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Electrochimica Acta 116 (2014) 188– 193

Contents lists available at ScienceDirect

Electrochimica Acta

jou rn al hom ep age: www.elsev ier .com/ locate /e lec tac ta

ynthesis of mesoporous birnessite-MnO2 composite as a cathodelectrode for lithium battery

rjun Kumar Thapaa,∗, Bill Pandita, Rajesh Thapab, Tulashi Luitelb, Hem Sharma Paudelc,amini Sumanasekeraa, Mahendra K. Sunkaraa, Nanda Gunawardhanad,atsumi Ishiharae,f, Masaki Yoshiod,1

Conn Center for Renewable Energy Research, University of Louisville, Louisville, KY 40292, USADepartment of Chemistry, University of Louisville, Louisville, KY 40292, USADepartment of English, University of Louisville, Louisville, KY 40292, USAAdvanced Research Center, Saga University, Saga 840-0047, JapanDepartment of Applied Chemistry, Kyushu University, Fukuoka 819-0395, JapanInternational Institute for Carbon Neutral Energy Research (WPI-I2CNER), Kyushu University, Fukuoka 819-0395, Japan

r t i c l e i n f o

rticle history:eceived 3 September 2013eceived in revised form 22 October 2013ccepted 12 November 2013vailable online 20 November 2013

a b s t r a c t

Mesoporous polythiophene birnessite (b)-MnO2 has been synthesized by a modified interfacial methodto develop cathode electrode materials for lithium batteries. The N2 adsorption/desorption isothermtest of mesoporous polythiophene MnO2 shows a type IV hysteresis loop, which is characteristic ofa mesoporous structure. Mesoporous polythiophene MnO2 has a high surface area of 226 m2 g−1

with a pore diameter of 5.2 nm The mesoporous polythiophene b-MnO2 cathode electrode for Li-−1 −1

eywords:esoporousanganese oxide

olythiopheneathode electrodeurface area

ion battery exhibited an initial discharge capacity of 305 mAh g at a current density of 10 mA g ,which is almost equal to its theoretical capacity. When applying a current of 300 mA g−1, meso-porous polythiophene MnO2 electrode shows the initial discharge capacity of 211 mAh g−1, whichis 69% of its theoretical capacity. Mesoporous polythiophene MnO2 shows high capacity and goodcycle stability even at high current densities due to its high surface area and fast Li-ion diffusionpath.

. Introduction

Mesoporous MnO2 has attracted substantial research atten-ion in recent years due to its application to various technologies,ow cost, non-toxicity, and environmental friendliness. MnO2 cane used as a catalyst [1] in sensors, solar cells, electrochemi-al capacitors [2–5], Li-ion [6–11], and Li-Air batteries [12–16]ecause of its high electrochemical activity and variable oxida-ion states. Several research studies have focused on preparingarious mesoporous metal oxides with impressive electrochemicalroperties through synthesis, which requires complicated proce-ures of employing template/surfactant-based MnO2 due to its

ighly active electrical properties. MnO2 exists in various crys-allographic forms such as �, �, �, �, �, etc., depending on how

nO6 octahedral subunits are inter-linked. The electrochemical

∗ Corresponding author.E-mail addresses: arjunthap@gmail.com, akthap01@louisville.edu (A.K. Thapa),

oshio@cc.saga-u.ac.jp (M. Yoshio).1 Tel.: + 81 952 20 4729, fax: +81 952 20 4729.

013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved.ttp://dx.doi.org/10.1016/j.electacta.2013.11.032

© 2013 Elsevier Ltd. All rights reserved.

properties of mesoporous MnO2 are highly influenced by particledimension, sample morphology, structure, and synthesis process.Therefore, various morphologies such as rods, belts, wires, ribb-ons, and tubes of low dimensional nanostructured MnO2 havebeen developed. Numerous reports describe template/surfactant-based mesoporous MnO2 due to its impressive electrical properties[17–19]. The birnessite-type manganese oxides with different mor-phologies such as nanobelt [20,21], flower-like microsphere [22],nanobundles [23], flower-like nanowhisker [24], and others havebeen prepared, but the obtained materials have less specific surfacearea in the range of 20-150 m2 g−1[25]. The aim of the present studyis to prepare a facile one-pot synthesis of polythiophene/MnO2(PTh/MnO2) nanocomposite with a submicron-sphere/nanosheethierarchical structure via a modified inorganic/organic interfacialroute [26–28]. The synthesis route does not involve any expen-sive templates, additional directing agents, or complex precursors.The microstructure as well as electrochemical properties of the

prepared composite are thoroughly investigated. The enhancedelectrochemical properties of the resulting product are discussedin terms of its microstructure and the synergic effects of the com-ponent.

imica Acta 116 (2014) 188– 193 189

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A.K. Thapa et al. / Electroch

. Experimental

In a typical synthesis of mesoporous polythiophene MnO2, 0.5 gf KMnO4 was dissolved in 100 ml of distilled water. The pH valuef the solution was adjusted at 2 by slowly adding drops of 0.02 Milute HCl solution. The organic phase was formed by adding 2 mLf thiophene in 100 mL of dichloromethane solution. Both solutionsere cooled to 4 ◦C before the reaction. Finally, a KMnO4 solutionaving pH value of 2 was poured into a beaker with the organicolution of dichloromethane and thiophene. The two-phase layeras clearly observed in a beaker. The mixed solution was kept at◦C for 24 h. After 24 h of reaction, a dark purple-brown powderas collected by centrifugation from the upper aqueous suspen-

ions and then washed five times with distilled water and onceith ethanol, before vacuum drying at room temperature.

Powder X-ray diffraction (XRD, Rigaku Rint 2500) ofolythiophene-coated MnO2 was carried out with CuK� (� =.541 A) at a low-angle scanning range of 0.5-5◦ and a wide-angleange of 10 − 80◦ (2�), with a scan speed of 0.02 degree sec−1.hermal behaviors of samples were examined by thermogravi-atric analysis (TGA, Shimadzu DTG-60) from room temperature

p to 600 ◦C at a heating rate of 5 ◦C min−1 in air. The bondingroperties of the samples were characterized with attenuated totaleflectance fourier transform infrared spectroscopy (ATR-FTIRerkin-Elmer, Spectrum series 100 spectroscopy). Powderedample was placed above ZnSe crystal and pressed with a pressuref 80 psi. Each FTIR spectrum was collected after 50 scans at aesolution of 2 cm−1 from 500 to 4000 cm−1. Morphologies ofamples were examined with field-emission scanning electronicroscopy (FESEM; Hitachi, S-8000) and transmission electronicroscope (TEM, JEOL JEM 2100, 200 kV) imaging. The Brunauer-

mmett-Teller (BET) specific surface area was determined fromitrogen adsorption-desorption isotherm using Nippon Bell, BEL-ORP 18PLUS-FS, Japan. The pore-size distribution was calculatedsing adsorption curve by Barrett-Joyner-Halenda (BJH) method.

The electrochemical characterizations were carried out using aR2032 coin-type cell. The cathode was formed by mixing 10 mgf active electrode with 3 mg of teflonized acetylene black (TAB-2)s a conducting binder. This mixture was pressed onto a stainlessteel mesh. The average thickness of the cathode electrode wasround 18-20 �m (10 mg per active mass of cathode electrode at

cm2), and lithium foil was used as an anode electrode separatedy a porous polypropylene film (ADVANTEC GB-100R). The cathodeas dried at 160 ◦C for 5 h under vacuum. The amount of electrolyteas 0.5 mL. The electrolyte used was 1 M LiPF6-EC: DMC (1:2 by vol-me), supplied by Ube Chemicals Co. Ltd., Japan. Discharge-chargeerformance was carried out in the voltage range of 1.5 − 4.0 V atonstant current densities of 10, 100, 200, and 300 mA g−1. Theyclic voltammetry study was carried out using three cell elec-rode with 1 M LiPF6-EC:DMC (1:2) electrolyte, where mesoporous-MnO2 was used as a working electrode and Li metal was useds both counter and reference electrode. The cyclic voltammetryeasurement was carried out at the voltage range of 4.0 V- 1.5 Vith scan speed of 5 mV s−1 using eDAQ e-corder potentiostat.

. Result and discussion

The synthesized composite was heat-treated in air at 400 ◦C for h with a heating rate of 5 ◦C min−1 to prepare mesoporous poly-hiophene MnO2. Low angle XRD pattern of prepared mesoporousolythiophene MnO2 is shown in Fig. 1a. The characteristic sharp

iffraction peak at 1.02◦ indicates ordered mesoporous structureas successfully synthesized. The wide angle XRD pattern of as-

ynthesized polythiophene MnO2 is shown in Fig. 1b. Powder X-rayiffraction analysis confirms that the as-synthesized material is

Fig. 1. (a) Low-angle, and (b) wide angle XRD of mesoporous polythiophene MnO2

powder.

layered birnessite-type MnO2 (JCPDS No. 80-1098, monoclinic,C2/m, a = 5.15 A, b = 2.84 A, c = 7.17 A), which has a two-dimensional(2D) lamellar structure with an interlayer spacing of about 0.73 nm[29]. The increased relative intensity of the peak located at 37.02◦

may be related to scattering from polymers. The relatively largeinterlayer gap is suitable for insertion/extraction of alkali cationswithin the oxide lattices [30].

The existence of polymer in the prepared MnO2 sample wasconfirmed by ATR-FTIR, as shown in the Fig. 2. The bands from 500-600 cm−1 can be assigned to Mn − O stretching vibrations. The largecharacteristic band appearing at 3390 cm−1 is attributed to O − Hstretching vibration; while the other band at 1625 cm−1 is ascribedto the O − H bending vibration from crystal or adsorbed watermolecules [31]. Vibration bands between 950 cm−1 and 1450 cm−1

originate from stretching or bending bands of polythiophene, sug-gesting the presence of polythiophene in the sample [31,32].

The TGA curve of the prepared sample powder is shown in Fig. 3.The weight loss from room temperature to 200 ◦C can be attributedto the dehydrating of physically adsorbed and chemically bonded

hydrates. The weight loss between 200 ◦C and 490 ◦C is assignedto the pyrolysis of polymer. Another weight loss between 490 ◦Cand 600 ◦C is derived from the self-decomposition from MnO2 to

190 A.K. Thapa et al. / Electrochimica Acta 116 (2014) 188– 193

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ig. 2. ATR-FTIR spectra of polythiophene b-MnO2 nanocomposite before (a), andfter (b) annealing at 400 ◦C for 1 h.

n2O3[33]. The weight fraction of polythiophene in the preparednO2 composite is about 3.76 wt% based on TGA result.The SEM image of the as-synthesized mesoporous polythio-

hene MnO2 is shown in Fig. 4a. It shows that the sample havingound-shaped particles with the size of 50-100 nm aggregatedach others to form large secondary particles of around 200-00 nm with wires coming out from the particle. The FESEM

mage in Fig. 4b shows that the sample is composed of non uni-orm submicron-spheres with diameters between 400-900 nm.he high magnification image clearly shows that the submicron-pheres consist of radial nanosheet at thickness less than 100 nm.he submicron-sphere hierarchical microstructure of the as-ynthesized polythiophene MnO2 is also verified by the highesolution TEM images shown in Fig. 4c. From the TEM images,he observed porous structure from the self-assembly of ultrathinanosheets exists in the entire sphere.

Fig. 5a shows N2 adsorption-desorption isotherms of meso-orous polythiophene MnO composite powder. The isotherm

2urve of polythiophene MnO2 is a typical type IV classification withn H3 hysteresis loop, which suggests the synthesized compositeas a disordered mesoporous structure, in which the mesopores

ig. 3. TGA curves of the prepared polythiophene b-MnO2 nanocomposite powder.

Fig. 4. (a) SEM, (b) FESEM, and (c) High resolution TEM images of polythiopheneb-MnO2 nanocomposite powder.

A.K. Thapa et al. / Electrochimica Acta 116 (2014) 188– 193 191

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and porosity of the structure. The cathode materials in the pres-

ig. 5. (a) Nitrogen adsorption-desorption isotherms of polythiophene b-MnO2, andb) pore size distribution of mesoporous polythiophene b-MnO2.

rise from the packing of ultrathin nanosheets. The mesoporousolythiophene MnO2 has a high BET surface area of 226 m2 g−1.he pore size distribution of polythiophene b-MnO2 is narrow andentered around the diameter of 5.2 nm, as shown in Fig. 5b. Theeasured BET-specific surface area is the highest reported so far for

emplate-free polythiophene MnO2 and, in certain cases, is greaterhan those observed in mesoporous MnO2 prepared by expensiveemplate synthesis [8,18,19,34–36].

Some literature reported interfacial synthesis of nanocompos-tes of polyaniline and MnO2, in which the synthesized nanocom-osite shows irregular microstructures. In organic/aqueous inter-acial synthesis, thiophene is dissolved in organic phase, whileotassium permanganate is from the aqueous phase [37–40]. Therowth mechanism of mesoporous polythiophene b-MnO2 struc-ure can be briefly described as follows. At the beginning, theedox reaction occurs at the interface to form stoichiometric MnO2nd redical cations. Then, chemical coupling of redical cationsccurs. Most of the generated radical cations couple into short-hain oligomers, and only a few fractions of them transfer intoong-chain polymers, due to the extremely high oxidation ability

f the MnO4

−. The generated oligomers then dissolve into organichase while the generated polymers are attached on the surfacef the MnO2 nuclei. Then, the high surface energy of the MnO2

Fig. 6. First discharge-charge curve of mesoporous polythiophene b-MnO2 elec-trode in the voltage range of 1.5–4.0 V with a constant current of 10 mA g−1.

nuclei facilitates anisotropic growth and self-assembly process toreduce surface energy. The coincidence of the anisotropic growthand the self-assembly process bring about the formation of meso-porous b-MnO2 hierarchial structure [41]. Importantly, the insitugenerated polythiophene can act as structure-directing agents toretard the agglomeration of MnO2 nuclei, resulting in the genera-tion of ultrathin mesoporous b-MnO2. The hydrophilic nature of thenewly formed MnO2 and polymers makes the generated compos-ite move away from the interface and then diffuse into the upperaqueous phase [42,43].

The galvanostatic first discharge and charge curves of the poly-thiophene mesoporous MnO2 electrode at the voltage range of1.5-4.0 V with a constant current density of 10 mA g−1 is shownin Fig. 6. The first discharge curve of polythiophene MnO2 showsdifferent, small, flat plateaus in the voltage range of 2.9 V and 2.5 V.However, common MnO2 shows a large discharge plateau around3.0 V during the discharge state [44,45]. Here, in case of poly-thiophene MnO2, the composite electrode having small potentialplateaus at different voltage ranges during discharge may be theresult of some organic impurities within the mesoporous polythio-phene b-MnO2. It has an initial discharge capacity of ca. 305 mAhg−1 during the first cycle, which is nearly equal to its theoreticalcapacity of 308 mAh g−1. The discharge and charge mechanismof polythiophene b-MnO2 shows that the Li-ion intercalation intothe MnO2 electrode at a discharge state is followed by Li- ion de-intercalation at the charge state by the following equation:

xLi+ + xe− + MnO2⇔ LiMnO2

The large surface area of the mesoporous polythiophene b-MnO2 composite cathode electrode enlarges the contact areabetween active materials and electrolyte with Li-ions, which allowsfor a fast Li-ion diffusion path.

Fig. 7 shows the charge-discharge curve of mesoporous poly-thiophene MnO2 cathode electrode in the voltage range of 4.0-1.5 Vwith a constant current of 300 mA g−1. It had an initial dischargecapacity of 211 mAh g,−1 with a highly reversible capacity. Theinitial capacity of 211mAh g−1 even at high current correspondsto a Li-ion insertion ratio of 0.69 in mesoporous polythiopheneMnO2. Generally, the performance of electrode materials was verystrongly influenced by the morphology, purity, crystalline phase,

ence of anion adsorbents (like polyaniline, polythiophene etc.,)give a high power density and sometime give a large capacity dueto the adsorbents [46]. Induced effect by polythiophene (anion

192 A.K. Thapa et al. / Electrochimica Acta 116 (2014) 188– 193

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the reduction peak at 2.7 V vs. Li/Li+ corresponds to the insertionof lithium into octahedral sites between sheets of MnO6 octahedra.

ig. 7. Several discharge-charge curves of mesoporous polythiophene b-MnO2 elec-rode in the voltage range of 1.5–4.0 V with a constant current of 300 mA g−1.

s adsorbed on polythiophene at the charged state), lithium ions strongly pulled down from the MnO2 surface. So, polythio-hene participates the electrochemical reaction and gives a goodycleability and gives a large capacity due to the existence ofolythiophene. We are now conducting a detailed study of thelectrochemical performance of several cathode materials in theresence of anion adsorbents.

The discharge capacity is a function of cycle numbers for theesoporous polythiophene MnO2 electrode at constant current

ensities of 100, 200, and 300 mA g−1, as shown in Fig. 8. Theischarge capacity decreased with increased current density. The

nitial discharge capacity of the mesoporous polythiophene b-nO2 electrode at constant current of 100 mA g−1 is 262 mAh g−1,hile at 200 and 300 mA g−1, it has an initial discharge capacities

f 258 and 211 mAh g−1, respectively. After 50 cycles, dischargeapacities of 123, 119, and 100 mAh g−1 were observed at a cur-ent of 100, 200, and 300 mAh g−1, respectively. The capacityetention rates were 47, 46.2, and 47.2% after 50 cycles at 100,00, and 300 mA g−1, respectively. The coulombic efficiency of the

esoporous polythiophene MnO2 electrode at constant current

ensities of 100, 200, and 300 mA g−1 shows the first initial coulom-ic efficiencies of ca. 86%. After second cycle all the mesoporousolythiophene MnO2 electrodes show almost 100% of coulombic

ig. 8. Discharge capacities versus cycle numbers of mesoporous polythiophene-MnO2 electrode at the current densities of 100, 200, and 300 mA g−1.

Fig. 9. Cyclic voltammetry of mesoporous b-MnO2 electrode in the voltage range of4.0–1.5 V with scan speed of 5 mV s−1.

efficiencies indicating good revesibility. It is well known that bulkbirnessite MnO2 reported so far usually undergoes phase trans-formation to spinel structure after cycling, which brings a largecapacity fade. In the case of nanocomposite birnessite MnO2, thistransformation is usually completed within a few charge dischargecycles [47]. In spite of high initial specific capacities, the rapiddegradation of capacity on cycling of the mesoporus polythiopheneMnO2 electrode results mainly from deintercalation of lithium ionsfrom the interlayer space during cell charge followed by migrationof manganese from the MnO2 layers to the interlayer region. Thesedata of high current densities of the mesoporous polythiopheneMnO2 electrode show the highest values among other manganesedioxides [48].

Fig. 9 shows a typical cyclic voltammogram of 1-3 cycles formesoporous b-MnO2 electrode for Li-ion battery. The cyclic voltam-mogram of the cells were recorded with lithium metal as bothcounter and reference electrode in the voltage range of 4.0 - 1.5 V atthe scan rate of 5 mV s−1. The cyclic voltammetry result shows that

Although capacity is lost on the initial cycle, the cyclic voltammetry

Fig. 10. Rate capability test of mesoporous b-MnO2 electrode at different currentdensities.

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A.K. Thapa et al. / Electroch

ndicates that the process is at last partially reversible. The oxida-ion peak at 3.2 V vs. Li/Li+ shows extraction of lithium from the

nO2 electrode.In order to investigate possible application in high power

ensity devices, the rate capability of the mesoporous b-MnO2 elec-rode was studied in the voltage range of 4.0–1.5 V with differenturrent densities, as shown in Fig. 10. The discharge capacities at5, 50, 200, 500, and 600 mA g−1 are approximately 239, 191, 172,64, 128, and 95 mAh g−1, respectively. When the current density

s reduced to 200 mA g−1 after the rate performance testing, thenO2 can provide discharge capacity close to formerly measured

alue of 160 mAh g−1, indicating its good reversibility and high rateapability.

. Conclusion

Mesoporous b-MnO2 samples having high surface area haveeen successfully synthesized by using aqueous/organic medium.he electrochemical measurement of lithium-ion batteries showshat prepared mesoporous b-MnO2 exhibit high discharge capac-ty, high rate capability, and excellent cycling stability. Even at highurrent density of 200, 500, and 600 mA g−1, mesoporous b-MnO2lectrode can deliver high discharge capacities of 164, 128, and5 mAh g−1, respectively. The mesoporous b-MnO2 structure canrovide high surface area, leading to high current density and ahin pore wall, reducing the lithium ion diffusion path. Addition-lly, mesoporous structure can provide a new approach to reactionctivity in materials for batteries, electrocatalysts for oxygen reduc-ion, and sensors.

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