1.1. GENERAL INTRODUCTION

Conduction of electric c u m t is of two form a) e1ectmnic conduction and

b) ionic conduction. The former case is due to the flow of electrons and later one

is due to flow of ions i.e., electric charges accompanied by mass transport. Liquid

electrolyte solutions exhibit ionic conduction, on applied field, by the dissociation

of electrolytes. Similarly, some solid substances exhibit high ionic conductivity of

the order of liquid electrolytes are called Solid Electrolytes (SEsj or Fast Ion

Conductors (FICs) or Superionic Conductors (SICs) and these materials come

under the field of Solid State Ionics [ l - 71.

Solid state ionics is an interdisciplinary area of science, which encloses the

fundamental and application of physics, chemistry and material science involving

all kinds of ionic transports in solid state. Wide range of SIC materials are known

starting from crystalline to organic macmmolecules of different microstructures,

includes single & polycrystaUine, composites, glass/amorphous and polymers of

different forms like bulk, powder, monolith, thin films and etc. The SICs possess

ion transport phenomena in solid state resulting in high value of conductivity (a)

in the range of 1 - 10d - 10-9 Sun1 comparable to that of the order of Liquid

Electrolytes (LEs) with neghgible electronic conductivity and low activation energy

for ion migration \I - 3). The h i ion conduction is due to the migration of ions

through inter-granular or inter-particle boundaries. The SIC materials are applied in elecb-ochemical systems to form solid state ionic devices such as solid

state battaies, capacitors, gas sensors, fuel cell, e l m e m i c a l miniature cells

etc. 11 - 19) and these are shown in the scheme 1.1 [3]. In focus of all these

features, the chemists are enabled to synthesis many newer superionic

conductme materials to sohe the major problem of energy conversion, storage

and other applications [ I - 251.

w n d a r y (-blegeable) battery electric

T cleclrolysis primary w full cell

c power

m t

coloration electric power electrochrornic display (EW photo cell

electric spa1 chemical sensors

Schcmc 1 1 ELbXTKOCt1EMICAL DEVICES

Ttus chapter p m d e s a brid description on the cladication of ion

conducting solids, the types of superionic conductors and the various models for

explaining thc ion transport mechanism m &ses of solid ionic conductors.

~ ~ y , a brid now on various applications of solid electrolytes and the aims of

the present invcsbgation an pnsented.

The phawmnon of ionic conduction is that if an dcctric fi& is applied

acms the adid dectdyte, the randomly oriented ions m m as a who& along the

direction of the appliai Ud. The ionic conduction talas place through the

pmicncr of dekts or imperfections to sustain. The ionic solids were classfied

into two types besed on type of defect or disorder as a) point defect and b) molten

sub-lattice. The point defect types of solids possess ionic mnduction through

l,'mnkel or Schottky defects and hence, the conductivity in- with

tc.mperaturr (1 , 251. In molten sub-lattice type d solids, all ions are available for

4 ontluction since the number of sites in the sublattice is more than the number

of ions and so ions can rnwe freely h r n one position to another with low

;~ct~vation energy psxsing high conductivity 111. The other method of

c~lassification is based on the mobile ions and defkrts concentration (n) into three

T y p e s : a) dilute point defects, n 5 loi8 crn considered to be normal ion

t-onduc-tors and b) txtended defects, n = loxJ cm3 & c) liquid-like molten sub-

latticr, n = 1 W cm 3 wns~derrd to be supenonn conductors

Normal ionic conductors (NICs) are malerials that exhibit low conductivity

of 10 lZ to 10 Ib Scm at ambient temperatuw e.g. NaCI, liCI. CaC12 etc. The ionic

cunduct~on mechanism m NlCs arise from the points defect or the imperfection in

the laturn s ~ t c cmted thermal& and the activation process involves both energy

due lo dckct formation (hd as weU as ion rmgration (hm) [I , 25, 261. The

conductivity expression is orpressed as follows

w h m the tx, prccxponenUal lactor, can be rpresented by

u, = e2vJ7.',4k 1 k 1.2

wherr e is the charge, vo jump or attempt frequency, f correlation factor, h jump

diapna, x fm3b of rnobik charge carriers and N charge canicr density.

1.2.3. Supsrionlc Conductors (SIC.)

Superionic conductors are the dass of ionic solids that exhibit very tugh ionic conductivity of the order 1.0 San-1 with n-ble electronic conductivity of

the order of 10.12 Scrn.1 at ambient as well--as hgh temperature [I]. The

conductivity of SIC mataials is based on the canier ion concentration of about

10" crn =, and hence. no thermally generated defccts i.e., hr = 0. Thus, its

conductivity is expressed as

Tablc 1.1. The d~tIerences between NICs and SICS

- - Ropem I I NICS I SICS I

I

Elecmn~c conduct~vlty , appreuable j n-ble

1

i , I just below the rneltmg / well below the melting 1 I High c o n d u m t y I I j pamt / ~ o m t I

Mobllc chargc carriers

and trmpaatun e&ct

low (1016 - 1018 an3] hgh ( l W cm9)

lughfy dependent / almost independent I I

k t i v a t h a w r ~ y

~=wb

I I ! high (hr and hm) 1 low (hm)

1-2 ev for NaCl 11, I1 / 0.1 ev for 111 i

From the early history of development of science, it is quite well known

that metals arc generally good conductors of electricity and most of the non-

metals are nonconductors or insulators. Later, it was rrahd that certain i

substances, such as common salt in aqueous solution, are good electrolytes of

electricity and other substances, like sugar in aqueous solution, are bad

rlrctrolytes. This led to the development of liquid electrolytes for ionic devices.

I fwever, it had various disadvantages Like

;I. limited Me time of the device because of the corrosion wactions occuning

txween the electrot).te solution and the electrodes

t). leakage of the Liquid electrolyte from the device due to corrosion

c. non-function of the devias below the freezing & above the b i h g points of the

electrolytes and

ti. dpcomprrition reaction occurring at high temperature of liquid electrolyte in

the device.

Iience, the unproved interest to search for newer materials that could

wemmc the above-mentioned disadvantages resulted in the dimvery of solid

electrolytes (St%). In 1833, Farday reported silver ion conductor in silver sulfide - 1301 and m 1899, Nemst found that the oxygen ion conductivity at lugh

temperatun in Yttria slatliked drcOnia ( 8 5 O . 0 Y203 - 1% ZrO?) 1311. In 1913,

Tubandt et al. have reported a h i electrical conductivity in qystalline a-AgI on

transformation of p to a phase at 422 K that has i n s p d large interest in the fieki

of solid-state electrochemistry, to synthesis a number of superionic conductors

126 - 291. During 1932, Tubandt reported a hlgh ionic conducting phase at high

ternperaturn in A&&. &Te, CuBr B Cul 1321. Later, in 1966 and 1%7, the

wpaiaslic muchng A&Sl and sodium p-alurninia wen reported as arhibiting

hi@ ion conductivity at ambient temperature 133, 341. Subsequently in 1967,

Bndly & Or&ne and Owens & Argue independently discovered the silver ion

CuCl

conduction in htAg.4~ (M - K*, Rb*, NH4') which was a break through in the

history of SICS (20a & b]. RbAg& synthesized based on the a - Agl model

consided as a good example of mattrial design, though the structures of these

two materials are not analogous. During the same period, Goodenough et al.

synthesized NASICON (Na,+xZrzPsxSi&,z], a - tailor made crystal in crystal

chemistry to understand the ionic conduction in thmdirnensional tunnel

suucture 1351. Since then, various kinds of newer solid state ion conducting

rlwterials have been synthesized and applied in the fabrication of suitable solid

state ionic devics.

1.4. CIASNFlCATIOll OF SUPERIONIC CONDUCTORS

Superionic Conducting materials are synthffized in large numbers and

c W 1 e d in diITercnt ways according to the properties possessed by the materials.

One of the ways is based on the microstructures and phases that can be broadly

dnided into four categories as follows

a) smp;k/po3rcrystalline, b] glasses/arnorphous, c) polymers and d) composites.

Superionic conducting crystalbe materials of ditferent cation (Ag, Cu*,

ti'. Na', H' etc] and anions (W . F-) conductors as charge canier ions have been

widely invcstigatal 8 reported 11, 3, 1 1 - 151. Examples of cmtalline superionic

conducting materiels are Listed in tabk 1.2 along with their conductivity at a

particular tunpaaturn 120 - 57). Fig 1.2 shous the temperature dependence of

ion conducdvity lor some of the SICS crystalline samples. S ik r ion cnnductors

arc rmet& b& on A@ and arr synthesized by substituting either cation or mion or bolh. The best rrportcd room temperature silver ion amductor is

RbAg& and it is prrpand by Rbl + 4Agl on the cation substitution [20e B b].

b t tims t n ~ t b n of cation conductors st!aned to concentrate main)y on

Large number of lithium based SICs materials because of the small Lit ionic radii

of with hlgh mobility, high energy density, Wtweight and htgh electrochemical

potential. Some of the distinctly known aystalline Li* ion conductors are LiI,

l,ixN. I.izSO4, Li4Si04, U~COJ, Li3Al04, LiCl-MnCh etc. 1211.

Superionic conducting glasses have been explored as materials for solid-

state electrical devim because of its thermodynamic properties of high random

fm energy for the motion of the carrier ion on compared to their respective

aystalline counterpart. In 1973, first, Kunze has reported the hgh ionic

conduction in Agl-AgzSeOs glassy system 1581. Later, large numbers of hgh ionic

conducting glassy compounds with Werent types of ionic species, like Ag, Cu*,

Li', Na' , W , F and 02., have been reported 13, 9 - 12, 591. The SIC glasses

possess not only htgh ionic conductivity but also a number of other inherent

advantagff wer their single/polycrystalline counter parts such as

ude range of selection of amposition.

chemical durability and thereby obtaining range of property control,

isompic propef es, tugh potential,

no grain bounday dect.

conGgurational Oadbility,

various f o m as tailor made,

btential chmxherrucal applications, etc.

Tabk 1.3 ~IVCS the some examples of the SIC ghsy compounds 159 - 731

and also hg. 1 3 shows the temperature dependence of conductivity of som of the

amorphous/glassy SIC materials. The Li' ion conducting glasses find mon

advantages L) the application point of view, since it posscsss high c n q

d d t y . H a r e , Li' ion conducting glasses arr more attention in sdentific ss wdl as technokgy. A d e w on the lithium based SICs arc Ma

discussed later in this chapter. 7

Fig. 1.3. h g o M; 1000/T pbts of some of the SIC glassy systems

l'ablr 1.2 l<xamples of single/ polycrystalline superionic conducting materials

Type of Sics

Silver Ion Conductors

a-A@

RbAg4ls

a - 4gdSI

Copprr ion Conductors

a - Cul

KCudls

a - Ch11Se Ru~CU 161-rCI 13

Litiuum Ion Conductors

Lbso4

Likli(Z1

LJhCh

W G

Sadiurn Ion Conductors

Nap--

Na1-.tlrU~.soO17

Na1.aRomMgD.nAl1osp017

Na. &.lYbZm&io IRPOI~

Conductivity (*I1

I

Temp (K)

420 26 - 29

20a&b

33

36

36

37

38

39

40

4 1

42

43

44

44

45

0.27 1 298

2

0 x 102

0.6

0.11

0.34

1

I x lo4

1 . 5 ~ 10.2

1 . 0 ~ 10'

1.4 x

2.4 x 10.1

4.5 x lo3

5.89 x 10.'

1 513

723

553

423

298

1073

673

723

543

298

643

643 . 673

Potassium Ion Conducton

w-m3

K B - alumina K30 - F a

Oxygen Ion Conductors

ZrOl - Y203

B i a s - W03

BiaNio IVO 9 0 s

B i h IVO 9 0 s ss

Fluorine Ion Conductors

P - %Fa

1x103

6.5 x 104

1.5 x 1B3

1.2 x 10.1

1.0 x 10-I

3.05 x 1 0'

1.28 x loJ

-1.5

53

54

55

56

57

573

,573

573

1273

1023

773

773

600

ci3.F~ 1 -3x104 600

l a F ~ 1 2 ~ 1 0 2 1 Roton Conductors

1

46

47

48

49

50

5 1

5 1

52

298

298

298

I

HUOaP04:4H20 i 4 ~ 1 0 . ~

Sth(%:4H10

hlytungsticacid (PWA)

3 x lo4 1.7 x 10.'

Tabk 1.3 Examples of superionic conducting glasses

59

60

60

GO

61

62

63

Typc of SICS

Silver glasses

60Agl-3OAg?O- 1 0 M s

55 Ag3S - 45 G c S ~

55 A&S - 45 P&

66.7 Ag?S - 33.3 A%%

70 A g P 0 3 - 30 &SO*

6OAgl-24AgaO-6PbO- lob03

30A&O-28&0:,-42T&

W u m Glasses

Na30 - 8 0 2

39 N d ) - 8 Y 2 0 3 - 53 SIO~

60 Na3S - 40 GeSl

Pottatsturn Glass

10W-90SiCb

Leadolessa,

4 o P b ( P C h ) a - 6 0 ~

FIuofdeOtesscj

Z r - a - a - F

Zt-Th-Be-U-F

Conductivity (Sari.')

8 . 5 ~ 1 0 4

1.4 x 10-3

2 . 6 8 ~ 10-=

1 x lo4

5 . 0 ~ 10"

9 x lo3

2.8 x 10"

I 2 8 x 105 I , 373 1 6'7

Temp. (9

298

298

298

298

298

298

373

I

68

69

70

71

72

73

3.39x103 1 573

1 . 5 ~ lo4

1 . 9 ~ 1 0 ~

7 . 0 8 ~ 10"

l.OxlOJ

1 . 0 ~ lo-'

373

748

473

473

473

High ionic condu~ting polymer solid electmlyte (PSEs) was first reported by

Fenton ct a1 [75]. In early seventies, Wnght et al. studied the cation based

polymer solid ekctmlyte such as alkali metal'-salts of LiCFaS03 8b NaSCN,

incorporated in p l y (ethykneaxide) (PEO) 86 poly (propylenecodde) (PPO) matrix, in

which the I' & Na' are the charge carrier ions 121, 761. In 1978, Armond et a1

proved the potenhalities of PSE as practical electrolyte materials in

elecunchemical devict 1771. The dominant class of polymer electrolytes comprises

of Ihe neurral plar poluvmer complexes with alkali metals/divalent/transition

metal/arnrnonium salts and acids. Thr pol}mer solid electrolytes consist of ionic

salts dissolved in a polymer matrix, and exist as solids but possess very high ionic

candumvity of the order of Quid electrolytes. The most common complexes of

poly(eth~1enctudde) (PEO) and W metal salts, MX is as follows

0 \

M'. . .. . ..X

The allrali salts used in the synthesis compcsed of anions of most@

monovalent bns that are large in size, soft and easily pokmed daived from

stmng Ebmstcd d. The most of the lithium salt anions are C104-, CF&-,

SCN., BR., hsFt,., PFs and etc. Table 1.4 gives some exampla of polymer solid

da%dytm with conductivity at W u l a r temperature 177 - 921. Polymer solid

ckcWy%s arc dessified as solvent free polymer salt complexes, solvent swollen

p@mm and p d y ~ l y t c with propaties that lie between those of a solid and

a Mgh viscws quid 178-801. The potyma solid e l m have a . .

and a good thmnel stability. Also, it can be easily prepared in the fonn of thin

Bhns. Thelithium~ekctrolytesystunsmofpracticalin~farthe dmbpnent dh&h magy density battaics. Lithium, copper, silw, and

I I

Table 1.4. Examples of superianic conducting polymer materials

Lithium Ion Conductors

(WdF-PECDME) - LiPF6 10.93xlW 1 - 2 9 3 / 81

I Sodium Ion Conductors 1 I I I

Type of Electrolyte

(PEO) 19 - Nd (PPO)I~ - N a C F S h (PEO)J s - NaSCN

WEER4 - NaCFSO3

Temp (K)

Conductivity ( S c m - I )

Proton Conducting bl.ymers

Reference

other ionic conduaing polymm an used in the solid-state devices and are

available commern'ally.

ionic conductors cont;urung dispersed semnd phase of e k t r d l y

insulating and chcnucally inert in the parent material to m c e the ionic

conductivity arc called composite ionic conductors. The composite electrolytes

arc in k t r n u l t i p k rnatcnals and typically of two-phase solid systems. Liang,

m 1973, h t obselved a remarkable ionic conductivity enhancement when A h 0 3

was added to Lid to form the Lil-A1103 composite 193). The dispersed second

p k particles neither reacted with nor dissolved in the matrix phase. Table 1.5

grves k t of composite ion~c matenals uith ion conductwty measured at a

paNcular tanpelaturr 193 - 97). The composite ionic materials were further

divided into Crystal-Crystal, Crystal-Poiyner. CytaKilass and Glass-Fblymer

Table 1.5 Examples or superionic conducting cornpasite materials

QP 0fEkctrolytc

Crysw-aysa ti1 - m C ~ I - AhOl

fw-Fhl 4-Abcb ~bso4-cas04

LiasCk-h4@,

Temp. (lq

298

298

298

298

773

673

Conductivity (sari.')

L O X 104

5.0 x lo6

1.2 x loJ 1.0 x loJ 1.0 x lo5 3.6 x lo3

Reference

93

94

95

95

%

97

1.6. REVEW ON IlTHIUM BASED GLASSEB

Idithiurn ion conducting glasses with hlgh conductivity were first

synthesized by Otto in 1%6 at relatively low temperatures. Otto reported the

synthesis of different compositions of the M2O - H 3 - SiO2 (M = ti, Na, ICj

systems and the tugh Li' ion conductivity was obtained for borate glasses, where

more than 40 mole% of lithium ion content was incorporated without

dwitrikation. The interest in achieving h~gh conductivity leads to the

incorporation of Large lithium ion content in the form of lithium salts Like LiiS04,

LiCl or t iF 1981. In 1972 and 1077, Jeitschko et al synthesized borate system

(LirROlzX (X - C1, Br, I)], with boracite structure in glass form 199-1011. Dunng

1978- 1981, a large number oflithiurn compounds like LizO - L i - 8 0 3 (n = 1, X

=C1,Br,I;n=2,X=S042;n=3,X=P043J,Li~O-LiF-Li~S03-Li~S04-&&r

LiF - LhO - AI(POJ]~ were prepared and found to &bit conductivity in the order

of - l o 2 to 103 San at 573K 1102 - 1071. During the same mod, Takahashi

and Yarnamoto studied the glass forming ability in h 0 - LiCl - A l z a - systems and pursued for increase in Li' ion concentration by a c h k h g

conductivity of the order 103 Sun1 at 573K [1081. In 1982, large interrst on

lithiurn &loroborate glass oompounds has increased due its hlgh conductivity of

S m 1 at 573K Further, in the WOs - h 0 systun, the rapidly quenched

materials found to exhibit aonductivity of lo4 S m l at 4%K, W S - L b 0 , hO-

SiG sh6wcd 103 Sun at 673K 1109, I 101. Later, in order to enhance the

conductivity of Li' ion conducting glass, the mixed former effect was attempted

and found it suassful in obtaining the conductivity of 1W Sari.' at 473K [I l l ] .

In 1980, lithium ion inun-poratcd sulphide glasses w m synthesi?;ed and found to

be high ion conducting 11 121. In general, sulphide glasses exhibit more than

thnx ordm af conductivity compared m the c w m p o n d q oxide glasses. M e tt el. &orted that &S-m-U system to possss conductivity of 10S

S W ~ at 293K 11 131. The sulfide glasses such as, && - hs s h d the

CoaduaMV of pLnost 104 San.1 at 298K 11 141. Fg. 1.4 shows conductivity vs.

Ng. 1.4. lag a M W / T pbts of some of the tithiurn based SIC glassy systems

tanperRNrC pbts of some Uthium based SIC glassy systems and were also

summarized in table 1.6 198 - 1441.

Table 1.6 Exampks of lithium based superionic conducting glassy materials

Type of electrolyte

42.5 W - 57.5 &OJ 35LbCblOI&C12-3OLi$iQ- 12.5&0~12.5SiO1

63 ti& - 37 W3

3 1.8 Liz0 - 12.3 LiCl - 59.9 &03

29Li30-24LiCl-3AlZOj-44&03

22.1L1&12LiF- 15.8Li&0~-6Li~S04-28.5&r03

2 5 W - 1OLhPO~-65&03

2 0 M - 3 6 L i F - 4 4 & 0 3

4OLhO - 60 S i a

39Li30 - 13&03 - 48SiOa

7 1.5LiSds - 28.5Li&W4

W - L i S C k - S i O l

67LiKl~ - 3 3 M 0 4

65 Wb0~ " 35 SiOa

60 Lip03 - 40 LiF

30W-70LiPO:,

15W-7OLiF- 15AI(POs)s

33 LhS-66 RSs

37!&3-45Lil-l8RSs

50!&3-50W

50Li30-5ONthCb

soIM)-5o'hdh

Conductivity

!%-I)

6.1 x 1V3

9.7 x lo-=

6.3 x 10"

3 . 2 ~ 10"

1 . 5 ~ 1 0 ~

2.3 x

1.0 x lo-3

3 . 1 4 ~ lo4

1 x.106

4 . 3 ~ 1V9

1.9 x

1x105

3 . 5 ~ 1V3

1 x lo4

6 . 7 ~ 1V9

1x106

1.3 x 1W

1 . 1 ~ 1 0 4

1 x lW

4x10J

5 . 3 ~ 1W

6.4 x 1W

Temp

(K)

623

623

298

298

623

473

603

473

373

298

623

373

523

473

298

298

493

298

298

298

623

623

Ref.

98

98

115

115

108

106

116

106

117

118

119

112

120

121

122

122

123

124

125

112

110

126,127

The experimentally detamined conductivity is generally arm@& as a product of a oania concentration and mobility. In the crystalline solids, the

can* concentration may be defined as the density of defects with n femce to

the perfect latikc. Since each carrier is identically situated and has the same

mobility, the m t may be due to the ensemble of ions and it is a simple

multiple of thr single ion cumnt. In case of glasses, it is not simple to explain

because of the lack of r e f m c e of a ptdect lattice and the definition of defect

becomes rather arbitmy. Thus, various theories have been proposed to explain

the bhavior of the conductivity in glass samples 191.

Random sitc model was proposed based on the consideration that all ions

of a partjrular type arc potent$l carriers with a gaussian dstribution of activation

eneves 11451. The potcnual canier ion mobility varies with the distriiution of

activation entrgy and in turn varies with glass composition. In general, the

variation in canier concentration with glass composition is almost constant, and

hcncr the conductivity variation with glass composition is controlled only by a

change in ion mobility. This model was successful in explamng the composition

dependen& of conductivity in superionic conducting glassy materials.

Ravh.re and Souquet haw proposed a theory of the weak electrolyte model

to a@& the km transport in SICS glasss 1146, 1471. In this model, it is

oonskkrrd that e ~~ of the total ions are mbile that amtn'bute to the

amducthi, whik other ions are immobile and canicr ion mobility is Mependent

d cuq&km, in turn with smcture. The basis of the model is the

cornlation of the ionic conductivity (a] with the thermodynamic activity (a) and

their dependence is given by a = K lajll2 where K is a constant (148- 1491. The

ionic conductivity (a) and the activity (a) are correlated in silica based glasses

1146, 1471.

Minami et al. proposed diffusion path model on the bas15 that the

depmdmct of ion mobity, canier ion concentration on glass compositions and

the distribution of conducting species in diaerent anionic environment. In this

model, there exists two types of ion population in the glass matrix as, ion located

in the W e surmundjngs, wtuch are found to be mobile and hence contribute to

Fig 1.5. Pormtia] energy dmgram~ cxpbmhg the conduction

'p~#rssinthtSlCs

conduction, thwc associated with the oxy-anion that are immobile and paml

covalena exists between canier ion & non-bridging oxygen ions 1150j. The

repmentation of the potential energy by a schematic diegram is shown in 6g 1.5.

The wide ahabw potential is formed of alkali ion wiq halide ions & the n m ,

deep potcntial weil is of alkali ion interaction with oxide ions. Since glass is

considemi to be highly disordered structure, it also possesses random

distribution of activation energy among the potential well formed as shallow-

s h a h , shallowdecp and deep - deep. The increase in the alkali halide

conantration inotases the mobile ion concentration present in shallow wells,

and the shallow wells arc connected for long period with the formation of

favorable diffusion path for the ion transport and thereby increase conductivity

11511. Several experimental EXAFS results support the existence of mobile and

immobik silver ions in these types of glasses 1 152- 1561.

Anderson and Stuart proposed a model for the microscopic transport

mechanism of ions m glasss. Accordingly, ionic transport in glasses occurs by

means of difusive motion of ions betwen encrgchcally stable sites located in the

glass s t rum. The potential barrier W that the ion must surmount can be

apParacd aa the sum of two emxgia W= Eb + Es. Eb is an elcamstatic binding

magy, whb3 is coulomb turn associated with the removal of an ion h m its

mtavailing charged environment at one site to a position midway between two

neighboring Bftek i3 is strain energy encountered by the ion when it pses

thmugh the gateway lormed by the fully bonded bridging ion sites (157-1591. ?he

~ a n d S n L a r t m o d e l a I l o w s ~ t i o n o f t h e t w o c o n t n ' b u t i o n S t o t h e

activatkm in a number of silicate glasses. Letex waal sevsaltions have

been mede to the Andem and Stuart model and this plwides a usdul

~~ errptanatlan tw the ion transport phenommn in glasses.

The duster bypass model considers that glass can be mgarded as a

cmgclation (fmEen state] or ordered microdomains or clustns corn& by a

connective tissue. On cooling the glass below T, the &ual liquid that inihally

sumunds the dusters solidifits to give a highly disordered phase refemd to as

connective tissues 1160, 1611. The essence of cluster bypass model, as suggested

ty Ingram et al, is that the preferred pathways for ion migration lie outside the

cluster but lie within the connective tissue. This model, besides explauung the

ton transport in various glasses, prwides a simple explanation for the mixed

alkali effect in ion conducting glasses.

Elliott & Owens proposed diffusion controlled relaxation model for the

r n i m p i c tmnspo~ mechanism of ions in glasses. The DCR model assumes

the applicability of an Andason-Stuart model i.e., in addition to the ekctmstatic

binding en= and strain energy contributions to the activation energy for

diffU.sion motion of ions, a contri%ution to the p h k a t i o n (a.c.) conductivity is

attniuted to the double occupancy effect [162, 1631. In thii modei, it is

wndlerrd that there are energctxdy stable sites (NBOs) for the mobile ions to

rrsidc in the oxide gla~sy mcture . Ionic transport occurs by means of diffusive

motion between the equivalent sites resulting in the primmy rrlaxational event

with a characteristic miaoscopic relaxation time r. Howeva, when a cation hops

into one of the vidnal equivalent sites will result in the a'cation of double

occupancy Imorm m intasti- effm on instantaneous d a x a t h process. 'l'hia modd qWns the bquency dependence of conductivity that what the c i r t r u ~ ~ I . e i a x a t i o n i s t q ~ e c t h v n s p o n s i b l e f o r ( ~ ) ~ n l a x a t i o n . Tbc

ch- Brom nonaponcntial (KWW) behavior to DebycWP (sin@

behavior both for very small ion concentrations and at high temperatures, can be

readily explained by this model.

Bunde et al. proposed a dynamic structure model based on the m t i o n of

fluctuating pathways, within a dynamically determined structure, for ion

transport in glasses (164, 1651. The main features of this model are i) the ion

transport is a hopping process ii) the mobile ions are acfive in creating glass

structure and iQ the glass structure continues to change at a local level even far

below TI. Based on the assumption that the process of ion hopping with bxaJ

stnrctural rrlaxation OCCUIS together, the ion mwcs on from its own site to the

naghboring site than the m m r y of the cation mts and so a empty site

remains there for sometime, I. Such empty site constitutes the p r e f d

pathways for ion migration and the effect caused is known as site memory effect

The succfirr of this model includes the dixovery of a simple power law (a- cv)

opaative in sin& cation glasses and the quantitafjve eluadation of many

featurn of the mixed alkali effect. The main feature of this model is that the glass

structure must p i s t for time scales much longer than the inverse hopping

ram of individual cations.

Funkc, Ingram and Bunde have developed a unikd site relaxation model

baaed on the jump relaxation model and dynamic saucture model (1661. The

beaic idea ol the model is that the ion transport is stmngly influenced by their

mutually rrplbive , c o u h b intaactions. As a amscquena, their hopping

motbnisnotarandomprocars,butkatunsastrongpnf~~sbcellcd

atm&aj --- hoppine acquenas. If an initial btward jump of an

bn k fdbud by a comlatcd backward jump, the former can k ngrrrQd s?r

irnsu~~~.ssTul. If however the ion manages to stay at the new site and neighboring

Ions rcanana: themselves causing site relaxation, then the initial fonvard hop is

called sumssful The later relaxation proctss (Coulomb relaxation] caum a

shifting of the Coulomb cage potential felt by the central ion and thereby initiates

the site nlaxation. Contrary to the jump relaxation model, which assumes that

;ill sites an geornetricaliy alike, the unified site relaxation model (like the dynamic

btructure model) considers at least two lands of sites.

SICS arr applied in the fabrication of ionic devices such as solid state batteries,

fuel cells, gas and humidity sensors, pressure gauges, timers, oxidation c a m ,

thennometers, thermoelectric generators, capacitors, coulometer, electIochrornic

displays etc. Kiukkola and Wagner applied oxygen ion conducting solid

dtctrotytc, for the fist time, in the fabrication of ionic devices to determine the

thermodynamic p r o w e s of tugh ternperaturP materials (3, 9-16]. Flood and

Boyc initiated the use of molten sulphates in e l m e m i c a 1 cells. Yao and

Kumma applied the sodium P-alumina, as solid electrolyte, in the h@ energy

density batteries. Owens Bb Argue and Bradley & Greene independently

discmud room temperature superionic solids M&& (M=K, Rb, NHd 1341. A

n u m b of Jolid electrolym and mixed mnductors are available for advanced

potcntiometrk solid state gas sensors. Liu and Woml have developed sensor

using multicxxnponent M g W 4 system, which is chemically stable and inherent

high ionic conductivity. Takada et al. developed lithium battery with mysulfide

glam with cspability up to almost 1 dm=. Polymer solid - l y t ~ ~ were

a m h the amsmctbn of ckmchmmic window applications. Gmblatt a al. dmbped lagt number oT solid state h d t y with various arddc

hydmtm~/hydddcs with layered network structures. m w , many new types of SQ syn- and applied in fabrication of various ionic devices

13, 1671. The tech* af syntheses and designing newer ionic deiies with

rnm etlkiency is being referred as Solid State lonics' (SSI) and the term SSI was

first used by Ruf. Takehiko Takehashi, Nagoya University, analogous to solid

state electronics [1,3]. Table 1.7 gives examples of application of dBmnt dass of

solid electrolyte materials 1168-1721. The pr&ciple, fabrication and

characteristics of solid state batteries will be discussed in 6fth chapter.

Table 1.7 Examples of Application of Superionic Conductors

Mataials

LiO - Vfi - S i a

Thin Glms

w 4 & h N b o 3

Lil

(EC-PC) +C104) - (-1

Gel membranes matrix

ATPiOs, WC and Sic seandphase&AbOs

Application

Rechargeable cell

Smart windows

Microfabricated solid state

secondary battery

Solid state supercapadtors

Sensors

Refmncc

168

169

170

17 1

172

The miew on the research work in the field of superionic conductors

inspired to investigate the synthesis of lithium based superionic conducting

glessy samples and also to study their properties. In b e present investigation,

the following three lithium based SIC glassy systems are undertaken for the

study.

SYSTEM PROCESS

W - & O s - S i Q lithium bomilimte (LBSj sol-gel

LiR - hOs - E b 0 3 - SiOl lithium phosphoborosilicate (LPBS) sol-gel

Li@ - V& - Pas lithium vanadophosphate (LVP) melt quench

All thne systems, different formers and m d i e r to formers compositions,

by vEuying molecuku wught perantages of various components, of glassy

compounds arc prepared by sol-gel Bs melt quench methods. The prepared LBS,

LWS & LVP samples are characterized by XRD, DSC, FTIR and electrical

conductivity studks. The impedance studies are canied out to Iind the h@

conducting coolposition, the appropriate equivalent drcuits, type of mobile

species and the distribution of relaxation process of the glass in each qmtem. The

obsavcd conductivity results in these glasses are an@yzed using the existing

theoreticel modds.

The LBS, LWS and LVP glassy systems, the hgh ionic and the negligible

electronic conducting composition of the compound is chosen from each system

for solid statc battay (SSB) applications. Fabrication of solid state battmk and

the open circuit v o l k art measund for all fabricatad sample battmk using the

folbwfnetnndconfigurPtion.

Anodc (Na) / SE (LBS or LWS or LVP) / Cathode (L+C+SE). 24

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