During the second year of research under NASA SBIR Contract NAS7-967, Covalent Associates and NASA contract monitors at the Jet Propulsion Laboratory agreed to perform an evaluation of the three best electrolytes developed during Phase 2. Due to the extensive period of time required to collect meaningful cycling data, we realized the study would extend well beyond the original formal end of the Phase 2 program (August 31, 1988). The substitution of this effort in lieu of an earlier proposed 20-cell final deliverable is formally documented in Modification No. 1 of Contract NAS7-967 as task 7. This Addendum contains the results of the cycling studies performed at Covalent Associates. In addition, sealed ampoules of each of these three electrolytes were delivered to the Jet Propulsion Laboratory Electrochemical Power Group. Their concurrent evaluation in a different test vehicle has also been recently concluded and their results are also summarized herein

Dominey, L. A.

Goldman, J. L.

Koch, V. R.

English

NASA Technical Reports Server

_JASA-CR-I 90 87,J
# %
SBIR- i0.08-1140
retease date 9/02/90 v"
THERMALLY STABLE ELECTROLYTES FOR RECHARGEABLE
LITHIUM BATTERIES - PHASE II
NASA SBIR Contract No. NAS7-967
_._
Final Report
Covering the Period January i, 1989 - August 31, 1989
Submitted by
L. A. Dominey, J. L. Goldman, and
V. R. Koch, Principal Investigator
Covalent Associates, Inc.
52 Dragon Court
Woburn, MA 01801
September 13, 1989
SBIR NOTICE (JUL
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This SBIR data is furnished with rights Contract No. NAS7-967.
For a period of two years after :eptance of items to be delivered under
this contract, the Government a_re to use tl _ data for Government purposes
only, and it shall not be disclose( outside Government during such period
without permission of the Contractor , subject to the foregoing use
and disclosure prohibitions, such dat disclosed for use by support con-
tractors. After the aforesaid two-ye_ the Government has a royalty-
free license to use and to authorize o to use on its behalf, this data for
Government purposes, but is relieved prohibitions and assumes
no liability for unauthorized use_ this by third parties. This Notice
shall be affixed to any reproduct_6ns of this i, in whole or in part.
/
https://ntrs.nasa.gov/search.jsp?R=19930004422 2020-03-17T10:19:12+00:00Z
ABSTRACT
During the second year of research under NASA SBIR Contract
NAS7-967, Covalent Associates and NASA contract monitors at the Jet
Propulsion Laboratory agreed to perform an evaluation of the three
best electrolytes developed during Phase II. Due to the extensive
period of time required to collect meaningful cycling data, we
realized the study would extend well beyond the original formal end
of the Phase II program (August 31, 1988). The substitution of this
effort in lieu of an earlier proposed 20-cell final deliverable is
formally documented in Modification No. 1 of Contract NAS7-967 as
Task 7.
This Addendum contains the results of the cycling studies per-
formed at Covalent Associates. In addition, sealed ampoules of each
of these three electrolytes were delivered to the Jet Propulsion
Laboratory Electrochemical Power Group. Their concurrent evaluation
in a different test vehicle has also been recently concluded and
their results are also summarized herein.
TABLEOF CONTENTS
ABSTRACT
OBJECTIVE
EXPERIMENTAL
RESULTS AND DISCUSSION
I. Covalent/JPL Concurrent Evaluation
2. Evaluation by Outside Vendors
CONCLUSION
REFERENCE
i
1
2
3
3
12
12
12
ii
OBJECTIVE
During Phase II a host of electrolytes were developed and compared in a
standardized 260 mAh TiSz test vehicle. While this procedure identified
several potentially important electrolytes, their capabilities remained to be
determined in electrochemical cells which would more surely achieve the NASA
goal of greater than 500 50% depth-of-discharge (DOD) cycles. We believed the
Li Figures of Merit were being compromised by overly thick cathodes, which may
develop concentration gradients and trap degradation products. Therefore, we
built a test vehicle with thinner cathodes and a higher Li/TiS2 ratio. In so
doing, we expected to achieve better Li utilization and a significantly
greater number of useful discharges.
Concurrently, the JPL team subjected the electrolytes to 100% DOD cycling
in their standard test vehicle with the primary goal of verifying state-of-
the-art Li utilization, i.e., high FOMs.
The overall goal of this Phase II effort was to develop the technology
base needed to fabricate high energy density/high cycle life rechargeable Li
cells which will fly on future NASA missions. Based upon the data presented
in the Final Report and in this Addendum, we believe that this goal has been
achieved.
1
EXPERIMENTAL
Below are the specifications for the Covalent Associates test vehicle
used in this set of experiments:
Discharge : 2 mA/cz_, Charge : 1 mA/cm 2
15 minute rests after charge and discharge
Voltage limits 2.7V- 1.6V
FOM cut off defined as 30_ DOD
Cycling will be at 50_ DOD at theoretical TiS2 capacity
Li will be i0 mil, i Ah/cell
TiS2 will be 6.5 mAh/cm2/cycling side and 15 mil thick in the cathodes
fabricated at Covalent (8 w/o Teflon, 8 w/o Shawinigan carbon black)
Electrodes will be i" x 1 1/2"
Total TiS2 capacity will be 130 mAh
Li/TiS2 = 7.7/1
5 ml of electrolyte/cell
LiAsF6 is Lithco tetrakisacetonitrile desolvate process (Lot 8801192H)
The JPL research group used their standard spiral-wound test vehicle (i)
with EPDM cathode binder and an approximate cathode capacity of 150 mAh and Li
capacity of i Ah. The cells were cycled initially to full i00_ DOD cycles.
Breakpoint values are defined as the cycle number when 40_ of the original
capacity was achievable. Current densities were 2 mA/cm 2 (discharge) and i
mA/c_ (charge).
The electrolytes prepared in the temperature range of -20 to +I0°C were:
1) 1.SM LiAsF6/THF (Li-Hg)/0.7 v/o 2-MeF/18Crown6/KOH
2) 1.5M LiAsFs 50-50 v/o THF + 2-MeTHF/0.7 v/o 2-HeF/KOH
3) 1.5M LiAsFs/DIOX (Li-Hg)/0.7 v/o 2-MeF/18Crown6/KOH
2
RESULTS AND DISCUSSION
Tables 1 and 2 summarize the results of the full cell cycling studies at
Covalent Associates and JPL.
TABLE 1
Summary of Final Cycling Program Results at Covalent
Electrolyte
1.SM LiAsFs in:
Blend 50/KOH/2-MeF
Current Cell 50_
Densities Number Cycles FOM
DIC (_I_) .....
2/1 026-124-13 542/581 39.3
026-124-14 104 14.4
1/0.5 026-125-37 401 32.6
026-125-38 600 >41"
THF/18Crown6/KOH/2-MeF 2/1 026-127-74 415/531 36.3
026-127-75 48 4.7
1/0.5 026-127-62 340 23.2
026-127-63 321 23.3
DIOX/18Crown6/KOH/2-MeF 2/1
1/0.5
026-128-98 67 7.4
026-128-99 35 4.7
026-128-86 232 15.3
026-128-87 125 13.5
*Still cycling at 48_ DOD at cycle 615
3
TABLE 2
Summary of Cycling Results at JPL
Electrolyte
1.5M LiAsFe in:
Blend 50/KOH/2-MeF
Total Cycles
100% DOD initial
40% DOD Cut-Off FOM
320* 39
320 39
THF/18Crown6/KOH/2-MeF 300 36
300 36
DIOX/18Crown6/KOH/2-MeF 300 34
250 24
*Average capacity per cycle = 120 mAh
Prior to these studies, Covalent Associates' best Li FOMs in cells with
thick (260 mAh) cathodes were in the range of 28 to 30. Our superior FOMs of
39 to 40 from the redesigned, thinner cathode (130 mAh) test vehicle clearly
confirm that electrolytes now exist which can fulfill the NASA goal of greater
than 500 50% DOD quality discharges. By adjusting the engineering parameters
we increased the Li efficiency by 33 percent.
The JPL team has confirmed our findings by achieving surprisingly similar
best FOMs with the Blend 50 and THF-based electrolytes. Additionally, they
also achieved what we believe is the best ever reported Li FOM with an LiAsFs/
DIOX-based system. Both groups found the Blend 50 and THF systems to be
somewhat better than DIOX. We suspect that a greatly reduced but steady low
level of DIOX polymerization may still proceed on the surface of the TiSz
crystallites and within the bulk electrolyte, thereby contributing to the
premature capacity fade relative to the THF and Blend 50 systems.
As determined at JPL, microcalorimetry of filled cells with all three
electrolytes favorably indicates low heat evolution during storage on open-
circuit. This is a necessary prerequisite for a battery to have a long shelf
life and is particularly important for multi-year NASA missions.
Figures 1 through 6 collect the voltages as a function of cycle number
for the cells fabricated at Covalent. All are at the 50_ DOD endpoint voltage
except Figure 3A which complements Figure 3B by comparing 25_ DOD voltages
with 50_ DOD voltages (v ida infra).
Despite the presence of identical electrolyte in duplicate cells, several
duplicate cells exhibited substantial variance from each other. We attribute
this to a combination of cell to cell hardware differences and the complicated
morphologies inherent in Li electrodeposition through subsequent charge cycles.
Figures 1 and 2 depict endpoint plots for the Blend 50 cells at 2mA/c_
and 1 mA/c_ discharge rates, respectively. The best results in each case
yielded FOMs of 40 or so and about 600 cycles.
In Figures 3A and 3B, the midpoint (25% DOD) and endpoint voltages for
the 2mA/c_ discharge rate are plotted for a cell containing the TItF-based
electrolyte. This cell's capacity became limited to 50% at cycle 415. A
subsequent 50% reduction in discharge and charge currents afforded a total of
about 531 50% DOD cycles. Figure 3A, the 25_ DOD voltage profile, is included
to demonstrate that during the early portion of the discharge the voltage
remains near 2.1V even as the voltage drops faster deeper in the discharge.
Figures 4A and 4B show very repeatable Li FOMs of 23.2 and 23.3 at the
lower, lmA/c_, discharge rate. A review of all the THF and Blend 50 results
shows little apparent difference in cycling efficiency at the two different
discharge rates.
Finally, DIOX cycling results are shown in Figures 5 and 6. These
results are inferior to both other electrolytes in this series; however, we
obtained significantly better results during the second year of Phase II with
our thicker cathodes. The major difference between the experiments is that
during Phase II we used TiS2 which had been pretreated by immersion in TItF
saturated with KOH.
This procedure eliminates residual acidic impurities in the TiS2 prior to
exposure to the electrolyte. It is quite likely that KOH treated TiS2 used in
the current test vehicle would afford comparable FOMs, as was the case in the
second year of Phase II.
5
CELL026-124-13
ENDPOIHTVOLTAGESQT 50_ DOD
1,1
KI
)-
,.J
0
> i,90
1.80
1.70
1.60
i i I I
28O 488
CYCLENU_ER
Figure IA
600
2,30
CELL026-124-14
EHDPOIIiTVOLTAGESAT50_DOD
W
0
C
)-
J
0
>
2,20
2,181 
\
1,6OI , -_ I u I
Figure IB
3_
CVCLEHUHBER
Figures IA (Top) and IB (Bottom): Endpoint Voltage Profile for
Blend 50/2-MeF/KOH Electrolyte Discharged at 2mA/cm 2.
I.SM LiAsF6/
CELL(_2&-425-_?
gIDPOIlfrVOLT_ES AT_k IX)O
Figure 2A
i.80
6BB
CELL_26-125-3B
• DPOIHTVOLTAGESAT 58% DOD
Figure 2B
4,80
CVCLENUMBER
Figures 2A and 2B: Endpoint Voltage Profile for 1.5M LiAsF6/Blend 50/2-MeF/KOH
Electrolyte Discharged at ImA/cm L.
CELl-B_-12_-_4
MID-(_.I_qR_E/DISO'kqRGEVOLT.IS
W
l-
-J
2,?B
2.60
2,5B
2,4B
2,3B
2,2g.
2,i0.
2,8B
i,90
Figure 3A
I I I |
CYCLEHUMBER
o DISCHARGE+ CHARGE
EII)POI_VOLTAGESAT5B'/,DO0
4_ 6@B
2,2_
2,1B.
Figure 3B
l,TB
4BB
CYCLEI'LtIBER
Figures 3A and 3B: Endpoint Voltage Profiles at 25% DOD (Top) and 50% DOD (Bottom)
for 1.5M LiAsF6/THF/2-MeF/18Crown6/KOH Electrolyte Discharged at 2mA/cm z.
8
CELL026-127-62
EY4)POIHTVOLTAGESAT59P.DO0
Figure 4A
1.78t
1.6e/
o
I
200 4OO
CELLG26-127-_3
E]_POIHTVOLTAGESAT_)_,DOD
Figure 4B
1,70
CYCLEIgJMSER
Figures 4A and 4B: Endpoint Voltage Profiles for 1.5M LiAsF 6/THF/2-MeF/18Crown6/
KOH Electrolytes Discharged at ImA/cm .
9
CELL026-128-98
EI'IDI::>OIHTVOLTAGESAT _P. DOD
2,20
2,18'
2,0_ '
1.98
1.80
1,78
1,60 I l
0 48
n
0
O
O
n
t
CYCLEHUMBER
Figure 5A
I
120 168
CELL026-428-99
EHDPOIHTVOLTAGESAT _% DOD
Figure 5B
W
o
_z
k-
.J
0
:>
0
=%
n
5
1,90
i.80
1.78
1.60 j
B
n
\
n
0
%
(:}
0
0
I I rl I I 1 t 1 I I I --
28 48 68 88 188 129
CVCLE11t_41_
Figures 5A and 5B: Endpoint Voltage PrRfiles for 1.5M LiAsF 6 /DlOX/2-MeF/18Crown6/
KOH Electrolytes Discharged at 2mA/cm z.
I0
, CELL826-129-86
ENDPOIHTVOLTAGESAT58_,DOD
Figure 6A
2,i8.
2,00'
i.98.
I.?0
1,68
] f
o
D
{3
O
I I i 8
O iOO 200
W
0
C
h
J
0
>
CELLB26-128-87
E_POIHTVOLTAGESAT58%DOD
2,30 Figure 6B
2'20k
!'_°I\
1'_1 "N
1,_°1 \
1,681 I I "_ i
CVCLEHU_ER
Figures 6A and 6B: Endpoint Voltage Profiles foK
18Crown6/KOH Electrolytes Discharged at lmA/cm L.
1.5M LiAsF6/DIOX/2-MeF/
11
DEvaluations by Outside Vendors:
In the last year, Covalent Associates has made small samples of the THF
and Blend 50-based electrolytes available to potential manufacturers of
Li/TiS2 batteries.
A series of cycling studies at Eveready Battery Company (Cleveland, OH)
concluded that the Covalent modified Blend 50 electrolyte afforded signifi-
cantly higher cathode utilization than other electrolytes with which they were
familiar when cycled at 60% DOD and imA/c_ discharge-O.5mA/c_ charge rates.
Li FOMs of 55.4 and 55.1 were achieved in duplicate cells. Cycling was halted
at this time. However, one cell was still demonstrating a 1.9V endpoint.
This indicates that substantial cycleability remained. A detailed summary of
this set of experiments has been reviewed by the JPL Electrochemical Power
Sources Group.
Additionally, a recent comparison of our improved Blend 50 and THF elec-
trolytes at W. R. Grace (Columbia, MD) has been completed. They found that
when compared with the Blend 50 electrolyte, our THF electrolyte afforded a
reproducibly higher cathode utilization under relatively high rate discharge
conditions.
CONCLUSION
With the development of these improved electrolytes, it is now feasible
to build a rechargeable Li/TiS2 battery capable of both good cycle life (>500
50% DOD) and improved rate capability (at least C13). While improvements over
present systems have been demonstrated and confirmed, further improvements are
highly likely. These will come about by electrolyte component optimization
(e.g., concentration/performance studies) and by further advances in cell
engineering.
REFERENCE
I. D. Shen, S. Subbarao, F. Deligiannis, and G. Halpert in Proc. Symp.
on Materials and Processes for Lithium Batteries, K. M. Abraham and
B. B. Owens, eds, Vol. 89-4 (The Electrochemical Society, 1989),
p. 223.
12


Thermally stable electrolytes for rechargeable lithium batteries, phase 2

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