1. Inves&ga&on
of
the
Influence
of
Electrolytes
on
the
Performance
of
Sodium-­‐Ion
Ba<eries
Ahmad
Ace
Haidrey
Department
of
Engineering
University
of
California,
Berkeley
Ahaidrey@berkeley.edu
Abstract
Energy
storage
has
become
a
front
and
center
issue
in
our
Ame,
causing
much
debate
and
deliberaAon.
To
meet
the
high
projected
demand
for
these
energy
storage
systems
such
as
smart
grids
and
electric
vehicles,
the
next
generaAon
of
secondary
baEeries
is
crucial.
Though
there
are
many
various
rechargeable
baEery
systems,
Na-­‐ion
are
one
of
the
leading
candidates
due
to
its
eco-­‐friendly
properAes,
its
natural
abundance,
and
its
chemical
similarity
to
Li-­‐ions,
the
current
standard
for
rechargeable
baEeries.
At
the
University
of
Singapore,
we
were
studying
the
general
similariAes
and
differences
of
Na-­‐ion
and
Li-­‐ion
baEeries
with
regards
to
solid-­‐state
chemistry
and
electrochemistry
of
the
posiAve
electrode,
the
negaAve
electrode
and
the
electrolyte
materials.
Due
to
the
larger
ionic
and
orbital
sizes
of
Na-­‐ions,
the
different
kineAc
and
thermodynamic
properAes
lead
to
new
phenomenon
never
encountered
with
Li-­‐ions.
Introduc&on
It
is
oNen
assumed
that
the
galvanostaAc
cycling
of
electrode
materials
used
for
Na-­‐ion
baEeries
is
due
to
the
working
electrode
while
the
counter
electrode
remains
at
a
fixed
potenAal,
but
Professor
Bayala’s
lab
at
the
NaAonal
University
of
Singapore
(NUS)
reported
a
new
phenomenon
never
seen
before
in
the
field:
a
voltage
step
present
in
the
discharge
profiles
at
high
rate
cycling.[1]
This
phenomenon
doesn’t
originate
from
a
potenAal
change
in
the
working
electrode
but
is
due
to
increased
polarizaAon
of
the
counter
electrode.
It
is
important
to
note
that
the
solvent
is
criAcal
for
the
step
formaAon.
If
pure
EC
or
PC
is
used,
there
is
no
response
but
using
EC:PC
(1v/1v)
has
an
evident
step,
which
can
be
seen
in
figure
1
below.
A
passivaAon
layer
grows
on
the
sodium
counter
electrode.
The
step
also
is
only
noAced
in
the
presence
of
sodium
metal
counter
electrode;
it
is
not
seen
in
a
Na-­‐ion
full
cell
or
in
any
Li-­‐
ion
cells.
In
recent
publicaAon,
a
voltage
step
was
spoEed
in
the
galvanostaAc
profiles
of
NaV2(PO4)3
(NVP)
during
sodium
inserAon
(or
discharge)
while
cycling
in
a
sodium
half
cell.
[2]
The
step
was
only
seen
at
high
rates
of
discharge,
and
never
during
the
sodium
extracAon
cycle
(charging).
The
step
is
not
due
to
changes
in
the
working
electrode
but
rather
due
to
the
passivaAon
layer
formed
on
the
Na-­‐ion
counter
electrode
caused
by
solvent
interacAons
that
produces
surface
species.
[3]
Results
The
experiments
were
conducted
using
a
sodium
metal
counter
electrode
with
NaTi2(PO4)3
(NTP)
working
electrode
due
to
its
flat
cycling
profile
and
its
redox
potenAal
lying
in
the
middle
of
the
electrochemical
window
of
electrolyte
soluAons
based
on
alkyl
carbonate
solvents.
[4]
ANer
noAcing
the
visible
step
in
an
EC:PC
solvent
and
not
in
the
pure
EC
or
pure
PC
solvents,
further
tesAng
went
on
to
observe
different
solvent
mixtures
as
can
be
seen
in
figure
2
and
3.
What
can
be
concluded
from
the
above
plots
is
that
EC
is
causing
a
barrier
which
results
in
the
voltage
step.
Altering
solvents
with
PC
mixtures
didn’t
produce
a
visible
voltage
step.
The
next
step
was
to
see
how
altering
the
concentraAon
of
EC
affects
the
locaAon
of
the
voltage
step
and
by
observing
figure
4,
it
can
be
seen
that
a
less
concentrated
EC
mixture
poses
an
earlier
step
while
a
more
heavily
concentrated
EC
mixture
poses
a
laEer
voltage
step.
While
all
of
these
phenomenon
were
occurring
in
NTP
using
NaClO4,
we
wanted
to
see
if
similar
results
would
be
shown
in
other
electrolytes:
NaPF6,
NaFSI,
and
NaTFSI.
In
figure
5,
the
results
seem
to
support
that
the
electrolyte
doesn’t
play
a
huge
impact
on
the
step
compared
to
the
solvent
mixture
it
is
in.
NaFSI
is
an
outlier
due
to
its
unstable
nature
and
high
polarizaAon.
Conclusions
In
this
set
of
experiments,
we
have
added
on
needed
informaAon
about
the
newly
uncovered
voltage
step
seen
in
cycling
profiles
for
NVP
or
NTP
when
cycled
against
sodium
metal
in
different
mixtures
of
soluAons.
By
looking
observing
only
the
data
extracted
from
the
graphs,
it
seems
that
EC
is
a
major
contributor
to
the
voltage
step,
which
is
most
likely
the
primary
source
to
the
polarizaAon
that
arises
due
to
a
passivaAon
layer
formed
on
the
sodium
counter
electrode.
In
past
arAcles,
EIS
processing
shows
that
passivaAon
layers
are
formed
with
PC
mixtures
as
well
but
just
different
in
nature
because
it
doesn’t
result
in
a
voltage
step.
[1]
Using
EC:DMC
or
EC:DEC
sAll
provide
a
voltage
step
while
using
PC:DEC
or
PC:DMC
does
not.
We
also
now
know
the
concentraAon
level
of
EC
does
play
an
impact
on
the
locaAon
of
the
step
and
that
changing
the
electrolyte,
from
the
ones
we
have
selected
and
tested,
don’t
seem
to
be
affected
in
regards
to
the
voltage
step
as
long
as
it
is
a
stable
soluAon.
The
next
set
of
work
would
be
to
create
three
electrode
cells
and
do
impedance
tesAng
to
be
able
to
explain
why
these
set
of
results
are
happening.
References
[1]
A.
Rudola,
K.
Saravanan,
S.
Devaraj,
H.
Gong,
P.
Balaya,
Chemical
CommunicaAons,
49
(2013)
7451-­‐
7453.
[2]
K.
Saravanan,
C.W.
Mason,
A.
Rudola,
K.H.
Wong,
P.
Balaya,
Advanced
Energy
Materials,
3
(2013)
444-­‐450.
[3]
Y.H.
Jung,
C.H.
Lim,
D.K.
Kim,
Journal
of
Materials
Chemistry
A,
1
(2013)
11350-­‐11354.
[4]
C.
Delmas,
F.
Cherkaoui,
A.
Nadiri,
P.
Hagenmuller,
Materials
Research
BulleAn,
22
(1987)
631-­‐639.
Figure 1: The voltage step is
evident in a solution mixture.
Figure 2: Fixed EC mixtures Figure 3: Fixed PC mixtures
Figure 4: Changing EC concentration
Figure 5: Changing the electrolyte in
EC:PC (1:1 v/v) solvent
Acknowledgements
I
would
like
to
thank
U.C.
Berkeley,
Cal
Energy
Corps,
Professor
Palani
Balaya,
Ph.D.
Ashisht
Rudola,
Professor
Paul
Wright,
Tracy
Turner,
Orion
Kew,
NUS,
my
family,
and
anyone
else
involved
to
make
this
experience
happen.
Thank
you.