Linden D., Reddy T.B. (eds.) Handbook of batteries

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SOLID-ELECTROLYTE BATTERIES 15.9

15.2.5 Storage

Long-term storage tests show that there is no loss of capacity after storage periods up to 1

year at 20, 45, and 60

⬚C (Fig. 15.7). The battery can withstand storage to 200⬚C, but for

best performance, such high temperatures should be avoided. Temperatures above 200

⬚C

may cause bulging and failure of the seal. The long shelf life results from the chemical

compatibility of the cell components, the absence of chemical reaction between the electrodes

and the electrolyte, and the low electronic conductivity of the solid electrolyte which mini-

mizes self-discharge. Longer-term tests show that there is no measurable loss of capacity

after storage periods of 4 year at 20

⬚C and up to 1 year at temperatures as high as 100⬚C.

On the basis of these tests it is projected that the shelf life of these batteries is at least 15–

20 years under normal storage conditions.

FIGURE 15.7 Storage tests of solid-state battery. Discharge after (a)no

storage: (b) 1 year at 45⬚C: (c) 1 year at 60⬚.(Courtesy of Duracell, Inc.)

15.2.6 Handling

Solid-state batteries are designed primarily for low power and long service life. These bat-

teries can withstand short circuit and voltage reversal, although these conditions should be

avoided. No explosion due to pressure buildup or chemical reaction is known to occur under

recommended operating temperatures. Prolonged short-circuiting will result in a separation

between the electrode and the electrolyte, making the cell inoperative.

15.3 THE LITHIUM/ IODINE BATTERY

15.3.1 General Characteristics

Lithium/ iodine batteries are based on the reaction

Li ⫹ ⁄

I → LiI

The speciﬁc reactions for the cell using poly-2-vinylpyridine (P2VP) in the cathode are

Anode 2Li → 2Li

⫹

⫹ 2e

Cathode 2Li

⫹

⫹ 2e ⫹ P2VP 䡠 nI

→ P2VP 䡠 (n ⫺ 1)I

⫹ 2LiI

Overall 2Li ⫹ P2VP 䡠 nI

→ P2VP 䡠 (n ⫺ 1)I

⫹ 2LiI

15.10 CHAPTER FIFTEEN

Lithium and iodine are consumed, and their reaction product, LiI, precipitates in the region

between the two reactants. The LiI not only is the discharge product, but also serves as the

cell separator and electrolyte. The theoretical energy density is 1.9 Wh/cm

(see Table 15.2).

Practical values approaching 1 Wh /cm

can be obtained at discharge rates of 1 to 2

␮

. Commercially available lithium/iodine batteries have a solid anode of lithium and a

polyphase cathode which is largely iodine. The iodine is made conductive by the addition

of an organic material. Pyridine-containing polymers are most often used for this purpose,

the additive in all present commercial batteries being P2VP. At ambient temperatures the

iodine/ P2VP mixtures are two-phase in undischarged batteries, liquid plus excess solid io-

dine.

The iodine content of the cathode decreases during discharge of the battery, and the

remaining cathode material becomes hard as the battery nears depletion. As discharge pro-

ceeds, the layer of lithium iodide becomes thicker. The resistance of the battery also increases

because of the growing amount of discharge product.

The volume change accompanying the cell discharge is negative. The theoretical value

for this volume change is

⫺15% for complete discharge of a balanced mixture of pure iodine

and lithium.

It is somewhat less when the chemical cathode is not pure iodine. For example,

a volume change of

⫺12% is expected if the cathode is 91% iodine by weight.

The volume

change may be accommodated by the formation of a porous discharge product or of mac-

roscopic voids in the cell.

Cells are formed by contacting the iodine-containing cathode directly with the lithium

anode. The chemical reaction between these two materials immediately forms a thin layer

of lithium iodide between anode and cathode. This layer serves to separate the two elec-

troactive materials electronically and prevents failure due to internal short-circuiting of the

anode and cathode. This makes them especially suitable for applications requiring very high

reliability.

Features of this system include low self-discharge, high reliability, and no gassing during

discharge. Shelf life is 10 years or longer, and the cells can take a considerable amount of

abuse without any catastrophic effects. Batteries of this type have found commercial appli-

cations powering various low-power devices such as cardiac pacemakers, solid-state mem-

ories, and digital watches. Power sources for portable monitoring and recording instruments

and the like are also possible applications.

All the currently available Li /I

batteries have a nominal capacity of 15 Ah or less, and

most have deliverable capacities under 5 Ah. All the Li /I

batteries intended for medical

applications are designed to be cathode-limited.

15.3.2 Cell Construction

Several generic types of Li /I

cells have been produced, three of which were designed for

medical applications such as cardiac pacemakers. Figure 15.8 shows the ﬁrst type, which

was phased out in the early 1980s. This unit had a case-neutral design and consisted of a

stainless-steel housing with a plastic insulator that lined the inside of the case. A lithium

envelope (the anode) ﬁtted inside the plastic and contained the I

(P2VP) depolarizer. The

cathode current collector was located in the center of the cell. Current collector leads from

both the anode and the cathode went through hermetic feed-throughs in the case. This cell

was formed by pouring molten iodine depolarizer into the lithium envelope. After the cathode

material solidiﬁed, the top of the lithium envelope was closed, the plastic cup added, and

the ﬁnal assembly completed. The construction used in this cell eliminates any contact be-

tween the case and the iodine depolarizer.

SOLID-ELECTROLYTE BATTERIES 15.11

FIGURE 15.8 Model 802/ 35 Li / I

cell. (Courtesy of Catalyst Research Corp.)

FIGURE 15.9 Cutaway view of typical can-positive

Li/I

cell. (Courtesy of Wilson Greatbatch, Ltd.)

A second construction uses a case-positive design of similar size. This cell is the type

used today for most medical applications. A cutaway view is shown in Fig. 15.9. This cell

is manufactured in a slightly different manner from that in Fig. 15.8. It contains a central

lithium anode and uses the stainless-steel case of the cell as the positive-current collector.

Most models are completely assembled with their header welded to the can before the cath-

ode is added to the cell. Hot depolarizer is poured into the cell can through a small ﬁll port,

which is later welded shut. The anode current collector is brought out via a glass-to-metal

feed-through.

15.12 CHAPTER FIFTEEN

FIGURE 15.10 Exploded view of case-positive

Li/I

cell. (Courtesy of Catalyst Research Corp.)

Manufacturers of these case-positive designs also precoat their anode assembly with a

layer of pure P2VP prior to assembly. This coating is designed to protect the anode from

the environment before assembly, but it also alters the electrical discharge behavior (see Sec.

15.3.4).

Another case-positive cell type has been used for medical applications. This unit is very

similar to the other case-positive designs, but the cathode is not poured into the battery can.

The iodine and P2VP are pelletized and then pressed onto the central anode assembly. After

the pressing operation, the entire unit is slipped into a nickel can. An exploded view of this

cell is shown in Fig. 15.10.

Case-neutral designs were developed to prevent corrosion of the exterior case and to

minimize leakage to the feed-through by the iodine depolarizer. However, 5 years of real-

time data have shown that no signiﬁcant corrosion of stainless steel in contact with the

cathode depolarizer or its vapor takes place in sealed can-positive cells. Tests show that

corrosion occurs during the ﬁrst few months after assembly and is limited to a 50-

␮

m layer.

Even at 60

⬚C, corrosion of stainless steel by the iodine depolarizer has not proved to be a

problem in the dry environment of the cell.

SOLID-ELECTROLYTE BATTERIES 15.13

Li/I

medical batteries are produced in a variety of sizes and shapes to meet speciﬁc

applications. Their proﬁles range from rectangular to semicircular, or a combination. All of

them are made quite thin and have ﬂat sides because their primary application is in cardiac

pacemakers. Cell thickness is typically 5 to 10 mm. The area of the lithium anodes ranges

between 10 and 20 cm

in current batteries. Some cells use a ribbed anode (see Fig. 15.9)

to increase the amount of active anode surface area in the battery.

The nonmedical batteries are made in more conventional button and cylindrical conﬁg-

urations. The hermetically sealed button-cell batteries are intended for powering digital

watches and serving as backup power for computer memories. These batteries are made by

pressing iodine cathode and lithium anode layers into a stainless-steel cup. The cup is the

positive-current collector. A glass-to-metal feed-through brings the negative terminal to the

exterior. Figure 15.11 shows a view of this cell type. The cylindrical (D-cell diameter) Li /

battery is welded hermetically, like the other batteries described. It is case-positive; the

negative connection is a button on the end of the cell. It is designed to withstand substantial

shock, vibration, and abuse without venting, swelling, leaking, or exploding. These button-

type batteries are no longer commercially available.

FIGURE 15.11 Button-type Li/ I

battery. (Courtesy of Catalyst Re-

search Corp.)

Manufacturing of all Li/ I

batteries is done in a dry environment (typically less than 1%

RH). In addition, all these batteries are sealed hermetically to prevent exchange of any

material with the environment. Good sealing is required in order to maintain the desired

electrical characteristics.

Connection to the medical-grade batteries is made by soldering or spot welding. The

case-positive varieties usually have a pin or wire welded to the case to facilitate making the

positive connection (see Figs. 15.9 and 15.10).

Manufacturers keep detailed records of the construction and manufacture of each battery

intended for medical applications and each unit is individually serialized. This procedure

allows the systematic tracing of the history and behavior of every battery, should the need

arise.

15.3.3 Commercially Available Batteries

Table 15.6 summarizes the manufacturer’s speciﬁcations for some typical pacemaker batter-

ies. Batteries for medical use are relatively expensive because of the low manufacturing

volume and demands for high reliability.

15.14 CHAPTER FIFTEEN

TABLE 15.6 WGL Speciﬁcations for Lithium/ Iodine Pacemaker Batteries

Type

Manufacturer’s

rated capacity,

Ah Wt., g Vol., cm

Size, mm

(length

⫻

width ⫻

height)

Energy

density

Wh/ cm

Speciﬁc

energy

Wh/g

9107

9114

9331

8711

8831

9412

9085

9438

9105

9236

8426

9074

8708

8431

8402

8843

8207

8950

8077

8206

8041

9111

0.43

0.56

0.82

0.86

0.89

0.92

0.98

0.99

1.00

1.07

1.15

1.21

1.28

1.29

1.36

1.55

1.58

1.78

1.98

2.10

2.36

5.3

7.0

9.1

9.5

10.7

10.2

10.9

10.7

10.6

11.0

11.6

12.9

13.4

13.8

14.4

13.6

17.2

15.9

18.9

20.4

22.0

22.8

1.40

1.82

2.34

2.52

2.75

2.20

2.93

2.95

2.85

3.02

3.28

3.52

3.67

3.72

3.73

4.70

4.45

5.11

5.54

5.81

6.24

⫻ 4.2 ⫻ 13

⫻ 6 ⫻ 14

⫻ 5 ⫻ 19

⫻ 5 ⫻ 22

⫻ 5 ⫻ 16

⫻ 5 ⫻ 21

⫻ 5 ⫻ 25

⫻ 6 ⫻ 21

⫻ 6 ⫻ 25

⫻ 5 ⫻ 23

⫻ 5 ⫻ 18

⫻ 5 ⫻ 22

⫻ 5 ⫻ 31

⫻ 5 ⫻ 22

⫻ 6 ⫻ 23

⫻ 7.8 ⫻ 25

⫻ 6 ⫻ 31

⫻ 7 ⫻ 22

⫻ 7.8 ⫻ 31

⫻ 8.6 ⫻ 26

⫻ 7 ⫻ 31

0.81

0.82

0.93

0.90

0.86

1.11

0.89

0.93

0.94

0.93

0.91

0.92

0.97

0.87

0.94

0.92

0.95

0.96

1.00

0.22

0.21

0.24

0.22

0.24

0.25

0.24

0.25

0.24

0.27

0.24

0.26

0.25

0.26

0.25

0.27

Courtesy Wilson Greatbatch Ltd.

15.3.4 Discharge Performance Characteristics

The open-circuit voltage of a Li /I

battery is very near 2.80 V. This value is maintained

through the useful life of the battery. The resistance of the lithium iodide discharge product

controls the load voltage for most of the discharge. The morphology of the lithium iodide

discharge product, in turn, depends on the way the battery is constructed. Only at the end

of the discharge cycle is the resistance of the iodine depolarizer signiﬁcant in the total internal

resistance of the battery. Because the battery resistance is quite high and increases throughout

life, the discharge curves are not ﬂat, even at moderate current drains. Two characteristic

types of discharge curves are observed.

Batteries made by putting the depolarizer against bare lithium anode metal discharge by

growing a nearly planar layer of lithium iodide. This layer becomes thicker in proportion to

the amount of discharge; hence the load voltage during this ﬁrst phase of the discharge

decreases linearly with the amount of charge removed from the battery, until all the available

iodine is consumed. At that time the voltage drops at a steeper rate, indicative of the in-

creasingly signiﬁcant resistance of the iodine-deﬁcient depolarizer. In the ﬁnal, third phase

the open-circuit as well as the load voltage drops as iodine is no longer available at unit

thermodynamic activity. The cells for medical use are made cathode-limited so their end-of-

life voltage change is much less abrupt than if they were lithium-limited. The normalized

slope (Volts per Ampere-hour) for the straight-line portion of the discharge curves is ap-

proximated by:

⌬E 3600Mi i

⫽ 䡠 ⫽ K

⌬QF

␴

dA A

SOLID-ELECTROLYTE BATTERIES 15.15

where ⌬ E/⌬Q ⫽ slope of discharge curve, V /Ah

⫽ molecular weight of LiI

⫽ density of LiI

␴

⫽ lithium ion conductivity of LiI

⫽ Faraday’s constant

⫽ discharge current

⫽ area of anode, cm

Literature values of the Li

⫹

conductivity in LiI range from 0.2 to 0.8 ⫻ 10 ⍀ 䡠 cm at

⫺

37⬚C.

16–18

K then lies between 1.5 and 6.0 ⫻ 10

. Experimental values of K measured from

discharge data have been found to be between 1.0 and 1.5

⫻ 10

. At high discharge rates,

fracturing of the LiI layer apparently occurs, and this equation holds only over small portions

of the discharge curve.

The resistance of these batteries behaves consistently with the load voltage curves. It

builds up linearly with discharge until the cells are depleted of iodine. At discharge rates

much above 10

␮

A/cm

a nonohmic component of the polarization begins to become in-

creasingly important.

The nonohmic component appears to originate near the discharge

product-cathode interface. The effects of this polarization may last a long time after high-

rate discharge. Thus extrapolation of performance between rates is not simple.

A different discharge curve is obtained from lithium /iodine batteries which are con-

structed with a coating of pure P2VP applied to the anode before the iodine depolarizer is

added to the battery. This coating alters the discharge behavior of these cells. The discharge

product no longer grows in a planar fashion but rather in columnar-type groupings. It has

been shown that a liquid breakdown reaction product of the P2VP anode coating has been

observed in special experimental cells constructed on microscope slides.

This liquid is

thought to be a liquid electrolyte which wets the lithium iodide discharge product and forms

a lower resistance path in parallel to the lithium iodide.

Plots of discharge voltage versus state of discharge for these batteries are not linear. The

battery resistance increases exponentially with the state of discharge. This dependence ex-

tends from the beginning of life throughout the region of discharge where the cathode is

two-phase, consisting of crystalline iodine plus a conductive liquid, which is a product of a

reaction between iodine and P2VP. During most of the discharge, the resistance is dominated

by the accumulating discharge product and the charge-transfer reaction. Once the crystalline

iodine is depleted, the resistivity of the cathode rises rapidly and soon dominates the resis-

tance of the battery. The crossover from electrolyte to cathode domination of battery resis-

tance gives the discharge curves of Li/ I

batteries their characteristic shape.

Detailed studies of Li/ I

batteries have shown that under conditions of mild discharge

(

ⱖ5

␮

A/cm

) the voltage loss below 2.8 V results from ﬁve physical processes: (1) a drop

in open-circuit potential as the discharge proceeds into the single-phase region of cathode

composition, (2) an exponential increase in electrolyte resistance throughout the discharge,

(3) bulk ohmic cathode resistance, which decreases slightly as the discharge proceeds in the

two-phase region of cathode composition, but which rises sharply after the crystalline iodine

is depleted, (4) a charge-transfer resistance at the case-to-cathode interface, and (5) concen-

tration polarization in the cathode. All of these processes have been characterized quantita-

tively in a general predictive steady-state discharge model of Li /I

battery discharge.

Figure 15.12 shows a typical plot of load voltage versus state of discharge for both coated-

and uncoated-anode groups of otherwise identical batteries at 37

⬚C. The voltage data are

shown between the initial voltage of 2.8 V and the cutoff voltage of 2.0 V. All data were

obtained at a current density of 6.7

␮

A/cm

. The voltage of the uncoated batteries decreases

linearly until iodine depletion starts to occur. At this time the slope of the curve changes.

15.16 CHAPTER FIFTEEN

FIGURE 15.12 Load voltage vs. discharge state for uncoated and

P2VP-coated anode Li/ I

batteries discharged at 6.7

␮

A/cm

.37⬚C.

(Courtesy of Medtronic, Inc.)

FIGURE 15.13 Load voltage and battery resistance vs. Q for coated-

anode Li / I

battery discharged at 100

␮

A, 37⬚C. (Courtesy of Med-

tronic, Inc.)

The same batteries with P2VP-coated anodes exhibit a higher load voltage at this current

density. However, as the current density decreases, the differences between the discharge

curves for the coated- and uncoated-anode batteries become smaller. Figure 15.13 shows the

discharge voltage data for the coated-anode battery in Fig. 15.12 to 0 V with the correspond-

ing resistance data. Log R versus Q is linear, and R varies between 100 and 8000

⍀ during

discharge. Typical discharge curves for a CRC 800 series battery, as well as changes in

SOLID-ELECTROLYTE BATTERIES 15.17

battery resistance, are shown in Fig. 15.14. Catalyst Research Corporation reported that its

900 series batteries (manufactured by pressing the cathode depolarizer onto a central lithium

anode assembly) exhibited discharge behavior very similar to that of the coated-anode bat-

teries.

The button and D-size batteries made by Catalyst Research Corporation were reported to

have discharge curves like the 900 series batteries. Figures 15.15 and 15.16 show projected

discharge curves for two types of button cells. Figure 15.17 is a similar discharge plot for

the D-size battery at 25

⬚C. This cell delivered 7 Ah (0.45 Wh /cm

) at the 2-month (5-mA)

rate; capacities and energy densities at lower rates are expected to be larger. The projected

discharge curves for the 1- and 2-mA rates are also shown.

FIGURE 15.14 800 series Li / I

battery (a) Typical discharge

curves. (b) Changes in cell resistance during discharge. (Courtesy

of Catalyst Research Corp.)

15.18 CHAPTER FIFTEEN

FIGURE 15.15 Projected discharge curves for S19P-20 button watch battery at 25⬚C. (Courtesy

of Catalyst Research Corp.)

FIGURE 15.16 Discharge characteristics for 2736 button-cell battery (a) Projected dis-

charge at 25⬚C. (b) Maximum continuous discharge current vs. temperature.

(Courtesy

of Catalyst Research Corp.)