Parasitic currents in electrochromic devices

.__ B

2%

SOLID STATE

d

__

ELSEVIER

IONICS

Solid State Ionics 86-88 (1996) 965-970

Parasitic currents in electrochromic F.M. Michalak”, Department

devices

J.R. Owen

of Chemistry, University of Southampton, Highjeld, Southumpton SOI 7 IBJ, UK

Abstract This study investigates the significance of parasitic currents at a WO, electrode, during cycling in a lithium non-aqueous electrolyte, which do not correspond to reversible ion-insertion reaction but result in a distortion of the expected Faradaic mass/charge relation as well as destruction of material. A measurement of the optical transmission monitors the lithium content of the electrode during cyclic voltammetry. During a cycle beginning and ending at the same transmission value the parasitic charge is equal to the charge imbalance during the cycle. A mathematical algorithm is then used to determine the parasitic current at each point in the cycle. The results indicate a predominant cathodic parasitic reaction with a Tafel slope of 300 mV per decade, reaching 0.1 PA cm-’ at 2 V versus Li’ /Li. Keywords:

Electrochromism; Parasitic currents; Charge imbalance

1. Introduction

This work is concerned with electrochromic devices which operate by the electrochemical transfer of guest ions between two insertion electrodes as in the “rocking chair” battery [l-3]. Parasitic currents in electrochromic devices are defined here as currents due to redox reactions other than the insertion and extraction of ions into and out of the host film structures. An example would be the current due to the process of hydrogen or oxygen evolution [4,5] instead of the intended insertion or extraction of protons in WO,. The effects of parasitic current include the following: (a) Gas evolution.

*Corresponding author. Tel:(441703) 59-4180; Fax: (441703) 67-6960; E-mail: [email protected]

(b) Formation of passivating layers of reaction product. (c) Corrosion of the electrode by discharge of host ions into the electrolyte. (d) Charge imbalance between the colouring and bleaching half-cycles. The avoidance of gas evolution is one reason for using non-aqueous electrolytes in electrochromic devices. Although the thermodynamic stability range of non-aqueous electrolytes is not wide, a kinetic “stability window” is often assumed to exist within which the reactions of the electrolyte occur at negligibly low rates. However, although short term stability may be assumed from the absence of gas bubbles at the electrode surface, slower reactions such as (b) and (c) above may limit the life of the device. A charge imbalance in itself is not a problem, although it is a symptom of a less benign parasitic current. Conversely, however, the absence of charge imbalance is not a convincing demonstration of the

0167.2738/96/$15.00 Copyright 0 1996 Elsevier Science B.V. All rights reserved PIf SO167-2738(96)00235-4

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FM. Michalak.

J.R. Owen I Solid State lonics 86-88

absence of parasitic currents since they may be equal and opposite for each half-cycle. Indeed, the latter case may develop as a steady state during constant charge cycling. The implications of electrolyte redox reactions are greater for non-aqueous systems than water. This is because water electrolysis is quite reversible; a wellengineered mechanism for gas recombination can return the system to its status quo after a temporary gas discharge, and the same phenomenon is put to good use as a protection against overcharge in aqueous batteries. However, in non-aqueous systems such as propylene carbonate, lithium battery studies have shown the redox reactions to be irreversible, producing undesirable products such as gas bubbles, passivating films as well as electrode corrosion. Although the intended regimes of operation of electrochromic devices limit the potential range such that degradation is minimal, there are some cases where the potential may enter regions where parasitic currents are significant. These are: (a) Fast switching using large overpotentials. (b) Switching of large area devices with lateral variations in interfacial potential. (c) Development of new electrodes which may be electrocatalysts toward electrolyte decomposition. In this work a sensitive technique is applied to the detection of parasitic currents on a WO, electrode. The method is based on slow scan cyclic voltammetry in conjunction with optical absorbance measurement. Slow scan rates help to distinguish the currents from the insertion reactions from the parasitic currents, which should be independent of scan rate. A further improvement is made in this work, which is the elimination of the insertion charge by subtracting the total charges required for insertion and extraction to corresponding levels of insertion, as determined by comparison of optical absorbance.

2. Experimental 2.1. Materials The tungsten

oxide samples

were deposited

to a

(1996) 965-970

thickness of ca. 3000 A on glass/IT0 (20 ohms per square) substrates by sputtering, and were supplied to us by Pilkington Technologry Centre. Electrical contacts were made with silver paint. The propylene carbonate (Aldrich, 99% + , anhydrous) was distilled under reduced pressure and then stored in an argon filled glove box (water content less than 10 ppm).The lithium triflate salt (Aldrich, 97%) was dried under vacuum at 90°C for 24 h, then at 140°C for 24 h and finally transferred into the glove box for storage and preparation of the 0.5 M solution in propylene carbonate to be used as the electrolyte. 2.2. Electrochemical

measurements

An IBM compatible PC was fitted with a CIODAS08-AOL (ComputerBoards, Inc.) ADC/DAC board controlled by program written in-house in Turbo-Pascal. The board was connected to a homebuilt potentiostat with a current follower with an input offset current of less than 1 nA. This potentiostat was connected to a hermetically sealed, twoelectrode cell having two quartz windows and a lithium foil counter/reference electrode situated in a side arm. The cell was fined in a Phillips UV-vis spectrophotometer model PU 8730 to record, in situ, the optical absorbency (or optical density designated in the following by O.D.) of the working electrode during the slow scan voltammetry experiment.

3. Results Fig. 1 shows four successive voltammograms (CV) of a WO, electrode at 0.1 mV/s. After the first cycle an approximately symmetrical pattern indicates reversible ion insertion with some hysteresis. The first cathodic scan shows a much larger peak at approximately 2700 mV vs. Li/Li+ as found by Burdis et al (61. Because there is no equivalent peak on the anodic scan the peak is probably due to an irreversible lithiation process. The high potential region shows a remarkable stability up to the experimental limit of 4.5V vs. Li+ /Li. Fig. 2 shows the potential as a function of the charge passed in the cell for the above CV’s. The first cycle shows a net deficit of ca 15 mC.cm-* mainly due to the first cathodic peak. Although the

F.M. Michalak,

J.R. Owen I Solid State Ionics 86-88

967

(1996) 965-970

3 34

2 t 1 ; ’ f z-1 t

-_i

1st cycie

N_ %2

E ; 0 -4 -5 ? -3 I+ -6 i -a

-7

_--.i

1500

4500

2500 3500 E I mV vs LilLi’

!

4

1800

2300 2800 E I mV vs LilLi’

Fig. 3. 1st estimate of the parasitic Fig. I. Cyclic voltammograms (cycles l-4) for a WO, electrode in PC / 0.5 M lithium triflate at a scan rate of 0.1 mV/s. Starting point 3100 mV, turn 1: 2000 mV, turn 2: 4500 mV

9 3000

t t

: 2 2500

2000

1500

L -60

Fig. 2. Potential

current by adding iLa,,ud,Land

~,““dl<.

states, since the potential is only a function of the composition at the electrode-electrolyte interface. Fig. 4 shows the optical density as a function of the charge passed for the cycles l-3. A visual comparison of the first cathodic sweep with subsequent cycles shows an irreversible charge of about 15 mC cm-*. The absence of a change in transmission suggests that this process is not associated with a change in the oxidation state of tungsten or with the population of the conduction band in WO,, but due to the reduction of impurities, such as peroxide or 0 species present in the pristine electrode. After this process, a constant electrochromic efficiency is

4500

‘2 3500 7

3300

-50

-40 -30 Charge

vs. charge

-20

-10

0

10

I mC.cm.’

for the CV’s represented

in Fig. 1.

following CV’s (cycles 2-4, Fig. 1) are similar, the potential/charge curve shifts progressively towards the cathodic region, showing that another irreversible cathodic process is occurring. A first estimate of the parasitic current may be obtained simply by subtracting the currents at corresponding potentials during insertion and extraction, respectively. Fig. 3 shows that this analysis is distorted by the fact that a comparison of electrode potential between points during insertion and extraction does not give a reliable indication of corresponding composition

1.6 I .4 __ 1.2

f3

__ OD 1st cycle ------OD2ndcycle -. -. OD 3rd cycle

1

0.8

0.6

0

-20 Charge

40

-60

mC.cm.*

Fig. 4. Optical density O.D. versus charge passed during the first three cycles at 0.1 mV/s.

FM.

968

Michalak.

J.R. Owen I Solid

obtained until the optical density reaches a value of ca. 0.8. Then the gradient decreases-showing that there can be two absorbing species in Li,WO,, one with a large extinction coefficient at lower insertion ratio, X, and another one with a smaller extinction coefficient at higher insertion ratio. The same behaviour is found in cycles two and three. During the extraction, the optical density decreases to zero.

4. Discussion Provided that the optical extinction coefficient, or elecrochromic efficiency, can be regarded as constant, application of the Beer-Lambert law results in an expression for the optical density which is a function of the average composition of a sample and independent of compositional inhomogeneities. Accordingly, the optical density is a far better indicator of the sample composition than the electrode potential, which only reflects the condition of the surface. The charge imbalance can be calculated for a cycle from, and back to, a given optical density as shown on Fig. 5 by subtracting the charge passed during the insertion scan from the charge passed during the extraction scan. The charge shift at the end of the cycle (calculated when O.D. = 0) is then ascribed to the accumulated parasitic charge along the whole cycle.

State

Ionics

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A determination of the parasitic current was made using the optical data as follows. First, a polynomial fit was made to the data points representing the relation between optical density and charge on the extraction half-cycle. This allowed the parasitic charge to be obtained as a function of optical density or charge by subtracting the charge values for insertion and extraction at states of corresponding optical density. The parasitic charge Q,, is illustrated by the distance between the insertion and extraction curves in Fig. 5 and plotted versus optical density on the same graph. Next, in Fig. 6, the derivative of the parasitic charge Q,, was taken by calculating the increments in parasitic charge “Qll for successive points during the insertion half-cycle. Since the data points were taken at a regular time interval at,, of 122 s, the derivative could be expressed as a current I,: I, = ~Q,/&i,

= LSQ,/ 122 s .

(1)

This current can he related to the parasitic current I,, by realizing that the latter should be obtained by dividing the increments in parasitic charge by the increments in total time, (at,, + &,,,) as shown in the diagram of Fig. 7. The ratios of Sti, to St,,, at given charge states were obtained as the reciprocal of the insertion and extraction currents observed in the current/charge plot of Fig. 8.

-2.0

-60

-1.5 “E -1.0 $ a” -0.5

-‘l--lo.o 0

0.5

OD

Fig. 5. Q, for a given optical density versus the O.D.

1

1.5

for the 3rd cycle, plotted

x

0 Parasitic charge

-0.5

0

Fig. 6. Determination versus the O.D.

0.5

O,-,

x

1

I

1.5

of SQP, for a given charge shift Q, plotted

FM. Michalak,

+

Insertion

+

Extraction

J.R. Owen I Solid State Ionics 86-88

(1996) 965-970

969

-0.04

-0.08 I -40

Fig. 7. Determination

of the total time Sf,,, + at,,,

-30 -20 -10 Charge I mC.cm.’

0

Fig. 9. Plot of the corrected parasitic current I,, versus the charged passed in one insertion scan. 3rd cycle at 0 1 mV/s

Potential I mV vs LilLi’ 2100

2200

2300

2400

2500 01

‘E 2 ; 0.01

yJ

“E Q

e 5 -2 0

x 0.001

=

-4 0.0001

-6

2000

-60

40

Finally,

I,, was obtained

I, = SQJSt,, =I,

of the correcrion

X Sti,l(St,,

x l/(1 +z,,,/z,,,>.

2400 2600 Potential

2800 LilLi’

I mVvs

-20

Charge I mC.cm”

Fig. 8. Determination

2200

3000

3200

Fig. IO. Plot of the corrected parasitic current I,, versus the average potential showing the exponential fit. The current in the 2000-2500 mV region is also plotted in the logarithmic plot.

factor i_/i,,,.

from I as follows: vs. Lif / Li. These parameters are not interpreted here but given as convenient inputs to models of the electrochemical behaviour of WO,.

+ Stext)

(2)

The parasitic current Z, was then plotted as a function of charge as shown in Fig. 9 and as a function of the average potential during insertion and extraction in Fig. 10. The plot is asymptotic to the zero current axis at positive potentials but shows a cathodic current increasing with decreasing potential. A logarithmic plot of the parasitic current versus potential gives a constant slope of about 300 mV per decade and an intercept of about 0.1 PA cmP2 at 2 V

5. Conclusions The observation of optical density during slow cycling of WO, shows that anodic parasitic currents are insignificant up to 4% vs. Li+/Li and gives important information regarding parasitic current at cathodic potentials. The mathematical algorithm developed in this work has led to the determination

FM.

970

Michalak,

J.R.

Owen

I Solid

of the value of 0.1 ,uA cm-’ tor the cathodic parasitic current at 2 V vs. Li+/Li and a constant Tafel slope of 300 mV per decade.

Technology

tonics

86-88

(1996)

965-970

References [II S.K. Deb, Applied. Optics Suppl. 3 (1969) 192. [21 C.M. Lampert, Solar Energy Mater. 1I (1984) 1 [31 C.G. Granqvist, Appl. Phys. A56 (1993) I. [4j B. VuilIemin and 0. Bohnke, 257.

Acknowledgments Pilkington gram.

State

Center,

EC Joule II pro-

Solid State tonics 68 (1994)

[51 0. Bohnke and M. Rezrari, Mat. SC. Eng. B 13 (1992) 323. [61 M.S. Burdis and J.R. Siddle, Thin Solid Films 237 (1994) 320.