polymer template-assisted microemulsion synthesis of large.pdf

ORIGINAL PAPER

Polymer template-assisted microemulsion synthesis of largesurface area, porous Li2MnO3 and its characterizationas a positive electrode material of Li-ion cells

Tirupathi Rao Penki & D. Shanmughasundaram &N. Munichandraiah

Received: 20 June 2013 /Revised: 6 August 2013 /Accepted: 9 August 2013# Springer-Verlag Berlin Heidelberg 2013

Abstract Lithium-rich manganese oxide (Li2MnO3) is pre-pared by reverse microemulsion method employing Pluronicacid (P123) as a soft template and studied as a positiveelectrode material. The as-prepared sample possesses goodcrystalline structure with a broadly distributed mesoporositybut low surface area. As expected, cyclic voltammetry andchargedischarge data indicate poor electrochemical activity.However, the sample gains surface area with narrowly distrib-uted mesoporosity and also electrochemical activity aftertreating in 4 M H2SO4. A discharge capacity of about160 mAh g1 is obtained. When the acid-treated sample isheated at 300 C, the resulting porous sample with a largesurface area and dual porosity provides a discharge capacity of240 mAh g1. The rate capability study suggests that the sampleprovides about 150 mAh g1 at a specific discharge current of1.25 A g1. Although the cycling stability is poor, the high ratecapability is attributed to porous nature of the material.

Keywords Lithium-ion cell . Mesoporous . Lithium excessmanganese oxide .Microemulsion route . Polymer template .

High rate capability

Introduction

Lithium-ion batteries have attracted global interest from bothconsumers and researchers during the past a couple of decades[1, 2]. The interest has arisen because of the extended appli-cations, which are successful in small sizes at present andanticipated in large sizes in the future. Although the energydensity of the present Li-ion batteries is greater than that of

Pb-acid batteries by about four times, future requirementssuch as electric vehicle applications require still greater energydensity. The next generation Li-ion batteries thus need novelelectrode materials which can provide greater discharge ca-pacity than the materials in use at present, in addition to theneed that they should be safe, inexpensive, non-toxic, andenvironmental-friendly.

The present Li-ion batteries employ positive electrode ma-terials of the categoryLiCoO2, LiMn2O4, and LiFePO4either in their pure state or with partial substitutions of thetransitional metals. The discharge capacity values of LiCoO2,LiMn2O4,and LiFePO4 are 140, 130, and 170 mAh g

1, re-spectively [3]. Li-ion batteries with greater energy densitythan the present batteries require positive electrode materialsof greater discharge capacity. Compounds which can storemore than one lithium atom per transition metal atom areexpected to provide enhanced discharge capacity. Li2MnO3belongs to this category of materials [4]. Li2MnO3 is consid-ered isostructural to layered LiCoO2 and its formula can alsobe represented as Li(Li0.33Mn0.67)O2. One third of the octahe-dral sites meant for Mn in the crystal lattice are occupied by Liatoms. On the basis of extraction of the total available Li inLi2MnO3, a discharge capacity of 456 mAh g

1 is expected,provided the compound is electrochemically active. Li2MnO3was synthesized in single-phase from the reaction of LiOHand MnO2 [5]. By treating the compound with H2SO4 orHNO3, a discharge capacity of 199 mAh g

1 was obtainedin the first chargedischarge cycle, which decreased rapidly to143mAh g1 in the eighth cycle. Following this report, severalpublications have appeared with varying capacity values[614]. Initial discharge capacity values are generally highfor the activated phases of Li2MnO3, but cycling instability isobserved in all reports.

In addition to the high discharge capacity, an electrodematerial needs to possess high rate capability for the purposeof fast charge or/and discharge. Porous materials are expected

T. R. Penki :D. Shanmughasundaram :N. Munichandraiah (*)Department of Inorganic and Physical Chemistry, Indian Institute ofScience, Bangalore 560012, Indiae-mail: [email protected]

J Solid State ElectrochemDOI 10.1007/s10008-013-2221-1

to possess high rate capability because the electrolyte cancreep into particles and enhance the contact area of theelectroactive surface with the electrolyte [15]. As a result,the material can withstand an enhanced specific current duringchargedischarge cycling. To the best of the authors' knowl-edge, there are no reports on the synthesis of porous Li2MnO3.In the present study, the lithium excess oxide is prepared byinverse microemulsion route assisted by soft polymer tem-plate, namely, Pluronic acid (P123). By treatment in diluteacid and heating, the compound gains a large surface area anddual mesoporosity, which result in providing a high initialdischarge capacity and also a high rate capability.

Experimental

High purity or analytical grade chemicals, namely lithiumnitrate (Aldrich), manganese nitrate tetrahydrate (Aldrich),

Pluronic acid [P123, poly(EO)20-poly(PO)70-poly(EO)20,where EO and PO are ethylene oxide and propylene oxideunits, respectively; molecular weight, 5,800] (Aldrich), lith-ium dodecylsulfate (LDS, Aldrich), cyclohexane (Merck), n -butanol (SD Fine Chemicals), H2SO4 (SD Fine Chemicals),lithium ribbon (0.75 mm thickness, Aldrich), acetylene black(AB, Alfa Aesar), poly(vinylidene fluoride) (PVDF, Aldrich),1-methyl-2-pyrrolidinone (NMP, Aldrich) and 1 M LiPF6dissolved in ethylene carbonate, diethyl carbonate anddimethyl carbonate (2:1:2v /v ) electrolyte (Chameleon) wereused as received.

Li2MnO3 was prepared by reverse microemulsion routeemploying P123 as a soft template. The oil and aqueousphases were prepared separately. For the oil phase, 1.0 gP123 was dissolved in a mixture consisting of 51.2 ml cyclo-hexane and 6.2 ml n -butanol by stirring for 2 h. Then, 0.225 gLDS was added and stirred for 3 h to get a transparentsolution. Lithium nitrate (0.84 g) and manganese nitratetetrahydrate (1.074 g) were dissolved in 15 ml double-distilled water. About 20 % of excess of lithium nitrate thanthe stoichiometric quantity was used. The aqueous phase wastransferred to the oil phase and stirred for 12 h at ambienttemperature. The emulsion was slowly evaporated at 110 C.Awhite gel was obtained. Samples of gel were calcined in airat 400, 500, 600, 800 C for 6 h. Red colored powder sampleswere obtained.

For activation of Li2MnO3, 1.0 g of a sample was added to100 ml of 4 M H2SO4 and stirred at ambient conditions fordifferent durations from 2 to 24 h. The powder was separatedfrom the acid by centrifugation and washed with double-distilled water thrice, finally rinsed with acetone, and driedat 110 C for about 12 h. A black colored powder wasobtained. The acid-treated Li2MnO3 samples were heated ateither 300 or 500 C in air for 4 h. The color of the samplesremained black.

The powder X-ray diffraction (XRD) patterns wererecorded using a Bruker AXS D8 Advance X-ray diffractom-eter at 40 kVand 30 mA using Cu Ka ( =1.5418 ) radiationsource. Nitrogen adsorptiondesorption isotherms wererecorded at 196 C by using Micromeritics surface areaanalyzer model ASAP 2020. The specific surface area wascalculated using the BrunauerEmmettTeller (BET) method

100 200 300 400 500 600 700 8000

20

40

60

80

100

170 C

9 % loss

455 C

8 % loss

II region

240 C

I region

59 % loss

(iv)

(iii)

(ii)

Weig

ht lo

ss / %

Temperature / C

(i)

10 20 30 40 50 60 70 80

Inte

nsity

/ a.

u.

2 / degree

(iv)

(iii)

(ii)

(i)

(330)/

(061)

(060)

(202)

(132)

(201)

(131)

(130)

(110)

(020)

(001)

a

b

Fig. 1 a Thermogravimetry recorded at a heating rate of 10 C min1 ofprecursor gel (i ), sample S5 (ii ), sample S5A6 (iii ), and sampleS5A6H3 (iv ); and b powder XRD pattern of samples S4 (i ), S5 (ii ),S6 (iii ), and S8 (iv )

Table 1 Unit cell parameters obtained from XRD pattern

Sample a () b () c () () Cell volume (3)

S4 4.942 (3) 8.523 (4) 5.008 (4) 109.14 (4) 199.29 (2)

S5 4.934 (4) 8.534 (5) 5.015 (4) 109.20 (2) 199.35 (2)

S6 4.929 (2) 8.530 (2) 5.022 (2) 109.12 (1) 199.49 (1)

S8 4.935 (3) 8.537 (5) 5.026 (4) 109.27 (2) 199.87 (2)

S5A6 5.042 (4) 8.672 (6) 5.045 (4) 110.29 (3) 206.90 (3)

J Solid State Electrochem

in the relative pressure (p /p0) range 0.050.25 from adsorp-tion branch of the isotherm. The pore size distribution wascalculated by BarrettJoynerHalenda (BJH) method fromdesorption isotherm. The morphology was examined using aFEI Co scanning electron microscope (SEM) model Sirion.The chemical composition was analyzed by inductive coupledplasma atomic emission spectroscopy using Varion inductive-ly couple atomic emission spectrometer model Vista-PRO.The elemental analysis for C and H was carried out by usinga Thermo Finnigan FLASH EA 1112 CHN analyzer.Thermogravimetric analysis (TGA) was recorded from ambi-ent temperature to 800 C at a heating rate of 10 C min1

under the flow of O2 gas by using thermal analyzerNETZSCH model TG 209 FI.

For fabrication of electrodes, the active material (80 wt%),AB (15 wt%) and PVDF (5 wt%) were mixed in a mortar andfew drops of NMP were added to obtain a slurry. Stainlesssteel disks (16 mm diameter) were cleaned with water, etchedin 30 % dilute HNO3, rinsed with double-distilled waterfollowed by acetone and air-dried. The slurry was applied ona pre-treated stainless steel disk and dried at 110 C undervacuum for 12 h. The mass of active material was 35 mg cm2. Lithium metal foil was used as a counter cumreference electrode and Celgard porous polypropylene

membrane (2400) was used as a separator. A commercialelectrolyte of 1 M LiPF6 dissolved in ethylene carbonate,diethyl carbonate and dimethyl carbonate (2:1:2v /v ) was usedas the electrolyte. Coin-type cells CR2032 (Hohsen Corpora-tion, Japan) were assembled in an argon-filled MBraun glovebox.

The cells were galvanostatically cycled in the voltage rangefrom 1.5 to 4.4 V at different current densities at room tem-perature. Cyclic voltammetry and chargedischarge cyclingexperiments were carried out using an EG&G potentiostatmodel Versastat and Biologic potentiostat/galvanostat modelVMP3. Rate capability with different current densities wasexamined by using Bitrode battery cycling unit in an air-conditioned room at 221 C.

Results and discussion

Soft chemical synthesis by inverse microemulsion route pro-vides a control over particle size of the product. By dispersinga small volume of aqueous phase consisting of the reactants ina large volume of non-aqueous phase, the reactants are con-fined to micrometer sized reaction zones and particles of theproducts are limited to the size of the aqueous droplets, which

Fig. 2 Scanning electronmicroscopy images of Li2MnO3samples a S4, b S5, c S6, and dS8


are stabilized by surfactant molecules. Sub-micrometer/nanosized product particles are synthesized by this route[16]. The presence of polymeric templates such as P123 inthe reaction medium facilitates the product particles to devel-op porosity. The presence of hydrophilic EO block and hy-drophobic PO block is considered to be responsible for gen-erating porosity on the product particles [17]. By combiningthe salient features of inverse microemulsion and polymerictemplates, synthesis of porous, sub-micrometer sizes cathodematerials, namely, LiFePO4 and LiNi1/3Mn1/3Co1/3O2, weresynthesized in our laboratory [1820]. A similar procedurewas adopted for preparation of Li2MnO3 in the present work.

The gel obtained after evaporation of solvents at 110 Cwas subjected to thermal analysis (Fig. 1a, curve i). There is acontinuous loss of mass between ambient and about 240 Cdue to the removal of solvents and decomposition of nitratesand organic matter. About 58 % of weight loss is observed at240 C. The mass of the sample is fairly constant between 240

and 450 C. There is about 8 % loss of mass between 240 and450 C followed by stability up to 800 C. Therefore, samplesof the gel were heated at several temperatures from 400 to800 C for 6 h. The samples prepared at 400, 500, 600, and800 C are hereafter referred to as S4, S5, S6 and S8, respec-tively. Thermogravimetric analysis of the heated samples(Fig. 1a, curve ii shown typically for sample S5) indicatesthermal stability of the compounds in the temperature rangefrom ambient to 800 C.

Powder XRD patterns of the samples prepared at differenttemperatures are shown in Fig. 1b, which are similar for allsamples. The structure of Li2MnO3 was determined by usingsingle crystal X-ray diffraction by Strobel and Lamber-Andron [21]. Li2MnO3 was described as Li[Li1/3Mn2/3]O2

Table 2 Conditions of preparation, BET surface area and pore diameterof different samples

Sample Conditions of preparation BET surfacearea (m2 g1)

porediameter(nm)

S4 Microemulsion400 C heating 5.8 1045S5 Microemulsion500 C heating 5.6 1025S6 Microemulsion600 C heating 5.3 1030S8 Microemulsion800 C heating 2.6 1050S5A2 Microemulsion500 C heating

2 h acid treatment115 3.9

S5A6 Microemulsion500 C heating6 h acid treatment

89 3.9


78 3.7


74 3.5

S5A2H3 Microemulsion500 C heating2 h acid treatment3 h heatingat 300 C

49 3.6 and 6.1

S5A6H3 Microemulsion500 C heating6 h acid treatment3 h heatingat 300 C

61 3.6 and 7

S5A12H3 Microemulsion500 C heating12 h acid treatment3 hheating at 300 C

44 2.4 and 3.7

S5A24H3 Microemulsion500 C heating24 h acid treatment3 hheating at 300 C

42 3.7

Table 3 Elemental analyses. Mn and Li were estimated by inductivecoupled plasma atomic emission spectroscopy; C and H by CHNSanalysis, and O is the balance

Sample Weight % of elements

Mn Li C H O

S5 39.09 7.78 1.28 0.78 51.07

S5A6 49.06 4.70 0.894 1.31 44.03

S5A6H3 51.79 4.55 0.407 0.67 42.57

0 30 60 90 120 150

0.0000

0.0003

0.0006

0.0009

dVol

/dD

ia /

cm3

g-1

nm

-1

Pore diameter / nm

(ii) (iii)(iv) (i)

0.0 0.3 0.6 0.9

0

20

40

(iv)

(ii)

(iii)

Quan

tity

of N

2 ads

orb

ed /

cm3

g-1

(i)

Relative pressure (p/po)

a

b

Fig. 3 a Nitrogen adsorption/desorption isotherms and b pore sizedistribution BJH curves of Li2MnO3 samples S4 (i), S5 (ii), S6 (iii),and S8 (iv) samples. In a , curves (ii), (iii), and (iv) are, respectively,vertically shifted by 5, 10, and 15 units of y-axis scale relative to theposition of curve (i)


structure of O3-type where the octahedral sites of inter-slabare occupied by lithium-ion and octahedral sites of the slab bylithium and manganese ions in 1:2 ratio. The XRD patterns(Fig. 1b) agree well with the standard pattern of layeredstructure (JCPDS file No. 841634). The (020) and (110)reflections in 2 range 2023 indicate LiMn ordering inthe mixed cation layer and these superstructure reflections aresignatures for Li2MnO3. The XRD patterns of all samples(Fig. 1b) were indexed to Li2MnO3 and lattice constants wereobtained (Table 1). Lattice constants are close to those report-ed in the JCPDS file 841634 (a =4.937 ; b =8.532 ; andc =5.03 ). Similar agreement in lattice constants wasreported for Li2MnO3 prepared from aqueous sol gel meth-od [22]. The average crystallite size of Li2MnO3 sampleswere calculated from diffraction peaks of (001), (201), and(131) planes using Scherrer equation [23] and the averagecrystallite size was 140 nm. The unit cell parameters andalso the crystallite size were nearly the same for all S4S8samples (Table 1).

SEM micrographs of the as-prepared samples of Li2MnO3are presented in Fig. 2. The S5 sample appears to have layer-like morphology with several layers aggregated together andedges projecting upwards. The thickness of layer is about23 nm and length is about 190 nm. With an increase intemperature of preparation, morphology changes to poroussponge-like at 600 C (S6 sample) and plate-like morphologyat 800 C (S8 sample). Thus, the temperature of preparationinfluences the morphology, particle nature, and as well as thesize.

Nitrogen adsorption/desorption isotherms and BJH poros-ity curves are presented in Fig. 3. The adsorption and desorp-tion branches do not merge in the pressure region p /p0 be-tween 0.50 and 0.99 for all samples suggesting porous natureof the samples. The amount of N2 adsorbed at p /p

0=0.99 isabout 40 cm3 g1, which is considered as high. This is attrib-uted to porosity of the materials. The porous nature is alsoreflected in BJH curves (Fig. 3b). There is a broad distributionof pores around 1040 nm diameter. The BETsurface area andaverage pore diameter obtained for all samples are listed inTable 2. The surface area of S5 samples is 5.5 m2 g1 withpore diameter of 1020 nm. There is a marginal decrease insurface area by increasing the temperature of preparation(Table 2). The porosity acquired by the Li2MnO3 samples isattributed to the presence of the polymeric template in thereaction medium of preparation.

Results of elemental analysis and the calculated composi-tion for S5 sample, typically, are provided in Table 3. It islikely that the origin for C and H is the polymer P123. Thecomposition is calculated assuming that Mn is present as perthe intended composition of Li2MnO3. The deficiency of Li(1.57 against intended 2.0) is probably due to loss of Li in theprocess of synthesis. It is reported that the XRD patterns ofcompounds with Li/Mn ratio less than 2 also correspond to thepattern of stoichiometric Li2MnO3 [24].

Cyclic voltammograms (not shown) of Li2MnO3 preparedat different temperatures suggested poor electrochemical ac-tivity of the compound. In general, the positive electrodematerials of Li-ion cells exhibit well-defined reduction andoxidation current peaks of cyclic voltammograms [25]. In thepresent study, redox current peaks were absent for all as-prepared samples. Poor electrochemical activity is alsoreflected in galvanostatic charge/discharge cycling (Fig. 4a).The electrodes were subjected to chargedischarge cyclingbetween 1.50 and 4.40 V at a specific current of 33 mA g1.Although Li2MnO3 was reportedly [14] cycled between 1.50and 4.80 V, the potentials greater than 4.50 V are expected tobe undesirable due to the possibility of decomposition of theelectrolyte and evolution of gases inside sealed cells. Hence,the upper limit of cycling is limited to 4.40 V in the presentstudy, similar to the studies reported on Li2MnO3 electrodeswhich were cycled between 1.50 and 4.50 V in order to avoiddecomposition of the electrolyte [26]. The discharge capacity

0 1 2 3 4 5 6 7 8 9 10 11

20

40

60

80

100

120

(i)

(iii)

(iv)Spec

ific d

ischa

rge c

apac

ity /

mAh

g-1

Cycle number

(ii)

0 20 40 60 80 100 1201.5

2.0

2.5

3.0

3.5

4.0

4.5

2st

ch

1st

dis

Pote

ntia

l / V

V

s Li

/Li+

Specific discharge capacity / mAh g-1

1st

ch

a

b

Fig. 4 a Chargedischarge curves of Li2MnO3 sample S5 at100 A cm2 (specific current 33 mA g1) and b discharge capacityvariation of Li2MnO3 samples S4, S5, S6, and S8 on repeated chargedischarge cycling at a specific current of 3033 mA g1


obtained at a specific current of 33 mA g1 rate is in the range95115 mAh g1 for S4, S5 and S6 samples (Fig. 4). Thecapacity of S8 sample is only about 22 mAh g1. On repeatedchargedischarge cycling (Fig. 4b), the discharge capacity ofS5 sample is fairly stable at 100 mAh g1, whereas thecapacity of S4 and S6 samples decreases gradually. Thecapacity of S8 sample is stable at about 10 mAh g1. Thesevalues are considerably lower than the theoretically expectedvalue of 456 mAh g1. The poor electrochemical of Li2MnO3samples is because Mn is already in +4 oxidation state anddelithiation of it during charging process necessitates an in-crease in the oxidation state to +5, which is unlikely to exist[5]. As the discharge capacity to the extent of 100 mAh g1 isobtained from the S4, S5, and S6 samples, it is presumed thatthese compounds are non-stoichiometric (Table 3) althoughthe XRD patterns (Fig. 1b) match with the standard pattern. Itis likely that the temperature of preparation influences stoichi-ometry of the compound. It was reported that discharge ca-pacity of Li2MnO3 depended on the method of synthesis [25].An initial capacity of about 70 mAh g1 was obtained whenLi2MnO3 was prepared from Mn3O4 at 900 C whereas lessthan 20 mAh g1 was obtained when it was prepared from -MnOOH precursor [26].

It is known that Li2MnO3 can be converted into electro-chemically active phase by treatment in acid [5]. By treatmentin acid, the Li2MnO3 undergoes a partial dissolution of Li2Othereby facilitating the insertion of Li+ ion into the resultingsample during discharge. Thus, the sample gains electrochem-ical activity. As the quantity of removable Li2O depends onduration of acid treatment, it was attempted to activate themesoporous Li2MnO3 samples by treatment in 4 M H2SO4solution for different durations. The S5 and S8 samples were

treated in 4 M H2SO4 for a few hours, and then tested forelectrochemical activity after washing and drying. It wasfound that both the samples delivered higher discharge capac-ity than the as-prepared samples, but the S5 sample deliveredhigher capacity than the S8 sample after acid treatment.Hence, detailed investigations were carried out with S5 sam-ple of Li2MnO3. Sample S5 was subjected to treatment in 4 MH2SO4 for different durations. Samples of S5, which weretreated for 2, 6, 12, and 24 h are hereafter referred to asS5A2, S5A6, S5A12, and S5A24, respectively. Thethermogravimetry data S5A6 sample (Fig. 1a, curve iii) sug-gests a mass loss of 10 wt% at about 170 C. It is thus inferredthat the sample gains protons or H2O to the extent of about10wt%. There is a gradual mass loss between 170 and 800 C.The sample retains about 75 % of its initial mass at 800 C.Powder XRD pattern of S5A6 sample is shown in Fig. 5a.There are some changes observed in the XRD pattern (Fig. 5a)in comparison with the patterns of as-prepared samples(Fig. 1b). The (001) reflection exhibits a split, the superlatticestructure is slightly altered at 2 =23, a new peak is devel-oped next to (130) reflection and the (131) peak is diminished(Fig. 5a). Nevertheless, the unit cell parameters of S5A6sample calculated on the basis of Li2MnO3 structure are listedin Table 1. There is a marginal increase in the values of a , b , , and unit cell volume. Significant changes are not observedin morphology (Fig. 5b) when compared with the data of theas-prepared S5 sample (Fig. 2). However, marked changes areobserved in N2 adsorption/desorption isotherms and BJHporosity curves (Fig. 6). BET surface area measured fromadsorption isotherms are significantly greater than the valuesmeasured for the as-prepared sample (Table 2). The loopbetween adsorption and desorption isotherms (Fig. 6a) is

10 20 30 40 50 60 70 80

Inte

nsity

/ a.u

.

2 / degree

a bFig. 5 a Powder XRD patternand b SEM image of sampleS5A6


wider in relation to the data of the as-prepared samples(Fig. 3). The quantity of N2 adsorbed by the acid-treatedsamples at p /p0=0.99 is about 120 cm3 g1 (Fig. 6a), whichis three times greater than the corresponding values for the as-prepared samples (Fig. 3). Pore diameter decreases to a narrowrange at about 4 and pore volume is also greater (Fig. 6b)than the as-prepared samples (Fig. 3). The increased surfacearea and pore volume are thus attributed to the acid treatmentof Li2MnO3, which is already mesoporous before subjectingto the acid treatment. The chemical analysis of S5A6sample (Table 3) indicates a decrease in the Li contentand also in C and O contents. The quantity of H has increaseddue to acid treatment, which is also reflected in TGA data(Fig. 1a, curve iii).

The electrochemical results of acid-treated samples ofLi2MnO3 are presented in Fig. 7. Cyclic voltammogram ofS5A6 sample (Fig. 7a) shows an oxidation current peak at3.20 Vand a reduction current peak at 2.80 V. Thus, the peak

potential separation is about 0.40 V, which is an indication ofan irreversible nature of electrode process. In addition to themajor oxidation current peak observed at 3.20 V, thereis a minor oxidation peak at 4.20 V. Thus, the cyclicvoltammogram indicates that the inactive phase of the as-prepared Li2MnO3 is converted into electrochemically activephase on treating in 4 M H2SO4 for a few hours, although thepeak potential separation is 0.40 V. The chargedischargecurves (Fig. 7b) contain potential plateaus at 3.20 and2.80 V for charge and discharge process, respectively. Thedischarge capacity calculated from Fig. 7b is 196 mAh g1 forsample S5A6. This value is significantly greater than the valueobtained from the as-prepared sample (Fig. 4b). The variationsof discharge capacity of acid-treated samples on repeatedcycling at a specific current of 30 mA g1 are shown inFig. 7c. The discharge capacity values of samples S5A2,S5A6, S5A12, and S5A24 in the first cycle are 179, 196,183, and 182 mAh g1, respectively, and the correspondingvalues in the tenth cycle are 135, 146, 161, and 110 mAh g1.The coulombic chargedischarge efficiency throughout thecycle-life test is greater than 95 % (Fig. 7c curve v, typicallyfor the sample S5A6). After mild acid treatment, thus,Li2MnO3 samples gain electrochemical activity, but the cy-cling stability is poor. A gradual change in crystallographicstructure is perhaps responsible for the cycling instability.

As thermogravimetry of the acid-treated sample (S5A6)indicated the presence of impurities which were removed at170 C (Fig. 1a, curve iii), attempts were made to heat thissample and to examine the electrochemical properties. Sam-ples of S5A6were heated for 4 h at 300 and 500 C, and testedfor chargedischarge capacity. The sample heated at 300 Cprovided greater discharge capacity than the sample heated at500 C. Therefore samples of S5A2, S5A6, S5A12, andS5A24 were heated at 300 C for 4 h. The resulting samplesare hereafter referred as S5A2H3, S5A6H3, S5A12H3, andS5A24H3, respectively. The thermogravimetry (Fig. 1a, curveiv) of sample S5A6H3 indicates that the sample is stable up to800 C and the impurities present in sample S5A6 wereremoved by heating at 300 C for 4 h.

Powder XRD pattern of the S5A6H3 sample, typically, isshown in Fig. 8a. The patterns of the other heated samples aresimilar to this pattern. The patterns of these samples aredifferent from the patterns of the as-prepared samples(Fig. 1b). Yu and Yanagida reported detailed structural analy-sis of Li2MnO3 and related compounds, recently [24]. Duringacid treatment, there was a gradual reduction of O3 peaks(ABCABC stacking) and an increase in the P3 peaks(AABBCC stacking) as supported by a shift of the mainXRD peak from 2 =18.7 to 19.15, and also by the emer-gence of a peak at 2 =38.3 [24]. On the basis of XRDpatterns, Raman spectra, and TGA results, it was concludedthat after acid treatment and heating, Li2MnO3 transforms intospinel Li4Mn5O12 phase [24]. The SEM image (Fig. 8b)

0 2 4 6 8 10 12 14

0.00

0.01

0.02

0.03

dVol

/dD

ia /

cm3

g-1

nm

-1

(iii)

(i)

(ii)

Pore diameter / nm

(iv)

0.0 0.2 0.4 0.6 0.8 1.00

20

40

60

80

100

120

140

(iii)(ii)

(iv)

Quan

tity

of N

2 ads

orbe

d / c

m3

g-1

Relative pressure (p/p0)

(i)

a

b

Fig. 6 a Nitrogen adsorption/desorption isotherms and b BJH curves ofsamples S5A2 (i), S5A6 (ii), S5A12 (iii), and S5A24 (iv). In a , curves(ii), (iii), and (iv) are, respectively, vertically shifted by 5, 10, and 15 unitsof y-axis scale relative to the position of curve (i)


shows that the morphology of S5A6H3 sample is nearly thesame as S5 and S5A6 samples (Fig. 2 and 5b). Nevertheless,significant changes are observed in N2 adsorption/desorption

isotherms and BJH curves (Fig. 9). The gap between theadsorption and desorption isotherms has increased (Fig. 9a).The quantity of N2 adsorbed at p /p

0=0.99 by S5A6H3

1.5 2.0 2.5 3.0 3.5 4.0 4.5

150

100

50

0

-50

-100

Curr

ent /

mA

cm-2

Potential / V vs Li/Li+

0 50 100 150 2001.5

2.0

2.5

3.0

3.5

4.0

4.5

2nd dis 1st

dis

2nd

ch

Pote

ntial

/ V V

s Li/L

i+


1st

ch

0 2 4 6 8 100

50

100

150

200

0 1 2 3 4 5 6 7 8 9 10 110

20

40

60

80

100(v)

(iv)

(i)(ii)(iii)

Coul

ombi

c effi

cien

cy /

%

Spec

ific d

ischa

rge c

apac

ity /

mAh

g-1

Cycle number

a

b

c

Fig. 7 a Cyclic voltammogramof sample S5A6 at a sweep rate of0.05 mV s1, b chargedischargecurves of sample S5A6 at aspecific current of 30mA g1, andc variation of specific dischargecapacity of samples S5A2 (i),S5A6 (ii), S5A612 (iii), andS5A24 (iv). Variation ofcoulombic efficiency is shown ascurve (v), typically, for sampleS5A6


sample is about 160 cm3 g1, which is greater than the volume(120 cm3 g1) adsorbed by S5A6 sample (Fig. 6a) and signif-icantly greater than the volume (40 cm3 g1) adsorbed by theas-prepared S5 sample (Fig. 3a). Furthermore, the presence oftwo kinds of pores is observed in BJH curves (Fig. 9b). Poresof narrow size distribution around 4 nm are present on allsamples S5A2H3S5A24H3, similar to the acid-treated sam-ples (Fig. 6b). Additionally, another pore with broad distribu-tion around 510 nm has evolved. Initiation of the secondarypore is clearly visible for S5A2H3 sample (Fig. 9b, curve i).Formation of the secondary pore around 7 nm is clearlyobserved for S5A6H3 sample (Fig. 9, curve ii). For the sampleS5A12H3, the secondary pore diameter decreases to 6 nmwith decreased pore volume (Fig. 9b, curve iii) and it de-creases further for the S5A24H3 sample (Fig. 9b, curve iv).Dual porosity is beneficial for electrode materials because thepores allow the electrolytes to creep and tolerate volumeexpansion/contraction during chargedischarge cycling.Thus, both the time of acid treatment and heating influenceto the formation of dual porosity. The chemical analysis ofS5A6H3 sample (Table 3) indicates a decrease in H and Ccontent in relation to S5A6 sample. However, the Mn and Licontents in the samples are nearly the same.

Electrochemistry results are presented in Fig. 10. Cyclicvoltammogram of S5A6H6 sample recorded at a sweep rate of0.05 mV s1 indicates sharp reductionoxidation pair of peaksin the potential region 2.803.20 V (Fig. 10a). In addition tothis pair of peaks, there is another pair of small broad peaksappearing at 4.204.50 V region. This is perhaps due to thepresence of some quantity of LiMn2O4 phase in the sample.The discharge profiles (Fig. 10b) of S5A6H3 sample providesa minor potential plateau at about 4.0 V and a major constant

10 20 30 40 50 60 70 802 / degree

Inte

nsity

/ a.u

a bFig. 8 a Powder XRD patternand b SEM image of sampleS5A6H3

0 2 4 6 8 10 12 14 16 18 20

0.000

0.007

0.014

dVol

/dD

ia /

cm3

g-1

nm

-1

(iii)

(ii)

(iv)

Pore diameter / nm

(i)

-

0.0 0.2 0.4 0.6 0.8 1.00

50

100

150

(iv)(iii)(ii)Qu

antit

y of

N2

ads

orb

ed /

cm3 g

-1

Relative pressure (p/p0)

(i)

a

b

Fig. 9 a Nitrogen adsorption/desorption isotherm and b BJH curves ofsamples S5A2H3 (i), S5A6H3 (ii), S5A12H3 (iii), and S5A24H3 (iv). Ina , curves (ii), (iii), and (iv) are, respectively, vertically shifted by 5, 10,and 15 units of y-axis scale relative to the position of curve (i)


0 50 100 150 200 2501.5

2.0

2.5

3.0

3.5

4.0

4.5

2nd dis

2nd ch

1st dis

Pote

ntia

l / V

V

s Li

/Li+


1st ch

0

50

100

150

200

250

300

0 5 10 15 20 25 30 35 40 45 500

10

20

30

40

50

60

70

80

90

100

0 1 2 3 4 5 6 7 8 9 10 110

50

100

150

200

250

300

(iv)

(iii)

(ii)

Spec

ific

disc

harg

e ca

pacit

y / m

Ah g

-1

Cycle number

(i)

Coul

ombi

c effi

cienc

y / %

Sp

ecifi

c disc

harg

e cap

acity

/ m

Ah g-

1

Cycle number

1.5 2.0 2.5 3.0 3.5 4.0 4.5

-400

-200

0

200

400

600

Potential / V vs Li/Li+

Curr

ent d

ensit

y / m

A cm

-2

a

b

c

Fig. 10 a Cyclic voltammogramof at a sweep rate of 0.05 mV s1,b chargedischarge curves at aspecific current of 30mA g1, andc cycle-life data of sampleS5A6H3. Cycle-life data ofS5A2H3 (i), S5A6H3 (ii),S5A12H3 (iii), and S5A24H3(iv) samples for 20 cycles at aspecific current of 30 mA g1 areshown as inset in c


potential plateau at 2.80 V, which are followed by a gradualpotential fall from 2.80 to 1.50 V. The major charge anddischarge plateaus observed in 2.803.00 V region agree withthe results reported by Yu and Yanagida [24] for acid-treatedand heated samples. The discharge capacity obtained from thefirst cycle is about 240 mAh g1. However, there is a rapidcapacity decrease on repeated chargedischarge cycling(Fig. 10c). Similar results are obtained from all heated samples(Fig. 10c inset).

The results of rate capability study are presented in Fig. 11.For each current density, a fresh cell was employed and it wassubjected to five chargedischarge cycles. At each current,there is a decrease in capacity similar to the data presented inFig. 10c. There is a gradual capacity decrease by increasingthe specific current. It is interesting to observe that about150 mAh g1 of initial capacity is delivered as a specificcurrent as high as 1.25 A g1. This high rate capacity isattributed to the porous nature of the samples.

Conclusions

Lithium-rich manganese oxide (Li2MnO3) was prepared byreverse microemulsion method employing P123 as a softtemplate and studied as a positive electrode material. The as-prepared sample possessed good crystalline structure with abroadly distributed mesoporosity with poor electrochemicalactivity. However, the sample gained surface area with nar-rowly distributed mesoporosity and also electrochemical ac-tivity after treating in 4 M H2SO4. A discharge capacity ofabout 160 mAh g1 was obtained. When the acid-treatedsample was heated at 300 C, the resulting sample with a largesurface area and dual porosity provided a discharge capacityof 240 mAh g1. The rate capability study suggested that the

sample provides about 150 mAh g1 at a specific dischargecurrent of 1.25 A g1. Further work is in progress on dual-porosity lithium-rich electrochemically stable composites ofLi2MnO3.

Acknowledgments The authors thank Renault Nissan Technology andBusiness Centre India Pvt. Ltd. for financial support, and Dr. Subramaniand Dr. Arockia Vimal for helpful discussions. The authors also thank Dr.C. Shivakumara for his help in analysis of XRD patterns.

References

1. Winter M, Brodd RJ (2004) What are batteries, fuel cells, andsupercapacitors. Chem Rev 104:42454269

2. Matsuki K, Ozawa K (2009) Lithium ion batteries. Wiley, Tokyo3. Wittingham MS (2004) Lithium batteries and cathode materials.

Chem Rev 104:427143014. Thackeray MM (1997) Manganese oxides for lithium batteries. Prog

Solid State Chem 25:1715. Rossouw MH, Thackeray MM (1991) Lithium manganese oxides

from Li2MnO3 for rechargeable lithium battery applications. MaterRes Bull 26:463473

6. Kalyani P, Chitra S, Mohan T, Gopukumar S (1999) Lithium metalrechargeable cells using Li2MnO3 as the positive electrode. J PowerSources 80:103106

7. Johnson CS, Korte SD, Vaughey JT, Thackeray MM, Bobinger TE,Shao YH, Hackney SA (1999) Structural and electrochemical anal-ysis of layered compounds from Li2MnO3. J Power Sources 8182:491495

8. Tang W, Kanoh H, Wang KOY (2000) Preparation of a new type ofmanganese oxide by selective lithium extraction from monoclinicLi2MnO3 for lithium rechargeable batteries. J Mater Sci Lett19:13611363

9. Robertson AD, Bruce PG (2002) The origin of electrochemicalactivity in Li2MnO3. Chem Comm 23:27902791

10. Armstrong AR, Robertson AD, Bruce PG (2005) Overchargingmanganese oxides: extracting lithium beyondMn4+. J Power Sources146:275280

11. Pasero D, McLaren V, De Souza S, West AR (2005) Oxygennonstoichiometry in Li2MnO3: an alternative explanation for itsanomalous electrochemical activity. Chem Mater 17:345348

12. Park SH, Sato Y, Kim JK, Lee YS (2007) Powder property andelectrochemical characterization of Li2MnO3 material. Mater ChemPhys 102:225

13. Yu DYW, Yanagida K, Kato Y, Nakamura H (2009) Electrochemicalactivities in Li2MnO3. J Electrochem Soc 156:A417A424

14. Lim J, Moon J, Gim J, Kim S, KimK, Song J, Kang J, ImWB, Kim J(2012) Fully activated Li2MnO3 nanoparticles by oxidation reaction.J Mater Chem 22:1177211777

15. Yang XY, Li Y, Lemaire A, Yu BJG, Su L (2009) Hierarchicallystructured functional materials: synthesis strategies for multimodalporous networks. Pure Appl Chem 81:22652307

16. Boutonnet M, Kizling J, Marie PSG (1982) The preparation ofmonodisperse colloidal metal particles from microemulsions. Col-loids and Surfaces 5:209

17. Kleitz F, Choi SH, Ryoo R (2003) Cubic Ia3d large mesoporoussilica: synthesis and replication to platinum nanowires, carbonnanorods and carbon nanotubes. Chem Comm 17:21362137

18. Sinha NN, Munichandraiah N (2009) Synthesis and characterizationof carbon-coated LiNi1/3Co1/3Mn1/3O2 in a single step by an inversemicroemulsion route. ACS Appl Mater Interface 1:12411249

0

50

100

150

200

250

300

5,05,05,05,05,05,05,05,0 5

125011217535183812491128457

5,0

Spec

ific d

ischa

rge c

apac

ity /

mAh

g-1

Cycle number0

28

Fig. 11 Rate capability of sample S5A6H6. Current density in milliam-pere per gram is indicated. For each current density, a fresh cell was used


19. Sinha NN, Shivakumara C, Munichandraiah N (2010) High ratecapability of a dual-porosity LiFePO4/C composite. ACS ApplMaterInterface 2:20312038

20. Sinha NN, Munichandraiah N (2010) High rate capability of porousLiNi1/3Co1/3Mn1/3O2 synthesized by polymer template route. JElectrochem Soc 157:A647A653

21. Strobel P, AndronBL (1988) Crystallographic andmagnetic structureof Li2MnO3. J Solid State Chem 75:9098

22. Jain G, Yang J, Balasubramanian M, Xu JJ (2005) Synthesis,electrochemistry, and structural studies of lithium intercalationof a nanocrystalline Li2MnO3-like compound. Chem Mater 17:38503860

23. Burton AW, Ong K, Rea T, Chan IY (2009) On the estimation ofaverage crystallite size of zeolites from the Scherrer equation: a criticalevaluation of its application to zeolites with one-dimensional poresystems. Microporous Mesoporous Mater 117:7590

24. Yu DYW, Yanagida K (2011) Structural analysis of Li2MnO3 andrelated LiMnO materials. J Electrochem Soc 158:A1015A1022

25. Park B, Kim YJ and Cho J (2004) in: Lithium batteries: Science andTechnology, G. A. Nazri, G. Pistoia. Kluwer, Boston

26. Park SH, Ahn HS, Park GJ, Kim J, Lee YS (2008) Cyclemechanism and electrochemical properties of lithium manganeseoxide prepared using different Mn sources. Mater Chem Phys112:696701


Polymer...AbstractIntroductionExperimentalResults and discussionConclusionsReferences

polymer template-assisted microemulsion synthesis of large.pdf

Documents