Alkaline earth metal anode-containing cell having electrolyte of organometallic alkaline earth metal salt and organic solvent

Alkaline earth metal anode cells having an intercalation cathode, a nonaqueous liquid electrolyte comprising (a) an organic solvent, for instance, an ether, an ester, a sulfone, an organic sulfide, an organic sulfate, a tertiary amine, an organic nitrite, and an organic nitrate, and (b) at least one of an electrolytically active alkaline earth metal salt comprising an organometallic alkaline earth metal salt represented by the formula: ##STR1## wherein Z is selected from the group consisting of boron and aluminum; X is selected from the group consisting of phosphorus and arsenic; M is an alkaline earth metal; and in which R.sub.1 -R.sub.6 are radicals independently selected from the following groups: alkyl, aryl, alkaryl, aralkyl, alkenyl, cycloalkyl, allyl, heterocyclic alkyl, and cyano, with the proviso that R.sub.1 -R.sub.6 cannot be all alkyl or all aryl and triarylalkylborate or aluminate anions, and trialkylarylborate or aluminate anions are excluded and M represents an alkaline earth metal.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to secondary electrochemical cells utilizing a 
nonaqueous, liquid electrolyte, an alkaline earth metal anode and a 
cathode capable of intercalation. 
2. Description of the Prior Art 
High energy density, rechargeable electrochemical cells have been recently 
developed having an alkali metal anode-active material, a transition metal 
chalcogenide cathode-active material, and a mixture of a lithium salt, 
such as lithium perchlorate, dissolved in an organic solvent as the 
electrolyte. 
While a rechargeable cell is theoretically capable of charging and 
discharging indefinitely, in practice this is not obtained because of 
dendritic growths on the anode and degradation of the cathode with 
cycling. The electrolyte can also be a limiting factor, particularly where 
a nonaqueous electrolyte is utilized. Certain nonaqueous electrolytes can 
provide good performance with a given anode-cathode couple and be 
ineffective or be less effective with other anode-cathode couples, either 
because the electrolyte is not inert or because it degrades during 
cycling. 
To obtain a battery system that is rechargeable at ambient temperatures, 
there are basically two directions that can be taken in selecting a 
cathode. The cathode can be a liquid so that reactions can readily take 
place; but when the cathode is a liquid, provision must be made to keep 
the cathode active material away from the anode, otherwise self-discharge 
will occur. The other alternative is to use a solid cathode that is 
essentially insoluble in the electrolyte but which will absorb and desorb 
the anode ion since solubility of the anode ion must occur reversibly 
during operation of the cell. Such a solid cathode, can be capable of 
intercalation of ions which are solubilized by the electrolyte. The 
electrolyte must also permit electroplating of solubilized ions at the 
anode; the plating of ions at the anode occurring during recharge of the 
cell and the intercalation of the cathode occurring during discharge of 
the cell. 
The research conducted on alkali metal batteries utilizing an intercalation 
cathode such as titanium disulfide has shown the desirability of utilizing 
a cathode which is capable of intercalating the solubilized anode ion. The 
bulk of the literature dealing with intercalation reactions in battery 
development focuses on the use of the alkali metals, specifically lithium 
as anodes. In comparison, very little work has been done with respect to 
the use of alkaline earth metals, such as magnesium, for use as anodes and 
the use of cathodes capable of intercalation of alkaline earth metal ions. 
With respect to insertion of magnesium ions into inorganic materials, there 
is disclosed in the literature the incorporation of magnesium into 
materials such as zeolites and graphite, particularly for use in the 
fields of catalysis and composite materials. In battery development, 
alkali metal ion intercalation is known to take place in the simple and 
complex transition metal oxides, sulfides, selenides, and tellurides. 
Layered transition metal disulfides have been extensively studied. Lithium 
is known to topochemically react with most of these disulfides, when used 
as cathodes, in stoichiometric ratios representing capacities of about 250 
milliampere hours per gram of cathode material. Laboratory cells in the 
ten ampere hour range incorporating titanium disulfide as the cathode and 
lithium as the anode have achieved specific energy densities of 55 watt 
hours per pound at moderate discharge rates over more than 100 cycles at 
50-80 percent discharge depth. Lithium anode cells having metal oxide 
cathodes have also been tested. Of particular interest as cathodes are 
molybdenum trioxide, magnanese dioxide, and chromium oxide (Cr.sub.3 
O.sub.8) because such cathodes offer an energy density of about 60 watt 
hours per pound which is similar to that obtained with titanium disulfide. 
Generally, intercalation chemistry is concerned with the insertion of metal 
guest ions into inorganic host structures. From a chemical standpoint 
intercalation is considered to be a reversible topotactic redox reaction 
by electron/ion transfer. Intercalation reactions are commonly viewed as 
correlating with a change in the electronic (oxidation) state of the host 
lattice. This oxidation change is typically nonintegral and 
non-stoichiometric in most compounds capable of intercalation. 
With regard to the structure of the host lattice, three basic types are 
known. (1) A three-dimensional framework structure containing 
interconnected or isolated empty channels as lattice sites which share 
polyhedral faces. Examples of this type are the complex vanadium oxides, 
the trioxides of molybdenum and tungsten and zeolites. (2) Another example 
of a host lattice structure is a two-dimensional structure consisting of a 
neutral layered unit as the building block. Between the layers a van der 
Waals gap exists representing, to a diffusing ion, an array of neighboring 
vacant lattice positions. Examples of this type of structure are the 
layered transition metal disulfides. (3) A third type structure is a 
one-dimensional structure composed of chain type units separated by a van 
der Waals gap providing neighboring lattice sites. Examples of this type 
are the transition metal trisulfides. Of the three types, the layered 
systems (2) offer the greatest flexibility for ion insertion. 
Klemann et al in U.S. Pat. No. 4,104,451 and U.S. Pat. No. 4,060,674 
disclose alkali metal anode/chalcogenide cathode reversible batteries 
having organometallic alkali metal salts in combination with organic 
solvents as electrolytes. Lamellar transition metal chalcogenides, 
particularly the dichalcogenides are preferred. Titanium disulfide is most 
preferred for use as a cathode in the disclosed cells. Nonaqueous 
electrolytes containing alkali metal salts of boron or aluminum containing 
organic groups are disclosed. 
In U.S. Pat. No. 4,069,372 to Voinov, cells are disclosed containing a 
solid mineral electrolyte capable of allowing selective migration of the 
anode metal in the form of cations. The electrolyte is coupled with a 
cathode capable of accepting electrons to form anions by cathodic 
reduction. Useful cathodes are disclosed as salts of transition metals 
such as a halide, an oxide, or a sulfide of a metal selected from iron, 
nickel, cobalt, chromium, copper or vanadium, i.e., ferrous chloride. The 
anode active material can be a metal from group Ia and IIa of the Periodic 
Table of the Elements. The electrolyte can be a salt of a metal of group 
Ia, IIa, IIb, or IIIb of the Periodic Table of the Elements. 
Higashi et al in U.S. Pat. No. 4,511,642 disclose organoborate salts of 
alkali metals represented by the formula: 
##STR2## 
in which R.sub.1 -R.sub.4 independently represent an alkyl group, an 
alkenyl group, a cycloalkyl group, an allyl group, an aryl group, a 
heterocyclic group, or a cyano group and M+ represents an alkali metal 
ion. 
Malpass in U.S. Pat. No. 4,231,896 and U.S. Pat. No. 4,325,840 discloses 
organomagnesium complexes which are hydrocarbon soluble and useful as 
co-catalysts in combination with conventional Zeigler catalysts for 
polymerizing olefins, etc. and as a source of ether-free diorganomagnesium 
compounds. 
In no one of these references is the electrochemical cell of the invention 
disclosed. In addition, there would be no suggestion for the use of an 
alkaline earth metal such as magnesium or calcium as an anode together 
with a nonaqueous organic solvent electrolyte containing an organometallic 
salt of an alkaline earth metal and a cathode capable of intercalation of 
an alkaline earth metal ion in view of the fact that the alkali metal 
anodes of the cells of the prior art are much more readily ionized than 
are alkaline earth metal anodes and therefore one skilled in the art would 
not expect that a cell containing an alkaline earth metal anode would 
provide suitable performance in comparison with a cell containing an 
alkali metal anode. Additionally, on recharge the cell must be capable of 
re-depositing the anode metal dissolved during discharge in a relatively 
pure state. 
SUMMARY OF THE INVENTION 
An electrochemical cell is disclosed which is capable of operation at 
ambient temperature and which contains an alkaline earth metal anode, an 
intercalation cathode capable of containing an intercalated ionic species, 
and a nonaqueous, liquid electrolyte wherein the electrolyte contains an 
organic solvent and an electrolytically active alkaline earth metal 
organometallic salt represented by the formulas: 
##STR3## 
wherein Z is selected from the group consisting of boron and aluminum and 
X is selected from phosphorus and arsenic, M is an alkaline earth metal, 
and R.sub.1 -R.sub.6 can be the same or different and are independently 
selected from the following unsubstituted or inertly substituted groups: 
alkyl, aryl, alkaryl, aralkyl, alkenyl, cycloalkyl, allyl, a heterocyclic 
alkyl group, and a cyano group with the proviso that R.sub.1 -R.sub.6 
cannot be all alkyl or all aryl and triarylalkylborate or aluminate anions 
and trialkylarylborate or aluminate anions are excluded.

DETAILED DESCRIPTION OF THE INVENTION AND THE PREFERRED EMBODIMENTS 
According to the present invention there is provided an electric current 
producing electrochemical cell which is capable of being recharged. The 
electrochemical cell contains an alkaline earth metal anode, a transition 
metal chalcogenide or boride cathode and a nonaqueous electrolyte 
containing an organometallic alkaline earth metal salt. The cell is 
operable at ambient temperature defined as about 0.degree. to about 
100.degree. C. The organometallic alkaline earth metal salt is a salt 
having the formula: 
##STR4## 
Z is selected from the group consisting of boron and aluminum and X is 
selected from phosphorus and arsenic, M is an alkaline earth metal, 
preferably magnesium or calcium, and R.sub.1 -R.sub.6 can be the same or 
different and are independently selected from the following unsubstituted 
or inertly substituted groups: alkyl, aryl, alkaryl, aralkyl, alkenyl, a 
cycloalkyl group, an allyl group, a heterocyclic group, and a cyano group 
with the proviso that R.sub.1 -R.sub.6 cannot be all alkyl or all aryl and 
triarylalkylborate or aluminate anions and trialkylarylborate or aluminate 
anions are excluded. By "inertly substituted" is meant radicals containing 
substituents which have no detrimental effect upon the electrolytic 
properties of the electrolyte composition with respect to effectiveness in 
an electrochemical cell, for instance, halogenated or partially 
halogenated derivatives of the above groups. Preferably said halogen is 
fluorine. Said aryl groups generally have 6-18 carbon atoms, preferably 6 
carbon atoms, and most preferably are phenyl groups, and said alkyl, 
alkenyl, cycloalkyl and allyl groups have 1-15, preferably 1-8 and, most 
preferably have 1-4 carbon atoms. Representative useful organometallic 
alkaline earth metal boron salts are as follows: magnesium 
dibutyldiphenylborate (1), magnesium (dicyclopentadienyl diphenyl) borate 
(2), magnesium dioctylbis (pentafluorophenyl) borate (3), magnesium 
dioctyl di(trifluoromethyl phenyl) borate (4). Homologous aluminate 
alkaline earth metal salts are useful. 
##STR5## 
Generally, the anode metal used can be an alkaline earth metal, preferably 
at least one of magnesium, calcium, strontium, or barium. It may be 
advantageous to use the anode metal in the form of an alloy with at least 
one other metal chosen from the metals belonging to groups Ia, IIa, IIb, 
and IIIb of the Periodic Table of the Elements. 
The cathode is generally a substance capable of accepting electrons to form 
anions by cathodic reduction and is characterized in that the cathode is a 
structure which is capable of accommodating, as an intercalated species, 
an ionized form of the metal from which the anode is formed. Thus the 
cathode can be characterized in that the active cathode material is a 
material capable of containing an intercalated species in its structure. 
The cathode generally can be at least one transition metal chalcogenide or 
transition metal boride including higher borides of transition metals of 
Groups IVa, Va, and VIa of the Periodic Table of the Elements where the 
boron to metal ratio is greater than 1.8. Preferably, the cathode is 
selected from the group consisting of RuO.sub.2, ZrS.sub.2, Mn2O.sub.3, 
Mn.sub.3 O.sub.4, V.sub.2 O.sub.5, Co.sub.3 O.sub.4, CrB.sub.2, VB.sub.2, 
NbB.sub.2, TiB.sub.2, ZrB.sub.2 and MoB.sub.2. Most preferably, the 
cathode active material is selected from the group consisting of 
RuO.sub.2, Co.sub.3 O.sub.4, ZrS.sub.2, and V.sub.2 O.sub.5. 
Where the cathode is not sufficiently capable of conducting electrons by 
itself, the cathode can be made electrically conducting by using an 
auxilliary conducting substance such as graphite or carbon. The cathode 
can be formed of at least one powdered transition metal chalcogenide or 
boride, as recited above, and a binder which is inert to the electrolyte, 
such as polytetrafluoroethylene. In forming the anode, the powdered 
materials, including a suitable proportion of binder, are formed into the 
anode structure by compressing the powdered materials, heating if 
necessary in order to flux the binder. 
The organic solvent employed as a portion of the electrolyte composition of 
the cell of the present invention is generally one having a dielectric 
constant of about 4 to about 10 and selected from substituted and 
unsubstituted ethers, esters, sulfones, organic sulfides, organic 
sulfates, organic nitrates, tertiary amines, and organic nitro compounds. 
Where substituted solvents are used, these are inertly substituted with 
respect to reaction conditions within the cell. Any of the foregoing 
solvents may function either as a diluent or as a complexing solvent with 
the organometallic alkaline earth metal salt portion of the electrolyte. 
Preferred useful solvents are selected from straight chain ethers, 
polyethers, and cyclic ethers and include such ethers as the acetals, 
ketals, and orthoesters. In addition organic esters, sulfones, organic 
nitro compounds, tertiary amines, and organic nitrites and organic 
sulfates and sulfites are useful. Representative useful solvents are 
furan, sulfolane, dimethylsulfite, nitrobenzene, N,N-dimethylaniline, and 
nitromethane. Representative examples of preferred organic solvents are 
tetrahydrofuran, 1,3-dioxolane, 1,2-dimethoxyethane, diethyl ether, 
trimethoxymethane and 2-methyl tetrahydrofuran. Most preferred solvents 
are 1,3-dioxolane, 1,2-dimethyloxyethane, tetrahydrofuran and mixtures of 
these solvents. An especially preferred solvent is one containing 
1,3-dioxolane. A more extensive listing of useful organic solvents can be 
found in U.S. Pat. No. 4,390,604, incorporated herein by reference. 
In the drawing, the FIGURE shows schematically one embodiment of the cell 
of the invention having a cell wall 1 in which a cathode 3 and an anode 2 
are contained. The cathode 3 comprises a cathode active material, Co.sub.3 
O.sub.4, which can intercalate magnesium ions. The anode 2 comprises 
magnesium. The nonaqueous electrolyte 4 is capable of allowing dissolving 
and replating of the magnesium anode during the operation of the cell. A 
specific example of a useful electrolyte is a 0.25 molar magnesium dibutyl 
diphenylborate salt dissolved in 70 percent by volume tetrahydrofuran and 
30 percent by volume dimethoxyethane. 
When a Co.sub.3 O.sub.4 cathode is coupled with a magnesium anode in one 
embodiment of an electrochemical cell of the invention, its half cell 
reaction is one of ion insertion. The reaction proceeds as follows: 
EQU XMg.sup.+2 +2Xe.sup.- +Co.sub.3 O.sub.4 .revreaction.Mg.sub.x Co.sub.3 
O.sub.4 
or more generally XM.sup.y+ +YXe.sup.- + Host.revreaction.M.sub.x.sup.y+ 
-Host.sup.(z-yx), where X=mole fraction or loading of the intercalated ion 
generally ranging up to 1.0, M=an alkaline earth metal ion, Y=oxidation 
number for the metal ion, Z=oxidation number for the host meal. 
Host=cathode material; for instance, Co.sub.3 O.sub.4, Mn.sub.2 O.sub.3, 
Mn.sub.3 O.sub.4, or V.sub.2 O.sub.5. M.sub.x.sup.y+ -Host.sup.(z-yx) 
=intercalated product. 
This reaction is reversible. The characteristic of an intercalation 
reaction is the formation of a ternary phase as the product. The phase 
formed during intercalation retains the physical structure of the parent, 
for instance, Co.sub.3 O.sub.4 with only minor variations being evident in 
the crystallographic lattice constants of the material. 
The intercalated product can comprise a ternary phase for magnesium ion 
proportions up to a maximum characteristic for each host structure and 
beyond which structural modifications take place in the host material 
producing multiple crystal phases with the intercalated ion. Structural 
charges are detrimental and hinder cathode reversibility. The operational 
range of magnesium loadings are expressed as moles magnesium per mole of 
cobalt oxide (Host). Throughout the compositional range of x=up to 0.9 
mole magnesium per mole of Co.sub.3 O.sub.4, a single ternary phase is 
produced. The preferred range for Co.sub.3 O.sub.4 is 0.7 to 0.8 mole of 
magnesium per mole of Co.sub.3 O.sub.4. 
The crystallographic structure of the ternary product can be an expanded or 
contracted form of its parent, i.e., Co.sub.3 O.sub.4 ; owing to the 
insertion or extraction of magnesium ions. The oxidation state of the 
transition metal (cobalt) can be reversibly altered by the amount of 
magnesium present in the ternary phase. For example, at the x=0.9 
composition, the cobalt ion has an effective oxidation state of +1.2 while 
at x=0 the ion is at its original oxidation state of +3. 
The cell discharge comprises the anodic oxidation of the Mg anode and the 
cathodic reduction of the cobalt oxide at the cathode. Magnesium dissolved 
at the anode diffuses into the electrolyte and through a porous separator 
(the process driven by magnesium ion concentration gradients). Magnesium 
ions at the surface of the cathode are absorbed to maintain electrical 
neutrality to counter the cathodic current at this electrode. The 
magnesium ion then diffuses into the bulk of the cathode to form a ternary 
phase with the cobalt oxide. This insertion of magnesium ions coupled with 
the cathodic current reduces the transition metal to complete the 
electrochemical mechanics of cell discharge. Cell discharge continues 
until the composition Mg .sub.0.9 Co.sub.3 O.sub.4 is reached. At about 
this point, the useful energy of the cell is exhausted. 
The following examples illustrate the various aspects of the invention but 
are not intended to limit its scope. Where not otherwise specified 
throughout this specification and claims, temperatures are given in 
degrees centigrade and parts, percentages, and proportions are by weight. 
EXAMPLE 1 
Magnesium dibutyldiphenylborate was prepared as follows: 
Diphenylbromoborate was dissolved in the amount of 7.4 milliliters in 40 
milliliters of tetrahydrofuran. The diphenylbromoborate was cooled to 
-78.degree. C. and the tetrahydrofuran was added. On warming, a reaction 
took place producing a solid tetrahydrofuran/diphenylbromoborate adduct. 
This product dissolves as the solution is warmed to room temperature with 
stirring. Thereafter, 56.3 milliliters of 0.71 molar dibutylmagnesium in 
hexane was added after first cooling the diphenylbromoborate solution in a 
dry ice/acetone bath. The reaction mixture was allowed to warm to room 
temperature with stirring and stirring was continued for one hour. The 
solvent was evaporated by distillation under reduced pressure and the 
residue was redissolved in 40 milliliters of tetrahydrofuran. Thereafter, 
20 milliliters of 1,4-dioxane were added and the mixture was stirred 
overnight. Substantial amounts of a white precipitate had formed after 
this time. The mixture was decanted onto a fine glass frit and the solvent 
was removed by filtration. As the filtrate was collected, two liquid 
phases formed and some of the original precipitate dissolved. The bottom 
layer appears to be the desired product. After stirring the filtrate 
mixture overnight, a precipitate formed. Analysis showed that this was 
primarily the desired product, magnesium diphenyldibutylboron. 
EXAMPLE 2 
A cell having a cathode material of Co.sub.3 O.sub.4, an anode consisting 
of magnesium sheet, and an electrolyte comprising magnesium 
dibutyldiphenylborate (0.25 molar in 70 percent by volume tetrahydrofuran 
and 30 percent by volume dimethyoxyethane was constructed to show that 
magnesium dibutyldiphenylborate as an electrolyte will support cell 
discharge, i.e., intercalation of magnesium ions into the cathode material 
and recharge, i.e., removal of intercalated magnesium ions from the 
cathode and plating of magnesium on the anode. The cell cathode of cobalt 
oxide consisted of 75 percent by weight cobalt oxide, 15 percent by weight 
carbon black, 10 percent by weight of polytetraflurorethylene. The cathode 
was rectangular in shape and had a area of 3.2 square centimeters, an 
active cobalt oxide mass of 36 milligrams and a capacity of 7.9 
milliampere hours. In constructing the cell, the cathode was wrapped in 
one layer of separator; the assembly consisting of one cathode and one 
anode. This assembly was inserted into a polyethylene bag which was heat 
sealed and sandwiched between two glass plates. The cell was tested at 
constant current in an inert atmosphere. Voltage versus time traces were 
recorded on a standard laboratory chart recorder. The pertinent cell 
parameters from this test are as follows: discharge current was 0.87 
milliamps. Open circuit voltage was 1.2 volts. The delivered capacity was 
6.9 milliampere hours. The current efficiency was 91 percent. The 
operational voltage was 0.37 volt. The cathode utilization was 88 percent. 
EXAMPLE 3 
(control forming no part of this invention) 
Magnesium tributylphenyl borate was prepared as follows: Diphenylmagnesium 
was prepared by the metathetical reaction of diphenylmercury with 
activated magnesium. 
##STR6## 
1.52 g of Mg (621/2 mmol) and 3.3 g of HgCl.sub.2 (121/2 mmole) were 
stirred together in THF. The following reaction was expected: 
EQU HgCl.sub.2 +Mg.fwdarw.MgCl.sub.2 +Hg.degree. 
The white precipitate which formed (MgCl.sub.2) was removed by washing the 
black residue with six 25-ml portions of tetrahydrofuran (THF) and drying 
under vacuum. 
To the activated magnesium (magnesium amalgam), thus produced, was added 
7.10 g (20 mmol) of diphenyl mercury, Ph.sub.2 Hg. THF was added to give a 
total volume of 25 ml. The mixture was stirred for 2 hours whereupon IR 
spectroscopy indicated complete reaction to diphenyl magnesium. (Some 
preparations required heating to 40.degree. to initiate the reaction.) The 
mixture was filtered through diatomaceous earth to yield a yellow 
solution, slightly cloudy with a black material which was thought to be 
magnesium. This was allowed to settle and the yellow solution was decanted 
off. All reactions were performed in an atmosphere of purified argon. 
Tributylboron solution 40 ml (BBu.sub.3 1 mola in Hexane) was pipetted into 
a reaction vessel. The diphenyl magnesium prepared above was slowly added 
with stirring. The mixture was stirred for one hour after the addition was 
complete. The slightly cloudy solution was filtered through diatomaceous 
earth and evaporated to dryness under reduced pressure yielding a white 
powder of magnesium tributylphenylborate. 
The compound Mg[R.sub.3 R'B].sub.2, where R-butyl and R'=phenyl when 
evaluated as an electrolyte salt in a cell made in accordance with that 
described in Example 1, does not permit the intercalation reaction of Mg 
into a Co.sub.3 O.sub.4 cathode material. With a RuO.sub.2 cathode, the 
intercalation of Mg ions does take place but only to an extent of about 5% 
of the potential capacity of the cathode, thus rendering this electrolyte 
salt essentially useless. Magnesium has however been successfully 
electrodeposited onto a magnesium anode from solutions of this compound in 
ethers. 
EXAMPLES 4-6 
(controls, forming no part of this invention) 
The compounds Mg[R.sub.3 R'B].sub.2 where R=phenyl and R'=butyl; R and 
R'=phenyl; and R and R'=butyl when evaluated as electrolyte salts in a 
cell made in accordance with that described in Example 1 do not permit 
both intercalation of magnesium ions into the cathode and plating of 
magnesium on the anode. The tetrabutylborate salt permitted plating of 
magnesium but not intercalation. The tetraphenylborate salt permitted 
intercalation but not plating. The triphenylbutylborate salt permitted 
intercalation but not plating. 
While this invention has been described with reference to certain 
embodiments, it will be recognized by those skilled in the art that many 
variations are possible without departing from the spirit and scope of the 
invention and it will be understood that it is intended to cover all 
changes and modifications of the invention disclosed herein for the 
purpsoes of illustration which do not constitute departures from the 
spirit and scope of the invention.