Potassium thiochromite, sodium thiochromite, and Ti 0.5V 0.5S 2 prepared and used in cycling studies. In addition, some electrochemical studies with polymer electrolyte (PEO-NaSCN) cells were also attempted

Mehrotra, G. M.

Worrell, W. L.

English

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Development of New Intercalated Cathode Materials
for Use in Sodium Batteries
NASA Grant NSG 3300
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s^	 Final Report
	 S rt'-'ce^ C! .-r/^.c.^. I ^u_.e^;.^
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This report describes progress during the past six months,
June-December 1981. In the Tune 1981 report, we observed that
after cell cycling using NaxAyTiS 2 cathodes (where A = Cs, Rb or
K), almost all the co-intercalant in these compounds is found in
'	 the auxiliary liquid electrolyte (lN1 NaI in triglyme). It was
f
also observed that the amount of calcium lost to the triglyme-
based liquid auxiliary electrolyte was much smaller (% 40) from
a Ca O 1TiS 2
 cathode. Therefore further cycling studies with
CayTiS 2
 (y = 0.1 and 0.2) cathodes have been undertaken.
Recently it was reported that more than 1 eq. Li + could
reversibly intercalate into a Na 0.1 CrS 2 cathode, with higher
voltage for Li/Li xNa O 1 CrS 2 cells (ti 0.67 - 0.2S v higher when
x = 0 - 0.7S) than those obtained from Li/Li xTiS 2 cells. There-
fore, as part of our investigation of other cathode materials,
potassium thiochromite, sodium thiochromite, and Ti0.5%O.SS2
have been prepared and used in cycling studies. In ^-.ddition,
some electrochemical studies , h polymer electrolyte (PEO-NaSCN)
cells have also been attempted.
A.	 Preparation and Characterization of the Cathode Materials
t
Equimolar mixture of K 2 S and Cr 2 S 3 was sealed in an evacuated ii
quartz tube and slowly heated (% SO°C/hr) in a tubular furnace to
ti 700 0C to prepare potassium thiochromite. After about 4 days at
(NASA-CR- 173 145)
	 DEVELOPMENT OF NEW
	 N84-16460
INTERCALATED CATHODE MATERIALS FOR USE IN
SODIUM BATTERIES Final Progress Report,
Ju-l. - Dec. 1981 (Pennsylvania Univ.)
	 14 p	 rinclas
HC A02/MF A01
	 CSCL 09C G3/33 11433
rw'I
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2
this temperature, the quartz tube containing the sulfide mixture
broke (in one experiment it exploded) when it was removed from.
the furnace. The K 2 S + Cr 2 S 3 mixture apparently reacts with the
quartz tube. Subsequently, potassium thiochromite was prepared
using the method described by Riidorff and Stegemann 2 . Cr 2 O 3 and
KCNS (weight ratio 1:30) mixture was heated to ,- 900 0C in an
alumina crucible covered with a lid and maintained at this
temperature for about one hour. The product was furnace cooled,
washed with a 10% KOH solution in water, filtered and then washed
with distilled water, ethyl alco:iol and ether, in the given
sequence. It was then dried at room temperature in vacuum
(ti 20-30 millitor) for about 4 hours.
Sodium thiochromite was prepared by heating a mixture of
K 2 CrO4 , Na 2 CO 3 and sulfur in the weight ratio 1:1S:1S to ti 8000C
and keeping it at this temperature for about one hour. After
furnace cooling, the product was washed with a dilute NaOH
solution in water, then with water and ethyl alcohol and finally
with ether. It was then dried, at room temperature, in vacuum -
for about 4 hours.
Ti0.5V0.552 was prepared by reacting titanium, vanadium
and sulfur in  required proportions, at % 900 0C for 3-4 days,P P
4
	
	 followed by annealing at ti S30 0 (" for about 3 days. The method of
preparation of Ca vTiS 2 compounds has already been described in
r 
the previous report.
The potassium - and sodium thiochromites were characterized
using Debye-Scherrer technique, atomic absorption spectroscopy and
X-ray fluorescence analysis. The Debye-Scherrer powder pattern of
i
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KCrS 2 " shows the reported reflections  for KCrS 2 and many more.
Some of these extra lines could be identified as KCr S S 9 1 The
additional phase(s) present in our compound could not be
identified. Radorff has reported that KCrS 2 can decompose during
washing of the powder and that there may exist other potassium
thiochromites. Atomic absorption spectroscopic analysis for
potassium determined the composition to be K0.61CrS2. However,
the analysis done using X-ray fluorescence technique gave
S = 58.59 at %, K = 22.39 at % and Cr = 19.02 at
	
In view of
our long experience of using atomic absorption spectroscopy, the
composition determined by this technique may be more reliable.
All the lines on the Debye-Scherrer pattern for the"NaCrS2"
matched with those reported 4 and no extra lines were observed.
The X-ray fluorescence technique pave its analysis as Na - 21.09
at %, Cr - 19.85 at % and S - 59.06 at %. The atomic absorption
spectroscopic analysis for sodium gave the composition as
T	
Na0 92CrS2'
	
B.	 Cycling Studies
The same cell design as described in our June 1981 report
was used in the cycling studies. All cycling studies were carried
c
out at 130 ± S0  iii a galvanostatic mode. The cell voltage during
discharge or charge was continuously measured with the help of a
Keithly 630 electrometer (input impedance no 10 14 ohms) hooked up
to a chart recorder or with a Tigraph 100 (6 channel chart
recorder, input impedance ti 10 7 ohms).
The discharge and charge behaviour of a Ca 0.1 TiS l
 cathode
7_
in a galvanostatic mode: at a current density of 0.9S mA/cm is
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shown in Fig. 1. During the discharge, the cell voltage drops
sharply by ti 0.2 V at the beginning of discharge and then drops
gradually until about x = 0.8, after which it changes very slowly
from 1.1 V at x = 0.8 to 1.02 V at x = 1.4S (the value of x in
Na xCa 0.1 TiS 2 has been calculated from coulombs passed) when
discharge was stopped and the charge cycle began. During recharge,
the cell voltage remains almosL unchanged when x < 0.4. A similar
observation was made earlier with recharge of other co-intercalated
electrodes (see June 1981 report).
Fig. 2 shows the discharge and charge behaviour of a Ca0.2TiS2
cathode in a galvanostatic mode. The features of discharge and
charge curves in this figure are similar to those of the discharge
and charge curves for a Ca 0 1 TiS 2 cathode (Fig. 1). After an
initial sharp drop of 0.1V at the beginning of the 1st disc-harge,
the cell voltage drops gradually until x t- 0.7S. The cell voltage
remains practically constant at ti 1 V from this point onwards until
x = 1.5. During the first charge half cycle, the voltage remains
practically unchanged at ti 2.5'V when x < 0.45. During the second
and third discharge, the cell voltage changes very slowly from
ti 1.3 V at x = 0.6 to \, 1.1 V at x = 1.4. During second recharge,
the cell voltage remains unchanged at ti 2.S V when x < 0.3. The
cell voltages during the first discharge of Ca 0 1 TiS 2 and Cao 2 T; S2
cathodes at x = 0.2, 0.4, 0.6, 0.8
	 ?d 1.0 are 1.76 and 1.81, 1.44
and 1.4S, 1.33 and 1.22, 1.11 and l.Gl, and 1.10 an-.L 1.00 V,
respectively. Thus, there is practically no difference in the
discharge behaviours of these two cathodes.
Fig. 3 shows the cycling behaviour, in a galvanostatic node
of the K0.61CrS2 electrode. The initial cell voltage of 2.33 V
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5
crops rapidly in the initial stases of discharge and reaches a
value of u 0.75Vat x = 0.16. After this the decrease in
voltage is slow until about x = 0.35, after which it drops
sharply to zero and becomes even negative (ti -0.2 V). During
the first charge, the cell voltage reaches a maximum (% 2.6 V)
when x = C.3 and then decreases slightly and remains almost
unchanged at 2.5 V when x < 0.4. During the second discharge,
the decrease in voltage is not as rapid, and the second recharge
curve does not show a plateau towards the end of recharge.
Analysis of the liquid electrolyte at the end of the second
recharge showed that ti 94 % of the potassium in K0.61CrS2 was
present in the electrolyte. The color of the electrolyte at the
ene of the experiment was like that of red wine. During experi-
ment, it was observed that the color becomes lighter (luring
discharge and deeper during charge half cycle.
Fig.	 4 shows the discharge and charge behaviour of K'G1CrS2
mixed with graphite (60 wt	 o	 K0.61CrS., +	 40 $	 graphite).	 The
features of the first discharge and charge curves are similar,
to those of first dischar-ge and cY arge curves in Fig. 3. But
the discharge proceeds at a high--r voltage, probabl ,r due to
improved conductivity of the cathode.
Cycling studies with Na 0 92CrS 2 cathode were also attempted.	
C
It has been reported 1that NaCrS 2 is not a good electronic con-
ductor (specific resistivity = 10 5 ohm.cm ). The cathode in the
cycling cell, therefore, consisted of 60 wt o Na 0.92 CTS 2 and
40 wt o graphite. When the cell was charged in a galvanostatic
rode at a current density of 0.63 mA/cm 2 , the cell voltage
i
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6increased instantaneously from the initial value of ti 2.4 V to
v 2.75 V and then decreased to \, 2.62 V and remained almost
unchanged for the remaining duration of the charge. The discharge
of the cell was then attempted when the value of x in NaxCrS2
was = 0 (as calculated from the coulombs passed). On beginning
the discharge of the cell, the voltage drop ped to zero in less
than one minute and then it became even negative. Similar
L	 behaviour was observed w;,^,n the cathode pellet did not contain
R 1
any graphite. The color of the electrolyte turned orange during
the experiment, which indicates a reaction with the electrolyte.
Fig. 5 shows a partial discharge of an NaxTi0.5V0.5S2
cathode. The cell voltage decreased to 0.86 V when the discharge
waS stopped at x = 0.7. No further studies have been made with
this cathode.
The characteristics of the discharge and charge curves shown
in Figs. 1-5, the coloration of the electrolyte and its chemical
analysis after cell cycling, all indicate that reactions occur
between the liquid triglyme-based electrolyte and the cathode
material. A cyclic voltammetric study was performed using the 1M
NaI solution in triglyme (vacuum distilled and then Passed through
neutral alumina) as the electrolyte, sodium reference and counter
electrodes and platinum as the working electrode (area = 1.96 x 10-3
cm 2 ). The result is shown in Fie. 6. Peaks (% 60 uA at 1.33V and
0.35 uA at 0 9 V) in the potential vs. current plot indicate
reactions within the electrolyte. it is not possible to say what
these reactions are. Fig. 6 also indicates that the electrolyte
cannot be used above % 2.65 V.
E:+
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7
C.	 Other Electrochemical Cell Studies
Open circuit voltages using Na 0 86TiS2, Na0.36TiS 2 1 Na0.32TiS2,
Na0.2TiS 2
 and Na0.03TiS2 cathodes were measured at 70-90 0C using
a PEO-NaSCN solid electrolyte. The results are in fair agreement
(} 0.005 to 0.05 V) with those obtained with V -Al 2 0 3 cells at
120-130 0C. Althou gh the transference number of Na + in the PEO-
NaSCN film has been reported  to be unity, the resistance of our
cells, obtained using a complex im pedance method, is % 10 5 ohms
at 93 0C. 'therefore these cells cannot be used for cycling studies
at current der.sities greater than ti 15-20 uA/cm2.
Summary
Cycling studies with K0.61CrS2 electrodes showed that
potassium is lost to the liquid, triglyme-based auxiliary electro-
lyte. The values of x in excess of unity in NaxCa yTiS. obtained
during discharge of cells with Ca 0 1 TiS 2 and Ca 0 2 TiS 2 cathodes
are most probably due to reactions with the liquid auxiliary
electrolyte, as indicated by the results of cyclic voltammetry
(Fig. 6). As this stage, it is unclear whether the loss of the
co-intercalant, A, from an AyTiS 2 cathode (where A = Cs, Rb^ K)
or of potassium from a KyCrS 2 cathode to the liquid auxiliary
electrolyte during; cell cycling is due to some intrinsic factor	 1',
related to the co-intercalated electrode material or to the
triglyme-based auxiliary electrolyte. Further cycling studies
with cells with another liquid auxiliary electrolyte or with
i
cells with only a solid electrolyte, e.g. V -Al 2 0 3 , may help
clarify this.
, D
.:.
8
O'
References
1.	 R. V. Moslitev, V. Manev, A. Nassalevska, G. PistoLa and
M. Icovi: J. F.leectrochem. Soc., 128 (1981) 1339.
V
	 IV. Rudorff and K. Stegemann: Z. Anorg. Allg. ChPm., 251
(1943) 376.
3. JCPDS	 Card No. 10-290.
4. JCPDS	 Card Nu. 10-292.
S.
	
	 R. Dupon, D. 11. Whitmore and D. F. Shriver: J. Electroc}em.
Foc., 128 (1981) 71S.
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Development of new intercalated cathode materials for use in sodium batteries

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