Various high energy density rechargeable batteries are being considered for future space applications. Of these, the sodium sulfur battery is one of the leading candidates. The primary advantage is the high energy density (760 Wh/kg theoretical). Energy densities in excess of 180 Wh/kg have been realized in practical batteries. More recently, cathodes other than sulfur are being evaluated. Researchers at JPL are evaluating various new cathode materials for use in high energy density sodium batteries for advanced space applications. The approach is to carry out basic electrochemical studies of these materials in a sodium cell configuration in order to understand their fundamental behaviors. Thus far studies have focused on alternate metal chlorides such as CuCl2 and organic cathode materials such as tetracyanoethylene (TCNE)

Bankston, C. P.

Distefano, S.

Ratnakumar, B. V.

English

NASA Technical Reports Server

N90-20459
ADVANCED RECHARGEABLE SODIUM BATTERIES WITH NOVEL CATHODES
S. Di Stefano, B.V. Ratnakumar and C.P. Bankston
Jet Propulsion Laboratory
California Institute of Technology
Pasadena, California 91109
Abstract
Various high energy density rechargeable batteries are being
considered for future space applications. Of these, the sodium
sulfur battery is one of the leading candidates. The primary
advantage is the high energy density (760 Wh/kg theoretical).
Energy densities in excess of 180 Wh/kg have been realized in
practical batteries. More recently, cathodes other than sulfur are
being evaluated. We at JPL are evaluating various new cathode
materials for use in high energy density sodium batteries for
advanced space applications. Our approach is to carry out basic
electrochemical studies of these materials in a sodium cell
configuration in order to understand their fundamental behaviors.
Thus far our studies have focused on alternate metal chlorides such
as CuCl 2 and organic cathode materials such as TCNE.
Introduction
Rechargeable batteries play a significant role in NASA's space
programs including planetary missions and satellite applications such
as Geosynchronous Earth Orbit (GEO) and Low Earth Orbit (LEO)
missions. There is an increasing need for a light weight high energy
battery with long active and cycle lives. Typically, the efforts at
various organizations are aimed at developing a battery system with
an energy density _ 200 Wh/kg at power densities _ i00 W/kg, cycle
life of i000 cycles and active life of ten years. The interest for
many years has been focussed on sodium-sulfur battery which has a
high formula energy density (-750 wh/kg) a good portion of which has
been realized in practical cells for several hundreds of
charge/discharge cycles (i). The battery typically operates at 300-
400°C where the 8" alumina solid electrolyte (BASE) is highly
conductive to sodium ions allowing high power densities drawn from
the battery. Together with these advantages, there are certain
deterrent factors associated with the use of the sodium sulfur
battery which partly protracted the development. R. M. Dell et
al. (2) have given an excellent account of the status of the NaS
battery in their recent review. Many of these operational and
safety problems stem from sulfur electrode, e.g., the excessively
corrosive nature of the discharge product, sodium polysulfide,
towards steels, the high vapor pressure of liquid sulfur above 700°C,
the possible degradation of BASE in polysulfide melts, (3) the
insulating nature of liquid sulfur, in particular the way in which it
33
https://ntrs.nasa.gov/search.jsp?R=19900011143 2020-03-19T22:44:05+00:00Z
isolates regions of BASE during recharge, etc. Alternatives to
liquid sulfur cathode are therefore being sought and there are some
examples in the literature based on fused salt electrolytes (4-6).
In this paper, we review our studies on alternate organic and
inorganic cathodes in sodium batteries. In both cases, the cell
contains sodium tetrachloroaluminate as the secondary molten
electrolyte and BASE as primary electrolyte.
z
Carbonitriles are a class of relatively stable organic compounds
that have high reduction potentials and energy densities. Tetra-
cyanoethylene (TCNE) in particular has an energy density of -620
Wh/kg at a high potential of -3.0 V vs. Na+/Na and a low melting
point. Therefore, TCNE has been examined as a possible liquid
cathode in sodium batteries. Among the inorganic cathode materials,
transition metal dichlorides (e.g., FeCl 2 and NiCI2) are gaining
increasing attention as solid, insoluble rechargeable cathodes in
place of liquid sulfur (8-11). These systems can operate at lower
temperatures -170°C, where the corrosion problems are proportionately
less severe. The safety problem is considerably reduced due to the
inherent sluggish kinetics of the chemical reaction between liquid
sodium and NaAICl 4 molten electrolyte in the event of ceramic failure
(12). Further, the molten electrolyte remains invariant during
charge/ discharge preventing polarization losses and localized high
current densities on the BASE surface. High energy densities -150
Wh/kg and long cycle life have been demonstrated with ferrous
chloride and more recently with nickel chlorides. We summarize below
our electrochemical studies on the ferrous chloride and nickel
chloride cathodes and also on another promising metal chloride,
copper chloride.
Experimental _;
,The experimental cell adopted for these studies is described in
detail in. (7) The cell essentially has a central-cathode design
with a provision to seal the anode half-cell. The cell is heated
electrically with a heating tape wound around a stainless steel tube
acting as a container for sodium. For the studies involving TCNE,
either Pt or C is used as the positive current Collector. Metal
chloride positive electrodes were initially fabricated by
impregnating commercially available sintered nickel grids (used in
Ni-Cd batteries) with the depolarizer by immersing the grid into a
concentrated chloride solution. Copper electrodes cannot however be
made in this manner for long-term studies due to the preferential
dissolution of nickel to copper in the impregnating solution.
Perforated copper grids were therefore used to make electrodes in the r
discharged state either by hot pressing With a binder (Teflon) or by _
sintering the powders in an inert atmosphere at temperatures below
the melting point of NaCl. Conventional electrochemical equipment
consisting of E S & S 273 potentiostat/Salvan0_tat _d E G & G 5301
lock-in amplifier interfaced with an Apple IIe computer were used.
AC and dc measurements were carried out using E G & G ac impedance - -
and Softcorr software. Various techniques such as potentiodynami 9
polarization, dc linear polarization and ac impedance were _emp[6yed
to obtain information on the kinetic parameters of the cathode
34
!
reactions. Discharge/charge cycling was also carried out
galvanostatically in a controlled fashion using the above
electrochemical equipment. All the chemicals TCNE, NaAICI4, NaCl and
metal powder were of analytical grade and were used as received.
BASE tubes were cleaned by etching in hot phosphoric acid. All the
cell fabrication operations were carried out in an argon-filled
glove box.
Results and Discussions
The findings of our studies on TCNE as a cathode material in
sodium batteries are described in detail elsewhere. (7) Briefly,
TCNE reduces at 3.0 V vs. Na+/Na reversibly with a coulombic yield of
>60% at 1 ma/cm 2 on either the Pt or C current collector. Fig. 1
gives the typical voltage-time curves of TCNE during discharge and
charging. The exchange current density on either Pt or C is of the
order of 10 -3 A/cm 2 in a catholyte containing typically 10% of TCNE.
The kinetics of TCNE reduction are essentially governed by the rate
of mass transfer in the cathode as evidenced from ac impedance and dc
potentiodynamic polarization data. In TCNE-rich ca£holytes there is
a kinetic hindrance to the reduction of TCNE reflected in increased
polarization (ohmic, concentration as well as charge transfer)
losses. As a result, the coulombic yield as well as diffusion
coefficient of TCNE decrease with increasing TCNE concentration in
the catholyte. The sluggish kinetics at higher TCNE concentrations
is related to the degradation processes occurring in TCNE at
operating temperatures of "230°C forming an adduct. FTIR studies
point to a possible polymerization of TCNE under the experimental
conditions. Due to the sluggish kinetics at higher TCNE
concentrations, it may not be possible to achieve high power
densities from TCNE. On the other hand, for medium to low power
applications, TCNE is a good cathode material, since the electro-
chemical activity and reversibility of TCNE are retained even after
polymerization.
Metal Dichloride Cathodes
Figure 2 shows the typical charge/discharge curves of a "16 mAh
NiCI 2 electrode (made b_ impregnating NicI 2 into sintered Ni plaque)
at 200°C and at 2 mA/cm z in the first two test cycles. The discharge
as well as charging curves are flat at nominal voltages of 2.56 and
2.63 V vs. Na+/Na respectively. There is not much polarization loss
from the open circuit voltage of 2.59 V.
The coulombic yield is nearly 100% and there is no loss in
capacity during cycling in the first few test cycles. Figure 3
illustrates the high rate discharge capability of the electrode. The
discharge curves at higher current densities, i.e., 4 mA/cm 2 and 8
mA/cm 2 are also flat with mid-point voltages of 2.54 and 2.5 V and
coulombic yields of 98% and 90% respectively. Ferrous chloride
electrodes (made by sintering dry powders of sodium chloride and
iron) exhibit a lower operating potential of "2.3 V as may be seen
from the discharge and charge curves of a "230 mAh electrode in
Figure 4. The discharge curve is flat in this case also, with a
35
coulombic yield in excess of 90%. The copper chloride cathode is
expected to give higher voltage than nickel chloride, from the
electrode potentials of Cu+2/Cu and Ni2+/Ni couples. Accordingly,
higher energy densities may be possible with copper chloride as
compared to ferrous chloride or nickel chloride especially at high
power densities due to a high electronic conductivity of copper
formed in the course of discharge. The essential requirement for a
successful operation of metal chloride cathode in the above sodium
batteries is total insolubiiity of the chloride in a NaAICI 4 melt.
Analytical studies carried out on the catholyte with CuCl 2 and NiCl 2
revealed no extra dissolution of copper over nickel. Initial studies
with copper chloride (electrode made by impregnation of CuCl 2 into
sintered Ni plaque) show a higher operating voltage than NiCl 2 or
FeCI 2 (Figure 5). The discharge (as well as charging) curve is,
however, not flat in this case as with FeCl 2 or NiCl 2 which may be
due to the mechanism of reduction being different with the possible
formation of a monovalent copper_
DC polariza£i0n studies were carried out on these cathode
materials to obtain information on the kinetics of their reduction.
The potentiodynamic polarlzation curves of NiCl2, Feel 2 and CuCl 2
(Figure 6) illustrate the high rate capability of these electrodes.
There exist mass transfer effects at high discharge rates interfering
with the charge transfer processes due to which limiting currents are
observed at high overpo£entials _ 300 mV. The limiting currents may
be due to a relatively slow diffusion processes inside the porous
cathode. Nevertheless, the limiting current densities are reasonably
high and are of the order of 15-25 mA/cm 2. The charge transfer
kinetics appear to be rapid as evidenced by high exchange current
densities of 2 mA/cm 2, 0.65 mA/cm 2 and 0.4 mA/cm 2 for NiCl2, FeCl 2
and CuCl2, respectively, based on the geometric areas of these
electrodes. Linear polarization experiments (i.e., at low
perturbations) were also carried out to derive similar information on
the exchange current densities. The current potential curves at
these potentials are linear (e.g., FeCl 2 in Figure 7) the slopes of
which give the polarization resistances. The exchange current
densities thus obtained are 4.6 mA/cm 2, 1.5 mA/cm 2 and 0.39 mA/cm 2
for NiCl2, FeCl 2 and CuCl2, respectively.
The complex plane impedance Spec£rum of NiCI 2 (Figure 8) shows a
large inductance at high frequencies not shown in the figure
attributed to the porous nickel grid and a low frequency semi-circle
ascribed to charge transfer kinetics. The exchange current density
calculated therefrom is 6.9 mA/cm 2 . There is no evidence of
diffusional impedance in the frequency range studied (i.e., Z 0.01
Hz). The complex plane impedance spectrum of CuCI 2 (sintered)
electrode, on the other hand, contains two relaxation loops. The
exchange current density evaluated from the first relaxation loop i s
0.9 mA/cm 2. it is _owever not ciear yet as to whether the second
relaxation loop is related to the formation, if any, of monovaient
copper during the reduction of CuCl 2.
36
m
Conclusions
Among the cathode materials studied at JPL as alternatives to
sulfur cathode in sodium batteries, TCNE is suitable for low to
medium power applications. Metal dichloride cathodes, on the other
hand can deliver energy densities comparable to sodium-sulfur
batteries at high power densities without many of the operational and
safety problems associated with the use of sodium-sulfur batteries
and are thus suitable for various applications including space, EV
load-levelling, etc. The electrochemical studies carried out so far
indicate high exchange current densities for the reduction of these
cathodes.
Acknowledgement
The work described here was carried out at the Jet Propulsion
Laboratory, California Institute of Technology, under contract with
the National Aeronautics and Space Administration. One of the
authors (B. V. Ratnakumar) acknowledges the National Research Council
for providing his Research Associateship during this work.
References
i. J. Sudworth and A. R. Tilley (Eds.), "The Sodium/Sulfur Battery,"
Chapman and Hall, New York 1985.
2. R. M. Dell and R. J. Bones, B. Electrochem., 4, 319 (1988).
3. M. Liu and L. C. De Jonghe, J. Electrochem. Soc., 135, 741
(1988).
4. J. Werth, U.S. Patent 3,877,984 (1975).
5. G. Mamantov, R. Marassi, M. Matsunaga, Y. Ogata, J. P. Wiaux and
E. J. Frazer, J. Electrochem. Soc., 127, 2319 (1980).
6. K. M. Abraham, L. Pitts and R. Schiff, J. Electrochem. Soc., 127,
2595 (1980).
7. B. V. Ratnakumar, S. Di Stefano, G. Nagasubramanian, R. M.
Williams and C. P. Bankston, Extd Abstr. #74, ECS Fall Meeting,
Chicago, IL, October 9-14, 1988. Manuscript submitted for
publication.
8. J. Coetzer and M. Thackeray, U.S. Patent 4,288,506.
9. J. Coetzer, J. Power Sources, 18, 377 (1986).
i0. R. C. Galloway, J. Electrochem. Soc., 134, 286 (1987).
II. B. J. Jones, J. Coetzer, R. C. Galloway and D. A. Teagle, J.
Electrochem. Soc., 134, 2379 (1987).
12. R. T. Wedlake, A. R. Tilley and D. A. Teagle, B. Electrochem., 4,
41 (1988).
37
zZ
!
4000
3560
3120
>
3>
E
. 2680
ILl
2240
1800
0
Figure i.
/
3
-\2
\\
I I I l 1
6 12 18 24 30
CAPACITY, mAh
Typical Charge/Discharge curves of TCNR
(I0 W%) at 230"C: (i) Fourth Discharge,
(2] Fifth Discharge, and (3) Charge
3000 I I I I
2800 -
z 2600-
g 2400
ILl
22OO
2000 __1 I I ....
0 30 60 90 120 150
t, rain
Figure 3. Discharge Curves Of 16 mAh NiCI 2 Electrode
at 200"C and at Current Density of
(I) 2 mA/cm 2 , (2) 4 mA/cm 2 , and (3) 8 mA/cm 2
3000
2800
2600
+
Z
g2400
2200 -
2OO0
0
Figure 2.
I I I I
I I I
30 60 90
t, rain
3
I
120 150
Typical Charge/DischargeCurve 16 mAh
Electrode at 200"C Durlng (1) Second,
(2) Third, (3) Fourth Discharge, and
(4) charge at 2 mA/cm 2
3.0 I I I I I "h '" I
z
Z
O3
>
LU
(9
O
>
2.8
24
2.0
_ : = = = = = _ = = _.E_--.._...._AFTER_._FIRST DISCHARG
AC AND OC EXPERIMENTS '_,AT DIFF POTENTIALS
1 L I I I I I __
30 60 90 120 150 180 210 240
CAPACITY, mAh
I'igure 4. Initial Charge/Discharge Curves of
FeCI 2 Cd rhode
|
=
!
38
L
3100
2860
2620
E 2380
2140
1900
Figure 5.
| I I I
__1-- z 1 J
0 80 160 240 320
t, rain
Charge/Discharge Curves of CuCl 2 Electrode
(Made by Impregnating CuCl 2 into a Sintered
Nickel Grid) at 200"C
28O0
2680
2560
+
z
_ 2440
g
w
2320
2200
2080
100
Figure 6.
........ I ........ i ........ l ........ i ........ I .......
i ® ® .-
1
--.LJ-LU,___L.L_JmL_t l*LuJ_u_]_.tz_ u_L
101 102 103 104 105 t06
I 0,Alcm 2)
Potentlodynamic PolarLzatlon Curves of
(I) CuC]2, (2} NICI2, And (3) FeCl 2
>
UJ
2384
2376
2368
2360
2352
I I I I
2344 I I I I
-6000 -3000 0 3000 6000
Figure 7.
/
I (IdVcm2)
Linear Polarization Curve for FeCI 2
from which an Exchange Current
Density of 1.5 mA/cm z is Calculated
_'o= 0.6f (a)
h ..
,==k
0.8
IQ •
I I I I I
2
Z' (ohms)
E
h
6O
5O
40
3O
2O
10
(b)
10 20 30 40 50 60
Z' (ohms)
Figure 8. Complex Plane Impedance of {a} NICl 2
and (b) CuCl 2
39
i!
|
i
=
i
E=
Z
B
........ ] __:}, T : 7 z:_-:: s : ....
9 : -£_-- ..... : z -_- -_
Z
=
i


Advanced rechargeable sodium batteries with novel cathodes

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server