CHAPTER VI

S W D STATE BATTERY APPLICATIONS

6.1 BASIC BATTERY CONCEPTS AND PRINCIPLES OF OPERATION .......................................................

6.1.1 Battery Parameters

6.1.2 Cell Classlflcat~on

6.1.3 Problems With Llquld Electrolyte Cells

6.2 SOLID STATE BATTERIES

6.2.1 Advantages of Solid State Batterlrs

6.2.2 L~mrtations

6.2.3 Criteria for the Deslgn of Solld State Batteries

6.3 EARLIER WORK ON SILVER SOLID STATE BATTERIES

6.4 PRESENT WORK

6.5 EXPERIHENTCIL -----------------

6.5.1 Fabrication of Solid Statc Battrricr

6.5.2 Srtup for Polarisation and Discharp. Charactrrlstics

6.6 RESULTS AND DISCUSSION ...........................

6.6.1 Cell Performance

Polarisation Characteristlcs

Discharge Characteristics

Temperature Dependence of OCV of the Cells

Time Deoendence of OCV of the Cells

6.6.2 Cells with Organic Ammonium Iodide Cathodes

OCV: Temperature and Time Dependence

Polarisation Characteristics

Discharge Characteristics

6.7 SUMMARY OND CONCLUSIONS ............................

REFERENCES

CHAPTER VI

SOLID STATE BATTERY APPUCATIONS

6.1 BASIC BATTERY CONCEPTS AND PRINCIPLES OF OPERATION

Battery is a device that ccmverts the chemical

energy, contained in its active materials, directly into

electrical energy by means of electrochemical oxidation -

reduction (redox) reactions. The basic electrochemical unit

of the battery system is called a qalvanic cell. A cell

consists of three maJor parts ( 1 ) Anode, ( 2 ) Cathode and ( 3 )

Electrolyte. The anode gives up the electron during

electrochemical reactron, while the cathode accepts it. The

electrolyte is an ion conductor, which provides the medium

for transfer of ions inside the cell between cathode and

anode. The basic cell scheme is shorn in Figurr 6.1(1). On

connecting the external load (L), the electroch@mical

reaction takes place and the current begins to flow In one

direction. The electron leaves the m o d e and flows through

the load to the cathode. The ions miprate through the

electrolyte t o complete the cell reaction. In Figure

b.l(b), the cell reaction in Mcrcad battery system is shown

a s an example.

lie-

F I Q . 6 . 1 ( a ) S k e t c h o f conventional 11qu1d e l e c t r o l y t e cell.

MERCAD BATTERY SYSTEM .....................

Anode I Cd

C a t h o d e r HQO

C e l l r e a c t i o n r Cd + HpO + HQ .r Cd ( 0 H ) r + HQ

F i p . 6 . 1 ( b ) C e l l r c a c t l o n ~ n Mercad b a t t e r y s y s t e m .

The basic criteria for the selection of anode, cathode

and electrolyte are a s followsr

Anode r

The anode should have hiqh efficiency as a reducinq

agent. Anode should also have very good conductivlty,

stability and ease in fabrication.

Cathode I

The cathode must have hiqh efficient oxifiisinq

properties and should be stable, when it is in contact w:th

electrolyte. The cathode should also provide a usei.,l

working v o l t a ~ e .

The electrolyte must have very high ionic conductiv~ty

and should have negligible electronic conductivity to avoid

internal short circuit. The electrolyte should b e non

reactive with the electrode materials. Tho thanqe in ... .

electrolyte properties with temperature must be low.

6.1.1 Battery Parameters

Some of the battery parameters, which are used to scale

the performance of the battery systems, are deflned here

Open-Circuit Voltage (0CV)r The potentla1 difference

between the terminals of cell or battery, when the clrcult

is ooen (no-load condition).

Current Density1 Current per unit area of the surface of the

electrode.

Discharge Ratel The rate, usually expressed In mil11

amperes, at whlch electrical current 1s taken from the cell

or battery.

Discharge Capacityc The product of discharge current and

discharge tlme taken for a particular drop in cell voltage

(usually 60% of OCV) and is expressed In mll11-ampere-hour

(mAh).

Energy Density: The ratio of the energy avarlable from a

cell o r battery t o its volume (expressed In Wh/L) or welpht

(expressed in (Whr/kg).

6.1.2 Cell Classification

Electrochemical cells are generally idrntlfied as

primary (nonrrchargeable) or secondary (rechargeable) cells.

Within this classification, other classif~catrons are used

t o identify particular structures or designs. Bared on

thxs, the battery systems are c l a s s l f ~ e d Into four malor

types. Table 6 . 1 91ves some examples o f drlferent class of

batterres wlth ~ t s parameters. An excellent text on

prlnclples of battery operatton, fabrrcatlon and performance

1s found In the lrterature [ I ] . The class~ficatlon o f

d ~ f f e r e n t types of electrochemical cells are described below

( 1 ) Primary Batteries: These batterres are once dlscharqed

and drscarded.

( 2 ) Secondary Batteries: These batterxes can be recharged

and reused for several cycles. These are also known as

storage batterles or accumulators.

( 3 ) Reserve Batterres: In these batterles, a key component

1s separated from the rest of the battery prlor to

actrvatlon. The reserve battery deslgn 1s used to meet

extremely long or environmentally severe storage

requirements.

( 4 ) Fuel cells: In these battery systems, the maln

elrctrochemlcal reactants are fed into the system and

cell functions as long as the reactants are supplred.

The fuel IS usually gaseous or aqueous.

6.1.3 Problems with Liquid Electrolyte Crlls

A large vartety o f llquld electrolyte battery systems

are being used today and all these battery systems have the

f o l l o w ~ n g d ~ s a d v a n t a g e s to a g$ter or lesser amount.

( 1 ) Short shelf ltfe due to corrosion o f electrode

materials.

(2) Leakage.

( 5 ) L ~ m i t e d temperature range of operation.

( 4 ) N o mlniaturlsatlon possible.

(3) Less rugged.

6.2 SOLID STATE BATTERIES

In conventional battery system, the Ion transport

m e d ~ u m (or) electrolyte IS a llquid. Recently, availabilrty

of fast ionic conducting solids stimulated the development

of Solid Electrolyte Batteries CZI. These S o l ~ d State Cells

have the following advantages over the convent~onal liquld

electrolyte cells.

6.2.1 Advantagcr of Solid State B a t t e r ~ e s

( 1 ) Absence of leakage or qasslng.

( 2 ) L o w self discharge and a very long shelf life.

(3) High thermal stability.

( 4 ) Abillty t o operate over a wide range of condltlons o f

temperature, pressure, acceleration, etc.

( 5 ) High energy dcnslty.

(6) Excellent packaglnq ++tlclency I wlthout the ie of

separators 1 .

( 7 ) Minlaturisatlon.

However, SOlld state battery systems have the toilowlng

l ~ m ~ t a t ~ o n s , whlch restrlrts their practical utillty.

( 1 ) Low power capacity.

( 2 ) Hlgh impedance of Solld Electrolyte at amblent

temperature.

( 3 ) Mechanical stress aue to volume changes associated

wlth discharge reactions.

( 4 ) Reduced electrode efficiency at high dxscharge rates.

( 5 ) Formation of hlgh resistance discharge product.

6 . 2 . 3 Crltwrla for the Deslpn of S o l ~ d State Battwrles

A cell or battery system consists of electrolyte and

electrode (cathode and anode) materials. Different types of

solld electrolyte materlals of d~fferent ion conducting

s p e c ~ e s have been syntheslsed. Variety of electrode

materlals have been syntheslsed and studled so far.

Excellent revlews and research articles about solrd

electrolyte materlals and electrode materlals are available

~n literature 13-10]. The following arc the important

criteria for the design of solld state batteries.

Solid Electrolyte

The ionic conductlvlty of the solld electrolyte should

be a s high as pas-ible at ambient temperatures, s o that, the

Internal resistance of the fabricated cell IS as minlmur as

posslble t o avoid lowering of potentlal of the cell. The

activation enerpy of the migrating ron should be as minxmum

as posslble. The electronic conductlvlty of the solid

electrolyte acts as an internal short c r r c u ~ t of the cells

and leads to the fast degradation of the cell. Hence, the

electronic conductlvtty must be nep11~1ble. The solid

electrolyte should be stable under arnblent condrtions such

as temperature, pressure, humldity and atmosphere to

facllltate mass production. The Gibbs free energy of the

total electrochem~cal reactions must be negative and large

to get a meaningful voltage.

Electrodes

The electrodes, both anode and cathode, should be good

electronic conductors to support the flow of current throuph

the load. The electrodes must have a good mechanical

stability and able to ulthCctand electrochemical potential

pradients. The anode should be highly electronapative and

c a t h o d e s h o u l d b e h l g h l y e l e c t r o p o s l t l v e .

E l e c t r o d e I E l r c t r o l y t r I n t r r t a c r

The e l e c t r o d e and e l e c t r o l y t e must b r c h r m l c a l l y and

p h y s r c a l l y compatible w l t h each o t h e r . They s h o u l d b e i n e r t

t o a v o l d damage o f I n t e r f a c e r e g l o n b y p r o c e s s e s l t k e I n t e r

d l f t u s l o n , formation o f v o x d s and d e n d r r t l c p r o w t h . The

l n t e r f a c l a l integrity between t h e e l e c t r o d e a n d e l e c t r o l y t e

mus t b e m a l n t a l n e d .

6.3 EARLIER WORK ON SILVER SOLID STATE BATTERIES

The f l r s t s i l v e r b a s e d s o l i d s t a t e c e l l

was r e p o r t e d b y W e l n l n g e r C113 i n 1959. T h l s c e l l h a s an

Open C l r c u l t V o l t a g e (OCV) o f 687 mV a t 2 5 O ~ . Though t h e

OCV 1 s h l g h f o r t h l s b a t t e r y , t h e I n t e r n a l resistance o f t h e

c e l l 1 s v e r y h l g h due t o t h e l o w c o n d u c t l v l t y o f t h e o f R g I

a t 25%. L a t e r , h i g h l o n l c c o n d u c t l v l t y o f t h e o r d e r o f

lo-' S cm-' I n P p S I was r e p o r t e d b y Takahash1 and Yamamato

~ n 1969 1121. The so11d s t a t e c e l l u s l n g PQsSI h a s b e e n

s t u d l e d w ~ t h t h e following c o n f l g u r a t l o n

T h e OCV o f t h e a b o v e c e l l w a s 675 mV a s c o m p a r e d w l t h t h e

theoretical t h e r m o d y n a r n l c a l v a l u e o f 687 mV. T h e I n t e r n a l

r e s l s t a n c e o f t h e c e l l was a b o u t 10 ohms a n d r t c o u l d w t t h .

s t a n d c u r r e n t - d e n s l t y o f 100 p ~ l c r n ? Owens a n d P r p u e C131

h a v e s h o w n t h a t t h e r e a c t ~ o n

may I n c r e a s e t h e l n t e r n a l r e s l s t a n c e o f t h e c e l l a n d

t h e r e f o r e t h e l o d l n e c a t h o d e 1 s n o t chemically c o m p a t ~ b l e .

L a t e r , s u l f u r w a s t r l e d I n t h e p l a c e o f l o d i n e , b u t t h e c e l l

g a v e v e r y l o w OCV o f 230 mV o n l y . C e l l s b a s e d o n o t h e r

s i l v e r h a l l d e s I l k e , A g B r , RgC1 w e r e a l s o s t u d l e d , b u t a l l

t h e s e e f f o r t s h a d a v e r y l l t t l e s u c c e s s a n d o n l y l ~ m l t e d

a p p l ~ c a t t o n o f t h e s e s y s t e m s r e s u l t e d . T h e historical

r e v l e w a b o u t t h e s e s y s t e m s w a s g l v e n b y M u g u d ~ c h 1964 a n d

F o l e y , 1969 C14,151.

A d r a m a t l c d e v e l o p m e n t h a s t a k e n p l a c e a f t e r t h e

d i s c o v e r y o f h l g h L o n l c c o n d u c t ~ o n In f l A ~ r I 5 (M = K, Rb,

NH41 a t r o o m t e m p e r a t u r e , b y Owens a n d A r g u e ~n 1967 C161

a n d B r a d l e y m d G r e r n e I n 1966 C171. Owens a n d c o - w o r k e r s

h a v m fabricated t h e s o l l d s t a t e b a t t e r i e s

w h i c h e r h l b l t e d OCV o f 687 mV. T h e s e c e l l s d e l ~ v e r e d h l g h e r

c u r r e n t b u t , t h e y h a d t h m r m o d y n a m l c t n s t a b ~ l l t ~ . T h e l o d t n e

reacts with the electrolyte to form Rb19 as

whlch agaln resulted In an increased cell resistance.

Later, the cells wlth Rbls cathode were t r ~ e d CIBI.

although these cells exhlblted q<ater s t a b l l ~ t y , the

discharge products were hlghly reslstlve. Besides these

cells, many other cathode materlals, such a s AgzSe-Se, etc.

C191 h a v e also been ~ n v e s t l g a t e d . These cells were found to

exhrblt lower potentials and lower discharge capacltles

compared t o other sllver cells.

The use o f tetra methyl poly-iodldes llke, Me4Nls and

MerNIo (Me = methyl group) as cathodes resulted ln stable

cells. These cells discharged to form lonlcally conductive

s o l ~ d reactron products between sllver lodlde and tetra

methyl ammonlum lodlde C201. Batteries utllrslng thrse

c a t h o d e s have exhlblted better d l s c h a r ~ e properties.

Although the sllver lodlne based battery systems

demonstrated e f f l c ~ e n t performance relatlve t o therr

rntrlnslc c a p a b ~ l l t l e s , thebr commercial usage 1s st111 very

I ~ m ~ t e d . T h ~ s 1s largely d u e t o the low energy densltles

and t h e materlals cost associated wlth the electrode and

electrolyte. However, actlve research IS s t 1 1 1 golng on due

t o t h e ~ r speclallsed appllcatlons, posslblllty o f maklng

m ~ c r o batterles, h ~ g h e f f ~ c i e n t performance, etc. C1,3-53

6.4 PRESENT WORK

In the present work, the hrghest c o n d u c t ~ n g

composltlons o f the glass in Sllver Molybdovanadate (SMV)r

AqI-F1gtO-(MoOP+V20S) and Sl lver Molybdoarsenate (SMPI):

&gI-AgtO-(flo01+AszOs) systems are used as solrd electrolytes

for the application of s o l ~ d state b a t t e r ~ e s . The transport

properties of the h ~ g h e s t conductlng compositlon o f the

glass ~n these two systems are summarlsed ~n Table 6.2

121,221. From the Table 6.2, lt 1s clear that these glasses

have very hlgh l o n ~ c c o n d u c t ~ v i t y and negllglble electronic

c o n d u c t ~ v l t y at amblent condltlons. Moreover, thelr

actlvatlon energy for the conductlng specles 1s also very

less. Thus, SflV and SMA glassy systems are s u ~ t a b l e for the

a p p l ~ c a t l o n o f solld state batterles.

In the present work, solld state b a t t e r ~ e s have been

fabricated using the hlphest conductinp c o m p o s ~ t ~ o n s o f the

glass ~n S M V and S M A systems (Ilsted in Table 6.2) a s solld

electrolytes. The cells are fabricated with different

cathode materials and battery performance 1s studled. In

thls chapter, the laboratory scale study o f the prlmary

batterles are presented and the battery performance 1 s

analysed.

6.5 EXPERIMENTAL

6.5.1 Fabrication o f Solid State Batteries

The solid state cells are fabricated according t o the

scheme,

Anode / Solld Electrolyte (SE) / Cathode

Ag+SE / SMV (or) SMA / C(I+C)SEI+TAAI

where, I - Iodine, C - Graphite and TAAI - Tetra Alkyl

Ammonium Iodide.

Solid Electrolyte: Flnely ground powders o f SMV (or) SMA

glasses, compositions listed In Table 6.2, have been chosen

a s solld electrolytes for the fabrication of batteries.

Anode: Mixer o f silver powder and solid electrolyte in the

weipht ratio 1 : l 1s used as anode material. Analar grade

silver powder (mesh size 400) and the solld electrolyte (SMV

o r SMA glass) are mixed in r e q u ~ r e d proportion and finely

grounded. Pressed pellets of the above mixture 1s used a s an

anode. The addition o f solid electrolyte t o rllver provides

more anode / electrolyte ~ n t e r f a c i a l contact area and hmnce

better interfacral pPOpcrtlcS.

Cathode: Analar grade chemicals are used for the

preparation of cathodes. Different cathode materials, ( 1 )

Iodine and ( 2 ) Mrxer o f iodine and Tetra Alkyl Ammonium

Iodide, have been used for the fabrication of solrd state

batterres. G r a p h ~ t e powder and solid electrolyte materials

are also added to the above cathode materrals In requlred

proportrons. Graphite powder IS added to the cathode to

increase rts electronrc conductrvrty and solrd electrolyte

is added to have better electrolytelcathode interfacral

properties and hence battery performance.

The cell performance is very sensitrve to the chemical

compositrons o f the cathode const~tuents. To select the

best cathode composition, the cells are fabrrcated with

various compositrons of the cathode constituents and the

battery performance is studied. The composrtions o f the

cathode lngredrents are varred accordrng to the following

scheme.

( 1 In the first step, the mrxture o f I+C 1s chosen a5

cathode materxal and the cells are fabricated for

varlous I:C ratios (10:0, 9:1, 8 : 2 , etc.). The

battery performance of these cells are studied and the

best I:C cathode comoorrtion IS chosen.

(iil T o this best IzC composition, solid electrolyte is

added in various weight ratios as (I+C)tSE and

batteries are fabricated and studied.

(iii) Finally, Tetra Alkyl Ammonrum Iodide (TAAI), (Alkyl =

Methyl (or1 Ethyl (or) Butyl) is added In varlous

rattos as C(I+C)+SEl:T&&I.

Cell Construction

The anode and electrolyte layers were pressed at a

pressure of 5000 kp/cme to form a pellet of 10 mm in dia.

and about 1 mm to 1.5 mm In thickness. Cathode pellets are

prepared separately by grlndzng Its constrtuents thoroughly

and pressing to form a pellet of 15 mm In daa. and 1 mm to

1.5mm rn thrckness. The cell 1s assembled by sandwlchlnp

the &node/Electrolyte and cathode pellets between graphlte

discs and copper falls, as shown In Figure 6.2. The cells

are then ~mmedlately sealed by Epoxy resln and stored at

amblent conditions. Set of slmllar cells are fabrrcated and

the Open Clrcult Voltage (OCV), polarlsatlon and dlscharpe

studles are carrxed out.

6 .3 .2 Setup for Polarisation and Discharge Characteristics

Polarisation characteristics are the plots of the

variation of the tcrmlnal voltape as a function of current

drawn from the cell. The circuit shown in Figure 6.3 1s

used to study the polarisation and discharge characteristics

of the fabracated batteries. In the polarisation studies,

the cell voltape is measured as a function of current drawn

Fig. 6.2 Cross sectional vlew of cell assembly. ( 1 ) Hylem holder, ( 2 ) Screw, ( 3 ) Copper foils, ( 4 ) Graphite dlscs, ( 5 ) Anode, ( 6 ) Solld electrolyte, (7)Cathode and ( 8 ) Epoxy sealing.

A

1

I I

, c

@ Fig. 6.3 Circult arrmpemmnt for mearurlng polarlsat~on and

dlscharpe c h a r a c t e r ~ s t ~ c s (C - cell under test).

f r o m t h e c e l l . Different c u r r e n t d r a ~ n s a r e o b t a i n e d b y

c o n n e c t i n g d i f f e r e n t l o a d resistances. The c e l l v o l t a g e IS

m e a s u r e d a f t e r 1 0 s e c o n d s o f c o n n e c t i n g t h e l o a d . The

v a r i a b l e l o a d 1 s p r o v l d e d b y t h e r e s i s t a n c e b o x RL. The

c u r r e n t 1 s measured u s l n g a Ph111ps ammeter ( a c c u r a c y o f

0 .1 nf l ) a n d t h e t e r m i n a l v o l t a g e i s measured u s l n g a m i c r o

v o l t m e t e r ( a c c u r a c y 0.1 p V ) .

The variation o f t e r m ~ n a l v o l t a g e w r t h t i m e a t a

c o n s t a n t l o a d o r c o n s t a n t c u r r e n t d e n s l t y IS known as t h e

d i s c h a r g e c h a r a c t e r ~ s t ~ c s o f t h e s o l i d s t a t e b a t t e r y . F o r

d l s c h a r g e s t u d i e s , t h e circuit shown l n F i g u r e 6.3 i s used .

The c u r r e n t d u r r n g t h e measurement IS monitored b y t h e

ammeter. The d l s c h a r g e s t u d l e s a r e c a r r l e d o u t f o r d i f f e r e n t

c u r r e n t densities (I.@. d ~ f f e r e n t d l s c h a r g e r a t e s ) . F o r

b o t h p o l a r ~ s a t l o n and d i s c h a r g e s t u d l e s , t h e c u t o f f v o l t a g e

was f i r e d a t 400mV, w h ~ c h i s approximately 60% o f t h e OCV.

6.6 RESULTS FIND DISCUSSION

In t h l s s e c t ~ o n , t h e r e s u l t s f o r b a t t e r ~ e s ( u s l n g SMV

a n d SM4 g l a s s e s a s s o l r d e l e c t r o l y t e s ) a r e p r e s e n t e d and

p e r f o r m a n c e r s d l s c u s s r d . T a b l e 6.3 summarlmes t h e r e s u l t s

f o r s e t s o f batteries s t u d i e d w l t h d l f t e r e n t c a t h o d e

materials. The open c r r c u l t v o l t a g e o f t h e c e l l s f o r t h e

c a t h o d e I + C i s f o u n d t o b e 687mV and IS e q u a l t o t h e

NNN- I

I 1 I

? ? ? ? n n a n I

I

b b o n i m m m m I Q Q Q Q I

I I

i no I I.."" I I r m r . 4 I

I I I I

u Y

0 n n n a r

I n I 1 n Y

t II N n

U Y n Y I 1 # I1 0 " I1

n n I 1 n I1 U 11 I 1 I1 ,I I1 11 I 1 U

0 n

9 11 11 I, n I, I! 11 I I, Y ,I 4,

n 11 . I, r. I1

Y U I1 ,I I1

Q a m II a u

U - H n 11

2; ; fi I1 I, 11 U I, W " "

[li- ll - .c 11 U u tl + .d r - 3 n " " I ,

thermodynamically calculated theoretical voltage. The

polrrisation characteristics of various SMV based cells,

differing in cathode composltlon l:C, are shown ~n Figure

6.4. The current drain for the drop In voltage of 0.4V is

about 2 to 5 ma, with the composition I:C = 7:s glving

maximum current o f 5 mA. The discharge character~stics with

the current density of 50 m l c m 2 for these cells are shown

In F ~ g u r e 6.5. From figure, it IS clear that cathode of

composltion I:C = 7:s gives higher d ~ s c h a r g e capaclty and

energy. The cathode of composition 6:4 also gives current

and energy equal to that of 7:3, but thls cathode pellet IS

h ~ g h l y b r ~ t t l e and is not suitable for practical use.

Hence, the cathode I:C = 7:3 IS found to be the best and ~t

1s ftred for further studies.

In the second step, the cells dlfferinp ~n cathode

compositions (I+C):SE are studied wlth I:C = 7 : s and SE =

EMV. The parameters of these cells are 11sted in Table 6.3

The OCV o f the cell wlth (I+C):SE = 9.1 1s found to be 687mV

and that of 7:s is b85mV. In Flgures 6.6 and 6.7, the

polarisat~on and discharge character~stics of these cells

are shown. 611 these cells exhibit almost similar

polar~sation characterlstrcs and gives around JwA current.

The cell with cathode composition (I+C)rSE = 7:s exhlblts

m ~ c e l l e n t discharge characteristics with the energy d e n s ~ t y

of 2.9 Whr/kg (refer Table 6.3). Thus, the addition of SE

700 - (I+C):SE

m +

>

GOO-

F ig . 6.7 D ~ s c h a r p e c h a r a c t e r ~ s t ~ c s fog cells w i t h (I+C)+SE cathodes in SMV a discharge r a t e o f 50 @/cm .

Improves t he performance c f t he b a t t e r y a p p r e c i a b l y and

energy d e n s i t y o f t h l s c e l l 1s h l g h compared t o t he c e l l s of

o t h e r ca thodes.

From these s t u d i e s . i t can be seen t h a t t he cathode o f

c o m p o s i t i o n 7:3 i s t he b e s t . S i m i l a r c e l l s o f ca thode

composition (I+C):SE = 7 : s a re f a b r i c a t e d and t h e i r

d i s c h a r g e c h a r a c t e r i s t i c s a t d i f f e r e n t d i scha rge r a t e s a re

s t u d i e d . The d l scha rpe p r o f i l e s o f these c e l l s a re shown i n

F i g u r e 6.B. The d i scha rge c a p a c ~ t y o f t h e c e l l a t d i f f e r e n t

d ~ s c h a r g e r a t e s a re a lmost equa l , wh ich shows t h a t t he

p r e s e n t b a t t e r y i s s t a b l e f o r v a r i a t i o n s i n c u r r e n t d r a i n s

I n t h e s t u d i e d range.

W i t h t he b e s t ca thode c o m p o s ~ t i o n . ( I+C):SE = 7:3, t he

s t u d y o f c e l l s o f SMA q l a s s as s o l i d e l e c t r o l y t e s a re

under taken. S i m i l a r c e l l s w i t h SnA g l a s s as s o l i d

e l e c t r o l y t e a re f a b r i c a t e d and t h e i r per formance i s s t u d l e d .

The r e s u l t s o f these s t u d i e s a r e summarised i n Tab le 6.3.

The OCV o f t he SMA based c e l l i s found t o be 683mV, which is

almost equa l t o t h e OCV o b t a i n e d f o r t he SMV based c e l l s .

I n F i p u r e s 6.9 and 6.10, t h e p o l a r i s a t i o n and diSCharQe

c h a r a c t e r i s t i c s ( a t d i f f e r e n t d i s c h a r p * r a t e s ) a re shown

r e s p e c t i v e l y . The c u r r e n t d r a i n f o r t h e d r o p i n v o l t a g e o f

0.4V f o r t h e p r e s e n t 6P1CI based c e l l 1s 7.5mA. The S W based

c e l l a l s o e x h i b i t s d i s c h a r p * p r o f i l e s s l m l l a r t o SMV based

cells, proving its stabrlrty for the different current

dralns ln the studled range. The energy dcnslty of the cell

IS about 2.7 Whr/kg.

In the above sectlon, the results for the set of

prlmary cells were presented. In thrs sectlon, the battery

performance is analysed based on results o f polarrsation and

discharge characterlstlcs, temperature and time dependence

of OCV of the cells.

Polarisation Characteristics

In the polarisation characteristics of the above cells,

inltlally, the voltage 1s independent upto a certain draln

current and thereafter ~t starts decreasing. The drop in

voltage of the cell 1s due to polarisatlon by the following

factors, ( 1 ) Ohmic or 1R drop in electrolytes and electrodes

and ( 2 ) Nucleation and crystallisation process at the

electrode / electrolyte interface. Durinp low current drain

these factors are insipnificant and the terminal voltage is

almost constant. Clt high current drains, the ohmic drop is

of the order of the terminal voltage. Further, high current

drain depletes rlectroactive material at electrodclelectro-

lyte interfacial area and increases the cell resistance.

Hence, v o l t a g e o f t h e c e l l d e c r e a s e s a t h r g h c u r r e n t d r a i n .

The c e l l s w r r e a b l e t o p r o v i d e h t g h v o l t a g e s u p t o a

c u r r e n t d r a l n o f n e a r l y 1mA and a f t e r t h a t v o l t a g e f a l l s o f f

d u e t o t h r above m r n t i o n e d f a c t o r s . T h l s 1 s m a i n l y d u e t o

t h e i R d r o p , n u c l r a t l o n a t t h e e l e c t r o d e / e l e c t r o l y t e

~ n t e r f a c e a n d d e p l e t i o n o f e l e c t r o a c t i v e m a t e r i a l s .

D i s c h a r g e C h a r a c t e r i s t i c s

The d i s c h a r g e c h a r a c t e r i s t i c s o f t h e c e l l s e x h i b i t a

s u d d e n d r o p i n t e r m i n a l v o l t a g e , t h e n a g r a d u a l d e c r e a s e I n

t h e v o l t a g e m d f i n a l l y a s t e e p f a l l i n t h e v o l t a g e . The

i n i t i a l f a l l 1 s due t o t h e e l e c t r o d e p r o c e s s m e n t i o n e d

e a r l l e r . I n t h e s e c o n d r e g i o n , d u r i n g discharge, t h e c e l l

r e a c t i o n r e s u l t s i n t h e f o r m a t i o n o f 6 9 1 l a y e r a t t h e

c a t h o d e / e l e c t r o l y t e i n t e r f a c e . A t a m b i e n t t e m p e r a t u r e , t h e

r e s i s t a n c e o f A g I i s h i g h when compared t o t h e I n t e r n a l

r e s i s t a n c e o f t h e c e l l . Hence, t h e i R d r o p a c r o s s t h e l a y e r

i s m o r e m d t h r r e f o r e t h e formation o f A91 l a y e r d e c r r a s e s

t h e t e r m x n a l v o l t a g e . As t h e d i s c h a r g e p r o c e e d s , t h e

t h i c k n r s s o f t h r A g I l a y e r i n c r e a s e s , w h i c h r e s u l t s i n t h e

p r o g r r s s i v c v o l t a g r d r o p a c r o s s t h e t e r m i n a l s .

T c m p c r a t u r r D c p c n d c n c r o f OCV o f thr C r l l s

T h r v a r i a t i o n o f OCV w i t h t r m p r r a t u r e f o r t h e c e l l s

wlth different cathode materials are shown in Figure 6.11.

The OCV of the cells increases wlth temperature. The

varlatlon o f OCV wlth temperature decreases progressively

for cells made up o f Iodine, I+C, and (I+C)+SE cathodes.

Thus, the additlon of graphlte and solld electrolyte not

only improves the polarlsation and dlscharpe characteristics

of the batteries but also stablllses the cells to posses

uninterrupted constant voltage source in the tested

temperature range ~ O ~ C - ~ O ~ C .

Time Deoendence of OCV of the Cells

The fabricated batteries are stored in dry ambxent

conditxon (at 30%) and the OCV of the cells is measured at

regular Intervals. The tlme dependence of the OCV of the

cell up t o one year is given in Figure 6.12. The shelf life

of the battery wlth iodine alone as cathode 1s very poor.

The tarnishing action of lodine molecule with the

electrolyte and other parts of the cells lead to the

decrease in shelf-life of the battery. The addition of

praphite (C) and solid electrolyte (M) to the cathode

reduces the above mentioned effects and stabilires the

cells. But, still there is a non-neglipible drop in the OCV

of the cell, reducinp its shelf-life.

FIQ. 6.11 Temperature dependence of OCV of the cells for different cathode compos~tlon. (1) I+C, ( 2 ) I+C+SE ( 5 ) I+C+SE+TMAI and ( 4 ) I+C+SE+TBAI.

F l g . 6.12 Time dependence o f OCV of the c e l l s fo r d i f f e r e n t cathode cornpositlon. ( 1 ) I, ( 2 ) I+C, ( 3 ) I+C+SE (41 I+C+GE+TNAI and ( 5 ) I+C+SE+TB&I.

6.6.2 Cells with Alkyl Ammonium Iodide Cathodes

In order to improve the performance o f these cells, the

other cathode materials w e experimented. B.B.Owens et a1.

C203 have suggested that some alkyl ammonium lodides nay

react with A91 to form conductive compounds at ambient

c o n d ~ t i o n s and reduce the activity of the iodine. On the

similar prounds, in order to improve the stability of the

present cells under study, the addition of Tetra !Alkyl

!Ammonium Iodides (TfIAIs) to the (I+C):SE cathode 1s

considered. The battery conflguratlon of these cells are

given below

Ag+SE / Solid Electrolyte / C(I+C)+SEI+TAAI

1: 1 / SflV or SMA /

where, Alkyl (A) = Nethyl tM) or Ethyl (E) or Butyl (8).

With (I+C):SE = 7:s and I:C = 7:3. Set of similar cells

were fabricated with various alkyl ammonium iodides in the

weight ratio, C(I+C)+SEl:TAAI = 9 : l .

OCV: Teqerature and Time Dependence

Table 6.4 sunmarises the results tor cells of S W and

SMA solid electrolyte systems respectively. The OCV ot the

SMV based cells are found to be 675mV, 673mV and 66OmV

respectively tor batteries with TAAI = T W I , TEA1 and T W I .

N Y)

? -

0

P

N 2

?

n 2 1 : F I I I Y I 21 1 r( l CI 0 1 - L I D Y I I u 1 - v I a

I I: w I b-

W h l l e , t h e OCV o f t h e SM& b a s e d c e l l s a r e f o u n d t o b e

670mV, 66OmV a n d 6 5 7 mV r e s p e c t r v e l y . The OCV 1 s r e d u c e d

c o m p a r e d t o t h e c e l l s o f C ( I + C ) + S E I c a t h o d e s ~n b o t h t h e

s y s t e m s . T h e t e m p e r a t u r e d e p e n d e n c e o f OCV o f t h e s e c e l l s

1 s s h o w n I n F t g u r e 6.11. F r o m f l g u r e , ~t IS c l e a r t h a t t h e

v a r l a t t o n o f OCV, f o r c e l l s w l t h TMAI a n d 1041, 1s v e r y l e s s

c o m p a r e d t o t h e o t h e r c e l l s . The a b o v e o b s e r v a t t o n s a r e

p r o m l s l n g f o r t h e s e b a t t e r i e s t o p o s s e s h l g h t e m p e r a t u r e

s h e l f l t f e a n d a l s o a n u n z n t e r r u p t e d c o n s t a n t v o l t a g e s o u r c e

tn t h e t e s t e d t e m p e r a t u r e r a n g e 3 0 % - 70°c. T h e OCV v e r s e s

t i m e f o r t h e s e batteries a r e s h o w n l n F l g u r e 6 .12. F o r a

s t o r a g e p e r l o d o f o n e y e a r , t h e OCV v a l u e s d r o p s o n l y b y 15

mV a n d t h l s s h o w s t h a t t h e s e c e l l s h a v e e x c e l l e n t s t a b l l t t y

a n d l o n g s h e l f I l f e .

P o l a r i s a t i o n C h a r a c t e r i s t a c s

F l g u r e s 6 . 1 3 a n d 6 . 1 4 s h o w t h e p o l a r l s a t i o n

c h a r a c t e r ~ s t i c s o f t h e c e l l s w i t h T A A I c a t h o d e s in SMV a n d

SMA s y s t r m s r e s p r c t i v e l y . T h e c u r r e n t d r a i n t h a t c o u l d b e

o b t a i n e d f r o m t h e s e c e l l s p r o g r e s s i v e l y i m p r o v e s b y t h e

a d d i t i o n of TMAI, T E A I anU TEA1 t o t h e c a t h o d e i n t h e same

o r d r r . T h e s e c e l l s c a n b e u s e d up to 1-5 ma w i t h o u t boy

s e r i o u s p o l a r i s a t i o n . T h e d r a i n c u r r s n t f o r t h e d r o p In OCV

o f u p t o 400mV a r e 9mA, 16mA a n d 37mA r e s p e c t i v e l y t o r

c a t h o d e s of TMAI . T E A I a n d T B A I i n SMV c r l l s a n d lOmA, Z O W

and 47me r e s p e c t l v e l y i n SMA c e l l s . The improvement ;-

p o l a r l s a t l o n c h a r a c t e r l s t l c s o f t hese c e l l s are a t t r l b ~ c t ~ d

t o l e s s iR d r o p s even a t h l g h e r c u r r e n t densities compared

t o c e l l s o f (I+C)+SE cathodes which may be due t o the

formation o f l e s s resistive complexes than AgI C203.

Discha rge C h a r a c t e r i s t i c s

The d ~ s c h a r q e p r o f l l e s o f c e l l s w i t h TAAI a t cons tan t

c u r r e n t d e n s i t y o f 5 0 1 r ~ / c m ~ i s shown i n Figures 6.15 and

6.16 f o r SMV and SMA s o l l d e l e c t r o l y t e s r e s p e c t l v e l y . The

d i s c h a r g e c h a r a c t e r l s t r c s o f these c e l l s show l e s s discharge

c a p a c l t y when compared t o t he c e l l s w l t h o u t TAAI I n b o t h SMV

and SMA s o l l d e l e c t r o l y t e s c e l l s . The use o f a l k y l ammonlum

p o l y xod ideo may y l e l d b e t t e r d i scha rge p r o f l l e s . Taking

I n t o c o n s l d e r a t l o n o f t h e l o n g s h e l f l l f e , p o l a r ~ s a t l o n and

d l s c h a r p e c h a r a c t c r i s t l c s , o n l y these c e l l s p r o v i d e us a

r e l ~ a b l e b a t t e r y per formance o f l o n g e r s t a b ~ l l t y .

6.7 SUMMARY AND CONCLUSIONS

The h i p h e s t conducting c o m p o s l t l o n o f t he g l a s s I n SMV

and S M g l a s s y systems a re used as s o l i d e l r c t r o l y t e s f o r

t h e f a b r i c a t i o n o f s o l i d s t a t e b a t t c r ~ e s . The p r t m a r y c e l l s

o f v a r y i n g cathode c o n p o s i t t o n s have been f a b r i c a t e d and

mtud ied . H l g h OCV o f around 687 mV t o 650mV a r e o b t a l n e d

SMV - [(I~C)+SE]:TAAI =9:1

2 - T E A 1

3 - T B A I

zoo0 I 1 I I I I

3 0 60 90 1 T i me(hrs.1

F l g . 6.15 Discharge c h a r a c t e r l r t ~ ~ s f o r t h e C ( I + C ) + S E l + T f l f i I cathodes in SHV based cells a t 50 M / c m .

f o r t h e s e c e l l s , w h l c h s u g g e s t s t h a t t h e fabricated

b a t t e r i e s a r e s u l t a b l e f o r e l e ~ t r o ~ h e m ~ c a l a p p l ~ c a t i o n s o f

c o m m e r c i a l I n t e r e s t . O u r r e s u l t s o n t h e c e l l s o f v a r y i n g

c a t h o d e composition indicates t h a t t h e b a t t e r y p e r f o r m a n c e

IS v e r y s e n s i t i v e t o t h e c o m p o s i t i o n o f t h e c a t h o d e a n d t h e

p r e s e n t s t u d y i s v e r y h e l p f u l t o c h o o s e p r o p e r c o r n p o s i t l o n

o f t h e c a t h o d e constituents. T h e s e t o f b a t t e r y s y s t e m s .

s t u d i e d , p r o v i d e s c u r r e n t d r a ~ n o f a b o u t 5 t o 0 mA a n d

e n e r p y d e n s i t y o f a b o u t 0.5 t o 3.0 W h r / k g . T h e s t a b x l i t y o f

t h e s e c e l l s a r e i m p r o v e d appreciably b y t h e a d d i t i o n o f

a l k y l ammon lum i o d l d e s t o t h e c a t h o d e s t r u c t u r e . F u r t h e r .

t h e p o l a r i s a t l o n characteristics o f t h e s e C e l l s a r e a130

l m p r o v e d b y t h e a d d i t ~ o n o f T 4 A I t o t h e c a t h o d e s .

T h e p r e s e n t s t u d y i n d ~ c a t e s t h a t t h e batteries b a s e d on

SMV a n d SMA s o l i d e l e c t r o l y t e s a r e s u l t a b l e f o r l o w p o w e r

d e v i c e a p p l i c a t i o n s . T h e p a r a m e t e r s o f t h e p r o d u c t i o n t y p e

b a t t e r l e r a r e e x p e c t e d t o b e s t i l l b e t t e r . I n v i e w o f thr

f a c t t h a t t h e p r m s c n t r e s u l t s a r e o b t a l n e d f r o m t h e h a n d

made l a b o r a t o r y c e l l s .

REFERENCES:

1. D a v ~ d Lxnden (Ed.) "Hand Book of Batteries and Fuel Cells" Mc-Graw H111, New York, 1984.

2. B.B.Owens, P.M.Skarstad and D.F.Untcreker In Ref.1, Pape no. 12-1.

3. C.A.C.Sequeria and A.Hooper (Eds.) "Solid State Battertes" NATO Odvanced Study Institute Series Martinuc Nijhoff Publishers, Dordrecht (1985).

4. B.V.R.Chowdar1 and S.Radhakrrshna (Eds.) "Materials for Solid State Batteries" World Scientific, (1906).

5. B.V.R.Chowdari and S.Radhakrlshna (Eds.) "Solid State Ionic Devlces" World Scientific, (1998).

6. R.G.Linford Solid State Ionlcs. 28-30 (1908) 831

7. C.A.Vrncent, F.Bonino, M.Lazzari and B.Scrosati (Eds.) "Modern Batteries" Edward Arnold, London (1984).

8. C.C.Liang Appl. Solid State Sci., 4 (1974) 95.

9. B.B.Ouens Fldvs. Electrochem. & Electrochcm Engp., B (1971) 1.

10. B.B.kholtenc and W.Van Gool In "Solid Electrolytes" P.Hagenmuller and W.Van Gool (Eds.1 academic Press, Neu York, (1978).

11. J.L.Weininper J. Electrocher. Soc., 106 (1959) 473.

12. T.Takrhashi and 0.Yamamoto Electrochem. kt., 11 (1969) 911.

13. B.B.Owens, B.R.Clrpue, J.J.Oroce and L.D.Hermo J. Electrocher. Soc., 116 (1969) 312.

14. J.N.Mugudich In "Encyclopedia of El@ctrochemlstry" Reinhold, New York, (1964).

15. R.T.Foley J. Electrochem. Soc., 116 (1969) 13C.

16. B.B.Owen8 and G.R.Argue Science, 157 (1967) 500.

17. J.N.Bradlry and P.D.Greene Tranc. Farad. Soc., 6 2 (19661 2069.

18. B.B.Owens and G.R.Arpue Abstract no. 281, Presented at 1331-d National Meeting o f Electrochcnical Society, Boston, mass, (may 1968).

19. T.Takahashi, S.Ikeda and 0.Yamamoto J. Electrochem. Soc., 117 (1970) 1.

20. B.B.Owens, J.S.Sprouse and D.L.Warburton Proc. 25th Annual Power Sources Conference Atlantic City, New Jercy, (1972) Page no. 8.

21. N.Satyanarayma, G.Govindara~ and A.Karth1key.n J. Non. Cryst. Solids, 136 (1991) 219.

22. A.Karthikeyan, G.Govindaraj and N.Satyanarayana Mater. Sci. & Enpg., B13 (1992) 295.