JOURNAL
OF
THE
ELECTROCHEMICAL
SOCIETY
ACCELERATED
BRIEF
~DECEMBER
COMMUNICATIONS
985
A Rechargeable All-Solid-State Sodium Cell with Polymer
Electrolyte
K. West, B, Zachau-Christiansen,T. Jacobsen,*and S. Atlung*
Fysisk-Kemisk Institut, The Technical University of Denmark, DK 2800 Lyngby, Denmark
INTRODUCTION
sodium with thicknesses between
and 100 ~m were obtained.
Secondary lithium cells with intercalation or insertion materials as positive
electrodes are now an extensively studied subject. However, only few p a p e r s
covering the corresponding sodium systerns have yet appeared. Ostensibly this
is due to the lack of reversible sodium
/electrolyte half-cells functioning
below 200~
higher t e m p e r a t u r e s
most
electrode
materials
will r e a c t
via
displacement reactions, which generally
have lower electrode potentials and do
not p o s s e s s the inherent reversibility
of intercalation reactions,
Liquid o r g a n i c electrolytes consisting
of NaI dissolved in propylene c a r b o n a t e
have been used for electrochemical prep a r a t i o n of s o d i u m i n t e r c a l a t e d
TiS 2
(1,2). The Exxon group has cycled both
Na/TiS~ and Na/MoS 3 using sodium triethyl [ N - p y r r o l )
b~orate d i s s o l v e d
in
1.3-dioxolane
as
electrolyte
(3,4).
Most other g r o u p s have chosen to use
two-electrolyte
systems
with an ionic
conducting ceramic
membrane. This membrane separates
molten sodium from the
liquid e l e c t r o l y t e f o r m i n g c o n t a c t to
the a c t i v e m a t e r i a l in the p o s i t i v e
electrode
(5).
this communication cycling of a cell
with solid s o d i u m in c o n t a c t
with a
polymer electrolyte and with a m o r p h o u s
MoS 3 as positive electrode is reported.
In
50 ;~m
Polymer
electrolyte
sheets
(-~30 ~m)
Were p r e p a r e d by evaporation of acetonitrile s o l u t i o n s
of p o l y - ( e t h y l e n e
oxide) (PEO) a n d the p r o p e r a m o u n t of
Nal, r e c r y s t a l l i z e d from a c e t o n i t r i l e
and v a c u u m d r i e d at l~0~
The PEO (WSR
301 Polyox, MW = 4.I0 ~, BDH) was
used
as received. The films w e r e p r e p a r e d
and handled in an argon filled dry box.
The Nal concentration in the films used
corresponded
to a N a / O r a t i o of 1:10.
With this electrolyte, the o p e r a t i n g
t e m p e r a t u r e of the cell is confined to
t h e i n t e r v a l b e t w e e n 98~
(melting of
sodium) a n d 65~
as the e l e c t r o l y t e
conductivity
decreases
r a p i d l y below
this temperature.
Composite MoS 3 electrode films (-~50 ~m)
with the overall composition:
72 w/o
MoS3, 20 w/o PEO, a n d 8 w/o Nal w e r e
p r e p a r e d by e v a p o r a t i n g
acetonitrile
suspensions/solutions
of the ingredie n t s o n t o nickel foils. In some c a s e s
graphite
was
added
as c o n d u c t i v e
diluent,
b u t no i m p r o v e m e n t
of cell
performance at these very low current
densities was achieved.
The cell was mounted with spring load
in a Ni-plated b r a s s container sealed
with a viton O-ring. This container was
heated in air to 90 ~ in a Buchi T0-50
oven.
RESULTS
EXPERIMENTAL
A m o r p h o u s MoS 3 was p r e p a r e d by thermal
decomposition
of (NH4)2MoS 4 as
described by Jacobson et aY (3):
The first two cycles of the cell
Sodium electrodes were made by pressing
a layer of sodium onto a Ni foil, using
polyethylene sheets to aid the spreading.
In this w a y s m o o t h l a y e r s of
are shown in fig. i. The current density usgd for discharge and c h a r g e was 15
~A/cm ~ in t h e s e cycles, c o r r e s p o n d i n g
to a stoichiometric discharge time of
45 h. The cell was discharged to 1.5 V
Na / PEO-NaI
(10:1)
/ HaS 3
*Electrochemical Society Active Member.
3061
Downloaded 28 Sep 2009 to 192.38.67.112. Redistribution subject to ECS license or copyright; see http://www.ecsdl.org/terms_use.jsp
3062
J. Electrochem. Soc.: A C C E L E R A T E D B R I E F COMMUNICATION
December 1985
I
2, 75
I
E / V va Na
I
I
l
X (Na/Mo)
§
I
TOTAL Na/Mo
E. 50
1,0
iO
+
......,...---'""-
2.25
<2. O0
....,'"
0.5
++
..
++;+
1, 75
+++++++§
++
.'
X
""
0.0
i. 50
0.0
Fig.
->
0.5
1,0
1.5
1. Initial cycles of the cell Na/
NaI-~EO
(10:1) / M o S 3 at 15
#A/cm ~. Charge recalculated as
X = Na/Mo b a s e d on total Moa n a l y s i s of cell.
and r e c h a r g e d to 2.6 V vs Na. It is
seen t h a t the coulombic cycling efficiency (the ratio between discharge and
charge capacity) is c o n s i d e r a b l y less
than unity, possibly due to disproportionation of the electrolyte. An advant a g e of using iodide-containing electrolytes
in alkali
metal
batteries
might
be t h a t
the iodine liberated
during overcharge recombines with the
alkali metal and thereby scavenges the
n e g a t i v e electrode of developing dendrites.
The discharge curve is smooth as expected for an amorphous electrode material, and 1.4 Na/Mo is inserted during
the first discharge. Previously discharge capacities of up to 3 Na/Mo have
been achieved (4), but the capacity has
been shown to be sensitive to the details of the MoS 3 s y n t h e s i s (6). The
stoichiometric cap~city calculated from
the first discharge is 290 Wh/kg.
The development of the discharge capacity upon cycling is s h o w n on fig. 2.
The charge a~d discharge current used
was 1~0 ~A/cm
in cycle 3 to 9, and 15
~A/cm = in the
other cycles, Despite
the rapid decrease in cell capacity, a
total n u m b e r
of 10 Na/Mo have been
cycled
during
the lifetime of this
cell, and the available a m o u n t of sodium has been cycled 1.5 times. This
shows that, even close to its melting
point,
sodium
forms
a sufficiently
Manuscript submitted July ii, 1985; revised
manuscript received Oct. 3, 1985.
0
Fig.
CYCLE++++++++++
I
I
I
I0
20
50
*++
40
2. Discharge capacity as function
of cycle n u m b e r .
The t o t a l
number of Na cycled per Mo is
also shown.
stable interface with a PEO-electrolyte
to allow reversible o p e r a t i o n of this
electrode. Possibly the formation of a
passive sodium ion conducting film on
the interface is responsible for this
stability.
It is thus concluded that solid state
sodium batteries might be feasible by
extension of lithium battery technology, especially when electrolytes with
better conductivity and more reversible
electrode materials are developed.
REFERENCES
i. D.A. Winn, J.M. Shemilt, a n d B.C.H.
Steele,
Mat.
Res.
Bull, 11, 559
(1976).
2. D. C h e r n s
and G.P. Ngo,
S t a t e Chem. 500, 7 (1983).
jr. Solid
3. A.J. Jacobson,
R.R. Chianelli,
Rich,
and M.S. W h i t t i n g h a m ,
Res. Bull. 1_.44, 1437 (1979).
4. G.H. N e w m a n
EJectz'ochem,
5. K.M. Abraham,
199 (1982).
S.M.
//at.
a n d L.D. Klemann,
..7.
Soc,, 127, 2097 (1980).
Solid S t a t e
Ioni'cs, 7--,
6. R. Williams,
D. Shen,
S.P.S. Yen,
and R.B. Somoano, E x t e n d e d 2 b s t r a c t
No. 74, Electrochem.
Soc. Meeting,
W a s h i n g t o n (1983).
The Technical University of Denmark assisted
in meeting the publication costs of this
article.
Downloaded 28 Sep 2009 to 192.38.67.112. Redistribution subject to ECS license or copyright; see http://www.ecsdl.org/terms_use.jsp