the cycles at 0.2 A g⁻¹ with a reversible capacity of 780 mA h g⁻¹ at the 200th cycle. The MnO₂–PPy/GF electrodes outperformed by a large margin the MnO₂/GF counterpart without a PPy coating at all current densities.

Fig. 5 (a) Cycling performance of the electrodes at a current density of 0.1 A g⁻¹, (b) discharge/charge profiles of the MnO₂–PPy/GF electrode, (c) rate performance at different current densities and (d) Nyquist plots of the electrodes over the frequency range between 100 kHz and 0.01 Hz.

To support the above findings, the electronic conductivity and the ion transfer behavior of the electrodes were studied using electrochemical impedance spectroscopy (EIS) and the Nyquist plots are shown in Fig. 5d. The corresponding impedance parameters were calculated using the Zview software. The R_s values, a measure of the electrolyte resistance and current collector/electrode contact resistance, were 13.7 and 7.7 Ω; and the R_ct values, the kinetic resistance of charge transfer at the electrode/electrolyte interface, were 299.6 and 121.8 Ω for MnO₂/GF and MnO₂–PPy/GF, respectively. The over twice differences in these impedance parameters signify the importance of the high electronic conductivity associated with fast ion/electron transfer arising from the PPy coating, which in turn facilitated enhanced electrochemical performance of the MnO₂–PPy/GF electrode.

The CV results shown in Fig. S3 (ESI†) indicate that the conversion reaction started at a potential below 0.4 V in the first lithiation process, characteristic of the electrochemical reduction of MnO₂ according to eqn (2) and the formation of a SEI layer on the electrode surface. This observation is consistent with a long, stable plateau in the discharge profile (Fig. 5b). In the following delithiation process, two distinct peaks with two sloping plateaus were identified in the charge profile: one appeared at a low potential of ∼0.3 V assigned to delithiation of graphene foam^4,44 and the other peak appeared at ∼1.2 V, corresponding to reversible oxidation of metallic Mn according to the reverse reaction of eqn (2).^45,46

MnO₂ + 4Li⁺ + 4e⁻ ↔ Mn + 2Li₂O (2)

The comparison of the CV curves between the two electrodes, i.e. Fig. S3a vs. b (ESI†), shows that except for the first lithiation in which the SEI layer was formed, the peaks in the following cycles overlapped for the MnO₂–PPy/GF electrode while it took 5 cycles for the MnO₂/GF electrode to achieve a stable lithiation/delithiation behavior. This finding verifies the ameliorating effect of the PPy coating on the electrochemical stability of the electrode during cycling.

3.3. In situ microscopy

In order to gain a better understanding of the influence of the PPy coating on the electrochemical behavior and microstructural stability, an in situ TEM examination was performed to evaluate the microstructural evolution of the electrodes during the initial lithiation process. The results for the MnO₂–PPy/GF and MnO₂/GF electrodes are shown in Fig. 6 and Fig. S4 (ESI†), respectively (see the full video images from Movies 1 and 2, ESI†).

Fig. 6 (a) Sequential in situ TEM images of the initial lithiation process of the MnO₂–PPy nanowire; TEM images and SAED patterns of MnO₂–PPy (b) before and (c) after complete lithiation; (d) schematic illustration of the full lithiation process of MnO₂–PPy; (e) lithiation length measured at the initial stage versus time for MnO₂ and MnO₂–PPy nanowires.

After −2 V was applied to the working electrode, Li ions started to propagate along the wire and initiated the conversion reactions with α-MnO₂, as shown in Fig. 6a. For both electrodes, the original shapes of the wires were maintained even after complete lithiation, and no wire failure or cracking due to volume expansion or mechanical deformation was observed, indicating the mechanical robustness of the α-MnO₂ nanowires against lithiation. The lithiation front was identified by the change in contrast (as marked with red arrows), which moved forwards in a uniform manner along the nanowire length with increasing time. According to the images taken before and after lithiation (Fig. 6b and c), the diameter of the α-MnO₂ single crystal coated with PPy increased from 26.7 to 41.0 nm, equivalent to a linear expansion of ∼ 53.5%, after full lithiation. In contrast, the electrode without the PPy coating experienced a much larger linear expansion of ∼121.8% in diameter, as shown in Fig. S4a to c (ESI†). The considerable difference in linear expansion signifies how effectively the PPy coating prevented the large volumetric expansion of the electrode due to lithiation, which may be one of the most important reasons explaining the enhanced reversible capacities of the MnO₂–PPy/GF composite electrode. The corresponding selected area electron diffraction (SAED) pattern taken from the unlithiated MnO₂ was consistent with the XRD analysis, where three sharp ring patterns are related to the Li₂O crystallographic planes. Another broad ring that freshly appeared after lithiation is attributed to the amorphous Mn, verifying the expected conversion reaction from the tunneling structure of α-MnO₂ nanowires to amorphous Mn. The reversible conversion reactions based on the aforementioned discussion of CV curves (Fig. S3, ESI†), known information from the literature,⁴⁶ and in situ TEM examinations (Fig. 6a–c and Fig. S4, ESI†), are schematically presented in Fig. 6d. The lithiation process took place according to eqn (2) in which Li⁺ ions first intercalated into the MnO₂ crystals to produce an intermediate phase LiMnO₂, followed by a conversion reaction to form amorphous Mn.⁴⁶ During this process, the conductive PPy coating played an important role to buffer the volume expansion in the radial direction while accelerating the lithiation rate along the nanowire length. The conversion reaction was reversible, and thus the abovementioned reactions were reversed in the following delithiation process as confirmed by the CV results. The lithiation lengths (L) were measured along the electrode wires from the in situ TEM images and are plotted versus time (t) in Fig. 6e. Both electrodes exhibited a linear relationship between L and t, and the slopes of these lines represent the lithiation rate at the initial stage. An ultrafast lithiation rate of 34.2 nm s⁻¹ was obtained for the α-MnO₂–PPy, which is almost three-fold faster than 9.3 nm s⁻¹ for the current, uncoated α-MnO₂ and also much higher than 21.2 nm s⁻¹ for similar α-MnO₂ nanowires.⁴⁶ This discovery reveals yet another ameliorating effect of the conductive PPy coating on reaction kinetics and ion/electron transport, which are directly relevant to the lithiation rate.

4. Conclusions

• Arrays of MnO₂–PPy core–shell structures were synthesized on a conductive, cellular GF substrate via hydrothermal deposition of single crystal α-MnO₂ nanowires, followed by a facile chemical polymerization of PPy to apply a thin coating on the nanowires.

• The freestanding composite anode presented remarkable electrochemical performance for LIBs with excellent mechanical stability and a high capacity of 945 mA h g⁻¹ after 150 cycles at 0.1 A g⁻¹.

• A thin layer of conductive and compliant PPy on the MnO₂ nanowires much enhanced the electrochemical performance of the electrode through a large increase in electronic conductivity as well as its capability to accommodate the volume expansion upon lithiation.

• The in situ TEM examination reveals that the MnO₂–PPy nanowires were lithiated over three times faster than the neat MnO₂ due to the ameliorating effect of the PPy coating on facilitating faster ion/electron transport along the wires, leading to a desirable augmentation in MnO₂ conversion reaction kinetics. Moreover, the PPy coating played a critical role in buffering the volume expansion of the wire during lithiation to enhance its structural integrity, partly responsible for the exceptional electrochemical stability and reversibility of the electrode.

• MnO₂ has been considered one of the most useful active materials for LIBs due to its natural abundance and low cost. However, the poor electrical conductivity and large volume expansion upon lithiation have limited its application as an anode in spite of its high theoretical capacity. The findings presented in this work shed new insights into how freestanding hybrid electrodes can be developed using MnO₂ nanowires grown on a conductive 3D GF substrate for LIB applications as well as for other energy devices such as supercapacitors.

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

This project was financially supported by the Research Grants Council (GRF Project codes: 16212814, 16208718) and the Innovation and Technology Commission (ITF Project code: ITS/001/17) of Hong Kong SAR. The authors also appreciate the technical assistance from the Materials Characterization and Preparation Facilities (MCPF) and the Advanced Engineering Materials facilities (AEMF) at HKUST.

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Footnote

† Electronic supplementary information (ESI) available: GF characterization, EDS analysis, CV curves, in situ TEM images of a MnO₂ nanowire and lithiation movies. See DOI: 10.1039/c8qm00153g

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