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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 .
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