Journal of New Materials for Electrochemical Systems 5, 273-279 (2002)
c J. New. Mat. Electrochem. Systems
Characterization of Plasma-Sprayed Pyrite/Electrolyte Composite Cathodes for Thermal
Batteries
Ronald A. Guidotti∗, Frederick W. Reinhardt
Jinxiang Dai1 , Jeff Roth1 , and David E. Reisner1
Sandia National Laboratories, P.O. Box 5800, Albuquerque, NM 87185-0614 U.S.A.
1
US Nanocorp , Inc.
74 Batterson Park Road, Farmington, CT 06032 U.S.A.
( Received June 9, 2001 ; received in revised form January 20, 2002 )
Abstract: A number of electrolytes were evaluated as co-spray additives for plasma spraying of pyrite powder to form composite cathodes for use
in thermally activated batteries. Initial work showed that the LiCl-KCl eutectic electrolyte was effective for this purpose. In this paper, the use
of alternative electrolytes is described and the effects on performance of the plasma-sprayed cathodes in Li(Si)/FeS2 single cells are presented.
Standard thermal-battery electrolytes such as the all-lithium (cation) LiBr-LiBr-LiF electrolyte and the low-melting LiBr-KBr-LiF eutectic were
evaluated. A nonhygroscopic electrolyte, the KCl-Li2 SO4 -NaCl eutectic, was also examined. These electrolytes encompass a range of melting
points and heats of fusion. Future research efforts in this area are also described.
Key words : Plasma-Sprayed pyrite composite cathodes, cost reduction
1.
INTRODUCTION
which is becoming increasingly important in many of today’s
military applications.
The standard technique for fabrication of electrodes for thermally activated (“thermal”) batteries involves cold pressing of
anolyte, separator, catholyte, and heat powder mixtures. The
pressures for pelletization increase as the square of the diameter of the pressed pellets. This results in the need for expensive,
high-capacity hydraulic presses for large-diameter (over 3”) pellets. Pressing large-diameter thin pellets is extremely difficult.
This is compounded by the fact that pellets of this size tend to be
quite fragile, which poses handling problems and reduces pelletization yields. Plasma spraying of electrodes offers a solution
to this problem for the cathode, which is normally pyrite (FeS2 )
based for most applications. Only the material that is needed for
a given application can be deposited with this technique, which
reduces the overall battery weight and volume. This, in turn,
increases the energy density and specific energy of the battery,
In earlier work, we demonstrated that FeS2 can be plasmasprayed onto a stainless steel substrate using elemental sulfur
as a co-spray agent [1]. The sulfur was originally designed to
serve as a flow enhancer for the plasma-spray feedstock. It also
acted as a thermal barrier, where the heat of sublimation served
to cool the pyrite particles as they traversed the plasma. Unfortunately, residual sulfur in the deposit interfered with the electrochemical discharge of the plasma-sprayed cathodes. During
activation of a thermal battery with these cathodes, the residual elemental sulfur would volatilize from the cathode and react
exothermically with the anode, which sometimes lead to a thermal runaway destroying the battery. Removal of the residual
sulfur avoided this difficulty, but at the expense of the mechanical integrity of the sample. The sulfur acted as an adhesive
bonding the pyrite to the substrate, so that removal of this material led to spalling of the plasma-sprayed pyrite during handling.
∗ To whom correspondence should be addressed: Phone: (505)844-1660;
fax: (505)844-6972; e-mail: [email protected]
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Ronald A. Guidotti et al./ J. New Mat. Electrochem. Systems 5, 273-279 (2002)
More recently, we were able to incorporate an alternate thermalbarrier coating by co-spraying the pyrite with LiCl-KCl eutectic
electrolyte [2]. The heat of fusion absorbs some of the heat from
the plasma, thereby reducing the tendency for thermal decomposition of the FeS2 via the reaction
F eS2(s) −→ F eS(s) + 1/2S2(v)
(1)
Since the LiCl-KCl electrolyte is used in many thermal batteries, the presence of free electrolyte in the deposit aids in the
electrochemical performance by providing a medium for Li+
transport during discharge. It also served to strongly bond the
pyrite to the substrate.
In this work, we will report on the evaluation of alternate electrolytes as co-spray feedstock additives that function as thermalbarrier coatings for plasma spraying of pyrite for thin-film electrodes for thermal batteries. Both halide-based systems as well
as those containing carbonate were evaluated. The intent was
to observe the effect of melting point and heat of fusion on the
physical, chemical, and electrochemical characteristics of the
deposits. The performance of materials made with alternative
electrolytes is compared to that of pyrite co-sprayed with LiClKCl eutectic electrolyte.
2.
EXPERIMENTAL
2.1
Plasma Spraying
Plasma spraying was carried out with a Metco 9MB plasma
spray system (Metco, Westbury, NY). A dc arc current of 188
A to 220 A was generated in the copper nozzle. Argon passing through the dc-arc was heated to >9,000o C and ionized.
The pyrite powder was fed radially into the plasma stream near
the nozzle exit at a rate of 0.7 to 0.9 kg/h. The plasma output was directed to a supported sheet of 0.005”-thick graphite
paper (Grafoil ) positioned 5 to 10 cm away. Typically, three
or more passes were used to prepare the deposits. The plasma
spraying was conducted under an atmosphere of dry argon, to
prevent sample oxidation. Exposure of the deposits to the ambient atmosphere was minimized, to avoid moisture pickup by
the hygroscopic lithium salts that were components of some of
the electrolytes.
the electrolyte for 3 h. The electrolytes that were examined in
this study are listed in Table 1 along with their measured melting
points and heats of fusion as determined by differential scanning
calorimetry (DSC).
Table 1: Electrolytes used as co-spraying agents for plasma
spraying of pyrite
Composition
Melting point
(w/o)
(o C)∗
45 LiCl/55 KCl
352
68.4 LiBr/
22.0 LiCl/
436
9.6 LiF
40.36 KCl/
53.79 Li2 SO4 /
425
16.51 NaCl
57.3 LiBr/
315
42.0 KBr/0.67 LiF)
* Measured by DSC at Sandia.
Heat of fusion,
(cal/g)∗
56.1
Literature
reference
5
70.2
6
70.3
7
32.0
8
Anode pellets were pressed from powders containing
44% Li/56% Si with 25% electrolyte [0.93 g, 0.042” (1.07 mm)
thick). Separators mixes were made by fusing the electrolytes
with 25% to 35% MgO (Maglite S, Merck) in a dry room maintained at <3% relative humidity. (The MgO acts as a binder
to immobilize the molten electrolyte by capillary action.) The
separator pellets weighed 1.00 g and were 0.027” (0.696 mm)
thick. The pellet densities were nominally 72.5% to 75.6% of
theoretical. For control purposes, standard lithiated catholytes
were used that contained 73.5% FeS2 /25% separator/1.5% Li2 O
[0.51 g, 0.0118” (0.300 mm) thick]. The Li2 O acts to fix the activity of Li in the catholyte to reduce the voltage transient that
normally occurs with this material on activation of a battery [3,
4]. All cell assembly and handling operations were conducted
in a dry room maintained at <3% relative humidity.
2.3
Deposit Characterization
Scanning electron microscopy (SEM) was used to gather morphological information on the plasma-sprayed deposits. In some
cases, the water-leached deposits were examined by x-ray diffraction (XRD) for phase identification.
2.2 Materials
2.4
The electrolytes were prepared by blending the reagent-grade
constituents together and fusing at 600o C or 75o C above the
melting point (whichever was greater) for 3 h in a quartz crucible. The pyrite powder (American Minerals) was –325 mesh,
or ~44 µm in diameter and purified by HCl leaching (1:1 v/v).
The pyrite powder was mixed with 15% to 20% of the various
electrolytes in a ball mill for 8 h under a dry argon cover and
was then fused under argon at 25o C above the melting point of
Electrochemical Characterization
Discs 1.25”(3.2 cm) in diameter were cut from the plasmasprayed deposits for testing in single cells. The thickness of
most of the plasma-sprayed electrodes ranged from 50 to 150µm.
The mass of active FeS2 in each sample was measured before
each test to allow calculation of the gravimetric capacity during discharge. The single cells were discharged galvanostatically to a cutoff voltage of 1.0 V between heated platens under
Characterization of Plasma-Sprayed Pyrite/Electrolyte Composite Cathodes. / J. New Mat. Electrochem. Systems 5, 273-279 (2002)
275
an applied pressure of 8.0 psig (55.2 kPa) in a glovebox under
high-purity argon. The cells were discharged between 400o and
550o C with a steady-state current of 1 A (~125 mA/cm2 ) and a
0.5-s to 1-s, 2-A (~250 mA/cm2 ) pulse applied every minute to
provide polarization data.
Generally, the plasma-sprayed test coupons were tested as prepared. Representative discs were leached with water to remove
the electrolyte constituent and reweighed afterwards to determine the electrolyte content. The active pyrite content was calculated by difference, subtracting the tare weight of the graphite
substrate. The melting points of the electrolytes were determined by DSC. The same technique was used with the deposits,
to measure any lowering of melting point caused by contaminants introduced during plasma spraying or by a change in
composition of the electrolyte. Select samples of the plasmasprayed deposits were subjected to chemical analysis of the electrolyte to determine if there were any compositional changes after plasma spraying. The water-leached pyrite from the deposit
was examined by x-ray diffraction (XRD) for phase identification.
3.
3.1
Figure 1: Response at 500o C and a background current density
of 125 mA/cm2 of Li(Si)/LiCl-KCl (MgO)/FeS2 single cells for
cathodes made by plasma spraying pyrite with LiCl-KCl eutectic and by cold pressing of powders.
RESULTS AND DISCUSSION
LiCl-KCl Eutectic
A typical response is shown in Figure 1 for single cells fabricated with several thicknesses of pyrite cathode prepared using
the LiCl-KCl eutectic electrolyte. The cell was tested at 500o C
with a background load of 125 mA/cm2 . The performance of a
cell with a conventional pressed-power cathode of comparable
thickness is included for comparison. Higher capacities were
realized for the cells with plasma-sprayed cathodes. Note the
absence of a hump or local maximum in the polarization trace
at about800 C/g FeS2 . This is typical for plasma-sprayed electrodes and is a consequence of the improved interparticle bonding that results from the high kinetic energy of the pyrite particles impinging on the substrate. This improved particle-particle
contact results in a lower resistance during discharge and more
effective utilization of active material. This is evident in Figure 2 that shows the SEM photomicrograph for this material.
The cathode made by cold pressing of powders was lithiated.
Thus, the cell with this cathode did not show a voltage spike at
the start of discharge that the cell with the unlithiated plasmasprayed electrode did. XRD analysis showed pyrite to be the
major phase in the deposit. Elemental analysis of the electrolyte
constituents showed no measurable change in composition.
One difficulty that was encountered is that the electrolyte content of the deposit (36% – 43%) was much greater than that of
the feedstock (20%). This indicates preferential loss of pyrite
during the spraying process. One reason for this behavior could
be that large particles of pyrite are bouncing off the substrate
during spraying. This would result in a build up of electrolyte
Figure 2: Cross-sectional view at a 60o tilt of a plasmasprayed deposit resulting from co-spraying LiCl-KCl eutectic
electrolyte with pyrite. (150 µm marker)
during multiple passes. This use of a finer-sized pyrite feed material would be expected to alleviate this difficulty. Future work
will examine nanostructured materials in an attempt to mitigate
this electrolyte-concentration problem.
With the exception of the initial voltage spike, the performance
of the plasma-sprayed electrodes in thermal cells is exceptional
when compared to its pressed-powder counterpart. The use
of thin-film electrodes would greatly reduce the size of many
short-lived thermal batteries by using only the required amount
of material in the cathode (and anode, for plasma-sprayed anodes), thus reducing the weight of pyrotechnic required for heating purposes. Current technology requires thicker (heavier) pellets than necessary strictly due to handling problems with thin
cells-especially for large-diameter ones (>5"). In addition, al-
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Ronald A. Guidotti et al./ J. New Mat. Electrochem. Systems 5, 273-279 (2002)
most any sized disc can be readily cut from sheets of plasmasprayed material, thus obviating the need for expensive dies and
hydraulic presses. The lower internal resistance that results with
the use of plasma-sprayed electrodes would translate into corresponding increases in power density and specific power for
batteries using these electrodes.
3.2
LiBr-LiCl-LiF Electrolyte
A cross-sectional view of the pyrite deposit co-sprayed with the
LiBr-LiCl-LiF eutectic is shown in Figure 3. The composition
of the deposit showed the same discrepancy from the feedstock
as was observed for the LiCl-KCl eutectic. Electrolyte levels
of up to 45% were found, compared to only 15% in the feedstock. The deposit was not as coherent or adherent as that made
with the LiCl-KCl eutectic. This material has a much higher
melting point and slightly higher heat of fusion than the binary
eutectic (Table 1). The performance of this material in single
cells at 500o C is shown in Figure 4 for material sprayed without
adequate protection from ambient atmosphere and with a protecting box flushed with dry argon during spraying. The nominal thickness of the deposit was ~130 µm. Data for a similar
pressed-powder cathode are included for comparison.
Figure 4: Response at 500o C and a background current density of 125 mA/cm2 of Li(Si)/LiCl-LiBr-LiF (MgO)/FeS2 single
cells for cathodes made by plasma spraying pyrite with LiClLiBr-LiF electrolyte and by cold pressing of powders.
agent was responsible for the initial voltage spike. Elimination
of the voltage spike for the plasma-sprayed electrode will result in an overall far superior performance relative to a similar
pressed-powder electrode due to the larger capacity and lower
impedance.
3.3
Figure 3: Cross-sectional view at a 60o tilt of a plasma-sprayed
deposit resulting from co-spraying LiCl-LiBr-LiF electrolyte
with pyrite (bottom part is Grafoil substrate). (150 µm marker)
The lack of adequate protection made a dramatic difference in
the electrochemical performance, drastically reducing by almost
a factor of four the critical upper-voltage plateau on which battery design is based. This material also contained measurable
amounts of FeSO4 , which contributed to the higher voltage transient at the beginning of discharge.
The cell with the cathode prepared using a protecting box has a
similar upper-voltage plateau as for the pressed-powder sample
and exhibited a reduced polarization. There was no indication
of even a slight hump in the polarization trace and the cell ran
longer on the lower-voltage plateau. The lack of any lithiation
LiBr-KBr-LiF Eutectic
A cross-sectional view of the pyrite deposit co-sprayed with the
LiBr-KBr-LiF eutectic is shown in Figure 5. The electrolyte
content of the deposit was 43% versus only 15% for the feedstock. The deposit was not as well consolidated as the ones
sprayed with other electrolytes. The single-cell performance of
this material at 500o C is shown in Figure 6 for material sprayed
with protection and for a corresponding pressed-powder cathode. The nominal thickness of the deposit was ~120 µm.
The high-voltage plateau for the plasma-sprayed cathode was
comparable to that obtained with pressed-powder cathodes under the same test conditions. However, the lower-voltage plateau
was much greater and the total polarization was less for the
plasma-sprayed cathode. The superior electrochemical performance of the plasma-sprayed cathode corroborates the improved
performance of plasma-sprayed electrode relative to pressedpowder counterparts that was demonstrated with the LiCl-KCl
and LiBr-LiCl-LiF electrolytes. The initial voltage spike remained since the plasma-sprayed cathode did not contain any
lithiation agent (e.g., Li2 O). This issue should be readily remedied by incorporation of an appropriate quantity of suitable lithiation agent in the plasma feedstock.
Characterization of Plasma-Sprayed Pyrite/Electrolyte Composite Cathodes. / J. New Mat. Electrochem. Systems 5, 273-279 (2002)
Figure 5: Cross-sectional view at a 60o tilt of a plasma-sprayed
deposit resulting from co-spraying LiBr-KBr-LiF electrolyte
with pyrite. (150 µm marker)
Figure 6: Response at 500o C and a background current density of 125 mA/cm2 of Li(Si)/LiBr-KBr-LiF (MgO)/FeS2 single
cells for cathodes made by plasma spraying pyrite with LiBrKBr-LiF electrolyte and by cold pressing of powders.
3.4 KCl-Li2 SO4 -NaCl Eutectic
A cross-sectional view of the pyrite deposit co-sprayed with
the KCl-Li2 SO4 -NaCl eutectic is shown in Figure 7. The electrolyte content of the deposit was 57% versus only 15% in the
feedstock. This is the highest increase in electrolyte concentration observed of all the electrolytes that were co-sprayed. The
deposit was fairly well consolidated, however. The performance
of this material at 500o C is presented in Figure 8 for single cells
that utilized the LiCl-KCl eutectic.
The use of a foreign electrolyte in the cathode for this test was
expected to potentially adversely impact performance because
of the higher melting point and lower ionic conductivity of Li+ .
Attempts to leach away the electrolyte and then test the plasma-
277
Figure 7: Cross-sectional view at a 60o tilt of a plasma-sprayed
deposit resulting from co-spraying pyrite with KCl-Li2 SO4 NaCl Eutectic. (150 µm marker)
sprayed cathode were not successful because of the loss of structural integrity of the sample afterwards. The lack of sample
protection in the one case dramatically adversely impacted performance, to the point that no higher-voltage plateau was observed. A reasonable upper-voltage plateau was obtained with
sample protection, however, although this plateau was somewhat shorter than that for the pressed-powder cathode. A much
longer lower-voltage plateau was obtained with the plasmasprayed cathode and the total polarization for the cell was much
less and did not show the hump evident for the cell with the
pressed-powder cathode. The initial voltage spike was still
present for the plasma-sprayed cathode, which was expected
in the absence of any lithiation agents in the deposit. While
these tests demonstrate that a nonhygroscopic electrolyte such
as the KCl-Li2 SO4 -NaCl eutectic can indeed be used as a cospray additive for plasma spraying of pyrite, the disadvantages
with respect to excessive electrolyte in the deposit and to overall
performance in single cells precludes its use.
4. FUTURE WORK
The need to reduce the initial voltage spike with the plasmasprayed deposits will be addressed by the incorporation of a
suitable lithiation agent in the plasma-spray feedstock. Candidates include Li2 O, Li2 S, and Li2 CO3 . The additive level will
be adjusted until the voltage transient is eliminated. The use of
nanostructured pyrite will also be explored in the place of the
current coarse material, in an attempt to achieve a deposit composition that is closer to that of the plasma-spray feedstock. This
may eliminate the buildup of excess electrolyte in the deposit,
which reduces the effective energy density and specific energy
of the thermal cell. Once these problems has been addressed,
additional single-cell tests will be carried out at other temperatures (to cover the temperature range expected during typical
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Ronald A. Guidotti et al./ J. New Mat. Electrochem. Systems 5, 273-279 (2002)
trochemical performance of plasma-sprayed cathodes is overall
superior to that of pressed-powder cathodes.
The build-up of electrolyte in the deposit may be a result of large
particles of pyrite bouncing from the substrate during spraying.
Experiments with nanostructured pyrite are planned to address
this issue. Several lithiation agents will be incorporated into
the plasma-spray feedstock to mitigate the unacceptable voltage
transient that is obtained for unlithiated cathodes at the onset of
discharge. Once these problems have been solved, the full potential of plasma-sprayed cathodes will be realized for thermal
batteries. That includes higher energy densities and specific energies as well as reduced overall battery costs.
Figure 8: Response at 500o C and a background current density
of 125 mA/cm2 of Li(Si)/LiCl-KCl (MgO)/FeS2 single cells for
cathodes made by plasma spraying pyrite with KCl-Li2 SO4 NaCl Eutectic and by cold pressing of powders.
thermal-battery operation) and under higher current densities.
These tests will then be followed by up small-scale (5-cell) battery tests over a range of activation temperatures and discharge
conditions.
5.
6.
ACKNOWLEDGMENTS
The authors are indebted to Bonnie McKenzie for the SEM photomicrographs.
Sandia National Laboratories is a multiprogram laboratory operated by Sandia Corp., a Lockheed Martin company, for the
United States Department of Energy under Contract DE-AC0494AL85000.
This work was performed as a joint effort under a Phase II SBIR
contract from the U.S. Army (DAAH01-98-C-R046).
CONCLUSION
REFERENCES
A number of electrolytes were examined as co-spray additives
for plasma spraying of pyrite to prepare cathodes for thermal
batteries. The effects of electrolyte composition on the physical and electrochemical properties of plasma-sprayed deposits
were evaluated. All the electrolytes function well as thermalbarrier coatings to protect the pyrite from thermal decomposition during plasma spraying, if adequate protection of the sample was provided (i.e., dry argon cover). The nonhygroscopic
KCl-Li2 SO4 -NaCl eutectic does not function as well in singlecell tests as the other electrolytes because of its high melting
point and low ionic conductivity. There is no obvious relationship between the melting points and heats of fusion for the various electrolytes on the physical and electrochemical properties
of the plasma-sprayed deposits. The viscosity and surface tension of the electrolytes may be more important for controlling
the deposit morphology and electrochemical performance.
All of the plasma-sprayed cathodes showed upper-voltage
plateaus that were comparable to those of pressed-powder cathodes tested under the same conditions. However, cells with the
plasma-sprayed cathodes showed much longer lower-voltage
plateaus (i.e., the capacities were higher) and lower overall polarization (impedance). They also did not show the maximum in
polarization during discharge exhibited by the pressed-powder
cathodes. This is a result of improved particle-particle contact imparted by the enhanced bonding that results during highenergy impact of the feedstock with the substrate. The elec-
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