AD-A2 6 1 321

V• Research and Development Technical ReportSLCET-TR-91-8

A Review of the Calcium Thionyl ChlorideElectrochemical System

Charles W. Walker, Jr.Electronics Technology and Devices Laboratory

April 1991

DTICS I.ECtEFEB?.. 6 '99

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4. TIRE AD SUBTITLE S. FUNDING .U&-AS

A REVIEW OF THE CALCIUM THIONYL CHLORIDE ELECTROCHEMICAL PI: ILISYSTEM PR: 61101

C__AUMRS) TA: A9IAWU: DA305576

Charles W. Walker, Jr.

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US Army Laboratory Command (LABCOM) SRIoT NUMER

Electronics Technology and Devices Laboratory (ETDL) SLCET-TR-91-8ATTN: SLCET-PRFort Monmouth, NJ 07703-5000

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13. ABSTRACT (Mxiw ,•200 .awo )

Citing work from the open literature and from Government-sponsored contractualefforts, research on the calcium thionyl chloride electrochemical system isreviewed. Progress has been made toward enhancing the performance at 25 deg Cby adding sulfur dioxide to the electrolyte, and by using cathodes comprised ofa blend of high- and low-surface area carbons. Sulfur dioxide raised electrolyteconductivity and improved low-temperature (-30 deg C) performance; still, low-temperature capacity remains poor. With strontium tetrachloroaluminate salt inthe electrolyte, the film that formed on calcium was altered, which reduced rapidcorrosion of calcium effectively, especially when used in conjunction with sulfurdioxide. With careful engineering, this may be a viable system for specialapplications.

14. SUB.JECT TERMS I. NUMBER Of PAGES

Calcium; thionyl chloride; corrosion protection; low-temperature 25performance 16. PRXI cOO3

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CONTENTS

Page

INTRODUCTION ............................................... 1

GENERAL CHEMISTRY .......................................... 2

ELECTROLYTE STUDIES ........................................ 2

CATHODE STUDIES ............................................ 5

ANODE AND ANODE CORROSION STUDIES .......................... 6

PERFORMANCE AND SAFETY ..................................... 10

CONCLUSIONS ................................................ 13

REFERENCES ...................... ....................... 14

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iii

INTRODUCTION

One of the primary goals in the development of new battery chem-istries is the safe use of high-energy systems. The lithium thionylchloride cell chemistry exhibits high energy density and high ratecapability over a wide range of temperatures (-30" to 55"C). Howev-er, abuse conditions may lead to fire or explosion in lithium thionylchloride batteries. When driven into reversal, lithium metal platesat the cathode and can create an internal short circuit. The heatgenerated from shorting may melt lithium (mp 180"C) and cause inter-nal cell pressure to increase, with serious consequences. The morebenign event is a "controlled" cell venting; the more serious is asudden cell explosion due to thermal runaway that is caused by re-action of molten lithium with sulfur (one of the discharge productsof SOC1 2 reduction).

Because it addresses these safety concerns, the calcium thionylchloride chemistry has attracted attention since the late 1970's.Initially, there was hope that this chemistry could be equal inenergy to lithium primary chemistries, yet far safer, because themelting point of calcium is 839"C. Plating of calcium in thionylchloride is extremely difficult, and safety studies have shown calci-um cells to be resistant to abuse. The goal was to develop an ultra-safe cell that would be so resistant to abuse that a ventless celldesign could be employed.

Although considered safe, several performance problems werereadily observed with Ca/SOCI 2 cells. Open-circuit potentials,although theoretically close to that of the Li/SOCI 2 couple (3.65 V),were several tenths of a volt lower. Capacities at ambient tempera-ture were poor, underutilizing the cathode capacity established inlithium systems. Performance at -30"C (a requirement in militaryspecifications) under moderate loads (5 mA cm-2) did not exist.Finally, there was a high rate of anode corrosion, which precludedcell storage.

One challenge facing investigators was to make a more conductiveelectrolyte that would improve low-temperature performance and pre-vent corrosion of the calcium. Anode activation and performance atlow temperatures were also major concerns. The improvement of cath-ode capacities was less troubling, but equally pursued. Most impor-tant, design improvements were not to compromise the inherent safetyfeatures of Ca/SOCd 2 .

This paper summarizes studies of the electrolyte, cathode, anode,and anode corrosion of Ca/SOCl2 cells. Some performance and safetytesting of laboratory cells and full hardware cells are also given;but due to the differing test vehicles and loads applied duringdischarge, comparisons between investigators are difficult.

GENERAL CHEMISTRY

Typically, the Ca/SOCI 2 cell consists of a calcium anode, carboncathode current collector, and a tbionyl chloride electrolyte com-posed of a tetrachloroaluminate salt (AlCl 4 -) with Ca 2 + or Sr2+ asthe counterion. Upon discharge, the reaction of calcium with thionylchloride is similar to that of lithium:

2Ca + 2SOC1 2 -- > S02 + S + 2CaCl 2 .

Because CaC1 2 is insoluble in a SOC1 2 electrolyte, ambient tem-perature cell failure is due to the passivation of the cathode withelectrically insulating CaCd 2 . Unlike LiCl, which forms a crystal-line deposit, CaCd 2 is glassy, amorphous, and evidently more effi-cient at passivating the cathode. 1 Thermodynamically, this reactionshould consume the reactants spontaneously, but CaCd 2 forms a passi-vating layer on the calcium anode that protects it from rapid corro-sion.

Although the theoretical potential of the Ca/SOCI 2 couple shouldbe between 3.6 and 3.7 V, observed values are about 3.0 to 3.2 V atroom temperature. In an interesting attempt to turn this discrepancyinto an advantage, calcium was considered for use in implantablemedical devices that require warning of end of life. 2 Because lithi-um cells have a flat voltage profile until very near their end oflife, a composite anode was fabricated with a calcium inner core.When the lithium was exhausted, the (lower) potential of calciumforeshadowed end of life before the remaining cell capacity becamedangerously low. Unfortunately, it was found that the higher thecalcium content, the higher the corrosion.

ELECTROLYTE STUDIES

An obvious starting place for improvement was the electrolyte,since Ca(AC1d 4 ) 2 -SOC1 2 is only 30 percent as conductive 3 as LiAlCl 4 -SOC1 2 . An approximately 1 M solution has a conductivity 3 -8 on theorder of 6 X 10-3 S cm- 1 , and 1.9 M electrolyte 7 4.1 X 10-3 S cm-1 .

2

Based upon conductivity data, it was determined that the optimal.concentration of Ca(AlC1 4 ) 2 -SOC1 2 for low-temperature discharge isapproximately 0.7 M, and for 10-60"C, it is about 1.25 M.7 ,9

The density of 2 M Ca(AlCl 4 ) 2 -SOCI 2 solution at temperatures from-20* to 60"C ranges from about 1.89 to 1.78 g cc-, and from 1.75 to1.61 g cc-' for a 0.4 M solution. 7 A 1.9 M solution is very viscous,even at room temperature. A 1.2 M solution shows a sharp increase inviscosity below 09C; whereas a concentration of 0.95 M shows gradualincrease in viscosity with decreasing temperature. 7 As temperatureswere decreased, no salt precipitation was noticed down to -30"C for a1.9 M solution, 9 and -40°C for 1.8 M solution. 1 0 At -500C, solutionsfrom 1.0 to 1.8 M froze, but no salt appeared to precipitate out. 1 0

An explanation for no salting out is based on studiesI1 of LiAlCl 4 -SOC1 2 , where it was proposed that the system behaves as a mixture oftwo miscible liquids, SOC1 2 and LiAlCI 4 .2SOC1 2 , rather than a saltsolution. It was supposed that Ca(AIC1 4 ) 2 -SOCl 2 behaves in a similarmanner.

Numerous salts and cosolvents were tried in order to improve onthe performance of Ca(AlCl 4 ) 2 . Although LiA1Cl 4 forms a conductiveelectrolyte, lithium is able to plate out, and therefore might causethe same hazards under abuse conditions as an all-lithium cell.Further, Ca/SOCl 2 cells with LiAlCl4 salt showed excessive corro-sion1 2 during storage and during discharge at temperatures greaterthan 100*C. The OCV of a calcium cell stored at 200"C was zero inless than 10 days (Ca - > CaCI 2 ), whereas no changes were observedin a lithium cell after 30 days. 1 2

It was found that additions of SO2 to Ca(AlCl 4 ) 2 electrolyteincreased conductivity 40 to 50 percent over temperatures 8 , 1 3 rangingfrom -40°C to 500C. Saturating a 1.2 M solution with S02 (approxi-mately 7% V/V) increased specific conductivity at room temperature 4 -6

from 0.006 to 0.009 S cm-I. The addition of 7 to 12 percent SO2reduced anode polarization at low temperatures and improved cathodecapacity and load voltage at room temperature. 4 -6, 1 0 , 1 4 , 1 5 Ramanspectroscopy 1 6 ' 1 7 in SOC1 2 , as well as Raman 1 8 and NMR 1 9 studies insulfuryl chloride (SO 2 C1 2 ) showed that the predominant interaction ofSO 2 is with Ca 2 + ions. Solvation of these cations is believed toaccount for better cell performance by decreasing viscosity, increas-ing cation mobility, and, possibly, by altering the morphology of theCaCI 2 deposited in the cathode. A 0.9 M Ca(AICI 4 ) 2 -SOCl 2 electrolytewith either 7, 11, or 20 percent S02 was evaluated in order to deter-mine optimum sulfur dioxide concentration. 2 0 The 20 percent (V/V)

3

concentration offered the best performance (actually making a 0.72solution). More recently, Peled et al. reported that a Sr(AIC14 ) 2electrolyte containing a combination of 20 percent SO2 and 0.3 per-cent of an undisclosed additive enhanced rate capability and reducedcorrosion to four percent per month 2 1 at 709C. Without these addi-tions, corrosion was 11 to 12 percent. Improved low-temperatureperformance was also achieved by adding 10 percent bromine to theelectrolyte.

2 2

Extensive work with many other cosolvents led to further study ofbenzoyl chloride as a cosolvent 2 3 and offered some interesting andpotentially promising results. However, additional studies 4 -6

indicated that this cosolvent gave lower electrolyte conductivity andpoor cathode capacity, compared with an electrolyte containing onlythionyl chloride as the solvent.

Although many alternative salts have been considered, only a fewwere studied in detail. A gallium salt, Ca(GaCl 4 ) 2 , was found toincrease both the open-circuit potential and the average voltage onload, but capacity was only 75 percent of the Ca(AlCl 4 ) 2capacity. 2 0 , 2 4 The gallium salt was also found to be ineffective atretarding corrosion of calcium. 2 3

Albeit less conductive than the calcium salt, the strontium andbarium tetrachloroaluminate salts have been used by Peled to retardcorrosion by altering the anode surface via exchange with the calciumin the CaC1 2 passivating layer. In low concentrations, conductiv-ity 1 6 , 1 7 of tetrachloroaluminate salts were ranked Ca>Ba>Sr. At lowtemperature and with high concentrations of SO2 , Ba(AIC1 4 ) 2 andSr(AlCl 4 ) 2 salts change position. The electrolyte containing thebarium salt possessed the lowest viscosity; the salts of str'ontiumand calcium were approximately equal. 1 7 Not only was corrosion inBa(AlCl 4 ) 2 and Sr(AlCl 4 ) electrolytes less than in Ca(AiC1 4 ) 2 , but itwas more homogeneous and free from pitting. 2 5 After storing for fourweeks, 1 M solutions of Ba, Sr, and Ca tetrachloroaluminate saltsshowed 8, 10, and 24 percent corrosion of calcium, respectively. 2 5

The composition of the passivating layer on calcium was analyzed asfollows 2 5 , 2 6 :

Electrolyte Salt ComDosition of Passivating Layer

Ba(AIC1 4 ) 2 96-99% CaCl 2 : 1-4% BaC12

Sr(AlCl 4 ) 2 5 20% CaC1 2 : > 80% SrCl 2

4

The ac spectra of SrCl 2 indicated its resistivity was greaterthan CaCl 2 , and C-size cells containing 0.7 M Sr(AlCI 4 ) 2 -SOC1 2 elec-trolyte did not lose capacity after four weeks storage2 5 at 70°C.Staniewicz found that cells with Sr(AlCI 4 ) 2 provided the same ambienttemperature performance as cells with Ca(AlCI 4 ) 2 electrolyte. 2 0

CATHODE STUDIES

Carbon cathode capacity in calcium cells is approximately 58percent that of analogous lithium cells. Based on the molar volumeof CaCI 2 , which passivates the cathode, one would expect cathodecapacity to be 80 percent that of lithium cells. 3 Compared to room-temperature performance of calcium cells discharged at 5 mA cL-2,more than 99 percent of the cathode capacity goes unused at -304C,but this is because severe anode polarization produces unusable loadvoltages. 1 4

Cathodes are usually fabricated with Teflon binder to hold thecarbon particles together. Teflon is electrically insulating, so thelower its content, the better the e'ectrical properties of the cath-ode. Cathodes become too fragile to provide services below someconcentration, but another way to improve their properties is tochange the carbon being used. Historically, low surface area (60 m2

g-1) carbons such as Shawinigan acetylene black have been used.Carbons with higher surface areas have been used successfully todecrease current density at the cathode and to provide more numerouscatalytically active reduction sites. Acetone-washed Black Pearls2000 (BP2000, 1500 m2 g-1 ) gave the longest cathode life and highestload voltages in lithium surfuryl chloride cells. 2 7 Walker et al.used this approach for calcium thionyl chloride cells. Pore volumeof BP2000 was 1OX higher than for Shawinigan. 4 Acetone washingfurther increased pore volume, probably by removing calcium andsulfur-containing impurities introduced during the manufacturingprocess. 4 A blend of 75 percent Shawinigan (for mechanical integri-ty) and 25 percent acetone-washed BP2000 allowed highest operatingvoltage and longest capacity when compared with either Shawinigan orBP2000 alone. 4 - 6 Adding SO 2 to Ca(AlCl 4 ) 2 -SOCl 2 electrolyte resultedin even further capacity improvement.

Conversely, Staniewicz found that low surface area Shawinigan,YS, and Ensagri carbons outperformed high surface area blacks. 2 8 A1:3 blend of YS:Shawinigan gave the highest electrical capacity (24mAh cm- 2 at 5 mA cm- 2 ) of carbons tested. 2 0 In his later studies,

5

however, a 1:3 mix of BP2000:YS with 12 percent Teflon binder wasused in an optimized D-size cell.

Early studies by Higgins noted safety consequences in using high-and low-surface area cathodes. 2 9 In abuse tests, cells with highsurface area cathodes burned violently and consumed the calcium.Fires in cells with low surface area cathodes were almost undetect-able from normal incineration. This was explained as resulting froma 5.2 X higher surface-to-volume ratio in high surface area cells. Athick cathode with a nickel foam current collector provided a lowsurface area design that reduced the danger of abuse situations. 3 0

The use of catalysts in the cathode has shown improved loadvoltages under high rate discharge. 4 - 6 However, at low temperatures,performance problems lie with the anode, so the added expense ofcatalysts would offset any marginal improvement, unless a very highrate cell were desired.

In studies by Peled et al., cell capacity was found to increasewith cathode porosity from 82 to 88 percent. 8 However, Staniewicz etal. concluded that altering cathode porosity had little influence onperformance. 2 4' 2 8 Since the glassy CaC 2 deposit precluded full

cathode utilization, it was concluded that thin, rather than thick,cathodes should be used. 2 4 , 2 8 With thinner cathodes, more cathodesurface area can be fit into a cell to yield increased cell capacity.For instance, it was calculated that at 0.024 Ah cm- 2 , a 300 cm2

"thick" cathode in a D-size cell would give 7.2 Ah, whereas a thinnerand larger cathode could provide 9 Ah. 2 0

ANODE AND ANODE CORROSION STUDIES

The most critical component of the calcium cell is the anode.Calcium is plagued by rapid corrosion in thionyl chloride electro-lytes and by severe polarization during the low-temperature dis-charge, which precludes useful load voltages. A black calcium oxidefilm is usually present when calcium is received; but in SOC1 2 solu-tions a new film is formed, which has been confirmed to be CaC1 2 byx-ray diffraction and x-ray emission techniques. 3

The kinetics of the passivating layer on calcium is described byPeled's Solid Electrolyte Interphase (SEI) model. 7 ' 3 1 ' 3 2 In essence,a metal-solution interaction forms an insoluble passivating layerthat acts as a solid electrolyte; ions, not electrons, are able to

6

pass through the film. The LiCI SEI in lithium -ells is a cationicconductor. Lithium has a transport number of about one. Therefore,during cathodic polarization (plating), Li+ is reduced at themetal/SEI interface while fresh Li+ from solution migrates throughthe SEI to the metal interface; the opposite occurs during celldischarge. On the other hand, CaCI 2 is an anionic conductor and thetransport number of Ca 2 + is close to zero. On cathodic polarization,calcium ions are reduced at the metal/SEI interface, and anions mi-grate from there through the CaCd 2 SEI into the solution and leavevoids in the crystal lattice. Negatively charged Cl- ions tend toaccumulate because they require combination with AlCl 3 to dissolve inSOC1 2 . Unless there are cracks in the SEI, Ca 2 + will not be able tomigrate from solution to the calcium anode to plate out. On dis-charge, CI- ions migrate to the SEI interface and fill conductingdefects to produce a more compact SEI and increase passivation. In-creasing the Ca(AlCl 4 ) 2 electrolyte concentration also increases thethickness of the passivating layer. 7

Further refinements of the SEI model indicate that the SEI filmis tens of angstroms thick and covered by a thicker porous passivat-ing layer, which may reach 50 microns. 1 6 During discharge, currentis carried through the SEI by Cl- ions; although the Ca 2 + ions thatare formed need to move away from the electrode and into solution.Therefore, it is expected that the SEI cracks and reheals continual-ly.16

It has been suggested that the open-circuit voltage (OCV) ofCa/SOCI 2 is lower than theoretical calculations due to electronicleakage through the SEI whereby the corrosion current causes thevoltage to drop. 7 It was noticed that after electrolyte is firstadded to a calcium cell, the OCV is <3.2 V and corrosion rates arerelatively high, 7 , 3 3 which coincides with the dissolution of calciumoxide and building of a new CaCl 2 SEI. Subsequently, the corrosionrate becomes less and cell OCV approaches '.2 V. The impurity con-tent of the SEI is critical for corrosion protection. 3 4

The SEI model has been used to explain the inefficiency of plat-ing calcium on calcium, since the SEI must be destroyed to allow Ca2 +

to reach the metal. Peled et al. reported that a Ca(AlCl 4 ) 2 -SOCI 2cell could not be charged, 3 5 and that no pra tical current was ob-served for charging voltages up to 40 V. 9 Meitav reported calciumovervoltages going to 20-40 V, with the accompanying cvolution of gasas thionyl chloride. 7 The SEI was reported to break down at about30 V, which allowed passage of some current; however, others have re-

7

ported the ability to plate calcium. Cyclic voltammetric studies byBehl on passivated glassy carbon electrode in Ca(AlCl 4 ) 2 -SOCl 2 showedcalcium deposition at -1.1 V versus a calcium reference. 3 6 Deposi-tion of calcium in sulfuryl chloride w;3 likewise seen at -1.1 V. 3 7

Also, Staniewicz et al. found that during charging the calcium over-potential of 24 V dropped to 6 V, which indicated probable crackingof the SEI to allow more efficient plating. 2 0 There is also evidencefor the depostion of calcium on stainless steel. In one case, farad-aic efficiency was approximately 7 percent 7 ; in another, 9 a smallamount of calcium was deposited at high overvoltage after prolongedelectrolysis at 2 mA cm- 2 . There was no evidence of calcium platingon nickel electrodes.- In contrast, lithium plates onto calciumeasily, with efficiency reported at 60 to 90 percent. 7

One attempt to alter the SEI for better corrosion protectionand/or better discharge performance was through the use of calciumalloys. Based on earlier work, 3 8 alloys with lithium were tried,which showed higher currents and load voltages than pure calcium,even though studies have shown corrosion in storage tests. 2 3 , 3 8

However, a two percent lithium alloy was later reported to reducecorrosion over pure calcium, 2 0 and by using Tafel slopes from impe-dence measurements, a shelf life of 5.5 years was calculated for0.02-inch thick electrodes. 3 4 Shelf life of pure calcium and a Ca/Sballoy were similarly calculated at 3 and 2.9 years, respectively. Acalcium-antimony alloy was investigated, because it also had previ-ously shown better current-carrying capability. 3 9 It was proposedthat the beneficial effect of lithium alloys was to incorporate LiClinto the CaCI 2 lattice, which would create immobile defects that pinC1- vacancies. This would reduce their mobility and the C - diffu-sion rate. 3 4 Also, because LiCl is a cationic conductor, it washoped that Ca 2 + transport might be improved.

In addition to alloys containing Li and Sb, alloys of Ag, Sr, andBa were also examined.10,14, 2 0 Walker concluded that there was noadvantage offered over pure calcium either at room temperature or lowtemperature, where performance is even worse than with pure cal-cium. 1 0, 1 4 A small capacity increase was seen with the Sb alloy withroom temperature discharge. 2 4 Severe corrosion was noticed1 0, 1 4 forall alloy samples stored 60 days at room temperature in ampules con-taining Ca(AlCl 4 ) 2 -SOCI 2 . In comparison, lithium showed no corro-sion, and it was concluded that one should use the purest calciumavailable.

8

An alternative approach to using alloys for changing the charac-ter of the SEI was by employing alternative salts. Strontium salts.were found 1 6 , 2 0 , 2 6 , 4 0 to be useful in altering corrosion characteris-tics. Peled reported that a calcium cell with Sr(AlCI 4 ) 2 -SOCI 2electrolyte and 7 percent S02 stored for four weeks at 70'C showed noloss in capacity. 1 6 , 2 6 By comparison, a cell with Ca(AICl 4 ) 2 storedonly two weeks at 70"C lost 25 to 100 percent of its capacity. Afterstoring for four week.,, cells with the calcium salt showed 155 percentcapacity loss. 4 0 In 0.4 M calcium electrolyte at 2100"C0 an 18 ± :ýpercent weight loss was measured after 100 hours. 4 1 Other studies byStaniewicz 2 0 showed reduced corrosion after storage at 55"C for fourweeks with 0.75 M Sr salt containing 20 percent V"V SC2, comparedwith the Ca salt with 20 percent SO2, which showed 50 percent capaci-ty loss. 2 4 Analysis of the SE126, 4 3 has shown that it is 80 percentor more SrCl 2 . Without added SO 2 , cells containing 1 M Sr salt haveretained only 59 percent of the fresh cell capacity after comparablestorage. 2 0 By increasing the SO 2 concentration from 7 to 20 percent,the self-discharge rate was decreased significantly. 4 3 Other stud-ies 4 4 with calcium foil discs have also shown reduced corrosion withthe addition of SO 2 .

When the barium salt was used, corrosion of calcium was reduced,although only one to four percent of the SE1 2 6 ' 4 2 was composed ofBaCl 2 . No further work has been published about this electrolyte,and cell performance is presumed to be poor. The gallium salts,Ca(GaCl 4 ) 2 and Sr(GaCl 4 ) 2 , reduced corrosion, but they provided only75 percent of the capacity realized with the tetrachloroaluminatesalt. 2 0 Other salts that were studied, but which did not reducecorrosion significantly, include Ca(SbCl 6 ) 2 , Ca(FeCl 4 ) 2 , (ethyl 4 N)(AlCl 4 ), Li(GaCl 4 ) 2 , LiAlCl 4 , and others. 3 , 2 3 , 2 9 , 4 5 LiAlCl4 was foundto be much less corrosive 2 3' 2 9 , 4 6 than Ca(AlCl 4 ) 2 , but the hazards ofplating lithium would probably offset this advantage.

Walker believed that anode polarization at low temperatures wasattributed primarily to mass transport properties in the electrolyte,rather than to kinetics in the SEI.10,14 Polarization curves at-30"C showed that as the Ca(AICl 4 ) 2 concentration was increased,severe polarization occurred at lower current densities. This is incontrast to 20"C data (where mass transport is relatively high) thatshowed about the same polarization of calcium in 1.0 and 1.8 M solu-tif-ns.I0,14 It was concluded that, although the SEI plays a signif-icant role because the anode does not polarize severely at 20'C,regardless of electrolyte concentration, factors associated withdecreasing temperature predominate; namely, increase in viscosity and

9

decrease in conductivity, mass transport, and salt solubility.Further evidence for this was obtained by following electrode OCVimmediately after polarization at -30'C. The calcium anode in 1.0 Melectrolyte quickly recovered to its original potential, which sug-gests that the effect was mainly iR drop. However, in 1 8 M electro-lyte, the calcium remained polarized on open circuit and took severalminutes to achieve its original OCV. The slow return was consistentwith mass-transport-related effects.

Several surface treatments have been tried, including coatingcalcium with cyanoacrylate. This was found to be a successful coat-ing in lithium systems, and seemed to reduce calcium corrosion aswell. 2 0 However, in a LiAICl 4 electrolyte, cyanoacrylate was foundnot to help storage of calcium. 2 3

Based on work 4 7 that showed decreased anion diffusion in SrCl 2 byincorporation of y3+, another suggestion (not tried) for reducingcorrosionI 0 was to substitute Ca 2 + with Y3+ in the SEI. Decreasedanion diffusion may in turn retard corrosion.

Formation of a synthetic SEI to overcome calcium corrosion wasattempted with an rf sputtered CaGeS coating. 4 8 Coatings containingfour percent Sr 2 + or one percent Y3+ were also evaluated and werefound to reduce corrosion; the Y3+ dopant appeared to enhance Ca 2 +

conduction through the SEI. Unfortunately, discharge performance incells was worse without the coating.

PERFORMANCE AND SAFETY

At ambient temperature, calcium cells were cathode limited, al-though at -30"C, cells were limited by severe anode polarization.With increasing electrolyte concentration at low temperature, polari-zation became severe, even at low current densities. 1 4 At 206C,anode polarization for 1.0 and 1.8 M Ca(AlC1 4 ) 2 electrolyte was aboutthe same, since mass transport is relatively high at ambient temper-ature1 4 Laboratory cell testing by Walker indicated1 0 , 1 4 that cellswith Ca(AlCl 4 ) 2 -SOCI 2 electrolyte had no usable life (load voltageabove 2 V) when discharged below -10°C at 5 mA cm- 2 . By adding SO2to reduce anode polarization and by using a cathode blend composed of75 percent Shawinigan and 25 percent BP2000, a few minutes of dis-charge above 2 V was possible. Raising the temperature to -20"Cprovided a half-hour discharge at this rate. Addition of 10 percentBr 2 was also shown to improve low-temperature performance. 2 2

10

Staniewicz found2 0 , 2 4 that at -30-C, OCV dropped to 2.55 V and loadvoltages above 2 V were achieved only by lowering 2 0 the load to 1.6mA cm- 2 . After storing for one month at 55"C, D-size cells had nousable capacity above 2.5 V at ambient temperature when discharged ata 2 A rate. 4 9 Capacity was 3.6 Ah to zero volts, a loss in capacityof more than 50 percent. It is apparent that use of Ca(AlC1 4 ) 2 -SOC1 2electrolyte will not permit a viable system.

Staniewicz et al. tested an optimized Ca/SOCl 2 system consistingof a calcium anode, 25/75 weight blend of BP2000 and YS carbon, and0.75 M Sr(AlCI 4 ) 2 -SOCI 2 with 20 percent V/V SO 2 . Fresh D-size cellsdischarged at 2 A constant current provided 8.9 Ah (19 mAh cm-2cathode area) at ambient temperature 2 0 and 5.3 Ah (11 mAh cm- 2 ) at-300C. Wade et al. 5 0 stored some of these cells either at roomtemperature or 55°C and then discharged them at low rate (1, 2, or 4mA cm-2) under ambient and -30"C conditions. Results (listed here)indicate significant corrosion of calcium had occurred, since poorcapacities, even under low drains, were observed at both ambient and-300C.

Storage RT Capacity/ -30"C Capacity/ CommentsAh Ah

1 month RT 5.6 3.35

4 month RT 0 to 5 0.6 to 3 some vented

2 month RT 1.1 not done low OCVs& 1 mo 55 0 C (only 1 of

7 cellsgavemeaningfuldata)

The conclusion was that this chemistry has a shelf life of lessthan 10 months, 5 0 which is in contrast to a report by Peled et al.,which projected2 6 an expected shelf life of longer than three yearsat 20'C with the Sr electrolyte + 7% SO 2 . He responded to this databy claiming that the use of nickel plated steel cans (used by Stanie-wicz) promoted corrosion, and only pure stainless steel should beused. As part of a European Research Office contract, Peled deliv-ered C-size cells for evaluation to the same group in the spring of

11

1990. The cells containing 0.84 M Sr(AlCl 4 ) 2 and 20 percent SO 2 weredischarged at a 0.7 A (approximately 3.2 mA cm- 2 ) rate at ambienttemperature, as recommended by Peled. Capacity to a 2.0 V cutoffaveraged 3.85 Ah for two fresh cells (4.0, 3.7 Ah). After storingfor one-month at 55", the average capacity was 3.3 Ah on a four-cellsample (4.0, 3.7, 3.5, 2.0 Ah). Two fresh cells that were dischargedat a 0.1 A rate at -30°C gave no capacity to a 2.0 V cutoff. Notethat, in addition to the can material, there were several otherdifferences between Staniewicz's and Peled's cells, which may alsoaccount for discrepancies in performance; specifically, the calciumstock and purity, selection of carbon in the cathode, electrolyteconcentration, and moisture content of cell assembly areas. Althoughthere was some variation in Peled's data, indications are that it ispossible to carefully engineer this cell chemistry to reduce corro-sion significantly.

Microcalorimetric studies by Peled on Ca(AlCl 4 ) 2 -SOCl 2 C-sizecells showed heat output of 450 AW one week after cell assembly. 5 1 Amodified cell of undisclosed chemistry, but which presumably con-tained the Sr electrolyte, showed lower heat output. 5 1 In cellscontaining the Sr electrolyte, discharge capacities after one monthstorage at 71°C were similar (only 6 percent loss) to those of freshcells.16, 2 6 , 4 0 However, at -30°C, cells with the Sr electrolyteprovided less capacity than the Ca(AlCl 4 ) 2 -containing cells. 1 6 , 4 0

Increasing electrolyte concentration and adding S02 increased thecapacity at -30°C, but it still fell short of that obtained with thecalcium salt.

In earlier safety tests, fires were reported2 9 , 3 0 from incinera-tion and penetration by bullets, but a thick cathode with a nickelfoam current collector reduced the danger. No condition produced ahazardous response in these cells in sizes as large as 600 Ah.30

Generally, in more recent studies with either the Ca or Sr elec-trolyte, the most serious result from abuse tests has been cellventing. This is an improvement over lithium cells, yet it suggeststhat development of a ventless cell may not be feasible without athicker can or additional safety improvements. The use of a Tefzelseparator can provide added safety on short circuit. 1 6 As tempera-ture goes up, the Tefzel will melt and the pores will close, whichwill increase cell resistance effectively and act as an internalfuse.

12

CONCLUSIONS

The premise that the Ca/SOCI 2 system is safer than Li/SOCl 2 hasbeen substantiated. The worst-case scenario from abuse appears to becell ventings.

Although cathode capacities are less than expected during ambienttemperature discharge, they are reasonably good. Best performance isachieved when using a cathode composed of a blend of high- and low-surface area carbons in combination with SO 2 added to the electro-lyte.

A persistent performance problem lies with the anode during low-temperature discharge. The reasons for severe polarization areattributable to the CaCd 2 SEI and the decreased mass transport.Attempts to modify the SEI to enhance protection from corrosion byuse of calcium alloys were unsuccessful. Modification through theuse of Sr(AlCl 4 ) 2 -SOCl 2 electrolyte has reduced corrosion, but hasalso diminished low-temperature performance. The addition of SO 2

improved electrolyte conductivity and enhanced mass transport todecrease anode polarization under load. Raman and NMR studies haveshown that SO2 acts to solvate Ca 2 +, similar to that observed for Li+in LiAlCI 4 -SOCl 2 solutions. 5 2 The addition of Br 2 also improvedlow-temperature anode performance. However, although slight improve-ments have been made, no one has been able to demonstrate reasonablelow-temperature performance. It is expected that battery packs thatare discharged at low temperatures should produce enough waste heatto increase cell temperature and thereby increase capacity and loadvoltage. 1 0 ,14, 2 0

The most useful cell chemistry seems to be one which contains apure calcium anode combined with a Sr(A1CI 4 ) 2 electrolyte containing20 percent added SO 2 for improved corrosion protection and perform-ance. A thin cathode of blended high- and low-surface area carbonsseems to offer best cathode performance.

Commercial development of a calcium-thionyl chloride battery hasbeen impeded by the shortfalls discussed, and enthusiasm for thissystem has dwindled. Considering its safety features, if a ventlesscell can be designed, it may have use in special applications such asclosed environments. This chemistry may also be useful in high-temperature environments (where lithium is molten) or in reserve cellapplications. However, use at low temperatures does not appear

13

and electrolyte chemistry. Although interest is limited, work onthis chemistry continues, primarily by Peled and coworkers.

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14

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15

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41. E. Peled, M. Brand, E. Elster, J. Kimel, and R. Cohen, AbstractNo. 26, 1986 Fall Meeting of the Electrochem Soc, San Diego, CA.

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16

43. R. Cohen, Y. Lavi, and E. Peled, J Electrochem Soc 137, 1999(1990).

44. J. Bradley, P. J. Mitchell, W. P. Hagan, and C. D. Tuck, J PowerSources, 2a, 361 (1989).

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47. G. Hood and J. Morrison, J Appl Phys, U, 4796 (1967).

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51. E. Peled, Proc 32d Power Sources SvmD, June 9-12, ElectrochemSoc, Pennington, NJ (1986), p. 445.

52. Y. Bedfer, J. Corset, M. C. Chamelincourt, R. Wallart, andP. Barbier, J Power Sources, 9, 267 (1983).

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55. R. Cohen, Y. Lavi, and E. Peled, J Electrochem Soc, 137, 2648(1990).

17

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