Cyclic voltammetry

In cyclic voltammetry experiments, a three electrode (working, reference, and

counter electrode) cell is used. The reactions under examination occur at a

working electrode. The basic operating principle o f cyclic voltammetry consists

o f a linear-potential ramp application between working electrode and reference

electrode. The experimentally measured response is the current passed between

working electrode and counter electrode.

The resulting cyclic voltammogram (i vs.E) indicates the potentials at which

electrochemical reactions occur at the working electrode. The movement o f

electroactive material to the surface and the electron transfer determine the

current response in the overall electrochemical process. The reaction rate is

proportional to the concentrations o f

A

and

B

at the electrode surface. Thus the

i'=»F{AriCA(D,<) - *fte B{0/}}

(2-1)

where C A(0,f) and C B(0,f) are the concentration o f A and B at the electrode

surface respectively.

The exponential dependence o f

kr

on the applied potential (.

E)

causes a very

steep increase in the current. However, in an unstirred solution diffusion is the

principal mode o f the movement o f the electroactive material. When diffusion

controls the overall reaction rate, the current falls w ith time. Hence a peak is

observed in the

i-E

trace. Cyclic voltammetry is very useful for mapping out the

electrochemical reactions occurring.

A teflon-bodied three-electrode cell with a spring to secure a tight contact

between electrodes and polymer electrolyte film w as used in this project. The cell

design is schematically illustrated in figure 2-1. It incorporated a Ni working

electrode, a lithium counter electrode, and a lithium reference electrode. Lithium

reference electrodes constructed by mechanically pressing lithium onto a thin Ni

wire were used. The reference electrode was situated close to the working

electrode to minimize ohmic drop by inserting it between polymer electrolyte

films. A BUCHI TI-50 heater controlled the cell operating temperature. All o f

the cyclic voltammetric experiments were carried out in an argon (BOC,

Research grade) filled dry box equipped with a re-circulation gas purifier

spe ... O-ring

.4

Spring 4

1

_

- S i m

^gasRSS “ * *•»«**•♦«>*(

E sam

v;V'< . .;■'.... *«■>•••• ■ *S

8

U - r ? ._________ Working electrode

► on the SUS current

collector

••■► Reference electrode

connected with Ni

wire

^ Counter electrode on the stainless steel (SUS)

current collector

Figure 2-1. Three-electrode cell used in electrochemical experiments in this project, Ni sheet was used as the working electrode and lithium as reference and co un ter electrode.

2.3.2. G alvanostatic ram p experiments

Critical current densities were obtained with the three-electrode cell (see

Ch.2.3.1). The potential difference across the cell (AE/mV) was plotted against

the applied current density (ifm A cm 2). The critical current density (i) was determined from the current density value showing a sudden increase in the

potential difference. The applied current density was increased in a linear ramp

W ave-function generator

Potentiostat

CE WE RE

0

Three-electrode test cell

High Im pedance DMM with

d ata logging system

AE

High Im pedance DMM with

(b)

R esistor

Figure

2

-

2

. (a) Electronic arran g em en t for m easurem ents of the critical

cu rren t and (b) scheme illustrating how to generate a ram p current.

2.3.3. Cycling efficiency

Cycling efficiency measurements o f lithium electrodes were performed by

electrochemically depositing lithium onto a nickel substrate followed by stripping

o f lithium at the same deposition current density. The total deposition charge

passed was 1.0 C/cm2. The overall cycling efficiency o f a cycle was calculated by

the following equation:

E ( % ) = ldist dis / /dept dep =

Qdis/Qdep

.-.(2-4)

where and i ^ denote the current for dissolution and deposition, t the time and

2 . 4 . M i c r o s c o p i c o b s e r v a t i o n

In-situ microscopic observation was carried out by depositing lithium galvanostatically on a Ni wire substrate to study the changing morphology of

deposits in polymer electrolytes. The corresponding critical current density was

also obtained with a Ni wire to study its relation with the deposition morphology.

In-situ FT-LR measurements were performed to characterize the passivating films

on the deposited lithium.

2.4.1. Optical microscopy

in-situ

Photographs of lithium deposits were taken using a two-electrode cell, which

could be mounted on a microscope stage (figure 2-3).

( a )

O-ring

'Sggjgg

Microscopic glass or

CaF2 window

►Ni wire electrode

Li

ggpgll

8 ^ Heater

(b) TV M onitor Video camera Test cell O bservation direction * -N i wire ^tmBBUSamasamtsx r'i-.--jB55Sg'~' Li • r r 5^ ____ deposited : - ' « - ' • ^ ":_^ ..' - > :S •: • -;v SSS®**'

P olym er

Electrolyte Film saMSM**''

Figure 2-3. (a) Two-electrode cell and (b) construction for

in-situ

observation

A smoother surface was achieved by using Ni wire as substrate. Since the cut-

edge o f the Ni sheet substrate was rough, the surface effect on the deposition

morphology could not be avoided. The Ni wire electrode was dipped in an

ultrasonic bath containing pure water. A fter washing, the Ni wire electrode was

dried and stored in vacuum chamber in an argon-filled dry box before cell

assembly. Prior to experiments, the test cell was held at 100°C for two hours in an

argon-filled dry box to secure better contact between electrodes and electrolyte.

Deposition was carried out at a given constant current. Photographs were taken at

various stages o f the deposition. The test cell was cycled as the same procedure

as described earlier. A Perspex box surrounded the microscope stage and optics

so that an argon shroud could be maintained around cell.

In document Lithium deposition in solid polymer electrolyte batteries (Page 63-70)